Evidence of redox cycling as a sub-mechanism in hydrogen production during ethanol steam reforming over La_0.7Sr_0.3MnO_3-x perovskite oxide catalysts

Ethanol steam reforming (ESR) is of societal interest. In this work, experiments were conducted to ascertain if some of the H₂ is produced by a redox cycle involving H₂O filling oxygen vacancies over reducible oxide catalysts. Redox cycling experiments were performed over La_0.7Sr_0.3MnO_3-x(1 0 0) in ultra-high vacuum. It was found that H₂ was produced from redox cycling with alternating ethanol and water exposures over La_0.7Sr_0.3MnO_3-x(1 0 0), with both half-cycles occurring at temperatures ≤800 K. In the first half-cycle, ethanol ‘directly’ reduced the surface to create oxygen vacancies (not by a CO intermediate), and in the second half-cycle water filled oxygen vacancies to make H₂. The H₂ production during the water exposure has a half-cycle turnover frequency of >3.2 × 10^-2 molecules site^-1 s⁻¹ in the temperature range of 700–800 K, which is fast enough to be part of the ESR full catalytic cycle. Flowing both reactant gases together, ethanol and water, over La_0.7Sr_0.3MnO_3-x(1 0 0) and La_0.7Sr_0.3MnO_3-x powders significantly increases hydrogen production compared to pure ethanol. The results suggest that steady state ESR includes a sub-mechanism of ethanol ‘directly’ reducing the surface to create oxygen vacancy, and water filling oxygen vacancy to make some of the H₂ by a Mars van Krevelen type mechanism.