柳树叶提取液还原制备水滑石负载纳米钯催化剂及催化Suzuki反应

以柳树树叶提取液作为还原剂和稳定剂,制备了水滑石负载纳米钯催化剂(Pd/HT)。采用IR、XRD、TEM、N-2吸附脱附等手段对制备的催化剂进行了表征,并研究了其催化Suzuki偶联反应的性能。表征结果显示:制备的Pd/HT中钯纳米粒子分散均匀,粒径分布较窄,平均粒径为3.8nm。提取液中的生物质不仅起到还原作用,而且还起到稳定钯纳米粒子的作用。催化Suzuki偶联反应结果表明:Pd/HT表现出优良的催化活性,重复使用5次,催化活性没有明显降低。     

成型TiO_2负载型纳米碳纤维催化材料的制备与表征

采用成型TiO2为载体,以甲烷为碳源,镍铜双金属催化剂,改变反应温度以及碳氢比(CH4/H2摩尔比),生长纳米碳纤维(CNF),制备出结构化复合纳米碳纤维催化材料-生长在成型TiO2上的纳米碳纤维材料(CNF/TiO2).扫描电镜(SEM)和物理吸附仪(BET)表征结果表明,CNF粗细均匀、直径～70 nm,而且与其他传统催化剂载体(活性炭)相比几乎没有微孔.并以CNF/TiO2为载体,采用浸渍法负载金属钯,制备出结构化纳米碳纤维负载型钯催化剂(Pd/CNF/TiO2),以苯乙烯加氢为模型反应进行活性评价,结果表明,其催化活性明显优于成型活性炭负载型Pd催化荆.结构化纳米碳纤维具有比表面适中,且不含微孔,是一种优良的催化剂载体,可望用于受内扩散制约的气液固三相催化反应.     

N-掺杂碳材料负载钯纳米催化剂的制备及其催化活性应用研究

研究了以3-MBP-dca(3-甲基-1-丁基吡啶二氰胺)离子溶液做前体,用煅烧法合成负载在SiO2颗粒表面的N-掺杂碳材料,在超声条件下,以汉斯酯作为还原剂将二价钯离子通过原位还原为钯纳米粒子并负载到制备的碳材料上,通过XRD、XPS及HRTEM等方法进行表征。以苄醇选择性催化氧化为苯甲醛为模型反应,研究纳米催化剂的催化活性,实验结果表明,该催化剂对苄醇的氧化反应有良好的催化活性,且对带有不同供电子基团的反应底物都有较好的适用性。     

制备不同Pd空间分布的Pd/UiO-66催化剂用于Heck反应

在金属有机框架(MOFs)生长过程中,通过调节事先合成的钯纳米粒子(Pd NPs)的添加时间顺序,将Pd NPs封装在MOFs中的中间和外围位置,成功地实现了在MOFs结构中对Pd NPs空间分布的调控。将制备得到的不同Pd空间分布的Pd/UiO-66催化剂用于Heck反应中,研究发现封装在UiO-66外围位置的Pd NPs展现更好的催化活性,表明了UiO-66中Pd NPs的空间分布对Heck反应的催化性能具有很大的影响。     

PVP保护制备水滑石负载纳米Pd催化剂及催化Suzuki反应中的应用

以水滑石(HT)为载体,在PVP保护下,制备了可重复使用的负载型纳米钯催化剂(Pd/HT)。通过UV-Vis、XRD、TEM、XPS等分析手段,对制备催化剂的结构进行了表征。分析结果表明:制备的Pd/HT结构稳定,钯纳米粒子平均粒径4nm左右,且分散均匀。考察了碱、溶剂、反应时间、反应温度等条件对Pd/HT催化Suzuki反应结果的影响。实验结果表明:当Pd/HT催化剂Pd的物质的量用量只为反应底物的0.5‰时,催化不同底物偶联反应的收率最高可达98.84%,反映出很高的催化活性。     

超临界流体沉积法对钯铜纳米粒子制备的影响

以乙酰丙酮钯、乙酰丙酮铜为金属前驱体，以氧化铝球为载体，采用超临界流体沉积法（SCFD）制备氧化铝球负载的平均粒径≤10 nm的钯铜纳米粒子。采用高角度环形暗场-扫描透射电子显微镜（HAADF-STEM）、X射线衍射（XRD）和称重法对钯铜纳米粒子的负载状况、粒径分布进行表征。结果表明，沉积温度与沉积压力对钯铜纳米粒子的粒径有重要影响，在65 ℃、15 MPa时，钯铜纳米粒子的平均粒径可达2.37 nm。在金属前驱体投料量一定时，沉积时间存在最佳值。助溶剂的选择影响钯铜纳米粒子的平均粒径与粒径分布，在65 ℃、15 MPa下，使用8 mL二氯甲烷作为助溶剂，钯铜纳米粒子的平均粒径降至1.81 nm。随着Pd理论负载量的增加，钯铜纳米粒子的平均粒径先降后增，当Pd理论负载量为0.50%时，出现最小值1.81 nm。     

Pd-Pb/SiO_2催化高浓度乙烯基乙炔加氢合成丁二烯

研究了硅胶负载钯铅双金属催化剂催化高浓度乙烯基乙炔加氢合成丁二烯过程。结果表明,加入适量的铅可起到分隔钯纳米粒子,阻碍钯纳米粒子团聚的作用,从而提高催化剂的催化活性,最佳Pb/Pd摩尔比为0.2。继续提高Pb/Pd摩尔比时,会生成铅钯合金相,造成催化剂活性降低。X射线光电子能谱结果表明,催化剂的催化活性与Pd 3d的电子结合能呈正相关关系。制备催化剂过程中,还原温度对催化剂的结构和催化性能影响显著。在350℃下还原得到的催化剂中金属氧化物还原不彻底,催化剂活性较低;还原温度为450℃时,则会引起钯纳米粒子烧结,造成催化剂的催化活性和对丁二烯的选择性同时降低;催化剂的最佳还原温度为400℃。在40℃催化乙烯基乙炔反应40 h后,积炭造成催化剂的孔道堵塞,催化剂失活。因此,需要进一步开展改善催化剂的抗积炭能力和使用寿命方面的研究。     

Pd-Pb/SiO2催化高浓度乙烯基乙炔加氢合成丁二烯

研究了硅胶负载钯铅双金属催化剂催化高浓度乙烯基乙炔加氢合成丁二烯过程。结果表明,加入适量的铅可起到分隔钯纳米粒子,阻碍钯纳米粒子团聚的作用,从而提高催化剂的催化活性,最佳Pb/Pd摩尔比为0.2。继续提高Pb/Pd摩尔比时,会生成铅钯合金相,造成催化剂活性降低。X射线光电子能谱结果表明,催化剂的催化活性与Pd 3d的电子结合能呈正相关关系。制备催化剂过程中,还原温度对催化剂的结构和催化性能影响显著。在350℃下还原得到的催化剂中金属氧化物还原不彻底,催化剂活性较低;还原温度为450℃时,则会引起钯纳米粒子烧结,造成催化剂的催化活性和对丁二烯的选择性同时降低;催化剂的最佳还原温度为400℃。在40℃催化乙烯基乙炔反应40 h后,积炭造成催化剂的孔道堵塞,催化剂失活。因此,需要进一步开展改善催化剂的抗积炭能力和使用寿命方面的研究。     

两种负载型钯基催化剂在Heck反应中的性能比较

制备了碳负载钯催化剂(Pd/C)以及氨基改性二氧化硅负载钯(Pd/Si O2-NH2)催化剂。两种催化剂应用于以溴苯为模型反应的Heck反应中,对比了两种催化剂的反应活性以及重复使用率。研究表明,Pd/C催化剂在Heck反应中有较高的催化活性,但是其重复使用寿命低于Pd/Si O2-NH2催化剂。     

氮化碳负载钯催化剂催化苯酚加氢制环己酮

为了开发苯酚加氢制环己酮高效催化剂,将脲在550 ℃高温聚合,制备了片层状氮化碳催化剂载体g-CN;负载钯纳米粒子后,得到Pd/g-CN催化剂。采用红外光谱、X射线粉末衍射、透射电镜和X射线光电子能谱对催化剂进行表征。将Pd/g-CN催化剂用于催化苯酚水相加氢,考察了不同载体和反应温度对催化性能的影响,并对催化剂重复使用性能进行研究。结果表明,载体g-CN含有大量的含N基团,能有效稳定金属纳米粒子,从而获得粒径较小、分散较好的Pd纳米粒子;同时,g-CN具有较强碱性,有利于苯酚的吸附,可提高苯酚的反应速率和环己酮选择性。采用负载Pd质量分数2%的Pd/g-CN催化剂,在反应温度80 ℃、反应压力0.1 MPa、n(Pd)∶n(苯酚)=0.02、苯酚1 mmol、水3 mL和反应时间3 h条件下,苯酚可完全转化,环己酮选择性高达99%。Pd/g-CN催化剂制备工艺简单,原料价廉,催化性能优异。     

结构化纳米碳纤维载体的制备与应用

研究了通过气相沉积法裂解甲烷在TiO2表面生长纳米碳纤维层,制备具有中孔孔径结构的结构化纳米碳纤维的方法。利用SEM-EDS和BET对该载体进行了表征。结果发现,该结构化纳米碳纤维载体的纳米碳纤维层厚度1.5～2.0μm,比表面积60.3 m2/g,其中外表面积为51.1 m2/g,只有很少的内表面积;平均孔径为5nm。在肉桂醛加氢反应中,该载体负载Pd催化剂能明显降低内扩散对反应选择性的影响,肉桂醛转化率低于56%时,氢化肉桂醛选择性达98%,明显高于常规活性炭负载型Pd催化剂。     

Comparison of CNF and XC-72 carbon supported palladium electrocatalysts for magnesium air fuel cell

Palladium catalysts supported on carbon nanofibers (CNFs) and XC-72 carbon were developed by chemically reducing palladium chloride with ethylene glycol. The morphologies and crystal structure of the Pd/CNF catalyst and Pd/XC-72 catalyst were investigated by TEM and XRD, respectively. The electrocatalytical activity of the catalysts was examined via cyclic voltammetry testing techniques. The performance of the air electrodes was examined by linear polarization methods. Magnesium air fuel cells with Pd/CNF catalyst and Pd/XC-72 catalyst were fabricated and characterized. The results showed that the Pd/CNF catalyst had higher catalytic activity for the oxygen reduction reaction and achieved better performance of the magnesium air fuel cell compared with the Pd/XC-72 catalyst.     

Carbon nanofiber-supported palladium nanoparticles as potential recyclable catalysts for the Heck reaction

5 wt% Pd catalysts supported on platelet carbon nanofibers has been prepared by incipient wetness impregnation. Both the calcination and the reduction temperature have a significant effect on the dispersion of palladium and it was found that about 3 nm sized Pd nanoparticles can be obtained at a calcination and reduction temperature of 250 °C and 150 °C, respectively. Pd catalysts have been applied to catalyze Heck reactions of various activated and non-activated aryl substrates. The activity increased exponentially with a decrease in Pd particle size. The high surface area, mesoporous structure of carbon nanofiber and highly dispersed palladium species on carbon nanofibers makes up one of the most active and reusable heterogeneous catalysts for Heck coupling reactions. Pd nanoparticles supported on platelet CNFs appear to be an excellent catalyst due to high activity, low sensitivity towards oxygen, almost no or low issues with leaching and high stability in multi-cycles.     

Simultaneous production of hydrogen and carbon nanostructures by decomposition of propane and cyclohexane over alumina supported binary catalysts

Nano-scale, binary, 4.5 wt.% Fe–0.5 wt.% M (M = Pd, Mo or Ni) catalysts supported on alumina have been shown to be very effective for the decomposition of lower alkanes to produce hydrogen and carbon nanofibers or nanotubes. After pre-reduction at 700 °C, all three binary catalysts exhibited significantly lower propane decomposition temperatures and longer time-on-stream performances than either the non-metallic alumina support or 5 wt.% Fe/Al₂O₃. Catalytic decomposition of propane using all three catalysts yielded only hydrogen, methane, unreacted propane, and carbon nanotubes. Above 475 °C, hydrogen and methane were the only gaseous products. Catalytic decomposition of cyclohexane using the (4.5 wt.% Fe–0.5 wt.% Pd)/Al₂O₃ catalyst produced primarily hydrogen, benzene, and unreacted cyclohexane below 450 °C, but only hydrogen, methane, and carbon nanotubes above 500 °C. The carbon nanotubes exhibited two distinct forms depending on the reaction temperature. Above 600 °C, they were predominantly in form of multi-walled nanotubes with parallel walls in the form of concentric graphene sheets. At or below 500 °C, carbon nanofibers with capped and truncated stacked-cone structure were produced. At 625 °C, decomposition of cyclohexane produced a mixture of the two types of carbon nanostructures.     

Effect of surface acid groups associated with amorphous and structured carbon on the catalytic hydrodechlorination of chlorobenzenes

BACKGROUND: Catalytic hydrodechlorination (HDC) is a progressive approach to treating chlorinated waste streams. While carbon is widely used as a catalyst support, the influence of carbon surface functionality on HDC performance has not been established. This work sets out to assess the impact of surface acid groups associated with activated carbon (AC), graphite and graphitic nanofibers (GNF) on Pd promoted gas phase HDC of chlorobenzene (CB) and 1,3‐dichlorobenzene (DCB). RESULTS: The acid groups were introduced by HNO₃ washing and the HDC reaction performed over bulk Pd and Pd physically mixed with each carbon. The carbon was subjected to a thermal treatment to remove the surface acidity. Characterization was by temperature programmed decomposition (TPD), temperature programmed hydrogen treatment (TPH), BET area, acid‐base titration, scanning and transmission electron microscopy. TPD, TPH and titration analyses served to establish the presence of surface oxygen groups after acid washing and facilitated an evaluation of the effectiveness of the thermal treatment to remove these groups. CONCLUSIONS: The surface acid groups inhibited HDC activity, a response most pronounced for Pd + AC, less so for Pd + graphite, while the effect was slight for Pd + GNF. HDC inhibition is attributed to chloroarene interaction with the surface functional (notably carboxylic) groups that impedes HDC. Fractional dechlorination of DCB was equivalent to or lower than CB HDC; there is some evidence of DCB interactions with heat treated graphite and GNF that served to raise HDC activity. Effective HDC over carbon based catalysts requires removal of surface acid groups. Copyright © 2008 Society of Chemical Industry     

Promotional SMSI Effect on Supported Palladium Catalysts for Methanol Synthesis

High-temperature reduction (HTR) of palladium catalysts supported on some reducible oxides, such as Pd/CeO₂, and Pd/TiO₂ catalysts, led to a strong metal-support interaction (SMSI), which was found to be the main reason for their high and stable activity for methanol synthesis from hydrogenation of carbon dioxide. But low-temperature-reduced (LTR) catalysts exhibited high methane selectivity and were oxidized to PdO quickly in the same reaction. Besides palladium, platinum exhibited similar behavior for this reaction when supported on these reducible oxides. Mechanistic studies of the Pd/CeO₂ catalyst clarified the promotional role of the SMSI effect, and the spillover effect on the HTR Pd/CeO₂ catalyst. Carbon dioxide was decomposed on Ce₂O₃, which was attached to Pd, to form CO and surface oxygen species. The carbon monoxide formed was hydrogenated to methanol successively on the palladium surface while the surface oxygen species was hydrogenated to water by spillover hydrogen from the gas phase. A reaction model for the hydrogenation of carbon dioxide was suggested for both HTR and LTR Pd/CeO₂ catalysts. Methanol synthesis from syngas on the LTR or HTR Pd/CeO₂ catalysts was also conducted. Both alcohol and hydrocarbons were formed significantly on the HTR catalyst from syngas while methanol formed predominantly on the LTR catalyst. Characterization of these two catalysts elucidated the reaction performances.     

Palladium catalyzed formation of carbon nanofibers by plasma enhanced chemical vapor deposition

A dc plasma enhanced chemical vapor deposition process is used to obtain vertically aligned carbon nanofibers (CNFs) from palladium catalysts using an ammonia-acetylene process gas mixture. Transmission electron microscopy is used to elucidate the microstructure of the as-grown fibers revealing different growth anomalies such as a new secondary growth phenomenon which we term hybrid tip growth. Also included in our analysis are conventional tip growth derived structures. In a few instances, the conventional tip growth derived structures possess elongated catalyst particles that impart small cone angles to the carbon nanofiber microstructure. Detailed microchemical analysis reveals that hybrid tip grown CNFs using thick Pd films are partially filled with Pd. Analysis of these growth phenomenon and implications for potential use as on-chip interconnects are discussed.     

CO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-free hydrogen and carbon nanofibers production by methane decomposition over nickel-alumina catalysts

Nickel catalysts supported on mesoporous nanocrystalline gamma alumina with various nickel loadings were prepared and employed for thermocatalytic decomposition of methane into CO _x -free hydrogen and carbon nanofibers. The prepared catalysts with different nickel contents exhibited mesoporous structure with high surface area in the range of 121.3 to 66.2m²g^?1. Increasing in nickel content decreased the pore volume and increased the crystallite size. The catalytic results revealed that the nickel content and operating temperature both play important roles on the catalytic performance of the prepared catalysts. The results showed that increasing in reaction temperature increased the initial conversion of catalysts and significantly decreased the catalyst lifetime. Scanning electron microscopy (SEM) analysis of the spent catalysts evaluated at different temperatures revealed the formation of intertwined carbon filaments. The results showed that increasing in reaction temperature decreased the diameters of nanofibers and increased the formation of encapsulating carbon.     

Carbon Nanofibers: Catalytic Synthesis and Applications

Carbon nanofibers (diameter range, 3-100 nm; length range, 0.1-1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (~200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.     

Index to Volume 31

Carbon nanofibers (diameter range, 3–100 nm; length range, 0.1–1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (?200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.