Noble Metal Particles Research Articles

Deceleration of SO2 poisoning on PtPd/Al2O3 catalyst during complete methane oxidation

The inhibiting effect of SO2 on the catalytic activity of the monometallic Pt/Al2O3 and Pd/Al2O3, as well as on bimetallic PtPd/Al2O3 catalyst for the complete oxidation of methane under lean conditions has been studied. Flow reactor experiments, in-situ DRIFT spectroscopy and characterization with XPS, STEM-EDX were performed. It was found that the addition of Pt to the Pd/Al2O3 resulted in a catalyst that was more robust towards sulfur poisoning. XPS results revealed residual sulfates on catalyst surface after regeneration. This was confirmed with EDX analysis, which demonstrated that sulfur was accumulated in noble metal particles and especially in the region of the particle rich in Pt. Although the catalyst has been deactivated in the presence of both SO2 and H2O, an additional presence of NO in the gas mixture of reactants resulted in an increased lifetime of the sample under reaction conditions. This NO effect strongly depends on the temperature of experiments and is most intense at a temperature close to 550 °C A postponed inhibition caused by the addition of NO may be explained by the DRIFTS results, which demonstrated that the presence of NO lowers the sulfate formation and mostly surface sulfites are observed that increase the lifetime of the catalyst during SO2 exposure.

Silica-Coated Plasmonic Metal Nanoparticles in Action.

Hybrid colloids consisting of noble metal cores and metal oxide shells have been under intense investigation for over two decades and have driven progress in diverse research lines including sensing, medicine, catalysis, and photovoltaics. Consequently, plasmonic core-shell particles have come to play a vital role in a plethora of applications. Here, an overview is provided of recent developments in the design and utilization of the most successful class of such hybrid materials, silica-coated plasmonic metal nanoparticles. Besides summarizing common simple approaches to silica shell growth, special emphasis is put on advanced synthesis routes that either overcome typical limitations of classical methods, such as stability issues and undefined silica porosity, or grant access to particularly sophisticated nanostructures. Hereby, a description is given, how different types of silica can be used to provide noble metal particles with specific functionalities. Finally, applications of such nanocomposites in ultrasensitive analyte detection, theranostics, catalysts, and thin-film solar cells are reviewed.

Preparation and photoelectric properties of Pt/TiO2 nanotube electrodes by a pre-doping method

A pre-doping method in which Pt nanoparticles (NPs) are introduced as dopants before the formation of a TiO2 nanotube (TNT) array was proposed in this paper. This method was intended to decorate a TNT array film with Pt nanoparticles (Pt/TNT), which effectively avoided the problem of blocking the top of the nanotubes by noble metal particles by post-doping methods. Pt/TNT array film was fabricated by anodizing the double Pt/Ti metal film, which was obtained by successive magnetron sputtering of Pt and Ti films onto FTO substrates. X-ray diffraction, X-ray photoelectron spectroscopy, and field emission scanning electron microscopy were used to characterize the microstructure of the Pt/TNT nanotubes array, which revealed that Pt NPs located at the bottom of the Ti film can diffuse into the upper TNT array film after anodization and annealing. Pt/TNT electrode exhibits a superior photoelectric property over that of TNT, of which the photocurrent value of the Pt/TNT electrode was more than twice that of the pure TNT electrode. It is therefore obvious that the pre-doping method is feasible and effective for fabricating a high-photocatalytic-efficient Pt/TNT electrode.

Advanced Insight into the Size Effect of PtPd Nanoparticles on NO Oxidation by in Situ FTIR Spectra

It is a recognized view that particle size could play a significant role in catalytic performance. To obtain the nanoparticles with the different size, a series of bimetallic PtPd/SiO2–Al2O3 catalysts subjected to hydrothermal treatment in different time were prepared and applied for NO oxidation under diesel exhaust. The experimental results confirmed that the average noble particle size was gradually increased at a different variation rate, along with aging time. An increasing of average particle size resulted in the reduction of the corresponding NO turnover frequencies (TOFs) by different degrees. Thus, a new sight on the relationship between particle size and NO catalytic activity is obtained. In the aging process, the overall trend of noble metal particles got larger along with receding NO catalytic performance, yet in a period, a few small particles presented more bridged/chelating nitrate and ionic nitrate species, further retarding its rate of loss activity.

Mechanistic insights into plasmonic photocatalysts in utilizing visible light.

The utilisation of sunlight as an abundant and renewable resource has motivated the development of sustainable photocatalysts that can collectively harvest visible light. However, the bottleneck in utilising the low energy photons has led to the discovery of plasmonic photocatalysts. The presence of noble metal on the plasmonic photocatalyst enables the harvesting of visible light through the unique characteristic features of the noble metal nanomaterials. Moreover, the formation of interfaces between noble metal particles and semiconductor materials further results in the formation of a Schottky junction. Thereby, the plasmonic characteristics have opened up a new direction in promoting an alternative path that can be of value to the society through sustainable development derived through energy available for all for diverse applications. We have comprehensively prepared this review to specifically focus on fundamental insights into plasmonic photocatalysts, various synthesis routes, together with their strengths and weaknesses, and the interaction of the plasmonic photocatalyst with pollutants as well as the role of active radical generation and identification. The review ends with a pinnacle insight into future perspectives regarding realistic applications of plasmonic photocatalysts.

The Effect of Annealing Treatment and Atom Layer Deposition to Au/Pt Nanoparticles-Decorated TiO₂ Nanorods as Photocatalysts.

The wide band gap of TiO2 hinders the utilization of visible light in high-performance photocatalysis. Herein, vertically aligned Ti nanopillar arrays (NPAs) were grown by the glancing angle deposition method (GLAD) and then thermally oxidized into TiO2 NPAs. The metallic nanoparticles (NPs) were fabricated by successive ion layer adsorption and reaction (SILAR) method. And we covered ultrathin TiO2 layer on Au/Pt NPs decorated NPA using atomic layer deposition (ALD) method and did annealing process in the end. The photoelectrochemical (PEC) performance and dye degradation have been studied. We find the dye degradation efficiency of best combination reaches up to 1.5 times higher than that of original Au/Pt-TiO2 sample under visible light irradiation. The TiO2 ALD layer effectively protects the nanostructure from corrosion and helps the transmission of electrons to the electrolyte. By controlling the annealing temperature we could achieve a matched band gap due to change in noble metal particle size. Our work demonstrates that rational design of composite nanostructures enhances the usage of broader wavelength range light and optimizes photocatalytic degradation of organic pollutants in practical applications.

A new alternative self-assembled-monolayer activation process for electroless deposition of copper interconnects without a conventional barrier

Amino-terminated, self-assembled monolayers (NH2-SAMs) are widely used for seeding in electroless metallization, and activation is typically performed in an acidic aqueous suspension of colloidal Pd or Au, generating noble-metal catalytic particles of sizes generally exceeding 10 nm. In this study, we develop a new activation process by which aliphatic NH2-SAMs immobilize Ni particles on low-k (SiOCH) dielectric layers, and which involves a cost-effective basic aqueous solution simply containing Ni2+ ions. Rutherford backscattering spectroscopy suggests that the activated SAMs work effectively as diffusion barriers, enabling integration of Cu with SiOCH without interposing a conventional barrier layer. Moreover, as revealed by transmission electron microscopy, the sizes of the Ni particles adsorbed are extremely small (2–3nm), leading to fast nucleation and the formation of void/seam-free Cu interconnects in 60-nm trenches. Based on the results of X-ray photoelectron spectroscopy, a new model of changes in the surface structure (and the phase transitions of the seeds) leading to effective seeding is proposed.

Synergistic effects of semiconductor substrate and noble metal nano-particles on SERS effect both theoretical and experimental aspects

As a means of chemical identification and analysis, Surface enhanced Raman spectroscopy (SERS), with the advantages of high sensitivity and selectivity, non-destructive, high repeatability and in situ detection etc., has important significance in the field of composition detection, environmental science, biological medicine etc. Physical model of coupling effect between different semiconductor substrates and noble metal particles were investigated by using 3D-FDTD method. Mechanism and the effects of excitation wavelength, particle spacing and semiconductor substrate types on the SERS effect were discussed. The results showed that the optimal excitation wavelengths of three noble metals of Ag, Au, Cu, were located at 510, 600 and 630nm, respectively; SERS effect of Ag, Au, Cu increases with the decreasing of the inter distance of particles, while the distance of the NPs reaches the critical value of 3nm, the strength of SERS effect will be greatly enhanced. For the four different types of substrate of Ge, Si, SiO2 (glass) and Al2O3, the SERS effect of Ag on SiO2>Ge>Al2O3>Si. For Au and Cu nanoparticles, the SERS effect of them on oxide substrate is stronger than that on non-oxide substrate. In order to verify FDTD simulations, taking silver nanoparticles as an example, and silver nanoparticles prepared by chemical method were spinning coating on the four different substrates with R6G as probe molecules. The results show that the experimental results are consistent with FDTD theoretical simulations, and the SERS enhancement effect of Ag-SiO2 substrate is best. The results of this study have important theoretical significance to explain the variations of SERS enhancement on different noble metals, which is also an important guide for the preparation of SERS substrates, especially for the microfluidics. The better Raman effect can be realized by choosing proper substrate type, particle spacing and excitation wavelength, result in expanding the depth and width of SERS application.

Composite of Au-Pd nanoalloys/reduced graphene oxide toward catalytic selective organic transformation to fine chemicals

A facile, stabilizing-molecules-free strategy has been utilized for anchoring Au-Pd alloy nanoparticles onto the flat surface of two-dimensional (2D) reduced graphene oxide (RGO) nanosheets. Formation of Au-Pd nanoalloys and loading onto the RGO are accomplished simultaneously. The Au-Pd/reduced graphene oxide (Au-Pd/RGO) exhibits higher catalytic activity than both Au/RGO and Pd/RGO, prepared by the same approach toward selective oxidation of benzyl alcohol and selective reduction of nitroaromatics, the catalytic activity order can be in good agreement with the noble metal particles size distribution of the Au, Pd and Au-Pd/RGO.

Plasmonic gold nanostars as optical nano-additives for injection molded polymer composites

Nanoscale engineering of noble metal particles has provided numerous material configurations to selectively confine and manipulate light across the electromagnetic spectrum. Transitioning these materials to a composite form while maintaining the desired resonance properties has proven challenging. In this work, the successful integration of plasmon-focusing gold nanostars (GNSs) into polymer nanocomposites (PNCs) is demonstrated. Tailored GNSs are produced with over a 90% yield and methods to control the branching structures are shown. A protective silica capping shell is employed on the nanomaterials to facilitate survivability in the high temperate/high shear processing parameters to create optically-tuned injection molded PNCs. The developed GNS PNCs possess dichroic scattering and absorption behavior, opening up potential applications in the fields of holographic imaging, optical filtering and photovoltaics.

Trace iron impurities deactivate palladium supported on nitrogen-doped carbon nanotubes for nitrobenzene hydrogenation

Nitrogen-doped carbons can effectively stabilize noble metal particles to achieve high catalytic performances. However, the metallic impurities in carbon nanomaterials, e.g. the residual growth catalysts of carbon nanotubes (CNTs), have some unforeseen effects on the catalysis involving nanocarbons. Herein, we demonstrate that the residual growth catalysts of N-doped CNTs (NCNTs) may significantly deactivate Pd catalysts for the hydrogenation of nitrobenzene. Through high-resolution transmission electron microscopy and CO-stripping, it was determined that the N dopants improved the dispersion of Pd nanoparticles. However, the iron, at ppm level, in residual catalysts encapsulated inside NCNTs can transfer onto the surface of Pd to block active sites so that the activity of Pd/NCNTs was much lower than that of Pd/CNTs. The similar effect was observed for most of the common metallic impurities in carbon materials, including Co, Ni, Mn, Cr, Cu, Zn, Mo, Al and Mg. To exploit the N-doped carbons, we deposited N-doped carbon layers on purified CNTs through pyridine pyrolysis and then supported Pd nanoparticles. By this means, the activity of Pd for nitrobenzene hydrogenation was improved by 3.85 folds compared to conventional CNTs, emphasizing the importance of controlling impurities in N-doped carbon materials for high performance catalysts.

Pd model catalysts: Effect of air pulse length during redox aging on Pd redispersion

Nano-sized noble metal particles are an essential component in automotive catalytic converters. However, during customer in-use operation they agglomerate into larger particles and the performance of the catalytic converter is negatively affected. Engine control methods are needed to provide an environment capable of redispersing noble metal catalyst particles while the particle size is still small. In this study, Pd model powder catalysts, supported on three-way catalytic converter washcoat components of ceria-zirconia (Pd/CZO), 4wt% La2O3 stabilized γ-Al2O3 (Pd/Al) and unstabilized γ-Al2O3 (Pd/A) were exposed to redox cycling at 700°C for 16h (to simulate customer in-use operation engine exhaust) either continuously or interrupted with brief air pulses. The two goals of this research were to determine the air pulse length required so that any Pd redispersion gains achieved by air treatment in each 20min redox cycle will be retained by the end of the 16h aging, and to determine if these supports enable rapid Pd redispersion on the time scale of a common ∼10s engine fuel cut. Pd particle size and dispersion measurements were performed with three complimentary techniques of H2 chemisorption, XRD and electron microscopy since not one technique could be used exclusively. Catalyst activity was determined by CO oxidation with the Water Gas Shift (WGS) reaction and Oxygen Storage Capacity (OSC) measurements to probe the contact between the noble metal and support at a given state of catalyst deterioration, and a CO light-off test to confirm trends observed with Pd metal dispersion. We found that air pulses lasting for 1/6min applied to interrupt short redox aging spans enabled Pd redispersion only on the Pd/Al catalysts. Strong metal support interaction on the Pd/CZO catalysts by decoration of the Pd surface with support oxide was suspected to slow Pd redispersion by limiting metal mobility. Support oxides that promote the Pd2+ state may have been required on the Pd catalysts in order to exhibit Pd redispersion. The insight gained from this work could be used to develop engine control and aftertreatment systems that actively intervene and regenerate a catalyst capable of rapid Pd redispersion.

Formation and deposition of platinum nanoparticles under boiling water reactor conditions

Stress corrosion cracking (SCC) is a well-known degradation mechanism for components of boiling water reactors (BWRs). Therefore the mitigation of SCC is important for ensuring the integrity of the reactor system. Noble metal chemical application (NMCA) has been developed by General Electric to mitigate SCC and reduce the negative side-effects of hydrogen water chemistry used initially for SCC mitigation. NMCA is now widely applied as an online process (OLNC) during power operation. However, the understanding of the parameters that control the formation and deposition of the noble metal (Pt) particles in a BWR was still incomplete. To fill this knowledge gap, systematic studies on the formation and deposition behaviour of Pt particles in simulated and real BWR environment were performed in the framework of a research project at PSI. The present paper summarizes the most important findings.Experiments in a sophisticated high-temperature water loop revealed that the flow conditions, water chemistry, the Pt injection rate, and the pre-conditioning of the stainless steel surfaces have an impact on the Pt deposition behaviour. Slower Pt injection rates and stoichiometric excess of H2 over O2 produce smaller particles, which may increase the efficiency of the OLNC technique in mitigating SCC. Surfaces with a well-developed oxide layer retain more Pt particles. Furthermore, the pre- and post-OLNC exposure times play an important role for the Pt deposition on specimens exposed at the KKL power plant. Redistribution of Pt in the plant takes place, but most of the Pt apparently does not redeposit on the steel surfaces in the reactor system. Comparison of lab and plant results also demonstrated that plant OLNC applications can be simulated reasonably well on the lab scale.

Modelling the radiolytic corrosion of α-doped UO2 and spent nuclear fuel

A model previously developed to predict the corrosion rate of spent fuel (UO2) inside a failed waste container has been adapted to simulate the rates measured on a wide range of α-doped UO2 and spent fuel specimens. This simulation confirms the validity of the model and demonstrates that the steady-state corrosion rate is controlled by the radiolytic production of H2O2 (which has been shown to be the primary oxidant driving fuel corrosion), irrespective of the reactivity of the UO2 matrix. The model was then used to determine the consequences of corrosion inside a failed container resealed by steel corrosion products. The possible accumulation of O2, produced by H2O2 decomposition, was found to accelerate the corrosion rate in a closed system. However, the simultaneous accumulation of radiolytic H2, which is activated as a reductant on the noble metal (ε) particles in the spent fuel, rapidly overcame this acceleration leading to the eventual suppression of the corrosion rate to insignificant values. Calculations also showed that, while the radiation dose rate, the H2O2 decomposition ratio, and the surface coverage of ε particles all influenced the short term corrosion rate, the influence of the radiolytically produced H2 was the overwhelming influence in reducing the rate to negligible level (i.e., <10−20 mol m−2 s−1).

Bio-oil hydrodeoxygenation catalysts produced using strong electrostatic adsorption

Our objective was to synthesize hydrothermally stable metal catalysts with controlled particle size and distribution, with the goal of determining which catalyst(s) can selectively produce aromatics. Both precious and transition metal catalysts (Ru, Pt, Ni, Cu, 2Pt1Ru, NiCu) were deposited on mesoporous alumina (mA) and carbon, respectively, using the strong electrostatic adsorption (SEA) method. Due to the alloying that occurs under SEA, our hypothesis was that controlled bimetallic combinations (precious and/or base metal) could enhance the HDO behavior. As verified by XRD, STEM, and TPR, the SEA method successfully deposited noble metal particles less than 2nm in size onto alumina. Alloying of bimetallic particles was also confirmed. Hydrodeoxygenation of pyrolysis bio-oil was run for 3 h at 300°C in an aqueous environment. While partial conversion of mesoporous alumina into boehmite phase occurred, the catalyst particles remained between 2 and 3nm post-reaction, indicating a high degree of anchoring. Over carbon, base metal particles were initially larger in size (about 3.3nm) and tended to sinter more. Bimetallic SEA catalysts produced aromatic hydrocarbons at a greater extent than those of Ni/C, or Cu/C, or commercial Ru/Al2O3, as evidenced by GC–MS and/or NMR. Bimetallic 2Pt1Ru/mA did not exhibit significantly greater activity than Pt or Ru or control catalysts made by dry impregnation (DI), whereas NiCu demonstrated improved oil quality and yields over single-metal Ni and Cu and DI-method control catalysts. With respect to product compounds, the effect of mA support is stronger than that of carbon support.

Ultra-sensitive graphene based mid-infrared plasmonic bio-chemical sensing using dielectric beads as a medium

Graphene is moving beyond the realm of simple electronic devices toward areas such as advanced bio-chemical sensing. The infrared (IR) response of graphene, characterized by collective long-lived charge-carrier oscillations, could be applied in IR-absorption spectroscopy, typically used for bio-chemical analysis. However, direct light absorption by propagating plasmons in graphene is forbidden due to the large momentum mismatch. Proposed methods to overcome this bottleneck come at a cost, the use of noble metal particles on graphene reduces the spectral bandwidth and nano-structuring graphene is expensive. Here, we propose a simple and cheap method to fabricate large scale ultra-sensitive graphene based mid-IR biosensors, by introducing dielectric beads to excite mid-IR range plasmons. Interference from waves scattered by the beads excite surface plasmon polaritons, which propagate several micrometers in graphene and enhance the interaction between the molecules and mid-IR light. This method opens an interesting window for the application of graphene in bio-chemical sensing.

Sulfur poisoning and regeneration of bimetallic Pd-Pt methane oxidation catalysts

The influence of SO2 on the total oxidation of methane over Pd-Pt/Al2O3 and Pd‐Pt/La2O3‐Y2O3‐CeO2-ZrO2 catalysts under typical lean burn gas engine conditions was investigated and compared to engine-aged samples. Differences in deactivation rate and in stability of poisoning species were identified at 400°C and 450°C. A positive effect of higher pressure, related to pre-turbo positioning of the catalyst, on the activity was observed. Realistic pre- turbo conditions including high temperature (600°C) and high pressure (4bar) were found to favor methane conversion but even under these conditions some deactivation took place due to formation of strongly bound sulfur species and probably due to sintering of the noble metal particles. Engine aged samples were obtained after 60h on an SO2 containing exhaust-gas of a lean‐burn gas engine at 450°C. Laboratory tests with these samples revealed high accumulation of sulfur species with a higher concentration at the catalyst inlet than the outlet. Different regeneration strategies for SO2 poisoned catalysts were successfully applied. Rich conditions led to very pronounced regeneration at 550°C of a poisoned Pd‐Pt/La2O3‐Y2O3‐CeO2-ZrO2 catalyst. Short rich pulses were even able to regenerate the catalyst during operation at the gas engine. Periodic switches to λ<1 finally stabilized a methane conversion above 65% for several hours demonstrating the good transferability of lab test data to the engine tests.

Surface plasmon resonance absorption of composite films doped with metal nanoparticles

Composite thin films formed by nanometer-sized metal particles embedded in dielectric matrices were fabricated by codepositing the metal and ceramic targets using a pulsed laser deposition technique. The optical absorption properties were measured from 350 to 800 nm, and the absorption peak due to the surface plasmon resonance of metal particles was found. The effects of different metal particles (Au, Ag, Fe, and Co) and embedding matrices (SrTiO3, Al2O3, and TiO2) on the optical absorption properties of the composite films were discussed. Strong absorption peaks can be found in composite films doped with noble metal particles, while most transition metal particles show ordinary absorption patterns. Dielectric properties of metal particles and the refractive index of embedding matrices were responsible for the observed results.

Metal–Dielectric Hybrid Dimer Nanoantenna: Coupling between Surface Plasmons and Dielectric Resonances for Fluorescence Enhancement

Dimers made of noble metal particles possess extraordinary field enhancements but suffer from large dissipation, whereas low-loss dielectric dimers are limited by relatively weak optical confinement. Hybrid systems could take advantages from both worlds. In this contribution, we study the mode coupling in a hybrid dimer with rigorous dipole–dipole interaction theory and explore its potential in fluorescence enhancement. We first discovered that the direct coupling between metal surface–plasmon resonance and dielectric electric–dipole mode creates a hybridized mode due to the strong electric–electric dipole–dipole interaction between the constituent nanoparticles, whereas the dielectric magnetic–dipole mode can only indirectly couple to the plasmons on the basis of the induced electric–magnetic dipole–dipole interaction. When an electric/magnetic quantum emitter couples to the hybrid dimer, the emitter selectively excites the electric/magnetic (magnetic/electric) resonant modes of the dimer for emitter ori...

UV and visible light driven H2 photo-production using Nb-doped TiO2: Comparing Pt and Pd co-catalysts

In this work we compare the performance of platinum and palladium co-catalysts supported on Nb-doped titania materials for the photo-generation of hydrogen using methanol as sacrificial agent. Liquid and gas phase processes are considered. The catalysts physico-chemical properties were characterized by a combination of X-ray diffraction, UV–vis and photoluminescence spectroscopies and transmission electron microscopy, and their photocatalytic performance assessed through the analysis of the optical properties, the measurement of the reaction rates and subsequent computation of the true quantum efficiency in both liquid and gas phase reactors. Using Pt and Pd containing catalysts having similar primary noble metal particle size we compare the two co-catalysts performance in terms of the carbon containing product selectivity of the methanol photo-transformation and the overall performance of the materials in the photon to chemical transformation as measured through the true quantum efficiency. The work shows that Pd displays competitive performance with respect to Pt under UV illumination but not under visible light. The physico-chemical analysis of the solids and particularly the measurement of the charge carrier handling and recombination through photoluminescence at several wavelengths indicate the important role played by charge recombination upon UV illumination but also that other phenomena need to be considered under visible illumination to fully explain the photo-catalytic evolution of hydrogen in noble metal containing catalysts.