Metal and metal sulfide nanoparticles are prepared using a method that is based on the rapid expansion of supercritical fluid solution (RESS) into a liquid solution and characterized using transmission electron microscopy and X-ray diffraction methods. The nanoparticles form solution-like stable suspensions in the presence of a stabilization agent such as poly(N-vinyl-2-pyrrolidone) (PVP) polymer. The stable suspensions allow systematic nonlinear optical measurements. The nanocrystalline silver metal and silver sulfide particles in PVP polymer-stabilized ethanol suspensions of high linear transmittance exhibit excellent optical limiting properties, with the optical limiting responses toward nanosecond laser pulses at 532 nm being much stronger than those of benchmark materials [60]fullerene and chloroaluminum phthalocyanine in solution. A comparison of the results with those of stable suspensions of other nanoparticles including cadmium sulfide, lead sulfide, and nickel suggests that the optical limiting properties are unique to the nanoscopic silver-containing materials. Mechanistic issues concerning the optical limiting performance of the silver-containing nanocrystalline particles are discussed, and a nonlinear absorption mechanism is proposed.

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The preparation and characterization of nanoscale silver sulfide and silver particles in Nafion and perfluorinated sulfonimide ionomer membranes are reported. The results show that the nanoparticles are hosted in the membrane structure in an isolated fashion, with no indication of channels-like domains as proposed in the ion cluster model for the ionomer membranes. These randomly dispersed nanoparticles are also significantly larger than the average size of the reverse micelle-like hydrophilic cavities estimated in the literature for Nafion membrane. The properties of the silver sulfide and silver nanoparticles are presented, and their implications to the understanding of membrane nanoscopic structural details are discussed.

In the development of catalytic materials, a set of standard conditions is needed where the kinetic performance of many samples can be compared. This can be challenging when a sample set covers a broad range of activity. Precise kinetic characterization requires uniformity in the gas and catalyst bed composition. This limits the range of convecting devices to low conversion (generally <20%). While steady-state kinetics offer a snapshot of conversion, yield and apparent rates of the slow reaction steps, transient techniques offer much greater detail of rate processes and hence more information as to why certain catalyst compositions offer better performance. In this work, transient experiments in two transport regimes are compared: an advecting differential plug flow reactor (PFR) and a pure-diffusion temporal analysis of products (TAP) reactor. The decomposition of ammonia was used as a model reaction to test three simple materials: polycrystalline iron, cobalt and a bimetallic preparation of the two. These materials presented a wide range of activity and it was not possible to capture transient information in the advecting device for all samples at the same conditions while ensuring uniformity. We push the boundary for the theoretical estimates of uniformity in the TAP device and find reliable kinetic measurement up to 90% conversion. However, what is more advantageous from this technique is the ability to observe the time-dependence of the reaction rate rather than just singular points of conversion and yield. For example, on the iron sample we observed reversible adsorption of ammonia and on cobalt materials we identify two routes for hydrogen production. From the time-dependence of reactants and product, the dynamic accumulation was calculated. This was used to understand the atomic distribution of H and N species regulated by the surface of different materials. When ammonia was pulsed at 550 °C, the surface hydrogen/nitrogen, (H/N), ratios that evolved for Fe, CoFe and Co were 2.4, 0.25 and 0.3 respectively. This indicates that iron will store a mixture of hydrogenated species while materials with cobalt will predominantly store NH and N. While much is already known about iron, cobalt and ammonia decomposition, the goal of this work was to demonstrate new tools for comparing materials over a wider window of conversion and with much greater kinetic detail. As such, this provides an approach for detailed kinetic discrimination of more complex industrial samples beyond conversion and yield.

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