Infrared studies on the interaction of N2O adsorbed in transition metal ion-exchanged zeolites A
Abstract
The suitability of nitrous oxide as probe for the characterization of sorption sites inside transition metal ion-exchanged zeolites A has been tested by FTIR spectroscopy. The ν3 and the ν1 fundamentals of non-dissociatively bound N2O appear in the 2300-2200 cm−1 and 1300-1200 cm−1 regions, the former being exclusively shifted upscale, the latter being situated on both sides of the gasphase frequency. The bands are tentatively assigned to surface compounds of N2O in linear arrangement with the interacting cation. The ν3 fundamental is supposed to indicate the different sorption species, while the ν1 vibration is sensitive to the orientation of adsorbed N2O. Zeolite CuA deviates from the other zeolites, which might be due to autoreduction.
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Cited by (10)
Reduction of N<inf>2</inf>O by NH<inf>3</inf> on polycrystalline copper and Cu(1 1 0): A combined XPS, FT-IRRAS and kinetics investigation
2006, Applied Catalysis B: EnvironmentalKinetics of N2O decomposition and catalytic reduction of N2O by NH3 in the presence or absence of oxygen have been studied on polycrystalline Cu planar chip (3 cm × 3 cm × 0.1 cm) or Cu(1 1 0) single crystal, using catalytic test equipment, XPS and FT-IRRAS techniques. It has been shown that N2O decomposes on metallic Cu, but gives then Cu2O, which is detrimental to N2O decomposition. The presence of a reductant, such as NH3, allowed N2O to react leading to its catalytic reduction to N2; 500 °C is the best temperature for catalytic reduction alone, i.e. with low additional self-decomposition of N2O or NH3. The presence of oxygen, in amount less than that of NH3, leads to more efficient NH3 oxidation, oxygen being observed to be more reactive than N2O on NH3. XPS results enabled to identify the active surface as metallic Cu and Cu3N for NH3 oxidation and NH2, NH, N adsorbed species as intermediates of the reaction. At room temperature, in the presence of N2O, O2 and NH3, FT-IRRAS allowed to show the formation of NH2 and NH species (bands at 1550 and 1440 cm−1, respectively) and of two N2δ− species (bands at 2170 and 2204 cm−1), the latter one corresponding to adsorbed N2δ− species close to adsorbed electron accepting O or OH species. This study demonstrated that N2O decomposed to N2 and O species during SCR reaction; it enabled to identify several adsorbed surface species (N, NH, NH2, N2δ−), both by XPS after catalytic reaction at 500 °C on the polycrystalline Cu chip and by IRRAS on Cu(1 1 0) single crystal in the presence of the reactants at room temperature. In addition, it was shown that N2 is a powerful IR probe to characterise the surrounding environment of surface sites that cannot be identified by any other way.
A drifts study of Cu-ZSM-5 prior to and during its use for N<inf>2</inf>O decomposition
2002, Journal of CatalysisCu–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.
Dynamics of NO and N<inf>2</inf>O decomposition over Cu-ZSM-5 under transient reducing and oxidizing conditions
2000, Journal of CatalysisN2O and NO decomposition pathways on Cu–ZSM-5 have been investigated by monitoring the adsorbate dynamics and changes in reactant and product concentrations using infrared spectroscopy (IR) and mass spectrometry (MS) under transient reducing and oxidizing conditions. Transient reducing and oxidizing conditions were produced by the H2, CO, and O2 pulses into either N2O or NO streams. IR and MS studies under the transient conditions revealed that adsorbed O produced from N2O exhibited different reactivity and dynamics from the adsorbed O produced during NO decomposition. The differences in reactivity and dynamics of the adsorbed O were evidenced by the fact that (i) adsorbed O from N2O interacted with CO/H2 to produce two humps in the H2O and O2 concentration profiles; adsorbed O from NO reacted with CO/H2 that led to only one hump in the H2O and O2 concentration profile, and (ii) addition of the O2 pulse led to reaction of O2 with adsorbed O from NO, resulting in oxidation of Cu+ in Cu+(NO) to Cu2+ in Cu2+(NO3−); addition of O2 did not lead to any reaction with adsorbed O from N2O decomposition. N2O decomposition is proposed to proceed via Cu+–ON2, Cu2+O−, and Cu2+O−–ON2 with Cu+–ON2 serving as a precursor for N2 formation and Cu2+O− as a precursor for O2 formation. NO decomposition proceeds via Cu+(NO), Cu2+O−, and Cu2+(NO3−) with Cu+(NO) serving as a precursor for NO dissociation. Cu+ in Cu+(NO) is different from that of Cu+ in Cu+–ON2. The former may be associated with Al(OH)4− of the zeolite, the latter with Si(OH)4−.
Infrared studies on nitrogen oxides adsorbed on alkali metal ion-exchanged ZSM-5 zeolites
1995, ZeolitesThe adsorption of nitrogen oxides (NOx) on a series of alkali metal ion-exchanged ZSM-5 zeolites at 226 K was investigated by in situ infrared spectroscopy. In the low NO pressure region, the main species found on each zeolites was an NO monomer linearly adsorbed onto the cation through an O atom (M+ ··· ON), but an M+ ··· NO species was not detected. On the other hand, a part of adsorbed NO disproportionated into N2O and N2O3 under high NO pressures (higher than 1.3 kPa). The formation of N2O3 was enhanced at a high density of nitrogen oxides (NOx) in the ZSM-5 pore.
An infrared study of adsorption of N<inf>2</inf>O on ZnO
1991, Spectrochimica Acta Part A: Molecular SpectroscopyThe adsorption of N2O on finely divided ZnO at room temperature shows two principal infrared absorption bands at 2237 cm−1 (strong) and 1255 cm−1 (weak), corresponding to the reversible adsorption of an N2O surface species. The N2O is postulated to be coordinated to Zn2+ cations by the oxygen atom. Water pre-treatment of the ZnO surface gives only weak bands from adsorbed N2O, indicating that the latter's adsorption site is taken up by adsorbed water. Spectroscopic experiments on ‘reduced’ surfaces of ZnO at 200°C show that limited reaction of N2O with the surface has occurred, presumably through decomposition to nitrogen and adsorbed oxygen. New adsorptions on the ZnO surface itself, and a reduced amount of reversibly adsorbed N2O, implied a reduction in pressure of the adsorbate. Such effects were not observed appreciably over ‘oxidised’ ZnO.
Self-Consistent-Charge X<inf>α</inf> Calculations on Sorption Complexes of Nitrous Oxide Attached To Transition Metal Occupied Zeolite Clusters
1989, Studies in Surface Science and CatalysisQuantum chemical SCC-Xα MO calculations were carried out on an aluminosilicate six-ring cluster occupied by nickel or copper ions, approached by a nitrous oxide molecule in N- or O-faced interaction in order to facilitate the assignment of infrared bands. In all cases stable sorption complexes are found with stronger bonding in the case of nickel and the Mn+-NNO species. Electronic structure and charge distribution are analyzed, revealing an N-N to N-O bond charge redistribution for both orientations on nickel, while a loss of electronic charge for the copper species results. The splitting of the v1 and the frequency shift of the v3 fundamental band of N2O on nickel ion-exchanged zeolite A may be explained by the different overlap populations of the bonds.