Low temperature oxidation and reduction of ceria particles on Rh(111)

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Abstract
X-ray photoelectron spectroscopy (XPS) was used to study the oxidation and reduction of Ce vapor deposited onto a Rh(111) single crystal in a system which couples a moderate pressure reactor to an ultrahigh vacuum (UHV) analysis chamber. This article compares the behavior of thick (≥50 Å) ceria layers to that of thin (4–8 Å) ceria islands. Using variable takeoff angle XPS and difference spectra we find that the interior of both the thick Ce layers and the thin Ce islands oxidize completely to CeO2 with 1 Torr of oxygen at 475 K; however, the surfaces of both of the cerias are a mixture of Ce+3 and Ce+4. As for reduction, under our conditions (≤525 K) both the thick layers and the thin islands undergo a surface reduction that generates Ce+3. This Ce+3 surface is quite stable and no reduction of the bulk of the material occurs. Whereas the thick ceria layers require high pressures (8 Torr) of CO in order to reduce the surface, the thin islands reduce under UHV conditions—CO adsorbed at 300 K forms CO2 at 475 K during desorption. Further reduction of the thin ceria islands does not occur if they are treated with 8 Torr of CO at 525 K. We conclude that intimate contact with Rh can enhance ceria reduction via reaction with CO which initially adsorbs on Rh. Further, these two cerias are in fact very similar in that the amount of oxygen that they store is proportional to their surface area.

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