On the origin of the high performance of MWNT-supported PtPd catalysts for the hydrogenation of aromatics Original Research Article

This work reports some surface and structural features of multi-wall carbon nanotubes (MWNT)-supported PtPd crystallites in order to explain their very good ability to hydrogenate aromatic rings in comparison with other catalyst systems consisting of the same metallic function but changing the support substrate (amorphous SiO2–Al2O3 (ASA) and silica-delaminated zirconium phosphate (ZrPSi)). The bimetallic PtPd catalysts were prepared by simultaneous impregnation of the respective supports with metal salt precursors and characterized using chemical analysis, nitrogen adsorption–desorption isotherms at 77 K, temperature-programmed desorption-mass spectroscopy (TPD-MS), DRIFT of adsorbed NH3 (DRIFT-NH3), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy measurements (XPS). The catalysts were tested in the vapor-phase simultaneous hydrogenation (HYD) of naphthalene and toluene in the presence of dibenzothiophene (DBT; 100 ppm of S) at 5 MPa of total pressure. Irrespectively of the reactant molecule, the PtPd/MWNT catalyst showed a higher initial turnover frequency (TOF) than the PtPd/ZrPSi and PtPd/ASA counterparts. The enhancement of activity observed with the PtPd/MWNT catalyst was related to the ensemble effect of Pt48Pd25 alloy located on the outer surface of the MWNT. Upon on-stream conditions the PtPd/MWNT showed the lowest thioresistence, sintering and coking among the catalysts studied. The improved resistance of this catalyst to metal sintering and coking is interpreted in terms of the reactivity changes of the coke precursors induced by adsorbed sulfur as well as the lowest acidity of this catalyst.