Nitrates adsorbed

The FTIR data reported in Figure 6.2b showed that only nitrate species were formed upon N02 adsorption, mainly of the ionic type (bands at 1320, 1420-1440 cm-1, vaSymN03 split for the partial removal of the degeneracy 1035-1020 cm-1, vsymN03) and in minor amounts of bidentate type (1560 cm-1, vasymN02 mode expected around 1300 cur1 obscured by the modes of ionic nitrates). Notably, the adsorbed nitrates were related to the Ba component as the surface of the alumina support was almost completely covered by Ba, as pointed out by FTIR data [25], which showed the disappearance of OH groups of the alumina support. 

The TPD data showed that decomposition of adsorbed nitrates occurs at temperatures very close to that of adsorption. The process was not yet completed at temperatures as high as 600°C. This was in line with literature indications [11], claiming that NO, spillover processes from Ba to Pt (not possible on the physical mixture) could affect the nitrate decomposition process. 

To darify better the role of Pt in the reduction mechanism, a physical mixture of the binary Pt/Y-Al2O3 (1 100 w/w) and Ba/Y-Al2O3 (20 100 w/w) samples was prepared and tested [117]. Also in this case NO was stored at 350 °C upon adsorption of NO in the presence of excess O2 [102], then the stability/reactivity of stored nitrates was investigated by means of TPD and H2 TPSR. The TPD data showed that decomposition of adsorbed nitrates occurs at temperatures very dose to that of adsorption the presence of H2 (TPSR run) does not decrease significantly the temperature threshold for nitrate decomposition (which is still observed near 350 °C), but instead provokes the reduction of the evolved NO to NH3 and N2. Hence the previously invoked Pt-catalyzed route is active only when Pt and Ba are dispersed over the same particle of the support. 

On the basis of these data, the following mechanism for reduction by hydrogen can be suggested. H2, activated over the Pt sites according to the Pt-catalyzed pathway discussed previously, reduces the stored nitrates directly to ammonia or, more likely, induces the decomposition of nitrates to gaseous NO, which are then reduced by H2 to NH3 over the Pt sites [overall reaction (13.47)]. Once ammonia has been formed, it can react with adsorbed nitrates and this reaction is very selective towards nitrogen. It is worth noting that the reaction of ammonia with NOx obeys the stoichiometry of reaction (13.49), which is different from that of the well-known NH3-NO SCR reaction because it implies the participation of nitrates. 

The adsorption and reduction of N03 ions at Au and Pt electrodes was studied by in situ fourier transform infrared (FTIR) spectroscopy [55]. Possible adsorption geometries were suggested for adsorbed nitrate ions and for nitrite ions formed by reduction. 

The ammonia, NO and nitrogen mass balance equations were modified with respect to those already presented in previous sections, in order to include all of the reactions observed in the NH3-N0/N02 reacting system. Moreover, additional mass balances for adsorbed nitrates ( hno3) ar d for gaseous N02, N20 and HN03 were introduced (a) adsorbed phase ... 

As for the surface characterization of the catalysts, FTIR is not yet a quantitative method to evaluate the number of several oxygen-containing surface groups of a support or a catalyst. As a consequence, FTIR is unable to distinguish a catalyst prepared by precipitation from another one prepared by incipient wetness impregnation. We have also shown that the absorption band observed at 1384 cm after oxidation is more probably due to adsorbed nitrate groups than carbonate or carboxylate functions. 

The kinetics of the process was described via the detailed kinetic model proposed by Visconti et al. (2013), which involves both gas-phase (oxygen, nitrogen monoxide, and nitrogen dioxide) and adsorbed species (nitrites, nitrates, and atomic oxygen). As per the adopted kinetic mechanism, NOx accumulation can follow two different paths, namely, the nitrite route and the nitrate route. The nitrite route proceeds with a stepwise oxidation mechanism occurring at Pt-Ba couples and leads to the formation of nitrite adspecies (reaction S3) from the gas-phase NO. The nitrate route instead involves the NO2 formed by NO oxidation (reaction S2) and leads to the formation of adsorbed nitrates (reaction S4). Nitrites can also be converted into nitrates by gas-phase NO2 (reaction S5) ... 

Periodically (e.g., every 60-120 s), the trap is regenerated either by introducing a rich pulse of reductant (e.g., diesel fuel) into the exhaust stream (157) or by switching the engine operating mode to stoichiometric or rich for 1-2 s. This is demonstrated in Figure 21. This rich pulse provides the necessary chemical reductant (H2 and CO) to convert the adsorbed nitrate to nitrogen over the Rh... 

The produced H2O is supposed to destabilize the adsorbed nitrates which are suggested to be decomposed on the free Pt surface. The dissociation of NOx species on platinum can thereafter leads to the recombination of Na atoms to form N2 (reactions 19.5 and 19.6), or to the reaction of these Na atoms with H2 to form NH, and, finally, NH3. 

Clayton et al. [22] also suggested that the ammonia formation mechanism includes the activation of H2 on Pt sites. They proposed that adsorbed nitrates are decomposed into NOx and released in the gas phase, due to hydrogen spill-over from the noble metal to the alumina support. NOx species are readily reduced to ammonia due to high local H/N ratio. 

The NO reduction by CsHs was recently studied on a series of La(Co, Mn, Fe)i j,(Cu, Pd)Os perovskites with high surface area and nanozised crystal domains [74,76]. An enhanced performance (100% NO conversion, 77% N2 yield) was achieved at 600 °C over LaFeo.8Cuo.2O3 under a dry feed stream, which was only slightly decreased in the presence of 10% water vapor. In a similar manner, Cu-substituted LaCoi- Cu Os [77] or LaMni, <,CU c03 [78] composites have also shown enhanced performance in this reaction. The better performance of Cu-substituted samples was attributed to the facility in the formation of adsorbed nitrate species via NO oxidation in addition to the intrinsic reactivity of Cu toward NO activation. 

As shown in Fig. 11.11, the obtained in the presence of NO is shifted considerably to lower temperatures with respect to that obtained in the presence of O2 only. It should also be observed that pretreatment with NO/O2 or with N02/02 also facilitates soot oxidation. A bimodal shape of the TPO profile is observed for the NO + O2 pretreated ceria with the low and high temperature peaks correlating well with those obtained in NO + Og and Og respectively (Fig. 11.11). This suggests that NO2 derived from adsorbed nitrates significantly promotes soot oxidation. 

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