carbon atoms, and α is the chain growth probability or the probability that a molecule will continue reacting to form a longer chain. In general, α is largely determined by the catalyst and the specific process conditions. Examination of the above equation reveals that methane will always be the largest single product so long as α is less than 0.5; however, by increasing α close to one, the total amount of methane formed can be minimized compared to the sum of all of the various long-chained products. Increasing α increases the formation of long-chained hydrocarbons. The very long-chained hydrocarbons are waxes, which are solid at room temperature. Therefore, for production of liquid transportation fuels it may be necessary to crack some of the FT products. In order to avoid this, some researchers have proposed using zeolites or other catalyst substrates with fixed sized pores that can restrict the formation of hydrocarbons longer than some characteristic size (usually n < 10). This way they can drive the reaction so as to minimize methane formation without producing many long-chained hydrocarbons. Such efforts have had only limited success. Catalysts Four metals are active as catalysts for the Fischer–Tropsch process: iron, cobalt, nickel, and ruthenium.Since FT process typically transforms inexpensive precursors into complex mixtures that require further refining, FT catalysts are based on inexpensive metals, especially iron and cobalt. Nickel generates too much methane, so it is not used. Typically, such heterogeneous catalysts are obtained through precipitation from iron nitrate solutions. Such solutions can be used to deposit the metal salt onto the catalyst support (see below). Such treated materials transform into active catalysts by heating under CO, H2 or with the feedstock to be treated, i.e., the catalysts are generated in situ. Owing to the multistep nature of the FT process, analysis of the catalytically active species is challenging. Furthermore, as is known for iron catalysts, a number of phases may coexist and may participate in diverse steps in the reaction. Such phases include various oxides and carbides as well as polymorphs of the metals. Control of these constituents may be relevant to product distributions. Aside from iron and cobalt, nickel and ruthenium are active for converting the CO/H2 mixture to hydrocarbons. Although expensive, ruthenium is the most active of the Fischer–Tropsch catalysts in the sense that It works at the lowest reaction temperatures and produces higher molecular weight hydrocarbons. Ruthenium catalysts consist