20 research outputs found
Coupling Methylammonium and Formamidinium Cations With Halide Anions: Hybrid Orbitals, Hydrogen Bonding, and the Role of Dynamics
The electronic structures of four precursors for organic-inorganic hybrid perovskites, namely, methylammonium chloride and iodide, as well as formamidinium bromide and iodide, are investigated by X-ray emission (XE) spectroscopy at the carbon and nitrogen K-edges. The XE spectra are analyzed based on density functional theory calculations. We simulate the XE spectra at the Kohn-Sham level for ground-state geometries and carry out detailed analyses of the molecular orbitals and the electronic density of states to give a thorough understanding of the spectra. Major parts of the spectra can be described by the model of the corresponding isolated organic cation, whereas high-emission energy peaks in the nitrogen K-edge XE spectra arise from electronic transitions involving hybrids of the molecular and atomic orbitals of the cations and halides, respectively. We find that the interaction of the methylammonium cation is stronger with the chlorine than with the iodine anion. Furthermore, our detailed theoretical analysis highlights the strong influence of ultrafast proton dynamics in the core-excited states, which is an intrinsic effect of the XE process. The inclusion of this effect is necessary for an accurate description of the experimental nitrogen K-edge X-ray emission spectra and gives information on the hydrogen-bonding strengths in the different precursor materials
Direct Observation of Reactant, Intermediate, and Product Species for Nitrogen Oxide-Selective Catalytic Reduction on Cu-SSZ-13 Using In Situ Soft X-ray Spectroscopy
Observation of Double Excitations in the Resonant Inelastic X-ray Scattering of Nitric Oxide
The nitrogen K-edge resonant inelastic x-ray scattering (RIXS) map of nitric oxide (NO) has been measured and simulated to provide a detailed analysis of the observed features. High-resolution experimental RIXS maps were collected using an in situ gas flow cell and a high-transmission soft x-ray spectrometer. Accurate descriptions of the ground, excited, and core-excited states are based upon restricted active space self-consistent-field calculations using second order multiconfigurational perturbation theory. The nitrogen K-edge RIXS map of NO shows a range of features that can be assigned to intermediate states arising from 1s→π* and 1s→Rydberg excitations; additional bands are attributed to doubly excited intermediate states comprising 1s→π* and π→π* excitations. These results provide a detailed picture of RIXS for an open-shell molecule and an extensive description of the core-excited electronic structure of NO, an important molecule in many chemical and biological processes
Operando investigation of Au-MnOx thin films with improved activity for the oxygen evolution reaction
Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry
Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel. This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on this technology. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calculate the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day production scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from 10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O_2 and H_2 in a single bed system and long molecular transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity analysis revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the production costs are consistent with the Department of Energy's targeted threshold cost of 4.00 per kg H_2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met
Finishing the euchromatic sequence of the human genome
The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead
Mapping Photoelectrochemical Current Distribution at Nanoscale Dimensions on Morphologically Controlled BiVO 4
Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction
Developing improved catalysts for
the oxygen evolution reaction
(OER) is key to the advancement of a number of renewable energy technologies,
including solar fuels production and metal air batteries. In this
study, we employ electrochemical methods and synchrotron techniques
to systematically investigate interactions between metal oxides and
noble metals that lead to enhanced OER catalysis for water oxidation.
In particular, we synthesize porous MnO<sub><i>x</i></sub> films together with nanoparticles of Au, Pd, Pt, or Ag and observe
significant improvement in activity for the combined catalysts. Soft
X-ray absorption spectroscopy (XAS) shows that increased activity
correlates with increased Mn oxidation states to 4+ under OER conditions
compared to bare MnO<sub><i>x</i></sub>, which exhibits
minimal OER current and remains in a 3+ oxidation state. Thickness
studies of bare MnO<sub><i>x</i></sub> films and of MnO<sub><i>x</i></sub> films deposited on Au nanoparticles reveal
trends suggesting that the enhancement in activity arises from interfacial
sites between Au and MnO<sub><i>x</i></sub>
Observing Local pH Changes Using a Rotating Ring-Disk Electrode Functionalized with a Potentiometric pH-Sensing Probe
Electrochemical reactions involving protons and hydroxide
ions
are significantly impacted by changes in the local pH near the catalyst
surface. Therefore, it is useful to quantify the catalyst local pH
to better understand the impact on overall reaction efficiency and
selectivity. While it is difficult to experimentally probe the catalyst/electrolyte
interface, this regime can be monitored indirectly using pH-sensitive
materials. In this work, we investigate the use of a rotating ring-disk
electrode coupled with a pH-sensing probe to track changes in proton
concentration near the catalyst surface for the oxygen reduction reaction
under well-defined mass transport conditions. We further examine the
limitations and describe methods for improving the robustness of this
experimental platform. Out of the electrode support and probe materials
examined, we find that iridium oxide electrodeposited using cyclic
voltammetry onto gold substrates exhibiting high surface area and
moderate porosity demonstrates the highest, fastest, and most stable
pH-potential response, enabling reliable measurements in under 10
s. Using an analytical convective-diffusion equation, we also estimate
the disk local pH under varied operating conditions (e.g., current
density and rotation rate) and reaction environments (e.g., bulk pH).
This work outlines best practices for applying this technique and
provides insights into the impact of relevant reaction environment
conditions on the catalytic performance