Acid sites on silica-supported molybdenum oxides probed by ammonia adsorbtion: 
Experiment and theory

Arnakawa, Kazuhik; Wang, Yuanqing; Kroehnert, Jutta; Schlögl, Robert; Trunschke, Annette

doi:10.1016/j.mcat.2019.110580

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Acid sites on silica-supported molybdenum oxides probed by ammonia adsorbtion: Experiment and theory

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Schlögl, Robert
Research Department Schlögl, Max Planck Institute for Chemical Energy Conversion, Max Planck Society;

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Citation

Arnakawa, K., Wang, Y., Kroehnert, J., Schlögl, R., & Trunschke, A. (2019). Acid sites on silica-supported molybdenum oxides probed by ammonia adsorbtion: Experiment and theory. Molecular Catalysis, 478: UNSP 110580. doi:10.1016/j.mcat.2019.110580.

Cite as: https://hdl.handle.net/21.11116/0000-0005-A91F-B

Abstract

The origin of Brdnsted 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 Bronsted acid site. A model is proposed that ascribes Bronsted 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 angstrom and a N-H(OSi) distance of < 1.1 angstrom 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.