Chemisorption of (CH_xand C₂H_y) Hydrocarbons on Pt(111) Clusters and Surfaces from DFT Studies

Abstract

We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH x and C 2 H y intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt 14.13.8 cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (σ-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt 3 )-CH prefers a µ 3 hollow site (fcc), (Pt 2 )CH 2 prefers a µ 2 bridge site, and PtCH 3 prefers µ 1 on-top sites. Vinyl leads to (Pt 2 )CH-CH 2 (Pt), which prefers a µ 3 hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt 2 )CdCH(Pt), retaining a CC π-bond while binding at a µ 3 hollow site (fcc), and HCCH, which binds as (Pt)HCdCH(Pt), retaining a π bond that coordinates to a third atom of a µ 3 hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (∆H f ) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH x species is methylidyne (CH, BE ) 146.61 kcal/mol), and ethylidyne (CCH 3 , BE ) 134.83 kcal/mol) among the C 2 H y molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.

The Mechanism of Forming H2O from H2 and O2 over a Pt Catalyst via Direct Oxygen Reduction

“…However, the aim now is to study the dissociation and reaction processes of more extended compounds, and hence, the Pt-cluster is enlarged to consider 35 atoms (hereafter Pt 35 ) with 14 atoms in the first, 13 in the second, and 8 atoms in the third layer. The reliability of this model has already been demonstrated for the adsorption of various hydrocarbons [23]. In addition, comparison of the O binding characteristics obtained with Pt 28 and Pt 35 shows the desired cluster-size convergence.…”

“…For the current systems this would be H 2 and O 2 at STP and bulk-fcc Pt. As discussed and applied in [23], a slightly different reference set was chosen, based on the following cluster-approximation: (i) The Pt 35 cluster is considered to be the standard state:…”

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

“…This can result in an adsorption energy for CH of more than 50 kJ mol À1 lower than that of isolated C atoms. [40][41][42]58] , smaller in comparison with past DFT studies [37,38] using the Perdew-Wang 91 (PW91) xc functional, [59] which is known to slightly overestimate the interaction between atoms. On the other hand, the RPBE functional often provides more accurate adsorption energies [45] and, in fact, the present adsorption energy of atomic H, 254 kJ mol…”

Molecular Dynamics Simulations of the Interactions between Platinum Clusters and Carbon Platelets

“…57 In these DFT studies, we found that all the hydrocarbons bind covalently such that a configuration is achieved in which each C is almost tetrahedrally structured with the missing H atoms replaced by covalent bonds to surface Pt atoms (see Figure 2). Thus, CH and vinyl prefer fcc hollow sites, CH 2 prefers a bridge site, and CH 3 prefers on-top binding.…”