Measuring Transient Reaction Rates from Nonstationary Catalysts

Vibrational energy transfer in collisions of molecules with metal surfaces

The Born-Oppenheimer approximation (BOA), which serves as the basis for our understanding of chemical bonding, reactivity and dynamics, is routinely violated for vibrationally inelastic scattering of molecules at metal surfaces. The title-field therefore represents a fascinating challenge to our conventional wisdom calling for new concepts that involve explicit electron dynamics occurring in concert with nuclear motion. Here, we review progress made in this field over the last decade, which has witnessed dramatic advances in experimental methods, thereby providing a much more extensive set of diverse observations than has ever before been available. We first review the experimental methods used in this field and then provide a systematic tour of the vast array of observations that are currently available. We show how these observations - taken together and without reference to computational simulations - lead us to a simple and intuitive picture of BOA failure in molecular dynamics at metal surfaces, one where electron transfer between the molecule and the metal plays a preeminent role. We also review recent progress made in the theory of electron transfer mediated BOA failure in molecule-surface interactions, describing the most important methods and their ability to reproduce experimental observation. Finally, we outline future directions for research and important unanswered questions.

Identification of reaction intermediates in the decomposition of formic acid on Pd

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. Identifying the reaction intermediates is often difficult because their short lifetimes and low concentrations make it difficult to observe them with surface sensitive spectroscopic techniques. In this paper we report a different approach to identify intermediates for the formic acid decomposition reaction on Pd(111) and Pd(332) based on accurate measurements of isotopologue specific thermal reaction rates. At low surface temperatures (∼400 K) CO2 formation is the major reaction pathway. The CO2 kinetic data show this occurs via two temporally resolved reaction processes. Thus, there must be two parallel pathways which we attribute to the participation of two intermediate species in the reaction. Isotopic substitution reveals large and isotopologue specific kinetic isotope effects that allow us to identify the two key intermediates as bidentate formate and carboxyl. The decomposition of the bidentate formate is substantially slower than that of carboxyl. On Pd(332), at high surface temperatures (643 K to 693 K) we observe both CO and CO2 production. The observation of CO formation reinforces the conclusion of calculations that suggest the carboxyl intermediate plays a major role in the water-gas shift reaction, where carboxyl exhibits temperature dependent branching between CO2 and CO.

Spiers Memorial Lecture: New directions in molecular scattering

The field of molecular scattering is reviewed as it pertains to gas-gas as well as gas-surface chemical reaction dynamics. We emphasize the importance of collaboration of experiment and theory, from which new directions of research are being pursued on increasingly complex problems. We review both experimental and theoretical advances that provide the modern toolbox available to molecular-scattering studies. We distinguish between two classes of work. The first involves simple systems and uses experiment to validate theory so that from the validated theory, one may learn far more than could ever be measured in the laboratory. The second class involves problems of great complexity that would be difficult or impossible to understand without a partnership of experiment and theory. Key topics covered in this review include crossed-beams reactive scattering and scattering at extremely low energies, where quantum effects dominate. They also include scattering from surfaces, reactive scattering and kinetics at surfaces, and scattering work done at liquid surfaces. The review closes with thoughts on future promising directions of research.

First-principles surface reaction rates by ring polymer molecular dynamics and neural network potential: role of anharmonicity and lattice motion

Elementary gas-surface processes are essential steps in heterogeneous catalysis. A predictive understanding of catalytic mechanisms remains challenging due largely to difficulties in accurately characterizing the kinetics of such steps. Experimentally, thermal rates for elementary surface reactions can now be measured using a novel velocity imaging technique, providing a stringent testing ground for ab initio rate theories. Here, we propose to combine ring polymer molecular dynamics (RPMD) rate theory with state-of-the-art first-principles-determined neural network potential to calculate surface reaction rates. Taking NO desorption from Pd(111) as an example, we show that the harmonic approximation and the neglect of lattice motion in the commonly-used transition state theory overestimates and underestimates the entropy change during the desorption process, respectively, leading to opposite errors in rate coefficient predictions and artificial error cancellations. Including anharmonicity and lattice motion, our results reveal a generally neglected surface entropy change due to significant local structural change during desorption and obtain the right answer for the right reasons. Although quantum effects are found to be less important in this system, the proposed approach establishes a more reliable theoretical benchmark for accurately predicting the kinetics of elementary gas-surface processes.

Adsorption and Absorption Energies of Hydrogen with Palladium

Thermal recombinative desorption rates of HD on Pd(111) and Pd(332) are reported from transient kinetic experiments performed between 523 and 1023 K. A detailed kinetic model accurately describes the competition between recombination of surface-adsorbed hydrogen and deuterium atoms and their diffusion into the bulk. By fitting the model to observed rates, we derive the dissociative adsorption energies (E _0, ads ^H₂ = 0.98 eV; E _0, ads ^D₂ = 1.00 eV; E _0, ads ^HD = 0.99 eV) as well as the classical dissociative binding energy ϵ_ads = 1.02 ± 0.03 eV, which provides a benchmark for electronic structure theory. In a similar way, we obtain the classical energy required to move an H or D atom from the surface to the bulk (ϵ_sb = 0.46 ± 0.01 eV) and the isotope specific energies, E _0, sb ^H = 0.41 eV and E _0, sb ^D = 0.43 eV. Detailed insights into the process of transient bulk diffusion are obtained from kinetic Monte Carlo simulations.

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.

Near-ambient pressure velocity map imaging

We present a new velocity map imaging instrument for studying molecular beam surface scattering in a near-ambient pressure (NAP-VMI) environment. The instrument offers the possibility to study chemical reaction dynamics and kinetics where higher pressures are either desired or unavoidable, adding a new tool to help close the “pressure gap” between surface science and applied catalysis. NAP-VMI conditions are created by two sets of ion optics that guide ions through an aperture and map their velocities. The aperture separates the high pressure ionization region and maintains the necessary vacuum in the detector region. The performance of the NAP-VMI is demonstrated with results from N2O photodissociation and N2 scattering from a Pd(110) surface, which are compared under vacuum and at near-ambient pressure (1 × 10−3 mbar). NAP-VMI has the potential to be applied to, and useful for, a broader range of experiments, including photoelectron spectroscopy and scattering with liquid microjets.

Application of an Event-Based Camera for Real-Time Velocity Resolved Kinetics

We describe here the application of an inexpensive event-based/neuromorphic camera in an ion imaging experiment operated at 1 kHz detection rate to study real-time velocity-resolved kinetics of thermal desorption. Such measurements involve a single gas pulse to initiate a time-dependent desorption process and a high repetition rate laser, where each pulse of the laser is used to produce an ion image. The sequence of ion images allows the time dependence of the desorption flux to be followed in real time. In previous work where a conventional framing camera was used, the large number of megapixel-sized images required data transfer and storage rates of up to 16 GB/s. This necessitated a large onboard memory that was quickly filled and limited continuous measurement to only a few seconds. Read-out of the memory became the bottleneck to the rate of data acquisition. We show here that since most pixels in each ion image contain no data, the data rate can be dramatically reduced by using an event-based/neuromorphic camera. The data stream is thus reduced to the intensity and location information on the pixels that are lit up by each ion event together with a time-stamp indicating the arrival time of an ion at the detector. This dramatically increases the duty cycle of the method and provides insights for the execution of other high rep-rate ion imaging experiments.

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.

NO Binding Energies to and Diffusion Barrier on Pd Obtained with Velocity-Resolved Kinetics

We report nitric oxide (NO) desorption rates from Pd(111) and Pd(332) surfaces measured with velocity-resolved kinetics. The desorption rates at the surface temperatures from 620 to 800 K span more than 3 orders of magnitude, and competing processes, like dissociation, are absent. Applying transition state theory (TST) to model experimental data leads to the NO binding energy E ₀ = 1.766 ± 0.024 eV and diffusion barrier D _T = 0.29 ± 0.11 eV on the (111) terrace and the stabilization energy for (110)-steps ΔE _ST = 0.060_-0.030 ^+0.015 eV. These parameters provide valuable benchmarks for theory.