Homogeneous Water Oxidation Catalysts Research Articles

Voltage-Driven Molecular Photoelectrocatalysis of Water Oxidation.

Molecular photocatalysis and photoelectrocatalysis have been widely used to conduct oxidation-reduction processes ranging from fuel generation to electroorganic synthesis. We recently showed that an electrostatic potential drop across the double layer contributes to the driving force for electron transfer (ET) between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. In this article, we report voltage-driven molecular photoelectrocatalysis with a prevalent homogeneous water oxidation catalyst, (bpy)Cu (II), which was covalently attached to the carbon surface and exhibited photocatalytic activity. The strong potential dependence of the photooxidation current suggests that the electrostatic potential drop across the double layer contributes to the driving force for ET between a water molecule and the excited state of surface-bound (bpy)Cu (II). Scanning electrochemical microscopy (SECM) was used to analyze the products and determine the faradaic efficiencies for the generation of oxygen and hydrogen peroxide. Unlike electrocatalytic water oxidation by (bpy)Cu (II) in the dark, which produces only O2, the voltage-driven photooxidation includes an additional 2e- pathway generating H2O2. DFT calculations show that the applied voltage and the presence of light can alter the activation energy for the rate-determining water nucleophilic attack steps, thereby increasing the reaction rate of photo-oxidation of water and opening the 2e- pathway. These results suggest a new route for designing next-generation hybrid molecular photo(electro)catalysts for water oxidation and other processes.

Exploring the Cooperation of the Redox Non-Innocent Ligand and Di-Cobalt Center for the Water Oxidation Reaction Catalyzed by a Binuclear Complex.

Water oxidation is a crucial reaction in the artificial photosynthesis system. In the present work, density functional calculations were employed to decipher the mechanism of water oxidation catalyzed by a binuclear cobalt complex, which was disclosed to be a homogeneous water oxidation catalyst in pH=7 phosphate buffer. The calculations showed that the catalytic cycle starts from the CoIII,III-OH2 species. Then, a proton-coupled electron transfer followed by a one-electron transfer process leads to the generation of the formal CoIV,IV-OH intermediate. The subsequent PCET produces the active species, namely the formal CoIV,V=O intermediate (4). The oxidation processes mainly occur on the ligand moiety, including the coordinated water moiety, implying a redox non-innocent behavior. Two cobalt centers keep their oxidation states and provide one catalytic center for water activation during the oxidation process. 4 triggers the O-O bond formation via the water nucleophilic attack pathway, in which the phosphate buffer ion functions as the proton acceptor. The O-O bond formation is the rate-limiting step with a calculated total barrier of 17.7 kcal/mol. The last electron oxidation process coupled with an intramolecular electron transfer results in the generation of O2.

Charge Transfer Mechanism on a Cobalt-Polyoxometalate-TiO2 Photoanode for Water Oxidation in Acid.

We constructed a photoanode comprising the homogeneous water oxidation catalyst (WOC) Na8K8[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (Co9POM) and nanoporous n-type TiO2 photoelectrodes (henceforth "TiO2-Co9POM") by first anchoring the cationic 3-aminopropyltrimethoxysilane (APS) ligand on a metal oxide light absorber, followed by treatment of the metal oxide-APS with a solution of the polyoxometalate WOC. The resulting TiO2-Co9POM photoelectrode exhibits a 3-fold oxygen evolution photocurrent enhancement compared to bare TiO2 in aqueous acidic conditions. Three-element (Co 2p, W 4f, and O 1s) X-ray photoelectron spectroscopy and Raman spectroscopy studies before and after use indicate that surface-bound Co9POM retains its structural integrity throughout all photoelectrochemical water oxidation studies reported here. Extensive charge-transfer mechanistic studies by photoelectrochemical techniques and transient absorption spectroscopy elucidate that Co9POM serves as an efficient WOC, extracting photogenerated holes from TiO2 on the picosecond time scale. This is the first comprehensive mechanistic investigation elucidating the roles of polyoxometalates in POM-photoelectrode hybrid oxygen evolution reaction systems.

Design principles for homogeneous water oxidation catalysts based on first–row transition metals

The development of homogeneous first–row transition metal (FRTM) catalysts for the water oxidation reaction is considerably more challenging than for second and third-row catalysts. Given that FRTM catalysts are, in general, more labile, additional design principles must be considered to develop robust and stable catalysts for the water oxidation reaction. In this review, we highlight important design criteria and summarize important lessons learned for FRTM water oxidation catalysts. • The design principles of homogeneous water oxidation catalysts are reviewed. • Additional design criteria that are required for first–row transition metal water oxidation catalysts are considered. • A link is made between operating conditions and catalyst homogeneity. • The utilization of redox-active ligands is discussed.

Computational Analysis of Structure - Activity Relationships in Highly Active Homogeneous Ruthenium-based Water Oxidation Catalysts.

Linear free energy scaling relationships (LFESRs) and regression analysis may predict the catalytic performance of heterogeneous and recently, homogenous water oxidation catalysts (WOCs). This study analyses twelve homogeneous Ru-based catalysts - some, the most active catalysts studied: the Ru(tpy-R)(QC) and Ru(tpy-R)(4-pic)2 catalysts, where tpy is 2,2:6,2-terpyridine, QC is 8-quinolinecarboxylate and 4-pic is 4-picoline. Typical relationships studied among heterogenous and solid-state catalysts cannot be broadly applied to homogeneous catalysts. This subset of structurally similar catalysts with impressive catalytic activity deserves closer computational and statistical analysis of energetics correlating with measured catalytic activity. We report general methods of LFESR analysis yield insufficiently robust relationships between descriptor variables. However, volcano plot-based analysis grounded in Sabatier's principle reveals ranges of ideal relative energies of the RuIV=O and RuIV-OH intermediates and optimal changes in free energies of water nucleophilic attack on RuV=O. A narrow range of RuIV-OH to RuV=O redox potentials corresponding with the highest catalytic activities suggests facile access to the catalytically competent high-valent RuV=O state, often inaccessible from RuIV=O. Our work introduces experimental oxygen evolution rates into approaches of LFESR and Sabatier principle-based analysis, identifying a narrow yet fertile energetic landscape to bountiful oxygen-evolution activity, leading future rational design.

On the Homogeneity of a Cobalt-Based Water Oxidation Catalyst

The homogeneity of molecular Co-based water oxidation catalysts (WOCs) has been a subject of debate over the last 10 years as assumed various homogeneous Co-based WOCs were found to actually form CoOx under operating conditions. The homogeneity of the Co(HL) (HL = N,N-bis(2,2′-bipyrid-6-yl)amine) system was investigated with cyclic voltammetry, electrochemical quartz crystal microbalance, and X-ray photoelectron spectroscopy. The obtained experimental results were compared with heterogeneous CoOx. Although it is shown that Co(HL) interacts with the electrode during electrocatalysis, the formation of CoOx was not observed. Instead, a molecular deposit of Co(HL) was found to be formed on the electrode surface. This study shows that deposition of catalytic material is not necessarily linked to the decomposition of homogeneous cobalt-based water oxidation catalysts.

Computational Scaling Relationships Predict Experimental Activity and Rate-Limiting Behavior in Homogeneous Water Oxidation.

While computational screening with first-principles density functional theory (DFT) is essential for evaluating candidate catalysts, limitations in accuracy typically prevent the prediction of experimentally relevant activities. Exemplary of these challenges are homogeneous water oxidation catalysts (WOCs) where differences in experimental conditions or small changes in ligand structure can alter rate constants by over an order of magnitude. Here, we compute mechanistically relevant electronic and energetic properties for 19 mononuclear Ru transition-metal complexes (TMCs) from three experimental water oxidation catalysis studies. We discover that 15 of these TMCs have experimental activities that correlate with a single property, the ionization potential of the Ru(II)-O2 catalytic intermediate. This scaling parameter allows the quantitative understanding of activity trends and provides insight into the rate-limiting behavior. We use this approach to rationalize differences in activity with different experimental conditions, and we qualitatively analyze the source of distinct behavior for different electronic states in the other four catalysts. Comparison to closely related single-atom catalysts and modified WOCs enables rationalization of the source of rate enhancement in these WOCs.

Cover Feature: Phosphate‐Templated Encapsulation of a {CoII4O4} Cubane in Germanotungstates as Carbon‐Free Homogeneous Water Oxidation Photocatalysts (12/2021)

The Cover Feature shows the cobalt(II)-substituted germanotungstate [Ge3PCo9(OH)5(H2O)4W30O115]20− (Co9), which was obtained by a phosphate-templated synthesis and exhibits high activity as a homogeneous water oxidation catalyst. More information can be found in the Full Paper by E. Al-Sayed, S. P. Nandan, E. Tanuhadi, et al.

Phosphate-Templated Encapsulation of a {CoII 4 O4 } Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts.

The ever‐growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO4 3− templated synthesis of three abundant‐metal‐based germanotungstate (GT) clusters Na15[Ge4PCo4(H2O)2W24O94] ⋅ 38H2O (Co4), Na2.5K17.5[Ge3PCo9(OH)5(H2O)4W30O115] ⋅ 45H2O (Co9), Na6K16[Ge4P4Co20(OH)14(H2O)18W36O150] ⋅ 61H2O (Co20) with non‐, quasi‐, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT‐scaffolds were active molecular WOCs, with Co9 and Co20 outperforming the well‐known Na10[Co4(H2O)2(PW9O34)2] {Co4P2W18} by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s−1 Co9 currently represents the fastest Co‐GT‐based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co9 was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co4.

Na3[Ru2(µ-CO3)4] as a Homogeneous Catalyst for Water Oxidation; HCO3− as a Co-Catalyst

In neutral medium (pH 7.0) [RuIIIRuII(µ-CO3)4(OH)]4− undergoes one electron oxidation to form [RuIIIRuIII(µ-CO3)4(OH)2]4− at an E1/2 of 0.85 V vs. NHE followed by electro-catalytic water oxidation at a potential ≥1.5 V. When the same electrochemical measurements are performed in bicarbonate medium (pH 8.3), the complex first undergoes one electron oxidation at an Epa of 0.86 V to form [RuIIIRuIII(µ-CO3)4(OH)2]4−. This complex further undergoes two step one electron oxidations to form RuIVRuIII and RuIVRuIV species at potentials (Epa) 1.18 and 1.35 V, respectively. The RuIVRuIII and RuIVRuIV species in bicarbonate solutions are [RuIVRuIII(µ-CO3)4(OH)(CO3)]4− and [RuIVRuIV(µ-CO3)4(O)(CO3)]4− based on density functional theory (DFT) calculations. The formation of HCO4− in the course of the oxidation has been demonstrated by DFT. The catalyst acts as homogeneous water oxidation catalyst, and after long term chronoamperometry, the absorption spectra does not change significantly. Each step has been found to follow a proton coupled electron transfer process (PCET) as obtained from the pH dependent studies. The catalytic current is found to follow linear relation with the concentration of the catalyst and bicarbonate. Thus, bicarbonate is involved in the catalytic process that is also evident from the generation of higher oxidation peaks in cyclic voltammetry. The detailed mechanism has been derived by DFT. A catalyst with no organic ligands has the advantage of long-time stability.

Covalent immobilization of ruthenium polypyridyl complex on multi-walled carbon nanotube supports for oxygen evolution reaction in an alkaline solution

Electrocatalytic water splitting to realize the sustainable production of hydrogen is one of the key ways to gain renewable clean energy. The development of the ruthenium-terpyridine-bipyridine (bpyRutpy) complexes used as efficient homogeneous water oxidation catalysts is one of the currently hot research fields. All catalytic active sites of the homogeneous molecular catalyst are fully utilized, and show a high catalytic efficiency. However, the homogeneous molecular catalyst has some intrinsic shortages such as difficulty on separation, recovery and regeneration, and high cost. In view of this, we synthesized the multi-walled carbon nanotube (MWCNT)-based heterogeneous catalyst, MWCNT-bpyRutpy, by covalently bonding the ruthenium-based complex onto the carbon nanotubes, in order to achieve the maximum molecular catalytic efficiency of the complex. The synthesized catalyst is fully characterized by mass spectrometry, nuclear magnetic resonance spectroscopy, infrared spectroscopy, ultraviolet–visible spectroscopy, photoelectron spectroscopy, Raman spectroscopy, and thermogravimetry. The formation of the nanocatalyst via fusing Ruthenium complexes to MWCNTs promotes the oxygen evolution reaction (OER) kinetics, enhances intrinsic activity, and increases the specific surface area of electrochemical activity, and achieve excellent OER performance with a low overpotential of 436 mV at 10mAcm2, a low Tafel slope of 77.2 mV, and a relatively high turnover frequency (TOF) of 0.5833 s−1 at an overpotential of 400 mV.

An Active‐Site Sulfonate Group Creates a Fast Water Oxidation Electrocatalyst That Exhibits High Activity in Acid

The storage of solar energy in chemical bonds will depend on pH-universal catalysts that are not only impervious to acid, but actually thrive in it. Whereas other homogeneous water oxidation catalysts are less active in acid, we report a catalyst that maintained high electrocatalytic turnover frequency at pH values as low as 1.1 and 0.43 (kcat =1501±608 s-1 and 831±254 s-1 , respectively). Moreover, current densities, related to catalytic reaction rates, ranged from 15 to 50 mA cm-2 mM-1 comparable to those reported for state-of-the-art heterogeneous catalysts and 30 to 100 times greater than those measured for two prominent literature homogeneous catalysts at pH 1.1 and 0.43. The catalyst also exhibited excellent durability when a chemical oxidant was used (CeIV , 7400 turnovers, TOF 0.88 s-1 ). Preliminary computational studies suggest that the unusual active-site sulfonate group acts a proton relay even in strong acid, as intended.

An Active‐Site Sulfonate Group Creates a Fast Water Oxidation Electrocatalyst That Exhibits High Activity in Acid

AbstractThe storage of solar energy in chemical bonds will depend on pH‐universal catalysts that are not only impervious to acid, but actually thrive in it. Whereas other homogeneous water oxidation catalysts are less active in acid, we report a catalyst that maintained high electrocatalytic turnover frequency at pH values as low as 1.1 and 0.43 (kcat=1501±608 s−1 and 831±254 s−1, respectively). Moreover, current densities, related to catalytic reaction rates, ranged from 15 to 50 mA cm−2 mM−1 comparable to those reported for state‐of‐the‐art heterogeneous catalysts and 30 to 100 times greater than those measured for two prominent literature homogeneous catalysts at pH 1.1 and 0.43. The catalyst also exhibited excellent durability when a chemical oxidant was used (CeIV, 7400 turnovers, TOF 0.88 s−1). Preliminary computational studies suggest that the unusual active‐site sulfonate group acts a proton relay even in strong acid, as intended.

Dinuclear nickel(II) complexes with ONO-pincer and coordinated aquo ligands as a robust homogeneous water oxidation catalyst

A nickel(II) complex with the newly synthesized dicarboxamide ligand [H2LBZ][(CF3SO3)Cl] was explored as a water oxidation catalysis. All the synthesized compounds were characterized by IR and NMR spectroscopy, thermogravimetry, and single-crystal XRD analysis. The superior kinetics of the water oxidation reaction in the presence of the NiII catalyst was shown by an enhanced current of 4535 µA (pH 8) or 8900 µA (pH 10) at an overpotential of 186 mV in a cyclic voltammogram. Controlled potential electrolysis demonstrated that the catalyst was stable during the water oxidation experiment. These results are akin to those for other reported nickel catalysts.

Theoretical Study of the Mechanisms of Two Copper Water Oxidation Electrocatalysts with Bipyridine Ligands

Hybrid density functional theory was employed to study the reaction mechanisms of two homogeneous copper water oxidation catalysts (WOCs), [(L22–)CuII(OH2)2] (H2L2 = 6,6′-dihydroxy-2,2′-bipyridine)...

Design of an inherently-stable water oxidation catalyst

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation. In response, we propose using oxidatively-inert ligands to harness the metal oxides themselves. This approach is demonstrated by covalently attaching entirely inorganic oxo-donor ligands (polyoxometalates) to 3-nm hematite cores, giving soluble anionic structures, highly resistant to aggregation, yet thermodynamically stable to oxidation and hydrolysis. Using orthoperiodate (at pH 8), and no added photosensitizers, the hematite-core complex catalyzes visible-light driven water oxidation for seven days (7600 turnovers) with no decrease in activity, far exceeding the documented lifetimes of molecular catalysts under turnover conditions in water. As such, a fundamental limitation of molecular complexes is entirely bypassed by using coordination chemistry to harness a transition-metal oxide as the reactive center of an inherently stable, homogeneous water-oxidation catalyst.

Homogeneous and heterogeneous photocatalytic water oxidation by polyoxometalates containing the most earth-abundant transition metal, iron

Reported here, inorganic molecular tetrairon(III)-substituted polyoxotungstate [FeIII4(H2O)2(P2W15O56)2]12− is firstly used to catalyze visible-light-driven oxygen evolution, which displays excellent catalytic activity (O2 yield of 48%, turnover number (TONcatalyst) of 900 and quantum (ΦQY) yield of 24%) in a homogeneous system. The O2 yield and TON of Fe4POM catalyst rank third and fourth among thirty documented homogeneous photocatalytic water oxidation catalysts based on POMs, respectively. In order to recover and reuse the homogeneous catalysts, it is realistic and necessary to heterogenize the homogeneous molecular catalyst. The electrostatic interaction is rationally designed as solid connection between ((3-aminopropyl)triethoxysilane) apts-modified mesoporous SBA-15 and polyoxoanion (Fe4POM). The catalytically active species of immobilized polyoxometalate maintains intact before and after the photocatalytic water oxidation, which is adequately confirmed by various spectroscopic techniques such as solid phosphorus-31 nuclear magnetic resonance spectra (31P MAS NMR), X-ray photoelectron spectra (XPS) and transmission electron microscopy (TEM) images.

Computational Modeling of Cobalt-Based Water Oxidation: Current Status and Future Challenges.

A lot of effort is nowadays put into the development of novel water oxidation catalysts. In this context, mechanistic studies are crucial in order to elucidate the reaction mechanisms governing this complex process, new design paradigms and strategies how to improve the stability and efficiency of those catalysts. This review is focused on recent theoretical mechanistic studies in the field of homogeneous cobalt-based water oxidation catalysts. In the first part, computational methodologies and protocols are summarized and evaluated on the basis of their applicability toward real catalytic or smaller model systems, whereby special emphasis is laid on the choice of an appropriate model system. In the second part, an overview of mechanistic studies is presented, from which conceptual guidelines are drawn on how to approach novel studies of catalysts and how to further develop the field of computational modeling of water oxidation reactions.

A low-overpotential nature-inspired molecular chromium water oxidation catalyst

The function of tyrosine Yz in photosystem II (PSII) has been considered as a model for establishing a new homogeneous water oxidation catalyst (WOC). In this work, we designed and synthesized a diphenoxy-based salen-type Schiff base ligand (L) in order to overcome the problems that so far have prevented the use of chromium complexes as water oxidation catalysts (WOCs). Cyclic voltammetry measurements performed in the alkaline solution of [Cr(L-2H)Cl] (1) displayed a large irreversible oxidation waves, corresponding to the catalytic water oxidation. The main species in the catalytically active solutions is found to be [Cr(L-2H)ClOH]-∙ The proposed catalytic system generates a current density (J) of 1.0 mA cm−2 at the relatively low overpotential (ƞ) of 0.426 V with a high turnover frequency (TOF) of 49.7 s−1. A comprehensive mechanistic study was carried out based on the results of experiments, calculations and according to previous works. The computational calculations supported the importance of the uncoordinated OH groups present in the ligand moiety of the complex for the oxidation of Cr3+ to Cr4+.

Homogeneous electrocatalytic water oxidation catalyzed by a mononuclear nickel complex

Three nickel (II) complexes of o-phenylenebis(N′-methyloxamidate) (L1), o-phenylene(N′-methyloxamidate)oxamate (L2) and o-phenylenebis(oxamate) (L3) with tetradentate ligands are synthesized and characterized. These four-coordinate complexes, (Me4N)2[NiLi] (i = 1–3 for complex 1–3), are investigated for electrocatalytic water oxidation in basic phosphate buffer solution. Experimental results show that (Me4N)2[NiL1] (1) is a homogeneous catalyst for electrochemical water oxidation. However, under the same condition, 2 and 3 decompose into NiOx nanoparticles, which acts as precatalyst for the electrocatalytic water oxidation. The catalytic mechanism for electrocatalytic water oxidation by 1 is proposed using cyclic voltammetry experiments, kinetic isotope effect and Pourbaix diagram. By constructing the relationship between the molecular structure and stability of these catalysts, nitrogen atom is found to be more beneficial than carboxyl group for the stability of nickel based homogeneous electrochemical water oxidation catalysts.