Coadsorption of Butadiene and Hydrogen on the (111) Surfaces of Pt and Pt₂Sn Surface Alloy: Understanding the Cohabitation from First-Principles Calculations

Abstract

Co-adsorption is a key initial step in heterogeneous catalysis, which by bringing the reactants together at high coverage on the surface of the catalyst has a clear implication on the catalytic reaction activity. We show that when using Density Functional Theory (DFT) calculations, the choice of the exchange correlation functional can have a qualitative influence on the nature of the obtained most stable coadsorption state. The coadsorption of butadiene and hydrogen, the precursor state for catalytic hydrogenation, is studied on Pt(111) and on the surface alloy Pt2Sn/Pt(111). At typical hydrogenation conditions, the PBE exchange correlation functional gives as most stable situation on both model catalysts a surface fully covered with hydrogen, butadiene remaining in gas phase. This behavior does not agree with available experimental data, and results from an incorrect balance between H and butadiene adsorption energy, mainly by a poor description of dispersion energy for butadiene adsorption. The non-local optPBE-vdW functional provides opposite and correct results, with a co-adsorption of butadiene and hydrogen as most stable situation. The butadiene adsorption energy is strengthened by the description of dispersive forces, hence modifying the nature of the energetic competition between the two adsorbates. The co-asorption energy difference between PBE and optPBE-vdW amounts to 1.04 (resp. 0.7) eV for Pt(111) (resp. Pt2Sn/Pt(111)) on the considered 3x3 unit cell. The computational study of co-adsorption systems from DFT is hence delicate. Errors do not only impact the quantitative adsorption energy of one adsorbate, but they might cumulate over several species, finally providing a qualitatively wrong picture of the optimal coadsorption situation. Going beyond the standard Generalized Gradient Approximation hence appears mandatory for a correct description of catalytic reactivity of unsaturated hydrocarbons.

Atomistic Models for Highly‐Dispersed PtSn/γ‐Al2O3 Catalysts: Ductility and Dilution Affect the Affinity for Hydrogen

“…The most stable structures are reported exhaustively in Supporting information S6, a selection of these being shown in Figure . It systematically appears that tin atoms are not favorable sites for hydrogen adsorption, in line with results obtained on flat alloyed surfaces . For the system investigated here (in particular, with the (100) alumina surface orientation), H spillover from the cluster to the support was not found to take place spontaneously.…”

“…The exchange‐correlation functional was treated within the generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerhof (PBE) and the electron‐ion interaction was described by the projector augmented wave (PAW) scheme with an energy cutoff of 400 eV. Note that corrections for dispersion forces were not included in the present work for two reasons: i) for the sake of consistency with previous studies that we want to make comparison with, ii) and because such refinements to the PBE energies and forces were shown to be rather low in the case of PtSn surfaces in interaction with hydrogen . The situation might be more critical for the quantification of the interaction with hydrocarbons, which we do not consider here, or for the description of the metal‐support adhesion in some cases .…”

“…Note that corrections for dispersion forces were not included in the present work for two reasons: i) for the sake of consistency with previous studies that we want to make comparison with, ii) and because such refinements to the PBE energies and forces were shown to be rather low in the case of PtSn surfaces in interaction with hydrogen . The situation might be more critical for the quantification of the interaction with hydrocarbons, which we do not consider here, or for the description of the metal‐support adhesion in some cases . Spin‐polarized calculations were performed for non‐supported clusters and used the interpolation formula of Vosko, Wilk and Nusair .…”

“…None of these studies includes at the same time support and environment effects. To the best of our knowledge, with the exception of an earlier study of H 2 interaction with isolated Pt−Sn dimers, hydrogen adsorption has been studied on ideal and infinite alloyed Pt 2 Sn surfaces only …”

“…In what follows, we will preferentially use the expression dilution (or ensemble) instead of geometric effect which may stand for a broader range of structural phenomena. For extended surfaces, both kinds of effects lead to a lower affinity of platinum‐tin for hydrogen as compared to platinum . It remains thus of crucial interest to combining this effect of Pt−Sn alloying with the nano‐size effect as previously studied for Pt 13 clusters, to predict how the structural, electronic, and hydrogen adsorption properties will be affected in the case of a “ductile” cluster, different from a “rigid” extended surface.…”