INVESTIGATING THE ELECTROCHEMICAL REDUCTION OF CO2 AT METAL SULFIDES PAN XIAORAN (B Eng., Guangxi University) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF MATERIALS SCIENCE NATIONAL UNIVERSITY OF SINGAPORE 2006 ACKNOWLEDGEMENTS First of all, I wish to express my most sincere gratitude to my supervisor, Associate Professor Daniel John Blackwood This project could not have been finished without his guidance, warm-hearted encouragement, significant patience and considerate personality I am exceedingly grateful to be his student as I have learnt so much, not only in academic field, but also about life in general, which includes humility, honesty and kindness I would also like to thank Dr Xue Junmin of the Department of Materials Science and Engineering for his valuable discussion and suggestion Special thanks go to both academic and non-academic members who have helped and supported me It is very pleasant to remember my group members Hu Xiaoping, Liu Minghui, Vijayalakshmi and Sudesh I am so glad to know all of them and will never forget the great time we had Special thanks go to my friends over here for the joyful days and friendship that I will cherish the rest of my life Last but not the least, I wish to express my most cordially gratitude to my parents and sisters for their years of patience, affection and encouragement Thank you my dearest family, I love you all I TABLE OF CONTENTS ACKNOWLEDGEMENTS I TABLE OF CONTENTS II SUMMARY VI LIST OF TABLES IX LIST OF FIGURES IX Chapter Introduction and Literature Review 01 1.1 General 01 1.2 Economic Interest of CO2 Reduction 02 1.3 Origin of Life: Iron Sulfur World Theory 03 1.4 A Brief Introduction to Mackinawite and Pyrite 05 1.4.1 Mackinawite 05 1.4.2 Pyrite 06 1.5 Electrochemical Reduction of CO2 07 1.6 Electrochemical Methods 10 1.6.1 Potential-pH Diagram (Pourbaix Diagram) 10 1.6.2 Cyclie Voltammotry 12 1.7 Raman Spectroscopy 14 1.7.1 A Brief Introduction to Raman Spectroscopy 15 1.7.2 Surface Enhanced Raman Spectroscopy (SERS) 19 1.8 Thesis Layout 21 II References 22 Chapter Experimental 26 2.1 Voltammetric Measurements at an Iron Electrode 26 2.1.1 Cell Setup and Electrolyte Preparation 27 2.1.2 Iron Electrode Preparation 28 2.2 Electrosynthesis of Iron Sulfide (FeS) 29 2.3 Characteriazation of Synthetic Iron Sulfide (FeS) 29 2.3.1 X-ray Diffraction (XRD) 29 2.3.2 Scanning Electron Microscopy (SEM) 30 2.4 Voltammetric Measurement at synthetic FeS and Natural Pyrite (FeS2) 30 Electrode 2.4.1 Cell Setup and Electrolyte Preparation 30 2.4.2 FeS Electrode Preparation 31 2.4.3 Natural Pyrite (FeS2) Electrode Preparation 32 2.5 Surface Enhanced Raman Spectroscopy (SERS) 33 References 37 Chapter Electrochemically Synthesis of Mackinawite 38 3.1 Potential-pH Diagram of the Fe-H2O-S System 39 3.2 Voltammogram of the Iron Plate 40 3.3 Synthetic FeS 43 3.4 Summary 50 III References 51 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes 52 4.1 Study of CO2 Electroreduction at Synthetic Mackinawite (FeS) Electrode 52 4.1.1 Voltammetric Measurement 52 4.1.2 Raman Spectroscopy 53 4.1.2.1 Background Peaks 54 4.1.2.2 In situ SERS at a FeS Electrode 55 4.2 Study of CO2 Electroreduction at Natural Pyrite (FeS2) Electrode 58 4.2.1 Physical Characterization of Natural Pyrite (FeS2) Electrode 58 4.2.2 Voltammetric Measurement 59 4.2.2.1 Cyclic Voltammetry of Natural Pyrite (FeS2) in Phosphate 59 Buffer 4.2.2.2 Cathodic Behavior of Natural Pyrite (FeS2) in Different 68 Gas-saturated Phosphate Buffers 4.2.3 Raman Spectroscopy 4.2.3.1 Background Peaks 72 72 4.2.3.2 Raman Spectra of CO Adsorption at a Natural Pyrite (FeS2) 73 Electrode 4.2.3.3 Raman Spectra of CO Adsorption at a Natural Pyrite (FeS2) 78 Electrode during CO2 Electroreduction References 84 IV Chapter Conclusion 86 Chapter Future Work 89 V SUMMARY The reduction of carbon dioxide to alcohols and other multi-carbon organic molecules is of great economic and scientific interest The economic aspect comes from the desire to use environmentally friendly fuel cells to power automobiles, an objective that will only be economically feasible with a liquid fuel (e.g alcohols) The scientific interest stems from the new theories, proposed by Wächtershäuser and named as “Iron Sulfur World Theory”, which claim that the reduction of carbon dioxide at iron sulphide surfaces under the high temperatures and pressures experienced at deep ocean volcanic vents is the origins of all life The work for this thesis consisted of two parts In the first part the process of electrochemical synthesis of FeS has been studied for the desire to obtain pure FeS which would serve as electrodes in the second part From the voltammograms of the Fe plate in electrolytes with various pH, it was found that the onset potential for the formation of the FeS layer at Fe electrodes shifted toward the negative direction with decreasing electrolyte pH Furthermore, the structures of FeS deposits showed an increasing porosity with reducing electrolyte pH Characterization of the synthetic FeS by XRD and SEM showed that the FeS produced at pH 11 and -0.5V (NHE) had the highest purity, so that the optimal conditions for preparation of FeS were pH 11 and -0.5V (NHE) VI The second part of the project involved investigating the mechanism of CO2 electroreduction at FeS and pyrite (FeS2) electrodes by using cyclic voltammetry and in situ surface enhanced Raman spectroscopy (SERS) This represented the first study of CO2 reduction at metal sulfides semiconductor electrodes Voltammograms from FeS electrodes illustrated the suppression of H2 evolution in the presence of CO or CO2 The fact that CO showed stronger restriction ability suggested the formation of adsorbed CO intermediate during the electroreduction of CO2 However, in situ SERS study did not lead to a confirmation of the CO-formation mechanism, as the characteristic CO stretching peak was not observed This may be due to the lost of the Ag needed to enhance the Raman signal during the experiments which is caused by the low stability of the FeS electrode itself At FeS2 electrodes, a phosphate deprotonation process which can be represented as the following reactions, H2PO4– + e–→ Had + HPO42– – – Had + H2PO4 + e → H2 ↑+ HPO42 – was observed It is the first time that this process is observed at a semiconductor electrode From the voltammograms at FeS2 electrodes, it was proposed that CO2 was reduced at FeS2 electrodes through a step of CO adsorption In situ SERS supported this proposal as Raman spectra collected in the CO2-saturated solution showed a characteristic CO VII stretching band at ~2080 cm-1, and this band increased in intensity and shifted to a higher wavenumber when the applied potential was decreased This observation implied that adsorbed CO may be produced in the CO2-saturated solution by the electroreduction of CO2 VIII LIST OF FIGURES AND TABLES TABLES Table 1.1 CO2 reduction at metal electrodes in 0.5M KHCO3 at 25℃ 08 (Adapted for Ref.[19]) Table 3.1 Summary of EDX reports for synthetic Mackinawite samples 47 synthesized at different pH values Table 3.2 Estimated particle and pore sized of the synthesized 50 mackinawite produced in different pH solutions Table 4.1 Dependence of the cathodic peak current density on the 67 concentration of phosphate buffer FIGURES Figure 1.1 Crystal structure of mackinawite (FeS) 06 Figure 1.2 Crystal structure of pyrite (FeS2) 07 Figure 1.3 Schematic cyclic voltammogram for a reversible process, 13 initially only O present in solution Figure 1.4 Theoretical cyclic voltammograms for the reduction of O 14 (adapted from Ref.[32]) Figure 1.5 Simplified presentation of the Raman mechanism 16 Figure 1.6 Energy level diagrams for Raman scattering 17 Figure 1.7 The CO2 molecule and its polarizability ellipsoid during the 18 IX Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes (Figure 4.16c and Figure 4.16d) The peak shifting is not very apparent from the direct comparison between Raman spectra under the two different potentials (-1.0V and -1.2V respectively) It is more visible if the spectrum of -1.0V is subtracted from the spectrum of -1.2V; as such species that are stronger at -1.2V result in upward pointing bands whilst downward pointing bands are from vibrations more prevalent at -1.0V (Figure 4.17) From Figure 4.17, it can be seen that the peak position of the CO stretching band shifted to higher wavenumbers when the pyrite electrode was polarized at -1.2V, representing a decrease in adsorption strength The increase of the peak density can be related to the higher CO coverage at the electrode surface, as more CO molecules are adsorbed under more negative potentials Meanwhile, since the negatively polarized electrode and electrons of adsorbed CO molecules repel each other, the electron density between C and O is increased, which causes an increased extent in the C-O bond stretching and hence an increase stretching frequency (wavenumber) This may explain the blueshift of the CO stretching band Raman peaks for the FeS2-CO vibration were expected to occur at the wavenumber range of 300-600cm-1, however, no salient peaks for this vibration were detected because the Raman peaks for quartz window and the pyrite electrode unfortunately overlap with and dominate this region As the potential reached -1.4V (Figure 4.15d), no valuable information was obtained because H2 evolved at this potential and the resultant bubbles destroyed the Raman spectrum (see Section 4.1.2.2) 76 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes 2100-2200 cm -1 d (-1.2V) Intensity (A.U.) 2000-2200 cm -1 c(-1.0V) b(-0.8V) a(open circuit) 1900 2000 2100 2200 2300 -1 W avenumber (cm ) Intensity (A.U.) Figure 4.16 The partially enlarged potential-dependent in situ Raman spectra of pyrite electrode in CO-saturated 0.1M K2HPO4+ 0.1M KH2PO4 solution (a) control spectrum;(b) -0.8V; (c) -1.0V; (d) -1.2V 19 0 20 0 21 0 22 0 23 0 -1 W a v e nu m b er (cm ) Figure 4.17 Subtractively normalized difference spectrum between -1.0V and -1.2V (Subtraction of Figure 4.16c from 4.16d 77 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes 4.2.3.3 Raman Spectra of CO Adsorption at a Natural Pyrite (FeS2) Electrode during CO2 Electroreduction In situ Raman spectra were also obtained from the CO2-saturated phosphate buffer in order to study the mechanism of CO2 reduction at the pyrite electrode As the concentration of the phosphate buffer (0.15M K2HPO4+0.05M KH2PO4) for saturating CO2 was different from that for saturating CO (0.1M K2HPO4+0.1M KH2PO4), another control spectrum (Figure 4.18) was collected from a pyrite electrode in the CO2-saturated 0.15M K2HPO4 + 0.05M KH2PO4 solution and under open circuit potential Band assignments are given below: 343cm-1, 379cm-1 and 431cm-1 peaks are from the pyrite electrode The assignments of these three peaks are the same as those obtained from the N2-saturated phosphate buffer (see Section 4.2.3.1) 880cm-1, 991cm-1 and 1074cm-1 peaks are from the phosphate buffer solution The peaks positions are identical to those from the CO-saturated phosphate buffer (see Section 4.2.3.1), so that the three peaks are attributed to the symmetric stretching of the two P-OH bonds in H2PO4- ions , the three P-O bonds in HPO42ions , and the two P-O bonds in H2PO4- ions respectively, too However, different relative intensities were observed For the CO-saturated 0.1M K2HPO4+0.1M KH2PO4 solution, the 991cm-1 peak (from HPO42- ions) is the strongest among the three peaks, whereas for the CO2- saturated 0.15M K2HPO4+0.05M KH2PO4 solution, the 991cm-1 peak becomes the weakest This can be explained as follows: 78 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes The introduction of CO2 releases protons by the reaction: CO2 + H2O → HCO3– + H+ (4.7) and the protons displaces the equilibrium H2PO4– HPO42– + H+ (4.6) to the left hand side So that the 991cm-1 peak which is caused by the HPO42ions loses its intensity due to the quantity decrease of the HPO42- ions, whilst the intensities of 880 and 1074 cm-1 peaks associated with H2PO4- increase 1640cm-1 peak is for the bending vibration of water molecules (H2O), the broad hump from 300cm-1 to 600cm-1 is caused by the quartz window and a very weak peak at 2330cm-1 is due to the symmetric stretching of gaseous N2 from atmosphere 79 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes 379 1600 343 2330 1640 880 800 991 1074 431 Intensity (A.U.) 1200 400 500 1000 1500 2000 2500 -1 W a v e n u m b e r (c m ) Figure 4.18 Control Raman spectrum from pyrite electrode and CO2-saturated 0.15M K2HPO4+ 0.05M KH2PO4 solution in the Raman cell Note that the spectral resolution of the Raman instrument is ±2.5cm-1 Figure 4.19 shows the in situ Raman spectra collected during CO2 reduction at a pyrite electrode in 0.15M K2HPO4+0.05M KH2PO4 solution The starting potential was -0.8V and the electrode potential was then altered to negative values in a staircase fashion with 0.2V increments For convenience of comparison, Raman spectra of the interested wavenumber region 1800-2300cm-1 are stacked upward toward decreasing potential in Figure 4.20 80 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes 2000 379 1600 (a) 2330 800 1640 880 991 1074 343 431 1200 400 500 1000 1500 2000 2500 2000 379 1600 2330 2000-2190 1640 880 991 1074 800 400 1600 500 1000 1500 2000 2500 (c) 2330 800 1970-2270 880 991 1074 343 431 1200 1640 2000 379 Intensity (A.U.) (b) 343 431 1200 400 500 1000 1500 2000 2500 2000 1600 (d) 1200 800 400 500 1000 1500 2000 2500 -1 Wavenumber (cm ) Figure 4.19 The potential-dependent in situ Raman spectra of pyrite electrode in CO2-saturated 0.15M K2HPO4+ 0.05M KH2PO4 solution (a) -0.8V;(b) -1.0V; (c) -1.2V; (d) -1.4V 81 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes The Raman spectrum at -0.8V (Figure 4.19a) was similar to the control one, indicating that no new substances are produced at this potential As the potential reached -1.0V, a broad and weak band arose, centralizing at about 2080cm-1 (Figure 4.19b and Figure 4.20c) This observation is similar to the Raman study for the CO-saturated phosphate buffer, suggesting that the CO2 it is the CO molecules produced by the CO2 reduction that give rise to this 2080cm-1 peak When the electrode was polarized at -1.2V, the 2080cm-1 peak increased in intensity and shifted to a higher wavenumber, 2206cm-1 (Figure 4.19c and Figure 4.20d) The subtraction between Raman spectra under -1.2V and -1.0V (Figure 4.21) illustrates this blueshift quite apparent The C-O stretching band shifted to higher wavenumber at -1.2V The intensity increasing and the peak shifting of the 2080cm-1 band are analogous to the corresponding band observed in the CO-saturated electrolyte, thus the behavior of the adsorbed molecules behind this observation may be similar as proposed for the CO adsorption study (see Section 4.2.3.2) The more negative potential further reduced CO2 and produced more CO molecules, thus the increase in the quantity of CO leads to the enhancement of the 2080cm-1 peak, and the stronger repellence between the electrode and the electrons of the adsorbed CO results in the blueshift of the 2080cm-1 peak The potential excursion was not able to go beyond -1.4V due to the evolution and accumulation of H2 bubbles 82 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes 1970-2270 cm Intensity (A.U.) 2000-2190 cm -1 -1 d(-1.2V) c(-1.0V) b(-0.8V) a(open circuit) 1800 2000 2200 2400 -1 W avenumber (cm ) Intensity (A.U.) Figure 4.20 The partially enlarged potential-dependent in situ Raman spectra of pyrite electrode in CO2-saturated 0.15M K2HPO4+ 0.05M KH2PO4 solution (a) control spectrum;(b) -0.8V; (c) -1.0V; (d) -1.2V 2000 2100 2200 2300 2400 -1 W avenum ber (cm ) Figure 4.21 Subtractively normalized difference spectrum between -1.0V and -1.2V (Subtraction of Figure 4.20c from 4.20d) 83 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes References [1] R P Van Duyne, in “Chemical and Biochemical Applications of Lasers”, Ed by C B Moore, 1979, Academic Press, New York, USA [2] V V Marinyuk, R M Lazorenko-Manevich, Y M Kolotyrkin, J Electroanal Chem., 110 (1980), 111 [3] J Lara, T Blunt, P Kotvis, A Riga, W T Tysoe, J Phys Chem B, 102 (1998), 1703 [4] H L Park, W B White, Phys Status Solidi., K69 (1987), 144 [5] G Niaura, A K Gaigalas, V L Vilker, J Phys Chem B, 101 (1997), 9250 [6] G Socrates, “Infrared and Raman Characteristic Group Frequencies”, 2001, John Wiley & Sons Ltd., New York, USA [7] A Wirasentana, “Characterization of Pyrite FeS2 Semiconductor Materials”, Hon Sc Thesis, National University of Singapore, 2003 [8] G H Kelsall, Q Yin, D J Vaughan, K E R England, N P Brandon, J Electroanal Chem., 471 (1999), 116 [9] P O’Neill, F Busi, V Concialini, O Tubertini, J Electroanl Chem., 284 (1990), 59 [10] K Takahara, Y Ide, T Nakazato, N Yoza, J Electroanal Chem., 293 (1990), 285 [11] S D Silva, R Basséguy, A Bergel, Electrochim Acta, 49 (2004), 4553 [12] W Jaegermann, H Tributsch, J Appl Electrochem., 13 (1983), 743 [13] Southampton Elctrochemsitry Group, “Instrumental Methods in Electrochemistry”, 1985, Ellis Horwood Ltd., Chichester, UK [14] I D Zaytsev, G G Aseyev, “Aqueous Solutions of Electrolytes”, 1992, CRC Press Inc., Boca Raton, USA [15] C Peigen, Z Qiling, S Yuhua, G Renao, Chem Phys Lett., 376 (2003), 806 [16] J J Kim, D P Summers, K W Frese Jr., J Electroanal Chem., 245 (1988), 223 [17] Y Hori, A Murata, R Takahashi, J Chem Soc., Faraday Trans 1, 85 (1989), 2309 84 Chapter Study of CO2 Electroreduction at Metal Sulfide Electrodes [18] Y Hori, K Kikuchi, S Suzuki, Chem Lett., (1985), 1695 [19] J Li, M E Wadsworth, Proceedings of 4th Milton E Wadsworth International Symposium on Hydrometallurgy, The Society for Mining Metallurgy, Exploration, Salt Lake City, USA, 1993, 127 [20] J Li, X Zhu, M E Wadsworth, Proceedings of TMS Annual Meeting, The Minerals Metallurgy, Materials Society, Denver, USA, 1993, 229 [21] S Ushioda, Solid State Commun., 10 (1972), 307 [22] H Vogt, T Chattopadhyay, H J Stolz, J Phys Chem Solids, 44 (1983), 869 [23] C Sourisseau, R Cavagnat, M Fouassier, J Phys Chem Solids, 52 (1991), 537 [24] B D Smith, D E Irish, P Kedzierzawski, J Augustynski, J Electrochem Soc., 144 (1997), 4288 [25] I Oda, H Ogasawara, M Ito, Langmuir, 12 (1996), 1094 [26] P Cao, Y Sun, R Gu, J Phys Chem B, 108 (2004), 4716 [27] A Kudelski, P Kedzierzawski, J Bukowska, M Janik-Czachor, Russ J Electrochem., 36 (2000), 1186 85 Chapter Conclusion CHAPTER CONCLUSION Electrochemical reduction of CO2 has attracted great interests for both economic and scientific sakes The economic aspect comes from the aspiration to have environmental friendly fuel via the production of methanol or ethanol by the reduction of CO2 or CO The scientific attention stems from the new theories, proposed by Wächtershäuser and named as “Iron Sulfur World Theory”, which claim that the reduction of carbon dioxide at iron sulfide is at the very heart of the origins of life In this work focus was predominately on the area of scientific interest The mechanism of CO2 electroreduction at metal sulfides, including electrochemically grown FeS and natural pyrite (FeS2), was studied by cyclic voltammetry and in situ Raman spectroscopy The work for this thesis consisted of mainly two parts In the first part, the process of electrochemically synthesizing FeS was studied The main conclusions from this part were: The onset potential for the formation of the FeS layer at Fe electrodes shiftsed toward the negative direction with decreasing electrolyte pH Structures of FeS deposits showed an increasing porosity with reducing electrolyte pH The formation rate of FeS deposit also increased with the decreasing electrolyte pH FeS formed at pH and 10 contained impurities for the electrolye, while FeS produced at pH 12 had impurities from the electrode 86 Chapter Conclusion The optimal conditions for preparation of FeS were pH 11 and -0.5V (NHE) The second part involved investigating the mechanism of CO2 electroreduction at FeS and natural pyrite (FeS2) electrodes by using cyclic voltammetry and in situ Raman spectroscopy This represented the first study of CO2 reduction at metal sulfides semiconductor electrodes The major findings were: At FeS (mackinawite) electrode Voltammograms from FeS electrodes illustrated the suppression of H2 evolution in the presence of CO or CO2 The fact that CO showed stronger restriction ability suggests the formation of adsorbed CO intermediate during the electroreduction of CO2 In situ Raman study did not lead to a confirmation of the CO-formation mechanism, as the characteristic CO stretching peak was not observed, even with the effort of enhancing the Raman signal by depositing a discontinuous layer of silver However, the existence of CO adsorption at FeS electrodes cannot be eliminated, as the absence of CO stretching band may be due to the lost of the Ag needed to enhance the Raman signal during the experiments which was caused by the low stability of the FeS electrode itself At FeS2 (natural pyrite) electrode A cathodic peak observed only in phosphate solution at -1.5V (NHE) was designated to a phosphate deprotonation process, with current density of the peak 87 Chapter Conclusion showing a proportional dependence on the concentration of the phosphate electrolyte This is the first time that the phosphate deprotontation process is observed at a semiconductor electrode CO2 was reduced at FeS2 electrodes through a step of CO formation; CO2-saturated solution was found to inhibit the adsorption of phosphate ions to a less extant than the CO-saturated solution In situ Raman spectroscopy supported above proposal In the CO-saturated solution, a broad peak at 2080cm-1 was observed at -1.0V (NHE), and this peak was designated to the C-O stretching vibration CO molecules may be adsorbed to the electrode by the formation of bonds between C atoms and the electrode The 2080cm-1 peak increaseed in the intensity and shifted to higher wavenumber when the electrode was polarized at -1.2V (NHE) The increase of the peak intensity can be related to the higher CO coverage at the electrode surface Likewise, the more negatively the electrode was charged, the more the electron cloud was repelled to the C-O bond, which causeed an increased extent in the C-O bond stretching and hence an increase stretching frequency (wavenumber) Raman spectra collected in the CO2-saturated solution were similar to those in the CO-saturated solution, implying that adsorbed CO may be produced in the CO2-saturated solution by the electroreduction of CO2 88 Chapter Future Work CHAPTER FUTURE WORK Although the current work has obtained some information about the mechanism of CO2 electroreduction at FeS and FeS2 electrodes, the thorough understand of this process still remains insufficiently investigated In order to get a clearer picture about the mechanism of CO2 reduction at FeS electrodes, there is a need to overcome the low stability of the FeS pellet electrode To prevent the lost of the Ag which is needed to enhance the Raman signal, more work should be carried out to improve the adhesion of the electrochemically grown FeS powder, for example, certain bonding additives could be added into the FeS powder, and a series of tests on the properties of the FeS pellet would be needed to determine the percentage of the additive required to achieve adhesion with a significant degradation in electronic properties Future work should focus on the electroreduction of CO2 or CO at both FeS and FeS2 electrodes, as the full scheme of the CO2 or CO reduction path ways could not be obtained without the analysis of the reduction product Gas or liquid chromatography would be a suitable candidate for the purpose of product analysis The ultimate goal of this project is to test the possibility of the cathode reduction of CO2 or CO at metal sulfides and the resulting formation of organic substance under model conditions simulating natural hydrothermal systems, in an attempt to support 89 Chapter Future Work the “Iron Sulfur World” theory which claims that the origin of life may lie in the formation of precursor chemicals of living cells at magmatic exhalations of volcano and hydrothermal vents An autoclave would be serving as the reaction container for the electrochemical reduction experiments 90 [...]... Figure 1.5 Simplified presentation of the Raman mechanism Two distinct events are possible for the Raman scattering The energy of the scattered radiation is less than the incident radiation for the Stokes line (Figure 1.6a) and the energy of the scattered radiation is more than the incident radiation for the anti-Stokes line (Figure 1.6b) Since at thermal equilibrium the number of molecules in a lower energy... anions at these later metals [28], so that after forming at the electrode, free radical anions are protonated to produce formate ions (Reaction 1.5) The development of this process, depending on electrolyte type and pH, may lead to the production of more reduced and complex organic molecules, such as oxalic acid and saturated hydrocarbons The mechanism of the electrochemical reduction of CO2 at different... Cyclic voltammograms of pyrite electrode in N2-saturated 64 0.1M K2HPO4+0.1M KH2PO4 (pH 6.8) at a scan rate of 20mV/s, (a) stirred; (b) stationary Figure 4.9 Voltammograms of pyrite electrode in different 66 concentration of K2HPO4+ KH2PO4 buffer saturated with N2 at a scan rate of 20mV/s Figure 4.10 Dependence of the observed cathodic peak current density 67 on the concentration of phosphate buffer according... the metal (M) to the adsorbate (A) and vice versa [55] 20 Chapter 1 Introduction and Literature Survey 1.8 Thesis Layout The overall objective of this project was to study the mechanism of the electrochemical reduction of CO /CO2 at metal sulfide electrodes which included anodically grown FeS and the natural mineral material FeS2 (pyrite) A range of electrochemical techniques were employed to achieve the. .. Since the appearance of these diagrams, 10 Chapter 1 Introduction and Literature Survey they have rapidly evolved towards problems of the electrochemistry of metals, and corrosion in particular The productiveness of the method caused them to be applied to all the elements-metals and non-metals, and its applications have developed greatly, spreading to other branches of electrochemistry and related fields... forward that metabolism was more fundamental than any of the cell machineries He calls his own proposal The Iron Sulfur World Theory” because he believes that metallic surfaces, particularly that of the common mineral iron sulfide, would have been promising facilitators, or catalysts, of the chemical reactions that created the precursor chemicals of living cells The known extant patterns of metabolism... [31] These findings all support the mechanism of CO2 reduction proposed by Jitaru [27] that CO formation is the first elemental step of the electroreduction of CO2 in aqueous electrolyte In the present project, the mechanism of CO2 electroreduction on pyrite (FeS2) and mackinawite (FeS) was studied by cyclic voltammotry and Raman spectroscopy This represents the first attempt to study this reaction at. .. spectra of FeS 56 electrode in CO-saturated 0.1M K2HPO4+ 0.1M KH2PO4 solution Figure 4.4 SEM images of the FeS electrode 58 Figure 4.5 XRD pattern of FeS2 59 Figure 4.6 Cyclic voltammogram of a pyrite electrode in 0.1M 61 K2HPO4+ 0.1M KH2PO4 (pH 6.8) saturated with N2 at a scan rate of 20mV/s Figure 4.7 Cyclic voltammograms of pyrite electrode in stationary 62 N2-saturated 0.1M KCl at a scan rate of 20mV/s... “pair of S atoms” located in the mid of the cube’s edges and in the center of the unit cell (Figure 1.2) 6 Chapter 1 Introduction and Literature Survey S Fe Figure 1.2 Crystal structure of pyrite (FeS2) Pyrite is the most abundant and widespread sulfide mineral in the Earth’s crust [20] The mechanism by which pyrite forms in nature is of scientific interest, as it has been suggested that the formation of. .. are the concentrations of oxidized (O) and reduced (R) species At 25ºC (298K), the numeric values of the constants and of the conversions from logarithms of base e (ln) to logarithms of base ten (log) can be combined to give a simpler form of the Nernst equation: Ee = Ee∅ + 0.059 cO log n cR (1.10) The E-pH diagrams give us a panoramic view of the “chemical configuration” of systems, predicted on a thermodynamic ... for the Raman scattering The energy of the scattered radiation is less than the incident radiation for the Stokes line (Figure 1.6a) and the energy of the scattered radiation is more than the. .. source of carbon for synthesis of secondary fuels and intermediates [4] The scientific attention stems from the theories which claim that the reduction of carbon dioxide at iron sulfide is at the. .. This observation implied that adsorbed CO may be produced in the CO2- saturated solution by the electroreduction of CO2 VIII LIST OF FIGURES AND TABLES TABLES Table 1.1 CO2 reduction at metal electrodes