In situ DRIFT studies of sulfided K-Mo/γ-Al2O3 catalysts

Abstract

Sulfided Mo/γ-Al2O3 catalyst and K-Mo/γ-Al2O3 catalysts with K/Mo=0.4 and 0.8 were investigated by in situ diffuse reflectance infrared Fourier transform (DRIFT) with NO and syngas adsorption and with high-pressure CO hydrogenation from syngas. On the sulfided Mo/γ-Al2O3 catalyst NO adsorption gives bands at 1790 and 1690 cm−1 and syngas adsorption gives bands at 2095, 2065 and 2010 cm−1. They are assigned to NO and CO adsorbed on the MoS2-like species on the surface, respectively. However, on the sulfided K-Mo/γ-Al2O3 catalysts, NO adsorption gives the only band at 1252 cm−1 because of the formation of NO2 or NO3− species; no band was observed by syngas adsorption. These results suggest that on the sulfided K-Mo/γ-Al2O3 catalysts, the sites for NO and CO adsorption over the MoS2-like species are addressed by K atoms to form K-MoS2 species, which effectively inhibits NO and CO adsorption. In addition, the K-MoS2 species are more likely to be oxidized than the MoS2-like species. DRIFT spectra recorded after the sulfided catalysts were used for high-pressure CO hydrogenation indicated that formate species are present on both the sulfided Mo/γ-Al2O3 catalysts and the sulfided K-Mo/γ-Al2O3 catalysts. They yielded bands at 1590, 1420 and 1385 cm−1 and at 1590, 1415 and 1352 cm−1, respectively. During high-pressure CO hydrogenation, formate species on the sulfided Mo/γ-Al2O3 catalyst are converted to CO2 and methane, whereas those on the sulfided K-Mo/γ-Al2O3 catalysts are converted to methanol, CO2, or CH4. On the sulfided K-Mo/γ-Al2O3 catalysts, the amounts of formate species increase with time through the initial 8 h of reaction. If 5000 ppm H2S in syngas is introduced on to the catalyst bed as an impurity during reaction, the amounts of the formate species greatly decrease, resulting in an increase in selectivity to hydrocarbons and a decrease in selectivity to mixed alcohols. Therefore, the loss of sulfur and partial oxidation of the K-MoS2 species are necessary for formation of active sites for alcohols synthesis. Our results indicate that during high-pressure CO hydrogenation, active sites for alcohol synthesis are on the partly oxidized K-MoS2 species.