Interaction of Methanol with Well-Defined Ceria Surfaces:  Reflection/Absorption Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and Temperature-Programmed Desorption Study

Abstract

The adsorption of methanol on ordered epitaxial layers of cerium oxides grown on a Cu(111) substrate has been studied using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and Fourier transform reflection/absorption infrared spectroscopy (FT-RAIRS) measurements. The oxide films exhibit a LEED pattern characteristic of a CeO2(111)-like structure, but the Ce/O stoichiometry achieved is strongly dependent on the exact pretreatment and film history. Grazing emission XPS also indicates that some Ce3+ ions are still present in the surface layers at 300 K after oxidation treatments. Methanol adsorbs dissociatively at 300 K, with a relatively high sticking probability, to yield surface methoxy species. The IR spectra of the methoxy species, in particular the CO stretch frequency, provide information about their coordination to the oxide surface, the presence of surface oxygen vacancies, and the general level of oxidation of the film. The methoxy species are stable on the (111)-type terraces of thicker (>5 ML) oxide films to temperatures in excess of 550 K but then decompose at about 585 K to yield predominantly H2 and CO with some simultaneous evolution of formaldehyde and water. A substantial number of more coordinatively unsaturated cerium ions exist at and near the periphery of oxide islands on films of a submonolayer oxide coverage and on aggregated films of higher oxide coverage (between 1 and 5 ML). When the substrate is well-oxidized, then some of the methoxy species adsorbed at such sites are readily oxidized to the formate species while the decomposition temperature of the remaining methoxy groups in this peripheral region is lowered to about 560 K and their decomposition yields a higher proportion of formaldehyde than is seen for the (111) terrace sites.

Direct CO Oxidation by Lattice Oxygen on Zr-Doped Ceria Surfaces

“…They are two stable low index surfaces and have been extensively studied [23,24]. It is known that CO takes the Ce site on the clean CeO 2 (111) surface but the adsorption energy is very small, 3.9 kcal/mol [25] from GGA calculations and 4.2 kcal/mol from the present GGA ?…”

A resonant photoelectron spectroscopy study of Sn(Ox) doped CeO2 catalysts

“…The preparation of ordered ultrathin cerium oxide overlayers on Pt(111), Rh(111) and Re(0001) single crystal surfaces was reported in Refs [6][7][8][9], and growth on Cu(111) in Ref. [10].…”

Methanol Reactivity on Silica-Supported Ceria Nanoparticles

“…To shed light on the structure and reactivity of ceria-based catalysts, thin CeO 2 (111) films grown on metal single crystals were used as planar model systems well-suited for vacuumbased, surface-sensitive techniques [17][18][19]. Ceria (CeO 2 ) crystallizes in the fluorite structure.…”

“…The latter species have been characterized by infrared reflection-absorption spectroscopy (IRAS) as on top, doubly bridged, and triply bridged entities with respect to the Ce cations [19,[22][23][24]. The surface methoxy species are quite stable and decompose at temperatures above 550 K to yield H 2 , CO, H 2 O, and formaldehyde (FA).…”

“…On reduced ceria, more methanol can be adsorbed, and it undergoes more extensive decomposition to produce CO and H 2 near 640 K in addition to H 2 O and FA. Ferrizz et al [25], using a CeO 2 (111) single crystal, argued that vacancies on the surface are necessary for methanol adsorption and dissociation, Mullins et al [26] and Siokou and Nix [19] found that those processes also take place on the fully oxidized surface, although the dissociation of methanol on the surface is considerably enhanced in the presence of vacancies [21]. In addition, Ferrizz et al [25] have examined ~ 4 nm-thick ceria films grown on the -Al 2 O 3 (0001) and yttria-stabilized zirconia (100) substrates.…”