Tuning Methanol Transformation Pathways for Sustainable Steam Reforming: Na-Promotion Effects on Ag/m-ZrO2 Catalysts

This work investigates the influence of sodium promotion on Ag/m-ZrO2 catalysts for methanol steam reforming (MSR), focusing on activity, selectivity, surface chemistry, and mechanistic pathways. Temperature programmed reduction (TPR), XANES/EXAFS, CO2 TPD, DRIFTS, and temperature programmed surface reaction methods were combined with fixed bed MSR testing to develop an integrated structure–function understanding of Na-modified Ag-ZrO2 interfaces. Na addition systematically increases surface basicity, stabilizes strongly basic O2− sites, and weakens the ν(CH) vibrational mode of surface formate, thereby facilitating C–H bond scission and accelerating decarboxylation to CO2. At moderate promoter levels (0.5–1.0 wt.% Na), the catalysts show significantly enhanced CO2 selectivity and increased conversion relative to unpromoted Ag/m-ZrO2, while CH4 formation remains negligible. Excessive Na (≥1.8 wt.%) leads to slower formate decomposition, greater carbonate stabilization, and suppressed conversion, revealing a narrow optimum around 1 wt.% Na. Short-term stability testing demonstrates steady conversion and product selectivity for both unpromoted and Na-promoted catalysts, with the latter maintaining markedly higher CO2 selectivity. Although Pt/YSZ retains far superior intrinsic activity at ~10× higher space velocity, Ag offers a cost-advantaged alternative where lower cost metals are desirable. Collectively, these findings show that Na promotion enables tunable MSR selectivity on Ag/m-ZrO2 by directing formate decomposition toward the CO2-forming pathway.