Synthesis of PtNi Nanoparticles to Accelerate the Oxygen Reduction Reaction.

We report the synthesis of core-shell Ni-Pt nanoparticles (NPs) with varying degrees of crystallographic facets and surface layers rich in Pt via a seed-mediated thermolytic approach. Mixtures of different surfactants used during synthesis resulted in preferential surface passivation, which in turn dictated the size, chemical composition, and geometric evolution of these PtNi NPs. Electrochemical investigations of these pristine core-shell Ni-Pt structures in the oxygen reduction reaction (ORR) show that their catalytic functionalities outperform the commercial Pt/C reference catalyst. The enhanced electrocatalytic ORR performances of these Pt-based PtNi NPs are correlated with the weakened oxygen binding strength or surface-adsorbed hydroxyl (OH) species on active Pt surface sites induced by the downshift of the d-band center as a result of compressive strain effects. Our studies offer a robust synthetic approach for the development of core-shell nanostructures for enhanced ORR catalysis.

Platinum-Catalysed Selective Aerobic Oxidation of Methane to Formaldehyde in the Presence of Liquid Water.

The aerobic, selective oxidation of methane to C1 -oxygenates remains a challenge, due to the more facile, consecutive oxidation of formed products to CO2 . Here, we report on the aerobic selective oxidation of methane under continuous flow conditions, over platinum-based catalysts yielding formaldehyde with a high selectivity (reaching 90 % for Pt/TiO2 and 65 % over Pt/Al2 O3 ) upon co-feeding water. The presence of liquid water under reaction conditions increases the activity strongly attaining a methane conversion of 1-3 % over Pt/TiO2 . Density-functional theory (DFT) calculations show that the preferential formation of formaldehyde is linked to the stability of the di-σ-hydroxy-methoxy species on platinum, the preferred carbon-containing species on Pt(111) at a high chemical potential of water. Our findings provide novel insights into the reaction pathway for the Pt-catalysed, aerobic selective oxidation of CH4 .

Formation of Pt-Based Alloy Nanoparticles Assisted by Molybdenum Hexacarbonyl.

We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fine-tuned monodisperse nanostructures (Pt(NiCo), PtNi and PtCo). The influence of different solute metal precursors and surfactants on the morphological evolution of homogeneous alloy nanoparticles (NPs) has been investigated. Molybdenum hexacarbonyl (Mo(CO)6) was used as the reductant. We demonstrate that this solution-based strategy results in uniform-sized NPs, the morphology of which can be manipulated by appropriate selection of surfactants and solute metal precursors. Co-surfactants (oleylamine, OAm, and hexadecylamine, HDA) enabled the development of a variety of high-index faceted NP morphologies with varying degrees of curvatures while pure OAm selectively produced octahedral NP morphologies. This Mo(CO)6-based synthetic protocol offers new avenues for the fabrication of multi-structured alloy NPs as high-performance electrocatalysts.

Topographical and compositional engineering of core-shell Ni@Pt ORR electro-catalysts.

Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance catalytic performance. We report on the preparation of binary PtNi NPs via a co-thermolytic approach in which we optimize the synthesis variables, which results in significantly improved catalytic performance. We used scanning transmission electron microscopy to characterise the range of morphologies produced, which included spherical and concave cuboidal core-shell structures. Electrocatalytic activity was evaluated using a rotating disc electrode (1600 rpm) in 0.1 M HClO4; the electrocatalytic performance of these Ni@Pt NPs showed significant (∼11-fold) improvement compared to a commercial Pt/C catalyst. Extended cycling revealed that electrochemical surface area was retained by cuboidal PtNi NPs post 5000 electrochemical cycles (0.05-1.00 V, vs. SHE). This is attributed to the enclosure of Ni atoms by a thick Pt shell, thus limiting Ni dissolution from the alloy structures. The novel synthetic strategy presented here results in a high yield of Ni@Pt NPs which show excellent electro-catalytic activity and useful durability.

Performance of a NiFe2O4@Co Core-Shell Fischer-Tropsch Catalyst: Effect of Low Temperature Reduction.

In situ TEM gas-cell imaging and spectroscopy with in situ XRD have been applied to reveal morphological changes in NiFe2O4@Co3O4 core-shell nanoparticles in hydrogen. The core-shell structure is retained upon reduction under mild conditions (180 °C for 1 h), resulting in a partially reduced shell. The core-shell structure was retained after exposing these reduced NiFe2O4@Co3O4 core-shell nanoparticles to Fischer-Tropsch conditions at 230 °C and 20 bar. Slightly harsher reduction (230 °C, 2 h) resulted in restructuring of the NiFe2O4@Co3O4 core-shell nanoparticles to form cobalt islands in addition to partially reduced NiFe2O4. NiFe2O4 underwent further transformation upon exposure to Fischer-Tropsch conditions, resulting in the formation of iron carbide and nickel/iron-nickel alloy. The turnover frequency in the Fischer-Tropsch synthesis over NiFe2O4@Co3O4 core-shell nanoparticles reduced in hydrogen at 180 °C for 1 h was estimated to be less than 0.02 s-1 (cobalt-time yield of 8.40 µmol.g-1.s-1) with a C5+ selectivity of 38 C-%. The low turnover frequency under these conditions in relation to the turnover frequency obtained with unsupported cobalt is attributed to the strain in the catalytically active cobalt.

Thermal Properties and Segregation Behavior of Pt Nanowires Modified with Au, Ag, and Pd Atoms: A Classical Molecular Dynamics Study.

Platinum nanowires (NWs) have been reported to be catalytically active toward the oxygen reduction reaction (ORR). The edge modification of Pt NWs with metals M (M = Au, Ag, or Pd) may have a positive impact on the overall ORR activity by facilitating diffusion of adsorbed oxygen, Oads, and hydroxyl groups, OHads, between the {001} and {111} terraces. In the present study, we have employed classical molecular dynamics simulations to investigate the segregation behavior of Au, Ag, and Pd decorating the edges of Pt NWs. We observe that, under vacuum conditions, Pd prefers to diffuse toward the core rather than stay on the NW surface. Ag and Au atoms are mobile at temperatures as low as 900 K; they remain on the surface but do not appear to be preferentially more stable at edge sites. To effect segregation of Au and Ag atoms toward the edge, we propose annealing in the presence of different reactive gas environments. Overall, our study suggests potential experimental steps required for the synthesis of Pt nanowires and nanoparticles with improved Oads and OHads interfacet diffusion rates and consequently an improved ORR activity.

Metal-support interaction on cobalt based FT catalysts - a DFT study of model inverse catalysts.

It is challenging to isolate the effect of metal-support interactions on catalyst reaction performance. In order to overcome this problem, inverse catalysts can be prepared in the laboratory and characterized and tested at relevant conditions. Inverse catalysts are catalysts where the precursor to the catalytically active phase is bonded to a support-like ligand. We can then view the metal-support interaction as a ligand interaction with the support acting as a supra-molecular ligand. Importantly, laboratory studies have shown that these ligands are still present after reduction of the catalyst. By varying the quantity of these ligands present on the surface, insight into the positive effect SMSI have during a reaction is gained. Here, we present a theoretical study of mono-dentate alumina support based ligands, adsorbed on cobalt surfaces. We find that the presence of the ligand may significantly affect the morphology of a cobalt crystallite. With Fischer-Tropsch synthesis in mind, the CO dissociation is used as a probe reaction, with the ligand assisting the dissociation, making it feasible to dissociate CO on the dense fcc Co(111) surface. The nature of the interaction between the ligand and the probe molecule is characterized, showing that the support-like ligands' metal centre is directly interacting with the probe molecule.

Size-dependent phase transformation of catalytically active nanoparticles captured in situ.

The utilization of metal nanoparticles traverses across disciplines and we continue to explore the intrinsic size-dependent properties that make them so unique. Ideal nanoparticle formulation to improve a process's efficiency is classically presented as exposing a greater surface area to volume ratio through decreasing the nanoparticle size. Although, the physiochemical characteristics of the nanoparticles, such as phase, structure, or behavior, may be influenced by the nature of the environment in which the nanoparticles are subjected1, 2 and, in some cases, could potentially lead to unwanted side effects. The degree of this influence on the particle properties can be size-dependent, which is seldom highlighted in research. Herein we reveal such an effect in an industrially valuable cobalt Fischer-Tropsch synthesis (FTS) catalyst using novel in situ characterization. We expose a direct correlation that exists between the cobalt nanoparticle's size and a phase transformation, which ultimately leads to catalyst deactivation.

Some insights in the sonochemical preparation of cobalt nano-particles.

The preparation of cobalt nano-particles from a solution of Co(CO)(3)(NO) in n-decane under ultrasonication with a frequency of 20 kHz yielded cobalt particles of a size of ca. 5 nm. The presence of either silica or oleic acid in the solution reduced the particle size to ca. 3 and 2 nm, respectively. The resulting particle size is independent of the ultrasonication time, initial Co(CO)(3)(NO) concentration, ultrasound intensity and solution temperature. It is postulated that bubble collapse generates multiple nucleation sites resulting in the formation of cobalt particles with a rather uniform particle size distribution.

Stability of nanocrystals: thermodynamic analysis of oxidation and re-reduction of cobalt in water/hydrogen mixtures.

The stability of nanosized materials differs significantly from the stability of bulk materials. In this study a thermodynamic analysis on the simultaneous oxidation and re-reduction of small metallic cobalt crystallites in the presence of water and hydrogen as a function of the crystallite diameter was performed as a model for catalyst deactivation in the Fischer-Tropsch synthesis. It is shown that spherical cobalt crystallites with a diameter less than 4.4 nm are likely to be oxidized under realistic Fischer-Tropsch synthesis conditions (p(H)(2)(O)/p(H)(2) < 1.5, T = 493 K).