Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Abstract

•A key superoxo intermediate for water oxidation was detected with SEIRAS-ATR•A potential-dependent O–O bond-formation mechanism is revealed•The intramolecular oxygen coupling mechanism dominates at low potentials•The water nucleophilic attack mechanism prevails at high potentials As the first step in natural photosynthesis, the oxidation of water is of paramount importance. It liberates electrons and protons that are required for downstream reactions such as CO2 and N2 reduction. Substantial research efforts have been devoted to understanding and, ultimately, performing this reaction at high efficiency with low-cost, long-lasting catalysts. Exciting progress notwithstanding, much remains poorly understood about the reaction, especially when it is performed on heterogeneous catalysts. A key elementary step of the water-oxidation reaction is the formation of the O–O bond. Herein, we report a potential-induced switch of the O–O bond-forming mechanism on Co-oxide-based catalysts. This mechanistic insight is expected to help advance the design of efficient water-oxidation catalysts. O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts. O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts. IntroductionIntense research on the water-oxidation catalyst (WOC) center in photosystem II (PSII) over the last decades has revealed deep insights on the mechanisms by which nature liberates electrons and protons from H2O, two critical ingredients for downstream reactions such as CO2 reduction and N2 fixation.1Shen J.R. The structure of photosystem II and the mechanism of water oxidation in photosynthesis.Annu. Rev. Plant Biol. 2015; 66: 23-48Crossref PubMed Scopus (404) Google Scholar,2Vinyard D.J. Brudvig G.W. Progress toward a molecular mechanism of water oxidation in photosystem II.Annu. Rev. Phys. Chem. 2017; 68: 101-116Crossref PubMed Scopus (121) Google Scholar This knowledge has propelled research on using molecular catalysts to oxidize water, and impressive progress has been made in terms of catalyst performance as measured by turn-over frequencies (TOFs) and turn-over numbers.3Blakemore J.D. Crabtree R.H. Brudvig G.W. Molecular catalysts for water oxidation.Chem. Rev. 2015; 115: 12974-13005Crossref PubMed Scopus (790) Google Scholar,4Shaffer D.W. Xie Y. Concepcion J.J. O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling.Chem. Soc. Rev. 2017; 46: 6170-6193Crossref PubMed Google Scholar From a technological development perspective, there is a strong incentive to perform the reaction on heterogeneous catalysts, especially on those of low-cost and outstanding stability. Indeed, recent years have witnessed a surge of such research activities.5Hunter B.M. Gray H.B. Müller A.M. Earth-abundant heterogeneous water oxidation catalysts.Chem. Rev. 2016; 116: 14120-14136Crossref PubMed Scopus (1007) Google Scholar, 6Suen N.T. Hung S.F. Quan Q. Zhang N. Xu Y.J. Chen H.M. Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives.Chem. Soc. Rev. 2017; 46: 337-365Crossref PubMed Google Scholar, 7Zhang M. Frei H. Water oxidation mechanisms of metal oxide catalysts by vibrational spectroscopy of transient intermediates.Annu. Rev. Phys. Chem. 2017; 68: 209-231Crossref PubMed Scopus (26) Google Scholar, 8Yu Z.Y. Duan Y. Gao M.R. Lang C.C. Zheng Y.R. Yu S.H. A one-dimensional porous carbon-supported Ni/Mo2C dual catalyst for efficient water splitting.Chem. Sci. 2017; 8: 968-973Crossref PubMed Google Scholar, 9Yu Z.-Y. Lang C.-C. Gao M.-R. Chen Y. Fu Q.-Q. Duan Y. Yu S.-H. Ni–Mo–O nanorod-derived composite catalysts for efficient alkaline water-to-hydrogen conversion via urea electrolysis.Energy Environ. Sci. 2018; 11: 1890-1897Crossref Google Scholar, 10Grimaud A. Diaz-Morales O. Han B. Hong W.T. Lee Y.L. Giordano L. Stoerzinger K.A. Koper M.T.M. Shao-Horn Y. Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution.Nat. Chem. 2017; 9: 457-465Crossref PubMed Scopus (928) Google Scholar, 11Negahdar L. Zeng F. Palkovits S. Broicher C. Palkovits R. Mechanistic aspects of the electrocatalytic oxygen evolution reaction over Ni−Co oxides.ChemElectroChem. 2019; 6: 5588-5595Crossref Scopus (39) Google Scholar Despite the apparent variety of these catalysts, they share important commonalities in the chemical mechanisms. For instance, it is generally believed that the reaction proceeds through a series of proton-coupled electron transfer steps that lead to the formation of M=O (where M represents an active metal center) intermediates.12Zandi O. Hamann T.W. Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy.Nat. Chem. 2016; 8: 778-783Crossref PubMed Scopus (256) Google Scholar,13Zhang M. de Respinis M. Frei H. Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst.Nat. Chem. 2014; 6: 362-367Crossref PubMed Scopus (573) Google Scholar It is also agreed upon that the subsequent O–O bond formation is of critical importance to the overall reaction.14Rosenthal J. Nocera D.G. Role of proton-coupled electron transfer in O–O bond activation.Acc. Chem. Res. 2007; 40: 543-553Crossref PubMed Scopus (311) Google Scholar However, the details of the O–O formation and the subsequent steps have been the subject of diverging views. At least two possible pathways have been proposed and supported.15Ullman A.M. Brodsky C.N. Li N. Zheng S.L. Nocera D.G. Probing edge site reactivity of oxidic cobalt water oxidation catalysts.J. Am. Chem. Soc. 2016; 138: 4229-4236Crossref PubMed Scopus (140) Google Scholar, 16Romain S. Vigara L. Llobet A. Oxygen−oxygen bond formation pathways promoted by ruthenium complexes.Acc. Chem. Res. 2009; 42: 1944-1953Crossref PubMed Scopus (244) Google Scholar, 17García-Melchor M. Vilella L. López N. Vojvodic A. Computationally probing the performance of hybrid, heterogeneous, and homogeneous iridium-based catalysts for water oxidation.ChemCatChem. 2016; 8: 1792-1798Crossref Scopus (23) Google Scholar, 18Chen X. Aschaffenburg D.J. Cuk T. Selecting between two transition states by which water oxidation intermediates decay on an oxide surface.Nat. Catal. 2019; 2: 820-827Crossref Scopus (27) Google Scholar One involves direct nucleophilic attack of water, followed by O2 release and regeneration of the catalyst. In the literature, this mechanism is referred to as water nucleophilic attack (WNA) (Figure 1, right pathway).4Shaffer D.W. Xie Y. Concepcion J.J. O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling.Chem. Soc. Rev. 2017; 46: 6170-6193Crossref PubMed Google Scholar,15Ullman A.M. Brodsky C.N. Li N. Zheng S.L. Nocera D.G. Probing edge site reactivity of oxidic cobalt water oxidation catalysts.J. Am. Chem. Soc. 2016; 138: 4229-4236Crossref PubMed Scopus (140) Google Scholar The other involves the coupling of two metal-oxo intermediates followed by O2 release, which is referred to as intramolecular oxygen coupling (IMOC) (Figure 1, left pathway).15Ullman A.M. Brodsky C.N. Li N. Zheng S.L. Nocera D.G. Probing edge site reactivity of oxidic cobalt water oxidation catalysts.J. Am. Chem. Soc. 2016; 138: 4229-4236Crossref PubMed Scopus (140) Google ScholarFor Ir- and Ru-based molecular catalysts, density-functional theory (DFT) calculations predicted that the IMOC pathway dominates at low overpotentials, whereas the WNA pathway becomes accessible at higher overpotentials.17García-Melchor M. Vilella L. López N. Vojvodic A. Computationally probing the performance of hybrid, heterogeneous, and homogeneous iridium-based catalysts for water oxidation.ChemCatChem. 2016; 8: 1792-1798Crossref Scopus (23) Google Scholar,19Craig M.J. Coulter G. Dolan E. Soriano-López J. Mates-Torres E. Schmitt W. García-Melchor M. Universal scaling relations for the rational design of molecular water oxidation catalysts with near-zero overpotential.Nat. Commun. 2019; 10: 4993Crossref PubMed Scopus (83) Google Scholar The two pathways were also predicted to be competitive on a heterogenized dinuclear Ir oxide cluster.17García-Melchor M. Vilella L. López N. Vojvodic A. Computationally probing the performance of hybrid, heterogeneous, and homogeneous iridium-based catalysts for water oxidation.ChemCatChem. 2016; 8: 1792-1798Crossref Scopus (23) Google Scholar With optical pump-probe spectroscopy, Cuk et al.18Chen X. Aschaffenburg D.J. Cuk T. Selecting between two transition states by which water oxidation intermediates decay on an oxide surface.Nat. Catal. 2019; 2: 820-827Crossref Scopus (27) Google Scholar monitored the microsecond decay of oxyl (Ti–O⋅) and bridge (Ti–O⋅–Ti) intermediates on SrTiO3 photoelectrodes. They found that the two species decay with distinct reaction rates on a microsecond timescale. It was suggested that Ti–O⋅’s convert to Ti–O–O–Ti by dimerization (IMOC pathway) and Ti–O⋅–Ti converts to Ti–OOH by nucleophilic attack of water (WNA pathway). Furthermore, it was found that the relative predominance of the two pathways was controlled by the ionic strength of the electrolyte, with the WNA pathway dominating at low ionic strength. However, how the relative predominance of these mechanisms depends on the applied electrode potential has not been investigated in experiments. Herein, we address this central question.Inspiration on how to further this understanding could be drawn from progress made in molecular WOC-based studies. To discern different pathways for the water-oxidation reaction by molecular catalysts, researchers have resorted to a strategy of correlating the reaction rate with the catalyst concentrations.4Shaffer D.W. Xie Y. Concepcion J.J. O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling.Chem. Soc. Rev. 2017; 46: 6170-6193Crossref PubMed Google Scholar With the help of additional experiments such as isotope labeling, significant knowledge has been gained.20Matheu R. Ertem M.Z. Gimbert-Suriñach C. Sala X. Llobet A. Seven coordinated molecular ruthenium–water oxidation catalysts: a coordination chemistry journey.Chem. Rev. 2019; 119: 3453-3471Crossref PubMed Scopus (100) Google Scholar, 21Duan L. Bozoglian F. Mandal S. Stewart B. Privalov T. Llobet A. Sun L. A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II.Nat. Chem. 2012; 4: 418-423Crossref PubMed Scopus (991) Google Scholar, 22Romain S. Bozoglian F. Sala X. Llobet A. Oxygen−oxygen bond formation by the Ru-Hbpp water oxidation catalyst occurs solely via an intramolecular reaction pathway.J. Am. Chem. Soc. 2009; 131: 2768-2769Crossref PubMed Scopus (138) Google Scholar However, similar approaches are challenging to implement for heterogeneous catalysts, because the active sites, including their structures and densities, are often poorly defined on a heterogeneous catalyst. The challenge could be circumvented using clever experimental designs. For instance, Durrant et al.23Le Formal F. Pastor E. Tilley S.D. Mesa C.A. Pendlebury S.R. Grätzel M. Durrant J.R. Rate law analysis of water oxidation on a hematite surface.J. Am. Chem. Soc. 2015; 137: 6629-6637Crossref PubMed Scopus (210) Google Scholar have identified a change of reaction orders relative to the hole concentration from the first to the third order on Fe2O3 using photoinduced absorption spectroscopy. Frei et al.13Zhang M. de Respinis M. Frei H. Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst.Nat. Chem. 2014; 6: 362-367Crossref PubMed Scopus (573) Google Scholar have succeeded in observing both the metal-oxo and superoxo species, using an infrared spectroscopy (IR) technique. In both studies, different reaction mechanisms were proposed for different light intensities. Nevertheless, owing to the lack of detailed information on the active centers, particularly their density under different conditions, it remains difficult to directly corroborate these early observations for an unambiguous understanding of water oxidation on heterogeneous catalysts. Although it is possible to address this challenge by synthesizing heterogeneous catalysts with well-defined active centers, as has been demonstrated recently by others and us,24Zhao Y. Yan X. Yang K.R. Cao S. Dong Q. Thorne J.E. Materna K.L. Zhu S. Pan X. Flytzani-Stephanopoulos M. et al.End-on bound iridium dinuclear heterogeneous catalysts on WO3 for solar water oxidation.ACS Cent. Sci. 2018; 4: 1166-1172Crossref PubMed Scopus (48) Google Scholar,25Zhao Y. Yang K.R. Wang Z. Yan X. Cao S. Ye Y. Dong Q. Zhang X. Thorne J.E. Jin L. et al.Stable iridium dinuclear heterogeneous catalysts supported on metal-oxide substrate for solar water oxidation.Proc. Natl. Acad. Sci. USA. 2018; 115: 2902-2907Crossref PubMed Scopus (160) Google Scholar the catalyst library remains limited, and significant work is needed before the values of such catalysts can be materialized. An alternative approach is to study how the reaction kinetics changes as a function of water activity, which is the main strategy for this present work.To appreciate the significance of this strategy, it helps to examine the proposed WNA and IMOC pathways on a heterogeneous Co phosphate (Co–Pi) catalyst (Figure 1). Previous studies have suggested that the initial electron/proton transfer steps (vertical arrow in the center) are fast in comparison with the O–O formation. Therefore, these steps are quasi-equilibrated, whereas O–O formation limits the rate of the reaction. From the oxidized state of the catalyst shown on the bottom of the scheme, the water-oxidation process can proceed through two distinct pathways: the WNA pathway involves a water molecule within the electric double layer in the rate-determining O–O forming step (right arrow). By contrast, the IMOC pathway only involves surface species in the rate-determining step (RDS) (left arrow). On the basis of this simplified mechanistic picture, the water-oxidation reaction is expected to be (pseudo) first order in the water activity when proceeding through the WNA pathway, whereas it is (pseudo) zeroth order when proceeding through the IMOC pathway. This simplified view assumes that the change in the water activity does not significantly affect the positions of the quasi-equilibria before the presumed RDS of O–O bond formation, as discussed later. Therefore, it is possible to discern the reaction mechanisms even without detailed knowledge of the active centers by altering the water activity, which has not been investigated previously.The problem is now reduced to how to alter water activity in a water-oxidation reaction. Indeed, most previous studies on this subject have treated water as a substrate of invariant activity, such that it was excluded in most kinetic considerations.26Wuttig A. Yoon Y. Ryu J. Surendranath Y. Bicarbonate is not a general acid in Au-catalyzed CO2 electroreduction.J. Am. Chem. Soc. 2017; 139: 17109-17113Crossref PubMed Scopus (149) Google Scholar,27Surendranath Y. Kanan M.W. Nocera D.G. Mechanistic studies of the oxygen evolution reaction by a cobalt-phosphate catalyst at neutral pH.J. Am. Chem. Soc. 2010; 132: 16501-16509Crossref PubMed Scopus (941) Google Scholar Only recently did we see advances where the water activity could be suppressed significantly in aqueous solutions.28Suo L. Borodin O. Gao T. Olguin M. Ho J. Fan X. Luo C. Wang C. Xu K. “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries.Science. 2015; 350: 938-943Crossref PubMed Scopus (1801) Google Scholar, 29Dong Q. Yao X. Zhao Y. Qi M. Zhang X. Sun H. He Y. Wang D. Cathodically stable Li-O2 battery operations using water-in-salt electrolyte.Chem. 2018; 4: 1345-1358Abstract Full Text Full Text PDF Scopus (56) Google Scholar, 30Dong Q. Zhang X. He D. Lang C. Wang D. Role of H2O in CO2 electrochemical reduction as studied in a water-in-salt system.ACS Cent. Sci. 2019; 5: 1461-1467Crossref PubMed Scopus (27) Google Scholar The so-called “water-in-salt” electrolyte containing high concentrations of salts (e.g., up to 21 m [mole per kg of H2O]) represents one such system. The strong solvation effect of the high-concentration ions renders its H2O behaviors drastically different from those in bulk H2O. It becomes possible to perform water-oxidation reactions in an aqueous system where the water activity is no longer unity. Therefore, we are offered an opportunity to test the hypothesis proposed in the previous paragraph. That is, we expect a different sensitivity of the kinetics on the water activity for different mechanisms.To prove this concept, we have chosen Co-oxide-based catalysts as a study platform because they represent a class of most studied heterogeneous WOCs, with Co–Pi receiving arguably the most attention. A broad knowledge base has already been generated.15Ullman A.M. Brodsky C.N. Li N. Zheng S.L. Nocera D.G. Probing edge site reactivity of oxidic cobalt water oxidation catalysts.J. Am. Chem. Soc. 2016; 138: 4229-4236Crossref PubMed Scopus (140) Google Scholar,27Surendranath Y. Kanan M.W. Nocera D.G. Mechanistic studies of the oxygen evolution reaction by a cobalt-phosphate catalyst at neutral pH.J. Am. Chem. Soc. 2010; 132: 16501-16509Crossref PubMed Scopus (941) Google Scholar,31Bergmann A. Jones T.E. Martinez Moreno E. Teschner D. Chernev P. Gliech M. Reier T. Dau H. Strasser P. Unified structural motifs of the catalytically active state of Co(oxyhydr)oxides during the electrochemical oxygen evolution reaction.Nat. Catal. 2018; 1: 711-719Crossref Scopus (258) Google Scholar, 32Pasquini C. Zaharieva I. González-Flores D. Chernev P. Mohammadi M.R. Guidoni L. Smith R.D.L. Dau H. H/D isotope effects reveal factors controlling catalytic activity in Co-based oxides for water oxidation.J. Am. Chem. Soc. 2019; 141: 2938-2948Crossref PubMed Scopus (42) Google Scholar, 33Kanan M.W. Nocera D.G. In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+.Science. 2008; 321: 1072-1075Crossref PubMed Scopus (3431) Google Scholar, 34Kanan M.W. Yano J. Surendranath Y. Dincă M. Yachandra V.K. Nocera D.G. Structure and valency of a cobalt−phosphate water oxidation catalyst determined by in situ X-ray spectroscopy.J. Am. Chem. Soc. 2010; 132: 13692-13701Crossref PubMed Scopus (585) Google Scholar, 35Li X. Siegbahn P.E.M. Water oxidation mechanism for synthetic Co–oxides with small nuclearity.J. Am. Chem. Soc. 2013; 135: 13804-13813Crossref PubMed Scopus (91) Google Scholar, 36Wang L.-P. Van Voorhis T. Direct-coupling O2 bond forming a pathway in cobalt oxide water oxidation catalysts.J. Phys. Chem. Lett. 2011; 2: 2200-2204Crossref Scopus (156) Google Scholar For example, the coordination environment of Co has been identified by a suite of spectroscopic techniques.34Kanan M.W. Yano J. Surendranath Y. Dincă M. Yachandra V.K. Nocera D.G. Structure and valency of a cobalt−phosphate water oxidation catalyst determined by in situ X-ray spectroscopy.J. Am. Chem. Soc. 2010; 132: 13692-13701Crossref PubMed Scopus (585) Google Scholar That the O–O formation is the RDS has been supported by numerous studies.15Ullman A.M. Brodsky C.N. Li N. Zheng S.L. Nocera D.G. Probing edge site reactivity of oxidic cobalt water oxidation catalysts.J. Am. Chem. Soc. 2016; 138: 4229-4236Crossref PubMed Scopus (140) Google Scholar,27Surendranath Y. Kanan M.W. Nocera D.G. Mechanistic studies of the oxygen evolution reaction by a cobalt-phosphate catalyst at neutral pH.J. Am. Chem. Soc. 2010; 132: 16501-16509Crossref PubMed Scopus (941) Google Scholar,31Bergmann A. Jones T.E. Martinez Moreno E. Teschner D. Chernev P. Gliech M. Reier T. Dau H. Strasser P. Unified structural motifs of the catalytically active state of Co(oxyhydr)oxides during the electrochemical oxygen evolution reaction.Nat. Catal. 2018; 1: 711-719Crossref Scopus (258) Google Scholar,32Pasquini C. Zaharieva I. González-Flores D. Chernev P. Mohammadi M.R. Guidoni L. Smith R.D.L. Dau H. H/D isotope effects reveal factors controlling catalytic activity in Co-based oxides for water oxidation.J. Am. Chem. Soc. 2019; 141: 2938-2948Crossref PubMed Scopus (42) Google Scholar,35Li X. Siegbahn P.E.M. Water oxidation mechanism for synthetic Co–oxides with small nuclearity.J. Am. Chem. Soc. 2013; 135: 13804-13813Crossref PubMed Scopus (91) Google Scholar,36Wang L.-P. Van Voorhis T. Direct-coupling O2 bond forming a pathway in cobalt oxide water oxidation catalysts.J. Phys. Chem. Lett. 2011; 2: 2200-2204Crossref Scopus (156) Google Scholar Both WNA and IMOC mechanisms have been proposed and supported by either experimental or computational studies for this catalyst.15Ullman A.M. Brodsky C.N. Li N. Zheng S.L. Nocera D.G. Probing edge site reactivity of oxidic cobalt water oxidation catalysts.J. Am. Chem. Soc. 2016; 138: 4229-4236Crossref PubMed Scopus (140) Google Scholar,32Pasquini C. Zaharieva I. González-Flores D. Chernev P. Mohammadi M.R. Guidoni L. Smith R.D.L. Dau H. H/D isotope effects reveal factors controlling catalytic activity in Co-based oxides for water oxidation.J. Am. Chem. Soc. 2019; 141: 2938-2948Crossref PubMed Scopus (42) Google Scholar,35Li X. Siegbahn P.E.M. Water oxidation mechanism for synthetic Co–oxides with small nuclearity.J. Am. Chem. Soc. 2013; 135: 13804-13813Crossref PubMed Scopus (91) Google Scholar, 36Wang L.-P. Van Voorhis T. Direct-coupling O2 bond forming a pathway in cobalt oxide water oxidation catalysts.J. Phys. Chem. Lett. 2011; 2: 2200-2204Crossref Scopus (156) Google Scholar, 37McAlpin J.G. Stich T.A. Casey W.H. Britt R.D. Comparison of cobalt and manganese in the chemistry of water oxidation.Coord. Chem. Rev. 2012; 256: 2445-2452Crossref Scopus (79) Google Scholar, 38Mattioli G. Giannozzi P. Amore Bonapasta A. Guidoni L. Reaction pathways for oxygen evolution promoted by cobalt catalyst.J. Am. Chem. Soc. 2013; 135: 15353-15363Crossref PubMed Scopus (197) Google Scholar, 39Risch M. Ringleb F. Kohlhoff M. Bogdanoff P. Chernev P. Zaharieva I. Dau H. Water oxidation by amorphous cobalt-based oxides: in situ tracking of redox transitions and mode of catalysis.Energy Environ. Sci. 2015; 8: 661-674Crossref Google Scholar Herein, we report the new observation of a switch from the IMOC pathway at low applied potentials to the WNA mechanism at high applied potentials.Results and discussionDetection of water-oxidation intermediatesPrevious studies have shown that various implementations of infrared and surface-enhanced Raman spectroscopies are powerful probes of water-oxidation intermediates.12Zandi O. Hamann T.W. Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy.Nat. Chem. 2016; 8: 778-783Crossref PubMed Scopus (256) Google Scholar,13Zhang M. de Respinis M. Frei H. Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst.Nat. Chem. 2014; 6: 362-367Crossref PubMed Scopus (573) Google Scholar,18Chen X. Aschaffenburg D.J. Cuk T. Selecting between two transition states by which water oxidation intermediates decay on an oxide surface.Nat. Catal. 2019; 2: 820-827Crossref Scopus (27) Google Scholar,40Nakamura R. Nakato Y. Primary intermediates of oxygen photoevolution reaction on TiO2 (rutile) particles, revealed by in situ FTIR absorption and photoluminescence measurements.J. Am. Chem. Soc. 2004; 126: 1290-1298Crossref PubMed Scopus (419) Google Scholar, 41Pavlovic Z. Ranjan C. van Gastel M. Schlögl R. The active site for the water oxidising anodic iridium oxide probed through in situ Raman spectroscopy.Chem. Commun. (Camb). 2017; 53: 12414-12417Crossref PubMed Google Scholar, 42Sivasankar N. Weare W.W. Frei H. Direct observation of a hydroperoxide surface intermediate upon visible light-driven water oxidation at an Ir oxide nanocluster catalyst by rapid-scan FT-IR spectroscopy.J. Am. Chem. Soc. 2011; 133: 12976-12979Crossref PubMed Scopus (90) Google Scholar, 43Zhang Y. Zhang H. Liu A. Chen C. Song W. Zhao J. Rate-limiting O–O Bond formation pathways for water oxidation on hematite photoanode.J. Am. Chem. Soc. 2018; 140: 3264-3269Crossref PubMed Scopus (94) Google Scholar, 44Liu H. Frei H. Observation of O–O bond forming step of molecular Co4O4 cubane catalyst for water oxidation by rapid-scan FT-IR spectroscopy.ACS Catal. 2020; 10: 2138-2147Crossref Scopus (17) Google Scholar, 45Chen X. Choing S.N. Aschaffenburg D.J. Pemmaraju C.D. Prendergast D. Cuk T. The formation time of Ti–O⋅ and Ti–O⋅–Ti radicals at the n-SrTiO3/Aqueous interface during photocatalytic water oxidation.J. Am. Chem. Soc. 2017; 139: 1830-1841Crossref PubMed Scopus (58) Google Scholar, 46Herlihy D.M. Waegele M.M. Chen X. Pemmaraju C.D. Prendergast D. Cuk T. Detecting the oxyl radical of photocatalytic water oxidation at an n-SrTiO3/aqueous interface through its subsurface vibration.Nat. Chem. 2016; 8: 549-555Crossref PubMed Scopus (93) Google Scholar, 47Yeo B.S. Bell A.T. Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen.J. Am. Chem. Soc. 2011; 133: 5587-5593Crossref PubMed Scopus (1090) Google Scholar, 48Rao R.R. Kolb M.J. Giordano L. Pedersen A.F. Katayama Y. Hwang J. Mehta A. You H. Lunger J.R. Zhou H. et al.Operando identification of site-dependent water oxidation activity on ruthenium dioxide single-crystal surfaces.Nat. Catal. 2020; 3: 516-525Crossref Scopus (90) Google Scholar To examine the mechanistic proposal (Figure 1), we employed surface-enhanced infrared absorption spectroscopy (SEIRAS) in the attenuated total reflection (ATR) geometry. In SEIRAS-ATR, the surface plasmon resonance of rough metal films locally enhances the evanescent field, rendering the technique sensitive to sub-monolayers of species adsorbed on the electrode.49Osawa M. Surface-enhanced infrared absorption.in: Kawata S. Near-Field Optics and Surface Plasmon Polaritons. Springer, 2001: 163-187Crossref Google Scholar With this work, we establish SEIRAS-ATR in the Kretschmann configuration as a tool for probing water-oxidation intermediates on metal oxide catalysts. For this purpose, we first electrochemically deposited a thin layer of CoOx(OH)y31Bergmann A. Jones T.E. Martinez Moreno E. Teschner D. Chernev P. Gliech M. Reier T. Dau H. Strasser P. Unified structural motifs of the catalytically active state of Co(oxyhydr)oxides during the electrochemical oxygen evolution reaction.Nat. Catal. 2018; 1: 711-719Crossref Scopus (258) Google Scholar onto a chemically deposited Au thin film (CoOx(OH)y-Au)50Miyake H. Ye S. Osawa M. Electroless deposition of gold thin films on silicon for surface-enhanced infrared spectroelectrochemistry.Electrochem. Commun. 2002; 4: 973-977Crossref Scopus (295) Google Scholar on a micro-machined Si-ATR crystal,51Morhart T.A. Unni B. Lardner M.J. Burgess I.J. Electrochemical ATR-SEIRAS using low-cost, micromachined Si wafers.Anal. Chem. 2017; 89: 11818-11824Crossref PubMed Scopus (26) Google Scholar which affords high infrared transparency below 1,200 cm−1. A scheme of the setup is shown in Figure S1 in the supplemental information. For SEIRAS-ATR, CoOx(OH)y instead of Co–Pi was employed as the prototypical catalyst because the latter would greatly complicate the interpretation of the IR spectra in the region around approximately 1,000 cm−1 owing to the phosphate anion and its response to the applied potentials. As will be discussed in detail later in this work, the electrochemical behaviors of CoOx(OH)y are comparable with Co–Pi. It also features structurally similar active sites and the same cobalt oxidation states under water-oxidation conditions as Co–Pi.31Bergmann A. Jones T.E. Martinez Moreno E. Teschner D. Chernev P. Gliech M. Reier T. Dau H. Strasser P. Unified structural motifs of the catalytically active state of Co(oxyhydr)oxides during the electrochemical oxygen evolution reaction.Nat. Catal. 2018; 1: 711-719Crossref Scopus (258) Google Scholar,52Du P. Kokhan O. Chapman K.W. Chupas P.J. Tiede D.M. Elucidating the domain structure of the cobalt oxide water splitting catalyst by X-ray pair distribution function analysis.J. Am. Chem. Soc. 2012; 134: 11096-11099Crossref PubMed Scopus (129) Google Scholar The CoOx(OH)y-Au film