Electroless deposition of platinum nanoparticles in room-temperature ionic liquids

The electroless deposition of Pt nanoparticles (Pt-NPs) could be carried out by dissolving potassium tetrachloroplatinate(II) (K2[PtCl4]) in 1-ethyl-3-methylimidazolium (EMI(+)) room-temperature ionic liquids (RTILs) containing bis(trifluoromethylsulfonyl) imide (NTf2(-)) or tetrafluoroborate (BF4(-)) anion and small cations, such as H(+), K(+), and Li(+). In this case, no deposition of Pt-NPs occurred in RTILs without such small cations. The formation of Pt-NPs was only observed in RTILs containing trifluoromethanesulfonimide (HNTf2) and protons at high temperature (≥80 °C) when potassium hexachloroplatinate(IV) (K2[PtCl6]) was dissolved in the RTILs. The obtained Pt-NPs gave a characteristic absorption spectrum of ultrasmall Pt-NPs. The ultrasmall and uniform Pt-NPs of ca. 1-4 nm in diameter were produced and the Pt-NPs/EMI(+)NTf2(-) dispersion was kept stably for several months without adding any additional stabilizers or capping molecules. The identified Fourier-transform patterns along the [0 1 1] zone axis were observed for the TEM images of Pt-NPs. On the basis of the results obtained, a probable mechanism of the electroless formation of Pt-NPs is discussed.

Nitrate Reduction on Platinum (111) Surfaces Modifiedl with Bi: Single Crystalsl and Nanoparticles

Nitrate reduction on well-oriented platinum surfaces modified with Bi adatoms has been studied. The quantification of the electrocatalytic enhancement of the reaction rate due to the presence of Bi at different coverages was made on Pt(111) and the vicinal surfaces Pt(554) and Pt(332). These contain 9 and 5 atoms-width (111) terraces, respectively, separated by (110) monoatomic steps. The study was then extended to preferentially {111}Pt oriented nanoparticles. In all cases, Bi catalyzes nitrate reduction at high potentials, but the catalytic current suddenly drops when Bi is reduced. The analysis of the variation of catalytic activity with Bi coverage reveals the participation of a third body effect, meaning that Bi impedes the NO formation on the surface that acts as a poison for the nitrate reduction.

Evidence of an Eley-Rideal mechanism in the stripping of a saturation layer of chemisorbed CO on platinum nanoparticles

The oxidative stripping of a saturation layer of CO(chem) was studied on platinum nanoparticles of high shape selectivity and narrow size distribution. Nanospheres, nanocubes, and nano-octahedrons were synthesized using the water-in-oil microemulsion or polyacrylate methods. The three shapes allowed examination of the CO(chem) stripping in relation to the geometry of the nanoparticles and presence of specific nanoscopic surface domains. Electrochemical quartz crystal nanobalance (EQCN) measurements provided evidence for the existence of more than one mechanism in the CO(chem) stripping. This was corroborated by chronoamperometry transient for a CO(chem) saturation layer at stripping potentials of E(strip) = 0.40, 0.50, 0.60, and 0.70 V. The first mechanism is operational in the case of CO(chem) stripping at lower E(strip) values; it proceeds without adsorption of anions or H(2)O molecules and corresponds to desorption of a fraction of CO(chem) in the form of a prepeak in voltammograms or in the form of an exponential decay in chrono-amperometry (CA) transients. The second mechanism is operational in the desorption of the remaining CO(chem) at higher E(strip) values and gives rise to at least two voltammetric peaks or two CA peaks. Analysis of the experimental data and modeling of the CA transients lead to the conclusion that the stripping of a saturation layer of CO(chem) first follows an Eley-Rideal mechanism in the early stage of the process and then a Langmuir-Hinshelwood mechanism.

In situ shaping of Pt nanoparticles directly overgrown on carbon supports

Pt cubes immobilized on carbon supports were synthesized by a one pot process in the presence of anchoring agents. The anchoring agents provide nucleating sites on the support, and also act as shape-controlling agents. In situ shaped cubic Pt/C showed superior activity and long-term stability for oxygen reduction reaction without an organic removal process.

Electro-oxidation of CO(chem) on Pt nanosurfaces: solution of the peak multiplicity puzzle

An understanding of the oxidation of chemisorbed CO (CO(chem)) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO(chem) oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO(chem) preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO(chem) oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO(chem) oxidative stripping unambiguously.

Electrodeposition of platinum nanoparticles in a room-temperature ionic liquid

The electrochemistry of the [PtCl(6)](2-)-[PtCl(4)](2-)-Pt redox system on a glassy carbon (GC) electrode in a room-temperature ionic liquid (RTIL) [i.e., N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEMEBF(4))] has been examined. The two-step four-electron reduction of [PtCl(6)](2-) to Pt, i.e., reduction of [PtCl(6)](2-) to [PtCl(4)](2-) and further reduction of [PtCl(4)](2-) to Pt, occurs separately in this RTIL in contrast to the one-step four-electron reduction of [PtCl(6)](2-) to Pt in aqueous media. The cathodic and anodic peaks corresponding to the [PtCl(6)](2-)/[PtCl(4)](2-) redox couple were observed at ca. -1.1 and 0.6 V vs a Pt wire quasi-reference electrode, respectively, while those observed at -2.8 and -0.5 V were found to correspond to the [PtCl(4)](2-)/Pt redox couple. The disproportionation reaction of the two-electron reduction product of [PtCl(6)](2-) (i.e., [PtCl(4)](2-)) to [PtCl(6)](2-) and Pt metal was also found to occur significantly. The electrodeposition of Pt nanoparticles could be carried out on a GC electrode in DEMEBF(4) containing [PtCl(6)](2-) by holding the potential at -3.5 or -2.0 V. At -3.5 V, the four-electron reduction of [PtCl(6)](2-) to Pt can take place, while at -2.0 V the two-electron reduction of [PtCl(6)](2-) to [PtCl(4)](2-) occurs. The results obtained demonstrate that the electrodeposition of Pt at -3.5 V may occur via a series of reductions of [PtCl(6)](2-) to [PtCl(4)](2-) and further [PtCl(4)](2-) to Pt and at -2.0 V via a disproportionation reaction of [PtCl(4)](2-) to [PtCl(6)](2-) and Pt. Furthermore, the deposition potential of Pt nanoparticles was found to largely influence their size and morphology as well as the relative ratio of Pt(110) and Pt(100) crystalline orientation domains. The sizes of the Pt nanoparticles prepared by holding the electrode potential at -2.0 and -3.5 V are almost the same, in the range of ca. 1-2 nm. These small nanoparticles are "grown" to form bigger particles with different morphologies: In the case of the deposition at -2.0 V, the GC electrode surface is totally, relatively compactly covered with Pt particles of relatively uniform size of ca. 10-50 nm. On the other hand, in the case of the electrodeposition at -3.5 V, small particles of ca. 50-100 nm and the grown-up particles of ca. 100-200 nm cover the GC surface irregularly and coarsely. Interestingly, the Pt nanoparticles prepared by holding the potential at -2.0 and -3.5 V are relatively enriched in Pt(100) and Pt(110) facets, respectively.

Exploring the first steps in core-shell electrocatalyst preparation: in situ characterization of the underpotential deposition of Cu on supported Au nanoparticles

The underpotential deposition (upd) of a Cu shell on a non-Pt nanoparticle core followed by galvanic displacement of the Cu template shell to form core–shell electrocatalyst materials is one means by which the Pt-based mass activity targets required for commercialization of PEM fuel cells may be reached. In situ EXAFS measurements were conducted at both the Au L₃ and the Cu K absorption edges during deposition of Cu onto a carbon-supported Au electrocatalyst to study the initial stages of formation of such a core–shell electrocatalyst. The Au L₃ EXAFS data obtained in 0.5 mol dm^–3 H₂SO₄ show that the shape of the Au core is potential dependent, from a flattened to a round spherical shape as the Cu upd potential is approached. Following the addition of 2 mmol dm^–3 Cu, the structure was also measured as a function of the applied potential. At +0.2 V vs Hg/Hg₂SO₄, the Cu²⁺ species was found to be a hydrated octahedron. As the potential was made more negative, single-crystal studies predict an ordered bilayer of sulfate anions and partially discharged Cu ions, followed by a complete/uniform layer of Cu atoms. In contrast, the model obtained by fitting the Au L₃ and Cu K EXAFS data corresponds first to partially discharged Cu ions deposited at the defect sites in the outer shell of the Au nanoparticles at −0.42 V, followed by the growth of clusters of Cu atoms at −0.51 V. The absence of a uniform/complete Cu shell, even at the most negative potentials investigated, has implications for the structure, and the activity and/or stability, of the core–shell catalyst that would be subsequently formed following galvanic displacement of the Cu shell.

The effect of Bi adsorption on CO oxidation inside 1.8 nm Pt pores

Modification of the surface of H(1)-e Pt with Bi causes significant changes in the CO stripping voltammetry; the pre-wave disappears and CO and Bi oxidation peaks appear. The absence of the pre-wave suggests that Bi preferentially adsorbs on the trough sites of the concave 1.8 nm diameter pore walls preventing oxygenated species from nucleating there.

Structure sensitivity and nanoscale effects in electrocatalysis

This review discusses the role of the detailed nanoscale structure of catalytic surfaces on the activity of various electrocatalytic reactions of importance for fuel cells, hydrogen production, and other environmentally important catalytic reactions, such as carbon monoxide oxidation, methanol and ethanol oxidation, ammonia oxidation, nitric oxide reduction, hydrogen evolution, and oxygen reduction. Specifically, results and insights obtained from surface-science single-crystal-based model experiments are linked to experiments on well-defined shape-controlled nanoparticles. A classification of structure sensitive effects in electrocatalysis is suggested, based both on empirical grounds and on quantum-chemical viz. thermochemical considerations. The mutual relation between the two classification schemes is also discussed. The review underscores the relevance of single-crystal modeling of nanoscale effects in catalysis, and points to the special role of two kinds of active sites for electrocatalysis on nanoparticulate surfaces: (i) steps and defects in (111) terraces or facets, and (ii) long-range (100) terraces or facets.