Site Requirements for the Adsorption and Reaction of Oxygenates on

Nov 26, 2012 - Site Requirements for the Adsorption and Reaction of Oxygenates on Metal Oxide Surfaces. John M. Vohs*. Department of Chemical and Biom...
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Site Requirements for the Adsorption and Reaction of Oxygenates on Metal Oxide Surfaces John M. Vohs* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States model catalysts have played a prominent role in helping to define the active sites for specific reactions on oxide surfaces. The study of model oxide surfaces has until recently, however, lagged somewhat behind that of metals. This is due in part to the formerly widely held belief that the insulating nature of most metal oxides and difficulties associated with heating and cooling them in UHV would render surface science studies of oxide single crystals intractable. Fortunately, this has largely proven not to be the case, and today their surface properties are routinely studied using the full range of surface sensitive spectroscopic and kinetic probes, including photoelectron spectroscopy (X-ray (XPS) and ultraviolet (UPS)), low energy CONTENTS electron diffraction (LEED), ion scattering spectroscopy (ISS), 1. Introduction A high-resolution electron energy loss spectroscopy (HREELS), 2. Wurtzite Zinc Oxide B and temperature programmed desorption (TPD). 2.1. ZnO(0001) and ZnO(0001̅) Polar Surfaces B The first big flourish of studies of the adsorption and reaction 2.2. ZnO(101̅0) D of oxygenates on metal oxide single-crystal surfaces appeared in 3. Metal Oxides with Rock Salt Structure F the literature in the late 1980s. The bulk of this work focused 3.1. MgO F on ZnO and TiO2, with a few studies of MoO3, MgO, and 3.2. NiO G SnO2 also appearing at this time. These studies provided 4. Oxides with the Fluorite Structure H considerable insight into the reaction of oxygenates on oxide 4.1. CeO2 H surfaces and provide the basis for many of the models of active 4.2. UO2 J sites on oxides that are still used today. Overviews of much of 5. Oxides with the Rutile Structure M this early work can be found in several previous reviews.1−3 It 5.1. Titanium Dioxide M should be noted that these early studies of the site requirements 5.1.1. TiO2(001) M for the reaction of oxygenates on oxides were hampered by a 5.1.2. TiO2(110) P lack of knowledge of the local atomic structure of the surface. 5.2. SnO2 S At this time, LEED was the primary technique used to infer the 6. Layered MoO3 U structure of the low index planes of oxides, especially for those 7. Concluding Remarks W cases where reconstructions occurred, and in many cases bulk Author Information X ideal termination was simply assumed. In the intervening years, Corresponding Author X the rapid development of scanning probe techniques has led to Notes X a wealth of new insight into the surface structure of oxides. Biography X Scanning tunneling microscopy (STM) has now been used Acknowledgments X extensively to characterize the structure of the surfaces of Abbreviations X semiconducting oxides, such as ZnO and TiO2 (the latter References X becomes semiconducting upon slight bulk reduction), and, more recently, noncontact atomic force microscopy (AFM) has been used to provide atomic resolution images of many 1. INTRODUCTION insulating oxide surfaces. Indeed, several reviews of the use of The diversity of the structure and composition of metal oxides these techniques to characterize oxide surface structure and to a allow these materials to have a wide range of surface reactivities, lesser extent adsorption on oxide surfaces can now be found in and they are used extensively as catalysts for reactions involving the literature.4−8 oxygenates (i.e., alcohols, aldehydes, and carboxylic acids) The scanning probe studies of oxide surfaces have provided including selective and complete oxidations, dehydrogenation, many valuable insights into the active sites for adsorption on dehydration, and reductive coupling. They are also used as oxides surfaces, especially for oxygenates. In some cases, these catalysts for photoxidations and as chemical sensors. These applications have motivated numerous studies into the Special Issue: 2013 Surface Chemistry of Oxides relationships between their structure and reactivity and the site requirements for specific reactions. As has also been the Received: August 10, 2012 case for metals, surface science studies using single crystals as © XXXX American Chemical Society

A

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example, on many oxide surfaces, alkoxide groups are oxidized to carboxylates by reaction with lattice oxygen. This pathway ultimately results in reduction of the surface, which is then reoxidized by oxygen from the gas phase.

scanning probe results have helped to confirm the active site models that were proposed in earlier reactivity studies, while in other cases, like ZnO, these studies have led to a reinterpretation. Thus, one of the goals of this Review is to update the earlier reviews of the reaction of oxygenates on single-crystal oxide surfaces and include a discussion of the local atomic structure of the surface as determined by scanning probe techniques and where appropriate, to discuss how this new structural information has led to more accurate models of the active sites. In the last 10−15 years, the range of metal oxides for which surface science techniques have been used to study the adsorption and reaction of oxygenates has also been greatly expanded. Many of these systems are also included in this Review, but again the focus is only on those metal oxides for which scanning probe techniques have provided insight into the surface structure. Before proceeding to discuss the reaction of oxygenates on individual metal oxides, it is useful to first give a brief overview of how these reactions have been traditionally categorized. Generally these reactions are described in terms of acid−base or redox type mechanisms.1,2,9−15 A Brønsted acid−base formalism provides a good description of the dissociative adsorption of alcohols and carboxylic acids. As will be illustrated below, on most oxides, exposed cation−anion pairs are the active sites for this type of reaction, which proceeds via abstraction of the acidic proton by a surface O2− anion to form an adsorbed hydroxyl group, with the conjugate base anion of the parent acid (e.g., an alkoxide or carboxylate) bonding to an exposed metal cation. The relative acid−base strengths of oxide surfaces can, therefore, be related to their ability to dissociate Brønsted acids. The local coordination environment of the cations and anions is also important, and some degree of coordinative unsaturation is generally required for catalytic activity. In many cases, this requirement can lead to large structural sensitivities, including dramatic variations in reactivities for different exposed crystal planes in a single metal oxide. Lewis acid−base descriptions of the reactivity of oxides are also important. The electron-deficient metal cations on oxide surfaces can be considered to be Lewis acids, especially for highly ionic materials. Indeed, the aforementioned bonding of the conjugate base of an organic acid to a surface metal cation can be described in terms of an electron-donating Lewis base, the conjugate base anion, reacting with an electron-accepting Lewis acid, the metal cation. This formalism is also useful when describing the adsorption of aldehydes on oxide surfaces where bonding can occur via interaction of the lone pair electrons on the aldehyde oxygen with a surface metal cation. It is also common practice to define the acid−base properties of an oxide based on the selectivity for dehydration versus dehydrogenation of alcohols and carboxylic acids. Dehydration that leads to the production of alkenes or ethers from alcohols is generally considered to be an acid-catalyzed pathway, while dehydrogenation is categorized as being base catalyzed. As many of the examples discussed below will illustrate, from a mechanistic point of view this categorization is harder to rationalize, because the reducibility of the oxide clearly plays a pivotal role in directing the selectivity for many systems. The redox properties of the oxide are also important in the complete and selective oxidation of oxygenates on oxide surfaces. These reactions often proceed via a Mars−van Krevelen type mechanism where oxidation occurs via nucleophilic attack of the adsorbed oxygenate by a surface lattice oxygen.16,17 For

2. WURTZITE ZINC OXIDE ZnO is active for the selective oxidation of alcohols, is an important component in methanol synthesis catalysts, and is used as an adsorbent for H2S in hydrodesulfurization processes. It is also used as a pigment in paints, as a UV adsorber in sunscreens, and as a chemical sensor. It was one of the fist oxides for which surface sensitive spectroscopic techniques, such as TPD, XPS, and UPS, were used extensively to study the adsorption and reaction of oxygenates on well-defined singlecrystal surfaces, and ZnO along with TiO2 have become the prototypical materials for studying structure−activity relationships for oxide catalysts. As discussed in the Introduction, the adsorption of alcohols and carboxylic acids on metal oxides is often described in terms of an acid−base reaction in which the molecule adsorbs dissociatively on a cation−anion site pair to form a hydroxyl group and the conjugate base anion of the parent acid, which bonds to the surface cation. Single-crystal surfaces of ZnO, especially the (0001) and (0001̅), are one of the most studied systems for this type of reaction. The initial studies of the reactions of alcohols on ZnO(0001) and ZnO(0001̅) surfaces were performed by Kung’s group at Northwestern University18−22 and Barteau’s group at the University of Delaware.23−27 We refer the reader to several excellent review articles by Barteau that describe this work in detail1,2 and will give only a brief overview of it here. Instead, we will focus on new insights that have been obtained in the past decade that provide a more complete description of the active sites for the reaction of oxygenates on these surfaces. A recent review article on the chemistry and physics of ZnO surfaces by Wöll28 is also a useful resource that the reader may wish to consult. 2.1. ZnO(0001) and ZnO(0001̅) Polar Surfaces

Under ambient conditions, ZnO has the noncentrosymmetric wurtzite crystal structure shown in Figure 1 in which the Zn and O ions are tetrahedrally bonded to each other. ZnO powders are generally composed of hexagonal rods or platelets, which preferentially expose the (0001), (0001)̅ , and (1010̅ ) surfaces, and as a result they have been the most studied. As shown in the figure, in the c-direction the Zn2+ and O2− ions are in alternating layers, and cleavage perpendicular to this axis results in two inequivalent surfaces, the (0001) surface that is terminated with Zn cations and the (0001̅) surface that is terminated with O anions. Ideal bulk termination models of these surfaces are also shown in Figure 1. An important aspect of the reaction of alcohols and carboxylic acids on ZnO that has been used to support the cation−anion site pair model is the large difference in reactivity that is observed between the ZnO(0001) and ZnO(0001̅) surfaces. On ZnO(0001), alcohols and carboxylic acids adsorb via dissociation of the acidic proton to form alkoxide and carboxylate intermediates, respectively.18,21,23,25,29,30 As shown by the TPD data for the reaction of CH3OH on ZnO(0001) in Figure 2,31 these species are stable up to ∼450 K, while at higher temperatures they undergo both dehydrogenation and oxidation via reaction with surface lattice oxygen. For methanol, both XPS and HREELS studies show that these reactions proceed via dehydrogenation of methoxide to produce B

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Figure 2. TPD spectra obtained from CH3OH-dosed ZnO(0001).31

that the subsurface O2− ions on the (0001) surface will be reactive toward the acidic proton on the alcohol or carboxylic acid, ultimately abstracting it to form a hydroxyl group, despite the fact that these cations are already fully coordinated. Furthermore, this model does not account for the experimental observation that the saturation coverage of dissociatively adsorbed organic Brønsted acids on ZnO(0001) rarely exceeds 10−20% of a monolayer,33,34 which indicates that only a relatively small fraction of the anions on the surface are active. While the cation−anion site pair model has lasted the test of time, as will be discussed below, a slightly more complex model of the active site emerges when one considers the local atomic structure of real (as opposed to ideal) ZnO surfaces. Despite the fact that for ideal bulk termination, the (0001) and (0001̅) polar surfaces have a net charge making them inherently unstable, the observation of hexagonal LEED patterns in early studies of the structure of these surfaces28,35−37 led to the conclusion that they did not reconstruct, and the models displayed in Figure 1 provide a good approximation of the surface structure. Detailed STM studies of ZnO surfaces performed in the early 2000s by the Diebold group, however, have shown that this is not the case.38−40 STM images of (0001) and (0001̅) surfaces that were prepared by sputtering and annealing in UHV reported in these studies are shown in Figure 3.38 These images clearly show that some reconstruction occurs on both surfaces. The STM image of a ZnO(0001) surface prepared by sputtering and annealing in UHV in Figure 3A shows that the surface consists of a mosaic of triangular-shaped terraces, each of which is separated by a single layer high (∼2.7 Å) step.38 A ball-and-stick model of a single triangular terrace is shown in panel B in the figure. Note that the edge of each step is terminated by under-coordinated O2− ions. Monte Carlo

Figure 1. Bulk crystal structure of wurtzite ZnO and models of the ideal ZnO(0001) and ZnO(0001)̅ surfaces.

formaldehyde, which then either desorbs at 500 K or reacts further with lattice oxygen to form formate, which decomposes to produce CO, CO2, and H2O at 575 K.23,32 The formation of these complete oxidation products demonstrates the rather facile removal of oxygen from this surface. In contrast to ZnO(0001), the ZnO(0001̅) surface is much less active for both the oxidative dehydrogenation and the oxidation of alcohols and carboxylic acids, with these species primarily adsorbing molecularly on this surface.19,21,23,25 This large variance in reactivity was initially ascribed to the structural differences between the two surfaces. As shown in Figure 1, the Zn cations on the ideal (0001) surface are undercoordinated with a single dangling bond directed normal to the surface. Because of the large difference in size of the Zn2+ and O2− ions, the subsurface layer of O2− ions is also largely exposed on this surface, thereby providing cation−anion site pairs. The positions of the Zn and O ions are reversed, however, on the ideal (0001̅) surface with the O2− anions being in the outermost layer. On this surface, the large O2‑ anions shield the subsurface layer of Zn2+ cations, making it effectively composed of only oxygen. This surface, therefore, lacks the exposed cation−anion site pairs that were thought to be required for dissociative adsorption of organic acids. While the simplicity of this explanation for the differences in reactivity of the polar surfaces of ZnO is appealing, it assumes C

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abstraction of oxygen from the ZnO lattice. The undercoordinated oxygen cations at the step edges would be more easily abstracted from the surface as compared to the fully coordinated cations on the terraces. The ideal termination of the ZnO(0001̅) surface (Figure 1) is composed only of oxygen ions. Because of the large size of these ions, the subsurface Zn layer is shielded from adsorbates. Thus, this surface does not have exposed cation−anion site pairs and would therefore not be expected to be active for the dissociative adsorption of Brønsted acids. Reports in the literature of the activity of the ZnO(0001̅) surface for the dissociative adsorption and further oxidation of Brønsted acids vary from the surface is completely inactive23,25,45 to the surface exhibits some activity, but is still much less active than ZnO(0001).18,19 While the experimental results are largely consistent with this picture of the surface chemistry, the variability in the reported activity of the (0001̅)) surface suggests that defect structures may again be important. The STM image of the sputtered and annealed (0001̅) surface from the Diebold group in Figure 3C38 shows that like ZnO(0001), this surface reconstructs to stabilize the surface charge. In this case, stabilization results in the formation of relatively large hexagonal terraces that are separated by doubleheight steps that intersect at an angle of 120°. As shown in panel D in the figure, a subset of the step edges are terminated by under-coordinated Zn2+ cations. These step edges contain the necessary cation−anion site pairs for the dissociative adsorption of Brønsted acids. The images in this figure show, however, that the concentration of the potentially active step edges on ZnO(0001̅) is significantly less than that on ZnO(0001), which again is in agreement with the reported relative activities of these two surfaces.

Figure 3. STM images of (A) ZnO(0001) and (B) ZnO(0001̅) surfaces that were prepared by sputtering and then annealing in UHV at 973 K. Models proposed by Dulub et al. for the structure of each surface are shown below each STM image. Reprinted with permission from ref 39. Copyright 2002 Elsevier.

simulations using empirical force fields41 and quantum chemical calculations39,42 also predict the formation of surfaces with a similar structure. Note that this reconstruction of the surface results in a decrease in the surface Zn2+ concentration relative to that for the ideal bulk termination and, therefore, lowers the surface charge producing a more stable surface. Thus, formation of these surface features appears to be driven primarily by electrostatics. On the basis of these STM results, Dulub et al. have suggested that the active sites for dissociative adsorption of alcohols and carboxylic acids on ZnO(0001) are Zn2+−O2− site pairs that exist at the perimeter of the triangular terraces.38 This scenario addresses the two aforementioned shortcomings of the proposal that all Zn−O sites on this surface are active. There are also some experimental data in the literature to support this hypothesis. For example, Grant et al. report that depositing 0.01 ML of Pt on ZnO(0001) results in severe attenuation of the dissociative adsorption of methanol on this surface.43 From this result, they concluded that methanol adsorption must occur at defect sites and that these sites are preferentially occupied by the Pt. Similar results have also been reported for Pd- and Co-covered ZnO(0001) surfaces.31,44 Because the step edges on ZnO(0001) (see Figure 3) would be expected to be the most energetically favorable bonding sites for the deposited metal atoms, these results are in concordance with the step edges also providing the sites for dissociative adsorption of methanol. The step-edge model for the active sites for dissociative adsorption of Brønsted acids on ZnO(0001) also helps to explain the high oxidation activity of this surface. As noted above, upon heating, methoxide species on ZnO(0001) undergo oxy-dehydrogenation to produce formaldehyde and water, with a portion of the former being further oxidized to formate. Note that both of these reactions require the

2.2. ZnO(101̅0)

In addition to the two polar surfaces, (0001) and (0001̅), the nonpolar (101̅0) prismatic surface is preferentially exposed in ZnO powders, and the reaction of oxygenates on this surface has also received considerable interest in the literature.18−21,28,45−49 As was the case for ZnO(0001̅), there is some variability in the reports of the activity of this surface for the dissociative adsorption of organic Brønsted acids and their subsequent oxidation. The general consensus from experimental studies, however, is that the surface is active for these reactions, although defect sites may play a dominant role.18,21,28,30,48,50 In contrast, DFT calculations for the adsorption of methanol on ZnO(101̅0) have predicted that molecular adsorption is preferred over dissociative adsorption to form a methoxide by ∼0.3 eV.46 A perspective view of the ZnO(101̅0) surface with the ideal bulk termination structure is shown in Figure 4. Note that the

Figure 4. Ball-and-stick model showing a perspective view of the ZnO(1010̅ ) surface. D

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surface is composed of alternating rows of three-coordinate O2− and Zn2+ ions, each of which has a dangling bond that is directed ∼40° from the surface normal. Because there is an equal number of oxygen and zinc ions, the surface is charge neutral and should be relatively stable. The unsaturated Zn2+ and O2− ions on the surface provide cation−anion site pairs that would be expected to be active sites for the dissociative adsorption of alcohols and carboxylic acids. Atomic resolution STM studies of this surface by Diebold et al.40 and more recently Shao et al.47 support the conclusion that the (101̅0) surface is relatively stable. Figure 5 displays STM

Figure 6. Structural model for dissociatively adsorbed CH3OH on ZnO(1010̅ ) proposed by Shao et al.47

Shao et al. also investigated the dissociative adsorption of methanol on ZnO(101̅0) surfaces that were slightly reduced by several repeated methanol TPD experiments.47 Line defects were observed on these surfaces, and partially on the basis of results from DFT studies,51,52 they were attributed to a missing row of Zn−O dimers. Shao et al. proposed the mechanism shown schematically in Figure 7 for the formation of these extended defect structures.47 According to this mechanism, methanol adsorbs dissociatively across the unsaturated, secondlayer, Zn−O site pair at the end of the missing dimer row to form methoxide and hydroxyl intermediates. Abstraction of one of the methyl hydrogens via reaction with the adjacent surface layer O2− anion results in the formation of formaldehyde, which

Figure 5. STM images of (a) 100 × 100 nm2, (b) 20 × 20 nm2, and (c) 3 × 6 nm2 regions of a ZnO(1010̅ ) surface that was prepared by sputtering and annealing in UHV. Panel (d) shows a model of the surface with the larger red and black circles corresponding to the O2− and Zn2+ ions, respectively.47

images of a ZnO(101̅0) surface prepared by Ar+ sputtering and annealing in UHV at 1000 K that were obtained in the Shao et al. study.47 These images show that the surface is composed of flat, relatively large, terraces that are separated by single height steps that run along the [0001] and [12̅10] crystallographic directions. Analysis of the STM images indicated that the surfaces also contained a very small number of Zn−O dimer vacancies, which have been predicted theoretically,51,52 but isolated oxygen vacancies were not observed; thus, the sputtered and annealed surface appears to be nearly stoichiometric. Shao et al. also used STM to determine the adsorption sites for methanol on stoichiometric ZnO(101̅0).47 After a 10 L methanol exposure at 300 K, they observed two ordered adsorption phases: an island structure covering ∼30% of the surface, and a linear chain structure that covered only 3% of the surface. The more stable island structure was assigned to dissociatively adsorbed methanol on the exposed cation−anion site pairs as shown schematically in Figure 6.47 A slightly different adsorption model has been proposed by Kiss et al.49 based on TPD, HREELS, He atom scattering data, and DFT calculations. They concluded that at high coverages, methanol adsorbs in rows consisting of alternating molecular methanol and methoxide groups. In both models, however, dissociative adsorption is proposed to occur across the Zn2+ and O2− rows with the methoxide groups bonding to the Zn2+ cation and the dissociated proton bonding to the O2− anion.

Figure 7. Model for the adsorption and reaction of methanol-derived species on ZnO(1010̅ ) proposed by Shao et al. According to this model, CH3OH adsorbs dissociatively at (a) oxygen vacancy sites and (b) the end of a line defect. (d) Reaction of the methoxide resulting in the formation of an oxygen vacancy at the end of a line defect. The red and black balls correspond to O2− and Zn2+, respectively.47 E

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room temperature to form methoxide groups bound to surface Mg2+ cations and surface hydroxyl groups. These species, however, recombine and desorb as methanol upon heating. Dissociative adsorption of carboxylic acids to form carboxylate intermediates has also been reported to occur.54,63,69−74 On the basis of ab initio molecular orbital calculations, Nakatsuji et al. concluded that carboxylate species bond in a bidentate, bridging configuration on MgO(100) with the oxygens bonded to adjacent Mg2+ ions;75 however, FTIR studies of acetate intermediates on MgO(100) provide strong evidence for a monodentate configuration with only one of the oxygens in the acetate bound to a surface Mg2+ cation.70 The dehydration activity of MgO is readily apparent in TPD studies of MgO dosed with C2 or higher carboxylic acids where dehydration to produce the corresponding unsaturated aldehyde occurs with high selectivity.61,63,70 For example, as shown in Figure 9, which displays CD3COOD/MgO(100) TPD data reported by Xu and Koel, acetate species on this surface undergo dehydration to produce exclusively water and ketene (H2CCO).70

desorbs near 400 K, and a second surface hydroxyl group. Subsequent reaction of the two adjacent hydroxyls produces gaseous water, which removes the O2− anion at the end of the missing dimer row. While not proposed in the Shao et al. work, the removal of the surface O2− anion would result in reduction of the adjacent top layer Zn2+ to Zn0. Under steady-state selective oxidation conditions, this reduced Zn may be rapidly reoxidized by oxygen from the gas phase, but during TPD in UHV it is likely to desorb. This latter conclusion is consistent with the observation of Zn as a desorption product near 600 K in TPD studies with ZnO single-crystal surfaces for reactions, such as methanol oxidation, which remove lattice oxygen from the surface.22,25,26 The studies of the interaction of oxygenates with ZnO singlecrystal surfaces support the cation−anion pair model of the active site for the dissociative adsorption of Brønsted acids on oxides. They also demonstrate, however, that one needs to be cautious when using an ideal bulk termination model to infer surface structure, because the actual surface may be much more complex. The studies of ZnO also demonstrate that defect structures, in this case step edges, may provide the active sites for both adsorption and reaction of oxygenates on oxide surfaces. The prominent role that defects, such as step edges and oxygen vacancies, play as active sites for the reaction of oxygenates on oxides will be a recurring theme for many of the oxides that are discussed in the remainder of this Review.

3. METAL OXIDES WITH ROCK SALT STRUCTURE 3.1. MgO

MgO has the rock salt crystal structure shown in Figure 8. Surface termination perpendicular to one of the unit cell axes

Figure 9. TPD spectra obtained from CD3COOD-dosed MgO(100) surface. The spectra for m/e values of 16, 20, and 46 correspond to D2O, CD2CO, and CD 3COOD, respectively. Reprinted with permission from ref 70. Copyright 1995 American Institute of Physics.

Because the cations and anions on the MgO(100) surface each have a single coordination vacancy, the cation−anion pair model of the adsorption site for Brønsted acids on metal oxide surfaces as described above suggests they are the most likely sites for dissociative adsorption of these molecules on MgO. A close examination of the available data suggests, however, that the situation is more complex. In UV−visible diffuse reflectance studies of MgO powders, Stone et al. have identified three adsorption bands that can be assigned to five-, four-, and threecoordinate surface Mg2+ cations.76,77 As noted above, the fivecoordinate cations are present on the (100) family of planes that are preferentially exposed in MgO powders. The fourcoordinate cations occur at the edges between these planes, and the three-coordinate sites occur either at the corners of the cubes or at kinks or other defects. Furthermore, Stone et al. found that the Mg2+ sites with the highest number of coordination vacancies were the most active for the heterolytic dissociative adsorption of Brønsted acids, and partially on the basis of this result it has been suggested that these lower

Figure 8. Ball-and-stick model of the rock salt lattice of MgO.

produces the charge neutral (100) family of surfaces, which are inherently stable and preferentially exposed in MgO powders. The highly ionic nature of the bonding in MgO renders the surface oxygen anions electron rich, and they are able to donate electrons to adsorbed species. Because of this strong Lewis base character, MgO is generally considered a highly basic oxide,53−58 and it is active for a variety of base-catalyzed reactions including the dehydration of alcohols and carboxylic acids.59−65 Detailed TPD and spectroscopic studies of the interaction of methanol with both MgO(100) single crystals62,66,67 and MgO powders68 indicate that methanol adsorbs dissociatively at F

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coordination Mg2+ sites contribute to a stronger acid−base interaction.76,78 This conclusion is supported by the work of Peng and Barteau who used XPS to measure the saturation coverage of a series of Brønsted acids on vacuum-annealed MgO(100), Ar+ sputtered MgO(100), and a polycrystalline MgO thin film.62 At 300 K, they observed that CH3OH did not adsorb on vacuumannealed MgO(100), but had a saturation coverage of 0.34 monolayer on the sputtered MgO(100). In contrast, HCOOH and CH3COOH were both found to adsorb dissociatively on the annealed and sputtered surfaces with similar saturation coverages of ∼0.7 monolayer. Subsequent to these studies of adsorption and reaction of oxygenates on MgO, AFM has been used to characterize the structure of single-crystal MgO surfaces, including several in which atomic resolution has been obtained.79−85 The atomic resolution AFM image of MgO(100) in Figure 10 reported by

The features observed in the AFM studies of MgO(100) surfaces appear to correlate reasonably well with the observed reactivity trends. The MgO(100) surface is active for dissociative adsorption of strong Brønsted acids such HCOOH and CH3COOH. The high coverage of the resulting carboxylates that have been observed on MgO(100) suggests that the cation−anion site pairs on this surface composed of five-coordinate Mg2+ and O2− ions are the active sites for this reaction. The base strength of these sites, however, appears to be insufficient to induce dissociation of weaker acids, such as alcohols. The relatively low saturation coverages of alkoxides produced by dissociative adsorption of alcohols on MgO(100) support the conclusion in previous studies that the fourcoordinate ions at the step edges are the active sites for adsorption of these species.76,78,62 While the results for MgO still support the idea that cation− anion pairs are the active sites for dissociative adsorption of alcohols and carboxylic acids, they suggest that the acid−base strengths of these sites depend on local atomic structure or coordination of the site and that a single material can have sites with a range of acid−base properties. While acid−base properties are clearly important in the initial adsorption of Brønsted acids, it is less clear what role they play in determining the selectivity for subsequent reactions of alkoxides and carboxylates. For example, is MgO highly selective for dehydration of alcohols and carboxylic acids rather than selective oxidation or dehydrogenation, because it is a good base, or due to the fact that oxidation ultimately requires extraction of lattice oxygen? On the basis of this and similar results from other oxides, Barteau has commented that dehydration/dehydrogenation selectivities for alcohols and carboxylic acids provide a rather dubious measure of the acid−base character of oxide surfaces, and that redox properties might provide a better starting point for understanding the selectivities of this surface chemistry.63 As will be discussed below, Onishi and Iwasawa have come to a similar conclusion based on their studies of the reaction of oxygenates on TiO2.86,87

Figure 10. Atomic resolution AFM image of a 3.2 × 2.2 nm2 region of a cleaved MgO(100) surface. The arrow points to a position in the image where a tip change occurred. The two line scans shown to the right of the image show a periodicity of 4.2 Å, which is in agreement with that expected for each ion sublattice in MgO. Reprinted with permission from ref 81. Copyright 2003 American Physical Society.

3.2. NiO

Like MgO, NiO has the rock salt structure with the nonpolar (100) family of planes being the most stable and preferentially exposed in NiO powders. The bonding is less ionic in NiO as compared to MgO, making its oxygen anions less electron rich than those in MgO. In light of this, one would expect NiO to be a weaker Lewis base than MgO, and studies of the adsorption and reaction on both NiO powders and NiO(100) surfaces support this conclusion.88−92 For example, Goodman et al. have reported that in contrast to MgO(100), methanol adsorbs only associatively on NiO(100).67,89 Formic acid, however, is observed to adsorb dissociatively, but this process appears to be activated and require temperatures in excess of 200 K.90,92 Motivated in large part by understanding spin dynamics in antiferromagnetic materials, several AFM studies of NiO(100) surfaces in which atomic resolution has been obtained have been reported in the literature in recent years.93−98 The AFM images obtained in these studies provide a picture of the NiO(100) surface that is similar to that described above for MgO(100). The NiO(100) surface is found to not reconstruct and has a periodicity consistent with ideal termination of the bulk.98 Wide scan images of cleaved NiO(100) surfaces show that the dominant defect structures are step edges aligned along the ⟨001⟩ and ⟨010⟩ crystallographic directions.94,97

81

Barth et al. shows a surface periodicity that is consistent with that expected for ideal termination of the bulk. Longer range scans, however, show that even vacuum-cleaved MgO(100) surfaces contain a high concentration of point defects and steps.81,83 The high density of step edges is apparent in the large area AFM scan in Figure 11 obtained by Perrot et al.83

Figure 11. (a) AFM topographic image, which shows steps aligned along the ⟨100⟩ direction on a cleaved MgO(100) surface. (b) Corresponding friction force image. Reprinted with permission from ref 83. Copyright 1994 IOP Science. G

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Review

The studies of the adsorption and reaction of alcohols and carboxylic acids on well-defined NiO single-crystal surfaces are much less extensive than those for MgO. This makes it more difficult to propose specific adsorption sites for these molecules on NiO. The similarity in the structure of NiO(100) and MgO(100), however, leads us to speculate that the sites are the same on these two surfaces with dissociative adsorption of carboxylic acids occurring on the five-coordinate Ni2+−O2− site pairs on the unreconstructed surface. As noted above, the stepedges appear to provide the sites for dissociative adsorption of alcohols on MgO(100). The lack of dissociative adsorption of methanol on NiO(100) suggests that both the five-coordinate Ni2+−O2− sites on the (100) surface and the four-coordinate Ni2+−O2− sites at the step edges have insufficient basicity to induce dissociation of weak acids, such as alcohols.

4. OXIDES WITH THE FLUORITE STRUCTURE Figure 12. Ball model of the fluorite metal oxide crystal lattice. The red and light colored balls correspond to the oxygen and metal ions, respectively.

4.1. CeO2

Ceria is used in a wide range of catalytic applications including wastewater treatment, combustion, and both automotive and stationary source emissions control.99 The latter are a particularly important class of applications, and ceria is currently used as an additive in automotive three-way emissions control catalysts. In addition to enhancing oxidation activity, ceria is thought to provide an oxygen storage function within the catalyst, which helps to extend the air-to-fuel operating window in which it can effectively operate.100 This function relies on the redox properties of ceria and the fact that both Ce4+ and Ce3+ cations are stable. In light of this, CeO2 is often referred to as highly reducible oxide, and catalytic oxidation reactions on its surfaces are generally described by redox mechanisms in which oxygen is removed or added to the lattice. While this description of ceria as a highly reducible oxide is widely used, it appears to be somewhat of misnomer if one bases this categorization on its bulk thermodynamics. Indeed, the differential enthalpy of reduction of CeO2 to CeO2−x is ∼780 kJ per mole of O2.101 There is evidence, however, that nanocrystalline ceria102 and ceria surfaces103 are much more reducible than the bulk material. Ceria−zirconia mixtures, a commonly used form of ceria in catalytic applications, are also much more reducible than pure ceria.101 CeO2 has the fluorite structure shown in Figure 12 in which the Ce4+ cations are coordinated to eight O2− anions. DFT studies consistently show that of the low index surface planes (i.e., (111), (110), (100)), the stoichiometric (111) surface is the most stable,104−107 and as expected on the basis of this result, this plane is preferentially exposed in ceria powders.108,109 The majority of the surface science studies of the adsorption and reaction of oxygenates on ceria have also focused on the (111) surface,110−123 although a few studies of the (100) surface can also be found in the literature.119,124 Ferrizz et al. were the first to investigate the adsorption and reaction of methanol on single-crystal CeO2(111).119 As shown by their TPD results displayed in Figure 13, nearly stoichiometric CeO2(111) exhibits low reactivity toward methanol. The saturation coverage of methanol for adsorption at 300 K was found to be