Effect of swelling agent in the synthesis of porous nanocrystalline nickel-zirconia-ceria composite

Abstract

Porous ceria-zirconia (Zr0 · 1Ce0 · 9O2 − δδ delete) powders were synthesized by a sol-gel route using inorganic chlorides (ZrCl4 and CeCl3·7H2O) as precursors, block copolymer P123 (PEO20PPO70PEO20) as the template and tri-isopropyl-benzene (TIPB) as the swelling agent. These porous materials show high surface area and gas permeability, important properties for Intermediate Temperature-Solid Oxide Fuel Cell (IT-SOFC) and catalytic applications. The samples were prepared with different P123:TIPB (w/w) % ratios (1:0, 1:1, 1:2 and 1:4), as a strategy to increase porosity. The samples were calcined at 400 °C in air to remove the template. Post-synthesis nickel impregnation was planned to obtain 3 and 10 (w/w) % of NiO after calcination. The structure and morphology of the samples were determined by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), nitrogen adsorption isotherms (NAI) and transmission and scanning electron microscopies (TEM and SEM). The resulting materials have high specific surface area (≈110 m2. g-1) and a wide pore size distribution of mesopores (3–50 nm). They are formed by nanocrystals (≈15 nm) of the predominant (≈90%) Fm3¯m cubic phase and by the P42/nmc tetragonal phase. The micrographs revealed that the nanocrystalline oxides have mesopores with slit shape and a secondary smaller mesoporosity with a narrow size distribution (≈4 nm). An increase of micropore volume (sizes < 2 nm) was observed with the presence of the swelling agent in the synthesis process, ideal for gas diffusion in catalysis and IT-SOFC processes. Temperature programmed reduction (TPR) analysis presented low temperature peaks, a marked increase in the total reduction value and H2 uptake of samples with 3 (w/w)% NiO in contrast to the bare supports. There was no significant further increase in the total reduction value when 10 (w/w)% NiO is incorporated. It is noteworthy that comparing the synthesis methods, all samples presented higher reduction values and H2 uptake.