Preferential oxidation of carbon monoxide over cobalt and palladium based catalysts supported on various metal oxides

Abstract

The interest on the use of proton exchange membrane (PEM) fuel cells for vehicle application has increase due to its efficiency, high power density and rapid start up. The on-board reforming process is used to generate hydrogen; however, this process simultaneously produces 1% CO which poisons Pt-based anode catalyst. Previous studies have shown that supported Pd-based catalysts have very good stability on preferential oxidation (PROX) of CO, but these catalysts suffer from lower selectivity. Metal oxides such as Co3O4 and CeO2 are known to have high oxygen vacancy which promotes CO oxidation. Furthermore, the pre-treatment of the catalysts by hydrazine as well as the addition of MnOx species have been shown to improve the surface properties of metal/metal oxides catalysts. The study envisages that the modification of PROX catalysts will improve the CO conversion and its selectivity while maintaining higher stability. In this work, as-prepared (Co3O4) and hydrazine treated cobalt (Co3O4(H)) based catalysts were prepared by precipitation method and investigated at temperature range of 40-220 oC for preferential oxidation (PROX) of CO in excess hydrogen. The FTIR and XPS data of hydrazine treated Co3O4 does not show peak ratio differences, indicating that usual amounts of Co3+ and Co2+ were formed. An improved surface reducibility with smaller crystallite size was noted on Co3O4(H) catalyst, which indicate some surface transformation. Interestingly, the in-situ treatment of standalone Co3O4(H) decreased the maximum CO conversion temperature (T100%) from 160 oC (over Co3O4) to 100 oC. The Co3O4(H) catalyst showed good stability, with approximately 85% CO conversion at 100 oC for 21 h, as compared to fast deactivation of the Co3O4 catalyst. However, the Co3O4(H) catalyst was unstable in both CO2 and the moisture environment. Based on the spent hydrazine treated (CoO(H)) cobalt catalyst, the high PROX is associated with the formation of Co3+ species as confirmed by XRD, XPS, and TPR data. The Pd species was incorporated on different Co3O4 by improved wet impregnation method and this has improved the surface area of the overall catalysts. However, the presence of Pd species on Co3O4(H) catalyst decreased the CO conversion due to formation of moisture. Although, the Pd on Co3O4(H) had lower activity, the catalyst showed better stability under both moisture and CO2 conditions at 100 oC for 21 h. vi The 2wt.% metal oxides (MnO2, CeO2, Cr3O4, TiO2, MgO) on cobalt, and Pd on CeO2- Co3O4 and MnO2-Co3O4 were prepared by co-precipitation method and the structural composition was confirmed by XRD, FTIR, XPS and TPR data. Although, 2wt.%MnO2 on Co3O4(H) showed higher activity at 80 oC, both MnO2 and CeO2 improved the activity of Co3O4(H) at 100 oC. The higher activity of MnO2 is attributed to the higher surface area of the composite catalyst, in relation to ceria composite catalyst. Although the MnO2 species transformed the structure of Co3O4 by lowering the oxidation state to Co2+, the spent catalyst showed transformation from Co2+ to Co3+ during PROX, as confirmed by TPR data. Studies on the effects of CeO2 loading on Co3O4 catalysts, showed an optimum activity over 2wt.%CeO2-Co3O4 as compared to other ceria loadings (i.e., 3, 5, 8, 10, 15, 30wt.%CeO2). However, upon addition of 0.5wt.%Pd species on 2wt.%CeO2- Co3O4(H) composite, the activity of the catalyst decreased slightly at 100 oC, which could be due to a decreased surface area. Although its activity is lower, the catalyst has shown good stability in dry, moisture and CO2 conditions at 100 oC for 21 h. In addition, studies were also undertaken on the effect of MnO2 concentration on Co3O4 catalysts. The data shows that 7wt.%MnO2 species improved the activity of Co3O4 catalyst at 60 oC, however, the catalyst could not improve the activities at higher temperatures. This low activity is associated with a decrease in surface area as concentration increases. The presence of 0.5wt.%Pd species on 7wt.%MnO2-Co3O4 increased the activity at 60 and 80 oC, which could be due to reduction of Co3+ to Co2+ in the presence of Pd, as confirmed by XPS data. The catalyst has shown good stability in dry, moisture, and CO2 conditions at 100 oC for 21 h. The hydrazine treated cobalt-based catalysts in the presence of palladium and manganese oxide is the promising catalysts for proton exchange membrane fuel cells technology.