Characteristics of Catalysts

A catalyst obeys a well-defined set of characteristics, regardless of the specific reaction it is accelerating (or retarding).

1. Only a very small quantity of catalyst is generally needed for a reaction run on a bulk scale — often a mere pinch is enough.
2. The catalyst may undergo minor physical changes (such as a change in particle size or crystalline form) during the reaction, but its mass and chemical composition remain unchanged at the end.
3. A catalyst cannot INITIATE a reaction that is not already occurring at all — it cannot start a reaction from nothing. What it CAN do is speed up a reaction that is already proceeding, even if only very slowly.
4. A solid catalyst is more effective when used in a finely divided form, since this maximises the exposed surface area (and hence the number of active centres) available to the reactants.
5. A catalyst is specific in nature — it catalyses only a particular type of reaction, not reactions in general.
6. In a reaction that reaches equilibrium, the presence of a catalyst reduces the TIME needed to reach that equilibrium, but it does not shift the POSITION of equilibrium and does not change the value of the equilibrium constant — the catalyst speeds up the forward and reverse reactions by exactly the same factor.
7. A catalyst is most effective at one particular temperature, called its optimum temperature.
8. The presence of a catalyst generally does not change the nature of the products formed — for example, $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$ is slow in the absence of a catalyst but proceeds fast in the presence of a platinum catalyst, without changing what the product actually is.

Promoters and catalytic poisons. A promoter is a substance that, though not itself a catalyst, increases the activity of a catalyst when present alongside it. In the Haber process, molybdenum acts as a promoter that boosts the activity of the iron catalyst; $\text{Al}_2\text{O}_3$ can similarly serve as a promoter for the same iron catalyst. A catalytic poison, conversely, is a substance that decreases or completely destroys a catalyst's activity when added to the reaction. In the oxidation of $\text{SO}_2$ to $\text{SO}_3$ over a platinum catalyst, $\text{As}_2\text{O}_3$ acts as a poison by blocking the catalyst's active sites; in the Haber process, the iron catalyst is poisoned by the presence of $\text{H}_2\text{S}$; and in the catalytic combination of $\text{H}_2$ and $\text{O}_2$ to give water over a platinum catalyst, carbon monoxide (CO) acts as a catalytic poison. …