Hydrocarbon rearrangements on Pt surfaces

Platinum catalysts and their alloys remain the catalyst of choice for many hydrocarbon reforming and conversion processes so important in chemical and petrochemical applications. In addition, there is increased interest in improving Pt-based catalysts for such critical applications as fuel cells. The chemistry of C₁ and C₂ hydrocarbons is important for many industrial reactions on Pt and these reactions serve as prototypes for many others. Knowledge of the structures, energetics, and barriers should be useful in characterizing the mechanisms and improving the processes. Although many experiments have characterized small hydrocarbon species on various metal surfaces, there remains considerable uncertainty about the structures and energetics of the chemisorbed intermediates and the role they play in various reaction pathways. To aid in developing improved catalysts, it is useful to know these quantities for a complete set of reaction intermediates.

Using the B3LYP flavor of density functional theory (DFT) we studied the chemisorption of all CH_x and C₂H_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, (Pt₃)CH prefers a μ₃ hollow site (fcc), (Pt₂)CH₂ prefers a μ₂ bridge site, and PtCH₃ prefers μ₁ on-top sites (see Figure 1).
Figure 1: Different CH_x species on Pt₃₅.

Vinyl leads to (Pt₂)CH–CH₂ (Pt), which prefers a μ₃ hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt₂)C=CH(Pt), retaining a CC π-bond while binding at an μ₃ hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a π-bond that coordinates to a third atom of an μ₃ hollow site (fcc) to form an off center structure (see Figure 2). These structures are in good agreement with available experimental data.


Figure 2: Different C₂H_y species on Pt₃₅.

For all species we calculated heats of formation ΔH_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 (CCH₃, BE=134.83\,kcal/mol) among the C₂H_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.


Figure 3: Decomposition of ethylene with di-σ-ethylene as initial compound.

Afterwards we used the heats of formation to analyze reactions such as hydrogenation and decomposition of ethylene (see Figure 3), but also the conversion of ethene to ethane (see Figure 4). Both chemically important reactions are still under discussion regarding the different involved reaction steps. For the decomposition of ethylene we were able to exclude CHCH₂ and CCH₂ as intermediates.

Figure 4: Stepwise hydrogenation of ethylene.