Reduction of nickel oxide on alpha alumina: coalescence of nickel to metallic crystallites

Reduction of alpha-alumina-supported nickel oxide was studied with hydrogen consumption, magnetization and XRD measurements. Rupture of Ni-O bonds at 270–350 ° C is much faster than nucleation of metallic nickel and precedes growth into crystallites. Water vapor and low hydrogen flows retard both processes. At higher temperatures, growth is more rapid than Ni-O bond rupture.     

氢气高温还原制备的负载型镍基催化剂用于苯酚加氢的性能

通过等体积浸渍法分别将Ni(NO₃)₂、NiCl₂、NiSO₄ 3种镍前体浸渍于A1₂O₃或SiO₂载体上,然后通过H₂高温还原法制备了负载型镍基催化剂,考察了镍前体、载体种类、镍负载量、反应条件等对镍基催化剂苯酚加氢性能的影响。结果表明,对比3种镍前体,在H₂高温还原体系中Ni(NO₃)₂最容易被还原,制备的镍基催化剂苯酚加氢活性最高。SiO₂负载的镍基催化剂活性远高于γ-Al₂O₃催化剂。适宜的Ni负载量有助于活性组分的分散和催化活性的提高。镍基催化剂的苯酚加氢产物以环己醇为主,相对缓和的反应条件更容易生成环己酮。在非极性溶剂正庚烷或环己烷存在下,苯酚加氢反应速率远远高于极性溶剂水或乙醇存在下的结果,而且环己酮的选择性更高。     

Influence of alumina binders on adhesion and cohesion during preparation of Cu‐SAPO‐34/monolith catalysts

The adhesion and cohesion between the coating layers and the ceramic honeycombs are usually one of the key issues in the preparation of high‐performance zeolite‐based/Monolith catalysts. In this work, we investigate the deposition of high‐efficiency Cu‐SAPO‐34 catalyst on a cordierite monolith with special focus on the impact of alumina binders on the structure, mechanical adhesion and cohesion, and catalytic performance of the monolithic catalyst. Two kinds of alumina nanoparticles, α‐alumina and γ‐alumina, have no significant impacts on the Cu‐SAPO‐34 crystal structure, micropore morphology and catalytic activity. But washcoating experiments showed that the mass loss rate of the coating was less and the loading of the catalyst was higher when α‐alumina was used as the binder. The reason of improving the adhesion and cohesion strength is the suitable thermal shrinkage, the uniform distribution and densification structure of α‐alumina. Furthermore, optimization of the coating formulation with α‐alumina as the binder has been performed. Finally, the catalyst prepared under the optimal conditions was tested by SCR and a maximum conversion rate of 97.4% was obtained. It was concluded that the use of the washcoating method with α‐alumina does not affect catalytic performance while it increases bonding strength between substrate and catalysts.     

The selective hydrogenation of acetonitrile on supported nickel catalysts

The catalytic behavior of several supported nickel catalysts in the hydrogenation of acetonitrile was studied. It was established that the selectivity of this process is greatly influenced by the nature of the support used. Catalysts consisting of nickel supported on acidic supports catalyzed the formation of condensation products, diethyl- and triethylamine. Nickel supported on basic supports was highly selective with respect to the formation of the primary amine, ethylamine. It was shown that modification of the intrinsic acidity of alumina-based supports by the application of alkaline additives has a large impact on the selectivity of the resulting catalyst. Based on the results obtained from measurements on a basic catalyst diluted with either an acidic or a basic support, a dual-function mechanism is suggested. The mechanism implies that the hydrogenation function of the catalyst is located on the metal, while the acid function, responsible for the condensation reactions, is located on the support. A mechanism, accounting for the occurrence of the acid-catalyzed condensation reactions, is proposed.     

Control of the product distribution in the hydrogenation of vegetable oils over nickel on silica catalysts

Several nickel on silica catalysts, prepared by impregnation or precipitation/deposition, and a commercial catalyst were tested for activity and selectivity in the sunflower seed oil hydrogenation. An average turn-over frequency of 2.57 s^?1 was found for the catalysts, assuming inaccessibility of nickel in pores smaller than 2 nm and a constant nickel surface concentration poisoned by the reaction mixture. After studying the mass-transfer steps, the effect of temperature (373-453 K) and pressure (101-608 kPa) on reaction rates in the kinetic regime was analyzed, and the corresponding apparent activation energies and reaction orders were obtained. Conclusions on the effect of temperature and pressure on the selectivity to the preferential hydrogenation of polyunsaturates (S_o) and to the formation of trans-isomers ((S_trans)₀) in the kinetic regime were derived from the results. Finally, a similar analysis was carried out when diffusion limitations were known to be present.     

The effect of sodium on the methanation activity of nickel/alumina coprecipitated catalysts

The influence of sodium impurities on the methanation activity of coprecipitated Ni/Al₂O₃, catalysts was investigated. The specific methanation activity was found to decrease substantially upon addition of relatively small quantities of sodium ions, whereas the nickel surface area decreased only slowly with increasing sodium content.     

Deactivation of fixed-bed nickel hydrogenation catalysts by sulfur

A series of fixed-bed nickel hydrogenation catalysts was tested in the dearomatization of hydrocarbon solvents. The mechanism of catalyst deactivation by aromatic sulfur compounds was studied in high-pressure micro-flow equipment by variation of the experimental conditions and the sulfur content of the feed. It is concluded that catalyst deactivation proceeds under mild but realistic conditions through formation of a surface sulfide which blocks the active surface. The rate of the disappearance of the active sites is a first-order process with rate constant 1.0 × 10⁻³ (ppm S)⁻¹ h⁻¹. Under more severe conditions, more sulfide layers are formed, but bulk Ni₃S₂ was not observed even after full deactivation of the catalysts. The poisoning of the active sites in the latter case is no longer a first-order process. Consequently, under the circumstances investigated, the sulfur resistance of nickel catalysts is determined by the nickel surface area per unit weight of catalyst.     

碳酸钠对镍渣碳热还原的催化作用

以全铁含量39.40wt%和SiO2含量32.50wt%的镍渣为原料,针对其中的铁以铁橄榄石形式存在难以直接还原磁选提铁的问题,在镍渣中添加不同质量比的碳酸钠促进镍渣碳热还原,进行了热力学计算和实验验证。结果表明,碳酸钠添加量由0增加至6wt%时,还原产物中铁的金属化率和回收率不断增大,继续增大碳酸钠添加量至8wt%时,铁的金属化率和回收率略有减小。不添加碳酸钠的还原产物中铁粒径很小,平均粒径为6 ?m,难还原的铁橄榄石大量存在,而加入6wt%碳酸钠的还原产物中铁粒径粗大,平均粒径增大至17 ?m,铁橄榄石含量明显降低,金属铁的XRD衍射峰强度明显增加。     

Effect of fluorination of alumina support on activity of platinum catalysts for complete oxidation of benzene

Platinum supported on fluorinated alumina is more active for the total oxidation of benzene than is the catalyst with the same Pt loading supported on hydrophilic unfluorinated alumina. The Pt-F/alumina catalyst contains well-dispersed small Pt particles, in contrast to Pt/alumina. The high dispersion is a consequence of a strong metal-support interaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

覆炭载体及镍覆炭催化剂的积炭行为研究

用恒温热重法，以正己烷脱氢反应为指示反应，研究了γ-Al2O3覆炭载体（CCA0，γ-Al2O3，Ni/Al2O3及Ni/CCA催化剂的抗结焦性能；考察了覆炭原料，覆炭量对CCA的抗结焦性能的影响及积炭温度，正己烷浓度和覆炭量对Ni/CCA催化剂的抗结焦性能的影响。结果，CCA，Ni/CCA催化剂的抗结焦性能优于γ-Al2O3，Ni/Al2O3，以环烷烃为覆炭原料得到的CCA的抗结焦性能最好。     

Catalytic oxidation of CO over Ag-Co/alumina catalysts

Low-loading silver cobalt catalysts on alumina for the catalytic oxidation of carbon monoxide were synthesized following different preparation methods: the single-step sol-gel method and the impregnation method. A catalyst synthesized by the single-step sol-gel method gave high surface area values. The activities of alumina-supported silver-cobalt catalysts were studied to obtain the effects of the calcination temperature, metal loading, and preparation methods on CO oxidation at low temperature. The catalysts prepared by impregnation metals on the alumina synthesized by the sol-gel method gave the best activity for the CO oxidation. Catalytic activity conversion of (5 wt% Ag-5 wt% Co)/Al 2 O 3 catalysts prepared by the sol-gel-impregnation method reached 100% at 200°C. Increasing silver loading over the catalysts gave rise to more active catalysts. The sol-gel catalysts had poor activity at low temperature.     

Stability of metal nanoparticles formed during reduction of alumina supported nickel and cobalt catalysts

The formation of nickel and cobalt nanoparticles in hydrogenation catalysts and their stability against sintering during the reduction of the oxidic precursors were investigated. The morphology of the catalysts was manipulated by varying the reduction conditions. The catalysts were characterized using temperature programmed reduction (TPR), hydrogen chemisorption, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HREM). The transformation of the oxidic precursor into the active phase was monitored using quasi in situ HREM, which proved to be an excellent technique to visualize the formation of metal nanoparticles. For the nickel catalyst the reduction temperature plays a crucial role, whereas time is more critical for the cobalt catalyst. The sintering rate of cobalt is considerably lower than that of nickel during reduction. It is concluded that the activation energy for sintering is significantly higher for nickel than for cobalt. A model is proposed which depicts the structure of both types of catalysts in their oxidic and reduced state. TPR and XPS results indicate that the passivated catalysts contain approximately two oxygen atoms per surface Ni or Co atom.     

磷对原位晶化催化剂抗镍性能的影响

考察磷元素对原位晶化催化剂抗镍性能的影响,借助等温N₂吸附-脱附、IR、XRD、MAT等分析表征磷改性后的催化剂样品。在固定流化床装置上评价催化剂反应性能。结果表明,采用浸渍法将磷负载于催化剂上,并对催化剂进行一定量的镍污染后,随着磷的质量分数的增加,催化剂的微反活性降低,重油转化率下降,C₅汽油收率降低,未表现出良好的抗镍性能。     

A study of the promoting effect of noble metal addition on niobia and niobia alumina catalysts

The catalytic activity of Nb₂O₅ and Nb₂O₅/Al₂O₃-supported metal catalysts was evaluated in the n-heptane conversion, CO hydrogenation and butadiene hydrogenation. After high temperature of reduction (HTR), the metal adsorption capacity decreases on all the samples, due to the reduction of Nb₂O₅ with subsequent blocking of metal atoms and bimetallic effect.

It was also observed that the activity decay caused by metal-support interaction was remarkably inhibited on the bimetallics with respect to the monometallics by comparing reaction rates after HTR. Thus, the addition of Rh to Co, Cu to Pd and Sn to Pt on niobia catalysts significantly altered the product distribution in Fischer–Tropsch synthesis (FTS) and in the hydrogenation and dehydrogenation of hydrocarbons, respectively. In addition, an unusual bifunctional effect was obtained in Pt/Nb₂O₅/Al₂O₃ catalyst.     

Pretreatment effects and NO x decomposition on alumina supported Rh/MoO3 catalysts

Rh-MoO₃/Al₂O₃, Rh/Al₂O₃ and MoO₃/Al₂O₃ catalysts have been prepared and subjected to various pretreatments including high temperature reduction, high temperature oxidation and oxidation/ reduction cycles. After each series of treatments, surfaces were analysed by XPS and FTIR of adsorbed NO. The effectiveness of these surfaces in dissociating NO was studied by TPRS in the temperature range 300–773 K. Reduction rates of Mo oxides in H₂ were determined gravimetrically while rhodium dispersion was determined from H₂ adsorption isotherms at 298 K. Results indicate that molybdenum and rhodium exist in close contact in both oxidised and reduced forms. H₂ chemisorption was suppressed following HTR of the catalyst due to coverage of rhodium by molybdenum oxide species but this could be reversed by HTO (773 K) followed by low temperature reduction. Although a small proportion of Mo could be reduced following HTR, cycles of HTR/HTO produced Rh/Mo oxide phases in which a proportion of Mo could be reduced to Mo° at 473 K. The presence of reduced Mo would appear to play an important role in the improved performance of Rh-MoO₃/Al₂O₃ catalysts.     

Activity during reduction of NO by CO over bimetallic palladium-rhodium/silica catalysts

A study of the activity of bimetallic Pd-Rh catalysts supported on silica in the reduction of NO by CO is presented. The catalysts were prepared by three different methods: (1) Pd and Rh were coimpregnated on the support, (2) Rh was impregnated first and, after calcining, the sample was impregnated with Pd, (3) the monometallic Pd and Rh catalysts were physically mixed. The results showed that the activity of the catalysts prepared by coimpregnation was much lower than that of the other two catalysts.     

Surface area reduction during the sintering of alumina

Surface area reduction trajectories of two well characterized calcined alumina powders were analyzed over a range of thermal profiles. Surface area reduction trajectories were confirmed to be independent of initial green density. Variations in trajectory were explained through experimental demonstrations of blended coarse and fine particle alumina systems. This work demonstrates that (1) nanoparticles do not appear to assist conventional sintering, because the nanoparticle surface area drops precipitously with minimal increase in density; and (2) the initial compact density does not contribute, nor control, surface area reduction during alumina sintering.     

Role of supported metals in the selective reduction of nitrogen monoxide with hydrocarbons over metal/alumina catalysts

The promoting effect of supported metals on alumina catalyst was investigated for the reduction of nitrogen monoxide in oxygen-rich atmospheres. For NO reduction with propene over impregnated CoO/A1₂O₃, the first reaction step was found to be the oxidation of NO to NO₂ probably catalyzed by dispersed cobalt species. The next reaction step, which is the reaction of NO₂ with propene to form N₂, was considered to take place on the alumina surface. Although the activity of impregnated FeO/A1₂O₃ was low because of the presence of large iron oxide particles catalyzing propene oxidation with dioxygen, FeO/A1₂O₃ prepared with sol-gel method showed excellent deNO_x activity.     

钴钼催化剂中钼的流失研究

本文对硫化态钴钼催化剂中活性组分钼的物理升华及化学升华流失的机理及动力学进行了研究。结果表明, 水蒸汽的存在会大大加快钼的化学升华流失。催化剂中钼含量及载气空速也会影响钼的流失速率。得出了钼升华流失的动力学方程。     

载镍金属有序介孔氧化铝催化一氧化碳 甲烷化反应性能研究

采用传统浸渍法制备出两种负载镍金属有序介孔氧化铝,并采用BET法比表面积测定、X射线衍射 （XRD）、程序升温还原（TPR）、X射线光电子能谱分析（XPS）、透射电镜（TEM）、电感耦合等离子体发射光谱仪（ICP-OES）等多种测试技术对合成的催化剂进行表征。通过一氧化碳甲烷化反应测定其催化活性并与负载镍金属普通氧化铝进行对比。实验结果表明,镍在两种有序介孔氧化铝上主要呈两种形态,即氧化镍和偏铝酸镍。合成的介孔氧化铝比普通氧化铝镍负载量大且分散性好,这主要归结于其具有的高比表面积（300~500 m²/g）及窄孔径分布（4~ 10 nm）的稳定有序结构。催化结果表明：由于具有较高的镍负载量及较好的镍金属分散性能,因而负载镍金属有序介孔氧化铝比普通氧化铝具有较好的催化活性。