Comprehensive evolved gas analysis (EGA) of amorphous precursors for S-doped titania by in situ TG–FTIR and TG/DTA–MS in air: Part 2. Precursor from thiourea and titanium(IV)-n-butoxide

Thermal decomposition of an amorphous precursor for sulfur-doped titania (S:TiO₂) nanopowders, prepared by controlled sol–gel hydrolysis-condensation of titanium(IV) tetrabutoxide and thiourea in aqueous butanol, has been studied in situ up to 850 °C in flowing air by simultaneous thermogravimetric and differential thermal analysis coupled online with quadrupole mass spectrometer (TG/DTA–MS) and FTIR spectrometric gas cell (TG–FTIR) for analysis of gases and their evolution dynamics in order to explore and simulate thermal annealing processes of fabrication techniques aimed S:TiO₂ photocatalysts with photocatalytic activities under visible light.The studied S-doped precursor's decomposition course remembers to that of non-doped xerogel from Ti(IV)-n-butoxide, which seems to retard a considerable amount of organics in the solid phase even at high temperature, probably in polymeric forms, proven by evolution of CO₂ in several temperature regions of decomposition stages. The incorporation form of thiourea in the original xerogel seems to be chemically bounded, resulting lower decomposition temperature than that of pure thiourea, and producing evolution of carbonyl sulfide (COS) already between 120 and 190 °C. Nevertheless, evolution of SO₂, and that of CO₂ is also observed above 500 °C by both EGA detection methods. The latter observation implies that the blackish grey samples obtained even at 750 °C might be simultaneously S- and C-doped ones.     

Evolved gas analysis of dichlorobis(thiourea)zinc(II) by coupled TG-FTIR and TG/DTA-MS techniques

Identification and monitoring of gaseous species released during thermal decomposition of the title compound 1, Zn(tu)₂Cl₂, (tu=thiourea, (NH₂)₂C=S) have been carried out in flowing air atmosphere up to 800°C by both online coupled TG-EGA-FTIR and simultaneous TG/DTA-EGA-MS. The first gaseous products of 1, between 200 and 240°C, are carbon disulfide (CS2) and ammonia (NH₃). At 240°C, an exothermic oxidation of CS₂ vapors occurs resulting in a sudden release of sulphur dioxide (SO₂) and carbonyl sulphide (COS). An intense evolution of hydrogen cyanide (HCN) and beginning of the evolution of cyanamide (H₂NCN) and isothiocyanic acid (HNCS) are also observed just above 240°C. Probably because of condensation and/or polymerization of cyanamide vapors on the windows and mirrors of the FTIR gas cell optics, some strange baseline shape changes are also occurring above 330°C. Above 500°C the oxidation process of organic residues appears to accelerate which is indicated by the increasing concentration of CO₂, while above 600°C zinc sulfide starts to oxidize resulting in the evolution of SO₂. All species identified by FTIR gas cell were also confirmed by mass spectrometry, except for HNCS. This revised version was published online in July 2006 with corrections to the Cover Date.     

High temperature reactions of titanium(IV) oxide with some alkali persulfates and their decomposition products

The solid state reactions between TiO₂ and Na₂S₂O₈ or K₂S₂O₈ have been investigated using TG, DTG, DTA, IR, and X-ray diffraction studies in the range of 20 to 1000°C.It has been shown that TiO₂ reacts stoichiometrically (1 : 1) with Na₂S₂O₈ in the range of 160 and 220°C forming the complex sodium monoperoxodisulfato—titanium(IV) as characterized by IR and X-ray analysis. The new complex then decomposes into the reactants above 190°C.An exothermic reaction has been observed between TiO₂ and molten K₂S₂O₇ at mole ratio 1:2 respectively and higher, in the range of 280 and 350°C. The IR and X-ray analyses have shown the formation of a complex namely, potassium tetrasulfato titanium(IV) for which the formula and structure have been proposed. This complex decomposes at higher temperatures into K₂SO₄ and a mixed sulfate of potassium and titanium. The mixed sulfate melts at 620°C and decomposes into K₂SO₄, TiO₂, and the gaseous SO₃.On the other hand, Na₂S₂O₈ decomposes in a special mode producing a polymeric product of Na₁₀S₉O₃₂. Decomposition of this species occurs after melting at 560°C into Na₂SO₄ and sulfur oxides. The decomposition reaction has been proved to be catalysed by TiO₂ itself.     

Comparative evolved gas analyses on thermal degradation of thiourea by coupled TG-FTIR and TG/DTA-MS instruments

Identification and monitoring of gaseous species released during thermal decomposition of pure thiourea, (NH₂)₂C=S in argon, helium and air atmosphere have been carried out by both online coupled TG-FTIR and simultaneous TG/DTA-MS apparatuses manufactured by TA Instruments (USA). In both inert atmospheres and air between 182 and 240°C the main gaseous products of thiourea are ammonia (NH₃) and carbon disulfide (CS₂), whilst in flowing air sulphur dioxide (SO₂) and carbonyl sulphide (COS) as gas phase oxidation products of CS₂, and in addition hydrogen cyanide (HCN) also occur, which are detected by both FTIR spectroscopic and mass spectrometric EGA methods. Some evolution of isothiocyanic acid (HNCS) and cyanamide (NH₂CN) vapours have also observed mainly by EGA-FTIR, and largely depending on the experimental conditions. HNCS is hardly identified by mass spectrometry. Any evolution of H₂S has not been detected at any stage of thiourea degradation by either of the two methods. The exothermic heat effect of gas phase oxidation process of CS₂ partially compensates the endothermicity of the corresponding degradation step producing CS₂.     

High temperature stability and photocatalytic activity of nanocrystalline anatase powders with Zr and Si co-dopants

TiO₂ nanopowders doped by Si and Zr were prepared by sol–gel method. The effects of Si and Zr doping on the structural, optical, and photo-catalytic properties of titania nanopowders have been studied by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and UV–Vis absorption spectroscopy. XRD results suggest that adding impurities has a significant effect on anatase phase stability, crystallinity, and particle size of TiO₂. Titania rutile phase formation in ternary system (Ti–Si–Zr) was inhibited by Zr⁴⁺ and Si⁴⁺ co-doped TiO₂ in high temperatures (500–900 °C) and 36 mol% anatase composition is retained even after calcination at 1,000 °C. The photocatalyst activity was evaluated by photocatalytic degradation kinetics of aqueous methylen orange under visible radiation. The results show that the photocatalytic activity of the 20 %Si and 15 %Zr co-doped TiO₂ nanopowders have a larger degradation efficiency than pure TiO₂ under visible light.     

Structure and evolved gas analyses (TG/DTA-MS and TG-FTIR) of <Emphasis Type="Italic">mer</Emphasis>-trichlorotris(thiourea)-indium(III), a precursor for indium sulfide thin films

The indium complex, mer-trichlorotris(thiourea)-indium(III) (In(tu)₃Cl₃, 1), crystallized from aqueous solution of InCl₃ and SC(NH₂)₂ (tu) with molar ratio of 1:3, is a single-source precursor for In₂S₃ films by chemical spray pyrolysis. The structural model of the triclinic crystal 1 (space group P-1 with a = 8.4842(2) Å, b = 10.5174(2) Å, c = 13.1767(2) Å, α = 111.1870(10)°, β = 98.0870(10)°, γ = 97.889(2)°) has been improved by single crystal X-ray diffraction analysis through successful separation of the disordered positions of the asymmetric complex molecule situated on the inversion centre into two spatial arrangements. Thermal decomposition of 1 occurs with very similar mass loss courses till 400 °C in both nitrogen and air, anyhow the DTA curve indicates a gas-phase oxidation with an additional exothermic heat effect at 255 °C in air. Partial or more advanced oxidation of the initially evolved CS₂ has taken place in both atmospheres, as its oxidation products, SO₂, COS, CO₂ are accompanied by the release of NH₃, HCl in temperature range of 205–275 °C, while H₂NCN and HCN evolve in air. In the third mass loss step, in the temperature interval of 405–750 °C in nitrogen and 405–700 °C in air, two processes, evaporation and oxidation of the solid residues are competing with each other, resulting in final decomposition product of 1 in air In₂O₃, while also some In₂O₃ in inert atmosphere beyond the main phase of In₂S₃ where, in addition considerable extent of loss of indium occurs, probably through volatile dimeric indium chloride species, which could not be detected either by EGA-MS or EGA-FTIR systems of ours. Nevertheless, evolution of HNCS is confirmed by EGA-FTIR, and release of CO₂, H₂NCN, SO₂, and a little HCl is detected at temperatures above 450 °C in both atmospheres.     

Evaluation of N719 amount in TiO2 films for DSSC by thermogravimetric analysis

The aim of this work is to evaluate the amount of N719 dye in TiO₂ films for DSSCs by thermogravimetric analysis coupled with mass spectrometry (TG-MS) in comparison with the traditional method based on the dye extraction in NaOH solutions. The characterization was carried out on TiO₂ films applied on FTO glasses by automatic screen printing method. For all the samples, TG-MS showed three well defined steps. The first, below 100 °C and coupled to an endothermic signal was due to water release. From 200 to about 300 °C, there was the release of CO₂ coming from decarboxylation reaction of N719. The last exothermic step was due to the combustion of organic residues. As the decarboxylation reaction occurs with release of 4 moles of CO₂ per mole of N719, it was used to determine the dye loading of the samples that resulted in the range 7?15 wt% well agreeing the relevant content of dye obtained by desorption with NaOH.     

Structure and thermal decomposition of ammonium metatungstate

The structure and morphology of ammonium metatungstate (AMT), (NH₄)₆[H₂W₁₂O₄₀]?4H₂O, and its thermal decomposition in air and nitrogen atmospheres were investigated by SEM, FTIR, XRD, and TG/DTA-MS. The cell parameters of the AMT sample were determined and refined with a full profile fit. The thermal decomposition of AMT involved several steps in inert atmosphere: (i) release of crystal water between 25 and 200 °C resulting in dehydrated AMT, (ii) formation of an amorphous phase between 200 and 380 °C, (iii) from which hexagonal WO₃ formed between 380 and 500 °C, and (iv) which then transformed into the more stable m-WO₃ between 500 and 600 °C. As a difference in air, the as-formed NH₃ ignited with an exothermic heat effect, and nitrous oxides formed as combustion products. The thermal behavior of AMT was similar to ammonium paratungstate (APT), (NH₄)₁₀[H₂W₁₂O₄₂]?4H₂O, the only main difference being the lack of dry NH₃ evolution between 170 and 240 °C in the case of AMT.     

Effect of synthesis conditions on the preparation of titanium dioxides from peroxotitanate solution and their photocatalytic activity

Nanosized TiO₂ particles were prepared by the hydrothermal method from the amorphous powders which were precipitated in an aqueous peroxotitanate solution. The physical properties of the nanosized TiO₂ particles prepared were investigated. We also examined the activity of TiO₂ particles as a photocatalyst on the decomposition of orange II. The titania sol can be successfully crystallized to the anatase phase through hydrothermal aging at temperatures higher than 160°C. The particle size increases from 18 to 26 nm as the synthesis temperature increases from 140 to 200°C. Titania particles prepared at 180°C show the highest activity, and titania particles calcined at 400°C show also the highest activity on the photocatalytic decomposition of orange II.     

Sol–gel synthesis of mesoporous WO3–TiO2 composite thin films for photochromic devices

Mesoporous WO₃–TiO₂ composite films were prepared by a sol gel based two stage dip coating method and subsequent annealing at 450, 500 and 600 °C. An organically modified silicate based templating strategy was adopted in order to obtain a mesoporous structure. The composite films were prepared on ITO coated glass substrates. The porosity, morphology, and microstructures of the resultant products were characterized by scanning electron microscopy, N₂ adsorption–desorption measurements, μ-Raman spectroscopy and X-ray diffraction. Calcination of the films at 450, and 500 °C resulted in mixed hexagonal (h) plus monoclinic phases, and pure monoclinic (m) phase of WO₃, respectively. The degree of crystallization of TiO₂ present in these composite films was not evident. The composite films annealed at 600 °C, however, consist of orthorhombic (o) WO₃ and anatase TiO₂. It was found that the o-WO₃ phase was stabilized by nanocrystalline anatase TiO₂. The thus obtained mesoporous WO₃–TiO₂ composite films were dye sensitized and applied for the construction of photochromic devices. The device constructed using dye sensitized WO₃–TiO₂ composite layer heat treated at 600 °C showed an optical modulation of 51 % in the NIR region, whereas the devices based on the composite layers heat treated at 450, and 500 °C showed only a moderate optical modulation of 24.9, and 38 %, respectively. This remarkable difference in the transmittance response is attributed to nanocrystalline anatase TiO₂ embedded in the orthorhombic WO₃ matrix of the WO₃–TiO₂ composite layer annealed at 600 °C.     

Erratum to: Sol–gel synthesis of mesoporous WO3–TiO2 composite thin films for photochromic devices

Mesoporous WO₃–TiO₂ composite films were prepared by a sol gel based two stage dip coating method and subsequent annealing at 450, 500 and 600 °C. An organically modified silicate based templating strategy was adopted in order to obtain a mesoporous structure. The composite films were prepared on ITO coated glass substrates. The porosity, morphology, and microstructures of the resultant products were characterized by scanning electron microscopy, N₂ adsorption–desorption measurements, μ-Raman spectroscopy and X-ray diffraction. Calcination of the films at 450, and 500 °C resulted in mixed hexagonal (h) plus monoclinic phases, and pure monoclinic (m) phase of WO₃, respectively. The degree of crystallization of TiO₂ present in these composite films was not evident. The composite films annealed at 600 °C, however, consist of orthorhombic (o) WO₃ and anatase TiO₂. It was found that the o-WO₃ phase was stabilized by nanocrystalline anatase TiO₂. The thus obtained mesoporous WO₃–TiO₂ composite films were dye sensitized and applied for the construction of photochromic devices. The device constructed using dye sensitized WO₃–TiO₂ composite layer heat treated at 600 °C showed an optical modulation of 51 % in the NIR region, whereas the devices based on the composite layers heat treated at 450, and 500 °C showed only a moderate optical modulation of 24.9, and 38 %, respectively. This remarkable difference in the transmittance response is attributed to nanocrystalline anatase TiO₂ embedded in the orthorhombic WO₃ matrix of the WO₃–TiO₂ composite layer annealed at 600 °C.     

Contribution of CuxO distribution,shape and ratio on TiO2 nanotubes to improve methanol production from CO2 photoelectroreduction

Many studies are focused on the development of materials for converting carbon dioxide into multicarbon oxygenates such as methanol and ethanol, because of their higher energy density and wider applicability. In this work, TiO₂ nanotubes (NT/TiO₂) were modified with Cu_xO nanoparticles in order to investigate the contribution of different ratio of Cu₂O/CuO and its distribution over NT/TiO₂ for CO₂ photoelectro-conversion to methanol. The photoelectrodes were built by anodization process to obtain NT/TiO₂ layer, and the decoration with Cu_xO hybrid system was carried out by electrodeposition process, using Na₂SO₄ or acid lactic as electrolyte, followed by annealing at different temperatures. X-ray photoelectron spectroscopy analysis revealed the predominance of Cu⁺¹ and Cu⁺² at 150 °C and 300 °C, respectively. X-ray diffraction and scanning electron microscopy indicated that under lactic acid solution, the oxide nanoparticles exhibited small size, cubic shape, and uniform distribution on the nanotube wall. While under Na₂SO₄ electrolyte, large nanoparticles with two different morphologies, octahedral and cubic shapes, were deposited on the top of the nanotubes. All modified electrodes converted CO₂ in methanol in different quantities, identified by gas chromatograph. However, the NT/TiO₂ modified with CuO/Cu₂O (80:20) nanoparticles using lactic acid as electrolyte showed better performance in the CO₂ reduction to methanol (0.11 mmol L⁻¹) in relation to the other electrodes. In all cases, a blend among the structures and nanoparticle morphologies were achieved and essential to create new site of reactions what improved the use of light irradiation, minimization of charge recombination rate and promoted high selectivity of products.

     

Synthesis of high surface area nanocrystalline anatase-TiO2 powders derived from particulate sol-gel route by tailoring processing parameters

Stabilised titania sols were prepared using an additive free particulate sol-gel route, via electrostatic stabilisation mechanism, with various processing parameters. Peptisation temperature, 50°C and 70°C, and TiO₂ concentration, 0.1, 0.2 and 0.4 molar, were chosen as processing parameters during sol preparation. Results from TiO₂ particle size and zeta potential of sols revealed that the smallest titania hydrodynamic diameter (13 nm) and the highest zeta potential (47.7 mV) were obtained for the sol produced at the lower peptisation temperature of 50°C and lower TiO₂ concentration of 0.1 M. On the other hand, between the sols prepared at 70°C, smaller titania particles (20 nm) and higher zeta potential (46.3 mV) were achieved with increasing TiO₂ concentration up to 0.4 M. X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) results of produced powders annealed at different temperatures showed that the 300°C annealed powder made from 0.1 M sol prepared at 50°C was a mixture of anatase and brookite, corresponding to a major phase of anatase (∼95% estimated), with the smallest average crystallite size of 1.3 nm and the highest specific surface area (SSA) of 193 m²/g. Furthermore, increasing TiO₂ concentration up to 0.4 molar for the sols prepared at 70°C resulted in decreasing the average crystallite size (1.9 nm at 300°C) and increasing SSA (116 m²/g at 300°C) of the powders annealed at different temperatures. Anatase-to-rutile phase transformation temperature was increased with decreasing peptisation temperature down to 50°C, whereas TiO₂ concentration had no effect on this transition. Anatase percentage increased with decreasing both peptisation temperature and TiO₂ concentration. Such prepared powders can be used in many applications in areas from photo catalysts to gas sensors.     

Fabrication of self-organized TiO2 nanotube arrays for photocatalytic reduction of CO2

The photocatalytic conversion of CO₂ and H₂O to alcohols was achieved using self-organized TiO₂ nanotube arrays (TNAs), which were prepared by electrochemical anodization of Ti foils in 1 M (NH₄)₂SO₄ electrolyte containing 0.5 wt% NH₄F. Experimental results revealed that the morphology and structure of self-organized TNAs could be strongly influenced by the applied voltage and anodization temperature, and the optimized TNAs were prepared by electrochemical anodization of Ti foils under optimal conditions (i.e., at 20 V for 2 h at 30 °C). The as-prepared TNAs were amorphous and could be transformed to anatase phase during the thermal treatment at 450 °C in air for 3 h. By using the annealed TNAs as a photocatalyst, the photocatalytic reduction of CO₂ to alcohol, predominately methanol and ethanol, was demonstrated under Xenon lamp illumination. Based on the photocatalytic measurements, the production rates of methanol and ethanol were calculated to be ～10 and ～9 nmol cm^?2 h^?1, respectively. In addition, the formation mechanism of methanol and ethanol was also tentatively proposed.     

Influence of alkali-treatment temperature on the one-dimensional structure of nanosized TiO2

One-dimensional titania nanomaterials were synthesized by soft chemical processes in an alkali medium. The effect of alkali-treatment temperature on the morphology, porosity, and crystalline phase of TiO₂ nanomaterials was investigated. Nanotubes having an opening end were observed when aging the sample at 130 °C, while conducting the process at 180 °C resulted in nanoribbons with a high aspect ratio. Post-treatment at 300 °C led to the partial transformation from tubular structures into nanoparticles or ribbons into nanobelts. While the monoclinic sodium hexatitanate and anatase crystal had a tubular structure, nanoribbons and nanobelt TiO₂ also showed the presence of hydrogen titanate and sodium trititanate. High surface areas were achieved in as-prepared nanotubes and nanoribbons, 456.5 and 72.1 m²/g, respectively, and a drastic reduction was obtained upon post-treatment at 300 °C to 72.1 and 19.2 m²/g, respectively.     

TG–MS–FTIR (evolved gas analysis) of kaolinite–urea intercalation complex

The products evolved during the thermal decomposition of kaolinite–urea intercalation complex were studied by using TG–FTIR–MS technique. The main gases and volatile products released during the thermal decomposition of kaolinite–urea intercalation complex are ammonia (NH₃), water (H₂O), cyanic acid (HNCO), carbon dioxide (CO₂), nitric acid (HNO₃), and biuret ((H₂NCO)₂NH). The results showed that the evolved products obtained were mainly divided into two processes: (1) the main evolved products CO₂, H₂O, NH₃, HNCO are mainly released at the temperature between 200 and 450 °C with a maximum at 355 °C; (2) up to 600 °C, the main evolved products are H₂O and CO₂ with a maximum at 575 °C. It is concluded that the thermal decomposition of the kaolinite–urea intercalation complex includes two stages: (a) thermal decomposition of urea in the intercalation complex takes place in four steps up to 450 °C; (b) the dehydroxylation of kaolinite and thermal decomposition of residual urea occurs between 500 and 600 °C with a maximum at 575 °C. The mass spectrometric analysis results are in good agreement with the infrared spectroscopic analysis of the evolved gases. These results give the evidence on the thermal decomposition products and make all explanation have the sufficient evidence. Therefore, TG–MS–IR is a powerful tool for the investigation of gas evolution from the thermal decomposition of materials and its intercalation complexes.     

Applying thermodynamics to digestion/gasification processes: the Attainable Region approach

The research shows theoretical calculations on the thermodynamics of digestion/gasification processes where glucose is used as a surrogate for biomass. The change in Enthalpy (?H) and Gibbs Free Energy (?G) is used to obtain the Attainable Region (AR) that shows the overall thermodynamic limits for digestion/gasification from 1 mol of glucose. Gibbs Free Energy and Enthalpy (G–H) plots were calculated for the temperature range 25–1500 °C. The results show the effect of temperature on the AR for the processes when water is in both liquid and gas states using 25 °C, 1 bar as the reference state. The AR results show that the production of CO, H₂, CH₄ and CO₂ are feasible at all temperatures studied. The minimum Gibbs Free Energy becomes more negative from ?418.68 kJ mol^?1 at 25 °C to ?3024.34 kJ mol^?1 at 1500 °C while the process shifts from exothermic (?141.90 kJ mol^?1) to endothermic (1161.80 kJ mol^?1) for the respective temperatures. Methane and carbon dioxide are favoured products (minimum Gibbs Free Energy) for temperatures up to about 600 °C, and this therefore includes Anaerobic Digestion. The process is exothermic below 500 °C, and thus Anaerobic Digestion requires heat removal. As the temperature continues to increase, hydrogen production becomes more favourable than methane production. The production of gas is endothermic above 500 °C, and it needs a supply of heat that could be done, either by combustion or by electricity (plasma gasification). The calculations show that glucose conversion at temperatures around 700 °C favours the production of carbon dioxide and hydrogen at minimum G. Generally, the results show that the gas from high-temperature gasification (>~800 °C) typically carries the energy mainly in syngas components CO and H₂, whereas at low-temperature gasification (<500 °C) the energy is carried in CH₄. The overall analysis for the temperature range (25–1500 °C) also suggests a close relationship between biogas production/digestion and gasification as biogas production can be referred to as a form of low-temperature gasification.     

Thermal analysis of the pyrolytic behaviors of a highly crosslinked polymer

Thermogravimetric-mass spectrometric (TG/MS) and differential scanning calorimetric (DSC) techniques were used in the characterization of oxidative and nonoxidative degradation reactions of a highly crosslinked divinylbenzene/styrene copolymer. When the copolymer was subjected to a temperature-programmed air environment, four exothermic reactions were detected. The initial small exothermic reaction, starting at ca. 125°C and reaching its maximum at ca. 180°C, was presumed to result from the decomposition of peroxides. The second exothermic reaction, which overlapped with the initial one and peaked at ca. 270°C, was attributed to oxidation with a significant amount of oxygen uptake and liberation of some gaseous products such as CO₂, styrene, benzaldehyde, ethylstyrene, and ethylbenzaldehyde. The strongest exothermic reaction took place at ca. 290–380°C and had its peak at ca. 360°C. Associated with this reaction was the generation of many gaseous pyrolysates, as given above. The exothermic reaction continued at a relatively constant rate from ca. 380°C to the maximum temperature of the experiment (500°C) with the release of only one gaseous product (CO₂). The initial exothermic reaction can be eliminated by controlled thermal decomposition of peroxides; therefore, a more thermally stable polymer can be obtained. Exothermic reactions, starting at ca. 170°C, were observed. Pyrolytic reactions in an inert gas were also studied.     

Short time deposition of TiO2 nanoparticles on SiC as photocatalysts for the degradation of organic dyes

The deposition of TiO₂ nanoparticles on SiC was carried out by mechanical milling under different conditions. SiC–TiO₂ samples were used as photocatalysts for the degradation of organic dyes such as methylene blue and rhodamine B. A short time deposition of TiO₂ nanoparticles was observed during mechanical milling (2 min at 200 rpm) to cover the SiC particles. The presence of SiC and TiO₂ (anatase and rutile) was confirmed by means of X-ray diffraction after thermal treatment at 450 °C. The deposition of TiO₂ on SiC was corroborated by scanning electron microscopy analysis; the thickness of the thin layer of TiO₂ deposited on SiC increases as the proportion of TiO₂ increases. The energy band gap values obtained for these compounds were around 3.0 eV. SiC–TiO₂ photocatalysts prepared by mechanical milling exhibited better activity under UV-light irradiation for the degradation of methylene blue and rhodamine B than commercial TiO₂ powder (titania P25).     

Kinetics and mechanisms of the oxidation of sulfur dioxide over titanium dioxide and nickel oxide

The reactions between SO₂ and O₂ were carried out in the presence of TiO₂ and NiO under various partial pressures of SO₂ and O₂ at temperatures from 240 to 330°C. TiO₂ and NiO were pretreated by applying an annealing effect from which the catalysts would have the different activity. The rates are the highest for TiO₂ pretreated at high temperature in the region of 400 to 600deg;C in vacuum, 1.21 × 10^?4 mmHg. In contrast, the rates are the lowest for NiO pretreated at high temperaturefrom 350 to 550°C. The data have been correlated with 1.4 and first-order kinetics for TiO₂ and NiO, respectively. A reaction mechanism to explain the data was suggested. The quantities of anionic vacancies in TiO₂ surfaces and of positive holes in NiO appeared to be paramount in determining the type of kinetics.