Synthesis of TiO2 nanotube arrays on 3D-printed structures for application as Fischer–Tropsch synthesis catalysts

Abstract

In this work, 3D-printed Ti6Al4V structures are used as substrates to synthesize TiO₂ nanotube arrays by electrochemical anodization. These nanostructured materials are used as supports for bimetallic FeCo catalysts in Fischer–Tropsch synthesis to produce hydrocarbons from syngas. These structures are annealed to assess the influence of phase transformations in the development of TiO₂ nanotubes. Field-emission scanning electron microscopy (FESEM) images of the untreated structures reveal needle-like formations in their microstructure, characteristic of Ti in its α′-phase resulting from the 3D printing process. X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDXS) are used to compare the specimens before and after annealing. The results suggest that annealing at 850 °C before anodization introduces an undesirable oxide layer, impeding the formation of TiO₂ nanotubes. This phenomenon is attributed to the complex crystallographic features of the phases formed during annealing, specifically Ti-β and TiO₂-rutile, which prevent fluoride ions in the electrolyte from penetrating the structure. The results suggest that the optimal synthesis process is a two-step electrochemical treatment followed by low-temperature annealing at 450 °C. This sequence produces a desirable crystalline morphology due to the phase transformation from TiO₂-rutile into TiO₂-anatase, as shown by XRD. EDXS data shows that the fluorine content from residual ions from the anodizing solution is significantly reduced after annealing. Fischer–Tropsch catalysts are synthesized using a FeCo (2.0 wt%) active phase on the optimized TiO₂ nanotube arrays and tested in a packed-bed reactor. These materials display catalytic activity, comparable to nanoparticulate TiO₂ supported catalysts, with considerable selectivity for lighter hydrocarbons.