Quantitative description of metal center organization and interactions in single-atom catalysts

Ultra-high-density single-atom catalysts (UHD-SACs) present unique opportunities for harnessing cooperative effects between neighboring metal centers. However, the lack of tools to establish correlations between the density, type, and arrangement of the isolated metal atoms with the support surface properties hinders efforts to engineer advanced material architectures. Here, we precisely describe the metal center organization in various mono- and multimetallic UHD-SACs based on nitrogen-doped carbon (NC) supports by coupling transmission electron microscopy with tailored machine-learning methods (released as a user-friendly web app) and density functional theory simulations. Our approach quantifies the non-negligible presence of multimers with increasing atom density, characterizes the size and shape of these low-nuclearity clusters, and identifies surface atom density criteria to ensure isolation. Further, it provides previously inaccessible experimental insights into coordination site arrangements in the NC host, uncovering a repulsive interaction that influences the disordered distribution of metal centers in UHD-SACs. This observation holds in multimetallic systems, where chemically-specific analysis quantifies the degree of intermixing. These fundamental insights into the materials chemistry of single-atom catalysts are crucial for designing catalytic systems with superior reactivity.This publication was created as part of NCCR Catalysis (grant number 180544), a National Centre of Competence in Research funded by the Swiss National Science Foundation. A. R.-F. acknowledges funding from the Generalitat de Catalunya and the European Union under Grant 2023 FI-3 00027. N.L. acknowledges support from the Ministerio de Ciencia e Innovación, ref. no. RTI2018-101394-B-100, and the Severo Ochoa Grant, MCIN/AEI/10.13039/501100011033-CEX2019-000925-S. The authors thank BSC-RES for generously providing computational resources.Peer ReviewedPostprint (published version

Novel Tools for the Design of Single-Atom Catalysts

Single-atom heterogeneous catalysts (SAC) represent the frontier in nanostructured catalyst design, permitting the effective manipulation of catalytic properties with the promise of unlocking more atom-efficient and sustainable chemical transformations. The recent impetus in exploring SAC has been driven by (i) advancements in analytical and synthesis capabilities enabling the preparation and understanding of new structural motifs, and (ii) the potential of SAC to enhance catalytic applications. Consequently, this thesis aims to expand the horizon of SAC research by exploring tools and use cases that address current fundamental challenges. Achieving high metal content (i.e., >1 wt.%) in SAC is challenging due to the tendency for metal clustering, yet essential for maximizing productivity in technological applications. To overcome this hurdle, a scalable two-step annealing method to synthesize mono and multimetallic ultra-high-density SAC (UHD-SAC) with unprecedented metal contents (23 wt.%) on diverse carriers is developed. Transfer to an automated synthesis platform demonstrated the reproducibility of the presented approach. Experimental and theoretical insights elucidated the central role of the step-wise ligand removal mechanism that effectively prevented metal agglomeration. Catalytic tests in three electro- and thermocatalytic applications showcased the technological potential of UHD-SAC. Assessing single atom dispersion in SAC with current characterization protocols is challenging, yet necessary to optimize materials synthesis. Atomic-resolution scanning transmission electron microscopy (STEM) is crucial in confirming metal site isolation in SAC, but deriving representative structural information remains a laborious effort. To address this gap, an automated method for the detection of platinum atoms in STEM images of a carbon-derived SAC was developed. The use of a customized convolutional neural network architecture permitted statistical analysis of surface density, atom proximity, and metal dispersion. Furthermore, the approach was generalizable to other carbon-related SAC and it enabled standardized uncertainty analysis in STEM image processing. The automated detection of metal centers in STEM images of SAC permits unprecedented insights into their surface spatial arrangements. A combination of STEM imaging, machine learning tools, and simulations elucidated the organization of metal atoms in distinct UHD-SAC based on nitrogen-doped carbon (NC). The analysis, while ensuring the isolation of metal centers, distinguished multimers from monomers at a high density regime. Quantitative analysis of nearest-neighbor distances in mono- and multimetallic UHD-SAC revealed that the distribution of atoms on the catalyst surface is not random but rather mediated by repulsive interactions. Simulations pinpointed that these patterns originated from the preferred arrangement of nitrogen-containing anchoring sites within the host, which are likely driven by electronic charge stabilization. Similar to assessing and understanding spatial arrangements of metal centers, controlling the size of supported metal species to form single atom or low-nuclearity clusters catalysts can significantly impact their performance. In general, for precious metal catalysts, large nanoparticles are favored in hydrogenation reactions. For earth-abundant metals such as iron that are less efficient at activating hydrogen, however, there is limited exploration of speciation trends. Controlling the preparation of low-nuclearity iron species and their characterization comprise key challenges. To address this, iron catalysts with distinct nuclearity were prepared via pre-selected precursor synthesis and evaluated in the continuous liquid-phase semihydrogenation of alkynes. Surprisingly, contrary to observations for palladium catalysts, single atoms of iron exhibited higher activity than larger clusters. Atomistic simulations predicted that residual carbonyl species from the metal precursor play a central role in stabilizing single atoms on the surface and enabling low-energy paths across these isolated sites. SAC have promising prospects for other fine-chemical transformations such as the Suzuki- Miyaura cross-coupling reaction. However, their potential for the mechanistically more complex, yet equally important Pd–Cu catalyzed Sonogashira coupling has not been explored. While SAC promise to ameliorate concerns about the reusability of conventionally used molecular catalysts and downstream palladium removal for product purification, quantitative metrics for the comparison of the ecological impact of homogeneous and (single-atom) heterogeneous catalysts are lacking. A life-cycle assessment (LCA) demonstrated that optimal heterogeneous catalysts can reduce the Sonogashira process footprint mainly through efficient palladium reuse. Anchoring palladium atoms on an nitrogen-doped carbon carrier synergized the advantages of solid and molecular catalysts and enabled high activity, complete metal recovery, and reuse for multiple cycles reflecting in 6-fold improved LCA-based metrics. Environmental impact is a crucial driver for optimizing mercury-free catalysts for vinyl chloride (VCM) production via acetylene hydrochlorination. While most efforts focus on nanostructured platinum-group metals (PGM), the potential of earth-abundant copper catalysts, known for their high stability under reaction conditions, remains underexplored. Catalytic evaluation of copper nanoparticles and single atoms deposited on activated carbon revealed that all materials converged in terms of performance irrespective of initial architecture, matching the stable VCM productivity achieved with the SAC. Integration of advanced characterization techniques and simulations were key to shed light into the reaction-induced formation of low-valent, single atom Cu(I)Cl sites, which was likely promoted by surface oxygen groups. When assessed under optimal conditions, the copper SAC, although less active, exhibited tenfold higher stability in comparison to PGM-based catalysts, resulting in a 100-fold reduced environmental impact of the catalyst, as estimated through a LCA. In conclusion, this thesis illustrates novel concepts across various facets of SAC design and their potential contributions to future materials discovery. It presents approaches to synthetically control surface atom densities in SAC, standardize quantification of single-atom dispersion and spatial arrangements via electron microscopy images. Furthermore, it sheds light on the underexplored interplay between nuclearity and ligands in iron hydrogenation catalysts, as well as the structural evolution and resulting active site speciation of copper catalysts in acetylene hydrochlorination. The described design strategies and tools, including the use of quantitative sustainability metrics to assess the ecological potential of SAC, can serve as blueprints for a more comprehensive and holistic development of this versatile family of catalytic materials

Single-atom heterogeneous 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.ISSN:0009-2665ISSN:1520-689

Dataset for Quantitative description of metal center organization in single-atom catalysts

Dataset for Quantitative description of metal center organization in single-atom catalysts by by K. Rossi, A. Ruiz-Ferrando, D. Faust Akl, V. Gimenez Abalos, J. Heras-Domingo, R. Graux, X. Hai, J. Lu, D. Garcia-Gasulla, N. López, J. Pérez-Ramírez, and S. Mitchell. The data is structured as follows: 01_Micrographs: all micrographs employed in .tif and .png format. An imageJ macro to overlay coordinate files with images is attached. 02_Ground_truth: contains the manually-labeled and predicted xy-coordinates of atomic positions including probabilities. 03_All_detection_data: contains all automated predictions of atomic positions in the images of this study (uhd), and of Mitchell et. al in JACS, 144, 8018-8029 (2022) (jacs_train, jacs_test). This folder further contains model weights and area segmentations needed to estimate the surface atomic densities. 04_Trimetallic_analysis: Figures complementing Supplementary Figure S21

Droplet-Based Microfluidics Platform for the Synthesis of Single-Atom Heterogeneous Catalysts

Wet chemical approaches are among the most versatile and scalable strategies for preparing single-atom heterogeneous catalysts (SACs). However, despite their broad application, the synthesis of SACs via these routes remains largely ad hoc, with limited attention to the effect of different synthetic parameters on the stabilization of metal species. As a proof of concept, herein, a microfluidic platform is demonstrated for short-timescale (<10 s), systematic syntheses of SACs via wet impregnation using a range of metal precursor-carrier combinations. The microfluidic environment within rapidly mixed, nanoliter droplets ensures precise control of the concentrations and residence times of the support particles in the metal precursor solutions. This enables the rapid assessment of the influence of the metal precursor concentration on the uptake and dispersion of the adsorbed metal species, as demonstrated for the synthesis of palladium and platinum SACs based on a high-surface form of graphitic carbon nitride (C3N4). Extension to SACs based on other metals (Ni) and relevant carriers (N-doped carbon, gamma-alumina) confirms the generality of the synthesis method. The microfluidic approach opens possibilities for high-throughput parameter screening and mechanistic studies in the design of heterogeneous single-atom catalysts.ISSN:2688-406

Structure sensitivity of nitrogen–doped carbon–supported metal catalysts in dihalomethane hydrodehalogenation

Nanostructuring metal catalysts has been demonstrated as an attractive strategy to enable selective hydrodehalogenation of CH2X2 (X = Cl, Br) to CH3X, but active phase size effects of promising metals and the role of the halogen are still poorly understood. Herein, the impact of these parameters on performance (activity, selectivity, and stability) is systematically assessed by employing a platform of N–doped carbon–supported metal nanostructures (Ir, Pt, Ru, and Ni), ranging from single atoms (SA) with defined coordination environment to nanoparticles (NP) of ca. 3.0 nm. Catalytic tests reveal that when compared to single atoms, highest reaction rates are attained over NP–based systems, which also exhibit improved stability ranking as Ir ≈ Pt > Ru ≫ Ni, independent of the halogen. The product distribution was markedly affected by the nanostructure and speciation of the active center as well as the dihalomethane type. Specifically, CH3Cl is the main reaction product over SA in hydrodechlorination, achieving an exceptional selectivity over Ir (up to 95%). In contrast, NP mainly generated CH4 or coke. Comparable patterns were observed in hydrodebromination, except over Ru, which exhibited an inverse structure–selectivity trend. Density Functional Theory simulations shed light on the speciation of the active phase and identified the adsorption and dissociation energies of CH2X2 and H2 as descriptors for catalytic reactivity. These findings elucidate hydrodehalogenation performance patterns, highlighting the impact of nanostructuring and the halogen type to advance future catalyst design.ISSN:0021-9517ISSN:1090-269

Reaction environment design for multigram synthesis via Sonogashira coupling over heterogeneous palladium single-atom catalysts

Single-atom heterogeneous catalysts (SACs) attract growing interest in their application in green chemistry and organic synthesis due to their potential for achieving atomic-level precision. These catalysts offer the possibility of achieving selectivity comparable to the traditionally applied organometallic complexes, while enhancing metal utilization and recovery. However, an understanding of SAC performance in organic reactions remains limited to model substrates, and their application as drop-in solutions may not yield optimal activity. Here, we investigate the previously unaddressed influence of the reaction environment, including solvent, base, cocatalyst, and ligand, on the performance of a palladium SAC in Sonogashira–Hagihara cross-couplings. By examining the effects of different solvents using the established criteria, we find that the behavior of the SAC deviates from trends observed with homogeneous catalysts, indicating a distinct interplay between heterogeneous systems and the reaction environment. Our results illustrate the satisfactory performance of SACs in cross-couplings of aryl iodides and acetylenes with electron-withdrawing and -donating groups, while the use of bromides and chlorides remains challenging. Extending the proof-of-concept stage to multigram scale, we demonstrate the synthesis of an intermediate of the anticancer drug Erlotinib. The catalyst exhibits high stability, allowing for multiple reuses, even under noninert conditions. Life-cycle assessment guides the upscaling of the catalyst preparation and quantifies the potential environmental and financial benefits of using the SAC, while also revealing the negligible impact of the PPh3 ligand and CuI cocatalyst. Our results underscore the significant potential of SACs to revolutionize sustainable organic chemistry and highlight the need for further understanding the distinct interplay between their performance and the reaction environment

Methanol Synthesis by Hydrogenation of Hybrid CO2−CO Feeds

The impact of carbon monoxide on CO2-to-methanol catalysts has been scarcely investigated, although CO will comprise up to half of the carbon feedstock, depending on the origin of CO2 and process configuration. In this study, copper-based systems and ZnO−ZrO2 are assessed in cycling experiments with hybrid CO2−CO feeds and their CO sensitivity is compared to In2O3-based materials. All catalysts are found to be promoted upon CO addition. Copper-based systems are intrinsically more active in CO hydrogenation and profit from exploiting this carbon source for methanol production, whereas CO induces surplus formation of oxygen vacancies (i. e., the catalytic sites) on ZnO−ZrO2, as in In2O3-based systems. Mild-to-moderate deactivation occurs upon re-exposure to CO2-rich streams, owing to water-induced sintering for all catalysts except ZnO−ZrO2, which responds reversibly to feed variations, likely owing to its more hydrophobic nature and the atomic mixing of its metal components. Catalytic systems are categorized for operation in hybrid CO2−CO feeds, emphasizing the significance of catalyst and process design to foster advances in CO2 utilization technologies.ISSN:1864-564XISSN:1864-563

Precursor Nuclearity and Ligand Effects in Atomically-Dispersed Heterogeneous Iron Catalysts for Alkyne Semi-Hydrogenation

Nanostructuring earth-abundant metals as single atoms or clusters of controlled size on suitable carriers opens new routes to develop high-performing heterogeneous catalysts, but resolving speciation trends remains challenging. Here, we investigate the potential of low-nuclearity iron catalysts in the continuous liquid-phase semi-hydrogenation of various alkynes. The activity depends on multiple factors, including the nuclearity and ligand sphere of the metal precursor and their evolution upon interaction with the mesoporous graphitic carbon nitride scaffold. Density functional theory predicts the favorable adsorption of the metal precursors on the scaffold without altering the nuclearity and preserving some ligands. Contrary to previous observations for palladium catalysts, single atoms of iron exhibit higher activity than larger clusters. Atomistic simulations suggest a central role of residual carbonyl species in permitting low-energy paths over these isolated metal centers.ISSN:1867-3880ISSN:1867-389

Quantitative Description of Metal Center Organization and Interactions in Single-Atom Catalysts

Ultra-high-density single-atom catalysts (UHD-SACs) present unique opportunities for harnessing cooperative effects between neighboring metal centers. However, the lack of tools to establish correlations between the density, types, and arrangements of isolated metal atoms and the support surface properties hinders efforts to engineer advanced material architectures. Here, this work precisely describes the metal center organization in various mono- and multimetallic UHD‑SACs based on nitrogen-doped carbon (NC) supports by coupling transmission electron microscopy with tailored machine-learning methods (released as a user-friendly web app) and density functional theory simulations. This approach quantifies the non-negligible presence of multimers with increasing atom density, characterizes the size and shape of these low‑nuclearity clusters, and identifies surface atom density criteria to ensure isolation. Further, it provides previously inaccessible experimental insights into coordination site arrangements in the NC host, uncovering a repulsive interaction that influences the disordered distribution of metal centers in UHD-SACs. This observation holds in multimetallic systems, where chemically-specific analysis quantifies the degree of intermixing. These fundamental insights into the materials chemistry of single-atom catalysts are crucial for designing catalytic systems with superior reactivity.ISSN:0935-9648ISSN:1521-409