Different supporting procedures were followed to alter the nanoparticle–support interactions (NPSI) in two Co3O4/Al2O3 catalysts, prepared using the reverse micelle technique. The catalysts were tested in the dry preferential oxidation of carbon monoxide (CO-PrOx) while their phase stability was monitored using four complementary in situ techniques, viz., magnet-based characterization, PXRD, and combined XAS/DRIFTS, as well as quasi in situ XPS, respectively. The catalyst with weak NPSI achieved higher CO2 yields and selectivities at temperatures below 225 °C compared to the sample with strong NPSI. However, relatively high degrees of reduction of Co3O4 to metallic Co were reached between 250 and 350 °C for the same catalyst. The presence of metallic Co led to the undesired formation of CH4, reaching a yield of over 90% above 300 °C. The catalyst with strong NPSI formed very low amounts of metallic Co (less than 1%) and CH4 (yield of up to 20%) even at 350 °C. When the temperature was decreased from 350 to 50 °C under the reaction gas, both catalysts were slightly reoxidized and gradually regained their CO oxidation activity, while the formation of CH4 diminished. The present study shows a strong relationship between catalyst performance (i.e., activity and selectivity) and phase stability, both of which are affected by the strength of the NPSI. When using a metal oxide as the active CO-PrOx catalyst, it is important for it to have significant reduction resistance to avoid the formation of undesired products, e.g., CH4. However, the metal oxide should also be reducible (especially on the surface) to allow for a complete conversion of CO to CO2 via the Mars–van Krevelen mechanism.

The preferential oxidation of carbon monoxide has been identified as an effective route to remove trace amounts of CO (approx. 0.5-1.0 vol%) in the H-rich reformate gas stream after the low-temperature water-gas shift. Instead of noble metal-based catalysts, CoO-based catalysts were investigated in this study as cheaper and more readily available alternatives. This study aimed at investigating the effect of crystallite size on the mass- and surface area-specific CO oxidation activity as well as on the reduction behaviour of CoO. Model CoO catalysts with average crystallite sizes between 3 and 15 nm were synthesised using the reverse micelle technique. Results from the catalytic tests revealed that decreasing the size of the CoO crystallites increased the mass-specific CO oxidation activity in the 50-200 °C temperature range. On the other hand, the surface area-specific CO oxidation activity displayed a volcano-type behaviour where crystallites with an average size of 8.5 nm were the most active within the same temperature range. In situ characterisation in the magnetometer revealed that the CoO crystallites are partially reduced to metallic Co above 225 °C with crystallites larger than 7.5 nm showing higher degrees of reduction under the H-rich environment of CO-PrOx. In situ PXRD experiments further showed the presence of CoO concurrently with metallic fcc Co in all the catalysts during the CO-PrOx runs. In all experiments, the formation of fcc Co coincided with the formation of CH. Upon decreasing the reaction temperature below 250 °C under the reaction gas, both in situ techniques revealed that the fcc Co previously formed is partially re-oxidised to CoO.

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.

Different morphologies of Co3O4 were synthesized and tested for their performance in the preferential oxidation (PrOx) of carbon monoxide to investigate the effect of preferentially exposed crystal planes.

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