Monoxide-Induced Reorganizations of Metal Atoms

Carbon Monoxide-Induced Reorganizations of Metal Atoms 

Recent uses of EXAFS, X-ray scattering, and FTIR spectroscopies have revealed that exposure of zeolite-entrapped Pd or Rh often induces thorough reorganizations of the metal atoms. The combination of CO and zeolite protons can sometimes even lead to changes of the oxidation state of the entrapped metal. 

When calcination is carried out at 5(X)°C and reduction is at 2(X)°C, the original nuclearity n is 1. For room temperature and zeolite Y the final nuclearity an is 13. The migration and coalescence of the mobile primary Pd carbonyl clusters leading to the formation of PdnfCO) , clusters, with concomitant release of zeolite protons, are schematically illustrated in Fig. 8. 

The release of zeolite protons due to the cutting of the original anchors has been independently detected by FTIR. It was found that the intensity of the supercage OH band between 3640 and 3650 cm increases on admission of dry CO to the reduced sample, in agreement with Eq. (7) and Fig. 8 (752). The effect of CO on the migration of monoatomically dispersed Pd in NaHY is clearly demonstrated by the EXAFS functions shown in Fig. 9 and their Fourier transforms shown in Fig. 10. 

The dynamic model for the formation of Pdn(CO) is confirmed by an experiment with zeolite 5A as a support. This zeolite is known to have supercages and sodalite cages similar to those of NaY, but it has smaller supercage windows ( 5 A) than NaY (7.5 A). Indeed, as shown by EXAFS (790), Pd6(CO)y clusters are exclusively formed in zeolite 5A. The CO 

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