Nature of the carbon by-product

The formation of filamentous carbon deposits on transition metal catalysts (Fe, Co, Ni) and their alloys have been investigated in some detail over the past two decades.21,38-40 Among them, nickel is the most promising candidate since it forms carbon deposits at temperatures as low as 723-823 K using CH4, C2H6 or CO + H2 feeds. Carbon fibres are usually produced during these reactions. Typical forms of the carbon produced from CH4 decomposition on silica-supported Ni catalysts are shown in Fig. 7.1. The pyrolysis of methane at temperatures somewhat lower than 873 K produces fish-bone type nanofibres.41 The Ni metal particles are present at the tip of each carbon fibre, and catalyse methane decomposition as well as growth 

However, the shapes of the carbon structures appear quite different on Pd-Ni bimetallic catalysts.43 For a Pd-Ni bimetallic catalyst with a Pd/(Pd + Ni) atomic ratio of 0.5, branched carbon nanofibres with a large variety of diameters ranging from 10 to 300 nm are developed, in contrast with the carbon nanofibres formed on supported nickel catalyst (Fig. 7.2). 

Some structural differences are clearly differentiated when comparing the monometallic Ni-catalyst and the bimetallic Pd-Ni system. The Ni K-edge XANES spectrum of the deactivated bimetallic Pd-Ni catalyst virtually coincides with that of the fresh counterpart, indicating that the local structure of the PdNi alloy remains unchanged during CH4 decomposition. The longer life of the Pd-Ni catalyst for CH4 decomposition may be ascribed to the PdNi alloy itself, which is resistant to reaction with carbon to form a stable carbide phase. 

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