Field emission techniques for studying surface reactions: applying them to NO-H2 interaction with Pd tips

Area of Science:

• Surface Science
• Heterogeneous Catalysis
• Chemical Kinetics

Background:

• Understanding the reaction between nitric oxide (NO) and hydrogen (H2) on palladium (Pd) is crucial for catalysis.
• Previous studies have explored NO-H2 reactions, but detailed kinetic mechanisms under varying conditions remain complex.

Purpose of the Study:

• To investigate the adsorption of NO and its reaction with H2 over Pd tips.
• To elucidate the kinetic phase diagram and hysteresis effects in the NO+H2/Pd system.

Main Methods:

• Field Ion Microscopy (FIM) for surface imaging.
• Pulsed Field Desorption Mass Spectrometry (PFDMS) for chemical analysis.
• Controlled variation of H2 partial pressure and temperature (400-600K).

Main Results:

• A strong hysteresis effect was observed in the NO+H2 reaction over Pd crystallites, dependent on H2 pressure and temperature.
• FIM revealed abrupt changes in surface patterns correlating with the hysteresis.
• Subsurface diffusion of H atoms occurred at high H2 pressures, blocked by NO/O adsorption at lower pressures.
• Evidence of Pd surface oxidation was detected via PdO2+ species.

Conclusions:

• The NO+H2/Pd system exhibits complex kinetic behavior with distinct phases.
• Hysteresis is driven by surface coverage and subsurface hydrogen dynamics.
• A novel kinetic phase diagram for this system was established.