Problem 76 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

(a) Particle size in commercial heterogeneous catalysts is important to increase the available surface area, providing more active sites for reactants to interact and leading to higher catalytic activity. Smaller particles also improve the overall efficiency and dispersion of the catalyst. (b) Adsorption plays a vital role in heterogeneous catalysis by providing a surface for reactants to come into close contact, allowing an efficient reaction. The process involves adsorption of reactants, formation of new intermediates or products, and desorption of products from the surface. Adsorption helps lower the activation energy required for the reaction, ensuring efficient formation of products and minimizing side reactions.

Step 1: (a) Importance of particle size in heterogeneous catalysts

Particle size is crucial in most commercial heterogeneous catalysts because the catalytic reactions take place on the surface of the catalyst particles. When the particle size is smaller, the surface area of the catalyst increases, providing more active sites for the reactants to interact with, leading to higher catalyst activity. A finely divided catalyst also has higher dispersion, which means that the reactants have easier access to the active sites, enhancing the overall efficiency of the catalyst. Small particle size ensures better utilization of the catalyst material and improves the overall productivity of the catalytic process.

Step 2: (b) Role of adsorption in heterogeneous catalysis

Adsorption plays a vital role in the action of a heterogeneous catalyst by providing a surface where the reactants can come into close contact, allowing the chemical reaction to take place efficiently. The process can be explained in the following steps: 1. Reactants adsorb onto the surface of the catalyst by forming bonds with the active sites. This adsorption helps to concentrate the reactants near the catalyst, increasing the likelihood of a reaction. 2. The adsorbed reactants undergo a chemical reaction on the surface of the catalyst to form new intermediates or products. 3. Finally, the products desorb from the catalyst surface and are released into the bulk of the reaction medium. Adsorption helps in lowering the activation energy required for the reaction, making it proceed at a faster rate. It spatially confines the reactants and intermediates near the catalyst's surface, ensuring that the products are formed efficiently. In essence, adsorption acts as a driving force for heterogeneous catalysis, promoting the desired reaction while minimizing side reactions.