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The adsorption–reduction by H 2 of NO x stored on a model LNT Pt–Ba/Al 2 O 3 catalyst sample and on a combined LNT + Fe-ZSM5 SCR catalyst is investigated in this work. The storage of NO x over the LNT Pt–Ba/Al 2 O 3 catalyst leads to nitrite formation at low temperatures, and to nitrate at high temperatures (above 300 °C). The stored NO x are reduced mainly to NH 3 at low temperatures, and to N 2 at high temperatures. In line with previous data and according to experiments carried out with H 2 and NH 3 , a two-step in-series pathway for nitrogen formation is proposed involving at first the fast formation of ammonia upon reaction of NO x with H 2 , followed by the slower reaction of the so-formed ammonia with the stored NO x leading to N 2 . This two-step in-series pathway together with the integral behaviour of the reactor accounts for the temporal evolution of the products which is observed during regeneration of LNT catalysts. A Fe-ZSM5 SCR placed downstream the LNT catalyst stores NH 3 released during the rich phase from the LNT sample. The stored ammonia is effectively converted to N 2 during the subsequent lean phase, due to the occurrence of the SCR reaction over the Fe-ZSM5 catalyst bed. Hence the presence of a SCR catalyst layer placed downstream a LNT sample leads to the twofold benefit of reducing the NH 3 slip and to increase the NO x removal efficiency. These advantages are most important at low temperatures, and tend to vanish at high temperatures where the NH 3 slip is less important.

The reduction by H2 of nitrates stored on a model Pt–Ba/Al2O3 catalyst under isothermal conditions was investigated. An in-series two-step process involving at first the formation of ammonia on reaction of nitrates with H2 (step 1), followed by the reaction of ammonia with residual stored nitrates to give N2 (step 2) is proposed. Different temperature thresholds for the two steps are identified, with step 1 being much faster than step 2. The reaction of ammonia with nitrates to give nitrogen (step 2) is very selective, is rate-determining, and represents the major route for nitrogen formation. Due to the fast reduction of adsorbed nitrates by hydrogen (step 1) and to the “plug-flow” integral behavior of the reactor, an H2 front develops that travels along the reactor. Ammonia is readily produced at the front; if the temperature is high enough, then the regeneration process continues downstream of the hydrogen front due to the reaction of NH3 with the stored nitrates, leading to N2 production. This fully explains the change in product distribution with time observed at the reactor outlet during regeneration of LNT catalysts, as well as the effect of temperature on the selectivity of the process. The effects of water and CO2 on the reduction process also are addressed. Water favors the first step of the reduction process but has no significant impact on the subsequent reaction of ammonia with stored nitrates, whereas CO2 has a strong inhibiting effect both on the reduction of nitrates to ammonia by H2 (step 1) and on the subsequent reaction of ammonia with the residual stored nitrates (step 2), due to poisoning of Pt by CO.

The NO x desorption profiles obtained in O 2 –He and n -C 10 –O 2 –He were compared on γ-Al 2 O 3 and Ag/γ-Al 2 O 3 . On Ag/γ-Al 2 O 3 , the low-temperature NO x desorption profiles obtained in n -C 10 –O 2 –He were significantly different from those obtained in O 2 –He. In particular, at 190–220 °C excess release of NO, instead of NO 2 , was observed concomitantly with n -C 10 consumption. This is interpreted as the result of the formation–decomposition of organo-NO x species issued from the interaction of NO 2 and hydrocarbons initially chemisorbed on Al 2 O 3 and activated on Ag species, respectively. The occurrence of such a phenomenon at temperatures close to those at which the n -C 10 -SCR reaction starts provides support for the involvement of the organo-NO x species as intermediates.

Mechanistic aspects of the reduction of stored NO x with hydrogen were analyzed over model Ba/ γ -Al 2 O 3 and Pt–Ba/ γ -Al 2 O 3 LNT trap catalysts and over a Ba/ γ -Al 2 O 3 –Pt/ γ -Al 2 O 3 physical mixture. NO x species were stored and stabilized at 350 °C under controlled conditions, and their stability/reactivity was then analyzed under different atmospheres (He and hydrogen) either at constant temperature or with temperature programming under nearly isothermal conditions, that is, in the absence of significant thermal effects. The collected data indicate that over the Pt–Ba/Al 2 O 3 catalyst, the reduction of stored NO x occurs at temperatures below those corresponding to their thermal stability. This indicates that the reduction of stored nitrates occurs in typical NSR catalysts according to a Pt-catalyzed surface reaction, which does not involve the thermal decomposition of the adsorbed NO x species as a preliminary step. The occurrence of such a pathway also requires the co-presence of the storage element and of the noble metal on the same support, because this route does not operate when they are dispersed on different support particles.

International audience; The selective catalytic reduction (SCR) of NOx assisted by propene is investigated on Pd/Ce0.68 Zr0.32 O2 catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO2 reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C3 H6 . These catalysts display very stable deNOx activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N2 selectivity obtained on Pd/ CZ suggests that the lean deNOx mechanism occurring on these catalysts is different from that occurring on Pd0/SiO2 . A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO2 and NO decomposition to N2 , whereas PdOx activates C3H6 via ad-NO2 species, intermediately producing R-NOx compounds that further decompose to NO and CxHyOz . The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C3H6 -assisted NO decomposition mechanism stresses the key role of NO2 , R-NOx and CxHyOz as intermediates of the SCR of NOx by hydrocarbons.

Adsorption of NO, NO/O 2 , and NO 2 on Pt–Ba–Al–O system is investigated at 350 °C by the transient response method (TRM) and FT-IR spectroscopy. The data suggest that in the presence of oxygen, NO is effectively adsorbed (at a Ba site in proximity of a Pt site) through stepwise oxidation to form at first nitrites that are progressively transformed into nitrates. NO is also oxidized to NO 2 over Pt in the presence of oxygen. NO 2 is directly adsorbed to form Ba nitrates according to a disproportionation reaction, which occurs with the evolution of NO. The stepwise oxidation route seems to play a major role in NO x storage from NO/O 2 mixtures.