A Reconsideration of the Conventional Rule in Catalysis and the Consequences

The conventional rule that a catalyst increases a reaction rate by lowering the activation energy according to Arrhenius’ law is the starting point of this article. However, this rule is incomplete, because the corresponding assignment of the true and the apparent activation energies is missing. The general validity of the rule can be determined by considering the entire reaction route depending on the temperature level. It forms an S-shaped curve, starting from the lowest and going to the highest conversion. In the middle of the curve, there is a turning point, which in catalysis is called the “isokinetic point”. This turning point divides the curve into two parts: Below this point, the curve is exponential, and therefore, the Arrhenius equation and even the conventional rule can be applied. This means that the conventional rule does not have a general validity that can be applied to the whole curve. For this reason, an additional rule is introduced for the upper operating state: high activation energy is the condition for very high activity. The further point is the activation energy, which is regarded as an important term in catalysis. According to its definition, the “activation energy” is the “energy barrier” that a reaction must overcome. But this definition does not agree with the roots of this term. In reality, the Arrhenius energy is the temperature coefficient connected with the energy term. The catalyst reduces the temperature of the homogeneous reaction (that means the reaction without the catalyst) to the reaction temperature, and this results in a gain in energy, which will be called “reaction energy” to have a clear distinction with the Arrhenius energy. It is shown that the two energies significantly differ in their magnitudes.