Synthesis Gas Conversion over Rh-Mn-W_xC/SiO₂ Catalysts Prepared by Atomic Layer Deposition

The catalytic conversion of synthesis gas to value-added oxygenates and light hydrocarbons was studied on Rh and Rh-Mn clusters on tungsten carbide-overcoated silica (W_xC/SiO₂) at 523 K, 580 psi, and CO/H₂ = 1/1. The W_xC-modified SiO₂ support was prepared by the overcoating of WO_x on SiO₂ using atomic layer deposition (ALD) followed by carburization. The reactivities of Rh/W_xC/SiO₂ catalysts with varying number of ALD cycles (number of cycles = 2, 5, 10, 20, and 30) were measured and showed that 5 ALD cycles of W_xC suppressed the formation of methane. All W_xC-modified catalysts showed enhancement in selectivity toward methanol and ethanol through CO hydrogenation and acetaldehyde hydrogenation, respectively. These catalysts also improved the overall turnover frequency (TOF). The addition of Mn species further promoted the activity and the selectivity toward ethanol and C₂₊ hydrocarbons, especially light alkenes. The best performing Rh-2Mn/5cycle-W_xC/SiO₂ catalyst (Rh:Mn molar ratio = 1:2) achieved 84.7% selectivity toward valuable oxygenates and C₂₊ hydrocarbons with a ratio of alkenes to alkanes equal to 1.7, compared to Rh/SiO₂ which exhibited 80.4% selectivity toward the products aforementioned with a ratio of 0.5. The overall TOF was 20 times higher than that over Rh/SiO₂ (i.e., 5.6 × 10^-2 s^-1 vs 2.9 × 10^-3 s^-1). X-ray diffraction revealed that the existence of W₂C, which was the dominant phase in Rh/5cycle-W_xC/SiO₂, favored the suppression of methane and enhanced the production of alcohols and C₂₊ hydrocarbons compared to the WC support. Density functional theory calculations for Rh₁₉, Rh₃₁, and Rh₃₇ clusters on various WC surfaces suggested that the shape of Rh clusters is condition dependent and subject to H₂ pressure. The C- and O- binding energies on various sites for the clusters were used with scaling relations to probe their catalytic activity. With the use of this approach, the increase in the O binding energy when moving from the SiO₂-supported Rh₃₇ cluster to the WC-supported cluster leads to an increase in the activity of Rh/W_xC/SiO₂ at the expense of the reduction in the number of sites that are selective toward C₂ oxygenates.