Surface Chemistry, Catalysis, and Heterogeneous Reactivity
Reactions at Interfaces
Adsorption and Surface Energetics
Heterogeneous catalysis begins with adsorption. Adsorption may be physisorption or chemisorption, and its strength is often described through adsorption energy, surface coverage, and site specificity. The Langmuir model treats monolayer adsorption on equivalent sites with no lateral interactions.
Catalysis Modes
Heterogeneous catalysis
- Reactants bind to a solid surface
- Easy separation of catalyst
- Site heterogeneity and diffusion matter
Homogeneous catalysis
- Catalyst and reactants share phase
- Precise molecular control
- Mechanistic characterization often easier
Scaling relations
On many catalyst surfaces, adsorption energies of intermediates correlate with each other, producing volcano plots that reveal an optimal binding strength for activity.
What does a Langmuir isotherm assume?
The Langmuir model assumes uniform, independent sites and a monolayer limit.
Correct answer: Equivalent independent adsorption sites and monolayer coverage
Surface Reaction Mechanisms
Mechanistic descriptions include Langmuir–Hinshelwood, Eley–Rideal, Mars–van Krevelen, and spillover processes. Surface reconstruction, coverage effects, and defect chemistry can dramatically alter apparent activity and selectivity.
What is the key idea behind a volcano plot in catalysis?
Too-weak or too-strong adsorption both reduce turnover.
Correct answer: Catalytic activity is maximized at an intermediate binding strength between weak adsorption and overstrong poisoning.
Evaluating a Surface Catalyst
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Step 1: Identify the active site and likely adsorbates.
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Step 2: Estimate adsorption strengths and coverage.
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Step 3: Determine the rate-limiting surface step.
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Step 4: Check for diffusion or transport limitations.
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Step 5: Compare predicted trends with volcano or scaling relations.
Which statement best describes chemisorption?
Chemisorption involves chemical bonding between adsorbate and surface.
Correct answer: Formation of stronger surface bonds with electronic structure changes
Why can defects be catalytically important on surfaces?
Steps, vacancies, and edges can be more reactive than ideal terraces.
Correct answer: They often create unique coordination environments and electronic states that lower activation barriers.