Noble Metal Particles Research Articles

Surface states modulation of topological insulator Bi2Se3 by noble metal decoration for gas sensing kinetic engineering

The chemical reaction of gas adsorption/desorption is strongly correlated with the surface state of sensitive films. Thus, understanding the surface state and kinetic process of gas-sensing reactions is crucial to design highly active sensing films. In this study, bismuth selenide (Bi2Se3) was selected as the platform to clarify those matters. The noble metal nanocrystals (Au, Ag and Pt) were decorated on the (001) surface of layered Bi2Se3 to tune the surface state and improve the dynamical gas sensing parameter. The response and response rate toward 10 ppm of NO2 were respectively improved by 2.5 and 3.8 times by the Au decoration, and 2.3 and 3.2 times by the Ag decoration, and 1.3 and 2.7 times by the Pt decoration. The gas adsorption/desorption isotherm was established to analyze the reaction kinetic parameters of the response (S), kinetic reaction rate (kr and k-r) and beta value (β). These results demonstrate that noble metals can greatly promote the reaction kinetic parameters by modulation the surface state. Especially, the Au10 sample acquired maximum kr values of 633.963, which is 9.3 times higher than pure Bi2Se3. This work provides an important strategy for designing gas sensing performance between topological insulators and noble metal particles.

Drastic Gas Sensing Selectivity in 2-Dimensional MoS2 Nanoflakes by Noble Metal Decoration.

Noble metal nanoparticle decoration is a representative strategy to enhance selectivity for fabricating chemical sensor arrays based on the 2-dimensional (2D) semiconductor material, represented by molybdenum disulfide (MoS2). However, the mechanism of selectivity tuning by noble metal decoration on 2D materials has not been fully elucidated. Here, we successfully decorated noble metal nanoparticles on MoS2 flakes by the solution process without using reducing agents. The MoS2 flakes showed drastic selectivity changes after surface decoration and distinguished ammonia, hydrogen, and ethanol gases clearly, which were not observed in general 3D metal oxide nanostructures. The role of noble metal nanoparticle decoration on the selectivity change is investigated by first-principles density functional theory (DFT) calculations. While the H2 sensitivity shows a similar tendency with the calculated binding energy, that of NH3 is strongly related to the binding site deactivation due to preferred noble metal particle decoration at the MoS2 edge. This finding is a specific phenomenon which originates from the distinguished structure of the 2D material, with highly active edge sites. We believe that our study will provide the fundamental comprehension for the strategy to devise the highly efficient sensor array based on 2D materials.

Alkaline Electro-Sorption of Hydrogen Onto Nanoparticles of Pt, Pd, Pt80Pd20 and Cu(OH)2 Obtained by Pulsed Laser Ablation.

Recently, hydrogen evolution reaction (HER) in alkaline media has received a renewed interest both in the fundamental research as well as in practical applications. Pulsed Laser Ablation in Liquid (PLAL) has been demonstrated as a very useful technique for the unconventional preparation of nanomaterials with amazing electro-catalyst properties toward HER, compared to those of nanomaterials prepared by conventional methods. In this paper, we compared the electro-sorption properties of hydrogen in alkaline media by Pt, Pd, Pt80Pd20, and Cu(OH)2 nanoparticles (NPs) prepared by PLAL. The NPs were placed onto graphene paper (GP). Noble metal particles have an almost spherical shape, whereas Cu(OH)2 presents a flower-bud-like shape, formed by very thin nanowalls. XPS analyses of Cu(OH)2 are compatible with a high co-ordination of Cu(II) centers by OH and H2O. A thin layer of perfluorosulfone ionomer placed onto the surface of nanoparticles (NPs) enhances their distribution on the surface of graphene paper (GP), thereby improving their electro-catalytic properties. The proposed mechanisms for hydrogen evolution reaction (HER) on noble metals and Cu(OH)2 are in line with the adsorption energies of H, OH, and H2O on the surfaces of Pt, Pd, and oxidized copper. A significant spillover mechanism was observed for the noble metals when supported by graphene paper. Cu(OH)2 prepared by PLAL shows a competitive efficiency toward HER that is attributed to its high hydrophilicity which, in turn, is due to the high co-ordination of Cu(II) centers in very thin Cu(OH)2 layers by OH- and H2O. We propose the formation of an intermediate complex with water which can reduce the barrier energy of water adsorption and dissociation.

Interfacial elaborating In2O3-decorated ZnO/reduced graphene oxide/ZnS heterostructure with robust internal electric field for efficient solar-driven hydrogen evolution

Solar-driven hydrogen evolution over ZnO-ZnS heterostructures is considered as a promising strategy for sustainable-energy issues. However, the industrialization of this strategy is still constrained by suppressed carrier migration, rapid charge recombination, and the inevitable utilization of noble-metal particles. Herein, we envision a novel strategy of successfully introducing In2O3 into the ZnO-ZnS heterostructure. Benefiting from the optimized internal electric field and the charge carrier migration mode based on the direct Z-scheme, the interfacial elaborating In2O3-decorated ZnO/reduced graphene oxide (rGO)/ZnS heterostructure manifests smooth charge migration, suppressed electron–hole pair recombination, and increased surface active sites. More importantly, the in situ introduction of In2O3 optimizes the construction of the internal electric field, favoring directional light-triggered carrier migration. As a result, the light-induced electrons generated from the heterostructure can be efficiently employed for the hydrogen evolution reaction. Hence, this work would shed light on the in situ fabrication of noble-metal-free photocatalysts for solar-driven water splitting.

Influencia del pretratamiento en los nanocompuestos de CeO2 y Au/CeO2 para mejorar la creación de defectos superficiales que permitan modificar la interbanda óptica

Cerium oxide as a material within heterogeneous catalysis remains a relevant research topic due to its redox behavior, directly related to the presence of oxygen vacancies (Vo-) which are responsible for ceria’s high oxygen storage capacity. CeO2 also have been employed as a support for active noble metal particles; in particular, supported gold catalyst has been proposed as a candidate to be used in different reactions such as CO oxidation, catalytic combustion of hydrocarbons, WGS reaction, among others. Despite its relevance, few works have a detailed understanding of the pretreatment effect on these catalysts. For this reason, in this work, we study the in situ behavior of CeO2 and Au/CeO2 during different pretreatments in temperature and atmosphere by Raman and Diffuse reflectance UV-Vis. These techniques allow us to follow in real-time the surface changes of Au nanoparticles and CeO2. We demonstrate a direct correlation lattice structural defects of CeO2 with modifications formed by electronic states in the optical interband and, the deposition of Au nanoparticles on the surface of CeO2 allows to improve the properties formed by the electronic states between the valence band and the conduction band by increasing more than twice the structural defects compared to CeO2 alone.

The kinetics of hydrogen peroxide reduction on rare earth doped UO2 and SIMFUEL

The electrochemical reduction of hydrogen peroxide has been studied in sodium chloride solutions containing various anions (bicarbonate/carbonate, sulphate) on Gd-UO2, Dy-UO2 and a SIMFUEL (UO2 doped to simulate spent nuclear fuel). The reaction was observed to proceed via the chemical oxidation of the surface to produce UV followed by its subsequent electrochemical reduction. The reaction was faster on the SIMFUEL surface due to the availability of the oxygen vacancies required to incorporate the OII ions necessary to maintain charge balance. Bicarbonate/carbonate, but not sulphate, was found to suppress peroxide reduction. This could be caused either by peroxide decomposition in solution or by the catalysis of peroxide reformation via the reaction of surface hydroxyl radicals with bicarbonate/carbonate to form carbonate radicals which subsequently decompose by reaction with water. The noble metal particles present in the SIMFUEL appear to play only a minor role in the reduction process.

Stabilization of metallic nickel particles obtained by chemical reduction in liquid by introducing high-molecular compounds at synthesis stage

Highly dispersed nickel metal particles are used as a filler in electrically conductive adhesives. In this application, they partially or completely replace particles of noble metals. However, the size of the filler particles in electrically conductive adhesives should be in the range from 5 to 15 μm. The method of chemical reduction of salts in liquid was used to obtain particles of this type. As the synthesis of nickel particles with hydrazine is a patented technology, nickel chloride was used as a salt and sodium borohydride was used as a reducing agent. Various polymer stabilizers are used in this method to control the size of the resulting particles. Polyvinyl alcohol, polyethylene glycol, polyacrylic acid (and its salts) were used for nickel particles stabilization. Different concentrations of the above polymers were introduced at the synthesis stage. The sizes of the nickel particles that were obtained using polyvinyl alcohol and polyethylene glycol did not exceed 50 nm. The use of high-molecular polyacrylic acid (M = 200,000 g/mol) ensures production of 2±1 μm nickel particles. The most suitable size of nickel particles for the specified purposes is obtained by adding low-molecular polyacrylic acid (M= 3,000 g/mol) and is 6±1 μm at a 0.15 wt. % stabilizer concentration. A study of the electrokinetic potential of the stabilized systems showed that the electrostatic component does not play any major role and that the systems are stabilized due to the adsorption film of polyacrylic acid that forms on the surface of metallic nickel particles.This research was funded by the Moscow Polytechnic University in the framework of V. E. Fortov Grant.The authors express gratitude to Research sharing center named after D. I. Mendeleev of the Mendeleev university of chemical technology of Russia

Ultrastable Anti-Acid "Shield" in Layered Silver Coordination Polymers.

Surface passivation technology provides noble-metal materials with limited chemical stability, especially under highly acidic condition. To design effective strategy to enhance stability of noble-metal particles, an understanding of their surface anticorrosion mechanism at the atomic level is desirable by using two-dimensional (2D) noble-metal coordination polymer (CP) as an ideal model for their interfacial region. With the protection of 2-thiobenzimidazole (TBI), we isolated two Ag-based 2D CPs, {Ag14 (TBI)12 X2 }n (S-X, where S denotes sheet and X=Cl or Br). These compounds exhibited excellent chemical stability upon immersion in various common solvents, boiling water, boiling ethanol, 10 % hydrogen peroxide, concentrated acid (12 M HCl), and concentrated alkali (19 M NaOH). Systematic characterization and DFT analyses demonstrate that the superior stability of S-X was attributed to the hydrophobic organic shell and dynamic proton buffer layer acting as a double protective "shield".

Enhanced photocatalytic performance of TiO2 nanowires by substituting noble metal particles with reduced graphene oxide

Noble metal particles have been embedded in semiconductors to improve photocatalysis efficiently, but the high cost made this approach difficult to apply widely in industry. Herein titanium dioxide/reduced graphene oxide (TiO2/rGO) nanowires in a core-shell structure were prepared. The physicochemical properties and photocatalytic performance of the specimen were characterized in comparison with TiO2 and TiO2/Pt nanowires. The rGO layer and Pt nanoparticles increased chemical states of the components, reduced bandgap energy of the nanowires, enhanced visible light absorption, improved conductance and capacitance significantly. The methylene blue as catalyzed by TiO2/Pt and TiO2/rGO nanowires was degraded to 7.9% and 8.4% in an hour, but retained 25.7% by the TiO2 nanowires. The properties and function of TiO2/rGO nanowires were close to those of TiO2/Pt nanowires, while the rGO price was much lower than that of Pt, which was of great significance for the photocatalytic application of TiO2 heterojunction materials in industry.

Estimates of noble metal particle growth in a molten salt reactor

Within the H2020-Euratom project SAMOSAFER, research is currently ongoing into the Molten Salt Fast Reactor (MSFR) concept in which nuclear fuel is dissolved in a liquid salt. A major challenge in the MSFR is the online and offline treatment of salt in order to remove a plethora of fission products. Noble metals are a type of such fission products, the atoms of which are thought to coagulate into particles as a result of diffusion. Due to their low solubility, noble metal particles will deposit onto interfaces such as solid walls but also on bubbles, as was observed in the Molten Salt Reactor Experiment (MSRE) at Oak Ridge national laboratory. The relatively large interfacial area generated by bubbles makes online or offline bubbling, therefore, an interesting option to extract noble metals from the MSFR and to limit wall deposition. However, to understand the performance of the bubbling process, particle sizes must be known. Particle size distributions arise from a complex interplay of atomic formation by fission and decay, growth by coagulation and removal by interfacial deposition. In this paper, theory is developed to understand these mechanisms on a semi-analytical level, in order to provide estimates of noble metal particle growth in the MSFR. The governing partial differential equation is reduced to a set of ordinary differential equations using the method of moments, revealing the dynamics of the underlying physics. We show that large noble metal particles, up to the micrometer scale, can arise in the reactor, but only at very long operational times. Moreover, we show that the previously assumed ‘cycle time’ of noble metal particle removal of 30 s is only feasible at very high bubbling void fractions. The theory developed in this work contributes significantly to a qualitative understanding of the behavior of noble metals in an MSR. The theory can potentially help to explain certain phenomena observed in the MSRE and can assist in the future design of safe and reliable MSRs.

Tuning Strong Metal–Support Interaction Kinetics on Pt-Loaded TiO2(110) by Choosing the Pressure: A Combined Ultrahigh Vacuum/Near-Ambient Pressure XPS Study

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the 'pressure gap' between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O$_2$ and H$_2$ on the SMSI-state of well-defined Pt/TiO$_2$(110) catalysts at pressures of up to 0.1 Torr. By tuning the O$_2$ pressure, we can either selectively oxidize the TiO$_2$ support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1x10$^{-5}$ Torr O$_2$, but orders of magnitudes less effective at higher O$_2$ pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.

NiO-Based Gas Sensors for Ethanol Detection: Recent Progress

In this review, we summarized the state-of-the-art progress on the ethanol performance of NiO by means of morphology, doping, loading noble metal particles, and forming heterojunctions. We first introduced the effect of modulating NiO morphology on ethanol performance that has been reported in recent years. The morphology with large specific surface area and high porosity was considered to be the one that can bring high gas response. Then, we discussed the enhanced effect of the doping of metal cations and noble metal particle loading on the ethanol-sensitive properties of NiO. Doping ions increased the ground-state resistance and increased the oxygen defect concentration of NiO. The effects of noble metal particles on the performance of NiO included chemical sensitization and electronic sensitization. Finally, the related contents of NiO forming complexes with metal oxides and bimetallic oxides were discussed. In this section, the specific improvement mechanism was discussed first, and then, the related work of researchers in recent years was summarized. At the same time, we presented a reasonable outlook for NiO-based ethanol sensors, imagining future directions.

Effects of thermal aging on the electronic and structural properties of Pt-Pd and toluene oxidation activity

Catalytic oxidation is a feasible method for remediating volatile organic compounds (VOCs), due to its lower energy consumption and mineralization of VOCs into H2O and CO2. Noble metal-based catalysts are preferred for the catalytic oxidation of VOCs because of their superior activity, but they are usually deactivated by thermal aging which sinters the metal particles. Here, we report that Pt-Pd/Al2O3 thermally aged at 700–900 °C in air showed enhanced catalytic activity for toluene oxidation in humid conditions. There were electronic and structural changes in the thermally aged Pt-Pd/Al2O3, as confirmed by numerous analyses. Both Pt and Pd existed in a metallic rather than oxidized state without additional reduction steps. The noble metal particles were assembled to form Pt-Pd alloy, in the form of isolated Pd atoms surrounded by Pt atoms. This specific alloy structure was found to be crucial to the observed enhancement in catalytic toluene oxidation at low temperature.

Molecular fluorescence significantly enhanced by gold nanoparticles@zeolitic imidazolate framework-8

Noble metal nanoparticles exhibit unique surface plasmon resonance dependent optical properties. On this basis, gold nanoparticles (AuNPs) encapsulated in metal–organic frameworks (MOFs) can form AuNPs@MOFs composites to modulate the optical properties of fluorescent molecules, which is less reported. In this paper, based on the fluorescence enhancement effect of AuNPs on 2-(2-hydroxyphenyl)-1H-benzimidazole (HPBI) molecules, zeolitic imidazolate framework-8 (ZIF-8) crystals with structural stability were introduced. AuNPs@ZIF-8 exhibited a significantly pronounced fluorescence enhancement of the HPBI molecules. In addition, by comparing the fluorescence characteristics of the HPBI molecules adsorbed on AuNPs@ZIF-8 and those captured in AuNPs@ZIF-8, we found that the ZIF-8 can act as a spacer layer with highly effective near-field enhancement. All our preliminary results shed light on future research on the composite structures of noble metal particles and MOFs for fluorescent probes and sensing applications.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions.

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)-induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal-oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.

One‐pot Synthesis of Hierarchical, Micro‐macroporous Zeolites with Encapsulated Metal Particles as Sinter‐resistant, Bifunctional Catalysts

AbstractWe report a new one‐pot synthesis procedure for hierarchical zeolites with intracrystalline macropores and metal particles encapsulated within the zeolitic walls. The synthesis allows to prepare macroporous zeolites of MFI topology with different heteroatoms (silicalite‐1, ZSM‐5 and TS‐1) and different encapsulated noble metal particles, such as gold, platinum and palladium. The hierarchically structured zeolites contain large macropores with diameters around 400 nm, which are well distributed and interconnected and should significantly enhance mass transport properties. The encapsulation of metal nanoparticles within the zeolitic walls leads to remarkable sinter resistance of the particles. Encapsulated gold nanoparticles (2.6 nm) do not significantly change in size during an 18‐hour treatment at 600 °C under air, while non‐encapsulated gold particles sinter heavily during the same treatment. Catalytic experiments for the direct epoxidation of propene with hydrogen and oxygen show that both catalytic functions of a macroporous TS‐1 sample that encapsulates gold particles are accessible and active. This catalyst displays high activity, although PO selectivity could still be improved. These materials show great potential for use in catalytic applications, due to their bifunctional nature, high sintering resistance, shape selective properties and hierarchical structure.

A Photocatalytic Hydrolysis and Degradation of Toxic Dyes by Using Plasmonic Metal–Semiconductor Heterostructures: A Review

Converting solar energy to chemical energy through a photocatalytic reaction is an efficient technique for obtaining a clean and affordable source of energy. The main problem with solar photocatalysts is the recombination of charge carriers and the large band gap of the photocatalysts. The plasmonic noble metal coupled with a semiconductor can give a unique synergetic effect and has emerged as the leading material for the photocatalytic reaction. The LSPR generation by these kinds of materials has proved to be very efficient in the photocatalytic hydrolysis of the hydrogen-rich compound, photocatalytic water splitting, and photocatalytic degradation of organic dyes. A noble metal coupled with a low bandgap semiconductor result in an ideal photocatalyst. Here, both the noble metal and semiconductor can absorb visible light. They tend to produce an electron–hole pair and prevent the recombination of the generated electron–hole pair, which ultimately reacts with the chemicals in the surrounding area, resulting in an enhanced photocatalytic reaction. The enhanced photocatalytic activity credit could be given to the shared effect of the strong SPR and the effective separation of photogenerated electrons and holes supported by noble metal particles. The study of plasmonic metal nanoparticles onto semiconductors has recently accelerated. It has emerged as a favourable technique to master the constraint of traditional photocatalysts and stimulate photocatalytic activity. This review work focuses on three main objectives: providing a brief explanation of plasmonic dynamics, understanding the synthesis procedure and examining the main features of the plasmonic metal nanostructure that dominate its photocatalytic activity, comparing the reported literature of some plasmonic photocatalysts on the hydrolysis of ammonia borane and dye water treatment, providing a detailed description of the four primary operations of the plasmonic energy transfer, and the study of prospects and future of plasmonic nanostructures.

Noble metals Pt, Au, and Ag as nucleating agents in BaO/SrO/ZnO/SiO2 glasses: formation of alloys and core\u2013shell structures

Noble metals such as Ag can be used as nucleation agents in glass ceramics. In glasses, it is incorporated predominantly as AgI. At temperatures slightly above the glass transition temperature, Tg, AgI reacts with SbIII to SbV and metallic Ag. Usually, face-centered cubic Ag particles are nearly spherical and get facetted during crystal growth. By contrast, in the case of BaO/SrO/ZnO/SiO2 glasses, silver has, in comparison to other noble metals, another significant, yet different effect. It forms metallic particles (hexagonal phase) with plate-like morphology during thermal treatment at 675 °C. In the second step of thermal treatment at 760 °C, this phase most probably expels some metallic Sb, which is oxidized by SbV (present in the surrounding glass phase) to SbIII. As a result, the plate-like morphology is maintained and a crystalline shell around the metallic core is formed, mainly consisting of ZnO with some SiO2 and antimony oxide, as proved by scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. This shell triggers the volume crystallization of Ba0.5Sr0.5Zn2Si2O7, a phase with low thermal expansion. By comparison, alloying of Au with Sb does not occur according to the phase diagram. Instead, a thermal treatment at temperatures slightly above Tg leads to nanocrystalline, spherical Au particles. Hence, alloying and subsequent decomposition of the alloy is a prerequisite for the formation of plate-like noble metal particles.

The Effect of Resorcinol on the Kinetics of Underpotentially Deposited Hydrogen and the Oxygen Evolution Reaction, Studied on Polycrystalline Pt in a 0.5 M H2SO4 Solution

This article reports on the influence of resorcinol (RC) on the kinetics of underpotential deposition of hydrogen (UPD of H) and the oxygen evolution reaction (OER), studied on a polycrystalline Pt electrode in a 0.5 M sulphuric acid supporting solution. It is well known that both PEM fuel cells and water electrolysers’ electrodes often contain significant amounts of nanostructured Pt or other types of noble metal particles. These materials provide the superior catalytic activity of electrochemical reactions such as OER (oxygen evolution reaction), HER (hydrogen evolution reaction) and ORR (oxygen reduction reaction). The trace amounts of phenolic substances contained in air or water could be harmful (when in contact with a fuel cell/water electrolyser’s working environment) to the abovementioned catalytic surfaces. Hence, they could potentially have severe detrimental effects on the kinetics of these processes. The results obtained in this work provided evidence for the detrimental role of Pt surface-adsorbed resorcinol molecules (or their electrodegradation products) on the kinetics of UPD of H and the oxygen evolution reaction. The above was revealed through evaluation of the associated charge-transfer resistance and capacitance parameters, comparatively derived on a platinum electrode, for the initial and the resorcinol-modified H2SO4 electrolyte.

Continuous synthesis of TiO2-supported noble metal nanoparticles and their application in ammonia borane hydrolysis

A stabilizer-free method based on segmented flow for the continuous synthesis of TiO2 supported noble metal nanoparticles (M/TiO2-MR, M = Pd, Pt or Au) was proposed. Due to the enhanced mixing performance arising from the internal circulation in the discrete plugs, the particle size of noble metal nanoparticles could be well controlled by reducing the metal precursors with NaBH4 just in the presence of TiO2 without using any stabilizer. In comparison with the batch method, the as-prepared M/TiO2-MR had smaller noble metal particle size and better dispersity. Experimental results showed that adjusting the oil-to-water phase ratio or increasing the total volume flow rate and synthetic temperature could lead to smaller average particle size with narrower distribution. The as-prepared M/TiO2-MR possessed higher catalytic activities in the hydrolysis of ammonia borane than those prepared by the batch method, which could be ascribed to smaller noble metal nanoparticles, exposing more active sites.