It is well known that noble metal promoter plays a vital role in accelerating the reduction rate of cobalt oxides in Co/Al2O3 based Fischer-Tropsch synthesis (FTS) catalysts and, consequently, exposing more active Co metal surface sites. The addition of platinum may promote the reducibility of cobalt oxides by a H2 dissociation and spillover mechanism or possibly a chemical effect. Either way, it is generally believed that good contact at the atomic level between promoter and cobalt is important. For economic reasons, silver metal as a promoter is attractive because it is significantly less expensive than platinum. In this work, both Pt and Ag promoter were studied in order to compare their local atomic structures and their ability to facilitate the reduction of cobalt oxides in Co/Al2O3 catalyst. The local atomic structure of each catalyst was examined by in-situ Extended X-ray Absorption Fine Structure (EXAFS) at the Pt LIII-edge for Pt-promoted catalyst and at the Ag K-edge for Ag-promoted catalyst. The EXAFS results reveal that platinum is well dispersed in the catalyst and isolated Pt atoms interact with cobalt cluster to form Pt-Co bonds, without evidence of Pt-Pt bond formation, even at levels as high as 5%Pt. In Ag-promoted Co/Al2O3 catalyst, not only were the Ag-Co bonds observed at the first shell coordination as preferred, but also Ag-Ag bonds were noticeably present, even at levels as low as 0.276% Ag. The degree of coordination of Ag atoms with Ag increased as a function of Ag loading. Therefore, although both promoters initially facilitate reduction of cobalt oxides, their local atomic structures are fundamentally different, and this may have consequences for long-term catalyst usage.

Fischer-Tropsch synthesis: Investigation of the impact of Pt and Ag promoter loading on the local atomic structure of Co/alumina catalysts using an in-situ XAS

245th National Meeting (2013)

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