Nonmonotonic dissociative adsorption of vibrationally excitedH2on metal surfaces

Abstract

Spurred by recent experimental developments in the production of vibrationally excited molecular beams to study reactive scattering on metal surfaces, we have carried out six-dimensional quasiclassical and quantum dynamics simulations for several H 2 ͑v Ͼ 0͒-surface systems. We predict a general nonmonotonic behavior of dissociative adsorption of vibrationally excited H 2 ͑v , J =0͒ molecules as a function of incidence energy whenever this process becomes nonactivated. This prediction is based on results obtained for up to five different activated systems, including pure ͓Pt͑111͒ and Cu͑110͔͒, bimetallic alloy ͓NiAl͑110͔͒, and bimetallic pseudomorphic ͓Pd/Ru͑0001͒ and Cu/Ru͑0001͔͒ surfaces.Over the last decade, surface science theorists have made a substantial progress in the description of reactive scattering of diatomic molecules from metal surfaces ͑see Refs. 1 and 2 and references therein͒. This has been motivated by the numerous industrial processes that involve heterogeneous catalysis by a metal surface, in which molecular dissociative adsorption is the first and usually the rate-limiting step. 3 Methods of increasing complexity, based on accurate density-functional theory ͑DFT͒ descriptions of the molecule/surface interaction, 4,5 have been developed to take into account all molecular degrees of freedom ͑DOFs͒ and even phonons 6,7 and electron-hole pair excitations. 8,9 So far, six-dimensional ͑6D͒ dynamical calculations, usually within the Born-Oppenheimer and rigid surface approximations, have challenged experimental measurements to an unprecedented degree of accuracy and have provided meaningful interpretations of, sometimes striking, observations such as: the dominant out-of-plane diffraction measured for H 2 / Pt͑111͒ ͑Ref. 10͒ and H 2 / Pd͑111͒ ͑Refs. 11 and 12͒; the lack of diffraction peaks in H 2 / Pd͑110͒ due to dynamic trapping; 13 the surprisingly low reaction probability observed in N 2 / Ru͑0001͒ for incidence energies well above the minimum reaction barrier ͑MRB͒ ͑Ref. 14͒; the dramatic difference in reactivity between chemically similar systems such as N 2 / W͑100͒ and N 2 / W͑110͒ ͑Ref. 15͒; or the strong nonadiabatic effects that govern the interaction of O 2 with Al͑111͒. 16,17 Most existing experiments have exclusively considered molecules in their ground vibrational state ͑v =0͒ since they employ supersonic molecular beams in which vibrationally excited states are barely populated except at relatively high kinetic energy. 18 Vibrational excited molecules can be more efficiently obtained by using stimulated Raman pumping. [19][20][21][22] However, only very recently, the advent of experimental setups based on pulsed narrow bandwidth laser Raman excitation have made it possible to perform quantum state resolved reactivity measurements with vibrationally excited polyatomic molecules, 23 e.g., methane, 24-28 on singlecrystal surfaces. The latter experiments have provided direct evidence of the central role played by vibrational energy in activating gas-surface reactions ͑the so-called vibr...

Quantum and classical dynamics of reactive scattering of H2 from metal surfaces

“…A very interesting finding is that for high enough v the reaction probability curve of these five systems (Fig. 244 However, for the five systems discussed the low energy increase of the reaction probability with decreasing E i is not due to increased trapping, Fig. 35f).…”

“…244 However, for the five systems discussed the low energy increase of the reaction probability with decreasing E i is not due to increased trapping, Fig. 244 The predicted effect is not an artefact of the use of the QCT method, as QD calculations on H 2 (v, j = 0) + Pd/Ru(0001) and Cu/Ru(0001) reproduce the QCT results quite well. The reaction probabilities are compared to the sticking probability R b (E av ; T n ) measured with seeded beams for the same nozzle temperature.…”

“…The ''gold standard'' in methods for fitting or interpolating PESs for diatomic molecules scattering from metal surfaces, which has been used the most 11,48,53,54,56,109,114,116,128,129,131,133,145,148,151,156,161,170,174,176,181,[183][184][185][186]188,[192][193][194][195][196][197]199,200,208,216,217,[242][243][244][245][246][247][248][249] in studies of H 2 -metal surface scattering that appeared over the last decade, is the corrugation reducing procedure (or the CRP method) developed by Busnengo et al 27 and extended by Olsen et al 250 The CRP method performs so well because it takes ...…”

“…244 Other alloy surfaces included pseudo-morphic monolayers of Pd and Cu on Ru(0001). 244 Other alloy surfaces included pseudo-morphic monolayers of Pd and Cu on Ru(0001).…”

“…To understand this behavior, one should realize that under grazing conditions the collision proceeds in the channeling regime, 345 i.e., the projectile is guided by the row formed by the surface atoms in its motion along the surface. 244,246 Here, we should remark that, at present, grazing incidence experimental setups do not allow one to carry out state-resolved scattering measurements. Overall, the agreement is best close to the saturation value of the reaction probability (Fig.…”