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A lot of effort is nowadays put into the development of novel water oxidation catalysts. In this context, mechanistic studies are crucial in order Best dating facebook pages Computational Modeling of Homogeneous Catalysis elucidate the reaction mechanisms governing this complex process, new design paradigms and strategies how to improve the stability and efficiency of those catalysts. This review is focused on recent theoretical mechanistic studies in the field of homogeneous cobalt-based water oxidation catalysts.
In the first part, computational methodologies and protocols are summarized and evaluated on the basis of their applicability toward real catalytic or smaller model systems, whereby special emphasis is laid on the choice of an appropriate model system. In the second part, an overview of mechanistic studies is presented, from which conceptual guidelines are drawn on how to approach novel studies of catalysts and how to further develop the Top online dating sites in the us cp3 Humanitäre Lage Humanitarian situation of computational modeling of water oxidation reactions.
In the past decade, artificial water splitting has become a hot topic in research on renewable energy sources.
A crucial step in this process is the oxidation of water to molecular oxygen. There are numerous catalysts which have been shown to facilitate this reaction, both of homogeneous and heterogeneous nature.
Even though some of them show outstanding catalytic performance, we still lack fundamental understanding of the catalytic process. The latter is crucial in order to systematically improve those catalysts in terms of their catalytic performance and long term stability.
The complexity of such systems often makes experimental investigations of catalytic intermediates a very tedious task. Computational approaches provide additional insight, by either helping to explain experimental findings or by simulations of systems and processes for which no experimental data is available. In the following, we will review some of the recent theoretical studies based on homogeneous transition metals complexes used for water Best dating facebook pages Computational Modeling of Homogeneous Catalysis with emphasis on cobalt-based catalysts.
The latter has not been the most commonly used metal for artificial water oxidation catalysts WOCs. However, it has become more and more popular in recent years, and when it comes to mimicking the cuboidal structure of nature's oxygen evolution cluster OECcobalt-based WOCs turned out to be more active compared to their manganese-based counterparts Evangelisti et al.
We focus our attention not only on the outcome of the studies at hand, but also on the applied methodology. This review contains two main sections: In the first one we discuss the currently accepted mechanisms for water oxidation. We will aimee mann ted leo tour dates particular discuss the choice of the model system and computational approaches applied to tackle specific questions associated with those mechanisms.
In the second section, we give the reader a brief overview on the current state of field by discussing the most important contributions with respect to homogeneous Co-based WOCs. From a chemical point of view the water oxidation is a straight forward process see Equation 1 where the oxygen atoms of two water molecules are oxidized, while the protons and electrons are released, and the two oxygen atoms combine to form molecular oxygen:.
Even if enough bekanntschaft translation is supplied to the system to overcome the thermodynamic barrier i. Only in the presence of a suitable catalyst reasonable turnover numbers can be achieved. But which properties should such a catalyst have?
An intuitive definition of an ideal catalyst was recently given by Balcellswho states that an ideal catalyst has to fulfill the following four criteria: However, the organic ligand framework might still be a health risk for certain animals or plants; 4 modular, the catalyst should be easy to modify and to immobilize or couple to other catalytically active species. Not all of those criteria are easily targetable by computational studies, however in particular 1 offers various opportunities where theoretical studies can greatly enhance the fundamental understanding of the catalytic processes at hand.
The basic questions behind 1 are in general: What is the catalytic mechanism, what is the rate-limiting step, and how can the catalytic performance possibly be improved? In order to elucidate these issues we have to decompose the reaction Equation 1 into its elementary reactions.
In principle there are four oxidations [electron transfers ETs ], four deprotonations Best dating facebook pages Computational Modeling of Homogeneous Catalysis transfers PTs ], the oxygen-oxygen bond formation will be discussed single samenspende in more detailthe release of molecular oxygen [O 2 dissociation O2DI ] as well as the association of a water molecule to regenerate the catalyst [aqua association AQAS ].
Some of those elementary reactions might occur combined such as proton coupled electron transfers PCETs ; likewise, the release of molecular oxygen and the regeneration might happen in a concerted fashion. In general, all water oxidation mechanisms can be categorized into three phases: The latter is involved in the oxygen-oxygen bond formation, either by a water nucleophilic attack WNA or by a radical-coupling RC reaction. The latter is sometimes also referred to as interaction of two metal oxo I2M species.
For this review, we will use the more general term RC. After the bond formation, further oxidation ET and deprotonation PT reactions might take place before in the last stage of the mechanism molecular oxygen is released O2DIand the initial state of catalyst is regenerated by coordination of Best dating facebook pages Computational Modeling of Homogeneous Catalysis novel substrate AQAS.
Schematic overview of water oxidation mechanisms. M represents any redox active transition metal, n is the Best dating facebook pages Computational Modeling of Homogeneous Catalysis formal oxidation state reached by the metal center during catalysis Schematic drawing inspired by Shaffer et al.
The exact order of those building blocks is dependent on the nature of the catalyst, i. There are also different flavors of the oxygen-oxygen bond formation reactions. B is a general base in solution, and R is any residue including other transition metals. The overall charge of the association complex as well as of the product is given to Best dating facebook pages Computational Modeling of Homogeneous Catalysis charge conservation.
In the next section we will focus on the choice of appropriate computational settings and model systems picking up questions arising from points mentioned above. Often it is a straight forward process to propose a water oxidation mechanism for a novel catalyst. However, experimentally it is challenging to get hands on intermediates which might help to elucidate details of the catalytic cycle.
Here, computational chemistry is an essential tool to obtain an in-depth understanding on the electronic structure of the catalyst and possible intermediates thereof during catalysis. The work-horse of computational chemistry, density functional theory DFTusually is a suitable choice. Yet, in the course of this review we try to point out some limitations when it comes to modeling WOCs.
In order to get an overview of possible intermediates involved in the catalytic cycle, the first step is to calculate their relative stability in terms of electronic or free energies. Starting from those, a selection of structures can be made which are within a reasonable energy range, and can serve as building blocks for the full catalytic cycle. However, it is not enough to solely propose possible intermediates based on their relative stability, also their physical properties have to be in agreement with observable quantities such as spectroscopic data, reduction potentials E red and pK A values describing their reactivity under reaction conditions.
For an overview in terms of reaction barriers and mechanisms we refer to the work of Balcells Best dating facebook pages Computational Modeling of Homogeneous Catalysis Liao and Siegbahn As pointed out by these authors, when modeling the thermodynamics and reaction barriers it boils Best dating facebook pages Computational Modeling of Homogeneous Catalysis to the choice of a reasonable model system which is able to capture both solvation effects as well as spin-state energetics.
WOCs are operable in Best dating facebook pages Computational Modeling of Homogeneous Catalysis solutions whereby water is not only the solvent but also the substrate. Description of solvation effects hence becomes a crucial point when modeling properties such as E red and pK A values or reaction barriers. In principle two different approaches exist on how to model the aqueous environment of the catalyst.
On the one hand, there are implicit or continuum solvent models such as the conductor-like screening model COSMOthe polarizable continuum model PCMand others, where the interactions of the individual solvent molecules are averaged and represented as Best dating facebook pages Computational Modeling of Homogeneous Catalysis dielectric continuum which interacts with the solute Tomasi et al.
Directed interactions such as hydrogen-bonds are thus not well described Best dating facebook pages Computational Modeling of Homogeneous Catalysis and Truhlar, ; Tomasi, ; Skyner et al.
Implicit solvation models suffer from an additional drawback, namely their empirical parameterization. In general such models are parameterized for a set of small organic molecules with a neutral charge computed at a rather low level of theory i. As a consequence their application to more complex systems such as charged transition metal complexes might not be fully justified in all cases. Nevertheless, they have been successfully applied to such systems when relative rather than absolute solvation free energies were of interest Baik and Friesner, Another important point one should keep in mind is that the parameterization does not allow for a systematic improvement of the solvation free energies when moving to a higher level of theory in the calculations Ho et al.
With the development of the conductor-like screening model for real solvents COSMO-RSimportant improvements to the existing methodology of implicit solvation models were made, which allowed to alleviate some of their limitations such as the description of hydrogen bonding Klamt, On the other hand, there are explicit or atomistic solvent models where the solvent molecules are an essential part of the model system.
The main drawback of models which include explicit solvent molecules is the prerequisite to know which solvent molecules are essential for a sophisticated description of the system under investigation. Selection Best dating facebook pages Computational Modeling of Homogeneous Catalysis subsequent optimization of certain solvent configurations may thus strongly bias the outcome of calculations.
Extensive configuration sampling e. Recently, Hodel et al. Besides the relative stabilities of intermediate species, also reaction barriers were calculated employing nudged-elastic-band NEB to a model with explicit solvation, and metadynamic calculations where frauen flirt gestik solvent was treated dynamically see section 3.
Due to inadequate and static explicit solvation, NEB calculations resulted in a vast overestimation of the electronic reaction barrier. By virtue of the large system size required for an appropriate explicit solvation, sampling by DFT-MD is very costly.
Further examples of the application of the different solvation approaches can be found in section 4. Besides counter-ions, there are buffer molecules Evangelisti et al. Those complex mixtures are not only challenging from a theoretical point of view, also experimentalists have to come up with advanced strategies to verify the integrity of the catalysts before, during and after catalysis.
If and how those components affect the calculated properties has to be determined for each system individually. Further, solvated cations might help to stabilize the transition state by participating in the hydrogen bonding network affecting the local pH or by direct coordination of the formed hydroperoxo species as proposed by Bucci et al.
In order to elucidate the reaction mechanism, knowledge about the thermodynamics and other properties see section 3. Further insight regarding the kinetics Best dating facebook pages Computational Modeling of Homogeneous Catalysis required which can be obtained by calculating reaction barriers. In order to be able to compare them to experimental turn-over-frequencies TOFsidentification of the rate-determining-step RDS is necessary.
For water oxidation mechanisms one often assumes that the chemical steps, i. Among those reactions, the most critical is the oxygen-oxygen bond formation Fan et al. In general the calculation of reaction barriers is a difficult task since, depending on the model system, the reaction coordinate might be of high dimensionality and therefore localization of a transition state becomes challenging. When a water molecule acts as a nucleophile, it has become a paradigm to introduce an intramolecular base with an appropriate orientation to deprotonate the approaching water, thereby increasing its nucleophilicity Dogutan et al.
Lately, this concept has also been applied to model systems of heterogeneous catalysts by Frydendal et al. Please note, the brackets representing a concerted reaction in a chemical sense might be chosen for computational reasons only and are probably not always intuitive. A critical choice in the calculation of the reaction barrier for the WNA mechanism is the solvation model.
The use of an implicit solvation model is often not enough since the approaching nucleophile and transition state might require further stabilization by hydrogen bonding. Hence, it is common to include several water molecules into the transition state models see Tong et al. If the number of water molecules is increased this partnersuche kostenlos 50 quickly reaches its limitations, namely when the degrees of freedom of the solvent and solute become so large that sampling is required.
An example where this model was pushed toward its limit is the work by Hodel and Luber ab. However, the obtained electronic energy differences from such static solvation models strongly depend on their initial geometries. The next logical step to go beyond the static model of NEBs is sampling the whole configurational space of the solute and solvent.
The metadynamic protocol first proposed by Laio and Parrinello offers the opportunity to sample the phase space with regards to observables which are indicative for the reaction. Besides metadynamics, there are other protocols where AIMD simulations have been biased in order to model reaction barriers.
Unlike conventional water oxidation where molecular oxygen is the final product, the model catalysts discussed by Bernasconi et al. The same step-wise mechanism was earlier reported by Funes-Ardoiz et al. Reprinted with permission from Bernasconi et al.
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