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The formation of N2O has been studied by means of isothermal lean-rich experiments at 150, 180 and 250 °C over Pt-Ba/Al2O3 and Pt/Al2O3 catalysts with H2 and/or C3H6 as reductants. This allows to provide further insights on the mechanistic aspects of N2O formation and on the influence of the storage component. Both gas phase analysis and surface species studies by operando FT-IR spectroscopy were performed. N2O evolution is observed at both lean-to-rich (primary N2O) and rich-to-lean (secondary N2O) transitions. The production of both primary and secondary N2O decreases by increasing the temperature. The presence of Ba markedly decreases secondary N2O formation. FT-IR analysis shows the presence of adsorbed ammonia at the end of the rich phase only for Pt/Al2O3 catalyst. These results suggest that: (i) primary N2O is formed when undissociated NO in the gas phase and partially reduced metal sites are present; (ii) secondary N2O originates from reaction between adsorbed NH3 and residual NOx at the beginning of the lean phase. Moreover, N2O reduction was studied performing temperature programming temperature experiments with H2, NH3 and C3H6 as reducing agents. The reduction is completely selective to nitrogen and occurs at temperature higher than 250 °C in the case of Pt-Ba/Al2O3 catalyst, while lower temperatures are detected for Pt/Al2O3 catalyst. The reactivity order of the reductants is the same for the two catalysts, being hydrogen the more efficient and propylene the less one. Having H2 a high reactivity in the reduction of N2O, it could react with N2O when the regeneration front is developing. Moreover, also ammonia present downstream to the H2 front could react with N2O, even if the reaction with stored NOx seems more efficient. [ABSTRACT FROM AUTHOR]

The detailed reaction mechanism of Di-Air, which showed the unprecedented high deNO performance at high temperature conditions, was studied in this work. Since the Di-Air phenomenon occurs with continuous short pulse injections of hydrocarbons over NSR (NO storage and reduction) catalyst, this study focused on the specific function of HC as an effective reductant for NO reduction reaction. As a first step, the deNO performance was compared with three different reductant gases including CO, H and CH in a modified synthetic gas bench equipped with a gas injector which enables continuous small amount of injections. At inlet gas temperature of 450 °C CH showed the best deNO performance whereas H was the best at 150 °C. Moreover the result of temperature programmed desorption showed that intermediate species represented by -NCO (isocyanates) produced from CH was thermally more stable than that produced from CO. These results confirmed that the injected HC reacts with adsorbed NO on NSR catalyst generating thermally stable intermediates, which could contribute the high deNO performance at high temperature conditions. [ABSTRACT FROM AUTHOR]

This study is devoted to an operando study of Pt-Rh/AlO-BaO lean-NO trap catalyst during the regeneration with H/CO reaction mixture. Particular attention was paid to the influence of CO coexisting with H during the regeneration that can simulate the regeneration step by using reformate composed of CO and H. In rich H mixture ammonia predominantly forms. As expected, strongly chemisorbed CO molecules over noble metals lower the efficiency of the trap at 150 °C. Successive hydrogenation of N atoms to ammonia predominates in our conditions. However, the comparison of the outlet gas composition with infrared spectral features also suggests a minor participation of isocyanate species (NCO) as possible intermediates in the production of ammonia especially for long regeneration duration in the absence of water. Interestingly, ammonia formation as reducing agent for the selective reduction of NO can stimulate practical applications for further coupling lean-NO trap with SCR catalysts. [ABSTRACT FROM AUTHOR]

Effects of Pt and Ba content of Pt/Ba/Al2O3 on NO x storage reduction (NSR) catalyst were investigated by means of kinetic analysis of surface nitrates using in situ FT/IR. Turnover frequencies of both storage and reduction of nitrate were significantly enhanced at higher Ba content, while those were higher at lower Pt content below 0.2 wt%. [ABSTRACT FROM AUTHOR]

Effects of Pt and Ba content of Pt/Ba/Al2O3 on NO x storage reduction (NSR) catalyst were investigated by means of kinetic analysis of surface nitrates using in situ FT/IR. Turnover frequencies of both storage and reduction of nitrate were significantly enhanced at higher Ba content, while those were higher at lower Pt content below 0.2 wt%. [ABSTRACT FROM AUTHOR]

The reduction of NO x stored over a Pt–K/Al2O3 Lean NO x Trap is analyzed when H2 is used as reductant. An in series 2-steps process is herein proposed, involving at first the formation of NH3 upon reaction of nitrates with H2 (step 1), followed by the reaction of NH3 with residual nitrates to give N2 (step 2). [ABSTRACT FROM AUTHOR]

The NO x reduction with n-C6H14 was studied over a 3% Ag/Al2O3 catalyst in the presence of hydrogen. The catalyst performance was evaluated by varying the H2 concentration from 0 to 1600 ppm and by comparing the results with blank runs in which an empty reactor with no catalyst was used. Two distinct reaction pathways were revealed: one at low-temperature (Treact < 370 °C) and another one at high-temperature (Treact > 370–390 °C). Co-feeding of H2 promotes the reaction within the 150–360 °C interval. The high-temperature pathway (Treact > 370–390 °C) seems to be almost independent of hydrogen co-feeding. The homogeneous gas-phase NO x oxidation initiated by NO presumably plays an important role in this high-temperature pathway. [ABSTRACT FROM AUTHOR]

A 2D axisymetric model of a NO x storage and reduction catalyst monolith channel combining mass transport with a detailed kinetic model was created to evaluate the importance of mass transport in a Pt/BaO/Al2O3 washcoat. Results show that there are small radial gradients in stored species concentration early during transients. The Sherwood number calculated during the transient storage phase will not be constant in time nor space as a film correlation would predict, but instead shows a region of higher Sherwood number propagating through the channel as the storage reaches completion. It is concluded that incorporating detailed mass transport provides a better spatially resolved picture of the dynamics of the proposed reaction mechanism and minimises the risk of arriving at false intrinsic kinetics during the development of a microkinetic model. [ABSTRACT FROM AUTHOR]

The relationship between the product selectivity for the NO–H2–O2 reaction and characteristics of the catalyst, Pt/ZrO2, was investigated. From the results of activity tests and characterizations, such as CO adsorption and TEM, the catalysts with high Pt dispersions showed high NH3 selectivities, and those with significantly agglomerated Pt particles by high temperature calcination exhibited higher N2 formation. The effect of the reduction and oxidation pretreatment was also investigated. The reduced Pt sites promoted the formation of NH3. The pretreatment condition influenced not only on the amount of accumulated nitrogen-containing species during the reaction but also on the decomposition of ammonium nitrate, which was suspected to be an accumulated species. The decomposition of ammonium nitrate would be involved in the NO–H2–O2 reaction. [ABSTRACT FROM AUTHOR]