Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT

Simple item page
dc.contributor.advisorVan Steen, Ericen_ZA
dc.contributor.advisorPetersen, Melissaen_ZA
dc.contributor.authorCilliers, Pierre Louisen_ZA
dc.date.accessioned2018-05-03T12:28:00Z
dc.date.available2018-05-03T12:28:00Z
dc.date.issued2018en_ZA
dc.description.abstractThe Langmuir adsorption isotherm is often used to model molecular adsorption on catalyst surfaces. The model assumes that adsorption occurs on a homogenous energy surface at specific localized sites with no lateral interactions between adsorbents. This simplification causes some concerns when considering adsorption at higher coverages as species have been observed to have a maximum coverage less than one monolayer (ML), such as O and OH on platinum (Pt) surfaces for use in direct methane to methanol synthesis. It has been suggested that the maximum coverages are due to repulsive lateral interactions which limit coverages on Pt to 0.50 ML and 0.75 ML for O and OH respectively, weakening the Langmuir assumption. For reactions sensitive to coverage it is useful to have a model representation of these interactions and the obtainable coverages. This would require determining the effect these interactions have on obtainable coverages and whether possible hydrogen bonding could allow for co-adsorption to fully saturate Pt catalysts. Thus, this study focuses on the coverage of Pt surfaces with O, OH and co-adsorbed O/OH adsorbents as a function of temperature and partial pressure with particular interest given to full coverage conditions. To determine the obtainable coverages on the dominant Pt surfaces, namely Pt(100) and Pt(111), a Density Functional Theory (DFT) study was done using a GGA-PBE and GGA-optB88 model utilising VASP. The coverages were modelled on a p(2x2) Pt cell which could model 0.25, 0.50, 0.75 and 1.00 ML. The relative Gibbs free energies were then calculated for all adsorbent combinations on the surface with oxygen and water as the gas phase reference. The change in Gibbs free energy upon adsorption was calculated across a chemical potential range of -0.22 eV, corresponding to the critical point for O2 (-118.6 °C, 50.06 bar), up to -3.5 eV. These chemical potentials were then related to specific temperatures and partial pressures. It was found that only full coverage with OH was achievable on Pt(111). In contrast, Pt(100) yielded several full coverage combinations. The generation of these phase diagrams showed a trend of increasing lateral interactions that prevent full coverage with a single O adsorbent species. As shown, by co-adsorbing OH it could be possible to achieve higher coverages through attractive lateral interactions. This weakens the lateral interaction assumption used in the Langmuir model and indicates the possibility of low temperature direct methane to methanol synthesis, around 80 °C, due to the formation of a fully saturated Pt surface.en_ZA
dc.identifier.apacitationCilliers, P. L. (2018). <i>Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT</i>. (Thesis). University of Cape Town ,Faculty of Engineering & the Built Environment ,Department of Chemical Engineering. Retrieved from http://hdl.handle.net/11427/27897en_ZA
dc.identifier.chicagocitationCilliers, Pierre Louis. <i>"Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT."</i> Thesis., University of Cape Town ,Faculty of Engineering & the Built Environment ,Department of Chemical Engineering, 2018. http://hdl.handle.net/11427/27897en_ZA
dc.identifier.citationCilliers, P. 2018. Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Cilliers, Pierre Louis AB - The Langmuir adsorption isotherm is often used to model molecular adsorption on catalyst surfaces. The model assumes that adsorption occurs on a homogenous energy surface at specific localized sites with no lateral interactions between adsorbents. This simplification causes some concerns when considering adsorption at higher coverages as species have been observed to have a maximum coverage less than one monolayer (ML), such as O and OH on platinum (Pt) surfaces for use in direct methane to methanol synthesis. It has been suggested that the maximum coverages are due to repulsive lateral interactions which limit coverages on Pt to 0.50 ML and 0.75 ML for O and OH respectively, weakening the Langmuir assumption. For reactions sensitive to coverage it is useful to have a model representation of these interactions and the obtainable coverages. This would require determining the effect these interactions have on obtainable coverages and whether possible hydrogen bonding could allow for co-adsorption to fully saturate Pt catalysts. Thus, this study focuses on the coverage of Pt surfaces with O, OH and co-adsorbed O/OH adsorbents as a function of temperature and partial pressure with particular interest given to full coverage conditions. To determine the obtainable coverages on the dominant Pt surfaces, namely Pt(100) and Pt(111), a Density Functional Theory (DFT) study was done using a GGA-PBE and GGA-optB88 model utilising VASP. The coverages were modelled on a p(2x2) Pt cell which could model 0.25, 0.50, 0.75 and 1.00 ML. The relative Gibbs free energies were then calculated for all adsorbent combinations on the surface with oxygen and water as the gas phase reference. The change in Gibbs free energy upon adsorption was calculated across a chemical potential range of -0.22 eV, corresponding to the critical point for O2 (-118.6 °C, 50.06 bar), up to -3.5 eV. These chemical potentials were then related to specific temperatures and partial pressures. It was found that only full coverage with OH was achievable on Pt(111). In contrast, Pt(100) yielded several full coverage combinations. The generation of these phase diagrams showed a trend of increasing lateral interactions that prevent full coverage with a single O adsorbent species. As shown, by co-adsorbing OH it could be possible to achieve higher coverages through attractive lateral interactions. This weakens the lateral interaction assumption used in the Langmuir model and indicates the possibility of low temperature direct methane to methanol synthesis, around 80 °C, due to the formation of a fully saturated Pt surface. DA - 2018 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2018 T1 - Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT TI - Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT UR - http://hdl.handle.net/11427/27897 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/27897
dc.identifier.vancouvercitationCilliers PL. Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT. [Thesis]. University of Cape Town ,Faculty of Engineering & the Built Environment ,Department of Chemical Engineering, 2018 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/27897en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Chemical Engineeringen_ZA
dc.publisher.facultyFaculty of Engineering and the Built Environment
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherChemical Engineeringen_ZA
dc.titlePhase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFTen_ZA
dc.typeMaster Thesis
dc.type.qualificationlevelMasters
dc.type.qualificationnameMSc (Eng)en_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
Files
Original bundle
Now showing 1 - 1 of 1
Thumbnail Image
Name:
thesis_ebe_2018_cilliers_pierre_louis.pdf
Size:
6.29 MB
Format:
Adobe Portable Document Format
Description:
Download
Collections
Masters