Structurally Precise Two-Transition-Metal Water Oxidation Catalysts: Quantifying Adjacent 3d Metals by Synchrotron X-Radiation Anomalous Dispersion Scattering

Mixed 3d metal oxides are some of the most promising water oxidation catalysts (WOCs), but it is very difficult to know thelocations and percent occupancies of different 3d metals in theseheterogeneous catalysts. Without such information, it is hard toquantify catalysis, stability, and other properties of the WOC as afunction of the catalyst active site structure. This study combines thesite selective synthesis of a homogeneous WOC with two adjacent 3dmetals, [Co2Ni2(PW9O34)2]10-(Co2Ni2P2) as a tractable molecularmodel for CoNi oxide, with the use of multiwavelength synchrotron X-radiation anomalous dispersion scattering (synchrotron XRAS) thatquantifies both the location and percent occupancy of Co (similar to 97%outer-central-belt positions only) and Ni (similar to 97% inner-central-beltpositions only) inCo2Ni2P2. This mixed-3d-metal complex catalyzeswater oxidation an order of magnitude faster than its isostructuralanalogue, [Co4(PW9O34)2]10-(Co4P2). Four independent and complementary lines of evidence confirm thatCo2Ni2P2andCo4P2are the principal WOCs and that Co2+(aq) is not. Density functional theory (DFT) studies revealed thatCo4P2andCo2Ni2P2havesimilar frontier orbitals, while stopped-flow kinetic studies and DFT calculations indicate that water oxidation by both complexesfollows analogous multistep mechanisms, including likely Co-OOH formation, with the energetics of most steps being lower forCo2Ni2P2than forCo4P2. Synchrotron XRAS should be generally applicable to active-site-structure-reactivity studies of multi-metal heterogeneous and homogeneous catalysts.