Dissociative chemisorption and energy transfer for methane on Ir(111)

Area of Science:

• Chemical kinetics
• Surface science
• Catalysis

Background:

• Understanding gas-surface reactivity is crucial for catalysis.
• Methane (CH4) dissociative sticking on iridium (Ir) is important for industrial processes.
• Non-equilibrium conditions can significantly alter reaction dynamics.

Purpose of the Study:

• To analyze and predict methane dissociative sticking coefficients on Ir(111).
• To investigate the role of non-equilibrium conditions and energy transfer in gas-surface reactions.
• To compare theoretical predictions with experimental data and implications for catalysis.

Main Methods:

• A 3-parameter local hot spot model was used.
• 14-dimensional microcanonical kinetics considered active degrees of freedom (surface oscillator, molecular vibrations, rotations, normal translation).
• Analysis of molecular beam experiments under varied conditions.

Main Results:

• The threshold energy for CH4 dissociative chemisorption on Ir(111) was determined to be 39 kJ/mol.
• A discrepancy of 88% was observed between experimental and theoretical sticking coefficients over 4 orders of magnitude.
• An enhancement in sticking at low translational energies (<10 kJ/mol) suggests a dynamical trapping/energy transfer channel.

Conclusions:

• A trapping-mediated sticking mechanism, prominent under non-equilibrium conditions, is proposed.
• This mechanism is expected to be less significant at thermal equilibrium.
• Predicted equilibrium sticking coefficients suggest potential for significant improvements in nanoscale Ir catalyst turnover rates.