The Nature of Catalytic Materials

There are different catalysts in nature. Molecules or ions are found in solution and are the simplest. Biological catalysts (enzymes) are large molecules present in the soft tissues or solutions. On the other hand, industries use robust inorganic catalysts which are capable of withstanding high temperatures. Most catalysts used in large scale natural gas processing or petroleum are metal oxides, metal sulfides, or metals. Solids catalyst offer more advantages than liquid catalysts in that they are noncorrosive, and separates easily from fluid-phase products. In addition, solid catalyst can be used in high temperature and low pressure because the reactants can be in gas phase. On the other hand, liquid catalysts are more costly since they require high pressures when used in high temperatures. Reactions catalyzed by solid catalysts take place on the surface. To ensure maximum rates of catalytic activity, the surface area per unit volume should be high with the solids having some pores. Various physical properties affecting the catalyst performance include surface area, pore volume, and pore distribution in the surface. These regulate the reactants entry and products molecules out of the pores. Catalysts have different shapes determined by the cost of manufacturing and the requirement of the process. Mostly used catalysts are cylindrical and spherical in shape. Spherical catalysts (particles) are more advantageous in that they are easily made and transferred into and out of the reactor. Cylindrical particles are formed by intrusion, whereas pellets are formed by compaction and granulation. Many catalysts are composite in nature which consists of support combined with components having catalytic activity or promoters. Some examples of the typical supports include aluminas, silica gel, zirconia, magnesia, zeolites, and carbon. The widely used support is alumina, which is also inexpensive, robust, and stable as well as having the advantage of having wide range of shapes, internal surface areas, and pore size distribution. Metal oxides act as either catalysts or sometimes supports. The catalyst incorporates only a small amount of catalytically active components dispersed on the internal surface of the support