A DRIFTS Study of Cu–ZSM-5 Prior to and during Its Use for N2O Decomposition

Abstract

Cu–ZSM-5 and Na–ZSM-5 were characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) prior to and during N2O decomposition at various temperatures to learn more about the state of the Cu–ZSM-5 catalyst and to better understand its very high activity for this reaction. In addition to IR bands also exhibited by Na–ZSM-5, the spectrum of Cu–ZSM-5 at 300 K prior to any thermal treatment displayed an additional band between 900 and 1000 cm−1, which was assigned to a stretching mode of an Si–O− bond perturbed by Cu2+ species as opposed to a similarly perturbed Si–O–Si or Si–O–Al zeolite vibration. Purging this sample in Ar at 300 K removed both adsorbed water and water coordinated in octahedral [Cu(H2O)6]2+ complexes. Heating to 773 K resulted in the thermal reduction of Cu2+ to Cu+, and a substantial fraction of the copper ions was stabilized as Si–O−Cu+. When N2O was introduced to Cu–ZSM-5, transient bands were observed which were assigned to N2O adsorbed on Cu+ via the O atom. Only Cu–ZSM-5 exhibited a slight but rapid decrease in the 3597-cm−1 υOH band for bridging Si(OH)Al groups and it was accompanied by the appearance of a band near 910 cm−1, indicating oxidation of Cu+ to Cu2+. No unequivocal bands for adsorbed N2O were detected under steady-state decomposition conditions with any catalyst, indicating very low steady-state surface concentrations of adsorbed N2O. These DRIFTS results are consistent with the following catalytic redox mechanism proposed for N2O decomposition over Cu–ZSM-5. Gas-phase N2O adsorbs molecularly via the O end onto a Cu+ ion at a Si–;O−Cu+ site maintained in a highly dispersed state. This adsorbed N2O species then irreversibly decomposes in a rate-determining step to form gaseous N2 and an adsorbed O atom. In the process Cu+ is oxidized to Cu2+, which is stabilized as either Si–O−[Cu2+(O−)]+ or Si–O−[Cu2+(OH−)]+. When two such sites are located in close proximity, such as at opposite corners of a four-membered ring having Al tetrahedra at the T9 sites of ZSM-5, oxygen recombination can readily occur to form O2, and in the process Cu2+ is reduced to Cu+, thus completing the redox cycle. This mechanism incorporates aspects specific to both the copper ions and the zeolite structure to explain the uniquely high activity of this particular catalyst, and the rate expression derived from this model fits the data well over a wide temperature range.