Transferable basis sets of numerical atomic orbitals

We show that numerical atomic orbital basis sets that are variationally optimized for specific compounds are biased and not fully transferable to other compounds. The situation improves when the basis sets are optimized for several compounds and a compromise is made. We demonstrate this for basis sets for compounds containing H, O, N, Al, Cl, and Cu atoms, using the SIESTA code. Reaction energies are compared to benchmark calculations with the ADF code. Our double-zeta basis sets give a root mean square error of 0.2 eV. With triple-zeta basis sets, we obtain a root mean square error of only 0.1 eV. Finally, for the case of mixed phase systems, we propose that optimizing basis sets for gaseous molecules and then using them also for bulk compounds, is better than the other way around

Diffusion Mechanisms for Ions over Hydroxylated Surfaces: Cu on Î-Al2O3

The energies involved in the diffusion of Cu2+ and Cu+ over hydroxylated Î-alumina were modeled with density functional theory using explorative molecular dynamics. This is the first time that the mechanism for diffusion of ions over hydroxylated surfaces is studied. It is found that the crucial requirement for feasible activation energies for ion diffusion is the prevention of charge separation. This can be realized either by counterion codiffusion or proton contradiffusion. Furthermore, the effects of the cation valency, hydroxylation level, and nature of the counterions were studied and general trends for diffusing cations on hydroxylated surfaces were postulated. At full hydroxylation, all charge compensation is performed by proton contradiffusion, while at intermediate hydroxylation levels, a combination of proton contradiffusion and counterion codiffusion occurs. Finally, energy barriers for codiffusion are related to the bonding strength of the counterions to the surface, which depends on the counterion and the hydroxylation level

The merits of the frozen-density embedding scheme to model solvatochromic shifts

We investigate the usefulness of a frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] for the calculation of solvatochromic shifts. The frozen-density calculations, particularly of excitation energies have two clear advantages over the standard supermolecule calculations: (i) calculations for much larger systems are feasible, since the time-consuming time-dependent density functional theory (TDDFT) part is carried out in a limited molecular orbital space, while the effect of the surroundings is still included at a quantum mechanical level. This allows a large number of solvent molecules to be included and thus affords both specific and nonspecific solvent effects to be modeled. (ii) Only excitations of the system of interest, i.e., the selected embedded system, are calculated. This allows an easy analysis and interpretation of the results. In TDDFT calculations, it avoids unphysical results introduced by spurious mixings with the artificially too low charge-transfer excitations which are an artifact of the adiabatic local-density approximation or generalized gradient approximation exchange-correlation kernels currently used. The performance of the frozen-density embedding method is tested for the well-studied solvatochromic properties of the n-->pi* excitation of acetone. Further enhancement of the efficiency is studied by constructing approximate solvent densities, e.g., from a superposition of densities of individual solvent molecules. This is demonstrated for systems with up to 802 atoms. To obtain a realistic modeling of the absorption spectra of solvated molecules, including the effect of the solvent motions, we combine the embedding scheme with classical molecular dynamics (MD) and Car-Parrinello MD simulations to obtain snapshots of the solute and its solvent environment, for which then excitation energies are calculated. The frozen-density embedding yields estimated solvent shifts in the range of 0.20–0.26 eV, in good agreement with experimental values of between 0.19 and 0.2

Diffusion Mechanisms for Ions over Hydroxylated Surfaces: Cu on Î-Al2O3

The energies involved in the diffusion of Cu2+ and Cu+ over hydroxylated Î-alumina were modeled with density functional theory using explorative molecular dynamics. This is the first time that the mechanism for diffusion of ions over hydroxylated surfaces is studied. It is found that the crucial requirement for feasible activation energies for ion diffusion is the prevention of charge separation. This can be realized either by counterion codiffusion or proton contradiffusion. Furthermore, the effects of the cation valency, hydroxylation level, and nature of the counterions were studied and general trends for diffusing cations on hydroxylated surfaces were postulated. At full hydroxylation, all charge compensation is performed by proton contradiffusion, while at intermediate hydroxylation levels, a combination of proton contradiffusion and counterion codiffusion occurs. Finally, energy barriers for codiffusion are related to the bonding strength of the counterions to the surface, which depends on the counterion and the hydroxylation level

Size and Promoter Effects in Supported Iron Fischer-Tropsch Catalysts: Insights from Experiment and Theory

The fundamentals of structure sensitivity and promoter effects in the Fischer–Tropsch synthesis of lower olefins have been studied. Steady state isotopic transient kinetic analysis, switching 12CO to 13CO and H2 to D2, was used to provide coverages and residence times for reactive species on supported iron carbide particles of 2–7 nm with and without promoters (Na + S). CO coverages appeared to be too low to be measured, suggesting dissociative adsorption of CO. Fitting of CH4 response curves revealed the presence of parallel side-pools of reacting carbon. CHx coverages decreased with increasing particle size, and this is rationalized by smaller particles having a higher number of highly active low coordination sites. It was also established that the turnover frequency increased with CHx coverage. To calculate H coverages, new equations were derived to fit HD response curves, again leading to a parallel side-pool model. The H coverages appeared to be lower for bigger particles. The H coverage was suppressed upon addition of promoters in line with lower methane selectivity and higher lower olefin selectivity. Density functional theory (DFT) was applied on H adsorption for a fundamental understanding of this promoter effect on the selectivities, with a special focus on counterion effects. Na2S is a better promoter than Na2O due to both a larger negative charge donation and a more effective binding configuration. On the unpromoted Fe5C2 (111) surface, H atoms bind preferably on C after dissociation on Fe. On Na2S-promoted Fe5C2 surfaces, adsorption on carbon sites weakens, and adsorption on iron sites strengthens, which fits with lower H coverage, less CH4 formation, and more olefin formation

Modeling solvent effects on electron spin resonance hyperfine couplings by frozen-density embedding

In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc's) of the H2NO molecule. The hfcc's for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle

Promoted cobalt metal catalysts suitable for the production of lower olefins from natural gas

Due to the surge of natural gas production, feedstocks for chemicals shift towards lighter hydrocarbons, particularly methane. The success of a Gas-to-Chemicals process via synthesis gas (CO and H 2 ) depends on the ability of catalysts to suppress methane and carbon dioxide formation. We designed a Co/Mn/Na/S catalyst, which gives rise to negligible Water-Gas-Shift activity and a hydrocarbon product spectrum deviating from the Anderson–Schulz–Flory distribution. At 240 °C and 1 bar, it shows a C 2 -C 4 olefins selectivity of 54%. At 10 bar, it displays 30% and 59% selectivities towards lower olefins and fuels, respectively. The spent catalyst consists of 10 nm Co nanoparticles with hcp Co metal phase. We propose a synergistic effect of Na plus S, which act as electronic promoters on the Co surface, thus improving selectivities towards lower olefins and fuels while largely reducing methane and carbon dioxide formation

Promoted cobalt metal catalysts suitable for the production of lower olefins from natural gas

Due to the surge of natural gas production, feedstocks for chemicals shift towards lighter hydrocarbons, particularly methane. The success of a Gas-to-Chemicals process via synthesis gas (CO and H 2 ) depends on the ability of catalysts to suppress methane and carbon dioxide formation. We designed a Co/Mn/Na/S catalyst, which gives rise to negligible Water-Gas-Shift activity and a hydrocarbon product spectrum deviating from the Anderson–Schulz–Flory distribution. At 240 °C and 1 bar, it shows a C 2 -C 4 olefins selectivity of 54%. At 10 bar, it displays 30% and 59% selectivities towards lower olefins and fuels, respectively. The spent catalyst consists of 10 nm Co nanoparticles with hcp Co metal phase. We propose a synergistic effect of Na plus S, which act as electronic promoters on the Co surface, thus improving selectivities towards lower olefins and fuels while largely reducing methane and carbon dioxide formation