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

Abstract

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.

The Positive Effects of Nanoclay on the Hydraulic Conductivity of Compacted Kahrizak Clay Permeated With Landfill Leachate

“…14 found to be low on the under‐coordinated site. This result is supported by other ultra high vacuum studies of methane adsorption on PdO(101) 15, 16.…”

“…Methane adsorption and dissociation was investigated over the (101) surface cut from a tetragonal bulk lattice. As only PdO adopts this structure, the other oxides should be considered as model systems that allow investigation of the electronic reasons for the low activation energy previously reported for methane dissociation over PdO(101) 14–16. The investigated metals are chosen to probe how the filling of the d‐shell affects the methane dissociation energy.…”

Strong Kinetic Isotope Effect in the Dissociative Chemisorption of H2 on a PdO(101) Thin Film

“…We have recently reported evidence of σ complex formation in the adsorption of small alkanes on PdO(101) as well. 22 A similar type of dative bonding has also been observed and examined in detail for H 2 on the RuO 2 (110) surface, 14 and Blanco-Rey and co-workers have presented details of the bonding of H 2 on the PdO(101) surface. 23 We will report our details of the nature of H 2 bonding on the PdO(101) thin film elsewhere.…”

Adsorption of small hydrocarbons on rutile TiO2(110)

“…The offset for the highly active PdO(101), which efficiently oxidizes all alkanes except methane and ethane [47,48], was found to be even higher, 25 kJ/mol [74,77], and was attributed to the dative bonding interactions between the alkanes and the cus-Pd atoms of the PdO(101) surface [47,50,51,74]. The alkane length independent enhancement suggested that the molecule-surface dative bonding produces a similar contribution to the total binding energy for all the n-alkanes (C 1 to C 5 ) studied.…”

“…Further comparisons can be made for small n-alkanes where absolute coverages were reported on Pt(111) and PdO(101)[15,49,52,74]. For example, using molecular beam techniques in combination with a calibrated leak, Carlsson and Madix found that the saturation monolayer coverages of C 1 -C 4 n-alkanes on Pt(111) change only slightly with chain length.…”