(528g) Different Dependency on Copper Oxidation States between H2 and CO2 Production during Partial Oxidation of Methanol

The increasing impact of climate change and pollution necessitates the need to replace our current fossil fuel based energy supply with cleaner energy production methods, such as fuel cells. Methanol is of particular interest due to its relatively high energy density and safe handling. Methanol can be directly or inderectly oxidized to power fuel cells. The partial oxidation of methanol (POM, CH₃OH + 0.5O₂ → CO₂ + 2H₂) catalyzed by a copper based catalyst can provide on-board generation of high purity H₂ streams. Although copper-containing catalysts has been invesigated for decades, the reaction mechanism and active sites are still poorly understood. In particular, the chemical nature of the active phase (Cu⁰, Cu⁺ or Cu²⁺) is unclear and their impacts on activity and selectivity of POM are unknown.

Results from the study of partial oxidation of methanol (POM) catalyzed by Cu/ZnO were presented here with the objective to identify correlations between POM reactivity and Cu oxidation state. A 30wt% Cu/ZnO nanoparticle catalyst was prepared by a co-precipitation synthesis. Catalyst morphology and elemental distribution on nanoscale were determined by transmission electron microscopy (TEM). The catalytic performance was measured at different O₂ /methanol molar ratios in a home-built micro-reactor. The Cu oxidation state was evaluated at different time-points via ex-situ X-ray photoelectron spectroscopy (XPS). We found that both reactivity for POM and the oxidation state of copper change with reaction time and with O₂ to methanol feed ratio. Most importantly, a strong correlation between H₂ selectivity and (metallic) Cu⁰ content of the catalyst was observed. Surprisingly, the CO₂ selectivity was not significantly impacted by the oxidation state of the catalyst, but showed a strong correlation with the O₂ partial pressure. Based on the observed correlations, we propose a mechanism for POM including different bonding configurations of reaction intermediates between metallic Cu and Cu₂O surfaces. We are currently in the process of verifying key reaction steps by first—principle calculations. The knowledge we gain from this study will benefit the optimization of current Cu-based catalysts which may lead us to a promising methanol based energy economy.