Effect of Support and Promoter on Activity and Selectivity of Gold Nanoparticles in Propanol Synthesis from CO2, C2H4, and H2

Direct propanol synthesis from CO2, H-2, and C2H4 was investigated over TiO2- and SiO2-based catalysts doped with K and possessing Au nanoparticles (NPs). The catalysts were characterized by scanning transmission electron microscopy and temperature-programmed reduction of adsorbed CO2. Mechanistic aspects of CO2 and C2H4 interaction with the catalysts were elucidated by means of temporal analysis of products with microsecond time resolution. CO2, which is activated on the support, is reduced to CO by hydrogen surface species formed from gas-phase H-2 on Au NPs. C2H4 adsorption also occurs on these sites. In comparison with TiO2-based catalysts, the promoter in the K-Au/SiO2 catalysts was found to increase CO2 conversion and propanol production, whereas Au-related turnover frequency of C2H4 hydrogenation to C2H6 decreased with rising K loading. The latter reason was linked to the effect of the support on the ability of Au NPs for activation of C2H4 and H-2. The positive effect of K on CO2 conversion was explained by partial dissolution of potassium in silica with formation of surface potassium silicate layer thus inhibiting formation of potassium carbonate, which binds CO2 stronger and therefore hinders its reduction to CO.