NOx Storage-reduction Process Research Articles

N2O formation during NOx storage and reduction using C3H6 as reductant

N2O formation over Pt-BaO/Al2O3 catalysts was found to be involved in a complex reaction network induced by C3H6 evolutions during NOx storage reduction process. C3H6 evolutions in the rich period travelled along cracking to CxHy(ad), partial oxidation to COad and followed by NCOad production. In the lean period, residual NCOad reacting with the introduced NO/O2 led to the formation of N2O. In the rich period, N2O was originated from the chemical adsorption of NO on Pt0 surface caused by C3H6 reduction. In addition, both H2O and CO2 in the feed had significant influences on N2O formation in rich period. In the presence of H2O, high performance of water-gas shift was found at T≥300°C, and steam reforming of C3H6 occurred at T≥350°C. These reactions produced a large number of Had that facilitated NH3 production and thereby decreased N2O formation. When CO2 was introduced to the feed, CO2 reforming of C3H6 appeared at T≥300°C as a result of COad production. These parts of COad were responsible for the increase in N2O formation.

Composition dependent performance of alumina-based oxide supported WO3 catalysts for the NH3-SCR reaction and the NSR + SCR coupled process

Among the specific technologies to reduce NOx from automobile lean exhaust gases, the NOx storage reduction (NSR) and the selective catalytic reduction (SCR) by urea/NH3 are the two main effective proposed processes. During the short rich excursions of the NSR process, emissions of undesired NH3 can occur and the combination of the NSR and a NH3-SCR catalyst has been proposed to improve the global treatment efficiency. With the aim to develop non zeolite SCR catalysts, materials based on modified alumina were prepared by sol–gel method. Necessary acidic and redox properties were expected by incorporation of metal ions such as cerium (for oxygen mobility), zirconium, titanium, and silica (acidic behavior). The influence of each element incorporated in the host alumina and their combination were evaluated in NH3-SCR before and after 9wt% WO3 impregnation. Among the 15 synthetized supports, three WO3 supported catalysts exhibited promising SCR activities were selected (WO3/Al0.2Ce0.4Ti0.4, WO3/Al0.2Ce0.16Zr0.32Ti0.32 and WO3/Al0.1Si0.1Ce0.16Zr0.32Ti0.32) and characterized (N2 adsorption, XRD analysis, NH3-SCO reactivity, NH3 storage capacity, pyridine adsorption monitored by IR spectroscopy, H2-TPR, OSC) and then associated downstream to a model Pt/Ba/Al2O3 NSR catalyst. The use of ammonia emitted from the NSR catalyst during the rich pulses was studied taking into accounts three possibilities: the NH3-SCR reaction (NH3+NOx→N2), the NH3-SCO reaction (NH3+O2→N2) and the unconverted NH3. Only the NH3-SCR reaction occurs at 200°C; whereas the three pathways take place at 300 and 400°C. In addition, a lack of strong acidic storage sites results in unconverted ammonia at high temperature.

Non-thermal plasma-assisted NOx storage and reduction over cobalt-containing LNT catalysts

Based on their high NO oxidation capacity (NOC) and NOx storage capacity (NSC), cobalt-containing LNT catalysts of the type Co/Ba/Al and Pd/Co/Ba/Al were tested under NOx storage-reduction (NSR) conditions, as well as in an H2-plasma assisted NSR process. For comparison purposes, a traditional Pt/Ba/Al catalyst was tested under the same conditions. The Co/Ba/Al catalyst exhibited similar catalytic properties to those of the Pt/Ba/Al catalyst when an H2-plasma was employed in the rich phase to assist reduction of the stored NOx. However, the NOx removal efficiency was greatly depressed by the presence of H2O and CO2 in the feed. Notably, the addition of Pd to the Co/Ba/Al sample greatly lessened the inhibiting effects of H2O and CO2 on plasma-assisted cycle-averaged NOx conversion, this being due to the excellent NOx storage performance of the Pd/Co/Ba/Al sample even in the presence of H2O and CO2. The results of the present study show that by combining the high NOx storage capacity of Pd/Co/Ba/Al in the lean phase with non-thermal plasma-assisted activation of the reductant in the rich phase, high NOx conversion can be obtained over a broad temperature window of 150–350°C.

From the powder to the honeycomb. A comparative study of the NSR efficiency and selectivity over Pt–CeZr based active phase

The efficiency and the selectivity of a model platinum based catalyst supported on a modified ceria–zirconia oxide was evaluated in the NOx storage-reduction (NSR) process at four catalytic scales: powder, (0.5″×1.5″) flow-through monolith (FTM) system, small size (1″×2″) and full size (5.66″×10″) catalysed Diesel Particulate Filter (DPF).The washcoating of the active phase over FTM affects both the NOx storage properties and the NOx reduction step. The reduction step efficiency is especially decreased at low temperatures. It is associated with an incomplete regeneration of the storage sites and with a strong NOx desorption peak during the rich pulses of the NSR process for the FTM supported system. The NOx reduction selectivity is also strongly affected by the upscale, with an important N2O selectivity detected over FTM. The recorded NOx profiles during NSR cycles indicate a probable diffusion limitation. However, same trends were observed for both powder and FTM systems concerning the effect of the reductant mixture, for both NSR efficiency and N-compounds selectivity.After incorporation of the active phase in the porosity of the DPF, a sharp drop in NOx storage properties and subsequently in NSR efficiency are observed. Supplementary tests suggest that the diffusion from the platinum oxidizing sites to the storage sites is again very affected by the upscale. Finally, the engine bench tests confirm the low DeNOx activity of the DPF system.

NOx removal efficiency and ammonia selectivity during the NOx storage-reduction process over Pt/BaO(Fe, Mn, Ce)/Al2O3 model catalysts. Part I: Influence of Fe and Mn addition

NOx storage-reduction process was studied using lean/rich cycling condition over Pt/BaO/Al2O3 model catalyst, with a special attention to the ammonia emission. The NOx reduction selectivity strongly depends on the hydrogen conversion introduced during the rich pulses. NH3 is emitted while hydrogen is not fully converted. It was concluded that the NH3 formation rate via the NOx reduction by H2, is higher than the NH3 reaction rate with NOx to form N2. The effect of H2O and CO2 on the reduction step was also examined and results were explained mainly taking into account the reverse water gas shift reaction.Fe addition in Pt/BaO/Al2O3 leads to a strong deactivation of the catalyst, probably due to interaction between iron and platinum. Mn is a poison for the reduction step at 200 and 300°C, but it significantly enhances the NOx reduction at 400°C (conversion and selectivity). Mn favors the NOx reduction with ammonia, even if the introduced hydrogen is not fully converted.

NOx removal efficiency and ammonia selectivity during the NOx storage-reduction process over Pt/BaO(Fe, Mn, Ce)/Al2O3 model catalysts. Part II: Influence of Ce and Mn–Ce addition

It was previously demonstrated in the first part of this work that NOx storage-reduction process over Pt/BaO/Al2O3 model catalyst is limited by the reduction step, with ammonia emission since H2 is not fully consumed. The stored NOx reacts preferentially with the introduced H2 giving NH3, than with NH3 in order to produce N2. Mn addition favors the NOx reduction with ammonia leading to better conversion and selectivity, but only at 400°C. In Part II, a special attention was focused on the role of Ce and Mn–Ce addition in regard to the NOx conversion and the ammonia emission in the 200–400°C temperature range. With ceria modified Pt/20Ba/Al catalyst, significant improvements are obtained from 300°C. In addition to the enhancement of the NOx+NH3 reaction, the ammonia selectivity is maintained at a lower level compared with Pt/Ba(Mn)/Al catalysts, even in the case of a large H2 excess. It is attributed to the ammonia oxidation into N2 via the available oxygen at the catalyst surface. A synergetic effect is observed between Mn and Ce when they are added simultaneously in Pt/Ba/Al catalyst.

Influence of Pt–Ba Proximity on NO x Storage–Reduction Mechanisms: A Space- and Time-Resolved In Situ Infrared Spectroscopic Study

The influence of Pt–Ba proximity on the performance and mechanism of NO x storage–reduction (NSR) was investigated by a comparative study of Pt–Ba/CeO2, Pt/CeO2 and mechanically mixed Pt/CeO2–Ba/CeO2. NO x storage capacity, regeneration activity and selectivity to nitrogen and ammonia during periodic lean (NO + O2)–rich (H2) cycles were evaluated and the chemical gradients along the axial direction of the catalyst beds were monitored by space- and time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The presence of Ba and its proximity to Pt greatly influenced the NSR process. In particular, the proximity was crucial to achieve better utilization of bulk Ba components as well as enhancing selectivity to N2. The space-resolved approach is shown to be a powerful tool to understand the impact of the proximity of Pt and Ba constituents on the final NSR performance.

Use of potassium conductors in the electrochemical promotion of environmental catalysis

This study summarizes the effect of the electrochemical promotion of catalysis (EPOC) by using potassium conductors in environmental catalytic reactions applied to the removal of several automotive pollutants, such us CO, C 3H 6 and nitrogen oxides (N 2O and NO x ). It has been shown the extraordinarily potential of using Pt/K–βAl 2O 3 in a wide variety of environmental reactions (oxidation, reduction), activating the catalyst at lower reaction temperatures and decreasing the inhibitory effect of poisons such as water in the reaction atmosphere. In addition, a new application of potassium conductors-based electrochemical catalysts has been developed for the NO x storage/reduction process (NSR). The idea of coupling catalysis and solid-state electrochemistry on this process would allow to improve and simplify the current NSR technology. Finally, the results have been obtained under reaction conditions compatible with the treatment of automotive exhaust emissions. This demonstrates the potential for the practical use of the phenomenon of electrochemical promotion by using potassium conductors on this kind of process.

In situ FTIR studies of the mechanism of NO x storage and reduction on Pt/Ba/Al 2O 3 catalysts

The mechanism of NO x adsorption and reaction on a typical 1 wt.% Pt/20 wt.% BaO/Al 2O 3 NO x storage reduction (NSR) catalyst was studied by the use of in situ Fourier Transform InfraRed (FTIR) spectroscopy. Parallel studies were also conducted with the individual components of the NSR system (i.e., 1 wt.% Pt/Al 2O 3 and 20 wt.% BaO/Al 2O 3) in an effort to identify the role and importance of each component in the NSR process. Finally, the role of SO 2 was also investigated, since it has been reported that it represents a poison for the NSR process. The results indicate that at 350 °C NO x is stored on the NSR catalyst primarily in the form of barium nitrite and nitrate species. In the presence of Pt a conversion of the surface barium nitrite to bulk barium nitrate is taking place during the storage phase. This step is accelerated in the presence of gas phase oxygen and at elevated temperatures. Both barium nitrite and nitrate species are reduced in the presence of propylene, although at different rates. These reactions are catalyzed by Pt and do not proceed at any significant rate over the BaO/Al 2O 3 sample. Finally, the formation of surface and bulk barium sulfates was observed when SO 2 was introduced as a component of the NO-containing gas phase mixture. These species are stable on the catalyst surface under the reduction conditions studied and inhibit the NO x storage process.