Heterogeneous materials, bulk properties

Studies under categories ii and iii provide more poignant examples of the power of Car-Parrinello methods. Because of the magnitude of the literature on applications of Car-Parrinello simulations, 1 have chosen to focus on a few case studies to iUustrate the potential of simulations under these categories for problems of interest to chemical engineers. The areas that 1 have chosen are (A) gas-phase processes (B) processes in bulk materials (C) properties of liquids, solvation, and reactions in liquids (D) heterogeneous reactions and processes on surfaces (E) phase transitions and (F) processes in biological systems. 

As a result of the mode of formation, coal is an extremely complex heterogeneous material that is difficult to characterize in a chemical sense, although methods of classification have evolved based on bulk properties (Chapter 9). 

There are two good reasons to study and understand this intrinsic difference between surface and bulk properties (i) sdentific curiosity and (ii) the desire to gain control over the so important interface processes such as corrosion, passivation, materials processing such as film growth and etching, and heterogeneous catalysis, etc. 

The valence band structure of very small metal crystallites is expected to differ from that of an infinite crystal for a number of reasons (a) with a ratio of surface to bulk atoms approaching unity (ca. 2 nm diameter), the potential seen by the nearly free valence electrons will be very different from the periodic potential of an infinite crystal (b) surface states, if they exist, would be expected to dominate the electronic density of states (DOS) (c) the electronic DOS of very small metal crystallites on a support surface will be affected by the metal-support interactions. It is essential to determine at what crystallite size (or number of atoms per crystallite) the electronic density of sates begins to depart from that of the infinite crystal, as the material state of the catalyst particle can affect changes in the surface thermodynamics which may control the catalysis and electro-catalysis of heterogeneous reactions as well as the physical properties of the catalyst particle [26]. 

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