Substrate Vibrations as Promoters of Chemical Reactivity on Metal Surfaces

Abstract

Studies exploring how vibrational energy (Evib) promotes chemical reactivity most often focus on molecular reagents, leaving the role of substrate atom motion in heterogeneous interfacial chemistry underexplored. This combined theoretical and experimental study of methane dissociation on Ni(111) shows that lattice atom motion modulates the reaction barrier height during each surface atom's vibrational period, which leads to a strong variation in the reaction probability (S0) with surface temperature (Tsurf). State-resolved beam-surface scattering studies at Tsurf = 90 K show a sharp threshold in S0 at translational energy (Etrans) = 42 kJ/mol. When Etrans decreases from 42 kJ/mol to 34 kJ/mol, S0 decreases 1000-fold at Tsurf = 90 K, but only 2-fold at Tsurf = 475 K. Results highlight the mechanism for this effect, provide benchmarks for DFT calculations, and suggest the potential importance of surface atom induced barrier height modulation in heterogeneously catalyzed reactions, particularly on structurally labile nanoscale particles and defect sites.

Quantum state resolved molecular beam reflectivity measurements: CH4 dissociation on Pt(111)

“…For the laser-off measurements, the sticking coefficient for T S = 500 K is lower for E trans less than 90 kJ/mol, whereas increasing T S at higher energies does not change S laser-off 0 significantly. This is consistent with previous experiments performed by Utz et al for the dissociative chemisorption of CH 4 on Ni(111), 22,25 where it was found that S ν 3 0 increased more as T S was increased when the total energy (E trans + E vib ) was lower than the barrier height, and S ν 3 0 changed less when the total energy was greater than the barrier height.…”

“…First principles theory combined with classical and quantum dynamics simulations is able to reproduce at least qualitatively many of the experimental observations. [15][16][17][18][19][20][21][22] Several techniques have been developed to measure sticking coefficients with quantum state resolution, for example, by detecting the surface bound products using Auger electron spectroscopy, 5,6,23,24 reflection absorption infrared spectroscopy, 7,11,15,21 or titration of the adsorbates to make molecules that desorb and can be detected in the gas phase mass spectrometrically. 9,12,22,25 Whilst these have proved to be powerful techniques for determining quantum state resolved sticking coefficients, they share a common disadvantage in that calibrations are necessary to quantify the adsorbate coverage and number of molecules incident on the surface, both of which need to be known to accurately determine the sticking coefficient.…”

Quantum-state-resolved reactivity of overtone excited CH4 on Ni(111): Comparing experiment and theory

“…Note that a purely vibrationally adiabatic model predicts that molecules in the ν 3 state are about as reactive as those in the ground state. In fact, experiment, like our theory, finds a strong vibrational enhancement; 25,46,47 the ν 1 enhancement is simply larger.…”

Incident Angle Dependence of CHD3 Dissociation on the Stepped Pt(211) Surface

“…At a surface temperature of 150 K and at lower incident energies, the branching ratio for the dissociation of CHD 3 , CH 2 D 2 , and CH 3 D on Pt(111) has been shown to be statistical under laser-off conditions, 49 whereas when a quantum of C–H stretch was added to the molecule, only C–H cleavage was observed. 49,50 Increasing the surface temperature (to 650 K) lowers the effective activation barrier to the dissociative chemisorption due to the thermal motion of the atoms in the surface, 14,51,52 which when combined with a higher incident translational energy is likely to make CHD 3 dissociation less bond selective for υ 1 = 1, as is seen to be the case in the AIMD calculations.…”