Cu-ZSM

Comparative kinetic and in-situ DRIFT studies of the N2O decomposition over Co-, Fe- and Cu-ZSM-5 have been performed. The implications of the presence of O2, CO, NO, H2O and SO2 on the catalyst activity and stabilitiy and on the mechanism are evaluated. 

Figure 7 Effect of CO on the N2O conversion over Cu-ZSM-5 at 0.1 kPa N2O and space time W/Fn20= 1.52 10 g.s/mol.

SO2 nearly completely deactivated the Cu-ZSM-5, resulted in an inhibition for Co-ZSM-5 and an enlargement of the N2O conversion over Fe-ZSM-5 (figure 9). Both the Fe and the Co systems returned to their original activity after removal of the SO2, this took several hours. 

Characterization of the Cu-ZSM-5 catalyst by in-situ diffuse reflectance FTIR spectroscopy after treatments in CO, air and NjO is presented in figure 10, the CO adsorption in figure 11. 

Figure 11 DRIFT spectra of Cu-ZSM-5 at 450 K in 5 kPa CO in Ar. Dashed line is a Fourier self-deconvolution spectrum.

Figure 12 Change in CO vibration on Cu-ZSM-5 upon readsorption of water at RT in 10 kPa, after CO reduction at 770 K.

Water exerts both a deactivating and inhibiting influence on Cu and Fe samples, while the reaction over Co is only inhibited. The deactivation of Fe- and Cu-ZSM-5 is clearly due to migration and the sintering of the active component in H2O atmospheres [34]. The Co-ZSM-5 catalyst is much more hydrotheimally stable in wet gas conditions [34,35]. The inhibition by water can be accounted for in a similar way as for CO via competitive adsorption on active sites, like in selective NO reduction studies [34]. For N2O decomposition this yields an expression like eq. (12). At 793 K Kn amounts to about 0.7 kPa". ... 

Evaluating the results a clear kinetic picture of the catalysts has been obtained. In the steady state the active sites in Fe- and Cu-ZSM-5 are nearly fully oxidized, while for Co only -50% of the sites are oxidized. The former catalysts oporate in an oxidation reduction cycle, Fe /Fe and CuVCu. Coi in zeolites is hardly oxidized or reduced, but ESR studies on diluted solid solutions of Co in MgO indicate that Co -0 formation is possible, rapidly followed by a migration of the deposited oxygen to lattice oxygen and reduction back to Co [36]. For Fe-ZSM-5 such a migration has been observed, so a similar model can be proposed for the zeolitic systems. Furthermore, it is obvious that application of these catalysts strongly depends on the composition of the gas that has to be treated. 

Based on previous studies [15, 22-25], the band at 1941 cm-i is assigned to Co2+(NO), and the pair of bands at 1894 and 1815 cm-i, to Co2+(NO)2- The shoulders at 1874 and 1799 cm may be due to a second dinitrosyl species. While little is known about the location and coordination of the Co 2+ in ZSM-5, it is likely that cobalt ions are associated with both [Si-0-Al]- and [Al-0-Si-0-AI]2- structures in the zeolite. In the former case, the cobalt cations are assumed to be present as Co2+(OH-) cations and in the latter case as Co2+ cations. The presence of cobalt cations in different environments could account for the appearance of two sets of dinitrosyl bands. The band at 2132 cm-> is present not only on Co-ZSM-5 but also on H-ZSM-5 and Na-ZSM-5, and has been observed by several authors on Cu-ZSM-5 [26-28]. 

A careful investigation of this feature suggests that it is attributable to N02 associated with both Bronsted acid and M+ cations [29]. The doublet at 1400 cm is identical in position and appearance to that observed in a sample of Na-Y doped with NaNOs [30]. We therefore assign this peak to a NO3- species affiliated with the residual sodium in the catalyst. The position of the band at 1528 cm- is very similar to that for nitrito species in Co-A and Co-Y [22, 23] and is, therefore, assigned to Co-ONO. The features at 1599, and 1574 cm- are best assigned to C0-O2NO [30]. The band at 1633 cm- is similar to that observed on H-, Na-, and Cu-ZSM-5. We believe that this feature is best assigned to nitrito (NO2) or nitrate (NO3-) species. 

Figures I and 2 show the NO and propene conversions of these three groups of catalysts as a function of temperature. For comparison, the NO conversion of a 3.2 wt.% Cu-ZSM-5 (Si/Al=70) catalyst is also shown, which was obtained at twice the space velocity as the Au catalysts. It can be seen that the NO conversions on Au/Al Oj of high C.F. s were comparable to those on Cu-ZSM-5 under these conditions.

Fig. I NO conversion in NO reduction over Au/y-AljO, and Cu-ZSM-5 catalysts. Reaction conditions as in Table I...

One of the most interesting results of this work is that properly prepared AU/Y-AI2O3 are effective lean NO, reduction catalysts in the presence of 1.5 % H2O and 4.7 % O2. Their activities are stable, and comparable or higher than a Cu-ZSM-5 catalyst under similar reaction conditions. Another interesting result is the observation that the activity depends strongly on the preparation procedure, which must be related to the detailed structure of the catalyst and the nature of the active sites. 

It has been shown that on Cu-ZSM-5 and Cu-ZrOj catalysts, reduction of NO and NOj in the presence oflarge excess of Oj proceed at about the same rate [20,21]. This is because over these catalysts, NO2 is rapidly reduced to NO (and not NO being rapidly oxidized to NOj) [20,22,23]. On the other hand, on catalysts that do not contain transition metal ions, such as Na-ZSM-5 [24], GajOj [25], AljO, [26], and H-ZSM-5 [26], NO2 reduction to Nj proceeds much... 

Figure 2.15. X-band EPR spectra recorded after NO adsorption (1 -5 torr) onto ConZSM-5, FenZSM-5 (after [64]), and Cu ZSM-5 (after [41]) zeolites.

Taking into account the electron density relocation (Table 2.4) two routes of NO adsorption can be distinguished. Thus, the nitric oxide coordinates to the monovalent Cr, Ni, and Cu ions in an oxidative way (A<2M > 0), whereas for the rest of the TMIs in a reductive way (AgM < 0). Although this classification is based on the rather simplified criteria, it is well substantiated by experimental observations. Examples of reductive adsorption are provided by interaction of NO with NinSi02 and NinZSM-5, leading at T > 200 K to a Ni -NOs+ adduct identified by the characteristic EPR signal [71]. At elevated temperatures, similar reduction takes place for ConZSM-5 [63], whereas in the case of Cu ZSM-5 part of the monovalent copper is oxidized by NO to Cu2+, as it can readily be inferred from IR and EPR spectra [72,73], This point is discussed in more detail elsewhere [4,57],... 

Figure 2.23. IR spectra obtained after subsequent 10 (rmol pulse adsorption of NO on a thermally preactivated Cu ZSM-S sample recorded at (a) 303 K and (b) 423 K (after [75]).

Figure 2.25. Energy landscape (BP/DNP) for the Cu ZSM-5 + 2NO- Cu—0 ZSM-5 + N20 reaction, showing all associated spin and conformation isomers calculated for the M5 site. The values are given in kcal x mol-1. The letters S, D and T indicate the singlet, doublet, and triplet states, respectively (after [75]).

Sojka, Z., Che, M. and Giamello, E. (1997) EPR investigation of the electronic structure of mononuclear copper(I) nitric oxide adduct formed upon low-pressure adsorption of NO onto Cu/ZSM-5 zeolite, J. Phys. Chem. B, 101, 4831. 

Another way to work in transient conditions is to stop suddenly (or conversely to instantaneously introduce) one of the reactants, in order to destabilize the system and to enhance the concentration of labile species. With this method, for example, Poignant et al. studied the DeNO. reaction mechanism on a H—Cu-ZSM-5 catalyst, using propane or propene as reducing agents. The introduction of 2000 ppm of hydrocarbon in a flow of NO (2000 ppm) + 5% 02 allowed to evidence the formation of acrylonitrile, which behaved as an intermediate. Its reactivity with NO+ species constituted a fundamental point to describe a detailed SCR mechanism for NO removal on zeolitic compounds [137],... 

The TPSR technique has also been used by Konduru and Chuang [160] in order to investigate N20 and NO decomposition pathways on Cu-ZSM-5. The infrared monitoring of the adsorbed species during the sample heating under NO showed that the Cu+(NO) intensity parallels the rate of N20 formation. This TPSR result allowed the authors to suggest that NO adsorbed on Cu+ acts as precursor for N20 formation. 

Groothaert et al., using operando UV-vis spectroscopy combined with online GC analysis [176] and operando X-ray absorption fine structure (XAFS) [177], presented the first experimental evidence for the formation of the bis( x-oxo)dicopper core in Cu-ZSM-5 and for its key role of intermediate in the sustained high activity of Cu-ZSM-5 in the direct decomposition of NO into N2 and 02. In particular, monitoring the catalytic conversion of NO and N20 above 673 K, they found that the bis( x-oxo)dicopper core is formed by the O abstraction of the intermediate N20 (Figure 4.14). Subsequently,... 

Pieplu, T., Poignant, F., Vallet, A. et al. (1995) Oxidation state of copper during the reduction of NOx with propane on H-Cu-ZSM-5 in excess oxygen, Stud. Surf. Sci. Catal., 96, 619. 

Praliaud, H., Mikhailenko, S., Chajar, Z. et al. (1998) Surface and bulk properties of Cu—ZSM-5 and Cu/A1203 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons, Appl. Catal. B, 16, 359. 

Konduru, M.V. and Chuang, S.S.C. (1999) Active and spectator adsorbates during NO decomposition over Cu—ZSM-5 Transient IR, site-poisoning, and site-promotion studies, J. Catal., 187, 436. 

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