Research Summary
Overview
Our research focuses on the development and application
of molecular based computational and theoretical
methods for the design of chemical systems and processes.
The overall objective is to develop a theoretical
understanding of important chemical systems in order
to advance beyond empirical approaches which are
based on heuristics and conceptual understanding.
Our theories must be verifiable and predictive.
The projects described below, incorporate the development
and use of Monte Carlo simulations, molecular dynamics
methods, ab initio and density functional theory
calculations, QM/MM methods, and first-principles
molecular dynamics.
Water in Inhomogeneous Environments
Nucleation of Hydrate Clathrates and Ice:
Hydrate-clathrates are ice-like materials, in which
cages of water molecules arrange themselves in regular
patterns around small guest molecules, such as CO2
and/or methane. They are involved in the sequestration
and ocean storage of CO2, and they host vast resources
of natural gas. The current understanding of nucleation,
the labile cluster hypothesis, is merely conceptual
at best. It states that under hydrate-forming conditions,
solid cages of water molecules are formed first,
and these then connect with each other to build
the lattice. We have developed a new approach to
study nucleation based on the Landau Free Energy
method, combined with the choice of suitable order
parameters that we developed. Using this method
to study CO2-hydrate clathrates, we showed that
the labile cluster hypothesis is not feasible. More
importantly, we developed a new theory, the local
structuring hypothesis, in which the critical nucleus
(~18 ‰ in diameter) must be formed completely via
local fluctuations. We intend to extend this theory
to the study of methane hydrates, to heterogeneous
nucleation, and to the development of kinetic inhibitors
to be using in gas pipelines.
Stabilization of Therapeutic Proteins via Solvent
Formulation: Therapeutic proteins, which are
used to treat an enormous variety of diseases, are
generally stored in aqueous media before they are
used. Unfortunately, they degrade over time from
the oxidation of sulfur atoms in methionine residues.
We hypothesize that local structuring of solvent
in addition to solvent accessibility controls the
rates of oxidation of various sites, and we are
developing theories to be used to add benign solutes
which will stabilize these proteins.
Modeling of the Dissolution of CO2 under Hydrate-Forming
Conditions: We have developed the first verifiable
macroscopic model that can describe the dissolution
of droplets of CO2 in the ocean. This has been accomplished
by using molecular simulations to compute model-independent
transport properties, such as diffusivities.
Modeling of the Phase Behavior of Hydrate-Clathrates:
We have used ab initio methods to compute 18,000
points on the methane-water potential energy hypersurface
with high accuracy. In order to accomplish this,
we developed a method to correct calculations performed
using small basis sets (~1 min./calc.) to those
using large basis sets (~1 day/calc.). We have validated
this potential for use in making predictions by
computing accurately the phase behavior of methane-hydrates.
We have also developed an analytical method for
fitting phase data. We have shown that our analytical
method is not only simpler than the numerical methods
currently used by hundreds of researchers but also
gives more meaningful physical understanding than
those methods.
Theoretical Heterogeneous Catalysis
Understanding Stratospheric Heterogeneous Catalysis
Responsible for Ozone Depletion: Ice in the
stratosphere, normally thought to be chemically
inactive, is an extremely good catalyst for activating
chlorine, which is a key component in cycles leading
to ozone depletion. We have used order parameters
to characterize the degree of disorder on ice surfaces
as a function of temperature and adsorbate concentration,
and we have helped to explain the high uptake of
HCl. We have concluded that HCl is most likely molecularly
adsorbed. In future work, we wish to study HCl uptake
on nitric acid trihydrate and futher reactions involving
HCl.
Reactions of Sulfur Oxides on Metal Surfaces:
Sulfur plays a key role in deactivating automotive
catalysts and components of fuel cells. In particular,
it can (reversibly) form metal sulfides under reducing
conditions, and (irreversibly) form sulfates from
metal oxides under lean conditions. We have been
quantifying the rates of elementary reactions involving
sulfur poisoning and plan to continue to do so.
Our objective is to use this understanding in order
to develop more sulfur resistant catalysts.
Development of Measures of Reactivity of Solid
Acid Catalysts: No scale currently exists to
quantify solid acidity, such as Br¯nsted acidity
in zeolites. Furthermore, simple measures, such
as the energy of adsorption of bases, sometimes
correlates to reactivity and sometimes does not.
We are performing quantum density functional theory
calculations on periodic models of chabazite in
order to develop a measure of solid acidity and
to address the question of whether different acid
sites in a particular zeolite have different activities.
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