Hydrogen peroxide is a valuable chemical oxidant with a wide range of applications in a variety of industrial processes, especially in water sanitization. Electrochemical synthesis of hydrogen peroxide (H 2 O 2 ) through two-electron oxygen reduction reaction (2e-ORR) or two-electron water oxidation reaction (2e-WOR) has emerged as an appealing process for onsite production of this chemically valuable oxidant. On-site produced H 2 O 2 can be applied for wastewater treatment in remote locations or any applications where H 2 O 2 is needed as an oxidizing agent. This contribution studies the theoretical efforts in understanding the challenges in catalysis for electrochemical synthesis of H 2 O 2 as well as providing design principles for more efficient catalyst materials.

Electrochemical synthesis of H 2 O 2 through a selective two-electron (2e −) oxygen reduction reaction (ORR) is an attractive alternative to the industrial anthraquinone oxidation method, as it allows decentralized H 2 O 2 production. Herein, we report that the synergistic interaction between partially oxidized palladium (Pd δ+) and oxygen-functionalized carbon can promote 2e − ORR in acidic electrolytes. An electrocatalyst synthesized by solution deposition of amorphous Pd δ+ clusters (Pd 3 δ+ and Pd 4 δ+) onto mildly oxidized carbon nanotubes (Pd δ+-OCNT) shows nearly 100% selectivity toward H 2 O 2 and a positive shift of ORR onset potential by~320 mV compared with the OCNT substrate. A high mass activity (1.946 A mg −1 at 0.45 V) of Pd δ+-OCNT is achieved. Extended X-ray absorption fine structure characterization and density functional theory calculations suggest that the interaction between Pd clusters and the nearby oxygen-containing functional groups is key for the high selectivity and activity for 2e − ORR.

Methane, the main component of natural gas, is widely utilized for energy consumption applications. The abundance of natural gas has driven many researchers to focus on the conversion of methane...

Selective electrochemical oxygen reduction (ORR) toward a two-electron (2e À ) pathway is an eco-friendly alternative method for H 2 O 2 synthesis to replace the energy-intensive anthraquinone oxidation process. Carbon-based electrocatalysts (CBEs) show great potential for practical H 2 O 2 synthesis. However, their complex structures make it challenging to determine the nature of active sites and to precisely control them. Herein, we show that precise modulation of the chemistry and structures of holey graphene with edge sites enriched by oxygen-containing functional groups can facilitate 2e À ORR. These combined functionalities could improve ORR performance under various pH conditions, for example, resulting in an average of 95% H 2 O 2 selectivity, ~97% Faraday efficiency, high productivity of 2360 mol kg cat À1 h À1 in alkaline media. Density functional theory calculations on the oxygen functional groups at the edge sites revealed the most active site for 2e À ORR is a synergy between ether (C O C) and carbonyl (C O) functional groups with nearly zero overpotential.

As one of the most important family of porous materials, metal-organic frameworks (MOFs) have well-defined atomic structures. This provides ideal models for investigating and understanding the relationships between structures and catalytic activities at a molecular level. However, the active sites on the edges of twodimensional (2D) MOFs have rarely been studied as they are less exposed to the surfaces. Here, for the firsttime, we synthesize and observe that the 2D layers can align perpendicular to the surface of a 2D zeolitic imidazolate framework L (ZIF-L) with a leaf-like morphology. Owing to this unique orientation, the active sites in on the edges of the 2D crystal structure can mostly be exposed to the surfaces. More interestingly, when another layer of ZIF-L-Co was heteroepitaxially grown onto ZIF-L-Zn

Forming pits on molybdenum disulfide (MoS2) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS2, and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS2 monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS2 monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS2, rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS2 interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS2.

Transition metal oxides have emerged as promising costeffective alternatives to Pt catalysts for oxygen reduction reaction (ORR) in fuel cell applications. However, their low stability under harsh electrochemical conditions hinders their widespread applications. Tantalum pentoxide (Ta 2 O 5 ) has proven to be a stable material under ORR conditions, but its activity is limited. In this work, we incorporate single atom catalysts (SACs) in Ta 2 O 5 to resolve the limited ORR activity of this material by altering the electronic structures of surface Ta atoms. We use density functional theory (DFT) calculations to identify the most promising SACs with enhanced ORR activity. Pt, Rh, and Ir, are found to be the most promising SACs with improved ORR activity and high stability. This work suggests that SACs are effective in enhancing Ta 2 O 5 catalytic activities for ORR.

Generating pits and thereby active edge sites of molybdenum disulfide (MoS2) monolayers is desirable for many electrochemical catalytic reactions including hydrogen evolution reaction (HER). Thermal oxidation is one of the potentially scalable and facile methods to effectively create the pits on MoS2 monolayers. Therefore understanding the thermal oxidation mechanism is very important to precisely control the generation of active edge sites of MoS2-based electrocatalysts. To date, pits are assumed to be favorably formed on MoS2 at undercoordinated sites such as sulfur (S) vacancies at high temperatures. However, the thermal oxidation studies have not considered the existence of adventitious carbon (C) that exists almost everywhere and interact with oxygen at elevated temperatures. Herein, we investigated the influence of adventitious C on the thermal oxidation of MoS2 monolayers. We employed in situ environmental transmission electron microscopy (ETEM) to demonstrate the pit formation mechanism with the presence of adventitious C at the oxidation temperature of 300 °C. The in situ ETEM results show that the adventitious C is agglomerated at high temperatures. Then, the interfaces between monolayer MoS2 and C nanoparticle provide preferred sites for thermal oxidation of MoS2 and thus pit formation while the individual S vacancies exist intact. Density functional theory (DFT) calculations show the interfaces between MoS2 and C nanoparticle make the sequential adsorption of oxygen atoms thermodynamically favorable, unlike only S vacancy sites. We also tested the electrochemical performance of MoS2 with pits for HER and it reduced the overpotential by ~ 130 mV. These results demonstrate the combination between ETEM experiment and DFT calculation can be effectively used to study the chemical reaction mechanism between 2-dimensional materials such as MoS2 and gaseous species. Furthermore, we provide the potential to control the active edge site of MoS2-based electrocatalyst by understanding the fundamental pit formation mechanism by thermal oxidation.