Role of citric acid in enhancing aromatic hydrogenation performance in Ni-HPW/SiO2 catalysts

In this study, we utilized a sol-gel method to prepare Ni-HPW/SiO 2 catalysts with varying citric acid (CA):Ni molar ratios ranging from 0 to 5.0. The catalysts demonstrated different levels of activity in the hydrogenation of ethylbenzene , which correlated to their respective CA:Ni ratios. Introducing citric acid in the preparation process of nickel-based catalysts yielded mesopore formation and reduced NiO particle size, producing a more uniform NiO dispersion on SiO 2 , which was confirmed by BET, XRD, and TEM analyses. The results of H 2 -TPR, H 2 -TPD, NH 3 -TPD, and Py-IR analyses suggested that the addition of citric acid augmented the interaction between phosphotungstic acid (HPW), Ni species, and SiO 2 . Citric acid improved Ni species and HPW dispersion, fostering increased HPW-Ni species formation. This, in turn, generated additional electron-deficient Ni ions , stemming from semi-reduced HPW-Ni species, which acted as Lewis acid sites, enhancing Ni + species effectiveness in activating aromatic rings. The better-dispersed, non-HPW-bound NiO, which, upon complete reduction to Ni, led to a substantial rise in H 2 bond cleavage, thereby facilitating more hydrogen spillover. Overall, the infusion of citric acid into fabricating Ni-HPW/SiO 2 catalysts sparked a significant surge in their aromatic hydrogenation ability.