Shape-controlled Synthesis Of Platinum Nanocrystals And The Study Of Their Properties

As an important catalyst, Platinum(Pt) has excellent catalytic activity and selectivity, being widely applied in industry. Since its price rockets in recent years, most of the efforts have focused on decreasing platinum utilization via increasing its catalytic efficiency. Being compared to the conventionally known Pt catalysts, well-defined Pt nanocrystals(Pt NCs) with large surface area, owning rough surfaces with terraced, stepped and kinked sites, or exposing high-index facets are expected to exhibit highly enhanced catalytic activity and selectivity, and the shape-controlled synthesis is the prerequisite to obtain those Pt NCs. We used the peptide as the capping agent, and changed serials of reaction conditions, such as Pt precursors, reducers, reaction temperature and time to obtain Pt NCs with different morphology, and studied their catalytic properties on cinnamaldehyde hydrogenation and methanol electro-oxidation.The mechanism of Ac-SSFPQPN-NH2(S7) and Ac-TLTTLTN-NH2(T7) capable of interacting with Pt NCs was investigated. Our work revealed that those peptide can adsorb onto the crystals with functional group C=O and N-H. From the thermodynamic and kinetic perspective, we discussed the possible mechanism of obtaining Pt NCs with different morphology. We thought in the system with S7, the crystal growth was a kinetically controlled process, thus the crystal preferred anisotropy and the nano-flower shaped Pt NCs were synthesized. While with T7, the growth was a thermodynamically controlled process, and the regular polyhedra were obtained.A comprehensive set of systematic studies on the synthesis factors(the kind of Pt precursors and reducers, reaction temperature and time) was reported, and the mechanism was studied with UV-vis method. We found that the precursor with stronger oxidizing ability led to larger size of Pt NCs, while the one with weaker oxidizing ability, formed smaller ones, and when the oxidizing ability of precursors were the same, the one with higher pH would lead to the regular polyhedra, whereas the one with lower pH would lead to nano-flower shaped Pt NCs. Since ascorbic acid has weaker reducing ability, they would form tiny Pt NCs which were inclined to aggregate. On the contrast, stronger reducer agent, NaBH4 would lead to mono-dispersed and narrow-sized regular polyhedra. Cooperating the two reducers, the nanoflower shaped Pt NCs were obtained. Through modulating the temperature, different shaped Pt NCs were synthesized. In 0℃, the crystals aggregated, while in 20℃ and 30℃, the crystals were mono-dispersed and flower-alike, and in 40℃, the regular polyhedra were obtained, and the core-shell Pt NCs appeared in 50℃.The impacts of the surface volume ratio, size, surface active sites and capping agents on the catalytic ability of Pt NCs were further studied. We found the capping agent with stronger adsorbing ability had lower their catalytic activity in cinnamaldehyde hydrogenation and methanol electro-oxidation, while the one had weaker adsorbing ability owning higher activity. Nanoflower shaped crystals selectively hydrogenated C=C in cinnamaldehyde, and capped with S7, Pt NCs had stronger CO-tolerance capability.