Optimal ionic strength for nonionically initiated polymerization

Surfactant-free emulsion polymerization involving a nonionic, and hence uncharged initiator presents a new approach towards environmentally friendly procedures to synthesize latex particles. Under optimal solvent conditions, notably pH and ionic strength, the latex particles are stabilized by the natural development of ionic charge at the surface of the particles. We emphasize that the present process does not at all involve the addition of stabilizers such as surfactants or the creation of surface-active species from ionic initiators. The width of the size distribution is found to vary strongly with experimental conditions, notably the ionic strength and to a much lesser extent pH. The phenomenon is explained by a critical ionic strength dependence of the aggregation of the just nucleated primary particles into larger secondary particles, the so-called “coagulative nucleation” step.Chemical EngineeringApplied Science

How to Determine the Core-Shell Nature in Bimetallic Catalyst Particles?

Nanometer-sized materials have significantly different chemical and physical properties compared to bulk material. However, these properties do not only depend on the elemental composition but also on the structure, shape, size and arrangement. Hence, it is not only of great importance to develop synthesis routes that enable control over the final structure but also characterization strategies that verify the exact nature of the nanoparticles obtained. Here, we consider the verification of contemporary synthesis strategies for the preparation of bimetallic core-shell particles in particular in relation to potential particle structures, such as partial absence of core, alloying and raspberry-like surface. It is discussed what properties must be investigated in order to fully confirm a covering, pin-hole free shell and which characterization techniques can provide such information. Not uncommonly, characterization strategies of core-shell particles rely heavily on visual imaging like transmission electron microscopy. The strengths and weaknesses of various techniques based on scattering, diffraction, transmission and absorption for investigating core-shell particles are discussed and, in particular, cases where structural ambiguities still remain will be highlighted. Our main conclusion is that for particles with extremely thin or mono-layered shells—i.e., structures outside the limitation of most imaging techniques—other strategies, not involving spectroscopy or imaging, are to be employed. We will provide a specific example of Fe-Pt core-shell particles prepared in bicontinuous microemulsion and point out the difficulties that arise in the characterization process of such particles.ChemE/Chemical EngineeringApplied Science

Proton binding by linear, branched, and hyperbranched polyelectrolytes

This article reviews our understanding of ionization processes of weak polyelectrolytes. The emphasis is put on a general introduction to site binding models, which are able to account for many experimental features of linear and branched polyelectrolytes, including dendrimers. These models are fully compatible with the classical description of acid-base equilibria. The review further discusses the nature of the site-site interaction and role of conformational equilibria. Experimental charging data of numerous weak polyelectrolytes are discussed in terms of these models in detail.Chemical EngineeringApplied Science

Stimulated-healing of proton exchange membrane fuel cell catalyst

Platinum nanoparticles, which are used as catalysts in Proton Exchange Membrane Fuel Cells (PEMFC), tend to degrade after long-term operation. We discriminate the following mechanisms of the degradation: poisoning, migration and coalescence, dissolution, and electrochemical Ostwald ripening. There are two ways to tackle this problem. The first option involves formulation of durable catalyst, which can resist harsh fuel cell conditions, and this is the conventional route. The second option is reactivation by dissolution and then redeposition of the catalyst nanoparticles, which is an unprecedented method for platinum catalyst regeneration/stimulated-healing and the one we shall discuss. Dissolution of platinum can be achieved electrochemically, by potential cycling of the fuel cell electrode impregnated with platinum nanoparticles in oxygen enriched acidic electrolyte according to following reactions [1]: Pt + H2O?PtO + 2H+ + 2e- (1) PtO + 2H+?Pt2+ + H2O (2) During the potential cycling, platinum oxides are formed at each positive cycle and subsequently dissolved as platinum ions in the electrolyte on the negative cycle. These cycles are alternated continuously. The partial dissolution of platinum nanoparticles results in a decrease in particles size and oxidation of the poisonous species on the platinum surface. The process of dissolution is monitored in-situ via cyclic voltammetry technique. The concentration of dissolved platinum is measured with Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The next step of the regeneration procedure is redeposition of the dissolved platinum back onto the carbon support of the fuel cell electrode. This can be realized by means of electrodeposition. A negative potential is applied to an electrode from where the platinum was dissolved and this results in a reduction of the dissolved platinum ions. Regenerated nanoparticles are characterized by AFM, TEM and XRD. The activity of the catalyst will be checked via voltammetric techniques.ChemE/Chemical EngineeringApplied Science

Electrode compartment for an electrochemical cell, a refreshing system for it and an emulsion to be used therefore

The invention relates to an electrode compartment for an electrochemical cell, including a bicontinuous micro-epulsion, wherein catalytic parts are generated in-situ in a fluid, which can act as a cathode as well as an anode. The electrode compartment comprises a connection to supply fuel or an oxidator, for example oxygen, to the compartment. The electrode compartment is part of a refreshing system with a reserve container for an emulsion and a storage container for used emulsion, conduits to connect each of the containers with the electrode compartment and a transport unit, for example a pump, to move the emulsion.DelftChemTechApplied Science

The Effect of Magnetic Field on Catalytic Properties in Core-Shell Type Particles

Magnetic field effects can provide a handle on steering chemical reactions and manipulating yields. The presence of a magnetic field can influence the energy levels of the active species by interacting with their spin states. Here we demonstrate the effect of a magnetic field on the electrocatalytic processes taking place on platinum-based nanoparticles in fuel cell conditions. We have identified a shift in the potentials representing hydrogen adsorption and desorption, present in all measurements recorded in the presence of a magnetic field. We argue that the changes in electrochemical behavior are a result of the interactions between the magnetic field and the unpaired spin states of hydrogen.RST/Storage of Electrochemical EnergyChemE/Advanced Soft Matte

Bicontinuous microemulsions for high yield, wet synthesis of ultrafine nanoparticles: A general approach

The design of a synthesis strategy for metal nanoparticles by templating dense microemulsions is proposed. Particle size is controlled by surfactant size rather than by microemulsion composition. The strategy was demonstrated with various systems with different surfactant: cationic, anionic and non-ionic and of different sizes. Formulations were determined using the microemulsion phase diagrams. Synthesis was demonstrated for platinum nanoparticles with some examples for gold. The nanoparticles were subsequently extracted from the microemulsion by absorption onto a carbon support, after which the surfactant was recycled.Chenical EngineeringApplied Science

Synthesis of Durable Platinum Catalyst for Proton Exchange Membrane Fuel Cells in Bicontinuous Microemulsion (abstract)

ChemE/Chemical EngineeringApplied Science

The effect of lattice strain on catalytic activity

We report on the effect of lattice strain in three different types of core-shell electrocatalyst particles on their catalytic activity towards the oxygen reduction reaction. We decouple the changes in catalytic activity with respect to a geometrical and an energetic contribution, both of electronic origin.ChemE/Advanced Soft Matte

Reversible Nanoparticle Formation As a Potential Strategy for PEMFC Catalyst Regeneration (abstract)

ChemE/Chemical EngineeringApplied Science