Redox Pathways for HCOOH Decomposition over CeO₂ Surfaces

The reaction of HCOOH, the simplest C-1 carboxylic acid, has been studied on oxidized (CeO 2 ) and reduced cerium oxide (CeO X ) thin films. HCOOH has been proposed as an important C1 precursor to the formation of CO 2 and H 2 in the water-gas-shift (WGS) reaction, in which ceria is also used as a stable support rich in oxygen. The HCOOH is observed to adsorb by way of a formate intermediate species (HCOO − ) through the dissociation of the acidic H over both CeO 2 (Ce 4+ ) and CeO X (Ce 4+ /Ce 3+ ) surfaces. On the fully oxidized CeO 2 surface, this H species reacts with surface O and desorbs as H 2 O when the temperature is <300 K. In addition to water, temperature programmed desorption spectra indicate the evolution of CO 2 and H 2 between 350 and 400 K followed by CO desorption in two regimes at 450 and 525 K. On reduced ceria, only CO and H 2 were produced between 450 and 600 K. The reaction of HCOOH with ceria therefore leads to both the reduction of the oxide, through the formation of H 2 O and CO 2, and oxidation of the oxide, through decomposition to H 2 and CO, depending on the initial state of the ceria. C K-edge near-edge X-ray absorption fine structure and soft X-ray photoelectron spectroscopy indicate that formate and −OH were the only surface intermediates.