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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

High-performance of CrOx/HZSM-5 catalyst on non-oxidative dehydrogenation of C 2 H 6 to C 2 H 4 : Effect of supporting materials and associated mechanism

Ethane (C 2 H 6 ) is an important inexpensive and widely available fuel resource. High-value use of C 2 H 6 has become increasingly important. Catalytic dehydrogenation of C 2 H 6 to ethylene (C 2 H 4 ) has attracted much attention in recent years due to its high energy efficiency. The direct non-oxidative ethane dehydrogenation (EDH) to ethylene is a promising strategy to produce ethylene and hydrogen at the same time. In this research, Cr/HZSM-5 catalyst with superior stability was synthesized and exhibited an C 2 H 6 converting activity of 1.47 µmol/(g·s) with the corresponding C 2 H 6 conversion and C 2 H 4 selectivity of 37.3% and 90%, respectively. Herein, the synergistic effects of Si and Al in supporting materials were investigated by comparing Cr/HZSM-5 with SBA-15, SiO 2 and Al 2 O 3 supported ones, which contains either Al or Si with different structures. Characterization results indicated that the intimate interactions between Cr and support significantly improved the catalytic performance. The presence of Al in the support promoted the formation of more active Cr 6+ species by forming the aluminum-chromium-chromate (Cr-O-Al) structures which were more efficient to active C-H bond and form (Cr, Al)-OH groups during the reaction. Meanwhile, the formation of internal silanol group with the dissociative adsorbed H* could stabilized the active Cr phase to achieve a stable dehydrogenation activity.

03 NATURAL GAS↗
DOE OSTI2018

Mechanical properties of aluminosilicate fiber heat-treated from 800 °C to 1400 °C: Effects of phase transition, grain growth and defects

Highlights: • As reaction between amorphous SiO{sub 2} and γ-Al{sub 2}O{sub 3}, phase transition was triggered at ≈1150 °C in AS fiber, thus forming mullite. • Grain size of formed mullite phase was evidently increased as the heat-treated temperature increased from 1200 °C to 1400 °C. • Higher density flaws were formed at higher temperatures, which led to significant degradation of strength of the AS fiber. - Abstract: Thermal stability of oxide fiber is a key property for high temperature structural applications of typical oxide fiber reinforced oxide matrix composites. This work contributes to provide insight into the mechanical properties of a typical aluminosilicate (AS) fiber heat-treated from 800 °C to 1400 °C in air, with the emphasis on the effects of phase transition, grain growth and defect formations. The results showed a phase reaction occurring at 1150 °C, which formed an orthorhombic structured mullite phase. The grain size of the newly formed mullite was observed increased significantly at higher temperatures. Both the phase transition and grain size led to more flaw formations in the microstructure, which influenced significantly the mechanical property of the AS fiber. Specifically, the Young's modulus and tensile strength were both degraded remarkably from ≈145 GPa to ≈110 GPa, and from ≈1.03 GPa to ≈0.52 GPa, respectively, as the heat-treatment temperature increased from 1200 °C to 1400 °C. Finally, the formations of more flaws yielded a higher dispersion of tensile strength of AS fiber, due to the volume dependent flaw distributions, as revealed by the Weibull statistical analysis.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Grain boundary precipitation phenomena in an alloy 33 (Cr-Fe-Ni-N) subjected to direct-aging treatments (700 °C and 900 °C)

Highlights: • Complex GB precipitation occurs in Alloy 33 upon aging at 700 °C: conventional and DP modes. At 900 °C only the former occurs. • Two types of DP colonies were observed at 700 °C: unidirectional/parallel and multidirectional/irregular precipitates. • X-ray micro analyses reveal that irregular DP products develop more than one precipitate phase within the same DP colony. - Abstract: Precipitation behavior of alloy 33 upon isothermally aging at 700 °C and at 900 °C was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (XEDS). The name, alloy 33, reflects the quasi-equal atomic content of the base elements Fe, Ni and Cr close to 33%, with the addition of Mo and N. The most remarkable features of the microstructural evolution resulting from the aging heat treatments are described as follows: aging at 700 °C leads to homogeneous and heterogeneous grain boundary precipitation, where the discontinuous mode is dominant at this temperature. Conversely, the opposite behavior was observed when the alloy is aged at 900 °C: predominantly homogeneous precipitation together with grain boundary precipitation, however with rare evidence for discontinuous precipitation (DP), even after longer aging times. In the present study, the topology and content of the DP colonies generated by the combined effect of grain boundary migration diffusion and precipitation under isothermal aging at 700 °C were particularly investigated. A variety of morphological features were observed: mainly (1) classical-type microstructure with straight and parallel lamellar precipitates with regular interlamellar spacing and (2) another microstructure with irregular and complex DP colonies. According to our analytical electron microscopy observations, the latter shows that there are more than one precipitate phase within the same colony, which is new to the current theory and experimental evidence on this subject.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of carbon content and size on Ti-C reaction behavior and resultant properties of Cu-Ti-C alloy system

Highlights: • TiC-strengthened Cu alloys were synthesized by an in situ powder metallurgy route. • Effect of C/Ti ratio on electrical and mechanical properties was studied. • Ti-C reaction behavior during powder metallurgy process was discussed. • Microstructure evolution during present process was analyzed. - Abstract: TiC-strengthened Cu alloys with various nominal compositions were prepared by a high-energy ball-milling process, followed by consolidation using spark plasma sintering (SPS), and a subsequent heat treatment. The results indicated that the lower atomic ratio of C/Ti, the more residual Ti in copper matrix, resulting in a lower electrical conductivity but higher microhardness of alloy. The optimal atomic ratio was found to be close to1.0. The reaction behavior of Ti-C in the Cu-Ti-C system during this process was investigated. Microstructural results suggested that TiC formation is through a diffusion-controlled mechanism. A thin TiC layer was formed at the surface of C particles, and the growth of TiC layer was controlled by interdiffusion of C and Ti atoms across the TiC layer. An incomplete chemical reaction between Ti and C, characterized by a core-shell structure, was observed when the initial C particles were large, and the TiC particle size was found to strongly depending on the C particle size.

36 MATERIALS SCIENCE↗
DOE OSTI2022

13 C-Isotope-Assisted Assessment of Metabolic Quenching During Sample Collection from Suspension Cell Cultures

Metabolomics and fluxomics are core approaches to directly profile and interrogate cellular metabolism in response to various genetic or environmental perturbations. In order to accurately measure the abundance and isotope enrichment of intracellular metabolites, cell culture samples must be rapidly harvested and cold-quenched to preserve the in vivo metabolic state of the cells at the time of sample collection. When dealing with suspension cultures, this process is complicated by the need to separate the liquid culture media from cellular biomass prior to metabolite extraction. Here, we examine the efficacy of several commonly used metabolic quenching methods, using the model cyanobacterium Synechocystis sp. PCC 6803 as an example. Multiple 13 C-labeled compounds, including 13 C-bicarbonate, 13 C-glucose and 13 C-glutamine, were used as tracers during the sample collection and cold-quenching process to assess the extent of metabolic turnover after cells were harvested from culture flasks. We show that the combination of rapid filtration followed by 100% cold (–80°C) methanol quenching exhibits the highest quenching efficiency, while mixing cell samples with a partially frozen 30% methanol slurry (–24°C) followed by centrifugation is slightly less effective at quenching metabolism but enables less laborious sample processing. By contrast, rapidly mixing the cells with a saline ice slurry (~0°C) is less effective as indicated by high isotope-labeling rates after sample harvest, while mixing the cells with 60% cold methanol (–65°C) prior to centrifugation causes significant metabolite loss. Furthermore, this study demonstrates a rigorous, quantitative, and broadly applicable method for assessing the metabolic quenching efficacy of protocols used for sample collection in metabolomics and fluxomics studies.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2026

Directed Gas-Phase Formation of The Propargyl Family of Resonance-Stabilized Radicals in The Reactions of Ground-State Carbon Atoms (C; 3 P j ) with Butene Isomers (C 4 H 8 ): Dimethylpropargyl and Ethylpropargyl

The propargyl radical (C 3 H 3 ) is the simplest resonance-stabilized free radical (RSFR), but how does stepwise methyl substitution in the alkene reactant affect its dynamics of their formation? We report a crossed molecular beam study of the reactions of atomic carbon (C, 3 P j ) with four butene isomers (C 4 H 8 ) under single collision conditions at a collision energy of 28 ± 2 kJ mol −1 . Barrierless addition of atomic carbon to the alkene C=C bond triggers ring opening to triplet substituted allenes—a de facto insertion mechanism—followed by unimolecular decomposition via atomic hydrogen (H), methyl (CH 3 ) or ethyl (C 2 H 5 ) loss, yielding a family of propargyl‑type RSFRs. RRKM calculations reveal that the branching ratios are highly sensitive to the alkene structure. While the methyl loss channel, affording 1‑methylpropargyl, dominates for 2 butenes (80–90%), the predicted hydrogen atom loss channel (≈10%) leading to 1,3 dimethylpropargyl is identified in the experiment by comparison with theoretical energetics. For isobutene, a near‑equal competition is seen, with the reaction producing 3‑methylpropargyl (≈50%) and 1,1‑dimethylpropargyl (≈40%), along with 2‑vinylallyl (≈5%), whose formation is supported by the experimental data. Most notably, the reaction with 1 butene uniquely favors an enthalpically driven hydrogen shift, eventually producing 1 vinylallyl (≈38%), which is assigned based on the excellent agreement between the measured and calculated reaction exothermicity. Rapid entropically favored fragmentation channels yield ≈40% of propargyl type species (propargyl, 1- and 3-ethylpropargyls), slightly outcompeting the allyl type product. Furthermore, these results establish a systematic progression from C 2 H 4 to C 4 H 8 , where the increasing alkyl substitution unlocks new fragmentation channels, providing a versatile gas phase route to alkylated RSFRs—key intermediates in the growth of methylated and ethylated PAHs and aliphatic chains in combustion and cold interstellar environments (molecular clouds).

Alkyls↗
DOE OSTI2016

Identification of Possible Pathways for C–C Bond Formation during Electrochemical Reduction of CO 2 : New Theoretical Insights from an Improved Electrochemical Model

Here, we have carried out a periodic Kohn-Sham density functional theory investigation of the pathways by which carbon-carbon bonds could be formed during the electrochemical reduction of CO 2 on Cu(100) using a model that includes the effects of the electrochemical potential, solvent, and electrolyte. The electrochemical potential was set by relating the applied potential to the Fermi energy and then calculating the number of electrons required by the simulation cell for that specific Fermi energy. The solvent was included as a continuum dielectric, and the electrolyte was described using a linearized Poisson-Boltzmann model. The calculated potential of zero charge for a variety of surfaces agrees with experiment to within a mean average error of 0.09 V, thereby validating the assumptions of the model. Analysis of the mechanism for C–C bond formation revealed that at low-applied potential, C–C bond formation occurs through a CO dimer. However, at high applied potentials, a large activation barrier blocks this pathway; therefore, C–C bond formation occurs through reaction of adsorbed CHO and CO. Rate parameters determined from our calculations were used to simulate the kinetics of ethene formation during the electrochemical reduction of CO over a Cu(100) surface. An excellent match was observed between previously reported measurements of the partial current for ethene formation as a function of applied voltage and the variation in the partial current for C–C bond formation predicted by our microkinetic model. The electrochemical model reported here is simple, fairly easy to implement, and involves only a small increase in computational cost over calculations neglecting the effects of the electrolyte and the applied field. Therefore, it can be used to study the effects of applied potential and electrolyte composition on the energetics of surface reactions for a wide variety of electrochemical reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Machine Learning-Accelerated First-Principles Molecular Dynamics Reveals C–C Coupling Mechanisms toward Ethylene on Cu(100)

Here, the Cu(100) termination has been identified as the most effective facet for converting CO and CO 2 into ethylene. To enhance both the activity and selectivity of ethylene production, we perform machine-learning-accelerated, first-principles molecular dynamics simulations at 298 K in an explicit solvent at pH 7 to elucidate the C–C coupling mechanism─the critical reaction step in forming C 2+ products. Among the six potential C–C coupling pathways, the most feasible are CO* dimerization and CO – CHO* and CHO* – CHO* couplings. Using the computational hydrogen electrode method, we demonstrate that all three pathways are equally accessible at −0.6 V vs RHE. At a potential below −1.0 V vs RHE, the thermodynamic barriers for the CO – CHO* and CHO* – CHO* pathways become negligible. Our computational findings explain the experimental observations, particularly the absence of C 2+ products above −0.4 V vs RHE and the peaks in ethylene production near −0.6 and −1.0 V vs RHE. Since CHO* acts as a key intermediate common to both C–C coupling and CH 4 formation, we propose that suppressing CHO* hydrogenation would inhibit CH 4 pathways, thereby maximizing ethylene selectivity.

CO2 reduction↗
DOE OSTI2016

The conversion of CO 2 to methanol on orthorhombic β-Mo 2 C and Cu/β-Mo 2 C catalysts: Mechanism for admetal induced change in the selectivity and activity

Here, the conversion of CO 2 into methanol catalyzed by β-Mo 2 C and Cu/β-Mo 2 C surfaces has been investigated by means of a combined experimental and theoretical study. Experiments have shown the direct activation and dissociation of the CO 2 molecule on bare β-Mo 2 C, whereas on Cu/β-Mo 2 C, CO 2 must be assisted by hydrogen for its conversion. Methane and CO are the main products on the clean surface and methanol production is lower. However, the deposition of Cu clusters avoids methane formation and increases methanol production even above that corresponding to a model of the technical catalyst. DFT calculations on surface models of both possible C- and Mo-terminations corroborate the experimental observations. Calculations for the clean Mo-terminated surface reveal the existence of two possible routes for methane production (C + 4H → CH 4 ; CH 3 O + 3H → CH 4 + H 2 O) which are competitive with methanol synthesis, displaying slightly lower energy barriers. On the other hand, a model for Cu deposited clusters on the Mo-terminated surface points towards a new route for methanol and CO production avoiding methane formation. The new route is a direct consequence of the generation of a Mo 2 C–Cu interface. The present experimental and theoretical results entail the interesting catalytic properties of Mo 2 C as an active support of metallic nanoparticles, and also illustrate how the deposition of a metal can drastically change the activity and selectivity of a carbide substrate for CO 2 hydrogenation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

{sup 12}C+{sup 12}C reactions at astrophysical energies: Tests of targets behaviour under beam bombardment

{sup 12}C({sup 12}C,α){sup 20}Ne and {sup 12}C({sup 12}C,p){sup 23}Na are the most important reactions during the carbon burning phase in stars. Direct measurements at the relevant astrophysical energy (E=1.5±0.3MeV) are very challenging because of the extremely small cross sections involved and of the high beam-induced background originating from impurities in the targets. In addition, persistent resonant structures at low energies are not well understood and make the extrapolation of the cross section from high energy data very uncertain. As a preliminary step towards the measurements of the {sup 12}C({sup 12}C,α){sup 20}Ne and {sup 12}C({sup 12}C,p){sup 23}Na reactions we intend to investigate the behaviour of targets under beam bombardment, specifically the quantitative measurement of hydrogen and deuterium content of highly pure stable carbon targets in relation to target temperature. Experiments are taking place at the CIRCE accelerator in Caserta, Italy and preliminary results are presented here.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Possible contribution of low shear modulus C{sub 44} to the low Young's modulus of Ti-36Nb-5Zr alloy

Despite the importance of single-crystal elastic constants of β-phase titanium alloys in understanding their low Young's modulus—a property crucial for many applications, such data are often difficult to obtain when the alloy composition is close to the instability limit of the β phase, where the presence of α' martensite precludes the fabrication of β-phase single crystal. In the present study, we extracted the single-crystal elastic constants of such a β-phase titanium alloy with low Young's modulus, Ti-36Nb-5Zr (wt. %), from polycrystalline specimens by using an in-situ synchrotron X-ray diffraction technique. It is indicated that the low Young's modulus of the alloy originates from the anomalously low shear modulus C{sub 44} as well as the low shear modulus C′, which is different from a common viewpoint that the Young's modulus of β-phase titanium alloys is dominantly controlled by the C′. This suggests that low C{sub 44} is an important contributor to low Young's modulus for instable β-phase titanium alloys.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Alkali metal mediated C–C bond coupling reaction

Metal catalyzed carbon-carbon (C–C) bond formation is one of the important reactions in pharmacy and in organic chemistry. In the present study, the electron and hole capture dynamics of a lithium-benzene sandwich complex, expressed by Li(Bz){sub 2}, have been investigated by means of direct ab-initio molecular dynamics method. Following the electron capture of Li(Bz){sub 2}, the structure of [Li(Bz){sub 2}]{sup −} was drastically changed: Bz–Bz parallel form was rapidly fluctuated as a function of time, and a new C–C single bond was formed in the C{sub 1}–C{sub 1}′ position of Bz–Bz interaction system. In the hole capture, the intermolecular vibration between Bz–Bz rings was only enhanced. The mechanism of C–C bond formation in the electron capture was discussed on the basis of theoretical results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Tungsten impurity transport experiments in Alcator C-Mod to address high priority research and development for ITER

Experiments in Alcator C-Mod tokamak plasmas in the Enhanced D-alpha H-mode regime with ITER-like mid-radius plasma density peaking and Ion Cyclotron Resonant heating, in which tungsten is introduced by the laser blow-off technique, have shown that accumulation of tungsten in the central region of the plasma does not take place in these conditions. The measurements obtained are consistent with anomalous transport dominating tungsten transport except in the central region of the plasma where tungsten transport is neoclassical, as previously observed in other devices with dominant neutral beam injection heating, such as JET and ASDEX Upgrade. In contrast to such results, yet, the measured scale lengths for plasma temperature and density in the central region of these Alcator C-Mod plasmas, with density profiles relatively flat in the core region due to the lack of core fuelling, are favourable to prevent inter and intra sawtooth tungsten accumulation in this region under dominance of neoclassical transport. Simulations of ITER H-mode plasmas, including both anomalous (modelled by the Gyro-Landau-Fluid code GLF23) and neoclassical transport for main ions and tungsten and with density profiles of similar peaking to those obtained in Alcator C-Mod show that accumulation of tungsten in the central plasma region is also unlikely to occur in stationary ITER H-mode plasmas due to the low fuelling source by the neutral beam injection (injection energy ~ 1 MeV), which is in consistent agreement with findings in the Alcator C-Mod experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Electronic transitions and band offsets in C{sub 60}:SubPc and C{sub 60}:MgPc on MoO{sub 3} studied by modulated surface photovoltage spectroscopy

Electronic transitions at interfaces between MoO{sub 3} layers and organic layers of C{sub 60}, SubPc, MgPc, and nano-composite layers of SubPc:C{sub 60} and MgPc:C{sub 60} have been studied by modulated surface photovoltage (SPV) spectroscopy. For all systems, time dependent and modulated SPV signals pointed to dissociation of excitons at the MoO{sub 3}/organic layer interfaces with a separation of holes towards MoO{sub 3}. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps (E{sub HL}) of C{sub 60}, SubPc, and MgPc and the effective E{sub HL} of SubPc:C{sub 60} and MgPc:C{sub 60} were measured. The offsets between the LUMO (ΔE{sub L}) or HOMO (ΔE{sub H}) bands were obtained with high precision and amounted to 0.33 or 0.73 eV for SubPc:C{sub 60}, respectively, and to −0.33 or 0.67 eV for MgPc:C{sub 60}, respectively. Exponential tails below E{sub HL} and most pronounced sub-bandgap transitions were characterized and ascribed to disorder and transitions from HOMO bands to unoccupied defect states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A first-principles study of cementite (Fe{sub 3}C) and its alloyed counterparts: Elastic constants, elastic anisotropies, and isotropic elastic moduli

A comprehensive computational study of elastic properties of cementite (Fe{sub 3}C) and its alloyed counterparts (M{sub 3}C (M = Al, Co, Cr, Cu, Fe, Hf, Mn, Mo, Nb, Ni, Si, Ta, Ti, V, W, Zr, Cr{sub 2}FeC and CrFe{sub 2}C) having the crystal structure of Fe{sub 3}C is carried out employing electronic density-functional theory (DFT), all-electron PAW pseudopotentials and the generalized gradient approximation for the exchange-correlation energy (GGA). Specifically, as a part of our systematic study of cohesive properties of solids and in the spirit of materials genome, following properties are calculated: (i) single-crystal elastic constants, C{sub ij}, of above M{sub 3}Cs; (ii) anisotropies of bulk, Young’s and shear moduli, and Poisson’s ratio based on calculated C{sub ij}s, demonstrating their extreme anisotropies; (iii) isotropic (polycrystalline) elastic moduli (bulk, shear, Young’s moduli and Poisson’s ratio) of M{sub 3}Cs by homogenization of calculated C{sub ij}s; and (iv) acoustic Debye temperature, θ{sub D}, of M{sub 3}Cs based on calculated C{sub ij}s. We provide a critical appraisal of available data of polycrystalline elastic properties of alloyed cementite. Calculated single crystal properties may be incorporated in anisotropic constitutive models to develop and test microstructure-processing-property-performance links in multi-phase materials where cementite is a constituent phase.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Charting C–C coupling pathways in electrochemical CO 2 reduction on Cu(111) using embedded correlated wavefunction theory

The electrochemical CO 2 reduction reaction (CO 2 RR) powered by excess zero-carbon-emission electricity to produce especially multicarbon (C 2+ ) products could contribute to a carbon-neutral to carbon-negative economy. Foundational to the rational design of efficient, selective CO 2 RR electrocatalysts is mechanistic analysis of the best metal catalyst thus far identified, namely, copper (Cu), via quantum mechanical computations to complement experiments. Here, we apply embedded correlated wavefunction (ECW) theory, which regionally corrects the electron exchange-correlation error in density functional theory (DFT) approximations, to examine multiple C–C coupling steps involving adsorbed CO (*CO) and its hydrogenated derivatives on the most ubiquitous facet, Cu(111). We predict that two adsorbed hydrogenated CO species, either *COH or *CHO, are necessary precursors for C–C bond formation. The three kinetically feasible pathways involving these species yield all three possible products: *COH–CHO, *COH–*COH, and *OCH–*OCH. The most kinetically favorable path forms *COH–CHO. In contrast, standard DFT approximations arrive at qualitatively different conclusions, namely, that only *CO and *COH will prevail on the surface and their C–C coupling paths produce only *COH–*COH and *CO–*CO, with a preference for the first product. This work demonstrates the importance of applying qualitatively and quantitatively accurate quantum mechanical method to simulate electrochemistry in order ultimately to shed light on ways to enhance selectivity toward C 2+ product formation via CO 2 RR electrocatalysts.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2021

The photosynthetic response of C 3 and C 4 bioenergy grass species to fluctuating light

Abstract Bioenergy grass species are a renewable energy source, but their productivity has not been fully realized. Improving photosynthetic efficiency has been proposed as a mechanism to increase the productivity of bioenergy grass species. Fluctuating light, experienced by all field grown crops, is known to reduce photosynthetic efficiency. This experiment aimed to evaluate the photosynthetic performance of both C 3 and C 4 bioenergy grass species under steady state and fluctuating light conditions by examining leaf gas exchange. The fluctuating light regime used here decreased carbon assimilation across all species when compared to expected steady state values. Overall, C 4 species assimilated more carbon than C 3 species during the fluctuating light regime, with both photosynthetic types assimilating about 16% less carbon than expected based on steady state measurements. Little diversity was observed in response to fluctuating light among C 3 species, and photorespiration partially contributed to the rapid decreases in net photosynthetic rates during high to low light transitions. In C 4 species, differences among the four NADP‐ME species were apparent. Diversity observed among C 4 species in this experiment provides evidence that photosynthetic efficiency in response to fluctuating light may be targeted to increase C 4 bioenergy grass productivity.

09 BIOMASS FUELS↗
DOE OSTI2016

GAS PHASE ABSORPTION SPECTROSCOPY OF C{sub 60}{sup +} AND C{sub 70}{sup +} IN A CRYOGENIC ION TRAP: COMPARISON WITH ASTRONOMICAL MEASUREMENTS

Recent low-temperature laboratory measurements and astronomical observations have proved that the fullerene cation C{sub 60}{sup +} is responsible for four diffuse interstellar bands (DIBs). These absorptions correspond to the strongest bands of the lowest electronic transition. The gas phase spectrum below 10 K is reported here for the full wavelength range encompassed by the electronic transition. The absorption spectrum of C{sub 70}{sup +}, with its origin band at 7959.2 A-ring , has been obtained under similar laboratory conditions. Observations made toward the reddened star HD 183143 were used in a specific search for the absorption of these fullerene cations in diffuse clouds. In the case of C{sub 60}{sup +}, one further band in the astronomical spectrum at 9348.5 A-ring is identified, increasing the total number of assigned DIBs to five. Numerous other C{sub 60}{sup +} absorptions in the laboratory spectrum are found to lie below the astronomical detection limit. Special emphasis is placed on the laboratory determination of absolute absorption cross-sections. For C{sub 60}{sup +} this directly yields a column density, N(C{sub 60}{sup +}), of 2×10{sup 13} cm{sup −2} in diffuse clouds, without the need to rely on theoretical oscillator strengths. The intensity of the C{sub 70}{sup +} electronic transition in the range 7000–8000 Å is spread over many features of similar strength. Absorption cross-section measurements indicate that even for a similar column density, the individual absorption bands of C{sub 70}{sup +} will be too weak to be detected in the astronomical spectra, which is confirmed giving an upper limit of 2 m A-ring to the equivalent width.

79 ASTRONOMY AND ASTROPHYSICS↗