Photocatalytic Hydrogen Production from H2S

  • Habeebllah B. Oladipo

Student thesis: Doctoral Thesis

Abstract

Hydrogen sulfide (H2S) is a noxious gas that confronts industries with a serious challenge in different areas such as environmental pollution and pipeline corrosion. It is produced in abundance in oil and gas industry, coal mining industry and wastewater treatment plant. Presently, Claus process represents the technology being used to dispose of H2S. However, it underutilizes H2S in the sense that the gas is converted to water and elemental sulfur. Recently, there is a growing interest in the replacement of fossil fuel with a clean fuel such as hydrogen. Thus, hydrogen production from H2S has a potential of solving environmental pollution caused by both H2S on one hand and burning of fossil fuel on the other hand. It is the aim of this work to prepare novel nanomaterials capable of generating hydrogen from H2S, study the mechanism of hydrogen production and perform techno-economical analysis of photocatalytic H2S splitting. In the first part of the work, anaerobic photocatalytic conversion of aqueous H2S to H2 was tested on Pt-decorated N-doped TiO2 grown on graphene. Hydrogen production was found to increase with HS‒ concentration until the critical value of 0.029 mol/dm3, after which it declined to negligible values. The latter was attributed to competitive adsorption between HS- ions and S2- ion, thus limiting the amount of adsorbed HS- ion available for hydrogen production. Similar tests performed at constant HS‒ concentration and variable pH did not yield a correlation, thus suggesting HS‒ as the major source of H2, and not H+. Consequently, a new kinetic model based on Langmuir-Hinshelwood mechanism reiterated the important role of sulfide ionic species in hydrogen production. A strong correlation between the latter and HS‒ concentration was established based on reactivity results and proposed mechanism. In the second part, a novel CdS nanocomposite grown on TiO2 was employed for hydrogen production from H2S. The preparation procedure affords a good contact between CdS and TiO2 as highlighted by UV-Vis Diffuse Reflectance Spectroscopy (DRS), X-Ray Diffraction (XRD) and Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) results. Comparison of hydrogen production from bare catalyst and the composites showed that hydrogen production rate was higher in all the synthesized composites of varying Ti to Cd molar ratio. Finally, an enhancement of photocatalytic activity of the composite was found when the suspension pH was increased to a value between 12.1 and 12.3 beyond which increase in S2- concentration became detrimental to hydrogen production. In the concluding part of the work, a process flow diagram (PFD) was proposed for photocatalytic H2S splitting. Economic analysis revealed that a significant portion of the CAPEX goes to the cost of purchasing the photocatalytic reactors needed in the process, thus indicating that further research is required in the synthesis of highly efficient photocatalysts for H2S splitting. With regard to OPEX, the cost of H2S solvent, namely NaOH, could create an economic challenge in scaling up photocatalytic H2S splitting.
Date of AwardMay 2020
Original languageAmerican English

Keywords

  • H2S Splitting
  • Photocatalysis
  • Bisulfide ions
  • Kinetic Modeling
  • Hydrogen production
  • Techno-economic analysis.

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