Synthesis of Highly Loaded Highly Dispersed Nickel on Carbon Nanofibers by Homogeneous Deposition–Precipitation

Highly loaded (45 wt%) Ni on graphitic carbon nanofiber (diameter 50nm) catalysts were prepared by means of homogeneous deposition–precipitation (HDP) from an aqueous solution. The obtained Ni metal particles were small (9nm). This shows clearly that HDP can be used for the preparation of catalysts based on inert supports, like carbon nanofibers, which cannot form surface co-precipitates between the catalyst support and the metal precursor. The latter is known to be a crucial step in the preparation of metal-on-oxidic support catalysts. The oxygen-containing groups on the surface of the carbon nanofiber act as nucleation and anchoring sites in the deposition of highly dispersed Ni.     

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.     

催化剂制备与研究 预处理对钯炭催化剂及其在肉桂醛加氢中性能的影响

采用不同预处理方式对活性炭表面进行改性,并制备钯炭催化剂。对活性炭的比表面积、孔结构等物理性质和含氧官能团种类以及钯炭催化剂进行表征,探究预处理对活性炭表面物化性能产生的变化和对钯炭催化剂在肉桂醛加氢反应中催化活性的影响。结果表明,氧化预处理使活性炭的比表面积和孔容下降,使钯炭催化剂上的钯粒子尺寸相对更小;碱预处理有一定的扩孔作用,使钯炭催化剂上的钯粒子尺寸更大。在肉桂醛加氢反应中,钯粒子尺寸越小的催化剂催化活性更高,而钯粒子尺寸更大的催化剂上肉桂醇选择性更高。     

Preparation and Study of Pd Catalysts Supported on Activated Carbon Cloth (ACC) for Direct Synthesis of H2O2 from H2 and O2

Activated carbon cloths (ACCs) were used as supports for Pd catalysts. The catalyst preparation was carried out by the impregnation method using acidic solution of palladium dichloride (PdCl₂) as metal precursor. The effects of the oxidation state of the loaded metal, heat treatment of the catalysts in different atmosphere (H₂, air) at different temperatures and surface chemistry of the support on the catalyst characterizations and the catalytic activities were investigated. Wet oxidation of ACC was done by nitric acid in order to induce oxygen-containing surface functional groups. Surface chemistry of the support and oxidation state of the metallic phase was investigated by means of XPS, TPD, SEM, DTA and TGA tests. Direct synthesis of hydrogen peroxide from H₂ and O₂ was performed batch wise in a stainless steel autoclave. The reactions were conducted under high pressure (38 bar) at 0 °C and methanol was used as reaction medium. The direct synthesis results showed that the oxygen-containing surface functional groups increase the selectivity of the catalysts by reducing the rate of water production. Existence of the oxidized state of Pd (PdO) also makes the catalyst more selective than the corresponding zerovalent state (Pd0). PdO affected on selectivity by increasing the rate of H₂O₂ production and reducing the amount of production of water, simultaneously.     

气相氧化处理对纳米碳纤维载体及其负载钌催化剂性质的影响

采用气相氧化方法对纳米碳纤维(CNFs)的表面进行改性,利用X射线衍射(XRD)、N_2物理吸附、Boehm酸碱滴定、CO化学吸附和程序升温还原等手段表征了不同温度气相氧化处理的CNFs载体及相应的负载Ru催化剂,并考察了Ru/CNFs催化剂在山梨醇氢解制备低碳多元醇过程中的催化性能。结果表明。气相氧化预处理对CNFs的晶体结构影响较小,但是大大增加了CNFs比表面积和表面酸性含氧基团;经气相氧化处理的CNFs负载的Ru催化剂更易还原,Ru金属分散度增加。催化剂考评结果表明,载体的气相氧化处理对Ru/CNFs催化剂在山梨醇氢解过程中反应活性的提高不利,但有利于低碳多元醇产物选择性的提高。研究认为CNFs表面酸性含氧基团是决定Ru/CNFs催化剂在山梨醇氢解过程催化性能的主要因素。     

载体预处理对Pd/C催化剂催化性能的影响

研究了活性炭硝酸表面改性对以其为载体制备的负载钯催化剂性能的影响。利用表面官能团滴定、N₂物理吸附和扫描电镜对催化剂进行表征，以邻硝基氯苯催化加氢制备2，2'-二氯氢化偶氮苯反应为模型反应对催化剂的性能进行评价。结果表明，经过不同浓度的HNO₃处理，活性炭孔结构性能变化不大，但是活性炭表面酸性含氧基团的浓度有了较大程度的增长，为Pd金属粒子的沉积提供了大量的吸附位，提高了Pd金属的分散度，从而制得高活性的Pd/C催化剂。通过30%HNO₃ 60 ℃水浴中回流4 h处理的活性炭可以达到最佳效果，所制得催化剂的活性是以未经硝酸处理过的活性炭载体制备的催化剂活性的2.3倍。     

Carbon-based catalytic briquettes for the reduction of NO: Effect of H2SO4 and HNO3 carbon support treatment

The influence of treating carbon with sulphuric and nitric acids on the activity of a carbon-based briquette catalyst for NO reduction with NH₃ was examined in a fixed-bed reactor at low temperature (150 °C). The briquette catalysts were prepared from a low-rank coal and a commercial tar pitch. The active phase was impregnated from a suspension of ashes of coke petroleum by means of an equilibrium adsorption method. The catalytic behaviour of NO reduction over acid treated briquettes was found to vary with the surface characteristics of the carbon support. This suggests that the number of oxygen-containing sites as well as vanadium load and dispersion affect the reaction activity. In the presence of oxygen, the SCR activity is enhanced with a nitric acid treatment, activity is promoted by the presence of acidic surface groups such as carboxyl and lactone, which can help not only to create a reservoir of reactants on the catalysts surface but also to improve the dispersion or even increase the amount of vanadium loading. Therefore, the results of this study suggest that the formation of acidic sites on the surface is an important step for NO reduction with NH₃ over carbon-based catalysts. Additional techniques such as XPS and TPD to characterize the oxygen surface and those such as N₂ adsorption to characterize the textural properties were also used in this study.     

Controlled preparation of Pd/AC catalysts for hydrogenation reactions

A commercial activated carbon was submitted to different liquid oxidizing treatments in order to create a high density of carboxylic groups on the carbonaceous surface. Characterization of the modified activated carbon shows that it is possible to generate a large range in acidic group concentration without significant change of the textural properties. This ability allows us to introduce, by ion exchange, palladium cations up to 7.3 wt.%. This way of preparation, coupled to soft formaldehyde reduction, leads to highly dispersed metallic palladium catalysts. The resulting catalysts were tested in hydrogenation of cinnamaldehyde and exhibited a high hydrogenation efficiency compared to activities reported in the literature.     

Production of COX Free Hydrogen by Catalytic Decomposition of Methane Over Ni/HY Catalysts

In this present paper, we report catalytic decomposition of methane over Ni/HY catalysts, with varying Ni loading at 550 °C and atmospheric pressure. The relationships between catalyst performance and characterization of the fresh and used form of catalysts are discussed from the data obtained by scanning electron microscopy, X-ray diffraction analysis, temperature programmed reduction, O₂ pulse chemisorption and carbon elemental analyses. It is observed that, the catalytic activity of Ni/HY catalysts is high at initial stages and gradually decreased with time and finally deactivated completely. The yield of hydrogen and carbon nanofibers is strongly dependent on Ni loading. It is found that 20 wt% Ni/HY catalyst showed higher hydrogen yield over the other loadings.     

Methanol oxidation activities of Pt nanoparticles supported on microporous carbon with and without a graphitic shell

Platinum nanoparticles were prepared as catalysts supported on microporous amorphous carbon with and without a graphitic carbon shell. The electro-oxidation of methanol in acidic solutions at room temperature was used as a probe reaction to explore the effect of the carbon structure on catalysis. CO anodic stripping voltammetry and cyclic voltammetry both recorded enhanced performance for the catalyst supported on the carbon with a graphitic shell. Some rationalizations of the possible roles of the graphitic shell are provided.     

Preparation, characterization and growth mechanism of platelet carbon nanofibers

The synthesis of platelet carbon nanofibers (PCNFs) on a silicon substrate using chemical vapor deposition method is reported. Scanning electron microscope, high-resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the nanofibers. It is found that these platelet nanofibers are of the order of 10 μm long, and most have a nearly rectangular transverse section with several hundreds nm wide and several tens of nm thick. Structure analysis reveals that the carbon layers of platelet nanofibers are parallel to each other, and have a uniform (0 0 2) orientation that is perpendicular to the fiber axis. Many faults and nanodomain have been found in the nanofibers. It is suggested that the PCNF grow in tip growth mechanism by the precipitation of carbon from the side facet of catalyst flakes.     

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.     

炭载体前处理对Pt-Ru/C催化剂性能影响

为研究水蒸气处理后热处理对炭黑表面特性的影响,提高DMFC阳极催化剂的催化活性,利用先水蒸气处理后热处理的Vnlcan XC-72炭黑为载体制备Pt-Ru/C催化剂,与水蒸气处理的和未经处理的炭载体制备Pt-Ru/C催化剂的性能进行比较.采用XPS和BET测试了处理后的炭粉表面的含氧浓度和比表面,结果表明：水蒸气处理后,炭载体比表面积增大,含氧浓度降低;水蒸气处理后热处理,炭载体比表面积进一步减小,含氧浓度增加.用XRD对催化剂的结构进行了表征,结果表明：水蒸气处理后热处理的炭黑为载体制备Pt-Ru/C催化剂结晶状态良好,催化剂颗粒较小.在0.5mol/L CH3OH和0.5mol/L H2SO4混合溶液中,利用玻炭电极测试了循环伏安曲线和阶跃电位曲线,结果表明：用先水蒸气处理后热处理的炭粉为载体制备的催化剂比仅水蒸气处理和未经处理的炭粉为载体制备的催化剂的活性最高.     

Prediction of elastic properties of carbon fibers and CVI matrices

The mechanical properties of carbon materials are highly dependent upon nanostructure and orientation distribution of graphitic layer planes. A model of deformations based upon theory of elasticity for anisotropic solids is proposed. Then it is used for prediction of elastic modulus and Poisson coefficient from intrinsic elastic constants for particles and orientation distribution of graphitic planes. It was applied to carbon fibers and to carbon matrices produced via Chemical Vapor Deposition based techniques. The orientation distribution of graphitic planes was determined using the distribution of intensity of X-ray scattering I(?). The predictions were compared to Young’s moduli measured on single fibers and matrices deposited on single fibers (microcomposites). The results underline the key role played by the modulus for shear between the graphitic layer planes. Influence of graphitic layer Poisson coefficient and Young’s modulus and nanostructure parameters is discussed.     

Influence of the surface chemistry of multi-walled carbon nanotubes on their activity as ozonation catalysts

Multi-walled carbon nanotubes (MWCNTs) with different surface chemical properties were prepared by oxidative treatments with HNO₃, H₂O₂ and O₂ to introduce oxygen-containing surface groups and by thermal treatments for their selective removal. The texture and surface chemistry of the MWCNTs were characterized by nitrogen adsorption, temperature programmed desorption (TPD) and pH at the point of zero charge. A deconvolution procedure of the TPD spectra was used to quantify the oxygenated surface groups. These materials were used as catalysts for ozone decomposition, and for the ozonation of oxalic and oxamic acids. Generally, all these catalytic processes are favoured by carbon nanotubes with low acidic character. MWCNTs were shown to exhibit higher activity for the ozonation of oxalic and oxamic acids, compared to activated carbon. Successive experimental runs of oxalic acid removal carried out with a selected MWCNT sample show that the catalyst suffers some deactivation as a result of the introduction of oxygenated groups on the surface. Therefore, the effect of the surface chemistry is mainly observed for the fresh catalysts.     

The influence of oxidation on the texture and the number of oxygen-containing surface groups of carbon nanofibers

The effect of liquid-phase oxidation on the texture and surface properties of carbon nanofibers has been studied using XRD, TEM, SEM, N₂-physisorption, TGA-MS, XPS and acid-base titrations. Oxidation was performed by refluxing the nanofibers in HNO₃ and mixtures of HNO₃/H₂SO₄ for different times. The graphite-like structure of the treated fibers remained intact, however, the specific surface area and the pore volume increased with the severity of oxidation treatment. For the first time it is shown that the most predominant effect that gives rise to these textural modifications is the opening of the inner tubes of the fibers. Moreover, it is demonstrated that both the total oxygen content (O/C=0.02-0.07 at/at) as well as the number of acidic groups (1-3 nm⁻²) are a function of the type of oxidizing agent used and the treatment time. The total oxygen content of the oxidized samples turns out to be substantially higher than can be accommodated in the form of oxygen-containing groups at the exterior surface.     

Hydrogen Oxidation Catalyzed by Pt Supported on Carbon Nanofibers with Different Graphite Sheet Orientations

The kinetic study of hydrogen oxidation with or without the presence of CO has been used as a tool to study the relative oxygen and CO adsorption strength on Pt nanoparticles, which are important parameters for fuel cell catalysts. It was found that the activation energy, which is determined by the oxygen binding energy, is influenced by the CNF graphite sheet orientation, CNF oxygen groups and catalyst preparation method. A weaker bonding of oxygen was indicated for Pt nanoparticles supported on platelet compared to Pt on fishbone CNFs. Moreover, oxygen seemed to be more strongly bonded to Pt particles on CNFs prepared by deposition–precipitation compared to those prepared by incipient wetness impregnation and a metal-oxide colloid method. Enhanced CO-adsorption was indicated for Pt supported on carbon nanofibers with introduced oxygen groups.     

Effect of the Acidity of the Groups of Functionalized Silicas on the Direct Synthesis of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Three catalysts consisting of palladium supported on functionalized silica with different acid groups (namely aryl sulfonic, alkyl phosphonic and alkyl carboxylic groups) were prepared and tested in the direct synthesis of hydrogen peroxide in neutral methanol medium at an overall pressure of 5.0 MPa. These catalysts can produce hydrogen peroxide. In addition, the activity results indicated correlations among the acid strength of the acidic group on the supports, the proportion of high binding energy palladium species and the selectivity for hydrogen peroxide.     

以KMnO4改性炭黑为载体的Pd-Ni催化剂的制备和性能

以不同浓度的KMnO4溶液预处理的炭黑为载体，通过沉淀共还原法合成了3种Pd1Ni1/ACx (x=3,5,7)催化剂，并将3种催化剂与商业Pd/C催化剂进行了性能对比。用XPS、ICP、XRD和TEM对催化剂了进行表征。结果表明：Pd1Ni1/AC5的Pd负载量（质量分数，下同）最大（3.66%），Pd晶粒的平均粒径最小（4.71 nm），且均匀地分布在KMnO4氧化处理后的碳载体上，活性位点较多；XRD显示，3种Pd1Ni1/ACx催化剂中的Ni均以无定形存在。在电化学性能测试中，3种催化剂均表现出比商业Pd/C更好的电化学稳定性和存活率；其中，Pd1Ni1/AC5电化学活性表面积达62.21 m2/gPd，且乙醇催化活性为1797.85 A/gPd。