Photoadsorptive effect

The photoadsorption effect as such does not constitute the subject matter of the present article. We shall consider it very briefly, only to the extent necessary to allow one to draw analogies between the mechanisms of the photoadsorptive and photocatalytic effects. The photoadsorptive effect has been studied sufficiently well. A brief summary of the experimental data will be given below. The mechanism of the phenomenon has been thoroughly discussed in a number of theoretical works from the standpoint of the electronic theory of chemisorption and catalysis C3,4,6-8). 

These data are consistent with the results obtained by Barry (16) who investigated the influence of the preliminary treatment of a specimen of ZnO on the sign of the photoadsorption effect with respect to oxygen. The specimen was first calcined at a high temperature in oxygen and then cooled to room temperature, at which adsorption was subsequently carried out. The untreated specimens showed photodesorption, while on the samples treated by the procedure indicated (saturation with oxygen) there was observed photoadsorption. 

The same result has been obtained by Terenin and Solonitzin (17) the reduced ZnO specimens showed a negative, and the oxidized ones a positive photoadsorption effect with respect to oxygen. 

The photoadsorptive effect is here characterized by the quantity 4 which is the relative change of the adsorption capacity of a surface caused by illumination ... 

If illumination enhances the adsorption capacity of the surface (i.e., N > No), the photoadsorption effect is positive (4> > 0) if, conversely, it causes a fall in the adsorption capacity (N < No), then the negative photoadsorption effect is observed ( < 0) if, finally, N = No, the absorption of light in this case is photoadsorptionally inactive (4> = 0). 

Let us calculate the value of the photoadsorption effect 4>. For this purpose, we determine N and No. Consider the case of steady-state adsorption equilibrium on a homogeneous surface. In this case (under the assumption that the adsorption is not accompanied by dissociation), we have aP(N - N) = b°N° exp (—

[Pg.173]

It is seen that the sign and absolute magnitude of the photoadsorption effect depend on the position of the Fermi level at the surface and in the bulk of the unilluminated specimen. 

Photoabsorption cross section, 34 213 Photoactivation, lattice oxygen, 31 123 Photoadsorption, 26 360 of oxygen, 32 106 Photoadsorptive effect... 

Thus, we have determined ij°, i), and i>+ [see formulas (33) ] and also expressed these quantities in terms of ij0°, m, and ij0+ [see formulas (12)]. This is quite sufficient, as will be seen later, to determine the magnitude and sign of the photoadsorption and photocatalytic effects. 

Romero-Rossi and Stone (11, 12) have studied the adsorption of 02 on ZnO. They observed, at room temperatures and low pressures of oxygen, the positive effect (photoadsorption) decreased with increasing pressure and was replaced at fairly high pressures by the negative effect (photodesorption). 

The data of Haber and Kowalska (20) are in agreement with the results obtained by Romero-Rossi and Stone. Using the same system (02 on ZnO), they found that the positive effect (photoadsorption) was replaced by the negative one (photodesorption) after the specimen had been oxidized. 

The effect of illumination on the adsorptivity of the surface has been observed by a number of authors and studied in detail on various adsorbents for different adsorbates and in different frequency intervals (e.g., 76-84 a review of the experimental work is given in Sf). In some cases photodesorption was observed, in other cases, on the contrary, photoadsorption. Why one or another of these two opposite effects takes place has not yet been experimentally elucidated. This remains a problem for a further experimental investigation. 

Photocatalytic decomposition of water on semiconductors is usually conducted under reduced pressure. There are arguments that 02 production is much less than stoichiometric when water photolysis is carried out under atmospheric pressure.20-34,353 Such nonstoichiometric 02 evolution is often ascribed to the photoadsorption of 02 onto semiconductor particles20,34,353 or the formation of peroxides.363 On the other hand, the electrochemical potential of H2 evolution shifts to the positive direction with increasing ambient pressure according to the Nemst equation. Therefore, pressure effect may be negatively significant for water photolysis by Ti02 photocatalysts, since tne flat band potentials of TiO are close to the potential of NHE.23,243... 

These measurements cannot be used to quantify the electron transfer from the semiconductor to the metal deposit, but an estimate has been drawn from studies of oxygen photoadsorption on Pt/Ti(>2 samples in a pressure range such that nearly all of the free electrons are captured to form adsorbed 05 ion-radicals. Increasing Pt contents corresponded to decreasing amounts of photoadsorbed oxygen, which corroborates the effect of deposited Pt on the Ti(>2 free electron density. For Ti(>2 samples evacuated at 423 K and... 

The second important effect is that irradiation absorption generates active states of the photoadsorption centers with trapped electrons and holes. By definition (Serpone and Emeline, 2002) the photoadsorption center is a surface site which reaches an active state after photoexcitation and then it is able to form photoadsorbed species by chemical interaction with substrate (molecules, or atoms, or ions) at solid/fluid interface. In turn, the active state of a surface photoadsorption center is an electronically excited surface center, i.e. surface defect with trapped photogenerated charge carrier that interacts with atoms, molecules or ions at the solid/gas or solidfiquid interfaces with formation of chemisorbed species. ... 

This complexity determines that investigations on heterogeneous photo-catalytic processes sometimes report information only on dark adsorption and use this information for discussing the results obtained under irradiation. This extrapolation is not adequate as the characteristics of photocatalyst surface change under irradiation and, moreover, active photoadsorption centers are generated. Nowadays very effective methods allow a soimd characterization of bulk properties of catalysts, and powerful spectroscopies give valuable information on surface properties. Unfortunately information on the photoadsorption extent under real reaction conditions, that is, at the same operative conditions at which the photoreactivity tests are performed, are not available. For the cases in which photoreaction events only occur on the catalyst surface, a critical step to affect the effectiveness of the transformation of a given compound is to understand the adsorption process of that compound on the catalyst surface. The study of the adsorbability of the substrate allows one to predict the mechanism and kinetics that promote its photoreaction and also to correctly compare the performance of different photocatalytic systems. 

The effect of catalyst amount on photoadsorption capacity is shown in Figure 9. This figure reports the benzyl alcohol moles photoadsorbed per unit mass of catalyst vs. the catalyst amount the reported data refer to runs carried out at equal initial benzyl alcohol concentration and lamp power. From the observation of data of Figure 9, a decrease of specific photoadsorption capacity by increasing the catalyst amount, differently from that expected on thermodynamic basis for which an increase of catalyst amount determines a corresponding increase of adsorbed substrate, may be noted. 

Bickley, R.I. Photoadsorption and Photodesorption at the Gas-Solid Interface Part 11 Photo-electronic Effects Relating to Photochromic Changes and to Photosorption in M. SchiaveUo (Ed.), Photocatalysis and Environment Trends and AppUcations . Kluwer, Dordrecht (1988b), pp. 233-239. 

The effect of ultraviolet and visible radiation on carbon monoxide adsorption on zinc oxide has also been studied (118). Both photoadsorption and photodesorption have been found, depending on the temperature, and there is a quite different pattern of behavior if oxygen has been preadsorbed. Reaction to give oxidized complexes, presumably of the COa,., or... 

A non-photocatalytic reaction occurring on the surface of an irradiated wide bandgap metal oxide such as ZrOa can also affect the process of photoinduced formation of Zr F- and V-type colour centres. The effect of such reactions is seen as the influence of photostimulated adsorption on the photocolouration of the metal-oxide specimen. Photoadsorption of electron donor molecules leads to an increase of electron colour centres, whereas photoadsorption of electron acceptor molecules leads to an increase of hole colour centres. Monitoring the photocolouration of a metal-oxide specimen by DRS spectroscopy during surface photochemical reactions can provide a further opportunity to evaluate whether the reactions are photocatalytic. 

---