Question

(a) Most commercial heterogeneous catalysts are extremely finely divided solid materials. Why is particle size important? (b) What role does adsorption play in the action of a heterogeneous catalyst?

Short answer

(a) Particle size is important in commercial heterogeneous catalysts because smaller particles provide a larger surface area per unit mass, leading to a greater number of active sites for catalytic reactions. Consequently, this results in higher catalytic activity and efficiency. (b) Adsorption plays a crucial role in heterogeneous catalysis by increasing reactant concentrations on the catalyst surface and promoting more effective collisions between reactants, thus enhancing the overall reaction rate. It consists of three steps: adsorption of reactants, reaction on the catalyst surface, and desorption of products.

Step-by-step solution

(a) Importance of particle size in heterogeneous catalysts

Particle size plays a major role in heterogeneous catalysts because it directly affects the surface area. When the particle size is smaller, the surface area of the catalyst per unit mass increases, leading to a larger number of active sites available for catalytic reactions. This increased surface area results in a higher catalytic activity for reactions occurring on the surface of the catalyst. Therefore, commercial heterogeneous catalysts are designed to have small, finely divided particles to maximize their activity, effectiveness, and overall efficiency.

(b) Role of adsorption in heterogeneous catalysis

Adsorption is the process where a substance, in this case, the reactants, form a thin layer on the surface of a solid or a liquid, such as the heterogeneous catalyst. In heterogeneous catalysis, the catalyst is typically a solid, while the reactants are in a gas or liquid phase. The role of adsorption in heterogeneous catalysis can be divided into the following steps: 1. Adsorption of reactants: Reactant molecules are adsorbed onto the surface of the catalyst, which increases their concentration on the catalyst's surface. This causes the probability of effective collisions between the reactants to increase, thus increasing the reaction rate. 2. Reaction on the catalyst surface: Within the adsorbed layer, the reactant molecules are brought closer together, allowing them to form bonds more easily. The energy required for the reaction to proceed is often lower due to the orientation and proximity of the reactants, which enhances the reaction rate. 3. Desorption of products: After the reaction occurs, the product molecules no longer need to remain on the catalyst's surface, so they desorb and are released into the surrounding medium. Overall, adsorption plays a vital role in the action of a heterogeneous catalyst by increasing reactant concentrations on the catalyst surface and promoting more effective collisions between the reactants, which in turn enhances the overall reaction rate.