Research PaperActive and durable alkaline earth metal substituted perovskite catalysts for dry reforming of methane
Graphical abstract
Introduction
The world is preparing to overcome the possible effects of global warming attributed to green house gas (GHG) emissions, particularly carbon dioxide emissions, as its level has crossed the 400 ppm level [1]. Hence, numerous initiatives are being taken for the development of CO2 capture and utilization technologies that can help either to cap or reduce its level in the atmosphere. Similarly, CH4 being another GHG; its release to the atmosphere is also a cause of concern. In recent times, various research programmes are proposed or initiated to utilize both these green house gases to obtain fuels and chemicals. Dry reforming of methane is one such programme that utilizes both CO2 and CH4 to give synthesis gas (syngas). Syngas is an important input for many chemicals and fuels. In dry reforming reaction, H2/CO ratio of product syngas is low (≈1) and hence suitable for production of long chain hydrocarbons through Fischer-Tropsch (FT) synthesis [2], [3].CH4 + CO2 ↔ 2CO + 2H2 (ΔH°298K = +247 kJ/mol)
In addition to main dry reforming reaction, reverse water gas shift reaction is also dominant under these conditions;CO2 + H2 ↔ CO + H2O (ΔH°298K = +40.6 kJ/mol)
However, to practice dry reforming commercially, the process faces some major challenges. Being highly endothermic reaction, it has to be conducted at high temperatures. As a consequence, catalysts are deactivated due to active metal sintering and coke formation [4]. Numerous investigations attempted to address these twin problems, mostly by using supported precious metal catalysts [5], [6]. However, use of supported precious metal catalysts commercially involves high costs. Hence, nickel based catalysts are the best alternative, due to their comparable activity in dry reforming, abundant availability and relatively lower price. But, Ni based catalysts are more prone to deactivation due to carbon deposition and metal sintering. There were many reports on understanding of coke formation on Ni catalysts and how to fortify them to resist coke formation, inorder to get durable catalysts for dry reforming reaction [7].
The dry reforming reaction proceeds through CH4 decomposition followed by oxidation of the carbon species. This mechanism requires bi-functional metal supported catalysts that can effectively catalyze CH4 cracking and simultaneous removal of carbon species. In order to meet these requirements, high metal dispersion and resistance to metal sintering are essential, while redox property of the support helps to oxidize the carbon formed during the reforming process. Structured oxides like perovskites and hydrotalcites have been investigated for this reaction [8]. It is also reported that the metal catalysts can be promoted by using supports that consist of basic oxides/redox oxides [9], [10]. Bimetallic systems were also explored for improved activity and long term durability [11]. Addition of basic oxides to the support is expected to enhance the CO2 activation and also helps in gasification of deposited carbon on the active sites [12]. In general, ZrO2 is considered as a good support, because of its higher thermal stability and unique chemical properties like redox and acid-base functionality [13]. There were several reports dealing with Ni supported on ZrO2, but most of them deactivated due to coke formation and sintering. Addition of promoters like CaO, MgO and CeO2 is reported to help improve the catalyst stability [14].
Perovskite type oxides have a well defined structure and they can facilitate high metal dispersion, even on subjecting to severe oxidation-reduction processes at high temperatures [15]. Hence, efforts were made to synthesize catalysts that have active metals (transition metals like Ni or precious metals) incorporated in to the perovskite lattice and use them for DRM. In addition, substitution of Ca, Sr and Ba in the A site of perovskite lattice is expected to improve and help stabilize dry reforming activity of the catalyst. In this report, Ni is substituted in MZrO3 (M = Ca, Sr and Ba) family of perovskite type oxides and their activity and durability was investigated for dry reforming (CH4 + CO2) reaction to obtain syngas. The perovskite oxides were synthesized using citrate gel method and characterized using XRD, Raman spectroscopy, BET surface area, CO chemisorption, CO2-temperature programmed desorption (CO2-TPD), O2 −temperature programmed desorption (O2-TPD), temperature programmed reduction (TPR), transient pulse technique and in-situ FTIR. Among the catalysts studied, Ca substituted perovskites were found to be more active in terms of CH4 and CO2 conversions and the activity was extraordinarily stable even after long hours (500 h) on stream. The carbon formed on these catalysts was investigated by XRD, TEM, TGA, XPS and Raman spectroscopy.
Section snippets
Preparation of the catalysts
The MZr1-xNixO3-δ (M = Ca, Sr and Ba; x = 0 and 0.2) perovskite type oxides were prepared by conventional citrate gel method. Stoichiometric quantities of the corresponding metal nitrates were dissolved in minimum required water and added drop wise to the citric acid solution under constant stirring at 353 K. Following complexation, the solution was evaporated and dried at 453 K for 12 h to obtain spongy amorphous citrate gel. This fluffy material was crushed and calcined at 1023 K for 6 h in air flow
Textural and structural characterization of catalysts
The BET surface areas and active metal (Ni) dispersions obtained through CO chemisorption of MZr1-xNixO3-δ (M = Ca, Sr and Ba; x = 0 and 0.2) perovskite type catalysts has been listed in Table 1. Specific surface area of the MZrO3 perovskite type oxides has increased with variation in alkaline earth cation down the group from Ca to Ba substituted oxides, with similar trend being observed even in case of Ni substituted samples. As a result, BET surface areas followed the order CaZr0.8Ni0.2O3-δ < SrZr
Conclusions
Perovskite oxides MZr1-xNixO3-δ (M = Ca, Sr and Ba; x = 0 and 0.2) were synthesized and evaluated as catalyst precursors in the dry reforming of methane. Changes in the lattice parameters, determined by XRD, confirmed that Ni is incorporated into MZrO3 lattice. TPR of H2 reveals that Ni in CaZr0.8Ni0.2O3-δ perovskites was difficult to reduce as compared to corresponding Sr and Ba substituted samples. The results show that Ni reducibility, oxygen storage, Ni dispersion and surface area of catalyst
Acknowledgments
Srikanth Dama, Richa Bobade and Hanmant R. Gurav acknowledge Council of Scientific and Industrial Research, New Delhi, for providing senior research fellowship. Authors also acknowledge financial support by CSIR, New Delhi through network project CSC-0102
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