(690e) Elucidation of Structure-Reactivity Relationships in Supported Mixed Metal Oxide Nanostructures of Mo O3 and W O3

Supported catalysts containing mixed nanostructures of tungsten, molybdenum and vanadium oxide are active for the selective oxidation of hydrocarbons and their reactivity characteristics can be tuned by varying the domain size and composition to achieve reaction properties superior to catalysts containing a single metal oxide. We will present methods to characterize the structure of catalysts containing nanostructures of mixed composition using UV-visible spectroscopy and describe how molecular level interactions between dissimilar metal oxides affects reactivity for the partial oxidation of ethanol. This fundamental structure-function assessment of mixed composition supported metal oxide catalysts is being used to aid in the design of catalysts for selective VOC sensing.

We report the first systematic investigation of structure-activity relationships for binary supported oxide catalysts containing submonolayer molybdenum and tungsten oxides in comparison to similar studies of the corresponding single metal oxides and heteropolyacid analogs. Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) was used to characterize the domain size of each metal oxide within binary catalysts and the ethanol oxidation reaction was used to probe redox and acid surface sites. Two distinct absorption edges were measured from physical mixtures of either bulk or supported Mo O_x and W O_x catalysts, demonstrating that isolated metal oxide domains of pure composition within a binary catalyst can be identified using this spectroscopic technique. Two distinct absorption edges at 3.7 and 4.1 eV, corresponding to monomeric Mo O_x and W O_x, were observed for a low loading alumina-supported catalyst containing 0.5 /nm² and 0.5 Mo/nm², suggesting the presence of compositionally segregated domains on the support surface. At higher surface densities (2-8 total metal atoms / nm²) only one edge was observed, suggesting that Mo O_x and W O_x are molecularly mixed and form a unique metal oxide nanostructure with a band gap different from W O_x or Mo O_x single metal oxide catalysts of the same domain size.

Partial oxidation of ethanol shows that at surface densities ranging from 1.0-8.0 total metal atoms / nm², tungsten and molybdenum oxides interact to form promoted W O_x-Mo O_x sites on alumina. The selectivity at 180 ºC to acetaldehyde relative to diethyl ether (the only two reaction products) is larger on all mixed W O_x-Mo O_x-Al₂ O₃ catalysts relative to Mo O_x-Al₂ O₃ metal oxides of similar domain size. Diethyl ether is the only product observed on W O_x-Al₂O₃ catalysts (acid function only). Therefore, the increased electronegativity provided by molecular interactions with W O_x clearly promotes the redox properties of Mo O_x.