Browsing by Author "Kooyman, Patricia J"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Open Access
    Atomic Scale Characterisation of Pt-Re/CeO2 for the Water Gas Shift Reaction
    (2022) Martin, Richard B; Kooyman, Patricia J
    Inexpensive hydrogen gas is needed to transition to a hydrogen based economy. For this to happen, economically feasible, large scale production of hydrogen gas is required. Currently, electrolysis is unable to meet this demand economically, so steam reforming of hydrocarbons (e.g. methane) is required to produce large volumes of hydrogen. To be used for mobile applications, such as in hydrogen fuel cells, the hydrogen fuel must be free of CO which is poisonous to the fuel cell catalyst. The water gas shift (WGS) reaction is used to convert CO and H2O to CO2 and H2, cleaning the fuel stream and producing additional hydrogen. A medium temperature shift catalyst, operating in a temperature range of 280 to 350 °C can be used to perform the WGS in one step, simplifying the complex two step process used industrially and enabling small scale hydrogen production. The catalytic material used for the medium temperature shift is platinum on a metal oxide support, such as CeO2, however this is prone to deactivation by sintering. It has been shown in literature that the addition of Re enhances the activity and stability of the catalyst, however it requires close contact with the platinum for promotion to take place. Traditional impregnation methods are unable to guarantee this close contact due to the non-uniform distribution of both metals on the catalyst support. Colloidal synthesis methods provide a way to ensure the platinum and rhenium are in close contact with each other. Literature has reported a method to produce homogeneously alloyed Pt3Re nanoparticles. It was hypothesised that using the colloidal method for synthesising Pt-Re/CeO2 catalysts would improve the activity and stability over Pt-Re/CeO2 catalysts synthesised by the traditional impregnation method. In this study the method reported in literature was optimised for small metallic nanoparticles with a spherical shape, which could then be supported on CeO. The optimised method was used to produce Pt/CeO2 and Pt-Re/CeO2 catalysts, while incipient wetness impregnation was used to produce a Pt-Re/CeO2 catalyst for comparison. These catalysts were physically characterised used X-Ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and transmission electron microscopy (TEM). Operando TEM was used to observe the Pt and Pt-Re nanoparticles under water gas shift conditions. The nanoparticles exhibited oscillations between facetted and spherical shapes, which was not present under normal TEM imaging conditions. The cause of these oscillations was attributed to the presence of reactive gases and elevated temperature. The catalytic performance of the catalysts was evaluated at 320 °C and 350 °C, and the Pt-Re/CeO2 synthesised by the colloidal method had the largest activity of all the catalysts, however, it showed more deactivation at 320 °C than the Pt-Re/CeO2 catalyst prepared by impregnation. At 350 °C, all of the catalysts showed an increase in stability. In conclusion, Pt-Re/CeO2 prepared by the colloidal method had a higher activity (at all temperatures) than Pt-Re/CeO2 prepared by the impregnation method, due to the closer contact between Pt and Re, with increasing stability at higher temperatures, the cause of which is currently unknown.
  • Thumbnail Image
    Item
    Open Access
    Carbon-Supported KCoMoS2 for Alcohol Synthesis from Synthesis Gas
    (Multidisciplinary Digital Publishing Institute, 2021-10-30) Osman, Mohamed E; Maximov, Vladimir V; Dorokhov, Viktor S; Mukhin, Viktor M; Sheshko, Tatiana F; Kooyman, Patricia J; Kogan, Viktor M
    KCoMoS2 was supported on various carbon support materials to study the support effect on synthesis gas conversion. Next to two activated carbons with high micropore volume, a traditional alumina (γ-Al2O3) support and its carbon coated form (CCA) were studied for comparison. Coating alumina with carbon increases the selectivity to alcohols, but the AC-supported catalysts show even higher alcohol selectivities and yields, especially at higher temperatures where the conversions over the AC-supported catalysts increase more than those over the γ-Al2O3-based catalysts. Increasing acidity leads to decreased CO conversion yield of alcohols. The two activated-carbon-supported catalysts give the highest yield of ethanol at the highest conversion studied, which seems to be due to increased KCoMoS2 stacking and possibly to the presence of micropores and low amount of mesopores.
  • Thumbnail Image
    Item
    Open Access
    Hydrocracking of n-C16 over MFI Zeolite Nano-sheets - Effect of the Si/Al Ratio
    (2018) Parker, Mohamed Habeeb; Kooyman, Patricia J; Brosius, Roald
    The combination of MFI zeolite nano-sheets with competitive adsorption of water (H2O) in hydrocracking of long-chain paraffins presents a promising opportunity to produce diesel with high yield and with high cetane number. Thus, in wet hydrocracking of a long-chain paraffin (n-hexadecane (n-C16)) over MFI nano-sheets, it was investigated whether catalytic activity increased with increasing number of Brønsted acid (H + ) sites (decreasing silicon-to-aluminium (Si/Al) ratio), while secondary cracking remained completely suppressed. Also, it was investigated whether more Al atoms could be incorporated into the framework of MFI nano-sheets by modifying the new synthesis method. It was demonstrated that the new synthesis method, which utilizes C22H45–N + (CH3)2–C6H12–N + (CH3)2–C6H13 (C22-6-6) as structure-directing agent (SDA), could be extended to various Si/Al ratios in the range 25 – 100. The nano-sheets exhibited extra-framework Al (EFAl) species. Nano-sheets with Si/Al = 75 exhibited an oddly large amount of EFAl species compared to the other nano-sheets. For nano-sheets with Si/Al = 75, a high fraction of the EFAl species may have formed during calcination of the ammonium form and may encompass flexible Al species with predominantly Al in octahedral coordination (AlVI). Nano-sheets were loaded with 0.9 wt% platinum (Pt) via incipient wetness impregnation (IWI). Pt/nano-sheets with Si/Al = 25, 50 and 100 exhibited similar and high Pt dispersion (γPt). In contrast, Pt/nano-sheets with Si/Al = 75 exhibited a very low γPt, which was probably a result of the abundance, nature and/or location of EFAl species present in the support. In dry hydrocracking of n-C16, the catalytic activity increased with decreasing Si/Al ratio, strongly suggesting that the number of H+ sites increased with decreasing Si/Al ratio. Nano-sheets with Si/Al = 75 most likely contained AlVI species associated with Brønsted acidity, supporting the presence of flexible AlVI species. In wet hydrocracking of n-C16, at a constant and sufficiently high γPt, the activity increased with increasing number of H+ sites (decreasing Si/Al ratio), while secondary cracking remained completely suppressed. Pt/nano-sheets with Si/Al = 75 displayed a lower activity than 2 Pt/nano-sheets with Si/Al = 100, which may be a result of the very low γPt of Pt/nano-sheets with Si/Al = 75, underlining the importance of high γPt. For Pt/nano-sheets with Si/Al = 25, 50 and 100, H2O favoured linear cracking products at low cracking yields. In contrast, for Pt/nano-sheets with Si/Al = 75, H2O favoured branched cracking products, which may be a result of Pt sites on the external surface of the support being too far from the H+ sites inside the micropores. The new synthesis method could be extended to a modified SDA, namely C22H45–N + (CH3)2–C6H12–N + (CH3)2–C3H7 (C22-6-3), at various Si/Al ratios in the range 25 – 100. Replacing the terminal –C6H13 group in C22-6-6 with –C3H7 resulted in an increase in the framework Al (FAl) content of MFI nano-sheets with Si/Al ≥ 50, with the increase being the most pronounced for nano-sheets with Si/Al = 50. This was due to the increased occupancy of the zeolite framework by the hydrophilic region of C22-6-3 in comparison to the hydrophilic region of C22-6-6 under the given set of synthesis conditions, since –C3H7 was less bulky than –C6H13. Calcined nano-sheets were loaded with 1 wt% Pt via competitive ion exchange (CIE). In dry and wet hydrocracking of n-C16, the activity increased with decreasing Si/Al ratio and in wet hydrocracking, secondary cracking was not completely suppressed up to high conversions. This was probably due to the presence of additional H+ sites generated after SDA removal. H2O favoured linear cracking products at low cracking yields. Sodium (Na+ ) ion-exchanged nano-sheets were loaded with 1 wt% Pt via CIE. The average Pt size (dPt) of the Pt/Na+ nano-sheets were larger than the dPt of the Pt/calcined nano-sheets, which may be a result of the nature and/or location of EFAl species present in the Na+ supports. In dry and wet hydrocracking of n-C16, differences in activity were observed and in wet hydrocracking, secondary cracking was not completely suppressed up to high conversions. This was probably due to insufficient intimacy between H+ sites and Pt sites such that the rate was controlled by diffusion of olefinic intermediates from H+ sites to Pt sites and vice versa. H2O favoured linear cracking products at low cracking yields.
  • Thumbnail Image
    Item
    Open Access
    Study of the effects of promoters (MnOx and FeOx) on Co3O4/CeO2 during Preferential Oxidation of Carbon monoxide
    (2022) Ndila, Noluvuyo; Kooyman, Patricia J
    The provision of sustainable, clean, safe, reliable, and affordable energy towards the advancement of the quality of life in all societies is goal 7 of the United Nations sustainable development goals . Currently, most energy production relies on the use of fossil fuels (Büyüközkan, Karabulut and Mukul, 2018). However, these have negative environmental impacts such as global warming, land degradation, and water pollution (Haryanto et al., 2005). Hence there is an urgent need for improving access to eco-friendly and reliable energy alternatives. One of the leading and promising pollution-free high-energy carriers is hydrogen for fuel cells. Currently, hydrogen gas is mostly produced via steam reforming followed by the water-gas shift reaction (Edwards et al., 2008). However, the concern with using this hydrogen in fuel cells is the carbon monoxide content which is between 0.5-1.0 vol % (Liu, Song & Subramani, 2010). Carbon monoxide is poisonous to the platinum anode of the fuel cell so it needs to be reduced to below 10 ppm before being introduced in the fuel cell (Liu, Song & Subramani, 2010). One of the methods for carbon monoxide removal is preferential oxidation (PROX) of carbon monoxide to carbon dioxide. This process has been identified as being cost-effective, much easier to implement, and ensures minimum loss of hydrogen when compared to other techniques such as selective CO methanation and pressure swing adsorption (Park, Lee and Lee, 2009). Catalysts used for CO-PROX are either noble metal or transition metal oxide based. Noble metal-based catalysts have a low CO oxidation activity at temperatures below 200 °C, higher cost, and lower availability when compared to transition metal-based catalysts (Kahlich, Gasteiger & Behm, 1997; Grisel et al., 2002). Of the transition metal oxides, Co3O4 and CuO are the most active catalysts for CO-PROX (Woods et al., 2010). The active species for CO oxidation in Co3O4 catalysts is the Co3+/Co2+ redox pair. However, during CO-PROX at temperatures above 200 °C, Co3O4 reduces to metallic Co which is active for CO methanation (Nyathi, 2016; Khasu, 2017). This is undesirable as CO methanation results in H2 consumption, hence the stability of the Co3O4 phase above 200 °C is crucial. In this study, our objective is to investigate the effects of the promoters MnOx and FeOx on the stability of the Co3O4 phase supported on CeO2 and CO-PROX activity. Monodispersed Co3O4 nanoparticles were synthesized via a surfactant-free non-aqueous thermal method. 10 wt.% Co3O4 was supported and promoted via wet impregnation using ultrasonication. The addition of the promoters improves the redox properties of Co3O4. TPR showed an increase in the reduction temperature of Co3O4, while TPO showed a decrease in the re-oxidation temperature of metallic Co. Additionally, COTPRE showed that the promoted catalysts have a higher oxygen storage capacity when compared to the unpromoted catalyst. Catalytic performance evaluation was performed in a fixed bed reactor in the feed gas 1 % CO, 1 % O2, 47 % H2, and 51 % N2 from 50 to 450 °C. The onset temperature for CO methanation on Co3O4/CeO2 was 275 °C and the highest CO conversion achieved was 70% at 220 °C. The addition of 0.3 wt.% MnOx does not change the onset temperature for CO methanation. However, CO conversion increased to 84 % between 240-260 °C. An increase in the MnOx content to 0.5 wt.% increases the onset temperature for CO methanation to 300 °C and the highest CO conversion achieved was 98 % from 200-240 °C. A further increase in MnOx to 1 wt.% increases the CO methanation onset temperature to 310 °C, however, the CO conversion decreases to 92 % from 180-200 °C. The decrease in the optimum temperature for the 0.5 wt.% MnOx promoted catalysts might be beneficial Addition of 0.3 wt.% FeOx increases the onset temperature for CO methanation when compared to the unpromoted catalyst to 300 °C. Furthermore, CO conversion increased to 84% between 240-260 °C. An increase in the FeOx content to 0.5 wt.% does not change the onset temperature for CO methanation but the highest CO conversion achieved was 100 % from 210-220 °C. A further increase in FeOx to 1 wt.% increases the CO methanation onset temperature to 310 °C, however, the CO conversion decreases to 88 % from 220-230 °C.