Sub-nanometer Clusters Research Articles

Controlled-Release Mechanism Regulates Rhodium Migration and Size Redistribution Boosting Catalytic Methane Conversion

The migration of Rh atoms under a gas/reactive environment has a great impact on the dynamic restructuring and size redistribution of Rh catalysts in a variety of structure-sensitive catalytic reactions. To date, regulating the size distribution of active Rh species via controlled atomic migration remains challenging. Here, we show a controlled-release mechanism to regulate Rh atom migration through two-dimensional (2D) zeolite nanosheets, enabling quasi-continuous size redistribution of active Rh species from a single atom to nanoparticles with reversibility. Utilizing state-of-the-art in situ characterizations, a reversible aggregation/redispersion of Rh catalysts in/out of the 2D zeolite was directly observed under H2 or CO environment. The interplay between gas environment and support confinement was disclosed that substantially restrained the dynamic migration of Rh species and enabled a quasi-continuous control of Rh size distribution over a wide temperature window and size range. The catalytic testing for mild oxidation of methane demonstrated a volcano correlation of methanol activity with increasing particle size. The sub-nanometer Rh clusters with an average diameter of 0.9 nm exhibited the highest methanol activity of 39.7 molCH3OOH·molRh–1·h–1 with remarkable selectivity as high as 73.2%, far beyond that of single atom species and larger particles. Density functional theory calculations and electron paramagnetic resonance spectroscopy further revealed that this size dependency is related to the preferential formation of •OH radicals on Rh clusters with different sizes, which acts as a key initiator for methane activation. Our results thus provide a practical approach to synthesize size-specific metal catalysts via controlled atomic migration.

Synergistic Mechanism of Sub-Nanometric Ru Clusters Anchored on Tungsten Oxide Nanowires for High-Efficient Bifunctional Hydrogen Electrocatalysis.

The construction of strong interactions and synergistic effects between small metal clusters and supports offers a great opportunity to achieve high-performance and cost-effective heterogeneous catalysis, however, studies on its applications in electrocatalysis are still insufficient. Herein, it is reported that W18 O49 nanowires supported sub-nanometric Ru clusters (denoted as Ru SNC/W18 O49 NWs) constitute an efficient bifunctional electrocatalyst for hydrogen evolution/oxidation reactions (HER and HOR) under acidic condition. Microstructural analyses, X-ray absorption spectroscopy, and density functional theory (DFT) calculations reveal that the Ru SNCs with an average RuRu coordination number of 4.9 are anchored to the W18 O49 NWs via RuOW bonds at the interface. The strong metal-support interaction leads to the electron-deficient state of Ru SNCs, which enables a modulated RuH strength. Furthermore, the unique proton transport capability of the W18 O49 also provides a potential migration channel for the reaction intermediates. These components collectively enable the remarkable performance of Ru SNC/W18 O49 NWs for hydrogen electrocatalysis with 2.5 times of exchange current density than that of carbon-supported Ru nanoparticles, and even rival the state-of-the-art Pt catalyst. This work provides a new prospect for the development of supported sub-nanometric metal clusters for efficient electrocatalysis.

Role of Fluxionality and Metastable Isomers in the ORR Activity: A Case Study

Understanding the dynamic reconstruction of active sites and the fluxional behavior of subnanoclusters has become the heart of operando modeling techniques. An accurate description of catalysis phenomena extends toward incorporating the activity of thermally accessible ensembles of metastable isomers in addition to the global minima (GM). Considering the Pt13 cluster as a model catalyst for gas-phase oxygen reduction reaction (ORR), we have studied the role of metastable isomers and unravelled their distinct catalyst dynamics leading to high fluxionality. The different adsorption energetics and ORR activity featured by the metastable isomers reveal the importance of addressing the fluxional behavior of subnanoclusters. A detailed mechanistic ORR investigation provided isomers possessing an activity comparable to GM. Furthermore, a statistical ensemble representation demonstrates the substantial role of metastable isomers within 0.4 eV relative energy difference from GM, with negligible activity enhancement of isomers with high energy at 300 K. Ab initio thermodynamics-based stability analysis represents different energetics associated with the Pt13 clusters at high oxygen coverage. This study highlights the importance of exploring the role of metastable isomers in determining the cumulative catalytic activity of subnanometer clusters.

Visualizing formation of high entropy alloy nanoparticles with liquid phase transmission electron microscopy.

High entropy alloy (HEA) nanoparticles hold promise as active and durable (electro)catalysts. Understanding their formation mechanism will enable rational control over composition and atomic arrangement of multimetallic catalytic surface sites to maximize their activity. While prior reports have attributed HEA nanoparticle formation to nucleation and growth, there is a dearth of detailed mechanistic investigations. Here we utilize liquid phase transmission electron microscopy (LPTEM), systematic synthesis, and mass spectrometry (MS) to demonstrate that HEA nanoparticles form by aggregation of metal cluster intermediates. AuAgCuPtPd HEA nanoparticles are synthesized by aqueous co-reduction of metal salts with sodium borohydride in the presence of thiolated polymer ligands. Varying the metal : ligand ratio during synthesis showed that alloyed HEA nanoparticles formed only above a threshold ligand concentration. Interestingly, stable single metal atoms and sub-nanometer clusters are observed by TEM and MS in the final HEA nanoparticle solution, suggesting nucleation and growth is not the dominant mechanism. Increasing supersaturation ratio increased particle size, which together with observations of stable single metal atoms and clusters, supported an aggregative growth mechanism. Direct real-time observation with LPTEM imaging showed aggregation of HEA nanoparticles during synthesis. Quantitative analyses of the nanoparticle growth kinetics and particle size distribution from LPTEM movies were consistent with a theoretical model for aggregative growth. Taken together, these results are consistent with a reaction mechanism involving rapid reduction of metal ions into sub-nanometer clusters followed by cluster aggregation driven by borohydride ion induced thiol ligand desorption. This work demonstrates the importance of cluster species as potential synthetic handles for rational control over HEA nanoparticle atomic structure.

Subnanometric Pt clusters supported on MgO-incorporated porous carbon as efficient metal–base bifunctional catalysts for reductive heterocyclization reactions

The developed Pt/MgO@C with ultralow Pt loading (0.078 wt%) exhibited superior catalytic performance for synthesizing 2,1-benzisoxazole derivatives by a reductive heterocyclization route under very mild conditions (1 bar hydrogen pressure at 303 K).

Metal and metal oxide sub nano cluster; emerging aspirant for catalytic applications

The usage of catalysis at the articulation is growing in a variety of industries, including the chemical industry and the production of valuable compounds. For the swiftly developing fields of energy generation and storage, the fabrication of a highly effective, reliable, and selective catalyst with the replacement of noble metals by the harmless yet helpful to the economy transition metal is very desirable. Reducing metal particle size to the subnanometer level is a crucial strategy for increasing activity per mass of metal. Because of this, sub-nanometer metal oxide clusters are becoming more significant materials, employed in a variety of catalytic processes as well as electronic devices like sensors. The current thorough literature study and rigorous assessment work emphasises the significance of sub-nanometer-sized clusters of various metals and metal oxides in the development of effective and reasonably priced methods to the various catalytic reactions. However, because of their great mobility and agglomerative nature, sub-nanoscale clusters continue to provide a significant challenge to synthesis chemists. The various sub-nano clusters and the advancements in their catalytic applications with regard to particle size are summarised here in the provided review. The current review article may pave the way for a more reliable design of a noble catalyst at the sub-nanoscale by allowing us to critically assess the structure–activity relationship and obtain the necessary catalytic efficiency.

Differentiating supported platinum single atoms, clusters and nanoparticles by styrene hydrogenation

Supported metal catalysts often consist of metal sites ranging from nanoparticles to subnanometer clusters and single atoms. It remains a necessity to differentiate these sites to guide design of optimal catalysts. Here we report a simple method to assess the distribution of metal active sites in catalyst samples. The method takes the advantage of the structure sensitivity of styrene hydrogenation over titania supported platinum (Pt) catalysts with Pt aggregates varied from single atom to ∼1.40 nm nanoparticles. The physicochemical properties were characterized by STEM, XPS, XANES, H2-TPR and CO-chemisorption measurements. The reactivity of Pt sites was quantified by styrene hydrogenation at ambient conditions. The nanometer-sized Pt clusters have significantly higher activity than Pt nanoparticles, sub-nanometer clusters or isolated single atoms. The relationship between activity and structural/electronic properties of Pt sites influenced by particle sizes was discussed. Similar relationship was found in the carbon supported Pt catalysts.

Direct activation of PMS by highly dispersed amorphous CoOx clusters in anatase TiO2 nanosheets for efficient oxidation of biomass-derived alcohols

Developing a green and cost-effective catalytic system for the selective oxidation of biomass-derived alcohols is vital for the sustainable synthesis of fine chemicals. Herein, highly dispersed subnanometric amorphous CoOx clusters in anatase TiO2 nanosheets (Co-TiO2) fabricated by green solvent CO2-assisted approach could directly activate peroxymonosulfate (PMS) for the highly selective oxidation of various biomass-derived alcohols. Advanced characterizations (e.g., EXAFS, EPR, AC HAADF-STEM) reveal that a strong interaction of CoOx clusters and the anatase TiO2 support exist in Co-TiO2 and Co atom in Co-TiO2 is mainly consisted of Co2+ and Co3+. The Co-TiO2 catalyst offers superior catalytic performance in the conversion of six types of alcohols (e.g., benzyl alcohol (BAL), 5-hydroxymethylfurfural (HMF)) with high selectivity to produce corresponding aldehydes. Highly dispersed CoOx clusters and the interaction between CoOx clusters and TiO2 support contribute to the superior performance. Mechanism studies show that SO4•− radicals play the dominant role in the selective oxidation of model reactant BAL and 1O2 participates in the non-radical pathway. DFT calculations are well matched with experiment and decipher that the strong interaction between CoOx clusters and TiO2 support promotes the formation of SO4•– and SO5•–.

Role of bimetallic Au-Ir subnanometer clusters mediating O2 adsorption and dissociation on anatase TiO2 (101).

A comprehensive computational study on the oxygen molecule (O2) adsorption and activation on bimetallic Au-Ir subnanometer clusters supported on TiO2(101)- up to five atoms in size-is performed. A global optimization density functional theory-based basin-hopping algorithm is used to determine putative global minima configurations of both mono- and bimetallic clusters supported on the metal oxide surface for all sizes and compositions. Our results indicate a strong cluster-oxide interaction for monometallic Ir clusters with calculated adsorption energy (Eads) values ranging from -3.11 to -5.91eV. Similar values are calculated for bimetallic Au-Ir clusters (-3.21 up to -5.69eV). However, weaker Eads values are calculated for Au clusters (ranging from -0.66 to -2.07eV). As a general trend, we demonstrate that for supported Au-Ir clusters on TiO2(101), those Ir atoms preferentially occupy cluster-oxide interface positions while acting as anchor sites for the Au atoms. The overall geometric arrangements of the putative global minima configurations define O2 adsorption and dissociation, particularly involving the monometallic Au5 and Ir5 as well as the bimetallic Au2Ir3 and Au3Ir2 supported clusters. Spontaneous O2 dissociation is observed on both Ir5 and on the Ir-metallic part of Au3Ir2 and Au2Ir3 supported clusters. This is in sharp contrast with supported Au5, where a large activation energy is needed (1.90eV). Interestingly, for Au5, we observe that molecular O2 adsorption is favorable at the cluster/oxide interface, followed by a smaller dissociation barrier (0.71eV). From a single cluster catalysis point of view, our results have strong implications in the ongoing understanding of oxide supported bimetallic while providing a useful first insight into the continuous in silico design of novel subnanometer catalysts.

Theoretical study on the cluster–surface interaction: The case of subnanometer Pt–Re clusters supported on MgO(100)

Bimetallic clusters in gas phase display novel physicochemical properties that are different from those present in their pure metal analogs. When bimetallic clusters are deposited on a substrate, such properties may change depending on the type and strength of the metal–support interaction. Here, we report a theoretical study, based on density functional theory (DFT), for sub-nanometer clusters of Pt–Re on the pristine MgO(100) surface, using the Basin-Hopping DFT algorithm to find the global minima. Then, their structural, energetic, electronic, and vibrational properties are calculated, and a comparison between gas- and supported-phase behavior is performed. It is obtained that the MgO(100) surface has a more relevant effect on the structural and vibrational properties of the Pt–Re bimetallic clusters and pure Pt clusters. For example, the metal–metal bond lengths of the supported clusters increase with respect to those observed in gas phase, also giving rise to red shifts in the corresponding vibrational frequencies. On the other hand, the structural and vibrational properties of pure gas- and supported-phase Re clusters are quite similar. These results are consistent with the adsorption energy calculations indicating a strong interaction between Pt–Re clusters and pure Pt clusters with the MgO(100) surface. Moreover, the metal–support interaction leads to a charge transfer from the metal oxide to the metal clusters and to the hybridization of the d- and s-states of the metal atoms with the p-states of the oxygen atoms present in the substrate. This work contributes to the understanding of the role of the support in bimetallic Pt–Re clusters on metal oxides, which would be of potential interest in the design of novel nanocatalysts.

Sub‐Nanometer Fe Clusters Confined in Carbon Nanocages for Boosting Dielectric Polarization and Broadband Electromagnetic Wave Absorption (Adv. Funct. Mater. 31/2022)

Subnanometer Clusters In article number 2204370, Yixing Li, Xuefeng Zhang, and co-workers fabricate sub-nanometer iron clusters within nitrogen-doped carbon nanocages. Benefitting from their unique structures, the energy transformation between the relatively complex permeability and permittivity can be realized, which thus boosts the dielectric polarization and improves the electromagnetic wave absorption performance.

Highly Efficient MOF-Driven Silver Subnanometer Clusters for the Catalytic Buchner Ring Expansion Reaction.

The preparation of novel efficient catalysts—that could be applicable in industrially important chemical processes—has attracted great interest. Small subnanometer metal clusters can exhibit outstanding catalytic capabilities, and thus, research efforts have been devoted, recently, to synthesize novel catalysts bearing such active sites. Here, we report the gram-scale preparation of Ag20 subnanometer clusters within the channels of a highly crystalline three-dimensional anionic metal–organic framework, with the formula [Ag20]@AgI2NaI2{NiII4[CuII2(Me3mpba)2]3}·48H2O [Me3mpba4– = N,N′-2,4,6-trimethyl-1,3-phenylenebis(oxamate)]. The resulting crystalline solid catalyst—fully characterized with the help of single-crystal X-ray diffraction—exhibits high catalytic activity for the catalytic Buchner ring expansion reaction.

Subnanometric Ru clusters with upshifted D band center improve performance for alkaline hydrogen evolution reaction

Subnanometric metal clusters usually have unique electronic structures and may display electrocatalytic performance distinctive from single atoms (SAs) and larger nanoparticles (NPs). However, the electrocatalytic performance of clusters, especially the size-activity relationship at the sub-nanoscale, is largely unexplored. Here, we synthesize a series of Ru nanocrystals from single atoms, subnanometric clusters to larger nanoparticles, aiming at investigating the size-dependent activity of hydrogen evolution in alkaline media. It is found that the d band center of Ru downshifts in a nearly linear relationship with the increase of diameter, and the subnanometric Ru clusters with d band center closer to Femi level display a stronger water dissociation ability and thus superior hydrogen evolution activity than SAs and larger nanoparticles. Benefiting from the high metal utilization and strong water dissociation ability, the Ru clusters manifest an ultrahigh turnover frequency of 43.3 s−1 at the overpotential of 100 mV, 36.1-fold larger than the commercial Pt/C.

Facile Solid-State Synthesis of Supported Ptm-Nps for the Oxygen Reduction Reaction

Supported bimetallic catalysts composed of platinum and transition metals are highly investigated electrocatalysts for the oxygen reduction reaction (ORR) because of their enhanced activity and stability. However, common routes to synthesize these materials are often laborious and not scalable, employing organic solvents or uneconomical metal deposition methods [1-2]. Here, we present a mechanochemistry-assisted, dry and scalable synthesis route towards supported bimetallic catalysts, which consists of only two steps: First, metal salts are dispersed on a carbon support in a planetary mill without further additives. Following, the powder is reduced with hydrogen and annealed to yield alloyed catalysts. With this process, we are able to synthesize PtM/C catalysts where M was Ni, Co or Ru. Both metal loading and ratio could be adjusted by changing the amount of metal salt. The average size of the PtM-NPs was similarly controlled by changing the annealing temperature.With X-ray diffraction (XRD), we show that no metal or salt reflexes are present after milling and that the XRD pattern matches the carbon support. After reduction and annealing, clear reflexes of the targeted alloy composition are visible. The latter was confirmed by scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDX). After milling, ionic metal species are present as sub-nanometer clusters evenly dispersed over the carbon support. After reduction and annealing, PtM nanoparticles have formed. Neither by XRD nor by STEM-EDX was the presence of unalloyed Pt or M detected. Optical emission spectrometry was used to confirm the targeted loading and bulk composition of the catalysts.Due to the known high activity for catalyzing the ORR, PtNi/C and PtCo/C were subjected to electrochemical evaluation in a rotating disc electrode setup [2]. In both cases, specific activities surpassing 1 mA/cm2 Pt at 0.9 V were reached. The electrochemically active surface area (ECSA) and mass activity were also in the range expected for the composition and particle size. To assess the stability, the catalysts were cycled from 0.4 to 1.0 V (RHE) with a scan rate of 1 V/s for 10800 cycles. The loss in ECSA of approximately 5 % was also in line with expectations. Since iron impurities are known to reduce the lifetime of a proton exchange membrane fuel cell, the milling process was adapted to a Si3N4-mill instead of a steel mill, again demonstrating the flexibility of this synthesis route [3].To conclude, the reported solid-state procedure allows the dry synthesis of supported bimetallic catalysts over a wide range of compositions. The materials show high activity and stability catalyzing the ORR. Due to its simplicity and flexibility, we expect this synthesis approach to be applied in other fields of catalysis within a short period of time.

Boosting toluene oxidation by the regulation of Pd species on UiO-66: Synergistic effect of Pd species

Supported single-atoms and sub-nanometre clusters have exhibited superb catalytic performance toward many reactions. However, inactivation of single-atom or cluster catalysts in complex reactive conditions poses major challenge for their practical application. Herein, we demonstrate that the prepared Pd-UiO-66 with ultra-low Pd loading (0.05 wt%) contains three robust active Pd species, (isolated Pd atom (Pd1), sub-nanometre Pd clusters (Pdc) and Pd nanoparticles (Pdn)) and presents superb activity for toluene oxidation and water resistance (10.0 vol%). Experiments and theoretical calculations firstly confirm that consecutive H2-O2 and reaction gas treatment (1000 ppm toluene in 20 vol.%O2/Ar) induce residual N species from solvent N, N-dimethylformamide to enter UiO-66 skeleton forming Pd1-N1 structures. DFT calculations reveal that the synergistic effect of Pd species (namely, the enhanced activation of O2 and H2O by Pd1 and the improved adsorption of toluene by Pdc and Pdn) is the main factor for the excellent activity of Pd-U-H-O-reused catalyst.

Subnanometric Pt‐Sn Monolayers Over a Rod‐Shaped Al2O3 for Propane Dehydrogenation

AbstractAtomic layer deposition was applied to precisely disperse Pt on Sn‐, K‐modified Al2O3, while the dispersion of Pt depended strongly on the atmosphere. Pt presented as single‐atoms in oxidative atmosphere but both single‐atoms and clusters under reductive gas. After H2‐treatment at 873 K, Pt single‐atoms, clusters and particles co‐existed in the catalysts, but their population varied considerably. The typical nanoparticles featured by crystallized Pt3Sn alloys, while the subnanometric clusters contained amorphous Pt−Sn alloys in a monolayer geometry. When tested for propane dehydrogenation at 873 K, the monolayers displayed outstanding performance; the mass specific activity approached 5.1 molC3H6 molPt−1 s−1, which was 2.7 times greater than that of particles. Structural analysis revealed that the amorphous Pt−Sn monolayer exposed more surficial Pt atoms surrounded by Sn atoms, which finely tuned the electronic property of Pt and thus favored the activation of C−H bond in propane.

Subnanometric Cu clusters on atomically Fe-doped MoO2 for furfural upgrading to aviation biofuels

Single cluster catalysts (SCCs) are considered as versatile boosters in heterogeneous catalysis due to their modifiable single cluster sites and supports. In this work, we report subnanometric Cu clusters dispersed on Fe-doped MoO2 support for biomass-derived furfural upgrading. Systematical characterizations suggest uniform Cu clusters (composing four Cu atoms in average) are homogeneously immobilized on the atomically Fe-doped ultrafine MoO2 nanocrystals (Cu4/Fe0.3Mo0.7O2@C). The atomic doping of Fe into MoO2 leads to significantly modified electronic structure and consequently charge redistribution inside the supported Cu clusters. The as-prepared Cu4/Fe0.3Mo0.7O2@C shows superior catalytic performance in the oxidative coupling of furfural with C3~C10 primary/secondary alcohols to produce C8~C15 aldehydes/ketones (aviation biofuel intermediates), outperforming the conventionally prepared counterparts. DFT calculations and control experiments are further carried out to interpret the structural and compositional merits of Cu4/Fe0.3Mo0.7O2@C in the oxidative coupling reaction, and elucidate the reaction pathway and related intermediates.

Experimental and Theoretical Advances on Single Atom and Atomic Cluster‐Decorated Low‐Dimensional Platforms towards Superior Electrocatalysts

AbstractThe fundamental relationship between structure and properties, which is called “structure‐property”, plays a vital role in the rational designing of high‐performance catalysts for diverse electrocatalytic applications. Low‐dimensional (LD) nanomaterials, including 0D, 1D, 2D materials, combined with low‐nuclearity metal atoms, ranging from single atoms to subnanometer clusters, are currently emerging as rising star nanoarchitectures for heterogeneous catalysis due to their well‐defined active sites and unbeatable metal utilization efficiencies. In this work, a comprehensive experimental and theoretical review is provided on the recent development of single atom and atomic cluster‐decorated LD platforms towards some typical clean energy reactions, such as water‐splitting, nitrogen fixation, and carbon dioxide reduction reactions. The upmost attractive structural properties, advanced characterization techniques, and theoretical principles of these low‐nuclearity electrocatalysts as well as their applications in key electrochemical energy devices are also elegantly discussed.

Adsorption and activation of CO2 on a Au19Pt subnanometer cluster in aqueous environment

We employ ab initio density functional theory based method to investigate the ability of a subnanometer bimetallic Au19Pt cluster to adsorb and activate a CO2 molecule in an aqueous electrochemical environment. We find that, in water, Au19Pt gets negatively charged at zero bias and selectively promotes the adsorption and activation of the CO2 molecule via electron transfer and through the hybridization of oxygen p-orbitals and partially filled platinum d-orbitals. Notably, Pt acts as a collector of negative charge and behaves as a CO2-activating single-atom catalyst embedded within a robust Au20-like framework, thus suggesting Au19Pt as a potential candidate for CO2 mitigation.

Size-dependent phase transitions boost catalytic activity of sub-nanometer gold clusters.

The characterization and identification of the dynamics of cluster catalysis are crucial to unraveling the origin of catalytic activity. However, the dynamical catalytic effects during the reaction process remain unclear. Herein, we investigate the dynamic coupling effect of elementary reactions with the structural fluctuations of sub-nanometer Au clusters with different sizes using ab initio molecular dynamics and the free energy calculation method. It was found that the adsorption-induced solid-to-liquid phase transitions of the cluster catalysts give rise to abnormal entropy increase, facilitating the proceeding of reaction, and this phase transition catalysis exists in a range of clusters with different sizes. Moreover, clusters with different sizes show different transition temperatures, resulting in a non-trivial size effect. These results unveil the dynamic effect of catalysts and help understand cluster catalysis to design better catalysts rationally.