Abstract
The influence of nickel on the acid strength distribution of a γ-Al 2O 3 support and a series of alumina-supported hydrotreating catalysts such as Ni&z.sbnd;Mo/γ-Al 2O 3and Ni&z.sbnd;P/γ-Al 2O 3 in their oxidic form in the concentration range close to commercial catalysts were examined by temperature-programmed desorption of ammonia. A substantial increase in the concentration of strong acid sites was observed when nickel was added to γ-Al 2O 3. On the other hand, addition of molybdenum to γ-Al 2O 3 resulted in the neutralization of the strong acid sites with a significant increase in the number of acid sites of medium strength. The influence of phosphorus on the acidity was similar to that of molybdenum. Addition of nickel to MoO 3/γ-Al 2O 3 and PO 4/γ-Al 2O 3 systems was found to increase the number of strong acid sites considerably, with a corresponding decrease in concentration of the acid sites of moderate strength. A mechanism based on the formation of a nickel aluminate spinel on the surface of alumina and its effect in increasing the net positive charge or the coordinatively unsaturated sites of the surface is proposed in order to rationalize the influence of nickel on increasing the acid strength.
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