Nanosized iron and chromium oxides supported on mesoporous CeO₂ and SBA-15 silica: Physicochemical and catalytic study

Abstract

Mesoporous ceria and SBA-15 silica were modified with iron and chromium oxide nanoparticles and characterized by XRD, N₂-physisorption, FTIR, UV–vis, Moessbauer spectroscopy and TPR–TG in hydrogen. Their catalytic behaviour in methanol decomposition to CO and hydrogen was also studied. Stabilization of mono- and bi-chromate species, FeO_x patches or isolated iron ions as well as Fe₂O₃ and Cr₂O₃ nanoparticles in different ratio depending on the nature of the porous matrix was observed. The simultaneous presence of iron and chromium oxides lead to change in their dispersion, providing easier reducibility, higher catalytic activity and stability of the obtained materials in comparison with the corresponding mono-component ones. The “intimate contact” at the interface of both loaded metal oxide nanoparticles and the support was discussed with respect to explain the differences in the state of the active ingredient and its specific catalytic behaviour.

Introduction

Multi-component metal oxide materials reveal a great potential for a development of catalysts with desired design and tunable properties. Their usage in nanosize scale significantly extends the region of application due to the unique electronic, optic, magnetic, sorption, reduction and catalytic properties, which the materials exhibit in this state [1], [2], [3]. Recently, mesoporous materials have been considered as suitable supports for preparation and stabilization of metal oxide nanoparticles due to their high specific surface area and well developed pore structure of ordered mesopores with tunable size, shape and connectivity [4], [5]. Among them, mesoporous metal oxides gain a considerable interest as catalysts supports due to their own catalytic activity [6], and CeO₂ is often studied with respect to its high oxygen storage–release capacity [7], [8], [9], [10], [11]. On the other hand, supported Cr₂O₃ materials are important catalysts for various industrial processes such as oxidation, polymerization, dehydrogenation–hydrogenation, reforming, DeNO_x etc. [12], [13]. It has been widely discussed that the state of supported chromium species and their catalytic behaviour strongly depends on the structural and chemical peculiarities of the support. In the case of amorphous [12], [13], [14], [15], [16] and ordered mesoporous (MSU [17], SBA-15 [18], [19], [20], MCM-41 [21] and MCM-48 [22]) silicas co-existence of Cr(VI) mono- and polychromate species as well as Cr₂O₃ nanoparticles with different degree of crystallinity has been established, while stabilization of highly dispersed Cr(VI) species is mainly observed on ceria support [23], [24]. Furthermore, owing to their specific optical and magnetic properties and superior catalytic behaviour, nanoscale iron particles have been in the focus of both fundamental and technological interest [5], [25], [26], [27], [28]. In our previous study, we have demonstrated formation of homogeneously dispersed iron oxide nanoparticles with high catalytic activity by their deposition on mesoporous silica supports with 3D-cubic structure of cylindrical mesopores with optimum size of 7 nm [29]. Size-determined effect on the reduction and catalytic behaviour in methanol decomposition supported on mesoporous silicas iron oxide nanoparticles has also been established [30]. Formation of complex catalytic centers including ions from the active ingredient and the support was discussed for iron oxide modified mesoporous oxides [31], [32], [33]. Recently, bi-component iron–chromium mixed oxide materials have been intensively studied due to their importance as catalysts for high temperature water gas shift reaction, oxidative dehydrogenation of butane, NO reduction with NH₃, etc. [34], [35], [36], [37], [38], [39]. However, to the best of our knowledge, only few data for the Fe–Cr–Ce–oxides multi-component system have been reported [34].

The present paper is aimed at comparing of the state of supported on SBA-15 silica and mesoporous CeO₂ iron and/or chromium oxide species. Their reduction and catalytic properties are also in the focus of investigation. Methanol decomposition to hydrogen and CO is used as a test reaction due to its potential importance as a source of alternative clean and efficient fuel for vehicles, gas turbines and fuel cells and as a “heat pump”, utilizing the waste heat in the industrial processes as well [40], [41], [42].