Catalytic activity of semiconductors

Korsunovsky (65-68), Gross weiner (77), Stephens and co-workers (69) and also Markham and Laidler (70) point out that the catalytic activity of semiconductor catalysts in relation to the reaction of oxidation of water under illumination with light from the fundamental absorption band first increases with increasing radiation dose and then attains saturation at sufficiently high doses. 

For the same reason, there are no grounds for seeking a relation between the catalytic activity of semiconductors and their conductivity type (n- or p-type conductivity) when dealing with semiconductors of different chemical nature. 

This result, obtained for the case L specific catalytic activity of semiconductors with an increase in the degree of dispersion, so frequently mentioned in the literature. The effect should evidently become important the earlier (i.e., the less the degree of dispersion) the greater the screening length I in the specimen. One could expect to observe it at S/V > 10 cm. i, i.e., when the surface area is of the order of tens of square meters per gram. 

In conclusion, over 130 semiconductors are known to catalyze the photochemical water-splitting reaction according to eq 1 or either water oxidation or reduction in the presence of sacrificial agents. Even though the principle activitycontrolling factors in semiconductor-heterostructures have been identified, many aspects of the function of inorganic photocatalysts are still unclear. Most importantly, the molecular mechanism of water reduction and oxidation on the semiconductor surface has not yet been elucidated in sufficient detail. ° Many questions about charge transfer between semiconductor and cocatalysts, and its dependence on the structural and electronic features of the interface are still open. The effect of variable material preparations and surface impurities on the catalytic activity of semiconductors (e.g. sulfur and oxide on... 

The catalytic activity of PCSs results from their semiconductor properties. The first studies in this field date from 1959—1961. Thus, we have demonstrated catalytic activity of products of the thermal transformation of PAN in the decomposition reactions of hydrogen peroxide, hydrazine hydrate, and formic acid270, 271. There is an indication of catalytic activity of poly(aminoquinone) in the reactions of the hydrogen peroxide decomposition272. ... 

A number of works have been devoted to the effect of preadsorbed foreign gases on the catalytic activity of a semiconductor in relation to the hydrogen-deuterium exchange reaction. 

VIII. Factors Affecting the Adsorptivity and Catalytic Activity of a Semiconductor 241... 

The catalytic activity of the semiconductor is determined by the position of the Fermi level on the surface of the crystal ,+. Here, as we have seen (Sec. V,B), it is necessary to differentiate between two classes of reactions those which are accelerated and those which are retarded as the Fermi level rises (i.e., as e,+ increases see Fig. 22). We have called these reactions n-type and p-type reactions, respectively. 

The existence of a correlation between the catalytic activity and the electrical conductivity which follows from the theory was indicated by us back in 1950 (37, 6S), when there were as yet no measurements available that could either corroborate or refute this theoretical prediction. To date we have already a whole series of experimental work in which such a correlation has been observed (e.g., 36, 56, 66-70). A number of authors have measured the electrical conductivity and the catalytic activity of various samples of a semiconductor which differed in the method of preparation and have discovered that these two properties of the semiconductor vary in the same or in opposite directions from one sample to another. The results of some of these experiments are presented in Table II. 

The fifth consequence of the theory is that the adsorptivity and catalytic activity of a semiconductor are affected by illumination. When a crystal absorbs light waves of photoelectrically active frequencies (i.e., frequencies exciting the internal photoeffect), this leads, generally speaking, to a change... 

In conclusion we stress once more that the above-considered mechanism of the effect of illumination on the adsorptivity and catalytic activity of a semiconductor holds in the case when the absorption of light increases the number of free electrons or holes (or both) in the crystal. This, however, does not always take place. The absorption of light by the crystal may proceed by an exciton mechanism. This seems to be the case in the region of intrinsic absorption, which is as a rule photoelectrically inactive. 

This effect predicted by the theory (effect of an external electric field on the adsorptivity and catalytic activity of a semiconductor) has not been... 

The position of the Fermi level determines, all other conditions being equal, the adsorptivity of the surface for each species of particles (Sec. IV,B) and the catalytic activity of the semiconductor for the given reaction (Sec. V,B). 

Correlations between catalytic activity and a variety of bulk properties of semiconductors have been reported (i) the average band gap of III-V and II-VI semiconductors and activity towards hydrogenation of isopropanol (ii) enthalpy of oxides and their activity towards oxidation of propylene and (iii) number of d-electrons (and crystal field stabilization energy) or 3rf-metal oxides and their activity towards N2O decomposition. The last correlation, due to Dowden (1972), is important since it provides a connection between heterogeneous catalysis and coordination chemistry of transition-metal compounds. A correlation between the catalytic activity of transition-metal sulphides towards hydrodesulphurization of aromatic compounds and the position of the transition metal in the periodic table has been made by Whittingham ... 

From a similar viewpoint, Clark (93) attempts to give an interpretation of the catalytic activity of oxides of the transition metals using the simplified band model of semiconductors. The important mechanism of the electron transfer in a space-charge boundary layer is not discussed in Clark s publication. 

NiO is known to work as a good co-catalyst for water photocleavage by semiconductor photocatalysts in an aqueous suspension.18) The catalytic activity of NiOx is very low for the reaction of H2 with 02, while it appears to produce efficiently H2 from protons. NiO/ri02 was used for water photolysis in NaOH solution under the same conditions as shown in Fig. 13.6. H2 and 02 are produced in a stoichiometric ratio even when the amount of NaOH solution is relatively large.15) The yield increases with decreasing amount of NaOH solution and maximized at ca. 0.12 ml quite similarly to the case of Pt/Ti02.15 Since the reverse reaction is very slow on NiOx/Ti02, this result indicates that some factors other than the reverse reaction suppress the yield of the water photolysis in suspension. 

The hypothesis can be tested if the catalytic activity of a metal can be modified by a controlled shift of the Fermi level of the support. With semiconducting supports such a shift is readily achieved by doping additions of cations of higher charge than that of the matrix cations produces quasi-free electrons and/or removes defect electrons and raises the Fermi level addition of lower charged cations has the opposite effect. This calls for investigation of metal catalysts on doped semiconductors as supports. 

Several excellent reviews have been written recently about the catalytic activity of organic semiconductors 1<2 >. These and also older review articles 3 4> emphasize one particular property of organic materials that have an extended system of conjugated double bonds, namely their semiconductivity. 

The catalytic activity of these simple oxides has been correlated [38,39] with its ability to chemisorb simple molecules such as CO and N20 via an electron transfer, which results in a change in the electron transport properties in the semiconductor solid oxide. In this sense, the catalytic activity of simple oxides has been correlated with the band gap, and the number of d-electrons for 3d metal oxides [22],... 

THE RELATIONSHIP BETWEEN infrared spectra of chemisorbed carbon monoxide and the catalytic activity of metais for the methanization reaction is discussed in conjunction with experiments dealing with the effect of dissolved hydrogen on the catalytic activity of nickel. The purpose of this discussion is to illustrate the type of reasoning involved in seeking a relationship between spectra of chemisorbed molecules and catalytic activity. The underlying concepts of this relationship are extended to include recent advances made in studies of the effect of the semiconductor properties of the carrier on the activity of sup -ported metal catalysts. 

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