Catalysis Science of Bulk Mixed Oxides

Abstract

Bulk mixed oxide catalysts are widely used for various applications (selective oxidation catalysts, electrocatalysts for solid oxide fuel cells, and solid oxide electrolyzers for the production of hydrogen), but fundamental understanding of their structure–performance relationships have lagged in the literature. The absence of suitable surface composition and surface structural characterization techniques and methods to determine the number of catalytic active sites, with the latter needed for determination of specific reaction rates (e.g., turnover frequency (1/s)), have hampered the development of sound fundamental concepts in this area of heterogeneous catalysis. This Perspective reviews the traditional concepts that have been employed to explain catalysis by bulk mixed oxides (molybdates, vanadates, spinels, perovskites, and several other specific mixed oxide systems) and introduces a modern perspective to the fundamental surface structure–activity/selectivity relationships for bulk mixed oxide catalysts. The new insights have recently been made available by advances in surface characterization techniques (low-energy ion scattering, energy-resolved XPS, and CH3OH-IR) that allow for direct analysis of the outermost surface layer of bulk mixed metal oxide catalysts. The new findings sound a note of caution for the accepted hypotheses and concepts, and new catalysis models need to be developed that are based on the actual surface features of bulk mixed oxide catalysts.

Promoted Hexagonal Tungsten Bronzes as Selective Catalysts in the Aerobic Transformation of Alcohols: Glycerol and Methanol

“…At the lower TF values methanol was converted to dimethyl ether -DME-and only traces of COx, whereas formaldehyde -FA-was not detected. This stresses the strong acid properties of h-WOx, particularly of the Brɸnsted type [2,3,21,22]. Indeed, strong acid sites hamper the desorption of the adsorbed methoxy intermediate, favoring the molecular encounter with a second molecule of ethanol to form DME, as reported previously [2].…”

“…Promoted mixed metal oxides bronzes have attracted the attention in the last years since they are effective catalysts in acid and oxidation reaction [1][2][3][4]. As suggested recently, redox and acid-base properties of metal oxides can be strongly changed by tailoring composition and catalyst preparation procedure which determines not only crystal sizes but also the local structure and the electronic/catalytic properties [3].…”

A Comparison of the Redox Properties of Bulk Vanadium Mixed Oxide Catalysts

“…The catalysts primarily give rise to HCHO, which reflects the dominance of the redox sites on its surface. The decomposition Tp value of the surface methoxide species is directly related to their thermal stability [19]. As shown in Fig.…”

Effect of Co/Ni ratios in cobalt nickel mixed oxide catalysts on methane combustion

“…Cobalt oxide exhibits weak oxygen bond strength and high turnover frequency for redox reaction [23,24]. Liotta et al reported that the morphological and redox properties of cobalt cerium mixed oxides are strongly dependent on the Co/Ce ratio for methane combustion [25].…”