A density functional study of C1-C4 alkyl adsorption on Cu(111)

To better understand the nature of alkyl intermediates often invoked in reactions involving hydrocarbon reactants and products, the adsorption of linear and branched C(1)-C(4) alkyls on Cu(111) at 1/4 ML and 1/9 ML coverages was studied using density functional theory. The adsorption energy and site preference are found to be coverage-dependent, and both direct alkyl-alkyl interactions and changes in the Cu electronic structure play a role in these trends. It was found that methyl strongly prefers the hollow sites, the branched alkyls strongly prefer the top site, and the linear C(2)-C(4) alkyls have weak site preferences that change with coverage. To explain these differences, rationalize alkyl adsorption trends, and predict the binding energy of other alkyls, a simple model was developed in which the binding energy is fit as a linear function of the number of C-Cu and C-H-Cu interactions as well as the C-H bond energy in the corresponding alkane. Site preference can be understood as a compromise between C-Cu interactions and C-H-Cu interactions. Density of states analysis was used to gain a molecular-orbital understanding of the bonding of alkyls to Cu(111).     

Mechanistic study of the electrochemical oxygen reduction reaction on Pt(111) using density functional theory

Density functional theory (DFT) was used to study the electrolyte solution effects on the oxygen reduction reaction (ORR) on Pt(111). To model the acid electrolyte, an H(5)O(2)(+) cluster was used. The vibrational proton oscillation modes for adsorbed H(5)O(2)(+) computed at 1711 and 1010 cm(-1), in addition to OH stretching and H(2)O scissoring modes, agree with experimental vibrational spectra for proton formation on Pt surfaces in ultrahigh vacuum. Using the H(5)O(2)(+) model, protonation of adsorbed species was found to be facile and consistent with the activation barrier of proton transfer in solution. After protonation, OOH dissociates with an activation barrier of 0.22 eV, similar to the barrier for O(2) dissociation. Comparison of the two pathways suggests that O(2) protonation precedes dissociation in the oxygen reduction reaction. Additionally, an OH diffusion step following O protonation inhibits the reaction, which may lead to accumulation of oxygen on the electrode surface.     

Interfaces in bulk thermoelectric materials: A review for Current Opinion in Colloid and Interface Science

We review current progress in the understanding of interfaces in bulk thermoelectric materials. Following a brief discussion of the mechanisms by which embedded interfaces can enhance the electronic and thermal transport properties, we focus on emerging routes to engineer the nanoscale grain and interfacial structures in bulk thermoelectric materials. We address in particular (i) control of crystallographic texture, (ii) reduction of grain size to nanocrystalline dimensions, and (iii) formation of nanocomposite structures. While these approaches are beginning to yield promising improvements in performance, continued progress will require an improved fundamental understanding of the mechanisms governing the formation, stability, and properties of thermoelectric interfaces.     

Surface-level mechanistic studies of adsorbate-adsorbate interactions in heterogeneous catalysis by metals

Despite being considered a mature field, recent developments in experimental and theoretical techniques have greatly increased the fundamental understanding of complex surface processes in catalysis. One area of particular interest is the effect of co-adsorbed species on reactivity in heterogeneous systems. Experiments have demonstrated that co-adsorbed species, both organic and inorganic, can improve surface activity and selectivity. We begin by classifying adsorbate-adsorbate interactions that have been shown to alter the reactivity of a metal surface. We then review numerous systems where such effects have been observed using experiment or theory. Systems such as the hydrogenation of olefins with other carbonaceous adsorbates present, the chiral templating of surfaces, and the co-adsorption of alkalis, halides, and other inorganic “poisons” to improve selectivity are discussed in detail. Finally, future directions of study and outstanding questions are addressed.