X-射线吸收谱原位表征Cu-Zn/SiO₂催化剂Cu价态

The valence states of copper-based catalyst are still controversial for whether Cu⁺ exists or even dominates the corresponding catalysis performance, especially for the catalyst prepared by unconventional method. For example, Cu/SiO₂ catalyst prepared by deposition-precipitation exhibits different ratio of Cu⁰ and Cu⁺ in the literatures. Zinc-doping Cu/SiO₂ catalyst is primarily chosen for carbonyl hydrogenation. Additionally, the copper catalysts employed in the industrial reactor require a doping of zinc for catalyst stability and relatively low temperature reduction, which makes the valence states of copper catalyst under working conditions even more complicated. In this paper, copper phyllosilicate, 15 wt% Cu-5.0 wt% Zn/SiO₂, was prepared by ammonia evaporation method with the following procedures:(1) complexation of Cu/Zn to silicasol by mixing of an amount of copper ammonia solution (0.5 mol/L) and zinc ammonia solution (0.5 mol/L) with silicasol (5%, SiO₂) for 5 h at room temperature, and then decomposition of metal ammonia under 95℃; (2) dried, crushed and calcined at 450℃ for 4 h. The Cu/SiO₂ catalyst precursor without the addition of zinc was prepared with the same procedures as Cu-Zn catalyst precursor. The valence states of copper catalyst of Cu-Zn/SiO₂ and Cu/SiO₂ catalyst during hydrogen reduction in the temperature range of 20℃ to 350℃ has been studied by in situ X-ray absorption spectroscopy characterization under the atmosphere of 10% H₂ in He, 50 mL/min. Valence state composition was analyzed by using Linear Combination Fitting (LCF) method based on X-ray Absorption Near Edge Spectroscopy (XANES) spectra, which indicated:(1) the reduction degree of Cu²⁺ to Cu⁰ is increased with the addition of zinc; (2) Cu⁺ occurred and mainly existed at low reduction temperature (<250℃) with a low concentration, but it diminished completely at high reduction temperature; (3) Cu⁰ could be the active center for hydrogenation of methyl acetate to ethanol.