Maximising carbon nanofiber and hydrogen production in the catalytic decomposition of ethylene over an unsupported Ni-Cu alloy Original Research Article

A Ni-Cu (4:1 wt/wt) metal alloy catalyst has been studied for the catalytic decomposition of ethylene to produce hydrogen and carbon nanofibers. To maximise ethylene conversion in a horizontal tube reactor with high residence times, hydrogen yields and selectivities, the temperature, sample mass and ethylene flow rate were varied. Both high ethylene conversions (>80%) and high hydrogen selectivities (75%) were obtained at 873 and 923 K. The highest coke yield obtained after just 3 h was around 480 gC/gNi+Cu at 873 K (86% ethylene conversion) and was significantly higher than the value of 250 gC/gNi+Cu which was previously reported as the maximum coke yield for the decomposition of ethylene over Ni-Cu catalysts at this temperature. At 923 K, the optimum conditions were achieved with 50 mg of Ni-Cu, which gave ethylene conversions of 74%, a hydrogen selectivity of 71%, hydrogen yields in excess of image and even higher coke yields of 1112 gC/gNi+Cu after 15 h with the catalyst still possessing high catalytic activity. The Ni-Cu (4:1 wt/wt) catalyst behaved similarly to a Ni-Pd (4:1 wt/wt) catalyst, which suggests that unsupported Ni-Cu alloys can replace the more expensive Ni-Pd alloys for the catalytic decomposition of hydrocarbons. In addition, there were no obvious structural differences between the carbon nanofibers obtained with the Ni-Cu and Ni-Pd alloys, which were twisted, branched and straight nanofibers composed of crystalline and amorphous carbon with diameters ranging from 30 to 300 nm.