Photoinduced reactivity of titanium dioxide

The interest in heterogeneous photocatalysis is intense and increasing, as shown by the number of publications on this theme which regularly appear in this journal, and the fact that over 2000 papers have been published on this topic since 1981. This article is an overview of the field of semiconductor photocatalysis : a brief examination of its roots, achievements and possible future. The semiconductor titanium dioxide (TiO 2 ) features predominantly in past and present work on semiconductor photocatalysis; as a result, in the most of the examples selected in this overview to illustrate various points the semiconductor is TiO 2 .

An overview of semiconductor photocatalysis

Sol-gel chemistry of transition metal oxides

In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

Aluminas have been used extensively as adsorbenu and active catalysrs and catalyst supponsm the pas. Already in 1197 the aluminadyzed dehydration of ettllnoi was dtscavered by Dutch chermsts: and S;rbatier [3] remewed the use of dumlnas as active cazaiysrs far vanous reacttons UI the second decade of thu century. She that time the applicazions of aluuuas m dycic pmcesses have mcreased tremendously. In tndustrral cualytic pmcesses, alumuus are mostiy used as catalyst suppons [4]. Oxides a d mued oxides ap well as tracuuion mauls and noble meare supported oa alumma. Thuscb. romaa-elumana catalysts are ktng used for the conversion of parafdns to olailnrc hydrocarbons, 10 hydrodealkplation of aromatics. and to a lesser exzm in catalyzic reforming. The larter process LS also caralyzed by molybdena-alumina, a cavlyst system whid is also active for malang toluene and ocher aromatics from satwed hydrocarc bons. It also dyzes the Isomerhation of pm. Great efions are presently be-made to nudy the surface c...

Catalytic Aluminas: Surface Models and Characterization of Surface Sites

Inorganic oxide aerogels

Changes in the surface structure and composition of an iron catalyst of reduced or unreduced Fe2O3 during the reaction of carbon monoxide and hydrogen

On pure ZrO2 and on ZnO/ZrO2 aerogels carbon monoxide interacts with the surface hydroxyl groups to form surface formate species. This species is relatively stable on pure ZrO2 where it is decomposed at increasing temperatures either resulting in regeneration of carbon monoxide and of surface OH's or in carbon dioxide and hydrogen. Only this second path is observed on ZnO/ZrO2 aerogel and it is paralleled by the formation on the surface of carbonate type species like unidented carbonate, carboxylate and ionic carbonate. This difference in the fate of the formate species onto the previous catalysts may bear some connection with the controversial problem of the pivotal species in the methanol synthesis, the formate of the carbonate type species.

Intermediate species on zirconia supported methanol aerogel catalysts: II. Adsorption of carbon monoxide on pure zirconia and on zirconia containing zinc oxide

Less methanol is being adsorbed at 25°C, and decomposed during temperature-programmed desorption (TPD) on copper-containing catalysts CuO/ZrO2 and CuO/ZnO/ZrO2 than on the copperless catalysts ZrO2 and ZnO/ZrO2. On this last class of solids the concentration of hydroxyl groups decreases by adsorption of methanol whereas the reverse is observed on copper-containing catalysts, where also lower temperatures are required for the transformation of the methoxy groups into more oxidized species such as formate and carbonates. The influence of copper on these processes is explained by the spillover of hydrogen, through copper ions, from the methoxy groups to oxygen ions of zirconia.

Intermediate species on zirconia supported methanol aerogel catalysts V. Adsorption of methanol

Divided metal oxides (Al 2 O 3 , Fe 2 O 3 , GeO 2 , SiO 2 , TiO 2 , V 2 O 5 , ZrO 2 ) are prepared by carrying vapor of metal chloride into the flame of an oxygen-hydrogen burner. Collected oxide particles do not have internal porosity and exhibit a nearly constant diameter in the range of 100–2000 A. The geometry of these particles and their crystalline structure depend on the flame temperature, the flow rate of carrier gas, and the linear velocity of the chloride vapor in the burner. Binary, ternary, or doped oxides may also be prepared by this method from the appropriate mixture of metal chlorides. Oxygen at 25°C is adsorbed on some oxides, in particular on TiO 2 , in the presence of uv radiation, as a labile species O 2 − which reacts with paraffins, olefins, CO, SO 2 , and NO, under irradiation, to give products of partial or total oxidation. The registered quantum yield is equal to one for wavelengths not exceeding 3400 A. The reactivity of O 2 − species toward isobutane is investigated by ESR for different wavelengths.

Preparation in a hydrogen-oxygen flame of ultrafine metal oxide particles. Oxidative properties toward hydrocarbons in the presence of ultraviolet radiation

Stanislas Jean Teichner

Papers

Inorganic oxide aerogels

Changes in the surface structure and composition of an iron catalyst of reduced or unreduced Fe2O3 during the reaction of carbon monoxide and hydrogen

Intermediate species on zirconia supported methanol aerogel catalysts: II. Adsorption of carbon monoxide on pure zirconia and on zirconia containing zinc oxide

Intermediate species on zirconia supported methanol aerogel catalysts V. Adsorption of methanol

Preparation in a hydrogen-oxygen flame of ultrafine metal oxide particles. Oxidative properties toward hydrocarbons in the presence of ultraviolet radiation