Supported perovskites for total oxidation of toluene

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Abstract

Supported LaCoO3 perovskites with 10 and 20 wt.% loading were obtained by incipient wetness impregnation of different Ce1−xZrxO2 (x = 0–0.3) supports with a “citrate” solution, prepared from La and Co nitrates, and citric acid. Ce1−xZrxO2 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 evidenced the formation of a pure perovskite phase. These materials were tested in toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T50 than the corresponding Ce1−xZrxO2 supports or pure LaCoO3 perovskite. The activity of the catalysts was found to increase with the perovskite loading, irrespective of the support composition. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO3.

Introduction

The need for clean, breathable and non-noxious atmosphere became a stringent request, regulated by the strict legislation in the last three decades. The large amounts of vapors of different proveniences (domestic, automotive exhaust, industry of almost any type) purged in the atmosphere are responsible for the global warming by means of the well-known “green-house effect” as well as the depreciation of the atmosphere quality, especially in and above urban and industrialized areas.

As CO2 production cannot be diminished, the efforts are focused on complete oxidation of gaseous pollutants. Direct thermal combustion with a flame has the major drawback of NOx production and is applicable only for concentrated streams. Therefore, alternative technologies are needed. Catalytic combustion (or catalytic total oxidation) appears to be a convenient way for the pollution control and prevention. Employed catalysts for VOC removal are mainly noble metals, very active at low temperatures. However, their limitations arise from the high price, their increase tendency to sinter and volatility. Therefore there is still a demand for alternative catalysts. Metal oxides of transitional metals (simple or mixed) have been studied for VOC removal from gaseous streams [1], [2], [3], [4]. Among mixed oxides, perovskite-type structures received a constant attention since the early 1970s when Voorhoeeve pointed to their potential use as total oxidation catalysts [5], [6]. Perovskites are a class of mixed oxides with a specific structure, having the general formula ABO3, where lanthanide elements usually are at the A-site position and first row transition metals at the B-site. They are less expensive and thermally more stable than noble metals. Some compositions were reported to possess similar or higher activity than Pt-supported catalysts [7], [8]. However, in spite of their good catalytic activity towards total oxidation (100% selectivity to CO2 for hydrocarbons) the use of perovskites is generally limited by the low surface area.

The most common approach to increase the contact surface between the perovskite and the reactant is the deposition on supports. With this respect, up to date, deposition of perovskites on thermally stable oxides seems to be the apparent solution. Since Gallagher and Johnson reported the preparation of supported LaMnO3 [9], [10], [11], other attempts were made, changing the nature or the shape of the support [8], [12], [13], [14]. When obtaining supported perovskites, two factors concur: the nature of the support and the preparation method chosen for active phase deposition. Unfortunately, the most common and largely accessible supports such as Al2O3 and SiO2 are reactive towards the elements contained in the perovskites, resulting in catalytically inert compounds such as spinnels [15]. This is mainly the case of Co-containing perovskites, Co easily diffusing into alumina network yielding CoAl2O4.

Fujii et al. [16] showed that if LaCoO3 or La0.8Sr0.2CoO3 were loaded on different oxides, the reaction rate for propane oxidation varied greatly with the nature of the oxide carrier. High catalytic activities, exceeding even the one of bulk La0.8Sr0.2CoO3 were obtained by using ZrO2 and CeO2. Unfortunately, the authors did not propose any explanation for this difference in behavior. Concerning ZrO2, it was also found that it enhanced the activity of LaNiO3 [17] for NOx reduction with CO and that of LaCoO3 and LaFeO3 [18], [19] for CO oxidation.

We have shown previously that the thermal decomposition of citrate precursors offers the possibility to obtain perovskite-type oxides at relatively low temperatures, such as 600–700 °C [20]. Zhang et al. [21] also reported that the citrate process leads to perovskites with small particle size and suggested that this process was promising for the preparation of supported perovskites [22].

The aim of the present paper is to explore the potential of Ce containing materials as carriers for LaCoO3 perovskite and to look for the influence of the support composition and of the perovskite loading. Both Ce and Zr are inert towards the components of the perovskite. Moreover, the presence of Ce might be helpful for Co-based perovskites as shown by previous papers on this subject [20], [23]. Toluene catalytic combustion was chosen as the test reaction.

Section snippets

Preparation of the support

The Ce1−xZrxO2 supports with x = 0–0.3 were prepared by the “citrate method” extensively described elsewhere [24]. Briefly, Ce(NO3)3 (from Fluka) and ZrO(NO3)2 (from Across) were dissolved in distilled water in appropriate amounts, mixed with citric acid monohydrate (from Merck), dried in a vacuum oven at 60 °C until the obtaining of a sponge-like material, and calcined at 700 °C for 3 h prior perovskite deposition.

Supported perovskites

La(NO3)3·6H2O (from Aldrich), Co(NO3)2·6H2O (from Merck), and citric acid monohydrate

Specific surface area and XRD investigations

Surface areas of the supports and catalysts are listed in Table 1. As observed before, the presence of zirconia in the matrix of ceria led to an increase of the surface area most probably due to a texture stabilization [24]. As expected, all catalysts presented lower surface areas than the corresponding supports. This drop in SSA increased with the perovskite loading, showing that the deposited material blocks the entrance of the pores. However, supported perovskites showed clearly larger

Discussion

The impregnation determined a reduction of the SSA of the support by 25–50%, proportional with the LaCoO3 loading. This decrease in SSA occurred very likely by the pore blocking by perovskite deposition. This fact was not unexpected since unsupported LaCoO3 has lower SSA than the support (Table 1) and XPS data from Fig. 3 pointed out to a preferential accumulation of the deposited perovskite at the external surface of the carrier.

XRD measurements performed on supports showed only the cubic

Conclusions

Supported LaCoO3 perovskites have been obtained by ceria–zirconia solid solutions impregnation via citrate method. From the structural point of view no formation of new phases, that may suggest an interaction of La or Co with the elements of the support, has been detected. This behavior certifies the suitability of the preparation method and of support choice.

Formation of LaCoO3 perovskite phase was detected at the surface of the supports, and SSA and XPS showed that this phase tends to

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