Preparation of nano-gold in zeolites for CO oxidation: Effects of structures and number of ion exchange sites of zeolites

Nano-gold was prepared in different types of zeolite (Y, β and mordenite) in pH-adjusted chloroauric acid solution. The effects of the pore structures and the aluminum content on the loading of gold and on the reaction activity for CO oxidation at 0 °C were investigated. The gold loading in each zeolite was strongly related to the aluminum content; it was also affected by the structure of the zeolite. HRTEM results revealed that a few particles of gold around 1 nm in diameter were formed in the cage of Y-type zeolite with less aluminum loading (1.8 wt.%), and there was aggregation of gold species on the exterior surface of the zeolite; in contrast, more particles of gold around 1 nm were formed in Y-type zeolite with more aluminum loading (12.4 wt.%), no aggregation was observed. The results from the temperature-programmed reduction in hydrogen indicated that the gold species were reduced in the smaller pores (in β and mordenite) only at lower temperatures. Nevertheless, gold was easily sintered in the channels of β and mordenite during CO oxidation, causing severely reducing the activity. The cage-like pore in Y-type zeolite with 12.4 wt.% aluminum loading prevented the sintering of gold, and its hydrophilic surface favored the activation of gold species; therefore, this Au/Y catalyst exhibited high catalytic activity and stability for CO oxidation.