Microcanonical unimolecular rate theory at surfaces. II. Vibrational state resolved dissociative chemisorption of methane on Ni(100)

Abstract

A three-parameter microcanonical theory of gas-surface reactivity is used to investigate the dissociative chemisorption of methane impinging on a Ni(100) surface. Assuming an apparent threshold energy for dissociative chemisorption of E(0)=65 kJ/mol, contributions to the dissociative sticking coefficient from individual methane vibrational states are calculated: (i) as a function of molecular translational energy to model nonequilibrium molecular beam experiments and (ii) as a function of temperature to model thermal equilibrium mbar pressure bulb experiments. Under fairly typical molecular beam conditions (e.g., E(t)>/=25 kJ mol(-1), T(s)>/=475 K, T(n)</=400 K), sticking from methane in the ground vibrational state dominates the overall sticking. In contrast, under thermal equilibrium conditions at temperatures T>/=100 K the dissociative sticking is dominated by methane in vibrationally excited states, particularly those involving excitation of the nu(4) bending mode. Fractional energy uptakes f(j) defined as the fraction of the mean energy of the reacting gas-surface collision complexes that derives from specific degrees of freedom of the reactants (i.e., molecular translation, rotation, vibration, and surface) are calculated for thermal dissociative chemisorption. At 500 K, the fractional energy uptakes are calculated to be f(t)=14%, f(r)=21%, f(v)=40%, and f(s)=25%. Over the temperature range from 500 K to 1500 K relevant to thermal catalysis, the incident gas-phase molecules supply the preponderance of energy used to surmount the barrier to dissociative chemisorption, f(g)=f(t)+f(r)+f(v) approximately 75%, with the highest energy uptake always coming from the molecular vibrational degrees of freedom. The predictions of the statistical, mode-nonspecific microcanonical theory are compared to those of other dynamical theories and to recent experimental data.

Six-dimensional quasiclassical and quantum dynamics of H2 dissociation on the c(2 × 2)-Ti/Al(100) surface

“…The fact that the H 2 + Ti/Al(100) system, which has a deep well in front of the barrier, has a rotational and vibrational efficacy close to 1.0 suggests that the total (internal and translational) energy undergoes complete randomization in the initial conditions (translational, rotational, and vibrational energy), making the reaction rate solely dependent on the energy. This agrees with the microcanonical unimolecular rate theory of dissociative chemisorption for CH 4 on Ni(100) developed by Harrison and co-workers, 22 suggesting that it should be possible to compute accurate reaction rates with this theory.…”

“…Both experimental 16,17 and theoretical 19 studies show that this system has rotational and vibrational efficacies of 0.40 and 0.65, respectively. Thus, the statistical model 22 with a randomization of the initial conditions cannot be applied to this direct reaction. One question addressed here is whether the same conclusion can be drawn for the late barrier system H 2 + Ti/Al(100), which has a deep molecular adsorption well in front of the barrier.…”

On the origin of mode- and bond-selectivity in vibrationally mediated reactions on surfaces

“…21 In solution, quenching limits IVR, but on a metal surface efficient quenching imposes an even stricter window on the time available for IVR, 18,30 with vibrational lifetimes ranging from about 1 ps for CO chemisorbed on Pt(111) to more than 10 ps for other small molecules adsorbed on metal surfaces. 58,59 These lifetimes are shorter than, or comparable to those for the fastest IVR processes observed in large molecules.…”

“…Harrison and co-workers adapted microcanonical unimolecular rate theory to describe the dissociative chemisorption of methane and other small molecules on surfaces. [19][20][21][22][23] By adjusting the number and frequency of surface phonon modes that participate in reaction and neglecting electronically non-adiabatic processes, they reproduced a wide range of internal-state-averaged bulb and beam-surface scattering data. Statistical theories are useful benchmarks for experimental measurements of reaction dynamics.…”

Mode-specific reactivity ofCH4onPt(110)−(1Bisson1, Sacchi2, Beck3 2010Phys. Rev. B553310View full textAdd to dashboardCite

“…These observations provide clear evidence for mode-specific reactivity for CH 4 on Pt͑110͒-͑1 ϫ 2͒ which contradicts a basic assumption of statistical reaction rate theory. 13 Furthermore, the different efficacies for promoting the reaction of these four vibrational states provide information on the relevant degrees of freedom that theoretical models should consider to give a realistic description of CH 4 dissociation dynamics.…”