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Applied Catalysis B, Environmental (v.57, #2)
Photoelectrochemical degradation of humic acid on a (TiO2)0.7(RuO2)0.3 dimensionally stable anode by L. Pinhedo; R. Pelegrini; R. Bertazzoli; A.J. Motheo (pp. 75-81).
This study reports the results obtained for the degradation of humic acid (HA) in aqueous solution, by electrochemical and photoelectrochemical oxidation. The experiments were carried out using electrolysis and photo-assisted electrolysis on a thermally prepared oxide electrode mounted in a flow cell reactor. Results showed that both process are effective for the degradation of humic acid. However, by assisting electrolysis with UV radiation, the total organic carbon (TOC) reduction rate was greatly improved. For instance, at 20mAcm−2, electrolysis was responsible for 25% of TOC reduction after 180min of reaction time. On the other hand, the decrease during the photo-electrolysis reached 65% at the same current density and processing time.
Keywords: Humic acid; DSA; ®; electrodes; Water treatment; Photoelectrooxidation
Modified TiO2–SiO2 mixed oxides by Santosh Kumar Samantaray; Kulamani Parida (pp. 83-91).
Catalytic combustion of acetone, methanol and 2-propanol as model volatile organic compounds (VOCs) has been measured over manganese-modified, hydrophobic TiO2–SiO2 mixed oxides in the temperature range 373–573K. The complete decomposition of VOCs at relatively low temperature range and no loss of catalytic activity in time-on-stream study depends on the surface area, surface oxygen and hydrophobic properties of the samples. CO2 and H2O are detected as the only combustion products of acetone and methanol at all reaction temperatures. In case of 2-propanol, acetone and di-isopropylether are detected as by-products. It is observed that manganese-modified, hydrophobic TiO2–SiO2 mixed oxides are effective enough for the destruction of VOCs and can be considered as low-temperature oxidation catalysts.
Keywords: Catalytic combustion; Volatile organic compounds; Mn(III); TiO; 2; –SiO; 2; mixed oxides
Photodegradation of ethylene using visible light responsive surfaces prepared from titania nanoparticle slurries by Surajit Kumar; Andrei G. Fedorov; James L. Gole (pp. 93-107).
An efficient nanoscale-based synthesis is used to form photocatalytically active slurries of titanium oxynitride compounds responsive to activation by visible photons and readily amenable to the surface deposition. The photocatalysts were synthesized in seconds at room temperature via direct nitridation of anatase TiO2 nanostructures with alkyl ammonium salts. With TiO2 nanoparticle agglomeration, TiO2− xN x anatase structured particles were obtained whose absorption onset extends well into the visible region at λ∼550nm. The introduction of a small quantity of palladium as PdCl2 or Pd(NO3)2 extends absorption to near-infrared region and enhances the photocatalytic activity of the nanoparticles. Tests in a stop-flow quartz reactor of concentric tubular configuration were run at room temperature (25°C), in a water-free environment and the reactants and products were detected using a mass spectrometer. The catalyzed ethylene oxidation process was observed in the stop-flow reactor in response to excitation provided by UV and incandescent lamps, and the catalyst response to visible light excitation was quantified. Extensive tests performed under a variety of conditions demonstrate photocatalytic activity in the order TiO2− xN x+Pd(NO3)2≥TiO2− xN x+PdCl2>TiO2− xN x>Colloidal TiO2 for nanoparticle assembled sites under visible light illumination. An increase in coating thickness enhances the photocatalysis under UV light illumination, but does not show as pronounced an effect when visible light is used for excitation. The synthesized photocatalysts exhibit excellent long-term stability, which is essential for their practical application.
Keywords: TiO; 2; Titania; Nanoparticle; Visible light responsive; Nitridation; Nitrogen doping; Metal seeding; Photocatalysis; Ethylene oxidation; Stop-flow reactor
Photocatalytic degradation of gaseous sulfur compounds by silver-deposited titanium dioxide by Shinji Kato; Yuji Hirano; Misao Iwata; Taizo Sano; Koji Takeuchi; Sadao Matsuzawa (pp. 109-115).
Deposition of ultrafine particle of silver on titanium dioxide (TiO2) significantly improved the photocatalytic activity for degradation of gaseous sulfur compounds, such as hydrogen sulfide (H2S) and methylmercaptan (CH3SH). The silver-deposited photocatalytic filter (Ag-PCF) was prepared by coating TiO2 powder on the porous ceramics substrate and successively depositing nano-sized silver particles on TiO2 by means of photodeposition method. The photocatalytic degradation rates of hydrogen sulfide and methylmercaptan by Ag-PCF were 7 times and 14 times larger, respectively, than those by conventional photocatalytic filter. The sulfur atom of hydrogen sulfide was oxidized into sulfate ion, which was accumulated on the filter. However, the degradation rate did not decline in the experimental period. The deposited silver seems to act as a co-catalyst that enhances the photocatalytic oxidation of sulfur compounds.
Keywords: Titanium dioxide; Silver; Photocatalytic degradation; Photodeposition; Sulfur compounds; Hydrogen sulfide; Methylmercaptan; Photocatalytic filter
Catalytic oxidation of volatile organic compounds by J. Tsou; P. Magnoux; M. Guisnet; J.J.M. Órfão; J.L. Figueiredo (pp. 117-123).
The catalytic oxidation of methyl-isobutyl-ketone (MIBK) in low concentration was investigated over several Pt/zeolite catalysts, namely Pt/HFAU(5 and 47) and Pt/HBEA(50), as well as Pt/γ-Al2O3, which is the most widely used catalyst for VOC oxidation. The Si/Al ratio of the zeolite was found to be an important parameter for this reaction: catalysts with high Si/Al (i.e., more hydrophobic) present a better performance than those with low Si/Al. On the other hand, the zeolite structure (BEA or FAU) does not have a major influence on the catalyst performance. However, any one of the dealuminated zeolites is a better support than conventional alumina. Pt/HFAU(47) was selected as the most active catalyst.The Pt/HFAU(47) catalyst suffered some deactivation in long duration experiments, probably as a result of coke accumulation. It was found that there is an optimal oxygen partial pressure in the feed: at low oxygen content the accumulated coke compounds are less oxidised; at high oxygen contents platinum oxidation can occur. The presence of water vapour did not have a major effect on the catalyst performance; however, the presence of o-xylene inhibited the oxidation of MIBK, while the oxidation of o-xylene was not affected.
Keywords: VOCs; MIBK; o; -Xylene; Oxidation; Platinum/zeolite; Platinum/alumina
Novel Zn–Ti-based mixed metal oxides for low-temperature adsorption of H2S from industrial gas streams by K. Polychronopoulou; J.L.G. Fierro; A.M. Efstathiou (pp. 125-137).
The efficiency of various Zn–Ti-based mixed metal oxides prepared by the sol–gel method towards H2S adsorption from a gas mixture containing 0.06vol.% H2S, 25vol.% H2, 7.5vol.% CO2 and 1vol.% H2O was studied in the 25–100°C range. The effects of the chemical nature of a third metal additive (M=Mn, Cu, Mo) and its at.% composition in the M–Zn–Ti–O solid on the H2S adsorption and regeneration performance of the latter were studied. The nominal chemical composition (metal at.%) of the solid adsorbent was found to have an important effect on the number, chemical composition and particle morphology of the crystal phases formed. It was found that the mixed metal oxides with compositions 20Zn–80Ti–O and 40Zn–60Ti–O present higher maximum H2S uptakes than ZnO and TiO2 solids also prepared by the sol–gel method. In addition, the Zn–Ti–O mixed metal oxides showed higher H2S uptakes after regeneration with 20% O2/He in the 500–750°C range compared to the ZnO and TiO2 solids. It was found that the10Mn–45Zn–45Ti–O solid results in higher H2S uptakes than a commercial Ni-based H2S adsorbent in the 25–50°C range. The effectiveness of the regeneration procedure of the 10Mn–45Zn–45Ti–O solid following sulfidation was found to be in the 45–170% range depending on the sulfidation temperature and regeneration conditions applied. The solid with composition 10Cu–45Zn–45Ti–O calcined at 200°C (after synthesis) exhibited three times higher H2S uptakes at 25°C than a commercial Ni-based adsorbent, result not obtained at higher calcination temperatures.
Keywords: H; 2; S adsorption; Zn–Ti-based mixed metal oxides; Regenerable H; 2; S adsorbents; Transient adsorption; H; 2; -TPR
Effect of the Cr6+ concentration in Cr-incorporated TiO2-loaded MCM-41 catalysts for visible light photocatalysis by Bo Sun; Ettireddy P. Reddy; Panagiotis G. Smirniotis (pp. 139-149).
The activity of 25% TiO2-loaded various amounts of Cr-incorporated MCM-41 (25%TiO2/Cr-MCM-41) was studied as a catalyst for photodegradation of 4-chlorophenol under visible light (400–800nm). Cr-MCM-41 with different ratios of Si to Cr was synthesized by hydrothermal method and loaded with 25wt.% TiO2 utilizing sol–gel method. The surface areas of Cr-MCM-41 and 25% TiO2/Cr-MCM-41 samples decreased with increasing the amount of Cr incorporated. XRD analysis showed that the MCM-41 structure was formed only in the samples with Si/Cr ratio larger than 20. Temperature program reduction (TPR) and UV–vis spectra results clearly indicated that only Cr6+ was present in the samples with the ratio of Si to Cr more than 20. Whereas the material with the ratio of Si to Cr less than 20 contained chromium with mixed oxidation states. It was found that the photodegradation ability of 25% TiO2/Cr-MCM-41 was highly related to the amount of Cr6+ which strongly interacted with TiO2; the optimum atomic ratio of Si to Cr was 20.This optimum catalyst was compared with two kinds of 25wt.% TiO2-loaded CrSi20O x bulk oxides. The overall photodegradation ability of these titania loaded bulk oxide catalysts under visible light was about 45% less active than 25% TiO2/Cr-MCM-41 (Si/Cr=20). TPR results clearly showed that the interaction of Cr6+ and TiO2 in the bulk oxide catalysts was weaker than that in TiO2/Cr-MCM-41. A good correlation was demonstrated between the Cr6+–TiO2 interaction (CrOTi) and catalyst's photoactivity. As a result, the MCM-41 structure can enhance the interaction between TiO2 and Cr6+.All the TiO2/Cr-MCM-41 catalysts deactivate gradually under visible light due to the reduction of Cr6+ in the catalysts. TPR and temperature program oxidation (TPO) results clearly indicated that the average oxidation state of chromium in TiO2/Cr-MCM-41 (Si/Cr=20) was +2 after 23h of photoreaction. The reduced chromium in the photocatalyst can be completely reoxidized by heating the catalyst at 450°C for 3h.
Keywords: TiO; 2; /Cr-MCM-41; UV–vis; TPR; 4-Chlorophenol; Photocatalysis; Deactivation; Reactivation