Paper SS-FrM3
Variations in the Orientation and Thermal Stability of the Carbonate Reaction Intermediate with the Surface Structure and Stoichiometry of Cerium Oxide Thin Films

Friday, November 1, 2013, 9:00 am, Room 201 A

Carbonate, [CO₃]^2- is a stable intermediate in the water-gas shift reaction (CO + H₂O --> H₂ + CO₂) over inverse CeO_X/Au(111) catalysts, relevant to efficient H₂ production [1]. In addition, carbonate has recently been proposed as the high-temperature (600-700 K) intermediate in the dehydrogenation of methanol [2], and possibly in the complex reaction of acetaldehyde, on CeO₂(100).

Here, we probe the formation and reaction of the carbonate species on CeO_X(100) as a function of temperature and Ce oxidation state by adsorbing CO₂ at 180 K. There is a marked variation in the stability of the carbonate species with the crystallographic orientation of the ceria thin film. Temperature-programmed desorption (TPD) identifies CO₂ as the sole product, with desorption states at 230 K, 410 K, 510 K, and 655 K in the case of stoichiometric CeO₂(100). On CeO₂(111), carbonate desorbs entirely as CO₂ by 300 K [1, 3].

On CeO₂(100), soft X-ray photoelectron spectroscopy (sXPS) confirms the persistence of carbonate up to 600 K. Near-edge X-ray absorption fine structure (NEXAFS) at the C k-edge shows a strong angular dependence that becomes more pronounced at higher temperature (lower coverage). The excitation of the pi* resonance at grazing incidence indicates that the carbonate is lying flat on CeO₂(100).

For partially reduced CeO_1.67(100), we observe: (1) A substantial decrease in low-temperature (< 250 K) desorption, (2) The appearance of an intense CO₂ desorption peak at higher temperature (765 K), and (3) The absence of well-defined peaks in the CO₂ desorption between 300 K and 600 K.

The integrated intensity of the [CO₃]^2- peak in the C 1s spectrum following CO₂ adsorption on CeO_1.67(100) at 180 K is a factor of two greater than that for fully oxidized CeO₂(100). On the reduced surface there is also no loss of this initial intensity upon heating to 300 K. At 700 K the carbonate coverage on the reduced surface is one-third of its starting value, whereas on the oxidized surface complete desorption (as CO₂) has occurred. Furthermore, resonant photoemission spectroscopy (RPES) indicates negligible re-oxidation of the CeO_1.67(100) surface. In contrast, an RPES study of CO₂ on reduced CeO_X(111) demonstrated partial re-oxidation at temperatures as low as 300 K [4].

Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy.

[1] S. D. Senanayake et al., J. Catal. 271, 392 (2010).

[2] P. M. Albrecht and D. R. Mullins, Langmuir 29, 4559 (2013).

[3] S. D. Senanayake and D. R. Mullins, J. Phys. Chem. C 112, 9744 (2008).

[4] T. Staudt et al., J. Catal. 275, 181 (2010).