Effect of Pretreatment Method on the Nanostructure and Performance of Supported Co Catalysts in Fischer−Tropsch Synthesis

ABSTRACT: Understanding precursor transformation to active catalysts is crucial to heterogeneous Fischer−Tropsch (FT) catalysis directed toward production of hydrocarbons for transportation fuels. Despite considerable literature on FT catalysis, the effect of pretreatment of supported cobalt catalysts on cobalt dispersion, dynamic atomic structure, and the activity of the catalysts is not well understood. Here we present systematic studies into the formation of active catalyst phases in supported Co catalyst precursors in FT catalysis using in situ environmental (scanning) transmission electron microscopy (E(S)TEM) with single-atom resolution under controlled reaction environments for in situ visualization, imaging, and analysis of reacting atomic species in real time, EXAFS, XAS, DRIFTS analyses, and catalytic activity measurements. We have synthesized and analyzed dried reduced (D) and dried calcined reduced (DC) Co real catalysts on reducible and nonreducible supports, such as SiO2, Al2O3, TiO2, and ZrO2. Comparisons of dynamic in situ atomic structural observations of reacting single atoms, atomic clusters, and nanoparticles of Co and DRIFTS, XAS, EXAFS, and catalytic activity data of the D and DC samples reveal in most cases better dispersion in the D samples, leading to a larger number of low-coordination Co0 sites and a higher number of active sites for CO adsorption. The experimental findings on the degree of reduction of D and 27 DC catalysts on reducible and nonreducible supports and correlations between hexagonal (hcp) Co sites and the activity of the catalysts generate structural insights into the catalyst dynamics, important to the development of efficient FT catalysts.