Stephen D. Cameron

Surface modification of platinum by titanium dioxide overlayers: A case of simple site blocking

Abstract The suppression of CO and H 2 chemisorption on Pt surfaces when supported on TiO 2 carriers is well established. Recently, it has been suggested that migration of the support material onto the Pt surface is responsible for this effect. To test this hypothesis, the suppression of CO chemisorption on TiO 2 modified Pt foil has been studied. A combination of X-ray photoelectron spectroscopy (XPS), low energy inelastic ion scattering (LEISS) and temperature programmed desorption were used to fully characterize the composition, electronic structure and chemisorption properties of the modified surface. Surface TiO 2 was prepared by in situ evaporation of Ti from TaTi alloy in a background of 1 × 10 −6 mbar O 2 . Subsequent annealing (770 K) of this surface produced a stoichiometric TiO 2 . When oxygen vacancies (Ti 3+ centers) were introduced by chemical treatment, an ohmic contact was formed between the reduced TiO 2 and the Pt surface. However, no new major chemisorption states were observed in CO TPD on this modified Pt surface. Rather, a simple suppression of the total amount of chemisorption was observed. Combining LEISS with TPD established a clear linear relationship between the extent of chemisorption suppression and the number of sites physically blocked by the TiO 2 .

Preparation of sulfated zirconia catalysts with improved control of sulfur content II. Effect of sulfur content on physical properties and catalytic activity

Abstract The sulfur content of sulfated zirconia can be varied by changing the quantity of sulfuric acid used for impregnation in the controlled impregnation method, in which a pre-set quantity of sulfuric acid is added and evaporated without filtration or decanting. The material is then dried and calcined (in this work, for 5 h at 610°C). The sulfur content of the calcined material increases with the increase in the quantity of sulfuric acid, but the ratio of sulfate retained after calcination to sulfate impregnated decreases at first and then increases. The surface area gradually increases with the quantity of the acid up to 3 ml of acid per gram of zirconium hydroxide (2.5% S in SZ after calcination), then decreases abruptly. The crystal structure of the sulfated zirconia is also affected by the sulfur content. XRD data show that the sulfated zirconia with a low or medium sulfur content crystallizes only in the tetragonal form, whereas at higher sulfur content a minor fraction of monoclinic zirconia begins to be seen, in addition to the major tetragonal form. XPS data indicate that at the higher sulfate loading part of the sulfate is present in the bulk phase, rather than on the surface. Thus, the increase in sulfur uptake, decrease in surface area and change in crystallinity appear related to sulfate migration inside the particles. The catalytic activity for the isomerization of methylcyclopentane at 65°C shows a maximum for a sulfur content around 3%. It appears that the optimum catalyst should contain the maximum amount of sulfate near the surface, but not necessarily on the surface, and crystallize in the tetragonal form.

A tilted precursor for CO dissociation on the Fe(100) surface

Near edge X-ray absorption fine structure (NEXAFS) has been used to study the molecular orientation of the α3 state of CO on the Fe(100) surface. It is found that the molecule is tilted by 45° ± 10° with respect to the surface normal, allowing direct interaction of the oxygen end of the molecule with the iron surface. The C-O bond is found to be elongated by 0.07 ± 0.02 A in the α3 state, relative to the other molecularly adsorbed CO states on this surface.

The interaction of CO and O2 with the (111) surface of Pt3Ti

Abstract The electronic properties of clean and partly oxidized Pt 3 Ti(111) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt 3 Ti(111) is quasi-pure platinum. Scarcely any (θ ≈ 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(111) and a p(2 × 2) “reconstruction” of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm −1 which coincides with infrared absorption spectra of TiO 2 . Flashing with CO shifts the band to 500 cm −1 . Correlated with this shift we observe (i) CO 2 desorption at a temperature well above that observed for Pt(111)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same c(4 × 2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates “islands of TiO 2 ”, segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti 4+ ions on the surface of these islands are reduced by CO to pentacoordinated Ti 3+ species. The vibrational shift, 650 to 500 cm −1 , can be understood by the dipole active bands of a triatomic O−Ti 4+ −O vibrator compared to a diatomic Ti 3+ −O vibrator.

The mechanism of the decomposition of methanethiol on Fe(100)

Abstract The mechanism of decomposition of methanethiol on the clean Fe(100) surface, and the Fe(100) surface modified by sulfur has been investigated. Temperature programmed reaction spectroscopy, high resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy have been used to investigate the adsorption and decomposition of methanethiol on these surfaces. At low coverage, methanethiol undergoes SH bond scission upon adsorption at temperatures as low as 102 K. In the case of multilayer adsorption, associative CH 3 SH is also detected. Thermal decomposition of methanethiol on the Fe(100) surface results in the formation of methane (CH 4 ) and hydrogen (H 2 ). The amount of CH 4 formed increases as methanethiol coverage is increased, while the production of H 2 shows a maximum as a function of coverage. On a c(2 × 2)S-Fe(100) surface almost all adsorbed CH 3 SH desorbs without decomposition. Thiomethoxy (−SCH 3 ) and methyl (−CH 3 ) are identified as surface intermediates during thermal decomposition of CH 3 SH, using HREELS. The existence of these intermediates is supported by XPS data. A mechanism for the decomposition involving the thiomethoxy and methyl intermediates is proposed.

Fluorescence yield near edge spectroscopy of π-bonded CO on Fe(100)

Abstract Near edge X-ray absorption fine structure (NEXAFS) spectra of CO adsorbed on the Fe(100) surface are reported. Spectra, obtained by fluorescence yield (FYNES), are presented for each of the four individual CO adsorption configurations observed on this surface. The π-bonded state exhibits an unusual FYNES spectrum and polarization dependence which indicates that the molecule is either extensively rehybridized or tilted with respect to the surface normal. The FYNES spectra of each of the adsorption states directly reflect the perturbation of the carbon-oxygen bond by the surface and track systematically with the heat of adsorption.

Surface core level shifts in Pt3Ti(111)

Abstract A surface core level shift in the Pt4f 7 2 XPS core level binding energy has been observed for the Pt 3 Ti(111) surface. The magnitude of the shift is -0.50±0.05 eV and is assigned to an increase in the electron density on the Pt atoms in the outermost atomic layer of the single crystal. The response of the surface peak position to a chemically interacting adsorbate (CO) and a chemically inert adsorbate (Xe), confirms the initial state origin of the surface core level shift. No surface components associated with Ti were resolved, however the FWHM of the Ti2p 3 2 did increase 0.2 eV during low take-off angle measurements. UPS measurements showed that the bulk DOS at the Fermi level of Pt 3 Ti(111) are depleted relative to clean Pt(111). However, low take-off angle measurements suggest that a surface state is located at the Fermi level of the Pt 3 Ti. It is suggested that this surface state is associated with the increased d electron density on the surface Pt atoms.