Step Hydrothermal Treatment Research Articles

Hierarchically Assembled One-Dimensional TiO2 Nanostructures: Echinoid- and Labyrinth-Shaped TiO2 Crystals for Dye Sensitized Solar Cell

TiO2 nanostructures have been studied intensively for decades for their photocatalytic properties. Recently, several interesting TiO2 nanostructures with controlled surface facets or shapes were reported. However, systemic approaches to obtain designed nanostructures are still rare. Here, various hierarchical 1D TiO2 nanostructures, including TiO2 nanorods, echinoid-shaped TiO2, and labyrinth-structured TiO2, were fabricated by a one-pot hydrothermal process. Concentrated HCl was added to a solution having a Ti4+/H+ ratio that ranged from 1/4 to 1/8. The highly concentrated acid stabilized hydrolysis and hindered condensation, thereby balancing nucleation and growth of TiO2 nanostructures in the hydrothermal treatment step. Dye-sensitized solar cells prepared using the hierarchical 1D TiO2 nanostructures have shown higher photon to current conversion efficiency in the wavelength over 600 nm compared to those prepared with TiO2 nanoparticles.

Optimization of hydrothermal synthesis of pure phase zeolite Na-P1 from South African coal fly ashes

This study was aimed at optimizing the synthesis conditions for pure phase zeolite Na-P1 from three coal fly ashes obtained from power stations in the Mpumalanga province of South Africa. Synthesis variables evaluated were: hydrothermal treatment time (12–48 hours), temperature (100–160°C) and varying molar quantities of water during the hydrothermal treatment step (H2O:SiO2 molar ratio ranged between 0–0.49). The optimum synthesis conditions for preparing pure phase zeolite Na-P1 were achieved when the molar regime was 1 SiO2: 0.36 Al2O3: 0.59 NaOH: 0.49 H2O and ageing was done at 47°C for 48 hours. The optimum hydrothermal treatment time and temperature was 48 hours and 140°C, respectively. Fly ashes sourced from two coal-fired power plants (A, B) were found to produce nearly same high purity zeolite Na-P1 under identical conditions whereas the third fly ash (C) lead to a low quality zeolite Na-P1 under these conditions. The cation exchange capacity for the high pure phase was found to be 4.11 meq/g. These results highlight the fact that adjustment of reactant composition and presynthesis or synthesis parameters, improved quality of zeolite Na-P1 can be achieved and hence an improved potential for application of zeolites prepared from coal fly ash.

Bioresistance of poplar wood compressed by combined hydro-thermo-mechanical wood modification (CHTM): Soft rot and brown-rot

This work studied fungal bioresistance of combined hydro-thermo-mechanically modified (CHTM) poplar wood. The CHTM technique, introduced by Mohebby et al. (2009), is a combination of two wood modification techniques–hydrothermal wood modification and densification of wood. Blocks of poplar wood were initially treated hydrothermally at temperatures of 120, 150, and 180 °C for holding times of 0, 30, and 90 min. Afterwards, the treated blocks were compressed by a hot press (160 and 180 °C) for 20 min with a compression set of 60%. After the CHTM-treated blocks were dried, small specimens were cut for soft-rot and brown-rot decay tests according to ENV 807 and EN 113. Mass losses as well as metabolic moisture contents were determined in the decayed samples. Results revealed that the combination of wood modification techniques showed fungal suppression. It was also found that the hydrothermal treatment step could significantly reduce fungal attack in comparison with densification. Reduction of the mass losses was associated with the hydrothermal treatment temperature. Also, the level of metabolic moisture content was correlated with the mass losses for both fungi. Any reduction of the mass loss decreased the moisture content in the wood.

Understanding the Formation of Iron Oxide Nanoparticles with Acicular Structure from Iron(III) Chloride and Hydrazine Monohydrate

Iron oxide nanoparticles, nanowires, and nanobelts were prepared from a 50 vol % ethanol solution of 28 mM iron(III) chloride and 0.3 mM polyethylene glycol (20 kDa) by hydrothermal treatment at 120 °C, using hydrazine monohydrate (N2H4) as a reducing agent. Factors governing phase and morphology of the iron oxide nanoparticles were studied. The amount of hydrazine added affected the hydrolysis of iron(III); therefore, the type of iron oxyhydroxide formed; this in turn controlled whether akaganeite, goethite, hematite, or magnetite was obtained after the hydrothermal treatment step. For conditions that favored the formation of magnetite, the addition rate of hydrazine was shown to affect the reduction of iron(III) to iron(II) and, hence, the relative amount of magnetite and goethite in the final product. Increasing the pH of the precursor solution to 14 resulted in the formation of magnetite nanobelts. Phase analysis of the intermediate products showed that the magnetite nanobelts were formed from the dis...

Short-period synthesis of high specific surface area silica from rice husk char

Porous silica with high specific surface areas is prepared from rice husk char within a short period by avoiding the commonly used hydrothermal treatment step and using polyethylene glycol (PEG, molecular weight = 20,000) as a template. The preparation process mainly involves sodium silicate production, precipitation with ortho-phosphoric acid, and calcination. The amount of used PEG significantly affects the silica textural properties like the specific surface area, total pore volume and mesopore volume. The reason should be attributed to the amount of PEG removed from the PEG-silica composites by calcination. By varying the PEG dosage from 100 to 176 mg, porous silica with specific surface areas ranging from 709 to 936 m 2/g could be successfully prepared within 10 h.

Effect of hydrothermal treatment of coal on the oxidation susceptibility and electrical resistivity of HTT coke

The influence of hydrothermal treatment of coal prior to carbonization, on the oxidation susceptibility of resultant coke/char, calcined at 1350, 1800 and 2200 °C has been investigated. The non-isothermal thermogravimetric analysis technique has been employed, and parameters such as onset, DTG peak temperatures, and cumulative oxidation loss (wt.%) at different temperatures have been utilized to compare proneness to oxidation with respective untreated samples apart from electrical resistivity. Data suggest that all the cokes/chars samples produced from hydrothermally treated coals are less reactive and more electrically conductive (less resistive) than their respective untreated counterparts. But the extent of improvement of oxidation resistance and electrical conductivity appears to be coal-specific. The kinetic parameters obtained by non-linear regression analysis on multi-curve reveal that the n th order reaction model (where ‘ n’ was found to vary from 0.9 to 1.3) is the best-fitted model. The higher activation energy values observed for hydrothermally treated coke samples are in agreement with the observation of TG analysis data. Overall results indicate the importance of introducing a hydrothermal treatment step for the improvement of oxidation resistance as well as electrical conductivity of the coke samples.

Synthesis and characterization of nanocrystalline potassium tantalum oxide by sol–gel processing

Nanocrystalline potassium tantalum oxide (KT) colloids have been synthesized using sol–gel technique, followed by the peptization and the growth of the precipitates under hydrothermal conditions. The peptization of the precipitates occurs easily by heating at 110 °C for 6 h in a 0.5 M tetramethylammonium hydroxide solution. The size of primary particles exhibiting nanometer order has been controlled by autoclaving temperature between 160 and 210 °C during the hydrothermal treatment step. X-ray diffraction and UV–vis spectrophotometer analyses show the pyrochlore phase transforms into perovskite phase at about 800 °C, which is consistent with a shift of absorption edge.

Effects of Acetylation and Hydrothermal Treatment on Lignin as Revealed by Cellular UV-Spectroscopy in Norway Spruce (Picea abies [L.] Karst.)

Summary The effect of two different wood modification treatments on the lignin of Norway spruce wood was studied by means of cellular UV-spectroscopy. The modification treatments chosen were acetylation using acetic anhydride and a two step hydrothermal treatment. Small cross sectional thin slides from the wood surface were taken and were subsequently analysed using a UV-micro-spectro-photometer. The modified samples were compared with a non-treated control sample. The results revealed changes in the absorption spectra of lignin after both modification treatments. Spectra of the compound middle lamella (CML) of acetylated wood revealed a decrease in the lignin-specific absorption maximum at 280nm, which is considered to be caused by superficial leaching of lignin due to acetylation. The retarding effect of acetylation on UV-degradation of wood is discussed. In hydrothermal-treated wood the 280nm maximum was increased within the S2 layer. This maximum remained almost unchanged in the CML while a lower maximum was observed at about 330nm. Changes in the chromophoric behaviour of lignin are probably caused by de-masking reactions within the S2 layer resulting from hydrolysis of carbohydrates and side chain reactions. Both treatments lead to a severe decrease in absorption at the lower end of the spectrum towards 250nm which is possibly attributed to changes or even splitting of biphenyls.

Effect of Calcium Sulfate Addition on the Activity of the Absorbent for Dry Flue Gas Desulfurization

The addition of calcium sulfate in the procedures of preparing flue gas desulfurization absorbent from calcium oxide and coal fly ash causes a negative effect in the slaking step and a positive effect in the following hydrothermal treatment step. The role of calcium sulfate added in each step was investigated. For the slaking step, XRD, XAFS, and Raman spectroscopic studies revealed that the addition of calcium sulfate in the step of slaking CaO with coal fly ash resulted in the coverage of calcium hydroxide with calcium sulfate to decrease the reactivity of the calcium hydroxide toward coal fly ash to form calcium silicate. For the hydrothermal treatment step, measurements of XRD, surface area, and Raman spectrum revealed that the addition of calcium sulfate promotes the formation of calcium silicate by suppressing crystal growth of calcium hydroxide to keep the reactivity of the calcium hydroxide toward coal fly ash.

Co(II)–Co(III) hydrotalcite-like compounds

The synthesis of monometallic hydrotalcite-like compounds (MHLC) type Co(II)–Co(III) materials was achieved by precipitation of a solution of cobalt nitrate(II) followed by microwave irradiation during the hydrothermal treatment step. The effects of irradiation time, precipitant agent and atmosphere were studied in order to establish the preparation conditions and the stability of these materials. As expected, these materials required careful preparation conditions, mainly a very slow addition of the precursors. NH4OH had to be used as precipitant agent in an air atmosphere. Short microwave irradiation times during the hydrothermal treatment provided a better crystallization of MHLC. The low stability of these compounds (∼200°C) was explained by the presence of several Co-complexes between the layers of the laminar structure. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal analysis and thermogravimetric analysis (DTA/TGA), and thermoprogrammed oxidation (TPO) were used as characterization techniques.

Thin molecular sieve films on noble metal substrates

A novel method whereby dense thin microporous films less than 300 nm thick may be prepared on noble metal substrates is exemplified by the deposition of silicalite-1 on a gold surface in a three-step process. The gold surface is modified with a silane coupling agent, gamma-mercaptopropyltrimethoxysilane, after which the silane is hydrolyzed to yield a negatively charged interface. Positively charged colloidal silicalite-1 crystals are readily adsorbed as a monolayer upon this surface and are induced to crystallize further in a separate hydrothermal treatment step to yield a dense thin silicalite-1 film.