Catalytic activity describes a catalyst’s ability 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 a catalyst’s effectiveness and plays a crucial role in selecting and optimizing catalysts for industrial applications. Various methods exist 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 to either the volume of the reactor, the mass of the catalyst, or the specific surface area of the catalyst. These specific measures allow for a standardized representation and a precise comparison of catalysts of different types.
The reaction rate can be defined, for example, as the change over time in the amount of substance of a reactant in the reaction volume:
Activation energy
The catalytic activity is strongly influenced by the temperature T, as the reaction rate increases exponentially with temperature. This is described by the Arrhenius equation:
Here, is the reaction rate constant, is the activation energy, is the pre-exponential factor corresponding to the maximum reaction rate at infinitely high temperature, and is the universal gas constant. The activation energy, on the other hand, is a measure of the energy barrier reduced by the catalyst. A catalyst that leads to a lower activation energy exhibits higher activity because it facilitates the reaction. For 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 sites. Therefore, it is useful to normalize the catalytic activity to the specific surface area to create an objective basis for comparison.
Units of measurement
Turnover frequency (TOF) is a precise measure of catalytic activity, as it describes the number of molecules converted per second per active site. This quantity is independent of the catalyst mass and reveals the intrinsic efficiency of the active sites. Therefore, TOF is a suitable metric 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 area in m²/g.
Measurement method
A commonly used, simpler method for determining activity is the measurement of turnover, i.e., the proportion of converted reactants compared to the originally used quantity:
A catalyst with a higher TOF will also achieve a higher conversion rate. The conversion rate can be determined as a function of temperature or space velocity (GSVH) to identify the parameters at which the catalyst achieves the desired conversion rate.
Conclusion
Catalytic activity is a property that depends on both the physical and chemical properties of the catalyst and the process conditions. Various approaches to characterizing this activity allow for the comparison of catalysts and the evaluation of the efficiency of catalytic processes. A deep understanding of the underlying mechanisms is essential for the development and optimization of high-performance catalysts.
