The Sabatier principle is a qualitative concept in chemical

heterogeneous catalysis In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. ...

named after the French chemist

Paul Sabatier Paul Sabatier may refer to: *Paul Sabatier (chemist) (1854–1941), French chemist and Nobel Prize winner *Paul Sabatier (theologian) Charles Paul Marie Sabatier (3 or 9 August 1858 – 5 March 1928), was a French clergyman and historian who prod ...

. It states that the interactions between the catalyst and the substrate 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. 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 A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensional grap ...

and is called a volcano surface. Volcano plots were introduced by Balandin. The figure on the right shows a volcano plot for the decomposition of formic acid using different

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...

s as catalysts. In this case, the

heat of formation In chemistry and thermodynamics, the standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their reference state, wit ...

(Δ''H''_f) 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 Δ''H''_f, the reaction is slow (in other words, requires higher temperatures) because the rate of adsorption is slow and rate-limiting. At high values of Δ''H''_f, desorption becomes the rate-limiting step. The maximum rate, which is observed for the

platinum group The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...

metals in this case, requires intermediate values of Δ''H''_f, with the rate being a combination of the rate of adsorption and the rate of desorption.

References

{{reflist Catalysis