Paper SS1-WeM10
Redox Properties of HCOOH over CeO₂ Surfaces: Pathways to Surface Oxidation and Reduction

Wednesday, October 17, 2007, 11:00 am, Room 608

This study undertakes a close scrutiny of the reaction of HCOOH, the simplest C-1 carboxylic acid, with the surfaces of CeO₂, a well defined (111) oriented lanthanide oxide system. HCOOH is an important precursor to the formation of CO₂ and H₂ in the water-gas-shift (WGS) reaction, in which ceria (in combination with nobel metal particles) is also used as a stable support rich in oxygen storage capacity. The HCOOH is observed to adsorb by way of a formate intermediate species (HCOO^-) through the dissociation of the acidic H over both CeO₂ (Ce⁺⁴) and CeO_x (Ce⁺⁴/Ce⁺³) surfaces. This species will be compared to other C1 adsorbates observed such as methanol¹ and formaldehyde² reacting over ceria, which yield methoxy (CH₃O) and dioxymethelene (CH₂O₂) species, respectively. The dissociated H species recombines with surface O and desorbs as H₂O <300K. At 300K polarization dependent C K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) data suggest that the formate is adsorbed in a bi-dentate structure with the O-C-O plane oriented normal to the surface. In addition to water, Temperature Programmed Desorption (TPD) spectra indicate the evolution of CO₂ (m/z 44) and H₂ (m/z 2) around 350-400K followed by only CO desorption in two regimes at 450 and 525K. The net result is a slight reduction of the ceria substrate. In a reversal of roles, formic acid oxidizes the reduced CeO_x surface. No H₂O or CO₂ desorbs at lower temperatures but is replaced with desorption of only CO and H₂ between 450-600K. Soft X-ray Photoelectron Spectroscopy (sXPS) indicates that formate is again the only surface intermediate. With the introduction of Rh nanoparticles to the reduced and oxidized Ceria surfaces the formate decomposition is observed over ceria. Furthermore, sXPS also shows CO adsorption on Rh (C1s ~286eV) that decomposes further to give Rh-C species (284.5eV) which can be compared to CO reaction over Rh / CeO_x surfaces.³

¹J. Phys. Chem. B., 110 (32) 15994 (2006).
²Surf. Sci., 600 1540 (2006).
³J. Catal., 188 340 (1999).
^*Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.