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 nanopartulate TiO₂ supported catalysts, with considerable selectivity for lighter hydrocarbons.