THEORETICAL-STUDIES OF CO AND NO ADSORPTION ON CU-ZSM-5 ZEOLITE()

Molecular models of the form (H2O)(n) ... Cu+... X, where n = 1, 2, 3 and X = CO, 2(CO), NO, (NO)(2), and H2O are employed to investigate th e catalytically active copper sites in Cu+-ZSM-5 zeolite. Structures, binding energies and vibrational frequencies are calculated for these molecular models by density functional theory with gradient-corrected functionals. The calculated vibrational frequencies are compared to ex perimental infrared spectra of CO and NO adsorbed in Cu+-ZSM-5 for eac h n value. The absence (presence) of an unpaired electron in the CO (N O) molecule is found to have effects in the structures, binding energi es, and vibrational frequencies of the adsorbed species. Upon adsorpti on at a copper cation site, we find that, in agreement with experiment , (1) the stretching frequency of CO undergoes a small blue shift wher eas that of NO is red shifted; (2) the formation of adsorbed dicarbony l species is not as favored as the formation of adsorbed dinitrosyl sp ecies; and (3) the frequency separation between the antisymmetric and symmetric stretching modes is much smaller in the dicarbonyl species t han in the dinitrosyl species. The choice of gradient-corrected functi onals and the basis set used in this study is found to reproduce accur ately the successive binding energies of (H2O)(n) ... Cu+ when three o r more Ligands to the copper cation are present.