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A catalyst, conventionally, is defined as a substance that speeds up the rate of a thermodynamically feasible chemical reaction by providing an alternative route with lower activation energy. A catalyst takes part in the chemical reactions and catalyzes all or several of the possible reactions. Only a small quantity of catalyst has to be used since the amount and nature of which remains basically unchanged.
However, in practice, a catalyst can be
Location of Pt in the nanoparticle
Judging by the ADF-STEM image of the Pt/Pd nanoparticle, depending on the miller index, it is suspected that Pt is located at the top right corner of the nanoparticle as shown in the figure. According to the working principle of the dark field microscopy, ADF detector responses to the intensity of scattered electrons, in the other words, the higher the atomic number of an element is, the brighter the 'dots' are. The given information suggests that the 'bright dots' at the top right corner could be Pt since Pt has larger relative atomic mass than Pd.
Due to the intrinsic uncertainties, detailed study on the structure of bimetallic nanoparticles must be done in order to verify the hypothesis. An important clue from the given information is that the Pt/Pd catalyst is used in hydrogen fuel cell system, in which a heterogeneous catalyst is a distinct possibility.
Heterogeneous electrocatalyst for hydrogen fuel cell system is commonly produced by depositing Pt/Pd bimetallic nanoparticles on electrode surfaces. (Page 50 metal clusters in catalysis….)The synthesis of bimetallic nanoparticles is mainly divided into two methods: chemical and physical method. The synthesis involves:
Self-assembly of bimetallic nanoparticle by physically mixing two kinds of pre-prepared monometallic nanoparticles with or without after-treatments.
Pt/Pd bimetallic nanoparticles are usually prepared by simultaneous reduction though successive reduction is still a possible synthetic route.
Successive Reduction Synthesis
Nobel metal ions are strong oxidizing agents. Therefore, alcohol reduction method can be used to reduce noble metal ions to zero-valent state. The Toshima's alcohol reduction process of metal ions Mn+ using ethanol is shown by Equation (1).(citation)
Pt/Pd bimetallic nanoparticles can be prepared by refluxing alcohol/water (1:1 v/v) solution of palladium(II) chloride and chloroplatinic(IV) acid in the presence of poly(N-vinyl-2-pyrrolidone) (PVP) at about 95â„ƒ for one hour.(Page 52 metal nanoclusters in catalysis…)The main advantage of Toshima's alcohol reduction method is that alcohol is served both as a solvent and a reducing agent. The corresponding aldehyde produced from reduction can easily be removed by distillation
Formation mechanism of Pt/Pd nanoparticle by simultaneous alcohol reduction in the presence of PVP is shown in Fig. x:
Step 1: Pt4+ and Pd2+ are coordinated to PVP by weak π-π interactions.
Step 2: Pt4+ having a higher redox potential is reduced first while Pd2+ remains as an ion.
Step 3a: Pd2+ is reduced to Pd and coexists with Pt.
Step 4a: Pt atoms aggregated to form Pt nanocluster, probably because the Pt-PVP coordinate bond is weaker than Pd-PVP coordinate bond.
Step 3b: Pt atoms coagulate to form Pt nanocluster while Pd2+ exists as ions
Step 4b: Pd2+ ions, which coordinated to PVP protecting Pt nanocluster are reduced to form Pd atoms
Step 5: Pd deposits on seed Pt nanoclusters to from Pt/Pd bimetallic nanoparticles.
The resulting bimetallic nanoparticles have a Pt-core/Pd-shell structure. The average diameter of the particles with a composition of Pd/Pt = 1:4 was as small as 1.5nm (55 atoms). The core/shel1l structure was confirmed by the technique of EAXFS. The result indicates that the Pd atoms are catalytically active while Pt atoms are inactive.
Similar process was carried out by Bronstein et al. The Pt/Pd bimetallic nanoparticles are prepared in molar ratio of 1:4. (cite nanoparticle and catalysis source 44)Palladium(II) acetate was reduced by sodium borohydride in the presence of polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) in toluene and THF. However, the resulting nanoparticles have a cluster-in cluster structure instead of a core/shell structure as expected. (cit. Nano and catal p.97) The cluster-in-cluster structure was confirmed by CO-FTIR spectroscopy.(nanoparticle and catalysis p.124) It is proposed that the redox potentials for Pd and Pt are close so a core/shell structure is hardly possible.
Structural transformation of bimetallic Pd/Pt nanoparticles can also be achieved by a sequential loading of chloroplatinic acid onto the Pd loaded catalyst, was investigated with EXAFS at high temperature.(cite p77 MNiCMS) The structure of the obtained bimetallic Pd/Pt nanoparticles seemed to be retained upon heating up to 1273K under ambient condition. (cite p77 MNiCMS) CO-FTIR spectroscopic measurement on Pd/Pt bimetallic nanoparticles at different composition raito with Pd-core/Pt-shell structure also showed when Pd:Pt = 1/4, CO adsorbed on Pd atomes at 1941 cm-1 is completely absent, which proved that Pd-core has been completely coverd by Pt-shell.(MNiCMS p.77)
Structural Analysis of Bimetallic Nano-structure
The structure of bimetallic nanoparticles mainly depends on five factors:
Molar ratio of both element
Bond strength between two kinds of metals
Coordinate bonds between polymer stabilizer and metal ions
Redox potential of both elements
As mentioned above, there are mainly three synthetic routes among which two are feasible to synthetize Pd/Pt bimetallic nanoparticles. Simutaneous reduction flavors core/shell structure while successive reduction flavors cluster-in-clusters structure.
Molar ratio of both elements is a dominant factor on the structure of bimetallic nanoparticles. CO-FTIR analysis showed that Pt/Pd at molar ratio of 1:1 and 1:2, a Pt-rich core/Pd-rich shell is adopted. Only if Pt/Pd at a molar ratio of 1:4, to be precise 13 Pt atoms and 42 Pd atoms in a 1.5nm (55-atom) nanocluster, a three-layered (two-shell type) fcc-core/shell structure can be obtained.
Bonding between Pt-Pt, Pd-Pd and Pt-Pd determines the extent of alloying in the Pt/Pd bimetallic nanoparticle. In A-B bimetallic nanoparticles, if A-A bond is preferred to B-B bond, instead of forming a homogeneous alloy structure, an A-core/B-shell structure is favoured, but the inverted structure can still be constructed. Nevertheless, the inverted core/shell structure is believed to be thermodynamically unstable. In 2012, the latest research conducted by Huang et al. (cite) presented a systematic study on structural and thermal stabilities of Pt/Pd bimetallic nanoparticles with core/shell and alloyed structures by using atomistic simulations. It was revealed that that the Pd-core/Pt-shell structures are the least structurally stable, while the Pt-core/Pd-shell nanoparticles are more stable than the alloyed ones when the Pt percentage exceeds 42%. Furthermore, Pt-core/Pd-shell structures exibit enhanced thermal stability as compared to the alloyed ones for Pt composition more than ca. 30%. In addition, the analyses of diffusion behaviour and atomic distribution suggest that the minimization of surface energy tends to form Pd surface segregation.
However, K. Sasaki et al. managed to develop a new hydrogen fuel cell electrocatalyst based on Pd-core/Pt-shell nanoparticles. The characterized nanoparticles have a single layer of Pt over a Pd-core. The bimetallic electrocatalyst showed promising reactivity and durability during charge/discharge test. The reactivity of the Pt/Pd electrocatalyst only dropped 37% after 100,000 cycles.
Recent research on Pd/Pt bimetallic nanoparticles reveals that common preparation method of Pd/Pt nanoparticls by simultaneous reduction would give a Pt-core/Pd-shell structure. The structure is proven to be structurally and thermodynamically stable. However, ad hoc characterization of Pd/Pt bimetallic nanoparticles with inverted core/shell structure for hydrogen fuel cell electrocatalysis illustrated excellent performance and durability. Therefore, I strongly believe the Pd/Pt bimetallic nanoparticle in the ADF-STEM image has a Pt-