Surface Chemistry of Oxygenated Molecules on CeO2(111) by RAIRS

CALAZA, FLORENCIA C.; CHEN, TSUNG-LIANG; XU, YE; MULLINS, DAVID R.; OVERBURY, STEVEN H.

Upton, NY

Congreso; The 4th International Congress on Operando Spectroscopy; 2012

Operando IV Organizing Comitee

Introduction and Objectives The study of adsorption and reactivity of oxygenated molecules on model oxide catalysts is of great interest to gain a better understanding of the mechanism of industrial reactions. Cerium oxide is commonly used in three-way auto exhaust and WGS catalysts. Our experimental design is intended to find the conditions where UHV experiments could mimic the real catalytic processes and from these results explain the reaction pathway from the atomistic level. Reflection Absorption Infrared Spectroscopy (RAIRS) is a very important and useful technique that studies the structure of adsorbed species on surfaces and two examples of its capabilities are presented here.     Results and Discussion We have examined these adsorption and decomposition pathways for two ethers, diethyl ether (DEE) and dimethyl ether (DME). Both ethers adsorb on fully oxidized CeO2(111) and highly reduced CeOx(111) under UHV conditions at low temperature without showing decomposition. If the catalyst surface is pre-covered by hydroxyls, then the adsorption geometry of the ethers on this hydroxylayted surface changes, indicating interaction with OH groups. Regarding their reactivity towards decomposition, the two ethers behave differently when exposed to hydroxylated CeO2-x(111) at 300-400 K. DEE promptly reacts by breaking the C-O bond presenting a very interesting chemistry. By using RAIRS and XPS, we could detect ethoxide and possibly carboxylate species as adsorbed intermediates for the reaction. However, when the hydroxylated CeO2-x(111) is exposed to DME at same conditions, the ether shows no reactivity, indicating the importance of H on a carbon atom â to oxygen. [1] RAIRS was coupled with density functional theory (DFT) to study the adsorption of acetaldehyde, the simplest C2 aldehyde. The molecule adsorbs weakly on the fully oxidized surface at low temperatures, and desorbs without further reaction near 215 K, by bonding to Ce4+ cations through the oxygen lone pair electrons in the carbonyl group with its C-C bond perpendicular to the surface plane and the acyl hydrogen tilted slightly towards one of the lattice oxygen anions of the first layer. On the reduced surfaces, acetaldehyde interacts more strongly with the surface upon adsorption at low temperatures by losing its carbonyl bond character and adsorbing as diacetal species by dimerization of two acetaldehyde molecules forming an ether linkage between the two. Heating the surface to 400 K leads to desorption of some amount of di-acetal adsorbed species as acetaldehyde and the appearance of hydroxyl and enolate species (CH2=CHO-Ce). The identities and structures of the different intermediates on the CeO2 and CeO2-x surfaces have been determined by their characteristic signatures in RAIRS and by DFT calculations.[2] The assignment of the enolate species is furthermore consistent with C1s XPS and C k-edge NEXAFS results.[3]  Conclusions UHV-RAIRS studies of adsorbates on ceria films gives great insights into the chemistry of oxygenated molecules to further understand their chemistry in heterogeneous catalytic processes. This technique can provide IR characterization of adsorbates and reaction intermediates geometry and their interaction with the surface. These results coupled to other surface science techniques and specially with DFT calculations can explain reactivity pathways and the role of co-adsorbates (vacancies and hydroxyls) on the catalytic surface.    References [1]   “Structure and reactivity of alkyl ethers adsorbed on CeO2(111) model catalysts” Calaza, F.; Chen, T.; Mullins, D.; Overbury, S. Topics in Catalysis 54 (2011) 56-69. [2]   “Oxygen vacancy-assisted coupling and enolization of acetaldehyde on CeO2-x(111)” Calaza, F.; Xu, Y.; Mullins, D.; Overbury, S. in preparation. [3]   “Adsorption and Reaction of Acetaldehyde over CeOX(111) Thin Films” Chen, T.; Mullins, D. J. Phys. Chem. C, 2011, 115, 3385-3392.