David S. Sholl, Ph.D - US grants

Proposal No.: CTS-9813937
PI: David S. Sholl, Andrew J. Gellman
Institution: Carnegie Mellon University
Title: Catalysis with One Hand

Professors David Sholl and Andrew Gellman seek to understand the characteristics of chiral surfaces that determine enantioselectivity in heterogeneous catalysis. They will pursue this goal using surface science, quantum chemical and Monte Carlo techniques. Their first objective is to study chemisorption and surface reactions on naturally chiral surfaces, such as Ag(643)S, Cu(643)S, Pt(643)S and Ag(643)R, Cu(643)R, Pt(643)R, which have S and R configurations. The differences in desorption energies, adsorbate orientation angles and reaction rate constants will be calculated and measured, in order to isolate and quantify the role of each type of interaction in adsorption-desorption enantiodifferentiation. Enantiomerically pure adsorbates will consist of alkyl chlorides, secondary or tertiary alcohols, alkenes, nitriles, and substituted aromatics. LEED will be used to characterize the surfaces, IRAS to determine adsorbate orientation, and TPD to measure desorption energy. Utilizing semiempirical potentials, molecule-surface and intermolecular interactions will be parametrized and adjusted to experimental measurements of alkyl chloride adsorption. Monte-Carlo simulations will afford the energy and orientation of adsorbed species in order to study surface coverage and adsorbate size effects on enantiodifferentiation. Extended Huckel Molecular Orbital (EHMO) theory will be used in the case of alcohol, ketone and alkene adsorption on naturally chiral Cu and Pt surfaces.
The second objective is to modify achiral surfaces (such as Pt(111) and Cu(111)) by adsorption of a chiral template, to model and quantify the types of interaction producing enantiospecific behavior. Templates will consist of R or S configurations of thermally stable molecules bound to the metal through oxygen or methylene groups, and containing side groups such as alkyl, vinyl, phenyl, etc.. Co-adsorbates will consist of reversible bound monofunctional species such as primary alcohols, ketones and nitriles. Interaction energies will be measured using TPD. The interaction between functional groups will be studied theoretically utilizing semiempirical potentials for group-surface and function-function interactions, as well as EHMO theory and Monte-Carlo simulation to obtain adsorbate orientation and desorption energy.
The third objective is to investigate the surface chemistry of enantiospecific surfaces. Series of alcohols on well-characterized kinked and stepped surfaces will be used to study structural factors such as relative size of adsorbate with respect to spacing between selective sites. Theoretical prediction of energies and molecular orientation for various combinations of surfaces, templates and co-adsorbents will be used to guide the experimentation.
The final goal is to generate fundamental knowledge of the surface and intermolecular interactions that produce enantiodifferentiation. Such knowledge could be used to guide the design of heterogeneous enantioselective catalysts. This theorico-experimental research will involve the training of students.

Professors Paul Sides and David Sholl will investigate the origin of the morphological instability occurring during homoepitaxial chemical vapor deposition of CdTe thin films from organometallic precursors. They will use Monte Carlo Simulations and continuum growth models as well as experiments of CdTe deposition on (100), (110), (111)CD and (111)Te. Morphology will be determined with nomarsky and scanning tunneling microscopy in a time-resolved manner, and the observations will be compared with the simulations. A new monoloayer-sentitive technique will be developed based on scattered total internal reflection infrared spectroscopy. Radiaiton below 900 nm will be directed through the crystal towards the CdTe-vapor interface with the purpose of detecting the appearance of steps. The hypothesis that Schwoebel barriers are responsible for the morphological instability of CdTe deposited by OMVPE will be tested.

ABSTRACT

Proposal Number: CTS-9983647
Principal Investigator: David S. Sholl
Institution: Carnegie Mellon University
Title: CAREER: Atomically Detailed Modeling of Transport through Zeolite Membranes


Zeolites are solid, crystalline materials that are permeated by networks of ordered nanometer-scale pores. These pores are approximately the same size as many small molecules. Because molecules adsorbed inside zeolite pores are always in intimate contact with the walls of the pore, the diffusion rates, and adsorption energies of various chemical species can depend strongly on the size, shape, and functionality of the species. These facts have long been exploited in the applications of zeolites to shape-selective catalysis and adsorption-based separations. Recent years have seen dramatic advances in the reliable synthesis of ultra-thin membranes made from zeolite crystals. An important challenge in the continuing development of these membranes is the need for accurate theoretical models of their performance. This project will focus on developing hierarchical models of molecular transport through zeolite membranes that combine detailed atomic-scale simulations with macroscopic nonequilibrium thermodynamic theories. A vital part of this process will be a series of direct comparisons between theoretical predictions from this research and experimental results obtained by experimental collaborators.

Zeolite membranes have a number of attractive properties. From a macroscopic point of view, they have excellent thermal, mechanical, and chemical stability. These properties allow zeolite membranes to be used under conditions that are too harsh for traditional membrane materials such as polymers. Microscopically, the atomically ordered pore structure of zeolites means that zeolite membranes can, in principle, be extremely selective even for mixtures of molecules with very similar shapes and sizes. By developing theoretical models that explicitly account for the atomic-scale structure of zeolite pores, this project will develop a framework for computationally screening libraries of zeolite structures to choose membrane materials that optimize throughput and selectivity for a desired chemical separation. In the shorter term, this work will clarify the fundamental microscopic processes that control molecular transport through zeolite membranes and provide insight into selecting efficient operating procedures for existing membranes.

ABSTRACT

PI: David S. Sholl, Lorenz T. Biegler and Steinar Hauan
Institution: Carnegie Mellon University
Proposal Number: 0094407

This is an equipment grant to provide funds to purchase a 32 node Beowulf cluster to support research in advanced computing to be used by three research groups at Carnegie Mellon University. Beowulf clusters represent an advantageous architecture for advanced computing: they are inexpensive to construct, flexible to configure, and provide a powerful computing environment for a broad variety of scientific computing tasks. The three research groups that will share the cluster will be conducting research on molecular dynamics and computational chemistry, modeling and visualization to support process synthesis and large-scale discrete and continuous process optimization. The cluster will thus serve computations that require high peak performance on time scales of minutes and hours, along with sustained computational performance on time scales of days. These complementary profiles mean that by sharing the cluster, its capabilities will be used more fully than they would be by any group individually. The cluster can also easily be operated in a way that distributes resources between different usage modes without significant computational overhead. The cluster will initially increase the PIs' computational capabilities by about a factor of three and will be expandable over succeeding years and will increase their computing power by over an order of magnitude within the next five years, without rendering existing nodes obsolete.

Specifically, the research of the three groups consists of the development of new algorithms for:
Calculation in computational chemistry, particularly in molecular dynamics and Monte Carlo simulations to determine macroscopic material properties from details of atomic-scale structure;
Accurate process modeling for process design and synthesis that includes phase equilibrium, representation and evaluation of process alternatives and visualization of design insights; and
Optimization of large-scale steady state and dynamic processes that involve both discrete and continuous decisions along with advanced decomposition strategies.

The objective of this project is to explore the fundamental aspects of enantioselective
chemistry on chiral metal surfaces. The investigation is focused on naturally chiral surfaces formed from high Miller index planes of metals such as Cu and Pt. The chirality of these surfaces arises from the kink-step-terrace structures that are formed by the intersections of three low Miller index microfacets. In this sense this type of surface chirality is distinct from the chirality imparted by the traditional approach of templating a surface with chiral ligands. Past work has probed the structure of naturally chiral surfaces, enantiospecific adsorbate geometries, and enantiospecific surface reaction energetics. These properties of chiral surfaces determine the enantioselectivities of catalytic reactions and chromatographic separations. The work proposed focuses on the exploration of naturally chiral surfaces of various structures using a combination of experimental and theoretical methods. Although there are an infinite number of chiral high Miller index surfaces, they are composed of a finite number of types of kink-step-terrace structures. The proposed investigation will explore the enantiospecificity of this set of structures on Cu single crystal surfaces by using chiral adsorbates that have previously exhibited enantiospecific surface chemistry. The educational activities are expected to be the traditional mentoring of graduate students. The fundamental goal of the proposed work is to provide a basic understanding of the roots of enantioselectivity on a set of highly characterized, naturally chiral surfaces.

The Midwest Thermodynamics and Statistical Mechanics Meeting will be held in Pittsburgh, PA on May 13-14, 2002. Although, most of the speakers are faculty, this meeting also emphasizes presentations and participation by graduate students and postdoctoral scholars working in all areas of thermodynamics, including experimental and classical thermodynamics, molecular simulation, and statistical mechanics. Funding is requested to reimburse graduate students for travel and registration costs associated with the meeting. Funds for other aspects of running this meeting have been arranged from other sources, primarily donations from Carnegie Mellon University and the University of Pittsburgh. NST-CTS has provided student travel funds for this meeting several times in the past, and this action has been important in making attendance at this meeting widely accessible to graduate students.

Abstract


Proposal Title: A Combined Theoretical and Experimental Study of Transport of Molecular Mixtures in Zeolite Membranes
Proposal Number: CTS-0413027
Principal Investigator: David Sholl
Institution: Carnegie Mellon University


This research will focus on using atomically-detailed simulations of diffusion in
zeolites in close connection with experimental studies to advance the understanding of macroscopic diffusion in nanoporous materials. Three closely linked topics will be examined: single-component diffusion of n-alkanes in silicalite, mixture diffusion of n-alkanes in silicalite, and effects of pore size and shape on mixture diffusion. Extensive atomistic simulations will be combined with membrane permeation experiments and neutron scattering experiments to self-consistently describe self and transport diffusion of n-alkanes in silicalite. Atomistic simulations will be combined with membrane permeation and neutron scattering experiments to assess the diffusion of multiple binary mixtures in silicalite. An extensive set of atomistic simulations will be performed to examine binary diffusion in a set of silica zeolites chosen to systematically vary pore size and connectivities. Similar simulations will be performed for carbon nanotubes, which are known to have very smooth pore walls, to probe the influence of pore wall corrugation. These simulations will greatly expand the variety of binary adsorbed mixtures for which data are available and provide stringent tests for the predictive models of mixture diffusion. In terms of the broader impacts, the proposed research will engage graduate students in an environment where they will develop strong communication skills and where close collaboration between theoretical and experimental participants is required. This work may lead to industrial applications in zeolite-based membrane separations.

Abstract


Proposal Title: NER: Carbon Nanotube/Polymer Composties for High Flux/High Selectivity Gas Separation Membranes
Proposal Number: CTS-0403692/0406855
Principal Investigator: Eva Marand/David Sholl
Institution: Virginia Polytechnic Institute and State University/
Carnegie Mellon University

This proposal was received in response to Nanoscale Science and Engineering initiative, NSF 03-043, category NER. The proposed research program will combine experimental and theoretical modeling activities to fabricate and characterize high quality nanotube/polymer composite membranes. Recent theoretical work by Sholl has predicted that carbon nanotubes, if used as membranes, have the flux/selectivity properties that far exceed those of any other known inorganic or organic material. Currently gas separation membranes are fabricated from polymeric materials that are typically processed as hollow fibers. However, despite the ability to produce robust, large-area membranes at relatively low cost, a wider implementation of polymer membranes is hindered by their intrinsic permeability and selectivity limitations. In this exploratory research project the development of novel nano-composite membranes could overcome such limitations. The PIs propose to fabricate and characterize nano-composite membranes, consisting of open-ended carbon nanotubes incorporated in specifically designed polymer matrices. Atomistic and multi-scale modeling of the nano-composite system will provide a fundamental feedback towards understanding and optimization of the transport properties of the nano-composite systems. The assembled team combines research expertise in molecular modeling, interfacial characterization and nano-composite processing, which will allow the PIs to effectively carry out the activities outlined in the proposal. In terms of the broader impacts, the research program may lead to the development of robust, highly permeable membranes for selective gas separations, as well as to the introduction of new chemistries and methods, which can be employed in the fabrication of high performance composites, sensors and other chemical devices incorporating carbon nanotubes. The research program has a strong educational component aimed at both graduate and undergraduate students, including unique interdisciplinary and inter-university research experience, inclusion of research examples into the undergraduate curriculum and other educational outreach activities.

This collaborative research project combines experimental studies carried out in the laboratory of Charles Sykes at Tufts University, experimental work from the laboratory of Andrew Gellman at Carnegie Mellon, and theoretical studies by David Scholl and coworkers, also at Carnegie Mellon. With the support of the Analytical and Surface Chemistry Program, this collaborative team is investigating the structure and reactivity of naturally chiral metal surfaces. Copper single crystals cut to expose chiral kink sites are the substrates being examined, and the adsorption and reaction of chiral and achiral small molecules on these surfaces are studied. A combination of scanning tunneling microscopy (STM), vibrational STM, photoemission of adsorbed Xe, thermal desorption spectroscopy, and density functional calculations are the tools used to examine these reactions. A fundamental understanding of structure and reactivity of chiral surfaces helps to develop enantioselective catalytic and separation processes, of considerable value in the pharmaceutical industry.

A collaboration between surface science experimentalists and theoreticians at Tufts University and Carnegie Mellon University addresses the question of chiral reactivity of miscut copper surfaces. By cutting the surfaces to expose chiral kink sites, the detailed effects of structure on adsorption and reaction of chiral molecules can be examined. A suite of experimental and theoretical probes are brought to bear on this question, with the goal of providing fundamental understanding for the design of enantioselective reaction and separation processes.

PROPOSAL NUMBER: 0651182
PRINCIPAL INVESTIGATOR: Gellman, Andrew J.
INSTITUTION: Carnegie-Mellon University

Intellectual Merit
The proposed research effort couples experiment and computational theory to study the transition states to catalytic surface reactions. The activation barriers, Et , to several elementary reactions will be measured using sets of selectively fluorinated reactants. Fluorine substituent effects on the Et will be used as experimental probes of the transition states to those reactions. In parallel, density functional theory (DFT) will be used to predict the transition state structures, electron density distributions and Et for the same reactions. By combining computational theory and experiment, the proposed work will provide the most accurate and well-benchmarked descriptions of catalytic transition states. The computed differences in electron density distributions between reactant and transition state will be compared with predictions based on substituent effects; used to validate some of the basic assumptions used in the interpretation of substituent effects on surfaces; used to probe the origins of phenomena such as structure sensitivity; and used to probe electrostatic screening effects in reactions on metals versus oxides. These address several fundamental phenomena that influence catalytic activity. The substituent effect methodology has been developed in prior work and is now being extended to probe the origins of structure sensitivity and the effects of surface composition of s urface reaction kinetics. Substituent effects will be used to probe several reactions:
ii-hydride elimination in alkyl and alkoxy groups on Cu(100), Cu(111) and Cu(110),
halkyl hydrogenation on Pt(111), Pt(100), and Pt(110), and dehalogenation reactions on Cr O 101 2 2 3 .
Infrared absorption will be used to determine the orientations of the reactant alkyl and alkoxy groups on the various surfaces. Temperature programmed reaction spectroscopy will be used to measure the Et to p-hydride elimination, hydrogenation and dehelogenation in sets of fluorine substituted reactants. For quantitative interpretation, the results of these measurements will be compared directly to the results of DFT simulations of the same reactants on the same surfaces. Using measurements to benchmark theory will provide an unprecedented level of confidence in the analysis of the transition states predicted by DFT.

Broad Impact
The proposed effort will have broad impact by generating a fundamental insight into the nature of transition states for surface reactions and into the influences of surface structure and composition on catalytic reaction kinetics. Because substituent effects have a fairly simple physical interpretation, the insights into the nature of surface transition states are amenable to presentation in the classroom. Students working on this research will be exposed to both experiment and computational theory. This and the fact that the work develops physical insight into fundamental catalytic phenomena will help to generate interest in catalytic phenomena among students. The deeper insights based on the use of density functional theory will be of interest to those trying to understand and model catalytic phenomena at a deeper level.
The results and concepts generated by the proposed research will be disseminated widely in the scientific community through publication and through presentations by the PIs and the students working on the project. In addition, the PIs are both active within several professional societies and will generate opportunities for broad dissemination of results through the organization of symposia in relevant areas.