Structural effect of ZrO2 on supported Ni-based catalysts for supercritical water gasification of oil-containing wastewater

Supercritical water gasification (SCWG) is a promising technology to treat oil-containing wastewater. This study reports a Ni/ZrO₂ catalyst with mixed phase of tetragonal ZrO₂ and monoclinic ZrO₂ synthesized by a one-step sol-gel method, which shows good catalytic activity and stability in SCWG of oil mixture. The evolution of particle size, crystal structure, and pore size caused by increasing calcination temperature is demonstrated by X-ray diffraction and transmission electron microscopy. When calcined at 700 °C, Ni/ZrO₂ shows superior catalytic activity and stability in SCWG reaction, with a high carbon gasification efficiency of 91.3% at 500 °C. The characterizations of fresh and spent catalysts suggest that Ni/ZrO₂ catalyst with the fraction of monoclinic ZrO₂ around 75% would be highly active and stable in SCWG reaction. In addition, two sets of optimal gasification conditions of SCWG of oil are obtained by varying operating parameters, which provides guidance for different concentrations of oil-containing wastewater treatment.     

A facile nonaqueous route for preparing mixed-phase TiO2 with high activity in photocatalytic hydrogen generation

A facile and simple method was developed to prepare amorphous titanium oxalate from nonaqueous reaction of tetrabutyl titanate and oxalic acid in ethanol at room temperature. This complex was converted to mixed-phase TiO₂ (anatase/rutile) by calcinations. The mixed-phase TiO₂ obtained at the optimum calcination temperature (600 °C) consisting of 67 wt% anatase and 33 wt% rutile exhibited superior photocatalytic hydrogen production activity (1026 μmol h^?1) with high stability, which can be ascribed to the phase-junctions (anatase/rutile) and high crystalline.     

Corrosion characteristics of a nickel-base alloy C-276 in harsh environments

The oxidation behaviors of alloy C-276 in two harsh environments: high-temperature air and supercritical water (SCW), respectively representing the working conditions of the external and internal surfaces of reactors for SCW gasification biomass to produce H₂, were investigated. In two environments, all oxidation kinetics followed parabolic laws, while the corrosion rate of alloy C-276 exposed to supercritical water gasification (SCWG) environments was 2.5–3 times higher than that in high-temperature air. The oxide scale formed in air at 500 °C consisted of an outer Fe-rich layer (Fe₂O₃ and NiCr₂O₄) and an inner layer of Cr₂O₃ and NiCr₂O₄, while the outer Fe-rich layer disappeared as the temperature increased to 550 °C. Compared to the scales formed on nickel-base alloys in near-pure SCW, the absence of NiO and Ni(OH)₂ phases within the scales formed on the C-276 samples in present SCWG environment may be due to higher molar proportion of hydrogen.     

Investigation of the conversion mechanism for hydrogen production by coal gasification in supercritical water

The technology of supercritical water gasification (SCWG) of coal has a great prospect because it converts coal into hydrogen-rich gas products efficiently and cleanly. However, there are bottlenecks affecting the complete gasification of coal in supercritical water (SCW) without catalyst under moderate conditions. This work is to explore the restricted factor for complete gasification of coal in SCW by investigating the conversion mechanism. The conversion mechanism of SCWG of coal with and without K₂CO₃ is proposed. Polycyclic aromatic hydrocarbons (PAHs) with graphite phase structures are formed by the condensation of aromatic structures at 550–750 °C. It is the restricted factor due to its characteristic of difficulty to be gasified. There is no condensation of aromatic structures in the process of SCWG of coal with K₂CO₃, which effectively inhibited the formation of PAHs with graphite phase structures. K₂CO₃ dramatically promoted the SCWG of coal, leading to carbon gasification efficiency (CE) reaching 98.43%.     

Influences of oxygen on corrosion characteristics of TiO2/316L stainless steel in supercritical water

Supercritical water gasification (SCWG) is a promising technology for converting organic wastes to hydrogen. Less amount of oxygen is beneficial for increasing hydrogen generation rate. However, the corrosion rate of reactor material would be accelerated. TiO₂ coating with a thickness of 0.1 mm was prepared on the surface of 316L stainless steel (SS316L) to improve its corrosion resistance in supercritical water (SCW). The corrosion performances of TiO₂/SS316L were tested in a bath SCW reactor at 400 °C, 25 MPa. The influences of oxygen concentration (0–1000 mg/L) on surface morphologies and corrosion depths were studied thoroughly. Results indicated that the surface of TiO₂/SS316L exhibited cracks and pores after exposed in SCW. And the average corrosion rates accelerated at higher oxygen concentrations. The interface between the coating and medium was relatively smooth and there was no obvious change in the thickness of the coating with oxygen concentration of 0 and 500 mg/L. While for that with 1000 mg/L oxygen, the surface of TiO₂/SS316L exhibited reticulate crack. The cross section showed a serrate structure, and only 0.08 mm thick of the coating was remained. In addition, the corrosion mechanism of coating was discussed.     

Synthesis of lithium insertion material Li4Ti5O12 from rutile TiO2 via surface activation

The synthesis of spinel-type lithium titanate, Li₄Ti₅O₁₂, a promising anode material of secondary lithium-ion battery, from “inert” rutile TiO₂, is investigated. On the purpose of increasing the reactivity of rutile TiO₂, it is treated by concentrated HNO₃. By applying such activated rutile TiO₂ as the titanium source in combination with the cellulose-assisted combustion synthesis, phase-pure Li₄Ti₅O₁₂ is successfully synthesized at 800 °C, at least 150 °C lower than that based on solid-state reaction. The resulted oxide shows a reversible discharge capacity of ～175 mAh g^?1 at 1 C rate, near the theoretical value. The resulted oxide also shows promising high rate performance with a discharge capacity of ～100 mAh g^?1 at 10 C rate and high cycling stability.     

Thermally stable Pt/Ti mesh catalyst for catalytic hydrogen combustion

In this study, platinum (Pt) supported on titanium (Ti) mesh catalysts for catalytic hydrogen combustion were prepared by depositing Pt as a thin-layer on metallic or calcined Ti mesh. The Pt thin-layer could be stabilized as uniformly distributed, near nano-sized particles on the surface of calcined Ti mesh by exposing the freshly sputtered Pt to hydrogen. Temperatures between 478 and 525 °C were reached during hydrogen combustion and could be maintained at a hydrogen flow rate of 0.4 normal liter (Nl)/min for several hrs. It was determined that Ti mesh calcination at ≥900 °C formed an oxide layer on the surface of Ti wires, which prevented significant Pt aggregation. X-ray photoelectron spectroscopy revealed that the surface of Ti mesh was fully converted to TiO₂ at ≥900 °C. Raman spectroscopy showed that the majority of TiO₂ was present in the rutile phase, with some minor contribution from anatase-TiO₂. The calcined Ti support was stable through all investigations and did not indicate any signs of degradation.     

Experimental study on oil-containing wastewater gasification in supercritical water in a continuous system

Recently, promising multi-fluid thermal stimulation technology has been studied and applied successfully to the offshore heavy oil recovery process. However, there are still some shortcomings, including heavy reliance on diesel, heavy and bulky water pretreatment plant, etc., which hinders the further industrialization process of the current generation system of thermal multi-fluid. A novel thermal multi-fluid generation technology to decrease reliance on diesel was presented in this paper. In this technology, oil-containing wastewater can be used directly as the material and energy source. Increased attention should be given to the supercritical water gasification (SCWG) process as the core of this novel technology. An experimental study on SCWG of diesel, a representative of oil-containing wastewater, was described detailly in this paper. Experiments were conducted in a continuous SCWG system under different experimental conditions, as follows: pressure of 23 MPa; reaction temperature of 600 °C–680 °C; mass ratios of water to diesel in emulsification of 1:1, 1:2.5, and 1:3.5; and reactor diesel concentration (R_DC) of 4.70 wt% to 7.70 wt%. The effects of alkali catalyst (K₂CO₃) on gaseous and liquid products were also investigated. A simplified gasification mechanism in SCW was obtained, which may provide a theory basis for the novel thermal multi-fluid generation technology based on oil-containing wastewater gasification in supercritical water.     

Effects of temperature on corrosion performances of TiO2/SS316L in supercritical water for hydrogen production

Temperature is the most important factor for hydrogen generation during supercritical water gasification process. However, the increasing temperature could accelerate the corrosion of the reactor material, at the presence of oxygen, as less amount of oxygen can promote the hydrogen production. In this study, we prepared a 0.1 mm thick of TiO₂ coating on the surface of 316L stainless steel (SS316L) to enhance the corrosion resistance of SS316L during hydrogen production process in supercritical water. The influences of temperature (400–500 °C) on surface morphologies and corrosion depth and rate of TiO₂/SS316L were evaluated at 25 MPa with 1000 mg/L oxygen for 80h. Results showed that cracks and pores were present on the surface of TiO₂/SS316L after corroded in SCW for 80h. The crack width and corrosion rate was aggravated at higher temperature. The remained thickness of the coating at 400 °C, 450 °C, 500 °C were 0.08 mm, 0.05 mm and 0.03 mm, respectively. NiO and NiFe₂O₄ were generated around the crack on the surface of TiO₂/316L at 500 °C, the coating had a tendency to peel off the substrate.     

Comprehensive experimental study on supercritical water gasification of oilfield sludge: Effect of operation parameters,and catalysts on the products

Supercritical water gasification (SCWG) was adopted to treat oilfield sludge and produce syngas. The effect of temperature (400–450 °C), reaction time (30–90 min) and catalyst addition on syngas production and residual products during SCWG of oilfield sludge was studied. When increasing SCWG temperature from 400 to 450 °C with reaction time of 60 min, the H₂ yield and the selectivity of H₂ increased significantly from 0.53 mol/kg and 75.53% to 0.98 mol/kg and 78.09%, respectively. It is noteworthy that when the reaction time was too long, CO₂ and CO were converted to CH₄ with the consumption of H₂ via methanation reaction. The addition of Ni/Al₂O₃ catalyst can substantially promote the production of high-quality syngas from SCWG of oilfield sludge. The H₂ yield and its selectivity at 450 °C and 60 min were as high as 1.37 mol/kg and 84.05% with 10Ni/Al catalyst. Moreover, the catalysts with bimetal loading (Fe–Ni, Rb–Ni or Ce–Ni) were found to be beneficial for improving gasification efficiency, H₂ yield, and the degradation of organic compounds. Among them, 5 wt% Rb on 10Ni/Al catalyst performed the best catalytic activity for SCWG at 450 °C and 60 min, which had the highest H₂ yield of 1.67 mol/kg and selectivity of 86.09%. More than 90% of total organic carbon in sludge was decomposed after the SCWG with all the catalysts. These findings indicated that catalytic SCWG is a promising alternative for efficiently dealing with oilfield sludge.     

Methanol steam reforming over highly efficient CuO–Al2O3 nanocatalysts synthesized by the solid-state mechanochemical method

CH₃OH steam reforming is an attractive way to produce hydrogen with high efficiency. In this study, CuO.xAl₂O₃ (x = 1, 2, 3, and 4) were fabricated based on the solid-state route, and the calcined samples were employed in methanol steam reforming at atmospheric pressure and in the temperature range of 200–450 °C. The results revealed that all samples have a high BET area (173–275 m² g⁻¹), and their crystallinity was reduced by increasing the alumina content in the catalyst formulation. The catalytic activity tests showed that the CH₃OH conversion and H₂ selectivity decreased by rising the Al₂O₃·CuO molar ratio. The methanol conversion enhanced from 13% to 85% by increasing the reaction temperature from 200 °C to 450 °C over the CuO·Al₂O₃ catalyst, due to the higher reducibility of this catalyst at lower temperatures compared to other prepared samples. The influence of calcination temperature (300–500 °C), GHSV (28,000–48000 ml h⁻¹. g⁻¹_cat), feed ratio (C:W = 1:1 to 1:9), and reduction temperature (250–450 °C) was also determined on the yield of the chosen sample. The results revealed that the maximum methanol conversion decreased from 90 to 79% by raising the calcination temperature from 300 to 500 °C due to the reduction of surface area and sintering of species at high calcination temperatures.     

Catalytic supercritical water gasification of black liquor along with lignocellulosic biomass

Supercritical water gasification (SCWG) is a novel technology for environmental pollution management and hydrogen production from biomass and wastes. In this study, the SCWG of black liquor (BL) which is high-potential biomass and rich in alkalis was investigated. The experiments were conducted in a batch reactor at 350–400 °C, reaction time of 1–60 min, and constant concentration of 9 wt% of BL in the absence and presence of heterogeneous catalysts (3–5 wt%), lignocellulosic biomass, and formic acid (5 and 7 wt %) in three parts. First, the SCWG of BL was performed without any additive. The experimental results showed that the maximum production of H₂, CO₂, and CH₄ was obtained at the highest temperature and reaction time; 400 °C and 60 min. The hydrogen yield was also enhanced by increasing the temperature, and reached 3.51 mol H₂/kg dry ash free-black liquor (DAF-BL) at 400 °C. Reaction time increment improved the gas product and gasification efficiency up to 28.03 mmol and 21.73%, respectively. Subsequently, three heterogeneous catalysts (MnO₂, CuO, and TiO₂) were used, however 5 wt% of MnO₂ was the best catalyst, significantly improving the hydrogen yield compared to the same condition of BL gasification without a catalyst. Hydrogen yield reached 5.09 mol H₂/kg (DAF-BL) at 400 °C and the reaction time of 10 min. Finally, BL with poplar wood residue as a lignocellulosic biomass and formic acid was gasified separately and the highest hydrogen yield was obtained in the case of 5 wt% of formic acid (10.79 mol H₂/kg (DAF-BL)). Overally, SCWG dramatically reduced the chemical oxygen demand of BL to 76% using 5 wt% of formic acid.     

Efficient hydrogen production from ammonia borane hydrolysis catalyzed by TiO2-supported RuCo catalysts

Hydrolysis of ammonia borane provides a reliable pathway for hydrogen production, while suitable catalysts are indispensable to make the hydrolysis reaction reach a considerable rate. In the present work, a series of TiO₂-supported RuCo catalysts have been fabricated by coprecipitation and subsequent reduction of Ru³⁺ and Co²⁺ on the surface of TiO₂ nanoparticles. Transmission electron microscopy and elemental mapping have verified the good distribution of metal species in the catalysts. The fabricated catalysts have shown excellent performance for catalyzing ammonia borane hydrolysis, especially in alkaline solutions with 0.5 M NaOH. For Ru₁Co₉/TiO₂ in which Ru/Co molar ratio is 1:9, the active energy of catalyzed ammonia borane hydrolysis is 33.25 kJ/mol, and a turnover frequency based on Ru as high as 1408 mol_H2/(mol_Ru·min) is obtained at 25 °C. Moreover, when different types of TiO₂ substrates are used, anatase TiO₂-supported catalysts show better catalytic activity than their counterparts with rutile TiO₂ as substrate or mixture of anatase and rutile TiO₂ as substrate.     

Supercritical water gasification of food waste: Effect of parameters on hydrogen production

Food waste is a type of municipal solid waste with abundant organic matter. Hydrogen contains high energy and can be produced by supercritical water gasification (SCWG) of organic waste. In this study, food waste was gasified at various reaction times (20–60 min) and temperatures (400 °C-450 °C) and with different food additives (NaOH, NaHCO₃, and NaCl) to investigate the effects of these factors on syngas yield and composition. The results showed that the increase in gasification temperature and time improved gasification efficiency. Also, the addition of food additives with Na⁺ promoted the SCWG of food waste. The highest H₂ yield obtained through non-catalytic experiments was 2.0 mol/kg, and the total gas yield was 7.89 mol/kg. NaOH demonstrated the best catalytic performance in SCWG of food waste, and the highest hydrogen production was 12.73 mol/kg. The results propose that supercritical water gasification could be a proficient technology for food waste to generate hydrogen-rich gas products.     

Cu-doped GdFeO3 perovskites as electrocatalysts for the oxygen evolution reaction in alkaline media

Development of electrocatalysts for the oxygen evolution reaction (OER) plays a critical role in electrochemical water splitting systems. In this aim, iron based perovskite oxides with composition GdFe_1-x Cu_x O₃ (0 ≤ x ≤ 0.3) have been investigated. The effect of copper doping, calcination temperature on the OER performance in alkaline media was studied. The incorporation of Cu²⁺ (0 ≤ x ≤ 0.3) decreases the activity of calcined electrodes at 800 °C from 6.33 to 2.79 mA cm⁻², while that containing 0.2 mol of copper calcined at 600 °C, exhibits the higher activity (9.66 mA cm⁻²) at 0.66 V. The stability study during 8 h indicates that the undoped electrode calcined at 800 °C, exhibits relatively a better stability of the OER performance compared to that doped with 20% of copper calcined at 600 °C. The achieved results show promising potential for cost-effective hydrogen generation using earth-abundant materials and cheap fabrication processes.     

Catalytic gasification of food waste in supercritical water over La promoted Ni/Al2O3 catalysts for enhancing H2 production

Ni/Al₂O₃ catalyst is the one of promising catalysts for enhancing H₂ production from supercritical water gasification (SCWG) of biomass. However, due to carbon deposition, the deactivation of Ni/Al₂O₃ catalyst is still a serious issue. In this work, the effects of lanthanum (La) as promoter on the properties and catalytic performance of Ni/Al₂O₃ in SCWG of food waste were investigated. La promoted Ni/Al₂O₃ catalysts with different La loading content (3–15 wt%) were prepared via impregnation method. The catalysts were characterized using XRD, SEM, BET techniques. The SCWG experiments were carried out in a Hastelloy batch reactor in the operating temperature range of 420–480 °C, and evaluated based on H₂ production. The stability of the catalysts was assessed by the amount of carbon deposition on catalyst surface and their catalytic activity after reuse cycles. The results showed that 9 wt% La promoter is the optimal loading as Ni/9La–Al₂O₃ catalyst performed best performance with the highest H₂ yield of 8.03 mol/kg, and H₂ mole fraction of 42.46% at 480 °C. La promoted Ni/Al₂O₃ catalysts have better anti-carbon deposition properties than bare Ni/Al₂O₃ catalyst, resulting in better gasification efficiency after reuse cycles. Ni/9La–Al₂O₃ catalyst showed high catalytic activity in SCWG of food waste and had good stability as it was still active for enhancing H₂ production when used in SCWG for the third time, which indicated that La promoted Ni/Al₂O₃ catalysts are potential additive to improve the SCWG of food waste.     

Direct Z-scheme anatase/rutile bi-phase nanocomposite TiO2 nanofiber photocatalyst with enhanced photocatalytic H2-production activity

Design and preparation of direct Z-scheme anatase/rutile TiO₂ nanofiber photocatalyst to enhance photocatalytic H₂-production activity via water splitting is of great importance from both theoretical and practical viewpoints. Herein, we develop a facile method for preparing anatase and rutile bi-phase TiO₂ nanofibers with changing rutile content via a slow and rapid cooling of calcined electrospun TiO₂ nanofibers. The phase structure and composition, surface morphology, specific surface area, surface chemical composition and element chemical states of TiO₂ nanofibers were analyzed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). By a rapid cooling of 500 °C-calcined electrospun TiO₂ precursor, anatase/rutile bi-phase TiO₂ nanofibers with a roughly equal weight ratio of 55 wt.% anatase and 45 wt.% rutile were prepared. The enhanced H₂ production performance was observed in the above obtained anatase/rutile composite TiO₂ nanofibers. A Z-scheme photocatalytic mechanism is first proposed to explain the enhanced photocatalytic H₂-production activity of anatase/rutile bi-phase TiO₂ nanofibers, which is different from the traditional heterojunction electron–hole separation mechanism. This report highlights the importance of phase structure and composition on optimizing photocatalytic activity of TiO₂-based material.     

Fabrication of TiO2 nanoflowers with bronze (TiO2(B))/anatase heterophase junctions for efficient photocatalytic hydrogen production

Light harvesting and charge separation are both significant in the photocatalysis, but it is challenging to synchronously realize both in a single-component material. The novel porous TiO₂ nanoflowers (NFs) photocatalysts with stable bronze (TiO₂(B))/anatase heterophase junctions and large pore sizes are prepared via a hydrothermal/annealing method. The presence of porous nanoflower structure enhances the light absorption through reflection/refraction of light. The stable TiO₂(B)/anatase heterophase junctions can efficiently promote the separation of photoinduced electrons and holes pairs and therefore suppress the charge recombination. The large pore sizes provide multi-level channels for the absorption and diffusion of reactants. With the increase of annealing temperatures from 350 to 550 °C, the H₂ evolution activity is promoted. However, overhigh annealing temperature (650 °C) cause the broken of nanoflower structure and TiO₂(B)/anatase heterophase junctions, thus inducing even decrease of H₂ evolution activity. As a consequence, the obtained TiO₂ NFs exhibit an enhanced photocatalytic H₂ evolution activity at the optimal annealing temperature (550 °C) with Pt as co-catalysts (5.013 mmol h⁻¹g⁻¹), exceeding that of TiO₂ NFs without annealing (0 mmol h⁻¹g⁻¹) and pure anatase TiO₂ NFs (TiO₂ NFs-650 °C, 4.722 mmol h⁻¹g⁻¹), respectively. Interestingly, TiO₂ NFs-550 °C still show a high hydrogen evolution rate of 4.317 mmol h⁻¹g⁻¹ in the absence of co-catalysts.     

Self-assembled ionic liquid synthesis of nitrogen-doped mesoporous TiO2 for visible-light-responsive hydrogen production

Nitrogen-doped mesoporous TiO₂ has been synthesized by a simple solvent evaporation-induced self-assembly method using a nitrogen-containing ionic liquid concurrently as a nitrogen source and mesoporous template. After being evaporated and subsequently calcined at various temperatures (300–900 °C), the synthesized samples were thoroughly characterized by X-ray diffraction (XRD), Raman, small-angle X-ray scattering patterns (SAXS), N₂ adsorption-desorption isotherms, X-ray photoelectron (XPS) and UV–Vis diffuse reflectance (UV–Vis DR) spectroscopies. The obtained results suggest that the calcination temperature greatly influences the crystallization of TiO₂, formation of mesoporous structure, specific surface area and N-doping amounts. Among the fabricated photocatalysts, the samples calcined at 600 °C exhibit superior photocatalytic performance for hydrogen production in water/methanol solution under visible light illumination if compared to other synthesized samples and commercial TiO₂ (Degussa P25). The finding is possibly due to the synergy of more N-doping amounts on the well-defined mesoporous TiO₂ with highly anatase crystal phase and moderate surface area in the catalysts.     

Remarkable charge separation and photocatalytic efficiency enhancement through TiO2(B)/anatase hetrophase junctions of TiO2 nanobelts

Light harvesting and charge separation are both significant to the photocatalysis, but it is challenging to synchronously realize both in a single-component material. The surface coarsened TiO₂ nanobelts with TiO₂(B)/anatase hetrophase junctions and large BET surface area are prepared via a hydrothermal/annealing method. The presence of surface coarsened nanobelt structure enhances the light absorption through reflection/refraction of light. The TiO₂(B)/anatase hetrophase junctions can efficiently promote the separation of photoinduced electrons and holes pairs and therefore decrease the charge recombination. The large BET surface area provides abundant active sites for the absorption and diffusion of reactants. As a consequence, the obtained TiO₂ nanobelts exhibit an enhanced photocatalytic H₂ evolution activity at the optimal annealing temperature (450 °C) with Pt as co-catalysts (0.786 mmol h⁻¹g⁻¹), exceeding that of pure anatase TiO₂ nanobelts (TiO₂ nanobelts-600 °C, 0.265 mmol h⁻¹g⁻¹). Interestingly, TiO₂ nanobelts-450 °C still show a high hydrogen evolution rate of 0.601 mmol h⁻¹g⁻¹ in the absence of co-catalysts.