Effect of fly ash composition on the synthesis of carbon nanomaterials
Date
2016-05-10
Authors
Matshitse, Refilwe Manyama Stephina
Journal Title
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Abstract
Fly ash is a by-product generated during the combustion of coal for electricity gen-
eration. Previous studies have shown that various waste fly-ashes (Japanese, Saudi
Arabian, and Australian) contain trace quantities of transition metal elements which
can be used in the synthesis of shaped carbon nanomaterials. A survey of the litera-
ture has shown that no attempts to correlate the composition of a particular coal fly
ash and the type or quantity of carbon nanomaterials (CNMs) that can be synthesized
has been made. Neither has the effect of leached fly ash been tested for the synthesis
of CNMs. Hence a study on the effect of the chemical composition of South African
fly ash (collected from ESKOM’s Duvha power station in Mpumalanga) upon the
chemical vapour deposition (CVD) synthesis of carbon nanostructures is justified.
Untreated and chemically treated fly ash samples were used as catalysts in the CVD
method to synthesize CNMs. In the latter case selective leaching experiments were
conducted on the fly ash samples under acidic, basic and neutral conditions. Op-
timal CNM synthetic conditions were achieved by initially flowing H2 gas to re-
duce the metal oxides within the fly ash catalyst followed by the introduction of
the carbon source (C2H2) at a temperature range of 600 - 800 ◦C. All samples were
quantitatively and/or qualitatively characterized. Inductively coupled plasma optical
emission spectrometry (ICP-OES) and X-ray fluorescence (XRF) techniques were
used to quantify the metal ions which were removed from the fly ash samples. Fur-
thermore, qualitative studies were conducted with (PXRD, and laser Raman spec-
troscopy), morphological and surface area characterization techniques (SEM, TEM
and BET) were used to investigate the synthesis of CNMs from the untreated and
chemically treated fly ash samples.
Results have shown that carbon nanofibers (CNFs) of different geometric morpholo-
gies were synthesized at an optimal yield temperature of 700◦C. A combination of
smooth, thin, wide, spiral platelet-like, stacked cup, and fishbone morphologies were
reported when the untreated fly ash catalyst was used. Fly ash catalysts under acidic,
basic and neutral treatments showed CNFs of varying sizes and specific morpholo-
gies. Smooth graphitic platelet-like, stacked cup and platelet-like CNFs were re-
ported when the fly ash catalyst was leached with neutral, basic and acidic solutions.
Carbon nanofibre sizes with the IG
ID
ratios were reported as follows 115 nm (1.092),
52 nm (0.799), and 200 nm (0.960) under neutral, basic and acidic mediums respec-
tively. Surface areas (41, 14 and 7) m2/g for the CNFs that were synthesised from
the neutral, basic and acidic treated fly ash catalysts were related to the selective
leaching of metals.
The quality and quantity of CNFs obtained under acidic medium were associated
with the leaching of iron (5.6%), cobalt (1.7%), calcium (20.4%), copper (12.5%),
chromium (4.6%), magnesium (23.3%), manganese (15.2%) and nickel (2%) from
the fly ash catalyst. Under a basic medium only chromium (0.2%), calcium (0.3%)
and copper (7.4%) were removed. Significantly the best quality of CNFs was ob-
tained when fly ash was treated under neutral conditions. Metal ions such as: cal-
cium (3.7%), copper (3.8%), chromium (0.1%), and magnesium (1.3%) were mod-
erately removed from the ash matrix. Therefore, composition and quantity of the fly
ash catalyst had an effect on the synthesis of CNFs.
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the Degree of Master of
Science.
Johannesburg, 2015.