Theoretical Investigation of the Hydrogenation of Cyclohexene Catalyzed by Supported Single-Atom Sites on Redox Noninnocent LiMn₂O₄ and Li₂Mn₂O₄ Surfaces

Abstract

The tuning of catalyst activity via stereoelectronic modulation of the active-site structure remains a grand challenge in heterogeneous catalysis. In homogeneous catalysis, the redox noninnocent ligands can be introduced to organometallic fragments to donate electrons to the metal center and fine-tune the catalytic activity. Analogously, lithium-ion battery materials, such as lithium manganese oxide (LMO), lithium titanium oxides (LTO), etc., can serve as redox noninnocent catalyst support to modulate the electronic structures of the active site via lithiation, hence tuning the catalytic activity of supported active sites. Experimentally, the Ni single-atom site was supported on LiMn 2 O 4 via oxidative grafting and exhibited no catalytic activity toward the hydrogenation of cyclohexene. After introducing additional Li into the catalyst support forming Li 2 Mn 2 O 4 , the Ni single-atom site becomes active, with the catalytic rates increasing as a function of the lithiation for Li/Mn ratios >0.9. In this paper, density functional theory (DFT) calculations are performed to study the Ni site structure via X-ray absorption near edge structure (XANES) simulations and investigate the electronic properties of Ni single-atom site before and after the addition of intercalated lithium in the LMO spinel structure. Furthermore, the study of the reaction mechanism is also carried out to understand the thermodynamically and kinetically favored pathways. XANES simulation suggests that the Ni single-atom site is likely to stay in the Li channel of the spinel support structure and form an octahedral structure. After Li intercalation, the Ni site becomes less positively charged, indicating the partial reduction of the Ni site. The simulated reaction energy profile over the LiMn 2 O 4 support exhibits high-energy barriers (1.07 eV) for the hydrogenation of cyclohexene; however, the Li 2 Mn 2 O 4 support is able to better stabilize low-coordinated ion sites and improve ion mobility, leading to lower overall energy barriers (0.64 eV). The reduced and low-coordinated ion site, thus, can better stabilize the reaction intermediates and promote hydrogenation reaction. Similarly, other transition metal ions (Fe, Co, and Cu) are also considered over the LiMn 2 O 4 and Li 2 Mn 2 O 4 catalyst supports for hydrogenation reaction.

Recent Advances on Computational Modeling of Supported Single-Atom and Cluster Catalysts: Characterization, Catalyst–Support Interaction, and Active Site Heterogeneity

“…Based on the mechanistic study, Ni/Li 2 Mn 2 O 4 showed much lower reaction energies compared with Ni/LiMn 2 O 4 , which validated the experimental observation. The follow-up study [127] demonstrated the reduction of supported single-atom sites (Fe, Co, Ni, and Cu) over Li 2 Mn 2 O 4 via Li intercalation into catalyst support utilizing Bader charge analysis. Furthermore, based on the reaction mechanism study, the considered active sites are more active towards cyclohexene hydrogenation than the reduced support, Li 2 Mn 2 O 4 , with low reaction thermodynamics and kinetic barriers.…”

“…Figure 4. Experimental Ni K-edge XANES (a) compare with calculated Ni K-edge XANES (b).Printed with permission from Xu et al[127] The linear combination fitting of Cu(111), Cu55, and Cu3 simulated in comparison with experimentally measured in situ Cu XANES (c). Printed with permission from Xu et al[32].…”