Asymmetric Oxygen Vacancies: the Intrinsic Redox Active Sites in Metal Oxide Catalysts

Abstract

stability in many important reactions, [1] especially in some redox reactions, such as CO oxidation, [2] water-gas shift (WGS) reaction, [3] selective catalytic reduction of NO x , [4] oxidation of volatile organic compounds (VOCs), [5] and soot combustion. [6,7] It is commonly known that the catalytic reaction takes place at defect sites of these oxide catalysts. [8] However, it is not always found a monotonous relation between the catalytic performance and the concentration of a certain kind of defect sites, such as oxygen vacancies, [9,10] lattice distortion [11] and defect generated Lewis acid/base sites, [12,13] as well as metastable valence states of cations. [14] As discussed by McFarland and Metiu, [15] heterogeneous catalytic reactions are run under steady-state conditions instead of at equilibrium. It is difficult to accurately describe the catalytic performance of oxide catalysts using the property parameters of as-prepared catalysts, which are more likely to be in thermodynamic equilibrium during the preparation process. For example, according to many reports, the redox properties of catalysts were believed to relate to the amount of oxygen vacancies (regarded as active sites) in oxide catalysts. However, the steady-state concentration of oxygen vacancies is related to the rate of vacancy formation and that of vacancy vanishing. In other words, a real active oxygen vacancy should have high abilities of adsorbing the reactant molecules and desorbing the product molecules, allowing molecules easy come, easy go. To keep a balance of these two opposite abilities, the local environment of the oxygen vacancy plays an important role.It has been well accepted that doping of low-valence dopants in oxides can create oxygen vacancies due to a charge compensation effect, [16,17] accompanied by improved oxygen activation ability, which is usually considered as the key factor in promoting catalytic activity for redox reactions. However, when the oxides are doped by same-valence or high-valence dopants, the effect of doping on their catalytic performance will become much more complicated. Fortunately, for most oxidation reactions catalyzed by metal oxides, the lattice oxygen atoms at/near the surface of oxides are the active species based on a generally accepted Mars− van Krevelen (MvK) mechanism. [18] Therefore, the bonding situations of these surface oxygen atoms should be the key factors affecting the catalytic performance.In the last few years, researchers have paid more attention to the chemical environment of oxygen vacancies, including To identify the intrinsic active sites in oxides or oxide supported catalysts is a research frontier in the fields of heterogeneous catalysis and material science. In particular, the role of oxygen vacancies on the redox properties of oxide catalysts is still not fully understood. Herein, some relevant research dealing with M 1 -O-M 2 or M 1 -□-M 2 linkages as active sites in mixed oxides, in oxide supported single-atom catalysts, and at metal/oxide interfaces of oxide supporte...

Insights into the Mechanism of Methanol Steam Reforming Tandem Reaction over CeO2 Supported Single-Site Catalysts

“…[15][16][17] Recent research demonstrated that "ensemble effects" between the metal single atoms and neighboring oxygen vacancies can favor the oxygen transfer and reactants adsorption, promoting the catalytic reaction efficiency with large molecules. [18][19][20] In our previous studies, we demonstrated the enhanced activity and selectivity of methanol dehydrogenation and n-hexane reforming when combining an oxygen vacancy with noble metal atoms. [21][22][23] The mechanism study also showed that the integration of metal single-sites and neighboring oxygen vacancies creates a special synergy that accelerates the reaction rates by facilitating the adsorption and activation of reactants and the transformation of intermediate species.…”

Praseodymia–titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides

“…Recently, Zhou and co-workers proposed that the asymmetric A metal -O-(B metal ) x led to easier O vacancy formation, which is accompanied by electron transformation between metals. 17 XPS spectroscopy was also used to study changes in surface oxygen vacancies, induced by mixing of Ti and Pr. According to Hyuntae et al, the O 1s could be deconvoluted into 3 peaks.…”

“…12 Zhou et al also nd the catalytic performances of some catalysts are not always increase with the concentration of O defects. 16,17 One possible reason is that not all the O vacancies are the active sites, only those can balance the easy adsorbing of reactant molecules and desorbing of product molecules are the active sites. 17 In fact, both the STM and DFT studies already prove that the O vacancies can be divided into different types according to their arrangements.…”

“…16,17 One possible reason is that not all the O vacancies are the active sites, only those can balance the easy adsorbing of reactant molecules and desorbing of product molecules are the active sites. 17 In fact, both the STM and DFT studies already prove that the O vacancies can be divided into different types according to their arrangements. 18,19 For example, in the CeO 2 single crystal, O vacancies are in the forms of an isolated single point, linear clusters and triangular trimers, which also correspond to different activities and stabilities.…”

Reversible dehydrogenation and rehydrogenation of cyclohexane and methylcyclohexane by single-site platinum catalyst

“…However, the lack of an ensemble of other active sites adjacent to the single noble metal atom may prevent surface reactions that involve large molecules 15,16 . Recent studies demonstrated that "ensemble effects" between the single metal atom and neighboring oxygen vacancies can favor oxygen transfer and adsorption of reactants, enabling catalysis reactions of the functional groups on large molecules with higher efficiency [17][18][19] . The beneficial effect of oxygen vacancy sites next to the noble metal 12,14 motivated us to further explore its possibilities with the goal to discover new reactions and to accelerate the development of next-generation single-site catalysts.…”