Preparation And Catalytic Property Studies Of The Composites Based On Gold Or Platinum Nanoparticles

With the rapid development of the economy,the consumption for energy has also been increasing quickly.The traditional fossil fuels can't meet the huge energy demand for the economy growth,meanwhile our environment is not able to continute to support the large scale consumption of pollutive energy.The sustainable development of the economy is significantly restricted to the traditional developing mode of highly consumption and heavy contamination for fossil fuels.Therefore,it is necessary to prepare suitable catalysts for improving the energy efficiency and exploit a new renewable clean energy for substituting traditional fossil fuels.In this thesis,we have studied the composites based on noble metal nanoparticles which are applied to catalyze the reaction of small organic molecules and the oxygen reduction reaction in the cathode of fuel cells,and the correlation between noble metal nanoparticles and the catalytic activity has also been examined.The thesis includes the following three aspects:Firstly,structural similar molecular clusters [Au11(PPh3)8Cl2]Cl and Au11(PPh3)7Cl3 were successfully synthesized and employed as catalysts for selective oxidation of styrene.The products were analysed by GC-MS and 1H NMR to examine the effect of the nanocluster structural difference upon the catalytic activity and selectivity.Interestingly,the main product was benzaldehyde when oxygen was employed as oxidant,and the catalytic activity of Au11(PPh3)7Cl3 was higher than that of [Au11(PPh3)8Cl2]Cl.Compared with oxygen,the reaction conversion demonstrated a sharp increase when tert-butyl hydroperoxide(TBHP)was employed as the oxidant.The catalytic activity of [Au11(PPh3)8Cl2]Cl and Au11(PPh3)7Cl3 are comparable,but styrene epoxide was the major product.Meanwhile,the cyclic voltammogram and differential pulse voltammogram revealed the electrochemical band gap of [Au11(PPh3)8Cl2]Cl and Au11(PPh3)7Cl3 was ~2.28 eV and ~2.44 eV,respectively.The large band gap indicate [Au11(PPh3)8Cl2]Cl and Au11(PPh3)7Cl3 possess obvious nonmetallic property,which originate from the strong quantum-confinement effects.Such high band gap is probably favorable for activating styrene and oxygen molecule hence facilitate the selective oxidation process of styrene.Secondly,glutathione-capped AuNPs of ca.4 nm in diameter were prepared and embedded into an ordered mesoporous carbon,and the obtained nanocomposites exhibited apparent ORR activity in alkaline media.Au25,Au38,and Au144 nanoclusters were employed as electrocatalysts for ORR.Interestingly,the electrocatalytic activity of three molecular nanoclusters increased from Au144 to Au25.However,their activity are not desirable due to the thiol ligand which restricted the active sites of the gold core.Subsequently,ordered porous carbon-supported gold nanoparticles of varied sizes were prepared by using molecular Au25,Au38,and Au144 nanoclusters as precursors,and the thiol ligands were removed by pyrolysis at controlled temperatures.The resulting nanocomposites displayed apparent catalytic activity and robust durability in the ORR in alkaline solution,which increased with the decreasing of nanoparticle dimension.Among the series of samples tested,the nanocomposite prepared with Au25 nanoclusters displayed the best activity,as manifested by the positive onset potential at +0.95 V vs RHE,remarkable long-term stability,and high numbers of electron transfer at(3.60-3.92)at potentials from +0.50 to +0.80 V.The above results demonstrated the small-sized Au particles are beneficial for the activation of molecular oxygen,as the low-coordination surface Au atoms can significantly decrease the activation energy of chemisorption of oxygen molecule,which facilitates the 4-electron transfer and leads to a more positive onset potential;Meanwhile,the integration of porous carbon and Au atoms offers a synergetic platform for improved ORR activity,as porous carbon not only act as a support and but also hold a positive influence on metal-support interactions.Thirdly,nanocomposites based on Co@Pt core@shell nanoparticles encapsulated in nitrogen-doped porous carbons were prepared as a new type of high-performance electrocatalysts for ORR.Controlled pyrolysis of zeolitic imidazolate framework 67(ZIF-67)led to the formation of Co nanoparticles encapsulated in nitrogen-doped porous carbon(Co-NC),which underwent galvanic replacement reactions with K2PtCl4 forming Co@Pt core@shell nanoparticles.With the Co@Pt particles encapsulated in nitrogen-doped porous carbon,the hybrids exhibited abundant catalytically active sites for ORR,remarkably higher durability and more robust tolerance against methanol.For Co@Pt nanoparticles,the outer layer of Pt contributed to various catalytically active surface sites,and the synergistic interactions between the Co core and the Pt shell tuned the electronic effects on the Pt shell,which facilitated the 4-electron transfer kinetics.For nitrogen-doped porous carbon,it not only served as a support to stabilize Co@Pt nanoparticles,but also the metal-nitrogen-carbon interactions can significantly lower the oxygen dissociation energy by accelerating the charge transfer from Pt atoms to oxygen molecules.The remarkably high ORR activity of Co@Pt-NC may be accounted for by those specific properties.