Catalyst pellets are usually granular, with diameters from 1 - 5 mm. They can be loaded into the reactor in several ways: as a single bed, separate shells, or in tubes. Catalysts are typically made from nickel, copper, osmium, platinum, and rhodium. The catalysts shown below are made of precious metal on 3.175 mm ceramic beads and are used in an electric catalytic oxidizer that treats air streams contaminated with volatile organic compounds (VOCs).