Using Ex Situ and In Situ HERFD-XANES to Reveal the Superior Oxidation and Reduction Cycling of Ceria Nanocubes Dispersed in Silica Aerogel

Abstract

The oxygen storage capacity of ceria-based catalytic materials is influenced by their size, morphology, and surface structure, which can be tuned using surfactant-mediated synthesis. In particular, the cuboidal morphology exposes the most reactive surfaces; however, when the capping agent is removed, the nanocubes can agglomerate and limit the available reactive surface. Here, we study ceria nanocubes, lanthanum-doped ceria nanocubes, and ceria nanocubes embedded inside a highly porous silica aerogel by high-energy resolution fluorescence detection-X-ray absorption near edge spectroscopy at the Ce L₃ edge. In situ measurements showed an increased reversibility of redox cycles in ceria nanocubes when embedded in the aerogel, demonstrating enhanced reactivity due to the retention of reactive surfaces. These aerogel nanocomposites show greater improvement in the redox capacity and increased thermal stability of this catalytic material compared to the surfactant-capped nanocubes. Ex situ measurements were also performed to study the effect of lanthanum doping on the cerium oxidation state in the nanocubes, indicating a higher proportion of Ce⁴⁺ compared to that of the undoped ceria nanocubes.

Figure 1

HERFD-XANES Ce L₃-edge spectra of ceria nanocubes at room temperature (green) and thermally treated ex situ at 450 °C (purple), along with the Ce³⁺ (dotted red) and Ce⁴⁺ (dotted blue) standards. The inset presents a magnified pre-edge region.

Figure 2

HERFD-XANES Ce L₃-edge spectra of 7.5% La-doped ceria nanocubes as-synthesized (green), thermally treated ex situ at 450 °C (purple), and the Ce³⁺ (dotted red) and Ce⁴⁺ (dotted blue) standards. The inset presents a magnified pre-edge region.

Figure 3

HERFD-XANES Ce L₃-edge spectra of 6 wt % ceria–silica aerogel nanocomposite as-synthesized (green), thermally treated ex situ at 450 °C (purple), thermally treated ex situ at 750 °C (orange), and the Ce³⁺ (dotted red) and Ce⁴⁺ (dotted blue) standards. The inset presents a magnified pre-edge region.

Figure 4

HERFD-XANES Ce L₃-edge spectra of ceria nanocubes under in situ oxidation at 275 °C (green), followed by reduction at 275 °C (purple), then reduction at 400 °C (orange), and the Ce³⁺ (dotted red) and Ce⁴⁺ (dotted blue) standards. The inset presents a magnified pre-edge.

Figure 5

HERFD-XANES Ce L₃-edge spectra of the ceria–silica aerogel nanocomposite under in situ oxidation at 275 °C (green), followed by reduction at 275 °C (purple), then reduction at 400 °C (orange), and the Ce³⁺ (dotted red) and Ce⁴⁺ (dotted blue) standards. The inset presents a magnified pre-edge.

Figure 6

Estimated relative concentrations of Ce³⁺ for unsupported CeO₂ nanocubes (dashed line) and ceria–silica aerogel nanocomposites (solid line) during in situ measurements taken at increasing temperatures and under alternating reduction/oxidation conditions (see Tables S1, 3, and 4).