Pt/UiO-66 Nanocomposites as Catalysts for CO₂ Methanation Process

Pt/UiO-66 nanocomposites with platinum target concentration of 3 wt.% were prepared by 3 preparation methods, characterized and tested in the CO₂ methanation process. Choice of the microporous UiO-66 metal-organic framework (Zr₆O₄(OH)₄ with 1,4-benzene-dicarboxylate ligand) as catalytic support was motivated by the CO₂ chemisorption capacity (proven by CO₂-TPD profiles), large specific surface area (1477 m²/g) which favors a high dispersion of metal nanoparticles and good thermal stability. The preparation methods for the Pt/UiO-66 nanocomposites are: (1) wetimpregnation followed by reduction in H₂ at 200 °C for 2 h; (2) wet-impregnation followed by reduction with an aqueous solution of NaBH₄; and (3) "double-solvent" method, followed by reduction with NaBH₄. The UiO-66 based nanocomposites were characterized by N₂ adsorption-desorption (BET method), XRD, and SEM/TEM. The Pt/UiO-66 catalyst prepared by method 3 was chosen for catalytic testing due to its highest surface area, smallest platinum nanoparticles (PtNPs) size, the localization of PtNPs both on the grain's internal and external surface and best thermal stability in the desired temperature range. Its capacity to adsorb and activate CO₂ and H₂ was evaluated in thermo-programmed desorption experiments (H₂-TPD and CO₂-TPD). Hydrogen is molecularly adsorbed, while CO₂ is adsorbed both molecularly and dissociatively. The catalytic performance in the CO₂ methanation process was evaluated by Temperature Programmed Reactions (TPRea, 2 °C/min, 30-350 °C), at atmospheric pressure. The best results were obtained at 350 °C, CO₂:H₂ molar ratio of 1:5.2 and GHSV ═ 1650 h^-1. In these conditions CO₂ conversion is almost 50% and CH₄ selectivity is 36%, the rest of the converted CO₂ being transformed in CO.