Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (39 results)

  • Article
  • Peer Reviewed
Lawrence Berkeley National Laboratory (2009)
    Cover page: On the evaporation of ammonium sulfate solution
    • Article
    Lawrence Berkeley National Laboratory (2008)
      Cover page: Determination of the Evaporation Coefficient of D2O
      • Thesis
      • Peer Reviewed
      UC Berkeley Electronic Theses and Dissertations (2010)

      The kinetics of vapor-liquid exchange in water are poorly understood, yet may be critically important in predicting changes in Earth's climate and understanding the water isotope record preserved in ice cores . In this thesis we present measurements of the kinetics of water evaporation. In Chapter 1 we review recent work on the subject, including our own liquid microjet technique which has higher precision than other methods.

      In Chapter 2 we extend our earlier measurements of the evaporation kinetics of H2O by studying pure D2O. We find that the evaporation coefficient, which can be thought of as the percentage of evaporation "attempts" which succeed, is identical for the two isotopomers. We interpret this result using a previously developed transition state theory (TST) model of evaporation, which predicts the respective evaporation coefficients to be equal due to competing energetic and entropic effects.

      In Chapter 3, we examine the evaporation kinetics of H2O evaporating from 3M ammonium sulfate solution. Ammonium sulfate was selected as it is the largest inorganic component of anthropogenic aerosol in the atmosphere. Again we find that the evaporation coefficient is unchanged relative to pure water. This is consistent with theoretical and experimental studies suggesting that both the ammonium ion and sulfate ion are repelled from the air-water interface, implying that these ions will not directly interact with evaporating water molecules. This result also suggests that inorganic components of atmospheric aerosol are unlikely to significantly affect evaporation kinetics.

      In Chapter 4 we examine the evaporation kinetics of H2O from 4M sodium perchlorate solution. Perchlorate was selected as it is expected to be strongly enhanced in concentration at the air-water interface, and therefore more likely to directly influence the evaporation process. We find that the evaporation coefficient for this system is ~25% smaller than that for pure H2O, indicating that the perchlorate ions do indeed impede evaporation. Given experimental evidence for the perchlorate ion slowing the rotational motions of H2O molecules in its first solvation shell, and our TST predictions indicating that the evaporation kinetics of water are highly sensitive to the hindered rotational motions of surface water molecules, we suggest that perchlorate ions at the interface are inhibiting the evaporation of H2O molecules with which they are in direct contact. This result suggests that other surface-enhanced ions may also affect the evaporation kinetics through direct interactions with evaporating molecules and opens several interesting new avenues of study, which are discussed in Chapter 5.

        Cover page: Measurements of Evaporation Kinetics of Pure Water and Salt Solutions
        • Article
        • Peer Reviewed
        LBL Publications (2015)
        Condensation of ortho-phenylenediamine derivatives with ortho-quinone moieties at edge planes of graphitic carbon generates graphite-conjugated pyrazines (GCPs) that are active for oxygen reduction electrocatalysis in alkaline aqueous electrolyte. Catalytic rates of oxygen reduction are positively correlated with the electrophilicity of the active site pyrazine unit and can be tuned by over 70-fold by appending electron-withdrawing substituents to the phenylenediamine precursors. Discrete molecular analogs containing pyrazine moieties display no activity above background under identical conditions. This simple bottom up method for constructing molecularly well-defined active sites on ubiquitous graphitic solids enables the rational design of tunable heterogeneous catalysts.
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Electrochemical reduction of CO2 provides an opportunity to produce fuels and chemicals in a carbon-neutral manner, assuming that CO2 can be captured from the atmosphere. To do so requires efficient, selective, and stable catalysts. In this study, we report a highly mesoporous metallic Cu catalyst prepared by electrochemical reduction of thermally nitrided Cu foil. Under aqueous saturated CO2 reduction conditions, the Cu3N-derived Cu electrocatalyst produces virtually no CH4, very little CO, and exhibits a faradaic efficiency of 68% in C2+ products (C2H4, C2H5OH, and C3H7OH) at a current density of ∼18.5 mA cm-2 and a cathode potential of -1.0 V versus the reversible hydrogen electrode. Under these conditions, the catalyst produces more oxygenated products than hydrocarbons. We show that surface roughness is a good descriptor of catalytic performance. The roughest surface reached 98% CO utilization efficiency for C2+ product formation from CO2 reduction and the ratio of oxygenated to hydrocarbon products correlates with the degree of surface roughness. These effects of surface roughness are attributed to the high population of undercoordinated sites as well as a high pH environment within the mesopores and adjacent to the surface of the catalyst.
            Cover page: Production of C2/C3 Oxygenates from Planar Copper Nitride-Derived Mesoporous Copper via Electrochemical Reduction of CO2
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2021)

            In situ near edge X-ray absorption fine structure spectroscopy directly probes unoccupied states associated with backbonding interactions between the open metal site in a metal–organic framework and various small molecule guests.

              Cover page: Backbonding contributions to small molecule chemisorption in a metal–organic framework with open copper(i) centers
              • Article
              • Peer Reviewed
              LBL Publications (2017)
              Constrained-occupancy delta-self-consistent-field (ΔSCF) methods and many-body perturbation theories (MBPT) are two strategies for obtaining electronic excitations from first principles. Using the two distinct approaches, we study the O 1s core excitations that have become increasingly important for characterizing transition-metal oxides and understanding strong electronic correlation. The ΔSCF approach, in its current single-particle form, systematically underestimates the pre-edge intensity for chosen oxides, despite its success in weakly correlated systems. By contrast, the Bethe-Salpeter equation within MBPT predicts much better line shapes. This motivates one to reexamine the many-electron dynamics of x-ray excitations. We find that the single-particle ΔSCF approach can be rectified by explicitly calculating many-electron transition amplitudes, producing x-ray spectra in excellent agreement with experiments. This study paves the way to accurately predict x-ray near-edge spectral fingerprints for physics and materials science beyond the Bethe-Salpether equation.
                Cover page: Accurate X-Ray Spectral Predictions: An Advanced Self-Consistent-Field Approach Inspired by Many-Body Perturbation Theory
                • Article
                • Peer Reviewed
                LBL Publications (2015)
                The development of low-cost catalysts with oxygen reduction reaction (ORR) activity superior to that of Pt for fuel cells is highly desirable but remains challenging. Herein, we report a bimetal-organic framework (bi-MOF) self-adjusted synthesis of support-free porous Co-N-C nanopolyhedron electrocatalysts by pyrolysis of a Zn/Co bi-MOF without any post-treatments. The presence of initial Zn forms a spatial isolation of Co that suppresses its sintering during pyrolysis, and Zn evaporation also promotes the surface area of the resultant catalysts. The composition, morphology, and hence ORR activity of Co-N-C could be tuned by the Zn/Co ratio. The optimal Co-N-C exhibited remarkable ORR activity with a half-wave potential of 0.871 V versus the reversible hydrogen electrode (RHE) (30 mV more positive than that of commercial 20 wt % Pt/C) and a kinetic current density of 39.3 mA cm-2 at 0.80 V versus RHE (3.1 times that of Pt/C) in 0.1 M KOH, and excellent stability and methanol tolerance. It also demonstrated ORR activity comparable to and stability much higher than those of Pt/C in acidic and neutral electrolytes. Various characterization techniques, including X-ray absorption spectroscopy, revealed that the superior activity and strong stability of Co-N-C originated from the intense interaction between Co and N, the high content of ORR active pyridinic and pyrrolic N, and the large specific surface area.
                  Cover page: Bimetal–Organic Framework Self-Adjusted Synthesis of Support-Free Nonprecious Electrocatalysts for Efficient Oxygen Reduction
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2015)
                  [Mn4O4{O2P(OtBu)2}6] (1), an Mn4O4 cubane complex combining the structural inspiration of the photosystem II oxygen-evolving complex with thermolytic precursor ligands, was synthesized and fully characterized. Core oxygen atoms within complex 1 are transferred upon reaction with an oxygen-atom acceptor (PEt3), to give the butterfly complex [Mn4O2{O2P(OtBu)2}6(OPEt3)2]. The cubane structure is restored by reaction of the latter complex with the O-atom donor PhIO. Complex 1 was investigated as a precursor to inorganic Mn metaphosphate/pyrophosphate materials, which were studied by X-ray absorption spectroscopy to determine the fate of the Mn4O4 unit. Under the conditions employed, thermolyses of 1 result in reduction of the manganese to Mn(II) species. Finally, the related butterfly complex [Mn4O2{O2P(pin)}6(bpy)2] (pin = pinacolate) is described.
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2024)
                    The L2,3-edge X-ray absorption spectra of late transition metals such as Cu, Ag, and Au exhibit absorption onsets lower in energy for higher oxidation states, which is at odds with the measured spectra of earlier transition metals. Time-dependent density functional theory calculations for Cu2+/Cu+ reveal a larger 2p core-exciton binding energy for Cu2+, overshadowing shifts in single-particle excitation energies with respect to Cu+. We explore this phenomenon in a Cu+ metal-organic framework with ∼12% Cu2+ defects and find that corrections with self-consistent excited-state total energy differences provide accurate XAS peak alignment.
                      Cover page: Understanding 2p core-level excitons of late transition metals by analysis of mixed-valence copper in a metal–organic framework
                      Top