Regenerable and durable catalyst for hydrogen production from ethanol steam reforming

https://doi.org/10.1016/j.ijhydene.2011.01.126Get rights and content

Abstract

In this work, perovskite-type oxides La1−xCaxFe0.7Ni0.3O3 were prepared by using a citrate complex method. The catalysts were employed in the reactions of steam reforming of ethanol (SRE) and oxidative steam reforming of ethanol (OSRE) to produce hydrogen. A reduction–oxidation cycle was proposed to overcome the problems of active component sintering and carbon deposition encountered in SRE reaction. In the ex-situ reactions, highly dispersed surface nickel particles formed during the reduction of La1−xCaxFe0.7Ni0.3O3, while during the introduction of an oxidative atmosphere these particles could be oxidized and restored back into the perovskite bulk. Owing to the existence of this segregation–incorporation cycle of nickel species in the perovskite oxides, the sintering of nickel particles under OSRE was found depressed effectively. Besides, this work proved that the oxygen in the feed is helpful to the elimination of deposited carbon. It seems promising for overcoming the problems of the active component sintering and carbon deposition in SRE reaction by regulating the redox ability of the perovskite-type oxides and the feed composition.

Introduction

Hydrogen production from bio-ethanol for fuel cells is an environmentally begin route to sustainable energy. Steam reforming of ethanol (SRE) is attractive owing to its high hydrogen yield and renewability of ethanol. For SRE catalyst, nickel [1], [2], [3], [4], cobalt [5], [6], [7] and noble metals [8], [9], [10], [11] are often used as the active components. Catalysts with high activity have been reported in many works. However, challenges in catalyst stability due to sintering of the active component and coke formation during the reaction still persist [12], [13]. We previously reported that the lattice oxygen vacancies in perovskite-type oxides (PTOs) were helpful in suppressing the carbon deposition [14]. The sintering of the active component can be moderated by improving the interaction between the support and the active component [15]. Besides, many works with different ideas have been reported recently on improving the catalyst stability, such as applying the support with proper acidity/basicity [1], [16], [17], [18], adjusting redox and oxygen mobility properties of the supports [19], [20], [21], [22], [23], [24] and tunning the catalyst by means of modification or doping [25], [26], [27], [28], [29], [30], [31]. Although some progresses have been made, improvement on the stability for SRE catalyst is still desiring.

Nickel has been widely used as the active component for SRE catalyst because of its high activity and low price. However, metallic Ni sintering at high reaction temperatures is a limiting factor. Carbon deposition is another major problem, but it can be suppressed by adding oxygen into the feedstock to oxidize the deposited carbon [32]. Nevertheless, the extra heat evolved from the carbon oxidation may accelerate the sintering of the active component [33], [34]. Therefore, a hypothesis is proposed that both the problems of sintering and carbon deposition can be solved, if a catalyst which is thermally stable even in the presence of oxygen can be developed.

PTOs, with a general formula of ABO3, are a sort of well characterized oxides and have been widely used in catalytic field. In the unit formula of perovskite-type oxides ABO3, as indicated in Fig. 1, A is the larger cation and B is the smaller cation. In this structure, the B cation is 6-fold coordinated and the A cation is 12-fold coordinated with the oxygen anions. It was pointed out that [35] around 90% of the metallic elements in the Periodic Table can be served as the cations in a perovskite-type oxide structure. In the structure, both A and B site cations can be partially substituted. These characteristics facilitate the modification of its structure as well as its applications in a variety of areas.

Here, we propose a reduction–oxidation cycle as shown in Scheme 1, viz. during reduction, nickel ions migrate out from the perovskite structure in the form of metallic nickel particles of nano-size; when the atmosphere is oxidative, the nickel particles are oxidized and restored back into the perovskite lattice. With such a cycle, metallic Ni with high dispersion can be maintained. Based on this idea, perovskite oxides with the composition of La1−xCaxFe0.7Ni0.3O3 are developed to explore the cycle movement of nickel inside and outside of the perovskite lattice under redox atmosphere. Iron ion is employed as the B site ion, because the PTOs with iron ion at the B site are very stable. For the A site, several metal ions, such as Sr2+, Ba2+ and Ca2+, have been investigated and Ca2+ was the most favorable one for the cycle. So the catalyst compositions are determined as La1−xCaxFe0.7Ni0.3O3 in this work. Their stability performance in SRE and OSRE and the structural cycle effect on catalyst stability are investigated.

Section snippets

Catalyst preparation

La1xCaxFe0.7Ni0.3O3 (x = 0, 0.1 and 0.3) were prepared by one step citrate method. Nitrates of nickel, iron, lanthanum and calcium with a proper molar ratio were dissolved in deionized water. Then citrate acid (with 20% excess of the total moles of the cations) and polyethylene glycol 400 (with 20% mole amount of the citrate acid) were added into the solution. The obtained solution was stirred for 5 h and then was concentrated by vaporization at 80 °C until the formation of a spongy solid. The

Temperature programmed reduction

The temperature programmed reduction curves of La1−xCaxFe0.7Ni0.3O3 with different “x” values are presented in Fig. 2. When x = 0, the material exhibits two peaks at around 410 and 900 °C, labeled as α and γ, which are attributed to the reduction of Ni3+ to Ni2+ and Ni2+ to metallic nickel, respectively [36]. For the other two perovskite oxides with Ca2+ introduction, an extra reduction peak in the temperature range of 580–800 °C (labeled as β) is observed. The β peaks are caused by the

Conclusion

In order to overcome the problems of active metal sintering and carbon deposition for the SRE catalyst, the perovskite structure oxides of La1−xCaxFe0.7Ni0.3O3 were prepared with citrate complexation method. By shifting the reducing and oxidizing atmosphere, a cycle movement of nickel species in La1−xCaxFe0.7Ni0.3O3 can be realized, i.e. nickel ions separation from the perovskite structure to form nano-size metallic nickel particles in reducing atmosphere and restoration of the metallic nickel

Acknowledgements

The financial support from the NSF of China (20963005) and the NSF of Tianjin, China (10JCZDJC23800) are gratefully acknowledged.

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