Insight of the structure fabrication of nano Ag and Au catalysts by pretreatment and its unique catalytic activity
Z. P. Qu
Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Sciences and Technology, Dalian University of Technology, Dalian, 116024, China
Nano silver and gold catalysts are gaining attention in the field of heterogeneous catalysis. Also we all know that the defects, recrystallization and the morphology change of the solids play an important role in the gas adsorption, diffusion and the surface reaction in metal solids. The ability of a solid to attain its thermodynamically predetermined equilibrium state needs significant mass transfer. Surface transitions at the gas-metal interface often occur at significantly lower temperatures than bulk transformations. Bulk transitions typically require temperatures in excess of the Tammann temperatur in order to produce structural changes. The changes in morphology may either be thermally induced or reaction-induced. The process is driven in such a way as to result in a decrease of the surface-free energy. Reaction-induced morphological changes take place in the presence of a reacting atmosphere, which results in the formation of a structure different from those formed after treatment in inert or vacuum. However, due to the low melting point of Ag and Au, and in most cases low-temperature treatments were undertaken to activate the Ag and Au catalysts the nanoparticles to prevent the sinter of nanoparticle at higher temperatures.
Here the surface restructure, evaporation–deposition-diffusion for nano-Ag and carbonization-oxidation induction process for nano-Au catalysts by changing pretreatment temperature (400-900oC) and atmosphere (H2, O2) were proposed. The oxygen can induce a decrease in the surface free energy at high temperature and restructure for nanosilver particle, which will be in favor for the following H2 reduction to obtain he high dispersion and high activity for CO oxidation. Differently, it was found that the carbonization process under the H2-pretreatment was the key first step to control the growth of Au nanoparticles when the following high-temperature oxidation treatment was conducted. The carbonization-oxidation induction favored the formation of the highly dispersed and defective Au NPs with large amounts of low-coordinated Au atoms and Aunδ+ clusters and effectively enhance the HCHO oxidation activity at RT.
Keywords: Nano Ag, Au; Structure fabrication; High temperature; Pre-treatment