Hirokazu Ueta

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

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 CHD3 + Pt(111). The comparison suggests the use of the sudden approximation for treating the rotations even though CHD3 has large rotational constants and yields an estimated reaction barrier of 0.9 eV for CH4 + Pt(111).

Vibrationally bond-selected chemisorption of methane isotopologues on Pt(111) studied by reflection absorption infrared spectroscopy

Reflection absorption infrared spectroscopy (RAIRS) was used to probe for vibrational bond-selectivity in the dissociative chemisorption of three partially deuterated methane isotopologues on a Pt(111) surface. While a combination of incident translational energy and thermal vibrational excitation produces a nearly statistical distribution of C-H and C-D bond cleavage products, we observe that laser excitation of an infrared active C-H stretch normal mode leads to highly selective dissociation of a C-H bond for CHD3, CH2D2, and CH3D. Our results show that vibrational energy redistribution between C-H and C-D stretch modes due to methane/surface interactions is negligible during the sub-picosecond collision time which indicates that vibrational bond-selectivity may be the rule rather than the exception in heterogeneous reactions of small polyatomic molecules.

Quantum state-resolved CH4 dissociation on Pt(111): coverage dependent barrier heights from experiment and density functional theory

The dissociative chemisorption of CH4 on Pt(111) was studied using quantum state-resolved methods at a surface temperature (T(s)) of 150 K where the nascent reaction products CH3(ads) and H(ads) are stable and accumulate on the surface. Most previous experimental studies of methane chemisorption on transition metal surfaces report only the initial sticking coefficients S0 on a clean surface. Reflection absorption infrared spectroscopy (RAIRS), used here for state resolved reactivity measurements, enables us to monitor the CH3(ads) uptake during molecular beam deposition as a function of incident translational energy (E(t)) and vibrational state (ν3 anti-symmetric C-H stretch of CH4) to obtain the initial sticking probability S0, the coverage dependence of the sticking probability S(θ) and the CH3(ads) saturation coverage θ(sat). We observe that both S0 and θ(sat) increase with increasing E(t) as well as upon ν3 excitation of the incident CH4 which indicates a coverage dependent dissociation barrier height for the dissociation of CH4 on Pt(111) at low surface temperature. This interpretation is supported by density functional calculations of barrier heights for dissociation, using large supercells containing one or more H and/or methyl adsorbates. We find a significant increase in the activation energies with coverage. These energies are used to construct simple models that reasonably reproduce the uptake data and the observed saturation coverages.

Vibrational Activation of Methane Chemisorption: The Role of Symmetry

Quantum state-resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4 on Pt(111). IR-IR double resonance excitation in a molecular beam is used to prepare CH4 in all three different vibrational symmetry components A1, E, and F2 of the 2ν3 antisymmetric stretch overtone vibration. Methyl dissociation products chemisorbed on the cold Pt(111) surface are detected via reflection absorption infrared spectroscopy (RAIRS). We observe similar reactivity for CH4 prepared in the A1 and F2 sublevels but up to a factor of 2 lower reactivity for excitation of the E sublevel. It is suggested that differences in the localization of the C-H stretch amplitudes for the three states at the transition state leads to the observed difference in reactivity rather than state-specific vibrational energy transfer to electronic excitation of the metal.

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

We report the design and characterization of a new molecular-beam/surface-science apparatus for quantum state-resolved studies of gas/surface reaction dynamics combining optical state-specific reactant preparation in a molecular beam by rapid adiabatic passage with detection of surface-bound reaction products by reflection absorption infrared spectroscopy (RAIRS). RAIRS is a non-invasive infrared spectroscopic detection technique that enables online monitoring of the buildup of reaction products on the target surface during reactant deposition by a molecular beam. The product uptake rate obtained by calibrated RAIRS detection yields the coverage dependent state-resolved reaction probability S(θ). Furthermore, the infrared absorption spectra of the adsorbed products obtained by the RAIRS technique provide structural information, which help to identify nascent reaction products, investigate reaction pathways, and determine branching ratios for different pathways of a chemisorption reaction. Measurements of the dissociative chemisorption of methane on Pt(111) with this new apparatus are presented to illustrate the utility of RAIRS detection for highly detailed studies of chemical reactions at the gas/surface interface.

Bond-Selective and Mode-Specific Dissociation of CH3D and CH2D2 on Pt(111)

Infrared laser excitation of partially deuterated methanes (CH3D and CH2D2) in a molecular beam is used to control their dissociative chemisorption on a Pt(111) single crystal and to determine the quantum state-resolved dissociation probabilities. The exclusive detection of C-H cleavage products adsorbed on the Pt(111) surface by infrared absorption reflection spectroscopy indicates strong bond selectivity for both methane isotopologues upon C-H stretch excitation. Furthermore, the dissociative chemisorption of both methane isotopologues is observed to be mode-specific. Excitation of symmetric C-H stretch modes produces a stronger reactivity increase than excitation of the antisymmetric C-H stretch modes, whereas bend overtone excitation has a weaker effect on reactivity. The observed mode specificity and bond selectivity are rationalized by the sudden vector projection model in terms of the overlap of the reactants normal mode vectors with the reaction coordinate at the transition state.

Quantum Tunneling Hydrogenation of Solid Benzene and Its Control via Surface Structure.

Despite the rapid accumulation of structural information about organic materials, the correlation between the surface structure of these materials and their chemical properties, a potentially important aspect of their chemistry, is not fully understood. Here, we show that the amorphous or crystalline structure of a solid benzene surface controls its chemical reactivity toward hydrogen. In situ infrared spectroscopy revealed that cold hydrogen atoms can add to an amorphous benzene surface at 20 K to form cyclohexane by tunneling. However, hydrogenation is greatly reduced on crystalline benzene. We suggest that the origin of the high selectivity of this reaction is the large difference in geometric constraints between the amorphous and the crystalline surfaces. The present findings can lead us to a more complete understanding of heterogeneous reaction systems, especially those involving tunneling, as well as to the possibility of nonenergetic surface chemical modification without undesired side reactions or physical processes.

Quantum tunneling observed without its characteristic large kinetic isotope effects

Significance Quantum tunneling, an important phenomenon in many surface and interfacial chemical processes, is strongly dependent on the isotope of the tunneling atom. However, surface tunneling during the hydrogenation/deuteration of solid benzene at 15–25 K is accompanied by an almost semiclassical kinetic isotope effect (KIE) of 1–1.5, which is much lower than that intrinsic to tunneling (≳100), because isotopically insensitive surface diffusion of the adsorbed atoms controls the chemical kinetics. Our results suggest that tunneling has been unrecognized in studies of the chemistry of condensed phases, and small-KIE tunneling may account for the unexplained fast reactions of hydrogen and deuterium observed in surface/interface chemical systems such as aerosols, enzymes, and interstellar dust grains. Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle’s ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1–1.5) despite the large intrinsic H/D KIE of tunneling (≳100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system.

Evidence of stable high-temperature Dx-CO intermediates on the Ru(0001) surface

We demonstrate the formation of complexes involving attractive interactions between D and CO on Ru(0001) that are stable at significantly higher temperatures than have previously been reported for such intermediate species on this surface. These complexes are evident by the appearance of new desorption features upon heating of the sample. They decompose in stages as the sample temperature is increased, with the most stable component desorbing at >500 K. The D:CO ratio remaining on the surface during the final stages of desorption tends towards 1:1. The new features are populated during normally incident molecular beam dosing of D(2) on to CO pre-covered Ru(0001) surfaces (180 K) when the CO coverage exceeds 50% of the saturation value. The amount of complex formed decreases somewhat with increasing CO pre-coverage. It is almost absent in the case of dosing on to the fully saturated surface. The results are interpreted in terms of both local and long-range rearrangements of the overlayer that give rise to the observed CO coverage dependence and limit the amount of complex that can be formed.