The Generation Of Oxygen-containing Groups On The Surface Of MWNTs And The Hydrogenation Activity Of Naphthalene On The Supported Pd-Pt/MWNTs Catalysts

Catalytically grown multi-walled carbon nanotubes (MWNTs) were activatated to improve its texture structure and surface hydrophilicity with the four different oxidants, acidic KMnO₄ solution, mixed concentrated HNO₃/H₂SO₄, diluted H₂O₂ in neutral water and melting KOH, respectively. N₂ physisorption, high-resolution transmission electron microscopy (HRTEM), Fourier transformation infrared (FTIR), chemical quantitative titration, and x-ray diffraction (XRD) were employed to investigate the activation method and the activation conditions on the surface and texture properties of MWNTs, and the hydrophilicity of MWNTs was observed and recorded and the pH values of zero charge (PZC) were determined by a experimental analysis. The analysis results showed that a big number of surface oxygen-containing groups were formed on the surface of MWNTs after activation in the acidic KMnO₄ or in HNO₃/H₂SO₄, however, much less in the neutral H₂O₂ and only a few after treatment in melting KOH. Furthermore, the proportion of carboxylic groups was high for the former two methods and was low for the third one, however, the majority of them was phenolic hydroxyl groups. Additionally, no clear changes of the bulk structure and morphology of MWNTs were observed and the BET specific surface areas were increased by about 50% after the activation in the acidic KMnO₄ or in HNO₃/H₂SO₄, but melting KOH made the bulk graphite layers of MWNTs damaged and the specific surface area was greatly increased by 150%. After the treatment, the PZC of MWNTs was declined, especially with the content of carboxylic groups and the lower its PZC, the more hydrophilic the MWNTs surface.Pd(NH₃)₄²⁺-Pt(NH₃)₄²⁺ as the precursors of the alloy catalysts were loaded on the surface of MWNTs by incipient wetness impregnation and ion-exchange methods and HRTEM and temperature-programed reduction (TPR) were used to investigate the interaction of the precursor Pd and Pt oxides particles with the MWNTs surface and the morphology of reduced Pd-Pt alloy particles. The hydrogenation actictivity of naphthalene to tetralin on Pd-Pt/MWNTs was measured in a high-pressured autoclave with 500 ppm of thiophene or without it. The results showed that the above issues were dependent on the amount of oxygen-containing groups, especially the carboxylic groups, and the prepared methods of catalysts. With the increase of the content of carboxylic groups, the interaction became strong and the dispersion of metals on the surface of MWNTs was improved and the particles of Pd-Pt alloy had a tendency to...