Selective methanation of CO over supported Ru catalysts

Abstract The catalytic performance of supported ruthenium catalysts for the selective methanation of CO in the presence of excess CO 2 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 (Al 2 O 3 , TiO 2 , YSZ, CeO 2 and SiO 2 ). Experiments were conducted in the temperature range of 170–470 °C using a feed composition consisting of 1%CO, 50% H 2 15% CO 2 and 0–30% H 2 O (balance He). It has been found that, for all catalysts investigated, conversion of CO 2 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 CO 2 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/CO 2 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/TiO 2 catalysts, TOFs of both CO (at 215 °C) and CO 2 (at 330 °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 Al 2 O 3 . 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 TiO 2 , compared to YSZ or SiO 2 , whereas CeO 2 - and Al 2 O 3 -supported catalysts exhibit intermediate performance. Optimal results were obtained over the 5%Ru/TiO 2 catalyst, which is able to completely and selectively convert CO at temperatures around 230 °C. Addition of water vapor in the feed does not affect CO hydrogenation but shifts the CO 2 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/TiO 2 catalyst is very stable and, therefore, is a promising candidate for use in the selective methanation of CO for fuel cell applications.