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Applied Catalysis B, Environmental (v.68, #1-2)
Size dependency of nanocrystalline TiO2 on its optical property and photocatalytic reactivity exemplified by 2-chlorophenol by H. Lin; C.P. Huang; W. Li; C. Ni; S. Ismat Shah; Yao-Hsuan Tseng (pp. 1-11).
Anatase TiO2 nanocrystallines (17–29nm) were successfully synthesized by the metal–organic chemical vapor deposition method (MOCVD). Moderate manipulation of system parameters of MOCVD can control the particle size. The electro-optical and photocatalytic properties of the synthesized TiO2 nanoparticles were studied along with several commercially available ultra-fine TiO2 particles (e.g., 3.8–5.7nm). The band gap of the TiO2 crystallines was determined using the transformed diffuse reflectance technique according to the Kubelka–Munk theory. Results showed that the band gap of TiO2 monotonically decreased from 3.239 to 3.173eV when the particle size decreased from 29 to 17nm and then increased from 3.173 to 3.289eV as the particle size decreased from 17 to 3.8nm. The results of band gap change as a function of particle size agreed well with what was predicted by the Brus’ equation, i.e., the effective mass model (EMM). However, results of the photocatalytic oxidation of 2-chlorophenol (2-CP), showed that the smaller the particle size, the faster the degradation rate. This is attributed in part to the combined effect of band gap change relative to the spectrum of the light source and the specific surface area (or particle size) of the photocatalysts. The change of band gap due to particle size represents only a small optical absorption window with respect to the total spectrum of the light source, i.e., from 380 to 400nm versus >280nm. Consequently, the gain in optical property of the larger particles was severely compromised by their decrease in specific surface area. Our results clearly indicated the importance of specific surface area in controlling the photocatalytic reactivity of photocatalysts. Results also showed that the secondary particle size grew with time due mainly to particle aggregation. The photocatalytic rate constants decreased exponentially with increase in primary particle size. Primary particle size alone is able to predict the photocatalytic rate as it is closely related to the electro-optical properties of photocatalysts.
Keywords: Size effect; TiO; 2; Photocatalyst; Particle size; 2-Chlorophenol; Size quantization effect; Electro-optical property
Mechanistic study of cyclohexene oxidation and its use in modification of industrial waste organics by Sanghamitra Mukherjee; Sujit Samanta; Asim Bhaumik; Bidhan Chandra Ray (pp. 12-20).
Mechanistic study of oxidation of cyclohexene was carried out over a copper complex and its immobilized analogue supported on organically modified silica. The immobilized complex has been characterized by using atomic absorption spectrophotometry (AAS), FT-IR, EPR and UV–vis spectroscopic studies and SEM image analysis. Catalytic oxidation of cyclohexene was carried out over this copper complex and the immobilized analogue with different oxidants like dilute aqueous hydrogen peroxide and tert-butyl hydroperoxide at ambient conditions. Acetonitrile and water was used as solvent and dispersion medium, respectively, with or without additional acid in different sets of oxidation reactions. The mechanism of hydride shift initiated the study of benzene oxidation, which gave quinone as major product. The activity of the immobilized catalyst remains nearly the same after four cycles, suggesting the true heterogeneous nature of the catalyst.
Keywords: Allylic oxidation; Copper-complex; Benzene; Oxidation; Immobilization; Peroxides; 2-Cyclohexene-1-one
Two-stage catalyst system for selective catalytic reduction of NO x by NH3 at low temperatures by Min Kang; Dae Jung Kim; Eun Duck Park; Ji Man Kim; Jae Eui Yie; Seoung Hyun Kim; Louisa Hope-Weeks; Edward M. Eyring (pp. 21-27).
Three catalysts, 1wt.% Pt/Al2O3, 20wt.% Cu/Al2O3, and 1wt.% Pt–20wt.% Cu/Al2O3 were evaluated for the oxidation of NO by O2 and the selective catalytic reduction (SCR) of NO x by NH3 in the absence and presence of H2O. The Pt/Al2O3 catalyst exhibited the largest values in NO oxidation efficiency and NO x removal efficiency among the three catalysts. The presence of H2O caused a drop in the NO oxidation and the SCR activities. The SCR performance was significantly improved when Pt/Al2O3 and Cu/Al2O3 catalysts were employed sequentially in a reactor. The highest SCR performance was achieved using two separate stages: oxidation of NO by O2 over the Pt/Al2O3 catalyst followed by reduction of NO2 by NH3 over the Cu/Al2O3 catalyst. Under the condition with H2O, the two-stage catalyst system showed NO x removal efficiency over 80% at low temperatures below 200°C, which was the highest value among the catalyst systems used in this study. The physical and chemical properties of the catalysts were determined using nitrogen adsorption, XRD, TPR, and pulse CO chemisorption experiments.
Keywords: NH; 3; SCR; NO oxidation; Cu catalyst; Pt catalyst; Two-stage catalyst system
Catalytic wet air oxidation of p-coumaric acid on CeO2, platinum and gold supported on CeO2 catalysts by C. Milone; M. Fazio; A. Pistone; S. Galvagno (pp. 28-37).
p-Coumaric acid is representative of the polyphenolic fraction typically found in olive milling waste waters (OMWW). The catalytic wet air oxidation of p-coumaric acid has been investigated at 353K at P=2MPa, using CeO2, Pt and Au supported on CeO2 catalysts. The influence of the metal and of the preparation method of the catalysts on the catalytic activity has been investigated.Upon addition of Pt to CeO2, the rate of oxidation of p-coumaric acid increases whereas addition of gold do not lead to a significant difference of the activity of CeO2. On all the catalysts investigated, the abatement of total organic carbon (TOC) was ≥80% after 300min of reaction. Catalysts containing metallic platinum are the most effective towards the mineralization of the organic carbon to CO2 and the degree of mineralization (DM%) was higher than 50%. On CeO2 and Au–CeO2 catalysts a great contribute to the abatement of TOC is given from a significant adsorption of the organic substrates on the solid.
Keywords: CWAO; p; -Coumaric acid; Ceria; Platinum catalysts; Gold catalysts
HDN and HDS of different gas oils derived from Athabasca bitumen over phosphorus-doped NiMo/γ-Al2O3 carbides by V. Sundaramurthy; A.K. Dalai; J. Adjaye (pp. 38-48).
A series of phosphorous-doped γ-Al2O3 supported Ni–Mo bimetallic carbide catalysts (PNiMo2C/Al2O3) with 0–4.5wt.% Ni, 13wt.% Mo and 2.5wt.% P were synthesized and characterized by elemental analysis, pulsed CO chemisorption, surface area measurement, X-ray diffraction (XRD), near-edge X-ray absorption fine structure (NEXAFS), DRIFT spectroscopy of CO adsorption and H2 temperature programmed reduction. XRD indicated the formation of pure β-Mo2C phase in these catalysts, whereas the near edge X-ray absorption fine structure of C K-edge confirmed the formation of carbidic carbons. DRIFT spectra of adsorbed CO revealed that Ni or P addition to Mo2C/Al2O3 catalyst not only increases the number of surface Mo sites, but also promotes the reducibility of Mo. The partial sulfidation of Mo2C phase in the presence of H2S/H2 gas mixture at 370°C was evidenced by DRIFTS of adsorbed CO. The HDN and HDS activities of these PNiMo2C/Al2O3 catalysts were performed in a trickle bed reactor using light gas oil (LGO) and heavy gas oil (HGO) derived from Athabasca bitumen at 8.8MPa and compared with the unpromoted Mo carbide (Mo2C/Al2O3), P doped Mo carbide (PMo2C/Al2O3) and Ni promoted Mo carbide (NiMo2C/Al2O3). The P doped Ni–Mo bimetallic carbide catalysts showed enhanced HDN activity compared to the Mo2C/Al2O3, NiMo2C/Al2O3 and PMo2C/Al2O3 catalysts. The maximum N and S conversions, respectively, were obtained over PNiMo2C/Al2O3 and NiMo2C/Al2O3 catalysts containing 2.5wt.% Ni.
Keywords: Mo; 2; C/γ-Al; 2; O; 3; P doped Ni–Mo carbide; DRIFTS of CO adsorption; C K-edge NEXAFS; Hydrodesulfurization; Hydrodenitrogenation; Gas oil
Photooxidation of dichloroacetic acid controlled by pH-stat technique using TiO2/layer silicate nanocomposites by Robert Kun; Márta Szekeres; Imre Dékány (pp. 49-58).
Nanocomposites containing anatase nanoparticles were prepared by heterocoagulation, using Na-montmorillonite and titanium dioxide obtained by hydrothermal sol–gel method. Heterocoagulation was carried out at pH 1 and 4. Based on X-ray diffraction measurements, an average particle size of 3.8–4.0nm was calculated by the Scherrer equation for the particles intercalated between the silicate lamellae. Nitrogen adsorption studies revealed that the specific surface area of nanocomposites prepared at pH 1 varies in the range of 157–284m2/g, depending on the TiO2 content. After preparation at pH 4, the specific surface area of the samples is lower (123–248m2/g). UV–vis analyses of the nanocomposites showed that as TiO2 content is increased, band gap energies relative to TiO2 decrease and gradually approach the value obtained for the pure sol–gel TiO2 sample ( Eg=3.12eV). The nanocomposites obtained were tested in photocatalytic degradation of dichloroacetic acid (DCA) in a suspension photoreactor. The reaction was quantitatively monitored during the entire irradiation time using the pH-stat technique. We found that higher catalytic efficiencies could be achieved when increasing sample TiO2 content. The photocatalytic efficiency of composites prepared at pH 1 was well below that of the samples prepared at pH 4, which was attributed to structural changes in the support brought about by the highly acidic medium. When photocatalytic degradation data were normalized to pure TiO2, composite samples containing 47% and 57% TiO2 were found to be the most efficient as compared to the 100% TiO2 sample prepared by the sol–gel method.
Keywords: TiO; 2; Layer silicate; Photooxidation; pH-stat; Dichloroacetic acid
Novel doubly-promoted catalysts for the lean NO x reduction by H2+CO: Pd(K)/Al2O3–(TiO2) by M. Konsolakis; M. Vrontaki; G. Avgouropoulos; T. Ioannides; I.V. Yentekakis (pp. 59-67).
The reduction of NO by CO+H2 in the presence of excess O2 has been studied on dispersed, low loading 0.5wt.% Pd-based catalysts. Two different methods of promotion were imposed separately and together, in order to investigate the feasibility of their synergy to enhance de-NO x efficiency of the catalytic system. Therefore, the Pd-catalysed NO+CO+H2+O2 reaction was gradually studied over: (i) Pd/Al2O3–(TiO2) catalysts (support-mediated promotion by modifying the Al2O3 support with TiO2), (ii) K-dosed Pd(K)/Al2O3 catalysts (surface-induced promotion by modifying the Pd surface with the addition of K), and (iii) doubly-promoted Pd(K)/Al2O3–(TiO2) catalysts. Both (i) and (ii) methods of promotion were found to significantly promote the system, when acting individually. However, the de-NO x efficiency and N2-selectivity of the doubly-promoted Pd(K)/Al2O3–(TiO2) catalysts were found to be better than both of the other two cases, indicating a synergy during the simultaneous imposition of the two methods of promotion. The resulted benefits with the optimal doubly-promoted catalyst were very high, ∼85%, NO x conversion in a very wide temperature range (100–400°C), accompanied by very good N2-selectivities of ∼85%, i.e., 15–30 additional percentage units in comparison to the selectivity of the original un-promoted Pd/Al2O3 catalyst.
Keywords: Palladium; Potassium; Promotion; Al; 2; O; 3; TiO; 2; NO; H; 2; CO; Lean NO; x; reduction
Gas-phase photocatalytic decontamination using polymer supported TiO2 by Matheus P. Paschoalino; John Kiwi; Wilson F. Jardim (pp. 68-73).
An alternative method to the conventional sol–gel process is proposed to incorporate P-25 TiO2 (Degussa) onto polydimethylsiloxane (PDMS) and orthophthalic polyester (OP) polymers. These TiO2-incorporated films were characterized using SEM–EDS and showed a homogenous distribution of TiO2. The catalytic activity of the films was tested in aqueous phase under black light (365nm) illumination, using solutions containing dichloroacetic acid, salicylic acid and phenol. OP/TiO2 films presented the best performance, showing no loss of catalytic activity after reuse. Further tests were performed using this film in the form of fixed bed catalyst coated onto the internal walls of a plug-flow concentric gas-phase reactor (767mL) provided with a black light lamp (365nm) and an internal area of ∼900cm2. Ethanol vapor was used as the model compound at 0.05 or 0.02Lmin−1 mutually with humid synthetic air at 0.15 or 0.18Lmin−1, totaling a 0.2Lmin−1 flow rate. A continuous monitoring of the photoreactor outlet using GC–MS showed total mineralization of the model compound within 25min at a concentration of 126ppmv. The polymer substrate showed to be resistant to degradation and is a promising material for the decontamination of confined atmospheres.
Keywords: TiO; 2; Heterogeneous photocatalysis; Gas-phase; Polymer substrate
Microstructure and environmental functionalities of TiO2-supported photocatalysts obtained by suspension plasma spraying by Filofteia-Laura Toma; Ghislaine Bertrand; Sylvie Begin; Cathy Meunier; Odile Barres; Didier Klein; Christian Coddet (pp. 74-84).
This paper deals with the elaboration of titanium dioxide coatings, designed for photocatalytic applications, obtained by a non-conventional method of deposition, suspension plasma spraying (SPS), an alternative of the atmospheric plasma spraying (APS) in which the material feedstock is a suspension of the material powder to be sprayed. TiO2 P25 powder (Degussa AG) mechanically dispersed in distilled water and/or ethanol was injected in argon–hydrogen and argon–hydrogen–helium plasma under atmospheric conditions. Scanning electron microscopy and X-ray diffraction were performed to study the morphology and the crystalline phases of the titania coatings. Surface features were investigated by Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). Photocatalytic efficiency of the elaborated samples was tested in an environmental test chamber set-up and evaluated from the conversion rate of nitrogen oxides.The results showed that the coating obtained from the injection of an alcoholic suspension contained 23% of anatase ratio and ensured a very low photocatalytic decomposition of nitrogen oxides. In contrast, the injection of an aqueous suspension into the plasma permitted to obtain deposits where the anatase phase and the crystallites size were preserved. Also, a conversion rate of the pollutants of about 40%, slightly better compared to that of the initial P25 powder (around 32%) was noticed. This slightly higher efficiency was correlated with a cleaning of the particles surfaces when crossing the plasma and a higher hydroxylation of the coating surface.
Keywords: Photocatalysis; TiO; 2; Suspension plasma spraying; Nitrogen oxides; FT-IR; XPS