Effect of Bi2O3 content on the microstructure and electrical properties of SrBi2Nb2O9 piezoelectric ceramics

Lead-free ceramics, SrBi₂Nb₂O₉–xBi₂O₃ (SBN–xBi), with different Bi contents of which the molar ratio, n(Sr) : n(Bi) : n(Nb), is 1 : 2(1 + x/2) : 2 (x = −0.05, 0.0, 0.05, 0.10), were prepared by conventional solid-state reaction method. The effect of excess bismuth on the crystal structure, microstructure and electrical properties of the ceramics were investigated. A layered perovskite structure without any detectable secondary phase and plate-like morphologies of the grains were clearly observed in all samples. The value of the activation energy suggested that the defects in samples could be related to oxygen vacancies. Excellent electrical properties (e.g., d₃₃ = 18 pC N⁻¹, 2P_r = 17.8 μC cm⁻², ρ_rd = 96.4% and T_c = 420 °C) were simultaneously obtained in the ceramic where x = 0.05. Thermal annealing studies indicated the SBN–xBi ceramics system possessed stable piezoelectric properties, demonstrating that the samples could be promising candidates for high-temperature applications.

Lead-free ceramics, SrBi₂Nb₂O₉–xBi₂O₃ (SBN–xBi), with different Bi contents of which the molar ratio, n(Sr) : n(Bi) : n(Nb), is 1 : 2(1 + x/2) : 2 (x = −0.05, 0.0, 0.05, 0.10), were prepared by conventional solid-state reaction method.     

Highly efficient selective hydrogenation of levulinic acid to γ-valerolactone over Cu–Re/TiO2 bimetallic catalysts

Highly active and thermally stable Cu–Re bimetallic catalysts supported on TiO₂ with 2.0 wt% loading of Cu were prepared via an incipient wetness impregnation method and were applied for liquid phase selective hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) in H₂. The effect of the molar ratios of Cu : Re on the physico-chemical properties and the catalytic performance of the Cu–Re/TiO₂ catalysts was investigated. Moreover, the influence of various reaction parameters on the hydrogenation of LA to GVL was studied. The results showed that the Cu–Re/TiO₂ catalyst with a 1 : 1 molar ratio of Cu to Re (Cu–Re(1 : 1)/TiO₂) exhibited the highest performance for the reaction. Complete conversion of LA with a 100% yield of GVL was achieved in 1,4-dioxane solvent under the reaction conditions of 180 °C, 4.0 MPa H₂ for 4 h, and the catalyst could be reused at least 6 times with only a slight loss of activity. Combined with the characterization results, the high performance of the catalyst was mainly attributed to the well-dispersed Cu–Re nanoparticles with a very fine average size (ca. 0.69 nm) and the co-presence of Cu–Re bimetal and ReO_x on the catalyst surface.

Herein, we report a highly efficient and recyclable Cu–Re(1 : 1)/TiO₂ bimetallic catalyst for liquid phase hydrogenation of levulinic acid to γ-valerolactone.     

Mn-based catalysts supported on γ-Al2O3, TiO2 and MCM-41: a comparison for low-temperature NO oxidation with low ratio of O3/NO

Mn-Based catalysts supported on γ-Al₂O₃, TiO₂ and MCM-41 synthesized by an impregnation method were compared to evaluate their NO catalytic oxidation performance with low ratio O₃/NO at low temperature (80–200 °C). Activity tests showed that the participation of O₃ remarkably promoted the NO oxidation. The catalytic oxidation performance of the three catalysts decreased in the following order: Mn/γ-Al₂O₃ > Mn/TiO₂ > Mn/MCM-41, indicating that Mn/γ-Al₂O₃ exhibited the best catalytic activity. In addition, there was a clear synergistic effect between Mn/γ-Al₂O₃ and O₃, followed by Mn/TiO₂ and O₃. The characterization results of XRD, EDS mapping, BET, H₂-TPR, XPS and TG showed that Mn/γ-Al₂O₃ had good manganese dispersion, excellent redox properties, appropriate amounts of coexisting Mn³⁺ and Mn⁴⁺ and abundant chemically adsorbed oxygen, which ensured its good performance. In situ DRIFTS demonstrated the NO adsorption performance on the catalyst surface. As revealed by in situ DRIFTS experiments, the chemically adsorbed oxygen, mainly from the decomposition of O₃, greatly promoted the NO adsorption and the formation of nitrates. The Mn-based catalysts showed stronger adsorption strength than the corresponding pure supports. Due to the abundant adsorption sites provided by pure γ-Al₂O₃, under the interaction of Mn and γ-Al₂O₃, the Mn/γ-Al₂O₃ catalyst exhibited the strongest NO adsorption performance among the three catalysts and produced lots of monodentate nitrates (–O–NO₂) and bidentate nitrates (–O₂NO), which were the vital intermediate species for NO₂ formation. Moreover, the NO–TPD studies also demonstrated that Mn/γ-Al₂O₃ showed the best NO desorption performance among the three catalysts. The good NO adsorption and desorption characteristics of Mn/γ-Al₂O₃ improved its high catalytic activity. In addition, the activity test results also suggested that Mn/γ-Al₂O₃ exhibited good SO₂ tolerance.

The Mn/γ-Al₂O₃ catalyst exhibited excellent performance for NO conversion in the presence of a low ratio of O₃/NO, which was due to the coexistence of Mn³⁺ and Mn⁴⁺ and abundant chemically adsorbed oxygen.     

Cerium and tin oxides anchored onto reduced graphene oxide for selective catalytic reduction of NO with NH3 at low temperatures

A series of cerium and tin oxides anchored on reduced graphene oxide (CeO₂–SnO_x/rGO) catalysts are synthesized using a hydrothermal method and their catalytic activities are investigated by selective catalytic reduction (SCR) of NO with NH₃ in the temperature range of 120–280 °C. The results indicate that the CeO₂–SnO_x/rGO catalyst shows high SCR activity and high selectivity to N₂ in the temperature range of 120–280 °C. The catalyst with a mass ratio of (Ce + Sn)/GO = 3.9 exhibits NO conversion of about 86% at 160 °C, above 97% NO conversion at temperatures of 200–280 °C and higher than 95% N₂ selectivity at 120–280 °C. In addition, the catalyst presents a certain SO₂ resistance. It is found that the highly dispersed CeO₂ nanoparticles are deposited on the surface of rGO nanosheets, because of the incorporation of Sn⁴⁺ into the lattice of CeO₂. The mesoporous structures of the CeO₂–SnO_x/rGO catalyst provides a large specific surface area and more active sites for facilitating the adsorption of reactant species, leading to high SCR activity. More importantly, the synergistic interaction between cerium and tin oxides is responsible for the excellent SCR activity, which results in a higher ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺), higher concentrations of surface chemisorbed oxygen and oxygen vacancies, more strong acid sites and stronger acid strength on the surface of the CeSn(3.9)/rGO catalyst.

A series of cerium and tin oxides anchored on graphene oxide (CeO₂–SnO_x/rGO) catalysts are synthesized for selective catalytic reduction of NO with NH₃ in the temperature range of 120–280 °C.     

Ce regulated surface properties of Mn/SAPO-34 for improved NH3-SCR at low temperature

Ce modified MnO_x/SAPO-34 was prepared and investigated for low-temperature selective catalytic reduction of NO_x with ammonia (NH₃-SCR). The 0.3Ce–Mn/SAPO-34 catalyst had nearly 95% NO conversion at 200–350 °C at a space velocity of 10 000 h⁻¹. Microporous SAPO-34 as the support provided the catalyst with increased hydrothermal stability. XPS and H₂-TPR results proved that the Mn⁴⁺ and O_α content increased after incorporation of Ce, this promoted the conversion of NO at low temperature via a ‘fast SCR’ route. NH₃-TPD measurements combined oxidation experiments of NO, NH₃ indicated the reduction of both the surface acidity and the amount of acid sites, which effectively decreased the NH₃ oxditaion to NO or N₂O at elevated temperature and promoted the catalytic selectivity for nitrogen. A redox cycle between manganese oxide and Ce was assumed for the active oxygen transfer and facilitated the catalyst durability.

Ce modified MnO_x/SAPO-34 was prepared and investigated for low-temperature selective catalytic reduction of NO_x with ammonia (NH₃-SCR).     

Selective hydroconversion of coconut oil-derived lauric acid to alcohol and aliphatic alkane over MoOx-modified Ru catalysts under mild conditions

Molybdenum oxide-modified ruthenium on titanium oxide (Ru–(y)MoO_x/TiO₂; y is the loading amount of Mo) catalysts show high activity for the hydroconversion of carboxylic acids to the corresponding alcohols (fatty alcohols) and aliphatic alkanes (biofuels) in 2-propanol/water (4.0/1.0 v/v) solvent in a batch reactor under mild reaction conditions. Among the Ru–(y)MoO_x/TiO₂ catalysts tested, the Ru–(0.026)MoO_x/TiO₂ (Mo loading amount of 0.026 mmol g⁻¹) catalyst shows the highest yield of aliphatic n-alkanes from hydroconversion of coconut oil derived lauric acid and various aliphatic fatty acid C6–C18 precursors at 170–230 °C, 30–40 bar for 7–20 h. Over Ru–(0.026)MoO_x/TiO₂, as the best catalyst, the hydroconversion of lauric acid at lower reaction temperatures (130 ≥ T ≤ 150 °C) produced dodecane-1-ol and dodecyl dodecanoate as the result of further esterification of lauric acid and the corresponding alcohols. An increase in reaction temperature up to 230 °C significantly enhanced the degree of hydrodeoxygenation of lauric acid and produced n-dodecane with maximum yield (up to 80%) at 230 °C, H₂ 40 bar for 7 h. Notably, the reusability of the Ru–(0.026)MoO_x/TiO₂ catalyst is slightly limited by the aggregation of Ru nanoparticles and the collapse of the catalyst structure.

Ru–(y)MoO_x/TiO₂ catalysed the hydroconversion of lauric acid to allow a remarkable yield of n-dodecane (up to 80%) under mild reaction conditions.     

Promotion effect of urchin-like MnOx@PrOx hollow core–shell structure catalysts for the low-temperature selective catalytic reduction of NO with NH3

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃. The structural properties of the catalyst were characterized by FE-SEM, TEM, XRD, BET, XPS, H₂-TPR and NH₃-TPD analyses, and the performance of the low-temperature NH₃-SCR was also tested. The results show that the catalyst with a molar ratio of Pr/Mn = 0.3 exhibited the highest NO conversion at nearly 99% at 120 °C and NO conversion greater than 90% over the temperature range of 100–240 °C. Also, the MnO_x@PrO_x catalyst presented desirable SO₂ and H₂O resistance in 100 ppm SO₂ and 10 vol% H₂O at the space velocity of 40 000 h⁻¹ and a testing time of 3 h test at 160 °C. The excellent low-temperature catalytic activity of the catalyst could ultimately be attributed to high concentrations of Mn⁴⁺ and adsorbed oxygen species on the catalyst surface, suitable Lewis acidic surface properties, and good reducing ability. Additionally, the enhanced SO₂ and H₂O resistance of the catalyst was primarily ascribed to its unique core–shell structure which prevented the MnO_x core from being sulfated.

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃.     

VOX supported on TiO2–Ce0.9Zr0.1O2 core–shell structure catalyst for NH3-SCR of NO

In this experiment, a TiO₂–Ce_0.9Zr_0.1O₂ support with core–shell structure was successfully prepared by a precipitation method and VO_X/TiO₂–Ce_0.9Zr_0.1O₂ catalyst was prepared by an impregnation method, and the catalyst was used to catalyze the NH₃-SCR of NO. Based on the results of HRTEM, XRD, BET, H₂-TPR, NH₃-TPD, XPS, Py-IR, it was speculated that due to the interaction between TiO₂ and Ce_0.9Zr_0.1O₂, more oxygen vacancies and Ce³⁺ are generated, which are beneficial to the existence of low-valence V by electron transfer between high valence state V and Ce³⁺and increase the acidic sites on the catalyst surface. The catalytic activity (>97%) of the VO_X/TiO₂–Ce_0.9Zr_0.1O₂ catalyst is superior to the current commercial catalyst (V₂O₅–WO₃/TiO₂) and has a higher N₂ selectivity (>97.5%) at 40 000 h⁻¹ GHSV and 250–400 °C.

VO_X/TiO₂–Ce_0.9Zr_0.1O₂ catalyst exhibits high activity and selectivity in a wide temperature window.     

Comparative study of mesoporous NixMn6−xCe4 composite oxides for NO catalytic oxidation

In this work, a series of mesoporous Ni_xMn_6−xCe ternary oxides were prepared to investigate their NO catalytic oxidation ability. The sample Ni₂Mn₄Ce₄ showed a 95% NO conversion at 210 °C (GHSV, ∼80 000 h⁻¹). Characterization results showed the good catalytic performance of Ni₂Mn₄Ce₄ was due to its high specific surface area, more surface oxygen and high valance manganese species, which can be ascribed to the incorporation of three elements. Based on the results of XRD, H₂-TPR, O₂-TPD and XPS, we confirmed the existence of Ni³⁺ + Mn³⁺ → Ni²⁺ + Mn⁴⁺, Ce⁴⁺ + Ni²⁺ → Ce³⁺ + Ni³⁺ in Ni₂Mn₄Ce₄, and the oxidation–reduction cycles were proved to be helpful for NO oxidation. The results from an in situ DRIFTS study indicated the presence of bidentate nitrate and monodentate nitrate species on the catalyst''s surface. The nitrate species were proved to be intermediates for NO oxidation to NO₂. A nitrogen circle mechanism was proposed to explain the possible route for NO oxidation. Nickel introduction was also helpful to improve the SO₂ resistance of the NO oxidation reaction. The activity drop of Ni₂Mn₄Ce₄ was 13.15% in the presence of SO₂, better than Mn₆Ce₄ (25.29%).

In this work, a series of mesoporous Ni_xMn_6−xCe ternary oxides were prepared to investigate their NO catalytic oxidation ability.     

Improvement of low-temperature NH3-SCR catalytic performance over nitrogen-doped MOx–Cr2O3–La2O3/TiO2–N (M = Cu,Fe, Ce) catalysts

A series of MO_x–Cr₂O₃–La₂O₃/TiO₂–N (M = Cu, Fe, Ce) catalysts with nitrogen doping were prepared via the impregnation method. Comparing the low-temperature NH₃-SCR activity of the catalysts, CeCrLa/Ti–N (xCeO₂–yCr₂O₃–zLa₂O₃/TiO₂–N) exhibited the best catalytic performance (NO conversion approaching 100% at 220–460 °C). The physico-chemical properties of the catalysts were characterized by XRD, BET, SEM, XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS. From the XRD and SEM results, N doping affects the crystalline growth of anatase TiO₂ and MO_x (M = Cu, Fe, Ce, Cr, La) which were well dispersed over the support. Moreover, the doping of N promotes the increase of the Cr⁶⁺/Cr ratio and Ce³⁺/Ce ratio, and the surface chemical adsorption oxygen content, which suggested the improvement of the redox properties of the catalyst. And the surface acid content of the catalyst increased with the doping of N, which is related to CeCrLa/TiO₂–N having the best catalytic activity at high temperature. Therefore, the CeCrLa/TiO₂–N catalyst exhibited the best NH₃-SCR performance and the redox performance of the catalysts is the main factor affecting their activity. Furthermore, in situ DRIFTS analysis indicates that Lewis-acid sites are the main adsorption sites for ammonia onto CeCrLa/TiO₂–N and the catalyst mainly follows the L–H mechanism.

A series of MO_x–Cr₂O₃–La₂O₃/TiO₂–N (M = Cu, Fe, Ce) catalysts with nitrogen doping were prepared via the impregnation method.     

Effect of initial support particle size of MnOx/TiO2 catalysts in the selective catalytic reduction of NO with NH3

A series of manganese-based catalysts supported by 5–10 nm, 10–25 nm, 40 nm and 60 nm anatase TiO₂ particles was synthesized via an impregnation method to investigate the effect of the initial support particle size on the selective catalytic reduction (SCR) of NO with NH₃. All catalysts were characterized by transmission electron microscopy (TEM), N₂ physisorption/desorption, X-ray diffraction (XRD), temperature programmed techniques, X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared transform spectroscopy (DRIFTS). TEM results indicated that the particle sizes of the MnO_x/TiO₂ catalysts were similar after the calcination process, although the initial TiO₂ support particle sizes were different. However, the initial TiO₂ support particle sizes were found to have a significant influence on the SCR catalytic performance. XPS and NH₃-TPD results of the MnO_x/TiO₂ catalysts illustrated that the surface Mn⁴⁺/Mn molar ratio and acid amount could be influenced by the initial TiO₂ support particle sizes. The order of surface Mn⁴⁺/Mn molar ratio and acid amount over the MnO_x/TiO₂ catalysts was as follows: MnO_x/TiO₂(10–25) > MnO_x/TiO₂(40) > MnO_x/TiO₂(60) > MnO_x/TiO₂(5–10), which agreed well with the order of SCR performance. In situ DRIFTS results revealed that the NH₃-SCR reactions over MnO_x/TiO₂ at low temperature occurred via a Langmuir–Hinshelwood mechanism. More importantly, it was found that the bridge and bidentate nitrates were the main active substances for the low-temperature SCR reaction, and bridge nitrate adsorbed on Mn⁴⁺ showed superior SCR activity among all the adsorbed NO_x species. The variation of the initial TiO₂ support particle size over MnO_x/TiO₂ could change the surface Mn⁴⁺/Mn molar ratio, which could influence the adsorption of NO_x species, thus bringing about the diversity of the SCR catalytic performance.

Support particle size could influence the surface Mn⁴⁺/Mn ratio of catalysts, promoting the reactivity of bridge nitrate, therefore enhancing SCR performance.     

Reaction induced robust PdxBiy/SiC catalyst for the gas phase oxidation of monopolistic alcohols

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity (92% conversion of benzyl alcohol and 98% selectivity of benzyl aldehyde) and stability (time on stream of 200 h) in the gas phase oxidation of alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species. TEM indicates that the agglomeration of the 5.8 nm nanoparticles is inhibited under the reaction conditions. The transformation from inactive PdO–Bi₂O₃ to active Pd⁰–Bi₂O₃ under the reaction conditions is confirmed elaborately by XRD and XPS.

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity and stability in the gas phase oxidation of monopolistic alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species.     

Effects of WO3 and SiO2 doping on CeO2–TiO2 catalysts for selective catalytic reduction of NO with ammonia

A series of CeO₂–WO₃/SiO₂–TiO₂ (CeW_xTiSi_y) catalysts with different loading amounts of WO₃ were synthesized by wet co-impregnation of ammonium metatungstate and cerium nitrate on a SiO₂–TiO₂ support, and were employed for the selective catalytic reduction (SCR) of NO by NH₃. The catalytic activity of the CeO₂/SiO₂–TiO₂ (CeSiTi) catalyst was enhanced by the addition of WO₃, and the W-containing catalysts showed higher hydrothermal stability especially between 550 and 600 °C. The introduction of WO₃ to the CeSiTi catalyst could produce more chemisorbed oxygen species, reducible subsurface oxygen species, acid sites and ad-NO_x species. Moreover, the modification of CeO₂–WO₃/TiO₂ (CeWTi) by SiO₂ could enhance the specific surface area, especially the aged specific surface area, thus improving the hydrothermal stability of the catalyst.

A series of CeO₂–WO₃/TiO₂–SiO₂ catalysts were prepared for SCR of NO_x with NH₃, and the effect of WO₃ and SiO₂ doping on the activity and stability of the catalysts were discussed.     

Praseodymia–titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides

Modulating the active sites for controllable tuning of the catalytic activity has been the goal of much research, however, this remains challenging. The O vacancy is well known as an active site in reducible oxides. To modify the activity of O vacancies in praseodymia, we synthesized a series of praseodymia–titania mixed oxides. Varying the Pr : Ti mole ratio (2 : 1, 1 : 2, 1 : 1, 1 : 4) allows us to control the electronic interactions between Au, Pr and Ti cations and the local chemical environment of the O vacancies. These effects have been studied study by X-ray photoelectron spectroscopy (XPS), CO diffuse reflectance Fourier transform infrared spectroscopy (CO-DRIFTS) and temperature-programmed reduction (CO-TPR, H₂-TPR). The water gas shift reaction (WGSR) was used as a benchmark reaction to test the catalytic performance of different praseodymia–titania supported Au. Among them, Au/Pr₁Ti₂O_x was identified to exhibit the highest activity, with a CO conversion of 75% at 300 °C, which is about 3.7 times that of Au/TiO₂ and Au/PrO_x. The Au/Pr₁Ti₂O_x also exhibited excellent stability, with the conversion after 40 h time-on-stream at 300 °C still being 67%. An optimal ratio of Pr content (Pr : Ti 1 : 2) is necessary for improving the surface oxygen mobility and oxygen exchange capability, a higher Pr content leads to more O vacancies, however with lower activity. This study presents a new route for modulating the active defect sites in mixed oxides which could also be extended to other heterogeneous catalysis systems.

Schematic illustration of H₂O activation on the Pr-TiO_x support and the following reaction with CO in the Au–oxide interface.     

Efficient and stable catalyst of α-FeOOH for NO oxidation from coke oven flue gas by the catalytic decomposition of gaseous H2O2

Goethite (α-FeOOH) possesses excellent catalytic activity, high selectivity and good stability as a catalyst for NO oxidation through the catalytic decomposition of gaseous H₂O₂. as the primary reactive oxygen species is involved in the NO oxidation process together with ·OH, and N₂O₅ is found for the first time in the products of NO oxidation.

A catalyst α-FeOOH possesses excellent catalytic activity for NO oxidation, and N₂O₅ is first found in NO oxidation products.

Sulphur dioxide (SO₂) and nitrogen oxides (NO_x) yielded by fossil fuels and ore combustion are the major air pollutants produced by traditional chemical industries, including iron and steel, coking, and boilers, and these pollutants cause acid rain, fog, and haze. Wet flue gas desulphurisation (WFGD) and selective catalytic reduction (SCR) are efficient methods used in power plants for desulphurisation and denitration, respectively.¹ However, in traditional chemical industries, for example, coke oven flue gas yielded by coke oven gas combustion has a flue gas temperature of about 200–230 °C and the composition is complex,² which limits the utilisation of SCR.³Recently, we proposed a promising process of gas-phase oxidation combined with wet scrubbing using steel slag slurry to treat NO_x from low-temperature flue gas.^4,5 In this process, the sparingly soluble NO could be oxidised into soluble NO₂, HNO₃ or N₂O₅ through the gas-phase oxidation process, and then the oxidation products could be absorbed together with SO₂ in a wet scrubbing device. This method can achieve the simultaneous removal of SO₂ and NO_x using the oxidisers. Some oxidisers, such as O₃,^6,7 H₂O₂,^8–12 NaClO₂,¹³ NaClO,¹⁴ persulphate salt (S₂O₈²⁻)¹⁵ and ferrate (Fe[vi]),¹⁶ can be used as the gas-phase oxidiser for NO oxidation after being gasified if needed.H₂O₂ is a green and low-cost oxidizer, which can be used as the gas-phase oxidizer for NO oxidation after liquid H₂O₂ is gasified at low temperature. Furthermore, the oxidation potential of H₂O₂ (1.77 eV) is lower compared with that of O₃ (2.08 eV) and ·OH (2.80 eV) and thus its efficiency in oxidising NO is low.^12,17 Thermal decomposition of gaseous H₂O₂ can decompose H₂O₂ into radicals (·OH or ) with high oxidation potential for NO oxidation. However, this technology requires high H₂O₂ consumption (H₂O₂/NO = 80) and excessive residence time (34 s) and results in low NO oxidation efficiency (∼60%).¹⁸ Introducing catalysts into the H₂O₂ decomposition process can effectively decompose H₂O₂ into radicals and considerably reduce the consumption of H₂O₂.¹¹ Iron-based materials, such as hematite (α-Fe₂O₃),⁸ nanoscale zero-valent iron,⁹ Fe₃O₄,¹⁰ γ-Fe₂O₃@Fe₃O₄,¹¹ Fe₂(MoO₄)₃ ¹⁹ and Fe₂(SO₄)₃,¹² have been used as catalysts to decompose gaseous H₂O₂ for NO oxidation. These catalysts have high removal efficiencies as heterogeneous catalysts for the simultaneous removal of NO and SO₂ in an integrated catalytic oxidation/wet scrubbing process. However, the use of these catalysts results in relatively high H₂O₂ consumption^8,9,12,20 and relatively low gas hourly space velocity (GHSV)¹⁹ and catalytic stability.^9,12 Therefore, a catalyst with low H₂O₂ consumption and high GHSV and catalytic stability for NO oxidation through catalytic decomposition of gaseous H₂O₂ should be developed.Goethite (α-FeOOH) is a ubiquitous natural mineral in soils and sediments at the Earth''s surface that is widely used as a heterogeneous Fenton catalyst for wastewater treatment due to its abundance, availability, relative stability and low cost.²¹ He et al. explored the catalytic performance of α-FeOOH and found that it can be used to catalyse H₂O₂ vapour for NO oxidation under low-temperature (<160 °C) flue gas;²² however, coke oven flue gas has a higher flue gas temperature (200–230 °C), and the reaction products and catalytic mechanism may be different under different temperature regions. Therefore, the performance of α-FeOOH for NO oxidation through catalysing gaseous H₂O₂ under high flue gas temperature should be investigated, and the SO₂ oxidation efficiency of this process should also be evaluated. Herein, NO and SO₂ conversions and NO₂ yield using α-FeOOH with gaseous H₂O₂ were performed under the wide temperature range of 100–350 °C, and the catalytic stability and reaction mechanism were determined.Fresh α-FeOOH catalyst was prepared via precipitation reaction of Fe³⁺ followed by crystal transformation according to the method of Böhm.²³ All chemicals used in the experiments were of analytical grade, and deionised water was used. NO, NO₂, and SO₂ concentrations in simulated flue gas were analysed using a UV differential optical absorption spectroscopy (DOAS) flue gas comprehensive analyser (Laoying3023, Qingdao Laoying Environmental Technology Co., Ltd.). The presence of HNO₃ and N₂O₅ in the flue gas were detected using a Fourier transform infrared (FTIR) spectrometer (Tensor 27, Bruker Optik, Inc.), which was equipped with a 2.4 m gas cell. Catalysts were characterised via X-ray diffraction (XRD) spectroscopy (Empyrean, PANalytical Instruments) and FTIR spectrometry. Hydroxyl radical (·OH) and superoxide anion radical decomposed by H₂O₂ were detected via electron paramagnetic resonance (EPR) spectroscopy (A300-10/12, Bruker Optik Inc.), and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used for capturing ·OH and . Specifically, DMPO–H₂O solvent was used to capture the ·OH generated after H₂O₂ was catalysed, whereas DMPO–CH₃OH was used to capture The experimental equipment for evaluating the catalytic performance of α-FeOOH was established (Fig. S1, ESI^†). In brief, the bypass of N₂ carried the gaseous H₂O₂ generated by the evaporation of H₂O₂ solution and mixed with the simulated flue gas, and then the mixture gas contacted with the catalyst, in which gaseous H₂O₂ was decomposed into radicals over the catalyst and oxidised NO and SO₂. At the outlet of the catalytic reactor, the simulated flue gas after being oxidised was detected using a UV DOAS flue gas comprehensive analyser and an FTIR spectrometer equipped with a gas cell. Each experiment was conducted for 20 min after the temperature was stabilised.According to Christensen et al., α-FeOOH is transformed to hematite (α-Fe₂O₃) within the temperature range of 171–311 °C, and α-FeOOH is totally converted to α-Fe₂O₃ at 350 °C.²⁴ The fresh catalyst and the catalyst calcinated at 350 °C were characterised via XRD and FTIR spectrometry. The FTIR and XRD spectra (Fig. S2 and S3, ESI^†) indicated that the fresh catalyst was α-FeOOH and the catalyst calcinated at 350 °C was α-Fe₂O₃ in an N₂ atmosphere. The colour of the fresh catalyst (yellow) and the calcinated catalyst (red) also proved the above results (Fig. S4, ESI^†). An FTIR spectrometer equipped with a gas cell was used to analyse the water vapour and further investigate the transformation temperature of α-FeOOH. Results (Fig. S5, ESI^†) showed that α-FeOOH began to decompose into α-Fe₂O₃ and H₂O when the temperature was above 200 °C in an N₂ atmosphere. However, when H₂O(g) and H₂O₂(g) were existed in N₂ atmosphere, they could improve the thermal stability of α-FeOOH. The reason may be that (1) multiple types of surface hydroxyls (–OHs) generated by the adsorption of water on α-FeOOH surface prevented the dehydration of –OHs;^25,26 and (2) H₂O(g) from the injected H₂O₂ solution were adsorbed on the reduced Fe(ii) and generated Fe(ii)–OH viaeqn (1) and (2), and Fe(ii)–OH was converted to Fe(iii)–OH viaeqn (3).^{12,19,22,27,28} The catalyst α-FeOOH was still stable when temperature was up to 225 °C, and little α-FeOOH close to the wall of the reactor was converted to α-Fe₂O₃ (the color of catalyst was changed from yellow to red) when temperature was up to 350 °C. Therefore, the catalyst α-FeOOH possesses great thermal stability under the simulated flue gas condition (e.g., the experimental condition in Fig. 1).1 Fe(ii) + V_o + H₂O → Fe(ii)–OH₂⁺ → Fe(ii)–OH + H⁺2 Fe(ii)–OH + H₂O₂ → Fe(iii)–OH + ·OH3Open in a separate windowFig. 1Effects of temperature on NO and SO₂ conversions and NO₂ yield (H₂O₂/NO, 2.0; H₂O₂ solution feeding rate, 148.9 μL min⁻¹; catalyst dosage, 0.5 g; GHSV, 137 747 h⁻¹; NO concentration, 200 ppm; SO₂ concentration, 660 ppm; O₂ concentration, 6%).As shown in Fig. 1, NO conversion and NO₂ yield were both lower than SO₂ conversion when gaseous H₂O₂ only was used for NO oxidation. This result indicated that SO₂ was more easily oxidised by gaseous H₂O₂ than NO, which consumed a large amount of H₂O₂. NO conversion and NO₂ yield were higher, but SO₂ conversion was considerably lower than using gaseous H₂O₂ only when α-FeOOH was used to catalyse gaseous H₂O₂ for NO oxidation. Therefore, α-FeOOH performed excellent catalytic activity and high selectivity for NO oxidation. Specifically, NO conversion achieved 98.8% and NO₂ yield reached 77.0% at H₂O₂/NO of 2.0, reaction temperature of 225 °C, catalyst dosage of 0.5 g and GHSV of 137 747 h⁻¹. The catalyst of α-FeOOH achieved a higher NO oxidation efficiency under low H₂O₂ consumption and high GHSV compared with those reported by previous studies (Table S1, ESI^†).^{8,9,12,17,19,20} Fig. 1 shows that NO conversion was higher than NO₂ yield within the temperature range of 100–225 °C, and NO conversion was closer to NO₂ yield when the temperature further increased from 250 °C to 350 °C. This trend indicated that the products of NO oxidation was not just NO₂ within the temperature range of 100–225 °C, and the product of NO oxidation might only be NO₂ within the temperature range of 250–350 °C. The products of NO oxidation were determined via FTIR spectroscopy. As shown in Fig. 2, the peaks for NO₂ (1599 cm⁻¹), HNO₃ (886 cm⁻¹) and N₂O₅ (1719 cm⁻¹) were observed in the FTIR spectra within the temperature range of 100–200 °C.^29,30 However, no obvious peaks for NO₂, HNO₃, and N₂O₅ were detected in the FTIR spectra when H₂O₂ was not added. This result indicated that NO₂, HNO₃ and N₂O₅ were the main products of NO oxidation through the catalytic decomposition of gaseous H₂O₂ over α-FeOOH in low-temperature (100–200 °C) region. Furthermore, the peaks for HNO₃ and N₂O₅ in the FTIR spectra disappeared when the temperature increased from 225 °C to 350 °C. The reason is that HNO₃ and N₂O₅ decomposed in the high-temperature region (225–350 °C) (eqn (4) and (5)).²⁹ In summary, NO₂, HNO₃ and N₂O₅ were the main products of NO oxidation in the low-temperature region (100–200 °C), whereas NO₂ was the main product in the high-temperature region (225–350 °C).N₂O₅ → NO + NO₂ + O₂42HNO₃ → NO + NO₂ + O₂ + H₂O5Open in a separate windowFig. 2FTIR spectra of simulated flue gas oxidised by gaseous H₂O₂ over α-FeOOH under different temperature conditions. (Experimental condition was the same as that in Fig. 1.) Fig. 1 shows that NO conversion sharply increased when the temperature increased from 100 °C to 200 °C and then decreased when the temperature further increased from 225 °C to 350 °C. The increase in NO conversion with temperature was because high temperatures enhance H₂O₂ decomposition into radicals according to the Arrhenius law,¹² which further accelerates NO conversion. The decrease in NO conversion at temperatures above 200 °C could be explained by (1) the thermal decomposition of gaseous H₂O₂ under high temperature and/or (2) the transformation of α-FeOOH under high temperature. On the one hand, gaseous H₂O₂ can be decomposed under high temperature. An FTIR spectrometer equipped with a gas cell was used to detect gaseous H₂O₂ under different temperature conditions (free of catalyst) to investigate the decomposition temperature of gaseous H₂O₂,³¹ and the variations in H₂O₂ content were measured by their homologous infrared absorption characteristic peaks (Fig. S6, ESI^†). Results (Fig. S7, ESI^†) indicated that H₂O₂ almost did not decompose at 100–300 °C, and the thermal decomposition of gaseous H₂O₂ occurred when the temperature increased further to 300 °C. On the other hand, NO-TPD (Fig. S8, ESI^†) showed that the absorbed NO began to desorb when the temperature was above 200 °C. Therefore, when the temperature was above 200 °C, NO conversion decreased when the temperature was above 200 °C. Overall, the transformation of α-FeOOH under high temperature (>200 °C) led to the decrease in NO conversion when the temperature was above 200 °C, and the thermal decomposition of gaseous H₂O₂ also resulted in the decrease in NO conversion when the temperature was above 300 °C.The catalytic stability of α-FeOOH was also investigated. As shown in Fig. 3, NO conversion was maintained at a high level (>97.0%) within the first 15 h and then fluctuated slightly but remained at >90.0% at 15–45 h. SO₂ conversion remained stable at about 2.0%. Results indicated that α-FeOOH possesses excellent catalytic activity, good stability and high selectivity for NO oxidation. According to the results in Fig. 1 and ​and3,3, the catalyst has an ideal temperature window of 175–250 °C; therefore, α-FeOOH can be applied in coke oven flue gas (200–230 °C) treatment.Open in a separate windowFig. 3Catalytic stability of α-FeOOH. (H₂O₂/NO, 2.0; H₂O₂ solution feeding rate, 148.9 μL min⁻¹; catalyst dosage, 0.5 g; GHSV, 137 747 h⁻¹; temperature, 225 °C; NO concentration, 200 ppm; SO₂ concentration, 660 ppm; O₂ concentration, 6%.)Fresh and used (after the 45 h test) α-FeOOH were characterised via FTIR and XRD spectroscopy. The bands at 3363, 3127 and 1628 cm⁻¹ are the O–H stretching mode in α-FeOOH, the stretching mode of surface water and the bending mode of H₂O, respectively. The characteristic strong bands at 795 and 891 cm⁻¹ were assigned to the Fe–O–H bending vibrations of α-FeOOH. The band at 638 cm⁻¹ was assigned to the Fe–O stretching vibration of pure α-FeOOH. The characteristic absorption peaks of α-FeOOH in the catalyst after the 45 h test did not change compared with that of the fresh catalyst (Fig. S9, ESI^†), indicating that the catalyst maintained the structure of α-FeOOH even after the 45 h test. The band at 999 cm⁻¹ was assigned to ν₁(SO₄) frequency, and the bands at 1076, 1136 and 1229 cm⁻¹ were interpreted as ν₃(SO₄) frequencies. These vibrational frequencies are attributed to specifically adsorbed SO₄²⁻ ions on the external and internal surfaces of catalyst particles after the 45 h test.^32,33 The XRD patterns showed that the phase of the catalyst before and after the stability test was still α-FeOOH (Fig. S10, ESI^†). This result also indicated that α-FeOOH possessed good stability in the NO oxidation process.EPR test was conducted to detect the radicals generated by H₂O₂ decomposition and analyse the oxygen species of the α-FeOOH/H₂O₂ system. Fig. 4 shows the 4-fold characteristic peak of DMPO–·OH adducts with an intensity ratio of 1 : 2 : 2 : 1 and four notable signals ascribed to adducts. These peaks and signals indicate the formation of ·OH and in the α-FeOOH/H₂O₂ system.^21,34 Furthermore, the intensity of the 4-fold characteristic peak of DMPO–·OH adducts was significantly weaker than that of adducts, indicated that the quantity of produced by the α-FeOOH/H₂O₂ system was larger than that of ·OH. Therefore, α-FeOOH can decompose H₂O₂ into ·OH and and may be dominant among ·OH and for NO oxidation. Radical trapping experiments (Fig. S11, ESI^†) further proved the roles of radicals (·OH and ) in the NO oxidation process. Benzoquinone (BQ, scavenger²⁰) was added into the H₂O₂ solutions, and H₂O₂ and BQ in the mixture solution was vapoured together. NO conversion substantially decreased from 92.9% to 6.1% when the molar ratio of BQ to H₂O₂ increased from 0 to 1.0. The reason was that the addition of BQ captured the generated by gaseous H₂O₂ decomposition over α-FeOOH, thereby decreasing NO conversion. NO conversion decreased from 92.9% to 8.0% when isopropanol (i-PrOH, ·OH scavenger²⁰) was introduced into the same system as the molar ratio of i-PrOH to H₂O₂ increased from 0 to 8.0. The reason was that ·OH generated in the α-FeOOH/H₂O₂ system was captured by i-PrOH instead of oxidised NO. These results indicated that when the addition concentration of the ·OH scavenger (i-PrOH) was eight times that of scavenger (BQ), both systems obtained the similar decrease in NO conversion. Therefore, as the primary reactive oxygen species was involved in the NO oxidation process together with ·OH.Open in a separate windowFig. 4EPR spectra of the α-FeOOH/H₂O₂ system.In this catalytic process, the reaction between H₂O₂ as an oxidant and iron ions as a catalyst to produce highly active species (·OH and ).³⁵ The conversion between Fe²⁺ and Fe³⁺ was proceed according to the Haber–Weiss mechanism. Fe(iii)–OH and Fe(iii) are reduced by H₂O₂ and generate Fe(ii)–OH and Fe(ii) (eqn (6) and (1)), the resulting Fe(ii)–OH and Fe(ii) also can be oxidized by H₂O₂ and produce Fe(iii)–OH and Fe(iii) (eqn (3) and (7)).^{12,19,22,27,28,36}6 Fe(ii) + H₂O₂ → Fe(ii) + ·OH + OH⁻7The proposed mechanism of NO oxidation by H₂O₂ decomposition over α-FeOOH is presented in Fig. 5 based on the study of the products of NO oxidation and reactive oxygen species ( and ·OH) in the NO oxidation process. (1) Gaseous H₂O₂ decomposed into ·OH and on the active sites ( Fe(ii)–OH and Fe(ii), Fe(iii)–OH and Fe(iii)) of the catalyst (eqn (1)–(3), (6) and (7)) according to the Haber–Weiss mechanism, a result that was proved through EPR analysis and scavenger experiments. (2) NO was oxidised by the generated ·OH and and produced NO₂ and HNO₃viaeqn (8) and (9). (3) The produced HNO₃ reacted with ·OH and produced NO₃ or NO₂ (eqn (10) and (11)). (4) NO₂ reacted with NO₃ and produced N₂O₅viaeqn (12).³⁷ The production of NO₂, HNO₃ and N₂O₅ during the NO oxidation process was proved by the FTIR spectra (Fig. 2).NO + 2·OH → NO₂ + H₂O89HNO₃ + ·OH → NO₃ + H₂O10HNO₃ + ·OH → NO₂ + H₂O₂11NO₂ + NO₃ → N₂O₅12Open in a separate windowFig. 5Catalytic mechanism of NO oxidation by H₂O₂ decomposition over α-FeOOH ( Fe^III: Fe(iii), Fe(iii)–OH; Fe^II: Fe(ii), Fe(ii)–OH).In summary, α-FeOOH can be used as an efficient catalyst for coke oven flue gas to enhance NO oxidation efficiency through the catalytic decomposition of gaseous H₂O₂. Moreover, α-FeOOH showed high selectivity for NO oxidation in the presence of SO₂. In this study, NO conversion achieved 98.8% under the following experimental conditions: H₂O₂/NO of 2.0, reaction temperature of 225 °C, catalyst dosage of 0.5 g and GHSV of 137 747 h⁻¹. The 45 h test indicated that α-FeOOH has good catalytic stability. The EPR test and radical trapping experiments revealed that as the primary reactive oxygen species was involved in the NO oxidation process together with ·OH. Furthermore, NO₂, HNO₃ and N₂O₅ were the products of NO oxidation through the catalysis of gaseous H₂O₂ over α-FeOOH within the temperature range of 100–200 °C, and NO₂ was the only oxidation product within the temperature range of 225–350 °C.The catalyst α-FeOOH can be used to decompose gaseous H₂O₂ into radicals and oxidise NO into high-valence NO_x, and then the oxidation products can be absorbed together with SO₂ in existing industrial-scale WFGD systems.^4,5 This process can achieve the simultaneous removal of SO₂ and NO_x from coke oven flue gas.     

Synergetic effect over flame-made manganese doped CuO–CeO2 nanocatalyst for enhanced CO oxidation performance

CuO–CeO₂ nanocatalysts with different amounts of Mn dopping (Mn/Cu molar ratios of 0.5 : 5, 1 : 5 and 1.5 : 5) were synthesized by flame spray pyrolysis (FSP) method and tested in the catalytic oxidation of CO. The physicochemical properties of the synthesised samples were characterized systematically, including using X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), oxygen-temperature programmed desorption (O₂-TPD), hydrogen-temperature programmed reduction (H₂-TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The results showed that the 1Mn–Cu–Ce sample (Mn/Cu molar ratio of 1 : 5) exhibited superior catalytic activity for CO oxidation, with the temperature of 90% CO oxidation at 131 °C at a high space velocity (SV = 60 000 mL g⁻¹ h⁻¹), which was 56 °C lower than that of the Cu–Ce sample. In addition, the 1Mn–Cu–Ce sample displays excellent stability with prolonged time on CO stream and the resistance to water vapor. The significantly enhanced activity was correlated with strong synergetic effect, leading to fine textual properties, abundant chemically adsorbed oxygen and high lattice oxygen mobility, which further induced more Cu⁺ species and less formation of carbon intermediates during the CO oxidation process detected by in situ DRIFTS analysis. This work will provide in-depth understanding of the synergetic effect on CO oxidation performances over Mn doped CuO–CeO₂ composite catalysts through FSP method.

The synergetic effect is promoted on Mn doped CuO–Ce O₂ catalyst to induce less carbon intermediates to enhance CO oxidation performance.     

Trapping of different stages of BaTiO3 reduction with LiH

We investigated the hydride reduction of tetragonal BaTiO₃ using LiH. The reactions employed molar H : BaTiO₃ ratios of 1.2, 3, and 10 and variable temperatures up to 700 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy, thermogravimetric analysis (TGA), X-ray fluorescence (XRF), and ¹H magic-angle spinning (MAS) NMR spectroscopy. Effective reduction, as indicated by the formation of dark blue to black colored, cubic-phased, products was observed at temperatures as low as 300 °C. The product obtained at 300 °C corresponded to oxyhydride BaTiO_∼2.9H_∼0.1, whereas reduction at higher temperatures resulted in simultaneous O defect formation, BaTiO_2.9−xH_0.1□_x, and eventually – at temperatures above 450 °C – to samples void of hydridic H. Concomitantly, the particles of samples reduced at high temperatures (500–600 °C) display substantial surface alteration, which is interpreted as the formation of a TiO_x(OH)_y shell, and sintering. Diffuse reflectance UV-VIS spectroscopy shows broad absorption in the VIS-NIR region, which is indicative of the presence of n-type free charge carriers. The size of the intrinsic band gap (∼3.2 eV) appears only slightly altered. Mott–Schottky measurements confirm the n-type conductivity and reveal shifts of the conduction band edge in the LiH reduced samples. Thus LiH appears as a versatile reagent to produce various distinct forms of reduced BaTiO₃ with tailored electronic properties.

Different forms of reduced BaTiO₃, which include oxyhydride BaTiO_2.9H_0.1 and O-deficient BaTiO_2.9−xH_0.1□_x, were obtained from reactions with LiH at various temperatures.     

Selective catalytic reduction of NO over W–Zr-Ox/TiO2: performance study of hierarchical pore structure

A series of W–Zr-O_x/TiO₂ catalysts with hierarchical pore structure were prepared and used for selective catalytic reduction of NO by NH₃. Results showed that the 5C-WZ/T had a hierarchical pore structure, and exhibited high catalytic activity and good resistance to water and sulfur poisoning. The activity of the 5C-WZ/T catalyst was close to 100% in the range of 350–500 °C. The hierarchical pore structure not only improved the specific surface area, redox performance, and acid quantity, but also enabled the catalyst to expose more active sites and improved the catalytic performance. Although the reducing atmosphere caused by citric acid monohydrate reduced the chemical adsorption oxygen concentration, it increased the oxygen mobility and Ti³⁺ ion concentration, which was more conducive to the improvement of catalytic activity. Finally, the NH₃-SCR reaction over 5C-WZ/T catalyst followed L-H and E-R mechanisms. The monodentate nitrite, bidentate nitrate, gas-phase NO₂, coordinated NH₃ and NH₄⁺ were the main reaction intermediates.

A series of W–Zr-O_x/TiO₂ catalysts with hierarchical pore structure were prepared and used for selective catalytic reduction of NO by NH₃.     

The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na3V2O2x(PO4)2F3−2x (0 < x ≤ 1) cathode materials for Na-ion batteries

NASICON-structured Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for x = 1 and 0.4 by reacting V₂O₅, NH₄H₂PO₄, and NaF precursors at temperatures as low as 180–200 °C for less than 15 min. Various available and inexpensive reducing agents were used to control the vanadium oxidation state and final product morphology. The vanadium oxidation state and O/F ratios were assessed using electron energy loss spectroscopy and infrared spectroscopy. According to electron diffraction and powder X-ray diffraction, the Na₃V₂O_2x(PO₄)₂F_3−2x solid solutions crystallized in a metastable disordered I4/mmm structure (a = 6.38643(4) Å, c = 10.62375(8) Å for Na₃V₂O₂(PO₄)₂F and a = 6.39455(5) Å, c = 10.6988(2) Å for Na₃V₂O_0.8(PO₄)₂F_2.2). With respect to electrochemical Na⁺ (de)insertion as positive electrodes (cathodes) for Na-ion batteries, the as-synthesized materials displayed two sloping plateaus upon charge and discharge, centered near 3.5–3.6 V and 4.0–4.1 V vs. Na⁺/Na, respectively, with a reversible capacity of ∼110 mA h g⁻¹. The application of a conducting carbon coating through the surface polymerization of dopamine with subsequent annealing at 500 °C improved both the rate capability (∼55 mA h g⁻¹ at a discharge rate of 10C) and capacity retention (∼93% after 50 cycles at a discharge rate of C/2).

NASICON-structured Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique.     

Porous niobia spheres with large surface area: alcothermal synthesis and controlling of their composition and phase transition behaviour

Submicron-sized niobia (Nb₂O₅) porous spheres with a high specific surface area (300 m² g⁻¹) and nano concave–convex surfaces were synthesized via a rapid one-pot single-step alcothermal reaction. Prolonged reaction time or high reaction temperatures resulted in a morphology change of Nb₂O₅ from amorphous sphere to rod crystals with hexagonal crystal phase. A similar alcothermal reaction yielded TiO₂–Nb₂O₅ composite porous spheres, whose Ti : Nb molar ratio was controlled by changing the precursor solution component ratios. A simple thermal treatment of amorphous TiO₂–Nb₂O₅ porous spheres consisting of 1 : 2 (molar ratio) Ti : Nb at 600 °C for 2 h induced crystal phase transfer from amorphous to a monoclinic crystal phase of submicron-sized TiNb₂O₇ porous spheres with a specific surface area of 50 m² g⁻¹.

Nb₂O₅, TiO₂–Nb₂O₅, and TiNb₂O₇ porous spheres with large surface area were synthesized by alcothermal reaction and calcination.