Competitive Adsorption of H₂O and SO₂ on Catalytic Platinum Surfaces: a Density Functional Theory Study

Abstract

Platinum has been widely used as the catalyst of choice for the production of hydrogen in the hybrid sulphur (HyS) cycle. In this cycle, water (H₂O) and  sulphur dioxide (SO₂) react to form sulphuric acid and hydrogen. However, the surface reactivity of platinum towards H₂O and SO₂ is not yet fully  understood, especially considering the competitive adsorption that may occur on the surface. In this study, we have carried out density functional theory  calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the competitive effect of both H₂O and SO₂ on the Pt (001), (011) and (111)  surfaces. Comparing the adsorption of a single H₂O molecule on the various Pt surfaces, it was found that the lowest adsorption energy (E_ads =  –1.758 eV) was obtained for the dissociative adsorption of H₂O on the (001) surface, followed by the molecular adsorption on the (011) surface (E_ads =  –0.699 eV) and (111) surface (E_ads = –0.464 eV). For the molecular SO₂ adsorption, the trend was similar, with the lowest adsorption energy (E_ads = –2.471  eV) obtained on the (001) surface, followed by the (011) surface (E_ads = –2.390 eV) and (111) surface (E_ads = –1.852 eV). During competitive adsorption by  H₂O and SO₂, the SO₂ molecule will therefore preferentially adsorb onto the Pt surface. If the concentration of SO_{2 i}ncreases, self-reaction between two  neighbouring SO₂ molecules may occur, leading to the formation of sulphur monoxide (SO) and -trioxide (SO₃) on the surface, which could lead to  sulphur poisoning of the Pt catalytic surface