Multiple Reaction Sites Research Articles

Temperature-programmed hydrogenation of carbon monoxide on supported nickel catalyst, multiple reaction sites and their interactions on Ni/TiO 2

The catalytic behaviors of Ni/TiO 2 and Ni/SiO 2 for CO hydrogenation were investigated using the temperature-programmed reaction (TPR) of pre-adsorbed carbon monoxide with hydrogen. The TPR spectra of Ni/SiO 2 exhibits a single peak with the characteristic shape of first-order kinetics. In contrast, two distinct peaks are observed for Ni/TiO 2, which indicates that there are two different reaction sites. The high temperature peak shows behavior similar to that of Ni/SiO 2, and it is believed that the corresponding sites are composed of nickel atoms. The low temperature peak is attributed to the hydrogenation of CO adsorbed on Ni-TiO x sites. Some interrupted experiments indicate the formation of a common intermediate species from both reaction sites. Thus, a mechanistic model is proposed on the basis of experimental observation. Computer simulation of the TPR spectra shows that the model fits the system well. By comparison with carbon hydrogenation experiments, the intermediate is believed to be the active surface carbon. This mechanism satisfactorily explains the high activity of CO hydrogenation on Ni/TiO 2 catalyst.

Steady state kinetics of enzymes with substrates having multiple reaction sites

An enzyme catalysing the reaction of a substrate with multiple reaction sites may display steady state kinetics described by a Michaels-Menten equation. The Km is identical for all sites considered individually and all sites together. The maximum velocity for a single site depends on the rate constants for reaction at that site and at all of other sites.

Detection of activated adsorption sites and a COH surface complex on [formula omitted

Temperature-programmed reaction and desorption (TPR, TPD), in combination with isotope labeling of CO, identified the existence of two distinct reaction sites for CO hydrogenation on Ru Al 2O 3 catalysts. These sites are similar to those previously reported on Ni Al 2O 3 catalysts. Carbon monoxide adsorbs on Ru crystallites at 300 K; at higher temperatures, in an activated process that requires H 2, CO transfers to the Al 2O 3 to form a COH complex, with the stoichiometry of a methoxy species. This transfer process is significantly slower than that on Ni Al 2O 3 . The decomposition of the COH complex limits the rate of CH 4 formation for the second peak seen in TPR. The first peak is due to hydrogenation of the CO absorbed on Ru. The similarities to Ni Al 2O 3 catalysts imply that an aluminate is not required for multiple reaction sites on Ni Al 2O 3 .

Laser Induced Decomposition of Triethylgallium and Trimethylgallium Adsorbed on GaAs(100)

ABSTRACTWe report X-ray photoelectron spectroscopy (XPS) studies of excimer laser stimulated decomposition of triethylgallium (TEGa) and trimethylgallium (TMGa) adsorbed on Gastabilized GaAs(100) surfaces in ultrahigh vacuum. TEGa and TMGa dissociatively chemisorb on GaAs at room temperature, whereupon irradiation by an excimer laser (at 193 or 351 nm) leads to further dissociation and desorption of carbon-containing species. The carbon removal rate (per laser pulse) decreases as carbon is removed suggesting multiple reaction sites, coverage dependent Arrhenius parameters, or second-order reactions. Based on the dependence of the rate on laser wavelength and fluence, we conclude that at low fluence, a two-photon electronic excitation of the adsorbate occurs, while at high fluence, laser induced pyrolysis dominates.

CO methanation on low-weight loading [formula omitted]: Multiple reaction sites

Temperature-programmed reaction (TPR) with mass spectrometric detection was used to study the hydrogenation of adsorbed carbon monoxide on low-weight loading Ni Al 2O 3 catalysts. These catalysts are shown to differ from high-weight loading nickel/alumina catalysts and are shown to have two distinct pathways for carbon monoxide methanation. These pathways are attributed to two forms of adsorbed CO. The CO site which reacts at low temperature (446 K) appears to be from carbon monoxide adsorbed on nickel atoms that are bonded to other nickel atoms. It has an activation energy for methanation of 51 kJ/mol. The CO site that reacts at high temperature (546 K) has an activation energy of 145 kJ/mol. The high-temperature site, which is sensitive to the pretreatment temperature, results from CO adsorbed on nickel atoms that are interacting strongly with an oxide phase of the catalyst. Interconversion of CO between the two sites occurs and is influenced by hydrogen. Neither of the pathways appears limited by direct hydrogenation of surface carbon; this step is shown to be slightly faster under TPR conditions. The ability of TPR to measure specific reaction rates and to separate reaction pathways that might be obscured in steadystate kinetic measurements is demonstrated.

The base promoted oligomerization of 15-dehydro-prostaglandin B 1: dimer formation and structural implications for a complex mixture termed PGB x

The base promoted oligomerization of 15-dehydro-PGB 1 ( 1a) results in the formation of six dimers via a Michael addition pathway. The simultaneous operation of multiple reaction sites as required by dimer formation has major consequences for the structural complexity of the oligomeric mixture PGBx.

OXIRON Epoxy Resins Offer Multiple Reaction Sites Plus Unique Vinyl Reactivity

RETURN TO ISSUEPREVAdvertisementNEXTOXIRON Epoxy Resins Offer Multiple Reaction Sites Plus Unique Vinyl ReactivityCite this: Chem. Eng. News 1960, 38, 29, 24Publication Date (Print):July 18, 1960Publication History Published online6 November 2010Published inissue 18 July 1960https://doi.org/10.1021/cen-v038n029.p024Copyright © 1960 AMERICAN CHEMICAL SOCIETYArticle Views60Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (58 KB) SUBJECTS:Vinyl,Reactivity,Organic polymers Get e-Alerts