Sol-gel preparation of pure and doped TiO₂ films for the photocatalytic oxidation of ethanol in air

Stable sols of TiO2 were synthesized by a non-aqueous sol-gel process using titanium (IV) isopropoxide as precursor. The microstructure, optical and morphological properties of the films obtained by spin-coating from the sol, and annealed at different temperatures, were investigated using scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy and ellipsometry. The crystalline structure of the films was characterized by X-ray diffraction and their photocatalytic activity was evaluated for the oxidation of ethanol in air. The influence of the calcination temperature, pre-heat treatment and the number of layers was studied. Simultaneous thermo-gravimetric and differential thermal analysis measurements were carried out to ascertain the thermal decomposition behavior of the precursors. In order to obtain a higher photoresponse in the visible region, a series of vanadium-, niobium- and tantalum-doped TiO2 catalysts was synthesized by the same sol-gel method. For V doping two different precursors, a vanadium alkoxide and V2O5, were used. The effect on the crystallization and photocatalytic activity of the doped TiO2 films was investigated. Furthermore, to identify the effective composition of the samples, they were characterized by X-ray photoelectron spectroscopy and the surface area of the powders was measured by N-2 adsorption. The 10 wt.% doped catalysts exhibit high photocatalytic activity under visible light and among them the best performance was obtained for the sample containing Ta as dopant. The crystallite sizes are closely related to the photocatalytic activity

Coating process of honeycomb cordierite support with Ni/boehmite gels

This study presents the development of a method for the washcoating of Ni/boehmite gels, prepared by sol-gel process, onto the surface of a commercial ceramic monolith. Indeed, cordierite monolith in honeycomb shape was used as substrate for Ni/Al2O3 deposition. An experimental assembly was made in order to apply the coating on the cordierite surface. Different suspensions were used with various viscosities and multiple coating parameters were tested as the withdrawal speed, or the number of impregnations. It was observed that the simple deposition of the Ni/boehmite gel led to the formation of coating. Different morphologies were observed, and defects were highlights as cracks, coating-free areas or aggregates. Among the various studied parameters, the pH of the sol appeared to play a role even more important than the viscosity. Indeed, the sol acidified with nitric acid showed a coating which was almost free of cracks or of large aggregates. Moreover, the use of a mix of a slurry of calcined alumina particles and of colloidal boehmite appeared also as an interesting path. The beneficial influence of a slurry was attributed to a better resistance of the coating against the stresses induced during drying, and a deviation of the cracks in the gels by slurry grains

Ni-doped γ-Al2O3 as secondary catalyst for bio-syngas purification: influence of Ni loading, catalyst preparation and gas composition on catalytic activity

peer reviewedIn this work, Ni/γ-Al2O3 catalysts were prepared by sol-gel methods with different Ni loadings (10 to 50 wt.%) and used as secondary catalyst for the steam reforming of toluene. A sample prepared by wet impregnation with 10 wt.% of Ni was also synthesized and compared to the corresponding sol-gel sample. This study was divided in three main parts: the comparison of catalysts prepared by sol-gel process and impregnation, the influence of the gas composition on the catalytic performance of the sol-gel 10 wt.% Ni/γ-Al2O3 catalyst and the influence of Ni loading on the catalytic activity. When sol-gel and impregnated samples are compared, the impregnated catalyst showed a high initial toluene conversion followed by a consequent and progressive deactivation. Contrarily, the sol-gel catalyst showed a stable catalytic activity and relatively low carbon deposit. Indeed, before the steam reforming of toluene at 650 °C, the sol-gel catalyst was only calcined and no reduction step was realized to reduce nickel oxide. So this sample was reduced during the catalytic test at 650 °C. Moreover, it was observed that, if toluene was withdrawn from the syngas mixture during the catalytic test, the sol-gel sample was progressively re-oxidized by CO2 and H2O, leading to higher deactivation. As the Ni loading increased, the nickel oxide with strong interactions (NiAl2O4) was progressively joined by nickel oxide with low interactions (NiO/Al2O3) and bulk nickel oxide (NiO). This led to a high initial conversion of toluene, but also to a progressive loss of the catalytic activity throughout the catalytic test. It was shown that the sol-gel method developed throughout this work allowed preparing micro/mesoporous Ni/γ-Al2O3 catalysts with a high dispersion of Ni nanoparticles

Sol-gel Ni/γ-Al2O3 material as secondary catalyst for toluene reforming: Tailoring the γ-Al2O3 substrate with stearic acid

In this work, Ni-based catalysts supported on alumina were studied for toluene reforming with a particular attention on the substrate texture. Indeed, the influence of stearic acid addition during the synthesis process of Ni/Al2O3 catalysts was study on the material texture and catalytic activity. All the samples were also characterized thanks to nitrogen adsorption-desorption isotherm measurements, mercury porosimetry and X-ray diffraction. It was shown that the step of addition of the surfactant during the synthesis process and the composition of the solvent medium (water/ethanol vs. pure ethanol) strongly modified the texture of the samples. Indeed, the sample prepared with addition of stearic acid before the precipitation step and inside a pure ethanol medium showed the highest increase of micro- and mesoporosity. The modification of the textural properties also proved to influence the size of the Ni particles, and thus the reducibility of the catalysts. The sample which showed the most interesting textural properties was tested for the reforming of 24.000 ppmv of toluene at 650 °C. Results showed that the conversion of toluene strongly increased when the samples were tailored but these changes also increased the catalyst sensitivity to carbon contamination

Synthesis, Surface characterization and photocatalytic activity of TiO2 supported on almond shell activated carbon

Three types of photocatalysts were synthesized by metal organic chemical vapor deposition and impregnation methods using the almond shell activated carbon as support. These photocatalysts denoted by (TiO2/ASAC (V), TiO2/ASAC (I1) and TiO2/ASAC (I2)) were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS) and nitrogen adsorptionedesorption isotherms. SEM observation shows that TiO2 was deposited on activated carbon surface. XRD results confirm that TiO2 existed in a mixture of anatase and rutile phases. The DRS spectra show the characteristic absorption edge of TiO2 at approximate 380 nm corresponding to the optical band gap of 3.26 eV. Besides, FTIR spectrum indicated the presence of (TieO) groups. The specific surface area of photocatalysts decreased drastically in comparison with the original activated carbon. The catalysts were very efficient for the photodegradation of total organic carbon (TOC) from industrial phosphoric acid solution under UV irradiation. The kinetics of photocatalytic TOC degradation was found to follow a pseudofirst- order model. The prepared TiO2/ASAC showed high photoactivity for the photodegradation of TOC in the following order: TiO2/ASAC (V) > TiO2/ASAC (I1) > TiO2/ASAC (I2) > ASAC > TiO2 (P25)

Synthesis of Ni/γ-Al2O3-SiO2 catalysts with different silicon precursors for the steam toluene reforming

This work investigated the intrinsic properties and performances of Ni/γ-Al2O3-SiO2 modified with different silicon precursors for the steam reforming of toluene in biomass gasifier exit conditions (8000 ppmv of toluene, 650 °C). Samples were synthesized by an aqueous sol-gel process in presence of aluminum nitrate, nickel nitrate and the use of either tetramethoxysilane (TMOS, Si-(OCH3)4) or tetraethoxysilane (TEOS, Si-(OC2H5)4) in order to understand the influence of the reactivity of the silicon precursor. Furthermore, the use of N-[3-(trimethoxysilyl)propyl]ethylenediamine (EDAS, (OCH3)3-Si-(CH2)3-NH-(CH2)2-NH2) is also investigated in order to understand the influence of a silicon precursor containing an ethylenediamine group, able to complex Ni2+ ions. By the chelation of Ni2+ ions by ethylenediamine groups during the synthesis, Ni/γ-Al2O3-SiO2 samples modified with EDAS showed higher dispersion of the metallic Ni nanoparticles and the higher resistance against the sintering of Ni particles. This was also attributed to the high microporous volume and the narrow mesoporous distribution of the support, which could also prevent the migration of the metallic Ni nanoparticles. The catalytic tests revealed that the reactivity of the silicon precursor played a major role on the conversion of toluene. For alumina supports modified with a silicon precursor with a low reactivity such as TEOS, the low integration of the Si atoms inside the bulk alumina lead to a slightly higher catalytic activity, but also to a high formation of structured carbon. The opposite effect was observed for the samples modified with a highly reactive silicon precursor, such as TMOS or EDAS, which showed a slightly lower catalytic activity, but a higher resistance against coking compared to pure Ni/γ-Al2O3 catalysts

Catalytic reforming of biomass gasification tars with bimetallic catalysts

The thermochemical method called “biomass gasification” is generating emphatic interest for the production of bio-Syngas (CO + H2) since this process presents the advantage of being renewable without emitting CO2. However, in practical applications, there are still some technical problems due to high tars concentration in the outlet gas which can condensate and clog the pipes. The objectives of this project is to synthesize, characterize and evaluate the catalytic performances of Ni/γ-Al2O3 catalysts doped with different metals (Cu, Co, Mo, Mn, Fe)

Synthesis, Surface characterization and photocatalytic activity of TiO2 supported on almond shell activated carbon

Three types of photocatalysts were synthesized by metal organic chemical vapor deposition and impregnation methods using the almond shell activated carbon as support. These photocatalysts denoted by (TiO2/ASAC (V), TiO2/ASAC (I1) and TiO2/ASAC (I2)) were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS) and nitrogen adsorptionedesorption isotherms. SEM observation shows that TiO2 was deposited on activated carbon surface. XRD results confirm that TiO2 existed in a mixture of anatase and rutile phases. The DRS spectra show the characteristic absorption edge of TiO2 at approximate 380 nm corresponding to the optical band gap of 3.26 eV. Besides, FTIR spectrum indicated the presence of (TieO) groups. The specific surface area of photocatalysts decreased drastically in comparison with the original activated carbon. The catalysts were very efficient for the photodegradation of total organic carbon (TOC) from industrial phosphoric acid solution under UV irradiation. The kinetics of photocatalytic TOC degradation was found to follow a pseudofirst- order model. The prepared TiO2/ASAC showed high photoactivity for the photodegradation of TOC in the following order: TiO2/ASAC (V) > TiO2/ASAC (I1) > TiO2/ASAC (I2) > ASAC > TiO2 (P25)