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Conversion of carbon dioxide via electrochemical method using a trimetallic cobalt complex
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i
Conversion of Carbon Dioxide via Electrochemical Method Using a Trimetallic
Cobalt Complex
By
Erin Joy E. Araneta
A Dissertation Presented to the
FACULTY OF THE USC CHEMISTRY GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CHEMISTRY)
May 2024
Copyright 2024 Erin Joy E. Araneta
ii
To God, you have ignited my passion to protect Your creation. You have worked through
me, and I am forever grateful to experience this joy in my life.
For you created my inmost being; you knit me together in my mother’s womb.
To Mom and Dad, thank you for supporting me in my educational path and for the
unconditional love you have given me. I couldn't have made it this far without you, and
I'm grateful for every sacrifice you've made for my education.
To my sisters and Wanda, thanks for always being there with your love and patience,
even when I'm rambling about chemistry. You're the best listeners and supporters a
person could ask for.
To my grandparents, your wisdom and love have shaped who I am today. I hope I've
done you proud.
To my amazing friends, your friendship and encouragement have been a lifeline. I'm
inspired by each of you and grateful for your role in my journey.
And Zach, you've been my rock through it all. Your love and support have kept me
grounded and am so grateful for the blessing of having you in my life.
iii
Acknowledgments
I am deeply grateful to my family for their unwavering love and support, which have
been my. Their belief in me, even in the face of failure, has been a guiding light. Wanda, you
are the sweetest little puppy. Thank you for all the fun hikes and love you have given me
throughout this time. Elaine and Erish, your presence in my life is a constant source of
inspiration, urging me to strive for excellence each day and providing the fortitude to
persevere. Mom and Dad, we cherish your selfless dedication to our well-being and your knack
for organizing rejuvenating getaways beyond words. To Grandma and Grandpa, your legacy
of love and encouragement lives on in our hearts. Mahal na mahal ko kayo.
The National Science Foundation Graduate fellowship has given me support to focus
on my research endeavors in graduate school. I have had the opportunity to work with
incredible people. I thank my advisor, Professor Smaranda Marinescu, and the Marinescu
group for teaching me chemistry. I also want to thank Dr. Jo-Hoo Lee and Dr. Shawn Wagner
for all the help given when I couldn’t detect the undetectable and have my NMRs work.
Thank you to all the people who have influenced my pre-USC career. My high-school
chemistry teachers, Ms. Hirang and Ms. Battig, thank you for sparking my love for chemistry
at a young age. My science coach, Mr. Gino Robledo, thank you for the time you spent
volunteering to educate young minds and teach them to be resilient. I am who I am today
because of Golden West College. I have to thank specifically some spectacular professors
who have impacted so much in my career: Professor Shimazu, Professor Shiroishi, Professor
Lodato, Professor Bayz, Dr. Green, Dr. Plaster, Dr. Pappano, and Dr. Egan for allowing me to
assist in teaching your class and for helping me find my path around chemistry. I also thank
Dr. Xianhui Bu and my mentor Dr. Yang for letting a nursing major synthesize complex
molecular orbital frameworks and for giving me exposure to how exciting a research lab is. I
am grateful for the Yang Lab, especially my mentors, Alissa and Tyler. Thank you for helping
me find comfort with the uncertainty by teaching me how to learn different lab techniques.
Where I am now is not possible without Professor Jenny Yang, thank you for always being so
willing to support me with all my goals.
I thank ACS, especially Jen, Carol, Sandra, Lori Ana, Natalie, Katie, & Julian, for
helping me develop my leadership skills & their support in this journey. I thank Katherine,
Leeor, Trishia, and Amy for the space they have shared with me in explaining my faith, myself,
and navigating graduate school.
I am grateful for my friends, Kim, Vivian, Assa, Yeshvi, Ashley, and Lucy. I can’t imagine
doing this degree without you, and I am eternally grateful for all the late-night study sessions,
Saturday coffee shop runs, and dinners we have shared. Thank you to my friends outside of
chemistry. Specifically, I would like to mention some special gals: Anita, Priscilla, Kara, Isa,
Nat, Carrie, Metzli, and Sydney. Thank you for trying to keep me human, for listening to my
frustrations and always sharing a great time with me. I treasure all our parties, brewery runs,
outdoor and city adventures, concerts, and screaming over Taylor Swift driving sessions.
I would like to extend my heartfelt gratitude to my second family, Denise, Jefferey,
Elijah, and the rest of the Schwartz squad. Your openness in sharing your insights on
chemistry, support in pursuing this degree, and the warmth of your welcome have meant the
world to me. I cannot overstate the pivotal role of Zachary Schwartz in both my life and
completing this thesis. Zachary, your unwavering support and encouragement have helped to
keep me motivated and focused throughout this journey. I am profoundly thankful for your
presence in my life; you have consistently challenged me to strive for excellence, both as a
person and as a scientist. I love you all so much.
iv
Table of Contents
Dedication…………………..…………………………………………………………………..…ii
Acknowledgements……………………………...………………………………………...…….iii
List of Figures ....................................................................................................................v
List of Tables .................................................................................................................. viii
Abstract............................................................................................................................. ix
Chapter 1 : Introduction .....................................................................................................1
1.1 Global Energy...........................................................................................................2
1.2 Renewable Chemistry ..............................................................................................3
1.3 Catalyst Study ..........................................................................................................5
Chapter 2 : Electrocatalytic Study of [Co(triphos)(tht)] Monometallic Cobalt Complex......7
2.1 Synthesis and Characterization................................................................................8
2.2 Cyclic Voltammetry...................................................................................................9
2.3 Controlled Potential Electrolysis .............................................................................10
Chapter 3 : [Co3(triphos)3(tht)]3+ reduction from CO2 to Formate ....................................15
3.1 Synthesis and Characterization..............................................................................16
3.2 Cyclic Voltammogram.............................................................................................20
3.3 Controlled Potential Electrolysis .............................................................................21
3.4 Future work.............................................................................................................40
Bibliography .....................................................................................................................41
v
List of Figures
Figure 1.1: Figure of the energy consumption source per year in quadrillion BTU. It is
projected that renewable energy will become the most used energy source over time. ...2
Figure 1.2: Exponential increase of carbon dioxide concentration continual increase in
parts per million. For sixty years, researchers at NOAA’s Mauna Loa Observatory in
Hawaii observed this continual increase in parts per million.3 ...........................................3
Figure 1.3: Conversion of CO2 and addition of hydrogen to produce formic acid and
methanol. ...........................................................................................................................4
Figure 1.4: CO2 undergoes the reduction process using 2e-, 4e-, or 6e- with the
standard rate potentials, in aqueous solutions. .................................................................5
Figure 1.5: Chemdraw illustrations of the monometallic [Co(triphos)(bdt)]+and
trimetallic complex of [Co3(triphos)3(tht)]3+ . .......................................................................6
Figure 2.1: 400 MHz 1H NMR spectrum of in Acetonitrile-d6.............................................9
Figure 2.2: CV of [Co(triphos)(BDT)][BF4] Phenol titration and -2.15V with
ferrocene reference in CO2. The working and counter electrode is glassy carbon. .......10
Figure 2.3: Controlled potential electrolysis of 0.45M [Co(triphos)(BDT)][BF4] with
0.5M Phenol and -2.15V with ferrocene reference. Reference Electrode is Ag wire.
The working and counter electrode is glassy carbon.......................................................11
Figure 2.4: 0.45M [Co(triphos)(BDT)][BF4] in acetonitrile and silver reference
electrode, glassy carbon working electrode, and platinum counter electrode. ................12
Figure 2.5: BE of 0.45 mM [Co(triphos)(BDT)][BF4] with 0.5M TFE, -2.15V potential vs
ferrocene, AgNO3 reference electrode, carbon working and counter electrode. .............13
Figure 2.6: 1HNMR in D2O of the working solution of the Bulk electrolysis form the
0.5M TFE with [Co(triphos)(BDT)][BF4] in MeCN. ...........................................................14
Figure 3.1: Synthesis scheme of [Co3(triphos)3(tht)]3+ .....................................................17
Figure 3.2: Trimetallic complex [Co3(triphos)3(tht)]3+ .......................................................18
Figure 3.3: 1H-NMR of the filtrate in d-DMSO after washing with 750mL THF................18
Figure 3.4: 31P-NMR of the filtrate in d-DMSO after washing with 700mL THF...............19
vi
Figure 3.5: 19F-NMR of the filtrate in d-DMSO after washing with 700mL THF. ..............20
Figure 3.6: 0.45M [Co3(triphos)3(THT)][BF4]3 in acetonitrile and silver reference
electrode, glassy carbon working electrode, and platinum counter electrode. ................21
Figure 3.7: a) Controlled potential electrolysis of the first 30 minutes and b) controlled
potential electrolysis of the following 30 minutes. 0.45mM [Co3(triphos)3(THT)][BF4]3
with AgNO3 reference, Glassy Carbon WE and CE, MeCN solvent and no acid
source,-2.1V. ...................................................................................................................23
Figure 3.8: Working electrode solution in a 1:10 dilution in D2O in a 400 MHz
1H NMR............................................................................................................................24
Figure 3.9: A) The complex was subjected to cyclic voltammetry under nitrogen, with
carbon dioxide, and after conducting controlled potential electrolysis. B)
Controlled potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4] 3+ with no acid
and -2.1V with ferrocene reference. Reference Electrode is Ag wire. The working and
counter electrode is glassy carbon. .................................................................................25
Figure 3.10: A) After performing the controlled potential electrolysis, we observed
the cyclic voltammogram of the complex under nitrogen, with carbon dioxide. B)
Controlled potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]3 with 0.3M TFE
and -2.1V with ferrocene reference. Reference Electrode is Ag wire. The working and
counter electrode is glassy carbon. .................................................................................27
Figure 3.11: Working electrode solution in a 1:10 dilution in D2O in a 400 MHz
1H NMR............................................................................................................................28
Figure 3.12: A) Under nitrogen and with carbon dioxide, the complex exhibited a cyclic
voltammogram, followed by the performance of controlled potential electrolysis. B)
Controlled potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]3
with 0.3M TFE
and -2.6V with ferrocene reference. Reference Electrode is Ag wire. The working and
counter electrode is glassy carbon. .................................................................................30
Figure 3.13: A) The complex was subjected to cyclic voltammetry under nitrogen,
with carbon dioxide, and after performing the controlled potential electrolysis. B)
Controlled potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]3
with 0.1M H2O
and -2.6V with ferrocene reference. Reference Electrode is Ag wire. The working and
counter electrode is glassy carbon. The absence of formate is indicated by 1H NMR,
but a peak at 3.0 ppm suggests the presence of oxalate. In the future, we can improve
this 1H NMR by using a 1:10 dilution. ..............................................................................32
vii
Figure 3.14: Working electrode solution in a 2:10 dilution in D2O in a 400 MHz
1H NMR............................................................................................................................33
Figure 3.15: A) The complex was subjected to cyclic voltammetry under nitrogen,
with carbon dioxide, and after performing the controlled potential electrolysis. B)
Controlled potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4] 3+ with 0.3M H2O
and -2.1V with ferrocene reference. Reference Electrode is Ag wire. The working and
counter electrode is glassy carbon. .................................................................................34
Figure 3.16: A) Cyclic voltammogram of the complex under nitrogen, with carbon
dioxide, and after the controlled potential electrolysis was performed. B) Controlled
potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]
3+ with 0.3M H2O and -2.1V
with ferrocene reference. Reference Electrode is Ag wire. The working and counter
electrode is glassy carbon. 1H NMR shows no sign of formate and oxalate....................36
Figure 3.17: Working electrode solution in a 2:10 dilution in D2O in a 400 MHz
1H NMR...........................................................................................................................37
Figure 3.18: A) The complex underwent cyclic voltammetry under nitrogen, with carbon
dioxide, and after performing controlled potential electrolysis. B) Controlled potential
electrolysis of 0.45M [Co3(triphos)3(THT)][BF4] 3+ with 0.3M H2O and -2.6V with
ferrocene reference. Reference Electrode is Ag wire. The working and counter
electrode is glassy carbon. 1H NMR shows signs of oxalate at 3.2 ppm and a small
formate peak at 7.0 ppm..................................................................................................38
Figure 3.19: Working electrode solution in a 2:10 dilution in D2O in a 400 MHz
1H NMR............................................................................................................................40
viii
List of Tables
Table 2.1: Faradaic efficiency results phenol with 0.45M [Co(triphos)(BDT)][BF4] with
0.5M Phenol and -2.15V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................11
Table 2.2: Faradaic efficiency results of phenol with 0.45M [Co(triphos)(BDT)][BF4]
with 0.5M TFE and -2.15V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................14
Table 3.1: Faradaic efficiency results of TFE with 0.45M [Co3(triphos)3(THT)][BF4]3
with 0M acid and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................24
Table 3.2: Faradaic efficiency results of H2O with 0.45M[Co3(triphos)3(THT)][BF4]3
with no acid and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................26
Table 3.3: Faradaic efficiency results of TFE with 0.45M [Co3(triphos)3(THT)][BF4]3
with 0.3M TFE and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................28
Table 3.4: Faradaic efficiency results of TFE with 0.45M [Co3(triphos)3(THT)][BF4]3
with 0.3M TFE and -2.6V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................30
Table 3.5: Faradaic efficiency results of H2O with 0.45M [Co3(triphos)3(THT)][BF4]3
with 0.1M H2O and -2.6V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................32
Table 3.6: Faradaic efficiency results of H2O with 0.45M[Co3(triphos)3(THT)][BF4]3
with no acid and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................35
Table 3.7: Faradaic efficiency results of H2O with 0.45M [Co3(triphos)3(THT)][BF4]3 3+
with 0.1M H2O and -2.6V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2...........................................................................................39
ix
Abstract
Sustainable chemistry entails devising solutions to halt global warming and mitigate
its environmental impact. Human activity has been the primary driver of the increased CO2
levels in the atmosphere over the past 150 years, primarily due to the utilization of fossil
fuels. The burning of coal and natural gas by power plants, refineries, and industrial
manufacturers is chiefly responsible for the significant release of CO2. While efforts to
convert and capture carbon dioxide have been ongoing, the technology remains immature.
This thesis focuses on carbon capture through electrochemistry. A cobalt
phosphino-thiolate complex developed by our group has demonstrated promising results,
achieving a conversion rate as high as 91% with H2O and maintaining a stable current for
over 8 hours. However, it was observed to have a considerable overpotential of 750 mV.
To address this issue, we are investigating a trimetallic complex species in an attempt to
reduce the overpotential. The presence of three cobalt atoms facilitates electron
distribution through conjugation within the complex, potentially aided by the conjugation of
THT, leading to a decreased overpotential.
In this thesis, cyclic voltammetry experiments were conducted to identify optimal
conditions in terms of acid type, concentration, and solvent source. Additionally, controlled
potential electrolysis (CPE) experiments were performed to analyze the catalyst's
selectivity, stability, and activity under varying conditions. This work delves into CPE
studies involving different acid concentrations and potentials to gain insights into the
selectivity, activity, and stability of the catalyst.
1
Chapter 1 : Introduction
2
1.1 Global Energy
Climate change is reshaping our future. Energy consumption has been on the rise
and an increasing demand requires the need for more energy sources. In 2019, the
primary sources of energy consumption is petroleum and natural gas. This poses issues
to our society because petroleum and natural gasses are finite.1 Petroleum and natural
gas also produce an excess of CO2 which leads to problematic results such as global
warming. Contrary to this, it is important to find solutions to store energy.2 Scientists are
exploring using molecular bonds to do this.
Figure 1.1: Figure of the world energy consumption source per year in quadrillion BTU. It
is projected that renewable energy will become the most used energy source over time.1
3
Figure 1.2: Exponential increase of carbon dioxide concentration continual increase in
parts per million. For sixty years, researchers at NOAA’s Mauna Loa Observatory in
Hawaii observed this continual increase in parts per million.3
1.2 Renewable Chemistry
Renewable chemistry is an important concept to understand providing an alternate
source of energy to the growing population that the world is facing and the environmental
impact of fossil fuels. Converting carbon dioxide to formic acid and methanol is extremely
beneficial due to easier storage, safety, and practical use. 5
4
Figure 1.3: Conversion of CO2 and addition of hydrogen to produce formic acid and
methanol.4
Carbon dioxide is a molecule of interest not only for sustainability purposes and to
lower carbon dioxide emissions. Different reactions can reduce carbon dioxide with 2, 4,
6, and 8 electrons to produce a variety of different desired products. For instance, formic
acid is a product of the 2-electron reaction and methane for the full eight-electron
reduction. However, researchers have observed that aqueous solutions exhibit high
reduction potentials, which is why there is an advantage in developing catalysts that can
lower the reduction potential. Additionally, exploring different catalysts will allow for the
design of unique pockets where CO2 can bind. 8
5
Figure 1.4: CO2 undergoes the reduction process using 2e-, 4e-, or 6e- with the standard
rate potentials, in aqueous solutions.8
This phenomenon is something that plants in the periplasm or cytoplasm already
are performing using iron [FeFe]-hydrogenase is used to convert CO2 to CO or HCOO-
.
7
A redox-active complex in the form of FeS was studied in CO dehydrogenase and
molybdopterin ligand, which stabilizes the reaction intermediates through electron
delocalization. 2,6
1.3 Catalyst Study
It is necessary to perform a fundamental study of the catalyst to optimize the
conditions and bring about improvements. The Mougel group conducted a study on cobalt
and achieved a 64% faradaic efficiency with E½ as -1.93 V. However, we observed a large
overpotential of 750 mV. Our group is studying a trimetallic complex species in an effort to
lower the overpotential. The presence of three cobalt distributes the electrons through
6
conjugation within the complex, along with the conjugation of THT may lead to a lower
overpotential. We performed cyclic voltammograms to observe optimal conditions in acid
type, concentration, and solvent source. To understand selectivity, stability, and activity in
various conditions, we conducted controlled potential electrolysis (CPE) experiments. This
work explores CPE studies with varying acid concentrations of acid and different potentials
to understand the selectivity, activity, and stability of the catalyst.
Figure 1.5: Chemdraw illustrations of the monometallic [Co(triphos)(bdt)]+and trimetallic
complex of [Co3(triphos)3(tht)]3+.
8
7
Chapter 2 : Electrocatalytic Study of [Co(triphos)(tht)]
Monometallic Cobalt Complex
8
2.1 Synthesis and Characterization
[Co(triphos)(tht)]+ was synthesized in a glove box by adding a 1:1 equivalence of cobalt
chloride (0.0014 moles) dissolved in 25mL of ethanol and 1,1-
tris(diphenylphosphinomethyl)ethane (triphos) (0.0014 moles), along with 20mL of
acetone in a 250mL schlenk flask. Then, we mixed the solutions for one hour to produce
Co(triphos)Cl2, a deep red color. The flask is taken out of the glove box and then brought
to the Schenk line. On a new 100 ml Schlenk flask taken to the glove box, dissolve NatBuO
in 20mL of dry methanol and add BDT. Bring this solution to a Schlenk line and cannula
transfer this solution to the deep red Co(triphos)Cl2 solution dropwise. Leave this to mix
overnight to make Co(triphos)(BDT) + 2NaCl. After mixing overnight, take the Schlenk
flask with mixing Co(triphos)(BDT) + 2NaCl to the glove box. Decant the solution by
filtering it in vaccuo and wash with water three times to remove the NaCl product. Wash
the product with ethanol to remove unreacted reagents and starting materials. Wash the
product with pentane to remove and dry the organics. To oxidize Co(triphos)(BDT),
dissolve Co(triphos)(BDT) and FeBF4 in a 1:1 molar equivalent in dichloromethane for two
hours. We used the round bottom flask to crash out hexanes until we removed all the
ferrocene. After that, we transfer the products to the fine frit and prepare a recrystallization
chamber with MeCN/Et2O. 1H NMR (500 MHz, DMSO-d6) δ 8.29 (m, 2H, C6H4S2), δ 7.55
(m, 2H, C6H4S2), 7.26-7.05 (m, 30H, P(C6H5)2), 2.95 (s, 6H, PCH2), 1.98 (s, 3H, CH3)
9
Figure 2.1: 400 MHz 1H NMR spectrum of in Acetonitrile-d6.
2.2 Cyclic Voltammetry
We synthesized [Co(triphos)(BDT)][BF4] using the reported literature procedure in
Chapter 2.1. In phenol and CO2 atmosphere, we performed Cyclic Voltammetry studies
of [Co(triphos)(BDT)][BF4] . We observed that the current density peaked at 0.5M. We
observed a change in shape at 0.7M and 1.2M phenol. This change may be because of
more products being formed that are not of interest, such as hydrogen. Based on this
information, we conducted a controlled potential electrolysis experiment to understand
the stability of the complex in the condition of 0.5M phenol, due to the threefold increase
from 0.2M PhOH to 0.5M PhOH. During the controlled potential electrolysis, a stable
current ranging from -5 to -5.5 mA was observed (Figure 1.3).
10
Figure 2.2: CV of [Co(triphos)(BDT)][BF4] Phenol titration and -2.15V with ferrocene
reference in CO2. The working and counter electrode is glassy carbon.
2.3 Controlled Potential Electrolysis
We performed a controlled potential electrolysis to evaluate the catalyst’s stability under
different acid conditions and currents, to comprehend its stability and the current density.
Additionally, we obtained an overview of the quantitative analysis of the product over a
certain period.
11
Figure 2.3: Controlled potential electrolysis of 0.45M [Co(triphos)(BDT)][BF4] with 0.5M
Phenol and -2.15V with ferrocene reference. Reference Electrode is Ag wire. The
working and counter electrode is glassy carbon.
Table 2.1: Faradaic efficiency results phenol with 0.45M [Co(triphos)(BDT)][BF4] with
0.5M Phenol and -2.15V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under
an atmosphere of CO2.
12
Cyclic voltammogram studies are performed to analyze how the addition of TFE can affect
the current density of the experiment with the [Co(triphos)(BDT)][BF4] and
tetrafluoroethylene (TFE). Based on the performed cyclic voltammogram, we observe that
the shape of the cyclic voltammogram remains consistent after 0.5M to 0.9M.
Observations from Figure 1.4 indicate an increase in current density at 1.2M.
Figure 2.4: 0.45M [Co(triphos)(BDT)][BF4] in acetonitrile and silver reference electrode,
glassy carbon working electrode, and platinum counter electrode.
Based on the CV, it is determined that 0.5M of TFE is where the acid concentration started
reaching a constant current density. Therefore, the controlled potential electrolysis was
13
performed at 0.5M TFE. The cyclic voltammogram studies have shown a stable current
being pulled from -1 to -1.1 mA with a lower current density at the end.
Figure 2.5: BE of 0.45 mM [Co(triphos)(BDT)][BF4] with 0.5M TFE, -2.15V potential vs
ferrocene, AgNO3 reference electrode, carbon working and counter electrode.
After this bulk electrolysis experiment, we measured a 33% faradaic efficiency. The 0.5M
TFE experiment with 0.45mM [Co(triphos)(BDT)][BF4] observed a 33% faradaic efficiency.
Observations indicate faradaic efficiencies of 16% for hydrogen, 16% for carbon
monoxide, 9% for formate, and 7% for oxalate. In addition, we used nuclear magnetic
resonance spectroscopy to investigate the production of formate or oxalate. However, due
to the low faradaic efficiency and charge in the system, there is no detectable oxalate and
formate in the 1HNMR (Figure 2.6).
14
Table 2.2: Faradaic efficiency results of phenol with 0.45M [Co(triphos)(BDT)][BF4] with
0.5M TFE and -2.15V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
Figure 2.6: 1HNMR in D2O of the working solution of the Bulk electrolysis form the 0.5M
TFE with [Co(triphos)(BDT)][BF4] in MeCN.
15
Chapter 3 : [Co3(triphos)3(tht)]3+ reduction from CO2 to Formate
16
3.1 Synthesis and Characterization
The trimetallic complex, [Co3(triphos)3(tht)]3+, was used for the possibility of increasing
catalytic activity and lower overpotential. The triphenylene-2,3,6,7,10,11-hexathiol (THT)
ligand is unique due to its strong conjugation and its contribution to a significant electronic
conjugation between cationic cobalt.
To synthesize [Co3(triphos)3(tht)]3+, the initial step involved synthesizing
[Co(CH3CN)6][BF4]2 in a dry grove box. To achieve this, we dissolved 503 mg of NOBF4
(12.841 mmol) in 40 mL of MeCN and then added 756.74 mg of cobalt (4.2805 mmol).
The resulting mixture was thoroughly stirred inside the glove box before being exposed to
a vacuum until a vigorous bubbling was observed. Subsequently, the flask underwent five
cycles of backfilling and vigorous vacuuming. The flask was then left under a static vacuum
for 12 hours with continuous stirring. The solution was concentrated to 10 mL through a
vacuum the following day, and the liquid was carefully transferred via cannula to a new
flask. Simultaneously, 50 mL of cold, dry diethyl ether was cannulated into the new flask
containing the 10 mL solution, with the new flask being chilled in an ice bath. After this
step, we subjected the flask to vacuum drying. Finally, the synthesized [Co(CH3CN)6][BF4]2
was stored in a dry box for further use. We brought 620mg of [Co(CH3CN)6][BF4]2 (1.29
mmol) to the wet glove box and added 582 mg 1,1,1-tris(diphenylphosphinomethyl)ethane
(triphos) (1.24 mmol) to the flask along with 50mL of acetonitrile. This was left to stir for 2
hours until the solution was lime green. 180 mg of THT (0.428 mmol) was added after 2
hours, and then the solution turned wine muddy red. The solution was left to mix overnight
17
and then taken out of the glove box. The solution was bubbled in syngas for 20 minutes
with a syringe to oxidize the complex and this should turn the solution blue. This solution
is filtered through a fine frit with green powder on the frit. The filtrate is saved and dried
out and the product is collected and washed with 750mL of THF. This is recrystallized by
vapor diffusion for a week using acetonitrile and diethyl ether. The yield was measured at
73%. 1H NMR (500 MHz, DMSO-d6): δ 9.93 (s, 2H, C18H6S6), 7.29 (t, 6H, P(C6H5)2), 7.18
(m, 12H, P(C6H5)2)), 7.10 (t, 12H, P(C6H5)2), 2.98 (s, 6H, PCH2), 2.00 (s, 3H, CH3). 31P-
{
1H} NMR (202 MHz, DMSO-d6): δ 33.8 (br s). 19F-{
1H} NMR (470 MHz, DMSO-d6):
Figure 3.1: Synthesis scheme of [Co3(triphos)3(tht)]3+
18
Figure 3.2: Trimetallic complex [Co3(triphos)3(tht)]3+
Figure 3.3: 1H-NMR of the filtrate in d-DMSO after washing with 750mL THF.
19
Figure 3.4: 31P-NMR of the filtrate in d-DMSO after washing with 700mL THF.
20
Figure 3.5: 19F-NMR of the filtrate in d-DMSO after washing with 700mL THF.
3.2 Cyclic Voltammogram
During the cyclic voltammogram studies of [Co3(triphos)3(THT)][BF4]3, we determined that
the optimal concentration is at 0.3M H2O in DMF and CO2 atmosphere. We observed a
reversible voltammogram and a peak at -2.5 V in order to study the effect of TFE on H2O
in a DMF solvent and atmosphere. Figure 3.5 shows that there is a significant gap of 0.2
between 0.3M and 0.5M H2O, indicating a potential hydrogen evolution. We chose 0.3M
of H2O for a controlled potential electrolysis study to determine if it is the optimal water
concentration and to observe the products of the reaction.
21
Figure 3.6: 0.45M [Co3(triphos)3(THT)][BF4]3 in acetonitrile and silver reference
electrode, glassy carbon working electrode, and platinum counter electrode.
3.3 Controlled Potential Electrolysis
We performed controlled potential electrolysis experiments to gain a better
understanding of the current and faradaic efficiency of the reaction. Controlled potential
electrolysis started with 0M acid and 0.45mM [Co3(triphos)3(THT)][BF4]3 with Ag
reference, Glassy Carbon WE and CE, MeCN solvent and no acid source, -2.1V V vs
Fc/Fc+. The current seems highly stable until it starts going down after 30 minutes. We
observe that this reaction has a 0.33% faradaic efficiency for hydrogen and 0% faradaic
efficiency for CO. We performed 1H NMR experiments to investigate the possibility of
formate and oxalate, but we did not observe any evidence of these traces in the 1H NMR
22
spectra. (Figure 3.8) Table 3.1 shows the faradaic efficiency of this experiment, and a
30% faradaic efficiency of formate is present along with a 20% faradaic efficiency in
oxalate. This may be because of hydrogen in the solvent or a machine error in the ion
chromatography.
A.
23
B.
Figure 3.7: a) Controlled potential electrolysis of the first 30 minutes and b) controlled
potential electrolysis of the following 30 minutes. 0.45mM [Co3(triphos)3(THT)][BF4]3 with
AgNO3 reference, Glassy Carbon WE and CE, MeCN solvent and no acid source, -2.1V.
24
Figure 3.8: Working electrode solution in a 1:10 dilution in D2O in a 400 MHz 1H NMR
Table 3.1: Faradaic efficiency results of TFE with 0.45M [Co3(triphos)3(THT)][BF4]3
with
0M acid and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
The experiment was repeated, using only Ag and not AgNO3.
25
A.
B.
Figure 3.9: A) The complex was subjected to cyclic voltammetry under nitrogen, with
carbon dioxide, and after conducting controlled potential electrolysis. B) Controlled
26
potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4] 3+ with no acid and -2.1V with
ferrocene reference. Reference Electrode is Ag wire. The working and counter electrode
is glassy carbon.
Table 3.2: Faradaic efficiency results of H2O with 0.45M[Co3(triphos)3(THT)][BF4]3 with
no acid and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
After obtaining information on the reaction of [Co3(triphos)3(THT)][BF4]3 without an acid
source, we conducted experiments with an acid source to observe how hydricity can
impact the faradaic efficiency of the controlled potential electrolysis (Figure 3.10). We
conducted experiments using 0.3M of TFE at a -2.1 V potential for 30 minutes.
27
A.
B.
Figure 3.10: A) After performing the controlled potential electrolysis, we observed the
cyclic voltammogram of the complex under nitrogen, with carbon dioxide. B) Controlled
28
potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]3 with 0.3M TFE and -2.1V with
ferrocene reference. Reference Electrode is Ag wire. The working and counter electrode
is glassy carbon.
Table 3.3: Faradaic efficiency results of TFE with 0.45M [Co3(triphos)3(THT)][BF4]3
with
0.3M TFE and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
Figure 3.11: Working electrode solution in a 1:10 dilution in D2O in a 400 MHz 1H NMR.
29
Unusual high of formate at 96% and oxalate at 45% may be due to a machine error and
experiments will be conducted to repeat this experiment with dry solvents under sieves.
Under the same conditions as Figure 3.9, we conducted experiments in -2.6 V. -2.6 V is
the point where the current goes up. In -2.6 V, it is also observed that the current is
lower, but a higher charge was generated.
A.
30
B.
Figure 3.12: A) Under nitrogen and with carbon dioxide, the complex exhibited a cyclic
voltammogram, followed by the performance of controlled potential electrolysis. B)
Controlled potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]3
with 0.3M TFE and -
2.6V with ferrocene reference. Reference Electrode is Ag wire. The working and counter
electrode is glassy carbon.
Table 3.4: Faradaic efficiency results of TFE with 0.45M [Co3(triphos)3(THT)][BF4]3 with
0.3M TFE and -2.6V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
31
We also investigated water as an acid source. 0.45mM [Co3(triphos)3(THT)][BF4]3 in
MeCN under CO2 and 0.1M TBAPF6 with an Ag reference, glassy carbon working and
counter electrode in a 30-minute electrolysis at -2.6V. Water as an acid source showed a
current at around -3 mA and 4.96 C. In the previous experiment, we observed an
unusually high amount of formate and oxalate at 178.15% formate and 145.5% oxalate.
This could be attributed to the solvent containing nitrogen or the solution being left to
wait for a while before taking an ion chromatography sample.
A.
32
B.
Figure 3.13: A) The complex was subjected to cyclic voltammetry under nitrogen, with
carbon dioxide, and after performing the controlled potential electrolysis. B) Controlled
potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]3
with 0.1M H2O and -2.6V with
ferrocene reference. Reference Electrode is Ag wire. The working and counter electrode
is glassy carbon. The absence of formate is indicated by 1H NMR, but a peak at 3.0 ppm
suggests the presence of oxalate. In the future, we can improve this 1H NMR by using a
1:10 dilution.
Table 3.5: Faradaic efficiency results of H2O with 0.45M [Co3(triphos)3(THT)][BF4]3 with
0.1M H2O and -2.6V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
33
Figure 3.14: Working electrode solution in a 2:10 dilution in D2O in a 400 MHz 1H NMR.
34
Figure 3.15: A) The complex was subjected to cyclic voltammetry under nitrogen, with
carbon dioxide, and after performing the controlled potential electrolysis. B) Controlled
35
potential electrolysis of 0.45M [Co3(triphos)3(THT)][BF4] 3+ with 0.3M H2Oand -2.1V with
ferrocene reference. Reference Electrode is Ag wire. The working and counter electrode
is glassy carbon.
Table 3.6: Faradaic efficiency results of H2O with 0.45M[Co3(triphos)3(THT)][BF4]3 with
no acid and -2.1V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
We repeated the experiments without adding NO3 around the Ag wire.
A.
36
B.
Figure 3.16: A) Cyclic voltammogram of the complex under nitrogen, with carbon dioxide,
and after the controlled potential electrolysis was performed. B) Controlled potential
electrolysis of 0.45M [Co3(triphos)3(THT)][BF4]
3+ with 0.3M H2O and -2.1V with ferrocene
reference. Reference Electrode is Ag wire. The working and counter electrode is glassy
carbon. 1H NMR shows no sign of formate and oxalate.
37
Figure 3.17: Working electrode solution in a 2:10 dilution in D2O in a 400 MHz 1H NMR.
Experiments were performed in -2.6V in the same conditions as Figure 1.17. -2.6V is the
point where the current starts going up. In -2.6V, we also observed that the current is
lower, but a higher charge is generated.
38
Figure 3.18: A) The complex underwent cyclic voltammetry under nitrogen, with carbon
dioxide, and after performing controlled potential electrolysis. B) Controlled potential
39
electrolysis of 0.45M [Co3(triphos)3(THT)][BF4] 3+ with 0.3M H2O and -2.6V with
ferrocene reference. Reference Electrode is Ag wire. The working and counter electrode
is glassy carbon. 1H NMR shows signs of oxalate at 3.2 ppm and a small formate peak at
7.0 ppm.
Table 3.7: Faradaic efficiency results of H2O with 0.45M [Co3(triphos)3(THT)][BF4]3 3+
with 0.1M H2O and -2.6V in a solution of acetonitrile containing 0.1 M [nBu4N][PF6] under
an atmosphere of CO2.
40
Figure 3.19: Working electrode solution in a 2:10 dilution in D2O in a 400 MHz 1H NMR.
3.4 Future work
To progress the experiment, we plan to repeat the controlled potential experiments in 0.3M
and 0.1M with both TFE or H2O for [Co3(triphos)3(THT)][BF4]
3+ using a clean solvent.
Because the catalyst decays after thirty minutes of electrolysis, we will explore pulse
electrolysis of the [Co3(triphos)3(THT)][BF4]
3+ complex to see if we can regenerate it. It will
also be interesting to understand how the catalyst looks like before and after electrolysis,
so taking an NMR will be advantageous.
41
Bibliography
1) U.S. Energy Information Administration, Annual Energy Outlook 2020
2) The atmosphere: Getting a handle on carbon dioxide. Nasa.gov
3) A. M. Appel, J. E. Bercaw, A. B. Bocarsly, H. Dobbek, D. L. Dubois, M. Dupuis, J.
G. Ferry, E. Fujita, R. Hille, P. J. A. Kenis, C. A. Kerfeld, R. H. Morris, C. H. F.
Peden, A. R. Portis, S. W. Ragsdale, T. B. Rauchfuss, J. N. H. Reek, L. C. Seefeldt,
R. K. Thauer and G. L. Waldrop, Chem. Rev., 2013, 113, 6621–6658.
4) Sordakis, K.; Tang, C.; Vogt, L. K.; Junge, H.; Dyson, P. J.; Beller, M.; Laurenczy,
Chem. Rev. 2018, 118 (2), 372–433.
5) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.;
Jaramillo, T. F. Science 2017, 355 (6321).
6) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Chem. Rev. 2014, 114, 4081.
7) Vignais, P. M.; Billoud, B.; Meyer, J. FEMS Microbiol. Rev. 2001, 25 (4), 455–501.
8) Francke, R.; Schille, B.; Roemelt, M. Chem. Rev. 2018, 118 (9), 4631–4701
9) Mougel, V. et al. Angew. Chem. 2020, 59, 15726.
10)J. A. Intrator, N. M. Orchanian, A. J. Clough, R. Haiges and S. C. Marinescu, Dalt.
Trans., 2022, 51, 5660–5672.
Abstract (if available)
Abstract
Sustainable chemistry entails devising solutions to halt global warming and mitigate its environmental impact. Human activity has been the primary driver of the increased CO2 levels in the atmosphere over the past 150 years, primarily due to the utilization of fossil fuels. The burning of coal and natural gas by power plants, refineries, and industrial manufacturers is chiefly responsible for the significant release of CO2. While efforts to convert and capture carbon dioxide have been ongoing, the technology remains immature.
This thesis focuses on carbon capture through electrochemistry. A cobalt phosphino-thiolate complex developed by our group has demonstrated promising results, achieving a conversion rate as high as 91% with H2O and maintaining a stable current for over 8 hours. However, it was observed to have a considerable overpotential of 750 mV. To address this issue, we are investigating a trimetallic complex species in an attempt to reduce the overpotential. The presence of three cobalt atoms facilitates electron distribution through conjugation within the complex, potentially aided by the conjugation of THT, leading to a decreased overpotential.
In this thesis, cyclic voltammetry experiments were conducted to identify optimal conditions in terms of acid type, concentration, and solvent source. Additionally, controlled potential electrolysis (CPE) experiments were performed to analyze the catalyst's selectivity, stability, and activity under varying conditions. This work delves into CPE studies involving different acid concentrations and potentials to gain insights into the selectivity, activity, and stability of the catalyst.
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Araneta, Erin Joy Esposo
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Conversion of carbon dioxide via electrochemical method using a trimetallic cobalt complex
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Chemistry
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2024-05
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catalyst activity
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