Ultrasmall gold nanoparticles confined in zeolite Y: Preparation and activity in CO oxidation

In this study, proton-type zeolites Y (HY) with Al/Si ratio of 0.36 were used as a support for deposition of HAuCl₄ (in pH 6 solution) to prepare Au/Y catalysts for CO oxidation at 25 °C. The low isoelectric point of HY (∼pH = 2) made this support inappropriate for loading negatively charged gold complexes, and caused the generation of large gold particles with low catalytic activity. We found that through a Na⁺ pretreatment of HY to form H(Na)Y, the surface charge can be reversed to positive, thus favoring the deposition. The resulting Au/H(Na)Y, without any high temperature treatment, gave excellent activity in CO oxidation comparable to Au/TiO₂. The CO oxidation rate of 0.21 mol CO/mol Au∙s remained stable at 25 °C. HRTEM images of the catalyst show highly dispersed gold nanoparticles ∼1 nm confined in the nanocage of H(Na)Y. XPS spectra show high proportion of Au(III) associated with Au-O. The weak Au-O in Au/H(Na)Y is found responsible for the easy reduction in TPR. For comparison, the other alternative procedures of gold deposition were also tried to reverse the surface charge of HY by decreasing the pH of solution. However, high contents of residual Cl in the nano-gold catalyst lead to almost no activity in CO oxidation.