Reduction of NO by CH4 with Microwave Heating

The reduction of NO by CH₄ in the presence of excess O₂ over Co/HZSM-5, Ni/HZSM-5 and Mn/HZSM-5 catalysts with microwave heating was studied. By comparing the activities of the catalysts in the microwave heating mode with that in the conventional reaction mode, it was demonstrated that microwave heating could greatly reduce the reaction temperature, and could clearly expand the temperature window of the catalysts. Especially for the Co/HZSM-5 catalyst, the maximum conversion of NO to N₂ in the conventional reaction mode was consistent with that in the microwave heating mode. However, the temperature window for the maximum conversion in the microwave heating mode was from 260 to 360 °C instead of a temperature of 420 °C in the conventional reaction mode. The results suggest that microwave heating has a novel effect in the reduction of NO.     

Facile preparation and characterization of lanthanum oxide powders by the calcination of lanthanum carbonate hydrate in microwave field

Micron-sized lanthanum oxide powders are prepared by the calcination of lanthanum carbonate hydrate in microwave field. The decomposition process of lanthanum carbonate hydrate was analyzed by TG-DSC and indicates the reaction undergoes three stages, resulting in the generation of lanthanum oxide at 770 °C. For microwave assisted calcination, XRD patterns demonstrate that hexagonal La₂O₃ structure is initially formed after calcination at 650 °C for 2 h, and FT-IR analyses confirm the decomposition of precursor is complete after calcination at 750 °C for 2 h. SEM investigations reveal that 800 °C is the optimal calcination temperature to generate La₂O₃ powders with uniform morphologies. In comparison, conventionally calcination experiments are carried out in electrical furnace. Both XRD and FT-IR analyses are in consistence with TG-DSC, which indicate the temperature required for fully decomposition of lanthanum carbonate hydrate by conventional heating is higher than that of microwave heating. SEM images present irregular morphologies and wide particle size distribution of conventionally prepared samples. All the techniques are utilized to prove the feasibility of decomposing La₂(CO₃)₃ to generate La₂O₃ in microwave field and highlight the advantages of microwave heating.     

Hydrothermal microwave-assisted synthesis of LaFeO3 catalyst for N2O decomposition

Lanthanum orthoferrite powders were synthesized via one-step hydrothermal reactions under mild conditions using microwave and conventional heating. The use of microwave irradiation during the synthesis allows one to obtain nanocrystalline LaFeO₃ with a higher yield and reduced crystallite and particle size within a 16 times shorter duration (3 hours) at a lower temperature of 220°C as compared to the conventional heating. The catalytic decomposition of nitrous oxide was performed over both samples, it was shown that the sample obtained under microwave conditions demonstrates enhanced activity as a catalyst: N₂O decomposes completely at 700°C over the catalyst formed at microwave conditions, while the comparative catalyst prepared by conventional heating reaches a lower conversion of only 60% at the same temperature and catalytic reaction conditions.     

Catalytic decomposition of TCE under microwave

The decomposition of trichloroethylene (TCE) by hydrogen over Co/γ-Al₂O₃ and Ni/γ-Al₂O₃ catalysts under microwave heating was studied. The comparison between the catalytic activity in the microwave heating mode and that in the conventional thermal mode demonstrated that the microwave heating could greatly reduce the reaction temperature, accelerate the TCE decomposition speed and improve the TCE decomposition ratio. The results suggest that the microwave heating has a novel effect in the decomposition of TCE.     

XPS evidence for molybdenum nitride formation in ZSM-5

Microwave plasma-assisted catalytic reduction of SO₂ by CO was studied over four catalysts. The activities of the four catalysts under microwave plasma decreased in the order of CoO/γ-Al₂O₃>>SnO₂> copper wires > iron wires, which was consistent with the results under conventional heating. By comparing the activity of CoO/γ-Al₂O₃ catalyst in the microwave plasma mode with that in the conventional mode, it is demonstrated that the temperature at which the full SO₂ conversion was obtained in the microwave plasma mode was about 200 °C lower than that under the conventional heating mode. Moreover, an increase of space velocity had little effect on SO₂ conversion and sulfur selectivity under microwave plasma; while under conventional heating mode, both SO₂ conversion and sulfur selectivity significantly decreased with an increase of space velocity.     

Microstructure and phase transformation behavior of Al2O3–ZrO2 under microwave sintering

Zirconia is an inorganic, nonmetallic material with excellent properties. However, the brittleness of the zirconia, resulting from the thermal performance during the heating and cooling process, seriously limits the application of zirconia in the metallurgical, military, and aerospace industries. Al₂O₃ doped ZrO₂ was developed to improve the potential material's toughness. This paper studied the evolution of the surface functional groups, phase composition, toughening mechanism, and particle morphology of Al₂O₃ doped ZrO₂ during the heating process. Especially microwave heating was selected as the heating method during the experiments to save energy consumption. The results showed that the phase transition temperature was reduced by the microwave sintering technique, which also promoted the transformation between the m-ZrO₂ and t-ZrO₂, advancing the crystallinity and structural properties of the samples. The specific surface area shows a positive relationship with the microwave heating temperature, while the particle size of the powder decreased with the temperature increase. The optimized sintering effect appears at 1000 °C in the studied roasting temperature range (800 °C–1200 °C) for Al₂O₃–ZrO₂ powders. With the optimized sintering temperature, the void of the granular zirconia material was controlled, and the best micromorphology was obtained. In practical production, the application of microwave sintering and alumina doping is beneficial to saving costs and protecting the environment.     

Preparation of TiO2-pillared montmorillonite as photocatalyst Part I. Microwave calcination, characterisation, and adsorption of a textile azo dye

Two photocatalysts based on TiO₂-pillared intercalated montmorillonite have been prepared by microwave for 10 min at 700 W or by furnace heating at 673 K. Montmorillonite pillaring with TiO₂ increased the basal spacing to 14.7 Å (conventional heating) and 17.6 Å (microwave heating). XRD patterns of both materials showed the presence of 100% anatase with a slightly higher rate of crystallinity obtained through microwave calcination than by conventional heating at 673 K. The BET specific surface area of the microwave prepared photocatalyst (151 m² g^− 1) was 3 fold higher than those of the Degussa TiO₂ P25. At pH = 5.8, the maximum adsorption capacity of Solophenyl red 3BL (a textile azo dye) on the TiO₂-pillared montmorillonite calcined by microwave was 185 mg g^− 1, whereas it was 1.4 and 3 fold lower on the TiO₂-pillared montmorillonite calcined at 673 K, and on the Degussa TiO₂ P25 respectively. The influence of pH on the adsorption of the dye depended on the pH_ZPC of the pillared montmorillonites.     

Conventional and microwave-heated oxygen pulsing techniques on metal-doped activated carbons

Conventional and microwave-heated oxygen pulsing techniques on metal-doped activated carbons to achieve a controlled meso/micropore structure were investigated. The gas pulsing experiments consisted of repeated cycles. Each cycle consists of an oxidising stage, under O₂/Ar atmosphere and constant temperature, and a burn-off stage, under N₂ atmosphere at variable temperature (heating and cooling). The porosity of the carbons was analysed by nitrogen adsorption at ?196 °C. Two different activated carbons, Cu-doped BPL (BPL-Cu) and ASC, were used as raw materials. The commercial activated carbon ASC showed higher reactivity towards O₂, due to the catalytic effect of the metals (mainly, Cu and Cr) that are on the carbon surface and their better dispersion. After several pulses, ASC underwent a moderate increase in the micropore volume and a significant increase in mesopore volume. BPL-Cu showed a higher increase in microporosity than mesoporosity, giving rise to a meso/micropore volume ratio lower than that of the original BPL-Cu. Oxygen pulsing technique was carried out in a conventional furnace and in a microwave oven. Conventional and microwave-heated oxygen pulsing on ASC yielded similar textural development. However, the time required under microwave heating was remarkably reduced respect to conventional heating (around 2.5 times less), which suggests that microwave-heated oxygen pulsing technique would be an interesting alternative to conventional activation.     

Synthesis,characterization and properties evaluation of copolymers of 2,3,4,5,6‐pentafluorostyrene and N‐phenylmaleimide

Copolymers of 2,3,4,5,6‐pentafluorostyrene (PFS) having a combination of high hydrophobicity and high glass transition temperature (T_g) are reported here for the first time. The copolymerization was carried out using N‐phenylmaleimide (NPM) as the comonomer and azobisisobutyronitrile (AIBN) as the initiator under both conventional thermal heating and microwave heating. The initial copolymerization rate was found to be higher under microwave heating than under thermal heating. The copolymerization parameters were determined using the Fineman–Ross method and were found to be r₁ (NPM) = 0.28 and r₂ (PFS) = 0.86. Increased incorporation of NPM in the copolymers led to an increase in T_g of the copolymers without significantly affecting the hydrophobicity of poly(2,3,4,5,6‐pentafluorostyrene). Thermal stability of the copolymers is also reported. Copyright © 2005 Society of Chemical Industry     

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k _m and k _s ) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

Amorphous Ni–B/SiO2 catalyst prepared by microwave heating and its catalytic activityin acrylonitrile hydrogenation

Microwave heating was utilized in the preparation of amorphous Ni–B/SiO₂ catalysts. After wetness impregnation with NiCl₂ aqueous solution, the catalyst precursor was dried at 383 K, followed by microwave heating and KBH₄ reduction. Liquid phase hydrogenation of acrylonitrile was employed as the probe reaction. While ICP, XRD, XPS and turnover frequency results did not reveal discernable structural and electronic differences between the active components of the catalysts with or without microwave heating, SEM uncovered the formation of smaller Ni–B alloy particles with microwave heating, leading to a higher concentration of active sites and greater thermal stability as verified by H₂ chemisorption and DSC. Copyright © 2003 Society of Chemical Industry     

Rapid microwave sintering of zinc oxide-based varistor ceramics

Samples of ZnO + Bi₂O₃ + Sb₂O₃ varistor ceramics were microwave sintered using gyrotron systems operating at a frequency of 24 GHz. The microwave power was automatically regulated to implement heating at a constant heating rate of 10–130 °C/min up to a temperature of 1100–1300 °C with no isothermal hold. The final sintered density of the samples was 95–96 % of the theoretical value. Manifestations of the thermal instability associated with the liquid phase formation were observed at a temperature of about 600 °C. The estimated volumetrically absorbed power density at the onset of instability was ≥20 W/cm³, and the temperature difference measured between the center and periphery of the samples reached 200 °C. Correlation has been revealed between the thermal instability occurrence and the shift of densification curves towards lower temperatures. A mechanism underlying enhanced densification in electromagnetic field-assisted sintering processes is suggested.     

Structural aspects of AlPO<Subscript>4</Subscript>-5 zeotypes synthesized by microwave-hydrothermal process. 1. Effect of heating time and microwave power

AlPO4-5 with AFI structure containing 12-membered rings was prepared using the aluminum isopropoxide precursor as a source of alumina and TEA as the structure directing agent via microwave technique. The influence of microwave power and heating time on the dimensions of AlPO₄-5 crystals formed in the system Al₂O₃:P₂O₅:(C₂H₅)₃N (or (C₃H₇)₃N):H₂O:HF has been studied systematically. It was found that the morphology of the AlPO₄-5 depended on the microwave power and heating time. Several mechanisms of fast crystallization existed in the microwave radiation, due to increased dissolution of the gel by lonely water molecules in almost temperature gradient-free and convection-free in situ heating.     

High-performance Al2O3 ceramics prepared by DCC-HVCI via microwave heating

A novel and rapid fabrication method for Al₂O₃ ceramics by the DCC-HVCI method via microwave heating was proposed. Effects of microwave heating temperature on coagulation time, micromorphology, as well as performance of the green body and ceramic sample were studied. As the microwave heating temperature rises, the coagulation time gradually reduced and compressive strength of green sample decreased while relative density and flexural strength of ceramics rose at the beginning and then dropped. The 50 vol.% Al₂O₃ suspension was coagulated and demolded after treating at 60°C for 800 s by microwave heating. The compressive strength of green samples reached 1.12 ± 0.13 MPa. The relative density of Al₂O₃ ceramic samples reached 99.39%. And the flexural strength of Al₂O₃ ceramics reached 334.55 ± 26.41 MPa. The Weibull modulus of Al₂O₃ ceramics reached 19. In contrast with the ceramic samples heated through water bath, the ceramic samples treated through microwave possessed uniform microstructures. Microwave heating could reduce the coagulation time by 77%. Meanwhile, it could significantly raise the compressive strength of green bodies by 65%. Additionally, it could increase the flexural strength of ceramics by 30%.     

Comparison between microwave and conventional thermal reactivations of spent activated carbon generated from vinyl acetate synthesis

Microwave and traditional thermal reactivations of activated carbon (AC) used as catalyst support in vinyl acetate synthesis have been investigated. Experiments have been carried out by using a single mode microwave device (MW) operating at 2450 MHz and a conventional electric furnace (CF) under steam and CO₂ atmosphere, respectively. The surface properties of the spent AC and the reactivated samples were characterized by means of N₂ adsorption and SEM, and compared the effects of different heating mechanisms and activating agents on the adsorption capacities and pore structures of the reactivated AC. These results indicated that the AC obtained by microwave irradiation showed higher adsorption capacities for iodine, methylene blue (MB) and acetate acid, higher BET surface areas and mesoporosity than those obtained by conventional thermal heating. The reactivated samples activated by steam had a narrower and more extensive microporosity as well as higher BET than those activated by carbon dioxide under the same heating equipment. From the results, it was concluded that microwave heating combined with steam as an activating agent could remarkably increase the reactivating efficiency compared to the traditional thermal heating.     

Influence of microwave and ultrasonic treatment on the formation of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> under hydrothermal conditions

The formation of CoFe₂O₄ nanocrystals under hydrothermal conditions at a temperature of 130°C is investigated. The average size of CoFe₂O₄ particles varies from 6 to 11 nm depending on the synthesis time. The hydrothermal medium is heated by two different methods, i.e., the microwave (with a synthesis time from 1 min to 2 h) and conventional (with a synthesis time from 30 min to 45 h) methods. It is demonstrated that the use of microwave heating considerably accelerates the formation of CoFe₂O₄ particles. Preliminary ultrasonic treatment for 3 min increases the phase formation rate in the case of microwave heating and hardly affects the occurrence of the process upon conventional heating. It is revealed that the exposure of the initial mixture (preliminarily treated with ultrasound) to room temperature for 2 h or longer almost completely reduces the efficiency of the action of ultrasonic treatment on the phase formation process under hydrothermal conditions.     

Synthesis and reaction evolution of high-purity Ti2SC powder by microwave hybrid heating at low temperature

Highly pure Ti₂SC was synthesised by microwave hybrid heating. The effects of composition and processing parameters, such as microwave output power, sintering temperature and dwell time on the formation process and morphology of Ti₂SC were studied. The purity of Ti₂SC is sensitive to the content of sulphur and sintering temperature. The results indicated that the optimum condition for synthesising Ti₂SC was using a mixture of 1.15TiS₂/2.85Ti/2C powders irradiated under a 100% microwave output power sintering at 1100°C for 3?min at Ar atmosphere. In addition, the reaction evolution of Ti₂SC was discussed in details.     

Apparent viscosity reduction during microwave sintering of amorphous silica

The sintering of arrays of spherical particles of amorphous SiO₂ was investigated experimentally under microwave (24 GHz) and conventional heating. The materials under study were compacts formed by gravitational sedimentation of monodisperse silica microspheres (1 µm in diameter). The kinetics of neck growth between individual particles was investigated by analyzing SEM images. It was found that the rates of viscous mass transport under microwave heating were significantly higher than those under conventional heating. The values of the viscosity obtained under microwave heating were significantly (by more than an order of magnitude) lower compared to conventional heating. Possible reasons for the viscosity decrease observed under microwave heating may be associated with the influence of water vapor and the action of the electromagnetic field on impurity ions. The interrelation of the observed effect with flash sintering and microwave-enhanced mass transport is discussed.     

Study on drying kinetics of calcium oxide doped zirconia by microwave-assisted drying

Ca–ZrO₂ is an essential structural and functional material, which is commonly used in refractories, electronic ceramics, and functional ceramics. The properties of Ca–ZrO₂ materials are depending on the quality of Ca–ZrO₂ powders. The main factors affecting the quality of powder are sintering temperature and the drying effect. This paper applied modern microwave drying technology to dry Ca–ZrO₂ powder. The impact of initial mass, microwave heating power, and initial moisture content on the drying of Ca–ZrO₂ were explored. The results showed that the average drying rate increased with the rise of initial mass, microwave heating power, and initial moisture content. Wang and Singh, Page, and Quadratic Model were applied to fit Ca–ZrO₂ with an initial moisture content of 5.6%, mass of 30 g, and microwave output power of 400 W. The results displayed that the Page model had a better fitting effect. It was also applicable to other different initial moisture content, original mass, and microwave heating power. The diffusion coefficient calculated by Fick's second law displayed that with the increase of initial mass, initial moisture content, and microwave heating power of Ca–ZrO₂, the effective diffusion coefficient increased first and then declined. When the Ca–ZrO₂ of microwave heating power was 640 W, mass was 30 g, and the moisture content was 5.65%, the effective diffusion coefficients of zirconia were 1.42533 × 10^?13, 2.91806 × 10^?13, 5.652.2471 × 10^?13 m²/s, respectively. To determine the activation energy of microwave dried zirconia, using the relationship between microwave power and activation energy, the activation energy of microwave dried zirconia was calculated to be ?23.39 g/W. This paper aims to rich experimental data for the industrial application of microwaves to strengthen dried zirconia and propose a theoretical basis.     

Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina

Microwave sintering of alumina has been carried out using SiC and Y‐ZrO₂ based susceptors. The microstructure of the sintered samples has been rigorously compared and correlated to the heating behaviour of the susceptor used. It was found that the nature of the susceptor highly influences the sintering behaviour of alumina. The results show that at high temperatures, the electrical conductivity of the SiC susceptor tends to screen the incident electric field, thus resulting in a surface heating of the alumina sample material. When using ZrO₂ susceptor, the microstructure analysis of the sintered alumina samples reveals a volumetric heating, which is a signature of the microwave dielectric loss mechanism. This could be explained by the lower ZrO₂ electrical conductivity compared to the SiC one. The simulation results confirm this behaviour, particularly showing that in the presence of ZrO₂, the intensity of the electric field within the sample is higher than the one when SiC susceptor is used. Basically, the results of the simulation data are in good agreement with our experimental results. Although the SiC based susceptor is usually used in the microwave heating of materials, the ZrO₂ based susceptor presents numerous advantages over the SiC one, especially in term of direct microwave heating contributions.