We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fin... more We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fine-tuned monodisperse nanostructures (Pt(NiCo), PtNi and PtCo). The influence of different solute metal precursors and surfactants on the morphological evolution of homogeneous alloy nanoparticles (NPs) has been investigated. Molybdenum hexacarbonyl (Mo(CO)6) was used as the reductant. We demonstrate that this solution-based strategy results in uniform-sized NPs, the morphology of which can be manipulated by appropriate selection of surfactants and solute metal precursors. Co-surfactants (oleylamine, OAm, and hexadecylamine, HDA) enabled the development of a variety of high-index faceted NP morphologies with varying degrees of curvatures while pure OAm selectively produced octahedral NP morphologies. This Mo(CO)6-based synthetic protocol offers new avenues for the fabrication of multi-structured alloy NPs as high-performance electrocatalysts.
Benzyl alcohol can be oxidized selectively to benzaldehyde over platinum-based catalysts using ei... more Benzyl alcohol can be oxidized selectively to benzaldehyde over platinum-based catalysts using either oxygen (O2, supplied in the form of synthetic air) or the more powerful hydrogen peroxide (H2O2) as the oxidant. Here we compare these oxidants in the aqueous phase oxidation of benzyl alcohol in a batch reactor at 363.15 K or 393.15 K over monodisperse Pt and Pt–Ni nanostructures synthesized with molybdenum hexacarbonyl (Mo(CO)6) as a reductant. The initial catalytic activity of either Pt or a Pt–Ni alloy anchored on titania support (TiO2) is much higher when using H2O2 than when using O2 (supplied in the form of synthetic air). However, the high initial activity using H2O2 is accompanied by a strong decrease in the activity over Pt. Alloying Pt with Ni results in a reduction in the activity in the benzyl alcohol oxidation when using O2 but enhances the initial activity when using H2O2. The results are rationalized based on a change in the relative surface concentration of oxygen-c...
In situ magnetometer study shows that high conversions facilitate sintering, reversible Co(ii)O a... more In situ magnetometer study shows that high conversions facilitate sintering, reversible Co(ii)O and irreversible CoAl2O4 formation within cobalt-based Fischer–Tropsch systems.
Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance ... more Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance catalytic performance.
Developing solid solution nanoparticles with complex faceted geometries and unusual composition s... more Developing solid solution nanoparticles with complex faceted geometries and unusual composition sectoral zoning can enhance their catalytic performance. In a solution-phase synthesis of PtNi nanopa...
In order to establish the effect of alumina modification and calcination temperature on the reduc... more In order to establish the effect of alumina modification and calcination temperature on the reducibility of cobalt oxide, alumina-modified cobalt oxide crystallites containing less than 2.5 wt% Al were prepared via incipient wetness impregnation, and calcined at 300 °C or 500 °C. The catalysts were characterised using X-ray diffraction, scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, temperature programmed reduction (TPR) and X-ray absorption near edge spectroscopy. The alumina modification was found to prevent sintering during calcination and decrease the reducibility of the catalysts. With increasing alumina loading, and increasing calcination temperature, reduction peaks shift to higher temperatures and peaks above 400 °C appear in the TPR. The kinetic evaluation shows that the decreased reducibility is due to a decrease in the pre-exponential factor, which suggests that the alumina modification hinders hydrogen activation and the nucleation of reduced cobalt phases. The catalysts are completely reduced below 800 °C, and no CoAl2O4 phase formation was observed. TPR peaks between 400 and 650 °C were assigned to the formation of a non-stoichiometric cobalt–alumina phase with cobalt ions in octahedral coordination, while peaks between 650 and 800 °C correspond to cobalt ions in tetrahedral coordination. The results show that introduction of small amounts of alumina to cobalt oxide can have drastic effects on the rate of reduction and sintering.Graphical Abstract
The stability and mobility of atomic cobalt and of cobalt subcarbonyl species on γ-Al2O3 surfaces... more The stability and mobility of atomic cobalt and of cobalt subcarbonyl species on γ-Al2O3 surfaces have been investigated using density functional theory (DFT) with a view to elucidate possible mobile species on these surfaces, which can act as agents in the Ostwald ripening process. The two most stable alumina surfaces γ-Al2O3(100) and γ-Al2O3(110) were probed at different levels of hydration. The stability of cobalt subcarbonyl species on γ-Al2O3(100) at high partial pressure of CO (10 bar) increases with increasing number of CO ligands attached to the central cobalt atom up to Co(CO)3 but exhibits a more complex behavior on γ-Al2O3(110). The effect of the hydration level on the stability of cobalt subcarbonyls was investigated. The interpretation of the DFT results in a thermodynamic model shows that at equilibrium the main cobalt subcarbonyl species present on the alumina surface at ca. 500K in the presence of CO are Co(CO)3 and Co(CO)4, with Co(CO)3 being the dominant species on dry γ-Al2O3(100) and w...
ABSTRACT While carbides are always present in iron-based Fischer-Tropsch synthesis, very little i... more ABSTRACT While carbides are always present in iron-based Fischer-Tropsch synthesis, very little is known about the presence and the role of carbides in cobalt-based CO hydrogenation. Cobalt carbide, CO2C, has been reported in catalysts where operational upsets occurred and it is associated with low catalyst activity and increased methane selectivity. In this study, a novel in situ magnetometer was used to study the formation and the stability of cobalt carbide during the Fischer-Tropsch synthesis at fully relevant conditions. The formation of cobalt carbide was confirmed by means of in situ XRD and synchrotron XRD measurements. Cobalt carbide is relatively stable at typical reaction conditions, but a rapid decomposition into hcp cobalt occurs in hydrogen above 150 degrees C. Cobalt carbide formation is inversely proportional to the H-2/CO ratio and the reaction temperature. However, the amounts of cobalt carbide formed are small and the impact on deactivation at realistic Fischer-Tropsch conditions should generally be negligible.
ABSTRACT The addition of a hydrocarbon, n-hexane, to the feed of the iron-catalyzed Fischer–Trops... more ABSTRACT The addition of a hydrocarbon, n-hexane, to the feed of the iron-catalyzed Fischer–Tropsch synthesis in a fixed bed reactor operating at a total pressure of 60 or 90bar is shown to be beneficial, i.e. resulting in a higher activity with only a slight increase in methane selectivity. The thermodynamic properties of pseudo-binary systems containing H2:CO:n-hexane (2:1:y)–long chain hydrocarbon were investigated using the Peng–Robinson equation of state. The chain length of the long chain hydrocarbon is systematically varied to investigate the thermodynamic behaviour of the system in real waxes. It is concluded that supercritical conditions require pressures in excess of those applied in this study (and those reported in the literature). Thus, a supercritical phase is not formed. It is further shown that a three-phase system is likely to be present in the reactor under the typically applied reaction condition for the so-called ‘supercritical’ Fischer–Tropsch synthesis. The reported beneficial effect of the added hydrocarbon to the feed of the Fischer–Tropsch synthesis might be attributed to the change in the partial pressure of the reactants, hydrogen and carbon monoxide. The addition of a hydrocarbon can further aid in obtaining primary products of the Fischer–Tropsch synthesis due to increase in the liquid flow rate through the reactor.
Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained ... more Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained show that in VSbO 4 mixed oxide catalyst systems, substitution of vanadium by small quantities of iron has a strongly positive effect on the activity of the catalysts. With such a ...
Introduction High activity of supported metal catalysts is usual ly obtained by providing high me... more Introduction High activity of supported metal catalysts is usual ly obtained by providing high metal surface area or small metal crystallites resp ectively. The stability of supported metal catalysts in Fischer-Tropsch synthesis can be negat iv ly affected as deactivation can result, inter alia, from sintering and phase changes such a s oxidation [1,2,3]. Characterization of such catalysts, in particular used ones, is notoriously difficult as exposure to air can cause dramatic changes of their physico-chemical properties. Moreo ver, they are often surrounded by a wax layer which can further complicate analysis by conv entional techniques such as XRD and TEM characterization. Ultimately therefore in-situ char acterization techniques of these catalysts are becoming crucially important in order to study the relation of catalyst changes and their performance.
A recent development in photocatalytic reactor systems is the introduction of catalyst coated opt... more A recent development in photocatalytic reactor systems is the introduction of catalyst coated optical fibre for improved illumination efficiency. A high catalyst loading per unit reactor volume would require a large amount of coated fibre. Hence, an automated optical fibre catalyst coating process (AOFCCP) was developed. Here, we report on the effect of process variables in the deposition of TiO2 (P25 and anatase) on the coating thickness and surface morphology. The thickness of the photocatalytic layer increases with an increase in the withdrawal speed, which is attributed to a reduced flow back of the slurry prior to the drying process. This is further substantiated by the observed increase the catalyst layer thickness upon increasing the drying/calcination temperature (200 – 500 °C) upon coating P25 on to the optical fibre. The surface of the layer created by dip-coating through a P25-slurry was often corrugated, whereas the anatase surface obtained by gel-coating was smooth.
Introduction Supported cobalt catalysts are the preferred catalyst for the low temperature Fische... more Introduction Supported cobalt catalysts are the preferred catalyst for the low temperature Fischer-Tropsch process (i.e. for the production of long chain hydrocarbons with a view to maximize the diesel yield) because of their stability, high hydrocarbon productivity particularly at high conversion, and low water-gas shift activity [1]. Supported cobalt catalysts are typically prepared by impregnation of oxidic supports with aqueous solutions of cobalt salts which upon reduction should result in the formation of dispersed Co crystallites [2]. Addition of noble metals, such as Pt, to supported cobalt Fischer-Tropsch catalysts has been shown to have an effect on the reduction of these catalysts, and thus an increased activity (although sometimes a shift in selectivity is observed as well)[3]. Noble metal promoters are typically brought on to the support by co-impregnation together with the aqueous cobalt salt precursor solution, which may lead to a random distribution of the noble meta...
Introduction Materials, such as natural gas, oil residue, coal, or biomass, can be converted into... more Introduction Materials, such as natural gas, oil residue, coal, or biomass, can be converted into easily transportable liquid products using the Fischer-Tropsch process. This process is typically optimized for the production of liquid fuels only. Hence, the carbonaceous feedstock is reformed/gasified using pure oxygen to avoid the introduction of the inert nitrogen into the process and thus enabling the recycle of unconverted synthesis gas. Recycle of synthesis gas enables the use of a moderately sized reaction vessel operating at intermediate conversion, whilst achieving a high overall conversion. The required air separation unit can make up ca. 15-25% of the overall capital cost of the Fischer-Tropsch plant [1,2]. Omission of this unit in the process, i.e. generating synthesis gas with air, results in a synthesis gas with a large amount of nitrogen. The effect of the in the synthesis gas, whose effect on the reaction kinetics can be countered by increasing the reaction pressure [3...
This paper describes the re-structuring of the chemical engineering curriculum at the University ... more This paper describes the re-structuring of the chemical engineering curriculum at the University of Cape Town, in order to respond both to a changing student population as well as the changing world of work. The basis for the changes proposed was an analysis of problems within the existing curriculum. The changes are based on educational research, particularly as it pertains to quality student learning, as well as models of curriculum innovation at other reputable institutions. Key features of the new curriculum are discussed.
Steam reforming of glycerol is often applied for the generation of green hydrogen; however, here,... more Steam reforming of glycerol is often applied for the generation of green hydrogen; however, here, an effort has been made to produce an efficient syngas composition that can utilize effectively for...
We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fin... more We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fine-tuned monodisperse nanostructures (Pt(NiCo), PtNi and PtCo). The influence of different solute metal precursors and surfactants on the morphological evolution of homogeneous alloy nanoparticles (NPs) has been investigated. Molybdenum hexacarbonyl (Mo(CO)6) was used as the reductant. We demonstrate that this solution-based strategy results in uniform-sized NPs, the morphology of which can be manipulated by appropriate selection of surfactants and solute metal precursors. Co-surfactants (oleylamine, OAm, and hexadecylamine, HDA) enabled the development of a variety of high-index faceted NP morphologies with varying degrees of curvatures while pure OAm selectively produced octahedral NP morphologies. This Mo(CO)6-based synthetic protocol offers new avenues for the fabrication of multi-structured alloy NPs as high-performance electrocatalysts.
Benzyl alcohol can be oxidized selectively to benzaldehyde over platinum-based catalysts using ei... more Benzyl alcohol can be oxidized selectively to benzaldehyde over platinum-based catalysts using either oxygen (O2, supplied in the form of synthetic air) or the more powerful hydrogen peroxide (H2O2) as the oxidant. Here we compare these oxidants in the aqueous phase oxidation of benzyl alcohol in a batch reactor at 363.15 K or 393.15 K over monodisperse Pt and Pt–Ni nanostructures synthesized with molybdenum hexacarbonyl (Mo(CO)6) as a reductant. The initial catalytic activity of either Pt or a Pt–Ni alloy anchored on titania support (TiO2) is much higher when using H2O2 than when using O2 (supplied in the form of synthetic air). However, the high initial activity using H2O2 is accompanied by a strong decrease in the activity over Pt. Alloying Pt with Ni results in a reduction in the activity in the benzyl alcohol oxidation when using O2 but enhances the initial activity when using H2O2. The results are rationalized based on a change in the relative surface concentration of oxygen-c...
In situ magnetometer study shows that high conversions facilitate sintering, reversible Co(ii)O a... more In situ magnetometer study shows that high conversions facilitate sintering, reversible Co(ii)O and irreversible CoAl2O4 formation within cobalt-based Fischer–Tropsch systems.
Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance ... more Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance catalytic performance.
Developing solid solution nanoparticles with complex faceted geometries and unusual composition s... more Developing solid solution nanoparticles with complex faceted geometries and unusual composition sectoral zoning can enhance their catalytic performance. In a solution-phase synthesis of PtNi nanopa...
In order to establish the effect of alumina modification and calcination temperature on the reduc... more In order to establish the effect of alumina modification and calcination temperature on the reducibility of cobalt oxide, alumina-modified cobalt oxide crystallites containing less than 2.5 wt% Al were prepared via incipient wetness impregnation, and calcined at 300 °C or 500 °C. The catalysts were characterised using X-ray diffraction, scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, temperature programmed reduction (TPR) and X-ray absorption near edge spectroscopy. The alumina modification was found to prevent sintering during calcination and decrease the reducibility of the catalysts. With increasing alumina loading, and increasing calcination temperature, reduction peaks shift to higher temperatures and peaks above 400 °C appear in the TPR. The kinetic evaluation shows that the decreased reducibility is due to a decrease in the pre-exponential factor, which suggests that the alumina modification hinders hydrogen activation and the nucleation of reduced cobalt phases. The catalysts are completely reduced below 800 °C, and no CoAl2O4 phase formation was observed. TPR peaks between 400 and 650 °C were assigned to the formation of a non-stoichiometric cobalt–alumina phase with cobalt ions in octahedral coordination, while peaks between 650 and 800 °C correspond to cobalt ions in tetrahedral coordination. The results show that introduction of small amounts of alumina to cobalt oxide can have drastic effects on the rate of reduction and sintering.Graphical Abstract
The stability and mobility of atomic cobalt and of cobalt subcarbonyl species on γ-Al2O3 surfaces... more The stability and mobility of atomic cobalt and of cobalt subcarbonyl species on γ-Al2O3 surfaces have been investigated using density functional theory (DFT) with a view to elucidate possible mobile species on these surfaces, which can act as agents in the Ostwald ripening process. The two most stable alumina surfaces γ-Al2O3(100) and γ-Al2O3(110) were probed at different levels of hydration. The stability of cobalt subcarbonyl species on γ-Al2O3(100) at high partial pressure of CO (10 bar) increases with increasing number of CO ligands attached to the central cobalt atom up to Co(CO)3 but exhibits a more complex behavior on γ-Al2O3(110). The effect of the hydration level on the stability of cobalt subcarbonyls was investigated. The interpretation of the DFT results in a thermodynamic model shows that at equilibrium the main cobalt subcarbonyl species present on the alumina surface at ca. 500K in the presence of CO are Co(CO)3 and Co(CO)4, with Co(CO)3 being the dominant species on dry γ-Al2O3(100) and w...
ABSTRACT While carbides are always present in iron-based Fischer-Tropsch synthesis, very little i... more ABSTRACT While carbides are always present in iron-based Fischer-Tropsch synthesis, very little is known about the presence and the role of carbides in cobalt-based CO hydrogenation. Cobalt carbide, CO2C, has been reported in catalysts where operational upsets occurred and it is associated with low catalyst activity and increased methane selectivity. In this study, a novel in situ magnetometer was used to study the formation and the stability of cobalt carbide during the Fischer-Tropsch synthesis at fully relevant conditions. The formation of cobalt carbide was confirmed by means of in situ XRD and synchrotron XRD measurements. Cobalt carbide is relatively stable at typical reaction conditions, but a rapid decomposition into hcp cobalt occurs in hydrogen above 150 degrees C. Cobalt carbide formation is inversely proportional to the H-2/CO ratio and the reaction temperature. However, the amounts of cobalt carbide formed are small and the impact on deactivation at realistic Fischer-Tropsch conditions should generally be negligible.
ABSTRACT The addition of a hydrocarbon, n-hexane, to the feed of the iron-catalyzed Fischer–Trops... more ABSTRACT The addition of a hydrocarbon, n-hexane, to the feed of the iron-catalyzed Fischer–Tropsch synthesis in a fixed bed reactor operating at a total pressure of 60 or 90bar is shown to be beneficial, i.e. resulting in a higher activity with only a slight increase in methane selectivity. The thermodynamic properties of pseudo-binary systems containing H2:CO:n-hexane (2:1:y)–long chain hydrocarbon were investigated using the Peng–Robinson equation of state. The chain length of the long chain hydrocarbon is systematically varied to investigate the thermodynamic behaviour of the system in real waxes. It is concluded that supercritical conditions require pressures in excess of those applied in this study (and those reported in the literature). Thus, a supercritical phase is not formed. It is further shown that a three-phase system is likely to be present in the reactor under the typically applied reaction condition for the so-called ‘supercritical’ Fischer–Tropsch synthesis. The reported beneficial effect of the added hydrocarbon to the feed of the Fischer–Tropsch synthesis might be attributed to the change in the partial pressure of the reactants, hydrogen and carbon monoxide. The addition of a hydrocarbon can further aid in obtaining primary products of the Fischer–Tropsch synthesis due to increase in the liquid flow rate through the reactor.
Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained ... more Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained show that in VSbO 4 mixed oxide catalyst systems, substitution of vanadium by small quantities of iron has a strongly positive effect on the activity of the catalysts. With such a ...
Introduction High activity of supported metal catalysts is usual ly obtained by providing high me... more Introduction High activity of supported metal catalysts is usual ly obtained by providing high metal surface area or small metal crystallites resp ectively. The stability of supported metal catalysts in Fischer-Tropsch synthesis can be negat iv ly affected as deactivation can result, inter alia, from sintering and phase changes such a s oxidation [1,2,3]. Characterization of such catalysts, in particular used ones, is notoriously difficult as exposure to air can cause dramatic changes of their physico-chemical properties. Moreo ver, they are often surrounded by a wax layer which can further complicate analysis by conv entional techniques such as XRD and TEM characterization. Ultimately therefore in-situ char acterization techniques of these catalysts are becoming crucially important in order to study the relation of catalyst changes and their performance.
A recent development in photocatalytic reactor systems is the introduction of catalyst coated opt... more A recent development in photocatalytic reactor systems is the introduction of catalyst coated optical fibre for improved illumination efficiency. A high catalyst loading per unit reactor volume would require a large amount of coated fibre. Hence, an automated optical fibre catalyst coating process (AOFCCP) was developed. Here, we report on the effect of process variables in the deposition of TiO2 (P25 and anatase) on the coating thickness and surface morphology. The thickness of the photocatalytic layer increases with an increase in the withdrawal speed, which is attributed to a reduced flow back of the slurry prior to the drying process. This is further substantiated by the observed increase the catalyst layer thickness upon increasing the drying/calcination temperature (200 – 500 °C) upon coating P25 on to the optical fibre. The surface of the layer created by dip-coating through a P25-slurry was often corrugated, whereas the anatase surface obtained by gel-coating was smooth.
Introduction Supported cobalt catalysts are the preferred catalyst for the low temperature Fische... more Introduction Supported cobalt catalysts are the preferred catalyst for the low temperature Fischer-Tropsch process (i.e. for the production of long chain hydrocarbons with a view to maximize the diesel yield) because of their stability, high hydrocarbon productivity particularly at high conversion, and low water-gas shift activity [1]. Supported cobalt catalysts are typically prepared by impregnation of oxidic supports with aqueous solutions of cobalt salts which upon reduction should result in the formation of dispersed Co crystallites [2]. Addition of noble metals, such as Pt, to supported cobalt Fischer-Tropsch catalysts has been shown to have an effect on the reduction of these catalysts, and thus an increased activity (although sometimes a shift in selectivity is observed as well)[3]. Noble metal promoters are typically brought on to the support by co-impregnation together with the aqueous cobalt salt precursor solution, which may lead to a random distribution of the noble meta...
Introduction Materials, such as natural gas, oil residue, coal, or biomass, can be converted into... more Introduction Materials, such as natural gas, oil residue, coal, or biomass, can be converted into easily transportable liquid products using the Fischer-Tropsch process. This process is typically optimized for the production of liquid fuels only. Hence, the carbonaceous feedstock is reformed/gasified using pure oxygen to avoid the introduction of the inert nitrogen into the process and thus enabling the recycle of unconverted synthesis gas. Recycle of synthesis gas enables the use of a moderately sized reaction vessel operating at intermediate conversion, whilst achieving a high overall conversion. The required air separation unit can make up ca. 15-25% of the overall capital cost of the Fischer-Tropsch plant [1,2]. Omission of this unit in the process, i.e. generating synthesis gas with air, results in a synthesis gas with a large amount of nitrogen. The effect of the in the synthesis gas, whose effect on the reaction kinetics can be countered by increasing the reaction pressure [3...
This paper describes the re-structuring of the chemical engineering curriculum at the University ... more This paper describes the re-structuring of the chemical engineering curriculum at the University of Cape Town, in order to respond both to a changing student population as well as the changing world of work. The basis for the changes proposed was an analysis of problems within the existing curriculum. The changes are based on educational research, particularly as it pertains to quality student learning, as well as models of curriculum innovation at other reputable institutions. Key features of the new curriculum are discussed.
Steam reforming of glycerol is often applied for the generation of green hydrogen; however, here,... more Steam reforming of glycerol is often applied for the generation of green hydrogen; however, here, an effort has been made to produce an efficient syngas composition that can utilize effectively for...
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