Ammonia Decomposition over Ni Supported on Multi-Wall Carbon Nanotubes: Effect of Surface Functional Groups

摘要

Graphene, single-wall carbon nanotubes (SWCNTs) and three types of commercial MWCNTs with different functional surface groups (MWCNTs-COOH, MWCNTs - OH, MWCNTs-NH_2) and non-functionalized mutiwall carbon nanotubes (MWCNTs) were used as supports to prepare nickel based catalysts. All catalysts were preapred by the incipient wetness method. Catalysts were characterized by using powder X-ray diffraction (XRD), Temperature programmed reduction (TPR), and X-ray Photoelectron Spectrometry (XPS). Ammonia decomposition (AD) reaction was carried out in a fixed bed quartz microreactor at atmospheric pressure and reaction temperatures from 400-500°C The gas hourly space velocity (GHSV) was varied between 6000 to 24000 h~(-1) Activity tests revealed that both Ni/Graphene and Ni/SWCNTs are essentially inactive with NH_3 conversions not exceeding 1.0% at 500°C and GHSV of 6000 h~(-1). Ni supported on MWCNTs with different surface functional groups showed that the Ni/MWCNTs-COOH catalyst was most active with ammonia conversion as high as 25.4% at 500°C and 6000 h~(-1) Comparable conversions were obtained with Ni/MWCNTs-OH (23.5%) and Ni/MWCNTs-NH_2 (24.6%). Non-functionalized, Ni/MWCNTs exhibited a much lower conversion of ammonia (13.2%) compared to Ni supported on functionalized MWCNTs. It was demonstrated that the presence of the attached surface functional groups on MWCNTs is essential for preparation of active Ni catalyst. The NiO crystallites sizes obtained from XRD diffraction of calcined catalysts were similar in size for both functionalized and non-functionalized supports. XPS measurements did not show significant difference in binding energies (Ni2p3/2 peaks of NiO), however, in agreement with XRD results, it showed that NiO was the major surface phase for all catalysts. TPR measurements indicated the presence of two reduction peaks in the temperature intervals from 100-500°C and 500-700°C. The first peak could be assigned to the reduction of the NiO particles to Ni. The TPR measurements indicated that the functionalized MWCNTs are much easier to reduce which could partialy explain their higher activity in AD reaction.