Surface transformation

In non-fluoride-containing solutions, silicon is stable due to the presence of an oxide film and the electrode behavior can remain constant under a continuous cathodic polarization. The surface of a silicon electrode in fluoride-containing aqueous solution at the open circuit potential is also stable due to hydrogen adsorption. However, surface transformation can occur at cathodic potentials due to formation of hydrides. Thermodynamically, silicon hydride can be a stable phase at certain cathodic potentials as shown in Fig. 2.2. 

FIGURE 6.6. Evolution of the reverse current on p-Si in dark and during a prolonged photocathodic polarization at 3 After de Mierry et al (Reproduced by permission of The Electrochemical Society, Inc.) 

Formation of an amorphous silicon hydride surfaee layer ean also oeeur during anodic photoetching of n-Si. For example, surface Si-H bonds corresponding to about 40 monolayers of hydrogen on an n-Si have been observed to form at a photocurrent of tph = 0.5 mA/cm in 0.5mM NH4F.  

FIGURE 6.7. Schematic illustration of the surface condition of Si as a function of potential. 

The driving force for metal deposition on a semiconductor electrode is determined by the difference between the Fermi level of the electrode and the reversible potential of the metal species in the solution. For the noble metals this difference at the OCP is generally negative indicating that the deposition process is spontaneous, which is the basis for electroless deposition. As shown in Fig. 6.8, the redox potentials of the 

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E. V. Albano. On the influence of reactant s induced surface transformations in the behavior of a heterogeneously catalyzed dimer-monomer reaction model. J Chem Phys 709 7498-7505, 1998. 

Fig. 5. Dependences of relative concentrations Cj on time variable r (arbitrary units) for consecutive catalytic reactions according to scheme (III) for various values of rate constants of the adsorption k,(ub and desorption fcduB of the intermediate B. Left-hand column (fcdesB/fcs = 0.1) desorption of B is slower than its surface transformation. Middle column (fcde.B/fcs = 1) equal rates of desorption of B and of its surface transformation. Right-hand column (fcdesB/fcj = 10) desorption of B is faster than its surface transformation. From G. Thomas, R. Montarnal, and P. Boutry, C.R. Acad. Sri., Ser. C 269, 283 (1969).

Menezes S, Lewerenz HJ (1984) Heterojunction by photoelectrochemical surface transformation n-CulnSe2/p-CulSe3Se°. J Electrochem Soc 131 2462-2463... 

The surface transformations of propylene, allyl alcohol and acrylic acid in the presence or absence of NHs over V-antimonate catalysts were studied by IR spectroscopy. The results show the existence of various possible pathways of surface transformation in the mechanism of propane ammoxidation, depending on the reaction condition and the surface coverage with chemisorbed NH3. A surface reaction network is proposed and used to explain the catalytic behavior observed in flow reactor conditions. 

Because of all these uncertainties and the questionable relationships found between bulk composition and activity, or even selectivities, we followed a different approach in order to gain an understanding and formulate a theory on surface composition responsible for FT and WGS reactivity. A logical sequence of catalyst surface transformations based on scientific principles was proposed and adapted to form a general model that can be used to explain our experimental observations. These proposed catalyst transformations will be discussed in a little more detail below. 

The carbonyl cluster Rh,5(CO)i,5 was initially stable as such on the completely dehydroxylated alumina surface. But as soon as hydroxyl groups were generated (e.g., by adding traces of water) it decomposed to give various surface transformations. First, the cluster structure was dismpted, with breakage of the core cluster frame, into (Al-0-)(Al-0H)Rh (C0)2, Rh > monoatomic species sigma and n-bonded to the oxygens atoms of the alumina surface, with formation of molecular... 

In only a few cases have the spectra of adsorbed ethene been explored much above room temperature, in contrast to the situation on single-crystal metals where measurements up to 600 K. are commonplace. The metal-dependent temperature for some surface transformations are summarized in Table VIII, in comparison with similar reactions of the linear butenes (Section VI.C.2.C). At sufficiently high temperatures, e.g., ca. 430 K on Ni/Si02 (32) or at ca. 500 K on Pt/Si02 (240), the CC bond of ethylidyne is broken to give CH4. 

Note that many of these surface reactions involve the conversion of a hydrophophic polymer to one with a hydrophilic surface or vice versa. For example, the replacement of trifluoroethoxy groups at the interface by hydroxyl units changes a non-adhesive, highly hydrophobic surface to an adhesive hydrophilic one. Variations in the reaction conditions allow both the depth of transformation and the ratios of the initial to the new surface groups to be controlled. A possible complication that needs to be kept in mind for all of these surface transformations is that polymer molecular motions may bury the newly introduced functional units if the polymer comes into contact with certain media. For example, a hydrophilic surface on a hydrophobic polymer may become buried when that surface is exposed to dry air or a hydrophobic liquid. But this process can be reversed by exposure to a hydrophilic liquid. 

The curves plotted on Fig la illustrate the variation of the rate of site blockage inside the pore, calculated on the assumption that coke precursors arise by a simple parallel mechanism, through surface transformations of intermediates [AZ].. The plots below on Fig. lb show the corresponding fractions of blocked sites. The curves plotted on all the figures represented, refer to values of the parameters D- 0.001 [cm /s], P (gas) = 0.72, — 0.4. The curve labels 1, 2 and 3 are relevant to time intervals characterized by the dimensionless parameters tl=0.14 t2 = 0.5, t3 = 1.0. 

While identifying the rate determining parameters of the catalytic trans formation schemes under consideration, we understand better the reason for possible energy correlations in catalysis, the Broensted Polanyi relations for the transition state energies of surface transformations being typically apphed. Such an approach was intensively used in 1960s—1970s to... 

B. A. Dubrovin, A. T. Fomenko, and S. P. Novikov, Modern Geometry—Methods and Applications Part I The Geometry of Surfaces Transformation Groups and Fields, Springer Verlag, New York, 1984. 

The in situ UV-VIS spectroscopic measurement showed that upon adsorption of propene/benzene=l/6 mixture on HP at 295 K, bands at 210, 260 and 340 nm appeared (Fig. 8), whose intensities did change neither -with time nor upon evacuation at room temperature. When evacuation was performed at 373 K the band at 340 nm increased gradually, while at 473 K new bands appeared at 400-500 nm indicating the surface transformation became more complex. 

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