Effect of metal nanoparticles on TiO2 for enhanced photocatalytic N-methylation of piperazine

Direct N-alkylation of amines, such as piperazine, with alcohols via photocatalysis is a promising method, where the incorporation of metal nanoparticles (NPs) into photocatalysts significantly influences catalytic performance. Here, we report TiO₂ photocatalysts impregnated with various NPs using the photodeposition method (labelled as M/TiO₂, where M includes Pt, Pd, Au, Ag, Cu, and Co) for the N-methylation of piperazine with methanol. Specifically, Pt/TiO₂ and Pd/TiO₂ catalysts achieved conversions of 64.3% and 50.9%, respectively, within 4 h, displaying distinct selectivities. Pt/TiO₂ predominantly yielded enamine intermediate with a selectivity of 95.4%, while Pd/TiO₂ mainly produced N-methylpiperazine with a selectivity of 83.6%. Through comprehensive characterizations and control experiments, we found that both Pt NPs and Pd NPs enhance the light absorption and promote photogenerated electron-hole separation. However, Pd NPs facilitate the formation of Pd–H active species and contain more sites for the adsorption of enamine, thereby favoring the hydrogenation of the enamine intermediate to produce N-methylpiperazine. This study underscores the pivotal role of metal NPs in modulating the outcomes of photocatalytic N-methylation reactions on TiO₂.