Elementary steps of NOx adsorption and surface reaction on a commercial storage–reduction catalyst
Introduction
Conventional Pt–Rh-based three-way catalysts (TWCs) are very efficient in reducing nitrogen oxide (NOx), CO, and unburned hydrocarbon (HC) emissions from gasoline engines. However, the general demand for lower CO2 emissions and, thus, the requirement of more fuel-efficient gasoline engines led to the development of lean-burn engines operating at significantly higher air-to-fuel ratios then used by traditional engines [1]. Under these oxygen-rich exhaust-gas conditions three-way catalysts cannot efficiently reduce NOx. The most promising approach to the reduction of NOx under lean-burn conditions is based on the concept of NOx storage-reduction (NSR), where the engine is operated in a mixed lean/rich operation mode [2], [3]. NSR catalysts contain storage components, typically alkali or alkaline earth metals, such as barium, and noble metal components providing oxidation/reduction functionality. Under lean operation conditions (i.e., oxidizing atmosphere), NO is oxidized to NO2 over the noble metal component and stored on the storage component. By periodic changes to short cycles of rich operating conditions (i.e., reducing atmosphere), NO2 is released from the storage component and converted to N2 over the metal component. The major drawback of this process at present is the high susceptibility of the NSR catalysts to sulfur poisoning, which lowers the NOx storage capacity [4], [5].
A detailed understanding of the overall NOx storage mechanism is one of the basic steps required to improve the efficiency, as well as the sulfur resistance, of the catalysts. Several studies have focused recently on the mechanism of NOx storage on model NSR catalysts [5], [6], [7], [8], [9], [10], [11]. In an early study, Takahashi et al. [5] investigated commercial NSR catalysts and found indications of oxidation of NO on metal sites and subsequent storage on adjacent storage sites in the form of nitrates. Mahzoul et al. [8] suggested that two different Pt sites are involved in the storage process, one close to BaO related to the nitrate formation and another (further away) acting as oxidation catalysts for NO. The authors also reported the formation of nitrates in the absence of gas phase oxygen and in the absence of Pt.
Fridell et al. [10] suggested two possible reaction pathways for NOx storage with NO2 as the primary oxidizing agent during NOx storage. The first proposed reaction path includes the oxidation of BaO to BaO2 by NO2, whereas the second pathway proposed involves the initial formation of nitrites with subsequent oxidation to nitrates by NO2. In more detail, Prinetto et al. [11], as well as Westerberg et al. [6], studied the interaction of NO and NO2 in the presence of O2 with model storage catalysts such as Pt/BaO/Al2O3 by IR spectroscopy. The main surface species observed were hyponitrites, nitrites, and nitrates on alumina and Ba oxide. Furthermore, Westerberg et al. [6] proposed that Al2O3 could play an important role as a storage site at temperatures below 300 °C.
As most of the studies were performed on model systems, detailed investigations of the NOx storage mechanism on commercial NSR catalysts are required. The purpose of the present study is to obtain an improved understanding of the NOx storage mechanism by identifying the surface species and reaction intermediates on such a catalyst under reaction conditions closely related to the practical application conditions. The catalyst was exposed to different gas compositions typical of the NSR process (i.e., NO, NO/O2, and NO2), and the NOx species formed on the catalyst surface were investigated by in situ IR spectroscopy, leading to a sequence of reaction steps for the NOx storage process.
Section snippets
Experimental
The catalyst studied in this work was a commercial NOx storage–reduction catalyst containing ∼1 wt% noble metals (Pt and Rh) as oxidation/reduction component and BaO/BaCO3 (∼8 wt%) as storage component, deposited on an Al2O3 support. From EXAFS analysis of the fresh sample the dispersion of the Pt particles was estimated to be 95%. The specific surface area of the catalyst, determined by N2 sorption (BET method), was 110 m2/g.
XRD measurements were performed on a Rigaku powder diffractometer
Catalyst characterization
The XRD patterns of the fresh and activated catalyst are given in Fig. 1. Besides the reflections typical for the support [13], the XRD pattern of the catalyst before activation exhibited reflections characteristic for BaCO3 at 2θ values of 24°, 24.3°, 34.1°, 34.7°, 42.1°, and 44.8° (indicated by ) and for BaO at 46.2° (indicated by ○). In the XRD pattern recorded after thermal treatment up to 700 °C the presence of BaCO3 was observed at a much lower concentration. Characteristic peaks for
Discussion
During the exposure of the catalyst to NO, predominantly nitrite surface species are formed on Ba oxide and Al oxide sites. Initially, linear and bridged nitrites are formed, which were preferably located on Ba oxide surface sites, due to their higher basicity [23]. It is likely that the nitrite first interacts via the positively charged N atom with the negatively charged oxygen atom next to Ba creating a linear nitrite species [24] as shown in Scheme 1.
If linear Ba nitrites are formed on less
Conclusions
Surface species and reaction intermediates on a commercial NOx storage–reduction catalyst during exposure to NO, NO2, and NO/O2 have been identified by in situ IR spectroscopy. During exposure of the catalyst to NO, mainly linear and bridged nitrites were formed by molecular adsorption of NO. After longer exposure, small concentrations of bridged and chelating nitrates on Al oxide sites were seen, which were formed via the oxidation of NO by reactive oxygen and via oxidation of adsorbed
Acknowledgements
The authors thank the European Union for funding this project, STORECAT: Brite/EuRam BRPR-CT98-0613.
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