Browsing by Author "Petersen, Melissa"

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    Open Access
    Adding ammonia during Fischer-Tropsch Synthesis: Pathways to the formation of N-containing compounds
    (2017) De Vries, Christian; Claeys, Michael; Petersen, Melissa
    The Fischer-Tropsch synthesis (FTS) process, better known for its ability to produce synthetic fuel via the hydrogenation of CO, has shown potential to produce valuable chemicals when ammonia is added to the feed. In this work certain aspects of the pathway to the formation of N-containing compounds that form when NH₃ is added during FTS, using mostly iron based catalysts is investigated. In addition, the effect this has on the FTS reaction itself is evaluated. To achieve this goal, both theoretical and experimental techniques are used in this study. The CO adsorption and dissociation reactions are assumed to be important elementary reactions for many proposed FTS pathways. In the theoretical part of this thesis, spin-polarized periodic density functional theory (DFT) calculations are employed to study aspects of the initial stage of the pathway on a model Fe(100) surface. Considering the formation of N-containing hydro- carbons, one would assume that NH₃ initially adsorbs and dissociates on the catalyst surface, which could take place in the presence of CO. The surface chemistry of these adsorbates is well studied both experimentally and theoretically, but their co-existence has not yet been evaluated on model Fe surfaces. Initially a platform is generated by calculating the individual potential energy surfaces (PES) for the decomposition of CO and NH₃ on Fe(100) at a coverage of ϴ = 0.25 ML. These calculations provided the basis for comparing the adsorption and dissoci- ation profiles of CO and NH₃ on the Fe(100) surface via the use of the same computational methodology, and importantly making use of the same exchange correlation functional (RPBE) for both adsorbates. Furthermore, it was desired to evaluate the kinetics and thermodynamics of the NH₃ decomposition on the Fe(100) surface at relevant temperatures and pressures (by combining the DFT results with statistical thermodynamics) to better understand the role of NHₓ surface species involved in the pathway to the formation of the N-containing compounds on a model catalyst surface. The DFT results that are reported for the individual decomposi- tion PES for CO and NH₃ were generally found to be in close agreement with what has been reported in previous DFT studies and deduced experimentally for the relevant adsorption and decomposition pathways. The resulting Gibbs free energies for the PES suggests that NH₂ may be kinetically trapped on the Fe(100) surface at a coverage of ϴ = 0.25 ML and the reaction conditions (T = 523 K and p*NH₃ = 0.2 bar) where NH₃ is co-fed with synthesis gas during FTS. The individual adsorptions of CO and NHₓ (with x = 3, 2, 1, 0) were compared to their coadsorbed states, by calculating the heat of mixing (ΔEmix) and the activation barriers (Eₐ) for CO dissociation in the presence and absence of the NHₓ surface species on the Fe(100) sur- face. Similar to the individual adsorption of NH₃, the 0 K regime inherent to DFT calculations is bridged by calculating the Gibbs free energy of mixing for CO + NH₃ on Fe(100) at higher temperatures. Both repulsive and attractive interaction energies were calculated for the various coadsorbed states (CO + NHₓ on Fe(100)) and similarly some configurations resulted in an energetically favored or unfavored heat of mixing. The activation barrier for CO dissociation was lowered when coadsorbed with NH₃ and NH₂, and raised when coadsorbed with NH and N. With all the coadsorbed structures the CO dissociation reaction became more endothermic. Previous experimental studies have shown a concomitant reduction in oxygenate selectivity with an increase in the selectivity for N-containing compounds, when NH₃ is added during FTS. It is well-known that oxygenates undergo secondary reactions when using iron-based catalysts in FTS. In addition, the catalyst used in aforementioned studies (precipitated Fe/K) are active for the amination reactions of oxygenates. It is therefore hypothesized that some oxygenates pro- duced via the primary FTS pathway are converted to N-containing compounds via a secondary reaction. The experimental part of this thesis is therefore aimed at testing this hypothesis. A base case study included a comparison between a Fe-catalyzed slurry phase FTS reaction and a FTS reaction with all parameters remaining unchanged, except for the addition of 1 vol % NH₃ to the syngas (CO + H₂) feed. The activity (CO and H₂ conversion) data collected did not reveal any appreciable loss in the rate of the FTS reaction when 1 vol % NH₃ was added and steady state was reached (, that is after 48 hours time on stream (TOS)). A slower carburization period was however observed when comparing the CO conversion during the first 24 hours TOS, and further supported by the slow increase in CO₂ selectivity during the same period. The use of two-dimensional gas chromatography (GC × GC-TOF/FID) allowed for the discovery of a formation of a range of secondary and tertiary amines, not reported in previous studies. The expected loss in oxygenate selectivity was observed and further probed by co-feeding 1-octanol with the feed (CO + 2H₂ + 1 vol % NH₃) via a saturator. These results clearly indicated a significant loss in oxygenate formation as a result of secondary conversion to N-containing compounds. Questions regarding the stability of aliphatic nitriles prompted the co-feeding of nonanitrile under similar conditions. The results obtained after co-feeding nonanitrile, sug- gests that nonanitrile is readily converted to secondary and tertiary amines and that the ratios of aliphatic alcohols and nitriles are close to equilibrium. The use of CO₂ as carbon source, the investigation of the product spectrum at higher space velocities and the use of Rh-based catalysts, when NH₃ is added during FTS were included in shorter studies. The combination of these results, adds to the knowledge pool for the case where NH₃ is present in the FTS regime, as a poison or reactant. Additional information regarding the path to the formation of N-containing compounds was obtained via the detailed analysis of the product spectra with two-dimensional gas chromatography and the subsequent co-feeding reactions. The results ob- tained via co-feeding reactions, can be used to devise strategies to increase the selectivity of the desired N-containing compounds.
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    Open Access
    Adsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study
    (2012) Cariem, Muhammad Junaid; Van Steen, Eric; Petersen, Melissa
    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.
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    DFT insight into the oxygen reduction reaction (ORR) on the Pt₃Co(111) surface
    (2012) Matsutsu, Molefi; Van Steen, Eric; Petersen, Melissa
    Proton exchange membrane fuel cells (PEMFC) are identified as future energy conversion devices, for application in portable and transportation devices. The preferred catalyst for the PEMFC is a Pt-catalyst. However, due to the slow oxygen reduction reaction (ORR) kinetics, high Pt loadings have to be used. The high Pt loadings lead to high costs of the PEMFC. Pt-Co alloys have been identified as catalysts having higher ORR activity higher than of a Pt-catalyst. Therefore, in the present study, the Density Functional Theory (DFT) technique is used to gain fundamental insight into the ORR on the Pt₃Co(111) surface. The calculations have been performed using the plane wave based code, the Vienna ab-initio Simulation Package (VASP). DFT spin-polarized calculations, utilizing the GGA-PW91 functional, have been used to study the adsorption of the ORR intermediates, viz. O₂, O, OOH, OH, H₂O and HOOH on the Pt₃Co(111) surface. The results obtained on the Pt₃Co(111) surface are compared to the results obtained on the Pt(111) surface. The adsorption strength of the ORR intermediates has been shown to be affected by the presence of Co to varying extents on the Pt₃Co(111) surface relative to adsorption on the Pt(111) surface. The most strongly stabilised ORR intermediate on the Pt₃Co(111) surface relative to adsorption on the Pt(111) surface is O: on the Pt₃Co(111) surface O is 0.45 eV more strongly adsorbed than on the Pt(111) surface. The least affected ORR intermediate is H₂O: H₂O adsorption on the Pt₃Co(111) surface is 0.20 eV more stable than on the Pt(111) surface. The energetically favorable, i.e. most strongly bound adsorption configurations for all the ORR intermediates involves a configuration in which the ORR intermediate is bonded to a surface Co atom. Therefore, the surface Co atom stabilizes the adsorption of the ORR intermediates, relative to adsorption on the Pt(111) surface. Coadsorbed configurations have been used to study the formation and dissociation of the ORR intermediates. From the coadsorption studies, it is shown that there is an energy cost associated with moving the adsorbates from their lowest energy sites, while separately adsorbed, to the higher energy coadsorbed state, prior to reaction. Hence, adsorbate-adsorbate interactions are expected to destabilize the coadsorbed state at the coverages considered in the present study. The Climbing Image Nudged Elastic Band (CI-NEB) method has been used to locate the transition states and to calculate the activation energies of the different elementary reaction steps. The calculated dissociation reaction activation energies for the Pt₃Co(111) surface are found to be lower than the dissociation activation energies calculated on the Pt(111) surface. The most lowered dissociation activation energy is for the dissociation of O₂: on the Pt₃Co(111) surface the activation energy is 0.08 eV, whilst on the Pt(111) surface the activation energy is 0.59 eV. For the hydrogenation reaction steps, only the hydrogenation of O to form OH occurs with a lower activation energy of 0.86 eV on the Pt₃Co(111) surface, compared to 0.95 eV on the Pt(111) surface. For other hydrogenation reaction steps, the activation energies on the Pt₃Co(111) surface are higher than those on the Pt(111) surface. Based on the calculated activation energies of the elementary ORR reaction steps, the dissociative and the O-assisted H₂O dissociation mechanisms are identified as the mechanisms most likely to be dominant on the Pt₃Co(111) surface, due to having lower activation energies relative to the associative mechanisms. For both mechanisms, the reaction step with the highest activation energy is the step involving O, i.e. O hydrogenation to form OH for the dissociative mechanism, and the O* + H₂O* --> 2OH* reaction for the O-assisted H₂O dissociation mechanism. Thus, the reaction step involving the reaction of the strongly adsorbed O species, is identified as the potential rate limiting step of the ORR. Both the dissociative and the O-assisted H₂O dissociation mechanisms are expected to be in competition on the Pt₃Co(111) surface, since the potential rate limiting step for both mechanisms have similar activation energies. Hence, the preferred mechanism will depend on the relative abundances of the H species and H₂O on the Pt₃Co(111) surface. A microkinetic analysis would be need needed to fully account for concentration and entropic contributions to the rate of reaction for the different ORR elementary reaction steps.
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    A DFT study of the interaction of Ox with Pt nanorod edge sites : a model for the ORR activity on Pt nanoparticle edges
    (2015) Gambu, Gorden Thobani; Van Steen, Eric; Petersen, Melissa
    Proton exchange membrane fuel cells (PEMFCs) are an attractive energy conversion technology, this due to their high theoretical fuel utilization efficiencies compared to Carnot engines. However, due to potential losses, the operational efficiencies achieved in state-of-the-art PEMFCs are only between 45% and 55%. The slow kinetics of the oxygen reduction reaction (ORR) over a platinum based electrode accounts for ca. 70% of the potential losses. As a result of the sluggish ORR kinetics, high platinum loadings are required. The high cost of platinum has made it crucial to improve the ORR activity and hence reduce platinum loading. The surface-area-specific ORR activity has been reported to decrease with platinum particle size. This places a limitation to the degree to which platinum loading can be reduced by increasing metal dispersion. To understand the origin of this behaviour, experimental studies have measured the ORR activity over different single crystalline surfaces and used model nanoparticle shapes to elucidate the overall ORR activity. Theoretical studies use density functional theory (DFT) to investigate the ORR activity on various site-types present on assumed model particle shapes. Thermodynamically, the exposed surface terminations aught to be predominantly Ptf111g and Ptf100g separated by edges and corners. It has been postulated that the overall ORR activity can be calculated as a weighted average of the activity of exposed surface terminations. Using DFT calculations and nanorod models the above postulations are tested for the edge sites between a Pt(111) and Pt(100) surface. A rhombic nanorod model is used due to its computational efficiency compared to model nanoparticle clusters which are generally large and computationally expensive models. Furthermore, the use of rhombic nanorod model enables the investigation of the connection and communication between the Pt(111) and Pt(100) facets, this is difficult to investigate with stepped-surface models. It is argued that if, (i) the edge has insubstantial effect on the adsorption strength of adsorbed ORR intermediates as a function of distance from the edge and (ii) the diffusion of ORR intermediates between adjacent surface planes is limited, then the above postulation does hold.
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    Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT
    (2018) Cilliers, Pierre Louis; Van Steen, Eric; Petersen, Melissa
    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.
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    Pt and Pt-Pd cluster interaction with graphene and TiO₂ based supports: A DFT study
    (2016) Matsutsu, Molefi; Van Steen, Eric; Petersen, Melissa
    Density functional theory (DFT) calculations have been performed to gain insight into the role of defects present on the surface of graphene and TiO₂ based supports on supported metal clusters. The clusters considered are a Pt₃₈ cluster and a bimetallic Pt₃₂Pd₆ alloy. The defects considered on graphene based supports are monovacancy defective graphene, OH and COOH functionalised graphene. The defects considered on TiO₂ based supports are a partially reduced TiO₂(110) surface with a surface oxygen bridge vacancy and hydroxylated TiO₂(110) surface with surface OH groups. The defect free graphene and TiO₂ surfaces were also considered. For both the Pt₃₈ and Pt₃₂Pd₆ cluster, and on both defect containing graphene and TiO₂ (except on hydroxylated TiO₂(110) surface) the binding of the clusters is enhanced relative to binding on the defect free supports. Enhanced binding at the defects imply that the clusters are bound strongly to the support and thus less likely to detach from the support material relative to binding on the defect free supports. Therefore, the defects may improve the durability of the catalyst by limiting particle detachment. The electronic properties of the cluster are modified differently depending on the identity of the defect present on the support. On the graphene based supports, OH functionalisation is expected to result in weaker binding energy of adsorbate molecules, whereas COOH functionalisation is expected to result in stronger binding energy of adsorbates for the supported Pt₃₈ cluster. This is due to different shifts in d-band centre of the facets on the cluster supported on these supports. Therefore, it can be expected that the Pt₃₈ cluster supported on OH functionalised graphene will be more tolerant to poison molecules. This is due to reduced binding strength of adsorbates on OH functionalised graphene compared to adsorption on COOH functionalised graphene. For the Pt₃₂Pd₆ cluster the OH and COOH functional groups do not appreciably modify the d-band centre of the facets available to reactants, and thus is expected not to significantly modify the binding strength of adsorbate molecules relative to binding on the free unsupported Pt₃₂Pd₆ cluster. The binding energy of adsorbate molecules on the Pt₃₈ cluster supported on defect containing TiO₂ is expected to be stronger than on the Pt₃₈ cluster supported on defective graphene based supports, due to higher extent of upward shift of the d-band centre of the exposed facets. The enhanced binding energy of adsorbates on the Pt₃₈ cluster supported on TiO₂ supports may be detrimental to catalyst durability and activity. This can be due to strong binding of poison molecules and reaction intermediates which maybe too strongly bound on the surface such that they cannot participate in further reaction steps. Overall it might turn out that the functionalised graphene based supports are better support materials over the TiO₂ based materials for particular reactions. The Nb-doped partially reduced TiO₂(110) surface attaches the Pt₃₂Pd₆ cluster strongly to the support compared to the functionalised graphene supports. Furthermore, the binding energy of adsorbate molecules is expected to be weaker on the Pt₃₂Pd₆ cluster supported on the Nbdoped partially reduced TiO₂(110) surface compared to the functionalised graphene supports. This might be beneficial as poison molecules may be weakly bound to the cluster resulting in high resistance to poisoning which can also have a positive effect on catalyst activity. In addition to enhancing binding of the cluster to the support and affecting the binding energy of adsorbates on the supported clusters, some of the defects can also alter the ordering pattern of Pd and Pt atoms within the Pt₃₂Pd₆ cluster. OH functionalised graphene and Nbdoped partially reduced TiO₂(110) surface result in segregation of Pd towards the clustersupport interface, thereby exposing more Pt atoms at the surface facets of the cluster. The modified arrangement of Pt and Pd atoms may result in modification of the reactivity of the Pt₃₂Pd₆ cluster. The results of this study indicate that the defects can play a vital role in determining the activity and durability of the catalyst. By having the right combination of defects on the support material, the durability and catalytic activity of the catalyst can be fine-tuned simultaneously. This can lead to better design of catalysts.
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    Reaction pathways for the formation of hydrogen peroxide in fuel cells : a DFT study
    (2013) Madala, Thendo; Van Steen, Eric; Petersen, Melissa
    The world's dependency on fossil fuel will come to an end in the near future due to depletion of natural resources. Thus, research into alternative energy carriers becomes imperative. Furthermore, the use of fossil fuels is associated with environmental problems, which might be minimised by using alternatives. Hydrogen technology in the form of fuel cells can be a reliable and clean technology to minimise the problems associated with the use of fossil fuels. Fuel cells utilise hydrogen and air, and convert them to electrical energy through electro-catalysis with a co-product being water and heat.
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    The mobility of oxygen containing species (OCS*) over Pt-based catalyst surfaces: Impact on the oxygen reduction reaction (ORR) activity
    (2020) Gambu, Thobani G; van Steen, Eric; Petersen, Melissa
    The growing need to curb greenhouse gas emissions has made low-temperature proton exchange membrane fuel cells (PEMFCs) more attractive for automotive application. One of the major problems facing PEMFCs is the sluggish kinetics of the oxygen reduction reaction (ORR). To further enable wide-scale commercialisation of PEMFCs for automotive applications, major improvements in the ORR catalyst are therefore needed. An in depth understanding of the ORR mechanism over Pt surfaces can enable rational approaches in the search for more active ORR catalysts. The ORR occurs over multi-faceted Pt nanoparticles which predominantly expose Pt{111} and Pt{100} facets. Most studies have modelled the overall ORR activity over multi-faceted surface assuming that the Pt{111} and Pt{100} facets are kinetically isolated. Density functional theory (DFT) studies have shown that Pt(111) surfaces can efficiently facilitate OH* hydrogenation to H2O* but not the hydrogenation of O* to OH*, whereas Pt(100) surfaces can facilitate O* hydrogenation to OH* better than OH* hydrogenation to H2O*. If O* intermediates can readily diffuse from Pt{111} to Pt{100} facets and OH* from Pt{100} to Pt{111} facets, the ORR activity on Pt{111} and Pt{100} facets of multi-faceted surfaces may no longer be limited by O* and OH* hydrogenation steps, respectively. This study uses DFT and microkinetic models to investigate the nature of inter-facet cooperation and how it influences the ORR activity under dry conditions, i.e. catalyst surface exposed to a gas mixture of 33% O2 and 67% H2 at 1 bar. Under these conditions, it is assumed that the Langmuir-Hinshelwood kinetics are dominant. Using DFT, the adsorption, diffusion and reaction energetics of various reaction intermediates and reaction steps were calculated. The Pt{111} and Pt{100} facets were modelled as Pt(111)-p(3x3) and Pt(100)-p(3x3) slabs, respectively. The edge was modelled using a rhombic nanowire model with alternating Pt{111} and Pt{100} facets. Edge sites were found to adsorb oxygen containing species strongly. Consequently, the diffusion barriers of O* and OH* from edge sites towards terrace sites were much higher than the diffusion on the terraces and even higher than the activation barrier for reaction in the ORR. Replacing the edge Pt atoms with Au and Ag atoms weakens the adsorption of both O* and OH* on edge sites. Microkinetic analyses of ORR requires the inclusion of lateral interactions, since otherwise a full coverage of the surface with O* is predicted. Higher ORR rates are obtained on Pt(100) surfaces and --(vi)-- Pt{100} facets than on Pt(111) surfaces and Pt{111} facets. The ORR activity on Pt(111) and Pt(100) is limited by O* hydrogenation at T < 480 K and O2* dissociation at high temperatures. The ORR pathway varies greatly over these surfaces. On Pt(111), the ORR follows a peroxyl pathway at T < 500 K and a dissociative pathway at T > 700 K. On Pt(100) surface H2O* is formed via O* hydrogenation to OH* followed by 2OH* coupling to H2O* and O*. The ORR activity on multifaceted Pt surfaces was shown to be dependent on the ratio of edge sites to Pt{111} sites Modelling the inter-facet exchange of ORR intermediates based on data generated using Au and Ag modified nanowires could improve inter-facet cooperation. The most interesting case was Ag modified systems where inter-facet exchange of OH* occurs at temperatures as low as 360 K. On these systems, the ORR pathway on Pt{111} involves OH* diffusion from edge followed by OH* hydrogenation to H2O*. No O2 adsorbs on the Pt{111} facet. Edge modification has the ability to selectively enable inter-facet exchange of some reaction intermediates whilst inhibiting others. Therefore, it should be explored in rational catalyst design.