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Author:Lehman, Lauri
Title:Ab initio calculations of the adsorption of carbon monoxide on metal-promoted metal oxide surfaces
Metalliatomien vaikutus hiilimonoksidin adsorptioon metallioksidipinnoilla ab initio -menetelmällä tutkittuna
Publication type:Master's thesis
Publication year:2008
Pages:55 (+25)      Language:   eng
Department/School:Teknillisen fysiikan laitos
Main subject:Fysiikka   (Tfy-3)
Supervisor:Nieminen, Risto
Instructor:Salo, Petri
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark T80     | Archive
Keywords:adsorption
metal oxide surface
carbon monoxide
adsorptio
metallioksidipinnat
hiilimonoksidi
Abstract (eng): The adsorption of carbon monoxide (CO) was studied computationally on the palladium (Pd) or silver (Ag) promoted magnesium oxide (MgO(001)) and titanium dioxide (TiO2(110)) surfaces using the periodic density functional theory approach.
The promotormetal oxide system is a simplication of a surface catalyst in which metal nanoclusters are adsorbed on an oxide surface and which introduces an alternative path to various reactions, particularly to the oxidation of CO.

The co-adsorption scheme of CO and the metal promotor was such that the singular metal atom was placed on its most adhesive adsorption site and the adsorption of CO was studied at seven distinct sites on MgO(001) and at eight distinct sites on TiO2(110).
On each site, the adsorption of CO was studied with either of the atoms in the molecule pointing toward the surface, the molecular axis being initally inclined perpendicularly to the surface.
Relaxations with respect to the forces on the atoms were carried out for each of the sites.

The study revealed that when the CO molecule and the Pd or Ag promotor are further apart from each other than the radius of 34 Ångstroms, they can be considered as being virtually isolated on the surface, both particles adsorbed as if on the clean surface.
When the two particles are closer to each other, their interaction is dirent from the case of the clean surface.
On the Pd promoted surfaces, the perpendicular adsorption of CO on top of the palladium atom the C end of the molecule closer to the surface is clearly favoured over the adsorption on the clean surface, the adsorption energy of CO at this site being 2.54 eV on MgO(001) and 2.01 eV on TiO2(110), as opposed to the 0.18 and 0.32 eV on the respective clean surfaces.
The adsorption energy of CO on top of the Ag atom was only 0.01 eV on MgO(001) and 1.27 eV on TiO2(110).
The adsorption on the Pd atom is thus favoured over the Ag atom on both surfaces.

The analysis of energy states showed that the s- and p-states of CO which are located at the Fermi level in vacuum and the d-states of the metal promotors are altered as a consequence of the adsorption of CO on top of the metal atom.
No signicant net charge transfer between the adsorbates or the surface was observed, and the maximum increase in the CO bond length was 2.9 % on the Ag-promoted magnesium oxide surface.
Abstract (fin): Hiilimonoksidin adsorptiota on tutkittu palladium- ja hopeapitoisilla magnesiumoksidin (001)- ja titaanidioksidin (110)-pinnoilla laskennallisella menetelmällä käyttäen tiheysfunktionaaliteoriaa ja jaksollista pintamallia.
Metalliedistäjämetallioksidi-yhdistelmä on yksinkertaistettu malli pintakatalyytista, jossa pienet metallihiukkaset ovat adsorboituneet oksidipinnalle ja joka tarjoaa vaihtoehtoisen reaktiopolun erilaisissa kemiallisissa reaktioissa, erityisesti hiilimonoksidin hapettamisessa.

Metallin ja hiilimonoksidin yhteisadsorptiota tutkittiin siten, että yksittäinen metalliatomi asetettiin sen sitovimmalle adsorptiopaikalle ja hiilimonoksidin adsorptiota tarkasteltiin seitsemällä eri paikalla magnesiumoksidipinnalla ja kahdeksalla eri paikalla titaanidioksidipinnalla.
Hiilimonoksidimolekyylin akseli oli alussa kohtisuorassa pintaa vastaan, ja kullakin paikalla tutkittiin tapaukset, joissa molekyylin hiili- tai happiatomi on lähempänä pintaa.
Systeemi relaksoitiin atomeihin kohdistuvien voimien suhteen kaikilla paikoilla.

Tutkimuksen mukaan hiilimonoksidi ja palladium- tai hopeaedistäjä ovat eristettyjä pinnalla silloin kun niiden välinen etäisyys on 34 Ångstromia.
Tällöin molempien voidaan katsoa adsorboituneen pinnalle samalla tavalla kuin puhtaalle pinnalle.
Kun mainittu etäisyys on pienempi, adsorboituneet hiukkaset vuorovaikuttavat eri tavalla kuin puhtaalla pinnalla.
Palladiumpitoisilla pinnoilla hiilimonoksidin sitoutuminen palladium-atomin päälle kohtisuorassa asennossa molekyylin hiilipää pintaa kohti on selvästi vahvempaa kuin puhtaalle pinnalle sitoutuminen, sillä adsorptioenergia palladiumin päällä on 2.54 eV magnesiumoksidipinnalla ja 2.01 eV titaanidioksidipinnalla, kun adsorptioenergiat vastaaville puhtaille pinnoille ovat 0.18 ja 0.32 eV.
Adsorptioenergia hopea-atomin päällä oli ainoastaan 0.01 eV magnesiumoksidilla ja titananidioksidilla 1.27 eV.
Sitoutuminen palladium-atomin päälle on siis voimakkaampaa kuin hopea-atomin päälle kummassakin tapauksessa.

Energiatilojen tarkastelussa havaittiin, että ne hiilimonoksidin s- ja p-tilat, jotka sijaitsevat Fermitasolla tyhjiössä, sekä metalliedistäjän d-tilat muuttuvat silloin, kun hiilimonoksidi on adsorboitunut metalliatomin päälle.
Pinnalla olevien hiukkasten tai pinnan välillä ei havaittu merkittävää varauksen kokonaissiirtymää, ja hiilimonoksidin sidosetäisyys kasvoi suurimmillaan 2.9 % hopeapitoisella magnesiumoksidipinnalla.
ED:2009-02-18
INSSI record number: 36771
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