Dinitrosyl species

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]. 

Since the formation of NO2 can occur homogeneously, it was of interest to establish whether adsorbed NO could be oxidized. NO was adsorbed at 225 C, after which the infrared cell was purged with He and subsequently a stream of 10.1% O2 in He was allowed to flow over the catalyst. Prior to the introduction of the 02-containing stream, the only features evident were those for mono- and dinitrosyls. In the presence of O2 at 225 °C, the intensities of the bands for both mono- and dinitrosyl species attenuated and new features appeared at 1628 and 1518 cm-, corresponding to nitrate and nitrito species, respectively. A similar experiment carried out in the absence of O2, showed only a small decrease in the intensity of the nitrosyl bands due to NO desorption and the absence of bands for nitrate and nitrito species during a 30 min purge in He at 225 °C. 

Surface nitrosyl complexes of TMI have been thoroughly investigated by the computational spectroscopy [22,23,32,33,36,49], and their molecular structure has been ascertained by a remarkable agreement between the theory and experiment of both vibrational (oscillation frequencies and intensities) and magnetic (g and A tensors) parameters. The calculated pNO values for the examined mononitrosyls along with the experimental frequencies are listed in Table 2.6. Analogous collation of the IR data for dinitrosyl species is shown in Table 2.7. 

It occurs via two adjacent adsorbed NO molecules, leading to an adsorbed dinitrosyl species. These last two co-adsorbed NO species made the two N-O bonds weaker, and the successive two N-O bond scissions led to N2. According to a general kinetic model [12], the NzO intermediate can desorb before dissociating to N2, if the desorption rate constant, kdes, is higher than the reaction (dissociation) rate constant, k, as presented in the following set of rate constants (Figure 5.2) ... 

Figure 5.1 shows that function 1 is therefore the oxidation of NO to N02, this last one being subsequently delivered to function 2 to oxidize HC to CxHyOz which will be delivered to function 3 to scavenge the adsorbed oxygen species left by the (NO)2 -adsorbed dinitrosyl species - decomposition. 

The KOCAT Society (no patent reference known by authors), in South Korea, solved the DeNO problem of big burners, by directly injecting oxygenates on the catalyst at the outlet of the burner. This process involves the third function of our model. This example shows that only one model (the present one) for DeNO reaction can be used for either mobile or stationary sources. Pathways are the same what is changing is the nature of the reductant, which has to be activated, through its partial oxidation, at the temperature when N—O bonds (dinitrosyl species) are broken. 

Such a reaction pathway has been earlier discussed on Rh according to the usual observation of gem-dinitrosyl species on isolated Rh1 species [36], On the other hand, such a proposal could be more controversial on metallic Pt sites [37,38],... 

In summary, a complex chemistry could be envisaged for explaining the formation of N2 and N20 under lean conditions. In the particular case of N20, nitrosyl, dinitrosyl species as well as nitrates and nitrites species could be considered as intermediates particularly under lean conditions. 

Non-noble metals such as Ni, Co, Mo, W, Fe, Ag and Cu have been added to zeolites for use in catalysis. In addition to CO, nitric oxide (NO) has been shown to be a good adsorbate for probing the electronic environment of these metals. When NO chemisorbs on these metals, it can form mononitrosyl (M-NO) and dinitrosyl species (ON-M-NO). The monontrosyl species has a single absorption band and the dinitrosyl species has two bands due to asymmetric and symmetric vibrational modes of the (ON-M-NO) moiety. Again, there have been many studies reported in the literature on the use of NO and/or CO adsorption on non-noble metals supported on zeolites and they are too numerous to list here. Several examples have been selected and summarized to provide the reader with the type of information that can be provided by this method. 

Sometimes a formal disproportionation of NO occurs. For example reaction with Co(NO)(PPh3)3 yields the nitrito dinitrosyl species Co(N02)(NO)2(PPh3) and nitrogen gas is evolved.82 The initial step in this reaction may be described as in equation (7). 

Dinuclear and tetranuclear complexes are also suitable starting materials, and much of the synthetic and substitutional chemistry of these species reflects the ability of the systems to sustain monomeric solvated dinitrosyl species such as [Fe(NO)2(solvent)2]+ or [Fe(NO)2(SR)(solvent)]. The latter tend to be formed in solvents having poor and the former in ones having good tt-acceptor capabilities. A review of nitrosyl complexes of iron-sulfur complexes is available. 

Adsorbed NO can be even more sensitive than adsorbed CO to the oxidation state of the cations. The use of NO as a probe allows, for example, the determination of the oxidation state of Co ions in zeoHtes better than CO [159], showing the existence of both Co and Co species. In fact, the bands observed at 1901, 1817cm are due to [Co(NO)2] gem-dinitrosyl species, while that at 1945cm is assigned to [Co-NO] mononitrosyl. The three bands due to adsorbed CO at 2204,... 

R. Ramprasad, K. C. Hass, W. F. Schneider, and J. B. Adams, /. Phys. Chem. B, 101, 6903 (1997). Cu-Dinitrosyl Species in Zeolites A Density Functional Molecular Cluster Study. 

Figure 6 shows the integrated intensities of bands characteristic of the different surface species generated upon NO adsorption. A comparison of the Cu(L)- and Cu(S)ZSM-5 samples demonstrates that NOj is formed on both samples, though its formation is more enhanced on Cu(L)ZSM-5. The band of the dinitrosyl species at 1730 cm decreases rapidly with the adsorption time on Cu(L)ZSM-5, while on Cu(S)ZSM-5 this band is hardly detectable. On both zeolites, the intensity of the band due to the mononitrosyl species at 1890 cm passes through a maximum. In general, the intensities of the bands are lower on Cu(S)ZSM-5 than on Cu(L)ZSM-5. 

Different surface species are formed from NO on Fe(L)ZSM-5. Here, both dinitrosyl vibrations at 1810 (asym) and 1920 (sym) cm are clearly seen. The intensities of these first increase and then remain constant within experimental error. It is noteworthy that only the band of weakly bound NOj but not that of strongly bound NOj appeared. In the range of the mononitrosyl vibration, two or three overlapping bands with maxima at 1897 and 1875 cm developed. More complex spectral changes are found for Ni(L)ZSM-5. Besides the NOj bands (weakly bound at 2120 cm, strongly bound at 1637 cm ) the band of NjO also appeared at 2241 cm. The latter band had constant intensity, while that of the weakly bounded NOj decreased, and that of the strongly bound NOj increased with time. The intensity of the dinitrosyl species increased, while that of the mononitrosyl remained constant. 

As concerns both discussed features, the Fe- and NiZSM-5 samples are intermediate between the other two. This is reflected in their spectral character. For both samples, bands due to the mono- and dinitrosyl species are observed, but their intensities differ substantially. 

Alloyed Sn completely changed the selectivity of NO decomposition reactions, primarily forming N O as the primary desorption product in TPD. Based on HREELS spectra, NO bonds in the same bent-atop configuration on both Pt(lOO) and the Sn/Pt(100) alloys at low NO coverages. At monolayer coverage, two molecules of NO bond to one Pt atom (dinitrosyl species), which is an intermediate in... 

This band is adsorbed on the same sites as the dinitrosyl species. 

These bands have been assigned according to Yuen et al. (22). The bands at 1815 and 1755 cm are assigned to mononitrosyl species, while those at 1897 and 1791 cm are assigned to the symmetric and asymmetric stretching modes, respectively, of an iron dinitrosyl species. The dinitrosyl species is converted to the mononitrosyl species at 1755 cm by evacuation. This will be discussed in more detail below. 

The 1% Fe/Si02 sample displayed bands for both mononitrosyl and dinitrosyl species. The dinitrosyl species were indicated by bands at ca. 1900 cm" and 1800 cm" which decreased in intensity upon evacuation, accompanied by a corresponding increase in the intensity of the mononitrosyl peak at ca. 1750 cm . Therefore, the mononitrosyl band at 1750 cm" and the dinitrosyl bands are thought to be adsorbed on the same site. A second mononitrosyl band was observed at 1815 cm". Unlike mononitrosyl species, for which the maximum intensities were attained rapidly, the intensities of the dinitrosyl species increased over a 2 h period. This is in agreement with the work of Segawa et al. (31) for Fe " in y"zeolite. 

The large quadrupole splittings and isomer shifts observed for the iron cations on 1% Fe/Ti02 and 1% Fe/Al203 and the absence of dinitrosyl species suggest that in surface compounds the iron cations are present in sites of high coordination (e.g., 5- or 6-fold). In the bulk compound FeAl20 the iron cations are located in tetrahedral sites (45) however, surface cations are expected to be... 

The Mbssbauer spectra of Fe on Si02> and the infrared spectra for NO on these samples, were similar to those reported in the literature. Two distinct types of iron sites were observed. These are characterized in the Mbssbauer spectra by two doublets, a doublet with a smaller isomer shift and quadrupole splitting denoted as an "inner doublet" and a second doublet denoted as an "outer doublet". The outer doublet iron has been assigned to sites of higher coordination (e.g., 5- or 6-fold), while the inner doublet has been assigned to sites of lower coordination (e.g., 3- or 4-fold). Dinitrosyl species may be formed on the "inner doublet sites" which are lower in coordination. 

The ability of Co° ions in X- and Y-zeolites to form dinitrosylic species has been investigated by means of spectroscopic methods. It has been concluded that, in the complex, the unpaired electron is essentially localized on the Co centre, whose formal oxidation state is very close to zero. The formation of [Co(en)202] adducts by interaction of ethylenediamine and O2 with Co ions in X- and Y-zeolites has been investigated by i.r. and e.s.r. techniques. Due to the presence of the strong i.r. active modes of the zeolite lattice, the stretching mode of the O2 species in the mixed oxygen-ethylenediamine adducts could not be observed. ... 

Other papers deal with tin oxide as a matrix for transition-metal ions and have already been discussed in Section 2 of this review. Adsorption of NO on H2 reduced samples leads to the reoxidation of the sample (N2 and N2O are formed in the gas phase). On evacuated samples, a chelating NO2 species is formed, together with a dinitrosylic species. 

Contact of NO with metal subcarbonyls invariably yields dinitrosylic species where the central atom has been oxidized by NO itself. In the case of Cr, valence states of two or three are likely to be attainedin the case of Mo and W the tetravalent state is favoured. The equivalence of the NO oscillators has been proved (in the case of Mo complexes) by isotopic exchange. Millman and Hall have studied the same system and assigned the observed bands to dimeric dinitrosyl species. 

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