A Density-Functional Theory Study of the Water−Gas Shift Mechanism on Pt/Ceria(111)

Density-functional theory has been used to model the interactions between ceria(111) and CO adsorbed on platinum in order to provide insights into the mechanism behind the water−gas shift reaction on this material. Morphological studies on ceria show the presence of various defect sites, both cerium and oxygen vacancies, which are believed to be the reason for the catalytic activity of the substrate. Of these defect sites, the reported calculations clearly show the preference for platinum to bind in cerium vacancies, as opposed to a defect-free surface or an oxygen deficiency site. Currently there are two main pathways proposed for this mechanism: a direct redox pathway and a formate-intermediate pathway. In both scenarios the initial step is the same: the oxygen storage capacity of ceria provides the oxygen necessary to oxidize CO into CO2; water then adsorbs and is dissociated in these oxygen vacancies helping reform the ceria surface. The key difference between the two mechanisms is either the desorption of CO2 and the formation of H2 when a second water molecule adsorbs at a site where water had been previously dissociated (catalyzed by platinum), or the formation of a formate species from the interaction of the adsorbed CO2 with adjacent water molecules. Energy pathways are mapped for both processes, demonstrating the preference toward the initial direct redox pathway. However, the resulting formation of adsorbed hydroxide species in the oxygen vacancies adjacent to the platinum hinders this pathway. A bifunctional mechanism is then presented as a means to remove these species and thereby form formate. The activation of these adsorbed hydroxides is also studied

A Density-Functional Theory Study of the Water−Gas Shift Mechanism on Pt/Ceria(111)

Density-functional theory has been used to model the interactions between ceria(111) and CO adsorbed on platinum in order to provide insights into the mechanism behind the water−gas shift reaction on this material. Morphological studies on ceria show the presence of various defect sites, both cerium and oxygen vacancies, which are believed to be the reason for the catalytic activity of the substrate. Of these defect sites, the reported calculations clearly show the preference for platinum to bind in cerium vacancies, as opposed to a defect-free surface or an oxygen deficiency site. Currently there are two main pathways proposed for this mechanism: a direct redox pathway and a formate-intermediate pathway. In both scenarios the initial step is the same: the oxygen storage capacity of ceria provides the oxygen necessary to oxidize CO into CO2; water then adsorbs and is dissociated in these oxygen vacancies helping reform the ceria surface. The key difference between the two mechanisms is either the desorption of CO2 and the formation of H2 when a second water molecule adsorbs at a site where water had been previously dissociated (catalyzed by platinum), or the formation of a formate species from the interaction of the adsorbed CO2 with adjacent water molecules. Energy pathways are mapped for both processes, demonstrating the preference toward the initial direct redox pathway. However, the resulting formation of adsorbed hydroxide species in the oxygen vacancies adjacent to the platinum hinders this pathway. A bifunctional mechanism is then presented as a means to remove these species and thereby form formate. The activation of these adsorbed hydroxides is also studied

Metallic BSi₃ Silicene: A Promising High Capacity Anode Material for Lithium-Ion Batteries

Very recently, intrinsically metallic B-substituted silicenes, namely, <i>H</i>-BSi₃ and <i>R</i>-BSi₃ (<i>H</i> and <i>R</i> denote the hexagonal and rectangular symmetry), have been predicted as the global minimum structures of the BSi₃ monolayer (<i>J. Phys. Chem. C</i> <b>2014</b>, DOI: 10.1021/jp507011p). With unusual planar geometry and better electronic conductivity relative to the buckled and semimetallic pristine silicene sheet, the B-substituted silicenes are expected to have good applications in high capacity lithium-ion batteries (LIBs) anodes. By means of density functional theory (DFT) computations, we systematically investigated the adsorption and diffusion of Li on <i>H</i>-BSi₃ and <i>R</i>-BSi₃, in comparison with silicene and graphite. Their exceptional properties, including good electronic conductivity, very high theoretical charge capacity (1410 and 846 mA·h/g for single- and double-layer, respectively), fast Li diffusion, and low open-circuit voltage (OCV), suggest that the BSi₃ silicene could serve as a promising high capacity and fast charge/discharge rate anode material for LIBs

Graphene-Supported Pt–Au Alloy Nanoparticles: A Highly Efficient Anode for Direct Formic Acid Fuel Cells

Graphene-supported Pt and Pt–Au alloy electrocatalysts are prepared by ethylene glycol reduction method and characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). XRD reveals the face-centered cubic structure of Pt in the materials. SEM and TEM images show the good spatial distribution of metal nanoparticles on layered graphene sheets. EDX reveals that the average composition of elements in the Pt–Au alloy catalyst is approximately 1:1. Electrocatalytic performance of the prepared materials toward formic acid oxidation (FAO) is investigated using cyclic voltammetry. FAO activity of the Pt–Au/graphene is found to be ten times higher than that of Pt/graphene. The prepared electrocatalysts are used as anode in a direct formic acid fuel cell and tested at 303 and 333 K. An increase in the performance with increasing temperature is observed. A maximum power density of 185, 70, and 53 mW/cm2 is observed with Pt–Au/graphene, Pt/graphene, and commercial Pt/C anodes, respectively, at 333 K. The high electrocatalytic performance of Pt–Au/graphene is attributed to the change in the electronic structure of Pt by the presence of alloying element, Au

A Density-Functional Theory Study of the Water−Gas Shift Mechanism on Pt/Ceria(111)

Density-functional theory has been used to model the interactions between ceria(111) and CO adsorbed on platinum in order to provide insights into the mechanism behind the water−gas shift reaction on this material. Morphological studies on ceria show the presence of various defect sites, both cerium and oxygen vacancies, which are believed to be the reason for the catalytic activity of the substrate. Of these defect sites, the reported calculations clearly show the preference for platinum to bind in cerium vacancies, as opposed to a defect-free surface or an oxygen deficiency site. Currently there are two main pathways proposed for this mechanism: a direct redox pathway and a formate-intermediate pathway. In both scenarios the initial step is the same: the oxygen storage capacity of ceria provides the oxygen necessary to oxidize CO into CO2; water then adsorbs and is dissociated in these oxygen vacancies helping reform the ceria surface. The key difference between the two mechanisms is either the desorption of CO2 and the formation of H2 when a second water molecule adsorbs at a site where water had been previously dissociated (catalyzed by platinum), or the formation of a formate species from the interaction of the adsorbed CO2 with adjacent water molecules. Energy pathways are mapped for both processes, demonstrating the preference toward the initial direct redox pathway. However, the resulting formation of adsorbed hydroxide species in the oxygen vacancies adjacent to the platinum hinders this pathway. A bifunctional mechanism is then presented as a means to remove these species and thereby form formate. The activation of these adsorbed hydroxides is also studied

In Search of the Active Site in Nitrogen-Doped Carbon Nanotube Electrodes for the Oxygen Reduction Reaction

Nitrogen-doped carbon nanomaterials are known to exhibit good electrocatalytic activity for the oxygen reduction reaction (ORR). However, the structure of the ORR active site and optimum content of nitrogen in the carbon lattice for ORR activity remains unknown. In this study, a series of vertically aligned carbon nanotubes (VA-CNTs) with a surface nitrogen concentration of 0, 4.3, 5.6, 8.4, and 10.7 atom % is prepared by the alumina template technique and characterized with XRD, Raman spectroscopy, SEM, and XPS. Electrocatalytic ORR activity is investigated by rotating disk electrode (RDE) voltammetry. Among them, VA-CNTs with a nitrogen concentration of 8.4 atom % exhibited the best ORR performance. This is ascribed to a greater number of pyridinic-type nitrogen sites. The good performance of less expensive nitrogen-doped CNTs makes the ORR electrodes a viable alternative to platinum for energy conversion device applications

Palladium Nanoshell Catalysts Synthesis on Highly Ordered Pyrolytic Graphite for Oxygen Reduction Reaction

A novel approach for the synthesis of palladium (Pd) nanoshells on highly ordered pyrolytic graphite (HOPG) surfaces for the oxygen reduction reaction (ORR) is described. Magnetron sputtering deposition was used to synthesize Pd thin films and nanoshells of different thicknesses on HOPG surfaces. Electrospun polymer fibers mats of poly­(ethylene) oxide (PEO) were used as templates for the Pd nanoshells formation. The palladium thicknesses between 25 and 95 nm were deposited by magnetron sputtering. Scanning electron microscopy and energy-dispersive X-ray fluorescence spectroscopy were used to study the morphology and composition of the Pd nanoshells. Electrocatalytic activity toward the ORR and methanol tolerance in oxygen saturated 0.5 M H₂SO₄ solution was determined. Palladium nanoshells presented higher electrocatalytic activity toward ORR than Pd thin films of similar electrodes thicknesses and geometric area. Since palladium has higher methanol tolerance than platinum, the Pd nanoshells are promising electrode materials for direct methanol fuel cells (DMFC)

RoDSE Synthesized Fine Tailored Au Nanoparticles from Au(X)₄^– (X = Cl^–, Br^–, and OH^–) on Unsupported Vulcan XC-72R for Ethanol Oxidation Reaction in Alkaline Media

An electrosynthesis method to obtain Au nanoparticles dispersed on carbon Vulcan XC-72R support material was done using AuX4– (X = Cl–, Br–, and OH–) as molecular precursors and different electrolyte media. The Au surface structure was significantly enhanced using KOH as an electrolyte as opposed to KBr and H2SO4. Cyclic voltammetry was used as a surface sensitive technique to illustrate the Au/Vulcan XC-72R catalytic activity for the ethanol oxidation reaction (EOR). The Au electroactive surface areas obtained were 1.88, 5.83, and 13.96 m2 g–1 for Au/C–H2SO4, Au/C–KBr, and Au/C–KOH, respectively. The latter compares to chemically reduced Au/C–spheres that had an electroactive surface area of 15.0 m2 g–1. The electrochemical Au electrodeposition, in alkaline media (Au/C–KOH), exhibited the highest catalytic activity for the EOR with a 50% increase in peak current density when compared with Au nanoparticles prepared by the chemical reduction route. Raman and X-ray photoelectron spectroscopies analyses of the Au/Vulcan XC-72R nanomaterials revealed a restructuring of the carbon functionalities responsible for the metal nanoparticle anchoring. Our results strongly suggest that the enhanced EOR catalytic activity is related to the presence of oxygen functional groups on the carbon surface, particularly ketonic groups on the carbon Vulcan XC-72R substrate

Enhanced Li Adsorption and Diffusion on MoS₂ Zigzag Nanoribbons by Edge Effects: A Computational Study

By means of density functional theory computations, we systematically investigated the adsorption and diffusion of Li on the 2-D MoS₂ nanosheets and 1-D zigzag MoS₂ nanoribbons (ZMoS₂NRs), in comparison with MoS₂ bulk. Although the Li mobility can be significantly facilitated in MoS₂ nanosheets, their decreased Li binding energies make them less attractive for cathode applications. Because of the presence of unique edge states, ZMoS₂NRs have a remarkably enhanced binding interaction with Li without sacrificing the Li mobility, and thus are promising as cathode materials of Li-ion batteries with a high power density and fast charge/discharge rates

Synthesis and Characterization of Palladium and Palladium–Cobalt Nanoparticles on Vulcan XC-72R for the Oxygen Reduction Reaction

A single-source approach was used to synthesize bimetallic nanoparticles on a high-surface-area carbon-support surface. The synthesis of palladium and palladium–cobalt nanoparticles on carbon black (Vulcan XC-72R) by chemical and thermal reduction using organometallic complexes as precursors is described. The electrocatalysts studied were Pd/C, Pd₂Co/C, and PdCo₂/C. The nanoparticles composition and morphology were characterized using inductively coupled plasma mass spectrophotometer (ICP–MS), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray fluorescence spectroscopy (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. Electrocatalytic activity towards the oxygen reduction reaction (ORR) and methanol tolerance in oxygen-saturated acid solution were determined. The bimetallic catalyst on carbon support synthetized by thermal reduction of the Pd₂Co precursor has ORR electrocatalytic activity and a higher methanol tolerance than a Pt/C catalyst