Catalysis by Doped Oxides - Chemical Reviews (ACS Publications)

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Catalysis by Doped Oxides Eric W. McFarland† and Horia Metiu*,‡ Department of Chemical Engineering, and ‡Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States

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9.1. Introduction 9.2. The Effect of LVDs on the Energy of OxygenVacancy Formation 9.3. The Presence of an LVD Makes the Surface More Reactive 9.3.1. General Comments 9.3.2. Methane Activation: Does the Brønsted−Evans−Polanyi Rule Work For It? 9.4. The Electronic Structure of an Oxide Doped with an LVD 9.5. Comparison with Experiments 10. The Effect of High-Valence Dopants (HVDs) 10.1. Introduction 10.2. HVDs in Irreducible Oxides 10.3. HVDs in Reducible Oxides 10.4. The Stability of High-Valence Dopants 11. Same-Valence Dopants (SVDs) and FlexibleValence Dopants (FVDs) 12. Two Case Studies: Oxidative Dehydrogenation of Ethylbenzene and Ethane 12.1. Introduction 12.2. Oxidative Dehydrogenation: Generalities 13. The Oxidative Dehydrogenation of Ethylbenzene to Styrene 14. Oxidative Dehydrogenation of Ethylbenzene to Styrene Using CO2 as an Oxidant 14.1. Introduction 14.2. Ethylbenzene Dehydrogenation in the Presence of CO2, Catalyzed by Doped Oxides 14.2.1. Experimental Results 14.2.2. Are These Results Consistent with What We Know from Calculations? 15. Oxidative Dehydrogenation of Ethylbenzene to Styrene by Using Oxygen and Doped Oxide Catalysts 16. Oxidative Dehydrogenation of Ethane to Ethylene 17. Background Information on NiO 18. Ethane Oxidative Dehydrogenation with Oxygen Catalyzed by Zr-Doped NiO 19. Ethane Oxidative Dehydrogenation to Ethylene Catalyzed by Nb-Doped NiO 19.1. Background Information about Nb and Its Oxides

CONTENTS 1. Introduction 2. Historical Note 3. Synthesis Methods for Doped Oxides 3.1. Introduction 3.2. Solid-State Synthesis 3.3. Incipient Wet Impregnation 3.4. Coprecipitation 3.5. Sol−Gel Synthesis 3.6. Combustion Synthesis 3.7. Ultrasonic Spray Pyrolysis (USP) 3.8. Electrochemical Synthesis 4. Characterization Methods for Doped Oxides 4.1. Introduction 4.2. X-ray Diffraction (XRD) 4.3. Synchrotron Radiation 4.4. X-ray Photoelectron Spectroscopy (XPS) 4.5. X-ray Fluorescence Spectroscopy (XRF) 4.6. Atomic Absorption Spectroscopy (AAS) 4.7. Ultraviolet Optical Spectroscopy (UV−Vis) 4.8. Raman Spectroscopy 4.9. Fourier Transformed Infrared Spectroscopy (FTIR) 4.10. Probing Metal Oxides with Electrons 4.11. Low-Energy Ion Scattering (LEIS) 4.12. Secondary Ion Mass Spectrometry (SIMS) 4.13. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) 4.14. Temperature Programmed Desorption (TPD) and Temperature Programmed Reaction (TPR) 5. Computational Methods 6. Strong Acid−Base Interactions 7. A Classification of Dopant−Oxide Pairs 8. The Mars−Van Krevelen Mechanism, the Oxygen Vacancy Formation, and Doping 9. Oxide Catalysts Doped with Low-Valence Dopants (LVDs) © 2013 American Chemical Society

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Special Issue: 2013 Surface Chemistry of Oxides

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Received: October 19, 2012 Published: January 27, 2013 4391

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Chemical Reviews 19.2. Computational Results on Nb-Doped NiO 19.3. The Preparation of NiO and Nb-Doped NiO 19.4. Catalytic Performance of NiO for ODH of Ethane to Ethylene 19.5. Catalytic Performance of Nb-Doped NiO for Ethane ODH 19.6. NbOx Clusters Supported on NiO 19.7. Long-Term Stability of the Nb-Doped NiO Catalyst 19.8. The Influence of Doping and of the Method of Preparation on Surface Area per Gram 19.9. Chemical Characterization of Nb-Doped NiO 19.10. Ethane ODH by NiO Doped with Li, Mg, Al, Ga, Ti, Ta, or Nb 19.11. Ethane ODH on Doped NiO: Conclusions 20. Brief Concluding Remarks Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

In this Review, we discuss only oxidation catalysis by substitutional cation doping of binary oxides (oxides with the formula MexOy), and leave out complex oxides (e.g., perovskites, vanadates). This is an emerging field, which is rapidly evolving and is very far from maturity; therefore, it is not advisable to attempt the kind of exhaustive review typical of mature fields. Instead, we concentrate on the main ideas and the major difficulties of the field, try to sort out its prospects, suggest future lines of research, and examine the extent to which computations aid the search for better doped-oxide catalysts. Rather than attempt to cover all work on catalysis by doped oxides, which is rather vast and fragmentary, we examine in some depth a few examples on which data are more abundant and the research is more extensive and more systematic. One example deals with the oxidative dehydrogenation (ODH) of ethylbenzene to styrene, by using oxygen or CO2 as oxidant. The other example examines the ODH of ethane with oxygen. We decided that piling up more examples would not bring more clarity to our understanding of the subject. This decision was made easier by our awareness of the finite nature of our time, energy, and intellectual resources. We note that we present scientific research rather than focus on efforts directed toward finding the best commercial catalyst. This means that some aspects that are essential for deciding whether a catalyst is commercially promising are not addressed in this article. An excellent discussion of the criteria that a commercial catalyst for oxidative dehydrogenation must meet can be found in Chapter 5 of the book by Centi, Cavani, and Trifiro.7 As we worked on this article, several themes became central. On the experimental side two facts imposed themselves on us: The performance depends strongly on the method of preparation; and it is difficult to provide convincing proof that a doped oxide has been prepared. For this reason, we devoted a section to the methods of preparation and one to the methods of characterization. There has been considerable computational work on various properties of doped oxides. There are inherent limitations in this work. First, we know that the computational methods usable in practice do not give accurate total energies. Second, we do not know, at the atomic level, the morphology or the composition of an oxide catalyst under working conditions. The latter is a shortcoming of the experiments. We could perform useful calculations on any morphology or any surface composition, if we knew what they are. Unfortunately, experiments are not likely to provide this information soon, despite the great progress made lately in operando microscopy and spectroscopy. For these reasons, we believe that computations are most useful, at this time, if they look for qualitative trends and qualitative rules for catalyst design. They should help increase the probability of finding better oxide catalysts. This opinion has guided the way we examined the computations and organized the presentation of their results. The calculations performed so far provide a classification of the dopant−oxide pairs based on the common properties obtained by computations. One of the questions we ask in this Review is: Are there hints in the experiments that these rules are obeyed? Given the uncertainties in the experiments, we can only say that many experimental results are in accord with the qualitative suggestions made by computations.

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1. INTRODUCTION Metal oxide catalysts have numerous industrial applications.1−7 In addition, oxides catalyze many important reactions whose conversion or selectivity is too low to be commercially interesting. We review here experiments and calculations whose goal was to improve the catalytic activity of an oxide by substituting a small fraction of the cations of a “host oxide” with a different cation. We call this substitutional doping or doping. The substitution disrupts chemical bonding at the surface of the host oxide, and optimists hope that this will modify favorably its catalytic activity. The active centers in such a system could be either the oxygen atoms near the dopant or the dopant itself. This is not the only kind of “creative disturbance” that can activate the surface of an oxide. The catalytic activity of a small (molecular size) cluster MeOx (where Me is a cation, O is oxygen, and x is usually unknown) supported on the surface of another oxide (or any other insulator) is often very different from that of the bulk oxide of Me. For example, VOx clusters supported on ceria8,9 are more active for methanol oxidation (and many other reactions) than either V2O5 or CeO2. The socalled “inverse catalysts” are similar. They consist of a very thin oxide film supported on a metal surface.10−30 In this system, as in the other two mentioned above, we place the oxygen atoms in unusual bonding situations with the intention of improving their ability to catalyze oxidation reactions. Substitutional cation doping is not the only possibility. One can imagine that replacing some anions with other anions may also be beneficial. There is evidence that the presence of small amounts of halogen in the feed or on the oxide surface improves its catalytic activity (more often the selectivity).31−53 We do not know of any work that has proven that the improvements come from the substitution of some of the oxygen atoms with halogens. Anion doping is beneficial for changing the optical properties of photocatalysts as reviewed recently by Kudo.54 4392


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2. HISTORICAL NOTE It is very likely that doped oxide catalysts have been used before the concept was formulated explicitly. Most oxide catalysts have low levels of impurities that may be substitutional dopants; if they segregate at the surface, they can affect the catalytic activity without our knowledge even though their net concentration is very low. Furthermore, most industrial catalysts have small amounts of “additives” that prevent coarsening, provide mechanical stability, prevent the evaporation of the catalyst, increase conversion, increase selectivity, or “poison” an unwanted reaction (one way of achieving selectivity). It is possible that catalysis by doped oxides is as old as catalysis by oxides, but we were not aware of the fact. It is not our intention to provide an accurate or comprehensive history of the subject. Paravano55−57 seems to be the first scientist who prepared substitutionally doped oxide intentionally, to boost the performance of an oxide catalyst. He studied the oxidation of CO by NiO with a variety of dopants. While at that time it was not possible to prove that doped oxides were prepared, at least the intention was to do so. The inspiration came from work that showed that doping changes the electronic properties of oxides (mainly the conductivity and the adsorption spectrum). The premise was that an additive that changed conductivity was likely to affect catalytic chemistry, especially for reactions in which electron transfer might be involved. Cimino et al.58,59 studied N2O decomposition on “solid-solutions” of NiO and MgO; by solidsolutions, they meant substitutional doping. They hoped that by “diluting” the Ni ions in the surface of NiO, they can modify the catalytic activity of Ni. The idea of using substitutional doping of oxides in catalysis languished until around 1985 when Lunsford60 showed that Lidoped MgO is a selective catalyst for oxidative coupling of methane to ethylene. This was followed by a large number of studies61−77 in which very stable oxides (e.g., MgO, CaO, La2O3, Sm2O3, etc.) were doped with low-valence dopants to perform oxidative coupling of methane. Several older reviews of this subject are available.78−80 We will not review oxidative methane-coupling by oxides containing alkali dopants for two reasons. First, this reaction is performed at very high temperature (between 600 and 900 °C), and the structure of the doped-oxide catalyst is affected by this.81 Therefore, this is not a good system for comparison to theory. Second, two excellent papers have reviewed the subject recently,81,82 and we have nothing to add to their analysis. We are aware of only two articles that review catalytic chemistry of doped oxides (besides the ones mentioned in connection with oxidative methane coupling). One, by Cimino and Stone,83 is mainly concerned with oxides more complex than the binary oxides studied here. The other, by Hegde,84 concentrates on the considerable amount of work done in his group in Bangalore.

No matter which method is used, it is difficult to prove that the material synthesized is a doped oxide with the dopant in the surface or the subsurface layer (which is the model used in calculations). While doped oxides are targeted, the preparation method may produce a system whose surface consists of very small oxide clusters of the dopant, supported on the surface of the host oxide. Such clusters are catalytically active.8,9 Being very small, they have physical and chemical properties different from those of the dopant’s bulk oxide. Because of this, it is difficult to distinguish them from a doped oxide. It is also possible that the “as-prepared” catalyst is a doped oxide that, under reducing reaction conditions, is converted to very small metallic dopant clusters supported on the host oxide. The physical and chemical properties of such clusters are different from those of a bulk metal, and it is difficult to distinguish them from a doped oxide. Our preferred strategy, for establishing that a doped oxide was obtained, is to use three methods of preparation: one that aims at making a doped oxide, one that intends to make small dopant−oxide clusters on the host oxide, and one that makes very small metallic clusters of the dopant, supported on the host oxide. If these three preparations have different chemical and physical properties, we are more confident that one of them is a doped oxide. It is likely that many doped oxides are thermodynamically metastable. If this is the case, the preparation method should be designed to prevent the system from going to equilibrium. It is more likely that this goal is reached if the preparation method starts with a homogeneous mixture of the precursors and the calcination temperature is low enough to prevent dopant diffusion and aggregation. Finally, we note that the “same” doped oxides (same dopant, same host oxide, same proportions) synthesized using different precursors (e.g., chloride, or nitrate, or acetate, or oxalate, etc.), or different calcination temperature and time, often have very different catalytic activities.90,91 These uncertainties in the outcome of the preparation also appear in work in which the bulk properties of a doped oxide are utilized. Work with doped YAG (yttrium aluminum garnet) metal oxide phosphors, made by a solid-state method, or by coprecipitation, or by sol−gel, has shown that even if a “homogeneous” doped oxide is produced, the impurities, the defects, and the morphology are different and depend on the precursors used and the synthesis method. These often lead to differences in physical properties and performance.92 3.2. Solid-State Synthesis

Molecular compounds (salts, oxides, etc.) of the host oxide and of the dopant are mechanically intermixed as solid particulates and then heated to high temperature, in oxygen, to allow the dopant to diffuse in the host oxide. This method is suitable when the doped oxide is the thermodynamically stable phase. The method starts with two phases, and the rate of forming a homogeneous phase increases when the particles are smaller and the temperature is higher. There are only a few examples where a substantial effort was made to prove that a doped oxide was prepared. In one example,93 a mixture of La2O3, SrCO3, TiO2, and Ta2O5 was prepared in the desired stoichiometry, then ball-milled in ethanol with zirconia balls for 12 h. The wet mixture was dried and calcined in air at 1350 °C for 6 h. Further ball-milling for 12 h followed by a final heating in air at 1460 °C formed a solid solution of Sr0.9La0.1Ti1−xTaxO3. XRD measurements for materials with x = 0.00, 0.01, 0.03, 0.05

3. SYNTHESIS METHODS FOR DOPED OXIDES 3.1. Introduction

There is an extensive literature on catalyst preparation85−89 to which we add several books edited by Pernicone and collaborators. We review here those methods of preparation that were used to make doped oxides and selected examples in which an effort was made to prove that doped oxides were obtained and that their physical and chemical properties differ from those of the undoped host oxide. 4393


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of the combustion method) and compared the catalytic performance of the solids obtained by the two methods, for the selective oxidation of CO. The coprecipitation method started with an aqueous solution of Cu(NO3)2·3H2O and Ce(NO3)2·6H2O to which a Na2CO3 solution was added dropwise, under continuous stirring. The pH was kept below 6.0. The precipitate was filtered, washed, dried at 110 °C for 12 h, and calcined for 4 h in flowing air at 650 °C. While the Cu/ (Cu+Ce) atomic ratio in the precursor solution was 0.143, the same ratio in the surface region (derived from XPS) was 0.61; Cu accumulates in the surface region probed by XPS. CuO XRD peaks are observed in the sample prepared by coprecipitation or impregnation, but not in the one prepared by combustion. Other experiments have shown that one is not guaranteed to obtain a doped oxide by coprecipitation. Muhammed and coworkers97 prepared 10% doped ceria catalysts for oxygen storage by coprecipitation of oxalates. Eleven different dopants were investigated. All samples formed doped oxides except for the one that attempted to use Co as a dopant but ended with a separate Co3O4 phase. Teller et al.98 synthesized Sb2O4 doped with Mo (1.5 at. %) or V (5 at. %) by coprecipitation and by intermixing and grinding the two oxides. They found no difference in the properties of the catalysts prepared by the two methods. Neutron diffraction showed that the Sb2O4 lattice was unchanged, and EXAFS showed the Mo atoms were surrounded by 3−5 nearest O atoms and gave bond lengths that excluded the possibility that Mo is sitting at Sb site. They concluded that Mo is trapped interstitially in the Sb oxide. The same measurements were used to conclude that the V atom substitutes the asymmetrical site of Sb. The fact that intermixing and grinding produces a material with the same properties as coprecipitation is not entirely unexpected; one can think that coprecipitation is just another way of mixing the two solids.

detected a single-phase strontium titanate structure, with a lattice constant shift consistent with substitutional doping. 3.3. Incipient Wet Impregnation

Wet impregnation exposes the host oxide to a liquid containing the precursor of the dopant. The mixture is then dried and heated in air. It is hoped that heating causes surface doping rather than a separate phase of the oxide of the dopant or a submonolayer of the dopant oxide on the host oxide. Copper-doped ceria catalysts were prepared by impregnating ceria (prepared by calcining ceria nitrate in air) with a copper nitrate solution. The particles were dried at 120 °C for 12 h and calcined in air, at 500 °C, for 4 h.94 When the Cu loading was below 3% by weight, no crystalline copper oxide phases were observed in XRD. Rietveld analysis determined that the material had ceria structure with a small lattice constant shift, which is considered evidence that some Cu ions substituted Ce ions in the CeO2 lattice. Above 3% Cu loading, phase separation was observed. The amount of copper oxide phase increased with the calcination temperature. The doped material was more active for CO oxidation than the undoped oxide. This method of preparation is not always successful. An example of “failure” is provided by the following preparation of Ni-doped ceria.95 A 0.5 M solution of Ni(NO3) in water was added to ceria powder. The suspension was stirred at room temperature for 0.5 h to make sure that the pores of ceria are filled with liquid. After being stirred, the solvent was evaporated at 110 °C and dried for 12 h. The resulting solid was calcined at 450 °C in air for 5 h. For low nominal Ni concentration, the XRD showed only the fluoride structure on CeO2 with no shift. This was considered an indication that no Ni is present in the bulk of the particles. The XPS signals are similar to those of bulk NiO. The catalytic activity of the Ni−CeO2 system for ethane ODH is much higher than that of CeO2 or NiO. It is assumed that the catalyst consists of NiOx spread on the ceria surface, although the presence of substitutionally doped Ni could not be excluded.

3.5. Sol−Gel Synthesis

3.4. Coprecipitation

In this method, the precursors are chosen so that they make a gel having a uniform distribution of the cations (of the host oxide and of the dopant). Liu et al.99 have prepared Ce1−xFexO2 (x = 0.1−0.5) by the citric acid sol−gel method. Citric acid powder was added to a solution of cerium and iron nitrate. The water was evaporated at 80 °C with stirring, and the resulting gel was dried (in this particular example, microwaves were used) and calcined at 700 °C, in air, for 4 h. The XRD measurements showed that the material with x = 0.1 has ceria structure. The UV−vis spectra had no signal typical of iron oxide. Segregation of Fe3O4 is observed for x > 0.1. Temperature programmed reduction with hydrogen shows that Ce0.9Fe0.1O2 is more reducible than CeO2 and is a better methane combustion catalyst than either undoped ceria or all of the other iron containing preparations (i.e., those with x > 0.1). Another interesting example is provided by work in Leffert’s group. Solutions of Mg(OCH3)2 in methanol (0.4 M), containing sufficient LiNO3 to obtain 0, 1, 3, and 5 wt % Li in MgO, were mixed with a water−methanol solution (0.8 M of water). The mixture was kept for 24 h at room temperature to form a wet gel. The gel was dried at 50 °C, in vacuum, for 7 h, then calcined in air for 1 h. The XRD measurements detected a Li2CO3 phase, which is hardly detectable in the 1 wt % material but clearly visible in the others. However, not all Li forms a carbonate. For the 1 wt % sample, 40% of the Li present in precursor was incorporated in MgO, presumably as a

This method starts with a solution of salts of the host cation and of the dopant, which are treated with a chemical that precipitates simultaneously both cations. The precipitate is washed, dried, and calcined at high temperature. An alternative is to precipitate the solid salts by evaporating the solvent. An interesting example of this method is provided by the work of Zhao and Gorte.96 They prepared a large number of doped ceria catalysts and used them for n-butane combustion. The coprecipitation was carried out by dissolving Ce(NO3)4 together with a precursor for the dopant, which was either a nitrate (for Gd, Y, La, Sm, Yb, Pr dopants), or a chloride (for Nb, Ta dopants), or an oxynitrate ZrO(NO3)2 (for Zr dopant). The solvent was evaporated, which caused coprecipitation, and the residue was calcined at 600 °C in air. The Nb-doped ceria was also prepared by the coprecipitation of an aqueous solution of Ce(NO3)4 and NbCl5 by treatment with NH4OH. The precipitate was washed, dried, and calcined in air at 600 °C. The two Nb-doped ceria samples have different n-butane combustion rates: the rate for the sample made from hydroxide was ∼5% higher. The effective activation energies were also different (85 kJ/mol for the sample obtained from the coprecipitated hydroxides and 105 kJ/mol for the other sample). Avgouropoulos et al.91 prepared Cu-doped CeO2 by coprecipitation and by combustion (see below a description 4394


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solution containing the desired precursors. This can be rapidly heated in a furnace (see Figure 2) to form doped oxides. Misch

substitutional dopant. The 5 wt % sample incorporated 16% of the Li. The 1 wt % material is a fairly good catalyst for propane ODH at 650 °C, having 45% conversion of propane and 73% selectivity for a mixture of propene and ethylene. A particularly valuable aspect of this work is the analysis of the process of gel formation. 3.6. Combustion Synthesis

In this method, a suitable organic fuel is added to a saturated aqueous solution of the desired metal salts. This mixture is heated until it ignites, causing a rapid, self-sustaining combustion reaction (see Figure 1), which produces a fine,

Figure 2. The apparatus for ultrasonic spray pyrolysis (USP). Reprinted with permission from ref 107. Copyright 2006 American Chemical Society.

et al.108 prepared, by this method, Pd-doped ceria and used it to catalyze methane partial oxidation and dry reforming. The precursor solution consisted of Ce(NO3)3·6H2O and Pd(NO3)2·2H2O dissolved in water in the appropriate molar ratios. The solution was nebulized, and the mist was carried by compressed air into a furnace held at 500 °C. The powders produced in the furnace were collected in sparged liquid traps containing 4:1 H2O/EtOH. The solvent was evaporated at 80 °C overnight. The XRD measurements indicated that Pd-doped ceria was produced. However, under reaction conditions the catalyst evolved into metallic Pd clusters supported on ceria.

Figure 1. Combustion synthesis. Reprinted with permission from ref 84. Copyright 2009 American Chemical Society.

3.8. Electrochemical Synthesis

This method is not commonly employed for synthesizing solid heterogeneous catalysts, but it has been used to produce doped metal−oxide photoelectrocatalysts, and different characterization methods have “proved” that doped oxides can be produced. Both cathodic and anodic depositions have been used, and the final doped oxide is formed by calcination. In an investigation of iron oxide doped with 0−0.5 at. % aluminum, Kleiman-Shwarsctein et al. deposited films from electrolytes with mixtures of iron and aluminum chloride salts using cyclic voltammetry with the applied voltage ramped.109 A hydroxide solid film was deposited, which was calcined at 700 °C in air. Xray diffractometry showed that the Fe2O3 films were singlephase hematite. Considerable morphological difference between the doped and undoped films was seen in SEM. A similar synthesis was performed by the same authors110 to produce Fe2O3 doped with Mo and Cr and by Hu111 for Fe2O3 doped with Pt. In Hu’s work, in addition to XRD, depth profiling by ion etching of the material while performing XPS showed that Pt was homogeneously distributed in the oxide. None of these methods are guaranteed to produce doped oxides, but all of them will work if the preparation conditions are chosen with care. It seems to us that the combustion method and the ultrasonic spray pyrolysis are more likely to make a doped oxide, because the solid is formed very rapidly and there is no time for dopant segregation. In most cases, the chance of making a doped oxide is substantially increased if the dopant concentration is low. While for every system there is an optimum dopant concentration, in most cases concentrations higher than 0.3 molar fraction lead to ill-defined and inactive materials.

dry, powder. Patil and other research groups have carried out the synthesis of various single, doped, and mixed ceramic oxides using as fuels urea, hydrazines, glycine, methylpyrazoles, or diformylhydrazine.100,101 Hegde and colleagues have used combustion synthesis to prepare several doped ceria catalysts.102,103 One of them, Pd-doped ceria, was characterized by XPS (ESCA), XRD, and EXAFS, and no evidence of mixed phases was detected.102 Bera et al.104 synthesized 3 at. % Cudoped ceria catalysts for NO reduction with NH3 or CO, using combustion synthesis from solutions of (NH4)2Ce(NO3)6, Cu(CO3)2, Cu(OH)2, and C2H6N4O2 (oxalyldihydrazide, as the fuel). Characterization using Rietveld refined XRD, EPR, XPS, and EXAFS was consistent with the presence of a doped oxide alone. Copper-doped ceria was also synthesized by Avgouropoulos et al.105 using urea as the fuel for a nitratecontaining solution, and by Rao et al.106 using hydrogen peroxide for fuel. The materials made by Rao’s team were characterized by XRD, EPR, and temperature programmed reduction (TPR) and, in contrast to the materials made using oxalyldihydrazide as the fuel, showed finely dispersed Cu2+ species on the surface at low copper content, but Cu2+ in the form of dimers and clusters at large Cu concentrations. It was concluded that little of the Cu2+ was in the bulk. Athawale et al. used microwave heating to initiate combustion synthesis of silver-doped lanthanum chromite catalysts, with urea as fuel.101 Urea fuel is appealing because it contains no carbon. 3.7. Ultrasonic Spray Pyrolysis (USP)

Ultrasonic nebulization was introduced by Skrabalak and Suslik107 who used ultrasound to produce droplets of the 4395


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Table 1. Characterization Methods for Doped Metal Oxides surface specific photons

electrons

atoms/chemical

XRD EXAFS, NEXAFS/XANES XPS XRF AAS UV−vis Raman FTIR DSC/TGA SEM TEM EELS EDS SIMS LEIS TPR/TPD

no no yes no no no no adsorbates − yes no yes yes yes yes yes yes yes

resolution

notes

synchrotron ΔE < 0.1 eV inelastic MFP ∼1−10 nm, >1 at. % Z > 10

0.1 eV (TEM) inelastic MFP

112, 114 115, 116 117, 119 101, 122

113 116 118 120, 121

123

NH4,H2,CO,18O

4. CHARACTERIZATION METHODS FOR DOPED OXIDES

refs

124 115, 125 126, 127

that a doped oxide has been prepared. XRD can identify whether or not a single crystalline phase is present to within the resolution of the instruments (typically >1%). Peak widths are fit to the Debye−Scherrer equation to determine crystallite size. Shifts due to substitutional doping are often interpreted using Vegard’s rule, which states that there is a linear relationship between the concentration of a substitutional dopant and the change in the lattice parameter. There is no theoretical basis for Vegard’s rule, and many violations have been documented. As an added complication, the lattice parameters of small nanocrystals depend on crystallite size, and one is not sure whether the observed lattice parameters shift is due to doping or to the small size of the crystallites.

4.1. Introduction

The substitution of a dopant atom in a metal oxide host, even at very small concentrations, can have profound effects on the properties of the host, including alterations in electronic and transport properties, morphology changes, and lower phase transition temperature. Ultimately, when making doped-oxide catalysts, we hope to improve their functional performance. If the performance is improved, we would like to show that the improvement is due to dopant substitution so that we might develop a more general understanding of the connection between dopant−oxide pairing and catalytic activity. The methods of characterization need to answer the following questions. Is the active catalyst a homogeneously doped oxide with some dopant in the surface layer? Are we sure that the dopant did not make very small oxide clusters on the surface? Can we rule out the presence of two phases? Are we sure that catalytic activity is not due to impurities in the precursors? What differences in activity are caused by different methods of preparation of doped oxides with the same nominal composition? It is difficult to establish beyond doubt that (1) a true homogeneously “doped” oxide has been synthesized, and (2) it is the homogeneously doped phase that is responsible for the observed catalytic activity. Below we discuss methods that have been frequently used in research on catalysis by doped oxides to help answer the questions posed above. Because we are interested in catalysis, our primary interest is in the properties of the surface. Nevertheless, it is helpful to show (i) that a material that is homogenously doped in the bulk has been synthesized (e.g., by XRD), and (ii) that the surface composition is the same or different from that of the bulk (e.g., by XPS). We will mention only characterization methods that use photons, electrons, or chemical behavior (Table 1).

4.3. Synchrotron Radiation

The synchrotron provides tunable X-ray radiation having 6 orders of magnitude greater intensity than laboratory sources. The frequency is tunable, and ultrafast time-resolved pulses of X-ray beams with small angular divergence are available. The beamline spectrometers are routinely capable of resolution below 0.1 eV and below 1 meV in certain cases. High intensity allows the performance of X-ray diffraction in situ, on a time scale comparable to that on which structural changes occur in a catalyst. For example, Rodriguez’s group128 has used this technique to show that metallic Ni clusters are formed from NixCe1−xO2, under reaction conditions. The monochromatic, high-intensity, coherent X-ray beams allow the investigation of solid-state catalysts both ex situ and in situ by examining the various aspects of energy dependent Xray absorption. Different techniques explore different energy regimes. XANES (X-ray absorption near-edge spectroscopy) and NEXAFS (near edge X-ray adsorption fine structure) refer to the same measurements but different energy ranges.129,130 An X-ray photon is adsorbed to make a core hole, and as a result the sample emits photons and Auger electrons, whose energy and intensity are measured. The technique provides information about the empty states of the system, the oxidation state of the atom, and its coordination. EXAFS (extended X-ray absorption fine structure) is used to determine the geometry and the chemical composition of the atoms surrounding the atom that emitted the photoelectron.129,130 In studies of ceria doped with Ba, La, Y, Hf, or Zn, used as a catalyst for the water−gas shift reaction, Linganiso and co-

4.2. X-ray Diffraction (XRD)

It is generally accepted that a doped oxide should have the structure of the host oxide (e.g., Ti-doped ceria should have the fluoride structure of the undoped ceria). Rietveld analysis is used to determine whether doping caused a small shift in the lattice parameter. These are necessary conditions for “proving” 4396


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4.6. Atomic Absorption Spectroscopy (AAS)

workers used XANES under TPR reaction conditions to show that the dopants increased the reducibility of ceria with hydrogen.112 In work with K-doped MoVSbO, for selective oxidation of propane and propylene, Balsko et al.113 were able to demonstrate with XANES that the presence of the potassium on the surface resulted in partial oxidation of surface Sb3+ to Sb5+; however, despite the use of FTIR, XRD, XPS, and EPR, in addition to XANES, there was no unequivocal evidence presented that the catalyst was atomically doped with K rather than intermixed with potassium oxides present on the surface. EXAFS has been used to study the bulk properties of doped oxides, and it is capable of providing the location of the dopant in the solid (e.g., substitutional or interstitial) and the positions of its neighbors.102,104,131−134

In this technique, light absorption is used to identify the atoms produced by the atomization (e.g., by using a flame) of the doped oxide. In their investigations of doped rare earth oxides, Korf and colleagues used AAS to identify alkali promoters added by wet impregnation of Sm2O3, CaO, or MgO used for the oxidative coupling of methane.65,116 In both studies, the volatility and migration of monovalent Na and Li species downstream in the reactors and the stability of the divalent Ca promoters were established by AAS of the catalysts before and after reaction. 4.7. Ultraviolet Optical Spectroscopy (UV−Vis)

UV−vis can be used to establish the presence of isolated dopant cations within a metal oxide. The spectra of the metal ions are interpreted by ligand field theory: ions with tetrahedral coordination are readily differentiated from octahedrally coordinated ones. Brückner used in situ UV−vis and EPR to characterize lanthanum- and chromium-doped alumina and zirconia, which catalyze the dehydrogenation of propane to propene.117 The UV−vis data showed that the as-synthesized catalyst exhibited Cr3+ d−d UV−vis absorbance at 630 nm in addition to a charge transfer bands from Cr6+ at 370 nm. Furthermore, EPR indicated the presence of Cr5+. Under the reducing environment of the propane dehydrogenation, even below reaction temperatures, the in situ characterization showed that both the Cr5+ and the Cr6+ were reduced to Cr3+. Zirconia stabilization in the cubic and tetragonal phases rather than monoclinic is of importance in maintaining an activated catalyst. Lopez et al. used UV−vis characterization of Mn-doped zirconia catalysts, prepared by coprecipitation, for the oxidation of phenanthrene and isopropanol.118 At 10% doping, XRD confirmed that the Mn4+ substitutes for the Zr4+ and stabilized the catalyst against the monoclinic phase transition. The UV−vis data support the presence of a solid solution of Mn in ZrO2 at 10% substitution and show the presence on Mn2O3 at loadings greater than 10%.

4.4. X-ray Photoelectron Spectroscopy (XPS)

This technique does not require synchrotron radiation and is widely used for the characterization of doped oxide catalysts. It provides information on atoms located within 2−4 times the electron inelastic mean free path (∼1−10 nm).114 The surface composition and oxidation state may be determined if the species of interest are present at the surface in concentration greater than approximately 1 at. %. XPS is generally performed in vacuum and is typically ex situ; thus the oxidation states measured may not reflect the electronic environment under reaction conditions. When used in conjunction with XRD and other means of determining bulk ratios of host:dopant atoms, XPS data can (i) determine the surface concentration of the dopant, and (ii) provide the oxidation states of the atoms located on the surface or in several layers near the surface. In an investigation of Pt-doped Fe2O3, Hu et al. synthesized doped polycrystalline films by electrochemical codeposition, and then confirmed that homogeneous solid solution was obtained by both XRD and by performing XPS as a function of time while etching the material with an ion beam.111 Recent work by Rajesh et al. demonstrated that ionic Pt was substituted on Ce sites of a doped BaCeO3 perovskite prepared by coprecipitation for the water−gas shift reaction.135 High-resolution electron microscopy along with XRD with Rietveld refinement and neutron diffraction were used to confirm the structure. The XPS data supported the presence of Pt2+ substituting Ce4+ sites, which was consistent with the observed linear decrease in cell volume and increase in oxygen vacancies with increase in Pt concentration. No metallic Pt was observed either pre- or postreaction, but an increase in the surface concentration of Pt4+ after the reaction was noted.

4.8. Raman Spectroscopy

In studies of Ce-doped CoCr2O4 catalyst for methane oxidation, Cr was substituted by Ce from 2% to 100%, and the material was characterized by Raman, XRD, and XPS. XRD suggested that a single phase solid solution was produced up to 10% substitution. Raman spectra were used to identify vibrations associated with pure phase materials and with the doped chromite.119 In situ measurements of Raman spectra under reaction conditions were combined with catalyst reactivity characterizations to evaluate the effect of alkali doping on the reducibility of vanadia supported on titania.136 It was found that reducibility of the vanadia correlates with the Raman spectrum of the vanadyl group.

4.5. X-ray Fluorescence Spectroscopy (XRF)

In XRF, an X-ray beam is used to excite core electrons of the atomic constituents in the sample, which decay through the emission of X-rays with energies characteristic of the element. Use of energy dispersive spectrometers (EDX, EDS) or wavelength dispersive spectrometers (WDX, WDS) allows energy resolution sufficient for quantitative elemental characterization, provided sufficient source intensity is available. XRF is used for establishing the concentration of dopants in the bulk of the catalyst. The technique can detect strongly absorbing atomic species at low (ppm) concentrations within metal oxides. In their investigation of methane partial oxidation on yttrium-stabilized zirconia, Zhu et al. found by XRF that all of their metal oxide catalysts had 0.005−1.17 mol % titania and hafnium oxide contamination.115 Surface analysis using LEIS did not have sufficient sensitivity to detect whether or not the Ti or Hf species were present on the surface.

4.9. Fourier Transformed Infrared Spectroscopy (FTIR)

The vibrational frequencies of metal−oxygen bonds is sensitive to the presence of dopant species at modest concentrations, and infrared spectroscopy has been used to support the fact that a doped oxide was prepared.101 Athawale and colleagues compared the IR spectra of silver-doped lanthanum chromite catalysts, prepared by combustion synthesis, to undoped samples where literature values for the frequencies of the O− M and O−M−O vibrations were known. They inferred from the presence of new and shifted peaks in the absorption spectrum that Ag was present as a substitutional dopant. In situ FTIR has been used to determine catalytic mechanism by 4397


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4.11. Low-Energy Ion Scattering (LEIS)

observation under reaction conditions, and various applications of this technique have been reviewed by Vimont120 and by Lamberti et al.137 Sharma et al.138 showed the methanation of carbon oxides on ruthenium-doped ceria. The Ru-doped ceria catalyst did make methane from CO2 and H2 but not from CO and H2. Hydroxyls were not observed by FTIR, when the feed was CO and H2, whereas they were abundantly present when the feed was CO2 and H2.138 Infrared spectroscopy can be used in the study of doped oxides by adsorbing molecules on the doped and the undoped oxides. Shifts in the vibrational frequencies of the adsorbate document the presence of the dopant in the surface. An example is provided by the work of Corma,139,140 who used shifts in the vibrational frequency of adsorbed CO to propose that CO oxidation by Au−ceria is due to ionic rather than metallic gold. It was proposed later that the ionic Au is likely to be a substitutional dopant.141,142 The in situ IR is one of the many “operando” methods (reviewed by Bañares121), which combine the spectroscopic characterization of the material and adsorbed species with simultaneous measurement of catalytic activity/selectivity, and which was reviewed by Bañares.121

LEIS is capable of determining the mass of the atoms in the surface layer by analyzing the energy loss of ions that have collided with the surface. Calibration relationships are constructed to obtain quantitative results. The experiments require vacuum, and the sample must be free of all environmental contamination to obtain meaningful data. Often the surface is cleaned by sputtering prior to the LEIS measurement. Less than 1% of a monolayer is inadvertently sputtered in the time it takes to obtain LEIS data. In their evaluation of yttrium-stabilized zirconia catalysts for methane partial oxidation, Zhu et al. used LEIS on the freshly prepared catalysts to determine the surface composition.115 They found that the surfaces of several samples were contaminated with F, Na, K (or Ca), Al (or Si), and Cr (or Mn). The elements in parentheses indicate an uncertainty in the mass of the surface impurity. None of the contaminants were intentionally added. Because the precursors used to make doped oxides are often 99.99% pure, it is possible that the 0.01% impurities may accumulate at the surface and inadvertently dope the oxide. These unknown dopants can interfere with the ones added intentionally. Moreover, the unknown dopants can act to compensate the chemical effect of the dopants added intentionally.145 LEIS is a particularly useful probe for doped oxide, and it is unfortunate that it is not more widely available.

4.10. Probing Metal Oxides with Electrons

Electron microscopy provides morphological information and under favorable conditions good instruments can detect single atoms.143 Many oxides are insulators, and care must be taken, when electrons are used to probe them, to avoid surface charging and surface damage. In an investigation of barium promoters on Ru metal catalysts, the Ba atoms were imaged in situ, and it was shown that it forms a BaO phase on the Ru metal nanoparticle, rather than dope the metal.144 Typically, in doped oxides the dopants are present in concentrations around 5%. Promoters such as alkali metals are frequently applied by investigators, and high-resolution electron microscopy has shown that alkali-metal promoters can form isolated islands on metal nanoparticle catalysts and are not be dopants.144 Wu et al., in a study of Zr-doped NiO for oxidative dehydrogenation of ethane, verified by XRD that below 10% Zr there was no ZrO2 phase separation, and the material had the NiO structure with a changed lattice parameter, which suggested that Zr is a dopant.126 The authors then performed HR-TEM and were able to visualize the atomic lattice of the nanocrystalline materials and measure the lattice spacing to show that it agreed with the XRD observations. At lower spatial resolutions, scanning electron microscopy (SEM) can be used to identify distinct crystalline structures or, qualitatively, the presence of multiple phases. The monochromatic electron beams from TEM or SEM can be used for spectroscopic measurements at different sites of the sample to determine the local electronic and atomic structure and composition. Electron diffraction (ED, EDS) and electron stimulated X-ray emission spectroscopy (EDX) can be performed as well as measurements of the inelastic electron energy loss (EELS) from the incident beam. High-performance electron spectrometers are limited to an overall energy resolution (determined by the electron sources) of 0.5−2 eV. In a systematic study of a large group of V/Mn/Fe oxides, EELS spectra, obtained from a 500 keV SEM, were correlated to the metal ion oxidation states.123 The results were sensitive to model fits and showed that care is required for getting meaningful data from SEM electron sources. No method exists that can unambiguously and universally determine oxidation states from EELS for all compounds.

4.12. Secondary Ion Mass Spectrometry (SIMS)

Elemental composition as a function of distance from the surface can be obtained by bombarding the sample with energetic ions. A pulsed beam of ions (typically Cs or Ga) bombards the surface, and the mass of the ejected ions is measured. Successive layers are removed and analyzed in the process. Debecker et al. used a combination of XRD and SIMS to evaluate the differences in ternary Si/Al/Mo mixed oxide catalysts prepared by sol−gel and wet impregnation methods.124 XRD showed the samples prepared by wet impregnation to be phase separated, when Mo content was as low as 8%, whereas even at 20% Mo the sol−gel samples were homogeneous. Oxide clusters containing more than one Mo were removed by SIMS from the surface of the wet impregnation samples, while only isolated Mo atoms were detected from the sol−gel prepared samples. 4.13. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC)

Besides being used to examine coke production and desorption of impurities, the examination of phase transition temperatures and crystallization phenomena using DSC/TGA can help identify the formation of mixed phase materials and provide support that a doped oxide has been synthesized. De la Rosa and co-workers122 used DSC to study La-, Mn-, and Fe-doped zirconia prepared by sol−gel synthesis and used as trichloroethylene combustion catalysts. A shift in the crystallization temperature of the doped material, determined by DSC, indicates that the dopant is incorporated in the bulk.122 4.14. Temperature Programmed Desorption (TPD) and Temperature Programmed Reaction (TPR)

TPD can be used by adsorbing molecules on the doped oxide, heating the sample, and measuring the composition of the gas desorbed from the surface. TPD with the as-prepared catalyst is used to determine the temperature at which O2 desorbs, or the temperature at which carbonates or hydroxyls present on the surface produce CO2 and water, respectively. Furthermore, 4398


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almost all cases, the surface is modeled by a slab of finite thickness in one direction and periodic in the other two. It is also possible to make small oxide cluster models, which allow the use of more accurate quantum chemistry methods. Experiments on gas-phase clusters146 and on mass-selected clusters supported on solid surfaces147−158 show that the catalytic activity of very small clusters depends strongly on their size. Therefore, a calculation using a cluster model has to increase the cluster size until its catalytic properties no longer change with size. If the size becomes too large, one has to resort to DFT. If it is too small, one can do accurate calculations, but the model is inaccurate. The “ordinary” GGA-DFT (GGA stands for generalizedgradient approximation) does not give the correct electronic structure for most oxides of transition metals and rare-earth metals. Some objections to GGA-DFT have to do with the values of the orbital energies. The energy difference between LUMO and HOMO (the band gap) given by GGA-DFT is always substantially smaller than the measured value. Experiments (UPS, EELS) observe that filled states are present in the gap when oxygen vacancies are present, while in GGA-DFT calculations the vacancy-induced states are at the bottom of the conduction band. One could argue that orbital energies should not be compared to observable quantities. Strictly speaking this is true; however, one must be concerned when GGA-DFT calculations predict that Ti2O3 is a metal, while experiments show that it has a band gap of 1 eV. In addition to the above shortcomings, we add the fact that the d- or f-bands in these oxides are much wider than in experiments. This means that, in the calculations, the electrons in the d- or f-orbitals are delocalized instead of being localized on the cations as the experiments assert. Because we have very few precise binding energy measurements on well-defined, clean oxide surfaces,30,159−164 it is not clear to what extent GGA-DFT makes errors when calculating total energy differences, which are the quantities most important to catalysis. A temporary palliation is provided by the DFT+U method165−167 in which a Hubbard-like correction is added to the DFT theory to prevent the delocalization of the d- or forbitals of the cations. Many versions of the theory have been published,168−174 but the implementation available in the VASP programs is most widely used. Unfortunately, there is no agreement on how to pick the value of the Hubbard parameter U. Because catalysis is controlled by total energy differences, we prefer to chose the value of U that fits the energy of some reaction involving the oxide.175,176 In principle, one expects U to depend on the cation−cation distance and on the positions and the nature of the surrounding anions. In practice, one assumes that U is a property of the cation, which is a ill-defined approximation. Despite these shortcomings, the consensus is that calculations on transition metal oxides or rare-earth oxides must use spin-polarized, DFTGGA+U, not GGA. It is prudent, however, to test that the qualitative conclusions reached by the calculations do not change when the value of U changes within a reasonable range. Watson and Nolan have suggested that a Hubbard correction ought to be used for the p-orbitals of oxygen. Because this is important in connection with low-valence dopants, we defer the discussion of this issue for that section. An alternative to GGA+U is to use a hybrid functional such as B3LYP177 or HSE.178−186 Both methods add some Hartree− Fock exchange energy to the GGA functional. This removes

specific reactions (e.g., the oxidation of an alkane by a doped oxide) can be monitored as the temperature is ramped up. Such measurements are particularly useful when a doped oxide is compared to the undoped one, or when comparing samples having the same host-oxide and various dopants. Wu and co-workers126 showed that O2 desorbs from Zrdoped NiO at much lower temperature than from undoped NiO. They also used TPR to monitor the oxidative dehydrogenation of ethane and found that Zr-doped NiO is more selective than NiO but has lower ethane conversion. Temperature programmed reaction provides a quick, preliminary way of testing whether a dopant affects strongly the reducibility of an oxide, its acidity, or its ability to perform a given reaction.

5. COMPUTATIONAL METHODS Computing accurately the catalytic properties of doped oxides requires a detailed knowledge of the morphology and the composition of the surface under reaction conditions. At this time, the experiments are unable to provide this information. One could imagine trying to use computations to determine the state of the catalyst by creating a large number of models, calculating their free energy, and studying catalysis with the model having the lowest free energy. This is impractical: a lot of computer time is needed because we have little guidance from experiments, and too many possibilities will have to be considered. In addition, this will give the equilibrium state of the catalyst, which is of limited usefulness; we need the state of the catalyst under steady-state reaction conditions, and this is governed by kinetics. Trying to calculate activation energies and reaction rate for all possible structures and reactions in a given system is possible in principle, but it is far beyond the current computing capabilities. Chemists have lived with uncertainty throughout their history, and the situation described above is not rare nor does it imply that we should abandon using computations. A possibility, which we follow, is to look for situations in which knowledge of simple rules or trends can improve our ability of predicting whether a catalyst or a class of catalysts is promising for a particular reaction. For example, alkane activation catalyzed by doped oxides is fertile ground for this approach. The rate-limiting step is the breaking of the C−H bond. One can compare the activity of different catalysts for this process by calculating the activation energy for C−H bond breaking. This makes the time needed for screening catalysts for this reaction comparable to the time of doing the experiments, which is an important practical consideration. In the case of doped oxide catalysts, there are a number of fundamental, qualitative questions. Among the many possible oxide−dopant combinations, which one has the lowest activation energy for breaking the C−H bond? Can we classify the dopant−oxide pairs in a useful way that might allow us to guess, without elaborate calculations, what catalyst to try for alkane activation? Is there a simple understanding of how a dopant affects catalytic activity? Finally, computations have the advantage that they can easily “make” new materials and explore their catalytic activity. If they are promising, the experimentalists might attempt to synthesize them. To do useful computations on a catalytic system, one has to deal with systems with a large number of electrons and perform a large number of calculations for all possible adsorption sites and reaction paths. In practice, only density functional theory (DFT) is efficient enough to satisfy these requirements. In 4399


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low energies. An example of this is provided by calculations done by Goodrow et al.211 who showed that in the conversion of methanol to formaldehyde, the dehydrogenation of the methoxide is much slower than a spin-forbidden transition invoked in another step of the mechanism. Therefore, the spinforbidden transition is not rate-limiting for that particular reaction. A long-debated issue in quantum chemistry is how to assign electron charges to an atom that is part of a molecule. There is no perfect method for doing this, but we prefer the one proposed by Bader,212 which was implemented efficiently by Henkelman, Jonsson et al.213,214 To the extent that this definition is accepted (and we find it logically satisfying), the Bader charges are the real charges of the atom in a solid or a molecule. We emphasize this because we also use formal charges in this Review, following the bad but useful habit of many people in the field. Formal charges do not exist. They are defined by deciding arbitrarily that oxygen in an oxide has the charge −2 and then determining the charge of the cation so that the oxide is electrically neutral.

some of the interaction of the electron with itself, a feature that plagues GGA-DFT. Both methods have been tested extensively for molecules and less so for oxides. The conclusion is that they are better than GGA and probably than GGA+U. Unfortunately, they require more computer time than GGA+U and are less practical (although possible) if one plans to perform all of the calculations needed when one studies adsorption and reactions on the surface of a slab. When possible, they should be used to test the key reaction steps in the GGA+U calculations. Some comparisons between GGA+U and HSE are available.187−190 For example, the energy of oxygen vacancy formation obtained by HSE differs from that given by PBE+U by 0.3 eV for CeO2(110), by 0.4 eV for CeO2(100), and by 0.8 eV on CeO2(111). These results suggest again that one should use calculations mostly for qualitative results and trends. In the example above, the qualitative conclusion that CeO2(111) is not as good an oxidant as Ce(100) or CeO2(100) is likely to be safe. Another shortcoming of GGA-DFT is its inability to describe van der Waals interactions. This is particularly important for binding of polarizable molecules to metals (which are very polarizable)191−195 and less so for oxides that have a large band gap. However, in the case of alkane interactions with siliceous zeolites, for example, the van der Waals interactions are the only interactions, and obviously they cannot be ignored. In calculating reaction energies, some of the van der Waals interactions cancel out but not completely. It is therefore preferable to use versions of DFT that include the van der Waals interaction, but this has been done infrequently for oxides. The quantum calculations performed on heterogeneous catalysts use a Hamiltonian that does not contain spindependent terms (such as the spin−orbit coupling). In this case, the exact wave function of the system must be an eigenstate of the square of the total spin operator, S2, of the electrons and of the projection Sz of the same operator on the z-axis. Quantum mechanics also tells us that if the Hamiltonian is truly spin independent, then the rate of transition between electronic states with different spin (e.g., from a singlet to a triplet) must be zero. We say that such transitions are spinforbidden.196,197 Transitions that are spin-forbidden take place in practice because the correct Hamiltonian contains spindependent operators. However, these operators have small matrix elements, and therefore the rates of spin-forbidden transitions are small. The consequences of these statements are well understood by people who apply quantum chemistry to radicals, or to organometallic compounds, or to enzymes.198−208 With few exceptions,209−211 they have been ignored in surface science and catalysis. Because the exact wave function must be an eigenstate of S2 and Sz, density functional should use only densities derived from wave functions satisfying this restriction. Unfortunately, we do not know how to impose this constraint. We have suggested209 that, as a temporary solution, one should assume that reactions in which the spin polarization (i.e., the number of spins “up” minus the number of spins “down”) of the reactants is different from that of the products are slow. This is based on the observation that such reaction would require “flipping” one or more spins and there are no magnetic fields in the system strong enough to make this process fast. Of course a reaction that does not require spin-flipping but has a very high activation energy can be slower than a reaction in which the spin is flipped but the crossing of the states with different spins takes place at

6. STRONG ACID−BASE INTERACTIONS Over the years, many people have attempted to correlate catalytic activity with the acidity or the basicity of oxides.215−219 There is no doubt that in cases where hydrocarbon cations are intermediates, Bronsted acidity of a catalyst is essential.220 In other cases, one constructs a basicity scale by using the binding energy of CO2. The stronger it binds CO2, the more basic is the oxide. The acidity is determined by measuring the binding energy of ammonia or pyridine or by measuring the shift in the vibrational frequency of CO.137 One then attempts to correlate the catalytic activity of a variety of oxide catalysts, for a given reaction, with the basicity or the acidity of the surface.216,217 In a recent article,221 we have shown that a large number of computational results can be qualitatively rationalized by a simple rule: the interaction between a Lewis acid and a Lewis base coadsorbed on the surface is surprisingly strong. We will not repeat the arguments here, but we will make use of this rule to guide us. While the definition and the classification of various aspects of Lewis acidity and basicity can become very complex,222 we use here the simplest possible definition. If two compounds come in contact or react, the one that receives electrons is a Lewis acid and the one that donates them is a Lewis base. While the concept of Lewis acid and base is very old,223 its use was limited by the fact that in most cases we did not know how the electrons flow when two compounds come in contact. Because of the work of Bader,212 we now have the means of calculating213,214 how the electrons are distributed over each atom in a molecule and therefore determine which molecule is an acid and which is a base. We emphasize that acidity and basicity are properties of a pair: a compound can be acid when interacting with one molecule and a base when interacting with another. For physicists, the name of electron donor (base) or electron acceptor (acid) may be more familiar. This concept is of interest here because the fragments formed by the dissociative adsorption of alkanes, the highvalence dopants, the oxygen vacancies, and the Ti3+ and Ce3+ cations, are bases and the low-valence dopants, the Ce4+ and Ti4+ cations, and O2, Br2, Cl2, and other electronegative adsorbates, are acids. The rule we found by examining the results of many calculations is that if we coadsorb on the surface of an oxide a Lewis base with a Lewis acid, the binding energy 4400


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There are of course the cases in which the dopant and the cation it substitutes have the same valence. We do not have sufficient computations on the chemistry of such systems, and our tentative suggestion is that they do not have a very large effect on the chemistry of the host oxide. An exception may be the case when the oxide of the dopant is much more stable than the host oxide and the oxygen−cation bond length in the two oxides are different. An example of this might be NiO doped with Mg.

of the pair exceeds substantially the sum of the binding energy of the compounds alone on the same surface. Thus, the presence of a Lewis base on a surface (e.g., a hydroxyl, an adsorbed CH3 radical, a K atom) increases the binding energy of a Lewis acid (e.g., a Au atom or an oxygen molecule). The presence of a Lewis acid on the surface (e.g., a dopant having a lower valence than the cation it substitutes in the host oxide) lowers the energy of formation of an oxygen vacancy because the oxygen vacancy is a base and its formation benefits by a large acid−base interaction. We will make use of this concept throughout this Review.

8. THE MARS−VAN KREVELEN MECHANISM, THE OXYGEN VACANCY FORMATION, AND DOPING It is widely accepted224,225 that most oxidation or oxidative dehydrogenation reactions catalyzed by oxides take place through a Mars−van Krevelen226 mechanism. In such reactions, the gases introduced in the reactor contain a reductant (e.g., an alkane, CO, H2) and an oxidant (e.g., O2, CO2, N2O). In the Mars−van Krevelen mechanism, the reductant is oxidized by the oxygen atoms from the oxide’s surface layer (not by adsorbed oxygen or by gas-phase oxygen). When the oxidation products desorb, they leave behind oxygen vacancies. The role of the gas-phase oxidant, introduced with the feed, is to annihilate the oxygen vacancies (to reoxidize the surface). In most cases, the reoxidation of the surface is very fast, and therefore the rate-limiting step is the reaction of the reductant with the surface oxygen. We assume that there is a connection between the ability of surface oxygen to act as an oxidizing agent and the energy ΔEv required for forming an oxygen vacancy. To be precise, ΔEv is the energy of the reaction

7. A CLASSIFICATION OF DOPANT−OXIDE PAIRS As we have explained already, the role of a dopant is to disrupt the chemical bonds in the surface of the doped oxide to make either the dopant or the oxygen atoms near the dopant engage more readily (than the undoped oxide) in chemical reactions. By reviewing the existing DFT calculations, we have come up with a tentative classification of the dopant−oxide pairs. We call a dopant a low-valence dopant (LVD) when its valence in its own stable oxide is lower than the valence of the cations it substitutes in the host oxide. For example, a La atom substituting a Ce atom in CeO2 is an LVD because lanthanum oxide has the formula La2O3. The chemistry of doped oxides in which the dopant’s valence is lower than that of the substituted cation by one is predictable. Their most interesting feature is that they activate the oxygen atoms near them and make them more reactive. The LVDs make the doped oxide a better oxidant than the undoped host oxide, and therefore they promote the first step in the Mars−van Krevelen catalytic oxidation mechanism. We call a dopant a high-valence dopant (HVD) when its valence in its stable oxide exceeds that of the cation it substitutes. An example is La dopant in CaO, because La is trivalent in its own oxide. The HVD’s chemical properties depend on whether the host oxide is reducible or not. If the host oxide is irreducible (MgO, La2O3, Ta2O5, etc.) and the dopant’s valence is larger by one unit than the cation it substitutes, the dopant makes the oxygen atoms in the surface of the host less reducible and therefore less active. However, the dopant adsorbs O2 from the gas phase and activates it. A reductant can react with this oxygen and undergo oxidation. Unlike in the Mars−van Krevelen mechanism, the oxygen atom in the oxidation product originates from the gas phase, not from the surface. If the valence of the dopant is much higher than that of the replaced cation, complications arise, and we examine these in the section dedicated to HVDs. If the host oxide is reducible (CeO2, TiO2, etc.), the dopant may donate electrons to the host cations (e.g., reducing Ce4+ to Ce3+), and this will affect the properties of the system. Because of this, we classify the HVDs in reducible oxides separately from those in the irreducible ones. To decide a priori the valence of a dopant, we use the oxides the dopant forms. Mg is divalent because the only stable oxide is MgO, and Ta is pentavalent because Ta2O5 is its only stable oxide. On the other hand, Nb has three stable oxides, Nb2O3, NbO2, and Nb2O5. We cannot decide a priori whether Nb in Nb-doped ZrO2 is a low-valence or a high-valence dopant. Of course, one can analyze the results of the DFT calculations to assign a valence to Nb in Nb-doped zirconia. However, this defeats our purpose, which is to provide a classification that allows us to predict qualitatively the effect of a dopant without performing calculations.

Ox → Ox v + 1/2 O2 (g)

where Ox is the oxide (doped or not) and Oxv is the oxide with an oxygen vacancy on the surface. The assumption is that the smaller is ΔEv, the better oxidant is the surface. Once we accept this idea, we can calculate ΔEv for a quick screening of the oxidative power of various oxides (doped or undoped). Moreover, if we want to make an oxide a better oxidant, we should look for modifications that make ΔEv smaller. However, one must keep in mind that a very small ΔEv means that the surface is a good oxidant, but it is not necessarily a good oxidation catalyst. To complete the catalytic cycle, the gaseous oxidant (oxygen, CO2, N2O) must be able to reoxidize the surface. If oxygen is too easy to remove from the surface layer, it will be difficult to put back. Therefore, in designing an oxidation catalyst, we must follow a “moderation principle”: we should modify the surface to make ΔEv small, but not too small.141 For this and other reasons, there is an extensive literature in which ΔEv was calculated.141,142,145,187−189,221,227−262 Pirovano, Hoffmann, and Sauer263 have written an excellent review of these calculations summarizing the situation up to 2007. According to the crude electron-pair Lewis theory of the chemical bond,264 removing an oxygen atom from the surface leaves behind two unpaired electrons. This means that an oxygen vacancy is a strong Lewis base. The fate of these electrons depends on whether the oxide is irreducible or reducible. Here, we say that an oxide is irreducible if the cation in it does not make an oxide in which it has a lower valence (e.g., CeO2 is reducible because Ce2O3 exists and is stable; MgO is irreducible because Mg2O does not exist). This means that the cations in the irreducible oxides will not lower their formal charge in the presence of electron donors. When an oxygen vacancy is made on the surface of an irreducible oxide, 4401


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optimizes the geometry to obtain a minimum energy. The polarons will be made at some location that is determined by the initial state prior to geometry optimization. To locate a polaron on a specific Ti atom, we displace slightly the neighboring oxygen atoms away from it and optimize the electronic energy (while keeping the oxygen atoms fixed in the displaced position).259 After optimizing the electronic energy, we allow the oxygen atoms to adjust their position to minimize the total energy. The end result is that this particular Ti atom has the formal charge +3 and the polaron is stabilized at the desired location. One can then use the same procedure to form a second polaron (the O-vacancy provides two electrons) on another Ti atom of our choice. Polaron formation lowers the energy of vacancy formation by an amount that depends on the location of the polarons. This dependence is fairly dramatic on rutile.259 Until recently, it was assumed that the unpaired electron reduce two Ti4+ ions located next to the oxygen vacancy, which is a plausible but incorrect. It turns out that 80 possible polaron-pair locations have been studied,259 and all have lower energy than in the case when both electrons are located on the two Ti atoms neighboring the vacant site. Depending on the value of U, the polaron formation lowers the formation energy of an oxygen vacancy by at least 0.5 eV.259 If one performs GGA calculations, the two unpaired electrons in TiO2(110) are spread out over several atoms surrounding the vacancy,270 and the cations are no longer Lewis acids. As a result, the energy of vacancy formation is higher. One of the most puzzling results of the calculations is the large magnitude of ΔEv. A few examples from our work are ΔEv = 3.3 eV for ZnO (101̅0),271 3.28 eV for NiO(011), 6.44 eV for La2O3(001),145,272 5.77 eV for CaO(001),145 ∼3.5 eV for TiO2(110),259 3.00 eV for CeO2(111),145 and 6.38 eV for LaOCl(001). The order of magnitude of the values reported here is consistent with that reported in calculations by other authors on these and other oxides. The last step in the synthesis of almost all oxide catalysts is calcination in air or oxygen, for 4 or 5 h, at temperatures between 400 and 500 °C. The large values of ΔEv suggest that there should hardly be any oxygen vacancies on the surfaces of most oxides. However, this statement is softened if we consider the thermodynamic equilibrium between a surface with oxygen vacancies on it and gaseous oxygen. The entropy of the final state (reduced oxide plus O2 in the gas) is much larger than the entropy of the initial state (unreduced oxide), because the translational and the rotational entropies of gaseous O2 are large.273 As a result, the entropy ΔSv of the reaction forming oxygen vacancies is positive. Because the Gibbs free energy of the reaction is ΔGv = ΔHv − TΔSv, the free energy of the reaction that makes oxygen vacancies is smaller than ΔEv. A thermodynamic calculation therefore makes vacancy formation less forbidding than a calculation of the total energy. It is likely that stable oxides such as CaO, MgO, La2O3, or Sm2O3 do not have oxygen vacancies on their surface as long as sufficient oxygen is available during their preparation and no reductants are present. The large values of ΔEv for these oxides explain why they are so hard to reduce. It also explains why they are so popular as catalysts for the oxidative methane coupling reaction (2CH4 + O2 → C2H4 + 2H2O), which takes place above 600 °C (it is often carried out at 700−800 °C).81,82,274 They are the oxides that interact with methane without combusting it. In this case, their ability to hold tightly to the oxygen atoms at their surface is an asset.

the unpaired electrons, left behind when the oxygen atom is removed, are localized at the vacancy site.258,260 This of course assumes that the oxide does not have electron-accepting impurities. In the presence of O2 in the gas phase, the oxygen vacancy in an irreducible oxide adsorbs O2 strongly. We expect this, because O2 is a strong Lewis acid and the vacancy is a strong Lewis base. In the materials science literature, there have been extensive discussions regarding the charge of an oxygen vacancy in the bulk.265−267 The model used in these discussions assumes that when the oxygen vacancy is formed the oxide is in contact with an electron reservoir whose properties are described solely by its Fermi level (chemical potential). The vacancy is in chemical equilibrium with a gas-phase oxygen reservoir and the electron reservoir. If the Fermi level of the electrons is very low, the vacancy will donate electrons to the electron reservoir and the vacancy is positively charge. In the opposite limit, it is negatively charged. The concentration of vacancies and their charge depends on oxygen pressure and temperature and on the Fermi level of the reservoir. The model is silent regarding the physical nature of the reservoir. Unless the oxide is in contact with a bulk metal, which will provide the reservoir, we must assume that the reservoir consists of impurities (electron donors or acceptors) or other “defects”. The question is whether the model is useful for the study of oxygen vacancies at the surface of an oxide. We believe that it is not, for two reasons. First, in the presence of oxygen (which is the only case of interest for oxidation catalysis), the vacancies will adsorb an O2 molecule, and this is the stable structure. Such adsorption is not relevant to bulk vacancies. Second, if the electron reservoir consists of electron donors or acceptors, the energy of vacancy formation will depend on how far the vacancy is from the nearest donor. This is a “local (chemical) effect”: the energy needed to make a vacancy near the donor is different from the energy to make it far from the donor. This means that the Fermi energy is not the only parameter in the problem. Calculations of ΔEv at different distances from various donors or acceptors show that this distance dependence if very substantial, with the oxygen atoms near the dopant being the easiest to remove.258 Oxygen vacancies in reducible oxides have interesting properties. They have been studied for CeO2 and TiO2, which are oxides that can change their oxidation state easily from 4+ to 3+. This means that the Ce4+ or the Ti4+ cations act as Lewis acids. Because an oxygen vacancy is a Lewis base and the cations are Lewis acids, the formation of an oxygen vacancy on these oxides is facilitated by a strong acid−base interaction. The unpaired electrons created when the vacancy is formed will occupy orbitals localized on the cations, reducing them.188,259,268,269 The oxygen atoms neighboring the reduced cation are displaced from their normal locations (which they have when the cation is Me4+). The “ensemble” consisting of the reduced ion plus the displaced oxygen atoms is called a polaron. In DFT calculations, polarons are formed only if one uses GGA+U theory, with a sufficiently high value of U (usually between 3 and 5 eV), or hybrid functionals (e.g., HSE or B3LYP). Thus, the creation of an oxygen vacancy causes the formation of two polarons. This polaron pair can be located on any pair of cations, and we describe next how one can control these locations. We use TiO2 as an example, but the procedure can be applied to ceria and presumably to any other reducible oxide. First, one makes an oxygen vacancy in the surface and one 4402


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The situation is more confusing for oxides for which ΔEv is not as large. Quite a bit of information regarding oxygen vacancies is available for TiO2(110), for which one can perform high-quality STM measurements in UHV.147,160−162,164,240,275−290 These measurements find that there are always oxygen vacancies on the surface. Their concentration can be as high as 16% (a vacancy concentration of ∼8% is more common), but achieving this requires heating at high temperature in UHV. One creates vacancies by heating TiO2 to high temperature, and one annihilates them by exposure to oxygen. These results seem inconsistent with the high values of the energy of vacancy formation. It is very difficult to compare these numerical values of ΔEv to experimental values for real catalysts for a number of reasons. The precursors used to make oxide catalysts have a purity of 99.99%. The 0.01% impurities may segregate at the surface and change dramatically ΔEv. For example,291−293 STM experiments found that Ca segregates on the surface of TiO2. Lefferts115 used low-energy ion scattering experiments and showed that F, Na, Al, or Si (the mass difference is too small to decide which one), K or Ca, and Cr or Mn, from unknown sources, were present on the surface of some commercial yttrium-stabilized zirconia, while X-ray fluorescence detected the presence of Hf and Ti in the bulk. All calculations show that if these dopants are LVDs, the energy of vacancy formation will be diminished substantially. Any acidic adsorbate, such as H (in the form of hydroxyls), will also lower the energy of vacancy formation.221 Calculations show that the value of ΔEv on different crystal faces can vary substantially. Because the grains of the catalysts are amorphous or have many facets, we can at best measure an average value of ΔEv. All of these make it difficult to compare the experimental values to those provided by calculations. The situation is complicated by the fact that experiments with “the same oxide” prepared by different methods give very different results. The temperature at which O2 starts to desorb from ceria, in temperature programmed desorption experiments, depends strongly on the dispersion of the oxide: the finer is the powder, the smaller is ΔEv.294−298 Information about the energy of forming oxygen vacancies on ceria can also be obtained from temperature programmed reduction with H2 or CO. TPR with hydrogen gives a widespread of the light-off temperature, hence a spread in the stability of surface oxygen.299−308 Temperature programmed reduction with CO leads to similar conclusions.309−311 As a result, we cannot tell which measurement of ΔEv should be compared to the calculated values. We repeat here that we are interested in the energy of oxygen vacancy formation ΔEv because it is a descriptor of the ability of an oxide to function as an oxidant. We show below that the ΔEv is strongly affected by substitutional cationic dopants.

create this deficit by replacing some of the cations of the host oxide with cations having a lower valence. The same strategy is suggested from another point of view. An oxygen vacancy is a Lewis base. If we make a chemical modification that creates a Lewis acid on the surface, the subsequent formation of an oxygen vacancy will benefit by a strong acid−base interaction and the energy to make a vacancy will lowered by this interaction.221 Doping with an LVD creates a Lewis acid on the surface and therefore facilitates oxygen vacancy formation and increases the binding energy of Lewis bases to the surface. Because the presence of an LVD in the surface layer makes it easier to make oxygen vacancies, the oxygen atoms that are easier to remove are also more reactive; an oxide doped with an LVD is a better oxidant than the undoped oxide. However, it is important to keep in mind that a better oxidant is not necessarily a better oxidation catalyst. In addition, because a surface doped with an LVD is a Lewis acid, it will bind strongly Lewis bases, such as H or CH3.221,312 It follows that doping with an LVD will increase the energy of the dissociative adsorption of an alkane because the fragments produced by dissociation are Lewis bases and they interact strongly with the Lewis acid surface. This makes such surfaces promising candidates for breaking the C−H bond in alkanes. If the Brønsted−Evans−Polanyi rule313−326 holds for alkane activation on doped oxides, increasing the binding energy of the fragments to the surface leads to lower activation energy for dissociation. 9.2. The Effect of LVDs on the Energy of Oxygen-Vacancy Formation

A large number of calculations studied the chemical effects of doping an oxide surface with an LVD.141,142,145,190,228,234,238,243,258,260,261,271,272,327−339 We enumerate below the conclusions of these calculations and propose that these are general and valid for any oxide doped with any LVD. (a) The energy ΔEv to make an oxygen vacancy near the location of an LVD is much smaller than ΔEv for the undoped oxide. Doped oxides are better oxidants than the undoped host. (b) The presence of an LVD affects oxygen atoms several sites away from the dopant, making them easier to remove than those of the undoped oxide. The oxygen atom easiest to remove is adjacent to the dopant.258 (c) The presence of an LVD makes the doped oxide a strong Lewis acid.221 As a result, the bond of any Lewis base (e.g., H, CH3, NH3, CO) with the oxygen atoms in the surface layer of the doped oxide is much stronger than the same bond with the undoped oxide. (d) The adsorption of a Lewis base on the surface of an oxide doped with an LVD will tend to cancel the properties conferred by the dopant, as if the base neutralizes the acidity caused by the LVD. We have called this a chemical compensation effect.145 The compensation effect may play an important role in alkane activation. When the C−H bond is broken by dissociative adsorption on an oxide surface doped with an LVD, the fragments (H and CH3) are Lewis bases. They neutralize the effect of the dopant (whose presence makes the surface an acid) making the activity of the doped oxide comparable to that of the undoped oxide. The activity of the doped oxide is restored only after H

9. OXIDE CATALYSTS DOPED WITH LOW-VALENCE DOPANTS (LVDS) 9.1. Introduction

We accept the “majority view”224,225 that most oxidation reactions catalyzed by undoped oxides take place by a Mars− van Krevelen (MvK) mechanism. This means that any oxide modification that facilitates the removal of surface oxygen will increase the efficiency of the MvK process. Because oxygen is electrophilic, we can remove it easier from the surface layer if we create an electron deficit at the surface. We propose to 4403


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have the same valence (e.g., CeO2 doped with In, Ga, Al). Some correlations have been found between the magnitude of ΔEv and other properties (e.g., the energy of formation of the oxide of the dopant, or the cation− oxygen bond length of the oxide of the dopant). However, most of them have been tested only on a small number of systems. (iii) The ability of a dopant to affect ΔEv may vary with the crystalline face of the guest oxide. In the case of La2O3, we found relatively small differences between the ΔEv on the (011) and the (001) face. Large differences have been observed for ceria doped with Au.261,338 The manner in which the effect of an LVD varies from one crystal face to another has not been sufficiently explored, and no qualitative explanation for these differences has been proposed. (iv) In the literature on materials chemistry, it is often stated that doping with an LVD will generate spontaneously an oxygen vacancy for “charge compensation”. This view is reinforced by measurements that show that an oxide doped with an LVD (in the bulk) has more oxygen vacancies and higher oxygen mobility than the undoped oxide. The calculations show that for the majority of oxides whose surface is doped with an LVD, the formation of an oxygen vacancy in the surface layer is endoergic. Often the magnitude of ΔEv ranges between 1 and 2.5 eV, which means that doping with an LVD facilitates the formation of an oxygen vacancy, but oxygen vacancies are not made spontaneously. However, for some crystalline faces of some oxides, with some LVDs in the surface layer, ΔEv is positive (the oxygen vacancy formation is exoergic). This happens, for example, for ceria doped with Au.142,261 This spontaneous vacancy formation is more likely to be observed when the valence of the dopant is lower by 2 or 3 than the valence of the cation it replaces. The number of oxygen vacancies at a surface (or in the bulk) of an oxide doped with an LVD depends on the partial pressure of the oxygen gas and the temperature of the surface used during the preparation of the oxide. For this reason, some workers use thermodynamic equilibrium calculations,335,343 in computations that examine oxygen-vacancy formation. This is useful for characterizing the “as-prepared” catalyst, which is likely to be in thermodynamic equilibrium with the oxygen gas present during calcination. However, catalytic reactions are run under steady-state conditions, not at equilibrium. The surface is exposed simultaneously to an oxidant (e.g., O2, CO2, N2O, H2O2) and a reductant (e.g., an alkane, CO, H2). The reductant makes vacancies, and the oxidant annihilates them. The steadystate concentration of vacancies is controlled by the interplay between the rate of vacancy formation and that of vacancy annihilation and can be very different from the concentration at equilibrium between oxygen gas and the oxide surface. For example, in alkane oxidation, the surface is reduced by the alkane and oxidized by the gas-phase oxygen (or whatever oxidant is used). At high alkane partial pressure and low oxidant pressure, the catalyst surface is likely to be reduced (a surface with many oxygen vacancies). In the opposite limit, the surface is close to being fully oxidized. If we use CO2 as an oxidant, the surface has substantially more oxygen vacancies than in the case when the oxidant is O2. In all cases, the oxygen leaving the surface, to make a vacancy, comes off as H2O, or CO, or CO2, etc. Forming oxygen vacancies by using a reductant is much

and CH3 are removed from the surface by subsequent chemistry. This raises the possibility that in some cases the limiting step will be the rate of this removal. Otsuka et al.340 and Sokolovskii et al.341 suggested that their experiments support this idea. In what follows, we present some of the computational results that were extrapolated to formulate these rules. Early examples of low-valence dopants dealt with very stable oxides (e.g., alkali-earth oxides, La2O3, Sm2O3) doped with alkali atoms, which catalyze oxidative coupling of methane to ethane and ethylene. We do not discuss this system here because excellent recent reviews are available.81,82 In addition, it is doubtful that the doped oxides catalyzing this reaction are stable under the reaction conditions used for oxidative methane coupling (very high temperature). In our earliest work,141 we have shown that the energy of oxygen vacancy formation in the surface layer of rutile TiO2(110) is substantially diminished by substitutional doping with, Au, Cu, Ag, Ni, Pd, or Pt. Similar results were obtained for CeO2 doped with Au,142,243,261,329,338 Ag,142 Cu,142,228,334,335,339 La,145,331,332 Y,190,258 Fe,329,333 Al,190 Sc,190 Mn,342 and In.190 This behavior is not confined to ceria. The same conclusion was reached in studies of ZnO271 doped with Li, Na, K, Rb, Cs, Cu, Ag, Au; of La2O3(100) and La2O3(011) doped with Cu, Zn, Mg;260,272 of CaO doped with Li, Na, or K;145 of NiO doped with alkali (Sun and Metiu, in preparation); and of TiO2 doped with Fe,234 Au, Ag, Cu, Ni.141 We are not aware of any exception to this rule. For this reason, we postulated that the rule is valid for any oxide doped with any LVD. In all cases, the lowering of ΔEv by the presence of an LVD is substantial. We give below a few examples. (i) The computed energy ΔEv of oxygen-vacancy formation at the surfaces of very stable oxides, such as CaO, MgO, or La2O3, is very large. For example, the surface of La2O3(001) has six-coordinated and four-coordinated oxygen atoms exposed to the vacuum. The energy to make a vacancy on this surface is260 6.44 eV (for the four-coordinated O) and 5.77 eV (for the six-coordinated O). Doping La2O3(001) with an LVD lowers the energy needed for removing a four-coordinated O atom to ∼0.58 eV for a Cu dopant, to ∼2.38 eV for a Zn dopant, and to ∼2.57 eV for a Mg dopant. A collapse in ΔEv of the same order of magnitude was obtained for the energy of removing a six-coordinated O atom from the doped surface. The same behavior was observed for La2O3(001)260 and other very stable, irreducible oxides such as MgO258 and CaO. It is likely that the same results will be observed for other stable oxides such as Sm2O3, alkali earth oxides, Al2O3, SiO2. (ii) The decrease of ΔEv by low-valence dopants, in less stable oxides (e.g., CeO2, TiO2, NiO, ZnO), is also large but not as dramatic as at (i). For example, the energy of vacancy formation on CeO2(111) is ∼3.00 eV.258 For CeO2(111) doped with La, ΔEv ≈ 1.36 eV;145 with Y, ΔEv ≈ 1.06 eV;258 with Fe, ΔEv ≈ 2.73 eV.234,333 This behavior is not caused by the fact that CeO2 is a reducible oxide. Calculations in our group have shown that doping NiO(011), which is not reducible, with Li reduces ΔEv from 3.28 to 2.65 eV. The lowering of ΔEv is a consistent trend. However, there is no qualitative explanation for the differences in ΔEv between samples having the same host oxide and different dopants that 4404


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Figure 3. The location of CH3 and H formed by the dissociative adsorption of CH4 on Zn-doped La2O3(001) (see Table 2). Top and side views of (a) the structure with the lowest energy, and (b) the structure with the next-lowest energy, in which CH3 binds to the Zn dopant.

decomposed to form CO2 and heal the oxygen vacancy. The last step closes the catalytic cycle. This is a typical Mars−van Krevelen mechanism. The role of the LVD is to make the surface oxygen atom more reactive; it is not the ionic dopant that is the active site. Therefore, these calculations provide no support for the old idea that one should think of doping with an LVD as a way of tuning the activity of the dopant (e.g., that a Ni dopant will do catalytic chemistry similar to that of metallic Ni but modified by being imbedded in the surface of the oxide). 9.3.2. Methane Activation: Does the Brønsted− Evans−Polanyi Rule Work For It? A more interesting reaction is the activation of alkanes for conversion to syngas, alkenes, or valuable oxygenates. An appealing feature of such reactions, as far as computational screening is concerned, is that the rate-limiting step is the breaking of the C−H bond. Therefore, one can screen the ability of a catalyst for activating methane by calculating the activation energy for this step. Such calculations can help us find active catalysts but tell us nothing about selectivity because this would require computing the activation energies for all possible steps in alkane oxidation. Because such calculations would be extremely time-consuming, our strategy has been to use computations to find catalysts that break the C−H easily and then perform experiments to determine what products are formed. Aside from the early work on oxidative methane coupling by doped oxides, Mayernik and Janik229,344 seem to be the first to have used computations to calculate the activation energy for methane activation by a doped oxide. They showed that CeO2 doped with Pd or Zr is a better methane activation catalyst than ceria. Subsequent work showed that the same is true for La2O3 doped with Cu, Mg, or Zn272 and for ceria doped with Pt.337 We propose that, in general, oxides doped with low-valence atoms are better alkane activation catalysts than the undoped oxides. We need however to qualify this statement. In most cases, alkane activation takes place with a useful conversion at temperatures above 400 °C. If the dopant makes the surface oxygen too easy to remove, then the doped oxide in contact with a reductant (CO, H2, an alkane) may lose the activated

more favorable energetically than desorbing surface oxygen atom as O2. The fact that ΔEv is 2 eV or higher does not mean that there are no vacancies on the surface because much energy is gained if the surface oxygen is removed by formation of water (rather than 1/2O2) or by formation of CO2 (in a reaction with CO). 9.3. The Presence of an LVD Makes the Surface More Reactive

9.3.1. General Comments. One of the guiding rules in working with doped oxide catalysts is that the easier it is to remove a surface oxygen atom, the more reactive is that atom. If this is true, a surface doped with LVDs ought to be more reactive than the undoped surfaces. We show below that this is the case. Because doping with an LVD turns the oxide surface into a Lewis acid, the oxygen on the surface of such oxide will bind strongly Lewis bases. This means that the presence of the LVD will activate the oxygen to bind more strongly CO, H, or an alkyl (which are Lewis bases). In particular, the doped surface will break a C−H in an alkane because the fragments formed by dissociation are Lewis bases. Calculations confirm these qualitative rules. For example, rutile TiO2(110) doped with Au is a much better CO oxidant than the undoped rutile.141 This early work made a number of observations about CO oxidation catalyzed by oxides doped with LVDs that are likely to be general.333,334,338,339,342 It showed that CO reacts with a surface oxygen atom located near the dopant to form a Os−CO group (Os is an oxygen atom in the surface layer) whose C−O bond lengths are close to those of CO2. CO binds most strongly to the oxygen atom that is easiest to remove. The CO2 molecule formed in this way desorbs easily from the surface, leaving behind an oxygen vacancy. Because the vacancy is a Lewis base and gaseous O2 is a Lewis acid, our rules suggest that the vacancy adsorbs O2 readily. Moreover, the vacancy donates electrons to the adsorbed O2, weakening the O−O bond. The calculations show that this is true: O2 adsorbed at the vacancy site reacts readily with CO and forms a “carbonate”, which is easily 4405


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experience shows that it often provides a useful rationalization of the experimental or computational results. The most relevant change, when an oxide is doped with an LVD, is the appearance of an empty orbital (hole) in the valence band; the LVD is a p-type dopant. This hole is responsible for the acidity of the doped oxide: when a Lewis base binds to the doped oxide, it donates an electron, which fills the hole. The energy gained in this hole-filling process contributes to the increase in the binding energy of the base. A much-debated question is whether the empty orbital is located around one oxygen atom (the hole is localized) or it is spread over many oxygen atoms neighboring the dopant (the hole is delocalized) or over the whole solid (the hole is completely delocalized). The answer to this question may be important: a localized hole converts (formally) an O2− into an O−, and a chemist will think, rightfully, that O− is more reactive than O2−. We have detailed information on this issue for Al-doped αSiO2 (smoky quartz). Al atoms are a common impurity in silica, and they are low-valence dopants. In what follows, we denote Al and the four atoms bound to it by the symbol [AlO4h]0, where h signifies the presence of a hole caused by the replacement of a tetravalent Si atom with a trivalent Al atom. The superscript 0 indicates that the AlO4 group is electrically neutral. GGA-DFT calculations on this system found that the hole is delocalized over the four oxygen atoms neighboring Al.327,345−347 Calculations on SiO2 clusters doped with Al and capped with H atoms,348−350 which allow the use of more reliable methods of computation, show that the hole is localized on one oxygen atom. This localization breaks the tetrahedral symmetry: one of the oxygen atoms is closer to Al than the other three. This is the oxygen atom on which the hole is localized. In particular, Pacchioni et al.350 have calculated the coupling parameters in the EPR Hamiltonian and obtained results that are close to the experimental ones.351−353 Prompted by this situation, Nolan and Watson328 proposed that whenever hole formation is suspected (e.g., when an LVD is present), one should use DFT+UO where UO is a U parameter for oxygen (in addition to the more common U parameter for the cations). They varied the value of UO to obtain a distorted geometry for [AlO4h]0 and localize the hole on one of the oxygen atom. They found that UO = 7 eV gives the desired result and smaller values of UO do not. Subsequently, they recommended that UO = 7 eV should be used by all studies of oxides containing low-valence dopants.190,328,330,332 One would infer that if U for oxygen is needed when an LVD is present, it should also be used in calculations on undoped oxides. We are not aware of any work in which such a procedure was used. To better understand what is involved, we summarize some of the facts about the [AlO4h] system, presented in several review articles.351−353 [AlO4h] is not normally present in smoky quartz; instead, one has either AlO4H or AlO4Li. This is not a surprise because Al-doped siliceous zeolites are wellknown to always come with a “charge compensating ion”. This is in keeping with the “chemical compensation” concept145 that proposes that whenever a surface is a Lewis acid, like the surface of an oxide doped with an LVD, energy is gained if a Lewis base binds to it. H and Li are the Lewis bases available when the quartz is synthesized, and, as a result, AlO4H or AlO4Li is present in the material. One has to resort to extreme measures to prepare [AlO4h]0: irradiate Al-doped silica351−353 for a long time with X-rays, or electrons, or γ-rays, or neutrons,

oxygen at a temperature that is lower than that of C−H bond activation. This is more likely to happen when the oxidant is CO2 than when it is O2. In this case, the oxide is reduced at the reaction temperature, and this reduced oxide might or might not be a good oxidation catalyst. Experiments underway in our group show that this is the case for dry reforming of methane by Ru-doped ceria: the catalyst is the reduced oxide. We believe that this is the case because pulsing O2 through the system under steady-state reaction conditions reduces the methane conversion. An interesting example of alkane activation by oxides doped with an LVD is provided by Zn-doped La2O3(001). The undoped oxide reacts with methane at very high temperatures, close to those at which CH4 dissociates into H and CH3 in the gas phase. The DFT calculations show that dissociative adsorption of CH4 is extremely exothermic (4.45 eV).145 The dissociative chemisorption of methane on Zn-doped La2O3(001) is slightly exoergic. In Figure 3, we show the two lowest energy states of the dissociated methane on this system (all other binding schemes we have studied have higher energies). In Figure 3a, the H and CH3 fragments bind to surface oxygen atoms neighboring the Zn dopant. In Figure 3b, CH3 binds to the dopant and H to an oxygen atom nearby. In what follows, we call these states 3a and 3b. In Table 2, we give Table 2. Information Regarding the Dissociative Adsorption of CH4 on Zn-Doped La2O3(001)a Figure 3a Figure 3b

ΔEr, eV

Ea, eV

dCH, Å

−0.45 −0.12

1.3 0.54

3.74 3.12

ΔEr is the reaction energy, Ea is the activation energy, and dCH is the distance between the C atom in the chemisorbed CH3 and the H atom in the hydroxyl formed by disociation.

a

the dissociation energies and the activation energies (calculated with the NEB method) for reaching 3a and 3b. Clearly the presence of the dopant has a dramatic effect on the reaction energy and on the activation energy for CH4 dissociation. The table also shows the distance dCH between the carbon atom in the adsorbed CH3 radical and the hydrogen atom forming the hydroxyl. This is relevant because of a very simple rule that we use for a preliminary screening of the activation energy for reaching various possible product state. The activation energy of a bond-breaking reaction is lowered when the fragments are able to start making bonds with the surface before one stretches too much the bond to be ruptured. This allows us to eliminate a priori final states in which the distance between the chemisorbed fragments is substantially larger than the length of the bond to be ruptured. Table 2 indicates that the distance dCH between the fragments in 3b is smaller than that in 3a. According to the rule just stated, the activation energy for reaching 3b should be smaller than that for reaching 3a, and it is. This runs counter to the Brønsted−Evans−Polanyi rule, which states that the activation energy is controlled by the binding energy of the final state. In the present example, the state 3a is bound more strongly to the surface than 3b, but the activation energy to reach 3a is higher. 9.4. The Electronic Structure of an Oxide Doped with an LVD

Chemists often use orbital energies and shapes to explain qualitatively the properties of chemical bonds. While there is no rigorous theoretical justification for such analysis, extensive 4406


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or heat the silica sample for 10 h at 1400 °C, and a pressure of ∼10−3 Torr.354 After one of these procedures is used, EPR measurements confirm that the oxygen geometry around Al is distorted and a localized hole (an O−) is present. Schnadt and Schneider, 352 however, found that the EPR spectrum disappears at a temperature above 170 K because the hole moves rapidly from one oxygen atom to another. It was estimated that the barrier preventing this motion is ∼0.03 eV. This means that at the temperatures at which catalysis takes place, the hole is very mobile and the total energy difference between a system with a localized hole and one with a delocalized hole is very small. At the temperatures used in catalysis, the localization or the delocalization of the hole is an irrelevant issue. We are therefore of the opinion that one should not use a UO parameter for oxygen if the reason is to localize the hole, because this will alter all other results of the computation. The question is whether the same small localization energy is observed in other systems. Chrétien has performed calculations on La-doped ceria and found that one can obtain a hole localized on oxygen by performing PBE-GGA as follows. Perform the electronic-energy optimization with a geometry in which one O atom is slightly shifted toward the La dopant to break the symmetry. After the electronic energy is optimized, release the constraint on O positions and optimize both the electronic energy and the geometry. The final state obtained by this procedure maintains the distorted structure, and the hole in this state is localized on two oxygen atoms (forming a polaron). A PBE-GGA starting with symmetric oxygen positions will maintain symmetry and will produce a delocalized hole. The energy difference between the two states is 0.03 eV; within the accuracy of DFT these states are equally probable. Chrétien has also performed calculations of the binding energy of hydrogen to various O atoms on the La-doped Ce(111) surface. The binding energies when using for oxygen UO = 7 eV (recommended by Nolan and Watson) differ from those obtained by using UO = 0 eV. This difference is of ∼0.7 eV, which is much larger than the localization energy. By using UO for oxygen, we are correcting an unimportant (i.e., small) energy error (that of localization) and causing large energy changes in a quantity (H-binding energy) that is important to chemistry. We do not know whether the binding energy obtained when UO is used is more accurate than the one obtained when UO = 0. Given this situation, we think that one should not use U for oxygen. However, this is an open question, and it will surely be taken up again in the future.

Fe2O3. At a given temperature, the conversion by the doped oxide catalyst is higher than on either one of the undoped oxides. This is what one expects from calculations. Wilkes et al.356 studied methane oxidation by ceria doped with trivalent La, Pr, and Gd and found that all of these dopants facilitate methane oxidation. This is another success for the calculations. However, Wilkes et al.357 tested the same doped ceria catalysts for CO oxidation and found that doping inhibits the reaction: the doped oxide is less reactive than the undoped one. This is not what calculations suggest. These two reactions are carried out at different temperatures, and CO and CH4 have different “reducing power” so it is likely that the surface of the doped oxide catalyst under reaction conditions is not the same for the two reactions. Zhao and Gorte have presented results on alkane combustion on doped and undoped ceria, which seem to be at odds with what is inferred from computations on other doped-oxide systems as well as with other measurements on doped ceria. In a study of n-butane combustion catalyzed by ceria and by Me0.2Ce0.8O2 with Me = Yb, Y, Sm, Gd, La, Nb, Ta, or Pr, Zhao and Gorte96 found that all doped-ceria oxides are less efficient than the undoped ceria, except for Zr0.2Ce0.8O2 whose activity is comparable to that of ceria. The difference in rate (measured in a differential reactor and given in units of molecules/(s m2)) is a factor of ∼100, and it is well outside the margin of error of the experiments. The amount of oxygen that can be removed by reacting with CO is larger on the undoped ceria than in the doped one, contrary to what the rules on LVDs’ effects would suggest. Another “anomaly” is that all doped-ceria samples had a smaller area per gram than ceria, contrary to what is frequently observed on other doped oxide systems. Note that some of the dopants are LVDs, some are HVDs, and one is an FVD; nevertheless, they all lower the rate of the combustion reaction. This is not what one would expect from theory. Zhao and Gorte358 focused next on the combustion of methane, ethane, propane, and n-butane on Sm0.2Ce0.8O2. They found that doping with Sm does not affect the rate of combustion of methane and ethane but lowers the rate of combustion of propane and n-butane. This is not what one would expect if one extrapolates what is known from calculations on other systems. It is however unwise to conclude that calculations are misleading without performing detailed calculations on these specific systems. We note that all dopants used in these studies form very stable and unreactive oxides, and therefore it is possible that they formed (during calcination) small unreactive oxide clusters at the surface, blocking ceria reactive sites. It is also conceivable that the LVDs used as dopants facilitate the removal of the oxygen from ceria: the catalyst is then the reduced oxide, and this may be less active than ceria.

9.5. Comparison with Experiments

We have already pointed out many of the difficulties related to comparing these calculations with experiments: we do not know to what extent the materials we prepare are similar to the models used by the calculation. The qualitative conclusion from calculations is that doping with an LVD in the surface layer will make the doped oxide a better oxidant than the undoped oxide. This means that we expect that temperature programmed desorption of oxygen, temperature programmed reduction by CO or H2, and the light-off temperature for alkane activation will occur at lower temperature for an oxide doped with an LVD than for the undoped host oxide. We give here a few examples to emphasize the difficulties we encounter. Qiao et al.355 prepared FexCe1−xO2 and showed that, as long as x is less than 0.2, CO oxidation and CH4 oxidation start (in TPR experiments) at lower temperature than on either CeO2 or

10. THE EFFECT OF HIGH-VALENCE DOPANTS (HVDS) 10.1. Introduction

The effect of high-valence dopants on the chemistry of the host oxide is more complex than that of LVDs because it depends on whether the host is reducible or not. It is easiest to understand why this is so in terms of Lewis acid−base interactions.221 In an irreducible oxide (e.g., ZnO), a high-valence dopant, such as Al, has one excess electron (two out of the three valence electrons of Al are used to “replace” Zn2+). This formal electron counting is not rigorous, but it is very helpful in understanding the behavior of an oxide doped with an HVD. If the host oxide is irreducible, this additional electron is localized on the 4407


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dopant.260,272 This means that the dopant is a Lewis base, and this controls some of its chemistry. A high-valence dopant in a reducible oxide (e.g., Ta dopant in CeO2) behaves differently.258 Because Ce4+ cations are easily reduced to Ce3+ (Ce4+ is a Lewis acid), an HVD can donate its “excess” electrons to reduce Ce ions in the host oxide. For example,258 hexavalent W or Mo dopants donate two electrons and reduce two Ce4+ ions; pentavalent Ta or Nb dopants donate one electron and reduce one Ce4+ ion. Thus, these high-valence dopants are formally tetravalent, and as a result they do not affect reducible oxides in the same way they would affect the irreducible ones. Because of this situation, we treat high-valence dopants in irreducible oxides separately from those in which the host oxide is reducible. We generalize the results of a small number of calculations to propose some qualitative rules that need to be tested by further calculations on similar systems. 10.2. HVDs in Irreducible Oxides

Figure 4. Top and side views of the structure of a Zr-doped La2O3(001) surface exposed to gaseous O2. The oxygen molecule shown in dark blue adsorbs on the dopant. The light-blue spheres are La atoms, and the red spheres are surface oxygen atoms. The dopant is dark gray.

271

Pala et al. examined the effect of all substitutional dopants on the energy of oxygen-vacancy formation on the surface of ZnO. They showed that all HVDs increased the energy needed for removing a surface oxygen atom located near the dopant. This suggests that the HVDs can be used to hinder oxidation by the Mars−van Krevelen mechanism. However, this does not necessarily mean that their presence converts the host oxide into a poor oxidation catalyst. If an oxidation reaction is performed with O2 (as opposed to other oxidants such as CO2), the oxygen, which is a strong Lewis acid, will bind to the high-valence dopant, which is a Lewis base. At this point, we need to distinguish two limiting cases. If the valence of the dopant exceeds that of the cation it substitutes by 1 (we call this an HVD1), then the adsorbed oxygen molecules gain one electron (formally) in an antibonding orbital. As a result, the bond length of the adsorbed species is substantially longer than that of gaseous O2, and the adsorbed species gains electron charge and closely resembles O2−. This is a reactive species, and when exposed to CO it makes a carbonate that decomposes to release CO2. This leads to what Pala et al.359 called a “nonMars−van Krevelen” mechanism: the oxygen atom from the oxidized product originates from the gas, not from the surface of the oxide. Experiments on CO oxidation by Al- or Ti-doped ZnO359 confirmed this prediction: when the oxidation was performed with short pulses of gaseous 18O2, the CO2 produced in the reaction contained 18O. A limited number of calculations on other systems suggest that the activation of the oxygen adsorbed on an HVD1 is likely to be general. For example, Zr-doped La2O3(001) binds an oxygen molecule (see Figure 4) with an energy release of 2.63 eV. The O2 molecule is not dissociated, but its bond length is 1.35 Å, which is longer than the bond length of gas-phase O2 and comparable to that of the gas-phase O2− ion. The adsorbed O2 makes use of the extra electrons on Zr; therefore, the ZrO2 group no longer has three electrons to form bonds with the oxide. Thus, ZrO2 creates an electron deficit in the system, which means that it acts as a low-valence dopant and turns the surface into a Lewis acid. If this qualitative model is correct, then the presence of ZrO2 should facilitate the formation of oxygen vacancies. Calculations show that indeed it does: it takes only 0.96 eV to remove an oxygen atom next to the ZrO2 group and form 1/2O2 in the gas. There are now two kinds of active oxygen atoms in the system: the adsorbed O2 and any one of the three surface oxygen atoms neighboring the ZrO2 group. This is reflected in the manner in which methane

adsorbs dissociatively in this system. In Figure 5, we show one final state for dissociative adsorption, in which H and CH3 bind

Figure 5. The dissociative adsorption of CH4 on a surface consisting of Zr-doped La2O3(001) on which an O2 molecule was preadsorbed on the dopant. The system consists of a methoxide and hydroxyl, both bound to the Zr dopant. The oxygen atoms originating from the gas are blue, and the oxygen atoms connected to the gray Zr dopant are red. The La atoms are located at the blue corners and the rest of the surface oxygen atoms at the red corners. Carbon is the smaller gray sphere, and the H atoms are white. The lower figure is the side view of the upper figure.

to the oxygen atoms adsorbed from the gas phase (which, in turn, are bonded to the Zr dopant). This reaction is exoergic by 1.20 eV. A second final state is shown in Figure 6, where the methyl radical binds to one of the oxygen atoms previously adsorbed from the gas (to form a methoxide bound to Ru) and the other binds to one of the oxygen atoms in the surface layer, near to Ru, to form a hydroxyl. This reaction is exoergic by 1.04 eV. Similar results are obtained for La2O3 doped with Ti, and we propose that this kind of behavior is general for HVD1 dopants in irreducible host oxides. However, more calculations 4408


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believe that this very strong bond with oxygen is typical for HVD2 or HVD3. This means that these dopants bind oxygen too strongly and the complex formed in this way is too low in energy to participate in redox reactions. More studies are needed to confirm these qualitative statements. 10.3. HVDs in Reducible Oxides

Very few calculations have examined this kind of system. They allow us to make some guesses and extrapolations, but they need to be reinforced (or modified) by further work. By definition, the cation in a reducible oxide (e.g., CeO2, TiO2) can easily change its formal charge while interacting with electron-donating species (Lewis bases). The few existing calculations suggest that Lewis bases adsorbed on the surface are able to reduce Ce4+ to Ce3+ (or Ti4+ to Ti3+). For example, H (a Lewis base) reduces a Ce ion when it is adsorbed on CeO2360 and so does a Cu atom adsorbed on ceria.335 Nolan331 has examined oxygen-vacancy formation at the surface of CeO2 doped with La. Because La is an LVD, its presence creates a hole located on an oxygen atom, which acts as an acid site. When an oxygen vacancy is formed, two unpaired electrons are created: one fills the hole and the other reduces Ce4+ to Ce3+. In all calculations, the reducibility of Ce4+ is present only in DFT+U (with sufficiently large U) or when hybrid functionals are used. These statements are consistent with experiments, which show that the adsorption of H (a Lewis base) or the formation of an oxygen vacancy (a Lewis base) is accompanied by the appearance of Ce3+ or Ti3+ in the XPS or in the EPR spectra. On the basis of these observations, we accept the suggestion that Ce4+ and Ti4+ can act as Lewis acids. It is also reasonable to assume that in the presence of oxygen in the gas, the highvalence dopant will adsorb oxygen and form either an MeO2 group or an MeO group.262 This should happen because oxygen is a much stronger acid than Ti4+ or Ce4+. We are not aware of the existence of calculations that have examined whether such behavior extends to other reducible oxides (V2O5, Nb2O5, SnO2, MoO3, etc.). We hope that the conjectures made here will stimulate more calculations on HVDs in reducible oxides.

Figure 6. The dissociative adsorption of CH4 on a surface consisting of Zr-doped La2O3(001) on which an O2 molecule was preadsorbed on the dopant. The system consists of a methoxide bound to the Zr dopant and a hydroxyl formed with a surface oxygen atom near the dopant. The oxygen atoms originating from the gas are blue, and the oxygen atoms connected to the gray Zr dopant are red. The La atoms are located at the blue corners and the rest of the surface oxygen atoms at the red ones. Carbon is the smaller gray sphere, and the H atoms are white. The lower figure is the side view of the upper one.

and more experiments are needed for establishing this statement firmly. The situation is different in the limiting case of HVD2. An example is provide by Ta-doped La2O3(001) exposed to oxygen. The O2 molecule dissociates: one atom binds to Ta, and the other binds to a surface oxygen atom forming a sort of O2 molecule (dark yellow in Figure 7). The binding energy of O2 is 4.58 eV. Similar results were obtained for Nb-doped La2O3. Although not many studies of this kind are available, we

10.4. The Stability of High-Valence Dopants

This is a question that has received little attention so far even though it is on the mind of anyone performing experiments with doped oxides. Consider a high-valence dopant that forms a very stable oxide (e.g., Ta, Nb, La) with a high coordination to oxygen. Imagine that such a cation is used to dope an oxide (e.g., NiO, PbO2, CuO) that is less stable than the dopant’s oxide. One would suspect that such a dopant in contact with gas-phase oxygen might prefer to leave the cationic site, move on top of the surface, and adsorb oxygen from the gas phase to form an oxide cluster. If this structure has lower energy it is likely that it will become prevalent during calcination (when the doped oxide is prepared). The intended doped oxide ends up being a submonolayer (or a layer) of small dopant−oxide clusters covering a surface with cationic defects. A few exploratory calculations in our group show that this is the case for Cr-doped CeO2 and for Nb-doped NiO. As we have discussed earlier, it is very difficult to determine by experiments whether this “segregation” of the dopant takes place.

Figure 7. The interaction of Ta-doped La2O3(001) with gas-phase oxygen. The Ta dopant is bright yellow. The four purple spheres are oxygen atoms bound to Ta; three are surface oxygen atoms, and one (sticking into the vacuum) comes from the gas phase. The dark yellow spheres are oxygen atoms. One is a surface oxygen atom, and the other came from the gas phase. The rest of the oxygen atoms are red or magenta, and La is gray. 4409


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11. SAME-VALENCE DOPANTS (SVDS) AND FLEXIBLE-VALENCE DOPANTS (FVDS) Quite a bit of work has been devoted to the situation when the dopant and the cation being substituted have the same valence. Calculations have been performed to study the properties of TiO2 doped with Ce,361 Zr,361 and Pt141 and of CeO2 doped with Zr,229,235,255,258,334,361,362 or with Ti,362 or Hf,362 or Pd,229 or Pt,258 or Si, Ge, Sn, and Pb.363 We note in particular a study by Mayernick and Janik229 on methane activation by Zr-doped ceria. All of these calculations show that the dopant affects favorably the properties of the host oxide. We mention a few experimental papers, many of which investigated the modification of the oxygen release in CeO2 doped with samevalence dopants, such as Zr,364−367 Ti,365,368−374 Sn,375 or Sn.376 Other studies investigated the redox properties of CeO2 doped with Ti,377 CO oxidation by CeO2 doped with Pt,378 water−gas shift by Pt-doped CeO2,379 and methane oxidation by ZrCeO2.380 In all cases, doping with an SVD improves performance. However, we were unable to formulate any simple rules regarding the SVDs in catalysis other than to say that they should be tried, especially if one tunes the properties of an oxide that is already an efficient catalyst. Because we define the valence of a dopant by examining the stoichiometry of its stable oxides, we cannot assign a unique valence to dopants that have several stable oxides. For example, V2O5 and V2O3 are stable oxides, VO2 and VO are also stable but less so, and compounds with formulas VnO2n+1 and VnO2n−1 have also been prepared. Obviously vanadium is a very versatile atom capable of taking a variety of valences when binding to oxygen. If we dope an oxide whose cations are tetravalent (e.g., SnO2) with vanadium, should we assume that vanadium is an HVD (because V2O5 is stable) or an LVD (because V2O3 is stable)? There is no a priori answer to this question. More complications arise if one considers that tetravalent and divalent vanadium are also possible. This is why we classify a V-dopant in a MeO2 host oxide as being a flexible-valence dopant (FVD). Not much is known about the general behavior of such systems. Nb is an FVD (Nb2O5 is the most stable Nb oxide but NbO2 and NbO are also stable), and if it dopes CeO2 it will donate an electron and reduce a cerium ion.258,330 Therefore, the formal valence of Nb is IV. It is interesting that if NO2 is adsorbed on the Nb-doped ceria surface an NO2− ion is formed.330 NO2 is apparently a stronger acid than Ce4+, and it manages to capture the electron that Nb (a Lewis base) makes available. The conclusions drawn from these calculations are supported by experiments on V-doped SnO2 catalysts, which consist, when the concentration of V is low, of a solid solution in which vanadium is V4+. Another interesting example of flexible-valence dopant is provided by Cr-doped ceria. Experiments in Hegde’s group381,382 indicated that this doped oxide contains Ce3+ and Ce4+ as well as Cr4+ and Cr6+. Cr-doped ceria is a very good oxygen storage material (when heated it releases O2 at ∼350 °C). The as-prepared compound has an excess of oxygen (the proposed formula is Ce0.67Cr0.33O2.11). While the amount of Cr in Hegde’s study is uncomfortably high (most solid solutions are not stable at such high concentrations of dopant), the XRD measurements show that no Cr oxide is present and the system has the fluorite structure of CeO2. Calculations in our group found that this system has very peculiar behavior. In general, doping CeO2(111) with a high-valence dopant distorts

slightly the position of the oxygen atoms surrounding the dopant. Cr makes three stable oxides, CrO3, CrO2, and Cr2O3, so it is therefore capable of being hexa-, tetra-, or trivalent, which makes it a flexible-valence dopant. Calculations in which one Ce ion in the CeO2(111) surface is replaced with a Cr atom show that this system has six different geometries whose energy differs by at most 0.26 eV from the lowest energy one. Two of the structures are practically degenerate and have the lowest energy. In one of these Cr binds to five oxygen atoms, one of which is pulled out of its lattice site by ∼1 Å. We call this the CrO5 structure. When the oxygen atom is pulled out of its lattice site, a vacant oxygen-lattice site is created, and we call this a pseudo oxygen vacancy (“pseudo” because the O atom has been removed from its lattice site but not from the surface). In addition to having a pseudo vacancy, the system also has a reduced Ce atom. In another structure, with practically the same energy as CrO5, the Cr dopant binds to four oxygen atoms, makes one pseudo oxygen vacancy, and reduces two Ce ions to Ce3+. A dopant causing such large distortions of oxygen positions is unusual. Both the CrO4 and the CrO5 structures adsorb oxygen from the gas phase, and the reaction Cr‐doped ceria + 1/2 O2 (g) → Cr‐doped ceria with an additional O atom

is exoergic by 1 eV for CrO4 and 0.7 eV for CrO5. This means that excess oxygen is present in the system, and this might explain why Hegde et al. found that the stoichiometric formula of the compound is Ce0.67Cr0.33O2.11. The calculations show that the compound formed by oxygen adsorption on Cr-doped ceria is a strong oxidant. The examples presented here suggest that it may be difficult to find simple rules for the effects of flexible-valence dopants, especially if the host oxide is a reducible. More computational and experimental work is needed for clarifying the behavior of these systems.

12. TWO CASE STUDIES: OXIDATIVE DEHYDROGENATION OF ETHYLBENZENE AND ETHANE 12.1. Introduction

While the literature on catalysis by doped oxide is extensive, there are very few cases in which such catalysts were studied systematically, by preparing one host oxide with a variety of dopants, by a variety of methods, and by using them for the same reaction under a variety of conditions. For this reason, we thought that it would be useful to concentrate on two examples for which substantial information exists. Both examples are oxidative dehydrogenation reactions: one converts ethyl benzene to styrene, and the other converts ethane to ethylene. 12.2. Oxidative Dehydrogenation: Generalities

The dehydrogenation reactions examined here are both described by the formula RCH 2CH3 → RCHCH 2 + H 2

In one case R is a hydrogen atom, and in the other it is a phenyl group C6H5. Both reactions are endothermic, equilibrium limited, and take place at sufficiently high temperatures to require expensive technology. It is therefore tempting to perform the dehydrogenation reaction in the presence of a compound that reacts with H2, thereby shifting favorably the 4410


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of ethylbenzene (136.2 °C) is very close to that of styrene (145.2 °C), and separation is difficult. This is a demanding process that could stand improvement, and much work has been devoted to finding good catalysts for oxidative dehydrogenation (ODH) of ethylbenzene to styrene. As discussed in section 12.2, this can be done by using either O2 or CO2. In what follows, we discuss the use of doped oxides to perform these reactions.

thermodynamic equilibrium and diminishing the amount of heat necessary for the reaction. A much-pursued variant of this scheme is oxidative dehydrogenation (ODH) using oxygen to form water: RCH 2CH3 + 1/2 O2 → RCHCH 2 + H 2O

An alternative is to use CO2, to react with H2 and make CO and water (the reverse water−gas shift reaction, RWGS): RCH 2CH3 + CO2 → RCHCH 2 + CO + H 2O

14. OXIDATIVE DEHYDROGENATION OF ETHYLBENZENE TO STYRENE USING CO2 AS AN OXIDANT

To our knowledge, the first coupling of a dehydrogenation reaction with a hydrogen consuming reaction is the Deacon process,383,384 which can be viewed as a oxidative dehydrogenation of HCl (2HCl + 1/2O2 → Cl2 + H2O). A detailed comparison between ODH with O2 and with CO2 is best made separately, for each compound being dehydrogenated. There are however several aspects that are common to all dehydrogenation reactions. The use of CO2 has the advantage of producing CO and thus recovering some of the fuel capacity of the disappearing hydrogen; ODH with O2 produces water and no fuel. One can think of the ODH with CO2 as using CO2 as an oxidant, instead of O2. CO2 is less aggressive than oxygen, and it is less likely to react with the alkene produced by dehydrogenation. The amount of oxygen in the ODH feed with O2 is limited by the danger of explosion. This is not a problem in ODH with CO2. Unfortunately, ODH with CO2 is an endothermic reaction and requires net heat addition. While the idea of coupling dehydrogenation with a hydrogen consuming reaction is appealing, the performance of the catalysts studied so far is not good enough for commercial use.

14.1. Introduction

In 1968, Olson was granted a patent388 suggesting that the conversion of ethylbenzene by dehydrogenation can be improved by coupling it with the reverse water−gas shift (RWGS) reaction. Towler and Lynn387 have calculated that at 600 °C and a CO2-to-ethylbenzene ratio of 5:1, the equilibrium conversion of ethylbenzene is 95%. If the dehydrogenation is performed under the same conditions, but in the absence of CO2, the equilibrium conversion is 67%. These calculations were made by assuming that no other reactions, besides eq 1 and RWGS, take place in the system, which is true only if the catalyst is very selective. Because of this assumption, the equilibrium yield of styrene calculated in this way is an upper limit. The reverse water−gas shift reaction is endothermic, so heat must be provided externally, but the cost of heating CO2 is smaller than that of heating steam, which is a significant advantage. Producing CO recovers to some extent the fuel lost by consuming the hydrogen, which is not the case when O2 is used as oxidant. A large number of papers have used oxides389−416 or “mixed oxides”390−392,394,395,399−401,403,407−411,413,415,417−427 as catalysts for the conversion of ethylbenzene to styrene. Many of the papers using mixed oxides did not intend to make substitutionally doped oxides. In most cases, their goal was to create two phases of two oxides, one basic (to adsorb and perhaps activate CO2) and one acidic (to activate the dehydrogenation of the ethylbenzene). Many examples are offered by the work in Park’s group390−392,394,403,424−426 that studied ZrO2 mixed with TiO2, or with MnO2,391 with SnO2,392 or with vanadium oxide394 or with TiO2 or with K2O,390 and vanadia supported on Al2O3 mixed with MgO, P2O5, Cr2O3, MoO3, Sb2O3.426 Reviews of this work are available.424,425 As a rule, the mixed oxides (two phase system) performed better than the individual, unmixed oxides. It is possible that these catalysts contained some amounts of doped oxides, but this has not been documented or suggested. We will not discuss work on mixed oxides further and concentrate on papers that intended to prepare doped oxide catalysts or are likely to have done so. Given the fact that so many oxides have fairly good activity for styrene preparation by ODH, it is natural to ask whether doping them might increase their activity.

13. THE OXIDATIVE DEHYDROGENATION OF ETHYLBENZENE TO STYRENE Styrene, C6H5CHCH2, is an important alkene used to make a variety of polymers.385 It is prepared industrially by dehydrogenating ethylbenzene: C6H5CH 2CH3 → C6H5CHCH 2 + H 2 ΔH 0 = 123.6 kJ/mol

(1)

a reaction catalyzed by a mixture of Fe2O3 (40−90%) and K2O (5−30%) promoted with Cr, Ce, Mo, Ca, and Mg oxides.385 There are many catalyst preparations that are variations on this theme, and Lloyd2 mentions that 12−15 different iron oxide catalysts for this reaction are provided by various suppliers. The promoters are added in small amounts, and we do not know if they are dopants or small oxide clusters supported on the surface of the main oxides. The role of these promoters is explained in a review by Cavani and Trifiro.386 The dehydrogenation reaction is endothermic, equilibrium-limited, and thermodynamically favored by high temperature and low pressure. Because the temperature used in industry ranges between 550 and 680 °C, one cannot use molten salts as a heat transfer agent, and the reactants are heated by injecting hot steam in the feed. Besides serving as a heat-transfer medium, the steam reduces coking, acts as a diluent (which favors product formation according to the Le Chatelier principle), and prevents the reduction of the iron oxide catalyst. The need to use at least 6−8 mol of steam per mole of ethylbenzene387 is a major expense in styrene manufacturing. The conversion is 60− 70%, and molar selectivity is 90−95%.387 High conversion is particularly important for this system because the boiling point

14.2. Ethylbenzene Dehydrogenation in the Presence of CO2, Catalyzed by Doped Oxides

14.2.1. Experimental Results. There is little work that attempted deliberately to prepare doped oxides and test them for ethylbenzene ODH with CO2. Ren et al.415 studied TiO2 and TiO2 doped with Fe, V, Zr, and Mg, and Gao et al.420 used ZrO2 and Ga-doped ZrO2. We examine Fe-doped TiO2 and 4411


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catalysts). The best styrene yield is 34.3%, catalyzed by 15GZ. The yield increases with increased amount of Ga up to 15GZ and then decreases for 20GZ. This is common with many doped oxides: when too much dopant is added, the activity is lost. Doping with Ga increases the conversion, however, as the area per gram of 15GZ is 53.9 m2/g, while that of ZrO2 is 19.0 m2/g. The conversion per area of catalysts is higher for ZrO2 (0.88) than for 15GZ (0.63). The selectivity on ZrO is slightly better. We conclude therefore that ZrO2 doped with Ga is a worse catalyst than ZrO2, and both have very low performance. It is unfortunate that the ethylbenzene conversion decreases with the time on stream, and after 4 h it is ∼15%. Ren et al.415 prepared TiO2 doped with Fe, V, Zr, and Mg. Among these, Fe3Ti is most active. The measurements on V3Ti, Zr3Ti, and Mg3Ti show that all of them have higher conversion and better selectivity than TiO2. When compared to each other, they have similar performance. In light of the computational results in other systems, this is surprising, given the fact that the V is a high-valence dopant, Mg is a low-valence dopant, and Zr has the same valence as Ti. We caution that the experiments described here were very limited in scope and look only at a narrow range of parameters (dopant concentration, method of preparation, gas composition, temperature, etc.). The conclusions we draw are temporary, and further experiments may change the situation. 14.2.2. Are These Results Consistent with What We Know from Calculations? Unfortunately, we have no systematic calculations for the systems discussed in this section. From calculations on other systems, we know that low-valence dopants lower the energy of vacancy formation and will make the oxide a better oxidant. Corma et al.234,329,428 have shown that this is also true for anatase doped with Fe, which is close to one of the systems we discuss here. We also know that the easier it is to remove an oxygen atom from the surface layer, the more reactive is that atom. In particular, the oxide doped with a low-valence dopant (LVD) should break a C−H bond more efficiently than the undoped one. This should be the case for Ga-doped zirconia and Fe-doped TiO2. This prediction would be best tested by doing TPR experiments with ethylbenzene to determine the temperature of the onset of reaction with the doped oxide and the undoped one. Such measurements are not available for the systems reviewed in this section. The situation is more complicated for steady-state experiments. The oxide catalyst is reduced by ethylbenzene, and it is oxidized by CO2. Because CO2 is a poor oxidant, the rate of reduction is likely to be faster than the rate of reoxidation of the surface, and the catalyst under working condition is likely to be the reduced doped oxide. Doping with an LVD makes the reduction of the oxide more facile. This qualitative prediction could be tested by running the reaction at steady state and then suddenly stopping the feed, sending oxygen pulses through the catalyst bed, and measuring the oxygen consumption. As far as we know, such experiments have not yet been performed on these systems. The next question is why Mg-, Zr-, and V-doped titania are less active than the Fe-doped sample. By analogy with calculations performed on other systems, we expect that doping with Zr will have a small effect on catalytic chemistry because its oxide is very similar to TiO2 (same valence, same structure). Calculations on CeO2 doped with a high-valence dopant have shown that the dopant loses electrons, which reduce a Ce atom from a formal charge of 4+ to 3+. If this happens for TiO2, which is a reducible oxide similar to CeO2, the V dopant will be tetravalent, and this means that the

Ga-doped ZrO2 together because in both cases the dopants are LVDs. TiO2 is reducible and ZrO2 is not, but this should not play an essential role in the chemical effects caused by the lowvalence dopants. Doping with Fe increases substantially the area per gram even though the doped and the undoped oxide were prepared by the same sol−gel method, under identical conditions (except for the presence of the precursor for the dopant). For example, the surface of TiO2 is 21.7 m2/g, that of Fe1Ti is 98.0 m2/g, and that of Fe3Ti is 61.1 m2/g. Here, we use the notation of Ren et al.,415 and Fe3Ti means that 3% of the Ti atoms were replaced with Fe. The XRD measurements detected no crystalline iron oxide. The undoped TiO2 made by the sol−gel method has an anatase structure. The presence of Fe results in a compound whose structure is a mixture of anatase and rutile; Fe7Ti is almost entirely rutile. This suggests that some of the Fe is atomically dispersed inside the TiO2 grains because it is unlikely that Fe will induce this phase transformation if all Fe atoms introduced in the system form small iron oxide clusters on the surface (which are not detectable in XRD). The reactivity of these catalysts, for ethylbenzene ODH with CO2 as oxidant, was measured at a temperature of 550 °C, a pressure of 0.1 MPa, W/F = 40 gcat h/mol, a CO2/ethylbenzene ratio of 1:1, and a flow rate of 50 mL/min. The best ethylbenzene conversion, obtained with Fe3Ti, was 19.3% with a styrene yield of 18.6%. The undoped TiO2 converted 5.4% of the ethylbenzene, with a styrene yield of 5.4%. If we do not take into account the change of surface area caused by doping, it would seem that Fe3Ti is more effective in converting ethylbenzene than TiO2. However, the area per gram of Fe3Ti is 98 m2/g, while that of TiO2 is 21.7 m2/g. If we calculate the ethylbenzene conversion per unit area then, by this measure, TiO2 is about as active as Fe3Ti; the main effect of the dopant on conversion is to increase the area per gram. However, the dopant improves selectivity, at the same conversion per area, which is 83.0% for TiO2 and 96.4% for Fe3Ti. The styrene yield is independent of area, and the better styrene yield on Fe3Ti indicates that selectivity benefits from doping. Aside from low conversion, a further disappointment is the low stability of the catalyst: the conversion drops from 19.4% to 8.6% after 2 h on stream. TGA experiments on the catalyst after the reaction show that the catalyst coked. However, reoxidizing the catalyst did not restore the ethylbenzene conversion fully, which suggests that additional factors are involved in deactivation. Gao et al.420 prepared Ga-doped ZrO2 by coprecipitation. We follow below their notation, in which 10GZ means ZrO2 doped with Ga and having 10% Ga2O3 by weight. The properties of ZrO2 and those of 5GZ, 10GZ, 15GZ, and 20GZ were studied. XRD, Raman, and UV−vis measurements support the assumption that Ga-doped zirconia has been prepared even though one has not ruled out the possibility that Ga is present in the form of very small GaOx clusters on the surface. Reactivity tests were performed with a gas mixture with the molar ration of N2/CO2/ethylbenzene = 361/19/1, at a temperature of 600 °C, with a flow rate of 60 mL/min. All catalysts tested were heated for 2 h at 600 °C in N2 flow before performing the ODH reaction. The conversion of ethylbenzene catalyzed by ZrO2 is 16.8%, and the selectivity is 100%, which gives a styrene yield of 16.8%. The ethylbenzene conversion increases by adding Ga, and the selectivity decreases slightly (it is in the nineties for all GZ 4412


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that the structure of the working catalyst is converted entirely to the rutile, which does not affect performance. No V2O5 was detected in the XRD spectra of the spent catalyst, and the XPS measurements detected coke on the surface. Calcination in air for 20 min at 500 °C restores activity, which suggests that the main reason for deactivation is coking. These experiments show that doping TiO2 with V improves performance, but the lack of stability is a serious limitation. If process economics justifies the added cost, the catalyst can be moved continuously out of the reactor, decoked, and moved back into the reactor. Such a procedure is used for several industrial catalytic processes. Fan’s group413 studied ODH of ethylbenzene to styrene with oxygen, catalyzed by CeO2 doped with Al, Sn, Zr, Mn, and Ni. All doped oxides had a 10% molar ratio of dopant to cerium. They were prepared by a template-assisted method,429,430 which makes a disordered mesoscopic solid. The Ni-doped ceria catalyst was also prepared by oxalate-gel coprecipitation to determine how the preparation method affects catalytic performance. The best catalyst in this group is Ni0.1Ce0.9O2, which gives an ethylbenzene conversion of 70% and a selectivity to styrene of 75%, at a temperature of 450 °C, O2/ethylbenzene ratio = 0.8, and space velocity = 250 mL/gcat/min. Changing the space velocity to 418 mL/gcat/min lowers the conversion to 64%, but increases the selectivity to 86%. Increasing the temperature increases the conversion but decreases selectivity. The performance of the catalyst decreases if the Ni content is lower or higher than 10% molar. It is likely that the performance can be improved by a more thorough optimization of the catalyst and of the reaction conditions. The performance of all other materials tested by Fan et al. is worse than that of Ni0.1Ce0.9O2. The selectivity is roughly the same, but the conversion is lower. The order of catalysts by their conversion at 450 °C is

chemistry of the V-doped TiO2 surface is similar to that of TiO2. The formal charge of Mg is 2+, and a few calculations on other systems of this type showed that when the valence of the dopant is lower by 2 than the valence of the cation it substitutes, the energy for making an oxygen vacancy near the dopant is either very small or exothermic. Therefore, the doped oxide may have lost the active oxygen by the time the system reaches the temperature at which ethylbenzene is activated. These speculations may explain why Zr, V, and Mg dopants are less efficient than Fe. Coupling the dehydrogenation of ethylbenzene with the reverse water−gas shift reaction is worth exploring further. Doping oxide catalysts, to improve their performance for this reaction, has received little attention. Very few oxide−dopant pairs have been examined, and no attempt was made to optimize the performance of those studied so far. No computations have been performed specifically for this reaction system, and the analysis performed here is based on analogy with other systems.

15. OXIDATIVE DEHYDROGENATION OF ETHYLBENZENE TO STYRENE BY USING OXYGEN AND DOPED OXIDE CATALYSTS As we have emphasized, the use of oxygen to react with the H2 shifts the equilibrium of the dehydrogenation reaction favorably. An obvious disadvantage is that an aggressive molecule such as oxygen might react with styrene, reducing selectivity. Danger of explosion and fire limits the oxygen/ hydrocarbon ratios that can be used. Burning hydrogen, which is a valuable product, may also diminish the economic appeal of the process. Sivarajani et al.412 prepared vanadium-doped titania, Ti1−xVxO2, by the combustion method. The values of x in the above formula were 0.05, 0.10, and 0.15, but these are calculated on the basis of the amount of V present in the precursors. The authors caution that some V may have been lost by evaporation during calcination. In their preparation method, they used urea as a fuel to avoid carbon contamination. No vanadium oxide peaks were observed in XRD, and the material is a mixture of rutile and anatase structures. Neither Raman nor IR measurements find evidence for the formation of VOx clusters supported on TiO2. The electron binding energies of Ti2p3/2, V2p, and O1s were measured by XPS. The Ti peaks were assigned to Ti4+, and there is no mention of the presence of Ti3+, which would be expected if the V dopant donates electrons to Ti. The XPS measurements were interpreted to mean that V dopants have a formal charge of 5+. Calculations262 have shown that a V atom substituting a Ti atom in the surface layer will bind oxygen from the gas phase, and the dopant is the VO group formed during calcination. In this compound, V is 5+ and it is possible that the surfacesensitive XPS measurements detect this group. This is consistent with the fact that no Ti3+ was detected. Another way of saying the same thing is to note that a V atom substituting a Ti atom is a Lewis base; given the chance to transfer an electron to Ti4+ or to oxygen, it will chose the oxygen, which is the stronger Lewis acid.221 Doping with V causes an increase in area per gram, as compared to that of TiO2 prepared by the same method. The styrene yield on all doped oxides is higher than that of undoped TiO2, but unfortunately the performance deteriorates rapidly with the time on stream. Sivarajani et al. performed a careful analysis of the catalyst after the reaction. They found

Ce0.9Ni 0.1O2 > Ce0.9Mn 0.1O2 > Ce0.9Sn 0.1O2 > CeO2 > Ce0.9Zr0.1O2 > Ce0.9Al 0.1O

However, this scale was made without normalizing the conversion to surface area. If we perform that normalization (we divide the conversion by the specific area), Zr-doped and Al-doped ceria are slightly more active than CeO2, because CeO2 has the highest area per gram of all of the materials studied here. It is interesting to note that for other methods of preparation, doping increases, almost always, the specific area of the catalyst; the method of preparation used by Fan et al. is an exception from this rule. The method of preparation makes a difference. Ce0.9Ni0.1O2 made by oxalate-gel coprecipitation has a conversion of 40.8%, which is much lower than that of the same composition prepared by the template-assisted method. However, the BET area of the material made by coprecipitation is 71 m2/g, and that of the material made by the template method is 134 m2/g. If one scales the conversion by the area, the sol−gel material has better performance. All Ni-doped catalysts are stable for 12 h on stream: neither the conversion nor the selectivity change, and the mesostructure of the catalyst is maintained. It is interesting that the activity of the mesoscopic undoped CeO2 decreases due to structural deterioration414 and doping with Ni suppresses or at least slows this process. 4413


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numerous shortcomings of the process: it is performed at high temperature (∼850 °C), it is endothermic and requires prodigious amounts of heat, it produces significant amounts of CO2, it is economical only if the units are of enormous size, the technology is complicated by a residence time of milliseconds (the gas velocity exceeds the speed of sound), and the composition of the products cannot always be adjusted to track changes in the demand. These difficulties stimulate the need for new processes for ethylene production, such as oxidative dehydrogenation with oxygen or CO2. Several reviews of alkane ODH are available.7,432−434 To be competitive with steam cracking, the oxidative dehydrogenation of ethane to ethylene ought to have433 high selectivity and productivity higher than 1.0 kg C2H4/gcat/h. The best catalyst found so far is Mo−V−Te−Nb oxide,435,436 which achieves 87% conversion and 89% selectivity. We are not aware of a commercial application of this catalyst. We dedicate substantial space to ethane ODH on NiO or doped NiO catalysts because they are the most extensively studied doped-oxide catalysts for this reaction.126,127,437−441 These articles provide information on ethane ODH catalysis by undoped NiO and NiO doped with Li, Mg, Al, Ga, Ti, Zr, Ta, and Nb. We have thus an opportunity to study, for one reaction and one host oxide, how catalytic performance changes with the dopant and the method of preparation of the catalyst. We do not review here work442−445 on undoped NiO supported on other oxides; we are interested in undoped and unsupported NiO for comparing it to unsupported, doped NiO.

XRD measurements on Ni-doped ceria show no peaks corresponding to NiO. The structure is that of the CeO2 with a smaller lattice spacing (the ionic radius of Ni is smaller than that of Ce). H2-TPR measurements show a dramatic change upon doping with Ni. For CeO2, the TPR signal (hydrogen consumption) is very broad, while that for Ni-doped CeO2 has a sharp peak at a much lower temperature. While we are not aware of any calculations of Ni-doped ceria surfaces, all calculations in which the dopant has a lower valence than the cation it replaces show that the dopant makes the oxygen atoms in the surface more reactive. This means that the dissociative adsorption of H2 and the removal of water (with the creation of an oxygen vacancy) are facilitated by the presence of the LVD. This is consistent with the H2-TPR results. The temperature programmed reaction of oxygen with the Ni-doped ceria catalyst that spent 12 h on stream was accompanied by TGA measurements. These show that no carbon deposits were present on the catalyst. The reaction is not catalyzed by carbon deposits, as has been sometimes suggested. Both Ce3+ and Ce4+ are observed in the XPS spectrum, and this is consistent with a Mars−van Krevelen mechanism. The hydrogen released by the hydrocarbon reacts with a surface oxygen atom desorbing as water and leaving behind an oxygen vacancy. Calculations show188,268 that an oxygen vacancy formation is accompanied by the reduction of two Ce4+ ions to Ce3+, and this will account for the presence of both ions in the catalyst (after use). As a rule, we expect that replacing Ce with a tetravalent dopant does not have a dramatic effect on the energy of oxygen vacancy formation or on the reactivity of the oxide. Calculations on Zr-doped ceria229,255,334,362 show that the presence of Zr makes it easier to make oxygen vacancies and make the oxygen atoms near the dopant more reactive. If the rate-limiting step in ODH is the breaking of a C−H bond in ethylbenzene, then the presence of Zr should improve the catalytic activity. This is not what the experiments find. However, if we normalize for differences in surface area, then Zr-doped ceria has roughly the same activity as CeO2. In general, the presence of an LVD, like Al, is expected to make the surface a better oxidant. This is true, but Al overdoes it: calculations by Nolan190 show that the formation of an oxygen vacancy on the surface of Al-doped ceria is exothermic. This means that the catalyst loses one oxygen atom next to the dopant when it is heated. We are not aware of any calculations that examine whether the oxygen atoms remaining on the surface (after on O-vacancy has been made) are still activated by the presence of Al. The acid−base rules proposed by us221 suggest that this will not be the case.

17. BACKGROUND INFORMATION ON NiO Nickel forms446 one stable oxide, NiO, which has the sodium chloride structure. Ni2O3 exists, is a strong oxidant, and is much less stable than NiO. The heat of formation of NiO is small, and it should be a good oxidant as compared to most other oxides. NiO prepared at 700 °C is green and is stoichiometric; we have not found any work that uses this oxide as a catalyst. NiO prepared at 450 °C is black, has Ni vacancies, and is a pconductor.58,447 The presence of a Ni vacancy creates (formally) a deficit of two electrons, which is equivalent to two holes and is believed to be responsible for the pconductivity of the material.58,447 The two holes may be localized on either two oxygen atoms or on two Ni atoms.448,449 In a chemist’s language, the presence of a Ni vacancy will create either O− or Ni3+ ions. Density functional theory has been used to examine Ni vacancies in the surface of NiO. The difficulties encountered by DFT in describing NiO correctly have been discussed by Pacchioni and Illas.450,451 The calculations on NiO (doped or undoped) that we report here used PBE+U with the U recommended by Rohrbach et al.452,453 Because of these uncertainties in the accuracy of DFT, we only emphasize here the trends suggested by the calculations. Pacchioni’s calculations454 show that if a Ni vacancy is present on the (100) surface of NiO, two holes are localized on the oxygen atoms. However, NiO is particularly easy to cleave159 along the (100) surface, and this suggests that this is not a reactive face. For this reason, we have examined Ni vacancies on the Ni(011) face. When a Ni vacancy is present on the surface of NiO(011), the Fermi level shifts toward the top of the valence band, which indicates that the vacancy is a pdopant, as expected. The empty orbitals at the top of the valence band are localized on the oxygen atoms next to the Ni vacancy. Therefore, formally the formation of a Ni vacancy at

16. OXIDATIVE DEHYDROGENATION OF ETHANE TO ETHYLENE Ethylene is produced by steam cracking of naphtha, or light petroleum products or natural gas rich in ethane.431 Early contributions to steam cracking were made by Dubbs father and son. In a moment of excessive enthusiasm for industrial organic chemistry, Dubbs father named his son Carbon. One would think that upon reaching maturity, the son might want to change his name: he did and became Carbon Petroleum Dubbs. His daughters were named Methyl and Ethyl. More ethylene is produced in the world than any other organic compound. The value of the product compensates for 4414


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the surface results in the creation of two O− ions. We note that by creating two holes localized on the oxygen atoms, a Ni vacancy is equivalent to an LVD. This means that the surface having a Ni vacancy is a strong Lewis acid and is a better oxidant than NiO that has no Ni vacancies. As in the case of all LVDs, it is possible that the presence of a Lewis base (e.g., H or C2H5) on the surface will compensate the effect of the Ni vacancy.145,221 In particular, it is reasonable to expect that H binds to the two O− atoms near the Ni vacancy to form two hydroxyls. We know no experimental evidence for or against this possibility.

18. ETHANE OXIDATIVE DEHYDROGENATION WITH OXYGEN CATALYZED BY Zr-DOPED NiO Wu et al.126 have prepared Zr-doped NiO and undoped NiO by a sol−gel method. Citric acid was added slowly to an aqueous solution of Ni(NO3)2·6H2O and Zr(NO3)4·5H2O. The solvent was evaporated at 70 °C until a gel was formed. This was dried at 120 °C for 24 h and then calcined in air at 450 °C for 6 h. Pure ZrO2 was not tested, but it is unlikely that it will catalyze ethane ODH at the temperature used in this work (T ≤ 400 °C). The amount of precursors was chosen so that the molar ratio 100 Zr/(Zr+Ni) was equal to 5, 10, 20, 40, and 75. The XRD measurements found that 5Zr and 10Zr have the structure of NiO. The notation 5Zr indicates Zr-doped NiO with the molar ratio 100 Zr/(Zr + Ni) = 5. The lattice parameter of NiO increased with the addition of Zr, whose ionic radius (0.72 Å) is larger than that of Ni (0.69 Å). No ZrO2 diffraction pattern was observed for Zr molar fractions less or equal to 10%. These results suggest that a Zr-doped NiO has been prepared when the dopant concentration is less or equal to 10%. In the XRD of 20Zr, one can see ZrO2 diffraction peaks. Because the catalytic activity of 20Zr is essentially the same as that of 10Zr, we suggest that 20Zr consists of a Zrdoped NiO phase (rather than an undoped NiO phase) and a ZrO2 phase. The NiO prepared by the sol−gel method mentioned above has a high specific area of 90 m2/g. Doping with Zr increases the specific area to 122 m2/g (for 5ZrNiO) and 170 m2/g (for 10ZrNiO). Later in this section, we show that the catalytic activity of undoped NiO depends strongly on the method of preparation.126,438,440,441 Reactivity experiments were performed126 at temperatures between 250 and 450 °C. The feed was a mixture of ethane, oxygen, and nitrogen (C2H6/O2/N2 = 1/1/8) with a space velocity of 30 000 mL/h·gcat. NiO starts converting ethane126 at ∼275 °C, and between 270 and 350 °C, it converts more ethane than any of the Zr-doped catalysts. The ethylene yield on NiO at 350 °C is 23% with ethylene selectivity of 47%; CO2 and ethylene were the only products. Unfortunately, at temperatures above 350 °C, the selectivity for ethylene is rapidly and completely lost. One suspects that the oxide also deactivates at lower temperatures, but it does so more slowly. The performance of the NiO and ZrNiO catalysts can be best judged from a plot (Figure 8) of selectivity to C2H4 versus ethane conversion.126 A point in the graph gives the conversion and the selectivity at a given temperature. Different points correspond to different temperatures. The conversion increases with temperature (up to 400 °C). At any given conversion, the ethylene selectivities of 5ZrNiO, 10ZrNiO, and 20ZrNiO are close to each other and substantially higher than that of NiO. For example, at a 50% ethane conversion the ethylene selectivity of NiO is ∼47%, while that of 10ZrNiO is ∼66%.

Figure 8. Ethylene selectivity as a function of ethane conversion for different concentrations of the Zr dopant in NiO. Reprinted with permission from ref 126. Copyright 2012 Elsevier.

It is clear that doping improves the selectivity and therefore the overall performance for ethane ODH. The best performance was obtained for 10ZrNiO at 430 °C with an ethylene yield of 40% and an ethylene selectivity of 66%. While 5ZrNiO and 10ZrNiO consist of one phase with the NiO structure, 20ZrNiO consists of a phase with NiO structure and a second phase which is ZrO2. It appears that the phase with NiO structure, in this two-phase 20ZrNiO system, is a doped nickel oxide because the activity of 20ZrNiO is very close to that of 10ZrNiO (which consists of one phase with NiO structure) and because a mixture of NiO and ZrO2 has the same activity126 as NiO, which indicates a lack of synergy between NiO and ZrO2. Zr-doped NiO behaves as calculations lead us to expect from an irreducible oxide doped with a high-valence dopant. Doping with Zr improves the selectivity to ethylene substantially, makes the oxide less reducible, and binds surface oxygen more strongly to the oxide, just as the calculations predict. We are not aware of any calculations on Zr-doped NiO. However, on the basis of calculations on similar systems (e.g., high-valence dopants (HVDs) in irreducible oxides), we expect the following. Because the only stable oxide of zirconium is ZrO2, Zr is a high-valence dopant (in NiO) and will act as a strong Lewis base. This can affect the system in several ways. Because NiO has a tendency to have Ni vacancies, they may be present even though we doped the oxide. In the absence of Zr, these Ni vacancies make NiO a better oxidant, and some authors assume that this is why NiO is an effective alkane combustion catalyst. A Zr dopant, which is a strong Lewis base, is very likely to transfer electrons to the electron−holes associated with the Ni vacancies. Put in another way, Zr will transfer electrons to the O− centers created when the Ni vacancy was formed, converting it to O2−. Thus, the presence of Zr is likely to decrease ethane oxidation. In addition to diminishing the reactivity caused by the Ni vacancies, Zr will increase the binding energy of the neighboring oxygen atoms to the oxide surface. This is equivalent to making these oxygen atoms less reactive. Temperature programmed hydrogen reduction126 experiments support this suggestion. Doping with even a small amount of Zr increases substantially the temperature at which hydrogen 4415


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system is complex. Calculations on a variety of systems show three possible scenarios. (a) A high-valence dopant may adsorb O2 from the gas phase and it may activate it.359 Ethane may react dissociatively with the adsorbed O2 to make an ethoxide and a hydroxyl. We rule out this mechanism for Nb-doped NiO(011) because our calculations find that the adsorption energy of O2 on top of the Nb dopant is −8.40 eV. The distance between the two oxygen atoms is much longer than any reasonable O2 bond length, and therefore the NbO2 group contains two O atoms (not a “weakened” O2 molecule). The adsorbed oxygen atoms are so strongly bound to Nb they cannot possibly be involved in the ODH reaction (Sabatier principle). (b) Because O2 binds so strongly to Nb and the O atoms are unreactive, it seems reasonable to consider that the dopant is NbO2. This changes the formal electron counting. If we assume that the formal charge on each O adsorbed on Nb is −2, then binding two oxygen atoms from the gas phase uses four of the Nb electrons and only one is left for making bonds with the oxide. Therefore, NbO2 should be viewed as a low-valence dopant similar to an alkali or a Ag atom. We have used calculations to test this hypothesis: if NbO2 is an LVD, then its presence will lower the energy of oxygen-vacancy formation at a neighboring oxygen site. DFT calculations in our group show that the presence of the NbO2 dopant lowers the energy of oxygen-vacancy formation in the top layer of the doped NiO(011): this energy is 3.28 eV for the undoped Ni(011), and it is 2.31 or 2.67 eV for the NiO(011) doped with NbO2 (depending on the position of the removed oxygen with respect to the Nb site). This is comparable to the value of 2.65 eV we obtained when the dopant is Li, and it is typical behavior for a lowvalence dopant. We also know that the presence of an LVD in the surface of an oxide lowers the energy of dissociative adsorption of an alkane (see section 9.3). Our calculations find that this is the case for ethane dissociative adsorption on NiO(011) doped with NbO2. The activation energy for breaking the C−H bond is low enough to explain why this system activates ethane. We would have suggested this mechanism for breaking of the C−H bond (the rate-limiting step in ODH), except for the next possible scenario. (c) Nb has a high affinity for oxygen and a propensity to make compounds in which it is pentavalent. Because of this, we investigated whether in contact with a gas-phase oxygen the dopant may come out of the Ni lattice site and form an oxide cluster on top of the surface. Our calculations show that when the doped surface is exposed to oxygen, the NbO2 cluster gains energy by reacting with 1/2O2 and forming the NbO3 cluster shown in Figure 9. In this final state, there is a NbO3 cluster on the surface (bound to the oxygen atoms on the surface of NiO) and a Ni vacancy in the surface layer. This is likely to happen with other high-valence dopants whose oxides are very stable, when they substitute lower valence cations in a less stable host oxide. When doped oxides are prepared, it is common to calcine them at temperatures around 450 °C for 4 or 5 h in the presence of oxygen. It is possible that Nb atoms are mobile at these temperatures and have enough time to reach the surface,

consumption by the oxide begins. Therefore, if the rate-limiting step in ethane ODH is the dissociative adsorption of ethane, to make a hydroxyl and an ethoxide with the oxygen atoms on the surface (Mars−-van Krevelen mechanism), the presence of Zr should make this reaction more difficult. This is not necessarily bad, because Zr will also limit the reactivity of the doped oxide with ethylene: a loss in conversion may be a gain in selectivity. Our ability to anticipate the mechanism without detailed calculations on this specific system is diminished by the fact that the Mars−van Krevelen mechanism is not the only possibility. As we have already explained, high-valence dopant adsorbs O2 from the gas phase. If the binding energy of the O2 to the dopant is not very large, the adsorbed O2 is reactive and can break the C−H bond in an alkane. We are not aware of any calculation in which this possibility has been examined for Zrdoped NiO. Experiments with 18O2 are sometimes able to distinguish which of these two mechanisms is operative (i.e., reaction with O2 adsorbed on Zr or reaction with surface O), but they have not been performed for this system. Experiments on hydrogen TPR and oxygen desorption from NiO and Zrdoped NiO are of help. They show that doping with Zr makes the oxide less able to consume hydrogen or to release oxygen. This suggests that Zr acts as a typical high-valence dopant (without O2 adsorbed on it).

19. ETHANE OXIDATIVE DEHYDROGENATION TO ETHYLENE CATALYZED BY Nb-DOPED NiO 19.1. Background Information about Nb and Its Oxides

We are interested in the oxides formed by Nb because they may allow us to guess some of the chemical properties of the Nb dopant. Niobium is a flexible atom when it combines with oxygen. It makes three stable oxides, Nb2O5, NbO2, and NbO, as well as compounds with the stoichiometry Nb3n+1O8n−2 with 5 ≤ n ≤ 8. Among these oxides, Nb2O5 has the highest energy of formation, and it is very unreactive. NbO has the lowest heat of formation (−406 kJ/mol) and is a stable oxide. It has a sodium chloride structure, just like NiO, except that 25% of the NaCl lattice sites are unoccupied; occasionally the formula Ni0.75O0.75 is used to draw attention to this fact. In binding to oxygen Nb prefers the oxidation state +5 if enough oxygen is available. However, when Nb is a substitutional dopant in another oxide, its oxidation state is likely to depend on its immediate environment. It is therefore difficult to guess a priori the oxidation state of a Nb atom that substitutes a Ni atom in the surface layer. 19.2. Computational Results on Nb-Doped NiO

Among all rocksalt oxides, NiO is most easily cleaved along the (100) plane.159 This suggests that the (100) plane is likely to be the least reactive face of the oxide, and this is what the calculations find. Therefore, we report here only results for the Nb-doped NiO(011). The Nb dopant has five valence electrons and when it substitutes a divalent cation (such as Ni in NiO) three of its electrons that are not tied up in chemical bonds. These “unused electrons” make the dopant a strong Lewis base. According to the rules we proposed, as a result of calculations on a variety of systems,141,142,145,221,262,271 a high-valence dopant in an irreducible oxide will increase the binding of neighboring surface oxygen atoms to the oxide, and this will hinder oxidation by a Mars−van Krevelen mechanism. However, in the presence of oxygen in the gas phase, which is always the case in an ODH reaction, the behavior of the 4416


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the same elemental content, are likely to evolve and end up having the same structure and surface composition. Wu et al.455 prepared NiO by a sol−gel method, a precipitation method, a microemulsion method, and by solidstate milling. Heracleous and Lemonidou438 dissolved Ni(NO3)2·6H2O and ammonium niobium oxalate in water by stirring and heating at 70 °C for 1 h. The solvent was then evaporated and the solid dried and calcined in synthetic air at 450 °C for 5 h. The pure NiO was obtained by the same procedure, in the absence of the Nb precursor. Caps and co-workers440 prepared NiO by three methods and showed that their sol−gel method provides the best NiO catalyst for ethane ODH, which is consistent with the results of Wu et al.455 Therefore, they prepared the Nb-doped NiO by the same sol−gel method. In this procedure, NbCl5 was added to water, which caused the formation of a Nb2O5·nH2O precipitate. This was washed to remove the chloride. After that, the precipitate was dissolved in a warm citric acid solution, and Ni(NO3)2·6H2O was added to it. The gel was slowly dried in several steps440 and then calcined at 450 °C for 4 h in air. Millet et al.441 prepared Nb-doped NiO by using the oxalatebased method developed in Lemonidou’s group (see above).456 Three other preparations used various amounts of oxalic acid (because the precursor for Nb is an oxalate) to test whether the amount of oxalate matters. The preparation having the most oxalic acid had the highest catalytic activity, and, in what follows, we discuss only the activity of this NiO sample from Millet’s group.

Figure 9. In the presence of oxygen, the Nb dopant (light purple) is pulled out of the Ni lattice site (the circle) to form a NbO3 cluster, where the three oxygen atoms (blue) are from the gas phase. The Ni atoms in the top layer are green, and the ones in the second layer are cyan. The oxygen atoms in the top surface layer are red, and the ones in the second layer are yellow.

react with the gas-phase oxygen, and form Nb oxide clusters or perhaps a Ni niobate (our calculations have not explored the latter possibility). The properties of the structure shown in Figure 9 have not been adequately explored. It is well-known8,9 that submonolayers of an oxide A (a generic name) supported on an oxide B (another generic name) are often better catalysts than the bulk oxide A or bulk oxide B. In an important experiment, Millet et al.441 have prepared, by an impregnation method (see below), submonolayers of NbOx supported on NiO. The activity of this catalyst is less than that of the samples that were prepared with the intention of making doped oxides. One infers that small NbOx clusters supported on NiO are not as active as Nb-doped NiO. However, this does not rule out the presence of a Nb-rich phase in Nb-doped NiO. While attempting to prepare doped oxides, Heracleous and Lemonidou439 found that besides a phase with NiO structure, a second phase is present that is amorphous and has an excess of Nb (determined by EDS microscopy). They believe that this phase is a precursor to the formation of NiNb2O6. Millet et al.441 found that some Nb0.15Ni0.85O seems to be embedded in Nb2O5. Caps440 saw regions with high Nb concentration in EDS and showed that NiNb2O6 is present. These findings agree qualitatively with the suggestion made by calculations that in the presence of oxygen some of the Nb substitutional dopant will segregate to the surface and be oxidized. The kinetics of this segregation is likely to be slow and might explain why some of the Nb-doped catalysts lose activity in time.

19.4. Catalytic Performance of NiO for ODH of Ethane to Ethylene

There are no widely accepted methods for ranking catalysts even when we disregard some of the practical factors such as heat management, equipment cost, and separation cost. Because conversion and selectivity are very important, we compare the ODH catalysts based on the conversion of C2H6 and the yield of ethene (the percent of the converted ethane that produces ethene). Physical chemists may prefer to use conversion per unit area (which we use here occasionally) or even the elusive turnover frequency. A comparison between the results of Millet441 and Caps440 is made in Table 3. In addition, we note the results of Wu et al.455 who give only the ethylene yield but not the conversion. They found that the ethylene yield is highest for the NiO prepared by a sol−gel method (∼15%), decreases to ∼8% for the one prepared by the precipitation method, and decreases further for the one prepared by a microemulsion method (3%). The specific area of the sol−gel sample, which had the highest Table 3. A Comparison of Results Obtained for Ethane ODH Catalyzed by NiO and Nb-Doped NiOa % yield

19.3. The Preparation of NiO and Nb-Doped NiO

Experiments have found that the catalytic activity of NiO and Nb-doped NiO depends substantially on the method of preparation. It is not clear whether such differences persist for a long time: one might think that under steady-state reaction conditions catalysts prepared differently, but having

441

catalyst

Millet

NiO Nb0.03 Nb0.15 Nb0.20

3.7 15 26

% conversion Caps

440

24 25 14 8

Millet441

Caps440

12 22 33

42 41 19

The temperature is 350 °C, the ethane to oxygen ratio is 1:1, and W/ F = 0.6 g s/mL for Caps and 0.54 g/s mL for Millet. The subscript “x” in Nbx indicates the atomic fraction of Nb in the Nb-doped NiO catalyst.

a

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conversion and 39% selectivity (9% yield). A suspension of this NiO oxide powder in a solution of ammonium niobium oxalate was stirred at room temperature for an hour, then the water was evaporated at 70 °C and the powder left behind was calcined in air, for 5 h, at 450 °C. We denote the resulting compound by NbOx/NiO. This catalyzes ethylene formation with an ethane conversion of 32% and a selectivity of 40%. NbOx/NiO is more active than the NiO support, but it is less active than Nb0.15Ni0.85O, or Nb0.07Ni0.93O, which are believed to be doped oxides. This does not rule out that when doped oxides are made some NbOx is formed on the surface, but indicates that it is unlikely that all or most of the Nb is in this form.

ethylene yield, was much smaller than the area of the other samples. The NiO prepared by Caps gives a substantially higher ethane conversion and ethylene yield than that prepared by Millet or Wu. The NiO prepared by Lemonidou has 80% CO2 selectivity439 (the ethylene yield on NiO is not reported in their paper but it is likely to be the same as in Millet’s work who used the same method of preparation), so one should think of it as more of a combustion catalyst than an ODH catalyst. This comparison shows that both conversion and selectivity in the catalytic oxidation of ethane on a NiO catalyst depend on the way in which the catalyst was prepared. The difference is both qualitative and quantitative: in one preparation, NiO is a fair catalyst for ethylene production, while in another, it combusts ethane.

19.7. Long-Term Stability of the Nb-Doped NiO Catalyst

Lemonidou438 found that Nb0.15Ni0.85O is stable on stream for 20 h at 350 °C. The conversion of the doped catalyst prepared by Caps440 drops slowly and steadily for 70 h, while selectivity increases. The change is most rapid in the first 5 h. Millet441 performed more extensive stability studies and found that at a temperature of 380 °C the conversion of ethane on Nb0.15Ni0.85O drops from 58% to ∼35% after 325 h on stream, while selectivity increases from 67% to ∼78%. The degradation of the catalyst is not due to coking because heating the degraded catalyst to 450 °C in air, for 30 min, and then using it again as a catalyst at 380 °C, did not restore its performance. Nor is the degradation caused by coarsening becauase the area did not change substantially. One possible explanation439 for deactivation is the formation of NiNb2O6.

19.5. Catalytic Performance of Nb-Doped NiO for Ethane ODH

In Millet’s experiments, adding a very small amount of Nb (to make Nb0.03Ni0.97O) improves dramatically both the ethane conversion and the ethylene yield as compared to the performance of NiO (see Table 3). However, such a small amount of Nb makes no difference in the experiments reported by Caps. Increasing the amount of Nb to make Nb0.15Ni0.85O increases ethane conversion and ethylene yield in the case of Millet and decreases them for the catalyst made by Caps. These are qualitative differences: in Millet’s experiments, doping with Nb improves the performance, while in Caps’s experiment it decreases it; the method of catalyst preparation is clearly important. Lemonidou’s experiments438 for Nb0.15Ni0.85O at T = 350 °C, C2H6/O2 = 1/1, and W/F = 0.54 g s/cm3 (the W/F is the same as Millet’s and slightly different from Caps’s 0.6 g/s cm3) give an ethylene selectivity of 81% and an ethane conversion of 35%, which corresponds to a yield of 28%. This is close to Millet’s results but different from those of Caps. Because Millet used the same method of synthesis as Lemonidou, we expect their results to be very similar. There are some data at 400 °C that show differences between Nb0.15Ni0.85O prepared by Caps440and that prepared by Lemonidou.127,437−439 Millet did not publish data at this temperature. The conversions are 25% (Caps) and 65% (Lemonidou), and the selectivities are 74% (Caps) and 66% (Lemonidou).

19.8. The Influence of Doping and of the Method of Preparation on Surface Area per Gram

We have compared the performance of NiO and the Nb-doped NiO catalysts prepared by various methods without taking into account their specific area. It is often preferable to know the conversion per unit area rather than the conversion per gram of catalyst. The specific area of the NiO is 9.4 m2/g for Lemonidou catalyst and 90 m2/g for that of Wu. The specific areas of the other NiO preparations mentioned here are between these values. Because of this, it is important to note that the area depends strongly on the method of preparation. As we have already pointed out, these preparations have very different catalytic activity. A possible reason for this is that the smaller particles are likely to have more defects (steps, kinks, etc.), and these are chemically active. NiO doped with Nb or Zr has a much larger specific area than undoped NiO. The specific area increases with the amount of Nb, as long as Nb concentration does not exceed 20%. For example, the area in m2/g is 16.7 for NiO, 55.9 for Ni0.9Nb0.01O, and 85.1 for Ni0.85Nb0.15O. This is often the case: a doped oxide has a substantially larger specific area than the host oxide prepared by the same method. There are however exceptions to this rule.96

19.6. NbOx Clusters Supported on NiO

Millet et al.441 performed an important experiment, which should become standard in research on catalysis by doped oxides. Our calculations suggest that, in the presence of oxygen in the gas phase, Nb is likely to segregate and make niobium oxide clusters on the NiO surface. The binding energy in the XPS spectrum of Nb atom in the material will be different from that of Nb in Nb2O5 for any of the following: for NbOx supported on NiO, for the substitutional Nb dopant in the surface of NiO, or for NbO2 that substitutes a Ni atom in the surface. Therefore, the shift in the XPS spectrum cannot be used to distinguish between these three possibilities. When physical methods give ambiguous results, it is often useful to appeal to chemistry. Millet et al.441 prepared NbOx clusters supported on NiO and tested the catalytic activity of this material for ethane ODH. The supported NbOx clusters were prepared by impregnation of a NiO powder prepared by the oxalate method. This NiO sample (with no Nb) is active for ODH and gives 23%

19.9. Chemical Characterization of Nb-Doped NiO

Temperature programmed reduction with H2 shows that doping with Nb reduces the hydrogen uptake.438,440,441 Temperature programmed desorption shows that less oxygen is released from the Nb-doped NiO than from the undoped NiO. These observations are consistent with the fact that Nb is a high-valence dopant that reduces the reactivity of surface oxygen and increases the energy of oxygen-vacancy formation. 4418


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19.10. Ethane ODH by NiO Doped with Li, Mg, Al, Ga, Ti, Ta, or Nb

Mars−van Krevelen mechanism. However, we are left with the possibility that O2 adsorbs on the dopant and ethane reacts with it. There are no calculations regarding this possibility. The amount of desorbed oxygen from various samples is also qualitatively consistent with the calculations. Li-doped NiO releases the largest amount of oxygen (per gram of catalyst), while the catalysts containing Al, Ga, Ti, Ta, or Nb release less (as predicted by calculations). The XRD measurements find that all samples contain a phase with the NiO structure with slightly shifted lattice constant. This is taken to signify that all samples consist of doped NiO. However, this is not always the only phase present. Anatase is observed in Ti-doped NiO, a phase tentatively assigned to NiTa2O6 is seen in the Ta-doped NiO, and either a Nb-rich phase or a NiNb2O6 phase is seen in the Nb-doped NiO sample. It is not clear how these phases affect catalytic performance. This presents us with additional complications when we compare the behavior of the doped NiO with the results of exploratory calculations.

Heracelous and Lemonidou127 prepared NiO doped with Li, Mg, Al, Ga, Ti, and Nb. These dopants were chosen because their ionic radius is similar to that of Ni and their valence in their stable oxides varies from one to five. All doped oxides were prepared under identical conditions, and all had the same nominal dopant to Ni ratio of 0.176 (the stoichiometry is Me0.15Ni0.85O, where Me stands for the dopant). The precursors for the dopants were nitrates, and the preparation was the same as in the early work of Lemonidou.438 The ODH of ethane was investigated in the temperature range from 300 to 425 °C, with a W/F ratio of 0.54 g s/cm3 and a feed containing 9.1% ethane, 9.1% O2, and 81.8% He. Ethane conversion, per gram of catalyst, decreased in the order: NbNi ≫ MgNi > LiNi > GaNi = NiO ≥ AlNi ≥ TiNi ≫ TaNi

19.11. Ethane ODH on Doped NiO: Conclusions

Here, the symbol MeNi (where Me is the dopant) stands for Me0.15Ni0.85O. According to this metric, NiO doped with Nb is the most efficient catalyst for breaking the C−H bond in ethane. Of course, it is difficult to judge the quality of a catalyst by using one simple metric. Selectivity is also of paramount importance, and we will examine this later. It seems reasonable, however, to examine ethane conversion per unit area because there are large differences in the surface areas per gram between various doped NiO catalysts. The BET measurements of the specific area give (in units of m2/g) 16.7 for NiO, 85.1 for NbNi, 7.6 for LiNi, 19.2 for MgNi, 67.8 for AlNi, 45.3 for GaNi, 18.6 for TiNi, and 78.9 for TaNi. With the exception of Li-doped NiO, all doped oxides have larger specific area than NiO. If we normalize the conversion to surface area per gram, the order of the efficiency of the catalysts for ethane activation (conversion per area) changes to

This detailed examination of ethane ODH by doped NiO illustrates the difficulties encountered by both theory and experiments when studying catalysis by doped oxides. It is impossible at this time to construct realistic models of the catalytic surface, and we are limited, in both computations and experiments, to making educated guesses. However, several broad conclusions can be reached. The trends predicted by computations compare fairly well with the experimental observations. The distinction between the effect of high- and low-valence dopants, so prevalent in calculations, is observed in experiments. The computed trends in the ability of a doped oxide to break the C−H bond are in agreement with the observations and so are the trends in reducibility (whether measured by H2-TPR or oxygen thermal desorption). The strong dependence on the method of preparation is a surprise. It might appear that this provides an additional parameter in the optimization of the catalyst. However, it is not clear whether this dependence on the method of preparation is of practical importance: after several days on stream, various morphologies, generated by different methods of preparation, may evolve into the same steady-state morphology. It is therefore important, for this and other reasons, to perform long time experiments. The dependence on the method of preparation points to a shortcoming of the computations: there is practically no computational work in which surface morphology is systematically varied and its influence on the ability to break the C−H bond is studied. Such calculations are possible and should be performed.

LiNi (0.139) ≫ MgNi (0.055) ≈ NiO (0.049) > NbNi (0.028) > GaNi (0.018) = TiNi (0.017) ≫ AlNi (0.011) ≫ TaNi (0.0004)

The numbers in parentheses are μg of C2H6 consumed per m2 per second.On the basis of these results, Li-doped NiO is by far the catalyst most able to break the C−H bond. To assess the reactivity of the oxygen atoms in the surface of doped and undoped NiO, we have calculated the energy of oxygen vacancy formation in the surface layer. For NiO(011), we obtained 3.28 eV. The energies to make an oxygen vacancy in the surface layer of a doped oxide, near the dopant, are 2.65 eV for Li, 2.74 eV for Mg, 4.6 eV for Al, 4.25 eV for Ti, and 4.13 for Nb. The reliability of these numerical values is not certain, and therefore we concentrate on trends. In general, a low-valence dopant will decrease the energy of vacancy formation, and this is what we see for Li-doped NiO. Mg has the same valence as Ni, and its ionic radius is comparable to that of Ni, and we expect it to have a small effect on ΔEv, which is the case. Al, Ti, and Nb are high-valence dopants, and they increase the energy of vacancy formation. The values of these energies of oxygen-vacancy formation suggest that doping with Li and Mg will activate the oxide and it will convert more ethane than NiO. This is what the experiments found. The calculations suggest that the high-valence dopants Al, Ti, and Nb will make the doped surface less active than NiO, for the

20. BRIEF CONCLUDING REMARKS Catalysis by doped oxides is an emerging field, which is still awaiting consolidation and rationalization. The number of possible oxide−dopant pairs is large, and none has been studied exhaustively by a combination of computations and experiments. Much more work is needed before we can form a clear picture of the effects of doping and of its utility. The characterization methods are unable to prove beyond doubt that the method of synthesis has produced a substitutionally doped oxide with the dopant imbedded in the surface layer. At this time, and for some time in the future, we have to rely on circumstantial evidence. We have no experimental methods that will tell us the morphology and the composition 4419


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of the surface at an atomic level, under reaction conditions. Because of this, the models used in computations are approximate. To bridge this “model gap”, we need to perform calculations of catalysis on surfaces having a variety of “defects” to obtain an estimate of their importance. Experiments on model systems that resemble more closely the models used in the calculations, such as those developed by Freund,10−14,249 for example, will also be very helpful. Furthermore, it is necessary to perform experiments specifically designed to test the predicted trends, which has not been done so far. Because of these uncertainties, one should use computations, at this time, mainly to obtain qualitative trends and design rules. The most important question is: does doping improve the catalytic activity of an oxide? With a few exceptions, both computations and experiments answer yes. In most cases, doping improved the catalytic performance, as compared to the undoped host oxide, by increasing the conversion, or the selectivity, or the area per gram of catalyst. Despite such improvements, we are not aware of any example where a deliberate doping of an oxide leads to a commercial catalyst. A second question is: Are computations helpful in our search for better doped-oxide catalysts? So far they helped us divide oxide−dopant pairs into classes based on commonality in the calculated chemical behavior. They have explained some aspects of surface chemistry on the basis of strong interaction between Lewis acids and bases. They are likely to be useful in prospecting for promising dopant−oxide pair catalysts, if it is understood that they provide probable behavior rather than absolute answers. The best we can hope, at this time, is to derive qualitative design rules that will allow us to guess a priori which doped-oxide systems are most promising catalysts for each specific reaction. The work performed so far is incomplete, fragmented, and exploratory, but the results obtained are promising and much remains to be done. There is also no reason to confine doping to the binary oxides examined here. Doped perovskites have shown interesting catalytic properties.108,135,457−490 MoS2 doped with Co or Ni is an industrial catalyst for oil hydrotreating.6,491,492 One can also imagine doping halide catalysts used, for example, in oxichlorination reactions,3 or any other inorganic material that has catalytic activity (e.g., vanadates, molybdates, nitrides, carbides). Finally, one can imagine using anion doping such as replacing some oxygen in an oxide with a halogen, or sulfur, etc. Finally, we note that doping is starting to be used in electrochemistry where doped oxides and doped sulfides have been explored as electrocatalysts493−495 and in surface photochemistry where doping can change the absorption spectrum and the catalytic properties.54

Biographies

AUTHOR INFORMATION

ACKNOWLEDGMENTS We are grateful to numerous students and postdocs with whom we have collaborated while working on doped oxide catalysts: Steeve Chrétien, Alan Derk, Zhenpeng Hu, Bo Li, Lauren Misch, Raj Pala, Vladimir Shapovalov, Sudhanshu Sharma, XiaoYing Sun, and Wei Tang, and to Professors Galen Stucky

Eric McFarland studied Nuclear Engineering at the University of California, Berkeley. After obtaining a Ph.D. from MIT and an M.D. from Harvard Medical School, McFarland joined the MIT Nuclear Engineering faculty. In 1991, he moved to the University of California, Santa Barbara where he is a Professor of Chemical Engineering. McFarland works closely with industry on problems related to fuel production. His recent research interests focus on heterogeneous and photoelectrochemical catalytic processes and production of fuels and chemicals from light alkanes.

Horia Metiu graduated from the Chemical Engineering Department of the Polytechnic Institute in Bucharest in 1961, received his Ph.D. in theoretical chemistry from MIT, in 1974 working with John Ross and Robert Silbey, and then was a postdoctoral fellow at the University of Chicago with Karl Freed. He has been a professor in the Department of Chemistry at the University of California, Santa Barbara since 1976. In the past, he performed theoretical work on surface-enhanced spectroscopy, nonadiabatic processes at surfaces, correlation-function theory of rate constants, quantum dynamics with Gaussian wavepackets, spectroscopy in time domain, femtochemistry, genetic programming, crystal growth, thermoelectric materials, and polymeric membranes for fuel cells. He has also performed experimental work on Penning spectroscopy of adsorbates, polymeric membranes for fuel cells, and mass selected clusters on surfaces. He is currently involved in experimental and computational investigations of catalysis and electrocatalysis.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4420


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and Ram Seshadri. We are grateful for numerous and useful discussions with Matthias Scheffler and Jens Nørskov. Funding was provided by the Air Force Office of Scientific Research (FA9550-09-1-0333), the U.S. Department of Energy (DEFG02-89ER140048), the University of California Lab Fees Program (09-LR-08-116809), and by the National Science Foundation through TeraGrid resources provided by Ranger@ TACC under grant number TG-ASC090080. We acknowledge support from the Center for Scientific Computing at the California NanoSystems Institute and the UCSB Materials Research Laboratory: an NSF MRSEC (DMR-1121053) and NSF CNS-0960316 and Hewlett-Packard. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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Catalysis by Doped Oxides - Chemical Reviews (ACS Publications)

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