Dimensionality dependent water splitting mechanisms on free manganese oxide clusters

Cluster size dependent coordination of formate to free manganese oxide clusters

The interaction of free manganese oxide clusters, MnxOy+ (x = 1-9, y = 0-12), with formic acid was studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO2+ and MnO4+, bind formic acid as an intact molecule in both the cis- and trans-configuration. In contrast, all clusters containing two or more manganese atoms deprotonate the acid's hydroxyl group. The coordination of the resulting formate group is strongly cluster-size-dependent according to supporting DFT calculations for selected model systems. For Mn2O2+ the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for Mn2O4+ the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for Mn4O4+. This binding motif stabilizes the 2D ring structure of the core of the Mn4O4+ cluster with respect to the 3D cubic geometry of the Mn4O4+ cluster core.

Water activation and splitting by single anionic iridium atoms

Mass spectrometric analysis of anionic products that result from interacting Ir^- with H₂O shows the efficient generation of [Ir(H₂O)]^- complexes and IrO^- molecular anions. Anion photoelectron spectra of [Ir(H₂O)]^-, formed under various source conditions, exhibit spectral features that are due to three different forms of the complex: the solvated anion-molecule complex, Ir^-(H₂O), as well as the intermediates, [H-Ir-OH]^- and [H₂-Ir-O]^-, where one and two O-H bonds have been broken, respectively. The measured and calculated vertical detachment energy values are in good agreement and, thus, support identification of all three types of isomers. The calculated reaction pathway shows that the overall reaction Ir^- + H₂O → IrO^- + H₂ is exothermic. Two minimum energy crossing points were found, which shuttle intermediates and products between singlet and triplet potential surfaces. This study presents the first example of water activation and splitting by single Ir^- anions.

Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric CaxMn5-xO5+ Clusters

We report on the first preparation of isolated ligand-free CaMn₄O₅⁺ gas-phase clusters, as well as other pentameric Ca_xMn_5-xO₅⁺ (x = 0-4) clusters with varying Ca contents, which serve as molecular models of the natural CaMn₄O₅ inorganic cluster in photosystem II. Ion trap reactivity studies with D₂O and H₂¹⁸O reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling. The critical coupling reaction step entails a single electron transfer from the oxyl radical to the accommodating cluster core with a concurrent O/OH coupling forming an adsorbed OOH intermediate group. The spin-conserving electron transfer step takes place when the spin of the transferred electron is aligned with the spins of the d-electrons of the Mn atoms in the cuboidal high-spin cluster isomer. The d-electrons provide a ferromagnetically ordered environment that facilitates the spin-gated selective electron transfer process, resulting in parallel-spin-exchange stabilization and a lowered transition state barrier for the coupling reaction involving the frontier orbitals of the oxyl and hydroxy reactant intermediates.

Water Splitting by C60 -Supported Vanadium Single Atoms

Water splitting is an important source of hydrogen, a promising future carrier for clean and renewable energy. A detailed understanding of the mechanisms of water splitting, catalyzed by supported metal atoms or nanoparticles, is essential to improve the design of efficient catalysts. Here, we report an infrared spectroscopic study of such a water splitting process, assisted by a C₆₀ supported vanadium atom, C₆₀ V⁺ +H₂ O→C₆₀ VO⁺ +H₂ . We probe both the entrance channel complex C₆₀ V⁺ (H₂ O) and the end product C₆₀ VO⁺ , and observe the formation of H₂ as a result from resonant infrared absorption. Density functional theory calculations exploring the detailed reaction pathway reveal that a quintet-to-triplet spin crossing facilitates the water splitting reaction by C₆₀ -supported V⁺ , whereas this reaction is kinetically hindered on the isolated V⁺ ion by a high energy barrier. The C₆₀ support has an important role in lowering the reaction barrier with more than 70 kJ mol^-1 due to a large orbital overlap of one water hydrogen atom with one carbon atom of the C₆₀ support. This fundamental insight in the water splitting reaction by a C₆₀ -supported single vanadium atom showcases the importance of supports in single atom catalysts by modifying the reaction potential energy surface.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H₂O into CO₂ and H₂ is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh₂MnO_1,2^- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H₂O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh₂MnO_1,2^- is paramount for the generation of an electron-rich Mn⁺-Rh^- bond that is critical to capture and reduce H₂O and giving rise to a polarized Rh⁺-Rh^- bond that functions as the real redox center to drive the WGS reaction.

Size, Stoichiometry, Dimensionality, and Ca Doping of Manganese Oxide-Based Water Oxidation Clusters: An Oxyl/Hydroxy Mechanism for Oxygen-Oxygen Coupling

Gas-phase ion-trap reactivity experiments and density functional simulations reveal that water oxidation to H₂O₂ mediated by (calcium) manganese oxide clusters proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O-O coupling. This mechanism is predicted to be energetically feasible for Mn₂O_y⁺ (y = 2-4) and the binary CaMn₃O₄⁺, in agreement with the experimental observations. In contrast, the reaction does not proceed for the tetramanganese oxides Mn₄O_y⁺ (y = 4-6) under these experimental conditions. This is attributed to the high fluxionality of the tetramanganese clusters, resulting in the instability of the terminal oxyl radical as well as an energetically unfavorable change of the spin state required for H₂O₂ formation. Ca doping, yielding a symmetry-broken lower-symmetry three-dimensional (3D) CaMn₃O₄⁺ cluster, results in structural stabilization of the oxyl radical configuration, accompanied by a favorable coupling between potential energy surfaces with different spin states, thus enabling the cluster-mediated water oxidation reaction and H₂O₂ formation.

Infrared Spectroscopy of Gas-Phase MnxOy(CO2)z+ Complexes

The interaction of manganese oxide clusters Mn_xO_y⁺ (x = 2-5, y ≥ x) with CO₂ is studied via infrared multiple-photon dissociation spectroscopy (IR-MPD) in the spectral region of 630-1860 cm^-1. Along with vibrational modes of the manganese oxide cluster core, two bands are observed around 1200-1450 cm^-1 and they are assigned to the characteristic Fermi resonance of CO₂ arising from anharmonic coupling between the symmetric stretch vibration and the overtone of the bending mode. The spectral position of the lower frequency band depends on the cluster size and the number of adsorbed CO₂ molecules, whereas the higher frequency band is largely unaffected. Despite these effects, the observation of the Fermi dyad indicates only a small perturbation of the CO₂ molecule. This finding is confirmed by the theoretical investigation of Mn₂O₂(CO₂)⁺ revealing only small orbital mixing between the dimanganese oxide cluster and CO₂, indicative of mainly electrostatic interaction.

Infrared photodissociation spectroscopy of di-manganese oxide cluster cations

Infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations have been employed to elucidate the geometric structure of a series of di-manganese oxide clusters Mn₂O_x⁺ (x = 4-7). The theoretical exploration predicts that all investigated clusters contain a rhombus-like Mn₂O₂ core with up to four, terminally bound, oxygen atoms. The short Mn-O bond length of the terminal oxygen atoms of ≤1.58 Å indicates triple bond character instead of oxyl radical formation. However, the IR-MPD spectra reveal that higher energy isomers with up to two O₂ molecules η²-coordinated to the cluster core can be kinetically trapped under the given experimental conditions. In these complexes, all O₂ units are activated to superoxide species. In addition, the sequential increase of the oxygen content in the cluster allows for a controlled increase of the positive charge localized on the Mn atoms reaching a maximum for Mn₂O₇⁺.

Hydrophilicity and oxophilicity of the isolated CaMn4O5 cationic cluster modeling inorganic core of the oxygen-evolving complex

Isolated CaMn₄O₅⁺ was hardly formed in the presence of oxygen, whereas CaMn₄O₄(OH)₂⁺ was formed stably in the presence of water.

Cluster size and composition dependent water deprotonation by free manganese oxide clusters

Vibrational spectroscopy and first-principles calculations reveal basic concepts of the interaction between manganese oxide clusters and water which could aid the future design of artificial water-splitting molecular catalysts.

Uncovering structure-activity relationships in manganese-oxide-based heterogeneous catalysts for efficient water oxidation

Artificial photosynthesis by harvesting solar light into chemical energy could solve the problems of energy conversion and storage in a sustainable way. In nature, CO2 and H2 O are transformed into carbohydrates by photosynthesis to store the solar energy in chemical bonds and water is oxidized to O2 in the oxygen-evolving center (OEC) of photosystem II (PS II). The OEC contains CaMn4 O5 cluster in which the metals are interconnected through oxido bridges. Inspired by biological systems, manganese-oxide-based catalysts have been synthesized and explored for water oxidation. Structural, functional modeling, and design of the materials have prevailed over the years to achieve an effective and stable catalyst system for water oxidation. Structural flexibility with eg(1) configuration of Mn(III) , mixed valency in manganese, and higher surface area are the main requirements to attain higher efficiency. This Minireview discusses the most recent progress in heterogeneous manganese-oxide-based catalysts for efficient chemical, photochemical, and electrochemical water oxidation as well as the structural requirements for the catalyst to perform actively.