Effect of the electronic state and copper localization in ZSM-5 pores on performance in NO selective catalytic reduction by propane

摘要

This study was devoted to determination of localization of the individual copper species in the pores of ZSM-5 zeolite after post-synthetic zeolite modification by incipient wetness impregnation with CuCl2 solution followed by hydrolytic polycondensation of the ions in the pores. A series of Cu(n)ZSM-5 samples, where n is the copper concentration varied from 0.5 to 5.0 wt.%, were synthesized using this method. A commercial HZSM-5 zeolite having Si/Al = 17 was used for the synthesis of the catalysts. After modification the Cu(n)ZSM-5 samples were dried and calcined in air at 110 and 450 °C, respectively. The samples were studied by UV-vis-NIR DR spectroscopy, XRD and by measuring and processing the low-temperature (77 K) adsorption of nitrogen and hydrogen. It was shown for the first time that at the concentration of 1.0 wt.% or lower copper was localized in the form of isolated Cu~(2+)_(Oh) cations in the ion-exchange positions in the zeolite micro- and mesopores as well as on oxygen complexes of extraframework aluminum in thin mesopores with D<3.2nm in the form of superficial spinel-like structures in octahedral oxygen coordination Cu~(2+)_(Oh). At the copper concentration above 1.0 wt.% linear associates of weakly bound Cu~(2+)_(Oh) ions with unusual orbital ordering were formed in addition to the two forms discussed above. These associates existing in the forms of one- and two-dimensional nanohydroxocompounds were localized partially in the zeolite micropores but mostly in thin mesopores. The effect of the individual copper species on the catalytic properties of Cu(n)ZSM-5 samples in selective catalytic reduction of NO by propane was also studied. It was found that isolated Cu~(2+)_(Oh) cations in the ion-exchange positions and Cu~(2+)_(Oh) cations localized on the oxygen complexes of extraframework aluminum had the highest catalytic activity in NO SCR by propane. The weakly bound linear associates of Cu~(2+)_(Oh) ions had lower catalytic activity, compared with isolated Cu~(2+)_(Oh) ions. When their amount was high, they caused steric hindrance for contact of the reagents with the more active sites. The Cu~(2+)_(Oh) ion associates were found to transform to nanodispersed Cu2(OH)3Cl during catalytic experiments.