Single Pd Research Articles

Facet-dependent synthesis of H2O2 from H2 and O2 over single Pt atom-modified Pd nanocrystal catalysts

An electron-rich catalyst (Pt1Pd(111)/TiO2) with a prominent facet dependence displays extraordinary catalytic properties in the direct synthesis of H2O2.

Realizing new 2D spintronic materials from the non-magnetic 1T-PdO2 monolayer through vacancy defects and doping.

In this work, vacancy- and doping-based magnetism engineering in a non-magnetic 1T-PdO2 monolayer is explored in order to realize new two-dimensional (2D) spintronic materials. The pristine monolayer is an indirect gap semiconductor with a band gap of 1.45 (3.20) eV obtained using the PBE (HSE06) functional. Half-metallicity with a total magnetic moment of 3.95 μB is induced by creating a single Pd vacancy, where the magnetic properties are produced mainly by O atoms around the vacancy site. In contrast, the non-magnetic nature is preserved under the effects of a single O vacancy, however a band gap reduction in the order of 37.93% is achieved. Further doping with transition metals (TMs = V, Cr, Mn, and Fe) in the Pd sublattice and with non-metals (B, C, N, and F) in the O sublattice is investigated. TM impurities lead to the emergence of a diluted magnetic semiconductor nature, where total magnetic moments of 1.00, 2.00, and 3.00 μB are obtained in the V-, Cr(Fe)-, and Mn-doped systems, respectively. In these cases, the TMs' 3d electrons mainly originate the system's magnetism. Significant magnetization of the PdO2 monolayer is also achieved by doping with B, N, and F atoms, where either half-metallic or diluted magnetic semiconductor natures are induced. Herein, electronic and magnetic properties are regulated mainly by the interactions between the 2p orbital of the dopant, 4d orbital of the first neighbor Pd atoms, and 2p orbital of the second neighbor O atoms. Meanwhile, C impurity induces no magnetism in the PdO2 monolayer because of the strong electronic hybridization with their neighbor atoms. Results presented herein may introduce efficient approaches to engineer magnetism in a non-magnetic PdO2 monolayer, such that the functionalized systems are further recommended for prospective spintronic applications.

Pd single-atom decorated OMS-2 nanorod for efficient silane oxidation

Single Pd atom-anchored OMS-2 material was synthesized and exhibited excellent catalytic performance for silane oxidation with H2O. Note that Pd species was trapped into frameworks of OMS-2 by substituting Mn ions and formed strong Pd–O bonds.

Surface hydroxyl group dominating aerobic oxidation of methane below room temperature

Titanium silicate-supported single Pd atoms coated with ultrathin N-doped carbon (Pd1/TS-1@CN) as an efficient and stable catalyst enabling direct oxidation of methane to value-added products with 100% selectivity by O2 and H2 at 15 °C.

Pro‐catalytic Role of Transition Metal/Metal Oxide Blend Low Pd Catalysts for Ethanol Oxidation: Diminution of Activation Energy and Promoting Electrode Kinetics at Low Temperature

AbstractIn this investigation the effect of temperature on catalytic oxidation of ethanol on multi‐metallic PdCoNi/C nano‐composite bearing reduced palladium loading, have been studied over the temperature range 20 °C to 80 °C. The structure, composition and surface morphology of the catalyst regime were determined through the respective techniques like XRD, EDX and TEM while electro‐catalytic parameters were evaluated by the help of voltammetry, chronoamperometry, impedance spectroscopy and ion‐chromatography. The apparent activation energies (Ea) of ethanol oxidation reaction (EOR) were drastically reduced over PdCoNi/C compared to Pd/C reflecting the energy efficiency of ternary matrix furnished with only 35 % Pd loading and the rest 65 % being cheaper transition metals. The long term poisoning rates warrant improved stability of PdCoNi/C and also figure out the robustness of the catalyst even at 20 °C. The Ea value is remarkably reduced, rendering appreciable mass activity in terms of current density per mg of Pd at each of the temperatures studied. The incorporation of transition metals Co and Ni onto Pd becomes particularly effective in orienting the EOR proceed through carbonate pathway as reflected by the columbic efficiency in terms of oxidation product yield ~5 times higher, compared to single Pd catalyst throughout the temperature range.

Tuning of catalysts for the selective hydrogenation of acetylene with high stability and selectivity: Ligand coordination effects

Selectivity of acetylene over CeO2 supported single Pd catalyst (Pd1/CeO2(111)) and 1,10-phenanthroline-5,6-dione (PDO)-liganded Pd single catalyst (Pd1-PDO/CeO2(111)) was studied by density functional theory calculations combined with microkinetic modeling as well as ab initio molecular dynamics (AIMD) in this work. The present results show that the PDO-ligand can stabilize the single Pd catalyst through strong Pd-O/Pd-N interaction, which is further conformed by AIMD simulation. In the presence of PDO ligand, which contains Pd-O/Pd-N bonds, the single atom Pd is oxidized and becomes more stable. It was also found that the adsorption strength of C2H2 is weaker than C2H4 on single Pd, whereas opposite trend holds for PDO-liganded Pd single catalyst, indicating potential higher C2H4 formation electivity for Pd1-PDO/CeO2(111). Reaction mechanism analysis and microkinetic modeling demonstrate that the catalytic activity of C2H2 hydrogenation decreases but the C2H4 formation selectivity increases when PDO ligand was introduced, which agrees with the experimental observation. Moreover, the free energy change of C2H4 desorption as well as its further hydrogenation was quantitative studied by AIMD simulation, verifies that the PDO ligand indeed helps ethene desorption at high reaction temperature. It is hoped the present work may extend to other ligands which contain -O or -N groups that can bind with single metal atom strongly like –OH groups.

A Lantern‐Shaped Pd(II) Cage Constructed from Four Different Low‐Symmetry Ligands with Positional and Orientational Control: An Ancillary Pairings Approach

AbstractOne of the key challenges of metallo‐supramolecular chemistry is to maintain the ease of self‐assembly but, at the same time, create structures of increasingly high levels of complexity. In palladium(II) quadruply stranded lantern‐shaped cages, this has been achieved through either 1) the formation of heteroleptic (multi‐ligand) assemblies, or 2) homoleptic assemblies from low‐symmetry ligands. Heteroleptic cages formed from low‐symmetry ligands, a hybid of these two approaches, would add an additional rich level of complexity but no examples of these have been reported. Here we use a system of ancillary complementary ligand pairings at the termini of cage ligands to target heteroleptic assemblies: these complementary pairs can only interact (through coordination to a single Pd(II) metal ion) between ligands in a cis position on the cage. Complementarity between each pair (and orthogonality to other pairs) is controlled by denticity (tridentate to monodentate or bidentate to bidentate) and/or hydrogen‐bonding capability (AA to DD or AD to DA). This allows positional and orientational control over ligands with different ancillary sites. By using this approach, we have successfully used low‐symmetry ligands to synthesise complex heteroleptic cages, including an example with four different low‐symmetry ligands.

A Lantern-Shaped Pd(II) Cage Constructed from Four Different Low-Symmetry Ligands with Positional and Orientational Control: An Ancillary Pairings Approach.

One of the key challenges of metallo-supramolecular chemistry is to maintain the ease of self-assembly but, at the same time, create structures of increasingly high levels of complexity. In palladium(II) quadruply stranded lantern-shaped cages, this has been achieved through either 1) the formation of heteroleptic (multi-ligand) assemblies, or 2) homoleptic assemblies from low-symmetry ligands. Heteroleptic cages formed from low-symmetry ligands, a hybid of these two approaches, would add an additional rich level of complexity but no examples of these have been reported. Here we use a system of ancillary complementary ligand pairings at the termini of cage ligands to target heteroleptic assemblies: these complementary pairs can only interact (through coordination to a single Pd(II) metal ion) between ligands in a cis position on the cage. Complementarity between each pair (and orthogonality to other pairs) is controlled by denticity (tridentate to monodentate or bidentate to bidentate) and/or hydrogen-bonding capability (AA to DD or AD to DA). This allows positional and orientational control over ligands with different ancillary sites. By using this approach, we have successfully used low-symmetry ligands to synthesise complex heteroleptic cages, including an example with four different low-symmetry ligands.

Single-atom Pd loaded on nitrogen-doped carbon as a bifunctional catalyst for the electrochemical degradation of 2,4-dichlorophenol

A nitrogen-doped carbon-anchored single atom Pd (SA-Pd/NC) electrocatalyst was prepared by pyrolysis of Pd-doped zeolite imidazolite framework (ZIF) precursors. The SA-Pd/NC catalyst was characterized by high-resolution transmission electron microscopy, aberration-corrected high-angle annular dark field scanning transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectrometry and applied in the degradation of 2,4-dichlorophenol (2,4-DCP). We demonstrate that the SA-Pd/NC catalyst has dual functions, with both electrocatalytic hydrodechlorination (ECH) activity and in situ heterogeneous electro-Fenton (hetero-EF) activity even at low current density. The ECH process was the main pathway of 2,4-DCP degradation without oxygen aeration. Under aerobic conditions, hetero-EF coupling with ECH occurred simultaneously on the SA-Pd/NC cathode, i.e., the dechlorination product phenol was oxidatively degraded with the reactive oxygen species produced on the electrode. The degradation rate of 2,4-DCP and TOC removal rate reached 95.1% and 37.6%, respectively, with a current density of 2.5 mA cm−2 and an initial pH of 3.0. This work provides the design of bifunctional single-atom catalysts for 2,4-DCP degradation, including dechlorination and mineralization.

Immobilization of Pd(0) nanoparticles on polyaza-ligand-functionalized polyacrylonitrile fiber as highly active catalyst for the Heck reaction

An novel aza-ligand functionalized polyacrylonitrile fiber (PANPALF) with amino groups and quaternary ammonium salt moieties was synthesized, and then a highly active catalyst for the Heck reaction was obtained by immobilizing Pd(0) nanoparticles on PANPALF through chelation and in situ reduction. The functionalized fibers were characterized by appropriate instruments, such as inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), etc., in order to further confirm that the modification process has been successfully carried out. The catalytic properties of the prepared fiber were evaluated in Heck reaction. The experimental results show that PANPALF-Pd(0) can catalyze Heck reactions and achieving 42 kinds of products with yields ranging from 43% to 99% under solvent-free condition, and verifies that the reactive activation of iodized aromatics is better than that of brominated aromatics. Noteworthily, the TON value of the reaction catalyzed by PANPALF-Pd(0) can reach up to 89000, showing that the fiber catalyst has excellent catalytic activity. Moreover, after PANPALF-Pd(0) reused 11 times, the isolated yield was still as high as 93%, and the average single Pd(0) leaching is only 0.075%, indicating that PANPALF-Pd(0) has satisfactory recyclability. Finally, a rational reaction mechanism was proposed to explain the role of fiber catalyst in the reaction.

(Invited) Electrocatalytic Conversion of CO2 to Syngas with Controlled CO/H2 Ratios

One of the main research challenges for electrocatalysis that produces carbon-containing products from CO2 is avoiding the competing hydrogen evolution reaction. Instead of totally eliminating hydrogen, our approach makes use of the readily available protons in aqueous electrolyte to co-produce CO and H2, making synthesis gas (syngas) with a tunable CO:H2 ratio. The resulting syngas can then be used as feedstock for existing thermocatalytic processes, such as Fischer–Tropsch and methanol synthesis reactions [1]. We will present our results in identifying palladium hydride (PdH), formed under electrocatalytic reaction conditions, as an effective electrocatalyst that enables the syngas production [2]. We will also report our efforts in reducing the loading of Pd by alloying Pd with inexpensive secondary metals, supporting Pd on transition metal carbides and nitrides, and utilizing single atom Pd catalysts. For each type of the catalysts, we monitor the phase transition from Pd to PdH under reaction conditions with in-situ synchrotron-based X-ray absorption and X-ray diffraction techniques. We also identify descriptors for syngas production on PdH, bimetallic PdH, and supported PdH catalysts by performing DFT calculations of the effect of PdH formation on the binding strength of reaction intermediates. The research methodology established here should be useful not only for continued optimization of Pd-based syngas-producing electrocatalysts, but also for enhancing activity while reducing the loading of precious metals for other electrocatalytic applications. Furthermore, we will discuss our recent results in utilizing a tandem scheme of electrocatalytic-thermocatalytic processes to convert CO2 to C3 oxygenates [3].[1] B.M. Tackett, E. Gomez and J.G. Chen, “Net reduction of CO2 via its thermocatalytic and electrocatalytic transformation reactions in standard and hybrid processes”, Nature Catalysis, 2 (2019) 381.[2] B.M. Tackett, J.H. Lee and J.G. Chen, “Electrochemical Conversion of CO2 to Syngas with Palladium-Based Electrocatalysts”, Accounts of Chemical Research, 53 (2020) 1535.[3] A.N. Biswas, Z. Xie, R. Xia, S. Overa, F. Jiao and J.G. Chen, “Tandem Electrocatalytic-Thermocatalytic Reaction Scheme for CO2 Conversion to C3 Oxygenates”, ACS Energy Letters, 7 (2022) 2904

Palladium and Ruthenium Dual‐Single‐Atom Sites on Porous Ionic Polymers for Acetylene Dialkoxycarbonylation: Synergetic Effects Stabilize the Active Site and Increase CO Adsorption

AbstractHeterogeneous single‐metal‐site catalysts usually suffer from poor stability, thereby limiting industrial applications. Dual Pd1−Ru1 single‐atom‐sites supported on porous ionic polymers (Pd1−Ru1/PIPs) were constructed using a wetness impregnation method. The two isolated metal species in the form of a binuclear complex were immobilized on the cationic framework of PIPs through ionic bonds. Compared to the single Pd‐ or Ru‐site catalyst, the dual single‐atom system exhibits higher activity with 98 % acetylene conversion and near 100 % selectivity to dialkoxycarbonylation products, as well as better cycling stability for ten cycles without obvious decay. Based on DFT calculations, it was found that the single‐Ru site exhibited a strong CO adsorption energy of −1.6 eV, leading to an increase in the local CO concentration of the catalyst. Notably, the Pd1−Ru1/PIPs catalyst had a much lower energy barrier of 2.49 eV compared to 3.87 eV of Pd1/PIPs for the rate‐determining step. The synergetic effect between neighboring single sites Pd1 and Ru1 not only enhanced the overall activity, but also stabilized PdII active sites. The discovery of synergetic effects between single sites can deepen our understanding of single‐site catalysts at the molecular level.

Palladium and Ruthenium Dual-Single-Atom Sites on Porous Ionic Polymers for Acetylene Dialkoxycarbonylation: Synergetic Effects Stabilize the Active Site and Increase CO Adsorption.

Heterogeneous single-metal-site catalysts usually suffer from the poor stability, limiting its industrial applications. Herein, the dual-sites Pd1-Ru1 supported on porous ionic polymers (Pd1-Ru1/PIPs) was constructed using wetnessimpregnation method. The two isolated metal species in the form of binuclear complex was immobilized on the cationic framework of PIPs through ionic bonds. Compared to the single-Pd or Ru-site catalyst, it exhibits higher activity with 98% acetylene conversion and near 100% selectivity of dialkoxycarbonylation products as well as better cycling stability for ten times cycles without obvious decay. Based on DFT calculations, it was found that the single-Ru-site exhibited a strong CO adsorption energy of -1.6 eV, leading to an increase in the local CO concentration of the catalyst. Notably, the Pd1-Ru1/PIPs catalyst had a much lower energy barrier of 2.49 eV compared to 3.87 eV of Pd1/PIPs for the rate-determining step. The synergetic effect between neighbouring single-sites Pd1 and Ru1 not only enhanced the overall activity, but also stabilized Pd(II) active sites. The discovery of synergetic effect between single-sites can deepen our understanding of single-site catalysts at the molecular level.

Insight into the Effects of Pd Single-atom Particles in the Activation of the Carbon-Chloride Bond

Palladium (Pd) shows high activity in a variety of catalysis reactions that refer to the activation of the carbon-halogenation bond, and single-atom Pd catalysts have received significant attention as new technology. Nevertheless, there is a huge knowledge gap regarding which Pd sites could form highly reactive centers. It is unclear whether single-atom Pd sites are active in this process or not. In this study, we synthesized Pd/TiO2 nanosheets with Pd loadings of 1%, 5%, and 10% and designed single atom Pd sites and Pd nanoparticles on similar substrates. A scanning transmission electron microscope was used to characterize the synthesized Pd catalysts. As a catalyst, Pd/TiO2 featuring Pd NPs showed high activity in activating the carbon-halogen bond in the activation of carbon-chloride bond, whereas Pd/TiO2 with single Pd atoms showed very low activity in this pros. Accordingly, our results demonstrate that the activation of carbon-halogen bonds cannot be catalyzed by the single-atom Pd sites and emphasize the significance of the ensemble effect in determining the activity of Pd sites.

Exploring the Strain Effect in Single Particle Electrochemistry using Pd Nanocrystals.

Tuning the surface strain of heterogeneous catalysts are recognized as a powerful strategy for tailoring their catalytic activity. However, a clear understanding of the strain effect in electrocatalysis at single-particle resolution is still lacking. Here, we explore the electrochemical hydrogen evolution reaction (HER) of single Pd octahedra and icosahedra with the same surface bounded {111} crystal facets and similar sizes using scanning electrochemical cell microscopy (SECCM). It is revealed that tensely strained Pd icosahedra display significantly superior HER electrocatalytic activity. The estimated turnover frequency at -0.87 V vs RHE on Pd icosahedra is about two times higher than that on Pd octahedra. Our single-particle electrochemistry study using SECCM at Pd nanocrystals ambiguously highlights the importance of tensile strain on electrocatalytic activity and may offer new strategy for understanding the fundamental relationship between surface strain and reactivity.

Exploring the Strain Effect in Single Particle Electrochemistry using Pd Nanocrystals

AbstractTuning the surface strain of heterogeneous catalysts is recognized as a powerful strategy for tailoring their catalytic activity. However, a clear understanding of the strain effect in electrocatalysis at single‐particle resolution is still lacking. Here, we explore the electrochemical hydrogen evolution reaction (HER) of single Pd octahedra and icosahedra with the same surface bounded {111} crystal facet and similar sizes using scanning electrochemical cell microscopy (SECCM). It is revealed that tensilely strained Pd icosahedra display significantly superior HER electrocatalytic activity. The estimated turnover frequency at −0.87 V vs RHE on Pd icosahedra is about two times higher than that on Pd octahedra. Our single‐particle electrochemistry study using SECCM at Pd nanocrystals unambiguously highlights the importance of tensile strain on electrocatalytic activity and may offer new strategy for understanding the fundamental relationship between surface strain and reactivity.

Pd-Ceria/CNMs Composites as Catalysts for CO and CH4 Oxidation.

The application of composite materials as catalysts for the oxidation of CO and other toxic compounds is a promising approach for air purification. In this work, the composites comprising palladium and ceria components supported on multiwall carbon nanotubes, carbon nanofibers and Sibunit were studied in the reactions of CO and CH4 oxidation. The instrumental methods showed that the defective sites of carbon nanomaterials (CNMs) successfully stabilize the deposited components in a highly-dispersed state: PdO and CeO2 nanoparticles, subnanosized PdOx and PdxCe1-xO2-δ clusters with an amorphous structure, as well as single Pd and Ce atoms, are formed. It was shown that the reactant activation process occurs on palladium species with the participation of oxygen from the ceria lattice. The presence of interblock contacts between PdO and CeO2 nanoparticles has an important effect on oxygen transfer, which consequently affects the catalytic activity. The morphological features of the CNMs, as well as the defect structure, have a strong influence on the particle size and mutual stabilization of the deposited PdO and CeO2 components. The optimal combination of highly dispersed PdOx and PdxCe1-xO2-δ species, as well as PdO nanoparticles in the CNTs-based catalyst, makes it highly effective in both studied oxidation reactions.

Plasmon driven super-high HER activity of electronic structure and lattice strain engineered single atomic layer Pd@Au nanorods

The development of efficient catalysts with low energy consumption has always been a key issue for sustainable hydrogen energy. Here, we report a catalyst comprising a single atomic Pd layer deposited on Au nanorods (ML-Pd@Au NRs) that shows a super-high HER activity and ultra-low overpotential under localized surface plasmon resonance condition. The Au NR plays triple determining roles in HER performance via adjusting the electronic structure and lattice strain of Pd that effectively reduces the adsorption energy of H-adatoms, eliminating the penetration of H adatoms in multilayer Pd crystal that decreases the H adatoms diffusion kinetics, and providing high energetic hot charge carriers under near-IR light irradiation that decreases the activation energy and onset potential of HER. The single atomic Pd layer on Au NRs ensures the hot electron transfer as the main dissipation pathway of the plasmonic hot charge carriers for HER. Thus, under near-IR light irradiation, HER on ML-Pd@Au NRs occurs with a high mass activity of 17.47 mA mg−1 at 100 mV and a very low overpotential of 13 mV at 10 mA cm−2, which are superior to the commercial Pd/C and commercial Pt/C catalysts under the same conditions.

Structure-dependence and metal-dependence on atomically dispersed Ir catalysts for efficient n-butane dehydrogenation

Single-site pincer-ligated iridium complexes exhibit the ability for C-H activation in homogeneous catalysis. However, instability and difficulty in catalyst recycling are inherent disadvantages of the homogeneous catalyst, limiting its development. Here, we report an atomically dispersed Ir catalyst as the bridge between homogeneous and heterogeneous catalysis, which displays an outstanding catalytic performance for n-butane dehydrogenation, with a remarkable n-butane reaction rate (8.8 mol·gIr−1·h−1) and high butene selectivity (95.6%) at low temperature (450 °C). Significantly, we correlate the BDH activity with the Ir species from nanoscale to sub-nanoscale, to reveal the nature of structure-dependence of catalyst. Moreover, we compare Ir single atoms with Pt single atoms and Pd single atoms for in-depth understanding the nature of metal-dependence at the atomic level. From experimental and theoretical calculations results, the isolated Ir site is suitable for both reactant adsorption/activation and product desorption. Its remarkable dehydrogenation capacity and moderate adsorption behavior are the key to the outstanding catalytic activity and selectivity.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water–Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO2-anatase SAC (Pd1/TiO2) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO2 by H2 alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H2 is accompanied by the partial sintering of single Pd atoms (Pd1) into disordered, flat, ∼1 nm diameter clusters (Pdn). The highly active Pd sites in the new coordination environment under H2 are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pdn and facilitates the reduction of TiO2. In contrast, Pd1 sinters into crystalline, ∼5 nm particles (PdNP) during CO treatment, deactivating Pd1/TiO2. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H2 dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H2. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.