Pt-CoOx nanoparticles supported on ETS-10 for preferential oxidation of CO reaction
Introduction
Among the many approaches to reduce CO concentration in the reformed gas mixture to the levels required for the use in low temperature Proton Exchange Membranes Fuel Cell (PEMFC), preferential catalytic oxidation of CO to CO2 (PROX) has been considered as the most promising [1]. An efficient catalyst for this reaction should convert CO avoiding the competing oxidation of H2. In fact, such an ideal catalyst should convert CO molecules in the presence of a large excess of H2, together with other components that can negatively affect the activity, like H2O or CO2 [2]. Typical reformate gas compositions at the exit of the water gas shift reactor include 15–20 vol.% CO2, 0.5–2 vol.% CO and about 15–25 vol.% H2O and H2 in high concentrations [3]. Due to the presence of these molecules in the reactor feed, the influence of H2O and CO2 on Pt group catalysts on different supports has been studied. On Pt-Co catalysts supported on alumina it was found that the presence of water showed a positive effect on CO conversion but only below 120 °C [4]. When aluminumphosphates were used as support a slight negative effect of CO2 on CO conversion was ascribed to the adsorption on the active sites for activating oxygen. In this case water also was found to have a negative effect on CO conversion probably because of the adsorption on the support [5]. Since the discovery of ETS-10 and ETS-4 by Kuznicki [6] these titanosilicates have attracted increasing interest and they have been studied extensively. In particular ETS-10 presents high capacity ion exchange isomorphic substitution and low acidity. We have previously studied the effect of water and CO2 on Pt–ETS-10 catalysts [3] and a strong inhibition on CO conversion was observed after the introduction of CO2. This was explained by taking into account the basic nature of ETS-10 that gives rise to a strong interaction with CO2, a reactant of acid nature. However, in the presence of water this effect was completely reversed. The water favored the formation of surface OH groups, which enhanced the Brønsted acidity of ETS-10 and compensates the strong inhibition effect of CO2. These observations make ETS-10 a good candidate as support for PROX reaction in the presence of water and CO2.
It is known that Pt catalysts are excellent for hydrogen oxidation, but in the presence of CO it is inhibited due to the strong CO adsorption and high CO coverage hindering the available sites for hydrogen and oxygen adsorption and further dissociation [7]. With increasing temperature the CO coverage decreases and the hydrogen oxidation starts competing for oxygen. The modification of the surface electronic structure and chemical properties on Pt, by the introduction of other subsurface 3d transition metals, was theoretically demonstrated at the beginning of this century by Kitchin et al. [8]. Since then the preparation and study of Pt bimetallic catalyst has grown exponentially and several reviews have been published in this field recently [9], [10]. To enhance the activity in the PROX reaction several non-noble metals have been added to the Pt catalysts such as Co, Ni, Cu, Fe, Mn, Sn [1], which clearly increase the activity of platinum in this reaction, among them Co and Fe seem to be the most promising.
The addition of cobalt to Pt nanoparticles on different supports was found to improve the catalytic activity for the PROX of CO [4], [5], [11], [12], [13], [14], [15], [16]. However only some of the catalysts were tested under simulated reformate streams containing all the components [4], [5], [11], [15]. There is a debate about the active phase in these catalysts for PROX. The group of Komatsu claimed that the active phase corresponds to the Pt3Co intermetallic compound [11], [17]. On the contrary Xu et al. proposed an architecture of the Pt-Co bimetallic catalyst consisting of Pt nanoparticles decorated with highly dispersed CoO nanostructures [18].
The reported Pt-Co catalysts tested in the PROX reaction were prepared by simultaneous or successive impregnation of precursor salts on the support followed by thermal treatments [4], [5], [10], [11], [13], [15]. By employing recent methods, developed in the field of nanotechnology for the synthesis of colloidal suspensions of metallic nanoparticles, Pt-Co nanoparticles can be synthesized in a controlled manner as preformed nanocatalysts before they are applied on support materials. This approach has been followed by several authors for different reactions such as Fischer-Tropsch [19], methanol oxidation reaction [20], CO2 hydrogenation [21] and selective carbonyl reduction in α,β-unsaturated aldehydes [22]. However, to the best of our knowledge, this strategy has not been pursued for the preparation of bimetallic Pt-Co catalysts tested in PROX reaction.
In this work Pt-Co nanoparticles were synthesized in solution and afterwards supported on microporous titanosilicate ETS-10. The materials were characterized by SEM, TEM, XRD, XPS and tested in PROX in a stream containing carbon dioxide and water besides a high concentration of hydrogen. The goal is to develop a two-step preparation method for controlling the synthesis of the bimetallic active phase, followed by deposition in an appropriate support. Furthermore we will get insight into the catalytic active phase thanks to the characterization of the bimetallic nanoparticles and the catalytic activity tests for different nanoparticle loadings and reaction conditions.
Section snippets
Nanoparticles synthesis
The synthesis of the nanoparticles was divided in two successive reduction steps for Co and Pt salts, using NaBH4 as reducing agent. The synthesis was performed at 0 °C under N2 atmosphere to avoid any oxidation. This procedure is based on a synthesis protocol previously reported by Du et al. [23]. In a typical synthesis 0.5 g of PVP (MW = 10000, Sigma-Aldrich) were dissolved in 100 mL of ethanolic solution 3.6 mM in CoCl2 (Sigma-Aldrich, anhydrous, purity ≥98%). Then 100 mL ethanol solution 1.35 mM in
Pt-Co nanoparticles characterization
Fig. 1 shows the TEM images (a) and particle size histogram (b) of the synthesized nanoparticles. The particles have spherical shape with diameter distribution obtained by measuring 100 particles in different images of 3.2 ± 0.6 nm. HAADF-STEM elemental mapping images clearly show that Pt and Co atoms are distributed throughout the whole particle (Fig. 1c). The coexistence of Pt and Co elements in the particles was further confirmed by the EDS analysis (Fig. 1d).
The bulk atomic Pt to Co ratio
Discussion
Pt-Co catalysts obtained by deposition of synthesized Pt-Co nanoparticles on ETS-10 with low platinum loading (1.4 wt%) achieve the complete CO conversion at 125 °C, working at WHSV = 30 L g−1 h−1 in presence of water and CO2. This activity was maintained for 50 h on reaction stream. In the literature there are works reporting the activity in the presence of H2O and CO2 of Pt-Co catalysts with low Pt loading. Ko et al. [16] reported a Pt-Co catalyst (0.5%Pt-Co/YSZ, Co/Pt = 10) that maintains the
Conclusions
Pt-Co catalysts were obtained by deposition of synthesized Pt-Co nanoparticles consisting of metallic platinum and cobalt and Pt oxides (Pt/Co bulk atomic ratio = 3) on ETS-10 support. The catalysts showed high activity for the selective oxidation of CO in a simulated reformate gas stream with high hydrogen concentration (74 vol%). The solid containing 1.4 wt% of nanoparticles can achieve complete CO conversion in the temperature range 120–150 °C working at WHSV = 30 L h−1 g−1. The stability of this
Acknowledgements
Financial support from MINECO (Spain)PRI-PIBAR-2011-1349 is acknowledged. The microscopy works have been conducted in the “Laboratorio de Microscopias Avanzadas” at “Instituto de Nanociencia de Aragon − Universidad de Zaragoza”. Authors acknowledge the LMA-INA for offering access to their instruments and expertise.
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