XPS measurements

Table 4. XPS measurements of HOPG after one cycle in 1.2 mol L 1 LiAsF6 /EC-DEC electrolyte...

The surface analyses of the Co/MgO catalyst for the steam reforming of naphthalene as a model compound of biomass tar were performed by TEM-EDS and XPS measurements. From TEM-EDS analysis, it was found that Co was supported on MgO not as particles but covering its surface in the case of 12 wt.% Co/MgO calcined at 873 K followed by reduction. XPS analysis results showed the existence of cobalt oxide on reduced catalyst, indicating that the reduction of Co/MgO by H2 was incomplete. In the steam reforming of naphthalene, film-like carbon and pyrolytic carbon were found to be deposited on the surface of catalyst by means of TPO and TEM-EDS analyses. 

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

The spectroscopic investigations were carried out in a modified LHS 12 MCD system. For the XPS measurements (Mg Ka 1253.6 eV, 240 W power) a fixed analyser pass energy of 108 eV... 

We express our thanks to Prof M.Yamada (Tohoku University) for the FTIR measurements and Mr. M.Matsuo (Rigaku Ltd.) for the XPS measurements. We are also grateful to Prof M.Nomura and staff of the Photon Factory, National Laboratory for High Energy Physics, for assistance in measuring EXAFS spectra (F oposals 89138 and 93G163). 

A third class of catalysts was prepared by electron beam induced deposition of XiCl4 on a polycrystalhne Au foil. Deposition of TiCU at 300 K leads to films which comprise Ti + and Ti species as inferred from XPS measurements [90]. Depending on the experimental parameters (background pressure of TiCU, electron flux, electron energy) different composition of Ti oxidation states are observed [23]. From angular-dependent measurements it was concluded that the Ti + centers are more prominent at the surface of the titanium chloride film, while the Xp+ centers are located in the bulk [90]. 

In the XRD patterns of the unprotected metal nanoclusters prepared by the alkaline EG synthesis method, no signals derived from their corresponding oxides could be detected. XPS measurements also revealed that the prepared metals nanoparticles had the binding energies close to those of their corresponding zero valence species, 70.9 eV for Pt 4f7/2, 280.0 eV for Ru 3ds/2, 307.1 eV for Rh 3d5/2 and 50.0 eV for Os 4f7/2, respectively, indicating that the metal species were in a metallic state. 

Skotheim et al. [286, 357, 362] have performed in situ electrochemistry and XPS measurements using a solid polymer electrolyte (based on poly (ethylene oxide) (PEO) [363]), which provides a large window of electrochemical stability and overcomes many of the problems associated with UHV electrochemistrty. The use of PEO as an electrolyte has also been investigated by Prosperi et al. [364] who found slow diffusion of the dopant at room temperature as would be expected, and Watanabe et al. have also produced polypyrrole/solid polymer electrolyte composites [365], The electrochemistry of chemically prepared polypyrrole powders has also been investigated using carbon paste electrodes [356, 366] with similar results to those found for electrochemically-prepared material. 

As might be expected, the properties of polythiophene show many similarities with those of polypyrrole. As with polypyrrole, polythiophene can be prepared via other routes than electrochemical oxidation both as the neutral material [390-392] or in the p-doped form [393]. This material is produced as an infusible black powder which is insoluble in common solvents (and stable in air up to 360°C), with conductivities ranging from approximately 10 11 Scm-1 in the neutral form [390] to 102 Scm-1 when doped [19, 393, 394]. Early work on thiophene polymers showed that the p-doped material is air-sensitive in that the conductivity decreases on exposure to the atmosphere [20, 395] although no evidence of oxygen-containing species was seen in XPS measurements [19],... 

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Especially in conjunction with the detection of water or OH species, ex situ XPS measurements have been critizised because of possible changes occurring during transfer and exposure of the sample to UHV. Kuroda et al. have demonstrated that structural changes of the passive film indeed occur when electron diffraction studies are performed in a hydrated and subsequently in a dehydrated environment. Structural changes, however, do not necessarily cause changes in elemental composition as determined by XPS. 

The anchoring and the reduction methods of precious metal precursors influence the particle size, the dispersion and the chemical composition of the catalyst. The results of SEM and H2 chemisorption measurements are summarised in Table 3. The XPS measurements indicate that the catalysts have only metallic Pd phase on their surface. The reduction of catalyst precursor with sodium formate resulted in a catalyst with lower dispersion than the one prepared by hydrogen reduction. The mesoporous carbon supported catalysts were prepared without anchoring agent, this explains why they have much lower dispersion than the commercial catalyst which was prepared in the presence of a spacing and anchoring agent (15). 

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