Spatial Profiling of a Pd/Al₂O₃ Catalyst during Selective Ammonia Oxidation.

Area of Science:

• Heterogeneous Catalysis
• Materials Science
• Chemical Engineering

Background:

• Operando spectroscopy enables observation of dynamic supported metal nanoparticles.
• Catalyst structure is sensitive to environmental conditions, necessitating spatial analysis in fixed-bed reactors.
• Understanding structure-activity relationships is crucial for optimizing catalytic processes.

Purpose of the Study:

• To spatially profile a Pd/Al₂O₃ catalyst during ammonia oxidation.
• To correlate catalyst structure with activity and selectivity along the reactor bed.
• To elucidate reaction pathways governing selective ammonia oxidation.

Main Methods:

• Spatial profiling of a Pd/Al₂O₃ catalyst using operando X-ray absorption spectroscopy (XAS) and mass spectrometry (MS).
• Data collection at discrete axial positions along the fixed-bed reactor.
• Analysis of structural changes and reaction products across the catalyst bed.

Main Results:

• Observed correlation between a specific palladium nitride (PdN) structure and selectivity towards N₂ production, even after bulk PdN signatures disappear.
• Proposed two simultaneous reaction pathways at high temperatures (≥400 °C): NH₃ oxidation to NO by PdO and subsequent NO reduction to N₂ by NH₃.
• Confirmed structural and catalytic diversity within the catalyst bed during ammonia oxidation.

Conclusions:

• Spatial analysis provides critical insights into structure-activity relationships for selective ammonia oxidation.
• Understanding catalyst diversity is essential for advancing emission control technologies.
• The identified PdN structure is key to selective N₂ formation.