Soil microbiology

RESP-385

Switch in photocatalytic reaction selectivity: The effect of oxygen partial pressure on carbon-carbon bond dissociation over hydroxylated TiO₂(1 1 0) surfaces

Photocatalytic oxidation of ethanol over rutile TiO₂ (1 1 0) in the presence of O₂ have been studied with scanning tunneling microscopy and on-line mass spectrometry to elucidate the reaction mechanisms. The O₂ partial pressure has a direct impact on C–C bond cleavage, resulting in a shift of selectivity in gas phase products from acetaldehyde (dehydrogenation) to methyl radicals (C–C bond dissociation) with increasing pressure. This differs from the behavior of anatase TiO₂(1 0 1) single crystal, where at all investigated pressures negligible C–C bond dissociation occurs. The prevalence of the methyl radical species at high oxygen pressures is correlated with an increase in the surface population of an adsorbed species bound to Ti₅_c after the reaction, which are identified as formate moieties. Parallel XPS C1s, Ti2p and O1s further confirmed the assignment of surface population, by STM, to ethoxides at 300 K, in dark conditions (C1s at 286.7 and 285.4 eV attributed to –CHO₂–and–CH₃ groups respectively). After photoreaction, a large fraction of the surface was covered by formates (XPS C1 at 289.7 eV). This also correlated with the STM assignment where species spaced by 6 Å along the [0 0 1] direction and with a height of ca. 1.1 Å attributed to formates. Moreover the profile for CH₃ radical desorption in the gas phase as a function O₂ partial pressures correlated with the increasing surface population of formates. Analysis of the rate of methyl radical formation reveals fast and slow regimes, with photoreaction cross-sections between 10⁻¹⁷ cm² and 10⁻¹⁹ cm². The parallel channel of acetaldehyde production has a non-varying cross-section of ca. 2 × 10⁻¹⁹ cm². A schematic description of the two different reaction channels (dehydrogenation and C–C bond dissociation) is given and discussed

Fossil biomass preserved as graphitic carbon in a late paleoproterozoic banded iron formation metamorphosed at more than 550°C

Metamorphism is thought to destroy microfossils, partly through devolatilization and graphitization of biogenic organic matter. However, the extent to which there is a loss of molecular, elemental and isotope signatures from biomass during high-temperature metamorphism is not clearly established. We report on graphitic structures inside and coating apatite grains from the c. 1850 Ma Michigamme silicate banded iron formation from Michigan, metamorphosed above 550°C. Traces of N, S, O, H, Ca and Fe are preserved in this graphitic carbon and X-ray spectra show traces of aliphatic groups. Graphitic carbon has an expanded lattice around 3.6 Å, forms microscopic concentrically-layered and radiating polygonal flakes and has homogeneous δ13C values around −22‰, identical to bulk analyses. Graphitic carbon inside apatite is associated with nanometre-size ammoniated phyllosilicate. Precursors of these metamorphic minerals and graphitic carbon originated from ferruginous clayrich sediments with biomass. We conclude that graphite coatings and inclusions in apatite grains indicate fluid remobilization during amphibolite-facies metamorphism of precursor biomass. This new evidence fills in observational gaps of metamorphosed biomass into graphite and supports the existence of biosignatures in the highly metamorphosed iron formation from the Eoarchean Akilia Association, which dates from the beginning of the sedimentary rock record

Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis

Thermalized Epoxide Formation in the Atmosphere

Epoxide formation was established a decade ago as a possible reaction pathway for beta-hydroperoxy alkyl radicals in the atmosphere. This epoxide-forming pathway required excess energy to compete with O-2 addition, as the thermal reaction rate coefficient is many orders of magnitude too slow. However, recently, a thermal epoxide forming reaction was discovered in the ISOPOOH + OH oxidation pathway. Here, we computationally investigate the effect of substituents on the epoxide formation rate coefficient of a series of substituted beta-hydroperoxy alkyl radicals. We find that the thermal reaction is likely to be competitive with O-2 addition when the alkyl radical carbon has a OH group, which is able to form a hydrogen bond to a substituent on the other carbon atom in the epoxide ring being formed. Reactants fulfilling these requirements can be formed in the OH-initiated oxidation of many biogenic hydrocarbons. Further, we find that beta-OOR alkyl radicals react similarly to beta-OOH alkyl radicals, making epoxide formation a possible decomposition pathway in the oxidation of ROOR peroxides. GEOS-Chem modeling shows that the total annual production of isoprene dihydroxy hydroperoxy epoxide is 23 Tg, making it by far the most abundant C-5-tetrafunctional species from isoprene oxidation.Peer reviewe

Warm perineal compresses during the second stage of labor for reducing perineal trauma: A meta-analysis

OBJECTIVE: Perineal trauma may have a negative impact on women's lives as it has been associated with perineal pain, urinary incontinence and sexual dysfunction. The aim of this systematic review and meta-analysis of randomized controlled trials was to evaluate the effectiveness of warm compresses during the second stage of labor in reducing perineal trauma. METHODS: Electronic databases were searched from inception of each database to May 2019. Inclusion criteria were randomized trials comparing warm compresses (i.e. intervention group) with no warm compresses (i.e. control group) during the second stage of labor. Types of participants included pregnant women planning to have a spontaneous vaginal birth at term with a singleton in a cephalic presentation. The primary outcome was the incidence of intact perineum. Meta-analysis was performed using the Cochrane Collaboration methodology with results being reported as relative risk (RR) with 95% confidence interval (CI). RESULTS: Seven trials, including 2103 participants, were included in this meta-analysis. Women assigned to the intervention group received warm compresses made from clean washcloths or perineal pads immersed in warm tap water. These were held against the woman's perineum during and in between pushes in second stage. Warm compresses usually started when the baby's head began to distend the perineum or when there was active fetal descent in the second stage of labor. We found a higher rate of intact perineum in the intervention group compared to the control group (22.4% vs 15.4%; RR 1.46, 95% CI 1.22 to 1.74); a lower rate of third degree tears (1.9% vs 5.0%; RR 0.38, 95% CI 0.22 to 0.64), fourth degree tears (0.0% vs 0.9%; RR 0.11, 95% CI 0.01 to 0.86) third and fourth degree tears combined (1.9% vs 5.8%; RR 0.34, 95% CI 0.20 to 0.56) and episiotomy (10.4% vs 17.1%; RR 0.61, 95% CI 0.51 to 0.74). CONCLUSION: Warm compresses applied during the second stage of labor increase the incidence of intact perineum and lower the risk of episiotomy and severe perineal trauma

Force distributions in 3D granular assemblies: Effects of packing order and inter-particle friction

We present a systematic investigation of the distribution of normal forces at the boundaries of static packings of spheres. A new method for the efficient construction of large hexagonal-close-packed crystals is introduced and used to study the effect of spatial ordering on the distribution of forces. Under uniaxial compression we find that the form for the probability distribution of normal forces between particles does not depend strongly on crystallinity or inter-particle friction. In all cases the distribution decays exponentially at large forces and shows a plateau or possibly a small peak near the average force but does not tend to zero at small forces.Comment: 9 pages including 8 figure