Structure sensitive adsorption of hydrogen on ruthenium and ruthenium-silver catalysts supported on silica

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Supported metal catalysts typically consist of particles with sizes less than 10 nm, and because of the small crystallite size, low coordination number sites (edges and corners) represent a significant fraction of all surface sites. Furthermore, it has been demonstrated that adsorption rates can be much greater at these low coordination sites than on basal plane sites. What has not been generally appreciated, however, is that preferential adsorption at edge and corner sites may explain the mechanism by which a promoter, or the addition of a second metal to form a bimetallic, can alter the selectivity and rate of reaction. ... continued below

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56 p.

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Kumar, N. February 12, 1999.

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This thesis or dissertation is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided by UNT Libraries Government Documents Department to Digital Library, a digital repository hosted by the UNT Libraries. More information about this document can be viewed below.

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  • Ames Laboratory
    Publisher Info: Ames Lab., IA (United States)
    Place of Publication: Iowa

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Description

Supported metal catalysts typically consist of particles with sizes less than 10 nm, and because of the small crystallite size, low coordination number sites (edges and corners) represent a significant fraction of all surface sites. Furthermore, it has been demonstrated that adsorption rates can be much greater at these low coordination sites than on basal plane sites. What has not been generally appreciated, however, is that preferential adsorption at edge and corner sites may explain the mechanism by which a promoter, or the addition of a second metal to form a bimetallic, can alter the selectivity and rate of reaction. For example, the measurements of hydrogen adsorption onto supported Ru-Ag catalysts show marked decreases in the amount of hydrogen adsorbed relative to the amount adsorbed on Ru catalysts. Although it is known that Ag does not dissociatively adsorb hydrogen, this decrease cannot be explained by a simple one-to-one site blocking mechanism unless Ag preferentially populates edges and corners, thereby reducing the number of Ru edge sites. Indeed, Monte Carlo simulations of Ru-Group IB metal catalysts predict that Group IB metal atoms preferentially populate corner and edge sites of ruthenium crystals. This evidence, taken together, suggests that adsorption occurs preferentially at Ru corner and edge sites, which act as portals onto basal planes. A model based on this portal theory for hydrogen adsorption onto supported ruthenium bimetallic catalysts has been developed using a rate equation approach. Specifically, the model accounts for the following features: (1) preferential adsorption through portals, (2) basal plane site-energy multiplicity, and (3) hydrogen spillover onto the support. A comparison of model predictions with experiment is presented for different concentration of Ag in Ru-Ag catalysts. The portal model of hydrogen adsorption can explain the observed decreased in the amount of hydrogen adsorbed on Ru-Ag catalysts. The model can be applied to understand the kinetics of ethane hydrogenolysis on Ru-Ag catalysts. The model is able to explain the change in the apparent order of hydrogenolysis reaction with respect to hydrogen from {minus}1.4 to {minus}2.4 when Ag is added to Ru/SiO{sub 2} catalyst.

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56 p.

Notes

OSTI as DE99002517

Source

  • Other Information: DN: Thesis submitted to Iowa State Univ., Ames, IA (US); TH: Thesis (Ph.D.)

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  • Other: DE99002517
  • Report No.: IS-T--1848
  • Grant Number: W-7405-ENG-82
  • Office of Scientific & Technical Information Report Number: 348888
  • Archival Resource Key: ark:/67531/metadc678583

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  • February 12, 1999

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  • July 25, 2015, 2:20 a.m.

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  • Nov. 6, 2015, 8:59 p.m.

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Kumar, N. Structure sensitive adsorption of hydrogen on ruthenium and ruthenium-silver catalysts supported on silica, thesis or dissertation, February 12, 1999; Iowa. (digital.library.unt.edu/ark:/67531/metadc678583/: accessed September 26, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.