Full Length ArticleEvidence of redox cycling as a sub-mechanism in hydrogen production during ethanol steam reforming over La0.7Sr0.3MnO3-x perovskite oxide catalysts
Graphical abstract
Introduction
Ethanol Steam Reforming (ESR) can be utilized to produce acetone and hydrogen,[1], [2], [3], [4], [5] which are both of societal interest. Acetone is used industrially,[6], [7] and hydrogen is a potential energy carrier for clean and sustainable energy infrastructure. Today, acetone[6], [7] and hydrogen[8] are mainly produced from fossil feedstocks, which is not sustainable. Ethanol is an attractive feedstock candidate because of its potentially renewable production.[9], [10] ESR has the potential for large-scale hydrogen production.[10] Reducible oxides, including perovskites and cerium oxides, can be used as catalysts for ESR.[11], [12] We and others have studied the mechanism for ESR over reducible oxides. In the present study, we present evidence that redox cycling, with ethanol ‘directly’ reducing the surface, followed by H2O filling oxygen vacancies created during steam reforming to create H2, is part of the chemistry that occurs in steam reforming over reducible oxides. As will be described in the background and discussion sections, the existence of this sub-mechanism during alcohol steam reforming over reducible oxides is not presently widely recognized, and is not commonly proposed in alcohol steam reforming studies.
Water filling oxygen vacancies to produce hydrogen is known to occur over reducible oxides, as seen during studies of water splitting over metal oxides through thermochemical cycling.[13] Most attempts to produce hydrogen from water with thermochemical cycling focus on the use of partially reducible oxides, and for these the re-oxidation by water generally occurs at temperatures above 800 K.[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34] Cerium oxide is one of the reducible oxides that has often been looked at for water splitting.[13], [15], [20], [21], [24], [26], [27], [31] Lanthanide based perovskites have also shown promise for redox cycling reactions.[35] In addition to thermochemical cycling, isothermal processes for water splitting over reducible oxides have also been considered for efficiency reasons, though generally at over 800 K.[23], [28], [36] Water splitting during isothermal ESR would be expected to have analogous behavior.
La0.7Sr0.3MnO3-x catalysts (which shares some chemistry with cerium oxide[37], [38], [39], [40] for conversion of organic oxygenates) have been shown to be able to catalyze ESR.[37] This ‘metal free’ steam reforming should be distinguished from the most common cases of ESR in the literature, where hydrogen production often has Rh or Ni present in the catalysts on supports such as ZnO, CeO2, and La2O3.[10], [41] In the present study, the catalyst does not include transition metals such as Ni, Rh, or Co. Thus, the H2 must be formed from the oxide catalyst, and it is conceivable that some of the H2 could come from the same mechanism as thermochemical cycling with H2O filling vacancies.
As explained in Section 2, earlier studies have not proposed that a redox cycle could occur by ‘direct’ reduction of the surface with ethanol (rather than by a gas phase intermediate such as CO, produced downstream from ethanol). Further, only a few studies have proposed that H2 from ethanol steam reforming may be partially from H2O filling oxygen vacancies at the oxide surface. In this study, we sought to answer the question as to whether a full redox cycle could be completed where (a) ethanol could ‘directly’ reduce the surface without going through a CO intermediate, and (b) whether H2O filling these oxygen vacancies to make hydrogen would then occur. If both half-cycles are shown to occur at < 800 K, this indicates that such a Mars van Krevelen type sub-mechanism may contribute to H2 production during steam reforming of ethanol over reducible oxides. La0.7Sr0.3MnO3-x was chosen for study because high surface La0.7Sr0.3MnO3-x powder as well as a La0.7Sr0.3MnO3-x(1 0 0) single crystal sample could be studied. The (1 0 0) face is the most thermodynamically stable face, thus should dominate the powder surfaces, and our earlier studies have shown that the chemistry on the single crystal sample surface did in fact match the chemistry of the powder surface.[37], [38] Thus, we sought to use the single crystal sample with ultra-high vacuum methods to study the redox cycling behavior, as well as experiments with simultaneous reactant exposures, and finally to compare the data to that over powders.
We first investigated several cycles of exposing the reductant (ethanol) followed by the oxidant (water) to a La0.7Sr0.3MnO3-x(1 0 0) single crystal sample, with studies under ultra-high vacuum conditions to ensure ‘direct’ reduction of the surface by ethanol. Hydrogen production from water was observed during this redox cycling, below 800 K. We then looked at co-feeding of ethanol and water over the La0.7Sr0.3MnO3-x(1 0 0) single crystal sample in the temperature range of 400–800 K. The La0.7Sr0.3MnO3-x(1 0 0) data was then compared to the data collected from catalytic conversion over La0.7Sr0.3MnO3-x powders at 400–800 K under atmospheric pressure flow experiments. Comparing the reaction data from the single crystal samples under ultra-high vacuum conditions and the reaction data from the powder sample at atmospheric pressure conditions, the results are consistent with ESR involving a step of water filling oxygen vacancies in a redox reaction that partially contributes to the H2 production. This creating and filling of oxygen vacancies within a catalytic cycle is considered a Mars van Krevelen type mechanism.
Section snippets
Mechanistic background
In steam reforming of alcohols, the goal is often to convert the reactants to solely H2 and CO2. In practice, thermodynamics and catalytic performance both make the conversion incomplete, and the carbon ends up distributed amongst CO, CO2, and other molecules including the alcohol.[42], [43] Various other gas products can be produced either as intermediates or alternate gas products, such as alkenes and acetone.[37], [38] In the ESR for this study, alkenes are not the desired product, though
Methods
La0.7Sr0.3MnO3-x(1 0 0) thin films with an average thickness of 20 nm were grown on 0.05 % Nb-doped SrTiO3(1 0 0) crystal via pulsed laser deposition (PLD) at the Oak Ridge National Laboratory Center for Nanophase Materials Sciences through a user proposal. Details about PLD deposition conditions and film quality characterization can be found in the recent publication from our group.[40].
Surface chemistry experiments were performed in an ultra-high vacuum chamber, as described previously.[86] The
Investigation of redox cycling on La0.7Sr0.3MnO3-x(1 0 0) film
Reducible oxides can undergo redox cycling by exposure to a reductant and then an oxidant at elevated temperatures. During the reduction half-cycle, oxygen is removed to make oxygen vacancies. During the oxidation half-cycle, oxygen is re-introduced into the vacancies by the oxidant. For our redox cycling, sequential cycles of ethanol gas flux (reduction) and water gas flux (oxidation) over La0.7Sr0.3MnO3-x(1 0 0) surface were investigated. Before and after each water exposure, the relative
Conclusions
In this study, we considered the hypothesis that H2 production is partially from H2O filling oxygen vacancies with H2 as a co-product during steam reforming of ethanol over reducible oxides. La0.7Sr0.3MnO3-x was chosen for study because a La0.7Sr0.3MnO3-x(1 0 0) single crystal sample could be studied as well as a high surface area La0.7Sr0.3MnO3-x powder. The single crystal sample was subjected to several redox cycles by exposing the reductant (ethanol) followed by the oxidant (water) to the
Notice
This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally
Data availability statement
Data may be extracted from the images in the figures and may be provided upon request.
Funding
This research was sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science program.
CRediT authorship contribution statement
Bo Chen: Conceptualization, Methodology, Formal analysis, Data curation, Writing – original draft. Shane Rickard: . Zhenghong Bao: . Zili Wu: Supervision, Project administration, Funding acquisition. Michelle K. Kidder: Methodology. Aditya Savara: Conceptualization, Methodology, Formal analysis, Data curation, Writing – original draft, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science program. Thin film growth and characterization research conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.
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Current Affiliation: Department of Pathology, Stony Brook University School of Medicine, Stony Brook, NY 11794-8691, United States.