Au Pd Research Articles

Integration of Plasmonic Metal and Cocatalyst: An Efficient Strategy for Boosting the Visible and Broad‐Spectrum Photocatalytic H2 Evolution

AbstractThe combination of plasmonic metal and cocatalyst with semiconductor has been widely used to simultaneously improve the efficiencies of light absorption, charge separation, and surface reaction for high‐performance photocatalysis. Despite the tremendous efforts, the ternary nanostructures suffer from two bottlenecks: 1) long distance between plasmonic metal and cocatalyst increases the chance of hot electron loss; 2) opposite transmission directions between plasmon‐induced hot electrons and semiconductor‐generated free electrons disfavor the practical application of broad‐spectrum photocatalysis. In this paper, a novel photocatalytic nanostructure, in which plasmonic Au and Pd cocatalysts are integrated into Janus heterojunction on the TiO2, is demonstrated. The advantages of the designed photocatalyst include: 1) the shortened distance between Au and Pd as well as the formation of additional channel of Au–Pd interface accelerates the hot electron transfer; 2) the opposite directions of visible‐light‐induced hot electrons and ultraviolet‐light‐excited free electrons are mediated with a unified TiO2 → Au → Pd transmission route during the broad‐spectrum photocatalysis. As a result, 15 mg of the designed photocatalyst achieves average H2 production rates of 0.79 and 7.59 µmol h−1 under visible and broad‐spectrum irradiation, respectively, much higher than those of other binary and ternary architectures consisting of TiO2, Pd, and Au.

Direct Synthesis of Hydrogen Peroxide over Au–Pd Supported Nanoparticles under Ambient Conditions

The direct synthesis of hydrogen peroxide (H2O2) from molecular H2 and O2 over Au-Pd / TiO2 catalysts prepared using a modified impregnation methodology is studied. Initially we investigated the role of Au: Pd ratio on the catalytic activity towards H2O2 formation and its subsequent degradation under conditions that have previously been found to be optimal for the formation of H2O2 using high pressure reactants and low temperatures. Subsequently, we have conducted a detailed study of H2O2 synthesis under ambient conditions which are both more challenging for achieving high yields of H2O2 and are more likely to be used for future industrial applications.

Alloying of gold with palladium: An effective strategy to improve catalytic stability and chlorine-tolerance of the 3DOM CeO2-supported catalysts in trichloroethylene combustion

The three-dimensionally ordered macroporous (3DOM) CeO2-supported Au–Pd alloys (xAuPdy/3DOM CeO2, x is the total loading (wt%) of Au and Pd, and y is the Pd/Au molar ratio) were synthesized using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods. The samples were characterized by a number of analytical techniques, and their catalytic performance was evaluated for the combustion of trichloroethylene (TCE). It is found that the xAuPdy/3DOM CeO2 samples displayed a good-quality 3DOM architecture, and the noble metal nanoparticles (NPs) with a size of 3–4 nm were uniformly dispersed on the skeleton surface of 3DOM CeO2. Among all of the samples, 2.85AuPd1.87/3DOM CeO2 exhibited the highest catalytic activity, with the temperature at a TCE conversion of 90% (T90%) being 415 °C at a space velocity of 20,000 mL/(g h). Furthermore, the 2.85AuPd1.87/3DOM CeO2 sample possessed the lowest apparent activation energy (33 kJ/mol), excellent catalytic stability, and good moisture- and chlorine-tolerant behaviors. Alloying of Au with Pd changed the pathway of TCE oxidation and reduced formation of perchloroethylene. We conclude that the excellent catalytic performance for TCE combustion of 2.85AuPd1.87/3DOM CeO2 was associated with the highly dispersed AuPd1.87 alloy NPs, high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between AuPd1.87 NPs and 3DOM CeO2 as well as the high-quality 3DOM structure and high surface acidity.

Nitrogen-Rich Porous Organic Polyamines for Stabilization of Highly Dispersed Metal Nanoparticles and Catalytic Application.

Nitrogen-rich triazine-based porous organic polyamines (POPa) synthesized via a one-step polycondensation of melamine and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde is employed to synthesize Au and Pd nanoparticles well-dispersed on POPa. The as-prepared POPa-supported Au NPs and Pd NPs (AuNPs@POPa, PdNPs@POPa) with a narrow size distribution show remarkable catalytic activity for the reduction of nitrobenzene compounds and organic dyes and the Suzuki-Miyaura coupling reaction, respectively. Benefitting from POPa the AuNPs@POPa and PdNPs@POPa catalysts can be readily recovered and reused almost without loss of activity. The nitrogen-rich porous organic polyamines provide great opportunities to prepare functional metal nanocatalysts with potential in the heterogeneous catalysis field.

Optical Sensing of Humidity Using Polymer Top-Covered Bragg Stacks and Polymer/Metal Thin Film Structures.

Thin films with nanometer thicknesses in the range 100–400 nm are prepared from double hydrophilic copolymers of complex branched structures containing poly(N,N-dimethyl acrylamide) and poly(ethylene oxide) blocks and are used as humidity sensitive media. Instead of using glass or opaque wafer for substrates, polymer thin films are deposited on Bragg stacks and thin (30 nm) sputtered Au–Pd films thus bringing color for the colorless polymer/glass system and enabling transmittance measurements for humidity sensing. All samples are characterized by transmittance measurements at different humidity levels in the range from 5% to 90% relative humidity. Additionally, the humidity induced color change is studied by calculating the color coordinates at different relative humidity using measured spectra of transmittance or reflectance. A special attention is paid to the selection of wavelength(s) of measurements and discriminating between different humidity levels when sensing is performed by measuring transmittance at fixed wavelengths. The influence of initial film thickness, sensor architecture, and measuring configuration on sensitivity is studied. The potential and advantages of using top covered Bragg stacks and polymer/metal thin film structures as humidity sensors with simple optical read-outs are demonstrated and discussed.

Ring-Opening Transformation of 5-Hydroxymethylfurfural Using a Golden Single-Atomic-Site Palladium Catalyst

The development of function-integrated catalysts to enable scalable and efficient biomass valorization is an important topic. Here, a stepwise methodology was developed to fabricate novel supported Au–Pd bimetallic catalysts featuring immobilized well-defined on-surface nanoarchitectures (Pd∧Au). This was achieved by an atomic decoration of a trace amount of Pd (Pd/Au molar ratio of 0.01–0.02) on the surface of 2 nm Au nanoparticle (NP, 0.86 wt %) preanchored to titania. Benefiting from their unique structural merits and the cooperative interplay among the selective hydrogenation activity furnished by the ultrasmall Pd∧Au NPs and prominent Lewis acidity endowed by TiO2, Pd0.02∧Au/TiO2 exhibits excellent activity with 100% conversion and over 90% selectivity and stability (turnover number up to 48 300) toward controlled ring-opening conversion of 5-hydroxymethylfurfural and related furanic compounds to the corresponding di- and triketone-type products under mild conditions (120 °C, 10 bar of H2). The resul...

Formation of PdO on Au–Pd bimetallic catalysts and the effect on benzyl alcohol oxidation

A series of catalysts consisting of gold–palladium bimetallic nanoparticles (Au–Pd NPs in the range of 1–6 nm) anchored on foamlike mesoporous silica were used for the aerobic oxidation of benzyl alcohol. A remarkable synergistic effect was observed on these Au–Pd NP catalysts prepared by a one-pot method. Both experimental and theoretical study revealed a close relationship between the surface PdO species on the catalysts and their catalytic performance; that is, higher surface PdO content leads to lower catalytic activity. The surface content of PdO species on the catalysts could be tuned by controlling the Au/Pd ratios, because the formation of Au–Pd alloy NPs and electron transfer between surface Au and Pd atoms prevented the oxidation of surface Pd and retarded the formation of PdO species. An optimal Au/Pd ratio of 1/4.5 on the foamlike mesoporous silica support was obtained, with nearly no surface PdO species formed, and resulted in the highest benzyl alcohol conversion of 96%. The bimetallic Au–Pd catalysts exhibited much higher catalytic activity for benzyl alcohol oxidation (TOF = 50,000–60,000 h−1) than the monometallic Pd catalyst (TOF = 12,500 h−1), on which surface Pd is easily oxidized to PdO. These results provide direct evidence for the synergistic effect of Au–Pd bimetallic catalysts in benzyl alcohol oxidation.

Size, nanostructure, and composition dependence of bimetallic Au–Pd supported on ceria–zirconia mixed oxide catalysts for selective oxidation of benzyl alcohol

A bimetallic Au–Pd catalyst supported on ceria–zirconia with Au:Pd molar ratio 0.8 has been synthesized using a simultaneous deposition–precipitation method and oxidized at 250, 450, and 700 °C in order to modify its particle size, nanostructure, and composition. Combined X-ray energy dispersive spectroscopy (XEDS) and X-ray photoelectron spectroscopy (XPS) analysis clearly evidence that the bimetallic Au–Pd catalyst oxidized at 250 °C is made up of a mixture of monometallic Au and Pd and bimetallic Au–Pd nanoparticles with Au:Pd ratios varying over a wide range. Increasing oxidation temperature leads to a stronger interaction between Au and Pd. Meanwhile, a slight increase of particle size and a narrowing of the Au:Pd ratio in the bimetallic nanoparticles take place. Compared with titania and activated carbon supports, the resistance against sintering at high temperatures of Au–Pd metal particles supported on ceria–zirconia is proven to be higher. A synergistic effect has been observed for selective oxidation of benzyl alcohol on these catalysts. The catalytic activity decreases only slightly after oxidation at 450 °C. However, oxidation at 700 °C results in much lower catalytic activity. Migration of Pd onto Au particles during oxidation of benzyl alcohol enhances the catalytic activity of a physical mixture of monometallic Au and Pd supported on ceria–zirconia catalysts. This fact, jointly with an analysis of the intrinsic activity, reveals the influence of the actual nature of Au–Pd interactions in the bimetallic particles, which points to higher activity of Au@Pd or AuPd@Pd nanostructures on ceria–zirconia supports.

Evaluation, of the bimetallic photocatalytic performance of Resin–Au–Pd nanocomposite for degradation of parathion pesticide under visible light

In this work, Pd-Au nanoparticles supported on Amberlyst® resin (Pd-Au/Amberlyst®) as bimetallic photocatalyst with super adsorption performance, plasmonic effect, and fast interfacial charge migration was synthesized. The nano photocatalyst was characterized by FT-IR, ICP, XRD, FESEM, EDS mapping analysis, and TEM. Photocatalytic degradation under visible light of parathion as an organophosphorus pesticide by Pd-Au/Amberlyst® nanocomposite was studied. UV–Vis absorption spectroscopic technique studies were employed to investigate the performance of the nanocatalyst. This nanocatalyst has several fascinating advantages including easy preparation method, good catalytic activity, environmentally benign reaction conditions and was recovered with no considerable decrease in activity, it could be reused 6 times without obvious decrease in the yield of reactions.

Pd–Au–Y as Efficient Catalyst for C–C Coupling Reactions, Benzylic C–H Bond Activation, and Oxidation of Ethanol for Synthesis of Cinnamaldehydes

Pd–Au nanoalloy supported on zeolite-Y (Pd–Au–Y) matrix was found to be an effective catalyst for C–Cl bond activation and oxidative coupling of 2-naphthol, leading to the formation of various biar...

A Fermi-Level-Pinning-Free 1D Electrical Contact at the Intrinsic 2D MoS2 -Metal Junction.

Currently 2D crystals are being studied intensively for use in future nanoelectronics, as conventional semiconductor devices face challenges in high power consumption and short channel effects when scaled to the quantum limit. Toward this end, achieving barrier-free contact to 2D semiconductors has emerged as a major roadblock. In conventional contacts to bulk metals, the 2D semiconductor Fermi levels become pinned inside the bandgap, deviating from the ideal Schottky-Mott rule and resulting in significant suppression of carrier transport in the device. Here, MoS2 polarity control is realized without extrinsic doping by employing a 1D elemental metal contact scheme. The use of high-work-function palladium (Pd) or gold (Au) enables a high-quality p-type dominant contact to intrinsic MoS2 , realizing Fermi level depinning. Field-effect transistors (FETs) with Pd edge contact and Au edge contact show high performance with the highest hole mobility reaching 330 and 432 cm2 V-1 s-1 at 300 K, respectively. The ideal Fermi level alignment allows creation of p- and n-type FETs on the same intrinsic MoS2 flake using Pd and low-work-function molybdenum (Mo) contacts, respectively. This device acts as an efficient inverter, a basic building block for semiconductor integrated circuits, with gain reaching 15 at VD = 5 V.

Insight into restructuring of Pd-Au nanoparticles using EXAFS

We acquired Pd K-edge and Au L3-edge EXAFS spectra of two libraries of titania-supported Pd-doped Au nanoparticle catalysts representing the as-prepared and post-catalysis state. The nanoparticles have narrow particle size distributions, in the range 2.8–6.6 nm, and were synthesized to preferentially locate the palladium atoms on the surface of a gold core, with Pd mole fractions of 1.4–38%. EXAFS spectra of both the as-prepared samples and post-catalysis Pd-Au particles were collected and modeled. EXAFS refinement of all samples showed contributions from both Pd-Au and Pd-Pd scattering paths, revealing that Pd and Au components alloyed together. Pd was in a partially oxidized state as evidenced by a Pd-O scattering contribution, unlike Au. Overlaying measured coordination numbers from the libraries with predicted values, using models of surface and bulk alloying, showed that Pd restructures among Au as a result of exposure to the reaction conditions.

Green Photocatalytic Oxidation of Benzyl Alcohol over Noble-Metal-Modified H2Ti3O7 Nanowires

One-dimensional H2Ti3O7 nanowires (NWs) supported by Au, Ag, and Pd monometallic nanoparticles (NPs) and Au–Pd bimetallic NPs were prepared and used for photochemical benzyl alcohol oxidation. Tech...

Self-limiting synthesis of Au–Pd core–shell nanocrystals with a near surface alloy and monolayer Pd shell structure and their superior catalytic activity on the conversion of hexavalent chromium

In eliminating environmental chromium pollution, hexavalent chromium (Cr(VI)) reduction by HCOOH on Pd-based bimetallic/multimetallic nanocatalysts is attracting an increasing attention. However, it is still a challenge by precisely controlling the surface interface structure of the nanocrystals (NCs) to obtain highly efficient nanocatalysts on Cr(VI)) conversion. Here, a simple and robust synthesis method for Au-Pd core-shell NCs with a special interface structure has been developed by a novel self-limiting strategy. This interesting structure has a near surface alloy (NSA) of AuPd layer on Au core with outer surface of monolayer (ML) of Pd shell (denoted as NSA/ML), which is achieved only by one-pot synthesis. The self-limiting strategy is developed based on the unique catalysis reduction reaction of Au core, with which the growth of Pd shell is internally regulated by the surface Au atoms until the Au–Pd NCs with NSA/ML structure i.e. Au@AuPd@PdML NCs are formed. Furthermore, the Au–Pd core–shell NCs with the shell from NSA to NSA/ML structure can be simply obtained by adjusting the added amount of Pd precursor. More importantly, the catalytic activities of Au–Pd NCs for the conversion of Cr(VI) to Cr(III) present a notable difference for the Pd in Au@AuPd and Au@AuPd@PdML structures, and the Au@AuPd@PdMLNCs exhibited the superior catalytic activity for the conversion of Cr(VI). The present study demonstrates the core@NSA@shellML is an efficient nanostructure to promote the catalytic performance of core–shell NCs for practical applications.

Oxidative Cross-Esterification and Related Pathways of Co-Adsorbed Oxygen and Ethanol on Pd–Au

Ethanol is one of the most important industrial chemicals considering its wide range of uses. From ultrahigh vacuum (UHV) experiments, we were able to effectively produce acetaldehyde and ethyl acetate on a Pd–Au model catalyst. The presence of oxygen at the Pd–Au interface sites was found to be highly reactive for ethanol dehydrogenation, significantly increasing acetaldehyde production and promoting cross-esterification to produce ethyl acetate. Density functional theory calculations show that compared to the bare PdAu(111) surface, the presence of preadsorbed oxygen and the subsequently formed hydroxyl group leads to highly active and selective initial ethanol dehydrogenation at the O–H and α–C-H position, which was corroborated by isotope-labeling experiments. Specifically, using temperature-programmed desorption with the isotope CD3CH2OH, we identified C–C bond breakage through methane production. Furthermore, the simultaneous desorption of D2 with ethyl acetate from the oxidation of CD3CD2OD suggest...

Selective Oxidation of Acetaldehyde to Acetic Acid on Pd–Au Bimetallic Model Catalysts

Acetic acid is a widely employed reactant in the chemical industry, and it is also used as a food ingredient. Here, we report a catalytic reaction pathway for the gas-phase selective oxidation of acetaldehyde to acetic acid on a Pd–Au(111) heterogeneous model catalyst. On an oxygen precovered Pd–Au surface, acetaldehyde is selectively oxidized to acetic acid from 250 to 340 K. Using FT-IR, the formation of acetate species is detected from 160 to 260 K on this surface, which is a widely known adsorbate derived from acetic acid on metal surfaces. With higher Pd coverages, the acetaldehyde is less selectively oxidized to acetic acid, and near 375 K, CO2, H2O, CH4, and H2 are evolved, evidence for the decarboxylation of acetate. In our density functional theory calculations, we confirm that the relative energy difference between the acetate state and the decarboxylated state decreases as Pd ensemble size increases.

A Gold–Palladium Nanoparticle Alloy Catalyst for CO Production from CO2 Electroreduction

Electrochemical CO2 reduction reaction (CO2RR) is a promising technology for combining CO2 reutilization and renewable electricity storage. For economic feasibility, better catalysts are required to overcome current limitations such as high overpotentials, poor faradaic efficiencies (FEs), and low current densities. Herein, size‐ and composition‐controlled gold (Au)–palladium (Pd) bimetallic alloy nanoparticles prepared by a metal vapor synthesis technique together with the monometallic Au and Pd equivalent materials are investigated. X‐ray diffraction and high‐angle annular dark‐field scanning transmission electron microscopy energy‐dispersive X‐ray spectroscopy analyses confirm the Au–Pd alloy formation with an average atomic ratio of 76 Pd wt% and 24 Au wt%. These bimetallic and the monometallic catalysts are characterized and tested for the electroreduction of CO2 in electrochemical cells and also in a complete CO2 electrolysis cell. Analysis of the reduction products shows a 100% CO2RR selectivity for CO. The FE for CO with respect to H2 as high as 90% is obtained with Au–Pd/C by tuning the electrode structure. The Au/C and Pd/C catalysts also show a better selectivity for CO with no evidence of other CO2RR products. The Au–Pd alloy formation improves the FE of Pd for CO by suppressing the parasitic H2 evolution reaction.

Self‐Aligned Anisotropic Plasmonic Nanostructures

Great opportunities emerge not only in the generation of anisotropic plasmonic nanostructures but also in controlling their orientation relative to incident light. Herein, a stepwise seeded growth method is reported for the synthesis of rod-shaped plasmon nanostructures which are vertically self-aligned with respect to the surface of colloidal substrates. Anisotropic growth of metal nanostructure is achieved by depositing metal seeds onto the surface of colloidal substrates and then selectively passivating the seed surface to induce symmetry breaking in the subsequent seed-mediated growth process. The versatility of this method is demonstrated by producing nanoparticle dimers and linear trimers of Au, Au-Ag, Au-Pd, and Au-Cu2 O. Further, this unique method enables the automatic vertical alignment of the resulting plasmonic nanostructures to the surface of the colloidal substrate, thereby making it possible to design magnetic/plasmonic nanocomposites that allow the dynamic tuning of the plasmon excitation by controlling their orientation using an external magnetic field. The controlled anisotropic growth of colloidal plasmonic nanostructures and their dynamic modulation of plasmon excitation further allow them to be conveniently fixed in a thin polymer film with a well-controlled orientation to display polarization-dependent patterns that may find important applications in information encryption.

AuPd/C core–shell and alloy nanoparticles with enhanced catalytic activity toward the electro-oxidation of ethanol in alkaline media

Carbon-supported Pd, Au@Pd core–shell and Au1–xPdx-alloyed nanoparticles were prepared by a chemical reduction method and characterized by different experimental techniques, including X-ray powder diffraction, transmission electron microscopy, scanning-transmission electron microscopy using bright-field and high-angle annular dark field detectors and X-ray energy dispersive spectroscopy. The catalytic mass activity toward ethanol oxidation was assessed by cyclic voltammetry and chronoamperometry at room temperature. The measurements showed that the addition of Au enhances remarkably the electrocatalytic activity of the material, due to the bifunctional effect of Au1–xPdx/C alloys, and the synergetic effect on Au@Pd/C, resulting in a dissolution resistance of core–shell catalysts at potentials of 1.5 V versus reversible hydrogen electrode. In situ Fourier transform infrared spectroscopy measurements showed that the mechanism for ethanol oxidation depends on the electrocatalyst structure and morphology. Acetate was identified as the main product of ethanol electro-oxidation on the studied electrocatalysts. However, the presence of a core–shell structure on Au@Pd/C resulted in enhanced ethanol oxidation selectivity toward CO2. The improvement of activity is attributed to the interaction between Pd shell and Au core.

Thermal properties and segregation phenomena in transition metals and alloys: modeling based on modified cohesive-energies

In spite of free-atom electronic-relaxation contributions to transition-metal cohesive-energies, numerous studies have misused the latter instead of using the solid-state interatomic bond-energy in modeling bulk and surface properties. This work reveals that eliminating the free-atom contributions from experimental cohesive-energies leads to highly accurate linear correlations of the resultant bond-energies with melting temperatures and enthalpies, as well as with inverse thermal-expansion coefficients, specifically for the fcc transition-metals. Likewise, predictions of surface segregation phenomena in Cu–Pd and Au–Pd alloys on the basis of the modified energetics are in much better agreement with reported low-energy ion scattering spectroscopy (LEISS) experimental results, as compared to the use of cohesive-energy values. A last demonstration of the problem and its solution involves the significant impact of the modification on segregation (separation) phase transitions in Cu–Ni model nanoparticles.