OpenUCT :: Browsing by Author "Claeys, Michael"

Browsing by Author "Claeys, Michael"

Now showing 1 - 20 of 38

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.
Open Access
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.
Open Access
Application of Molybdenum Carbide Catalysts for the CO2-assisted Oxidative Dehydrogenation of Ethane
(2022) Marquart, Wijnand; Fischer, Nico; Claeys, Michael
The rising demand for light olefins is at present mainly met via catalytic/thermal dehydrogenation of alkanes at temperatures of up to 900 °C. Under these severe process conditions, competing side reactions and catalyst deactivation via coking are the major challenges. Co-feeding an oxidant significantly decreases the reaction temperature. The oxidative dehydrogenation of ethane to ethylene, using CO2 as the oxidant (CO2-ODH), has earned a lot of interest in the past decade. The use of CO2, a soft oxidant in comparison to O2, prevents the overoxidation reaction of the paraffin to CO2 and allows for improved heat control. Besides that, the coking effect, which is believed to be the main catalyst deactivation pathway during these high temperature processes, could be significantly lowered due to the reverse Boudouard reaction. The most common catalytic materials reported are reducible metal oxides (MOx) due to their redox properties; a key concept to activate the C-H bond of the alkane and subsequently activate CO2. Besides metal oxides, transition metal carbides have also shown to be active for the CO2-ODH, reaching high yields of ethylene. Specifically, molybdenum carbide (MoxCy) has shown to be a highly efficient catalyst for CO2 activation and alkane dehydrogenation, demonstrating its ability to cleave C-H bonds. These characteristics are important in making a MoxCy-based catalyst a serious candidate for the CO2- ODH of light alkanes. This work entails the design of novel MoxCy-based catalysts for application in the CO2-ODH of C2H6. Previous work on MoxCy-based catalysts found that the bulk material has limited activity and selectivity towards producing C2H4 but is significantly improved once dispersed on a support material. The type of support material dictates whether the CO2-ODH reaction takes place, or if one of the major side reactions, the dry-reforming of C2H6 to synthesis gas, is preferred. In this study, MoxCy nanoparticles were prepared via various (novel) synthesis techniques, dispersed on a variety of MOx support materials as well as modified with various promoters. Besides the exploratory nature of this study, gaining knowledge on the activity of the various formulations of MoxCy-based catalysts, the preparation conditions of the carbide materials were investigated. To prepare MoxCy, the precursor samples (in the molybdate or oxide phase) are exposed to a temperature programmed treatment (carburization) in the presence of a carbonaceous and reductive gas mixture. The carbide formation, in terms of crystallite structure, surface composition as well as potential fouling mechanisms is highly dependent on the heating rate, gas mixture, final temperature and precursor composition. Various experiments utilizing in situ characterization techniques, such as in situ X-ray diffraction, X-ray adsorption and Raman spectroscopy as well as online product analysis techniques were employed to gain knowledge on the carburization process, the structural and chemical properties and their effect on the activity of the various prepared catalysts in the CO2-ODH as well as the reverse water-gas-shift reaction. The use of MoxCy-based catalysts in the CO2-ODH reaction has not been thoroughly investigated in literature before and is still a very new topic to the scientific community. The presented research can contribute on various aspects of the use and viability of MoxCy-based catalysts in CO2 utilizing reactions and can be extended to dry-reforming or CO2 hydrogenation to fuels. In terms of catalyst synthesis, the extensive characterization exposing the various possible crystal structures of MoxCy nanoparticles and application of surface sensitive techniques, allowed for a better understanding of the possible active phases responsible for CO2 and alkane activation. Besides the identification of the active phase, the deactivation mechanism for MoxCy-based catalysts in the CO2-ODH reaction is studied in more detail by focusing on the crystal structure and the presence of carbon on the catalyst surface. By varying catalyst compositions as well as reaction conditions, including the use of various co-feeding experiments, an increase in catalytic stability, while maintaining high yields of the desired product from CO2-ODH (ethylene), was achieved.
Open Access
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.
Open Access
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.
Open Access
[Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ electride as a novel support system for iron-based Fischer-Tropsch synthesis
(2018) Motala, Muhammad; Fischer, Nico; Claeys, Michael
Iron-based Fischer-Tropsch (FT) technology is well established in the industrial production of hydrocarbon resources. The use of a stable inorganic electride as a support system, with potential promoter effects which could replace commonly used alkali metals, may have the potential to open a new facet within the research and development of iron-based FT technology. The hydrothermally produced [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ electride material as carrier for Ru nanoparticles for the synthesis of ammonia from its elements has been reported in literature as support system and substitute to conventional alkali promotion. This is due to highly localized electrons present in the crystallographic cages of the material that may serve as a Lewis base and facilitate bond breaking of feedstock molecules adsorbed to the catalytic active sites. The hydrothermal preparation of this electride previously resulted in a surface area of up to 50m²·g⁻¹ and a two-fold rate increase in ammonia production. The low work function of [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ is comparable to that of potassium, therefore the iron loading onto the electride is postulated to mimic the promoting effects of potassium(oxide) in the FT synthesis. Following the hydrothermal synthesis of the electride produced at varying evacuation temperatures (800 °C and 1000 °C), supported iron catalysts were prepared using a stoichiometric amount of Fe(acac)₃ precursor to decompose as iron oxide onto the novel support. Iron supported on the unreduced mayenite precursor, [Ca₂₄Al₂₈O₆₄]⁴⁺(O²⁻)₂, and unpromoted precipitated iron diluted in γ-alumina were used as control/baseline catalysts. The testing of these catalysts was conducted at 240 °C and 15 bar with a 2:1 H₂/CO ratio. The electride and mayenite catalysts performed similarly with regards to hydrocarbon selectivities with the precipitated iron bulk catalyst. However, significantly larger CO₂ selectivity and olefin/paraffin ratios were observed for the supported catalysts showing little difference between the electride and its unreduced counterpart.
Open Access
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.
Open Access
The development of an in-situ X-ray diffraction cell for Fischer-Tropsch catalyst characterisation
(2012) Clapham, Brett; Claeys, Michael
In the current study, the successful development of a novel in-situ X-ray diffraction cell is described. This cell allows the direct observation of crystallite changes to be made under reaction conditions and in real time. The cell permits operation up to 500°C and 25 bar to be realised, with more severe conditions being achievable upon changing the reactor component. The design is such that it can be mounted to any commercial, laboratory-scale X-ray diffractometer or synchrotron facility.
Open Access
Effect of ammonia co-feeding on oxygenates over K-Mo2C in the Fischer-Tropsch synthesis
(2018) Marquart, Wijnand; Fischer, Nico; Claeys, Michael
The Fischer-Tropsch (FT) process, producing long chained waxes and transportation fuels, is competing with fuels derived from crude oils and its profitability is therefore dependent on the global oil price. However, increasing the value of synthesized products could render the profitability of the FTS independent of fluctuations in the oil price (which are mostly due to global political trends). One way to achieve this, is to target fine chemicals instead of fuels. At the Catalysis Institute, this has been investigated by adding ammonia to the feed gas stream and obtaining highly valuable amines, amides and nitriles. It has been shown that the so-called nitrogen containing compounds are formed instead of the Fischer-Tropsch typical albeit minor products alcohols, aldehydes and carboxylic acids, i.e. oxygenates. Increasing the oxygenate selectivity was investigated in numerous studies as no commercial FT based process exists which produces oxygenates at a significant yield. Typically, transition metals such as Fe, Co, Rh and Ni are active for the FT synthesis. Based on reaction conditions employed, commercial Fe and Co based catalysts have been shown to produce between 6 and 12 C% oxygenates. Rh has been shown to have a high oxygenate selectivity, but the associated high raw material cost becomes prohibitive for use as a commercial FT catalyst. Catalysts other than the traditionally known FT active transition metals have shown promising results in terms of oxygenate selectivity. Transition metal carbides such as Mo2C, have been investigated under Fischer-Tropsch conditions. While the bare catalyst produces mainly methane and other hydrocarbons, upon promotion with potassium the selectivity shows a significant shift towards oxygenates. This project investigates the use of potassium promoted molybdenum carbide as a catalyst for high oxygenate selectivity in the Fischer-Tropsch synthesis. β-Mo2C was synthesized and subsequently promoted with different levels of potassium and its Fischer-Tropsch synthesis performance was evaluated in a stainless steel fixed bed reactor. The influence of catalyst synthesis protocols, reactor pressure and temperature, feed gas space velocity, and K/Mo wt.% promotion on catalyst activity and selectivity were studied. At a stable CO conversion (±10%) and its related oxygenate selectivity (±35 C%) ammonia was co-fed to the catalyst to study the conversion of oxygenates to nitrogen containing compounds. In summary, an unpromoted β-Mo2C catalyst reached CO conversions to ±40% at the conditions applied. Initial promotion of the catalyst with potassium showed a significant drop in catalyst activity, however, an increase in potassium content did not further decrease catalyst activity. The selectivity towards oxygenates was greatly enhanced from 10 C% up to 42 C% (CO2-free) at similar reaction conditions. Simultaneously, the oxygenate distribution shifted towards higher alcohols. The initial methanol content in the total oxygenate slate was around 60 C%, decreasing to about 20 C% upon potassium promotion. During co-feeding of ammonia, N-containing compounds were observed in the form of nitriles (±9 C%, CO2-free) and small traces of amides (±0.1 C%, CO2-free). Acetonitrile was the most dominating formed N-containing compound (≥58 C%). Upon the co-feeding of ammonia, the oxygenate selectivity decreased by roughly 10 C% points (CO2-free) but did not reach zero. Catalyst activity was slightly affected but recovered with time on stream. A slowly building up blockage appeared after 1-3 hours TOS simultaneously with a decreasing CO2 selectivity, suggesting the reaction with NH3 forming ammonium carbonate. This could however not be confirmed. The benefits of producing N-containing compounds using a potassium promoted β-Mo2C needs to be further investigated, trying to avoid the blockage by suppressing the WGS-activity of the catalyst. It is promising that the activity is hardly affected and that in the short period of time on stream N-containing compounds were observed.
Open Access
Effects of Crystallite size and water partial pressure on the activity and selectivity of low temperature iron-based fischer-tropsch catalysts
(2009) Cheang, Virginia; Claeys, Michael
Fischer-Tropsch synthesis is a reaction between hydrogen and carbon monoxide to produce long-chained hydrocarbons and water. It has been stated that the Fischer-Tropsch synthesis reaction is a surface phenomenon, thus for optimum catalyst performance, maximum metal usage must be achieved. Therefore it is expected that the smaller the crystallite size of the metal, the more active the catalyst will be under test conditions. This work investigates the influence of iron crystallite size and water co-feed on the activity and selectivity of low temperature Fischer-Tropsch synthesis. In heterogeneous catalysis it is well known that with smaller crystallites, more active surface area is exposed allowing for higher overall activities. However the subject of size sensitivity remains an issue as research has shown that nano-meter sized crystallites below a certain point (ie. 6 nm) can behave differently to larger crystallites in terms of activity and selectivity in Fischer-Tropsch synthesis. It has been suggested that this behaviour may be due to the effect of crystallite size on iron phase changes such as oxidation by product water during the Fischer-Tropsch (FT) reaction. Alternatively, effects of structure sensitivity may play a role. This work can essentially be divided into 3 sections; firstly, the preparation and characterisation of a model alumina supported iron catalyst with iron crystallites in the nano-meter range between 2 and 16 nm. Secondly, Fischer-Tropsch testing in a fixed bed reactor of model catalyst systems, testing the effect of differing crystallite size on the activity and selectivity of Fischer-Tropsch synthesis, followed by the characterisation of spent catalyst samples. Finally, Fischer-Tropsch testing of model catalyst systems under different conditions of water addition simulating conditions under high syniii thesis gas conversion, again followed by the characterisation of spent samples. The first major objective of this work was the preparation of a model catalyst sample, where a narrow size distribution was required as well as a good distribution of the metal crystallites onto the support material. A narrow size distribution was successfully achieved through the utilisation of the reverse micelle technique. Size control can be achieved through variation of the water to surfactant ratio, with a high ratio leading to larger crystallites being formed. An even distribution of metal crystallites onto the support material was harder to achieve. Four different support addition methods were tested as well as different support materials, drying conditions and calcination conditions. It was shown that the support addition method previously used by Mabaso (2005) was the one that achieved the best dispersion. Furthermore, the variation of support, drying and calcination conditions can have a large impact on the final catalyst with different supports and conditions leading to increased clustering and sintering. Alumina was found to be the best support material and a threshold temperature of 300 C existed for calcination past which severe sintering took place. Model catalysts were successfully prepared with a narrow size distribution and a good dispersion of metal crystallites onto the support material. Six catalyst samples were prepared with crystallite sizes ranging between 2 and 16 nm with a metal loading of 13 wt%. Examination of the synthesis gas conversion showed that the specific Fischer-Tropsch rate shows a decrease with decreasing crystallite size. This change in rate has been theorized to be due to either the thermodynamically simpler oxidation of smaller crystallites or the lack of ensembles of atoms required for Fischer-Tropsch synthesis on smaller crystallites. Through the characterisation of spent samples, it has been shown that the âEnsemble Effectsâ theory to be the more likely one as bulk oxidation was not observed. In terms of product formation, smaller crystallites showed a higher inclination for the production of methane. This is further support for the âEnsemble effectsâ theory, as this result leads to the conclusion that less chain growth sites and more methanation sites may be available on smaller crystallites leading to an increased methane selectivity on small crystallites. iv Chapter 0. Synopsis Other product selectivities such as olefin and oxygenate formation did not follow the trend of hydrogen richer products obtained on smaller crystallites, instead it was the catalyst samples in the middle metal crystallite size range that showed increased propensity toward secondary reactions. This was believed to be due to either an extension of the âEnsemble Effectsâ theory or the electronic effects between readsorbing olefins and the metal surface. A conversion level of below 10% was chosen for this work in order to fully and directly compare the activity and selectivity results of the various catalyst samples. However this condition means that the effect of higher conversion levels are not shown. In order to overcome this problem the water partial pressure was increased, where a higher water partial pressure simulates the conditions of higher conversion via addition of water. The âbasecaseâ water partial pressure was set at 0 bar, while water addition conditions had water partial pressures of 3 and 6 bar. In terms of specific Fischer-Tropsch rate it was shown that the addition of water leads to deactivation of the catalyst irrespective of crystallite size. This deactivation has been theorized by previous work to be either due to oxidation or sintering. Again characterisation results, including an in-situ method, show that oxidation is not the likely cause of the deactivation, instead the clustering and agglomeration of metal crystallites show that sintering is the more likely candidate. Product formation results show that the addition of water leads to a decrease in methane selectivity and an increase in olefin production. It is theorized that these selectivity results are caused by water inhibiting desorption and readsorption mechanisms leading to increased chain growth and decreased secondary reactions respectively.
Open Access
Hydrogen spillover in the Fischer-Tropsch synthesis: the role of platinum and gold as promoters in cobalt-based catalysts
(2015) Nabaho, Doreen; Van Steen, Eric; Claeys, Michael
The Low Temperature Fischer-Tropsch (LTFT) synthesis involves the catalytic hydrogenation of carbon monoxide with the aim to produce long-chained hydrocarbons. Commercial cobalt-based catalysts incorporate oxidic supports that are known to negatively affect the reducibility and hinder formation of the active phase. Consequently, reduction promoters such as Pt are introduced to facilitate the reduction of cobalt during catalyst pretreatment. However, synergistic and adverse effects of the promoter have been reported under reaction conditions including a higher site-time yield and higher selectivity towards hydrogenated products. The perspective on the operation of the Pt promoter is polarised between 'Hydrogen spillover', which is a so-called remote-control effect that could otherwise occur in the absence of Pt-Co contact, and 'ligand/electronic effects' that require direct Pt-Co coordination. The objective of this study was to explicate the operation of Pt and Au as promoters of the Co/Al2O3 catalyst by decoupling hydrogen spillover from effects that require direct promotercobalt coordination. The analysis was subdivided into the reduction process and the Fischer- Tropsch reaction, which are the two arenas in which the actions of these promoters have been claimed. The employment of model 'hybrid' catalysts, which are mechanical mixtures of the monometallic constituents of the promoted catalyst, presents a novel way to investigate the role of spillover hydrogen in the Pt-Co and Au-Co catalyst systems. Thus far, no systematic investigation of the hydrogen spillover phenomenon using these catalyst systems during both reduction and under commercially relevant LTFT conditions has been encountered in the published literature. Furthermore, this study serves to contribute to the limited body of literature on the role of Au as a potential promoter for the commercial cobalt-based catalyst.
Open Access
Hydrothermal Sintering and Oxidation of an Alumina-Supported Nickel Methanation Catalyst Studied Using In Situ Magnetometry
(2021-05-16) Maphutha, Malebelo; de Oliveira, Dominic; Nyathi, Thulani M; Fadlalla, Mohamed I; Henkel, Robert; Fischer, Nico; Claeys, Michael
The presented study investigated the effects of temperature (350–650 ◦C) and gas environment (pure Ar versus a H2O/H2 partial pressure ratio (PH2O/PH2) of 5) on the extent of sintering and oxidation of Al2O3 -supported Ni0 nanoparticles (≈4 nm). We note that a PH2O/PH2 of 5 corresponds to a simulated CO conversion of 94% during methanation. Sintering and oxidation were studied using in situ magnetometry, while ex situ TEM analyses confirmed the particle sizes before and after the magnetometry-based experiments. It was found that increasing the temperature from 350 to 650 ◦C in Ar at atmospheric pressure causes a negligible change to the average size and degree of reduction (DOR) of the starting Ni0 nanoparticles. However, studying the same temperature window under hydrothermal conditions at 10 bar causes significant particle growth (≈9 nm) and the development of a bimodal distribution. Furthermore, the presence of steam decreases the DOR of Ni0 from 86.2% after initial activation to 22.2% due to oxidation. In summary, this study reports on the expected sintering and oxidation of Ni-based catalysts under high CO conversion conditions at elevated temperatures during methanation. Importantly, we were able to demonstrate how magnetometry-based analyses can provide similar size information (and changes thereof) as those observed with TEM but with the added advantage that this information can be obtained in situ.
Open Access
In situ sintering study of model nickel catalysts
(2014) Maphutha, Malebelo; Claeys, Michael
Lipid catabolism plays a significant role in the survival of M.tb inside the host. The development of analytical techniques such as gas chromatography mass spectroscopy (GCMS) and liquid chromatography mass spectroscopy (LC-MS) has become popular as metabolomics tools in the study of such catabolic pathways. The development of biomarkers and internal standards to perform quantitative and qualitative analysis of metabolites in the catabolic pathway would be an attractive tool. Thus, cholesterol derivatives were synthesized as thia-, fluoro- and deuterium labeled analogs. Sulfur was incorporated into cholesterol at positions, C3 as well as C23. The 3â-mercaptocholest-5-ene was synthesized to block the initial stage of cholesterol catabolism and evaluate whether side chain degradation can still occur. Fluorine was integrated into the cholesterol backbone at C3 to evaluate the side-chain degradation in the absence of cholesterol oxidase activity. Steroids with fluorine at C6 are known to have good biological activity and were for this reason also synthesized. Deuterium labeled compounds were synthesized and used as internal standards for GC-MS analysis. As an alternative to cholesterol catabolism, fatty acids like stearic acid are important in producing building blocks for long chain mycolic acids which provides protection to the mycobacterium. For this reason thiastearic acid derivatives were synthesized and evaluated as biomarkers.
Open Access
In situ study of Co₃O₄ morphology in the CO-PROX reaction
(2017) Khasu, Motlokoa; Fischer, Nico; Claeys, Michael
The preferential oxidation (PROX) reaction is an effective process for the removal of trace amounts of carbon monoxide from a reformate stream. Tricobalt tetraoxide (Co₃O₄) is the candidate for CO-PROX in a H₂ rich gas and could be an alternative to the rare and expensive PGMs. This study investigates the effect of different Co₃O₄ morphologies in the preferential oxidation of carbon monoxide in H₂ rich gas. Reports have shown morphology dependency in CO oxidation in the absence of hydrogen, no study has investigated the morphology dependency in H₂ rich atmospheres. Different morphologies of nanocubes, nanosheets and nanobelts were prepared using hydrothermal mn and precipitation. Conventional spherical nanoparticles from our group were included to compare the activity of conventional nanoparticles with nanoparticles of different morphology. The model catalysts were supported on silica spheres which were also prepared. The CO-PROX experiments were conducted in the in situ UCT-developed magnetometer and PXRD capillary cell instruments by induced reduction at temperatures between 50 and 450°C. Catalyst tests showed two distinct temperature regions with maximum activity. In the range of 150 – 175ᵒC, activity decreased from nanoparticles > amine nanosheets > nanobelts. However, the surface area specific rate of CO₂ formation displayed an inverse trend. In the region of 225 – 250ᵒC, nanocubes > NaOH nanosheet > HCl nanocubes showed maximum activity. The surface area specific rate was the same for amine nanocubes and NaOH nanosheets. None of the model catalysts retained their morphology after the temperature was ramped from 50ᵒC to 450ᵒC, and back to 50ᵒC. The catalysts were partially reduced to metallic Coo (other phase being CoO). Figure 1: In situ PXRD analysis and kinetics of CH4, CO and CO₂ showing the behaviour of Co₃O₄/SiO₂ (amine nanocubes) under CO-PROX conditions
Open Access
Influence of basicity in Fischer-Tropsch synthesis over supported iron-based catalysts
(2007) Blignaut, Annalie; Van Steen, Eric; Claeys, Michael
The Fischer-Tropsch synthesis catalyzed by iron is a well-established process for the production of synthetic fuels, waxes and high-value chemicals, such as α-olefins. A draw-back of the currently used iron-based catalysts is their short lifetime, caused by sintering and particle break-up. These disadvantages might be overcome by utilizing a supported iron-based catalyst. However, supported iron Fischer-Tropsch synthesis, which has been tested up to now, show a high methane selectivity. This might be caused by a lack of alkali near the catalytic site, which can be alleviated by using a basic support. Classical basic supports such as CaO and MgO will react with CO2 (a major by-product in iron-catalyzed Fischer-Tropsch synthesis) yielding carbonates and can therefore not be used, since the formation of carbonates will result in a large particle expansion. An alternative would be to generate a silica-based basic support by attaching basic groups to the silica. In this study iron Fischer-Tropsch catalysts supported on silica were tested for conversion of synthesis gas to hydrocarbon products. Silica was modified with aminopropyltriethoxysilane (APTeS) by impregnation followed by calcination to provide basic surface groups onto the silica surface. The CHN analysis and IR-analysis indicate the presence of amine groups in the APTeS-modified silica. The pore radius distribution of silica is slightly shifted towards higher pore radii in comparison to APTeS-modified silica. It might thus be stated that aminopropyltriethoxysilane covers the pore walls and does not seem to result in pore blockage. Thermal gravimetric analysis indicates that the thermal stability of APTeS-modified silica is low. A major difference between silica and APTeS-modified silica was their zeta-potential. Whereas the surface of silica is mainly negatively charged in the pH-range of interest during impregnation, the surface of APTeS-modified silica is mainly positively charged. This is attributed to the presence of amine groups on the surface. Iron was brought onto the support by impregnation. The surface modification of silica with APTeS seems to be destroyed upon calcination of the impregnated catalysts. The iron phase in the calcined iron catalyst supported on silica catalysts is mainly hematite (Fe203), whereas the iron phase in the calcined iron catalyst supported on APTeS-modified silica catalysts is mainly iron oxide hydroxide FeOOH. The presence of basic amine groups may favour the formation of FeOOH crystallites during the impregnation/calcination on the APTeS-modified silica. The FeOOH-crystallites on the APTeS-modified silica support are typically smaller than the Fe203 crystallites on silica. The maximum catalytic activity is obtained at 0.01 mol K I mol Fe for the iron catalyst supported on silica and at 0.02 mol K I mol Fe for the APTeS-modified catalyst, indicating the optimum potassium loading. The difference in the optimum potassium loading might be linked to the smaller crystallite sizes obtained with the APTeS-modified catalyst. All the potassium promoted catalysts show a lower methane selectivity compared to the 0 K iron catalyst supported on silica and the 0 K iron catalyst supported on APTeS-modified silica. The 1-olefin and n-olefin content in the fraction of linear hydrocarbons increase with increasing potassium loading over all the iron catalyst supported on silica promoted with potassium except for the catalysts 0.005 K and 0.01 K. Increasing potassium content on the catalyst resulted in higher 1-olefin content in the fraction of linear olefins. The trend suggests that potassium promotion suppresses secondary double bond isomerisation of 1-0lefin into internal olefins. The high degree of branching obtained with the 0.005 K catalyst and the 0.01 K catalyst, is characteristic of weak alkali promotion. The iron catalysts supported on APTeS-modified silica indicate an increase in the degree of branching with increasing potassium content.
Open Access
Influence of preparation techniques on the Fischer-Tropsch performance of supported cobalt catalysts
(2003) Chirinos Maruri, Ada Elida; Claeys, Michael; Fletcher, Jack; Van Steen, Eric
Cobalt based catalysts are generally used for the FT synthesis due to their high activity and selectivity for linear hydrocarbons, low activity for the water gas shift reaction and lower price compared to noble metals [22]. There can, however, be a large effective loss of active metal due to strong metal-support interaction forming complexes that are not reduced at temperatures below 400°C.
Open Access
Investigation of the promotional effect of Cu and Ag on iron-based Fischer-Tropsch catalysts using ferrites as model catalysts
(2014) Chonco, Zandile Hlengiwe; Van Steen, Eric; Claeys, Michael
The Fischer-Tropsch synthesis is regarded as a stepwise polymerisation reaction between adsorbed hydrogen, carbon monoxide and monomers formed from the reaction of hydrogen and carbon monoxide. The catalytically active metals for industrial application are cobalt and iron. The commercially used iron-based Fischer-Tropsch catalyst is supported on silica (Si), to improve the dispersion of the active metal and is promoted with small amounts of potassium to enhance the activity and selectivity of the catalyst and copper to enhance the reducibility of the iron oxide. However, the effect of copper on the iron catalyst on the product activity and selectivity remains elusive. A number of studies that have been conducted on the promotional effect of copper on iron-based Fischer-Tropsch catalysts have mainly been focused on fully promoted iron-based FT catalyst (Fe/Cu/K/Si), thus making it difficult to exclusively study the effect of the overall promotional effect of copper on the FT performance of iron-based catalysts. Additionally, minimal work has been conducted on the promotional effect of metals (i.e. silver) in the same group in the periodic table as copper. A previous study further showed that silver had no effect on the FT performance of the iron catalysts. These results were ascribed to the lack of intimate contact between the promoter and the catalytically active phase. In this study, copper and silver ferrites which are model iron catalysts composed of Cu or Ag as promoters (CuFe2O4, CuFeO2 and AgFeO2) will be prepared via the co-precipitation method. The model catalysts will then be activated in H2 and CO reaction environment and exposured to Fischer-Tropsch conditions in an attempt to understand the influence of the copper (Cu) as well as silver (Ag) on the iron catalyst. The results are compared to maghemite (Î³-Fe2O3) and hematite (Î±-Fe2O3). The presence of group 11 metals in the crystal structure facilitates the reduction of trivalent iron into magnetite during catalyst activation in either hydrogen or carbon monoxide and the consecutive conversion of Fe3O4 to Î±-Fe under H2-activation implying the ability of these metals to spillover hydrogen to Fe3O4. The conversion of Fe3O4 to predominantly Ï‡-Fe5C2 under CO-treatment is not facilitated by the presence of the promoter element. The amount of carbide in the catalyst under Fischerâ€“Tropsch conditions is dependent on the presence of the promoter (Cu and Ag) in close proximity to the iron phases. An increase in the FT activity is observed for the promoted iron catalysts, and this is primarily attributed to the increased carbide surface area within the catalyst. Carbon dioxide (CO2) in the Fischer-Tropsch synthesis is formed either in the oxygen removal from the catalytic surface or in the carburization of particularly superparamagnetic Fe3O4. It is further shown that the olefin selectivity in the Fischer-Tropsch synthesis over the catalyst AgFeO2 (ex) is higher than that obtained over the catalyst CuFe2O4 (ex) and CuFeO2 (ex), which can be ascribed to a lower hydrogenation activity of silver in comparison to copper ((ex) is in reference to the model catalyst after Fischer-Tropsch synthesis). Furthermore, copper seems to facilitate secondary olefin hydrogenation.
Open Access
Iron modification of rhodium nano-crystallites for CO hydrogenation
(2013) Nozonke, Dumani; Claeys, Michael
The aim of this study was to investigate the effect of iron on alumina-supported well defined nano-sized rhodium crystallites on the activity and selectivity for CO hydrogenation. The objective was to prepare model catalysts with similar average crystallite size and narrow size distribution.
Open Access
Iron-Based Alloys as Catalysts for CO2 Hydrogenation
(2022) Mullins, Christopher; Claeys, Michael; Fischer, Nico
Use of CO2 as a chemical feedstock in a wide range of applications has been postulated as a method to reduce its concentrations in the atmosphere, in an effort to combat climate change. An especially attractive use of CO2 is its hydrogenation to hydrocarbon fuels. If coupled with a source of renewably generated H2, this reaction could provide a source of carbon neutral energy that can be readily integrated with current infrastructure. This study looked at the performance of a range of iron-based bimetallic catalysts in promoting CO2 hydrogenation. Specifically, iron-nickel, iron-cobalt and iron-copper supported on βsilicon carbide were studied. It had been reported that these materials were more active and selective towards long chain hydrocarbons than their pure metal counterparts, although the reason was unclear. It was hypothesized that alloy formation in these materials would supresses carbide formation, in turn enhancing CO2 activation and hence reaction performance. The catalysts were synthesized using an ammonium hydroxide modified benzyl alcohol technique, which yielded ferrite nanoparticles below 10 nm with narrow size distribution. These ferrites were supported on silicon carbide via a suspension-deposition technique. In total five catalysts were synthesized – two iron-cobalt, two iron-nickel and one iron-copper. All catalysts were synthesized with a molar ratio of two iron to one counter-metal. The catalysts generally had average particle diameters of 6 nm, with one of the iron-nickel catalysts and the ironcopper catalyst slightly smaller at 3 nm and 2 nm respectively. The supported ferrites were reduced in order to yield the active metallic phase. It was shown via in situ characterization that a body centred cubic (BCC) alloy formed in the iron-cobalt samples (final size of 15 nm), while the iron-nickel samples were comprised of two alloy allotropes, with BCC and face centred cubic (FCC) crystalline structures (final size of 10 nm). The iron-copper sample reduced into pure iron (final size 20 nm) and copper phases. The increased size of the metallic phases compared to the freshly synthesized catalysts was due to sintering of the nanoparticles during reduction. In situ reaction studies showed that the iron-cobalt alloys were remarkably stable, with almost no changes in metallic phase seen. The iron-nickel samples were more readily changed by the reactant gases, with the BCC iron-nickel alloy converted to nickel-containing Hägg carbide. The FCC iron-nickel alloy remained unchanged, however. The iron-copper sample, which demonstrated no alloy formation, had its iron phase completely converted to Hägg carbide. Alloying of iron was thus shown to supress carbide formation. Reaction performance of all catalysts to long-chain hydrocarbons was poor when compared to similar materials tested in the literature, with conversions in the range of 4% - 8%. The product distribution was also undesirable, with the majority of product carbon reporting to CO in all five catalysts. Of the hydrocarbons formed, 80% - 96% reported to undesirable methane, depending on the counter metal used. It seemed that iron carbide in the iron-copper catalyst favoured longer chain hydrocarbon production when compared to the more metallic cobalt- and nickel-containing samples (which produced far more methane), but struggled to activate CO2 past CO. While the iron-cobalt catalysts seemed to facilitate more activation of CO2 to hydrocarbons, they showed less potential in forming longer chain hydrocarbons. The two iron-nickel catalysts behaved differently; one catalyst had a stable FCC phase, while its BCC alloy phase was completely converted to carbides, and favoured mostly methane formation. The other catalyst had a similarly stable FCC phase, but also maintained an appreciable BCC alloy fraction, and showed far more propensity to form longer chain hydrocarbons. This catalyst was still not as successful in promoting chain growth as its ironcopper counterpart, however. When comparing performance of the iron-cobalt and -copper catalysts, it seemed that carbide formation was beneficial in encouraging hydrocarbon chain growth, but detrimental to CO2 activation. On the other hand, the iron-nickel catalysts demonstrated that the BCC alloy phase was required to encourage chain growth, while the carbide resulting from its conversion diminished this. These results indicated that an improvement in the activation of CO2 did not necessarily increase hydrocarbon chain length, and that while carbides may be desirable for encouraging longer chain molecules, the presence of nickel in the carbide spoils the effect, at least in the range of temperatures tested. These results led to rejection of the hypothesis that alloys resulted in bimetallic catalysts' improved performance. They indicated that iron carbides are required for stable conversion of CO2 to longer chain hydrocarbons, but that the carbides alone were not extremely active nor selective. It is therefore likely that the counter metal's role in enhancing activity and selectivity at more dilute concentrations is by modulating the carbide phase. It is thus suggested that the impacts of counter metals in more iron-rich systems be studied, where carbide formation would be more facile. Additionally, the difficulty which the catalysts had in activating CO2 could be mitigated by promotion and use of an active support.
Open Access
Low frequency, in-situ vibrating sample magnetometer : electrical systems and control software design
(2006) Kelly, Simon Gilbert Daneel; Claeys, Michael; Wilkinson, Andrew John
A low frequency vibrating sample magnetometer has been built to measure the in-situ properties of ferromagnetic catalysts. The instrument allows measurements to be taken during an experimental catalyst test run (in-situ). The vibration is performed by a motor crank arrangement frequency of 2 Hz. The software designed to control the instrument and the reaction was written in Lab View which enabled a rapid prototyping approach. This thesis focuses on the software and electrical systems of the setup. Results of research conducted using this system are published separately however this document shows the relationship between the magnetic saturation and remnance and the mass of ferromagnetic material present in the reference material as well as the effect of temperature on this material.