Oxidation State Of Palladium Research Articles

High-Temperature Behavior of Pd/MgO Catalysts Prepared via Various Sol-Gel Approaches.

A series of Pd/MgO catalysts based on nanocrystalline MgO were prepared via different sol-gel approaches. In the first two cases, palladium was introduced during the gel preparation, followed by drying it in supercritical or ambient conditions. In the third case, aerogel-prepared MgO was impregnated with an ethanol solution of Pd(NO3)2. The prepared catalysts differ in particle size and oxidation state of palladium. The catalytic performance and thermal stability of the samples were examined in a model reaction of CO oxidation at prompt thermal aging conditions. The as-prepared and aged materials were characterized by low-temperature nitrogen adsorption, UV-vis spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and ethane hydrogenolysis testing reaction. The highest initial activity (T50 = 103 °C) was demonstrated by the impregnated sample, containing Pd0 particles of 3 nm in size. The lowest T50 value (215 °C) after aging at 1000 °C was demonstrated by the impregnated Pd/MgO-WI sample. The high-temperature behavior of the catalysts was found to be affected by the initial oxidation state and dispersion of Pd. Two deactivation mechanisms, such as the agglomeration of Pd particles and migration of small Pd species into the bulk of the MgO support with the formation of Pd-MgO solid solutions, were discussed.

Deciphering the influence of PdII and PdIV oxidation states on non-standard chemical bonds within bis(jN-heterocyclic carbene) complexes: insights from DFT.

Bis-N-heterocyclic carbene ligands (bis(NHC)) have introduced a new approach to designing homogeneous and heterogeneous catalysts, demonstrating the versatility of ligand concepts in catalysis. This study presents a computational analysis of palladium (+ii and +iv) complexes containing either a normally (bis(nNHC)) or an abnormally (bis(aNHC)) bound CH2-bridged bis-N-heterocyclic carbene ligand; in addition, ancillary ligands are permuted from chlorides (X = Cl) to bromides (X = Br). Density functional theory at the B3PW91/6-31G(d)/Lanl2DZ level in the gas phase was used to investigate the electronic structure and bonding properties of bis(NHC)PdIIX2 and bis(NHC)PdIVX4 for bis(NHC) palladium(ii) dihalide and palladium(iv) tetrachloride complexes, respectively. Results indicate that all of the palladium complex structures prefer a flexible boat-type conformation with an average C 2v symmetry, according to bond property (Ccarbene-Pd and Pd-Cl[Br]) analysis. The strength of these bonds depends on coordinating halide ions (Cl- and Br-), the type of ligand (bis(nNHC) and bis(aNHC)), and the palladium oxidation state (+ii and +iv). Analysis of thermodynamic parameters (ΔH 0, ΔG 0, and ΔE bind) shows an increase in values from an abnormal to normal chelating mode in tetrahalides, whereas the opposite is observed for dihalide complexes. The lower π-backbonding ability of the metal, which is influenced by the quantity and size of halide ions involved, could be one possible explanation for this deficiency.

Efficient aqueous media Suzuki–Miyaura cross-coupling reaction using ligand-free and environmentally-friendly palladium-doped ceria–zirconia mixed metal oxide nanocatalyst

A palladium ceria–zirconia Pd0.05Ce0.5Zr0.475O2 catalytic based nanomaterial was produced by co-precipitation method. To compare the effect of each atom, Pd0.1Ce1.45O3 and Pd0.1Zr1.45O3 were synthesized. Several techniques were used (TGA, XRD, N2 adsorption/desorption (BET theory), XPS, SEM and TEM and HR-TEM microscopy) to characterize the samples. Pd0.05Ce0.5Zr0.475O2 exhibits the highest BET surface area, 117 m2/g compared to the two mentioned materials. According to XRD and XPS analysis, the oxidation state of palladium was +2 in this oxide. Compared to other materials, Pd0.05Ce0.5Zr0.475O2 was the most efficient catalyst for a wide range of aryl bromides and aryl boronic acids in aqueous media without the use of any ligand. Very good yields were achieved and could easily reach a value of 93% in the case of iodobromides. Pd0.05Ce0.5Zr0.475O2 showed also a good stability and could be easily separated and reused with high catalytic activity after three cycles. In addition, no leaching of Pd2+ was observed which confirmed the highly heterogeneous nature of our catalyst.

Low-resistivity Pd nanopatterns created by a direct electron beam irradiation process free of post-treatment steps

The ability to create metallic patterned nanostructures with excellent control of size, shape and spatial orientation is of utmost importance in the construction of next-generation electronic and optical devices as well as in other applications such as (bio)sensors, reactive surfaces for catalysis, etc. Moreover, development of simple, rapid and low-cost fabrication processes of metallic patterned nanostructures is a challenging issue for the incorporation of such devices in real market applications. In this contribution, a direct-write method that results in highly conducting palladium-based nanopatterned structures without the need of applying subsequent curing processes is presented. Spin-coated films of palladium acetate were irradiated with an electron beam to produce palladium nanodeposits (PdNDs) with controlled size, shape and height. The use of different electron doses was investigated and its influence on the PdNDs features determined, namely: (1) thickness of the deposits, (2) atomic percentage of palladium content, (3) oxidation state of palladium in the deposit, (4) morphology of the sample and grain size of the Pd nanocrystals and (5) resistivity. It has been probed that the use of high electron doses, 30000 μC cm−2 results in the lowest resistivity reported to date for PdNDs, namely 145 μΩ cm, which is only one order of magnitude higher than bulk palladium. This result paves the way for development of simplified lithography processes of nanostructured deposits avoiding subsequent post-treatment steps.

Cover Picture: Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2 Catalysts (Angew. Chem. Int. Ed. 23/2022)

Operando spectroscopy captures the dynamic behavior of ceria-supported Pd single atoms, clusters, and nanoparticles during CO oxidation. In their Research Article (e202200434), Emiel J. M. Hensen et al. elucidate the catalytic role of different Pd species using a combination of in situ spectroscopy with transient and steady-state kinetic analysis. Highly dispersed Pd–oxo species and interfacial Pd sites catalyze low-temperature CO oxidation, while metallic Pd contributes to the activity at elevated temperature.

Titelbild: Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2 Catalysts (Angew. Chem. 23/2022)

Operando-Spektroskopie wurde genutzt, um das dynamische Verhalten von Pd-Einzelatomen, Pd-Clustern und Pd-Nanopartikeln auf Ceroxidträgern während der CO-Oxidation zu erfassen. In ihrem Forschungsartikel (e202200434) identifizieren Emiel J. M. Hensen et al. die katalytische Rolle verschiedener Pd-Spezies durch eine Kombination von In-situ-Spektroskopie mit transienter und stationärer kinetischer Analyse. Hochdisperse Pd-Oxo-Spezies und Pd-Grenzflächen katalysieren die CO-Oxidation bei niedrigen Temperaturen, während metallisches Pd zur Aktivität bei höheren Temperaturen beiträgt.

Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2 Catalysts

AbstractAiming at knowledge‐driven design of novel metal–ceria catalysts for automotive exhaust abatement, current efforts mostly pertain to the synthesis and understanding of well‐defined systems. In contrast, technical catalysts are often heterogeneous in their metal speciation. Here, we unveiled rich structural dynamics of a conventional impregnated Pd/CeO2 catalyst during CO oxidation. In situ X‐ray photoelectron spectroscopy and operando X‐ray absorption spectroscopy revealed the presence of metallic and oxidic Pd states during the reaction. Using transient operando infrared spectroscopy, we probed the nature and reactivity of the surface intermediates involved in CO oxidation. We found that while low‐temperature activity is associated with sub‐oxidized and interfacial Pd sites, the reaction at elevated temperatures involves metallic Pd. These results highlight the utility of the multi‐technique operando approach for establishing structure–activity relationships of technical catalysts.

Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2 Catalysts.

Aiming at knowledge‐driven design of novel metal–ceria catalysts for automotive exhaust abatement, current efforts mostly pertain to the synthesis and understanding of well‐defined systems. In contrast, technical catalysts are often heterogeneous in their metal speciation. Here, we unveiled rich structural dynamics of a conventional impregnated Pd/CeO2 catalyst during CO oxidation. In situ X‐ray photoelectron spectroscopy and operando X‐ray absorption spectroscopy revealed the presence of metallic and oxidic Pd states during the reaction. Using transient operando infrared spectroscopy, we probed the nature and reactivity of the surface intermediates involved in CO oxidation. We found that while low‐temperature activity is associated with sub‐oxidized and interfacial Pd sites, the reaction at elevated temperatures involves metallic Pd. These results highlight the utility of the multi‐technique operando approach for establishing structure–activity relationships of technical catalysts.

Flavin Metallaphotoredox Catalysis: Synergistic Synthesis in Water

Combining a transition metal with a photocatalyst can drive modern synthetic chemistry. For transformations performed in water, this concept has been largely unexplored. We report the successful merger of a biocompatible flavin photocatalyst with a palladium catalyst to build isotopically enriched peptidomimetics, to mediate conjugate addition and C-H functionalization reactions, and to assemble unprotected proteinogenic and nonproteinogenic peptides, in water. We detail the important role of the ligand and the palladium oxidation state for controlling product selectivity when constructing synthetic peptides.

Defining the Qualities of High-Quality Palladium on Carbon Catalysts for Hydrogenolysis.

Palladium-catalyzed hydrogenolysis is often the final step in challenging natural product total syntheses and a key step in industrial processes producing fine chemicals. Here, we demonstrate that there is wide variability in the efficiency of commercial sources of palladium on carbon (Pd/C) resulting in significant differences in selectivity, reaction times, and yields. We identified the physicochemical properties of efficient catalysts for hydrogenolysis: (1) small Pd/PdO particle size (2) homogeneous distribution of Pd/PdO on the carbon support, and (3) palladium oxidation state are good predictors of catalytic efficiency. Now chemists can identify and predict a catalyst’s efficiency prior to the use of valuable synthetic material and time.

Ce and Ca/Nb doped Pd-mesocellular foam catalysts for gas-phase conversion of acetone to methyl isobutyl ketone

Herein, we report a study on the design of new palladium containing catalysts based on Ce- and Ca–Nb-mesocellular foam (MCF) supports for gas-phase conversion of acetone to methyl isobutyl ketone (MIBK). A comparison to other catalyst materials already described in the literature shows that the role of material structure (mesostructured cellular foams, hydroxyapatite or mesoporous aluminophosphate), acid-base character of the support, state of palladium species and the support metal dopants play important roles for the catalytic performance in the conversion of acetone to MIBK. Thus, the new catalytic system Pd/Ce-MCF with 11% of conversion and 94% of selectivity to MIBK (250 °C) is demonstrated to be more attractive in term of stability for the reaction over time than previously described in literature for Pd/hydroxyapatite. • Palladium was impregnated on cerium and calcium-niobium doped MCF support. • The oxidation state of palladium was influenced by the MCF support composition. • Both the state of palladium and sample basicity had impact on catalytic performance. • Pd/Ce-MCF demonstrated superior catalyst stability over prolonged reaction time.

Inherent Selectivity of Pd C-H Activation from Different Metal Oxidation States.

In investigating potential control factors that would permit a palladium-catalyzed benzylic vs arene C-H activation as previously reported by our group, it was discovered that the oxidation state of the homogenous palladium species influences the selectivity of C-H activation. DFT calculations show that Pd0 and PdI preferentially activate the sp3 C-H bond in toluene, whereas PdII and PdIII preferentially activate the sp2 C-H bond. This selectivity appears to originate from the steric environment created by the ligand framework on the palladium. As the palladium oxidation state increases, the number of ligand sites increases, which decreases the energetic favorability for activation of the weaker, yet more hindered sp3 C-H bond.

Investigation of the Deactivation and Reactivation Mechanism of a Heterogeneous Palladium(II) Catalyst in the Cycloisomerization of Acetylenic Acids by In Situ XAS .

A well-studied heterogeneous palladium(II) catalyst used for the cycloisomerization of acetylenic acids is known to be susceptible to deactivation through reduction. To gain a deeper understanding of this deactivation process and to enable the design of a reactivation strategy, in situ X-ray absorption spectroscopy (XAS) was used. With this technique, changes in the palladium oxidation state and coordination environment could be studied in close detail, which provided experimental evidence that the deactivation was primarily caused by triethylamine-promoted reduction of palladium(II) to metallic palladium nanoparticles. Furthermore, it was observed that the choice of the acetylenic acid substrate influenced the distribution between palladium(II) and palladium(0) species in the heterogeneous catalyst after the reaction. From the mechanistic insight gained through XAS, an improved catalytic protocol was developed that did not suffer from deactivation and allowed for more efficient recycling of the catalyst.

Palladium supported aminobenzamide modified silica coated superparamagnetic iron oxide as an applicable nanocatalyst for Heck cross-coupling reaction

An applicable palladium-based nanocatalyst was constructed through the immobilization of palladium onto 2-aminobenzamide functionalized silica coated superparamagnetic iron oxide magnetic nanoparticles. The nanocatalyst (named as Pd@ABA@SPIONs@SiO2) was characterized by several characterization methods, including scanning electron microscope (SEM), transmission electron microscopy (TEM), vibrating-sample magnetometry (VSM), energy-dispersive X-ray spectroscopy (EDS), dynamic light scattering (DLS), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS) analyses. Microscopy results showed that the nanoparticles are spherical in shape with 20–25 nm size. The size of the nanoparticles was confirmed by the DLS method. The superparamagnetic nature of the catalyst was confirmed by the VSM method. The successful functionalization of SPIONs@SiO2 was confirmed by FT-IR spectroscopy. The presence of palladium in the structure of the nanocatalyst was illustrated by XRD and EDS analysis. Also using XPS technique, the oxidation state of palladium in Pd@ABA@SPIONs@SiO2 was determined zero before and after the catalyst was applied in Mizoroki-Heck reaction. Several aryl halides and alkenes were reacted in the presence of the nanocatalyst and formed the corresponding products in high isolated yields. The nanocatalyst showed very good reusability and did not decrease its activity after 10 sequential runs. Density functional theory (DFT) calculation was performed to provide a mechanism for the reaction and confirmed the role of the palladium catalyst in the reaction function.

A fluorescent probe for the discrimination of oxidation states of palladium.

Palladium-based catalysts are widely used in pharmaceutical industries, which can sometimes cause palladium contamination in pharmaceutical drug manufacture. It is important to separately quantify the different oxidation states of palladium (Pd0 and Pd2+) in pharmaceuticals as they react with scavengers differently. Although palladium sensors have been under intense investigation, oxidation state differentiators are very rare. Here, we report a simple porphyrin–coumarin conjugate, PPIX-L2, that can selectively discriminate between the oxidation states of palladium. The reaction of PPIX-L2 with Pd0 showed a 24-fold fluorescence increase of the coumarin emission, meanwhile, the presence of Pd2+ led to a 98% quenching of the porphyrin emission. Fluorescent responses of PPIX-L2 towards Pd0 and Pd2+ are specific, and its sensitivity towards both palladium species is significantly increased with a detection limit of 75 nM and 382 nM for Pd0 and Pd2+ respectively.

Progress in designing novel single-phase room temperature multiferroics

Magnetoelectric multiferroics, because of their coupled electric and magnetic ordering in a single phase, are eminent for applications in logic and nonvolatile memory devices. Recent advances in computational modeling, synthesis, and characterization techniques are spurring significant advances in the study of these materials. During last several years, our group at University of Puerto Rico has been involved in synthesizing novel single-phase room temperature multiferroic materials that included not only solid solutions of lead zirconate titanate with lead iron tungstate, lead iron niobate, and lead iron tantalate, but also recently discovered palladium substituted room temperature multiferroics, namely PbPd0.3Ti0.7O3 and Pb(Zr0.2Ti0.8)Pd0.3O3. In this review, we have reported thin films of the above materials that were synthesized using pulsed laser deposition technique on various substrates at Speclab, University of Puerto Rico. We describe in detail thin film fabrication, structural diffraction analyses (XRD, TEM), dielectric, ferroelectric, piezo-response force microscopy, and magnetization results on recently grown Pd-substituted lead titanate and lead zirconate titanate thin films in an elaborate manner. The films were phase pure and stabilized in c-axis oriented tetragonal phase (P4mm) with surface roughness RQ of 2–4 nm. Temperature-dependent dielectric studies on metal-dielectric-metal heterostructure capacitors indicate that ferroelectric to paraelectric phase transition is above 500 K. A proof of ferroelectricity was confirmed from strong domain switching responses using piezo-response force microscopy. These films showed ferromagnetic ordering below 395 K. The origin of magnetic ordering in these materials was attributed to the existence of Pd2+ and Pd4+ mixed oxidation states of palladium dispersed in the polar matrices. We conclude with the above findings that these multiferroics may have potential applications.in nonvolatile memory and other multifunctional devices.

Lead palladium zirconate titanate: A room temperature nanoscale multiferroic thin film

The discovery of single-phase multiferroic materials and the understanding of intriguing physics of the coupling mechanisms between their spin and polarization are important for the next generation of multifunctional devices. In this work, we report dielectric, ferroelectric, and magnetization results of Pd-substituted room-temperature magnetoelectric multiferroic Pb(Zr0.20Ti0.80)0.7Pd0.3O3 (PZTPd) thin films. Highly c axis oriented PZTPd thin films were grown on {(LaAlO3)0.3(Sr2AlTaO6)0.7} substrates using the pulsed laser deposition technique. These films were phase pure and stabilized in a tetragonal phase. Atomic force microscopic studies indicated a homogeneous distribution of grains on the films' surface with a surface roughness (RQ) of ∼5.4 nm. A large dielectric constant of ∼1780 and a low loss tangent value of ∼0.52 were observed at 1 kHz measured at room temperature. Temperature dependent dielectric studies on Pt/PZTPd/La0.7Sr0.3MnO3 metal–insulator–metal heterostructure capacitors between 80 and 660 K indicated a diffused ferroelectric to paraelectric phase transition around 515 K. The polarization hysteresis loops observed at room temperature were attributed to its ferroelectric behavior. Saturated magnetization hysteresis loop with remanent magnetization of 1.54 emu/cm3 was obtained at room temperature. Ferromagnetic ordering in thin films was found to sustain in the entire temperature range from 5 to 395 K, as revealed from the constant behavior of the dM/dT curve with temperature. The mixed oxidation states of palladium (Pd2+/Pd4+) dispersed in the polar Pb(Zr0.20Ti0.80)O3 matrix were revealed from our high resolution x-ray photoelectron spectroscopic studies and is ascribed to the origin of ferromagnetic ordering in the film. These findings suggest that PZTPd thin films are multiferroic (ferroelectric–ferromagnetic) at room temperature.

Precatalyst or dosing-device? The [Pd2{μ-(C6H4) PPh2}2{μ-O2C(C6H5)}2] complex anchored on a carboxypolystyrene polymer as an effective supplier of palladium catalytically active nanoparticles for the Suzuki-Miyaura reaction

A new catalyst has been synthesized from the precursor [Pd2{μ-(C6H4) PPh2}2 {μ-O2C(C6H5)}2] immobilized on a carboxypolystyrene polymer that exhibits an excellent dispersion of the Pd (II) centers, reusability, and catalytic activity in front of phenyl bromides. The activity of this new material was studied in detail for the Suzuki-Miyaura reaction and compared to that of Pd nanoparticles (NPs) supported on UVM-7 (a mesoporous silica), and Pd NPs stabilized with polyvinylpirrolydone. The homogeneous/heterogeneous character of the catalytic process was determined from the results of the hot-filtration, centrifugation, poisoning, three phases tests, and from differential sensitivity kinetic assays. Changes in the palladium oxidation state, and the size and morphology of the Pd NPs formed during several reusing catalytic cycles were determined by XPS, and by SEM and STEM-HAADF respectively. Kinetic and structural analyses concluded that the new material behaved as a supplier of active Pd(0) NPs to the Suzuki-Miyaura catalytic cycle, which was determined to be driven mainly by the palladium fraction present in solution.

Oxidized Palladium Supported on Ceria Nanorods for Catalytic Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde in Protic Solvents

In the present study, the catalytic activity of palladium oxide (PdOx) supported on ceria nanorods (CeO2-NR) for aerobic selective oxidation of benzyl alcohol (BnOH) to benzaldehyde (PhCHO) was evaluated. The CeO2-NR was synthesized hydrothermally and the Pd(NO3)2 was deposited by a wet impregnation method, followed by calcination to acquire PdOx/CeO2-NR. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), transmission electron microscopy (TEM), Brunauer–Emmet–Teller (BET) surface area analysis, and X-ray photoelectron spectroscopy (XPS). In addition, the TPR-reduced PdOx/CeO2-NR (PdOx/CeO2-NR-Red) was studied by XRD, BET, and XPS. Characterizations showed the formation of CeO2-NR with (111) exposed plane and relatively high BET surface area. PdOx (x > 1) was detected to be the major oxide species on the PdOx/CeO2-NR. The activities of the catalysts in BnOH oxidation were evaluated using air, as an environmentally friendly oxidant, and various solvents. Effects of temperature, solvent nature and palladium oxidation state were investigated. The PdOx/CeO2-NR showed remarkable activity when protic solvents were utilized. The best result was achieved using PdOx/CeO2-NR and boiling ethanol as solvent, leading to 93% BnOH conversion and 96% selectivity toward PhCHO. A mechanistic hypothesis for BnOH oxidation with PdOx/CeO2-NR in ethanol is presented.

Synthesis, stability and activity of palladium supported over various inorganic matrices in the selective hydrogenation of nitroaniline

This work aimed to study the influence of catalyst supports on their catalytic activity and physico-chemical properties of resulting material. Such supports as SiO2, γ-Al2O3 and carbon containing 3, 4 or 5% palladium are used for the experiments. The hydrogenation of p-nitroaniline is chosen as a model reaction for testing the synthesized catalysts. Catalysts were studied by different physico-chemical methods in order to reveal the structure and its changes in reaction. XPS data shows the catalyst surface changed its chemical structure. The palladium oxidation state strongly depends on the nature of the matrix. For γ-Al2O3- and SiO2-supported catalysts, the formation of palladium(II) oxide was found to take place, at the same time, the carbon support is characterized by the presence of both palladium(II) oxide and palladium(IV) oxide. Such a difference shows that the nature of the support influences the oxidation state of active metal. The application of carbon matrix provides a lower oxidation state of the active metal due to electron density shift to the support. Temperature programmed desorption of NH3 demonstrates γ-Al2O3 to contain weak, medium and strong acid sites, while activated carbon has only medium and weak ones. Silica has only weak acid sites. A correlation between catalyst activity and acid–base properties was shown.