Elsevier

Applied Catalysis A: General

Volume 550, 25 January 2018, Pages 184-197
Applied Catalysis A: General

Interconnection between feed composition and Ni/Co ratio in (La-Ni-Co-O)-based perovskites and its effects on the stability of LPG steam reforming

https://doi.org/10.1016/j.apcata.2017.11.011Get rights and content

Highlights

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    Catalysts derived from (La-Ni-Co-O)-based perovskite precursors are stable in steam reforming of LPG.

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    Stability achieved under conditions of low water/LPG feed ratio, which prevents surface oxidation of metal particles.

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    A suitable water/LPG feed ratio coupled with the proper Ni/Co ratio is essential to keep catalysts active longer.

Abstract

Nickel-based perovskites (La-Ni-O) partially substituted with cobalt were tested for steam reforming of Liquefied Petroleum Gas (LPG), and analyzed by a series of in situ techniques. In situ X-ray diffraction (XRD) analyses showed that, depending on the fraction of substituted Ni, the perovskites go through one or two reduction stages before decomposing into Ni0/Co0 and La2O3. Extended X-ray absorption fine structure (EXAFS) data indicated that the nanoparticle’s surface are enriched with Co after reduction, but the Ni atoms migrate to the surface and overlay the former Co shell soon after feeding the LPG steam reforming mixture. In samples synthesized with a low Co/Ni ratio, the Ni surface layer oxidizes after a few hours under reaction conditions, showing signs of deactivation. The oxidation could be prevented by decreasing the water/LPG feed ratio, which reactivated the catalyst. The use of a proper water/LPG feed ratio coupled with the suitable Co/Ni ratio in the synthesis of perovskite precursors is essential to keep the catalysts active during LPG steam reform.

Introduction

Steam reforming of Liquefied Petroleum Gas (LPG) is a viable option for producing hydrogen in isolated regions where there is no pipeline natural gas supply [1]. Hydrogen could then be used as a reagent in fuel cells for converting chemical energy into electricity, fueling vehicles, providing home power, heating, cooling, among other applications.

Noble metals such as Rh, Ru and Pt have already proved to have good activity for LPG reforming processes [2], [3], [4], [5], but their high cost has shifted attention to transition metals such as Ni [6], [7], [8], [9], [10], which are also highly active for these processes. A major problem with using Ni-based supported catalysts is the high deactivation rate caused by carbon deposition and accumulation on the surface of metal particles [6], [11]. Carbon deposition strongly depends on the reaction conditions, such as reaction temperature, water/LPG molar ratio and oxygen/LPG molar ratio. For instance, co-feeding oxygen in the reactor inlet stream usually increases the oxidation rate of carbon deposits, improving the catalyst stability [4], but the appropriate oxygen/LPG molar ratio should be respected to avoid side effects such as the oxidation of metal particles [12].

It was reported that the chemical binding between metal and support minimizes the sintering of Ni and decreases the process of carbon dissolution and diffusion to the nickel particles, protecting them from lifting up from the surface during carbon filament growth [6]. It is also known that catalysts prepared from perovskite precursors may retain the metal particles dispersed and decrease sintering due to the high interaction particle-support [13], but even so the oxidation rate of carbon deposits is still high for these catalysts [14]. Both LaNiO3 and LaCoO3 perovskites have been reported to be good catalysts precursors for higher chain hydrocarbons reforming processes aiming to produce hydrogen [15], [16], [17]. The partial substitution of Ni or Co in the B-site of the perovskite ABO3 structure has also been investigated with interesting results in terms of increasing reducibility and stability while maintaining high yield of hydrogen [15], [18]. The literature also reports the use of Ni/Co bimetallic catalysts for reforming of biogas [19], [20], ethanol [21], acetic acid [22] and glycerin [23].

In order to understand the reasons for high carbon deposition rates on catalysts derived from perovskites and to attempt to control the deposition process, Ni/Co bimetallic catalysts synthesized from La-based perovskite precursors were tested for LPG steam reforming. The 24 h stability tests were carried out using different water/LPG molar ratios in the reactor inlet stream. In parallel, the catalysts were characterized by temperature-dependent X-ray Absorption Spectroscopy (XAS) and X-ray Diffraction (XRD) techniques, which allowed structural changes to be observed both during reduction and during the reaction.

Section snippets

Synthesis of perovskite precursors

Nickel, cobalt and lanthanum salts (nickel nitrate hexahydrate, cobalt nitrate hexahydrate and lanthanum nitrate hexahydrate) were added in stoichiometric proportions to small amounts of deionized water, and the solutions were transferred to a larger beaker. Following, citric acid was added to the saline solution in the proportion of 1.2 mol of citric acid per mole of metals (Ni, Co and La), with subsequent stirring until complete homogenization. While still under stirring, the solution was

Temperature-resolved XRD

Fig. 1A shows the temperature-resolved XRD patterns of samples LaNi1 and LaNi0.8 during reduction in a 5 vol% H2/He atmosphere. The temperature values shown in the graph are approximate values, with a maximum deviation of 10 K. At 368 K, the LaNi1 and LaNi0.8 samples present diffraction lines around 32.8°, 33.3° and 47.4 (α symbols), corresponding to lattice planes (1 1 0), (1 0 4) and (0 2 4) of the LaNiO3 perovskite-type structure, where Ni atoms have valence 3+ [28], [29]. Sample LaNi1 also

Discussion

The reduced monometallic catalyst LaNi1 has metallic Ni crystals with coordination number of 11.8 (Table 1). The temperature-resolved XRD data obtained during the reduction process (Fig. 1A, solid line) suggest that the average particle diameter of this catalyst is around 18.4 nm (refer to Section 3.1 for more details). Under butane steam reforming conditions, the coordination number decreases to 10.5 (Table 1). We speculate that the Ni particles should not undergo sintering or oxidation

Conclusions

Perovskite-type precursors synthesized with different nickel and cobalt contents were characterized in situ by X-ray diffraction and X-ray absorption techniques during reduction and under LPG steam reforming conditions. The reduction of Ni-richer perovskites occurred in two stages, with initial loss of lattice oxygen atoms and irreversible decomposition into Ni0 and La2O3 at higher temperatures. In Co-richer perovskites, the reduction occurred in three stages, passing through metastable phases

Acknowledgements

The authors would like to thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for financial support (grant numbers 470711/2013-2 and 307937/2015-1) and for the Ph.D. scholarship granted to Dr. Rondinele A. R. Ferreira. The authors would also like to thank FAPEMIG (Fundação de Amparo à Pesquisa do Estado de Minas Gerais, grant number BPD-00493-13) and CAPES (PNPD program, grant 2694/2011) for financial support, and the Brazilian Synchrotron Light Laboratory (LNLS) for

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    Current address: Faculty of Civil Engineering, Federal University of Uberlândia, Av. João Naves de Ávila, 2121, Campus Santa Mônica - Bloco 1Y, 38400-902, Uberlândia, MG, Brazil.

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