CO Dissociation on Rh Deposited on Reduced Cerium Oxide Thin Films
Journal of Catalysis, 188 (1999) 340
The absorption and reaction of CO on submonolayer films of Rh deposited on cerium oxide thin films has been studied by soft x-ray photoelectron spectroscopy (SXPS). Stubenrauch and Vohs have previously demonstrated by thermal desorption and isotopic labeling that CO will dissociate on Rh deposited on a partially reduced cerium oxide surface but does not dissociate on Rh deposited an a fully oxidized CeO2 surface[1]. Using SXPS it is possible to monitor the state of the CO, Rh and CeOX with very high sensitivity and resolution.
The figure on the left shows the C 1s spectra for CO adsorbed at 100 K on Rh deposited on fully oxidized CeO2. The peak at 286.9 eV is assigned to molecular CO and is unchanged from 100 - 300 K. The molecular CO completely desorbs upon annealing to 500K. The C 1s spectra for CO absorbed at 100 K on Rh deposited on partially reduced CeO1.6 is shown on the right. Only molecular CO is again observed between 100 K and 300 K. After annealing to 400 K there is a decrease in the molecular CO peak at 286.9 eV and a new peak occurs at 284.6 eV due to atomic C. The disappearance of the CO signal is mirrored in the O 1s spectra. There are no new peaks in the O 1s spectra following dissociation indicating that the atomic O cannot be distinguished from the lattice O in CeOX in the O 1s spectra. At 500 K the CO peak is almost totally gone and a new peak occurs at 284.6 eV due to atomic C . The atomic C peak is unchanged at 600 K but is virtually gone at 700 K.
The Rh 3d and Ce 4d spectra indicate that the C and O stay on the Rh surface throughout the adsorption and decomposition cycle. There is a positive binding energy shift in the Rh 3d after CO adsorption due to the polarization of electron charge away from the Rh toward the CO. After CO dissociation, the Rh 3d signal undergoes hardly any shift back to lower binding energy and the Ce 4d indicates no oxidation of the CeOX which suggest that the O stays on the Rh.
[1] J. Stubenrauch and J. M. Vohs, Catal. Lett., 47 (1997) 21.
