Christa Ros
of hydrocarbons from carbon dioxide and water
CO 2 + H 2 O → hydrocarbons + O 2
dioxide and water
CO 2 + H 2 O → hydrocarbons + O 2
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen 10 maart 2016 om 10.00 uur
door
Cornelia Hester Ros
scheikundig ingenieur geboren te Zoetermeer, Nederland
Samenstelling promotiecommissie:
Rector Magnificus Voorzitter
Professor dr. F. Kapteijn TU Delft, promotor
Professor dr. G. Mul Universiteit Twente, promotor Professor dr. P.J. Kooyman University of Cape Town, promotor
Independent members:
Professor dr. J. Gascon TU Delft
Professor dr. M.T.M. Koper Universiteit Leiden Professor dr. J.J.C. Geerlings TU Delft
Professor em. dr. J.A.R. van Veen TU Eindhoven
Reservelid: Professor dr. E.J.R. Sudhölder TU Delft
The research reported in this thesis was performed in the Catalysis Engineering section of the Chemical Engineering department (ChemE) of the faculty of Applied Sciences (TNW) of Delft University of Technology, with financial support of the Shell Mobility program.
Thesis, Delft University of Technology
Met samenvatting in het Nederlands / with summary in Dutch
ISBN: 978-94-028-0074-6
© 2016 C.H. Ros
All rights reserved
Art direction & lay-out: Esther Beekman (www.estherontwerpt.nl) Printed by: Ipskamp drukkers BV, Enschede
Problem Statement
The electrocatalytic reduction
Research topics
1.3.1 Electrode material
1.3.2 Potential 1.3.3 Temperature 1.3.4 Pressure 1.3.5 Electrolyte 1.3.6 Copper Catalyst 1.3.7 Reaction Mechanism 1.3.8 Reactor Design
The research objective for the current project Outline of the thesis
Reactor design and experimental basics The first reactor design
Changes in the reactor design
2.2.1 Positioning of the reference electrode
2.2.2 Positioning of the working- and counter electrode 2.2.3 The effect of stirring
2.2.4 Temperature control 2.2.5 Manual analysis The new reactor design The standard experiment
Product analysis by gas chromatography 2.5.1 Data processing
2.5.2 Production or Faraday efficiency Statistics
2.6.1 Variance in current
2.6.2 Variance in methane production 2.6.3 Variance in ethylene production 2.6.4 Variance in the Faraday efficiency 2.6.5 Conclusions on variance in data points Surface analysis techniques
1.1 1.2 1.3
1.4 1.5
2 2.1 2.2
2.3 2.4 2.5
2.6
2.7
10 12 13 15 15 18 18 19 21 24 28 29 29
33 34 35 35 35 38 39 39 39 40 42 43 43 45 45 46 48 49 50 51
3.2
3.3
3.4
4 4.1 4.2
4.3
63 63 64 64 64 66 72 73 73 74 75 78 80 82 82
85 86 89 91 93 95 96 97 97 100 101 101 102 113 115 3.1.4 Conclusions
Temperature 3.2.1 Literature 3.2.2 Experimental 3.2.3 Constant current 3.2.4 Constant potential 3.2.5 Conclusions Electrolyte 3.3.1 Literature 3.3.2 Electrolyte types
3.3.3 Faraday efficiency vs. concentration 3.3.4 Faraday efficiency vs. cation size 3.3.5 Faraday efficiency vs. pH 3.3.6 Conclusions
The effect of external parameters
Copper plate catalysts: Effect of morphology Literature review of studies on copper electrodes
The effect of surface structure and purity on the productivity and selectivity 4.2.1 Analysis of the surface
4.2.2 The effect of potential on the electrocatalytic reduction 4.2.3 Catalyst batches
4.2.4 Conclusions
The deposition of copper on a copper substrate 4.3.1 Theoretical background of electrodeposition 4.3.2 Experimental
4.3.3 Controlled copper electrode production by electrodeposition 4.3.3.1 Reproducibility
4.3.3.2 Sulfuric acid and sodium chloride addition 4.3.3.3 Hydrochloric acid
4.3.3.4 Effect of copper substrate and anode material
4.4.1 Reproducibility of production and selectivity 2
4.4.2 Production and selectivity versus surface roughness and crystal orientation 4.4.3 Different substrate and counter electrode effects
4.4.4 Conclusions
Overall conclusions on copper catalysts
Carbon-based electrodes
Graphite plate electrodes with deposited copper 5.1.1 Deposition of copper on graphite plates
5.1.2 CO2 reduction with copper deposited graphite plates 5.1.3 Conclusions
Carbon nanofiber electrodes
5.2.1 Production of carbon nanofibers 5.2.1.1 Carbon nanofiber growth
5.2.1.2 Nickel deposition on graphite sheets 5.2.1.3 Growing carbon nanofibers
5.2.2 Copper on carbon nanofiber substrates 5.2.2.1 Homogeneous deposition precipitation 5.2.2.2 Impregnation
5.2.2.3 Electrodeposition
5.2.3 CO2 reduction with copper on carbon nanofibers 5.2.4 Conclusions on carbon nanofibers
Carbon Xerogel Electrodes 5.3.1 Xerogel production 5.3.2 Xerogel characterization
5.3.3 CO2 reduction with copper-doped carbon xerogels 5.3.4 Conclusions on copper- doped carbon xerogels Conclusions on carbon-based catalysts
Summary
External parameters Copper catalysts Carbon-based catalysts Recommendations 4.5
5 5.1
5.2
5.3
5.4
116 119 123 124 124
127 128 128 132 135 135 135 135 136 138 139 139 140 141 144 144 145 146 146 147 149 149
151 153 154 155 156
Bibliography Acknowledgement List of publications Curriculum vitae
167 175 179 182
Introduction
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It is 1944, Pierre van Rysselberghe is doing experiments measuring polarographic waves.
At a certain moment he decides to take a 0.1 M solution of tetramethylammonium chloride saturated with carbon dioxide to measure the polarographic waves at the dropping mercury cathode. To his surprise he sees a well-defined reduction wave appear with a half-wave potential of -2.24±0.01 V referred to the saturated calomel electrode. When he repeats the experiment, the same wave is observed and also the third, fourth and six more experiments show exactly the same result. When he does the experiment with hydrogen instead of carbon dioxide, also a well-defined wave is observed, but at a less negative potential of -2.18 V. Together with his partner Alkire, van Rysselberghe writes a letter to the American Chemical Society to inform them of his findings: “We have arrived at the following definite conclusions: carbon dioxide exhibits well-defined reduction waves with a half-wave potential of remarkable constancy: -2.24±0.01 V referred to the saturated calomel electrode”[1]. Two years later they publish two additional papers on this matter with a mathematical theory [2] and a more precise description of the experimental work, where the unknown mathematical parameters are estimated from the experiments [3].
In 1953, almost 10 years after van Rysselberghe’s first publication, A.A. Vlcek writes him that he repeated the experiment using absolute (99.8%) ethyl alcohol with 0.3 M tetramethylammonium chloride. Vlcek finds a well-defined wave with a maximum at -2 V. He concludes: “The behavior shows it to be due to the evolution of hydrogen from carbonic acid, the carbon dioxide molecule being regarded as not reducible” [4].
Determined to settle the matter once and for all, van Rysselberghe and his colleague Truman Teeter set themselves at the equipment again to do various experiments on polarization curves and electrolysis with different electrolytes, saturated with different gases. These experiments proved that carbon dioxide is definitely reducible [5].
1.1 Problem statement
“Transport represents a significant threat to long-term sustainable development, and is one of the fastest-growing consumers of energy and sources of greenhouse gas emissions. Moreover, transport is heavily reliant on petroleum, a limited resource that is also associated with geopolitical risks to security of supply”[6]. This citation of Hal Turton (2006) is the introduction of an article with the title “Sustainable global automobile transport in the 21st century: An integrated scenario analysis” which covers the three major issues mankind is facing in the coming decades.
The first problem is the growing demand for energy all over the world. Within the coming 10 years an increase in demand of around 20% is predicted [7]. The transport sector is only one of many sectors consuming more and more energy [8].
The second problem is the limited availability of fossil fuels. In 2013 the estimated consumption of oil all over the world was about 9.2·107 BBL per day [9]. The proven reserves on the first of January 2014 were 1.66·1012 BBL [10]. A quick calculation gives an availability of oil for about 50 years. Not only will the transport sector have a problem, also the chemical industry, which uses crude oil for the production of compounds like plastics, will suffer from the loss of crude oil as a feedstock.
ˇ ˇ
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Last but not least, burning these fossil fuels to generate energy brings a lot of CO2 into the atmosphere (figure 1.1). CO2 is one of the greenhouse gases and although there is no consensus that greenhouse gases cause global warming [12,13], decreasing the emissions will not harm the environment anyway.
To solve the problem of increasing energy demand, much research is focused on wind, tidal and solar energy. These sources of energy are sustainable (there will always be wind and solar radiation), so this is an advantage. The largest disadvantage is the discrepancy between demand and supply. The sun is most powerful in daytime, but most energy is needed during the evening and night. The wind is not always blowing or sometimes is too strong to let the windmills do their job [14]. The energy provided by these sustainable sources is mainly in the form of electricity. Storage of electricity is necessary in times when more is produced than used and vice versa.
One of the ways to store electricity is the use of batteries. But batteries are expensive and the materials used to make batteries are very heavy and a concern for the environment [15]. The weight of batteries is especially a disadvantage for the transport sector, where weight = power. To lower the weight of batteries, lithium is used. The trouble is that lithium is also used for medical applications, so a conflict in material use should be taken into account since lithium is a scarce material. Electric vehicles do exist already, but action radius (the distance a car can drive on one battery charge) and recharging times are still problems of concern [15,16]. Also, the lifetime of a battery is about 1000 cycles [17], which is not very long. So the search for alternatives is important.
Figure 1.1: CO2 emissions from transportation and industry [11].
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Many scenarios have been written on how to reduce the CO2 emissions [18-22], by storing carbon dioxide for future use [18] or switching to an economy without the use of hydrocarbons in transport: the hydrogen economy [23].
The hydrogen economy is a scenario where hydrogen is used as a fuel. Hydrogen can be produced in different ways, but the idea is to use the electricity from renewable sources to convert water into oxygen and hydrogen: storage of energy in chemicals.
This can be done in two ways, either using the electricity generated from renewable sources to electrolyze water, or directly using sunlight in a photocatalytic reaction. [24]
Although promising results have been booked with the production of hydrogen in large quantities, there is another problem that needs to be solved before a switch towards a hydrogen economy can be made, viz. the storage of hydrogen. Storing under high pressure costs a lot of energy and special storage tanks are necessary to prevent leakage. Storage in metal hydrides faces problems with both loading and unloading, and again the weight of the metal is playing an important role [23]. Also the full infrastructure now available for gasoline and diesel will have to be changed to make it suitable for the distribution of hydrogen.
The suggested solutions can solve the problems for energy demand and the reduction of CO2 emissions, but they still do not solve the problem of the depletion of crude oil.
Crude oil is not only a source for energy production, but also a supplier of raw materials for the chemical industry. The latter problem will not be solved by transition to a hydrogen economy.
But other alternatives are emerging. Especially carbon dioxide is changing reputation nowadays from a useless greenhouse gas to a useful raw material. There are increasing attempts to use CO2 as a feedstock to produce fuels or chemicals, rather than considering CO2 as a waste that costs lots of money to dispose of. CO2 can be used as a weak acid or as an oxidation agent. Furthermore CO2 can be reduced by chemical, electrochemical, photochemical or syngas routes into useful products e.g.
acids, alcohols, esters, hydrocarbons and/or polymers. Xu and Moulijn [18] and Centi and Perathoner [25-27] extensively describe different processes for the usage of CO2 as a feedstock, where the (photo)electrochemical conversion of CO2 to hydrocarbons is suggested as a good opportunity.
The electrochemical reduction of CO2 to generate hydrocarbons at ambient pressure and temperature is an attractive route for the conversion of this greenhouse gas into useful raw materials for the process industry and for transportation fuels. This process can be driven using excess energy from wind, solar and tidal sources so it will not compete with other energy needs.
1.2 The electrocatalytic reduction
The interest in the CO2 reduction process started with the curiosity for the process that takes place in plants. Green vegetation reduces CO2 into oxygen, carbohydrates and hydrocarbons (used for its growth) with sunlight as a source of energy [28]. To mimic this process, an external energy source (the electricity grid) is used.
13
The reactor for this operation uses three electrodes (a working, a reference and a counter electrode) and an electrolyte. At the working electrode, the important reaction of CO2 conversion is taking place. The metal that is used as working electrode is also the catalyst for this process.
The main reactions (1.1-1.3) that take place at the working electrode are listed with their formation energies (Gibbs free energy; ΔG0, T = 298 K and P = 1 bar) [29].
CO2 + 8H+ + 8e- CH4 + 2H2O (l) ΔG0 = -131 kJ/mol (1.1) CO2 + 6H+ + 6e- ½C2H4 + 2H2O (l) ΔG0 = -114 kJ/mol (1.2) CO2 + 2H+ + 2e- CO + H2O (l) ΔG0 = 20 kJ/mol (1.3) An undesired side reaction that takes place is the formation of hydrogen:
2H+ + 2e- H2 ΔG0 = 0 kJ/mol (1.4)
Which of these reactions is dominant depends on many factors that will be discussed later.
The required protons for the reactions at the working electrode are produced from the water splitting reaction that occurs at the counter electrode:
2H2O (l) O2 + 4H+ + 4e- ΔG0 = 474 kJ/mol (1.5) The reactions described above do not occur spontaneously, either because the reaction has a positive Gibbs free energy or because the reaction has a high activation energy to be overcome. Therefore, an external energy source is necessary. A potential sufficient to form the preferred products is applied to the system and a current will flow through the system to rearrange the electrons from reactants to products. The minimum potential at which a reaction runs is called the half-cell standard potential, which can be calculated from the Gibbs free energy:
E0 = − ΔG0 / nF (1.6)
Where E0 is the half-cell potential, ΔG0 is the Gibbs free energy, n is the number of electrons involved and F is the Faraday constant (F = 96485.3 C/mol).
The efficiency of the CO2 reduction reaction is often expressed in the number of electrons that goes into a certain product divided by the total number of electrons that went into the system, and is called the Faraday Efficiency (FE). The mathematical expression is given by:
Faraday Efficiency = n*Na*prod / (I*t/e) (1.7) Where n is the number of electrons involved, Na is Avogadro’s constant (6.022*1023 mol-1), prod is the total production of a compound in mol, I is the current in Ampère, t is the reaction time in seconds and e is the elementary charge (1.602*10−19 Coulomb).
1.3 Research topics
A lot of research has been performed on CO2 reduction; already in the 1940s people were studying CO2 reduction as described in the introduction. But the first introduction to this research topic dates from the end of the 19th century when scientists found that
14
it was possible to reduce CO2 towards formic acid by applying a current over two metal plates. In 1870 the first article was published on the electrochemical conversion of CO2 into products, written by M.E. Royer [30]. Until around 1980, mainly mercury was used as cathode material, pure or in some cases mixed with zinc or gold (amalgams) [31-35].
The reduction of CO2 did take place, but much hydrogen production (an undesired side- reaction) was seen for currents above 1 A. In the 1980s, interest in the topic increased and many groups studied different aspects of the CO2 reduction reaction. These parameters are described one by one in the next sections.
Table 1.1: Faraday efficiency (%) towards different products for some metals [37]
(V) CO CH4 C2H4 C2H6 EtOH PrOH HCOO- H2 Total
Cu -1.41 2.0 29.4 30.1 0.0 6.9 2.1 9.7 10.9 91.0
Ag -1.37 81.5 0.0 0.0 0.0 0.0 0.0 0.8 12.3 94.6
Au -1.14 87.1 0.0 0.0 0.0 0.0 0.0 0.7 10.2 93.0
Zn -1.54 79.4 0.0 0.0 0.0 0.0 0.0 6.1 9.9 95.4
Pb -1.76 Tr 0.0 0.0 0.0 0.0 0.0 97.5 1.2 98.7
In -1.55 2.1 0.0 0.0 0.0 0.0 0.0 94.9 3.3 100.3
Sn -1.48 7.1 0.0 0.0 0.0 0.0 0.0 88.4 4.6 100.1
Cd -1.63 13.9 1.3 0.0 0.0 0.0 0.0 78.4 9.4 103.0
Ni -1.48 0.0 1.8 0.1 0.2 0.0 0.0 1.4 88.9 92.4
Fe -0.91 0.0 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8
Pt -1.07 0.0 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8
Ti -1.60 tr 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7
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1.3.1 Electrode material
Hori et al. [36] showed in 1985 that the metal used for the working electrodes has a large influence on the products that are formed. They showed that copper is very effective for the formation of methane and ethylene, while all other metals mainly give formic acid, carbon monoxide or hydrogen. Table 1.1 shows the Faraday efficiency towards different products for several metals.
Ikeda et al. [38,39] classified the metals into three groups according to their main products from CO2 reduction (either CO, (COO-)2 or both) and also tried to relate (unsuccessfully) the symmetry in the periodic system to the product selectivity. Hori et al. divided the metals into 4 groups (hydrocarbons, CO, HCOO- or no carbon-containing products) [40]. Continuing the search for a correlation, Chaplin and Wragg [41] linked the product formation to sp-metals or d-metals, which came close to linking the product formation to the periodic system, but still no clear conclusions could be drawn.
The conclusion is that the nature of the metal determines the surface coverage of different reduction species. If the surface adsorbs hydrogen very easily, mainly hydrogen and methane are formed, whereas surfaces that predominantly adsorb carbon-containing species lead to the production of hydrocarbons and alcohols [42,43] (figure 1.2).
The most effective metals are those with a small number of electrons in the s- and p-orbital and/or full d-orbitals [41]. Copper is rather unique in this matter, with a full 3d-orbital and only one electron in the 4s- and 4p-orbital. Thereby, copper is the only metal with the proper adsorption specifications for the production of hydrocarbons.
Indeed, copper shows distinct activity for the formation of CH4 and C2H4 and ethanol, while the other metals mainly produce CO, HCOOH and/or H2.
If pressure [44,45] and temperature [46,47] are taken into account, the metals seem to react differently. At high CO2 pressures (above 30 atm) also other metals are able to produce hydrocarbons, although the productivity is still very low. Especially CO production increases at higher pressures for different metal catalysts. Figures 1.3 a and b show the periodic table in which color-codes give the products that are formed in CO2 reduction at 220C and 20C, respectively.
Most metals show the same product formation at the two different temperatures, but titanium, nickel and platinum show a large change in selectivity with temperature.
A more detailed description of the effect of temperature and pressure on copper electrodes is given in section 1.3.3 and 1.3.4, respectively. For the other parameters under investigation, only papers related to copper electrodes are discussed here.
1.3.2 Potential
The potential influences the product distribution, the current determines the amount of electrons and thus the amount of product formed. At potentials less negative than -1.0 V, CO and CHOO- are the main products, while a potential more negative than -1.4 V is required to enhance the formation of CH4 and C2H4 [49].
Frese and co-workers [50] measured the effect of potentials between -1.2 and -2.0 V vs.
SCE in a 0.5 M KHCO3 electrolyte solution at 220C on the selectivity towards methane, CO and ethylene.
16
Figure 1.2: Reaction path as a function of electrode adsorption properties [41].
Figure 1.3: Product formation for different metals in the periodic system. A) At ambient temperature [48, page 9]; B) At 20C [47].
17
The methane production increases with more negative potential to about 50% Faraday efficiency. CO and ethylene have an optimum production of about 2-3% FE at -1.6 V and -1.9 V vs. SCE respectively. The hydrogen production decreases from 80% to 20% FE between -1.4 and -1.9 V and increases again to 50% at -2.0 V vs. SCE. In a later article, Frese [51] reports an increasing hydrogen production rate with decreasing (more negative) potential, which is in line with the observations in the first article. Frese’s second article shows an increasing production rate with more negative potentials for CO, while the first article speaks of an optimum. Frese does not report any Faraday efficiencies in the second article, so it is impossible to say if the data agree with each other in terms of energy efficiency.
Ikeda et al. [52] did the reduction reaction in a 0.1 M KHCO3 electrolyte solution at 250C.
They used a potential range between -1.4 and -1.7 V vs. Ag/AgCl reference electrode and found a FE between 35 and 40% for all three products at different optimal potentials (CO and ethylene at -1.5 V, methane at -1.7 V vs. Ag/AgCl). Hydrogen shows the same trend as described by Frese and co-workers [50], the minimum is around -1.6 V vs. Ag/
AgCl. In the same year Hori et al. reported experimental results for the reduction reaction performed at 190C [53]. The potential range lies between -0.8 and -1.4 V vs. the normal hydrogen electrode (NHE). Those conditions show an increase in both methane (from 0 to 40%) and ethylene (from 0 to 20%), and a strong decrease in hydrogen production (from 90% to 20%) for more negative potentials. CO has an optimal Faraday efficiency of 20% at -1.2 V vs. NHE.
Momose et al. [54] also performed the electrocatalytic reduction of CO2 in a 0.1 M KHCO3 electrolyte solution at 250C. In the range from -1.6 V to -2.2 V vs. SCE the gas production ratio of hydrogen decreased, methane increased and ethylene has an optimum at -2.0 V vs. SCE.
With new and better measuring techniques to analyze the liquid phase, new products formed by CO2 reduction are found. By using NMR, Kuhl et al. [55] found five products that were not reported before on any metal. The five new products were all C2 and C3 oxygenates, found in very low quantities in the liquid phase. All C2 products found had an optimum production around -1.2 V vs. RHE; for the C3 products optimum production was seen at -1.05 V. For the already familiar products H2, CO and CH4 the same trend was observed as reported in other papers.
In 2010 Peterson et al. [56] published data from DFT calculations, where they explain why certain products are formed at different potentials. Two years later [57] an explanation was found for the high over-potential that is required to perform different intermediate reaction steps (section 1.3.7). The equilibrium potential for the production of methane from CO2 is 0.17 V vs. RHE. Each intermediate reaction step has its own limiting potential.
The distance between the equilibrium potential and the potential of the intermediate step with the most negative potential determines the over-potential. The step from intermediate species •CO into intermediate species •CHO is mainly responsible for the large over-potential in the CO2 reduction reaction.
The effect of potential on other metals [47, 58-62] and alloys [43] has also been investigated, but will not be further discussed here, since this thesis will only focus on copper catalysts.
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1.3.3 Temperature
Hori et al. [63] investigated the effect of temperature in the range from 00C to 400C in a 0.5 M KHCO3 electrolyte solution at a constant current of -5 mA/cm2. The Faraday efficiency towards methane at 00C was 65% and decreased linearly to 0% at 400C.
The Faraday efficiency towards ethylene was 0% at 00C and increased to about 20%
at 400C. The FE for hydrogen was 20% from 00C to 200C and then increased with increasing temperature.
Kaneco et al. [64] also used a 0.5 M KHCO3 electrolyte solution, but did the experiments at a constant potential of -1.9 V vs. SCE in the range from 00C to 600C. For increasing temperature, the Faraday efficiency for methane decreases from 40% to 7%. The FE for CO decreases to half its value at 600C compared to 00C (4% vs. 8%). The hydrogen formation increases with increasing temperature from 35% to almost 90%. The ethylene production in this case is independent of the temperature. They used ultrasonic irradiation to maintain the potential. In a later paper [65], the effect of lower temperature is investigated. But it is difficult to compare the results, since another concentration of electrolyte (1.1 M) and another potential (-2.0 V vs. Ag/AgCl) were used. Low temperature plays a significant role in improving the selectivity of CO2 reduction over H2 evolution [66]; higher temperatures increase H2 production and decrease methane production.
The decrease of hydrocarbon formation at higher temperatures could be due to the lower CO2 solubility (figure 3.14).
Not many papers are found on the effect of temperature for copper electrodes. The conclusion is that higher temperatures favor the production of C2H4 over CH4, but also increase the undesired production of hydrogen.
The effect of temperature for other metals is not further discussed here and can be found elsewhere [47, 67-71].
1.3.4 Pressure
The effect of high pressure on CO2 reduction at copper electrodes is only discussed in three papers.
Kaneco et al. [72] only performed the reaction under 10 bar CO2-pressure and used 0.500 M LiCl dissolved in methanol as the electrolyte. At an applied potential of -3.0 V vs.
an Ag rod quasi-reference electrode and a temperature of 15°C, a FE towards methane of 8%, ethylene of 1% and formic acid of 19% was observed. The specific electrolyte makes it difficult to compare these results with experiments from other research groups.
Hara et al. [45] investigated the effect of 30 bar CO2 pressure for different metals. For the copper electrode, a Faraday efficiency of 10% for CH4 and 4% for C2H4 was found. The main product was HCOOH (54%) followed by CO (20%). Only 3% Faraday efficiency for H2 was found. The reaction was performed at -163 mA/cm2 (-1.6 V vs. Ag/AgCl) in 0.1 M KHCO3. Compared to results from other groups at ambient pressure, the production of methane and ethylene is much lower but that of formic acid is much higher at 30 bar.
Recently, Mul and co-workers [73] performed the reaction at pressures between 1 and 9 atm. in 0.5 M KHCO3 at -1.8 V vs. Ag/AgCl under constant CO2 flow at copper nano- particle catalyst material. At 1 and 2 atm. the Faraday efficiency towards methane was 20%, while at 4, 7 and 9 atm. methane efficiency decreased to 4, 3 and 2%, respectively.
The Faraday efficiency towards ethylene increased from 10% (1 atm) and 15% (2 atm)
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to as much as 44% (9 atm). The CO2 pressure has a large influence on the product selectivity. They suggest that high local pH at the surface favors the C2H4 production. At high pressure, the CO2 concentration increases and lowers the local pH, which would suggest lower ethylene production, but the local CO concentration increases, which enhances the CO surface coverage and thus increases the formation of the CO dimer, responsible for C2H4 formation.
The effect of pressure for other metals can be found elsewhere [44, 45, 68, 74]. Also experiments with CO instead of CO2 as reactant have been carried out at high pressure [75].
1.3.5 Electrolyte
Another parameter of influence is the kind of electrolyte that is used for the electrocatalytic reduction reaction. In nature, plants make use of salt-solutions as “electrolyte” for the conversion of CO2, but also acids seem to play an important role [28], since protons are involved in the reaction.
Especially in the early years of research, aprotic (non-aqueous) electrolytes were used. Examples of aprotic electrolytes are methanol and tetraethylammonium or tetrabutylammonium perchlorate in acetonitrille [60, 76-79]. In the absence of protons, only coupling of two CO2 molecules (e.g. CO2 + 2e- à C2O42-) or the dissociation of CO2 into CO and O2 are possible [80]. Without a source of hydrogen it is not possible to produce hydrocarbons from CO2. When some water is present, also formic acid can be one of the products.
Kaneco et al. [81, 82] performed the reaction at -4.0 V vs. Ag/AgCl reference electrode.
The anolyte (electrolyte at the anode) was KOH/methanol. CsOH/methanol, KOH/
methanol or RbOH/methanol were used as a catholyte (electrolyte at the cathode).
The Faraday efficiency for ethylene was 32%, 38% and 30% respectively. Methane efficiency was much lower: 8%, 16% and 5%, respectively. The hydrogen production is suppressed (it is only between 4 and 12%), due to the lack of hydrogen ions.
Noda and co-workers [77] used an aprotic catholyte and a water-based anolyte separated by a Nafion sheet (membrane). The reaction was performed at 00C and -150C at an applied potential of -2.3 V vs. SCE. Due to the water-based electrolyte at the anode side, water could be split into protons and oxygen gas. The protons can go through the membrane (between catholyte and anolyte) and therefore, protons are available at the copper cathode. Methane and ethylene were formed up to almost 40% and 17%
Faraday efficiency respectively, depending on the temperature.
For water-based electrolytes there are three parameters for the electrolyte that influence the performance of the reaction, namely pH, the nature of the electrolyte (used anion/
cation) and the molarity.
The pH of the electrolyte can have a large influence on the product distribution. Hori et al. [83] did CO reduction experiments at a pH of 6.0 (phosphate), 9.6 (KHCO3) and 13 (KOH) for three different fixed currents. At -1 mA/cm2 using the phosphate buffer no hydrocarbon formation took place, while for the KHCO3-solution almost 22% ethylene formation was seen (and hardly any methane). At the more negative current of -2.5 mA/
cm2, the phosphate buffer shows methane production (9% FE) and also for the KHCO3- solution methane is now formed (16% FE). Ethylene production is still 22% FE. For an
20
even more negative current of -5 mA/cm2, the phosphate buffer shows 16% FE towards methane, and for the KHCO3-solution also the methane is still 16% FE, but the ethylene production collapses to 5.5% FE. At -5 mA/cm2 also a solution of KOH was used, where hardly any methane was formed and around 14% FE for ethylene is detected.
Depending on the applied current, the results for methane and ethylene production are very different at the various pH levels. The measured potentials at constant current for the three electrolytes are very different. Since the potential determines the product distribution (section 1.2), no clear conclusions can be drawn.
Salimon and Kalaji [84] show that the pH of the electrolyte does affect the electrocatalytic reduction of CO2 at a copper electrode. Voltammetric measurements show that the anodic peak shifts to more positive and the cathode peak to more negative potential with an increase in pH. Since no Faraday efficiencies are given, results cannot be related to the paper of Hori et al. [83].
Schouten et al. [85] investigated CO2 reduction on copper single crystals in 0.2 M phosphate buffers of pH 1 and pH 7. Higher pH could not be investigated, since CO2 strongly influences the pH. At pH 1, methane formation starts at -0.55 V vs. RHE for (100) while for (111) no reduction products were observed. At pH 7, (100) showed ethylene formation at lower potentials than methane formation, while (111) showed similar potential dependence for methane and ethylene.
Also other electrolyte types were used at the same pH [86]. Electrolytes with 0.1 M KCl, KClO4 and K2SO4, all having a pH around 5.9, were investigated. For all three electrolytes, methane production was around 11% and ethylene production around 47%. The hydrogen production was 6 to 8%. This indicates that the type of electrolyte has no influence on the product selectivity.
Friebe et al. [87] investigated the effect of (the size of) the cation (Li+ < Na+ < K+ < Rb+
< Cs+)on the reduction reaction at a copper deposited membrane electrode at a fixed potential of -2.2 V vs. NHE. The products were measured with a mass spectrometer, in all cases bicarbonate (HCO3-) was the anion. For Li+ and Na+ a small amount of methane was observed and ethylene was not detectable. K+ and Rb+ both show mainly methane, but also ethylene is observed. Cs+ shows equal mass signals for methane and ethylene.
The highest mass signal for methane and ethylene was observed for Rb+. CH4/C2H4 Faraday efficiency ratio increases in the order of Cs+ < Rb+ = K+ < Na+ = Li+.
Murata and Hori [88] used the same electrolytes, but performed the experiments at a fixed current of -5 mA/cm2. In all cases, both methane and ethylene were formed. The highest methane efficiency was observed for Na+ (55% efficiency), contradicting the results found by Friebe et al. [87]. Ethylene shows about 30% efficiency for both K+ and Cs+. CH4/C2H4 Faraday efficiency ratio increases in the order of Cs+ < K+ < Na+ < Li+, which shows the same trend as observed by Friebe.
The effect of HCO3- concentrationwas investigated by Hori et al. [53, 89]. For the reduction of CO at a copper electrode, a higher HCO3- concentration gives more methane formation. For the ethylene production, no clear relation was found. In the case of CO2-reduction [53, 86], a more clear relation is reported. Ethylene decreases linearly (from 40 to 5% FE) and hydrogen increases (from 10 to 45% FE) with an increase in HCO3- concentration (0.05 – 2 M). Methane production reaches an optimum of 30% FE for an electrolyte concentration of 0.3 M.
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Hori et al. also investigated the CO2 reduction in different concentrations of KCl [88].
In the range between 0.05 and 1 M the current efficiency1 towards methane is almost constant, as is the current efficiency towards ethylene (around 45%). Also a relatively high amount of ethanol is found (18%). Hydrogen production is low, around 10%.
In 1971 Udupa et al. [59] concluded that using only sodium sulfate on an amalgamated copper cathode gives a low current efficiency (around 18%), while adding sodium bicarbonate increased the current efficiency up to 80%. If no CO2 was added to the sodium bicarbonate, current efficiency was low again (18%), indicating that it was really the CO2 that was reacting.
From literature it is not directly clear which electrolyte is the best one to use, so a more structured study still needs to be done (see chapter 3.3).
The effects of different electrolytes on the productivity for other metals are discussed elsewhere [61, 90-95].
1.3.6 Copper Catalyst
The catalyst for the reduction of CO2 into hydrocarbons is an electrode made of a metal.
In this thesis the focus is on copper as electrode and serving as the catalyst for the production of hydrocarbons.
Frese and co-workers [50] showed that the pre-treatment of copper foil to remove the oxide layer has an effect on the methane production. They pretreated the copper in three different ways; I: scrubbing with “micro” brand aqueous detergent and then dipping in 10% HNO3; II: dipping in 10% HCl; III: dipping in 10% HCl and subsequently heating at 1000C (forming an oxide layer). Pretreatment I gave methane and CO as the only products, pretreatment II showed a higher methane production and also gave ethylene as a product. Oxidizing the surface (pretreatment III) killed the conversion of CO2. Although a clear influence of the pretreatment is seen, no analyses were done on the surface to see what the differences in morphology or impurities were.
Momose and co-workers [96] pre-treated the surface by applying a potential of -3.0 V vs. SCE for 5 minutes to remove the naturally formed oxide layer. Then electrocatalytic reduction was performed at -1.9 V vs. SCE. The clean copper surface gave higher hydrocarbon production (methane and ethylene) compared to the copper surface that was not treated before the catalytic reduction reaction. XPS analysis was done before and after the electrolysis, but not before and after the pre-treatment, so no conclusions can be drawn in terms of surface structural differences.
Also the effect of an oxide layer on the product formation was investigated. Frese [51]
found that if an oxide layer is present, methanol is formed at a rate of 4·10-6 mol/(cm2*h).
The methane production rate was at least one order of magnitude lower. Different oxidation times and oxidation temperatures were used, but no clear relation can be determined. In the article no results for a pure copper plate are shown, which makes it difficult to compare what the precise effect of the oxide layer is. Momose and co- workers [96] confirm the result of Frese: an oxide layer reduces the Faraday efficiency
1 In some papers the term current efficiency rather than Faraday efficiency is used. No clear definitions are given in any of those papers.
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towards methane and ethylene to less than 5%. XPS analysis of the surface before the catalytic reduction shows that the whole surface is covered with CuO and Cu2O, while after reduction peaks for metallic copper appear, indicating that the surface oxide is unstable and is reduced electrochemically during reaction. The instability of the copper oxide surface is confirmed by DFT calculations [97].
Li and Kanan [98] are very positive about a thick oxide layer before the start of CO2 reduction. They concluded that the thicker the oxide layer was, the lower the overpotential needed for the CO2 reduction to take place. However, this only holds for CO and HCOOH production; the production of methane and ethylene is completely absent when starting with an oxide layer on the surface.
Hori et al. investigated the effect of Cu single crystal structure on the CO2 reduction reaction for a 0.1 M KHCO3 electrolyte and a fixed current of -5 mA/cm2 at ambient temperature and pressure [83, 99]. They conclude that a (100) surface gives rise to ethylene production and (111) favours methane production. (110) gives a combination of both. Considering the data shown (Table 1.2), however, other conclusions could be drawn, namely that (100) yields both products, (110) mainly methane and some ethylene and (111) gives mainly hydrogen and methane. Eight years later Hori et al. published three articles [100-102] where more single crystals were investigated. Although they concluded that electrodes based on (100) terraced surfaces give ethylene as a major product and also (111) or (110) steps on a (100) plane enhance ethylene formation, the data given are not very conclusive. Gattrell et al. [103] and Lim et al. [104] combined the published data in review articles, but this did not lead to clearer conclusions.
Peterson et al. [105, 106] performed DFT calculations on different surface morphologies.
A very smooth surface should produce mainly hydrogen, whereas copper nanoparticles deposited on a copper substrate should give rise to ethylene production (36% FE).
They concluded that roughened surfaces should show higher selectivity towards hydrocarbons. The morphology effect is explained by the greater abundance of sites with a lower coordination, which are more likely to be active sites for CO2 reduction.
Instead of using flat copper plates as a catalyst, nowadays novel copper electrode architectures are designed, e.g. copper mesh, electrodeposited copper layers, nanostructured copper and copper foams. A short overview of the results that are described in literature is given below.
Conςalves et al. [107] investigated the effect of copper mesh and two modified copper electrodes, where an unspecified deposit was added. The copper mesh showed methane and ethylene with Faraday efficiencies of 19.4% and 18.7%, respectively.
Modified copper 1 showed mainly ethylene (33.3%) and a small amount of methane (3.6%). Modified copper 2 gave rise to the production of a new component, ethane (C2H6), and showed a decrease in total Faraday efficiency towards hydrocarbons.
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They concluded that the change in surface area was responsible for the different productivity and selectivity, but did not further specify their conclusion. In another article [108], they reported ex-situ copper deposition on the copper mesh. By applying different potentials, different surface morphologies were created. The methane current efficiency became almost 0 when a more negative potential was applied in the electrodeposition and a high selectivity towards ethylene was observed.
Electrodeposition of copper onto a substrate is one of the possibilities to change the surface morphology of the copper catalyst. Wasmus et al. [109] compared a bulk copper electrode with an electrode of glassy carbon onto which copper was deposited prior to the CO2 reduction. The reactions are performed at the same potentials, but the deposited copper gives a much higher productivity. They also did an experiment with in-situ deposition and saw a change in selectivity from methane towards ethylene, probably due to the Cu2+ in the electrolyte solution. Cook et al. [110] also performed in-situ copper deposition experiments by adding 5·10-4 M CuSO4 to the electrolyte and claim that a Faraday efficiency of almost 100% (73% methane and 25% ethylene) is reached at -8.3 mA/cm2 after 15 minutes of reaction. For longer reaction times, the efficiency decreases, probably due to the depletion of Cu2+ in the electrolyte.
Kanan and co-workers [111] used oxide-derived nanocrystalline copper (OD-Cu) as a catalyst. The OD-Cu was formed by annealing polycrystalline copper foil in air at 500°C and subsequently reducing the formed oxide layer into metallic copper nanocrystallites.
Copper reduction was performed in two different ways: electrochemically in aqueous solution at ambient temperature and in H2 at 130°C. The main products observed in CO2 reduction are acetate and ethanol; around -0.4 V vs. RHE some ethylene/ethane is obtained. The reduction method influences the product distribution.
Table 1.2: Products from CO2 reduction on single crystal Cu surfaces [99]
Electrode Potential (V) versus NHE
Current efficiency (%)
CH4 C2H4 CO HCOO- MeCHO EtOH H2 Total
Polycrystal -1.44 33.3 25.5 1.3 9.4 1.1 5.7 20.5 103.5
(100) -1.42 25.0 31.7 0.0 5.1 1.9 9.8 23.3 96.9
(110) -1.55 49.5 15.1 0.0 6.6 3.1 7.4 18.8 100.4
(111) -1.56 38.9 4.7 0.0 4.8 0.0 0.9 56.5 105.7
Electrolyte solution: 0.1 M KHCO3. Current density: 5.0 mA cm-2. Temperature: 18°C.
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Mul and co-workers [112] produced copper nanoparticles by depositing a copper oxide layer on a copper plate under different conditions. The surface morphology depended on the amount of Coulombs passed through during deposition. CO2 reduction was performed at -1.1 V vs. RHE in 0.1 M KHCO3. Ethylene was the main product (Faraday efficiency around 30%). Methane production decreased (from 4 to 0%) and ethane production increased (from 0 to 8%) with the amount of Coulombs used for deposition (so the amount of copper deposited). The total Faraday efficiency towards hydrocarbons decreased as well.
Zhang and co-workers [113] created copper fibrous structures by combining high- temperature annealing and electroreduction. The electrodes were tested by cyclic voltammetry in 0.5 M KHCO3. A Faraday efficiency of 43% for HCOO- was observed, other products were not reported.
New developments in the field of catalysts are metal alloys [114], nanostructured metals [115], metals on a graphite substrate [116-124], metal organic frameworks [125] and combinations of these options [114].
The newest surface architecture is copper nano-foam deposited on a copper substrate [126]. The copper foams show formic acid formation at a lower onset potential with Faraday efficiency of 37%, depending on the surface structure (deposition time). CO, methane and ethylene production are lower, whereas ethane and C3 compounds are produced in small amounts (<1.5%).
Although the copper electrode is directly the catalyst for the reduction of CO2 into hydrocarbons, only the last two years (2013/2014) papers describe very precisely how the catalyst is structured in terms of surface morphology and crystal structure.
1.3.7 Reaction Mechanism
Also the exact reaction mechanism is still unresolved. In 1967, Haynes and Sawyer [90]
were among the first to propose a reaction mechanism for the reduction of CO2 at a mercury or gold electrode. The main products formed on gold in anhydrous media are CO and the carbonate ion, explained by the one-electron reaction:
CO2 + e- à •CO2- (1.8)
2 •CO2- à CO + CO32- (1.9)
According to Haynes and Sawyer, in the presence of water, the ·CO2- reacts into a formate-ion and a second ·CO2- forms a bicarbonate-ion:
CO2 + e- à •CO2- (1.8)
•CO2- + H2O à HCO2- + •OH (1.10)
•OH + •CO2- à HCO3- (1.11) For mercury a two-electron reaction was proposed since chronopotentiometric data show two waves, indicating that two electrons are involved in the reaction. In presence of water, the reaction mechanism was described as:
CO2 + 2e- à CO22- (1.12)
CO22- + H2O à HCO2- + OH- rate-determining (1.13)
CO2 + OH- à HCO3- (1.14)
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In 1971 a third possible reaction mechanism was added [59] where water is split into OH- and H+ and the H+ reacts with CO2 to formic acid.
2 H+ + CO2 + 2e- à HCOOH (1.15)
Since copper is the only metal electrode that shows production of hydrocarbons, this mechanism cannot be valid. In 1989 Hori et al. [53] published a scheme explaining different routes towards different forms of hydrocarbons (figure 1.4a). They described a Fischer-Tropsch-like mechanism where adsorbed CO-molecules are added one by one to a hydrocarbon-chain to form an alcohol, and then the oxygen-atom is removed with adsorbed H+ under formation of water. In 1997 [89] Hori changes his view regarding the formation of CH4, which is not any more via the CH2,ad intermediate, but by the release of HCO- from the surface that reacts with protons and electrons into methane (figure 1.4c).
Noda and co-workers [77] described a different route (compared to Hori) for the formation of CO. They reasoned along the same lines as Haynes and Sawyer did. Adsorbed CO2 reacts with an electron to form CO2- and then either reacts with a second CO2-molecule to form CO and CO32- or with a proton to form CO and OH-. The CO then reacts with 4 protons to form carbene (•CH2) on the surface and a water molecule.
Wasmus et al. [109] suggested that two carbene species (•CH2) react with each other to form ethylene, and that this only happens at (electrochemically) ‘high’ temperatures (around 48°C). He agrees with Noda and co-workers on the intermediate-species CO and the way methane is formed, where he adds that this reaction happens at 0°C.
Figure 1.4: A) Hori’s mechanistic proposal for the production of different hydrocarbons at a copper electrode in 1989 [53]. B) A more detailed explanation for the formation of COads [40]. C) Hori’s proposed mechanism for the production of different hydrocarbons at a copper electrode in 1997 [89].
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In 1997, Friebe et al. [87] investigated the prevention of deactivation at the surface.
By applying anodic pulses, the copper surface can be cleaned and deactivation is postponed. They noted that the deactivation of methane and ethylene were both affected in the same way, and concluded that probably the carbene-species is removed from the surface by the anodic pulse and that the carbene-species indeed is an intermediate for both methane and ethylene formation. They also described that the cation of the electrolyte influences the choice of the reaction path after the carbene-species is formed, based on the different methane-to-ethylene ratios found for different cations (figure 1.5). They think that the reactive sites for the proton coupling are affected by the choice of electrolyte cation.
Frese [51] found methanol as a product and based on analysing the Faraday efficiencies he came to the following reaction mechanism where CO2 is converted to CO, which desorbs before reacting with water to form an HCO-species on the surface.
CO2,g à •CO + •O (1.16)
•CO à COg (1.17)
H2O + e- à OH- + •H (1.18)
COg + H2O à •HCO +•OH (1.19)
•HCO + 3•H à CH3OH (1.20)
Other reactions that take place are
•O + 2•H à H2O (1.21)
•OH +•H à H2O (1.22)
2•H à H2 (1.23)
Kaneco et al. [65] agree with the adsorption of CO2 on the surface, but have a different view on the formation of CH2,ad on the surface. Hori et al. [53] suggested that this species is formed via CHOH or C and O adsorbed, Kaneco describes the formation of CH2,ad by the reaction of COad with protons from the electrolyte (figure 1.6).
Figure 1.5: Reaction pathway scheme of the electrochemical CO2 reduction on copper according to Friebe et al. [87]. The cation determines which reaction path is preferred.
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In a review in 2003 Chaplin and Wragg [41] combined mechanistic proposals from different studies into one diagram. This diagram (figure 1.7) shows four principal pathways for CO2 reduction. Drawback of this diagram is that it only shows different pathways towards HCOOH.
Dubé and Brisard [127] investigated the reaction mechanism in acidic media by mass spectroscopy, which was connected to the electrochemical cell by a membrane. All products are formed from adsorbed species (CO2, CO, CH2 and/or H) on the surface.
Most important conclusion from this study is that the reduction products seem to depend on the pre-adsorbed sulfate on the surface, although how this influences the pathways towards different products is not discussed.
Jaramillo and co-workers [55] found many oxygenates (mainly containing two or three carbon atoms) and proposed that there might be an enol-like intermediate species on the surface. Because of the same potential at which all C2- and C3-products are formed, they suggested that this enol-like surface species is the key intermediate for the pathway to multi-carbon CO2 reduction.
New in the study on CO2 reduction is a more theoretical approach, where density functional theory (DFT) computer simulations are used to calculate the pathway with the least resistance to show which paths are favored at a certain potential, and thus influence productivity and selectivity. Peterson et al. [56] found that CHO is the important intermediate in the formation of ethylene.
The most recently suggested routes to methane and ethylene are proposed by Schouten et al. [128]. Where it was earlier suggested that the formation of methane and ethylene followed a different route after CH2,ad, they suspect a different route to methane and ethylene already after the formation of COad (figure 1.8). Ethylene is formed from the CO-dimer intermediate. In a later article [129] they propose that there are two separate pathways for the formation of ethylene, one at the (111) crystal surface sharing an intermediate with the pathway to methane, and a second pathway only occurring on the (100) crystal surface.
Figure 1.6: Reaction pathway according to Kaneco et al. [65].
28 Although a lot of literature can be found on this topic, each group gives a different pathway for the production of methane and ethylene from CO2. Some relate it to Schulz-Flory distributions [53], others claim that methane and ethylene are formed via completely different routes.
Literature about reaction mechanisms for other metals can be found elsewhere [38, 61, 68, 78, 79, 92, 93, 130, 131].
1.3.8 Reactor design
It is hard to compare results from different authors, since there are many operational parameters which influence the results as can be concluded from sections 1.3.1 to 1.3.6.
Another important variable is the reactor design. Depending on the research group, different reactors are used. The main parameters of the reactor design are not always properly described: is a membrane used, is there more than one reaction compartment, how are the electrodes put in place, where is the reference electrode positioned and how is the sampling taking place, is the liquid mixture stirred or is the electrolyte being circulated? Transport phenomena may induce resistances that affect local concentrations and gradients, but also the total electrical resistance of the system is of importance. Also operation in batch- or continuous mode can change the temporal conditions. All these kinds of aspects could be of importance for the performance of the catalyst, i.e. its productivity, selectivity and efficiency.
Figure 1.7: 4 reaction pathways to produce HCOOH according to Chaplin and Wragg [41].
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1.4 The research objective for the current project
The process to produce hydrocarbons electrochemically from carbon dioxide and water is a very complex reaction, where many parameters play an important role. So the main research question that will be addressed is:
“What is the influence of different parameters on the productivity and selectivity of the electrocatalytic reaction of carbon dioxide and water into hydrocarbons for copper catalysts?”
Productivity is defined as ‘the amount of hydrocarbons that is formed in a certain amount of time’. The selectivity is defined as the amount of electrons that are used for a specific hydrocarbon (e.g. methane, ethylene, etc.) in relation to the total amount of electrons.
As described above, many parameters influence the electrocatalytic reduction of CO2. Parameters can be divided into two groups: the external parameters that are based on the reaction conditions, e.g. applied potential or current, temperature, pressure and electrolyte; and the parameters that influence the activity of the catalyst, e.g. metal used, crystal structure, oxidation state and surface morphology.
This thesis will focus on both external parameters (current/potential, temperature and electrolyte) and the effect of catalyst morphology and purity for this process.
Since copper is the only metal that has been reported to produce hydrocarbons in the electrochemical CO2 reduction at ambient pressure and temperature, this thesis focuses on studying the reaction using copper as the basis for the working electrode.
1.5 Outline of the thesis
In chapter 2 a thorough description of the experimental aspects is given. The reactor design is presented together with important aspects to be considered when studying
Figure 1.8: Reaction pathway according to Schouten et al. [128].
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electrochemical reactions. In chapter 3 the effects of external parameters are discussed.
Current and potential are directly related and therefore discussed together, while temperature and the kind of electrolyte (pH and the size of cation and anion) are discussed separately. Chapters 4 and 5 focus on the copper electrode as a catalyst.
The performance of commercially available copper sheets (productivity, selectivity and efficiency) is presented in chapter 4, including the effect of surface morphology influenced by electrodeposition. Results with carbon as a substrate for the copper catalyst are presented in chapter 5. Carbon sheets, carbon nanofibers and carbon xerogels are investigated.
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Reactor design and
experimental basics
34
For improving the performance of chemical processes many challenges are faced, e.g. the search for a better catalyst, the challenge to optimize the preferred reaction conditions and minimize byproducts, as well as the design of the reactor in which the reaction takes place. Electrochemical reactions are even more complicated due to additional parameters like potential, current and resistance.
In this chapter, the design of an electrocatalytic reactor is described and special attention is paid to the challenges that have to be taken into account in this design. After the description of the reactor design, the standard conditions applied experimentally are discussed and statistically evaluated.
2.1 The first reactor design
The electrocatalytic conversion of CO2 was carried out in a three-electrode electrochemical cell. The first experiments were performed in the static reactor that was designed and used by Shibata [48, 132], and is shown in figure 2.1. A potentiostat, (HSV-100, Hokuto Denko) was used to apply a potential difference over the electrodes.
Figure 2.1: Schematic picture of Shibata’s electrochemical cell [48, 132].
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A platinum plate (Pt, Hokuto Denko, 10 x 20 x 0.1 mm3) was used as a counter electrode (anode), a copper plate as the working electrode (cathode) and a silver/silver chloride electrode as the reference electrode (Hokuto Denko HS205-C). The reference electrode was placed in a separate compartment and connected to the electrolyte by means of a capillary salt bridge of KCl and agar. The working- and counter electrode were clipped to electrical wires using crocodile clips and hung into the electrolyte.
2.2 Changes in the reactor design
The reactor design of Shibata [48, 132] was not yet optimal to perform reproducible experiments. Therefore, a few changes were made to the reactor design, to optimize reproducibility. These changes are described one by one, showing the problem regarding reproducibility and giving a solution to come to a new reactor design.
2.2.1 Positioning of the reference electrode
The position of the reference electrode was planned to be in a separate vessel. The connection between the electrolyte and the reference electrode was made using a capillary salt bridge. However, the capillary filled with electrolyte was slowly emptying, losing the connection with the reference electrode and therefore ruining the experiment.
By placing the reference electrode directly into the compartment with the other electrodes, the problem of a capillary salt bridge is eliminated. The only thing to keep in mind when the reference electrode is placed directly in the electrolyte, is if and how the reference electrode is influenced by the properties of the electrolyte and the conditions at which the reactions are taking place. For example, when the reference electrode is heated, a correction factor for the potential must be taken into account.
2.2.2 Positioning of the working- and counter electrode
The working- and counter electrodes were both connected to flexible and easily bendable electrical wires with crocodile clips. The clips are not allowed to touch the liquid, since they will work as reacting surfaces as well. Therefore, the surface area of both electrodes that is within the liquid was not reproducibly defined. The surface area is important, since this parameter is responsible for the availability of reaction sites and also determines the resistance of the system.
A holder was designed to clip the electrodes into, to create a fixed exposed surface area. The holder is made from Teflon, which is in itself non-conducting and therefore will not act as a reaction surface. The connection between the electrode material and the potentiostat is via an unbendable bar made out of stainless steel (for the copper plate working electrode) or brass (for the platinum plate counter electrode). The electrode plates are pressed to the bars by the Teflon holder to ensure electrical contact (figure 2.2). The holder also protects the unbendable bars from the electrolyte. The exposed surface areas of the copper and the platinum electrodes are 3.14 cm2 and 4.71 cm2, respectively.
When testing the effect of the holders, no products were detected during the first 2 hours. A closer look at the holders showed that gas bubbles were clinging to the
