In this chapter, we consider specific topics of significant commercial value where nonuniform distribution plays an important role in catalyst design. These include catalysts for automotive exhaust cleanup, petroleum refining operations such as hydrotreating and cracking, biotechnology, and acid catalysis. Particularly in the case of automotive catalysis, nonuniform distribution of noble metals provides critical advantages for pollution abatement reactions and has been employed extensively.

Automotive Exhaust Catalysts

Automobile exhaust is considered the main source of air pollution in urban areas. The major pollutants in exhaust gas are carbon monoxide, hydrocarbons, and oxides of nitrogen. Following the Federal Clean Air Act of 1970, which called for a drastic reduction in these emissions, on all cars made in the United States since 1975, catalysts have been used to convert the pollutants into harmless gases. The catalysts in pre-1981 automobiles were oxidation catalysts, which controlled carbon monoxide and hydrocarbons only, by oxidizing them to form carbon dioxide and water. Their active components were platinum and palladium, which were deposited on substrates with a large surface area, either a monolith or pellets packed in shallow, pancake-shaped converters. Starting with the 1981 model year, because of stricter nitrogen oxide controls and fuel economy requirements, oxidation catalysts were replaced by three-way catalysts (TWCs), which simultaneously control all three of the major pollutants. They oxidize carbon monoxide and hydrocarbons while reducing nitrogen oxides as well. To perform these tasks, commercial TWCs contain platinum, palladium, rhodium, and cerium oxide as active components. Platinum and palladium provide activity for carbon monoxide and hydrocarbon oxidation, while rhodium is excellent for NOx reduction.