1. Introduction
Many studies have been conducted about Dry Reforming of Methane (DRM) mechanism and is a consensus that this reaction follows a bifunctional mechanism where CH4 is activated preferentially on metallic sites and CO2 is activated by the support [1, 2]. Since the catalysts applied on this reaction are prone to deactivate, the support exhibits a fundamental role.
Several studies on the literature have proposed that the nature of the support effects on the mechanism of carbon species oxidation. For example Ni-based catalyst supported on CeO2, La2O3, CaO, MgO and BaO are alternatives to inhibit the deactivation by carbon deposition. However, for nickel based perovskites, few investigations have been done with barium as a promoter [3].
Therefore, the aim of this study was evaluating Ni-La based catalysts and the impact of barium on the DRM reaction. It was investigated the effect of the synthesis method (nickel perovskite reduction or wetness impregnation) on Ni/La2O3 catalysts properties as well as evaluate the influence of barium species on carbon deposition.
2. Experimental
In this work perovskite precursors La1-xBaxNiO3 (x = 0.0, 0.05, 0.1 and 0.2) were prepared using citrate method. The samples were characterized by X-ray fluorescence (XRF), nitrogen adsorption, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). In situ X-ray diffraction experiments were conducted at the XPD beam line using a radiation with λ = 1.2397Å at the Brazilian Synchrotron Light Source at Campinas (LNLS). DRM reaction was carried out in a stainless-steel tubular reactor at 973K during 20 h under 100 mL min-1 25% CH4/ 25% CO2/ Ar(balance). The gas effluent was analyzed using a GC Shimadzu 2014 equipped with TCD detector and column Carboxen 1010. Before the test the samples were pre-treated as follows: heated (10°C min-1) under H2 flow (30 mL min-1) to 1073 K during 1 h. Also aging tests were performed. The used samples were characterized by TGA and SEM to determine the amount and morphology of the coke formed.
3. Results
The results of in situ XRD experiments are shown in Figure 1 (a) and (b) and the results of DRM aging tests are presented in Figure 2.
Figure 1: In situ ]XRD under CH4 and CO2: (a) LaNiO3 and (b) La0.8Ba0.2NiO3
Figure 2: La0.8Ba0.2NiO3 aging tests at 973 K: CH4 conversion and H2 selectivity curves.
Table 1 – Carbon deposited after DRM catalytic test determined by TPO.
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4. Discussion
In situ XRD showed the phase evolution when the catalysts were exposed under CH4/CO2 mixture from room temperature to 800 ºC. For all catalysts, metallic nickel was oxidized to nickel oxide when exposed to reaction atmosphere. This event suggests that CO2 present on the feed was responsible for the nickel sites oxidation. Comparing the catalysts obtained via LaNiO3 and La0.8Ba0.2NiO3 (fig. 1a and 1b) it was verified that the nickel sites present on LaNiO3 catalyst are more susceptible to oxidation. At 289 ºC, the intensities of the diffraction lines associated to La2O3 phase decreased in intensity and the peak (111) attributed to metallic nickel disappeared, characterizing the total nickel oxidation to NiO. At the same temperature, the peaks characteristics of NiO (111) and (200), started to appear, reaching a maximum at 463 ºC. Moreover, at 463 ºC, the peak (111) attributed to metallic nickel becomes detectable, suggesting the catalyst was being regenerated. For the catalyst obtained from La0.8Ba0.2NiO3, the oxidation event takes place at higher temperatures, starting from 432 ºC and ending at 610 ºC. Nonetheless, the nickel sites were not totally oxidized, suggesting a partial oxidation process, which was characterized only by the decreasing of the metallic nickel peak (111). This behavior considered that BaCO3, during reduction process, may cover the nickel sites, at least partially, blocking such sites and making them less susceptible to oxidation.
Barium carbonate and carbon species can interact to form a carbonate-carbon complex that decomposes rapidly to carbon monoxide. If this process is taken into account, it is possible to explain the higher resistance of barium-content catalysts to carbon deposition observed in table 1. It is worth mentioning the CO2 present on the feed may promote the BaCO3 regeneration, since this phase is present in all temperature range in which the catalyst was exposed to CH4/CO2.
5. Conclusions
In situ XRD under reaction atmosphere suggested all catalysts were oxidized to NiO when exposed to reaction atmosphere and regenerated increasing the temperature. Nevertheless, barium catalysts showed more resistant to oxidation. Aging tests and TPO analysis suggested barium addition promotes higher resistance to catalysts deactivation since BaCO3 can act as a carbon oxidant in reaction conditions. Also, those catalysts may provide other reaction surface regeneration pathways.
References
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[3] D. Pakhare, J. Spivey, Chem. Soc. Rev. 43 (2014).
[2] L.M.T.S. Rodrigues, R.B. Silva, M.G.C. Rocha, P. Bargiela, F.B. Noronha, S.T. Brandao, Catal. Today. 197 (2012) 137–143.