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First-principles calculations have been performed to investigate CH4 dissociation and C diffusion during the Ni/Fe-catalyzed growth of carbon nanofibers (CNFs). Two bulk models with different Ni to Fe molar ratios (1:1 and 2:1) are constructed, and x-ray diffraction (XRD) simulations are conducted to evaluate their reliability. With the comparison between the calculated and experimental XRD patterns, these models are found to be well suited to reproduce the crystalline structures of Ni/Fe bulk alloys. The calculations indicate the binding of the C1 derivatives to the Ni/Fe closest-packed surfaces is strengthened compared to that on Ni(111), arising from the upshift of the weighted d-band centers of catalyst surfaces. Then, the transition states for the four successive dehydrogenation steps in CH4 dissociation are located using the dimer method. It is found that the energy barriers for the first three steps are rather close on the alloyed Ni/Fe and Ni surfaces, while the activation energy for CH dissociation is substantially lowered with the introduction of Fe. The dissolution of the generated C from the surface into the bulk of the Ni/Fe alloys is thermodynamically favorable, and the diffusion of C through catalyst particles is hindered by the Fe component. With the combination of density functional theory calculations and kinetic analysis, the C concentration in catalyst particles is predicted to increase with the Fe content. Meanwhile, other experimental conditions, such as the composition of carbon-containing gases, feedstock partial pressure, and reaction temperature, are also found to play a key role in determining the C concentration in bulk metal, and hence the microstructures of generated CNFs. [ABSTRACT FROM AUTHOR]

Intrinsic defect of electronic structure for the chlorine-type porphyrin 1, which was synthesized for use in dye-sensitized solar cell (DSSC), is found by theoretical calculation including density functional method (DFT), time-dependent DFT, and C+/C− function. It is believed that the limited cell performance obtained by using dye 1 as the sensitizer is due to the existence of this electronic defect. To avoid this defect, a series of novel molecules with electron deficient π bridge were designed. The subsequent theoretical calculation indicated that the electron deficient π bridge in the newly designed molecule is quite effective in offsetting the electronic defect observed for dye 1. The parameters for the designed molecules closely associated with open-circuit voltage and short-circuit current density including dipole moment of dye vertical to the surface of semiconductor and light-harvesting efficiency were then evaluated. By comparing these parameters of designed dyes with those of dye 1, we can predict that the DSSC based on dye 4 (2, 6-cyan benzoic acid as anchoring group) should possess enhanced performance, which would be a valuable theoretical guidance for the practical work. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: Catalytic hydrogenation of benzene to cyclohexene is of great importance in the petroleum industry. The introduction of Zn into Ru catalyst can significantly improve the selectivity toward cyclohexene. To elucidate the effect of Zn on the enhanced selectivity, the adsorption of all the possible reaction intermediates involved is investigated by performing density functional theory calculations, and the transition states for the hydrogenation steps are identified by using the constrained Broyden minimization method. With the introduction of Zn into Ru(0001) at high H surface coverage, the adsorption energies of the intermediates are much lower than those on bare Ru(0001), which indicates that the desorption of cyclohexene is promoted on the alloyed surface. The calculated energy barriers for the hydrogenation steps from benzene to cyclohexene are in the region of 0.58–1.04eV, and the hydrogenation of benzene on Zn/Ru(0001) follows the Horiuti-Polanyi mechanism with the successive hydrogenation of the neighboring C atoms in the hexagonal carbon ring. The deep hydrogenation of cyclohexene to produce cyclohexane is kinetically hindered with the introduction of Zn, which also gives rise to the improvement of the selectivity toward cyclohexene. [Copyright &y& Elsevier]

Abstract: First-principles calculations based on density functional theory have been performed to investigate the adsorption of CO and NO on Pd(111) surface. The average and differential adsorption energies of CO and NO at the high-symmetry adsorption sites on Pd(111) at different coverages, namely 0.11, 0.25, 0.50, and 0.75ML are calculated. The results show that at low coverages of 0.11 and 0.25ML, both CO and NO are preferentially adsorbed at the threefold hollow sites. At the medium coverage of 0.50ML, CO molecules occupy both hollow and bridge sites with almost degenerate adsorption energies, while NO molecules prefer the hollow sites to the bridge sites, which can be attributed to the more corrugated potential energy surface of NO on Pd(111) surface. At high coverage of 0.75ML, both CO and NO form the p(3×3)- 3CO structures with the adsorbates at the hollow and atop sites. The calculated stretching vibrational frequencies are in good agreement with the experimental values. The electronic structure analysis shows that the adsorption of CO and NO on Pd(111) surface follows a donation and back-donation mechanism, and the high-coordinated sites promote the donation and back-donation process. The energy decomposition scheme indicates that the site preference at higher coverage is a compromise between the adsorbate–substrate interaction and adsorbate–adsorbate interaction. The relationship between the adsorption energies and the surface coverage has been well established, and linear relations are identified for both CO and NO. We suggest that the coverage-dependent adsorption energies of CO and NO on Pd(111) are useful parameters for the kinetic studies on the relevant catalytic reaction systems. [Copyright &y& Elsevier]

Abstract: First-principles calculations have been performed to examine the effect of doped Ag on the kinetics of Ni-catalyzed methane dissociation and coke formation. The close-packed Ag/Ni(111) and stepped Ag/Ni(211) surfaces as well as the defect facets with step sites blocked by Ag or C atoms are constructed to investigate the role of the coordinatively unsaturated sites in the catalytic performance of Ni nanoparticles. The most stable CH x (x =0–4) adsorption configurations and transition states for methane dissociation have been identified on both Ni and Ag/Ni surfaces. The calculated results indicate that the activation energy for methane dissociation is increased with the Ag coverage on Ni(111), and the C atoms deposited on the catalyst surface can be readily separated into small islands by Ag. On Ni(211) Ag atoms are predicted to bind preferentially to the middle-step sites which act as the nucleation center for the growth of filamentous carbon and therefore have the potential to prevent catalyst particles from being destroyed. Meanwhile, as the energy barrier for methane dissociation on the Ag-blocked Ni(211) surface is even higher than that on pure Ni(111), the active center is transferred from the stepped surface to the close-packed surface. These findings provide a rational interpretation of the experimental observations that Ag/Ni catalyst exhibits lower catalytic activity towards steam methane reforming but high resistance to coke deposition. [Copyright &y& Elsevier]

Abstract: First-principles calculations based on density functional theory have been performed to elucidate the reaction mechanism for ethylbenzene dehydrogenation and the role of CO2 in H removal. On the basis of the experimental information and theoretical prediction, three model surfaces with Fe-, ferryl- and O-termination are constructed to represent the active Fe2O3(0001) surface. The calculated results indicate that on all of the three surfaces the C–H activation in the methylene group followed by the dehydrogenation of the methyl group is kinetically more favorable. The energy barriers for ethylbenzene dehydrogenation are lowest on the O-terminated surface, but the generated styrene is adsorbed too strongly to be released. As CO2 decomposition and the formation of HCOO are hindered by the relatively high activation energies, CO2 cannot serve as the oxidant to recover the O- and ferryl-terminated surfaces to keep the redox cycle. At the steady state of the reaction the coupling mechanism dominates on the Fe-terminated surface, with the synergistic effect between ethylbenzene dehydrogenation and the reverse water–gas shift reaction. Since the energy barrier for the formation of COOH is comparable to that for H2 formation, both the one-step and two-step pathways are predicted to contribute to the coupling mechanism, although the former is more probable. [Copyright &y& Elsevier]

Abstract: The catalytic hydrogenation of benzene on transition metal surfaces is of fundamental importance in petroleum industry. With the aim to improve its efficiency and particularly the selectivity to cyclohexene, in this contribution we perform periodic density functional theory calculations to determine the potential energy surface in the hydrogenation of benzene on Ru(0001). By following the Horiuti–Polanyi mechanism with a step-wise addition of hydrogen adatoms, we investigate the adsorption of all the possible reaction intermediates and identify the most favored adsorption configuration for each intermediate. In particular, the most stable isomer for the same C6H n (n =8, 9, 10) species are revealed as the most conjugated isomers, which are consistent with those in the gas phase. The elementary hydrogenation reactions of the most stable intermediates are then investigated under different H coverage conditions: the reaction barriers are calculated to be 0.68–0.97eV at the low H coverage and 0.32–1.14eV at the high H coverage. The high H coverage reduces significantly the overall barrier height of hydrogenation. With the determined pathway, we propose that the hydrogenation of benzene on Ru(0001) follows the mechanism with the step-wise hydrogenation of neighboring C atoms in the ring, i.e., 1–2–3… hydrogenation. The selectivity to cyclohexene on Ru is also discussed, which highlights the importance of the π mode adsorption of benzene and also the adverse effect of secondary reaction process involving the readsorption and hydrogenation of cyclohexene. [ABSTRACT FROM AUTHOR]

Density functional theory calculations have been conducted to explore the physical origin of the synergistic effect over Ni-based surface alloys using methane dissociation as a probe reaction. Some late transition metal atoms (M = Cu, Ru, Rh, Pd, Ag, Pt, and Au) are substituted for surface Ni atoms to examine the variation in electronic structure and adsorption property of Ni(111). Two types of threefold hollow sites, namely, the Ni2M and Ni3 sites, are taken into account. The calculated results indicate that the variation in the CHx adsorption energy at the Ni2M and Ni3 sites is dominated by the ensemble and ligand effect, respectively, and the other factors such as surface and adsorbate distortion and electrostatic interaction affect the catalytic properties of the bimetallic surfaces to a smaller extent. Both the Bro\nsted-Evans-Polanyi relationship and the scaling correlation hold true on the Ni-based bimetallic surfaces. With the combination of these two linear energy relations, the corrected binding energy of atomic C is found to be a good descriptor for representing the catalytic activity of the alloyed surfaces. Considering the compromise between the catalytic activity and catalyst stability, we suggest that the Rh/Ni catalyst is a good candidate for methane dissociation. [ABSTRACT FROM AUTHOR]

Abstract: First-principles calculations based on density functional theory (DFT) have been performed to study the adsorption and decomposition of NH3 on Ni(110). The adsorption sites, the adsorption energies, the transition states and the activation energies of the stepwise dehydrogenation of NH3 and the associative desorption of N are determined, and the zero point energy correction is included, which makes it possible to compute the rate constants of the elementary steps in NH3 decomposition. Combined DFT calculations and kinetic analysis at 350K indicate that the associative desorption of N has a reaction rate lower than NHx dehydrogenation and is therefore the rate determining step. The distinctly different rate constants over Ni(110), Ni(111) and Ni(211) imply that ammonia decomposition over Ni-based catalyst is a structure-sensitive reaction. [Copyright &y& Elsevier]

Abstract: DFT–GGA calculations have been used to study the chemisorption of CH x (x =0–3), C2H y (y =3–5) and C3H z (z =3–8) on Pt(111) at the coverage of 1/9 ML. The adsorption energies, favored binding sites and geometric parameters of adsorption configurations are determined. All the carbonaceous species are found to be adsorbed on Pt(111) with C atoms sp3-hybridized. The geometries of adsorption configurations have been compared with published experimental data to evaluate the reliability of our calculation method. The calculated adsorption energies of C1 species, ranked in descending order, are as follows, C>CH>CH2 >CH3, and the similar trend has been observed with respect to the chemisorption of C2 species. As for the C3 species, propylene prefers binding to the surface in the di-σ mode and the adsorption energy is calculated to be −0.93eV. The stepwise dehydrogenation of propane to propynyl (CCCH3) is investigated by calculating the change of reaction heat, which suggests that propylidyne (CCH2CH3) is the most stable species on Pt(111). Based on the adsorption energies and Brønsted–Evans–Polanyi (BEP) analysis, propyne is predicted to be the most likely starting point for the C–C scission. [Copyright &y& Elsevier]

Abstract: Ammonia decomposition on transition metal surfaces is of great importance in energy and environmental industries. With the aim to obtain a systematic knowledge about the reaction mechanism of NH3 decomposition on the close-packed surfaces of the 3d-late transition metals, we perform first-principles calculations to determine the preferred adsorption sites and the adsorption energies of NH x (x =0–3) and H, and identify the transition states of the NH3 stepwise dehydrogenation reactions and the N recombination reactions on the close-packed Fe(110), Co(111) and Ni(111) surfaces. The results show that the calculated adsorption energies and activated energies mainly depend on the d-band center of metal surfaces. Barrier decomposition analysis reveals that the interaction among binding species in transition states will increase the energy barrier while the bonding to the surface of the species in the initial states and the transition states will decrease the energy barrier. Moreover, a linear relationship, known as the Brϕnsted–Evans–Polanyi relation, also exists between the activation energy of the N recombination reaction and the adsorption energy of N on the close-packed surfaces of the 3d-late transition metals. [Copyright &y& Elsevier]

Abstract: The ground-state structure and electronic and vibrational spectra of octaethylporphyrin diacid (H4OEP2+) have been studied with the density functional theory. The geometrical parameters computed with B3LYP, PBE1PBE and mPW1PW91 functionals and 6-31G* basis sets are well consistent with the experimental values. Electronic absorption spectrum of H4OEP2+ has been studied with the time-dependent DFT method, and the calculated excitation energies and oscillator strengths are compared with the experimental results. The Raman and IR spectra of H4OEP2+ and the Raman spectrum of its N-deuterated analogue (D4OEP2+) were measured. The observed Raman and IR bands have been assigned based on the frequency calculations at the B3LYP/6-31G* level of theory. [Copyright &y& Elsevier]

The normal and UV near-resonance Raman (UVRR) spectra of 1,1‘-bi-2-naphthol (BN) in basic solution were measured and analyzed. Density functional theory (DFT) calculations were carried out to study the ground state geometry structure, vibrational frequencies , off-resonance Raman intensities I, and depolarization ratios of 1,1‘-bi-2-naphtholate dianion (BN2-). On the basis of the calculated and experimental results of , I, and , the observed Raman bands were assigned in detail. The 1612 cm-1Raman band of BN in basic solution was found dramatically enhanced in the UV resonance Raman spectrum in comparison with the normal Raman spectrum. Analyzing the depolarization ratios of the 1366 and 1612 cm-1bands in the RR spectra manifests that both the symmetric and antisymmetric parts of transition polarizabilities contribute to the 1366 cm-1band, but that only the symmetric part contributes to the 1612 cm-1band. [ABSTRACT FROM AUTHOR]

Abstract: The ground-state structure of the charge-transfer complex formed by pyridine (Py) as electron donor and chloranil (CA) as acceptor has been studied by full geometry optimization at the MP2 and DFT levels of theory. Binding energies were calculated and counterpoise corrections were used to correct the BSSE. Both MP2 and DFT indicate that the pyridine binds with chloranil to form an inclined T-shape structure, with the pyridine plane perpendicular to the chloranil. The CP and ZPE corrected binding energies were calculated to be 14.21kJ/mol by PBEPBE/6-31G(d) and 23.21kJ/mol by MP2/6-31G(d). The charge distribution of the ground state Py–CA complex was evaluated with the natural population analysis, showing a net charge transfer from Py to CA. Analysis of the frontier molecular orbitals reveals a σ–π interaction between CA and Py, and the binding is reinforced by the attraction of the O7 atom of CA with the H23 atom of Py. TD-DFT calculations have been performed to analyze the UV–visible spectrum of Py–CA complex, revealing both the charge transfer transitions and the weak symmetry-relieved chloranil π–π∗ transition in the UV–visible region. [Copyright &y& Elsevier]

The potential energy surface for the CH2O ClO reaction was calculated at the QCISD(T)/6-311G(2d,2p)//B3LYP/6-311G(d,p) level of theory. The rate constants for the lower barrier reaction channels producing HOCl HCO, H atom, OCH2OCl, cis-HC(O)OCl and trans-HC(O)OCl have been calculated by TST and multichannel RRKM theory. Over the temperature range of 200−2000 K, the overall rate constants were k(200−2000K) 1.19 × 10-13T0.79exp(−3000.00/T). At 250 K, the calculated overall rate constant was 5.80 × 10-17cm3molecule-1s-1, which was in good agreement with the experimental upper limit data. The calculated results demonstrated that the formation of HOCl HCO was the dominant reaction channel and was exothermic by 9.7 kcal/mol with a barrier of 5.0 kcal/mol. When it retrograded to the reactants CH2O ClO, an energy barrier of 14.7 kcal/mol is required. Furthermore, when HOCl decomposed into H ClO, the energy required was 93.3 kcal/mol. These results suggest that the decomposition in both the forward and backward directions for HOCl would be difficult in the ground electronic state. [ABSTRACT FROM AUTHOR]

Abstract: The vibrational spectra of meso-tetraphenylporphyrin diacid (H4TPP2+) have been studied with the density functional theory. Raman and IR spectra of H4TPP2+ and its N-deuterated analogue (D4TPP2+) are measured and compared with the computational results. Complete assignments of observed IR and Raman bands were proposed on the bases of calculation results. The DFT calculations reproduce 140 observed fundamentals with the RMS 8.6cm−1. The computational as well as the experimental results reveal that the saddle-distortion of porphyrin macrocycle for the diacid leads to a significant effect on its vibrational spectra. Especially, several out-of-plane skeletal modes, which were either unobserved or very weak in the Raman spectra of CuTPP and H2TPP, are activated in the Raman spectra of the diacids. In addition, enhancement for the Raman bands of phenyl CC stretching modes were observed and attributed to the conjugation effect of π-systems of the phenyl and the porphyrinato macrocycles. [Copyright &y& Elsevier]

Abstract: The ground-state geometries of monolithium porphyrin (LiP) π-radical and its three reduced/oxidized species ([LiP]−, [LiP]2−, and [LiP]+) have been calculated with the density functional theory. The results show that LiP, [LiP]− and [LiP]+ have planar D4h structures while the [LiP]2− dianion has a rectangular distorted D2h structure due to the ground-state Jahn-Teller effect. LiTPP and LiOEP radicals were also studied to assess the effect of peripheral substitutions. The ground-state LiTPP has a normal porphyrin ring (D4h effective symmetry), while the ground-state LiOEP has a porphyrin ring with pronounced bond length alternation (C4h effective symmetry) due to the pseudo-Jahn-Teller (pJT) distortion along an A2g-like coordinate. The spin densities of LiP, LiTPP, and LiOEP were analyzed, showing evident effects of peripheral substitution. Electronic absorption spectrum of LiP radical was calculated with the time-dependent DFT and was compared with the experimental results of LiTPP. [Copyright &y& Elsevier]