Influence of pH and soft templates on the hydrothermal synthesis of rod-shaped LaNiO3 catalytic precursors applied to dry reforming of methane for hydrogen production

Abstract

LaNiO₃ perovskite catalytic precursors synthesized via the hydrothermal method can exhibit different structures and morphologies depending on the synthesis factors, such as soft templates and pH. Depending on the structures and morphologies of the catalytic precursors, their catalytic performance (field, conversion, stability, etc.) in dry reforming of methane (DRM) may change. Thus, this study aimed to synthesize LaNiO₃ catalytic precursors with rod-like morphology by the hydrothermal method without and with soft templates (CTAB and citric acid), varying pH values (9 and 13), to evaluate the best catalytic precursors in DRM. The samples were characterized by energy-dispersive X-ray spectroscopy (EDXRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption-desorption analysis, H₂ temperature-programmed reduction (H₂-TPR), and thermogravimetric analyses (TGA/DTG/DTA). After activation, the catalysts' performance was evaluated based on a stability test in DRM for 15 h at 600 °C and with 96 L h⁻¹ g⁻¹ GHSV (gas hourly space velocity). Thus, the results showed that the LaNiO₃ catalytic precursors synthesized without soft template at pH 9 and 13 presented rod-like morphology and were chosen for DRM application. The reducibility test showed that the catalysts synthesized at pH 9 and 13 had a degree of reduction of 69.89 % and 77.18 %, respectively. The stability test showed that the catalyst synthesized at pH 13 had a greater CH₄ and CO₂ conversion and stability than the catalyst synthesized at pH 9, as well as the H₂ and CO yield, and H₂/CO ratio. Its greater stability was assigned to its nanorod-like morphology, which favored a more homogeneous Ni⁰ dispersion after activation stage. Furthermore, the spent catalysts showed whisker-like carbon deposition, in which the catalyst synthesized at pH 13 (90.36 wt%) had a greater amount than the catalyst synthesized at pH 9 (47.67 wt%). Its greater whisker-like carbon deposition was assigned to the lower amount of La₂O₂CO₃ phase formed after DRM.