PtFe alloy nanostructures enclosed by differently oriented facets, including polyhedrons, concave cubes, and nanocubes, were synthesized through the fine adjustment of specific surfactant−crystal facet bindings. PtFe nanostructures with various alloy compositions were then employed as the counter electrodes (CEs) for the redox reaction of iodide/ tri-iodide (I − /I 3 − ) in dye-sensitized solar cells. Devices with the Pt 9 Fe 1 polyhedrons and Pt 9 Fe 1 concave cubes produced better photovoltaic conversion efficiency (PCE) of 8.01% and 7.63% in comparison to the PCE of 7.24% achieved with Pt CE. The superiority is attributed to the rapid charge transfer, higher limit current, and better electronic conductivity and catalytic activity with respect to the Pt CEs. The photovoltaic and electrochemical results indicated the shape-and composition-dependent activity in the I − /I 3 − redox reaction, which obeys the sequence of polyhedrons > concave cubes > nanocubes and Pt 9 Fe 1 nanostructures > Pt 7 Fe 3 nanostructures. Further theoretical work indicated that the I 3 − reduction activity of the nanosurfaces was in the order of Pt 9 Fe 1 (111) > Pt(111) > Pt 9 Fe 1 (100). The combination of experimental and theoretical work thus clearly demonstrates the shape-and composition-dependence of PtFe nanostructures in terms of the I 3 − reduction activity.

No abstract

Our calculations with spin-polarized density functional theory were carried out to characterize the adsorption and dissociation of the NH3 molecule on the Fe(111) surface. The molecular structures and adsorbate/substrate interaction energies of NH3/Fe(111), NH2/Fe(111), NH/Fe(111), N/Fe(111), and H/Fe(111) configurations were predicted. In these calculations, four adsorption sites, such as top (T), bridge (B), 3-fold-shallow (S), and 3-fold-deep (D) sites, of the Fe(111) surface, were considered. It was shown that the barriers for the stepwise NH3 dissociation reaction, NH3(g) -> N-(a) + 3H((a)), are 28.32 kcal/mol (for H2N-H bond activation), 28.49 kcal/mol (for HN-H bond activation), and 25.34 kcal/mol (for N-H bond activation), and the entire process is 20.08 kcal/mol exothermic. To gain insight into the catalytic activity of the Fe(111) surface for the dehydrogenation of NH3, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis

We have studied the mechanism of the waterÀgas shift reaction (WGS, CO + H 2 O f CO 2 + H 2 ) catalyzed by nanosized gold particles by using density functional theory calculations. The molecular structures and adsorbate/ substrate interaction energies of H 2 O/Au 38 , CO/Au 38 , HO/Au 38 , and H/Au 38 configurations were predicted. Several adsorption sites on the Au 38 nanoparticle were considered in this study and characterized as top, bridge, hollow, and hcp sites. A potential energy surface for WGS reaction on the Au 38 nanoparticle has been constructed using the nudged elastic band method. It was found that water dissociation (H 2 O f H + OH) is the rate-limiting step, with an energy barrier of 31.41 kcal/mol. The overall reaction CO + H 2 O + Au 38 f CO 2 + H 2 + Au 38 is exothermic by 16.18 kcal/mol. To gain insights into the high catalytic activity of the gold nanoparticles, the nature of the interaction between adsorbate and substrate is also analyzed by the detailed electronic local density of states.

The interactions and reduction mechanisms of O2 molecule on the fully oxidized and reduced CeO2 surface were studied using periodic density functional theory calculations implementing on-site Coulomb interactions (DFT + U) consideration. The adsorbed O2 species on the oxidized CeO2 surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within -0.05 to 0.02 eV and 1530-1552 cm(-1), respectively. For the reduced CeO2 surface, the adsorption of O2 on Ce4+, one-electron defects (Ce3+ on the CeO2 surface) and two-electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O2, O2a- (0 < a < 1), superoxide (O2-), and peroxide (O(2)(2-)) species. Their corresponding adsorption energies are -0.01 to -0.09, -0.20 to -0.37, -1.34 and -1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O2a- (0 < a < 1) and physisorbed species are 897, 1234, 1323-1389, and 1462-1545 cm(-1), respectively, which are in good agreement with experimental data. Potential energy profiles for the O2 reduction on the oxidized and reduced CeO2 (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria-based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT-SOFC electrolyte materials.

Molecular dynamic simulation is used to investigate the adsorption mechanism of water molecules surrounding Au nanoparticles with different sizes. Our results show that the adsorption mechanism of the water molecules in the first water shell will be influenced by the size of the Au nanoparticle. For the larger Au nanoparticles, the hydrogen bonding of water molecules adsorbed on the surface of the Au nanoparticles are arranged in a two-dimensional structure, while those adsorbed on the edge of the surface of the Au nanoparticles are arranged in a three-dimensional structure. However, in the case of the smallest Au nanoparticle, the hydrogen bonding of the water molecules on the first adsorbed layer are arranged only in a three-dimensional structure. The arrangement of the water molecules in the first water shell can be determined by orientation order parameter. The water molecules that adsorb on the larger Au nanoparticles tend to arrange in an irregular arrangement, while those adsorbed on the smallest Au nanoparticle tend to arrange a regular arrangement. Interestingly, the water molecules adsorbed on the smallest nanoparticle are arranged in a bulklike structure in the first shell.

We have elucidated the mechanism of CO oxidation catalyzed by gold nanoparticles through first-principle density-functional theory (DFT) calculations. Calculations on selected model show that the low-coordinated Au atoms of the Au(29) nanoparticle carry slightly negative charges, which enhance the O(2) binding energy compared with the corresponding bulk surfaces. Two reaction pathways of the CO oxidation were considered: the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH). The overall LH reaction O(2(ads)) + CO((gas)) --> O(2(ads)) + CO((ads)) --> OOCO((ads)) --> O((ads)) + CO(2(gas)) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states (TS(LH1) and TS(LH2)) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au(29) + O(2(gas)) + CO((gas)) --> Au(29)-O(2(ads)) + CO((gas)) --> Au(29)-CO(3(ads)) --> Au(29)-O((ads)) + CO(2(gas)) requires an overall activation barrier of 0.19 eV, and the formation of Au(29)-CO(3(ads)) intermediate possesses high exothermicity of 4.33 eV, indicating that this process may compete with the LH mechanism. Thereafter, a second CO molecule can react with the remaining O atom via the ER mechanism with a very small barrier (0.03 eV). Our calculations suggest that the CO oxidation catalyzed by the Au(29) nanoparticle is likely to occur at or even below room temperature. To gain insights into high-catalytic activity of the gold nanoparticles, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis.

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically of vital importance. Recent experimental studies show that small gold nanoparticles supported on inert materials are efficient and robust catalysts for the selective oxidation of alkenes. The reasons for the outstanding catalyst of Au nanoparticles have been investigated and compared with the Au(111) surface by means of density functional theory. The nanoparticle is intrinsically much more selective than the surface in the epoxidation. The fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation. On the nanoparticle, epoxide formation is less activated than acetaldehyde formation, whereas the opposite is true on the (111) surface. This behavior is associated with a late transition state to epoxidation on the nanoparticle (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on the (111) surface.