Does CO₂ Oxidize Ni Catalysts? A Quick X-ray Absorption Spectroscopy Answer

Abstract

MgAl2O4-supported Ni materials are highly active and cost-effective CO2 conversion catalysts, yet their oxidation by CO2 remains dubious. Herein, NiO/MgAl2O4, prepared via colloidal synthesis (10 wt % Ni) to limit size distribution, or wet impregnation (5, 10, 20, and 40 wt % Ni), and bare, i.e., unsupported, NiO are examined in H2 reduction and CO2 oxidation, using thermal conductivity detector-based measurements and in situ quick X-ray absorption spectroscopy, analyzed via multivariate curve resolution-alternating least-squares. Ni reoxidation does not occur for bare Ni but is observed solely on supported materials. Only samples with the smallest particle sizes get fully reoxidized. The Ni-MgAl2O4 interface, exhibiting metal–support interactions, activates CO2 and channels oxygen into the reduced lattice. Oxygen diffuses inward, away from the interface, oxidizing Ni entirely or partially, depending on the particle size in the applied oxidation time frame. This work provides evidence for Ni oxidation by CO2 and explores the conditions of its occurrence and the importance of metal–support effects.

Modulation Engineering: Stimulation Design for Enhanced Kinetic Information from Modulation-Excitation Experiments on Catalytic Systems

“…Moreover, the analysis elucidates an FCC Fe structure, which originates from Ni–Fe alloy formation, proving that segregated Fe interacts with Ni on the support surface and engages in alloy formation, as previously reported for these Ni/MgFeAlO 4 materials after H 2 -TPR . From the Ni K edge EXAFS results, a similar evidence for alloy formation is not easy to discern (Figure S9D), yet mainly Ni 0 and only minor Ni–O contributions are found, which are likely located at the support interface . After activation, the material thus consists of MgFeAlO 4 with incorporated Fe 2+/3+ , ∼16% Ni–O, as well as ∼55% metallic Fe and ∼84% metallic Ni fractions, which are engaged in alloy formation.…”

“…In addition, periodic changes are resolved in the post-edge region (>8400 eV), which, based on their amplitude, equally correspond to reversible changes within the Ni fine structure. Although re-oxidation of supported Ni by CO 2 is not always recognized or considered as such, it has been reported for Ni/MgFeAlO 4 by Theofanidis et al and more recently, examined for Ni/MgAl 2 O 4 in a dedicated study . Comparison with an NiO–Ni difference spectrum (Figure S20) suggests Ni ↔ NiO redox behavior occurring in ∼1/1000 of Ni atoms.…”

“…A possible explanation is that, after Fe extraction and oxidation, Ni requires metal–metal oxide interface sites to undergo oxidation. The latter provide active sites for CO 2 activation, ,, dissociating it into CO and oxygen species that interact with Ni, ultimately resulting in its oxidation. As such, Ni oxidation occurs at a later instant relative to Fe.…”

“…Although re-oxidation of supported Ni by CO 2 is not always recognized or considered as such, it has been reported for Ni/MgFeAlO 4 by Theofanidis et al 36 and more recently, examined for Ni/MgAl 2 O 4 in a dedicated study. 56 Comparison with an NiO−Ni difference spectrum (Figure S20) suggests Ni ↔ NiO redox behavior occurring in ∼1/1000 of Ni atoms. However, the demodulated spectrum deviates considerably from the difference spectrum.…”

“…36 From the Ni K edge EXAFS results, a similar evidence for alloy formation is not easy to discern (Figure S9D), yet mainly Ni 0 and only minor Ni−O contributions are found, which are likely located at the support interface. 56 However, due to the inertia of the flow control system, the constituent modulation shapes of p CO2 are reminiscent of isosceles trapezoids, i.e., with visible "slopes" between setpoint changes, rather than true vertical sides. Additionally, p H2 (Figure 1B) exhibits small deviations from the intended constant value.…”