Surface chemistry deals with the phenomenon that occurs at the surfaces or interfaces. The interface or surface is represented by separating the bulk phases by a hyphen or a slash. For example, the inetrface between a solid and a gas may be represented by solid-gas or solid/gas. Due to complete miscibility, there is no interface between the gases. The bulk phases that we come across in surface chemistry may be pure compounds or solutions. The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallization occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.

The mechanism of heterogeneous catalysis involves the following steps:

A. Occurence of chemical reaction on the catalyst's surface through formation of an intermediate

B. Diffusion of reaction products away from catalyst's surface

C. Diffusion of reactants on the surface of the catalyst

D. Adsorption of reactant molecules on the surface of the catalyst

E. Desorption of reaction products from the catalysts surface and thereby, making the surface available again for more reaction to occur.

What is the correct sequence of the steps?

Choose the correct answers from the options given below:

CDAEB

The correct answer is option 3. CDAEB.

Let us explore the detailed mechanism of heterogeneous catalysis by going through each step in the correct sequence:

Heterogeneous catalysis refers to the process where the catalyst is in a different phase (usually solid) from the reactants (which are often gases or liquids). The reaction takes place on the surface of the solid catalyst

Explanation of Steps:

1. Diffusion of Reactants on the Surface of the Catalyst (C)

Reactant molecules from the bulk phase (gas or liquid) move towards the catalyst's surface. This step is influenced by factors like concentration gradient, temperature, and the thickness of the boundary layer surrounding the catalyst. This step involves the migration of adsorbed species across the surface, ensuring that they come into proximity with other reactant molecules or with reactive sites that facilitate the chemical transformation.

2. Adsorption of Reactant Molecules on the Surface of the Catalyst (D)

Adsorption is the process where reactant molecules adhere to the surface of the solid catalyst. The catalyst surface provides active sites, which are specific locations on the surface where the reactants can bind. This step is crucial as it brings the reactant molecules close to each other in a favorable orientation, thereby increasing the chances of a reaction.

Adsorption can be of two types:

Physisorption: Involves weak van der Waals forces.

Chemisorption: Involves the formation of stronger chemical bonds.

3. Occurrence of Chemical Reaction on the Catalyst's Surface through Formation of an Intermediate (A)

At this stage, the adsorbed reactant molecules interact with each other, and a chemical reaction takes place on the catalyst's surface. Often, the catalyst facilitates the formation of a reaction intermediate that lowers the activation energy of the reaction. The intermediate is typically more stable on the catalyst surface than in the gas or liquid phase, which makes the reaction proceed more easily. This step is where the actual chemical transformation occurs, converting reactants into products.

4. Desorption of Reaction Products from the Catalyst's Surface, Making the Surface Available Again for More Reaction to Occur (E)

After the reaction is complete, the newly formed product molecules must detach from the catalyst surface in a process called desorption.  Desorption is essential because it frees up the active sites on the catalyst surface, making them available for new reactant molecules to adsorb. Without desorption, the catalyst surface would become saturated with products, blocking further reactions.

5. Diffusion of Reaction Products Away from the Catalyst's Surface (B)

Finally, the product molecules that have desorbed from the catalyst surface must diffuse away into the surrounding medium (gas or liquid phase). This step ensures that the products are removed from the vicinity of the catalyst, allowing the process to continue with fresh reactant molecules. The diffusion of products away from the surface prevents the reverse reaction and helps in driving the overall process forward.

Each step is crucial and must occur in this specific order for the catalytic process to be efficient. This sequence ensures that reactants are properly activated, converted into products, and that the catalyst remains available to continue facilitating the reaction.

The correct sequence is CDAEB (option 3). This sequence illustrates the full cycle of heterogeneous catalysis, from the initial adsorption of reactants to the final diffusion of products away from the catalyst surface.