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INTEGRATED MASTERS IN ENVIRONMENTAL ENGINEERING
2014/2015
CO2 CONVERSION TO RENEWABLE FUELS
Nuno Nascimento Ciravegna da Fonseca
Dissertation submitted for the degree of:
MASTER IN ENVIRONMENTAL ENGINEERING
President of the Jury: ___________________________________________________________
Supervisor: Adrián M.T. Silva (Principal Investigator, Department of
Chemistry, Faculty of Engineering, University of Porto)
Co-Supervisor: Luisa M. Pastrana Martínez (Post-Doc Investigator,
Department of Chemistry, Faculty of Engineering, University of Porto)
Co-Supervisor: Joaquim L. Faria (Associate Professor, Department of
Chemistry, Faculty of Engineering, University of Porto)
Department of Chemical Engineering
Faculty of Engineering - University of Porto
February 2015
CO2 conversion to renewable fuels I
Agradecimentos
Esta página é dedicada às pessoas merecedoras de toda a minha admiração e
reconhecimento que, pelas mais variadas formas, deram o seu contributo na minha vida
e no meu percurso académico.
Quero agradecer, em primeiro lugar, aos meus orientadores Doutor Adrián Silva,
Doutora Luisa Martínez e Professor Doutor Joaquim Faria, pela oportunidade que me
deram de realizar este trabalho inovador. Quero deixar um agradecimento muito
especial à Doutora Luisa Martínez, pela sua simpatia, disponibilidade e o apoio total
prestado durante o projeto pois foi o pilar fundamental para o desenvolvimento desta
dissertação. Ao Dr. Carlos Sá do CEMUP (Centro Materiais da Universidade do Porto)
pela assistência técnica prestada com as análises de SEM e XPS.
Agradeço aos investigadores do Laboratório de Catálise de Materiais (LCM): Dra.
Salomé Soares, Dr. Sérgio Torres, Dra. Carla Fonseca, Dra. Alexandra Gonçalves, Dra.
Rita Ribeiro, Dra. Cláudia Silva e Dra. Marina González pela simpatia transmitida e
disponibilidade. Também quero dar o meu agradecimento aos colegas bolseiros
Ricardo, Raquel, Maria José S., Maria José L., Lucília, Teresa, Nuno, Diogo, Tânia,
Marta, Patrícia e Carla pela simpatia e acompanhamento.
Fora do campo académico, quero agradecer aos meus Pais pelo apoio
incondicional e incentivo prestado ao longo da minha vida académica, à minha irmã
Filipa, ao meu irmão Miguel, à minha madrinha Gabi e ao meu padrinho Pedro. Para
finalizar a esfera familiar, quero agradecer profundamente aos meus avós maternos por
todo o amor, dedicação e valores que me conseguiram transmitir, guardo-vos para
sempre no meu coração.
Quero agradecer à minha querida namorada, Cristiana, por todo o carinho,
paciência e apoio que me deu durante esta longa viagem académica. Aos meus amigos
de infância, Paulo, Daniel, Renato e Mariana por todos os inúmeros momentos de
convívio. Ao meu amigo Alain, pelos fantásticos momentos vividos e pela sua ímpar
visão que me transmitiu. Aos meus amigos do polo-aquático, onde incorporei um
verdadeiro espírito de equipa. A todos meus colegas de curso com quem tive a
oportunidade de conviver durante o percurso académico, em particular ao meu amigo
Samuel.
CO2 conversion to renewable fuels II
CO2 conversion to renewable fuels III
Abstract
One solar energy based technology to recycle carbon dioxide (CO2) into
transportable hydrocarbon fuels (i.e., a solar fuel) would help to reduce atmospheric
CO2 levels and partially fulfill energy demands within the present hydrocarbon based
fuel infrastructure.
Graphene oxide (GO) has stimulated the interest in the design of high-
performance photocatalysts with the aim to enhance the photoefficiency of the
semiconductors under visible light conditions. On the other hand, the presence of co-
catalysts such as copper plays a crucial role in semiconductor-based photocatalyst,
increasing CO2 reduction.
This work describes the photocatalytic reduction of CO2, in aqueous phase, into
hydrocarbons with high added value (e.g., methanol and ethanol). Composite
photocatalysts prepared from TiO2 and GO by the liquid phase deposition method were
used. Copper-loaded catalysts from different copper precursors were synthesized. The
photocatalysts were characterized by scanning electron microscopy (SEM), diffuse
reflectance ultraviolet-visible spectroscopy (DRUV) and X-ray photoelectron
spectroscopy (XPS). The experiments were carried out in alkaline solution (0.2 M
NaOH) and under ultraviolet-visible (UV/Vis) irradiation (> 350 nm).
The results revealed methanol and ethanol as the main reduction products. The
combination of GO with TiO2 exhibited higher photocatalytic activity for methanol
production than TiO2 alone. The high efficiency of this composite was attributed to the
optimal assembly between the TiO2 nanoparticles and GO sheets, since GO material can
act simultaneously as electron acceptor and donor, thus suppressing charge
recombination.
TiO2 loaded with Cu, using Cu(NO3)2 as copper precursor (TiO2_Cu(NO3)2),
showed the best performance for both methanol and ethanol production (23.9 and 124
µmol g-1
h-1
, respectively). The copper(I) species detected in the photocatalysts also
promoted a decrease of the charge recombination phenomenon. The mechanism
assumes the formation of formic acid (HCOOH) and oxalic acid (HOOC-COOH)
adsorbed on the surface as the primary products.
Keywords: CO2 photoreduction, solar fuels, TiO2, graphene oxide, copper.
CO2 conversion to renewable fuels IV
CO2 conversion to renewable fuels V
Resumo
As tecnologias baseadas na energia solar, capazes de reciclar o dióxido de carbono
(CO2) e transformá-lo num combustível facilmente transportável (i.e., um combustível
solar), ajudariam a reduzir os níveis de CO2 atmosféricos e a compensar parcialmente a
procura de energia no atual sector de combustíveis à base de hidrocarbonetos.
O óxido de grafeno (GO-graphene oxide) tem estimulado o interesse na área do
desenho de fotocatalisadores de elevado rendimento, de modo a evidenciar a
foto-eficiência de semicondutores sob condições de luz visível. Por outro lado, a
presença de co-catalisadores, tal como o cobre, desempenham um papel fundamental
nos fotocatalisadores à base de semicondutores, aumentando assim a redução do CO2.
Este trabalho descreve a redução fotocatalítica do CO2 aquoso em hidrocarbonetos
de elevado valor acrescentado (ex. metanol e etanol), utilizando H2O como agente
redutor. Os fotocatalisadores foram preparados utilizando TiO2 e GO pelo método de
deposição em fase líquida. Os catalisadores modificados com cobre foram sintetizados
utilizando diferentes precursores de cobre. Os fotocatalisadores foram caracterizados
por microscopia eletrónica de varrimento (SEM, scanning electron microscopy),
espectroscopia de refletância difusa no ultravioleta-visível (DRUV, diffuse reflectance
ultraviolet-visible spectroscopy) e espectroscopia de fotoeletrões de raios-X (XPS, X-
ray photoelectron spectroscopy). As experiências foram realizadas em solução alcalina
(0.2 M NaOH) e sob irradiação ultravioleta-visível (> 350 nm).
Os resultados revelaram que o metanol e o etanol foram os principais produtos de
redução do CO2. A combinação do GO com TiO2 exibiu maior atividade fotocatalítica
na produção de metanol que o TiO2. A elevada eficiência deste material compósito foi
atribuída ao ajuste ideal entre as nanopartículas de TiO2 e as folhas de GO, permitindo
ao material atuar simultaneamente como aceitador e dador de eletrões, evitando assim a
recombinação de carga.
O TiO2 modificado com cobre (TiO2_Cu(NO3)2), sintetizado a partir do percursor
Cu(NO3)2, demonstrou ter o melhor desempenho na produção de metanol e etanol (23.9
e 124 µmol g-1
h-1
, respectivamente). O cobre(I) detetado nos fotocatalisadores, também
promoveu uma diminuição do fenómeno de recombinação de carga. O mecanismo
assume a formação de ácido fórmico (HCOOH) e ácido oxálico (HOOC-COOH),
adsorvidos na superfície como produtos primários.
CO2 conversion to renewable fuels VI
Palavras-chave: Fotorredução do CO2, combustíveis solares, TiO2, óxido de
grafeno, cobre.
CO2 conversion to renewable fuels VII
Nomenclature
CB Conduction band
CCICED China Council for International Cooperation on
Environment and Development
DRUV Diffuse reflectance UV/Vis spectroscopy
e- Photogenerated electron
EFF Emissions from fossil fuels and cement production
EG Energy gap
IEA International Energy Agency
IFF Carbon intensity of the world economy
IPCC Intergovernmental Panel on Climate Change
FID Flame ionization detector
GC Gas chromatography
GDP Gross domestic product
GHG Greenhouse gas
GO Graphene oxide
GOSAT Greenhouse Gases Observing Satellite
h+ Photogenerated hole
LED Light-emitting diode
MLO Mauna Loa Observatory
NASA National Aeronautics and Space Administration
OCO Orbiting Carbon Observatory
SEM Scanning electron microscopy
TCD Thermal conductor detector
VB Valence band
NHE Normal hydrogen electrode
CO2 conversion to renewable fuels VIII
XPS X-ray photoelectron spectroscopy
CO2 conversion to renewable fuels IX
Table of Contents
AGRADECIMENTOS .................................................................................................... I
ABSTRACT .................................................................................................................. III
RESUMO ......................................................................................................................... V
NOMENCLATURE ................................................................................................... VII
1. INTRODUCTION ................................................................................................... 1
1.1. Overview ............................................................................................................... 1
1.2. Presentation of the Research Unit ...................................................................... 2
1.3. Structure of the thesis ......................................................................................... 3
2. STATE OF THE ART, MOTIVATION AND OBJECTIVES ........................... 5
2.1. Overview of the problematic of CO2 .................................................................. 5
2.2. Photocatalytic conversion of CO2 ....................................................................... 8
2.3. Enhancement of the photocatalytic activity of TiO2 ...................................... 10
2.3.1. Metal Co-catalysts ........................................................................................ 10
2.3.2. Nanostructured carbon materials .................................................................. 11
2.4. Methanol Applications ...................................................................................... 12
2.5. Motivation and objectives of the thesis ............................................................ 17
3. EXPERIMENTAL ................................................................................................ 19
3.1. Chemicals ........................................................................................................... 19
3.2. Synthesis of graphene oxide .............................................................................. 19
3.3. Preparation of GO-TiO2 composites ................................................................ 19
3.4. Preparation of Cu-loaded GOT........................................................................ 20
3.5. Catalysts characterization ................................................................................ 21
3.6. Photocatalytic experiments ............................................................................... 22
4. RESULTS AND DISCUSSION............................................................................ 25
4.1. Characterization of the materials .................................................................... 25
CO2 conversion to renewable fuels X
4.1.1. Thermogravimetric analysis and scanning electron microscopy (SEM) ..... 25
4.1.2. Diffuse reflectance UV/Vis spectroscopy (DRUV) ..................................... 26
4.1.3. X-ray photoelectron spectroscopy (XPS) ..................................................... 27
4.2. Photocatalytic reduction of CO2 ....................................................................... 30
4.2.1. Catalyst screening ........................................................................................ 30
4.2.1.1. Methanol production ................................................................................. 30
4.2.1.2. Ethanol production .................................................................................... 33
4.2.1.3. Acetic acid production. ............................................................................. 35
4.3. Mechanism of reaction ...................................................................................... 36
5. CONCLUSIONS, FUTURE WORK AND FINAL REMARKS ...................... 39
5.1. Conclusions ........................................................................................................ 39
5.2. Future work ....................................................................................................... 40
5.3. Final remarks ..................................................................................................... 40
6. REFERENCES ...................................................................................................... 41
APPENDIX A: CALIBRATION CURVES OF METHANOL, ETHANOL AND
ACETIC ACID ............................................................................................................. 47
List of Tables
Table 1 - Summary of recent studies based on semiconductor materials used in the
photocatalytic conversion of CO2 into methanol. ........................................................... 13
Table 2 - Preparation conditions of photocatalysts. ....................................................... 21
Table 3 - Copper content obtained from XPS analysis. ................................................. 28
Table 4 - Yield of the products formed. ......................................................................... 36
CO2 conversion to renewable fuels XI
List of Figures
Figure 1 - Global CO2 emissions from fossil fuel combustion and cement production
(black dots) and estimating to 2019 (red dots). Reprinted from [1]. ................................ 5
Figure 2 - The CO2 emissions from the top four emitters (China, USA, EU28, India)
and estimating to 2019 (red dots). Reprinted from [1]. .................................................... 7
Figure 3 - Schematic illustration of photoinduced generation of an electron–hole pair in
TiO2 semiconductor that transfers to the surface for CO2 reduction. ............................. 10
Figure 4 - Process of CO2 photoreduction on Cu/TiO2. ................................................ 11
Figure 5 - Schematic diagram of the lab-scale set-up for photoreduction of CO2. ....... 22
Figure 6 - Irradiance spectra of the Heraeus TQ-150 UV-Vis lamp equipped with a
DURAN 50® jacket and transmittance spectrum of the cut-off filter. .......................... 23
Figure 7 - Photocatalytic reactor. .................................................................................. 24
Figure 8 - SEM micrographs for (a) TiO2, (b) GOT, (c) TiO2_Cu(NO3)2, (d)
GOT_Cu(NO3)2, (e) TiO2_Cu2(AC), (f) GOT_Cu2(AC), (g) TiO2_Cu2O, (h)
GOT_Cu2O, (i) EDS spectrum of GOT_Cu(NO3)2. ....................................................... 26
Figure 9 - Diffuse reflectance UV/Vis spectroscopy of some photocatalysts materials.
........................................................................................................................................ 27
Figure 10 - XPS spectra of the Ti 2p and Cu 2p regions for (a and c) TiO2_(CuNO3)2
and (b and d) GOT_(CuNO3)2 composites, respectively. .............................................. 29
Figure 11 - Methanol yield evolution in the experiments performed with different
materials and blank for comparison................................................................................ 31
Figure 12 - Methanol yield at 240 min in the experiments performed with different
materials and blank for comparison................................................................................ 32
Figure 13 - Ethanol yield evolution in the experiments performed with different
materials and blank for comparison................................................................................ 33
Figure 14 - Ethanol yield at 240 min in the experiments performed with different
materials and blank for comparison................................................................................ 34
Figure 15 - Acetic acid yield evolution in the performed experiments. ........................ 35
Figure 16 - Schematic diagram showing mechanism of CO2 photoreduction on Cu-
loaded photocatalyst. Reprinted from [65]. .................................................................... 37
Figure A1 - Calibration curve of methanol. .................................................................. 47
Figure A2 - Calibration curve of ethanol. ...................................................................... 47
Figure A3 - Calibration curve of acetic acid. ................................................................ 48
CO2 conversion to renewable fuels XII
CO2 conversion to renewable fuels 1
1. Introduction
1.1. Overview
As a greenhouse gas, CO2 contributes to global climate change and anthropogenic
emission of CO2 is resulting in an increasing global concern [1]. In this sense, the
development of an efficient process for CO2 conversion is one of the biggest challenges
in environmental research [2]. Considering that solar energy is abundant, clean and a
sustainable energy resource [3], there is much current interest on developing fuels
obtained from sunlight [4].
Photocatalytic reduction of CO2 into fuels, by using water as the reductant and
natural sunlight as the photon source, offers an attractive way to decrease actual CO2
atmospheric concentrations [5]. Previous works already demonstrated that CO2 could be
photocatalytically reduced to hydrocarbons in liquid and gas phase, the efficiency of the
photocatalytic materials for the conversion of CO2, being of critical importance [6, 7].
Solar fuels derived from CO2 can be neutral from the CO2 balance point of view and
they can be also easily transportable as liquid chemicals, such as CH3OH, or even other
chemicals that have been already used in massive quantity (e.g., CH4 and CO) [8].
Titanium dioxide (TiO2) is a classic semiconductor material that has been widely
applied in the field of energy conversion and photocatalysis because of its effectiveness,
low cost, relatively low toxicity and chemical stability [9]. Numerous photocatalytic
applications for TiO2 have been studied, including degradation of pollutants [10], water
photolysis [11], and also CO2 reduction [12]. However, practical application of TiO2
photocatalytic materials is compromised by two inherent limitations, namely the low
quantum yield, which is primarily impaired by the recombination of photo-generated
charge carriers [13], and the poor light-harvesting ability that is restricted by the wide
band gap of TiO2 to the UV spectral range [14].
Graphene and graphene-based materials have stimulated the interest on the design
of high-performance photocatalysts since they can enhance the photoefficiency of the
semiconductors due to the excellent mobility of charge carriers, a large specific surface
area, flexible structure, high transparency and good electrical and thermal conduction
[15].
CO2 conversion to renewable fuels 2
On the other hand, co-catalysts (such as Cu, Pt, Au, among others) have shown to
increase the photocatalytic conversion of CO2, while disfavoring hydrogen generation
that is one of the main problems during CO2 reduction. It is generally accepted that the
co-catalyst may facilitates the separation of the photo-generated electrons and holes by
trapping electrons, enhancing the photocatalytic activity [16].
In this thesis, GO-TiO2 composites were prepared and applied to the
photocatalytic reduction of CO2 into renewable fuels. The effect of copper metal as co-
catalyst was assessed in the photocatalytic reaction. The main products obtained during
this process were methanol, ethanol and acetic acid.
1.2. Presentation of the Research Unit
The Laboratory of Catalysis and Materials (LCM) in partnership with the
Laboratory of Separation and Reaction Engineering (LSRE) became a national
Associate Laboratory in 2004, in recognition of the capacity of the two units to
cooperate in a stable, competent and effective way in the prosecution of specific
objectives of the National Scientific and Technological Policy. The Associate
Laboratory is based in the Chemical Engineering Department of the Faculty of
Engineering of University of Porto (FEUP), with two external Poles at Instituto
Politécnico de Bragança and Instituto Politécnico de Leiria. FEUP is a public institution
of higher education with financial autonomy and the largest Faculty of the University of
Porto.
The present work is in line with the objectives of the Research Group on Catalysis
and Carbon Materials, namely: development of new catalytic technologies for efficient
energy production and synthesis of high performance carbon-semiconductor
photocatalysts for solar fuels production (methanol from the photoreduction of CO2).
The work was carried out at the LCM laboratories located in the Department of
Chemical Engineering/FEUP (E-301 and E-302A). The most relevant equipment used
in the present work were a lab-scale set-up for the photocatalytic reduction of CO2
equipped with a Heraeus TQ 150 medium pressure mercury vapor lamp, which was
built and optimized in the framework of this MSc Thesis, and a gas chromatograph
system, using a flame ionization detector (FID) and a thermal conductor detector
(TCD).
CO2 conversion to renewable fuels 3
1.3. Structure of the thesis
The thesis is divided into five chapters:
(i) The first chapter considers the problem that is under investigation
in this work and describes the Research Unit.
(ii) The second chapter presents the state of the art, besides the
motivation and objectives of the thesis.
(iii) The third chapter details the preparation of the materials used in
this study, the description of characterization techniques as well
as the experiments performed.
(iv) The fourth chapter includes the results obtained during the work
performed and the corresponding discussion.
(v) The chapter five resumes the main conclusions resulting from this
work, some suggestions for future work and the final
appreciation.
CO2 conversion to renewable fuels 4
CO2 conversion to renewable fuels 5
2. State of the art, motivation and objectives
2.1. Overview of the problematic of CO2
Fossil fuels are currently the most important source of energy due to their
availability, stability, high energy density (e.g., 33 GJ/m3 for gasoline) and high added
value [12]. Nevertheless, the combustion of fossil fuels is accompanied by the
significant drawback of environmental pollution, mainly owing to CO2 emissions and
their associated impacts on the ecosystem [17]. The increase of CO2 emissions is widely
considered as the main driving factor that causes the greenhouse phenomenon leading to
an increase of temperature in the surface of Earth [18].
The rise in greenhouse gas concentrations is believed to result in climate changes
that will have serious environmental, social and economic effects such as: (i) rising sea
levels, (ii) frequency and intensity of extreme weather events, (iii) changes in
agricultural productivity, (iv) extinction of species and (v) number of disease vectors
[18]. In the last years, global CO2 emissions have been growing by ca. 2.5 % yr–1
(Figure 1). CO2 emissions from fossil fuels and cement production (EFF) are estimated
at 37.0 Gt CO2 in 2014 and the perspective is to reach the value of 43.2 Gt CO2, in 2019.
CO2 EFF are generally driven by the gross domestic product (GDP) and the decrease
(improvement) in the carbon intensity of the world economy (IFF) [1].
Figure 1 - Global CO2 emissions from fossil fuel combustion and cement production (black
dots) and estimating to 2019 (red dots). Reprinted from [1].
Besides Europe, there are three countries (China, India, USA) that play a critical
role in emissions growth (Figure 2). Since 2006, China was the world largest emitter of
CO2 conversion to renewable fuels 6
carbon dioxide, responsible for 28.1% of emissions, followed by USA (14.1%), then
Europe (9.19%), and finally India (6.77%) [1]. The growth of emissions from China
was 4.2% in 2013 [1] and the China Council for International Cooperation on
Environment and Development (CCICED) projected that emissions in China will start
to decrease only by 2050 [19]. China is currently the third largest producer of biofuels
in the world and the Chinese coal sector produces about 40% of the world coal [20].
The USA’s 2.9% growth in emissions in 2013 reversed the previous nation’s trend
of decreasing emissions since 2007 until 2012 as a result of a return to a stronger
economic growth rate (2.2%), and an unusual increase in IFF (0.7%). These results can
be explained because coal has regained some market share from natural gas in the
electric power sector. The decrease of 1.8% of CO2 emissions in Europe in 2013 was a
continued downward trend, due to the relatively low GDP growth rate (0.5%) and IFF
decrease (2.2%). The recent Indian emissions growth was driven by robust economic
growth and by an increase in IFF. This country is known as being the only major
economy with a sustained increase in IFF from 2010 to 2013 [1].
According to the International Energy Agency (IEA), in 2007, the main global
carbon dioxide emissions from fossil fuels combustion are by sector: 41% power
generation, 23% transportation and 17% industry [21].
Carbon footprint is defined as the total amount of carbon dioxide (CO2) produced
directly and indirectly by human activities, usually expressed in equivalent tons of CO2
[22]. This concept motivates companies to develop market products with environmental
attributes, promoting a green economy. Thus, it is expected that the carbon product
labelling makes a decisive pressure on manufacturers and major chains to reduce carbon
emissions from their stores. The government and international institutions have a key
role in defining the protocols and standards of carbon labelling [23].
CO2 conversion to renewable fuels 7
Figure 2 - The CO2 emissions from the top four emitters (China, USA, EU28, India) and
estimating to 2019 (red dots). Reprinted from [1].
According to the Mauna Loa Observatory (MLO), which has been monitoring
atmospheric CO2 since 1958 in Hawaii, the atmospheric CO2 concentration on Earth
exceeded 400 ppm, for the first time, in May 2013 [24, 25]. Furthermore, the
Intergovernmental Panel on Climate Change (IPCC) released an assessment in 2007,
which recommended to keep atmospheric greenhouse gases below 450 ppm in order to
keep the temperature rise under a crucial 2 ºC level [24].
In order to improve the knowledge about the concentration of atmospheric CO2,
two satellites were designed before 2010 to measure specifically the column-averaged
dry air mole fraction of CO2 (XCO2): (i) GOSAT, the Japanese satellite successfully
launched in January 2009 and (ii) OCO-1, a NASA satellite that got lost because of its
launch failure in February 2009. More recently, in July 2014, NASA has launched a
new satellite, OCO-2, with an instrument very similar to that of OCO-1. Nowadays,
atmospheric CO2 measurement with satellites is one of the most effective approaches
for monitoring the global distributions of greenhouse gases at high spatiotemporal
resolution and carry out a key role in the current study of CO2 [25].
CO2 conversion to renewable fuels 8
2.2. Photocatalytic conversion of CO2
CO2 reduction into hydrocarbons is a strategy to decrease the CO2 atmospheric
concentrations while producing a new renewable energy source. CO2 reduction can be
achieved by electrocatalysis [3, 26] or photocatalysis [3, 4, 6, 16, 27, 28] and in both
cases water is ideally used as reducing agent, providing the hydrogen atoms/ions
required to react with CO2 to form hydrocarbons. In this way, CO2 is removed from the
atmosphere while hydrocarbons are produced as chemical fuels. In fact, the synthesis of
chemical fuels is one of the most sustainable and practical ways to store energy [5].
The advantage, but also the challenge, of the photocatalytic process is the
possibility to employ natural sunlight as the photon source, offering an attractive and
economic way for the reduction of CO2 to hydrocarbons, in this case known as “solar
fuels” [5]. The concept of solar fuels has been created to denote the strategy based on
the conversion of sunlight into chemicals that could be readily stored and/or transported
[29, 30]. The photocatalytic reduction of CO2 into solar fuels demands a high input
energy to break C=O bond and form C–H bond, involving the participation of multiple
electrons and a corresponding number of protons in the process [16].
In photocatalysis, when a semiconductor is illuminated with photons of energy hν
that is equal to or higher than the semiconductor band-gap EG (hν ≥ EG), these photons
are absorbed and create high energy electron-hole pairs, which dissociate into free
photoelectrons (in the conduction band) and photoholes (in the valence band). The
photo-generated electrons and holes that migrate to the surface of the semiconductor
without recombination can, respectively, reduce and oxidize the reactants adsorbed on
the semiconductor surface [31].
The CO2 photoreduction is a complex series of multiple steps in which several
products may be formed simultaneously [28] (Eqs 1-10) [27]:
hehvTiO2 (1)
Oxidation reaction:
HOhOH 2 ½2 22 (2)
CO2 conversion to renewable fuels 9
Reduction reactions:
Hydrogen formation:
22 HeH
(3)
CO2 radical formation:
22 COeCO (4)
Formic acid formation:
HHCOeHCO 22 22
(5)
Carbon monoxide formation:
OHCOeHCO 22 22
(6)
Formaldehyde formation:
OHHCHOeHCO 22 44
(7)
Methanol formation:
OHOHCHeHCO 232 66
(8)
Methane formation:
OHCHeHCO 242 288
(9)
Ethanol formation:
OHOHHCeHCO 2522 312122
(10)
Different semiconductors have been applied in the photoreduction of CO2 [4, 16,
27], but TiO2-based materials are the most extensively studied due to of the TiO2
stability, relatively low toxicity and low cost [9]. During the process of CO2
photoreduction with H2O, photo-illumination of the catalyst surface induces the
generation of electron-hole pairs in TiO2. The excited electrons in the conduction band
(CB) of TiO2 could migrate to the surface and reduce CO2 to solar fuels (i.e., CO, CH4,
CH3OH, HCOOH). Meanwhile, the holes left in the valence band (VB) of TiO2 could
oxidize H2O into oxygen (general scheme shown on Figure 3) [4, 16, 27].
CO2 conversion to renewable fuels 10
Figure 3 - Schematic illustration of photoinduced generation of an electron–hole pair in TiO2
semiconductor that transfers to the surface for CO2 reduction.
The main reasons for the limited efficiency of photocatalytic reduction of CO2
with H2O are: (i) the highly unfavorable one-electron transfer to the formation of CO2
radical (
2CO ) that requires a very negative reduction potential (Eq. 4); (ii) the strong
oxidation power of the photoexcited holes that induce backward reactions, i.e.,
oxidizing the intermediates and products converted from CO2 (iii) the phenomenon of
recombination electrons (e-)/holes (h
+) (Eq. 1), and (iv) hydrogen generation as a
consequence of the presence of water (Eq. 3), that can compete for the electrons in the
conduction band [27].
2.3. Enhancement of the photocatalytic activity of TiO2
As previously mentioned, TiO2 presents some particular drawbacks namely the
low quantum yield [32], which is primarily impaired by the recombination of photo-
generated charge carriers as well as a limited photoactivity in the visible range of
Earth’s solar spectrum. In order to overcome these limitations, several approaches have
been attempted to modify TiO2 such as metal co-catalysts and nanostructured carbon
materials, among others.
2.3.1. Metal Co-catalysts
A metal co-catalyst has the ability to reduce the phenomenon of recombination of
electrons (e-) and holes (h
+) as well as the problem of photocatalytic water reduction
H2O
TiO2
VB
CB CO2
h+
e-
H+ + O2
CH4, CH3OH, etc
CO2 conversion to renewable fuels 11
(hydrogen generation) [4, 16, 27, 32-36]. By doping metals like copper (Cu), Platinum
(Pt) or Iron (Fe), in the TiO2 lattice, or by coating the TiO2 surface with these metals, a
better performance in the photocatalytic conversion of CO2 can be achieved. These
metals can operate as sinks for the excited TiO2 electrons. Thereby, the metal co-
catalysts promote a decrease of the phenomenon of e– / h
+ pairs recombination as well
as the problem of hydrogen generation [27].
Copper appears as the preferred metal to increase the reduction of CO2, among the
various metal studied. The deposition of copper (I) oxide (Cu2O) on TiO2 not only
reduces the amount of CO2, but also increases the yield of product formation,
particularly methanol [27, 32, 37]. This mechanism is illustrated in Figure 4.
Figure 4 - Process of CO2 photoreduction on Cu/TiO2.
2.3.2. Nanostructured carbon materials
Among different materials that can be selected to prepare composites with TiO2,
carbon materials offer unique advantages such as inert nature and stability, in both acid
and basic media, and the possibility to control their textural and chemical properties.
Recently, nanostructured carbon materials such as carbon nanotubes (CNT), fullerenes
(C60) and graphene have been used to produce carbon-based TiO2 composites [38]. In
particular, graphene oxide (GO) consists of graphene layers decorated with oxygen
functional groups on the graphene basal plane and on the edges, affording abundant and
reactive anchoring sites for the assembly of semiconductor materials [38]. GO can act as
CO2 conversion to renewable fuels 12
electron acceptor or electron donor, increasing the photocatalytic activity of the
semiconductor towards solar irradiation [16].
2.4. Methanol Applications
Methanol (CH3OH) is a liquid fuel with several applications [39]. For instance, it
can be used in fuel cells [39, 40] as well as a way to store and transport fuels [39, 41].
This high energy density alcohol can be also used as a solvent and anti-freezing agent
and it also presents good efficiency for fuel turbines applied in power generation.
Methanol has also excellent characteristics of storage and transportation [39-42] and can
be used as an alternative fuel for vehicles (mixture of 85% methanol with 15% unleaded
gasoline) [39, 41, 42]. It can be used as a feedstock for biodiesel production or in the
production of formaldehyde, methyl tertiary butyl ether (MTBE), acetic acid, methyl
methacrylate and dimethyl terephthalate. Moreover, methanol can replace oil as a
feedstock for the main products of petrochemical and chemical industry, due to its
ability to be transformed into ethylene and propylene [41].
Methanol demand has increased and it is foreseen that, in the next few years, its
price will rise converting this chemical in a more valuable liquid fuel [39].
CO2 conversion to renewable fuels 13
Table 1 shows some works related with the production of methanol by the
photocatalytic reduction of CO2. The methanol yield value was achieved by the
following equation (Eq. 11):
(11)
By the analysis of the different works, it was possible to realize that the majority
of photocatalysts with some efficiency for CO2 photoreduction, have a slight
photoresponse under sunlight.
Table 1 - Summary of recent studies based on semiconductor materials used in the
photocatalytic conversion of CO2 into methanol.
Catalyst Methanol yield
(µmol g-1 h-1) Light source Reaction Comments
Year
Ref.
TiO2 0.780
254 nm
8 W Hg lamp
138 µWcm-2
0.15 – 0.6 g catalyst
300 ml of 0.2 M
NaOH solution
6h irradiation
Constant
temperature of 50ºC
TiO2 and Cu/TiO2 were
synthesized by an improved sol-
gel process. Cu/TiO2 and
Cu/P25 were impregnated by
adding CuCl2. Formic acid,
formaldehyde and ethanol were
detected in some catalytic
reactions, in amounts that were
much lower than methanol.
2002
[37]
Degussa P25 6.40
2.0% Cu/P25 10.0
0.6% Cu/TiO2 6.30
1.0% Cu/TiO2 12.0
2.0% Cu/TiO2 20.0
3.3% Cu/TiO2 15.0
6.0% Cu/TiO2 3.30
TiO2 (P25) 135
UV black light
lamps
6 × 10 W
2450 µWcm-2
0.3 g catalyst
300 ml of 1 M
KHCO3 solution.
6h irradiation
Temperature
between
43ºC-100 ºC
Modified catalysts were
prepared by impregnating P25
with a copper nitrate solution.
The methanol yield rises as the
temperature increases
2005
[32]
0.5% CuO/TiO2 197
1.0% CuO/TiO2 265
3.0% CuO/TiO2 442
5.0% CuO/TiO2 213
10% CuO/TiO2 127
3.0% Cu/TiO2 194
3.0% Cu2O/TiO2 224
CO2 conversion to renewable fuels 14
Table 1 - (Continued)
Catalyst Methanol yield
(µmol g-1 h-1) Light source Reaction Comments
Year
Ref.
P25 0.176
> 300 nm UV-
Vis
500 W Xe
lamp
0.05 g catalyst
30 ml of 0.08 M
NaHCO3
3h irradiation
Constant
temperature
FeTiO3/TiO2 composites were
synthesized by an hydrothermal
method.
2012
[36]
TiO2 0.175
10% FeTiO3/TiO2 0.338
20% FeTiO3/TiO2 0.462
50% FeTiO3/TiO2 0.298
P25 0.0450 > 400 nm
Visible light
500 W Xe
lamp
TiO2 0.141
10% FeTiO3/TiO2 0.319
20% FeTiO3/TiO2 0.432
50% FeTiO3/TiO2 0.352
InTaO4 62.5* 390 - 770 nm
100 mW
Xe lamp
0.1g catalyst
50 mL of desionized
water
2 h irradiation
Constant
temperature of 25ºC
Ni@NiO/InTaO4-N
photocatalyst was obtained by
loading 3.2 wt % Ni on InTaO4-
N by impregnation of an
aqueous Ni(NO3)2 solution.
2011
[43]
InTaO4-N 137*
Ni@NiO/InTaO4-N 175*
TiO2 2.16
254 nm
UV lamp
10 W 0.4 g catalyst
400 ml of 0.2 M
NaOH and 0.2 M
Na2SO3
8 h irradiation
Constant
temperature of 75ºC
N-Ni/TiO2 were prerared by an
improved sol-gel method.
2011
[44]
N/TiO2 30.7
Ni/TiO2 26.8
N-Ni/TiO2 60.3
TiO2 0.450
365 nm UV
lamp 10 W
N/TiO2 21.0
Ni/TiO2 7.50
N-Ni/TiO2 31.7
TiO2 0.130 400 - 780 nm
Incandescent
lamp
N/TiO2 7.58
Ni/TiO2 3.59
N-Ni/TiO2 15.1
CdS 40.2
> 400 nm
UV light 500
W Xe lamp
0.2 g catalyst
0.8 g of sodium
hydroxide and 2.52
g of absolute sodium
sulfite were
dissolved in 200
distilled water
5 h irradiation
The photocatalysts were
prepared the hydrothermal
method using the corresponding
salt and thiourea in an ammonia
bath.
2011
[45] Bi2S3 62.8
Bi2S3/CdS 123
Bi2WO6 SSR 0.640 Visible light
irradiation 2 h irradiation
An anion exchange strategy is
explored to synthesize Bi2WO6
hollow microspheres that show
a high CO2 adsorption capacity.
2012
[46]
Bi2WO6 HMSs 16.3
CO2 conversion to renewable fuels 15
Table 1 - (Continued)
Catalyst Methanol yield
(µmol g-1 h-1) Light source Reaction Comments
Year
Ref.
ZnS deposited on
MMT
1.41*
254 nm
8 W Hg lamp
ZnS-MMT catalyst
(1 g l−1
) was suspended
in the 0.2 M NaOH
solution
24 h irradiation
Two stirred batch annular
reactors with three quartz glass
tubes of different diameters
(3.5, 4.0 and 4.5 cm).
It was found that the formation
of the products depends on the
volume of liquid phase and the
reactor diameter.
2011
[47]
TiO2 14.4
400 nm
visible light
irradiation
500 W Xe
lamp
and
254 nm
UV irradiation
16 W Hg lamp
0.4 g catalysts
400 ml distilled
water
3 h irradiation
Temperature: 40ºC
Ag/TiO2 nanocomposites were
obtained by microwave assisted
chemical reduction method.
2014
[35]
0.5% Ag/TiO2 60.0*
1.0% Ag/TiO2 91.7*
1.5% Ag/TiO2 100*
2.0% Ag/TiO2 125*
2.5% Ag/TiO2 135
3.0% Ag/TiO2 130*
3.5% Ag/TiO2 113*
4.0% Ag/TiO2 108*
TiO2 0
Visible
irradiation 2.5% Ag/TiO2 29.9
TiO2 13.6
UV irradiation
2.5% Ag/TiO2 43.4
1.0% NiO-InTaO4 1.39
Visible light
irradiation
500 W
halogen lamp
0.14 g catalyst
50 ml of 0.2 M
KHCO3 solution
20 h irradiation
Room temperature
NiO was prepared by
impregnation and then by
reduction-oxidation.
2007
[48]
KATI66
0.190
254 nm
UV 8 W Hg
lamp
0.1 g catalyst
100 ml of 0.2 M
NaOH
24 h irradiation
KATI66 (kaolinite/TiO2)
composite was prepared using
thermal hydrolysis of
kaolinite/titanyl sulphate
suspension.
2011
[2]
DegussaP25 0.0300*
2%Cu/TiO2
20.0
254 nm
UV Hg lamp
0.3 g catalyst
300 ml 0.2 M NaOH
solution
30 h irradiation
The temperature was
monitored
continuously
Cu/TiO2 was synthesized with a
sol-gel process.
The precursor CuCl2 performed
better than the copper acetate.
2004
[49]
2%Cu/TiO2 0.330 365nm
UVA
CO2 conversion to renewable fuels 16
Table 1 - (Continued)
Catalyst Methanol Yield
(µmol g-1 h-1) Light source Reaction Comments
Year
Ref.
7.0% AgBr/TiO2 8.60* Visible light
irradiation
>420 nm
150 W Xe
lamp
0.5 g catalyst
100 ml of 0.2 M
KHCO3 solution
5 h irradiation
Room temperature
AgBr/TiO2 was prepared by the
deposition-precipitation method.
2011
[50]
11.6% AgBr/TiO2 13.6*
23.2% AgBr/TiO2 15.6
46.4% AgBr/TiO2 11.6*
175 % AgBr/TiO2 8.10*
TiO2 0.0300*
254 nm
UV 8 W Hg
lamp
1g l-1
catalyst
24 h irradiation
TiO2 and Ag/TiO2 catalysts
were prepared by the sol–gel
process.
2010
[34]
1% Ag/TiO2 0.0400*
3% Ag/TiO2 0.0400*
5% Ag/TiO2 0.0500*
5% Ag/TiO2 0.0800*
1.2% Cu/TiO2 0.460
365 nm
Hg lamp
16 W cm-2
75ºC constant
temperature
Methanol yield was studied with
the copper weight change and
also the light intensity (1-16 W
cm-2
).
2005
[51]
TiO2/N-100 10.0 UV-Vis lamp
300-600 nm
0.6 g catalyst
800 ml of water
2 h irradiation
65 ºC
TiO2/N-100 was synthesized by
hydrothermal reaction of
amorphous anatase TiO2 and
NH4OH at 100 ºC.
2014
[52]
TiO2 17.7
254 nm
UV-lamp
0.97 mW cm-2
0.5 g catalyst
500 ml of 0.1 M
NaHCO3
12 h irradiation
25ºC constant
temperature
Cu or Co incorporated
TiO2/ZSm-5 were prepared by a
sol-gel method.
2015
[53]
TiO2/ZSM5 26.5
Cu-TiO2/ZSM5 50.1
Co-TiO2/ZSM5 35.1
TiO2 0.0330
Visible light
irradiation
>400 nm
500 W Xe
lamp
0.05 g catalyst
50 ml distilled water
containing 0.08M
NaHCO3, 0 and 0.08
M HCl
9 h irradiation
Constant
temperature
Mesoporous graphene and
tourmaline single and co-doped
TiO2 composites were prepared
by the sol–gel method.
The yield of methanol was
studied by varying the amount
of catalyst (0.5 to 2.5 g L-1
) of
0:08 M NaHCO3.
G (Graphene) serves as the
transporter of photogenerated
electrons from anatase TiO2.
T (Tourmaline) serves as the
acceptor of photogenerated
electrons.
2014
[54]
1%G/TiO2 0.120
1%T/TiO2 0.240
1%G/1%T/TiO2 0.390
1%G/1.5%T/TiO2 0.450
1%G/2%T/TiO2 0.620
1%G/2.5%T/TiO2 0.720
1%G/3%T/TiO2 0.680
* these values have a small inaccuracy error because they were removed from the graph’s article since it
doesn’t show the numerical values of methanol yield.
CO2 conversion to renewable fuels 17
2.5. Motivation and objectives of the thesis
The Master in Environmental Engineering is a multidisciplinary course that
provides an integrated approach in order to develop scientific and technological
solutions to existing environmental problems, while promoting sustainable
development.
The CO2 photoreduction into renewable fuels is seen as a promising solution to
reduce the atmospheric concentrations of CO2 and a potential way of creating added-
value chemical products. The development of a selective and high yield photocatalytic
process, which uses CO2 as a feedstock to produce hydrocarbons has a growing
environmental importance. This work has the aim of developing different photocatalysts
and to study their performance in CO2 photoconversion into renewable fuels. In this
sense, the following specific objectives were defined:
Synthesis of TiO2 and graphene oxide-TiO2 (GOT) composites;
Synthesis of Cu loaded TiO2 and GOT catalysts;
Characterization of the prepared materials using different techniques;
To study the performance of all catalysts in the photoreduction of CO2 under
UV/Vis irradiation, aiming catalysts with high activity and stability;
To evaluate the performance of the photocatalysts, with the determination
and quantification of the main subproducts obtained during the process.
CO2 conversion to renewable fuels 18
CO2 conversion to renewable fuels 19
3. Experimental
3.1. Chemicals
Ammonium hexaflurorotitanate(IV) ((NH4)2TiF6, > 99.99%), boric acid (H3BO3,
> 99%), ammonium persulfate ((NH4)2S2O8, > 98%), sulphuric acid (H2SO4, > 95%),
copper(II) nitrate trihydrate (Cu(NO3)2 3H2O, > 98%) and copper(II) acetate
(Cu(CH3COO)2, 98%) were obtained from Sigma-Aldrich. Copper(I) oxide (Cu2O,
97%) and copper(II) chloride (CuCl2) were purchased from Riedel-de-Haën. Sodium
hydroxide (NaOH, 98 wt.%) was obtained from Panreac.
3.2. Synthesis of graphene oxide
GO was synthesized from graphite (particle size ≤ 20 µm, from Sigma-Aldrich)
by a modified Hummers method as described elsewhere [55, 56]. In a typical procedure,
50 mL of H2SO4 was added gradually with stirring and cooling to a 500 mL flask
containing 2 g of graphite. Then 6 g of potassium permanganate (KMnO4) was added
slowly to the mixture. The suspension was continuously stirred for 2 h at 35 ºC. After
that, it was cooled in an ice bath and subsequently diluted by 350 mL of deionized
water. Then H2O2 (30% w/v) was added in order to reduce residual permanganate to
soluble manganese ions, appearing a bright yellow color in the suspension. The
oxidized material was purified with a 10% HCl solution and then the suspension was
filtered, washed several times with water until achieve a neutral pH in the resulting
water, and dried at 60 ºC for 24 h to obtain graphite oxide. The resulting material was
dispersed in a given volume of water and sonicated in an ultrasound bath (ultrasonic
processor UP400S, 24 kHz) for 1 h. The sonicated dispersion was centrifuged for 20
min at 3000 rpm to remove unexfoliated graphite oxide particles from the supernatant
and the obtained suspension of graphene oxide was then used for the synthesis of GOT
composites.
3.3. Preparation of GO-TiO2 composites
GO-TiO2 (referred as GOT) was synthesized by the liquid phase deposition
(LPD) method at room temperature, as described elsewhere [57]. Ammonium
hexafluorotitanate (IV), (NH4)2TiF6 (0.1 mol/L), and boric acid, H3BO3 (0.3 mol/L),
CO2 conversion to renewable fuels 20
were added to a certain amount of the GO dispersion heated at 60 ºC for 2 h under
vigorous stirring. The material was separated by filtration, washed with water and dried
at 100 ºC under vacuum for 2 h. The post-treatment under N2 atmosphere at 200 ºC was
established in previous experiments, taking into account the crystallinity of TiO2
particles and the stability of the GO material at that temperature. The carbon loading (4
wt.%) was selected taking into account the highest photocatalytic activity obtained with
this GOT composite in previous works [57, 58]. During the thermal treatment, anatase
TiO2 particles were exclusively formed at such temperature. Bare TiO2 was prepared
using the same methodology but without the addition of GO (referred as TiO2). The
photocatalyst from Evonik Degussa Corporation (P25) was also used as reference
material.
3.4. Preparation of Cu-loaded GOT
The photocatalysts were prepared by adapting the procedure described by Wu et
al. for Cu-loaded TiO2 [51]. The copper precursor was directly added during the
preparation method of the GOT composite.
Different copper precursors, either copper chloride (CuCl2), copper acetate
(Cu2(Ac)), copper nitrate (Cu(NO3)2) or copper (I) oxide (Cu2O) were used. The amount
of copper precursor was adjusted to obtain the desired Cu loading of ca. 2 wt.%. The
resulting material was separated by filtration, washed with water and dried at 100 ºC.
Then, the catalysts were treated in a furnace at 150 ºC for 3 h by 3 ºC min-1
from room
temperature, then raised 2 ºC min-1
to 500 ºC and stayed for 5 h under air atmosphere.
Cu (I) specie is predicted to be formed during this calcination step [51].
The TiO2-Cu2O composites were prepared following the same procedure but
without the addition of GO. Table 2 summarizes the preparation conditions and the
label used for the prepared catalysts.
CO2 conversion to renewable fuels 21
Table 2 - Preparation conditions of photocatalysts.
Photocatalyst Cu (wt.%) Copper precursor Post-treatment
P25
TiO2
GOT
--
--
--
--
--
--
--
200 ºC (N2)
200 ºC (N2)
TiO2_CuCl2 ~ 2.0 CuCl2 150 ºC ( 3h, air) +
500 ºC (5h, air)
TiO2_Cu2(Ac) ~ 2.0 Cu2(OAc)4 150 ºC ( 3h, air) +
500 ºC (5h, air)
TiO2_Cu(NO3)2 ~ 2.0 Cu(NO3)2 150 ºC ( 3h, air) +
500 ºC (5h, air)
TiO2_Cu2O ~ 2.0 Cu2O --
GOT_CuCl2 ~ 2.0 CuCl2 150 ºC ( 3h, air) +
500 ºC (5h, air)
GOT_ Cu2(Ac) ~ 2.0 Cu2(OAc)4 150 ºC ( 3h, air) +
500 ºC (5h, air)
GOT_Cu(NO3)2 ~ 2.0 Cu(NO3)2 150 ºC ( 3h, air) +
500 ºC (5h, air)
GOT_Cu2O ~ 2.0 Cu2O --
3.5. Catalysts characterization
Thermogravimetric (TG) analysis of the composites were performed using a STA
490 PC/4/H Luxx Netzsch thermal analyser, by heating the sample in N2 flow from 50
ºC to 1000 ºC at 20 ºC min-1
.
The morphology of the materials was studied by scanning electron microscopy
(SEM) using a FEI Quanta 400FEG ESEM/EDAX Genesis X4M microscope.
The optical properties of the samples were analyzed by diffuse reflectance UV/Vis
spectroscopy using a JASCO V-560 UV/Vis spectrophotometer, equipped with an
integrating sphere attachment (JASCO ISV-469) and barium sulfate was used as a
reference. The reflectance spectra were converted by the instrument software (JASCO)
to equivalent absorption Kubelka-Munk units.
X-ray photoelectron spectroscopy (XPS) was performed in a Kratos AXIS Ultra
HSA using a monochromatic Al Kα X-ray source (1486.7 eV), operating at 15 kV (90
W), in FAT mode (Fixed Analyser Transmission), with a pass energy of 40 eV for
regions of interest and 80 eV for survey.
CO2 conversion to renewable fuels 22
3.6. Photocatalytic experiments
The photocatalytic runs were carried out in a cylindrical glass immersion photo-
reactor filled with 250 mL of 0.2 M NaOH solution and containing 250 mg of catalyst
(catalyst load of 1 g L−1
). A schematic diagram of the photocatalytic reactor is shown in
Figure 5.
Figure 5 - Schematic diagram of the lab-scale set-up for photoreduction of CO2.
A Heraeus TQ 150 medium-pressure mercury vapor lamp (exc = 254, 313, 365,
436 and 546 nm) was located axially in the reactor and held in a quartz immersion tube.
A DURAN glass cooling jacket was used for irradiation in the near-UV to visible light
H2O in
H2O out
Valve
Liquid Sample
Extracted Sample
Stirrer Bar
Photo ReactorFiltered Samples
Hg Lamp
CO2 + He
NaOH 0.2M + Catalyst
CO2 He
MFC MFC
Vent
GC-FID HPLC-UV
GC-TCD
Gas Sample
Vent
Power Supply
CO2 conversion to renewable fuels 23
range (exc > 350 nm) - Figure 6. During the photocatalytic experiments, the temperature
was kept at around 25 ºC. A photograph of the experimental set-up is shown in Figure 7.
Prior to the light irradiation, the above system was degased by using a He and
CO2 flow overnight (10 cm3 min
−1 and 2 cm
3 min
−1, respectively) to saturate the
solution. The initial pH value of 0.2 M NaOH solution was approximately pH 13 and
the pH value of the CO2-saturated NaOH solution was nearly pH 7. Then, the reactor
was irradiated during 360 min. To maintain CO2 saturation during measurements, He
and CO2 flow rates were maintained at 27 cm3 min
−1 and 3 cm
3 min
−1, respectively.
Figure 6 - Irradiance spectra of the Heraeus TQ-150 UV-Vis lamp equipped with a DURAN
50® jacket and transmittance spectrum of the cut-off filter.
The gas product was analyzed by an online gas chromatograph (GC) equipped
with a thermal conductivity detector (TCD), using a capillary column (Carboxen 1010
Plot. Supelco).
In order to determine the formation of methanol and ethanol, liquid samples were
periodically withdrawn and analysed by gas chromatography (DANI GC-1000) using a
capillary column (WCOT Fused Silica 30 m, 0.32 mm i.d., coated with CP-Sil 8 CB
low bleed/MS 1 m film) and a flame ionization detector (FID). All the photocatalytic
experiments were replicated two times. The reported data corresponds to the average
over two independent runs.
CO2 conversion to renewable fuels 24
Concentrations of some organic acids resulting from the CO2 photo degradation
were followed by HPLC using a Hitachi Elite LaChrom HPLC equipped with a UV
detector. An Alltech OA-1000 column (300 mm × 6.5 mm) was used as stationary
phase, working at room temperature, under isocratic elution with a solution of H2SO4 5
mM at a flow rate of 0.5 mL min−1
and using an injection volume of 15 μL.
Figure 7 - Photocatalytic reactor.
CO2 conversion to renewable fuels 25
4. Results and discussion
4.1. Characterization of the materials
4.1.1. Thermogravimetric analysis and scanning electron microscopy (SEM)
The copper content (wt.%) in the composite materials was determined by
thermogravimetric analysis (not shown); both GOT and the Cu-loaded composites were
submitted to a thermal treatment under N2 and the weight loss was monitored. The
copper content in the composite corresponds to the difference between the weight loss
observed for the Cu-loaded composite and that of GOT composite. The obtained results
are in a good agreement with the expected Cu content (c.a. 2 wt.%).
The representative SEM images of TiO2, GOT composite as well as the copper-
loaded photocatalysts are shown in Figures 8a-h. The morphology of TiO2 consists of
spherical particles that aggregate originating clusters of TiO2 particles (Figure 8a). The
GOT composite consists of TiO2 particles aggregated on the GO layers, forming GOT
platelets (Figure 8b) with a well TiO2 distribution on both sides of the graphene oxide
nanosheets [57].
The surface morphology of the Cu-loaded TiO2 photocatalysts prepared with
Cu2(OAc)4 and Cu2O (Figure 8e and 8g, respectively) seems similar to that of bare TiO2
(Figure 8a). On the contrary, the morphology observed for TiO2_Cu(NO3)2 (Figure 8c)
was different than that obtained for bare TiO2, presenting larger clusters. Regarding the
Cu-loaded GOT composites (Figures 8d, 8f and 8h), the morphologies obtained were
slightly different than that observed for GOT (Figure 8b), since the platelets were not so
notorious when Cu was loaded. All the materials presented a quite homogeneous
morphology, but the better assembling of TiO2 nanoparticles and GO platelets seems to
be achieved with GOT.
The EDS spectra obtained for the composites (as shown in Figure 8i for
GOT_Cu(NO3)2), revealed Ti, O and C peaks, that were associated to TiO2 and to GO
(the O peak resulting from TiO2 and from the oxygenated surface groups present in the
chemical structure of GO). It is difficult to infer about the presence of Cu, probably due
to the low amounts loaded, even so there is some indication of the Cu presence in the
EDS spectrum (Figure 8i).
CO2 conversion to renewable fuels 26
4.1.2. Diffuse reflectance UV/Vis spectroscopy (DRUV)
The optical properties of selected samples were analyzed using the diffuse
reflectance DRUV-Vis spectra expressed in terms of Kulbelka-Munk equivalent
absorption units, as shown in Figure 9 for P25, GOT, TiO2_Cu(NO3)2, GOT_Cu(NO3)2,
TiO2_Cu2O, GOT_Cu2O and TiO2_CuCl2.
For P25, the characteristic absorption edge rising at 330 nm is associated with
the excitation of the O 2p electron in the valence band (VB) to the Ti 3d state in the
conduction band (CB) [59]. Regarding the Cu-loaded TiO2 catalysts, the band edge is
slightly expanded into both UV and visible regions (Figure 9). This effect could be due
i) GOT_Cu(NO3)2
c) TiO2_Cu(NO3)2
d) GOT_Cu(NO3)2 e) TiO2_Cu2(AC) f) GOT_Cu2(AC)
g) TiO2_Cu2O h) GOT_Cu2O
b) GOT a) TiO2
Figure 8 - SEM micrographs for (a) TiO2, (b) GOT, (c) TiO2_Cu(NO3)2, (d)
GOT_Cu(NO3)2, (e) TiO2_Cu2(AC), (f) GOT_Cu2(AC), (g) TiO2_Cu2O, (h) GOT_Cu2O, (i)
EDS spectrum of GOT_Cu(NO3)2.
CO2 conversion to renewable fuels 27
to the additional energy levels created by the copper ions in the band gap of TiO2 [49,
60, 61].
Figure 9 - Diffuse reflectance UV/Vis spectroscopy of some photocatalysts materials.
As expected, for GOT and Cu-loaded GOT composites, a higher absorption in the
visible spectral range is clearly observed, in comparison with GO-free materials, i.e.,
P25, TiO2_Cu2O, TiO2_Cu(NO3)2 and TiO2_CuCl2, due to the introduction of the GO
material (Figure 9). This increase in the absorption promoted by GO has been ascribed
to both the capacity of the carbon phase to absorb light and also to the creation of an
electronic inter phase interaction between GO and TiO2 phase, as observed for other
carbonaceous materials combined with TiO2 [38, 62].
4.1.3. X-ray photoelectron spectroscopy (XPS)
XPS measurements were carried out over the photocatalysts in order to determine
the oxidation state of the copper species existing in the photocatalysts. As example,
figures 10a and 10b show the XPS spectra of the Ti 2p region for TiO2_Cu(NO3)2 and
GOT_Cu(NO3)2 composites, respectively. From the Ti 2p region, two peaks were
detected and centered at 459.1 and 464.8 eV, corresponding to Ti 2p3/2 and Ti 2p1/2,
0
1
2
3
4
250 300 350 400 450 500 550 600
Ku
belk
a M
un
k (
u.a
.)
λ (nm)
P25
TiO₂_Cu₂O
TiO₂_Cu(NO₃)2
TiO₂_CuCl2
GOT_Cu(NO₃)2
GOT
GOT_Cu₂O
CO2 conversion to renewable fuels 28
respectively. In addition, the splitting between both peaks was found at 5.7 eV. These
results indicate a Ti4+
chemical state, typical of TiO2 as previously reported [57, 60].
Table 3 presents the amounts of copper introduced on the surface of the materials
calculated from the XPS data. The photocatalysts prepared with Cu(NO3)2 as Cu
precursor present the largest amount of Cu in the surface of these materials (0.7% and
0.6% for TiO2_Cu(NO3)2 and GOT_Cu(NO3)2, respectively). The low amounts of Cu
detected (in comparison with the theoretical 2 wt.%) indicate that copper was dispersed
mostly inside the structure of the prepared catalysts, rather than in the catalyst surface.
The XPS Cu 2p spectra of the TiO2_Cu(NO3)2 and GOT_Cu(NO3)2 samples are
shown in Figures 10c and 10d, respectively. Cu 2p3/2 characteristic peaks for metallic
Cu, Cu2O and CuO appear at 932.0, 932.7 and 933.8 eV [63]. The Cu 2p3/2 and Cu 2p1/2
binding energies of the prepared materials were found to be respectively ca. 932.4 and
952.4 eV, confirming the presence of Cu(I). However the binding energies for CuO
were 1 eV above those for Cu2O which are 933.8 and 953.8, respectively [61]. It has
been reported that, according to the position and the shape of the peaks, the copper on
the surface of the catalyst may exist in multiple-oxidation states but Cu(I) is the primary
species [49]. These results are in agreement with previous works, where lower oxidation
state of Cu was observed at higher calcination temperatures [51, 60].
Table 3 - Copper content obtained from XPS analysis.
Photocatalyst CuXPS (%)
TiO2_CuCl2 ~ 0.5
TiO2_Cu2(Ac) ~ 0.3
TiO2_Cu(NO3)2 ~ 0.7
GOT_CuCl2 ~ 0.5
GOT_ Cu2(Ac) ~ 0.2
GOT_Cu(NO3)2 ~ 0.6
CO2 conversion to renewable fuels 29
Figure 10 - XPS spectra of the Ti 2p and Cu 2p regions for (a and c) TiO2_(CuNO3)2 and (b
and d) GOT_(CuNO3)2 composites, respectively.
454459464469
Inte
nsi
ty (
a.u
)
Binding Energy (eV)
Ti 2p
TiO₂_Cu(NO3)2
5.7 eV
454459464469
Inte
nsi
ty (
a.u
)
Binding Energy (eV)
Ti 2p
5.7 eV
GOT_Cu(NO3)2
928933938943948953958
Inte
nsi
ty (
a.u
)
Binding Energy (eV)
Cu 2p
TiO₂_Cu(NO3)2
20 eV
928933938943948953958
Inte
nsi
ty (
a.u
)
Binding Energy (eV)
Cu 2pGOT_Cu(NO3)2
20 eV
2p1/2 2p3/2
2p1/2 2p3/2
2p1/2
2p3/2
2p1/2
2p3/2
a)
b)
c)
d)
CO2 conversion to renewable fuels 30
4.2. Photocatalytic reduction of CO2
4.2.1. Catalyst screening
4.2.1.1. Methanol production
As previously indicated, water can act as the reducing agent in the photocatalytic
reduction of CO2. However, a NaOH aqueous solution is used normally in this reaction
due to the following reasons: (i) the amount of dissolved CO2 is higher because NaOH
solution dissolves more CO2 than pure water and (ii) the recombination of electron-
holes pairs is lower because the highly concentrated OH ions in aqueous solution could
act as strong holes scavengers favoring generation of •OH radicals. The most common
concentration of NaOH reported is 0.2 M [4, 49, 64, 65] and this concentration was
adopted in the present work.
The methanol yield evolution during the photocatalytic experiments for the ten
synthesized heterogeneous catalysts as well as for the commercial TiO2 (P25) is
depicted in Figure 11 and 12, while the results for ethanol are shown in Figures 13 and
14. Photocatalytic experiments were typically carried out during 6 h under near-
UV/Visible irradiation.
Blank experiments were also conducted to confirm that the hydrocarbon
production was due to the photoreduction of CO2. The first blank was UV/Vis light
irradiation without photocatalyst, the second blank was performed in dark conditions
with both photocatalyst and CO2 under the same experimental conditions (not shown),
and the third one was over the illuminated photocatalyst in the absence of CO2 (not
shown). Since the results show that no hydrocarbon production was detected in the
above blank tests, as previously reported [35, 44, 52, 66, 67], only the results obtained
with the first blank are shown in Figures 11 and 13, for methanol and ethanol
production, respectively).
In general, the photocatalysts were efficient for methanol generation with the
exception of bare TiO2, GOT_CuCl2 and the catalysts prepared using Cu2O as copper
precursor (i.e., TiO2_Cu2O and GOT_Cu2O, respectively).
CO2 conversion to renewable fuels 31
Figure 11 - Methanol yield evolution in the experiments performed with different materials and
blank for comparison.
The highest methanol production was observed at 240 min of UV-Vis light
irradiation and the best four catalysts were identified as follows: GOT > TiO2-Cu(NO3)2
GOT-Cu(NO3)2 > GOT-Cu2(Ac) with a maximum methanol yield of 163, 153, 146
and 138 µmol g-1
, respectively (Figure 12). The efficiency of the photocatalytic process
depends on the copper precursor used. Among the Cu-loaded composites,
TiO2_Cu(NO3)2 and GOT_Cu(NO3)2 exhibited the highest photocatalytic activity under
near-UV/Vis, i.e., these prepared with the Cu(NO3)2 precursor. This photocatalytic
activity may be related with the largest amount of Cu on the surface, in comparison to
the other Cu-loaded photocatalysts (derived from the XPS analysis, showed in Table 3).
However, among all the photocatalysts, the highest methanol formation was found
for the GOT composite under near-UV/Vis irradiation (exceeding that of P25 and bare
TiO2 photocatalysts). The high performance of GOT was attributed to the good
assembly and interfacial coupling between the GO sheets and TiO2 nanoparticles (as can
be observed in the SEM micrograph of this composite, Figure 8b), acting as efficient
electron acceptor and donor [38, 57]. Recently, Tan et al. [68] have reported the
photocatalytic reduction of CO2 over reduced graphene oxide (rGO)-TiO2 hybrid
nanocrystals synthetized though a solvothermal route. The reaction was carried out
under gas condition using water vapour as reductant. Under these conditions, CH4 was
obtained as main by-product of reaction. In that work the authors indicate that the
0
20
40
60
80
100
120
140
160
180
0 60 120 180 240 300 360
Me
tha
no
l yie
ld (
µm
ol g
-1)
Time (min)
P25
TiO₂
GOT
TiO₂_CuCl₂
TiO₂_Cu₂(Ac)
TiO₂_Cu(NO₃)₂
TiO₂_Cu₂O
GOT_CuCl₂
GOT_Cu₂(Ac)
GOT_Cu(NO₃)₂
GOT_Cu₂O
blank
CO2 conversion to renewable fuels 32
enhancement of the photocatalytic activity was attributed to the close contact between
TiO2 and rGO that can accelerate the transfer of photogenerated electrons on TiO2 to
rGO preventing efficiently charge recombination.
Figure 12 - Methanol yield at 240 min in the experiments performed with different materials
and blank for comparison
The methanol production with GOT strongly increased in the first 60 min of
reaction, after that, the increase was less pronounced, reaching the maximum at 240 min
of UV/Vis irradiation (Figure 11). It is also worth noting that the presence of copper in
Cu-TiO2 composites, that were not prepared with Cu2O, promotes a strong increase in
the efficiency of the photoreduction process when compared with bare TiO2 (Figure 12).
As previously mentioned, copper can act as an electron trapper and prohibits the
recombination of electrons and holes, increasing the photoefficiency. However, for the
GOT material, the presence of co-catalyst decreased the photoactivity in all the
composites. In general, the photocatalytic activity of the Cu-loaded GOT composites
seems to be related with the differences in the morphology derived from the SEM
analysis (Figure 8), indicating that the variation in the self-assembling of GO with TiO2
particles is crucial for their photocatalytic performance. For future work, is here
recommended to impregnate Cu over GOT rather than mixing the GO, TiO2 and Cu
precursor together.
P-2
5
TiO₂
GO
T
TiO₂_
Cu
Cl₂
TiO₂_
Cu₂(
Ac
)
TiO₂_
Cu
(NO₃)₂
TiO₂_
Cu₂O
GO
T_
Cu
Cl₂
GO
T_
Cu₂(
Ac
)
GO
T_
Cu
(NO₃)₂
GO
T_
Cu₂O
0
50
100
150
Meth
an
ol yie
ld (
µm
ol g
-1)
240 m
in
Catalysts
CO2 conversion to renewable fuels 33
It is also noticed that during the photocatalytic reaction, a slight decrease in the
CO2 formation was observed for GOT composite after 240 min of irradiation whereas
the photocatalytic activity for the Cu-loaded photocatalysts was maintained. In this
sense, the photocatalyst seems to deactivate after a long period of irradiation. Three
possible reasons are: (i) the adsorption or accumulation of intermediate products on the
semiconductor surface that could occupy the photocatalytic reaction centers and also
hinder the adsorption of CO2 or H2O; (ii) the adsorption of hydrocarbon fuel products
could affect the adsorption of the reactants, and (iii) the contamination of the surface
photocatalyst prevents the absorption of light and at the same time to leads less
electron/hole pairs.
4.2.1.2. Ethanol production
Regarding the ethanol production, Figure 13 shows the results obtained with all
materials tested. The production of ethanol under non-catalytic conditions (blank) was
negligible.
For the photocatalysts TiO2, GOT_CuCl2 and those prepared using Cu2O as
copper precursor (i.e., TiO2_Cu2O and GOT_Cu2O, respectively) any photocatalytic
activity was observed, as in the case of methanol.
Figure 13 - Ethanol yield evolution in the experiments performed with different materials and
blank for comparison.
0
200
400
600
800
1000
0 60 120 180 240 300 360
Eth
an
ol yie
ld (
µm
ol g
-1)
Time (min)
P25
TiO₂
GOT
TiO₂_CuCl₂
TiO₂_Cu₂(Ac)
TiO₂_Cu(NO₃)₂
TiO₂_Cu₂O
GOT_CuCl₂
GOT_Cu₂(Ac)
GOT_Cu(NO₃)₂
GOT_Cu₂O
blank
CO2 conversion to renewable fuels 34
The highest ethanol yield was found for TiO2_Cu(NO3)2, followed by the GOT
composite (872 and 479 µmol g-1
of catalyst, respectively at 240 min of irradiation,
Figure 14). Thus, these two catalysts performed better than all the other tested materials
regarding methanol and ethanol products. On the other hand, no ethanol formation was
observed when the reaction was carried out with P25, indicating that, in general, the
presence of GO and copper as co-catalysts lead to composites with improve
photoefficiency compared to both bare TiO2 and P25.
The main hydrocarbons detected during the reaction were both methanol and
ethanol, although the presence of other hydrocarbons such as propanol has been also
identified. These results are in agreement with literature where alcohols, such as
methanol and ethanol, were the major products of CO2 photoreduction in aqueous
solution [27].
Figure 14 - Ethanol yield at 240 min in the experiments performed with different materials and
blank for comparison
It has been also noticed that among all the prepared photocatalysts, the largest
activity for ethanol production was obtained with the catalysts prepared with Cu(NO3)2
as copper precursor, for both ethanol and methanol production. The lowest
photocatalytic activity was obtained with CuCl2 and Cu2O as copper precursors and
could be justified by the amount of the organic residues (e.g., carbon or hydrocarbons)
P-2
5
TiO₂
GO
T
TiO₂_
Cu
Cl₂
TiO₂_
Cu₂(
Ac
)
TiO₂_
Cu
(NO₃)₂
TiO₂_
Cu₂O
GO
T_
Cu
Cl₂
GO
T_
Cu₂(
Ac
)
GO
T_
Cu
(NO₃)₂
GO
T_
Cu₂O
0
200
400
600
800
1000
Eth
an
ol yie
ld (
µm
ol g
-1)
240 m
in
Catalysts
CO2 conversion to renewable fuels 35
or inorganic ions (e.g., Cl–) that may still be present on the catalyst surface even after
calcination. These surface contaminants could react with CO2 and H2O under light
irradiation, interfering with the photoinduced reactions and influencing the
photocatalytic activity [27]. Therefore, regarding future work, Cu(NO3)2 is
recommended as precursor over a GOT material prepared separately.
4.2.1.3. Acetic acid production.
Figure 15 shows the photocatalytic formation of acetic acid (undesired product)
by the tested materials. The formation of other organic acids such as oxalic acid, lactic
acid and formic acid have been also observed, but in lower quantities. In general, all the
tested catalysts produced a considerable amount of acetic acid. These observations are
in agreement with previous results reported for Cu-TiO2 [16] where acetic acid was the
main by-product of the reaction [69].
Figure 15 - Acetic acid yield evolution in the performed experiments.
Table 4 summarizes the yields of all the byproducts determined (methanol,
ethanol and acetic acid, µmol g-1
h-1
) after 360 min of irradiation. At the end of the
reaction, TiO2_Cu(NO3)2 was the best material for the production of both methanol and
ethanol production (23.9 and 124 µmol g-1
h-1
, respectively), and the acetic acid yield
was negligible (0.870 µmol g-1
h-1
). The largest amount of undesired acetic acid was
0
20
40
60
80
0 60 120 180 240 300 360
Ac
eti
c A
cid
yie
ld (
µm
ol g
-1)
Time (min)
P25
GOT
TiO₂_CuCl₂
TiO₂_Cu₂(Ac)
TiO₂_Cu(NO₃)₂
GOT_CuCl₂
GOT_Cu₂(Ac)
GOT_Cu(NO₃)₂
CO2 conversion to renewable fuels 36
produced by P25 (8.71 µmol g-1
h-1
) probably due to the high catalytic activity and low
selectivity of this material in photocatalytic reactions.
Table 4 - Yield of the products formed.
Catalyst Methanol yield
(µmol g-1
h-1
)
Ethanol yield
(µmol g-1
h-1
)
Acetic Acid yield
(µmol g-1
h-1
)
P25 20.4 1.79 8.91
TiO2 0 0 -
GOT 22.4 50.4 3.09
TiO2_CuCl2 14.9 2.18 5.71
TiO2_Cu2(Ac) 17.7 15.4 0.710
TiO2_Cu(NO3)2 23.9 124 0.870
TiO2_Cu2O 0 0 -
GOT_CuCl2 0 0 4.74
GOT_Cu2(Ac) 22.8 12.7 8.55
GOT_Cu(NO3)2 22.1 27.7 1.40
GOT_Cu2O 0 0 -
4.3. Mechanism of reaction
For the photocatalytic reduction of CO2 with H2O over semiconductors materials,
the reaction pathway generally includes the following steps: the adsorption of reactants
on the catalyst; activation of the adsorbed reactants by the photogenerated charge
carriers; formation of surface intermediates; conversion of intermediates to products;
desorption of products from catalyst surface; and regeneration of the catalyst. Each of
these steps determines the dynamics of the reaction process and affects the final
products from CO2 conversion [27, 37].
Under the conditions used in this work, i.e alkaline solution (0.2 M NaOH), the
possibility of more hydroxyl ions serves as hole scavengers which reduces the
recombination of holes and electrons [65].
The consideration in selecting copper oxide is based on its potential redox value,
which represents their ability to attack electrons. In this case, Cu+ specie has been
reported to be more active than Cu2+
and metallic Cu0, because Cu
+ has the highest
positive potential redox value (Cu+/Cu
0 = 0.52 V). Therefore, Cu2O should effectively
act as an electron trapper to inhibit electron-hole recombination [32] .
CO2 conversion to renewable fuels 37
Various reaction schemes for the photocatalytic reduction of CO2 with H2O on
different catalysts have been proposed in literature [4, 64, 65, 68]. In the present study,
methanol, ethanol and acetic acid were the three major identified products obtained
from CO2 photoreduction, although other product as formic acid and oxalic acid, among
others, were also detected. Figure 16 shows a mechanism route proposed for the
photocatalytic reduction of CO2. The formation of methanol and ethanol can be
explained by the electrochemical Eq. 8 and Eq. 10. Moreover, the oxalic acid formation
as well as the acetic acid formation are described in Eq. 12-13 and Eq. 14, respectively.
Figure 16 - Schematic diagram showing mechanism of CO2 photoreduction on Cu-loaded
photocatalyst. Reprinted from [65].
With respect to the route of CH3OH formation, the CO2 photoreduction
mechanism in the presence of water starts with the adsorption of both reactants, CO2
and H2O leading to adsorbed species, followed by their activation by one-electron and
one-hole transfer, respectively. Positive holes (h+) can oxidize water and/or carbonic
acid to produce H+ and HO
• radical on the surface of the catalyst, as depicted in Figure
16, while the CO2 molecule together with the electrons in the conduction band (e-)
results in the formation of the CO2 radical anion (
2CO ) according to Eq. 4.
The
2CO radical anion reacts with protons and electrons forming unstable
•COOH species (Eq. 12). These chemical species has two possible ways to achieve the
stability. On one hand, the coupling of two similar •COOH species gives rise to the
formation of oxalic acid (Eq. 12-13). The formation of formic (HCOOH) and/or oxalic
CO2 conversion to renewable fuels 38
acid (HOOC-COOH) as primary products, has been previously reported by monitoring
the FTIR spectra of the solid photocatalyst [70].
Oxalic Acid formation:
COOHHCO 2 (12)
422 OHCCOOHCOOH
(13)
Acetic acid formation:
OHCOOHCHHCOOHOHCH 233
(14)
CO2 conversion to renewable fuels 39
5. Conclusions, future work and final remarks
5.1. Conclusions
Graphene oxide (GO)-TiO2 (GOT) composite and Cu-loaded (2 wt.%) GOT
composites were prepared using different Cu-precursors upon thermal treatment (500
ºC).
The presence of GO and copper in the composites increased the absorption in the
visible spectral range, enhancing the CO2 photoreduction in aqueous phase, with
formation of methanol and ethanol, as main products under near-UV/Vis light
irradiation.
High methanol yield occurred with the GOT composite comprising 4 wt.% of GO,
which was attributed to the optimal self-assembly between GO and TiO2 particles, GO
acting as electron acceptor and donor.
Among all the prepared Cu-loaded composites, TiO2_Cu(NO3)2 was the best
material for both methanol and ethanol production. This effect could be related with Cu
(I) species acting as an effective electron trapper and able to inhibit the recombination
of electron-hole pairs. The largest amount of acetic acid was produced by P25 under
near-UV/Vis light irradiation (360 min) probably due to its high catalytic activity and
low selectivity.
The lowest photocatalytic activities were obtaining using CuCl2 and Cu2O as
copper precursor, due to organic residues or inorganic ions that may still be present on
the catalyst surface even after calcination.
The mechanism of reaction was based on the formation of •COOH species that
react with the protons generated to the formation of methanol, ethanol and acetic acid,
although mechanism study is still in progress to confirm this suggestion.
In general, it seems that the selectivity towards methanol formation arises from
the presence of Cu on the photocatalysts, while the visible light photoresponse would be
introduced by the presence of GO material.
CO2 conversion to renewable fuels 40
5.2. Future work
The conversion of CO2 by photocatalysis is seen nowadays as one of the most
promising approaches to solve CO2 footprint problems, since CO2 can be reduced to
useful compounds by irradiation with solar light. To improve the photocatalytic
performance of the materials presented in this MSc Thesis, the following possible future
studies are proposed:
- To prepare photocatalysts with the impregnation method over the photocatalysts
(TiO2 and GOT), using Cu(NO3)2 as Cu precursor;
-To add other type of co-catalysts (such as Pt or Au) to the semiconductors used
(TiO2 and GOT);
-To improve the thermal treatment conditions of the catalysts in order to enhance
their photocatalytic properties;
-To increase the time of the photocatalytic reactions in order to reach a better
understanding of the evolution of the products over time;
-To use different light sources, such as LEDs;
-To reuse the catalysts in consecutive runs and analyze their photocatalytic
behaviour.
5.3. Final remarks
This research was completed with much satisfaction since it is focused on an
extremely interesting technological solution, where CO2 is used as a feedstock for the
chemical industry and transformed into a chemical valuable product.
Throughout the development of this work, the new findings can contribute to the
scientific knowledge by improving and making this technology more viable in a near
future. At the moment, every advance in this area is very relevant.
This research was an enriching experience at the academic level because it
allowed me to understand new concepts, related to the field of environmental and
chemical engineering. This study was operated in a laboratory unit of excellence, where
I overcame challenges and obstacles that allowed me to develop technical skills, which
could be useful in my future.
CO2 conversion to renewable fuels 41
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CO2 conversion to renewable fuels 47
Appendix A: Calibration curves of methanol, ethanol and acetic acid
Figure A1 - Calibration curve of methanol.
Figure A2 - Calibration curve of ethanol.
y = 16850x - 1256.4R² = 0.993
0
20000
40000
60000
80000
100000
0 1 2 3 4 5
Are
a
Methanol concentration (mM)
y = 53082x + 2277.1R² = 0.9948
0
50000
100000
150000
200000
250000
300000
0 1 2 3 4 5
Are
a
Ethanol concentration (mM)
CO2 conversion to renewable fuels 48
Figure A3 - Calibration curve of acetic acid.
y = 842228x - 17931R² = 0.9995
0
250000
500000
750000
1000000
1250000
1500000
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Are
a
Acetic Acid concentration (mM)