Controlling Selectivity in Reactions of Complex Oxygenates over Metal Catalysts
J. Will Medlin
Dept. of Chemical & Biological Engineering, Univ. of Colorado, Boulder, CO 80309
Complex reactants such as biomass-derived oxygenates often have multiple reactive functional groups. A consequence of this multi-functionality is that these reactants can potentially adsorb in many configurations on catalyst surfaces. We have found that selectivity to desired reaction products is often closely related to the favorability of a particular configuration of the adsorbed reactant. To engineer more selective catalysts, it is therefore desirable to be able to predict how adsorbate geometry is influenced by catalyst structure and composition. This presentation will focus on our efforts to understand how adsorbate structure is influenced by catalyst properties, and to manipulate those properties to control surface-adsorbate interactions and thus improve catalyst performance. Our approach to developing an understanding of these issues is rooted in ultrahigh vacuum surface spectroscopies; the insights from those techniques are then used in design of technical catalysts.
Not surprisingly, key parameters in catalyst design include surface composition and catalyst nanostructure. However, in some cases (such as the hydrodeoxygenation of biomass pyrolysis compounds) these properties influence adsorbate configurations—and thus product selectivities—in surprising ways. Moreover, controlling the near-surface environment through coadsorption of organic species is seen to exert a powerful influence over reactant adsorption and product selectivity. Methods for engineering the near-surface environment through incorporation of organic ligands for chemoselective hydrogenation reactions will be discussed in detail.
Photoelectron spectroscopy under ambient relative humidity
Lawrence Berkeley National Laboratory, Berkeley, CA 94720
Solid/vapor, liquid/vapor and liquid/solid interfaces govern many processes in the environment, heterogeneous catalysis, and technology. Ambient pressure photoelectron spectroscopy is an excellent method for the characterization of these interfaces under operating conditions. This talk will focus on the application of APXPS to the investigation of surfaces under realistic relative humidity and discuss in particular the reaction of water vapor with oxide surfaces, the influence of adsorbates on the premelting transition at the ice surface, as well as the application of APXPS to the investigation of liquid/solid interfaces.
Observing Ion Interactions at Charged Solid-Liquid Interfaces using X-rays:
From Statics to Dynamics*
Argonne National Laboratory, Argonne IL 60439 USA
The interaction of ions with charged solid-liquid interfaces is a critical feature for understanding a number of important phenomena, ranging from the transport of contaminants in the environment (e.g., at mineral-water interfaces) and an understanding of capacitive energy storage technologies (e.g., the organization of ions at the electrode electrolyte interface). The actual distribution of ions at the interfaces is normally obscured by the presence of the liquid phase. I will present recent work where we use X-ray based probes (e.g., x-ray reflectivity and resonant scattering) to observe the structures and interactions of ions at solid liquid interfaces through direct in-situ measurements. Examples will include: 1) metal ion adsorption at mineral-water interfaces including the muscovite-water, quartz-water, and calcite-water interfaces. The results reveal the critical role of ion solvation in understanding adsorbed cation properties at mineral-water interfaces; and 2) the organization of room temperature ionic liquids at graphene surfaces, revealing the charge separated ion layers under applied potentials and insight into the unexpected slow processes and hysteresis observed with time-dependent potentials.
*This work was done in collaboration with Sang Soo Lee, Ahmet Uysal (Argonne National Laboratory), Erika Callagon, Kathryn Nagy and Neil Sturchio, (University of Illinois at Chicago) and others. Mineral-water interface work is supported by the DOE/BES/Geosciences Research Program, and the ionic liquid/graphene studies are funded by the Fluid Interface Structure Reactivity and Transport (FIRST) project, a DOE/BES Energy Frontier Research Center.
Challenges of 3D TOF-SIMS Analysis – Bumps, Humps and Holes
Nathan Havercroft1, 1ION-TOF USA, Felix Kollmer2, Rudolf Moellers2, Derk Rading2, Ewald Niehuis2
1ION-TOF USA, Inc., Chestnut Ridge, NY 10977
2ION-TOF GmbH, Muenster, Germany 48149
Since the commercialization of TOF-SIMS approximately 30 years ago its use has grown from static SIMS spectrometry to three-dimensional (3D) imaging. With these new capabilities TOF-SIMS has been used to analyze an ever increasing variety of samples and 3D analysis has become critical in many areas of research including, nanoscience, microelectronics and biomaterials.
Three-dimensional analysis is performed in the dual beam mode using a pulsed analysis beam and a low energy sputter beam. However, the topography of the initial sample surface, as well as the subsequent evolution of the topography by sputtering, cannot be identified by the technique and lead to distortions of the detected depth distribution. This is especially true for analyses requiring greater depths (>10μm) where the buildup of surface roughness at the crater bottom also limits the achievable spatial resolution.
In order to provide the complimentary surface topographic information a technique such as Scanning Force Microscopy (SFM) is needed. We have developed a unique UHV instrument that combines TOF-SIMS and SFM allowing for topography corrected 3D SIMS images to be obtained. Further, the SFM can be operated in a variety of modes, such as Magnetic Force Microscopy (MFM) and Kelvin Probe Force Microscopy (KPFM), so that physical properties can be directly related to chemistry.
For those samples where large 3D volumes are required, the combination of TOF-SIMS with Focused Ion Beam (FIB) milling provides a method to overcome the roughening issues. In this case the FIB beam (Bi clusters or Ga) is used to mill an initial crater in the sample. A 2D TOF-SIMS image is then obtained from the vertical crater wall. Since the crater wall is hardly affected by the aforementioned roughening problems this approach allows the in-depth distribution of elements to be determined by analyzing a plane perpendicular to the surface at high lateral resolution. Full 3D characterization is obtained by serial slicing of the crater wall followed by intermediate analysis steps.
Holes such as vias in the microelectronics field are an especially difficult kind of 3D structure to analyze due to typical aspect ratios where the depth is often greater than the diameter. However, by utilizing sample tilt and optimized extraction conditions, images from the bottom of these vias can be obtained.
Examples from all of these 3D analyses will be presented.
 F. Kollmer, W. Paul, M. Krehl and E,. Niehuis, Surf. Interface Anal. 45, 312 (2013)
 E. Niehuis, R. Moellers, D. Rading, H.-G. Cramer, R. Kersting, Surf. Interface Anal. 45, (2013) 158
 F. Kollmer, W. Paul, M. Krehl, E. Niehuis, SIMS XVIII proceedings paper, Surf. Interface Anal., 2012
Utilization of 3-D Atom Probe Tomography for Chemical Analysis of Buried Interfaces
Brian P. Gorman, George Burton, Adam Stokes, Rita Kirchhofer, David R. Diercks
Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO, USA 80401
Buried interfaces such as heterojunctions, homojunctions, and grain boundaries frequently define the transport properties of materials and devices. Heterojunctions are frequently found to control the minority carrier recombination rate in photovoltaic devices, phonon scattering in thermoelectrics, and light scattering in solid state light emitting devices. Understanding surface composition during growth is well established using in-situ characterization techniques such as XPS, AES, and optical probes. Post-growth processing (e.g., activation anneals) can frequently cause minority elements to segregate to low energy positions at heterojunctions and structural defects.
Characterization of elemental segregation post-growth is not straightforward. Few techniques have the ability to analyze with sub-nm spatial and ppm chemical resolution. While SIMS can measure elemental concentrations down to ~1015/cm3, its spatial resolution is limited by ion forward scattering in the z-direction and by the ion probe spot size in the x-y plane. Atom probe tomography (APT) has the ability to measure elemental concentrations down to ~1017/cm3 while maintaining sub-nm spatial resolution in 3-dimensions. However, spatial resolution at heterojunctions is frequently limited by the ability to field evaporate through materials with widely varying optical, thermal, and mechanical properties. In this work, we will illustrate several examples where APT successfully and unsuccessfully analyzed buried interfaces in ceramic and semiconductor devices.
Photovoltaic devices based upon CdTe frequently have limited efficiencies due to carrier recombination at the surface. Using double heterojunctions of (CdxMg1-x)Te has proven to give world record recombination rates when grown on InSb. APT was able to successfully quantify the Mg concentration in these <5 nm thick heterojunctions but was not as successful in measuring the Mg concentration of significantly thicker layers. This is most likely due to the changing thermal properties between the two materials causing mechanical stresses and different evaporation rates resulting in “hoodoo” specimen morphologies. APT was also successful in measuring dopant accumulation in As doped CdTe as well as phase segregation in Cd(TeySe1-y).
During laser pulsed APT, specimen quench rates as high at 1013 K/sec have been observed with temperatures ranging from ~30 to greater than 2000 K. Using these high temperatures and quench rates combined with the near atomic scale resolution of APT, it should be possible to measure the diffusion of different elemental species one atomic jump at a time. Using PLE processed (Zn,Mg)O / sapphire as a model buried interface, single atomic jump quantification of diffusion is illustrated with sub-ns temporal resolution. Interfacial formation of MgAl2O4 spinel is measured using in-situ transmission electron diffraction and subsequent sublimation of Zn from the specimen surface is also observed. Atomic scale quantification of diffusion should be possible in a wide range of oxides and semiconductors.
Oxidation/Reduction Reactions on Actinide Surfaces: An X-ray Photoelectron
Art J. Nelson
Lawrence Livermore National Laboratory, Livermore, CA 94550 USA
Oxidation/reduction reactions on actinide metals continue to garner interest for establishing safe handling and storage procedures. X-ray photoelectron spectroscopy (XPS) has long been used to provide information on oxidation state, chemical environment, and bonding characteristics of uranium and plutonium surfaces. XPS and X-ray excited Auger transitions can also be combined to characterize differences in the oxidation state and local electronic structure of actinide compounds. By measuring the X-ray excited Auger transitions and combining with the core level photoemission in a chemical state plot (Wagner plot) along with the Auger parameter (difference in the binding energy of the photoelectron and Auger lines), one can gain additional information on the initial state effects and final state relaxation energy. In this study, we have measured the chemical shift of core-valence-valence (CVV) and core-core-valence (CCV) Auger transitions and combined it with the measured chemical shift of the 4f photoelectron lines for a select set of U and Pu compounds. Results show that U and Pu species have definitive Auger line-shapes. These data were used to produce Wagner plots for select uranium and plutonium oxides. This methodolgy allowed us to distinguish between the trivalent Pu hydride and the trivalent Pu oxide, which cannot be differentiated by the Pu 4f binding energy alone.
This work was performed under the auspices of the U.S. Dept. of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.