Christopher T. Williams

Probing buried interfaces with non-linear optical spectroscopy

The importance of buried interfaces in our everyday lives and in current scientific research is highlighted, along with experimental difficulty associated with studying such systems. We present an overview of the application of second harmonic generation and sum-frequency spectroscopy to the study of buried interfaces. Several examples from the current literature are presented, ranging from chemical and biological, to electrical and magnetic interfaces. The importance of this work in the context of ongoing research in these areas is discussed. Finally, we provide a snapshot of the state of the art in non-linear optical spectroscopy by mentioning several new directions that are likely to have a large impact on future research into the physics and chemistry of buried interfaces.

How To Light Special Hot Spots in Multiparticle–Film Configurations

The precise control over the locations of hot spots in a nanostructured ensemble is of great importance in plasmon-enhanced spectroscopy, chemical sensing, and super-resolution optical imaging. However, for multiparticle configurations over metal films that involve localized and propagating surface plasmon modes, the locations of hot spots are difficult to predict due to complex plasmon competition and synergistic effects. In this work, theoretical simulations based on multiparticle-film configurations predict that the locations of hot spots can be efficiently controlled in the particle-particle gaps, the particle-film junctions, or in both, by suppressing or promoting specific plasmonic coupling effects in specific wavelength ranges. These findings offer an avenue to obtain strong Raman signals from molecules situated on single crystal surfaces and simultaneously avoid signal interference from particle-particle gaps.

Degradation Characteristics of Elastomeric Gasket Materials in a Simulated PEM Fuel Cell Environment

Polymer electrolyte membrane (PEM) fuel cell stack requires gaskets and seals in each cell to keep the reactant gases (hydrogen and oxygen) within their respective regions. The stability of the gaskets/seals is critical to the operating life as well as the electrochemical performance of the fuel cell. The time-dependant chemical and mechanical degradation of two commercially available silicones-based elastomeric gasket materials in a simulated fuel cell environment was investigated in this work. Two temperatures based on actual fuel cell operation were selected and used in this study. Using optical microscopy, the topographical damage on the sample surface due to the acidic environment was revealed. Atomic adsorption spectrometer analysis shows that silicon, calcium, and magnesium were leached from the materials into the soaking solution. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to study the surface chemistry of the elastomeric gasket materials before and after exposure to the simulated fuel cell environment over time. The ATR-FTIR and XPS test results indicate that the surface chemistry changed significantly and the chemical degradation mechanism is de-crosslinking and chain scission in the backbone. The microindentation test results show that the mechanical properties of the silicone materials changed significantly after exposure to the simulated PEM fuel cell environment over time.

Pd–Ag/SiO2 bimetallic catalysts prepared by galvanic displacement for selective hydrogenation of acetylene in excess ethylene

A series of bimetallic Pd–Ag/SiO2 catalysts were prepared by galvanic displacement with increasing loadings of Pd on Ag. The catalysts were characterized by atomic absorption spectroscopy, Fourier-transform infrared spectroscopy of CO adsorption and X-ray photoelectron spectroscopy. An actual Pd deposition beyond the theoretical limit for galvanic displacement suggested that the large difference in surface free energy for Pd and Ag resulted in Pd diffusion into the bulk of Ag particles, or Ag diffusion to the surface to provide fresh Ag atoms for further galvanic displacement. Characterization results indicated that on this series of catalysts the Pd atoms are distributed in very small ensembles or possibly even atomically on the Ag surface, and there was a transfer of electrons from Pd to Ag at all Pd loadings. For comparison, the catalysts were also evaluated for the selective hydrogenation of acetylene in excess ethylene at the conditions used in our previous study of the reverse Ag–Pd/SiO2 catalysts. The selectivities for C2H4 formation remained high and constant due to the geometric effects that Pd atom existed as small ensembles. However, the electronic effects resulted in lower selectivities for C2H4 formation than those from the catalysts with high coverage of Ag on Pd.

Silicon-nickel intermetallic compounds supported on silica as a highly efficient catalyst for CO methanation

Silicon–nickel intermetallic compounds (IMCs) supported on silica (Si–Ni/SiO2), as a highly efficient catalyst for CO methanation, have been prepared by direct silicification of Ni/SiO2 with silane at relatively low temperature in a fluidized bed reactor. The as-prepared materials were characterized by X-ray diffraction, transmission electron microscopy, in situ FT-IR of CO adsorption, and H2-temperature programmed reduction-mass spectrometry (TPR-MS) of CO. The results indicate that uniform NiSix nanoparticles with about 3–4 nm are evenly dispersed on silica. The combined in situ FTIR and TPR-MS results suggest that the Si–Ni/SiO2 catalysts afforded high activity in CO methanation, promoting the formation of CH4 at ca. 240 °C. The catalytic hydrogenation of CO on Si–Ni/SiO2 was investigated in a fixed-bed reactor at GHSVs 48 000 mL h−1 g−1 under 1 atm in the temperature interval 200–600 °C. In the higher temperature reaction region (500–600 °C), it is notable that the Si–Ni/SiO2 catalysts present high activity for CO methanation as compared to the Ni/SiO2 catalyst. More importantly, the Si–Ni/SiO2-350 catalyst containing thermally stabile Si–Ni IMCs shows significantly higher resistance to the sintering of Ni particles. Raman characterization of the spent materials qualitatively shows that carbon deposition observed on the conventional Ni/SiO2 catalyst is much higher than that of the used Si–Ni/SiO2-350. It is proposed that small amounts of silicon interacting with Ni atoms selectively prevent the adsorption of resilient carbon species.

In-situ Raman investigation of cinchonidine adsorption on polycrystalline platinum in ethanol

Abstract The vibrational properties of adsorbed cinchonidine on polycrystalline platinum in ethanol solutions have been probed in-situ using surface-enhanced Raman spectroscopy (SERS). Analysis of the SER spectra indicates that the modifier is adsorbed through the quinoline portion of cinchonidine by π-bonding with the Pt surface. Consideration of surface selection rules suggests that the cinchonidine is at least somewhat tilted with respect to the platinum surface over a range of cinchonidine liquid-phase concentrations (at least from 50 μM to 1.2 mM).

Controlled preparation and characterization of supported CuCr2O4 catalysts for hydrogenolysis of highly concentrated glycerol

Supported Cu–Cr catalysts were prepared by a non-alkoxide sol–gel route, and characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) measurement. Their structures were significantly tuned by the Cu–Cr molar ratio. CuCr2O4, CuCr2O4–CuO and CuCr2O4–Cr2O3 structures were confirmed in CuCr(0.5), CuCr(4) and CuCr(0.25) catalysts, respectively. A direct interaction between CuCr2O4 and CuO or Cr2O3 in CuCr(4) and CuCr(0.25) catalyst was observed by the H2-TPR and XPS results. The catalytic performance of Cu–Cr catalysts with various structures was examined by hydrogenolysis reaction of high concentrated glycerol. Under mild conditions (2.0 MPa and 130 °C) and high concentration (100 wt%), the maximum conversion (52%) was obtained over the CuCr(0.5) catalyst, while the CuCr(4) catalyst gave the highest selectivity of 1,2-PD (up to 100%). This finding will result in the production of less waste water and lower energy consumption in the following separation steps during glycerol hydrogenolysis.

A facile and novel approach to magnetic Fe@SiO2 and FeSi2@SiO2 nanoparticles

Fe@SiO2 and FeSi2@SiO2 nanoparticles with core@shell structure have successfully been synthesized by direct silane silicification of Fe2O3 nanoparticles. The as-prepared samples were characterized by N2 physisorption, X-ray diffraction, transmission electron microscopy, temperature programmed reduction of H2, X-ray photoelectron spectroscopy, Mossbauer spectroscopy, and superconducting quantum interference magnetometry. It was found that the amorphous SiO2 shell was formed to protect the core against oxidation when the reduced Fe2O3 nanoparticles were silicified by silane. When the reduced Fe2O3 nanoparticles were exposed to air, a Fe2O3 layer was formed. The structure of the core changed from cubic Fe to orthorhombic FeSi2 with increasing silicification temperatures from 350 to 550 °C, due to the dissolution of Si atoms into the iron lattice. The magnetic characterization showed that all samples have ferromagnetic nature and the saturation magnetization values drastically decreased with increasing silicification temperature. This novel methodology can be applied to synthesis of Co@SiO2 and Ni@SiO2 with core@shell structure. The as-prepared Fe@SiO2 and FeSi2@SiO2 nanoparticles with core@shell structure can find applications in magnetically separable catalysts, biomedicines, and magnetically recording materials.

In situ ATR-IR study of prochiral 2-methyl-2-pentenoic acid adsorption on Al2O3 and Pd/Al2O3

Adsorption and desorption of trans-2-methyl-2-pentenoic acid (MPeA) in dichloromethane (CH(2)Cl(2)) were investigated by using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. A liquid flow-through spectroscopic cell allowed for high quality spectra to be obtained from deposited thin films of Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) on a ZnSe internal reflection element. The MPeA molecules adsorb on both Al(2)O(3) and Pd surfaces molecularly and dissociatively under the concentration range examined (2-16 mM). In the case of molecular adsorption, both monomer (ν(C=O) ~ 1720 cm(-1)) and dimer (ν(C=O) ~ 1685 cm(-1)) species are observed to adsorb, with the relative amount of monomer to dimer dependent on the surface and the liquid phase acid concentration. In the case of dissociative adsorption, the acid adsorbs predominantly in a bridged bidentate configuration, as adjudged by the ca. 150-220 cm(-1) separation between asymmetric and symmetric vibrational bands. All of these species are found to be strongly adsorbed on both Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) surfaces, even under pure solvent flow after adsorption.

Preparation and magnetic properties of single phase Ni2Si by reverse Rochow reaction

Single phase Ni2Si nanoparticles (NPs) have been successfully synthesized by using the Rochow reverse reaction, in which organosilanes ((CH3)nSiCl4−n) are used as the silicon source. The results demonstrate that the crystalline size and phase of nickel silicide can be controlled through changing the organosilanes and reaction time. A formation mechanism of Ni2Si NPs has been proposed, which involved reaction deposition and subsequently diffusion of Si atoms. Magnetism performance tests indicate that the saturation magnetization and coercive field of Ni2Si NPs depend greatly on the environmental temperature and particle size. The blocking temperature (TB) of the materials was found to strongly depend on selecting the organosilanes precursor:  in the case of the Ni2Si-0 (148 K) and Ni2Si-2 (336 K). This novel methodology opens a route to prepare other classes of metal silicides with single phase and stoichiometry. The as-prepared single phase Ni2Si nanoparticles can find applications in ferrofluids, imaging, and magnetic separation due to its special magnetic behavior depending on the applied temperature (below or above TB).