(41c) Synthesis Gas Conversion over Rh-Based Catalysts Promoted By Fe and Mn

The direct conversion of synthesis gas to C₂₊ oxygenates has attracted considerable interest owing to their wide range of application in chemical, fuel and polymer industries.^1,2 Rhodium has been reported to be the best metal for the selective production of oxygenates,^3,4 and transition metals have been shown to be promoters to increase the selectivity for C₂₊ oxygenates. To date, experimental studies addressing the interaction among promoters and the resulting impacts on synthesis gas conversion are limited.

Here, we present our results on synthesis gas conversion to oxygenates and hydrocarbons over Rh-based catalysts promoted by Fe and Mn. We show that selective deposition of Fe and Mn species on Rh nanoparticles can be achieved by controlled surface reactions. By carrying out reactions at 523 K, 580 psi and CO/H₂=1/1, the catalytic results show that the interaction between Rh and Fe promotes the selective production of ethanol through hydrogenation of acetaldehyde and enhances the selectivity towards C₂ oxygenates, which include ethanol and acetaldehyde. Mn promotes the overall reaction rate and the selectivity towards C₂₊ hydrocarbons, primarily alkenes. Trimetallic Rh-Fe-Mn catalysts surpass the performance of their bimetallic counterparts. The highest selectivities towards ethanol (36.9%) and C₂ oxygenates (39.6%) were achieved over the Rh-Fe-Mn ternary system with a molar ratio of 1:0.15:0.10, as opposed to the selectivities obtained over Rh/SiO₂ which were 3.5% and 20.4%, respectively. Moreover, 55% of the total products are industrially valuable oxygenates and C₂₊ hydrocarbons. To understand the nature of Fe and Mn species, measurements obtained from X-ray photoelectron spectroscopy suggest that Fe and Mn exist as metallic iron and manganese oxides on Rh surface upon reduction. Through density functional theory (DFT) calculations, we reveal that manganese oxide is more stable on the Rh (211) step-edges whereas metallic iron has no preference over Rh (111) and (211). Fourier transform infrared spectroscopy of adsorbed CO and DFT calculations suggest that the adsorption of CO on Fe and/or MnO_x modified Rh (211) step-edges is altered compared to the binding of CO to clean Rh (211) surface. Accordingly, MnO_x species and Fe alter the bonding of CO on Rh step-edges and shift their selectivity away from CH₄, while reduced Fe species also alter the reactivity of Rh terrace sites.

1. Angelici, C.; Weckhuysen, B. M.; Bruijnincx, P. C. A. ChemSusChem 2013, 6, 1595–1614.

2. Goldemberg, J.; Goldemberg, J. Sci. 2007, 315, 808–810.

3. Ichikawa, M. J. Chem. Soc. Chem. Commun. 1978, 1500, 566.

4. Van der Lee, G.; Schuller, B.; Post, H.; Favre, T. L. F.; Ponec, V. J. Catal. 1986, 98, 522–529.