Ethanol Steam Reforming over Cobalt Based Catalysts: Roles of Zinc Oxide
Davidson, Stephen de Lemos
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The effects of zinc oxide (ZnO) addition to cobalt based catalysts was investigated over multiple supports. In each case the impacts on cobalt oxidation state and ESR activity were studied. In the case of cobalt supported on carbon (Co/C) it was found that ZnO addition inhibited reduction of cobalt oxides by H2 and created surface sites for H2O activation. At 250 °C cobalt reduction was minimal, in situ XANES demonstrated that ZnO addition significantly facilitated oxidation of Co0 under vapor phase reforming conditions, which was further demonstrated by lower C1 product yield. During vapor phase reforming at 450 °C the redox of cobalt, driven by steam oxidation and H2 reduction, trended to an equilibrium of Co0/Co2+. ZnO showed no significant effect on cobalt oxidation, inferred from the minor changes of C1 product yield. Surface sites created by ZnO addition enhanced water activation and oxidation of surface carbon species, increasing CO2 selectivity. In the case of cobalt supported on zirconia (Co/ZrO2) it was found that the addition of ZnO to cobalt supported on ZrO2 decreased the activity for H2O dissociation, leading to a lower degree of cobalt oxidation. The decreased H2O dissociation was also found to affect the reaction pathway, evidenced by a shift in liquid product selectivity away from acetone and towards acetaldehyde. The changes in H2O dissociation activity were further supported by in situ Raman spectroscopy, H2O-TPO, and pulsed H2O oxidation coupled with H2-TPR. Finally, for the case of cobalt supported on ceria (Co/CeO2) it was found that the addition of ZnO facilitates the oxidation of metallic Co0 via enhanced oxygen mobility of the CeO2 support which decreases the activity of Co/CeO2 in C–C bond cleavage of ethanol. 3 wt % ZnO promoted Co/CeO2 exhibits minimum CO and CH4 selectivity and maximum CO2 selectivity. Overall, it was found that ZnO addition can affect the activity for H2O dissociation and the oxidation stability of Co0. This creates a new control facet for more intelligent design of future cobalt based catalysts for ESR.