Reverse water-gas shift: Na doping of m-ZrO₂ supported Pt for selectivity control

Reverse water-gas shift (RWGS) is a vital step in producing syngas for the chemical conversion of CO₂ to liquid transportation fuels and chemicals. Na-doping of m-ZrO₂ supported Pt catalysts allowed selectivity control by systematically increasing the ratio of relative rates of r_CO/r_CH4. This was achieved by facilitating the formation of formate intermediate species, which precedes CO formation, and by suppressing the metallic Pt⁰ active sites responsible for CH₄ formation. A 2.5%Na-2%Pt/m-ZrO₂ catalyst was first tested for the forward water-gas shift (FWGS) reaction and found to have 50% higher CO conversion at 285 °C compared to the undoped catalyst. Results of DRIFTS spectroscopy of adsorbed CO confirmed a formate ν(CH) band shift to lower wavenumbers (2870–2802 cm⁻¹) with the addition of Na and more rapid forward formate decomposition in steam to H₂ and carbonate species, the precursor to CO₂. This is consistent with C-H bond breaking being the rate limiting step of a FWGS mechanism occurring at the metal-support junction. Consistent with this, DRIFTS of RWGS in 4%CO₂ + 60%H₂ showed more facile formation of formate for the Na-doped catalyst and, once again, the ν(CH) band was shifted to lower wavenumbers (2874–2803 cm⁻¹) with Na-doping. In addition, Na doping resulted in a systematic decrease in the Pt-carbonyl band in DRIFTS of adsorbed CO as well as DRIFTS of in-situ RWGS reaction tests, suggesting that Na blocked a fraction of on-top Pt sites, breaking up ensembles of Pt⁰ responsible for methanation. The selectivity of the 2.5%Na-doped catalyst, unlike its undoped counterpart, was remarkably resistant to methanation (e.g., selectivity < 0.2% CH₄ with pressures of up to 20 bar).