D.J. Dwyer

A study of the nickel-titanium oxide interaction

Previous studies have demonstrated that, when nickel supported on titanium oxide is reduced in hydrogen at 450 °C and higher, the system exhibits SMSI properties. We have employed several complementary experimental approaches in an attempt to gain an insight into the intimate details surrounding the nickel-titanium oxide interaction. High resolution transmission electron microscopy was used to examine the changes in morphology of nickel particles following reduction at increasing temperatures. In situ ferromagnetic resonance studies have provided characterization of the state of the nickel as a function of reduction temperature. A geometrically designed catalyst in combination with scanning Auger surface analysis was used to probe transport phenomena involving nickel and titanium oxide during treatment in hydrogen. The combined results of these studies have enabled us to develop a model which involves the migration of titanium-oxygen moieties onto the surface of the nickel particles during reduction in hydrogen. This decoration model provides a mechanism whereby SMSI properties are observed.

Activation of carbon monoxide on clean and sulfur modified Fe(100)

Chemisorption of CO on clean and sulfur modified Fe(100) has been investigated using X-ray photoelectron spectroscopy, temperature programmed desorption, high resolution electron energy loss spectroscopy and near edge X-ray absorption fine spectroscopy. On the clean iron surface, CO adsorbs in three sequentially filled molecular states. Two of these states desorb at or below 300 K. The third state ( α 3 ) partially desorbs at 440 K and is the precursor to CO dissociation on the Fe(100) surface. This precursor occupies the 4-fold hollow sites and has an extremely weak CO bond ( ν CO =1210 cm −1 ). The geometry of the adsorbed CO is a major factor in determining the unusually weak carbon-oxygen bond in the precursor state. Sulfur modification of the surface reduces CO dissociation by simple site blocking; no evidence for a long range electronic interaction was obtained.

Adsorption of CO on the clean and sulfur modified Fe(100) surface

Abstract The interaction of CO with clean and sulfur modified Fe(100) surfaces has been investigated using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). On the clean Fe(100) surface, CO adsorbs molecularly in three sequentially filled states. Dissociation of CO occurs at 440 K and is correlated with the most tightly bound molecular state. Investigation of the CO adsorption as a function of sulfur coverage on the Fe(100) surface indicated that sulfur reduces the CO dissociation by blocking the sites for dissociated carbon and oxygen atoms. No evidence for long-range electronic effects was obtained.

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 .

XPS identification of the chemical state of subsurface oxygen in the OPd(110) system

Abstract Adsorbed oxygen at room temperatures exists on Pd(110) in two states, a surface state ( β 2 ) and a more weakly bound state ( β 1 ), previously designated as “subsurface” oxygen. The “subsurface” state was postulated as responsible for the oscillations in the kinetics of catalytic CO oxidation observed on this surface. However, previous studies have cast doubt on this explanation. We have investigated the interaction of oxygen on Pd(110) by temperature programmed desorption (TPD), ultraviolet photoemission spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). The XPS combines conventional UHV with in-situ surface analysis in controlled atmospheres of up to 1 mbar. UPS work function measurements show a decrease with oxygen adsorption, coinciding with β 1 formation observed in TPD as reported previously. XPS measurements of the Pd 3 d 5 2 line show only oxyg induced surface core-level shifts (SCLS) for oxygen exposures as high as 22 800 L ( p o 2 = 1.0 × 10 −4 mbar and T = 400 K). surface-sensitive XPS measurements and TPD for higher oxygen exposures ( p o 2 = 4.0 × 10 −2 mbar for 20 min at T = 400 K) s oxide formation coinciding with a corresponding high increase in β 1 formation. These results for oxygen pressures and temperatures in the oscillation regime, suggest that the oscillatory behavior on this surface may be due to an oxidation and reduction mechanism.

The catalytic reduction of carbon monoxide over iron surfaces: A surface science investigation

Abstract The hydrogenation of carbon monoxide over iron surfaces has been investigated at medium pressure (7 atm) and over the temperature range 473–578 K. The study was performed in ultrahigh vacuum surface analytical system equipped with a high-pressure reactor capability. High-purity iron foils were found to deactivate rapidly under synthesis conditions. This deactivation appears to be associated with the deposition of a graphitic type of surface carbon. Under the same reaction conditions higher surface area iron powders promote the formation of carbidic rather than graphitic carbon. The carbided surfaces were more stable under reaction conditions but were also susceptible to poisoning by graphite deposition at temperature above 550 K. Changes in catalyst activity and selectivity can be correlated with changes in the amount and type of carbon on the surface.

Surface chemistry of H2S sensitive tungsten oxide films

We have applied X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to characterize thin tungsten oxide films and to investigate their interaction with hydrogen sulfide in view of their use as very sensitive hydrogen sulfide gas sensors. W 4f core-level spectra indicate a partial reduction of W6+ after the reaction. No evidence for band bending after H2S dosing could be found in the valence-band spectra. The results suggest that the primary sensing mechanism involves the formation of oxygen vacancies on the surface in the presence of hydrogen sulfide. Alternative mechanisms, such as the formation of a tungsten sulfide or a hydrogen tungsten bronze on the surface, are judged to be unlikely.

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 water dissociation reaction on clean and oxidized iron (110)

The interaction of H2O with the clean Fe(110) surface, Fe(110) surfaces with absorbed oxygen, and Fe(110) surfaces after bulk oxygen penetration were investigated using UPS. At 225 K, surface absorbed hydroxyl groups are present on all surfaces studied. On initially clean Fe(110) and surfaces with up to half monolayer of absorbed oxygen, heating the hydroxyl layer produced at 225 to 360 K causes hydrogen and water desorption and additional oxygen deposition relative to the starting surface. Beyond one‐half monolayer of initial oxygen coverage, dehydroxylation proceeded exclusively via water desorption with the quantitative restoration of the initial surface composition. These results demonstrate that the (110) surface of iron is passive toward oxidation by water alone below 360 K, after the first monolayer.

Characterization of Fe, Fe-Cu, and Fe-Ag Fischer-tropsch catalysts

Abstract Copper and silver were found to interact very differently with the oxidized iron catalyst. Copper is oxidized and highly dispersed on the passivated iron surface. The intimate contact between copper oxide and iron oxide and the facile reduction of copper oxide are responsible for the ability of copper to enhance the reduction of iron oxide. The metallic copper nuclei promote the nucl