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

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 Auδ+ 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.