The purification of the polymer membrane fuel cell (PEMFC) reformate as by the methanation reaction with the use of platinum group metals (PGMs) on TiO2 support

Abstract

The catalytic performance of TiO2 supported PGMs (Pd, Ru, Pt, Rh) was investigated for the CO , CO2 and CO/CO2 methanation reactions with respect to metal loading. The Pd/TiO2 and Ru/TiO2 catalysts were prepared by both the deposition-precipitation method and the wetincipient method. In the latter case catalysts were either subjected to calcination or were left uncalcined. From TEM and TPR analysis it was noted that the uncalcined Pd/TiO2 and Ru/TiO2 catalysts demonstrated different characteristics such as having smaller particle sizes and the presence of a second reduction peak which was not present in the other differently prepared catalysts. This peak was assumed to be due to metal-support interactions. The Pd/TiO2 catalysts did not display significant differences in activity. However the uncalcined Ru/TiO2 catalysts displayed higher activities for the CO methanation reaction; hence the Pt/TiO2 and Rh/TiO2 catalysts were prepared with this method. The catalytic activity for the CO methanation reaction of the catalysts was observed to vary in the order of Rh/TiO2 > Ru/TiO2 > Pd/TiO2 > Pt/TiO2. A similar ranking of aluminia supported PGMs has been reported in several studies.1,2 All of the investigated catalysts lead to the production of one hydrocarbon which was methane even at lower temperatures such as 240 0C. Higher hydrocarbons were not observed and the extent to which methane was produced increased with increasing temperature. For all of the investigated catalysts activity was observed to decrease with decreasing metal loading. During the CO2 methanation reaction compared to the Pd and Pt catalysts, the Rh and Ru catalysts displayed the highest relative activity for CO2 methanation. Plots which compared CO methanation with CO2 methanation showed that the Rh/TiO2 and Ru/TiO2 catalysts had the largest temperature window at which CO methanation was at a maximum while CO2 methanation was at a minimum. The Pt/TiO2 and Pd/TiO2 catalysts had the smallest temperature window of operation. The Pt/TiO2 catalysts appeared to have higher tendencies for CO2 methanation and not CO methanation. For all catalysts investigated the methanation of CO2 lead to the production of CO via the reverse water gas shift reaction (RWGS) at high temperatures. In order to determine the selectivity of the catalysts for the CO methanation reaction compared to ii CO2 methanation, R values were calculated and it was established that the Rh/TiO2 and Ru/TiO2 catalysts had the highest maximum R values and thus better selectivity towards CO methanation. Compared to the CO and CO2 methanation results, the CO/CO2 methanation results were different as the catalysts would reach maximum CO conversion at a certain temperature and then decrease with further increase of temperature due to the RWGS reaction. However the Pd and Pt catalysts displayed similar results as in the CO2 methanation reaction, because they demonstrated a higher affinity for the RWGS reaction during the CO/CO2 methanation reaction. The Rh and Ru catalysts displayed the highest activity for the selective methanation of CO in the CO/CO2 gas mixture. The formation of CH4 was determined for the Rh/TiO2 and Ru/TiO2 catalysts since they displayed the best performance for the selective methanation of CO in the CO/CO2 gas mixture. It was noted that the CH4 present also results from the methanation of CO2 and that a temperature range at which complete CO methanation and negligible CO2 methanation occurs is not present.