Editing Chemisorption

{{Short description|Phenomenon of surface adhesion}}
{{More citations needed|date=December 2021}}

'''Chemisorption''' is a kind of [[adsorption]] which involves a chemical reaction between the surface and the adsorbate.  New chemical bonds are generated at the adsorbent surface.  Examples include macroscopic phenomena that can be very obvious, like [[corrosion]]{{What|date=December 2021}}, and subtler effects associated with [[heterogeneous catalysis]], where the catalyst and reactants are in different phases. The strong interaction between the [[adsorbate]] and the [[Substrate (chemistry)|substrate]] [[Interface (matter)|surface]] creates new types of electronic [[chemical bond|bonds]].<ref name="oura">{{cite book |first1=K. |last1=Oura |first2=V.G. |last2=Lifshits |first3=A.A. |last3=Saranin |first4=A.V. |last4=Zotov |first5=M. |last5=Katayama  |title=Surface Science, An Introduction |publisher=Springer |year=2003 |isbn=3-540-00545-5}}</ref>

In contrast with chemisorption is [[physisorption]], which leaves the chemical species of the [[adsorbate]] and surface intact. It is conventionally accepted that the energetic threshold separating the [[binding energy]] of "physisorption" from that of "chemisorption" is about 0.5 eV per adsorbed [[Chemical species|species]].

Due to specificity, the nature of chemisorption can greatly differ, depending on the chemical identity and the surface structural properties.
The bond between the adsorbate and adsorbent in chemisorption is either ionic or covalent.

==Uses==
An important example of chemisorption is in [[heterogeneous catalysis]] which involves molecules reacting with each other via the formation of chemisorbed intermediates.  After the chemisorbed species combine (by forming bonds with each other) the product desorbs from the surface.
[[Image:Hydrogenation on catalyst.svg|thumb|[[Hydrogenation]] of an [[alkene]] on a solid catalyst entails chemisorption of the molecules of hydrogen and alkene, which form bonds to the surface atoms.]]

==Self-assembled monolayers==
[[Self-assembled monolayer]]s (SAMs) are formed by chemisorbing reactive reagents with metal surfaces.  A famous example involves [[thiol]]s (RS-H) adsorbing onto the surface of [[gold]].  This process forms strong Au-SR bonds and releases H<sub>2</sub>.  The densely packed SR groups protect the surface.

==Gas-surface chemisorption==

===Adsorption kinetics===
As an instance of adsorption, chemisorption follows the adsorption process.  The first stage is for the adsorbate particle to come into contact with the surface.  The particle needs to be trapped onto the surface by not possessing enough energy to leave the gas-surface [[potential well]].  If it elastically collides with the surface, then it would return to the bulk gas.  If it loses enough [[momentum]] through an [[inelastic collision]], then it "sticks" onto the surface, forming a precursor state bonded to the surface by weak forces, similar to physisorption.  The particle diffuses on the surface until it finds a deep chemisorption potential well.  Then it reacts with the surface or simply desorbs after enough energy and time.<ref name="rettner">{{cite journal |doi=10.1021/jp9536007 |first=C.T |last=Rettner |author2=Auerbach, D.J. |year=1996 |title=Chemical Dynamics at the Gas-Surface Interface |journal=Journal of Physical Chemistry |volume=100 |issue=31 |pages=13021–33}}</ref>

The reaction with the surface is dependent on the chemical species involved.  Applying the [[Gibbs energy]] equation for reactions:

:<math>\Delta G = \Delta H - T\Delta S</math>

General [[thermodynamics]] states that for spontaneous reactions at constant temperature and pressure, the change in free energy should be negative.  Since a free particle is restrained to a surface, and unless the surface atom is highly mobile, entropy is lowered.  This means that the [[enthalpy]] term must be negative, implying an [[exothermic reaction]].<ref name="gasser">{{cite book |last=Gasser |first=R.P.H. |year=1985 |title=An introduction to chemisorption and catalysis by metals |publisher=Clarendon Press |isbn=0198551630 }}</ref>

Physisorption is given as a [[Lennard-Jones potential]] and chemisorption is given as a [[Morse potential]].  There exists a point of crossover between the physisorption and chemisorption, meaning a point of transfer.  It can occur above or below the zero-energy line (with a difference in the Morse potential, a), representing an [[activation energy]] requirement or lack of.  Most simple gases on clean metal surfaces lack the activation energy requirement.
===Modeling===

For experimental setups of chemisorption, the amount of adsorption of a particular system is quantified by a sticking probability value.<ref name="gasser" />

However, chemisorption is very difficult to theorize.  A multidimensional [[potential energy surface]] (PES) derived from [[Effective medium approximations|effective medium theory]] is used to describe the effect of the surface on absorption, but only certain parts of it are used depending on what is to be studied.  A simple example of a PES, which takes the total of the energy as a function of location:

:<math>E(\{R_i\}) = E_{el}(\{R_i\}) + V_{\text{ion-ion}}(\{R_i\})</math>

where <math>E_{el}</math> is the [[Energy eigenstates|energy eigenvalue]] of the [[Schrödinger equation]] for the electronic degrees of freedom and <math>V_{ion-ion}</math> is the ion interactions.  This expression is without translational energy, [[rotational energy]], vibrational excitations, and other such considerations.<ref name="norskov">{{cite journal |doi=10.1088/0034-4885/53/10/001 |first=J.K. |last=Norskov |year=1990 |title=Chemisorption on metal surfaces |journal=Reports on Progress in Physics |volume=53 |issue=10 |pages=1253–95|bibcode=1990RPPh...53.1253N |s2cid=250866073 }}</ref>

There exist several models to describe surface reactions: the [[Langmuir adsorption model|Langmuir–Hinshelwood mechanism]] in which both reacting species are adsorbed, and the [[Reactions on surfaces|Eley–Rideal mechanism]] in which one is adsorbed and the other reacts with it.<ref name="gasser" />

Real systems have many irregularities, making theoretical calculations more difficult:<ref name="clark">{{cite book |last=Clark |first=A. |year=1974 |title=The Chemisorptive Bond: Basic Concepts |publisher=Academic Press |isbn=0121754405}}</ref>

* Solid surfaces are not necessarily at equilibrium.  
* They may be perturbed and irregular, defects and such.  
* Distribution of adsorption energies and odd adsorption sites.  
* Bonds formed between the adsorbates.

Compared to physisorption where adsorbates are simply sitting on the surface, the adsorbates can change the surface, along with its structure.  The structure can go through relaxation, where the first few layers change interplanar distances without changing the surface structure, or reconstruction where the surface structure is changed.<ref name="clark" /> A direct transition from physisorption to chemisorption has been observed by attaching a CO molecule to the tip of an atomic force microscope and measuring its interaction with a single iron atom.<ref>{{cite journal|last=Huber|first=F.|title=Chemical bond formation showing a transition from physisorption to chemisorption|journal=Science|date=12 September 2019|volume=365|issue=xx|pages=235–238|doi=10.1126/science.aay3444|bibcode = 2019Sci...366..235H|display-authors=etal|pmid=31515246|s2cid=202569091|doi-access=free}}</ref>

For example, oxygen can form very strong bonds (~4 eV) with metals, such as Cu(110).  This comes with the breaking apart of surface bonds in forming surface-adsorbate bonds.  A large restructuring occurs by missing row.

===Dissociative chemisorption===

A particular brand of gas-surface chemisorption is the [[Dissociation (chemistry)|dissociation]] of [[Diatomic molecule|diatomic]] gas molecules, such as [[hydrogen]], [[oxygen]], and [[nitrogen]].  One model used to describe the process is precursor-mediation.  The absorbed molecule is adsorbed onto a surface into a precursor state.  The molecule then diffuses across the surface to the chemisorption sites.  They break the molecular bond in favor of new bonds to the surface.  The energy to overcome the activation potential of dissociation usually comes from translational energy and vibrational energy.<ref name="rettner" />

An example is the hydrogen and [[copper]] system, one that has been studied many times over.  It has a large activation energy of 0.35 – 0.85 eV.  The vibrational excitation of the hydrogen molecule promotes dissociation on low index surfaces of copper.<ref name="rettner" />

==See also==
*[[Adsorption]]
*[[Physisorption]]

== References ==

{{Reflist}}

== Bibliography ==
*{{cite book |first=F.C. |last=Tompkins |title=Chemisorption of gases on metals |publisher=Academic Press |year=1978 |isbn=0126946507}}
*{{cite journal |first1=L. |last1=Schlapbach |last2=Züttel |first2=A.|title=Hydrogen-storage materials for mobile applications |journal=Nature |date=15 November 2001 |volume=414 |issue=6861 |pages=353–8 |doi=10.1038/35104634 |pmid=11713542 |bibcode=2001Natur.414..353S |s2cid=3025203 |url=http://doc.rero.ch/record/6025/files/zuttel_hsm.pdf }}

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[[Category:Physical chemistry]]
[[Category:Catalysis]]