Methanol Oxidation over Pd-Doped Co- and/or Ag-Based Catalysts: Effect of Impurities (H2O and CO)

The methanol oxidation reaction was investigated on Co- and/or Ag-based γ-Al2O3 catalysts, which were prepared by different methods (WI: wet impregnation and SI: spray impregnation) and further doped with noble metals (Pd, Pt). During the present study, three different reaction pathways were revealed. The complete oxidation of methanol to CO2 and H2O was achieved on Pd-doped catalysts prepared by the spray impregnation method (Pd-Co/Al-SI and Pd-Ag/Al-SI), while partial oxidation to intermediates such as formaldehyde was observed for Ag/alumina catalysts. The dehydration reaction of methanol to dimethyl ether was carried out on Co/alumina, Ag-Co/alumina, and Pt-Co/alumina catalysts. The improved reducibility of the 5Co/Al-SI catalyst with the incorporation of Pd, combined with the easier surface oxygen desorption, resulted in higher catalytic activity compared to the Pt-doped catalyst. On the other hand, the incorporation of Pd into Ag/Al-SI enhanced the well-dispersed Ag2O species, mainly affecting the structural properties of the catalyst, thus resulting in partial oxidation of methanol. The 0.5 wt.% Pd-5 wt.% Co/γ-Al2O3 catalyst, prepared by the spray impregnation method, exhibited the highest methanol oxidation efficiency (T50: 43 °C) and was further evaluated in the presence of H2O and CO in the feed for several hours on stream and at reaction temperature of 230 °C. The presence of impurities initially reduced the catalyst’s activity from 100% methanol conversion (in the absence of H2O and CO in the feed) to 80%; however, over time complete methanol oxidation was regained (achieving again 100% methanol conversion after 12 h on stream). Characterization of the used catalyst (after the stability experiment) revealed that in addition to the Co3O4 phase, initially formed in the fresh, as-prepared catalyst, some Co3O4 species were reduced to CoO under the reaction conditions, suggesting that the active phase of the 0.5Pd-5Co/Al-SI catalyst for the methanol oxidation reaction in the presence of the impurities (such as H2O and CO) is probably a mixture of Co3O4 and CoO phases.