Understanding the role of C3H6, CO and H2 on efficiency and selectivity of NOx storage reduction (NSR) process

Abstract

The NOx storage reduction (NSR) process is commonly envisaged for the NOx treatment of exhaust gas from lean-burn engine vehicles. NOx are firstly stored on the catalyst, which is periodically submitted to a reducing mixture for few seconds in order to reduce the stored NOx into N2. The on-board reducer is coming from the gasoline/Diesel fuel and, in fact, the NSR catalyst is submitted to a mixture of hydrocarbons, CO and H2 with various compositions depending on the lean/rich step. In this study, the influence of each reducer (C3H6, CO and H2) is evaluated separately, with a special consideration to the N2O selectivity. It is demonstrated that the N2O can be emitted during both lean and rich periods, with varying ratio depending on the considered reducer and the temperature of gas. For instance, at 300°C, a high N2O selectivity is observed when C3H6 is used, and near half of the N2O emission occurs during the storage phase in lean condition.

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

“…At 300°C, the NOx conversion is slightly higher than the NOx storage rate. This observation can be attributed to a partial NOx reduction during the lean phases including traces of reductants, as previously demonstrated in [12]. Table 1 (full gas) for Pt/CZ catalyst used as powder (A) and honeycomb (B).…”

“…In fact, on-board, the reductant is initially coming from the gasoline/Diesel fuel and the NSR catalyst is submitted to a mixture of hydrocarbons, CO and H2 in various amounts. In previous studies [11,12], we have demonstrated that using complex rich and lean mixtures, the N2O emission is mainly driven by H2 at 200 °C, and by C3H6 at 300 °C. Unexpectedly, a large part of the N2O emission at 300 °C was attributable to a NOx reduction occurring during the lean phases carried out with traces of reductant.…”

“…Catalyst as powder was characterized by means of nitrogen adsorption-desorption isotherms, XRD, H2 chemisorption, H2-TPR and NOx storage capacity measurements as previously reported in [12]. The obtained catalyst is noted Pt/CZ and exhibits BET specific surface area of 82 m 2 .g -1 with a platinum dispersion of about 16%.…”

“…The results obtained on powdered Pt/CZ are detailed in [12]. For further comparisons, the percentage of stored NOx in 60 s (duration of the lean period of the cycled NSR test) and NSR behaviors obtained with powder using the "full gas" rich mixture (Table 1) are summarized in Figure 1A.…”

Mechanistic Aspect of N2O Formation Over Pt–Ba/γ-Al2O3 Catalysts

“…at higher temperature for example, leading to high N 2 selectivity [30]. For example, H 2 leads to more N 2 O at lower temperatures, 200 °C, but propylene at 300 °C, where its activation begins [31]. In addition, the use of C 3 H 6 as a reducing agent involves a reaction network that includes NCO species, that leads to N 2 O during the lean phase, but this is reduced in the presence of water [32].…”