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Abstract

The origin of Brønsted acidity in a series of silica-supported molybdenum oxide catalysts with Mo loadings of 2.1–13.3 wt%, and apparent Mo surface densities of 0.2–2.5 nm⁻², respectively, was analyzed by ammonia adsorption investigated by temperature-programmed desorption, infrared spectroscopy, and DFT calculations. Every surface molybdenum atom in the molybdenum oxide (sub-)monolayer is involved in the interaction with ammonia, either as Lewis or as Brønsted acid site. A model is proposed that ascribes Brønsted acidity to the interaction between silanol groups and adjacent surface molybdate species under formation of pseudo-bridging Si—O(H)---Mo(=O)₂ species with a Mo---O(Si) distance of 2.1 Å and a N-H(OSi) distance of <1.1 Å in the formed adsorption complex of the ammonia molecule. The combined experimental and computational study contributes to an improved fundamental understanding of acidity in amorphous mixed metal oxides.