Porous materials for steady-state NO conversion: Comparisons of activated carbon fiber cloths, zeolites and metal-organic frameworks

It is important to develop new materials/methods to remove NO from gas streams to protect the environment. Use of porous materials to catalytically convert NO to NO₂ may improve existing NO_x control technologies. In this work, three porous materials (i.e., activated carbon fiber cloths (ACFCs), zeolites, and metal-organic frameworks (MOFs)) were evaluated as NO oxidation catalysts at 25 °C. Maximum included sphere diameter (D_i) between 5.5 and 11.0 Å is shown to result in higher steady-state NO conversion (23%−71.5 ± 2.5%) by providing appropriate channels/cages that accommodate and stabilize the first transition state (TS1) through van der Waals interactions. For all considered catalysts but MIL-53 (Al), steady-state NO conversion initially increases, then decreases, with increasing pore volume contributed by D_i (5.5–11.0 Å). Maximum free sphere diameter (D_max) for the selected catalysts with steady-state NO conversion between 23% and 71 ± 2.5% is between 3.7 and 9.0 Å allowing transport of reactants and products. Extra-framework cations (H⁺, NH₄⁺, and Na⁺) and coordinatively unsaturated sites (Cu²⁺, Fe³⁺, and Cr³⁺) increase steady-state NO conversion by providing TS1 stabilization through electrostatic and coordinative interactions, respectively. However, appropriate D_i and D_max values are identified as the most influential variables for steady-state NO conversion using these catalysts. For NO conversion in the presence of SO₂, no impact is observed for steady-state NO conversion of a MOF sample, but decreases of 100% and 36% for ACFC and zeolite samples, respectively. These results provide direction for designing NO oxidation catalysts to improve steady-state NO conversion.