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Chemical modifiers enhance the efficiency of metal catalysts in numerous applications, but their introduction often involves toxic or expensive precursors and complicates the synthesis. Here, we show that a porous boron nitride carrier can directly modify supported palladium nanoparticles, originating unparalleled performance in the continuous semi‐hydrogenation of alkynes. Analysis of the impact of various structural parameters reveals that using a defective high surface area boron nitride and ensuring a palladium particle size of 4–5 nm is critical for maximizing the specific rate. The combined experimental and theoretical analyses point towards boron incorporation from defects in the support to the palladium subsurface, creating the desired isolated ensembles determining the selectivity. This practical approach highlights the unexplored potential of using tailored carriers for catalyst design. [ABSTRACT FROM AUTHOR]

Graphitic carbon nitride (g-C3N4) exhibits unique properties as a support for single-atom heterogeneous catalysts (SAHCs). Understanding how the synthesis method, carrier properties, and metal identity impact the isolation of metal centers is essential to guide their design. This study compares the effectiveness of direct and postsynthetic routes to prepare SAHCs by incorporating palladium, silver, iridium, platinum, or gold in g-C3N4 of distinct morphology (bulk, mesoporous and exfoliated). The speciation (single atoms, dimers, clusters, or nanoparticles), distribution, and oxidation state of the supported metals are characterized by multiple techniques including extensive use of aberration-corrected electron microscopy. SAHCs are most readily attained via direct approaches applying copolymerizable metal precursors and employing high surface area carriers. In contrast, although post-synthetic routes enable improved control over the metal loading, nanoparticle formation is more prevalent. Comparison of the carrier morphologies also points toward the involvement of defects in stabilizing single atoms. The distinct metal dispersions are rationalized by density functional theory and kinetic Monte Carlo simulations, highlighting the interplay between the adsorption energetics and diffusion kinetics. Evaluation in the continuous three-phase semihydrogenation of 1-hexyne identifies controlling the metal-carrier interaction and exposing the metal sites at the surface layer as key challenges in designing efficient SAHCs. [ABSTRACT FROM AUTHOR]

Ligand-modified palladium nanoparticles deposited on a carbon carrier efficiently catalyze the direct synthesis of H2O2 and the unique performance is due to their hybrid nanostructure. Catalytic testing demonstrated that the selectivity increases with the HHDMA ligand content from 10 % for naked nanoparticles up to 80 %, rivalling that obtained with state-of-the-art bimetallic catalysts (HHDMA=C20H46NO5P). Furthermore, it remains stable over five consecutive reaction runs owing to the high resistance towards leaching of the organic moiety, arising from its bond with the metal surface. As rationalized by density functional theory, this behavior is attributed to the adsorption mode of the reaction intermediates on the metal surface. Whereas they lie flat in the absence of the organic shell, their electrostatic interaction with the ligand result in a unique vertical configuration which prevents further dissociation and over-hydrogenation. These findings demonstrate the importance of understanding substrate-ligand interactions in capped nanoparticles to develop smart catalysts for the sustainable manufacture of hydrogen peroxide. [ABSTRACT FROM AUTHOR]

Site modification and isolation through selective poisoning comprise an effective strategy to enhance the selectivity of palladium catalysts in the partial hydrogenation of triple bonds in acetylenic compounds. The recent emergence of supported hybrid materials matching the stereo- and chemoselectivity of the classical Lindlar catalyst holds promise to revolutionize palladium-catalyzed hydrogenations, and will benefit from an in-depth understanding of these new materials. In this work, we compare the performance of bare, lead-poisoned, and ligand-modified palladium catalysts in the hydrogenation of diverse alkynes. Catalytic tests, conducted in a continuous-flow three-phase reactor, coupled with theoretical calculations and characterization methods, enable elucidation of the structural origins of the observed selectivity patterns. Distinctions in the catalytic performance are correlated with the relative accessibility of the active site to the organic substrate, and with the adsorption configuration and strength, depending on the ensemble size and surface potentials. This explains the role of the ligand in the colloidally prepared catalysts in promoting superior performance in the hydrogenation of terminal and internal alkynes, and short-chain alkynols. In contrast, the greater accessibility of the active surface of the Pd-Pb alloy and the absence of polar groups are shown to be favorable in the conversion of alkynes containing long aliphatic chains and/or ketone groups. These findings provide detailed insights for the advanced design of supported nanostructured catalysts. [ABSTRACT FROM AUTHOR]

Palladium nanoparticles are deposited on the surface of highly magnetic carbon-coated cobalt nanoparticles. In contrast to the established synthesis of Pd nanoparticles via reduction of Pd(II) precursors, the microwave decomposition of a Pd(0) source leads to a more efficient Pd deposition, resulting in a material with considerably higher activity in the hydrogenation of alkenes. Systematic variation of the Pd loading on the carbon-coated cobalt nanoparticle surface reveals a distinct trend to higher activities with decreased loading of Pd. The activity of the catalyst is further improved by the addition of 10 vol% Et2O to iso-propanol that is found to be the solvent of choice. With respect to activity (turnover frequencies up to 11 095 h−1), handling, recyclability through magnetic decantation, and leaching of Pd (≤6 ppm/cycle), this novel magnetic hybrid material compares favorably to conventional Pd/C or Pd@CNT catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Invited for this month′s cover is the group of Javier Pérez‐Ramírez at ETH Zurich. The image shows the Pisa tower supported by an ant standing on carbon nitride and a palladium nanoparticle, which symbolizes the CO2‐based production of formic acid through bifunctional catalysis. The Full Paper itself is available at 10.1002/cssc.201801920. [ABSTRACT FROM AUTHOR]

Aufgestanden für die Selektivität! In ihrer Zuschrift auf S. 1801 ff. zeigen Pérez‐Ramírez et al. die hervorragende Selektivität von Liganden‐modifizierten Pd‐Nanopartikeln bei der direkten H2O2‐Synthese. Die Hybridstruktur macht die Absorption des OOH‐Intermediats auf der Oberfläche möglich und verhindert ungewünschte Dissoziations‐ und Überhydrierungspfade. Durch elektrostatische Wechselwirkungen, dargestellt von den haltenden Fäden der Pinocchio‐Marionette, mit dem Liganden wird die Konfiguration stabilisiert. [ABSTRACT FROM AUTHOR]

The evolutionary tale of palladium‐based catalysts for selective alkyne hydrogenation, from bare to lead‐alloyed, and, most recently, ligand‐modified nanoparticles, is unlocked using flow chemistry. Distinctions in the performance of each catalyst system as well as accessibility constraints arising from the use of ligands are identified and rationalized. For more details, see the Full Paper by J. Pérez‐Ramírez, N. López et al. on page 5926 ff. [ABSTRACT FROM AUTHOR]

Abstract: DFT simulations have uncovered the origin of the highly selective character of gold nanoparticles in hydrogenation of triple bonds. This is ascribed to the better adsorption of Cedges of Au nanoparticles compared with Cthe barriers for hydrogenation of triple and double bonds on gold are comparable, selectivity is determined by the binding energy of the reactants. The situation is less favorable over palladium (commercial hydrorefining catalyst), because both Ce:glyph name="tbnd" />C bonds are adsorbed on the Pd surface. The outcome of the simulations was demonstrated experimentally in mixtures with both propyne and propylene over a Au/CeO2 catalyst, where C3H6 selectivities up to 95% were attained. Our finding opens a new path for chemoselective hydrogenation of molecules containing -yne and -ene groups on gold, with prospective application in petrochemical operations and in the fine chemical industry. [Copyright &y& Elsevier]

Abstract: We introduce a novel ‘spray deposition’ method as a continuous, reproducible, versatile, simple, and scalable method for the preparation of highly-dispersed, uniformly-sized, and sintering-resistant supported metal nanoparticles on porous supports. Instant drying of aqueous aerosol droplets containing the metal precursor (e.g. palladium nitrate) and the support (e.g. ferrierite zeolite) in a commercial bench-scale spray drier proves the key for maximizing the metal dispersion. In contrast, the much lower rates of solvent removal characteristic of dry and wet impregnation methods using classical drying or even freeze drying led to broad particle size distributions. The spray-dried Pd/ferrierite catalyst exhibited high alkene selectivity in the gas-phase hydrogenation of propyne. These results demonstrate that spray drying is not only a highly suited unit operation to prepare catalyst supports but also functional catalysts in one step. The method offers attractive opportunities for large-scale catalyst manufacture and as well as for preparation of well-defined systems for academic studies. [Copyright &y& Elsevier]