Surfaces at High Pressures

The above studies show that the chemisorptions on metals could often alter the composition and structure of metal surfaces. To bridge the pressure gap, in situ STM has played a critical role in observing the dynamic behavior of catalytic surfaces from UHV to atmospheric pressures. 

The CO-covered Pt(l 1 0) surface was then exposed to a mixture of CO/02 gases, with the ratio of CO/02 adjusted by flow meters. The pressure changes of CO, 02, and the reaction product, C02, were monitored by leaking the gases from the flow reactor to a quadrupole mass spectrometer (QMS) attached to the flow-reactor STM. The surface structure and reactivity of Pt(l 1 0) could be measured simultaneously with the combination of STM and QMS. 

Combined with their kinetic measurements, the authors proposed CO from the gas phase could directly react with oxygen atoms in the surface oxides, accounting for relatively high reactivity of this phase for CO oxidation. This mechanism, termed as Mars-Van Krevelen mechanism, challenges the general concept that CO oxidation on Pt group metals is dominated by the Langmuir-Hinshelwood mechanism, which proceeds via (1) the adsorption of CO and the dissociative adsorption of 02 and (2) surface diffusion of COa(j and Oa(j atoms to ultimately form C02. 

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. 

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J. Phys. Chem. B, 106, 5143-5154. Somorjai, G. A. and Rupprechter, G. (1999) Molecular studies of catalytic reactions on crystal surfaces at high pressures and high temperatures by infrared-visible sum frequency generation (SFG) surface vibrational spectroscopy. J. Phys. Chem., 103, 1623-1638. 

Mobility and Flexibility of Catalyst Surfaces at High-Pressure High-Temperature Reaction Conditions... 

D. Hydrocarbon Reactions on Platinum Crystal Surfaces at High Pressures... 

The most important effect of concentration polarization is to reduce the membrane flux, but it also affects the retention of macromolecules. Retention data obtained with dextran polysaccharides at various pressures are shown in Figure 6.12 [17]. Because these are stirred batch cell data, the effect of increased concentration polarization with increased applied pressure is particularly marked. A similar drop of retention with pressure is observed with flow-through cells, but the effect is less because concentration polarization is better controlled in such cells. With macromolecular solutions, the concentration of retained macromolecules at the membrane surface increases with increased pressure, so permeation of the macromolecules also increases, lowering rejection. The effect is particularly noticeable at low pressures, under which conditions increasing the applied pressure produces the largest increase in flux, and hence concentration polarization, at the membrane surface. At high pressure, the change in flux with... 

There is considerable uncertainty in the contact conductance data reported in the literature, and care should be exercised when using them. In Table 3-2 some experimental results are given for the contact conductance between similar and dissimilar metal surfaces for use in preliminary design calculations. Note that the thermal contact conductance is highest (and thu.s the contact resistance is lowest) for soft metals with smooth surfaces at high pressure. 

The available experimental data show a wide diversity of lower and upper critical solution temperature variation with increasing pressure. If the critical curves Li = L2 do not have the hypercritical solution point, they should intersect a crystallization surface at high pressures and end in the nonvariant critical point Li = L2-S (Valyashko, 1990a), as it was found for acetonitrile (C2H3N) - H2O and other binary mixtures (Schneider, 1964, 1970). 

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 popularity of the Langmuir equation is explained by the fact that it is the simplest dependence predicting linear (Henry-type) behaviour at low partial pressures and the appearance of saturation of the surface at high pressures, which is the expected physical behaviour for most of the systems. 

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