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

摘要

The catalytic performance of TiO2 supported PGMs (Pd, Ru, Pt, Rh) was investigated for the COud, CO2 and CO/CO2 methanation reactions with respect to metal loading. The Pd/TiO2 andudRu/TiO2 catalysts were prepared by both the deposition-precipitation method and the wetincipientudmethod. In the latter case catalysts were either subjected to calcination or were leftuduncalcined. From TEM and TPR analysis it was noted that the uncalcined Pd/TiO2 and Ru/TiO2udcatalysts demonstrated different characteristics such as having smaller particle sizes and theudpresence of a second reduction peak which was not present in the other differently preparedudcatalysts. This peak was assumed to be due to metal-support interactions. The Pd/TiO2 catalystsuddid not display significant differences in activity. However the uncalcined Ru/TiO2 catalystsuddisplayed higher activities for the CO methanation reaction; hence the Pt/TiO2 and Rh/TiO2udcatalysts were prepared with this method. The catalytic activity for the CO methanation reactionudof the catalysts was observed to vary in the order of Rh/TiO2 > Ru/TiO2 > Pd/TiO2 > Pt/TiO2. Audsimilar ranking of aluminia supported PGMs has been reported in several studies.1,2 All of theudinvestigated catalysts lead to the production of one hydrocarbon which was methane even atudlower temperatures such as 240 0C. Higher hydrocarbons were not observed and the extent toudwhich methane was produced increased with increasing temperature. For all of the investigatedudcatalysts activity was observed to decrease with decreasing metal loading.udDuring the CO2 methanation reaction compared to the Pd and Pt catalysts, the Rh and Ruudcatalysts displayed the highest relative activity for CO2 methanation. Plots which compared COudmethanation with CO2 methanation showed that the Rh/TiO2 and Ru/TiO2 catalysts had theudlargest temperature window at which CO methanation was at a maximum while CO2udmethanation was at a minimum. The Pt/TiO2 and Pd/TiO2 catalysts had the smallest temperatureudwindow of operation. The Pt/TiO2 catalysts appeared to have higher tendencies for CO2udmethanation and not CO methanation. For all catalysts investigated the methanation of CO2 leadudto the production of CO via the reverse water gas shift reaction (RWGS) at high temperatures. Inudorder to determine the selectivity of the catalysts for the CO methanation reaction compared toudiiudCO2 methanation, R values were calculated and it was established that the Rh/TiO2 and Ru/TiO2udcatalysts had the highest maximum R values and thus better selectivity towards CO methanation.udCompared to the CO and CO2 methanation results, the CO/CO2 methanation results wereuddifferent as the catalysts would reach maximum CO conversion at a certain temperature and thenuddecrease with further increase of temperature due to the RWGS reaction. However the Pd and Ptudcatalysts displayed similar results as in the CO2 methanation reaction, because they demonstrateduda higher affinity for the RWGS reaction during the CO/CO2 methanation reaction. The Rh andudRu catalysts displayed the highest activity for the selective methanation of CO in the CO/CO2udgas mixture. The formation of CH4 was determined for the Rh/TiO2 and Ru/TiO2 catalysts sinceudthey displayed the best performance for the selective methanation of CO in the CO/CO2 gasudmixture. It was noted that the CH4 present also results from the methanation of CO2 and that audtemperature range at which complete CO methanation and negligible CO2 methanation occurs isudnot present.