Heterogeneous catalysis 1427685 206030530 2008-04-16T15:04:02Z 130.79.66.210 [[Image:Hydrogenation on catalyst.png|thumb|[[Hydrogenation]] of [[ethene]] on a solid surface]] '''Heterogeneous catalysis''' is a [[chemistry]] term which describes [[catalysis]] where the [[catalyst]] is in a different phase (ie. [[solid]], [[liquid]] and [[gas]], but also [[oil]] and [[water]]) to the [[reactants]]. Heterogeneous catalysts provide a surface for the chemical reaction to take place on. In order for the reaction to occur, one or more of the reactants must [[surface diffusion|diffuse]] to the catalyst surface and [[adsorb]] onto it. After reaction, the products must desorb from the surface and diffuse away from the solid surface. Frequently, this transport of reactants and products from one phase to another plays a dominant role in limiting the [[reaction rate]]. Understanding these transport phenomena and [[surface chemistry]] such as [[Dispersion (materials science)|dispersion]] is an important area of heterogeneous catalyst research. Catalyst surface area may also be considered. [[Mesoporous silicates]], for example, have found utility as catalysts because their surface areas may be in excess of 1000 m<sup>2</sup>/g, which increases the probability that a reactant molecule in solution will come in contact with the catalyst surface and adsorb. If diffusion rates are not taken into account, the reaction rates for various [[reactions on surfaces]] depend solely on the rate constants and reactant concentrations. '''Asymmetric heterogeneous catalysis''' can be used to synthesize enantiomerically pure compounds using chiral heterogeneous catalysts.<ref>Heitbaum, Glorius, Escher, Asymmetric heterogeneous catalysis, ''Angew. Chem. Int. Ed.'' '''2006''', ''45'', 4732.</ref> The field is of great industrial and environmental importance. It has attracted two [[Nobel prize|Nobel prizes]] for [[Irving Langmuir]] in 1932 and [[Gerhard Ertl]] in 2007. ==Examples of Heterogeneous Catalysis Reactions== The synthesis of Ammonia is an example of heterogeneous catalysis: 3H<sub>2</sub>(g) + N<sub>2</sub>(g) ↔ 2NH<sub>3</sub>(g) - ''catalysed by Fe(s).'' The use of [[Nickel]] in the [[hydrogenation]] of vegetable oils to produce [[margarine]]. The [[unsaturated fat]] present in the vegetable oils are converted to [[saturated fat]] by the addition of hydrogen. This in turn breaks the [[Carbon-carbon double bond]]s. In order for this reaction to be catalysed effectively the [[nickel]] must present a large surface area so therefore must be finely divided. -CH=CH- + H<sub>2</sub> → -CH<sub>2</sub>-CH<sub>2</sub>- The cracking, [[isomerisation]] and re-forming of [[hydrocarbons]] to form appropriate and useful blends of Petrol. '''Catalytic Converters''' These are often used in automobiles. Three main reactions are catalysed by [[Catalytic converters]]. The [[oxidation]] of [[carbon monoxide]] to [[carbon dioxide]]. 2CO(g) + O<sub>2</sub>(g) → 2CO<sub>2</sub>(g) The [[reduction]] of [[nitrogen monoxide]] back to [[nitrogen]]. 2NO(g) + 2CO(g) → N<sub>2</sub>(g) + 2CO<sub>2</sub>(g) The [[oxidation]] of [[hydrocarbons]] to water and carbon dioxide. This can occur on any of the [[hydrocarbons]] however primarily [[Petrol]] or [[Diesel]]. 2C<sub>6</sub>H<sub>6</sub>(g) + 15O<sub>2</sub> → 12CO<sub>2</sub>(g) +6H<sub>2</sub>O(l) ==See also== *[[Homogeneous catalysis]] [[Category:Catalysts]] ''' '''References'''''' <references/> [[de:Heterogene Katalyse]] [[fr:Catalyseur hétérogène]]