Increasing Oxide Reducibility: The Role of Metal/Oxide Interfaces in the Formation of Oxygen Vacancies

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Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via R. Cozzi, 55 I-20125 Milano, Italy
*E-mail for G.P.: [email protected]
Cite this: ACS Catal. 2017, 7, 10, 6493–6513
Publication Date (Web):August 18, 2017
https://doi.org/10.1021/acscatal.7b01913
Copyright © 2017 American Chemical Society
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Abstract

Reducibility is an essential characteristic of oxide catalysts in oxidation reactions following the Mars–van Krevelen mechanism. A typical descriptor of the reducibility of an oxide is the cost of formation of an oxygen vacancy, which measures the tendency of the oxide to lose oxygen or to donate it to an adsorbed species with consequent change in the surface composition, from MnOm to MnOmx. The oxide reducibility, however, can be modified in various ways: for instance, by doping and/or nanostructuring. In this review we consider an additional aspect, related to the formation of a metal/oxide interface. This can be realized when small metal nanoparticles are deposited on the surface of an oxide support or when a nanostructured oxide, either a nanoparticle or a thin film, is grown on a metal. In the past decade, both theory and experiment indicate a particularly high reactivity of the oxygen atoms at the boundary region between a metal and an oxide. Oxygen atoms can be removed from interface sites at much lower cost than in other regions of the surface. This can alter completely the reactivity of a solid catalyst. In this respect, reducibility of the bulk material may differ completely from that of the metal/oxide surface. The atomistic study of CO oxidation and water-gas shift reactions are used as examples to provide compelling evidence that the oxidation occurs at specific interface sites, the actual active sites in the complex catalyst. Combining oxide nanostructuring with metal/oxide interfaces opens promising perspectives to turn hardly reducible oxides into reactive materials in oxidation reactions based on the Mars–van Krevelen mechanism.

1 Introduction: Oxides in Heterogeneous Catalysis

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Metal oxides are widely used in catalysis, as they represent essential components of an active heterogeneous catalyst. Searching the Web of Science for “oxides AND catalysis OR catalyst” provides about 300000 papers. Due to the huge variety of compositions and electronic and geometrical structures, metal oxides offer a very broad spectrum of properties and behaviors that can result in specific functionalities and chemical activities. Oxides can be used as “inert” supports of finely dispersed active metal nanoparticles or directly as catalysts. In this latter case, the oxide surface must be able to exchange chemical species with the liquid- or gas-phase surroundings or to adsorb chemical species and promote dissociation and regeneration of chemical bonds. The great flexibility of oxide surfaces stems from the presence on the surface of both Lewis and Brønsted acid and basic sites, sometimes acting in a cooperative way. (1)
Oxides can be divided into two main classes, depending on their chemical behavior: nonreducible and reducible oxides. Nonreducible oxides consist of materials that do not easily lose oxygen, due to the intrinsic resistance of the corresponding metal cations to change oxidation state. Since oxygen is formally in a −2 oxidation state, the excess electrons that are left on the material by removal of a neutral O atom cannot be accommodated in the cation empty states which lie too high in energy, contributing to the formation of the conduction band of the material. Oxides such as SiO2, MgO, Al2O3, and many other main-group oxides belong to this class. Usually these materials are characterized by a very large band gap (typically >3 eV) separating the valence band (VB) from the conduction band (CB). The excess electrons left on the material when oxygen is removed in the form of O2 or H2O are trapped in specific sites (e.g., an oxygen vacancy) and give rise to new defect states in the band gap. (2) This process is energetically very costly, and therefore these as-prepared materials are highly stoichiometric, stable, and chemically inert. (3) The group of reducible oxides, in contrast, is characterized by the capability to exchange oxygen in a relatively easy way. This is because the lowest empty states available on the material (CB) consist of cation d orbitals which lie at not overly high energy with respect to the VB. These oxides usually have semiconductor character, with band gaps <3 eV. The removal of oxygen results in excess electrons that are redistributed on the cation empty levels, thus changing their oxidation state from Mn+ to M(n–1)+. Transition-metal oxides such as TiO2, WO3, NiO, Fe2O3, CeO2, etc., just to mention a few, belong to this category. (4)
The difference between nonreducible and reducible oxides is of fundamental importance for the chemical reactivity of these materials. A substantial fraction of industrial catalysis is dealing with oxidation reactions or oxidative dehydrogenation processes. In most cases the active catalyst is an oxide, and the reaction follows a mechanism originally described by Mars and van Krevelen (5) and thus is named after the two authors (MvK). The key of the MvK mechanism is that the oxide surface is not just a spectator of the reaction but rather is directly involved via its most reactive oxygen atoms. An organic substance can react with these specific sites at the oxide surface (e.g., oxygen atoms located at low-coordinated sites). A weakly bound surface O atom is added to the reactant forming the oxygenated compound, leaving behind an oxygen vacancy on the surface, hereafter referred to as VO. This results in a MOnx compound, with a stoichiometry which is no longer that of the starting MOn oxide since oxygen has been lost in the process. In order to be catalytic, the reaction must occur under oxygen pressure so that molecular oxygen can interact with the surface, dissociate, and eventually refill the vacancy created in the oxidative process. In this way, the original stoichiometry and composition of the catalyst are restored, closing the catalytic cycle. Using isotopically labeled oxygen, it is possible to prove that the oxygen atom incorporated in the organic reactant does not come from the gas phase but rather is directly extracted from the solid surface. (6)
Clearly, in this kind of process a very good descriptor of the reaction is the cost of removing an oxygen atom from the clean surface of the catalyst. This in fact determines both the kinetics and the thermodynamics of the overall reaction. This simple example already shows that the generation of oxygen vacancies on the surface of an oxide is a very important aspect of its chemistry. The problem is that the identification and characterization of oxygen vacancies on oxide surfaces are far from trivial. In fact, one is dealing with the identification of a missing atom. For this reason, some years ago some of us discussed the role of oxygen vacancies on oxide surfaces as “the invisible agent on oxide surfaces”. (7) In recent years several techniques have been developed to better identify and characterize these centers. Among others, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have progressively grown in importance for the atomistic characterization of these defects. (1, 3, 8, 9) In particular, enormous progress has been made in the use of AFM which, at variance with STM, can be employed also on nonconducting supports. Since many oxides are insulating, in particular the nonreducible ones, this has opened the perspective to better characterize these centers on all forms of oxides, reducible and nonreducible. A combination of AFM and STM measurements can provide information on the localization and charge state of defects in oxides. (10, 11)
A better knowledge of the nature and characteristics of anion vacancies on oxide surfaces is important because it allows the design of materials with tailored properties, in particular to prepare oxides that can be more or less reactive and that can (or cannot) exchange oxygen with adsorbed chemical species. This has prompted several researchers to explore methods or procedures to improve the reducibility of a metal oxide. In general, there are three main conceptual approaches to this target: (i) doping the material with heteroatoms, (ii) producing the oxide in form of nanocrystallites or thin films, and (iii) depositing metal nanoparticles on the surface of an oxide. All of these procedures are relatively novel, but probably, the importance of the last method, i.e. the formation of metal/oxide interfaces, has been fully realized only very recently. This is also the topic of this perspective article.

1.1 Oxide Reducibility: Effect of Doping

Doping oxides by heteroatoms and the effect on oxide reducibility have been studied recently both experimentally and theoretically with the hope of producing better catalytic materials, in particular for oxidation reactions. (12-14) The chemical, electrical, and optical properties of an oxide largely depend on the presence and the concentration of intrinsic or extrinsic defects. Defect engineering is the science that aims at manipulating the nature and the concentration of defects in a solid, as well as tuning its properties in a desired manner. For instance, defects can turn a colorless insulator into a black material with metallic conductivity or improve the photoactivity of an oxide semiconductor, making it able to absorb solar light. (15) In a similar way, one can modify the chemistry of an oxide surface by introducing heteroatoms in the structure. (14) One example is the substitution of a metal cation in an oxide of formula MxOy with another dopant, D. This results in a direct modification of the electronic structure of the oxide, with important consequences on its properties. If the doping heteroatom D has the same valence of the metal cation M that is replaced, then the modifications are restricted to size effects or, at most, to a shift of the frontier orbitals with consequent changes in the strength of the M–O and D–O bonds. Much more complex is the situation occurring when the dopant D has a different valence in comparison to the replaced cation. In this case, the simple substitution of the cation M results in a charge imbalance that needs to be compensated by the creation of other defects in the structure. The number of possibilities is extremely large, since in principle all atoms of the periodic table can be used to replace a metal cation in an oxide. The possibility to explore theoretically with first-principles simulations the effect of doping is thus of great help for the design of new catalytic materials. This problem has been extensively investigated and reviewed by Metiu and co-workers in a series of DFT calculations where several dopants have been introduced in an oxide matrix. (16) Both non-transition-metal and transition-metal atoms have been tested. (16) The general assumption is that any oxide modification that facilitates the removal of surface oxygen will facilitate the MvK mechanism in oxidation reactions. Since oxygen is electrophilic, one can expect to lower the cost to remove it from the surface by making the surface more electron deficient. This can be done, at least in principle, by replacing some of the cations of the host oxide with cation dopants D having a lower valence. Indeed, several theoretical studies indicate that the cost to create a VO center near a low-valence dopant is lower than that on the undoped surface. It has also been observed that the presence of a low-valent dopant affects oxygen atoms several sites away from the dopants, with a relatively long-range effect. However, the easiest oxygen atom to remove remains that in direct contact with the dopant D. These conclusions appear to be rather general, as they have been obtained for several oxides, such as TiO2, CeO2, ZnO, La2O3, CaO, and NiO. (16)
The theoretical conclusions are supported by some experimental evidence that, indeed, doping improves the catalytic performances, by improving the conversion, the selectivity, or the surface area of the catalyst.
Oxide anions can also be replaced with direct effects on the properties of the material and in particular on its reducibility. A classic example is that of doping TiO2 with nitrogen. Doping semiconductor oxides with nonmetal atoms has attracted huge interest in the last 15 years, in the attempt to improve the solar light absorption and to increase the generation of electron–hole pairs. (17-20) The N dopant can either replace an O atom in the lattice or take an interstitial position. In both cases, it introduces new defect states in the gap, just above the top of the valence band, thus lowering the energy required to excite an electron into the CB. An important secondary effect observed in N-doped titania is that the presence of partially filled N 2p states deep in the gap favors the formation of oxygen vacancies and hence the reducibility of the oxide. (19, 20) The partially filled low-lying acceptor N 2p orbitals are able to trap the electrons released when an O atom is removed from the surface. The excess electrons, instead of being accommodated in the high-lying Ti empty 3d states (Ti4+(3d0) + e → Ti3+(3d1)), close to the bottom of the CB, fill the low-lying singly occupied N 2p orbitals, with a large gain in stability. As a consequence, the cost to create a vacancy in bulk TiO2 drops from 4.63 to 0.27 eV. (21) This effect is general and has been observed for many other oxides, including materials with high band gap and low reducibility such as ZrO2 (22) and MgO. (23) Thus, the presence of heteroatoms can have unexpected but profound consequences on the stability of the oxide and on its capability to release oxygen.
While conceptually very attractive, the idea to selectively dope an oxide material is difficult to realize in practice. The incorporation of heteroatoms, in fact, requires a specific chemical synthesis designed for this purpose, and the product has to be carefully characterized at an atomistic level to make sure that the preparation results in a doped material. This is far from simple. With a chemical synthesis, it is hard to know a priori the kind and concentration of dopants in the structure. This depends on the external conditions of temperature, pressure, purity of reactants, annealing, etc. Second, there is a large number of possible mechanisms for charge compensation, which opens a wide spectrum of combination of defects. For all these reasons, while doping with heteroatoms can significantly help the catalyst reducibility, our present understanding and control of the doping process for the rational design of new efficient and practical catalysts are still rudimentary.

1.2 Oxide Reducibility: Effect of Nanostructuring

Another way to improve oxide reducibility is to control the morphology and dimensionality of oxide particles. Oxygen ions at low-coordinated sites of an oxide surface can behave very differently from the corresponding bulk counterparts. One of the first oxides considered in this context was MgO. (24) MgO is an ionic oxide with a rock salt structure, a wide band gap, and very low chemical activity when it is prepared in single-crystal form. The (001) surface of MgO is notoriously inert and defect free. CO adsorption on a single crystal of MgO can only be realized at very low temperatures; in fact, the CO molecule is weakly bound by van der Waals forces to the surface and easily desorbs as the temperature exceeds 57 K. (25, 26) Slightly higher desorption energies are measured for CO bound to Mg cations at steps and edges of the surface. Virtually no chemistry occurs on a single-crystal MgO surface. In contrast, if MgO nanocrystals are exposed to CO at 60 K, this results in a rich and complex chemistry, as shown, for instance, by a multitude of features due to the formation of carbonates and other [O–CO]n2– species appearing in the infrared spectrum. (27-29) This is due to the much higher reactivity of the low-coordinated O sites at steps and corners, a result that has been fully rationalized in terms of the different Madelung potential at these sites in comparison to the regular MgO (001) surface. (30) In a similar way, generation of oxygen vacancies has a very different energy cost at the surface, where the cost is prohibitively high, and along steps and corners, where it decreases significantly. (24) Thus, oxide nanostructuring, which increases the ratio of surface versus bulk atoms, and in particular of the number of low-coordinated ions on the surface, can result in deeply modified properties of oxide materials.
The problem has been recently investigated for the case of ZrO2. ZrO2 is a highly ionic insulating oxide with a band gap of ∼6 eV; (31, 32) it is considered a nonreducible oxide like MgO. (33) The removal of a O4c ion from bulk has a energy cost of 6.16 eV. This is computed at DFT level with respect to 1/2O2, the standard reference for O vacancy formation energies. This reduces to 5.97 eV when a O3c atom is removed from the (101) surface, a very small change (notice that the values of the VO formation energy in ZrO2 reported in this review may change by ±0.1 eV due to the different sizes of the adopted supercells). Instead, when ZrO2 is nanoscaled down to nanoparticles of 1–2 nm size, the formation energy of an O vacancy is significantly lower. For instance, removing a corner O2c atom from the Zr16O32, Zr40O80, and Zr80O160 nanoparticles has energy costs of 3.94, 3.70, and 2.26 eV, respectively; 4–5 eV is required to remove O3c atoms at facets of the nanoparticles. (34, 35) The O2c and O3c sites in the nanoparticles become important catalytic centers in reactions involving oxygen transfer. The possibility of modifying the reducibility of zirconia may have important implications in catalysis. For example, prereduced solid catalysts based on ZrO2 nanoparticles display a better activity in the transformation of biomass into fuels. (36, 37) In addition, reduced ZrO2–x is shown to be photocatalytically active in H2 production under solar light while stoichiometric ZrO2 is inactive. (38)
A similar response has been found for CeO2. Differently from zirconia, CeO2 is a reducible oxide, whose catalytic activity is closely related to the change from +4 to +3 in the oxidation state of the cerium cations and its capacity to release oxygen according to the reaction 2CeO2 → Ce2O3 + 1/2O2. (39) The energy required to create a O4c vacancy in bulk CeO2 is 4.73 eV; (40) this reduces to 2.25 eV on the (111) surface (O3c). (41, 42) However, when Ce21O42, Ce40O80, and Ce80O160 nanoparticles are considered, the formation energies of a corner O2c vacancy decrease to 1.67, 0.80, and 1.20 eV, respectively. (43) The enhanced reducibility at the nanoscale is observed also when a metal cluster is deposited on a CeO2 nanoparticle. It has been demonstrated, both by experiment and by DFT calculations, that O spillover occurs spontaneously at room temperature from the supporting CeO2 nanoparticle to a deposited Pt cluster, while on the extended CeO2 surface this process is highly endothermic (this topic will be reconsidered below, at the end of this Perspective). (41, 42) Thus, CeO2 nanoparticles can dramatically improve the catalytic activity in comparison to bulk ceria.
Therefore, as for ZrO2, also for CeO2 the redox behavior changes dramatically at the nanoscale, where formation of VO centers becomes easier. A deep understanding of these phenomena is essential for the use of these oxides in catalysis. (44) The phenomenon can be rationalized in two ways. First, nanoparticles contain a large fraction of undercoordinated cations such as corners and edges, whose empty d or f states are stabilized with respect to the bottom of the CB. Thus, empty Mnd defective electronic states are introduced in the band gap that lower the energy cost to accommodate the extra charge associated with an O vacancy. (34, 35) Consequently, the electron density of the reduced system tends to be localized in low-coordinated Zr3+ and Ce3+ centers that, trapping a single electron, become magnetic and can be detected by electron paramagnetic resonance (EPR). (34, 35, 40, 33, 45, 46) The second aspect is the increased structural flexibility. Nanoparticles and in general nanostructures are more flexible than extended surfaces or the bulk material. (47) Atomic relaxation around the created O vacancy occurs at much lower cost, thus contributing to stabilize the defect. (34, 35, 45) In short, both the size and morphology of oxide supports are very important in determining the catalytic processes.

1.3 Oxide Reducibility: Effect of Metal/Oxide Interface

The interface between an oxide surface and a metal is of crucial importance in several modern technologies. Metal/oxide interfaces find application in microelectronic devices to create Schottky contacts and metal/oxide resistive random access memories, in electrochemistry for contact electrodes, in corrosion protection as oxide films to form a protective barrier against the oxidation of the underlying metal, in catalysis for supported metal nanoparticles, etc. Despite the technological importance of metal/oxide interfaces, our knowledge of the interface between a metal and an oxide is still very unsatisfactory. The reason is that the interface is difficult to access experimentally with the typical techniques available in surface science and is also difficult to describe theoretically, as it involves two materials with completely different properties: a conductive metal on one side and an insulator or semiconductor phase on the other. DFT has problems in treating on an equal footing these two classes of materials: methods that work very well for one category (e.g., standard GGA functionals for metals) usually perform poorly for the second component (the insulating phase); vice versa, self-interaction corrected functionals work well for semiconductors and insulators but may fail when it comes to describing a metallic phase. (48)
Despite these problems, it is becoming increasingly clear that the contact region between an oxide and a metal is chemically very active. One aspect of key importance is the occurrence of a direct charge transfer between the metal and the oxide. (49) Metals with low work function deposited on oxides with high electron affinity can induce the reduction of the oxide by direct electron transfer. The oxide surface becomes electron rich and thus changes its chemical properties. Of course, oxygen exchange can be affected by this phenomenon as well, although this has not been studied in detail until very recently.
The interface between a metal and an oxide is strongly affected by the presence of oxygen vacancies. So far, most of the attention has been dedicated to the fact that vacancies, when present, play an important role in stabilizing deposited metal nanoparticles and eventually in tuning their chemical activity. (50-56) Theory has shown that metal atoms and clusters bind much more strongly to these defect sites. New experiments have been designed to nucleate clusters under controlled conditions or even to deposit via soft-landing techniques mass-selected clusters generated in the gas phase. (57) These experiments offer a unique opportunity to study the cluster reactivity as a function of the particle size and of the deposition site, (56, 58, 59) opening new perspectives for the understanding of the basic principles of catalysis by small metal particles. For very small cluster sizes, in the nanometer size regime, the strong interaction with a surface defect may lead to a modification of the chemical activity of the particle. (50, 51, 56) The literature about the role of surface oxygen vacancies in promoting nucleation and growth of deposited metal atoms and clusters is very abundant and covers several oxides and virtually every metal.
What has attracted much less attention, however, is the fact that by depositing a metal cluster on an oxide surface one may also favor the formation of oxygen vacancies. The idea is that the metal/oxide interface, in particular the periphery between a metal nanoparticle and the oxide support, are regions where the oxide reactivity is significantly enhanced and where oxygen can be easily removed. This is also the main topic of this review. By considering a number of examples, from both theory and experiment, we will demonstrate that the reducibility of an oxide can be deeply modified by creating contacts between metals and oxide surfaces. Table 1 reports a noncomprehensive overview of direct experimental evidence that the presence of a metal facilitates the formation of oxygen vacancies on oxide surfaces. (60-78) This will be further commented upon in the next sections.
Table 1. Overview of Direct Experimental Evidence That the Presence of a Metal Facilitates the Formation of Oxygen Vacancies on an Oxide Surface
yearreftitleexptl methods useda and short description
2016 60Formation and removal of active oxygen species for the noncatalytic CO oxidation on Au/TiO2 catalystsTAP, TPD
distinction between noncatalytic “irreversible” oxygen species and catalytically active “reversible” oxygen species for the CO oxidation reaction over Au/TiO2, the latter being titania lattice oxygen
2016 61How temperature affects the mechanism of CO oxidation on Au/TiO2: a combined EPR and TAP reactor study of the reactive removal of TiO2 surface lattice oxygen in Au/TiO2 by COEPR, TAP
removal of TiO2 surface lattice oxygen from a Au/TiO2 catalyst and Ti3+ formation upon exposure to CO
2016 62Mild activation of CeO2-supported gold nanoclusters and insight into the catalytic behavior in CO oxidationTAP
redox cycle in which CO could reduce the surface of CeO2 to produce oxygen vacancies
active oxygen species present on the surface of the pretreated catalyst react with CO pulses to generate CO2
    
2016 63Direct evidence for the participation of oxygen vacancies in the oxidation of CO over ceria- supported gold catalysts by using operando Raman spectroscopyoperando Raman, IR
direct spectroscopic evidence for the participation of oxygen vacancies in the oxidation of CO over ceria-supported gold
2016 64Catalytic oxidation of carbon monoxide over of gold-supported iron oxide catalystFTIR, TEM, XRD
the reducibility of the support is greatly enhanced and shifted to lower temperatures; this shift is due to strong interaction between the support and Au nanoparticles
    
2015 65An atomic-scale view of CO and H2 oxidation on a Pt/Fe3O4 model catalystSTM
CO extracts lattice oxygen atoms at the cluster perimeter to form CO2, creating large holes in the metal oxide surface
    
2015 66High activity of Au/γ-Fe2O3 for CO oxidation: effect of support crystal phase in catalyst designCO-TPR, sequential pulse reaction, in situ Raman spectroscopy
Au/γ-Fe2O3 shows higher activity for CO oxidation than Au/α-Fe2O3
systematic study shows that this higher-redox-property-based higher activity could be extended to γ-Fe2O3-supported Pt-group metals and to other reactions that follow Mars–Van Krevelen mechanism
2014 67On the origin of the selectivity in the preferential CO oxidation on Au/TiO2–Nature of the active oxygen species for H2 oxidationTAP
absolute amount of active oxygen for H2 oxidation is identical to that in CO oxidation, and is positioned at the Au/TiO2 interface perimeter.
    
2013 68Origin of the high activity of Au/FeOx for low-temperature CO oxidation: Direct evidence for a redox mechanismFTIR, Raman spectroscopy, microcalorimetry, DFT
unambiguous evidence that the surface lattice oxygen of the FeOx support participates directly in the low-temperature CO oxidation
verification via DFT: oxygen vacancy formation at Pt8–10/FeOx perimeter
2013 69Generation of oxygen vacancies at a Au/TiO2 perimeter interface during CO oxidation detected by in situ electrical conductance measurementin situ ECM
detection of Ti3+ formation under reaction conditions for CO oxidation with O2 over Au/TiO2 catalyst
    
2011 70Active oxygen on a Au/TiO2 catalyst: Formation, stability, and CO oxidation activityTAP
correlation between oxygen storage capacity and CO oxidation activity
results allow clear identification of the nature of the active oxygen species and their location on the catalyst surface
2011 41Support nanostructure boosts oxygen transfer to catalytically active platinum nanoparticlesRPES and DFT
electron transfer from the Pt nanoparticle to the support and oxygen transfer from ceria to the Pt nanoparticle.
oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt.
    
2010 71Support effects in the Au-catalyzed CO oxidation–Correlation between activity, oxygen storage capacity, and support reducibilityTAP
four different metal oxide supported Au catalysts with similar Au loading and Au particle sizes (Au/Al2O3, Au/TiO2, Au/ZnO, Au/ZrO2) are compared
oxygen storage capacity and activity for CO oxidation differ significantly for these catalysts and are correlated with each other and with the reducibility of the respective support material, pointing to a distinct support effect and a direct participation of the support in the reaction
    
2010 72The interplay between structure and CO oxidation catalysis on metal–supported ultrathin oxide filmsSTM, AES, TDS, LEED, DFT
oxygen enrichment of FeO/Pt(111) to FeOx/Pt(111) (with x → 2) and subsequent CO oxidation via the Mars–van Krevelen mechanism
    
2009 73Reactive oxygen on a Au/TiO2 supported catalystTAP
both the oxygen storage capacity and the activity for CO oxidation scale linearly with the Au/TiO2 perimeter length
2009 74Kinetic and mechanistic studies of the water-gas shift reaction on Pt/TiO2 catalystSSITKA, DRIFTS
redox mechanism demonstrated as the prevailing mechanism on a Pt/TiO2 catalyst, where labile oxygen and oxygen vacancies of TiO2 near the metal–support interface can participate in the reaction path of the water-gas shift reaction
2007 75Activation of a Au/CeO2 catalyst for the CO oxidation reaction by surface oxygen removal/oxygen vacancy formationTAP
first experimental verification of a CO oxidation rate enhancement by oxygen surface vacancies on a realistic oxide-supported Au catalyst
2004 76Nanocrystalline CeO2 increases the activity of Au for CO oxidation by 2 orders of magnitudeTEM, IR, XPS, GC
formation of Ce3+ under exposure of the catalyst to CO at 60 °C
    
2004 77