Water dissociation and water-gas shift energetics on MgO, MgO/Ag and Au/MgO/Ag surfaces
Water dissociation and the energetics of the WGS reaction on a bulk MgO, an Ag supported MgO thin film and Au/MgO/Ag surfaces is studied based on the density functional theory. The literature results for water splitting on MgO and MgO/Ag surfaces with a step type defect are repeated and further anal...
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| Other Authors: | , , , , , |
| Format: | Master's thesis |
| Language: | eng |
| Published: |
2011
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| Subjects: | |
| Online Access: | https://jyx.jyu.fi/handle/123456789/37046 |
| Summary: | Water dissociation and the energetics of the WGS reaction on a bulk
MgO, an Ag supported MgO thin film and Au/MgO/Ag surfaces is
studied based on the density functional theory. The literature results for
water splitting on MgO and MgO/Ag surfaces with a step type defect are
repeated and further analysis on the origin of the energetically favored
dissociative water adsorption are made. The reactivity towards water
dissociation on the stepped MgO and MgO/Ag surfaces is attributed to
four contributing factors:
1. The change in the interlayer distance of the surface in dissociative
adsorption is diminished on the stepped surfaces compared to the
flat surfaces. On the flat surfaces the dissociative adsorption of
water leads to an increased interlayer distance between the first
and second MgO layer while the molecular adsorption does not
change the distance.
2. A surface OH group is formed in the H adsorption. On the stepped
surface the adsorbed OH and surface OH form aligned dipoles. On
the flat surface the adsorbed OH and the surface OH group do not
align.
3. The density difference analysis shows that the charge transfer in
the co-adsorption of H and OH on the stepped MgO/Ag surface
is mainly between the adsorbed H and OH. This leads to an at-
tractive Coulomb interaction between the adsorbed species.
4. The p-band center of the edge O atoms show a strong shift towards
the Fermi energy compared to the average surface O atoms. On
the bulk MgO surfaces the p-band center of the O atom at the H
adsorption site is shown to correlate with the H adsorption energy.
Similar trend is seen in the MgO/Ag surfaces.
The energetics of a WGS reaction on a surface with large Au clusters
deposited on an Ag supported MgO thin film is studied. The large Au
cluster is modeled with an Au stripe set with the periodic boundary
conditions to produce similar distances between the Au atoms as for a
large Au cluster. The Bader charges of the edge atoms of the Au stripe
are similar to the charges for the edge atoms of a large Au cluster. Water
dissociation at the edge of the Au stripe is not deemed probable due to
the weak binding of OH to the Au/MgO/Ag surface.
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