Kinetics on Model Systems

Selectivity in Hydrogenation Catalysis with Unsaturated Aldehydes: Parallel versus Sequential Steps

“…This discrepancy highlights the different chemical regimes probed by the two approaches. In TPD, the temperature of the surface is ramped as the experiment proceeds, and that promotes the desorption of adsorbates. , It is difficult to reproduce steady-state hydrogenation conditions that way, even if the surface is predosed with H 2 before adding the reactant, because the adsorbed atomic hydrogen recombines and desorbs at relatively low temperatures; it has not been possible, to the best of our knowledge, to mimic steady-state hydrogenation conditions, which require high hydrogen surface coverages at the temperature of the conversion of the organic reactant, using such transient experiment. ,,,− Our high-flux molecular beam arrangement, on the other hand, provides the means to sustain high hydrogen surface coverages as the catalytic conversion takes place in an steady-state regime. , Such conditions not only promote hydrogenation reactions but also inhibit decomposition pathways, in part at least because the adsorbed hydrogen also promotes weaker (π rather than di-σ bonding) adsorption of the reactants. ,− …”

Monte Carlo Simulations of the Uptake of Chiral Compounds on Solid Surfaces

“…This is because the methodology used to analyze most chemical kinetics in homogeneous media are not adequate to address the additional complications introduced by the presence of a surface, which include a change in dimensionality (from 3D to 2D), possible strong interactions among adsorbed reactants (and products), which can be long-range, and spatial heterogeneity in the way the different species are distributed on the surface. 1,2 These issues are not easy to emulate using simple rate laws, and typically cannot be reproduced using conventional microkinetics or mean-field kinetic models. 3−6 Instead, they can be visualized by using a stochastic approach, specifically employing Monte Carlo algorithms.…”

“…The kinetics of chemical reactions on solid surfaces are central to many areas of practical importance, including catalysis, electrochemistry, tribology, film deposition, crystallization, and etching, yet they are often difficult to describe. This is because the methodology used to analyze most chemical kinetics in homogeneous media are not adequate to address the additional complications introduced by the presence of a surface, which include a change in dimensionality (from 3D to 2D), possible strong interactions among adsorbed reactants (and products), which can be long-range, and spatial heterogeneity in the way the different species are distributed on the surface. , These issues are not easy to emulate using simple rate laws, and typically cannot be reproduced using conventional microkinetics or mean-field kinetic models. − Instead, they can be visualized by using a stochastic approach, specifically employing Monte Carlo algorithms. − Here we describe the use of such an approach to develop a molecular-level understanding of the adsorption of chiral (and prochiral) molecules on solid surfaces.…”

The Surface Chemistry of Catalytic Reactions: Progress and Challenges

“…Some of the most significant advances in the study of surface reactions in recent years come from theory. Over the years, experimentalist have been able to collect a reasonably large database for the energetics of simple adsorbates on well-defined (i.e., single-crystal) surfaces, mainly by using temperature-programmed desorption but more recently also employing microcalorimetry, and related experiments have yielded complementary information on activation energies of surface reactions . Density functional theory (DFT) has now been refined to the point of being able to provide good estimates for binding energies and to favorably compare those to experimental values, but the calculation of kinetic data such as activation energies and other details of the potential energy surface has proven more elusive .…”