Assessing Crucial Parameters for Catalyst Performance: TON & TOF, How to Calculate?

TON

Turnover Number (TON) represents the number of catalytic reactions or the amount of target product generated or reactant consumed per active site under certain temperature, pressure, reactant ratio, and a specific extent of reaction. It is a measure of the stability of the photocatalyst, indicating the turnover number that active sites can sustain before complete deactivation.

As the number of active sites in photocatalysts is usually difficult to determine, the ratio of reaction electrons to the atomic or surface atomic number of the photocatalyst is commonly used for TON calculations. The number of reaction electrons is calculated based on the amount of released products.

Since the atomic number of the photocatalyst is greater than the number of active sites, the calculated TON value < real TON. Therefore, normalizing catalytic activity to the mass of the photocatalyst is inaccurate, and mmol·h⁻¹·g⁻¹ cannot entirely measure the catalytic activity of the photocatalyst. Although the amount of photocatalyst is suitable for specific experimental conditions, photocatalytic activity is usually not proportional to the quantity of the photocatalyst. Thus, the amount of photocatalyst should be optimized for each different experimental system.

TOF

Turnover Frequency (TOF) represents the number of catalytic reactions or the amount of target product generated or reactant consumed per unit time per active site under certain temperature, pressure, reactant ratio, and a specific extent of reaction. It is a crucial indicator parameter that reflects the intrinsic activity of the catalyst.

Currently, in systems for evaluating catalyst activity, the number of active sites of the catalyst can be determined using Brunauer–Emmett–Teller (BET) tests or electrochemical measurements.

TOF values cannot be simply calculated by the ratio of TON to time. Data used to calculate TOF values need to consider the following issues:

1. The generally accepted values are for data where the conversion rate is <10%, i.e., the reactor is equivalent to a differential reactor at this point;

2. The influence of the diffusion of reactants or products inside and outside the catalyst should be excluded;

3. Instantaneous reaction rates can be used to calculate TOF values.

In homogeneous electrochemical reactions, TOF only represents the activity of the catalyst near the electrode surface in the reaction diffusion layer and is independent of the catalyst in the bulk electrolyte.

The formula is more accurate when applied to structurally single and uniformly surfaced materials. However, for actual catalysts with complex compositions and surface structures, obtaining accurate TOF values remains a significant challenge. Nevertheless, TOF can still serve as an essential parameter for comparing the activity of catalysts of the same type (Information compiled based on literature; corrections are welcome).

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