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

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 alpha '-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 degrees 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-beta 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 degrees 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.