Kinetics of Methane Catalytic Combustion on Mn-Substituted Barium Hexaaluminate Catalysts By Veronique Pitchon and Dmitry Yu. Murzin* The oxidation of methane has been studied in the temperature range 475 C £ T £ 660 C isothermally using Mn-substituted barium hexaaluminate catalysts. The solid material was characterized by BET, XRD and TPD of oxygen. Reproducible oxidation rates are provided and the effects of varying both methane (3.8±18.8 Torr) and oxygen (22.8±114 Torr) partial pressure at various reaction temperatures are presented to generate experimental databases. Thesuitability of several kinetic expressions, which is based on reaction mechanisms that have been advanced in the literature to represent the experimental data was assessed. On the whole, several mechanistic models provided an adequate fit to the entire experimental data sets. Appreciable deviations of the predicted from the actual reaction rates have allowed us to discriminate between the various mechanistic models. The best description was obtained for a model featuring a two-step reaction sequence on a nonuniform surface. 1 Introduction The principal interest of catalytic combustion over flame combustion is the fact that the temperature is considerably lower therefore preventing the reaction between nitrogen and oxygen to form undesirable NO x by classical radical mecha- nism [1±3]. One of the very promising catalysts is the Mn- substituted hexaaluminate because the retention of large surface area is the most prominent feature for high- temperature application. The crystal structure of hexaaluminate is of a b-alumina or magnetoplumbite type. This structure consists of oxygen in close-packed spinel blocks separated by mirror planes along the c-axis. These mirror planes contain large cations, such as Ba 2+ and loosely packed oxygen atoms [4]. The large surface area after high calcination temperature is related to the suppression of crystal growth along the c-axis [5]. Also, the insertion of Mn ions in the structure induces a reversible reduction/oxidation behavior of this transition element in the crystal lattice between the di- and trivalent state leading to an enhanced activity for methane combustion [6]. The catalytic activity increases with Mn substitution, this increase being according to some authors linear with an incorporation of Mn ions comprised between 0 and 3 ions, or more important between 0 and 2 ions and less significant for an incorporation of 3 Mn according to other authors [7,8]. The value of 3 Mn is the limit number of Mn ions that the hexaaluminates can accept and above which substitution of Al 3+ is not possible anymore. The nature of the interaction between the hydrocarbon molecule and the oxygen at the surface of the substituted hexaaluminate is very likely to be affected by the modification induced into the bulk structure due the Mn substitution. Kinetic studies provide a very powerful tool to address mechanisms of heterogeneous catalytic reactions [9], e.g., to detect very subtle changes of the surface. A thorough knowledge of the kinetics and mechanism of the gas-phase methane combustion over noble metals and perovskite catalysts [10,11] was acquired. At the same time it is fair to state that kinetic studies on combustion are usually limited to investigation of light-off behavior and very rare steady-state kinetics over a range of parameters (T, pressures of organic substance and oxygen) are reported. In a thorough kinetic study of methane oxidation on LaCr 1±x Ni x O 3 [11] a coupling between the gas composition and temperature dependence was observed, e.g., the apparent activation energy depended on gas composition and the effective reaction orders were temperature-dependent. A power-law rate expression with effective methane order from 0.5 to unity and oxygen orders from nil to 0.25 provided adequate fits. Note that methane and oxygen orders were anticorrelated. In a later study of methane combustion over Mg doped LaMnO 3 perovskites [10] the best kinetic law for data fitting was an Eley-Rideal equation (e.g. first order in methane). The underlying mechanism was based on the dissociative oxygen chemisorption, which was followed by a reaction with methane. To the best of our knowledge isothermal (e.g. non- temperature-programmed) kinetics of methane combustion over hexaaluminates was not studied so far. Therefore, it is the purpose of this paper to put in evidence the variation of the reaction mechanism as a function of the main parameters which include the variation of the amount of Mn, temperature, and both methane and oxygen partial pressures. In this paper, we also assess the applicability of several kinetic models to an array of gathered methane combustion data over Mn-substituted barium hexaaluminate catalysts. 2 Experimental 2.1 Preparation of the Catalysts Barium aluminates have been developed as high-tempera- ture materials for methane combustion especially by the team Chem. Eng. Technol. 24 (2001) 12, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/1212-1301 $ 17.50+.50/0 1301 ± [*] Dr. V. Pitchon, LERCSI, Laboratoire d'Etudes de la RØactivitØ Catalytique, Surfaces et Interfaces, UMR 7515 du CNRS-ECPM, 25, rue Becquerel, 67087 Strasbourg Cedex 2, France, e-mail: pitchon@ chimie.u-strasbg.fr; Prof. Dr. D. Yu. Murzin, Laboratory of Industrial Chemistry, Process Chemistry Group, Faculty of Chemical Engineering, bo Akademi University, Turku, Finland, e-mail: dmurzin@abo.fi. 0930-7516/01/1212-1301 $ 17.50+.50/0 Full Paper