Physical adsorption is the process by which a substance adheres to the surface of another substance through weak intermolecular forces. Chemical adsorption, on the other hand, involves the formation of chemical bonds between the adsorbate and the adsorbent. Let's analyze each statement to determine their accuracy.

a) Physical adsorption is multi-layer, non-directional, and non-specific

- Multi-layer: Physical adsorption can occur in multiple layers, meaning that the adsorbate molecules can stack on top of each other on the surface of the adsorbent. This is due to the weak van der Waals forces that are involved in physical adsorption.
- Non-directional: The forces involved in physical adsorption are non-directional, meaning that they act equally in all directions. This lack of directionality allows the adsorbate molecules to be easily removed from the surface of the adsorbent.
- Non-specific: Physical adsorption is a non-specific process, meaning that it does not depend on the specific chemical properties of the adsorbate or the adsorbent. Any molecule that can interact with the surface through weak intermolecular forces can undergo physical adsorption.

b) Chemical adsorption is due to valence of atoms

- Chemical adsorption involves the formation of chemical bonds between the adsorbate and the adsorbent. These bonds are typically formed through the sharing or transfer of valence electrons between the atoms of the adsorbate and the adsorbent. Therefore, it is correct to say that chemical adsorption is due to the valence of atoms.

c) Physical adsorption is due to valence of atoms

- This statement is incorrect. Physical adsorption is not due to the valence of atoms. Instead, it is primarily governed by weak intermolecular forces such as van der Waals forces, dipole-dipole interactions, and hydrogen bonding. These forces arise from temporary fluctuations in electron distribution within molecules and do not involve the sharing or transfer of valence electrons.

d) Chemical adsorption is stronger than physical adsorption

- This statement is correct. Chemical adsorption is generally stronger than physical adsorption due to the formation of chemical bonds. Chemical bonds are typically stronger than intermolecular forces, resulting in a stronger adsorbate-adsorbent interaction. Chemical adsorption is often more difficult to reverse or remove compared to physical adsorption.

In summary, the correct statements are a) Physical adsorption is multi-layer, non-directional, and non-specific, b) Chemical adsorption is due to the valence of atoms, and d) Chemical adsorption is stronger than physical adsorption.