John N. Kuhn

Sub-10 nm Platinum Nanocrystals with Size and Shape Control: Catalytic Study for Ethylene and Pyrrole Hydrogenation

Platinum nanocubes and nanopolyhedra with tunable size from 5 to 9 nm were synthesized by controlling the reducing rate of metal precursor ions in a one-pot polyol synthesis. A two-stage process is proposed for the simultaneous control of size and shape. In the first stage, the oxidation state of the metal ion precursors determined the nucleation rate and consequently the number of nuclei. The reaction temperature controlled the shape in the second stage by regulation of the growth kinetics. These well-defined nanocrystals were loaded into MCF-17 mesoporous silica for examination of catalytic properties. Pt loadings and dispersions of the supported catalysts were determined by elemental analysis (ICP-MS) and H(2) chemisorption isotherms, respectively. Ethylene hydrogenation rates over the Pt nanocrystals were independent of both size and shape and comparable to Pt single crystals. For pyrrole hydrogenation, the nanocubes enhanced ring-opening ability and thus showed a higher selectivity to n-butylamine as compared to nanopolyhedra.

Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles

A continuing goal in catalysis is to unite the advantages of homogeneous and heterogeneous catalytic processes. To this end, nanoparticles represent a new frontier in heterogeneous catalysis, where this unification can also be supplemented by the ability to obtain new or divergent reactivity and selectivity. We report a novel method for applying heterogeneous catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles. These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl(2), and catalyse a range of π-bond activation reactions previously only catalysed through homogeneous processes. Multiple experimental methods are used to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously inaccessible in heterogeneous catalysis. Furthermore, a size and capping agent study revealed that Pt PAMAM dendrimer-capped nanoparticles demonstrate superior activity and recyclability compared with larger, polymer-capped analogues.

Dendrimer Templated Synthesis of One Nanometer Rh and Pt Particles Supported on Mesoporous Silica: Catalytic Activity for Ethylene and Pyrrole Hydrogenation.

Monodisperse rhodium (Rh) and platinum (Pt) nanoparticles as small as approximately 1 nm were synthesized within a fourth generation polyaminoamide (PAMAM) dendrimer, a hyperbranched polymer, in aqueous solution and immobilized by depositing onto a high-surface-area SBA-15 mesoporous support. X-ray photoelectron spectroscopy indicated that the as-synthesized Rh and Pt nanoparticles were mostly oxidized. Catalytic activity of the SBA-15 supported Rh and Pt nanoparticles was studied with ethylene hydrogenation at 273 and 293 K in 10 torr of ethylene and 100 torr of H 2 after reduction (76 torr of H 2 mixed with 690 torr of He) at different temperatures. Catalysts were active without removing the dendrimer capping but reached their highest activity after hydrogen reduction at a moderate temperature (423 K). When treated at a higher temperature (473, 573, and 673 K) in hydrogen, catalytic activity decreased. By using the same treatment that led to maximum ethylene hydrogenation activity, catalytic activity was also evaluated for pyrrole hydrogenation.

Highly Selective Synthesis of Catalytically Active Monodisperse Rhodium Nanocubes

Monodisperse sub-10 nm Rh nanocubes were synthesized with high selectivity (>85%) by a seedless polyol method. The {100} faces of the Rh NCs were effectively stabilized by chemically adsorbed Br- ions from trimethyl(tetradecyl)ammonium bromide (TTAB). This simple one-step polyol route can be readily applied to the preparation of Pt and Pd nanocubes. Moreover, the organic molecules of PVP and TTAB that encapsulated the Rh nanocubes did not prevent catalytic activity for pyrrole hydrogenation and CO oxidation.

Purification Process for Single-Wall Carbon Nanotubes

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electrical conductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamental challenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs, along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screen and compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluating their purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward, inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotube carbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pure SWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which take only a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relative purity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.

EDTA functionalized silica for removal of Cu(II), Zn(II) and Ni(II) from aqueous solution.

Ethylenediaminetetraacetic acid (EDTA) functionalized silica adsorbent has been synthesized using (3-aminopropyl) triethoxylsilane (APTES) as a bridging link between silanol groups (SiOH) of silica and carboxylic group of EDTA. Fourier transform infrared spectroscopy (FTIR) and Temperature-programmed oxidation (TPO) analysis confirmed the grafting of EDTA onto the silica. The synthesized EDTA-silica was investigated as an adsorbent for removal of Cu(II), Zn(II) and Ni(II) from aqueous solution. The effect of solution pH, initial solution concentration, and contact time were studied. The removal of metal ions increased with the increase in solution pH, contact time and concentration. The maximum equilibrium time was found to be 45min for all three metal ions. Kinetics studies revealed that the adsorption of Cu(II), Zn(II) and Ni(II) onto EDTA-silica followed the pseudo-second order kinetics and film diffusion and intra-particle diffusion mechanism were involved. Adsorption equilibrium data were well fitted to Langmuir isotherm model and maximum monolayer adsorption capacity for Cu(II), Zn(II) and Ni(II) was 79.36, 74.07 and 67.56mg g(-1), respectively. Thermodynamic results reveal that the removal of metals onto EDTA-silica was endothermic and spontaneous in nature.

CO2 conversion by reverse water gas shift catalysis: comparison of catalysts, mechanisms and their consequences for CO2 conversion to liquid fuels

Current society is inherently based on liquid hydrocarbon fuel economies and seems to be so for the foreseeable future. Due to the low rates (photocatalysis) and high capital investments (solar-thermo-chemical cycles) of competing technologies, reverse water gas shift (rWGS) catalysis appears as the prominent technology for converting CO2 to CO, which can then be converted via CO hydrogenation to a liquid fuel of choice (diesel, gasoline, and alcohols). This approach has the advantage of high rates, selectivity, and technological readiness, but requires renewable hydrogen generation from direct (photocatalysis) or indirect (electricity and electrolysis) sources. The goal of this review is to examine the literature on rWGS catalyst types, catalyst mechanisms, and the implications of their use CO2 conversion processes in the future.

Remediation of Cu(II), Ni(II), and Cr(III) ions from simulated wastewater by dendrimer/titania composites.

Generation 4 polyamidoamine (PAMAM) dendrimers with ethylenediamine cores (G4-OH) were immobilized on titania (TiO(2)) and examined as novel metal chelation materials. Characterization results indicate both the effective immobilization of dendrimers onto titania and retention of the dendrimer on titania following remediation. The effective remediation of Cu(II), Ni(II), and Cr(III), which are model pollutants commonly found in industrial electroplating wastewater, is demonstrated in this work. Important parameters that influence the efficiency of metal ion removal were investigated; e.g. solution pH, retention time, metal ion concentration, and composite material dosage. Metal ion removal was achieved over a wide metal concentration range within a 1 h equilibration time. Maximum metal ion removal was achieved at pH ≥7 for both Cu(II) and Cr(III), and pH ≥9 for Ni(II). Further, the dendrimer/titania composite materials were even more effective when metal ion mixtures were tested. Specifically, a dramatic increase was observed for Ni(II) chelation when in a mixture was compared to a pure nickel solution. These findings suggest new strategies for improving metal ion removal from industrial wastewater.

Iron Chelation by Polyamidoamine Dendrimers: A Second-Order Kinetic Model for Metal–Amine Complexation

This study presents a kinetic model of the chelation of iron ions by generation 4 hydroxyl-terminated polyamidoamine (PAMAM) with ethylenediamine core (G4-OH). The coordination processes of iron ions from ferric chloride, FeCl(3), and ferrous bromide, FeBr(2), to G4-OH dendrimers were analyzed using ultraviolet-visible (UV-vis) spectroscopy, proton nuclear magnetic resonance ((1)H NMR) spectroscopy, and liquid chromatography-mass spectrometry (LC-MS). In the visible region, a charge-transfer was observed when the dendrimer was added to a ferric chloride solution. This phenomenon is a ligand-to-metal charge-transfer (LMCT) between the free electron group of the dendrimers internal amines and the dehalogenated iron ion that takes 2 h to complete at room temperature. Analysis of potential rate laws and diffusion effects led to a second-order kinetic model for this reaction. By measuring the rate coefficients as a function of temperature (22-37 °C), an apparent activation energy of 41.5 kJ/mol was obtained using the Arrhenius method. The results of this study will fuel research of PAMAM dendrimers for environmental, pharmaceutical, and materials applications.

Deactivation Mechanisms of Ni-Based Tar Reforming Catalysts As Monitored by X-ray Absorption Spectroscopy†

Deactivation mechanisms of alumina-supported, Ni-based catalysts for tar reforming in biomass-derived syngas were evaluated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Catalysts were characterized before and after catalytic reaction cycles and regeneration procedures, which included oxidation by a mixture of steam and air, and reduction in hydrogen. Qualitative analysis of the EXAFS spectra revealed that oxidation of a portion of the Ni in the catalysts to form an oxide phase and/or a sulfide phase were likely scenarios that led to catalyst deactivation with time-on-stream and with increased reaction cycles. Deactivation through carbon deposition, phosphorus poisoning, or changes in particle size were deemed as unlikely causes. Quantitative analysis of the EXAFS spectra indicated sulfur poisoning occurred with time-on-stream, and the contaminating species could not be completely removed during the regeneration protocols. The results also verified that Ni-containing oxide phases (most likely a spinel also containing Mg and Al) formed and contributed to the deactivation. This study validates the need for developing catalyst systems that will protect Ni from sulfur poisoning and oxide formation at elevated reaction and regeneration temperatures.