3 Search Results

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to investigate methanol (CH₃OH) adsorption and reaction on the GaP(110) surface. Exposure of CH₃OH to GaP(110) at room temperature led to the formation of at least four different surface species as indicated by analysis of C 1s and O 1s XPS features. By combining AP-XPS data with density functional theory calculations, the surface species were identified as methoxy (CH₃O*), formaldehyde (CH₂O*), and paired methanol (p-CH₃O*H) and methoxy (p-CH₃O*) species, where “paired” means that they belong to a hydrogen-bonded methoxy-methanol complex. Asterisk * here indicates an adsite. The formation of CH₂O* via themore » dehydrogenation of CH₃O* was shown to be limited by the availability of vacant phosphorus (P) sites on GaP(110). With an increase in CH₃OH pressure, the fractional coverage of CH₃O* species reached 0.55, and the surface P sites were completely saturated with hydrogen. Under a constant CH₃OH pressure of 0.5 Torr, the surface concentration of the paired species and of CH₂O* remained constant until 400 K. At higher temperatures, thermally driven reactions led to a significant increase in the concentration of surface CH_x* species, which suggests that C-O bond cleavage of the CH₃O group is the dominant decomposition mechanism on GaP(110). In conclusion, based on the reactivity of GaP(110) toward CH₃OH dehydrogenation, elevated temperatures and CH₃OH pressures may be used to functionalize this surface.« less

Recent experiments revealed the possibility of using titanium disulfide (TiS₂) as the cathode material in capacitive deionization (CDI) devices for water desalination. Although it performed stably up to 70 cycles with a salt removal capacity of 14 mg/g (corresponding to a sodium ion removal capacity of 35.8 mg/g) at high molar concentration (600 mM NaCl), the maximum capacity of TiS₂ as a CDI electrode was much lower (~70 mAh/g) than as a supercapacitor (239 mAh/g). Understanding why the ion capacities of these two configurations of the same material differ will entail elucidating detailed charge/discharge mechanisms at the atomic scale. Here,more » we present density functional theory simulations of sodium intercalation into TiS₂ interlayers in order to gain such understanding. We systematically investigated TiS₂ stacking patterns and ion intercalation sites, energetics of the intercalated compounds, and phase transformation during sodium intercalation. The calculated structural evolution and capacitance–voltage curve agree quite well with previous measurements. We conclude that the different maximum capacities of TiS₂ measured in aqueous and dry environments originate from weaker interlayer interactions with respect to shear strain after ~33% intercalation of Na⁺–H₂O pairs, which is detrimental to the mechanical stability of TiS₂. In conclusion, this study sheds light on the underlying mechanisms of ion intercalation into layered materials and contributes to understanding requirements for future design and optimization of CDI electrode materials for water desalination.« less

Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe²⁺in bridgmanite synthesized at 40–70 GPa and 2,000 K, while it is in mixed oxidation state (Fe³⁺/ΣFe = 60%) in the samples synthesized below and above the pressuremore » range. Little Fe³⁺in bridgmanite combined with the strong partitioning of Fe²⁺into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Further, our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant changes in bulk composition for most of the mantle, but possible changes in physical properties and processes (such as viscosity and mantle flow patterns) in the midmantle.« less