Influence of the solution pH in impregnation with citric acid and activity of Ni/W/Al₂O₃ catalysts

The impregnation parameters setting is an essential strategy to find efficient active phase precursors in heterogeneous catalysts due to the close connection between the chemistry of impregnated aqueous solution and the nature of metal-support interactions. In this work, the effect of impregnation solution pH on active NiWS phase formation was studied. Ni/W/Al₂O₃ catalysts were prepared by impregnation of a W/Al₂O₃-based catalyst using aqueous nickel-citric acid (CA) solutions (CA/Ni = 2) at different pH values (0.3, 3.4, 5.1 and 8.7). The citric acid presence and the impregnation pH influenced the NiWS phase formation. Citric acid provided isolation to supported metal species avoiding strong metal-support interactions, delaying nickel reduction at higher temperatures and decreasing WS₂ sintering when compared with a free-citric acid catalyst. An increase of 2.2 times in NiWS fraction was related to these citric acid features. Furthermore, the solution pH influenced the aqueous and supported nickel speciation. Dispersed nickel species were associated with AlO-Ni-Cit surface complexes generated by the adsorption of [Ni(HCit)(Cit)(H₂O)₄]^3- or [Ni(Cit)₂(H₂O)₄]^4- over alumina surface. In turn, these surface complexes were associated with a higher W sulfidation. 3-fold increases in Ni/W/Al₂O₃ catalytic activity for hydrodesulfurization of 4,6-dimethyldibenzothiophene were reached by the presence of citric acid and a pH of nickel-citric acid solution around 5. Therefore, the active NiWS species formation in Ni/W/Al₂O₃ catalysts was improved with citric acid addition and furthermore, it was notably optimized by the control of the nickel species via tuned impregnation pH.