Semiconductor catalysts supporting

T. Ioannides, and X.E. Verykios, Charge transfer in metal catalysts supported on Doped Ti02 A Theoretical approach based on metal-semiconductor contact theory, J. Catal. 161,560-569 (1996). 

Among the various types of composite systems, that of the metal-support ranks as one of the most important, because of its crucial role in catalysis. The situation under consideration is that of chemisorption on a thin metal him (the catalyst), which sits on the surface of a semiconductor (the support). The fundamental question concerns the thickness of the film needed to accurately mimic the chemisorption properties of the bulk metal, because metallization of inexpensive semiconductor materials provides a means of fabricating catalysts economically, even from such precious metals as Pt, Au and Ag. 

Ueno A, N. Kakuta N, Park KH, Einlayson ME, Bard AJ, Campion A, Fox MA, Webber SE, White JM (1985) Silica supported ZnS-CdS mixed semiconductor catalysts for photogeneration of hydrogen. J Phys Chem 89 3828-3833... 

The philosophy of membrane-mimetic chemistry may be illustrated by a comparison of plant photosynthesis with sacrificial water photoreduction in artificial systems the process has been mediated by metal-catalyst-coated semiconductor colloids supported on polymerized vesicles (Fig. 4a) [59-64]. 

The low reproducibility of certain of the systems involving heterogeneous catalysts, described above, as well as the potential for charge separation using semiconductor catalysts has led to the use of heterogeneous catalysts supported on semiconducting materials, e.g. Ti02. 

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. 

The inverse case, a semiconducting catalyst supported by a metal, termed inverse supported catalyst, has been studied systematically only in the last few years. Here, even more drastic effects can be expected because normally the number of free electrons in a metal is several orders of magnitude higher than in semiconductors. The effects are indeed considerably larger as will be shown below. However, the principles and the theory involved are more complex (6-8). 

An important consideration for the electronics of semiconductor/metal supported catalysts is that the work function of metals as a rule is smaller than that of semiconductors. As a consequence, before contact the Fermi level in the metal is higher than that in the semiconductor. After contact electrons pass from the metal to the semiconductor, and the semiconductor s bands are bent downward in a thin boundary layer, the space charge region. In this region the conduction band approaches the Fermi level this situation tends to favor acceptor reactions and slow down donor reactions. This concept can be tested by two methods. One is the variation of the thickness of a catalyst layer. Since the bands are bent only within a boundary layer of perhaps 10-5 to 10 6 cm in width, a variation of the catalyst layer thickness or particle size should result in variations of the activation energy and the rate of the catalyzed reaction. A second test consists in a variation of the work function of the metallic support, which is easily possible by preparing homogeneous alloys with additive metals that are either electron-rich or electron-poor relative to the main support metal. 

In a few cases (70) catalysts have been studied that consist of mixtures of two semiconductors of the same chemical composition but with different levels of doping. The considerations here are analogous to those presented above. Of course, the ultimate proof as to whether electronic factors are indeed responsible for the catalytic effects discussed above will have to come through physical measurements of the electronic properties of catalyst/support systems. 

Room Temperature Oxidations, Isotopic Exchanges, and Dehydrogenations over Illuminated Neat or Metal-Supporting Semiconductor Catalysts... 

Tin (IV) oxide, Sn02, (rutile-type structure), a well-established n-type semiconductor with a wide band gap ( gap = 3.6 eV at 300 K) also has potential applications as a catalyst support,as transparent conducting electrodes,and as a gas sensor.i This material possesses many advantages, such as (i) high thermodynamic stability in air (at least up to 500 °C), (ii) low cost, and (iii) the possibility of the introduction of catalysts or dopants to enhance the sensitivity or selectivity. ... 

Tungsten-based materials as n-type semiconductor with nonstoichiometric compositions are extremely stable under electrochemical oxidation conditions and could be used as a non-carbon support for catalyst. The interest in their use as catalyst support is due to the possible synergetic effect between metal catalyst and support. 

Carbon nanotubes. Carbon nanotubes have the nature of metallic or semiconductor due to its curvature like graphite layer and fiber structure. In addition, the geometric structure of carbon nanotube is beneficial to the formation of ammonia and desorption of products. The active components and promoter can be distributed on them very well due to the large surface, which is beneficial to the activation of N2 and H2 and the electron transfers. Therefore, some researchers have investigated the ruthenium catalysts supported on the carbon nanotubes. Due to its high price, it now only has the theoretical value, probably without any industrialization prospect. 

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