Sonochemical synthesis of thermally stable hierarchical Ce_1-xM_xO_2-δ (M = Pt or Pd, 0 ≤ x ≤ 0.10) nanocrystallites: redox properties and methanol electro-oxidation activity

Abstract

Hierarchical Ce_1-xM_xO_2-δ (M = Pt or Pd, (0 ≤ x≤ 0.10) nanocrystallites of ~5 nm sizes have been synthesized by a sonication route using diethylenetriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy. Ce_1-xM_xO_2-δ (M = Pt or Pd, 0 ≤ x ≤ 0.1) crystallize in fluorite structure. Pt exists in the +4 state and Ce in the mixed valent (+3, +4) state in Ce_1-xPt_xO_2-δ and Pd exists in the +2 state and Ce in the mixed valent (+3, +4) state Ce_1-xPd_xO_2-δ. TEM, scanning electron microscopy (SEM), and BET surface and porosity measurements confirmed that the as-prepared compound is hierarchical structured with individual crystallites of ~5 nm size, and even after sintering the sample at 600 °C, hierarchical structure is retained with individual crystallites of ~7 nm size. Substitution of the Pt and Pd ion in CeO₂ activated the lattice oxygen, and hydrogen spillover or a higher H/Pt ratio ~8.1 and H/Pd ratio ~4.2 is observed. Reversible nature of higher oxygen storage capacity or higher H/P, H/Pd ratio is due to interaction of redox couples of Pt^4+/2+(0.91 V), Pt^2+/0 (1.18 V), Pd^2+/0 (0.92 V), and Ce^4+/3+ (1.61 V). Because of the participation of lattice oxygen, Ce_0.95Pt_0.05O_1.95 and Ce_0.95Pd_0.05O_1.90 have shown higher electro-oxidation of methanol compared to the same moles of Pt in 5% Pt/C.