Selective methanation of CO over supported Ru catalysts

The catalytic performance of supported ruthenium catalysts for the selective methanation of CO in the presence of excess CO2 has been investigated with respect to the loading (0.5-5.0 wt.%) and mean crystallite size.(1.3-13.6 nm) of the metallic phase as well as with respect to the nature of the support (Al2O3, TiO2, YSZ, CeO2 and SiO2). Experiments were conducted in the temperature range of 170-470 degrees C using a feed composition consisting of 1%CO, 50% H-2 15% CO2 and 0-30% H2O (balance He). It has been found that, for all catalysts investigated, 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 becomes 100% when the CO2 methanation reaction is initiated. Increasing metal loading results in a significant shift of the CO conversion curve toward lower temperatures, where the undesired reverse water-gas shift reaction becomes less significant. Results of kinetic measurements show that CO/CO2 hydrogenation reactions over Ru catalysts are structure sensitive, i.e., the reaction rate per surface metal atom (turnover frequency, TOF) depends on metal crystallite size. In particular, for Ru/TiO2 Catalysts, TOFs of both CO (at 215 degrees C) and CO2 (at 330 degrees C) increase by a factor of 40 and 25, respectively, with increasing mean crystallite size of Ru from 2.1 to 4.5 nm, which is accompanied by an increase of selectivity to methane. Qualitatively similar results were obtained from Ru catalysts supported on Al2O3. Experiments conducted with the use of Ru catalyst of the same metal loading (5 wt.%) and comparable crystallite size show that the nature of the metal oxide support affects significantly catalytic performance. In particular, the turnover frequency of CO is 1-2 orders of magnitude higher when Ru is supported on TiO2, compared to YSZ or SiO2, whereas CeO2- and Al2O3-supported catalysts exhibit intermediate performance. Optimal results were obtained over the 5%Ru/TiO2 catalyst, which is able to completely and selectively convert CO at temperatures around 230 degrees C. Addition of water vapor in the feed does not affect CO hydrogenation but shifts the CO2 conversion curve toward higher temperatures, thereby further improving the performance of this catalyst for the title reaction. In addition, long-term stability tests conducted under realistic reaction conditions show that the 5%Ru/TiO2 catalyst is very stable and, therefore, is a promising candidate for use in the selective methanation of CO for fuel cell applications. (C) 2008 Elsevier B.V. All rights reserved.