Questions: Surface Chemistry and Heterogeneous Catalysis

Question 1

A research team tests a series of transition metals for a catalytic reaction and finds that osmium (which binds the substrate very strongly) shows very low activity despite high surface coverage of the reactant. How does the Sabatier principle explain this?

• A. Osmium has too few surface atoms to provide adequate active sites for the reaction
• B. Osmium's strong binding prevents product desorption, leaving active sites permanently blocked
• C. Osmium binds the substrate too weakly, so surface coverage remains negligible
• D. Osmium does not participate in the Langmuir-Hinshelwood mechanism and thus cannot catalyze the reaction

Answer: (B)

The Sabatier principle states that optimal catalysts bind adsorbates with intermediate strength. Osmium sits on the right (strong-binding) side of the volcano plot. While it adsorbs and activates the substrate readily — which is why coverage is high — the products also bind strongly and cannot desorb, leaving active sites permanently occupied. The rate-limiting step becomes product release, not substrate activation. This is the opposite failure mode from weak binding (left of volcano), where the substrate barely sticks. Strong binding sounds beneficial but poisons the catalyst.

Question 2

In the industrial oxidation of SO₂ to SO₃ on vanadium oxide, an oxygen atom from the catalyst lattice is incorporated into the SO₃ product, leaving an oxygen vacancy that is subsequently refilled by gas-phase O₂. Which heterogeneous catalysis mechanism does this describe?

• A. Langmuir-Hinshelwood — both SO₂ and O₂ adsorb on the surface and then react with each other
• B. Eley-Rideal — SO₂ adsorbs on the surface and then reacts with gas-phase O₂ directly
• C. Mars-van Krevelen — a lattice atom from the catalyst itself participates in the reaction
• D. Sabatier mechanism — the reaction proceeds through a surface intermediate of optimal binding strength

Answer: (C)

Mars-van Krevelen is distinctive because the catalyst is a reactant, not merely a surface. Lattice oxygen from vanadium oxide becomes part of the product (SO₃), leaving a vacancy, which is then re-oxidized by gas-phase O₂. This differs from LH (both reactants adsorb first, then react on the surface) and ER (one adsorbs, the other reacts from the gas phase without adsorbing). 'Sabatier mechanism' is not a named mechanism — the Sabatier principle is a design guideline about optimal binding strength, not a description of how reactants meet.

Question 3

A catalyst on the weak-binding (left) side of a volcano plot can in principle be improved by adding electron-donating promoters or alloying with a more reactive metal.

• T. True
• F. False

Answer: True

True. If a catalyst is on the weak-binding side, low surface coverage is the limiting factor — reactants barely stick, so few are available to react. Electron-donating promoters strengthen the metal–adsorbate bond, increasing coverage and moving the catalyst toward the volcano peak. Alloying with a more reactive metal has the same effect. This is the practical payoff of the Sabatier principle and volcano framework: knowing which side of the peak you're on reveals the direction to push binding energy for improvement.

Question 4

Two heterogeneous catalysts with identical turnover frequencies (TOF) will necessarily show the same catalytic activity per gram of material.

• T. True
• F. False

Answer: False

False. Turnover frequency measures rate per active site — it is a measure of intrinsic site quality. Activity per gram also depends on the number of active sites, which is determined by surface area and active site density. A catalyst with the same TOF but 10× higher BET surface area (e.g., from smaller particle size) will produce 10× more product per gram per second. TOF separates these two contributions: a catalyst can have excellent intrinsic activity (high TOF) but poor industrial performance due to low surface area, or vice versa. Conflating TOF with overall activity is a common source of error in comparing catalysts.

Question 5

Explain why volcano plots peak at intermediate binding energy rather than at the maximum binding strength, using the distinction between adsorption and desorption steps.

Model answer: At very low binding energy, reactants barely adsorb and surface coverage is negligible — almost no molecules are available on the surface to react. Activity is low because the catalyst cannot hold onto its reactants. At very high binding energy, reactants adsorb strongly and coverage is high, but products also bind strongly and cannot desorb, permanently blocking active sites. Activity is again low because the catalyst cannot release its products. Peak activity occurs at the intermediate binding energy where there is sufficient coverage to drive the reaction AND products desorb quickly enough to regenerate active sites for the next cycle. The volcano reflects a fundamental kinetic competition between activation and regeneration.

The volcano shape is an empirical confirmation that catalysis requires cycle completion — after each reaction, the product must leave so a new reactant can take its place. Both extremes break the catalytic cycle in different ways: weak binding breaks it at the adsorption step, strong binding breaks it at the desorption step. The Sabatier principle names the optimum; volcano plots locate it experimentally for different metal–adsorbate systems.