Carbon dioxide adsorption on flat Pt3Sn surfaces

Abstract

The experimental studies have shown that the PROX activity of the Pt{Sn/AC catalyst is either unaffected or positively affected from the presence of carbon dioxide in the feed mixture. The catalyst characterization studies revealed that Pt{Sn/AC system has Pt3Sn alloy as one of the active metallic phases and the alloy plays a dominant role in the PROX activity. Thus, the unaffected or enhanced PROX activity levels by the presence of carbon dioxide can be explained by either the stabilization of carbon dioxide on AC surface or limited interaction between carbon dioxide and Pt3Sn alloy surface. In this computational work, the proposed mechanism on the alloy surface was investigated by reproducing the periodic vacuum slabs of the Pt3Sn alloy. Thus, carbon dioxide adsorption on the Pt3Sn (111), (110), and (001) surfaces was studied at atomic level by utilizing semi ab-initio quantum mechanical code, CASTEP. The surfaces of Pt3Sn alloy were generated with all possible bulk terminations, and all types of active sites on these terminations were tested for carbon dioxide adsorption. The adsorption energies of carbon dioxide molecule at all adsorption sites were calculated. The Local Density of States (LDOS) analysis was used for understanding the electronic interaction between the atoms of the adsorption sites and carbon dioxide molecule. In order to identify whether the carbon dioxide adsorption is stable, geometries of the carbon dioxide molecule as well as LDOS of both the carbon dioxide molecule and binding atom of the surface, Pt and/or Sn, were analyzed for all of the sites on each termination. The LDOS profiles for each active site were compared with those of the corresponding bare surfaces and free carbon dioxide to determine the new mixed orbitals. The study showed that (i) carbon dioxide adsorption on Pt3Sn surfaces is not thermodynamically favorable, (ii) the adsorbed carbon dioxide assumes two different geometries dependent on the presence of a Sn atom at the site of adsorption, (iii) the adsorbed carbon dioxide molecule does not dissociatively adsorb on Pt3Sn surfaces, and (iv) the adsorbed carbon dioxide molecule resembles the structure of the carbon dioxide anion (CO¡2 ).