Nanoparticle proximity controls selectivity in benzaldehyde hydrogenation

Disentangling the effects of nanoparticle proximity and size on thermal catalytic performance is challenging with traditional synthetic methods. Here we adapt a modular raspberry-colloid-templating approach to tune the average interparticle distance of PdAu alloy nanoparticles, while preserving all other physicochemical characteristics, including nanoparticle size. By controlling the metal loading and placement of pre-formed nanoparticles within a 3D macroporous SiO2 support and using the hydrogenation of benzaldehyde to benzyl alcohol and toluene as the probe reaction, we report that increasing the interparticle distance (from 12 to 21 nm) substantially enhances selectivity towards benzyl alcohol (from 54% to 99%) without compromising catalytic performance. Combining electron tomography, kinetic evaluation and simulations, we show that interparticle distance modulates the local benzyl alcohol concentration profile between active sites, consequently affecting benzyl alcohol readsorption, which promotes hydrogenolysis to toluene. Our results illustrate the relevance of proximity effects as a mesoscale tool to control the adsorption of intermediates and, hence, catalytic performance. (Figure presented.)

Identifying the Optimal Pd Ensemble Size in Dilute PdAu Alloy Nanomaterials for Benzaldehyde Hydrogenation

Unraveling metal nuclearity effects is central for active site identification and the development of high-performance heterogeneous catalysts. Herein, a platform of nanostructured palladium (Pd) in gold (Au) dilute alloy nanoparticles supported on raspberry-colloid-templated (RCT) silica was employed to systematically assess the impact of the Pd ensemble size for the low-nuclearity regime in the Au surface layer, from single atoms to clusters, on the catalytic performance in the liquid-phase hydrogenation of benzaldehyde to benzyl alcohol. Combining catalyst evaluation, detailed characterization, and mechanistic studies based on density functional theory, we show that Pd single atoms in the Au surface plane (corresponding to samples with 4 atom % Pd in Au) are virtually inactive in this reaction and benzyl alcohol production is optimal over small Pd clusters (corresponding to samples with 10-12 atom % Pd in Au) due to superior benzaldehyde adsorption and transition state stabilization for the C-H bond formation step. For larger Pd ensembles (samples with ≥10 atom % Pd in Au), C-O bond hydrogenolysis occurs, promoting toluene formation and decreasing the selectivity toward benzyl alcohol, in line with a relatively lowered C-O bond cleavage barrier. Nevertheless, the nanostructured bimetallic Pd13Au87/SiO2-RCT catalyst still outperforms monometallic Pd counterparts in terms of selectivity for benzyl alcohol over toluene at comparable conversion and rate. Furthermore, the stability is improved compared to pure Pd nanoparticles due to inhibited particle agglomeration in the RCT silica matrix

Descriptors for High-Performance Nitrogen-Doped Carbon Catalysts in Acetylene Hydrochlorination

ISSN:2155-543

Nitrogen-doped carbons with hierarchical porosity via chemical blowing towards long-lived metal-free catalysts for acetylene hydrochlorination

Porous nitrogen‐doped carbons (NCs) are sustainable alternatives to the toxic mercury‐based acetylene hydrochlorination catalysts applied in the manufacture of polyvinyl chloride. However, the application of NCs as metal‐free catalysts is hampered by their insufficient durability under industrially relevant process conditions. In particular, pore blockage leads to accelerated deactivation of NCs compared to the state‐of‐the‐art precious metal‐based systems. Herein, we develop a salt template‐ assisted synthesis strategy coupled with chemical blowing to tune the textural properties of NCs, while preserving the N‐content and speciation. The addition of metal salts (i. e., Mg(OAc)2 or CaCO3) enhances gas evolution, leading to an increased formation of micro‐ and mesopores, while the in‐situ generated CaO/CaCl2 and MgO/MgCl2 develop auxiliary pore networks. Micropores are easily blocked during acetylene hydrochlorination, but meso‐ and macropores are structurally stable, enhancing the lifetime of hierarchical NCs by ca. 50 times compared to their non‐ templated analogues, rivaling the stability of benchmark metal‐based catalysts

Preserved in a Shell: High-Performance Graphene-Confined Ruthenium Nanoparticles in Acetylene Hydrochlorination

ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

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

Sustainable synthesis of bimetallic single atom gold-based catalysts with enhanced durability in acetylene hydrochlorination

Gold single‐atom catalysts (SACs) exhibit outstanding reactivity in acetylene hydrochlorination to vinyl chloride, but their practical applicability is compromised by current synthesis protocols, using aqua regia as chlorine‐based dispersing agent, and their high susceptibility to sintering on non‐functionalized carbon supports at >500 K and/or under reaction conditions. Herein, a sustainable synthesis route to carbon‐supported gold nanostructures in bimetallic catalysts is developed by employing salts as alternative chlorine source, allowing for tailored gold dispersion, ultimately reaching atomic level when using H2PtCl6. To rationalize these observations, several synthesis parameters (i.e., pH, Cl‐content) as well as the choice of metal chlorides are evaluated, hinting at the key role of platinum in promoting a chlorine‐mediated dispersion mechanism. This can be further extrapolated to redisperse large gold agglomerates (>70 nm) on carbon carriers into isolated atoms, which has important implications for catalyst regeneration. Another key role of platinum single atoms is to inhibit the sintering of their spatially isolated gold‐based analogs up to 800 K and during acetylene hydrochlorination, without compromising the intrinsic activity of Au(I)‐Cl active sites. Accordingly, exploiting cooperativity effects of a second metal is a promising strategy towards practical applicability of gold SACs, opening up exciting opportunities for multifunctional single‐atom catalysis.ISSN:1613-6810ISSN:1613-682

Design of carbon supports for metal-catalyzed acetylene hydrochlorination

For decades, carbons have been the support of choice in acetylene hydrochlorination, a key industrial process for polyvinyl chloride manufacture. However, no unequivocal design criteria could be established to date, due to the complex interplay between the carbon host and the metal nanostructure. Herein, we disentangle the roles of carbon in determining activity and stability of platinum-, ruthenium-, and gold-based hydrochlorination catalysts and derive descriptors for optimal host design, by systematically varying the porous properties and surface functionalization of carbon, while preserving the active metal sites. The acetylene adsorption capacity is identified as central activity descriptor, while the density of acidic oxygen sites determines the coking tendency and thus catalyst stability. With this understanding, a platinum single-atom catalyst is developed with stable catalytic performance under two-fold accelerated deactivation conditions compared to the state-of-the-art system, marking a step ahead towards sustainable PVC production.This work was supported by ETH research grant (ETH-40 17-1) and GV (PROMETEO/2018/076)

Design of single gold atoms on nitrogen-doped carbon for molecular recognition in alkyne semi-hydrogenation

ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

Design of Single Gold Atoms on Nitrogen-Doped Carbon for Molecular Recognition in Alkyne Semi-Hydrogenation

Single-atom heterogeneous catalysts with welldefined architectures are promising for deriving structure– performance relationships, but the challenge lies in finely tuning the structural and electronic properties of the metal. To tackle this point, a new approach based on the surface diffusion of gold atoms on different cavities of N-doped carbon is presented. By controlling the activation temperature, the coordination neighbors (Cl, O, N) and the oxidation state of the metal can be tailored. Semi-hydrogenation of various alkynes on the single-atom gold catalysts displays substratedependent catalytic responses; structure insensitive for alkynols with g-OH and unfunctionalized alkynes, and sensitive for alkynols with a-OH. Density functional theory links the sensitivity for alkynols to the strong interaction between the substrate and specific gold-cavity ensembles, mimicking a molecular recognition pattern that allows to identify the cavity site and to enhance the catalytic activity