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23 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Dynamics of HCN, HNC, and HNCO Formation in the 193 nm Photodissociation of Formamide

Formamide (NH 2 CHO) is the simplest molecule containing a peptide linkage [-NH-C($=$O)-], and it plays an essential role in the study of prebiotic chemistry. Exposure to UV irradiation allows formamide to decompose and act as a prebiotic feedstock in the formation of nucleobases and other necessary starting materials. The photodissociation mechanism of gaseous formamide at 193 nm is studied using (a) chirped-pulse Fourier transform millimeter-wave spectroscopy in the 260-290 GHz spectral region in a room-temperature flow-tube reactor at 1 mu bar pressure, (b) a combination of electronic structure theory, transition state theory, and quasiclassical trajectories, and (c) the Active Thermochemical Tables. The HCN and HNC photoproducts of hydrogenated (NH 2 CHO) and deuterated (NH 2 CDO and ND 2 CHO) formamide precursors are examined to gain insight into the photodissociation mechanism. Here, the theoretical investigation has characterized the main pathway leading to each of the HCN/HNC isomers from the precursor isotopologues. The theoretical branching ratio [HNC]/[HCN] = 2.1 for nascent photofragments agrees with the experiment. The effect of the postphotolysis HNC ↔ HCN isomerization on the [HNC]/[HCN] ratio is predicted. We report the experimental branching ratio [HNCO]: ([HNC] + [HCN]) = 12 ± 3 and propose that most of HNCO originates from dissociation on the S-1 electronic state of formamide.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Aldol Condensation and Esterification over Ti-Substituted *BEA Zeolite: Mechanisms and Effects of Pore Hydrophobicity

Aldol condensation and esterification reactions provide paths to upgrade ethanol and acetaldehyde to higher- value molecules useful as fuels or intermediates for the synthesis of polymers. Transition-metal-substituted BEA zeolites (M-BEA) catalyze these reactions; however, the mechanisms for these processes in M-BEA and the effects of incidental or purposefully included silanol groups are not reported. Here, we combine kinetic and spectroscopic measurements obtained during catalytic reactions of acetaldehyde (CH 3 CHO), ethanol (C 2 H 5 OH), and hydrogen (H 2 ) mixtures over a series of Ti-BEA catalysts that possess a known range of silanol group densities to examine the kinetic relevance of intervening steps and the impact of silanol groups on catalytic rates. Across all Ti-BEA, rates for aldol condensation and esterification increase with the pressure of CH 3 CHO; however, C 2 H 5 OH and H 2 O weakly inhibit the rates of these reactions. The substitution of CD 3 CDO for CH 3 CHO decreases aldol condensation rates slightly (~10%) but leads to greater esterification rates (2- to 5-fold). The kinetic isotope effects together with the measured dependence of rates on reactant pressures suggest that aldol condensation and esterification occur on unoccupied Ti sites and involve multiple kinetically relevant steps. CH 3 CHO deprotonates irreversibly, and the kinetically relevant nucleophilic attack of the enolate to CH 3 CHO* (i.e., adsorbed CH 3 CHO on Ti sites) leads to aldol products, while the nucleophilic attack of the enolate to C 2 H 5 OH* gives esters. Selectivities toward aldol condensation increase with the ratio of CH 3 CHO to C 2 H 5 OH pressure and with increases in the silanol density of the as-synthesized Ti-BEA. During catalysis, in situ infrared spectroscopy demonstrates that these silanol groups react with C 2 H 5 OH to form ethoxysilane groups (i.e., SiOC 2 H 5 ) that modify the polarity of the environment near Ti active sites. As initial silanol densities increase, steady-state turnover rates for aldol condensation and esterification increase by factors of 5 and 2, respectively. The changes in rates and selectivities among Ti-BEA catalysts likely reflect changes in excess free energies of transition states for enolization and nucleophilic attack of the enolate to adsorbed coreactants. The differences in excess stability report on the interactions among reactive intermediates at framework Ti atoms and the ethoxysilane and remaining silanol groups present. Furthermore, the in situ modification of these pore environments confers changes in the stability of reactive species in a manner that contradicts intuition when considering the initial state of the catalyst but can be reconciled after accounting for the formation of persistent alkoxy surface moieties in the pores.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Gravimeter Search for Compact Dark Matter Objects Moving in the Earth

Dark matter could be composed of compact dark objects (CDOs). These objects may interact very weakly with normal matter and could move freely inside the Earth. A CDO moving in the inner core of the Earth will have an orbital period near 55 min and produce a time-dependent signal in a gravimeter. Data from superconducting gravimeters rule out such objects moving inside the Earth unless their mass m D and or orbital radius a are very small so that m D a < 1.2 × 10 - 13 M ⊕ R ⊕ . Here, M ⊕ and R ⊕ are the mass and radius of the Earth, respectively.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Materials Data on CdHgO2 by Materials Project

CdO(HgO) crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Hg2+ is bonded in a linear geometry to two equivalent O2- atoms. Both Hg–O bond lengths are 2.02 Å. Cd2+ is bonded to six equivalent O2- atoms to form edge-sharing CdO6 octahedra. All Cd–O bond lengths are 2.36 Å. O2- is bonded to one Hg2+ and three equivalent Cd2+ atoms to form a mixture of distorted edge and corner-sharing OCd3Hg tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Oxides related to cadmium telluride solar cells

Polycrystalline cadmium telluride (CdTe) is a leading material in photovoltaic technology due to its high absorption coefficient and near-optimum bandgap of 1.44 eV. It is known that CdTe film processing can promote surface oxidation depending on the growth environment and upon exposure to different oxidation conditions. For example, CdTeO3 forms when the CdTe film is treated in heated dry air, while in humid air, CdTe2O5 is detected. These oxides feature tellurium in the oxidation state +4 compared to the +2 state in CdTe. Other possible relevant oxides are CdO, TeO2, and TeO3. Using hybrid density functional calculations, we studied the electronic structure of these oxide materials and their band alignment to CdTe, which are essential parameters in the characterization of the interfaces at grain boundaries. The goal is to understand their stability and possible effects on passivating grain boundaries. The results are compared to the available experimental data.

14 SOLAR ENERGY↗