Quantum-chemical calculations of CO and OH interacting with bimetallic surfaces
Introduction
Most currently considered CO tolerant catalysts for low-temperature polymer membrane fuel cell catalysts consist of platinum and a second or even third metal to improve overall performance [1], [2]. The most important issues in contemporary fuel cell electrocatalysis are related to developing a material that either oxidizes CO (as present in fuel reformate or as an intermediate in direct methanol oxidation) at low overpotential, or that adsorbs a limited amount of CO while still capable of oxidizing hydrogen at an acceptable rate [1], [2], [3].
There are many fundamental surface-science studies devoted to the elucidation of the factors involved in CO adsorption and oxidation on well-defined electrode surfaces [3], [4], [5], [5](a), [5](b), [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. These studies have mainly focused on bimetallic Pt systems, i.e. Pt either alloyed or modified with other elements such Ru, Sn, Mo, Re, Pd, and Rh. The system that has been studied in the greatest detail is PtRu, as this is still one of the most stable and active CO tolerant catalysts under practical conditions. It is generally believed that the mechanism by which CO is removed from the PtRu surface is the so-called ‘bifunctional mechanism’ [20]. In this mechanism, CO adsorbed on Pt is oxidized preferentially by an oxygen-containing surface species on Ru. Most authors write the oxygen-containing surface species as OH, even if the exact nature of this species is still unknown. In reaction equations, the bifunctional mechanism reads as:
However, this mechanism does not take into account a possible change in the CO binding energy on Pt induced by Ru (often referred to as an electronic effect), nor does it directly describe the effect of CO and OH competitive adsorption on the Ru. For instance, recent experiments using well-defined PtSn and PtMo electrodes [7], [8], [21], [22] have shown that both surfaces are better CO oxidation catalysts than PtRu, even though both may not be particularly useful as practical CO tolerant hydrogen or methanol oxidation catalysts. This enhanced performance compared with PtRu was ascribed to a lack of adsorption of CO on either Sn or Mo, leaving more adsorption sites for oxygen-containing species. In a sense, they should correspond more closely to ‘true’ bifunctional catalysts, with one element interacting preferentially with the first reactant, and the other element with the second reactant.
In this paper, we describe the results of quantum-chemical calculations of CO and OH interacting with a variety of bimetallic surfaces, in order to assess from the computational viewpoint the molecular nature of the bifunctional mechanism and the electronic effects involved. We will consider in some detail the results of density-functional calculations of CO and OH adsorption on PtRu, PtMo, PtSn, and a number of Pt-modified transition metals such as Rh and Ir. All surfaces are modeled as semi-infinite slabs in a periodic supercell. The advantage of using quantum-chemical calculations is that they give direct information on the binding energetics of the different species, which are relatively difficult to extract from electrochemical measurements. Moreover, calculations permit to establish the quantum-chemical nature of the surface bond and the different factors involved, and how these might be related to measurable properties of chemisorbates such their vibrational characteristics. In this way, quantum-chemical calculations provide invaluable information to complement and correctly interpret experimental data, even if the calculated systems considered may appear rather idealized compared with the experimental catalysts. This computational approach to understanding catalytic and electrocatalytic systems is obviously not new (for recent reviews, see Refs. [23], [24]), and we will compare our results to other calculations where appropriate. In particular, there have been some pioneering semi-empirical cluster calculations by Anderson related to fuel cell catalysis [25], [26], [27]. More recently, DFT cluster and slab calculations for CO and OH on PtRu have appeared and we will compare our results to those computations [28], [29], [30], [31], [32]. Especially the slab calculations (as considered here) are of interest as these are now well established to give the most reliable results.
Section snippets
Computational methods
DFT-GGA periodic slab calculations were performed using the Vienna Ab Initio Simulation Package VASP [33], [33](a), [33](b), [33](c). Unless specified otherwise, the (√3×√3)R30° unit cell was used to model the adsorption of CO and OH, corresponding to a 1/3 ML coverage on the surface. The surfaces were modeled as four-layer slabs, with the atomic distances fixed to the calculated bulk lattice distance, and a vacuum, corresponding to five equivalent layers. The top metal layer was relaxed in all
CO and OH on PtRu
The current state-of-the-art CO tolerant electrocatalyst is a platinumruthenium bimetallic alloy. As mentioned in the introduction, the mechanism for CO tolerance is believed to involve the adsorption of oxygen containing species (OHads) on the ruthenium sites such that CO adsorbed on platinum can participate in a bimolecular reaction with the activated oxygen species.
Recently we reported a detailed study of CO and OH adsorption on PtRu alloy surfaces [32]. The binding energies and
Conclusions
In this paper, we have presented the results of a periodic DFT-GGA study of CO and OH interacting with variety of bimetallic surfaces.
As our most important conclusions, concerning the PtRu alloy, we found that mixing of Pt by Ru weakens the binding of both CO and OH to the Pt surface sites. By contrast, the CO and OH binding gets stronger as Pt is mixed in. The surface with the weakest CO binding energy in our calculation is PtML–Ru(0001). Interestingly, a similar surface was recently found to
Acknowledgements
This work was financially supported by the Energy research Center of the Netherlands (ECN) and the Royal Netherlands Academy of Arts and Sciences (KNAW).
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2020, Journal of CatalysisCitation Excerpt :Bimetallics in which the secondary site is selective for OH* adsorption – as is usually argued to be the case for Pt-Sn and Pt-Mo systems – contain secondary sites (i.e. Sn, Mo) that have a low affinity for CO* binding, but a high affinity for OH* [45]. This differs from the Pt-Ru system, where the Ru sites bind both CO* and OH* more strongly than Pt, and therefore the ability of Ru to provide OH* to remove CO* from the Pt sites could, under certain conditions, be precluded by CO* poisoning on the Ru sites themselves [28,44]. This distinction highlights the importance of accounting for the coverages of all species on both site types when performing a kinetic analysis of potentially-bifunctional catalysts.