Fabrication of the heterogeneous metal-free carbo-catalysts for the removal of organic pollutants

Carbonaceous materials have shown undisputable promise as catalysts in various applications. The ability to tune key features such as specific surface area and functionality to improve their activity represents a key advantage. This requires the development of general, facile and cost-effective synthetic methods to deliver these multifunctional materials for efficiency, reproduceability and affordability purpose. This work describes the fabrication of a series of graphene-chromophore (dye) systems, fabrication of graphene-chitosan (two-component), graphene-chromophore-chitosan (three-component), as well as the application of graphene-chromophore hybrids using physisorption and photocatalytic activity studies. Two photochemical methods involving 2-step procedure; diazonium salt formation, isolation and storage, followed with functionalization of graphene using with diazonium salt. Other method is in-situ production of diazonium salt and subsequent functionalization of graphene in one-pot reaction mixture was employed in the fabrication of the graphene-chromophore. It was found that the two-step procedure is limited by the stability of the diazonium salts and thus has a limited application. The one-pot method allows preparation of a wide range of different composites in an easy and inexpensive manner. To compare the the grafting rate and performance efficiency of the catalysts, functionalisation of graphene-chromophore-chitosan was conducted in one-pot and 2-step covalent radical procedure, while fabrication of graphene-chitosan was conducted through one-pot covalent and non-covalent pathways. Synthesised samples were characterised by a range of methods including electron microscopy and optical analysis such as, X-ray diffraction, gas adsorption, infra-red, UV-Vis and fluorescence spectroscopy. Optical analysis of the UV-vis absorption spectra shows substantial alteration through both bathochromic (red) and hypsochromic (blue) shifts due to introduced functionalities proving successful surface modification. Changes in a peak pattern have been observed in the FTIR spectra of the modified graphene. Further analyses using PXRD, BET and TGA shows evidence of functionalised graphene surface. Lorentz peak fitting and BET surface analysis show about 12-38% increase in the FWHM values and up to 65% decrease in the surface area of the graphene-chromophores compared to unmodified graphene. TEM analysis reveals an ordered array of chromophores on the surface of the graphene nanoplatelets. Adsorption experiments of the modified graphene using two most common basic dyes (cationic) and one most common weak acidic (anionic) dye as model dyes: methyl blue (MB), rhodamine B (RhB) and methyl orange (MO) were used and showed different range of adsorption capacities: GP-Chr; (38.3 mg/g, 58.19 mg/g and 21.40 mg/g), GP-Acr; (47.9 mg/g, 63.0 mg/g and 33.50 mg/g), GP-Acn; (30.0 mg/g, 61.77mg/g and 16.19 mg/g), GP-Pnz; (32.6 mg/g, 33.15 mg/g and 9.97 mg/g), GP-Fsc; (49.6 mg/g, 63.5 mg/g and 24.75 mg/g) and GP-Thio; (43.5 mg/g, 45.05 mg/g and 14.79 mg/g). Kinetics studies of materials using the same dye models (MB, RhB and MO) were conducted under dark and light illuminations. Performance efficiency of all the catalysts was higher using RhB and MB than MO, this is because the structure of MO is not flat and therefore not favourable for efficient electrostatic interactions such as π-π stacking. However, some catalysts manifested higher removal rate with illumination than in the dark due to photocatalytic activity.