CO2-Assisted Propane Dehydrogenation over Lithium-Promoted PtZn4@S-1: Unraveling the Active Sites

Abstract

Constructing a bifunctional active site to boost propane dehydrogenation (PDH) in tandem with the reverse water gas reaction (RWGS) showed great potential in meeting the supply of olefins, while the crucial role of the bifunctional active sites is still unclear. Herein, a combination of kinetic and spectroscopic evidence was utilized to confirm the nonuniform bifunctional site distribution of the PtZn₄@S-1 catalyst. Then, lithium atoms were incorporated to tune the bifunctional sites. A series of characterizations indicated that the introduction of lithium suppressed the incorporation of Zn atoms into the framework of silicalite-1 zeolite. As a result, the nanosized Pt_xZn_y clusters were converted into larger Li_zPt_xZn_y clusters with a higher Pt–Pt coordination number. This led to decreased PDH activity and stability, which confirmed that the real active sites of PDH are nanosized Pt_xZn_y clusters anchored by the framework Zn atoms. However, the lithium atoms showed a volcano curve with CO₂ conversion. When the atomic mole ratio of Li to Pt was 34, it showed the highest CO₂ conversion, indicating that the Si-OLi species and the Li_zPt_xZn_y clusters were directly involved in the CO₂ activation process. Further treatment of the Li₃₄PtZn₄@S-1 catalysts with water washing converted the Si-OLi species back to Si–OH species. This change had little influence on the PDH activity, while the RWGS showed nearly no activity, confirming that the real active site of CO₂ activation is the synergy between the Li_zPt_xZn_y clusters and Si-OLi species.