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We report the preparation and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Single‐atom heterogeneous catalysts (SACs) hold promise as sustainable alternatives to metal complexes in organic transformations. However, their working structure and dynamics remain poorly understood, hindering advances in their design. Exploiting the unique features of droplet‐based microfluidics, we present the first in‐situ assessment of a palladium SAC based on exfoliated carbon nitride in Suzuki–Miyaura cross‐coupling using X‐ray absorption spectroscopy. Our results confirm a surface‐catalyzed mechanism, revealing the distinct electronic structure of active Pd centers compared to homogeneous systems, and providing insights into the stabilizing role of ligands and bases. This study establishes a valuable framework for advancing mechanistic understanding of organic syntheses catalyzed by SACs. [ABSTRACT FROM AUTHOR]

Solar‐driven transfer hydrogenation of unsaturated bonds has received considerable attention in the research area of sustainable organic synthesis; however, water, the ultimate green source of hydrogen, has rarely been investigated due to the high barrier associated with splitting of water molecules. We report a carbon‐nitride‐supported palladium single‐atom heterogeneous catalyst with unparalleled performance in photocatalytic water‐donating transfer hydrogenation compared to its nanoparticle counterparts. Isotopic‐labeling experiments and operando nuclear magnetic resonance measurements confirm the direct hydrogenation mechanism using in situ‐generated protons from water splitting under visible‐light irradiation. Density functional theory calculations attribute the high activity to lower barriers for hydrogenation, facilitated desorption of ethylbenzene, and facile hydrogen replenishment from water on the atomic palladium sites. [ABSTRACT FROM AUTHOR]

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]

The application of nitrogen-doped carbons (NDCs) as host materials for single-atom catalysts (SACs) is increasing because of the strong binding affinity of the heteroatom with transition metals. Establishment of the relation between the properties of NDCs, their interaction with metals, and the performance of the resulting catalysts is crucial to guide the design of more effective SACs but has not been critically addressed. Here, a series of NDCs is prepared and studied as hosts for palladium atoms. The amount of nitrogen incorporated primarily depends on the choice of carbon followed by the doping temperature and nitrogen source. Pyridinic and pyrrolic species predominate in all cases, especially at lower doping temperatures. The stabilization of palladium atoms is successful above a critical nitrogen content that depends on the carbon type. Evaluation in the catalytic semi-hydrogenation of 2-methyl-3-butyn-2-ol readily distinguishes the reactivity of single atoms and nanoparticles, which correlates with the electronic properties of palladium described by the average oxidation state. Comparison with SACs based on compositionally-related carbon nitride hosts shows that similarly high stability and tunability of the metal is achievable over NDCs, despite their lower nitrogen contents and greater heterogeneity of coordination sites. [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]

We report the preparation and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites. [ABSTRACT FROM AUTHOR]

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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: The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary and secondary () interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N2O, and N2 are equivalent on the three noble metals. NO is a primary reaction product, while N2 and N2O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH3, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH3 conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of , both N2O and N2 production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage. [Copyright &y& Elsevier]

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N2 adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co based HTlc's improves considerably the catalytic activity. Co–Rh,Al HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex Co,Pd–La,Al HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh containing catalyst, both in a N2O containing stream and in one containing also SO2 and O2, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO2 and O2 are considered. [ABSTRACT FROM AUTHOR]

Metal promotion is broadly applied to enhance the performance of heterogeneous catalysts to fulfill industrial requirements. Still, generating and quantifying the effect of the promoter speciation that exclusively introduces desired properties and ensures proximity to or accommodation within the active site and durability upon reaction is very challenging. Recently, In2O3 was discovered as a highly selective and stable catalyst for green methanol production from CO2. Activity boosting by promotion with palladium, an efficient H2-splitter, was partially successful since palladium nanoparticles mediate the parasitic reverse water–gas shift reaction, reducing selectivity, and sinter or alloy with indium, limiting metal utilization and robustness. Here, we show that the precise palladium atoms architecture reached by controlled co-precipitation eliminates these limitations. Palladium atoms replacing indium atoms in the active In3O5 ensemble attract additional palladium atoms deposited onto the surface forming low-nuclearity clusters, which foster H2 activation and remain unaltered, enabling record productivities for 500 h. Generating and quantifying the effect of the promoter speciation in heterogeneous catalysts is very challenging. Here, the authors show that the precise palladium atoms architecture reached by controlled co-precipitation overcomes selectivity and stability limitations associated with palladium nanoparticles for CO2-based methanol synthesis. [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]

Most birds build nests to incubate their eggs and raise their young in a protective environment. In their Communication on page 11265 ff., J. Pérez‐Ramírez, N. López et al. apply a nesting concept to anchor isolated palladium atoms within the cavities of graphitic carbon nitride. This leads to a stable single‐site heterogeneous catalyst with outstanding performance in hydrogenation reactions (artwork concept: Amalia Gallardo, illustration: Marcel Reich). [ABSTRACT FROM AUTHOR]

Viele Vögel bauen Nester …︁ …︁, um ihre Eier auszubrüten und ihre Jungen in geschützter Umgebung großzuziehen. In ihrer Zuschrift auf S. 11417 ff. verwenden J. Pérez ‐ Ramírez, N. López et al. ein “Nestbau ‐ Konzept” für die Einlagerung isolierter Palladiumatome in den Poren von graphitischem Kohlenstoffnitrid. Dies führt zu einem hervorragenden heterogenen “Single ‐ Site” ‐ Katalysator für Hydrierungsreaktionen (Titelbildkonzept: Amalia Gallardo, Illustration: Marcel Reich). [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: Alkyne hydrogenation, a widely used process in industry to purify olefin streams, comprises a prototype reaction to understand selectivity in heterogeneously catalyzed reactions. The selectivity of the reaction on palladium catalysts to the alkene, alkane, or oligomers strongly depends on the state of the (sub)surface; i.e., the occurrence of complex carbide/hydride phases. In practice, hydrogenation reactors in C2 and C3 cuts of steam crackers require continuous CO feeding in order to enhance the alkene selectivity of palladium-supported catalysts. In the present work, we have studied the impact of carbon monoxide on the formation of carbide and hydride phases as a standpoint to derive structure–performance relationships under realistic process conditions. For this purpose, catalytic tests on a standard 1wt.% Pd/Al2O3 and Density Functional Theory on Pd(111) were combined. The influence of: (i) the alkyne (ethyne and propyne), (ii) the hydrogen:alkyne ratio (1–10), (iii) the carbon monoxide:hydrogen ratio (0–0.2), and (iv) the catalyst pretreatment on the product distribution was assessed in a continuous flow fixed-bed reactor at ambient pressure. In absence of CO, subtle changes in the hydrogen:alkyne ratio generate undesired products. Carbon monoxide enables the external control of the catalyst state by suppressing the formation of subsurface hydride and carbide phases, thereby stabilizing a high alkene yield in a broad range of feed hydrogen:alkyne ratios. This scenario contrasts with the more fragile regime of the hydride–carbide phases under CO-free conditions. DFT calculations obtained a single Brønsted–Evans–Polanyi relationship independently of the state of the catalyst (carbide, hydride, CO-covered) and the alkyne–alkene–alkane set (C2, C3). [Copyright &y& Elsevier]

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]