Adsorption dynamics of O2 on Cu(111): a supersonic molecular beam study.

We have studied the adsorption of O2 on Cu(111) using supersonic molecular beam techniques. For incident energies ranging between 100 and 400 meV, we have determined the sticking probability as a function of angle of incidence, surface temperature, and coverage. Initial sticking probabilities range from near 0 to 0.85 with an onset near 100 meV, making Cu(111) considerably less reactive than Cu(110) and Cu(100). Normal energy scaling applies and reactivity increases appreciably over the entire range of surface temperatures from 90 to 670 K. A strictly linearly decreasing coverage dependence on sticking precludes adsorption and dissociation via an extrinsic or long-lived mobile precursor state. We cannot exclude that sticking also occurs molecularly at the lowest surface temperatures. However, all tell tales from our experiments suggest that sticking is predominantly direct and dissociative. Comparison to earlier data shows implications for the relative reactivity of Cu(111) vs. Cu/Ru(0001) overlayers.

Characterization of CO Adsorbed to Clean and Partially Oxidized Cu(211) and Cu(111).

Copper-based catalysts gain activity through the presence of poorly coordinated Cu atoms and incomplete oxidation at the surface. The catalytic mechanisms can in principle be observed by controlled dosing of reactants to single-crystal substrates. However, the interconnected influences of surface defects, partial oxidation, and adsorbate coverage present a large matrix of conditions that have not been fully explored in the literature. We recently characterized oxygen and carbon monoxide coadsorption on Cu(111), a nominally defect-free surface, and now extend our study to the stepped surface Cu(211). Temperature-programmed desorption of CO adsorbed to bare metal surfaces confirms that two sites dominate desorption from a saturated layer: atop terrace atoms of local (111) character and atop step edge atoms with CO bound more strongly to the latter. At low coverage, discrete CO resonances in reflection adsorption infrared spectra can be assigned to these sites: 2077 cm-1 for extended (111) terraces, 2093 cm-1 for step sites, and additional kink-adsorbed molecules at 2110 cm-1. With increasing coverage, in contrast to Cu(111), the infrared spectral features on Cu(211) evolve and shift as a consequence of dipole-dipole coupling between differentially occupied types of sites. Auger electron spectroscopy shows that exposure to background O2 oxidizes the (211) surface at a rate nearly 1 order of magnitude greater than (111); we argue that the resulting surface is stoichiometric Cu2O, as previously found for Cu(111). This oxide binds CO less strongly than the bare metal and the underlying crystal cut continues to influence the adsorption sites available to CO. On oxidized (111) terraces, broad absorption peaks at 2115-2120 cm-1; on oxidized Cu(211), CO adsorbed to step sites appears as a resolved secondary peak at 2144 cm-1. This suite of spectroscopic signatures, obtained under carefully controlled conditions, will help to determine the origin and fate of adsorbed species in future studies of reaction mechanisms on copper.

RAIRS Characterization of CO and O Coadsorption on Cu(111).

In a study preliminary to investigating CO2 dissociation, we report our results on oxygen and carbon monoxide coadsorption on Cu(111). We use reflection adsorption infrared spectroscopy and Auger electron spectroscopy to characterize and quantify adsorbed species. On clean Cu(111), the CO internal stretch mode appears initially at 2077 cm-1 for a surface temperature of ∼80 K. We accurately reproduce the previously determined redshift of the absorption band with increasing CO coverage. We subsequently oxidize the surface by exposure to O2 at 300 K to ensure O2 dissociation. The band's frequency and line shape of subsequently adsorbed CO at ∼80 K are not affected. However, the maximum absorbance and integrated peak intensities drop with increasing O coverage. The data suggest that CO is not adsorbed near O, likely as a consequence of the mechanism of Cu(111) surface oxidation by O2 at 300 K. We discuss whether our RAIRS results may be used to quantify CO2 dissociation in the zero-coverage limit.

Mobilization of soil-bound residue of organochlorine pesticides and polycyclic aromatic hydrocarbons in an in vitro gastrointestinal model.

A previous study on mobilization of organochlorine pesticides (OCPs) in contaminated soils from the field revealed that the total amount of OCPs measured in digestive fluid and chyme of an in vitro gastrointestinal model was higher than the quantity directly extracted using a solvent extraction without digestion, providing a clue that the bound residue of OCPs might be mobilized. This hypothesis was tested in this study for both OCPs and polycyclic aromatic hydrocarbons (PAHs). Three contaminated surface soil samples with different organic carbon (OC) contents were collected from the field, and extracted with a solvent with and without digestion in an in vitro gastrointestinal model. It was found that bound residues of OCPs and PAHs were mobilized to a certain extent during digestion. The ratios of the mobilized bound residues over the total quantities extracted after digestion (R(b)) varied from 0 to 0.96 for individual compounds. The R(b) was positively correlated with OC content. Among the five constitutes of digestive juice, bile salt was the only one that served to mobilize the bound residues and the extractability of bile salt was constant over a concentration range from 2 to 20 mg/mL. The mobilization process followed typical first-order kinetics. The calculated rate constants suggest that mobilization was fast and 90% of extracted bound residues of OCPs and PAHs were mobilized within 2.4 and 4.8 h, respectively.

Enantioselective behavior of alpha-HCH in mouse and quail tissues.

alpha-HCH (hexachlorocyclohexane) is chiral and can still be detected in almost all environmental media. In this study, the enantioselective behavior of alpha-HCH in mice (CD1) and quail (Coturnix japonica) was investigated and compared after a single dose of exposure. The primary nerve cell culture was conducted to evaluate the enantioselective metabolic capacity of nerve cells of mouse and quail for alpha-HCH. In various tissues of the mice and quail, the alpha-HCH concentrations showed a typical pattern of first-order dynamics after exposure. The enantiomeric fractions (EFs) in nonbrain tissues of mice decreased substantially, indicating continuous depletion of (+)-alpha-HCH in mice. Tissue-specific EF trends in quail and enantioselective degradation of (-)-alpha-HCH in quail liver were observed. These observations indicated that the dynamic changes of EFs in mice and quail were independent of concentration changes in the same tissues. In brain tissues, the enantioenrichment of (+)-enantiomer was totally independent of their concentrations in blood. The in vitro metabolism of alpha-HCH in the primary nerve cells were negligible, and the slight EF changes in primary nerve cells demonstrated that metabolism, uptake, and excretion in the brain cells would not lead to the observed dramatic enantioenrichment of (+)-alpha-HCH in the brain tissues of the two animals. The enantioselective transport across the blood-brain barrier was the primary cause for the enantioenrichment of (+)-alpha-HCH in the brain tissues.

Dynamic changes of α-hexachlorocyclohexane and its enantiomers in various tissues of Japanese Rabbits (Oyctolagus cuniculus) after oral or dermal exposure.

Japanese Rabbits (Oyctolagus cuniculus) were exposed to α-hexachlorocyclohexane (α-HCH) either orally or dermally and concentrations of α-HCH and its two enantiomers were measured at different time intervals in the blood, intestine, liver, kidney, fat, brain, and muscle. The time trends were quantified using toxicokinetic models. It was found that absorption and elimination of α-HCH in blood were first-order processes which can be characterized by a single compartmental kinetic model. The absorption of dermally exposed α-HCH in blood was more than one order of magnitude faster than that of orally exposed α-HCH. The transport of α-HCH from the blood to other tissues was characterized using a two-compartment model. The accumulation rates were different among tissues depending on blood flow rate and fat content. Significant correlation was revealed between α-HCH concentration and fat content for various tissues either before or 1 d after the exposure. However, there was no such correlation at 10 min immediately after the exposure. The enantiomeric fraction (EF) of α-HCH in rabbit blood was nearly racemic before the exposure and increased to 0.73 and 0.82 after oral or dermal exposure, respectively. The result of a toxicokinetic modeling suggested that the strong enantioenrichment of (+)-α-HCH was primarily because the elimination rate of (-)-α-HCH was more than two times higher than that of (+)-α-HCH. The EFs for other tissues also increased dramatically after the exposure and the EFs in brain reached as high as 0.99 due to enantioselective transport across the blood brain barrier.

Mobility of polycyclic aromatic hydrocarbons in the gastrointestinal tract assessed using an in vitro digestion model with sorption rectification.

In a previous study, it was demonstrated that mobilization of organochlorine pesticides would be underestimated by an in vitro gastrointestinal model if the sorption of the mobilized pollutants on the digestive residue was not taken into consideration. A multiple fluid/solid ratio procedure was developed to characterize the sorption. In this study, the sorption hypothesis was further tested for polycyclic aromatic hydrocarbons (PAHs), and the sorption of the mobilized PAHs on digestive residue was directly characterized by spiking the gastrointestinal digest with several deuterated PAHs. It was demonstrated that 10-41% of the spiked deuterated PAHs were sorbed on the assimilated residue, which would remain in the solid phase after separation. It appears that the mobility of PAHs would be underestimated if only those dissolved in the fluid is measured. The total mobilized fraction of a PAH compound was defined as a sum of that dissolved in the fluid and that sorbed on the residue. The average mobilized PAH fractions in the studied soils was 70 +/- 24% which was significantly higher than 47 +/- 19% in the fluid. It was also found that the sorption of the mobilized PAHs on the digestive residue was positively correlated with both soil organic carbon (SOC) and molecular weight (MW(t)) of PAHs, and a regression model was developed so that the sorption of different PAHs on soils with different SOCs could be estimated.

Assessment of oral bioaccessibility of organochlorine pesticides in soil using an in vitro gastrointestinal model.

A static in vitro gastrointestinal model was used to investigate the oral bioaccessibility of organochlorine pesticides (OCPs) including hexachlorocyclohexane isomers (HCHs) and dichlorodiphenyltrichloroethane and metabolites in contaminated soils with different organic matter contents. A key hypothesis tested was that a fraction of the mobilized contaminants is sorbed on the solid after digestion, and this fraction could be desorbed and become bioaccessible in the intestinal tract due to absorption of the dissolved fraction by the small intestine. The bioaccessibility would be underestimated if the sorbed fraction was separated from the fluid by centrifugation or filtration in an in vitro test. In our experiment, a procedure using multiple fluid-to-solid ratios was developed to characterize the sorption. It was found that 8-38% of the mobilized OCPs were sorbed on the residue and remained in the solid phase after the separation. Taking into consideration the sorbed fraction, the measured bioaccessibilities of the OCPs varied from 4% to 97% depending on the properties of the soils and the compounds. The enantiomeric ratios of chiral compounds studied were also determined to distinguish biotic (enzymatic) and abiotic (chemical extraction) processes in the in vitro digestion model. It was found that although alpha-HCH in the raw soils was racemic, (+)-alpha-HCH was enriched in the gastric digestive fluid but depleted in the intestinal digestive fluid.