1 Electrochemical Studies of Catalysed Aqueous Sulphide Oxidation A thesis submitted for the degree of Doctor of Philosophy of the University of London and The Diploma of Imperial College by Ian Thompson Department of Mineral Resources Engineering Sept. 1987 Imperial College of Science and Technology University of London LONDON SW7 2BP But it's all right now, In fact it's a Gas Gas Gas! Mick Jagger (1968) Abstract 3 Abstract Electrochemical Studies of Catalysed Aqueous Sulphide Oxidation This thesis concerns the mechanism of oxidation of aqueous sulphide solutions in the British Gas Stretford Process, which uses atmospheric oxygen to achieve the partial oxidation of hydrogen sulphide, producing elemental sulphur and water. Hydrogen sulphide is absorbed in an alkaline solution (pH 8.5-9.5) containing vanadium (V) salts and anthraquinone derivatives which act as oxidation catalysts. The important methods of removing hydrogen sulphide from fuel gases were reviewed, and a detailed description of the Stretford Process was provided. The thermodynamic data on sulphur species were presented in the form of Eh/pH diagrams, and the literature relating to the oxidation of sulphide solutions was surveyed. The redox behaviour of sulphide and polysulphide solutions were investigated using electrochemical techniques such as cyclic and pulse voltammetry at gold ring-disc electrodes. It was shown that polysulphide species were important intermediates in the oxidation of HS" ions. The aqueous chemistry of vanadium was described and the electrochemical behaviour of vanadium (V) and (IV) solutions at pH 9.2 were investigated at mercury, carbon and gold electrodes. The electrochemical reduction of vanadium (V) was shown to be irreversible and to lead to vanadium oxide films, rather than to solution species. The redox chemistry of anthraquinone was reviewed and electrochemical studies were made of the compound anthraquinone 2,7-disulphonate. The reduced species and intermediates were identified using UV-visible spectrophotometry and ESR spectroscopy. The reduction pathways of oxygen in alkaline solution were reviewed, and the role of hydrogen peroxide as a possible reactive intermediate was investigated. The Stretford Process chemistry was examined using stopped-flow spectroscopic methods; these enabled the courses of the redox reactions between sulphide solutions and solutions containing the catalysts to be followed. A mechanism was proposed for the Stretford Process, and possible process improvements were discussed. Acknowledgements 4 Acknowledgements I would like to thank Dr. G. H. Kelsall for his supervision during the course of this work. Thanks must also go to Dr. T. Ritter from the British Gas London Research Station and to the other staff there who have given me help; Dr. D. Keene, Dr R. Mounce, Dr R. Gibbons, Roy Lowry, Lucien Anthony, and Susan Mahony. From Imperial College I would particularly like to thank Gordon "the glass" as well as the other technical and academic staff in the Mineral Resources Engineering and the Chemistry Departments. Research is not carried out alone. The present and past members of the research group have both aided my studies and, through their company, made them more enjoyable. Thank you. I would also like to acknowledge the financial help of British Gas, the Science and Engineering Research Council and last but definitely not least, Corina Thompson. Contents 5 Contents Abstract 3 Acknowledgements 4 Contents 5 List of Figures 9 List of Tables 12 1. Introduction: The Importance of Sulphide Oxidation 14 1.1 The Removal of Hydrogen Sulphide 14 1.1.1 Absorption by liquids 15 1.1.2 Adsorption by solids 16 1.1.3 Electrochemical oxidation of hydrogen sulphide 17 1.1.4 Aqueous oxidation of hydrogen sulphide 18 1.2 The Stretford Process 19 1.2.1 Historical development 2 0 1.2.2 Operational Problems 21 1.2.3 Mechanistic studies 23 1.3 Objectives of the Present Study 25 1.3.1. Research Approach 2 5 2. Review of Sulphide Oxidation 2 6 2 .1 The Oxidation States of Sulphur 2 9 2.1.1 Sulphide (-II) 2 9 2.1.2 Poly sulphides (-1 to 0) 3 0 2.1.3 Elemental Sulphur (0) 3 2 2.1.4 Polythionates (0 to IV) 3 3 2.1.5 Thiosulphate (II) 33 2.1.6 Sulphite (IV) 3 4 2.1.7 Sulphate (VI) 3 4 2.2 Electrochemical Studies of Sulphide Oxidation 3 4 2.3 Chemical Oxidation of Sulphide using Oxygen 3 7 2.3.1 Rate of Reaction of Sulphide Solutions with Oxygen 3 8 2.3.2 Effect of Temperature and pH on Reaction Rate 3 9 2.3.3 Catalysis of Sulphide Oxidation 3 9 2.3.4 B acterial Action in S ulphide Oxidation 4 0 2.4 The Production of Elemental Sulphur 4 0 Contents 6 3. Sulphide Electrochemistry 41 3.1 Thermodynamic Calculations 43 3.2 Experimental 44 3.2.1 Solution Preparation 45 3.2.2 Electrochemical Instrumentation 46 3.2.3 Electrode Pretreatment 47 3.2.4 Experimental: Ion chromatography 4 8 3.3 Sulphide Voltammetry: Results and Discussion 5 0 3.4 Thiosulphate Voltammetry: Results and Discussion 51 3.5 Polysulphide Voltammetry: Results and Discussion 53 3.6 Ring-Disc Studies: Results and Discussion 56 3.7 Calculated Polysulphide Concentrations vs. Potential 62 3.8 Detection of Polysulphides Using Ion Chromatography 64 3.8.1 Ion Chromatography: Results and Discussion 6 4 3.9 Summary 65 4. Vanadium 66 4.1 Vanadium (V) 67 4.2 Vanadium (IV) 7 0 4.3 Vanadium (V)/(IV) Compounds 72 4.4 Vanadium (III) 74 4.5 Vanadium (II) 7 4 4.6 Vanadium Electrochemistry 75 4.6.1 The Vanadium (V)/(IV) Couple 7 5 4.6.2 Vanadium (TV) reduction 76 4.6.3 The Vanadium (ffl)/(II) Couple 76 4.7 Oxidation of Vanadium (IV) solutions using Oxygen 76 4.8 Vanadium Sulphides 7 7 4.8.1 V3S,V5S4,VS 77 4.8.2 V2S5 78 4.8.3 VS2andVS4 78 4.9 Vanadium-Sulphur Complexes 7 8 4.10 Summary 80 5. Vanadium Electrochemistry 82 5.1 Vanadium Electrochemistry: Experimental 82 5.1.1 Solution Preparation 83 5.2 Vanadium (V) Voltammetry: Results and Discussion 84 5.3 Summary 91 Contents 7 6. Review of Anthraquinone Redox Chemistry 9 2 6.1 Anthraquinone Reduction 9 2 6.1.1 Substituent effects 9 4 6.1.2 Photo-reduction 9 5 6.2 Anthraquinones in the Production of Hydrogen Peroxide 9 6 7. Redox Chemistry of Anthraquinone 2,7-disulphonate 9 7 7.1 Purification of Anthraquinone 2,7-disulphonate 9 7 7.1.1 Analysis of the Purified Material 9 7 7.2 Experimental: Voltammetry 9 8 7.3 Experimental: Exhaustive Electrolysis 9 9 7.3.1 Calculations: Exhaustive Electrolysis 100 7.3.2 Calibration of Exhaustive Electrolysis Apparatus 101 7.4 Voltammetry: Results and Discussion 103 7.5 Exhaustive Electrolysis: Results and Discussion 108 7.6 UV-Visible Spectrophotometry: Experimental 111 7.7 Results and Discussion: UV-Visible Spectrophotometry 113 7.7.1 Spectral Assignments 114 7 .8 ESR Spectroscopy: Experimental 116 7 .9 ESR Spectroscopy: Results and Discussion 117 7 .1 0 ESR Spectral Structure 119 7.11 Summary 120 8. Oxygen Reduction 122 8.1 The Oxygen / Water Couple 124 8.1.1 The Evolution of Oxygen 126 8.2 Hydrogen Peroxide 127 8.3 Superoxides 129 8.4 Experimental 129 8.5 Oxygen Reduction: Results and Discussion 130 8.6 Summary 132 Contents 8 9. The Redox Chemistry of the Stretford Process 133 9.1 Experimental 133 9.1.1 S topped Flow Apparatus 134 9.1.2 Experimental: Measurement of Solution Potential 135 9.1.3 Experimental: Preparation o f51V NMR Samples 136 9.2 Reaction of AQ27DS and HS“: Stopped Flow Results 137 9.2.1 Rate Studies 138 9.2.2 S olution Potential Measurements 140 9.3 Reaction of V(V) and HS“: Stopped Flow Results 142 9.3.1 Vanadium (V) Reduction 144 9.4 Interaction of AQ27DH" ions with Oxygen 147 9.5 Stretford Solution Chemistry: Electrochemical Results 148 9.6 The Stretford Process: Possible Mechanism 150 10. Conclusions 152 10.1 The S(-II)/S(0) Redox Couple 152 10.2 The V(V)/V(IV) Redox Couple 152 10.3 The Anthraquinone/Anthraquinol Redox Couple 153 10.4 The C^/OH- Redox Couple 154 10.5 The Redox Chemistry of the Stretford Process 154 10.6 The Mechanism of the Stretford Process 155 10.7 Concluding Remarks 156 Appendix: Thermodynamic Data Used in Eh-pH Diagrams 158 References 161 Figures 9 List of Figures 1 .1 The Stretford Process. 19 2 .1 Possible valence states of sulphur in aqueous media. 2 6 2 .2 Efo-pH diagram for the sulphur/water system at 298 K. 27 2 .3 . Efo-pH diagram for metastable sulphur system at 298 K. 2 8 2 .4 E^-pH diagram for the sulphur/water system at 298 K. 2 9 (Oxy-sulphur anions not considered.) 3 .1 Eh-pH Diagram for the Au/Cl/S System. 4 2 3 .2 Eh-pH diagram for the sulphur/water system at 298 K. 43 3 .3 Metastable Eh-pH diagram for the S/H20 system at 298 K. 44 3 .4 A Rotating Ring Disc Electrode. 4 4 3 .5 Ion Chromatography Apparatus. 4 9 3 .6 Voltammogram of HS~ on Gold Plated Disc Electrode. 5 0 ([HS_] = 10 mol m-3, pH = 9.2, nth. cycle, 20 mV s_1.) 3 .7 Cyclic Voltammograms of Sodium Thiosulphate. 5 2 ([Na2S2C>3] = 10 mol m~3. 1st Scans. 100 mV s-1. pH = 8.2.) 3 .8 Voltammograms of Polysulphide Solution at a Gold Disc. 5 3 ([Sx] = 1 mol m-3.