Dynamics of hydrogen dissociation on stepped platinum

Electronically Nonadiabatic H Atom Scattering from Low Miller Index Surfaces of Silver

The reactivity of a surface depends strongly on the surface structure. To study the influence of surface structure on H atom adsorption, we performed inelastic scattering experiments and complementary electronically nonadiabatic molecular dynamics (MD) simulations for H atoms colliding with the three low Miller index surface facets of silver. Experiment reveals very similar energy loss distributions for all three investigated facets. However, for the (100) facet a dependence on the surface orientation is observed that is absent for the other two facets. The nonadiabatic MD simulations manage to describe the experiments well. Despite the observed insignificant influence of the surface geometry on the energy loss distributions, our simulations predict that the capability of the H atoms to penetrate the surface critically depends on the surface structure. The observed crystal orientation dependence of the energy loss distributions in the experiment for Ag(100) cannot be explained with our simulations, and we provide a discussion for a better theoretical description of this system to stimulate future computational investigations.

Alignment and impact angular dependence to O2 sticking and dissociation on Pt(111) and close-packed steps

Oxygen's interaction with Pt surfaces serves as a model system in the development of an accurate theoretical description of reaction mechanisms that involve multiple precursor states. To benchmark the influence of surface structure on the dynamics of this interaction, we report absolute values of the initial sticking probability of O₂ onto Pt(111) and two vicinal surfaces for state-selected and rotationally-aligned O₂ molecules. Sticking probabilities vary significantly for helicoptering and cartwheeling molecules. Our data can be understood if normal energy scaling holds for all molecular orientations relative to the surface. Vicinal surfaces are much more reactive than Pt(111) with little to no dependence on the molecule's alignment and a more complex angular dependence. At low incident energies, sticking probabilities are highest for incidence into step facets. The weak alignment dependence points toward predominant scattering into a physisorbed state preceding chemisorbed states over a wide angular range.

Chiral Surface Characterisation and Reactivity Toward H–D Exchange of a Curved Platinum Crystal

Understanding heterogeneous catalysis at the atomic level requires detailed knowledge of the reactivity of different surface sites toward specific bond breaking and bond making events. We illustrate a new method in such investigations. We use a macroscopically curved Pt single crystal containing a large variation in density of highly kinked steps of two different chiralities. Scanning tunneling microscopy maps the entire range of surface structures present on the 31° section surrounding the Pt(111) apex. Whereas most of the surface shows the expected characteristic arrays of parallel steps, hexagonally-shaped, single-atom deep pits remain after cleaning procedures near the apex. Their orientation is indicative of the different chiralities present on the two sides of the crystal’s apex. These unintended defects locally raise the surface defect concentration, but are of little consequence to subsequent reactivity measurements for $$\text {D}_2$$ D 2 dissociation and H–D exchange as probed by supersonic molecular beam techniques. We quantify absolute elementary dissociation and relative isotopic exchange rates across the surface with high spatial resolution. At low incident energies, elementary dissociation of the homonuclear isotoplogues is dominated by the kinked steps. H–D exchange kinetics depend also mostly linearly on step density. The changing ratio of D2 dissociation to H–D formation, however, suggests that anisotropic diffusion of H(D) atoms is of influence to the measured HD production rate.

Steps on Pt stereodynamically filter sticking of O2

Platinum is widely applied as an oxidation catalyst. The surfaces of metallic catalyst particles are generally composed of atomically flat planes connected by edges. In this paper, we unveil how Pt edges affect O₂ sticking. Opposing types of behaviors are observed for high and low incident energies. The dependencies identify the mechanisms by which O₂ molecules stick, which is the first step in the oxidation reactions.

Low coordinated sites on catalytic surfaces often enhance reactivity, but the underlying dynamical processes are poorly understood. Using two independent approaches, we investigate the reactivity of O₂ impinging onto platinum and resolve how step edges on (111) terraces enhance sticking. At low incident energy, the linear dependence on step density, independence of step type, and insensitivity to O₂’s molecular alignment show that trapping into a physisorbed state precedes molecular chemisorption and dissociation. At higher impact energies, direct molecular chemisorption occurs in parallel on steps and terraces. While terraces are insensitive to alignment of the molecule within the (111) plane, steps favor molecules impacting with their internuclear axis parallel to the edge. Stereodynamical filtering thus controls sticking and dissociation of O₂ on Pt with a twofold role of steps.

Transferability of the Specific Reaction Parameter Density Functional for H2 + Pt(111) to H2 + Pt(211)

The accurate description of heterogeneously catalyzed reactions may require chemically accurate evaluation of barriers for reactions of molecules at the edges of metal nanoparticles. It was recently shown that a semiempirical density functional describing the interaction of a molecule dissociating on a flat metal surface (CHD₃ + Pt(111)) is transferable to the same molecule reacting on a stepped surface of the same metal (Pt(211)). However, validation of the method for additional systems is desirable. To address the question whether the specific reaction parameter (SRP) functional that describes H₂ + Pt(111) with chemical accuracy is also capable of accurately describing H₂ + Pt(211), we have performed molecular beam simulations with the quasi-classical trajectory (QCT) method, using the SRP functional developed for H₂ + Pt(111). Our calculations used the Born-Oppenheimer static surface model. The accuracy of the QCT method was assessed by comparison with quantum dynamics results for reaction of the ro-vibrational ground state of H₂. The theoretical results for sticking of H₂ and D₂ on Pt(211) are in quite good agreement with the experiment, but uncertainties remain because of a lack of accuracy of the QCT simulations at low incidence energies and possible inaccuracies in the reported experimental incidence energies at high energies. We also investigated the nonadiabatic effect of electron-hole pair excitation on the reactivity using the molecular dynamics with the electron friction (MDEF) method, employing the local density friction approximation (LDFA). Only small effects of electron-hole pair excitation on sticking are found.

Site-specific reactivity of molecules with surface defects—the case of H2 dissociation on Pt

The classic system that describes weakly activated dissociation in heterogeneous catalysis has been explained by two dynamical models that are fundamentally at odds. Whereas one model for hydrogen dissociation on platinum(111) invokes a preequilibrium and diffusion toward defects, the other is based on direct and local reaction. We resolve this dispute by quantifying site-specific reactivity using a curved platinum single-crystal surface. Reactivity is step-type dependent and varies linearly with step density. Only the model that relies on localized dissociation is consistent with our results. Our approach provides absolute, site-specific reaction cross sections.

Hydrogen adsorption and desorption from Cu(111) and Cu(211)

We present a combined experimental-theoretical study on structural and coverage dependences of the adsorption and desorption of molecular hydrogen on atomically flat Cu(111) and highly stepped Cu(211) surfaces. For molecules with identical incident energy from supersonic molecular beams, we find a reduced dissociative sticking probability for the stepped surface compared to Cu(111). DFT calculations of activation barriers to dissociation for the clean and partially precovered surfaces, as well as quantitative analysis of TPD spectra, support that the A-type step of the (211) surface causes an upward shift in activation barriers to dissociation and lowering of the desorption barrier. The new data allow us to determine low sticking probabilities at conditions where King and Wells measurements fail to determine the reactivity. They are also fully consistent with the unexpected observation that monoatomic steps on a surface lower the reactivity toward the dissociation of a diatomic molecule.

Anomalous Dependence of the Reactivity on the Presence of Steps: Dissociation of D2 on Cu(211)

Stepped metal surfaces are usually assumed to exhibit an increased catalytic activity for bond cleavage of small molecules over their flat single-crystal counterparts. We present experimental and theoretical data on the dissociative adsorption of molecular hydrogen on copper that contradicts this notion. We observe hydrogen molecules to be more reactive on the flat Cu(111) than on the stepped Cu(211) surface. We suggest that this exceptional behavior is due to a geometric effect, that is, that bond cleavage on the flat surface does not occur preferentially over a top site.

Two-dimensional nanomaterial-based field-effect transistors for chemical and biological sensing

This review highlights the recent progress in graphene-, 2D transition metal dichalcogenide-, and 2D black phosphorus-based FET sensors for detecting gases, biomolecules, and water contaminants.

Quantum and classical dynamics of reactive scattering of H2 from metal surfaces

State-of-the-art theoretical models allow nowadays an accurate description of H₂/metal surface systems and phenomena relative to heterogeneous catalysis. Here we review the most relevant ones investigated during the last 10 years.