Atom-Coordinated Structure Triggers Selective H2O2 Production

The family of atomic catalysts in defective carbons is now of increasing interest for diverse reactions. In this issue of Chem, Liu and co-workers unveil the nature of M–N–C (M = Mn, Fe, Co, Ni, and Cu) as active sites that can finely tune the oxygen reduction reaction (ORR) pathways spanning 1e–4e transfer processes. The family of atomic catalysts in defective carbons is now of increasing interest for diverse reactions. In this issue of Chem, Liu and co-workers unveil the nature of M–N–C (M = Mn, Fe, Co, Ni, and Cu) as active sites that can finely tune the oxygen reduction reaction (ORR) pathways spanning 1e–4e transfer processes. With an annual global demand of about four million tons, hydrogen peroxide (H2O2) is one of the most important chemicals because it plays a crucial role in the chemical and medical industries.1Perry S.C. Pangotra D. Vieira L. Csepei L.-I. Sieber V. Wang L. de León C.P. Walsh F.C. Electrochemical synthesis of hydrogen peroxide from water and oxygen.Nat. Rev. Chem. 2019; 3: 442-458Crossref Scopus (307) Google Scholar Compared with the current industrial production of H2O2 through the energy-intensive and costly anthraquinone cycling process with noble-metal (e.g., Pd-Au alloy)-based catalysts, utilizing renewable electricity for on-site H2O2 generation via electrocatalytic processes has recently emerged as a promising alternative.2Siahrostami S. Verdaguer-Casadevall A. Karamad M. Deiana D. Malacrida P. Wickman B. Escudero-Escribano M. Paoli E.A. Frydendal R. Hansen T.W. et al.Enabling direct H2O2 production through rational electrocatalyst design.Nat. Mater. 2013; 12: 1137-1143Crossref PubMed Scopus (723) Google Scholar,3Freakley S.J. He Q. Harrhy J.H. Lu L. Crole D.A. Morgan D.J. Ntainjua E.N. Edwards J.K. Carley A.F. Borisevich A.Y. et al.Palladium-tin catalysts for the direct synthesis of H2O2 with high selectivity.Science. 2016; 351: 965-968Crossref PubMed Scopus (363) Google Scholar Notably, there are two ways for electrochemical H2O2 generation: the first starts from O2 via the two-electron oxygen reduction reaction (2e− ORR), and the second starts from H2O via the 2e− water oxidation reaction. However, both of these pathways inevitably suffer from the competing reaction of the 4e pathway toward H2O or O2. For example, in the ORR, increasing the selectivity of H2O2 production requires, in principle, that O–O bond breaking be minimized. To this end, it is of paramount importance to design and synthesize rational active sites in electrocatalysts to optimize the binding energy with oxygen-containing intermediates. In recent years, so-called single-atom catalysts (SACs), which comprise single-metal atoms stabilized on a support, have provided a representative platform for studying the cooperative interactions between metal centers and the surrounding atomic configurations.4Wang A. Li J. Zhang T. Heterogeneous single-atom catalysis.Nat. Rev. Chem. 2018; 2: 65-81Crossref Scopus (1917) Google Scholar In particular, nitrogen-doped carbon materials featuring atomically dispersed transition-metal cations (M–N–C) are an emerging family of materials and are a mainstream in research for replacing platinum-group-metal (PGM)-based catalysts toward the 4e− ORR to H2O for fuel-cell applications. However, the general principle that governs M–N–C catalysts’ selectivity specifically toward the 2e− ORR to H2O2, which is a green “dream” process for future chemical industry, still remains poorly understood. Now, writing in this issue of Chem, Liu, Huang, and co-workers report an elegant and important step toward the fundamental understanding of the nature of M–N–C (M = Mn, Fe, Co, Ni, and Cu) as active sites that can finely tune ORR pathways spanning 1e–4e transfer processes (Figure 1A).5Gao J.J. Yang H.B. Huang X. Hung S.-H. Cai W.Z. Jia C.M. Miao S. Chen H.M. Yang X.F. Huang Y.Q. et al.Enabling direct H2O2 production in acidic media through rational design of transition metal single atom catalyst.Chem. 2020; 6: 658-674Abstract Full Text Full Text PDF Scopus (249) Google Scholar Theoretically, density functional theory calculations revealed that the binding energies of *OOH, *O, and *OH generally scale with the number of valence electrons in the M atom from manganese to copper. The d-band center of the M atom shifts down in energy relative to the Fermi level from Mn to Cu, leading to a larger number of valence electrons in M and weaker binding of these intermediates to the M atom (Figure 1B). In detail, the anti-bonding states derived from the coupling between d orbitals of the M atom and 2p orbitals of the bonded O atom of intermediates are shifted down in energy and thus are more filled, which weakens the M–O bonding from Mn to Cu. Experimentally, ORR electrochemical measurements demonstrated that the cobalt SAC (Co-SAC) anchored in nitrogen doped graphene exhibited superb H2O2 synthesis (1 mA/cmdisk2 at 0.6 V versus reversible hydrogen electrode [RHE] in 0.1 M HClO4, >90% faradic efficiency, 2.5 S−1 turnover frequency at 0.5 V versus RHE) to outperform other M-SACs, as well as the benchmark noble-metal-based (e.g., Pd-Hg alloy) catalysts (Figure 1C). To track the dynamic change of the nitrogen-coordinated cobalt active center under reaction conditions, the authors conducted operando X-ray absorption spectroscopy to probe the change of the Co K-edge under H2O2 synthesis acidic conditions. Figure 1D shows that the Co–N bonding distance experienced elastic compression with the shortened ∼0.03 Å from 0.6 to 0.3 V versus RHE and then bounded back to its initial value (1.35 Å) after the potential returned to that of open-circuit conditions, suggesting that molecular O2 adsorption on the cobalt center in the Co–N4 motif is in the form of the end-on type rather than the bridging coordination (the latter could increase the possibility of O–O dissociation and therefore turn into the 4e transfer pathway). This is a vivid demonstration that the electronic structure of atomic Co can be finely tuned by bonding to diverse types of transition metals in the form of M–N4 motifs and lead to different target reaction pathways in the ORR. In fact, more M-atom-coordinated structures tailored by the defect engineering strategy can be employed as proper active sites, such as Co–N4-xCx, in di-vacancy defects6Yang Q. Jia Y. Wei F. Zhuang L. Yang D. Liu J. Wang X. Lin S. Yuan P. Yao X. Understanding the activity of Co-N4-xCx in atomic metal catalysts for oxygen reduction catalysis.Angew. Chem. Int. Ed. 2020; https://doi.org/10.1002/anie.202000324Crossref Scopus (109) Google Scholar or even in single-vacancy or edge defects.7Jia Y. Jiang K. Wang H.T. Yao X.D. The role of defect sites in nanomaterials for electrocatalytic energy conversion.Chem. 2019; 5: 1371-1397Abstract Full Text Full Text PDF Scopus (200) Google Scholar,8Jia Y. Zhang L.Z. Zhuang L.Z. Liu H.L. Yan X.C. Wang X. Liu J.D. Wang J.C. Zheng Y.R. Xiao Z.H. et al.Identification of active sites for acidic oxygen reduction on carbon catalysts with and without nitrogen-doping.Nat. Catal. 2019; 2: 688-695Crossref Scopus (288) Google Scholar Moreover, the highly dense distribution of defects could provide the possibility of atomic metal-metal synergistic interaction (e.g., Co and Pt “trapped” in adjacent di-vacancy carbon defects), thus promoting the catalytic activity and completely changing the reaction pathways.9Zhang L. Fischer J.M.T.A. Jia Y. Yan X. Xu W. Wang X. Chen J. Yang D. Liu H. Zhuang L. et al.Coordination of atomic Co-Pt coupling species at carbon defects as active sites for oxygen reduction reaction.J. Am. Chem. Soc. 2018; 140: 10757-10763Crossref PubMed Scopus (328) Google Scholar Therefore, future research on metal-metal and metal-support interfaces at the atomic scale is essential and extremely significance not only for the fundamentals of catalysis but also for the design and target synthesis of new catalysts. Enabling Direct H2O2 Production in Acidic Media through Rational Design of Transition Metal Single Atom CatalystGao et al.ChemJanuary 16, 2020In BriefBy combining theoretical and experimental methods, Gao et al. systematically studied the relationship between the structure of transition metal (Mn, Fe, Co, Ni, and Cu) single-atom catalyst anchored in nitrogen-doped graphene and the catalytic performance of hydrogen peroxide (H2O2) synthesis via electrochemical two-electron oxygen reduction reaction (ORR) (2e− ORR). The thus designed Co single-atom catalyst can function as a highly active and selective catalyst for H2O2 synthesis and even slightly outperforms state-of-the-art noble-metal-based electrocatalysts in acidic media. Full-Text PDF Open Archive