排序方式: 共有9条查询结果,搜索用时 265 毫秒
1
1.
Merte LR Peng G Bechstein R Rieboldt F Farberow CA Grabow LC Kudernatsch W Wendt S Lægsgaard E Mavrikakis M Besenbacher F 《Science (New York, N.Y.)》2012,336(6083):889-893
The diffusion of hydrogen atoms across solid oxide surfaces is often assumed to be accelerated by the presence of water molecules. Here we present a high-resolution, high-speed scanning tunneling microscopy (STM) study of the diffusion of H atoms on an FeO thin film. STM movies directly reveal a water-mediated hydrogen diffusion mechanism on the oxide surface at temperatures between 100 and 300 kelvin. Density functional theory calculations and isotope-exchange experiments confirm the STM observations, and a proton-transfer mechanism that proceeds via an H(3)O(+)-like transition state is revealed. This mechanism differs from that observed previously for rutile TiO(2)(110), where water dissociation is a key step in proton diffusion. 相似文献
2.
Zhai Y Pierre D Si R Deng W Ferrin P Nilekar AU Peng G Herron JA Bell DC Saltsburg H Mavrikakis M Flytzani-Stephanopoulos M 《Science (New York, N.Y.)》2010,329(5999):1633-1636
We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H(2)O + CO → H(2) + CO(2)) used for producing H(2). The alkali ion-associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-O(x)(OH)(y) species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria. 相似文献
3.
4.
Background Biocommunities of phototrophic microorganisms (principally cyanobacteria) developing on the surface sediments of shallow,
warm and semi-isolated coastal environments, bind or trap small particles on their polysaccharide sheaths creating organosedimentary
structures. Those structures are called algal mats and they are commonly flat and laminar in shape. They are also called recent
stromatolites due to their remarkable similarity to the fossil ones.
Objectives Not many geochemical studies exist concerning cyanobacterial mats in Mediterranean coasts, in comparison with the biological
ones. Considering the importance of nitrogen and phosphorus in the cyanobacteria mat development, the present study aims to
contribute to the knowledge of their chemical behaviour in such environments.
Methods Sediment cores of about 10 cm length along with water samples were collected during the course of a year from two Greek coastal
areas. Sediments were fractionated in three distinct layers according to their structure. A wet oxidation method for the simultaneous
determination of total nitrogen and total phosphorus was applied to all sediment samples. The total organic carbon (TOC),
the total inorganic phosphorus and the natural pigment content in the sediment samples were also determined. Standard spectrophotometric
methods were used for the determination of the concentration of dissolved nutrients. In situ measurements of pH, temperature
and salinity were also carried out.
Results and Discussion The sampling areas were found to be in the mesotrophic level. The group of aerobic filamentous cyanobacteria dominated the
sediment biocommunities. The pH of the sediment showed a gradual reduction downwards. The highest content of TOC, total nitrogen
and total phosphorus were detected in the surface layer of the sediment samples. The contribution of organic phosphorus to
the total phosphorous ranged between 0% and 50% with an average value of 26.6%. A depletion of organic phosphorus in the anoxic
middle layer of the sediment was detected. A good positive correlation between total nitrogen and chlorophyll-a, as well as
total nitrogen and TOC in the surface sediment layer, was observed. A good negative correlation between total phosphorous
in sediments and dissolved phosphates in seawater was also observed.
Conclusions The high content of total nitrogen recorded in the surface sediments indicate that cyanobacteria accumulate nitrogen acquiring
it from the sediment, the marine environment and the air (in the form of N2). A proportional accumulation in phosphorus also
happens, although to a lesser degree. The temporal fluctuation of nitrogen content in the sediment's surface layer agrees
with the temporal fluctuation of primary production. The cyanobacterial mat acts as a natural pump that transfers phosphorus
from seawater to the surface of the sediment.
Recommendations and Outlook A further, more extensive, multidisciplinary study of cyanobacterial mats is needed for the determination of the biogeochemical
processes that take place there and for the effective environmental management of the areas where these forms develop. 相似文献
5.
Xiaomei Wei Tim S. Bugni Mary Kay Harper Imelda T. Sandoval Elizabeth J. Manos Jennifer Swift Ryan M. Van Wagoner David A. Jones Chris M. Ireland 《Marine drugs》2010,8(6):1769-1778
Three new minor components, the pyridoacridine alkaloids 1-hydroxy-deoxyamphimedine (1), 3-hydroxy-deoxyamphimedine (2), debromopetrosamine (3), and three known compounds, amphimedine (4), neoamphimedine (5) and deoxyamphimedine (6), have been isolated from the sponge Xestospongia cf. carbonaria, collected in Palau. Structures were assigned on the basis of extensive 1D and 2D NMR studies as well as analysis by HRESIMS. Compounds 1–6 were evaluated in a zebrafish phenotype-based assay. Amphimedine (4) was the only compound that caused a phenotype in zebrafish embryos at 30 μM. No phenotype other than death was observed for compounds 1–3, 5, 6. 相似文献
6.
Michael Scoullos Manos Dassenakis Christina Zeri 《Water, air, and soil pollution》1996,88(3-4):269-295
Trace metals (Al, Pb, Zn, Cr, Cu, Ni, Fe, Mn) were studied in waters (dissolved and particulate phases) and sediments of the Louros Estuary in the Amvrakikos Gulf, one of the most important European wetlands located at the NW coast of Greece. The study system is small, with a relatively narrow mixing zone, typical for Mediterranean estuaries. Particular emphasis was given to understanding the conditions prevailing in summer. During this season saline water intrudes the estuary along the river bed, despite the existing shallow sill, and forms a thin salt-wedge water mass, which occupies the near bottom layer with its thin end pointed upstream. Particulate metal concentrations within this saline bottom layer are considerably higher than in the riverine and marine sections of the estuary. Since the metal content of particles collected upstream is higher than that of the marine ones, there is a clear evidence that the salt-wedge acts as a ‘sink’ for most metals during the summer. Coexistance in the same zone of high dissolved metal concentrations indicate that loosely associated metals are desorbed from riverine particles, whereas newly formed suspended matter is deposited together with particles, transported by the river. The accumulation of metals in the near bottom layer affects directly their distribution in the sediments. The maximum concentrations of the metal fraction which is loosely held in sediments, are found primarily at the same site. The distribution of the ‘non-labile’ metal fraction of the sediments (particularly for Cu and Pb) is broadly constant throughout the estuary, confirming the absence of any significant natural or industrial point sources, at the lower part of the river. However, the analysis of sediment cores reveals an enrichment of this metal fraction at the top, near surface sections of the mouth area, indicating relatively recent pollution. 相似文献
7.
Gel permeation chromatography was used for monitoring the changes in humic molecular weight distribution after contact with kaolin clay and activated carbon. The high molecular weight fractions excluded from Sephadex G-50 were less removed by carbon indicating the exclusion effect of pores, but were preferentially adsorbed by clay. The aliphatic portion of humic acids is effectively removed by carbon as suggested by the decrease in E4: E6 ratio. The Freundlich and the Langmuir isotherms showed curvilinearity which was resolved into two straight lines. The sorption effect in humic molecular weight distribution suggests the necessity of batch adsorption test with respect to the treated humic material. 相似文献
8.
Humic and fulvic acids extracted from alkali treated municipal sludge contain more higher molecular weight materials than from secondary wastewater effluents. This suggests that higher molecular weight materials (greater than 30 000)' are more easily entrapped on the proteinaceous surface of the biomass. About 28% of the sludge COD is humic material. The humic acid fraction is of a higher molecular weight than the fulvic acid fraction. The value of theE 4:E 6 ratio indicates that humic materials of raw sludges are of recent origin. 相似文献
9.
1