Browsing by Author "Van Steen, Eric"

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    Activity and selectivity of transition metal (Fe, Mo and W) carbides in the Fischer-Tropsch synthesis
    (2007) Patterson, Veronica A; Van Steen, Eric
    This study focused on the Fischer-Tropsch activity and selectivity of transition metal (iron, molybdenum and tungsten) carbides. The carbide catalysts were prepared by a temperature programmed method. The properties of the materials were characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), BET measurements and temperature programmed desorption of carbon monoxide (CO-TPD). The performance of the materials was tested in a Berty reactor. A reduced, precipitated iron oxide catalyst was used as a reference catalyst.
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    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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    Alumina-modified cobalt catalysts for the Fischer-Tropsch synthesis
    (2018) Petersen, Anna Paula; Van Steen, Eric; Claeys, Michael
    In the Fischer-Tropsch process, valuable hydrocarbons are produced using the basic starting materials hydrogen and carbon monoxide, which can be derived from alternative carbon sources such as coal, gas or biomass [1]. Although this process has been studied for almost a century, the effects of the support material on activity, selectivity and stability of the catalyst remain obscure. This study aims to gain fundamental insights into the effect of metal-support interactions in cobalt alumina based Fischer-Tropsch catalysts. To accomplish this, the effects of metal-support interactions have to be isolated from possible convoluting effects of the metal crystallite size and support porosity. This is achieved by preparing inverse-model catalysts, in which the support is deposited onto the metal, in contrast to conventional supported catalysts, in which the metal phase is deposited onto a porous support [2]. Cobalt alumina inverse-model catalysts were prepared by incipient wetness impregnation of cobalt oxide with aluminium sec-butoxide. The alumina loading was varied systematically between 0 and 2.5 wt% Al. The catalysts were characterised by X-ray diffraction (XRD), Transmission electron microscopy (TEM), H2 -chemisorption, and X-ray absorption near edge spectroscopy (XANES). The catalyst reducibility was studied by temperature programmed reduction (TPR), in situ (XRD) and in situ (XANES) experiments. The catalytic performance for the Fischer-Tropsch synthesis was studied in a slurry reactor under industrially relevant conditions. The alumina modification was found to prevent sintering and decrease the reducibility of the catalysts. With increasing alumina loading, and increasing calcination temperature, reduction peaks shifted to higher temperatures and peaks with maxima above 400 ˝C appeared in the TPR. The kinetic evaluation showed that the decreased reducibility was due to a decrease in the pre-exponential factor, which suggests that the alumina modification hindered hydrogen activation and/or nucleation of reduced cobalt phases. The activity of the catalysts for the FT reaction was found to increase with increasing alumina loading. This was likely an effect of the increase in metal dispersion upon alumina modification. Furthermore, alumina-modified catalysts had a higher C5+ and olefin selectivity, and lower methane selectivity. Pyridine-TPD experiments showed that the alumina modification introduced Lewis acid sites to the cobalt catalysts. Lewis acid sites may interact with adsorbed CO thereby weakening the C-O bond and facilitating CO dissociation. This was supported by CO-TPR experiments, which revealed that alumina-modified catalysts had an increased activity for the surface catalysed Boudouard reaction. It is concluded that the alumina modification increased the rate of CO dissociation on metallic cobalt. An increased rate of CO dissociation may lead to coverage of the metal surface with carbon thereby decreasing hydrogenation and shifting the product selectivity towards high molecular weight products. Hence, alumina may promote the selectivity of cobalt catalysts via a synergistic effect.
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    Ammoxidation of propene over iron promoted bismuth molybdate for the production of acrylonitrile
    (2001) Maripane, Kgolole David; Van Steen, Eric
    The fundamental aim of this study is to prepare pure bismuth molybdate (α-phase) followed by iron impregnation at different ratios of iron to bismuth. The influence of iron in the ammoxidation of propene for the formation of an acrylonitrile is to be investigated.
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    AuPt nano-alloys as reduction promoters for Co/TiO₂ Fischer-Tropsch catalysts
    (2014) Kunene, Avela; Van Steen, Eric; Claeys, Michael
    Cobalt-based catalysts for the Fischer-Tropsch synthesis are typically promoted with noble metals to achieve a more facile reduction of Co₃O₄ to the catalytically active metal, Co⁰. Hydrogen spillover is thought to be the dominant mechanism for the functioning of noble metals during the reduction process. Platinum is a well-known reduction promoter and its functioning as a reduction promoter is thought to occur via H₂ - spillover mechanism. This process is switched off during the Fischer-Tropsch synthesis, when platinum is used as a reduction promoter, since platinum has been shown to be catalytically inert under these conditions, due to strong adsorption of CO. Some hydrogen spill-over during the Fischer-Tropsch synthesis might be desired to obtain more stable catalysts (less coking), but this effect has to be balanced against increased methanation activity.
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    Carbidization and size effects of unsupported nanosized iron in the low temperature Fischer-Tropsch process
    (2012) Amod, Muhammad Ali; Claeys, Michael; Van Steen, Eric
    In the process of developing the most efficient production of fuels from coal or natural gas, there have been major advances in the development of the catalysts used. Previous work at the Centre for Catalysis Research, at the University of Cape Town, has shown great potential and provided a much deeper under- standing of the workings of the Fischer-Tropsch catalyst. The research has found that the catalyst crystallite size plays a crucial part in the product selectivity and requires strict control in order to obtain a certain desired product spectrum. The aim of this project is to provide insight on the behavior of various iron oxide crystallite sizes when placed in a CO concentrated environment during catalyst pretreatment. It will also clarify whether the sizes of the nano-crystallites will increase or decrease when the different phases form and which size carbides faster.
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    Characterization of gold catalysts for methanol synthesis
    (2012) Van Heerden, Tracey; Van Steen, Eric; Case, Jenni
    The activity (per mass of catalyst) of supported gold catalysts across a range of reduction and oxidation reactions is significantly affected by the average crystallite size of the gold crystallites in the catalyst. Supported gold catalysts are most commonly characterized for particle size using TEM and XRD. Both of these methods can have a large degree of inaccuracy associated with them at low metal loadings and for catalysts containing small gold crystallites. In this study oxygen chemisorption was used as an additional method to characterize supported gold catalysts to complement electron microscope techniques. Agreement between the results from these different methods was obtained only with regard to the order of magnitude range of crystallite size. The oxygen chemisorption can be used to estimate the mass fraction of gold present as nano-crystallites (typically less than 1%) implying a large room for improvement in catalyst preparation technique. In this study a range of supported gold catalysts were prepared by ion exchange, varying a range of preparation variables, including gold concentration in the precursor solution, washing procedure using an aqueous ammonia solution, as well as drying and calcination procedures. The washing procedure and in particular the concentration of ammonia and the duration affected the final metal loading of the catalyst. TEM analyses show crystallite size distributions between 2-5nm for all catalysts excepting those which were not washed using an aqueous ammonia solution and which did not show any small crystallites. Only the total omission of the ammonia wash resulted in a significant change in the gold crystallite sizes observed on TEM-images. Further characterization with SEM showed that catalysts that appeared identical on the TEM-images also contained large 50-500nm crystallites. This additional method of characterisation using SEM allowed for the identification of significant differences between catalysts upon varying the preparation method. Catalyst drying was also shown to be a crucial step in the catalyst preparation method, with SEM images displaying only small well-distributed gold crystallites for catalysts dried in the rotary evaporator. Two of the catalysts were then tested for their activity and selectivity in the hydrogenation of CO or CO 2. Although it has been shown that the production of methanol from CO (and CO 2) can be catalysed by gold particles with crystallite sizes below 5nm (Haruta, 1997), this reaction has received comparatively little attention compared to the more extensively studied CO oxidation reaction. Testing was done over a range of temperatures (200 - 350°C) at a pressure of 30bar. The obtained methanol yields and selectivities are comparable to reported values in literature. The hydrogenation of CO 2 was shown to have higher yields and selectivities to methanol than the hydrogenation of CO over the same catalyst. The preparation of the catalyst was shown to have an effect on the activity and selectivity, with the catalyst dried in the rotary evaporator having a higher yield and selectivity to methanol, while also forming a larger variety of products than the catalyst dried in the oven.
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    Cobalt core-shell nanoparticles as precursors for cobalt-based Fischer-Tropsch synthesis catalysts
    (2018) Govender, Alisa; Van Steen, Eric; Olivier, Jaco; Forbes, Roy; Neethling, Jan
    Core-shell nanoparticles may have an economic advantage over traditional nanoparticles as a catalyst, since the expensive, catalytically active material, which is subsurface, may be replaced with a cheaper counterpart. Furthermore, core-shell nanoparticles may be tailored to have a specific structure and composition at the nanoscale, due to a mixing of electronic properties of each phase and/or geometric effects. In this study, nickel ferrite (NiFe2O4) and zinc ferrite (ZnFe2O4) were chosen as core materials around which a cobalt (II, III) oxide (Co3O4) shell was grown. These ferrites were chosen due to their structural similarity to Co3O4 as this was expected to allow an epitaxial growth of the Co3O4 shell onto the ferrite core. Additionally, the difference in the lattice parameter between each ferrite core and the Co3O4 shell was postulated to introduce a varying degree of strain onto the shell, particularly after reduction when metallic cobalt should be present. Core-shell nanoparticles with either a nickel ferrite (NiFe2O4) core or a zinc ferrite (ZnFe2O4) core and a cobalt (II, III) oxide (Co3O4) shell (NiFe2O4@Co3O4 and ZnFe2O4@Co3O4 respectively) were synthesized, characterized and tested for their performance in the Fischer-Tropsch synthesis. These core-shell systems were compared to each other to evaluate the influence of the core and the applicability of NiFe2O4 or ZnFe2O4 as core nanoparticles in a cobalt-based Fischer-Tropsch catalyst. NiFe2O4@Co3O4 core-shell nanoparticles were also supported on Stöber silica spheres to determine the effect of the support on its properties and performance. The influence of two different reduction conditions, viz. 180°C (1 hour) or 230°C (2 hours), on the structure and Fischer-Tropsch synthesis performance of unsupported and Stöber silica spheres supported NiFe2O4@Co3O4 core-shell nanoparticles was also studied. Prior to the preparation of the core-shell nanoparticles, each ferrite core was prepared using the citrate precursor method. A Fe/M mole % ratio (where M is Ni or Zn) of 2.3 and calcination temperature of 450°C yielded phase pure NiFe2O4 or ZnFe2O4 nanoparticles with an average size of 14 nm. Using nickel ferrite (NiFe2O4) nanoparticles as a core, the growth of cobalt (II, III) oxide (Co3O4) around the core was studied by following a homogeneous precipitation synthesis. It was established that a two-step synthesis route was needed to synthesize the core-shell material with a fairly uniform Co3O4 shell. It was found that for both NiFe2O4@Co3O4 and ZnFe2O4@Co3O4 core-shell nanoparticles, the assynthesized materials had a Co3O4 shell around the ferrite core with an average thickness of 2 nm. NiFe2O4@Co3O4 and ZnFe2O4@Co3O4 core-shell nanoparticles were compared to each other as precursors for Fischer-Tropsch synthesis catalysts. Here, the first report on the nanoscale restructuring during reduction of these core-shell nanoparticles in pure hydrogen at 230°C and 250°C, respectively, was observed. This resulted in the formation of small cobalt islands on the ferrite surface. Catalytic testing of the core-shell materials, NiFe2O4@Co3O4 and ZnFe2O4@Co3O4, after reduction showed a cobalt-time yield of 13.64 µmolCO .gCo -1.s -1 and 4.27 µmolCO .gCo -1.s -1 and a C5+ selectivity of 47 C-% and 68 C-%, respectively. The observed difference in cobalt-time yield and selectivity between NiFe2O4@Co3O4 and ZnFe2O4@Co3O4 core-shell nanoparticles was due to a combination of effects that included the presence of cobalt islands over the surface of the core and the difference in extent of reduction of each core under Fischer-Tropsch synthesis conditions. The core-shell structure in NiFe2O4@Co3O4 core-shell nanoparticles was found to be retained with the use of mild reduction conditions of 180°C (1 hour). Thus, the performance in the Fischer-Tropsch synthesis of a system with a true core-shell structure with a cobalt shell was established. The former has not been reported to date. Owing to the former, strain effects may have contributed to NiFe2O4@Co3O4 core-shell (reduced at 180°C, 1 hour) having a low cobalt-time yield of 8.40 µmolCO .gCo -1.s -1 and a C5+ selectivity of 38 C-% during the Fischer-Tropsch synthesis. It was also shown that NiFe2O4@Co3O4 core-shell nanoparticles reduced at 180 °C (1 hour) had a similar activity to unsupported Co3O4, however, the former had a higher C5+ selectivity. The differences in the performance between NiFe2O4@Co3O4 core-shell (reduced at 180°C, 1 hour) and unsupported Co3O4 may have been due to strain effects. The nanoscale structural and compositional differences induced by each reduction condition applied may have been the cause for the inferior Fischer-Tropsch synthesis performance of these core-shell nanoparticles after reduction at 180°C for 1 hour than 230°C for 2 hours. The effect of a Stöber silica spheres support on the characteristics and Fischer-Tropsch synthesis behavior of NiFe2O4@Co3O4 core-shell nanoparticles was also investigated. Prior to characterization and the Fischer-Tropsch synthesis, NiFe2O4@Co3O4/SiO2 was reduced at either 180°C for 1 hour or 230°C for 2 hours. A higher cobalt-time yield (23.80 µmolCO .gCo -1.s -1) with a lower C5+ selectivity (44 C-%) was obtained with reduction at 230°C (2 hours) than 180°C (1 hour). After reduction at 230°C (2 hours), the influence of the support was clearly seen due to the higher activity obtained with NiFe2O4@Co3O4/SiO2. However, the unsupported and supported NiFe2O4@Co3O4 nanoparticles had similar product selectivities. After reduction at 230°C for 2 hours and exposure to Fischer-Tropsch synthesis conditions, the core-shell structure was retained in NiFe2O4@Co3O4/SiO2 possibly due to reducing the contact between the individual core-shell nanoparticles due to the presence of the support. This would be enhanced by anchoring the core-shell nanoparticles onto the Stöber silica spheres support.
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    Deposition of Au/Pt on Co/SiO 2 for Fischer-Tropsch synthesis
    (2014) De Beer , Martin Patrick; Van Steen, Eric; Claeys, Michael
    Cobalt Fischer-Tropsch catalysts, which are used when the desired products are long chain, linear waxes and diesel [1], are promoted with noble metals [2].This is to primarily increase the reducibility of the cobalt oxide (Co3O4) phase present in the supported catalyst during preparation but also has also been seen to effect the Co3O4 crystallite size (i.e. dispersion) and intrinsic activity of these catalysts [3]. These promoted catalysts are typically prepared by co-impregnation [4] or sequential impregnation [5, 6] with a noble metal precursor. This study investigates the preparation of cobalt Fischer-Tropsch catalysts. The effect of using mixed cobalt precursors (i.e. cobalt nitrate and cobalt acetate) in the preparation of unpromoted 10 wt% Co/SiO2 catalysts is investigated. The incorporation of higher amounts of cobalt nitrate is found to result in larger Co3O4 particles with higher reducibility and higher metallic Co-surface area after reduction. The formation of large amounts of hardly-reducible cobalt species (possibly cobalt silicates) are suspected from the reduction behaviour of catalysts prepared with higher amounts of cobalt acetate. The use of some cobalt acetate, however, in the promoted catalyst (which is expected to have an increased reducibility) may derive greater benefit than the catalyst prepared from pure cobalt nitrate by enhancement of the reduction of these hardly-reducible cobalt species. The promotion of the calcined cobalt acetate-cobalt nitrate catalyst with platinum and gold by strong electrostatic adsorption (SEA) is investigated. The promotion with these catalysts with platinum and gold by this method is achievable however subsequent calcination results in extensive sintering of gold particles (this was not observed in the platinum case). The pH during SEA is found to have an effect on the adsorption of platinum and gold species with the adsorption of platinum decreasing and that of gold increasing with increasing pH. This is possibly explained by different adsorption mechanisms for the AuCl4-and PtCl62- species. The physical characteristics of these promoted catalysts are investigated. Promotion with platinum results in a significant enhancement of the degree of reduction and a decrease in the reduction temperatures of the processes associated with Co3O4 reduction as well as the hardly-reducible species present on these catalysts. These catalysts show a higher metallic Co-surface area than the unpromoted case. The pH of the SEA solution seems to have a significant effect on the reaction performance of these catalysts. The Pt promoted catalysts promoted at low pH and high pH both demonstrated significantly higher mass specific activity than the unpromoted catalyst with the vcatalyst promoted at low pH having the highest activity. These catalysts showed comparable methane selectivities and chain growth probabilities to the unpromoted catalyst. The promotion with gold is, unfortunately, much less promising. Promotion by SEA (and subsequent calcination) results in very large gold particles. The presence of these particles on the catalyst has some effect on the reduction of the catalyst, but it unlikely any positive effect on the degree of reduction is derived from this effect as the degree of reduction in fact decreases in these catalysts. These catalysts have a marginally higher or slightly lower metallic Co-surface depending on the pH of the SEA solution. The gold promoted catalyst prepared at low pH had a slightly higher mass specific activity than the unpromoted catalyst however the catalyst promoted at high pH in fact had a decrease in activity. The gold-promoted catalysts generally had higher methane selectivity and lower chain growth probabilities than the unpromoted catalysts. The hypothesis of this work was: “The pH of the solution in which Co3O4/SiO2 is promoted by SEA has an effect on the position at which the noble metal complex adsorbs and will thus ultimately have an effect on the properties of the promoted catalyst” It is difficult to state conclusively whether the pH of the SEA solution had an effect on the position of the noble metal but it is apparent that the pH has a significant effect on the catalytic performance of both the platinum- and gold-promoted catalysts.
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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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    DFT modelling on the effect of manganese in cobalt-based FT-catalysts
    (2023) Ragoo, Yatheshthrao; Van Steen, Eric
    The Fischer-Tropsch synthesis (FTS) process can be described as a combination of reactions that convert synthesis gas (essentially CO and H2) into long chain hydrocarbons (syncrude), which in turn is refined into transportation fuels, lubricants, and other petrochemicals. Among the commercially implemented catalysts such as Fe and Co, the latter is considered as a successful candidate for catalysing FTS reactions towards long chain hydrocarbons (C5+) primarily due to its high activity and longer lifespan. To further drive the selectivity towards longer chain hydrocarbons, manganese may be added to the cobalt-based catalyst as a promoter. To this date, experimental studies have suggested that Mn exists as MnOx in the working catalyst, and it has been proposed that manganese facilitates the dissociation of CO. To investigate the promotional effect of Mn, suitable DFT-based MnOx models were devised on the basis of their formation under Fischer-Tropsch conditions. These were modelled on the Co(111) surface owing to it being amongst the densest surface planes of cobalt. Thermodynamically, the OMn and O2Mn ligands, are the most likely form of Mn on a Co(111) surface under Fischer-Tropsch conditions with OMn being the ligand present at low ratios of H2O to H2 and O2Mn becoming the dominant species at high conversion. An essential feature of the Fischer-Tropsch synthesis is the adsorption and dissociation of adsorbed CO and the removal of surface oxygen. The presence of the dominant forms of the manganese complex (OMn and O2Mn) on Co(111) on these reaction steps was probed. It was found that the presence of OMn on Co(111) stabilised the adsorption of CO further and both the presence of OMn and O2Mnduces an elongation of the C-O bond in adsorbed CO. The presence of OMn also stabilises the dissociation products, co-adsorbed carbon and oxygen on Co(111), whereas the presence of O2Mn does not seem to affect the dissociation equilibrium significantly. The presence of these ligands slightly enhances the rate of dissociation of CO by lowering the barrier for CO-dissociation from 2.59 eV on Co(111) to 2.55 eV in the presence of OMn and to 2.37 eV in the presence of O2Mn. Hence, the presence of MnOx on Co(111) results in a slightly faster direct CO dissociation than on a clean slab. It should however be noted that the direct CO-dissociation on Co(111) is very slow at typical Fischer-Tropsch conditions, even in the presence of MnOx ligands. Hence, hydrogen-assisted CO dissociation is typically considered on these dense surfaces. The hydrogen assisted CO-dissociation over Co(111) proceeds with an activation barrier of 0.18 eV. The presence of MnOx ligands does not seem to facilitate the hydrogen-assisted dissociation based on the pathways considered as elevated barriers were determined. The disproportionation of surface hydroxyl species was considered as the key reaction for the removal of surface oxygen as water, and the systems involving the precursors of the reaction were compared. The presence of MnOx ligands on Co(111) offers new pathways for oxygen removal involving hydroxylated surface manganese complexes, i.e., OMn(OH) and O2Mn(OH), which can act as reactive intermediates in the oxygen removal. A microkinetic analysis shows that the oxygen removal in the presence of O2Mn on Co(111) was ca. 104.2 times faster than in the absence of this ligand under Fischer-Tropsch conditions. It is thus concluded that manganese as a promoter for cobalt-based catalysts may affect the CO adsorption but may not affect the dominant indirect hydrogen assisted CO-dissociation. The promotional effect of manganese may be related to the oxygen removal from Co(111)
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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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    The effect of activation on the deactivation of a precipitated iron based catalyst
    (2009) Cloete, Jezreel; Van Steen, Eric
    There has been renewed interest in the Fischer-Tropsch process since diesel fuel derived from natural gas Fischer-Tropsch processes can meet the proposed future legislation standards for sulphur emissions without further processing. Oil refineries producing diesel fuel on the other hand would have to invest $4-13 billion to meet the proposed sulphur content requirements. Also, Fischer-Tropsch processes using coal as a feedstock could become and attractive alternative to oil refining due to the large differential between crude oil and coal prices...A study was undertaken in order to investigate the effect of low hydrogen content synthesis gas (as would be derived from coal) on the performance and lifetime of an iron-based catalyst during the low temperature Fischer-Tropsch process. In particular, the effect of the activation procedure on the lifetime of the iron-based catalyst (deactivation behaviour) was investigated...
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    The effect of CO2 on the activation of a precipitated iron Fischer-Tropsch catalyst
    (2000) Harding, Samantha; Van Steen, Eric
    The effect of CO2 on the activation of a precipitated iron catalyst by hydrogen and carbon monoxide was investigated. The iron-based catalyst was precipitated from a mixture of iron nitrate and copper nitrate then bounf by the addition of potassium waterglass to achieve a final catalyst precursor composition of 3.8K2O/4.8Cu/26.0SiO2/100Fe. The activation procedures used four different gas compositions.
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    The effect of copper as a chemical promoter on the performance of a cobalt based Fischer-Tropsch synthesis catalyst
    (2000) Van Wyk, Gert Stefanus; Dry, Mark; Van Steen, Eric
    In the light of the fact that copper is used as a promoter for the iron catalyst and that the industry is looking at a commercial FT synthesis cobalt catalyst, it is essential to reinvestigate the effect of copper on a cobalt catalyst. Copper has an effect on the overall activity of iron and cobalt [Kolbel, 1951, J. Schwank, 1991], but the reasons are not clearly understood. In the present study the role of copper as a promoter on the performance of 16.67% (wt) Co/Si02 is investigated with TPR, CO chemisorption, TEM and FT synthesis.
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    The effect of temperature and crystallite size on the growth and morphology of carbon nanotubes
    (2005) Kleinsmidt, Jacques N; Van Steen, Eric
    The aim of the study was to synthesise iron oxide crystallites with different crystallite sizes supported on y-A120 3 using the reverse micelle technique. It was hypothesised that changing the crystallite size of the synthesised iron oxide crystallites could lead to the control of the external nanotube diameter. The effect of temperature on the external diameter and productivity was also investigated. It was found through titration and AAS that the iron loading was lower than the expected 15 wt.-%. Furthermore, it was observed that the loading was not consistent through different catalyst samples. This was attributed to incomplete precipitation of iron using the reverse micelle technique, the rigorous cleaning regime implemented and weak metal-support interaction. It was found through XRD and TEM that crystallites in the nanometre range were produced although they were not well distributed over the support. It was also found that the expected linear relationship between water to surfactant ratio and crystallite size was not achieved. Hence the obtained crystallite sizes were significantly different from those obtained in the work by Mabaso.
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    The effect of temperature on the Fischer-Tropsch selectivity and further mechanistic insights
    (2009) Fletcher, Jack Vincent; Van Steen, Eric
    Concern’s that the world’s energy supply will not be able to keep pace with rising energy demands, have surfaced periodically for much of the petrochemical industry’s nearly 150 year history, but each time the industry has responded with technological advances and innovations to satisfy the global energy needs. Future advances will most likely include the enhanced recovery of conventional oil, the production of extra-heavy oil / tar sands and the utilization of alternative energy production technologies (technologies other than crude oil refining). The Fischer-Tropsch Synthesis (FTS) discovered in 1923 by Fischer and Tropsch, is one of these alternative fuel production technologies and can briefly be defined as the means used to convert synthesis gas containing hydrogen and carbon monoxide over a group VIII metal catalyst to hydrocarbon products and water. Given the vast product spectrum possible for the FTS (paraffins, olefins, alcohols, carbonyls, acids and aromatics), a great deal of controversy still exists as to the chemical identity of the monomeric building block and the propagation of the hydrocarbon chain on the catalyst surface [van Dijk., 2001]. Several mechanisms have been published with the four most popular (alkyl, alkenyl, enol and CO-insertion), recently reviewed by Claeys and van Steen (2004). It must however, be appreciated that given the complexity of the FT reaction it is generally accepted that more than one mechanism may operate on the catalyst surface at any one time. Furthermore, process parameters such as temperature, total pressure, partial pressure, hydrogen to carbon monoxide ratio, space velocity and residence time all have an influence on the FT product selectivity. Because of this it becomes exceptionally complicated to determine the effects of just one parameter while taking the effects of the additional parameters into account.
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    The effect of water partial pressure on low temperature iron Fischer-Tropsch reaction rate, selectivity and catalyst structure
    (2004) Biel, Herbert Benjamin; Van Steen, Eric
    The Fischer-Tropsch synthesis catalysed by iron is a well-established process for the production of synthetic fuels, waxes and many other chemicals, yet there are many aspects that are still not totally understood. Controversy still exists as to what the active phase(s) is of the iron Fischer-Tropsch catalyst. A big drawback of the iron based Fischer-Tropsch synthesis is that one of its primary products, wate, changes the structure and stability of the catalyst. Little is know about the effect that water partial pressure has on the phases present in the working catalyst.
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    Fischer-Tropsch synthesis over SiO2, ZnO and MnO supported cobalt catalysts
    (1999) Walsh, Richard; Van Steen, Eric
    Silica is well known as a support for cobalt supported Fischer-Tropsch catalysts. Silica has a high surface area with an amorphous structure that promotes dispersion of the active cobalt phase over the support surface. This dispersion is vital in terms of catalyst performance and derives from the strength of interaction between the cobalt and the support. However, the stronger the metal support interaction, the greater is the loss of active cobalt through formation of cobalt support species that are hard to reduce. Consequently ZnO and MnO were evaluated in comparison to Si02 as supports for cobalt supported Fischer-Tropsch catalysts. The aim of the study was to characterise the interaction between cobalt and the three supports (Si02, ZnO and MnO) in terms of the cobalt reducibility as visualised using TPR, exposed cobalt surface area and cobalt dispersion as evaluated using hydrogen chemisorption, and catalytic performance under Fischer-Tropsch synthesis conditions.