Ammonia Hydrothermally Treated SiO2 Supported Rh-Based Catalyst for CO Hydrogenation to C2 Oxygenates: Remarkable Effect of Support Pore Size

Silica is a traditional carrier for Rh-based catalyst for CO hydrogenation to C-2 oxygenates, and its pretreatment with various organic solvents under specified conditions usually affects the surface and textural properties of silica and thus the catalytic performance of Rh-based catalyst. In this paper, silica samples were hydrothermally treated with different concentrations of ammonia and were used to support Rh-based catalysts. The as-made supports and the respective catalysts were characterized by N-2 adsorption-desorption, TEM, XRD, FT-IR, H-2-TPR, CO chemisorption, CO-TPD, and temperature-programmed surface reaction (TPSR). The results showed that the hydrothermal treatment enlarged the pore size of silica gradually from 8.1 nm on parent sample to 26.5 nm on that treated by 10% ammonia. Smaller pore size usually with higher surface area of silica was beneficial to the formation of highly dispersed Rh species; however, the mean Rh particle sizes here were within the optimal range of 2-4 nm. It was also demonstrated that, with the increasing of pore size of silica, the CO insertion ability weakened, while the CO dissociation capability strengthened relatively; meanwhile, the diffusion limitation of CO could be eliminated gradually, leading to a decrease of the local H-2/CO ratio during CO hydrogenation reaction. This could restrain the hydrogenation ability but facilitate the CO dissociation or insertion. Consequently, carbon chain propagation was much promoted to give more C-2(+) hydrocarbons, while the hydrogenation of C-2 oxygenate precursors was prohibited to yield much less ethanol. Moreover, Rh-based catalysts supported on larger pore sizes of silica were more stable than that on the untreated one. This finding provides a useful strategy to effectively adjust the catalytic performance of Rh-based catalyst through controlling internal diffusion by selecting a support with the proper pore size.