Pd/Co双金属纳米颗粒的制备及催化制氢性能

摘要

采用聚乙烯吡咯烷酮(PVP)保护的化学共还原法制备了Pd/Co双金属纳米颗粒，研究了PVP及还原剂(NaBH4)的用量、金属盐浓度、金属比例等对Pd/Co双金属纳米颗粒催化NaBH4制氢性能的影响.透射电子显微镜(TEM)的结果表明，所制备的Pd/Co双金属纳米颗粒的平均粒径在1.5-2.8 nm之间. Pd/Co双金属纳米颗粒(BNPs)的催化活性远高于Pd与Co单金属纳米颗粒的活性；当Pd/Co的理论原子比为1/9时，双金属纳米颗粒的催化活性最高可达15570 mol∙mol-1∙h-1(文中纳米颗粒的催化活性均为每摩尔Pd的活性).密度泛函理论(DFT)的计算结果表明， Pd 原子与Co原子之间发生电荷转移，使得 Pd 原子带负电而Co原子带正电，荷电的Pd和Co原子进而成为催化反应的活性中心.所制备的Pd/Co双金属纳米颗粒具有很好的催化耐久性，即使重复使用5次后，该催化剂仍具有较高的催化活性，且使用后的纳米颗粒催化剂也没有出现团聚现象.双金属纳米颗粒催化NaBH4水解反应的活化能约为54 kJ∙mol-1.%Reported here is a facile route for the synthesis of polyvinyl pyrrolidone (PVP)-stabilized Pd/Co bimetallic nanoparticles via a chemical co-reduction process. The effects of molar ratio of PVP and reducing reagent (NaBH4) to the total metal ions, and metal ion concentration and composition on the catalytic activity for hydrogen generation from NaBH4 over Pd/Co bimetal ic nanoparticles (BNPs) were studied. The transmission electron microscopy (TEM) results indicated that the prepared Pd/Co bimetallic nanoparticles, which had an average size of 1.5-2.8 nm, showed much higher catalytic activity than Pd and Co monometal ic nanoparticles (MNPs). The highest catalytic activity of al the prepared bimetallic nanoparticles was 15570 mol∙mol-1∙h-1 (the activity was normalized by the content of Pd in the BNPs), which was achieved with Pd/Co theoretical atom ratio of 1/9. The higher catalytic activity of the Pd/Co BNPs compared with the corresponding MNPs was ascribed to electronic charge transfer effects; this hypothesis was validated using density functional theory (DFT) calculations, which showed that the Pd atoms were indeed negatively charged, while the Co atoms were positively charged because of electron donation from the Co atoms to the Pd atoms. The positively charged Co atoms and negatively charged Pd atoms acted as catalytic active sites for the hydrolysis reaction of the alkaline NaBH4 solution. Good catalytic stability was observed with the existing high catalytic activity, even after five runs of evaluating the catalytic activity. Moreover, no clear agglomeration was observed in the nanoparticle catalyst used. The corresponding apparent activation energy was determined as 54 kJ∙mol-1, based on the kinetic study of the hydrogen generation achieved via the NaBH4 hydrolysis over the PVP-protected Pd10Co90 bimetal ic nanoparticles.