Capturing the Genesis of an Active Fischer–Tropsch Synthesis Catalyst with Operando X-ray Nanospectroscopy

Abstract

A state-of-the-art operando spectroscopic technique is applied to Co/TiO 2 catalysts, which account for nearly half of the worlds transportation fuels produced by Fischer-Tropsch catalysis. This allows determination of, at a spatial resolution of approximately 50 nm, the interdependence of formed hydrocarbon species in the inorganic catalyst. Observed trends show intra-and interparticular heterogeneities previously believed not to occur in particles under 200 mm. These heterogeneities are strongly dependent on changes in H 2 /CO ratio, but also on changes thereby induced on the Co and Ti valence states. We have captured the genesis of an active FTS particle over its propagation to steady-state operation, in which microgradients lead to the gradual saturation of the Co/TiO 2 catalyst surface with long chain hydrocarbons (i.e., organic film formation). Heterogeneous catalytic reactions such as the Fischer-Tropsch synthesis (FTS) of long-chain hydrocarbons are dynamic and complex, often comprised of inorganic (catalyst) and organic parts (reactants and products). Conventional and widely applied spectroscopic techniques often focus on either the organic part (e.g., vibrational spectroscopy) or on the inorganic part (e.g., X-ray spectroscopic techniques). The combination of both inorganic and organic information shows great promise to answer long-standing questions about complex catalytic reactions. The FTS propagates through a complex surface polymerization process of adsorbed C 1 reaction intermediates derived from synthesis gas (a mixture of CO and H 2). [1-7] Cobalt-based FTS catalysts are an integral part of this gas-to-liquid (GTL) process because of their high wax selectivity and relatively high stability. [8, 9] The activation and deactivation of these cobalt nanoparticles supported on an inorganic oxide, such as Al 2 O 3 or TiO 2 , is believed to occur through a multitude of mechanisms, however consensus in literature is often still lacking. While the literature is imbued with proposed deactivation mechanisms, [5, 10, 11] the equally interesting catalyst activation period is often overlooked. [12, 13] During the day(s)-long activation or induction period (which is highly dependent on reaction conditions and the catalyst), FTS catalyst particles are believed to be gradually saturated by a film of long-chain hydrocarbons, followed by pore filling through which further reactants must diffuse. [12, 14, 15] The complexity of the FTS process is also captured in the myriad of proposed deactivation mechanisms, which are generally related to the conversion of the active phase, considered as metallic cobalt, into an inert phase. For example, cobalt reoxidation or carburization, [16, 17] the formation of support oxide-cobalt species occurring through strong metal-support interactions (SMSI), [8, 18, 19] the loss of active cobalt surface area arising from crystalline growth (i.e., metal sintering), [11, 20-22] and finally fouling for example by hydrocarbon deposition in the form of various carbon species formed at the cobalt surface. [11, 23-25] The dynamic interplay of these different activation and deactivation mechanisms necessitates studies under-or approaching-true reaction conditions. Passivation, or for example, changes in the samples gaseous environment can significantly alter the state of a FTS catalyst, and hence have to date prevented a complete understanding of the catalyst material under relevant conditions. [26-29] Operando characterization studies can greatly advance our knowledge of working catalyst systems providing nanoscale chemical information on both the organic (products and reaction intermediates) and the inorganic (metal and support) constituents of the catalyst material under operating conditions (i.e., high temperatures and pressures and reactants). [30] The development of scanning transmission X-ray microscopy (STXM), which is a combination of microscopy and X-ray absorption spectroscopy, under operating conditions and with on-line activity data, presents the imperative qualities necessary for understanding complex catalytic systems. In this work, we present an operando STXM study of a Co/TiO 2 FTS catalyst operated under various FTS conditions (493 K, 1-4 bar and 1:1, and 2:1 H 2 /CO feed) over extended periods of time (i.e., 3 days). The technique allows mapping of single catalyst particles (i.e., several Co nano-particles supported on grains of TiO 2, schematic in Figure 1 and STEM-EDX in Figure S2) at a spatial resolution of approximately 50 nm in the soft X-ray regime (200-2000 eV), which offers the unique ability to detect both the full range of [*] I.