Sub-nanometer Clusters Research Articles

Rapid synthesis of Pd single-atom/cluster as highly active catalysts for Suzuki coupling reactions

Palladium (Pd)-based catalysts are essential to drive high-performance Suzuki coupling reactions, which are powerful tools for the synthesis of functional organic compounds. Herein, we developed a solution-rapid-annealing process to stabilize nitrogen-mesoporous carbon supported Pd single-atom/cluster (Pd/NMC) material, which provided a catalyst with superior performance for Suzuki coupling reactions. In comparison with commercial palladium/carbon (Pd/C) catalysts, the Pd/NMC catalyst exhibited significantly boosted activity (100% selectivity and 95% yield) and excellent stability (almost no decay in activity after 10 reuse cycles) for the Suzuki coupling reactions of chlorobenzenes, together with superior yield and excellent selectivity in the fields of the board scope of the reactants. Moreover, our newly developed rapid annealing process of precursor solutions is applied as a generalized method to stabilize metal clusters (e.g. Pd, Pt, Ru), opening new possibilities in the construction of efficient highly dispersed metal atom and sub-nanometer cluster catalysts with high performance.

A consistent formation free energy definition for multicomponent clusters in quantum thermochemistry

Quantum chemical calculations have proven to describe the thermodynamics of sub-nanometre clusters more accurately than the liquid drop model used in classical nucleation theory (CNT). However, the standard quantum chemical free energies of multicomponent clusters are fundamentally incompatible with the quantity appearing in the exponential of the CNT expression for the nucleation rate. The origin of this incompatibility is known to be in the statistical thermochemistry, but it is also connected to a much debated issue of self-consistency within CNT. Although these issues do not affect the main results of previous theoretical studies of nucleation, a thorough analysis and discussion about the nature of free energy in quantum chemistry of cluster formation has been missing. In this study, we present a consistent definition for the formation free energy using the law of mass action and a general equilibrium cluster distribution function, and apply this definition to gas-phase quantum thermochemical calculations. This internal consistency allows us to integrate the high-level thermochemical data to the basic framework of CNT and the liquid drop model contained therein. Moreover, based on our analysis, we derive a simple analytical expression for the steady-state nucleation rate compatible with the widely used numerical acdc model. As an illustrative example, the consistent formation free energies and nucleation rates are presented and analysed for atmospherically relevant H2SO4–NH3 clusters at various temperature and monomer vapour pressure conditions.

Relativistic Effects in Platinum Nanocluster Catalysis: A Statistical Ensemble-Based Analysis.

Nanoclusters are materials of paramount catalytic importance. Among various unique properties featured by nanoclusters, a pronounced relativistic effect can be a decisive parameter in governing their catalytic activity. A concise study delineating the role of relativistic effects in nanocluster catalysis is carried by investigating the oxygen reduction reaction (ORR) activity of a Pt7 subnanometer cluster. Global optimization analysis shows the critical role of spin-orbit coupling (SOC) in regulating the relative stability between structural isomers of the cluster. An overall improved ORR adsorption energetics and differently scaled adsorption-induced structural changes are identified with SOC compared to a non-SOC scenario. Ab initio atomistic thermodynamics analysis predicted nearly identical phase diagrams with significant structural differences for high coverage oxygenated clusters under realistic conditions. Though inclusion of SOC does not bring about drastic changes in the overall catalytic activity of the cluster, it is having a crucial role in governing the rate-determining step, transition-state configuration, and energetics of elementary reaction pathways. Furthermore, a statistical ensemble-based approach illustrates the strong contribution of low-energy local minimum structural isomers to the total ORR activity, which is significantly scaled up along the activity improving direction within the SOC framework. The study provides critical insights toward the importance of relativistic effects in determining various catalytic activity relevant features of nanoclusters.

Tuning the C/N Ratio of C-Rich Graphitic Carbon Nitride (g-C3 N4 ) Materials by the Melamine/Carboxylic Acid Adduct Route.

C-rich graphitic carbonitride materials (CNx ) with a large range of compositions have been prepared thanks to the self-assembly, in different ratios, of melamine (M) and a panel of polycarboxylic acids (A) such as oxalic, tartaric and citric acid. The thermal conversion of the formed adducts (MAy ), led to CNx phases, with x ranging from 0.66 to 1.4 (x=1.33 for g-C3 N4 for comparison). The properties of these materials were examined by different techniques (XRD, Raman spectroscopy, TEM, TGA, XPS and DRIFT). It appears that the increase in the C content is associated with the disappearance of the long-range order of heptazine units and an increase in the sub-nanometer carbon-rich cluster size within the graphitic g-C3 N4 structure. This trend is followed by a significant increase in the interlayer spacing and a lower proportion of N=C-N bonds compared to C=C bonds. The thermal stability under an inert atmosphere of these phases and their UV-Visible absorbance properties were also investigated.

Size-dependent selectivity and activity of highly dispersed sub-nanometer Pt clusters integrated with P25 for CO2 photoreduction into methane fuel

Integration of sub-nanometer–sized Pt cocatalysts on semiconductor materials is a promising approach to improve the activity and selectivity of CO2 photoreduction. However, the practical realization of such an integrated catalyst is a challenging task. We rationally integrated sub-nanometer Pt nanoclusters (Pt NCs) on the surface of commercial P25 catalyst by adjusting the amount of Pt loading. The lateral reduction in the size of Pt NCs resulted in high metal dispersion, increased active metal surface area, better CO2 adsorption, and hindered photoexcited charge-carrier recombination. The sub-nanometer Pt NCs integrated with P25 with a weight percentage of 0.5% (Pt-NC/P25-0.5) exhibited exceptional CO2 photoreduction activity for CH4 production and excellent stability during successive test runs. This catalyst outperformed its counterparts with larger Pt NCs and several previously reported state-of-the-art photocatalysts in terms of CH4 generation. Importantly, compared to its counterparts, Pt-NC/P25-0.5 had a higher proportion of edge and corner sites, which have a strong affinity for CO molecules, resulting in a high CH4 selectivity of 95% against H2. The size-dependent selectivity and activity of Pt NCs for CO2 reduction demonstrated in this study offer insights into the development of sub-nanometer metal-based catalysts for photocatalytic energy applications.

First-principles modelling of the new generation of subnanometric metal clusters: Recent case studies

The very recent development of highly selective techniques making possible the synthesis and experimental characterization of subnanometric (subnanometer-sized) metal clusters (even single atoms) is pushing our understanding far beyond the present knowledge in materials science, driving these clusters as a new generation of quantum materials at the lower bounds of nanotechnology. When the size of the metal cluster is reduced to a small number of atoms, the d-band of the metal splits into a subnanometric d-type molecular orbitals network in which all metal atoms are inter-connected, with the inter-connections having the length of a chemical bond (1–2 Å). These molecular characteristics are at the very core of the high stability and novel properties of the smallest metal clusters, with their integration into colloidal materials interacting with the environment having the potential to further boost their performance in applications such as luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Through the presentation of very recent case studies, this Feature Article is aimed to illustrate how first-principles modelling, including methods beyond the state-of-the-art and an interplay with cutting-edge experiments, is helping to understand the special properties of these clusters at the most fundamental level. Moreover, it will be discussed how superfluid helium droplets can act both as nano-reactors and carriers to achieve the synthesis and surface deposition of metal clusters. This concept will be illustrated with the quantum simulation of the helium droplet-assisted soft-landing of a single Au atom onto a titanium dioxide (TiO2) surface. Next, it will be shown how the application of first-principles methods have disclosed the fundamental reasons why subnanometric Cu5 clusters are resistant to irreversible oxidation, and capable of increasing and extending into the visible region the solar absorption of TiO2, of augmenting its efficiency for photo-catalysis beyond a factor of four, also considering the decomposition and photo-activation of CO2 as a prototypical (photo-) catalytic reaction. Finally, I will discuss how the modification of the same material with subnanometric Ag5 clusters has converted it into a “reporter” of a surface polaron property as well as a novel two-dimensional polaronic material.

Subnanometric CoSn clusters embedded in N-doped carbon nanoboxes (SN-CoSn@C NBs) realize high efficiency adsorption–conversion of polysulfides

SN-CoSn@C NBs as sulfur cathodes for lithium–sulfur batteries. The subnanometric CoSn clusters with single atom and atomic cluster sites were able to boost the chemisorption and electrocatalytic conversion dynamics of polysulfides.

Cytotoxic sub-nanometer aqueous platinum clusters as potential antitumoral agents.

Ligand-free sub-nanometer metal clusters (MCs) of Pt, Ir, Rh, Au and Cu, are prepared here in neat water and used as extremely active (nM) antitumoral agents for HeLa and A2870 cells. The preparation just consists of adding the biocompatible polymer ethylene-vinyl alcohol (EVOH) to an aqueous solution of the corresponding metal salt, to give liters of a MC solution after filtration of the polymer. Since the MC solution is composed of just neat metal atoms and water, the intrinsic antitumoral activity of the different sub-nanometer metal clusters can now fairly be evaluated. Pt clusters show an IC50 of 0.48 μM for HeLa and A2870 cancer cells, 23 times higher than that of cisplatin and 1000 times higher than that of Pt NPs, and this extremely high cytotoxicity also occurs for cisplatin-resistant (A2870 cis) cells, with a resistance factor of 1.4 (IC50 = 0.68 μM). Rh and Ir clusters showed an IC50 ∼ 1 μM. Combined experimental and computational studies support an enhanced internalization and cytotoxic activation.

Pd speciation on black phosphorene in a CO and C2H4 atmosphere: a first-principles investigation.

Deposited transition metal clusters and nanoparticles are widely used as catalysts and have long been thought stable in reaction conditions. We investigated the electronic structure and stability of freestanding and black phosphorene supported small Pd clusters containing 1 to 6 atoms in a CO or C2H4 atmosphere by extensive first-principles based calculations. We showed that, driven by the thermodynamics, subnanometric Pd clusters and single Pd atom on phosphorene may evolve for a better balance among metal-metal, metal-support and metal-adsorbate interactions, etc., resulting in atomic dispersion of Pd in reaction conditions. The strong interfacial Pd-P interactions would deform preformed Pd clusters into atomic strips of various lengths with enhanced stability comparable to bulk Pd. The diffusion barriers of terminal Pd atoms in the zigzag direction on phosphorene are small (<0.3 eV) and vary within ∼0.1 eV with the length of these atomic strips. Further adsorption of CO or C2H4 alters the Pd-P and Pd-Pd interactions and forms thermodynamically stable surface Pd species with decreased diffusion barriers, suggesting that atomic dispersion of Pd can be achieved on phosphorene, especially in a CO or C2H4 atmosphere. The current work may help to understand the superior catalytic performance of supported subnanometric transition metal catalysts in reaction conditions and pave the way for fabrication of single atom catalysts with the desired performance.

A subnanometric material reveals new quantum-chemical insights into surface polarons

The recent advent of cutting-edge experimental techniques allows for a precise synthesis of monodisperse subnanometric metal clusters composed by just a few atoms, and opens new possibilities for subnanometer science. The decoration of titanium dioxide surfaces with Ag5 atomic clusters has enabled the stabilization of surface polarons and provided new quantum-chemical insights into an electron polarization phenomenon revealed by their formation.

Exploring the materials space in the smallest particle size range: from heterogeneous catalysis to electrocatalysis and photocatalysis.

Ultrasmall clusters of subnanometer size can possess unique and even unexpected physical and chemical propensities which make them interesting in various fields of basic science and for potential applications, such as catalysis, photocatalysis, electrocatalysis, and optical and chemical sensors, just to name a few examples. These small particles often offer the tunability of their performance in an atom-by-atom fashion and an economic atom-efficient use of the metal loading. In this paper we review recent progress in the characterization and theory of well-defined subnanometer clusters in catalytic processes, and discuss their optical properties and stability, along with the potential of the size-selected clusters for the understanding of catalytic processes and for the development of new classes of catalysts.

Understanding anharmonic effects on hydrogen desorption characteristics of MgnH2n nanoclusters by ab initio trained deep neural network.

Magnesium hydride (MgH2) has been widely studied for effective hydrogen storage. However, its bulk desorption temperature (553 K) is deemed too high for practical applications. Besides doping, a strategy to decrease such reaction energy for releasing hydrogen is the use of MgH2-based nanoparticles (NPs). Here, we investigate first the thermodynamic properties of MgnH2n NPs (n < 10) from first-principles, in particular by assessing the anharmonic effects on the enthalpy, entropy and thermal expansion by means of the stochastic self consistent harmonic approximation (SSCHA). This method goes beyond previous approaches, typically based on molecular mechanics and the quasi-harmonic approximation, allowing the ab initio calculation of the fully-anharmonic free energy. We find an almost linear dependence on temperature of the interatomic bond lengths - with a relative variation of few percent over 300 K - alongside with a bond distance decrease of the Mg-H bonds. In order to increase the size of MgnH2n NPs toward experiments of hydrogen desorption we devise a computationally effective machine learning model trained to accurately determine the forces and total energies (i.e. the potential energy surfaces), integrating the latter with the SSCHA model to fully include the anharmonic effects. We find a significative decrease of the H-desorption temperature for sub-nanometric clusters MgnH2n with n ≤ 10, with a non-negligible, although little effect due to anharmonicities (up to 10%).

CO oxidation on MgAl2O4 supported Irn: activation of lattice oxygen in the subnanometer regime and emergence of nuclearity-activity volcano

Ir subnanometer clusters activate surface oxygen on a traditionally non-reducible MgAl2O4 support which leads to a more active two-site mechanism during low-temperature CO oxidation.

Enhanced hydrogen evolution reaction in alkaline solution by constructing strong metal-support interaction on Pd-CeO2-x-NC hybrids

Diminishing the size of metal nanostructures can significantly improve the performance of catalysts. However, the self-aggregation of small particles is still an insurmountable obstacle, resulting in the unfavorable stability and recyclability. Herein, we designed and fabricated the Pd-CeO2-x-NC catalyst though an accurate deposition strategy to downsize the Pd particle to sub-nanometer level and enhance its running stability. The CeO2-x nanoclusters were firstly dispersed on the nitrogen-doped carbon nanosheets. Further, the active Pd sub-nanoclusters were accurately scattered on the surface of CeO2-x ascribing to the strong metal-support interaction (SMSI) between Pd and CeO2-x, which was beneficial to promote the catalytic activity. Subsequently, the high oxidation state Pdn+ species were formed due to the electron transfer from Pd to CeO2-x caused by the SMSI effect. Strikingly, the HER performance of Pd-CeO2-x-NC was surprisingly correlated with the ratio of Pdn+, suggesting Pdn+ acted as the dominant active species. Besides, the SMSI effect stabilized the valence state of active Pdn+ species and prevented the sub-nanometer Pd clusters from aggregation, which played a vital role for the enhanced stability of the hybrid catalyst. This synthetic process described here is contributed to prepare various nanostructured catalysts with satisfactory stability through the direct targeting strategy.

Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters

<h2>Summary</h2> Ammonia synthesis is structure sensitive, and a minute change in the catalyst structure would cause a dramatic change in activity. To date, none of the studies reveal the metal size effect at a subnanometer scale on NH₃ synthesis, and such investigation remains a challenge. Here, we report the synthesis of Ru catalysts with sizes ranging from single atoms, atomic clusters (ACCs), sub-nanometric clusters, to nanoparticles (NPs) by adjusting precursor and/or loading of Ru. Sub-nanometric Ru catalysts not only exhibit performance different from that of NPs but also follow a different route for N₂ activation. The strong intra-cluster interaction of Ru atomic clusters enables the formation of strong interactions of Ru <i>d</i>-orbitals with the <i>σ</i> and <i>π</i> orbitals of N₂ molecules, resulting in N₂ activation over Ru ACCs to occur more easily than that over Ru NPs. Consequently, Ru ACCs display an unprecedentedly high NH₃ synthesis rate and large turnover frequency at mild conditions.

Assessment of metal-metal interactions and catalytic behavior in platinum-tin bimetallic subnanometric clusters by using reactive characterizations

Several reactive characterizations were used to study the effect that reduction duration of PtSn-K/MFI has on hydrocarbon catalysis, including propane dehydrogenation, propane hydrogenolysis, and ethane H/D exchange. Propane dehydrogenation results demonstrate that increased reduction time causes the PtSn-K/MFI to have higher selectivity towards propylene, lower rates of catalyst deactivation, and lower rates of dehydrogenation. Consistent with higher selectivity towards propylene with longer catalyst reduction times, slower rates of hydrogenolysis are also observed over a large range of H2:propane ratios. H/D exchange experiments to assess rates of C–H activation were carried out on PtSn-K/MFI by using ethane and D2, which demonstrate that longer reduction time significantly inhibits the rate of C–H activation on PtSn-K/MFI. The substantial effects observed by using reactive characterizations were difficult to be interpreted by more advanced characterization techniques such as EXAFS, because the structural changes of the PtSn nanoclusters are quite subtle. Indeed, in a previous report on the PtSn-K/MFI material, Pt and Sn EXAFS were statistically indistinguishable with different reduction times, illustrating that combining reactive characterizations, particularly with the application of H/D exchange, provides the ability to differentiate fundamental effects that impact catalyst performance. The importance of C–H activation was confirmed by isotopic experiments for propane dehydrogenation, which indicate that the first C–H activation step is rate-limiting during dehydrogenation, with a kinetic isotope effect of ∼2.5. These results together show that increased reduction time gives a catalytic material with superior catalytic properties (i.e. higher propylene selectivity and lower deactivation rates), and this is proposed to be due to increased platinum-tin interactions, which modifies and weakens hydrocarbon adsorption, thereby limiting strongly adsorbed intermediates that are precursors to hydrogenolysis products and coke.

Nonadiabatic Effects in the Molecular Oxidation of Subnanometric Cu5 Clusters.

The electronic structure of subnanometric clusters, far off the bulk regime, is still dominated by molecular characteristics. The spatial arrangement of the notoriously undercoordinated metal atoms is strongly coupled to the electronic properties of the system, which makes this class of materials particularly interesting for applications including luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Opposing a common rule of thumb that assumes an increasing chemical reactivity with smaller cluster size, Cu5 clusters have proven to be exceptionally resistant to irreversible oxidation, i.e., the dissociative chemisorption of molecular oxygen. Besides providing reasons for this behavior in the case of heavy loading with molecular oxygen, we investigate the competition between physisorption and molecular chemisorption from the perspective of nonadiabatic effects. Landau–Zener theory is applied to the Cu5(O2)3 complex to estimate the probability for a switching between the electronic states correlating the neutral O2 + Cu5(O2)2 and the ionic O2– + (Cu5(O2)2)+ fragments in a diabatic representation. Our work demonstrates the involvement of strong nonadiabatic effects in the associated charge transfer process, which might be a common motive in reactions involving subnanometric metal structures.

Ru0.05Ce0.95O2 Solid Solution Derived Ru Catalyst Enables Selective Hydrodeoxygenation of m‐Cresol to Toluene

AbstractSelective hydrodeoxygenation (HDO) of biomass lignin‐derived phenolic compounds to aromatics is a demanding reaction. Oxophilic Ru based catalysts can catalyze direct deoxygenation but suffer from competing C−C hydrogenolysis to CH4 at 350 °C and hydrogenation of phenyl ring at 250 °C under atmospheric pressure. In this work, a solid solution Ru0.05Ce0.95O2 catalyst was hydrothermally synthesized, characterized and tested for HDO of m‐cresol. The Ru species is highly dispersed in the solid solution as isolated atoms and/or subnanometer clusters as a result of strong Ru−O−Ce interaction, and the catalyst shows high concentration of oxygen vacancies. In contrast to nonselective Ru/SiO2, Ru0.05Ce0.95O2 selectively catalyzes deoxygenation to toluene while significantly inhibits C−C hydrogenolysis and hydrogenation. Moreover, the overall intrinsic HDO rate on Ru0.05Ce0.95O2 is 2.2 and 8.8 times higher than those on Ru/CeO2 and Ru/SiO2, respectively. The Ru−OV−Ce sites were proposed to be the active sites for deoxygenation. Inhibition of C−C hydrogenolysis and phenyl hydrogenation can be ascribed to the reduced size of the Ru ensembles due to the strong interaction in the solid solution.

Interfacial Proton Transfer for Hydrogen Evolution at the Sub-Nanometric Platinum/Electrolyte Interface.

Understanding the dynamic process of interfacial charge transfer prior to chemisorption is crucial to the development of electrocatalysis. Recently, interfacial water has been highlighted in transferring protons through the electrode/electrolyte interface; however, the identification of the related structural configurations and their influences on the catalytic mechanism is largely complicated by the amorphous and mutable structure of the electrical double layer (EDL). To this end, sub-nanometric Pt electrocatalysts, potentially offering intriguing activity and featuring fully exposed atoms, are studied to uncover the elusive electrode/electrolyte interface via operando X-ray absorption spectroscopy during the hydrogen evolution reaction (HER). Our results show that the metallic Pt clusters derived from the reduction of sub-nanometric Pt clusters (SNM-Pt) exhibit excellent HER activity, with an only 18 mV overpotential at 10 mA/cm2 and one-magnitude-higher mass activity than commercial Pt/C. More importantly, a unique Pt-interfacial water configuration with a Pt (from Pt clusters)-O (from water) radial distance of approximately 2.5 Å is experimentally identified as the structural foundation for the interfacial proton transfer. Toward high overpotentials, the interfacial water that structurally evolves from "O-close" to "O-far" accelerates the proton transfer and is responsible for the improved reaction rate by increasing the hydrogen coverage.

Real-time dynamics and structures of supported subnanometer catalysts via multiscale simulations

Understanding the performance of subnanometer catalysts and how catalyst treatment and exposure to spectroscopic probe molecules change the structure requires accurate structure determination under working conditions. Experiments lack simultaneous temporal and spatial resolution and could alter the structure, and similar challenges hinder first-principles calculations from answering these questions. Here, we introduce a multiscale modeling framework to follow the evolution of subnanometer clusters at experimentally relevant time scales. We demonstrate its feasibility on Pd adsorbed on CeO2(111) at various catalyst loadings, temperatures, and exposures to CO. We show that sintering occurs in seconds even at room temperature and is mainly driven by free energy reduction. It leads to a kinetically (far from equilibrium) frozen ensemble of quasi-two-dimensional structures that CO chemisorption and infrared experiments probe. CO adsorption makes structures flatter and smaller. High temperatures drive very rapid sintering toward larger, stable/metastable equilibrium structures, where CO induces secondary structure changes only.