Catalyst Characterisation and Surface Reactivity
A catalyst is used to increase the rate of a particular chemical reaction. This can have a number of benefits:
- Increase the rate of a reaction, without requiring a higher temperature.
- Allow the use of a lower temperature, for the same reaction rate.
- Selectively enhance one particular reaction pathway, to avoid other, undesirable reactions from taking place.
A more technical definition is: “A substance that increases the rate of a reaction without modifying the overall standard Gibbs energy change in the reaction” (from the IUPAC Gold Book)
In industrial applications, most catalysts are solids, usually finely divided metals, and often dispersed on a highly porous support, such as a zeolite. The chemical reaction takes place at active sites on the surface, where the energetic barrier which determines the reaction rate is reduced.
However, for a catalyst to be efficient, a number of other processes must also occur, to provide the reactants to the active site, and to remove the reaction products.
The steps required for a catalytic reaction can be divided into the following stages:
- Adsorption of reactants to the surface
- Surface diffusion to the reaction site
- Surface diffusion of the product molecules
- Desorption of the product molecules
To maximise the efficiency of the catalyst, all of these processes should be as fast as possible. However, the rates of the different processes depend on both the conditions of Temperature, Pressure, Concentration, and on each other, so optimisation of a catalytic process often requires a careful balancing of different effects. Additionally, the properties of the catalyst can change, for example, due to changes in physical structure or chemical state of the support or active component, presence of impurities or by-products which bind tightly to the reactive sites or block the diffusion of the reactants or products.
At its simplest, characterisation of a catalyst involves measuring the rate of formation of the reaction products under varying conditions and treatments. This allows comparison of different catalyst formulations, and choice of the correct operating conditions.
More complex methods allow the quantification of a variety of properties of the catalyst. Some of these methods are detailed below.
Experimental Methods of Catalyst Characterisation
Temperature Programmed Methods
Some related possibilities include:
- CATLAB-PCS for Pulsed Reaction Mass Spectrometry (PRMS)
- Multicomponent gas and vapour sorption: gravimetric measurements with the IGA-003
- Thermal Desorption Analysis for Hydrogen Storage materials with IMI-FLOW
- Sorption of vapours with IGA-002
If you would like to discuss your application, please contact us.