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H Over

Publications and source records attributed to H Over.

At least 19 recordsLinked to original sources
PubMed2006

Unusual process of water formation on RuO2(110) by hydrogen exposure at room temperature.

The reduction mechanism of the RuO(2)(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO(2)(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (O(br)-H) species and subsequently part of these O(br)-H groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.

Journal Article↗
PubMed2005

Ru(0001) model catalyst under oxidizing and reducing reaction conditions: in-situ high-pressure surface X-ray diffraction study.

With surface X-ray diffraction (SXRD) using a high-pressure reaction chamber we investigated in-situ the oxidation of the Ru(0001) model catalyst under various reaction conditions, starting from a strongly oxidizing environment to reaction conditions typical for CO oxidation. With a mixture of O(2) and CO (stoichiometry, 2:1) the partial pressure of oxygen has to be increased to 20 mbar to form the catalytically active RuO(2)(110) oxide film, while in pure oxygen environment a pressure of 10(-5) mbar is already sufficient to oxidize the Ru(0001) surface. For preparation temperatures in the range of 550-630 K a self-limiting RuO(2)(110) film is produced with a thickness of 1.6 nm. The RuO(2)(110) film grows self-acceleratedly after an induction period. The RuO(2) films on Ru(0001) can readily be reduced by H(2) and CO exposures at 415 K, without an induction period.

Journal Article↗
PubMed2004

Visualization of atomic processes on ruthenium dioxide using scanning tunneling microscopy.

The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO2(110) and RuO2(100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO2. CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO2 at the oxide surface. In contrast to the RuO2(110) surface, the RuO2(100) surface stabilizes also a catalytically inactive c(2 x 2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO2(100)-c(2 x 2) phase may play an important role in the deactivation of RuO2 catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions.

Journal Article↗
PubMed2001

Bonding mechanism and atomic geometry of an ordered hydroxyl overlayer on Pt(111).

Exposing water to a (2 x 2)-O precovered Pt(111) surface at 100 K and subsequently annealing at 155 K led to the formation of a well-ordered (square root 3 x square root 3)R30 degrees overlayer. The structure of this overlayer is determined by DFT and full dynamical LEED calculations. There are two O containing groups per (square root 3 x square root 3)R30 degrees unit cell and both occupy near on-top positions with a Pt-O bond length of (2.11 +/- 0.04) A. DFT calculations determined the hydrogen positions of the OH species and clearly indicate hydrogen bonds between the neighboring adsorbed OH groups whose interaction is mainly of electrostatic nature. A theoretical comparison with H(2)O shows the hybridization of OH on Pt(111) to be sp(3).

Journal Article↗
PubMed2001

Effects of carbamide peroxide containing bleaching agents on the morphology and subsurface hardness of enamel.

The purpose of this study is to evaluate the effects of nightguard bleaching agents (Karisma and Yotuel) on the enamel surface of forty anterior teeth. Ten teeth of each group were evaluated with SEM and ten teeth of each group were tested with a microhardness tester Morphologic alterations were observed on the enamel surfaces with SEM. Karisma group showed a significant decrease in enamel hardness (p<0.05) and microhardness values of enamel were increased significantly in Yotuel group (p<0.05).

Carbamide Peroxide↗
PubMed2000

Atomic-scale structure and catalytic reactivity of the RuO(2)(110) surface

The structure of RuO(2)(110) and the mechanism for catalytic carbon monoxide oxidation on this surface were studied by low-energy electron diffraction, scanning tunneling microscopy, and density-functional calculations. The RuO(2)(110) surface exposes bridging oxygen atoms and ruthenium atoms not capped by oxygen. The latter act as coordinatively unsaturated sites-a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces-onto which carbon monoxide can chemisorb and from where it can react with neighboring lattice-oxygen to carbon dioxide. Under steady-state conditions, the consumed lattice-oxygen is continuously restored by oxygen uptake from the gas phase. The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.

Journal Article↗
PubMed1999

Transrectal ultrasound in the diagnosis and management of inflammatory bowel disease.

BACKGROUND AND STUDY AIMS: To aim of the present study was to determine the value of transrectal ultrasonography (TRUS) in the assessment of disease activity in ulcerative colitis patients, and in differentiating between mucosal inflammation and transmural inflammation. PATIENTS AND METHODS: TRUS examinations were used to study 30 control individuals and 76 patients with inflammatory bowel disease, including 50 cases of ulcerative colitis and 26 of Crohn's disease. A rigid linear endorectal probe was used to examine the rectal wall. RESULTS: In the 30 control individuals, the rectal wall showed five layers, with a mean total diameter of 2.6 mm. There were significant differences between patients with quiescent ulcerative colitis, active ulcerative colitis, and control individuals with regard to the total rectal wall thickness (P<0.001), submucosal thickness (P<0.001) and mucosal thickness (P<0.001). Using cut-off values, differentiation between active ulcerative colitis and remission ulcerative colitis was found to be 100% specific and 73 % sensitive for submucosal thicknesses. TRUS revealed a 100% specificity in differentiating between remission ulcerative colitis and control cases based on the total rectal wall thickness, submucosal, and mucosal thicknesses. In the differential diagnosis of active and remission ulcerative colitis, an increase in submucosal wall thickness and the existence of arterial and venous capillary flow in the submucosa were found to be specific and more sensitive than the other parameters. TRUS examination revealed transmural inflammation in 21 of the 26 Crohn's disease patients, and mucosal inflammation in all 50 of the ulcerative colitis patients. CONCLUSION: TRUS is a reliable and easy method of assessing ulcerative colitis activity and differentiating between rectal diseases.

Adult↗