Can Hakanoglu

Molecular adsorption of small alkanes on a PdO(101) thin film: Evidence of σ-complex formation

We investigated the molecular adsorption of methane, ethane, and propane on a PdO(101) thin film using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. The TPD data reveal that each of the alkanes adsorbs into a low-coverage molecular state on PdO(101) in which the binding is stronger than that for alkanes physically adsorbed on Pd(111). Analysis of the TPD data using limiting values of the desorption prefactors predicts that the alkane binding energies on PdO(101) increase linearly with increasing chain length, but that the resulting line extrapolates to a nonzero value between about 22 and 26 kJ/mol at zero chain length. This constant offset implies that a roughly molecule-independent interaction contributes to the alkane binding energies for the molecules studied. DFT calculations predict that the small alkanes bind on PdO(101) by forming dative bonds with coordinatively unsaturated Pd atoms. The resulting adsorbed species are analogous to alkane sigma-complexes in that the bonding involves electron donation from C-H sigma bonds to the Pd center as well as backdonation from the metal, which weakens the C-H bonds. The binding energies predicted by DFT lie in a range from 16 to 24 kJ/mol, in good agreement with the constant offsets estimated from the TPD data. We conclude that both the dispersion interaction and the formation of sigma-complexes contribute to the binding of small alkanes on PdO(101), and estimate that sigma-complex formation accounts for between 30% and 50% of the total binding energy for the molecules studied. The predicted weakening of C-H bonds resulting from sigma-complex formation may help to explain the high activity of PdO surfaces toward alkane activation.

Dispersion-corrected density functional theory calculations of the molecular binding of n-alkanes on Pd(111) and PdO(101)

We investigated the molecular binding of n-alkanes on Pd(111) and PdO(101) using conventional density functional theory (DFT) and the dispersion-corrected DFT-D3 method. In agreement with experimental findings, DFT-D3 predicts that the n-alkane desorption energies scale linearly with the molecule chain length on both surfaces, and that n-alkanes bind more strongly on PdO(101) than on Pd(111). The desorption energies computed using DFT-D3 are slightly higher than the measured values for n-alkanes on Pd(111), though the agreement between computation and experiment is a significant improvement over conventional DFT. The measured desorption energies of n-alkanes on PdO(101) and the energies computed using DFT-D3 agree to within better than 2.5 kJ/mol (< 5%) for chain lengths up to n-butane. The DFT-D3 calculations predict that the molecule-surface dispersion energy for a given n-alkane is similar in magnitude on Pd(111) and PdO(101), and that dative bonding between the alkanes and coordinatively unsaturated Pd atoms is primarily responsible for the enhanced binding of n-alkanes on PdO(101). From analysis of the DFT-D3 results, we estimate that the strength of an alkane η(2)(H, H) interaction on PdO(101) is ~16 kJ/mol, while a single η(1) H-Pd dative bond is worth about 10 kJ/mol.

High Selectivity for Primary C–H Bond Cleavage of Propane σ-Complexes on the PdO(101) Surface

We investigated regioselectivity in the initial C-H bond activation of propane σ-complexes on the PdO(101) surface using temperature programmed reaction spectroscopy (TPRS) experiments. We observe a significant kinetic isotope effect (KIE) in the initial C-H(D) bond cleavage of propane on PdO(101) such that the dissociation yield of C(3)H(8) is 2.7 times higher than that of C(3)D(8) at temperatures between 150 and 200 K. Measurements of the reactivity of (CH(3))(2)CD(2) and (CD(3))(2)CH(2) show that deuteration of the methyl groups is primarily responsible for the lower reactivity of C(3)D(8) relative to C(3)H(8), and thus that 1° C-H bond cleavage is the preferred pathway for propane activation on PdO(101). By analyzing the rate data within the context of a kinetic model for precursor-mediated dissociation, we estimate that 90% of the propane σ-complexes which dissociate on PdO(101) during TPRS do so by 1° C-H bond cleavage.

Effects of non-local exchange on core level shifts for gas-phase and adsorbed molecules

Density functional theory calculations are often used to interpret experimental shifts in core level binding energies. Calculations based on gradient-corrected (GC) exchange-correlation functionals are known to reproduce measured core level shifts (CLS) of isolated molecules and metal surfaces with reasonable accuracy. In the present study, we discuss a series of examples where the shifts calculated within a GC-functional significantly deviate from the experimental values, namely the CLS of C 1s in ethyl trifluoroacetate, Pd 3d in PdO and the O 1s shift for CO adsorbed on PdO(101). The deviations are traced to effects of the electronic self-interaction error with GC-functionals and substantially better agreements between calculated and measured CLS are obtained when a fraction of exact exchange is used in the exchange-correlation functional.