Surface density of sites

Information about the number of surface sites required for an oxidation reaction, or activation of the reactant molecule, can be obtained by examination of the variation of the TOF with surface vanadia coverage. In general, reactions requiring only one surface site will exhibit a TOF that is independent of the surface vanadia coverage (surface density of sites) and reactions requiring multiple surface sites will exhibit a TOF that increases with the surface vanadia coverage (surface density of sites). From such an analysis, the number of surface vanadia sites required for various oxidation reactions is presented in Table 3. 

As deduced from the TD studies reported in lisowski et at 78 K and % - 0.2 torr, hydrogen dissociation could only be observed on films grown at 78K (unsintered films), whereas on those annealed at T> 320Kit does not occur. TD techniques have also been used to investigate the adsorption of atomic hydrogen on sintered films at 78K In accordance with Lisowski et al, the total amount of hydrogen desorbed from the latter experiment is about 50 times larger than that determined from the former, which clearly indicates that the surface density of sites able to dissociate ffg molecules on the surface of an unsintered thin film is rather low. 

Oxidation of polyethylene by sulphuric acid and potassium chlorate [9,10] improves its adhesiveness. The free energy of adhesion of the polymer is found to increase linearly with the surface density of the hydrophyl-lic sites created by oxidation. 

The gas phase partition function Qg s of the atom is the same however, since the atoms are immediately immobilized on a two-dimensional surface, we need to take the configuration of the adsorbed atoms into account in the transition state. Again we consider a surface containing M sites each with an area of a. The density of sites per area is Nq = M/A = 1 /a. The M sites are not necessarily free as some could be occupied already hence, the number of free sites will be M and 0 = M /M = (1-0a)-If we have N atoms adsorbed on these sites (we again use for the transition state Airmobile), the partition function for this system is given by... 

Bioremediation of sites that are contaminated with toxic metals is an important issue in environmental restoration. Bacteria have long been known for their ability to Itake up metals from their immediate environment (Borrok and Fein 2004). The efficiency of bacterial cells in concentrating metals is related to their large surface area-to-volume ratio and high surface density of charge. The cell surfaces of all bacteria are negatively... 

If the fraction of surface that is covered with protons is smaller than 1 (%h 1), the surface density of singly, doubly, and triply protonated surface sites (B, C, and D, respectively (see Fig. 5.6)) can be described as probability functions of the surface protonation Cf. ... 

At this point, the problem has been reduced to solving the surface chemistry equations to calculate the E-function. It is convenient to introduce dimensionless notations by normalizing all surface concentrations to the total density of sites Ns -... 

A variation of this method was used to control the surface properties of aluminum oxide particles, particularly the surface density of Lewis acid sites. Instead of using a long-chain amine surfactant, the solution of aluminum alkoxide precursor was mixed with a small amine to convert the alkoxide dimer (or oligomer in general) into monomeric alkoxide-amine adduct. Controlled hydrolysis of this adduct produces an aluminum oxyhydroxide in which the surface A1 ions are coordinated to amine... 

The catallytic metallic Pd covers only a small fraction of the surface. The amount of Pd on the glass substrate (11) is 0.04 to 0.05 ixglcm . Assuming uniform distribution, this amount corresponds roughly to 0.3 of a monolayer of Pd on a glass substrate. The surface density of catalytic sites a depends on substrate material. For glass the maximum value of a was found to be 10 sites/cm. ... 

The TOFs for methanol oxidation to formaldehyde (95-99% selectivity), butane oxidation to maleic anhydride and C0/C02 (30% maleic anhydride selectivity) and S02 oxidation to SO3 are independent of surface vanadia coverage. This observation suggests that these oxidation reactions do not depend on the surface concentration of bridging V-O-V bonds since the reaction TOFs do not correlate with the surface density of bridging V-O-V bonds. Furthermore, the constant TOFs with surface vanadia coverage suggest that only one surface vanadia site is required for the activation of these molecules during the oxidation reactions. 

It is not a requirement that the number of sites of type n balance in a given reaction. If surface sites are not conserved, the density of sites T is not necessarily constant. The production rate r (mol/m2-s) for each surface phase is... 

General Crystal Surfaces. General surfaces possess a high density of sites where atoms from the vapor can be incorporated in the crystal and are therefore expected... 

Having identified the existence of separate oxidation sites, it is desirable to determine the densities of sites which contribute to the different selectivities among different catalysts. Since the products of the thermal desorption experiments described above are directly correlated with the active sites, it is possible to measure the number of active sites by measuring the quantities of desorbed products, provided that each and every active site produces only one molecule of reaction product. To achieve this condition, it must be established that the adsorbate is fully equilibrated with the surface, that there is no multiple reaction per site during equilibration, that there is no readsorption and reaction of desorbed products, and that all reaction products are quantitatively determined. 

Nearly all theories to date predict that IETS intensities should be proportional to n, the surface density of molecular scatterers. Langan and Hansma (21) used radioactively labeled chemicals to measure a surface concentration vs solution concentration curve ( Fig. 10 ) for benzoic acid on alumina using the liquid doping technique. The dashed line in Fig. 10 is a 2 parameter fit to the data using a simple statistical mechanical model by Cederberg and Kirtley (35). This model matched the free energy of the molecule on the surface with that in solution. The two parameters in this model were the surface density of binding sites ( 10" A )... 

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

Chiche et a/.[56] have studied the oligomerization of butene over a series of zeolite (HBeta and HZSM-5), amorphous silica alumina and mesoporous MTS-type aluminosilicates with different pores. The authors found that MTS catalyst converts selectively butenes into a mixture of branched dimers at 423 K and 1.5-2 MPa. Under the same reaction conditions, acid zeolites and amorphous silica alumina are practically inactive due to rapid deactivation caused by the accumulation of hydrocarbon residue on the catalyst surface blocking pores and active sites. The catalytic behaviour observed for the MTS catalyst was attributed to the low density of sites on their surface along with the absence of diffusional limitations due to an open porosity. This would result in a low concentration of reactive species on the surface with short residence times, and favour deprotonation and desorption of the octyl cations, thus preventing secondary reaction of the olefinic products. 

Finally, the (1120) face is characterized by the highest density of sites (10.88 Cr ions/100 A2). These Cr3+ sites are all fivefold coordinated and present in pairs 0.265 nm apart, and the distance between the pairs is 0.415 nm. Because the Cr3+ ions are greatly shielded by the surrounding oxygen ions (which are located at an upper level), this surface appears quite homopolar, and it is expected to be less reactive or even unreactive with probe molecules. 

As expected for silica-alumina as a mixed oxide (see also Section IV.B.5), the PyH+ and PyL species are observed simultaneously (160, 205,206,221-223). Two distinct types of Lewis acid sites could be detected (19b mode at 1456 and 1462 cm-1, respectively) on a specially prepared aluminum-on-silica catalyst (160). On water addition, the Lewis sites can be converted into Br nsted sites (160, 205, 221), The effect of Na+ ions on the acidity of silica-aluminas has been studied by Parry (205) and by Bourne et al. (160). It can be concluded from Parry s results that Na+ ions affect both types of acid sites, so that alkali poisoning does not seem to eliminate the Br nsted sites selectively. For quantitative determination of the surface density of Lewis and Br nsted acid sites by pyridine chemisorption, one requires the knowledge of at least the ratio of the extinction coefficients for characteristic infrared absorption bands of the PyH+ and PyL species. Attempts have been made to evaluate this ratio for the 19b mode, which occurs near 1450 cm-1 for the PyL species and near 1545 cm-1 for the PyH+ species (160,198,206,221,224,225). The most reliable value as calculated from the data given by Hughes and White (198) seems to be... 

In this paper, a two-dimensional model is proposed, in which the polyelectrolyte chains are treated as rigid cylinders normally grafted to each of the plates. On the surface of the cylinders, ionizable sites are uniformly distributed and the surface charge of the cylinder is generated through their dissociation. Using this two-dimensional model, the repulsive force between two plates is calculated as a function of electrolyte concentration, surface density of polyelectrolyte chains on the plate, thickness of the polyelectrolyte brushes, and bulk pH (denoted in what follows as pHo). 

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