Plane-Wave DFT Investigations of the Adsorption, Diffusion, and Activation of CO on Kinked Fe(710) and Fe(310) Surfaces

The adsorption, diffusion, and dissociation properties of CO on kinked Fe(710) and Fe(310) surfaces have been analyzed using spin-polarized plane-wave density functional theory (DFT) calculations within the generalized gradient approximation (GGA). Several one-, two-, three- and 4-fold binding configurations have been identified among which the preferential adsorption takes place at a 4-fold hollow site near the top of the step while the one with the smallest activation energy for dissociation is located at a 4-fold site near the bottom of the step. In the case of the individual atomic species, the adsorption takes place preferentially at the hollow site for the C atom and at the pseudo 3-fold site on the step for the O atom. By the increase in coverage, there is an overall decrease of the adsorption energies for either molecular (CO) or atomic (C,O) species. The diffusion barriers among different local minima at the steps or on the terraces of both surfaces have been determined, and their values were found to be smaller than the barriers for CO dissociation. The most activated configuration at the bottom of the step can be populated by direct diffusion over the step, from the upper terrace, or by reorientation of the CO molecule within the same hollow site. This last process requires an activation energy of 6.5 kcal/mol on Fe(710) and 9.3 kcal/mol on the Fe(310) surface. The barrier heights for dissociation of CO molecules on Fe(710) (Fe(310)) were found to vary between 15.4 and 20.5 (16.7 and 20.9) kcal/mol depending on the specific location of the hollow site relative to the step edge and to the particular molecular orientation within a given hollow site. For the set of crystallographic Fe surfaces (110), (100), (211), (710), (310), and (111), we found that dissociation on Fe(710) and Fe(310) requires the smallest activation energies in the regime of low coverages. The analysis of the activation properties of CO on this six-member set of flat, stepped, and kinked surfaces indicates the existence of a direct correlation between the apparent activation energy and the rebonding energy of the noninteracting products of the reaction

Plane-Wave Density Functional Theory Investigations of the Adsorption and Activation of CO on Fe₅C₂ Surfaces

A systematic analysis of the adsorption properties of CO on a set of seven low Miller index surfaces ((010) 0.25, (111̅) 0.00, (110) 0.00, (111) 0.00, (111̅) 0.50, (110) 0.50, and (100) 0.00)) of the Hägg iron carbide (Fe5C2) phase has been performed. Calculations were based on spin-polarized plane-wave density functional theory (DFT) within the generalized gradient approximation (GGA). Three general groups of adsorption configurations have been identified corresponding to CO binding exclusively to surface Fe atoms (Fe-only states), to mixed Fe and C(s) atoms (mF−C states), and exclusively to surface C(s) atoms (0F−C states), respectively. Among these, the most stable adsorption configurations correspond to adsorption at Fe-only sites with maximum binding energies ranging from 44.4 to 48.5 kcal/mol, depending on the crystallographic orientation. A diverse bonding scheme for CO was found to exist with formation of one up to six different bonds to the Fe atoms. In the case of CO adsorption at mixed mF−C states or exclusively on top of C(s) atoms, lower adsorption energies are observed ranging from 18.5 to 35 kcal/mol. Despite the lower binding energies, adsorption at mF−C states is shown to lead to significant weakening of the CO bonds, as reflected by large bond elongations and red shifts of the vibrational frequencies. The analysis of the dissociation properties of CO indicates that the most stable adsorption configurations at Fe-only sites have also large activation energies for dissociation, in excess of 40 kcal/mol. Decrease of the activation energy of dissociation was found to take place only for a limited number of cases in which the molecule adsorb in a lying down configuration, where both the C and O ends are bonded to the surface by a total of at least five bonds. Molecular dissociation from mixed mF−C states requires significantly lower activation energies, consistent to the weakening of the CO bonds observed in adsorption studies. In such instances activation energies as low as 15.6 kcal/mol have been determined. Formation of small carbon chains is preferential upon molecular dissociation from such states

Assessing a Generalized CHNO Intermolecular Potential through ab Initio Crystal Structure Prediction

We have analyzed a previously proposed [J. Phys. Chem. B 1997, 101, 798] Buckingham repulsion-dispersion intermolecular potential originally developed for the nitramine explosive RDX using ab initio crystal prediction methods. A total of 174 crystals whose molecules contain functional groups common to CHNO energetic materials were subjected to this methodology. This database includes acyclic and cyclic nitramines, nitrate esters, nitroaromatics, and nitroaliphatic systems. The results of these investigations have shown that for 148 of the 174 systems studied the predicted crystal structures matched the experimental configurations; 75% of these corresponded to the global energy minimum on the potential energy surface. Root-mean-square percent differences between the predicted and the experimental values for the cell edge lengths and densities are about 2 and 4%, respectively. Root-mean-square deviations of rigid body rotational and translational displacements are 2° and 0.07 Å, respectively. Additionally, these same statistics are applicable to the nitramine, nitroaliphatic, nitroaromatic, and nitrate ester classes, suggesting that this interaction potential is transferable across these classes of compounds. The success rate in predicting crystals with structural parameters and space group symmetries in agreement with experiment indicates that this method and interaction potential are suitable for use in crystal predictions of similar CHNO systems when the molecular configuration is known

Molecular Simulations of CO₂ and H₂ Sorption into Ionic Liquid 1-<i>n</i>-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide ([hmim][Tf₂N]) Confined in Carbon Nanotubes

Atomistic simulations are used to study the ionic liquid (IL) 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]) confined into (20,20) and (9,9) carbon nanotubes (CNTs) and the effect of confinement upon gas sorption. The cations and the anions exhibit highly ordered structures in the CNT. There are more cations adsorbed close to the (20,20) tube wall while more anions adsorb in the tube center at high IL loadings. The IL molecules in the CNT exhibit self-diffusivity coefficients about 1−2 orders of magnitude larger than the corresponding bulk IL molecules. Sorption of CO2 and H2 gases in the composite material consisting of CNT and IL indicates that H2 molecules diffuse about 1.5 times faster than the CO2. In contrast, H2 diffuses about 10 times faster than CO2 in both the CNT and in bulk IL. The CNT exhibits the largest amount of sorption for both CO2 and H2, followed by the composite material, and the IL exhibits the least gas sorption. When the temperature is increased, the amount of sorbed CO2 decreases in all three types of systems (IL, CNT, and the composite material) while the H2 sorption increases in [hmim][Tf2N], decreases in the CNT, and does not change significantly in the composite material. The composite material exhibits higher sorption selectivity for CO2/H2 than both the IL and the CNT. It is very interesting to note that the IL molecules can be dissolved in the CO2 molecules under confinement due to a favorable negative transferring energy. However, in the absence of confinement the IL molecules will not dissolve in the CO2 due to a very large unfavorable positive transferring energy

Assessing a Generalized CHNO Intermolecular Potential through ab Initio Crystal Structure Prediction

We have analyzed a previously proposed [J. Phys. Chem. B 1997, 101, 798] Buckingham repulsion-dispersion intermolecular potential originally developed for the nitramine explosive RDX using ab initio crystal prediction methods. A total of 174 crystals whose molecules contain functional groups common to CHNO energetic materials were subjected to this methodology. This database includes acyclic and cyclic nitramines, nitrate esters, nitroaromatics, and nitroaliphatic systems. The results of these investigations have shown that for 148 of the 174 systems studied the predicted crystal structures matched the experimental configurations; 75% of these corresponded to the global energy minimum on the potential energy surface. Root-mean-square percent differences between the predicted and the experimental values for the cell edge lengths and densities are about 2 and 4%, respectively. Root-mean-square deviations of rigid body rotational and translational displacements are 2° and 0.07 Å, respectively. Additionally, these same statistics are applicable to the nitramine, nitroaliphatic, nitroaromatic, and nitrate ester classes, suggesting that this interaction potential is transferable across these classes of compounds. The success rate in predicting crystals with structural parameters and space group symmetries in agreement with experiment indicates that this method and interaction potential are suitable for use in crystal predictions of similar CHNO systems when the molecular configuration is known

Theoretical Predictions of Energetic Molecular Crystals at Ambient and Hydrostatic Compression Conditions Using Dispersion Corrections to Conventional Density Functionals (DFT-D)

Theoretical predictions of the crystallographic properties of a series of 10 energetic molecular crystals have been done using a semiempirical correction to account for the van der Waals interactions in conventional density functional theory (termed DFT-D) as implemented in a pseudopotential plane-wave code. This series contains compounds representative for energetic materials applications, that is, hexahydro-1,3,5-trinitro-1,3,5-s triazine (α- and γ-RDX phases), 1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane (β-, α-, and δ-HMX phases), 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL20) (ε-, β-, and γ-HNIW phases), nitromethane (NM), trans-1,2,-dinitrocyclopropane, 1,2,3,5,7-pentanitrocubane (PNC), pentaerythritol tetranitrate (PETN), 2,4,6-trinitro-1,3,5-benzenetriamine (TATB), 2,4,6-trinitrotoluene (TNT-I phase), and 1,1-diamino-2,2-dinitroethylene (FOX-7), systems belonging to diverse chemical classes that encompass nitramines, nitroalkanes, nitroaromatics, nitrocubanes, nitrate esters, and amino-nitro derivatives. At ambient pressure, we show that the DFT-D method is capable of providing an accurate description of the crystallographic lattice parameters with error bars significantly lower than those obtained using conventional DFT. Practically, for all crystals considered in this study the predicted lattice parameters are within 2% from the corresponding experimental data [α-RDX (1.58%), β-HMX (0.64%), ε-HNIW (1.42%), NM (0.75%), DNCP (1.99%), TATB (1.74%), TNT-I (0.92%), PNC(0.78%), PETN(1.35%), FOX-7(1.57%)], with the best level of agreement being found for systems where experimental data have been collected at low temperatures. A similar good agreement of the predicted and experimental crystallographic parameters was obtained under hydrostatic compression conditions as demonstrated for the cases of RDX, HMX, CL20, NM, TATB, and PETN crystals. These results indicate that the DFT-D method provides significant improvements for description of intermolecular interactions in molecular crystals at both ambient and high pressures relative to conventional DFT. In this last case, large errors of the predicted lattice parameters have been found at low pressures; theoretical values approach the experimental results only at pressures in excess of 6 GPa

Assessing a Generalized CHNO Intermolecular Potential through ab Initio Crystal Structure Prediction

We have analyzed a previously proposed [J. Phys. Chem. B 1997, 101, 798] Buckingham repulsion-dispersion intermolecular potential originally developed for the nitramine explosive RDX using ab initio crystal prediction methods. A total of 174 crystals whose molecules contain functional groups common to CHNO energetic materials were subjected to this methodology. This database includes acyclic and cyclic nitramines, nitrate esters, nitroaromatics, and nitroaliphatic systems. The results of these investigations have shown that for 148 of the 174 systems studied the predicted crystal structures matched the experimental configurations; 75% of these corresponded to the global energy minimum on the potential energy surface. Root-mean-square percent differences between the predicted and the experimental values for the cell edge lengths and densities are about 2 and 4%, respectively. Root-mean-square deviations of rigid body rotational and translational displacements are 2° and 0.07 Å, respectively. Additionally, these same statistics are applicable to the nitramine, nitroaliphatic, nitroaromatic, and nitrate ester classes, suggesting that this interaction potential is transferable across these classes of compounds. The success rate in predicting crystals with structural parameters and space group symmetries in agreement with experiment indicates that this method and interaction potential are suitable for use in crystal predictions of similar CHNO systems when the molecular configuration is known

Assessing a Generalized CHNO Intermolecular Potential through ab Initio Crystal Structure Prediction

We have analyzed a previously proposed [J. Phys. Chem. B 1997, 101, 798] Buckingham repulsion-dispersion intermolecular potential originally developed for the nitramine explosive RDX using ab initio crystal prediction methods. A total of 174 crystals whose molecules contain functional groups common to CHNO energetic materials were subjected to this methodology. This database includes acyclic and cyclic nitramines, nitrate esters, nitroaromatics, and nitroaliphatic systems. The results of these investigations have shown that for 148 of the 174 systems studied the predicted crystal structures matched the experimental configurations; 75% of these corresponded to the global energy minimum on the potential energy surface. Root-mean-square percent differences between the predicted and the experimental values for the cell edge lengths and densities are about 2 and 4%, respectively. Root-mean-square deviations of rigid body rotational and translational displacements are 2° and 0.07 Å, respectively. Additionally, these same statistics are applicable to the nitramine, nitroaliphatic, nitroaromatic, and nitrate ester classes, suggesting that this interaction potential is transferable across these classes of compounds. The success rate in predicting crystals with structural parameters and space group symmetries in agreement with experiment indicates that this method and interaction potential are suitable for use in crystal predictions of similar CHNO systems when the molecular configuration is known

RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations

Prediction of the density and lattice compression properties of the α and γ phases of the hexahydro-1,3,5-trinitro-1,3,5-<i>s</i>-triazine (RDX) crystal and of the low-pressure α → γ phase transition upon pressure increase are general tests used to assess the accuracy of density-functional-theory- (DFT-) based computational methods and to identify the essential parameters that govern the behavior of this high-energy-density material under extreme conditions. The majority of previous DFT studies have analyzed such issues under static optimization conditions by neglecting the corresponding temperature effects. In this study, we extend previous investigations and analyze the performance of dispersion-corrected density functional theory to predict the compression of RDX in the pressure range of 0–9 GPa and the corresponding α → γ phase transition under realistic temperature and pressure conditions. We demonstrate that, by using static dispersion-corrected density functional theory calculations, direct interconversion between the α and γ phases upon compression is not observed. This limitation can be addressed by using isobaric–isothermal molecular dynamic simulations in conjunction with DFT-D2-calculated potentials, an approach that is shown to provide an accurate description of both the crystallographic RDX lattice parameters and the dynamical effects associated with the α→ γ phase transformation. An even more comprehensive and demanding analysis was done by predicting the corresponding shock Hugoniot curve of RDX in the pressure range of 0–9 GPa. It was found that the theoretical results reproduce reasonably well the available experimental Hugoniot shock data for both the α and γ phases. The results obtained demonstrate that a satisfactory prediction of the shock properties in high-energy-density materials undergoing crystallographic and configurational transformations is possible through the combined use of molecular dynamics simulations in the isobaric–isothermal ensemble with dispersion-corrected density functional theory methods

D₂O Interaction with Planar ZnO(0001) Bilayer Supported on Au(111): Structures, Energetics and Influence of Hydroxyls

We investigate the interaction between D2O and the planar ZnO(0001) bilayer grown on Au(111) with temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We show that D2O molecules adsorbed on this planar surface form two ordered overlayers, a (3 × 3) and a (√3 × √3)R30°, not seen before on any of the bulk ZnO single crystal surfaces. The apparent activation energies of desorption (Ed) estimated from TPD peaks are 15.2 and 16.7–17.3 kcal/mol for (3 × 3) and (√3 × √3)R30°, respectively, which agree well with the adsorption energy values calculated from DFT (14.9–15.6 kcal/mol and 16.8–16.9 kcal/mol, respectively). The DFT calculations reveal that the formation of the overlayers takes place at different packing densities and is mediated by extensive hydrogen bonding among the molecules. The hydroxyl groups, which accumulate very slowly on the ZnO(0001) bilayer surface under the standard ultrahigh vacuum (UHV) environment, strongly suppress the formation of the (√3 × √3)R30° overlayer but have less impact on the (3 × 3) overlayer. These findings are explained based on the difference in packing densities of the overlayers such that only the (3 × 3) overlayer with a more open structure can accommodate small amounts of the adsorbed hydroxyl groups