Paper SS-MoA10
Probing Charge Transfer Following Molecular Adsorption on CeO₂(100) using Resonant Photoemission

Monday, October 28, 2013, 5:00 pm, Room 201 A

Resonant photoemission excited at the Ce N_IV,V–edge was used to study electron transfer from molecular adsorbates to the Ce cation on fully oxidized CeO₂(100). Two features near the top of the valence band in CeO₂ are extremely sensitive to the oxidation state of the Ce cation. The O 2p feature at a binding energy of 4 eV is proportional to the Ce⁴⁺ content in the surface. The Ce 4f peak at a binding energy of 1.5 eV is proportional to the Ce³⁺ content. The ratio of these two peaks at their respective resonance maxima provides a measure of the Ce³⁺/Ce⁴⁺ content. This ratio is extremely sensitive because the Ce 4f feature rises from a flat, low count background in fully oxidized CeO₂ to a peak with thousands of counts at even modest levels of Ce reduction. The kinetic energy of the valence electrons at resonance is ca. 120 eV which places them near the minimum for the electron mean free path and therefore provides excellent surface sensitivity.

A variety of molecules were adsorbed on CeO₂(100) at 190 K. CeO₂ is a reducible oxide and removal of O from the surface will result in a reduction of Ce⁴⁺ to Ce³⁺. At 190 K none of the molecules studied resulted in desorption from the surface and therefore reduction did not result from the removal of O but from charge transfer from the adsorbate to the Ce. At elevated temperatures products such as water and CO₂ were observed that clearly resulted in reduction of the Ce due to O removal.

Methanol and water undergo similar adsorption processes by breaking an O-H bond, depositing H on a surface O anion and adsorbing R-O- on the Ce cations. For methanol the methoxy adsorbate produced ca. 10% Ce³⁺ following adsorption whereas the hydroxyl adsorbate resulting from water exposure produced a negligible change in the Ce³⁺ content. This suggests that the C in the alcohol is partially oxidized following adsorption whereas there is no net transfer of charge from the H and O in the water to the Ce.

Acetaldehyde does act as a Bronsted acid by breaking an O-H bond yet it also produces a 10% reduction of the Ce. This may result from a partial oxidation of the C in the dioxy / η₂ adsorption state and also from an oxidation of a small amount of the acetaldehyde to acetate. Pyridine, a classic probe of acid sites through the lone-pair electrons on the N, results in no change in the Ce³⁺ content. N 1s XPS and C k-edge NEXAFS suggest that the pyridine may not bond to the surface through the N atom.

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