Browsing by Author "Piet, Marvin"

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    Synthesis and characterization of cathode catalysts for use in direct methanol fuels cells
    (University of the Western Cape, 2010) Piet, Marvin; Green, I.R.; Khotseng, Lindiwe; Dept. of Chemistry; Faculty of Science
    In this work a modified polyol method was developed to synthesize in-house catalysts. The method was modified for maximum delivery of product and proved to be quick and efficient as well as cost effective. The series of IH catalysts were characterized using techniques such as UV-vis and FT-IR spectroscopy, TEM, XRD, ICP and CV. The polyol method developed effectively reduced and deposited Pt nanoparticles onto different carbon supports. Functionalization of some of the supports was also successfully carried out through acid oxidative treatment which introduced carboxylic acid and hydroxyl groups onto the surface of the supporting material, which was supported by FT-IR which demonstrated that there was a relative increase in absorbance of functionalities viz., carboxylic acid (1270 cm-1) and hydroxyl groups on the surface of the acid treated MWCNT’s-F compared to the untreated MWCNT’s. Addition of a sedimentation promoter proved to increase the amount of metal deposition on the support thereby improving the loading dramatically. It was also found that addition of a specific amount of water in the polyol method allowed one to control the particle growth during the deposition phase on the various carbon supports investigated namely; XC-72 carbon, MWCNT’s and MWCNT’s-F. The in-house catalysts synthesized namely; Pt/C-IH, Pt/MWCNT’s-IH and Pt/MWCNT’s-F-IH all displayed narrow particle size distributions with average mean particle sizes of 2-5 nm, 2-6 nm and 3-6 nm respectively which was in good agreement with particle size measurements obtained from XRD using the Scherrer formula. All measured CV’s obtained for the series of IH catalysts prepared by this protocol were comparable with the commercial catalyst. The IH catalysts displayed the characteristic XRD peaks associated with Pt on carbon supports in acidic media. The ORR measurements for Pt/MWCNT’s-F-IH (functionalised) proved to be slightly superior (0.058 A/cm2) compared to that of the commercial catalyst (0.047 A/cm2) at a potential of 0.3V. This was attributed to the fact that the addition of water effectively controlled particle growth and deposition onto the support, and the fact that MWCNT’s-F offered a larger surface area and with the functionalized surface offering anchorage sites for Pt nanoparticles through carboxylic and hydroxyl functional groups. Further attempts to modify the developed polyol method using NaOAc as an electrostatic stabilizer to control the growth of Pt nanoparticles were made. It was found that although the stabilizer employed effectively stabilized Pt nanoparticles supported on XC-72 carbon, in our hands we were not able to produce the same results in the case of Pt supported on the MWCNT’s. TEM images revealed that Pt/CNaOAc displayed narrow particle size distribution with a mean particle size of 2-5 nm whereas particle agglomeration was observed for Pt/MWCNT’s-NaOAc and Pt/MWCNT’s-F-NaOAc with a broader particle size distribution and average mean particle sizes of 2-7 and 2-8 nm in diameter respectively. The CV’s obtained for these modified IH catalysts were in the main comparable to that of the commercial catalyst. However ORR activities of the IH Pt/C-NaOAc catalysts when carefully compared to the commercial catalyst revealed that it was indeed slightly superior (0.064 A/cm2). This is mainly attributed to the narrow particle size and even distribution of particles on the carbon support and resembled the best particle required to provide maximum activity in the ORR as a consequence of the facial kinetics involved.
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    Synthesis and characterization of new adsorbents for CO2 capturing
    (University of the Western Cape, 2014) Piet, Marvin; Titinchi, Salam; Abbo, Hanna
    Carbon dioxide emissions have become a major concern as they are one of the contributing factors to the “green-house” effect. Recently, much effort has been put into separating carbon dioxide (CO2) from flue gases linked to the combustion processes at fixed point sources. The development of solid sorbents for adsorption based on CO2 capture has attracted much attention. Ordered Mesoporous Silica (OMS) materials have recently attracted much attention as solid adsorbents for capturing CO2. OMS have been investigated for this purpose owing to their high pore volume, large surface area and ease of functionalizationIn this work we report on the synthesis of OMS viz. MCM-41 and SBA-15 along with amorphous silica as adsorbents for CO2 capture. MCM-41 was prepared with surfactants having different alkyl chain lengths (C14TABr, C16TABr and C18TABr) where TABr is trimethylammonium bromide. SBA-15 was prepared using a Triblock copolymer as a structure directing agent for the organization of polymerizing silica species. Commercial amorphous silica gel was used for comparative purposes. Initial characterization OMS with powder X-Ray diffraction (XRD) and small angle diffraction (SAXS) yielded diffraction patterns which may be associated with well-ordered structures of hexagonal mesoporous material. Ease of preparation for MCM-41 materials allowed for convenient scale- up, obtaining highly ordered mesoporous silica MCM-41 at room temperature. SBA-15 was also found to be scaled up with considerable ease through increasing the volume of the autoclave during hydrothermal treatment. Structural, morphological and textural properties of the adsorbents were characterized by N2 physisorption measurements, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Thermogravimetric Analysis (TGA). TEM confirmed the hexagonal structure; SEM showed that C14MCM-41 had spheroidal particle morphology whereas SBA-15 displayed rod-like structures. High surface areas of up to 1302, 1186, 1211 and 1024 m2/g for C14MCM-41, C16MCM-41, C18MCM-41 and SBA-15, respectively were obtained. The pore size of MCM-41 materials was increased from 2.6 nm for C14MCM-41 to 4.4 nm in diameter for C18MCM-41 using surfactants with different alkyl chain lengths. CO2 adsorption characteristics of OMS were studied using CO2-temperature programmed desorption (TPD). The results showed that C14MCM-41, C16MCM- 41, C18MCM-41 and SBA-15 desorbed 0.19, 0.16, 0.11 and 0.26 mmol/g respectively. The synthesized OMS were then further modified by grafting various amine moieties on their surfaces in order to increase their CO2 adsorptive capabilities. 3-(Aminopropyl)triethoxysilane (APS), N-[3-(trimethoxysilyl)propyl]ethylene- diamine (TPED), 3-chloropropyl)-trimethoxysilane (CPS), ethyl 2-bromopropanoate, tris(2-aminoethyl)amine (TREN) and guanidine. Several characterization techniques such as XRD, SAXS, HRTEM, HRSEM, TGA, Fourier Transform Infra-Red (FT-IR) spectroscopy, CO2 isotherms and CO2-TPD were used to analyze amine grafted solid sorbents for CO2 capture. The results revealed that the structural integrity of the amine modified sorbents was not compromised during the grafting process. The structural properties of the supports, such as surface area and pore size, nature of amine and the number of amine groups, affected the loading and CO2 adsorption capacity of chemically grafted sorbents. APS grafted amorphous silica gel adsorbed 0.67 mmol/g CO2, which proved to be the highest compared to C18MCM-41 and SBA-15 grafted with APS and TPED. C18MCM-41, SBA-15 and amorphous silica gel were also grafted with TREN and adsorbed 0.42, 0.51 and 0.27 mmol/g of CO2, respectively. A systematic study of guanidine grafted on C14MCM-41, C16MCM-41 and C18MCM-41 was investigated, for the first time, to the best of our knowledge. Structural properties like variation in pore size, proved to enhance the adsorption capacity of the adsorbent, coupled with the guanidine molecules grafted on MCM-41 materials. C18MCM-41-guanidine showed the highest CO2 uptake of the guanidine grafted MCM-41 materials, adsorbing 0.30 mmol/g. A novel synthetic route to TPED-TREN grafted C18MCM-41 and SBA-15, using ethyl 2-bromopropanoate as a linker, was investigated as a potential adsorbent for CO2 capture, for the first time. C18MCM-41-TPED-TREN and SBA-15-TPED-TREN CO2 adsorption capacity were found to be 0.14 and 0.14 mmol/g, respectively.