Processing of hot pressed Al2O3–Cr2O3/Cr-carbide nanocomposite prepared by MOCVD in fluidized bed

Nanosized particles dispersed uniformly on Al₂O₃ particles were prepared from the decomposition of precursor Cr(CO)₆ by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr₂O₃, CrC_1−x, and C. A solid solution of Al₂O₃–Cr₂O₃ and an Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al₂O₃–Cr₂O₃/Cr-carbide (Cr₃C₂ and Cr₇C₃) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr₃C₂ and Al₂O₃ is non-coherent, while the interface between Cr₇C₃ and Al₂O₃ is semi-coherent.     

Effect of Al2O3 on mechanical properties of Ti3SiC2/Al2O3 composite

The effect of Al₂O₃ on mechanical properties of Ti₃SiC₂/Al₂O₃ composite fabricated by SPS was studied systematically. The results show that the hardness of the Ti₃SiC₂/Al₂O₃ composite can reach 10.28 GPa, 50% higher than that of pure Ti₃SiC₂. However, slight decrease in the other mechanical properties was observed with Al₂O₃ addition higher than 5–10 vol.%, which is believed to be due to the agglomeration of Al₂O₃ in the composite.     

Electrochemical impedance studies of molten (0.9Na,0.1K)2SO4-induced hot corrosion of the Ni-based superalloy M38G at 900 °C in air

The hot corrosion behavior of the Ni-based superalloy M38G covered with a film of molten 0.9Na₂SO₄–0.1K₂SO₄ (mole fraction) has been studied by electrochemical impedance spectroscopy at 900 °C in air. For comparison, the corrosion of the alloy immersed in molten (Na,K)₂SO₄ was also examined. The electrochemical impedance spectra in deep molten salt during an initial stage consisted of a semicircle at high frequency and a line at low frequency indicating a diffusion-controlled reaction. With extended immersion the impedance spectra were composed of two capacitive loops, i.e., a small semicircle at high-frequency port, and a large one at low-frequency port. The change of the impedance spectra is related to the formation of a protective scale. Contrary to the corrosion in deep molten salt, the corrosion of the alloy in the presence of a film of fused salt presented the characteristics of two capacitive loops for all the duration of the experimental test. Equivalent circuits representing the corrosion of the alloy in both corrosion conditions are proposed to fit the impedance spectra and electrochemical parameters in the equivalent circuits are also calculated.     

Na2SO4掺杂含MgO铝酸钙熟料的结构表征及浸出性能

使用分析纯MgO、CaCO₃、SiO₂、Al₂O₃与Na₂SO₄在1350℃保温1 h合成了掺杂Na₂SO₄的含MgO铝酸钙熟料,在Na₂CO₃溶液体系下研究了其氧化铝浸出性能,通过XRD等分析手段对其晶体结构和自粉化性能进行了研究。结果表明,Na₂SO₄可以显著提升铝酸钙熟料的浸出性能,Na₂SO₄掺杂量由0%提高到4%,熟料的氧化铝浸出率由61.89%提高到92.01%,继续添加Na₂SO₄,浸出性能趋于稳定。由XRD结果可知,Na₂SO₄促使20CaO·13Al₂O₃·3MgO·3SiO₂（Q相）发生分解并使其转变为12CaO·7Al₂O₃（C₁₂A₇）。Na⁺进入C₁₂A₇晶格引起晶格畸变,从而提高C₁₂A₇的氧化铝浸出性能。Na₂SO₄的加入降低了熟料的自粉化性能,Na₂SO₄掺杂量由0%提高到6%,熟料的自粉率由97.46%下降到85.34%,当Na₂SO₄掺杂量达到10%后,熟料自粉率仅为36.3%。     

片状氧化铝的熔盐法制备关键参数

高径厚比片状Al₂O₃在高端珠光颜料片状基材应用方面潜力巨大,但是其制备技术为国外所垄断,开发片状Al₂O₃商用生产技术迫在眉睫。熔盐法是制备片状Al₂O₃的理想方法,目前尚无系统研究熔盐法关键工艺参数对片状Al₂O₃影响规律的报道。本文通过片状Al₂O₃的形成机理,系统研究了TiO₂添加剂、Na₃P₃O₉添加剂、煅烧温度和熔盐用量4种关键工艺参数对制备片状Al₂O₃的影响规律,并进一步优化制备工艺,提出最优工艺参数。利用扫描电子显微镜和X射线衍射仪分析Al₂O₃的形貌、粒度和物相。结果表明：采用熔融盐（Na₂SO₄-K₂SO₄）添加NaCO₃为凝胶剂,再加入质量分数分别为3.0%的Na₃P₃O₉和2.0%的TiO₂为添加剂,在煅烧温度为1200℃、保温时间5h时,所制得的六角片状Al₂O₃粉体具有优异的品质,其平均粒径约为4μm,厚度为50～200nm,径厚比为20～80。     

Formation of Y3Al5O12–Al2O3 eutectic microstructure with off-eutectic composition

Homogeneous-eutectic microstructure of Y₃Al₅O₁₂–Al₂O₃ system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y₂O₃ and Al₂O₃ having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y₃Al₅O₁₂ and Al₂O₃ phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al₂O₃), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed.     

Study of n-hexane isomerization on mixed Al2O3–ZrO2/SO4 catalysts

Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr₂O were synthesized by means of sol–gel methods. The catalysts were sulfated with H₂SO₄ 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N₂-adsorption at 78 K, and SEM. The catalytic properties of the Al₂O₃–ZrO₂ series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al₂O₃–ZrO₂ solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane.     

Phase diagram of the Al2O3–ZrO2–Nd2O3 system

The phase diagram of the Al₂O₃–ZrO₂–Nd₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1675 °C and it corresponds to the ternary eutectic Nd₂O₃·11Al₂O₃ + F-ZrO₂ + NdAlO₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–Nd₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system.     

GO/Al2O3复合纳滤膜的制备及其稳定性能研究

以平均孔径为20 nm的Al₂O₃管式超滤膜为载体,经多巴胺改性后,利用压力驱动沉积法成功制备出能在水溶液中长期稳定的GO/Al₂O₃复合纳滤膜,并通过改变负载量实现了对GO层厚的调控。结果表明,随错流时间的延长,不同GO负载量下GO/Al₂O₃复合纳滤膜的纯水渗透系数均呈现先降低后稳定的趋势。且随着GO负载量的增加,稳态纯水渗透系数逐渐降低;当GO负载量增加到90 mg/m²后,GO/Al₂O₃复合纳滤膜对一二价盐的渗透系数与截留率均无显著变化。同时,由于盐测试过程中残余的盐离子在GO片层间产生了交联作用,从而导致随着在纯水中存放时间的延长,不同GO负载量的GO/Al₂O₃复合纳滤膜对一二价盐的截留率均呈上升趋势。GO负载量为140 mg/m²的GO/Al₂O₃复合纳滤膜在水中浸泡680 h后对1 mmol/L Na₂SO₄的截留率可达到91.0%。GO/Al₂O₃复合纳滤膜对四种一二价盐的截留率满足：R(Na₂SO₄) > R(MgSO₄) > R(NaCl) > R(MgCl₂)。     

Strength degradation of Si3N4---SiC-based ceramics in salt environments

The mechanism and kinetics of molten salt corrosion of reaction-sintered Si₃N₄---SiC-based ceramics, as well as the effect of these salts on its strength and fracture toughness at 20°C and 1200°C, were investigated. It was found that this material is highly resistant to corrosion in NaCl and sea salt up to 1100°C but degrades easily in Na₂SO₄ being a strong oxidant. Molten salt corrosion and salt-assisted oxidation cause mechanical properties' deterioration. Thus, NaCl effects involve up to 25% and 30% strength decrease at room temperature and at 1200°C, respectively. The results are explained by the etching of grain boundaries and by the formation of sodium silicate glassy layer during oxidation with it further cracking after cooling.     

Preparation of novel TiP2O7 carbon composite using ion-exchanged resin (C467) and evaluation for photocatalytic decomposition of 2-propanol

TiP₂O₇ carbon composite photocatalyst was successfully prepared by using ion-exchanged resin (C467) containing amino phosphate by metal ion-exchanged carbothermal reduction (MIER-CTR) method using TiCl₃ and TiCl₄. During the carbonization process in nitrogen, the pre-oxidation (300–350 °C) in air is essential for producing homogeneously dispersed TiP₂O₇ on the carbon matrix. In the absence of pre-oxidation, the resin was melted. The carbonization temperature 500 °C was found to be suitable for producing single phase TiP₂O₇ with higher yields. Powder X-ray diffraction (XRD) and Raman spectroscopic results suggest the formation of TiP₂O₇, while X-ray diffraction results reveal that the crystallite size was less than 35 nm. UV-Vis studies show that the band gap of TiP₂O₇ was 3.32 eV. The TiP₂O₇ carbon composite catalyst was applied for the photocatalytic decomposition of 2-propanol at 30 °C using a mercury lamp (365 nm).     

ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming

Surface-phase ZrO₂ on SiO₂ (SZrOs) and surface-phase La₂O₃ on Al₂O₃ (SLaOs) were prepared with various loadings of ZrO₂ and La₂O₃, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH₄/CO₂ reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al₂O₃ and Pt/SiO₂ catalysts. CO₂ or CH₄ pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides.

The zirconia can be homogeneously dispersed on SiO₂ to form a stable surface-phase oxide. The lanthana cannot be spread well on Al₂O₃, but it forms a stable amorphous oxide with Al₂O₃. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO₂ and Pt/Al₂O₃ by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO₂ and Pt/Al₂O₃ catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO₂ adsorbed on the basic sites of SZrOs and SLaOs.     

Al18B4O33 aluminium borate: A new efficient support for palladium in the high temperature catalytic combustion of methane

Well crystallised aluminium borate Al₁₈B₄O₃₃ has been synthesised from alumina and boric acid with a BET area of 18 m²/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al₁₈B₄O₃₃ was prepared by conventional impregnation of Pd(NO₃)₂ aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al₁₈B₄O₃₃ in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al₂O₃. Pd/Al₁₈B₄O₃₃ exhibited a much lower activity than Pd/Al₂O₃ when treated in hydrogen at 500 °C or aged in O₂/H₂O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al₁₈B₄O₃₃ which became as active as Pd/Al₂O₃. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al₂O₃ and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al₁₈B₄O₃₃. The existence of two distinct types of PdO species formed on Al₁₈B₄O₃₃ and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O₂.     

Investigation of CoNiMo/Al2O3 catalysts: Relationship between H2S adsorption and HDS activity

Sulfidation of trimetallic CoNiMo/Al₂O₃ catalysts was studied by thermogravimetry at 400 °C under flow and pressure conditions. Results were compared with those obtained on prepared and industrial CoMo/Al₂O₃ and NiMo/Al₂O₃ catalysts. The amount of sorbed H₂S on the sulfided solids was measured at 300 °C in the H₂S pressure range 0–3.5 MPa at constant H₂ pressure (3.8 MPa). The adsorption isotherms were simulated using a model featuring dissociated adsorption of H₂S on supported metal sulfides and bare alumina. The amount of sulfur-vacancy sites could thus be determined under conditions close to industrial practice. A relationship with activity results for thiophene HDS and benzene hydrogenation was sought for.     

Quantified NOx adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O

A multi-component NO_x-trap catalyst consisting of Pt and K supported on γ-Al₂O₃ was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H₂O and CO₂ on NO_x storage. The Al₂O₃ support was shown to have NO_x trapping capability with and without Pt present (at 250 °C Pt/Al₂O₃ adsorbs 2.3 μmols NO_x/m²). NO_x is primarily trapped on Al₂O₃ in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO_x/m², and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al₂O₃ catalyst are coordinated on the Al₂O₃ support at saturation.

When 5% CO₂ was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H₂O was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al₂O₃-based nitrates decreased by 92%. Interestingly, with both 5% CO₂ and 5% H₂O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al₂O₃ destabilized the K–CO₂ bond; specifically, H₂O mitigates the NO_x storage capacity losses associated with carboxylate competition.     

Effects of alumina incorporation in coprecipitated NiO–Al2O3 catalysts

The effect of Al₂O₃ levels on the properties of NiO in coprecipitated NiO–Al₂O₃ samples were investigated, using samples with up to 60.7 wt.% Al₂O₃ that had been calcined in the range 300–700°C. Characterization techniques included BET surface area of fresh and reduced catalysts, X-ray diffraction analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H₂. Only NiO was detected in samples with up to 4.1 wt.% Al₂O₃ for all sample calcination temperatures. Surface areas were similar for all fresh samples but decreased rapidly after calcination at high temperatures. The surface area loss was less for the higher Al₂O₃-containing samples. Nickel oxide crystallite sizes increased at higher calcination temperatures, but remained approximately the same for each Al₂O₃ level.

The NiO was nonstoichiometric (NiO_1+x), with x decreasing at higher calcination temperatures and increasing with small amounts of added Al₂O₃ through a maximum at about 3 wt.% Al₂O₃. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al₂O₃ addition. At higher levels of Al₂O₃ (13.6 wt.% and above), surface areas increased with higher Al₂O₃ loadings, but NiO crystallite sizes remained approximately the same, independent of both Al₂O₃ content and calcination temperature. X-ray diffraction patterns were very diffuse, and it was not possible to rule out the presence of pseudo-spinel combinations of NiO and Al₂O₃. Reducibility was more difficult than with low Al₂O₃ levels, and nonstoichiometry was low and independent of Al₂O₃ content.

Reducibilities of all samples calcined at 300°C correlated well with the final BET surface area of the reduced samples, indicating that more dispersed NiO crystallites are more difficult to reduce, a conclusion that supports a model for reduction proposed previously.     

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NO_x by hydrogen under lean burn conditions over Pt/Al₂O₃ is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO_x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al₂O₃. In this case, although pure H₂ and pure CO are ineffective for NO_x reduction under lean burn conditions, H₂/CO mixtures are very effective. With a realistic (1:3) H₂:CO ratio, typical of actual exhaust gas, Pd/Al₂O₃ is significantly more active than Pt/Al₂O₃, delivering 45% NO_x conversion at 160 °C, compared to >15% for Pt/Al₂O₃ under identical conditions. The nature of the support is also critically important, with Pd/Al₂O₃ being much more active than Pd/SiO₂. Possible mechanisms for the improved performance of Pd/Al₂O₃ in the presence of H₂+CO are discussed.     

A comparison study on non-thermal plasma-assisted catalytic reduction of NO by C3H6 at low temperatures between Ag/USY and Ag/Al2O3 catalysts

A comparison study was carried out on non-thermal plasma (NTP)-assisted selective catalytic reduction (SCR) of NO_x by propene over Ag/USY and Ag/Al₂O₃ catalysts. Ag/USY was almost inactive in thermal SCR while it showed obvious activities in NTP-assisted SCR at 100 °C–200 °C. Although the NO_x conversion over Ag/Al₂O₃ was also enhanced at 300 °C–400 °C by the assistance of NTP, it was ineffective below 250 °C. The intermediates over Ag/USY and Ag/Al₂O₃ were investigated using in situ DRIFTS method. It was found that key intermediates in HC-SCR, such as NCO, CN, oxygenates and some N-containing organic species were enriched after the assistance of NTP. The differences in the behaviors of above intermediates were not found between these two kinds of catalysts. However, some evidences suggested that different properties of the absorbed NO_x species resulted in the distinction of SCR reactions over Ag/USY and Ag/Al₂O₃. TPD profiles of Ag/Al₂O₃ showed that nitrates formed over the catalyst were quite stable at low temperatures, which might occupy the active sites and were unfavorable to SCR reactions. The nitrates over Ag/USY were unstable, among which the unidentate nitrate species is probably contributed to the SCR reactions at low temperatures.     

Na2O-Al2O3-SiO2-P2O5玻璃的结构和析晶性能研究

采用熔融-淬冷法制备了不同(Al₂O₃+P₂O₅)含量的碱铝硅酸盐玻璃,通过拉曼光谱、X射线衍射光谱、扫描电镜研究了其结构特征和析晶性能。发现随着(Al₂O₃+P₂O₅)含量减少,玻璃中Na₂O含量增加,玻璃化转变温度从685 ℃降低到622 ℃,当减少至摩尔分数为22%时,出现析晶峰且起始析晶温度降低。拉曼光谱表明Q⁴_P对应的拉曼峰强度变低且逐渐向低波数方向移动,说明Na₂O作为网络修饰体使硅酸盐玻璃结构逐步解聚,玻璃的析晶能力逐渐增强。结果表明:当(Al₂O₃+P₂O₅)摩尔分数为22%时热处理后的样品存在晶型转变,700 ℃热处理时以NaAlSiO₄霞石晶体为主,900 ℃时转变为以Na_6.8Al_6.3Si_9.7O₃₂霞石晶体为主。当(Al₂O₃+P₂O₅)的摩尔分数为21%和20%时,热处理后的样品能稳定析出Na₃PO₄和Na_6.8Al_6.3Si_9.7O₃₂晶体。热处理后的样品析出了耐酸侵蚀性较差的富磷相和Na₃PO₄晶体,导致化学稳定性变差。     

The role of lanthana loading on the catalytic properties of Pd/Al2O3-La2O3 in the NO reduction with H2

The role of La₂O₃ loading in Pd/Al₂O₃-La₂O₃ prepared by sol–gel on the catalytic properties in the NO reduction with H₂ was studied. The catalysts were characterized by N₂ physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO.

The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La₂O₃ inclusion. The selectivity depends on the NO/H₂ molar ratio (GHSV = 72,000 h⁻¹) and the extent of interaction between Pd and La₂O₃. At NO/H₂ = 0.5, the catalysts show high N₂ selectivity (60–75%) at temperatures lower than 250 °C. For NO/H₂ = 1, the N₂ selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H₂O vapor. The high N₂ selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdO_x phase interacting with La₂O₃. The formation of this phase is favored by the spreading of PdO promoted by La₂O₃. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al₂O₃ the O₂ uptake is consistent with the oxidation of PdO to PdO₂, and when La₂O₃ is present the O₂ uptake exceeds that amount (1.5 times). La₂O₃ in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N₂.