Heterogeneous catalysis: For a solid-catalyzed reaction, the overall effectiveness and activity of the solid catalyst most directly depend on which type of adsorption at the catalyst surface?

Introduction / Context:
In heterogeneous catalysis, molecules from the fluid phase must interact with the solid catalyst surface before reaction can proceed. This interaction is governed by adsorption phenomena, which directly influence reaction rate, selectivity, and overall effectiveness of the catalyst pellet.

Given Data / Assumptions:

• Solid catalyst with active sites on its surface.
• Reactants approach from a gas or liquid phase and must first adsorb.
• We are comparing physical adsorption versus chemical adsorption.

Concept / Approach:
Physical adsorption (physisorption) involves weak van der Waals forces and does not substantially alter the electronic structure of the adsorbate. Chemical adsorption (chemisorption) forms stronger, localized chemical bonds, creating activated surface complexes that can proceed to products with lower activation barriers. For most catalytic reactions, the rate-determining elementary steps involve surface reaction of chemisorbed species rather than physisorbed multilayers.

Step-by-Step Solution:
Recognize that catalytic activity requires reactant activation at active sites.Activation typically arises when reactants chemisorb, altering bond strengths and orientations.Therefore, catalyst effectiveness tracks with the extent and appropriateness of chemisorption (coverage, strength, and turnover of adsorbed species).Physisorption may aid initial capture at low temperatures, but it is generally insufficient for bond-breaking/forming steps that define catalysis.

Verification / Alternative check:
Rate expressions derived from Langmuir–Hinshelwood or Eley–Rideal mechanisms assume chemisorbed intermediates; measured apparent activation energies and spectroscopic evidence (e.g., IR of adsorbates) support chemisorption as the kinetic prerequisite.

Why Other Options Are Wrong:

• Physical adsorption: usually too weak to generate reactive surface intermediates.
• Both equally: in most cases, chemisorption dominates the kinetic step; physisorption plays a lesser, precursor role.
• Neither/Capillary condensation only: contradicts established catalytic mechanisms that rely on surface binding and reaction.

Common Pitfalls:
Confusing high surface area (which enhances adsorption capacity) with the nature of adsorption; catalytic turnover requires the right strength of chemisorption, not just more physisorption.

Final Answer:
Chemical adsorption (chemisorption)