A Kinetic Study of Oxygen Adsorption/Desorption and NO Oxidation over Pt/Al 2 O 3 Catalysts Louise Olsson,* ,†,‡ Bjo 1 rn Westerberg, †,‡ Hans Persson, ‡ Erik Fridell, ‡ Magnus Skoglundh, ‡ and Bengt Andersson †,‡ Department of Chemical Reaction Engineering, Chalmers UniVersity of Technology, SE-412 96 Go ¨ teborg, Sweden, and Competence Centre for Catalysis, Chalmers UniVersity of Technology, SE-412 96 Go ¨ teborg, Sweden ReceiVed: June 8, 1999; In Final Form: September 15, 1999 Laboratory tests and kinetic modeling were carried out in order to provide kinetic input data to a systematic investigation of the mechanism of nitrogen oxides (NO x ) storage in catalysts used for lean-burn engines. In particular, we present a kinetic model of the NO oxidation to NO 2 over a Pt/Al 2 O 3 model catalyst for the temperature range 250-450 °C. Since the oxygen behavior at atmospheric pressure is critical for such a model, we have also studied the adsorption/desorption of oxygen by temperature-programmed desorption (TPD) experiments. The experiments show that oxygen starts to desorb at about 300 °C. Furthermore, the NO oxidation was studied in a temperature ramp with NO and oxygen in the gas feed. The data from this experiment and the above-determined values for the oxygen adsorption/desorption were used to construct a kinetic model for the NO oxidation. Finally, the model was validated with some transient experiments with either NO or NO 2 and different oxygen concentrations in the gas feed. We found a good agreement between these experiments and the model. Introduction Cars equipped with lean-burn engines show a better fuel economy compared to conventional engines operated at stoi- chiometric conditions. However, the catalytic reduction of nitrogen oxides in oxygen excess poses a major problem. One possible solution to this problem is to use so-called NO x storage catalysts in combination with mixed-lean engine operation. 1-3 The NO x storage concept is based on the idea to add a specific NO x adsorbent, e.g. BaO or SrO, to the catalyst. By tuning the engine in alternating lean and rich periods, nitrogen oxides can then be stored under relatively long lean periods and then during short rich intervals the stored NO x is released. The NO x is then further reacted with hydrocarbons and CO to produce N 2 , CO 2 and H 2 O. Even though the NO x storage concept has been known for some years, there is still a lack of detailed knowledge about the reaction mechanisms. Further knowledge is of importance for improving temperature stability, NO x storage ability, regen- eration times, and hindering of sulfur deactivation. To analyze the NO x storage phenomenon it is appropriate to consider a synthetic exhaust gas feed (e.g. NO, O 2 ,C 3 H 6 in an inert gas) over a Pt/BaO/Al 2 O 3 model catalyst at temperatures between 300 and 500 °C (this is the temperature interval where NO x storage is substantial for these model systems 3 ). The rich and lean periods can be simulated by turning the flow of oxygen on and off. Investigations of such systems indicate three important reaction steps: Under lean conditions, NO is oxidized to NO 2 on Pt and subsequently stored 2 on BaO in some nitrite or nitrate form. During rich periods, the stored NO x species decompose and are reduced on Pt to N 2 . Thus, an important key step in a detailed mechanistic investigation of NO x storage is to understand the NO oxidation. Following the idea to decrease the number of active components in the model system, it is adequate to analyze a gas consisting of NO and O 2 over a Pt/Al 2 O 3 catalyst. From the literature we find some kinetic models on NO oxidation over Pt/Al 2 O 3 at atmospheric pres- sures, 4,5 but usually they are constructed with hydrocarbons present in the experiments. There are also some purely experimental studies that present data for NO oxidation. 6,7 An important part in the NO oxidation model is to understand the interaction between oxygen and the active surface for the relevant temperature interval (300-500 °C). For these temper- atures the alumina support can be considered to be quite inert. There are several investigations of oxygen adsorption/desorption on Pt under UHV conditions. 8,9 The maximum surface coverage in those experiments, with O 2 on Pt, was found to be 0.25 oxygen atoms per surface Pt atom. 10-12 However, the coverage of oxygen on Pt can be higher. Parker et al. 10 obtained a high coverage of oxygen on Pt(111) by exposing the catalyst to NO 2 and Saliba et al. 13 by exposing the surface to ozone. Parker et al. 10 also found that the activation energy for desorption of oxygen decreases with increasing oxygen coverage. Moreover, oxygen adsorption studies at atmospheric pressure 13-15 show the formation of different platinum oxides. In this paper, we construct a kinetic model for NO oxidation, over a Pt/Al 2 O 3 coated monolith in a flow reactor, where the parameters obtained can be used as input for a later more complex model describing NO x storage over a Pt/BaO/Al 2 O 3 catalyst. Specifically, to obtain independent oxygen adsorption and desorption data at atmospheric pressures, TPD studies are performed. Further, for NO oxidation a temperature ramp and transient validation experiments are presented. * Corresponding author. E-mail: louise@cre.chalmers.se. Fax: +46(0)- 31-772 3035 (Department of Chemical Reaction Engineering), +46(0)31- 772 3134 (Competence Centre for Catalysis). † Department of Chemical Reaction Engineering. ‡ Competence Centre for Catalysis. 10433 J. Phys. Chem. B 1999, 103, 10433-10439 10.1021/jp9918757 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/10/1999