Specific Role of Potassium in Promoting Ag/Al2O3 for Catalytic Oxidation of Formaldehyde at Low Temperature

Ag-based catalysts are efficient materials for formaldehyde (HCHO) oxidation in the low-temperature range. It has been reported that K addition can increase the catalytic performance of Ag-based catalysts, whereas the specific role of K doping remains ambiguous. In this work, Ag/Al2O3 and K–Ag/Al2O3 catalysts with different K doping levels were prepared and subsequently tested for catalytic HCHO oxidation. It was observed that K doping has a dramatic promotion effect on the activity of the Ag/Al2O3 catalyst. The 2K–Ag/Al2O3 sample was the most active catalyst, over which HCHO could be completely converted into CO2 and H2O in a gas hourly space velocity of 100 000 mL/(gcat h) at 65 °C. The catalysts were next characterized by X-ray powder diffraction, Brunauer–Emmett–Teller, transmission electron microscopy, H2-temperature-programmed reduction, UV–vis, X-ray absorption fine structure, and in situ diffuse reflectance infrared Fourier transform spectroscopy methods. The characterization results show that addition of K species increased the content of metallic Ag species in Ag/Al2O3 catalysts and promoted the adsorption and activation of oxygen species, whereas excessive doping with K+ resulted in the agglomeration of Ag particles and loss of some metallic sites. Therefore, the 2K–Ag/Al2O3 catalyst demonstrated the best performance among the series of K-doped catalysts. Promotion of O2 activation by K doping, rather than enhanced Ag dispersion or surface OH activation or exposure of more Ag(111) facets, was found to be the key factor in the improved catalytic performance of K–Ag/Al2O3 catalysts.