Conversion of synthesis gas into value-added products, including oxygenates and C2+ hydrocarbons, was studied at 523 K, 580 psi, and a 1/1 CO/H2 ratio over Rh catalysts on catalyst supports prepared by atomic layer deposition (ALD) of molybdenum and tungsten species on silica. The reactivity measurements showed that coating the silica support with molybdenum and tungsten species helped to suppress the methane selectivity and promote the overall conversion rate. When the silica support was coated with five cycles of β-Mo2C, the methane selectivity decreased from 32% (1% Rh/SiO2) to 13% (1% Rh/5c-Mo2C/SiO2), and the overall product rate increased 33 times from 0.4 to 12.7 mmol min–1 (g of Rh)−1. CO-FTIR results showed that supporting Rh on silica led to the formation of Rh(211) facets, which favored the formation of methane and had a higher CO conversion rate. Rh on a MoO3/SiO2 support prepared by ALD was found to be the most active catalyst while maintaining the suppression of methane selectivity, showing an overall rate ∼60 times higher than that of 1% Rh/SiO2. A reaction pathway is proposed, in which hydrogenation steps are promoted most significantly by Mo and W species, followed by promotion of CO insertion steps for ethanol synthesis and C–C coupling steps for hydrocarbon formation. CO-FTIR results showed that 1% Rh/MoO3/SiO2 has the highest proportion of gem-dicarbonyl adsorption sites and the lowest proportion of bridge-bonded CO adsorption sites. The rate of methanol formation shows a positive correlation with the number of sites that form gem-dicarbonyl species.

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 x C/SiO 2 ) at 523 K, 580 psi, and CO/H 2 = 1/1. The W x C-modified SiO 2 support was prepared by the overcoating of WO x on SiO 2 using atomic layer deposition (ALD) followed by carburization. The reactivities of Rh/W x C/SiO 2 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 x C suppressed the formation of methane. All W x C-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 C 2+ hydrocarbons, especially light alkenes. The best performing Rh-2Mn/5cycle-W x C/SiO 2 catalyst (Rh:Mn molar ratio = 1:2) achieved 84.7% selectivity toward valuable oxygenates and C 2+ hydrocarbons with a ratio of alkenes to alkanes equal to 1.7, compared to Rh/SiO 2 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/SiO 2 (i.e., 5.6 × 10 −2 s −1 vs 2.9 × 10 −3 s −1 ). X-ray diffraction revealed that the existence of W 2 C, which was the dominant phase in Rh/5cycle-W x C/SiO 2 , favored the suppression of methane and enhanced the production of alcohols and C 2+ hydrocarbons compared to the WC support. Density functional theory calculations for Rh 19 , Rh 31 , and Rh 37 clusters on various WC surfaces suggested that the shape of Rh clusters is condition dependent and subject to H 2 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 SiO 2 -supported Rh 37 cluster to the WC-supported cluster leads to an increase in the activity of Rh/W x C/SiO 2 at the expense of the reduction in the number of sites that are selective toward C 2 oxygenates.

Catalysts consisting of transition metals (Ni, Co, Cu, and Ru) supported on molybdenum oxide synthesized by atomic layer deposition (ALD) on silica were studied for synthesis gas conversion at 523 K and a pressure of 580 psi. Transition-metal-promoted Mo-based catalysts (M/MoO3/SiO2) showed different selectivity patterns from the transition metal supported on silica (M/SiO2). All molybdenum-based catalysts displayed a similar selectivity pattern, consisting of 15–20% of CH4, 30–40% of C2+ hydrocarbons, and 35–40% of oxygenates. The addition of transition metals to molybdenum oxide promoted the catalytic activity by an order of magnitude. Temperature program reduction indicated hydrogen spillover from the transition metals to molybdenum species. H2–D2 exchange rate measurements showed that the addition of the transition metal enhanced the rate of H2 dissociation on the catalyst. CO chemisorption measurements indicated that transition-metal-promoted molybdenum catalysts consist of a similar amount of strong adsorption sites, which may originate from the reduced transition metal, and weak adsorption sites, which may originate from reduced molybdenum oxides. A dual-site mechanism is suggested in which low-valent molybdenum species dissociate CO and generate CH x groups that are hydrogenated to hydrocarbons or react with adsorbed CO on higher-valent Mo sites to form higher alcohols. Ni, Co, and Ru are able to generate CH x groups and enhance the production of C2+ oxygenates, whereas all of the transition metals studied are able to provide sites for H2 dissociation and H spillover to molybdenum oxide, leading to further enhancement in catalytic activity compared to MoO x /SiO2.

Base catalysts were studied for the dehydration of fatty alcohols to linear alpha olefins (LAOs). For the dehydration of 1-octanol to 1-octene, 15%Cs/SiO2 catalyst was 56% selective at 10% conversion....