Ionic Nickel Embedded in Ceria with High Specific CO₂ Methanation Activity

Abstract

CO 2 hydrogenation to methane is gaining increasing interest as one of the most promising ways to store intermittent renewable energy in the form of chemical fuels. Ni particles supported on CeO 2 represent a highly efficient, stable and inexpensive catalyst for this reaction. Herein, Ni-doped CeO 2 nanoparticles were tested for CO 2 methanation showing an extremely high Ni mass-specific activity and CH 4 selectivity. Operando characterization reveals that this performance is tightly associated with ionic Νi and Ce 3 + surface sites, while formation of metallic Ni does not seem to considerably promote the reaction. Theoretical calculations confirmed the stability of interstitial ionic Ni sites on ceria surfaces and highlighted the role of Ce-O frustrated Lewis pair (FLP), Ni-O classical Lewis pair (CLP) and Ni-Ce pair sites to the activation of H 2 and CO 2 molecules. To a large extent, the theoretical predictions were validated by in situ spectroscopy under H 2 and CO 2 : H 2 gaseous environments.

Niobium Modification of CeO2 Tuning Electron Density of Nickel–Ceria Interfacial Sites for Enhanced CO2 Methanation

“…X-ray photoelectron spectroscopy (XPS) analysis was performed on Ni/Ce and Ni/10NbCe catalysts to gain further insights into their chemical compositions and valence states. 45 This shift provides evidence that Nb 5+ effectively replaces Ce 4+ in the CeO 2 lattice during catalyst synthesis, regulating the electronic properties of the Ni cluster. Figure 2d presents the high-resolution spectra of the O 1s spectrum for Ni/Ce and Ni/10NbCe.…”

“…Additionally, the interaction between Ni and NbCe enhances the electron transfer to the active sites on the NiCe surface, optimizing the electronic properties of the Ni cluster and Ni−Ce interface and thereby improving the CO 2 methanation activity of the catalyst. 21,33,45 Textural parameters were also determined by nitrogen physisorption (Figure S5). Ni/10NbCe exhibits nearly the same BET surface area (87 m 2 /g) as Ni/Ce (90 m 2 /g), which had no contribution to the CO 2 methanation.…”

“…This difference indicates that the Nb 5+ has been successfully incorporated into the CeO 2 lattice, resulting in the formation of a Nb–Ce–O solid solution, therefore changing the electronic state of the Nb element. , This results in an increase in the conduction band electron concentration within the carrier. Additionally, the interaction between Ni and NbCe enhances the electron transfer to the active sites on the NiCe surface, optimizing the electronic properties of the Ni cluster and Ni–Ce interface and thereby improving the CO 2 methanation activity of the catalyst. ,, Textural parameters were also determined by nitrogen physisorption (Figure S5). Ni/10NbCe exhibits nearly the same BET surface area (87 m 2 /g) as Ni/Ce (90 m 2 /g), which had no contribution to the CO 2 methanation.…”

“…Figure c exhibits the high-resolution spectrum of Ni 2p for both the Ni/Ce and Ni/10NbCe catalysts, revealing the coexistence of Ni 2+ (∼852.5 eV) and Ni–O–Ce (represents close contact between NiO and CeO 2 ) on the catalyst surface. Notably, a noticeable shift of the Ni peak toward higher energy is observed in Ni/10NbCe compared to Ni/Ce, indicating the ionic nature of the Ni species and the strong interaction between Ni and the NbCe support . This shift provides evidence that Nb 5+ effectively replaces Ce 4+ in the CeO 2 lattice during catalyst synthesis, regulating the electronic properties of the Ni cluster.…”

“…This observation suggests that electron transfer occurs from Ni to the metal− support interface through strong metal−support interactions via Ni−O-M bonding (where M represents the metal of the support), resulting in a positive charge on the nickel metal. 45,53 Importantly, the Ni 4 cluster on Nb−CeO 2 exhibited a higher electron transfer (0.84 e − ) to the Ni-support interface compared to the Ni 4 /CeO 2 catalyst (0.57 e − ), indicating a stronger metal−support interaction in Ni 4 /Nb−CeO 2 . 54 These findings align with the HRTEM and H 2 -TPR results, further corroborating the enhanced metal−support interaction in the Ni 4 /Nb−CeO 2 catalysts.…”