LB Sites Research Articles

Frustrated Lewis Pairs on Zr Single Atoms Supported N‐Doped TiO2‐x Catalysts for Electrochemical Nitrate Reduction To Ammonia

AbstractThe electrochemical reduction of nitrates (NO3RR) for ammonia synthesis at room temperature holds immense potential. One key challenge is the adsorption and activation of NO3−, along with the provision of sufficient active hydrogen to accelerate the hydrogenation process. Here, the study prepares N‐doped TiO2‐x supported by Zr single atoms (Zr‐TiON) with rich oxygen vacancies (Ov), in which unsaturated Zr (Lewis acidic, LA) sites together with oxygen atoms around Ov (Lewis base, LB) form frustrated Lewis acid‐base pairs (FLPs). At −60 mA cm−2, NH3 Faradaic efficiency reaches 94.8%, corresponding to the production rate of 663.15 µmol h−1 mgcat−1. The yield rate is up to 26.16 mmol h−1mgcat−1 at −1 A cm−2 in flowing electrolyzer. Theoretical calculations and in situ spectroscopy analysis reveal that the interaction between LA and LB sites in FLPs plays a crucial role in facilitating adsorption and activation of electron‐rich NO3− and electron‐deficient *H. The presence of enhanced FLPs significantly reduces the energy barrier for H2O dissociation, lowering it to 0.20 eV, which facilitates subsequent hydrogenation reactions. The abundance of *H accelerates hydrogenation process, thereby enhancing the activity of NO3RR. This FLP design offers a promising approach for paving the way for the development of highly efficient NO3RR catalysts.

High-density frustrated Lewis pairs based on Lamellar Nb2O5 for photocatalytic non-oxidative methane coupling

Photocatalytic methane conversion requires a strong polarization environment composed of abundant activation sites with the robust stretching ability for C-H scissoring. High-density frustrated Lewis pairs consisting of low-valence Lewis acid Nb and Lewis base Nb-OH are fabricated on lamellar Nb2O5 through a thermal-reduction promoted phase-transition process. Benefitting from the planar atomic arrangement of lamellar Nb2O5, the frustrated Lewis pairs sites are highly exposed and accessible to reactants, which results in a superior methane conversion rate of 1456 μmol g−1 h−1 for photocatalytic non-oxidative methane coupling without the assistance of noble metals. The time-dependent DFT calculation demonstrates the photo-induced electron transfer from LA to LB sites enhances their intensities in a concerted way, promoting the C-H cleavage through the coupling of LA and LB. This work provides in-depth insight into designing and constructing a polarization micro-environment for photocatalytic C-H activation of methane without the assistance of noble metals.

Various Activities above Sunspot Light Bridges in IRIS Observations: Classification and Comparison

Abstract Light bridges (LBs) are among the most striking substructures in sunspots, where various activities have been revealed by recent high-resolution observations from the Interface Region Imaging Spectrograph (IRIS). Based on the variety of their physical properties, we classified these activities into four distinct categories: transient brightening (TB), intermittent jet (IJ), type-I light wall (LW-I), and type-II light wall (LW-II). In IRIS 1400/1330 Å observations, TBs are characterized by abrupt emission enhancements, and IJs appear as collimated plasma ejections with a width of 1–2 Mm at some LB sites. Most observed TBs are associated with IJs and show superpositions of some chromosphere absorption lines on enhanced and broadened wings of C ii and Si iv lines, which could be driven by intermittent magnetic reconnection in the lower atmosphere. LW-I and LW-II are wall-shaped structures with bright fronts above the whole LB. An LW-I has a continuous oscillating front with a typical height of several Mm and an almost stationary period of 4–5 minutes. On the contrary, an LW-II has an indented front with a height of over 10 Mm, which has no stable period and is accompanied by recurrent TBs in the entire LB. These results support that LW-IIs are driven by frequent reconnection occurring along the entire LB due to large-scale magnetic flux emergence or intrusion, rather than the leakage of waves producing LW-Is. Our observations reveal a highly dynamical scenario of activities above LBs driven by different basic physical processes, including magnetoconvection, magnetic reconnection, and wave leakage.

Adsorption mechanism of H2S and CH3SH on Fe(110) surface: A density functional theory study

Based on the first-principles method of density functional theory (DFT), the adsorption properties, bond strength and electronic structure of two sulfur-containing compounds (H 2 S and CH 3 SH) in oil and natural gas were analyzed by means of adsorption energy, partial density of states (PDOS) and charge difference density. The adsorption energy of H 2 S and CH 3 SH molecules on the LB site of Fe (110) surface is the lowest. Their adsorption energies are −2.88 eV and −3.13 eV, respectively. When the H 2 S symmetry axis is perpendicular to the surface, the S–H bond is easy to break. The generated S atom is firmly adsorbed on the hollow site. The results of partial density of states show that the hybrid conjugation peaks of the three adsorbed species appear on the substrate surface, and new bonds are formed between the surface molecules and the iron substrate. The charge around the three adsorbates is redistributed, and the order of interaction size between these molecules and the Fe (110) surface is S > CH 3 SH > H 2 S. This also shows that in oil and gas, H 2 S, CH 3 SH and S atoms generated after dissociation are very easy to adsorb on the surface of iron pipes, which directly affects the accuracy of the determination of sulfide gas in oil and natural gas. • The adsorption of H 2 S and CH 3 SH on four highly symmetric sites of Fe (110) surface is calculated. • The adsorption configurations of hydrogen sulfide and methyl mercaptan on Fe (110) surface were constructed. • The adsorption structure, adsorption energy, bonding and the change of electronic structure were discussed in detail. • The H 2 S molecules adsorbed horizontally are easy to dissociate.

Diurnal and seasonal variations in carbon fluxes in bamboo forests during the growing season in Zhejiang province, China

Bamboo forest is an important forest type in subtropical China and is characterized by fast growth and high carbon sequestration capacity. However, the dynamics of carbon fluxes during the fast growing period of bamboo shoots and their correlation with environment factors are poorly understood. We measured carbon dioxide exchange and climate variables using open-path eddy covariance methods during the 2011 growing season in a Moso bamboo forest (MB, Phyllostchys edulis) and a Lei bamboo forest (LB, Phyllostachys violascens) in Zhejiang province, China. The bamboo forests were carbon sinks during the growing season. The minimum diurnal net ecosystem exchange (NEE) at MB and LB sites were − 0.64 and − 0.66 mg C m−2 s−1, respectively. The minimum monthly NEE, ecosystem respiration (RE), and gross ecosystem exchange (GEE) were − 99.3 ± 4.03, 76.2 ± 2.46, and − 191.5 ± 4.98 g C m−2 month−1, respectively, at MB site, compared with − 31.8 ± 3.44, 70.4 ± 1.41, and − 157.9 ± 4.86 g C m−2 month−1, respectively, at LB site. Maximum RE was 92.1 ± 1.32 g C m−2 month−1 at MB site and 151.0 ± 2.38 g C m−2 month−1 at LB site. Key control factors varied by month during the growing season, but across the whole growing season, NEE and GEE at both sites showed similar trends in sensitivities to photosynthetic active radiation and vapor pressure deficit, and air temperature had the strongest correlation with RE at both sites. Carbon fluxes at LB site were more sensitive to soil water content compared to those at MB site. Both on-year (years when many new shoots are produced) and off-year (years when none or few new shoots are produced) should be studied in bamboo forests to better understand their role in global carbon cycling.

Marker-free PLRV resistant potato mediated by Cre-loxP excision and RNAi.

An inverted repeat construct corresponding to a segment of the potato leaf roll virus coat protein gene was created under control of a constitutive promoter and transferred into a transformation vector with a heat inducible Cre-loxP system to excise the nptII antibiotic resistance marker gene. Fifty-eight transgenic events were evaluated for resistance to PLRV by greenhouse inoculations, which lead to the identification of 7 highly resistant events, of which 4 were extremely resistant. This resistance was also highly effective against accumulation in subsequent tuber generations from inoculated plants, which has not been reported before. Northern blot analysis showed correlation of PLRV specific siRNA accumulation with the level of PLRV resistance. Heat mediated excision of the nptII antibiotic resistance gene in PLRV resistant events was highly efficient in one event with full excision in 71 % of treated explants. On the other hand 8 out of 10 analyzed events showed truncated T-DNA insertions lacking one of the two loxP sites as determined by PCR and confirmed by sequencing flanking regions in 2 events, suggesting cryptic LB sites in the non-coding region between the nptII gene and the flanking loxP site. Accordingly, it is proposed to modify the Cre-loxP vector by reducing the 1 kb size of the region between nptII, loxP, and the LB.Electronic supplementary materialThe online version of this article (doi:10.1007/s11248-016-9976-y) contains supplementary material, which is available to authorized users.

Kinetic Monte Carlo simulation of hydrogen diffusion on tungsten reconstructed (0 0 1) surface

A kinetic Monte Carlo (KMC) algorithm has been developed to study the hydrogen diffusion on tungsten reconstructed (001) surface in the temperature range 220–300K. The hydrogen diffusion coefficients predicted by the developed KMC code match the experimental values very well at low hydrogen coverage of a fraction of monolayer. A diffusion coefficient formula as a function of temperature and hydrogen coverage was derived from KMC simulations. Due to the very low probability of hydrogen occupying the long bridge adsorption sites, the rates of hydrogen atom having 3 or 4 neighbors are found to be zero for hydrogen coverage much less than a monolayer, while the rates of hydrogen atom having 0–2 neighbors are linear with respect to the hydrogen coverage. The calculated average rates of hydrogen located at the LB sites are very close to zero for low hydrogen coverage. Hydrogen only starts to occupy the LB sites after almost all SB sites are occupied.

Adsorption and Vibration of O Atoms on Fe Low-index and Fe(211) High-index Surfaces

The 5-parameter Morse potential (5-MP) of the interaction between oxygen atoms and iron surfaces was constructed. The adsorption and diffusion of O atoms on Fe low-index and Fe(211) high-index surfaces were investigated by using 5-MP. All the critical characteristics of the system, such as adsorption site, adsorption geometry, binding energy, and eigenvalues for vibration, were calculated. The calculation results show that O atoms are located at the fourfold hollow site of the Fe(100) surface with an eigenvibration at 437 cm −1 . These results are in good agreement with the experimental and theoretical results obtained previously. With regard to the adsorption site of O—Fe(110) system, the authors of this study assume that the preferential adsorption state is the H 3 site and not the LB site, which is not in agreement with the experimental inferences obtained earlier. However, on the Fe(111) and Fe(211) surfaces, O atoms predominantly occupy the quasi-3-fold site.

Soil nutrient status after bamboo flowering and death in a seasonal tropical forest in western Thailand

AbstractWe have examined the surface (0–10 cm) soil characteristics of sites after bamboo (Cephalostachyum pergracile) mass flowering and death (DB sites) in comparison with sites with living bamboo (Bambusa tulda) (LB sites) in a seasonal tropical forest in Thailand. One year after bamboo flowering the DB sites were acidic with lower concentrations of exchangeable Ca and Mg and soil nitrogen than the LB sites. Therefore, although leaf and root litter of the dead bamboo was deposited in the DB sites after bamboo flowering, soil nutrient status decreased.

Theoretical study on adsorption and diffusion of N atoms on Cu low‐index surface

AbstractThe adsorption and diffusion of N atoms on the three low‐index Cu planes were studied using 5‐parameter Morse potential (5‐MP) method, and the best theory‐experiment agreement was obtained. N atom of Cu(100) surface sit on the fourfold hollow site with the vertical height of 0.018 nm closely coplanar with the topmost copper layer, and the four CuN bond lengths are 0.182 nm and the fifth CuN distance is 0.199 nm. For Cu(111) system, the existence of aberrant Cu(100) reconstructed structure is approved at higher coverage, and at low coverage the structure is almost an ideal Cu(111) surface structure. With respect to Cu(110) system, the N atoms are adsorbed at LB and H3 sites, not at SB site. The diffusion passage and diffusion barrier of adsorbed N atoms were also studied.

Benthic sulfate reduction along the Chesapeake Bay central channel. II. Temporal controls

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 260:55-70 (2003) - doi:10.3354/meps260055 Benthic sulfate reduction along the Chesapeake Bay central channel. II. Temporal controls M. C. Marvin-DiPasquale1,*, W. R. Boynton2, D. G. Capone3 1Water Resource Division, US Geological Survey, MS 480, 345 Middlefield Road, Menlo Park, California 94025, USA 2Chesapeake Biological Laboratory, Center for Environmental Science, University of Maryland, 1 Williams St., Solomons, Maryland 20688, USA 3Department of Biological Sciences, University of Southern California, AHF107, Los Angeles, California 90089-0371, USA *Email: mmarvin@usgs.gov ABSTRACT: Seasonal and interannual controls of benthic sulfate reduction (SR) were examined at 3 sites (upper [UB], mid- [MB] and lower [LB] bay) along the Chesapeake Bay central channel, from early spring through fall, for 6 yr (1989 to 1994). The combined influences of temperature, sulfate, organic loading and bioturbation affected seasonal SR rates differently in the 3 regions. Consistently low SR rates at UB resulted from low overlying-water sulfate concentrations and the dominance of refractory organic terrestrial material. Combined seasonal variation in temperature and sulfate accounted for 50% of the annual variability in 0 to 2 cm depth interval SR rates, while sediment organic content had no significant seasonal influence. In contrast, MB and LB sites had high rates of SR fostered by high levels of overlying water SO42- and organic input dominated by labile phytoplankton detritus. New organic loading (measured as chl a) stimulated 0 to 2 cm SR during spring at both sites. Combined organic quantity (as particulate C and/or N) and temperature accounted for >75% of the variability in 0 to 2 cm SR at MB during spring and fall. Molecular diffusion supplied 25 to 45% of the SO42- needed to fuel 0 to 12 cm depth interval SR at MB, with the balance presumably supplied by S-recycling. Interannual differences in summertime SR rates were linked to the extent of freshwater flow during spring, with high-flow years associated with high SR rates at UB and MB, and low rates at LB. The negative trend between benthic SR and river flow at LB may result from the up-estuary transport of senescing organic matter in bottom water, which increases in the lower reach of the estuary with increasing freshwater inflow. KEY WORDS: Sulfate reduction · Benthic metabolism · Sediment · Estuary · Chesapeake Bay Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 260. Online publication date: September 30, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

DFT Calculation Analysis of the Infrared Spectra of Ethylene Adsorbed on Cu(110), Pd(110), and Ag(110)

A density functional theory (DFT) calculation was performed on the cluster models of ethylene on Cu(110), Ag(110), and Pd(110) to clarify the correlation between the IR spectra of the adsorbate and the modes of ethylene−surface interaction. The metal surfaces were modeled by two- or three-layered clusters consisting of 13−34 metal atoms. Four kinds of adsorption sites were considered: atop bonding sites with the CC bond parallel and perpendicular to the 〈11̄0〉 direction (ST and LT sites), a short bridge site with the CC bond parallel to the 〈11̄0〉 direction (SB site), and a long bridge site with the CC bond perpendicular to the 〈11̄0〉 direction (LB site). The results of calculations for three-layered models consisting of more than 20 metals could be compared reasonably with the experimental data. The comparison indicated that (i) upon increasing surface coverage, ethylene on Cu(110) converts its adsorption site from an SB to an ST site, (ii) ethylene adsorbs at an LT site of Ag(110), and (iii) ethylene on Pd(110) takes on an ST site. These conclusions are consistent with those derived from STM and other spectroscopic measurements including UPS and NEXAFS, indicating that the DFT calculation on the cluster models is efficient for the analysis of the IR spectra of ethylene adsorbed on metal surfaces, which delineates the adsorption modes. The contribution of donation and back-donation of electrons to the ethylene−metal bonding was estimated by calculating the projections to the π-bonding and π*-antibonding orbitals of the isolated ethylene in the adsorbed geometries. The results proved that both the π donation and π* back-donation make appreciable contributions to the ethylene−surface interaction on Cu(110), whereas the π* back-donation is negligible in the ethylene−Ag(110) interaction. It was suggested that the frequency increase of the CH2 out-of-plane wagging vibration from that of the free ethylene observed for ethylene on Ag(110) is a measure of the contribution of the π donation to the ethylene−surface interaction.

Sulfur cycling in wetland peat of the New Jersey Pinelands and its effect on stream water chemistry

The dynamics of sulfur cycling in wetland peat along an elevational transect at high, intermediate, and low locations (MS, NW, and LB sites, respectively) was investigated in the watershed of McDonalds Branch within the New Jersey Pinelands, by utilizing both soil incubation experiments (with 35SO 4 2− as a radiotracer) and stable isotope (δ 34S and δ 18O) analyses of soil, rain, and streamwater. The results indicate that sulfur cycling can vary greatly among different portions of the watershed and this can have large effects on streamwater chemistry and δ 34S values over distances as short as 1 km. Laboratory incubations of peat samples collected in July 1993 revealed the co-occurrence of dissimilatory sulfate reduction (DSR; rates ranging from 0.2 to 22.1 nmol/wet g/day) and net generation of sulfate (NaH 2PO 4 extractable) in the porewater. Generation of sulfate, which was most pronounced at the LB site, may involve oxidation of reduced sulfur in the peat and/or hydrolysis of ester sulfates (ES). Seasonal changes in streamwater SO 4 2−/Cl − molar ratios were similar at LB and NW, being low during the summer and high in the winter, probably a result of higher rates of DSR within the peat during the summer. Consistent with this, higher δ 34S values of sulfate in streamwater at NW during the summer are attributable to kinetic isotope effects associated with DSR. In contrast to NW, δ 34S values of streamwater sulfate at LB were consistently lower, fluctuated little throughout the year, and were most negative during the summer (as much as 9‰ lower than streamwater at NW 1 km upstream). In comparison to NW, SO 4 2−/Cl − ratios were lower in streamwater at LB throughout most of the year except for reversals during the summer, which coincided with the lowest δ 34S values. In addition, there was a marked difference in the relationship of δ 18O vs. δ 34S of sulfate in LB and NW streamwater, further suggesting that sulfur cycling varies greatly over relatively short distances within this watershed. In order to explain some of these site-specific differences in streamwater SO 4 2−/Cl − ratios, δ 34S, and δ 18O values, we hypothesize that the ES pool at LB may, by means of hydrolysis or isotopic exchange with streamwater sulfate, serve as an additional source of isotopically light sulfate to streamwater throughout most of the year. During the summer, drier conditions lower the water table at LB and enhance oxidation of reduced sulfur which releases a pulse of even isotopically lighter sulfate to the stream.

Chemisorption of O and O2 on Ag(110): An LCGTO‐LSD cluster study

AbstractAs part of a study aimed at better understanding of molecular and dissociative chemisorption of oxygen on Ag(110), linear combinations of Gaussian type orbitals‐local spin density (LCGTO‐LSD) calculations have been performed for O, O−, O2, O−2, O2−2 and a variety of silver clusters interacting with O or O2.For atomic O adsorption a very small cluster, Ag4, chosen to model the long‐bridge site already affords very good agreement with both recent EXAFS experiments and recent ab initio calculations. We calculate O to be 0.25 Å above the surface (exp. 0.2 Å). The Ag4O vibrational frequency is estimated to be 400 cm−1, in reasonable accord with the experimental EELS value of 325 cm−1.Determination of the geometry for O2 (ads.) and, ultimately, of the dissociation path are far more difficult tasks. An extensive search for local minima in the vicinity of the LB site is being carried out. Results to date for small, Ag2 and Ag4, clusters have furnished insight into the factors influencing the structure. Overlap between the π* orbital of the O2 moiety and Ag s functions is a key factor; that is, there is an important covalent component of the binding. For geometries with O2 parallel to the surface, this is achieved by twisting the O2 fragment with respect to the [11¯0] grooves (geometries either parallel or perpendicular to the grooves yield zero π‖*−s overlap by symmetry). The structure with O2 perpendicular to the surface also achieves reasonable overlap and lies close in energy to the most stable ‘parallel’ geometry.