Questions: Heterogeneous Catalysis on Metal Surfaces

Question 1

The Haber-Bosch process for ammonia synthesis (N₂ + 3H₂ → 2NH₃) uses iron as the catalyst. Why is iron near the peak of the volcano plot for this reaction?

• A. Iron is the cheapest transition metal available in industrial quantities
• B. Iron binds N₂ strongly enough to dissociate the N≡N triple bond but not so strongly that the nitrogen atoms cannot be hydrogenated and the product NH₃ cannot desorb — it balances the competing requirements of activation and product release
• C. Iron has the highest surface area of any metal
• D. Iron is the most electronegative transition metal, enabling it to donate electrons to nitrogen

Answer: (B)

The Sabatier principle states that optimal catalysis requires intermediate binding strength. For ammonia synthesis, the rate-limiting step changes across the volcano plot: metals that bind N₂ too weakly (like Cu, Ag) cannot dissociate the extremely strong N≡N triple bond (945 kJ/mol). Metals that bind nitrogen too strongly (like Mo, W) can dissociate N₂ but hold the nitrogen atoms so tightly that hydrogenation and NH₃ desorption become prohibitively slow. Iron sits near the peak — it dissociates N₂ with modest activation energy and releases NH₃ at a practical rate. Ruthenium is actually slightly better but too expensive for industrial use.

Question 2

Chemisorption involves the formation of chemical bonds between the adsorbate and the metal surface, while physisorption involves only weak van der Waals interactions.

• T. True
• F. False

Answer: True

The distinction between chemisorption and physisorption is fundamental to heterogeneous catalysis. Chemisorption involves the formation of genuine chemical bonds (covalent or ionic) between the substrate and surface metal atoms, with adsorption energies typically 40-400 kJ/mol. The substrate's electronic structure is significantly perturbed, often leading to bond weakening or dissociation. Physisorption involves only van der Waals forces (5-40 kJ/mol), is non-specific, and does not significantly alter the substrate's bonds. Only chemisorption activates substrates for catalytic reaction. The d-band model (Hammer and Norskov) explains trends in chemisorption strength across the transition metals.

Question 3

The d-band center model predicts that metals with d-band centers closer to the Fermi level will have stronger chemisorption of adsorbates.

• T. True
• F. False

Answer: True

The Hammer-Norskov d-band model provides a simple electronic structure explanation for trends in chemisorption. When an adsorbate orbital interacts with the metal d-band, it forms bonding and antibonding states. If the d-band center is high (close to the Fermi level), more of the antibonding states lie above the Fermi level and are empty — resulting in stronger net bonding (more bonding electrons, fewer antibonding). If the d-band center is low (far below the Fermi level), the antibonding states are filled, weakening the bond. This model explains why early transition metals (high d-band center) bind adsorbates strongly while late transition metals (low d-band center) bind weakly, and it provides the electronic-structure basis for the volcano plot.

Question 4

Explain why heterogeneous catalysts are often prepared as nanoparticles on a support rather than as bulk metal, and describe two properties of nanoparticles that enhance catalytic activity.

Model answer: Nanoparticles maximize the fraction of metal atoms exposed at the surface (the surface-to-volume ratio increases as particle size decreases). A 2 nm nanoparticle has ~50% of its atoms on the surface, while a 20 nm particle has only ~5%. Since catalysis occurs exclusively at the surface, smaller particles provide more active sites per gram of metal. Two additional properties enhance activity: (1) Nanoparticles have a high density of under-coordinated atoms at edges, corners, and steps — these low-coordination sites bind adsorbates more strongly and often have lower activation barriers for bond-breaking. (2) The electronic structure of nanoparticles differs from bulk metal — quantum confinement and surface effects shift the d-band center, tuning adsorption strengths. The support (typically alumina, silica, or carbon) prevents nanoparticles from sintering (aggregating into larger, less active particles) at reaction temperatures.

This is why modern heterogeneous catalyst development focuses on controlling nanoparticle size, shape, and composition. Single-atom catalysts (isolated metal atoms on a support) represent the extreme limit, maximizing atom efficiency and offering unique selectivity due to their distinctive electronic environment.