Effect of manganese on the selective catalytic hydrogenation of COx in the presence of light hydrocarbons over Ni/Al₂O₃: An experimental and computational study

The promoting effect of manganese on Ni/Al₂O₃ catalyst for the hydrogenation of carbon oxides, in presence of light hydrocarbons, was studied. Ni/Al₂O₃ displayed a high activity for the complete conversion of CO and CO₂ to methane and C₂₊ hydrocarbons. Moreover, over a discrete and relatively narrow temperature range, the net concentration of light C₂₊ hydrocarbons was elevated, with the exit stream containing a higher concentration of C₂₊ species than was present in the feed stream and the product stream being virtually free of carbon oxides. It is found that the addition of manganese can enhance the selectivity toward the production of light hydrocarbons. A series of Ni-Mn/Al₂O₃ catalysts, prepared with different Ni/Mn ratios, was studied. Various characterisation techniques such as XRD, CO and H₂ chemisorption, in situ NO-FTIR and TPR were performed to gain insight into how the addition of Mn to the primary catalyst enhances the yield of light hydrocarbons. The origin of Mn-promotion was demonstrated through density functional theory (DFT) calculations, which revealed the favorable Mn substitution at the Ni(₂₁₁) step edge sites under reducing conditions. The affinity of these Mn species toward oxidation stabilizes the CO dissociation product and thus provides a thermodynamic driving force that promotes C-O bond cleavage as compared to the Mn-unmodified catalyst surface.