Revealing the Active Sites of Pd Nanocrystals for Propyne Semihydrogenation: From Theory to Experiment

A highly efficient synthetic strategy for de novo NP encapsulation into metal-organic frameworks: enabling further modulated control of catalytic properties

De novo encapsulation is a prevalent method to prepare composite materials where the structure-tunable metal nanoparticles (NPs) are holistically coated with metal-organic frameworks (MOFs). This method has been demonstrated to have promise in various fields but the extensive application of this approach is still challenging. This study proposed, for the first time, leveraging a specific surface-energy-dominated (SED) mechanism to achieve a highly efficient synthetic strategy for de novo NP encapsulation. The generality of this strategy is proved in applying to various MOFs, reaction conditions and the use of capping agents. By applying the strategy, Pd NPs with different morphologies are encapsulated in UiO-67, which is prone to self-assembly without coating, and an interesting enhancement is investigated in the selective semihydrogenation of alkynes on different Pd surfaces. These results demonstrate that the control of surface energy is a feasible method for efficient NP encapsulation which sheds light on the rational design of MOF-based composites for future applications.

Controllable Deposition of Bi onto Pd for Selective Hydrogenation of Acetylene

The rational regulation of catalyst active sites at atomic scale is a key approach to unveil the relationship between structure and catalytic performance. Herein, we reported a strategy for the controllable deposition of Bi on Pd nanocubes (Pd NCs) in the priority order from corners to edges and then to facets (Pd NCs@Bi). The spherical aberration-corrected scanning transmission electron microscopy (ac-STEM) results indicated that Bi₂O₃ with an amorphous structure covers the specific sites of Pd NCs. When only the corners and edges of the Pd NCs were covered, the supported Pd NCs@Bi catalyst exhibited an optimal trade-off between high conversion and selectivity in the hydrogenation of acetylene to ethylene under ethylene-rich conditions (99.7% C₂H₂ conversion and 94.3% C₂H₄ selectivity at 170 °C) with remarkable long-term stability. According to the H₂-TPR and C₂H₄-TPD measurements, the moderate hydrogen dissociation and the weak ethylene adsorption are responsible for this excellent catalytic performance. Following these results, the selectively Bi-deposited Pd nanoparticle catalysts showed incredible acetylene hydrogenation performance, which provides a feasible perspective to design and develop highly selective hydrogenation catalysts for industrial applications.

Increasing the Distance of Adjacent Palladium Atoms for Configuration Matching in Selective Hydrogenation

Precisely tailoring the distance between adjacent metal sites to match adsorption configurations of key species for the targeted reaction pathway is a great challenge in heterogeneous catalysis. Here, we report a proof-of-concept study on the atomically sites-tailored pathway in Pd-catalyzed acetylene hydrogenation, i.e., increasing the distance of adjacent Pd atoms (d_Pd-a-Pd ) for configuration matching in acetylene semi-hydrogenation against coupling. d_Pd-a-Pd is identified as a structural descriptor for describing the competitiveness for reaction pathways, and the increased d_Pd-a-Pd prefers the semi-hydrogenation pathway due to simultaneously promoted C₂ H₄ desorption and the destabilized transition state of the C₂ H₃ * coupling. Spectroscopic, kinetics and electronic structure studies reveal that increasing d_Pd-a-Pd to 3.31 Å delivers superior selectivity and stability due to energy matching and appropriate hybridization of Pd 4d with In 2s and, especially, 2p orbitals.

Unveiling the Local Structure and Electronic Properties of PdBi Surface Alloy for Selective Hydrogenation of Propyne

Building a reliable relationship between the electronic structure of alloyed metallic catalysts and catalytic performance is important but remains challenging due to the interference from many entangled factors. Herein, a PdBi surface alloy structural model, by tuning the deposition rate of Bi atoms relative to the atomic interdiffusion rate at the interface, realizes a continuous modulation of the electronic structure of Pd. Using advanced X-ray characterization techniques, we provide a precise depiction of the electronic structure of the PdBi surface alloy. As a result, the PdBi catalysts show enhanced propene selectivity compared with the pure Pd catalyst in the selective hydrogenation of propyne. The prevented formation of saturated β-hydrides in the subsurface layers and weakened propene adsorption on the surface contribute to the high selectivity. Our work provides in-depth understanding of the electronic properties of surface alloy structure and underlies the study of the electronic structure-performance relationship in bimetallic catalysts.

Research on alkali metal-modified Pd catalyst for oxygen removal from propylene

A series of alkali metal (Li, Na, and K)-modified Pd catalysts and Pd/Al₂O₃ were prepared and used to remove oxygen in a propylene flow with hydrogen’s existence. The results showed that the alkali metals could enhance the performance of the Pd catalysts and the effect followed the order of K > Na > Li. X-Ray diffraction (XRD), N₂-physisorption, transmission electron microscopy (TEM), hydrogen temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS) were carried out to investigate the alkali metal-modified Pd catalysts and the promotional effect mechanism was explained. The results showed that alkali metal modification increased the electron density of Pd atoms to induce the negatively charged Pd species, which could enhance the adsorption of oxygen while weakening the adsorption of propylene, and then enhance the performance of the modified catalysts for oxygen removal from unsaturated hydrocarbon. The Pd-K/A catalyst performed the best on both oxygen removal and propylene hydrogenation inhibition.

Dynamic Modification of Palladium Catalysts with Chain Alkylamines for the Selective Hydrogenation of Alkynes

Selective hydrogenation of alkynes plays a pivotal role in the field of chemical production but still suffers from restrained catalytic activity and low alkene selectivity. Herein, a dynamic modification strategy was utilized by preferentially attaching diethylenetriamine (DETA) to the surface of the support to modify the Pd catalyst. The DETA-modified Pd catalyst demonstrates unprecedented reactivity (14,412 h^-1) and selectivity as high as 94% for the semihydrogenation of 2-methyl-3-butyn-2-ol at 35 °C, presenting a 36-fold higher reactivity than the Lindlar catalyst. Moreover, the yield exceeds 98.2% at full conversion under no solvent and organic adsorbate conditions, indicating the potential applications for industrial production. Systematic studies reveal that flexible DETA serves in a reversible "breathing pattern" for the molecular discrimination by constructing dynamic metal-support interaction (DMSI), enabling selective exclusion of alkenes from the Pd surface. DETA is competent to dynamically adjust the adsorption behaviors of reactants and effectively boost the intrinsic activity of the modified catalyst. Impressively, the DETA-modified Pd catalyst exhibits exceptional stability even after being recycled 20 times. This work sheds light on a novel and applicable method for the rational design of heterogeneous catalysts via DMSI.

Facet-controlled palladium nanocrystalline for enhanced nitrate reduction towards ammonia

Electrochemical nitrate reduction reaction (NO₃^-RR) is considered an appealing way for producing ammonia (NH₃) under ambient conditions and solving environmental problems caused by nitrate, whereas the lack of adequate catalysts hampers the development of NO₃^-RR. Here, we firstly demonstrate that the Pd nanocrystalline with a well-desired facet can act as a highly efficient NO₃^-RR electrocatalyst for ambient ammonia synthesis. Pd (1 1 1) exhibits excellent activity and selectivity in reducing NO₃^- to NH₄⁺ with a Faradaic efficiency of 79.91% and an NH₄⁺ production of 0.5485 mmol h^-1 cm^-2 (2.74 mmol h^-1 mg^-1) in 0.1 M Na₂SO₄ (containing 0.1 M NO₃^-), which is 1.4 times higher than Pd (1 0 0) and 1.9 times higher than Pd (1 1 0), respectively. Density functional theory (DFT) calculation reveals that the superior NO₃^-RR activity of Pd (1 1 1) originates from its optimized activity of NO₃^- adsorption, smaller free energy change of the rate-limiting step (*NH₃ to NH₃), and poorer hydrogen evolution reaction activity (HER, competitive reaction). This work not only highlights the potentials of Pd-based nanocatalysts for NO₃^-RR but also provides new insight for the applications in NO₃^-RR of other facet-orientation nanomaterials.

Revealing the Correlation between Catalytic Selectivity and the Local Coordination Environment of Pt Single Atom

Single atom catalysts (SACs) have recently attracted great attention in heterogeneous catalysis and have been regarded as ideal models for investigating the strong interaction between metal and support. Despite the huge progress over the past decade, the deep understanding on the structure-performance correlation of SACs at a single atom level still remains to be a great challenge. In this study, we demonstrate that the variation in the coordination number of the Pt single atom can significantly promote the propylene selectivity during propyne semihydrogenation (PSH) for the first time. Specifically, the propylene selectivity greatly increases from 65.4% to 94.1% as the coordination number of Pt-O increases from ∼3.4 to ∼5, whereas the variation in the coordination number of Pt-O slightly influences the turnover frequency values of SACs. We anticipate that the present work may deepen the understanding on the structure-performance of SACs and also promote the fundamental research in single atom catalysis.