Sabatier principle

{{short description|Chemistry rule regarding heterogenous catalysis}}

File:Volcano plot.png

In chemistry, the Sabatier principle is a qualitative concept in heterogeneous catalysis named after the French chemist Paul Sabatier. It states that the interactions between the catalyst and the reactants should be "just right"; that is, neither too strong nor too weak. If the interaction is too weak, the molecule will fail to bind to the catalyst and no reaction will take place. On the other hand, if the interaction is too strong, the product fails to dissociate.{{cite book |title=Catalysis: Concepts and Green Applications |author=Gadi Rothenberg |publisher=Wiley-VCH |year=2008 |isbn=978-3-527-31824-7 |pages=65}}

The principle can be shown graphically by plotting the reaction rate against a property such as the heat of adsorption of the reactant by the catalyst. Such plots pass through a maximum, looking roughly like a triangle or an inverted parabola, and are called volcano plots because of their shape. Analogous three-dimensional plots can also be built against two different properties, such as the heats of adsorption of the two reactants for a two-component reaction. In that case the plot is generally shown as a contour plot and is called a volcano surface.{{cite journal |journal=J. Am. Chem. Soc. |volume=130 |issue=33 |pages=10868–10869 |year=2008 |doi=10.1021/ja803555g |author=Jun Cheng; P. Hu |title=Utilization of the Three-Dimensional Volcano Surface To Understand the Chemistry of Multiphase Systems in Heterogeneous Catalysis |pmid=18651740|bibcode=2008JAChS.13010868C }} Volcano plots were introduced by Balandin.{{cite book |title=Ullmann's Encyclopedia of Industrial Chemistry |chapter=Heterogeneous Catalysis and Solid Catalysts |author=Helmut Knözinger; Karl Kochloefl |publisher=Wiley-VCH Verlag |year=2005 |doi=10.1002/14356007.a05_313|isbn=3527306730 }}{{cite journal |doi=10.1016/S0360-0564(08)60029-2 |author=Balandin, A. |title=Modern State of the Multiplet Theory of Heterogeneous Catalysis1 |journal=Adv. Catal. Rel. Subj. |series=Advances in Catalysis |volume=19 |pages=1–210 |year=1969|isbn=9780120078196 }}

The figure on the right shows a volcano plot for the decomposition of formic acid using different transition metals as catalysts.{{Cite journal|last1=Rootsaert|first1=W. J. M.|last2=Sachtler|first2=W. M. H.|date=1960|title=Interaction of Formic Acid Vapour with Tungsten|journal=Zeitschrift für Physikalische Chemie|language=en|volume=26|pages=16–26|doi=10.1524/zpch.1960.26.1_2.016|issn=0942-9352}} In this case, the heat of formation ({{math|Δ_fH}}) of the metal formate salt was used for the x axis because studies showed that the reaction intermediate was a surface formate. For the y axis, the temperature at which the reaction reaches a specific rate was used (the y axis is plotted in reverse to preserve the conventional "volcano" shape). At low values of {{math|Δ_fH}}, the reaction is slow (in other words, requires higher temperatures) because the rate of adsorption is slow and rate-limiting. At high values of {{math|Δ_fH}}, desorption becomes the rate-limiting step. The maximum rate, which is observed for the platinum group metals in this case, requires intermediate values of {{math|Δ_fH}}, with the rate being a combination of the rate of adsorption and the rate of desorption. Catalysts can exceed the Sabatier limit via catalytic resonance.