Xuming Wei

Interactions in the Fe/TiO2(110) system

Abstract Fe deposited on TiO2(110) has been studied using AES, LEED, ISS, UPS and HREELS. The deposited Fe overlayer is found to grow in a layer-by-layer mode. Oxygen migrates from the subsurface layers of TiO2(110) to the deposited Fe during the deposition process. The chemical species of the deposited Fe depends on the state of oxidation of TiO2(110) and the amount of Fe deposition. The state of the adsorbed CO in this system is found to depend remarkably on the amount of Fe deposited on the TiO2(110) substrate. CO is found to adsorb weakly on a low Fe coverage system of 0.2 ML Fe/TiO2(110), while it is strongly chemisorbed on a high Fe coverage system of 1.7 ML Fe/TiO2(110). Difference in the CO chemisorption behavior is attributed to the different structural characteristics of these two states of the Fe/TiO2(110) system.

Interaction of oxygen with silver surface at high temperature

The interaction between oxygen and silver surface at high temperature was investigated by Auger electron spectrometer (AES), ISS, X-ray photoelectron spectra (XPS), thermal desorption spectra (TDS) and high-resolution electron energy loss spectra (HREELS). When dosing oxygen to silver surface at 873 K, a new atomic oxygen species forms on the silver surface. This species of oxygen has very high thermal stability, and the TDS experiment shows no detectable desorption peak appearing below 973 K. The O-1s binding energy of this oxygen species is 529.3 eV and HREELS result gives a peak at 352 cm(-1). All these properties show that this kind of atomic oxygen species is different from those previously found on silver surface

An AES, UPS and HREELS study of the oxidation and reaction of NB(110)

Abstract The oxidation of Nb(110) upon oxygen adsorption has been investigated using AES, UPS and HREELS. For low and medium exposures of oxygen, two new loss features at 720 and ∼950 cm −1 develop, which are assigned to NbO and NbONb stretching vibrations, respectively. When oxygen exposure is high, a broad and strong loss peak at 600–900 cm −1 appears, which is due to bulk-like NbONb stretching vibrations, the changes in the vibrational spectroscopy suggest that, with increasing oxidation of the surface, NbO 4 tetrahedra transform into NbO 6 octahedra. Experiments involving methanol adsorption further confirm these results concerning the process of oxidation.

Oxidation of ethylene on the ordered alloy surface Pd{001}c(2 × 2)-Mn

Abstract Ethylene adsorption and coadsorption with oxygen on the ordered alloy surface Pd{001}c(2 × 2)-Mn were studied with high resolution electron energy loss spectroscopy (EELS). The vibrational spectrum for C 2 H 4 adsorbed on Pd{001}c(2 × 2)-Mn at 140 K is similar to that for C 2 H 4 adsorbed on Pd{001} at 80 K, which has been ascribed to di-σ-bonded C 2 H 4 . In contrary to the observation on Pd{001}, where oxygen preadsorption inhibits C 2 H 4 by limiting the formation of di-σ-bonded C 2 H 4 , on Pd{001}c(2 × 2)-Mn, the vibrational spectrum of C 2 H 4 adsorbed at 140 K is not affected by the preadsorbed oxygen. At 230 K, the coadsorbed C 2 H 4 and O 2 react with each other and form CO and H 2 O on the surface.

An anomalous adsorption curve: Cl on Ag(111)

Abstract ISS characterization of Cl from Cl adsorption on Ag(111) showed a variation with exposure that is anomalous: the Cl ISS signal first increases and then decreases to undetectable levels. The Cl AES signal showed a normal adsorption curve. It is suggested that at higher exposures structural changes occur that render Cl undetectable by ISS, but which leaves Cl AES eelctrons detectable.

The adsorption and reaction of cyclohexanone on the Pt(111) surface

Abstract The interaction of cyclohexanone with the Pt(111) surface has been investigated by HREELS and TDS. At 160 K, the chemisorption of cyclohexanone induces a “red-shift” of the C O stretch band and a decrease in work function. Heating the sample from 220 to 370 K leads to complete disappearance of the dominant CH 2 rocking band at 720 cm −1 and the C O stretching band at 1650 cm −1 , indicating the dehydrogenation of the entire adlayer to form intermediate species. Strong bands of the dehydrogenated species appear at 860 and 3000 cm −1 which is different from phenoxy on Pt(111). Above ∼ 380K, the dehydrogenated species is decomposed to form hydrocarbon fragments and C O which directly desorbs into the vacuum.

The role of dioxygen in methanol oxidation on Ag

Abstract An Ag surface with a dioxygen species, stable to 380 K, as the only oxygen species was used to investigate the roles of this dioxygen in methanol oxidation with the techniques of HREELS and TDS. The roles of this dioxygen were found to be similar to those of atomic oxygen: as a base in the deprotonation of methanol, dehydrogenation of adsorbed methoxide and as a nucleophile in an attack on the carbon of adsorbed formaldehyde. The temperatures of gaseous product evolution are different with these two oxygen species and is possibly due to the coverage of oxygen in excess.

CL ADSORPTION ON AG(111) AND ITS PROMOTER ACTION

Cl adsorption on Ag(111) is described with reference to AES, HREELS, LEED, UPS, and ISS data. An important feature is the creation of Ag defect sites. Oxygen adsorption on a Cl modified surface changes from atomic to molecular form. This same altered behavior of oxygen adsorption was also seen when Cl was removed after it had modified the surface. This suggests that Cl modifies the Ag surface via structural changes.

HREELS study on coadsorption of oxygen and cyclohexanone on the Pt(111) surface

The coadsorption of cyclohexanone and oxygen on Pt(111) has been investigated by HREE-LS and TDS. At 205 K the presence of oxygen induces an “extra” red-shift of the C = O stretching of coadsorbed cyclohexanone. Heating this coadsorbed surface from 205 to 250 K leads to further dehydrogenation to form intermediate species and to complete disappearance of the C = O stretching band. Above 300 K, the molecule rings of dehydrogenation species cleave to form hydrocarbon fragments and CO molecules which directly desorb into the vacuum or react with preadsorbed oxygen to produrn CO2 which leave the surface immediately. TDS results provide further evidence that the preadsorbed oxygen promotes the decomposition of cyclohexanone.