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It is widely recognized that Pt nanostructures exhibit favorable catalytic properties for several important technological reactions. Furthermore, selecting an appropriate support has the potential to enhance the catalytic activity of these materials. In this study, we investigate Pt nanoparticles deposited on quantum dots using quantum chemical calculations. We explore the utilization of low-dimensional carbonaceous support by employing graphene quantum dots (GQDs), which offer abundant active sites, such as edges, and diverse conformations. This provides excellent tuning possibilities for both chemical and physical properties. Our goal is to gather information on the alterations in electronic properties, charge redistribution and reactivity of platinum particles on GQD, also analyzing their potential role as catalysts in the water dissociation reaction. Based on thermodynamic and kinetic considerations, our calculations suggest that a Pt3nanoparticle adsorbed on the edge of the GQD exhibits favorable energetics, leading to a promising catalytic material.
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Using DFT computational methods, single-walled carbon nanotubes (CNT) are explored in different geometric configurations (armchair, chiral and zigzag) doped with Fe. Geometry, electronic structure and magnetic properties are investigated for all systems, in order to evaluate a potential application of these structures as electrocatalysts in efficient and low-cost fuel cells. In search for a better electrode material, we turn our attention on nature for help. Oxygen molecules are well-known to reveal a remarkable affinity to the heme group. Therefore, we model the adsorption/dissociative behavior of oxygen molecules on carbon nanotubes doped with Fe atoms. We analyze in detail the effect of the chiral nature of carbon nanotubes that governs their electric, magnetic and chemical behavior. Our results indicate that the dissociation phenomenon involving the armchair (5,5) Fe@CNT is more favored than other chiralities and other doped CNT systems, leading to the lowest activation barrier.
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In the present contribution we have focused on the electrochemical adsorption of a proton from the solution-the Volmer reaction-on a variety of systems based on bimetallic nanostructures-clusters and wires-of Pd and Pt deposited on a surface of Au(111). We have calculated the free energy surface for the electron transfer step by a combination of DFT calculations, MD simulations and the theory of electrocatalysis. We analyze in detail the interaction of the metal d band with the valence orbital of the hydrogen and its effect on the catalytic activity as well as several aspects that influence the electrode reactivity such as spatial arrangements of the nanostructures, the solvation shell and chemical factors. We found that the mixed Pd2Pt wire interacts strongly with hydrogen, and retains an almost complete solvation shell, which is reflected in a substantially reduced activation energy for the Volmer step. Thus, Pd2Pt wires on Au(111) are predicted to be efficient electrocatalysts for the reaction.
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The structure of bimetallic NiCu nanoparticles (NP) is investigated as a function of their composition and size by means of Lattice MonteCarlo (LMC) and molecular dynamics (MD) simulations. According to our results, copper segregation takes place at any composition of the particles. We found that this feature is not size-dependent. In contrast, nickel segregation depends on the NP size. When the size increases, Ni atoms tend to remain in the vicinity of the surface and deeper. For smaller NPs, Ni atoms are located at the surface as well. Our results also showed that most of the metal atoms segregated at the surface area were found to decorate edges and/or form islands. Our findings agree qualitatively with the experimental data found in the literature. In addition, we comment on the reactivity of these nanoparticles.
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We consider the insertion of alkali-halide ion pairs into a narrow (5,5) carbon nanotube. In all cases considered, the insertion of a dimer is only slightly exothermic. While the image charge induced on the surface of the tube favors insertion, it simultaneously weakens the Coulomb attraction between the two ions. In addition, the anion experiences a sizable Pauli repulsion. For a one dimensional chain of NaCl embedded in the tube the most favorable position for the anion is at the center, and for the cation near the wall. The phonon spectrum of such chains shows both an acoustic and an optical branch.
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We have investigated the electrocatalytic properties of multilayers of Pd epitaxially deposited on Au(111). In contrast to the numerous previous works in this area, we have focused on the kinetics of the electrochemical step for hydrogen adsorption (Volmer reaction) and determined its energies of activation. We have used a combination of density functional theory calculations and our own theory of electrocatalysis, which allows us to investigate the systems in an electrochemical environment. Contrary to our previous work with a submonolayer of Pd in Au(111), the activation barrier for the hydrogen adsorption process from proton is very low or almost zero for all bimetallic systems investigated. It is about 0.2 eV for pure Pd(111). In the case of two layers of Pd on Au(111) containing absorbed hydrogen in the subsurface, the adsorption free energy is less negative and the barrier lower than for the other investigated systems. This is in agreement with experimental data that shows a larger activity for hydrogen oxidation with hydride Pd systems.
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The two faces of gold: the reduction of oxygen on gold electrodes in alkaline solutions has been investigated theoretically. The most favorable reaction leads directly to adsorbed O(2)(-), but the activation energy for a two-step pathway, in which the first step is an outer-sphere electron transfer to give solvated O(2)(-), is only slightly higher. d-band catalysis, which dominates oxygen reduction in acid media, plays no role. The reason why the reaction is slow in acid media is also explained.
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Density functional theory (DFT) by itself is insufficient to model electrochemical reactions, because the interface is too large, and there is no satisfactory way to incorporate the electrode potential. In our group we have developed a theory of electrocatalysis, which combines DFT with our model for electrochemical electron transfer, and thereby avoids these difficulties. Our theory explains how a metal d band situated near the Fermi level can lower the energy of activation for a charge transfer reaction. An explicit application to the hydrogen evolution reaction gives results that agree very well with experimental data obtained both on plain and on nanostructured electrodes. Finally, we outline how our method can be extended to other reactions and present first results for the adsorption of OH on Pt(111).
RESUMO
On a number of electrodes the second step in hydrogen evolution is the reaction of a proton with an adsorbed hydrogen intermediate to form a molecule, which is also known as the Heyrovsky reaction. We have developed a model Hamiltonian for this reaction, which for concrete applications requires extensive calculations on the basis of density-functional theory. Explicit results are presented for a Ag(111) electrode. The rate-determining step is electron transfer to the proton that approaches the electrode from the solution. At the saddle point for this reaction the adsorbed hydrogen atom has moved a little away from the surface in order to reduce the repulsion of the product molecule. Electron transfer to the proton occurs when the distance between the two particles is close to the bond distance of the hydrogen molecule.
