Investigation of hydrogen transfer reaction mechanisms over supported oxide catalysts.

摘要

The reaction mechanism of NH₃ with NO over titania-supported vanadia catalysts was investigated using FTIR, laser Raman spectroscopy, and quantum chemical calculations based on density functional theory. Hydrogen atoms were found to bond more strongly to terminal (V = 0) oxygen atoms than to bridging (V-O-V) oxygen atoms, thus suggesting that terminal OH groups act as the active Brønsted acid sites for this reaction. Ammonia was found to adsorb as NH4 only on polymeric vanadia species in which the central vanadium atoms were maintained at a +5 oxidation state. The heat of ammonia adsorption was calculated as −110 kJ/mol, which is consistent with the value reported in the literature. A reaction mechanism involving the reaction of NH₄ species with gas-phase NO was investigated. NH₄ reacts with gas-phase NO in a series of elementary steps to yield a NH ₂NO species, which isomerizes to form N₂ and H₂O. The rate limiting step occurs during the formation of the NH₂NO species, which is in agreement with the observation that NH₂NO has been observed only in trace amounts as a reaction product.; The dehydration of methyl lactate was carried out over a series of silica-supported sodium and hydrogen phosphate catalysts. Although all catalysts showed activity for methyl lactate dehydration, NaH₂PO₄/SiO₂ showed a 69% selectivity towards acrylic acid, whereas H₃PO ₄/SiO₂ showed a 89% selectivity towards acetaldehyde. Laser Raman studies showed that among the sodium phosphate catalysts tested, the extent of phosphate polymerization was a good predictor of selectivity towards acrylates. Ab initio quantum calculations based on density functional theory were used to model the dehydration of methyl lactate. Acetaldehyde and acrylic acid are formed via a series of surface intermediate species; methyl acrylate (and acetaldehyde) may be formed directly from the dehydration of adsorbed methyl lactate. The role of sodium was found to be twofold: to stabilize the surface intermediate species responsible for acrylic acid formation, and to allow dehydration reactions to take place on POH and P=O pairs belonging to adjacent, rather than the same, PO₄ groups, which selectively promotes the formation of acrylic acid.