State-Resolved Gas-Surface Reactivity of Methane in the Symmetric C-H Stretch Vibration on Ni(100)

Abstract

The state-resolved reactivity of CH 4 in its totally symmetric C-H stretch vibration ( 1 ) has been measured on a Ni(100) surface. Methane molecules were accelerated to kinetic energies of 49 and 63:5 kJ=mol in a molecular beam and vibrationally excited to 1 by stimulated Raman pumping before surface impact at normal incidence. The reactivity of the symmetric-stretch excited CH 4 is about an order of magnitude higher than that of methane excited to the antisymmetric stretch ( 3 ) reported by Juurlink et al. [Phys. Rev. Lett. 83, 868 (1999)] and is similar to that we have previously observed for the excitation of the first overtone (2 3 ). The difference between the state-resolved reactivity for 1 and 3 is consistent with predictions of a vibrationally adiabatic model of the methane reaction dynamics and indicates that statistical models cannot correctly describe the chemisorption of CH 4 on nickel.Activated dissociation of molecules on a metal surface is a fundamental step in many catalytic processes. An important example is the chemisorption of methane on nickel to form surface-bound methyl and hydrogen; this reaction is the rate-limiting step in steam reforming, which is the principal process for industrial hydrogen production. The importance of this process has incited a number of studies, both theoretical and experimental, directed towards understanding the detailed mechanism of methane chemisorption [1-15]. Molecular-beam experiments have shown that methane chemisorption on nickel is a direct process, activated by translational and vibrational energy [1,2]. More recent state-resolved experiments investigating the reactivity of CH 4 excited to the antisymmetric stretch fundamental vibration ( 3 ) [6] and first overtone (2 3 ) [9] on Ni(100) have found that energy in 3 promotes the reaction with similar efficacy as kinetic energy along the surface normal. Furthermore, Juurlink et al. [6] have shown that CH 4 with excitation in 3 contributes less than 2% to the activated chemisorption of thermally excited methane [2]. They conclude that vibrational modes other than 3 must play a significant role in methane reactivity under thermal conditions. Theoretical treatments of methane chemisorption include statistical [11,12] as well as dynamical models [3,4,7,8,13]. While the statistical approach excludes the possibility of mode-specific reactivity, it has been claimed to reproduce the results of both thermally averaged and eigenstate-resolved measurements for CH 4 on Ni(100) [11,12]. On the other hand, simplified dynamical models for gas-surface reactions suggest the possibility of mode specificity [7,8]. For reactions that occur entirely in the gas phase, more realistic dynamical calculations find that the symmetric-stretch vibration is generally more efficient than the antisymmetric stretch in promoting reaction [16 -22], and this has been confirmed, in part, by experiments [18,23].We have previously reported vibrational mode-specific chemisorption of CD 2 H 2 on Ni(100), where we demon-strated the difference in re...

Enhancing dissociative chemisorption of H 2 O on Cu(111) via vibrational excitation

“…The dissociative chemisorption of CH 4 is more complicated because of its many vibrational modes (22). On Ni, for example, the excitation of the symmetric stretching (v 1 ) mode of CH 4 led to significant enhancement relative to the same amount of translational energy (18), while the antisymmetric stretching (v 3 ) mode offered similar efficacy as translational motion (14,15,17). Similar observations have been reported for the gas phase reaction between CH 3 D and Cl (11).…”

“…This mode selectivity and related bond selectivity have been experimentally demonstrated for only a few reactive systems in the gas phase (6-13) and on surfaces (14)(15)(16)(17)(18)(19)(20)(21). For example, it was shown that both the kinetics and dynamics of the H þ H 2 O reaction depend sensitively on the vibrational state of the H 2 O reactant (6).…”

Quantum state-resolved gas/surface reaction dynamics probed by reflection absorption infrared spectroscopy

“…1 The emergence of quantum state-resolved experimental techniques using state-specific reactant preparation by laser excitation of the incident species has greatly increased the level of detail available from molecular beam experiments. [2][3][4] Stateresolved reactivity measurements have demonstrated both mode-specificity and bond-selectivity [5][6][7][8][9] as well as stereodynamical control 10 for chemisorption of methane on transition metal surfaces. Highly detailed data provided by state-resolved experiments enable stringent tests of ab initio theory of gas/surface reaction dynamics, 11,12 helping experimentalists and theorists to work towards the ultimate goal of using computers to predict surface reactivity with chemical accuracy in order to design and optimize heterogeneous catalysts.…”

Ab Initio Molecular Dynamics Calculations versus Quantum-State-Resolved Experiments on CHD3 + Pt(111): New Insights into a Prototypical Gas–Surface Reaction

“…For some vibrational modes, the vibrational efficacy, which measures how effective putting energy into vibration is at promoting reaction relative to increasing the incidence energy (E i ), is even larger than one. 7,10 In addition, the dissociation of partially deuterated molecules shows bond selectivity; for instance, in CHD 3 , the CH bond can be selectively broken upon excitation to an appropriate initial vibrational state. 11,12 Finally, dissociative chemisorption of methane on metal surfaces represents a current frontier in the theoretical description of the dynamics of reactions of gas-phase molecules on metal surfaces, 15−24 with much current efforts now being aimed at achieving an accurate description of this reaction through high-dimensional quantum dynamics calculations.…”