Charge distribution analysis of catalysts under simulated reaction conditions. Final report, October 1, 1993--June 30, 1995 Page: 3 of 46
This report is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to UNT Digital Library by the UNT Libraries Government Documents Department.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
Contract DE-AC 22 92 PC 92106
Background
Every catalytic reaction entails some amount of transfer of charges, either electrons (e') or holes (h'),
between the solid surface and the reactants during transient adsorption or chemisorption. The transfer may
be transient or permanent for a given reaction step. On the basis of this argument we may predict that
catalytic activity should be related to the capacity of a solid surface to donate electrons to or accept
electrons from molecules in the gas or liquid phases with which it is brought in contact. Two such
processes involving species A and B are outlined schematically in Figure 1.Surface Charge Transfer Processes
A A* B* B
[n t]
orhFigure 1:
Schematic representation of a
electron or hole transfer between
a catalytically active surface and
two molecules A or B. Through
the transfer of charges A and B
become activated A* and B* or
chemically modified as radicals.
e' and h* stand for an electron
and hole, respectively. n and
stand for the number density
and mobility of charges.
Subscripts s and b designate
surface and bulk, respectively.A specific case is the hydrogen abstraction from CH4 on oxide contacts at temperatures above 600 C, a
reaction that is fundamental to oxidative coupling of methane and its conversion into higher hydrocarbons.
A large number of oxides have been found to be catalytically active including such very simple non-
transition metal oxides like MgO and CaO. On the catalyst side the reaction is controlled by the availability
of holes h* at the oxide surface, e.g. y the availability of O- states on the surface primarily composed of
02-. As shown by Figure 2, a reaction then occurs between CH4 and O- by which an H atom is
abstracted from the CH4 molecule and transferred onto the oxide surface, forming an OH- at the surface.
The CH4 molecule becomes a methyl radical, -CH3, which remains in the gas phase, CH4 + 0-= -CH3 +
OH-, while the methyl radicals continue to react to give higher hydrocarbons such as ethane: -CH3 +
-CH3 = C2H6-
Though the oxidative coupling of methane is not of direct interest to coal liquefaction, the example shows
how, in the case of a simple oxide, its surface may become reduced by hydrogenation while the gas phase
reactants become oxidized.3
Surface/Interface:
Final Report
Upcoming Pages
Here’s what’s next.
Search Inside
This report can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Report.
Freund, F. Charge distribution analysis of catalysts under simulated reaction conditions. Final report, October 1, 1993--June 30, 1995, report, February 1, 1996; United States. (https://digital.library.unt.edu/ark:/67531/metadc664158/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.