Dissociative Chemisorption Of CH4 Research Articles

Site Selective Detection of Methane Dissociation on Stepped Pt Surfaces

We report a combined experimental and theoretical study comparing methane dissociation on three different platinum surfaces Pt(111), Pt(211), and Pt(110)-(1 × 2). Reflection absorption infrared spectroscopy (RAIRS) was used to detect chemisorbed methyl species formed by dissociative chemisorption of CH4 on specific surface sites and to measure surface-site-specific sticking coefficients of CH4 on the terrace, step, and ridge sites as function of incident translational energy. Methane dissociation is observed to be direct on all sites and diffusion of the chemisorbed methyl species is absent for surface temperature below 150 K. The experimental data are compared with the results of density functional (DFT) calculations that give minimum energy barriers for CH4 chemisorption that properly account for the experimental relative site-specific reactivities. Also in agreement with experiments, DFT results predict a negligible effect of co-adsorbed H and CH3 species on the vibrational frequency of a methyl group chemisorbed on terrace and step sites of Pt(211). However, the origin of the red-shift of the RAIRS peak of CH3 chemisorbed on terrace sites compared with that on step sites of Pt(211) remains elusive and still demands further investigation.

Mode selectivity in methane dissociative chemisorption on Ni(111)

Dissociative chemisorption of CH4 on transition-metal surfaces, representing the rate-limiting step in methane steam reforming, has been shown experimentally to be strongly mode selective. To understand the mode selectivity, a twelve-dimensional global potential energy surface is developed for CH4 interacting with a rigid Ni(111) surface based on a large number of density functional theory points. The reaction dynamics is investigated using an eight-dimensional quantum model, which includes representatives of all four vibrational modes of methane. After correcting for surface effects, key experimental observations, including the mode selectivity, are well reproduced. These theoretical results, along with mechanistic analysis, provide insights into this industrially important heterogeneous reaction.

Quantum state-resolved CH4 dissociation on Pt(111): coverage dependent barrier heights from experiment and density functional theory

The dissociative chemisorption of CH4 on Pt(111) was studied using quantum state-resolved methods at a surface temperature (T(s)) of 150 K where the nascent reaction products CH3(ads) and H(ads) are stable and accumulate on the surface. Most previous experimental studies of methane chemisorption on transition metal surfaces report only the initial sticking coefficients S0 on a clean surface. Reflection absorption infrared spectroscopy (RAIRS), used here for state resolved reactivity measurements, enables us to monitor the CH3(ads) uptake during molecular beam deposition as a function of incident translational energy (E(t)) and vibrational state (ν3 anti-symmetric C-H stretch of CH4) to obtain the initial sticking probability S0, the coverage dependence of the sticking probability S(θ) and the CH3(ads) saturation coverage θ(sat). We observe that both S0 and θ(sat) increase with increasing E(t) as well as upon ν3 excitation of the incident CH4 which indicates a coverage dependent dissociation barrier height for the dissociation of CH4 on Pt(111) at low surface temperature. This interpretation is supported by density functional calculations of barrier heights for dissociation, using large supercells containing one or more H and/or methyl adsorbates. We find a significant increase in the activation energies with coverage. These energies are used to construct simple models that reasonably reproduce the uptake data and the observed saturation coverages.

Dissociative chemisorption of CH4 on a cesiated Pt(111) surface studied by supersonic molecular beam scattering techniques

We have clarified the dynamics and mechanism of CH4 dissociative chemisorption processes on a cesiated Pt(111) surface by means of molecular beam scattering techniques. A comparison is made of similar processes on a Pt(111) surface. It is found that the CH4 molecules with enough energy to overcome the activation barrier for dissociation can no longer dissociate on a cesiated surface of Pt(111). Time-of-flight and angular intensity distribution measurements of the reflected CH4 molecules have revealed a similarity in inelastic collision dynamics both on the Pt(111) and the cesiated Pt(111) surfaces. Even a low concentration of the Cs layer as 0.06 of coverage is found to modify the surface electronic structure such that the activation barrier is enhanced as a result of an increased Pauli repulsive potential component in the potential energy surface. It is also found from time-of-flight measurements that direct dissociation is the only pass way and precursor mediated dissociation is not possible both on the Pt(111) and the cesiated Pt(111) surfaces in our experimental conditions.

Improved Properties of the Catalytic Model System Ni/Ru(0001)

The dissociative chemisorption of CH4 on Ni overlayers on Ru(0001) has been investigated. It is found that the initial sticking probability at T=530 K is approximately a factor of 20–30 higher on a pseudomorphic overlayer of Ni than on Ni(111) and a factor of two higher than on Ru(0001) illustrating the unique properties of metal-on-metal systems. The effect of enhanced reactivity is primarily ascribed to electronic effects induced by a straining of the Ni overlayer. The enhanced reactivity towards CH4 is accompanied by new features in the thermal desorption spectra of CO. The reactivity of the system depends strongly on the annealing temperature. Molecular beam experiments at high translational energy are qualitatively different from thermal data showing a monotonic decrease of the CH4 sticking probability as Ni is added.

Electronic structure calculations and dynamics of methane activation on nickel and cobalt

The dissociative chemisorption of CH4 on nickel and cobalt has been studied using different cluster models. Density functional theory is used to determine the structure and potential energy surface in the reactant-, transition state-, and product region. The transition state is explicitly determined on a single atom, a one layer 7-atom cluster and a spherical 13-atom cluster. We find transition state barriers of 41 kJ/mol for a single nickel atom, 79 kJ/mol for a single cobalt atom, 214 kJ/mol for the Ni7-cluster, 216 kJ/mol for the Co7-cluster, 121 kJ/mol for the Ni13-cluster, and 110 kJ/mol for the Co13-cluster. The overall reaction energies are −34, 6, 142, 135, 30, and 8 kJ/mol, respectively. The higher barrier for the single cobalt atom in comparison with the nickel atom can be attributed to the difference between both atoms in the occupation of the s-orbital in the lowest lying states. The higher and almost the same barrier for the 7-atom clusters can be attributed to the intrinsic lower reactivity of the central atom embedded in the cluster and the similar electronic nature of the atoms in the clusters; in both clusters the atoms have open s- and d-shells. The lower barrier for the 13-atom clusters compared with the 7-atom clusters is a result of each surface atom now having 5 bonds, which gives a more balanced description of the substrate model.

Theoretical investigation of CH4 decomposition on Ni: electronic structure calculations and dynamics

The dissociative chemisorption of CH4 on an Ni(111) surface has been studied using different cluster models. Density functional theory is used to determine the transition state and the dissociated state of CH4 on the substrate. The transition state is explicitly determined on a one-layer Ni7 cluster. We find a transition-state barrier of 210.3 kJ mol–1, which is much higher than our one-atom result of 40.7 kJ mol–1. The higher barrier can be understood in terms of the intrinsic lower reactivity of the central nickel atom in the cluster and a more extended CH bond. If we use as a substrate an Ni13 cluster, the barrier drops to 99.7 kJ mol–1. Vibrational frequencies are abstracted from the potential-energy surface at the transition state and the dissociated state. We have used transition-state theory to compute rate constants in terms of rotational, vibrational and translational partition functions. We have also determined sticking coefficients and activation energies for CH4 decomposition as well as CH3–H recombination of the surface. Sticking coefficients are small, which is consistent with experimental values. At 500 K a kinetic isotope effect of 6.2 is found for CH4 adsorption, while only a factor of 2.0 is found for CH3–H association at this temperature. The isotope effect on the activation energy for adsorption is 6.3 kJ mol–1, while essentially no effect for the activation energy for association is found.

A molecular beam investigation on activated dissociative adsorption of CH4 and N2 on Ni surface

The dissociative chemisorption of CH4 and N2 on Ni(poly) has been studied using molecular beam techniques. Chemisorption probabilities have been measured as a function of translational kinetic energy. The initial sticking probability was found to depend strongly on the incident energy Ek, scaling En =Ek cos 2θi. No dissociative chemisorption was observed for CH4 below En =44.0 kJ/mol and for N2 below En =46.1 kJ/mol. Above these threshold values, the initial sticking probability increases linearly from 0 to 0.54 as En increases from 44.0 to 47.9 kJ/mol for CH4 and from 0 to 0.03 as En increases from 46.1 to 83.3 kJ/mol for N2 . These results indicate that these systems proceed by a rather direct chemisorption process, rather than by a classical precursor mechnanism. The variation of the initial sticking probability with ‘‘normal’’ kinetic energy s0(En ) is analyzed to obtain the barrier height or apparent activation energy as 46.6 and 50.2 kJ/mol for CH4 /Ni(poly) and N2 /Ni(poly) systems, respectively. Finally, comparison with previous results and pressure gap effects in heterogeneous catalysis are discussed.

Dissociative chemisorption of CH4 on W(110): Dramatic activation by initial kinetic energy.

Etudes de l'energie d'incidence et des coefficients de collage en fonction de l'angle d'incidence, dans le cas de CH 4 et de CD 2 sur W(110)