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A comparison between water adsorbed on Rh(111) and Pt(111), with and without predosed oxygen
A515 Surface Science 191 (1987) 121-146 North-Holland. Amsterdam A COMPARISON BETWEEN WATER ADSORBED AND 1~(111), WITH AND WITHOUT PREDOSED F.T. WAGNER
121 ON Rh(lll) OXYGEN
and T.E. MOYLAN
Physical Chemistry Department, General Motors Research Laboratories, Warren, M I 48090-9055, USA Received 3 November 1986; accepted for publication 19 June 1987 The adsorption of water and its interactions with oxygen on R h ( l l l ) were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS), and low energy electron diffraction (LEED); and comparison was made with similar data for P t ( l l l ) . On R h ( l l l ) water absorbs molecularly in hydrogen-bonded clusters; no evidence for dissociation was seen on the clean surface. Reaction of water with adsorbed oxygen on R h ( l l l ) produces hydrated surface hydroxyls. While the gross features of adsorption and hydroxyl formation are similar to those previously reported on Pt(lll), significant differences in detail were found. In particular, the complex librational and OH-stretching regions of the HREELS spectra for H 2 0 / R h ( l l l ) more closely resemble those for other noble metal surfaces than the sharp, single features observed for Pt(111). HREELS peaks at 970, 1020 and 1950 cm -1 seen for H20/Pt(111 ) were absent on Rh(111). The middle (3al) molecular orbital for molecular water on Rh(111) is shifted towards the Fermi level, while on P t ( l l l ) the spacing between the three orbitals is the same as in water vapor. Comparison with spectral data for bulk phases suggests that water on Pt(111) exists primarily in a state with O - O nearest neighbor distances closer to those of liquid water than of ice, allowing better match with the Pt(111) surface mesh. Additional minority species account for the additional EELS peaks specific to Pt(111). Water on Rh(111) is a mixture of ice-like water and water similar to the majority species on Pt(111). The structural differences lead to different chemistry. On both surfaces adsorbed oxygen and water react to yield a surface phase which evolves water upon heating to 210 K. On Pt(111) this phase contains OH but no H20. On Rh(111) this phase contains both OH and H 2 0 in association. The differences in the interactions between water and the (111) surfaces of these two catalytically and electrochemically similar metals may help explain electrochemical effects peculiar to the (111) face of Pt.
Surface Science 191 (1987) 147-156 North-Holland, Amsterdam
147
LOW TEMPERATURE DIFFUSION OF Pt AND Au ATOMS THROUGH THIN TiO2 FILMS ON A Ti SUBSTRATE Carmen
OCAL
and Salvador FERRER
Departamento de Fisica Fundamental, C-Ill, Universidad Autbnoma de Madrid, Cantoblanco, 28049-Madrid, Spain Received 9 March 1987; accepted for publication 9 June 1987 We have characterized with XPS and UPS thin films of titanium oxide grown by oxidizing a Ti(0001) surface. The oxide films are easily reduced by thermal treatments. Typically, annealing to 500 K causes - 20% of anionic vacancies. We present evidences for the existence of anionic overlayers on the surface of TiO 2 films that are stable at or below room temperatures. Deposition of Pt or Au atoms on thin (few atomic layers) TiO 2 films at 80 K results in diffusion of the deposited metals through the oxide to the metal-oxide interface. This result may be interpreted on the basis of the Cabrera-Mott theory for oxidation of metals at low temperatures.
121 ON Rh(lll) OXYGEN
and T.E. MOYLAN
Physical Chemistry Department, General Motors Research Laboratories, Warren, M I 48090-9055, USA Received 3 November 1986; accepted for publication 19 June 1987 The adsorption of water and its interactions with oxygen on R h ( l l l ) were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS), and low energy electron diffraction (LEED); and comparison was made with similar data for P t ( l l l ) . On R h ( l l l ) water absorbs molecularly in hydrogen-bonded clusters; no evidence for dissociation was seen on the clean surface. Reaction of water with adsorbed oxygen on R h ( l l l ) produces hydrated surface hydroxyls. While the gross features of adsorption and hydroxyl formation are similar to those previously reported on Pt(lll), significant differences in detail were found. In particular, the complex librational and OH-stretching regions of the HREELS spectra for H 2 0 / R h ( l l l ) more closely resemble those for other noble metal surfaces than the sharp, single features observed for Pt(111). HREELS peaks at 970, 1020 and 1950 cm -1 seen for H20/Pt(111 ) were absent on Rh(111). The middle (3al) molecular orbital for molecular water on Rh(111) is shifted towards the Fermi level, while on P t ( l l l ) the spacing between the three orbitals is the same as in water vapor. Comparison with spectral data for bulk phases suggests that water on Pt(111) exists primarily in a state with O - O nearest neighbor distances closer to those of liquid water than of ice, allowing better match with the Pt(111) surface mesh. Additional minority species account for the additional EELS peaks specific to Pt(111). Water on Rh(111) is a mixture of ice-like water and water similar to the majority species on Pt(111). The structural differences lead to different chemistry. On both surfaces adsorbed oxygen and water react to yield a surface phase which evolves water upon heating to 210 K. On Pt(111) this phase contains OH but no H20. On Rh(111) this phase contains both OH and H 2 0 in association. The differences in the interactions between water and the (111) surfaces of these two catalytically and electrochemically similar metals may help explain electrochemical effects peculiar to the (111) face of Pt.
Surface Science 191 (1987) 147-156 North-Holland, Amsterdam
147
LOW TEMPERATURE DIFFUSION OF Pt AND Au ATOMS THROUGH THIN TiO2 FILMS ON A Ti SUBSTRATE Carmen
OCAL
and Salvador FERRER
Departamento de Fisica Fundamental, C-Ill, Universidad Autbnoma de Madrid, Cantoblanco, 28049-Madrid, Spain Received 9 March 1987; accepted for publication 9 June 1987 We have characterized with XPS and UPS thin films of titanium oxide grown by oxidizing a Ti(0001) surface. The oxide films are easily reduced by thermal treatments. Typically, annealing to 500 K causes - 20% of anionic vacancies. We present evidences for the existence of anionic overlayers on the surface of TiO 2 films that are stable at or below room temperatures. Deposition of Pt or Au atoms on thin (few atomic layers) TiO 2 films at 80 K results in diffusion of the deposited metals through the oxide to the metal-oxide interface. This result may be interpreted on the basis of the Cabrera-Mott theory for oxidation of metals at low temperatures.