Alumina-Supported NiMo Hydrotreating CatalystsAspects\nof 3D Structure, Synthesis, and Activity

Abstract

Preparation conditions\nhave a vital effect on the structure of\nalumina-supported hydrodesulfurization (HDS) catalysts. To explore\nthis effect, we prepared two NiMoS/Al₂O₃ catalyst\nsamples with the same target composition using different chemical\nsources and characterizing the oxidic NiMo precursors and sulfided\nand spent catalysts to understand the influence of catalyst structure\non performance. The sample prepared from ammonium heptamolybdate and\nnickel nitrate (sample A) contains Mo in the oxidic precursor predominantly\nin tetrahedral coordination in the form of crystalline domains, which\nshow low reducibility and strong metal–support interactions.\nThis property influences the sulfidation process such that the sulfidation\nprocesses of Ni and Mo occur tendentially separately with a decreased\nefficiency to form active Ni–Mo–S particles. Moreover,\ninactive unsupported MoS₂ particles or isolated NiS<i>_x</i> species are formed, which are either washed\noff during catalytic reaction or aggregated to larger particles as\nseen in scanning transmission electron microscopy/energy-dispersive\nX-ray spectroscopy (STEM/EDX). The oxidic precursor of the sample\nsynthesized using nickel carbonate and molybdenum trioxide as metal\nsources (sample B), however, contains Mo in octahedral coordination\nand shows higher reducibility of the metal species as well as weaker\nmetal–support interactions than that of sample A; these properties\nallow an efficient sulfidation of Mo and Ni such that formation of\nactive Ni–Mo–S particles is the main product. Ptychographic\nX-ray computed tomography (PXCT) and STEM and EDX measurements show\nthat the structure formed during sulfidation is stable under operation\nconditions. The structural differences explain the HDS activity difference\nbetween these two samples and explain why sample B is much active\nthan sample A.