MAS NMR Studies on the Formation and Structure of Oxygen Vacancy on the CeO₂ {110} Surface under a Reducing Atmosphere

Abstract

The surface oxygen vacancy of metal oxide is a type of important catalytic active site. To investigate the origin and nature of oxygen vacancy, CeO 2 nanorods with exposed {110} facets were treated at different temperatures under H 2 atmosphere. 1 H MAS NMR, 31 P MAS NMR with trimethylphosphine (TMP) as the probe molecule, in situ XPS, and in situ EPR were employed to characterize the surface oxygen vacancy of the reduced samples. The results show that the decrease of the hydroxyl group content and the increase of Ce 3+ concentration occur gradually with the increase of reduction temperature. When the temperature reaches 350 °C, the surface and bulk oxygen vacancies of CeO 2 are generated. The longitudinal relaxation time (T 1 ) and the motion of 31 P nuclei from TMP molecules adsorbed on the CeO 2 surface were found to be significantly reduced with the formation of surface oxygen vacancies. Based on the characterization results, the adsorption models of probe molecules on the surface of the samples are established and the amount of surface oxygen vacancies can be estimated quantitatively.

MAS NMR Studies on the Formation and Structure of Oxygen Vacancies on the CeO2 {111} Surface Under Reducing Atmosphere

“…The 1 H MAS NMR spectra of CeO 2 -rod samples only exhibit the peaks in the range of 0.9 ∼ 8.0 ppm with no peak around 12 ppm. 25 These results suggest that the hydroxyl group H b3 at 12 ppm on the exposed {111} facets of CeO 2 -polyhedron are able to facilitate the oxidation of TMP to TMPO. Both bridge hydroxyl H b3 and surface nonhydroxyl oxygen are believed to cause the generation of TMPO.…”

“…Nanosized CeO 2 rods were prepared according to a previous report. 25 The synthesis procedure of nanosized CeO 2 rods was the same as that of CeO 2 cubes, except that the hydrothermal temperature and time were 140 °C and 18 h, respectively. The obtained nanosized CeO 2 polyhedrons, CeO 2 cubes, and CeO 2 rods are denoted as CeO 2 -polyhedron, CeO 2cube, and CeO 2 -rod, respectively.…”

“…The peak around 50 ppm is assigned to TMPO chemisorbed on the CeO 2 surface, which is due to oxidization of TMP by CeO 2 . 25 The peaks in the range of −30 ∼ −60 ppm are attributed to TMP molecules adsorbed on the surface of CeO 2polyhedron. 48 It is worth noting that there is no peak appearing in the range of 0 ∼ −5 ppm in the 31 P MAS NMR spectra attributed to TMP bound to Bronsted acid sites although 1 H MAS NMR spectra show a peak around 12 ppm (H b3 , Figure 5) assigned to bridge hydroxyl with strong acidity.…”

“…31 P MAS NMR with TMP as probe molecule is an effective technique for quantitative characterization of surface oxygen vacancies. 25,45−47 TMP may adsorb on Ce sites Although CeO 2 -polyhedron mainly exposes {111} facets, it also exhibits a small amount of {100} facets. In order to eliminate the influence of {100} facets, the CeO 2 -cube only with exposed {100} facets is used as a reference sample and its morphological characterization results are shown in Figures S5 and S6.…”

Oxygen-vacancy-rich CeO2/Ru nanoparticles enable a high-performance catalyst for alkaline hydrogen oxidation

“…S32†), which further confirmed the adsorption capacity of oxygen-vacancy-rich CeO 2 for hydroxyl groups. 55,56 CeO 2 has been reported to be a hydroxyl adsorption site in the HOR process. 51 According to the literature, the oxophilicity of Ce is stronger than that of Ru; 57 thus, at the interface between Ru and CeO 2 (v), hydroxyl groups are more likely to be adsorbed on the CeO 2 (v) surface.…”