Structure-activity relationships in NH3-SCR over Cu-SSZ-13 as probed by reaction kinetics and EPR studies

Abstract

Cu-SSZ-13 catalysts with various Cu loadings were prepared via aqueous solution ion-exchange. The hydrated samples were characterized with Electron Paramagnetic Resonance (EPR). Cu2+ ion coordination numbers were obtained by analyzing the hyperfine structures, while Cu-Cu distances were estimated from line broadening of the EPR features. By examining EPR and temperature-programmed reduction (TPR) results, two Cu2+ ion locations are suggested. Standard NH3-SCR, as well as non-selective NH3 oxidation reaction with O2, were carried out over these catalysts at high-space velocities. For the SCR reaction, intra-particle diffusion limitations are found. The kinetic data allow for reactant diffusivities to be estimated. However, clear structure-activity relationships for the SCR reaction cannot be derived due to this diffusion limitation. The slower NH3 oxidation reaction, on the other hand, is kinetically limited at low temperatures, and, therefore allows for a correlation between Cu2+ ion location and reaction kinetics to be made.