Catalytic activity and structural change in Ni-W catalysts supported on $Al_2O_3$ were investigated with respect to the various contents of nickel (atomic ratio of Ni/(Ni+W) = 0 - 1) in order to investigate the effect of the promoter, Ni, in the hydrodesulfurization (HDS) reaction. TPR(Temperature Programmed Reduction), DRS(Diffuse Reflectance Spectroscopy), DOC(Dynamic Oxygen Chemisorption), DNOC(Dynamic NO Chemisorption) and sulfur content analysis were used for the characterization of catalysts. Catalytic activity was analyzed by measuring pseudo-1st order reaction rate constant in the atmospheric HDS reaction of thiophene.
As the content of promoter was increased, catalytic activity increased rapidly until the atomic ratio reached to 0.2. Beyond the atomic ratio of 0.2, catalytic activity increased slowly upto the atomic ratio of 0.6, where the maximum catalytic activity was achieved, and then started to decrease.
TPR showed three peaks of NiOII (dispersed surface nickel species), NiWOAl(active phase precursor) and NiOIIIb (dilute $NiAl_2O_4$ spinel-like species). The peak of NiWOAl was mainly observed until the atomic ratio reached to 0.2. As the atomic ratio was increased further, the intensities of these three peaks increased until atomic ratio reached to 0.6. Above atomic ratio of 0.6 the intensity of NiOII peak increased rapidly. DRS analysis revealed that KM remission function ratio, $F(R)_720nm/F(R)_635nm$, indicating the ratio of octahedral Ni species to tetetrahedral Ni species, decreased with the increase of atomic ratio until 0.6, then increased again. These results indicated that dilute $NiAl_2O_4$ spinel-like species (NiOIIIb) was saturated because of the total occupation of the tetrahedral hole of the alumina. The increase of octahedral species above the ratio of 0.6 indicated by DRS spectrum suggested that NiWOAl synergic phase may be covered by NiOII phase results in the decrease in activity.
Catalytic activity and structural change of Ni-W catalysts supported on $Al_2O_3$ with respect to various sulfiding temperatures (400 - 800℃) were also investigated. Atomic ratio was set at 0.5. The increase in sulfiding temperature decreased the catalytic activity and rapid decrease in activity was observed between 500 - 600℃. Analysis of catalysts by XRD (X-Ray Diffraction), XPS (X-ray Photoelectron Spectroscopy), sulfur content analysis, DOC and DNOC showed that tungsten sulfide on alumina support became more crystalline as the sulfiding temperature increased. More nickel was diffused into support at higher sulfiding temperature. These two phenomena decreased the formation of synergistic Ni-W-S phase.
