Direct NO decomposition over conventional and mesoporous Cu-ZSM-5 and Cu-ZSM-11 catalysts: Improved performance with hierarchical zeolites

Abstract

Conventional Cu-ZSM-5 has for many years been recognized as a unique catalyst for direct NO decomposition. Zeolite-based catalysts have a crystallographically well-defined microporous structure. In such microporous catalysts, the creation and accessibility of the active sites is often influenced by the geometry of the pore system. To improve these catalysts, secondary mesoporous systems can benefitially be introduced into the structure of conventional zeolites. Here, this approach was used with Cu-containing ZSM-5 and ZSM-11 type mesoporous zeolite catalysts, which were used as catalysts for direct NO decomposition. It was discovered that introducing mesoporosity into the conventional materials leads to a significant improvement of the catalytic activity. Additionally, mesoporous Cu-ZSM-11 catalyst was found to be about twice as active as mesoporous Cu-ZSM-5. This difference is attributed to the fact that ZSM-11 has only straight microporous channels, while ZSM-5 has both straight and sinusoidal channels. Apparently, there is a preferential formation of active sites and/or improved accessibility in the straight channels compared to the sinusoidal channels, which make the ZSM-11 material a better catalysts than ZSM-5. All samples were characterized by XRPD, SEM, TPD-NH3, in situ EPR and N2 physisorption measurements. Thus, this confirms our previous observation that the special pore structure of Cu-ZSM-5 is not a decisive factor for the catalytic activity in NO decomposition.