Role of the nanoparticles of Cu-Co alloy derived from perovskite in dry reforming of methane
Introduction
One of the highly attractive ways for the production of syngas (CO + H2) is steam reforming of methane (Eq. (1)). The synthesis gas produced by this process at a high H2/CO (>3) ratio cannot be converted to long-chain hydrocarbons via Fischer-Tropsch synthesis due to the excess hydrogen which inhibits chain growth and decreases selectivity of higher hydrocarbons [1,2]. For this reason, together with both environmental and economic concerns, production of syngas via carbon dioxide reforming of methane (Eq. (2)), also called dry reforming of methane (DRM), has attracts much attention in the last few years [2]. This reaction also has very important role in the industries due to lower H2/CO ratio in product gas, which can be preferentially used for production of liquid hydrocarbons in Fischer-Tropsch synthesis [2,3]. A main concern of DRM catalysts is the carbon deposition during the reaction, which causes catalyst deactivation, catalyst destruction, and reactor blockage. The carbon deposition on the active sites of catalyst is mainly due to the CH4 decomposition (Eq. (3)) and inverse Boudouard reaction (Eq. (4)). These reactions occur on the catalyst surface and form solid carbon [4].
On the other hand, under the high temperature operation condition, metal catalyst tends to deactivate due to the sintering of the active phase or the reaction between metal and support forming inactive species. Thus, a thermal stable catalyst that can resist degradation and carbon deposition is essential. Although noble metals like Pt, Rh and Ru are more active towards DRM and more resistant to carbon deposition than transition metals like Ni, Co and Fe. However, the high cost of noble metals have drastically limit their applications on the industrial scale.
Therefore, a good dispersion of the active phase and interaction between support and metal favored by the formation of fine metallic particles that can contribute to reducing the phenomenon of deactivation. For this reason, researchers hope to improve particle size, by deposit the nickel on a support (Al2O3, SiO2 ... .etc.) [5,6]. Another possibility for increasing the metal dispersion on the surface of a catalyst is to incorporate the active phase in a well-defined structure such as hydrotalcite, spinel, perovskite, core-shell structures … etc [[7], [8], [9], [10]].
Perovskite structure defined by the general formula ABO3, where the A and B -site cations represent a 12 and 6 coordination respectively. A wide variety of perovskites has already used as catalysts for CO2 reforming of CH4 [[11], [12], [13], [14], [15]]. However, the perovskite structure can be stabilized by partial substitution of A- and/or B-site cations under DRM reaction conditions, which is able to enhance the metal-support interaction [12,16].
According to recent investigations regarding self-regeneration properties of perovskite catalyst, the sintering and agglomeration of metal particles is inhibited by a reversible evolution between metal particle onto the surface and the inside of perovskite in redox process [[17], [18], [19]]. Sagar et al. [20] recently used two series of perovskite based catalyst LaNixAl1-xO3 (0 ≤ x ≤ 1) prepared by hydrothermal and sol-gel methods. The results showed that the catalysts containing trimetallic perovskite showed higher CH4 and CO2 conversions than the bimetallic perovskite, due to the strong interaction of Ni with the former. Strong interaction increased the reduction temperature of the active species and reduced the sintering of metallic particles. Arandiyan et al. [21] synthesized LaNixFe1-xO3 (0 ≤ x ≥ 1) perovskite metal-oxides by a sol–gel method and investigated for the reforming of CH4 to syngas. They found that the substitution of Ni with Fe in LaNiO3 perovskite catalyst promote the stability and showed fewer carbon deposits were detected on the spent catalyst after 1800 min evaluation on TOS. Provendier et al. [22] also reported that a LaNixFe1-xO3 perovskite structure enhances the metal-support interaction and effectively suppresses carbon deposition and the agglomeration of Ni particles. The effect of modification Co with Fe in LaFe1-xCoxO3 perovskites as catalysts was investigated in the combined reforming of methane with CO2 and O2 for the production of syngas. They discovered that introducing Fe in LaCoFe perovskite enhanced the activity efficiency by avoiding the sintering of active phase and coke formation on the surface of the catalysts. The authors explain these findings by the existence of strong Fe–Co interaction in the LaFe1-xCoxO3 with x = [0.4–0.6] produces a synergetic effect improving methane conversion and H2 production [23].
The purpose of this study is to explore the effect cobalt substituting by a small amount of copper in LaCoO3 perovskite for improving of the resistance to coking and sintering of active phase in DRM reactions.
Section snippets
Catalyst synthesis
A catalysts LaCoO3 and LaCu0.55Co0.45O3, were prepared by the sol-gel method described elsewhere [9]. Initially, Aqueous solutions (0.5 M, 50 mL) of La(NO3)3·6H2O were added quickly at room temperature to solutions of Cu(NO3)2.6H2O (0.25 M, 25 mL) and Co(NO3)2.6H2O (0.25 M, 25 mL). Finally, 1.5 mol equivalents of citric acid was then added with vigorous stirring and the mixture was evaporated under reduced pressure. The samples dried in oven for 5 h at 100 °C and calcined under constant flow of
Catalyst composition (ICP)
Chemical composition of synthesized samples are regrouped in Table 1. The values of elemental analyses of the perovskite type oxides was relatively in good agreement with the experimental values initially started with.
BET surface area
Table 1 summarized the surface area values (S.BET) of the solids after calcination at 800 °C. Low values of BET specific surface area were obtained in the case of two solids. According to the literature [26,27], the small value of surface area is probably attributed to the
Activity of the catalysts at different reaction temperature
The catalytic activities of LaCoO3 and LaCu0.55Co0.45O3 for DRM were tested under identical experimental conditions, with the molar ratio CH4/CO2 = 1, under atmospheric pressure in the range of 400–700 °C. The experimental results conversion of CH4 and CO2 as a function of reaction temperature are regrouped in Fig. 8(A) and (B) respectively, and syngas H2/CO molar ratio is shown in Fig. 8(C).
The CH4 conversions (Fig. 8(A)) of LaCoO3 were 15%, 23%, 45% and 57%, while those of CO2 (Fig. 8 (B))
Conclusion
LaCoO3 and LaCu0.55Co0.45O3 solids derived from perovskite were obtained by sol-gel method namely the citrate acid complexing method. Several characterization methods have been used to determine the structure of perovskite and physico-chemical properties of the solids. The results of TPR, XRD, SEM-EDX and TEM characterization confirmed that the formation of Co0, Cu0 and Cu-Co alloy after reduction. Prior to reaction, the synthesized solids were reduce at 700 °C to form Co0 as the active phase
Acknowledgements
We gratefully thank Research and Development Center (RDC) Boumerdes, Algeria and the Ministry of Higher Education Scientific Research (MESRS) Algeria for their financial support.
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