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. [Display omitted] •TiO2 with metal nanoparticles are prepared by photodeposition method.•Pt/TiO2 and Pd/TiO2 show varied outcomes in piperazine N-methylation.•Pt/TiO2 yields 95.4 % selectivity for enamine; Pd/TiO2 83.6 % for N-methyled product.•Pd/TiO2 generates Pd-H species, facilitating hydrogenation of enamine.