Large-scale engineered synthesis of BaTiO3 nanoparticles using low-temperature bioinspired principles.

We report here a robust, large-scale synthesis of BaTiO3 nanopowders using a bioinspired process that first was developed on a much smaller scale. The most advantageous points of this protocol are that it takes place at nearly room temperature (25°C), overcomes many limitations encountered in other scale-up processes (such as the need for external drivers, e.g., heat, radiation or pressure), bypasses the use of surfactants and templates and does not necessitate pH adjustment. The use of a single-source, bimetallic alkoxide with the vapor diffusion of a hydrolytic catalyst (H2O) provides the necessary conditions for facile crystallization and growth of small, well-defined BaTiO3 nanoparticles at mild temperatures, yielding batches of up to 250 ± 5 g in a green process. Extension of this method to kilogram-scale production of BaTiO3 nanocrystals in semicontinuous and continuous processes is feasible.

High-efficiency aminocarbonylation by introducing CO to a pressurized continuous flow reactor.

Halogenated aryl carboxylic acids were efficiently converted to the corresponding dicarboxylic acid monoamides by a one-step Pd-catalyzed aminocarbonylation in a micro/meso fluidic continuous flow reactor (X-Cube) operated at high pressure and high temperature with CO gas introduction. Reaction parameters (solvent, base, catalyst, pressure, temperature) were rapidly optimized in the reactions, which required less than 2 min. The method gave improved results over comparable batch techniques and is also suited to automated parallel syntheses of compound libraries.

Adsorption and co-adsorption of ethylene and carbon monoxide on silica-supported monodisperse Pt nanoparticles: volumetric adsorption and infrared spectroscopy studies.

The adsorption of carbon monoxide and ethylene, and their sequential adsorption, was studied over a series of Pt/SBA-15 catalysts with monodisperse particle sizes ranging from 1.7 to 7.1 nm by diffuse-reflectance infrared spectroscopy and chemisorption. Gas adsorption was dependent on the Pt particle size, temperature, and sequence of gas exposure. Adsorption of CO at room temperature on Pt/SBA-15 gives rise to a spectroscopic feature assigned to the C-O stretch: nu(CO) = 2075 cm-1 (1.9 nm); 2079 cm-1 (2.9 nm); 2082 cm-1 (3.6 nm); and 2090 cm-1 (7.1 nm). The intensity of the signal decreased in a sigmoidal fashion with increasing temperature, thereby providing semiquantitative surface coverage information. Adsorption of ethylene on Pt/SBA-15 gave rise to spectroscopic features at approximately 1340, approximately 1420, and approximately 1500 cm-1 assigned to ethylidyne, di-sigma-bonded ethylene, and pi-bonded ethylene, respectively. The ratio of these surface species is highly dependent on the Pt particle size. At room temperature, Pt particles stabilize ethylidyne as well as di-sigma- and pi-bonded ethylene; however, ethylidyne predominated on the surfaces of larger particles. Ethylidyne was the only identifiable species at 403 K, with its formation being more facile on larger particles. Co-adsorption experiments reveal that the composition of the surface layer is dependent on the order of exposure to gases. Exposure of a C2H4-covered Pt surface to CO resulted in an approximately 50% decrease in chemisorbed CO compared to a fresh Pt surface. The nu(CO) appeared at 2050 cm-1 on Pt/SBA-15 pretreated with C2H4 at room temperature. The di-sigma-bonded and pi-bonded species are the most susceptible to displacement from the surface by CO. The formation of ethylidyne appeared to be less sensitive to the presence of adsorbed carbon monoxide, especially on larger particles. Upon exposure of C2H4 to a CO-covered Pt surface, little irreversible uptake occurred due to nearly 100% site blocking. These results demonstrate that carbon monoxide competes directly with ethylene for surface sites, which will have direct implications on the poisoning of the heterogeneously catalyzed conversion of hydrocarbons.

Rhodium nanoparticles from cluster seeds: control of size and shape by precursor addition rate.

The size-tunable synthesis of poly(vinylpyrrolidone)-stabilized cuboctahedral rhodium nanoparticles with mean diameters ranging between 3-7 nm and multipod structures was accomplished using seeded growth methods. Isotropic PVP-capped 2.9 nm seeds were prepared by ligand exchange on rhodium-triphenylphosphine metal-organic clusters. Quantitative investigation of reaction parameters in ethylene glycol revealed that size and shape could be controlled at a single reaction temperature of 120 degrees C. The rate of rhodium monomer addition was found to be critical for monodispersity and shape control, regardless of thermodynamic factors. Solvent viscosity, varied by changing the polyol solvents, indicated that autocatalytic addition kinetics are responsible for isotropic versus anisotropic growth.

Hydrothermal growth of mesoporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: synthesis, characterization, and catalytic properties.

A novel high surface area heterogeneous catalyst based on solution phase colloidal nanoparticle chemistry has been developed. Monodisperse platinum nanoparticles of 1.7-7.1 nm have been synthesized by alcohol reduction methods and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis. Characterization of the Pt/SBA-15 catalysts suggests that Pt particles are located within the surfactant micelles during silica formation leading to their dispersion throughout the silica structure. After removal of the templating polymer from the nanoparticle surface, Pt particle sizes were determined from monolayer gas adsorption measurements. Infrared studies of CO adsorption revealed that CO exclusively adsorbs to atop sites and red-shifts as the particle size decreases suggesting surface roughness increases with decreasing particle size. Ethylene hydrogenation rates were invariant with particle size and consistent with a clean Pt surface. Ethane hydrogenolysis displayed significant structure sensitivity over the size range of 1-7 nm, while the apparent activation energy increased linearly up to a Pt particle size of approximately 4 nm and then remained constant. The observed rate dependence with particle size is attributed to a higher reactivity of coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in large particles. The most reactive of these unsaturated surface atoms are responsible for ethane decomposition to surface carbon. The ability to design catalytic structures with tunable properties by rational synthetic methods is a major advance in the field of catalyst synthesis and for the development of accurate structure-function relationships in heterogeneous reaction kinetics.

Precise control of the Pt nanoparticle size by seeded growth using EO13PO30EO13 triblock copolymers as protective agents.

Here, we report an efficient way to produce homogeneous Pt nanoparticles within a well-defined size range (3.5-6.6 nm) as a result of the seeded growth procedure using Pluronic L64 polymer capping agent. First, small seeds (3.5 nm) were prepared by the reduction of H2PtCl6.6H2O in water with NaBH4 in the presence of the capping poly(ethylene oxide)13-poly(propylene oxide)30-poly(ethylene oxide)13 triblock copolymer at room temperature. Additional anionic Pt salt was then introduced under flowing H2 to obtain larger nanoparticles.

Sol-gel synthesis of ordered mesoporous alumina.

Well-ordered mesoporous alumina materials with high surface area and a narrow pore size distribution were synthesized using a sol-gel based self assembly technique.

Radial anisotropic growth of rhodium nanoparticles.

In this contribution, we report the synthesis of rhodium multipods that result from a homogeneous seeded growth mechanism. Small Rh nanocrystal seeds were synthesized by the reduction of RhCl3 in ethylene glycol in the presence of PVP. These seed particles could be subsequently used, without isolation, to form larger rhodium nanoparticles. A reaction temperature of 190 degrees C led to isotropic cubic Rh particles. Lowering the reaction temperature resulted in more anisotropic growth, which gave Rh cubes with horns at 140 degrees C, and Rh multipods at 90 degrees C. The anisotropic growth occurred in the (111) direction, as determined by high-resolution TEM (HRTEM). Anisotropic growth proceeds via a seeded growth mechanism, and not by oriented attachment.

Structure sensitivity of vibrational spectra of mesoporous silica SBA-15 and Pt/SBA-15.

The vibrational properties of mesoporous silica (SBA-15) were investigated by deep ultraviolet (UV) Raman and infrared spectroscopies with and without the presence of platinum nanoparticles in the mesopores that were incorporated by sonication. Raman and IR spectral line assignments were made by comparison to amorphous silicas. This procedure permitted identification of vibrations of longitudinal (LO) and transverse (TO) optical lattice modes, the presence of Si-OH, and vibrational modes associated with the presence of three-, four-, and six-membered siloxane rings. Hydraulic pressing of the mesoporous silica with pressure in the range 3-7 tons cm(-2) destroys the X-ray diffraction pattern and strongly decreases the Raman peak (D2) associated with three-membered rings at the surface. In the presence of platinum nanoparticles in the silica mesopores, a peak attributed to a Pt-O stretching vibration appears at between 530 and 580 cm(-1) in the UV-Raman spectrum, which can be used to monitor the presence of the platinum particles and their interaction with the support. The D2 feature in the UV-Raman spectra also decreases with increasing Pt loading, which is attributed to interactions of the Pt nanoparticles with the silica surface.