Abstract
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.
Highlights
Heterogeneous catalysts commonly comprise metal or metal oxide nanoparticles anchored on mechanically and thermally stable carriers referred to as supports.[1]
CO-PrOx is a promising final step for the removal of trace amounts of CO in H2-rich streams before being fed into proton exchange membrane fuel cells (PEMFCs) for power generation, as the CO poisons the Pt-based anode catalyst of PEMFCs.[10−12] We were able to show that Co3O4 reduces to CoO and to metallic Co at high reaction temperatures and that the reduction is influenced by crystallite size
The present study has addressed the challenges faced when using the transition-metal oxide Co3O4, regarding its catalytic performance and phase stability under the reducing environment of CO-PrOx
Summary
Heterogeneous catalysts commonly comprise metal or metal oxide nanoparticles anchored on mechanically and thermally stable carriers referred to as supports.[1]. We have previously conducted an in situ study investigating the effect of the crystallite size of Al2O3-supported Co3O4 nanoparticles on the preferential oxidation of CO (CO-PrOx) in a H2-rich gas mixture.[9] CO-PrOx is a promising final step for the removal of trace amounts of CO in H2-rich streams (e.g., originating from the consecutive CH4 steam reforming and the water−gas shift processes) before being fed into proton exchange membrane fuel cells (PEMFCs) for power generation, as the CO poisons the Pt-based anode catalyst of PEMFCs.[10−12] We were able to show that Co3O4 reduces to CoO and to metallic Co at high reaction temperatures and that the reduction is influenced by crystallite size This catalyst phase change proved to be unfavorable as less CO2 was formed, and instead, CH4 was produced due to the presence of the metallic Co. Co3O4 or the Co3+−Co2+ redox pair is believed to play an important role in the oxidation of CO.[13−16]. Quasi-in-situ X-ray photoelectron spectroscopy (XPS) was carried out to study the nature of the surface of each catalyst at selected reaction temperatures
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