The most commonly used catalyst is iron, often treated with sulfur, hydrogen sulfide, etc. to lower the melting point and facilitate its penetration into the pores of carbon and hence, to produce more growth sites. Fe/Ni, Ni, Co, Mn, Cu, V, Cr, Mo, Pd, MgO, and Al2O3 are also used as catalyst. Acetylene, ethylene, methane, natural gas, and benzene are the most commonly used carbonaceous gases. Often carbon monoxide (CO) is introduced in the gas flow to increase the carbon yield through reduction of possible iron oxides in the system.