Browsing by Author "Fischer, Nico"
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- ItemOpen AccessApplication of Molybdenum Carbide Catalysts for the CO2-assisted Oxidative Dehydrogenation of Ethane(2022) Marquart, Wijnand; Fischer, Nico; Claeys, MichaelThe 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.
- ItemOpen Access[Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ electride as a novel support system for iron-based Fischer-Tropsch synthesis(2018) Motala, Muhammad; Fischer, Nico; Claeys, MichaelIron-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.
- ItemOpen AccessEffect of ammonia co-feeding on oxygenates over K-Mo2C in the Fischer-Tropsch synthesis(2018) Marquart, Wijnand; Fischer, Nico; Claeys, MichaelThe 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.
- ItemOpen AccessHydrothermal 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, MichaelThe 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.
- ItemOpen AccessIn situ study of Co₃O₄ morphology in the CO-PROX reaction(2017) Khasu, Motlokoa; Fischer, Nico; Claeys, MichaelThe 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
- ItemOpen AccessIron-Based Alloys as Catalysts for CO2 Hydrogenation(2022) Mullins, Christopher; Claeys, Michael; Fischer, NicoUse 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.
- ItemOpen AccessLow PGM PtCo alloy catalysts for low loading Polymer Electrolyte Membrane Fuel Cells(2023) Munro, Tiaan; Fischer, NicoPolymer Electrolyte Membrane Fuel Cells (PEMFCs) are a promising means of generating clean energy in especially transportation applications. The electrochemical conversion of energy is driven within membrane electrode assemblies (MEAs) over costly Pt catalyst which limits PEMFC cost viability. PtCo alloy catalysts are attractive alternatives to Pt, with similar or higher catalytic activity at a reduced cost. The catalyst is combined with a perfluorosulfonic acid (PFSA) ionomer which serves as the proton conductor and binder to form the catalyst layer (CL). The ionomer contains both hydrophilic and hydrophobic groups, and therefore plays a key role in water management in the fuel cell. In every MEA design, ionomer content needs to be optimised to maximise fuel cell performance, ensuring adequate reactant-catalyst interaction, without flooding the MEA. This optimal ionomer content depends on the properties of the ionomer as well as catalyst characteristics (such as platinum loading, carbon to metal ratio and particle size), as well as fuel cell operating conditions. To establish an ideal low PGM MEA design, this thesis investigated two commercial PtCo catalysts (PtCoU30 and PtCoT50). Physical and electrochemical characterisation of the PtCo catalysts were investigated and compared to an in-house Pt benchmark (PtH40). The morphology, composition, size distribution, physical surface area, electrochemical surface area, performance and durability of these catalysts were established. Electrochemical characterization of the two catalysts showed that the higher Pt loading catalyst (PtCoT50) had lower mass activity compared to the lower Pt loading catalyst (PtCoU30). The PtCoT50 catalyst, however, exhibited similar specific activity. The electrochemical surface area of PtCoT50 was smaller than that of PtCoU30, which was consistent with the larger particle sizes observed for PtCoT50. Ex-situ accelerated stress tests showed that the higher metal loading catalyst was more susceptible to PtCo metal degradation but had greater carbon support durability. The ionomer contents for PtCoU30 and PtCoT50 MEAs were optimised at 30 wt% and 24 wt% respectively. This suggested that the higher metal content catalyst (PtCoT50) required less ionomer than the PtCoU30, due to the thinner catalyst layer thickness. Both optimised PtCo MEAs outperformed the PtH40 benchmark. This trend was consistent with the activities seen in ex-situ RDE. The impact of incorporating ionomer in different ink preparation stages/phases on performance and durability was determined. A 1-phase catalyst ink was prepared by adding the ionomer to catalyst prior to heat treatment, and solvent addition. A 2-phase ink was made by adding additional ionomer to the 1-phase ink, without being heat treated. It was shown that the performance for the PtCo catalysts was higher when all the ionomer was added in the heat treatment phase. However, the 2-phase MEAs showed slightly improved carbon corrosion and particle stability. It was suggested that the minor improvement in stability for 2-phase MEAs did not override the 1-phase MEA performance benefit . The recommended design for a low PGM PtCo MEA based on these findings is a 30 wt% PtCo/C MEA with 30 wt% ionomer content added during the heat treatment phase.
- ItemOpen AccessMetal oxide supported iron-nickel nano-alloys in the reverse water-gas shift reaction(2023) De Kock, Karina; Fischer, Nico; Claeys MichaelRising atmospheric CO2 concentrations pose an existential threat to human life. The development of carbon capture and utilization (CCU) technologies which can consume CO2 at the same scale that it's being produced are necessary to limit the global mean surface temperature increase to 2 °C, in accordance with the Paris Agreement. A challenge in developing these technologies is to activate CO2, which is thermodynamically low in energy and needs to be reacted with high energy molecules. The Power-to-X (PtX) concept considers various pathways and technologies for the conversion of CO2 to different target products. PtX assumes the availability of cheap renewable energy and green H2 in the future and in many of these scenarios, CO2 is first activated to carbon monoxide via the reverse water- gas shift (RWGS) reaction. The RWGS is thermodynamically limited. CO and CO2 methanation are favoured over the RWGS at industrially preferred reaction conditions and there is a need for low temperature catalysts with a high CO selectivity. Noble metal catalysts have been well studied for the RWGS, though affordable and abundant metals are required for large-scale applications. Iron-based catalysts have been widely explored due to their high stability, while nickel catalysts are rarely considered for the RWGS due to their known hydrogenation activity. Metal oxide (MOx) catalysts, such as ceria, have also been studied for the RWGS due to their high oxygen storage capacity and reducibility at high temperatures. In recent work from the group of Fischer at the University of Cape Town, supported Fe-Ni nanoparticles showed promising CO2 activation potential. It was found that upon reduction of the oxidic Fe-Ni precursor nanoparticles, a mixture of fcc and bcc alloy phases was formed. An increase in Fe content increased the concentration of the bcc phase in the alloy. It was further demonstrated that the bcc surface of the Fe-Ni alloy activates CO2 while the fcc surface of the alloy appears to be inert. This work further studies the FexNiy/MOx catalysts developed by the group of Fischer, which make use of novel synthesis approaches. The reducibility of two different MOx support materials is studied (M = Cr and Ga). To limit the effect of the varying physical properties of the respective bulk oxides on catalyst performance, bespoke support materials were prepared by impregnating a common γ-Al2O3 carrier with MOx overlayers. The surface of the prepared materials has the chemical and electronic properties of the respective MOx, but the pore geometry of the γ-Al2O3 is maintained. The effect of iron content on catalytic performance is of interest, and to obtain the Fe-Ni alloy phase, oxidic (NixFe1-x)Fe2O4 precursor nanoparticles of varying composition are synthesized (Fe:Ni = 3, 5 and 7, as well as pure iron oxide). This approach increases the chance of alloy formation upon reduction of the precursor particles compared to techniques such as co-impregnation, as the Fe and Ni are well mixed in the precursor particle. A hydrothermal synthesis technique in benzyl alcohol is employed to produce nanoparticles with a narrow size distribution without the use of surfactants, which are difficult to remove and can influence catalytic performance. The nanoparticles are deposited onto the two MOx@Al2O3 overlayer materials, as well as onto inert SiO2 to isolate the performance of the metallic Fe-Ni phases without support effects. Characterizations of the unsupported (NixFe1-x)Fe2O4 confirm that nanoparticles of varying composition are successfully synthesized in the correct phase with satisfactory overlap in particle size distribution. Characterizations of the prepared MOx@Al2O3 overlayer support materials confirm that the desirable textural properties of the underlying γ-Al2O3 support are maintained. No bulk MOx crystallites are detected, suggesting the MOx exists as a 2D overlayer covering the γ-Al2O3 surface. H2- TPR studies in in-situ XRD confirm that the reduced catalysts contain a mixture of a bcc alloy phase and an fcc alloy phase in agreement with the previous work and irrespective of the support material. The relative concentrations of each phase are a function of iron content, with an increase in iron content increasing the concentration of the bcc alloy phase. In-situ XRD temperature-programmed CO2 activation experiments confirm that the bcc phase has a high affinity towards re-oxidation but, unlike in the previous work, the fcc phase was found to be partially re-oxidized at elevated temperatures (> 600 °C). Catalytic performance evaluation was carried out in a dual quartz tube fixed-bed reactor set-up at 600 °C. All samples tested show > 99% CO selectivity but, using Fe100/SiO2 as a reference catalyst, it was found that alloying Fe with Ni and the use of an active support has significant impacts on catalytic activity and stability. The SiO2-supported samples all deactivated rapidly and in general, the CrOx- supported samples have the best activity, and the GaOx-supported samples have the best stability. Catalytic performance is dependent on both the alloy composition and the MOx support, with the surprising observation made of a reversal of the trend in activity with iron content between CrOx@Al2O3 and GaOx@Al2O3. Spent catalyst characterization showed that the rapid deactivation seen on SiO2 cannot be explained by sintering, oxidation, or carbon deposition. The deactivation is instead credited to the consumption of the bcc phase under reaction conditions. The results show that there is some interaction between the fcc phase and an active supports which enhances RWGS performance. Possible untested explanations for this could be enhanced H2 activation on the fcc phase which boosts CO2 activation on the support through a H+ spillover effect, or the creation of new active sites at the metal-metal oxide interface which are RWGS active.
- ItemOpen AccessMetallic cobalt derived from cobalt nitrides for Fischer-Tropsch Synthesis(2022) Padayachee, Veroushia; Claeys, Michael; Fischer, Nico; Fadlalla, MohamedThe Fischer-Tropsch synthesis (FTS) is a catalytic surface polymerisation reaction that converts synthesis gas, a mixture of carbon monoxide (CO) and hydrogen, into long chain hydrocarbons, namely α-olefins and n-paraffins. The product selectivity of this reaction is dependent on the nano effects of the catalyst used in addition to the reaction conditions, making FTS a highly structure-sensitive reaction. The transition elements namely ruthenium, iron, cobalt and nickel are highly active catalysts in FTS, however cobalt will be the focus of this research. Cobalt (Co) has two common crystalline phases: hexagonal close-packed (hcp) and face-centred cubic (fcc). Both these phases are active under FTS conditions, however, theoretical and reactor testing studies have found the hcp Co phase to be more active as a result of the hcp crystal structure having more favourable facets and active sites available for CO dissociation than the fcc Co phase. The conventional route for synthesising hcp Co nanoparticles is through a reduction carburization-reduction (RCR) route or via the reduction of hexagonal cobalt oxide. The former route produces a catalyst that is predominantly hcp Co, however not all of the carbon is able to be removed, especially at lower second reduction end temperatures of the RCR process. The latter route produces a reduced catalyst with an intergrowth (which occurs when there is more than one way in which a close packed layer of atoms can be put together) and could be an indication of a mixture of hcp and fcc Co. As a result, the objective of this study was to produce pristine hcp Co without an intergrowth or unfavourable surface species and determine the selectivity of this pure phase under Low temperature FTS (LTFTS) conditions. To complete this, a novel technique to producing metallic cobalt catalysts in FTS was proposed and implemented: cobalt nitride decomposition. This novel route involved producing a cobalt nitride with the desired crystal phase (e.g. hexagonal cobalt nitride, Co3N) using ammonia, and decomposing this nitride in a hydrogen atmosphere to the metallic phase that retained that crystal structure (e.g. hcp Co). Using this method, a pure hcp Co phase of an applicable FT crystal size of 17 nm was synthesised. Interestingly, cobalt hydroxide (α-Co(OH)2), the precursor to the Co3N, also reduced to a pure hcp Co phase of 32 nm. Both hcp Co catalysts had no indication of an intergrowth, nor were there any nitrogen species detected on the surface of the catalyst (according to the PXRD and SEM-EDX data), indicating a successful decomposition or reduction. The fcc Co catalyst was derived from exposing cobalt oxide (Co3O4) to ammonia, which reduced the catalyst to pure fcc Co instead of nitriding the catalyst to the fcc cobalt nitride, Co4N. When Co3O4 was reduced in hydrogen, the final catalyst was a mixture of hcp and fcc Co, with a predominant fcc Co phase (90 %). Thus, the four catalysts studied in this research are the two fcc Co and two hcp Co catalysts derived from the four different reduction techniques: CAT 1: fcc Co – H, through a pure hydrogen reduction of Co3O4, CAT 2: fcc Co – NH, through an ammonia treatment followed by a hydrogen reduction of Co3O4, CAT 3: hcp Co – H, the direct reduction of α-Co(OH)2 and finally CAT 4: hcp Co – NH, through the thermal decomposition of Co3N. These catalysts were tested under low temperature Fischer-Tropsch conditions (T=220 °C, P=20 bar, H2: CO =2 and GHSV = 0.5 NL/ gcat hr-1 ). The FT results revealed that CAT 2: fcc Co – NH achieved the highest CO conversion of 31 %, followed by CAT 1: fcc Co – H at 23 %, CAT 3: hcp Co – H at 5 %, and CAT 4: hcp Co – NH at 3 %. CAT 2: fcc Co – NH achieved the highest α value of 0.69, with the largest selectivity to long chain hydrocarbons and the smallest selectivity to methane, possibly as a result of the high water formation at the relatively high CO conversion (Claeys & van Steen, 2004). CAT 1: fcc Co – H had an α value of 0.62 and was more selective to methane. This indicated CAT 1: fcc Co – H had more favourable active sites available for methanation instead of chain growth. The hcp Co catalysts appeared to deactivate quickly and thus have little to no activity for chain growth, and as a result the selectivity towards methane was high. The α value for CAT 3: hcp Co – H and CAT: hcp Co – NH was 0.64 and 0.54 respectively. CAT 3: hcp Co – H did have a comparable alpha value to CAT 1: fcc Co – H, however as a result of the large difference in their CO conversions, the yield for the product classes for CAT 1: fcc Co – H was much higher than that of the hcp Co catalyst. CAT 2: fcc Co – NH subsequently achieved the highest turnover frequency (TOF) at the end of time on stream (TOS). The PXRD of the spent catalyst showed that wax was formed on this catalyst, which supported the selectivity to the longer hydrocarbon chains. The major difference between the preparation methods between CAT 1: fcc Co – H and CAT 2: fcc Co – NH, is that the former catalyst used solely a hydrogen atmosphere, whilst the latter used an ammonia and then a hydrogen atmosphere for the reduction. The difference in the reduction atmospheres influenced the purity of the resulting crystal phase achieved (CAT 1: fcc Co – H: 90% fcc Co, CAT 2: fcc Co – NH: 100 % fcc Co), and as a result, the activity of the catalyst. When the spent catalysts were characterised to determine the source of the deactivation, an increase in the crystal size of the catalysts was noticed. The samples did appear to have sintered, but to a certain extent. The XRD patterns show only the active metallic phase present on the catalyst after 48 hours, with the absence of carbon species and oxidized cobalt. Surface nitrogen was not detected according to the SEM-EDX results of both the catalysts that required an ammonia step during their preparation (CAT 2: fcc Co – NH and CAT 4: hcp Co – NH). It was proposed that the deactivation of the catalysts were perhaps a result of the choice in the metal salt used during synthesis preparation. According to a report conducted by Rosynek & Polansky (1991), residual chlorine ions from the cobalt chloride salt (similar to the precursor salt used in this research) blocked a considerable amount of the reduced metallic catalyst during FT studies. The SEM-EDX conducted on the spent catalysts confirmed the presence of chlorine on all four catalysts. The results showed that the fcc Co catalysts derived from Co3O4 (CAT 1: fcc Co – H and CAT 2: fcc Co – NH) had a lower chlorine concentration than the hcp Co catalysts derived from α-Co(OH)2 (CAT 3: hcp Co – H and CAT 4: hcp – NH). The calcination step in synthesising Co3O4 helped remove some of the chlorine ions on the surface of the catalyst, similarly suggested by Panpranot et al. (2003) and Rosynek & Polansky (1991). The α-Co(OH)2 precursor was deliberately not calcined in air in order to maintain the stacking sequence and purity of the hcp phase, which was the main aim of this research. Cobalt nitrides as FT catalyst themselves have not been thoroughly investigated. However, the nitrogen content on the catalyst has been reported to present additional active sites that aids in the desorption of reactants and thus improving the activity of catalysts in CO methanation (Razzaq et al., 2015). Two cobalt nitrides, Co3N (hexagonal) and Co2N (orthorhombic), were successfully synthesised to be tested under LTFT conditions. However, when their stabilities were investigated using the in situ XRD under LTFT conditions, it was found that the nitrides were unstable. Neither catalyst was able to remain a nitride for more than 2 hours at 190 °C before hcp Co was detected. Decreasing the temperature would not have offered useful results as the activity would be extremely low at those conditions. As a result, the cobalt nitrides were not tested under LTFT.
- ItemOpen AccessNovel empowered supports for iron-based Fischer-Tropsch in a power-to-liquids process(2023) Ketlogetswe, Oaitse; Fischer, Nico; Claeys MichaelFischer-Tropsch (FT) synthesis is a process that converts carbon monoxide and hydrogen into liquid hydrocarbons 1 . In the past, FT synthesis has been used to produce gasoline and diesel from coal and natural gas. However, with increasing demand for renewable and sustainable energy sources, there has been renewed interest in using FT to produce clean fuels 2 . The FT synthesis process takes place in the presence of a catalyst, at moderate temperatures and pressures. Iron-based catalysts are frequently used in commercial FT as they are quite useful in hydrogen lean synthesis gas because of their high water-gas shift activity. Nevertheless, these catalysts often need to be promoted to optimize their performance and they are prone to deactivation under FT conditions 3 . Promotion with potassium is often done to shift the FT performance and selectivities, while manganese and copper are often added to stir catalyst reduction 4 . Potassium is often used to modify iron-based catalysts to improve their performance. It has been suggested that potassium can impact the rate at which carbon is formed during FT by facilitating CO dissociation on the catalyst surface. Potassium also inhibits hydrogen chemisorption resulting in more carbon on the catalyst surface, longer chain products and higher olefin production 4,5 . The large ionic radius of potassium, however, has been reported to prevent its incorporation into iron oxide during synthesis, resulting in its mobility at the catalyst surface. The potassium mobility during FT results in enrichment of carbon on the catalyst surface and consequently catalyst deactivation 6 . In this study, we explored the possibility of using perovskites materials as support structures for the iron catalyst, and potassium incorporated within their structures. The perovskites allowed for better dispersion of the catalysts and improved control over the contact between the iron catalyst and potassium promoter. This in turn suppressed the movement of the promoter species, leading to improved catalytic performance. Two lanthanum-based perovskites were examined: LaAlO3-δ and LaTiO3-δ. The LaAlO3-δ catalysts were modified by substituting potassium to the A site and substituting 20 atom-% manganese to the B site. A range of materials with varying amounts of potassium, La1- xKxAl0.8Mn0.2O3-δ, (x = 0, 2, 4, 6, 8, 10 atom-%) were prepared via the citrate method 7 . The LaTiO3-δ catalysts were only modified by substituting potassium on the A site. A series of potassium substituted La1- xKxTiO3-δ perovskites (x = 0, 5, 10, 15, 20 and 30 atom-%) were prepared using a wet chemical technique 8 . Iron nanoparticles prepared using the coprecipitation method 9 , were deposited onto these supports as catalyst for FT synthesis process. The effect of potassium promotion on the properties and performance of these catalysts were evaluated. These catalysts were activated under H2 flow at 80 ml/min at 450 °C for 15 hours. Fischer-Tropsch synthesis was conducted in a 600 ml continuously stirred reactor tank at 240 °C, 15 bar with synthesis gas ratio of 2 (H2/CO = 2) and a space velocity of 2.4L/(h.gcatalyst). The catalysts performance was evaluated over a 48-hour period using online GC-TCD and offline GC-FID. In this study, the Fe-La1-xKxAl0.8Mn0.2O3-δ catalysts with potassium promotion showed higher catalytic activity than the unpromoted catalyst. The activity of the catalysts increased with increasing potassium loading until a maximum was reached at 8 atom-% potassium loading. The higher activity of the potassium promoted catalysts was attributed to the enhanced formation of iron carbide phases, which have been shown to be active in FT synthesis. Potassium promotion also led to lower methane selectivity, which was not dependent on the amount of potassium added. The results were ascribed to the water-gas shift activity of the iron catalyst. There was also a decrease in selectivity of C2 – C4 hydrocarbons and increased selectivity of C5+ hydrocarbons, while reducing the selectivity of oxygenates. Potassium promotion on the Fe-La1-xKxTiO3-δ catalysts resulted in structural change from orthorhombic at low potassium promotion to cubic structures at higher potassium promotion. The in situ XRD results revealed that potassium results in reduced onset reduction temperature from Fe3O4 to wüstite (FeO) and retards the reduction process from the FeO to metallic Fe. Nonetheless, it enhanced the initial formation of carbides through its influence on CO and H2 chemisorption. There was improved catalytic activity of the catalysts until a maximum at 10 atom-% potassium loading. On the other hand, this promotion led to a decrease in methane selectivity over time. The water-gas shift activity was higher on the promoted catalysts with no clear trend on the effect of amount of potassium added. Furthermore, potassium promotion resulted in a decrease in the selectivity of C2 – C4 hydrocarbons, slight increase in C5+ hydrocarbons selectivity and little impact on the productivity of the oxygenates. These developed catalysts had shown stability under FT reaction conditions as there was no observed structural change after FT synthesis. Due to their demonstrated stability at higher temperatures, these materials may be suitable for use in other catalytic processes such as reverse water-gas-shift reaction. Additionally, the lanthanum titanate perovskites could be promoted with manganese to investigate their reducibility and potentially improve their performance.
- ItemOpen AccessOxidative dehydrogenation of ethane with carbon dioxide over iron-nickel nano-alloys supported on metal oxide overlayers(2022) Raseale, Shaine; Fischer, Nico; Claeys, MichaelThe CO2-mediated oxidative dehydrogenation of ethane (CO2-ODHE) is one of the promising alternative routes for simultaneous conversion of ethane and the greenhouse gas (GHG) CO2 into high-demand monomers: ethylene and CO [1,2]. It has been extensively investigated on oxide catalysts with those based on Cr, V and Ga exhibiting high yields and selectivity for ethylene but those deactivate rapidly [3–12]. The reaction follows a Mars van Krevelen mechanism where the oxide is partially reduced upon ethane activation and subsequently re-oxidized upon CO2- activation [13]. Thus, suitable redox properties as well as Lewis acidity of metal oxides are some essential properties governing catalytic performance [13–15]. The lower stability of these materials is linked to their failure to act as multifunctional catalysts that can prevent carbon buildup via the dissociation of CO2 while co-activating ethane. Promoters such as Fe-based oxides have been studied to enhance the CO2-activation functionality and stability of the oxides [5]. A Density Functional Theory (DFT) study suggested that bimetallic alloys can have superior activity for CO2-activation compared to their monometallic counterparts [16]. Therefore, bimetallic alloys in conjunction with metal oxides may serve as stable bifunctional CO2-ODHE catalysts with enhanced CO2-activation ability. Improved CO2-activation can boost the re-oxidation rate of the metal oxide via a spillover-type mechanism and aid in coke removal via the reverse Boudouard reaction [5,11]. Recent studies have already demonstrated this experimentally with a series of supported FexNiy catalysts prepared via impregnation which, depending on Fe : Ni ratio and support, can promote either the CO2-ODHE or the competing ethane dry-reforming (DRE) reactions with enhanced CO2-activation [17,18,18–20]. However, composition and crystallite size uniformity are difficult to attain via impregnation as Ni and Fe oxide phases present in parallel make it challenging to extract the influence of the FexNiy alloy metallic composition [21]. This work aims to synthesize catalytically active metal oxide overlayers of a comparable pore structure, anchor FexNiy nano-alloys of uniform composition with Fe and Ni in close proximity and investigate the combined effect of the overlayer support's acidity/reducibility and alloy composition on the catalytic performance under CO2-ODHE conditions. The close proximity of Fe and Ni in the alloy is introduced by the use of oxidic Ni-ferrite spinel structures as precursors of the alloy which is formed upon their reduction in H2 atmosphere and was found to exist as a mixture of a bcc and an fcc phases depending on Fe : Ni ratio. A higher Fe content in the alloy increases the fraction of the bcc phase as confirmed via H2-TPR studies in in situ XRD. In situ XRD temperature-programmed CO2-activation studies also revealed that CO2 is only able to react with the bcc phase of the alloy which is re-oxidised into the oxidic Niferrite spinel while the fcc phase is stable against re-oxidation. While they deactivate rapidly due to a limited re-oxidation and coking caused by insufficient CO2- activation, the bare metal oxide overlayers exhibit an initial activity that reduces with a decrease of the surface acid site strength until a minimum is reached and then slightly increases with increasing basicity under CO2-ODHE conditions. Their catalytic stability increases with weakening of the acid site strength. Decreasing the overlayer acidity enhances the CO2-ODHE/DD (DD : direct dehydrogenation) activity resulting in increased ethylene and decreased CO selectivity. Spent catalyst analysis revealed the formation of surface carbonaceous deposits suspected to cause catalyst deactivation. Increasing the concentration of CO2 in the feed results in improved and sustained CO2-activation which enhances the reverse Boudouard reaction and improves the catalyst stability by reducing carbon deposition while reducing ethylene and increasing CO selectivity. Deposition of the FexNiy nano-alloys of Fe : Ni atomic ratios of 1, 3 and 5 on the reducible and acidic CrOx@Al2O3 results in an alloy composition-dependent catalyst performance, while the alloys are essentially inactive over the less acidic and unreducible ZrOx@Al2O3. This clearly confirms a bifunctional character of these materials and reveals that their catalytic performance depends on both the overlayer reducibility/acidity and the metallic composition of the alloy. Over CrOx@Al2O3, the alloy enhances the CO2-activation functionality with increasing Ni-content boosting the overall activity and stability of the catalyst. However, with increasing Ni-content, the CO selectivity increases while ethylene selectivity reduces due to the suppressed CO2-ODHE/DD activity and promotion of the competing DRE reaction. The target CO2-ODHE/DD reaction activity is maximal at an overall Fe : Ni atomic ratio of 5, about 10% at a ratio of 3 and completely suppressed at a ratio of 1. Spent catalyst analysis revealed formation of surface carbonaceous deposits and that the bcc phase of the alloy is re-oxidised into the Ni-ferrite oxidic spinel phase while the fcc phase of the alloy is stable against re-oxidation during the reaction. Increased CO2 concentration in the feed has similar effects as described for the bare overlayers. Deposition the Fe3Ni1 and Fe5Ni1 nano-alloys on GaOx@Al2O3, VOx@Al2O3, SmOx@Al2O3 and TiOx@Al2O3 revealed that despite the alloy composition, a predominant DRE activity is observed over the highly acidic and reducible VOx@Al2O3. While CrOx@Al2O3 and GaOx@Al2O3 show a similar performance when tested bare, the addition of the Fe3Ni1 and Fe3Ni1 alloys on the GaOx@Al2O3 overlayer results in a high CO2-ODHE/DD activity with stability decreasing with increasing Fe-content. Over TiOx@Al2O3, the Fe3Ni1 nano-alloy exhibits a similar behaviour as over GaOx@Al2O3 while higher iron contents resulted in an inactive catalyst. Over SmOx@Al2O3 no alloy composition yields appreciable catalytic activity. The CO2-ODHE/DD behaviour of Fe3Ni1/GaOx@Al2O3 is in stark contrast to the high DRE activity over Fe3Ni1/CrOx@Al2O3 emphasising that a specific alloy composition exists for each overlayer to yield a stable and dominating CO2-ODHE/DD or DRE activity. For a high and stable CO2-ODHE/DD activity, the optimum in atomic Fe : Ni ratio was found to be between the 3 and 5 at intermediate-intermittent overlayer acidity. In addition to improving stability, increased CO2 concentration in the feed was found to significantly accelerate an observed active site re-construction during reaction which results in formation of more CO2-ODHE/DD sites.
- ItemOpen AccessPerovskites as New Support Materials for the Iron Based Fischer-Tropsch Synthesis(2022) Khasu, Motlokoa; Fischer, Nico; Claeys, MichaelIn the Fischer-Tropsch process, valuable products consisting mainly of long chain hydrocarbons and water as by-product are synthesized from the basic starting materials hydrogen and carbon monoxide, derived from sources such as coal, gas or biomass [1]. Iron catalysts and cobalt based systems are the preferred catalysts for the commercial Fischer-Tropsch synthesis (FTS) employed at PetroSA and Sasol in South Africa [2]. The use of iron catalysts is not only influenced by the low price of the active material compared Co and Ru, but also by higher activity and a wide window of FT operating conditions. Though the product selectivity of iron-based FT synthesis can, to a certain extent, be controlled by changing the process temperature, promoters are added to improve selectivity and activity of the catalyst by modifying structural and/or electronic properties of the active phase [3]. Potassium is the most used among alkali promoters in iron-catalyzed FTS because of its Lewis acid nature [4–6]. Alkali promoters simultaneously promote dissociative CO chemisorption and inhibit chemisorption of H2 by a charge transfer to the surface of the iron based active phase, resulting in a higher C/H ratio on the catalyst surface and faster carburization rates, longer chain products and higher olefin content [5,7,8]. The addition of promoters is beneficial up to a certain amount, above which it becomes detrimental to the overall activity of the catalyst due to coverage of active sites and/or excessive coking [9–11]. Its mobility under reaction conditions is another challenge as it results in a highly dynamic system. The goal of the here proposed study is to investigate the possibility of using a new class of supports (perovskites) that do not only provide high surface area and enhance the dispersion of the active phase but also adopt the role currently played by promoting elements without suffering from the disadvantages reported above (mobility on catalyst, blocking active sites, etc.) [12]. Perovskites are mixed-metal oxides that have attracted much scientific attention due to their low price, adaptability and thermal stability. They generally have the formula ABO3 or A2BO4 (A and B are cations of different sizes, and O is the anion that binds them), exhibiting a range of stoichiometries and crystal structures. Their structural features make it possible to accommodate around 90% of the metallic elements from the periodic table in positions A and/or B, without destroying the matrix structure. ‘A' can be a rare earth alkali (La, Ce, Pr) or alkaline earth metals (Cs, K, Ca) of a larger diameter than ‘B' which is normally a transition metal (Co, Fe, Cu, Mn, Cr) [13,14]. This study aims to incorporate selectivity and activity promoters into the perovskite structure, anchor iron particles on the perovskite and investigate the reducibility and activity/selectivity of the obtained model catalyst under Fischer-Tropsch conditions. The model catalysts are developed by preparing a series of LaAlO3 perovskites with partial substitution of lanthanum with 10 atom-% potassium, and 0, 10, 20, 60, and 100 atom-% substitution of aluminum with manganese. The performance of iron supported on these materials is compared with the one on LaAlO3 support with potassium promotion via impregnation (2, 1 and 0.5 wt.-%). EDS-STEM and XANES results confirm the successful incorporation of potassium and manganese into the matrix of the LaAlO3. In situ studies highlight the effect of potassium on the activation of the oxidic iron precursor. The iron supported on the potassium incorporated perovskite exhibits a lower reduction temperature and a faster subsequent formation of Hägg carbide (χ-Fe2C5). It is well reported that iron carbide is the active phase for the FischerTropsch synthesis [15–17]. The Fischer-Tropsch activity of the iron supported on the unmodified LaAlO3 perovskite support compares well with common supports such as SiO2, Al2O3, ZrO2 and TiO2 in terms of CO conversion and selectivity towards FTS products. The incorporation of low concentrations of manganese into the perovskite support results in a slight decrease in CO conversion but no significant change of the product selectivity was observed. With a further increase in manganese concentration, CO conversion decreases accompanied by an increase in C1 and C2–C4 product selectivity. The same phenomenon is observed with these high manganese containing samples when the perovskite support is further modified with potassium. Incorporation of potassium into the LaAlO3 and LaMn0.2Al0.8O3 perovskites results in a substantial increase in CO conversion, beyond 70%. At a CO conversion comparable to the promoter free perovskite catalyst, the novel materials show a comparable CH4 selectivity with lower CO2 formation, resulting in a lower overall undesired C1 product fraction suggesting a decoupling of the potassium enhancement of the FTS activity versus the enhancement of the water gas shift reaction. Additionally, these materials also show good stability under FT reaction conditions as no significant change is observed in the perovskite structure post reaction. It can therefore be confirmed that perovskites can double as both the support and electronic promoter via the incorporation of the known promoter elements within their structure, and their electronic effects can surpass those of conventional promoters.
- ItemOpen AccessPhase and structural changes of nickel catalysts as a function of reaction conditions(2019) De Oliveira, Dominic; Claeys, Michael; Fischer, NicoCarbon dioxide hydrogenation is a route for the production of methane from hydrogen and carbon dioxide, which has attracted increased attention in recent times. It provides a means for both energy storage through substitute natural gas (SNG) production and as a process for the conversion of carbon dioxide into valuable products. Ni is the most widely used metal for SNG production due to its high activity, high selectivity towards methane and low cost in comparison to the other active metals. Ni catalysts have been extensively studied due to their uses in steam reforming and CO methanation, and it is known that deactivation by sintering, sulphur poisoning and carbon formation are the most common deactivation mechanisms for Ni catalysts. Some deactivation of Ni by oxidation has been reported, despite the fact the oxidation of Ni to NiO is not thermodynamically feasible at reaction conditions relevant to Ni catalysts. This has also been observed with Co catalysts for Fischer-Tropsch synthesis applications, where it has been explained that the size dependent oxidation, by produced water, can occur due to the higher surface energy contributions of smaller crystallites. The aim of this project is to study the phase changes of nickel catalysts, specifically through the oxidation of Ni in the presence of steam using in situ magnetic techniques to identify the loss of metallic Ni. As this oxidation is thought to be a size dependent effect, a set of catalysts with narrow crystallite size distributions and tuneable size is required for the experimental testing. The use of organometallic precursor reduction (OPR) and homogeneous deposition precipitation (HDP) was investigated for the synthesis in this study. OPR produced unsupported nanoparticles with a suitable size, however the occurrence of sintering during the anchoring and supporting of these nanoparticles on silica spheres, due to the high temperature calcination step, made these catalysts unsuitable for use in the oxidation study. The catalysts synthesised by HDP produced supported nickel catalysts with high loadings and sizes of 3.6 and 7.5 nm, with minimal overlap of size distributions, making them suitable for oxidation testing. The size of these nanoparticles was controlled by varying the reduction temperature. The HDP catalysts were tested at model conditions (i.e. in the absence of CO2) where the partial pressure ratios of steam to hydrogen, simulating different conversion levels, were increased up to a steam to hydrogen ratio of 400, to determine at what ratio the catalysts would begin to oxidise. The smaller catalysts showed significant oxidation at lower partial pressure ratios and to a greater extent than the larger particle size. These results showed the size dependence of the oxidation, with the large particles showing greater resistance to oxidation. These results were compared to iii thermodynamic calculations made for the size dependent oxidation of Ni, and good agreement between the experimental and predicted results was observed. The use of magnetic characterisation of the particle size was conducted by application of the Langevin equation as well as by a dispersion measurement, carried out by the titration of the Ni surface with H2. These in situ characterisation techniques showed consistency with the conventional external characterisation techniques and also showed that no size changes occurred throughout the testing, indicating that the results are truly due to size effects. Upon re-reduction of the oxidised catalysts, the full recovery of oxidised Ni was achieved with the large sample, whereas the smaller sample only achieved 60 % recovery of oxidised material. This is thought to be due to the formation of a less reducible phase, specifically metal-support compounds such as nickel silicate.
- ItemOpen AccessPreferential oxidation of carbon monoxide in hydrogen-rich gases over supported cobalt oxide catalysts(2016) Nyathi, Thulani Mvelo; Claeys, Michael; Fischer, NicoThe preferential oxidation of CO (CO-PROX) has been identified as one route of further reducing the trace amounts of CO (approx. 0.5 - 1 vol%) in the H2-rich reformate gas after the high- and low-temperature water-gas shift reactions. CO-PROX makes use of air to preferentially oxidise CO to CO₂, reducing the CO content to below 10 ppm while minimising the loss of H₂ to H₂O. In this study, a Co₃O₄/γ-Al₂O₃ model catalyst was investigated as a cheaper alternative to the widely used noble metal-based ones. The CO oxidation reaction in the absence of hydrogen has been reported to be crystallite size-dependent when using Co₃O₄ as the catalyst. However, studies looking at the effect of crystallite size during the CO-PROX reaction are very few. Metal-support interactions also play a significant role on the catalyst's performance. Strong metal-support interactions (SMSI) in Co₃O₄/Al2o₃ catalysts give rise to irreducible cobalt aluminate-like species. Under CO oxidation and CO-PROX reaction conditions, such strong interactions in a similar catalyst can have a negative effect on the performance of Co₃O₄ but can keep its chemical phase intact i.e., help prevent the reduction of the Co₃O₄ phase. The catalysts used to investigate these two effects (i.e., crystallite size and metal-support interactions) were synthesised using the reverse micelle technique from which nanoparticles with a narrow size distribution were obtained. Certain properties of the microemulsions prepared were altered to obtain five catalysts with varying Co₃O₄ crystallite sizes averaging between 3.0 and 15.0 nm. Four other catalysts with different metal-support interactions were also synthesised by altering the method for contacting the support with the cobalt precursor. The crystallite size of Co₃O₄ in these four catalysts was kept in the 3.0 - 5.0 nm size range.
- ItemOpen AccessPreparation of nano and Angstrom sized cobalt ensembles and their performance in the Fischer-Tropsch synthesis(2011) Fischer, Nico; Claeys, Michael; Van Steen, EricIn Fischer-Tropsch synthesis carbon monoxide and hydrogen are converted in a surface polymerisation reaction over a heterogeneous catalyst to mainly long chain hydrocarbons and water. Although all group VIII metals are reported to show activity in this process, only iron and cobalt are used on an industrial scale due to availability and costs. In order to minimise the costs of these catalysts it is generally important to increase the mass specific active surface area by dispersing the active material on an inert carrier. Recent studies on nano sized iron, cobalt, ruthenium and rhodium crystallites indicate that below a certain crystallite size they display a decrease in surface specific activity. This work aims to study the crystallite size effect of cobalt, supported on an industrially relevant carrier material, on activity and selectivity in the Fischer-Tropsch synthesis.
- ItemOpen AccessSelective production of nitrogen-containing compounds via a modified Fischer-Tropsch process(2021) Goho, Danielle Sympathie; Claeys, Michael; Fischer, Nico; Fadlalla, MohamedResearch on the co-feeding of ammonia into the Fischer-Tropsch (FTS) process over ironbased catalysts revealed that the presence of ammonia during the FTS leads to the formation of nitrogen-containing compounds (NCCs). Recent studies on the addition of ammonia to the FTS process, now known as the Nitrogen Fischer-Tropsch (NFTS) process, reported that the production of NCCs during the NFTS process is enhanced by the presence of oxygenates. The studies, therefore, suggested that oxygenates are the primary precursors of NCCs. However, due to the gap in knowledge related to the NFTS reactions mechanisms, the validity of this assumption is still unknown. In this thesis, the aim was to investigate the correlation between the presence of oxygenates under the FTS conditions and the formation of NCCs under the NFTS conditions and check the suitability of various iron-based catalysts for the NFTS process. From literature, four ironbased catalysts, known for yielding a high percentage of oxygenates, were identified, synthesised, characterised and then tested under FTS conditions to determine the optimum reaction conditions for oxygenates formation. It was found that high oxygenates selectivity can be achieved at low temperature and high space velocity as at these operating conditions the occurrence of secondary reactions involving oxygenates are limited. Furthermore, the catalysts were tested under NFTS conditions to determine their catalytic performance and their selectivity towards NCCs. During the NFTS process, in addition to the decrease in the CO conversion, a significant drop in the oxygenates and CO2 selectivity followed by the formation of NCCs were observed. These results confirmed a sight activity inhibiting effect of ammonia and pointed out the correlation between the presence of oxygenates and the formation of NCCs under FTS and NFTS processes respectively. At the conditions applied, selectivities of up to 17.9 C% of NCCs (predominantly nitriles) could be obtained. This modified process may therefore be considered as an important variation of the FTS process with greatly enhanced chemicals production potential.
- ItemRestrictedStrong-metal–support interaction by molecular design: Fe–silicate interactions in Fischer–Tropsch catalysts(Elsevier, 2012) Mogorosi, Ramoshibidu P; Fischer, Nico; Claeys, Michael; van Steen, EricMetal–support interactions in the form of iron–silicate were investigated by an inverse approach, that is, modification of nano-sized iron oxide with surface silicate groups. The presence of surface silicate groups in the calcined catalyst precursor was confirmed using diffuse reflectance infra-red Fourier transform analysis. The genesis of the various iron phases in the presence of surface silicate groups after H2-activation and the Fischer–Tropsch synthesis was followed. The surface silicate groups are preserved after a hydrogen treatment at 350 C for 16 h, and these surface ligands are associated with the residual iron oxide phase, wüstite. During the Fischer–Tropsch synthesis, a-Fe is mostly converted into v-Fe5C2, whereas FeO is the main source for e-Fe2C. The activity per unit surface area of hexagonal carbide, eFe2C, is ca. 25% higher than that of v-Fe5C2. The presence of surface silicate ligands on e-Fe2C results in a further enhancement of the rate per unit surface area of e-Fe2C by a factor of ca. 3. This is being ascribed to the enhanced availability of hydrogen on the surface due to the presence of the surface silicate groups, which also results in an increase in the methane selectivity, a decrease in the olefin content and a decrease in formation of branched product compounds.
- ItemOpen AccessSupported Cobalt Oxide Catalysts for the Preferential Oxidation of Carbon Monoxide: An in situ Investigation(2021) Nyathi, Thulani Mvelo; Claeys, Michael; Fischer, NicoThe study presented in this thesis has placed great focus on Co3O4-based catalysts for producing CO-free H2-rich gas streams for power generation using proton-exchange or polymer electrolyte membrane fuel cells (PEMFCs). The removal of CO (0.5 – 2%) is essential as it negatively affects the performance of the Pt-based anode catalyst of PEMFCs. Among the various CO removal processes reported, the preferential oxidation of CO (CO-PrOx) to CO2 is a very attractive catalytic process for decreasing the CO content to acceptable levels (i.e.,< 10 ppm) for operating the PEMFC. Co3O4-based catalysts have shown very good catalytic activity for the total oxidation of CO in the absence of H2, H2O and CO2. More specifically, the performance of Co3O4 is known to be influenced by numerous factors such as particle size, particle shape, and the preparation method. As a result, there has also been growing interest in Co3O4 as a cheaper alternative to noble metals for the CO-PrOx reaction. However, the H2 (40 – 75%) in the CO-PrOx feed can also react with O2 (0.5 – 4%) to produce H2O, which consequently decreases the selectivity towards CO2 (based on the total O2 conversion). Aside from H2, the CO-PrOx feed also contains H2O and CO2 which may affect the CO oxidation process as well. The use of Co3O4 as the active catalyst for CO-PrOx can have shortcomings – the main one being its relatively high susceptibility to reduction by H2, forming less active and selective Co-based phases (viz., CoO and metallic Co). Particularly over metallic Co, the conversion pathway of CO can change from oxidation to hydrogenation, forming CH4 instead of CO2. Therefore, the first objective of the work carried out was to investigate the effect of the gas feed components (viz., H2, H2O and CO2; co-fed individually and simultaneously) on the progress of the CO oxidation reaction and the phase stability of Co3O4 over a wide temperature range (50 – 450 °C). It should be noted that the presence of these three gases can also introduce more side reactions, viz., the forward and reverse water-gas shift, respectively, as well as CO and CO2 methanation, respectively. In the supported state, the choice of support, as well as the nature and/or strength of the interaction between the Co3O4 nanoparticles and the support can influence catalytic performance and phase stability. CO oxidation over metal oxides such as Co3O4, is believed to proceed via the Mars-van Krevelen mechanism, which depends on the surface of the catalyst being reducible in order to release lattice oxygen species. Generally, strong metal-support interactions (MSIs) or nanoparticlesupport interactions (NPSIs) can hinder the removal of surface (and bulk) oxygen species, which can negatively affect the catalytic performance. Strong interactions can also promote the solidstate reaction between the species from the nanoparticle with those from the support, leading to the formation of metal-support compounds (MSCs). The supports SiO2, TiO2 and Al2O3 are well known for this phenomenon, and consequently, allow for the formation of silicates, titanates and aluminates, respectively. Support materials such as CeO2, ZrO2 and SiC, are not known for interacting strongly with nanoparticles and often do not react to form MSCs. Therefore, the second objective of this Ph.D. study was to investigate the effect of different support materials (viz., CeO2, ZrO2, SiC, SiO2, TiO2 and Al2O3) on the catalytic performance and phase stability of Co3O4 under different CO-PrOx reaction gas environments. Before carrying out the lab-based experiments, theoretical evaluations were performed by means of thermodynamic calculations based on the Gibbs-Helmholtz Equation. The calculations helped determine the equilibrium conversions of each gas-phase reaction, revealing the extent to which a certain reaction can be expected to take place between 0 and 500 °C. Thermodynamic calculations were also performed to predict the stability of Co3O4, CoO and metallic Co at different temperatures and partial pressure ratios of H2-to-H2O. In the case of supported nanoparticles, the formation of the Co-support compounds - Co2SiO4, CoTiO3 and CoAl2O4 from SiO2, TiO2 and Al2O3, respectively - was shown to be thermodynamically feasible in H2-H2O mixtures. Unsupported Co3O4 nanoparticles were synthesised using the reverse microemulsion technique, while supported Co3O4 nanoparticles were prepared using incipient wetness impregnation. In situ PXRD- and magnetometry-based CO-PrOx catalytic testing was performed in different gas environments as depicted in Figure S.1. The different conditions chosen allowed for the effect of H2, H2O and CO2 on the progress of the CO oxidation reaction and on the reducibility of Co3O4 to be studied. For the first time, this work has identified all the possible gas-phase side reactions (in addition to CO oxidation) that can take place under CO-PrOx conditions. Each reaction could be linked to a specific Co-based phase which is responsible for its occurrence. Furthermore, the temperatures and the extent to which these reactions take place were in-line with the predictions from the thermodynamic calculations. The presence of a support does stabilise the Co3O4 (and CoO) phase over a wide temperature range. Over the weakly-interacting supports (i.e., ZrO2 and SiC), high CO conversions (91.5% and 80.8%, respectively) and O2 selectivities (55.2% and 55.9%, respectively) to CO2 could be obtained, in addition to the improved phase stability of Co3O4. In agreement with the thermodynamic predictions, the presence of Co2SiO4 (7.7%), CoTiO3 (13.8% (from TiO2- anatase) and 8.9% (from TiO2-rutile)), and CoAl2O4 (26.6%) was confirmed using ex situ X-Ray Absorption Spectroscopy in the spent samples of Co3O4/SiO2, Co3O4/TiO2-anatase, Co3O4/TiO2- rutile and Co3O4/Al2O3, respectively, after CO-PrOx. These three samples also exhibited relatively low CO oxidation activities and selectivities, as well as low Co3O4 reducibility.
- ItemOpen AccessThe Leaching of Cobalt in Platinum-Cobalt Fuel Cell Catalysts(2022) Ranganthan, Omishka; Claeys, Michael; Fischer, NicoPlatinum-cobalt catalysts have proved to be promising alternatives to the conventional platinum catalyst used for the ORR reaction at the cathode of PEM fuel cells. These catalysts, however, undergo leaching of cobalt in the harsh environment of the fuel cell during its operation as well as during catalyst ink preparation and conditioning. Several methods have been developed and are currently being used to characterise this leaching, however, these methods are long and complicated or only quantify cobalt loss before and after operation and not during the process. In this study a unique method of testing cobalt leaching was developed, which involves utilizing the magnetic properties of the alloy through the use of an in-situ magnetometer. This method is able to characterise the cobalt mass during catalyst ink preparation as well as during fuel cell operation. Furthermore, it may allow for uninterrupted catalyst characterisation during fuel cell operation. The leaching occurring during electrode preparation was tested using this methodology in this study. The method involved removing the reactor typically placed in the magnetometer with a titanium sample holder providing minimal background signal. The detection limits of the magnetometer were investigated, and it was found that the minimum amount of cobalt loaded on an individual electrode was 0.5 mg. It was also determined that the electrode surface area should not exceed 6.25 cm2 in order to obtain a high magnetic signal. The electrodes were taped to the sample holder which was clamped to the arm of the oscillating arm of the magnetometer and aligned. Data of magnetisation as a function of magnetic field strength were used for calibration as well as electrode leaching tests. The calibration curve was constructed by varying amounts of catalyst loaded on individual electrodes and determining the magnetic signals for each. Electrode leaching tests involved catalyst and ionomer forming an ink which was coated on electrodes with different loadings. The magnetic responses were obtained for each electrode. In addition, tests involving a similar methodology with the catalyst or catalyst ink were conducted, which allowed accurate time on stream data of the leaching process. Three different catalysts – PtCo/C, PtCo2/C and Pt3Co/C were synthesised using an organometallic chemical deposition synthesis method and characterised XRD and SEM-EDS analyses. Different masses of these catalysts were dusted on individual Toray paper gas diffusion electrodes and the saturation magnetisation for each loading of all catalysts was used to construct calibration curves. Superparamagnetic behaviour was observed, therefore sizing of the crystallites was possible. Three catalyst inks were made for each alloy catalyst consisting of Nafion solution, water and isopropanol. These were brush coated on electrodes that were tested in the magnetometer. These results showed that the PtCo2/C catalyst experienced a high degree of leaching during electrode preparation, which increased with an increase in initial cobalt loaded. The leaching, or cobalt loss was between 28% - 50%. The Pt3Co/C catalyst experienced the least amount of leaching with a maximum of 13%. The PtCo/C catalyst experienced 26 – 55%, which was similar to the PtCo2/C catalyst. Leaching was analysed with time and temperature, and it was found that an increase in temperature increased the rate of leaching. Leaching occurred immediately when the Nafion ionomer was in contact with the catalyst. Miniature fuel cells were designed to implement this methodology in order to test leaching occurring during the other stages of the fuel cell – conditioning and operation. These cells are designed to contain minimal ferromagnetic material to minimise background signal.
- ItemOpen AccessThe suitability of Ni-Co catalysts in the dry reforming reaction(2022) Mtetwa, Bongani Leslie; Claeys, Michael; Fischer, NicoIn recent times, the dry reforming of methane has received significant interest as an alternative process through which synthesis gas can be produced. This is because dry reforming combines methane and carbon dioxide which are both greenhouse gases into synthesis gas which is used in the production of synthetic fuels and chemicals. The main problem faced by the dry reforming reaction is the formation carbon which causes catalyst deactivation. Noble metal catalysts such as ruthenium and rhodium have shown great promise as dry reforming catalysts because of their resistance to carbon formation, but they are expensive making their use on an industrial scale unlikely. This has led to nonnoble metals such as nickel and cobalt being considered as potential catalysts. Bimetallic nickel-cobalt (Ni-Co) catalysts have garnered a lot of interest as dry reforming catalysts as combining these two metals is believed to produce catalysts that would be more stable than monometallic nickel and cobalt catalysts. The objectives of the project were to investigate the suitability of nickel-cobalt (Ni-Co) alloy catalysts with different compositions as well as monometallic nickel and cobalt catalysts as dry reforming catalysts. In doing so, special emphasis was placed on understanding the effect of the Ni-Co ratio on catalyst activity, stability, and deactivation mechanisms. In the study, seven catalysts with varying Ni-Co ratios were prepared. The catalysts had a 10 wt.% active metal (nickel and cobalt combined) loading and were supported on magnesium aluminate (MgAl2O4). Catalyst testing was carried out on all the catalysts at 700 °C for a period of 12 hours to compare their performance in the dry reforming reaction. The results from catalyst testing showed that the Ni-Co catalysts that were nickel rich (70% and 90% nickel in terms of active metal) were the most active catalysts. This was because these catalysts achieved higher methane and carbon dioxide conversions in comparison to the rest of the catalysts. The most surprising result from catalyst testing was that the monometallic nickel catalyst showed very limited activity and was unstable. Post run catalyst characterisation using Raman spectroscopy showed that the Ni-Co composition of the catalysts influenced the type of carbon deposited on the catalysts during catalyst testing. This was because the carbon deposits on the cobalt rich Ni-Co catalysts were found to be more graphitic in nature compared to those on the nickel rich Ni-Co catalysts. However, the Ni-Co composition of the catalysts was found to have no influence on the amount of carbon deposited on the catalysts based on the results obtained from TGA analysis. In addition, post run catalyst characterisation showed that there was carbon formation on all the catalysts studied except for the monometallic nickel catalyst. This showed that there is a need to investigate additional means through which the carbon formation can be limited during catalyst testing. The co-feeding of water in the dry reforming of methane is one such measure that should be investigated.