Nanosized iron and iron–cobalt spinel oxides as catalysts for methanol decomposition

Abstract

Nanosized iron and mixed iron–cobalt oxides supported on activated carbon materials and their bulk analogues prepared by thermal synthesis are studied by X-rays diffraction, Mössbauer spectroscopy, magnetic measurements and temperature programmed reduction. Their catalytic behavior in methanol decomposition to H₂, CO and methane is tested. Phase transformations in the metal oxides affected by the reaction medium are also investigated. Changes in the reaction mechanism of the methanol decomposition after the metal oxides deposition on the support as compared to the bulk phases are discussed.

Introduction

Nanosized spinel ferrite particles have attracted in the past considerable attention and research efforts. Because of their technological importance in microwave industries, high-speed digital tape or disk recording, magnetic refrigeration systems and ferrofluids they are still objects of intensive investigations [1], [2], [3], [4]. It is well established that various binary and ternary spinel ferrites are effective catalysts for a number of industrial processes such as oxidative dehydration of hydrocarbons, decomposition of alcohols, alkylation reaction, hydrodesulfurization of crude petroleum, Fischer-Tropsch reaction etc. [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Binary oxide 2–3 spinels may be described by the general formula Me²⁺Me₂³⁺O₄. The cation distribution in these spinels can be: (i) “normal”, i.e. the divalent metal ions are located on the tetrahedral (A)-sites—(Me²⁺)_A[Me₂³⁺]_BO₄; (ii) “inverse”, i.e. the divalent metal ions occupy octahedral [B]-sites—(Me³⁺)_A[Me²⁺Me³⁺]_BO₄ and (iii) “intermediate” (partially inverse) between normal and inverse—(Me_x²⁺Me_λ³⁺)_A[Me_1−x²⁺Me_2−λ³⁺]_BO₄. For spinels, where only divalent and trivalent cations are present, the inversion degree (λ) is defined as a fraction of (A)-sites occupied by trivalent ions [21]. It was also reported that, in the case of ferrites, Fe³⁺ ions could be easily shifted either to octahedral or to tetrahedral sites by varying stoichiometric ratio with the other cations. As a result, the physical and catalytic properties of the spinel oxides might be influenced not only by the nature and the oxidative state of the transition metal ions, but also by their distribution in the spinel structure [21], [22], [23], [24], [25]. In this aspect, the determination of cation distribution in the spinels gains a considerable interest because of its influence on their physical and chemical properties.

Methanol is expected to become one of the new liquid energy carriers because it can be synthesized from biomass, coal and natural gas, all of them being more abundant resources than the crude oil. In the last two decades among the various procedures of methanol conversion (steam reforming, partial oxidation, etc.) the methanol decomposition has received growing attention as a source of hydrogen and/or synthesis gas for chemical processes or as an ecological fuel for gas turbines, vehicles and fuel cells [26], [27], [28], [29], [30]. Since the methanol decomposition to hydrogen and carbon monoxide is an endothermic process, it is also suitable for chemical storage of heat. However, a significant improvement of catalysts for the methanol decomposition is desired. Various metals and metal oxides are reported to be effective catalysts for this reaction [31], [32], [33], [34], [35]. We established that nanoparticles of iron or iron oxide supported on mesoporous molecular sieves could substantially change the reaction selectivity, at that hydrogen and methane/or carbon monoxide being the main products [36], [37], [38], [39], [40]. It has been shown as well that the selectivity of methanol conversion to CO and methane could be easily controlled by varying of the supported iron oxide dispersion and its transformations provoked by the reaction medium [38]. The role of the support pore architecture on the state of the supported iron species was also widely discussed. However, only a few data on the methanol decomposition using mixed iron–cobalt oxides catalysts have been published so far. In our previous study we investigated the catalytic behavior of mechanochemically synthesized nano-dimensional iron cobalt spinel oxides in methanol decomposition. A well-defined effect of the preparation conditions and the Fe/Co ratio on the reduction and catalytic properties of iron–cobalt catalysts is established [41]. However, the obtained nanoparticles usually show a strong tendency to aggregate. The latter makes it very difficult to exploit their unique physical properties. Dispersion of the nanoparticles in a matrix [42], [43] as well as their deposition on various supports [44], [45], [46], [47], [48], [49], [50], [51] are approaches to reducing their agglomeration.

The aim of the present paper is to elucidate the changes in the catalytic behavior of iron and mixed iron–cobalt oxides in methanol decomposition after their deposition on a support. For this purpose spinel ferrites with various Co/Fe ratios were supported on activated carbon and their catalytic properties were measured. Activated carbon was chosen as a support, because of its high specific surface area, well-developed pore structure and catalytic inertness [52], [53]. Special attention is paid on the phase transformations during the catalytic process and their relation to the catalytic efficiency of the samples.