Bimetallic Ag-Pt Sub-nanometer Supported Clusters as Highly Efficient and Robust Oxidation Catalysts

Customizing a Hyperbranched Ligand Confers Supported Platinum Nanoclusters with Unexpected Catalytic Activity toward the Reduction of 4-Nitrophenol

We here show that a dendritic molecule combined with ligand merit confers supported platinum nanoclusters (PtNCs) with unprecedented catalytic performance. Branched polyethylenimine (PEI, Mn = 2000 D) patched on a porous bead is modified with 2-(diphenylphosphino)benzaldehyde (dppb) before being used to mediate a platinum nanoparticle/nanocluster (Pt0). The catalytic activity of Pt0 toward the reduction of 4-nitrophenol (4-NP) is evaluated from the parameter of Pt-normalized rate constant (kc). Optimization of the dppb level along with transformation of the PEI hydrogens into diol or trimethylammonium groups imparts supported Pt0 unprecedented activity (kc = 19.2 L mmol-1 s-1 and turnover frequency (TOF) = 1041 h-1). The supported Pt0 at an extremely low dosage of 0.1 ppm promotes 98% conversion of 4-NP within minutes and is well recyclable. The striking catalytic activity is attributed to the combination of orthogonal ligand properties such as weak ligand nature, catalyst-activating ability, excellent substrate affinity, and effect on PtNC-size mediation of the ligand.

The Role of In Situ/Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis

Heterogeneous catalysts undergo thermal- and/or adsorbate-induced dynamic changes under reaction conditions, which consequently modify their catalytic behavior. Hence, it is increasingly crucial to characterize the properties of a catalyst under reaction conditions through the so-called "operando" approach. Operando IR spectroscopy is probably one of the most ubiquitous and versatile characterization methods in the field of heterogeneous catalysis, but its potential in identifying adsorbate- and thermal-induced phenomena is often overlooked in favor of other less accessible methods, such as XAS spectroscopy and high-resolution microscopy. Without detracting from these techniques, and while aware of the enormous value of a multitechnique approach, the purpose of this Review is to show that IR spectroscopy alone can provide relevant information in this field. This is done by discussing a few selected case studies from our own research experience, which belong to the categories of both "single-site"- and nanoparticle-based catalysts.

Scalable Synthesis of Flexible Single-Atom Monolithic Catalysts for High-Efficiency, Durable CO Oxidation at Low Temperature

The creation of single-atom catalysts in a large-size, high-yield, and stable form represents an important direction for high-efficiency industrial catalysis in the future. Herein, we report a strategy to synthesize flexible single-atom monolithic catalysts (SAMCs) based on the hierarchical 3D assembly of single-atom-loaded oxide ceramic nanofibers. The nanofibers, which can be produced in a continuous and scalable manner, serve as an ideal support for single atoms spontaneously and almost completely exposed at the surface through the Kirkendall effect-enabled in situ ion migration during the spinning process, resulting in both high yield and large loading quantity. Moreover, the hierarchical 3D assembly of these nanofibers into a porous, flexible structure endows the SAMCs with the advantages of sufficient infiltration and oscillation tolerance when faced with high-throughput gaseous media, leading to both high catalytic efficiency and excellent durability. As a proof-of-concept demonstration, a Pt SAMC is synthesized, which exhibits 100% CO oxidation at low temperature (∼170 °C), excellent invariance toward high-frequency (10 Hz) oscillation, and high structural stability from 25 to 300 °C. This work is beneficial for the large-scale production of SAMCs in broad industrial applications.

Surface Atomic Arrangement of Aluminum Ultra-Thin Layers Grown on Si(111)

Surface atomic arrangement and physical properties of aluminum ultrathin layers on c-Si(111)-7 × 7 and hydrogen-terminated c-Si(111)-1 × 1 surfaces deposited using molecular beam epitaxy were investigated. X-ray photoelectron spectroscopy spectra were collected in two configurations (take-off angle of 0° and 45°) to precisely determine the surface species. Moreover, 3D atomic force microscopy (AFM) images of the air-exposed samples were acquired to investigate the clustering formations in film structure. The deposition of the Al layers was monitored in situ using a reflection high-energy electron diffraction (RHEED) experiments to confirm the surface crystalline structure of the c-Si(111). The analysis of the RHEED patterns during the growth process suggests the settlement of aluminum atoms in Al(111)-1 × 1 clustered formations on both types of surfaces. The surface electrical conductivity in both configurations was tested against atmospheric oxidation. The results indicate differences in conductivity based on the formation of various alloys on the surface.

Oxidative dehydrogenation of cyclohexene on atomically precise subnanometer Cu4-nPdn (0 ≤ n ≤ 4) tetramer clusters: the effect of cluster composition and support on performance

The pronounced effects of the composition of four-atom monometallic Cu and Pd and bimetallic CuPd clusters and the support on the catalytic activity and selectivity in the oxidative dehydrogenation of cyclohexene are reported. The ultra-nanocrystalline diamond supported clusters are highly active and dominantly produce benzene; some of the mixed clusters also produce cyclohexadiene, which are all clusters with a much suppressed combustion channel. The also highly active TiO₂-supported tetramers solely produce benzene, without any combustion to CO₂. The selectivity of the zirconia-supported mixed CuPd clusters and the monometallic Cu cluster is entirely different; though they are less active in comparison to clusters with other supports, these clusters produce significant fractions of cyclohexadiene, with their selectivity towards cyclohexadiene gradually increasing with the increasing number of copper atoms in the cluster, reaching about 50% for Cu₃Pd₁. The zirconia-supported copper tetramer stands out from among all the other tetramers in this reaction, with a selectivity towards cyclohexadiene of 70%, which far exceeds those of all the other cluster-support combinations. The findings from this study indicate a positive effect of copper on the stability of the mixed tetramers and potential new ways of fine-tuning catalyst performance by controlling the composition of the active site and via cluster-support interactions in complex oxidative reactions under the suppression of the undesired combustion of the feed.

Au3-Decorated graphene as a sensing platform for O2 adsorption and desorption kinetics

The adsorption and desorption kinetics of molecules is of significant fundamental and applied interest. In this paper, we present a new method to quantify the energy barriers for the adsorption and desorption of gas molecules on few-atom clusters, by exploiting reaction induced changes of the doping level of a graphene substrate. The method is illustrated for oxygen adsorption on Au₃ clusters. The gold clusters were deposited on a graphene field effect transistor and exposed to O₂. From the change in graphene's electronic properties during adsorption, the energy barrier for the adsorption of O₂ on Au₃ is estimated to be 0.45 eV. Electric current pulses increase the temperature of the graphene strip in a controlled way and provide the required thermal energy for oxygen desorption. The oxygen binding energy on Au₃/graphene is found to be 1.03 eV and the activation entropy is 1.4 meV K^-1. The experimental values are compared and interpreted on the basis of density functional theory calculations of the adsorption barrier, the binding energy and the activation entropy. The large value of the activation entropy is explained by the hindering effect that the adsorbed O₂ has on the fluxional motion of the Au₃ cluster.

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

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...

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.

Synthetic strategies of supported atomic clusters for heterogeneous catalysis

Supported atomic clusters with uniform metal sites and definite low-nuclearity are intermediate states between single-atom catalysts (SACs) and nanoparticles in size. Benefiting from the presence of metal–metal bonds, supported atomic clusters can trigger synergistic effects among every metal atom, which contributes to achieving unique catalytic properties different from SACs and nanoparticles. However, the scalable and precise synthesis and atomic-level insights into the structure–properties relationship of supported atomic clusters is a great challenge. This perspective presents the latest progress of the synthesis of supported atomic clusters, highlights how the structure affects catalytic properties, and discusses the limitations as well as prospects.

Supported atomic clusters with precise nuclearity are intermediate states between single-atom catalysts and nanoparticles in size. Here the authors summarize and discuss synthetic strategies of supported atomic clusters with unique catalytic properties for heterogeneous reactions.

γ-Alumina-supported Pt17 cluster: controlled loading, geometrical structure, and size-specific catalytic activity for carbon monoxide and propylene oxidation

Although Pt is extensively used as a catalyst to purify automotive exhaust gas, it is desirable to reduce Pt consumption through size reduction because Pt is a rare element and an expensive noble metal. In this study, we successfully loaded a Pt₁₇ cluster on γ-alumina (γ-Al₂O₃) (Pt₁₇/γ-Al₂O₃) using [Pt₁₇(CO)₁₂(PPh₃)₈]Cl _n (n = 1, 2) as a precursor. In addition, we demonstrated that Pt is not present in the form of an oxide in Pt₁₇/γ-Al₂O₃ but instead has a framework structure as a metal cluster. Moreover, we revealed that Pt₁₇/γ-Al₂O₃ exhibits higher catalytic activity for carbon monoxide and propylene oxidation than γ-Al₂O₃-supported larger Pt nanoparticles (Pt_NP/γ-Al₂O₃) prepared using the conventional impregnation method. Recently, our group discovered a simple method for synthesizing the precursor [Pt₁₇(CO)₁₂(PPh₃)₈]Cl _n . Furthermore, Pt₁₇ is a Pt cluster within the size range associated with high catalytic activity. By combining our established synthesis and loading methods, other groups can conduct further research on Pt₁₇/γ-Al₂O₃ to explore its catalytic activities in greater depth.

Geometry control of size selected Pt clusters bound to Si substrate surface by cluster impact deposition

Geometry of platinum clusters, Pt_N (N = 30-71), supported on a silicon substrate was investigated, aiming to control the geometry. The supported clusters were prepared by the impact of size-selected Pt_N ⁺ onto the substrate at a given collision energy (cluster-impact deposition), and their geometry was observed by means of a scanning-tunneling microscope. Even at the collision energy of 1 eV per Pt atom, sufficiently strong Pt-Si interaction between Pt_N (N = 30 and 45) and the Si substrate allows them to be supported as close-packed monatomic-layered Pt disks, while at N = 60, multilayered shapes exist besides the monatomic-layered shape, the fraction of which increases at N = 71. When the collision energy is increased, Si atoms located at the interface between the cluster and Si substrate dissolve into the cluster, and with further increase in the collision energy, the Pt-Si cluster is partially implanted into the substrate. The transition in the shape of the supported clusters with the collision energy and the cluster size was explained according to the deformation of the clusters and the substrate surface by the cluster impact. It is proposed that the momentum of Pt_N ⁺ per its cross section is a good index to control the geometry in the case of strong cluster-support interaction such as Pt and Si.

Tuning the structure of bifunctional Pt/SmMn2O5 interfaces for promoted low-temperature CO oxidation activity

The interfacial structure of metal-oxide composite catalysts plays a vital role in heterogeneous catalysis, which is crucial to the adsorption and activation of reactants. Herein, the interfacial effects of bare and Fe/Co/Ni doped SmMn2O5 mullite oxide supported Pt clusters on CO oxidation have been investigated by first-principles based microkinetics analysis. A robust formation of Pt/Mn2 trimer structures is demonstrated at the bifunctional interfaces irrespective of the Ptn cluster's size, which can provide spatially separated sites for CO adsorption and O2 dissociation. The binding strength of CO at the interfacial Pt sites is in the optimal range due to the charge transfer from Pt clusters to oxide, while the strong polarization of Mn2 dimers induced by Pt clusters with stable three-dimensional morphologies can lower the energy barrier of O2 dissociation. Based on the microkinetics analysis, the O2 dissociation is the rate-determining step in the full CO oxidation cycle, and the introduction of Mn-Fe hetero-dimers at the interface is predicted to further enhance the low temperature CO oxidation activity of Pt/SmMn2O5 catalysts.

Atmosphere-dependent stability and mobility of catalytic Pt single atoms and clusters on γ-Al2O3

Atomically dispersed metals promise the ultimate catalytic efficiency, but their stabilization onto suitable supports remains challenging owing to their aggregation tendency. Focusing on the industrially-relevant Pt/γ-Al2O3 catalyst, in situ X-ray absorption spectroscopy and environmental scanning transmission electron microscopy allow us to monitor the stabilization of Pt single atoms under O2 atmosphere, as well as their aggregation into mobile reduced subnanometric clusters under H2. Density functional theory calculations reveal that oxygen from the gas phase directly contributes to metal-support adhesion, maximal for single Pt atoms, whereas hydrogen only adsorbs on Pt, and thereby leads to Pt clustering. Finally, Pt cluster mobility is shown to be activated at low temperature and high H2 pressure. Our results highlight the crucial importance of the reactive atmosphere on the stability of single-atom versus cluster catalysts.

2D oxides on metal materials: concepts, status, and perspectives

Two-dimensional oxide-on-metal materials: concepts, methods, and link to technological applications, with 5 subtopics: structural motifs, robustness, catalysis, ternaries, and nanopatterning.

Single atom alloy catalyst for SO3 decomposition: enhancement of platinum catalyst's performance by Ag atom embedding

Recently, single atom alloy catalysts (SAA) have shown improved catalytic activity in numerous catalytic reactions. However, to date, single atom alloy (SAA) catalyst is not available for SO3 decomposition reaction, which is a key reactions in the hydrogen economy. Using state of the art density functional theory, we report a novel single Ag atom alloy Pt catalyst in the sub-nanometer length scale (AgPt9@Al2O3) showing superior catalytic behavior for SO3 decomposition. It was found that alloying the alumina-supported platinum nanocluster with a single Ag atom lowers the activation barrier for S-O bond breaking by more than 50% in comparison with the pristine platinum counterpart. Activation barrier for AgPt9@Al2O3 catalyst is 0.52 eV, which is the lowest of any platinum based catalyst reported so far. At variance with pure Pt10@Al2O3, which tries to detach from the support during decomposition reaction, single atom alloy (SAA) nanocluster AgPt9@Al2O3 enhances binding with support, thus strengthening sintering resistance. Notably, influence of single Ag atom is also observed at larger length scale, i.e., at Pt(111) slab, where single Ag atom substituted surface Ag1Pt(111) shows ∼30% reduction in activation barrier in contrast to a pristine surface. Single Ag atom works in bifunctional mode as it not only reduces the activation barrier, but also simultaneously weakly adsorbs the reaction product SO2, signifying relatively easier desorption and better recyclability. Deeper location of silver d-electrons and lesser electronegativity of silver is responsible for the better performance of single Ag atom alloyed Pt catalyst. We strongly believe that these remarkable results will open new avenues for future designing and fabrication of cost-effective catalysts for SO3 decomposition.

Active Sites in Heterogeneous Catalytic Reaction on Metal and Metal Oxide: Theory and Practice

Active sites play an essential role in heterogeneous catalysis and largely determine the reaction properties. Yet identification and study of the active sites remain challenging owing to their dynamic behaviors during catalysis process and issues with current characterization techniques. This article provides a short review of research progresses in active sites of metal and metal oxide catalysts, which covers the past achievements, current research status, and perspectives in this research field. In particular, the concepts and theories of active sites are introduced. Major experimental and computational approaches that are used in active site study are summarized, with their applications and limitations being discussed. An outlook of future research direction in both experimental and computational catalysis research is provided.

Polydopamine Induced in-Situ Formation of Metallic Nanoparticles in Confined Microchannels of Porous Membrane as Flexible Catalytic Reactor

Oxidant-regulated polymerization of dopamine was exploited, for the first time, for effective surface engineering of the well-defined cylindrical pores of nuclear track-etched membranes (NTEMs) to develop novel catalytic membrane reactor. First, in the presence of a strong oxidant, controlled synthesis of polydopamine (PDA) with tunable particle size was achieved, allowing a homogeneous deposition to the confined pore channels of NTEMs. The PDA interfaces rich in catechol and amine groups provided enhanced hydrophilicity to promote mass transport across the membrane and abundant nucleation sites for formation and stabilization of metallic nanoparticles (NPs). In-situ reductive growth of multiple metallic NPs, including Pd, Ag, and Au, was then achieved inside the cylindrical pores of NTEMs. Using the functionalized membrane as a catalytic reactor, efficient reduction of 4-nitrophenol (4-NP) was demonstrated in a flow-through mode. Moreover, after dissolution removal of the NTEMs, self-sustained one-dimensional (1D) PDA/M (M = Pd, Ag, or Au) hybrid nanotubes (NTs), with determined aspect ratio and a length reaching up to 10 μm, were obtained for catalysis of 4-NP in a batch reaction mode. This study established a facile and versatile method, by rational tuning of the polymerization behavior of dopamine, for effective modification of confined microscale/nanoscale cavities with different surface characteristics. The integration of PDA chemistry with NTEMs would provide more opportunities for development of novel catalytic membrane reactors as well as for the tailored synthesis of functional 1D nanotubes for broadened applications.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Local Fluxionality of Surface-Deposited Cluster Catalysts: The Case of Pt7 on Al2O3

Subnano surface-supported catalytic clusters can be generally characterized by many low-energy isomers accessible at elevated temperatures of catalysis. The most stable isomer may not be the most catalytically active. Additionally, isomers may interconvert across barriers, i.e., exhibit fluxionality, during catalysis. To study the big picture of the cluster fluxional behavior, we model such a process as isomerization graph using bipartite matching algorithm, harmonic transition state theory, and paralleled nudged elastic band method. All the minimal energy paths form a minimum spanning tree (MST) of the original graph. Detailed inspection shows that, at temperatures typical for catalysis, the cluster geometry changes frequently within several regions in the MST, while transition across regions is less likely. As a further confirmation, the structural similarity analysis was additionally performed based on molecular dynamics trajectories. This local fluxionality picture provides a new perspective on understanding finite-temperate catalytic processes.

Bifunctional CO oxidation over Mn-mullite anchored Pt sub-nanoclusters via atomic layer deposition

CO oxidation is a widely used model system for understanding fundamental aspects of heterogeneous catalysis. While platinum (Pt) continues to be a reference material for CO oxidation catalysis, poisoning of Pt catalysts presents a critical issue that blocks reaction sites and impedes subsequent reaction steps. Fabrication of CO poison-free Pt catalysts remains a great challenge due to its CO-philic nature. Herein, we report a Pt based catalyst to effectively tackle CO poisoning by tightly anchoring Pt sub-nanoclusters onto Mn-mullite oxide (SmMn₂O₅) via atomic layer deposition. Superior CO oxidation activity has been observed with a significantly lowered light-off temperature and apparent activation energy. In situ diffuse reflectance infrared Fourier transform spectroscopy analysis, oxygen isotope experiments and density functional theory calculations confirm that the low-temperature activity originates from active oxygen atom sources at the bifunctional interface structure.

Computational screening of a single transition metal atom supported on the C2N monolayer for electrochemical ammonia synthesis

The single Mo atom supported by C₂N layer is an effective electrocatalyst for NH₃ synthesis from N₂.

ORR viability of alumina-supported platinum nanocluster: exploring oxidation behaviour by DFT

Despite abundant use of alumina-supported platinum nanoclusters as catalyst for various chemical reactions, their potential as an ORR catalyst is yet to be explored. Therefore, the present study aimed to assess the viability of alumina supported platinum clusters as ORR catalysts.