Photocatalytic degradation rate of oxalic acid on a semiconductive layer of n-TiO2 particles in a batch mode plate photoreactor Part II: Light intensity limit

The effect of light intensity, flow rate and oxygen bubbling on the photocatalytic degradation rate of oxalic acid on a layer of TiO2 particles was investigated. At higher concentrations of oxalic acid ( 0.005 M) and photon flux intensity in the range 3.26 × 10–10 to 1.07 × 10–8 einstein cm–2 s–1, the rate of photocatalysis was controlled simultaneously by the flux of both oxygen and photons. This is probably caused by the ability of oxygen to react with photogenerated electrons and thus suppress the electron-hole recombination and increase the efficiency of the photocatalytic degradation.     

Nitrous oxide flux from a solid dairy manure pile measured using a micrometeorological mass balance method

A micrometeorological mass balance technique was used to quantify the N₂O flux from a solid dairy manure pile under field conditions. Flux was determined using time-averaged measurements of wind speed, and nitrous oxide concentration using a tunable diode laser trace gas analyzer. A total of 66 hourly flux averages were collected and values were never lower than 200 ng N₂O-N m^–2 s^–1. The mean hourly N₂O flux was 4865 ng N₂O-N m^–2 s^–1 (0.42 g N m^–2 day^–1), which is of the same order of magnitude, albeit higher, as previously observed for a similar solid pig manure storage.     

Effects of rice cultivars on methane fluxes in a paddy soil

CH₄ emission and its relevant processes involved (i.e. CH₄ production, rhizospheric CH₄ oxidation and plant-mediated CH₄ transport) were studied simultaneously to comprehensively understand how rice cultivars (Yanxuan, 72031, and 9516) at growth stages (early and late tillering, panicle initiation, ripening, and harvest stage) affect CH₄ emission in a paddy soil. Over the entire rice-growing season, Yanxuan had the highest CH₄ emission flux with 5.98 g CH₄ m^–2 h^–1 followed by 72031 (4.48 g CH₄ m^–2 h^–1) and 9516 (3.41 g CH₄ m^–2 h^–1). The highest CH₄ production rate of paddy soils planted to Yanxuan was observed with 18.0 g CH₄ kg_{{ (d.w.soil)}} h^–1 followed by the soil planted to 9516 (17.5 g CH₄ kg_{{ (d.w.soil)}} h^–1). For each cultivar, both rhizospheric CH₄ oxidation ability and plant-mediated CH₄ transport efficiency varied widely with a range of 9.81–76.8% and 15.5–80.5% over the duration of crop growth, respectively. Multiple regression analyses showed that CH₄ emission flux was positively related with CH₄ production rate and rice plant-mediated CH₄ transport efficiency, but negatively with rhizospheric CH₄ oxidation (R ²=0.425 for Yanxuan, P<0.01; R ²=0.426 for 72031, P<0.01; R ²=0.564 for 9516, P<0.01). The contribution of rice plants to CH₄ production seems to be more important than to rhizospheric CH₄ oxidation and plant-mediated transport in impact of rice plants on CH₄ emission.     

Impact of supercritical drying and heat treatment on physical properties of titania/silica aerogel monolithic and its applications

TiO₂–SiO₂ monolithic aerogels were homogeneously prepared using sol–gel method. Critical point of drying of TiO₂–SiO₂ gels with ethanol was studied for 30, 60, 90 and 120 min. Subsequently, the gels were dried with supercritical ethanol, resulting in amorphous aerogels that crystallized following heat treatment at 550 °C from 1 to 5 h. The TiO₂–SiO₂ aerogels were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements. The molar ratio of SiO₂:TiO₂ was 6 and the synthetic strategy revealed that TiO₂–SiO₂ aerogel, had a surface area 868 m²/g, particle size 40 nm, density 0.17 g/cm³ and 80% porosity. The finding indicated that from economic point of view, TiO₂–SiO₂ gel should be supercritical dried for 30 min and heat-treated for 5 h. The TiO₂–SiO₂ aerogel monoliths photocatalyst synthesized using sol–gel method provided insight into the characteristics that make a photocatalyst material well-suited for photodegradation of phenol and cyanide in an industrial waste stream containing Cl⁻, S²⁻ and NH₄⁺. Interestingly, after multiple reuse cycles (i.e. ≥7), photodegradation systems with regenerated photocatalyst showed a slightly decreasing of photoactivity 2–4%. The overall kinetics of photodegradation of either phenol or cyanide using TiO₂–SiO₂ aerogel photocatalyst was found to be of first order.     

Synergistic effect of TiO2 and iron oxide supported on fluorocarbon films. Part 1: Effect of preparation parameters on photocatalytic degradation of organic pollutant at neutral pH

Iron oxide and TiO₂ were immobilized on modified polyvinyl fluoride films in a sequential process. Synergic effects of iron oxide and TiO₂ on the polymer film were observed during the heterogeneous degradation of hydroquinone (HQ) in the presence of H₂O₂ at pH close to neutrality and under simulated solar irradiation. Within the degradation period, little iron leaching (<0.5 mg/L) was observed.The surface of commercial polyvinyl fluoride (PVF) film was modified by TiO₂ under light inducing oxygen group (C–OH, CO, COOH) formation on the film surface. During this treatment, TiO₂ nanoparticles simultaneously bind to the film, leading to PVF^f–TiO₂. The possible mechanistic pathway for the TiO₂ deposition and the nature of the polymer–TiO₂ interaction are discussed. Furthermore PVF and PVF^f–TiO₂ were immersed in an aqueous solution for the deposition of iron oxide layer by hydrolysis of FeCl₃, leading to PVF–Fe oxide and to PVF^f–TiO₂–Fe oxide respectively.HQ degradation and mineralization mediated by PVF^f–TiO₂, PVF–Fe oxide and PVF^f–TiO₂–Fe oxide were investigated under different conditions. Remarkable synergistic effects were observed for PVF^f–TiO₂–Fe oxide possibly due to Fe(II) regeneration, accelerated by electron transfer from TiO₂ to the iron oxide under light.     

Supported Heteropolyacids for NOx Storage and Reduction

TiO₂ and SnO₂ were studied as possible supports for HPW in order to produce a new type of sorbent for NO _x trap applications. SnO₂ was synthesised by various inorganic methods while TiO₂ was purchased commercially. The sorption capacities and efficiencies depend upon the BET surfaces. Best performances were obtained with support with large pores where the crystalline structure of HPW is maintained. The optimal loading of HPW is ca. 50% with BET surface in the range 20–80 m²g^–1.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

Comparison of the photocatalytic degradation of 2-propanol in gas–solid and liquid–solid systems by using TiO2–LnPc2 hybrid powders

Photocatalytic degradation of 2-propanol was carried out as a probe reaction both in gas–solid and in liquid–solid systems in the presence of TiO₂ both bare and impregnated with lanthanide (Sm, Gd, Ho) bis-phthalocyanines (LnPc₂) used as sensitizers. Continuous and batch photo-reactors, irradiated with an equal flux of photons, were used in gas–solid and in liquid–solid systems, respectively. Propanone and acetaldehyde were the main intermediates found in both systems during 2-propanol oxidation, whereas carbon dioxide and water were the final oxidation products exclusively in the gas–solid regime. The photocatalysts exhibited significantly higher activity in the liquid–solid system than in the gas–solid one. Samples impregnated with the sensitizers were apparently more photoactive than bare TiO₂. The highest photoactivity was shown by the 1.85 HoPc₂–TiO₂ sample.     

Revisiting Ga2O3/H-ZSM-5 propane aromatization catalysts

Oxidative or reducing treatments at 873 K of Ga₂O₃/H-ZSM-5 (Si/Al = 13 or 60) cause a change in H⁺ concentration as evidenced by IR spectroscopy of OH groups at 3610 cm^–1; this change is nicely correlated to the variation ofm-xylene isomerisation rate.     

Highly active TiO2−x−yNxFy visible photocatalyst prepared under supercritical conditions in NH4F/EtOH fluid

A novel N and F co-doped TiO₂ (TiO_2−x−yN_xF_y) photocatalyst is prepared by treating the TiO₂ precursor in NH₄F/ethanol fluid under supercritical conditions. During photocatalytic degradation of methylene blue under visible light irradiation, the as-prepared TiO_2−x−yN_xF_y exhibits higher activity than the undoped TiO₂, N-doped TiO₂ (TiO_2−xN_x), and F-doped TiO₂ (TiO_2−yF_y). Based on the characterizations including XRD, Raman, FTIR, TEM, PLS, UV–vis DRS, N₂ adsorption–desorption isotherms, XPS and NH₃-TPD, the synergetic promotions of N- and F-dopants incorporated into the TiO₂ lattice are discussed based on the enhanced spectral response in visible region, oxygen vacancies, and surface acidic sites. Meanwhile, the supercritical treatment also promotes the activity owing to the increase in both the surface area and the crystallization degree of anatase, and the enhanced incorporation of N- and F-dopants into the TiO₂ lattice.     

Tungsten and nitrogen co-doped TiO2 nano-powders with strong visible light response

A two-step method, combining with sol–gel and mechanical alloying (MA) method, was used to fabricate the tungsten and nitrogen co-doped TiO₂ nano-powders ((W, N) co-doped TiO₂ NPs). The (W, N) co-doped TiO₂ NPs showed strong absorbance in visible range, as long as 650 nm. Enhanced photocatalytic activities under visible light irradiation were also observed from the results of photodegradation experiments and chemical oxygen demand (COD) analysis. Physical, chemical, and optical properties of the samples were investigated. Possible reasons for the enhanced photocatalytic activities were analyzed based on the experimental results. Oxygen vacancies detected by electron spin response (ESR) spectra, acting as trapping agencies for electrons (e⁻) to produce active oxygen species (O²⁻), were proved to be the main cause for the improved photocatalytic performances.     

Structure and performance of a novel TiO2-phosphonate composite photocatalyst

An innovative SiO₂-PO₄³⁻-TiO₂ photocatalyst is presented which is able to bond TiO₂ to Raschig rings (RR). Evidence for the formation on the catalyst surface of PO stretching bands near 1200–1250 cm⁻¹ is presented by FTIR spectroscopy. The TiO₂ Degussa P25 on the catalyst surface (RR) was further characterized by high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction showing that the composite catalyst prepared at 500 °C does not alter the particle size or crystallographic composition of the TiO₂ Degussa P25 particles. The Ti- and P-distribution of the catalyst surface overlayers was obtained by Ar-sputtering eroding up to 100 topmost catalyst layers. By atomic force microscopy (AFM) the root mean square roughness (Rq) or rugosity of 771 nm and an average height of the catalyst layer of 1.52 μm were found on the glass surface. The root mean square roughness Rq varies very little in value before and after the photocatalysis indicating that the sample porosity is conserved during 4-CP photodegradation. The disappearance kinetics of 4-chlorophenol (4-CP) on the SiO₂-PO₄³⁻-TiO₂ composite occurred within 15 min and was faster than the 45 min needed with suspensions of TiO₂ Degussa P25 (1 g L⁻¹). The SiO₂-PO₄³⁻-TiO₂ photocatalyst was able to degrade repetitively 4-CP solutions without loss of activity. The effect of the light intensity, oxidant concentration and 4-CP concentration on the photodegradation kinetics was investigated and is reported in this study.     

Nitrous oxide emissions from paddy soil in three rice-based cropping systems in China

Rice-flooding fallow, rice-wheat, and double rice-wheat systems were adopted in pot experiment in an annual rotation to investigate the effects of cropping system on N₂O emission from rice-based cropping systems. The annual N₂O emission from the rice-wheat and the double rice-wheat cropping systems were 4.3 kg N ha^–1 and 3.9 kg N ha^–1, respectively, higher than that from rice-flooding fallow cropping system, 1.4 kg N ha^–1. The average N₂O flux was 115 and 118 g N m^–2 h^–1 for rice season in rice-wheat system and early rice season in double rice-wheat system, respectively, 68.6 and 35.3 g N m^–2 h^–1 for the late rice season in double rice-wheat system and rice season in rice-flooding fallow, respectively, and only 3.1–5.3 g N m^–2 h^–1 for winter wheat or flooding fallow season. Temporal variations of N₂O emission during rice growing seasons differed and high N₂O emission occurred when soil conditions changed from upland crop to flooded rice.     

Photocatalytic disinfection ofE. Coli in a suspended TiO2/UV reactor

Disinfection capability using both TiO₂ and UV radiation was more than 27 times as that by using only the UV light. Optimal TiO₂ concentration and UV light intensity were 0.1 g TiO₂ l ^-1 and 50W m^-2 with which 100% reduction time was just 2–3 min in a batch-type slurry reactor.     

Measurement of nitrous oxide emissions from two rice-based cropping systems in China

A field experiment was conducted to investigate the effects of winter management and N fertilization on N₂O emission from a double rice-based cropping system. A rice field was either cropped with milk vetch (plot V) or left fallow (plot F) during the winter between rice crops. The milk vetch was incorporated in situ when the plot was prepared for rice transplanting. Then the plots V and F were divided into two sub-plots, which were then fertilized with 276 kg urea-N ha^–1 (referred to as plot VN and plot FN) or not fertilized (referred to as plot VU and plot FU). N₂O emission was measured periodically during the winter season and double rice growing seasons. The average N₂O flux was 11.0 and 18.1 g N m^–2 h^–1 for plot V and plot F, respectively, during winter season. During the early rice growing period, N₂O emission from plot VN averaged 167 g N m^–2 h^–1, which was eight- to fifteen-fold higher than that from the other three treatments (17.8, 21.0 and 10.8 g N m^–2 h^–1 for plots VU, FN, and FU, respectively). During the late rice growing period, the mean N₂O flux was 14.5, 11.1, 12.1 and 9.9 g N m^–2 h^–1 for plots VN, VU, FN and FU, respectively. The annual N₂O emission rates from green manure-double rice and fallow-double rice cropping systems were 3.6 kg N ha^–1 and 1.3 kg N ha^–1, respectively, with synthetic N fertilizer, and were 0.99 kg N ha^–1 and 1.12 kg N ha^–1, respectively, without synthetic N fertilizer. Generally, both green manure N and synthetic fertilizer N contribute to N₂O emission during double rice season.     

Ceria modified MnOx/TiO2 as a superior catalyst for NO reduction with NH3 at low-temperature

A series of cerium modified MnOx/TiO₂ catalysts were prepared by sol–gel method and used for low-temperature selective catalytic reduction (SCR) of NOx with ammonia. The experimental results showed that NO conversion could be improved by doping Ce from 39% to 84% at 80 °C with a gas hourly space velocity (GHSV) of 40,000 h⁻¹. This activity improvement may be contributed to the increase of chemisorbed oxygen and acidity after Ce doping. TPR results also verified that the redox property of Ce modified MnOx/TiO₂ was enhanced at low-temperature.     

Electrical properties of V2O5–WO3/TiO2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNOx reaction

Electrical conductivity measurements on EUROCAT V₂O₅–WO₃/TiO₂ catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH₃ to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH₃ without O₂, and (ii) 2000 ppm NO + 2000 ppm NH₃ + 500 ppm O₂) to put in evidence redox processes in SCR deNO_x reaction.It was first demonstrated that titania support contains certain amounts of dissolved W⁶⁺ and V⁵⁺ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO₃/TiO₂ precursor of V₂O₅–WO₃/TiO₂ EUROCAT catalyst. It contained an average amount of 0.37 at.% W⁶⁺dissolved in titania, i.e. 1.07 × 10²⁰ W⁶⁺ cations dissolved/cm³ of titania. For the fresh catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved in titania were found to be equal to 1.07 × 10²⁰ and 4.47 × 10²⁰ cm⁻³, respectively. For the used catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved were found to be equal to 1.07 × 10²⁰ and 7.42 × 10²⁰ cm⁻³, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10²⁰ additional V⁵⁺ cations in titania.After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO⁻ and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH₃ reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N^o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to

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. In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.     

CO hydrogenation to iso-C4 hydrocarbons over CeO2–TiO2 catalysts

CeO₂–TiO₂ (Ce:Ti = 0.25–9, molar ratio) catalysts were synthesized by a sol–gel method and the catalytic performances were evaluated in the selective synthesis of isobutene and isobutane from CO hydrogenation under the reaction conditions of 673–748 K, 1–5 MPa and 720–3000 h⁻¹. The physical properties, such as specific surface area, cumulative pore volume, average pore diameter, crystal phase and size, of the catalysts were characterized by N₂ adsorption/desorption and XRD. All the CeO₂–TiO₂ composite oxides showed higher surface areas than pure TiO₂ and CeO₂. No TiO₂ phase was detected on the samples of CeO₂–TiO₂ in which TiO₂ contents were in the range of 10–50 mol%. Crystalline Ce₂O₃ was detected in CeO₂–TiO₂ (8:2). The reaction conditions, temperature, pressure and space velocity, had obvious influences on the CO conversion and distribution of the products over CeO₂–TiO₂ (8:2) catalyst.     

Solid acidities of SiO2–TiO2/montmorillonite composites synthesized under different pH conditions

SiO₂–TiO₂/montmorillonite composites were prepared under acidic, neutral and basic conditions and the solid acidity of the resulting composites were determined. All the SiO₂/TiO₂ ratio of the colloidal particles was set at 10 but the resulting SiO₂/TiO₂ ratios were significantly richer in TiO₂. The XRD patterns of the acidic composite showed expanding and broadening of the (001) reflection by intercalation of colloidal SiO₂–TiO₂ particles, but the neutral and basic composites showed only broadening of the reflections and no intercalation. The specific surface areas of the acidic, neutral and basic composites (375, 237 and 247 m²/g, respectively) were much larger than of montmorillonite (6 m²/g). The average pore sizes were about 4, 15 and 50 nm, and the amounts of solid acidic sites measured by the NH₃-TPD were 178, 95 and 86 µmol/g for the acidic, neutral and basic composites, respectively. The solid acid amount of the acidic composite was twice that of a commercial catalyst, K-10, (85 µmol/g) and much higher than the guest phase SiO₂–TiO₂ gel (16 µmol/g) or the host phase montmorillonite (6 µmol/g). The TPD peak temperatures reflect the acid strength, and were similar in all the samples, ranging from 175° to 200 °C.     

Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions

Annual cycles of NO, NO₂ and N₂O emission rates from soil were determined with high temporal resolution at a spruce (control and limed plot) and beech forest site (Höglwald) in Southern Germany (Bavaria) by use of fully automated measuring systems. The fully automated measuring system used for the determination of NO and NO₂ flux rates is described in detail. In addition, NO, NO₂ and N₂O emission rates from soils of different pine forest ecosystems of Northeastern Germany (Brandenburg) were determined during 2 measuring campaigns in 1995. Mean monthly NO and N₂O emission rates (July 1994–June 1995) of the untreated spruce plot at the Höglwald site were in the range of 20–130 µg NO-N m^-2 h^-1 and 3.5–16.4 µg N₂O-N m^-2 h^-1, respectively. Generally, NO emission exceeded N₂O emission. Liming of a spruce plot resulted in a reduction of NO emission rates (monthly means: 15–140 µg NO-N m^-2 h^-1) by 25-30% as compared to the control spruce plot. On the other hand, liming of a spruce plot significantly enhanced over the entire observation period N₂O emission rates (monthly means: 6.2–22.1 µg N₂O-N m^-2 h^-1). Contrary to the spruce stand, mean monthly N₂O emission rates from soil of the beech plot (range: 7.9–102 µg N₂O-N m^-2 h^-1) were generally significantly higher than NO emission rates (range: 6.1–47.0 µg NO-N m^-2 h^-1). Results obtained from measuring campaigns in three different pine forest ecosystems revealed mean N₂O emission rates between 6.0 and 53.0 µg N₂O-N m^-2 h^-1 and mean NO emission rates between 2.6 and 31.1 µg NO-N m^-2 h^-1. The NO and N₂O flux rates reported here for the different measuring sites are high compared to other reported fluxes from temperate forests. Ratios of NO/N₂O emission rates were >> 1 for the spruce control and limed plot of the Höglwald site and << 1 for the beech plot. The pine forest ecosystems showed ratios of NO/N₂O emission rates of 0.9 ± 0.4. These results indicate a strong differentiating impact of tree species on the ratio of NO to N₂O emitted from soil.