Cobalt-modified mesoporous MgO, ZrO2, and CeO2 oxides as catalysts for methanol decomposition

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Abstract

Cobalt oxide-modified mesoporous CeO2, ZrO2, and MgO oxides and their SBA-15 silica analogues were prepared and characterized by XRD, TEM, N2 physisorption, FTIR, and TPR–TG analysis. Their catalytic activity in methanol decomposition to CO and hydrogen was tested. The support effect on the state and catalytic behavior of the loaded cobalt oxide nanoparticles is discussed. The best catalytic activity and selectivity of methanol decomposition to CO and hydrogen are registered for Co/CeO2.

Graphical abstract

Mesoporous oxides are suitable hosts for the preparation and stabilization of cobalt oxide nanoparticles. Their catalytic behavior could be successfully tuned, changing the nature of the support.

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Introduction

Cobalt oxide-based materials are well known as promising catalysts in various processes, such as automobile exhaust gas treatment, volatile organic compound (VOC) elimination, and Fischer–Tropsch synthesis [1], [2], [3], [4], [5], [6], [7], [8]. Recently, these materials have been investigated widely with respect to improving their catalytic properties. These efforts have led to applications of cobalt oxide particles in the nanoscale range and mesoporous silicas have been considered as a suitable host for the preparation and stabilization of such nanometer particles [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Nowadays, mesoporous metal oxides offer new prospects in catalysis [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. On the one hand, these materials could be successfully used as catalysts, because they usually possess their own catalytic activity, which could be tuned via adjustment of their surface properties and textural characteristics. On the other hand, the relatively high specific surface area and well-developed porous structure of these mesoporous oxides reveal possibilities for their application as catalyst supports of various metal or metal oxide species. However, the catalytic behavior of such obtained composites is far from being predicted, because the mesoporous oxide support usually participates in the catalytic process. It could directly interact with the reactants and with the product intermediates or could regulate the oxidative state and dispersion of the loaded nanoparticles.

Various methods for the preparation of mesoporous metal oxides have been developed, and among them, ligand-assistant template synthesis is most frequently used [29], [30], [31], [32], [33], [34], [35], [36]. The main disadvantage of this technique is the collapse of the mesoporous structure during the thermal removal of the template. Recently, these disadvantages were overcome using porous solids as a rigid template. A series of mesoporous oxides, such as SiO2, Al2O3, TiO2, MgO, ZrO2, has been developed using mesoporous carbon, type CMK, as a template [19], [22], [23], [24], [37], [38]. In fact, the obtained mesoporous oxide represents a negative replica of CMK, and a positive replica of the corresponding mesoporous silica, which is the matrix for the CMK material.

In the present study, we focus our attention on the catalytic behavior of cobalt oxide nanoparticles, supported on various mesoporous oxides. For this purpose, mesoporous silica type SBA-15 is compared with some metal oxide mesoporous analogues. Among them, MgO is well known with its strong basicity, ZrO2 is characterized with bifunctional basic and acidic properties, and CeO2 is intensively studied because of its high oxygen lattice mobility. To the best of our knowledge, there are only a few data for the catalytic behavior of such type of materials, where the favorable catalytic effect of mesoporous support is discussed [39], [40], [41], [42]. In the current study we use methanol decomposition to CO and hydrogen as a catalytic test, taking into account its potential application as an alternative carrier to liquefied natural gas. Catalysts, working at relatively low temperatures with good activity and high selectivity to CO and H2, are needed for its successful application as a fuel in vehicles, gas turbines, and fuel cells, or for recovering the waste heat from industry [43].

Section snippets

Materials

Mesoporous SBA-15 silica was obtained using Pluronic P123 triblock copolymer (SiO2/P123 molar ratio (r) of 60) as a structure-directing agent according to the procedure described in [44]. The template was removed by ethanol–HCl extraction followed by calcination at 823 K. The mesoporous MgO and CeO2 materials were prepared as described in Refs. [23], [24], respectively, using mesoporous CMK-3 carbon as a structure-directing matrix. In the case of MgO, the mesoporous carbon was impregnated with

XRD and TEM study

In Figs. 1a and 1b are presented XRD patterns of parent mesoporous oxides. Small-angle X-ray diffraction patterns of SBA-15 (Fig. 1a) represent intensive well-resolved peaks, which can be indexed as (100), (110), and (200) reflections of 2-D hexagonal lattice with p6mm symmetry. Much broader (100) reflection is observed for all nonsiliceous oxides that could be assigned to less ordered in comparison with the SBA-15 structure of pores, which are in random position with respect to each other [31]

Conclusion

Mesoporous oxides are suitable hosts for the preparation and stabilization of cobalt oxide nanoparticles. Their catalytic behavior could be successfully tuned, changing the nature of the support. Formation of catalytic active complexes with the participation of sharing with the mesoporous support cobalt species is assumed. The mobility of the oxygen in these complexes seems to be of primary importance for the catalytic process. The easier release of oxygen, which is realized in the case of

Acknowledgments

Dr. M. Tiemann and Dr. J. Roggenbuck are acknowledged for the mesoporous oxide preparation. Financial support of the DAAD and Bulgarian Ministry of Education and Science, project DAAD-04/2007, is acknowledged.

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