(411f) Effects of Reduction Temperature on Negatively Charged Gold Species over Nonreducible Supports

Highly dispersed Au nanoparticles (NPs) performed excellently in low-temperature CO oxidation. However, opinions regarding the nature of the catalytically active gold species (Au^δ+, Au⁰, Au^δ-) are still controversial. It is reported that in the CO oxidation reaction, the activity is in good correlation with the reduction degree of Au species and negatively charged gold (Au^δ-) species are the most active. Au^δ- species can be easily obtained on reducible supports as the electron can be transferred from the reduced supports. However, a small number of works report Au^δ- species on “inert” and nonreducible supports (SiO_2,Al₂O₃) which cannot transmit electrons.

In this study, we synthesized ultra-small gold NPs on SiO₂ and Al₂O₃ through strong electrostatic adsorption (SEA) and demonstrate how the reduction temperature relates to the formation of Au^δ- species. Both XRD and STEM characterization showed the particle size of nanoparticulate gold sample as 1.6 nm and TPR plots suggested no organic ligand left. It is evaluated for room temperature CO oxidation.

Initially CO adsorption over Au samples was investigated at different in-situ reduction temperatures, as depicted in figure 1. The introduction of CO onto ex-situ reduced gold sample led to the appearance of Au⁰-CO species (2107 cm^-1 to 2094 cm^-1)(Fig. 1a). Treated the gold sample with an elevated in-situ reduction temperature, new bands assigned to assigned to Au^δ- species (<2080 cm^-1)developed. This is the first report that reduction temperature can affect Au^δ- on nonreducible supports. Gold sample prepared by DI (10.2nm) in-situ reduced at 400⁰C (Fig.1f) displayed decreased intensity. Hence, Au^δ- species on nonreducible supports only occurred on highly dispersed Au particles with high temperature reduction.

References

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