Simulated Temperature Programmed Desorption of Acetaldehyde on CeO 2 (111): Evidence for the Role of Oxygen Vacancy and
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ORIGINAL PAPER
Simulated Temperature Programmed Desorption of Acetaldehyde on CeO2(111): Evidence for the Role of Oxygen Vacancy and Hydrogen Transfer Chuanlin Zhao1 • Ye Xu1
Ó Springer Science+Business Media New York 2016
Abstract The temperature programmed desorption of acetaldehyde adsorbed on partially reduced CeO2(111) has been studied in detail using microkinetic modeling based on self-consistent, periodic density functional theory calculations at the GGA-PW91?U level. Previous experimental studies (Chen et al. J. Phys. Chem. C 115: 3385, 2011; Calaza et al. J. Am. Chem. Soc. 134: 18034, 2012) have shown that, whereas on fully oxidized CeO2(111) acetaldehyde desorbs molecularly with a peak temperature of 210 K, the polymerization and enolization of acetaldehyde dominate the surface reactivity on partially reduced CeO2(111), resulting in acetaldehyde desorption at higher temperatures. Here we propose a comprehensive reaction mechanism that is consistent with the spectroscopic evidence of the identities of the surface intermediates and with the observed desorption activities, including the formation of ethylene and acetylene. Besides acetaldehyde (CH3 CHO) and its enolate (CH2CHO), several other C2HxO species are proposed as key intermediates which are not seen spectroscopically, including ethoxy (CH3CH2O), ethyleneoxy (CH2CH2O), and formylmethylene (CHCHO). Our study suggests that oxygen vacancies play the critical role of activating the carbonyl bond and thereby facilitating b C–H bond scissions in acetaldehyde, leading to enolization, intermolecular hydrogen transfer, deoxygenation, and potentially C–C coupling reactions.
Electronic supplementary material The online version of this article (doi:10.1007/s11244-016-0703-y) contains supplementary material, which is available to authorized users. & Ye Xu [email protected] 1
Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA
Keywords CeO2(111) Ceria Oxygen vacancy Acetaldehyde Enolization Transfer hydrogenation DFT
1 Introduction Ceria is the most abundant of the rare earth oxides and is well known for its ability to readily convert between the 3? and 4? oxidation states chemically and structurally. It has been widely used as a catalyst support and promoter because it enhances the redox activity of metals for a number of technological reactions, including the automotive three-way catalysis [1], water–gas shift (WGS) [2, 3], hydrocarbon reforming [4, 5], and CO2 reduction [6]. Ceria has also been investigated as a catalyst in its own right for the combustion of soot and volatile organic compounds [7, 8]. Recently, ceria and other reducible oxides, such as titania and zirconia, have been considered for catalytic biomass conversion [9–11]. Typical biomass processing routes such as pyrolysis, fermentation, and reforming produce mixtures containing large fractions of carboxylic acids, aldehydes and ketones, and alcohols [12–14]. They are typically of low molecular weight and high oxygen content, neither of which is
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