ACKNOWLEDGEMENT

I would like to express my deepest gratitude to the Almighty God for blessing me with this topic. Firstly,
I would love to express my sincere gratitude to my supervisor Prof. Dr. Nor Aishah Saidina Amin for her
continuous support throughout my research, for her patience, motivation, enthusiasm, wealth of
knowledge and experience. Her guidance helped me a lot during my research and the writing of this
dissertation. It is a great honor working as your student during my research. I would also like to
appreciate my co-supervisor Dr. Muhammad Tahir for introducing me to this topic. For always being
there any time I needed an advice or assistance, for the drive towards research and publication that you
instilled in me. Aside that, I express my sincere appreciation to the Chemical Reaction Engineering Group
(CREG) members and other UTM friends for their support and valuable inputs regarding this research.
Words cannot express how grateful I am to my mother, father and siblings for all of the sacrifices that
they made on my behalf. Your prayers for me was what sustained me thus far. I would also like to thank
all my family members, especially my dear Aunty, Toyin Adegborioye and her husband for supporting
me throughout my academic journey.

ABSTRACT

Photoreduction of CO2 to useful chemicals have shown promising results from the research on CO2
conversion and utilization. The objective of this study is to synthesize copper and carbon nitride based
titanium dioxide nanocomposites for selective photoreduction of carbon dioxide to methanol under
visible light irradiations.

The nanocomposites were synthesized by a chemical precipitation method and characterized using XRD,
FT-IR, FESEM, TEM, DRS, BET and XPS. The XRD results confirmed the presence of TiO2, g-C3N4 and Cu in
the nanocomposite by their characteristic peaks. The doping of Cu metal reduced the intensity of the PL
emission and the rate of recombination. The most effective catalysts was g−C3N4/(3% Cu/TiO2) which
gave a maximum methanol yield of 948.14µmol/g.cat after 2 h. Cu doped TiO2 enhanced its
photoactivity by fostering carrier separation. The position of Cu in the composite affected the
distribution of electrons and hence the photo-activity.

Parameters investigated were weight percent ratio, effect of time and stability. The position of Cu in the
composite affected the distribution of electrons and hence the photo-activity. After 8 h of
photoreaction, a maximum CH3OH yield of 2574 µmol/g. cat was obtained using visible light. The ratio
of g-C3N4 to Cu/TiO2 dictated the efficiency of the composite and the visible light was seen to
demonstrate higher efficiency compared to the ultraviolet light. The higher emitting power UV light
provided more photons for photoexcitation of more electrons, but photo-oxidation of CH3OH to HCOOH
affected the product yield while using UV light. The low band gap, electronic structure and light
absorption capacity of g-C3N4 assisted in the transfer of photogenerated electrons to Cu/TiO2 in the
composite thereby aiding maximal usage of the irradiated light. Cu/TiO2 demonstrated a high selectivity
for photoreduction of CO2 to CH3OH in the nanocomposite. The photostability of the composite was
maintained even after three cycles. Possible reaction mechanisms were proposed to understand the
type of catalysts and light irradiations on yield and selectivity.

1.0 INTRODUCTION

1.1 Problem Background

Global warming is considered to be one of the major environmental concerns of mankind today (Tahir
and Amin, 2013).

One of the major hazards from industrialization and technological advancement is the unguarded
release of carbon dioxide (CO2). Combustion of fossil fuel is the main source of greenhouse gas
emission, which ultimately leads to global warming. It is gradually destroying the earth’s climate and
making survival tougher than ever (Olah et al., 2006).

CO2 can be perceived to be a safe gas to some extent since it is exhaled by man and animals and
absorbed by plants but if its percentage in the atmosphere is not checked it could become a potential
threat to the ecosystem and its occupants. This has generated massive attentions as it is a problem that
has ripple effects such as global warming which is the major challenge in the world at the moment. The
environment is under a lot of stress and a sustainable immediate solution is essential (Ali et al., 2015).

Several options exist for global warming resolution and they can be categorized into two alternatives:
eliminating the sources of greenhouse gases and capture of the gases. The first option cannot totally be
accomplished because the comfort of man, industrial development and advancement are tied to most of
these sources (Jiang et al., 2010).

This leaves us with the option of reducing the concentration of CO2 in the atmosphere by capturing the
released CO2 and providing other alternatives which are 2 not CO2 producing. One way of doing this is
to capture the CO2 and store it in oceans, depleted coal seams etc. This option is CO2 capture and
storage also known as Sequestration, but it is expensive therefore unsustainable. The alternative and
preferred option is to convert the captured CO2 into valuable bulk chemicals such as methanol etc.
Technologies for capturing CO2 from flue gas includes absorption & adsorption of gases, the use of
permeable membranes, cryogenic distillation etc. Many of these methods are not economically feasible
(Cheah et al., 2016).

Although it is obvious that CO2 is a major cause of global warming and other environmental mishaps,
another issue of concern in the world today is energy and its conservation. As of today, the largest
percentage of the world energy demand is met through the deployment of fossil fuels and if more
alternatives are not focused on this might not change in decades to come. The worlds reserve of natural
gas is approximately 1014 m3 which is a large portion of the worlds energy in total. In comparison to
crude oil reserve, natural gas storage will remain longer and is hence a better option. Nations who are
major consumers of petroleum and petroleum products constantly face problems due to the use of
fossil fuels, geological spread and political supremacy of key petroleum raw materials (Aruchamy et al.,
1982; Moritis, 2004).

One other viable substitute for fossils is nuclear energy except it is non-renewable and is destructive. To
this end, an alternative source of energy which provides a simpler and cleaner fuel is a better option.
CO2 conversion and utilization provides us this alternative – turning CO2 into a raw material for useful
chemicals.

1.2 Photocatalysis as a viable route for CO2 Conversion and Utilization

The conversion and utilization of captured CO2 is a better option compared to sequestration, as it is a
win-win approach. CO2 conversion techniques include: - electrochemical (Li et al., 2016), photochemical
(Grebenshchikov, 2016), thermochemical (Dufour, 2016), radio-chemical (Yadav and Purkait, 2016),
biochemical (Cheah, et al., 2016), photoreduction and photo-electrochemical reduction processes
(Apaydin et al., 2016; Prasad et al., 2016).

For certain reasons such as cost of 3 electricity (electrochemical), low efficiency (photochemical), one of
the best methods for CO2 conversion is the photocatalytic method in which solar energy is transformed
and stored as chemical energy. Photocatalytic reduction of CO2 is a clean, low cost and environmentally
safe process (Cybula et al., 2012).

The photocatalytic process involves direct absorption of photons by the photocatalyst. These photons
must have band gap energies equal or greater than that of the photocatalyst in order to generate
electron hole pairs. This is the initial step followed by reactions which will take place as a result of the
excitation and energy transfer of the electrons to the reactants adsorbed on the photocatalyst. Although
photoreduction of CO2 is a multi-step reaction which is thermodynamically uphill it remains a very
feasible and promising process (Indrakanti et al., 2009).

Photocatalysts provide the most viable method for harvesting solar energy with their reversible
oxidation-reduction capabilities. They reduce CO2 to form hydrocarbons such as methane and ethanol
and essentially take exhaust and turn it back to fuel (Graham et al., 2012).

Considering the numerous benefits that can be derived from photocatalytic conversion of CO2 to useful
chemicals, this study focuses on the photocatalytic reduction of CO2 to CH3OH. The research on
photoreduction of CO2 to methanol is a progressive one, a number of researches have been conducted
using various photocatalysts. In terms of photocatalysts, TiO2 remains the most researched of all
photocatalysts owing to its exceptional properties though it is limited by its large band gap (3.2 eV)
(Tahir and Amin, 2013).

This limitation-necessitated modification of TiO2 and one of the common modification methods is
doping with metals (Cu (Slamet et al., 2009), Ag (Liu et al., 2014), and Au (Neaţu et al., 2014) etc.).

One notable research on CO2 photoreduction to CH3OH is that of (Slamet, et al., 2009) involving the use
of Cu doped TiO2. A very good yield of methanol was obtained using 3% Cu/TiO2 to photoreduce CO2.
Other alternative methods for modification include - non-metal (N) doping (Tahir and Tahir, 2016), co-
catalyst (Prasad, et al., 2016), formation of heterostructures (Li et al., 2015), use of nanocomposites
(Gusain et al., 2016) etc.

The results obtained from the photocatalysts modified using these methods are better than that of pure
TiO2. Recently, the use of g-C3N4 as a photocatalyst have increased due to 4 its unique properties. It is
thermally and chemically stable, can be prepared easily from nitrogen containing precursors, it is non-
toxic, possess a low band gap of approximately 2.7 eV and it is active in the visible region (Yin et al.,
2015).

Nanocomposites of g-C3N4 and TiO2 have been used to photoreduce CO2 to other hydrocarbons such
as CH4, CO (Zhou et al., 2014) and H2 (Chai et al., 2012).

Therefore considering the work of (Slamet, et al., 2009) and previous works done on g-C3N4 and TiO2
nanocomposites, the focus of this research is to investigate the prospects of gC3N4/(Cu/TiO2) for
photoreduction of CO2 to CH3OH. The doping with copped helps in creation of more active sites for
adsorption of CO2, (2) enables TiO2 to absorb and utilize visible light and (3) creates a Schottky barrier,
which promotes separation of electron and hole pairs hence inhibiting recombination (Slamet, et al.,
2009).

In conclusion, the yield of the product is of major concern in photocatalytic reduction of CO2 and the
yield depends on: - the type of photocatalyst, nature of the light used, reductant and type of reactor
used. The nanocomposite synthesized (gC3N4/(Cu/TiO2) is expected to fulfill the material requirements
to obtain a yield that is better than that of pure TiO2. This is because the nanocomposite utilizes the
unique properties of each of its constituents (g-C3N4, TiO2 and Cu) to provide the necessary band
structure required for effective charge separation, light absorption and utilization. It is expected that the
use of NaOH as the reductant, two different light sources (UV and Visible) and the slurry type
photoreactors would improve the yield of CH3OH produced.

1.3 Problem Statement and Research Hypothesis

Though photoreduction of CO2 to hydrocarbons is getting increased attention in research there are still
certain limitations faced and the main challenges are low yield and selectivity of the products. To this
end, the problems and possible solution approach are:

1. There is a need for a photocatalyst that is photo-stable, possess high light absorption and utilization
efficiency, has high charge separation, inhibits recombination, absorbs in both the UV and visible region
and has a large surface area to adsorb enough CO2. The constituents of the g−C3N4/(Cu/TiO2)
nanocomposite possess these characteristics hence it is expected these problems will be solved by
synthesizing it.

2. The solubility of CO2 in the reductant used dictates the amount of CO2 available for the photocatalyst
and the photoreduction process. A reductant that is environmentally benign, affordable and dissolves
CO2 very well is one of the focus of CO2 photoreduction. The use of NaOH as a reductant would improve
the solubility of CO2 into the system and give the desired result during photo splitting as opposed to
using pure water.

3. The selectivity of the product from photoreduction of CO2 depends on the choice of dopant or co-
catalyst used. For example, Pt. is known to possess a high affinity for H2 and CH4 during photoreduction
of CO2. Therefore, the type of cocatalyst to use is paramount. The use of Cu in the photoreduction of
CO2 is expected to give high selectivity for CH3OH production.

4. The knowledge of how type of light affects the mechanism of CO2 photoreduction is a topic of debate
and research is focused on understanding more about this. The use of both UV and visible light in this
research is expected to shed more light to this issue and give better understanding on the effect of light
intensity.

1.4 Research Objective

The objectives of this research include: -

1. To synthesize and characterize copper and graphitic carbon nitride based TiO2 nanocatalysts for CO2
conversion to methanol;

2. To study and compare the performance of nanocatalysts for selective photocatalytic CO2 conversion
to methanol under UV and visible light irradiations;

3. To study the effect of operating parameters and propose reaction mechanisms for the catalyst having
maximum yield and selectivity.

1.5 Research Scope

The research focus is summarized in detail. The photocatalysts to be used for the photoreduction
process were synthesized i.e. (g−C3N4, g−C3N4/TiO2, Cu/TiO2, Cu/g−C3N4, (Cu/g−C3N4)/TiO2,
g−C3N4/(Cu/TiO2) using the appropriate methods. The characterization of the catalysts was carried out
using the following technologies XRD, FTIR, FESEM, BET, XPS, TEM, UV-VIS and PL. These analysis were
done to determine the crystalline nature, the organic and inorganic bands of functional groups,
morphology, surface area, porosity and pore dimension, oxidative state, atomic structure and the
formation of heterostructure, absorption region of each catalyst in the spectrum and identify the
catalyst sample with the lowest PL emission intensity and recombination rate respectively. The catalysts
were then used to photoreduce CO2 to obtain CH3OH using both UV &Visible light and their
performances were compared based on the yield of CH3OH. The catalyst with the optimum yield of
methanol was used to study the operating parameters (time, % weight ratio and photostability test).
After proper analysis and study of the results obtained, reaction mechanisms for both UV and Visible
light were proposed.

1.6 Significance of Study

This study has immense contribution to researchers in photocatalysis, the scientific community and the
public for the following reasons. Firstly, the research on the g-C3N4/(Cu/TiO2) nanocomposite provides
more insight and direction on the mechanism of composites during CO2 photoreduction. In addition, the
effect of type of light on the efficiency of photocatalysis can be better understood from this research. A
photocatalyst that is photo-stable, has high charge separation and is environmentally benign has been
introduced. The process of CO2 utilization and conversion has been accomplished through this study.

1.7 Outline of Thesis

This thesis is divided into five chapters excluding all introductory pages, table of content and abstract.
The first chapter (Chapter 1) contains the introduction, problem statement and research hypothesis,
objectives, research scope, significance of study and outline of thesis. The literature survey, basics of
photocatalysis and CO2 photoreduction, previous works in photoreduction of CO2, the photoreactor
setups, and characterization techniques were discussed in Chapter 2. Chapter 3 gives a detailed
representation of the research methodology and order of the research, details of the methods used to
synthesize the catalysts and carry out the photoreduction process. The results obtained from the
experiments and analysis of characterization are discussed in Chapter 4. Chapter 5 concludes the thesis
with inferences drawn and recommendations for further research.

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