Partial Oxidation of Ethanol to Acetaldehyde over LaMnO 3 -Based Perovskites: A Kinetic Study Bing-Shiun Jiang, Ray Chang, and Yu-Chuan Lin* Department of Chemical Engineering and Materials Science, Yuan Ze University, Chungli, Taoyuan 32003, Taiwan * S Supporting Information ABSTRACT: A set of experiments was carried out in a continuous fixed-bed reactor to investigate the relative catalytic activities of LaMnO 3 and LaMn 0.95 Pd 0.05 O 3 for the partial oxidation of ethanol to acetaldehyde. Both catalysts were prepared by the sol- gel method. The resultant data have indicated that LaMn 0.95 Pd 0.05 O 3 is more active than LaMnO 3 and that acetaldehyde selectivities of both are close at around 90%. To gain an in-depth understanding of perovskite’s chemistry involved, kinetic analysis of the data has been conducted with the differential method. Accordingly, eight elementary reactions have been proposed by resorting to the Mars-van Krevelen redox cycle. Subsequently, these elementary reactions have been lumped into five steps comprising ethanol adsorption, oxygen adsorption, surface reaction, acetaldehyde desorption, and water desorption. This has rendered it possible to derive a set of rate equations based on the Langmuir-Hinshelwood-Hougen-Watson formalism. The exploration of these rate equations has revealed that surface reaction, evolving chemisorbed ethanol and oxygen, is rate-limiting. The estimated activation energies of 11.72 kcal/mol for LaMnO 3 and 12.44 kcal/mol for LaMn 0.95 Pd 0.05 O 3 are nearly identical; however, the pre-exponential factor of the latter is about twice the value of the former. This can be attributed to the better reducibility of Pd-promoted LaMnO 3 , thereby leading to greater reactivity in ethanol partial oxidation. 1. INTRODUCTION Perovskite has an ABO 3 -type crystal structure and has been widely applied. 1 Sometimes, for example, in automotive emission control, the catalytic activity of the perovskite is more pronounced than those of noble metal catalysts. 2 We have recently reported that by substituting manganese with trace Pd cations in the B-site position of LaMnO 3 perovskite, its catalytic activity and formaldehyde selectivity in methanol partial oxidation can be substantially improved. 3,4 This is attributed to the enhanced reducibility and oxygen mobility by Pd substitution in LaMnO 3 . Similar to methanol partial oxidation, partial oxidation of ethanol (POE, eq 1) to acetaldehyde has been known to follow a redox route, 5,6 also known as the Mars-van Krevelen mechanism. 7 At the outset, ethanol chemisorbs on the catalyst surface to form an ethoxyl and a hydroxyl. Subsequently, the ethoxyl dehydrogenates as an acetyl and desorbs as an acetaldehyde; the hydroxyl reduces the surface by dehydration, thus leaving an oxygen vacancy. Finally, the reduced surface regains its oxidation state by dissociative chemisorption of O 2 . Figure 1 portrays the Mars-van Krevelen mechanism of POE. + → + C H OH 1 2 O CHO H O 2 5 2 2 4 2 (1) Table 1 lists earlier studies of POE. It is usually conducted in the mild temperature regime (100-300 °C) under atmospheric pressure. The table reveals that vanadium-based catalysts are most frequently deployed. This can probably be attributed to the unique active phase-support interaction of these cata- lysts, 25,26 which enhances both activity and selectivity in POE. Compared to the vanadium catalysts, relatively little effort has been devoted to the exploration of perovskite as catalysts in POE although the redox chemistry of perovskite is highly likely to play a central role. The current work appears to be the first to deploy both pure and Pd-doped LaMnO 3 -based catalysts for POE to gain insight into redox chemistry. The rate equations have been derived from the resultant experimental data according to the Mars- van Krevelen redox mechanism. This has rendered it possible to identify the rate-determining step. Moreover, the values of parameters in the rate equations involving this rate-determining step can be corroborated with the performances of the aforementioned catalysts in POE. 2. EXPERIMENTAL SECTION Two catalysts, LaMnO 3 and LaMn 0.95 Pd 0.05 O 3 , were prepared via the sol-gel method. Metal acetylacetonates (acac) (La-, Mn-, and Pd-(acac) 3 ) served as the precursors. Specific Special Issue: L. T. Fan Festschrift Received: February 8, 2012 Revised: April 18, 2012 Accepted: April 20, 2012 Published: April 20, 2012 Figure 1. Mars-van Krevelen mechanism of POE. Article pubs.acs.org/IECR © 2012 American Chemical Society 37 dx.doi.org/10.1021/ie300348u | Ind. Eng. Chem. Res. 2013, 52, 37-42