Redox Chemistry of CoZSM-5 Zeolite

Abstract

The redox chemistry and the changes that occurred within the solids were studied for an excessively exchanged and a nearly stoichiometric CoZSM-5 zeolite as a function of pretreatment, reduction, and reoxidation conditions. Both gravimetric and volumetric techniques were used to determine this chemistry and the stoichiometry of the processes, which became reversible after the first couple of oxidation/reduction cycles. The catalyst was examined in its several states by diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), X-ray photoelectron spectroscoy (XPS), and both the Faraday and Gouy magnetic susceptibility methods. With the excessively exchanged sample, it was found that Co 3 O 4 formed after pretreatment with O 2 and that Co metal was produced on reduction with either CO or H 2 at 773 K in amounts corresponding to only a portion of the Co 2+ originally present. The stoichiometric sample showed evidence for the presence of Co 3+ , but crystalline Co 3 O 4 was not detectable. However, two different products were formed from the original freshly exchanged (and hydrated) zeolite, depending upon whether it was dried in an inert gas or in O 2 as the temperature was raised to 773 K.

Cu and Co exchanged ZSM-5 zeolites: activity towards no reduction and hydrocarbon oxidation

“…The latter, which is present only in samples with a high metal content, is attributed to Co 3 O 4 oxide 26 . The bands at 210 and 255 nm are associated to the electronic transitions of the zeolite structure and to Co 2+ species compensating the negative charge of the zeolite, respectively.…”

Electron spin resonance and optical spectroscopic studies of Co-ZSM-5 with nitric oxide

“…In this geometry an ESR line at g = 5.5 is observed [29]. Fierro et al [30] assigned three bands at 525, 588 and 625 nm to tetrahedral Co(II) in Co-ZSM-5 dehydrated in helium at 500°C overnight. Optical data on Co(II) in other zeolites is similar [31][32][33].…”

“…Cu(II) ions in Cu-ZSM-5 are easily reduced at 270°C by hydrogen, but Co(II) ions in Co-ZSM-5 are only reduced at 720°C by hydrogen [35]. Fierro et al [30] report that the X-ray photoelectron spectrum of dehydrated Co-ZSM-5 (dehydrated at 500°C in flowing He overnight) is very similar to that for fully hydrated Co-ZSM-5 [30]. Therefore we also suggest that Co(II) ions in dehydrated Co-ZSM-5 are very stable.…”

Improved stability of Co-Ferrierite catalyst by Mn in dry–wet cycles of lean CH4-SCR of NOx

“…In the spectra of figure 3a the two absorptions at 444 and 525 have been assigned to Co1b and Co1a sites, respectively, the band at 667 nm to Co2a sites, and the two absorptions below 400 nm and above 700 nm to Co oxides species [20]. After catalytic tests (figure 3b), Co1a absorption increased, indicating the migration of Co 2+ ion to tetrahedral sites, and Co2a population decreased.…”