Zero-emission NO₂ capture using divalent metal cation-exchanged zeolites for air purification

Area of Science:

• Environmental Chemistry
• Materials Science
• Chemical Engineering

Background:

• Decarbonization efforts using hydrogen and biofuels may increase nitrogen dioxide (NO2) pollution due to higher combustion temperatures and excess oxygen, leading to elevated nitrogen oxides (NOx) emissions.
• Effective removal of ambient NO2 is crucial for public health, but current adsorbents often emit toxic nitrogen monoxide (NO), hindering their practical application.
• Existing NO2 removal technologies face limitations in efficiency and safety, necessitating the development of advanced materials.

Purpose of the Study:

• To develop novel adsorbents for efficient NO2 capture with minimal or zero NO emission.
• To investigate the performance of divalent metal cation-exchanged zeolites for NO2 removal under various conditions.
• To demonstrate the practical application of these zeolites in personal air purification devices.

Main Methods:

• Synthesis and characterization of divalent metal cation-exchanged zeolites (e.g., Ca2+ and Mn2+ exchanged zeolites).
• Testing of NO2 adsorption capacity and selectivity under dry and humid conditions (up to 70% RH) and varying NO2 concentrations (up to 500 ppm).
• Mechanistic studies using surface active sites to understand the NO formation suppression pathway.
• Integration of developed zeolites into a wearable air-purifying respirator prototype.

Main Results:

• Divalent metal cation-exchanged zeolites demonstrated complete NO2 capture with negligible NO emission.
• The materials maintained high performance in both dry and humid environments and across a range of NO2 concentrations.
• Mechanistic investigations revealed that divalent cations stabilize intermediates, effectively suppressing NO formation.
• Successful integration into a wearable respirator prototype showcased potential for real-world personal protection.

Conclusions:

• Divalent metal cation-exchanged zeolites offer a highly effective and safe solution for ambient NO2 removal.
• These materials overcome the critical challenge of NO emission associated with traditional adsorbents.
• The developed technology presents a scalable pathway towards advanced air purification and a 'zero-NOx shield' for personal safety.