Characterization of precursors and reactivity of LaNi1−xCoxO3 for the partial oxidation of methane
Introduction
The partial oxidation of methane (POM):has been extensively investigated in last years, because it offers some advantages with respect to the conventional technology of steam reforming (MSR) [1], [2], [3]. POM reaction is exothermic and is very fast and can be run in small reactors. However, the resulting H2/CO = 2, which is appropriate for methanol synthesis and Fischer–Tropsch reactions, is lower than the theoretical one (H2/CO = 3) obtained by the MSR reaction [4]. For the POM reaction two different mechanisms have been proposed: (i) CO and H2 are produced directly from CH4 and (ii) CH4 is first oxidized into H2O and CO2 and then these intermediate products reformate the excess of CH4 to yield CO and H2 [5], [6].
Many catalysts have been studied in the POM reaction. Most of them include non-noble (Ni, Co, Fe) and noble (Ru, Pt, Ir, Os, Rh, Pd) transition metals, usually deposited as fine particles over porous substrates [7], [8].
A particularly attractive option of a catalyst for a high temperature reaction such as POM is to use mixed metal oxide precursors with a perovskite structure, which also display an exceptional high thermal stability [9], [10], [11], [12], [13]. The general formula of these oxides is ABO3, in which the cation A of a larger size is responsible for the thermal resistance of the catalyst whereas the cation B of smaller size accounts for the catalytic performance. These compounds also offer the possibility of partially substitute both A and B cations by other kind of cations, thus allowing to tailor both thermal stability and catalytic performance [4]. The peculiarities of these oxides make them excellent candidates to be used in oxidation–reduction reactions, such as partial oxidation of methane, which requires a catalyst with redox functionality [4]. Due to the importance of these oxides as redox catalysts, the type and the nature of BO bonds in ABO3 systems have long been studied [1], [4]. All these studies showed that the particular behaviour of the ABO3 perovskites as oxidation catalysts is mainly due to the relative ease with which the oxygen species can be released from the catalyst surface [1], [14].
Taking into account the peculiarities of the ABO3 systems, this work was undertaken with the aim to prepare LaNi1−xCoxO3 catalysts with the perovskite structure, which generate a highly dispersed transition metal (Ni and Co) on the lanthanum oxide matrix during the activation process so that they can be used in the POM reaction [8]. Another objective was to investigate whether Co and Ni reduced phases display a similar performance in the target reaction because cobalt tends to form a very stable LaCoO3 structure even under a reducing environment. The proposed bulk oxides in principle offer important advantages with respect to supported perovskites on an alumina substrate. In line with this, it was found that Co/Al2O3 catalysts could be used in the POM reaction [15], [16]. However, they form coke deposits and also become deactivated by the loss of metal cobalt and by the solid-state reaction between cobalt and the alumina substrate to form an inactive CoAl2O4 phase.
Section snippets
Catalyst preparation
The LaNi1−xCoxO3 oxides were prepared by the thermal decomposition of the amorphous citrate precursors [17]. For this purpose, concentrated solutions of lanthanum, nickel and cobalt nitrates (Merck, p.a) were prepared. The concentrations were selected in order to obtain a La:Ni:Co atomic ratio of 1:(1 − x):x, where x is the substitution degree. In parallel a concentrated solution of citric acid (Aldrich) was prepared and then mixed with the previous solutions. The excess of water was removed at
Elemental chemical analysis
The results of elemental chemical analyses of the samples are reported in Table 1. It is seen that experimental values are very close to the theoretical ones in all the samples. This is expected using the method of decomposition of amorphous citrate precursors, since the cations are incorporated quantitatively to the precursor and no losses are produced. Besides, this finding suggests that under the experimental conditions used in this study, the formation of mixed oxides of cobalt, nickel and
Conclusions
The effect of substitution degree (x) on the partial oxidation of methane over LaNi1−xCoxO3 oxides was investigated. Although the specific surface area of these perovskites is rather low, they are promising candidates for this reaction because they are highly stable and are able to stabilize the metal particles (Ni) in well-dispersed state after the activation process. It was shown that the LaCoO3 system is inactive in the POM reaction because this mixed oxide is much more stable than its LaNiO3
Acknowledgements
GCA and SML gratefully acknowledge fellowship granted by CAPES and CNPq (Brazil), respectively. This work was partly supported by MCYT (Spain) (Project MAT2001-2215-C03-01) and EU (Project ENK-2002-00682).
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