![SC-XRD structures of the Ni II 2{Ni II 4[Cu II 2(Me3mpba)2]3}·54H2O (a) and the {Ca II Cu II 6[(S,S)-methox]3(OH)2(H2O)} (b) 3D networks. Copper, nickel and calcium atoms from the coordination networks are represented by cyan, pink and dark blue polyhedral whereas nickel cations and sulphur atoms within the pores are depicted by pink and blue spheres.](https://www.researchgate.net/profile/Emilio-Pardo/publication/339084733/figure/fig3/AS:870082867642372@1584455329992/SC-XRD-structures-of-the-Ni-II-2Ni-II-4Cu-II-2Me3mpba2354H2O-a-and-the-Ca-II-Cu_Q320.jpg)
![(a) Views of the crystal structures of Ni II 2{Ni II 4[Cu II 2(Me3mpba)2]3}·54H2O (left) and the resulting [M(H2O)6] 3+ -MOF (M = Fe and Ru) after the cation exchange. (b and c) Views of one channel of [M(H2O)6] 3+ -MOF emphasizing the stabilizing interactions (dotted lines). Copper(II) and nickel(II) cations from the coordination network are represented by cyan and blue polyhedra, respectively, whereas the ligands are depicted as gray sticks. M 3+ cations and water molecules hosted in the channels are represented by gold and red spheres with surface, respectively. Adapted with permission from refs. 39 and 40. Copyright 2018 ACS.](https://www.researchgate.net/profile/Emilio-Pardo/publication/339084733/figure/fig5/AS:870082871836672@1584455330104/a-Views-of-the-crystal-structures-of-Ni-II-2Ni-II-4Cu-II-2Me3mpba2354H2O-left_Q320.jpg)
![(Top) Acetylene (blue squares) and ethane (red rhombuses) amounts in ppm during the continuous hydrogenation of acetylene (1.2%) in an ethylene flow catalyzed by [Fe(H2O)6] 3+ -MOF at 150 C. (Bottom) Reaction scheme for the hydrogenation of acetylene (A and B) with a plausible mechanism (C). Adapted with permission from ref. 39. Copyright 2018 ACS.](https://www.researchgate.net/profile/Emilio-Pardo/publication/339084733/figure/fig6/AS:870082871840768@1584455330440/Top-Acetylene-blue-squares-and-ethane-red-rhombuses-amounts-in-ppm-during-the_Q320.jpg)
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Metal–Organic Frameworks as Chemical Nanoreactors: Synthesis and Stabilization of Catalytically Active Metal Species in Confined Spaces
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Since the advent of the first metal-organic frameworks (MOFs), we have witnessed an explosion of captivating architectures with exciting physicochemical properties and applications in a wide range of fields. This, in part, can be understood under the light of their rich host-guest chemistry and the possibility to use single-crystal X-ray diffraction (SC-XRD) as a basic characterization tool. Moreover, chemistry on preformed MOFs, applying recent developments in template-directed synthesis and postsynthetic methodologies (PSMs), has shown to be a powerful synthetic tool to (i) tailor MOFs channels of known topology via single-crystal to single-crystal (SC-SC) processes, (ii) impart higher degrees of complexity and heterogeneity within them, and most importantly, (iii) improve their capabilities toward applications with respect to the parent MOFs. However, the unique properties of MOFs have been, somehow, limited and underestimated. This is clearly reflected on the use of MOFs as chemical nanoreactors, which has been barely uncovered. In this Account, we bring together our recent advances on the construction of MOFs with appealing properties to act as chemical nanoreactors and be used to synthesize and stabilize, within their channels, catalytically active species that otherwise could be hardly accessible. First, through two relevant examples, we present the potential of the metalloligand approach to build highly robust and crystalline oxamato- and oxamidato-MOFs with tailored channels, in terms of size, charge and functionality. These are initial requisites to have a playground where we can develop and fully take advantage of singular properties of MOFs as well as visualize/understand the processes that take place within MOFs pores and somehow make structure-functionalities correlations and develop more performant MOFs nanoreactors. Then, we describe how to exploit the unique and singular features that offer each of these MOFs confined space for (i) the incorporation and stabilization of metals salts and complexes, (ii) the in situ stepwise synthesis of subnanometric metal clusters (SNMCs), and (iii) the confined-space self-assembly of supramolecular coordination complexes (SCCs), by means of PSMs and underpinned by SC-XRD. The strategy outlined here has led to unique rewards such as the highly challenging gram-scale preparation of stable and well-defined ligand-free SNMCs, exhibiting outstanding catalytic activities, and the preparation of unique SCCs, different to those assembled in solution, with enhanced stabilities, catalytic activities, recyclabilities, and selectivities. The results presented in this Accounts are just a few recent examples, but highly encouraging, of the large potential way of MOFs acting as chemical nanoreactors. More work is needed to found the boundaries and fully understand the chemistry in the confined space. In this sense, mastering the synthetic chemistry of discrete organic molecules and inorganic complexes has basically changed our way of live. Thus, achieving the same degree of control on extended hybrid networks will open new frontiers of knowledge with unforeseen possibilities. We aim to stimulate the interest of researchers working in broadly different fields to fully unleash the host-guest chemistry in MOFs as chemical nanoreactors with exclusive functional species.
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... Oxamate ligands constitute a remarkable family of ligands for the preparation of appealing molecular materials, such as sieves, sensors or nanoreactors. [1][2][3] They also play an important role in the design and synthesis of molecular magnetic materials, [4][5][6][7] successfully leading to the preparation of singlemolecule magnets, 8 spintronic candidates, 9 single-chain magnets, 10 supramolecular magnets, 11,12 2D/3D coordination network magnets, 13,14 and porous magnets. [15][16][17] This ability the oxamate ligands have in facilitating the preparation of magnetic molecular materials originates from the unique combination of two features. ...
... 30,31 To increase the dimensionality of our edifices, we have naturally moved to a parasubstitued phenyloxamate, and started to investigate the 4-(ethyloxamate)benzoic acid, H 2 Et-paba. We present here its use in solvothermal conditions and in the presence of copper(II) to prepare the heterobimetallic (TMA) 3 [CuMn(paba) 2 (OAc)]·8H 2 O (1) 2D coordination network. ...
... Further washing was done with cold 96% ethanol and the solid was dried with ether and then in air. The final product was collected as a white powder (9.41 g, Yield: 90.7%, M=237.2 gmol -1 ). 1 Synthesis of (TMA) 3 2 .4H 2 O (0.5 mmol, 122 mg) and 0.36 mL of TMAOH (25% wt. in water, 1 eq.) were placed in a 23 mL Teflon-lined autoclave reactor filled with DMF (4 mL). The reactor was heated to 120 °C in 2 h, kept at this temperature for 12 h and then cooled down to room temperature in 10 h. Green crystals of 1 were collected and washed with EtOH for single-crystal X-ray diffraction studies. ...
... Metal organic frameworks (MOFs), an intriguing class of inorganic-organic hybrid porous crystalline materials, have recently showed tremendous promise in these well-known applications such as environmental remediation, adsorption and separation, drug delivery, sensing, catalysis, and so on [1][2][3][4][5][6][7][8][9][10][11][12]. For example, some MOFs could be used not only as adsorbents for the decolorization of dye-containing wastewater due to the presence of permanent porosity and/ or special functional groups but also as catalysts to degrade inorganic or organic pollutants due to the reversible redox activity [13][14][15][16][17]. Generally, the judicious selection of both metal center and organic linker is very key for synthesizing various MOFs with excellent catalytic performance [18,19]. ...
... For example, some MOFs could be used not only as adsorbents for the decolorization of dye-containing wastewater due to the presence of permanent porosity and/ or special functional groups but also as catalysts to degrade inorganic or organic pollutants due to the reversible redox activity [13][14][15][16][17]. Generally, the judicious selection of both metal center and organic linker is very key for synthesizing various MOFs with excellent catalytic performance [18,19]. The Fe-MOFs using earth-abundant, low-cost, non-toxic and life-essential element Fe (including Fe(II)/ Fe(III)) as the metal centers have been recognized as one of the most promising MOF-based photocatalysts and/or Fenton-like catalysts because of their obvious advantages including: (1) superior photocatalytic and/or Fenton-like catalytic efficiency, (2) green and safe to human body and environment [17][18][19][20]. The literature survey found that most of the reported Fe-MOFs were constructed from organic linkers containing carboxylate groups possibly due to strong coordination ability and diverse coordination modes [4,19,20]. ...
A two-dimensional (2D) pillared-bilayer iron-based metal–organic framework (MOF), [Fe2(aip)2(bpy)2]·(H2O)(DMF) (1) (where H2aip = 5-aminoisophthalic acid, bpy = 4,4'-bipyridine, DMF = N,N-dimethylformamide), was solvo/hydrothermally synthesized using H2aip, bpy and ferrous sulfate heptahydrate as raw materials in DMF/H2O mixed solvent. The structure and properties of 1 were characterized by single-crystal X-ray diffraction, powder X-ray diffraction, elemental analysis, thermogravimetric analysis, infrared spectroscopy and UV–vis diffuse reflectance spectroscopy. In 1, each Fe(II) ion exhibits a distorted octahedral geometry, while the aip²⁻ and bpy ligands adopt the chelating/bridging bis-bidentate (μ3-η¹:η¹:η²) and bridging bidentate (μ2-η¹:η¹) coordination modes, respectively. Infinite one-dimensional [Fe2(aip)2]n chains extending along the a axis are formed through bridging and chelating carboxylate groups of aip²⁻ ligands and are further cross-linked by bpy ligands along the b axis to generate an infinite 2D pillared-bilayer framework with an interdigitated structure. Fe-MOF 1 shows both UV-light photocatalytic activity and Fenton-like catalytic activity toward the degradation of rhodamine B solution, which can be potentially applied in environmental remediation such as industrial wastewater treatment.
Since the emergence of metal-organic frameworks (MOFs), a myriad of thrilling properties and applications, in a wide range of fields, have been reported for these materials, which mainly arise from their porous nature and rich host-guest chemistry. However, other important features of MOFs that offer great potential rewards have been only barely explored. For instance, despite the fact that MOFs are suitable candidates to be used as chemical nanoreactors for the preparation, stabilization and characterization of unique functional species, that would be hardly accessible outside the functional constrained space offered by MOF channels, only very few examples have been reported so far. In particular, we outline in this feature recent advances in the use of highly robust and crystalline oxamato- and oxamidato-based MOFs as reactors for the in situ preparation of well-defined catalytically active single atom catalysts (SACS), subnanometer metal nanoclusters (SNMCs) and supramolecular coordination complexes (SCCs). The robustness of selected MOFs permits the post-synthetic (PS) in situ preparation of the desired catalytically active metal species, which can be characterised by single-crystal X-ray diffraction (SC-XRD) taking advantage of its high crystallinity. The strategy highlighted here permits the always challenging large-scale preparation of stable and well-defined SACs, SNMCs and SCCs, exhibiting outstanding catalytic activities.
In this work, we describe the synthesis, crystal structure and magnetic properties of the neutral hexacopper(II) complex of formula {[Cubpca)]2[Cu(dmopba)(H2O)]}2·4H2O (1), where Hbpca = bis(2-pyridylcarbonyl)-amide and dmopba = 4,5-dimethyl-1,2-phenylenebis(oxamato). Single crystals of 1 were obtained from the stoichiometric reaction (1:2 molar ratio) of the mononuclear copper(II) complexes (n-Bu4N)2[Cu(dmpba)] and [Cu(bpca)(H2O)2]NO3·2H2O through slow diffusion techniques in water as a solvent. The crystal structure of 1 shows that two neutral {[Cu(bpca)]2[Cu(dmopba)(H2O)]} trinuclear units are connected through double out-of-plane copper to outer carboxylate oxygen atoms resulting in a unique oxamate-bridged linear hexanuclear complex. Hydrogen bonds among adjacent entities involving the non-coordinated water molecules result in a supramolecular 3D network. Magnetic measurements on 1 show the occurrence of moderate antiferromagnetic intratrinuclear interactions between the copper(II) ions from the [Cu(bpca)]+ and [Cu(dmopba)(H2O)]2– fragments across the oxamate bridge and a weak intertrinuclear ferromagnetic interaction between the copper(II) ions that occurs between the two central [Cu(bpca)]+ fragments mediated by the carboxylate groups from the oxamate bridge [J = –31.96(2) cm–1 and J′ = +1.34(2) cm−1; H = J (S1·S2 + S2·S3 + S1’·S2’ + S2’·S3’) + J′ (S1·S1’)].
Photocatalysis is a fundamental and promising strategy for solar energy conversion. Photocatalytic reactions are divided into three basic steps: light absorption, photogenerated charge separation and migration, and surface redox reactions. Generally speaking, nanoreactor is a reactor based on atomic, nanometer or submicrometer scales designed to meet practical application, e.g., [email protected] microstructure. Rational design of nanoreactors for photocatalytic reactions is of great importance to effectively enhance the photocatalytic activity. In this review, the nanoreactors include metal [email protected] [email protected] structure nanoreactors, metal [email protected] [email protected] structure nanoreactors as well as covalent–organic framework/covalent–organic frameworks-based nanoreactors are summarized. Firstly, this article briefly reviews the general synthesis strategies of nanoreactors. Secondly, various strategies for enhancing the photocatalytic performance of nanoreactors are summarized and discussed, including metal [email protected] [email protected] structure nanoreactors, metal [email protected] [email protected] structure nanoreactors and covalent–organic frameworks/covalent–organic frameworks-based nanoreactors. Furthermore, some metal–organic frameworks/metal–organic frameworks-based nanoreactors are also discussed. Finally, the challenges and future directions of nanoreactors for photocatalysis are proposed.
Metal–organic frameworks (MOFs) as a class of unique porous crystalline materials have developed rapidly in the past decades. The unique features of component diversities, structural designability, and tunable porosities of MOFs endow them inherent superiorities as hosts to accommodate diverse species of guests. The resulting host–guest MOFs reveal a variety of properties and applications, which could be readily modulated through the tuning of the host, the guest, and the host–guest interactions. Therefore, the host–guest MOFs have attracted much attention and become a hot topic for research. This review will concentrate on the recent progress of host–guest MOFs. The achievements and understanding of host–guest MOF, including the components of host–guest systems, the types of species interactions, and the characterization methods have been reviewed and discussed. Based on this, the design and tuning principles for the construction of host–guest MOFs have been highlighted. The recently typical examples of function-oriented construction of host–guest MOF for various properties including optical, electrical, and catalytic properties have been overviewed. Furthermore, the challenges and perspectives of developing host–guest MOF materials have been generalized.
Nanosized metal aggregates (MAs), including metal nanoparticles (NPs) and nanoclusters (NCs), are often the active species in numerous applications. In order to maintain the active form of MAs in "use", they need to be anchored and stabilised, preventing agglomeration. In this context, metal-organic frameworks (MOFs), which exhibit a unique combination of properties, are of particular interest as a tunable and porous matrix to host MAs. A high degree of control in the synthesis towards atom-efficient and application-oriented MA@MOF composites is required to derive specific structure-property relationships and in turn to enable design of functions on the molecular level. Due to the versatility of MA@MOF (derived) materials, their applications are not limited to the obvious field of catalysis, but increasingly include 'out of the box' applications, for example medical diagnostics and theranostics, as well as specialised (bio-)sensoring techniques. This review focuses on recent advances in the controlled synthesis of MA@MOF materials en route to atom-precise MAs. The main synthetic strategies, namely 'ship-in-bottle', 'bottle-around-ship', and approaches to achieve novel hierarchical MA@MOF structures are highlighted and discussed while identifying their potential as well as their limitations. Hereby, an overview of standard characterisation methods that enable a systematic analysis procedure and state-of-art techniques that localise MA within MOF cavities are provided. While the perspectives of MA@MOF materials in general have been reviewed various times in the recent past, few atom-precise MAs inside MOFs have been reported so far, opening opportunities for future investigation.
Space confined reactions have emerged as a viable strategy for achieving important and fascinating properties in functional materials. Various scaffolds have been reported so far for confinement and it gives rise to the phenomenon of nanoconfinement, where the energetics and kinetics of catalytic reactions can be modulated upon confining the catalysts in a particular site. Although various systems have been reported so far for confinement, emphasis has been placed on the concept of space confinement, and the changes in the confined space itself are neglected. Strikingly, this critical issue would be touched on and revealed by supercritical CO2 (SC CO2) that is used in confined geometries. Herein, we define the structural changes of confined spaces induced by SC CO2 as an anti-nanoconfinement effect, which can bring about a series of variations together with electronic band and structural transformation. Moreover, progress in the design and applications of the anti-nanoconfinement effect is traced, and there is a discussion of emerging issues that have yet to be explored to achieve a future direction to develop more novel two-dimensional (2D) structures.
We report a new water-stable multivariate (MTV) Metal-Organic Framework (MOF) prepared by combining two different oxamide-based metalloligands derived from the natural amino acids L-serine and L-methionine. This unique material features hexagonal channels decorated with two types of flexible and functional “arms” (–CH2OH and –CH2CH2SCH3) capa-ble to act, synergistically, for the simultaneous and efficient removal of both inorganic (heavy metals like Hg2+, Pb2+ and Tl+) and organic (dyes such as Pyronin Y, Auramine O, Brilliant Green and Methylene Blue) contaminants and, in addi-tion, this MTV-MOF is completely reusable. Single-crystal X-ray diffraction (SCXRD) measurements allowed to solve the crystal structure of a host-guest adsorbate, containing both HgCl2 and Methylene Blue, and offered unprecedented snap-shots about this unique dual capture process. This is the very first time that a MOF can be used for the removal of all sorts of pollutants from water resources, thus opening new perspectives for this emerging type of MTV-MOFs.
Subnanometric metal species (single atoms and clusters) have been demonstrated to be unique compared with their nanoparticulate counterparts. However, the poor stabilization of subnanometric metal species towards sintering at high temperature (>500 °C) under oxidative or reductive reaction conditions limits their catalytic application. Zeolites can serve as an ideal support to stabilize subnanometric metal catalysts, but it is challenging to localize subnanometric metal species on specific sites and modulate their reactivity. We have achieved a very high preference for localization of highly stable subnanometric Pt and PtSn clusters in the sinusoidal channels of purely siliceous MFI zeolite, as revealed by atomically resolved electron microscopy combining high-angle annular dark-field and integrated differential phase contrast imaging techniques. These catalysts show very high stability, selectivity and activity for the industrially important dehydrogenation of propane to form propylene. This stabilization strategy could be extended to other crystalline porous materials.
Supramolecular Coordination Compounds (SCCs) represent the power of Coordination Chemistry methodologies to self-assemble discrete architectures with targeted properties. SCCs are generally synthesised in solution, with isolat-ed fully-coordinated metal atoms as structural nodes, thus severely limited as metal-based catalysts. Metal-Organic Frameworks (MOFs) show unique features to act as chemical nanoreactors for the in-situ synthesis and stabilization of otherwise not accessible functional species. Here, we present the self-assembly of PdII SCCs within the confined space of a preformed MOF ([email protected]) and its post-assembly metalation to give a PdII-AuIII supramolecular assembly, crys-tallography underpinned. These [email protected] catalyse the coupling of boronic acids and/or alkynes, representative multisite metallic-catalysed reactions in which traditional SCCs tend to decompose, and retain its structural integrity as consequence of the synergetic hybridization between SCCs and MOF. These results open new avenues in both the synthesis of novel SCCs and their use on heterogeneous metal-based Supramolecular Catalysis.
Catalysis has always been part of the development of mankind; from the fermentation of alcoholic drinks, through the development of fertilisers in the agricultural revolution and production of bulk chemicals in the 20th Century. Today, society demands improved production routes with greater product output and energy efficiency; the ultimate goal to achieving this would be having all catalytic reactions in concert, effectively functioning like a biological cell.
Metal organic frameworks (MOFs) are a relatively new type of hybrid material. Their crystalline porous structure, built up from organic and inorganic building blocks, presents a vast array of composition, porosity and functionality offering enormous potential in catalytic systems.
This book examines the latest research and discovery in the use of MOFs in catalysis, highlighting the extent to which these materials have been embraced by the community. Beyond presenting a digest of recent research by major players in the field, the book presents the strategies behind recent developments, providing a lasting reference for seasoned researchers and newcomers to the field.
The synthesis and reactivity of single metal atoms in low–valence state bound to just water, rather than to organic ligands or surfaces, is a major experimental challenge. Here, we show a gram–scale wet synthesis of Pt11+ stabilized in a confined space by a well–defined crystallographic first water sphere, and with a second coordination sphere linked to the Metal–Organic Framework (MOF) through electrostatic and H–bonding interactions. The role of the water cluster is not only isolating and stabilizing the Pt atoms, but also regulating the charge of the metal and the adsorption of reactants. This is shown for the low–temperature water–gas shift reaction (WGSR: CO + H2O → CO2 + H2), where both metal coordinated and H–bonded water molecules trigger a double water attack mechanism to CO and give CO2 with both oxygen atoms coming from water. The stabilized Pt1+ single sites allow performing the WGSR at temperatures as low as 50 ºC.
Imines are ubiquitous intermediates in organic synthesis and the metal–mediated imination of alcohols is one of the most direct and simple method for their synthesis. However, reported protocols lack compatibility with many other functional groups since basic supports/media, pure oxygen atmospheres and/or released hydrogen gas are required during reaction. Here we show that, in contrast to previous metal–catalyzed methods, hexa–aqueous Ru(III) catalyzes the imination of primary alcohols with very wide functional group tolerance, at slightly acid pH and under low oxygen atmos-pheres. The inorganic metal complex can be supported and stabilized, integrally, within either a faujasite–type zeolites (Y and X) or a metal organic framework (MOF), to give a reusable heterogeneous catalysts which provides an industrially viable process well–below the flammability limit of alcohols and amines.
Beyond conventional porous materials, metal–organic frameworks (MOFs) have aroused great interest in the construction of nanocatalysts with the characteristics of catalytically active nanoparticles (NPs) confined into the cavities/channels of MOFs or surrounded by MOFs. The advantages of adopting MOFs as the encapsulating matrix are multifold: uniform and long‐range ordered cavities can effectively promote the mass transfer and diffusion of substrates and products, while the diverse metal nodes and tunable organic linkers may enable outstanding synergy functions with the encapsulated active NPs. Herein, some key issues related to MOFs for catalysis are discussed. Then, state‐of‐the art progress in the encapsulation of catalytically active NPs by MOFs as well as their synergy functions for enhanced catalytic performance in the fields of thermo‐, photo‐, and electrocatalysis are summarized. Notably, encapsulation‐structured nanocatalysts exhibit distinct advantages over conventional supported catalysts, especially in terms of the catalytic selectivity and stability. Finally, challenges and future developments in MOF‐based encapsulation‐structured nanocatalysts are proposed. The aim is to deliver better insight into the design of well‐defined nanocatalysts with atomically accurate structures and high performance in challenging reactions. Compared with conventional supported catalysts, controllable encapsulation of catalytically active nanoparticles by porous metal–organic frameworks can exhibit intriguing features, such as uniform catalytic interface, strong interactions, the pore confinement effect, high stability, and outstanding designability, all of which contribute to the design and construction of high‐performance nanocatalysts.
We report the application of a post-synthetic solid-state cation-exchange process to afford a novel 3D MOF with hydrated barium cations hosted at pores able to trigger a selective and reversible...
Metal organic frameworks (MOFs) are a class of porous crystalline materials that feature a series of unique properties, such as large surface area and porosity, high content of transition metals, and possibility to be designed and modified after synthesis, that make these solids especially suitable as heterogeneous catalysts. The active sites can be coordinatively unsaturated metal ions, substituents at the organic linkers or guest species located inside the pores. The defects on the structure also create these open sites. The present review summarizes the current state of the art in the use of MOFs as solid catalysts according to the type of site, making special emphasis on the more recent strategies to increase the population of these active sites and tuning their activity, either by adapting the synthesis conditions or by post-synthetic modification. This review highlights those reports illustrating the synergy derived from the presence of more than one of these types of sites, leading to activation of a substrate by more than one site or to the simultaneous activation of different substrates by complementary sites. This synergy is frequently the main reason for the higher catalytic activity of MOFs compared to homogeneous catalysts or other alternative solid materials. Besides dark reactions, this review also summarizes the use of MOFs as photocatalysts emphasizing the uniqueness of these materials regarding adaptation of the linkers as light absorbers and metal exchange at the nodes to enhance photoinduced electron transfer, in comparison with conventional inorganic photocatalysts. This versatility and flexibility that is offered by MOFs to optimize their visible light photocatalytic activity explains the current interest in exploiting these materials for novel photocatalytic reactions, including hydrogen evolution and photocatalytic CO2 reduction.