Methanol Steam Reforming on Ru/m-ZrO2: Sodium Promotion of the CO2-Forming Pathway

Sodium (Na) promotion of Ru/m-ZrO2 was investigated to elucidate how an alkali modification tunes selectivity in methanol steam reforming (MSR). H2-TPR/XANES/EXAFS show that Na increases surface basicity and strengthens Ru–O interactions, shifting RuOx reduction and H2 spillover to a higher temperature. DRIFTS reveals Na-induced red shifts of the formate ν(CH) band and changes in OCO vibrational splitting, consistent with weakening of the formate C–H bond and an altered binding geometry. CO2-TPD confirms a monotonic shift toward stronger basic sites with increasing Na concentrations. Under MSR conditions, Na selectively increases CO2 concentration at the expense of CO. At ~80% conversion and 325 °C, CO2 selectivity increases from 12.0% (unpromoted) to 16.2, 21.0, and 26.5% for 0.5, 1.0, and 1.8% Na, respectively; at ~300 °C and ~66–69% conversion, CO2 selectivity increases from 8.6% to 23.7% at 1.8% Na. Transient MSR experiments further show earlier and larger H2 evolution upon Na addition, corroborating the promotion of the dehydrogenation/decarbonylation route to CO2 + H2. We propose that Na increases basicity and modifies the Ru–support interface to favor formate dehydrogenation/decarboxylation, thereby increasing the H2 yield and lowering CO formation. Ru’s higher-energy, less occupied d-band stabilizes CO and oxygenated intermediates more strongly in the reforming environment, making the CO-forming pathway more resistant to suppression than on Pt.