Surface Noble Metal Research Articles

Enhancing Stability of Surface Au under Oxidizing Conditions through Reduced Bulk Au Content.

Contrary to the common assumption that a higher bulk content of precious metals facilitates the preservation of more surface noble metal by serving as a reservoir for surface enrichment, we demonstrate that a lower bulk content of Au results in a more stable arrangement of Au atoms at the surface of Cu-Au nanoparticles when exposed to an O2 atmosphere. Using ambient pressure X-ray photoelectron spectroscopy, we investigate the surface segregation and oxidation behavior of Cu-Au nanoparticles across various compositions. Our results reveal that in Au-rich nanoparticles exposed to an H2 atmosphere, surface segregation prompts the formation of a continuous Au-enriched shell, which subsequently oxidizes into a complete CuOx shell upon transitioning to an O2 atmosphere. Conversely, in Au-poor nanoparticles during H2 treatment, segregation results in the emergence of Au clusters embedded within the surface layer, persisting upon exposure to O2. This unexpected phenomenon shows that reducing the bulk content of precious metals can enhance the surface stability of noble atoms under oxidizing conditions, as further demonstrated by comparing the catalytic performance of Cu-Au nanoparticles with varying Au bulk contents in CO oxidation.

Highly Active Oxidation Catalysts through Confining Pd Clusters on CeO2 Nano‐Islands

AbstractCeO2‐supported noble metal clusters are attractive catalytic materials for several applications. However, their atomic dispersion under oxidizing reaction conditions often leads to catalyst deactivation. In this study, the noble metal cluster formation threshold is rationally adjusted by using a mixed CeO2‐Al2O3 support. The preferential location of Pd on CeO2 islands leads to a high local surface noble metal concentration and promotes the in situ formation of small Pd clusters at a rather low noble metal loading (0.5 wt %), which are shown to be the active species for CO conversion at low temperatures. As elucidated by complementary in situ/operando techniques, the spatial separation of CeO2 islands on Al2O3 confines the mobility of Pd, preventing the full redispersion or the formation of larger noble metal particles and maintaining a high CO oxidation activity at low temperatures. In a broader perspective, this approach to more efficiently use the noble metal can be transferred to further systems and reactions in heterogeneous catalysis.

Highly Active Oxidation Catalysts through Confining Pd Clusters on CeO2 Nano-Islands.

CeO2-supported noble metal clusters are attractive catalytic materials for several applications. However, their atomic dispersion under oxidizing reaction conditions often leads to catalyst deactivation. In this study, the noble metal cluster formation threshold is rationally adjusted by using a mixed CeO2-Al2O3 support. The preferential location of Pd on CeO2 islands leads to a high local surface noble metal concentration and promotes the in situ formation of small Pd clusters at a rather low noble metal loading (0.5 wt %), which are shown to be the active species for CO conversion at low temperatures. As elucidated by complementary in situ/operando techniques, the spatial separation of CeO2 islands on Al2O3 confines the mobility of Pd, preventing the full redispersion or the formation of larger noble metal particles and maintaining a high CO oxidation activity at low temperatures. In a broader perspective, this approach to more efficiently use the noble metal can be transferred to further systems and reactions in heterogeneous catalysis.

Surface Noble Metal Concentration on Ceria as a Key Descriptor for Efficient Catalytic CO Oxidation

During the CO oxidation over metallic Pt clusters and Pt nanoparticles in Pt/CeO2 catalysts, we found that the Pt surface concentration is a key descriptor for the reaction rate. By increasing the surface noble metal concentration (SNMC) of a Pt/CeO2 catalyst by a factor of ∼4, while keeping the weight hourly space velocity constant, the ignition temperature of CO oxidation was decreased by ∼200 °C in the as-prepared state. Moreover, the stability was enhanced at higher SNMC. Complementary characterization and theoretical calculations unraveled that the origin of this improved reaction rate at higher Pt surface concentrations can be traced back to the formation of larger oxidized Pt-clusters and the SNMC-dependent aggregation rate of highly dispersed Pt species. The Pt diffusion barriers for cluster formation were found to decrease with increasing SNMC, promoting more facile agglomeration of active, metallic Pt particles. In contrast, when Pt particle formation was forced with a reductive pretreatment, the influence of the SNMC was temporarily diminished, and all catalysts showed a similar CO oxidation activity. The work shows the general relevance of the proximity influence in the formation and stabilization of active centers in heterogeneous catalysis.

Direct Microscopic Proof of the Fermi Level Pinning Gas-Sensing Mechanism: The Case of Platinum-Loaded WO3.

It is widely known that the sensing characteristics of metal oxides are drastically changed through noble metal oxide surface additives. Using operando infrared spectroscopy it was identified that the Fermi level pinning mechanism dominates the sensor response of platinum-loaded WO3. Spectroscopy, however, provides information about the sample only on average. Traditional microscopy offers structural information but is typically done in vacuum and on unheated sensors, very different than the operation conditions of metal oxide gas sensors. Here, state-of-the-art in situ scanning transmission electron microscopy offers spatially resolved information on heated samples at atmospheric pressure in varying gas atmospheres. As a result it was possible to directly couple microscopically observed structural changes in the surface noble metal nanoclusters with IR spectra and sensor responses. On the basis of the findings, the dominant Fermi level pinning mechanism could be validated. The presented work demonstrates the benefits of coupling in situ microscopy with operando spectroscopy in order to elucidate the sensing mechanism of metal oxides.

Preparation of cyclonic Co3O4/Au/mesoporous SiO2 catalysts with core–shell structure for solvent-free oxidation of benzyl alcohol

• A core–shell catalyst was prepared by cyclonic Co 3 O 4 as core and m-SiO 2 as shell for selective oxidation of benzyl alcohol. • 58% conversion and 82% selectivity were obtained together with the excellent reusability and stability. • A plausible oxidation reaction mechanism over SiO@Co 3 O 4 /Au@m-SiO 2 catalysts was tentatively proposed. As one of the ideal catalysts, noble metal materials can realize the conversion from benzyl alcohol to benzaldehyde. However, in previous reports, the loss of surface noble metal is one of the important reasons for the decrease in reaction performance. Here, a simple method was reported for stepwise fabrication of SiO 2 @Co 3 O 4 /Au@m-SiO 2 catalysts with core–shell structure. On the one hand, the core–shell structure provided abundant reaction sites for the oxidation of benzyl alcohol to benzaldehyde. On the other hand, the outer layer of m-SiO 2 effectively prevents the loss of Au nanoparticles. In this process, we studied the effects of multiple factors on the target reaction by controlling a single variable. The experimental results show that under the optimal conditions (the catalyst dosage, reaction temperature, reaction time and O 2 flow rate are 40 mg, 160 °C, 6 h, 60 ml/min, respectively), the conversion rate of the target reaction reaches 58% and the selectivity is as high as 82%. In the end, a mechanism was put forward to illustrate the underlying reaction pathway in benzyl alcohol oxidation.

Pd- and Au-Decorated MoS2 Gas Sensors for Enhanced Selectivity

To date, chemoresistive gas sensors based on metal oxide semiconductors (MOS) have been the most attractive sensor types for the practical application. However, with the emerging concept of Internet of Everything, high operating temperatures over 300 °C of the gas sensors based on MOS must be reduced to achieve low power consumption and overcome a limited battery capacity of mobile devices. The 2-dimensional materials like MoS2 have been, therefore, one of the most recently studied materials for gas sensors with capability of operation at significantly lower temperatures. However, lacking selectivity toward various target gas species limited their application to gas sensors. Herein, we investigated the effects of noble metals (Pd and Au) decoration on the gas sensing properties of MoS2 thin films. Due to the electronic sensitization of the noble metal catalysts and the formation of Pd hydride, overall gas responses and selectivity were significantly improved toward tests gas species. These results provide clear understandings on the effects of the surface noble metal catalysts on gas sensing properties of MoS2.

Quantum mechanical studies of full‐shell noble metal nanoclusters in water

AbstractWater–metal interfaces are ubiquitous, but their underlying physicochemical principles remain elusive. In this work, we performed scalar‐relativistic and dispersion‐corrected first‐principle calculations for full‐shell noble metal nanoclusters in water for characterizing their electronic interactions. With increasing nanocluster (NC) size, the average bond lengths of Ag and Au NCs decrease due to their strong relative effects, while those of Pd and Pt NCs increase normally. The calculated Löwdin net atomic charges show that naked full‐shell M x (M is denoting Ag, Au, Pd, and Pt with x = 13, 55, 147, respectively) nanoclusters have core‐shell charge separations with positively charged core and negatively charged surface. Ag and Au NCs have higher charge separations with their well delocalized s‐electrons as compared with Pd and Pt NCs. The charged surface noble metal atoms of nanoclusters gain a small amount of electrons from neighbor oxygen atoms of water. The electronic structure analysis manifests a mixing state of hybrid s‐p‐d orbitals of noble metal nanoclusters with O_p states of water, which gives rise to the electronic interactions of dative‐bond character between noble metal nanoclusters and water. Such a kind of electronic interaction leads to charge transfer from neighbor oxygen atoms of water to surface noble metal atoms. These findings have an implication for those researches and applications in association with noble metal nanoclusters in liquid environments.

Surface noble metal modified PdM/C (M = Ru, Pt, Au) as anode catalysts for direct ethanol fuel cells

In this article, we studied the surface noble metal modification on Pd nanoparticles, other than the homogeneous or core-shell structure. The surface modification will lead to the uneven constitution within the nanoparticles and thus more obvious optimization effect toward the catalyst brought by the lattice deformation. The surface of the as-prepared Pd nanoparticles was modified with Ru, Pt or Au by a moderate and green approach, respectively. XPS results confirm the interactive electron effects between Pd and the modified noble metal. Electrochemical measurements show that the surface noble metal modified catalysts not only show higher catalytic activity, but also better stability and durability. The PdM/C catalysts all exhibit good dispersion and very little agglomeration after long-term potential cycles toward ethanol oxidation. With only 10% metallic atomic ratio of Au, PdAu/C catalyst shows extraordinary catalytic activity and stability, the peak current reaches 1700 mA mg−1 Pd, about 2.5 times that of Pd/C. Moreover, the PdAu/C maintains 40% of the catalytic activity after 4500 potential cycles.

Au sensitized ZnO nanorods for enhanced liquefied petroleum gas sensing properties

The zinc oxide (ZnO) nanorods have grown on glass substrate by spray pyrolysis deposition (SPD) method using zinc acetate solution. The phase formation, surface morphology and elemental composition of ZnO films have been investigated using X-ray diffraction, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and energy dispersive X-ray (EDX) techniques. The liquefied petroleum gas (LPG) sensing response was remarkably improved by sensitization of gold (Au) surface noble metal on ZnO nanorods film. Maximum LPG response of 21% was observed for 1040ppm of LPG, for pure ZnO nanorods sample. After Au sensitization on ZnO nanorods film sample, the LPG response greatly improved up to 48% at operating temperature 623K. The improved LPG response is attributed Au sensitization with spill-over mechanism. Proposed model for LPG sensing mechanism discussed.

Research on Fretting Performance of Silver Conducting Adhesive

Fretting is one of the main reasons for electric contact failure. Metal contact pairs have long been used, generally with the structure of copper alloy base, Ni interlayer and surface noble metal (such as gold) plating. The influences of plating material and plating thickness on fretting property have been researched deeply, meanwhile, new materials have been tried. In this paper, a new conductive material, silver conducting adhesive is used as the surface coating of contact pair, and a series of tests have been done to study the performance of fretting wear by compare with the coupons without silver conducting adhesive. The results show that the use of silver conductive adhesive improves the fretting property of electrical contacts, especially with small normal force.

Photocatalysts for Solar-Induced Water Disinfection: New Developments and Opportunities

Recent years have seen a surge of interest in the application of solar energy for water disinfection by using nanostructured photocatalysts elaborately designed and fabricated. Photocatalysis has its unique advantage for utilizing sunlight to drive the disinfection process. The highly reactive oxygen species (ROS) serve as the main oxidants and are capable of inactivating microorganisms, including viruses, bacteria, spores and protozoa. This chapter presents an overview of current research activities that center on the preparation, characterization and application of highly efficient photocatalysts for water disinfection under both UV and visible light irradiation. It is organized into two major parts. One is the development of TiO2-based photocatalysts including surface noble metal modified, ion doped, dye-sensitized, and composite TiO2. The other part is the introduction of new types of photocatalysts and advanced technologies that have recently fascinated the scientific community. Particular attention is given to the pioneering fields such as graphene-based photocatalysts, plasmonic-metal nanostructures and naturally occurring photocatalysts. Finally, we conclude with a discussion of what major advancements are needed to move the field of photocatalytic water disinfection forward.

Study on the Adhesion Between Epoxy Molding Compound and Nanocone-Arrayed Pd Preplated Leadframes

Since Au is used as the end finish on Pd preplated leadframes (Pd PPFs), the weak adhesion between the surface noble metal Au and the epoxy molding compound (EMC) often causes delamination and reduces the reliability of integrated circuit (IC) packages, which greatly limits the application of Pd PPFs. This paper reports on a novel application of arrays of nanocones prepared by a special electrodeposition method to improve the adhesion between the leadframe and EMC. Nanocone-arrayed structures greatly increase the area of the interface between the Pd PPF and the EMC. Shearing strength tests indicate that the adhesion between the EMC and nanocone-arrayed PPF is three to four times higher than that of the conventional PPF. Apart from increased interfacial area, the effect of a physical inlay may also increase the adhesion strength.

Iridium oxide-based nanocrystalline particles as oxygen evolution electrocatalysts

Iridium-based oxides are highly active as oxygen evolving electrocatalysts in PEM water electrolyzers. In this work XRD reveals that Ir-Sn oxides contain a single rutile phase with lattice parameters between those of pure IrO2 and SnO2. Addition of Ru leads to the synthesis of a core-shell type material due to the strong agglomeration of Ru colloids during the preparation procedure. The shell of this material consists of an Ir-Sn-Ru oxide deficient in Ru relative to the bulk. This leads to a decrease in the surface noble metal concentration (as found by XPS), which in turn results in a significant reduction in electrochemically active surface area. Polarization analysis indicates that the addition of Ru can influence the rate-determining step or mechanism by which oxygen is evolved. In a PEM water electrolysis cell, small additions of Sn do not significantly reduce the operating performance, however larger additions cause a performance loss due to a reduction in active surface area and increased ohmic resistance. When a pure IrO2 anode is used, a cell voltage is 1.61 V at 1 A cm−2 and 90°C.

Application of surface photovoltage technique in photocatalysis studies on modified TiO2 photo-catalysts for photo-reduction of CO2

Surface photovoltage spectrum (SPS) is a useful technique in photocatalysis studies. Here, the relationship between the catalytic activities of modified TiO2, which are composed of Pd/RuO2/TiO2 (A), Pd/TiO2 (B), a-TiO2 (C) and b-TiO2 (D) for photoreduction of CO2 and surface photovoltage (SPV) response was studied by means of this technique. The SPV response increased after surface noble metal (Pd and Ru) depositing and a new SPV response band appeared in the SPS curve of A and B in near IR, which was attributed to the surface state population transition formed by Pd deposition. Corresponding to SPV response, the catalytic activity of photoreduction of CO2 improved after surface noble metal (Pd and Ru) depositing. Moreover, the stronger SPV response is, the higher the catalytic activity of photoreduction of CO2 is. The effect of metal depositing on the behavior of photo-generated carriers in TiO2 semiconductor photocatalysts, as well as the mechanism on photoreduction CO2 of A was discussed.