Physicochemical properties and photocatalytic activities of TiO2-films prepared by sol–gel methods

The physicochemical properties, the photocatalytic activities in aqueous solution and the adhesion properties of supported TiO₂ films prepared by different sol–gel methods have been studied. The thickness, the TiO₂ loading and the photocatalytic activities are influenced by the nature of the stabilising agent. By contrast, the nature of the organic titanium precursor, as well as the solvent and the absence of stabilising agents are determining for the resulting photocatalytic activities. Titania-sol generated by non-controlled hydrolysis of titanium isopropoxide was used to determine the influence (i) of number of coating, (ii) of the calcination temperature and (iii) of the nature of support on the photocatalytic activity under direct and backside irradiation. The higher the coating number, the thicker the TiO₂-film and the higher the photocatalytic efficiency. the optimum calcination temperature was found to be 400 °C. Migration of cationic species into TiO₂-films and the decrease of thickness at higher temperatures lead to the decline of activity.     

Catalytic and photocatalytic ozonation of phenol on MnO2 supported catalysts

Different series of manganese-supported catalysts containing 10 wt.% of manganese, as oxide, on TiO₂ have been prepared by the sol–gel method and by the traditional method based on the impregnation of the support with the metal precursor on commercial and sol–gel supports. The samples were characterized by measuring the specific area (S_BET), temperature-programmed desorption (TPD), Fourier transform infrared (FTIR) spectroscopy, temperature-programmed reduction (TPR), electrophoretic migration (IP) and X-ray diffraction (XRD). The catalytic and photocatalytic activity was measured in a batch reactor using ozone as the oxidizing agent. The catalytic behavior, expressed as constant rate, in absence of irradiation did not show significant changes for the manganese-supported catalysts. The only exception was the cogelated Mn/TiO₂ catalyst, which showed higher degradation activity, the main product being benzoquinone. On the other hand, all the irradiated systems showed an increase in the phenol degradation, being CO₂ and small organic acids the final product.     

Studies on structural properties, surface acidity and benzene isopropylation activity of sulphated ZrO2–TiO2 mixed oxide catalysts

A series of sulphated ZrO₂–TiO₂ mixed oxide with different nominal sulphate loadings in the range of 2–15 wt.% was prepared and characterized for their structural properties, surface acidity and benzene isopropylation activity. The catalyst with 10 wt.% nominal sulphate loading showed highest surface area and uniform pore size distributions. Surface acidity, measured by NH₃–TPD method, showed increase in acidity with sulphate loading and the 10 wt.% sulphate loaded catalyst showed highest acidity. The activities of these catalysts were tested for isopropylation of benzene to cumene using 2-propanol as the alkylating agent. The 10 wt.% sulphate-loaded catalyst also showed highest activity for this reaction with 97% cumene selectivity. The higher activity of this catalyst was attributed to its higher acidity.     

The role of H2O in the photocatalytic oxidation of toluene in vapour phase on anatase TiO2 catalyst: A FTIR study

Photocatalytic oxidation of toluene has been carried out in a gas–solid regime by using polycrystalline anatase TiO₂ in a fixed-bed continuous reactor. Air containing toluene and water vapours in various molar ratios was fed to the photoreactor irradiated by a medium pressure Hg lamp. Toluene was mainly photo-oxidised to benzaldehyde, and small amount of benzene, benzyl alcohol and traces of benzoic acid and phenol were also detected. In the presence of water, no decrease of photoreactivity was observed at steady-state conditions. By removing water vapour from the feed, the conversion of toluene to benzaldehyde was almost completely inhibited, and an irreversible deactivation of the catalyst occurred. FTIR investigations indicated that benzaldehyde is photoproduced on the TiO₂ surface even in the absence of water vapour, but exposure of the catalyst to the UV light in a dry atmosphere results in an irreversible consumption of surface hydroxyl groups. As these species play a key role in the photoreactive process, this dehydroxylation should be the reason of the catalyst deactivation observed in the catalytic runs carried out in the absence of water vapour.     

Support effects in Pt/TiO2–ZrO2 catalysts for NO reduction with CH4

ZrO₂–TiO₂ mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH₄ were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773 K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f_7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773 K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O₂ co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH₄ reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface.     

CO oxidation on Pd/CeO2–ZrO2 catalysts

The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO₂–Pd on the kinetics of CO oxidation over Pd/CeO₂–ZrO₂. Under reducing-to-moderately oxidizing conditions, a zero-order O₂ pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO₂–ZrO₂ support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior.     

Synthesis, structure and photocatalytic properties of Fe(III)-doped TiO2 prepared from TiCl3

Iron(III)-doped titanium dioxide photocatalysts were prepared from aqueous titanium(III) chloride solution in the presence of dissolved FeCl₃ (0–10.0 at.% relative to TiCl₃) by co-precipitation method. The precipitate was completely oxidized in the aerated suspension, hydrothermally treated, washed and calcinated. The structure of the powders was characterized by thermoanalysis (TG-DTA), diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), nitrogen adsorption and transmission electron microscopy (TEM). The light absorption of the iron-containing powders is red shifted relative to the bare sample. The particle size and anatase content were found to significantly decrease at iron contents ≥6.0 at.% which is accompanied with the increase of their specific surface area. XANES measurements showed that the local structure of iron systematically changes with the variation of the dopant concentrations: at higher Fe-contents, hematite- or goethite-like environments were observed, consistent with the formation of separate X-ray amorphous Fe(III)-containing phases. The local structure of iron gradually transformed with decreasing dopant concentrations, possibly due to substitution of Fe(III) in the titania (TiO₂) crystal lattice. Energy dispersive X-ray analysis (EDX) and chemical analysis was used to characterize the iron content of the samples in the bulk and X-ray photoelectron spectroscopy (XPS) in the surface layer of the particles. The photocatalytic performance of the prepared photocatalysts was compared with the activity of Aldrich anatase under UV–vis and VIS irradiation in two different photoreactors. Maximum photocatalytic performance was found at 3.0 at.% iron concentration for UV–vis and at 1.2 at.% for VIS irradiation. Doping with iron(III) ions increased the photodegradation rate of phenol by a factor of three for UV–vis irradiation and by a factor of two for VIS irradiation, relative to the bare photocatalyst.     

Low temperature synthesis of MgxAl2(1−x)Ti(1+x)O5 films by sol–gel processing

Aluminium titanate films thicker than 0.5 μm have been synthesized by sol–gel methods. The films have been deposited via repetitive dip-coating on silicon wafers and their thermal stability has been tested as a function of the annealing time and temperature. The sol–gel approach has allowed the formation of the aluminium titanate phase at temperatures (700 °C) much lower than those necessary for solid-state reactions (1450 °C). Magnesium oxide has been used to improve the thermal stability of the films at high temperatures. The behavior of samples prepared with two different Mg content, i.e. Mg_0.2Al_1.6Ti_0.8O₅ and Mg_0.6Al_0.8Ti_1.6O₅, has been studied. The films have proven to be stable at 1150 °C, for up to 90 h. X-ray photoelectron spectroscopy has shown that after firing at 500 °C the surface chemical composition of the films is in accordance with the nominal one, whilst at higher annealing temperatures some differences, attributed to diffusion effects, have been observed.     

Study of n-hexane isomerization on mixed Al2O3–ZrO2/SO4 catalysts

Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr₂O were synthesized by means of sol–gel methods. The catalysts were sulfated with H₂SO₄ 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N₂-adsorption at 78 K, and SEM. The catalytic properties of the Al₂O₃–ZrO₂ series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al₂O₃–ZrO₂ solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane.     

ZrO2对Al2TiO5-板状刚玉复合材料性能的影响

以α-Al2O3微粉、板状刚玉和TiO2为主要原料,添加不同含量的ZrO2,通过高温烧成以固相反应直接合成Al2TiO5-板状刚玉-ZrO2复合材料,研究了ZrO2含量对复合材料的烧结收缩率、显气孔率、强度等的影响。结果表明:添加ZrO2的复相材料的收缩率和体积密度明显增加。ZrO2含量为4%时,可以制备出高致密度高强度的Al2TiO5-板状刚玉-ZrO2复合材料,其显气孔率为9.31%,抗折强度为25.6MPa,抗压强度为225MPa。     

Active sites in Cu/ZnO/ZrO2 catalysts for methanol synthesis from CO/H2

Among various Cu/ZnO/ZrO₂ catalysts with the Cu/Zn ratio of 3/7, the one with 15 wt.% of ZrO₂ obtains the best activity for methanol synthesis by hydrogenation of CO. The TPR, TPO and XPS analyses reveal that a new copper oxide phase is formed in the calcined Cu/ZnO/ZrO₂ catalysts by the dissolution of zirconium ions in copper oxide. In addition, the Cu/ZnO/ZrO₂ catalyst with 15 wt.% of ZrO₂ turns out to contain the largest amount of the new copper oxide phase. When the Cu/ZnO/ZrO₂ catalysts is reduced, the Cu²⁺ species present in the ZrO₂ lattice is transformed to Cu⁺ species. This leads to the speculation that the addition of ZrO₂ to Cu/ZnO catalysts gives rise to the formation of Cu⁺ species, which is related to the methanol synthesis activity of Cu/ZnO/ZrO₂ catalyst in addition to Cu metal particles. Consequently, the ratio of Cu⁺/Cu⁰ is an important factor for the specific activity of Cu/ZnO/ZrO₂ catalyst for methanol synthesis.     

Mechanical properties of Al2O3/ZrO2 composites

In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al₂O₃/(t-ZrO₂+m-ZrO₂) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO₂ inclusions are close to each other in the Al₂O₃ matrix, the yttrium ion originally in t-ZrO₂ particles can diffuse to nearby m-ZrO₂ particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al₂O₃/(t-ZrO₂+m-ZrO₂) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al₂O₃ alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al₂O₃ alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation.     

ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming

Surface-phase ZrO₂ on SiO₂ (SZrOs) and surface-phase La₂O₃ on Al₂O₃ (SLaOs) were prepared with various loadings of ZrO₂ and La₂O₃, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH₄/CO₂ reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al₂O₃ and Pt/SiO₂ catalysts. CO₂ or CH₄ pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides.

The zirconia can be homogeneously dispersed on SiO₂ to form a stable surface-phase oxide. The lanthana cannot be spread well on Al₂O₃, but it forms a stable amorphous oxide with Al₂O₃. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO₂ and Pt/Al₂O₃ by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO₂ and Pt/Al₂O₃ catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO₂ adsorbed on the basic sites of SZrOs and SLaOs.     

Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts supported on ZrO2–Al2O3–WOx prepared by sol–gel

The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO₂ (70, 85 wt.%)–Al₂O₃ (15, 0 wt.%)–WO_x (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N₂ physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel favored the high dispersion of WO_x and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of monoclinic-WO₃ bulk phase. The catalysts increased their S_BET for about 15% promoted by Al₂O₃ addition. Various oxidation states of WO_x species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WO_x species is constituted mainly of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated to the coexistence of W⁶⁺ and reduced-state WO_x species (either W⁴⁺ or W⁰). The addition of Pt to the Pd/ZrAlW catalyst does not modify significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) isomers.     

Oxidative dehydrogenation of ethylbenzene to styrene with CO2 over SnO2–ZrO2 mixed oxide nanocomposite catalysts

SnO₂–ZrO₂ nanocomposite catalysts with different compositions ranging from 0 to 100% of SnO₂ were prepared at room temperature by co-precipitation method using aqueous ammonia as a hydrolyzing agent. X-ray diffraction, transmission electron microscopic characterization revealed the SnO₂–ZrO₂ nanocomposite behavior. Acid–base properties of these catalysts were ascertained by temperature-programmed desorption (TPD) of NH₃ and CO₂. Both acidic and basic sites distribution of the nanocomposite catalysts is quite different from those of respective single oxides (SnO₂ or ZrO₂). Catalytic activity of these nanocomposite catalysts for ethylbenzene dehydrogenation (EBD) to styrene in the presence of excess CO₂ was evaluated. The change in the acid–base bi-functionality of the nanocomposite catalysts in comparison with single oxides had profound positive influence in enhancing the catalytic activity.     

Local structure and photocatalytic activity of B2O3–SiO2/TiO2 ternary mixed oxides prepared by sol–gel method

The local structure and the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides (SiO₂ content was fixed as 30 at.% with respect to TiO₂) was investigated by using XRD, FT-IR, BET, UV-vis spectra, and electron paramagnetic resonance (EPR) measurement. In FT-IR analysis, boron was incorporated into the framework of titania matrix with replacing Ti---O---Si with Si---O---B or Ti---O---B bonds. Also, paramagnetic species such as O⁻ and Ti³⁺ defects were formed by the boron incorporation. In SiO₂/TiO₂ mixed oxides, a blue shift in the light absorption band was observed due to the quantization of band structure. All B₂O₃–SiO₂/TiO₂ samples had pure anatase phase and no rutile phase was formed even though the calcination temperature was over 900 °C. Incorporating boron oxides of more than 10% enlarges the grain size of anatase phase and causes a red shift of the light absorption spectrum. The surface area was monotonically decreased with increasing the content of boron content. As a result, the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides was greatly influenced by the content of boron oxide. The highest photoactivity (g moles/min l) was obtained when the boron content was 5% and seven times higher than that of silica/titania binary mixed oxide. In addition, the specific photoactivity (g moles/m² l) was maximum still at 5%. It was concluded that the large reduction of surface area, the change of band structure, and more formation of bulk Ti³⁺ sites are responsible for the deterioration in the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides when the content of boron is over 10%, although their crystallinity was enhanced by increasing the calcination temperature with keeping anatase phase.     

Preparation of Cu/ZrO2 catalysts for methanol synthesis from CO2/H2

Cu/ZrO₂ catalysts for methanol synthesis from CO₂/H₂ were respectively prepared by deposition coprecipitation (DP) and solid state reaction (SR) methods. There is an intimate interaction between copper and zirconia, which strongly affects the reduction property and catalytic performance of the catalysts. The stronger the interaction, the lower the reduction temperature and the better the performance of the catalysts. Surface area, pore structure and crystal structure of the catalysts are mainly controlled by preparation methods and alkalinity of synthesis system. The conversion of CO₂ and selectivity of methanol are higher for DP catalysts than for SP catalysts.     

Gd,B共掺杂改性TiO2纳米颗粒的可见光光催化活性

采用溶胶-凝胶法制备了Gd和B共掺杂的TiO2纳米颗粒,研究了TiO2纳米颗粒在可见光下的光催化活性。应用XRD、TEM和UV-Vis等手段对TiO2纳米颗粒的物相、粒径、形貌及光学性能进行了表征。结果表明,掺杂可以抑制TiO2晶粒增长,阻碍TiO2由锐钛矿相向金红石相的转变。紫外-可见吸收光谱显示,共掺杂纳米颗粒在可见光区吸收有较强提高,共掺杂离子以协同作用拓展TiO2光谱响应,使吸收带产生红移,提高光生载流子的分离效率。光催化降解实验表明,共掺杂TiO2纳米颗粒有很高的可见光光催化活性,以500℃热处理的共掺杂摩尔比为0.005 Gd和0.04 B的TiO2纳米颗粒光催化效果最好,在可见光下对甲基橙的降解率为98.9%。     

A novel preparation of high surface area TiO2 nanoparticles from alkoxide precursor and using active carbon as additive

TiO₂ nanoparticles have been prepared by a novel alkoxide sol–gel precipitation. The presence of active carbon in different percentages could act as an interesting template. Upon calcination, carbon is eliminated leaving surface features significantly different from TiO₂ prepared in the absence of carbon. Wide surface and structural characterisation of samples have been carried out. Correlations with carbon percentage is pointed out from this characterisation. Interesting spherical aggregates of nanosized TiO₂ are observed from TEM images probably stabilised by the presence of carbon. Physicochemical correlations made will be very useful in further application of these TiO₂ to be used as potential high surface area photocatalyst.     

Features of the transformation of Hg by heterogeneous photocatalysis over TiO2

UV/TiO₂ photocatalysis of 0.5 mM mercuric aqueous solutions has been analyzed starting from Hg(NO₃)₂, Hg(ClO₄)₂ and HgCl₂ at different pH (3, 7 and 11) and in the presence or absence of oxygen. Profiles of Hg^II concentration with time were characterized by a relatively rapid initial conversion followed by a decrease or an arrest of the rate, the shape of profiles changing with the conditions. Conversions at 60 min and initial quantum efficiencies have been found dependent on the initial conditions and type of mercuric salt. The faster transformation took place at pH 11 for all salts. A good transformation yield is observed also for HgCl₂, which behaves differently to the other two salts, at pH 3 under nitrogen and pH 7 (N₂ or O₂). Inhibition by oxygen was observed in acid and neutral media but not at basic pH. When the conversion was 50% or more, pale or dark gray solids were deposited on the catalyst, identified as mixtures of Hg⁰, HgO or Hg₂Cl₂. A unique kinetic scheme could not be defined, which seemed to depend on the nature of the mercury salt, the ambient conditions and the type of deposit. Implications of the application of the technique to real systems are discussed.