- Vishal Agarwal - Google Scholar Citations
- Materials Chemistry and Physics
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This should be possible for all possible arrangements of the electrons: in chemical bonds, in excited states, in dissociated states, at transition states for chemical reactions, etc. It should also be possible for all atoms of the periodic systems. The addition of dynamic electron correlation is as crucial here as it is in the HF method.
The suggested solution is to compute this energy using second order perturbation theory CASPT2 because it is relatively simple and allows applications to a wide variety of systems and many electrons.
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The review will focus on the methods themselves. Applications will be described in other chapters of the book. Unable to display preview. Download preview PDF. Skip to main content. Advertisement Hide. Multiconfigurational quantum chemistry for ground and excited states. This is a preview of subscription content, log in to check access. Among the surfaces studied, we have found that s and s faces have the lowest oxygen-vacancy formation energies, and s has the highest.
Abstract: Oxygen vacancy formation energies are often used as a descriptor of the catalytic activity of metal oxides for oxidation reactions having the Mars—van Krevelen mechanism. When these energies are calculated, it is often assumed that they depend only on the concentration of the vacancies in the top oxygen layer. Previous work has shown that in the case of TiO2 and V2O5, the energy of vacancy formation depends not only on their concentration but also on the manner in which they are distributed on the surface. However, the energy change due to the change of configuration in these systems is very small.
One of the questions we wanted to answer is whether the fact that zeolite cages are made of a silica monolayer plays any role in their catalytic activity.
We find no important difference. Abstract: We use the Potts-lattice gas model to study nucleation at and near the eutectic composition.
We use rare-event methods to compute the free energy landscape for the competing nucleation products, and short trajectories at the barrier top to obtain prefactors. We introduce a procedure to tune the frequency of semigrand Monte Carlo moves so that the dynamics of a small closed system roughly resemble those of an infinite system. The non- dimensionalized nucleation rates follow trends as predicted by the classical nucleation theory.
Vishal Agarwal - Google Scholar Citations
Finally, we develop corrections that convert free energy surfaces from closed canonical simulations into free energy surfaces from open semigrand simulations. The new corrections extend earlier corrections to now address situations like nucleation at the eutectic point where two products nucleate competitively. Abstract: This chapter focuses on a frontier for molecular simulation: nucleation of solute precipitates from solution. Two simple cases of heterogeneous nucleation are considered: i a spherical cap model for nucleation on a hard flat surface and ii a lens model for nucleation at fluid—fluid interfaces.
Over the decade following the discovery of two'step nucleation, several computational studies have pointed to two'step nucleation as a more general phenomena occurring even at points where there is no metastable fluid—fluid critical point. Rare events simulation methods can relax many of the assumptions made by classical nucleation theory CNT. The chapter concludes with case studies on laser'induced nucleation, and on nucleation of methane hydrates and calcium carbonate.
To obtain initial conditions at reasonable densities, we extracted coordinates from larger classical NPT simulations at the target temperatures. CPMD-metadynamics implemented with various sets of collective variables, such as coordination numbers of the glycosidic oxygen, yielded a variety of chemical reactions such as depolymerization, fragmentation, ring opening, and ring contraction. These reactions yielded precursors to levoglucosan LGA —the major product of pyrolysis—and also to minor products such as 5-hydroxy-methylfurfural HMF and formic acid. We performed several comparison calculations to gauge the accuracy of CPMD-metadynamics barriers with respect to basis set and level of theory.
Computed IR spectra were found to agree semi-quantitatively with experiment, especially in the O—H stretching region. We assigned O—H stretches using a novel synthesis of normal mode analysis and power spectrum methods. Simulated IR spectra at elevated temperatures suggest a structural transformation above K, a result in agreement with experimental IR results.
A three-dimensional hydrogen bonding network emerges at high temperatures due to formation of new interchain hydrogen bonds, which may explain the stability of the cellulose structure at such high temperatures.
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Abstract: We have studied base strengths of nitrogen-substituted nitrided zeolites with faujasite FAU structure by calculating sorption energies of probe molecules BF 3 and BH 3 using density functional theory with mixed basis sets applied to embedded clusters. Sorption energy and hence base strength was found to decrease with increasing Si:Al ratio from 1 to 3 beyond which the base strength was found to increase again.
Abstract: We have modeled the formation kinetics of nitrogen-substituted nitrided zeolites HY and silicalite; we have also modeled the stability of nitrided sites to heat and humidity. These kinetic calculations are based on mechanisms computed from DFT-computed pathways reported in our previous work. Reactant ammonia and product water concentrations were fixed at various levels to mimic continuous nitridation reactors.
These threshold temperatures are in good agreement with experiments. Nitridation yields were found to be sensitive to water concentration, especially for silicalite where nitridation is more strongly endothermic. As a result, overall nitridation yields in silicalite are predicted to be much lower than those in HY.
The stability of nitrided sites was investigated by modeling the kinetics of nitridation in reverse, going back to untreated zeolite plus ammonia. As such, our calculations suggest that nitrided silicalite and HY zeolites require high temperatures to form, but once formed, they remain relatively stable, auguring well for their use as shape-selective base catalysts. Abstract: We have performed embedded-cluster calculations using density functional theory to investigate mechanisms of nitrogen substitution nitridation in HY and silicalite zeolites.
To reveal mechanistic details, we have combined for the first time the nudged elastic band method of finding elusive transition states, with the ONIOM method of treating embedded quantum clusters. We predict that nitridation of silicalite proceeds via a planar intermediate involving a ring with pentavalent Si, whereas nitridation of HY is found to proceed via an intermediate similar to physisorbed ammonia.
Although the overall nitridation barriers are relatively high, requiring high temperatures for substitution, the overall barriers for the reverse processes are also high. As such, we predict that once these catalysts are made, they remain relatively stable. Abstract: Nanoporous acid catalysts such as zeolites form the backbone of catalytic technologies for refining petroleum. Citations per year.
Materials Chemistry and Physics
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