Ligand-coordination effects on the selective hydrogenation of acetylene in single-site Pd-ligand supported catalysts

Hemilabile Coordination in Single-Atom Catalyst: A Strategy To Overcome the Limitation of the Scaling Relationship

Traditional catalyst optimization, based on the Sabatier principle, encounters performance limits due to the scaling relationship between binding energies for a series of adsorbates. This restriction prevents independent optimization of the reactant activation and product desorption. Single-atom catalysts (SACs) offer a unique advantage, with their ability to dynamically adjust the metal-support coordination environment. This flexibility allows us to apply hemilability, a concept from homogeneous catalysis, to modulate catalytic activity. Hemilability, which involves the reversible opening and closing of the coordination site, enables SACs to dynamically alter their electronic structure, effectively decoupling the competing requirements of activation and desorption. In this Perspective, we highlight how SACs, with hemilabile metal-support coordination, represent a promising strategy to bypass the limitations imposed by the scaling relationship. We also discuss the experimental challenges and future opportunities for directly observing and controlling these dynamic processes in SACs, thus presenting a powerful way for developing more efficient catalytic systems.

Selective Hydrogenation of Furfural Under Mild Conditions Over Single-Atom Pd1/α-MoC Catalyst

The selective activation of C=O bonds was the key challenge in the field of biomass utilization. Researchers worked on this purpose by developing high-active and high-selective catalysts. In this study, a Pd1/α-MoC single-atom catalyst was synthesized and applied in selective hydrogenation of biomass-derived furfural with 96.7 % conversion and 92.4 % selectivity under a near-room temperature. With various characterizations, the formation of Pd single-atom sites over the surface of α-MoC was confirmed. Then, the dominant structure of Pd single-atom site and the reaction pathway were proposed with experimental and Density Functional Theory (DFT) studies. Compared with undecorated α-MoC, the introduction of Pd single-atom species significantly altered the reaction mechanism from Meerwein-Ponndorf-Verley (MPV) process. Moreover, the Pd single-atoms loading on α-MoC(111) surface notably reduced the energy barriers of H2 activation and C=O bond hydrogenation, which may lead to the improving catalytic performance of α-MoC based catalyst. Hence, this investigation could provide a new strategy and understanding for the development of high-active and low-cost catalysts.

Active-Site Ensemble for the Reverse Water-Gas Shift Reaction over Pd/TiO2: Two Is Better than One or More

Determining whether the active-site ensemble number for a given reaction is a single atom, dual atom, or subnanocluster remains a challenge for both experimental and theoretical studies. This work presents a systematic theoretical study, employing density functional theory and mean-field microkinetic modeling, on the reverse water-gas shift (rWGS) reaction over the anatase TiO2-supported Pd cluster, Pdn/A-TiO2 (n = 1, 2, 3, 4) to explore the optimal active ensemble number required for rWGS. Our results show that Pd2 shows the best catalytic activity for rWGS, while neither Pd1 nor Pd3(Pd4) demonstrates high catalytic activity. This is due to either the limited active sites for carboxyl formation, as observed with Pd1, or excessively strong binding of H* species, which hinders the carboxyl formation, as seen with Pd3 (or Pd4). We found that Pd2with the most stable position for [COOH* + H*] species along the rWGS reaction energy diagrams, which maybe one of the possible reasons for its high rWGS catalytic activity. Moreover, H* binding energy can be used as a descriptor of rWGS activity, in line with the Sabatier principle, neither too strong nor too weak binding is favorable. It is hoped that the present findings can be extended to other reaction types, such as the water-gas shift (WGS) reaction, where Pd2 may outperform either a single Pd atom or small Pd clusters.

Investigation of Sn Promoter on Ni/CeO2 Catalysts for Enhanced Acetylene Semihydrogenation to Ethylene

Ethylene, as an important chemical raw material, could be produced through the coal-based acetylene hydrogenation route. Nickel-based catalysts demonstrate significant activity in the semihydrogenation reaction of acetylene, but they encounter challenges related to catalyst deactivation and overhydrogenation. Herein, the effect of Sn promoter on Ni/CeO2 catalysts has been comprehensively explored for acetylene semihydrogenation. The optimized Ni/8%Sn-CeO2 catalytic performance was significantly improved, with 100% acetylene conversion and 82.5% ethylene selectivity at 250 °C, and the catalyst maintained high catalyst performance within a 1000 min stability test. A series of characterization tests show that CeO2 modified by moderate Sn4+ doping is more conducive to modulating the charge structure and geometry of the Ni active center. Additionally, the in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy and density functional theory results indicated that catalysts doped with Sn4+ facilitated more efficient desorption of ethylene from the catalyst surface compared to Ni/CeO2 catalysts, thus improving ethylene selectivity and yield. This study highlights an effective strategy for improving the catalytic performance of rare-earth-based catalysts through the incorporation of effective metal promoters.

Laser-Induced Pd-PdO/rGO Catalysts for Enhanced Electrocatalytic Conversion of Nitrate into Ammonia

Electrochemical reduction of nitrate to ammonia (eNO3RR) is proposed as a sustainable solution for high-rate ammonia synthesis under ambient conditions. The complex, multistep eNO3RR mechanism necessitates the use of a catalyst for the complete conversion of nitrate to ammonia. Our research focuses on developing a novel Pd-PdO doped in a reduced graphene oxide (rGO) composite catalyst synthesized via a laser-assisted one-step technique. This catalyst demonstrates dual functionality: palladium (Pd) boosts hydrogen adsorption, while its oxide (PdO) demonstrates considerable nitrogen adsorption affinity and exhibits a maximum ammonia yield of 5456.4 ± 453.4 μg/h/cm2 at -0.6 V vs reversible hydrogen electrode (RHE), with significant yields for nitrite and hydroxylamine under ambient conditions in a nitrate-containing alkaline electrolyte. At a lower potential of -0.1 V, the catalyst exhibited a minimal hydrogen evolution reaction of 3.1 ± 2.2% while achieving high ammonia selectivity (74.9 ± 4.4%), with the balance for nitrite and hydroxylamine. Additionally, the catalyst's stability and activity can be regenerated through the electrooxidation of Pd.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.