Browsing by Author "Kooyman, Patricia"
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- ItemOpen AccessCuAg bimetallic nanoparticles for the electrochemical reduction of carbon dioxide(2021) Dlamini, Gcinisizwe; Kooyman, Patricia; Levecque, PieterThe electrochemical reduction of carbon dioxide is a surface reaction, involving the conversion of carbon dioxide and water to hydrocarbons and oxygenates in an electrolytic environment. This reaction grants an opportunity for the rerouting of carbon dioxide from expulsion to the atmosphere towards the production of chemical products. Due to the stable C-O bond in carbon dioxide, this reaction requires a catalyst and an external energy source to activate it. The use of renewable energy as an energy source would ensure that the electrochemical reduction process is carbon neutral. Cu has been identified as a promising catalyst for the electrochemical reduction of carbon dioxide, as it is more active and produces higher amounts of hydrocarbons and oxygenates relative to other transition metals,. However, Cu is unselective towards a specific product, and it highly active for the undesirable hydrogen evolution reaction (Kuhl et al., 2014). On the other hand, under electrochemical conditions, Ag yields mainly CO, which has been shown to compete with the hydrogen evolution reaction (Hori, Murata & Takahashi, 1989). This study focuses on the synthesis of different ratios of CuAg bimetallic nanoparticles, and their electrocatalytic performance evaluation for the electrochemical reduction of carbon dioxide. Bimetallic nanoparticles were synthesised via a wet chemical method using two synthesis routes. One synthesis was performed in the presence of hexadecylamine (HDA), surfactant, while the other was performed in its absence. The electrocatalytic performance evaluation was conducted using two reactors, a batch reactor with a gas diffusion electrode, and a rotating disc electrode reactor. It was found that catalysts synthesised in the absence of HDA had a phase-separated atomic arrangement, forming islands of Cu and Ag. On the other hand, synthesis conducted in the presence of HDA culminated in a CuAg solid solution. The two synthesis routes resulted in catalysts that had distinct product distributions. Catalysts prepared in the absence of HDA predominantly formed formate, with catalysts that had a higher Cu content forming methanol and CO. The yield of formate for catalysts synthesised under the absence of HDA did not decline at higher potentials relative to Cu catalysts which suffered from hydrogen production. On the other hand, bimetallic catalysts synthesised in the presence of HDA demonstrated behaviour similar to monometallic catalysts. Catalysts with a higher Cu content predominantly produced formate, while catalysts with a high Ag content produced a CO rich stream. This study indicates a profound dependency of the catalyst activity and product distribution on the CuAg bimetallic ratio and atomic arrangement. This study adds knowledge on the synthesis of CuAg bimetallic nanoparticles, and the design of catalysts for the electrochemical reduction of carbon dioxide.
- ItemOpen AccessPlatinum Nanoparticles to Study Metal Location in Shape-Selective Hydrocracking(2018) Nel, Dayle; Brosius, Roald; Kooyman, Patricia; Fletcher, JackIn the petrochemical industry, bifunctional hydrocracking is used to further refine long chain hydrocarbon product molecules into smaller, more valuable fragments (such as diesel and aviation fuel). The current study is focused on using hydrocracking to increase diesel yield, as the preferred automotive fuel in terms of efficiency and environmental impact. However, even though conventional hydrocracking can significantly increase diesel yield, it is not selective to improving diesel quality. In a recent study by Brosius et al., 2016, the use of a Pt/MFI bifunctional hydrocracking catalyst in the presence of H2O has shown, for the first time, selective hydrocracking to produce not only a high yield but also high cetane number diesel product. The current study is a continuation of the work by Brosius et al., 2016, with a focus on improving the catalyst design and, in particular, the metal function, as in this study the Pt metal was supported using wet impregnation which does not allow for control of metal size and location. The hydrocracking mechanism consists of a (de)hydrogenation function at the metal sites and cracking/isomerisation steps at the acid sites. The reason a high yield of high cetane diesel is produced from hydrocracking over Pt/MFI in the presence of H2O is twofold: firstly, hydrocracking inside the MFI micropores is selective to C-type β-scission which results in linear cracking products, and secondly, H2O competes at the acid sites which allows for the desorption of the linear primary cracking products. From previous literature it is evident that metal location affects both activity and selectivity during hydrocracking over various catalyst types due to diffusion limitations. Furthermore, with hydrocracking over MFI, metal location is also expected to influence the amount of reactant molecules which crack inside the micropores. Therefore, the aim of this study was to quantify the extent that metal located inside or outside the micropores of MFI influences the cracking and isomerisation steps in the hydrocracking of n-C16 in the presence of H2O. Pt metal was selectively placed inside and outside the MFI micropores using different preparation techniques. To place the metal exclusively outside the micropores, monodisperse Pt NPs, which were bigger than the micropore diameter of 0.55 nm, were pre-synthesised and then contacted with the MFI support. To place the metal exclusively inside the micropores, a competitive ion exchange (CIE) method was used where Pt ions were chemically bonded within the MFI structure. Besides bulk MFI, the Pt NPs were also supported on MFI nanosheets (NS), which have been used in previous research to decrease diffusion limitations between metal and acid sites, to assess the effects of metal location. The prepared catalysts were characterised using TEM, ICP-OES, CO chemisorption, and NH3-TPD to ensure metal location was the only varied parameter. The catalysts were then analysed by dry and wet hydrocracking of n-C16 in a fixed-bed, trickle reactor, where the product composition was determined using an online GC. The performance of PtNPs/MFI and Pt-CIE/MFI were compared directly with Pt/MFI from Brosius et al., 2016. Catalyst activity was first analysed using conversion versus temperature plots and Arrhenius plots, primary hydrocracking was then assessed using C4/C12 ratio and once achieved, branched versus linear product selectivity could be compared. Looking at activity, H2O decreased the activity by ~30 °C, which is expected as H2O blocks acid sites. Comparing the individual samples showed relatively similar results in wet and dry hydrocracking, showing metal location does not play a major role in overall activity. From the Arrhenius plots, the activation energies could be compared and showed that the difference between wet and dry hydrocracking was relatable to the isosteric heat of adsorption of H2O on MFI. Primary hydrocracking was achieved for all three catalyst samples in the presence of H2O, again showing that metal location does not largely influence the yield of primary cracking products. However, when comparing branched versus linear primary cracking products, a significant difference was observed based on metal location. At low conversion, the greatest differences are seen, with Pt/MFI and Pt- CIE/MFI achieving ~100 % n-alkanes in C1-C15 cracked products, whereas PtNPs/MFI achieved only ~40 %. The reason for the vast differences is because having the metal inside the micropores, firstly, provides a non-diffusion hindered steady supply of reactant molecules to encourage desorption of the primary cracked products and, secondly, the cracking/isomerisation steps need to occur inside the MFI micropores which is more likely with internal metal. Furthermore, placing Pt NPs on NSs showed that even with thin NSs of 2 nm thickness, if the metal is placed outside, the hydrocracking products are just as branched as those from PtNPs/MFI. Post-mortem TEM characterisation of the catalyst samples revealed that no significant changes to the Pt metal sites were seen after ~10 days of reaction under varying hydrocracking conditions, ensuring that metal location was the only parameter varied in the study. In conclusion, regardless of metal location, primary hydrocracking can be achieved in the presence of H2O. Furthermore, placing metal exclusively inside the MFI micropores results in a significant increase in linear primary cracking products in comparison to placing metal completely outside the micropores, which results in mostly branched primary cracking products.
- ItemOpen AccessSynthesis of monodisperse silver nanoparticles for antibacterial purposes(2018) Adam, Sarah; Kooyman, Patricia; Smart, Mariette; Harrison, SueSafe drinking water is a scarcity for many in the developing world. Currently, 884 million people, 48% of whom live in sub-Saharan Africa, are without access to even basic drinking-water services (WHO/UNICEF Joint Monitoring Programme for Water Supply and Sanitation, 2017). This has a severe impact on the health of those living in such communities, which is why the universal access to safe and affordable drinking water has been made a priority by the United Nations. There is an undeniable need for change so that the lives of these many millions of people may be improved. Silver nanoparticles have great potential in being used in water disinfection applications because of their high antibacterial activity and broad antimicrobial spectrum (Qu, Alvarez, & Li, 2013). Development in this area is critical, particularly in advancing technology to allow greater accessibility to clean drinking water for people in poor, rural areas in developing countries. Incorporating nanotechnology into current water disinfection systems, as well as developing new water treatment nanotechnology, shows promise in addressing this issue. However, much research needs to be done first before this can become a reality (Q. Li et al., 2008). There is particular concern about the toxicity aspects of silver nanoparticles, both in humans and towards the environment. Whilst the current study does not investigate their toxicity, it is important to highlight the need to fully understand the human and environmental impacts nanoparticles may have in assessing their applicability in microbial control. Literature indicates that, although the role of silver nanoparticles themselves in the antibacterial mechanism cannot be excluded entirely, it is the silver ions that are mostly responsible for their antibacterial activity (Foldbjerg, Jiang, Miclăuş, et al., 2015; Le Ouay & Stellacci, 2015; Panacek et al., 2006; Xiu, Zhang, Puppala, Colvin, & Alvarez, 2012). Sotiriou & Pratsinis (2010) found that silver nanoparticles of smaller than 10 nm had a negligible antibacterial effect in comparison to the ions they released. Thus, to isolate just the effect of the released silver ions, it was desired to prepare uniformly sized particles of smaller than 10 nm. Controlling the size of the formed particles requires consideration of parameters that affect their nucleation and growth (Thanh et al., 2014). These can be thermodynamic, kinetic or stoichiometric parameters. It is on this basis that the work described herein was developed. This study aimed to synthesise silver nanoparticles suitable for use in water disinfection applications by exploring how preparation conditions affect the particle size and distribution. To do this, two different aqueous chemical reduction preparation methods were performed and reaction conditions such as surfactant concentration, agitation rate, synthesis temperature, and method of chemical addition were varied to produce monodisperse silver nanoparticles with an average size of smaller than 10 nm. This study also aimed to investigate the antibacterial efficacy of silver nanoparticles deposited on quartz fibre filters against E. coli. Two silver nanoparticle syntheses procedures were extensively investigated. Method One (AL-Thabaiti et al, 2008) uses ascorbic acid as the reducing agent and SDS (sodium dodecyl sulphate) as the surfactant whilst Method Two (Yang, Yin, Jia, & Wei, 2011) uses aniline as the reducing agent, DBSA (dodecylbenzenesulfonic acid) as the surfactant and NaOH (sodium hydroxide) as the ‘activating’ chemical. The surfactant concentrations, agitation rates, synthesis temperature, reducing agent concentrations and methods of chemical addition were varied for each of these synthesis procedures and the effect thereof on particle size was investigated. Both synthesis methods produced fcc metallic silver nanoparticles with (111) and (200) lattice planes, confirmed by studying nanoparticle d-spacings. For Method One, the unaltered synthesis procedure produced the smallest particles with a numberbased mean size of 3.6 ± 3.8 nm and a volume-based mean particle size of 15.4 ± 6.4 nm. For Method Two, which is performed at 90 °C, the ‘hot’ injection of NaOH into the system resulted in the production of the smallest nanoparticles with a number-based mean particle size of 6.7 ± 5.4 nm and a volumebased mean particle size of 22.3 ± 10.9. Removing excess surfactant and collecting these nanoparticles in powder form would facilitate antibacterial efficacy studies, however this proved to be difficult. Additionally, the presence of large nanoparticles in both samples, as evidenced from the volume-based size distributions, means that in assessing antibacterial activity of the nanoparticles, it will be difficult to interpret whether the bactericidal effect is due to silver ions or because of an interaction between the bacteria and the actual nanoparticles. Antibacterial efficacy studies were therefore not performed on these synthesised silver nanoparticles. Silver nanoparticles deposited on quartz fibre filters via spark ablation were prepared at Delft University of Technology. SEM revealed that the deposited nanoparticles on the filters had a mean particle size ranging from 25 to 70 nm. Studies using E. coli (ATCC® 25922™) did not conclusively demonstrate antibacterial activity of the filters. It is believed the large particle size, and thus slow dissolution into silver ions, may be the reason for the lack of evidence of bactericidal activity over the 24-hour experimental period. The results of this study indicate how small changes in synthesis parameters can have a significant effect on nanoparticle size and uniformity, morphology, and degree of agglomeration. This reveals the importance in specifying exact parameters used in nanoparticle preparation to allow for better reproducibility, including vessel size, mixing speed, and rate of chemical addition. This work also showed that it is important to quantify the release of silver ions from silver nanoparticles before performing antibacterial efficacy assessments. Since silver ions are the most important factor in the antibacterial action of silver nanoparticles, understanding their rate of release will allow for improved experimental design thus producing useful results. There is great potential for the use of silver nanoparticles for disinfection, as evidenced particularly by the antibacterial efficiency of Ag+ against E. coli (ATCC® 25922™). However, improvements in both the synthesis of silver nanoparticles and methods of assessing their bactericidal efficacy are clearly necessary. This study has highlighted the challenges that may be faced in the pursuit of efficiently and safely using silver nanoparticles for water treatment and disinfection. Numerous recommendations for future studies have been put forward. These include: further optimisation of the nanoparticle synthesis procedure so as to produce particles of the desired size and acquire them in powder form, performing a thermodynamic estimation of the equilibrium silver ion concentration as a function of silver nanoparticle size to quantify the effect nanoparticle size will have on bactericidal activity, and using more realistic water conditions for antibacterial efficacy experiments to simulate the environment in which silver nanoparticles will be applied.