Catalytic methane activation over La1−xSrxScO3−α proton-conducting oxide surface: A comprehensive study
Graphical abstract
Introduction
The growing interest in the chemical industry to the development of highly efficient technologies for hydrocarbon reforming has instigated the search for new catalytic materials. The technologies of light hydrocarbon reforming, such as steam reforming, dry reforming and partial oxidation have many advantages e.g. lower temperature of reaction, high efficiency of conversion and high rates [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. However, the strict catalytic requirements i.e. a high catalytic activity to the reaction of the light hydrocarbon conversion a high stability in the reductive condition, and a high tolerance to the carbon coke formation and gas impurities, limit the commercial application of these technologies.
Recently, new types of materials named proton-conducting oxides e.g. perovskites based on cerates, zirconates and titanates [[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]], have drawn a lot of attention due to their possible application in proton ceramic fuel cell electrochemical devices.
The proton ceramic fuel cell (PCFC) is the sort of solid oxide fuel cell (SOFC) device that transforms the energy of the chemical reaction between oxygen and hydrogen in the gas phase to electrical power [[36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]]. The main advantages of these devices are low temperatures of operation, high efficiency, and the possibility of direct hydrocarbon use as hydrogen containing gases [[36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]]. The major feature of PCFCs, amongst other solid oxide fuel cells, is the presence of a proton-conducting oxide membrane as the main component of the cell. This allows hydrogen to permeate through the electrolyte towards the cathode side and react with oxygen from the gas phase. Due to this feature, it is easier to reduce the operating temperature of PCFC devices compared to other SOFCs, which is one of the major problems that have prevented the commercial application of SOFCs thus far.
Among other works related to PCFC devices, recent research [[45], [46]] has demonstrated PCFCs based on La0.9Sr0.1ScO3 proton-conducting electrolyte and shown that the PCFC device is a promising way of SOFC development.
Amongst various types of proton-conducting oxide materials, the most extensive studies have been carried out with ABO3 perovskite materials based on doped BaZrO3, BaCeO3, SrCeO3 [[23], [24], [25], [26], [27], [28], [29], [30], [31], [32]]. Up-to-date the majority of the studies with these materials have been conducted using water containing atmospheres and they have shown that proton-conducting oxides were able to incorporate hydrogen from the water vapour [[23], [24], [25], [26], [27], [28], [29], [30], [31]],[32] according to reaction (1):
H. Malerød-Fjeld and coworkers [47], reported the results of catalytic methane conversion on a BaZrO3 based proton-conducting electrolyte, showing that proton-conducting oxides are promising materials for methane reforming devices.
Apart from water there are other hydrogen containing gases such as H2, H2S, NH3, and CH4 as well as other hydrocarbons. To date, the process of hydrogen incorporation from these atmospheres has been poorly studied.
The first experiments with dry molecular hydrogen were carried out using proton-conducting oxide materials La1−xSrxScO3−α.These oxides showed the greatest stability in a highly reductive atmosphere [[48], [49], [50], [51], [52], [53], [54], [55], [56], [57]] and acceptable values of proton conduction amongst other perovskite-based proton conducting oxides. A. R. Farlenkov and coworkers [54], authors showed that La1−xSrxScO3−α oxides can incorporate hydrogen from water vapour, as was previously found in the BaZrO3-BaCeO3 system. These properties make it favourable to apply these materials in PCFCs and proton ceramic electrolyser devices, as was recently reported [45,46].
Experiments with dry molecular hydrogen showed that strontium doped lanthanum scandates could incorporate hydrogen directly from molecular hydrogen in the gas phase [[55], [56], [58]],[]. In the case of molecular hydrogen, the incorporation process can be described with reaction (2):
Despite extensive studies of strontium doped lanthanum scandates, the behaviour of these oxides in a dry reductive methane atmosphere is still unclear. In a previous study [59] we firstly reported the methane activation process over the surface of La0.95Sr0.05ScO3−α and suggested a two-step methane activation mechanism with two subsequent elementary steps i.e. dissociative adsorption of methane with the formation of a methyl-type adsorbed form and a hydrogen adatom, and the subsequent incorporation of hydrogen into the oxide structure. This process can be represented by eqs. (3), (4):
However, the nature of the active sites, as well as the influence of strontium and the vacancy concentration on the methane activation mechanism, still needs to be clarified. Moreover, aside from steps (3), (4), there are other possible steps that should be considered e.g.:
In fact, steps (5), (6), (7) can be written as (3) if the process of dissociation is fast, and step (4) can represent the direct incorporation of hydrogen from methane if the incorporation is also a fast process.
Thus, this paper aims at studying the influence of the strontium content on the mechanism of methane activation over the surface of La1−xSrxScO3−α , with respect to the different channels of hydrogen incorporation into the structure by means of a H/D isotopic exchange experiment between methane in the gas phase and a proton-conducting oxide, and the determination of the active sites and the nature of the adsorbed species over the surface of La1−xSrxScO3−α with Raman Spectroscopy, 1H NMR spectroscopy, and DFT modelling methods.
Section snippets
Materials and methods
The La1−xSrxScO3−α (with × = 0.02, 0.05, 0.1) materials were prepared by combustion synthesis with ethylene glycol as fuel. The stoichiometric amounts of La2O3, Sc2O3, SrCO3 (high purity grade) were dissolved in a solution containing the required amount of nitric acid (reagent grade). After the dissolution of all the precursors was completed, ethylene glycol (reagent grade) was added and the mixture evaporated prior to the combustion reaction. The ratio of LSS oxide mass/nitric acid
The two-step methane activation mechanism
In our previous work [59], we suggested a two-step mechanism of methane activation: at the first step a methane molecule dissociatively adsorbs at the oxide surface forming a surface methyl-like adsorbed form. At the second step, the hydrogen atoms from the methane adsorbed form undergo an exchange with incorporated hydrogen from the oxide. These elementary steps can be represented with the eqs:
where X is a hydrogen isotope (H or D); /
Sample characterisation
The results of XRD showed that all the studied samples were single phase and crystallised in an orthorhombic structure with the space group Pnma (see Fig. 6). The microstructural analysis revealed that all the samples have an average particle size about 0.2 μm (see Fig. 7). The BET measurements indicated that the La1−xSrxScO3−α (x = 0.02, 0.05, 0.1) specific surface area was 6.05 – 7.50 m2/g.
H/D isotopic exchange in the system “methane – oxide” experiment results
The results of the H/D isotopic exchange in the system “methane – oxide” experiment with samples La1−xSrx
Conclusions
The methane activation process was investigated over La1−xSrxScO3−α (x = 0.02, 0.05, 0.1) oxide surface with H/D isotopic exchange in the system “methane – oxide” with a gas phase composition equilibration method. The kinetics of the methane activation was described by the five types of exchange model, the two-step mechanism, and the two-step mechanism with parallel channels of hydrogen adsorption and incorporation. The increase of strontium content causes an increase of the isotopic exchange
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 work was supported by the Russian Science Foundation under grant No. 16-13-00053 using instruments of Shared Access Center «Composition of Compounds» and Unique Scientific Setup «Isotopic Exchange». The DFT calculations were supported by the Ministry of Science and Higher Education of the Russian Federation (theme “Electron” No. AAAA-A18-118020190098-5). The authors are grateful to A. V. Khodimchuk for the XRD analysis and Dr. N.M. Porotnikova for BET measurements.
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