Ultrasound and microwave assisted synthesis of high loading Fe-supported Fischer-Tropsch catalysts

Microwave modification of iron supported on beta silicon carbide catalysts for Fischer–Tropsch synthesis

Beta silicon carbide is a good microwave absorber support. Microwave irradiation can improve the surface properties of Fe/β-SiC catalysts. Microwave irradiation can be used to improve the catalytic performance of Fe/β-SiC catalysts.

The effect of microwave irradiation on heterogeneous catalysts for Fischer–Tropsch synthesis

The work that has been carried out on microwave irradiation applied in catalyst preparation for drying, calcination or postsynthesis methods, and as a heating source for the Fischer–Tropsch reaction has been reviewed. It has been found that microwave irradiation can, in some cases, greatly enhance the performance of heterogeneous catalyst systems for Fischer–Tropsch synthesis. We have also summarized the advantages and drawbacks of using microwave irradiation in Fischer–Tropsch catalyst preparation and postsynthesis, and identified opportunities for future investigation.

Ultrasound assisted synthesis of Ag-decorated TiO2 active in visible light

Titanium dioxide is the most popular photocatalyst to degrade organic pollutants in air, as well as in water. The principal drawback preventing its commercial application lies in its limited absorption of the visible light (400-700nm), while it is active under UV irradiation (≤387nm). Supporting noble metals in the form of nanoparticles on TiO₂ increases its activity in the visible range. However, both the synthesis of noble metal nanoparticles and their deposition on TiO₂ are multi-step processes that often require organic solvents. Here, we deposit Ag nanoparticles from AgNO₃ on the surface of micrometric TiO₂ with H₂O as a solvent and under ultrasound irradiation at 30Wcm^-2. Ultrasound increases the surface amount of Ag on TiO₂ with heterogeneous size distribution of Ag nanoparticles, which are bigger and overlaid (1-20nm vs. 0.5-3nm) compared to the sample obtained in traditional conditions (TEM images). While this change in morphology had no effect on acetone photodegradation under UV light, the 5%, 10%, and 20% Ag-TiO₂ degraded 17%, 20% and 24% acetone under visible light, respectively. The 10% by weight Ag-TiO₂ sample obtained in absence of ultrasound only degraded 14% acetone in 6h, while the bare TiO₂ was not active.

Ultrasonic impregnation of MnO2/CeO2 and its application in catalytic sono-degradation of methyl orange

MnO₂/CeO₂ catalyst was prepared by ultrasonic impregnation method. The traditional and stirring impregnation methods were used as control. Results showed that ultrasonic impregnation was the best synthesis method. The impregnation time was shortened from 120 min (traditional method) to 20 min, the specific surface area of catalyst was three times larger, and the catalytic activity of catalyst was also the highest. Furthermore, MnO₂ had crystalline structure and distributed uniformly on the support, CeO₂. The preparing conditions were further examined and the optimal conditions were found to be: 20 min of ultrasonic impregnation, 4.3 mol/L of manganese nitrate concentration and 450 °C of calcination temperature. The so prepared catalyst removed 94% of methyl orange in 30 min with a dosage of 0.5 g/L. The efficiency was 77.7% and 85.9% for traditional and stirring impregnation method under the same experimental conditions. The reaction process involved two stages: adsorption-dominated and degradation-dominated stages. The reaction rate constant of adsorption-dominated stage had little difference. However, compared with traditional impregnation, the reaction rate constant of degradation-dominated stage improved from 0.01 to 0.14 min^-1 by ultrasonic impregnation. Mechanism analysis showed that the activity of ultrasonic impregnation MnO₂/CeO₂ was improved by the effects of acoustic cavitation and ultrasound oscillation on solid-liquid transport and distribution status.

Non-Noble Metal Oxide Catalysts for Methane Catalytic Combustion: Sonochemical Synthesis and Characterisation

The aim of this study was to obtain nanocrystalline mixed metal-oxide–ZrO₂ catalysts via a sonochemically-induced preparation method. The effect of a stabiliser’s addition on the catalyst parameters was investigated by several characterisation methods including X-ray Diffraction (XRD), nitrogen adsorption, X-ray fluorescence (XRF), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM) and µRaman. The sonochemical preparation method allowed us to manufacture the catalysts with uniformly dispersed metal-oxide nanoparticles at the support surface. The catalytic activity was tested in a methane combustion reaction. The activity of the catalysts prepared by the sonochemical method was higher than that of the reference catalysts prepared by the incipient wetness method without ultrasonic irradiation. The cobalt and chromium mixed zirconia catalysts revealed their high activities, which are comparable with those presented in the literature.

Fe-based heterogeneous catalysts for the Fischer-Tropsch reaction: Sonochemical synthesis and bench-scale experimental tests

The sonochemical synthesis of nanostructured materials owes its origins to the extreme conditions created during acoustic cavitation, i.e., the formation of localized hot spots in the core of collapsing bubbles in a liquid irradiated with high intensity ultrasound (US). In particular, in the present work a sonochemical synthesis has been investigated for the production of three different iron-based samples supported on SiO₂ and loaded with different metals and promoters (10 %wt of Fe; 30 %wt of Fe; 30 %wt of Fe, 2 %wt of K and 3.75 %wt of Cu) active in the Fischer-Tropsch (FT) process. Sonochemically synthesized heterogeneous catalysts were characterized by BET, XRPD, TPR, ICP, CHN, TEM, SEM and then tested in a fixed bed FT-bench-scale rig fed with a mixture of H₂ and CO at a H₂/CO molar ratio equal to 2, at activation temperatures of 350-400°C and reaction temperatures of 250-260°C. The experimental results showed that the ultrasonic samples are effective catalysts for the FT process. Notably, increasing the activation temperature increased CO conversion, while product selectivity did not diminish. All the sonochemically prepared samples presented in this work provided better catalytic results compared to the corresponding traditional FT impregnated catalysts.

CO2-enhanced dehydrogenation of ethane over sonochemically synthesized Cr/clinoptilolite-ZrO2 nanocatalyst: Effects of ultrasound irradiation and ZrO2 loading on catalytic activity and stability

CO2-enhanced oxidative dehydrogenation of ethane was investigated over sonochemically synthesized Cr/clinoptilolite-ZrO2 nanocatalyst with the aim of assessing the effect of composite support and ultrasonic irradiation on the nanocatalyst reactivity and stability. To this aim, ZrO2 promoted clinoptilolite supports varying in zirconia content (0, 25, 50wt%) were synthesized by hydrothermally precipitation method and impregnated with chromium nitrate under ultrasound irradiation. The samples were characterized by XRD, FESEM, EDX, TEM, ICP, BET, FTIR, TPR-H2 and TPD-NH3 techniques. The characterization results indicated that ultrasound irradiation could not only reduce the formation of Cr2O3 and decrease submicron particle size of chromium oxide to nanometer scale, but also promote the distribution of metallic particles and strengthen the chromium-support interaction. As a result, utilizing ultrasound irradiation in the synthesis of Cr/Clinoptilolite helped to maintain a high and stable catalytic activity. These features were more prominent in the presence of zirconia. It was found that the metal oxide nanoparticles with about 4-8nm are dispersed uniformly on the surface of composite support containing 25wt% ZrO2 (CLT-Z25). Moreover, the addition of ZrO2 resulted in the formation of new strong acid sites and a significant modification in the reducibility of chromium species, which alongside homogenous and small Cr nanoparticles account for the superior catalytic performance of ZrO2 containing samples. However, excessive loading of ZrO2 (50wt%) severely covered the surface of clinoptilolite, afforded the aggregations of metallic particles and thereupon, weakened the contact between clinoptilolite and ZrO2, which together with more acid strength seriously resulted in the deactivation of catalyst. In spite of superior initial activity of ZrO2-rich sample among the catalysts tested, ultrasonic synthesized Cr/CLT-Z25 nanocatalyst showed the best catalytic performance after 5h-catalytic reaction.

Highly loaded Ni-based catalysts for low temperature ethanol steam reforming

This paper describes the design of high-loading Ni/Al2O3 catalysts (78 wt% Ni) for low temperature ethanol steam reforming. The catalysts were synthesized via both co-precipitation (COP) and impregnation (IMP) methods. All the catalysts were measured by N2 adsorption-desorption, XRD, H2-TPR, and H2 pulse chemisorption. The characterization results demonstrated that the preparation method and the loading significantly affected the nickel particle size, active nickel surface area and catalytic performance. Over COP catalysts, large nickel particles were presented in nickel aluminum mixed oxides. In comparison, IMP catalysts gained more "free" NiO particles with weak interaction with the aluminum oxide. Consequently, COP catalysts yielded smaller nickel particles and larger active nickel surface areas than those of IMP catalysts. High loading is beneficial for obtaining sufficient active nickel sites when nickel particles are dispersed via COP, whereas excessive nickel content is not desired for catalysts prepared by IMP. Specifically, the 78 wt% nickel loaded catalyst synthesized by COP possessed small nickel particles (∼6.0 nm) and an abundant active nickel area (35.1 m(2) gcat(-1)). Consequently, COP-78 achieved superior stability with 92% ethanol conversion and ∼35% H2 selectivity at 673 K for 30 h despite the presence of a considerable amount of coke.

Microwave-ultrasound assisted synthesis of β-FeOOH and its catalytic property in a photo-Fenton-like process

In this study, nanoparticles of single-phase akaganeite (β-FeOOH) were synthesized by an ultrasound-microwave assisted method. The synthesis parameters were optimized by means of response surface methodology. X-ray diffractions (XRD), Fourier transform infrared spectrum (FTIR), UV-vis diffused reflection spectra (UV-vis DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and specific surface area were used to characterize the as-prepared samples. The catalytic activity of the prepared β-FeOOH was evaluated in a heterogeneous photo-Fenton-like process using methyl orange as target pollutant. It is found that the reaction temperature and interaction between microwave and ultrasound have a significant influence on the catalytic properties of the prepared β-FeOOH samples. The β-FeOOH prepared at microwave power of 400 W, ultrasound power of 200 W, reaction temperature of 70°C and reaction time of 3 h, exhibited considerable catalytic activity in weak alkaline solution and under visible light irradiation, which would be of great promise for the industrial application of this catalyst to oxidize organic pollutants for wastewater treatment.

A novel method of microwave heating mixed liquid-assisted regeneration of V₂O₅-WO₃/TiO₂ commercial SCR catalysts

An experimental study on the regeneration of deactivated SCR catalysts was carried out using a microwave-assisted method containing three steps of washing with mixed liquid of ethanol and water, impregnating, and drying. After the regeneration treatment, NO conversion at 320 °C increased from 39 to 90% and vanadium content increased by 62.2%, which were much higher than those regenerated by the traditional method. The more impregnated vanadium was due to the fact that the rapid evaporation of mixed liquid inside the catalyst channels led to the enlargement of surface areas by creating more pores on the catalysts. Meanwhile, with the increasing concentrations of ethanol, the heating rate of the mixed liquid increased, and the volume after complete evaporation of the mixed liquid was gradually reduced. Since higher heating rate and lager volume after the liquid evaporation could help to create more pores, therefore, when the volume ratio of ethanol/mixed solution was 20%, the catalyst obtained the maximum specific surface area, which significantly increased to ca. 123% compared with the deactivated catalyst. In addition, the catalyst dried by microwave exhibited better catalytic activity than that dried in conventional oven. Therefore, this method showed great potential in industrial applications.

Ultrasound-assistant preparation of Cu-SAPO-34 nanocatalyst for selective catalytic reduction of NO by NH3

The influence of the various preparation methods of Cu-SAPO-34 nanocatalysts on the selective catalytic reduction of NO with NH3 under excess oxygen was studied. Cu-SAPO-34 nanocatalysts were prepared by using four techniques: conventional impregnation (IM), ultrasound-enhanced impregnation (UIM), conventional deposition precipitation (DP) using NaOH and homogeneous deposition precipitation (HDP) using urea. These catalysts were characterized in detail by various techniques such as N2-sorption, XRD, TEM, H2-TPR, NH3-TPD and XPS to understand the catalyst structure, the nature and the dispersed state of the copper species, and the acid sites for NH3 adsorption. All of the nanocatalysts showed high activities for NO removal. However, the activities were different and followed the sequence of Cu-SAPO-34 (UIM)>Cu-SAPO-34 (HDP)>Cu-SAPO-34 (IM)>Cu-SAPO-34 (DP). Based on the obtained results, it was concluded that the NO conversion on Cu-SAPO-34 nanocatalysts was mainly related to the high reducibility of the isolated Cu(2+) ions and CuO species, the number of the acid sites and the dispersion of CuO species on SAPO-34.

The beneficial use of ultrasound in synthesis of nanostructured Ce-doped SAPO-34 used in methanol conversion to light olefins

Methanol to olefins process is an interesting route for synthesis of light olefins over nanostructured catalysts. The present research deals with catalyst development by sonochemical method for methanol to olefins reaction with the aim of reaching the most efficient catalyst. The CeSAPO-34 catalyst was prepared via ultrasound assisted hydrothermal method and characterized by XRD, FESEM, PSD, EDX, BET and FTIR techniques. The characteristics and performance of this sample were compared to the catalyst prepared by conventional hydrothermal method. XRD patterns reflected the higher crystallinity of the catalyst synthesized by ultrasound application. In comparison, particles with smaller sizes obtained by applying ultrasonic irradiation. The catalyst obtained using ultrasound had the longer lifetime and sustained desired light olefins at higher values.

Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants

Heterogeneous photocatalysis is becoming increasingly important due to the variety of its applications and multidisciplinary aspects. Applications such as water/air detoxification-disinfection, solar-energy storage, high-value-chemical production, optoelectronics, and sensors are some of the most promising. In recent years, the development of environmentally-friendly and cost-efficient procedures for material synthesis that could substitute the old ones has been on demand. Unconventional and soft techniques, such as sonication, offer enormous possibilities for the synthesis of a broad spectrum of nanostructured materials, among them photocatalysts. This Review will focus the readers' attention on ultrasound-induced methodologies used for the preparation of nanostructured photocatalysts (e.g., supported nanoparticles, semiconductors) and their application in the photocatalytic oxidation of organic contaminants.

Direct conversion of syngas to dimethyl ether as a green fuel over ultrasound-assisted synthesized CuO–ZnO–Al2O3/HZSM-5 nanocatalyst: effect of active phase ratio on physicochemical and catalytic properties at different process conditions

Hybrid co-precipitation–ultrasound synthesis of CuO–ZnO–Al₂O₃/HZSM-5 used in direct conversion of syngas to dimethyl ether as a green fuel.