Catalysis Letters 66 (2000) 71–74 71 The mechanism for NO x storage Erik Fridell ∗ , Hans Persson, Bj¨ orn Westerberg, Louise Olsson and Magnus Skoglundh Competence Centre for Catalysis, Chalmers University of Technology, SE 412 96 G¨ oteborg, Sweden E-mail: fridell@fy.chalmers.se Received 26 October 1999; accepted 25 February 2000 The mechanisms for storing of NOx in platinum–barium–alumina catalysts during lean–rich transients are investigated. Oxidation of NO to NO 2 is found to be an important step. NO 2 is found to be important for oxidation of the catalyst or of nitrites to form nitrates. NOx is then stored in the form of surface nitrates. FTIR studies show no formation of bulk nitrates in these experiments. Keywords: lean burn, nitrogen oxides, NOx storage catalyst, platinum, barium, NO 2 , surface nitrates It is essential to reduce the emissions into the atmosphere of the greenhouse gas CO 2 from fossil sources [1]. Since traffic is a major source it is important to find ways to im- prove the fuel economy of cars. One way to contribute to this is to replace the normal petrol engines, running at sto- ichiometric conditions, with lean burn engines. However, this leads to the problem of reducing nitrogen oxides in the lean exhaust emitted by these engines. The three-way cata- lysts used today are not able to do this and, therefore, new catalytic systems are needed. So-called selective catalytic reduction using ammonia as reducing agent, seems to work relatively well [2]. There is, however, great resistance to implement this kind of solution, where a separate tank with ammonia or urea solution is needed in the vehicle. Con- tinuous reduction with hydrocarbons as the reducing agent is therefore more attractive but has not yet proven to give large enough conversions to solve the problems with NO x emissions even though several different types of catalysts have been investigated [3,4]. A different way to solve this problem is to use a so- called NO x storage catalyst in combination with mixed lean operation of the engine. In this case, NO x is trapped in the catalyst under lean conditions. As the catalyst becomes saturated with NO x it needs to be regenerated. This is achieved by tuning the engine to rich conditions for a short period so that the stored NO x is released and reduced to N 2 . These types of systems are sold by Toyota for the Japanese market [5]. The main obstacle for introduction in Europe is presently the relatively high level of sulphur in gasoline. Sulphur will bind to the catalyst in the form of sulphates and deactivate the NO x storage function [6,7]. However, it is possible to produce low sulphur gasoline and a stricter future legislation is expected in Europe opening up for NO x storage systems to be implemented. 1 ∗ To whom correspondence should be addressed. 1 Low levels of sulphur originating from lubricants will still slowly de- activate the catalyst. It is therefore essential to find suitable sulphur regeneration strategies for these systems. This letter deals with the mechanisms for storage of NO x . In order to investigate this more closely it is use- ful to simplify the system. The essential functions of the catalyst are (1) oxidation of hydrocarbons and NO, (2) stor- age of NO x and (3) release and reduction of stored NO x under rich conditions. The model samples we have used contain Pt for oxidation and reduction, barium as storage compound and alumina as support material. Some experi- ments on samples without either Pt or Ba have also been performed for comparison. Simplified gas mixtures with oxygen, NO x and reducing agents were used. Monolith samples with 2 wt% Pt and 20 wt% BaO in the washcoat were prepared by wet deposition methods as described in [8]. Most experiments were performed in a flow reactor sys- tem, described elsewhere [9], equipped with a chemilumi- nescence detector for NO and NO 2 and IR instruments for CO 2 and N 2 O. In order to mimic the mixed lean condi- tions, transient experiments where the gas mixture is reg- ularly altered between lean and rich conditions have been performed. Normally this was achieved by turning on and off the oxygen in the gas flow and replacing it with inert gas. Typically the gas mixture then contains O 2 , NO, C 3 H 6 and inert gas under lean conditions and NO, C 3 H 6 and inert gas for the rich period. However, several variations on this have also been used. An important parameter is the amount of NO x that is stored in each lean period. This amount is obtained from the NO x signal measured after the catalyst during the transients as described in [8]. The first step in the NO x storage sequence is the oxida- tion of NO to NO 2 taking place on Pt sites. The kinetics for this over supported Pt have been studied by Olsson et al. [10] and it is found that the rate-limiting step is the re- action between NO and adsorbed atomic oxygen. Figure 1 shows the NO 2 signal after the NO x storage catalyst during a heating ramp in a lean gas mixture containing 600 ppm NO, 900 ppm C 3 H 6 and 8% O 2 . At low temperatures NO 2 is favoured over NO from thermodynamic considera-  J.C. Baltzer AG, Science Publishers