Langmuir 2002, 18, 423-432
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Pore Structure-Chemical Composition Interactions of New High Surface Area Manganese Based Mesoporous Materials. Materials Preparation, Characterization, and Catalytic Activity Constantinos E. Salmas,† Vassilis N. Stathopoulos,‡ Philippos J. Pomonis,‡ and George P. Androutsopoulos*,† Department of Chemical Engineering, Section II, Chemical Process Engineering Laboratory, National Technical University of Athens, 9 Heroon Polytechniou Street, GR 157 80 Athens, Greece, and Department of Chemistry, Section of Industrial and Food Chemistry, Laboratory of Industrial Chemistry, University of Ioannina, GR 451 10 Ioannina, Greece Received March 6, 2001. In Final Form: September 25, 2001 The present article deals with pore structure-chemical composition interactions of new high surface area manganese based porous mixed oxides (MANPOs). These mixed oxidic materials were prepared by the dropwise addition of a solution containing metal nitrate into an acetonic solution containing the trinuclear complex of manganese [Mn3O(CH3COO)6(pyr)3]ClO4, which is water sensitive. The CPSM model (corrugated pore structure model) was employed in pore structure investigations of the MANPO materials containing Al, Fe, La, La + Ce, and La + Sr. Successful CPSM simulations of nitrogen sorption hysteresis loops for the MANPO solids enabled the evaluation of intrinsic pore size distributions (PSDs) and tortuosity factors. The latter vary over the approximate range τCPSM ) 3.0-5.7 (i.e. τCPSM ≈ 3 for materials containing Al or Fe and τCPSM ) 4.7-5.7 for those containing La, La + Ce, or La + Sr). These values are typical of porous catalysts and are higher than those reported for MCM-41 materials (ca. τCPSM ) 1.00-2.35) and anodic aluminum oxide films (e.g. τCPSM ) 2.60). The BET surface areas of the MANPO materials vary over the approximate range 180-900 m2/g, being higher than those reported for manganese oxide mesopore solids (MOMS), for example 59-170 m2/g. Intrinsic PSD predictions for the MANPO solids are of bimodal type with mean pore sizes systematically higher than the respective mean hydraulic diameters. The addition of Fe and La + Ce caused a PSD shift toward the micropore region. Such MANPO materials are amorphous after heating to 500 °C (except for the ones containing Fe) and extremely efficient catalysts for redox reactions such as lean de-NOx applications and NO-CO conversion, probably because of the extremely high dispersion of separate nanophases into the solid.
1. Introduction 1.1. General. A number of significant applications stress the importance of synthesizing high surface area mesoporous manganese based solids. The latter materials are employed in the catalysis of oxidation reactions taking place at milder temperature conditions than those for reactions catalyzed by noble metals.1,2 Manganese solids can also be used for the production of aqueous and nonaqueous batteries (either regenerable or nonregenerable type).1,2 A comprehensive review of various preparation procedures for Manganese catalysts is provided in ref 1 with special reference to the preparation of high surface area mesoporous manganese solids. 1.2. Catalytic Applications of Manganese Oxides. Methane and butane oxidation on oceanic manganese nodules showed a better catalytic activity than that of a commercial Pt-Al2O3 catalyst and demonstrated that manganese oxides possess satisfactory oxidation properties and offer the possibility of using these and similar materials in catalytic processes for the removal of volatile organic compounds (VOCs) as well as gaseous pollutants † ‡
National Technical University of Athens. University of Ioannina.
(1) Stathopoulos, V. N.; Petrakis, D. E.; Hudson, M.; Falaras, P.; Neophytides, S. G.; Pomonis, P. J. Characterization of Porous Solids-V; Unger, K. K., Kreysa, G., Baselt, J. P., Eds.; Elsevier: Amsterdam, 1999, p 593. (2) Stathopoulos, V. N.; Belessi, V. C.; Costa, C. N.; Neophytides, S.; Falaras, P.; Efstathiou, A. M.; Pomonis, P. J. Studies of Surface Catalysis; Corma, A., Melo, F. V., Mentioroz, S., Fierro, J. L. G., Eds.; Elsevier: Amsterdam, 2000; Vol. 130, p 1529.
from combustion gases.3,4 The combination of manganese oxide with other transition metals results in the formation of active catalytic materials suitable for the oxidation of a spectrum of VOC compounds. Moreover, the required process temperatures are lower than those for the corresponding catalytic materials containing noble metals.5 Additional applications of manganese based catalysts are (i) benzene oxidation,6 (ii) photocatalytic conversion of 2-propanol to acetone and the decomposition of halogenated hydrocarbons,7-9 (iii) selective catalytic reduction of NO in NH3 or hydrocarbons for the development of pollution abatement technologies against NOx,10 and (iv) oxidation of cyclohexane into cyclohexanone and hexane into hexanol isomers (i.e. 1-, 2-, 3-).11 In most of the abovementioned examples the oxidative manganese materials are nonporous with a low specific surface area (i.e.
