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ABSTRACT: In the present work, methanol oxidation reaction was investigated on Pt particles of various diameters on carbon-nanofibers and carbon-black supports with different surface-oxygen concentrations, aiming for a better understanding of the relationship between the catalyst properties and the electrochemical performance. The pre-synthesized Pt nanoparticles in ethylene glycol, prepared by the polyol method without using any capping agents, were deposited on different carbon supports. Removal of oxygen-groups from the carbon supports had profound positive effects on not only the Pt dispersion but also the specific activity. The edge structures on the stacked graphene sheets in the platelet carbon-nanofibers provided a strong interaction with the Pt particles, significantly reconstructing them in the process. Such reconstruction resulted in the formation of more plated Pt particles on the CNF than on the carbon-black and exposure of more Pt atoms with relatively high co-ordination numbers, and thereby higher specific activity. Owing to the combined advantages of optimum Pt particle diameter, an oxygen-free surface and the unique properties of CNFs, Pt supported on heat-treated CNFs exhibited a higher mass activity twice of that of its commercial counterpart.
Physical Chemistry Chemical Physics 02/2013; · 3.57 Impact Factor
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ABSTRACT: The impact of thermal treatment at various preparation stages of carbon supported Au/TiO2 catalysts prior to oxidation of CO in the presence and absence of hydrogen was studied. An increase in catalytic activity
for thermally treated samples due to a more ordered structure of TiO2 was observed. A reversible deactivation of the catalysts occurred in the absence of hydrogen. However, the activity was restored
at preferential CO oxidation conditions in presence of hydrogen.
KeywordsPROX–CO oxidation–Au/TiO2
–Structured catalyst–Carbon nanofibres
Topics in Catalysis 05/2012; 54(13):922-930. · 2.62 Impact Factor
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ABSTRACT: 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.
Topics in Catalysis 04/2012; 52(6):664-674. · 2.62 Impact Factor
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ABSTRACT: A series of Co–Ni catalysts, prepared from hydrotalcite (HT)-like materials by co-precipitation, has been studied for the
hydrogen production by ethanol steam reforming. The total metal loading was fixed at 40% and the Co–Ni composition was varied
(40–0, 30–10, 20–20, 10–30 and 0–40). The catalysts were characterized using X-ray diffraction, N2 physisorption, H2 chemisorption, temperature-programmed reduction, scanning transmission electron microscope and energy dispersive spectroscopy.
The results demonstrated that the particle size and reducibility of the Co–Ni catalysts are influenced by the degree of formation
of a HT-like structure, increasing with Co content. All the catalysts were active and stable at 575°C during the course of
ethanol steam reforming with a molar ratio of H2O:ethanol=3:1. The activity decreased in the order 30Co–10Ni>40Co~20Ni–20Co~10Co–30Ni>40Ni. The 40Ni catalyst displayed
the strongest resistance to deactivation, while all the Co-containing catalysts exhibited much higher activity than the 40Ni
catalyst. The hydrogen selectivities were high and similar among the catalysts, the highest yield of hydrogen was found over
the 30Co–10Ni catalyst. In general, the best catalytic performance is obtained with the 30Co–10Ni catalyst, in which Co and
Ni are intimately mixed and dispersed in the HT-derived support, as indicated by the STEM micrograph and complementary mapping
of Co, Ni, Al, Mg and O.
Topics in Catalysis 04/2012; 52(3):206-217. · 2.62 Impact Factor
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ABSTRACT: Concerning energy and environmental sustainability, it is appealing to produce hydrogen from sugars or sugar alcohols that are readily obtained from the hydrolysis of cellulosic biomass. Nevertheless, the conversion of such compounds for hydrogen production poses great technical challenges. In this paper, we report that hydrogen purity and yield can be significantly improved by integrating in situ CO(2) capture into the steam reforming reaction of the model compounds-glucose and sorbitol. The experimental assessment was conducted at a steam-to-carbon ratio of 1.8 for sorbitol and 6 for glucose from 450-625 °C. As predicted by thermodynamic analysis, combining CO(2) capture and reforming reactions at favorable operating conditions yielded very high purity hydrogen, for instance, 98.8 mol % from sorbitol and 99.9 mol % from glucose. However, there are trade-offs between hydrogen purity and yield in practice. The lower operating temperatures in the examined range helped to increase the hydrogen purity and reduce the CO content in the gas product, whereas a high hydrogen yield was more likely to be obtained at higher temperatures. Coupling CO(2) capture lowered the risk of coke formation during the steam reforming of glucose. Coke accumulated in the reactor for the sorption-enhanced steam reforming of glucose was mostly from the slow pyrolysis of glucose before it came into contact with the catalyst-acceptor bed. This problem may be solved by improving heat transfer or reconstructing the reactor, for instance, by using a fluidized-bed reactor.
ChemSusChem 02/2012; 5(3):587-95. · 6.83 Impact Factor
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Piotr Ochal,
Jose Luis Gomez de la Fuente,
Mikhail Tsypkin,
Frode Seland,
Svein Sunde,
Navaneethan Muthuswamy,
Magnus Rønning, De Chen,
Sergio Garcia,
Selim Alayoglu,
Bryan Eichhorn
Journal of electroanalytical chemistry 06/2011; 655(2):140-146. · 2.90 Impact Factor
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ABSTRACT: The hierarchically structured carbon nanofibers (CNFs)/carbon felt composites, in which CNFs were directly grown on the surface of microfibers in carbon felt, forming a CNF layer on a micrometer range that completely covers the microfiber surfaces, were tested as a novel support material for cobalt nanoparticles in the highly exothermic Fischer-Tropsch (F-T) synthesis. A compact, fixed-bed reactor, made of disks of such composite materials, offered the advantages of improved heat and mass transfer, relatively low pressure drop, and safe handling of immobilized CNFs. An efficient 3-D thermal conductive network in the composite provided a relatively uniform temperature profile, whereas the open structure of the CNF layer afforded an almost 100 % effectiveness of Co nanoparticles in the F-T synthesis in the fixed bed. The greatly improved mass and heat transport makes the compact reactor attractive for applications in the conversion of biomass, coal, and natural gas to liquids.
ChemSusChem 05/2011; 4(7):935-42. · 6.83 Impact Factor
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ABSTRACT: A platelet carbon nanofiber/carbon felt composite with a high surface area (>250 m2/g) is synthesized over a carbon felt supported Ni−Cu catalyst using ethane−hydrogen mixtures at 923 K. A uniform layer of carbon nanofibers is thus achieved via an in situ generation and dispersion of Ni−Cu nanoparticles induced by the surface reaction of hydrocarbon decomposition. This represents a cost-effective and flexible method without the requirement for presynthesis of uniformly distributed nanoparticles. Both thermodynamic analysis and experimental observation reveal that, in the initial growth period, formation of a molten phase of the nanosized Ni−Cu−C phase promotes the fragmentation of Ni−Cu parent particles of micrometer size. A unique octopus-like growth mode is observed during the formation of graphene layers from this liquid-like Ni−Cu−C system. A comparative study between Ni and Ni−Cu catalysts is performed. It is found that the addition of Cu in the Ni−Cu catalysts can enhance the fragmentation ability of the parent Ni−Cu particles when contacting with hydrocarbon. A treelike growth mode of carbon nanofibers grown on Ni−Cu catalysts is observed due to the further fragmentation of Ni−Cu particles during carbon nanofiber growth. In contrast to Ni−Cu catalysts, two different growth mechanisms on the Ni catalyst are found: a governing tip-growth mechanism by particles smaller than 50 nm and an octopus-like growth mechanism by larger particles (50−100 nm). The graphene sheet orientation in the carbon nanofibers depends on the composition of the metal particle; fishbone carbon nanofibers are obtained with carbon felt supported Ni catalysts, while platelet carbon nanofibers were obtained with Ni−Cu catalysts. The formation of platelet carbon nanofibers from the Ni−Cu catalysts is ascribed to a higher degree of interface wetting between the Ni−Cu particle and the produced graphene sheets.
12/2010;
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ABSTRACT: Co/γ-Al(2)O(3) catalysts with particle sizes in the range of 4-15 nm were investigated by isothermal hydrogenation (IH), temperature programmed hydrogenation (TPH), and steady-state isotopic transient kinetic analysis (SSITKA). Kinetic isotope effect experiments were used to probe possible mechanisms on Co/γ-Al(2)O(3) with different particle size. It was found that CO dissociated on Co/γ-Al(2)O(3) catalysts at 210 °C. The total amount of CO(2) formed following the dissociation depends on the cobalt crystal size. O-Co binding energy was found to be highly dependent on the Co metal particle size, whereas similar C-Co binding energy was found on catalysts with different Co particle size. Very strongly bonded carbon and oxygen surface species increased with decreasing particle size and acted as site blocking species in the methanation reaction. SSITKA experiments showed that the intrinsic activity (1/τ(CH(x))) remained constant as the particle size increased from 4 to 15 nm. The number of surface intermediates (N(CH(x))) increased with increasing particle size. The apparent activation energies were found similar for these catalysts, about 85 kJ/mol. D(2)-H(2) switches further confirmed that the particle size did not change the kinetically relevant steps in the reaction. The reactivity of the active sites on the 4 nm particles was the same as those on the 8, 11, and 15 nm particles, and only the number of total available surface active sites was less on the 4 nm particles than on the others.
Langmuir 11/2010; 26(21):16558-67. · 4.19 Impact Factor
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ChemSusChem 10/2010; 3(10):1169-71. · 6.83 Impact Factor
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ABSTRACT: In this work, a hierarchical multiscale modeling approach is demonstrated. Models at the atomic and molecular level, on Ni crystal, in catalyst pellets and reactor tubes in a steam methane reformer are included. The kinetics of steam reforming, including carbon formation on the supported Ni catalyst, was studied experimentally in a tapered element oscillating microbalance (TEOM) reactor at relevant industrial conditions. A predictive microkinetic model of steam reforming including filamentous carbon formation was developed. The activation energy and pre-exponential factor of each elementary step were estimated using the unity bond index−quadratic exponential potential (UBI−QEP) approach and transition-state theory, respectively. Only a few parameters in the model were refined based on the experimental results and DFT calculations. The hybrid kinetic model combining a traditional kinetic model and a microkinetic model was used in simulations to significantly reduce the computational load. Maps of the kinetic carbon potential in the catalyst pellets and tubular reformer were established at different operating conditions. It was found that intraparticle diffusion resistance increases the carbon potential. High carbon potentials were found near both the inlet and the outlet of the reactor.
09/2010;
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ABSTRACT: LaFeO3 perovskites were prepared by solution combustion method using Dextro d-(−) Levo-(+) tartaric acid as a complexing agent. Characterization by different techniques such as X-ray diffraction, Brunauer−Emmett−Teller surface area, and scanning electron microscopy revealed relatively small crystals of perovskites. A relatively high capacity of reversible oxygen storage (3 mmol/gcat) of LaFeO3 has been evidenced. High activity and high selectivity to synthesis gas make LaFeO3 attractive as a catalyst and oxygen carrier for methane partial oxidation by the chemical looping process. The study of the reaction rate as function of oxygen site coverage reveals a kinetic relevant step in methane partial oxidation involving a pair of surface oxygen and oxygen vacancy. Removal of lattice oxygen generated vacancy sites which increase the reaction rate at relatively high oxygen concentrations, while the availability of surface oxygen determines the reaction rate at relatively low surface oxygen concentrations. The surface adsorbed oxygen is highly active to complete combustion while the lattice oxygen is very selective to synthesis gas.
07/2010;
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ABSTRACT: The effect of the curvature of the carbon support material has been investigated by molecular dynamics
simulations of a Pt_(100) and a Ni_(100) cluster adsorbed on fishbone-like carbon nanofibers using the ReaxFF reactive
force field. Carbon nanocones both with and without hydrogen termination have been considered. Without
hydrogen termination, significant differences are found between adsorbed Pt and Ni clusters for the dependence
of the bond strain on the curvature. For instance, the support curvature does not seem to have an appreciable
effect on the interatomic distances in adsorbed Pt clusters, while increased bond strain is observed for Ni
clusters with increasing curvature. Since the bond length is related to a d-band shift as well as to a change
in the bond order, it is concluded that the catalytic performance of Ni clusters can be enhanced by optimizing
the curvature of the support material. In general, hydrogen termination attenuates the degree of metal-metal
bond strain.
The Journal of Physical Chemistry C 02/2010; 114:3522-3530. · 4.80 Impact Factor
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ABSTRACT: It was found that the Pt loading obtained by immobilization of colloidal Pt oxide particles on carbon nanofibers (CNFs) at a pH of 9−10 varied from 1.4 to 19.6 wt % (nominal loading 19.4 wt %). The amount of Pt deposited depended significantly on the CNF properties. The study targeted the identification of the determining carbon support properties for the successful preparation of Pt catalysts. CNFs, multiwall carbon nanotubes (MWNTs), and Vulcan XC-72R were utilized as supports. Platelet and fishbone CNFs with different surface areas, graphene layer stacking angles, and amount of surface oxygen groups were obtained by catalytic chemical vapor deposition. The carbon supports were thoroughly characterized by transmission electron microscopy (TEM), N2-adsorption measurements, X-ray diffraction (XRD), temperature-programmed oxidation (TPO), ζ-potential measurements, and X-ray photoelectron spectroscopy (XPS). Characterization of the deposited Pt particles by TEM revealed similar sizes but differences with respect to particle location. Based on TEM, BET, XRD, and XPS, a clear indication of the importance of surface defects and edge sites for successful immobilization of Pt oxide colloid particles was found for all carbon supports. From XPS a linear relationship was found between the fraction of species originating at a binding energy of 285.1 eV and the final Pt loading. These species can be sp3-hybridized carbon, defects, and/or dangling bonds (edge structure). The surface oxygen groups were found to have a decisive effect on the immobilization of Pt. Negative linear trends were found between the Pt loading obtained on CNFs and the O 1s/C 1s ratio and number of carboxylic groups determined from XPS. It is based on the results believed that the oxygen-free defect and edge structure can play a vital and important role in the preparation of more effective CNF-supported catalysts.
01/2010;
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Journal of Physical Chemistry A - J PHYS CHEM A. 01/2010; 114(11):3834-3844.
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ABSTRACT: Sorption enhanced steam reforming of ethanol (SESRE), featured by yielding high purity of H(2) from one single reaction unit, is a new reaction process with a great potential for realizing sustainable H(2) production. The potential of such process with a CaO-based acceptor has been assessed by thermodynamic analysis and experimental demonstration. As predicted, ethanol can be reformed at relatively low temperatures (500-600 degrees C), still yielding high-quality H(2). Another major advantage of coupling CO(2) capture to the reforming process is predicted to be low risk in carbon formation. The SESRE reaction was carried out over a mixture of hydrotalcite-like material derived Co-Ni catalysts (Co-Ni/HTls) and calcined dolomite with a steam to carbon (S/C) ratio of 3 and temperatures ranging from 500 to 650 degrees C. The chosen reaction system was able to yield H(2) with purity fairly close to the theoretical prediction. Particularly, the best result was obtained over 40Ni and 20Co-20Ni/HTls at 550 degrees C, where the product gas had composition of more than 99 mol % H(2), ca. 0.4 mol % CH(4), 0.1 mol % CO, and 0.2 mol % CO(2). Special emphasis was put on the effect of steam on the stability of the CO(2) acceptor during the SESRE reaction. Hydration of CaO in the acceptor did not cause appreciable induction period, even at the low operating temperatures. However, different from a test under dry atmosphere (CO(2)/argon), the acceptor showed rapid deactivation in a multicycle operation of SESRE. A similar deactivation tend was given by a comparative test in a steam/CO(2)/Ar atmosphere.
The Journal of Physical Chemistry A 10/2009; 114(11):3834-44. · 2.95 Impact Factor
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ABSTRACT: Carbon nanofibers (CNFs) are grown by catalytic CO disproportionation over ultrafine Fe3O4 catalyst at a hydrogen concentration of 0–29.17%, and the time-depending rates of CNFs growth are continuously monitored and the morphologies of the as-synthesized CNFs are analyzed. Increasing H2 concentration will lower CO dissociation energy and assist catalyst reconstruction so as to shorten the induction period and increase the growth rate of CNFs, but it will also increase the rate of catalyst deactivation because carbon hydrogasification is not possible and carbon diffusion in the catalyst particle is rate limiting. As a result of H2-induced catalyst reconstruction and carbon deposition, the morphology of the CNFs changes from twisty to helical and to straight and becomes less entangled when the H2 concentration is increased. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering 05/2009; 4(5):590 - 595. · 0.76 Impact Factor
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ABSTRACT: Carbon nanofibers (CNFs) were immobilized on carbon microfiber (CMF) felt by chemical vapor deposition with Ni catalysts. A carbon coating, which was derived from a viscoid phenolic resin thin film involved with Ni salts, was introduced as a protect layer to stabilize the Ni nanoparticles for CNF growth, and as an interface to enhance the interaction between the CNFs and the CMFs, thus improving the uniformity and anchorage of the CNFs on the felt. The uniformly dispersed Ni nanoparticles in the carbon coating homogenously covering the CMF surfaces have resulted in a uniform layer of grown CNFs throughout the felt. The interlocked network of the entangled CNFs combined with possible penetration of the roots of CNFs into the carbon coating has enhanced the anchorage of CNFs on the surface of CMFs.
01/2009;
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Microporous and Mesoporous Materials 01/2009; 126:152. · 3.29 Impact Factor
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ABSTRACT: Molecular dynamics simulations have been performed using a Reax force field for C/H/Ni systems to study the structural changes of an Ni100 cluster adsorbed on a carbon platelet. Three different edges of a carbon platelet are considered. We find a complete restructuring of the initial structure of the Ni100 clusters adsorbed on the armchair and zigzag edges. Nonetheless, the mean Ni−Ni bond length hardly changes. Several preferential sites on each of the graphite edges are identified. Diffusion of the entire cluster is found both for adsorption on the basal plane and for binding to a hydrogen terminated graphite edge.
The Journal of Physical Chemistry C 07/2008; 112:12663-12668. · 4.80 Impact Factor
