The activity of Endogas catalysts is determined by the surface area of the active metal. For example, in environmental catalysts, platinum (Pt) is frequently used as the precious metal component, converting unwanted substances such as VOCs, CO, and others into CO2 and water. To achieve the largest possible distribution, or surface area, of the metal, it is deposited onto an unreactive (inert) support material such as aluminum oxide (Pt-Al) or activated carbon (Pt-C).
Figure 1. Gas flow movements (blue) and distribution of metal particles of a catalyst
The degree of this distribution can be measured, among other methods, by chemisorption. For this, a reactive gas, such as hydrogen (H₂), is passed over the catalyst, where it binds irreversibly to the metals. The amount of active metal can then be determined from the amount of gas consumed. The ratio of (surface-)active metal to the total metal content is expressed as the dispersion – derived from the Latin word for distribution. This allows, among other things, the determination of the catalyst’s effectiveness compared to other catalysts or the degree of deactivation.
Figure 2. Pulse Chemisoptions Plot. Plot of the detector signal against the measurement time.
At fixed time intervals, a defined amount of H₂ gas is passed over the sample. When all metal atoms bound to the surface have been reduced, the reactive gas (H₂) is registered by the detector as a pulse.
In addition to dispersion, the reduction and oxidation temperatures at which the catalyst operates optimally can also be determined using temperature-programmed reduction (TRP) and oxidation (TPO), respectively. Hydrogen (H2) is usually used for reduction and oxygen (O2) for oxidation.
Figure 3. Temperature-programmed reduction (TPR) of copper oxide (CuO) with hydrogen (H2).
Summary
Chemisorption, using a reactive gas, allows the surface area of the metal particles responsible for the catalyst’s activity to be determined. In contrast, BET analysis uses an inert gas (N2) which adsorbs non-specifically onto both the support and the metals via physisorption, thus determining the total surface area of the catalyst. Furthermore, TPX analysis allows for a more detailed determination of the catalyst’s reaction behavior.
