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 the Ni/Al2O3 catalyst for the hydrogenation of carbon oxides, in the 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, were studied. Various characterization techniques such as X-ray diffraction (XRD) analysis, CO and H-2 chemisorption, in situ nitric oxide adsorption Fourier transform infrared spectroscopy (NO-FTIR), and temperature-programmed reduction (TPR) were performed to gain an 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 compared to the Mn-unmodified catalyst surface.