The reactivity of heterogeneous catalysts under working conditions may be strongly influenced by adsorbate-adsorbate interactions which alter reaction barriers and lead to the formation of ordered adsorbate structures like islands. To predict catalytic reactivity, accurate knowledge of adsorbate-adsorbate interaction energies is required, but it is rarely available. One challenge arises from the surprisingly long range over which these interactions exert influence. We show in this work-using DFT with periodic boundary conditions and an (8 × 8) unit cell-that a single O-atom adsorbed at Pt(111) induces significant Pt atom displacements out to more than 7 Å. This adsorbate induced surface strain allows O*-atoms at distances of 14 Å to experience repulsion between one another due to the adsorbate induced displacement of the Pt atoms between them. Similar calculations using smaller unit cells overestimate repulsion due to interactions between O* atoms in neighboring periodic images. The use of an (8 × 8) unit cell removes this error, revealing short range attractive interactions between 3rd nearest neighbors. We have used these improved DFT interaction energies to perform kinetic Monte Carlo simulations of oxygen island formation and to show how the interplay of short- and long-range forces determines the sizes and shapes of these islands. Neglect of long-range interactions leads to round and compact island structures, which are in conflict with STM experiments. Including all interactions out to the 9th nearest-neighbor results in simulations that eerily resemble the STM observations.

Despite its immense practical importance in industrial production of nitric acid, the mechanisms of catalytic ammonia oxidation on platinum group metals remain controversial. In this work, we employ velocity-resolved kinetics to study ammonia oxidation on a model Pd(332) catalyst between 600 and 700 K. We obtain the temporal evolution of gas-phase reactants (NH3), products (NO, H2O) andwith the help of femtosecond laser-induced desorptionof a reaction intermediate, N*. The reaction exhibits the prompt appearance of H2O and the delayed formation of NO; the rate-determining step is the reaction N* + O* → N*O occurring at step sites. This means that N* is the longest-lived reaction intermediate, an insight that helps explain formation of byproducts like N2 and N2O. We present a mechanism that explains all experimental observations, based on transition-state theory calculations and using input from density functional theory. We also show that N*O desorption is accelerated by coadsorbed oxygen.

Velocity-resolved kinetics (VRK) employs a pulsed molecular beam that initiates reactions at a surface and a pulsed ionization-laser that detects desorbed products, exploiting ion-imaging-based, velocity-sensitive detection to derive the product-flux vs reaction time. In its original form, kinetics was observed by scanning the delay between the molecular beam and laser while obtaining ion-images for ions with a single mass-to-charge ratio (m/q). Here, we demonstrate a dramatically improved version of VRK, where data are obtained quasi-continuously with a 100 kHz ionization laser and an event camera based on CERN Timepix3 technology (Amsterdam Scientific Instruments TPX3CAM). This improved version provides the full temporal behavior of the reaction products for every molecular beam pulse, resulting in a duty cycle advantage ranging from ≈ 300 to 105 in the examples given. Furthermore, the new technique allows us to simultaneously follow the kinetics of multiple species with different m/q by using the event camera’s ability to provide pixel-specific timestamps. Data for three example systems, hydrogen atom recombination, hydrogen oxidation, and ammonia oxidation on Pt(332), illustrate the improvements in the acquisition rate and signal-to-noise ratio obtained. We also show that it is possible to extend the application of the VRK technique to non-stationary catalysts.

Developing predictive theories for the rates of reactions between surface-bound molecules is a central challenge to understanding many important phenomena including: heterogeneous catalysis, electrocatalysis, nanofabrication, and corrosion. To meet this challenge, chemically accurate benchmarks to test theoretically derived reaction rates and barrier heights are essential, but few exist. Here, we determine from experiment an accurate zero-point-energy corrected barrier height, 0.76 ± 0.03, for the reaction O* + H* → OH* occurring at atomic Pt B-type step sites, the rate limiting step of hydrogen oxidation on Pt. This experimental benchmark agrees with density functional theory (DFT) predictions made at the level of the generalized gradient approximation (GGA) for five different functionals, exhibiting a mean absolute error (MAE) of 25 meV. This is far better agreement than commonly expected for this level of theory. We speculate that this level of agreement may be a common feature for reactions that involve only species adsorbed on surfaces.

The emergence of high-power ionizing light sources with repetition rates greater than 100 kHz promises vastly improved data acquisition times for electron and ion imaging experiments; however, optimal means to record ion images at high average ion flux are challenging. A popular ion imaging detector uses a double (chevron) microchannel plate (MCP) stack operating at high gain (∼106) and a phosphor screen floated to 4 kV while images are recorded using a CCD or CMOS camera. Such imaging detectors tend to exhibit severe inhomogeneity at high incidence ion flux due to saturation effects in the MCP, even when the amplified current is far below the strip-current limit. This inhomogeneity arises from a local loss of gain in channels experiencing a high frequency of ion-amplification events, even when most channels are behaving normally. Here, we describe an alternative ion imaging scheme using a detector based on a single microchannel plate, a phosphor screen that can be floated to 20 kV, and a Timepix3 based event camera and demonstrate its performance in an ion-imaging experiment performed at a repetition rate of 100 kHz. The reduced gain of the single MCP avoids gain inhomogeneity up to higher count-rates, while the high-voltage phosphor helps maintain high single ion detection efficiency. The detector performs well at an ion flux of 107 ions cm−2 s−1.

Although the dynamics of collisions between a molecule and a solid surface are ultimately quantum mechanical, decohering effects owing to the large number of interacting degrees of freedom typically obscure the wavelike nature of these events. However, a partial decoupling of internal molecular motion from external degrees of freedom can reveal striking interference effects despite significant momentum exchange between the molecule and the bath of surface vibrations. We report state-prepared and state-resolved measurements of methane scattering from a room-temperature gold surface that demonstrate total destructive interference between molecular states related by a reflection symmetry operation. High-contrast interference effects prevail for all processes investigated, including vibrationally excited and vibrationally inelastic collisions. The results demonstrate the distinctly quantum mechanical effect of discrete symmetries in molecular collision dynamics.

Atomic-scale structures that account for the acceleration of reactivity by heterogeneous catalysts often form only under reaction conditions of high temperatures and pressures, making them impossible to observe with low-temperature, ultra-high-vacuum methods. We present velocity-resolved kinetics measurements for catalytic hydrogen oxidation on palladium over a wide range of surface concentrations and at high temperatures. The rates exhibit a complex dependence on oxygen coverage and step density, which can be quantitatively explained by a density functional and transition-state theory-based kinetic model involving a cooperatively stabilized configuration of at least three oxygen atoms at steps. Here, two oxygen atoms recruit a third oxygen atom to a nearby binding site to produce an active configuration that is far more reactive than isolated oxygen atoms. Thus, hydrogen oxidation on palladium provides a clear example of how reactivity can be enhanced on a working catalyst.

Uncovering the role of reaction intermediates is crucial to developing an understanding of heterogeneous catalysis because catalytic reactions often involve complex networks of elementary steps. Unfortunately, the short lifetimes and...

Improved catalysts and electrocatalysts composed of transition metal nanoparticles dispersed on high-area supports are essential for energy and environmental technologies. The chemical potential of the metal atoms in these supported nanoparticles is an important descriptor that correlates with both their catalytic activity and deactivation rate. This descriptor (mu(M)) is predictably determined by the particle size and the adhesion energy per unit area at the metal/support interface (E-adh). We show here that the adhesion energies for different metals on a given support scale linearly with a simple property of the metal: for oxides, it is proportional to the metal oxophilicity, and for the carbon support, it increases linearly with metal carbophilicity (both divided by the area per metal atom). These relationships allow predicting E-adh for other metal/support combinations, thus allowing estimation of mu(M) versus particle size and thereby better structure-based predictions of catalysts' performance, which can aid in designing improved catalysts.

Studying dynamics of the dissociative adsorption and recombinative desorption of hydrogen on copper surfaces has shaped our atomic-scale understanding of surface chemistry, yet experimentally determining the thermal rates for these processes, which dictate the outcome of catalytic reactions, has been impossible so far. In this work, we determine the thermal rate constants for dissociative adsorption and recombinative desorption of hydrogen on Cu(111) between 200 and 1000 K using data from reaction dynamics experiments. Contrary to current understanding, our findings demonstrate the predominant role of quantum tunneling, even at temperatures as high as 400 K. We also provide precise values for the reaction barrier (0.619 ± 0.020 eV) and adsorption energy (0.348 ± 0.026 eV) for H2 on Cu(111). Remarkably, the thermal rate constants are in excellent agreement with a first-principles quantum rate theory based on a new implementation of ring polymer molecular dynamics for reactions on surfaces, paving the way to discovering better catalysts using reliable and efficient computational methods.

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.

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.

Supported nanoparticles are of great importance to many technologies like fuel processing and chemical synthesis using catalysts and electrocatalysts, energy storage and generation using fuel cells and batteries, electrochemistry, magnetic noble gas ions like He+ is a powerful tool for the characterization of nanoparticles dispersed across flat support surfaces due to its ability to probe the elemental composition in the topmost atomic layer of a surface, providing quantitative information regarding the size and number density of nanoparticles. In this work, we present a derivation of the LEIS intensities expected from nanoparticles and the support material as a function of the average particle size, their number per unit area, and their contact angle with the support when modeled as spherical caps of the nanoparticle material dispersed over the surface of a flat support. The model assumes that the ion intensities are determined only by the physical blocking of linear ion trajectories and independent of the tilt angle of the local surface relative to the incident and scattered ion directions, an assumption we support by quantitative modeling of published data which tested tilt-angle effects. The model is a generalization to arbitrary contact angles of the hemispherical cap model, which assumes 90 degrees contact angle and has been widely used to model spectroscopic signals in LEIS (and also in Auger and photoelectron spectroscopies) during nanoparticle growth. This new model quantitatively reveals how LEIS signals are sensitive not only to the diameter and number density of the nanoparticle but also to their contact angle (or height/diameter ratio). With the use of additional data (e.g., from microscopy or adsorption microcalorimetry), the model presented here will enable more accurate determination of the average size, shape, and number density of supported nanoparticles based on LEIS intensity measurements.

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.

The chemical potential of metal atoms, mu(M), in supported metal nanoparticles is an important descriptor related to both the catalytic activity and the stability of the nanoparticles. Here, we derive an expression relating mu(M) to the radius of the particle's contact area with the support and the adhesion energy at the metal/support interface (E-adh) that assumes the particles have the shape of spherical caps but of arbitrary contact angle with the support (theta(c)) and includes an empirical correction for the increase in metal surface energy and adhesion energy with decreasing radius of curvature. We then show that, at any assumed contact angle, we can simultaneously fit previously reported measurements of both calorimetric mu(M) (from heats of metal vapor adsorption during nanoparticle growth by vapor deposition) versus metal coverage data and the He+ low-energy ion scattering (LEIS) intensities for the metal and/or support versus metal coverage (using our recently developed spherical cap model for quantitative LEIS intensities), to determine the particle size versus coverage and E-adh. Only one choice of contact angle gives a pair of values for contact angle and E-adh, which is consistent with the Young-Dupre equation for the equilibrium shape of a spherical particle. At this equilibrium shape, we then applied this spherical cap model (SCM) to reanalyze microcalorimetric metal chemical potentials and LEIS signals versus coverage data for nine metal/support combinations that were previously analyzed by assuming that the particles had the shape of hemispherical caps, i.e., with a contact angle of 90 degrees. We show that this revised approach gives close agreement with the calorimetric and LEIS data; the best-fit contact angles vary from 64 to 84 degrees, correcting the earlier assumption of 90 degrees. These results provide significant accuracy improvements in particle size versus coverage, metal chemical potential versus size and coverage, metal/support adhesion energies and contact angles for Cu, Ag and Au on CeO2(111), Ni on MgO(100), Ag on Fe3O4(111) and TiO2(100), and Ag, Ni and Pd on Ni-supported graphene. This revised approach is much more broadly applicable than the earlier hemispherical cap model (HCM).

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.

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.

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.