title: Ni/SiO2 promoted growth of carbon nanofibers from chlorobenzene: characterization of the active metal sites.
abstract: The temporal changes to supported Ni sites during the growth of graphitic carbon nanofibers (GCNs) via the decomposition of chlorobenzene over Ni/SiO2 at 873 K have been investigated. The reaction of chlorobenzene with hydrogen also generated benzene, via catalytic hydrodechlorination, as the principal competing reaction. Reaction selectivity was found to be time dependent with a switch from a preferential hydrodechlorination to a predominant decomposition that generated an increasingly more structured carbon product over prolonged time-on-stream. These findings are discussed in terms of Cl/catalyst interaction(s) leading to metal site restructuring, the latter manifest in a sintering and faceting of the Ni metal particles. The pressure exerted on the metal/support interface due to fiber formation was of sufficient magnitude to extract the Ni particle from the support; the occurrence of an entrapped Ni particle at the fiber tip is a feature common to the majority of GCNs with the incorporation of Ni fragments along the length of the GCN. Metal site restructuring has been probed by temperature-programmed reduction of the passivated samples, H2 chemisorption/temperature-programmed desorption (TPD) and XANES/EXAFS analyses. This restructuring serves to enhance destructive chemisorption and/or facilitate carbon diffusion to generate the resultant GCN. The nature of the carbonaceous product has been characterized by a combination of TEM-EDX, SEM, XRD and temperature-programmed oxidation (TPO).
authors.affiliation: Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, UK. m.a.keane@hw.ac.uk
authors.forename: Mark A
authors.initials: MA
authors.lastname: Keane
authors.forename: Gary
authors.initials: G
authors.lastname: Jacobs
authors.forename: Patricia M
authors.initials: PM
authors.lastname: Patterson
mesh_terms: D000327:Adsorption; D002244:Carbon / Q000737:chemistry*; D000077482:Carbon Fiber; D002384:Catalysis; D002722:Chlorobenzenes / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D008855:Microscopy, Electron, Scanning / Q000379:methods; D046529:Microscopy, Electron, Transmission / Q000379:methods; D049329:Nanostructures / Q000737:chemistry*; D009532:Nickel / Q000737:chemistry*; D010316:Particle Size; D012680:Sensitivity and Specificity; D012822:Silicon Dioxide / Q000737:chemistry*; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D000077482:Carbon Fiber; D002722:Chlorobenzenes; D002244:Carbon; D012822:Silicon Dioxide; D009532:Nickel; D006859:Hydrogen; C031294:chlorobenzene