Ceria-based oxides as supports for LaCoO₃ perovskite; catalysts for total oxidation of VOC

Abstract

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.

Introduction

Increasing pollution problem needs a fast and effective answer [1]. The progress made in the last 30 years for atmosphere quality preservation since the large-scale application of TWC were accompanied by a parallel development of non-environmental-friendly industries. Among various types of pollutants, volatile organic compounds (VOC) are of particular concern due to their high toxic potential and their easiness of spreading over large areas through the atmosphere. With this respect, drastic international regulations were applied for different sources of such compounds [2]. Usually, VOC streams are lean. This requires very active catalysts and if possible with a high heat capacity, especially when reverse-flow reactors are used [3], [4]. Noble metals are the most used catalysts for environmental protection against VOCs but in view of economical and practical features the trend is their replacement with cheaper oxides [5], [6], [7]. Improved properties of mixed oxides against simple ones are well known, especially in the environmental protection field [7], [8]. Tunable properties or synergetic effects due to composition places these materials on top of preferences for large scale applications. Among mixed oxides, perovskites occupy an important place both for fundamental studies (usually physics, solid physics) and applicability [9]. Perovskites were largely studied as substitutes for the actual total oxidation catalysts. However, the major limitation of these results were their lower surface area and their increased tendency to sinter. In this view, the only solution of increasing the contact surface between the VOC and the perovskite is to disperse it on a large surface area and thermally stable support. Two problems arise when supporting an active phase: the preservation of the structure or accepting/generating a controlled alteration and a good and uniform dispersion. With respect to the first requirement different attempts have been made, using mainly alumina-based supports [10], [11], [12], [13]. However, in spite of the advantages offered by alumina, the drawback of catalytically inactive spinel-phase formation with the transitional metals usually used for catalytic purposes focused the research to other directions. Other papers claim that magnesia [14], [15] offers a good matrix for perovskite dispersion. Ceria is a basic component of various catalytic converters due to its properties of oxygen buffer [16], [17]. The use of ceria and zirconia as suitable supports for La–Co-perovskites is previously reported and interesting in the presented works are the enhanced catalytic activities with respect to the corresponding bulk perovskites. High catalytic activities for propane combustion, exceeding even the one of bulk LaCoO₃ or La_0.8Sr_0.2CoO₃ were obtained by using inert ZrO₂ and CeO₂ as supports [18]. LaCoO₃ and LaFeO₃ supported on ZrO₂ were more active than the corresponding bulk perovskites for CO oxidation [19], [20]. Apart from increased activity, such materials possess a good heat capacity pointing to their possible employment in adiabatic reverse-flow reactors and were successfully tested for methane combustion [3], [21].

With respect to the second requirement for a supported catalyst, namely a uniform coating of the carrier, some authors pointed to the use of the citrate method for obtaining mixed oxides supported phases with a good control of stoichiometry and small particle size and suggested that this approach is promising for both the preparation of supported perovskites and ceria–zirconia supports [22], [23], [24]. Previous results reported by our team for toluene complete oxidation were obtained on catalysts prepared by incipient wetness impregnation [25]. The catalytic results were promising but, however, the physico-chemical characterizations pointed to certain heterogeneity of the coating with the dispersed perovskite phase. Aiming at obtaining a uniform coating of the support, in the present study another approach for perovskite dispersion, namely wet impregnation by allowing a longer contact time between the precursor and the carriers, is used [25].

Present work deals with preparation of supported LaCoO₃ by wet impregnation on different Ce_1−xZr_xO₂ supports and comparative investigation of the catalytic behaviour for benzene and toluene complete oxidation.