Single gold atoms stabilized on nanoscale metal oxide supports are catalytic active centers for various reactions

Coadsorption Interfered CO Oxidation over Atomically Dispersed Au on h-BN

Similar to the metal centers in biocatalysis and homogeneous catalysis, the metal species in single atom catalysts (SACs) are charged, atomically dispersed and stabilized by support and substrate. The reaction condition dependent catalytic performance of SACs has long been realized, but seldom investigated before. We investigated CO oxidation pathways over SACs in reaction conditions using atomically dispersed Au on h-BN (AuBN) as a model with extensive first-principles-based calculations. We demonstrated that the adsorption of reactants, namely CO, O₂ and CO₂, and their coadsorption with reaction species on AuBN would be condition dependent, leading to various reaction species with different reactivity and impact the CO conversion. Specifically, the revised Langmuir–Hinshelwood pathway with the CO-mediated activation of O₂ and dissociation of cyclic peroxide intermediate followed by the Eley–Rideal type reduction is dominant at high temperatures, while the coadsorbed CO-mediated dissociation of peroxide intermediate becomes plausible at low temperatures and high CO partial pressures. Carbonate species would also form in existence of CO₂, react with coadsorbed CO and benefit the conversion. The findings highlight the origin of the condition-dependent CO oxidation performance of SACs in detailed conditions and may help to rationalize the current understanding of the superior catalytic performance of SACs.

3D printing of recombinant Escherichia coli/Au nanocomposites as agitating paddles towards robust catalytic reduction of 4-nitrophenol

Three-dimensional (3D) printing technology has received remarkable attention in manufacturing catalysts with tailored shapes and high precision, particularly facilitating catalyst recovery, maximizing heat/mass transfer, as well as enhancing catalytic performance. Herein, an engineered recombinant Escherichia coli strain (denoted as e-E. coli) with overexpressing metallothionein (a metal-binding protein) was explored to synthesize Au nanoparticles serving as both reducing and stabilizing agents. Then, the mixed inks containing e-E. coli/Au composite and biocompatible polymers (sodium alginate and gelatin) were extruded based on a direct ink writing method followed by chemical crosslinking to form robust 3D grids with square symmetry. To boost the mass transfer and minimize pressure drop, the monolith catalysts were assembled into agitating paddles and used for liquid-phase batch reactions (volume: 1 L). As such, the reaction solutions were mixed internally via the powered "catalytic paddles" with high mechanical strength, excellent reactivity, and easy recyclability, which could be reused at least 7 cycles without performance loss. Our work provides a novel strategy for the fabrication of supported Au catalysts, and the proof-of-concept "catalytic paddles" by 3D printing technology can be applied to other industrial solution-based reactions.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

CO direct esterification to dimethyl oxalate and dimethyl carbonate: the key functional motifs for catalytic selectivity

The direct esterification of CO involves processes using CO as the starting material and ester chemicals as products. Dimethyl oxalate (DMO) and dimethyl carbonate (DMC) are two different products of the direct CO esterification reaction. However, the effective control of the reaction pathway and direct synthesis of DMO and DMC are challenging. In this review, we summarize the recent research progress on the direct esterification of CO to DMO/DMC and reveal the functional motifs responsible for the catalytic selectivity. Firstly, we discuss the microstructure of catalysts for the direct esterification of CO to DMO and DMC, including the valence state and the aggregate state of Pd. Then, the influence of characteristics of the support on the selectivity is analyzed. Importantly, the aggregate state of the active component, Pd is deemed as a vital functional motif for catalytic selectivity. The isolated Pd is conducive for the formation of DMC, while the aggregated Pd is beneficial for the formation of DMO. This review will provide rational guidance for the direct esterification of CO to DMO and DMC.

Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts

In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO₂ hydrogenation, C–C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts.

Theoretical insights into single-atom catalysts

Schematic diagram of theoretical models and applications of single atom catalysts. A review on the theoretical models, intrinsic properties, and the related application of SACs.

Supported Noble-Metal Single Atoms for Heterogeneous Catalysis

Single-atom catalysts (SACs), with atomically distributed active metal sites on supports, serve as a newly advanced material in catalysis, and open broad prospects for a wide variety of catalytic processes owing to their unique catalytic behaviors. To construct SACs with precise structures and high density of accessible single-atom sites, while preventing aggregation to large nanoparticles, various strategies for their chemical synthesis have been recently developed by improving the distribution and chemical bonding of active sites on supports, which results in excellent activity and selectivity in a variety of catalytic reactions. Noble-metal-based SACs are discussed, and their structural properties, chemical synthesis, and catalytic applications are highlighted. The structure-activity relationships and the underlying catalytic mechanisms are addressed, including the influences of surface species and reducibility of supports on the activity and stability, impact of the unique structural and electronic properties of single-atom centers modulated by metal/support interactions on catalytic activity and selectivity, and how the modified catalytic mechanism obtained by inhibiting the multiatoms involves catalytic pathways. Finally, the prospects and challenges for development in this field are highlighted.

Strategy for stabilizing noble metal nanoparticles without sacrificing active sites

We report a facile surface fluorination strategy for restricting Pt nanoparticle sintering through providing anchoring sites on the TiO₂ support and enhancing metal–support interaction via improved electronic interaction without sacrificing the active sites.

First-principles study of single transition metal atoms on ZnO for the water gas shift reaction

A single Ni atom substituted on a ZnO surface is a promising catalyst for the water gas shift reaction.

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

Nanomaterials coated with atomically dispersed platinum on ceria are structurally dynamic and show high potential for applications in fuel cells.