Photosystem II (PSII) is the membrane-bound oxidoreductase peptide complex responsible for the oxidation of water to molecular oxygen and reduction of plastoquinone to plastoquinol. Primary electron transfer is initiated upon absorption of a photon by the primary donor chl resulting in electron transfer and production of a P680+QA charge separated state. P680+ is reduced by YZ (Y161 of the D1 polypeptide subunit), one of two redox-active tyrosine residues found in PSII. This produces a neutral tyrosyl radical (YZ ) which is subsequently reduced by electrons derived from water at the oxygen-evolving complex (OEC). The OEC is composed of four manganese, one calcium ion, and one chloride ion. Four photons are required to convert water to O2, each photon advancing the OEC through successive oxidation states or S states. The exact chemical mechanism of water oxidation in PSII is not known. However, proton-coupled electron transfer (PCET) is thought to be one of the fundamental steps in driving the extraction of electrons and protons from water. Here, the mechanism of water oxidation is investigated with focus on PCET events using vibrational spectroscopy. Vibrational spectroscopy is sensitive to changes in protein structure, charge, and hydrogen bonding, and is ideal for the study of fast events coupled with light-induced electron transfer. The results presented here demonstrate the utility of time-resolved infrared spectroscopy in the detection of intermediates of photosynthetic water oxidation. We suggest that proton transfer may precede manganese oxidation during water oxidation based on time-resolved infrared and difference FT-IR spectroscopic results. The mechanism of PCET associated with YZ reduction is investigated. Using reaction-induced difference FT-IR spectroscopy, the identity of the chloride binding site is speculated through the use of bromide exchange at the OEC. Also, proton transfer reactions at the OEC are investigated using azide as a vibrational probe. The advances in the understanding of photosynthetic water oxidation gained in this work will aid in the elucidation of the chemical mechanism of this important reaction. Understanding the details of photosynthetic water oxidation will assist in the development of technology aimed at harnessing the energy of the sun for the benefit of humankind.

No description available.

abstract: The utilization of solar energy requires an efficient means of its storage as fuel. In bio-inspired artificial photosynthesis, light energy can be used to drive water oxidation, but catalysts that produce molecular oxygen from water are required. This dissertation demonstrates a novel complex utilizing earth-abundant Ni in combination with glycine as an efficient catalyst with a modest overpotential of 0.475 ± 0.005 V for a current density of 1 mA/cm<super>2</super> at pH 11. The production of molecular oxygen at a high potential was verified by measurement of the change in oxygen concentration, yielding a Faradaic efficiency of 60 ± 5%. This Ni species can achieve a current density of 4 mA/cm<super>2</super> that persists for at least 10 hours. Based upon the observed pH dependence of the current amplitude and oxidation/reduction peaks, the catalysis is an electron-proton coupled process. In addition, to investigate the binding of divalent metals to proteins, four peptides were designed and synthesized with carboxylate and histidine ligands. The binding of the metals was characterized by monitoring the metal-induced changes in circular dichroism spectra. Cyclic voltammetry demonstrated that bound copper underwent a Cu(I)/Cu(II) oxidation/reduction change at a potential of approximately 0.32 V in a quasi-reversible process. The relative binding affinity of Mn(II), Fe(II), Co(II), Ni(II) and Cu(II) to the peptides is correlated with the stability constants of the Irving-Williams series for divalent metal ions. A potential application of these complexes of transition metals with amino acids or peptides is in the development of artificial photosynthetic cells. / Dissertation/Thesis / Doctoral Dissertation Biological Design 2014

Thesis advisor: Dunwei Wang / Atomically dispersed catalysts refer to substrate-supported heterogeneous catalysts featuring one or a few active metal atoms that are separated from one another. They represent an important class of materials ranging from single atom catalysts (SACs) and nanoparticles (NPs). The study of SACs has brought an attention of understanding the reaction mechanism at the molecular level. SACs is a promising field, however, there are still many challenges and opportunities in developing the next generation of catalysts. Catalysts featuring two atoms with well-defined structures as active sites are poorly studied. It is expected that this class of catalysts will show uniqueness in activity, selectivity, and stability. However, the difficulty in synthesizing such structures has been a critical challenge. I tackled this challenge by using a facile photochemical method to generate active metal centers consisting of two iridium metal atoms bridged by O ligands and bound to a support by stripping the ligands of the organometallic complex. My research also unveiled the structure of this dinuclear heterogeneous catalysts (DHCs) by integrating various characterization resources. Direct evidence unambiguously supporting the dinuclear nature of catalysts anchored on metal oxides is obtained by aberration-corrected scanning transmission electron microscopy. In addition, different binding modes have been achieved on two categories of metal oxides with distinguishable surface oxygen densities and interatomic distances of binding sites. Side-on bound DHCs was demonstrated on iron oxide and ceria where both Ir atoms are affixed to the surface with similar coordination environment. The binding sites on the OH-terminated surface of Fe2O3 and CeO2 anchor the catalysts to provide outstanding stability against detachment, diffusion and aggregation. The competing end-on binding mode, where only one Ir atom is attached to the substrate and the other one is dangling was observed on WO3. Evidence supporting the binding modes was obtained by in situ diffuse reflectance infrared Fourier transform spectroscopy. In addition, the synergistic effect between two adjacent Ir atoms and the uniqueness of different coordinative oxygen atoms around Ir atoms were investigated by a series of operando spectroscopy such as X-ray absorption spectroscopy and microscopy at atomic level under the reaction condition. The resulting catalysts exhibit high activities and stabilities toward H2O photo-oxidation and preferential CO oxidation. Density functional theory calculations provide additional support for atomic structure, binding sites modes on metal oxides, as well as insights into how DHCs may be beneficial for these catalytic reactions. This research has important implications for future studies of highly effective heterogeneous catalysts for complex chemical reactions. / Thesis (PhD) — Boston College, 2020. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

No description available.

Metal-organic frameworks (MOFs) have attracted much attention in the past few decades due to their ordered, crystalline nature, synthetic tunability, and porosity. MOFs represent a class of hybrid inorganic-organic materials that have been investigated for their applications in areas such as gas sorption and separation, catalysis, drug delivery, and electron or proton conduction. It has been the goal of my graduate research to investigate MOFs for their ability to transport electrons and store and separate gases for ultimate catalytic applications in alternative energy generation. I aim to provide new insight into the design and development of stable MOFs for such applications. We first investigated a cobalt(III) porphyrin based MOF comprised of Co(II)-carboxylate nodes, designated as CoPIZA, for its electron transport capabilities. Thin films of CoPIZA were formed solvothermally on conductive fluorine-doped tin oxide (FTO) substrates and used for electrochemical characterization. Electrochemistry coupled with spectroscopic analysis of the CoPIZA film revealed reversible reduction of the cobalt centers of the porphyrin linkers with maintenance of the overall framework structure. The mechanism of charge transport throughout the film was facilitated by redox hopping of electrons between the metal centers of the nodes and linkers. The ability to incorporate desired properties, such as pore functionalities or open metal centers, into frameworks makes them attractive for applications in separation of gaseous mixtures, such as CO2/N2 from combustion power plants. To investigate the selective adsorption properties, we performed gas sorption measurements on bulk MOF materials to determine their affinity toward CO2. Two Zn-based MOFs containing 2,5-pyridine dicarboxylate linkers were prepared in our laboratory and contained unsaturated Zn(II) metal centers, which possess a binding site on the metal without an activation procedure to remove bound solvent molecules. These MOFs were compared to the well-known Zn-based MOF-69C containing 1,4-benzene dicarboxylate linkers. Thermodynamic analysis of the gas sorption data revealed that the mechanism of CO2 binding involved the coordinatively unsaturated Zn(II) center. The microporous MOF also demonstrated selectivity for CO2 over N2 under the same conditions. As these materials were able to uptake CO2, their ability to transport electrons was also investigated for ultimate applications in catalysis. Electrochemical impedance spectroscopy was performed on the bulk MOF powders and was coupled with solid-state nuclear magnetic resonance spectroscopy. These results determined that the conduction mechanism proceeded via solvent molecules within the pores of the framework. The catalytic ability toward water oxidation of two MOFs was investigated electrochemically. Initial studies focused on a cobalt-based MOF comprised of 2-pyrimidinolate (pymo) linkers, designated as Co(pymo)2, which was prepared on FTO via drop-casting and used for electrochemical experiments. At applied anodic potentials, the CoII centers of Co(pymo)2 became oxidized to form a Co-oxide species on the electrode surface, which was found to be the active catalysis for water oxidation. Further investigations utilized a notably more stable Zr-based MOF with nickel(II) porphyrin linkers, designated as PCN-224-Ni. PCN-224-Ni was prepared solvothermally on FTO and used directly for electrochemical water oxidation. The mechanism of water oxidation at PCN-224-Ni proceeds via oxidation of the porphyrin macrocycle followed by binding of water to the Ni(II) center. Cooperative proton transfer to the Zr-oxo node facilitated water oxidation with the eventual release of O2. Thorough characterization revealed that PCN-224-Ni retained its structural integrity over the course of electrochemical catalysis. These results have allowed us a deeper understanding of the mechanisms of electron transport and conduction throughout frameworks. Specifically, the incorporation of metalloporphyrin molecules with redox active metal centers coupled with the presence of redox active metal nodes resulted in redox hopping charge transport throughout the MOF. In addition, the presence of solvent molecules in the pores of the framework provided an extended network for charge transport. We have gained insight into the structure-function relationship of MOFs for applications in selective gas sorption, where an unsaturated metal center serves as the binding site for gas molecules. Finally, through selection of the components that comprise the framework, a stable metalloporphyrin MOF was found to be capable of electrochemically facilitating the water oxidation reaction. As a result, we have gained valuable insight into the properties of frameworks necessary for charge transport and stability, which will allow for further improvements in the smart design of MOFs for catalytic applications. / Ph. D.

Este trabalho está focado no desenvolvimento de complexos de rutênio binucleares baseados no ligante ponte 2,6-bis(2-piridil)benzodiimidazol (dpimH2) com potencial aplicação como catalisadores para oxidação da água. O acoplamento eletrônico entre os centros metálicos bem como as propriedades eletrônicas e catalíticas podem ser controlados via reações ácido-base no ligante bis-bidentado. Dessa forma, neste trabalho descrevemos o preparo e a caracterização do respectivo composto mononuclear, bem como do complexo binuclear simétrico [{RuCl(phtpy)}2(dpimH2)](Otf) 2 (onde phtpy=4-fenil-2,2\':6\',\'\'-terpiridina), e do análogo assimétrico [{Ru(bpy)2}(dpimH2){Ru(phtpy)Cl}](ClO4)3 (onde bpy=2,2\'-bypiridina), que possui um centro catalítico e um grupo cromóforo na mesma molécula como esperado em um fotocatalisador, em que os dois centros catalíticos estão covalentemente conectados através do ligante ponte funcional. As caracterizações estrutural e eletrônica de ambos os complexos por 1H RMN, ESI-MS e espectroscopia de absorção UV-Vis indicaram a presença de isômeros geométricos com perfis eletrônicos similares. Por outro lado, a análise eletroquímica por voltametria cíclica demonstrou menores potenciais Ru(III/II) quando comparados a complexos polipiridínicos análogos. Este potencial redox pode ainda ser catodicamente deslocado através da remoção de prótons dos grupos imidazóis do ligante ponte, possibilitando, dessa forma, a modulação das propriedades eletrônicas e catalíticas destes complexos de rutênio através de reações de protonação/desprotonação dos grupos -NH. Além disso, neste trabalho é investigada a inesperada formação do complexo [Ru(phtpy)2] nas reações do complexo [RuCl3(phtpy)] puro com ligantes bidentados, utilizando-se espectroscopia UV-Vis e de 1H RMN. / This work is focused on the development of dinuclear ruthenium complexes with potential application as catalysts for oxidation of water, that are characterized by a benzobisimidazole 2,6-bis(2-pyridyl)benzodiimidazole (dpimH2) bridging ligand, whose interaction between the metal centers as well as the electronic and catalytic properties can be tuned by acid-base reactions in that moiety. Thus, the preparation and characterization of the respective mononuclear species are described. The dinuclear complex [2(dpimH2)](Otf)2(phtpy=4-phenyl-2,2\':6\',2\'\'-terpiridine), in which two catalytic centers are covalently linked through that bridging ligand, and of the [(dpimH2)](ClO4)3 complex (where bpy=2,2\'-bypiridine) integrating a chromophore and a catalytic center in the same molecule as expected for a photocatalyst. The structural and electronic characterization of both complexes by NMR, ESI-MS and UV-vis spectroscopy indicated the presence of geometric isomers with similar electronic profiles. On the other hand, the electrochemical analysis by cyclic voltammetry displayed redox potential values for the Ru3+/Ru2+ couples lower than the respective polypyridyl complex counterparts. This redox potential can be even more shifted to less positive potentials by removal of protons from the imidazole groups in the bridging ligand, opening the possibility of tuning the electronic and catalytic properties of those ruthenium complexes based on protonation/deprotonation of the -NH groups. Furthermore, in this work is analyzed the unexpected formation of the bisterpyridine [Ru(phpy)2] complex in reactions starting with pure [RuCl3(phtpy)] complex with bidentated ligands, as through UV-Vis spectroscopy and RMN.

Complexos polipiridínicos de rutênio mononuclares vem sendo ativamente estudados como catalisadores da reação de oxidação de água a oxigênio, mas o complexo ativado dos catalisadores mais eficientes envolve a formação de um dímero, indicando a importância da estrutura binuclear para ativação dos mesmos. Assim, nesta tese propomos o estudo dos possíveis efeitos sinérgicos em complexos binucleares de rutênio polipiridinas angulares para ativação das espécies de alta valência do tipo RuV=O e RuIV=O. Assim, foram preparadas séries de complexos polipiridínicos de rutênio empregando os ligantes tridentados derivados de terpiridinas e bidentados tipo bipiridina na forma cloro complexos e aqua complexos mono e binucleares, capazes de atuar como precursores das espécies ativas de alta valência por meio de reações de transferência de elétrons acoplado a transferência de prótons (PCET). Os complexos [RuCl(bpy)(phtpy)](PF6), [Ru2Cl2(bpy)2(tpy2ph)](PF6)2 e [Ru2Cl2(Clphen)2(tpy2ph)](PF6)2 (phtpy= 4\'-fenil-2,2\':6\',2\'\'-terpiridina, bpy= 2,2´-bipiriridina, Clphen= 5-cloro-1,10-fenantrolina e tpy2ph= 1,3-bis(4\'-2,2\':6\',2\'\'-terpiridil)benzeno) e seus aqua complexos foram sintetizados e caracterizados por técnicas espectroscópicas e eletroquímicas. Os complexos [RuCl(bpy)phtpy](PF6), [Ru2Cl2(bpy)2(tpy2ph)](PF6)2 e [Ru2Cl2(Clphen)2(tpy2ph)](PF6)2 apresentam apenas reações de transferência de elétrons onde o estado de oxidação máximo do íon rutênio é 3+. Todavia, os respectivos aqua complexos [Ru(H2O)(bpy)(phtpy)](PF6)2, [Ru2(H2O)2(bpy)(tpy2ph)](PF6)4 e [Ru2(H2O)2(Clphen)2(tpy2ph)](PF6)4 podem ser oxidados de modo a gerar complexos de alta valência com íon rutênio nos estados de oxidação 4+ e 5+ via reação de transferência eletrônica acoplada a transferência de prótons (PCET). Os complexos de RuIV=O são gerados em potenciais relativamente baixos e não apresentaram atividade eletrocatalítica significativa, enquanto que as espécies RuV=O ([RuV(O)(bpy)(phtpy)]3+ e [Ru2V(O)2(bpy)2(tpy2ph)]6+) atuam como catalisadores eficientes para a reação de oxidação da água a oxigênio. Os valores de TOF para os complexos binuclear (0,97 s-1) é cerca de três vezes maior que para o complexo mononuclear (0,32 s-1), confirmando a presença de efeitos sinérgicos que aceleram a liberação de oxigênio no complexo binuclear. As propriedades eletrocatalíticas dos complexos polipiridínicos de rutênio de alta valência foram transferidos para a superfície de eletrodos via eletropolimerização redutiva do complexo [Ru2(H2O)2(Clphen)2(tpy2ph)](TfO)4. Neste caso foram observadas a geração eletroquímica de espécies contendo o íon rutênio nos estados de oxidação 2+, 4+ e 5+, enquanto que a espécie no estado 3+ aparentemente não é estável e sofre desproporcionamento. O eletrodo modificado preservou a alta atividade eletrocatalítica do aqua complexo binuclear para a reação de oxidação da água (TOF de 0,80 s-1) e também para a oxidação de álcool benzílico a benzaldeído, com kRuIV= 14,70 L·mol-1 s-1 demonstrando o elevado potencial do material para a oxidação de substratos orgânicos. / Mononuclear ruthenium polypyridyl complexes have been studied as catalysts of oxygen evolution in water oxidation reaction, but the activated complex of most efficient catalysts assume the formation of dimers indicating the importance of the binuclear structure for their activation. Thereby, in this thesis we propose the study of possible synergistic effects in binuclear ruthenium polypyridyl complexes in order to activate species with high valence as RuV=O and RuIV=O for multi-electronic catalytic oxidation reactions. For this purpose, it was prepared a series of ruthenium polyppyridyl complexes using tridentate ligands based in terpyridine and bidentate bipyridine generating binuclear chloride complexes and aqua complexes which are able to act as precursors of the respective high valence active species generated by proton coupled electron transfer (PCET) reactions. The [RuCl(bpy)(phtpy)](PF6) and [Ru2Cl2(bpy)2(tpy2ph)](PF6)2 complexes (phtpy= 4\'-phenyl-2,2\':6\',2\'\'-terpyridine, bpy= 2,2´-bipyridine and tpy2ph= 1,3-bis(4\'-2,2\':6\',2\'\'-terpyridin-4-yl)benzene) and their respective aqua complexes were synthetized and characterized by spectroscopic and electrochemical techniques. The chloro complexes [RuCl(bpy)(phtpy)](PF6), [Ru2Cl2(bpy)2(tpy2ph)](PF6)2 and [Ru2Cl2(Clphen)2(tpy2ph)](PF6)2 (Clphen= 5-Chloro-1,10-phenanthroline) show only electron transfer reactions where the maximum oxidation state of the ruthenium ion is 3+. However, the respective aqua complexes [Ru(H2O)(bpy)(phtpy)](PF6)2, [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 and [Ru2(H2O)2(Clphen)2(tpy2ph)](PF6)4 can be oxidized further by proton coupled electron transfer (PCET), generating high valence complexes where the ruthenium oxidation state can be 4+ and 5+. Complexes of RuIV=O are generated in relatively low potentials and do not presented significant electrocatalytic activity for oxidation of water to dioxygen, whereas the RuV=O species ([RuV(O)(bpy)(phtpy)]3+ and [Ru2V(O)2(bpy)2(tpy2ph)]6+) showed to be efficient catalysts for the reaction of water oxidation. The values of TOF for the binuclear complexes (0,97 s-1) were about three times larger than for the mononuclear complex (0,32 s-1), confirming the presence of synergistic effects accelerating the formation and release of oxygen by the binuclear complex. The electrocatalytic properties of high valence ruthenium polypyridyl complexes were transferred to electrodes surface by reductive electropolymerization of the [Ru2(H2O)2(Clphen)2(tpy2ph)](TfO)4 complex. In this case the electrochemical generation of ruthenium 2+, 4+ and 5+ species were observed whereas the 3+ species was not stable and disproportionated. The modified electrodes preserved the high electrocatalytic activity of the binuclear aqua complexes for water oxidation reaction (TOF de 0,80 s-1), and also for oxidation of benzyl alcohol to benzaldehyde with kRuIV= 14,70 L mol-1 s-1 demonstrating the high catalytic efficiency for oxidation of organic substrates.

Este trabalho está focado no desenvolvimento de complexos de rutênio binucleares baseados no ligante ponte 2,6-bis(2-piridil)benzodiimidazol (dpimH2) com potencial aplicação como catalisadores para oxidação da água. O acoplamento eletrônico entre os centros metálicos bem como as propriedades eletrônicas e catalíticas podem ser controlados via reações ácido-base no ligante bis-bidentado. Dessa forma, neste trabalho descrevemos o preparo e a caracterização do respectivo composto mononuclear, bem como do complexo binuclear simétrico [{RuCl(phtpy)}2(dpimH2)](Otf) 2 (onde phtpy=4-fenil-2,2\':6\',\'\'-terpiridina), e do análogo assimétrico [{Ru(bpy)2}(dpimH2){Ru(phtpy)Cl}](ClO4)3 (onde bpy=2,2\'-bypiridina), que possui um centro catalítico e um grupo cromóforo na mesma molécula como esperado em um fotocatalisador, em que os dois centros catalíticos estão covalentemente conectados através do ligante ponte funcional. As caracterizações estrutural e eletrônica de ambos os complexos por 1H RMN, ESI-MS e espectroscopia de absorção UV-Vis indicaram a presença de isômeros geométricos com perfis eletrônicos similares. Por outro lado, a análise eletroquímica por voltametria cíclica demonstrou menores potenciais Ru(III/II) quando comparados a complexos polipiridínicos análogos. Este potencial redox pode ainda ser catodicamente deslocado através da remoção de prótons dos grupos imidazóis do ligante ponte, possibilitando, dessa forma, a modulação das propriedades eletrônicas e catalíticas destes complexos de rutênio através de reações de protonação/desprotonação dos grupos -NH. Além disso, neste trabalho é investigada a inesperada formação do complexo [Ru(phtpy)2] nas reações do complexo [RuCl3(phtpy)] puro com ligantes bidentados, utilizando-se espectroscopia UV-Vis e de 1H RMN. / This work is focused on the development of dinuclear ruthenium complexes with potential application as catalysts for oxidation of water, that are characterized by a benzobisimidazole 2,6-bis(2-pyridyl)benzodiimidazole (dpimH2) bridging ligand, whose interaction between the metal centers as well as the electronic and catalytic properties can be tuned by acid-base reactions in that moiety. Thus, the preparation and characterization of the respective mononuclear species are described. The dinuclear complex [2(dpimH2)](Otf)2(phtpy=4-phenyl-2,2\':6\',2\'\'-terpiridine), in which two catalytic centers are covalently linked through that bridging ligand, and of the [(dpimH2)](ClO4)3 complex (where bpy=2,2\'-bypiridine) integrating a chromophore and a catalytic center in the same molecule as expected for a photocatalyst. The structural and electronic characterization of both complexes by NMR, ESI-MS and UV-vis spectroscopy indicated the presence of geometric isomers with similar electronic profiles. On the other hand, the electrochemical analysis by cyclic voltammetry displayed redox potential values for the Ru3+/Ru2+ couples lower than the respective polypyridyl complex counterparts. This redox potential can be even more shifted to less positive potentials by removal of protons from the imidazole groups in the bridging ligand, opening the possibility of tuning the electronic and catalytic properties of those ruthenium complexes based on protonation/deprotonation of the -NH groups. Furthermore, in this work is analyzed the unexpected formation of the bisterpyridine [Ru(phpy)2] complex in reactions starting with pure [RuCl3(phtpy)] complex with bidentated ligands, as through UV-Vis spectroscopy and RMN.

<p>A molecular system capable of photoinduced water splitting is an attractive approach to solar energy conversion. This thesis deals with the functional characterization of molecular building blocks for the three principal functions of such a molecular system: Photoinduced accumulative charge separation, catalytic water oxidation, and catalytic proton reduction. </p><p>Systems combining a ruthenium-trisbipyridine photosensitizer with multi-electron donors in form of dinuclear ruthenium or manganese complexes were investigated in view of the rate constants of electron transfer and excited state quenching. The kinetics were studied in the different oxidation states of the donor unit by combination of electrochemistry and time resolved spectroscopy. The rapid excited state quenching by the multi-electron donors points to the importance of redox intermediates for efficient accumulative photooxidation of the terminal donor.</p><p>The redox behavior of manganese complexes as mimics of the water oxidizing catalyst in the natural photosynthetic reaction center was studied by electrochemical and spectroscopic methods. For a dinuclear manganese complex ligand exchange reactions were studied in view of their importance for the accumulative oxidation of the complex and its reactivity towards water. With the binding of substrate water, multiple oxidation in a narrow potential range and concomitant deprotonation of the bound water it was demonstrated that the manganese complex is capable of mimicking multiple aspects of photosynthetic water oxidation.</p><p>A dinuclear iron complex was investigated as biomimetic proton reduction catalyst. The complex structurally mimics the active site of the iron-only hydrogenase enzyme and was designed to hold a proton on the bridging ligand and a hydride on the iron centers. Thermodynamics and kinetics of the protonation reactions and the electrochemical behavior of the different protonation states were studied in view of their potential catalytic performance.</p>