Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

doi:10.1016/j.susc.2009.02.010

Surface Science

Volume 603, Issue 8, 15 April 2009, Pages 1087-1092

https://doi.org/10.1016/j.susc.2009.02.010 Get rights and content

Abstract

Adsorption and desorption of methanol on a CeO₂(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce⁴⁺ → Ce³⁺ transition and an increase of the Ce 4f electronic state occupancy.

Introduction

Cerium dioxide (CeO₂) is attractive for various catalytic, electronic and optical applications. Reduction of CeO₂ results in the formation of oxygen vacancies on the cerium oxide surface and in a CeO₂ → Ce₂O₃ phase transition, which corresponds in general to the crystal structure transition from the cubic fluorite lattice (Fm3m space group) of CeO₂ to the hexagonal crystal structure (P-3m1) [1].

The interaction of methanol with oxide surfaces is of considerable interest due to its large field of application in catalytic reactions, but there is some disagreement regarding its adsorption on ceria. Methanol has been reported to adsorb and dissociate to methoxy on oxidized CeO₂(1 1 1) surfaces [2], [3]. Siokou and Nix [4] have shown that methanol adsorbed dissociatively onto a CeO₂(1 1 1) like surface present in the form of thick films or oxide islands at 300 K. Adsorption gave predominantly surface methoxy species. Some of them were oxidized to surface formate mainly on lower coordination ceria sites. CeO₂(1 1 1) films prepared by stepwise oxidation of Ce films always exhibited some Ce³⁺ states at the surface. Ferrizz et al. found that oxygen vacancies were necessary for methanol adsorption and dissociation [5]. Mullins et al. showed by using photoelectron spectroscopy (PES) and temperature programmed desorption (TPD) [2] that methanol was an effective reductant for highly ordered cerium oxide films. It condensed on the surface below 110 K and dissociated by forming methoxy, hydroxyl and water above 140 K, resulting in a partially reduced cerium oxide surface. O 1s and C 1s PES and TPD spectra of CH₃OH on the ceria surface showed methanol desorption from 100 to 600 K. Significant cerium oxide reduction was observed, by investigating Ce 4d photoemission spectra, as a result of cyclic exposures of the CeO₂(1 1 1) surface to methanol at low temperature followed by annealing to 700 K.

X-ray photoelectron spectroscopy (XPS) is a powerful tool for investigating Ce 4f states. The Ce 3d core level spectrum of stoichiometric CeO₂ consists of three 3d_3/2–3d_5/2 spin–orbit-split doublets [6], [7], [8], [9], [10]. The Ce 3d signal of the CeO₂ surface containing surface oxygen vacancies is characterized by the appearance of two new spectral components, e.g. [9], [10]. The complex structure of the Ce 3d spectra makes the fitting procedure difficult, in particular due to ambiguous background subtraction and determination of the peak fitting parameters. Consequently the determination of the Ce³⁺/Ce⁴⁺ state ratio from the Ce 3d spectra is not very reliable, especially in the case of low concentration of Ce³⁺ ions. Resonance photoemission spectroscopy (RPES) in the Ce 4d–4f photoabsorption region [9], [11], using synchrotron radiation, is efficient in revealing the 4f occupancy of cerium oxide (Ce³⁺ formation) with very high sensitivity.

In this work, we investigated low temperature methanol adsorption by monitoring formation of the Ce³⁺ state and adsorption products due to interaction of methanol with a ceria surface. We used high sensitivity resonance photoelectron spectroscopy of the valence band and Ce 3d XPS to determine cerium oxide chemical state variations due to methanol dissociation and intermediate desorption during annealing of the CH₃OH/CeO₂(1 1 1)/Cu(1 1 1) system from 110 to 600 K. The results are in partial agreement with the previously published work on CeO₂(1 1 1)/Ru(0 0 0 1) thin film by Mullins et al. [2], and provide complementary information about methanol interaction with cerium oxide model catalysts. We did not observe methanol dissociation in the case of adsorption at 300 K. The CeO₂(1 1 1) model catalyst was prepared by using a preparation process different from those reported previously [2], [4].

Section snippets

Experimental

The experiments were performed at the Materials Science Beamline at the Elettra synchrotron light source in Trieste. The UHV experimental chamber was equipped with a 150 mm mean radius electron energy analyser, rear view LEED optics, a dual Mg/Al X-ray source and Ce evaporation source. The base pressure of the vacuum chamber was 1 × 10⁻¹⁰ mbar.

The photoelectron spectra were recorded at 20° emission of the photoelectrons with respect to the surface normal at different photon energies: Al Kα (hν =

Results and discussion

The 1.5 nm thick CeO₂(1 1 1) thin film model catalyst was grown on Cu(1 1 1) as described above. Layers prepared in this way exhibited good crystalline structure, negligible concentration of Ce³⁺ ions, high thermal stability up to 970 K and they are continuous for thicknesses above 1 nm [12], [13].

As determined from the O 1s XPS and C 1s SRPES spectra in Fig. 1, Fig. 2, 30 L methanol exposure of the CeO₂(1 1 1) surface at 110 K results in CH₃OH multilayer film formation which is characterized by single

Conclusion

We have investigated low temperature adsorption and gradual thermal desorption of methanol on CeO₂(1 1 1) thin film surfaces. A continuous cerium oxide film was grown by molecular epitaxy on the Cu(1 1 1) substrate. PES was used to measure the Ce 3d, O 1s, C 1s core level and valence band spectra. The resonance spectroscopy RPES was used to determine occupancy of the Ce 4f states with high sensitivity. The photoemission spectra provided information on chemical states of C, O and Ce permitting us to

Acknowledgements

This work is a part of the research programs Nos. MSM 0021620834, ME08056 and LC06058 that are financed by the Ministry of Education of the Czech Republic. Our thanks are also to Dr. S. Fabris for stimulating discussion concerning the mechanism of interaction of methanol with ceria.

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Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

Surf. Sci.

Surf. Sci.

Surf. Sci.

Appl. Surf. Sci.

Thin Solid Films

Appl. Surf. Sci.

Surf. Sci.

Chem. Eng. J.

Surf. Sci.

Surf. Sci.

Phys. Rev. B

J. Phys. Chem. B

Methanol adsorption on a CeO₂(1 1 1)/Cu(1 1 1) thin film model catalyst