Pressure-induced phase transitions in Al2(WO4)3

The high pressure behavior of aluminum tungstate [Al₂(WO₄)₃] has been investigated up to ∼18 GPa with the help of Raman scattering studies. Our results confirm the recent observations of two reversible phase transitions below 3 GPa. In addition, we find that this compound undergoes two more phase transitions at ∼5.3 and ∼6 GPa before transforming irreversibly to an amorphous phase at ∼14 GPa.     

Compressibility of CaZrO3 perovskite: Comparison with Ca-oxide perovskites

The evolution of the unit-cell parameters of CaZrO₃ perovskite, an orthorhombic perovskite belonging to space group Pbnm, have been determined to a pressure of 8.7 GPa at room temperature using single-crystal X-ray diffraction measurements. A fit of a third-order Birch-Murnaghan equation of state to the pressure-volume data yields values of V₀=258.04(2) Å³, K_T0=154(1) GPa and K₀′=5.9(3). Although CaZrO₃ perovskite does not exhibit any phase transitions in this pressure range, the compression of the structure is anisotropic with [010] approximately 20% less compressible than either [100] or [001]. Compressional moduli for the unit cell parameters are: K_a0=142(1) GPa and K_a0′=4.4(2), K_b0=177(2) GPa and K_b0′=9.4(5), K_c0=146(2) GPa and K_c0′=5.4(4). Comparison with other orthorhombic Ca-oxide perovskites shows that there is systematic increase in compressional anisotropy with increasing distortion from cubic symmetry.     

Pressure-induced amorphization in Y2(WO4)3: in situ X-ray diffraction and Raman studies

The high-pressure behavior of Y₂(WO₄)₃ has been investigated at room temperature by in situ X-ray diffraction and Raman scattering measurements. Both the studies show that beyond ∼3 GPa, this compound smoothly transforms from the ambient orthorhombic phase to a disordered phase. The structural modifications are found to be reversible up to ∼4 GPa but become irreversible at higher pressures. Low pressures of transformation imply that these changes are intrinsic and not due to non-hydrostatic stresses. In addition, the correlation between the stability range of orthorhombic phase and counter cation size supports that this compound has a large field of negative thermal expansion in this family of compounds.     

Crystal-chemistry of mullite-type aluminoborates Al18B4O33 and Al5BO9: A stoichiometry puzzle

Orthorhombic Al₂O₃-rich aluminoborate is an important ceramic material for which two slightly different compositions have been assumed: Al₅BO₉ (5Al₂O₃:B₂O₃) and Al₁₈B₄O₃₃ (9Al₂O₃:2B₂O₃). The formula Al₁₈B₄O₃₃ (=Al_4.91B_1.09O₉) was derived from results of chemical analyses when crystal structure data were not yet available. Subsequent structural investigations indicated Al₅BO₉ composition. Nevertheless, Al₁₈B₄O₃₃ was still accepted as the correct stoichiometry assuming that additional B replaces 9% Al.Powder samples of both compositions and ones with excess boron were prepared by solid state reactions between α-Al₂O₃ and B₂O₃/H₃BO₃ at temperatures above 1100 °C and single-crystals were grown from flux at 1100 and 1550 °C. Products were investigated by single-crystal and powder XRD, ¹¹B and ²⁷Al solid-state MAS-NMR, Raman and FTIR spectroscopy as well as Laser-ablation ICP-MS. No indication of the predicted 9% B→Al substitution was found. LA ICP-MS indicated 12.36(27) wt% B₂O₃ corresponding to Al_4.97B_1.03O₉. Hence, the suggested Al₁₈B₄O₃₃ stoichiometry can be excluded for all synthesized samples. A very low amount of Al vacancies at a five-fold coordinated site are likely, charge balanced by an additional nearby three-fold coordinated B site. All evidences indicate that the title compound should be reported as Al_5−xB_1+xO₉ with x<0.038(6), which is close to Al₅BO₉.     

High pressure behavior of α-NaVO3: A Raman scattering study

The reported pressure-induced amorphization in α-NaVO₃ has been re-investigated using Raman spectroscopy. Discontinuous changes are noted in the Raman spectrum above 5.6 GPa implying large structural changes across the transition. The decrease in frequency of the V-O stretching mode across the transition suggests that the vanadium atom may be in octahedral coordination in the high pressure phase. Excessive broadening of the internal modes is observed above 6 GPa. New peaks characteristic of a crystalline phase gain in intensity at higher pressures in the bending modes region; however, the transformation is not complete even at 13 GPa. Co-existence of phases is noted over a significant pressure range above the onset of transition. Pressure released spectrum is found to be a mixture of crystalline α-phase, traces of crystalline β-phase and highly disordered phase consisting of V-O units in five- and six-fold coordination.     

Preparation, characterization and application of KNH2/Al2O3 and KF/Al2O3 as strong solid bases

This review describes the preparation, characterization and application of KNH₂ loaded on alumina and KF loaded on alumina. These strong solid bases catalyze a variety of organic reactions in a very selective manner. The reactions include isomerizations of alkenes and alkynes, dimerization of alkynes, Tishchenko reaction, and the reaction of silanes to form of Si–C, Si–N and Si–O bonds.     

Ti∶Al2O3纳米粉体的共沉淀法合成及表征

本文采用共沉淀法合成了Ti∶Al2O3纳米粉体。利用热重/差热(TG/DTA)/X射线衍射(XRD)/红外光谱(FTIR)/扫描电镜(SEM)以及能谱(EDS)等分析方法对合成的Ti∶Al2O3纳米粉体进行了表征。结果分析表明:前驱体在1200℃下,保温1 h可以得到纯的α-Al2O3晶相;粉体的粒径均匀、分散性好,平均粒径在25~50 nm之间。     

Thermal Stability of Pt/Al2O3 Catalysts Prepared by Sol-Gel

A series of Pt/Al₂O₃ catalysts were prepared using a sol-gel method. The influence of several parameters used in the synthesis including: metal content, identity of the metal precursor, and the water/alkoxide ratio on the structural properties of the fresh (dried) and calcined samples were studied. It was found that the BET surface area decreased with an increase in the platinum content. A surface area of 500 m²/g was obtained following calcination at 773 K. The structure of fresh samples as determined by FTIR corresponded to that of a pseudoboehmite structure. Samples prepared using a water/alkoxide ratio (H₂O/ATB) of 9 showed a well-defined, uniform pore size distribution following calcination at 773 K. Metal dispersions comparable to those obtained using impregnation methods were obtained. Aging studies (calcination at 873 K for 24 h) performed on these catalysts, exhibited sintering behavior which were similar to Pt/Al₂O₃ catalysts prepared by other methods. The sample prepared using a H₂O/ATB ratio of 9 had the highest surface area and was more thermally resistant towards metal sintering. A bimodal metal particle size distribution was observed: some particles exhibited sintering while others of similar size showed a greater thermal stability to sintering. The sample having the largest surface area and the highest thermal stability following thermal treatment was a consequence of a more condensed structure and a higher pore roughness obtained after drying the gel. This enabled the formation of an alumina structure which was more amorphous and limited aggregation of platinum particles due to surface diffusion within the pore structure.     

添加Al2O3对Ni0.5Zn0.5Fe2O4纳米纤维结构和磁性能的影响

采用静电纺丝技术制备了添加0～20wt%Al2O3的Ni0.5Zn0.5Fe2O4纳米纤维。通过XRD、FESEM、TEM和VSM对样品的物相结构、形貌和磁性能进行了表征。结果表明,所合成的复合纳米纤维的直径都分布在40～150 nm之间,添加到纤维中的Al2O3主要以非晶态形式分布于铁氧体晶粒边界;随着Al2O3添加量的增加,可观察到γ-Fe2O3相逐渐析出,Ni-Zn铁氧体的晶格常数单调减小,说明有一些Al2O3进入到尖晶石晶格中取代了B位的Fe3+离子,Ni-Zn铁氧体的平均晶粒尺寸先增大后减小,在Al2O3添加量为8wt%时达到最大值39.2 nm;比饱和磁化强度和矫顽力随Al2O3添加量的增加呈现出相同的变化规律,先减小后增大,当Al2O3添加量超过5wt%时又开始变小。     

Mn-Fe/Al2O3催化剂的低温脱硝性能研究

以Al_2O_3为载体,Fe、Mn为活性组分,采用浸渍法制备了Mn-Fe/Al_2O_3催化剂,研究了Mn-Fe/Al_2O_3催化剂的低温脱硝性能.实验结果表明,Fe负载量为7%时,7Fe/Al_2O_3催化剂显示出较佳的低温脱硝性能;添加Mn能明显改变7Fe/Al_2O_3催化剂低温脱硝性能,其中当Mn/Fe质量比为11∶7时,11Mn7Fe/Al_2O_3催化剂获得最佳低温脱硝效率.对催化剂的表征结果表明,比表面积和孔径的大小不是决定催化剂性能高低的主要因素;Mn O2和Fe2O3在x Mn7Fe/Al_2O_3催化剂中具有较强的相互作用,影响活性组分微观晶体结构,改善活性组分分散程度,从而提高了催化剂的低温脱硝性能;Fe的添加使催化剂表面酸性位点数目增加,从而提高7Fe/Al_2O_3催化剂的低温脱硝效率.添加Mn不仅增多了11Mn7Fe/Al_2O_3催化剂表面酸性位点数目增加,而且使催化剂表面出现新的中强性酸位点,使其低温脱硝效率进一步提高;经过详细分析,结果表明表面吸附氧Oβ、Mn4+和Fe3+的含量较高是11Mn7Fe/Al_2O_3催化剂脱硝活性较高的主要原因.     

纳米膜组装介孔Al2O3：合成及Pt/Al2O3的硝基苯催化加氢性能

以"乙酸乙酯(EA)-偏铝酸钠-水"体系在室温下合成了纳米膜组装介孔Al2O3。研究发现:合成反应时间、静置前搅拌时间、NaAlO2用量、EA用量及反应温度等对合成产物的形貌有影响;另外,与用商品γ-Al2O3制备的Pt/γ-Al2O3催化剂相比,纳米膜组装介孔Al2O3制备的Pt/Al2O3催化剂含有部分易被还原的PtOx物种。在硝基苯催化加氢反应中,用合成Al2O3为载体制备的Pt/Al2O3催化剂,比用商品γ-Al2O3制备的Pt/γ-Al2O3催化剂具有更好的催化活性。     

Methane steam reforming over rhodium promoted Ni/Al2O3 catalysts

The effect of Rh addition upon catalyst characteristics and performance in methane steam reforming was investigated using Rh-promoted Ni/Al₂O₃ catalysts. The number of reduced metal atoms exposed on the surface increased for the Rh-promoted catalysts. Rh-promoted catalysts showed an increase in CH₄ reforming activity; however, constant turnover frequencies for promoted and unpromoted catalysts suggest that the increase in the number of metal surface atoms caused the activity enhancement. Rh also facilitated reduction of Ni/Al₂O₃.     

A new, efficient and simple method for the thionation of ketones to thioketones using P4S10/Al2O3

A simple, efficient and new method has been developed for the synthesis of thioketones by thionation of ketones using P₄S₁₀/Al₂O₃ in acetonitrile. It has been found that the P₄S₁₀/Al₂O₃ combination provides a simple and convenient method for the synthesis of thioketones.     

水合氧化铝的热处理及纳米氧化铝的颗粒特性

利用IR、XRD、TG-DTA和TEM等测试技术研究了醇铝的水解产物——水合氧化铝的特性及水合氧化铝的热处理过程，探讨了热处理工艺对纳米氧化铝颗粒特性的影响规律。结果表明，醇铝的水解产物水合氧化铝为富含吸附水和结构水的非晶态一水铝石，300 ℃煅烧后出现部分非晶态γ-Al₂O₃；煅烧温度、保温时间和冷却方式均能影响纳米氧化铝粒子的晶型、大小和形貌，其中煅烧温度影响最大，冷却方式影响次之，保温时间影响最弱；在750 ℃ / 3 h、800 ℃ / 4 h、900 ℃ / 4 h、950 ℃ / 4 h和1 000 ℃ / 4 h随炉冷却工艺条件下，分别得到粒径为3~5 nm、3~5 nm、5~10 nm、10~15 nm和50~100 nm的纤维状或球状、α相或γ相的纳米氧化铝粒子。     

Surface modification of thermoplastics by atomic layer deposition of Al2O3 and TiO2 thin films

Al₂O₃ and TiO₂ thin films were deposited by atomic layer deposition at 80-250 °C on various polymeric substrates such as polymethylmethacrylate (PMMA), polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE) and ethylenetetrafluoroethylene (ETFE). The films were studied with FESEM, EDX, XRD, contact angle measurements and adhesion tests. The film growth rates on the thermoplastics were close to the corresponding growth rates on Si substrates. The adhesion of the films was good on PEEK and poor on PTFE. All coated surfaces showed lower water contact angles than the uncoated thermoplastics. Furthermore, the water contact angles on all TiO₂-coated surfaces decreased upon UV illumination, most efficiently with crystalline TiO₂ coatings.     

Pressure-induced structural changes of the tetragonal Bi2CuO4

The tetragonal compound Bi₂CuO₄ was investigated at high pressures by using in situ Raman scattering and X-ray diffraction (XRD) methods. A pressure-induced structural transition started at 20 GPa and completed at ∼37 GPa was found. The high pressure phase is in orthorhombic symmetry. Raman and XRD measurements revealed that the above phase transition is reversible.     

K2CO3含量对Ni-Cu-Mn-K/Al2O3水煤气变换催化剂结构和性能的影响

以γ-Al₂O₃为载体，采用等体积浸渍法，制备了不同K₂CO₃含量的Ni-Cu-Mn-K/Al₂O₃水煤气变换催化剂，采用低温N₂吸附、XRD、TPD和TPR，考察了K₂CO₃含量对催化剂结构和性能的影响。结果表明:K₂CO₃的加入使催化剂的还原温度有所提高，适量的K₂CO₃能增加活性组分的电子密度，从而增强其给电子活化CO的能力，提高催化剂的活性。但过量的K₂CO₃使得催化剂比表面积和孔容降低，且导致催化剂对CO吸附过强，催化活性降低。当Ni-Cu-Mn-K/γ-Al₂O₃催化剂中K₂CO₃的添加量为7.5%时，且催化剂经530 ℃耐热15 h后，在350 ℃时水煤气变换反应中CO转化率达62.29%。     

Pressure-induced metastable phase transition in orthoenstatite (MgSiO3) at room temperature: a Raman spectroscopic study

Phase behavior of a synthetic orthoenstatite in a diamond-anvil cell has been studied up to ∼22 GPa by using Raman spectroscopy at room temperature. Under quasi-hydrostatic conditions, orthoenstatite undergoes a reversible phase transformation at an apparent transition pressure of ∼10 GPa for compression and ∼9.5 GPa for decompression. The 3d transition-metal cations, e.g., Fe²⁺ and Ni²⁺, show only a minor effect on the transition pressure within 10 wt% of addition. All the Raman frequencies in both orthoenstatite and its high-pressure phase increase monotonically with increasing pressure. The amount of forward or backward transition is fixed at a given pressure and forms a hysteresis loop in the transition %-pressure plan. The type for the present metastable phase transition is inferred to be of first order and the high-pressure polymorph may be the intermediate between orthoenstatite and the high-pressure clinoenstatite (i.e., the high-P C2/c phase). A mechanism based on Mnyukh's edgewise model of interface motion has been suggested to account for the observed phenomena.     

Hydrogenation of nitrate in water over bimetallic Pd-Ag/Al2O3 catalysts

Hydrogenation of nitrate (NO₃ ⁻) in water was studied with 0.8 ×10⁻³–3.2 ×10⁻³ mol/dm³ of reactant in the temperature range of 293–313 K over palladium promoted Ag catalysts. Pd-Ag catalysts with a low ratio of Ag/Pd were characterized by high efficiency in the reduction of nitrates. The degradation of nitrates followed approximately first order decay and the estimated apparent activation energy was about 4 kJ/mol.     

Fe2O3/Al2O3二元气凝胶合成高质量单壁纳米碳管

通过溶胶和超临界干燥方法制得了Fe₂O₃/Al₂O₃二元气凝胶，其比表面积和孔隙体积分别为246 m²·g^-1和1.89 cm³·g^-1，并具有较宽的孔径分布。以Fe₂O₃/Al₂O₃二元气凝胶作催化剂，通过甲烷催化裂解成功地合成了高质量的单壁纳米碳管。利用FESEM、TEM和HRTEM、Raman光谱等分析手段研究了反应温度对单壁纳米碳管生长的影响。结果表明在900 ℃时合成单壁纳米碳管的质量较高，并且合成的炭产物为毡状，该炭产物主要为高质量的单壁纳米碳管。