Determination of Surface Composition

Determination of the surface composition from the survey scan shown in Figure 1 and presented in Table 1, shows that the 1:1 Ce:Zr stoichiometry in the bulk is maintained at the surface. Carbon is also observed at the surface and is thought to be due to hydrocarbon contamination before introduction of the sample into vacuum.

Figure 1: X-Ray photoelectron survey scan from the as received catalyst surface.

Chemical State Analysis of Cerium

There has been considerable debate in the literature on the assignment of the CeO₂ 3d photoelectron peaks due to the complexity of the spectra. The peaks are spin-orbit split into a doublet, with each doublet showing further structure due to final state effects. Using the notation of Burroughs et al.¹ the peaks are assigned for Ce(IV)O₂ as v, v'' and v''' for Ce 3d_5/2, with the corresponding Ce 3d_3/2 peaks labelled u, u'' and u'''. An additional doublet is also observed due to the presence of Ce₂O₃ (Ce in oxidation state (III)) and is assigned v' and u'. The "as introduced" sample shows structure in the Ce 3d photoemission spectrum due to both CeO₂ and Ce₂O₃, implying that cerium is present at the surface in both Ce(IV) and Ce(III) oxidation states. The Ce 3d photoemission spectrum is shown in Figure 2(a) annotated to show peaks due to both CeO₂ and Ce₂O₃ for the "as introduced" catalyst sample.

Figure 2: High resolution scan over the Ce 3d region

Due to a rearrangement of bonding electrons, a progressive reduction of CeO₂ to Ce₂O₃ will affect the Ce 3d spectrum causing a decrease in the u'', u''', v'', v''' peaks with an associated increase in v' and u'. This effect is observed as the sample is Ar⁺ ion sputtered due to the preferential removal of oxygen from the surface. The relative peak areas of the features assigned u' and v' are seen to increase in Figure 2 after Ar⁺ ion sputtering for 450 seconds, indicating an increase in the surface content of Ce₂O₃