Accurately simulating heterogeneously catalyzed reactions requires reliable barriers for molecules reacting at defects on metal surfaces, such as steps. However, first-principles methods capable of computing these barriers to chemical accuracy have yet to be demonstrated. We show that state-resolved molecular beam experiments combined with ab initio molecular dynamics using specific reaction parameter density functional theory (SRP-DFT) can determine the molecule-metal surface interaction with the required reliability. Crucially, SRP-DFT exhibits transferability: the functional devised for methane reacting on a flat (111) face of Pt (and Ni) also describes its reaction on stepped Pt(211) with chemical accuracy. Our approach can help bridge the materials gap between fundamental surface science studies on regular surfaces and heterogeneous catalysis in which defected surfaces are important.
ABSTRACT:The dissociative chemisorption of methane on metal surfaces is of fundamental and practical interest, being a rate-limiting step in the steam reforming process. The reaction is best modeled with quantum dynamics calculations, but these are currently not guaranteed to produce accurate results because they rely on potential energy surfaces based on untested density functionals and on untested dynamical approximations. To help overcome these limitations, here we present for the first time statistically accurate reaction probabilities obtained with ab initio molecular dynamics (AIMD) for a polyatomic gas-phase molecule reacting with a metal surface. Using a general purpose density functional, the AIMD reaction probabilities are in semiquantitative agreement with new quantum-state-resolved experiments on CHD 3 + Pt(111). The comparison suggests the use of the sudden approximation for treating the rotations even though CHD 3 has large rotational constants and yields an estimated reaction barrier of 0.9 eV for CH 4 + Pt(111). SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis T he steam reforming process, in which methane and water react over a Ni catalyst, is the main commercial source of molecular hydrogen. The dissociation (or dissociative chemisorption) of CH 4 on the catalyst into CH 3 (ad) + H(ad) is a rate-determining step of the full process. 1 Moreover, dissociation of methane on metal surfaces is of fundamental interest. 2−13 Already from early molecular beam experiments, it is known that vibration is very effective in promoting reactivity. 3,4,14 More recently, it has been shown that the reaction is mode-specific, that is, the degree to which energizing the molecule promotes reaction depends on whether the energy is put in translation or vibration and even on which vibration it is put in (vibrational mode specificity). 5−8 These observations, which have been explained qualitatively on the basis of different models, 9,15 rule out the application of fully statistical models. 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. 16,23,24 A wealth of experiments exist for the methane + Pt(111) system. 3,8,12,17,25−29 There has been considerable debate 2,25 concerning the importance of tunneling in this and similar systems. Recent calculations 17,23,30 suggest only a...
New measurements of the differential steric effect for NO + Ar inelastic scattering highlight the importance of quantum interference.
Rotational angular momentum alignment effects in the rotationally inelastic collisions of NO(X) with Ar have been investigated at a collision energy of 66 meV by means of hexapole electric field initial state selection coupled with velocity-map ion imaging final state detection. The fully quantum state resolved second rank renormalized polarization dependent differential cross sections determined experimentally are reported for a selection of spin-orbit conserving and changing transitions for the first time. The results are compared with the findings of previous theoretical investigations, and in particular with the results of exact quantum mechanical scattering calculations. The agreement between experiment and theory is generally found to be good throughout the entire scattering angle range. The results reveal that the hard shell nature of the interaction potential is predominantly responsible for the rotational alignment of the NO(X) upon collision with Ar.
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