Electrochemical Reversible Reforming between Ethylamine and Acetonitrile on Heterostructured Pd-Ni(OH)2 Nanosheets

Rational design of electrocatalysts is essential to achieve desirable performance of electrochemical synthesis process. Heterostructured catalysts have thus attracted widespread attention due to their multifunctional intrinsic properties, and diverse catalytic applications with corresponding outstanding activities. Here, we report an in situ restoration strategy for the synthesis of ultrathin Pd-Ni(OH)2 nanosheets. Such Pd-Ni(OH)2 nanosheets exhibit excellent activity and selectivity towards reversible electrochemical reforming of ethylamine and acetonitrile. In the acetonitrile reduction process, Pd acts as reaction center, while Ni(OH)2 provide proton hydrogen through promoting the dissociation of water. Also ethylamine oxidation process can be achieved on the surface of the heterostructured nanosheets with abundant Ni(II) defects. More importantly, an electrolytic cell driven by solar cells was successfully constructed to realize ethylamine-acetonitrile reversible reforming. This work demonstrates the importance of heterostructure engineering in the rational synthesis of multifunctional catalysts towards electrochemical synthesis of fine chemicals.

A Surfactant-Free and General Strategy for the Synthesis of Bimetallic Core-Shell Nanocrystals on Reduced Graphene Oxide through Targeted Photodeposition

Tunable physicochemical properties of bimetallic core-shell heterostructured nanocrystals (HNCs) have shown enormous potential in electrocatalytic reactions. In many cases, HNCs are required to load on supports to inhibit catalyst aggregation. However, the introduction of supports during the process of growing core-shell HNCs makes the synthesis much more complicated and difficult to control precisely. Herein, we reported a universal photochemical synthetic strategy for the controlled synthesis of well-defined surfactant-free core-shell metal HNCs on a reduced graphene oxide (rGO) support, which was assisted by the fine control of photogenerated electrons directly transferring to the targeted metal seeds via rGO and the precisely tuned adsorption capacity of the added second metal precursors. The surface photovoltage microscopy (SPVM) platform proved that photogenerated electrons flowed through rGO to Pd particles under illumination. We have successfully synthesized 24 different core-shell metal HNCs, i.,e., M_A@M_B (M_A = Pd, Au, and Pt; M_B = Au, Ag, Pt, Pd, Ir, Ru, Rh, Ni and Cu), on the rGO supports. The as-prepared Pd@Cu core-shell HNCs showed outstanding performance in the electrocatalytic reduction of CO₂ to CH₄. This work could shed light on the controlled synthesis of more functional bimetallic nanostructured materials on diverse supports for various applications.

Elucidating the Role of Capping Agents in Facet-Dependent Adsorption Performance of Hematite Nanostructures

Organic capping agents are a ubiquitous and crucial part of preparing reproducible and homogeneous batches of nanomaterials, particularly nanocrystals with well-defined facets. Despite studies reporting surface ligands (e.g., capping agents) having a non-negligible role in catalytic behavior, their impact is less understood in contaminant adsorption, an important consideration given their potential to obfuscate facet-dependent trends in performance. To ascribe observed behaviors to the facet or the ligand, this report evaluates the impact of poly(N-vinyl-2-pyrrolidone) (PVP), a commonly utilized capping agent, on the adsorption performance of nanohematite particles of varying prevailing facet in the removal of selenite (Se(IV)) as a model system. The PVP capping agent reduces the available surface area for contaminant binding, thus resulting in a reduction in overall Se(IV) adsorbed. However, accounting for the effects of surface area, {012}-faceted nanohematite demonstrates a significantly higher sorption capacity for Se(IV) compared with that of {001}-faceted nanohematite. Notably, chemical treatment is minimally effective in removing strongly bound PVP, indicating that complete removal of surface ligands remains challenging.

Quantification, Exchange, and Removal of Surface Ligands on Noble-Metal Nanocrystals

ConspectusSurface ligands are vital to the colloidal synthesis of noble-metal nanocrystals with well-controlled sizes and shapes for various applications. The surface ligands not only dictate the formation of nanocrystals with diverse shapes but also serve as a colloidal stabilizer to prevent the suspended nanocrystals from aggregation during their synthesis or storage. By leveraging the facet selectivity of some surface ligands, one can further control the sites for growth or galvanic replacement to transform presynthesized nanocrystals into complex structures that are otherwise difficult to fabricate using conventional methods. Furthermore, the presence of surface ligands on nanocrystals also facilitates their applications in areas such as sensing, imaging, nanomedicine, and self-assembly. Despite their popular use in enhancing the properties of nanocrystals and thus optimizing their performance in a wide variety of applications, it remains a major challenge to quantitatively determine the coverage density of ligand molecules, not to mention the difficulty of substituting or removing them without compromising the surface structure and aggregation state of the nanocrystals.In this Account, we recapitulate our efforts in developing methods capable of qualitatively or quantitatively measuring, exchanging, and removing the surface ligands adsorbed on noble-metal nanocrystals. We begin with an introduction to the typical interactions between ligand molecules and surface atoms, followed by a discussion of the Langmuir model that can be used to describe the adsorption of surface ligands. It is also emphasized that the adsorption process may become very complex in the case of a polymeric ligand due to the variations in binding configuration and chain conformation. We then highlight the capabilities of various spectroscopy methods to analyze the adsorbed ligands qualitatively or quantitatively. Specifically, surface-enhanced Raman scattering, Fourier transform infrared, and X-ray photoelectron spectroscopy are three examples of qualitative methods that can be used to confirm the absence or presence of a surface ligand. On the other hand, ultraviolet-visible spectroscopy and inductively coupled plasma mass spectrometry can be used for quantitative measurements. Additionally, the coverage density of a ligand can be derived by analyzing the morphological changes during nanocrystal growth. We then discuss how the ligands present on the surface of metal nanocrystals can be exchanged directly or indirectly to meet the requirements of different applications. The former can be done using a ligand with stronger binding, whereas the latter is achieved by introducing a sacrificial shell to the surface of the nanocrystals. Furthermore, we highlight three additional strategies besides simple washing to remove the surface ligands, including calcination, heating in a solution, and UV-ozone treatment. Finally, we showcase three applications of metal nanocrystals in nanomedicine, tumor targeting, and self-assembly by taking advantage of the diversity of surface ligands bearing different functional groups. We also offer perspectives on the challenges and opportunities in realizing the full potential of surface ligands.

The role of crystallinity of palladium nanocrystals in ROS generation and cytotoxicity induction

Palladium (Pd) nanocrystals with different crystalline forms exhibit distinct enzyme-like activities in generating reactive oxygen species (ROS). How such crystallinity-dependent catalytic activity regulates potential cytotoxicity remains to be elucidated. In the present work, Pd nanocrystals with four different crystalline forms were synthesized, and the underlying mechanisms involved in ROS-mediated cytotoxicity were systematically revealed. Pd nanocrystals with the {100} (nanocubes) and {111} (nanooctahedrons and nanotetrahedrons) facets triggered cytotoxicity by generating singlet oxygen (¹O₂) and hydroxyl radicals (OH˙), respectively. Meanwhile, Pd nanoconcave-tetrahedrons, which had both the {110} and {111} facets, induced ROS-mediated cytotoxicity via activating the superoxide (O₂˙^-) pathway. Consumption of protons and generation of hydroxide during intracellular ROS conversion resulted in pH alkalization, eventually leading to cell death. Our findings emphasize the importance of facet-dependent ROS generation promoted by Pd nanocrystals. Furthermore, alkalization is identified as a new biomarker for analyzing ROS-mediated cytotoxicity.

Boosting the Electrocatalytic Formic Acid Oxidation Activity via P-PdAuAg Quaternary Alloying

Direct formic acid fuel cells (DFAFCs) are considered promising sustainable power sources due to their high energy density, nonflammability, and low fuel crossover. However, serious CO poisoning and activity attenuation of the anodic formic acid oxidation reaction (FAOR) greatly restrict the output and durability of DFAFCs. Inspired by the specific relationship between the composition, type, and property of alloys, in this work, we synthesize a series of hybrid substitutional/interstitial quaternary alloys P-PdAuAg by means of a novel polyphosphide route to address these issues. Due to the simultaneous interstitial P-doping and metal (Au, Ag, Pd) co-reduction, the P-PdAuAg quaternary alloy obtained is only 3 nm in diameter with abundant defects. It not only achieves a new high mass activity of 8.08 A mg_Pd^-1 (6.78 A mg_catalyst^-1) but also maintains high stability in the high potential range and harsh reaction conditions. Both the activity and anti-poisoning ability are far exceeding those of the currently reported FAOR catalysts. Detailed density functional theory (DFT) calculations reveal that the superb electrochemical performances originate from the shift of the d-band center of Pd as a result of the synergistic electronic/ligand effects between Pd, Au, Ag, and P. The introduction of interstitial P inhibits the occurrence of an indirect reaction pathway on Pd, while Au and Ag suppress the adsorption of CO and optimize the sequential dehydrogenation steps, leading to boosted reaction kinetics and CO tolerance. This work pioneered a facile way for the synthesis of Pd-based substitutional/interstitial hybrid alloys, providing a promising means of further improving the performance of alloying catalysts.

Metallene-related materials for electrocatalysis and energy conversion

As a member of graphene analogs, metallenes are a class of two-dimensional materials with atomic thickness and well-controlled surface atomic arrangement made of metals or alloys. When utilized as catalysts, metallenes exhibit distinctive physicochemical properties endowed from the under-coordinated metal atoms on the surface, making them highly competitive candidates for energy-related electrocatalysis and energy conversion systems. Significantly, their catalytic activity can be precisely tuned through the chemical modification of their surface and subsurface atoms for efficient catalyst engineering. This minireview summarizes the recent progress in the synthesis and characterization of metallenes, together with their use as electrocatalysts toward reactions for energy conversion. In the Synthesis section, we pay particular attention to the strategies designed to tune their exposed facets, composition, and surface strain, as well as the porosity/cavity, defects, and crystallinity on the surface. We then discuss the electrocatalytic properties of metallenes in terms of oxygen reduction, hydrogen evolution, alcohol and acid oxidation, carbon dioxide reduction, and nitrogen reduction reaction, with a small extension regarding photocatalysis. At the end, we offer perspectives on the challenges and opportunities with respect to the synthesis, characterization, modeling, and application of metallenes.

Atomically Reconstructed Palladium Metallene by Intercalation-Induced Lattice Expansion and Amorphization for Highly Efficient Electrocatalysis

As an emerging class of materials with distinctive physicochemical properties, metallenes are deemed as efficient catalysts for energy-related electrocatalytic reactions. Engineering the lattice strain, electronic structure, crystallinity, and even surface porosity of metallene provides a great opportunity to further enhance its catalytic performance. Herein, we rationally developed a reconstruction strategy of Pd metallenes at atomic scale to generate a series of nonmetallic atom-intercalated Pd metallenes (M-Pdene, M = H, N, C) with lattice expansion and S-doped Pd metallene (S-Pdene) with an amorphous structure. Catalytic performance evaluation demonstrated that N-Pdene exhibited the highest mass activities of 7.96 A mg^-1, which was 10.6 and 8.5 time greater than those of commercial Pd/C and Pt/C, respectively, for methanol oxidation reaction (MOR). Density functional theory calculations suggested that the well-controlled lattice tensile strain as well as the strong p-d hybridization interaction between N and Pd resulted in enhanced OH adsorption and weakened CO adsorption for efficient MOR catalysis on N-Pdene. When tested as hydrogen evolution reaction (HER) catalysts, the amorphous S-Pdene delivered superior activity and durability relative to the crystalline counterparts because of the disordered Pd surface with a further elongated bond length and a downshifted d-band center. This work provides an effective strategy for atomic engineering of metallene nanomaterials with high performance as electrocatalysts.

Boosting the anti-poisoning ability of palladium towards electrocatalytic formic acid oxidation via polyphosphide chemistry

In this work, we reported a novel polyphosphide strategy for the synthesis of phosphorus doped Pd (P-Pd) using red phosphorus as the starting material at quasi-ambient conditions. Polyphophide anions, as the key reaction intermediates, served as the reducing agent and phosphorus source to modulate the surface electronic structure of Pd. The P-Pd obtained exhibited topmost CO tolerance and electrocatalytic activity to formic acid oxidation among the state-of-arts reports. The mass activity and turnover frequency of P-Pd reached 4413 mA mg^-1_Pd and 16.04 s^-1 at 0.8 V, which were 23.7 and 6.4 times that of commercial Pd/C respectively. After 1000 repeated cycles, 82% initial activity was reserved. Combined with the electrochemical analysis and the density functional theory calculation, the boosted electrochemical performances can be attributed to the size and electronic effects induced by the P doping, which increase the surface actives sites, inhibit the adsorption of CO and change the reaction pathway to favorable CO₂ route. A full cell was also assembled to demonstrate the practical potential of the P-Pd, which showed a maximum power density of 21.56 mW cm^-2. This polyphophide-based reaction route provides a new strategy for the preparation of efficient and durable phosphorus doped alloys for electrocatalysis.

Solution-Phase Synthesis of PdH0.706 Nanocubes with Enhanced Stability and Activity toward Formic Acid Oxidation

Palladium is one of the few metals capable of forming hydrides, with the catalytic properties being dependent on the elemental composition and spatial distribution of H atoms in the lattice. Herein, we report a facile method for the complete transformation of Pd nanocubes into a stable phase made of PdH_0.706 by treating them with aqueous hydrazine at a concentration as low as 9.2 mM. Using formic acid oxidation (FAO) as a model reaction, we systematically investigated the structure-catalytic property relationship of the resultant nanocubes with different degrees of hydride formation. The current density at 0.4 V was enhanced by four times when the nanocubes were completely converted from Pd to PdH_0.706. On the basis of a set of slab models with PdH(100) overlayers on Pd(100), we conducted density functional theory calculations to demonstrate that the degree of hybrid formation could influence both the activity and selectivity toward FAO by modulating the relative stability of formate (HCOO) and carboxyl (COOH) intermediates. This work provides a viable strategy for augmenting the performance of Pd-based catalysts toward various reactions without altering the loading of this scarce metal.

Surfactant-Free Synthesis of Three-Dimensional Metallic Nanonetworks via Nanobubble-Assisted Self-Assembly

Three-dimensional metallic nanonetworks (3D-MNWs) demonstrate unique performances across a wide range of fields, and their facile and green synthetic method is of high significance. Herein, we report a self-generated-nanobubble scaffolding strategy for the fabrication of 3D-MNWs, which employs aqua ammonia (AA) as a nanobubble reservoir and avoids the use of any surfactants or polymeric capping agents. Benefiting from the interaction between ammonia and metallic nanoparticles, finely interlocked nanonetworks (Au, Pt, Ag, and Cu) with curved geometry and abundant pores are obtained by precisely controlling the anisotropic kinetic growth using a strong reducing agent and a high concentration of AA. As a demonstration, the methanol oxidation reaction (MOR) is tested to assess the electrocatalytic performance of the Pt 3D-MNWs. The peak current of Pt 3D-MNWs reaches 152 mA/mg^Pt, which is 2.5 times higher than that of commercial Pt black. This unique nanobubble-assisted strategy has great potential in the basic synthetic prototype for polyporous nanomaterials.

Using Reduction Kinetics to Control and Predict the Outcome of a Colloidal Synthesis of Noble-Metal Nanocrystals

Improving the performance of noble-metal nanocrystals in various applications critically depends on our ability to manipulate their synthesis in a rational, robust, and controllable fashion. Different from a conventional trial-and-error approach, the reduction kinetics of a colloidal synthesis has recently been demonstrated as a reliable knob for controlling the synthesis of noble-metal nanocrystals in a deterministic and predictable manner. Here we present a brief Viewpoint on the recent progress in leveraging reduction kinetics for controlling and predicting the outcome of a synthesis of noble-metal nanocrystals. With a focus on Pd nanocrystals, we first offer a discussion on the correlation between the initial reduction rate and the internal structure of the resultant seeds. The kinetic approaches for controlling both nucleation and growth in a one-pot setting are then introduced with an emphasis on manipulation of the reduction pathways taken by the precursor. We then illustrate how to extend the strategy into a bimetallic system for the preparation of nanocrystals with different shapes and elemental distributions. Finally, the influence of speciation of the precursor on reduction kinetics is highlighted, followed by our perspectives on the challenges and future endeavors in achieving a controllable and predictable synthesis of noble-metal nanocrystals.