Complex interplay of structural and surface properties of ceria on platinum supported catalyst under water gas shift reaction
Graphical abstract
Introduction
In the water gas shift (WGS) reaction, CO reacts with steam producing CO2 and H2. It is an important reaction to decrease the CO level in the syngas, deriving from the process of steam reforming of natural gas or other sources. In addition, WGS is one of the main reactions to produce H2 at industrial scale. Industrially, it is performed in two steps: the first one at high temperature, with iron-based catalysts, and the second one at low temperature, with copper-based catalysts [1]. Recently, the development of PEM (proton exchange membrane) fuel cell technology, which requires H2 with low CO levels, has boosted the search for new WGS catalysts.
Platinum is a non-pyrophoric WGS catalyst with high activity but the stability of Pt nanoparticles and their activity depends on the nature of the support [2], [3], [4], [5], [6]. One of the most studied supports is ceria. Thirty years ago, changes on ceria redox behavior caused by the interaction with platinum were demonstrated [7], [8]. Later, several studies pointed out that Pt is anchored on CeO2 through PtOCe bond, improving Pt stability and its catalytic properties [5], [9], [10], [11], [12].
Cerium has a low redox potential changing its oxidation state from Ce4+ to Ce3+ and leading to the formation of non-stoichiometric oxides (CeO2-x), with capacity to store and release oxygen dependent on the atmosphere [8]. It has been shown that several aspects such as crystallographic orientation in cerium oxide, particle size, presence of doping atoms (Ti4+, La3+, Zr4+, etc.), presence of alkali metals, among others, affect the formation of oxygen vacancies, the interaction with gases and/or the interaction with metallic particles impacting its catalytic performance [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Nevertheless, one important aspect that has not been deeply tackled is the direct impact of the structural characteristics of nm-sized ceria phase in the overall activity.
Despite extensive work, some issues about WGS on Pt-ceria remain under intense debate: (i) the main reaction mechanism, (ii) the reaction rate limiting step, (iii) the species that are real intermediates and those that are mere spectators and (iv) the adsorption sites (e.g., on metal, support or interface). Two main mechanisms have been discussed: redox and associative [15], [23], [24], [25], [26], [27], [28]. The redox pathway involves the participation of O atoms from the support while the associative mechanism the O is provided by H2O activation. Variants such as associative with redox regeneration involving OH species generated with the O from the support have been proposed [27]. The intermediaries such as formates, carboxylates, carbonates, etc, that are formed in the associative pathway have also been debated. It seems now consensus that a unified mechanism for all Pt catalysts does not exist, depending on the Pt size, support and interaction as well as on the catalytic conditions.
To address these questions, different approaches have been adopted. To evaluate the role of the metallic phase and support, for example, metal loading, particle size and composition are varied using the same support; or the metal loading is fixed and the support composition is varied. By tuning parameters such as size, shape, structure and composition of both phases, it has been possible to highlight not only the metal influence but also the support and the metal-support influence [17], [20], [22], [23], [29], [30], [31], [32], [33]. For example, by comparing Pt/CeO2 with Ru/CeO2 and Pt-Ru/CeO2 catalysts, Xu et al. [34] showed that the vacancies formation and the CO adsorption were favored by alloying Pt with Ru while the formation of formates and methane production decreased. However, alloy formation did not affect activity indicating that formates were spectators or were not involved in the main reaction pathway in the used conditions. When Pt and CeOx were co-deposited on TiO2 [6], [16], [22], a higher degree of reduction of the CeOx nanoparticles was observed under WGS and assigned as a key factor to explain the improved catalytic performance of this catalyst. Kalamaras et al. [17] studied Pt/CexZr1-xO2 catalysts and also correlated the increase in the number of vacancies in the CeO2 lattice due to Zr addition to the enhanced catalytic activity. The reaction pathway was extremely dependent on the reaction temperature, Ce/Zr atomic ratio and Pt particle size, the latter in accordance to the structure-sensitive nature of the WGS reaction. The correlation between catalyst activity and its reducibility was also studied on platinum ceria and ceria-gallia catalysts but different conclusions were obtained [15]. Although the data confirmed that activation of water involves oxygen vacancy filling in the support, this is a fast process and not the rate-determining step. This contrast with conclusions from earlier works by Rodriguez et al. [35] and Bruix et al. [29]. Concerning the reaction mechanism, Vecchietti et al. [15] suggested that monodentate formate and carboxylate are reaction intermediates and are located at the metal-support interface while carbonate and bidentate formate are spectators.
Important insights about the reaction mechanisms have been obtained by SSITKA (Steady-State Isotopic Transient Kinetics Analysis) coupled with spectroscopic techniques, such as DRIFTS (Difuse Reflectance Infrared Fourier Transform Spectroscopy) [25], [36], [37]. In the case of Pt supported on CeO2, extensive work in a broad set of catalyst [17], [18], [19], [20], [21], [22], [23], [38] clearly show a bifunctional mechanism operating in this system, in which both Pt particle size and the nature of the support directly impact on the number and kind of active sites. The main mechanism is also dependent on the catalytic conditions [23], [25]. In addition, DFT (Density Functional Theory) calculations are also helping to improve the knowledge about the WGS reaction mechanism. Recent work by Clay et al. [39] suggests that even in the case of Pt supported on Al2O3, considered an “inert” support, the support plays an active role. In the case of Pt on CeO2, this has been clearly demonstrated [27], [28], [29], [40]. Despite the kinetic parameters and energies found by DFT are still not fully consistent with the experimental ones [27], [28], [39], DFT is helping to point out the most energetically favored pathways. For example, Aranifard et al. [27], [28] showed the importance of associative carboxyl mechanism on Pt/CeO2 and that a formate mechanism is energetically unfavored in the analyzed framework.
Therefore, in the case of Pt supported on CeO2 a complex scenario takes place, and in this context, it is known that catalyst properties can be significantly affected under reaction conditions and in situ studies are critical to correctly access catalyst electronic and structural properties [11], [12], [41], [42], [43], [44], [45], [46], [47]. In addition, the use of pre-formed colloidal nanoparticles, with narrow size distribution, has provided an insightful approach to produce model catalysts, to address independently and in an uncorrelated way several parameters such as composition, size, shape and structure of either metallic or oxide phase [48], [49], [50].
In this work, we performed a detail characterization of platinum-cerium-alumina model catalysts under WGS conditions by using in situ techniques and pre-formed platinum nanoparticles (Pt-NPs), which were encapsulated in Al2O3 sol-gel modified with CeO2 or encapsulated in CeO2-Al2O3 sol-gel. Our results show a lack of direct dependence of the activity with the average ceria structural properties suggesting that the creation of the interfacial PtOCe sites is likely to be the limiting factor in these samples.
Section snippets
Catalysts preparation
The synthesis of Pt-alumina catalyst is described elsewhere [49]. Briefly, colloidal Pt-NPs protected by polivinilpirrolidone (PVP), with 2.8 nm diameter, were added during the sol-gel synthesis of bohemite to produced, after drying the gel and calcining at 500 °C, the PtAl catalyst (1.5 wt% Pt).
Promoted cerium PtAl catalysts, with 12 or 20 wt% of CeO2, were prepared by two methods, both using Ce(NO3)3·6H2O (99.9% Aldrich) as precursor. In the first one, an aqueous solution of cerium precursor was
Results
Table S2 shows the catalysts surface area and CeO2 crystallite size. The incorporation of Pt-NPs does not significantly affect the textural properties of the γ-Al2O3 [49] whereas adding cerium lead to a small decrease in surface area as previously reported [58], [59].
Fig. 1a presents the diffraction pattern of PtAl before calcination corresponding to the boehmite precursor. Similar XRD patterns (not shown) were obtained for all sample [51]. Diffraction patterns after calcination are shown in
Discussion
By preparing the catalysts from the same batch of colloidal nanoparticles it was possible to obtain Pt-Al2O3 and Pt-CeO2-Al2O3 catalysts with similar platinum dispersion as previously described [49], [50]. This strategy of synthesis allowed evaluating independently and in an uncorrelated manner the impact of the structural and redox properties of the CeOx on the alumina support in the WGS reaction. By changing the method used to produce the ceria-alumina support it was possible to raise
Conclusion
Catalysts are very complex and dynamic systems since their behavior depends on the structural and electronic properties of different species and on the temperature and atmosphere they are submitted to. In this work we show that the presence of Ce in PtAl catalysts increases the WGS specific reaction rates up to seven times in the WGS reaction. Several parameters impact the activity and the use of pre-formed Pt-NPs was the strategy adopted to prepare model systems focusing on the support and
Acknowledgements
The authors are grateful for the financial support of FAPESP (Fundação de Apoio à Pesquisa do Estado de São Paulo), proc. 2011/50727-9, 2010/52291-0, 2012/00523-0 (BEPE program), and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico). We thank the Brazilian Synchrotron Light Laboratory (LNLS) for the access to the XAFS1 beamline and XPS facilities and the Brazilian Nanotechnology National Laboratory (LNNano) for the access to the TEM. LNLS staff is acknowledged by the support
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