Synthesis In Supercritical Water Research Articles

Supercritical water synthesized Ni/ZrO2 catalyst and its application in the harmless treatment of oilfield wastewater in supercritical water

Supercritical water (SCW) catalytic gasification is a clean and efficient technology with prominent advantages in treating oily wastewater generated in oilfields. SCW synthesis is a promising one-step preparation method for nano-catalytic materials, while the influence of organic matter on SCW synthesis materials remains to be studied. This study focuses on the synthesis of the Ni/ZrO2 catalysts by the supercritical water synthesis (SCWS) method, particularly examining how different types and concentrations of organic matter affect the catalysts’ pore information and structural characteristics. It found that glycerol acted as a more effective precursor for generating reducing gases compared to glucose. This is attributed to glycerol’s capacity in SCW to generate reducing gases for activating a greater number of Ni site within the catalyst, and the 5 wt% glycerol should be a proper concentration for SCWS of Ni/ZrO2. When employed in SCW treatment of oilfield wastewater, the Ni/ZrO2 catalyst demonstrated impressive results, achieving the carbon gasification efficiency of 99.1 % and a chemical oxygen demand (COD) removal efficiency of 100 % at 500°C. These findings indicate that the SCW technique can effectively and harmlessly treat oilfield wastewater using the Ni/ZrO2 catalyst under relatively mild conditions, showing its potential as an innovative solution for wastewater management in the oil industry.

Thermodynamic and environmental analysis of solar-driven supercritical water gasification of algae for ammonia synthesis and power production

A novel system of solar-driven supercritical water gasification (SCWG) of microalgae for ammonia synthesis and power production was proposed to achieve chemical hydrogen storage with ammonia carrier. The system units mainly include SCWG, ammonia synthesis in supercritical water (SCW), power generation by supercritical water mixing fluid, organic Rankine circulating power generation, gas separation and circulating compression, and steam methane reforming. The system design and process simulation were implemented using Aspen plus. The thermodynamic performance of the system was analyzed. The life cycle environmental impact was evaluated with Simapro software. The results of thermodynamic analysis revealed that the energy efficiency, exergy efficiency, and energy consumption per ton of ammonia of the system were closely related to the microalgae concentration designed for the system. The simulation results presented the highest energy efficiency of 42.62% and exergy efficiency of 44.27% with the lowest energy consumption per ton of ammonia of 48.79 GJ/t. The circulating gas compressor was the largest electricity consumer and the heat loss of the coolers was the main reason for the energy loss of the system, indicating the direction of system optimization in the future. The evaluation of the life cycle environmental impact assessment showed that the plant operation stage contributed little to the overall environmental impact compared with the construction and dismantling stage. The values of the main midpoint indicator global warming potential (GWP) and endpoint indicator Human health could be as low as 0.69 kg CO2-eq·kg−1 NH3 and 8.17 × 10−5, respectively, under suitable design conditions. Compared with other ammonia synthesis routes, this work has great advantages in terms of thermodynamics and environmental impact, while it faces both opportunities and challenges. This research reveals the feasibility of SCWG of algae for ammonia synthesis and provides theoretical guidance for biomass cleaning conversion.

One-pot supercritical water synthesis of Bi2MoO6-RGO 2D heterostructure as anodes for Li-ion batteries

Two dimensional (2D) materials stacked with Van der waals bonding to obtain a heterostructure have always generated tailored physical and chemical properties. In this paper, the development and application of single-phase 2D Bi2MoO6-reduced graphene oxide (BMO-RGO) heterostructure by one-pot supercritical water (SCW) method is reported for the first time. The as-synthesized nanocomposite of BMO-RGO with single-phase orthorhombic crystal structure is confirmed by XRD. The 2D nanoflake morphology was observed under transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Fourier transform infrared spectroscopy is performed to confirm the presence of BMO-RGO, RGO, and other functional groups, respectively. The specific capacities obtained through charge-discharge measurements from BMO and BMO-RGO heterostructure nanocomposites are compared and the optimization of RGO wt% is established. After 50 cycles, the capacity retention is around 202 mA h g−1 for BMO with 24 wt% RGO (BMO-24RGO) nanocomposite. This value is higher in comparison to pure BMO nanoflakes which have a capacity retention of 25 mA h g−1. These results show the influence of RGO content on the performance of electrochemical measurements when compared to pure BMO and BMO-24RGO electrodes. One-pot SCW synthesis is found to be a reliable method for the synthesis of BMO-RGO nanocomposite.

Efficient and stable supercritical-water-synthesized Ni-based catalysts for supercritical water gasification

Catalyst sintering problem hinders the development of effective supercritical water gasification (SCWG) at milder supercritical water (SCW) temperature for gas fuel production from wet biomass resources. Herein, we prepared a series of supported Ni catalysts by hydrothermal synthesis method particularly operated at the SCW region, and studied the activity and property stability of these newly developed catalysts for SCWG. Supporting materials studied for Ni include ZrO2, Ce-promoted ZrO2, Al2O3, Mg-promoted Al2O3, active carbon and carbon nanotube, and the supercritical water synthesis (SCWS) was conducted at 500 °C, 23 MPa in batch reactor using metallic nitrates as starting feed. SCWG results and catalyst characterization (XRD, SEM-EDS, N2 adsorption/desorption and TGA) confirmed the high activity, superiorly stable crystalline structure and pore structure, and excellent regeneration ability of the SCWS-prepared catalysts (especially Ni/MgO-Al2O3) for SCWG reactions, showing that SCWS is a promising method to develop active and stable catalyst for SCWG process.

Facile sub-/supercritical water synthesis of nanoflake MoVTeNbOx-mixed metal oxides without post-heat treatment and their catalytic performance

A fast and simple sub-/supercritical water synthesis method is presented in this work in which MoVTeNbOx-mixed metal oxides with various phase compositions and morphologies could be synthesized without post-heat treatment. It was demonstrated that the system temperature for synthesis had a significant influence on the physico-chemical properties of MoVTeNbOx. Higher temperatures were beneficial for the formation of a mixed crystalline phase containing TeVO4, Te3Mo2V2O17, Mo4O11 and TeO2, which are very different from the crystalline phases of conventional Mo–V–Te–Nb-mixed metal oxides. While at lower temperatures, Mo4O11 was replaced by Te. At high temperature, the as-prepared samples presented distinct nanoflake morphologies with an average size of 10–60 nm in width and exhibited excellent catalytic performances in the selective oxidation of propylene to acrylic acid. It is illustrated that the large specific surface area, presence of Mo4O11 and superficial Mo6+ and Te4+ ions are responsible for the high propylene conversion, while suitable acidic sites and superficial Nb5+ ions improved the selectivity to acrylic acid.

Supercritical water synthesized Ni/ZrO2 catalyst for hydrogen production from supercritical water gasification of glycerol

Catalysts are crucial to promote the technical feasibility of supercritical water gasification (SCWG) for H2 production from wet biomass, yet catalysts prepared by conventional methods normally encounter sintering problems in supercritical water. Herein, a series of ZrO2-supported Ni catalysts were tried to be prepared by supercritical water synthesis (SCWS) and evaluated for SCWG in terms of activity and property stability. The SCWS was conducted at 500 °C and 23 MPa using metal nitrates as starting materials. Effect of precursor concentration on property and catalytic performance of the SCWS-prepared catalysts for SCWG of 20 wt% glycerol were systematically studied. XRD, SEM-EDS, TEM and TGA were applied for catalyst characterization. Results verified the successful obtaining of Ni/ZrO2 nanocatalysts with Ni crystals of 30–70 nm and ZrO2 crystals of ~11 nm by the SCWS process, which were found to be active on the WGSR for SCWG to increase the H2 yield as high as 155%. Importantly, the SCWS-prepared Ni/ZrO2 catalysts exhibited excellent property stability and anti-coking ability for SCWG of glycerol.

Hydrothermal synthesis of perovskite metal oxide nanoparticles in supercritical water

In this paper we report about the particle size controlling factors on various perovskite metal oxide, XTiO3 (X = Ca, Sr, Ba, Pb), nanoparticles by hydrothermal synthesis in supercritical water of 400 °C and 30 MPa. Experiments were performed the developed flow reaction system. Characteristics of the product were performed by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy equipped with energy dispersive X-ray spectrometry (SEM-EDS) and nitrogen adsorption. The behavior of particle growth of perovskite metal oxides depended on A-site metal ions and the order of the particle size was Sr < Ca < Ba < Pb. The particle growth of perovskite metal oxide in supercritical water could be affected by solubility of A site ions.

Ultrafine powder of barium titanate for production of lead-free ceramic piezomaterials

The paper is aimed to perform and compare the capabilities of the solid state technology and a new method of synthesis in supercritical water for the production of barium titanate powder. This material is of interest for applications such as dense piezoceramics. The new method of synthesis in an aqueous medium allowed obtaining barium titanate of higher phase purity and with a narrow crystal size distribution compared to the solid state method. The ceramics manufactured from the barium titanate powder synthesized in supercritical water possessed a consistently high dielectric constant (2400-2600) and a low loss tangent (∼ 10−2) at low and medium frequencies.

Dependence of the Physicochemical and Catalytic Properties of Ce0.5Zr0.5O2 Oxide on the Means of Synthesis

The effect the means of synthesis have on the texture, phase composition, redox properties, and catalytic activity of binary oxide systems with the composition Ce0.5Zr0.5O2 are studied. The obtained samples are characterized via BET, SEM, DTA, XRD, and Raman spectroscopy. A comparative analysis is performed of the physicochemical properties of biomorphic systems Ce0.5Zr0.5O2 obtained using wood sawdust and cellulose as templates and the properties of binary oxides of the same composition obtained by template-free means. The catalytic properties of the obtained oxide systems Ce0.5Zr0.5O2 are studied in the reaction of carbon black oxidation. It is shown that the texture of the oxide depends on the means of synthesis. When biotemplates are used, fragile porous systems form from thin binary oxide plates containing micro-, meso-, and macropores. Oxide obtained via coprecipitation consists of dense agglomerates with pores around 30 A in size. In supercritical water, nanoparticles of metal oxide form that are loosely agglomerated. The intermediate spaces between them act as pores more than 100 A in size. A system of single-phase pseudocubic modification is obtained using a cellulose template. The crystal lattices of all the obtained systems contain a great many defects. It is shown that the system prepared via synthesis in supercritical water has the best oxygen-exchange properties. A comparative analysis is performed of the effect the physicochemical properties of the samples have on their activity in the catalytic oxidation of carbon black.

Supercritical water synthesis of nano-particle catalyst on TiO2 and its application in supercritical water gasification of biomass

ABSTRACTHydrogen production by catalytic gasification in supercritical water (SCW) is a promising way to utilise biomass resource. Supercritical water not only provides homogeneous and rapid reaction environment for the biomass gasification but also causes catalyst agglomeration problems. In order to prepare activity and stable catalyst for biomass gasification in supercritical water, supercritical water synthesis method was utilised and the preparation method was investigated. Ni, Co, Zn and Cu metal elements were loaded on TiO2 particles which was proved to be hythothermally steady in supercritical water. And nano-particles were successfully made. Based on gas chromatography/mass spectrometer (GC/MS), scanning electron microscopy, energy dispersive spectrometer (EDS) and X-ray diffraction analysis methods, it turned out that metal catalysts have a uniform spherical structure with diameter around 30 nm. Metal catalysts synthesised with supercritical hydrothermal method showed certain catalytic effects. Ni catalyst had the best performance in stability while Zn catalyst possessed highest hydrogen yield.

Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water

Supercritical water not only provides good solvent of biomass gasification but also causes catalyst agglomeration. Therefore, the supercritical water synthesis method was utilized to guarantee the stability of the catalyst. In order to take into account the advantages of different metal catalysts, bimetallic catalysts were prepared based on Ni, Co, Zn and Cu, with TiO2 as carrier. Hydrogen production by furfural gasification in supercritical water was conducted with different catalysts. The experimental results turned out that all combinations showed remarkable stability except for the combination Co + Ni. Zn + Ni and Cu + Zn showed better selectivity on hydrogen, and Co + Ni showed better carbon gasification efficiency. Spherical coke particles were obtained after the reaction with bimetallic catalysts, while the coke particles were amorphous after the reaction with K2CO3.

Crystalline barium titanate synthesized in sub- and supercritical water

The promoting effect of water in different states of aggregation on the transformation processes in oxide mixture was studied in case of BaTiO3 formation. The reaction between barium and titanium oxide was carried out in liquid, in gaseous and in supercritical water. The state of aggregation of the reaction medium had an influence on the degree of transformation of the reagents and on the characteristics of produced BaTiO3 such as the crystal size, the content of volatile impurities and sinterability. On the basis of comparative investigation it was concluded that the synthesis in supercritical water appeared to be the most effective way for production of pure fine-crystalline BaTiO3 for the ceramics preparation.

Continuous Flow Supercritical Water Synthesis and Temperature-Dependent Defect Structure Analysis of YAG and YbAG Nanoparticles

Yttrium and ytterbium aluminum garnet (Y3Al5O12 and Yb3Al5O12) nanoparticles have been synthesized under sub- and supercritical water conditions in a continuous flow reactor. The particle size has been investigated by scanning and transmission electron microscopy. The heat-induced structural changes of the garnet structure have been investigated by multitemperature powder X-ray diffraction at a synchrotron source (100–1000 K). In combination, Rietveld refinement of these multitemperature data and thermal analysis indicate a proposed diffusion mechanism for the aluminum and yttrium/ytterbium atoms in the garnet structure, which leads to fewer defects at higher temperature. Hence, hydrothermally synthesized nanoparticles give novel knowledge about the disordered internal structure important for their use in optical applications.

Zirconia nanoparticle synthesis in sub and supercritical water — particle morphology and chemical equilibria

Zirconia nanoparticles were prepared in supercritical water using hydrothermal method, where particle morphology was found to depend on the reaction temperature and pressure, pH and precursor concentration. Having calculated the equilibrium constant using revised HKF (R-HKF) and Sue-Adschiri-Arai (SAA) thermodynamic models, the relationships between particle morphological characteristics and distribution of zirconium chemical species (including Zr4+, ZrOH3+, ZrO2, Zr(OH)22+, Zr(OH)3+, Zr(OH)4, Zr(OH)5−, NO3−) in subcritical and supercritical water were studied. Results indicated that Zr(OH)22+ was the dominant species at lower pH, while at higher pH, Zr(OH)5−− became the dominant one. Distribution of zirconium species as well as different phases of ZrO2, suggested that, Zr(OH)22+ and Zr(OH)5−− ions most likely play a significant role in the synthesis of monoclinic and tetragonal phases, respectively.

A rapid production of pure TMA–montmorillonite nanoclay in supercritical water: Study on powder crystallinity and adsorption capacity

Pure nano-montmorillonite (MMT) with interlayer cations of tetramethylammonium (TMA+) was prepared by the two different procedures in supercritical water (SCW). In single-step procedure, synthesis of a 1-month aged precursor was conducted directly in SCW medium; while, in two-step procedure, a temperature programmed-promotion approach was applied. For the later technique, first the aging of the fresh solution of reactants was carried out for 2h at 80°C followed by temperature increment to reach supercritical condition (T>372°C, P>221atm). The prepared powders were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The Al/Si ratio of synthesized montmorillonite was about 1.73 as determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Furthermore, the effects of operational parameters, i.e. temperature, reaction time, and precursor aging were studied on purity and crystallinity of product. In spite of the high crystalline montmorillonite obtained from the two procedures, it was revealed that the two-step procedure had a significant effect on the crystallinity, surface area, and consequently, adsorption capacity of products compared to MMT obtained from a single-step method. High adsorption capacity indicates that crystalline TMA–montmorillonite can be synthesized efficiently using two-stage procedure in supercritical water. Possible mechanism of montmorillonite synthesis in supercritical water was also suggested.

Supercritical water syntheses of CexTiO2 nano-catalysts with a strong metal-support interaction for selective catalytic reduction of NO with NH3

This paper reported the feasibility of a supercritical water synthesis route (referred as CHFS) in the application of selective catalytic reduction (SCR) of NO with NH3. A series of well-reported CexTiO2 SCR catalysts were synthesized and then subjected to a range of characterizations (e.g. XRD, XPS, H2-TPR, HADDF, etc.). Experimental results revealed that the unusual and rapid crystallizing environment of the CHFS route had resulted in a highly dispersed CeO2 and a strong metal-support interaction (SMSI) for CexTiO2 catalysts, which promoted the enriched surface oxygen vacancies and facilitated the formation of NO2 and nitrite species over the catalyst surface, effectively enhancing their SCR performances. Amongst the developed CexTiO2 catalysts, Ce0.25TiO2 revealed the highest SCR activity with more than 90% NO conversion being achieved in a broad temperature range of 240–440°C. Such broad temperature window was proven due to the SMSI, which lowered the NH3 oxidation ability of CeO2, retaining the effective utilization of NH3 throughout the SCR process. The work conducted herein demonstrates the feasibility of the CHFS route in the syntheses of SMSI catalysts for SCR application and also explores the SCR reaction mechanism over the well-reported CexTiO2 catalysts. We expected that this work could shed some lights on the development of feasible preparative routes for the syntheses of advanced SMSI catalysts for SCR application.

Colloidal monolayer titania quantum dots prepared by hydrothermal synthesis in supercritical water

Two-dimensional monolayer titania quantum dots (MTQDs) with ∼0.4nm thickness and ∼2nm lateral size are synthesized by supercritical water (SCW) treatment of titania nanotubes (TNTs). The morphology, chemical characteristics and the structure of MTQDs are studied. The formation mechanism of the MTQDs and the differences between SCW and low-temperature hydrothermal treatment are discussed. During the reaction, the high temperature, high pressure and high H+/OH− concentration of SCW dissolved TNTs into MTQDs, and the intercalation property of the “active” water clusters formed from the broken hydrogen bonding network facilitated the detachment of the MTQDs from the TNTs. The above two reasons lead to the capture of the dissolved tiny particles, which could hardly preserved in low-temperature hydrothermal treatment. The MTQDs may be the minimum constituent unit existing in the reality of the anatase TiO2. As a new member of the monolayer family, this new kind of 2D material may shed new light on the study of the monolayer materials.

Special features of the tungsten wire heat transfer and the WO3 nanoparticle synthesis in supercritical water

Special features of the tungsten wire (TW) heat transfer in supercritical water at P = 25 MPa and T ≤ 923 K are investigated, including the conditions of (WO3)n nanoparticle formation on the tungsten wire surface with the reaction kn〈W〉 + 3knH2O = k(WO3)n + 3knH2. Values of the heat transfer coefficient as a function of oxidation time and temperature were determined using the method of electric current flash heating of the tungsten wire. It has been found that due to WO3 dissolution in supercritical water, there occurs oxide recondensation from the tungsten wire surface onto other design elements of the setup, which is accompanied by formation of monoclinic and rhombic WO3, and also monoclinic FeWO4.

Continuous synthesis of lithium iron phosphate nanoparticles in supercritical water: Effect of process parameters

This study investigates the effect of various process parameters during continuous synthesis in supercritical water on the physicochemical and electrochemical properties of lithium iron phosphate (LiFePO4) for use in large-scale lithium 2nd battery applications. The process parameters include reaction temperature (300–400°C), precursor solution concentration (0.01–0.18M), precursor solution flow rate (1.5–3.0g/min), water flow rate (9.0–36.0g/min), and residence time (9–72s). Under subcritical water conditions, mixed Fe3(PO4)2⋅8H2O, Fe2O3, and Fe3O4 particles formed; in contrast, under supercritical fluid conditions, well-crystallized LiFePO4 particles with some Fe3+ impurities (i.e., Fe2O3 and Fe3O4) were obtained. In supercritical water, an increase in the precursor concentration leads to an increase in the Fe3+ impurity content. At a high water flow rate, a significant decrease in crystallinity and the formation of Fe3(PO4)2⋅8H2O and Li6P6O18⋅9H2O phases rather than LiFePO4 were observed. Highly crystalline LiFePO4 with good discharge capacity was obtained with high temperature, low precursor concentration, and low flow rate conditions. Depending on the synthetic conditions, bare LiFePO4 particles exhibit discharge capacities of 55–85mAh/g at 0.1 C-rate after 30 cycles. After carbon coating, a marginal capacity decay from 141 to 135mAh/g was observed during the 30 charge–discharge cycles.

Scale Up Production of Nanoparticles: Continuous Supercritical Water Synthesis of Ce–Zn Oxides

A new continuous supercritical water pilot plant was used for the large-scale production of nanomaterials in the Zn–Ce oxide system. Similar to an existing laboratory continuous process, the pilot plant mixes aqueous solutions of the metal salts at room temperature with a flow of supercritical water (450 °C and 24.1 MPa) in a confined jet mixer, resulting in the formation of nanoparticles in a continuous manner. The Zn–Ce oxide system, as synthesized here under identical concentration conditions than those used in our laboratory scale process (but 17.5 times total flow rate), has been used as a model system to identify differences in particle properties due to the physical enlargement of the mixer. The data collected for the nanoparticles from the pilot plant was compared to previous work using a laboratory scale continuous reactor. In the Ce–Zn binary oxide series, it was shown that Zn had an apparent solubility of about 20 mol% in the CeO2 (fluorite) lattice, whereafter a composite of the two phases was...