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

Water Flow-Driven Coupling Process of Anodic Oxygen Evolution and Cathodic Oxygen Activation for Water Decontamination and Prevention of Chlorinated Byproducts

Electrochemical advanced oxidation process (EAOP) is a promising technology for decentralized water decontamination but is subject to parasitic anodic oxygen evolution and formation of toxic chlorinated byproducts in the presence of Cl-. To address this issue, we developed a novel electrolytic process by water flow-driven coupling of anodic oxygen evolution reaction (OER) and cathodic molecular oxygen activation (MOA). When water flows from anode to cathode, O2 produced from OER is carried by water through convection, followed by being activated by atomic hydrogen (H*) on Pd cathode to produce •OH. The water flow-driven OER/MOA process enables the anode to be polarized at low potential (1.7 V vs SHE) that is lower than that of conventional EAOP whose •OH is produced from direct water oxidation (>2.3 V vs SHE). At a flow rate of 30 mL min-1, the process could achieve 94.8% removal of 2,4-dichlorophenol (2,4-DCP) and 71.5% removal of chemical oxygen demand (COD) within 45 min at an anode potential of 1.7 V vs SHE and cathode potential of -0.5 V vs SHE. To achieve the comparable 2,4-DCP removal performance, 4.3-fold higher energy consumption was needed for the conventional EAOP with titanium suboxide anode (anode potential of 2.9 V vs SHE), but current efficiency declined by 3.5 folds. Unlike conventional EAOP, chlorate and perchlorate were not detected in the OER/MOA process, because low anode potential <2.0 V vs SHE was thermodynamically unfavorable for the formation of chlorinated byproducts by anodic oxidation, indicated by theoretical calculations and experimental data. This study provides a proof-in-concept demonstration of water flow-driven OER/MOA process, representing a paradigm shift of electrochemical technology for water decontamination and prevention of chlorinated byproducts, making electrochemical water decontamination more efficient, more economic, and more sustainable.

Effective Electro-Activation Process of Hydrogen Peroxide/Peroxydisulfate Induced by Atomic Hydrogen for Rapid Oxidation of Norfloxacin over the Carbon-Based Pd Nanocatalyst

Peroxydisulfate (PDS) can be activated by electrochemistry, for which using atom H* as an activator is feasibly favorable in theoretical and experimental applications. Studies have shown that atomic H* can cleave the peroxide bond as a single-electron reducing agent in Na₂S₂O₈ to generate SO₄^•-, thus achieving the degradation of pollutants. Herein, Pd nanoparticles synthesized by in an in situ solution were dispersed in carbon black and then loaded on carbon felt, called Pd/C@CF, as the cathode for peroxydisulfate activation. This showed an ideal degradation effect on a small electrode (10 mm × 10 mm). Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests were taken to verify the significant increase in the yield of the reduction of Na₂S₂O₈ by H*. The degradation experiments and free-radical scavenging experiment confirmed that the atomic H* was the dominant component triggering the activation of PDS to generate SO₄^•-. A Pd/C@CF composite electrodes have low pH dependence, high stability and recyclability, etc., which has many potential practical applications in wastewater treatment. In addition, H* can also reduce H₂O₂ to •OH by breaking the peroxide bond, so the removal of pollutants by the same amount of H₂O₂ and Na₂S₂O₈ under the same conditions is compared, and their application prospects are analyzed and compared.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Synergistic effect of atomically dispersed Fe-Ni pair sites for electrocatalytic reactions to remove chlorinated organic compounds

Electrocatalysis is a promising and environmentally friendly technology for the removal of refractory organics. Diatomic catalysts with an increased number of active sites have emerged with further expansion of the field of atomic catalysts. Here, a metal diatomic FeNi supported graphene (FeNi/N-rGO) catalyst is successfully synthesized. The atomically dispersed Fe and Ni species on graphene is verified by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy. The pollutant degradation efficiencies for the cathode and anode are found to reach 97.6% and 95.8%, respectively, within 90 min in the diatomic catalytic system. According to DFT theoretical calculations, FeNi diatomic catalysts have a lower free energy (ΔG = -0.2 eV), and the higher adsorption energy for the active substance H* is -0.412 eV. This work presents a method for the preparation of high-performance diatomic catalysts and promotes their application in the electrochemical degradation of chlorinated organic pollutants.

Kinetics of Hydrogen Evolution Reactions in Acidic Media on Pt, Pd, and MoS2

Hydrogen evolution reaction (HER) are investigated on Pt, Pd, and MoS₂ in a 0.5 M H₂SO₄ electrolyte in a rotating disk electrode (RDE) configuration in the temperature range of 285-335 K. The reaction is temperature-sensitive on all of the three catalyst surfaces at their respective overpotential ranges. The kinetic parameters (activation enthalpy (ΔH^#), free energy of activation (ΔG^#), and pre-exponential factor (A_f)) toward HER are obtained from the Arrhenius and Eyring relations, and the overall kinetics on the catalyst surfaces is analyzed. ΔH^# for HER is a strong function of the overpotential in the case of both Pt and Pd. On the other hand, the trend in A_f suggests that the electrocatalysis of HER on MoS₂ originates from an increase in entropy factor, perhaps due to the solvent-dipole interaction at the interface. Such analysis is pivotal to the investigation of electrocatalysis of HER, especially on surfaces for which determination of active-site density is not established.

Catalysis of hydrogen evolution on Pt(111) by absorbed hydrogen

The activity of Pt(111) electrodes for the hydrogen evolution reaction (HER) in 0.5M H₂SO₄ solution is found to increase with continuous potential cycling in the HER potential region. In addition, the basic cyclic voltammograms obtained in 0.5M H₂SO₄ saturated with N₂ after HER show several characteristic changes: the current waves for hydrogen adsorption in the region of0.2 < E < 0.35 V and for sulfate adsorption at 0.35 < E < 0.5 V decrease and the current spike at 0.44 V for the phase transition of the sulfate adlayer gradually disappears. We suggest that these changes are caused by the absorption of a small amount of hydrogen in the subsurface layer and propose a mechanism by which this enhances hydrogen evolution.

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.

pH-Independent Production of Hydroxyl Radical from Atomic H*-Mediated Electrocatalytic H2O2 Reduction: A Green Fenton Process without Byproducts

Hydroxyl radical (^•OH) can hydroxylate or dehydrogenate organics without forming extra products and is thereby expediently applied in extensive domains. Although it can be efficiently produced through single-electron transfer from transition-metal-containing activators to hydrogen peroxide (H₂O₂), narrow applicable pH range, strict activator/H₂O₂ ratio requirement, and byproducts that are formed in the mixture with the background matrix necessitate the need for additional energy-intensive up/downstream treatments. Here, we show a green Fenton process in an electrochemical cell, where the electro-generated atomic H* on a Pd/graphite cathode enables the efficient conversion of H₂O₂ into ^•OH and subsequent degradation of organic pollutants (80% efficiency). Operando liquid time-of-fight secondary ion mass spectrometry verified that H₂O₂ activation takes place through a transition state of the Pd-H*-H₂O₂ adduct with a low reaction energy barrier of 0.92 eV, whereby the lone electron in atomic H* can readily cleave the peroxide bridge, with ^•OH and H₂O as products (ΔG_r = -1.344 eV). Using H⁺ or H₂O as the resource, we demonstrate that the well-directed output of H* determines the pH-independent production of ^•OH for stable conversion of organic contaminants in wider pH ranges (3-12). The research pioneers a novel path for eliminating the restrictions that are historically challenging in the traditional Fenton process.

Direct deposition of a nanoporous palladium electrocatalyst for efficient hydrogen evolution reaction

The porous palladium directly deposited on metallic substrates by aerosol-assisted chemical vapor deposition exhibits remarkable HER performance in an acidic electrolyte.

Defect Sites in Ultrathin Pd Nanowires Facilitate the Highly Efficient Electrochemical Hydrodechlorination of Pollutants by H*ads

Adsorbed atomic H (H*_ads) facilitates indirect pathways playing a major role in the electrochemical removal of various priority pollutants. It is crucial to identify the atomic sites responsible for the provision of H*_ads. Herein, through a systematic study of the distribution of H*_ads on Pd nanocatalysts with different sizes and, more importantly, deliberately controlled relative abundance of surface defects, we uncovered the central role of defects in the provision of H*_ads. Specifically, the H*_ads generated on Pd in an electrochemical process increased markedly upon introducing defect sites by changing the morphology to ultrathin polycrystalline Pd nanowires (NWs), while dramatically reducing upon decreasing the number of surface defects through an annealing treatment. Benefiting from a proportion of H*_ads up to 40% of the total H* species, the Pd NWs showed an electrochemical active surface area normalized rate constant of 13.8 ± 0.8 h^-1 m^-2, which is 8-9 times higher than its Pd/C counterparts. The pivotal role of defect sites for the generation of H*_ads was further verified by blocking such sites with Rh and Pt atoms, while theoretical calculation also confirms that the adsorption energy of H*_ads on these sites is much higher than that on the Pd{111} facet.

An overview on Pd-based electrocatalysts for the hydrogen evolution reaction

The electrochemical hydrogen evolution reaction (HER) is a well-studied reaction which involves the reduction of protons for hydrogen production. Pd-based compounds are expected to have activity on par with or better than the expensive state-of-the-art Pt and can be considered as the future materials for the HER.

Probing the Surface of Platinum during the Hydrogen Evolution Reaction in Alkaline Electrolyte

Understanding the surface chemistry of electrocatalysts in operando can bring insight into the reaction mechanism, and ultimately the design of more efficient materials for sustainable energy storage and conversion. Recent progress in synchrotron based X-ray spectroscopies for in operando characterization allows us to probe the solid/liquid interface directly while applying an external potential, applied here to the model system of Pt in alkaline electrolyte for the hydrogen evolution reaction (HER). We employ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify the oxidation and reduction of Pt-oxides and hydroxides on the surface as a function of applied potential, and further assess the potential for hydrogen adsorption and absorption (hydride formation) during and after the HER. This new window into the surface chemistry of Pt in alkaline electrolyte brings insight into the nature of the rate limiting step, the extent of H ad/absorption, and its persistence at more anodic potentials.

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.