Electrochemical interfacial influences on deoxygenation and hydrogenation reactions in CO reduction on a Cu(100) surface.

Electroreduction of CO2 to hydrocarbons on a copper surface has attracted much attention in the last few decades for providing a sustainable way for energy storage. During the CO2 and further CO electroreduction processes, deoxygenation that is C-O bond dissociation, and hydrogenation that is C-H bond formation, are two main types of surface reactions catalyzed by the copper electrode. In this work, by performing the state-of-the-art constrained ab initio molecular dynamics simulations, we have systematically investigated deoxygenation and hydrogenation reactions involving two important intermediates, COHads and CHOads, under various conditions of (i) on a Cu(100) surface without water molecules, (ii) at the water/Cu(100) interface and (iii) at the charged water/Cu(100) interface, in order to elucidate the electrochemical interfacial influences. It has been found that the electrochemical interface can facilitate considerably the C-O bond dissociation via changing the reaction mechanisms. However, C-H bond formation has not been affected by the presence of water or electrical charge. Furthermore, the promotional roles of an aqueous environment and negative electrode potential in deoxygenation have been clarified, respectively. This fundamental study provides an atomic level insight into the significance of the electrochemical interface towards electrocatalysis, which is of general importance for understanding electrochemistry

Elucidation of the surface structure-selectivity relationship in ethanol electro-oxidation over platinum by density functional theory

We have successfully built a general framework to comprehend the structure-selectivity relationship in ethanol electrooxidation on platinum by density functional theory calculations. Based on the reaction mechanisms on three basal planes and five stepped surfaces, it was found that only (110) and n(111) × (110) sites can enhance CO2 selectivity but other non-selective step sites are more beneficial to activity

Designing Pt-based electrocatalysts with high surface energy

The reactivity of an electrocatalyst depends strongly on its surface structure. Pt-based electrocatalysts of nanocrystals (NCs) enclosed with high-index facets contain a large density of catalytically active sites formed from step and kink atoms on the facets and exhibit intrinsically superior activity. However, the Pt-based NCs of high-index facets do possess a high surface energy and are thermodynamically metastable, leading to a big challenge in their shape-controlled synthesis. To overcome the challenge, kinetic–thermodynamic control of crystal growth is indispensable and is currently realized mainly by electrochemical methods and surfactant-based wet chemical approaches. This Perspective reviews recent progresses in Pt-based electrocatalysts of monometallic and bimetallic NCs of high surface energy with different morphologies of convex or concave tetrahexahedron, trapezohedron, trisoctahedron, hexoctahedron, etc. Remarkable electrocatalytic performance of these NCs has been demonstrated. Despite the considerable progress already made, the electrocatalysts of NCs with high surface energy still hold significant future opportunities in both fundamental understanding and practical applications

An insight into methanol oxidation mechanisms on RuO2(100) under an aqueous environment by DFT calculations

In this work, we have studied methanol oxidation mechanisms on RuO2(100) by using density functional theory (DFT) calculations and ab initio molecular dynamics (MD) simulations with some explicit interfacial water molecules. The overall mechanisms are identified as: CH3OH* → CH3O* → HCHO* → HCH(OH)2* → HCHOOH* → HCOOH* → mono-HCOO* → CO2*, without CO formation. This study provides a theoretical insight into C1 molecule oxidation mechanisms at atomic levels on metal oxide surfaces under an aqueous environment

Insight into CO activation over Cu(100) under electrochemical conditions

The reduction of CO2 on copper electrodes has attracted great attentions in the last decades, since it provides a sustainable approach for energy restore. During the CO2 reduction process, the electron transfer to COads is experimentally suggested to be the crucial step. In this work, we examine two possible pathways in CO activation, i.e. to generate COHads and CHOads, respectively, by performing the state-of-the-art constrained ab initio molecular dynamics simulations on the charged Cu(100) electrode under aqueous conditions, which is close to the realistic electrochemical condition. The free energy profile in the formation of COHads via the coupled proton and electron transfer is plotted. Furthermore, by Bader charge analyses, a linear relationship between C-O bond distance and the negative charge in CO fragment is unveiled. The formation of CHOads is identified to be a surface catalytic reaction, which requires the adsorption of H atom on the surface first. By comparing these two pathways, we demonstrate that kinetically the formation of COHads is more favored than that of CHOads, while CHOads is thermodynamically more stable. This work reveals that CO activation via COHads intermediate is an important pathway in electrocatalysis, which could provide some insights into CO2 electroreduction over Cu electrodes

Insights into the mechanism of Nitrobenzene reduction to aniline over Pt catalyst and the significance of the adsorption of phenyl group on kinetics

Aniline (C6H5NH2) plays a significant role in both industry and daily life, and can be synthesized via catalytic hydrogenation of nitrobenzene (C6H5NO2) over transition metals; however fundamental investigations on reaction mechanisms in the heterogeneous catalysis are still lacking. In this work, the nitrobenzene reduction reaction over the Pt(111) model catalyst was studied using density functional theory (DFT) with the inclusion of van der Waals interaction, for fundamentally understanding the mechanisms at atomic and molecular levels. It was found that the double H-induced dissociation of N-O bond was the preferential path for the activation of nitro group, having a much lower reaction barrier than that of the direct dissociation and single H-induced dissociation paths. The overall mechanisms have been identified as: C6H5NO2* → C6H5NOOH* → C6H5N(OH)2* → C6H5NOH* → C6H5NHOH* → C6H5NH* → C6H5NH2*. The overall barrier of the nitro group reduction was calculated to be 0.75 eV, which is much lower than that of the benzene reduction (1.08 eV). Our DFT data elucidates clearly the reason why the major product of nitrobenzene reduction reaction was aniline. Furthermore, the adsorption/desorption of phenyl group was found to have significant impacts on kinetic barriers. Generally, in the hydrogenation process (N-H or O-H bond association), the phenyl group preferred to adsorb on the surface; but in the dissociation process (N-O bond dissociation) it preferred to desorb transiently at the transition state and to adsorb again when the dissociation was completed. This study also provides a solid theoretical insight into the selective catalysis of the large aromatic compounds

Pd nanocrystals with continuously tunable high-index facets as a model nanocatalyst

Knowledge of the structure–reactivity relationship of catalysts is usually gained through using well-defined bulk single-crystal planes as model catalysts. However, there exists a huge gap between bulk single-crystal planes and practical nanocatalysts in terms of size, structural complexity, and local environment. Herein, we efficiently bridged this gap by developing a model nanocatalyst based on nanocrystals with continuously tunable surface structures. Pd nanocrystals with finely tunable facets, ranging from a flat {100} low-index facet to a series of {hk0} high-index facets, were prepared by an electrochemical square-wave potential method. The validity of the Pd model nanocatalyst has been demonstrated by structure–reactivity studies of electrocatalytic oxidation of small organic molecules. We further observed that Pd nanocrystals exhibited catalytic performance considerably different from bulk Pd single-crystal planes with the same Miller indices. Such differences were attributed to special catalytic functions conferred by nanocrystal edges. This study paves a promising route for investigating catalytic reactions effectively at the atomic level and nanoscales

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Insights into reaction mechanisms of ethanol electrooxidation at the Pt/Au(111) interfaces using density functional theory

Understanding ethanol electrooxidation reaction kinetics is fundamental to the development of direct ethanol fuel cells. The utilization of binary PtAu catalysts has been reported recently as an effective strategy to enhance ethanol electrocatalytic oxidation; however, the catalytic reaction mechanisms are still unclear. In this work, we systematically studied the ethanol electrooxidation reaction mechanisms on Pt/Au(111) model surfaces at an atomic level through high level density functional theory (DFT) calculations; particularly the flat (111) terrace and the stepped (111) × (110) and (111) × (100) interfaces with diverse surface atomic arrangements were considered, respectively. It was found that for ethanol dissociation, the flat (111) terrace is more active than the stepped (111) × (110) and (111) × (100) interfaces. The stepped interfaces, however, could activate water from the aqueous electrolyte solution to form adsorbed OH* at the electrode potential below 0.53 V vs. SHE (standard hydrogen electrode), which is of great importance in coupling with the CH3CO* intermediate formed from ethanol dissociation to produce acetic acid as the final product of the ethanol electrooxidation reaction without releasing CO2. The C–C bond splitting process for ethanol oxidation to form C1 products was very limited. The terrace sites can facilitate both ethanol decomposition and acetic acid formation at the electrode potential above 0.53 V vs. SHE. Our results clearly identify the fact that for ethanol electrooxidation reactions, with an increase in electrode potential, the active sites on Pt/Au(111) surfaces change from those at the stepped interfaces to the flat terrace sites.</p

Role of Water and Adsorbed Hydroxyls on Ethanol Electrochemistry on Pd: New Mechanism, Active Centers, and Energetics for Direct Ethanol Fuel Cell Running in Alkaline Medium

First principles calculations with molecular dynamics are utilized to simulate a simplified electrical double layer formed in the active electric potential region during the electrocatalytic oxidation of ethanol on Pd electrodes running in an alkaline electrolyte. Our simulations provide an atomic level insight into how ethanol oxidation occurs in fuel cells: New mechanisms in the presence of the simplified electrical double layer are found to be different from the traditional ones; through concerted-like dehydrogenation paths, both acetaldehyde and acetate are produced in such a way as to avoid a variety of intermediates, which is consistent with the experimental data obtained from <i>in situ</i> FTIR spectroscopy. Our work shows that adsorbed OH on the Pd electrode rather than Pd atoms is the active center for the reactions; the dissociation of the C–H bond is facilitated by the adsorption of an OH^– anion on the surface, resulting in the formation of water. Our calculations demonstrate that water dissociation rather than H desorption is the main channel through which electrical current is generated on the Pd electrode. The effects of the inner Helmholtz layer and the outer Helmholtz layer are decoupled, with only the inner Helmholtz layer being found to have a significant impact on the mechanistics of the reaction. Our results provide atomic level insight into the significance of the simplified electrical double layer in electrocatalysis, which may be of general importance