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

The adsorption of NO on Pd-exchanged H-ZSM-5 containing from 0.18 up to 1.5 6 wt.% Pd was studied by in situ FTIR spectroscopy and dynamic volumetric e xperiments. At room temperature, NO adsorbs as two distinct NO-containing s pecies (nu (NO) at 2136 and 1881 cm(-1)). These species exhibit different t hermal stability: the 2136 cm(-1) species easily decomposes in vacuo below 473 K while the 1881 cm(-1) species is stable up to 673 K. At 523 K in flow ing NO, only the 1881 cm(-1) species forms and NO2 is released. The strikin g feature is that the release of NO2 is delayed with respect to the NO cons umption 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 spectrom etry, IR spectroscopy and volumetric data, the transient species were unamb iguously associated with the 2136 cm(-1) species and attributed to NO+ occu pying 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 dispe rsion of Pd as isolated complexes at exchange sites in the zeolite channels was found to depend strongly on both Pd loading and activation conditions.