Pd K-edge Research Articles

Synthesis of Helional by Hydrodechlorination Reaction in the Presence of Mono- and Bimetallic Catalysts Supported on Alumina

Hydrodechlorination reaction of 3-(benzo-1,3-dioxol-5-yl)-3-chloro-2-methylacrylaldehyde in the presence of different low metal content heterogeneous mono- or bimetallic catalysts was tested for the synthesis of the fragrance Helional® (3-[3,4-methylendioxyphenyl]-2-methyl-propionaldehyde). In particular, mono Pd/Al2O3, Rh/Al2O3 or bimetallic Pd-Cu/Al2O3, Rh-Cu/Al2O3 catalysts were tested in different reaction conditions from which it emerged that mono-Rh/Al2O3 was the best performing catalyst, allowing achievement of 100% substrate conversion and 99% selectivity towards Helional® in 24 h at 80 °C, p(H2) 1.0 MPa in the presence of a base. To establish correlations between atomic structure and catalytic activity, catalysts were characterized by Cu, Rh and Pd K-edge XANES, EXAFS analysis. These characterizations allowed verification that the formation of Pd-Cu alloys and the presence of Cu oxide/hydroxide species on the surface of the Al2O3 support are responsible for the very low catalytic efficiency of bimetallic species tested.

Incorporation of palladium into pyrite: Insights from X-ray absorption spectroscopy analysis and modelling

Pyrite (FeS2) often accommodates elevated concentrations of platinum-group elements in ores of magmatic and hydrothermal origin. In order to elucidate the role of pyrite in concentrating Pd, Pd-doped synthetic crystals were studied via X-ray absorption spectroscopy (XAS). Crystals were obtained by salt-flux method in the system saturated with respect to Pd at the temperature of 580 °C and sulphur fugacity of log f (S2) = −0.4. Scanning electron microscopy, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry studies demonstrated a uniform distribution of Pd within the pyrite crystals. The median and average values of Pd content of ∼0.7 ± 0.1 wt% were measured. Comparison of the Pd K-edge X-ray absorption near edge structure (XANES) spectra with the spectra of standards revealed that the formal oxidation state of Pd was close to +2. Fitting of the extended X-ray absorption fine structure (EXAFS) and Finite Difference Method for Near-Edge Structure (FDMNES) theoretical simulations of XANES spectra showed that Pd substituted for Fe in the crystal structure of pyrite. The isomorphous Pd in pyrite was octahedrally coordinated by S atoms at ∼2.385 Å. The PdS interatomic distance was 5.6 % larger than that of FeS due to the difference in their covalent radii of ∼5.3 %. The expansion caused by the incorporation of Pd into the pyrite structure disappeared at the distance of R > 3 Å. The information on the state of Pd in pyrite, including the local atomic environment and formal oxidation state, is essential for scientific and industrial purposes, e.g. physical-chemical modelling and improvement of leaching and extraction processing respectively.

Palladium K-edge X-ray Absorption Spectroscopy Studies on Controlled Ligand Systems.

X-ray absorption spectroscopy (XAS) is widely used across the life and physical sciences to identify the electronic properties and structure surrounding a specific element. XAS is less often used for the characterization of organometallic compounds, especially for sensitive and highly reactive species. In this study, we used solid- and solution-phase XAS to compare a series of 25 palladium complexes in controlled ligand environments. The compounds include palladium centers in the formal I, II, III, and IV oxidation states, supported by tridentate and tetradentate macrocyclic ligands, with different halide and methyl ligand combinations. The Pd K-edge energies increased not only upon oxidizing the metal center but also upon increasing the denticity of the ligand framework, substituting sigma-donating methyl groups with chlorides, and increasing the charge of the overall metal complex by replacing charged ligands with neutral ligands. These trends were then applied to characterize compounds whose oxidation states were otherwise unconfirmed.

Formation and Segregation of a Pd-MgO Solid Solution Studied by X-ray Absorption Spectroscopy.

Thermal treatment of Pd nanoparticles or Pd(NH3)4(NO3)2 supported on MgO resulted in the formation of a solid solution of Pd-MgO, as evidenced by Pd K-edge X-ray absorption fine structure (XAFS). The valence of Pd in the Pd-MgO solid solution was determined to be 4+ from the comparison of X-ray absorption near edge structure (XANES) with reference compounds. A characteristic shrinkage of the Pd-O bond distance was observed in comparison with that of the nearest-neighboring Mg-O bond in MgO, which agreed with the density functional theory (DFT) calculations. The two-spike pattern was observed in the dispersion of Pd-MgO owing to the formation and successive segregation of solid solutions above 1073 K.

Synchrotron-based X-ray absorption spectroscopy of Pd nanoparticles decorated on multi-walled carbon nanotubes during successive cycles of He/H2 exposure.

The MWCNTs are decorated by Pd nanoparticles via various techniques including laser ablation in liquid, chemical reduction, and simultaneously both of them. To study the hydrogen adsorption mechanism, Pd K-edge X-ray absorption spectroscopy (XAS) is carried out via heating/cooling processes under He/H2 exposure. The Fourier transform X-ray absorption fine structure (FT-EXAFS) simulation indicates the presence of the Pd-Pd and Pd-C(O) bonds. Furthermore, during the successive cycles of He/H2 exposure, bond restructuring takes place. Moreover, the heating process under He/H2 exposure induces a destructive effect on the Pd-C(O) links. Furthermore, the Pd-Pd bond distance enlarges due to the hydrogen adsorption for all samples, however, in the case of PLAL, the change in the bond distances becomes dominant. XRD and XPS are applied to support the findings.

Temperature-dependent hydrogen storage mechanism in palladium nanoparticles decorated on multi-walled carbon nanotubes

In this study, Palladium (Pd) nanoparticles (NPs) are decorated on the multi-walled carbon nanotubes (MWCNTs) using the chemical reduction method. In situ extended x-ray absorption fine structure (EXAFS) spectroscopy of Pd NPs decorated on MWCNTs is carried out in different temperatures from 77 K up to 540 K to investigate the local structure around the absorber atom Pd, under molecular hydrogen, H2, pressure, and in a vacuum. The Pd K-edge EXAFS gives a deeper insight into the structural properties of Pd-MWCNT compounds under in-situ H2 treatment at various temperatures. EXAFS data show three mechanisms of the interaction of H2 with Pd: the first mechanism involves physical absorption of H2 at low temperatures from 110 to 190 K, the second mechanism includes the formation of PdHx compound at 300 K, and the third mechanism is attributed to the decomposition of PdHx compound at above 400 K. Furthermore, X-ray photoelectron spectroscopy (XPS) results verify the reduction of the palladium oxide after interaction of the sample with molecular hydrogen and the stability of the catalyst under several cycles of annealing and cooling in hydrogen.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

On the effect of metal loading on the reducibility and redox chemistry of ceria supported Pd catalysts.

The effect of Pd loading on the redox characteristics of a ceria support was examined using in situ Pd K-edge XAS, Ce L3-edge XAS and in situ X-ray diffraction techniques. Analysis of the data obtained from these techniques indicates that the onset temperature for the partial reduction of Ce(IV) to Ce(III), by exposure to H2, varies inversely with the loading of Pd. Whilst the onset and completion temperatures of the reduction of Ce(IV) to Ce(III) are different, both samples yield the same maximal fraction of Ce(III) formation independent of Pd loading. Furthermore, the partial reduction of Ce is found to be concurrent with the reduction of PdO and demonstrated that the presence of metallic Pd is necessary for the reduction of the CeO2 support. Upon passivation by room temperature oxidation, a full oxidation of the reduced ceria support was observed. However, only a mild surface oxidation of Pd was identified. The mild passivation of the Pd is found to lead to a highly reactive sample upon a second reduction by H2. The onset of the reduction of Pd and Ce has been demonstrated to be independent of the Pd loading after a mild passivation with both samples exhibiting near room temperature reduction in the presence of H2.

Incorporating Sulfur Atoms into Palladium Catalysts by Reactive Metal–Support Interaction for Selective Hydrogenation

Incorporating Sulfur Atoms into Palladium Catalysts by Reactive Metal–Support Interaction for Selective Hydrogenation

Simultaneous two-color X-ray absorption spectroscopy using Laue crystals at an inverse-compton scattering X-ray facility.

X-ray absorption spectroscopy (XAS) is an element-selective technique that provides electronic and structural information of materials and reveals the essential mechanisms of the reactions involved. However, the technique is typically conducted at synchrotrons and usually only probes one element at a time. In this paper, a simultaneous two-color XAS setup at a laboratory-scale synchrotron facility is proposed based on inverse Compton scattering (ICS) at the Munich Compact Light Source (MuCLS), which is based on inverse Compton scattering (ICS). The setup utilizes two silicon crystals in a Laue geometry. A proof-of-principle experiment is presented where both silver (Ag) and palladium (Pd) K-edge X-ray absorption near-edge structure spectra were simultaneously measured. The simplicity of the setup facilitates its migration to other ICS facilities or maybe to other X-ray sources (e.g. a bending-magnet beamline). Such a setup has the potential to study reaction mechanisms and synergistic effects of chemical systems containing multiple elements of interest, such as a bimetallic catalyst system.

Inclusion of bidentate phosphine molecules in the supercage of FAU zeolite

Bidentate phosphines (Ph 2 P(CH 2 ) n PPh 2 ) with different alkyl chain lengths (n = 1–10) were included in the supercage (SC) of FAU-zeolite. Thermal treatment of the mixture of a bidentate phosphines and FAU-zeolite at 443 K resulted in the inclusion of the former into the SC, with up to 0.75 molecules per SC. The included molecules were strongly bound by FAU-zeolite, with desorption observed at higher temperatures compared to those of unloaded ones. The specific surface area of the FAU-zeolite decreased linearly with an increase in the number of phosphines included. This tendency was more emphasized in phosphines with n = 6–10 compared with those with n = 1–5. A characteristic shift in the C–C stretching band of the benzene ring was observed in the Fourier-transform infrared spectroscopy (FT-IR) spectrum, due to the strong interaction between bidentate phosphines and FAU-zeolite; the extent was more when using *BEA-loaded ones. Formation of the inclusion compounds between bidentate phosphines and FAU zeolites was observed with the molecular dynamics (MD) calculations. The bidentate phosphines included in the SC of the FAU-zeolite were reacted with PdCl 2 to form Pd complexes. The detailed analysis of the Pd complexes with Pd K-edge EXAFS revealed a characteristic decrease in the Pd–Cl bond distance with the use of complexes with n = 3 and 4, suggesting that the local structure around Pd was distorted through the interaction of the Pd complexes with the pores of the FAU-zeolite. The Pd complexes included in the SC were active for allylic alkylation between allyl methyl carbonate and ethyl acetoacetate. • Bidentate phosphines could be directly included in the supercage of FAU zeolite. • Formation of the inclusion compound was simulated by MD calculation. • Pd complexes with bidentate phosphines in SCs of FAU zeolite exhibited activity in the allylic alkylation.

Investigation of Local Structure and Enhanced Thermal Stability of Ir-Doped PdRu Nanoparticles for Three-Way Catalytic Applications

The local structure and thermal stability of newly synthesized PdRuIr and PdRu alloy nanoparticles (NPs) were studied using X-ray absorption fine structure spectroscopy (XAFS) and compared with those of Ru NPs. Pd K-edge XAFS reveals that a significant fraction of Pd segregates, forming metal NP clusters. In the PdRuIr NPs, a small fraction of Pd forms an alloy with Ir, whereas a majority phase is Ru–Ir alloy having the novel face-centered cubic structure. Apart from the distinct local surroundings, XAFS analysis revealed the presence of an anharmonic disorder in the PdRu and PdRuIr NPs. The previously observed enhanced thermal stability with Pd and Ir doping was investigated using temperature-dependent in situ XAFS. In the Ru NPs, an abrupt change in the near-edge features was observed at 673 K, which was gradually suppressed for PdRu and PdRuIr NPs. At this temperature, the dynamical fluctuations were more pronounced in the pure Ru NPs, helping to convert surface-adsorbed O2 into the volatile RuO4 phase, thereby leading to earlier evaporation of Ru. Dynamical fluctuation suppresses with alloying elements or gets extended to a higher temperature, helping to delay the RuO4 formation process and enhancing the thermal stability of the PdRu and PdRuIr NPs in increasing order.

Mechanism of Hydrogen Storage and Structural Transformation in Bimetallic Pd–Pt Nanoparticles

The hydrogen storage capacity of Pd nanoparticles (NPs) decreases as the particles become smaller; however, this reduced capacity is ameliorated by addition of Pt. In the present work, the hydrogen storage mechanism and structural transformations of core (Pd)-shell (Pt) (CS) and solid-solution (SS) NPs during hydrogen absorption and desorption (PHAD) processes are investigated. In situ X-ray absorption spectroscopy measurements were performed to study the evolution of electronic and local structures around Pd and Pt during PHAD. Under ambient conditions, Pd and Pt have distinct local structures. The Pd atomic pairs are more strained in CS NPs than in SS NPs. A similar behavior has been seen in CS NPs after PHAD. The Pd K-edge extended X-ray absorption fine structure data indicate that in CS and SS NPs a substantial fraction of the signal derives from Pd-Pd atomic pairs, indicating that Pd clusters remain present even after PHAD. PHAD causes a rearrangement of the interfacial structure, which becomes homogeneously distributed. The higher coverage of active bimetallic sites results in a higher observed hydrogen storage capacity in the SS phase.

Latent Representation Learning for Structural Characterization of Catalysts.

Supervised machine learning-enabled mapping of the X-ray absorption near edge structure (XANES) spectra to local structural descriptors offers new methods for understanding the structure and function of working nanocatalysts. We briefly summarize a status of XANES analysis approaches by supervised machine learning methods. We present an example of an autoencoder-based, unsupervised machine learning approach for latent representation learning of XANES spectra. This new approach produces a lower-dimensional latent representation, which retains a spectrum-structure relationship that can be eventually mapped to physicochemical properties. The latent space of the autoencoder also provides a pathway to interpret the information content "hidden" in the X-ray absorption coefficient. Our approach (that we named latent space analysis of spectra, or LSAS) is demonstrated for the supported Pd nanoparticle catalyst studied during the formation of Pd hydride. By employing the low-dimensional representation of Pd K-edge XANES, the LSAS method was able to isolate the key factors responsible for the observed spectral changes.

(Invited) X-Ray Absorption Spectroscopy: Investigations of the Interfacial Chemistry of Batteries at APS Beamline 9-BM

As a beamline dedicated to the X-ray Absorption Spectroscopy, 9-BM is capable to cover very wide energy range (2145.5 eV P-K edge to 24350 eV Pd-K edge). In recent years more efforts have been devoted to the fields of catalysis and energy storage, focusing on catalysis for efficient conversion of energy resources into usable forms, and storage of such energy in efficient and safe capacitors. To understand and predict how catalysts and/or energy storage materials function, it’s very important to characterize the materials under actual reaction conditions (in-situ or operando). 9-BM has leveraged the advanced capabilities for in-situ catalysis and electrochemistry, as well as the ability to collect high quality spectroscopy data at a rapid rate (Quick EXAFS). Scientists using 9-BM are able to gain unique insight to how chemical processes affect and are affected by the materials under investigation. In this talk, the capabilities will be displayed with a few examples of the science enabled by them.

Engineering Electronic Structure of Single-Atom Pd Site on Ti 0.87 O 2 Nanosheet via Charge Transfer Enables C–Br Cleavage for Room-Temperature Suzuki Coupling

The palladium (Pd)-catalyzed Suzuki reaction is widely applied in the pharmaceutical industry, where constructing highly active and low-cost Pd sites are impendent. Here, we report the fabrication ...

Electronic and Catalytic Effects of Single-Atom Pd Additives on the Hydrogen Sensing Properties of Co3O4 Nanoparticle Films.

Atomically dispersed Pd additives significantly enhanced the hydrogen sensing performance of a Co3O4 nanoparticle film, and their electronic along with catalytic roles were comprehensively investigated based on a series of systematic experiments. Aggregates of Co3O4 nanoparticles (approximately 3 nm in size) with homogeneously dispersed Pd additives at concentrations in the range of 1-20% (on a molar basis with respect to Co) were generated in the gas phase via reactive pulsed laser ablation of Co-Pd alloy targets in He/O2 mixtures. The form of the Pd could be modified from single atoms to oxide clusters (1-2 nm), and the effects of these additives on the hydrogen sensing properties of thick films prepared by direct deposition were examined. The highest hydrogen sensing performance was obtained at 5% Pd loading, where single Pd atoms were present at the maximum density. Further Pd loading resulted in the formation of Pd oxide clusters and degraded the sensitivity. X-ray photoelectron spectroscopy and Pd K-edge X-ray absorption spectroscopy showed that single Pd atoms in the Pd4+ state at Co3+ sites on the Co3O4 nanoparticle surfaces donated electrons to the Co3O4 valence band. The greater concentration of free electrons led to an increase in the concentration of ionosorbed oxygen under dry air. Consequently, more ionosorbed oxygen was available for reaction with hydrogen, enhancing sensitivity. In situ X-ray absorption spectroscopy data confirmed that approximately 10% of the single Pd atoms in the Pd4+ state were reduced to Pd2+ during exposure to 1000 ppm H2, implying that a Pd4+ ↔ Pd2+ catalytic redox cycle accelerates the water formation reaction during hydrogen sensing. The present results provide deeper insights and understanding of the effects of noble metal additives on gas sensing, while highlighting the unique role of single-atom additives.

Absorption of Hydrocarbons on Palladium Catalysts: From Simple Models Towards Machine Learning Analysis of X-ray Absorption Spectroscopy Data

Element selectivity and possibilities for in situ and operando applications make X-ray absorption spectroscopy a powerful tool for structural characterization of catalysts. While determination of coordination numbers and interatomic distances from extended spectral region is rather straightforward, analysis of X-ray absorption near-edge structure (XANES) spectra remains a highly debated and topical problem. The latter region of spectra is shaped depending on the local 3D geometry and electronic structure. However, there is no straightforward procedure for the unambiguous extraction of these parameters. This work gives a critical vision on the amount of information that can be practically extracted from Pd K-edge XANES spectra measured under in situ and operando conditions, in which adsorption of reactive molecules at the surface of palladium with further formation of subsurface and bulk palladium carbides are expected. We investigate how particle size, concentration of carbon impurities, and their distribution in the bulk and at the surface of palladium particles affect Pd K-edge XANES features and to which extend they should be implemented in the theoretical model to adequately reproduce experimental data. Then, we show how the step-by-step increasing the complexity of the theoretical model improves the agreement with experiment. Finally, we suggest a set of formal descriptors relevant to possible structural diversity and construct a library of theoretical spectra for machine-learning-based analysis of the experimental data.

(Invited) X-Ray Absorption Spectroscopy: Investigations of Catalysis and Batteries at APS Beamline 9-BM

As a beamline dedicated to the X-ray Absorption Spectroscopy, 9-BM is capable to cover very wide energy range (2145.5 eV P-K edge to 24350 eV Pd-K edge). In recent years more efforts have been devoted to the fields of catalysis and energy storage, focusing on catalysis for efficient conversion of energy resources into usable forms, and storage of such energy in efficient and safe capacitors. To understand and predict how catalysts and/or energy storage materials function, it’s very important to characterize the materials under actual reaction conditions (in-situ or operando). 9-BM has leveraged the advanced capabilities for in-situ catalysis and electrochemistry, as well as the ability to collect high quality spectroscopy data at a rapid rate (Quick EXAFS). Scientists using 9-BM are able to gain unique insight to how chemical processes affect and are affected by the materials under investigation. In this talk, the capabilities will be displayed with a few examples of the science enabled by them.

Direct immobilization of triphenylphosphine palladium complexes on the external surface of zeolite β

Abstract The immobilization of a Pd complex on several supports was attempted using a simple method. For this purpose, bis(triphenylphosphine)palladium(II) dichloride(Pd(PPh 3 ) 2 Cl 2 ) was attached to several oxides including MgO,γ-Al 2 O 3 , TiO 2 , and zeolites. Pd(PPh 3 ) 2 Cl 2 supported on zeolite β remained mostly intact, whereas the complex completely decomposed to provide metal Pd or PdO on MgO and γ-Al 2 O 3 , as analyzed by Pd K-edge X-ray absorption fine structure spectroscopy (XAFS). The specific surface area was slightly reduced after Pd(PPh 3 ) 2 Cl 2 loading on zeolite β, probably due to the deposition of the complex on the external surface. From a comparison experiment using the adsorption of triphenylphosphine (PPh 3 ), it was supposed that the PPh 3 ligand of Pd(PPh 3 ) 2 Cl 2 was inserted into the pores of zeolite β located close to the external surface. The Pd complexes immobilized on zeolite β exhibited much higher activity for allylic alkylation compared with that of pristine Pd(PPh 3 ) 2 Cl 2 . Recycling of the zeolite β loaded Pd complex was possible after the used catalyst was rinsed with toluene. The activity of the Pd complex loaded on zeolite β was enhanced by the addition of a small amount of leucomalachite green (LMG) or leucocrystal violet (LCV).