RESUMO
Supporting silver and gold on mordenites by ion-exchange method with further reduction with H2 leads to formation of neutral and charged metal clusters inside zeolite channels as well as metal nanoparticles on external surface of mordenite. A portion of the cluster states of the metals and stability of the clusters depend strongly on acidity of zeolite (determined by SiO2/Al2O3 molar ratio) and nature of zeolite cation (H+, Na+, NH4+). The investigations of silver and gold samples after prolonged storage for 6 and 12 months revealed that silver clusters are comparatively stable while oxidation of gold clusters and nanoparticles by air is the probable cause of deactivation of gold catalysts. The comparison of the results for Au and Ag samples allow suggesting NaM15 and NaM24 mordenites for effective synthesis of complex Au-Ag clusters as active and stable species of catalytic reactions occurring at room temperature.
RESUMO
Gold species deposited on NH4(+)- and H(+)-mordenites were studied by X-ray diffraction, xenon adsorption, NMR of 27Al and 129Xe, UV-Visible spectroscopy and TPR. Mordenite ion-exchange cations exert a significant effect on size distribution and electronic state of gold species supported on the zeolites. AuNH4M contains larger nanoparticles of gold on its external surface than AuHM because protonic form of zeolite has stronger sites for stabilization of gold nanospecies. In the case of NH4(+)-mordenite, Au clusters inside zeolite channels have bigger size and fill the side-pockets completely after gold deposition due to more complete reduction of gold entities, while in H(+)-mordenite only half the side-pockets are filled by gold species. H(+)-mordenite favors the stabilization of both small gold clusters and cations. Even after the reduction of gold by hydrogen flow at 100 degrees C, some gold species are not completely reduced and have certain effective charge (Au(n)delta+ clusters).