Short CommunicationCobalt catalysts derived from hydrotalcite-type precursors applied to steam reforming of ethanol
Graphical abstract
Research Highlights
► XANES indicated Co interacting strongly with the Mg/Al oxide support. ► The presence of Ce and La weakened the interaction Co-support. ► Ce and La favored acetaldehyde production at the lower feed water content. ► Increasing the water content decreased the acetaldehyde formation on CoCe and CoLa. ► Ce and La favored the adsorption of water molecules and acetaldehyde steam reforming.
Introduction
Ethanol steam reforming (ESR, Eq. 1) has been suggested as an alternative route for H2 production, since this reaction predicts a high yield of H2 at low reaction temperatures.
In the literature, Rh, Co and Ni are described as promising metals for ESR catalysis [1]. Rh is the most active of these, but very costly, being a noble metal. Cheaper metals such as Ni and Co usually suffer deactivation by the deposition of carbon or sintering of the active phase [1]. The stability of such catalysts may be enhanced by careful choice of the constituents of the support and the method of preparation [2].
Hydrotalcites are known for their layered structure, which, on heat treatment, leads to a homogenous dispersion of the active phase [3]. Comas et al. [4] studied the performance of Ni–Al catalysts made from hydrotalcite-type precursors in the ESR reaction, carried out at 773 K, varying the molar ratio (R) of H2O:C2H5OH in the feed from 1 to 6. They reported that H2 selectivity increased with rising R. The authors suggested that the increased H2 selectivity could be related to the fine dispersion of Ni resulting from the method of preparation from hydrotalcite-type precursors.
Lucrédio et al. [5] tested catalyst precursors composed of Ni/Mg/Al oxides promoted with La and Ce in ESR reactions. The catalysts did not suffer significant carbon deposition throughout 6 h on stream and the reaction data showed that the addition of Ce and La resulted in greater H2 production at 550 °C.
Considering the promising results obtained with hydrotalcite-type precursors, the goal of this paper was to assess the effect of Ce and La addition on the performance of cobalt catalysts derived from hydrotalcite-type precursors, Co/Mg/Al, in the ESR reaction.
Section snippets
Synthesis
The preparation of the catalysts has already been described in detail in previous papers [6], [7].
Briefly, the hydrotalcite-type precursors were prepared in two ways: the standard Co-LDH (layered double hydroxide) catalyst by the traditional technique of precipitation of carbonates and the promoted catalysts, LaCo-LDH and CeCo-LDH, by anion exchange of Mg/Al/Co with EDTA chelation complexes of La and Ce, to favor the transport of these cations into the hydrotalcite layers.
All hydrotalcites were
Results
The characterization of these catalysts is presented elsewhere by the authors [7], [8]. Table 1 reproduces the molar composition of the catalysts [7] and according to this table, the concentrations of Co, La and Ce are higher on surface. According to the temperature-programmed reduction (TPR) and X-ray diffraction (XRD) analysis reported in these previous studies, the Co and Mg species are interacted strongly, probably in the form of a solid solution of CoO–MgO doped with Al3± cations [7], [8].
Conclusion
It has been demonstrated that Co/Mg/Al catalysts prepared via hydrotalcite-type precursors are active for ESR. The presence of the Ce and La promoters favored acetaldehyde production at the lower feed water content (H2O:ethanol = 3:1). Increasing the water content and thus decreasing the contact time improved the catalytic performance of the samples modified with Ce and La and decreased their acetaldehyde formation, probably by favoring the adsorption of water under these conditions.
Acknowledgements
The authors are grateful to the Brazilian state and federal research funding agencies, FAPESP, CNPq and CAPES, for the financial support and to the LNLS (Brazilian Synchrotron Light Source Laboratory) for the XANES facilities.
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