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Probing the Surface of Ceria-Zirconia Catalysts Using NOx Adsorption/Desorption: A First Step Toward the Investigation of Crystallite Heterogeneity Bruno Azambre,*,† Idriss Atribak,‡ Agustı´n Bueno-Lo´pez,‡ and Avelina Garcı´a-Garcı´a‡ UniVersite´ Paul Verlaine de Metz, Laboratoire de Chimie et Me´thodologies pour l’EnVironnement (LCME) EA 4164, Rue Victor Demange, IUT Chimie, F-57500 Saint-AVold, France, and MCMA Group, Department of Inorganic Chemistry, Faculty of Sciences, UniVersity of Alicante, Carretera de San Vicente del Raspeig s/n, 03690, San Vicente del Raspeig, Alicante, Spain ReceiVed: April 1, 2010; ReVised Manuscript ReceiVed: June 30, 2010
Ce-Zr mixed oxides with a targeted composition of Ce0.76Zr0.24O2 have been synthesized from nitrate precursors following different synthesis methods: coprecipitation, solid combustion synthesis with urea, citrate complexation, reverse microemulsion, and synthesis with an activated carbon fiber template. Though their real nominal compositions were nearly equal in most cases, the characterization by XRD, Raman, N2 adsorption, FTIR, and XPS revealed distinct surface and bulk properties. The monitoring of NO + O2 adsorption/desorption processes by in situ DRIFTS and TPD-MS allowed distinguishing the type, amount, and relative thermostabilities of nitrite/nitrate species on the different surfaces. A detailed analysis of these spectroscopic data was carried out in order to estimate the respective participation of Ce and Zr sites to the adsorption and further reaction of NOx on ceria-zirconia surfaces. In the most favorable cases, the adsorption of (NO + O2) or NO2 can be used to assess the relative enrichment of the different surfaces in Ce or Zr, but this method was found more difficult to apply than the FTIR of adsorbed methanol. 1. Introduction Nowadays, many automotive after-treatment technologies use oxygen storage components (namely, ceria-based) in catalytic formulations to perform oxidation reactions efficiently. In that respect, the NO oxidation reaction to NO2 is involved in the mechanisms of several depollution processes, such as in NOx storage and reduction (NSR) catalysis, selective catalytic reduction (SCR) of NOx, and NO2-assisted soot combustion.1-5 The NO oxidation reaction is kinetically limited at low temperatures and thermodynamically limited at high temperatures.4 For the most studied Pt-based catalysts, NO oxidation kinetics were satisfactorily modeled using both Eley-Rideal and Langmuir-Hinshelwood mechanisms,4 and this explains why a consensus was not reached on the elementary steps involved in the overall reaction. On some mixed metal oxides, such as ceria-zirconia, it has been shown that thermodynamic levels of NO2 can be reached around 400 °C even in the absence of noble metals.6-10 However, experimental studies on the mechanism of NO oxidation on these systems are further complicated due to the occurrence of several, often interdependent, issues: the effect of Zr content and local structure on oxygen vacancy generation/healing and oxygen mobility, interaction of (NO + O2) with the catalyst surface, trapping of ad-NOx species of different nature and stabilities, further redox reaction of these species, and final desorption as N2O, NO, NO2, or NO + O2 due to thermodynamics or redox decomposition reactions.4,7,11 In relation to the investigation of NOx sorption processes, the surface characterization of nanosized ceria-zirconia materials is itself still very challenging. As described in depth in * To whom correspondence should be addressed. E-mail: bazambre@ univ-metz.fr. † Universite´ Paul Verlaine de Metz. ‡ University of Alicante.
Kaspar’s review12 and in ref 1, the redox properties of these materials are very dependent on the composition, specific surface area, the preparation method, and the thermal history. Characterization of ceria-zirconia surfaces is further complicated by the lack of adequate analytical tools to address the issues relevant to nanoscale heterogeneity.12 In our last studies performed on commercial11 and coprecipitated ceria-zirconia catalysts,6-8 the respective effects of the composition/structure, degree of annealing, type of reactive gas, and the reaction temperature on NOx adsorption/desorption were investigated. By confronting our DRIFTS results7,8,11 with the literature,4,13-15 we drew the following conclusions: (i) Whatever the nature of the reactive gas (NO, NO + O2, NO2), IR spectra are primarily influenced by the adsorption and subsequent reaction of NO2 (obtained, for instance, by NO oxidation), which produces mainly negatively charged nitrites and nitrates in NO oxidation conditions. Their sorption kinetics on ceria-zirconia are, however, several degrees of magnitude slower than on barium storage components. Though molecular adsorption of NO is usually not observed in IR spectra above room temperature, the detection of neutral nitrosyls and/ or NO dimers at subambient temperatures13-15 could perhaps be involved in the formation of molecular NO2 via a Langmuir-Hinshelwood mechanism. (ii) In the initial adsorption steps, NO2 reoxidizes the defects present on ceria-zirconia surfaces, the main product observed being coordinated nitrites.11,15 (iii) These surface NO2- species are then progressively oxidized to nitrates having different kinds of bonding with the surface. The ease in oxidation increases with temperature, suggesting an activated process due to the necessity for the reaction to proceed with an activated form of oxygen, and loosely bound O-, O2-, and O22- have been postulated as intermediates.16
10.1021/jp102949r 2010 American Chemical Society Published on Web 07/16/2010
Probing the Surface of Ce-Zr Catalysts (iv) Some types of OH groups interact (or are consumed) with NO2, and this process possibly contributes to its storage as surface nitrates. Water and possibly nitrous acid are released as byproducts.11,13 (v) In a process rather similar to what was described for other kinds of metal oxides,13,17,18 NO2 sorption can additionally also take place via N2O4, disproportionation as NOx+-NO3- pairs, but this has to be confirmed for ceria-containing materials. In the present study, ceria-zirconia samples of nearly similar compositions have been prepared following different synthesis methods and characterized by different analytical techniques. The Ce0.76Zr0.24O2 composition was chosen because, in our previous studies, it represented the best compromise between an efficient NO and soot oxidation activity and resistance to thermal sintering. NOx sorption/desorption processes on these samples have been studied by in situ DRIFTS and TPD-MS with the double aim of (i) investigating the effect of the preparation method on the surface properties and sample heterogeneity at the crystallite scale and (ii) assessing to what extent NO2 (or NO + O2) can be used as an analytical probe to study the differences in surface composition and reactivity of ceria-zirconia nanopowders. 2. Experimental Section 2.1. Materials. The precursors used for the preparation of samples were Ce(NO3)3 · 6H2O and ZrO(NO3)2 · nH2O (supplied by Aldrich). In every synthesis of Ce0.76Zr0.24O2, the amounts of precursors are 1.516 g (for cerium) and 0.375 g (for zirconium). Eight samples have been used in the study, which are denoted by CeO2, ZrO2, RH, CO, SCS, CCR, RME, and ACF. Five different preparation routes were followed: (i) Precipitation/coprecipitation (samples CeO2, ZrO2, and CO): the Ce and/or Zr precursors were dissolved in water, and the hydroxides were precipitated/coprecipitated by dropping an ammonia solution to keep the pH about 9. (ii) Solid combustion synthesis with urea (sample SCS):19 the Ce and Zr precursors as well as urea, acting as a sacrificial fuel, were dissolved into the minimum volume of distilled watersnitrate/urea ratio of 1:2sand this solution was placed for 8 min into a muffle furnace previously heated at 500 °C. (iii) Citrate complexation (sample CCR), also known as “amorphous citrates method”: The Ce and Zr precursors were dissolved in 33 mL of distilled water in the appropriate amounts to get solutions with a citric acid/(Ce + Zr) molar ratio of 1.2. The resultant solution was stirred and dried at 110 °C to produce a xerogel.20 (iv) Reversed microemulsion (sample RME): A reversed microemulsion (water in organic) was prepared by mixing with stirring an aqueous solution of the Ce and Zr precursors with 86 g of heptane, 28 g of Triton X-100, and 22 g of hexanol, according to the reactant proportions used by Martinez-Arias et al.21 This emulsion was mixed while stirring with another emulsion having similar characteristics, except that the aqueous solution now contained tetramethylammonium hydroxide. The fluid was stirred for 24 h, after which the resulting suspension was centrifuged and decanted. The remaining solid was washed with ethanol and dried at 110 °C for 24 h. (v) Template synthesis (sample ACF): High surface area activated carbon fibers, supplied by Kynol, with a BET surface area of 2000 m2/g, were used as a template. The fibers were impregnated by incipient wetness with an aqueous solution of the Ce and Zr precursors, dried at 110 °C, and calcined at 300 °C for the required time to burn out the carbon template.
J. Phys. Chem. C, Vol. 114, No. 31, 2010 13301 (vi) A commercial catalyst (RH) with a nominal composition of Ce0.75Zr0.25O2, obtained from Rhodia (La Rochelle, France). The commercial (RH) and homemade samples (except SCS) were finally calcined at 500 °C for 3 h. 2.2. Bulk and Surface Characterizations. The catalysts were characterized by N2 adsorption at -196 °C in an automatic volumetric system (Autosorb-6B from Quantachrome) after degassing the samples at 250 °C for 4 h. Energy-dispersive X-ray analysis (EDX) were collected on a JEOL JEM 2010 microscope operating at 200 kV equipped with a PGT Imix PC system. X-ray diffractograms were measured on a Seifert powder diffractometer using the Cu KR radiation (λ ) 0.15418 nm). Spectra were recorded between 10 and 60° (2θ) with a step size of 0.05° and measuring for 3 s at each step. XPS characterization of the catalysts was performed using a VG-Microtech Multilab electron spectrometer equipped with a Mg KR (1253.6 eV) radiation source and a pressure of 5 × 10-10 mbar in the analysis chamber. The electronic transitions Zr 3d5/2, Ce 3d5/2, and Ce 3d3/2 and the satellite cerium peak centered at 917 eV were used to determine the surface concentration of zirconium and cerium. The proportion of Ce3+ cations with regard to the total cerium was calculated following the method reported elsewhere.22 2.3. In Situ DRIFTS. Infrared spectra were recorded in the 4000-700 cm-1 range (resolution ) 4 cm-1, 100 scans) on a Varian Excalibur 4100 spectrometer equipped with a MCT detector and the Graseby Specac “Selector” DRIFTS optical accessory (off-axis alignment). Pure catalysts (without diluents) were loaded into a Spectra-Tech environmental cell designed to work under controlled temperatures and flowing gases. Prior to reaction with NOx, freshly calcined samples were in situ thermally treated at 400 °C under He. Time-resolved spectra obtained during catalyst exposure to NO + O2 (2000 ppm NO/ He + 5% O2/He) at 350 °C were recorded in single-beam (SB) mode. They were then recalculated as difference pseudoabsorbance spectra using the SB spectrum of the catalyst just prior to NOx admission as reference. IR spectra representative of the OH groups present on the different catalysts surfaces prior to reaction with NOx were obtained using a KBr SB spectrum (also thermally treated in situ) as a reference. 2.4. He-TPD after NO2 Adsorption. Prior to TPD experiments, the catalysts were outgassed at 130 °C under high vacuum into a homemade adsorption chamber and were subsequently exposed to the reactive gas (2000 ppm NO2/He balance) at 60 °C during 12 h. Afterward, the samples were again outgassed under vacuum in order to remove the weakly bound physisorbed species. A 20 mg portion of catalyst was then placed inside a quartz tube coupled to an HP 5973 mass detector working in electron impact (EI) mode at 70 eV and heated from 25 up to 700 °C at a heating rate of 10 °C/min using He as the carrier gas. After normalization toward the sample mass, individual desorption profiles of NO2 and O2 were determined from their known relative contributions to the fragments m/z ) 46 and 32 amu, respectively. The desorption profile of NO was recalculated considering the relative contributions of NO, NO2, and N2O to m/z ) 30 (NO+). Concentration profiles of NO and NO2 in parts per million were obtained after calibration with standard gas mixtures. 3. Results and Discussion 3.1. Catalyst Characterization. The BET surface area, mean crystal size, lattice parameter, proportion of Ce3+, and Ce/Zr
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TABLE 1: N2 Adsorption at 77 K, EDX, XRD, and XPS Characterization Results sample CO SCS CCR RME ACF RH CeO2 ZrO2
preparation procedure coprecipitation solid combustion synthesis citrate complexation reverse microemulsion carbon fiber template commercial precipitation precipitation
nominal compositiona
BET (m2/g)
crystal sizeb (nm)
lattice parameter (nm)
Ce/Zr atomic surface ratiod
Ce3+/(Ce3+ + Ce4+) (%)
67 60 53 128 80 113 64 65
11 9 6 5 9 9 14
0.5398 0.5389 0.5357 0.5340 0.5419c 0.5348 0.5425
5.0 1.5 2.1 1.2 0.8 3.1 ∞ 0
27.5 39.7 34.7 38.7 45.1 34.0 31.4
Ce0.77Zr0.23O2 Ce0.77Zr0.23O2 Ce0.75Zr0.25O2 Ce0.63Zr0.37O2 Ce0.74Zr0.26O2 CeO2 ZrO2
a
Calculated by EDX (from 16 measurements). b Estimated from XRD (Debye-Scherrer equation). c Due to the heterogeneous character of ACF, a was calculated taking into account the CeO2-like crystallites only (see text for details). d Calculated by XPS.
Figure 1. XRD diffraction patterns of ceria-zirconia catalysts and their parent materials. The indexed reflections are all related to the cubic fluorite structure. Asterisks denote the existence of a tetragonal zirconia phase and triangles the existence of a monoclinic zirconia phase.
atomic surface ratio of the samples are presented in Table 1. Depending on the preparation method, the specific surface areas of the mixed oxides lie between 53 and 128 m2/g, the highest value being determined for the material synthesized using reversed micelles (RME). This value is even superior to that of the commercial sample (RH) after calcination under identical conditions. Accordingly and as estimated from the Debye-Scherrer method applied to X-ray line broadening, the RME material has also the lowest mean crystallite size (5 nm) and pure ceria the highest (14 nm). This was expected considering that, on the one hand, Zr doping inhibits crystallite growth in ceria1 and, on the other hand, that the specific surface area of ceria-zirconia solid solutions is mainly connected with the nanocrystallite size distribution. The X-ray diffractograms of all samples are compiled in Figure 1. All mixed oxides display the typical diffraction pattern of the ceria fluorite structure,1 and the introduction of Zr cations within the cubic ceria framework is confirmed by the values of the lattice parameters (Table 1). As expected, the lattice parameter of ceria (a ) 0.5425 nm) decreases in all mixed oxides upon doping the framework with the smaller Zr4+ cations.1 According to the literature data collected in Kaspar’s review12 for high specific surface area samples, the expected lattice parameter value for ceria-zirconia catalysts of an average composition of Ce0.76Zr0.24O2 should be about 0.536 nm. Hence, this suggests that the CCR and RH catalysts (with a < 0.536 nm) display an enhanced repartition of Ce and Zr at the atomic scale by comparison with CO and SCS (a . 0.536 nm). Phase
segregation is only obvious for the catalyst prepared using the activated carbon fiber (ACF) template because tetragonal zirconia reflections appear in the diffractogram of this sample along with the fluorite peaks of the cubic ceria phase (a ) 0.5419 nm, very close to that of CeO2, Table 1). The XRD diffractogram of ZrO2 shows a mixture of monoclinic and tetragonal phases. The comparison between nominal Ce/Zr ratios (as obtained from EDX analyses averaged over 16 measurements, Table 1) and surface Ce/Zr ratios (as obtained from XPS, Table 1) provides further information about the homogeneity of the mixed oxides at the nanoscale. Most mixed oxides present surfaces slightly enriched in Zr (SCS, CCR, RME, and ACF), whereas only CO (and also RH) presents a Ce-enriched surface. Accordingly, the latter material is also characterized by the lower proportion of surface reduced Ce3+ species. Here, it is worth recalling that Ce-based materials usually undergo a spontaneous reduction under the vacuum conditions used for XPS experiments. This may significantly enhance the proportion of surface Ce3+ species in comparison to the pretreatment conditions we used for NO oxidation/NOx sorption studies (350 °C under He). Nevertheless and though the determined ratios of Ce3+/(Ce3+ + Ce4+) in percent have not been considered as absolute, they may give a trend concerning the reducibility of the different samples. These Ce/Zr surface ratios, and also the proportion of surface Ce3+ cations (see Table 1), will be considered afterward because they provide important information to explain the interaction of NOx with each surface. 3.2. DRIFTS Characterization of Surface Hydroxyls. As stated above, the inhomogeneous distribution of Ce and Zr components in the synthesized materials induces a corresponding enrichment in one of the two components at the surface of the nanoparticles. Hence, a DRIFTS study was carried out in order to assess the effect of the preparation method on the surface properties of the different Ce0.76Zr0.24O2 materials. Because of the rather basic character of ceria and ceria-zirconia mixed oxides, it was indeed not possible to eliminate all the contamination in our conditions, which consists, namely, of C-Hcontaining species and bidentate carbonates. For that purpose, preliminary TPD-MS experiments have shown that temperatures above 600-700 °C are required, but this should also lead to some sintering and significant modification of the catalysts’ surface properties (calcined at 500 °C previously). IR spectra in the OH group region (referenced to KBr) were recorded in situ at 350 °C under He just prior to the admission of NOx into the reaction cell. DRIFTS spectra in the OH region of both the parent ZrO2 and CeO2 precipitated catalysts (also calcined at 500 °C) and the different Ce0.76Zr0.24O2 materials are compared in Figure 2. A summary of the main IR bands assignments is given in Table 2. Because the investigated
Probing the Surface of Ce-Zr Catalysts
Figure 2. DRIFTS spectra (referenced to KBr) recorded in the 3900-3400 cm-1 region at 350 °C under He.
samples had distinct specific surface areas and different populations of OH groups, spectra were normalized in intensity to facilitate the comparison between catalysts. In addition to coordinated OH groups, all spectra were found to present an absorption tail down to 2500 cm-1, indicative of the existence of associated hydroxyl species. The IR spectrum of ZrO2 displays four main overlapped bands located at ca. 3760, 3730, 3660, and 3647 cm-1. The 3760 and 3660 cm-1 bands are assigned, respectively, to terminal and bridged OH species on the monoclinic structure and the others to terminal and bridged species on the tetragonal one, in good agreement with the literature.23-25 By contrast, the IR spectrum of CeO2 displays two prominent bands at 3500 and 3646 cm-1, the latter with a shoulder around 3615 cm-1, and two bands of medium intensities at 3695 and 3680 cm-1, accompanied by a weak shoulder around 3715 cm-1. According to the nomenclature used by Daturi et al.,24 the shoulder at 3715 cm-1 is assigned to remaining monocoordinated OH (I) groups, presumably having basic properties, which were not totally removed by the thermal pretreatment at 350 °C. The broad band at 3500 cm-1 is related to oxy-hydroxy species or, possibly, to hydroxy carbonates if residual carbonates remain on the sample and the absorptions under 3620 cm-1 to triply bridging Ce3(OH) species.23 Other bands are more difficult to assign because their frequencies were found to be strongly shifted with the reduction state of the surface and/or the existence of H-bonding interactions. Nevertheless, the strong 3646 cm-1 band is assigned to doubly bridged OH species (II-B) interacting with O vacancies, whereas the bands at 3695 and 3680 cm-1 correspond to adsorbed molecular water and another kind of bridged OH species (II-A), respectively.24 The formation of reduced sites at the ceria surface in
J. Phys. Chem. C, Vol. 114, No. 31, 2010 13303 our conditions (also established by XPS in Table 1) is not considered as surprising because it can be explained at least by two different mechanisms. As the temperature increases to 350 °C, the less stable OH groups, most likely the monocoordinated ones, diffuse on the surface and interact with each other to produce H2O. This process is accompanied by the reduction of Ce4+ to Ce3+ and the formation of an oxygen vacancy. Additionally, the oxidative elimination of some carbon-containing impurities during the pretreatment should also involve the extraction of some labile oxygen species, leading to surface reduction.1 The stability of surface oxygen vacancies generated by these processes depends on both the temperature and the intrinsic oxygen diffusion of the material. Hence, the supply of oxygen species from the bulk to the surface is expected to be higher for solid solutions having an improved oxygen storage capacity than pure ceria.23,24 Interestingly, DRIFTS spectra of the Ce-Zr mixed oxides (Figure 2) displayed various degrees of resemblance with those of the pure oxides, which were further established by summing the spectrum of ZrO2 to that of CeO2 at increasing ratios. In general, perfect matches were not obtained because mixed oxide surfaces probably display increased surface disorder and/or may expose other surface planes than the (111) one, which is expected to be the major surface plane for polycrystalline ceria. Nevertheless, the ACF sample, which was found to exhibit phase segregation between an almost pure cubic ceria phase and tetragonal zirconia phase displayed an IR spectrum very similar to that of CeO2, except the absence of the 3500 cm-1 band and the presence of a weak shoulder at 3740 cm-1 characteristic of type I OH groups on tetragonal zirconia.23-25 The CCR mixed oxide and the commercial RH catalyst have also an IR spectrum somewhat close to that of CeO2, but the presence of Zr atoms or induced defects in the vicinity of Ce surface sites for the RH sample slightly shifted the main band toward higher frequencies (3651 cm-1 against 3646 cm-1 for CeO2). Under our conditions (pretreatment at 350 °C under He), such an upward shift was detected for all the mixed oxides (excepted CCR) and was noticeably more important for RME and CO (3660 and 3657 cm-1, respectively). It is worth noting that this shift seems typical of the surface structures of the mixed oxides, as it cannot be satisfactorily simulated by summing the two IR spectra of the parent materials whatever the ratio used (the summation does not appreciably change the frequency maxima for the ratios of CeO2/ZrO2 of 2, 1, and 0.67). Hence, it can be assumed that it can be connected either with the existence of some bridged Zr(OH)Ce species or to a higher initial degree of surface reduction.23,24 Additionally, the CO and SCS mixed oxides were characterized by spectral features corresponding somewhat to both of the parent materials with the presence of the 3500 cm-1 band (typical of ceria) and absorptions above 3700 cm-1 (typical of zirconia), which may be related to the existence of nanopatches of the parent structures on their surfaces. For the SCS mixed oxide, the summation of the two parent spectra at a CeO2/ ZrO2 ratio of 0.67 gives a rather good agreement with its observed spectral features, and this denotes an important surface enrichment in Zr, which was confirmed using XPS (Table 1). Finally, the catalyst prepared by a more sophisticated method (RME) displayed a rather distinct behavior, which may account either for an uneven repartition of Zr and Ce components at the surface or for an increased disorder at an atomic scale. In view of the very small crystal size in this material and the slightly different nominal and surface compositions (Table 1), it is possible that termination by more reactive surface planes (such as (100), (110), and (310)) may be promoted.
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TABLE 2: IR Assignments of OH Groups and ad-NOx Species Present at 350 °C on CeO2 and ZrO2 and the Different Prepared Ce0.76Zr0.24O2 Materialsb IR frequencies (cm-1)
surface species OH groups on m-ZrO2 OH groups on t-ZrO2 OH groups on CeO2
3760 3660 3730 3647 3715 3695 3680 3650
a OH groups on Ce0.76Zr0.24O2
nitrites on ZrO2 nitrites on CeO2
a
nitrites on Ce0.76Zr0.24O2
nitrates on ZrO2 nitrates on CeO2
a nitrates on Ce0.76Zr0.24O2
3620 3500 3760-3720
proposed assignments
references -
terminal Zr -OH (I) bridged OH (II) terminal Zr4+-OH (I) bridged OH (II) terminal OH (I) adsorbed water and/or bridged OH (II) bridged OH (II-A) on Cex+ sites bridged OH (II-B) on Cex+ sites interacting with O vacancies triply bridged OH (III) oxy-hydroxy or hydroxycarbonates terminal OH (I) on Zr4+ sites 4+
23 25 23-25 23-25 this work
23-25
terminal OH (I) on Cex+ sites this work bridged M2(OH) (M ) Zr4+ and/or Cex+) at regular/defective positions oxy-hydroxy or hydroxycarbonates on Ce sites bidentate Zr4+)O2N species 7, 11, 13 bidentate Ce4+)O2N species 7, 8, 11, 15 at regular positions this work 1160, 1105 bidentate Cex+)O2N species in the vicinity of O vacancies and/or at defective positions unresolved absorptions in the bidentate M4+)O2N species at regular positions and 7, 8, 11, this work range of 1200-1105 Cex+)O2N species in the vicinity of O vacancies and/or at defective positions 1620, 1230, 1000 bridged bidentate NO3- on Zr4+ sites 13 1583, 1242, 1030 chelated bidentate Zr4+)O2NOthis work 1550-1450, 1280, 1000 monodentate Zr4+-ONO21595, 1210, 1000 bridged bidentate NO3- on 7, 8, 11, 15, this work Cex+ sites 1585-1545, 1265-1225, 1030-1000 chelated bidentate Cex+)O2NOat different positions 1540-1500, 1275, 1030 monodentate Cex+-ONO2at different positions this work 1600 and above, 1230, 1000 bridged bidentate NO3-, mostly on Zr4+ sites 1600-1545, 1260-1230, 1030-1000 chelated bidentate, mostly of Cex+)O2NO- type in different environments/positions 1545-1500, 1270, 1030 mostly monodentate Cex+-ONO2- in different environments/positions 3720-3700 unresolved absorptions in the range of 3700-3620 3500 1190 1184
a IR frequencies and relative intensities are sensitive to the Ce/Zr surface ratio and the preparation method. regarding the reactivity of these species.
To sum up, the characterization techniques employed have revealed distinct properties for the different investigated mixed oxides despite their rather similar nominal compositions. Most peculiarly, the surface properties of these ceria-zirconia are found to be rather strongly tuned by their preparation method. Hence, different behaviors are also expected for their reactivity toward nitrogen oxides. 3.3. DRIFTS Study of NOx Sorption on Ceria and Ce0.76Zr0.24O2 Mixed Oxides. Time-resolved DRIFTS spectra obtained under a (NO + O2) flow at 350 °C were recorded for both the different Ce0.76Zr0.24O2 catalysts and the pure oxides. Before comparing the results in detail, it is worth mentioning that the overall progress of the reaction with NOx followed the same scheme for all the materials, only differing by the kinetics and discrete changes of spectroscopic data. Such a conclusion was also drawn from our recent studies and for a list of most band assignments presented in this paper; the reader may also refer to ref 11. As a matter of fact, molecular NO chemisorption does not take place on the terrace sites of many metal
b
Please see text for details
oxides.13,17,18 Because of the high mobility of NO radicals on the surface, stabilization as physisorbed N2O2 dimers is often preferred at low temperatures.14 By contrast, at low coordinated pair sites on steps and corners, activated sorption could take place as NO- or NO2- species and this process predominates above room temperature.13,14,17,18 For the sake of brevity, the whole set of time-resolved DRIFTS spectra is only presented for CeO2, which presents the most resolved spectral features (Figure 3). By contrast, ceria-zirconia catalysts displayed more blurred spectral features due to surface heterogeneity. This can be, for instance, observed in Figure 4, corresponding to adsorbed NOx species near saturation coverage and also in previous papers.7,8,11 The most relevant IR bands assignments can be found in Table 2. Adsorption as Nitrites and Their ReactiWity. In the first minutes of reaction and whatever the catalyst, the spectra were dominated by the presence of bidentate or chelated nitrites, as revealed by the existence of a broad and strong asymmetric N-O mode centered in the 1194-1160 cm-1 range (1160 cm-1
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Figure 3. Time-course difference DRIFTS spectra recorded during exposure of CeO2 to a NO + O2 flow (60 mL/min) at 350 °C in the (A) 1800-800 cm-1 region and (B) 3840-3540 cm-1 region. The reference is pure CeO2 at 350 °C under He.
for CeO2 in Figure 3). The symmetric mode is around 1270-1260 cm-113 but is not easily detected due to its weak intensity and overlapping with some vibrations of nitrates. On CeO2, one well-known way to form these surface nitrites is via electron transfer from Ce3+ reduced centers to NO2 molecules (formed by NO oxidation on another site) to give bonded NO2- entities (with one O healing the vacancy):
Ce3+-0 + NO2 f Ce4+ ) ONO-
(1)
The simultaneous consumption of OH groups detected during the first minutes of reaction (Figure 3B) points out that nitrites are also formed by direct interaction of NO with (negatively charged) hydroxyl groups, this process, in principle, releasing water:7,11
2Ce4+-OH- + NO f 2Ce4+-ONO- + H2O
(2)
On ceria-rich surfaces, both the electronic properties and the geometrical configuration of sorption sites are affected by Zr doping. Therefore, the observed frequencies of the corresponding surface nitrites are expected to change discretely depending on the negative charge carried by the NO2δ- entity, that is, depending on the extent of electron transfer with the surface. From our previous studies devoted on commercial11 and coprecipitated ceria-zirconia,7,8 it was noticed that the main nitrite band, which was initially sharp when formed around 30-100 °C in the presence of NOx, significantly decreased in intensity, broadened, and was shifted toward lower frequencies by 20-30 cm-1 when the temperature was further increased to 200-300 °C. In line with the idea that nitrites are highly reactive species,4,13 we believe that both the red shift and the broadening
Figure 4. Time-course DRIFTS spectra recorded during exposure of the different Ce0.76Zr0.24O2 catalysts under a NO + O2 flow (60 mL/ min) at 350 °C and t ) 65 min.
observed could be related to a weakening of one of the N-O bonds in the surface nitrites. This process represents the first step in the decomposition of nitrites as NO(g) and O(ads), which is followed by production of nitrates, as we will see later on. Rather similar conclusions were drawn in a theoretical study,26 where the nitrites present on reduced model ceria surfaces were described to display a severe elongation of one of the N-O bonds while the other was slightly shortened or remained unaffected. In the present study, it is worth noting that, depending on the catalyst investigated under a (NO + O2) flow at 350 °C, discrete changes of the nitrites’ frequencies were detected in the first minutes of the reaction. Hence, the comparison of the corresponding DRIFTS data between ceria-zirconia and pure oxide catalysts may be taken as an indicator of the relative stability/reactivity of adsorbed nitrites on the different surfaces. For CeO2, which has a high ability in producing and decomposing nitrites (as explained in section 3.4), the main features observed were a broad band centered at 1160 cm-1 accompanied by strong shoulders at 1105 and 1184 cm-1. These data probably have to be assigned to different types of surface nitrites with N-O bonds nonequivalent in length, for instance, located on different ceria planes or at special positions. As expected, similar spectral features were obtained for the ACF catalyst, which presents some phase segregation in CeO2 and ZrO2. Hence, this seems to confirm the presence of ceria-like domains (also detected by XRD) in the surface of ACF crystallites. By contrast, the DRIFTS spectrum of the less reactive ZrO2 displays a single band at 1194 cm-1 and the other ceria-zirconia catalysts several overlapped maxima in the
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Azambre et al.
1188-1160 cm-1 range, presumably due to adsorption on both Ce and Zr sites. Among them, the RH commercial catalyst has its main maximum rather shifted to 1160 cm-1, but the shoulder at 1105 cm-1 is not present as in CeO2, whereas for the others, it is increased to higher frequencies. The detection of an isosbestic point at 1200-1190 cm-1 in time-course DRIFTS spectra of ceria (and also for all the Ce-Zr catalysts) points out the existence of surface species transformation during the exposure to the (NO + O2) flow (Figure 3). Indeed, the intensity of the main nitrite band decreases quickly at the expense of very strong absorptions at 1620-1510, 1290-1210, and 1030-990 cm-1, corresponding to surface nitrates in a variety of structures/configurations13 (Figure 4), which will be commented with more details later on. The overall reaction taking place is NO2 (ads) + NO2(g) T NO(g) + NO3 (ads)
(3)
This scheme is consistent with the relative growth/consumption rate of the nitrites, which was found in our previous studies7,11 to decrease in the order NO2 > NO + O2 > NO. It is also worth noting that the rate of nitrate formation followed approximately the same trend. On the basis of DRIFTS data (and also the TPD data presented in section 3.4), one elemental step in this reaction is probably the decomposition of the nitrite as NO(g) and O(ads), followed by the reaction of NO2 with the activated oxygen released to produce a nitrate species. Alternatively, the direct oxidation of nitrites by an activated form of oxygen can also be considered, as reported in refs 7, 8, and 11: NO2 (ads) + O(ads) T NO3 (ads)
(4)
The time dependence of the whole population of nitrites and nitrates existing on the different catalysts is represented semiquantitatively by plotting, respectively, the absorbance or the area of these species versus time in Figure 5. Considering nitrites first, it can be seen that the investigated catalysts display different time profiles for the formation/decay of nitrites (these are consecutive reactions). According to eqs 1 and 2, surface defects and/or reactive OH groups are required to enhance the initial rate of nitrite formation. The decay rate of the nitrites is rather affected by both their intrinsic reactivity (such as those corresponding to the band at 1160 cm-1 should be the more reactive) and the concentration of gaseous NO2 around the sorption site, as suggested by eq 3. Hence, the NO oxidation activity of the catalysts at 350 °C plays a central role on the kinetics of the sorption processes, as NO2 is thought to enhance both the formation and the decay rates of the nitrites. For instance, ZrO2, which, according to ref 7, has a poor activity for NO oxidation in comparison to Ce-rich catalysts, only displays an isosbestic point in its (NO + O2) IR spectra after 25 min (still present at 65 min, witnessing that transformation to nitrates is not complete), whereas under a NO2 flow (not shown here), this point is detected between 2 and 8 min. Similar observations have been made for the CO catalyst in our previous study when NO was used instead of NO + O2 or NO2.7 This could be also rationalized considering that the formation of nitrites only requires an electron transfer or fast reactive hydroxyl consumption, whereas the nitrate formation requires several elemental steps, including O transfer, which require an additional energy cost.
Figure 5. Semiquantitative DRIFTS data representative of the evolution of (A) OH groups (consumption), (B) nitrites (formation + consumption), and (C) nitrates (formation) as a function of time during adsorption of NO + O2 at 350 °C on the different ceria-zirconia catalysts.
Adsorption as Nitrates. The types of nitrates found on the different surfaces (Figure 4) and their time dependence (Figure 5) are investigated now. By contrast with the free nitrate ion that has a strong absorption around 1380 cm-1, coordination of NO3- to a metal ion causes the antisymmetric stretch mode (υ3) to split into a low-frequency component (1300-1200 cm-1) due to the metal-bound O atoms (N-O moiety) and a high-frequency component due to the free O atoms (NdO moiety, 1640-1450 cm-1).13 Additionally, the detection of the symmetric stretch (υ1) in the range of 1040-990 cm-1 for all of the the catalysts is indicative of distorted surface structures. The usual way to distinguish the different kinds of coordinated nitrates is to consider both the positions of the two υ3 components and the ∆υ3 value. The latter parameter is indicative of the interaction strength between the metal and the nitrate ligand and should, in principle, increase in the order monodentate < chelated
