Kendall T. Thomson

Water in porous carbons

Abstract We present an overview of progress in understanding the behavior of water in porous carbons at the molecular level. We survey experimental investigations, semi-empirical approaches, and simulation studies. Experimental work faces a number of challenges: the determination of the distribution of carbon microcrystal sizes, the densities and species of surface groups, the topological nature of the connected pore structure, and pore size distributions. The lack of experimental characterization, together with the uncertainty in the intermolecular potentials involved, has thwarted molecular simulation efforts thus far. A concerted approach that links experimental and simulation efforts appears promising in gaining a better understanding of the behavior of water in porous carbons. Experimental results could aid in the development of realistic carbon models and improve the intermolecular potentials used in the simulation studies. In a complementary fashion, molecular simulation could help improve characterization methods of both the carbon structure and the surface chemistry.

CO adsorption on pure and binary-alloy gold clusters: a quantum chemical study.

We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters.

Density functional theory investigation of gold cluster geometry and gas-phase reactivity with O2

We have conducted a density functional theory investigation into the gas-phase reactivity of small gold cluster ions in the interest of understanding gold cluster reactivity in several catalytic systems. Previously unreported geometries for Au9− and Au10− anions are obtained and reported from geometry optimizations. Predicted values of the vertical detachment energy match well with experiment, as does a rough simulation of its ultraviolet photoelectron spectrum—we found that comparison of predicted spectra with experimental data is a more sensitive analysis of geometry differences. Several binding sites for O2 with different energies are identified on Au10−, but we show that the strongest binding site and orientation is predicted by frontier orbital theory. In addition, weakly stable adsorbed states for O2 on the anion clusters Au9−, Au10−, and Au11− are predicted in agreement with frontier orbital theory. The calculated binding energies are consistent with the experimentally observed patterns in adsorpti...

Polymorphs of Alumina Predicted by First Principles: Putting Pressure on the Ruby Pressure Scale

Fully optimized quantum mechanical calculations indicate that Al2O3 transforms from the corundum structure to the as yet unobserved Rh2O3 (II) structure at about 78 gigapascals, and it further transforms to Pbnm-perovskite structure at 223 gigapascals. The predicted x-ray spectrum of the Rh2O3 (II) structure is similar to that of the corundum structure, suggesting that the Rh2O3 (II) structure could go undetected in high-pressure x-ray measurements. It is therefore possible that the ruby (Cr3+-doped corundum) fluorescence pressure scale is sensitive to the thermal history of the ruby chips in a given experiment.

Catalyst design: knowledge extraction from high-throughput experimentation

We present a new framework for catalyst design that integrates computer-aided extraction of knowledge with high-throughput experimentation (HTE) and expert knowledge to realize the full benefit of HTE. We describe the current state of HTE and illustrate its speed and accuracy using an FTIR imaging system for oxidation of CO over metals. However, data is just information and not knowledge. In order to more effectively extract knowledge from HTE data, we propose a framework that, through advanced models and novel software architectures, strives to approximate the thought processes of the human expert. In the forward model the underlying chemistry is described as rules and the data or predictions as features. We discuss how our modeling framework—via a knowledge extraction (KE) engine— transparently maps rules-to-equations-to-parameters-to-features as part of the forward model. We show that our KE engine is capable of robust, automated model refinement, when modeled features do not match the experimental features. Further, when multiple models exist that can describe experimental data, new sets of HTE can be suggested. Thus, the KE engine improves (i) selection of chemistry rules and (ii) the completeness of the HTE data set as the model and data converge. We demonstrate the validity of the KE engine and model refinement capabilities using the production of aromatics from propane on H-ZSM-5. We also discuss how the framework applies to the inverse model, in order to meet the design challenge of predicting catalyst compositions for desired performance.  2003 Elsevier Science (USA). All rights reserved.

Mechanistic Detail Revealed via Comprehensive Kinetic Modeling of [rac-C2H4(1-indenyl)2ZrMe2]-Catalyzed 1-Hexene Polymerization

Thorough kinetic characterization of single-site olefin polymerization catalysis requires comprehensive, quantitative kinetic modeling of a rich multiresponse data set that includes monomer consumption, molecular weight distributions (MWDs), end group analysis, etc. at various conditions. Herein we report the results obtained via a comprehensive, quantitative kinetic modeling of all chemical species in the batch polymerization of 1-hexene by rac-C(2)H(4)(1-Ind)(2)ZrMe(2) activated with B(C(6)F(5))(3). While extensive studies have been published on this catalyst system, the previously acknowledged kinetic mechanism is unable to predict the MWD. We now show it is possible to predict the entire multiresponse data set (including the MWDs) using a kinetic model featuring a catalytic event that renders 43% of the catalyst inactive for the duration of the polymerization. This finding has significant implications regarding the behavior of the catalyst and the polymer produced and is potentially relevant to other single-site polymerization catalysts, where it would have been undetected as a result of incomplete kinetic modeling. In addition, comprehensive kinetic modeling of multiresponse data yields robust values of rate constants (uncertainties of less than 16% for this catalyst) for future use in developing predictive structure-activity relationships.

Molecular modeling of carbon aerogels

Abstract Carbon aerogels are prepared via pyrolysis of resorcinol-formaldehyde gels. The structure consists of a highly porous three-dimensional network made up of interconnected, roughly spherical carbon particles. The aerogel studied in this work was mesoporous and had carbon particles having a diameter of ≈6 nm, connected in an open-cell structure with a porosity of ≈0.55. In addition to the mesopores between the carbon particles, the carbon particles themselves possess slit-shaped micropores with a width of ≈0.7 nm. We present a molecular model of this material, consisting of carbon spheres of diameter 6 nm in a connected network. This matrix is prepared by first generating a random close-packed structure of slightly overlapping spheres, followed by random removal of spheres to match the targeted porosity. Structural characteristics of the model have been studied using different MC techniques and compare well with those for the laboratory material. Nitrogen adsorption in this model aerogel was studied using a parallelized Grand Canonical Monte Carlo algorithm based on a domain-decomposition scheme. Large systems are needed for this simulation in order to represent the pore network in a realistic fashion. Adsorption occurs in the micropores at very low pressure, followed by adsorption in the mesopores, with capillary condensation occurring at the higher pressures.

The effects of a dynamic lattice on methane self-diffusivity calculations in AlPO4-5

Canonical ensemble molecular dynamics simulations were conducted for methane diffusion in AlPO4-5 in order to assess the role of the lattice motion on adsorbate diffusivity in straight pore zeolites. Both a static lattice model and a full dynamic lattice model were used at a loading of 1.5 methane/unit cell at 295 K. Although recent simulation work has asserted that there should be a difference, we show that there is little difference in the observed methane diffusivity (1.26×10−7 m2/s) and passing frequency (0.305) when a static lattice approximation is used over a full dynamic lattice (1.33×10−7 m2/s and 0.328). Furthermore, we introduce a methodology for handling lattice motion in molecular simulations by utilizing the normal vibrational modes in a harmonic crystal approximation.

A density functional study of the electronic structure of sodalite

We have conducted a first principles density functional theory (DFT) calculation to explore the electronic structure of sodalite at various stages of Al-substitution. By calculating the electronic structure of both substituted and unsubstituted frameworks, with and without the presence of extra-framework atoms, we show that Al-substitution and cation compensation essentially affect the electronic structure only at the upper valence band edge (i.e., the frontier orbitals of reactivity theory). In addition, we show that the equilibrium positions of the extra-framework cations are located in the vicinity of the frontier orbitals which are preferentially localized near aluminum.

Lifting the Pt{100} surface reconstruction through oxygen adsorption: A density functional theory analysis

The adsorption of atomic oxygen on unreconstructed Pt[100]-(1 x 1) and reconstructed Pt[100]-(5 x 1) was modeled using density-functional theory in an attempt to understand the relative stability of the unreconstructed phase as a function of oxygen coverage. Our calculations showed that at zero temperature the (5 x 1) is more stable than the unreconstructed (1 x 1) phase at zero oxygen coverage. However, oxygen absorption on the Pt[100]-(5 x 1) phase removed the reconstruction, reversing the phase stability. Using thermochemical analysis, we show desorption of oxygen corresponding to a temperature near 730 K, consistent with experimentally observed desorption peaks for oxygen covered (1 x 1) surfaces. These results have ramifications for understanding the full Pt[100](1 x 1)-->Pt[100]hex-R0.7 degrees surface phase transition.