Chemisorption

Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate. New chemical bonds are generated at the adsorbant surface. Examples include macroscopic phenomena that can be very obvious, like corrosion, 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 surface creates new types of electronic bonds.^[1]

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 species.

Due to specificity, the nature of chemisorption can greatly differ, depending on the chemical identity and the surface structural properties.

Uses[edit]

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.

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[edit]

Self-assembled monolayers (SAMs) are formed by chemisorbing reactive reagents with metal surfaces. A famous example involves thiols(RS-H) adsorbing onto the surface of gold. This process forms strong Au-SR bonds and releases H₂. The densely packed SR groups protect the surface.

Gas-surface chemisorption[edit]

Adsorption kinetics[edit]

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.^[2]

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

${\displaystyle \Delta G=\Delta H-T\Delta S}$

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.^[3]

Figure 1 is a graph of physisorption and chemisorption energy curves of tungsten and oxygen. 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[edit]

For experimental setups of chemisorption, the amount of adsorption of a particular system is quantified by a sticking probability value.^[3]

However, chemisorption is very difficult to theorize. A multidimensional potential energy surface (PES) derived from 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:

${\displaystyle E(\{R_{i}\})=E_{el}(\{R_{i}\})+V_{\text{ion-ion}}(\{R_{i}\})}$

where ${\displaystyle E_{el}}$ is the energy eigenvalue of the Schrödinger equation for the electronic degrees of freedom and ${\displaystyle V_{ion-ion}}$ is the ion interactions. This expression is without translational energy, rotational energy, vibrational excitations, and other such considerations.^[4]

There exist several models to describe surface reactions: the Langmuir–Hinshelwood mechanism in which both reacting species are adsorbed, and the Eley–Rideal mechanism in which one is adsorbed and the other reacts with it.^[3]

Real systems have many irregularities, making theoretical calculations more difficult:^[5]

• 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.^[5] 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.^[6]

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 as seen in Figure 2.

Dissociation chemisorption[edit]

A particular brand of gas-surface chemisorption is the dissociation of 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 the translational energy and vibrational energy.^[2]

An example is the hydrogen and copper system, one that has been studied many times over. It has a large activation energy of .35 – .85 eV. The vibrational excitation of the hydrogen molecule promotes dissociation on low index surfaces of copper.^[2]

2. Chemical adsorption:

This type of adsorption is also known as chemisorption. It is due to strong chemical forces of bonding type between adsorbate and adsorbent. We can take the example involving the formation of iron nitride on the surface when iron is heated in N2 gas at 623 K. 

Adsorption of gas on a solid is a spontaneous exothermic reaction. Amount of heat liberated when a unit mass of a gas is adsorbed on the surface is called heat of adsorption.

Characteristics of chemical adsorption:

1. This type of adsorption is caused by chemical forces.
2. It is a very strong process.
3. This type of adsorption is almost a single-layered phenomenon.
4. Chemisorption is highly specific and takes place at reaction centres on the adsorbant.
5. Surface area, temperature, nature of adsorbate effects chemisorption.
6. Energy of activation is very high 40 – 400 kJ/mol.

Answer:

                    Chemisorption referred to as activated adsorption as it involves chemical bond formation between reactant and adsorbent molecules. Formation of chemical bond requires high activation energy So, it is activated on increasing temperature. 

Chemisorption involves the diffusion of chemical species from vapor (say) phase to a substrate surface (solid phase). During the process chemical species make chemical bonds with the substrate surface and eventually become a part of the substrate surface. The process is controlled by the diffusion length of chemical species involved and hence the pressure of ambient.

The rate of chemisorption depends on the flux of chemical species incident on the substrate surface. High incident flux results in high chemisorption rate. As we keep on increase the pressure the incident flux gets reduced due to low diffusion length of species and hence a decrease in the rate of chemisorption. Low pressures result in larger diffusion length of chemical species towards the substrate. As a result the flux incident on substrate surface increases which eventually results in high chemisorption rate.

since, adsorption process is exothermic . therefore , physical adsorption occurs readily at low temperature and decreases with increase in temperature. Acc to Le- chatelier's principle. if the temperature is increased , kinetic energy of the gas molecules increases and they leave the surface of the adsorbent. Therefore, rise in temp. decreases the extent of adsorption.

Chemical adsorption first increases with, increase in temperature upto a certain extent and then decreases regularly. A gas adsorbed at low temperature by physical adsorption may changes into chemisorption at a high temperature.for example, hydrogen is a first adsorbed on nickel by van der waals forces. Molecules of hydrogen then dissociate to form hydrogen atoms which are held on the surface by chemisorption. 

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Chemisorption is one of the type of adsorption.In chemisorption,the gas molecules or atoms are held to the solid surface by chemical bonds.Thus we can say that there is chemical forces operate between the adsobate and the adsorbent molecules.
Chemisorption involves a high activation energy,so also reffered to as activated adsorption.It is found in chemisorption that it first increases and than decreases with increase in temperature.When adsorption isobar is plotted the graph first increases and than decreases with T.The initial increase is due to the heat supplied,which act as activation energy required in chemisorption.But later it decreases,due to the exothermic nature of adsorption at equilibrium.

Answer:

Effect of temperature. Temperature has different effect in physisorption and chemisorption. (i) In Physisorption. Adsorption is exothermic process but reverse process i.e., desorption is endothermic. An increase in temperature will favourthe desorption process because molecules of the adsorbate get energy and leave the surface so rate of adsorption decreases with increase in temperature. A graph plotted between amount   of gas adsorbed and temperature at a constant pressure is known as Adsorption Isobar. (ii) In Chemisorption. With initial rise in temperature, the rate of adsorption increases but with further rise in temperature, after a certain limit, adsorption starts decreasing. This is because an initial rise in temperature will provide the molecules necessary activation energy for chemical bond formation so rate of adsorption increases. At a certain temperature all the bonds are formed and now the further increase in temperature will favourdesportion i.e., rate the adsorption now starts decreasing. Effect of pressure. Adsorption of a gas by an adsorbent (solid) depends upon the pressure of the gas. Initially the amount of gas adsorbed for a given amount of adsorbent (x/m) increases rapidly with the increase in pressure. However, as the pressure becomes high and almost the entire surface of the adsorbent gets saturated with the gas, the effect of pressure becomes very small. Ultimately a stage is reached when no more adsorption occurs even if the pressure is increased. This stage is known as saturation Stage and pressure applied is known as saturation pressure. 

Chemical adsorption, or chemisorption, is a process resulting from a chemical bond between adsorbate molecules and specific surface locations on a material, known as active sites. This interaction is much stronger than physical adsorption, or physisorption, which takes place on all surfaces if temperature and pressure conditions are favorable. Chemisorption only occurs on clean active sites and, unlike physisorption, ceases when the adsorbate can no longer make direct contact with the surface, making chemisorption a single layer process.

Chemisorption measurement techniques are useful for evaluating physical and chemical properties of materials that are critical for process / reaction performance. Primarily, chemisorption is used to evaluate the number of available active sites to increase the rate of, or catalyze, chemical reactions. Other properties can include the (reduction or oxidation) temperature at which catalysts become active, strength of specific types of active sites, or ability of materials to perform after reduction/oxidation cycles.

Chemisorption measurements are important for characterization of catalysts used in several industries including oil and gas (e.g. petroleum refining, syngas conversions, biofuel production, fuel cells), petrochemicals and fine chemicals (e.g. hydrogen production, polymers and plastics production), environmental (e.g. automotive catalytic converters, green chemistry),  and many others.

Analyses can be performed using either static or dynamic flow methods. Either method of chemisorption, conducted at a temperature of interest, can determine the number of accessible active sites, active surface area, degree of dispersion, and active particle (crystallite) size.  Pulse chemisorption is commonly used to probe strong active sites only, while the static technique can distinguish between strong or weak active sites.

Since industrial catalytic applications often involve changes in reaction temperature, non-isothermal methods are also available including Temperature-Programmed Reduction (TPR), Temperature Programmed Oxidation (TPO), and Temperature Programmed Desorption (TPD). TPR measurements are used to evaluate the reducible sites, such as metal oxides, while TPO measurements are used to evaluate the oxidative sites, such as metals or carbonaceous deposits on catalysts.  Temperature Programmed Desorption (TPD) is used to evaluate the number, relative strength and heterogeneity of the active sites, such as solid or supported acid catalysts.

PTL offers the following chemisorption analyses which are highly customizable to client specific parameters:

Chemisorption Tests:

Static (Volumetric) Chemisorption analysis
Dynamic or Pulse Chemisorption analysis
Pulse Chemisorption using liquid vapors


Temperature-Programmed Studies:

Temperature-Programmed Reduction (TPR)
Temperature-Programmed Desorption (TPD)
Temperature-Programmed Oxidation (TPO)


Other Chemisorption Experiments:

Heat of Desorption, first order Kinetics
Isosteric Heat of Adsorption





Q1: What is the main difference between chemisorption and physical
adsorption?
A1: Physical adsorption involves the forces of molecular interaction which
embrace permanent dipole, induced dipole, and quadruple attraction. For
this reason it is often termed van der
Waals adsorption. Chemisorption, on the other hand, involves the
rearrangement of the electrons of the interacting gas and solid, with
consequential formation and rupture of chemical bonds. Physical
adsorption is characterised by enthalpy changes that are small, typically in
the range -10 to -40 kJ mol-1
(heats of adsorption of 10- 40 kJ mol-1
),
whereas heats of chemisorption are rarely less than 80 kJ mol-1
and often
exceed 400 kJ mol-1
.
Q2: What is the significance of chemisorption in catalysis?
A2: 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. 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.
Q3: What makes zeolites special when compared with other inorganic
oxide materials?
A3: The combination of many properties, among them: the microporous
character of the uniform pore dimensions, the ion exchange properties, the
ability to develop internal acidity, the high thermal stability, the high internal
surface area. These make zeolites unique among inorganic oxides.
Q4: Are zeolites stable?
A4: Many zeolites are thermally stable up to 500 °C. Some are stable in an
alkaline environment, and some are stable in acidic media. They are also
stable to ionizing radiation and can be used to adsorb radioactive cations.
Q5: What quantitative information can be obtained from TPD?
A5: The area under a TPD curve is proportional to the initial coverage of
the adsorbate before desorption. If these areas can be calibrated, e.g.
against ordered patterns in LEED, or against a known saturation coverage,
TPD can be used to determine the surface coverage. A set of TPD curves
contains highly valuable information on the concentration or surface
coverage of species, and determining these, such that they
can be combined with structures, vibrations or reactivity patterns, is one of
the most useful applications of the technique.
Q6: What is meant by the term “texture” of a catalyst support?
A6: Specific surface area, pore volume, pore structure.




Definition

Adsorption Isotherm is a curve that expresses the variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature.

Freundlich Adsorption Isotherm

In 1909, German scientist Freundlich provided an empirical relationship between the amount of gas adsorbed by a unit mass of solid adsorbent and pressure at a particular temperature. It is expressed using the following equation –

x/m = k.P1/n (n > 1)

where ‘x’ is the mass of the gas adsorbed on mass ‘m’ of the adsorbent at pressure ‘P’. ‘k’ and ‘n’ are constants that depend on the nature of the adsorbent and the gas at a particular temperature.

The mass of the gas adsorbed per gram of the adsorbent is plotted against pressure in the form of a curve to show the relationship. Here, at a fixed pressure, physical adsorption decreases with increase in temperature. The curves reach saturation at high pressure. Now, if you take the log of the above equation –

log x/m = log k + 1/n log P

To test the validity of Freundlich isotherm, we can plot log x/m on the y-axis and log P on the x-axis. If the plot shows a straight line, then the Freundlich isotherm is valid, otherwise, it is not. The slope of the straight line gives the value of 1/n, while the intercept on the y-axis gives the value of log k.


Freundlich Isotherm


Limitations of Freundlich Isotherm

Freundlich isotherm only approximately explains the behaviour of adsorption. The value of 1/n can be between 0 and 1, therefore the equation holds good only over a limited range of pressure.

When 1/n = 0, x/m is constant, the adsorption is independent of pressure.
When 1/n =1, x/m = k P, i.e. x/m ∝ P, adsorption is directly proportional to pressure.


Experimental results support both of the above mentioned conditions. At high pressure, the experimental isotherms always seem to approach saturation. Freundlich isotherm does not explain this observation and therefore, fails at high pressure.

The Freundlich isotherm was followed by two other isotherms – Langmuir adsorption isotherm and BET adsorption isotherm. Langmuir isotherm assumed that adsorption is monolayer in nature whereas BET isotherm assumed that it is multi-layer.









The term Adsorption was first coined in 1881 by a German physicist named Heinrich Kayser. Adsorption is often described as a surface phenomenon where particles are attached to the top layer of material. It normally involves the molecules, atoms or even ions of a gas, liquid or a solid in a dissolved state that are attached to the surface.

Adsorption is mainly a consequence of surface energy. Generally, the surface particles which can be exposed partially tend to attract other particles to their site. Interestingly, adsorption is present in many physical, natural, biological and chemical systems and finds its use in many industrial applications. We will learn about the concept in detail below.

Table of Content

What is Adsorption?
Types of Adsorption
Physisorption and Chemisorption Adsorption Characteristics
Adsorption Isotherm
Differences Between Absorption and Adsorption
Applications of Adsorption
Adsorption Questions




What is Adsorption?

Adsorption is a process which involves the accumulation of a substance in molecular species in higher concentration on the surface. If we look at Hydrogen, Nitrogen and Oxygen, these gases adsorb on activated charcoal. Meanwhile, we have to note that adsorption is different from absorption. The two processes involve totally different mechanisms.

For the adsorption process, two components are required,

Adsorbate: Substance which is deposited on the surface of another substance. For example, H2, N2 and O2 gases.
Adsorbent: Surface of a substance on which adsorbate adsorbs. For example, Charcoal, Silica gel, Alumina.


Also Read: Surface Chemistry



Types of Adsorption

On the basis of interaction forces between adsorbate and adsorbent, adsorption is of two types. 

Physical adsorption:


This type of adsorption is also known as physisorption. It is due to weak Van der Waals forces between adsorbate and adsorbent. 

For example, H2 and N2 gases adsorb on coconut charcoal.

Chemical adsorption:


This type of adsorption is also known as chemisorption. It is due to strong chemical forces of bonding type between adsorbate and adsorbent. We can take the example involving the formation of iron nitride on the surface when iron is heated in N2 gas at 623 K. 

Adsorption of gas on a solid is a spontaneous exothermic reaction. Amount of heat liberated when a unit mass of a gas is adsorbed on the surface is called heat of adsorption.


Physisorption and Chemisorption Adsorption Characteristics

Characteristics of physical adsorption:

This type of adsorption is caused by physical forces.
Physisorption is a weak phenomenon.
This adsorption is a multi-layered process.
Physical adsorption is not specific and takes place all over the adsorbant.
Surface area, temperature, pressure, nature of adsorbate effects physisorption.
Energy for activation is low (20 – 40 kg/mol).


Characteristics of chemical adsorption:

This type of adsorption is caused by chemical forces.
It is a very strong process.
This type of adsorption is almost a single-layered phenomenon.
Chemisorption is highly specific and takes place at reaction centres on the adsorbant.
Surface area, temperature, nature of adsorbate effects chemisorption.
Energy of activation is very high 40 – 400 kJ/mol.




Adsorption Isotherm

Adsorption is usually described by isotherms. It is due to the fact that temperature plays an important role or that it has a great effect on the whole process. Moreover, there are several isotherm models that are used to describe the adsorption technique. These include;

Freundlich Theory

Freundlich adsorption isotherm is obeyed by the adsorption where the adsorbate forms a monomolecular layer on the surface of the adsorbent.

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x represents the amount of gas adsorbed on the m gram of adsorbent, K and n are adsorption constants, ‘p’ is pressure n always greater than one. 

A major drawback of Freundlich adsorption isotherm it fails at high pressure. It could not explain the multi-layered adsorption process.

Langmuir Theory

In 1916 Langmuir proposed theory of adsorption of a gas on the surface of the solid to be made up of elementary sites each of which would adsorb one gas. It is assumed that all adsorption sites are equivalent and the ability of a gas molecule to get bound to any one site is independent whether or not the neighbouring sites are occupied. Additionally, it is also assumed that dynamic equilibrium exists between adsorbed and non adsorbed gas molecule.

Following principles can be obtained from Langmuir adsorption isotherm 

The gas adsorbed behaves ideally in a vapour phase.
Only monolayer adsorption takes place.
The surface of the solid is homogeneous.
There are no lateral interactive force between the adsorbate molecule.
The adsorbed gas molecules are localized.


BET theory (after Brunauer, Emmett, and Teller)

The BET theory was proposed by Brunauer, Emmett and Teller in the year 1938. This theory explains the formation of multilayer adsorption during physisorption. This theory also talks about the uniformity in the sites of adsorption of solid surfaces. It assumes that when adsorption occurs at one site it will not affect adsorption at neighbouring sites.


Differences Between Absorption and Adsorption


Absorption
Adsorption
Complete deposition of a substance in another substance is absorption
Deposition of a substance on the surface is known as adsorption.
It is not a surface phenomenon.
It is a surface process.
It is not spontaneous.
Adsorption of gas on solid is spontaneous.
It takes place uniformly throughout.
It does not take place uniformly.
Greater molecular interaction.
Less molecular interaction.
It involves the application of potential in the absorption of water by root hairs.
There is no involvement of potentials during adsorption.
It is not subdivided.
Surface absorption is adsorption it is subdivided.





Applications of Adsorption

1) Air pollution masks:

These consists of silica gel or activated charcoal powder, when dust or smoke are paused through them, those particles get adsorbed on the surface of these materials.

2) Separation of noble gases by Dewar’s flask process:

A mixture of noble gases of Ne, Ar, Kr is passed through Dewar’s flask in presence of heated coconut charcoal. Argon and Krypton gels adsorbed leaving Neon.

3) Purification of water:

By the addition of alum stone to the water, impurities get adsorbed on the alum and water gets purified.

4) Removal of moisture and humidity:

Moisture in the air is removed by placing silica gel on which water molecular gets adsorbed.

5) Adsorption chromatography:

It is used to separate pigments and hormone.

6) Ion exchange method:

In this method of removing the hardness of water, calcium and magnesium ions get adsorbed on the surface of ion exchange resin

7) In metallurgy:

In froth floatation process of concentration of ore, the particle gets adsorbed on the froth.


Adsorption Questions

1. Adsorption of gas on a solid is always exothermic explain?

Solution:

Adsorption of gas on a solid is a spontaneous process. When a gas adsorbs on the solid, due to molecular interaction process, the entropy of gas molecules decreases. To make the process spontaneous, adsorption must be exothermic.

2. Among SO2, CH4, H2 gases which gas adsorbs more on the charcoal and why?

Solution:

Sulphur dioxide adsorbs more than methane and hydrogen since the critical temperature of sulphur dioxide is higher than methane and hydrogen.

3. What do you mean by critical temperature? What is the relation of critical temperature and the gas adsorbed?

Solution:

The temperature above which a gas cannot be liquefied even by the application of high pressure is known as critical temperature. Greater is the critical temperature (To) more in the gas adsorbed on the surface.

4. What are the factors effective adsorption of gases on solids?

Solution:

Nature of gases: Ease is liquefication more is gas adsorbed.
The surface area of adsorbent: More the surface area more gas is adsorbed.
Temperature: Physisorption increases with a decrease in temperature while chemisorption increases with an increase in temperature.
Pressure: It affects only physisorption if pressure increase amount of gas adsorbs also increase.


5. Define activation.

Solution:

When adsorbents are heated in a vacuum from 573 – 623 K, the surface area is increased and it is known as activation.

6. Write about adsorption from solutions?

Solution:

Charcoal pieces are added to the acetic acid solution. Some of the acetic acid molecules adsorb on charcoal. This depends upon,

Nature of adsorbate
The surface area of the adsorbent 
Temperature 
Concentration







    Improved design, effectiveness and performance of catalysts require an in-depth understanding of the surface structure and surface chemistry of the catalytic materials. Chemisorption is utilized to determine a catalyst’s efficiency in promoting a desired reaction as well monitoring the degradation of catalytic activity/regeneration over time.


Chemisorption is used to quantitatively measure the number of surface active sites which are used to promote a specified catalytic reaction. Critical parameters for chemisorption measurement are: the area of the active element, metal dispersion, surface acidity, exposed proportion of the active element.



During the chemisorption process the adsorbing gas or vapor molecule splits into atoms, radicals, or ions that form a chemical bond with the adsorption site. This interaction involves the sharing of electrons between the gas and the solid surface and may be regarded as the formation of a surface compound.

We offer two instrumental techniques for chemisorption analysis, the static volumetric technique and the dynamic (flowing gas) technique. The former technique is performed by the Micromeritics ASAP 2020Plus, which operates over a range of pressures. The dynamic chemisorption technique is utilized by the 3Flex Chemi, Chemisorb and AutoChem series of instruments. These three instruments utilize pulse chemisorption in a dynamic flow system to titrate the active surface.