Catalytic activity describes the ability of a catalyst to accelerate a chemical reaction by increasing the reaction rate compared to the uncatalyzed reaction under otherwise identical process conditions. Activity is a key measure of the effectiveness of a catalyst and plays a crucial role in the selection and optimization of catalysts for industrial applications. There are various methods to quantify catalytic activity and compare different catalysts.
Basic concepts
The activity of a catalyst is often expressed as a reaction rate, which is related either to the volume of the reactor, the catalyst mass or the specific surface area of the catalyst. These specific measurements allow a standardized representation and a precise comparison of catalysts of different nature.
The reaction rate can be defined, for example, as the change in the amount of a reactant in the reaction volume over time:
activation energy
The catalytic activity is strongly influenced by the temperature T, since the reaction rate increases exponentially with temperature. This is described by the Arrhenius equation:
The reaction rate constant, the activation energy, is the pre-exponential factor, which corresponds to the maximum reaction rate at infinitely high temperature, and the universal gas constant. The activation energy, on the other hand, is a measure of the energy barrier that is reduced by the catalyst. A catalyst that leads to a lower activation energy has a higher activity because it facilitates the reaction. In heterogeneous catalysts, the specific surface area is a key factor for catalytic activity. A higher surface-to-volume ratio increases the probability that reactants will interact with active centers. It is therefore useful to normalize the catalytic activity to the specific surface area in order to create an objective basis for comparison.
measurement units
The turnover frequency (TOF) is a precise measure of catalytic activity, as it describes the number of molecules converted per second and per active center. This value is independent of the catalyst mass and provides information about the intrinsic efficiency of the active centers. The TOF is therefore a suitable measurement for comparing the efficiency of different catalysts.
The determination of the active centers can be done indirectly via a BET or chemisorption analysis, which indicates the specific or active surface in m²/g.
Measurement method
A frequently used, simpler method for determining activity is to measure the conversion, i.e. the proportion of reactants converted compared to the amount originally used:
A catalyst with a higher TOF will also achieve a higher conversion. The conversion can be determined as a function of temperature or space velocity (GSVH) to determine the parameters at which the catalyst achieves a desired conversion.
Conclusion
Catalytic activity is a property that depends on both the physical and chemical properties of the catalyst and the process conditions. Different approaches to characterizing activity allow catalysts to be compared and the efficiency of catalytic processes to be evaluated. A deep understanding of the underlying mechanisms is essential for the development and optimization of high-performance catalysts.
