Conversion of syngas to C2+ oxygenates over Rh-based/SiO2 catalyst: The promoting effect of Fe

The effect of Fe promoter on the catalytic properties of Rh–Mn–Li/SiO2 catalyst for CO hydrogenation was investigated. The catalysts were comprehensively characterized by means of X-ray diffraction (XRD), N2 adsorption–desorption, temperature programmed reduction (TPR), temperature programmed desorption (TPD), temperature programmed surface reaction (TPSR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Activity testing results showed that low loading of Fe (≤0.1 wt%) improved the reactivity and yield of C2+ oxygenates; however, the opposite effect appeared at the high values of Fe (>0.1 wt%). Characterization results suggested that the addition of Fe strengthened the Rh–Mn interaction and increased the desorption/transformation rate of adsorbed CO, which could be responsible for the increase of CO conversion. But on the other hand, the existence of Fe might deposit over the Rh surface, and decreased the number of active sites, resulting in the decrease of CO conversion when the Fe amount was excessive. The selectivity to C2+ oxygenates varied inversely with the reducibility of Rh oxide species. Moreover, it is proposed that the transformation of dicarbonyl Rh+(CO)2 into H–Rh–CO is favorable for the formation of C2+ oxygenates, and the hydrogenation ability of Fe can increase the hydrogenation of acetaldehyde to ethanol.