Hot Adatom Diffusion Following Oxygen Dissociation on Pd(100) and Pd(111): A First-Principles Study of the Equilibration Dynamics of Exothermic Surface Reactions

We augment ab initio molecular dynamics simulations with a quantitative account of phononic dissipation to study the hyperthermal adsorbate dynamics resulting from a noninstantaneous energy dissipation during exothermic surface chemical reactions. Comparing the hot adatom diffusion ensuing O2 dissociation over Pd(100) and Pd(111) we find experimentally accessible product end distances to form a rather misleading measure for the lifetime of this hyperthermal state. The lifetime is particularly long at Pd(111) where a random-walk-type diffusion leads only to small net displacements. A detailed phonon analysis rationalizes the slow equilibration through long-lived Rayleigh mode excitations that spatially confine the released energy within a nanoscopic “hot spot” around the impingement region

Phononic dissipation during “hot” adatom motion: A QM/Me study of O₂ dissociation at Pd surfaces

We augment ab initio molecular dynamics simulations with a quantitative account of phononic dissipation to study the non-equilibrium aftermath of the exothermic oxygen dissociation at low-index (111), (100), and (110) Pd surfaces. Comparing the hyperthermal diffusion arising from a non-instantaneous dissipation of the released chemical energy, we find a striking difference in the resulting “hot” adatom lifetime that is not overall reflected in experimentally recorded product end distances. We rationalize this finding through a detailed mode-specific phonon analysis and identify the dominant dissipation channels as qualitatively different groups of localized surface modes that ultimately lead to intrinsically different rates of dissipation to the Pd bulk. The thus obtained first-principles perspective on non-equilibrium adsorbate-phonon dynamics thereby underscores the sensitive dependence on details of the phononic fine structure, while questioning prevalent assumptions about energy sinks made in commonly used model bath Hamiltonians

Energy dissipation at metal surfaces

Conversion of energy at the gas–solid interface lies at the heart of many industrial applications such as heterogeneous catalysis. Dissipation of parts of this energy into the substrate bulk drives the thermalization of surface species, but also constitutes a potentially unwanted loss channel. At present, little is known about the underlying microscopic dissipation mechanisms and their (relative) efficiency. At metal surfaces, prominent such mechanisms are the generation of substrate phonons and the electronically non-adiabatic excitation of electron–hole pairs. In recent years, dedicated surface science experiments at defined single-crystal surfaces and predictive-quality first-principles simulations have increasingly been used to analyze these dissipation mechanisms in prototypical surface dynamical processes such as gas-phase scattering and adsorption, diffusion, vibration, and surface reactions. In this topical review we provide an overview of modeling approaches to incorporate dissipation into corresponding dynamical simulations starting from coarse-grained effective theories to increasingly sophisticated methods. We illustrate these at the level of individual elementary processes through applications found in the literature, while specifically highlighting the persisting difficulty of gauging their performance based on experimentally accessible observables

Trends of Pd3Au(111) alloy surface segregation in oxygen, carbon, and nitrogen environments

Catalytic properties of alloys are largely determined by the specific chemical composition at the surface. Differences in composition between surface and bulk regions depend intricately on both the parent metals and surrounding environment. While a non-reactive environment favors surface segregation of the more noble alloy component, a reactive environment such as oxygen is expected to draw the more active component to the surface. Using ab initio thermodynamics, we explore here the structure and composition of the Pd3Au(111) alloy surface in oxygen, carbon, and nitrogen containing environments. An extensive and systematic search of the available phase-space shows the segregation profile in an oxygen atmosphere to follow the anticipated picture described above, with O preferentially staying at the surface. In contrast, carbon at low coverages burrows deeper into the alloy substrate, without significant effect on the segregation profile. A nitrogen environment induces an intermediate behavior to oxygen and carbon where the nitrogen atoms first favor either surface or subsurface sites depending on the detailed metallic composition profile. Our results overall demonstrate the complex response that has to be expected for an active alloy surface during catalysis, while assessing the level of detail that is required to be accounted for in corresponding reaction models

Near-infrared investigation of folding sepiolite

Sepiolite is an industrially important clay mineral of the palygorskite-sepiolite group with alternating 2:1 ribbons and hydrated tunnels. Dry sepiolite, Mg8Si12O30(OH)4(OH2)4, loses half of its OH2 content upon further heating and undergoes a structural collapse known as folding. This treatment is considered essential for enhancing the absorptive properties of the clay. In this paper, the folding process is studied by near-infrared (NIR) spectroscopy, mid-infrared attenuated total reflectance (ATR), and thermogravimetric analysis (TGA). The folded state, Mg8Si12O30(OH)4(OH2)2, reveals a new spectrum of fundamental and higher-order OH2 vibrations, as well as systematically split doublets of structural and surface O-H vibrations. Detailed assignments for the stretching, combination, and overtone O-H modes are proposed on the basis of the two non-degenerate populations of Mg3OH (and SiOH) present in the folded state. It is demonstrated that NIR is of particular diagnostic value in monitoring conveniently and non-invasively the folding process, which appears as a simple transition between well-defined dry- and folded-structures. At the level of elementary sepiolite particles (laths), folding is described as a cooperative process requiring the integrity of the ribbons and the inter-ribbon linkages (moderately acid-leached sepiolite does not fold). This is opposed to the skewed and sometimes complex OH2 desorption trace observed by high-resolution TGA, which appears to indicate a multimodal distribution of laths. It is proposed that the rate-determining step for a sepiolite (also, palygorskite) lath to fold is the creation of a critical zone at mid-particle length, which is OH2-deficient and contains unstable, fivefold-coordinated Mg2+. © 2015, Walter de Gruyter GmbH. All rights reserved

Enhancement of lithium-mediated ammonia synthesis by addition of oxygen

Owing to the worrying increase in carbon dioxide concentrations in the atmosphere, there is a need to electrify fossil-fuel–powered chemical processes such as the Haber-Bosch ammonia synthesis. Lithium-mediated electrochemical nitrogen reduction has shown preliminary promise but still lacks sufficient faradaic efficiency and ammonia formation rate to be industrially relevant. Here, we show that oxygen, previously believed to hinder the reaction, actually greatly improves the faradaic efficiency and stability of the lithium-mediated nitrogen reduction when added to the reaction atmosphere in small amounts. With this counterintuitive discovery, we reach record high faradaic efficiencies of up to 78.0 ± 1.3% at 0.6 to 0.8 mole % oxygen in 20 bar of nitrogen. Experimental x-ray analysis and theoretical microkinetic modeling shed light on the underlying mechanism