A accelerator, conventionally, is defined as a substance that speeds up the rate of a thermodynamically executable chemical reaction by supplying an alternate path with lower activation energy. A accelerator takes portion in the chemical reactions and catalyzes all or several of the possible reactions. Merely a little measure of accelerator has to be used since the sum and nature of which remains fundamentally unchanged.
However, in pattern, a accelerator 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. Harmonizing to the working rule of the dark field microscopy, ADF sensor responses to the strength of scattered negatrons, in the other words, the higher the atomic figure of an component is, the brighter the ‘dots ‘ are. The given information suggests that the ‘bright points ‘ at the top right corner could be Pt since Pt has larger comparative atomic mass than Pd.
Due to the intrinsic uncertainnesss, detailed survey on the construction of bimetallistic nanoparticles must be done in order to verify the hypothesis. An of import hint from the given information is that the Pt/Pd accelerator is used in H fuel cell system, in which a heterogenous accelerator is a distinguishable possibility.
Heterogeneous electrocatalyst for H fuel cell system is normally produced by lodging Pt/Pd bimetallistic nanoparticles on electrode surfaces. ( Page 50 metal bunchs in catalysis… . ) The synthesis of bimetallistic nanoparticles is chiefly divided into two methods: chemical and physical method. The synthesis involves:
Self-assembly of bimetallistic nanoparticle by physically blending two sorts of pre-prepared monometallic nanoparticles with or without after-treatments.
Pt/Pd bimetallistic nanoparticles are normally prepared by coincident decrease though consecutive decrease is still a possible man-made path.
Consecutive Reduction Synthesis
Nobel metal ions are strong oxidising agents. Therefore, intoxicant decrease method can be used to cut down baronial metal ions to zero-valent province. The Toshima ‘s intoxicant decrease procedure of metal ions Mn+ utilizing ethyl alcohol is shown by Equation ( 1 ) . ( commendation )
Pt/Pd bimetallistic nanoparticles can be prepared by refluxing alcohol/water ( 1:1 v/v ) solution of Pd ( II ) chloride and chloroplatinic ( IV ) acid in the presence of poly ( N-vinyl-2-pyrrolidone ) ( PVP ) at approximately 95a„? for one hr. ( Page 52 metal nanoclusters in catalysis… ) The chief advantage of Toshima ‘s intoxicant decrease method is that intoxicant is served both as a dissolver and a reduction agent. The corresponding aldehyde produced from decrease can easy be removed by distillment
Formation mechanism of Pt/Pd nanoparticle by coincident intoxicant decrease in the presence of PVP is shown in Fig. ten:
Measure 1: Pt4+ and Pd2+ are coordinated to PVP by weak ?-? interactions.
Measure 2: Pt4+ holding a higher redox potency is reduced foremost while Pd2+ remains as an ion.
Measure 3a: Pd2+ is reduced to Pd and coexists with Pt.
Measure 4a: Platinum atoms aggregated to organize Pt nanocluster, likely because the Pt-PVP co-ordinate bond is weaker than Pd-PVP co-ordinate bond.
Measure 3b: Platinum atoms coagulate to organize Pt nanocluster while Pd2+ exists as ions
Measure 4b: Pd2+ ions, which coordinated to PVP protecting Pt nanocluster are reduced to organize Pd atoms
Measure 5: Palladium sedimentations on seed Pt nanoclusters to from Pt/Pd bimetallistic nanoparticles.
The ensuing bimetallistic nanoparticles have a Pt-core/Pd-shell construction. The mean diameter of the atoms with a composing of Pd/Pt = 1:4 was every bit little as 1.5nm ( 55 atoms ) . The core/shel1l construction was confirmed by the technique of EAXFS. The consequence indicates that the Pd atoms are catalytically active while Pt atoms are inactive.
Similar procedure was carried out by Bronstein et Al. The Pt/Pd bimetallistic nanoparticles are prepared in molar ratio of 1:4. ( cite nanoparticle and contact action beginning 44 ) Palladium ( II ) ethanoate was reduced by Na borohydride in the presence of polystyrene-block-poly-4-vinylpyridine ( PS-b-P4VP ) in methylbenzene and THF. However, the ensuing nanoparticles have a cluster-in bunch construction alternatively of a core/shell construction as expected. ( cit. Nano and catal p.97 ) The cluster-in-cluster construction was confirmed by CO-FTIR spectrometry. ( nanoparticle and contact action p.124 ) It is proposed that the oxidation-reduction potencies for Pd and Pt are near so a core/shell construction is barely possible.
Structural transmutation of bimetallistic Pd/Pt nanoparticles can besides be achieved by a consecutive burden of chloroplatinic acid onto the Pd loaded accelerator, was investigated with EXAFS at high temperature. ( cite p77 MNiCMS ) The construction of the obtained bimetallistic Pd/Pt nanoparticles seemed to be retained upon heating up to 1273K under ambient status. ( cite p77 MNiCMS ) CO-FTIR spectroscopic measuring on Pd/Pt bimetallistic nanoparticles at different composing raito with Pd-core/Pt-shell construction besides showed when Pd: Pt = 1/4, CO adsorbed on Pd atomes at 1941 cm-1 is wholly absent, which proved that Pd-core has been wholly coverd by Pt-shell. ( MNiCMS p.77 )
Structural Analysis of Bimetallic Nano-structure
The construction of bimetallistic nanoparticles chiefly depends on five factors:
Molar ratio of both component
Chemical bond strength between two sorts of metals
Coordinate bonds between polymer stabilizer and metal ions
Redox potency of both elements
As mentioned above, there are chiefly three man-made paths among which two are executable to synthetize Pd/Pt bimetallistic nanoparticles. Simutaneous decrease spirits core/shell construction while consecutive decrease spirits cluster-in-clusters construction.
Molar ratio of both elements is a dominant factor on the construction of bimetallistic 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. Merely 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 construction can be obtained.
Adhering between Pt-Pt, Pd-Pd and Pt-Pd determines the extent of debasing in the Pt/Pd bimetallistic nanoparticle. In A-B bimetallistic nanoparticles, if A-A bond is preferred to B-B bond, alternatively of organizing a homogenous metal construction, an A-core/B-shell construction is favoured, but the upside-down construction can still be constructed. However, the upside-down core/shell construction is believed to be thermodynamically unstable. In 2012, the latest research conducted by Huang et Al. ( citation ) presented a systematic survey on structural and thermic stablenesss of Pt/Pd bimetallistic nanoparticles with core/shell and alloyed constructions by utilizing atomistic simulations. It was revealed that that the Pd-core/Pt-shell constructions are the least structurally stable, while the Pt-core/Pd-shell nanoparticles are more stable than the alloyed 1s when the Pt per centum exceeds 42 % . Furthermore, Pt-core/Pd-shell structures exibit enhanced thermic stableness as compared to the alloyed 1s for Pt composing more than ca. 30 % . In add-on, the analyses of diffusion behavior and atomic distribution suggest that the minimisation of surface energy tends to organize Pd surface segregation.
However, K. Sasaki et Al. managed to develop a new H fuel cell electrocatalyst based on Pd-core/Pt-shell nanoparticles. The characterized nanoparticles have a individual bed of Pt over a Pd-core. The bimetallistic electrocatalyst showed promising responsiveness and lastingness during charge/discharge trial. The responsiveness of the Pt/Pd electrocatalyst merely dropped 37 % after 100,000 rhythms.
Recent research on Pd/Pt bimetallistic nanoparticles reveals that common readying method of Pd/Pt nanoparticls by coincident decrease would give a Pt-core/Pd-shell construction. The construction is proven to be structurally and thermodynamically stable. However, ad hoc word picture of Pd/Pt bimetallistic nanoparticles with upside-down core/shell construction for H fuel cell electrocatalysis illustrated first-class public presentation and lastingness. Therefore, I strongly believe the Pd/Pt bimetallistic nanoparticle in the ADF-STEM image has a Pt-