Pt-substituted rutile SnO₂ (110) surface: first-principles study on its adsorption of formaldehyde

This work presents an extensive study for analyzing the adsorption mechanism of formaldehyde on pure and Pt-substituted rutile SnO₂ (110) surfaces via the Density Functional Theory (DFT) method. Out of the two suitable surface sites for Pt-substitution, namely, Sn_5c and Sn_6c, the latter was found to be more suitable. Three formaldehyde configurations were considered, monodentate η¹(O)-straight, monodentate η¹(O)-tilted, and bidentate η²(O, C)-tilted. It was found that after Pt-substitution, the adsorption energies for η¹(O)-tilted and η²(O, C)-tilted formaldehyde geometries were slightly improved. This improvement was found to be due to electron injection by Pt into the band gap of the surface combined with the stability of the formaldehyde orientation. Also, the overall adsorption energy values were better for the tilted configurations rather than those for the straight geometry. The changes in surface structure and bond lengths after optimization are seen to aid the adsorption mechanism by inducing steric effects and improved electrostatic interactions. Bader charge analysis shows two-fold interactions of charge transfer as well as only one-fold interaction in the cases of tilted and straight geometries respectively. Charge density difference (CDD) plots visually verified the above-stated results. The total density of states showed the injection of additional electronic states near zero energy (Fermi energy) level after Pt-substitution and an additional peak upon introduction of the gas molecule. PDOS plots were obtained to analyze the contribution of the orbitals of surface sites and the gas molecules to the gas adsorption mechanism. The recovery times for all the systems were obtained.