Single-Site Vanadyl Species Isolated within Molybdenum Oxide Monolayers in Propane Oxidation

Teng Fu,Anna Wernbacher,Robert Schlögl,Annette Trunschke,Yuanqing Wang

doi:10.1021/acscatal.9b00326

Abstract

The cooperation of metal oxide subunits in complex mixed metal oxide catalysts for selective oxidation of alkanes still needs deeper understanding to allow for a rational tuning of catalyst perform...

Highlights

Transition metal oxide-based catalysts containing molybdenum and vanadium have been intensively investigated in the activation of short-chain alkanes and their oxidative dehydrogenation or selective oxidation.[1−4] Generally, it has been assumed that vanadium is the element that is responsible for C−H activation and that dominates catalytic activity of Vcontaining mixed metal oxides.[2,4]
Decoration of the SBA-15 surface by molybdenum−vanadium oxide monolayers has an impact on the porosity of the support
The interaction between molybdenum and vanadium oxide species on the surface of mesostructured silica SBA-15 has been studied by synthesis of a series of nearmonolayer catalysts that contain Mo and V in a variable ratio between 10 and 1

Summary

■ INTRODUCTION

Transition metal oxide-based catalysts containing molybdenum and vanadium have been intensively investigated in the activation of short-chain alkanes and their oxidative dehydrogenation or selective oxidation.[1−4] Generally, it has been assumed that vanadium is the element that is responsible for C−H activation and that dominates catalytic activity of Vcontaining mixed metal oxides.[2,4] One prominent example of a highly active and selective catalyst in ethane oxidative dehydrogenation to ethylene, propane selective oxidation to acrylic acid, and ammoxidation to acrylonitril is the Mo−V− Te−Nb oxide composed of the so-called M1 structure.[5,6] The surface of crystalline mixed Mo−V−(Te−Nb) oxide catalysts under reaction conditions of propane oxidation to acrylic acid is enriched in V5+, whereas molybdenum is present in the oxidation state 6+.7−9 The presence of surface oxide species other than vanadia may, affect the reaction pathways of alkanes and reaction intermediates.[10]. The vanadium-only structure (model S4) clearly shows a shift of the edge position to lower energy compared to the simulated spectrum of model S2 that contains a single vandyl site, which matches the experimental spectrum of 4Mo-4V (inset in Figure 2c), indicating the presence of V−O V moieties in both 4V and 4Mo-4V catalysts.[63,64] The calculation result is in agreement with previous studies where a red shift of the absorption spectra has been predicted with an increasing degree of oligomerization and in particular upon the presence of V−OH terminations.[31]. The mixed metal oxide catalysts exhibit lower rate constants of reduction compared to the vanadia catalyst due to the presence of isolated single-site metal oxide species

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES