Ammonia adsorption on Keggin-type heteropolyacid catalysts explored by density functional quantum chemistry calculations

Density functional quantum chemical calculations have been used to compare the acid strengths of phosphotungstic and phosphomolybdic acids by computin g the adsorption energy of ammonia on model clusters of each heteropolyacid . The adsorption of ammonia on a phosphotungstic acid cluster was stronger than the adsorption on a phosphomolybdic acid cluster. The predicted adsorp tion energies were near -150 and -106 kJ mol(-1) for phosphotungstic and ph osphomolybdic acid, respectively. This compares well with the experimental heats of ammonia sorption determined from microcalorimetry. An analysis of different adsorption modes of ammonia on phosphotungstic acid showed that b identate adsorption of ammonia on the heteropolyacid clusters aided in prot on transfer and yielded stronger adsorption energies than did a monodentate adsorption mode. In addition, we report the computed adsorption energies o f pyridine on a heteropolyacid cluster.