Electron donor adsorption properties

Hence, in both, in bimolecular and in unimolecular heterogeneous photoredox reactions the rate of photoproduct formation depends on the surface concentration of an electron donor or acceptor and on the adsorption properties of the adsorbing species. 

Namely, when electron-transfer adsorbates such as the electron-acceptor tetra-cyanoethylene (TCNE) and electron-donor tetrathiafulvalene (TTF) molecules interact with the inorganic framework, the energy gap of the mesoporous NU-Ge-1 (1.87 eV) is red-shifted to 1.71 and 1.64 eV, respectively. Indeed, this change in electronic structure is reversible and the optical adsorption onsets going to 1.83 eV upon formation of the inactive TTF-TCNE complex inside the pores. Incorporation of molecules without electron-acceptor or electron-donor properties such as anthracene did not affect the electronic structure of NU-Ge-1. 

The first two terms represent van der Waals interactions between the adsorbed SOC and the surface, which would apply to all SOC. The second two terms represent Lewis acid-base interactions, which can be important for compounds containing O, N, or aromatic rings, for example, the adsorption of alkyl ethers on the polar surface of quartz. The y coefficients (in mJ m 2) describe the surface properties, where yvdw is associated with its van der Waals interactions with adsorbing gases, y describes its electron-acceptor interactions, and y describes the electron-donor interactions of the surface. On the other hand, the properties of the adsorbing species are described by In pL for the van der Waals interactions and by the dimensionless parameters ft and which relate to the electron-donor and electron-acceptor properties (if any), respectively, of the adsorbing molecule. 

Competitive reactions and essentially competitive hydrogenations were often used to discuss the extent of the electronic transfers induced by poison adsorption. For instance, two model molecules with different electronic densities are chosen, e.g., benzene and toluene. In this case the electronic donor properties of the methyl group increase the electronic density on the insaturated bonds. During competitive hydrogenation of benzene and toluene, sulfur adsorption poisons the two reactions but is less toxic for toluene than for benzene hydrogenation (94-96). Sulfur, by its adsorption as an electron acceptor, is able to decrease the electronic density of the unpoisoned metallic surface area and could favor the adsorption of the reactant with the highest donor properties enhancing the hydro-... 

The electronic properties of Pt particles on different supports and in zeolites of different proton concentrations were also probed with the competitive hydrogenation of toluene and benzene (276). It was found that the ratio of the adsorption coefficients of toluene and benzene hr/6b, which can be obtained from a kinetic analysis of the hydrogenation rate data, can be used as a convenient empirical index for the electronic environment of Pt particles. For Pt in zeolite Y, the ratio was found to increase with increasing acidity, as does the electron deficiency. This trend was rationalized by considering that toluene is a stronger electron donor than benzene. 

It has been suggested" that, for MgO heated at temperatures up to 523 K, the so called free OH groups are able to transfer one electron (OH groups type-A according to Horlock and Anderson ). For species heated at temperatures above 973 K the 0 anions have the same ability." Through successive adsorption of Hammett indicators and indicators having one-electron donor properties it was possible to establish that 0 " anions cannot simultaneously serve as one-electron and electron-pair sources. This phenomenon is completely different from that of alumina," and has so far not been explained, although it is very important from the catalytic point of view. 

Adsorptions on Pd and Pt-Y zeolites of tetracyano-ethylene and trinitrobenzene (indicative of electron donor property of zeolites) and perylene and anthracene (indicative of electron acceptor properties of zeolites) were then studied by ESR. Both radical cation and anion concentrations were seen to rise sharply with Pt content for Na-Y zeolite. 

The layer is thought to be doped with interstitial silver atoms which act as electron donors and endow the layer with n-type semiconducting properties. The work function of such a catalyst falls progressively, up to 7 mol % addition of barium. The authors have argued that this effect will favour the adsorption of the more strongly polarized oxygen species, i.e. atomic rather than molecular oxygen, which results in the observed loss of selectivity and increase in activity. 

Thus the thermodynamic characteristics of adsorption at small coverage of different classes organic compounds determined by gas chromatography show that surface of ful-lerene molecular crystals and surface of graphitized carbon black have essentially different adsorption properties. On adsorption on fullerene crystals the electron-acceptor and electron-donor properties of fullerene molecules are manifested. Adsorption data on fullerenes Ceo nd C70 show that properties of fullerene Ceo a-nd C70 molecules arranged in surface layer of crystals are different. 

Davydov et al. [46] used IGC to determine several adsorption thermodynamic properties (equilibrium constants and adsorption heats) for the adsorption of organic compounds on C q crystals, and compared them with those obtained for graphitized carbon black. The adsorption potential of the surface of fiillerene crystals was much lower than that of a carbon black surface. The dispersive interaction of organic molecules with C q is much weaker than with carbon black. The adsorption equilibrium constant for alkanes and aromatic compounds is therefore lower in the case of fullerenes. Aliphatic and aromatic alcohols as well as electron-donor compounds such as ketones, nitriles and amines were adsorbed more efficiently on the surface of fiillerene crystals. This was taken as proof that fiillerene molecules have electron-donor and electron-acceptor properties, which is in agreement with the results of Abraham et al. [44]... 

The activity of an adsorbed electron donor for the interfacial electron transfer depends on both its ability to be oxidized by the photogenerated hole (Fig. 1 ) j and its adsorption properties. The first is a thermodynamic, the second a kinetic requirement. Kinetic experiments performed at various concentrations of the electron donor can give information on its adsorption properties. Different authors have reported, that the rate of the photocatalytic oxidation of an electron donor at the Ti02 surface varied as a function of the dissolved concentration of the electron donor according to a Langmuir-type isotherm (l ig. 

Surface acidity and basicity were measured by adsorption of organic bases such as pyridine (PY, pk,=5.3), morpholine (MP, pk, = 8.33), piperidine (PP, pk, = 11.1) and acidic substrates like acrylic acid (AA, pk = 4.2), phenol (PH, pk, = 9.9), respectively by spectrophotometric method[15,16]. Redox properties (one electron donor and one electron acceptor) were determined by the same method using meta dinitrobenzene (DNB, electron affinity, EA = 2.21eV) and phenothiazine (PNTZ, ionisation energy, IE = 7.13eV) as the adsorbates. 

In the foregoing examples the spectral data indicated a Lewis acid-base reaction on the surface where the alkali and alkaline earth cations acted as the electron acceptors while the adsorbates were the electron donors. It is quite natural that the reverse situation might be possible that is, the adsorbent be basic while the adsorbate show acidic properties so that in the chemisorption electron transfer will occur in the reverse direction. Several examples of such adsorption have already been discussed in this chapter. Kortiim (22) found another example in the adsorption of symmetrical trinitrobenzene on magnesia and on alumina. Whereas trinitrobenzene adsorbed on calcium fluoride or silica was colorless, on magnesia it was red with an absorption maximum at 4650 A (Fig. 26) and the spectrum of the adsorbed species was very... 

Another article concerning liquid-phase reactions catalyzed by perovskites is that by Sugunan and Meera (1995). They studied the reduction of ketones and oxidation of alcohol using RBO3 (R = La, Pr or Sr, B = Cr, Mn, Co or Ni) perovskites. Their goal was, however, to correlate data from these test reactions with surface electron-donor properties of these oxides. The electron-donor properties were investigated by the adsorption of electron acceptors with different electron affinities such as para- and /n-dinitrobenzene, benzoquinone, etc. They adsorbed these electron acceptors on both the mixed and the individual oxides. The results obtained are not conclusive to explain the catalytic behavior of the solids studied on the basis of this single property, as is often the case in many catalytic systems. 

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