Structure and Reactivity of Alkyl Ethers Adsorbed on CeO2(111) Model Catalysts

Abstract

The effect of surface hydroxyls on the adsorption of ether on ceria was explored. Adsorption of dimethyl ether (DME) and diethyl ether (DEE) on oxidized and reduced CeO 2 (111) films was studied and compared with Ru(0001) using RAIRS and sXPS within a UHV environment. On Ru(0001) the ethers adsorb weakly with the molecular plane close to parallel to the surface plane. On the ceria films, the adsorption of the ethers was stronger than on the metal surface, presumably due to stronger interaction of the ether oxygen lone pair electrons with a cerium cation. This interaction causes the ethers to tilt away from the surface plane compared to the Ru(0001) surface. No pronounced differences were found between oxidized (CeO 2 ) and reduced (CeOx) films. The adsorption of the ethers was found to be perturbed by the presence of OH groups on hydroxylated CeOx. In the case of DEE, the geometry of adsorption resembles that found on Ru, and in the case of dimethyl ether DME is in between that one found on clean CeOx and the metal surface. Decomposition of the DEE was observed on the OH/CeOx surface following high DEE exposure at 300 K and higher temperatures. Ethoxides and acetates were identified as adsorbed species on the surface by means of RAIRS and ethoxides and formates by s-XPS. No decomposition of dimethyl ether was observed on the OH/CeOx at these higher temperatures, implying that the dissociation of the C-O bond from ethers requires the presence of b-hydrogen.

Oxygen Vacancy-Assisted Coupling and Enolization of Acetaldehyde on CeO2(111)

“…The peak at 1046 cm −1 is considered to be the C−O stretching mode of an ethoxy impurity. 70 Its intensity was small compared to that measured for an adsorbed ethoxide layer, and was found to be variable relative to the AcH features, consistent with its being due to a sporadic impurity.…”

“…In the literature this feature has been assigned to a C–C stretch but with contribution from rocking modes of the methyl group. ,,, Our calculations indicate an intense mode at 1092 cm –1 that corresponds to a methyl rock and C–C stretch with a change in the dipole moment perpendicular to the surface, consistent with the η 1 -O state (Figure a). The peak at 1046 cm –1 is considered to be the C–O stretching mode of an ethoxy impurity . Its intensity was small compared to that measured for an adsorbed ethoxide layer, and was found to be variable relative to the AcH features, consistent with its being due to a sporadic impurity.…”

The surface chemistry of cerium oxide

“…Vibrational spectroscopy and DFT calculations have shed additional light on the first acetaldehyde desorption peak near 400 K on CeO 2-X (111) (Figure 24, blue line). [77] This desorption feature results from the decomposition of a dimer formed from the coupling of the carbonyl O and the acyl C of two acetaldehyde molecules.As noted at the beginning of this section, dimethyl ether and diethyl ether interact weakly and do not decompose on either oxidized or reduced CeO X(111) [225]. Co-adsorption of diethyl ether with -OH on reduced CeO 2-X (111) resulted in decomposition of the ether into ethoxy and formate.…”

Improved Performance of Ru/γ‐Al2O3 Catalysts in the Selective Methanation of CO in CO2‐Rich Reformate Gases upon Transient Exposure to Water‐Containing Reaction Gas

“…The two bands appearing at ñ = 1590 and 1390 cm À1 are characteristicf or antisymmetric and symmetric bending of surface formate species, [16] respectively,a nd the band at ñ = 1190 cm À1 is attributed to metalÀOÀCs tretching vibrations in adsorbed methoxy or ethoxyg roups, which accumulate on the catalysts upport surfaced uring the reaction. [40] These bands are also much more pronouncedf or the catalystoperated under dry conditions only than forthose pretreated in wet reformate first.…”