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 Catalyst UnVeiled

One-fourth to one-half of all products produced worldwide depend on the use of catalysts in at least one step in their production. These production processes range from the synthesis of drugs to the manufacture of plastics.

Despite the tremendous importance of catalysts, the mechanisms by which these substances speed chemical reactions is not well understood. In most cases, catalystic reactions cannot be re-created experimentally under conditions that will provide the unambiguous, molecular-level information scientists need in order to modify and control the reactions. These catalyzed chemical events are complex and because catalysts are usually opaque solids, they are often impossible to study with typical spectroscopic techniques, which involve shining light through the material or reflecting light off the material. As a result, there are often few, if any, options available for studying catalysis in real time to obtain molecular-level information. In these cases, the computer is the best microscope available.

Harrell Sellers, a member of the Alliance's Chemical Engineering Applications Technologies Team and a chemist at South Dakota State University, leads a team that is studying catalytic processes on metal surfaces and is developing new computational methods and software tools that are helping scientists study these processes more thoroughly. Sellers's team has developed methods for simulating reactions on metal surfaces in order to obtain the information necessary to build computer models of the catalytic reactors that influence the catalysis process. These models can then simulate the behavior of catalytic reactors as scientists vary the temperature, pressure, and composition of the starting material. (See Shustorvich, E., and H. Sellers. 1998. Surf. Sci. Rep. 31:1-119.)

During this coming year, Sellers and his team will integrate their software, called Catalysis and Chemisorption Modeling Programs (CACMP), into the Chemical Engineering Workbench being developed by the Alliance's chemical engineering team. The workbench brings together a wide range of software tools used in chemical engineering research into an integrated, easy-to-use Web-based environment. Although Sellers's animations are relatively simple because of the visualization capabilities available at South Dakota State, CACMP can take advantage of sophisticated visualization tools and render images in 3D. A goal of Sellers's team is to disseminate this tool to Alliance partners with advanced visualization capabilities so that 3D modeling of the catalysis process becomes routine.

Below are animations that show the path through space traveled by individual atoms during catalysis. RealVideo Animation 1 These atomic trajectories were created using CACMP's molecular dynamics module. The first animation depicts the desorption, or evaporation, of an ethylene molecule (C2H4) from the surface of the metal palladium. This animation sequence resulted from the group's study of the addition of hydrogen molecules (hydrogenation) to olefin, an unsaturated hydrocarbon. The sequence shows how the rotational motion of a nonlinear molecule can either contribute to the desorption process or hinder the opposite process -- adsorption, or condensation. In nonlinear molecules, the atoms do not exist in a line and the molecules can rotate in more ways than can linear molecules. Knowing the rate at which the olefins adsorb and desorb determines how much of this substance will be present on the metal surface.

RealVideo Animation 2

The second animation shows the breaking of the carbon-oxygen (C-O) bond in methoxy (CH3O), on the surface of nickel (Ni). This sequence resulted from a study of the conversion of methane (CH4) to methanol (CH3OH) by way of the intermediate methoxy. An intermediate is a chemical compound that forms and is consumed in a reaction mechanism, in this case, catalysis. The methoxy intermediate occurs as the next to last step in the reaction that ends with the creation of methanol. Methoxy tends to decompose on the metal surface, but this decompostion should be avoided if possible because it will prevent the final step in the reaction -- the creation of methanol -- from occuring. This simulation helps researchers explore ways of inhibiting methoxy decompostion. (The apparent sudden movement of the methoxy from the edge of the metal cluster to the center is an artifact of the calculation.)