Silica Mixed Oxides Research Articles

A Kinetic Study of Photocatalytic Degradation of Phenol over Titania–Silica Mixed Oxide Materials under UV Illumination

A set of titania–silica mixed oxide materials were prepared by a cosolvent-induced gelation method using ethanol and toluene as solvent and cosolvent, respectively. These materials were extensively characterized by utilizing several characterization techniques and assessed for phenol degradation under UV illumination. The degradation of phenol follows first-order kinetics, and fragmented products formed during the phenol degradation were qualitatively identified by using high performance liquid Chromatographic (HPLC) and atomic pressure chemical ionization mass spectroscopic (APCI-MS) techniques. The complete mineralization of phenol was further evidenced by the measurement of the total organic contents that remained in the solution after irradiation. The pore diameter of the materials was found to be the key factor for phenol degradation, whereas surface area and pore volume play a role among the mixed oxide materials. In addition, in the mixed oxide system there was an inverse correlation obtained with the particle size of the materials and the degradation efficiency. The smaller particle size of titania in the mixed oxide material was found to be a requirement for an effective degradation of phenol.

Energy and electron transfer reactions on silica gel and titania\u2013silica mixed oxide surfaces

The energy and electron transfer reactions of anthracene co-adsorbed with an electron donor on silica gel and titania–silica mixed oxides have been studied by a combination of steady-state reflectance, emission spectroscopy, and nanosecond diffuse reflectance laser flash photolysis. Bimolecular rate constants for energy and electron transfer between anthracene and azulene have been measured; kinetic analysis of the decay of the anthracene triplet state and radical cation show that the kinetic parameters depend on the titania content of the sample and the azulene loading. The rate of energy and electron transfer reactions increases as a function of azulene loading and decreases with increasing titania content in titania–silica mixed oxides. These findings indicate that the observed rate of reaction is determined by the rate of diffusion of anthracene on the titania–silica surfaces, whereas, in contrast, the observed rate of reaction on silica gel is predominantly governed by the rate of diffusion of azulene.Graphic abstract

Elucidating the nature and role of copper species in catalytic carbonylation of methanol to methyl acetate over copper/titania–silica mixed oxides

In this study, a series of copper/titania–silica mixed oxide (Cu/TS) catalysts with different copper contents were prepared by a sol–gel method.

Catalytic hydrodeoxygenation of anisole over nickel supported on plasma treated alumina–silica mixed oxides

Hydrodeoxygenation (HDO) of anisole, a representative of lignin-derived bio oil, was investigated on plasma treated SiO2–Al2O3 supported Ni nanocatalyst at low hydrogen pressure.

Titania–silica mixed oxides investigated with density functional theory and molecular dynamics simulations

AbstractMixed phases of titania and silica have raised interest for their high activity as photocatalysts and for their optical properties, such as a high refractive index. To rationalize their properties, it is necessary to understand their atomic structure in crystalline and amorphous phases. We have investigated Ti–Si mixed oxide phases by density functional theory and molecular dynamics. A polarizable potential has been developed for and has been employed to sample the configuration space. Mixed phases are metastable with respect to separate and , but they are only slightly higher in energy, and therefore might form under technologically relevant conditions. All Ti–Si mixed oxides considered, regardless of cation coordination, display peaks in IR and Raman spectra in the region of frequencies 880–950, while Ti‐only phases do not, even if titanium is in a tetrahedral coordination. Our calculations thus confirm that these peaks are a signature of Ti–O–Si links, but they do not deliver information on the actual atomic structure of the material. Phases with titanium in a tetrahedral coordination display a higher electronic band gap than phases with titanium in octahedral coordination. This should have important consequences for the photocatalytic activity of mixed oxides of titania and silica.

Hafnia–Silica Cryogels: Solvent‐Assisted Textural and Catalytic Control in the Citronellal Cyclization

AbstractA series of hafnia–silica mixed oxides of HfO2(SiO2)8 stoichiometry were prepared by a sol–gel method followed by freeze‐drying. Two hafnium precursors, Hf(OiPr)4 and Hf(OtBu)4 were examined under a variety of aqueous–alcoholic systems. A decisive dependence of the alcoholic solvent on the gel formation and the surface and porosity of the final materials was found, which in turn were key to the enhancement of catalysis. Hence, acid hydrolysis and gelification of Hf(OiPr)4 in tert‐butyl alcohol, followed by lyophilization, yielded a material with the highest mesoporosity and specific surface area. This catalyst behaved most efficiently in the acid‐catalyzed cyclization of citronellal to isopulegol at room temperature. Furthermore, this cryogel also presented combined Lewis acidity and hydrogen bonding that resulted in higher diastereoselectivity for the above reaction. These results highlight the effect of how the textural and, therefore catalytic, properties of the cryogels depend decisively on the solvents chosen for their synthesis.

Effect of the type of acid used in the synthesis of titania–silica mixed oxides on their photocatalytic properties

TiO2–SiO2 mixed oxides were synthesized by the sol–gel technique using three different acids, i.e., acetic, sulfuric, or chlorhydric acid. Their photocatalytic behavior was evaluated on the phenol oxidation in liquid phase and correlated with the characterization results. It was found that the kind of acid used during the preparation strongly influences the phase composition and stability of the TiO2 phases incorporated in the silica structure as well as the photocatalytic activity. In all cases, silica introduced a dispersive effect that stabilized the TiO2 crystalline phases upon calcination at 700°C. SO42− and CH3COO− ions stabilized the anatase phase at high calcination temperatures (700°C) leading to samples with the highest photoactivities. Cl− ions induced the formation of traces of rutile and brookite resulting in a lower photoactivity. The highest photoactivity was achieved with the catalyst synthesized with acetic acid and calcined at 700°C (TS1-700-ace). The photocatalytic performance of this material was even better than that obtained with the commercial catalyst Degussa P-25.

Competitive role of structural properties of titania–silica mixed oxides and a mechanistic study of the photocatalytic degradation of phenol

TiO2–SiO2 mixed oxide materials were hydrothermally synthesized and the photocatalytic degradation of phenol under UV-irradiation was evaluated. We also demonstrated that varying the co-solvent, modulates the structural properties of the materials. In particular, the use of non-polar co-solvents such as toluene seemed to increase the crystallinity, surface area, and pore diameter while the crystallite size of titania seemed to change little. A comprehensive characterization using surface and bulk techniques evidenced the role of porosities, crystallinity, and Ti–O–Si linkages of the mixed oxides as significant factors that contribute to the degradation of phenol. The TiO2–SiO2 mixed oxide material prepared using only ethanol as the solvent showed 24% degradation of phenol after 120min of irradiation whereas other mixed oxide materials degraded phenol more efficiently (57% to 100%) in the same duration of time. The higher photocatalytic activities of the mixed oxide materials prepared using non-polar solvents is attributed to a combination of factors that include higher Apparent Surface Coverages of Ti–O–Si heterolinkages, larger pore sizes, and most importantly higher crystallinities of the titania phase. Larger pore sizes enabled better transport of reactant molecules and products to and from the active sites (Ti–O–Si heterolinkages) and the higher crystallinities of the titania phase helped in minimizing the electron–hole recombination in these photocatalysts, and thus resulted in high degradation efficiencies.

An investigation into the effect of porosities on the adsorption of rhodamine B using titania–silica mixed oxide xerogels

Aperiodic mesoporous titania–silica (TiO2·SiO2) xerogels with varying silica contents were synthesized under ambient conditions. The physico-chemical properties of the xerogels were examined by a variety of techniques that include powder X-Ray Diffraction (XRD), nitrogen adsorption, Fourier Transform-Infra-Red spectroscopy (FT-IR), Scanning and Transmission Electron Microscopies (SEM and TEM), Thermo Gravimetric Analysis (TGA), zeta potential, and Diffuse Reflectance Spectroscopic (DRS) studies. The adsorption of a model dye molecule, rhodamine B (RhB) was studied over the titania–silica xerogels and compared with titania and silica. It was determined that the pore volume of the xerogels mainly influences the adsorption of RhB. The xerogels exhibited good adsorption capacity with more than 90% dye removal at low dye concentrations. Our results suggest that low cost approaches to the synthesis of xerogels with tailored properties such as large pore volume could provide cost-effective solutions to mitigate environmental problems related to removal of water based toxic pollutants such as dyes by simple adsorption processes.

Detection of surface silanol groups on pristine and functionalized silica mixed oxides and zirconia

The surface hydroxyl content and surface structure of silica and other oxides with and without surface modification were systematically studied by solid state 29Si NMR, thermogravimetric analysis, and the lithium alanate method. Aerosil 90 as a well described reference system and functionalized zirconia–silica particles were used in the validation of the lithium alanate method. 3-Methacryloxypropyltrimethoxysilane and dodecylphosphonic acid were applied as surface modifiers. The determination of silanol content of Aerosil 90 by 29Si NMR and TGA confirms the results obtained by the lithium alanate method, which also allows for the determination of the remaining surface hydroxyl content after surface modification. For both silane coupling agents, the residual hydroxyl content of modified zirconia–silica is decreased by a factor of approximately 2 compared with that of the unmodified mixed oxide, whereas after modification with dodecylphosphonic acid, the hydroxyl content is slightly higher. These results are again in good agreement with those by 29Si NMR confirming that the lithium alanate method is a reliable and easily practicable method for surface hydroxyl determination.

Synthesis of titania–silica xerogels by co-solvent induced gelation at ambient temperature

A series of binary titania–silica mixed oxides was prepared by sol–gel synthesis at room temperature. The hydrolysis of titanium isopropoxide (Ti( i OPr) 4) and tetraethylorthosilicate (TEOS) was facilitated by co-solvent induced gelation in acidic media. The resulting gels were dried, calcined, and then characterized by powder X-ray diffraction analysis, nitrogen sorption studies (at 77 K), diffuse reflectance spectroscopy, Raman microscopy, and transmission electron microscopy. The nitrogen sorption studies indicate that the porosities could be tuned when simple aromatic solvents such as toluene, p-xylene, or mesitylene were added as a co-solvent to the synthesis gel. The binary mixed metal oxide materials obtained in this study showed high activity towards the degradation of phenol, and possessed high surface areas, and large pore volumes with narrow pore size distribution without the need for additional hydrothermal synthesis or supercritical drying.

Textural, structural, and morphological characterizations and catalytic activity of nanosized CeO 2–MO x (M = Mg 2+, Al 3+, Si 4+) mixed oxides for CO oxidation

The present work focuses on the combination of ceria with another oxide of different ionic valences from period 3 (Mg 2+, Al 3+, and Si 4+) using coprecipitation method, followed by calcination at 450 and 750 °C, respectively. The textural, structural, morphological and redox properties of nanosized ceria–magnesia, ceria–alumina and ceria–silica mixed oxides have been investigated by means of N 2 physisorption, XRD, Raman, HRTEM, DRS, FT-IR, and H 2-TPR technologies. XRD results of these mixed oxides reveal that only nanocrystalline ceria ( ca. 3–6 nm for the 450 °C calcined samples) could be observed. The grain size of ceria increases with the increasing calcination temperature from 450 to 750 °C due to sintering effect. The highest specific surface area is obtained at CeO 2–Al 2O 3 mixed oxides when calcination temperature reaches 750 °C. Raman spectra display the cubic fluorite structure of ceria and the existence of oxygen vacancies, and displacement of oxygen ions from their normal lattice positions in the ceria-based mixed oxides. DRS measurements confirm that the smaller the grain size of the ceria, the higher indirect band gap energy. H 2-TPR results suggest that the reductions of surface and bulk oxygen of ceria were predominant at low and high calcination temperature, respectively. Finally, CO oxidation were performed over these ceria-based mixed oxides, and the combination of CeO 2–Al 2O 3 exhibited highest activity irrespective of calcination temperature, which may due to excellent textural/structural properties, good homogeneity, and redox abilities.

High-Surface-Area Alumina-Silica Nanocatalysts Prepared by a Hybrid Sol-Gel Route Using a Boehmite Precursor

High-surface-area alumina–silica mixed oxide (Al2O3:SiO2) nanocatalysts have been prepared by a hybrid sol–gel method using boehmite (synthesized from aluminum nitrate) as the source of alumina and tetraethyl orthosilicate as the source of silica. The gels, after calcination at 400°C, result in mixed oxides with specific surface areas of 287 m2/g (Al2O3:SiO2=3:1) and 262 m2/g (Al2O3:SiO2=3:4). Further heating to 600°C produces materials with specific surface areas of 237 and 205 m2/g, respectively. The larger specific surface areas characteristic of the 3Al2O3:SiO2 samples are attributed, via transmission electron micrograph investigations, to the presence of ∼10 nm size, needle-like particles having an aspect ratio of 1:50. Further addition of silica leads to the formation of larger needles of 20–75 nm size. Calcination at 600°C induced an approximately 5% decrease in the total pore volume for the 3Al2O3:SiO2 sample. In contrast, the material with Al2O3:SiO2=3:4 showed an approximately 12% increase in pore volume when heated at 600°C. The pore-size distribution was in the range 1–3.5 nm with rmax at ∼2 and ∼2.5 nm at 600° and 800°C, respectively. Adsorption isotherms and pore-size distribution analyses are discussed in some detail for the aluminosilicates at different calcination temperatures. This discussion is supported by structural information determined from FTIR and 27Al MAS NMR studies. Relatively high acidity values (0.234 mmol/g for Al2O3: SiO2=3:4) are observed for silica-rich compositions consistent with their application as efficient acid catalysts.

ChemInform Abstract: Genesis and Structural Properties of Alkoxide-Prepared Titania-Silica Xerogels.

Titania–silica mixed oxide gels with a 1 : 4 Ti : Si molar composition were prepared from the hydrolysis of tetraethoxy orthosilicate (TEOS) and tetraisopropoxy orthotitanate (TIOT), using different amounts of water, acid or base, and hydrolysis procedure. Surface area, pore structure, and crystallinity were compared in relation to sol-gel process conditions. Gels in which the TEOS was hydrolysed using a basic medium possessed larger mesopores and correspondingly lower surface area than gels prepared with acid-hydrolysed TEOS, but were more resistant to mesopore collapse upon drying, owing to the spherically shaped silica subunits in these gels. Gels hydrolysed by a two-stage procedure, involving partial hydrolysis of TEOS in acid, followed by mixing with pure TIOT and completion of hydrolysis, are most susceptible to pore collapse upon drying, producing a completely microporous xerogel. Pore collapse did not occur when the wet-gel material was aged in slightly basic medium for several days. All gels prepared by the two-stage hydrolysis procedure remain X-ray amorphous after treatment in air at 870 K, whereas gels produced by mixing completely hydrolysed component sols show crystallization of the titania. Thermal analysis runs show water loss primarily results from weakly bound water. In general, a residual amount of unreacted alkoxy groups is present in gels prepared without a large excess of hydrolysis water. Highest residual organic amounts of 2–8 wt. % were found with the two-stage hydrolysed gels. Rutile was only detected in gels prepared in excess water, but the extent of rutile formation, as well as crystallite growth, is considerably less than in similarly prepared pure titania gels.

Characterisation and photocatalytic properties of titania–silica mixed oxides doped with Ag and Pt

TiO 2–SiO 2 mixed oxides have been synthesised and modified by Ag and Pt deposition. Due to the effect of the silica on the mixed oxide, the prepared materials presented high surface areas and stabilised anatase as the only crystalline phase after calcination at 700 °C. Even using the same photodeposition experimental conditions, the yield for metal deposition depended highly on the metal considered, being much lower for Ag deposition. XPS studies permitted to estimate metal dispersion and oxidation state of the different samples, being both factors of high importance regarding photocatalytic improvement by metal deposition.

ChemInform Abstract: Selectivity Enhancement in Heterogeneous Catalysis Induced by Reaction Modifiers

Abstract The application range of solid catalysts can be greatly extended by reaction or process modifiers , that is by simple addition of an inorganic or organic compound to the reaction mixture. The modifier, used in catalytic amounts, ideally interacts strongly with the active sites in a fashion which induces favorable changes in the outcome of the reaction. Evolution of the actual modified metal catalyst during reaction and the importance of in situ characterization in understanding these processes are illustrated using the examples of promotion by metal ions and nitrogen-containing bases. The major part of the review describes the advantages and limitations of employing N-base modifiers for tuning the performance of solid catalysts. Reactions discussed include chemo-, stereo-, enantio- and diastereoselective hydrogenations over metal catalysts, aerobic oxidation of alcohols with Pt and Pd, and epoxidation of allylic alcohols with titania–silica mixed oxides and alkyl hydroperoxides.

Structural characterization and dehydration activity of CeO 2–SiO 2 and CeO 2–ZrO 2 mixed oxides prepared by a rapid microwave-assisted combustion synthesis method

Catalytic performance of ceria–silica and ceria–zirconia mixed oxides for dehydration of 4-methylpentan-2-ol has been investigated. The reaction was carried out in the temperature region 523–673 K in a fixed-bed microreactor at normal atmospheric pressure. The investigated mixed oxides were prepared by a rapid microwave-assisted combustion synthesis method by using metal nitrates as precursors. X-ray diffraction (XRD), Raman spectroscopy (RS) and temperature programmed desorption (TPD) techniques were used to investigate the structural and surface properties of the mixed oxides. XRD studies revealed the formation of a specific Ce 0.5Zr 0.5O 2 solid solution in case of ceria–zirconia sample, and the ceria–silica mixed oxide is found to be in amorphous form. RS measurements suggest the presence of oxygen vacancies and lattice defects in the ceria–zirconia mixed oxide sample. TPD of ammonia and CO 2 results reveal that ceria–silica exhibits strong acid–base properties compared to ceria–zirconia mixed oxide. Good alcohol conversion and excellent 4-methylpent-1-ene selectivity were observed on both the mixed oxides.

Preparation of Titania−Silica Mixed Oxides by a Sol−Gel Route in the Presence of Citric Acid

Titania−silica mixed oxides (denoted as TiO2−SiO2) with different Ti contents are prepared by a sol−gel route in the presence of citric acid (CA). Various characterization techniques, including XRD, FT-IR, N2-adsorption, TEM, solid-state CP-MAS NMR spectroscopy, UV−vis, and NH3-TPD, have been carried out to investigate the physical−chemical properties, states of Ti species, and the formation process of TiO2−SiO2 materials. The characterization results show that the TiO2−SiO2 materials possess a mesoporous silicate framework with high specific surface area and thermal stability. Titanium species in the TiO2−SiO2 materials are mainly present in the form of two-dimensional low polymeric TiOx species, which are uniformly dispersed on the surface of mesoporous silica. Additionally, the titanium species have a strong resistence to high-temperature sintering, even for the sample with high Ti contents (25 wt %). Weak acid sites are observed over TiO2−SiO2 materials, and the amount of acidic sites increases with t...

Synthesis and characterization of water-soluble SiO 1.5/TiO 2 hybrid nanoparticles by hydrolytic co-condensation of triethoxysilane containing hydroxyl groups

Novel R–SiO 1.5/TiO 2 hybrid nanoparticles were synthesized by hydrolytic co-condensation of titanium alkoxides (Ti(OR′) 4, R′ = ethyl, isopropyl, and butyl) with a triethoxysilane precursor, R–Si(OCH 2CH 3) 3, R = –CH 2CH 2CH 2N(CH 2CH 2COOCH 2CH 2OH) 2, derived from 2-hydroxyethyl acrylate. Co-condensation of a titanium alkoxide with the triethoxysilane precursor was investigated at different feed ratios, suggesting that water-soluble nanoparticles were obtained only at less than 30% of Ti(OEt) 4 molar ratio in the feed. In contrast, the co-condensation of titanium tetraisopropoxide, Ti(O iPr) 4, with the triethoxysilane precursor in the presence of acetylacetone proceeded as a homogeneous system until 70% of Ti(O iPr) 4 molar ratio to afford water-soluble organic–inorganic hybrid nanoparticles containing titania–silica mixed oxides, as confirmed by NMR, FT-IR, elemental and ICP analyses. Scanning force microscopy (SFM) measurements of the product prepared at Ti(O iPr) 4/triethoxysilane = 50/50 mol% with acetylacetone indicated the formation of the nanoparticles having relatively narrow size distribution with average particle diameter less than 2.0 nm without aggregation. The refractive index of the hybrid nanoparticle was 1.571. The isolated nanoparticles distributed homogeneously were visualized by transmission electron microscopy (TEM), and the size of the hybrid nanoparticle (1.9 nm) was determined by X-ray diffraction (XRD).

Simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid by methylene blue adsorbed on a phosphorylated zirconia–silica composite electrode

This paper describes the preparation, characterization and application of a composite electrode based on methylene blue adsorption to phosphorylated zirconia–silica mixed oxide particles prepared by a sol–gel process. This electrode electrocatalytically oxidizes ascorbic acid (AA), dopamine (DA) and uric acid (UA), allowing their simultaneous voltammetric detection. Well-defined and -separated oxidation peaks were observed by differential pulse voltammetry in a 0.35 mol l −1 Tris–HCl buffer solution (pH 7.4) containing 0.5 mol l −1 KCl. The anodic peak currents observed at −74, 94 and 181 mV increased with increasing concentrations of AA, DA and UA, respectively. Linear calibration plots were obtained over the range of 100–1600 μmol l −1 for ascorbic acid, 6–100 μmol l −1 for dopamine and 22–350 μmol l −1 for uric acid with detection limits of 8.3 ± 0.1, 1.7 ± 0.1 and 3.7 ± 0.2 μmol l −1, respectively. DA and UA concentrations could also be determined under conditions of excess AA (1 mmol l −1).