Iron pentacarbonyl (IPC) gas forms upon the reaction of carbon monoxide with Fe containing metallic surfaces under gas reforming conditions. IPC formation can sometimes reach alarming levels that cause metal loss, pipeline thinning corrosion, catalyst poisoning, and contamination of sensitive industrial equipment. In this work, we demystify using multiscale computational modeling the mechanism of Iron pentacarbonyl formation: Density functional theory (DFT) is used to explore various catalytic reactions that involve a Fe adatom reacting with adsorbed carbon monoxide. Our calculated carbonyls desorption barriers on a perfect and clean Fe surface are too high to allow the carbonyls to form then desorb at temperatures <500 K at the rates reported experimentally. Most importantly, our calculations indicate that a high CO surface coverage, in addition to the presence of Fe adatoms, favors carbonyl formation and its desorption towards the flowing gas medium. Using insights extracted from ab initio molecular dynamics simulations, we propose that the most plausible IPC formation mechanism consists of: (1) on surface reactions of adsorbed CO molecules with an Fe adatom to form iron tricarbonyl (Fe(CO) 3 * ) molecules; (2) an adsorbate assisted movement of iron tricarbonyl on top of the CO adlayer; and (3) the interaction of iron tricarbonyl with CO molecules from the gaseous medium eventually leading to iron adatom removal as Fe(CO) 5 gas.

DFT functionals are of paramount importance for an accurate electronic and structural description of transition metal systems. In this work, a systematic analysis using some well-known and commonly used DFT functionals is performed. A comparison of the structural and energetic parameters calculated with the available experimental data is made in order to find the adequate functional for an accurate description of the TiO2 bulk and surface of both anatase and rutile structures. In the absence of experimental data on the surface energy, the theoretical predictions obtained using the high-accuracy HSE06 functional were used as a reference to compare against the surface energy values calculated with the other DFT functionals. A clear improvement in the electronic description of both anatase and rutile was observed by introducing the Hubbard U correction term to PBE, PW91, and OptPBE functionals. The OptPBE-U4 functional was found to offer a good compromise between accurately describing the structural and electronic properties of titania.

Under Deposit Corrosion (UDC) presents a significant issue to carbon steel tubes in industrial boiler systems, however, the fundamental mechanisms are not fully understood. This work uses time-resolved high-speed tomography to investigate iron corrosion products in-situ, in aerated and deaerated environments at 200°C in 0.1M NiCl2 solution. Laminated corrosion structures were observed, in agreement with previous observations and field examples. The oxidation rate increased by 55% in aerated specimens, confirming the presence of oxygen causes an increase in the oxidation rate. This experimental set up provides an opportunity for investigations into complex corrosion mechanisms, for a range of aggressive environments.

Methane has emerged as a promising fuel due to its abundance and clean combustion properties. It is also a raw material for various value-added chemicals. However, the conversion of methane to other chemicals such as olefins, aromatics, and hydrocarbons is a difficult task. In recent years, ionic iron carbide clusters have been explored as potential catalysts for efficient direct methane conversion. Herein, we have investigated the gas-phase methane conversion process on various neutral iron carbide clusters with different Fe:C ratios using density functional theory. Reaction pathways were studied on mononuclear and trinuclear iron carbide clusters in the three lowest energy spin multiplicity channels. Three descriptors - methane binding energy, the effective energy barrier for C-H bond activation, and the effective energy required for methyl radical evolution - were chosen to identify the best catalyst among the clusters considered. Isomers of Fe3C6 (Fe3C6-iso) and Fe3C9 (Fe3C9-iso) are recognized as being the most promising catalysts among all the clusters considered here because they require the least methyl radical evolution energy, a step that is crucial in methane conversion to higher hydrocarbon but also requires the most energy.

Isolated Fe-sites on silica substrate have recently been reported for direct and non-oxidativeconversion of gaseous methane with high selectivity. The activated catalyst was proposed to beFeC2 cluster embedded in silica. Using a combination of density-functional theoretic methodsand micro-kinetic modeling, we show that under the same reaction conditions (1223 K , 1 atm)FeC2 sites convert to FeC3 and the latter is instead responsible for the observed activity. Weinvestigate the detailed mechanism of conversion of methane to methyl radical and hydrogenon FeC3@SiO2 under different conditions of methane partial pressure. We find that methylradical evolution is the rate-determining step for the overall conversion. Our calculations alsoindicate that the conversion of embedded FeC3 to FeC4 competes with methyl radical evolutionfrom the active catalyst. However, due to the higher stability of FeC3 sites, we anticipate thatformation of higher carbides can be inhibited by controlling the hydrogen partial pressure.

The conventional corrosion management process consists of defining the expected process conditions, identifying potential corrosion threats, and estimating their likely rate, then using that information to develop mitigation plans and inspection schedules. The Virtual Corrosion Engineer (VCE) project aims to improve this process by utilizing online monitoring data to automate the running of the best available corrosion models and provide a continuously updated dashboard in real time. This paper provides an overview of the VCE, together with a brief discussion of the underlying models for two exemplar damage mechanisms, High-Temperature Hydrogen Attack (HTHA) and Under Deposit Corrosion (UDC) in steam generators.

Photosynthesis is one of the most essential biological processes in which specialized pigment-protein complexes absorb solar photons, and with a remarkably high efficiency, guide the photo-induced excitation energy toward the reaction center to subsequently trigger its conversion to chemical energy. In this work, we review the principles of optimal energy transfer in various natural and artificial light harvesting systems. We begin by presenting the guiding principles for optimizing the energy transfer efficiency in systems connected to dissipative environments, with particular attention paid to the potential role of quantum coherence in light harvesting systems. We will comment briefly on photo-protective mechanisms in natural systems that ensure optimal functionality under varying ambient conditions. For completeness, we will also present an overview of the charge separation and electron transfer pathways in reaction centers. Finally, recent theoretical and experimental progress on excitation energy transfer, charge separation, and charge transport in artificial light harvesting systems is delineated, with organic solar cells taken as prime examples.

Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

We have systematically assessed the influence of oxygen vacancy defects on the structural, electronic and magnetic properties of La2/3Sr1/3MnO3 via first-principles calculations using the bare GGA as well as the GGA + U formalism. The on-site Coulombic repulsion parameter U for Mn 3d orbital in the latter has been determined by the linear response theory. It is revealed that the introduction of the vacancy defects causes prominent structural changes in the microenvironment of a defect including the distortions of MnO6 octahedra. In contrast to the general notion, the GGA + U formalism is found to yield significantly more prominent structural changes than the bare GGA method. The octahedral distortion leads to a strengthening or weakening of the hybridization between Mn 3d and O 2p orbitals depending upon an increase or decrease, respectively, in the Mn-O distances as compared to the pristine system. The magnetic moments of the Mn atoms located in three typical sites of the vacancy-containing supercell are all larger than those in the pristine system. This enhancement for the Mn atoms located in the first- and third-nearest neighboring MnO6 octahedra of the vacancy defect originates from the electron transfer from 4s/3p to 3d orbitals. On the other hand, for the Mn atom located in the first-nearest neighboring site of the vacancy it is attributed to the increased total number of electrons in 3d orbitals due to the absence of one Mn-O bond. Furthermore, we have characterized the O-vacancy defect as a hole-type defect that forms a negative charge center, attracting electrons. (C) 2015 Elsevier B.V. All rights reserved.

We carry out a computational study of optical properties of two novel 5,6-difluorobenzo[c][1,2,5]-thiadiazole-based polymers, PFBT-T20TT and PFBT-T12TT, to elucidate the surprisingly superior performance of polymer solar cells based on the former, when it differs from the latter only in the alkyl side chains. Density Functional Theory (DFT) based geometry optimization at the B3LYP/6-31G(d) level reveals differences in internal coordinates, which are important in tuning the electronic and optical properties. We further calculate the electronic structure at room temperature by employing molecular dynamics (MD) simulations in combination with DFT techniques. The energies of the highest occupied molecular orbital (HOMO) are found to be in reasonable agreement with the available experimental data and the HOMO-lowest unoccupied MO energy gap is found to be similar for both the molecules. The electronic density of the HOMO in PFBT-T20TT is, however, found to be significantly more delocalized along the backbone, which is proposed to be conducive for the formation of charge separated states leading to an improved device performance. Furthermore, via fitting the absorption spectra calculated with the multi-mode Brownian Oscillator model, we have also extracted a weaker exciton-phonon coupling parameter in PFBT-T20TT, consistent with the trends revealed via the DFT-MD results.

We have computationally investigated the role of intramolecular vibrational modes in determining nonradiative relaxation pathways of photoexcited electronic states in isolated chlorophyll A (ChlA) molecules. To simulate the excited state relaxation from the initially excited Soret state to the lowest excited state Qy, the approach of nonadiabatic excited state molecular dynamics has been adopted. The intramolecular vibrational energy relaxation and redistribution that accompany the electronic internal conversion process is followed by analyzing the excited state trajectories in terms of the ground state equilibrium normal modes. The time dependence of the normal mode velocities is determined by projecting instantaneous Cartesian velocities onto the normal mode vectors. Our analysis of the time evolution of the average mode energies uncovers that only a small subset of the medium-to-high frequency normal modes actively participate in the electronic relaxation processes. These active modes are characterized by the highest overlap with the nonadiabatic coupling vectors (NACRs) during the electronic transitions. Further statistical analysis of the nonadiabatic transitions reveals that the electronic and vibrational energy relaxation occurs via two distinct pathways with significantly different time scales on which the hopping from Soret to Qx occurs thereby dictating the overall dynamics. Furthermore, the NACRs corresponding to each of the transitions have been consistently found to be predominantly similar to a set of normal modes that vary depending upon the transition and the identified categories. Each pathway exhibits a differential time scale of energy transfer and also a differential set of predominant active modes. Our present analysis can be considered as a general approach allowing identification of a reduced subset of specific vibrational coordinates associated with nonradiative relaxation pathways. Therefore, it represents an adequate prior strategy that can particularly facilitates mixed quantum-classical approaches.

We have developed an adiabatic connection to formulate the groundstate exchange-correlation energy in terms of pairing matrix linear fluctuations. This formulation of the exchange-correlation energy opens new a channel for density functional approximations based on the many-body perturbation theory. We illustrate the potential of such approaches with an approximation based on the particle-particle Random Phase Approximation (pp-RPA). This resulting method has many highly desirable properties. It has minimal delocalization error with a nearly linear energy behavior for systems with fractional charges, describes van der Waals interactions similarly and thermodynamic properties significantly better than the conventional RPA, and eliminates static correlation error for single bond systems. Most significantly, it is the first known functional with an explicit and closed-form dependence on the occupied and unoccupied orbitals, which captures the energy derivative discontinuity in strongly correlated systems. We also adopted pp-RPA and the particle-particle Tamm-Dancoff approximation (pp-TDA) to approximate the pairing matrix fluctuation and then determine excitation energies by the differences of two-electron addition/removal energies. This approach captures all types of interesting excitations: single and double excitations are described accurately, Rydberg excitations are in good agreement with experimental data and CT excitations display correct 1/R dependence. Furthermore, the pp-RPA and the pp-TDA have a computational cost similar to TDDFT and consequently are promising for practical calculations. To further explore the potential use of pairing matrix dependent functionals, we developed the linear-response time-dependent densityfunctional theory with pairing fields with both adiabatic and frequency-dependent kernels. The linear-response theory is established based on the representability assumption of the pairing matrix. The linear response theory justifies the use of approximated density functionals in the pp-RPA equation. This work sets the fundamentals for future density-functional development to enhance the description of ground state correlation energies and N ± 2 excitation energies.

Electronic structures of doped NaTaO3 compounds are of significant interest to visible light photocatalysis. This work involves the study of the band gap, band edge potentials, and thermodynamic stability of certain mono-doped and co-doped NaTaO3 systems, using DFT-PBE as well as hybrid (PBE0) functional calculations. Doping of certain non-magnetic cations (Ti, V, Cu, Zn, W, In, Sn, Sb, Ce, and La), certain anions (N, C, and I), and certain co-dopant pairs (W-Ti, W-Ce, N-I, N-W, La-C, Pb-I, and Cu-Sn) is investigated. Our calculations suggest that substitutional doping of Cu at the Ta site, Cu at the Na site, and C at the O site narrows the band gap of NaTaO3 to 2.3, 2.8, and 2.1 eV, respectively, inducing visible light absorption. Additionally, passivated co-doping of Pb-I and N-W narrows the band gap of NaTaO3 to the visible region, while maintaining the band potentials at favorable positions. Hybrid density of states (DOS) accurately describe the effective band potentials and the location of mid-gap states, which shed light on the possible mechanism of photoexcitation in relation to the photocatalysis reactions. Furthermore, the thermodynamic stability of the doped systems and defect pair binding energies of co-doped systems are discussed in detail. The present results provide useful insights into designing new photocatalysts based on NaTaO3.

We have carried out a theoretical and numerical study of disorder-induced changes in the absorption line shape of a chromophore embedded in a host matrix. The stochastic sudden jump model is employed wherein the host matrix molecules are treated as noninteracting two-level systems (TLSs) occupying points on a three-dimensional lattice with randomly oriented dipole moments. By systematically controlling the degree of positional disorder (α) attributed to them, a perfectly crystalline (α = 0) or a glassy environment (α = 1) or a combination of the two is obtained. The interaction between the chromophore and the TLSs is assumed to be of the dipole-dipole form. With an increase in α, the broadening of the absorption line shape was found to follow a power-law behavior. More importantly, it is revealed in the long-time limit that the resultant line shape is Gaussian in the absence of disorder but transforms to Lorentzian for a completely disordered environment. For an arbitrarily intermediate value of α, the resultant line shape can be approximately fitted by a linear combination of Gaussian and Lorentzian components. The Lorentzian profile for the disordered medium is attributed to the chomophore-TLS pairs with vanishingly small separation between them.

We have carried out analytical and numerical studies of the spin-boson model in the sub-ohmic regime with the influence of both the diagonal and the off-diagonal coupling accounted for, via the Davydov D1 variational ansatz. While a second-order phase transition is known to be exhibited by this model in the presence of diagonal coupling only, we demonstrate the emergence of a discontinuous first order phase transition upon incorporation of the off-diagonal coupling. A plot of the ground state energy versus magnetization highlights the discontinuous nature of the transition between the isotropic (zero magnetization) state and nematic (finite magnetization) phases. We have also calculated the entanglement entropy and a discontinuity found at a critical coupling strength further supports the discontinuous crossover in the spin-boson model in the presence of off-diagonal coupling. It is further revealed via a canonical transformation approach that for the special case of identical exponents for the spectral densities of the diagonal and the off-diagonal coupling, there exists a continuous crossover from a single localized phase to doubly degenerate localized phase with differing magnetizations.

Using a stochastic model, we examine disorder-induced changes in the absorption line shape of a chromophore embedded in a matrix of noninteracting two-level systems (TLSs) with randomly oriented dipole moments. By systematically controlling the degree of TLS positional disorder, a perfectly crystalline, glassy or a combination of the two environments is obtained. The chromophore is assumed to interact with TLSs via long-range dipole–dipole coupling. At long times and in the absence of disorder, Gaussian line shapes are found, which morph into Lorentzian for a completely disordered environment owing to strong coupling between the chromophore and a TLS in close vicinity.

The mathematics and algorithms of PCAThere are many areas of scientific explorations that lead to enormous quantities of data.Post-processing of such a huge data to extract only the most valuable information is often a

That a commensurate contact usually leads to greater friction than an incommensurate one is a commonly held view in nanotribology. However, this perception seems paradoxical as commensurability is found to have negligible effect on the energy dissipation in double-walled carbon nanotube (DWNT) based oscillators. Using molecular dynamics simulations, we investigate such a paradox from the viewpoint of the atomic origin of friction. It is revealed that the commensurability cannot have a pronounced effect on friction unless the intertube interaction strength and the energy corrugation exceed their critical values. Both the commensurate and incommensurate oscillators constructed from natural DWNTs with an intertube distance of about 3.4 Å, may thus exhibit similar performance.

We study the onset of electrical percolation in multiwalled carbon nanotubes (MWNTs)/epoxy nanocomposites. Experiments show a threshold value of 3.2 wt % of MWNTs for percolation to occur. Simulations based on two varied approaches are carried out to evaluate the conductivity characteristics resulting from increasing MWNT content Simple Monte Carlo simulations in which MWNTs are modeled as either 1D sticks or 2D narrow, rectangles dispersed in a 2D simulation volume are shown to yield a percolation threshold in dose agreement with experiments We find that a higher degree of anisotropy in the orientation. of nanotubes or of the waviness leads to an increase, in the percolation threshold. A more insightful approach encompassing the quantum tunneling effect is also undertaken using the tight-binding simulations. Consideration of the tunneling effect is found to be particularly important when the nanotube aspect ratio is small, the case in which simpler Monte Carlo simulations overestimate the Percolation threshold.

We investigate the phenomenon of actuation of relative linear motion in double-walled carbon nanotubes (DWNTs) resulting from a temperature gradient. Molecular dynamics simulations of DWNTs with short outer tube reveal that the outer tube is driven towards the cold end of the long inner tube. It is also found that the terminal velocity of the sleeve roughly depends linearly on the applied thermal gradient. We calculate the inter-tube interaction energy surface which is revealed to have a gradient depending upon the applied thermal gradient. Consequently, it is proposed that the origin of the thermophoretic motion of the outer tube may be attributed partially to the existence of such an energy gradient. A simple analytical model is presented accounting for the gradient in energy profile as well as the effect of biased thermal noise. It is shown that the proposed model predicts the dynamical behaviour of the long-time performance reasonably well.