Kinetics of NH₃ Desorption and Diffusion on Pt: Implications for the Ostwald Process

Area of Science:

• Surface science
• Chemical kinetics
• Catalysis

Background:

• Ammonia (NH3) desorption kinetics on platinum surfaces are critical for understanding catalytic processes like the Ostwald process.
• Existing models often use simplified partition functions, potentially limiting accuracy.

Purpose of the Study:

• To accurately measure NH3 desorption rates from Pt(111) and Pt(332) surfaces.
• To determine elementary rate constants for desorption and diffusion.
• To refine theoretical models for surface reactions.

Main Methods:

• Time-resolved desorption measurements.
• Transition state theory modeling.
• Development of coupled degrees of freedom partition functions.

Main Results:

• Accurate rate constants for NH3 desorption and diffusion on Pt(111) were determined.
• Coupled degrees of freedom partition functions accurately reproduced experimental data.
• NH3 binding energy to Pt(111) (1.13 ± 0.02 eV) and diffusion barrier (0.71 ± 0.04 eV) were established.
• NH3 shows a binding energy preference for step sites over terrace sites on Pt (0.23 ± 0.03 eV).

Conclusions:

• Conventional partition function models are insufficient for describing NH3 desorption.
• A refined theoretical approach incorporating coupled degrees of freedom is necessary.
• The findings challenge the 12% rule and explain limitations of current Ostwald process models, suggesting mean-field kinetics are not always applicable.