First-principle theoretical and topological characterization of catalytic systems. Anatase surface, doping, metal clusters, and ligands

Three main developing areas for catalysis can be termed as the following: I. Among efficient reactions catalysis, the CO and NO oxidation is one of the most versatile catalyzed reactions1,2, since the environmental impact of these reactive oxygen species3 is detrimental. Despite their apparent simplicity, CO and NO oxidation remains the subject of on-going research4, and understanding of their mechanisms is still being refined. II. More complex applications for catalysis has been developed in recent years5, the medical application for local treatment in damaged tissues that are unable to carry out efficient process in biological reactions. It is an opportunity for the simulation and theoretical development of more efficient catalytic systems for biological use.6 III. Another point of application for catalysis requiring a thorough development of the whole catalytic systems is the petrochemical and industrial process7. The efficiency under abrasive conditions is the key for a catalyzer8. Although the developing areas for catalysis are widespread and the theoretical treatment covers different approaches9¿14, we focus on the oxidation of gas pollutants, since this topic is afflicting large populations, mostly in developing countries such as industrial sections located near to urban centers, e.g. the main cities of Colombia: Bogota and Medellin15. Gas pollutants such as CO, NO, SO2, chlorofluorocarbons (CFC), among others, represent a global economic issue16. The need of efficiently design substances capable of not only capturing the pollutants but catalyzing them into less harmful compounds is a world concern17,18. This is why understanding the dynamics between the factors of the catalysis of these pollutants will lead us to theoretically propose more efficient catalysts that help to alleviate the pollution problems. That is how, 20 years ago, scientists turned their attention to a mineral formed by titanium dioxide19, named a hundred years ago as Anatase, from the Greek word `extension¿. The Anatase mineral and variations, such as rutile, have photo-catalytic properties able to oxidize pollutants20 as it has been researched and published in many papers from both experimental and theoretical points of view21,22. Therefore, the uses of the mineral span from electronic structure calculations and molecular dynamics simulations (nano-scale) to prototype full scale air-purifying pavement. For instance, covering an entire street with photocatalytic blocks built mostly with Anatase titanium dioxide in combination with cementitious materials showed an outdoor air quality improvement of an overall 60.2% reduction of NOx pollutant23. Nevertheless, the morphological variation of the mineral and the molecular disposition of the titanium dioxide in a block-cell make the theoretical simulations computationally expensive. Recent theoretical efforts to understand the reaction mechanisms and active sites of different size and type materials have not converged to an unanimous criterion24. Nonetheless, these efforts are necessary to build and develop theory with respect to catalytic phenomena i.e. computational material science. Different techniques focused on the simulation of the catalytic process cover first-principle calculations of considerable size structure, semi-empirical molecular dynamics or hybrid quantum mechanics / molecular mechanics (QM/MM) calculations11,25. The researcher must be aware of the limitations and advantages of each one of the techniques; the computation depends on the size and accuracy of the designed simulation. Moreover, metal clusters show unique properties such as the photo-catalytic reduction of CO2 and the production of H2 useful in power cells27 within the alternative fuel production. Thereby, to gain insights on the suggested catalytic phenomena to study, key factors must be taken into account related to the catalytic activity: a reliable characterization of the support materials i.e. the surface of Anatase, the metal clusters, the cluster-support interface, and ultimately, the whole interaction between the catalyzer and the reactants. Explicitly, the nature of the TiO2 surface polymorph (e.g.101 for Anatase and/or 110 for Rutile), the size and charge of the metal cluster, the nature of the metallic atoms and their combinations, such as bi-metallic clusters, and a correct mechanism of interaction between the reactants and the catalyzer. Determinations of the factors described above with the adequate methodology represent a challenge from the computational point of view11. The size of the TiO2 surface, the electron correlation and relativistic effects for the metal clusters and the inherent structural variation for a given set of isomers, and a correct description of the reaction mechanism, are issues to overcome during the theoretical investigation. Fortunately, current advances in Quantum Mechanics such as DFT-TB (see following paragraph) and Quantum Topology phase diagrams will allow us to thoroughly develop and propose efficient catalyzers28. The goal of computational materials science is pointing toward to improve the atomic scale understanding of materials structures and properties. Structure¿property correlations, that are never resolved in experiments on a molecular level, are thus becoming quantified. In this context, nano-scale simulations (100¿200 atoms) contribute to the optimization of materials properties by allowing tailoring of structures and chemical compositions for a broad range of applications. Looking for suitable methods that do not demand excessive computational cost, during the last decade the development of approximate methods has become a reality for treating large systems. In some cases, these approaches are very useful for getting starting geometrical points with high quality. For example, DFTB29, which tries to merge the spirit and reliability of Density Functional Theory (DFT) with the simplicity and efficiency of the Tight Binding (TB) physics. Thus, approximate and acceptable computation of small catalytic systems can be achieved30. In addition, a new Quantum Topology dimensionality theory for describing the Potential Energy Surface (PES) for a set of isomers and even compare catalytic reactions, has been developed recently by prof. S. Jenkins31. Hence, with an intuitive topological analysis of a few sets of isomers, a Quantum Topology Phase Diagram (QTPD) can be constructed spanning a wide set of different of non-considered and yet missing topologies32. Additionally, the application of QTPDs for non-isomeric exploration of the PES e.g. a catalyzed chemical reaction could lead to theoretically describe the efficiency28 of any proposed catalyzer within a specific reaction.