Pd Species Research Articles

Pd-Promoted Bi2Zr2O7 Pyrochlore for Soot Particulate combustion: the Impact of Pd Distributing Location and Valence State on the Reactivity

In this study, Pd has been adopted to modify Bi2Zr2O7 pyrochlore on the surface (Pd/Bi2Zr2O7) or in the bulk (Pd-Bi2Zr2O7) by using different methods. Compared with the unmodified Bi2Zr2O7, the addition of Pd significantly improves the reactivity, following the sequence of Bi2Zr2O7 < Pd-Bi2Zr2O7 < Pd/Bi2Zr2O7. For Pd/Bi2Zr2O7, both PdO and metallic Pd0 are formed and located mainly on its surface, which is favorable for the activity. In contrast, for Pd-Bi2Zr2O7, the Pd species are majorly doped into the pyrochlore matrix at B-site as Pd2+ and Pd4+, which in fact restricts the activity. Furthermore, Pd modification can promote the generation of active O2− and O22- species, especially when Pd locates on the surface. As discovered, the distribution locations and valence states of Pd, together with the number of reactive surface oxygen sites, determines the activity.

Radical-Dearomative Generation of Cyclohexadienyl Pd(II) toward the 3D Transformation of Nonactivated Phenyl Rings.

Traditional palladium-catalyzed dearomatization of (hetero)arenes takes place via an ionic pathway and usually requires elevated temperatures to overcome the energy barrier of the dearomative insertion step. Herein, a combination of the radical and two-electron pathways is disclosed, which enables room temperature dearomative 3D transformations of nonactivated phenyl rings with Pd(0) as the catalyst. Experimental results together with density functional theory (DFT) calculations indicate a versatile π-allyl Pd(II) species, cyclohexadienyl Pd(II), possibly is involved in the dearomatization. This species is generated by combining the cyclohexadienyl radical and Pd(I). The cyclohexadienyl Pd(II) provides chemoselective (carboamination and trieneylation), regioselective (1,2-carboamination), and diastereoselective (carbonyl-group directed face selectivity) conversions.

Suzuki-Miyaura Cross-Coupling Reaction Using Palladium Catalysts Supported on Phosphine Periodic Mesoporous Organosilica.

Phosphine periodic mesoporous organosilicas (R-P-PMO-TMS: R=Ph, tBu), which possess electron-donating alkyl substituents on the phosphorus atom, were synthesized using bifunctional compounds with alkoxysilyl- and phosphino groups, bis[3-(triethoxysilyl)propyl]phenylphosphine borane (1 a) and bis[3-(triethoxysilyl)propyl]-tert-butylphosphine borane (1 b). Immobilization of Pd(0) species was performed to give R-P-Pd-PMO-TMS: R=Ph (2 a), tBu (3 a), respectively. The Pd(0) immobilized 2 a and 3 a were applicable as catalysts for Suzuki-Miyaura cross-coupling reactions of aryl chlorides with phenylboronic acid. It was revealed that 3 a bearing more electron-donating tBu groups exhibited higher catalytic activity. Various functional groups including both electron withdrawing and donating substituents were compatible in the system. The recyclability of 3 a was examined to support its moderate utility for the recycle use.

DAPD Set of Pd-Containing Diatomic Molecules: Accurate Molecular Properties and the Great Lengths to Obtain Them.

In the present study, we obtained reliable bond energy, bond length, and zero-point vibrational frequency for a set of diatomic Pd species (the DAPD set). It includes PdH, Pd2, and PdX (X = B, C, N, O, F, Al, Si, P, S, and Cl). Our highest-level protocol (W4X-L) represents scalar and spin-orbit relativistic, valence- and inner-valence correlated, extrapolated CCSDTQ(5) energy. The DAPD set of molecules is challenging for computational chemistry methods in different manners; for Pd2, the spin-orbit contribution to the bond energy is fairly large, whereas for PdC and PdSi, the post-CCSD(T) correlation components are considerable. The diverse range of requirements represents a significant challenge for lower-level methods. While density functional theory (DFT) methods generally yield good agreements for bond lengths and vibrational frequencies, large deviations are found for bond energies. In general, hybrid DFT methods are more accurate than nonhybrid functionals, but the agreement in individual cases varies. This illustrates the critical role that new high-quality reference data would play in the continual development of lower-cost methods.

Photocatalytic production of ethylene and propionic acid from plastic waste by titania-supported atomically dispersed Pd species

Current chemical recycling of bulk synthetic plastic, polyethylene (PE), operates at high temperature/pressure and yields a complex mixture of products. PE conversion under mild conditions and with good selectivity toward value-added chemicals remains a practical challenge. Here, we demonstrate an atomic engineering strategy to modify a TiO 2 photocatalyst with reversible Pd species for the selective conversion of PE to ethylene (C 2 H 4 ) and propionic acid via dicarboxylic acid intermediates under moderate conditions. TiO 2 -supported atomically dispersed Pd species exhibits C 2 H 4 evolution of 531.2 μmol g cat −1 hour −1 , 408 times that of pristine TiO 2 . The liquid product is a valuable chemical propanoic acid with 98.8% selectivity. Plastic conversion with a C 2 hydrocarbon yield of 0.9% and a propionic acid yield of 6.3% was achieved in oxidation coupled with 3 hours of photoreaction. In situ spectroscopic studies confirm a dual role of atomic Pd species: an electron acceptor to boost charge separation/transfer for efficient photoredox, and a mediator to stabilize reaction intermediates for selective decarboxylation.

Mechanistic Insight into Palladium‐Catalyzed Asymmetric Alkylation of Indoles with Diazoesters Employing Bipyridine‐N,N’‐dioxides as Chiral Controllers

AbstractMetal‐catalyzed asymmetric alkylation of indoles with α‐diazoesters is well‐known, however, the underlying mechanisms of this reaction, particularly the origin of stereoselectivity, remain uncertain. For the Pd catalysis, we address this cutting‐edge challenge from two complementary viewpoints – i) the molecular level regarding a single catalytically active Pd center; and ii) nano‐level Pd species investigating the factors favoring the appearance of the preferred catalytic centers. The formation of the active catalytic species was monitored by structural methods (NMR and ESI‐MS), and metal particles were characterized with electron microscopy (SEM, EDX). On the molecular level, chiral bipyridine‐N,N’‐dioxides proved to be competent chiral controllers. The kinetic and DFT computational data revealed a crucial role of water in the rate and selectivity determining steps and showed that the enantioselectivity of the process is controlled by the protodepalladation step. On the nano‐scale, the important effect of catalyst precursor on the overall reaction performance was shown.

Valorization of olive stones for the synthesis of phosphorus-modified porous carbons and their use as support of Pd particles for the oxidative condensation of furfural with ethanol

Activated carbons have been synthesized by a pyrolytic treatment and a chemical activation with H3PO4 to give rise to amorphous graphite, with high micro-mesoporosity and Brönsted acid sites. These materials have been employed as supports to incorporate Pd species to be used as catalysts in the oxidative condensation of furfural with ethanol to obtain furan-2-acrolein (F2A), as main product. The catalytic results show that F2A is obtained at shorter reaction times, with a maximum yield of 45 % after 180 min at 150 °C. The catalytic studies at longer reaction time worsen the F2A yield due to undesired and uncontrolled condensation reactions, which lead to the formation of humins. On the other hand, it is demonstrated that the amount of oxidant (H2O2) and base (Na2CO3) must be modulated to attain an optimum F2A yield.

Effects of dealumination on the methane-combustion activity of Pd/SSZ-13 catalysts

This study investigated the effects of dealumination on Pd/SSZ-13 catalysts for methane combustion. Pd(1)/SSZ-13 (HTA), prepared by treating the catalyst hydrothermally at 750 °C, displayed extremely low catalytic activity. In contrast, Pd(1)/SSZ-13 (DeAl), prepared by treating the SSZ-13 support hydrothermally followed by Pd impregnation, exhibited superior catalytic activity for methane oxidation. Catalytic properties, including zeolite hydrophobicity and Pd location and states, were investigated by using various characterization techniques to clarify the differences between the catalysts. Most Pd species in the Pd/SSZ-13 (HTA) catalyst existed as Pd ions, which were not very active for the reaction. On the contrary, most Pd species in the Pd(1)/SSZ-13 (DeAl) catalyst existed as PdO nanoparticles at the external surface, which were highly active for methane oxidation. The study results suggested that the amount of external PdO nanoparticles should be maximized in order to make Pd/SSZ-13 catalyst highly active for methane combustion, which was achieved by dealuminating the SSZ-13 support before Pd impregnation.

One stone, three birds strategy for synthesis of N-doped activated carbon-supported surface-enriched and redispersed Pd NPs via plasma for formic acid dehydrogenation

Rational design of supported Pd catalysts with excellent performance for H2 production via inexpensive and sustainable formic acid (FA) dehydrogenation remains challenging. N-doped activated carbon (AC)-supported surface-enriched and redispersed Pd nanoparticles (Pd/ACCP (250 W, 10 min)) with enhanced performance for FA dehydrogenation were prepared via plasma-assisted thermal reduction (input power: 250 W, discharge time: 10 min). The TOFinitial (initial turnover frequency) of Pd/ACCP (250 W, 10 min) reached 1276 h−1, and gas generation over it for the first and third reaction cycles was 1.11 and 1.52 times higher, respectively, than that of Pd/ACC and 1.25 and 12.40 times higher, respectively, than that of Pd/AC (Sigma–Aldrich). Characterization results indicated that Pd/ACCP (250 W, 10 min) had surface-enriched and redispersed Pd species, owing to the plasma etching and strong electric field, large specific surface area, small Pd particle size, and high Pd/C atomic ratio. Enhanced N doping of AC was achieved via plasma treatment, and N mainly existed as highly active pyridine nitrogen. The plasma-based strategy yielded Pd/ACCP (250 W, 10 min) with excellent catalytic performance for FA dehydrogenation. This study provides insights into plasma-controllable synthesis of supported metal catalysts and delivers a new rational design concept of FA dehydrogenation catalysts.

Modification of Pillared Intercalated Montmorillonite Clay as Heterogeneous Pd Catalyst Supports.

Montmorillonite clay was modified by pillaring with AlMn oxides in different Al/Mn ratios and intercalation of two kinds of N-containing polymers (i.e., chitosan (CS) and polyvinyl pyrrolidinone (PVP)) chains. The modified pillared montmorillonite clay (PM) showed a rich two-dimensional layered porous structure with tunable parameters, such as large interlayer spacing, high specific area, and large porous volume. They were then used as supports for Pd nanoparticles. As applied in coupling reactions of aryl halides with terminal alkynes, Pd@CS/AlMn-PM showed better comprehensive catalytic performance than Pd@PVP/AlMn-PM. This was mainly attributed to its higher specific area, stronger chelation to Pd species, and better solvent resistance.

Na Cocations and Hydrothermal Aging Cooperatively Boost the Regeneration of Phosphorus-Poisoned Pd/SSZ-13 for Passive NOx Adsorption.

Pd/SSZ-13 has been proposed as a passive NOx adsorber (PNA) for low-temperature NOx adsorption. However, it remains challenging for Pd/SSZ-13 to work efficiently when suffering from phosphorus poisoning. Herein, we report a simple and efficient strategy to regenerate the phosphorus-poisoned Pd/SSZ-13 based on the cooperation between hydrothermal aging treatment and Na cocations. It was found that hydrothermal aging treatment enabled the redispersion of Pd and P-containing species in phosphorus-poisoned Pd/SSZ-13. Meanwhile, the presence of Na cocations significantly reduced the formation of AlPO4 and retained more paired Al sites for highly dispersed Pd2+ ions, which was of great importance for the recovery of adsorption performance. To our satisfaction, the restoration ratio of the adsorption capacity of poisoned Pd/SSZ-13 was >90% after regeneration. Strikingly, the NOx adsorption activities of phosphorus-poisoned Pd/SSZ-13 with phosphorus loadings of 0.2 and 0.4 mmol g-1 almost completely recovered upon regeneration. This study demonstrates the promoting effect of Na cocations on the regeneration of phosphorus-poisoned Pd/SSZ-13 by hydrothermal aging treatment, which provides useful guidance for the design of PNA materials with excellent durability for cold-start application.

In Situ Synthesis of Encapsulated Pd@silicalite-2 for Highly Stable Methane Catalytic Combustion.

Methane emissions from vehicles have made a significant contribution to the greenhouse effect, primarily due to its high global warming potential. Supported noble metal catalysts are widely employed in catalytic combustion of methane in vehicles, but they still face challenges such as inadequate low-temperature activity and deactivation due to sintering under harsh operating conditions. In the present work, a series of encapsulated structured catalysts with palladium nanoparticles confined in hydrophobic silicalite-2 were prepared by an in situ synthesis method. Based on various characterization methods, including XRD, HR-TEM, XPS, H2-TPR, O2-TPD, H2O-TPD, CH4-TPR, Raman, and in situ DRIFTS-MS, it was confirmed that PdOx nanoparticles were mainly encapsulated inside the silicalite-2 zeolite, which further maintained the stability of the nanoparticles under harsh conditions. Specifically, the 3Pd@S-2 sample exhibited high catalytic activity for methane oxidation even after harsh hydrothermal aging at 750 °C for 16 h and maintained long-term stability at 400 °C for 130 h during wet methane combustion. In situ Raman spectroscopy has confirmed that PdOx species act as active species for methane oxidation. During this reaction, methane reacts with PdOx to produce CO2 and H2O, while simultaneously reducing PdOx to metallic Pd species, which is further reoxidized by oxygen to replenish the PdOx catalyst.

Chitosan-Pd0 nanoparticles encapsulated in Al, Co-pillared montmorillonite by one-pot process

Chitosan-Pd0 (CS-Pd0) nano particles encapsulated in Al, Co-pillared montmorillonite (MMt) nanocomposites were synthesized by a facile one-pot heat treatment method. The MMt was impregnated with Al, Co pillaring reagent, protonated chitosan solution, and Na2PdCl4 solution, followed with a controlled low-temperature (200 °C) heat treatment, resulting in Pd@CS/Al, Co-MMt nanocomposites. Al, Co pillaring effectively expanded the basal spacing of MMt layers from 1.24 nm to 1.82 nm, and generated numerous mesopores in interlayer space, resulting in increase of surface area from 11.8 m2/g to 198.3 m2/g. Due to the well encapsulation of CS chains in the interlayer nano-spaces, the specific surface area of the hybrid nanocomposite underwent a reasonably decrease as the CS content increase. Most of nano-sized Pd species (2–3 nm) chelated with CS chains dispersed well in the interlayer space of the porous Al, Co pillared MMt matrix. The resultant nanocomposite prepared by such facial one-pot process method have similar high comprehensive catalytic performances as applied in coupling reactions to that prepared by regular divided-multistep method. Moreover, it is confirmed that Al pillaring doped with Co and encapsulation of CS chains have synergistic improving effects on the comprehensive catalytic performance of the nanocomposites.

Highly effective photoativation Pd catalyst for Suzuki coupling reaction

Suzuki coupling reaction is widely applied for constructing biaryl skeletons in various fields. Designing and developing highly effective catalysts are essential regarding the Suzuki coupling reaction. Here, we report a simple light irradiation approach to achieve the highly efficient Suzuki coupling reaction via the application of low valence Pd homogeneous catalyst. The catalyst exhibited a TOF value of 76,000 h−1. We systematically investigated the impact of various experimental conditions on catalytic performance and further clarify the evolution of Pd species during the light irradiation and catalytic reaction process. This work provides a facile strategy for designing highly effective catalysts for the Suzuki coupling reaction and understanding the detailed evolution process of Pd during the reaction process.

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes.

The selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)-based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization-reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd-C4 structure and, meanwhile, facilitate the desorption of alkenes by forming the Si-O-C structure with zeolite framework, as validated by the state-of-the-art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H2 ), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite-encapsulated carbon-free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite-confined Pd-carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes

AbstractThe selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)‐based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization‐reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd−C4 structure and, meanwhile, facilitate the desorption of alkenes by forming the Si−O−C structure with zeolite framework, as validated by the state‐of‐the‐art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H2), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite‐encapsulated carbon‐free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite‐confined Pd‐carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.

Atomically Precise Pd Species Accelerating CO2 Hydrodeoxygenation into CH4 with 100% Selectivity

Atomically Precise Pd Species Accelerating CO₂ Hydrodeoxygenation into CH₄ with 100% Selectivity

Atroposelective NiII -Catalyzed Cross-Coupling Reactions Enable a Deeper Understanding of Negishi Couplings: Isolation and Application of Solid Aryl Higher-Order Zincates.

The Negishi cross-coupling reactions involves the application of organozinc reagents and is a highly versatile reaction in synthetic organic chemistry. The transmetallation step plays a pivotal role in the mechanism of these types of cross-coupling reactions. In this study, mechanistic investigations are presented indicating that higher-order zincates are the transmetallating active species in Pd- and Ni-catalyzed Negishi cross-coupling reactions. These findings are supported by halide salt addition experiments and by obtaining a single X-ray crystal structure of the solid monoaryl higher-order zincate [1-NaphthylZnX3 ]2- Mg(THF)2 2+ . The procedure developed in this work was further applied to the synthesis of various monoaryl higher-order zincates, after which their synthetic usefulness in terms of high reactivity towards transmetallation in Negishi cross-couplings, as well as stability, was exemplified in several reactions.

Kinetics of methane combustion on model supported Pd/LaxMnO3 natural gas vehicle catalysts: Sensitivity of La-stoichiometry on the catalytic properties of Pd

The kinetics of the catalytic CH4/O2 reaction has been studied on 1 wt% Pd/LaxMnO3 with x = 0.7, 1.0 and 1.3. Steady-state kinetic measurements have been performed in lean conditions at 400 °C on pre-reduced and on aged catalysts after exposure to wet atmosphere at 750 °C. A single site reaction mechanism occurs preferentially on Pd/La1.3MnO3 exhibiting a La-rich surface. In contrast a dual-site reaction mechanism, combining Pd and surface reactive oxygen species from the support, is preferentially involved on Pd-La0.7MnO3 characterized by a Mn-rich surface. Pd/LaMnO3 exhibits a mixed regime but subsequent deterioration of the Pd-LaxMnO3 interface during thermal aging leads to increasing contribution of the single site reaction mechanism. High surface Pd/Mn ratio and high concentration of Pdn+ with n > 2 can be considered as good descriptors to probe the efficacy of Pd/LaxMnO3 catalysts. Remarkably, the high thermal stability of the Pd-La0.7MnO3 interface, preserving the Pd dispersion on Pd/La0.7MnO3, leads to lower sensitivity to deactivation.

Catalytic removal of harmful volatile organic compounds by reutilizing zinc rods waste from spent batteries as a palladium catalyst support

The emission of volatile organic compounds (VOCs) has led to significant deterioration in air quality, making it imperative to ensure that these compounds are removed from emission sources before they are released into the atmosphere. In this context, the present study recycled spent primary batteries to use their zinc rods waste (ZRW) as a palladium catalyst support for the removal of harmful VOCs. To this end, palladium supported on ZRW (Pd/ZRW) catalysts were prepared and tested for the catalytic oxidation of benzene, methylbenzene and 1,2-dimethylbenzene. The physicochemical properties of the Pd/ZRW catalysts were carefully characterized by ICP-OES, BET, SEM, XRD, FE-TEM, XPS, and H2-TPR analyses. The main component of ZRW was identified as ZnO. Consistent with expectations, increases in the loading of Pd from 0.1 to 1.0 wt% in the Pd/ZRW catalysts resulted in enhanced VOCs removal efficiency. The reaction temperature required for the complete oxidation (100% removal efficiency) of methylbenzene and 1,2-dimethylbenzene on the 1.0 wt% Pd/ZRW catalyst was below 340 °C at a gas hourly space velocity of 50,000 h−1. TEM, XPS, and H2-TPR results implied that the enhancement of catalytic activity with the addition of Pd could be attributed to the readily movable surface lattice oxygen as well as the active component (Pd species). Ultimately, ZRW of spent primary batteries appear to show promise as a catalyst support for VOCs removal. This study has introduced a novel strategy for reducing air pollutants by utilizing waste, which promotes the disposal of hazardous solid waste and ensures clean air quality.