Infrared studies on the interaction of ammonia and water on MoO3/SiO2 catalysts

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

“…We used ammonia and propene adsorption to analyze the 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 [25]. Both, Lewis acidity attributed to coordinative unsaturated molybdenum ions as well as Brønsted acidity was found in agreement with previous reports that indicated abundant Brønsted acidity at the surface MoO3-SiO2 catalysts based on adsorption of probe molecules or catalytic test reactions [17,[27][28][29][30][31][32].…”

“…Molybdenum oxide [33] and silica are predominantly covalent oxides, but the coordination chemistry of Mo and Si differ significantly. The Brønsted acidity of MoO3-SiO2 was generally attributed to the supported molybdenum oxide itself [17], either by formation of hydroxyl groups coordinated to surface molybdate species directly (Mo-OH) [34], traces of water that lead to formation of ammonium polymolybdate species [29], or the reaction between molybdenum oxide with the silica support that yields silicomolybdic acid [28,30,32]. Since the presence of Mo-OH as well as silicomolybdic acid was excluded by 1 H MAS-NMR and Raman spectroscopy, respectively, in the above mentioned study of a series of silica-supported molybdenum oxide catalysts [25], we initiated the present work that deals with ammonia adsorption investigated by temperature-programmed desorption, infrared spectroscopy, and DFT calculations with the aim to identify the origin of Brønsted acidity in silica-supported molybdenum oxide.…”