Density functional theory is used for the evaluation of surface segregation, trends for dissolution of Pt surface atoms in acid medium, and oxygen reduction reaction activity of core-shell materials, containing a monolayer of platinum over a monometallic or bimetallic core. Two groups of cores are investigated: Pt/X with X = Ir, Au; Pd, Rh, Ag; Co, Ni, Cu; and Pt/Pd(3)X, with X = Co, Fe, Cr, V, Ti, Ir, Re. It is found that all the 4d and 5d pure cores may serve as stable cores, and their beneficial effect on the Pt monolayer may be further tuned by alloying the core to another element, here chosen from 3d or 5d groups. The Pd(3)X cores enhance the stability of the surface Pt atoms both in vacuum and under adsorbed oxygen; however the high oxygen philicity of some of the X elements induces their surface segregation that may cause surface poisoning with oxygenated species and their dissolution in acid medium.
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"However, some of the Pt-TM alloys are electrochemically unstable due to the dissolution of the surface TM atoms   in acidic media. Thus, many Pt-alloys formed from Platinum Group Metals (PGMs) are proposed for MOR    and ORR  , demonstrating higher catalytic activities and improved electrochemical stability compared to bimetallic Pt-TM alloys . Among Pt-PGM alloys, Pt-Ir displayed improved activities toward "
[Show abstract][Hide abstract] ABSTRACT: The comprehensive investigation of ternary PtIrCo-alloy nanoparticles supported on 3D carbon aerogel matrix toward Methanol Oxidation Reaction (MOR) and Oxygen Reduction Reaction (ORR) is focused on CO tolerance in MOR, methanol vs. oxygen selectivity in ORR, and the structural changes within the metal alloy nanoparticles caused by the heat treatment and the electrochemical conditioning. Amorphous tri-metallic Pt59Ir34Co7-nanoparticles with mean size of 1.47 ± 0.21 nm were formed within 3D CA matrix in supercritical CO2 (scCO2) from the Pt, Ir, and Co organometallic precursors. Depending on the heat-treatment temperature (600 °C or 900 °C), the amorphous organometallic phase of Pt, Ir and Co was transformed into a single-phase fcc PtIrCo-alloy nanoparticles with a mean particle size of 1.68 ± 0.33 nm and 2.68 ± 0.61 nm, respectively. The fcc lattice parameters of the PtIrCo-alloy (0.389 nm at 600 °C and 0.386 nm at 900 °C) were smaller than that of the Pt fcc lattice (0.392 nm) and the lattice parameters reduced proportionally to the heat-treatment temperatures. After 50 electrochemical cycles, PtIrCo/CA catalysts display improved MOR specific activity (∼2.5 X Pt/C), mass activity (∼3 X Pt/C) at 0.7 V vs. SHE, higher CO-tolerance, and better electrochemical stability than Pt/C, but less CO-tolerance and stability than PtRu/C. Regarding ORR, the PtIrCo/CA catalysts demonstrate enhanced ORR specific activity (∼5.5 X Pt/C) and mass activity (∼6.3 X Pt/C) at 0.7 V vs. SHE along with superior methanol-tolerance in comparison to Pt/C. The structural changes associated with PtIrCo/CA fcc-lattice parameter with respect to Pt-lattice have contributed to better MOR and ORR activities. Faster surface diffusion associated with weak adsorption of hydroxide species and higher selective adsorption of oxygen in comparison to methanol onto PtIrCo-alloy surface could have contributed to their better CO-tolerant MOR and methanol-resistant ORR activity compared to Pt-catalyst.
"Moreover, we note that the effect of the reduced lattice constant of the Ir-Co cores by the Pd interlayer clearly is reflected in the adsorption energies of the adsorbates. Ramírez-Caballero et al.  recently reported adsorption energies of O and OH, respectively, on Pt/Pd (bulk) as −4.21 and −2.33 eV; the values of O and OH, correspondingly, for Pt/PdIr 3 Co are −3.62 and −2.47 eV, and for Pt/Pd/IrCo are −3.48 and −2.10 eV. To summarize, this section describes a simple method of improving Pt monolayer core-shell catalysts that have an inadequate Pt-core interaction causing their relatively low ORR activity. "
[Show abstract][Hide abstract] ABSTRACT: This paper demonstrates that the ORR activity of Pt ML electrocatalysts can be further improved by the modification of surface and subsurface of the core materials. The removal of surface low-coordination sites, generation (via addition or segregation) of an interlayer between Pt ML and the core, or the introduction of a second metal component to the subsurface layer of the core can further improve the ORR activity and/or stability of Pt ML electrocatalysts. These modifications generate the alternation of the interactions between the substrate and the Pt ML , involving the changes on both electronic (ligand) and geometric (strain) properties of the substrates. The improvements resulted from the application of these approaches provide a new perspective to designing of the new generation Pt ML electrocatalysts.
Full-text · Article · Nov 2011 · Advances in Physical Chemistry
[Show abstract][Hide abstract] ABSTRACT: Interfacial interactions between sub-4 nm metal alloy nanoparticles and carbon supports, although not well understood at the atomic level, may be expected to have a profound influence on catalytic properties. Pd3Pt2 alloy particles comprised of a disordered surface layer over a corrugated crystalline core are shown to exhibit strong interfacial interactions with a ∼20–50 nm spherical carbon support, as characterized by probe aberration corrected scanning transmission electron microscopy (pcSTEM). The disordered shells were formed from defects introduced by Pd during arrested growth synthesis of the alloy nanoparticles. The chemical and morphological changes in the catalyst, before and after cyclic stability testing (1000 cycles, 0.5–1.2 V), were probed with cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and pcSTEM. The strong metal–support interaction, along with the uniform alloy structure raised the mass activity by a factor of 1.8 versus pure Pt. The metal–support interactions also mitigated nanoparticle coalescence, dissolution, and ripening, resulting in only a 20% loss in mass activity (versus 60% for pure Pt on carbon) after the cyclic stability test. The design of alloy structure, guided by insight from atomic scale pcSTEM, for enhanced catalytic activity and stability, resulting from strong wetting with a deformable disordered shell, has the potential to be a general paradigm for improving catalytic performance.
No preview · Article · Mar 2012 · Electrochimica Acta