Surface structural changes

Table 26.7 gives a list of the boundary conditions which define a tire wear test simulation and in fact also an acmal road test. The road surface is the laboratory surface on which the abrasion data for the simulation were obtained. There is as yet no definition of a road surface and even if there were one, it would be of httle use since road surface structures change frequently along the road surface as pointed out earlier. 

The trend is illustrated for ammonia activation in Figure 1.17 [19]. In this figure, the activation energies of ammonia activation are compared for stepped and nonstepped surfaces of Pt. Similarly as also found for H2O activation [20], the dissociation barrier is found to be invariant to surface structural changes. This is very different compared to the earlier discussed activation of methane that shows a very strong structural dependence. 

Figure 5.11 Variation in the catalytic activity of an Mg(0001) surface when exposed to a propene-rich propene- oxygen mixture at room temperature. The surface chemistry is followed by XPS (a), the gas phase by mass spectrometry (b) and surface structural changes by STM (c, d). Initially the surface is catalytically active producing a mixture of C4 and C6 products, but as the surface concentrations of carbonate and carbonaceous CxHy species increase, the activity decreases. STM images indicate that activity is high during the nucleation of the surface phase when oxygen transients dominate. (Reproduced from Ref. 39).

Kokubo, T., Kushitani, H., Sakka, S., Kitsugi, T. and Yamamuro, T. (1990) Solution able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W. Journal of Biomedical Materials Research, 24, 721-734. 

Once the values of hkl are found, then the arrangement of atoms on these surfaces is easily obtained, and Figure 1.2 shows the commonest low-index form of these surfaces. If the common surfaces of the fee structure are examined, it will be seen that the surface structure changes quite remarkably. The (111) surface is clearly a close-packed structure but the (100) surface has a square arrangement of metal atoms and the fee (110) surface, which shows grooves running parallel to the c-axis, is even more remarkable. The coordination of the surface atoms clearly is also very different, with the coordination evidently 9 in the (111) surface, 8 in the (100) surface and a remarkable 6 in the (110) surface, as compared to 12 in the bulk. 

In situ dynamic surface structural changes of catalyst particles in response to variations in gas environments were examined by ETEM by Gai et al. (78,97). In studies of copper catalysts on alumina, which are of interest for the water gas shift reaction, bulk diffusion of metal particles through the support in oxygen atmospheres was shown (78). The discovery of this new catalyst diffusion process required a radical revision of the understanding of regeneration processes in catalysis. 

Aberration-corrected ETEM/STEM (130) is expected to offer superior (subatomic) resolution under catalytic reaction conditions furthermore, it will provide improved flexibility for tilting the sample to different crystallographic orientations to allow understanding of the geometry of surface structural changes, enable the use of complex sample stages, and perhaps higher gas pressures. 

Adsorbate Temp (X ) change Adsorbate diffraction features or surface structure change Substrate struct ure after adsorption Adsorbate diffraction features or surface structures... 

Topsoe N-Y, Topsoe H. On the nature of surface structural changes in Cu/ZnO methanol synthesis catalysts. Topics Catal. 1999 8 267. 

An induction period of 30 min was observed at the initial stage of the CO formation in the photocatalysis for CO2 reduction on CdS-DMF, suggesting that the surface structures change photochemically during this period, giving catalytically active sites. 

Secondary Ion Mass Spectrometry. The surface structural changes implied by the data discussed so far influence a variety of SIMS ions. We discuss here a SIMS experiment designed specifically to examine effects of temperature on carbon mobility and the formation of dense phase carbon islands on the surface by monitoring carbon isotope 2 mixing. The loy ss negative ion spectrum of 0.5 monolayers of C deposited from shows peaks at 12, 16, and 24 amu with... 

The change in catalytic activity is not due oply the surface structural change. As was mentioned in the oxidation in many cases the mechanism is also altered which results in the modification of the reaction rate and selectivity. 

Summary. Atomically resolved surface structures were observed at the van der Waals face of InSe and the (100) face of p- and n-GaAs electrodes in electrolyte solutions imder potential control by electrochemical atomic force microscope (AFM). The surface structure change of these electrodes during electrochemical reactions was followed. 

Modeling of surface reactions has kept pace with the new spectroscopic views of mineral surface structures. Chang et al. (30) and Van Riemsdik and Hiemstra (31) present excellent examples of the incorporation of new surface structure paradigms into computer modeling studies. 

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