Selective methanation of CO over supported noble metal catalysts: Effects of the nature of the metallic phase on catalytic performance

The catalytic performance of Al2O3-supported noble metal catalysts for the methanation of CO, CO2 and their mixture has been investigated with respect to the nature of the dispersed metallic phase (Ru, Rh, Pt, Pd). Experiments have been conducted using feed compositions relevant to those of reformate gas streams, both in the absence and in the presence of water. It has been found that, for all experimental conditions investigated, Ru and Rh are significantly more active than Pt and Pd. Selectivity toward hydrogenation products depends strongly on the noble metal catalyst employed, as well as on whether solo- or co-methanation of CO/CO2 is occuring. For hydrogenation of CO alone, selectivity toward CH4 increases with increasing temperature at the expense of higher hydrocarbons. In presence of water in the feed, catalytic activity of Ru is not affected, while that of Rh is reduced. On the other hand, the performance of Pt and Pd is poor since they promote the undesired water-gas shift (WGS) reaction. In hydrogenation of CO2 alone, selectivity toward higher hydrocarbons is negligible for all catalysts investigated but the reaction is accompanied by production of CO via the reverse WGS reaction. In all cases, addition of water in the feed results in decrease of catalytic activity. In combined hydrogenation of CO/CO2 mixtures, conversion of CO2 is completely suppressed until conversion of CO reaches its maximum value. Selectivity toward methane, which is typically higher than 70%, increases with increasing temperature and approaches 100% when CO2 conversion is initiated. Addition of 30% water vapor in the feed does not affect CO hydrogenation over Ru and Rh catalysts but retards CO2 hydrogenation, thereby expanding the temperature window of operation. (C) 2008 Elsevier B.V. All rights reserved.