Identifying the adsorbed active intermediates on Pt surface and promotion of activity through the redox CeO2 in preferential oxidation of CO in H2
Graphical abstract
Different mechanisms for activation of reagents molecules for non-promoted and promoted Pt catalysts with ceria in preferential oxidation of CO in H2.
Introduction
From environmental point of view, the hydrogen fuel cell is an efficient system to decrease CO2 in the atmosphere. The hydrogen feed produced by reforming of hydrocarbons, biogas reforming, steam reforming or partial oxidation reactions contains about 5–15% CO, CO2, H2O and CH4. So, the water-gas shift (WGS) reaction is carried out to reduce the CO amount to 0.5–1%. To avoid the poisoning of Pt anode by CO, the CO in H2-atmosphere is lowered to be less than 10 ppm by preferential oxidation reaction (PROX) of CO in H2 (CO + 1/2O2 → CO2). Great efforts have been spent to develop CO-tolerant Pt-based anode, since the poisoning of Pt anode continue to be object of many studies to resolve this problem.
Up to now a variety of catalysts have been developed for PROX-CO based on Pt-group metals such as Pt [1], [2], [3], [4], [5], Au [6], [7], [8], [9], Pd, Ru, Rh [10], [11], [12] and transition metals as Cu, Co, Fe and others [13], [14], [15]. Among the noble metals Au and Pt-based catalysts have shown very good catalytic properties. Since, the fuel cells utilize significant amount of Pt, it is necessary Pt content to be minimized in the catalyst for PROX. One alternative is to use promoters that allow decrease of Pt loading while maintaining a high activity and selectivity. The combination of Pt with redox oxides such as ceria or niobia could lead to obtain highly effective catalyst systems with high activity and CO oxidation selectivity in PROX of CO.
However, the catalysts for PROX are very complex and dynamic systems because their behaviors depend on the structural and electronic properties, as well as on the reaction temperature and atmosphere. In this context it is well known, that CeO2 due to its excellent properties is extensively used in heterogeneous catalysts, but these properties strongly depend on the used pretreatment conditions and on the exact composition of the oxide. Studies showed [1], [2], [3], [4], [5] that ceria acts as oxygen buffer for oxidation reactions due to its redox properties.
Pt/CeO2 catalyst within the catalysts with Pd, Rh, Ir or Ru was not only the most active in PROX, but also the most selective catalyst, reaching near 100% CO2 at low temperature of about 100 °C [1]. Ceria with the highest oxygen mobility at the oxide surface was shown to be the support for supported noble metals [16]. The linear relationship between the amount of adsorbed water and the CO2 production over Pt/CeO2 catalyst [17] indicated that the hydrated ceria supplies an oxidizing agent at the metal/support interface reacting with the nearby surface carbonyls on the Pt particles yielding CO2. Studying the mechanism of the PROX reaction on Pt/ceria catalysts by in situ DRIFT at different partial pressures of O2 and/or CO [18] it was found that the significantly oxygen vacancy formation was correlated with the enhanced CO oxidation activity and the water desorption was hindered. Therefore, the highly hydrated ceria with significant vacancy density was found to be beneficial for the PROX process.
Niobia-based materials are effective catalysts in several applications, such as in epoxidation, hydrocarbon conversion reactions or in photochemistry, but last decades the interest to niobium-based catalysts for PROX was increased due to their redox properties. It was shown [19] that at high Nb loading in Pt/Nb/Al2O3, the selectivity to CO2 decreases, while the selectivity to H2 oxidation increases in opposite to the observed selectivity at low Nb content. A low selectivity was observed for Pt-Sb/Nb2O5 and Pt/Nb2O3 catalysts in the reaction of CO oxidation in the presence of H2 [20]. Pt supported on mixed CeO2-Nb2O5 oxide also showed high CO conversion in PROX [21].
In spite of the great number of studies on the effect of niobia and ceria as supports or modifiers of supported Pt catalysts, especially in the case of CeO2, there is still no resolved problems and questions about the nature of the active sites in PROX and the reaction mechanism. Since several parameters impact on the catalyst activity such as synthesis method, pretreatment atmosphere and temperature, reducing agent that are related to the properties of metallic nanoparticles such as their particle size, morphology, local bonding and the number of surrounding atoms and the interaction with support. Therefore, there are different opinions about the nature of active sites in the PROX of CO.
In the present study we attempted to obtain a more detailed characterization of non- and modified with ceria or niobia alumina-supported Pt catalysts in relation to the effect of the content of Pt and of modifier on their structural and electronic properties, to identify the kind and nature of the active sites in the PROX of CO. Different techniques have been used for the physicochemical characterization of the catalysts such as: adsorption-desorption N2 isotherms, X-ray diffraction (XRD), Diffuse reflectance Fourier transform infrared spectroscopy (DRFTIR), temperature programmed reduction (TPR), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) spectroscopy and scanning transmission electron microscopy (STEM).
To our knowledge, for first time in this work ΔXANES was used to give information about the change in the strength of the bond of Pt with different adsorbates as H2, CO and O2 and the type of bonding sites at Pt surface as a function of temperature in different atmosphere of H2, H2/CO and H2/CO/O2 mixtures.
Section snippets
Samples preparation
Alumina support was prepared by the sol-gel method, based on the methodology proposed by Vasquez et al. [22]. Initially, 0.1 mol of aluminum tri-sec-butoxide (Merc Schuchardt OHG, Germany) was added to a 250 ml glass flask. Over this precursor, 0.8 mol of 2-Butanol solvent (Sigma Aldrich) were added. This mixture was maintained at 85 °C for 1 h under moderate agitation. Then, 0.4 mol of 1,3-butanediol (Sigma Aldrich) was added slowly for gel formation, and vigorously stirred for 2 h at 85 °C. After
Textural properties
The specific surface area (SBET) and pore volume (Vp) of supported Pt samples are summarized in Table 1. In addition, the metallic dispersion (D) obtained by the H2 chemisorption and the calculated hydrogen consumption during TPR are included in Table 1. It is important to note that the used sol-gel method for preparation of γ-Al2O3 leaded to obtain a carrier with a high surface area (318 m2/g). The surface areas and pore volumes of non-promoted alumina-supported Pt samples are almost similar,
Conclusions
γ-Al2O3 with a high surface of 318 m2/g was obtained by sol-gel method that was applied as a suitable carrier for alumina-supported Pt catalysts. Two series of supported Pt catalysts were prepared: non-modified and modified with reducible oxides of ceria and niobia with different content.
The efficacy of Pt catalysts in the process of preferential oxidation of CO in H2 is a result of the combined influence of their electronic and structural characteristics, which change as a function of the
Acknowledgments
The authors gratefully acknowledged the financial support from FAPESP (Project No 2011/50727-9 and 2010/11385-2 and fellowship grant No 2012/00567-8) and to CNPQ (Project No 407030/2013-1), as well as to Brazilian Synchrotron Light Laboratory (LNLS, Campinas-Brazil) for the XAFS beam line experiments. S.D. is acknowledged to contract DFNI E02/16/2014 for financial support.
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