Jeremy Kua, Francesco Faglioni and William A. Goddard III
Using first principles quantum mechanics (nonlocal density functional theory with exact exchange, B3LYP), we studied the bonding and electronic states for clusters of metal atoms to develop the interstitial electron model (IEM) in which
Using this model we have examined such reaction intermediates as H, CHx, C2Hx CHxOy, OHx, and selected C3 and C4 organics. These intermediates were examined for the on-top, bridge, and cap sites of Ru, Rh, Pd, Os, Ir, and Pt. We find some simple thermochemical concepts that apply fairly well across all of these systems. Thus in nearly every case these organics bond covalently to the surface atoms leading to tetrahedral hybridization of each carbon bonded to the surface. Thus, (i) CH3 prefers an on-top site (a bond energy of ~54 kcal/mol for Pt), (ii) CH2 prefers a bridge site (a bond energy of ~104 kcal/mol for Pt), and (iii) CH prefers a threefold site (a bond energy of ~167 kcal/mol for Pt). We also find that CH2CH2 forms a strong di-s bond (~36 kcal/mol for Pt) and CCH3 is the thermodynamic sink of the C2Hx species. These results are all in good agreement with experimental results on Pt(111). These results indicate that Pt is most favorable for dehydrogenation of organics while Ru is the least favorable. In addition, chemisorbed organic species display thermochemical additivity effects similar to Benson group additivities for gas phase organics. For example, the binding energies of CH3, CH2CH3, CH(CH3)2, and C(CH3)3 are 54, 49, 41 and 31 kcal/mol respectively, for CH2, CHCH3, and C(CH3)2 they are 104, 98 and 85 kcal/mol respectively, for CH and CCH3 they are 167 and 156 kcal/mol respectively. Trends due to adding CH3 groups include an increase of charge transferred to the metal, lengthening of M-C bonds, decrease in binding energies, increase in steric effects and increase in electronic excitation contribution to lowering of the binding energy.
We used these results to consider the electrocatalysis of methanol oxidation in direct methanol fuel cells (DMFCs). We find that methanol dehydrogenation is most facile on Pt, with the hydrogens stripped first off the carbon end. Water dehydrogenation is most facile on Ru. These results support the bifunctional mechanism of Pt-Ru alloys in DMFCs. They suggest that this reaction goes through the COOH intermediate. We find that pure Os is capable of performing both these functionalities without co-catalyst.
We believe that this approach of studying large numbers of intermediates at all feasible sites on a large number of metals will lead to the deeper understanding of the thermochemistry and mechanism required for designing new generations of catalysts. Such quantum chemical methods should provide data for combinatorial strategies to discover and design new catalysts.
The research was funded by
the NSF and ARO (in collaboration with Illinois Institute of Technology
and Schrodinger, Inc.). Some calculataions were carried out at the NCSA,
University of Illinois.
For slides of the full talk, see below
The Interstitial Electron Model/Choice of Cluster (6 pages) PDF
C1 and C2 organics on Pt group metals (12 pages) GIF
Application: Direct Methanol Fuel Cells (13 pages) PDF