Addition of CeO2 to enhance Ni dispersion and oxygen vacancies

Ce was added to Ni/CaO dual functional materials to enhance the performance of ICCU-RWGS.

https://doi.org/10.1016/j.apcatb.2018.11.040

“The porous structure and particle distribution are shown in Fig. 5. The pure sol-gel CaO as shown in Fig. 5a exhibits a porous structure without apparent particles, which is in agreement with the SEM results. There are irregular particles distributed on the sol-gel CaO matrix, which are assigned to NiO particles as shown in Fig. 5. As for the Ce-doped DFMs, the metal particle size exhibits a dramatic decrease. It is verified that nano-sized particles with the more coordinative unsaturated sites on the surfaces are easier to absorb CO₂, and they enable faster diffusion rate of CO₂ through the layer of formed CaCO₃ [57,58]. With further increase of Ce loading, the Ca₁Ni_0.1Ce_0.033 displays the obvious lattice fringes (Fig. 5e) indicating the high crystallinity of CeO₂ and NiO. The interplanar spacing of 0.312 nm and 0.209 nm represent (111) lattice plane of CeO₂ and (202) lattice plane of NiO, respectively. The elemental mapping of Ca₁Ni_0.1Ce_0.033 demonstrates the uniform distribution of Ca, O, Ni and Ce elements, which is effective to delay the sintering and prevent the aggregation of CaO particles, resulting in an outstanding stability.”

“Fig. 5. TEM images of fresh DFMs (a) CaO, (b) Ca₁Ni_0.1, (c) Ca₁Ni_0.1Ce_0.017, (d) Ca₁Ni_0.1Ce_0.033. High-resolution TEM (e) and elemental mappings (f–i) of Ca₁Ni_0.1Ce_0.033.”

“XPS analysis is performed on DFMs to identify the valence states and the chemical composition. The Ce3d spectra are complicated due to the existence of two different cerium oxidation states, the hybridization of the O2p valance band with the Ce4f level as well as spin-orbit coupling [56]. The main features of the Ce3d are composed of ten peaks marked as u and v, which are corresponding to five pairs of spin-orbit components attributed to 3d_3/2and 3d_5/2 [59,60]. Six of the peaks labelled as U, U″, U‴, V, V″, V‴ are associated to Ce⁴⁺ ions, and the other four peaks denoted by U°, U′, V°, V′ are connected to Ce³⁺ ions. As shown in Fig. 6a, six main binding energy peaks can be observed including 916.3, 907.4, 888.9 and 881.8 eV assigned to Ce⁴⁺3d_5/2, as well as 900.6 and 898.0 eV ascribed to Ce³⁺3d_3/2. Therefore, Ce mainly exists in +3 and +4 oxidation states in the DFMs. The Ce³⁺ to Ce⁴⁺ ratios of the Ca₁Ni_0.1Ce_0.017 and the Ca₁Ni_0.1Ce_0.033 are 0.52 and 0.58 (Table 2), respectively, indicating more oxygen vacancies created by the Ce³⁺ with the increase of Ce loading.”

“Fig. 6. XPS spectra of (a) Ce3d and (b) Ni2p of fresh DFMs.”