In chemistry, a molecule is adsorbed onto a surface when temporary bonds are formed between the surface and the molecule. These bonds are typically covalent bonds and once a chemical process occurs the bonds release the molecule. Heterogeneous catalysts typically work by having a substrate in the aqueous, liquid or gas phase adsorbing onto a solid surface. Transition elements are common catalysts in which this occurs, including Nickel, Platinum, Gold, Iron (Haber Process), Rhodium. They are often powdered and ground up to optimize their surface area.