Infrared and volumetric study of NO adsorption on Pd-H-ZSM-5

Abstract

The adsorption of NO on Pd-exchanged H-ZSM-5 containing from 0.18 up to 1.56 wt.% Pd was studied by in situ FTIR spectroscopy and dynamic volumetric experiments. At room temperature, NO adsorbs as two distinct NO-containing species (ν(NO) at 2136 and 1881 cm⁻¹). These species exhibit different thermal stability: the 2136 cm⁻¹ species easily decomposes in acuo below 473 K while the 1881 cm⁻¹ species is stable up to 673 K. At 523 K in flowing NO, only the 1881 cm⁻¹ species forms and NO₂ is released. The striking feature is that the release of NO₂ is delayed with respect to the NO consumption and that, after being formed, NO₂ is reversibly adsorbed with NO in a 1:1 ratio inside the channels of the zeolite. By combining mass spectrometry, IR spectroscopy and volumetric data, the transient species were unambiguously associated with the 2136 cm⁻¹ species and attributed to NO⁺ occupying exchange sites. All these data were consistent with the reduction of Pd(II) into Pd(I) ions dispersed in the zeolite channels and the subsequent formation of Pd(I) mononitrosyl complexes responsible for the 1881 cm⁻¹ feature. The Pd(I) nitrosyl complexes are monovalent and strongly anchored to the zeolite framework, as indicated by the appearance of a sharp IR band at 975 cm⁻¹ attributed to distorted T–O (T = Si, Al) vibrations. The dispersion of Pd as isolated complexes at exchange sites in the zeolite channels was found to depend strongly on both Pd loading and activation conditions.