Chemical adsorption, also known as chemisorption is the process that results from a chemical bond between adsorbate molecules and particular surface locations on a material which are called active sites.
This article will outline the process of chemisorption and its relationship to catalysis.
An Overview of Catalysis
Catalysis refers to the process in which a reaction is altered by the presence of a non-consumable substance called a catalyst. Catalytic agents may decrease or increase the rate of reaction, decrease the required temperature for the reaction or alter the selectivity of the reaction.
Why is Adsorption So Important in Catalysis
Adsorption is a key step in heterogeneous catalysis, being the process of a gas phase molecule (adsorbate) binding to solid surface atoms (adsorbent) before it can react to form more desirable products. In catalysis, the reactants are the adsorbate and the catalyst is the adsorbent. There are two types of adsorption: physical and chemical. Physical adsorption is weakly bound with a low enthalpy of adsorption (typically 20 kJ/mol) and chemical adsorption is stronger bound (typically 200 kJ/mol).
Chemisorption occurs when a molecule moves close enough to surface atoms so that their electron clouds overlap. The adsorbate and adsorbent share electrons, leading to the formation of chemical bonds.
In molecular adsorption, the adsorbate stays intact such as in alkene binding using platinum. Dissociation adsorption involves more than one bond-breaking concomitantly. The barrier to dissociation impacts the rate of adsorption with an example being binding H2 to a metal catalyst.
In physisorption, molecules are attracted to the surface atoms due to van der Waals forces. No chemical bonds are formed between the adsorbent and adsorbate and their electronic states are not altered.
In catalysis, when a reactant modules physisorbs to a catalyst, it is usually described as being in a precursor state.
Benefits of Chemisorption
Chemisorption methods with probe molecules such as CO, H2 and NH3 are well suited to the evaluation of chemical and physical properties of materials that are crucial for reaction/process performance. Chemisorption is primarily used to quantify the number of active sites available to create catalysis in chemical reactions.
Chemisorption measurements are used for catalysis characterization in a range of industries such as petroleum refining, biofuel production, plastic production, and environment as well as many others.
Catalysis can be studied using quadrupole mass spectrometers. Hiden Analytical has developed a range that would be suitable for your research needs. Their website contains vast amounts of information on catalysis and how their products can help.