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. 2022 Dec 21;3(1):36–46. doi: 10.1021/jacsau.2c00507

Learning from Nature’s Example: Repair Strategies in Light-Driven Catalysis

Alexander K Mengele †, Sven Rau †,*
PMCID: PMC9875256  PMID: 36711104

Abstract

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The continuous repair of subunits of the photosynthetic apparatus is a key factor determining the overall efficiency of biological photosynthesis. Recent concepts for repairing artificial photocatalysts and catalytically active materials within the realm of solar fuel formation show great potential in reshaping the research directions within this field. This perspective describes the latest advances, concepts, and mechanisms in the field of catalyst repair and catalyst self-healing and provides an outlook on which additional steps need to be taken to bring artificial photosynthetic systems closer to real-life applications.

Keywords: photocatalysis, repair, healing, artificial photosynthesis, solar fuels

1. Introduction

Natural oxygenic photosynthesis that enabled the evolution of complex life on earth has likely started ca. 2.4 billion years ago.1 The photosynthetic apparatus, i.e. the molecular machinery allowing the coupling of light absorption with water splitting and reductive assimilation of CO2 leading to the formation of oxygen and organic matter, has attracted the interest of scientists for generations. The study of natural photosynthesis prompted scientists to rethink the future energy landscape in general2−4 and also fruitfully induced research in specific fields such as artificial photosynthesis5−7 or the development of bioinspired electrocatalysts.8

A functional analysis of the molecular photosynthetic apparatus leads to the conclusion that individual components of a man-made photocatalytic system should carry out individual tasks such as light-absorption-induced charge separation, electron transport, and catalytic turnover. In addition, nature also teaches mankind that even the most sophisticated systems might suffer from unavoidable degradation.9 Green plants have developed complex repair strategies to cope with the continuous degradation of vulnerable subunits critical to the functioning of the whole photosynthetic machinery (see Scheme 1).10 Consequently, the longevity of the photosynthetic apparatus in total is enhanced by molecular repair. Following the path outlined above of transferring concepts and lessons from nature to artificial systems as well as already promising examples of self-healing materials for e.g. water oxidation,11 the next steps in improving many artificial (photo)catalytic systems is to better understand degradation pathways12 and consequently implement knowledge-guided repair and self-healing strategies.

Scheme 1. Simplified Scheme of Biological PSII Repair Representing the Elementary Steps of Photoinhibition by D1 Degradation (i), PSII Disassembly (ii), Implementation of De Novo Synthesized D1 (iii), and Photoinduced Reassembly of the OEC (iv).

Scheme 1

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Driven by the fact that natural photosynthesis has ever been a source for inspiration, the repair cycle of natural photosynthesis will be briefly outlined (see Scheme 1).9,10,13 The most vulnerable part of the photosynthetic apparatus is the D1 protein representing—along with the D2 protein—a core polypeptide structure within the reaction center of PSII (PS = photosystem). On illumination with high light intensity, the rate of the D1 replacement procedure can occur as often as twice per hour.10 Although the specific molecular processes ultimately triggering the photoinhibition of the photosynthesis via photodegradation of the D1 protein are still under debate (Φ ≈ 10–7),9 the processes leading to the reactivation of the photosynthetic apparatus have been clarified to a large extent.13 When the D1 protein is damaged (step i in Scheme 1), phosphorylation induces the disassembly of PSII as well as the degradation of the no longer functioning D1 protein by different proteases (step ii). This process is followed by the de novo synthesis of the D1 protein and the reassembly of the functional PSII (step iii).13

In contrast to degradation and resynthesis of the D1 protein, the final assembly step, i.e., the formation of the functional oxygen-evolving complex (OEC) using Mn(II) and Ca(II) ions (step iv), is directly triggered by light. The stepwise assembly of the OEC is initiated by the PSII-mediated oxidation of Mn(II) to Mn(III), which initiates the coordination of further Mn(II) and Ca(II) ions, finally leading to the formation of the functional CaMn4O5 cluster. It can thus be stated that the repair of the natural photosynthetic apparatus exhibits both photoindependent and photodependent steps. As will be described below, both strategies have also been utilized for repairing artificial photosynthetic systems.

Concerning the terminology used within this article, it should be noted that self-healing is a specific type of repair defined as the in situ repair of the catalyst under operating conditions without applying an additional external stimulus; for water-oxidizing catalysts self-healing has recently been extensively reviewed.11 Following this systematic definition, all other systems that need some kind of external input (chemicals, light, etc.) under nonoperating conditions to induce the reactivation of catalytic activity do not exhibit self-healing properties and must be (actively, e.g. by human action) repaired. Finally, within this perspective the term repair (process) will be applied to all processes where a chemical reaction takes place that (i) regenerates the catalyst in its initial (or only a slightly altered) form and (ii) restores catalytic activity.

Rather than focusing on a specific field of catalyst repair, the goal of this perspective is to present the breadth of possible repair strategies in different areas: i.e., covering reductive as well as oxidative catalysis and treating both molecular systems and material-based catalysts, respectively. Although a variety of different mechanisms can lead to the loss of photocatalytic activity, these detrimental processes have been tackled so far by a limited number of concepts by which these degradation processes are reversed or ideally even inhibited (see Scheme 2). These concepts will be discussed in the following sections.

Scheme 2. Pictorial Representation of Different Repair Strategies.

Scheme 2

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Green, orange, and gray represent stable, unstable, and inactivity-inducing ligand environments of metal catalyst M, respectively. Blue indicates a material capable of anchoring M, while reagents A are highlighted in red (S, substrate; P, product).

2. Repair Concepts for Molecular Catalysts

2.1. Concepts for Repairing the Loss of Structural Integrity of the Catalyst Unit Itself

Light-driven hydrogen evolution using molecular artificial photosynthetic systems is regularly enabled using α-diimine complexes of simple Pt and Pd dihalides. The literature suggests that special requirements for the transition metal coordinating ligand sphere are necessary to avoid metal particle formation:14 i.e., by offering electron-withdrawing substituents or electron reservoir moieties.15−17 Recently another strategy has been reported how metal colloid formation can be inhibited: i.e., how fast self-healing, occurring at a higher rate than colloid formation, is achieved (Scheme 2A). The concept is based on metal–organic frameworks (MOFs), being built up by carboxylic acid functionalized bpy ligands (see Scheme 3A; bpy = 2,2′-bipyridine).18 The MOFs contain only a limited number of [(ppy)2Ir(bpy)]+ chromophores (ppy = 2-phenylpyridine) and [(bpy)PtCl2] catalysts as well as a varying amount of vacant α-diimine binding sites. In presence of an 100-fold excess of vacant bpy moieties, the MOF did not show any signs of colloid formation. In contrast, a lower bpy excess leads to slow particle formation. Interestingly, the duration of molecular catalysis was also prolonged when an excess of free bpy ligand was added to the MOF-free homogeneous reference catalysis solution. However, the longevity-boosting effect was not as pronounced as for the MOF itself, where intermediately decoordinated Pt centers are more efficiently recaptured by the directly available α-diimine sites. This is enabled by their higher local concentration in the MOF architecture and the concomitantly constrained movement of the coordinatively labile Pt moieties. This strategy represents a very promising concept and should consequently be pursued further.

Scheme 3. Selection of Published Repair Strategies in the Realm of Molecular (Photo)catalysis: (A, B) Chromophore–Catalyst Pairs Which Are Attached to MOFs via Their CO2H Groups;25,26 (C) Reductive In Situ Repair of [(xantphos)Pd];27 (D) Repair of a RCM Ru Catalyst28.

Scheme 3

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This is also highlighted by another report where the self-healing strategy by offering vacant α-diimine sites (Scheme 2A) has recently been utilized for the recycling of very similar MOFs bearing MOF-linked [Ru(bpy)3]2+ chromophores and the same [(bpy)PtCl2]-based catalysts as described above.19 The recycled material did not show any signs of degradation for three consecutive runs of light-driven hydrogen evolution.

The self-healing concept for prolonging molecular catalysis by recoordination of labile metal centers via addition of excess ligand (Scheme 2A) has also been investigated in the area of [CoCl(dmgH)2(py)] complexes (dmgH2 = dimethylglyoxime; py = pyridine). When this Co catalyst and eosin Y were combined in the presence of triethanolamine (TEOA, an electron donor), light-driven hydrogen evolution was observed.20 The longevity of the photocatalytic system as well as the finally obtained turnover number (TON) could be increased stepwise by addition of 6 and 12 equiv of free dmgH2, respectively. In the latter case and based on the amount of utilized Co catalyst, a TON of 900 was obtained. In a follow-up publication Eisenberg and co-workers reported two further details: ligand exchanges (of py and dmgH) at the Co center are drastically increased when Co(III) (a d6 system) is reduced to coordinatively more labile Co(II) (a d7 system), e.g. under photoreductive conditions, and after reaching complete inactivity a repair process can be performed.21 This process is based on the hypothesis that during reductive photocatalysis the dmgH2 ligand became hydrogenated or decomposed otherwise so that the active Co(I) state can no longer be thermodynamically accessed using eosin Y as a chromophore. Exploiting the reduction-induced fast ligand exchange at the Co catalyst thus opens the possibility that the addition of fresh dmgH2 reactivates the catalytic system by replacing the hydrogenated bidentate ligand from the Co center (Scheme 2B; A = reduction equivalents). The repair concept of photochemistry-induced ligand exchanges via a light-driven variation of the metal’s redox state in the inactive decomposition product is thus a very elegant way to repair catalysts that do not suffer from metal–ligand bond dissociation loss but from disadvantageous ligand transformations.

In addition to the dynamic coordination chemistry of Co centers and dmgH2 ligands, also Co–Npy bonds were utilized as repair sites for light-driven hydrogen evolution. In a MOF equipped with a [CoCl(dmgH)2(py)]-like complex where the py ligand is covalently connected to the MOF skeleton, light-driven H2 formation of the functionalized MOF using eosin Y as a freely diffusing chromophore is accompanied by 90% Co loss after 20 h of irradiation (TON = 45 per Co center).22 However, the system could be repaired by linking new Co centers to the py moiety of the MOF via post mortem treatment with [CoCl2(dmgH)(dmgH2)] (Scheme 2C). By this, 60% of the initial activity could be restored. Furthermore, as the catalytic activity of the MOF-based photocatalytic system was notably higher than that of the homogeneous reference system, the authors concluded that the porous structure of the MOF partially inhibited leaching of the Co centers during catalysis, thus keeping them in close proximity to the unbound py ligands and consequently facilitating self-healing via recoordination. It should also be noted that in a heterodinuclear Fe–Co photocatalyst the light-independent dissociation of the Co center from the Npy atom occurred by simply diluting the catalyst in acetonitrile (MeCN).23 In such cases, recoordination-driven self-healing of the dinuclear species, i.e. the active catalyst, might only take place efficiently at high concentration. Finally, it should be mentioned that a hybrid system consisting of photosystem I (PSI) and [CoCl(dmgH)2(py)] yielded ca. 5200 molecules of H2 per PSI. Efforts to restore photocatalytic activity via addition of fresh [CoCl(dmgH)2(py)] have so far been unsuccessful.24

Pd-based systems have been shown to exhibit an even higher tendency to form metal particles than their Pt congeners, as evident from studies on, e.g., heterodinuclear Ru–BL–Pd complexes (BL = bridging ligand).17,29 Low-valent Pd species, which disadvantageously also tend to aggregate to metal clusters, are important intermediates in oxidative organic transformations as well, such as decarboxylative coupling of allylic alcohols or C–H alkenylation of 2-phenyl phenols. Jiang and co-workers developed bifunctional photocatalytically active MOFs based on carboxylic acid functionalized [(ppy)2Ir(bpy)]+ chromophores and [(bpy)PdX2] catalysts (X = OAc, TFA (=trifluoroacetate); see Scheme 3B) for the organic transformations named above.26 By illuminating the MOF samples during oxidative organic transformations, the Pd colloid formation was suppressed, and a strongly enhanced activity (25-fold compared to the homogeneous reference system) of the MOF material for the photooxidative substrate transformations was observed. This effect was ascribed to the efficient reoxidation of Pd(0) centers by electron transfer to nearby photoexcited Ir centers; oxygen serves as a terminal electron acceptor (Scheme 2D).

However, Pd-based catalysts for hydrogen formation tend to be more active than the related Pt analogues. It is therefore very relevant to investigate strategies which may lead to the stabilization of the Pd metal to ligand bond in all oxidation states of the metal center. Here, organometallic catalysis has developed some very interesting concepts. For the Pd-catalyzed azidocarbonylation of aryl iodides Grushin and colleagues have reported a repair process.27 In contrast to the aforementioned MOF system, the catalytically active species is a robustly phosphine bound Pd(0) center, and the poisoned catalyst was identified as [(xantphos)PdI2]. Based on the identification of the degradation product, a continuous repair process was developed (see Scheme 3C). The catalytic activity of the Pd-catalyzed cross-coupling reaction can be increased by addition of PMHS (polymethylhydrosiloxane), which reliably re-forms xantphos-stabilized Pd(0) from intermittently generated and catalytically inactive [(xantphos)PdI2] (Scheme 2B; A = PMHS). In the future this successful redox-chemistry-based repair strategy may also be applied to other systems. If the degradation products have been identified as structurally intact systems not being capable of returning back into the catalytic cycle due to the lack of energetically suitable redox equivalents, repair by suitable oxidants or reductants represents a viable option in reviving catalytic activity.

For the ring-closing metathesis (RCM) reaction of diallylmalonate using the first-generation Hoveyda–Grubbs catalyst a repair process has been developed as well.28 To study the repair of one of the most likely occurring catalyst degradation products, the first-generation Hoveyda–Grubbs catalyst was purposely inactivated with ethylene (see Scheme 3D). After analyzing the chemistry of this intermediate, treatment of the Ru-based decomposition product from RCM with 1-(3,5-diisopropoxy-phenyl)-1-phenylprop-2-yn-1-ol for 18 h in THF under reflux resulted in the reactivation of only 43% of the initially employed Ru catalyst (Scheme 2E). However, RCM still proceeded with 90% yield when the repaired system was allowed to react for longer times.

A detailed study into the light-driven formation of CO from CO2 offers an insight into another important aspect limiting catalytic activity. A simple repair/recycling process has been described for the photocatalytic CO2 reduction to CO using [Ru(bpy)3]2+ and [Co(bpy)3]3+ complexes as chromophores as well as catalysts, respectively.30 Irrespective of the linkage of [Ru(bpy)3]2+ to a Nafion membrane, a simple restoration of catalytic activity was obtained by thoroughly degassing the catalytic solution with fresh CO2 (Scheme 2F). The loss of catalytic activity after several hours of CO formation was assigned to poisoning of the Co catalyst by the diatomic product. Additionally, product trapping within polymer pores is a well-known effect.31 Although no specific information on the binding parameters for these Co catalysts and CO or CO2 were given for the [Co(bpy)3]3+-containing system, an analysis of similar Co-based CO2 reducing catalysts containing macrocyclic N-donor ligands revealed that CO binds to Co(I) centers ca. 104 times stronger than CO2 does.32 Although the authors claim that CO is removed efficiently from the Co center in the presence of excess CO2, thus allowing the simple recovery of catalytic activity, an analysis of the binding constants from related Co catalysts suggests that significantly more CO2 will have to be utilized for catalyst regeneration than the amount that has catalytically been converted (TON < 10). If the excess CO2 utilized for this repair process would not be used for any further processes, this repair strategy would likely have to be seen to be critical from an environmental point of view. A similar effect was also observed for a Ni cyclam complex showing CO2 reducing electrocatalytic activity. Only upon addition of a suitable CO scavenger is the catalyst efficiently freed from strongly bound CO and the electrocatalytic activity is increased by a factor of 10 (Scheme 2F).33

Following the route depicted in Scheme 2C, repair of a hydrogen-bonded organic framework containing [(bpy)Re(CO)3Cl] sites for the light-driven reduction of CO2 has been recently reported as well.34 After eight cycles, the drop of the material’s catalytic activity to ca. 50% of its initial value is assigned to Re leaching. Postfunctionalization of the material collected after eight cycles with fresh Re moieties lead to a return of its initial catalytic performance. Especially if less costly transition-metal catalysts would be utilized, these remetalation processes of sophisticated substrates (Scheme 2C) could become cost-efficient repair processes.

Mechanistically connected to these processes is a 2,2′:6′,2″-terpyridine (tpy)-functionalized polymer, in which the programmed assembly and disassembly with tpy-substituted [Ru(bpy)3]2+ complexes was investigated. Mg2+-induced assembly of polymer and Ru complex via a [(tpy)Mg(tpy)]2+ motif is associated with increased photo(electro)chemical activity: e.g., photooxidative dimerization of benzylamine.35 Programmed disassembly—and accordingly a decrease in photocatalytic activity—is observed when TBAF as a fluoride source is added due to the formation of MgF2. The Ru-functionalized polymer can be repaired after a washing step when fresh tpy-functionalized [Ru(bpy)3]2+ complex and Mg2+ ions are added to the solution.

A similar “disassembly and repair on purpose” process, typically employed to induce a specific catalyst deactivation process and probe repair strategies for the selected degradation pathway, has been described by Streb and colleagues (Scheme 2G; A = ethylenediaminetetraacetic acid (EDTA)).36 The Cu-functionalized polyoxovanadate {CuV12} was utilized for the oxidation of 1-phenylethanol to acetophenone using tert-butyl hydroperoxide (tBuOOH) as oxidant. Addition of EDTA led to an immediate stop of alcohol oxidation by removing Cu(II) from the polyoxometalate (POM) scaffold via precipitation of a Cu-EDTA complex. Targeted repair of the active catalyst is possible by addition of fresh Cu(NO3)2. The efficient remetalation of the polyoxovanadate {V12} under catalytic conditions was also confirmed by mixing it with Cu(NO3)2 at the start of the catalytic process and observing activity identical to that for the independently synthesized {CuV12}. This indicates that in a mixture of Cu(II) ions and the POM {V12} the active catalyst {CuV12} represents the thermodynamically most stable species.

The property of POM-based systems, i.e., the catalytically active state represents at the same time the thermodynamic sink of the system, even under catalytic conditions, has also been exploited by other groups. In fact, these characteristics would qualify a catalyst to be active indefinitely, even if it equilibrates to a certain extent into smaller (sub)structures during catalytic operation (Scheme 2H).37 Therefore, this self-healing-based longevity and the possibility to self-assemble from simple inorganic precursor salts rendering the presynthesis of the catalyst unnecessary make these POM catalysts highly attractive for industrial applications. The proposed self-assembly of active POMs during catalysis from simple inorganic salts has been utilized not only for their application in the selective epoxidation of cis-stilbene38 but also for the oxidation of lignin into CO2 and H2O39 as well as the oxidation of alcohols to the corresponding aldehydes, ketones, and carboxylic acids in the presence of H2O2.40

2.2. Reversal of Ligand Hydrogenations As Repair Strategy for Structurally Stable Photocatalysts

In the previous section many repair strategies were based on reestablishing the direct chemical environment of the catalytically active center (i.e., the first coordination sphere), which has been altered in the course of catalysis. Although to some extent intertwined (see the exploitation of dmgH2 ligand exchange reactions for the repair of [CoCl(dmgH)2(py)]), the often occurring ligand hydrogenation processes and their impact on ceasing catalytic activity as well as opening possibilities for molecular repair will be discussed in the following paragraphs.

Under the conditions necessary for reductive photocatalysis, π-extended aromatic ligand scaffolds containing N or other heteroatoms are prone to ligand hydrogenation41−51 by two or more sequential reduction and protonation events or by a reaction cascade involving a proton-coupled electron transfer (PCET), followed by the disproportionation of two semihydrogenated species.44,46 In contrast to all other complexes shown in Scheme 4, RuRu2 is the only system capable of not only storing two but rather four redox equivalents on its central BL.41 These ligand hydrogenation processes have mainly been investigated for [Ru(bpy)3]2+-like systems (see Scheme 4) and are known to alter the photophysical properties of the complexes, sometimes even dramatically.42,48 For example, the photochemical reduction of [Ru(bpy)2(bpp)]2+ (Ru3 in Scheme 4) by two electrons led to the formation of an NADH-like bppH2 ligand (bpp = benzo[b]pyrido[3,2-f][1,7]-phenanthroline). This diminished the luminescence quantum yield of the complex by more than 60%.48 Complete loss of luminescence upon formation of the hydrogenated state was observed for RuRu1 by offering a ligand-centered low-lying π–π* state.42 Very active and stable intramolecular photocatalysts for H2 generation suffer from a related deactivation pathway. For [(tbbpy)2Ru(tpphz)PtI2](PF6)2 (RuPt1 in Scheme 4) a gradual decrease of photocatalytic hydrogen evolution activity can be associated with the slow in operando formation of the tpphz-hydrogenated species (tbbpy = 4,4′-tert-butyl-2,2′-bipyridine, tpphz = tetrapyridophenazine).51 Light-driven hydrogenation of the phenazine moiety of the corresponding mononuclear Ru complex has previously been described as well.50

Scheme 4. Ligand Hydrogenation Processes Taking Place from Green to Orange as well as Reported Oxygen-Dependent Reoxidation Processes of Different Transition Metal Complexes41−49,51.

Scheme 4

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The technically simple reoxidation of different hydrogenated Ru complexes by reaeration of the solutions or in situ formation of 1O2 (see Scheme 4) allowed the development of an active repair cycle for RuPt1.51 It should be noted that the use of either 3O2 (Scheme 2B; A = 3O2) or 1O2 (Scheme 2I; A = 1O2 generated by a Ru complex) is associated with the fact that different hydrogenated ligand spheres exhibit varying reactivities resulting from their different thermodynamic stabilities.52 For rather stable systems such as Ru3 and RuPt1, an efficient repair process is only possible upon light-driven formation of more reactive 1O2 (E(1O2/O2•–) = +1.07 V vs NHE and E(3O2/O2•–) = +0.10 V vs NHE).53

Consequently, after a plateau for light-driven H2 formation by RuPt1 had been reached, evaporation of the solvent mixture used for photocatalysis (MeCN:TEA:H2O = 6:3:1, v:v:v) and subsequent addition of fresh, air-equilibrated MeCN along with irradiation of the MeCN solution of hydrogenated RuPt1 for several minutes led to the regeneration of the active catalyst and H2O2 as a byproduct (Scheme 2I; note that concomitant RuPt1 reduction as well as TEA oxidation by 1O2 prevents the light-driven repair process of hydrogenated RuPt1 in the solvent mixture used for photocatalysis). By repeating these processes at the end of each photocatalytic run, the 8-fold recycling of RuPt1 led to an increase of the total turnover number (TON) by a factor of ca. 7, reaching values of more than 3000. This highlights that the search for reasonable repair strategies is very important for increasing the catalytic output per molecule and consequently improving the cost efficiency of photocatalytic H2 evolution.54 Interestingly, RuPt1 can also be repaired by adding excess (NH4)2S2O8 to the catalytic solution after hydrogen formation has stopped (Scheme 2B, A = (NH4)2S2O8).51

3. Repair and Self-Healing of Catalytically Active Materials

A large number of materials used for the (photo)electrocatalytic splitting of water (mainly water oxidation) exhibit self-healing properties as defined by Thorarinsdottir et al.:11 i.e., without any human intervention the catalytically active species reestablishes itself under the operating conditions. These catalysts thus possess theoretical immortality when brought into the correct chemical environment.

Before discussing self-healing oxygen evolution catalysts, other material-applied repair strategies are described. Similar to the oxygen-driven reversal of ligand hydrogenation in Ru complexes—including hydrogen evolving photocatalyst RuPt1(51)—the material SION-X, i.e. Cu2[(BO)(OH)2](OH)3, can also be regenerated after quantitative catalytic H2 release from ammonia–borane to its initial chemical composition via simple reaeration of the reaction mixture (Scheme 2B; A = 3O2).55 By that, the intermediately formed and catalytically active Cu(0) nanoparticles (NPs) are converted to Cu(II), leading to the regeneration of SION-X. This repair procedure could be repeated for 50 cycles without significant loss of Cu.

Another unique repair strategy was reported for an α-FeOOH@g-C3N4 catalyst utilized for the radical-driven degradation of bisphenol A.56 The latter is decomposed by an Fe(II)-consuming reductive radical formation of persulfate. The continuous in situ “healing process” of the Fe centers is based on the light-driven reduction of Fe(III) to Fe(II) by g-C3N4 (Scheme 2I). As Fe leaching from the visible-light-absorbing g-C3N4 was low, recycling of the hybrid material and its use for several bisphenol A degradation runs could be demonstrated.

Although the vast majority of self-healing materials have been reported for the water oxidation reaction (see below), these interesting properties have also been identified in a system utilized for the photoelectrochemical hydrogen formation.57 2D WSe2 was used as a substrate for film-forming MonOxSy complexes. The combination of efficient hydrogen evolution as well as point defect covering was ascribed to a multicomponent behavior of various mono-, di-, and trimeric Mo complexes (Scheme 2C). Whereas some of the catalytically less active complexes showed a high tendency for adsorption on 2D WSe2 at point defects formed during catalysis, other Mo complexes being part of the film represented efficient hydrogen-evolving catalysts.

Self-healing properties were also observed in Co3O4 nanorods (NRs) used for the Fischer–Tropsch synthesis (FTS) of hydrocarbons.58 During FTS in the presence of water CoOOH species are formed on the material’s surface. In contrast to also investigated Co3O4 NPs, where an ever-increasing CoOOH peak was found by Raman spectroscopy during FTS, Co3N4 NRs exhibited limited and even reversible CoOOH formation. The mechanism behind the reversibility of CoOOH poisoning on Co3O4 NRs is the exergonic reduction of Co3+ centers to Co(0) at the NR surface (Scheme 2B; A = H2). Due to the fact that the NPs exhibit Co2+ centers on their surface as a consequence of another crystal facet pointing toward the surrounding medium as well as the endergonic formation of metallic Co(0) from these Co2+ centers, no self-healing properties were observed for Co3O4 NPs. As this crystal-facet-dependent effect is very prominent, self-healing induced by surface engineering might be another highly relevant aspect of future catalyst studies.

After highlighting some currently unique repair and self-healing properties of selected materials, the much more thoroughly investigated Co-, Ni-, and Mn-oxide systems for the (photo)electrochemical oxygen evolution reaction (OER) will be described in the following text.11,13 Although each of these systems has its own characteristics regarding the specific molecular steps involved in OER and self-healing, they all follow the very simplistic global steps depicted in Schemes 5 and 2J (A = oxidation equivalents at the electrode surface).11 As a consequence of the different redox states of the metal centers inevitably being part of the catalytic cycle of OER as well as the metals’ redox-state-dependent solubilities and dissolution rates, the breakup of the catalyst film via loss of reduced metal centers into the surrounding medium is observed. However, if reoxidation of the dissolved metal centers occurs at a lower anodic potential than the potential needed for performing OER and the rate of reassembly is equal to or larger than the rate of catalyst dissolution, self-healing of the OEC utilizing the dissolved metal ions can occur. Interestingly, the assembly of the natural blueprint, i.e. the OEC of PSII, occurs in a similar fashion.13 As described above, the high redox potential of PSII is utilized to (photo)oxidize Mn(II) to Mn(III), initiating the metal oxide cluster formation (see step iv in Scheme 1).

Scheme 5. Schematic Representation of Catalytically Active Materials for Water Oxidation and Their In Situ Repair by Redox-Chemistry-Driven Self-Healing.

Scheme 5

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For the Co-based systems atomistic insights into their working principle have been obtained.11,59−62 These self-healing Co catalysts are obtained as thin films via anodization of dilute Co2+ solutions containing suitable counterions such as phosphate, methylphosphonate, or borate (CoPi, CoMePi and CoBi catalysts).63,64 For CoPi the role of phosphate is (i) to prohibit the formation of giant three-dimensional Co-oxide domains (typical cobaltate clusters consist of 10–60 Co centers)11,65 via surface capping that blocks further cluster growth and (ii) to serve as a basic acceptor of those protons being released during the OER.11 The Co:P ratio is ca. 2:1.63 In neutral and only slightly basic solutions and in the absence of suitable proton-accepting electrolytes, the metal oxide itself would serve as the most basic sites in the reaction mixture, resulting in protonation-induced catalyst film dissolution.11,61

During the OER the octahedrally oxo-surrounded Co centers exist as Co2+, Co3+ (resting state), and Co4+.11,59,61,62 During the final reductive elimination step of the O2 molecule from the actual catalytically active Co2 edge sites of the cobaltate clusters,60 Co2+ centers are formed. Co2+ exhibits a d7 electron configuration and consequently monoelectronic population of the antibonding eg* orbital in an octahedral ligand environment is associated with these Co2+ centers being prone to ligand exchange and dissolution (replacement of bridging oxides or hydroxides in the solid vs water) if not rapidly reoxidized to Co3+.11

Self-healing of the active CoPi film occurs via reassembly of the dissolved [Co(OH)(H2O)5]+ ions by a disproportionation process.11 At the applied potential (Eapp) for the OER a significant amount of Co(IV) centers is present in the catalyst film. The presence of Co(IV) centers at potentials 0.2 V lower than Eapp represents the actual “trick” of CoPi self-healing as reassembly of Co2+ ions from solution results in the formation of two substitutionally inert Co3+ centers.

Due to its remarkable self-healing properties under neutral conditions and ion-containing water, the CoPi-based water oxidation has been interfered with several biological systems. The OER provided electrons not only for the CoP-catalyzed formation of H2 which later was consumed by Ralstonia eutropha for CO2 assimilation yielding biomass and fusel alcohols5 but also for the ambient N2 fixation using Xanthobacter autotrophicus.6 This solar-powered nitrogen fixation process served as a fertilizing system for the improved growth of radishes.

In contrast to the Co-based catalysts which typically show self-healing at pH values greater than 5.2, the related Mn-OECs are self-healing even in a very acidic environment (up to pH = −0.5). Catalyst dissolution likely occurs via high-spin d5 Mn(II) centers, but reoxidation to Mn(IV) again occurs at potentials lower than that used for OER.66 NiBi systems have also been reported as self-healing OER catalysts between pH 9 and 14.67 They outperform CoPi-OECs at basic pH, but CoPi remains the superior catalyst under neutral conditions.

Finally, in contrast to these monometallic systems described above, mixed-metal oxide OECs have been investigated as well. The idea behind these mixed systems is that either a structurally robust metal oxide is combined with a catalytically highly active species leading to both high activity and high stability68 or that several catalytically active centers beneficially influence each other.11 High robustness for OER has, for example, been shown in the case of NiFeOx films deposited from 2 M carbonate solutions69 and NiCoFe-Bi systems70 as well as NiFe-OECs assembled on BiVO4, where self-healing was assigned to site-specific redeposition of the active NiFe catalyst at point defects on bare BiVO4.71

4. Concluding Remarks

Various strategies for the repair of molecular- as well as material-based catalysts have been highlighted. In order to design rational repair and healing processes or even a complete inhibition of catalyst degradation for a specific catalytic system, the identification of the molecular processes leading to ceasing of catalytic turnover is necessary. The currently most advanced systems exhibit autonomous additive-free self-healing properties. This is due to the fact that under operating conditions the thermodynamically most stable configuration of the systems represents at the same time the active catalyst. On the molecular level this is true for different POM-based catalysts utilized for various oxidation reactions,37−40 whereas on the material level a larger number of self-healing systems are known: i.e., MonOxSy-based catalysts for photoelectrochemical hydrogen evolution,57 Co3O4 NRs for FTS,58 and various (mixed) metal oxides serving as electrocatalysts for water oxidation under different conditions.11 However, also in the case of systems where human intervention is necessary to restart catalysis, i.e., a stimuli-induced active repair process has to be performed to temporarily alter the conditions to which the catalyst is exposed, potentials are far from being exhausted. For such systems where the thermodynamics prohibit continuous in operando self-healing and limited longevity is only obtained by kinetic stabilization of the catalytically active configuration, technically simple and economically viable repair strategies can represent solutions for the problem of gradual catalyst degradation.

Therefore, further research on rational repair and healing strategies is highly recommended; as technically feasible repair processes offer the reactivation of the catalyst and its use for a second, third, fourth, and nth run, it will save precious resources. Only if the development of straightforward repair processes or self-healing (molecular) artificial photosynthetic schemes is successful will these systems come closer to their urgently needed real-life applications.

Promising strategies to also bring systems other than the self-healing water oxidizing catalysts closer to everyday life applications are, for example, the research on POMs to discover further molecular metal oxides combining high catalytic activity and the intriguing property that the catalytically active species represents the structural thermodynamic sink of the system. Furthermore, also the design of sophisticated but chemically stable substrates offering catalytic-activity-boosting binding sites for cost-efficient metal centers is highly interesting as well. The repair has been shown to proceed efficiently via remetalation in many systems. Also, the application of cheap oxidants or reductants to catalytic mixtures seems to be an important aspect for further developments in the field. As shown for e.g. the CoPi system (Scheme 5), RuPt1 (Scheme 4) or [(xantphos)Pd] (Scheme 3), (in situ) redox-chemistry-driven reintegration of the deactivated species into the catalytic cycle had a tremendous effect on the overall obtained catalytic output. To come closer to real-life applications it is a fact that, as in natural photosynthesis, also artificial systems need to have integrated repair or self-healing concepts that deal with the omnipresent redox changes during water oxidation/reduction and the associated instability/deactivation problems resulting thereof. Structurally simple or chemically robust catalysts may assist this development by limiting the number of possible degradation products that need to be considered in the design of suitable repair/self-healing strategies.

Acknowledgments

Financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)–Projektnummer 364549901, TRR 234 [A1]–is gratefully acknowledged.

Glossary

Abbreviations

BL

bridging ligand

bpp

benzo[b]pyrido[3,2-f][1,7]-phenanthroline

bpy

2,2′-bipyridine

dmgH2

dimethylglyoxime

EDTA

ethylenediaminetetraacetic acid

FTS

Fischer–Tropsch synthesis

MeCN

acetonitrile

MOF

metal–organic framework

NP

nanoparticle

NR

nanorod

OEC

oxygen-evolving complex

OER

oxygen evolution reaction

PCET

proton-coupled electron transfer

PMHS

polymethylhydrosiloxane

POM

polyoxometalate

PS

photosystem

ppy

2-phenylpyridine

py

pyridine

RCM

ring-closing metathesis

tbbpy

4,4′-tert-butyl-2,2′-bipyridine

tBuOOH

tert-butyl hydroperoxide

TEOA

triethanolamine

TON

turnover number

tpphz

tetrapyridophenazine

tpy = 2,2′:6′

2″-terpyridine.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. CRediT: Alexander Klaus Mengele project administration, writing-original draft, writing-review & editing; Sven Rau conceptualization, funding acquisition, project administration, resources, supervision, writing-review & editing.

The authors declare no competing financial interest.

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