Surface-induced vibrational energy redistribution in methane/surface scattering depends on catalytic activity.

Recent state-to-state experiments of methane scattering from Ni(111) and graphene-covered Ni(111) combined with quantum mechanical simulations suggest an intriguing correlation between the surface-induced vibrational energy redistribution (SIVR) during the molecule/surface scattering event and the catalytic activity for methane dissociation of the target surface (Werdecker, Phys. Rev. Res., 2020, 2, 043251). Herein, we report new quantum state and angle-resolved measurements for methane scattering from Ni(111) and Au(111) probing the extent of ν3→ν1 antisymmetric-to-symmetric conversion of methane stretching motion for two surfaces with different catalytic activities. Consistent with the expectations, the extent of SIVR occurring on the more catalytically active Ni(111) surface, as measured by the ν1:ν3 scattered population ratio, is found to be several times stronger than that on the more inert Au(111) surface. We also present additional insights on the rovibrational scattering dynamics contained in the angle- and state-resolved data. The results together highlight the power of state-resolved scattering measurements as a tool for investigating methane-surface interactions.

Quantum state-resolved methane scattering from Ni(111) and NiO(111) by bolometer infrared laser tagging: The effect of surface oxidation.

We describe a novel ultrahigh vacuum state-to-state molecule/surface scattering apparatus with quantum state preparation of the incident molecular beam and angle-resolved quantum state detection of the scattered molecules. State-resolved detection is accomplished using a tunable mid-infrared laser source combined with a cryogenic bolometer detector and is applicable to any molecule with an infrared-active vibrational transition. Results on rotationally inelastic scattering of CH4 methane from a Ni(111) surface and NiO(111)/Ni(111) oxide film, obtained by the new apparatus, are presented. Molecules scattering from the oxidized surface, compared to those scattering from the bare nickel surface, are more highly excited rotationally and scatter into a broader distribution of angles. The internal alignment of molecular rotation is in addition found to be stronger in molecules scattering from the bare surface. Furthermore, the maxima of the state-resolved angular distributions shift toward and away from surface normal with increasing rotational quantum number J for the oxidized and bare surface, respectively. Finally, the rotational state populations produced in scattering from the oxidized surface are well-described by a Boltzmann distribution, while those produced in scattering from the bare surface exhibit large deviations from their best-fit Boltzmann distributions. These results point toward a marked enhancement in molecule-surface collisional energy exchange induced by oxidation of the nickel surface.

Highly Rotationally Excited N2 Reveals Transition-State Character in the Thermal Decomposition of N2O on Pd(110).

We employ time-slice and velocity map ion imaging methods to explore the quantum-state resolved dynamics in thermal N2O decomposition on Pd(110). We observe two reaction channels: a thermal channel that is ascribed to N2 products initially trapped at surface defects and a hyperthermal channel involving a direct release of N2 to the gas phase from N2O adsorbed on bridge sites oriented along the [001] azimuth. The hyperthermal N2 is highly rotationally excited up to J = 52 (v″ = 0) with a large average translational energy of 0.62 eV. Between 35 and 79% of the estimated barrier energy (1.5 eV) released upon dissociation of the transition state (TS) is taken up by the desorbed hyperthermal N2. The observed attributes of the hyperthermal channel are interpreted by post-transition-state classical trajectories on a density functional theory-based high-dimensional potential energy surface. The energy disposal pattern is rationalized by the sudden vector projection model, which attributes to unique features of the TS. Applying detailed balance, we predict that in the reverse Eley-Rideal reaction, both N2 translational and rotational excitation promote N2O formation.

Spin-dependent reactivity and spin-flipping dynamics in oxygen atom scattering from graphite.

The formation of two-electron chemical bonds requires the alignment of spins. Hence, it is well established for gas-phase reactions that changing a molecule's electronic spin state can dramatically alter its reactivity. For reactions occurring at surfaces, which are of great interest during, among other processes, heterogeneous catalysis, there is an absence of definitive state-to-state experiments capable of observing spin conservation and therefore the role of electronic spin in surface chemistry remains controversial. Here we use an incoming/outgoing correlation ion imaging technique to perform scattering experiments for O(3P) and O(1D) atoms colliding with a graphite surface, in which the initial spin-state distribution is controlled and the final spin states determined. We demonstrate that O(1D) is more reactive with graphite than O(3P). We also identify electronically nonadiabatic pathways whereby incident O(1D) is quenched to O(3P), which departs from the surface. With the help of molecular dynamics simulations carried out on high-dimensional machine-learning-assisted first-principles potential energy surfaces, we obtain a mechanistic understanding for this system: spin-forbidden transitions do occur, but with low probabilities.

Binding Energy and Diffusion Barrier of Formic Acid on Pd(111).

Velocity-resolved kinetics is used to measure the thermal rate of formic acid desorption from Pd(111) between 228 and 273 K for four isotopologues: HCOOH, HCOOD, DCOOH, DCOOD. Upon molecular adsorption, formic acid undergoes decomposition to CO2 and H2 and thermal desorption. To disentangle the contributions of individual processes, we implement a mass-balance-based calibration procedure from which the branching ratio between desorption and decomposition for formic acid is determined. From experimentally derived elementary desorption rate constants, we obtain the binding energy 639 ± 8 meV and the diffusion barrier 370 ± 130 meV using the detailed balance rate model (DBRM). The DBRM explains the observed kinetic isotope effects.

Steric Hindrance of NH3 Diffusion on Pt(111) by Co-Adsorbed O-Atoms.

A detailed velocity-resolved kinetics study of NH3 thermal desorption rates from p(2 × 2) O/Pt(111) is presented. We find a large reduction in the NH3 desorption rate due to adsorption of O-atoms on Pt(111). A physical model describing the interactions between adsorbed NH3 and O-atoms explains these observations. By fitting the model to the derived desorption rate constants, we find an NH3 stabilization on p(2 × 2) O/Pt(111) of 0.147-0.014+0.023 eV compared to Pt(111) and a rotational barrier of 0.084-0.022+0.049 eV, which is not present on Pt(111). The model also quantitatively predicts the steric hindrance of NH3 diffusion on Pt(111) due to co-adsorbed O-atoms. The derived diffusion barrier of NH3 on p(2 × 2) O/Pt(111) is 1.10-0.13+0.22 eV, which is 0.39-0.14+0.22 eV higher than that on pristine Pt(111). We find that Perdew Burke Ernzerhof (PBE) and revised Perdew Burke Ernzerhof (RPBE) exchange-correlation functionals are unable to reproduce the experimentally observed NH3-O adsorbate-adsorbate interactions and NH3 binding energies at Pt(111) and p(2 × 2) O/Pt(111), which indicates the importance of dispersion interactions for both systems.

Quantum effects in thermal reaction rates at metal surfaces.

There is wide interest in developing accurate theories for predicting rates of chemical reactions that occur at metal surfaces, especially for applications in industrial catalysis. Conventional methods contain many approximations that lack experimental validation. In practice, there are few reactions where sufficiently accurate experimental data exist to even allow meaningful comparisons to theory. Here, we present experimentally derived thermal rate constants for hydrogen atom recombination on platinum single-crystal surfaces, which are accurate enough to test established theoretical approximations. A quantum rate model is also presented, making possible a direct evaluation of the accuracy of commonly used approximations to adsorbate entropy. We find that neglecting the wave nature of adsorbed hydrogen atoms and their electronic spin degeneracy leads to a 10× to 1000× overestimation of the rate constant for temperatures relevant to heterogeneous catalysis. These quantum effects are also found to be important for nanoparticle catalysts.

Kinetics of NH3 Desorption and Diffusion on Pt: Implications for the Ostwald Process.

We report accurate time-resolved measurements of NH3 desorption from Pt(111) and Pt(332) and use these results to determine elementary rate constants for desorption from steps, from (111) terrace sites and for diffusion on (111) terraces. Modeling the extracted rate constants with transition state theory, we find that conventional models for partition functions, which rely on uncoupled degrees of freedom (DOFs), are not able to reproduce the experimental observations. The results can be reproduced using a more sophisticated partition function, which couples DOFs that are most sensitive to NH3 translation parallel to the surface; this approach yields accurate values for the NH3 binding energy to Pt(111) (1.13 ± 0.02 eV) and the diffusion barrier (0.71 ± 0.04 eV). In addition, we determine NH3's binding energy preference for steps over terraces on Pt (0.23 ± 0.03 eV). The ratio of the diffusion barrier to desorption energy is ∼0.65, in violation of the so-called 12% rule. Using our derived diffusion/desorption rates, we explain why established rate models of the Ostwald process incorrectly predict low selectivity and yields of NO under typical reactor operating conditions. Our results suggest that mean-field kinetics models have limited applicability for modeling the Ostwald process.

The Barrier for CO2 Functionalization to Formate on Hydrogenated Pt.

Understanding heterogeneous catalysis is based on knowing the energetic stability of adsorbed reactants, intermediates, and products as well as the energetic barriers separating them. We report an experimental determination of the barrier to CO2 functionalization to form bidentate formate on a hydrogenated Pt surface and the corresponding reaction energy. This determination was possible using velocity resolved kinetics, which simultaneously provides information about both the dynamics and rates of surface chemical reactions. In these experiments, a pulse of isotopically labeled formic acid (DCOOH) doses the Pt surface rapidly forming bidentate formate (DCO*O*). We then record the (much slower) rate of decomposition of DCO*O* to form adsorbed D* and gas phase CO2. We establish the reaction mechanism by dosing with O2 to form adsorbed O*, which efficiently converts H* or D* to gas phase water. H2O is formed immediately reflecting rapid loss of the acidic proton associated with formation of formate, while D2O formation proceeds more slowly and on the same time scale as the CO2 production. The temperature dependence of the reaction rate yields an activation energy that reflects the energy of the transition state with respect to DCO*O*. The derived heat of formation for DCO*O* on Pt(111) agrees well with results of microcalorimetry. The maximum release of translational energy of the formed CO2 provides a measure of the energy of the transition state with respect to the products and the barrier to the reverse process, functionalization of CO2. The comparison between the results on Pt(111) and Pt(332) shows that the barrier for CO2 functionalization is reduced by the presence of steps. The approach taken here could provide a method to optimize catalysts for CO2 functionalization.

Random Force in Molecular Dynamics with Electronic Friction.

Originally conceived to describe thermal diffusion, the Langevin equation includes both a frictional drag and a random force, the latter representing thermal fluctuations first seen as Brownian motion. The random force is crucial for the diffusion problem as it explains why friction does not simply bring the system to a standstill. When using the Langevin equation to describe ballistic motion, the importance of the random force is less obvious and it is often omitted, for example, in theoretical treatments of hot ions and atoms interacting with metals. Here, friction results from electronic nonadiabaticity (electronic friction), and the random force arises from thermal electron-hole pairs. We show the consequences of omitting the random force in the dynamics of H-atom scattering from metals. We compare molecular dynamics simulations based on the Langevin equation to experimentally derived energy loss distributions. Despite the fact that the incidence energy is much larger than the thermal energy and the scattering time is only about 25 fs, the energy loss distribution fails to reproduce the experiment if the random force is neglected. Neglecting the random force is an even more severe approximation than freezing the positions of the metal atoms or modelling the lattice vibrations as a generalized Langevin oscillator. This behavior can be understood by considering analytic solutions to the Ornstein-Uhlenbeck process, where a ballistic particle experiencing friction decelerates under the influence of thermal fluctuations.

Highly Efficient Activation of HCl Dissociation on Au(111) via Rotational Preexcitation.

The probability for dissociation of molecules on metal surfaces, which often controls the rate of industrially important catalytic processes, can depend strongly on how energy is partitioned in the incident molecule. There are many example systems where the addition of vibrational energy promotes reaction more effectively than the addition of translational energy, but for rotational pre-excitation similar examples have not yet been discovered. Here, we make an experimentally testable theoretical prediction that adding energy to the rotation of HCl can promote its dissociation on Au(111) 20 times more effectively than increasing its translational energy. In the underlying mechanism, the molecule's initial rotational motion allows it to pass through a critical region of the reaction path, where this path shows a strong and nonmonotonic dependence on the molecular orientation.

CO2 conversion by plasma: how to get efficient CO2 conversion and high energy efficiency.

Conversion of CO2 into CO with plasma processing is a potential method to transform intermittent sustainable electricity into storable chemical energy. The main challenges for developing this technology are how to get efficient CO2 conversion with high energy efficiency and how to prove its feasibility on an industrial scale. In this paper we review the mechanisms and performance of different plasma methodologies used in CO2 conversion. Mindful of the goals of obtaining efficient conversion and high energy efficiency, as well as industrial feasibility in mind, we emphasize a promising new approach of CO2 conversion by using a thermal plasma in combination with a carbon co-reactant.

Following the microscopic pathway to adsorption through chemisorption and physisorption wells.

Adsorption involves molecules colliding at the surface of a solid and losing their incidence energy by traversing a dynamical pathway to equilibrium. The interactions responsible for energy loss generally include both chemical bond formation (chemisorption) and nonbonding interactions (physisorption). In this work, we present experiments that revealed a quantitative energy landscape and the microscopic pathways underlying a molecule's equilibration with a surface in a prototypical system: CO adsorption on Au(111). Although the minimum energy state was physisorbed, initial capture of the gas-phase molecule, dosed with an energetic molecular beam, was into a metastable chemisorption state. Subsequent thermal decay of the chemisorbed state led molecules to the physisorption minimum. We found, through detailed balance, that thermal adsorption into both binding states was important at all temperatures.

First principles rates for surface chemistry employing exact transition state theory: application to recombinative desorption of hydrogen from Cu(111).

We present first principles calculations of the reactive flux for thermal recombinative desorption of hydrogen from Cu(111). We follow a theoretical paradigm used successfully for gas phase reactions, where electronic structure theory (DFT-GGA) is combined with transition state theory (TST). Classical ab initio molecular dynamics trajectories initiated from a thermal distribution near the transition state provide dynamical corrections to the desorption rate. We use this to calculate and study the recrossing error of TST and to directly simulate thermal desorption experiments based on a high temperature permeation method. Transition state recrossing is strongly temperature dependent and is even important in a frozen Cu-atom model. It is not influenced by inclusion of electron-hole pair excitation at the level of the local density electronic friction approximation. We also present the kinetic energy resolved flux of desorbing H2 at elevated temperature. This provides a more direct way to compare first principles theory to experiment, with no need to invoke detailed balance.

The African Descent and Glaucoma Evaluation Study (ADAGES) III: Contribution of Genotype to Glaucoma Phenotype in African Americans: Study Design and Baseline Data.

PURPOSE: To describe the study protocol and baseline characteristics of the African Descent and Glaucoma Evaluation Study (ADAGES) III. DESIGN: Cross-sectional, case-control study. PARTICIPANTS: Three thousand two hundred sixty-six glaucoma patients and control participants without glaucoma of African or European descent were recruited from 5 study centers in different regions of the United States. METHODS: Individuals of African descent (AD) and European descent (ED) with primary open-angle glaucoma (POAG) and control participants completed a detailed demographic and medical history interview. Standardized height, weight, and blood pressure measurements were obtained. Saliva and blood samples to provide serum, plasma, DNA, and RNA were collected for standardized processing. Visual fields, stereoscopic disc photographs, and details of the ophthalmic examination were obtained and transferred to the University of California, San Diego, Data Coordinating Center for standardized processing and quality review. MAIN OUTCOME MEASURES: Participant gender, age, race, body mass index, blood pressure, history of smoking and alcohol use in POAG patients and control participants were described. Ophthalmic measures included intraocular pressure, visual field mean deviation, central corneal thickness, glaucoma medication use, or past glaucoma surgery. Ocular conditions, including diabetic retinopathy, age-related macular degeneration, and past cataract surgery, were recorded. RESULTS: The 3266 ADAGES III study participants in this report include 2146 AD POAG patients, 695 ED POAG patients, 198 AD control participants, and 227 ED control participants. The AD POAG patients and control participants were significantly younger (both, 67.4 years) than ED POAG patients and control participants (73.4 and 70.2 years, respectively). After adjusting for age, AD POAG patients had different phenotypic characteristics compared with ED POAG patients, including higher intraocular pressure, worse visual acuity and visual field mean deviation, and thinner corneas (all P < 0.001). Family history of glaucoma did not differ between AD and ED POAG patients. CONCLUSIONS: With its large sample size, extensive specimen collection, and deep phenotyping of AD and ED glaucoma patients and control participants from different regions in the United States, the ADAGES III genomics study will address gaps in our knowledge of the genetics of POAG in this high-risk population.

An ultrahigh vacuum apparatus for H atom scattering from surfaces.

We present an apparatus to study inelastic H or D atom scattering from surfaces under ultra-high vacuum conditions. The apparatus provides high resolution information on scattering energy and angular distributions by combining a photolysis-based atom source with Rydberg atom tagging time-of-flight. Using hydrogen halides as precursors, H and D atom beams can be formed with energies from 500 meV up to 7 eV, with an energy spread of down to 2 meV and an intensity of up to 108 atoms per pulse. A six-axis manipulator holds the sample and allows variation of both polar and azimuthal incidence angles. Surface temperature can be varied from 45 K up to 1500 K. The apparatus' energy resolution ( E / Δ E ) can be as high as 1000 and its angular resolution can be adjusted between 0.3° and 3°.

Associative desorption of hydrogen isotopologues from copper surfaces: Characterization of two reaction mechanisms.

We report quantum-state resolved measurements of angular and velocity distributions of the associative desorption of H2, HD, and D2 from Cu(111) and Cu(211) surfaces. The desorbing molecules have bimodal velocity distributions comprising a "fast" channel and a "slow" channel on both facets. The "fast channel" is promoted by both hydrogen incidence translational and vibrational energy, while the "slow channel" is promoted by vibrational energy but inhibited by translational energy. Using detailed balance, we determine state-specific reaction probabilities for dissociative adsorption and compare these to theoretical calculations. The results for the activation barrier for the "fast channel" on Cu(111) are in agreement with theory within "chemical accuracy" (1 kcal/mole). Results on the Cu(211) facet provide direct information on the effect of increasing step density, which is commonly believed to increase reactivity. Differences in reactivity on the (111) and (211) facets are subtle - quantum state specific reactivity on the (211) surface is characterized by a broader distribution of barrier heights whose average values are higher than for reaction on (111). We fully characterize the "slow channel," which has not been found in theoretical calculations although it makes up a large fraction of the reactivity in these experiments.

Velocity-resolved kinetics of site-specific carbon monoxide oxidation on platinum surfaces.

Catalysts are widely used to increase reaction rates. They function by stabilizing the transition state of the reaction at their active site, where the atomic arrangement ensures favourable interactions 1 . However, mechanistic understanding is often limited when catalysts possess multiple active sites-such as sites associated with either the step edges or the close-packed terraces of inorganic nanoparticles2-4-with distinct activities that cannot be measured simultaneously. An example is the oxidation of carbon monoxide over platinum surfaces, one of the oldest and best studied heterogeneous reactions. In 1824, this reaction was recognized to be crucial for the function of the Davy safety lamp, and today it is used to optimize combustion, hydrogen production and fuel-cell operation5,6. The carbon dioxide products are formed in a bimodal kinetic energy distribution7-13; however, despite extensive study 5 , it remains unclear whether this reflects the involvement of more than one reaction mechanism occurring at multiple active sites12,13. Here we show that the reaction rates at different active sites can be measured simultaneously, using molecular beams to controllably introduce reactants and slice ion imaging14,15 to map the velocity vectors of the product molecules, which reflect the symmetry and the orientation of the active site 16 . We use this velocity-resolved kinetics approach to map the oxidation rates of carbon monoxide at step edges and terrace sites on platinum surfaces, and find that the reaction proceeds through two distinct channels11-13: it is dominated at low temperatures by the more active step sites, and at high temperatures by the more abundant terrace sites. We expect our approach to be applicable to a wide range of heterogeneous reactions and to provide improved mechanistic understanding of the contribution of different active sites, which should be useful in the design of improved catalysts.

Observation of the adsorption and desorption of vibrationally excited molecules on a metal surface.

The most common mechanism of catalytic surface chemistry is that of Langmuir and Hinshelwood (LH). In the LH mechanism, reactants adsorb, become thermalized with the surface, and subsequently react. The measured vibrational (relaxation) lifetimes of molecules adsorbed at metal surfaces are in the range of a few picoseconds. As a consequence, vibrational promotion of LH chemistry is rarely observed, with the exception of LH reactions occurring via a molecular physisorbed intermediate. Here, we directly detect adsorption and subsequent desorption of vibrationally excited CO molecules from a Au(111) surface. Our results show that CO (v = 1) survives on a Au(111) surface for ~1 × 10-10 s. Such long vibrational lifetimes for adsorbates on metal surfaces are unexpected and pose an interesting challenge to the current understanding of vibrational energy dissipation on metal surfaces. They also suggest that vibrational promotion of surface chemistry might be more common than is generally believed.