CO adsorption on CoMo and NiMo sulfide catalysts: a combined IR and DFT study.

Experimental IR spectra of carbon monoxide adsorbed on a series of Mo/Al2O3, CoMo/Al2O3, and NiMo/Al2O3 sulfided catalysts have been compared to ab initio DFT calculations of CO adsorption on CoMo and NiMo model surfaces. This approach allows the main IR features of CO adsorbed on the sulfide phase to be assigned with an uncertainty of 15 cm(-1). On the CoMo system, the band at 2070 cm(-1) is specific of the promotion by Co and is assigned to CO interacting either with a Co atom or with a Mo atom adjacent to a Co atom. On the NiMo system, CO adsorption on Ni centers of the promoted phase leads to a high-wavenumber band at approximately 2120 cm(-1) that strongly overlaps the band at 2110 cm(-1) characteristic of nonpromoted Mo sites. For NiMo and CoMo catalysts, broad shoulders at low wave numbers (below 2060 cm(-1)) are characteristic of Mo centers adjacent to promoter atoms, indicating a partial decoration of the MoS2 edges by the promoter.

Ultrastructure in anterior and posterior stroma of perfused human and rabbit corneas. Relation to transparency.

PURPOSE: The authors sought to discover whether there are differences in the degree of spatial order in the fibrillar ultrastructure between anterior and posterior stroma. METHODS: Human corneas were obtained from eye bank eyes. Although they had been classified as normal, some swelling remained after 3 hours of deturgescence. Freshly excised, unswollen rabbit corneas also were used. Image analysis methods were applied to transmission electron micrographs of the anterior, middle, and posterior stroma of these corneas to determine the positions and radii of fibrils, the fraction of total area occupied by fibrils, and the fibril number density. Results were used to calculate the interference factor that appears in the direct summation of the fields for light scattering theory and to estimate the total scattering cross-section per fibril. The interference factor is a measure of the spatial order in the positions and sizes of the fibrils. RESULTS: Electron micrographs showed anterior-posterior variations in size and number density of fibrils. The interference factor at wavelengths of visible light was lower in posterior stroma than in anterior stroma for humans and rabbits. In some instances in humans, the anterior interference factor was characteristic of mildly swollen cornea. When averaged for the electron micrographs analyzed, the anterior stroma was predicted to scatter approximately twice as much light per unit depth as the posterior stroma in humans (at any given wavelength) and approximately three times as much in rabbits. CONCLUSIONS: Calculations of the interference factor showed that there were differences in the anterior-posterior spatial ordering of fibrils. In human corneas, the differences could have been caused by intrinsic in vivo differences between anterior and posterior stroma; however, possible anterior-posterior variations in swelling between the two regions in vitro also could have affected the results.

Corneal endothelial damage by air bubbles during phacoemulsification.

OBJECTIVE: To characterize the mechanism by which air bubbles damage the corneal endothelium during phacoemulsification. MATERIALS AND METHODS: A series of experiments was conducted to expose the corneal endothelium of New Zealand white rabbit and human eyes that were obtained from an eye bank to air under different conditions. Phacoemulsification at different power settings and irrigation with and without the introduction of air into the anterior chamber were performed. Corneal endothelial perfusion experiments were conducted with air bubbles that were introduced into the perfusion chamber for 2 seconds to 1 hour. Air was also injected into the anterior chambers of anesthetized rabbits for 2 minutes to 3 hours. Corneas were stained with nitrobenzo-xadiazole-phallacidin and examined with fluorescence microscopy. Selected corneas were also examined with scanning and transmission electron microscopy. RESULTS: Intracameral air bubbles during phacoemulsification, irrigation, and perfusion studies resulted in a severe injury to the corneal endothelium in as little as 20 seconds. Intracameral air bubbles in a living rabbit resulted in a slower injury that was morphologically different from the more rapid injury. CONCLUSIONS: Air bubbles in intraocular fluids with a high surface tension can cause a ring-shaped pattern of damage to the corneal endothelium. The mechanism that caused this pattern of damage appears to be a surface tension phenomenon.

Effects of laser in situ keratomileusis (LASIK) on the corneal endothelium.

PURPOSE: To assess the effects of laser in situ keratomileusis (LASIK) on the corneal endothelium. METHODS: In a prospective study, the corneal endothelium of 98 eyes of 65 consecutive patients (mean age, 41 years; range, 22 to 66 years) was photographed before, 2 weeks after, and 12 weeks after LASIK for the correction of 2.75 to 14.5 diopters of myopia. Theoretical ablation depths were 200 to 330 microm below the corneal surface. Cell density, coefficient of variation, and percent of hexagonal cells were determined using 150 to 200 cells from each image. Eighty-eight eyes (91%) of 59 patients had a history of contact lens wear. RESULTS: The mean+/-SD preoperative endothelial cell density was 2,549+/-365 cells per mm2, and the mean coefficient of variation was 0.35+/-0.06. There was no statistically significant change in the mean endothelial cell density or mean coefficient of variation of cell size at the 2-week (2,561+/-360 cells per mm2 and 0.35+/-0.06) or 12-week (2,541+/-364 cells per mm2 and 0.35+/-0.05) postoperative examinations. The percent of hexagonal cells was not significantly changed 2 weeks postoperatively; however, 12 weeks postoperatively (P=.0413, two-tailed t test), the percent of hexagonal cells was decreased by 1%. CONCLUSIONS: Corneal endothelial cell density and morphology were unchanged 2 and 12 weeks after LASIK for the correction of up to 14.5 diopters of myopia. In this LASIK study, the correction of up to 14.5 diopters of myopia appears to cause no clinically significant effect on corneal endothelial cell density or morphology.

Results of large penetrating keratoplasty in microbial keratitis.

Extensive corneal disease secondary to microbial keratitis can result in frank or impending corneal perforation requiring a large penetrating keratoplasty. In an 8-year period, 26 penetrating keratoplasties with recipient beds of > or = 9.5 mm were performed on 22 eyes: 11 for bacterial keratitis, 10 for fungal keratitis, and one for a mixed bacterial and fungal keratitis. The graft failed in 18 of 19 eyes (94.7%), with a median time to failure of 12.9 weeks in bacterial keratitis and 4.0 weeks in fungal keratitis. After large keratoplasty, 17 of 20 eyes (85.0%) maintained the structural integrity of the globe. The remainder became phthisical or required enucleation. With preservation of the structural integrity of the globe, a subsequent smaller optical penetrating keratoplasty is an option in some of these eyes.

The mucinous layer of corneal endothelial cells.

PURPOSE: The goal of this study was to characterize the morphology of the mucinous layer on rabbit, bovine, owl, and human corneal endothelial cells. METHODS: Corneoscleral buttons were fixed using cetylpyridinium chloride to stabilize "mucus" and the tissue was prepared for transmission electron microscopy. Photomicrographs were measured to determine the thickness of the endothelial and epithelial mucinous layer in the central cornea. RESULTS: The endothelial mucinous layer was seen as a nearly uniform electrodense region on the apical aspect of the endothelium. It was found to be 0.9 microm, 0.9 microm, 0.9 microm, and 0.5 microm thick in rabbit, bovine, owl, and human, respectively. The owl endothelium had an additional less electrodense layer with a granular appearance and a thickness of about 200 microm. The mucinous layer on the epithelium was similar in appearance to that on the endothelium and across species. CONCLUSIONS: The morphologic similarity of the endothelial and epithelial mucinous layers is a serendipitous finding that should prove valuable in experimental design. Ultimately, it is hoped that studies of the posterior corneal surface will deepen our knowledge of endothelial protection.

A new experimental cell for in situ and operandoX-ray absorption measurements in heterogeneous catalysis.

A new X-ray absorption cell dedicated to in situ and operando experiments in heterogeneous catalysis has been built and tested. The cell consists of several boron nitride and stainless steel plates linked together using graphite seals. It allows the measurement of XANES and EXAFS spectra of heterogeneous catalysts within a wide range of photon energies in transmission mode under the flow of various oxidative and reductive gas mixtures at elevated temperatures. The cell is compact and easy to build. Catalysts are loaded into the cell as powders. The use of boron nitride and a small beam pathlength in the cell result in a low absorption of the X-ray beam at lower energies. The cell was tested by in situ characterizing cobalt species during oxidative and reductive pre-treatments of a silica-supported Fischer-Tropsch catalyst. An operando study of methanol conversion over alumina-supported molybdenum catalysts was also carried out.

A comparison of corneal stromal edema induced from the anterior or the posterior surface.

BACKGROUND: Many differences between the anterior and posterior corneal stroma have been reported. The physiological and mechanical properties of the cornea are a summation of these properties across each of the corneal regions. This article investigates corneal stromal swelling that is experimentally induced through each surface. METHODS: Corneal stromal swelling was induced in human and rabbit corneas through either the anterior or posterior surface. The rate of stromal swelling was analyzed with a linear regression model. RESULTS: Swelling in the rabbit stroma was 3.65 x faster when induced through the posterior surface than through the anterior surface (p less than .0001), while the human stroma swelled 13.1 x faster through the posterior surface (p less than .0001). The hydration of the stroma increased during swelling through the posterior surface, but paradoxically decreased during swelling through the anterior surface. CONCLUSIONS: These experiments showed that stromal swelling occurs more rapidly through the posterior corneal surface than through the anterior surface. These results may have implications for the refractive surgeon performing laser ablative procedures on the anterior surface of the cornea.

5-Fluorouracil after trabeculectomy and the iridocorneal endothelial syndrome.

Nine patients (nine eyes) with iridocorneal endothelial (ICE) syndrome underwent trabeculectomy and received postoperative subconjunctival injections of 5-fluorouracil (5-FU) to enhance bleb formation (total 5-FU dose, 30-105 mg; mean, 53.8 mg). Eight eyes had undergone prior unsuccessful trabeculectomy. Four eyes had intraocular pressure (IOP) less than or equal to 21 mmHg on zero to two glaucoma medications after 6 to 54 months of follow-up (mean, 25.3 months). Five eyes required repeat surgery within 2 to 13 months and were considered failures. All five eyes that failed received a Molteno drainage implant. Progressive endothelial proliferation may explain late onset bleb failure and the relative ineffectiveness of 5-FU in this condition.