Bebensee, F.

Kropp, T.

Nefedov, A.

Paier, J.

Sauer, J.

Wöll, C.

Yang, C.

KITopen

KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Chengwu Yang,1,2 Fabian Bebensee,1 Alexei Nefedov,1 Thomas Kropp,3 Joachim Paier,3 Joachim Sauer,3 Christof Wöll1,2 IRRAS Studies of Methanol Adsorption on CeO2(111) 1 Institute of Functional Interfaces, Karlsruhe Institute of Technology 2 Helmholtz Research School “Energy-Related Catalysis”, Karlsruhe Institute of Technology 3Department of Chemistry, Humboldt University of Berlin Motivation  Due to the oxygen mobility and the reducibility of cerium ions, ceria and related materials have extensive catalytic applications.[1] Studying the interaction of methanol with ceria surfaces taking the so-called surface science approach is of crucial importance to gain a fundamental understanding of the oxidation of alcohols to aldehydes.[2] Here, we use a novel apparatus[3] combining a state-of-the-art FTIR spectrometer attached to a dedicated UHV-chamber to monitor the adsorption of methanol on CeO2(111) surfaces. The system allows acquiring spectra in both reflection at grazing incidence on single crystals and in transmission geometry on polycrystalline powders. A precise interpretation of the complicated infrared spectra for adsorbates on powders can only be reached from comparison to data obtained on well-defined single crystal model systems.  We find evidence for methanol adsorbing dissociatively at 120 K on the defective CeO2(111) surface to yield surface methoxy species. IR spectra, in particular the corresponding frequencies of the C-O stretch vibrations, provide information about the coordination of the methoxy species in respect to the oxide surface.  • Methanol adsorbs dissociatively  at CeO2(111) surfaces to yield surface methoxy species. • The sharp (O-H) band at 3650 cm-1 observed for methanol adsorption on the reduced surface is possibly due to the existence of a larger amount of isolated hydroxyl groups. These groups seem to be more abundant on the reduced surface compared to the oxidized surface, where they aren’t visible in the spectra.  • The bands in the (C-O) region for methanol adsorption on CeO2(111) were assigned based on theoretical calculations: the band at 1108 cm-1 is assigned to the CH3 rocking mode and the bands at 1057-1085 cm-1 are assigned to C-O stretch vibrations with different coordination numbers of hydrogen bonding. References [1]  A. Trovarelli, P. Fornasiero (Ed.), Catalysis by Ceria and Related Materials (2nd edition), Imperial College  Press, London, 2013. [2]  J. M. Vohs, Chemical Reviews, 2013, 113, 4136. [3]  C. Yang, L. Yin, F. Bebensee, M. Buchholz, H. Sezen, S. Heissler, J. Chen, A. Nefedov, H. Idriss, X. Gong,       C. Wöll, Physical Chemistry Chemical Physics, 2014, 16, 24165. [4]  A. Siokou, R. M. Nix, J. Phys. Chem. B, 1999, 103, 6984. [5]  A. Badri, C. Binet, J.-C. Lavelley, J. Chem. Soc., Faraday Trans., 1997, 93, 1159. Experimental  The infra-red reflection absorption spectroscopy (IRRAS) measurements were carried out using a novel UHV-IR/XPS apparatus at the KIT. A CeO2 single crystal was mounted on the sample holder with electron beam heating. A liquid helium cryostat connected to the sample holder via copper braids allowed for cooling the sample below 100 K. The CeO2(111) single crystal surface was prepared by repeated cycles of sputtering with 1 keV Ar+-ions and annealing at 800 K for 15 min in an O2 atmosphere (partial pressure of 1×10-5 mbar) to form a stoichiometric surface. Alternately, annealing without O2 was employed to create a reduced surface. For temperature monitoring, a K-type thermocouple was clamped directly onto the sample surface.   Methanol was cleaned via several pump-freeze-thaw cycles before use. Exposure to methanol was performed at low sample surface temperatures by achieved by backfilling the IR- chamber. IRRA spectra only were acquired after achieving the desired Ce oxidation state and high surface crystallinity as judged from XPS and LEED. Typical base pressures during acquisition of IRRA spectra were below 1×10-10 mbar.  Experimental Results 3800 3600 3400 3200 3000 280028202790 Absorbanceexposure time 10 sec 20 sec2x10-43650 CH3OH adsorption on the reduced surface (oxygen vacancy density ~10%) CH3OH adsorption on the oxidized surface CD3OD adsorption on the oxidized surface CH3OH/CD3OD multilayers on the oxidized surface  3800 3600 3400 3200 3000 2800 Absorbance 120 K 87 K 86 K 86 K29102795 5x10-51150 1100 1050 1000p-pol106010851108 1x10-42600 2500 2400 2300 2200 2100 200020382218 Absorbance5x10-51150 1100 1050 1000 87 K 86 K10201117 1040 5x10-5Assignments of bands in the (C-O) region  • Two intense bands at 1075 cm-1 and 1057 cm-1 are apparent in the (C-O) region. • A positive (O-H) band is visible at 3650 cm-1, especially after larger exposure. • The very strong feature at 1117 cm-1 is assigned to CD3 s-deform. • The bands at 1082 cm-1 and 1058 cm-1 for CH3OH adsorption shift isotopically to 1040 cm-1 and 1020 cm-1, respectively, for CD3OD. exposure pressure 1.3×10-10 mbar, exposure time 5 second. exposure pressure 1.3×10-10 mbar, time 5 second. UHV-IR/XPS apparatus  UHV-FTIR (Bruker Vertex 80v) XPS, UPS and AES analyzer (VG Scienta R4000) Analysis chamber Distribution chambers Preparation chamber with LEED, AES, TDS and effusion cells exposure pressure 1.3×10-10 mbar Wavenumber (cm-1) Wavenumber (cm-1) O-CH3 monodentate O-CH3 bidentate O-CH3 tridentate single crystal 1108 1082, 1058 — thin film[4] 1105 1054 1015-1025 powder[5] 1103 1061-1065 1015 Table 1. O-CH3 species adsorbed on ceria oxidized surfaces O-CH3 monodentate O-CH3 bidentate O-CH3 tridentate single crystal — 1075, 1057 — thin film[4] 1117 1067 1045 powder[5] 1115 1075-1083, 1065 1040 Table 2. O-CH3 species adsorbed on ceria reduced surfaces Comparison to previous assignments Theoretical calculations The experimental data for the  thin film do not agree with our data for ceria single crystal, but are consistent with those obtained for powders. This indicates that the thin film surface exhibits numerous defects. 3500 3000 2500 2000 1500 1000985112410371143147515152835296020702220236024503160 Absorbance CD3OD CH3OH5x10-43300/10MeOD 2450 and 2360 OD stretch 2220 CD3 as-stretch 2070 CD3 s-stretch 1124 CD3 s-deform 985 CO stretch MeOH 3300 and 3160 OH stretch 2960 CH3 as-stretch 2835 CH3 s-stretch 1515 CH3 d-deform 1475 CH3 s-deform 1143 CH3 rock 1037 CO stretch 1150 1100 1050 1000 5 s 10 s1057 5x10-51075Wavenumber (cm-1) • Three bands are visible in the (C-O) region at 1108 cm-1, 1082 cm-1 and 1058 cm-1. Tsample during exposure 120 K. Conclusions Wavenumber (cm-1) 

IRRAS studies of methanol adsorption on CeO₂(111)

https://publikationen.bibliothek.kit.edu/220102037/3816903

IRRAS studies of methanol adsorption on CeO₂(111)

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