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

2022; Elsevier BV; Volume: 650; Linguagem: Inglês

10.1016/j.apcata.2022.119000

1873-3875

Grant Seuser, Michela Martinelli, Elijah S. Garcia, Gabriel F. Upton, Martin Ayala, Jesus Villarreal, Zahra Rajabi, Donald C. Cronauer, A. Jeremy Kropf, Gary Jacobs,

Ammonia Synthesis and Nitrogen Reduction

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 rCO/rCH4. 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).