Reactive and Nonreactive Scattering of H ₂ from a Metal Surface Is Electronically Adiabatic

Abstract

The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool of the computational chemist. However, its validity for molecule-metal surface reactions, which are important to heterogeneous catalysis, has been questioned because of the possibility of electron-hole pair excitations. We have performed experiments and calculations on the scattering of molecular hydrogen from a catalytically relevant metal surface, obtaining absolute probabilities for changes in the molecule's velocity parallel to the representative Pt(111) surface. The comparison for in-plane and out-of-plane scattering and results for dissociative chemisorption in the same system show that for hydrogen-metal systems, reaction and diffractive scattering can be accurately described using the Born-Oppenheimer approximation.

Electronic Friction Dominates Hydrogen Hot-Atom Relaxation on Pd(100)

“…The correlation found between chemicurrent intensities and adsorption energies is a strong indication that a large fraction of the energy dissipated in both the dissociative and nondissociative adsorption processes is used to excite e-h pairs. However, this strongly contrasts with examples showing that the dissociative adsorption is reasonably well described within the electronically adiabatic approach, which neglects the coupling between electronic excitations and the nuclear motion [6,[18][19][20]23,25], and that the effect of electronic energy dissipation seems negligible [21,24,26]. Therefore, a question that is raised here is at what stage of the dissociative adsorption process e-h pair excitations do become relevant.…”

Understanding the electrocatalysis of oxygen reduction on platinum and its alloys

“…55 Catalytic reactions at the gas-solid interface are more straightforward to calculate; density functional theory (DFT) provides accurate values for the chemical potential of gas phase species. [56][57][58][59] By definition, when the electrode potential U ¼ 0 V with respect to a standard hydrogen electrode (SHE), the hydrogen evolution and oxidation reactions are at equilibrium: 2H + + 2e À # H 2 . Therefore, at U ¼ 0 V (RHE), the free energies of the solvated protons and the electrons in the solid are equal to the free energy of gas phase hydrogen at atmospheric pressure.…”

Chemistry at surfaces: from ab initio structures to quantum dynamics

“…electron-hole excitations, since interaction with metals always makes suspect the validity of the (fundamental) Born-Oppenheimer approximation. Despite in some cases combined experimental and theoretical efforts allowed to rule out such non-adiabatic effects in hydrogen scattering on a metal surface [5], there is evidence that in other systems they do play a role [156]. The second, important problem is the role of quantum mechanics in light atom dynamics.…”

“…Because of its attitude in explaining surface processes at the atomic-scale resolution, it is of valuable help in rationalizing experimental data, and with the ever increasing availability and reliability of predicting theories, models, software tools and computational power, its importance is being recognized even by the more skeptical ones. "Theoretical design" of a catalyst has been recently accomplished on a world-wide important catalytic reaction (steam reforming) [1], valuable theoretical advice has been given for optimal catalysts in ammonia synthesis [2], and advances have been made in the in Silico dream, the chemistry lab in a computer, with first-principle determination of reaction rates at surfaces [3][4][5]. At the same time, continuous progresses in experimental techniques give rise to new, unexpected results and pose new challenges to theory.…”