The effect of liquid-phase oxidation on the texture and surface properties of carbon nanofibers has been studied using XRD, TEM, SEM, N 2-physisorption, TGA-MS, XPS and acid-base titrations. Oxidation was performed by refluxing the nanofibers in HNO 3 and mixtures of HNO 3 /H 2 SO 4 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 ¼ 0.02-0.07 at/at) as well as the number of acidic groups (1-3 nm À2) 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.

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Catalytically grown fishbone carbon nanofibers (CNF), are prepared by the decomposition of carbon-containing gases (CH 4 , CO/H 2 or C 2 H 4 /H 2) over a silica-supported nickel catalyst and an unsupported nickel catalyst at 550 • C. It turns out that both the nickel particle size and the nature of the carbon-containing gas significantly affects the CNF growth process. We demonstrate that at the chosen temperature small supported nickel particles need a carbon-containing gas with a relatively low reactivity, like CH 4 or CO/H 2 , to produce CNF. The resulting fishbone CNF have a uniform and small diameter (25 nm). The CNF thus synthesized hold great potential, e.g. as catalyst support material. However, the large unsupported nickel particles only produce CNF using a reactive carbon-containing gas, like C 2 H 4 /H 2. The CNF thus obtained show a variety of morphologies with a large range of diameters (50-500 nm). The CNF yield is a subtle interplay between the nickel particle size and consequently the exposed crystal planes on the one hand and the reactivity of the carbon-containing gas on the other.

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The metal particle size and structure of the metal-support interface of platinum supported on Vulcan XC-72 (a commercial catalyst used in platinum fuel-cell electrodes) and on carbon nanofibers (CNF) have been determined with extended X-ray absorption fine structure spectroscopy (EXAFS). The CNF-supported Pt catalysts were synthesized using a homogeneous deposition precipitation (HDP) method. The amount of acidic oxygen groups on the CNF surface was modified by treatment in an inert atmosphere at different temperatures. The average first shell Pt-Pt coordination number (∼5.5) detected in Pt/CNF is much smaller than for Pt/ Vulcan XC-72 (∼8.2). The presence of oxygen-containing groups in the CNF support most probably leads to the stabilization of small Pt particles on the CNF support. A prominent interaction between the metal particles and the support atoms was detected on both kinds of catalysts, which confirms that the metal is in direct contact with the carbon support atoms. After reduction, a long metal-carbon distance around 2.62 Å was detected in both Pt/Vulcan XC-72 and Pt/CNF. After evacuation of Pt/CNF at higher temperatures, the distance between support and interfacial metal atoms decreased to 2.02 Å. Therefore, the long metal-carbon support distance is ascribed to the presence of atomic chemisorbed hydrogen in the interface between the Pt particles and the carbon support. According to the number of interfacial Pt-C bonds (four), the platinum particles supported on CNF are proposed to be in contact with a prismatic surface of the carbon support, on which oxygen groups have more stable bonds with carbon atoms. Six Pt-C bonds could be detected in the metal-support interface of Pt/Vulcan XC-72 with an even longer carbon shell at 3.62 Å, indicating that the metal particles are located on a more carbon-rich surface. This supports a structural model in which the platinum metal particles are epitaxially grown on the (0001) basal surface plane of carbon graphite.

Carbon-nanofiber-supported ruthenium catalysts were employed to study the influence of oxygen-containing surface groups on catalytic performance in the liquid-phase hydrogenation of cinnamaldehyde. The carbon nanofibers were oxidized to introduce oxygen-containing groups and the metal precursor was applied using homogeneous deposition precipitation. After reduction the catalysts were heat-treated in nitrogen at different temperatures to tune the number of surface oxygen functional groups. TEM and chemisorption studies showed the presence of a narrow and stable particle-size distribution (1-2 nm) even after heat treatment at 973 K. The overall specific activity increased by a factor of 22 after treatment at 973 K, which is related to the decreasing number of oxygen-containing groups. The cinnamyl alcohol selectivity decreases from 48 to 8% due to enhanced rate of hydrocinnamaldehyde production with increasing heat treatment. This unambiguously demonstrates the metal-support interaction, which involves support surface-oxygen functionalities that affect the metal activity and selectivity. The precise nature of this interaction has yet to be elucidated.

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Carbon nanofiber (CNF) supported platinum catalysts have been prepared using Pt(NH 3 ) 4 (NO 3 ) 2 as a precursor by two different ion adsorption techniques, one at a constant pH and one in which the pH is gradually and homogeneously increased from 3 to 6 by hydrolysis of urea. The latter method resembles the procedure of homogeneous deposition precipitation (HDP). Characterization of the CNF support was performed by acidbase titration, thermogravimetric mass spectrometry, and X-ray photoelectron spectroscopy, and for the various platinum catalysts, transmission electron microscopy, H 2 -chemisorption, and X-ray fluorescence/inductively coupled plasma-atomic emission spectrometry were utilized. With both synthesis techniques from diluted precursor solutions homogeneously distributed, highly dispersed and thermally stable metal particles were obtained with an average particle size of 1-2 nm. With the HDP method for the Pt/CNF catalysts, a linear relationship between the number of acidic oxygen-containing groups on the surface of activated CNF and the metal loading has been found. For the highest loaded catalyst, a platinum/adsorption site ratio of 0.5 was established, corresponding to about 0.7 Pt(NH 3 ) 4 2+ molecules/nm 2 . Furthermore, it has been established that with this procedure higher platinum loadings (∼4 wt %) can be achieved than with the ion adsorption procedure (<2 wt %). The HDP method using RuNO(NO 3 ) 3 (H 2 O) 2 also turned out to be suitable for the preparation of small (1-2 nm) uniform ruthenium particles on CNF with a high thermostability.