Pd Nanocatalysts Research Articles

Unexpected catalytic activity of Pd(II)-coordinated nucleotides in hydrogenation reduction

Controlled synthesis of noble metal nanocatalysts with desirable catalytic activities remains a great challenge. Inspired by electron-rich functional groups and supramolecular assembly properties of natural nucleotides, herein we propose a versatile approach to construct Pd nanocatalysts through employing seven nucleotides as both electron donors and stabilizers. The catalytic performance of nucleotide-Pd complexes are greatly associated with the chemical structures of nucleobases as well as the number of phosphate groups. Guanosine 5′-triphosphate-Pd complexes possess high activities in the hydrogenation reduction of 4-nitrophenol assisted by NaBH4, with the relative rate constant of 7770 min−1 mM−1 at 25 °C. Pd(II)-coordinated nucleotides are also promising in NaBH4-mediated degradation of organic dyes. This work demonstrates a green and facile process to construct Pd nanocatalysts containing abundant active sites under the assistance by natural nucleotides.

Facile coupling of content design and efficient modulation on the activity of CNT-supported PdAgCu nanoparticle electrocatalysts: Leaching lift-up and annealing fall-off

In the present study, carbon nanotubes (CNTs) decorated with Pd, PdAg and PdAgCu nanocatalysts with various Cu content have been synthesized by a wet chemical route. These catalysts are characterized by X-ray diffraction (XRD), transmission electron spectroscopy (TEM), high-resolution transmission electron spectroscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperometry (CA). After suffering an efficient electrochemical leaching activation, the post-activated Pd25Ag25Cu50/CNTs catalyst demonstrates the best catalytic activities toward common small organic molecules oxidation reaction including methanol, ethanol and formic acid. Apart from the triggered high electrochemical active surface area (SEAS) owing to surface reconstruction via Cu depletion, the widespread geometric donation (i.e., lattice mismatch, strain, defect and/or dislocations) and strengthened electronic interaction between Pd, Ag and residual buried Cu should play significant roles in the catalytic activity enhancement. It also evidences that the annealing toward as-synthesized catalyst results in a significant decrease of SEAS value, thus engendering a degradation on the mass activities of electrocatalytic oxidation reaction toward small organic molecules.

Facile synthesis of monodispersed Pd nanocatalysts decorated on graphene oxide for reduction of nitroaromatics in aqueous solution

We synthesized the reproducible heterogeneous catalyst of graphene oxide (GO)-supported palladium nanoparticles (NPs) via a simple and green process. The structure, morphology and physicochemical properties of the synthesized heterogeneous catalyst were characterized by the latest techniques such as high-resolution transmission electron microscopy (TEM), scanning TEM, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, and X-ray photoelectron spectroscopy. The GO-supported Pd NPs (Pd/GO nanocatalyst) exhibited excellent catalytic activity for the reduction of nitroaromatics to aminoaromatics in aqueous sodium borohydride. The nitroaromatics were converted to corresponding aminoaromatics with high yields (up to 99%) using Pd/GO nanocatalyst in aqueous solution. The hybrid heterogeneous catalyst showed 83% of conversion after six cycles in the reduction of nitrobenzene to aminobenzene. These features ensured the high catalytic activity of the introduced graphene oxide supported Pd nanocatalysts.

Highly Active Pd Nanocatalysts Regulated by Biothiols for Suzuki Coupling Reaction

A series of biothiols involving N-acetyl-l-cysteine (NAC), penicillamine (PEN), homocysteine (Hcys) and cysteine (Cys), were employed as nucleation templates to synthesize Pd nanocatalysts for Suzuki coupling reaction. It is the first report to comparatively study the physicochemical properties of Pd nanoparticles synthesized by biothiols with different chemical structures. Through the characterizations of UV–Vis, FT-IR, TEM, XPS and HPLC, it is demonstrated that Pd(II) precursors can coordinate with biothiols mainly through thiol, amino/amido and carboxyl groups. NAC molecules are capable of generating uniform Pd nanoparticles with the size distribution from 2.2 to 3.3 nm. At the [PdCl42−]/[biothiol] of 6, NAC- and Hcys-stabilized Pd displays spherical particles with the average size of 3.3 and 3.6 nm respectively, whereas Cys- and PEN-capped Pd exhibit worm-like morphology with poor dispersion. NAC-capped Pd nanoparticles with the mean diameter of 3.3 nm and the Pd0 proportion of 63%, show highly active in Suzuki cross-coupling between iodobenzene and phenylboronic acid, with the turn over frequency (TOF) of 38625 mol biphenyl (mol Pd × h)−1 in EtOH/water = 1:2 at 60 °C. This work paves a promising way to construct highly active Pd nanocatalysts, with a facile modulation of the morphology, the particle size and the charge property via different biothiols. A series of biothiols were employed as nucleation templates to synthesize Pd nanocatalysts for Suzuki coupling reaction. This work paves a promising way to construct highly active Pd nanocatalysts, with a facile modulation of the morphology, the particle size and the charge property via different biothiols.

Tailoring the 3D Structure of Pd Nanocatalysts Supported on Mesoporous Carbon for Furfural Hydrogenation

AbstractBy quantitative 3D structural characterization of Pd nanoparticles supported on mesoporous carbon, we identify the most important structural aspects of the catalysts, which are responsible for the activity, selectivity and stability in liquid phase furfural hydrogenation. A systematic series of catalysts were prepared by three synthetic methods: incipient wetness impregnation, wet impregnation and immobilization of preformed PVA stabilized nanoparticles in order to control the Pd nanoparticle distribution on the external and internal surfaces of the mesoporous carbon. The activity and selectivity of these catalysts are highly affected by the particle locations. More Pd nanoparticles located inside pores promote the formation of 2‐methyl furan, while selectivity to furfuryl alcohol and tetrahydrofurfuryl alcohol is suppressed correspondingly. Through recycling tests we find that leaching depends on the synthesis process and plays an important role with respect to catalytic stability.

Anchored complexes of Ni, Pt, and Pd on Fe3O4 nanoparticles as new and eco-friendly nanocatalysts in Suzuki and Heck coupling reactions

Three recoverable nanocatalysts were developed by immobilizing Ni, Pt, and Pd Schiff-base complexes on the magnetite nanoparticles. Successful preparation of the nanocatalysts was evidenced by Fourier transform infrared (FT-IR) and energy dispersive X-ray (EDX) spectroscopy. Nano-sized spherical structure of the nanocatalysts was indicated by scanning electron microscopy (SEM). X-ray powder diffraction (XRD) determined that the crystalline cubic spinel structure of Fe3O4 remained constant through the synthesis of three complexes on Fe3O4 nanoparticles. Magnetic properties of the nanocatalysts were analyzed by the vibration sample magnetometer (VSM). Thermostability of the nanocatalysts was studied by thermogravimetric analysis (TGA). Metal loading of these nanocatalysts was evidenced by inductively coupled plasma atomic emission (ICP-AES).Investigating the catalytic activity of these nanocatalysts in Suzuki and Heck reactions implicated that in the presence of Pd nanocatalyst coupling reactions proceeded efficiently. For Pt nanocatalyst, Suzuki reaction took place in longer time with moderate to good yield. For the Heck reaction, the desired products were achieved only for aryl iodide and some aryl bromides. The Ni nanocatalyst could just catalyze the Suzuki reaction.Relying on the magnetic characteristic, these nanocatalysts could be simply recovered and reused several cycles without significant loss in catalytic activity.

Multilayered Pd nanocatalysts with nano-bulge structure in a microreactor for multiphase catalytic reaction

Microreactors are promising for fine chemistry and value-added chemical synthesis, but the small dimension places significant challenges in catalyst layer fabrication. Meanwhile, enhancing catalyst dispersion to obtain smaller particle size are of great interests. We proposed a multilayered Pd nanocatalysts with nano-bulge structure inside a microreactor for multiphase catalytic reaction. The unique bulged nanostructure can provide more accessible sites for precursor (PdCl42−) adsorption, and extend the available area to reduce the Pd nanoparticle size. Experimental results show rugged Pd catalyst layer with the nano-bulge structure was prepared, and both the precursor utilization rate and Pd nanocatalysts dispersion were enhanced, resulting in 35.4% higher catalyst loading and 4-fold smaller particle size. Benefitting from these advantages, compared with conventional Pd coated polydopamine catalyst layer, the proposed multilayered Pd nanocatalyst can significantly intensify the reaction rate and obtain an order of magnitude longer durability in nitrobenzene hydrogenation. This versatile and convenient fabrication method can be used in fabricating high performance and durable catalyst layers in microreactors.

Laser Synthesis of Carbonaceous TiO2 from Metal–Organic Frameworks: Optimum Support for Pd Nanoparticles for C–C Cross-Coupling Reactions

In this work, we report the development of highly active Pd nanoparticle catalysts encapsulated within novel carbonaceous and N-doped carbonaceous TiO2 support nanomaterials for C–C cross-coupling reactions. The support materials are generated, for the first time, via the laser-vaporization-controlled condensation (LVCC) of Ti and amino-functionalized Ti metal–organic frameworks (MOFs): MIL-125(Ti) and NH2-MIL-125(Ti). Ultrasmall crystalline anatase phases with a small fraction of the rutile phase of TiO2 nanoparticles are well dispersed within C and N-doped C spheres, resulting in improved supports for the Pd nanocatalysts. The Pd nanoparticles are generated by the photocatalytic reduction of Pd4+ ions in aqueous solutions containing the MOF-derived carbonaceous or N-doped carbonaceous TiO2 nanostructures using pulsed-laser irradiation without any chemical reducing agents or surface capping materials. The laser photoreduction of the Pd ions is accompanied by oxidative etching of the C spheres, resulting ...

Palladium nanoclusters immobilized on defective nanodiamond-graphene core-shell supports for semihydrogenation of phenylacetylene

Abstract We report a nanocarbon material with nanodiamond (ND) core and graphene shell (ND@G) as a support for Pd nanocatalysts. The designed catalyst performed good selectivity of styrene (85.2%) at full conversion of phenylacetylene and superior stability under mild conditions. Supported Pd catalysts are characterized by means of high resolution transmission electron microscopy (HRTEM), Raman, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and H2 temperature-programmed reduction (H2-TPR). The results clearly show that formation of the strong metal-support interaction (SMSI) between Pd nanoclusters and the defective graphene shell helpfully modifies the selectivity and stability of the Pd-based catalysts.

H2O adsorption on the Au and Pd single atom catalysts supported on ceria: A first-principles study

The interaction of H2O molecules with the CeO2(1 1 1) and (1 1 0) surfaces with the supported Au and Pd single atom catalysts (Au/CeO2 and Pd/CeO2) has been studied based on the first-principles calculation. It is found that H2O energetically prefers to be dissociative adsorption on the Pd/CeO2(1 1 0), Au/CeO2(1 1 1) and Au/CeO2(1 1 0) surfaces with the adsorption energies of 1.71, 1.91 and 1.40 eV, respectively, while both molecular (1.06 eV) and dissociative adsorption (1.15 eV) may coexist for H2O on the Pd/CeO2(1 1 1) surface. H2O dissociative adsorption is much more exothermic on the Pd/CeO2(1 1 0) surface than on the Pd/CeO2(1 1 1) surface, while the opposite behavior has been observed for H2O adsorption on the Au/CeO2 surface. The relevant mechanism has been discussed based on the electronic structure analysis. In addition, we have made a comparison between the Pd(Au)/CeO2 surfaces and the pristine CeO2 surfaces in terms of the H2O adsorption exothermicity and the kinetic barrier of H2O dissociation, which indicate that the supported metal single atoms can greatly enhance the chemical activity of CeO2 toward H2O. Our study would give useful insights into the reactions involving H2O catalyzed by the CeO2-supported Pd and Au nanocatalysts, when the Pd and Au are existing in the form of single atoms.

Defect Sites in Ultrathin Pd Nanowires Facilitate the Highly Efficient Electrochemical Hydrodechlorination of Pollutants by H*ads.

Adsorbed atomic H (H*ads) facilitates indirect pathways playing a major role in the electrochemical removal of various priority pollutants. It is crucial to identify the atomic sites responsible for the provision of H*ads. Herein, through a systematic study of the distribution of H*ads on Pd nanocatalysts with different sizes and, more importantly, deliberately controlled relative abundance of surface defects, we uncovered the central role of defects in the provision of H*ads. Specifically, the H*ads generated on Pd in an electrochemical process increased markedly upon introducing defect sites by changing the morphology to ultrathin polycrystalline Pd nanowires (NWs), while dramatically reducing upon decreasing the number of surface defects through an annealing treatment. Benefiting from a proportion of H*ads up to 40% of the total H* species, the Pd NWs showed an electrochemical active surface area normalized rate constant of 13.8 ± 0.8 h-1 m-2, which is 8-9 times higher than its Pd/C counterparts. The pivotal role of defect sites for the generation of H*ads was further verified by blocking such sites with Rh and Pt atoms, while theoretical calculation also confirms that the adsorption energy of H*ads on these sites is much higher than that on the Pd{111} facet.

Palladium Nanoparticles Immobilized on an Aminopropyl‐functionalized Silica‐Magnetite Composite as a Recyclable Catalyst for Suzuki‐Miyaura Reactions

AbstractHerein, we describe the straightforward synthesis and thorough characterization of a magnetically‐separable heterogeneous catalyst comprised of 1–3 nm‐sized Pd nanoparticles immobilized on a mesoporous silica‐magnetite composite (Pd0‐AmP‐SMC). Catalytic evaluations were conducted using Suzuki‐Miyaura cross‐couplings as the model reactions, for which this Pd nanocatalyst exhibited high performance in an environmentally‐friendly solvent mixture. Additionally, this Pd nanocatalyst could be re‐used up to five cycles without any observable loss of activity, and separation of the catalyst could be conveniently done by a magnet.

Electrostatically mediated selectivity of Pd nanocatalyst via rectifying contact with semiconductor: Replace ligands with light

Noble metal-based heterogeneous catalysts usually suffered from limited strategies for modifying their selectivity as compared with the homogeneous counterparts. Herein, we reported the electrostatically mediated selectivity of Pd nanocatalyst of usual supported Pd nanoparticles for total hydrogenation of furfural by enhancing the interfacial electric field using visible light and a rectifying contact with semiconductive supports to promote preferred adsorption of COgroups of furfural to the Pd side. This concept-new approach showed the potential to replace traditional organic ligands with light. The Pd nanoparticles deposited on a semiconductor support, optimized to be g-C3N4 (Pd/CN) in this work, exhibits outstanding selectivity (≥ 97%) to tetrahydrofurfuryl alcohol, far surpassing the selectivity of reported single-phase Pd nanocatalysts.

Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic Aldehydes by Palladium Immobilized on Amine-Functionalized Iron-Based Metal–Organic Frameworks

Visible-light-induced selective transfer hydrogenation of aromatic aldehyde to the corresponding alcohol was achieved by using Pd nanocatalyst supported on amine-functionalized iron-based metal–organic frameworks [Pd/MIL-101(Fe)-NH2] with triethylamine (TEA) as an electron donor and HCOOH as a proton source. The Pd/MIL-101(Fe)-NH2, obtained by an in situ photodeposition method, showed homogeneously and highly dispersed Pd nanoparticles (NPs) with a uniform size throughout the MIL-101(Fe)-NH2 support due to an effective stabilization role of amine groups on the backbone linkage of MIL-101(Fe)-NH2. The resulting Pd/MIL-101(Fe)-NH2 exhibited excellent catalytic performance toward a visible-light-induced transfer hydrogenation of benzaldehyde by producing a benzyl alcohol yield of 77% with a full benzaldehyde conversion in the presence of TEA-HCOOH. In addition to benzaldehyde, biomass-based renewable platform molecules such as furfural and 5-hydroxymethylfurfural (HMF) were successively converted into the co...

Coenzyme A-regulated Pd nanocatalysts for formic acid-mediated reduction of hexavalent chromium

Coenzyme A (CoA), as a cofactor of enzymes in bacteria, is involved in various reactions of intermediary metabolism owing to its thiol groups. Herein we develop a facile CoA-assisted approach for the green synthesis of nonspherical Pd nanoparticles (NPs) with grain boundaries, which potentially serve as the catalytic hot spots. As-prepared CoA-Pd nanocomposites were applied for the catalytic reduction of highly toxic chromium (VI) by using formic acid (FA) as the reductant with the highest reduction rate of 2.45 mmol mg−1 min−1 (50 °C), which is superior to recently reported homogenous and heterogeneous Pd-based catalysts. Through the characterizations by UV–vis, FTIR, TEM, XRD and XPS in combination with DFT calculations, CoA plays an essential role in regulating the growth of Pd NPs, resulting in the formation of nonspherical morphology at high [Na2PdCl4]/[CoA]. The catalytic activity of CoA-Pd was also assessed for the hydrogenation reduction of p-nitrophenol and the degradation of trichloroethylene (TCE) in the presence of formic acid. This strategy expands eco-friendly synthetic techniques to design highly efficient nanocatalysts to meet the requirement of catalytic reduction of Cr(VI) and other types of FA-mediated hydrogenation reactions.

Role of hydrogen absorption in supported Pd nanocatalysts during CO-PROX: Insights from operando X-ray absorption spectroscopy

The nature of the active phase (metallic vs. oxidic, metal phase vs. concentrated hydride/ dilute solid solution with hydrogen) in heterogeneous catalysis by supported metals is still a matter of high debate. Here, we have monitored for the first time oxide-supported Pd nanocatalysts (average particle size 4.5 nm) during both CO oxidation (in H2-free atmosphere) and preferential oxidation of CO in H2 excess (PROX) by operando X-ray absorption spectroscopy. Under our conditions, the CO conversion in the absence of H2 is around 30% at 150 °C and reaches 100% at 200 °C, whereas in the presence of H2 the conversion reaches a maximum of 15% (at 250 °C), in agreement with our previous works using a conventional flow-fixed bed reactor. The active phase for CO oxidation below 200 °C is metallic Pd, whereas it is a solid solution of Pd with hydrogen during PROX below 300 °C. This work provides a direct evidence of the presence of subsurface/bulk hydrogen as a probable cause of the low PROX performance of supported Pd catalysts.

Magnetic Pd nanocatalyst Fe3O4@Pd for C–C bond formation and hydrogenation reactions

Small core-shell Fe3O4@Pd superparamagnetic nanoparticles (MNPs) were obtained with good control in size and shape distribution by metal-complex thermal decomposition in organic media. The role of the stabilizer in the synthesis of MNPs was studied, employing oleylamine (OA), triphenylphosphine (TPP) and triphenylamine (TPA). The results revealed that, among the stabilizer investigated, the presence of oleylamine in the reaction media is crucial in order to obtain an uniform shell of Pd(0) in Fe3O4@Pd MNPs of 7 ± 1 nm. The synthesized core-shell MNPs were tested in Pd-catalyzed Heck-Mizoroki and Suzuki-Miyaura coupling reactions and p-chloronitrobenzene hydrogenation. High conversion, good reaction yields, and good TOF values were achieved in the three reaction systems with this nanocatalyst. The core-shell nanoparticle was easily recovered by a simple magnetic separation using a neodymium commercial magnet, which allowed performing up to four cycles of reuse.

Pd(0) nanocatalyst stabilized on a novel agar/pectin composite and its catalytic activity in the synthesis of biphenyl compounds by Suzuki-Miyaura cross coupling reaction and reduction of o-nitroaniline

The most important parameters for the fabrication of metallic nanoparticles are their synthesis method and the selection of stabilizer. These properties influence durability, shape, size, distribution, and catalytic performance of metallic nanoparticles. Therefore, a novel agar/pectin composite (APC) was designed in this study as a stabilizer, and then palladium nanocatalyst (Pd NPs@APC) was synthesized using green protocol which contains no hazardous reducing agents. The results of the thermal analysis showed that Pd NPs@APC had highly thermal durability (239 °C). Palladium nanoparticles were in spherical shape and homogeneously dispersed on APC surface. In addition, the particle sizes of palladium nanoparticles were determined as 34–54 nm. Catalytic activity of Pd NPs@APC was investigated in (i) Suzuki-Miyaura cross coupling reactions by using microwave irradiation to decrease high reaction time and energy as well as to provide clean synthesis and (ii) the reduction of o-nitroaniline to 1,2-benzenediamine at room temperature in water. Catalytic tests showed that Pd NPs@APC had high activity and sustainability performance against both Suzuki-Miyaura reactions and the reduction of o-nitroaniline.

Cooperative communication within and between single nanocatalysts.

Enzymes often show catalytic allostery in which reactions occurring at different sites communicate cooperatively over distances of up to a few nanometres. Whether such effects can occur with non-biological nanocatalysts remains unclear, even though these nanocatalysts can undergo restructuring and molecules can diffuse over catalyst surfaces. Here we report that phenomenologically similar, but mechanistically distinct, cooperative effects indeed exist for nanocatalysts. Using spatiotemporally resolved single-molecule catalysis imaging, we find that catalytic reactions on a single Pd or Au nanocatalyst can communicate with each other, probably via hopping of positively charged holes on the catalyst surface, over ~102 nanometres and with a temporal memory of ~101 to 102 seconds, giving rise to positive cooperativity among its surface active sites. Similar communication is also observed between individual nanocatalysts, however it operates via a molecular diffusion mechanism involving negatively charged product molecules, and its communication distance is many micrometres. Generalization of these long-range intra- and interparticle catalytic communication mechanisms may introduce a novel conceptual framework for understanding nanoscale catalysis.

Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

Summary Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes.