Pd Catalysts Research Articles

Green, mild, and rapid plasma synthesis of nitrogen/amino-cofunctionalized MWCNTs-supported Pd catalyst in solution for boosting formic acid dehydrogenation

Developing Pd catalysts with high catalytic activity and stability for formic acid dehydrogenation still remains a great challenge. Herein, a nitrogen/amino-cofunctionalized MWCNTs-supported Pd catalyst (Pd/MWCNTs-AP) was prepared in solution by a green, mild, and fast plasma technology. At 333 K reaction temperature, the turnover frequency (8415 h−1) over Pd/MWCNTs-AP for formic acid dehydrogenation was 1.26 and 5.16 times of those using Sigma-Aldrich Pd/C and thermal reduction prepared Pd/MWCNTs-AH, respectively. Impressively, the activity of Pd/MWCNTs-AP was not reduced during the six reaction cycles. In contrast, after the sixth cycle, the volume of gas produced using Sigma-Aldrich Pd/C decreased to 48 % of the initial volume. The results of structural characterization showed that Pd/MWCNTs-AP possessed ultrasmall-sized Pd, high Pd/C and N/C atomic ratios, and abundant –OH, facilitating the breakage of C–H and O–H bonds in formic acid to produce H and thus H2. Therefore, Pd/MWCNTs-AP exhibited excellent catalytic performance for formic acid dehydrogenation.

Synergistic effects of La loading on furfural hydrogenation using wrinkled silica-supported Pd catalysts: Role of partially oxidized Pd and base sites

The catalytic hydrogenation of furfural (FF) to tetrahydrofurfuryl alcohol (THFAL) is complicated by competing hydrogenation reactions involving the carbonyl group and furan ring. To address this challenge, we developed Pd/xLa@wrinkled silica (Pd/xLa@KCC) catalysts exhibiting excellent efficiency via the optimization of La/Si molar ratios, improved Pd dispersion, and stabilization of partially oxidized Pd particles (Pdδ+). These catalysts were designed for the selective hydrogenation of FF to THFAL. Pd/0.2La@KCC, characterized by a high level of oxygen defects, good basicity, and abundant Pdδ+ species, achieved complete conversion and 91.2 % THFAL selectivity at 45 °C within 6 h. Pd/0.2La@KCC exhibited a THFAL formation rate of 100.9 mmol/g/h, 10.8 times higher than that of Pd/0.02La@KCC. Improved Pd dispersion increased the available metallic surface area for H2 dissociations, whereas abundant Lewis-base sites and Pdδ+ species served as adsorption sites for the carbonyl group and furan ring, respectively. The catalyst facilitated the activation and conversion of the furan ring and carbonyl group to THFAL owing to the parallel adsorption geometry of FF. This approach highlights the importance of the surface characteristics of the catalyst for achieving exceptional selectivity and efficiency. Our findings provide valuable insights into the development of advanced catalysts exhibiting excellent selectivity for biomass-derived chemicals.

Dissolution Stability of Graphene-Modified Pt(111)

Pt plays a leading role in electrocatalysis, being the best pure metal catalyst for the oxygen reduction reaction (ORR) taking place at the cathode of the commercial polymer electrolyte membrane fuel cells (PEMFCs). In addition, it is usually used in the cathode to catalyze the hydrogen evolution reaction (HER) in water electrolyzers for hydrogen production. Apart from activity, stability is another critical parameter to consider when evaluating the performance of electrochemical energy conversion devices. Even pure Pt catalysts degrade under real-life conditions due to electro-oxidation and dissolution processes. Fundamental studies with well-defined Pt surfaces are mandatory for understanding these phenomena and developing new electrocatalysts with improved activity and stability.1 Different types of protection on the surface of electrocatalysts, such as carbon layers or ceria surrounding metal nanoparticles,2-4 have been investigated to enhance their stability while maintaining or improving their activity towards a reaction of interest. Here, we propose the modification of Pt(111) single crystal electrode with an adlayer of graphene as a model system to evaluate the protection mechanism of carbon-capped catalysts.In this work, the stability of graphene-modified Pt(111) in 0.1 M HClO4 is studied by on-line inductively coupled plasma mass spectrometry (ICP-MS) measurements using an electrochemical scanning flow cell (SFC). The Pt(111) surface was modified by a fast and easy transfer method of a commercially available chemical vapor deposited (CVD) graphene single-layer. The cyclic voltammetry results are comparable to those previously reported for surfaces prepared by direct CVD on the Pt(111) electrode.5 To test the quality of the graphene adlayer we checked the electrocatalytic activities towards different reactions of interest, such as ORR, HER and hydrogen oxidation reaction (HOR), and compared the results with the previous works.6 The graphene-modified Pt(111) surface presents enhanced stability since no dissolution is detected for a cyclic voltammetry experiment at 10 mV s-1 with an upper potential limit (UPL) of 1.3 V vs. RHE. In contrast, evident Pt dissolution is observed for the unmodified surface. Different accelerated stress test (AST) protocols consisting in groups of 50 cyclic voltammetries at 200 mV s-1 were performed up to three different UPL, namely 1.3 V, 1.5 V and 1.7 V vs. RHE, showing almost no dissolution over the protocol when UPL = 1.3 V, while the integrity of the graphene adlayer was quickly compromised for UPL = 1.7 V as pointed out by the quick increase in Pt dissolution. The degradation of the graphene layer in the different conditions was additionally characterized by Raman spectroscopy and atomic force microscopy (AFM). The results from this work provide useful information about the behavior of carbon capping agents on Pt surfaces, which can help to design new electrocatalytic materials with improved stability. References (1) Fuchs, T.; Briega-Martos, V.; Drnec, J.; Stubb, N.; Martens, I.; Calle-Vallejo, F.; Harrington, D. A.; Cherevko, S.; Magnussen, O. M. Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species. Angew. Chem. Int. Edit. 2023, 62, e202304293.(2) Sgarbi, R.; Doan, H.; Martin, V.; Chatenet, M. Tailoring the Durability of Carbon-Coated Pd Catalysts Towards Hydrogen Oxidation Reaction (HOR) in Alkaline Media. Electrocatalysis 2023, 14 (2), 267-278.(3) Kim, H.; Kwon, G. H.; Han, S. O.; Robertson, A. Platinum Encapsulated within a Bacterial Nanocellulosic-Graphene Nanosandwich as a Durable Thin-Film Fuel Cell Catalyst. ACS Appl. Energy Mater. 2021, 4 (2), 1286-1293.(4) Miller, H. A.; Vizza, F.; Marelli, M.; Zadick, A.; Dubau, L.; Chatenet, M.; Geiger, S.; Cherevko, S.; Doan, H.; Pavlicek, R. K.; et al. Highly active nanostructured palladium-ceria electrocatalysts for the hydrogen oxidation reaction in alkaline medium. Nano Energy 2017, 33, 293-305.(5) Fu, Y. C.; Rudnev, A. V.; Wiberg, G. K. H.; Arenz, M. Single Graphene Layer on Pt(111) Creates Confined Electrochemical Environment via Selective Ion Transport. Angew. Chem. Int. Edit. 2017, 56 (42), 12883-12887.(6) Shih, A. J.; Arulmozhi, N.; Koper, M. T. M. Electrocatalysis under Cover: Enhanced Hydrogen Evolution via Defective Graphene-Covered Pt(111). ACS Catal. 2021, 11 (17), 10892-10901.

Elucidating the hydrogen adsorption mechanism in bare and TMs (Ni, Pd and Cd) decorated Al12P12, B12P12 and C24 nanocages for hydrogen storage via DFT study

This density functional theory investigation aims to assess the hydrogen adsorption potential of bare and metal catalysts (Ni, Pd, and Cd) decorated Al12P12, B12P12, and C24 nanocages for hydrogen storage applications. To find stability of the cages, cohesive energy and Atom-centered Density Matrix Propagation (ADMP) simulation is obtained. The computed adsorption energy (Ead) reveals the physisorption of H2 molecule in all bare cages. Thus, to improve hydrogen adsorption, Ni, Pd and Cd atoms are decorated in bare cages as a catalyst. After H and H2 adsorption in metal decorated cages, the highest Ead obtained is −3.49 and −0.91 eV in Ni decorated Al12P12 cages. The obtained gravimetric densities of ∼6 wt% for all Ni and Pd decorated Al12P12, B12P12, and C24 nanocages meet the Department of Energy (DOE) target. Hence, Ni and Pd catalysts decorated Al12P12, B12P12, and C24 nanocages are proposed as viable materials for hydrogen storage applications.

Mechanocatalytic Hydrogenolysis of the Lignin Model Dimer Benzyl Phenyl Ether over Supported Palladium Catalysts.

This work demonstrates the mechanocatalytic hydrogenolysis of the ether bond in the lignin model compound benzyl phenyl ether (BPE) and hardwood lignin isolated by hydrolysis with supercritical water. Pd catalysts with 4 wt % loading on Al2O3 and SiO2 supports achieve 100% conversion of BPE with a toluene production rate of (2.6-2.9) × 10-5 mol·min-1. The formation of palladium hydrides under H2 gas flow contributes to an increase in the turnover frequency by a factor of up to 300 compared to Ni on silica-alumina. While a near-quantitative toluene yield is obtained, some of the phenolic products remain adsorbed on the catalyst.

Deciphering the catalytic activity of nickel anchored on Fe3O4@SiO2@3-CPMS@L as a magnetically recoverable nanocatalyst for the efficacious reduction of 4-nitrophenol, nitrobenzene, and methyl orange

In this paper, a versatile heterogeneous nanocatalyst was fabricated employing a self-assembly technique. To commence, Fe3O4 MNPs were coated with a thin layer of SiO2 using the stobbers method. Subsequently, the surface was further functionalized with 3-CPMS, followed by a reaction with a Schiff base. Finally, nickel NPs were deposited on the surface through in situ deposition, forming the Fe3O4@SiO2@3-CPMS@L-Ni magnetic nanocatalyst. The architecture of this magnetic nanocatalyst was meticulously characterized through an array of sophisticated techniques: XRD, FT-IR, SEM, TEM, BET and VSM. The XRD diffraction pattern confirmed the presence of Fe3O4 MNPs, SiO2, and Ni peaks, providing evidence for successful synthesis. Moreover, the successful functionalization with a Schiff base was demonstrated by the presence of an azomethane peak in the FTIR spectra of the synthesized nanocatalyst. The fabricated nanocatalyst was adeptly utilized for the reduction of 4-NP, NB, and MO demonstrating a remarkably elevated rate of catalytic efficacy. Moreover, this catalyst was effortlessly retrievable through the application of an external magnet, and it maintained its catalytic prowess across at least six consecutive cycles. The utilization of water as an environmentally friendly solvent, coupled with the utilization of abundant and cost-effective nickel catalyst instead of the costly Pd or Pt catalysts, along with the successful recovery and scalability of the catalyst, render this method highly advantageous from both environmental and economic perspectives for the reduction of 4-NP, NB, and MO.

Palladium-Catalyzed Selective Benzylic C-H Alkylation of Aromatic Sulfonamides with Maleimides.

An efficient method for the selective benzylic C-H alkylation of sulfonamides using maleimides has been developed. The reaction proceeds via the benzylic C(sp3)-H bond activation of sulfonamide in the presence of a Pd(II) catalyst without requiring any oxidant, additive, or external ligand. This methodology is highly compatible with a wide variety of substituted maleimides. A plausible reaction mechanism is also proposed to account for this alkylation reaction.

A General Strategy Based on Hetero-Charge Coupling Effect for Constructing Single-Atom Sites.

Single-atom catalysts have emerged as cutting-edge hotspots in the field of material science owing to their excellent catalytic performance brought about by well-defined metal single-atom sites (M SASs). However, huge challenges still lie in achieving the rational design and precise synthesis of M SASs. Herein, we report a novel synthesis strategy based on the hetero-charge coupling effect (HCCE) to prepare M SASs loaded on N and S co-doped porous carbon (M1/NSC). The proposed strategy was widely applied to prepare 17 types of M1/NSC composed of single or multi-metal with the integrated regulation of the coordination environment and electronic structure, exhibiting good universality and flexible adjustability. Furthermore, this strategy provided a low-cost method of efficiently synthesizing M1/NSC with high yields, that can produce more than 50 g catalyst at one time, which is key to large-scale production. Among various as-prepared unary M1/NSC (M can be Fe, Co, Ni, V, Cr, Mn, Mo, Pd, W, Re, Ir, Pt, or Bi) catalysts, Fe1/NSC delivered excellent performance for electrocatalytic nitrate reduction to NH3 with high NH3 Faradaic efficiency of 86.6 % and high NH3 yield rate of 1.50 mg h-1 mgcat. -1 at -0.6 V vs. RHE. Even using Fe1/NSC as a cathode in a Zn-nitrate battery, it exhibited a high open circuit voltage of 1.756 V and high energy density of 4.42 mW cm-2 with good cycling stability.

Exploring the key role of electronic effects in Pd catalysts supported on Ni-modified N-doped porous carbon for direct synthesis of H2O2 under atmospheric pressure

Developing high-performance catalysts for direct synthesis of hydrogen peroxide (H2O2; DSHP) remains challenging. The paper proposes a strategy to optimize Pd catalysts by enriching Ni-modified N-doped porous carbon. The theoretical studies indicate that Ni modulates the electronic structure of Pd through charge transfer and strain effects, effectively reducing the adsorption energy of reaction intermediates. Furthermore, the Crystal Orbital Hamilton Population (COHP) analysis demonstrated that the energy-integrated COHP (ICOHP) of the OO bonds in O2* (* denotes the adsorbed state), OOH*, and HOOH* were positively correlated with dissociation activation energies and negatively correlated with hydrogenation activation energies. The modulation of the electronic structure of Pd by Ni reduces the hydrogenation activation energies of O2* and OOH* and increases the dissociation activation energies of O2*, OOH*, and HOOH*, thereby enhancing the selectivity and productivity of H2O2. The experimental results aligned with the theoretical predictions, showcasing that the optimized Pd-Ni7.5/NPCs catalyst exhibited remarkable H2O2 selectivity and productivity of 70.5 % and 230.31 mol kgcat-1·h-1, respectively, surpassing the performance of Pd/NPCs (47.1 % and 52.3 mol kgcat-1·h-1). This study offers profound insights into the crucial role of electron effects in DSHP using nickel-modified palladium catalysts, providing strong evidence into the design and optimization of Pd catalysts.

Single-Atom Alloy Formation via Reaction-Driven Catalyst Restructuring.

We demonstrate that single-atom alloy catalysts can be made by exposing physical mixtures of monometallic supported Cu and Pd catalysts to vinyl acetate (VA) synthesis reaction conditions. This reaction induces the formation of mobile clusters of metal diacetate species that drive extensive metal nanoparticle restructuring, leading to atomic dispersion of the precious metal, smaller nanoparticle sizes than the parent catalysts, and high activity and selectivity for both VA synthesis and ethanol dehydrogenation reactions. This approach is scalable and appears to be generalizable to other alloy catalysts.

Room temperature thermocatalytic removal of formaldehyde in air using a copper manganite spinel-supported palladium catalyst with ultralow noble metal content

The room temperature (RT)-catalytic oxidation of gaseous formaldehyde (FA) in the dark is regarded as one of the most viable technical options for its removal from indoor air. Herein, the copper manganite spinel (CuMn2O4) supported palladium (Pd) catalysts are firstly synthesized for the thermocatalytic oxidation of FA in air. In particular, 0.05% Pd-CuMn2O4-R (‘R’ denotes high-temperature reduction pre-treatment using hydrogen) achieves 100% conversion (XFA) of 50 ppm FA at RT (gas hourly space velocity of 4342 h−1). The performance of 0.05% Pd-CuMn2O4-R against FA, if assessed in terms of the reaction kinetic rate (r at 10% XFA), is estimated as 3.01E−02 mmol g−1h−1. The lowering of XFA with the increases in FA concentration, flow rate, and relative humidity indicates the detrimental effects of such variables on the catalytic activity. FA oxidation kinetics predominantly follows the Mars van Krevelen mechanism. In-situ diffuse reflectance infrared Fourier transform spectroscopy shows that FA oxidation proceeds through multiple reaction intermediates (e.g., dioxymethylene and formate). The density functional theory simulations indicate that the catalytic oxidation efficacy of FA over CuMn2O4 increases by the synergy between Pd loading and surface reduction. The present study represents the first report on the synthesis of CuMn2O4-supported Pd catalysts and their application for the RT oxidative removal of FA in air under dark conditions. The findings of the present work offer valuable insights into the construction of efficient and cost-effective catalysts with ultra-low noble metal content for the RT oxidative removal of VOCs in indoor environment.

Constructing Pd and Cu Crowding Single Atoms by Protein Confinement to Promote Sonogashira Reaction.

For multicenter-catalyzed reactions, it is important to accurately construct heterogeneous catalysts containing multiple active centers with high activity and low cost, whichis more challenging compared to homogeneous catalysts because of the low activity and spatial confinement of active centers in the loaded state. Herein, a convenient protein confinement strategy is reported to locate Pd and Cu single atoms in crowding state on carbon coated alumina for promoting Sonogashira reaction, the most powerful method for constructing the acetylenic moiety in molecules. The single-atomic Pd and Cu centers take advantage in not only the maximized atomic utilization for low cost, but also the much-enhanced performance by facilitating the activation of aryl halides and alkynes. Their locally crowded dispersion brings them closer to each other, which facilitates the transmetallation process of acetylide intermediates between them. Thus, the Sonogashira reaction is drove smoothly by the obtained catalyst with a turnover frequency value of 313 h-1, much more efficiently than that by commercial Pd/C and CuI catalyst, conventional Pd and Cu nanocatalysts, and mixed Pd and Cu single-atom catalyst. The obtained catalyst also exhibits the outstanding durability in the recycling test.

Magnetic yolk-shell structured periodic mesoporous organosilica supported palladium as a powerful and highly recoverable nanocatalyst for the reduction of nitrobenzenes

A novel palladium-loaded yolk-shell structured nanomaterial with magnetite core and phenylene-based periodic mesoporous organosilica (PMO) shell (Fe3O4@YS-Ph-PMO/Pd) nanocatalyst was synthesized for the reduction of nitrobenzenes. The Fe3O4@YS-Ph-PMO/Pd was prepared through cetyltrimethylammonium bromide (CTAB) directed condensation of 1,4-bis(triethoxysilyl)benzene (BTEB) around Fe3O4@silica nanoparticles followed by treatment with palladium acetate. This nanocatalyst was characterized by using Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), low-angle and wide-angle powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) analyses. These analyses showed a magnetic nanomaterial with high chemical and thermal stability for the designed composite. The Fe3O4@YS-Ph-PMO/Pd nanocomposite was employed as a powerful and highly recoverable catalyst in the green reduction of nitroarenes in H2O at room temperature. A variety of nitroarene derivatives were applied as substrate in the presence of 0.9 mol% of Fe3O4@YS-Ph-PMO/Pd catalyst. All nitroarenes were selectively converted to their corresponding amines with high to excellent yields (92–96%) within short reaction times (10–18 min). This catalyst was recovered and reused at least 11 times without significant decrease in efficiency and stability.

Significant Promotional Effect of Nb Doping in Bifunctional Pd/WOx Nanocatalysts on One‐Step Benzene Hydroalkylation

AbstractThe development of high‐efficiency heterogeneous bifunctional metal‐acid catalysts with an appropriate metal‐acid balance for the production of cyclohexylbenzene (CHB) via the one‐step tandem benzene hydroalkylation still faces a dilemma because of the difficulty in inhibiting the over hydrogenation of benzene substrate and cyclohexene intermediate and controlling target product selectivity. In this study, Nb‐doped WOx supported Pd nanocatalysts were developed. It has been shown that doping Nb into WOx supports altered surface properties and microstructures of catalysts, resulting in the generation of more surface acidic sites and defective W−Ov−Nb structures (Ov: oxygen vacancies). As‐constructed Pd‐based nanocatalyst with only 0.1 wt % Pd loading and a Nb/(Nb+W) molar ratio of 0.25 exhibited superior catalytic benzene hydroalkylation performance to undoped supported Pd catalyst, with a much higher yield of CHB (35.6 %) at 220 °C. It was authenticated that highly dispersed Pd sites facilitated the dissociation of hydrogen molecules, defective W−Ov−Nb structures were conductive to the surface transfer of active hydrogen species, a large number of acidic sites favored benzene/cyclohexene intermediate adsorption, and abundant Brønsted acidic sites promoted the alkylation between cyclohexene intermediate formed and benzene. Accordingly, excellent cooperative catalysis between Pd sites, acidic sites, and oxygen vacancies contributed to improved catalytic performance of Nb‐doped WOx supported Pd catalysts in benzene hydroalkylation. This research presents an alternative for the creation of low‐cost practical bifunctional metal‐acid catalysts for highly efficient benzene hydroalkylation.

Designing nanoporous Pd catalyst with dual enhancement of thermodynamics and kinetics for formic acid oxidation reaction

Conventional strategies for developing highly efficient electrocatalysts involve enhancing thermodynamics to minimize overpotential and improving kinetics to maximize reaction current. However, current research on Pd-based catalysts for formic acid oxidation reaction (FAOR) mainly focuses on accelerating its kinetics to increase the peak current. This study proposes a catalyst design that can simultaneously improve the kinetics and thermodynamics of FAOR. Specifically, the home-made catalyst (NPG-Pt1-Pd1) is obtained by sequentially depositing monolayer Pt and Pd shells on the surface of nanoporous gold (NPG) substrate. Compared with commercial Pd/C, this electrocatalyst exhibits a noticeable negative shift in onset potential (∼100 mV) and a significantly improved peak current (∼6 times). Experimental data and theoretical calculations demonstrate that NPG-Pt1-Pd1 exhibits a comparatively suitable adsorption energy for small molecules, not only reducing the overpotential but also accelerating the reaction rate of FAOR. Moreover, when applied in direct formic acid fuel cells (DFAFC), the NPG-Pt1-Pd1 anode with an exceptionally low Pd loading of 8.5 μg cm−2, achieves a remarkable power density of 141 mW cm−2. The power efficiency of Pd is 230 times that of Pd/C (1 mg cm−2, 73 mW cm−2). Therefore, this work provides a new methodology for developing efficient Pd-based FAOR electrocatalysts.

Ambient hydrogenation of solid aromatics enabled by a high entropy alloy nanocatalyst

Hydrogenation is a versatile chemical process with significant applications in various industries, including food production, petrochemical refining, pharmaceuticals, and hydrogen carriers/safety. Traditional hydrogenation of aromatics, hindered by the stable π-conjugated phenyl ring structures, typically requires high temperatures and pressures, making ambient hydrogenation a grand challenge. Herein, we introduce a PdPtRuCuNi high entropy alloy (HEA) nanocatalyst, achieving an exceptional 100% hydrogenation of carbon-carbon unsaturated bonds, including alkynyl and phenyl groups, in solid 1,4-bis(phenylethynyl)benzene (DEB) at 25 °C under ≤1 bar H2 and solventless condition. This results in a threefold higher hydrogen uptake for DEB-contained composites compared to conventional Pd catalysts, which can only hydrogenate the alkynyl groups with a ~ 27% conversion of DEB. Our experimental results, complemented by theoretical calculations, reveal that PdPtRu alloy is highly active and crucial in enabling the hydrogenation of phenyl groups, while all five elements work synergistically to regulate the reaction rate. Remarkably, this newly developed catalyst also achieves nearly 100% reactivity for ambient hydrogenation of a broad range of aromatics, suggesting its universal effectiveness. Our research uncovers a novel material platform and catalyst design principle for efficient and general hydrogenation. The multi-element synergy in HEA also promises unique catalytic behaviors beyond hydrogenation applications.

Kinetics of the CH4/O2 reaction over supported palladium catalysts on K-doped manganite perovskite: Impact of potassium on the nature of Pd-support interface and related reaction mechanisms

The kinetics of the CH4/O2 reaction, occurring on post-combustion catalysts to treat the exhaust gases of natural gas-powered engines, has been investigated in lean conditions on model Pd catalysts supported on K-doped LaMnO3. Experimental and predicted reaction rates have been compared according to a single site on Pd active sites and dual site reaction mechanism on active sites located at the Pd-perovskite interface. Comparisons with a benchmark Pd/LaMnO3 emphasized significant changes induced by thermal aging at 750 °C in wet atmosphere (10 vol% H2O in 5 vol% O2). Indeed, the contribution of the single site mechanism grows on aged Pd/LaMnO3 emphasizing a deterioration of the Pd-LaMnO3 interface. The opposite trend is observed on Pd/La1−xKxMnO3 with kinetics obeying to a single site mechanism on pre-reduced catalyst and then shifting to a quasi-exclusive dual site mechanism after aging. Such kinetic features have been discussed with respect to bulk and surface physicochemical characterization pointing out the key role of potassium in the stability of the Pd-support interface.

A green Heck reaction protocol towards trisubstituted alkenes, versatile pharmaceutical intermediates.

The Heck reaction is widely employed to build a variety of biologically relevant scaffolds and has been successfully implemented in the production of active pharmaceutical ingredients (APIs). Typically, the reaction with terminal alkenes gives high yields and stereoselectivity toward the trans-substituted alkenes product, and many green variants of the original protocol have been developed for such substrates. However, these methodologies may not be applied with the same efficiency to reactions with challenging substrates, such as internal olefins, providing trisubstituted alkenes. In the present work, we have implemented a Heck reaction protocol under green conditions to access trisubstituted alkenes as final products or key intermediates of pharmaceutical interest. A set of preliminary experiments performed on a model reaction led to selecting a simple and green setup based on a design of experiments (DoE) study. In such a way, the best experimental conditions (catalyst loading, equivalents of alkene, base and tetraalkylammonium salt, composition, and amount of solvent) have been identified. Then, a second set of experiments were performed, bringing the reaction to completion and considering additional factors. The protocol thus defined involves using EtOH as the solvent, microwave (mw) irradiation to achieve short reaction times, and the supported catalyst Pd EnCat®40, which affords an easier recovery and reuse. These conditions were tested on different aryl bromides and internal olefines to evaluate the substrate scope. Furthermore, with the aim to limit as much as possible the production of waste, a simple isomerization procedure was developed to convert the isomeric byproducts into the desired conjugated E alkene, which is also the thermodynamically favoured product. The approach herein disclosed represents a green, efficient, and easy-to-use handle towards different trisubstituted alkenes via the Heck reaction.

H2-D2 Exchange Activity and Electronic Structure of Ag x Pd1-x Alloy Catalysts Spanning Composition Space.

Many computational studies of catalytic surface reaction kinetics have demonstrated the existence of linear scaling relationships between physical descriptors of catalysts and reaction barriers on their surfaces. In this work, the relationship between catalyst activity, electronic structure, and alloy composition was investigated experimentally using a Ag x Pd1-x Composition Spread Alloy Film (CSAF) and a multichannel reactor array that allows measurement of steady-state reaction kinetics at 100 alloy compositions simultaneously. Steady-state H2-D2 exchange kinetics were measured at atmospheric pressure on Ag x Pd1-x catalysts over a temperature range of 333-593 K and a range of inlet H2 and D2 partial pressures. X-ray photoelectron spectroscopy (XPS) was used to characterize the CSAF by determining the local surface compositions and the valence band electronic structure at each composition. The valence band photoemission spectra showed that the average energy of the valence band, ε̅v, shifts linearly with composition from -6.2 eV for pure Ag to -3.4 eV for pure Pd. At all reaction conditions, the H2-D2 exchange activity was found to be highest on pure Pd and gradually decreased as the alloy was diluted with Ag until no activity was observed for compositions with x Pd < 0.58. Measured H2-D2 exchange rates across the CSAF were fit using the Dual Subsurface Hydrogen (2H') mechanism to extract estimates for the activation energy barriers to dissociative adsorption, ΔE ads ‡, associative desorption, ΔE des ‡, and the surface-to-subsurface diffusion energy, ΔE ss, as a function of alloy composition, x Pd. The 2H' mechanism predicts ΔE ads ‡ = 0-10 kJ/mol, ΔE des ‡ = 30-65 kJ/mol, and ΔE ss = 20-30 kJ/mol for all alloy compositions with x Pd ≥ 0.64, including for the pure Pd catalyst (i.e., x Pd = 1). For these Pd-rich catalysts, ΔE des ‡ and ΔE ss appeared to increase by ∼5 kJ/mol with decreasing x Pd. However, due to the coupling of kinetic parameters in the 2H' mechanism, we are unable to exclude the possibility that the kinetic parameters predicted when x Pd ≥ 0.64 are identical to those predicted for pure Pd. This suggests that H2-D2 exchange occurs only on bulk-like Pd domains, presumably due to the strong interactions between H2 and Pd. In this case, the decrease in catalytic activity with decreasing x Pd can be explained by a reduction in the availability of surface Pd at high Ag compositions.

Deconvoluting Substrates, Support, and Temperature Effects on Leaching and Deactivation of Pd Catalysts: An In Situ Study in Flow.

Leaching behavior of three different Pd heterogeneous catalysts (PdEnCat 30, FibreCat FC1001, and Pd/Al2O3) during the Heck reaction of iodobenzene and methyl acrylate, in the presence of triethylamine, was compared using a tandem flow reactor. While leaching was observed in all three cases, Pd/Al2O3 appeared to be the most robust, showing little/no leaching at ambient temperature. The leached Pd species also appear to display different catalytic activities. With a slight modification of the reactor, the leaching caused by individual components of the reaction mixture can be assessed separately. For the polymer-supported catalysts, triethylamine caused the largest amount of leaching, even at 30 °C. In contrast, the leaching from Pd/Al2O3 was observed only in the presence of iodobenzene at 90 °C. Variations in leaching behavior were ascribed to differences in Pd species and immobilization methods.