Two-nozzle frame spray analysis (FSP) synthesis of CoMo/Al2O3 where Co and Al are sprayed in separate flames was applied to minimize the formation of CoAl2O4 observed in one-nozzle flame spray pyrolysis (FSP) synthesis and the materials were characterized by N2-adsorption (BET), X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy, Raman spectroscopy, transmission electron microscopy, and catalytic performances in hydrotreating. By varying the flame mixing distances (81–175 mm) the amount of CoAl2O4 could be minimized. As evidenced by UV–vis spectroscopy, CoAl2O4 was detected only at short flame mixing distances, where the flame conditions resemble one-nozzle FSP. Raman spectroscopy revealed that β-CoMoO4 was a component of all the catalysts (in the as-prepared oxidic form) together with alumina supported MoO x surface species. The only phase detected with XRD was γ-Al2O3. The FSP synthesized oxidic catalysts were activated by sulfidation without further heat treatments. The hydrodesulfurization activity of the best two-nozzle FSP catalysts, compared to the one-nozzle FSP catalysts, improved from 75 to 91 % activity relative to a commercial reference catalyst and the hydrodenitrogenation activity improved from 70 to 90 % relative activity. This suggests that better promotion of the active molybdenum sulfide phase was achieved when using two-nozzle FSP synthesis, probably due to less formation of the undesired phase CoAl2O4, which makes Co unavailable for promotion.
Catalysis Letters, 2013, Vol 143, Issue 5, p. 386-394