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Applied Catalysis B, Environmental (v.83, #3-4)
Catalytic properties of goethite prepared in the presence of Nb on oxidation reactions in water: Computational and experimental studies by Luiz C.A. Oliveira; Teodorico C. Ramalho; Eugênio F. Souza; Maraísa Gonçalves; Diana Q.L. Oliveira; Márcio C. Pereira; José D. Fabris (pp. 169-176).
Nb-substituted goethites have been prepared and characterized by Mössbauer spectroscopy, XRD, FTIR and BET surface area measurements. The doublet formation in Mössbauer spectra and the decreasing of the crystallinity shown in XRD analyses indicated that the Fe domain size is small, which may be the result of either Fe3+ substitution for Nb5+ in the goethite structure or simply the formation of small particle-size goethite when Nb is present. FTIR analyses showed shifts and broadening of the bands as result of the incorporation of Nb5+ ions into the α-FeOOH structure. The insertion of Nb in the goethite structure caused a significant increase in the BET surface area of the material. The prepared materials were investigated for the H2O2 decomposition and the Fenton reaction in the oxidation of methylene blue dye. It was observed that the introduction of Nb during the synthesis of goethite produced a strong increase in the activity for the dye contaminant oxidation by H2O2. Theoretical quantum DFT calculations were carried out in order to understand the degradation mechanism for methylene blue with goethites.
Keywords: Goethite; Niobium; DFT; Fenton reaction
Tungsten and nitrogen co-doped TiO2 nano-powders with strong visible light response by Yanfang Shen; Tianying Xiong; Tiefan Li; Ke Yang (pp. 177-185).
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 650nm. 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.
Keywords: TiO; 2; Co-doped; Two-step method; COD
Deactivation of V2O5-WO3-TiO2 SCR catalyst at biomass fired power plants: Elucidation of mechanisms by lab- and pilot-scale experiments by Yuanjing Zheng; Anker Degn Jensen; Jan Erik Johnsson; Joakim Reimer Thøgersen (pp. 186-194).
In this work, deactivation of a commercial type V2O5-WO3-TiO2 catalyst by aerosols of potassium compounds was investigated in two ways: (1) by exposing the catalyst in a lab-scale reactor to a layer of KCl particles or fly ash from biomass combustion; (2) by exposing full-length monolith catalysts to pure KCl or K2SO4 aerosols in a bench-scale reactor. Exposed samples were characterized by activity measurements, SEM-EDX, BET/Hg-porosimetry, and NH3 chemisorption. The work was carried out to support the interpretation of observations of a previous study in which catalysts were exposed on a full-scale biomass fired power plant and to reveal the mechanisms of catalyst deactivation.Slight deactivation (about 10%) was observed for catalyst plates exposed to a layer of KCl particles at 350°C for 2397h. No deactivation was found for catalyst plates exposed for 2970h to fly ash (consisting mainly of KCl and K2SO4) collected from an SCR pilot plant installed on a straw-fired power plant. A fast deactivation was observed for catalysts exposed to pure KCl or K2SO4 aerosols at 350°C in the bench-scale reactor. The deactivation rates for KCl aerosol and K2SO4 aerosol exposed catalysts were about 1% per day and 0.4% per day, respectively.SEM analysis of potassium-containing aerosol exposed catalysts revealed that the potassium salt partly deposited on the catalyst outer wall which may decrease the diffusion rate of NO and NH3 into the catalyst. However, potassium also penetrated into the catalyst wall and the average K/V ratios (0.5–0.75) in the catalyst structure are high enough to explain the level of deactivation observed. The catalyst capacity for NH3 chemisorption decreased as a function of exposure time, which reveals that Brønsted acid sites had reacted with potassium compounds and thereby rendered inactive in the catalytic cycle. The conclusion is that chemical poisoning of active sites is the dominating deactivation mechanism, but physical blocking of the surface area may also contribute to the loss of activity in a practical application. The results support the observation and mechanisms of deactivation of SCR catalysts in biomass fired systems proposed in a previous study [Y. Zheng, A.D. Jensen, J.E. Johnsson, Appl. Catal. B 60 (2005) 253].
Keywords: Selective catalytic reduction (SCR); NO; x; V; 2; O; 5; -WO; 3; -TiO; 2; Potassium; Aerosol; Deactivation; Biomass combustion
Production of middle distillate in a dual-bed reactor from synthesis gas through wax cracking: Effect of acid property of Pd-loaded solid acid catalysts on the wax conversion and middle distillate selectivity by Kyung Min Cho; Sunyoung Park; Jeong Gil Seo; Min Hye Youn; Sung-Hyeon Baeck; Ki-Won Jun; Jin Suk Chung; In Kyu Song (pp. 195-201).
Selective production of middle distillate (C10–C20) from synthesis gas (CO+H2) was carried out in a dual-bed reactor. Co-based catalysts were used in the first-bed reactor to produce wax (C21+) from synthesis gas, and Pd-loaded solid acid catalysts were used in the second-bed reactor to produce middle distillate from wax through hydrocracking. ▪Selective production of middle distillate (C10–C20) from synthesis gas (CO+H2) was carried out in a dual-bed reactor. Co-based catalysts were used in the first-bed reactor to produce wax (C21+) from synthesis gas, and Pd-loaded solid acid catalysts were used in the second-bed reactor to produce middle distillate from wax through hydrocracking. Co/TiO2 catalyst in the first-bed reactor produced more than 35wt% middle distillate and more than 20wt% wax. These products served as a suitable feed for the production of middle distillate through hydrocracking in the second-bed reactor. Pd-loaded solid acid catalysts used in the second-bed reactor retained three types of acid sites (weak-, medium-, and strong-acid sites). Correlations between medium acidity and catalytic activity of Pd-loaded solid acid catalyst revealed that wax conversion and increment of middle distillate selectivity in the second-bed reactor were increased with increasing medium acidity of the catalyst. Total acidity of Pd-loaded solid acid catalyst was increased with increasing medium acidity of the catalyst. Thus, the medium acidity (total acidity) of Pd-loaded solid acid catalyst served as a crucial factor in hydrocracking of wax. Among the second-bed catalysts, the Pd/mesoporous alumina (Pd/MA) catalyst with the highest medium acidity (total acidity) showed the best catalytic performance (ca. 83% wax conversion and ca. 15wt% increment of middle distillate selectivity).
Keywords: Fischer–Tropsch synthesis (FTS); Synthesis gas; Middle distillate; Dual-bed reactor; Wax cracking
Efficient decomposition of organic compounds with FeTiO3/TiO2 heterojunction under visible light irradiation by Bifen Gao; Yong Joo Kim; Ashok Kumar Chakraborty; Wan In Lee (pp. 202-207).
FeTiO3/TiO2, a new heterojunction-type photocatalyst working at visible light, was prepared by a simple sol–gel method. Not only did FeTiO3/TiO2 exhibit greatly enhanced photocatalytic activity in decomposing 2-propanol in gas phase and 4-chlorophenol in aqueous solution, but also it induced efficient mineralization of 2-propanol under visible light irradiation ( λ≥420nm). Furthermore, it showed a good photochemical stability in repeated photocatalytic applications. FeTiO3 showed a profound absorption over the entire visible range, and its valence band (VB) position is close to that of TiO2. The unusually high photocatalytic efficiency of the FeTiO3/TiO2 composite was therefore deduced to be caused by hole transfer between the VB of FeTiO3 and TiO2.
Keywords: Photocatalyst; Visible light; FeTiO; 3; TiO; 2; FeTiO; 3; /TiO; 2; Heterojunction; Decomposition; CO; 2; evolution
Synthesis of nanosized biogenic magnetite and comparison of its catalytic activity in ozonation by Haeryong Jung; Jung-Woo Kim; Heechul Choi; Ji-Hoon Lee; Hor-Gil Hur (pp. 208-213).
Nanosized biogenic iron oxide was synthesized by dissimilatory iron-reducing bacterium, Shewanella sp. This biogenic iron oxide was evaluated as a catalyst in the heterogeneous catalytic ozonation of para-chlorobenzoic acid ( pCBA). XRD and TEM analyses showed that the biogenic iron oxide was magnetite phase (Fe3O4) and was composed of nanosized irregular particles in the range of 10.0±4.0nm in diameter. Catalytic ozonation was carried out at acidic pH levels (∼2.5) in the presence of the biogenic magnetite. It was clearly shown that the biogenic magnetite enhanced the degradation of pCBA by the production ofOH resulting from the catalytic decomposition of ozone on the surface of the particles. Functional groups on the surface of the biogenic magnetite played a role of catalytic active sites, and this was confirmed by FT-IR and titration analyses. However, the biogenic magnetite showed a lower catalytic efficiency than the commercial nanosized magnetite, resulted from the formation of 4 times bigger aggregates of the biogenic magnetite than the commercial one in aqueous solutions. The Rct values representing the ratio of hydroxyl radicals and ozone were found to be divided into two regions during reaction. The Rct values during first period (1min) were much greater than those during second period, and this was caused by initial rapid decrease of pCBA.
Keywords: Biogenic iron oxide; Magnetite; Ozonation; Catalytic reaction; p; CBA
Pd-doped LaCoO3 regenerative catalyst for automotive emissions control by Sina Sartipi; Abbas Ali Khodadadi; Yadollah Mortazavi (pp. 214-220).
The effect of partial substitution of Co by Pd in LaCoO3 perovskite structure (i.e., LaCo0.95Pd0.05O3) and the reductive diffusion of Pd from the bulk of perovskite to its surface, thus forming Pd nanoparticles, on CO and C3H8 oxidation present in air (simulated exhaust gas) are reported. X-ray powder diffraction (XRD) analyses confirm the perovskite structure for the catalysts. Scanning electron microscopy (SEM) and BET surface area measurements show that partial substitution of Co by Pd decreases the crystallite size of the perovskite and therefore increases its surface area. H2-temperature programmed reduction (TPR) experiments reveal that Pd reduces at 135°C and facilitates the reduction of Co in the perovskite structure. By partial reduction of the Pd containing catalyst at 180°C for 30min, the complete oxidation temperatures of CO and C3H8 decrease by about 70 and 50°C, respectively.The reduction duration of the Pd containing catalyst strongly affects the T50 and T90 temperatures (temperatures at which 50 and 90% conversion occurs, respectively) and has an optimum, where it decreases by increasing the reduction temperature of the catalyst.
Keywords: Pollution; Perovskite; Palladium; Regenerative; Reduction
Gel-combustion synthesis of nanocrystalline spinel catalysts for VOCs elimination by U. Zavyalova; B. Nigrovski; K. Pollok; F. Langenhorst; B. Müller; P. Scholz; B. Ondruschka (pp. 221-228).
Gel-combustion synthesis (GCS) in self-sustaining mode was used for preparation of mixed oxide with spinel structure AB2O4 (where A=Co, Cu and B=Cr, Co). To support these mixed oxides on CeO2 and ZrO2 powders, the GCS technique in self-propagating mode was developed. The synthesized materials were characterized by IR-spectroscopy, X-ray diffraction and transmission electron microscopy. The GCS preparation leads to the formation of nanocrystalline, single-phase spinel catalysts, which showed high activity in VOCs (hexane) elimination. The best catalytic performance was obtained over copper cobaltite catalyst prepared by GCS from glycerin-chelated precursor.
Keywords: Gel-combustion synthesis; Spinel catalyst; Nanocrystalline films; VOCs removal
Effect of sintering on the catalytic activity of a Pt based catalyst for CO oxidation: Experiments and modeling by J. Yang; V. Tschamber; D. Habermacher; F. Garin; P. Gilot (pp. 229-239).
The goal of this paper was to make the link between sintering of a 1.6% Pt/Al2O3 catalyst and its activity for CO oxidation reaction. Thermal aging of this catalyst for different durations ranging from 15min to 16h, at 600 and 700°C, under 7% O2, led to a shift of the platinum particle size distributions towards larger diameters, due to sintering. These distributions were studied by transmission electron microscopy. The number and the surface average diameters of platinum particles increase from 1.3 to 8.9nm and 2.1 to 12.8nm, respectively, after 16h aging at 600°C. The catalytic activity for CO oxidation under different CO and O2 inlet concentrations decreases after aging the catalyst. The light-off temperature increased by 48°C when the catalyst was aged for 16h at 600°C. The CO oxidation reaction is structure sensitive with a catalytic activity increasing with the platinum particle size. To account for this size effect, two intrinsic kinetic constants, related either to platinum atoms on planar faces or atoms on edges and corners were defined. A platinum site located on a planar face was found to be 2.5 more active than a platinum site on edges or corners, whatever the temperature. The global kinetic law { r (molm−2s−1)=103×exp(−64,500/ RT)[O2]0.74[CO]−0.5)} related to a reaction occurring on a platinum atom located on planar faces allows a simulation of the CO conversion curves during a temperature ramp. Modeling of the catalytic CO conversion during a temperature ramp, using the different aged catalysts, allows prediction of the CO conversion curves over a wide range of experimental conditions.
Keywords: Carbon monoxide; Oxidation; Modeling; Sintering; Platinum based catalyst
Au/Ce1− xZr xO2 as effective catalysts for low-temperature CO oxidation by Izabela Dobrosz-Gómez; Ireneusz Kocemba; Jacek M. Rynkowski (pp. 240-255).
The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO2 and ZrO2 in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce1− xZr xO2 ( x=0, 0.25, 0.5, 0.75, 1) oxides has been prepared by sol–gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO2 and Ce0.75Zr0.25O2, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce1− xZr xO2 catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO2-ZrO2 mixed oxides are promising supports for Au nanoparticles in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio.
Keywords: Au; Ce-Zr mixed oxide; Gold catalyst; CO oxidation
Catalytic ozonation for the degradation of nitrobenzene in aqueous solution by ceramic honeycomb-supported manganese by Lei Zhao; Jun Ma; Zhi-zhong Sun; Xue-dong Zhai (pp. 256-264).
Catalytic ozonation of nitrobenzene in aqueous solution has been carried out in a semi-continuous laboratory reactor where ceramic honeycomb and Mn–ceramic honeycomb have been used as the catalysts. The presences of the two catalysts significantly improve the degradation efficiency of nitrobenzene, the utilization efficiency of ozone and the production of oxidative intermediate species compared to the results from non-catalytic ozonation, and the improvement of them is even more pronounced in the presence of Mn–ceramic honeycomb. Adsorptions of nitrobenzene on the two catalytic surfaces have no remarkable influence on the degradation efficiency. Addition of tert-butanol causes the obvious decrease of degradation efficiency, suggesting that degradation of nitrobenzene follows the mechanism of hydroxyl radical (OH) oxidation. Some of the main operating variables like amount of catalyst and reaction temperature exert a positive influence on the degradation efficiency of nitrobenzene. Initial pH also presents a positive effect in the ozonation alone system while the optimum working initial pH is found to be around 8.83 and 10.67 to the processes of ozonation/ceramic honeycomb and ozonation/Mn–ceramic honeycomb, respectively. The surface characteristics measurement of the two catalysts indicates that the loading of Mn increases the specific surface area, the pH at the point of zero charge (pHPZC) and the density of surface hydroxyl groups, and results in the appearance of new crystalline phase of MnO2. The results of mechanism research confirm that the loading of Mn promotes the initiation ofOH.
Keywords: Catalytic ozonation; Nitrobenzene; Ceramic honeycomb; Mn; Hydroxyl radical (; OH)
Al-MCM-41 catalyzed decomposition of polypropylene and hybrid genetic algorithm for kinetics analysis by B. Saha; P. Chowdhury; A.K. Ghoshal (pp. 265-276).
Mesoporous catalysts (Al-MCM-41) are synthesized by sol–gel and hydrothermal methods to study their effects on the catalytic decomposition of polypropylene (PP) sample. The catalysts are characterized by X-ray diffraction (XRD) analysis and nitrogen adsorption study. Since sol–gel Al-MCM-41 catalyst shows better catalytic activity, further experimental studies were conducted to find its reusability and its activity at five different heating rates. The constant pattern behaviour of the TG curves for different catalyst percentages possibly suggests existence of similar reaction mechanism where large polymer fragments are cracked on the external surface of the catalyst and then enters into the mesopores for further cracking. Thus, presence of catalyst surfaces not only converts the polymer into comparatively smaller fractions, but also makes the decomposition of PP energy effective. Kinetics parameters are estimated based on 15 different decomposition models and the multi-heating rate experimental data both for catalytic and noncatalytic decomposition of PP using hybrid genetic algorithm (HGA). Suitability of the model is tested using corrected Akaike's Information Criteria (AICc). Results show that Nucleation and Growth model better predicted the experimental TGA data. However, nth order model also shows good AICc score and well predicted the experimental TGA data. Thus, though apparently it seems that Nucleation and Growth model controls the decomposition of PP sample, further investigation in detail including infrared or mass spectroscopy, morphology study using SEM or TEM during such decomposition is very much essential to conclude upon the actual reaction mechanism that controls decomposition of PP sample.
Keywords: Al-MCM-41 catalyst; Hybrid genetic algorithm; Kinetics parameters; Polypropylene; Reusability
The effect of Pt on the photocatalytic degradation pathway of methylene blue over TiO2 under ambient conditions by Zhiqiang Yu; Steven S.C. Chuang (pp. 277-285).
Photocatalytic degradation pathway of methylene blue (MB) has been studied over TiO2, 0.5wt.% Pt/TiO2, and 3wt.% Pt/TiO2 at ambient conditions (30°C and 1atm of air) by infrared (IR) spectroscopy. The reaction was proposed to be initiated via the abstraction of H from MB molecule by hydroxyl radical (·OH), followed by –CH3 elimination and CAr–N scission. The correlation in IR intensity between the decrease in C–H bond in MB molecule and the increase in hydroxyl group (–OH) at 3672cm−1 during the reaction (i) provides an indirect evidence to support the proposed ·OH-initiating pathway, (ii) suggests that the –OH sites could be related toOH generation sites, and (iii) offers new insights into the photoinduced hydrophilicity of the TiO2 surface. Subsequent breakup of the MB central ring via accepting protons and photogenerated electrons resulted in the formation of intermediates containing CO, COO−, and N–H groups.Deposition of 0.5wt.% Pt to TiO2 enhanced (i) the scission of ▪ and C–H bonds and (ii) the formation of intermediates containing CO and COO− groups. The latter suggests that Pt provided the sites for oxygen absorption, accelerating the formation rate of oxygen-containing intermediates. The comparable IR intensity of –OH at 3632cm−1 before reaction and –OH at 3672cm−1 during the reaction on TiO2 and 0.5wt.% Pt/TiO2 indicates thatOH generation sites were not blocked by low Pt loading (i.e., 0.5wt.%). The simultaneous presence ofOH generation, photoelectron generation, and Pt sites could play a synergetic role in enhancing ▪ and C–H bond scission and formation of CO and COO− species. Increasing Pt loading to 3wt.% resulted in the total elimination of –OH and the significant decrease in the MB degradation rate, indicating that the low activity of 3wt.% Pt/TiO2 could be attributed to the blocking ofOH generation sites by the high Pt loading.
Keywords: Photocatalytic degradation; Titanium dioxide; Hydroxyl group; Photogenerated electron; Infrared spectroscopy