Direct propanol synthesis from CO2, C2H4, and H2 over Cs–Au/TiO2 rutile: effect of promoter loading, temperature and feed composition

Abstract

Cs–Au/TiO₂ rutile catalysts have been prepared and tested for the direct synthesis of n-propanol from CO₂ in the presence of C₂H₄ and H₂. Irrespective of the Cs loading, two kinds of Au particles, i.e. Au nanoparticles (NPs) around 4 nm to 8 nm and below 2 nm (even atomic gold), were identified on the surface of the catalysts. No direct relationship between the size and propanol production was established, while Cs loading strongly affected these catalyst properties. HAADF-STEM and EDX proved that Cs was homogenously distributed on both TiO₂ and Au NPs and favoured stabilisation of the Au^δ+ species. Such species were suggested to increase the rate of CO insertion into C₂H₄ thus improving the selectivity to propanol. In addition to Cs loading, the temperature and the H₂/CO₂, H₂/C₂H₄ and CO₂/C₂H₄ ratios also affected both desired (propanol formation) and undesired (C₂H₄ hydrogenation to C₂H₆) reaction pathways. The conversion of CO₂ increased with an increase in the temperature from 473 K to 543 K, while the conversion of C₂H₄ was practically not affected but the selectivity to propanol decreased from 100% to around 30%. For all temperatures, the catalyst performance depended on the H₂/(CO₂ + C₂H₄) ratio. Based on the obtained results, optimal reaction conditions for propanol synthesis were identified.