Preferential CO oxidation in a hydrogen-rich stream over gold supported on Ni–Fe mixed metal oxides for fuel cell applications
Graphical abstract
Introduction
Recently, the oxidation of CO at low temperatures in the presence of hydrogen has received considerable attention in the fuel cell industry. In most cases, hydrogen used in fuel cells, especially the proton exchange membrane (PEM) fuel cells, is produced through the hydrocarbon reforming process. The resulting reformate gas mixture contains about 1% CO, in addition to CO2, H2O and H2 [1], [2]. CO is harmful to PEM fuel cells since it adsorbs on the surface of the platinum electro-catalysts causing its performance to diminish rapidly [3]. Gold supported as nanoparticles on metal oxides or mixed metal oxides is known to effectively remove CO in the presence of hydrogen at low temperatures [1], [3], [4]. It has been suggested that the activity of gold catalysts depend on a number of factors, such as particle size, nature of support, catalyst preparation method, calcination temperature and catalyst pre-treatment [5], [6], [7]. Studies conducted by Haruta [8], [9] and Haruta and Daté [10], showed that the particle size of gold is key in CO adsorption. They proposed that small gold particles provide the steps, corners and edges onto which CO adsorbs. Using FT-IR, they demonstrated that CO adsorption peaks disappeared as the particle size increased to larger than 10 nm and concluded that larger particles are smooth and do not favour the adsorption of CO. This was supported by Valden et al. [11] who suggested that the CO oxidation reaction catalysed by gold reaches a maximum when the diameters of Au nanoparticles are 3.5 nm. Apart from the particle size, Hutchings et al. [12] argued that the activity of gold catalysts depend on the presence of both cationic and metallic gold. In their experiments, calcined and uncalcined Au/Fe2O3 catalysts were used. The uncalcined catalyst exhibited better activity than the calcined catalyst. XPS analysis showed that calcined catalysts contained mostly, metallic gold, whereas, the uncalcined catalyst contained a combination of oxidation states.
Reducible oxide supports such as CeO2, TiO2, NiO, Fe2O3 and their binary mixed oxides have been extensively studied because of their ability to undergo reduction, thereby providing oxygen mobility [13], [14], [15], [16]. In general, the role played by reducible metal oxide supports has been understood to provide oxygen vacancies by undergoing a redox reaction during preferential CO oxidation (CO-PROX). In other studies, mixed metal oxides have been employed as supports since their surface are enhanced by synergistic effects [13], [15], [16], [17], [18], [19], [20], [21]. Among the reducible oxides and mixed oxides, NiO and Fe2O3 are the most extensively studied supports for gold in the preferential oxidation of CO [5], [22], [23], [24], [25], [26]. Schubert et al. [25] studied the effect of gold on a series of supports for CO PROX and concluded that Au/α-Fe2O3 was a much better catalyst than the others on account of its activity, selectivity and long term stability, whereas Au/NiO displayed reasonable activity with good stability. Avgouropoulos et al. [27] further demonstrated that the Au/α-Fe2O3 catalyst was an efficient catalyst in CO-PROX even in the presence of both CO2 and H2O in the feed.
In this report, NiO–Fe2O3 mixed oxides prepared from hydrotalcite-like precursors are used as supports for gold in CO-PROX. Hydrotalcites or hydrotalcite-like precursors are compounds that contain a double layered hydroxide structure that has a brucite-like network [28]. These compounds have a general formula of where M2+ = Zn, Ni, Cu, Mn, M3+ = Al, Fe, Cr and A = , , Cl− [28], [29] and often give medium to high surface area mixed metal oxides after calcination at moderate temperatures [30].
Section snippets
Catalyst preparation
Mixed metal oxide supports were prepared by co-precipitation according to the method described by Labajos et al. [28]. A solution containing nickel and iron salts, obtained by dissolving 87.24 g of Ni(NO3)2.6H2O and 60.6 g of Fe(NO3)3.9H2O in 200 mL of double distilled water and another 200 mL solution containing 40.6 g of Na2CO3.10H2O and 21.6 g NaOH were simultaneously added drop wise to a beaker containing a small amount of water. The pH of the mixture was maintained between 8 and 10. The
XRD
The hydrotalcite-like precursors were not well crystallized as shown in Fig. 1, but the formation of the layered structure was clearly evident [25].
The first three peaks at lower 2θ values at about 10, 23 and 35°, correspond to the harmonic planes (003), (006) and (009), respectively. The doublet at about 2θ = 60° is due to the (110) and (113) planes, respectively [25], [26]. After calcination at 550 °C, characteristic peaks at around = 36°, 43°, 63° for both samples are indexed to NiO
Summary and conclusion
Mixed metal oxides containing NiO and Fe2O3 were successfully prepared by the co-precipitation method and the deposition of gold on these oxides was close to nominal loadings. Higher gold loading and higher oxygen content improved catalytic performance. The reducibility of the supports was enhanced by the presence of gold and XPS results show that gold was in the metallic state for all the catalysts. The catalytic activity also depended on gold dispersion and particle size. The amount of Ni in
Acknowledgements
The authors would like to thank Hydrogen South Africa (HySA) and the National Research Foundation in South Africa for their financial support.
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