The role of Cu on the reduction behavior and surface properties of Fe-based Fischer–Tropsch catalysts

Abstract

The effect of Cu on the reduction behavior and surface properties of supported and unsupported Fe-based Fischer–Tropsch synthesis (FTS) catalysts was investigated using in situ X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS) in combination with ex situ bulk characterization. During exposure to 0.4 mbar CO–H₂ above 180 °C, the reduction of CuO to Cu⁰ marked the onset of the reduction of Fe₃O₄ to α-Fe. The promotion effects of Cu are explained by a combination of spillover of H₂ and/or CO molecules from metallic Cu⁰ nuclei to closely associated iron oxide species and textural promotion. XAS showed that in the supported catalyst, Cu⁺ and Fe²⁺ species were stabilized by SiO₂ and, as a result, Fe species were not reduced significantly beyond Fe₃O₄ and Fe²⁺, even after treatment at 350 °C. After the reduction treatment, XPS showed that the concentration of oxygen and carbon surface species was higher in the presence of Cu. Furthermore, it was observed that the unsupported, Cu-containing catalyst showed higher CO₂ concentration in the product gas stream during and after reduction and Fe surface species were slightly oxidized after prolonged exposure to CO–H₂. These observations suggest that, in addition to facilitating the reduction of the iron oxide phase, Cu also plays a direct role in altering the surface chemistry of Fe-based FTS catalysts.