Surface reactions monolayers

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

The above stm study also discovered a facile transport of surface gold atoms in the presence of the Hquid phase, suggesting that the two-step mechanism does not provide a complete picture of the surface reactions, and that adsorption/desorption processes may have an important role in the formation of the final equiHbrium stmcture of the monolayer. Support for the importance of a desorption process comes from atomic absorption studies showing the existence of gold in the alkanethiol solution. The stm studies suggest that this gold comes from terraces, where single-a tomic deep pits are formed (281—283). 

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer stmctures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer stmcture with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is - 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

Chidsey and coworkers made pioneering works in preparing covalently bonded monolayer films on silicon surfaces by the radical-initiated reaction of 1-alkenes with the H-Si(lll) surfaces. Reactions were carried out in neat deoxygenated alkenes using thermal decomposition of diacyl peroxides as the... 

Solution If the surface reaction controls the overall rate, all the active sites will be occupied. Assume that the entire surface is active so that it will be covered with a monolayer of A molecules. The surface area is 101(400) = 40,400m (4ha or 10 U.S. acres ). 

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

The reaction of solids occurs in the monolayer of molecules adsorbed on the surface of the solid B, as sketched in Figure 9-3. Therefore, this process can be regarded as a surface reaction with rate r", just hke the catalytic reactions of Chapter 7 except that now the molecules of the solid react with the gaseous molecules to form a gaseous product and remove solid molecules. The rate of the reaction should therefore be... 

If a surface reaction is to involve more than monolayer-chemisorption, then the species adsorbed on the surface must be able to migrate into the second and deeper layers forming new chemical bonds and often new molecular species. This is step 3, product formation, and it often requires an activation mechanism to proceed, i.e., a monolayer is formed and the reaction stops unless the substrate is held at elevated temperature or there is ion or electron bombardment. Damage-enhanced diffsusion, knock-on collisions, and bond breaking may promote the reaction in the presence of ion bombardment. Although the precise mechanisms are unclear, it is certain that electron and ion bombardment cause step 3 to occur in some instances where the chemical reaction does not proceed in the absence of radiation. 

Mass spectrum from V. J. Angelico, S. A. Mitchell, and V. H. Wysocki, "Low-Energy Ion-Surface Reactions of Pyrazine with Two Classes of Self-Assembled Monolayers, Anal. Chem. 2000, 72, 2603. 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

Schonherr, H., V. Chechik, C.J.M. Stirling, and G.J. Vansco. 2000. Monitoring surface reactions at an AFM tip An approach to follow reaction kinetics in self-assembled monolayers on the nanometer scale. J. Am. Chem. Soc. 122 3679-3687. 

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