Catalytic purification of hydrogen streams by PROX on Cu supported on an organized mesoporous ceria-modified alumina

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Abstract

In view of the stringent CO intolerance of the proton-exchange membrane fuel cells (PEMFCs), it is essential to eliminate even trace amounts of carbon monoxide from the reformate streams. The preferential oxidation (PROX) is considered to be a promising method for CO cleaning up. Catalytic systems, based on copper supported on ceria-modified organized mesoporous alumina, were investigated for the PROX of CO in hydrogen-rich gas stream. The catalysts showed an interesting activity in the CO-PROX. The sample with the best catalytic performance was characterized by N2 adsorption–desorption, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRPD), high-resolution transmission electron microscopy (HRTEM), temperature-programmed reduction by H2 (H2-TPR), and successive re-oxidation (TPO).

Introduction

Hydrogen, in combination with fuel cells, has been proposed, in the last years, as alternative energy source, for mobile and stationary applications, in order to substantially reduce the atmospheric pollution and the global dependency on fossil fuels [1], [2], [3]. In particular, hydrogen fed proton-exchange membrane fuel cells (PEMFCs) can produce electrical energy at low temperature with high efficiency and they are therefore particularly suitable for use on board of motor vehicles [4], [5], [6], [7].

Nevertheless, the hydrogen storage, distribution and transportation are, up to date, still major problems. A solution can be the on-site hydrogen generation from a suitable gas or liquid fuel and on board reforming of alcohols (mainly ethanol and methanol) is considered a technically feasible and convenient way to produce hydrogen suitable to feed a PEM fuel cell [8], [9], [10], [11], [12], [13], [14], [15], [16].

Conventional fuel reformers are based on complex multistage processes, such as steam reforming or oxidative steam reforming [8], [9], [10], [11], [12], [13], [14], [15], [16], where hydrogen-containing compounds (MeOH or EtOH) react with oxygen and water, producing hydrogen streams often containing variable amounts of carbon monoxide.

Reforming must be followed by gas conditioning, gas separation and purification stages because, under the operating temperatures, PEM fuel cells are extremely sensitive to even trace amounts (<50 ppm) of CO [17]. An effective route to CO removal is the preferential oxidation (PROX) of CO to CO2 in the presence of an excess of H2 that can reduce the carbon monoxide concentration in the feed to PEMFC down to few ppm [18], [19], [20].

The main reactions involved are:desiredreaction:CO+12O2CO2ΔH=67.6kcal/molundesiredreaction:H2+12O2H2OΔH=58.6kcal/mol

The reactions (1) and (2) are irreversible, exothermic and competitive.

The current catalysts for CO PROX are based on late transition metals such as Pt [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], Rh and Ru [31], [32], [33], [34], [35], [36], [37], [38] or coinage metals Au [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51] or Cu [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62] supported on oxides. Studies are also reported on nonprecious transition metals such as Co, Cr, Ni [63], [64]. One of the most interesting catalytic system is represented by the CuO-CeO2 association that resulted more selective and thermally stable than Pt or Au based catalysts [59], [60], [61], [62]. It is not entirely clear how the redox properties and the metal-ceria interaction affect the catalytic performance, although the facile Ce(III)-Ce(IV) conversion and the high oxygen storage capacity on the ceria appear to play significant roles [65], [66].

Moreover, the absence of precious metals in the composition of these catalysts constitutes a remarkable economic advantage, in view of their large-scale potential application in fuel cell-powered vehicles.

The very interesting properties of catalytic systems based on organized mesoporous oxides are also well known [67], [68], [69]. In this paper, we present our studies on hydrogen purification by CO-PROX catalyzed by copper supported on ceria-modified organized mesoporous alumina.

Section snippets

Reagents

All the materials used in this paper are Aldrich products and no further purification was carried out.

Catalyst preparation

The synthesis procedure, for the preparation of an organized mesoporous alumina, is a slight modification of that reported by Čejka et al. [70] and is based on the addition of aluminum tri-sec-butoxide (27.4 g) to a calculate amount of water (6.2 ml) and stearic acid (10.2 g), previously dissolved in 1-propanol (200 ml) by sonication, in order to reach the molar composition of: 1 Al(sec-BuO)3: 0.33

Catalytic activity measurements

The nomenclature used for the catalysts as well as their main compositional and textural properties are summarized in Table 1.

Conclusions

The use of a microcrystalline organized mesoporous γ-alumina, prepared by template sol–gel synthesis, allows one to obtain, after impregnation with metal salts and calcination, active Cu/Ce-based CO PROX catalysts with high surface area and with a good thermal stability. The synergistic interaction between Ce and Cu, that improves the reducibility of CuO, giving rise to the formation of surface Cu1+, as shown by TPR and XPS analyses, appears to play a significant role in determining the

Acknowledgements

MIUR-PRIN 2004 and INSTM are gratefully acknowledged for financial support by the Italian authors.

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