Abstract

UNGERER, Marietjie J.; SANTOS-CARBALLAL, David; VAN SITTERT, Cornelia G.C.E.  and  DE LEEUW, Nora H.. Competitive Adsorption of H₂O and SO₂ on Catalytic Platinum Surfaces: a Density Functional Theory Study. S.Afr.j.chem. (Online) [online]. 2021, vol.74, pp.57-68. ISSN 1996-840X.  http://dx.doi.org/10.17159/0379-4350/2021/v74a10.

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 SO₂ increases, 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.

Keywords : Platinum; water; sulphur dioxide; hydrogen; adsorption; density functional theory.

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