Behavior of S, SO, and SO₃ on Pt (001), (011), and (111) surfaces: A DFT study

In the hybrid sulfur (HyS) cycle, the reaction between SO₂ and H₂O is manipulated to produce hydrogen with water and sulfuric acid as by-products. However, sulfur poisoning of the catalyst has been widely reported to occur in this cycle, which is due to strong chemisorption of sulfur on the metal surface. The catalysts may deactivate as a result of these impurities present in the reactants or incorporated in the catalyst during its preparation and operation of the HyS cycle. Here, we report a density functional theory investigation of the interaction between S, SO, and SO₃ with the Pt (001), (011), and (111) surfaces. First, we have investigated the adsorption of single gas phase molecules on the three Pt surfaces. During adsorption, the 4F hollow sites on the (001) and (011) surfaces and the fcc hollow site on the (111) surface were preferred. S adsorption followed the trend of (001)_4F > (011)_4F > (111)_fcc, while SO adsorption showed (001)_4F > (011)_bridge/4F > (111)_fcc and SO₃ adsorption was most stable in a S,O,O bound configuration on the (001)_4F > (011)_4F > (111)_fcc sites. The surface coverage was increased on all the surfaces until a monolayer was obtained. The highest surface coverage for S shows the trend (001)_S = (111)_S > (011)_S, and for SO it is (001)_SO > (011)_SO > (111)_SO, similar to SO₃ where we found (001)_SO3 > (011)_SO3 > (111)_SO3. These trends indicate that the (001) surface is more susceptible to S species poisoning. It is also evident that both the (001) and (111) surfaces were reactive toward S, leading to the formation of S₂. The high coverage of SO₃ showed the formation of SO₂ and SO₄, especially on the (011) surface. The thermodynamics indicated that an increased temperature of up to 2000 K resulted in Pt surfaces fully covered with elemental S. The SO coverage showed θ ≥ 1.00 on both the (001) and (011) surfaces and θ = 0.78 for the (111) surface in the experimental region where the HyS cycle is operated. Lower coverages of SO₃ were observed due to the size of the molecule.