Utilizing Surface Enhanced Raman Spectroscopy to Monitor the Carbon Dioxide Electro‐Reduction Reaction on Copper at Low Overpotentials

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THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Katherine Ann Muhlenkamp

Graduate Program in Chemistry

The Ohio State University

2015

Master's Examination Committee:

Dr. Anne Co, Advisor

Dr. Terry Gustafson

Katherine Ann Muhlenkamp

2015

Abstract

Carbon dioxide is a common byproduct of industrial processes and the burning of fossil

fuels. It is also a greenhouse gas that is contributing to global warming. Research is

currently being done on ways to optimize the electro-reduction reaction of carbon dioxide

to energy rich fuels that can be recycled, making a much more sustainable energy source.

Copper metal is known to be the only metal to catalyze the electro-reduction of carbon

dioxide to hydrocarbons1, the most desired products possible from the carbon dioxide

electro-reduction. While the products of this reaction are often studied to determine the efficiencies of different copper catalysts, it is still unknown what the intermediate species that form on the copper surface are. In Situ Surface Enhanced Raman Scattering (SERS) was implemented to determine the species that are absorbed onto the metal catalyst surface during this electro-reduction of carbon dioxide. By correlating these intermediates to the possible species adsorbed, a better understanding of the mechanism of the carbon dioxide electro-reduction on copper can be obtained.

Dedication

This document is dedicated to my family and friends. They have supported me on the

days when everything went perfectly and the days that nothing went right.

“Research is formalized curiosity. It is poking and prodding with a purpose.”

-Zora Neale Hurston

“Research is what I am doing when I don’t know what I am doing”

-Wernher Von Braun

iii

Acknowledgments

I would like to thank Dr. Anne Co for her guidance and patience throughout the process of this research. Thanks to Dr. Asthagiri for his work on performing the DFT calculations. I would like to thank Kwan Leung for her willingness to obtain SEM images for this work and her willingness to listen to my frustrations when things weren’t working. I would like to thank Chibuokem Amuneke-Nze for teaching me all the skills to do this work, performing the foundation work for this research, and for making me laugh on the long nights of experiments. I would also like to thank all the members of the Co group, past and present, that have helped in the endless number of small ways to make me the person I am and the research I performed possible; from listening to my snoring after a night of experiments to helping me learn my way around the lab and OSU to helping prepare samples. I would also like to thank The Ohio State University

Department of Chemistry and Biochemistry for funding this research.

Vita

May 2008 ...... St. Marys Memorial High School

2012...... B.S. Chemistry, Ohio Dominican University

2012 to present ...... Graduate Teaching Associate, Department

of Chemistry, The Ohio State University

Publications

Hogarth, Lewis and Muhlenkamp, Katherine "An Unusual and Simply Prepared

Compound of Copper" Chem 13 News. Nov 2012. p. 6-9

Fields of Study

Major Field: Chemistry

Table of Contents

Abstract ...... ii

Dedication ...... iii

Acknowledgments...... iv

Vita ...... v

List of Tables ...... viii

List of Figures ...... ix

Chapter 1: Introduction and Background ...... 1

1.1 Energy Concerns ...... 1

1.2 Carbon Dioxide ...... 3

1.3 CO2 Electro-Reduction ...... 6

1.4 Raman and SERS ...... 11

1.5 Literature Review ...... 14

Chapter 2: Experimental ...... 18

2.1 Catalyst Preparation ...... 18

2.2 Solution Preparation ...... 18

2.3 Characterization ...... 19

2.4 In Situ electro-spectroscopy ...... 19

Chapter 3: Results and Discussion ...... 22

3.1 Roughening technique comparison ...... 22

3.2 SERS with pyridine ...... 24

3.4 Peak Assignments ...... 43

3.5 C-13 vs. C-12 Spectra ...... 45

Chapter 4: Conclusion...... 49

References ...... 51

vii

List of Tables

1 Table 1: The products of the CO2 electro-reduction process on different metals...... 7

Table 2: Possible Raman peak assignments from literature...... 40

viii

List of Figures

Figure 1: The total world energy supply by fuel.2 ...... 2

Figure 2: The world energy consumption by fuel.2 ...... 3

2 Figure 3: The measure of CO2 emission by fuels...... 5

7 Figure 4: The products of the CO2 electro-reduction on copper at different potentials. ... 8

Figure 5: Percent efficiency of CO2 electro-reduction products at -0.45 V vs. RHE with

CuSO4 r Cu foil as the catalyst...... 9

Figure 6: Proposed CO2 electro-reduction mechanism at low overpotentials based on

DFT calculations.16 ...... 10

Figure 7: An energy diagram depicting the different types of scattering...... 12

Figure 8: Image of the in situ cell used for electro-spectroscopy...... 20

Figure 9: Block diagram of in situ Raman set-up. CO2 is bubbled into the 0.1 M

KHCO3/0.01 M KCl solution that is pumped into the flow cell before flowing into waste.

The potentiostat is connected to the flow cell. The flow cell is inside the Raman

microprobe to perform in situ measurements...... 21

Figure 10: (a) SEM of CuSO4 r Cu (0.36 M H2SO4/0.2 M CuSO4 solution, Eapplied=0.711

V vs. Ag/AgCl, 120 sec potential hold, 295 K), (b) SEM of KCl r Cu (0.1 M KCl, 20 mV/s CV from -0.481 to 0.149 V vs. Ag/AgCl, 50 positive to negative scans)...... 22

Figure 11: Raman spectra of roughened copper foil comparing both KCl and CuSO4

roughening techniques. Spectra measured in air...... 23 ix

Figure 12: Raman spectra of pyridine adsorbed onto the roughened Cu (raw intensities) for both roughening techniques...... 24

Figure 13: The shift observed when the pyridine probe is adsorbed onto the roughened Cu

foil. The neat pyridine has peaks at 990 cm-1 and 1030 cm-1. While the roughened Cu

only shows a peak at 1018 cm-1...... 25

Figure 14: Raman spectra at several times over a potential hold of -0.4 V after a -0.5 V

potential hold. This shows the peak change that occurs. (These potentials are vs.

Ag/AgCl)...... 26

Figure 15: A plot of the peak intensity at the different potentials. This also shows how at low potentials the peak is at 1018 cm-1 but how it shifts to 1012 cm-1 at the higher

potentials...... 27

Figure 16: The potential range of the pyridine/KCl mixture. There is a peak shift that occurs between -0.4 to -0.5 V vs. Ag/AgCl. The peak changes from being at 1018 cm-1 to

1008 cm-1...... 28

Figure 17: Raman spectra of CO2 being introduced to the system and saturating the 0.1 M

KHCO3/0.01 M KCl solution...... 30

Figure 18: Raman spectra of CO2 being removed from the system and the 0.1 M

KHCO3/0.01 M KCl soln becoming unsaturated...... 31

Figure 19: Raman spectra of all the possible background components in the matrix, the roughened Copper foil (in air, with water, and with CO2 sat. bicarbonate/chloride

solution), solid bicarbonate, and the bicarbonate/chloride solution (sat and unsat)...... 32

Figure 20: CO2 saturated solution with no potential (at OCP). This spectrum demonstrates bicarbonate adsorption onto catalyst surface...... 33

Figure 21: A comparison of CO2 saturation and pH change to determine the source of the

bicarbonate adsorption on the catalyst surface...... 34

Figure 22: The potential ranges along with the OCP (just solution without applied potential). This demonstrates how the potential changes the spectra...... 36

Figure 23: The spectra over time as -0.4 V vs. RHE is applied, removed and applied to

show the change in spectra...... 38

Figure 24: A closer look at the first 1000 sec of the -4.0 V vs. RHE potential hold to

better observe the changes that occur over this time...... 39

Figure 25: In Situ Raman spectra of C-13 labelled CO2 electro-reduction...... 47

Figure 26: A comparison of the C-13 labelled CO2 to C-12 CO2 at 600 sec in the potential

hold at -0.4 V vs. RHE...... 48

Chapter 1: Introduction and Background

This chapter will give a brief review on the CO2 electro-reduction as well as Raman

spectroscopy and SERS. This will provide the necessary background to understand the

research presented.

1.1 Energy Concerns

A growing concern in today’s world is energy consumption and sustainability. The Key

World Energy Statistics2 gives out yearly reports on the energy supply and consumption

of the world. As seen in figure 1, well over 80 % of the energy supplied to the world is

from nonrenewable sources such as natural gas and coal. Notice that the needed energy

supply is also growing every year and has more than doubled between 1971 and 2012. To

maintain the amount of energy needed in the world with these nonrenewable resources,

the world energy supply can be sustained for a very limited amount of time. What needs

to be done is a switch to a more sustainable energy cycle. This sustainable energy cycle

will utilize renewable resources such as hydro or solar, or a recycling of sources, such as biofuels. It can be seen that the use of more sustainable sources such as biofuels and

hydro is slowly increasing with time. On the supply side it can be seen that in 1971, an

approximate amount of energy supplied by biofuels was 500 Mtoe, while in 2012

biofuels could account for approximately 1500 Mtoe, a noticeable increase. While this is

a start, the use of these renewable sources needs to increase much more and can only do

so with more knowledge of the processes as well as more advances in the technology using them to make them safer and cheaper options for industrial uses.

Figure 1: The total world energy supply by fuel.2

The consumption of this energy produced is also a great concern. In figure 2, we see that one of the most highly consumed forms of fuel is oil, in 2012 more than 3000 Mtoe of oil

was consumed, this accounts for approximately one-third of the energy consumed. Oil is

the main form of fuel for cars, which for any developed country is a daily tool, thus this

dependence and consumption of oil is one of the largest concerns for the world. As seen,

the consumption of oil has grown with time; however, the use of other fuels, such as

2 natural gas, has increased as well. Natural gas consumption has approximately doubled since 1971. Coal consumption has stayed fairly constant over the last 40 years, even though the amount of energy supplied by coal has increased over that time, indicating more efficient processes for coal burning. The burning of any fossil fuel (coal, oil, or natural gas) to obtain energy involves the emission of carbon dioxide (CO2), see figure 3.

Figure 2: The world energy consumption by fuel.2

1.2 Carbon Dioxide

As figure 3 demonstrates, the amount of CO2 being emitted is increasing every year. CO2 is a major byproduct of burning fossil fuels (coal, oil, and natural gas), but is also often a 3

byproduct in industrial processes such as iron and steel production or cement production.

3 Steel production gave off more than 60 Tg CO2 Eq. in 2011. Since the industrial

revolution, there has been an extremely large increase of CO2 in the atmosphere that is growing each year. While CO2 does need to be present in the atmosphere for plants to grow and the natural cycle to occur, the large upswing of CO2 emissions is also

corresponding to a decrease in the amount of vegetation. This decrease in vegetation is

due to the fact that humans are cutting down forests to make more room for homes,

stores, and other industrial plants that all lead to an increase in CO2 emissions. All of

these things are leading to no longer having the natural balance of the ecosystem that

sustains life. Because of this imbalance, there is a larger amount of CO2 staying in the

atmosphere than ever seen in history. In 1960 there was approximately 320 ppm CO2 in

4 the atmosphere, but this has increased to more than 400 ppm CO2 in 2015. CO2 is the main greenhouse gas in our atmosphere that is contributing to global warming. There is a push to decrease the CO2 emissions to decrease the greenhouse effect that is changing the climate worldwide. One method currently being developed in the United States is to use the non-agriculture land and develop grasslands and forests as a net sink to increase the storage of CO2 in vegetation. This led to the emissions of CO2 being offset by

approximately 13% in 2013.5

2 Figure 3: The measure of CO2 emission by fuels.

Several options are being utilized to reduce the CO2 emissions that come from both the

average human and the industrial giants. One such method is to replace the use of fossil

fuels with other renewable energy sources. Many people utilize the simple method of

energy conservation, turning off lights and using natural sunlight or riding a bike to work

instead of driving. Another method of decreasing the CO2 emissions into the atmosphere

is to capture the CO2 being emitted at the industrial plant that is burning fossil fuels and

storing underground then recycling it to other fuels.

While the energy conservation is a fairly simple lifestyle change, the CO2 capture and then recycle process is still a work in progress. Many industries are willing to look into being more energy conscience, but as of right now, it is still fairly new technology and

thus is rather expensive to employ. Also, it is still being researched as to which system would be the best for recycling and part of that depends on what products one wants from

the CO2 being recycled. The ideal recycling system would not use any energy, or very little energy, to reduce the carbon dioxide into large hydrocarbons that could then be used as fuels themselves; particularly hydrocarbons that contain multiple carbon bonds, as they give much more energy when broken. One very interesting way to accomplish the

6 reduction of CO2 is through the use of electrochemistry.

1.3 CO2 Electro-Reduction

1 Dr. Hori and co-workers have been doing a lot of research on the CO2 electro-reduction.

They have looked into the different metal catalysts, and what the different products are

on those catalysts and at what potential different products emerge. These researchers can

be attributed as laying the foundation for CO2 electro-reduction research. By looking at

many different transition metals as possible catalysts, Dr. Hori was able to determine that

the only metal that produced hydrocarbons, without treatment with other metals or

organic complexes, was copper. While the main products of all other metals were found

- 1 to be CO, formate (HCOO ), or H2 from the aqueous solution, see table 1.

1 Table 1: The products of the CO2 electro-reduction process on different metals. 7

From there Dr. Hori et al. were able to determine the potential range at which the copper produced the hydrocarbons of interest (figure 4). As figure 4 demonstrates, a potential of more than -1.3 V vs. SHE must be applied in order to create these hydrocarbons as the main product of the CO2 electro-reduction. However, some information about the mechanism of formation may be found by looking at the lower potentials and monitoring the surface species where formate and CO are the only carbonaceous products. Figure 5 depicts that at a potential of -0.45 V vs. RHE, formate is produced at about 2 % efficiency and CO is produced at approximately 1 % efficiency.

7 Figure 4: The products of the CO2 electro-reduction on copper at different potentials.

Figure 5: Percent efficiency of CO2 electro-reduction products at -0.45 V vs. RHE with CuSO4 r Cu foil as the catalyst.

Several reactions that are occurring are shown in equations 1-6 with their standard reduction potentials at a pH of 7.0 at 25 oC with respect to the standard hydrogen electrode (SHE). These standard potentials are in an ideal setting, which is not the case for the actual experiment, thus leading to the need for a larger over-potential to obtain the products.1, 7, 8, 9, 10, 11, 12, 13 Some DFT calculations have been done to begin looking at possible mechanisms for the production of the hydrocarbons that, ideally, will eventually be correlated to the adsorbed species found through experimentation and spectroscopy.14,

15, 16 As figure 6 demonstrates, the proposed mechanism at the low overpotentials observed in this research mostly attributes the rate determining step to be the step that follows the formation of the carboxyl group adsoorbed onto the Cu catalyst surface.16 This

9 means that a carboxyl group should be able to be observed when utilizing in situ techniques to monitor the surface species on the Cu catalyst.

Electrochemical Reactions of CO2 Potential at pH 7 vs. SHE - - CO2 + 6H2O + 8e  CH4 + 8OH (-0.25 V) (1) - - 2CO2 + 9H2O + 12e C2H5OH + 12OH (-0.33 V) (2)

- - 2CO2 + 8H2O + 12e  C2H4 + 12OH (-0.34 V) (3)

- - - CO2 +H2O + 2e  HCOO + OH (-0.43 V) (4) - - CO2 + H2O + 2e  CO + 2OH (-0.52 V) (5) - .- CO2 + e  CO2 Most probable limiting step (6)

Figure 6: Proposed CO2 electro-reduction mechanism at low overpotentials based on DFT calculations.16

1.4 Raman and SERS

While it is well established that copper is the only metal catalyst that produces hydrocarbons upon the electro-reduction of CO2, the mechanism as to which this reduction occurs is still unknown. The purpose of the research done in this paper is to give more insight into the possible mechanisms of products formed at low over- potentials. One way to do this is to monitor the surface species on the copper catalyst, in the case of this research, with Raman spectroscopy, more specifically surface enhanced

Raman scattering (SERS).14

Raman spectroscopy is a spectroscopic technique that utilizes light to excite vibrational energy levels within molecules. It is a useful tool in identifying molecular species since it probes vibrational energy changes within the molecule that have a polarizability change induced by the incident radiation. Raman scattering occurs from the inelastic scattering of the emitted photon upon energy relaxation of the electron cloud from the “virtual state” to a quantized vibrational state (either higher or lower energy than the incident photon) after colliding with the sample molecules. This virtual state is more accurately defined as the tail in energy distribution from the excited electronic state without the actual electronic state excitation occurring. The change in energy observed in the molecule is due to the transition between vibrational energy states within the molecule.

There are three types of scattering that may occur; Rayleigh, Stokes, and Anti-Stokes

Scattering (figure 7). Only Stokes and Anti-Stokes are Raman scatterings. If the emitted photon has a higher energy than that of the incident photon, it is called Anti-Stokes

Raman Scattering, while if the emitted photon has less energy, it is called Stokes Raman

Scattering. The other main type of scattering is Rayleigh scattering. Rayleigh scattering is

the elastic scattering, or rather, the scattering when the emitted photon has the same

energy as the incident photon. Rayleigh scattering is approximately a billion times more

intense than either Stokes or Anti-Stokes Scattering. Stokes Scattering is more intense

than Anti-Stokes Scattering since a greater number of molecules will be at a lower energy

state at room temperature as they are governed by a Boltzmann distribution.14, 15

Figure 7: An energy diagram depicting the different types of scattering.

While the signal for a bulk solid or liquid solution is easily measured with acceptable

intensity, single molecules in a system are impossible without some enhancement. Since

the catalyst of interest is copper, surface enhancement can be utilized because a piece of roughened copper foil is used as the catalyst and working electrode in the system. Surface enhanced Raman scattering (SERS) is a very useful technique that can enhance Raman signal by up to 106.15 Only specific metals can give SERS in the visible to IR region,

including copper, silver, and gold. While it has been shown that you can also get SERS

from Al supported Ni, Fe, and Cd 16, the majority of SERS is obtained by using Cu, Ag,

or Au. One drawback of SERS is that the surface must be roughened, and thus single

crystal studies are impossible. Another consideration is that SERS is wavelength

dependent. The excitation wavelength must be in resonance with the plasmon of the

metal surface. Every metal has plasmon and this is the collective oscillation of electrons at the surface. For example, in order to obtain SERS from Cu or Au surfaces, the excitation wavelength must be red (between 620 nm and 750 nm). However, the main benefit of utilizing SERS is that surface species can be observed.14, 15

The mechanism of surface enhancement in SERS is still uncertain. The two theories

describing surface enhancement are the electromagnetic (EM) mechanism and the charge

transfer (CT) mechanism. As seen in equation 7, the Raman intensity is proportional to

the induced dipole (µ) and relies on both the electric field (E) and polarizability (α).

μ (7)

The proposed EM mechanism of SERS attributes the increase in signal to the local

electric field that the molecules experience due to the small defects on the roughened

surface. As one would expect, due to the way the electric fields work, Au, Ag, and Cu

lead to the most significant signal increase. One attractive feature of this method is that it

can explain why only select metals are SERS active. It also takes the optical properties of

the metal into account when describing the signal increase.14, 15

The proposed CT mechanism of SERS attributes the increase in signal to an adsorbate on

the surface that is interacting with the atomic scale roughness features on the metal. This

feature allows for the preferential enhancement of the “first” layer adsorbates. In the CT

mechanism model, there is a creation of an adsorbate-surface complex. This complex is

able to possess electronic affinity levels which can be accessed by the photo-excited

electron.14, 15, 17

While both the CT and EM mechanisms have attractive features that describe the SERS

enhancement, there is still no single mechanism that describes what is occurring during

this phenomenon. The best way to describe the different enhancements observed is actually a mix of the EM and CT mechanisms. Different metals and molecules react

differently and thus the enhancement is very much experimentally dependent.16, 17

1.5 Literature Review

The CO2 electro-reduction process is something that has garnered a lot of research. While

the majority of published works on the system have focused solely on the products

formed and how to manipulate the system to obtain high energy compounds as products,

there is also some ongoing research into the intermediates of the reaction. Due to wanting to look at surface specific species, two main forms of spectroscopy are being utilized,

Raman and Infrared (IR). While these two techniques are very complimentary, they do

give different information since IR is dependent on a change in the dipole moment of a

molecule while Raman is dependent on a change in the polarizability.

Innocent et al. used FTIR to study the CO2 reduction on a lead electrode that led them to conclude that CO2 does not adsorb onto the electrode. This allowed them to then obtain a

- - likely mechanism as to how formate (HCOO ) may be a product by reducing the HCO3 in aqueous solution. In a second study, they found that if the solution was changed to propylene carbonate the main product was oxalate.18, 19

Hori et al. used IR to observe CO and other intermediates of the CO2 reduction on copper

and concluded that the first step in the CO2 reduction process is to form CO as they have

very similar spectra. They went on to study the process more rigorously and believe that

there is a complex ((CO)4H2) on the electrode surface that is a precursor to the formation

of the alcohols and hydrocarbons. With the knowledge Hori gained with these studies, he

went on to examine the process with single crystal electrodes, mostly through the use of

electrochemistry, rather than spectroscopy. This gave more insight into which crystal

facets preferentially gave the different products.9, 10, 12

Smith et al. studied the CO2 electro-reduction on a copper catalyst using SERS and found

poisoning species. Smith also demonstrated that some previous assignments of the

different peaks observed in the in situ spectra were arguably invalid assignments and

dismissed them when assigning peaks.15, 20

Batista et al. found SERS to be a helpful tool to observe the surface of the copper catalyst

during the CO2 electro-reduction. Specifically, they looked at the low cathodic potentials in acidic medium and found peaks indicative of hydrogenated species located at the surface of the copper catalyst. When the same experiment was performed in basic media,

Batista found that adsorbed CO characteristic band became larger indicating that in the basic media, CO formation is favored.21

Pohl et al. utilized the SERS technique to observe species present on an alkali-metal doped copper surface, specifically potassium-doped cold-deposited copper surfaces. This

gave no real insight to the mechanism or intermediates of the reaction. However, they were able to monitor the co-adsorption of CO2 and atomic H to find that surface bound

formate species were the most likely product, should H be allowed to adsorb to the

surface first.22

Kudelski et al. looked at CO intermediate adsorptions using SERS on a modified Cu-Zr

amorphous alloy during the CO2 electro-reduction. While they concluded that the Cu rich

areas of the electrode helped with the CO2 reduction, the main product in the area was

23 H2.

Oda et al. monitored the CO adsorbed during the CO2 electro-reduction process on both

Cu and Ag electrodes using SERS and IR. They were able to determine that the CO

adsorbed during the electro-reduction poisoned the surface of the Cu electrode, while the

Ag electrode was able to be cleaned by performing a positive potential sweep from -0.7 V

to -0.5 V.24

Hernandez et al. found that they could monitor the reaction using IR and altering the

isotopically labelled species. By altering the isotopically labelled species they were able

to state with certainty that the carbonate species at the surface of the electrode is from the

dissolved gas and not the bulk electrolyte. Also, unlike other researchers, they did not observe evidence of CO being adsorbed to the copper catalyst surface during the CO2

electro-reduction.25

While there is still more research being performed every day on this reaction, the reaction

mechanism is still out of reach. New insights are gained by altering individual variables

such as the electrolyte or the catalyst, as demonstrated by Hernandez.

Chapter 2: Experimental

2.1 Catalyst Preparation

Electrochemically roughening of the copper catalyst was performed in a three-electrode potentiostatic system, where a platinum wire was utilized as the counter electrode,

Ag/AgCl was utilized as the reference electrode, and the copper foil was utilized as the

working electrode. All potentiostatic measurements in this work (OCPs and potential

holds) were done using a CH Instrument 604D potentiostat.

Copper foil was electrochemically roughened using an anodic ‘active’ dissolution in 0.2

M copper sulfate (Fisher) and 0.36 M sulfuric acid (Fisher, trace metal grade) with an applied E of 0.711 V vs. Ag/AgCl (1.31 V vs. RHE) for 120 sec.26

Copper foil was also electrochemically roughened by using a 0.1 M KCl (Sigma) solution

and doing 50 positive-negative scans at 20 mV/s from -0.481 to +0.149 V vs. Ag/AgCl

2.2 Solution Preparation

To pre-electrolyze the KHCO3 and KCl stock solutions for use in in situ measurements,

Pt mesh (50 cm2 area) was used as counter and working electrodes. An EXTECH DC

Power Supply was attached and the output current was changed to be 0.12(5) A. This was

left for 12 hours. The solution was then vacuum filtered and stored in a plastic container.

2.3 Characterization

Catalysts were characterized using an FEI Quanta 200 SEM with a general purpose

tungsten source.

Catalysts were compared utilizing a Renishaw inVia Raman and Smiths IlluminatIR IR

Combined Microprobe to determine SERS activity. The excitation wavelength used was

633 nm from an Argon laser source with a CCD detector. Pyridine (Fisher) was used as a

probe molecule, both neat (no potential applied) and in a 0.05 M Pyridine/0.1 M KCl

solution (potential applied). For neat pyridine tests, the catalyst was soaked in pyridine

for 8-12 hrs prior to spectroscopic examination. Immediately prior to performing a scan,

the catalyst was removed from the pyridine, rinsed with nano-pure H2O, and dried with a

Kimwipe.

2.4 In Situ electro-spectroscopy

For the in situ experiments, a solution of pre-electrolyzed 0.1 M KHCO3 (Sigma-Aldrich)

and 0.01 M KCl (Sigma) aqueous (milli-Q water) was used as the electrolyte, and was

then saturated with CO2 (Praxair).

A Pt black mesh and an Ag/AgCl electrode were used as counter and reference

electrodes respectively, while the roughened Cu was used as the working electrode. All potentials reported here, unless otherwise stated, have been corrected to RHE. (See figure

8 for image of cell)

Figure 8: Image of the in situ cell used for electro-spectroscopy.

For in situ experiments, the KHCO3/KCl saturated CO2 solution was constantly pumped through the cell using a Thermo Scientific FH100 Peristaltic Pump at a 2 mL/min flow rate. SERS spectra were obtained with the Renishaw inVia Raman and

Smiths IlluminatIR IR Combined Microprobe equipped with a CCD detector, just as with the pyridine characterization.

Figure 9: Block diagram of in situ Raman set-up. CO2 is bubbled into the 0.1 M KHCO3/0.01 M KCl solution that is pumped into the flow cell before flowing into waste. The potentiostat is connected to the flow cell. The flow cell is inside the Raman microprobe to perform in situ measurements.

Chapter 3: Results and Discussion

3.1 Roughening technique comparison

a b

Figure 10: (a) SEM of CuSO4 r Cu (0.36 M H2SO4/0.2 M CuSO4 solution, Eapplied=0.711 V vs. Ag/AgCl, 120 sec potential hold, 295 K), (b) SEM of KCl r Cu (0.1 M KCl, 20 mV/s CV from -0.481 to 0.149 V vs. Ag/AgCl, 50 positive to negative scans).

SEM images of both KCl roughened copper (traditional method of roughening copper) and CuSO4/H2SO4 roughened copper (method utilized in this paper) were collected for

comparison. The methods result in similar roughening as observed, but when looking at

the background for the Raman spectra from the roughened copper, it is observed that

there is a broad peak from 1000 – 3500 cm-1 for the KCl roughened copper that is absent 22

in the CuSO4/H2SO4 roughened copper. This peak is attributed to some fluorescent

interference.

Figure 11: Raman spectra of roughened copper foil comparing both KCl and CuSO4 roughening techniques. Spectra measured in air.

With pyridine as a probe, the two roughening techniques were compared as well, with the

CuSO4/H2SO4 roughening technique leading to higher intensities than the KCl roughening technique. Based on this, the CuSO4/H2SO4 roughening technique was

utilized for all the in situ measurements done.

Figure 12: Raman spectra of pyridine adsorbed onto the roughened Cu (raw intensities) for both roughening techniques.

3.2 SERS with pyridine

The Raman system was optimized for the CuSO4/H2SO4 roughened copper by utilizing

pyridine as a probe molecule and altering the system parameters.

Figure 13: The shift observed when the pyridine probe is adsorbed onto the roughened Cu foil. The neat pyridine has peaks at 990 cm-1 and 1030 cm-1. While the roughened Cu only shows a peak at 1018 cm-1.

Figure 13 demonstrates that the roughened copper surface is SERS active by the peak

-1 -1 shift of the v1 (ring breathing mode) peak from 990 cm to 1018 cm with no potential

applied to the copper surface. The intensity of this peak changes as a potential is applied

to the roughened copper (a 0.1 M KCl/0.05 M pyridine solution was used for this as an electrolyte is needed to apply the potential). The peak shifts from 1018 cm-1 to 1012 cm-1

around 0.1V vs. RHE (-0.5 V vs. Ag/AgCl) and more negative potentials (see figures 14

and 16). Gao et al. found a similar shift with a change in the applied potential.27 In figure

13, it is seen that after applying a -0.5 V vs. Ag/AgCl, it takes more than two min. to get

25 back to the original signal of a -0.4 V vs. Ag/AgCl potential hold, though it does take less time to actually see the peak shift, though still more than 60 sec.

Figure 14: Raman spectra at several times over a potential hold of -0.4 V after a -0.5 V potential hold. This shows the peak change that occurs. (These potentials are vs. Ag/AgCl).

This shift is attributed to there being enough energy entering the system to change how the pyridine is binding to the surface of the roughened copper. This shift and change in intensity is compared directly in figure 15. There is a small peak at about 0.1 V vs. RHE

(-0.5 V vs. Ag/AgCl) which corresponds to the shift from one peak to the other.

However, it is interesting to note that both peaks reach a maximum at this same point. As

the overpotential is increased (the anodic potential is increased), the signal does increase

again, leading to an eventual plateau.

Figure 15: A plot of the peak intensity at the different potentials. This also shows how at low potentials the peak is at 1018 cm-1 but how it shifts to 1012 cm-1 at the higher potentials.

Figure 16: The potential range of the pyridine/KCl mixture. There is a peak shift that occurs between -0.4 to -0.5 V vs. Ag/AgCl. The peak changes from being at 1018 cm-1 to 1008 cm-1. 28

3.3 In Situ measurements

- - As a way to determine how the system changes as CO2 is introduced to the HCO3 /Cl solution, spectra were collected as CO2 was introduced into the system until saturation

(figure 17) and as CO2 was removed (figure 18). As the CO2 saturated the solution, the

spectra changed and specifically became more intense, but the peaks don’t really shift.

This indicates that while the adsorbed species aren’t changing, there is something that is interacting with the surface more. As the pH is known to change from 9 to 6.78 when

CO2 saturates the solution, we attribute these peaks to the bicarbonate in the solution interacting on the surface due to the change in equilibrium of the carbonate. Figure 21 depicts the effect of this pH change on the surface species adsorbed onto the Cu surface.

- -1 Bicarbonate (HCO3 ) is known to have several broad bands between 3300 – 2000 cm , as observed in figures 19 and 20, there is a very broad band that stretches from 2897-2808 cm-1, and a small broad peak at 3076 cm-1.28 The large, broad feature observed at 3750 –

2950 cm-1 is attributed to bulk water.29 The strongest peak observed (1386 cm-1) is a possible shifting from the 1370 - 1290 cm-1 that is normally associated with bicarbonate

as the species is adsorbed to the copper catalyst surface.28 As there is no potential being

applied, the species adsorbed on the surface should not be any limiting factors in the

electro-reduction of CO2, but may still be used to create the products of the reaction since

they are on and near the surface of the catalyst.

As can be seen in figure 19, there are only weak peaks observed for the roughened copper

foil (in air and in water), as well as the bulk solution (whether saturated or not).

Figure 17: Raman spectra of CO2 being introduced to the system and saturating the 0.1 M KHCO3/0.01 M KCl solution. 30

Figure 18: Raman spectra of CO2 being removed from the system and the 0.1 M KHCO3/0.01 M KCl soln becoming unsaturated. 31

Figure 19: Raman spectra of all the possible background components in the matrix, the roughened Copper foil (in air, with water, and with CO2 sat. bicarbonate/chloride solution), solid bicarbonate, and the bicarbonate/chloride solution (sat and unsat). 32

Figure 20: CO2 saturated solution with no potential (at OCP). This spectrum demonstrates bicarbonate adsorption onto catalyst surface. 33

Figure 21: A comparison of CO2 saturation and pH change to determine the source of the bicarbonate adsorption on the catalyst surface. 34

This gives the stark difference for when SERS enhancement is observed as the CO2

saturated solution adsorbs onto the copper surface, shown in figure 17 and labelled in figure 18.

The amount of potential applied to the catalyst can change the adsorbed species as it also affects the products that are formed. At low over-potentials, the main products have been shown to be mostly CO, formate, and H2. While this range is not where the products of

interest are, this paper is limited to this low over-potential range as the in situ cell has

limitations associated with it that does not allow for spectra to be collected above -0.4 V vs. RHE currently. Specifically, as the Raman Microprobe is focused through the solution, at the more negative over-potentials, too many gaseous products form on the catalyst surface and interfere with the focus of the Microprobe. Figure 22 shows the small potential range that was able to be collected (-0.4 to -0.1 V vs. RHE). All of the potentials exhibit extremely similar peaks, with an overall trend of an increase in signal at the more negative over-potentials. As all these are low over-potentials that would have similar products, this trend makes sense. The fact that there is definite deviation in the

spectra from the OCP (no potential applied to the system) to the applied potential spectra

demonstrates that even at these low over-potentials, the surface species changes with an

applied potential.

Figure 22: The potential ranges along with the OCP (just solution without applied potential). This demonstrates how the potential changes the spectra. 36

While all these potentials are interesting, this paper focuses on the most negative

potential that provided reasonable spectra, -0.4 V vs. RHE, as it is the closest to the range

that is of actual interest. Figure 23 shows in situ Raman spectra during a potential hold at

-0.4 V vs. RHE over time. The bottom spectrum is of the solution, then just the

roughened Cu foil, and the CO2 saturated KHCO3/KCl on the roughened Cu foil. The next four spectra are of the catalyst surface while the -0.4 V vs. RHE potential is applied.

The next two spectra are again using OCP, this demonstrates that the potential does need

to be applied to observe the peaks, as the peaks disappear in these and the spectra goes

back to being what is observed for the CO2 saturated solution on the roughened Cu foil

with no potential. The -0.4 V vs. RHE potential was once again applied after the OCP

(the next three spectra) and the peaks were once again observed. Again, this is good, as it

demonstrates that the catalyst remains active even after the potential was removed and is

able to be reused. The final two spectra are once again at OCP and the peaks have gone

back to the CO2 saturated solution on the roughened Cu foil without potential. Finally, looking at just the first applied potential over time (figure 24) we can see that as time

increases, the intensity of the peaks may increase slightly, but that no new peaks

appear. This indicates that the species that adsorb onto the catalyst surface do so early

and, at these low over-potentials, stay on the surface.

Figure 23: The spectra over time as -0.4 V vs. RHE is applied, removed and applied to show the change in spectra. 38

Figure 24: A closer look at the first 1000 sec of the -4.0 V vs. RHE potential hold to better observe the changes that occur over this time. 39

Table 2: Possible Raman peak assignments from literature.

Solution on -0.4 V Davis and Oliver Batista (experimental)21 Smith Asthagiri roughened vs. RHE (theoretical – (experimental)15 (theoretical – DFT copper in situ group theory)29 calculations)30 (cm-1) (cm-1) 290 Copper oxides (if wide band) Cu-Cl (possible also Cu-CO frustrated rotation) 399 Copper oxides (if wide band) 465 Copper oxides (if wide band) 533 Copper oxides (if wide band) Cu-O, Cu-C 40 549 Copper oxides (if wide band) Horizontally adsorbed C on Cu surface 559 Copper oxides (if wide band) 632 (OH) CO bend Copper oxides (if wide band) Horizontally (HCO3-) adsorbed C on Cu surface 782 785 Adsorbed hydrogenated species with C=C bond, carboxyl group, and C-H bond 1015 C-OH stretch Adsorbed ethylene C-O stretching - 1042 Adsorbed carbonate species HCO3 continued 40

C-O stretching Table 2 continued (possible C-O-H 1136 scissoring) 1196 1190 CO2 in H2O Adsorbed hydrogenated C-O-H scissoring species with C=C bond, carboxyl group, and C-H bond 1260 CO2 in H2O Adsorbed hydrogenated species with C=C bond, carboxyl group, and C-H bond 41 1285 Adsorbed hydrogenated C-O-H scissoring or species with C=C bond, C-O stretching carboxyl group, and C-H bond 1301

1365 CO2 in H2O Graphitic C C-O stretching or C-O-H scissoring 1386 V3 (E’) isolated Adsorbed ethylidyne Graphitic C C-O stretching carbonate ion 1391 Adsorbed hydrogenated Graphitic C species with C=C bond, carboxyl group, and C-H bond 1436 1433 Adsorbed hydrogenated Graphitic C C-O stretching species with C=C bond, carboxyl group, and C-H 41 continued

Table 2 continued bond 1489 Adsorbed hydrogenated Graphitic C C-O stretching species with C=C bond, carboxyl group, and C-H bond 1500 Graphitic C 1536 Graphitic C 1572 1572 Adsorbed ethylene Graphitic C 1619 s 1610 1689 2851 2849 C-H from formate, O- O-H stretching 42 C-O from formate 2923 2935 Multiple C-H bonds 3061 3061

3.4 Peak Assignments

The possible peak assignments based on literature can be found in table 2. Of the

literature used to determine possible assignment, two were theoretical (group theory and

DFT based on single crystal facets), 2 were purely experimental, and 1 is a renowned

textbook with both theoretical and experimental assignments (not included in table 2).

The peaks at 549 cm-1 and 632 cm-1 which could be attributed to a horizontally adsorbed

carbon on copper based on DFT calculations.31 Though, group theory allows a possible

assignment of an (OH)-CO bend in bicarbonate for the 632 cm-1 peak.29 However, as this

peak is much weaker, if present at all, in the spectra with no applied potential, it seems

much more likely to be the carbon horizontally adsorbed to the surface that allows for these peaks to be observed. The 785 cm-1 peak is more intense with the applied potential

indicating that the increase in energy in the system allows the specie(s) involved to have

an increased concentration on the surface of the roughened copper foil catalyst. Several

possibilities for the assignment of this peak include hydrogenated species that have a

C=C bond, carboxyl group, and/or C-H bond21; a characteristic carbonate stretch28; the

CCO stretch observed in primary alcohols28; or the CH out of plane vibration and C-C=O

in plane deformation vibration observed in formate.28 Considering the products of the

CO2 electro-reduction, the first possibility seems unlikely as the only product with a C=C

is ethylene. The carboxyl group is a very definite possibility as figure 6 depicts that the

step following the formation of a carboxyl group on the Cu surface may be a rate

determining step. As the peak is observed when there is no potential applied, the

possibility of it being a carbonate peak is a valid assignment. The third possibility would

demonstrate a shift from the typical range of 900-800 cm-1, but as it would be adsorbed

onto the copper catalyst, this is not completely unrealistic; the same is true of the final

option though it is a blue shift from the typical 775-620 cm-1 range from literature. The

1015 cm-1 peak is only observed when the potential is applied; the OCP spectra does have

a peak at 1042 cm-1 which is attributed to adsorbed and bulk carbonate species20, 21, 28, 30,

but this 1015 cm-1 peak has been attributed to a C-OH stretch29, adsorbed ethylene21, and

C-O stretching.30 All of these are very possible theoretical assignments for the peak. The

adsorbed ethylene is a little bit of a stretch as when we view the products of the reaction,

we do not have any measurable quantity of ethylene. That being said, it could still be

forming and adsorbed to the surface and as the peak is seen to grow with time, ethylene

could be stuck to the surface without enough energy to desorb and become a full product.

Both the C-OH and C-O stretching are very plausible as we know we can form alcohols

and aldehydes as products and that CO can be formed during the reduction process. The

next peak at 1136 cm-1 has only one possible assignment of being due to C-O stretching, with a small argument being made for possible C-O-H scissoring.30 However, it is more plausible that the C-O-H scissoring can be observed with the 1190 cm-1 peak. The

problem with this peak assignment is that this 1190 cm-1 peak may also be attributed to

29 -1 CO2 in H2O. Because there is a peak observed at 1196 cm at OCP, the CO2 in H2O

assignment is given to that. A potential being applied to the solution could shift this peak

a small amount, and thus this assignment is still possible to be valid for the 1190 cm-1 peak. Though, there is a distinct possibility of a carboxyl group stretch in this range as well. This could account for the increase in signal as well as some of the shifting

-1 observed for this peak. As for the 1285 cm peak, there is a possible assignment of a CH2

28 twisting vibration or CH2 wagging seen in primary alcohols , but it could also be

indicative of C-O-H scissoring, C-O stretching30, or even a carboxyl group. All of these

possibilities are reasonable assignments and it is difficult to determine with certainty

which is completely accurate. This also true for the 1386 cm-1 peak, it could possibly be

28, 29 15, 30 attributed to a carbonate ion (V3 (E’) isolated ion) , graphitic carbon , adsorbed

ethylidine21, C-O stretching30, or a primary alcohol.28 The rest of the peaks are very

similar to the spectra taken at OCP and thus are attributed to carbonate species adsorbed

on the copper catalyst surface.

3.5 C-13 vs. C-12 Spectra

To attempt to prove that the peaks observed are from the CO2, and not the bicarbonate in

solution, C-13 labelled CO2 was used to saturate the KHCO3/KCl solution (figure 25).

The peaks did change with the change of the isotopically labelled CO2, indicating that

they are in fact due to the CO2. Signal to noise for the C-13 CO2 was much smaller than was observed for the C-12 CO2, meaning it is much more difficult to distinguish the true

signal from the background noise. In Situ Raman spectra for the C-12 and C-13 CO2 at

approximately 600 sec in the potential hold are compared in figure 26. One definite peak

-1 -1 change is the 1015 cm peak of the C-12 CO2, shifted down to about 1004 cm with the

C-13 CO2, this peak is attributed to a C-O stretch on the surface. There is also shifting in

the 1136 cm-1 peak which is also attributed to a C-O stretch on the Cu surface. There is arguably a small shift in the 785 cm-1 and the 1572 cm-1 peaks as well; these peaks are 45

attributed to a carboxyl group adsorbed onto the Cu surface. Another noticeable change is the intensity of the broad hydrocarbon peak from 2849 – 2935 cm-1. For a very few

-1 spectra when the C-12 CO2 is used, there is a very small peak between 2020 cm and

-1 2080 cm ; however, for the C-13 CO2 this peak is consistently present. This very small

feature is indicative of CO on the surface of the catalyst. Most of the other peaks are

hard to determine exact assignments as they seem more likely to be an increase in the

signal of peaks that are very low intensity with the C-12 CO2. While these results are

very indicative and can be used to determine that the intermediate adsorbed species

observed are due to the CO2 and not just the bicarbonate solution, this experiment needs

to be ran several more times to ensure reproducibility.

Figure 25: In Situ Raman spectra of C-13 labelled CO2 electro-reduction. 47

Figure 26: A comparison of the C-13 labelled CO2 to C-12 CO2 at 600 sec in the potential hold at -0.4 V vs. RHE. 48

Chapter 4: Conclusion

The CO2 electro-reduction process at low overpotentials has two carbonaceous products,

formate and carbon monoxide. Based on previously proposed mechanisms (based on

DFT calculations), the main intermediate that should be observed for the formation of

both of these products is an adsorbed carboxyl group. The use of SERS to monitor the

intermediate species adsorbed onto the catalyst surface can give insight to the plausibility

of the proposed mechanism of the CO2 electro-reduction on copper at low overpotentials.

As demonstrated, there are multiple peaks observed that can be attributed to an adsorbed

carboxyl group. Even more indicative that this intermediate is observed is the shift in

peaks when a C-13 labelled CO2 is used. Spectroscopy, however, it is limited when used

on its own, as many of the observed peaks are difficult to assign accurately to one

specific species, and thus possible assignment errors may occur. When coupled with

other techniques, such as the DFT calculations, the results can be very explicit in the true

peak assignments.

While the peaks observed here do indicate that the potential affects the surface species,

the next step is to re-make the in situ cell in order to be able to collect spectra at high

over-potentials where the products are the hydrocarbons of interest. If possible, it would be ideal to also use other types of copper or even copper alloys to see how the surface species change with different selectivities. The surface species may change based on the

roughness of the copper, so it may be of interest to alter the roughening technique to higher over-potentials and longer times to observe how that affects the selectivity of the

surface species. In the future it would also be of great interest to correlate the results with

in situ IR spectroscopy.

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