Sulphur dioxide oxidation mechanism on Pt surfaces : a DFT study

Abstract

Lately, significant interest has risen towards the development of renewable energy in order to lower fossil fuel consumption and its environmental impact. Hydrogen (H2) has been proposed as a viable alternative to fossil fuels due to its high energy per unit mass, as well as its low environmental impact. Despite numerous methods that exist for obtaining H2, production of H2 is often difficult. Among various viable processes utilised to produce H2, is the Hybrid Sulphur (HyS) cycle wherein sulphur dioxide (SO2) and water (H2O) is thermo-electrochemically converted into sulphuric acid (H2SO4) and H2. Platinum (Pt) has proven to be an efficient catalyst for the HyS cycle, producing a high purity of H2. However, uncertainties regarding SO2-H2O interaction on the Pt surface persist, complicating efforts towards the development of catalyst with high efficiency. Although various experimental investigations have been performed, no absolute conclusions regarding the HyS cycle mechanism have thus far been made. Since little is still understood regarding the fundamental description of the HyS cycle, and in particular, the interaction between SO2 and H2O on the Pt surface, this dissertation fundamentally investigates the energetic and electronic properties of co-adsorbed SO2 and H2O on a Pt surface. Through density functional theory (DFT) analysis, it is observed that the adsorption of H2O on the Pt surface is significantly influenced by the presence of pre-adsorbed SO2, both in terms of adsorption energy and adsorption geometry. In addition, when comparing the influence of pre-adsorbed SO2 and preadsorbed H2O on the H2O adsorption process, it is observed that H2O adsorption is influenced more by pre-adsorbed SO2 than by pre-adsorbed H2O. Furthermore, the adsorption of other sulphur containing species such as sulphur monoxide SO and sulphuric acid H2SO4 (both cis- and trans-isomers), as well as the adsorption of H2 on Pt(111) are discussed. The co-adsorption of H2SO4 and H2 on Pt(111) is also discussed and compared to the co-adsorption of SO2 and H2O. Finally, possible SO2 oxidation mechanisms for the HyS reaction on Pt(111) were investigated. Both the Eley-Rideal and Langmuir-Hinshelwood mechanisms were evaluated on pristine Pt(111) with the former mechanism identified as occurring by preference.