A mean‐field modeling study of the interaction between hydrogen and a palladium (110) single crystal

A new interpretation of the temperature programmed desorption (TPD) spectra of hydrogen on a Pd(110) single crystal surface is presented. The transient mean‐field model developed accounts for both adsorption on the surface and diffusion of hydrogen into the lattice of the crystal. A new approach for modeling TPD spectra is developed, in which both the isothermal exposure as well as the temperature ramp of the TPD experiment are modeled in a consistent way. We demonstrate that for systems with substantial diffusion of the adsorbate into the substrate’s lattice it is not sufficient to report exposure values. For such systems, exposure time has a far more pronounced effect than the exposure pressure has on the TPD spectra, because diffusion is a strongly time‐dependent process. The modeling and experimental results for the H2/Pd(110) system are in good agreement and suggest the existence of a distinct subsurface state, in addition to the surface and bulk states of hydrogen. Low frequency factors derived from the model for the processes connecting the subsurface with the surface and bulk state emphasize the restricted nature of the corresponding transition states. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70193/2/JCPSA6-105-18-8398-1.pd

A Geometric Formulation of Quantum Stress Fields

We present a derivation of the stress field for an interacting quantum system within the framework of local density functional theory. The formulation is geometric in nature and exploits the relationship between the strain tensor field and Riemannian metric tensor field. Within this formulation, we demonstrate that the stress field is unique up to a single ambiguous parameter. The ambiguity is due to the non-unique dependence of the kinetic energy on the metric tensor. To illustrate this formalism, we compute the pressure field for two phases of solid molecular hydrogen. Furthermore, we demonstrate that qualitative results obtained by interpreting the hydrogen pressure field are not influenced by the presence of the kinetic ambiguity.Comment: 22 pages, 2 figures. Submitted to Physical Review B. This paper supersedes cond-mat/000627

Stability of gold nanowires at large Au-Au separations

The unusual structural stability of gold nanowires at large separations of gold atoms is explained from first-principles quantum mechanical calculations. We show that undetected light atoms, in particular hydrogen, stabilize the experimentally observed structures, which would be unstable in pure gold wires. The enhanced cohesion is due to the partial charge transfer from gold to the light atoms. This finding should resolve a long-standing controversy between theoretical predictions and experimental observations.Comment: 7 pages, 3 figure

Cataract surgery by appointment – a pilot study

BACKGROUND: "Cataract Surgery by Appointment" is a new method of delivery of cataract surgery that reduces the time a patient spends in hospital by their direct arrival at the operating theatre, having self-prepared for surgery, thus avoiding admission to the ward or time spent in the Day Case Unit. The patient can stay as little as 20 minutes from their arrival to going home. We describe the process in detail, and seek to evaluate the visual outcome, safety and patient satisfaction of same. METHODS: Visual outcome and safety data were obtained from patients' medical records, prospectively. Patients were also surveyed by a questionnaire to determine their satisfaction with the service and viability as a prospect for providing a more efficient cataract surgery service. RESULTS: In 2002, fifty-one eyes of 39 consecutive patients underwent "Cataract Surgery by Appointment". There were 16 male and 23 female. The pre-operative best-corrected visual acuity was 6/9 or better in 17 (33%) eyes. The post-operative best-corrected visual acuity was 6/9 or better in 44 (86%) eyes. There were no per-operative complications. Post-operative complications occurred in 3 (6%) eyes. The average number of days from surgery to final discharge was 14.5 days. Twenty-eight (72%) completed questionnaires were returned. The results show that the majority of patients were satisfied with their overall experience of this mode of delivery for cataract surgery. CONCLUSION: "Cataract Surgery by Appointment" performed under local anaesthesia by a skilled ophthalmic surgeon appears to be safe and effective for highly selected cases. This method of delivery gave a high level of patient satisfaction, and is the ultimate form of day case cataract surgery. The method may gain widespread use should per-operative intracameral pupil dilatation prove to be effective and acceptable. Attention should be paid to risk-stratification, so complex cases are allocated more time on the operating list

CO Adsorption on Supported Gold Nanoparticle Catalysts: Application of the Temkin Model

The adsorption of CO on the supported gold nanoparticle catalysts Au/TiO2, Au/Fe2O3, and Au/ZrO2 was examined using infrared transmission spectroscopy to quantify the isobaric CO coverage as a function of temperature. The Temkin adsorbate interaction model was then applied to account for the adsorption behavior. To test the general applicability of the Temkin model, this treatment was also applied to three data sets from the literature. This included another real-world catalyst and two model catalysts. All data sets were accurately represented by the Temkin adsorbate interaction model. The resulting thermodynamic metrics are consistent with previous determinations and reflect a particle size-dependence. In particular, the intrinsic adsorption enthalpy at zero CO coverage varies almost linearly with Au particle size, and this trend appears to be correlated with the abundance of low-coordinate Au sites (cf., CN = 6 and 7 for corners and edges, respectively). For very small particles with mostly CN = 6 corner sites, the enthalpy reflects strong binding (cf., −ΔH0 ≈ 78 kJ/mol), while for large particles with mostly CN = 7 edge sites, the enthalpy reflects weaker binding (cf., −ΔH0 ≈ 63 kJ/mol). The results also suggest that these sites are coupled. This study demonstrates that the Temkin adsorbate interaction model accurately represents adsorption data, yields meaningful metrics that are useful for characterizing nanoparticle catalysts, and should be applicable to other adsorption data sets