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1. Highly selective oxygen reduction to hydrogen peroxide on transition metal single atom coordination

   https://doi.org/10.1038/s41467-019-11992-2  

   Jiang, Kun ; Back, Seoin ; Akey, Austin J. ; Xia, Chuan ; Hu, Yongfeng ; Liang, Wentao ; Schaak, Diane ; Stavitski, Eli ; Nørskov, Jens K. ; Siahrostami, Samira ; et al ( September 2019 , Nature Communications)

   Shifting electrochemical oxygen reduction towards 2e^–pathway to hydrogen peroxide (H₂O₂), instead of the traditional 4e^–to water, becomes increasingly important as a green method for H₂O₂generation. Here, through a flexible control of oxygen reduction pathways on different transition metal single atom coordination in carbon nanotube, we discovered Fe-C-O as an efficient H₂O₂catalyst, with an unprecedented onset of 0.822 V versus reversible hydrogen electrode in 0.1 M KOH to deliver 0.1 mA cm⁻²H₂O₂current, and a high H₂O₂selectivity of above 95% in both alkaline and neutral pH. A wide range tuning of 2e^–/4e^–ORR pathways was achieved via different metal centers or neighboring metalloid coordination. Density functional theory calculations indicate that the Fe-C-O motifs, in a sharp contrast to the well-known Fe-C-N for 4e^–, are responsible for the H₂O₂pathway. This iron single atom catalyst demonstrated an effective water disinfection as a representative application.

    

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2. Computational Discovery of Active and Selective Metal‐Nitrogen‐Graphene Catalysts for Electrooxidation of Water to H 2 O 2

   https://doi.org/10.1002/cctc.202300055  

   Chaudhary, Payal ; Evazzade, Iman ; Belosludov, Rodion ; Alexandrov, Vitaly ( April 2023 , ChemCatChem)

   A direct electrosynthesis of H₂O₂from either O₂or H₂O is an attractive strategy to replace the energy‐intensive industrial anthraquinone process. Two‐electron water oxidation reaction (2e‐WOR) offers several advantages over the oxygen reduction reaction such as better mass transfer due to the absence of gas‐phase reactants. However, 2e‐WOR is a more challenging and less studied process with only a handful of metal oxides exhibiting reasonable activity/selectivity properties. Herein, we employ density‐functional‐theory calculations to screen a variety of metal‐nitrogen‐graphene structures for 2e‐WOR. As a consequence of scaling between the adsorption energies of reaction intermediates, we determine a linear relation between selectivities for the first and second reaction steps of 2e‐WOR, viz. that if selectivity toward adsorbed OH is improved, then selectivity toward H₂O₂at the subsequent step is decreased. We also find that selectivity and activity are linearly scaled in such a way that a higher activity (i. e., a lower overpotential) leads to a lower selectivity for the H₂O₂formation step. Based on the obtained results several chemistries, e. g., containing NiN_x−C moieties, are predicted to rival the best‐performing metal oxides such as ZnO and CaSnO₃in terms of combination of their activity/selectivity characteristics for 2e‐WOR.

    

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3. Microstructural origin of selective water oxidation to hydrogen peroxide at low overpotentials: a study on Mn-alloyed TiO 2

   https://doi.org/10.1039/D1TA05451A  

   Li, Jiahui ; Solanki, Devan ; Zhu, Qianhong ; Shen, Xin ; Callander, Grace ; Kim, Jaehong ; Li, Yaogang ; Wang, Hongzhi ; Hu, Shu ( August 2021 , Journal of Materials Chemistry A)

   One key objective in electrocatalysis is to design selective catalysts, particularly in cases where the desired products require thermodynamically unfavorable pathways. Electrochemical synthesis of hydrogen peroxide (H 2 O 2 ) via the two-electron water oxidation reaction (2e − WOR) requires a +0.54 V higher potential than four-electron O 2 evolution. So far, best-performing electrocatalysts require considerable overpotentials before reaching peak faradaic efficiency. We present Mn-alloyed TiO 2 coatings prepared by atomic layer deposition (ALD) and annealing as a stable and selective electrocatalyst for 2e − WOR. Faradaic efficiency of >90% at < 150 mV overpotentials was achieved for H 2 O 2 production, accumulating 2.97 mM H 2 O 2 after 8 hours. Nanoscale mixing of Mn 2 O 3 and TiO 2 resulted in a partially filled, highly conductive Mn 3+ intermediate band (IB) within the TiO 2 mid-gap to transport charge across the (Ti,Mn)O x coating. This IB energetically matched that of H 2 O 2 -producing surface intermediates, turning a wide bandgap oxide into a selective electrocatalyst capable of operating in the dark. However, the high selectivity is limited to the low overpotential regime, which limits the system to low current densities and requires further research into increasing turn-over frequency per active site. 

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4. Discovery of LaAlO3 as an efficient catalyst for two-electron water electrolysis towards hydrogen peroxide

   https://doi.org/10.1038/s41467-022-34884-4  

   Baek, Jihyun ; Jin, Qiu ; Johnson, Nathan Scott ; Jiang, Yue ; Ning, Rui ; Mehta, Apurva ; Siahrostami, Samira ; Zheng, Xiaolin ( November 2022 , Nature Communications)

   Electrochemical two-electron water oxidation reaction (2e-WOR) has drawn significant attention as a promising process to achieve the continuous on-site production of hydrogen peroxide (H₂O₂). However, compared to the cathodic H₂O₂generation, the anodic 2e-WOR is more challenging to establish catalysts due to the severe oxidizing environment. In this study, we combine density functional theory (DFT) calculations with experiments to discover a stable and efficient perovskite catalyst for the anodic 2e-WOR. Our theoretical screening efforts identify LaAlO₃perovskite as a stable, active, and selective candidate for catalyzing 2e-WOR. Our experimental results verify that LaAlO₃achieves an overpotential of 510 mV at 10 mA cm⁻²in 4 M K₂CO₃/KHCO₃, lower than those of many reported metal oxide catalysts. In addition, LaAlO₃maintains a stable H₂O₂Faradaic efficiency with only a 3% decrease after 3 h at 2.7 V vs. RHE. This computation-experiment synergistic approach introduces another effective direction to discover promising catalysts for the harsh anodic 2e-WOR towards H₂O₂.

    

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5. Electrocatalytic production of hydrogen peroxide enabled by post-synthetic modification of a self-assembled porphyrin cube

   https://doi.org/10.1039/d2qi02050e  

   Crawley, Matthew R. ; Zhang, Daoyang ; Cook, Timothy R. ( December 2022 , Inorganic Chemistry Frontiers)

   Self-assembled metallacyles and cages formed via coordination chemistry have been used as catalysts to enforce 4H + /4e − reduction of oxygen to water with an emphasis on attenuating the formation of hydrogen peroxide. That said, the kinetically favored 2H + /2e − reduction to H 2 O 2 is critically important to industry. In this work we report the synthesis, characterization, and electrochemical benchmarking of a hexa-porphyrin cube which catalyses the electrochemical reduction of molecular oxygen to hydrogen peroxide. An established sub-component self-assembly approach was used to synthesize the cubic free-base porphryin topologies from 2-pyridinecarboxaldehyde, tetra-4-aminophenylporphryin (TAPP), and Fe(OTf) 2 (OTf − = trifluoromethansulfonate). Then, a tandem metalation/transmetallation was used to introduce Co( ii ) into the porphyrin faces of the cube, and exchange with the Fe( ii ) cations at the vertices, furnishing a tetrakaideca cobalt cage. Electron paramagnetic resonance studies on a Cu( ii )/Fe( ii ) analogue probed radical interactions which inform on electronic structure. The efficacy and selectivity of the CoCo-cube as a catalyst for hydrogen peroxide generation was investigated using hydrodynamic voltammetry, revealing a higher selectivity than that of a mononuclear Co( ii ) porphyrin (83% versus ∼50%) with orders of magnitude enhancement in standard rate constant ( k s = 2.2 × 10 2 M −1 s −1 versus k s = 3 × 10 0 M −1 s −1 ). This work expands the use of coordination-driven self-assembly beyond ORR to water by exploiting post-synthetic modification and structural control that is associated with this synthetic method. 

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