Narrow Results

A Monte Carlo simulation technique has been used to investigate the effect of sulphur impurity and ceria support on the surface composition and catalytic activity of Pt–Rh/ceria nanocatalysts. For temperatures in the range of 400–1300 K the surface Pt concentration decreases with increase in sulphur coverage. The Pt concentration at the base of the nanocatalyst in contact with the support increases with metal-support interaction. However, for weak metal-support interaction (V_ms<-0.01 eV) the surface Pt concentration slightly increases with metal-support interaction. Overall, sulphur is found to influence the surface composition much more strongly than the metal-support interaction. The MC-simulated surface composition results have been used to study the energetics of the CO+NO reactions over Pt–Rh/CeO₂ catalysts. It is found that the CO₂ formation rate decreases with increase in sulphur coverage and marginally increases with metal-support interaction up to V_ms=-0.01 eV. All the results qualitatively agree with the experimental results.

Lithium-rich manganese-based layered oxides are of great interest as cathode materials for lithium ion batteries due to their high energy density. The voltage decay and capacity fading during prolonged charge/discharge cycling are the key obstacles for their practical usage. In this work, using density functional theory, we investigated the origin of the Ni surface segregation by calculating the defect formation energies of antisite defects, including Ni cation substituting a Li cation $({\mathrm{\text{Ni}}}_{\mathrm{\text{Li}}})$ and pairs of Ni cation swapping with Li cation (${\mathrm{\text{Ni}}}_{\mathrm{\text{Li}}}$–${\mathrm{\text{Li}}}_{\mathrm{\text{Ni}}}$) in ${\mathrm{\text{LiMnO}}}_2$, ${\mathrm{\text{LiNi}}}_{1/3}{\mathrm{\text{Mn}}}_{1/3}{\mathrm{\text{Co}}}_{1/3}{\mathrm{\text{O}}}_2$ and ${\mathrm{\text{LiNiO}}}_2$ to represent the Mn-rich, Mn–Ni equivalence and Ni-rich conditions, respectively. Results show that ${\mathrm{\text{Ni}}}_{\mathrm{\text{Li}}}$–${\mathrm{\text{Li}}}_{\mathrm{\text{Ni}}}$ defect is of energy favorable in Li-rich and Mn-rich layered oxide cathode. Al-doping is beneficial for improving the structure stability and preventing the surface segregation, thus Al-doping can improve cycle ability and rate capability of Li-rich and Mn-rich layered cathodes. This work provides deep insights for the design of layered cathode materials with both high capacity and voltage stability during cycling.

The adsorption of oxygen and potassium on the two-phase system: carbide-modified stepped-W(100) surface (CMT) in contact with the solid solution of carbon in bulk tungsten, was investigated by AES and WF measurements. The CMT surface shows metallic behavior judging from its interaction with K. The expected dissociative adsorption of oxygen appears to occur with 1 - θ kinetics, possibly via a molecularly chemisorbed state. The "dispersed phase — two-phase" model is clearly applicable when oxygen adsorbs on the K-pre-covered carbide. The initial sticking coefficient of oxygen increases drastically from the dispersed to the condensed phase, at least four-fold with respect to s₀ on the clean carbide. It is proposed that this two-phase carbon system can be advantageous compared with the bulk carbide since it can easily regenerate the surface if the latter is depleted from carbon.

We have investigated the effect of Bi on the heteroepitaxial growth of Co on Cu by reflection high-energy electron diffraction (RHEED) measurements. It was found that Bi enhanced the layer-by-layer growth of Co on the Cu(111) surfaces at 100°C. The dependence of the growth on Bi layer thickness suggested that there existed a suitable amount of Bi surfactant layer that enhanced smoother layered growth. On the contrary, for the case of Co growth on Cu(100), Bi depressed the layer-by-layer growth of Co on Cu(100). The surface segregation effect of Bi was also studied by Auger electron spectroscopy (AES).

The surface segregation, structure, and valence band density of states of Pt₃Ni(100), (110), and (111) single crystals are characterized with low energy ion scattering (LEIS), low energy electron diffraction (LEED), and ultraviolet photoemission spectroscopy (UPS). The results of LEIS clearly reveal the complete surface segregation of Pt to the top layer on all crystal alloys. LEED indicates the (5 × 1) surface reconstruction on the Pt₃Ni(100), while (110) and (111) surfaces show (2 × 1) and (1 × 1) patterns, respectively, identical to Pt single crystals. The valence bands density of states on Pt₃Ni alloys are compared to those of Pt single crystals via UPS measurements.

The surface concentrations and concentration depth profiles to the (110) surface of an Au₇₅Pd₂₅ alloy is studied by modified analytical embedded atom method (MAEAM) with the Monte Carlo simulations. The results indicate that Au enriched in the two topmost layers, but depleted in the third layer. The Au concentration in the non-reconstructed surface is less than that in the reconstructed surface. Au concentration in third layer of reconstructed surface, which is more agreement with experimental data in present simulations, is about 63% 61% and 55%, at 800K, 600K and 400K respectively. Thus the present simulations are helpful for a better understanding of surface segregation of AuPd alloys.

The Free-energy Concentration Expansion Method (FCEM) was utilized for the prediction of compositional structures in Ni–Cu–Rh cubo-octahedron nanoclusters in comparison to recently reported Ni–Cu–Pd data. While both systems exhibit site-specific, sequentially competitive surface segregation (and resultant core separations), remarkable differences governed by the opposite heteronuclear effective interactions, were noted in the surface compositional patterns. Thus, at relatively low temperatures "mixed" Cu/Pd ordering takes place at the Ni–Cu–Pd cluster surface, whereas in the Ni–Cu–Rh cluster Cu and Ni populate separate low and high-coordinated surface sites, thus forming a kind of "demixed surface order". Dissimilarities in the temperature dependence are discussed in terms of the interplay of segregation and compositional order. Such findings may have implications in heterogeneous catalysis and other technologies based on highly dispersed alloyed particles.