Direct decomposition of nitrogen monoxide on (Ho,Zr, Pr)2O3 + δ Catalysts

C-type cubic holmium oxide (Ho₂O₃), which has a large number of basic sites, was employed as a host material for NO decomposition catalysts, and zirconium and praseodymium ions were introduced into the lattice to increase the surface area and to control O₂ desorption ability of Ho₂O₃, respectively. The highest catalytic activity was obtained for (Ho_0.87Zr_0.05Pr_0.08)₂O_3.05 + δ, on which the NO conversion to N₂ was as high as 71% at 900 °C in the absence of coexisting gases (NO/He atmosphere). The catalytic activity was maintained at a high conversion ratio even in the presence of the coexisting gases; the N₂ yields of 50% and 47% were obtained even in the presence of 5 vol% O₂ and CO₂, respectively.     

SO42−–Mn–Co–Ce supported on TiO2/SiO2 with high sulfur durability for low-temperature SCR of NO with NH3

The catalysts SO₄^2 − Mn–Co–Ce/TiO₂/SiO₂ were investigated for the low-temperature SCR of NO with NH₃ in the presence of SO₂. An excellent SO₂ durability at low temperature was obtained with the catalyst used TiO₂/SiO₂ as support and modified with SO₄^2 −. The catalyst sulfated with 0.1 mol/L H₂SO₄ solution and then calcined at 300 °C exhibited the best NO_x conversion efficiency of 99.5% at 250 °C in the presence of 50 ppm SO₂. The conversion efficiency did not decrease after repeatedly used for 8 times.     

Selective reduction of NO by NH3 over Fe-zeolite-promoted V2O5-WO3/TiO2-based catalysts: Great suppression of N2O formation and origin of NO removal activity loss

Great depression of the formation N₂O in the selective catalytic reduction of NO by NH₃ (NH₃-SCR) has been studied by combining a V₂O₅-WO₃/TiO₂ (VWT) catalyst with a Fe-exchanged ZSM-5 zeolite (FeZ). At temperatures > 400 °C, N₂O formation was significant over VWT but < 5 ppm over FeZ/VWT catalysts with the FeZ ≥ 8%. Unfortunately, all these FeZ-promoted catalysts disclosed a decrease in deNO_xing performances, due to an enhanced NH₃ oxidation into NO. At temperatures > 350 °C, the chemically-combined VWT-based FeZ systems could facilitate both N₂O reduction with NH₃ and N₂O decomposition, thereby suppressing N₂O emissions in NH₃-SCR reaction.     

Effect of NO on the catalytic removal of N2O over FeZSM-5. Friend or foe

Selective catalytic reduction (SCR) of N₂O with C₃H₈ over FeZSM-5 under excess oxygen is strongly inhibited by NO. The assistance of the hydrocarbon in N₂O reduction vanishes at high partial NO pressures, approaching the activity of the N₂O + NO system at molar NO/N₂O ratios of 1.5–2, with N₂O/C₃H₈=1. This effect differs from the NO promotion in direct N₂O decomposition over Fe-zeolite catalysts. The negative effect of NO on the N₂O reduction has a significant impact in the design and operation of catalytic reactors in tail-gases of nitric acid plants and other sources where NO comes along with N₂O.     

Zr/HZSM-5 catalyst for NO reduction by C2H2 in lean-burn conditions

Zr-based zeolite catalysts were investigated for the first time in selective catalytic reduction of NO by hydrocarbon (HC-SCR). Highly dispersed zirconium species, especially the amorphous ultrafine zirconium oxide in the catalyst, considerably enhanced the activity for selective catalytic reduction of NO by acetylene (C₂H₂-SCR), both by accelerating the NO oxidation to NO₂ and enlarging the NO₂ adsorption capacity of the catalyst under the reaction conditions. Thus a durable and active Zr/HZSM-5 catalyst giving 89% of NO conversion to N₂ at 350 °C in 1600 ppm NO, 800 ppm C₂H₂, and 9.95% O₂ in helium was obtained. For the C₂H₂-SCR of NO, it was suggested that acidic sites with strong acidity on the Zr-based HZSM-5 catalysts are indispensable to initiate the aimed reaction via the route of NO oxidation to NO₂, which explains the higher activity for the reaction obtained over the Zr/HZSM-5 catalyst sample with lower SiO₂/Al₂O₃ ratio. The zirconium species could only functioned in the presence of protons in the C₂H₂-SCR of NO, so a synergistic effect between the zirconium species and protons of the Zr/HZSM-5 catalyst was proposed.     

Adsorption and temperature programmed desorption of NO,NO + O2, NO2 and NO + NO2 over CoH-ZSM-5

The NO₂, NO (O₂) adsorption and temperature programmed desorption (TPD) were studied systematically to probe into the selective catalytic reduction of NO by methane (CH₄–SCR) over CoH-ZSM-5 (SiO₂/Al₂O₃ = 25, Co/Al = 0.132–0.312). Adsorption conditions significantly affect the adsorption of NO, NO₂ and NO + O₂. Adsorbed NO species are unstable and desorbed below the reactive temperature 523 K. Increasing adsorption temperature results in the decrease of the adsorbed NO species amount. The amount of –NO_y species formed from NO₂ adsorption increases with the increase of NO₂ concentration in the adsorption process, while decreases significantly with the increase of adsorption temperature. Though NO species are adsorbed weakly on CoH-ZSM-5, competitive adsorption between NO and –NO_y species decreases the amount of adsorbed –NO_y species. Similar desorption profiles of NO₂ were obtained over CoH-ZSM-5 while they were contacted with NO₂ or NO + O₂ followed by TPD. If NO₂ was essential to form adsorbed –NO_y species, the amount of adsorbed –NO_y species for NO + O₂ adsorption should be the least among the adsorptions of NO₂, NO + O₂ and NO + NO₂ because of the lowest NO₂ concentration and highest NO concentration. In fact, the amount of adsorbed –NO_y species is between the other two adsorption processes. These indicate that the formation of adsorbed –NO_y species may not originate from NO₂.     

Catalytic gasification of lignin with Ni/Al2O3–SiO2 in sub/supercritical water

Lignin has been gasified with a Ni/Al₂O₃–SiO₂ catalyst in sub/supercritical water (SCW) to produce gaseous fuels. XRD pattern at 6θ angle shows characteristic peaks of crystalline NiO, NiSi, and AlNi₃, suggesting that Al₂O₃–SiO₂ not only offers high surface area (122 m² g) for Ni, but also changes the crystal morphology of the metal. 9 mmol/g of H₂ and 3.5 mmol/g of CH₄ were produced at the conditions that 5.0 wt% alkaline lignin plus 1 g/g Ni/Al₂O₃–SiO₂ operating for 30 min at 550 °C. A kinetic model was also developed, and the activation energies of gas and char formation were calculated to be 36.68 ± 0.22 and 9.0 ± 2.4 kJ/mol, respectively. Although the loss of activity surface area during reuse caused slight activity reduction in Ni/Al₂O₃–SiO₂, the catalyst system still possessed high catalytic activity in generating H₂ and CH₄. It is noted that sulfur linkage could be hydrolyzed to hydrogen sulfide in the gasification process of alkaline lignin. The stable chemical states of Ni/Al₂O₃–SiO₂ grants its insensitivity to sulfur, suggesting that Ni/Al₂O₃–SiO₂ should be economically promising for sub/supercritical water gasification of biomass in the presence of sulfur.     

Catalytic conversion of CO,NO and SO2 on the supported sulfide catalyst: I. Catalytic reduction of SO2 by CO

Catalytic reduction of SO₂ to elemental sulfur by CO has been systematically investigated over γ-Al₂O₃-supported sulfide catalysts of transition metals including Co, Mo, Fe, CoMo and FeMo with different loadings of the metals. The sulfided CoMo/Al₂O₃ exhibited outstanding activity: a complete conversion of SO₂ was achieved at a temperature of 300°C. The reaction proceeds catalytically and consistently over time and most efficiently at a molar feed ratio CO/SO₂ = 2. A precursor CoMo/Al₂O₃ oxide which experienced sulfurization through the CO–SO₂ reaction yielded a working sulfide catalyst having a yet lower activity than the CoMo catalyst sulfided before reaction (pre-sulfiding). The catalytic activity of various metal sulfides decreased in order of 4% Co 16% Mo > 4% Fe15% Mo > 16% Mo ≥ 25% Mo > 14% Co ≥ 4% Co > 4% Fe. A DRIFT study showed that CO adsorbs exclusively on CoMo phase and that SO₂ predominantly on γ-Al₂O₃. It is suggested that the Co–Mo–S structure is more adequate than the other metal-sulfur structures for the formation of a carbonyl sulfide (COS) intermediate because of the proper strength of metal–sulfur bond, and catalytically works with γ-Al₂O₃ for the COS–SO₂ reaction.     

Phase equilibria in the CaO·SiO2Na2O·SiO2Na2O·Al2O3·6SiO2 system

Equilibrium phase relations in the system CaO·SiO₂Na₂O·SiO₂Na₂O·Al₂O₃·6SiO₂ at 40–80 wt% Na₂O·Al₂O₃·6SiO₂ composition range have been experimentally studied at temperatures between 800 °C and 1200 °C. The liquidus temperature was determined with differential scanning calorimetry. The equilibrated samples were quenched with pressurized nitrogen, and examined with electron probe X-ray microanalysis and X-ray diffraction for identification of microstructure and phase relations. Five primary phase fields, CaO·SiO₂, Na₂O·SiO₂, Na₂O·2CaO·3SiO₂, 2Na₂O·CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ were established. The ternary eutectic point of CaO·SiO₂, Na₂O·2CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ was determined to be at 1030 °C with the composition of 29.0 wt% CaO·SiO₂, 12.0 wt% Na₂O·SiO₂ and 59.0 wt% Na₂O·Al₂O₃·6SiO₂. Peritectic reaction of Na₂O·2CaO·3SiO₂, 2Na₂O·CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ occurred at 930 °C with the composition of 13.0 wt% CaO·SiO₂, 29.0 wt% Na₂O·SiO₂ and 58.0 wt% Na₂O·Al₂O₃·6SiO₂. The liquidus surface projection of the ternary system has been constructed in the composition region important for the bottom ash application.     

Effect of post-sintering heat-treatment on thermal and mechanical properties of Si3N4 ceramics sintered with different additives

To improve the thermal conductivity of Si₃N₄ ceramics, elimination of grain-boundary glassy phase by post-sintering heat-treatment was examined. Si₃N₄ ceramics containing SiO₂–MgO–Y₂O₃-additives were sintered at 2123 K for 2 h under a nitrogen gas pressure of 1.0 MPa. After sintering, the SiO₂ and MgO could be eliminated from the ceramics by vaporization during post-sintering heat-treatment at 2223 K for 8 h under a nitrogen gas pressure of 1.0 MPa. Thermal conductivity of 3 mass% SiO₂, 3 mass% MgO and 1 mass% Y₂O₃-added Si₃N₄ ceramics increases from 44 to 89 Wm⁻¹ K⁻¹ by the decrease in glassy phase and lattice oxygen after the heat-treatment. Relatively higher fracture toughness (3.8 MPa m^1/2) and bending strength (675 MPa) with high hardness (19.2 GPa) after the heat-treatment were achieved in this specimen. Effects of heat-treatment on microstructure and chemical composition were also observed, and compared with those of Y₂O₃–SiO₂-added and Y₂O₃–Al₂O₃-added Si₃N₄ ceramics.     

Resistance to sulfidation of the catalytic reduction of NOx over Ag/Al2O3 by controlling the loadings of Ag and Ag+

SO₂ strongly decreased the catalytic activities of low loading Ag/Al₂O₃ below 500 °C in selective catalytic reduction (SCR) of NO_x by propene with or without the assistance of non-thermal plasma (NTP), which was mainly attributed to the competition between SO₂ and NO. By controlling the loadings of Ag and Ag⁺ over alumina, the resistance of SO₂ was remarkably enhanced between 400 °C and 500 °C in thermal SCR. In the NTP-assisted SCR, most of the NO_x conversions were also apparently recovered from 250 °C to 500 °C.     

A novel low-cost method for Hg0 removal from flue gas by visible-light-driven BiOX (X = Cl,Br, I) photocatalysts

BiOX (X = Cl, Br, I) photocatalysts were synthesized by a simple coprecipitation method and were characterized by SEM, TEM, HRTEM, XRD, TG, DRS, PL, and ESR techniques. The photocatalytic activity of Hg⁰ removal and the effects SO₂ and NO were investigated under fluorescent light. The Hg⁰ removal performance was in the sequence of BiOI > BiOBr > BiOCl. Compared with BiOBr, BiOI showed much excellent SO₂ resistance on Hg⁰ removal. In the BiOBr reaction system, h⁺ and O₂⁻ could play key roles in Hg⁰ removal, while for BiOI photocatalytic system, I₂ might be an important species for higher Hg⁰ removal.     

Conversions of fuel-N,volatile-N,and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N₂O during combustion of a single coal particle in O₂/N₂ and O₂/H₂O were studied in a tube reactor at low temperature. In O₂/N₂, NO reaches the maximum value in the devolatilization stage and N₂O reaches the maximum value in the char combustion stage. In O₂/H₂O, both NO and N₂O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N₂O in O₂/N₂ are obviously higher than those in O₂/H₂O, due to the reduction of H₂O on NO and N₂O. Temperature changes the trade-off between NO and N₂O. In O₂/N₂ and O₂/H₂O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N₂O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at < O₂ > = 30 vol% in O₂/N₂. In O₂/H₂O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at < O₂ > = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N₂O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N₂O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N₂O in both atmospheres. This work is to develop efficient ways to understand and control NO and N₂O emissions for a clean and sustainable atmosphere.     

Efficient and stable oxidative steam reforming of ethanol for hydrogen production: Effect of in situ dispersion of Ir over Ir/La2O3

La₂O₃-supported Ir catalyst was prepared by wetness impregnation method for the oxidative steam reforming of ethanol (OSRE). Fresh, reduced, and used catalysts were characterized by N₂ adsorption, H₂ chemisorption, XRD, FT-IR, TEM, and XPS. La₂O₃ would transform into hexagonal La₂O₂CO₃ during OSRE, which suppress coking effectively. Reduced Ir metal can interplay with La₂O₂CO₃ to form Ir-doped La₂O₂CO₃. It dynamically forms and decomposes to release active Ir nanoparticles, thereby preventing the catalyst from sintering and affording high dispersion of Ir/La₂O₃ catalysts at elevated temperatures. By introducing ultrasonic-assisted impregnation method during the preparation of a catalyst, the surface Ir concentration was significantly improved, while the in situ dispersion effect inhibited Ir from sintering. The Ir/La₂O₃ catalyst prepared by the ultrasonic-assisted impregnation method is highly active and stable for the OSRE reaction, in which the Ir crystallite size was maintained at 3.2 nm after 100 h on stream at 650 °C and metal loading was high up to 9 wt%.     

Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts

A series of vanadia doped TiO₂-pillared clay (TiO₂-PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V₂O₅/TiO₂-PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V₂O₅/TiO₂ catalyst and was the same as the commercial V₂O₅ + WO₃/TiO₂ catalyst. The V₂O₅/TiO₂-PILC catalysts also had higher N₂/N₂O product selectivities as compared to V₂O₅ doped TiO₂ catalysts. In addition, H₂O + SO₂ slightly increased the activities at high temperatures (>350°C) for the V₂O₅/TiO₂-PILC catalysts. Addition of WO₃ to V₂O₅ further increased the activities of the PILC catalysts. These results indicate that TiO₂-PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Brønsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Brønsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH₃ on the V₂O₅/TiO₂-PILC is similar to that on V₂O₅/TiO₂ catalyst, i.e., N₂ originates from the reaction between gaseous NO and NH₃ adspecies.     

Role of ceria in the improvement of SO2 resistance of LaxCe1 − xFeO3 catalysts for catalytic reduction of NO with CO

Perovskite-type catalysts with LaFeO₃ and substituted La_xCe_1 − xFeO₃ compositions were prepared by sol–gel method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), and SO₂ temperature-programmed desorption (SO₂-TPD). Catalytic reaction for NO reduction with CO in the presence of SO₂ has been investigated in this study. LaFeO₃ exhibited an excellent catalytic activity without SO₂, but decreased sharply when SO₂ gas was added to the CO + NO reaction system. In order to inhibit the effect of SO₂, substitution of Ce in the structure of LaFeO₃ perovskite has been investigated. It was found that La_0.6Ce_0.4FeO₃ showed the maximum SO₂ resistance among a series of La_xCe_1 − xFeO3 composite oxides.     

Generation of nitric oxide by photolysis of silver hyponitrite in suspension induced by LMCT excitation

The yellow colour of silver hyponitrite is attributed to an LMCT band at λ_max = 419 nm. Solid Ag₂N₂O₂ and its suspensions in water or acetonitrile are light sensitive. LMCT excitation leads to a photolysis according to the equation Ag₂N₂O₂ → 2Ag + 2NO. In the presence of air and water, NO is finally converted to HNO₂.     

Microwave dielectric properties of mineral sillimanite obtained by conventional and cold sintering process

The sillimanite (Al₂SiO₅) mineral has been sintered by conventional ceramic route and by cold sintering methods. The mineral has very poor sinterability and transformed to mullite on sintering above 1525 °C. The dielectric properties of sillimanite mineral (Al₂SiO₅) are investigated at radio and microwave frequency ranges. The mineral sintered at 1525 °C has low ε_r of 4.71 and tanδ of 0.002 at 1 MHz and at microwave frequency ε_r = 4.43, Q_u × f = 41,800 GHz with τ_f = −17 ppm/°C. The sintering aid used for cold sintering Al₂SiO₅ is sodium chloride (NaCl). The Al₂SiO₅NaCl composite was cold sintered at 120 °C. XRD analysis of the composite revealed that there is no additional phase apart from Al₂SiO₅ and NaCl. The densification of the Al₂SiO₅NaCl composite was confirmed by using microstructure analysis. The Al₂SiO₅NaCl composite has ε_r of 5.37 and tanδ of 0.005 at 1 MHz whereas at microwave frequency it has ε_r = 4.52, Q_u × f = 22,350 GHz with τ_f = −24 ppm/°C. The cold sintered NaCl has ε_r = 5.2, Q_u × f = 12,000 GHz with τ_f = −36 ppm/°C.     

Catalytic role of vanadium(V) sulfate on activated carbon for SO2 oxidation and NH3-SCR of NO at low temperatures

Carbon-supported catalyst with vanadium(V) sulfate (V₂O₃(SO₄)₂) as active component was prepared, characterized and tested for SO₂ and NO catalytic removal. Result shows that this catalyst is very active towards SO₂ oxidation and selective catalytic reduction of NO with NH₃ in the low temperature range of 100–250 °C.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.