Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study

 

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dc.contributor.advisor Van Steen, Eric en_ZA
dc.contributor.advisor Petersen, Melissa en_ZA
dc.contributor.author Cariem, Muhammad Junaid en_ZA
dc.date.accessioned 2016-04-01T06:51:44Z
dc.date.available 2016-04-01T06:51:44Z
dc.date.issued 2012 en_ZA
dc.identifier.citation Cariem, M. 2012. Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study. University of Cape Town. en_ZA
dc.identifier.uri http://hdl.handle.net/11427/18476
dc.description Includes bibliographic references. en_ZA
dc.description.abstract The Fischer-Tropsch synthesis catalysed by iron is a well-established process, used for the conversion of syngas (a mixture of CO and H₂ ) to long chain hydrocarbons. Potassium is typically added as a promoter in iron-based Fischer-Tropsch to improve activity, selectivity and product distribution. The mechanism behind potassium promotion has in the past been explained as a combination of electron donation and electrostatic interaction. However, despite the importance of potassium as a promoter, the nature of the potassium species on the surface; whether it is present as metallic potassium (K) or is present as another species has received relatively little investigation. No research has been published as of yet as to the effects of potassium adsorption on a Hägg iron carbide surface or the effects on CO adsorption when co-adsorbing CO with potassium on a Hägg iron carbide surface. In this study density functional theory (DFT) has been used to investigate * The adsorption of CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. * The adsorption of K, O and KO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. * The co-adsorption of K, O or KO with CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. A thermodynamic analysis was done to investigate the stability of K versus the stability of KO at Fischer-Tropsch conditions. The adsorption of CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces was done as a pre-cursor to investigating the effect of co-adsorbing K, O or KO with CO on the CO adsorption energy, CO stretching frequency and CO bond length. Subsurface carbon on the Fe₅C₂(100)₀.₀₀ surface caused a decrease in the CO8 adsorption energy of 0.38eV when compared to CO adsorption on a similar site with subsurface iron. On the Fe₅C₂(100)₀.₀₈₉ surface, the lack of subsurface carbon allowed for CO adsorption in the 1F adsorption configuration on top of a valley iron site. The strength of potassium adsorption on both surfaces was calculated to be similar to that of CO in its most stable state (~1.60eV). Potassium is highly mobile across the surface, with a maximum barrier for K diffusion of 0.02eV calculated on both surfaces. A Bader analysis revealed that potassium donates electrons to the surface (~0.72) and that the electron donation from the potassium to the surface is localised and affects only the iron atoms not the carbon atoms. The co-adsorption of O with K leads to a significant increase in the stability of O adsorption on both surfaces, with increases in the O adsorption energy of O of ~0.60eV on the Fe₅C₂(100)₀.₀₀ surface and ~0.40eV on the Fe₅C₂(100)₀.₀₈₉ surface. The O also stabilises the K with the maximum barrier for diffusion of K increasing to 0.07eV on the Fe₅C₂(100)₀.₀₀ surface and 0.15eV on the Fe₅C₂(100)₀.₀₈₉ surface. However, these maximum barriers for diffusion are still extremely low, indicating that potassium is still highly mobile on the surface. The charge density difference plot showed some polarisation of the O towards the K and vice versa, indicating interaction between the two species. No orbital overlap between the adsorbed O and adsorbed K was observed in the charge density difference plot. This together with the results from a local density of states (LDOS) plot indicates that the interaction between O and K on the surface is ionic in nature. The co-adsorption of CO with either K or KO on both the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces resulted in a significant increase in the calculated CO adsorption energy coupled with an increase in the CO bond length and a decrease in the CO stretching frequency. The magnitude of the increases in calculated CO adsorption energy and CO bond length as well as the magnitude of the decrease in the CO stretching frequency was virtually the same irrespective of whether CO was co-adsorbed with K or KO. The combination of these results shows that K and KO both enhance CO adsorption to a similar degree on Hägg iron carbide surfaces while possibly making CO dissociation more facile. Co-adsorbing CO with O on the Fe₅C₂(100) 0;00 surface lead to a significant decrease in the CO adsorption energies, an increase in CO bond length and an increase in the CO stretching frequency in certain cases. This negative effect on CO adsorption is very localised and restricted to CO adsorption sites which are near to the adsorbed O and have subsurface carbon which prevents CO migration away from the O to a more stable site. On the Fe₅C₂(100)₀.₀₈₉ surface where no subsurface carbon is present, the CO migrates away from the O to a site unaffected by the presence of O. en_ZA
dc.language.iso eng en_ZA
dc.subject.other Chemical Engineering en_ZA
dc.title Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study en_ZA
dc.type Master Thesis
uct.type.publication Research en_ZA
uct.type.resource Thesis en_ZA
dc.publisher.institution University of Cape Town
dc.publisher.faculty Faculty of Engineering and the Built Environment
dc.publisher.department Department of Chemical Engineering en_ZA
dc.type.qualificationlevel Masters
dc.type.qualificationname MSc (ChemEng) en_ZA
uct.type.filetype Text
uct.type.filetype Image
dc.identifier.apacitation Cariem, M. J. (2012). <i>Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study</i>. (Thesis). University of Cape Town ,Faculty of Engineering & the Built Environment ,Department of Chemical Engineering. Retrieved from http://hdl.handle.net/11427/18476 en_ZA
dc.identifier.chicagocitation Cariem, Muhammad Junaid. <i>"Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study."</i> Thesis., University of Cape Town ,Faculty of Engineering & the Built Environment ,Department of Chemical Engineering, 2012. http://hdl.handle.net/11427/18476 en_ZA
dc.identifier.vancouvercitation Cariem MJ. Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study. [Thesis]. University of Cape Town ,Faculty of Engineering & the Built Environment ,Department of Chemical Engineering, 2012 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/18476 en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Cariem, Muhammad Junaid AB - The Fischer-Tropsch synthesis catalysed by iron is a well-established process, used for the conversion of syngas (a mixture of CO and H₂ ) to long chain hydrocarbons. Potassium is typically added as a promoter in iron-based Fischer-Tropsch to improve activity, selectivity and product distribution. The mechanism behind potassium promotion has in the past been explained as a combination of electron donation and electrostatic interaction. However, despite the importance of potassium as a promoter, the nature of the potassium species on the surface; whether it is present as metallic potassium (K) or is present as another species has received relatively little investigation. No research has been published as of yet as to the effects of potassium adsorption on a Hägg iron carbide surface or the effects on CO adsorption when co-adsorbing CO with potassium on a Hägg iron carbide surface. In this study density functional theory (DFT) has been used to investigate * The adsorption of CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. * The adsorption of K, O and KO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. * The co-adsorption of K, O or KO with CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. A thermodynamic analysis was done to investigate the stability of K versus the stability of KO at Fischer-Tropsch conditions. The adsorption of CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces was done as a pre-cursor to investigating the effect of co-adsorbing K, O or KO with CO on the CO adsorption energy, CO stretching frequency and CO bond length. Subsurface carbon on the Fe₅C₂(100)₀.₀₀ surface caused a decrease in the CO8 adsorption energy of 0.38eV when compared to CO adsorption on a similar site with subsurface iron. On the Fe₅C₂(100)₀.₀₈₉ surface, the lack of subsurface carbon allowed for CO adsorption in the 1F adsorption configuration on top of a valley iron site. The strength of potassium adsorption on both surfaces was calculated to be similar to that of CO in its most stable state (~1.60eV). Potassium is highly mobile across the surface, with a maximum barrier for K diffusion of 0.02eV calculated on both surfaces. A Bader analysis revealed that potassium donates electrons to the surface (~0.72) and that the electron donation from the potassium to the surface is localised and affects only the iron atoms not the carbon atoms. The co-adsorption of O with K leads to a significant increase in the stability of O adsorption on both surfaces, with increases in the O adsorption energy of O of ~0.60eV on the Fe₅C₂(100)₀.₀₀ surface and ~0.40eV on the Fe₅C₂(100)₀.₀₈₉ surface. The O also stabilises the K with the maximum barrier for diffusion of K increasing to 0.07eV on the Fe₅C₂(100)₀.₀₀ surface and 0.15eV on the Fe₅C₂(100)₀.₀₈₉ surface. However, these maximum barriers for diffusion are still extremely low, indicating that potassium is still highly mobile on the surface. The charge density difference plot showed some polarisation of the O towards the K and vice versa, indicating interaction between the two species. No orbital overlap between the adsorbed O and adsorbed K was observed in the charge density difference plot. This together with the results from a local density of states (LDOS) plot indicates that the interaction between O and K on the surface is ionic in nature. The co-adsorption of CO with either K or KO on both the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces resulted in a significant increase in the calculated CO adsorption energy coupled with an increase in the CO bond length and a decrease in the CO stretching frequency. The magnitude of the increases in calculated CO adsorption energy and CO bond length as well as the magnitude of the decrease in the CO stretching frequency was virtually the same irrespective of whether CO was co-adsorbed with K or KO. The combination of these results shows that K and KO both enhance CO adsorption to a similar degree on Hägg iron carbide surfaces while possibly making CO dissociation more facile. Co-adsorbing CO with O on the Fe₅C₂(100) 0;00 surface lead to a significant decrease in the CO adsorption energies, an increase in CO bond length and an increase in the CO stretching frequency in certain cases. This negative effect on CO adsorption is very localised and restricted to CO adsorption sites which are near to the adsorbed O and have subsurface carbon which prevents CO migration away from the O to a more stable site. On the Fe₅C₂(100)₀.₀₈₉ surface where no subsurface carbon is present, the CO migrates away from the O to a site unaffected by the presence of O. DA - 2012 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2012 T1 - Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study TI - Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study UR - http://hdl.handle.net/11427/18476 ER - en_ZA


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