Kinetics on oxide surface

Majority of studies concerning organic compounds chemisorption kinetics on oxides surface are performed for the reaction mechanisms described by Schemes 1, 8 or 15 and mainly, for the first mechanism of organosilicon compounds interaction with OH groups of non-porous silica surface. 

The following models are used for description of organic compound chemisorption kinetics on oxides surface. 

The exponential distribution function on chemisorption activation energy conforms to Howar and Taylor s, Banham and Burt s as well as to Kwan s equations. They were used long ago for the description of small molecule chemisorption kinetics on oxide surfaces [116]. It is suggested that apparent rate constant depends on the coverage heterogeneous surface as follows [117] ... 

A set of simple physical and chemical principles can be used to understand and predict the surface reconstruction or faceting, which occurs on oxide surfaces.8,9 These include (1) autocompensation of stable surfaces, (2) rehybridization of the dangling bond charge density, (3) formation of an insulating surface, (4) conservation of near-neighbor bond lengths (which minimize the formation of surface strain fields), and (5) surface kinetic factors. We shall briefly discuss all of these factors governing the surface reconstruction of metal oxide. 

Fig. 16.6 Effects of pre-adsorbed Zn(II) vs. Zn(lI)/As(III) simultaneous treatment on As(III) oxidation kinetics on birnessite surfaces (pH 4.5, suspension density = 0.1 g L , in 0.01 M NaCl, total Zn(II) concentration of 100pM, and atmosphere). Percent As(III) depletion, As(V) release, and total As adsorption are shown as a function of time (hours), (a) Initial As(III) concentrations [As(III)] = lOOpM (b) [As(III)]j = 300pM. Reprinted with permission from Power LE, Aral Y, Sparks DL (2005) Zinc adsorption effect on arsenite oxidation kinetics at the birnessite water interface. Environ Sci Technol 39 181-187. Copyright 2005 American Chemical Society...

Compared with literature data for adsorption of surfactants from aqueous solutions on oxide surfaces(/4,75), the kinetic data obtained in this work for C18/glass are of similar orders of magnitude to the former systems. The values of 1/k and k.for CIg/Al are greater than those for oxide adsorbent studied, indicating the strong adsorbing ability of metallic aluminum (even oxidized) relative to the mineral oxides. 

Scanning probe microscopies are now able to study in situ the growth of metal clusters. These studies are performed sequentially after deposition. On metal/metal systems it has been possible to follow the nucleation kinetics and to derive the elementary energies like adsorption and diffusion energies (see the excellent review by Brune [68]). On oxide surfaces only recently such studies have been undertaken. STM can be only used on conducting samples, however it is possible to use as a support an ultrathin film of oxide grown on a metal. By this way it has been possible to study the nucleation of several... 

V. I. Bogillo, Kinetics of organic compounds chemisorption from the gas phase on oxides surface", in Adsorption on New and Modified Inorganic Sorbents (Eds. A.Dabrowski, V.A.Tertykh), Elsevier, Amsterdam, 1996, p.237. 

Kinetics of organic compounds chemisorption from the gas phase on oxides surface... 

KINETIC MODELS USED FOR DESCRIPTION OF CHEMISORPTION ON OXIDES SURFACE... 

Section 1 presents the studies both on the structure of the formed surface compounds and the main regularities of the chemisorption processes and on kinetics of chemisorption from the gas phase on oxide surfaces. Prospective ways for the immobillization of different active compounds have been also proposed. 

The classical treatment of Langmuir kinetics on uniform surfaces can be extended to the reactions that occur on two types of sites in close proximity. The reaction proceeds via interactions between species adsorbed on such distinct sites. Examples of such reactions are various oxidation reactions on metal oxides, which contain two types of sites, metal atoms sites and oxygen atom sites, each having a specific function. Another example is hydrogenolysis of haloarenes where spillover hydrogen reacts with an organic molecule adsorbed on the metal surface ... 

Pelmenchikov, A.G., Paukstis, E.A., Zhanpeisov, N.K., Pavlov, V.I., Zhidomirov, G.M. (1987) Comparative discussion of ammonium formation mechanisms on oxide surfaces and in acidic media . React. Kinet. Catal. Lett. 33, 423-428. 

Muzykantov, V.S., Popovsky, V.V., Boreskov, G.K. (1969) Reactivity of oxygen adsorbed on oxide surface 2. Isotopic exchange of oxygen in Fe-Sb-O catalysts. Kinet. Catal, 10, 1270-1277 (in Russian). 

A pletliora of different SA systems have been reported in tire literature. Examples include organosilanes on hydroxylated surfaces, alkanetliiols on gold, silver, copper and platinum, dialkyl disulphides on gold, alcohols and amines on platinum and carboxyl acids on aluminium oxide and silver. Some examples and references can be found in [123]. More recently also phosphonic and phosphoric esters on aluminium oxides have been reported [124, 125]. Only a small selection out of tliis number of SA systems can be presented here and properties such as kinetics, tliennal, chemical and mechanical stability are briefly presented for alkanetliiols on gold as an example. 

Finally, in 1985, the results of an extensive investigation in which adsorjDtion took place onto an aluminium oxide layer fonned on a film of aluminium deposited in vacuo onto a silicon wafer was published by Allara and Nuzzo 1127, 1281. Various carboxylic acids were dissolved in high-purity hexadecane and allowed to adsorb from this solution onto the prepared aluminium oxide surface. It was found that for chains with more than 12 carbon atoms, chains are nearly in a vertical orientation and are tightly packed. For shorter chains, however, no stable monolayers were found. The kinetic processes involved in layer fonnation can take up to several days. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

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