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.

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Bulk mixed metal oxides are widely used in industry for various oxidation reactions, but there is still debate in the heterogeneous catalysis literature about the nature of their catalytic active sites. In the present study, the two-component iron vanadate mixed metal oxide system is employed to investigate the outermost surface composition and surface chemistry of the bulk FeVO4 mixed metal oxide catalyst. The bulk V2O5, α-Fe2O3, and supported 4% V2O5/α-Fe2O3 metal oxide systems were also investigated to better understand the surface composition and surface chemistry of the bulk FeVO4 catalyst. Raman spectroscopy confirmed that the bulk FeVO4 was phase pure and has no contribution from excess V2O5 and α-Fe2O3 phases. IR spectroscopy confirmed that the model supported 4% V2O5/α-Fe2O3 catalyst consists of an amorphous surface VO x monolayer on the α-Fe2O3 support. The surface chemistry of the metal oxides was chemically probed with temperature programmed CH3OH-IR spectroscopy and revealed that both intact surface CH3OH* and CH3O* species are present on the catalysts. On acidic α-Fe2O3, the surface CH3OH* and CH3O* intermediates yield CH3OH and dimethyl ether (DME), respectively. For the redox V2O5, FeVO4 and supported 4% V2O5/α-Fe2O3 catalysts: however, both surface intermediates primarily give rise to HCHO. These results confirm that the surface of bulk FeVO4 and supported 4% V2O5/α-Fe2O3 catalysts are similar and that surface VO x species are the catalytic active sites for methanol oxidation to formaldehyde over bulk FeVO4 catalysts. This conclusion is supported by HR-TEM images that reveal an amorphous VO x enriched layer of ∼1 nm at the outer surface of the bulk FeVO4 catalysts. Methanol oxidation over bulk FeVO4 was found to proceed via a Mars−van Krevelen mechanism, where the reduced surface VO x species are reoxidized by bulk lattice oxygen rather than gas phase molecular O2. This study demonstrates that the catalytic active sites for oxidation reactions over the bulk FeVO4 mixed oxide reside in the outermost surface layer and not in the bulk lattice structure.

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The hydrodeoxygenation (HDO) of bio‐oil derived from white oak wood using non‐sulfided catalysts was studied in a two zone continuous flow trickle bed reactor system. The major organic components of the pyrolysis oil were pyrolytic lignin (large phenolic polymers), xylose, levoglucosan, organic acids (primarily acetic acid), and hydroxyacetaldehyde. The first zone was a low temperature zone (130 °C) that contained a Ru/C catalyst. In this zone, carbonyl groups were hydrogenated, producing propylene glycol (from hydroxyacetone), ethylene glycol (from hydroxyacetaldehyde), and sorbitol (from levoglucosan). A more severe hydrotreatment was performed in a second zone containing a bifunctional Pt/ZrP catalyst at a temperature between 300 and 400 °C. In the two‐stage HDO, an organic phase was produced that consisted of a distribution of hydrocarbons that were primarily cyclic alkanes (naphthenes) ranging from C7 to C24. The organic phase carbon yield decreased with increasing reaction temperature in the second zone. Catalyst deactivation and reactor plugging by coking occurred under all reaction conditions after 55–72 h time on stream (TOS). After ≈55 h TOS, more than 25 % of the carbon in the original bio‐oil was accumulated as coke, with increasing amounts for higher temperatures in the second zone. Hydrotreatment gave rise to >C5 hydrocarbon (gasoline and distillate‐range fuel) overall yields between ≈30 and 47 carbon % for all experiments compared to the 79.5 % theoretical yield calculated for the bio‐oil feedstock. Coke formation and undesired cracking to C1–C4 hydrocarbon gases were the main causes of lower fuel carbon yields.