Structure and electronic states of gold species in mordenites

Au-zeolites with SiO2/Al2O3 molar ratios 10 and 206 were prepared by ion exchange of the H-mordenites with [Au(NH3)4](NO3)3 complex solution followed by hydrogen reduction or calcination at different temperatures. The samples were characterized by UV-visible DRS, XPS, XRD, TEM and TPR in order to understand the effect of zeolite acidity and pretreatment conditions on formation of different gold states. Three kinds of gold species were observed in the samples: (1) Au3+cations, (2) partly reduced species with the diameter less than 1 nm assigned to charged clusters Aunδ+ (with n proposed ⩽8), suggested to be stabilized inside zeolite channels, and (3) Au0 nanoparticles with diameter varied within interval of 1.5-20 nm. The third type is located on the external surface of zeolite microcrystals. Aunδ+ clusters were observed in as-prepared samples just after ion-exchange. Acid site strength did not change significantly amount of the clusters but influenced the electronic state of gold in these clusters and redox properties of gold cations. The contribution of the clusters was not changed significantly with temperature treatment even up to 500 °C. Au0 nanoparticles were formed by reduction of gold ions at temperatures starting from 50 to 200 °C depending on the medium of treatment and the molar ratio of the zeolite. The obtained results are important for understanding the effect of multiplicity of catalytically active gold species in CO oxidation.