Catalytic filamentous carbon as supports for nickel catalysts
Introduction
The last decade showed an increasing fundamental and practical interest in carbon nanotubes and nanofibers. Among its potential practical applications are adsorbents for purification, magnetic and sensor materials [1], [2], [3]. However, the use of these materials as catalyst supports seems the most promising [4], [5], [6], [7], [8].
Catalytic filamentous carbon (CFC) is a new type of carbon materials. It is synthesized by decomposition of hydrocarbons over catalysts based on the iron group metals (Ni, Co, Fe) and alloys (Ni–Cu, Fe–Co, Fe–Ni) [9], [10], [11], [12], [13], [14]. Ni–Al2O3, Ni–Cu–Al2O3, Co–Al2O3 and Fe–Co–Al2O3 catalysts with high metal loadings studied before are capable of accumulating filamentous carbon in huge amounts (up to 500 g/gcat) during methane decomposition at 500–675 °C [12], [13], [14], [15]. CFC does not deactivate the catalysts for a long time due to its filamentous morphology.
Microstructural and textural properties of CFC are affected by the chemical nature of the metal catalyst and the methane decomposition conditions. The term microstructural properties means the structure of a CFC filament formed by graphite-like layers oriented in certain manner along the axis.
Transmission electron microscopy (TEM) studies demonstrate that the filaments grown on the Ni–Al2O3 catalyst during decomposition of methane are ≈50 nm in diameter and consist of coaxial cone-shaped graphitic planes. Graphene layers are oriented at an angle (α) ≈45° with respect to the fiber axis [12]. The filaments of Co–Al2O3 based CFC have hollow core structure, and graphite platelets stacked at a small angle to the fiber axis, ≈15° [13]. Another structure is observed for CFC deposited on the Ni–Cu–Al2O3 catalysts. In this case, graphene layers are oriented perpendicular to the fiber axis (α=90°), and the filament diameter varies between 50 and 200 nm [12], [15]. Multiwall carbon nanotubes (MWNTs) with an average outer diameter of 30 nm and inner channel of 8–10 nm are formed during methane decomposition over Fe-containing catalysts at 625 °C [14]. During methane decomposition carbon filaments usually interlace chaotically and form sponge-like granules 3–8 mm in size.
CFC is a predominantly mesoporous carbon material with a large surface area varying from 100 to 300 m2/g [1], [16]. Such unique properties of CFC make it an attractive catalyst support that can display unusual behavior compared to classic supports. At present, numerous studies are focused at investigating CFC as supports for metal catalysts for liquid-phase selective hydrogenation reactions [4], [17], [18] and for gas-phase reactions [5], [6], [19], [20], [21].
In our previous work [6] we have used methane decomposition as a model reaction to study the catalytic properties of nickel supported on two types of CFC: CFC (Ni–Al2O3) and CFC (Ni–Cu–Al2O3). The best performance in this reaction has been observed with the supported Ni/CFC (Ni–Cu–Al2O3) catalyst prepared from nickel chloride precursor. Its superior properties are due to the formation of large (30–70 nm) Ni particles leading to the growth of new “secondary” carbon filaments. The yield of the secondary carbon has been shown to reach 240 g/gNi at 550 °C [6].
Baker and co-workers have reported that nickel decorating carbon nanofibers exhibits higher catalytic activity in the gas-phase hydrogenation of light hydrocarbons at atmospheric pressure in comparison with classic supports (activated carbon, silica and Al2O3) [22]. Modification in the catalytic performance of the metal particles supported on filamentous carbon was attributed to different crystallographic orientations accepted by the metal crystallites on the surface of the filaments. When nickel is supported on graphite nanofibers, the metal crystallites have been suggested [17], [19], [20], [22] to adopt thin hexagonal morphology different from that observed when the nickel is supported on oxide supports such as alumina or silica. The peculiar metal morphology observed on the carbon nanofiber support is accounted for by strong metal–support interaction between nickel crystallites and exposed graphite prismatic planes. It was suggested that the graphite edge in the nanofiber support acted as templates for the dispersed nickel crystallites, which adopted a specific geometry. The same phenomenon has been also observed [18] with Pd supported on carbon nanofibers.
In continuation of our investigation, in this paper we have studied new CFC as a support for Ni catalyst for methane decomposition and the influence of textural and structural properties of CFC on the efficiency of supported Ni/CFC catalysts.
Section snippets
Experimental
CFC supports were produced by methane decomposition over coprecipitated Ni–Al2O3 (CFC (Ni)), Ni–Cu–Al2O3 (CFC (Ni–Cu)), Co–Al2O3 (CFC (Co)) and Fe–Co–Al2O3 (CFC (Fe–Co)) catalysts with high metal loadings, as described elsewhere [12], [13], [14]. Table 1 presents the conditions of methane decomposition and structural parameters of CFC. CFC samples were treated with concentrated hydrochloric acid for 72 h to remove metal catalyst particles from the fibers. The treatment was followed by careful
Results and discussion
We used the following parameters to compare the carbon formation efficiency of prepared Ni/CFC catalysts in the methane decomposition reaction: methane conversion (x, %), carbon accumulation until complete deactivation of the catalysts (the so-called carbon capacity G, calculated as gram of carbon per gram of nickel, g/gNi) and lifetime of the catalysts (t,h).
CFC samples of different types were used as nickel supports:
- 1.
CFC with similar surface areas (ABET) but differentstructures of the carbon
Conclusions
Nickel catalysts supported on the filamentous carbon with different structures and textures were studied in methane decomposition at 525 °C. The carbon capacity of 15 wt% Ni/CFC catalysts followed the order:
CFC(Ni–Cu, 625 °C) > CFC(Co) > CFC (Fe–Co)=CFC(Ni–Cu, 575 °C) > CFC (Ni–Cu, 675 °C) ≫ CFC (Ni).
The highest yield of secondary carbon was shown to reach 224 g/gNi on the Ni/CFC (Ni–Cu, 625 °C) catalyst. The stability and activity of the Ni/CFC catalysts for deposition of the secondary carbon at 525
Acknowledgements
We are grateful to Prof. V.B. Fenelonov and. T.Ya. Efimenko for the adsorption data, and Dr. V.A. Ushakov and Dr. S.V. Cherepanova for the XRD data. The research described in this publication was made possible in part by Award No. REC-008 from the US Civilian Research and Development Foundation for the Independent States of the Former Soviet Union (CRDF) and by the Netherlands Organization for Scientific Research (NWO) in the year of 2001.
References (30)
- et al.
Carbon
(1997) - et al.
Carbon
(2001) - et al.
J. Catal.
(1985) - et al.
Appl. Catal. A
(1996) - et al.
Appl. Catal. A
(1999) - et al.
Appl. Catal. A
(2002) - et al.
Appl. Catal. A
(2003) - et al.
Carbon
(2003) - et al.
J. Mol. Catal. A
(2001) - et al.
Carbon
(2003)