Catalytic oxidation of CO over ordered mesoporous platinum
Introduction
One of the most commonly used catalysts today is Pt/Al2O3 which consists of platinum particles supported on alumina. This type of catalyst is used to oxidize, e.g., volatile organic compounds in the exhausts from process industries, and hydrocarbons and CO in exhaust gases from diesel engines. The noble metal is typically present as small crystallites or clusters dispersed on the alumina surface providing a high active surface area and thus effective use of the expensive precious metal. These deposited platinum particles are typically a few nanometers in size and adopt a convex or possibly faceted surface.
Templated synthesis methods using self-organizing media such as lyotropic liquid crystals can be used to prepare ordered mesostructured materials. In 1997 Attard et al. reported that ordered mesoporous platinum could be prepared by chemical reduction of hexachloroplatinic acid (HCPA) in the presence of a liquid crystalline template [1]. Additionally, it was shown that these liquid crystal phases could be used to electrodeposit platinum films containing an ordered mesoporous structure [2], [3], [4]. Recently, Yamauchi et al. have shown that the choice of reducing agent is highly significant for the formation of the mesoporous network [5]. Accordingly, the nucleation process and hence the choice of reducing agent is crucial in soft templating synthesis of long-range ordered mesoporous platinum.
In contrast to conventional platinum nanoparticles, mesoporous platinum may thus contain an ordered arrangement of pores throughout the structure. Although, the ordered mesoporous platinum particles may have convex perimeter surfaces, the internal pore surface can be prepared such that it is mainly concave [5]. For structure sensitive reactions, in heterogeneous catalysis, the activity and/or selectivity vary with the surface structure of the active phase [6]. It is thus of interest to compare the catalytic performance of an ordered mesoporous hexagonal platinum phase (H1Pt) to that of conventional Pt nanoparticles. An interesting reaction for such an evaluation is the important CO oxidation reaction, which is known to be structure sensitive at low CO concentrations [6], [7], [8], [9], [10], [11], [12], [13], [14]. Recently, we reported that mesoporous H1Pt/Al2O3 exhibits different catalytic properties compared to more conventionally prepared Pt/Al2O3 catalysts [15]. Here, we further evaluate the catalytic and structural properties of the H1Pt/Al2O3 catalyst thus highlighting the differences between mesoporous and conventionally prepared platinum. The structure of the two platinum samples was analyzed using TEM, SEM-EDX, wide- and low-angle XRD and N2 physisorption. In addition, the influence of pre-treatment on the catalytic properties of the two different types of platinum was evaluated using oxygen step-response experiments and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT).
Section snippets
Preparation and characterization of hexagonal mesoporous (H1)-platinum
Ordered mesoporous platinum was prepared following the procedure described by Attard et al. [1]. A mixture containing 4.50 g nonionic surfactant Brij76 (Aldrich), 1.50 g hexachloroplatinic acid, HCPA (Aldrich), and 1.42 g H2O was prepared forming the H1 liquid crystalline phase, as confirmed using polarized light microscopy. After formation of the crystalline phase, the HCPA was reduced to metallic Pt using a thin sheet of steel. A thin layer of the mixture was thus spread onto a glass tile and
Characterization of platinum samples
The nitrogen adsorption/desorption isotherms for H1Pt and Pt-black are shown in Fig. 1. The pore-size distributions for the two samples, calculated from the nitrogen adsorption isotherms [20] are shown in the inset in Fig. 1. The H1Pt sample shows a narrow pore-size distribution with a mean pore diameter of 3.4 nm, whereas the Pt-black sample shows a broad peak with maximum at a considerably larger diameter, 15 nm. Moreover, the measured BET surface areas are 34.9 and 34.2 m2/g for H1Pt and
Conclusions
Hexagonal mesoporous H1Pt, prepared using the method developed by Attard et al. was deposited on Al2O3 and evaluated as catalyst for CO oxidation in comparison with Pt-black/Al2O3. The H1Pt/Al2O3 catalyst showed ignition at lower temperatures but extinction at higher temperatures compared to Pt-black/Al2O3. These findings were in agreement with results from oxygen step-response experiments at constant temperature, where the H1Pt/Al2O3 catalyst showed ignition at lower oxygen concentrations in
Acknowledgements
This work has been performed at the Competence Centre for Catalysis, which is financially supported by The Swedish Energy Agency, AB Volvo, Volvo Car Corporation, Scania CV AB, GM Powertrain Sweden AB, Haldor Topsøe A/S, and The Swedish Space Corporation. A.E.C.P. thanks the Swedish Research Council for a Senior Researcher grant. M.Sc. Elin Becker at Chalmers University of Technology is thanked for her assistance with the FTIR spectroscopy measurements. Ph.D. Kjell Wikander at Chalmers
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