STM at high pressure

Recently, in situ studies of catalytic surface chemical reactions at high pressures have been undertaken [46, 47]. These studies employed sum frequency generation (SFG) and STM in order to probe the surfaces as the reactions are occurring under conditions similar to those employed for industrial catalysis (SFG is a laser-based teclmique that is described in section A 1.7.5.5 and section BT22). These studies have shown that the highly stable adsorbate sites that are probed under vacuum conditions are not necessarily tlie same sites that are active in high-pressure catalysis. Instead, less stable sites that are only occupied at high pressures are often responsible for catalysis. Because the active... 

The autocatalytic reaction mechanism apparent at low temperatures is expected to apply to catalytic hydrogen oxidation at high pressures. In addition, the above study is the first to use STM to observe the formation of dynamic surface patterns at the mesoscopic level, which had previously been observed by other imaging techniques in surface reactions with nonlinear kinetics [57]. This study illustrates the ability of in situ STM to visualize reaction intermediates and to reveal the reaction pathway with atomic resolution. 

The authors further tested the Pt(l 11) and Pd(l 10) surfaces [71, 72] using in situ STM and SXRD. All these single crystals show a similar kinetic behavior in CO oxidation. The gradual roughening of the surface corresponds to the formation of surface oxides and a higher CO oxidation rate. The structure insensitivity observed at high pressure is in contrast with the results obtained in UHV, where the reactivity shows a strong orientational dependence. 

These systematic studies suggest that an intrinsic connection between the adsorbate structure, mobility, and the formation of product can be established with the aid of structural information obtained from high-pressure STM. It further demonstrated the importance of STM in studies of heterogeneous catalysis at high pressure. 

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). 

Many of the other techniques discussed in this section can be used for catalyst samples in reactive atmospheres, sometimes at high temperatures and high pressures. IR spectroscopy is the method used most successfully with samples in the presence of reactive atmospheres even at high pressure. Raman, EX AFS, Mossbauer, and STM spectroscopies can be also used under such conditions. 

In conclusion, strained surfaces can show very original structures and new catalytic properties. In order to associate the modified catalytic properties to the peculiar structures generated, one has to asume that these original structures are still present under the reactive mixture, at high pressure. Measurements under pressure of reactants are then necessary to measure both the surface structure and the surface species as reaction intermediates. Up to now, only very few data are available in that field. Recent developments around techniques such as STM [79-80], grazing X-ray Diffraction [81]. .. and optical vibrational spectroscopies such as IRRAS[82-83] using a polarized light and SFG [79] have demonstrated the possibility to realise such observations. 

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