Editing Heterogeneous catalysis (section)

== Adsorption ==
[[Adsorption]] is an essential step in heterogeneous catalysis. Adsorption is the process by which a gas (or solution) phase molecule (the adsorbate) binds to solid (or liquid) surface atoms (the adsorbent). The reverse of adsorption is [[desorption]], the adsorbate splitting from adsorbent. In a reaction facilitated by heterogeneous catalysis, the catalyst is the adsorbent and the reactants are the adsorbate.

=== Types of adsorption ===
Two types of adsorption are recognized: [[physisorption]], weakly bound adsorption, and [[chemisorption]], strongly bound adsorption. Many processes in heterogeneous catalysis lie between the two extremes. The [[Lennard-Jones potential|Lennard-Jones model]] provides a basic framework for predicting molecular interactions as a function of atomic separation.<ref name=":2">{{Cite book |title=Principles and practice of heterogeneous catalysis|first1=J. M. |last1=Thomas |last2=Thomas |first2=W. J.|isbn=9783527683789|edition=Second, revised |location=Weinheim, Germany|oclc=898421752|date=2014-11-19 }}</ref>

==== Physisorption ====
In physisorption, a molecule becomes attracted to the surface atoms via [[van der Waals force]]s. These include dipole-dipole interactions, induced dipole interactions, and London dispersion forces. Note that no chemical bonds are formed between adsorbate and adsorbent, and their electronic states remain relatively unperturbed. Typical energies for physisorption are from 3 to 10 kcal/mol.<ref name=":02" /> In heterogeneous catalysis, when a reactant molecule physisorbs to a catalyst, it is commonly said to be in a precursor state, an intermediate energy state before chemisorption, a more strongly bound adsorption.<ref name=":2" /> From the precursor state, a molecule can either undergo chemisorption, desorption, or migration across the surface.<ref name=":3">{{Cite journal|last=Bowker|first=Michael|date=2016-03-28|title=The Role of Precursor States in Adsorption, Surface Reactions and Catalysis|journal=Topics in Catalysis|volume=59|issue=8–9|pages=663–670|doi=10.1007/s11244-016-0538-6|pmid=21386456|issn=1022-5528|doi-access=free|url=https://orca.cardiff.ac.uk/id/eprint/91556/1/art_10.1007_s11244-016-0538-6.pdf}}</ref> The nature of the precursor state can influence the reaction kinetics.<ref name=":3" />

==== Chemisorption ====
When a molecule approaches close enough to surface atoms such that their [[Atomic orbital|electron clouds]] overlap, chemisorption can occur. In chemisorption, the adsorbate and adsorbent share electrons signifying the formation of [[chemical bond]]s. Typical energies for chemisorption range from 20 to 100 kcal/mol.<ref name=":02" /> Two cases of chemisorption are:

* Molecular adsorption: the adsorbate remains intact. An example is alkene binding by platinum.
* Dissociation adsorption: one or more bonds break concomitantly with adsorption. In this case, the barrier to [[Dissociation (chemistry)|dissociation]] affects the rate of adsorption. An example of this is the binding of H<sub>2</sub> to a metal catalyst, where the H-H bond is broken upon adsorption.