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Formation of Tilted FeN4 Configuration as the Origin of Oxygen Reduction Reaction Activity Enhancement on a Pyrolyzed Fe-N-C Catalyst with FeN4-Edge Active Sites
J. Phys. Chem. C 2021, 125, 36, 19682–19696
Abstract: We elucidate the origin of oxygen reduction reaction (ORR) activity enhancement on the FeN4 site at the edge of a pyrolyzed Fe-N-C catalyst (FeN4-edge) using a combination of density functional theory calculations and microkinetic simulations. The presence of graphitic edges facilitates the formation of FeN4 active sites. Majority of these edge active sites have quite similar activities as in the case of a typical FeN4 active site in the interior of graphene (FeN4G). The ORR activity enhancement of this catalyst originates from the formation of two unique tilted-FeN4 configurations at the edge of graphene, which have remarkable ORR activities. The first configuration is the tilted FeN4 site at the zigzag edge, which can positively shift the onset potential for the associative reduction mechanism about 0.13 V higher than that on the typical FeN4G system. The second configuration is the tilted FeN4 site at the armchair edge, which has a low O2 dissociation energy and a high onset potential for the dissociative reduction mechanism (0.26 V higher than the FeN4G system).
Distinct Behaviors of Cu- and Ni-ZSM-5 Zeolites toward the Post-activation Reactions of Methane
J. Phys. Chem. C 2021, 125, 35, 19333–19344
Abstract: The post-activation reactions of methane (CH4) to methanol (CH3OH), formaldehyde (CH2O), and dimethyl ether (C2H6O) are crucial issues in the CH4 selective oxidation to CH3OH over metal-exchanged zeolites. In the present work, we utilize density functional theory calculations to investigate several possible reactions following the CH4 activation on the mono(μ-O)Cu2II, bis(μ-O)Cu2III, and bis(μ-O)Ni2III active sites anchored in the ZSM-5 zeolite framework. In the mono(μ-O)Cu2 case, we found that a CH3 ligand formed during the CH4 activation is favorably oxidized to CH3OH or C2H6O when H2O or CH3OH are, respectively, present on the reduced (CH3)OF–CuI–OH–CuI site. Nonetheless, the reaction rates are predicted to be lower than the CH4 activation, confirming the fact that the CH3OH extraction step using steam requires a longer time. Similarly, although the bis(μ-O)Cu2 active site is reported to easily form and desorb CH3OH, the reduced CuII–O–CuII center is active to oxidize the formed CH3OH to CH2O with high exothermicity and reaction rate. The bis(μ-O)Ni2 active site, on the other hand, not only is reported to facilely form and desorb CH3OH but also is resistant to the overoxidation reaction forming CH2O, due to an early occupancy of the Ni δ* acceptor orbital at the H–CH2OH activation stage, resulting in a product-like (late) transition structure, where one of the Ni2+ centers is already reduced to a highly unstable Ni+. This work provides insights into the reaction mechanisms and elaborates the importance of the CH3O formation to achieve high-selectivity CH3OH.
Density Functional Theory Studies of the Direct Conversion of Methane to Methanol Using O₂ on Graphitic MN₄G-BN (M = Fe, Co, Cu) and CuN₄G-PN Single-Atom Catalysts
ACS Appl. Nano Mater. 2023, 6, 8, 6559–6566
Abstract: Graphene-based single-atom catalysts have attracted increasing interest due to their potential to catalyze the direct conversion of CH4 to CH3OH. In particular, the porphyrin-like FeN4 complex has recently been reported to convert CH4 to CH3OH at low temperatures with high selectivity. However, only N2O and H2O2, which are high-cost and scarce compared to O2, can be used as the oxidant of the reaction. In this paper, we perform density functional theory calculations on graphitic MN4G-BN (M = Fe, Co, Cu) and CuN4G-PN systems to evaluate the CH4 oxidation to CH3OH using O2. We found that the addition of B doping adjacent to the Fe and Co centers as well as P doing adjacent to the Cu center facilitates a facile O═O bond dissociation with an activation barrier of less than 0.4 eV, resulting in active M–O and inactive B/P–O sites. This low barrier is due to the early O═O bond elongation at the O2 adsorption step and the stability of the atomically adsorbed O atoms. In the subsequent CH4 oxidation, the resultant OCuN4G-OPN is found to be significantly more CH4-reactive than the OFeN4G-OBN and OCoN4G-OBN with a H–CH3 activation barrier of only 0.66 eV. Such high reactivity is due to the proximity of the electron-acceptor orbital (i.e., the Cu–O lowest unoccupied molecular orbital) toward the Fermi level. Moreover, the CH4 oxidation on CuN4G-PN is predicted to form CH3OH with high exothermicity and high resistance to overoxidation. This study suggests a high possibility for CuN4G-PN as a potential catalyst for the stepwise conversion of CH4 to CH3OH using O2 at low temperatures.
Rutile-type metal dioxide (110) surfaces for the cyclic oxidation of methane to methanol
Mater. Adv. 2024, 5, 22, 8961–8969
Abstract: The direct conversion of methane to methanol has attracted increasing interests, owing to the necessity for an abundant low-carbon source of energy. However, numerous challenges are encountered in attaining a high conversion rate and selectivity using the existing approach and catalysts. One of them is the need for a reaction halt and a reactivation of the catalyst using an oxidant at high temperature, which makes the whole process non-cyclic. In this study, we employ density functional theory calculations to evaluate rutile-type IrO2(110), β-PtO2(110), and β-MnO2(110) surfaces not only for cleaving the H–CH3 bond but also for forming methanol. We find that IrO2(110) and β-PtO2(110) thermodynamically and kinetically favor the C–H activation on the bridging μO-atom terminations via a heterolytic pathway. However, the formation of strong Ir–C and Pt–C bonds, which initially help the C–H bond scission, hinders the methanol formation. In the β-MnO2(110) case, in contrast, the Mn–C interaction is quite weak, and the Mn(μ-O)Mn active site is electrophilic, thus allowing the formation of a stable ˙CH3 radical intermediate state that becomes the driving force for a low-barrier homolytic C–H bond scission as well as a low-barrier and highly exothermic formation of methanol. This first cycle of methane oxidation results in a reduced β-MnO2(110) surface, where no more μ-O active sites are available for the subsequent cycles of methane activation. Nonetheless, this reduced surface can also oxidize methane to methanol when the H2O2 oxidant is inserted in the mid-way reaction and forms new active sites of μ-OH. The second reaction is also highly exothermic although the C–H activation barrier is not as low as that for the fresh stoichiometric surface. This study suggests the β-MnO2(110) surface as a potential catalyst for the cyclic oxidation of methane to methanol using the H2O2 oxidant without halting for reactivation.
CO2 hydrogenation to HCOOH on PdZn surface and supported PdZn Cluster: A Comparative DFT study
Appl. Surf. Sci. 2025, 685, 162095
Abstract: Modifying heterogeneous catalysts for supported cluster-based types is important to design catalysts with better activity, stability, and selectivity. Alloying Pd with Zn and supported by ZrO2 is a promising way to design catalysts for CO2 hydrogenation to HCOOH, but the nature of the active catalytic sites and the mechanism remain unknown. Two representative models have been investigated: subnanometer cluster Pd5Zn/ZrO2 and PdZn(101) surface. DFT calculations combined with microkinetic simulations are used to identify the optimum structure and configurations for the reaction. Compared to the PdZn(101) surface, the Pd5Zn/ZrO2 offers much more stable adsorption and formation of intermediate species. Moreover, the formate route is more likely to proceed on PdZn(101) surface from the viewpoint of thermodynamic and kinetic. In contrast, the supported Pd5Zn/ZrO2 cluster prefers the carboxyl pathway, where the interface site between cluster-support is ascribed to a far more stable configuration. Electronic structure analysis reveals the nature of the transition state on intermediate formation, particularly the role of Pd and Zn edge atoms on the selectivity towards the carboxyl pathway on Pd5Zn/ZrO2. Finally, the comparison of microkinetic simulation results shows a preference for HCOOH formation on Pd5Zn/ZrO2 than PdZn(101) surface at medium to higher temperature.
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