Alkane activation on crystalline metal oxide surfaces.

Advances in the fundamental understanding of alkane activation on oxide surfaces are essential for developing new catalysts that efficiently and selectively promote chemical transformations of alkanes. In this tutorial review, we discuss the current understanding of alkane activation on crystalline metal oxide surfaces, and focus mainly on summarizing our findings on alkane adsorption and C-H bond cleavage on the PdO(101) surface as determined from model ultrahigh vacuum experiments and theoretical calculations. These studies show that alkanes form strongly-bound σ-complexes on PdO(101) by datively bonding with coordinatively-unsaturated Pd atoms and that these molecularly adsorbed species serve as precursors for C-H bond activation on the oxide surface. In addition to discussing the binding and properties of alkane σ-complexes on PdO(101), we also summarize recent advances in kinetic models to predict alkane dissociation rates on solid surfaces. Lastly, we highlight computations which predict that the formation and facile C-H bond activation of alkane σ-complexes also occurs on RuO2 and IrO2 surfaces.

Selectivity in the initial C-H bond cleavage of n-butane on PdO(101).

We used temperature programmed reaction spectroscopy (TPRS) and molecular beam reflectivity measurements to investigate the initial dissociation of n-butane isotopologues on PdO(101) and determine kinetic parameters governing the selectivity of initial C-H(D) bond cleavage. We observe differences in the reactivity of the n-butane isotopologues on PdO(101) due to kinetic isotope effects, and find that the initial dissociation probability decreases with increasing surface temperature for each isotopologue. We performed an analysis of the dissociation kinetics using a model that is based on a precursor-mediated mechanism for n-butane dissociation and enables quantification of kinetic parameters for selective C-H bond cleavage by considering differences in the reactivity among the n-butane isotopologues. From the analysis, we estimate that 49% of the n-butane molecules which react during TPRS do so through 1° C-H bond cleavage when the initial coverage of n-butane lies between ∼40% and 100% of the saturation coverage of the molecular precursor state. For dissociation in the limit of zero coverage, we estimate that the conditional probability for 1° C-H bond cleavage is equal to ∼87% and varies only weakly with surface temperature from 300 K to 400 K. Analysis of the temperature dependent rate data further predicts that the barrier for 1° C-H bond cleavage is 3.5 kJ mol(-1) lower than that for 2° C-H bond cleavage for n-butane dissociation on PdO(101) in the limit of zero coverage. Our results provide evidence that the selectivity for 1° C-H bond cleavage on PdO(101) increases as the n-butane coverage decreases below ∼40% of the saturation value. We speculate that intermolecular interactions among the n-butane species are responsible for the apparent coverage dependence of the C-H bond selectivity for n-butane dissociation on PdO(101).

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.

Molecular adsorption of small alkanes on a PdO(101) thin film: Evidence of sigma-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.