Bare surface, heterogeneity

Uniform n-Si/CHsOH-dmFc contact (bare n-Si surface) Heterogeneous n-Si/Ni/E prepared using 174 nm diameter spheres Uniform n-Si/Ni contact (completely metallised surface) n-Si/Ni/E theory interacrting contact model n-Si/Ni/E theory independent contact model n-Si/Ni/E photoresponse predicted from dark response... 

A clear problem of heterogeneous versus homogeneous systems is the fact that there are often many different active sites on a metal particle. In regions of the particle, which have low densities of chiral modifiers, the racemic reaction over the bare surface can compete strongly with the enantioselective reaction, leading to poor ee values. In the Ni/tartrate-type system, the loss of modifier over the course... 

Frankenburger suggested a two-step mechanism for N2 adsorption involving a short lived physical adsorption on the surface followed by nitrogen dissociation. Another reason for the heterogeneous behaviour of the ammonia catalyst surface, in addition to the two already mentioned, was suggested by Frankenburger previously adsorbed particles influence the entire catalyst and particularly its surface in such a way that it exerts forces of attraction or repulsion toward N2 molecules from the gas phase different from those exerted by a completely bare surface. ... 

Such reduction In overpotentlal Is the largest observed for a bare glassy carbon electrode. The presence of surface qulnones may be Indicative of activation but does not appear to mediate the heterogeneous electron transfer. XFS results support the presence of qulnones as a minor constituent on the surface. 

The mobility of metal atoms in bare metal clusters and small metallic nanoparticles (NPs) is of fundamental importance to cluster science and nanochemistry. Atomic mobility also has significant implications in the reactivity of catalysts in heterogeneous transformation [6]. Surface restmcturing in bimetallic NP and cluster catalysts is particularly relevant because changes in the local environment of a metal atom can alter its chemical activity [7, 8]. 

From a consideration of the velocity of a number of heterogeneous gas reactions (Eideal and Taylor, Gatalysis in Theory and Practice) a certain number of conclusions may be drawn in respect to the valency of the adsorbate and the number of elementary spaces on the crystal lattice which the adsorbate occupies or adheres to. If we consider a unimolecular reaction to occur catalytically at a surface, e.g. Xg 2X, and that the reactant is but feebly adsorbed by the catalyst, then adopting the previous notation the rate of condensation of the gas on the surface of the catalyst (since the catalyst is almost bare) will be ay,. If the reactant occupies a fraction 6 and each molecule n elementary spaces on the lattice the rate of evaporation of the unchanged product will be vd -. Provided the chemical reaction occur but slowly we obtain (1) ay = vO. ... 

Although the surface of the cornea (1.3 cm2) represents barely one-sixth of the total surface area of the eyeball, it is the cornea through which the drug reaches the inner tissues of the eye. The cornea, which is 1 mm thick at its edges and 0.5 mm thick at its center, is classically described as a heterogeneous tissue composed of the following [16] ... 

Because of the extreme heterogeneity of 0 basic sites present on the surfaces of high-surface-area MgO crystals, Spoto et al. (26) used the simplest cluster model able to account for the strongly basic character of the 0 sites responsible for the CO oligomerization reactions on MgO. The cluster model consists of a bare, unconstrained neutral Na O Na" cluster. Of course, the limitations of this trivial cluster are evident relative to the more realistic structures adopted by Lu et al. (175), which incorporated three neutral (MgO) ( = 4, 6, and 8) three-dimensional clusters containing Of , O4J, Mgsc, and centers as reactive sites for the... 

For smaller clusters the primary route for dissociation is the heterogeneous dissociation of the molecule. Just as for isolated collisions in the gas-phase, the probability of dissociation of a diatomic molecule upon impact at the surface is enhanced by its vibrational excitation. As the small cluster impacts the surface, it can be that the halogen molecule reaches the surface immediately, so that it still has the same velocity as that of the cluster center of mass. If that velocity is above the threshold, it will dissociate with about a 40% probability, just as a bare molecule would. The other process that can happen is that the halogen molecule collides first with a cluster atom, an atom that has already hit the surface, and therefore lost a fraction of its translational energy to the surface. Such a collision does two things. It slows the halogen molecule and so reduces its probability to dissociate at the surface. At the same time, at the supersonic velocities of... 

We will attempt an analysis of the possible misconceptions. To begin with, let us consider only the heterogeneity, independent of whether the surface is bare or chemically modified. The topography of the adsorption centers is usually described as patch-wise, random and intermediate. It seems that the surfaces employed in the radiochemical studies have been almost solely of the random type. In the strict sense of the term, such a surface consists of sites with uncorrelated values of the adsorption energy. However, it seems that the potential barrier between two adjacent adsorption sites cannot be of completely random height it must be somewhat correlated with the depths of the partner wells. As to the adsorbate, we suppose that each molecular entity takes only one site and that the particles occupying adjacent sites do not interact with each other. 

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