Single-atom Dispersion Research Articles

Insight into the stability of binuclear Ir–La catalysts for efficient heterogeneous methanol carbonylation

Heterogeneous single-site catalysts are ideal alternatives to their homogenous counterparts. Methanol carbonylation to produce acetic acid is a typical homogeneous catalytic process. Here, we investigate the stability of a heterogeneous single-site catalyst for vapor methanol carbonylation constituted by an Ir–La binuclear complex on activated carbon (Ir1-La1/AC). Through the structure of the binuclear complex on the Ir1-La1/AC catalyst, the La promoter could protect the Ir+ species from reduction by H2, favor the single-atom dispersion of Ir metal, and also increase the reaction rate of methanol carbonylation. Furthermore, the apparent activation energy of the Ir1-La1/AC catalyst was 12.2 kJ/mol lower than that of the Ir1/AC catalyst. A density functional theory calculation showed that reductive elimination of acetyl iodide was the rate-determining step for the Ir1/AC catalyst, leading to a severe carbon deposition problem, while CO migration insertion was believed to be the rate-determining step on the Ir1-La1/AC catalyst, which can reduce the carbon deposition to a great extent.

High-performance single atom bifunctional oxygen catalysts derived from ZIF-67 superstructures

Single-atom catalysts (SACs) always exhibit distinctive catalytic activities for oxygen reduction/evolution reactions (ORR/OER), making SACs relevant in a number of crucial applications including fuel cells, metal-air batteries, as well as in industrial applications. Recently, fabricating SACs with rich multidimensional nanoarchitectures has become fascinating but challenging. Here, for the first time, we explore a facile and practicable “sacrificed-template” method to prepare cobalt single-atom electrocatalysts with Urchin-like Nano-Tube hierarchical structures (UNT Co SAs/N-C) derived from well-aligned metal-organic-frameworks (MOFs) superstructure (UNT ZIF-67). The as-prepared UNT Co SAs/N-C catalysts exhibit superior performance both in ORR/OER. XAFS and density functional theory (DFT) calculations reveal that outstanding catalytic activities stem from high-quality single-atom dispersion, precise local coordination of CoN4, and well-aligned carbon matrix based on MOFs superstructures. Our work provides new perspective in enriching SACs synthesis methodologies.

Single-atom Rh based bipyridine framework porous organic polymer: A high active and superb stable catalyst for heterogeneous methanol carbonylation

A novel Rh based porous composed N polymer with hierarchical pore, high surface area and high thermal stability (Rh-POL-2BPY) was successfully obtained by impregnation of Rh2(CO)4Cl2 solution of dichloromethane. And based on Rh species as the most active carbonylation site, Rh-POL-2BPY was selected as a catalyst applied in heterogeneous methanol carbonylation system and behaved excellent carbonylation activity (TOF ≈ 1400 h−1) under 195 °C, 2.5 MPa, higher than the corresponding homogeneous system without acetic acid. In addition, contrast to the most reported unstable heterogeneous methanol carbonylation systems, Rh-POL-2BPY could maintain incredibly outstanding stability for near 400 h owing to the firm coordination bond between Rh and N and high exposed N content in the polymer. Here, POL-2BPY apart from its heat-resisting property acts as massive rivets to immobilize the Rh species in its framework solidly during carbonylation process. XPS, XANES, HAADF-STEM and EXAFS results verify the N species of bipyridine, the single atom dispersion of Rh and RhN coordination bond. And above all, the novel five-coordinated Rh center is proposed as the real active site of methanol carbonylation by EXAFS spectra.

High-density dispersion of single platinum atoms on graphene by plasma sputtering in N2 atmosphere

Isolated single metal atoms have attracted significant attention as catalytic materials with maximum surface areas as well as high activity and selectivity. However, a dense dispersion of single atoms on supported materials has not been achieved. We applied plasma sputtering in N2 atmosphere to disperse single Pt atoms on graphene. Scanning transmission electron microscopy observations confirmed the high-density dispersion of single Pt atoms, without widespread sintering into three-dimensional nanoparticles. X-ray photoelectron spectroscopy measurements revealed that pyridinic nitrogen was selectively doped in graphene during the sputtering. Our novel method provides a simple approach to synthesize high-density single Pt atoms on graphene.

Catalyst Architecture for Stable Single Atom Dispersion Enables Site-Specific Spectroscopic and Reactivity Measurements of CO Adsorbed to Pt Atoms, Oxidized Pt Clusters, and Metallic Pt Clusters on TiO2.

Oxide-supported precious metal nanoparticles are widely used industrial catalysts. Due to expense and rarity, developing synthetic protocols that reduce precious metal nanoparticle size and stabilize dispersed species is essential. Supported atomically dispersed, single precious metal atoms represent the most efficient metal utilization geometry, although debate regarding the catalytic activity of supported single precious atom species has arisen from difficulty in synthesizing homogeneous and stable single atom dispersions, and a lack of site-specific characterization approaches. We propose a catalyst architecture and characterization approach to overcome these limitations, by depositing ∼1 precious metal atom per support particle and characterizing structures by correlating scanning transmission electron microscopy imaging and CO probe molecule infrared spectroscopy. This is demonstrated for Pt supported on anatase TiO2. In these structures, isolated Pt atoms, Ptiso, remain stable through various conditions, and spectroscopic evidence suggests Ptiso species exist in homogeneous local environments. Comparing Ptiso to ∼1 nm preoxidized (Ptox) and prereduced (Ptmetal) Pt clusters on TiO2, we identify unique spectroscopic signatures of CO bound to each site and find CO adsorption energy is ordered: Ptiso ≪ Ptmetal < Ptox. Ptiso species exhibited a 2-fold greater turnover frequency for CO oxidation than 1 nm Ptmetal clusters but share an identical reaction mechanism. We propose the active catalytic sites are cationic interfacial Pt atoms bonded to TiO2 and that Ptiso exhibits optimal reactivity because every atom is exposed for catalysis and forms an interfacial site with TiO2. This approach should be generally useful for studying the behavior of supported precious metal atoms.

Pd/ZnO catalysts with different origins for high chemoselectivity in acetylene semi-hydrogenation

The heterogeneity of active sites is the main obstacle for selectivity control in heterogeneous catalysis. Single atom catalysts (SACs) with homogeneous isolated active sites are highly desired in chemoselective transformations. In this work, a Pd1/ZnO catalyst with single-atom dispersion of Pd active sites was achieved by decreasing the Pd loading and reducing the sample at a relatively low temperature. The Pd1/ZnO SAC exhibited excellent catalytic performance in the chemoselective hydrogenation of acetylene with comparable chemoselectivity to that of PdZn intermetallic catalysts and a greatly enhanced utilization of Pd metal. Such unusual behaviors of the Pd1/ZnO SAC in acetylene semi-hydrogenation were ascribed to the high-valent single Pd active sites, which could promote electrostatic interactions with acetylene but restrain undesired ethylene hydrogenation via the spatial restrictions of σ-chemical bonding toward ethylene.

Water Adsorption and Dissociation on Ceria-Supported Single-Atom Catalysts: A First-Principles DFT+U Investigation.

Single-atom catalysts have attracted wide attention owing to their extremely high atom efficiency and activities. In this paper, we applied density functional theory with the inclusion of the on-site Coulomb interaction (DFT+U) to investigate water adsorption and dissociation on clean CeO2 (111) surfaces and single transition metal atoms (STMAs) adsorbed on the CeO2 (111) surface. It is found that the most stable water configuration is molecular adsorption on the clean CeO2 (111) surface and dissociative adsorption on STMA/CeO2 (111) surfaces, respectively. In addition, our results indicate that the more the electrons that transfer from STMA to the ceria substrate, the stronger the binding energies between the STMA and ceria surfaces. A linear relationship is identified between the water dissociation barriers and the d band centers of STMA, known as the generalized Brønsted-Evans-Polanyi principle. By combining the oxygen spillovers, single-atom dispersion stabilities, and water dissociation barriers, Zn, Cr, and V are identified as potential candidates for the future design of ceria-supported single-atom catalysts for reactions in which the dissociation of water plays an important role, such as the water-gas shift reaction.

Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis

Supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. The size of metal particles is a key factor in determining the performance of such catalysts. In particular, because low-coordinated metal atoms often function as the catalytically active sites, the specific activity per metal atom usually increases with decreasing size of the metal particles. However, the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters. Using an appropriate support material that strongly interacts with the metal species prevents this aggregation, creating stable, finely dispersed metal clusters with a high catalytic activity, an approach industry has used for a long time. Nevertheless, practical supported metal catalysts are inhomogeneous and usually consist of a mixture of sizes from nanoparticles to subnanometer clusters. Such heterogeneity not only reduces the metal atom efficiency but also frequently leads to undesired side reactions. It also makes it extremely difficult, if not impossible, to uniquely identify and control the active sites of interest. The ultimate small-size limit for metal particles is the single-atom catalyst (SAC), which contains isolated metal atoms singly dispersed on supports. SACs maximize the efficiency of metal atom use, which is particularly important for supported noble metal catalysts. Moreover, with well-defined and uniform single-atom dispersion, SACs offer great potential for achieving high activity and selectivity. In this Account, we highlight recent advances in preparation, characterization, and catalytic performance of SACs, with a focus on single atoms anchored to metal oxides, metal surfaces, and graphene. We discuss experimental and theoretical studies for a variety of reactions, including oxidation, water gas shift, and hydrogenation. We describe advances in understanding the spatial arrangements and electronic properties of single atoms, as well as their interactions with the support. Single metal atoms on support surfaces provide a unique opportunity to tune active sites and optimize the activity, selectivity, and stability of heterogeneous catalysts, offering the potential for applications in a variety of industrial chemical reactions.

A portable microevaporator for low temperature single atom studies by scanning tunneling and dynamic force microscopy

Here, we present a microevaporator setup for single adatom deposition at low temperature, which is a prerequisite for most single atom studies with scanning probe techniques. The construction of the microevaporator is based on the tungsten filament of a modified halogen lamp, covered with the required adsorbate. Very stable evaporation conditions were obtained, which were controlled by the filament current. The installation of this microevaporator on a manipulator enabled its transportation directly to the sample at the microscope kept at 5 K. In this way, the controlled deposition of Li onto Ag(100), Li, Pd, and Au onto MgO/Ag(001) as well as Au onto alumina/NiAl(110) at low temperature has been performed. The obtained images recorded after the deposition show the presence of single Li/Au atoms on the sample surfaces as a prove for successful dispersion of single atoms onto the sample surface using this technique.

Linewidth prevents macroscopic quantum superposition generation by single atom dispersion

A method for generating macroscopically distinguishable superposition states via single atom dispersion was proposed by Savage et al. see Optics Letters, vol.15, p.628, 1990. In that model, the atomic linewidth was not considered. However, including the atomic linewidth, the authors get two contradictory conditions for the generation of distinguishable superposition states. This shows that the atomic linewidth prevents the formation of macroscopically distinguishable superposition states by the proposed method.

Macroscopic quantum superpositions by means of single-atom dispersion

We show that macroscopic quantum superpositions of the electromagnetic field can be generated through amplification of microscopic quantum superpositions prepared in a single atom. Our scheme has the advantage that dissipation is negligible, and hence the superpositions are not rapidly destroyed.