On the electrocatalysis of nanostructures: Monolayers of a foreign atom (Pd) on different substrates M(111)

Electrochemical adsorption of hydrogen on mixed Pd2Pt nanostructures

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.

Facile Synthesis of Co3O4@CoO@Co Gradient Core@Shell Nanoparticles and Their Applications for Oxygen Evolution and Reduction in Alkaline Electrolytes

We demonstrate a facile fabrication scheme for Co₃O₄@CoO@Co (gradient core@shell) nanoparticles on graphene and explore their electrocatalytic potentials for an oxygen evolution reaction (OER) and an oxygen reduction reaction (ORR) in alkaline electrolytes. The synthetic approach begins with the preparation of Co₃O₄ nanoparticles via a hydrothermal process, which is followed by a controlled hydrogen reduction treatment to render nanoparticles with radial constituents of Co₃O₄/CoO/Co (inside/outside). X-ray diffraction patterns confirm the formation of crystalline Co₃O₄ nanoparticles, and their gradual transformation to cubic CoO and fcc Co on the surface. Images from transmission electron microscope reveal a core@shell microstructure. These Co₃O₄@CoO@Co nanoparticles show impressive activities and durability for OER. For ORR electrocatalysis, the Co₃O₄@CoO@Co nanoparticles are subjected to a galvanic displacement reaction in which the surface Co atoms undergo oxidative dissolution for the reduction of Pt ions from the electrolyte to form Co₃O₄@Pt nanoparticles. With commercial Pt/C as a benchmark, we determine the ORR activities in sequence of Pt/C > Co₃O₄@Pt > Co₃O₄. Measurements from a rotation disk electrode at various rotation speeds indicate a 4-electron transfer path for Co₃O₄@Pt. In addition, the specific activity of Co₃O₄@Pt is more than two times greater than that of Pt/C.

The effect of lattice strain on catalytic activity

We report on the effect of lattice strain in three different types of core–shell electrocatalyst particles on their catalytic activity towards the oxygen reduction reaction.

Understanding the structure and reactivity of NiCu nanoparticles: an atomistic model

Fine details of the surface structure of NiCu nanoparticles of different sizes and compositions are investigated by atomistic simulations. Their reactivity in electrochemical hydrogen oxidation is dicussed in terms of the density of electronic states.

Unravelling the hydrogen absorption process in Pd overlayers on a Au(111) surface

Shedding light on the mechanism of hydrogen absorption occurring in nano-structured materials using the power of modern computational chemistry.

Hydrogen evolution reaction on palladium multilayers deposited on Au(111): a theoretical approach

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.

Bimetallic alloys in action: dynamic atomistic motifs for electrochemistry and catalysis

Multifarious structural motifs, dynamic surface morphologies and novel reaction mechanisms are essential aspects of bimetallic alloys, making them promising candidates for diverse applications in electrochemistry and heterogeneous catalysis.

Platinum Monolayer Electrocatalysts for the Oxygen Reduction Reaction: Improvements Induced by Surface and Subsurface Modifications of Cores

This paper demonstrates that the ORR activity of PtML electrocatalysts can be further improved by the modification of surface and subsurface of the core materials. The removal of surface low-coordination sites, generation (via addition or segregation) of an interlayer between PtML and the core, or the introduction of a second metal component to the subsurface layer of the core can further improve the ORR activity and/or stability of PtML electrocatalysts. These modifications generate the alternation of the interactions between the substrate and the PtML, involving the changes on both electronic (ligand) and geometric (strain) properties of the substrates. The improvements resulted from the application of these approaches provide a new perspective to designing of the new generation PtML electrocatalysts.

Hydrogen Adsorption on Palladium and Platinum Overlayers: DFT Study

Hydrogen adsorption on twenty different palladium and platinum overlayer surfaces with (111) crystallographic orientation was studied by means of periodic DFT calculations on the GGA-PBE level. Palladium and platinum overlayers here denote either the Pd and Pt mono- and bilayers deposited over (111) crystallographic plane of Pd, Pt, Cu, and Au monocrystals or the (111) crystallographic plane of Pd and Pt monocrystals with inserted one-atom-thick surface underlayer of Pd, Pt, Cu, and Au. The attention was focused on the bond lengths, hydrogen adsorption energetics, mobility of adsorbed hydrogen, and surface reactivity toward hydrogen electrode reactions. Both the ligand and strain effects were considered, found to lead to a significant modification of the electronic structure of Pd and Pt overlayers, described through the position of the d-band center, and tuning of the hydrogen adsorption energy in the range that covers approximately 120 kJmol−1. Mobility of hydrogen adsorbed on studied overlayers was found to be determined by hydrogen-metal binding energy. Obtained results regarding Pd layers on Pt(111) and Au(111) surfaces, in conjunction with some of the recent experimental data, were used to explain its electrocatalytic activity towards hydrogen evolution reaction.

Recent Progress in Hydrogen Electrocatalysis

Recently, we have proposed a unified model for electrochemical electron transfer reactions which explicitly accounts for the electronic structure of the electrode. It provides a framework describing the whole course of bond-breaking electron transfer, which explains catalytic effects caused by the presence of surface d bands. In application on real systems, the parameters of this model—interaction strengths, densities of states, and energies of reorganization—are obtained from density functional theory (DFT). In this opportunity, we review our main achievements in applying the theory of electrocatalysis. Particularly, we have focused on the electrochemical adsorption of a proton from the solution—the Volmer reaction—on a variety of systems of technological interest, such as bare single crystals and nanostructured surfaces. We discuss in detail the interaction of the surface metal d band with the valence orbital of the reactant and its effect on the catalytic activity as well as other aspects that influence the surface-electrode reactivity such as strain and chemical factors.