The effect of Pd/Al2O3 pretreatment on catalytic activity in cyclopentane/deuterium exchange

Big variations in overall activity and product selectivity in the cyclopentane/deuterium exchange reaction were found in effect of various pretreatments of two chlorine‐free Pd/γ‐Al₂O₃ catalysts. The most important changes are observed when severely prereduced (at 600 °C) Pd/Al₂O₃ catalysts have been reoxidised and mildly rereduced: the multiple type of exchange, typical of mildly pretreated Pd catalysts, is replaced by a stepwise mode, and a big increase in catalytic activity occurs. At this state, the Pd/γ‐Al₂O₃ catalysts retain some water (as surface hydroxyls) generated by reoxidation and mild reduction. Deuterium spillover from Pd onto alumina and changes in acidity of alumina are invoked to rationalize the kinetic results. Changes in the state of Pd after various pretreatments, as probed by temperature‐programmed hydride decomposition, can hardly be correlated with changes in the catalytic behaviour in the exchange reaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Role of the surface state of Ni/Al2O3 in partial oxidation of CH4

The effect of gas phase O₂ and reversibly adsorbed oxygen on the decomposition of CH₄ and the surface state of a Ni/Al₂O₃ catalyst during partial oxidation of CH₄ were studied using the transient response technique at atmospheric pressure and 700°C. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H₂ can be produced from non‐selective oxidation of CH₄ by reversibly adsorbed oxygen which is more active in oxidizing CH₄ completely than NiO via the Rideal–Eley mechanism and both the conversions of CH₄ and O₂ and the selectivities to CO and H₂ are very low. Therefore, keeping the catalyst surface in the reduced state is the precondition of high conversion of CH₄ and high selectivities to CO and H₂. The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH₄. During partial oxidation of CH₄, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduced state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH₄, and the indirect oxidation mechanism is not able to gain dominance in the reaction under the experimental conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stable carbon dioxide reforming of methane over modified Ni/Al2O3 catalysts

CO₂ reforming of methane was studied over modified Ni/Al₂O₃ catalysts. The metal modifiers were Co, Cu, Zr, Mn, Mo, Ti, Ag and Sn. Relative to unmodified Ni/Al₂O₃, catalysts modified with Co, Cu and Zr showed slightly improved activity, while other promoters reduced the activity of CO₂ reforming. Mn-promoted catalyst showed a remarkable reduction in coke deposition, while entailing only a small reduction in catalytic activity compared to unmodified catalyst. The catalysts prepared at high calcination temperatures showed higher activity than those prepared at low calcination temperature. The Mn-promoted catalyst showed very low coke deposition even in the absence of diluent gas and the activity changed only slightly during 100 h operation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of space velocity on methanol synthesis from Co2/Co/H2 over Cu/ZnO/Al2O3 catalyst

The space velocity had profound and complicated effects on methanol synthesis from CO₂/CO/H₂ over Cu/ZnO/Al₂O₃ at 523 K and 3.0MPa. At high space velocities, methanol yields as well as the rate of methanol production increased continuously with increasing CO₂ concentration in the feed. Below a certain space velocity, methanol yields and reaction rates showed a maximum at CO₂ concentration of 5–10%. Different coverages of surface reaction intermediates on copper appeared to be responsible for this phenomenon. The space velocity that gave the maximal rate of methanol production also depended on the feed composition. Higher space velocity yielded higher rates for CO₂/ H₂ and the opposite effect was observed for the CO/H₂ feed. For CO₂/CO/H₂ feed, an optimal space velocity existed for obtaining the maximal rate.     

Catalytic Activities of Al2O3-promoted NiSO4/TiO2 for Acid Catalysis

A series of catalysts, NiSO₄/Al₂O₃–TiO₂, for acid catalysis was prepared by the impregnation method, where support, Al₂O₃–TiO₂ was prepared by the coprecipitation method using a mixed aqueous solution of titanium tetrachloride and aluminum nitrate solution followed by adding an aqueous ammonia solution. The addition of nickel sulfate (or Al₂O₃) to TiO₂ shifted the phase transition of TiO₂ from amorphous to anatase to higher temperature because of the interaction between nickel sulfate (or Al₂O₃) and TiO₂. 15-NiSO₄/5-Al₂O₃–TiO₂ containing 15 wt% NiSO₄ and 5 mol% Al₂O₃, and calcined at 400°C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The charge transfer from Ti atoms to the neighboring Al atoms strengthens the Al–O bond between Al and the surface sulfate species. The addition of Al₂O₃ up to 5 mol% enhanced the acidity, thermal property, and catalytic activities of NiSO₄/Al₂O₃–TiO₂ gradually due to the interaction between Al₂O₃ and TiO₂ and consequent formation of Al–O–Ti bond.     

On the development of metallic particles and their initial catalytic properties in the CO + H2 reaction over Co/Al2O3 catalyst

Formation of Co^o phases with different surface structure over 10 wt% Co/Al₂O₃ and their catalytic properties were induced by pretreatments in H₂ at 570 K for 1 h or 20 h. Electronic behaviour of the Co^o phase, which consists of small (after 1 h reduction) or large bulk-like particles (after 20 h reduction), did not change during the CO hydrogenation after 5 h on stream as was determined by XPS. On the basis of the measured C₂₊ hydrocarbon selectivities the CO molecules are suggested to dissociate on small Co particles to a larger extent than on large cobalt particles. The slight decrease in the catalytic activity with increasing time on stream obtained for the long-term reduced sample is explained by the change in the surface Co^o content detected by XPS. The increase in the catalytic activity along with the change in olefin selectivity, measured for the sample reduced for 1 h, is interpreted by the change of a reaction path involving the Co^o-support interface during the initial period of the reaction.     

Gadolinium oxide nanocrystal nonvolatile memory with HfO2/Al2O3 nanostructure tunneling layers

In this study, Gd₂O₃ nanocrystal (Gd₂O₃-NC) memories with nanostructure tunneling layers are fabricated to examine their performance. A higher programming speed for Gd₂O₃-NC memories with nanostructure tunneling layers is obtained when compared with that of memories using a single tunneling layer. A longer data retention (< 15% charge loss after 10⁴ s) is also observed. This is due to the increased physical thickness of the nanostructure tunneling layer. The activation energy of charge loss at different temperatures is estimated. The higher activation energy value (0.13 to 0.17 eV) observed at the initial charge loss stage is attributed to the thermionic emission mechanism, while the lower one (0.07 to 0.08 eV) observed at the later charge loss stage is attributed to the direct tunneling mechanism. Gd₂O₃-NC memories with nanostructure tunneling layers can be operated without degradation over several operation cycles. Such NC structures could potentially be used in future nonvolatile memory applications.     

Dehydrogenation of ethylbenzene to styrene over Fe2O3/Al2O3 catalysts in the presence of carbon dioxide

An Fe₂O₃ (10 wt%)/Al₂O₃ (90 wt%) catalyst prepared by a coprecipitation method was found to be effective for dehydrogenation of ethylbenzene to produce styrene in the presence of CO₂ instead of steam used in commercial processes. The dehydrogenation of ethylbenzene over the catalyst in the presence of CO₂ was considered to proceed both via a one-step pathway and via a two-step pathway. CO₂ was found to suppress the deactivation of the catalyst during the dehydrogenation of ethylbenzene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transformation of cyclopentanone into cyclopentanethiol over a sulfided CoMo/Al2O3 catalyst

The transformation of cyclopentanone in the presence of H₂S/H₂ was investigated at atmospheric pressure over a sulfided CoMo/Al₂O₃ catalyst. The main reaction products were cyclopentanethiol and cyclopentene, the relative amounts of which depended on the reaction temperature and on the H₂S to cyclopentanone molar ratio. The best results were obtained at 220°C, with a 2.5 H₂S to cyclopentanone molar ratio: under these conditions, the cyclopentanethiol molar selectivity remained at about 90%, in a range of cyclopentanone conversion of 10–70%.     

Pulse studies of CH4 interaction with NiO/Al2O3 catalysts

Pulse studies of the interaction of CH₄ and NiO/Al₂O₃ catalysts at 500°C indicate that CH₄ adsorption on reduced nickel sites is a key step for CH₄ oxidative conversion. On an oxygen-rich surface, CH₄ conversion is low and the selectivity of CO₂ is higher than that of CO. With the consumption of surface oxygen, CO selectivity increases while the CO₂ selectivity falls. The conversion of CH₄ is small at 500°C when a pulse of CH₄/O₂ (CH₄O₂=21) is introduced to the partially reduced catalyst, indicating that CH₄ and O₂ adsorption are competitive steps and the adsorption of O₂ is more favorable than CH₄ adsorption     

Analysis of structure-sensitivity in reactions of alkanes:n-butane hydrogenolysis on a Ru/Al2O3 catalyst

The rate ofn-butane hydrogenolysis on 1% Ru/Al₂O₃, the response of the rate to variation of H₂ pressure, and the product selectivities and their alteration with H₂ pressure, change with particle size and with the type of pretreatment used. The generality of particle size effects reported in the literature is rendered uncertain by these results; the predominant factor underlying them may be the strength of H₂ chemisorption.     

In situ synthesis,mechanical and cyclic oxidation properties of Ti3AlC2/Al2O3 composites

ABSTRACT

Ti₃AlC₂/Al₂O₃ composite materials were successfully fabricated from TiO₂/TiC/Ti/Al powders by the in situ reactive hot pressed technique. The microstructure, mechanical and oxidation properties of the composites were investigated in the paper. Vickers hardness increased with the Al₂O₃ content. The relative density of Ti₃AlC₂/Al₂O₃ composites exhibits a declining tendency with Al₂O₃ content especially exceeds 10 vol.?%. The Ti₃AlC₂/Al₂O₃ composites show excellent electrical conductivity. The flexural strength and fracture toughness of Ti₃AlC₂/10 vol. % Al₂O₃ are 461 ± 20?MPa and 6.2?±?0.2?MPa m^1/2, respectively. The cyclic oxidation behaviour of resistance of Ti₃AlC₂/10 vol. % Al₂O₃ composites at 800–1000°C generally obeys a parabolic law. The oxide scale of sample consists of a mass of α-Al₂O₃ and TiO₂, forming a dense and adhesive protect layer. The result indicates that the Al₂O₃ can greatly improve the oxidation resistance of Ti₃AlC₂.     

High performances of Pt-K/Al2O3 versus Pt-Ba/Al2O3 LNT catalysts in the simultaneous removal of NO x and soot

The potentiality of a Pt-K/Al₂O₃ catalyst in the simultaneous removal of particulate matter (soot) and NO _x is investigated in this work by means of Temperature Programmed Oxidation (TPO) experiments and Transient Response Method (TRM), and compared with Pt-Ba/Al₂O₃. The results point out the higher performances of K-based sample in the soot combustion as compared to the Ba-based catalyst, and similar behaviour in the NO _x -storage.     

Gold supported CeO2/Al2O3 catalysts for CO oxidation: influence of the ceria phase

A series of low loading gold supported ceria/alumina catalysts have been prepared by the deposition–precipitation method, varying the pH of the synthesis. The catalysts were characterised by means of XRD, TEM, S_BET, XRF and UV–Vis techniques, and their catalytic activity towards CO oxidation in the absence and in presence of water in the stream, were tested. It has been found that in this low loading gold catalysts, where the metallic particles are far away one from another and the oxygen transportation is not the limiting step of the reaction, the electronic properties of the ceria phase and the structure of the metal-support perimeter more than the diameter of the gold nanoparticles is the determinant factor in the catalytic performances of the solid.     

Synthesis of porous Al2O3/ZrO2 nanocomposites by chemical vapour deposition

Al₂O₃/ZrO₂ one-dimensional nanocomposite structures were synthesised by chemical vapour deposition using Al₂O₃ nanowires and a ZrCl₄ powder source at a temperature of 800?°C and a pressure of 130?Pa. The samples were characterised using X-ray diffraction, the scanning electron microscopy, the transmission electron microscopy, and N₂ adsorption–desorption. The results revealed that Al₂O₃/ZrO₂ composite nanowires coated with surface-embedded ZrO₂ nanocrystals were formed and that the ZrO₂ macroporous and mesoporous structures changed as the ZrO₂ deposition time increased. The pore structure and surface area were also elucidated from the N₂ adsorption–desorption measurements.     

Activity and stability of Cu/ZnO/Al2O3 catalyst promoted with B2O3 for methanol synthesis

The addition of B₂O₃ to a Cu/ZnO/Al₂O₃ catalyst increased the activity of the catalyst for methanol synthesis after an induction period during the reaction. The stability of the B₂O₃-containing Cu/ZnO/Al₂O₃ catalyst was greatly improved by the addition of a small amount of colloidal silica to the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mechanistic Insight in the Ethane Dehydrogenation Reaction over Cr/Al2O3 Catalysts

A Cr/Al₂O₃ alkane dehydrogenation catalyst exhibits a maximum in ethylene yield during an ethane dehydrogenation cycle. Isotopic labelling experiments with monolabelled ¹³C-ethane and deuterium were used to elucidate whether the initial activity increase could be due to formation of an active, larger hydrocarbon intermediate on the surface. The results strongly indicate that this is not the case, and instead point to a traditional reaction cycle involving adsorption of ethane to form an ethyl species, followed by desorption of ethene and hydrogen. Transient kinetic data suggest that ethane adsorption is the rate-determining step of reaction.     

Mechanical Sensitivity and Detonation Parameters of Aluminized Explosives

Experiments were performed to study the effect of the species particle size and structure of aluminized mixture samples on the sensitivity and detonation parameters of HMX, nitroguanidine, bis(2,2,2trinitroethyl)nitramine, and their mixtures with an Al powder with a mean particle size of 0.1 – 150 m. The addition of ultrafine Al to HMX and bis (2,2,2trinitroethyl)nitramine substantially increases the sensitivity to mechanical effects and decreases the detonation velocity. In compositions with nitroguanidine, the detonation velocity practically does not vary. For nitroguanidine, the width of the chemicalreaction zone and Chapman–Jouguet parameters were determined by recording the detonationpressure profiles. The pressure profiles for bis(2,2,2trinitroethyl)nitramine show that detonation decomposition can occur in two stages. A twopeak detonationwave structure was detected for mixtures of HMX with Al. Temperature measurements indicate that Al interacts with detonation products in the immediate proximity to the front. The highest temperature was recorded for compositions containing ultrafine aluminum and an aluminum dust.     

Deposition of CoO onto MoO3/Al2O3 hydrodesulfurization catalysts by solvent assisted spreading

Solvent assisted spreading of CoO over monolayer MoO₃/Al₂O₃ catalysts has been studied. CoCO₃ · Co(OH)₂ and CoCO₃ reacted with MoO₃/Al₂O₃ in water slurries. CoO deposition over MoO₃/Al₂O₃ extrudates was followed by EPMA. In the set of eleven MoO₃/Al₂O₃ catalysts, the amount of CoO adsorbed was roughly proportional to the surface area of MoO₃ monolayer. The adsorbed Co species efficiently enhanced the HDS activity.     

工业炸药的爆轰性能研究

用有机玻璃法测定了岩石膨化、煤矿膨化、铵梯以及乳化炸药的爆速和爆压，同时用VLWR爆轰程序对岩11石膨化硝铵和铵梯炸药的爆轰参数及C—J产物的平衡组成进行了计算。结果表明，工业炸药具有低爆速和低爆压的非理想爆轰特征，而理论计算值和实验值比较接近，获得了较好的结果。