Surface Sensitivity of Mesoporous Carbon-Supported Iron Catalysts in the Fischer–Tropsch Synthesis: An In Situ XPS Site Evolution at Distinct Reaction Conditions

Abstract

The Fischer–Tropsch process can be considered as an alternative route to convert fossil fuels such as crude oil, coal, and methane for the production of more environmentally friendly liquid fuels and chemicals. As recoverable sources of carbon, Fischer–Tropsch synthesis (FTS) converts a mixture of CO and H₂ to a range of hydrocarbons, which is free of sulfur and nitrogen and low in aromatics. The surface-sensitive investigation of the temperature and pressure effects on the FT synthesis performance over mesoporous carbon-supported iron catalysts was examined by in situ X-ray photoelectron spectroscopy analyses. Raman and Mössbauer spectroscopy measurements illustrated the structural properties of mesoporous Fe-based oxides. Under FTS reaction conditions of 20 and 30 atm, and temperatures of 240, 255, and 270 °C with a CO-to- H₂ ratio of 1, the solids were active with 38–45% of CO conversion and a rate of 1 × 10^–5 mol_CO·g^–1·s^–1. The product distribution gave C₁–C₄, C₅–C₉, and C₁₀⁺ products with the structure of the solid marginally affected by the type of product obtained. The in situ surface XPS analyses were conducted at ∼240–270 °C and 10 atm with a CO-to-H₂ ratio of 1 for 1 h. The α-Fe₂O₃ phase was reduced to Fe₃O₄ resulting in well-dispersed magnetite nanoparticles with further reduction to metallic iron on the mesoporous carbon support. Such α-Fe phase demonstrated accessibility of the syngas resulting in the activity of the solids. The chemical evolution of the Fe 2p, O 1s, C 1s, and K 2s core levels during the FTS with increasing the temperature up to 255 °C suggested that the surface carburization formed χ-Fe₅C₂ and θ-Fe₃C iron carbide phases along with Fe₃O₄ and, tentatively, the metallic iron phase. The mesoporous carbon-supported iron catalysts having χ-Fe₅C₂ carbide determined the activity and stability during the FTS synthesis.