Pd-doped chabazite (Pd/CHA) offers unique opportunities to adsorb and desorb NO x in the target temperature range for application as a passive NO x adsorber (PNA). The ability of Pd/CHA to trap NO x emissions at low temperatures (<200 °C) is facilitated by the binding of NO x species at various Pd sites available in the CHA framework. Density functional theory (DFT) simulations are performed to understand Pd speciation in CHA and the interaction of NO with Pd/CHA to explain the mechanisms of NO adsorption, oxidation, and desorption processes. The calculations are used to elucidate the important role of Pd1+ cationic species, anchored at 6MR-3NN, in providing a strong (E b = −272 kJ/mol) NO adsorption site in Pd/CHA. For NO release, the redox transformation of Pd species comes into play and Pd1+ species are suggested to transform into cationic Pd2+, [PdOH]+, or [Pd–O–Pd]2+ species, all of which show significantly reduced NO binding (−116, −153, and −117 kJ/mol, respectively) as compared to Pd1+. This enables NO desorption at the operating temperature of a downstream catalyst for subsequent catalytic reduction.

The extent to which solvent-mediated effective interactions between nanoparticles can be predicted based on structure and associated thermodynamic estimators for bulk solvents and for solvation of single and pairs of nanoparticles is studied here. As a test of the approach, we analyse the strategy for creating temperature-independent solvent environments using a series of homologous chain fluids as solvents, as suggested by an experimental paper [M. I. Bodnarchuk et al., J. Am. Chem. Soc. 132, 11967 (2010)]. Our conclusions are based on molecular dynamics simulations of Au140(SC10H21)62 nanoparticles in n-alkane solvents, specifically hexane, octane, decane and dodecane, using the TraPPE-UA potential to model the alkanes and alkylthiols. The 140-atom gold core of the nanocrystal is held rigid in a truncated octahedral geometry and the gold-thiolate interaction is modeled using a Morse potential. The experimental observation was that the structural and rheological properties of n-alkane solvents are constant over a temperature range determined by equivalent solvent vapour pressures. We show that this is a consequence of the fact that long chain alkane liquids behave to a good approximation as simple liquids formed by packing of monomeric methyl/methylene units. Over the corresponding temperature range (233-361 K), the solvation environment is approximately constant at the single and pair nanoparticle levels under good solvent conditions. However, quantitative variations of the order of 10%-20% do exist in various quantities, such as molar volume of solute at infinite dilution, entropy of solvation, and onset distance for soft repulsions. In the opposite limit of a poor solvent, represented by vacuum in this study, the effective interactions between nanoparticles are no longer temperature-independent with attractive interactions increasing by up to 50% on decreasing the temperature from 361 K to 290 K, accompanied by an increase in emergent anisotropy due to correlation of mass dipoles on the two nanoparticles. One expects therefore that during self-assembly using solvent evaporation, temperature can be used as a structure-directing factor as long as good solvent conditions are maintained. It also suggests that disordered configurations may emerge as solvent quality decreases due to increasing role of short-range attractions and ligand fluctuation-driven anisotropy. The possibilities of using structural estimators of various thermodynamic quantities to analyse the interplay of ligand fluctuations and solvent quality in self-assembly as well as to design solvation environments are discussed.

Solvent density fluctuations play a crucial role in liquid-vapor transitions in solvophobic confinement and can also be important for understanding solvation of polar and apolar solutes. In the case of ionic liquids (ILs), density fluctuations can be used to understand important processes in the context of nanoscale aggregation and colloidal self-assemblies. In this article, we explore the nature of density fluctuations associated with capillary evaporation of the IL 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF 4 ]) in the confined region of model solvophobic nanoscale sheets by using molecular dynamics simulations combined with non-Boltzmann sampling techniques. We demonstrate that density fluctuations of the confined IL play an important role in capillary evaporation, suggesting analogies to dewetting transitions involving water. Significant changes in the interfacial structure of the IL are also detailed and suggested to underlie a non-classical (non-parabolic) dependence of the free energy barrier to evaporation on the degree of confinement. Published by AIP Publishing. https://doi.

In the search for a "green gasoline", a new reformulation strategy, having no or reduced amount of aromatics, is proposed. Biomass-derived alkyl levulinates (ALs) are prospected as oxygenated additives as well as blending components to circumvent the use of aromatics and methyl tertiary butyl ether (MTBE) in gasoline. By utilizing molecular dynamics (MD) simulations, the thermophysical and dynamical behavior of gasoline blends with four alkyl levulinates, viz. methyl levulinate (ML), ethyl levulinate (EL), propyl levulinate (PL), and butyl levulinate (BL), was scrutinized and compared with those of MTBE−gasoline mixtures. It is shown that, at 300 K and 1 atm, ALs in conventional gasoline can be used for reformulation with amounts up to 18 mol % while maintaining the density, viscosity, and compressibility within the recommended limits. However, this amount can be further increased to 35 mol % by modification of aromatic content. Among the studied oxygenates, BL was observed to have the lowest miscibility in water as compared to other ALs studied. The methodology may be applied to study similar biomass-derived oxygenates for their applicability as a fuel additive or blend.

2-Pyrones have been identified as potential platform molecules derived from biomass to produce high-value chemicals. Theoretical studies utilizing density functional theory (DFT) simulations have shown the partially saturated form of these molecules undergoing a retro Diels−Alder (rDA) reaction in both vapor and solvent conditions. Developing an understanding of the effect of solvent media is crucial to improve the processing of these biomass-derived compounds. In this context, DFT simulations had shown limited access due to either implicit solvent or static explicit solvent environment applied in the model, which did not account for the contribution from the dynamics. Herein, Car−Parrinello molecular dynamics (CPMD) simulations, in conjunction with metadynamics, were performed to understand the effect of dynamics of the solvent on the rDA reaction of partially saturated 2-pyrones. For this, compounds containing both electron-donating and -withdrawing groups as substituent were studied. It was observed that, in the vapor phase, the results from CPMD simulations were in agreement with our previous DFT results. However, in water, a relative lowering in calculated activation barriers (by ∼10 kJ/mol) was observed, due to the dynamic behavior of solvent, differentially interacting with the reactant and the activated complex, along the minimum energy path. For hydroxyl substituted 2-pyrones, computed activation free energy of 63 ± 7 kJ/mol was in agreement with the apparent activation energy measured experimentally (42 ± 18 kJ/mol). CPMD-metadynamics simulations were further applied to study the reaction energetics of complex 2-pyrone molecule undergoing a similar rDA reaction.