High-Pressure Ammonia Adsorption and Dissociation on Clean Fe(111) and Oxygen-Precovered Fe(111) Studied by Sum Frequency Generation Vibrational Spectroscopy

Solid-Liquid-Solution Phases in Poly(diallyldimethylammonium)/Poly(acrylic acid) Polyelectrolyte Complexes at Varying Temperatures

The coacervation and complexation of oppositely charged polyelectrolytes are dependent on numerous environmental and preparatory factors, but temperature is often overlooked. Temperature effects remain unclear because the temperature dependence of both the dielectric constant and polymer-solvent interaction parameter can yield lower and/or upper critical solution phase behaviors for PECs. Further, secondary interactions, such as hydrogen bonding, can affect the temperature response of a PEC. That is, mixtures of oppositely charged polyelectrolytes can exhibit phase separation upon lowering and/or increasing the mixture's temperature. Here, the phase behavior of poly(diallylmethylammonium)/poly(acrylic acid) (PDADMA/PAA) complexes under varying KBr ionic strengths, mixing ratios, and temperatures at a fixed pH (in which PAA hydrogen bonding can occur) is examined. At room temperature, the PDADMA/PAA PECs exhibit four different phase states: precipitate, coexisting precipitate and coacervate, solid-like gel, and coacervate. Variable-temperature optical microscopy reveals the upper critical solution temperature (UCST) at which each phase transitioned to a solution state. Interestingly, the UCST value is highly dependent on the original phase of the PEC, in which solid-like precipitates exhibit higher UCST values. Large-scale all-atom molecular dynamics (MD) simulations support that precipitates exhibit kinetic trapping, which may contribute to the higher UCST values observed in the experiment. Taken together, this study highlights the significance of temperature on the phase behavior of PECs, which may play a larger role in stimuli-responsive materials, membraneless organelles, and separations applications.

Carbon-based catalysts for Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) is an essential approach to convert coal, biomass, and shale gas into fuels and chemicals, such as lower olefins, gasoline, diesel, and so on. In recent years, there has been increasing motivation to deploy FTS at commercial scales which has been boosting the discovery of high performance catalysts. In particular, the importance of support in modulating the activity of metals has been recognized and carbonaceous materials have attracted attention as supports for FTS. In this review, we summarised the substantial progress in the preparation of carbon-based catalysts for FTS by applying activated carbon (AC), carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon spheres (CSs), and metal-organic frameworks (MOFs) derived carbonaceous materials as supports. A general assessment of carbon-based catalysts for FTS, concerning the support and metal properties, activity and products selectivity, and their interactions is systematically discussed. Finally, current challenges and future trends in the development of carbon-based catalysts for commercial utilization in FTS are proposed.

Hydrogen Bonds and Molecular Orientations of Supramolecular Structure between Barbituric Acid and Melamine Derivative at the Air/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

We studied the supramolecular structure between barbituric acid (pyrimidine-2,4,6(1H,3H,5H)-trione, BA) and an amphiphilic melamine derivative at the air/water interface by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. HD-VSFG measurements in situ showed a positive broad band from 2300 to 2950 cm^-1. By comparing the experimental results with ab initio molecular dynamics (AIMD) simulations, we assigned the broad band to the NH stretching modes of BA strongly hydrogen-bonded to the melamine derivative. In addition, we report in situ HD-VSFG spectra of the interfacial supramolecular structure in the CO stretching region. Two CO stretching bands were identified. On the basis of the signs of the C=O bands, we uniquely determined the orientation of BA. The strong hydrogen bonds and the molecular orientations are direct evidence for the supramolecular structure based on complementary hydrogen bonds at the air/water interface.

Oxygen mediated synthesis of high quality InN nanowires above their decomposition temperature

A novel method for synthesis of high quality InN nanowires, at temperatures well above their decomposition temperature, has been demonstrated by utilizing controlled oxygen flow in the growth chamber. Detailed structural and chemical analyses indicate that the nanowires consist of pure InN, with no evidence of In2O3 detected by any of the characterization methods. It is proposed that the oxygen, pre-adsorbed on the Au catalyst surface, assists in accelerating the decomposition of NH3 at the growth temperature by providing high concentration of atomic nitrogen to assist in the growth, and prevent decomposition of the InN nanowires, without getting incorporated in them. The proposed role of oxygen is supported by improved material quality at higher oxygen flow rates.

Molecular studies of model surfaces of metals from single crystals to nanoparticles under catalytic reaction conditions. Evolution from prenatal and postmortem studies of catalysts

Molecular level studies of metal crystal and nanoparticle surfaces under catalytic reaction conditions at ambient pressures during turnover were made possible by the use of instruments developed at the University of California at Berkeley. Sum frequency generation vibrational spectroscopy (SFGVS), owing to its surface specificity and sensitivity, is able to identify the vibrational features of adsorbed monolayers of molecules. We identified reaction intermediates, different from reactants and products, under reaction conditions and for multipath reactions on metal single crystals and nanoparticles of varying size and shape. The high-pressure scanning tunneling microscope (HP-STM) revealed the dynamics of a catalytically active metallic surface by detecting the mobility of the adsorbed species during catalytic turnover. It also demonstrated the reversible and adsorbate-driven surface restructuring of platinum when exposed to molecules such as CO and ethylene. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) detected the reversible changes of surface composition in rhodium-palladium, platinum-palladium, and other bimetallic nanoparticles as the reactant atmosphere changed from oxidizing to reducing. It was found that metal nanoparticles of less than 2 nm in size are present in higher oxidation states, which alters and enhances their catalytic activity. The catalytic nanodiode (CND) confirmed that a catalytic reaction-induced current flow exists at oxide-metal interfaces, which correlates well with the reaction turnover.

Tuning of Redox Properties of Iron and Iron Oxides via Encapsulation within Carbon Nanotubes

We report the tuning of the redox properties of iron and iron oxide nanoparticles by encapsulation within carbon nanotubes (CNTs) with varying inner diameters. Raman spectroscopy was employed to investigate the interaction of the encapsulated nanoparticles with the CNTs. A red shift of the Fe-O mode is observed in the nanoparticles deposited on the outer CNT surfaces with respect to bulk Fe2O3. However, this mode is found to be stepwise blue-shifted with decreasing inner diameter in the CNT-encapsulated Fe2O3 nanoparticles, suggesting an enhanced interaction of Fe2O3 with the inner CNT surface as its curvature increases. The autoreduction of the encapsulated Fe2O3 is significantly facilitated inside CNTs with respect to the outside nanoparticles. Interestingly, it becomes more facile with decreasing CNT channel diameter as evidenced by temperature programmed reaction, in situ XRD, and Raman spectroscopy. The oxidation of encapsulated metallic Fe nanoparticles on the other hand is retarded in comparison to that of the outside Fe particles as shown by in situ XRD and gravimetrical measurements with an online microbalance. We attribute this tunable redox behavior of transition metal nanoparticles inside CNTs to a particular electronic interaction of the encapsulates with the interior CNT surface, which stabilizes the metallic state of Fe.

Carbon Monoxide Adsorption and Oxidation on Monolayer Films of Cubic Platinum Nanoparticles Investigated by Infrared−Visible Sum Frequency Generation Vibrational Spectroscopy

The adsorption and oxidation of CO on monolayer films of cubic Pt nanoparticles synthesized by a modified solution-phase polyol process were examined by sum frequency generation (SFG) vibrational spectroscopy in total internal reflection (TIR) geometry. Extremely low incident laser power (approximately 5 microJ/pulse of infrared) yields sufficient SFG intensity in TIR geometry and reduces destructive interference. Because TIR-SFG spectroscopy does not require correction for bulk gas absorption, CO spectra can be collected over a wide pressure range (<1 mTorr up to 700 Torr). Poly(vinylpyrrolidone)-capped Pt nanoparticles deposited on single-crystal sapphire were monitored under high-pressure reaction conditions in a combined spectroscopy-catalytic reactor cell. The effect of the capping polymer on the position and intensity of the CO peak was studied before and after low-temperature calcination. The polymer decreased the amount of CO adsorption and caused a slight red-shift of the atop CO band relative to a surface treated in oxygen at 373 K. Oxidation rates were determined by measuring the intensity of the atop CO peak as a function of time in the presence of flowing oxygen. The activation energy (approximately 19.8 kcal/mol) determined from the SFG data is close to that obtained from gas chromatography (GC) measurements of CO oxidation rates at different temperatures. The SFG and GC results are in good agreement with published data for Pt(100) surfaces.

Thermochemistry of the activation of N2 on iron cluster cations: Guided ion beam studies of the reactions of Fen+ (n=1–19) with N2

The kinetic energy dependences of the reactions of Fe(n)+ (n = 1-19) with N2 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-15 eV. In addition to collision-induced dissociation forming Fe(m)+ ions, which dominate the product spectra, a variety of Fe(m)N2+ and Fe(m)N+ product ions, where m < or = n, is observed. All processes are observed to exhibit thresholds. Fe(m)+ - N and Fe(m)+ - 2N bond energies as a function of cluster size are derived from the threshold analysis of the kinetic energy dependences of the endothermic reactions. The trends in this thermochemistry are compared to the isoelectronic D0(Fe(n)+ - CH), and to bulk phase values. A fairly uniform barrier of 0.48+/-0.03 eV at 0 K is observed for formation of the Fe(n)N2+ product ions (n = 12, 15-19) and can be related to the rate-limiting step in the Haber process for catalytic ammonia production.

Facile Autoreduction of Iron Oxide/Carbon Nanotube Encapsulates

Facile autoreduction of iron oxide encapsulated within carbon nanotubes has been observed at a temperature 200 degrees C lower than those on the outer surface. This opens a new route to tune the state of confined nanoparticles of d-band metals by the confinement of CNTs.

Preparation of Alkanethiol Monolayers on Mild Steel Surfaces Studied with Sum Frequency Generation and Electrochemistry

An n-alkanethiol, octadecanethiol (ODT), monolayer was successfully prepared onto an oxide-free mild steel (MS) surface under cathodic polarization in a 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT. Cyclic voltammetry (CV) and electrochemical impedance (EIS) and sum frequency generation (SFG) spectroscopy were applied to study and characterize the adsorption of ODT at a MS surface. In 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT, CV of the MS electrode shows a dramatic decrease in charging current and a positive shift in oxidation potential when compared to a solution without ODT. The interfacial capacitance was obtained as 2.52 microF/cm(2) from the impedance data. An average chain tilt angle of 48 degrees for the ODT molecules was deduced from the comparison of the interfacial capacitances of the ODT/MS and ODT/Au monolayers. X-ray photoelectron spectroscopy confirmed the formation of the ODT monolayer on mild steel. The ppp SFG spectrum of the ODT-modified MS features three strong methyl vibrational modes at 2877, 2943, and 2967 cm(-1), indicating the formation of the oriented and densely packed ODT monolayer. However, the appearance of the two weak CH(2) groups' vibrational modes at 2850 and 2914 cm(-1) implies the presence of defects in the ODT monolayer. ODT/Au films were prepared to compare with the ODT/MS films. Orientation analysis of the air/solid interface suggests that the methyl group of ODT/Au films has a tilt angle of 30 degrees , while the methyl group of ODT/MS films has a tilt angle of 23 degrees . Water was found to have an impact on the shape of the SFG spectra of ODT/MS. This suggests that the solution penetrated through the defects to reach the MS surface.