Evidence of redox cycling as a sub-mechanism in hydrogen production during ethanol steam reforming over La0.7Sr0.3MnO3-x perovskite oxide catalysts

Abstract

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 La0.7Sr0.3MnO3-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 La0.7Sr0.3MnO3-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 La0.7Sr0.3MnO3-x(1 0 0) and La0.7Sr0.3MnO3-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.