platinum based alloys remain the best catalysts for many hydrocarbon reforming
and conversion processes in fuel cell applications. 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 CHx and C2Hy intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt14.13.8 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 CHx 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 C2Hy species on Pt35.
For all species we calculated heats of formation ΔHf to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CHx species is methylidyne (CH, BE=146.61\,kcal/mol), and ethylidyne (CCH3, BE=134.83\,kcal/mol) among the C2Hy 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.
Personnel: Dr. Timo Jacob