Selective CO oxidation over CuO-CeO2 catalysts prepared via the urea–nitrate combustion method

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Abstract

CuO-CeO2 catalysts have been proposed as a promising candidate catalytic system for CO removal from reformed fuels via selective oxidation. In this work, the performance of CuO-CeO2 catalysts, synthesized via the urea–nitrate combustion method, in the reaction of selective CO oxidation in the presence of H2 has been investigated. The combustion method was found to be a simple and fast route for the synthesis of ultrafine, nanocrystalline CuO-CeO2 catalysts. The influence of the fuel/oxidant (urea/nitrate) ratio and the Cu content on the catalytic properties of CuO-CeO2 catalysts has been studied and optimal values of these parameters have been determined. Compared to CuO-CeO2 catalysts prepared with other techniques, the catalysts prepared via the combustion method exhibited similar catalytic performance, remaining very active and stable, remarkably selective and with good tolerance towards CO2 and H2O.

Introduction

H2-fuelled, polymer electrolyte membrane fuel cells (H2-PEMFC) hold considerable potential as a cleaner, more efficient system than the currently used compression engines in vehicles. As long as hydrogen is difficult to store on a vehicle, employment of liquid fuels and on-board fuel processors has been proposed for the production of H2-rich fuel as a feed for the fuel cell stack [1], [2], [3]. The main units of a fuel processor are the reformer/POX reactor, the shift converter and the CO removal unit. Steam reforming/partial oxidation of alcohols or hydrocarbons, followed by the water-gas shift reaction produces a gaseous stream containing 40–75 vol.% H2, 15–25 vol.% CO2, 0.5–2 vol.% CO, a few vol.% H2O, and N2 [4], [5], [6]. Carbon monoxide generated in the fuel processor, must be removed in order to avoid poisoning of the anode electrocatalysts resulting in the degradation of the cell performance [1]. Bimetallic alloy electrocatalysts can tolerate <100 ppm CO [1], [7]. Catalytic methanation, palladium-based membrane purification and catalytic selective CO oxidation are the options to reduce CO concentration to acceptable levels. Among the aforementioned methods, selective oxidation of CO with molecular oxygen appears to be the simplest and most cost effective method for removing CO [1], [8]. Therefore, the development of selective CO oxidation catalysts has stimulated considerable interest worldwide. The most important requirement for the selective oxidation reaction is a high oxidation rate combined with high selectivity, with respect to the undesired H2 oxidation side reaction. Another important requirement is that these catalysts should be able to tolerate the presence of CO2 and H2O in the reformed fuel.

The catalysts proposed in the literature for this process are noble metal-based, such as alumina-supported platinum-group metal catalysts [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], zeolite-supported platinum catalysts [20], [21], and metal oxide-supported gold catalysts [22], [23], [24], [25], [26]. Gold-based catalysts were found to be remarkably more active than platinum-group metal catalysts at relatively low temperatures (<120 °C), but not so resistant towards deactivation by CO2 and H2O present in the feed [27]. In any case, all these catalysts cannot avoid significant losses of hydrogen due to oxidation. It has been also reported that removal of CO can take place over a CoO catalyst at 100 °C with a selectivity of 90% [28]. CuO-CeO2 mixed oxide catalysts have been recently proposed as a promising candidate for the selective removal of CO from reformate streams [27], [29], [30]. These catalysts are able to operate at a temperature range of 100–200 °C with almost ideal selectivity. Their performance is superior to that of Pt-group-based catalysts, since they are more active and remarkably more selective while operating at a lower reaction temperature. Comparing their performance with that of gold-based catalysts, they are less active but much more selective. Although noble metal catalysts have desirable air and temperature stability, their high cost may limit their applicability for transportation applications. Development of oxide catalysts without any precious metal in their composition, such as CuO-CeO2, is therefore attractive. The CuO-CeO2 catalytic system has been examined for several processes including (selective) CO oxidation [27], [29], [30], [31], [32], [33], CWO of phenol [34], SO2 reduction [35], NO reduction [36], methane oxidation [31], and water-gas shift reaction [37]. Several chemical methods for the preparation of CuO-CeO2 catalysts have been reported [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], and among these methods, co-precipitation [29], [30], [33] and sol–gel [27] have been applied for the preparation of CuO-CeO2 catalysts for removal of CO present in reformed fuels. The combustion method [36], [38], [39], [40] is an attractive technique for catalyst synthesis due to its low cost and simple preparation route, leading to homogeneous, high-surface-area materials without the need of additional calcination steps. On the other hand, co-precipitation or sol–gel techniques are pH-sensitive or require expensive precursors, respectively. The combustion method involves the autoignition of an aqueous solution containing an oxidizer (the corresponding metal nitrates) and an organic fuel, such as urea. The resulting material characteristics, such as surface area and crystallite size, are strongly dependent on the fuel/oxidant ratio [38], [39]. The combustion reaction is most vigorous and reaches high temperatures, when the fuel/nitrate molar ratio is close to its stoichiometric value [39], leading to large particles or agglomerates. Rapid evolution of a large volume of gases during the process cools immediately the product, limits the occurrence of agglomeration, thus leading to nanocrystalline powders.

In this paper, we report on the catalytic performance of CuO-CeO2 catalysts, prepared via the combustion method, for the selective CO oxidation in simulated reformate gas. The combustion-synthesized catalysts have been characterized by N2 adsorption–desorption, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR) by H2. Parameters of this study were the urea/nitrate ratio and the Cu content of CuO-CeO2 catalysts.

Section snippets

Catalyst preparation

The urea–nitrate combustion method was used for the synthesis of CuO-CeO2 mixed oxide catalysts. Cerium nitrate (Ce(NO3)3·6H2O), copper nitrate (Cu(NO3)2·3H2O), and urea (CO(NH2)2) were mixed in the appropriate molar ratios in a minimum volume of distilled water to obtain a transparent solution. The initial urea/nitrate molar ratio was adjusted according to the principle of propellant chemistry [38], taking into account that the urea/nitrate stoichiometric molar ratio is equal to 5(3−x)/6,

Urea–nitrates redox reaction

The intensity of the combustion process was strongly affected by the initial urea/nitrate molar ratio. It was found that the closer the ratio is to the stoichiometric value, the more violent is the autoignition. A similar behavior has been observed in other catalytic systems, such as Ni-YSZ cermets [39]. The autoignition is possible for a limited range of fuel/oxidant molar ratio, above and below the stoichiometric value, depending on the nature of fuel and oxidants [38], [39]. In this work,

Conclusions

In this study, CuO-CeO2 catalysts were prepared via the urea–nitrate combustion method and tested for the selective oxidation of CO in the presence of hydrogen. The resulting material characteristics and its catalytic properties for the selective oxidation of CO were strongly influenced by the urea/nitrate molar ratio used in synthesis. The fuel-rich samples were found to have favorable characteristics and catalytic properties. In contrast, use of a stoichiometric urea/nitrate ratio resulted in

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