Optimization of S/N co-doped hollow carbon nanospheres encapsulating Ni NPs for enhanced phenol hydrogenation performance

Abstract

The development of catalysts exhibiting high activity and stability is of great significance for the directional hydrogenation of phenol. Ni@NHCs-X and Ni@NSHCs-4h-Y catalysts were prepared by adjusting the reduction time and the ratio of trithiocyanuric acid to 3-aminophenol (which controls the S/N doping ratio). The effects of catalysts composition, structure, and electronic properties on phenol hydrogenation were investigated. Catalytic activity depended on both the reduction treatment time applied to the in situ nitrogen-doped catalysts and the S/N doping ratio in the carbon matrix. Reduction treatment increased the content of reduced metallic nickel (Ni⁰) in the catalysts, which effectively enhanced catalytic hydrogenation activity. S/N co-doping further strengthened the interaction between the metal and charge carriers, optimizing the catalysts' composition and catalytic performance. However, an excessive S/N doping ratio disrupted the charge distribution within the carbon matrix, weakened the synergistic effect of S/N co-doping, and consequently reduced catalytic activity. Under reaction conditions of 100 °C, 1 MPa H₂, and 1.5 h, the Ni@NSHCs-4h-1 catalyst achieved a phenol conversion rate of 99.76% and a cyclohexanol selectivity of 99.53%. These results indicate that the combined effects of S/N co-doping and the hollow confinement structure provide abundant Ni⁰ active sites. These sites efficiently supply active hydrogen to the system, enabling rapid progression of the phenol hydrogenation tandem reaction and the highly efficient production of cyclohexanol.