Subnanometric Metal Research Articles

Silanol-Stabilized Atomically Dispersed Ptδ+-Ox-Sn Active Sites in Protozeolite for Propane Dehydrogenation.

Crystalline zeolites have been proven to be excellent supports for confining subnanometric metal catalysts to boost the propane dehydrogenation (PDH) reaction. However, the introduced metallic species may suffer from severe sintering and limited stability during the catalytic process, especially when utilizing an industrial impregnation method for metal incorporation. In this study, we developed a new type of support based on amorphous protozeolite (PZ), taking advantage of its adjustable silanol chemistry and zeolitic microporous characteristic for stabilizing atomically dispersed PtSn catalyst via a simple, cost-effective coimpregnation process. The combination of X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy under CO atmosphere, and density functional theory calculations confirmed the formation of highly dispersed active Ptδ+-Ox-Sn species in PtSn/PZ. The PtSn/PZ catalyst exhibited a high propane conversion of 45.4% and a high propylene selectivity of 99% (WHSV= 3.6 h-1, 550 °C), with a high apparent rate coefficient of 565 molC3H6·gPt-1·h-1·bar-1 at a high WHSV of 108 h-1, presenting a top-level performance among the state-of-the-art Pt-based catalysts prepared by in situ synthesis and impregnation methods. The silanol density determined the chemical state of PtSn species, showing a change from atomically dispersed Ptδ+-Ox-Sn sites to PtSn alloy with decreasing silanol density of supports. This work provides a general strategy using silanol-rich amorphous protozeolite as support for stabilizing various metal catalysts by the simple impregnation method and also offers an effective way for fine tailoring the chemical state of metallic species via a silanol-engineered approach.

Boosting the Stability of Subnanometer Pt Catalysts by the Presence of Framework Indium(III) Sites in Zeolite.

Subnanometer metal clusters show advantages over conventional metal nanoparticles in numerous catalytic reactions owing to their high percentage of exposed surface sites, abundance of under-coordinated metal sites and unique electronic structures. However, the applications of subnanometer metal clusters in high-temperature catalytic reactions (>600 °C) are still hindered, because of their low stability under harsh reaction conditions. In this work, we have developed a zeolite-confined bimetallic PtIn catalyst with exceptionally high stability against sintering. A combination of experimental and theoretical studies shows that the isolated framework In(III) species serve as the anchoring sites for Pt species, precluding the migration and sintering of Pt species in the oxidative atmosphere at ≥650 °C. The catalyst comprising subnanometer PtIn clusters exhibits long-term stability of >1000 h during a cyclic reaction-regeneration test for ethane dehydrogenation reaction.

Boosting the Stability of Subnanometer Pt Catalysts by the Presence of Framework Indium(III) Sites in Zeolite

AbstractSubnanometer metal clusters show advantages over conventional metal nanoparticles in numerous catalytic reactions owing to their high percentage of exposed surface sites, abundance of under‐coordinated metal sites and unique electronic structures. However, the applications of subnanometer metal clusters in high‐temperature catalytic reactions (>600 °C) are still hindered, because of their low stability under harsh reaction conditions. In this work, we have developed a zeolite‐confined bimetallic PtIn catalyst with exceptionally high stability against sintering. A combination of experimental and theoretical studies shows that the isolated framework In(III) species serve as the anchoring sites for Pt species, precluding the migration and sintering of Pt species in the oxidative atmosphere at ≥650 °C. The catalyst comprising subnanometer PtIn clusters exhibits long‐term stability of >1000 h during a cyclic reaction‐regeneration test for ethane dehydrogenation reaction.

Sub-nanometric ruthenium clusters prepared by solid-state dispersion for catalytic selective aerobic oxidation of aromatic alcohols

The selective aerobic oxidation of alcohols to corresponding aldehydes is one of the most important challenges in the green chemical industry. In this work, Ru/Al2O3-SSD catalyst with highly dispersed metallic ruthenium clusters (average particle size of 0.97 nm) was prepared by solid-state dispersion (SSD) for efficient selective aerobic oxidation of aromatic alcohols to aldehydes. Compared with the counterpart catalyst Ru/Al2O3-IM prepared by wet impregnation (IM), this catalyst has smaller metal particles (0.97 nm vs. 3.4 nm), resulting in a special electronic structure, and exhibits much higher TOF (234 h−1 vs. 98 h−1) in the aerobic oxidation reaction of benzyl alcohol at 80 °C. The catalyst also shows excellent catalytic activity in the preparation of vanillin by selective aerobic oxidation of vanillyl alcohol at 100 °C, delivering a vanillin yield of 99%. The solid-state dispersion strategy provides an efficient and facile way for the fabrication of high-performance sub-nanometric metal catalysts.

Unraveling the Unique Strong Metal-Support Interaction in Titanium Dioxide Supported Platinum Clusters for the Hydrogen Evolution Reaction.

Strong metal-support interaction (SMSI) is crucial to modulating the nature of metal species, yet the SMSI behaviors of sub-nanometer metal clusters remain unknown due to the difficulties in constructing SMSI at cluster scale. Herein, we achieve the successful construction of the SMSI between Pt clusters and amorphous TiO2 nanosheets by vacuum annealing, which requires a relatively low temperature that avoids the aggregation of small clusters. In situ scanning transmission electron microscopy observation is employed to explore the SMSI behaviors, and the results reveal the dynamic rearrangement of Pt atoms upon annealing for the first time. The originally disordered Pt atoms become ordered as the crystallizing of the amorphous TiO2 support, forming an epitaxial interface between Pt and TiO2. Such a SMSI state can remain stable in oxidation environment even at 400 °C. Further investigations prove that the electron transfer from TiO2 to Pt occupies the Pt 5d orbitals, which is responsible for the disappeared CO adsorption ability of Pt/TiO2 after forming SMSI. This work not only opens a new avenue for constructing SMSI at cluster scale but also provides in-depth understanding on the unique SMSI behavior, which would stimulate the development of supported metal clusters for catalysis applications.

Small luminescent silver clusters stabilized in porous crystalline solids.

Subnanometric or small metal clusters (SMCs) have been extensively researched due to their unique electronic, optical, catalytic, and magnetic properties, which differ from those of bulk samples. Among the SMCs, silver (Ag) clusters have received significant interest due to their affordability and unique luminescent properties. Currently, two major approaches, gas-phase and liquid-phase synthesis, have been employed to obtain Ag clusters with precise control of size and structure. More recently, attention has been directed toward the utilization of porous crystalline solids such as metal-organic frameworks (MOFs), zeolites, and porous ionic crystals (PICs) to synthesize and stabilize Ag clusters. In this review, we aim to provide a comprehensive overview of the synthesis, structures, and luminescent properties of Ag clusters in porous crystalline solids.

State-of-the-art strategies for the synthesis of zeolite-encapsulated subnanometric metal clusters

Novel and conventional strategies for the synthesis of zeolite-encapsulated subnanometric metal clusters are reviewed. Further perspectives for the rational design of zeolite-encapsulated subnanometric metal clusters are analyzed.

O2 activation by subnanometer Re-Pt clusters supported on TiO2(110): exploring adsorption sites.

Activation of O2 by subnanometer metal clusters is a fundamental step in the reactivity and oxidation processes of single-cluster catalysts. In this work, we examine the adsorption and dissociation of O2 on RenPtm (n + m = 5) clusters supported on rutile TiO2(110) using DFT calculations. The adhesion energies of RenPtm clusters on the support are high, indicating significant stability of the supported clusters. Furthermore, the bimetallic Re-Pt clusters attach to the surface through the Re atoms. The oxygen molecule was adsorbed on three sites of the supported systems: the metal cluster, the surface, and the interface. At the metal cluster site, the O2 molecule binds strongly to RenPtm clusters, especially on the Re-rich clusters. O2 activation occurs by charge transfer from the metal atoms to the molecule. The dissociation of O2 on the RenPtm clusters is an exothermic process with low barriers. As a result, sub-nanometer Re-Pt clusters can be susceptible to oxidation. Similar results are obtained at the metal-support interface, where both the surface and cluster transfer charge to O2. To surface sites, molecular oxygen is adsorbed onto the Ti5c atoms with moderate adsorption energies. The polarons, which are produced by the interaction between the metal cluster and the surface, participate in the activation of the molecule. However, dissociating O2 in these sites is challenging due to the endothermic nature of the process and the high energy barriers involved. Our findings provide novel insights into the reactivity of supported clusters, specifically regarding the O2 activation by Re-Pt clusters on rutile TiO2(110).

Migration of zeolite-encapsulated subnanometre platinum clusters via reactive neural network potentials.

The migration of atoms and small clusters is an important process in sub-nanometre scale heterogeneous catalysis, affecting activity, accessibility and deactivation through sintering. Control of migration can be partially achieved via encapsulation of sub-nanometre metal particles into porous media such as zeolites. However, a general understanding of the migration mechanisms and their sensitivity to particle size and framework environment is lacking. Here, we extend the time-scale and sampling of atomistic simulations of platinum cluster diffusion in siliceous zeolite frameworks, by introducing a reactive neural network potential of density functional quality. We observe that Pt atoms migrate in a qualitatively different manner from clusters, occupying the dense region of the framework and avoiding the free pore space. We also find that for cage-like zeolite CHA there exists a maximum in self diffusivity for the Pt dimer beyond which, confinement effects hinder intercage migration. By extending the quality of sampling, NNP-based methods allow for the discovery of novel dynamical processes at the atomistic scale, bringing modelling closer to operando experimental characterization of catalytic materials.

Generation of Subnanometer Metal Clusters in Silicoaluminate Zeolites as Bifunctional Catalysts.

Zeolite-encapsulated subnanometer metal catalysts are an emerging class of solid catalysts with superior performances in comparison to metal catalysts supported on open-structure solid carriers. Currently, there is no general synthesis methodology for the encapsulation of subnanometer metal catalysts in different zeolite structures. In this work, we will show a general synthesis method for the encapsulation of subnanometer metal clusters (Pt, Pd, and Rh) within various silicoaluminate zeolites with different topologies (MFI, CHA, TON, MOR). The successful generation of subnanometer metal species in silicoaluminate zeolites relies on the introduction of Sn, which can suppress the migration of subnanometer metal species during high-temperature oxidation-reduction treatments according to advanced electron microscopy and spectroscopy characterizations. The advantage of encapsulated subnanometer Pt catalysts in silicoaluminate zeolites is reflected in the direct coupling of ethane and benzene for production of ethylbenzene, in which the Pt and the acid sites work in a synergistic way.

Size-Dependent Reactivity of Con- (n = 5-25) Cluster Anions toward Carbon Dioxide.

A fundamental understanding of the reactivity evolution of nanosized clusters at an atomically precise level is pivotal to assemble desired materials with promising candidates. Benefiting from the tandem mass spectrometer coupled with a high-temperature ion-trap reactor, the reactions of mass-selected Con- (n = 5-25) clusters with CO2 were investigated and the increased reactivity of Co20-25- was newly discovered herein. This finding marks an important step to understand property evolution of subnanometer metal clusters (Co25-, ∼0.8 nm) atom-by-atom. The reasons behind the increased reactivity of Co20-25- were proposed by analyzing the reactions of smaller Co6-8- clusters that exhibit significantly different reactivity toward CO2, in which a lower electron affinity of Con contributes to the capture of CO2 while the flexibility of Con- could play vital roles to stabilize reaction intermediates and suppress the barriers of O-CO rupture and CO desorption.

Stability and Reversible Oxidation of Sub-Nanometric Cu5 Metal Clusters: Integrated Experimental Study and Theoretical Modeling.

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude "breathing" motions. A chemical phase diagram for Cu oxidation states of the Cu5 -oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu.

MOF-Triggered Synthesis of Subnanometer Ag02 Clusters and Fe3+ Single Atoms: Heterogenization Led to Efficient and Synergetic One-Pot Catalytic Reactions.

The combination of well-defined Fe3+ isolated single-metal atoms and Ag2 subnanometer metal clusters within the channels of a metal-organic framework (MOF) is reported and characterized by single-crystal X-ray diffraction for the first time. The resulting hybrid material, with the formula [Ag02(Ag0)1.34FeIII0.66]@NaI2{NiII4[CuII2(Me3mpba)2]3}·63H2O (Fe3+Ag02@MOF), is capable of catalyzing the unprecedented direct conversion of styrene to phenylacetylene in one pot. In particular, Fe3+Ag02@MOF─which can easily be obtained in a gram scale─exhibits superior catalytic activity for the TEMPO-free oxidative cross-coupling of styrenes with phenyl sulfone to give vinyl sulfones in yields up to >99%, which are ultimately transformed, in situ, to the corresponding phenylacetylene product. The results presented here constitute a paradigmatic example of how the synthesis of different metal species in well-defined solid catalysts, combined with speciation of the true metal catalyst of an organic reaction in solution, allows the design of a new challenging reaction.

Graphene-confined ultrafast radiant heating for high-loading subnanometer metal cluster catalysts.

Thermally activated ultrafast diffusion, collision and combination of metal atoms comprise the fundamental processes of synthesizing burgeoning subnanometer metal clusters for diverse applications. However, so far, no method has allowed the kinetically controllable synthesis of subnanometer metal clusters without compromising metal loading. Herein, we have developed, for the first time, a graphene-confined ultrafast radiant heating (GCURH) method for the synthesis of high-loading metal cluster catalysts in microseconds, where the impermeable and flexible graphene acts as a diffusion-constrained nanoreactor for high-temperature reactions. Originating from graphene-mediated ultrafast and efficient laser-to-thermal conversion, the GCURH method is capable of providing a record-high heating and cooling rate of ∼109°C/s and a peak temperature above 2000°C, and the diffusion of thermally activated atoms is spatially limited within the confinement of the graphene nanoreactor. As a result, due to the kinetics-dominant and diffusion-constrained condition provided by GCURH, subnanometer Co cluster catalysts with high metal loading up to 27.1wt% have been synthesized by pyrolyzing a Co-based metal-organic framework (MOF) in microseconds, representing one of the highest size-loading combinations and the quickest rate for MOF pyrolysis in the reported literature. The obtained Co cluster catalyst not only exhibits an extraordinary activity similar to that of most modern multicomponent noble metal counterparts in the electrocatalytic oxygen evolution reaction, but is also highly convenient for catalyst recycling and refining due to its single metal component. Such a novel GCURH technique paves the way for the kinetically regulated, limited diffusion distance of thermally activated atoms, which in turn provides enormous opportunities for the development of sophisticated and environmentally sustainable metal cluster catalysts.

Dynamic catalysis of sub-nanometer metal clusters in oxygen dissociation

When the catalyst size decreases down to the nanometer or even sub-nanometer scale, their geometric and electronic structures are very different from the bulk materials. Especially, their structural dynamics can greatly affect catalytic performances. Herein, we report a theoretical study on oxygen dissociation on Ag19 cluster, a critical step of ethylene epoxidation, to investigate the effect of structural dynamics on the catalytic activity. Combining ab initio molecular dynamics (AIMD) and free energy calculation, we obtain the free energy profiles under different temperatures and find sudden drops in the reaction free energy and barrier at certain temperature ranges. The reaction entropy change shows a positive peak because of the different melting temperatures between initial and final states. Additionally, we further investigate the oxygen diffusion process on the Ag cluster and compare it with the Au and Cu clusters. We find that weak adsorption of oxygen on the silver cluster leads to its facile diffusion and thus reduces the melting temperature of the cluster, therefore facilitating partial oxidation under mild conditions.

Synthesis of two-dimensional van der waals superlattices, heterostructures, and alloys from conversion of sequentially layered sub-nanometer metal films

Manipulation of bulk material properties by controlling layer-by-layer chemistry and structure of nanomaterials has remained an overarching goal of nanoscience and nanoengineering. In the case of two-dimensional (2D) materials, heterostructures consisting of different compositions, stacking, and orientation can serve as a platform for designing bulk material properties; however, fundamental challenges associated with materials processes have limited material quality and impeded scalability. Most attempts to overcome this limitation have relied on slow layer-by-layer growth or meticulous transfer of grown single layers. Our work describes a method to directly convert easily fabricated sub-nanometer metal multilayer heterostructures on both sapphire and SiO2 substrates followed by conversion to 2D van der Waals superlattices, heterostructures, and alloys. These materials exhibit novel bulk properties compared to individual 2D layers themselves including reduced bandgap, enhanced light-matter coupling, and improved catalytic performance. The process versatility enables tunable orientation, layer structure, and chemistry in an exciting class of 2D nanomaterials and provides an opportunity to generate a wide range of artificially stacked multicompositional 2D structures with controlled morphologies.

Formation of Au-pseudocarbynes by self-assembly of carbon chains and gold clusters

Au-pseudocarbynes are a new class of materials produced through non-covalent interactions between finite one dimensional sp-hybridized carbon chains and sub-nanometer metal clusters. These exciting and innovative materials are expected to approach the unprecedented theoretical strength, elastic modulus, and stiffness of carbyne, which has defied isolation and characterization for over a century. The current approach for synthesizing Au-pseudocarbynes relies on the laser ablation of a liquid/metal interface, and therefore limits its production to small yields through laser ablation. In this study, we synthesized Au-pseudocarbynes by a self-assembly of previously prepared gold clusters and polyynes in solution, which yields similar results under spectroscopic analysis and through x-ray diffraction to the products synthesized via laser ablation. Therefore, we obtain new insights toward their formation mechanism by showing that the extreme temperatures and pressures provided by laser ablation are not necessary for assembling gold particles with polyynes. This self-assembly approach also provides a new way to develop Au-pseudocarbynes for applications by enabling large-scale synthesis.

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.

Metal-organic frameworks as chemical nanoreactors for the preparation of catalytically active metal compounds.

Since the emergence of metal-organic frameworks (MOFs), a myriad of thrilling properties and applications, in a wide range of fields, have been reported for these materials, which mainly arise from their porous nature and rich host-guest chemistry. However, other important features of MOFs that offer great potential rewards have been only barely explored. For instance, despite the fact that MOFs are suitable candidates to be used as chemical nanoreactors for the preparation, stabilization and characterization of unique functional species, that would be hardly accessible outside the functional constrained space offered by MOF channels, only very few examples have been reported so far. In particular, we outline in this feature recent advances in the use of highly robust and crystalline oxamato- and oxamidato-based MOFs as reactors for the in situ preparation of well-defined catalytically active single atom catalysts (SACS), subnanometer metal nanoclusters (SNMCs) and supramolecular coordination complexes (SCCs). The robustness of selected MOFs permits the post-synthetic (PS) in situ preparation of the desired catalytically active metal species, which can be characterised by single-crystal X-ray diffraction (SC-XRD) taking advantage of its high crystallinity. The strategy highlighted here permits the always challenging large-scale preparation of stable and well-defined SACs, SNMCs and SCCs, exhibiting outstanding catalytic activities.

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.