Density functional theory calculations and rotating ring disk electrode experiments were performed to investigate the poisoning effects of sulfur species on the catalytic properties of elemental Pt and Pt3Co alloy surfaces. Experimental data indicates that there is a positive shift in the oxidation overpotential of Pt3Co accompanied by less oxidation/reduction cycles necessary in rotating ring disk electrode experiments (RRDE) in order to remove most of the sulfur species. Our theoretical calculations suggest that OH clustering is substantially reduced on the Pt3C(111) surface irrespective of the presence of Co atoms versus Pt(111). While the presence of Co does enhance adsorption of electronegative atoms/molecules on neighboring Pt sites, once Co atoms are oxidized or a Co-S bond is formed, they serve as a pin for the poison and subsequently reduce bonding of additional electronegative atoms/molecules at nearby sites. Additionally, our calculations indicate that a combination of effects due to less Pt3Co surface oxidation, more weakly adsorbed S species, and lower reaction barriers for SO2 oxidation on Pt3Co versus Pt subsequently leads to easier cleaning of the surface.

In adolescents, hallucinations can be a transient illness or can be associated with non-psychotic psychopathology, psychosocial adversity, or a physical illness. We present the case of a 15-year-old secondary-school student who presented with a 1-month history of first onset auditory hallucinations, which had been increasing in frequency and severity, and mild paranoid ideation. Over a 10-week period, there was a gradual diminution, followed by a complete resolution, of symptoms. We discuss issues regarding the diagnosis and prognosis of auditory hallucinations in adolescents.

Using density functional theory, we investigate the effect of exchanging Al for transition metal cations in the cathode material, LiNi1/3Co1/3Mn1/3O2 (NCM). We first detail the development of the cationic energy bands that determine the electrochemically active ion at various Li concentrations. We find that throughout most of the voltage range, electrons are drawn from Ni-derived bands. We find that substituting Al for Mn allows the higher voltage Co redox couple to be accessed earlier in the charge cycle. Our theoretical result indicates that Al doping could prove useful for raising the voltage of NCM cathodes.

Sulfur contaminants play a significant role in modifying the performance of transition metal fuel cell cathodes [1-3]. Sulfur blocks relevant reactants from adsorbing on the cathode side of the fuel cell thereby reducing the oxidation reduction reaction (ORR). The ORR proceeds as 2H + 2e + 1⁄2 O2 H2O. The most common catalyst being used in polymer electrolyte fuel cell cathodes is Pt. Recently however it has been found that using Pt alloys such as Pt3Ni and Pt3Co can increase the fuel cell cathode efficiency versus the pure Pt catalyst [4]. These catalysts increase the overall fuel cell efficiency by increasing reaction rate of the ORR. Despite the rate increase by using alternative Pt alloys these systems are still subject to sulfur contaminant exposure[2]. Indeed, experimental and theoretical works have investigated a variety of ways to remove sulfur. The most common is hydrogenation of S to H2S. Studies have found however that sulfur is extremely resistant to hydrogenation on a variety of transition metal surfaces[5, 6]. More specific to an alloy replacement for Pt a recent theoretical study investigated the hydrogenation process of S on a Pt3Ni(111) surface [7]. This study found that although the barriers for hydrogenation are decreased versus Pt(111) they were still significantly high and were unlikely to occur around room temperature. Similar to Pt3Ni(111), Pt3Co(111) surfaces have shown a higher resistance to sulfur poisons than Pt(111). A variety of surfaces with varying efficiencies towards the ORR can exist for Pt3Co(111) depending on preparation conditions[2]. If the Pt3Co(111) sample is annealed this results in a “Pt-skin” (a layer of pure Pt atoms) on top of a subsurface enriched with Co. However if the sample is not annealed it has a Ll2-Cu3Au type structure where Co atoms are present on the surface. Here we compare the interaction between OH and S on both Pt3Co(111) with and without the “Pt skin” as well as the Pt(111) surface. Fig. 1 shows the adsorption energy of S and OH on each of the surfaces considered. We find that on the Pt3Co(111)-Ll2 surface that those sites closest to Co surface atoms are the most favorable for OH and S adsorption. However, once these sites are “poisoned” additional adsorption of S species is significantly weakened. These results agree well with experimental observations that find initially S species are easier to remove from Pt3Co(111) versus Pt(111) surfaces [8]. In addition to adsorption properties of OH and S we also investigate the geometric and electronic properties of the Pt3Co(111) with and without the Pt-skin as well as the effect of S interaction with OH adsorption.

Interactions between hydrogen and sulfur on Ni(111) and Pt(111) were investigated using density functional theory. Hydrogen adsorption on Pt(111) indicates that the potential energy surface is smooth while on Ni(111) hydrogen prefers three fold hollow sites. Sulfur adsorption on both surfaces is energetically favored at the three fold hollow fcc site. The adsorption energies of H and S individually are greater on Ni(111) than Pt(111); this correlates well with their respective d-band center positions. On both surfaces, an increase in S coverage shifts the local d-band center towards a lower energy and gives the expected decrease in adsorption energy. The magnitude of the d-band shift as a function of S coverage was found to be non-linear and nearly identical on Pt and Ni surfaces, but the overall adsorption energy decrease for coadsorbed H was significantly greater on Ni(111). This indicates that coadsorbed species in close proximity interact directly (electrostatically) and not only through the electronic structure of the surface.

We examine the structure of small Aun, Agn, and Cun (n=2−4) clusters on rutile TiO2(110) surfaces using density functional theory calculations. Based on the comparison of supported and gas-phase clusters, we also discuss the effect of the cluster–substrate interaction on the atomic structure of small Au, Ag, and Cu clusters grown on TiO2(110).

First principles periodic slab calculations based on gradient-corrected density functional theory have been performed to investigate CO oxidation on rutile TiO2(110) at varying O2 coverages (theta = 1, 2, and 3, where theta is defined as the number of O2 per oxygen vacancy). For each coverage we only present the reaction of CO with oxygen species in the most stable configuration. Our results show a significant variation in the oxidation activation energy with O2 coverage.

In this paper, we propose a new adsorption model for molecular oxygen on reduced TiO2(110), based on extensive first principles density functional calculations. For the first time, our calculations predict formation of tetraoxygen (O4) anchored at the vacancy site, which in turn allows adsorption of three O2 molecules per vacancy in saturation coverage. We present the structure, bonding, and energetics of adsorbed oxygen species by changing the number of adsorbed oxygen molecules per vacancy. We also find that thermally activated O2 desorption may take place via two channels that require overcoming barriers of 0.41 and 1.25 eV, respectively. In addition, our study provides strong theoretical evidence for the change in O2 reactivity with O2 coverage. Our findings associated with tetraoxygen complexes are consistent with existing experimental results.

The adsorption properties of Au, Ag and Cu on TiO2 (110) rutile surfaces are examined using density functional theory slab calculations within the generalized gradient approximation. We consider five and four different adsorption sites for the metal adsorption on the stoichiometric and reduced surfaces, respectively. The metal-oxide bonding mechanism and the reactivity of metal atoms are also discussed based on the analyses of local density of states and charge density differences. This study predicts that Au atoms prefer to adsorb at the fourfold hollow site over the fivefold-coordinated Ti(5c) and in-plane and bridging O(2c) atoms with the adsorption energy of ≈0.6 eV. At this site, it appears that the covalent and ionic interactions with the Ti(5c) and the O(2c), respectively, contribute synergistically to the Au adsorption. At a neutral F s 0 center on the reduced surface, Au binds to the surface via a rather strong ionic interaction with surrounding sixfold-coordinated Ti(6c) atoms, and its binding energy is much larger than to the stoichiometric surface. On the other hand, Ag and Cu strongly interact with the surface bridging O(2c) atoms, and the site between two bridging O(2c) atoms is predicted to be energetically the most favorable adsorption site. The adsorption energies of Ag and Cu at the B site are estimated to be ≈1.2 eV and ≈1.8 eV, respectively. Unlike Au, the interaction of Ag and Cu with a vacancy defect is much weaker than with the stoichiometric surface.