Environment-Dependent Catalytic Performance and Phase Stability of Co₃O₄in the Preferential Oxidation of Carbon Monoxide Studied<i>In Situ</i>

Abstract

The preferential oxidation of CO (CO-PrOx) with co-fed O2 is an attractive route for removing trace amounts of CO in the H2-rich reformate gas prior to being used for power generation in proton-exchange membrane fuel cells. Despite being an affordable and very promising catalyst for CO-PrOx, the CO oxidation activity, selectivity, and phase stability of cobalt(II,III) oxide (Co3O4) may be affected by the other gas feed components, viz., H2, H2O, and CO2. However, the influence of these gases (individually and collectively) during CO-PrOx is still not understood. In the current study, in situ powder X-ray diffraction and magnetometry were used to systematically evaluate unsupported Co3O4 nanoparticles under different CO-PrOx environments, that is, with/without H2, H2O, and/or CO2. The presence of H2 in the feed (with no H2O and CO2) results in the unwanted competitive conversion of the co-fed O2 to H2O and in the reduction of Co3O4 to Co0. The presence of Co0 favors CH4 formation from CO, which is also undesired as it consumes valuable H2. Co-feeding H2O not only decreases Co3O4 reducibility but also decreases CO oxidation activity through competitive surface adsorption. The forward water−gas shift is possible in the absence of CO2 in the feed over CoO and Co0 at elevated reaction temperatures. Co-feeding CO2 has no effect on Co3O4 reducibility but leads to the unwanted CO2 methanation and reverse water−gas shift over Co0. For the first time, the presented work shows the individual and combined effect of H2, H2O, and CO2 on the progress of the targeted CO oxidation process through the occurrence of undesired side reactions. Also, for the first time, in situ catalyst characterization reveals the Co-based phase(s) responsible for the occurrence of each identified side reaction.

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