1,720,992 research outputs found
Nuclear Quantum Effects in OH*/OD* Formation Kinetics on Pd(332): A Ring-Polymer Molecular Dynamics Study
No full textFundamental mechanisms for molecular energy conversion and chemical reactions at surfaces.
No full textThe dream of theoretical surface chemistry is to predict the outcome of reactions in order to find the ideal catalyst for a certain application. Having a working <i>ab initio</i> theory in hand would not only enable these predictions but also provide insights into the mechanisms of surface reactions.The development of theoretical models can be assisted by experimental studies providing benchmark data. Though for some reactions a quantitative agreement between experimental observations and theoretical calculations has been achieved, theoretical surface chemistry is in general still far away from gaining predictive power. Here we review recent experimental developments towards the understanding of surface reactions. It is demonstrated how quantum-state resolved scattering experiments on reactive and nonreactive systems can be used to test front-running theoretical approaches. Two challenges for describing dynamics at surfaces are addressed: nonadiabaticity in diatomic molecule surface scattering and the increasing system size when observing and describing the dynamics of polyatomic molecules at surfaces. Finally recent experimental studies on reactive systems are presented. It is shown how elementary steps in a complex surface reaction can be revealed experimentally
Cooperative adsorbate binding catalyzes high-temperature hydrogen oxidation on palladium
No full textAtomic-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.Editor’s summary Hydrogen oxidation to form water on a palladium surface proceeds through cooperative binding of at least three oxygen atoms at step sites to form an active configuration. Velocity-resolved kinetics measurements by Schwarzer et al . revealed a complex dependence on oxygen coverage and step density. Density functional and transition-state theory showed that reactivity increased greatly after two unreactive adsorbate oxygen atoms were able to recruit a third oxygen atom to a nearby binding site. —Phil SzuromiAtomic-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.Editor’s summary Hydrogen oxidation to form water on a palladium surface proceeds through cooperative binding of at least three oxygen atoms at step sites to form an active configuration. Velocity-resolved kinetics measurements by Schwarzer et al . revealed a complex dependence on oxygen coverage and step density. Density functional and transition-state theory showed that reactivity increased greatly after two unreactive adsorbate oxygen atoms were able to recruit a third oxygen atom to a nearby binding site. —Phil Szurom
Adsorption and Absorption Energies of Hydrogen with Palladium
No full text[Image: see text] 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)(H(2)) = 0.98 eV; E(0, ads)(D(2)) = 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The barrier for CO2 functionalization to formate on hydrogenated Pt
No full textUnderstanding 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
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