Understanding Reaction Networks through Controlled Approach to Equilibrium Experiments Using Transient Methods

Area of Science:

• Heterogeneous catalysis
• Surface science
• Computational chemistry

Background:

• Understanding heterogeneous catalytic processes is crucial for chemical synthesis.
• Traditional methods often lack detailed insights into surface reaction mechanisms.
• Ammonia synthesis and decomposition are industrially significant reactions.

Purpose of the Study:

• To develop and demonstrate a combined experimental and theoretical approach for studying gas/solid catalytic reactions.
• To elucidate the role of individual surface reaction steps in ammonia synthesis and decomposition.
• To determine surface reaction pathways and intermediate lifetimes on model catalysts.

Main Methods:

• Low-pressure Temporal Analysis of Products (TAP) pulse response experiments were performed on polycrystalline iron and cobalt.
• Quantum mechanics (QM)-based calculations were used to determine reaction free energies on relevant metal facets (Fe-BCC, Co-FCC).
• Controlled pulsing of reactants (ammonia, deuterium) and varying delay times were employed to probe reaction mechanisms and approach to equilibrium.

Main Results:

• The combined approach successfully provided detailed information on surface reaction steps.
• The nitrogen formation barrier was identified as a key factor controlling surface intermediate concentrations.
• Surface lifetimes of key reaction intermediates were determined for iron and cobalt catalysts.

Conclusions:

• The developed experimental/theoretical methodology is effective for dissecting complex catalytic reaction mechanisms.
• Insights gained from monometallic catalysts were successfully applied to interpret results on a bimetallic CoFe catalyst.
• This approach offers a powerful tool for understanding and designing heterogeneous catalysts.