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Journal ArticleDOI

Gas phase synthesis of titania with aerogel character and its application as a support in oxidation catalysis

02 Nov 2010-Journal of Materials Chemistry (The Royal Society of Chemistry)-Vol. 20, Iss: 44, pp 10032-10040
TL;DR: In this paper, the aerogel-like TiO2 materials discussed here are prepared exclusively by a high-temperature aerosol process and the titania particles forming the network are highly pure, highly crystalline and their surfaces are free of any additives.
Abstract: Titania materials with structures resembling aerogels are presented in the current contribution. In contrast to the classical routes the aerogel-like TiO2 materials discussed here are prepared exclusively by a high-temperature aerosol process. Thus, the titania particles forming the network are highly pure, highly crystalline and their surfaces are free of any additives. It is shown that the structure of the materials (particle-size, porosity, surface area, crystallinity, and phase composition) depends on a complex interplay of different parameters (temperature gradients, precursor concentration, residence time, etc.). However, the understanding of those correlations enables the preparation of tailor made titania materials with aerogel-like structure. The size of the primary, nanocrystalline particles can be varied systematically from 13 nm to 70 nm. The composition regarding the two different crystalline phases anatase and rutile can be adjusted as an independent parameter. Finally, the aerogel-like TiO2 materials were loaded with Au clusters and the activity in oxidation catalysis was investigated.
Citations
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Journal ArticleDOI
TL;DR: The present work critically reviews the formation of crystalline nanoscale titania particles via solution-based approaches without thermal treatment, with special focus on the resulting polymorphs, crystal morphology, surface area, and particle dimensions.
Abstract: Titanium dioxide is one of the most intensely studied oxides due to its interesting electrochemical and photocatalytic properties and it is widely applied, for example in photocatalysis, electrochemical energy storage, in white pigments, as support in catalysis, etc. Common synthesis methods of titanium dioxide typically require a high temperature step to crystallize the amorphous material into one of the polymorphs of titania, e.g. anatase, brookite and rutile, thus resulting in larger particles and mostly non-porous materials. Only recently, low temperature solution-based protocols gave access to crystalline titania with higher degree of control over the formed polymorph and its intra- or interparticle porosity. The present work critically reviews the formation of crystalline nanoscale titania particles via solution-based approaches without thermal treatment, with special focus on the resulting polymorphs, crystal morphology, surface area, and particle dimensions. Special emphasis is given to sol–gel processes via glycolated precursor molecules as well as the miniemulsion technique. The functional properties of these materials and the differences to chemically identical, non-porous materials are illustrated using heterogeneous catalysis and electrochemical energy storage (battery materials) as example.

389 citations

Journal ArticleDOI
TL;DR: The preparation of materials characterized by three types of porosity could be prepared by a continuous chemical gas-phase method and show superior properties as gas sensors in comparison to materials constructed from compact nanoparticles.
Abstract: The preparation of materials characterized by three types of porosity could be prepared by a continuous chemical gas-phase method. The multistep formation mechanism involves a critical temperature gradient and occurs within seconds. The resulting hollow aerogel materials show superior properties as gas sensors in comparison to materials constructed from compact nanoparticles.

47 citations

Journal ArticleDOI
TL;DR: In this article, a gas phase synthesis method is employed for aerogel-like zinc oxide materials with a defined content of aluminum (n-doping), which were then used for the assembly of gas sensors.
Abstract: Atmospheric contamination with organic compounds is undesired in industry and in society because of odor nuisance or potential toxicity. Resistive gas sensors made of semiconducting metal oxides are effective in the detection of gases even at low concentration. Major drawbacks are low selectivity and missing sensitivity toward a targeted compound. Acetaldehyde is selected due to its high relevance in chemical industry and its toxic character. Considering the similarity between gas-sensing and heterogeneous catalysis (surface reactions, activity, selectivity), it is tempting to transfer concepts. A question of importance is how doping and the resulting change in electronic properties of a metal-oxide support with semiconducting properties alters reactivity of the surfaces and the functionality in gas-sensing and in heterogeneous catalysis. A gas-phase synthesis method is employed for aerogel-like zinc oxide materials with a defined content of aluminum (n-doping), which were then used for the assembly of gas sensors. It is shown that only Al-doped ZnO represents an effective sensor material that is sensitive down to very low concentrations (<350 ppb). The advance in properties relates to a catalytic effect for the doped semiconductor nanomaterial.

39 citations

Journal ArticleDOI
TL;DR: Research on homogeneous and heterogeneous catalysis is indeed convergent and finds subnanometric particles to be at the heart of catalytically active species and monodisperse gold clusters are deposited from the gas phase onto porous titania generating well-defined model systems.
Abstract: Research on homogeneous and heterogeneous catalysis is indeed convergent and finds subnanometric particles to be at the heart of catalytically active species. Here, monodisperse gold clusters are deposited from the gas phase onto porous titania generating well-defined model systems and the resulting composite materials exhibit a sharp size-dependency on the number of gold atoms per cluster and exceptionally high-turnovers toward the bromination of 1,4-dimethoxybenzene are observed. This indicates that the deliberate generation of active centres is of utmost importance for the creation of the most “gold-efficient” catalysts.

15 citations

Journal ArticleDOI
TL;DR: In this paper, a multiphase TiO2 aerogels were synthesized through a simple, environment friendly sol-gel route, where TiCl3 was used as nonalkoxide precursor with ethanol as solvent.
Abstract: Multiphase TiO2 aerogels were synthesized through a simple, environment friendly sol–gel route. TiCl3 was used as nonalkoxide precursor with ethanol as solvent. The formed alcogels were converted into aerogels by CO2 supercritical drying. The results revealed that the increase in TiCl3 concentration leads to the decay of the gelation time, possibly due to the availability of Ti+4 ions in the medium that led to the formation of the interconnected gel network more easily. The aerogels produced from 0.75 M TiCl3 and calcined at 450 °C exhibited the three main crystalline phases of titania (anatase, brookite, and rutile) and a specific surface area of 124 m2/g. Through transmission electron microscopy, it was possible to visualize the morphology of the nanostructured aerogels, comprised of a network of nanoparticles with sizes between 5 and 15 nm. The combination of high specific surface area, particle size in nanometer scale, and crystallinity of the material allowed greater photocatalytic reduction of 4-nitrophenol compared with commercial Degussa P-25.

12 citations

References
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Journal ArticleDOI
24 Oct 1991-Nature
TL;DR: In this article, the authors describe a photovoltaic cell, created from low-to medium-purity materials through low-cost processes, which exhibits a commercially realistic energy-conversion efficiency.
Abstract: THE large-scale use of photovoltaic devices for electricity generation is prohibitively expensive at present: generation from existing commercial devices costs about ten times more than conventional methods1. Here we describe a photovoltaic cell, created from low-to medium-purity materials through low-cost processes, which exhibits a commercially realistic energy-conversion efficiency. The device is based on a 10-µm-thick, optically transparent film of titanium dioxide particles a few nanometres in size, coated with a monolayer of a charge-transfer dye to sensitize the film for light harvesting. Because of the high surface area of the semiconductor film and the ideal spectral characteristics of the dye, the device harvests a high proportion of the incident solar energy flux (46%) and shows exceptionally high efficiencies for the conversion of incident photons to electrical current (more than 80%). The overall light-to-electric energy conversion yield is 7.1-7.9% in simulated solar light and 12% in diffuse daylight. The large current densities (greater than 12 mA cm-2) and exceptional stability (sustaining at least five million turnovers without decomposition), as well as the low cost, make practical applications feasible.

26,457 citations

Book
12 May 1990
TL;DR: Gel-Derived and Conventional Ceramics: as discussed by the authors Theoretical analysis of deformation and flow in gels and a comparison of gel-derived and conventional ceramics.
Abstract: Preface. Acknowledgments. Introduction. Hydrolysis and Condensation I: Nonsilicates. Hydrolysis and Condensation II: Silicates. Particulate Sols and Gels. Gelation. Aging of Gels. Theory of Deformation and Flow in Gels. Drying. Structural Evolution during Consolidation. Surface Chemistry and Chemical Modification. Sintering. Comparison of Gel-Derived and Conventional Ceramics. Film Formation. Applications. Index.

8,160 citations

Journal ArticleDOI
TL;DR: In this paper, the Gurevic and Judd formulas were derived from the Kubelka-Munk differential equations, and they are exact under the same conditions as in this paper, that is, when the material is perfectly dull and when the light, is perfectly diffused or if it is parallel and hits the specimen under an angle of 60° from normal.
Abstract: The system of differential equations of Kubelka-Munk, -di=-(S+K)idx+Sjdx, dj=-(S+K)jdx+Sidx(i, j⋯ intensities of the light traveling inside a plane-parallel light-scattering specimen towards its unilluminated and its illuminated surface; x⋯ distance from the unilluminated surface S, K⋯ constants), has been derived from a simplified model of traveling of light in the material. Now, without simplifying assumptions the following exact system is derived: -di=-12(S+K)uidx+12Svjdx,dj=-12(S+K)vjdx+12Suidx,u≡∫0π/2(∂i/i∂φ)(dφ/cosφ), v≡∫0π/2(∂j/j∂φ)(dφ/cosφ), φ≡angle from normal of the light). Both systems become identical when u=v=2, that is, for instance, when the material is perfectly dull and when the light, is perfectly diffused or if it is parallel and hits the specimen under an angle of 60° from normal. Consequently, the different formulas Kubelka-Munk got by integration of their differential equations are exact when these conditions are fulfilled. The Gurevic and Judd formulas, although derived in another way by their authors, may be got from the Kubelka-Munk differential equations too. Consequently, they are exact under the same conditions. The integrated equations may be adapted for practical use by introducing hyperbolic functions and the secondary constants a=12(1/R∞+R∞) and b=12(1/R∞-R∞), (R∞≡reflectivity). Reflectance R, for instance, is then represented by the formula R=1-Rg(a-b ctghbSX)a+b ctghbSX-Rg(Rg≡reflectance of the backing, X=thickness of the specimen) and transmittance T by the formula T=ba sinhbSX+b coshbSX.In many practical cases the exact formulas may be replaced by appropriated approximations.

2,322 citations

Journal ArticleDOI
TL;DR: Aerogels form a new class of solids showing sophisticated potentialities for a range of applications, and can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis.
Abstract: 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

1,773 citations

Book
01 Jan 1978
TL;DR: In this paper, the authors describe current industrial applications of organic chemistry and offer an excellent overview of industrial organic chemistry, covering the manufacture of the most important precursors and intermediates and their use in the development of commercial products.
Abstract: This book describes current industrial applications of organic chemistry and offers an excellent overview of industrial organic chemistry. It covers the manufacture of the most important precursors and intermediates and their use in the development of commercial products. It details chemical and physical data, aspects of energy and raw materials supply, and the production of chemicals in different countries. It also describes the possibilities for future developments in the manufacture of various chemicals.

1,529 citations