Novel Surfactant-Assisted Hydrothermal Fabrication of a Lignin Microsphere as a Green Reducer and Carrier for Pd Nanoparticles

The development and application of lignin materials with regular nano/microstructures have attracted great interest, as they offer emerging valorization of this currently underused biopolymer. In this work, we propose a novel surfactant-assisted hydrothermal strategy for fabrication of a lignin microsphere (LMS), avoiding the usage of high quantities of costly and environmentally problematic organic solvents in conventional methods. The as-prepared LMS was then applied as a green reducer and carrier for the synthesis of Pd nanoparticles (NPs) that were employed to catalyze hydrogen evolution from formic acid. Under hydrothermal treatment at 180 °C, LMS was formed from irregular bulk lignin particles in only 20 min. However, some irregular particles with large sizes (over 600 nm) were observed even under optimized hydrothermal conditions. Therefore, surfactant poly(vinylpyrrolidone) (PVP) was added to improve the size uniformity and spherical morphology of the LMS, thus obtaining LMS with narrow size distribution (100–500 nm) and a regular spherical morphology. Interestingly, the addition of PVP further promoted the dispersion of Pd NPs on LMS and decreased the Pd NP size. Hence, Pd@LMS-P (PVP added) presented substantially improved activity than Pd@LMS-0 (PVP free), and both catalysts exhibited significantly enhanced activity compared with bare Pd NPs. Consequently, this work demonstrates that the proposed surfactant-assisted hydrothermal method is feasible to prepare well-defined LMS and further contributes to the green formation of Pd NPs with reduced particle size and improved catalytic activity.