High-pressure XPS

Since the 1980s, the surface science community has developed techniques that probe the structure, composition, mechanical properties, and dynamics of surfaces at high pressure. The research activity on in situ surface characterization will be covered in Part 111. Several examples of high-pressure surface apparatuses are shown in Eig. 1.8. These include (a) high-pressure SFG vibration spectroscopy, (b) high-pressure STM, (c) high-pressure XPS, and (d) atomic force microscopy (AFM). 

The high pressure spectrometer should, therefore, enable an XPS study of weakly adsorbed species at room temperature or above by increasing their surface coverage through the dependence of coverage on pressure. 

In 1979, the construction of an XPS system for the investigation of solids in gas atmospheres at pressures of up to 1 mbar was reported by Joyner and Roberts (1979a) this was later commercialized. One differential pumping stage around the high-pressure sample cell was used in combination with the commercial hemispherical electron energy analyzer ESCALAB of V.G. Scientific Ltd. Two "high-pressure" spectrometers of this type were supplied to the University of Wales (Cardiff, UK) and the Boreskov Institute of Catalysis (Novosibirsk, Russia). 

We emphasize that XPS indicated the presence of CO adsorbed only at threefold hollow, bridge, and on-top sites—no signatures of high-pressure species were found. 

In summary, the results of this section demonstrate that by application of XPS, adsorbate structures and coverages can be obtained at pressures of the order of a millibar. Thus, the pressure range in XPS has been expanded by at least five orders of magnitude. Even at these relatively high pressures, CO structures were found to be similar to those known from UHV investigations, but obtained at low temperatures (<300 K). CO on Pd(lll) adsorbs in threefold hollow, bridge, and on-top sites. Bukhtiyarov and coworkers (Bukhtiyarov et al., 1994 Kaichev et al., 2003) did not find any indications of CO dissociation or metal carbonyl formation under the experimental conditions (Bukhtiyarov et al., 2005 Kaichev et al., 2003). 

Poiarization-Modulation IR Reflection Absorption Spectroscopy (PM-IRAS) High-pressure X-ray Photoeiectron Spectroscopy (HP-XPS)... 

Adsorbed moleeules and intermediates at high pressures can be detected by vibrational speetroseopies provided they can differentiate between gas phase and surfaee signals. For example, Fig. 4 shows a (conventional) IRAS spectrum of CO at 50mbar on Pd(l 11) at 300 K (113,114). The signal of adsorbed on-top CO at approximately 2060 cm is nearly obscured by the rovibrational absorption spee-trum of the CO gas phase. In contrast, as shown below, SFG and PM-IRAS selectively probe the adsorbed surface species and thus provide surface-sensitive information, even in the presence of a gas phase. Investigations of the catalyst structure and composition under working conditions can be earried out by high-pressure (HP-) STM and (HP-) XPS, provided that the instruments are properly modified (9,115). 

To combine optical SFG spectroscopy with the more traditional surface analysis methods (e.g., LEED, AES, TPD, XPS), the basic requirement is to simply add IR-transparent windows (e.g., CaF2 or BaF2) to a UHV chamber. However, if SFG spectroscopy is to be carried out at high pressure or during catalytic reactions, instruments combining a EIHV surface analysis system with an SFG-compatible... 

Fig. 11. (a) Experimental apparatus combining a UHV surface analysis chamber with a UHV-high-pressure reaction cell optimized for PM-IRAS spectroscopy. Pre- and post-reaction surface analysis under UHV can be performed by XPS, LEED, AES, and TDS. The optical equipment and the high-pressure cell used for the PM-IRAS experiments are shown in (b) 84,113,114,171). 

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