Pathways for C–H bond cleavage of propane σ-complexes on PdO(101)

Abstract

We used dispersion-corrected density functional theory (DFT-D3) calculations to investigate the initial C-H bond cleavage of propane σ-complexes adsorbed on the PdO(101) surface. The calculations predict that propane molecules adsorbed in η(1) configurations can undergo facile C-H bond cleavage on PdO(101), where the energy barrier for C-H bond activation is lower than that for desorption for each molecular complex. The preferred pathway for propane dissociation on PdO(101) corresponds to cleavage of a primary C-H bond of a so-called staggered p-2η(1) complex which initially coordinates with the surface by forming two H-Pd dative bonds, one at each CH(3) group. Among all of the adsorbed propane complexes, the staggered p-2η(1) complex has the highest binding energy and must overcome the lowest energy barrier for C-H bond scission. Analysis of the atomic charges reveals that propane C-H bond cleavage occurs heterolytically on PdO(101), and suggests that primary C-H bond activation is favored because a more stabilizing charge distribution develops within the 1-propyl transition state structures. Lastly, we conducted kinetic simulations using microkinetic models derived from the DFT-D3 structures, and find that the models reproduce the apparent activation energy for propane dissociation on PdO(101) to within 14% of that determined experimentally. We show that the entropic contributions of the adsorbed transition structures greatly exceed those predicted by the harmonic oscillator model, and that quantitative agreement with the apparent dissociation pre-factor may be obtained by approximating two of the frustrated adsorbate motions as free motions while treating the remaining modes as harmonic vibrations.

Kinetic Prefactors of Reactions on Solid Surfaces

“…2). A recent experimental study of the kinetic competition between dissociation and desorption of propane on PdO(101) also shows a ratio ν diss /ν des of ∼ 1/1000, that was also reproduced by DFT calculations when the extra entropy of the adsorbates mentioned above was approximately included [63].…”

“…This is the same reason that the hindered rotation of one methyl group about the C-C axis in gas-phase ethane contributes only low vibrational entropy at low temperature but, when k B T exceeds the barrier for that methyl rotation, that mode becomes a free rotor; but even when k B T is only 15% of the barrier, there is already a very large increase in entropy [11]. The large error of the HO approximation in estimating prefactors was recently pointed out for propane desorption from PdO(101) [42].…”

Adsorption of small hydrocarbons on rutile TiO2(110)

“…The work available in the literature in this regard has been largely limited to the adsorption of small nalkanes (C N H 2N+2 , N = 1 -12) on Al 2 O 3 (0001) [40], MgO(100) [41][42][43][44][45][46], and PdO(101) [47][48][49][50][51][52]. Using temperature programmed desorption (TPD), Slayton et al [40] first investigated the desorption of butane, hexane and octane from Al 2 O 3 (0001) under ultrahigh vacuum (UHV) conditions, and their data suggested that the relative magnitude of molecule-surface interactions compared to molecule-molecule interactions decreases with increased chain length.…”

“…A notable finding in this work was a linear increase in the desorption energy with a small y-intercept value, along with an observed six orders of magnitude increase in prefactor with increasing the molecular chain length [44]. In contrast with the physisorption of n-alkanes on Al 2 O 3 (0001) and MgO(100) surfaces, the adsorption of n-alkanes on the late transition oxide Pd(101) surface has been demonstrated to yield σcomplexes by datively bonding with the coordinatively-unsaturated Pd atoms, which act as precursors for the C-H bond activation of the n-alkanes larger than ethane [47][48][49][50][51][52]. Recent studies have predicted that other late transition-metal oxide surfaces should exhibit high reactivity toward alkanes due to the availability of both coordinately-unsaturated metal and oxygen surface sites [47].…”

Promotion of CO oxidation on PdO(101) by adsorbed H2O

“…The PdO(101) surface was modeled by a rectangular 4 × 1 unit cell, 172 with a corresponding 4 × 2 × 1 Monkhorst-Pack k-point mesh [21]. As 173 in our prior studies [33][34][35], the PdO(101) film was strained (a = …”