**1. Introduction**

Traditionally, the area of catalysis has been divided into three fields: heterogeneous, homoge‐ neous, and enzymatic catalyses. While heterogeneous catalysis has offered a great advantage

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in terms of the process design, the separation of products from a catalyst and its regeneration, it has usually suffered from a lack of understanding of the so‐called "active site." This fact has plausibly impeded the rational development of these systems. On the contrary, homogeneous catalysis can be designed in a more rational way as its properties are easily tuned via ligand designs, providing a substantial comprehension of elementary steps. Nevertheless, these processes have often required undertaking many technical advances to be competitive to heterogeneous catalysts. From the industrial point of view, this led in the final practice to the preference of heterogeneous from homogeneous catalysis. However, the development of enhanced heterogeneous catalysts has been hindered typically owing to their content of numerous variable active sites and their low concentration. Homogeneous catalysts, on the contrary, have been well‐defined systems that could be easily characterized and studied. Comparative studies of homogeneous and heterogeneous catalyses, providing a successful implementation of appropriate homogeneous models with molecular modeling, can yield a new insight into the complex of processes accompanying heterogeneous catalysis. In order to perceive the involved complex molecular events, it is also essential to construct a clear‐cut active site, test its catalytic performance, and assess the relationship between its structure and activity. Finally, the acquired picture can be utilized to design a new generation of catalysts applicable "as in homogeneous catalysis."

Typically, the determining characteristic of heterogeneous catalysis lays in the structural composition of its active sites and molecular structures, in the case of a metal catalyst and the reactant, respectively [1, 2]. In other words, the chemical and physical properties of the surface of the active sites and the molecule entering the reaction generate a generous number of parameters decisive for the catalytic mechanism. In order to predict the catalytic behavior, geometric and electronic structural properties have been traditionally studied [2], particularly based on "Theory of Electronic Effects" [2, 3]. The core of this study consists in the active site (catalyst side)‐reaction center (substrate side) whose behavior is determined by their interde‐ pendent electronic properties, with each component either donating or accepting electrons. Orbitals of both of the reaction players are thus directed by relatively strongly dependent interactions of their affinity and repelling forces. Last but not the least, geometric degree of freedom may have decisive impact on the surface complex (active site‐reactant interaction) [2, 4]. However, the above list of parameters is not complete as other potentially significant aspects could contribute to the problem: thermodynamics, reaction conditions, and hydrogen‐catalytic surface interaction [5].

Electronic effects and spatial geometry are reckoned to play the most prominent role in heterogeneously catalyzed processes. Catalytic properties, related particularly to the metal behavior, are listed below by their descending importance [2, 6–11]:

