Colloquia Abstracts

Understanding Active Sites and Activation Processes via the Interaction of Organometallics with Surfaces:
A Combined Spectroscopic-Computational Approach

Professor Susannah L. Scott
Department of Chemical Engineering
Department of Chemistry and Biochemistry
University of California, Santa Barbara

Depositing organometallic complexes on solid supports is a powerful way to create catalysts with uniform active sites. These catalysts have the potential to be more active and more selective than conventional heterogeneous catalysts, at the cost of more difficult synthesis and handling. These hybrid materials also offer the prospect of being able to probe reaction mechanisms on their surfaces using the tools of molecular chemistry. One of the most intriguing questions in heterogeneous catalysis is the mechanism of spontaneous activation of supported metal oxides. Catalysts such as Cr/SiO₂ and Re/Al₂O₃ generate the supported metal alkyls and alkylidenes that induce polymerization and metathesis of olefins without any added activator, via direct reaction with the olefin substrate, and they can be reactivated simply by removing accumulated catalyst poisons (e.g., by calcination). The first step in using a molecular complex as a precursor to one of these active sites is to understand its interaction with the surface. We have recently shown how these reactions inform us about the distribution of grafting sites, and thereby the interactions between active sites. For example, silica treated at an elevated temperature (e.g., 800 °C) has a low surface hydroxyl density, but this does not mean that the average interhydroxyl distance is very large, or that immobilized organometallic complexes are "isolated". The EXAFS of Ga(CH₃)₃ deposited on such a silica shows unequivocally that the hydroxyl sites are paired, and that the dimethylgallium(III) fragments are close enough to interact. A DFT model suggest that this interaction likely occurs via bridging surface oxygens. 

Catalytic Production of Liquid Fuels and Chemicals from Biomass-derived Oxygenated Hydrocarbons

James A. Dumesic
Department of Chemical and Biological Engineering
University of Wisconsin – Madison

Environmental and political issues created by our dependence on fossil fuels, such as global warming and national security, combined with diminishing petroleum resources are causing our society to search for new renewable sources of energy and chemicals, and an important sustainable source of organic fuels, chemicals and materials is plant biomass. We present results for aqueous-phase and vapor-phase reforming of oxygenated hydrocarbons, such as glycerol. Moreover, we show how gas mixtures of H₂ and CO can be produced at high rates and selectivities from glycerol over platinum-based bi-metallic catalysts at temperatures (e.g., 500-620 K) that are significantly lower compared to conventional gasification of biomass, allowing this gasification step to be coupled effectively with hydrocarbon production by Fischer-Tropsch synthesis. We will also report an integrated catalytic approach for the conversion of carbohydrates to specific classes of hydrocarbons for use as liquid transportation fuels. This approach is based on the integration of two flow reactors operated in a cascade mode, where the effluent from the first reactor is fed to the second reactor, and can be tuned either for production of highly branched hydrocarbons and aromatic compounds in gasoline, or for production of longer chain, less highly branched hydrocarbons in diesel and jet fuels. This two-reactor approach provides further processing flexibility because the effluent stream from the first flow reactor produces a liquid organic stream containing mono-functional compounds, such as alcohols, ketones, carboxylic acids, and heterocycles that can also be used to provide reactive intermediates for the lower-volume, but higher value, fine chemicals and polymers markets. Finally, we will show how hydroxymethylfurfural (HMF) can be formed in high yields by dehydration of carbohydrates in a biphasic reactor, and we illustrate how HMF can subsequently be used to produce liquid transportation fuels, such as dimethylfuran and alkanes ranging from C₈ to C₁₅.

Transport in Molecular Junctions: Thoughts Coherent and Incoherent

Mark A. Ratner
Chemistry Dept. and Center for Nanotechnology
Northwestern University

Current experimental efforts are clarifying quite beautifully the nature of charge transport in so-called molecular junctions, in which a single molecule provides the channel for current flow between two electrodes. The theoretical modeling of such structures is challenging, because of the uncertainty of geometry, the nonequilibrium nature of the process, and the variety of available mechanisms. The talk will center on the formulation of the problem in terms of non-equilibrium theory, and then on the generalizations needed to make that simple picture relevant to the real experimental situation. These include antiresonances, vibronic coupling and its control, structural disorder and representations for the electronic structure. Comments will be made on the measurements of inelastic spectra, and the information to be gained from them.

Highly Accurate Force Fields for F-elements and Their Use in Computational Environmental Chemistry

Aurora Clark
Washington State University

An algorithm has been developed for fitting classical force-fields, based upon the force matching method. It is interfaced with the electronic structure codes Gaussian03 and Crystal06 and the molecular dynamics codes DL_POLY, LAMMPS, and Amber. The quality of force-fields fit solely to the ab-initio PES (rather than experimental observables) has been examined with an emphasis upon the fitting of different functional forms with varying accuracy to the local minima vs. the entire dissociation curve for a given potential. Force-fields have explicitly been developed for trivalent lanthanide ions, enabling molecular dynamics simulations of their aqueous behavior and sorption characteristics to common minerals, and allowing for benchmarking to experimental data.

Defining Hydrogen Quality (for Fuel Vehicles) and Its Impact on Cost

Dennis Papadias
Chemical Sciences and Engineering Division
Argonne National Laboratory

Significant numbers of hydrogen fueled fuel-cell vehicles are expected to be deployed within the next several years. To develop the infrastructure to fuel these vehicles, several hydrogen refueling stations are being demonstrated at a number of locations in the U. S. and elsewhere. Typically, these stations are producing hydrogen on-site, either by the steam or autothermal reforming of natural gas, or by the electrolysis of water. The product hydrogen is further purified using pressure swing adsorption (PSA). As fuel cell vehicles approach wide-scale deployment, the issue of the quality of hydrogen dispensed to the vehicles has become increasingly important. The various factors that must be considered include the effects of different contaminants on fuel cell performance and durability, the production and purification of hydrogen to meet fuel quality guidelines, and the associated costs of providing hydrogen of that quality to the fuel cell vehicles.

This presentation will focus on our efforts to track contaminant levels through a near-term commercially promising hydrogen production/purification pathway for producing fuel cell quality hydrogen. This presentation will report on a process through which the hydrogen is obtained by steam reforming of natural gas and the hydrogen is then purified using PSA. By developing a model for the process, the effect of the operating conditions on the process efficiency, the level of key contaminants in the product hydrogen, and the cost of hydrogen have been calculated. The results indicate that at suggested hydrogen quality specifications, CO would limit the maximum hydrogen recovery from the PSA under typical design and operating conditions. Lowering the CO specification is not expected to significantly affect the cost of hydrogen.

Transport and Catalysis of Hydrocarbons in Confined Spaces

Johannes A. Lercher
Lerstuhl fuer Technische Chemie, Technische Univ.

Molecular sieves are key elements of catalysts in hydrocarbon catalysis helping to lead processes to long-term sustainability. To understand the elementary steps involved in these conversions is a prerequisite to be able to design and realize new generations of catalysts. The lecture will outline how new insight into transport and reaction processes has been used to develop materials allowing faster transport of molecules to the active sites and how the local environment of the catalytically active sites can be used to induce new catalytic chemistry. The roles of the steric control of the acid/active site and of bifunctionality in these processes will be discussed, analyzing how transition energies and entropies influence activity and selectivity.

Catalytic Autothermal Reforming of Renewable Fuels at Millisecond Times

Lanny Schmidt
Department of Chemical Engineering and Materials Science
University of Minnesota

We compare the reforming of different types of biofuels by autothermal reforming at millisecond contact times to produce synthesis gas, hydrogen, and chemicals. Fuels examined are alcohols, esters, carbohydrates, biodiesel, vegetable oil, and solid biomass.

Biofuels generally have higher conversions than fossil fuels because the hydroxyl and ester linkages in these fuels produce higher sticking coefficients than for saturated alkanes. Consequently, conversions of all biofuels in these processes are nearly 100%. Highly oxygenated feedstocks tend to produce mostly syngas with little olefins or oxygenated products because surface reactions dominate, and these larger products are formed predominantly by homogeneous reaction processes after all oxygen is consumed.

Recent results on production of syngas by reactive flash volatilization of nonvolatile liquids and solids will also be described. We show that, by impinging cold liquid drops or small solid particles onto the hot catalyst surface, the process can be operated in steady state with no carbon formation for many hours. This occurs because, while pyrolysis of vegetable oils and carbohydrates at low temperatures produces carbon, above ~600^oC the equilibrium shifts to produce syngas rather than solid carbon.

Recent results using fast photography at 1000 frames per second will be shown that examine the time dependence of solid and liquid particle decomposition and disappearance. Spatial profiles of temperature and species concentrations within the working catalyst will also be described.

Metals as Selective Catalysts

Robert Schlögl
Fritz-Haber-Institut der Max-Planck-Gesellschaft

Metals have been widely studied by model catalysis for their basic catalytic functions in oxidation and hydrogenation. The extremely important issue of selectivity has basically been treated for small molecule hydrogenation. A critical role was ascribed to carbonaceous species co-adsorbed with the substrate.

The presentation reviews the role of carbon in controlling the selectivity of hydrogenation and compares it to the role of oxygen in controlling the oxidation selectivity of metals. It will be demonstrated that the control of the third dimension of the catalytic material is a crucial requirement for which several strategies can be adopted.

Quantum Mechanics in Biology: Using Spectroscopy to Elucidate Design Principles in Photosynthesis

Greg Engel
The University of Chicago

Life on earth is effectively solar-powered, yet how energy moves through photosynthetic complexes prior to the biochemical steps of photosynthesis is still not completely understood. Evidence for a purely quantum mechanical mechanism of energy transfer in photosynthetic complexes was discovered in the Fenna-Matthews-Olson (FMO) complex of Chlorobium tepidum in 2007. The quantum beating phenomenon observed in this complex is now much better understood. Further, new data indicate that this mechanism is not specific to FMO, but manifests in reaction centers of purple bacteria and antenna complexes of higher plants. Having observed such a mechanism in disparate photosynthetic complexes, we are exploring what the minimal requirements are to support quantum coherence transfer in a biological environment and how such an environment might be reproduced synthetically. Emerging details in this story will be presented along with a preview of upcoming experimental efforts to dissect the details of energy transfer, the basis for the efficiency of the energy transfer process and efforts to isolate signals at room temperature.
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Coherent Ultrafast Multidimensional Spectroscopy of Chromophore Aggregates; from NMR to X-Rays

Shaul Mukamel
Department of Chemistry, University of California, Irvine

The multidimensional techniques which originated with NMR in the 1970s have been extended over the past 15 years to the optical regime. NMR spectroscopists have developed principles for the design of pulse sequences that resolve otherwise congested spectra, enhance selected spectral features and reveal desired dynamical events. These principles may be extended to the optical regime.

The response of photosynthetic light harvesting complexes to sequences of femtosecond optical pulses provides multidimensional snapshots of their structure and electronic dynamics. Two-dimensional (2D) signals show characteristic cross-peak patterns which carry information about structures, fluctuations and the entire pathways of energy and charge transfer. The signals reveal couplings between chromophores, and quantum coherence signatures of chromophore entanglement. Coherent quantum pathways and incoherent energy hopping processes may be disentangled. Pulse shaping coherent control strategies and specific polarization configurations of the optical fields that make use of molecular chirality enhance the resolution even further applications to Fenna-Matthews-Olson (FMO) antenna complex and the PSI complex are presented. 2D signals also provide unique information about electron correlations.

Future extensions to the attosecond regime using x-ray pulses will be discussed. Since core excitations are highly localized at selected atoms such signals can monitor the motions of valence electron wavepackets in real space with atomic spatial resolution. Common principles underlying coherent spectroscopy techniques for spins, valence electrons, and core electronic excitations, spanning frequencies from radiowaves to hard X-rays will be discussed.

“Coherent Multidimensional Optical Probes for Electronic Correlations and Exciton Dynamics; from NMR to X-rays”, S. Mukamel, D. Abramavicius, L. Yang, W.Zhuang, I.V. Schweigert and D. Voronine. Acct.Chem.Res. (April, 2009).

“Double-quantum Coherence Attosecond X-ray Spectroscopy of Spatially-separated, spectrally-overlaping core-election transitions,” I.V. Schweigert and S. Mukamel. Phys. Rev. A. 78, 052509(2008).

“Unraveling coherent dynamic and energy dissipation in photosynthetic complexes by 2D spectroscopy, D. Abramavicius, D. Voronine and S. Mukamel, Biophys. J.94, 3613-3619, 2008.

Electron Transfer Dynamics at the Mineral/Microbe Interface
Kevin M. Rosso
Pacific Northwest National Laboratory, Richland, WA 99352

The chemical behavior of mineral-water and mineral-microbe interfaces is central to aqueous reactivity in natural waters, soil evolution, atmospheric chemistry, and is of direct relevance for maintaining the integrity of waste repositories and remediating environmental pollutants. An important subset of reactions is the exchange of electron equivalents across these interfaces associated with natural variation in redox conditions or the activity of microorganisms at the earth’s near-surface. For example, microbially catalyzed reductive transformation of Fe(III)-oxides to solubilized Fe(II) by dissimilatory metal reducing bacteria is a process that can link to and control transport of redox-active contaminants. Detailed microbiologic study has revealed the presence of highly efficient biomolecular machinery for interfacial electron transfer localized on the outer-membranes of these microorganisms. Multi-heme cytochromes with high heme densities appear optimized for efficient interfacial electron transfer. Furthermore, some Fe(III)-oxides specifically utilized by these microorganisms, such as hematite, are natural electrical semiconductors with the propensity to accept and mobilize electrons in support of sustained microbiologic respiration. This presentation will center on current experimental and computational modeling research at PNNL focused on elucidating molecular-scale mechanisms and kinetics of electron exchange across this interface. In particular, the fundamental behavior of electrons in the mineral hematite and at key crystallographic terminations will be discussed. Single-molecule tunneling spectroscopy of microbial outer-membrane cytochromes will be compared with computational molecular modeling of cytochrome/hematite electron transfer. Common aspects of biomolecular and solid-state electron transfer processes at this environmental interface will be highlighted in terms of modern electron transfer theory.

Molecular Foundations of Surface Chemistry and Catalytic Selectivity by Metals

Gabor A. Somorjai
Department of Chemistry and Lawrence Berkeley National Laboratory
University of California, Berkeley

Heterogeneous metal catalysts are nanoparticles that carry out reactions at high reactant gas pressures or in the liquid phase. Instruments developed in Berkeley for molecular studies under these conditions are sum frequency generation vibrational spectroscopy, high pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy. Model surfaces were used to study heterogeneous catalytic reactions that permitted to control and monitor the atomic surface structure, composition and reaction intermediates and simultaneously measure reaction rates and selectivities. This way precise quantitative correlations could be obtained between catalytic reaction kinetics and the molecular factors that control reaction dynamics. Single crystal surfaces were used at first as model catalysts followed by the use of metal and bimetallic nanoparticles that were synthesized with precise size and shape using colloid techniques. Catalytic studies that produce a single molecule (ethylene hydrogenation, CO oxidation) were redirected to focus on reaction selectivity in multipath chemical processes. Reactions were found to induce restructuring of the metal surfaces and mobility of adsorbed molecules. Reaction selectivity and rates can be altered by changing the nanoparticle size in the 0.8 – 10 nm range and shape (surface structure). Transition metal catalysts that are nanosize achieve facile restructuring and rapid change in surface composition under reaction conditions as their low atom coordination permits rapid bond rearrangements. Exothermic surface reactions can cause the flow of hot electrons at oxide metal interfaces and the clustering of metal atoms at the interface, which dramatically increases the metal oxide interface area. Improvements of techniques for molecular studies of surfaces that provide better time resolution and spatial resolution will enhance our ability to study the dynamics of surfaces, which are key to both activity and selectivity during catalysis. The control of metal nanoparticle size and shape provides opportunities to achieve superior reaction selectivity. Combined studies of nanoparticle catalyst synthesis, characterization and reaction studies will accelerate developments of this important field of chemical sciences and chemical energy conversion.

Homogenous catalysts are easily tailored to accomplish a wide variety of reactions including asymmetric transformations. With the current developments in organometallic chemistry and organic chemistry many catalysts can be synthesized and due to the advances in high throughput screening techniques their performance can be evaluated systematically. The understanding of catalyst performance, deactivation pathways and the mechanisms responsible for their selectivity is still a challenge.

Adventures in Catalysis

Emilio Bunel, Director
Chemical Sciences and Engineering Division
Argonne National Laboratory

In the presentation we will illustrate the use of homogenous catalysts as they relate to:

• The development of catalysts for asymmetric hydroformylation
• Reaction mechanisms for Rh-catalyzed processes involving CO as a building block
• Catalyst deactivation reactions for hydroformylation of cyanide containing substrates
• Asymmetric transformations involving monodentate ligands
• Ligand design for biphasic catalysis