Deposition of Pt Nanoclusters on Au and Their Catalytic Activity

Abstract

Electrocatalytic studies of Pt nanoclusters (Pt-nc) distributed on foreign metals have shown to exhibit different and often much higher activity than the bulk Pt. In recent years, 2D Pt sub-ML catalysts designed using surface limited redox replacement reaction (SLRR) of an underpotentially deposited (UPD) monolayer of Cu have been used as model systems in studies of the coverage-dependant behaviour during formic acid, methanol, ethanol and hydrogen oxidation reactions.1-4 The effects of the substrate (ligand effect) as well as the size-dependant strain (geometric effect) have been suggested to play an important role in the Pt-nc activity. However, studies on single layered and multilayer Pt-nc suggests that there is an important correlation between the nanoclusters size (2D and 3D) and coverage (controlling the methods of deposition) that needs to be understood better.5 Formic acid oxidation (FAO) is an ideal reaction to study as a function of size and coverage of Pt-nc as many studies recently reported the low-coverage Pt-nc on Au as the ‘most active’ bimetallic system configurations for this reaction.2, 6-8 Here in this talk we will present a study of FAO activity and stability dependence on the size and coverage of 2D and 3D Pt-nc designed by two different methods: spontaneous deposition and the SLRR via Pb UPD. SD is a method that produces uniformly distributed multi-layered Pt-nc by reduction of the spontaneously adsorbed [PtCl4]2- complex.9A systematic study of Pt-nc coverage and distribution included the time of the Pt-complex adsorption and the number of the applied SD cycles on the evolution of the 3D nature of Pt-nc as well as the distribution i.e. on Au substrate coverage. A particular attention was given to the different surface processes used as analytical tools for the determination of Pt coverage: 1) oxidation/reduction of AuO and PtO surface oxides and 2) adsorption processes such as H-UPD and CO. The discrepancies in the values of Pt coverage obtained by different methods requires better understanding of these processes on Pt-nc/Au. The study has shown that up to 5 SD cycles the Pt-nc size and coverage increases up to ~ 60% of Au substrate coverage. Subsequent cycles results in the growth of exiting Pt-nc height but not in the formation of new clusters. The SLRR deposition of 2D Pt-nc using Pb UPD has been controlled by galvanic replacement of electrodeposited Pb-sub ML layer on Au. The study of isolated Pt-nc of comparable coverage (0.23 ±0.02) created by SD and SLRR methods showed the differences in their FAO behaviour that can be attributed to the clusters size (2D and 3D) and coverage. The analysis showed that SD Pt-nc with smaller diameter but larger variation of height than SLRR Pt-nc exhibit 5 times higher activity for FAO. Stability of SD and SLRR Pt-nc during FAO cycling showed that both Pt-nc structures follow a similar trend of changes that can be correlated with the dissolution of Pt. The SD Pt-nc however, showed higher (longer) stability during FAO that could be attributed to their size and larger percentage of 2ML height clusters compared to the SLRR Pt-nc.