Noble Metal Deposition Research Articles

A Perspective on the Flame Spray Synthesis of Photocatalyst Nanoparticles.

The synthesis of functional nanoparticles via one-step flame spray pyrolysis (FSP), especially those of catalytic nature, has attracted the interests of scientists and engineers, as well as industries. The rapid and high temperature continuous synthesis yields nanoparticles with intrinsic features of active catalysts, that is, high surface area and surface energetics. For these reasons, FSP finds applications in various thermally inducible catalytic reactions. However, the design and synthesis of photocatalysts by FSP requires a knowledge set which is different from that established for thermal catalysts. Unknown to many, this has resulted in frustrations to those entering the field unprepared, especially since FSP appears to be an elegant tool in synthesising oxide nanoparticles of any elemental construct. From simple oxide to doped-oxide, and mixed metal oxide to the in situ deposition of noble metals, this Perspective gives an overview on the development of photocatalysts made by FSP in the last decade that led to a better understanding of the design criteria. Various challenges and opportunities are also highlighted, especially those beyond simple metal oxides, which perhaps contain the greatest potential for the exploitation of photocatalysts design by FSP.

Synthesis and characterization of TiO2–graphene nanocomposites modified with noble metals as a photocatalyst for degradation of pollutants

TiO2–graphene (TiO2–GR) nanocomposites were synthesized using photocatalytic reduction method. TiO2–GR nanocomposites were thereafter doped with noble metals (Pt and Pd) by chemical reduction of the corresponding cations. The samples were characterized by different techniques. The addition of GR to TiO2 decreases the crystalline size of TiO2 due to the homogeneous dispersion of the TiO2 nanoparticles on GR sheets and prevention of coagulation of TiO2 nanoparticles during synthesis process. In addition, the surface area of TiO2 was increased by addition of GR and deposition of noble metals which helps to prevent agglomeration of graphene sheets and TiO2 nanoparticles. Red shifts to the higher wavelength have been detected for both TiO2–GR and M-TiO2–GR nanocomposites. X-ray photoelectron spectroscopy demonstrated that graphene oxide is reduced to GR by the UV irradiated TiO2 nanoparticles, TiC bonds are formed, and Ti3+ sites are also present on the surface of photocatalyst. The photocatalytic activity of all samples was evaluated for the photodegradation of two pollutants (2,4-dichlorophenoxyacetic acid and Reactive Red 195) in aqueous medium under UV and visible irradiations. The M-TiO2–graphene nanocomposites exhibited excellent photocatalytic activity within both UV and visible regions. The results confirm that the addition of GR as well as noble metal to TiO2 nanoparticles increases electron transport, thus impeding the charge recombination of the excited TiO2. The highest photocatalytic activity was observed for Pt–TiO2–GR nanocomposites due to its high photonic efficiency.

Hollow metallic pyramid plasmonic structures fabricated by direct laser writing and electron beam evaporation

Here we report a method that enables the fabrication of hollow metallic pyramid plasmonic structures on free-standing copper grids, which were fabricated by direct laser writing lithography followed by noble metal deposition. Gratings on the faces of these pyramids can translate incident light into plasmons that propagate toward the apex of the noble metal coated pyramids. Surface Enhanced Raman Scattering (SERS) measurements confirm that these hollow metallic pyramid structures have superior properties to the planar metal surface and the degree of enhancement can be tuned by the size and geometry of the fabricated structures, which potentially could be used for better understanding of surface enhanced plasmonics on free-standing three dimensional structures as well as find application in the research field of SERS.

Low temperature atomic layer deposition of noble metals using ozone and molecular hydrogen as reactants

Atomic layer deposition (ALD) of noble metals by thermal processes has relied mostly on the use of molecular oxygen as a reactant at temperatures of 200°C and above. In this study, the concept of using consecutive ozone and molecular hydrogen pulses with noble metal precursors in ALD is introduced for palladium, rhodium, and platinum metals. This approach facilitates the growth of noble metal thin films below 200°C. Also the ALD of palladium oxide thin films is demonstrated by the ozone-based chemistry. The growth rates, resistivities, crystallinities, surface roughnesses, impurity contents, and adhesion of the films to the underlying Al2O3 starting surface are reported and the results are compared with the most common noble metal ALD processes.

A facile one-step way to anchor noble metal (Au, Ag, Pd) nanoparticles on a reduced graphene oxide mat with catalytic activity for selective reduction of nitroaromatic compounds

A facile, one-step clean strategy has been utilized for anchoring noble metal (Au, Ag, Pd) nanoparticles onto the flat surface of two-dimensional (2D) graphene oxide (GO) nanosheets. Without using any additional reductants, surfactants or protecting ligands, the deposition of metallic noble metal (Au, Ag, Pd) nanoparticles on the partially reduced graphene oxide (PRGO) mat has been realized by a simple redox reaction between the noble metal precursors and GO in an aqueous solution. The main size distributions of the Au, Ag and Pd particles are centered on 1–20 nm, 3–10 nm and 0.5–3 nm, respectively. The as-obtained Au, Ag and Pd–PRGO nanocomposites display superb catalytic activities for the selective reduction of nitroaromatic compounds into the corresponding amino compounds under ambient conditions. The rates of reduction follow the sequence, Pd–PRGO > Ag–PRGO > Au–PRGO, suggesting that the smallest noble metal particles size affords the highest catalytic activity. In addition, considering that the Au, Ag and Pd–PRGO nanocomposites still hold the abundant functional groups of the original GO, these noble metal–PRGO composites maintain the excellent hydrophilic nature of GO, which provides a flexible platform for the further fabrication of PRGO–metal-based multicomponent functional materials by a wet-chemistry approach.

Catalytic partial oxidation of CH4–H2 mixtures over Ni foams modified with Rh and Pt

Abstract The catalytic partial oxidation (CPO) of methane–hydrogen mixtures in air, intended for the first stage of hybrid radiant catalytic burners, was investigated under self-sustained short contact time conditions on commercial Ni foam catalysts eventually modified with Rh and Pt. The modified catalysts were prepared by a simple novel method based on the spontaneous deposition of noble metals via metal exchange reactions onto those Ni foam substrates. SEM-EDS, electrochemical methods and H2-TPR analysis were integrated to characterize morphology, surface area of metal deposits and reducibility of foam catalysts before and after exposure to severe conditions in the CPO reactor. In particular Rh forms finely dispersed deposits that retain their high specific surface area at temperatures up ca. 1100 °C. Modification with noble metals enhances stability and reducibility of the Ni foam whereas the overall CPO performance is not significantly improved. Safe operation of the CPO reactor with up to 70% vol. H2 in the fuel mixture has been achieved by properly increasing the feed equivalence ratio to avoid catalyst overheating, while guaranteeing high methane conversions and a persistent net hydrogen production.

Photodecomposition of Formic Acid on N-Doped and Metal-Promoted TiO2Production of CO-Free H2

The photoinduced vapor-phase decomposition of formic acid was investigated on pure, N-doped and metal-promoted TiO2. The catalysts were characterized by bandgap determination, and by Fourier transformed infrared spectroscopy, the bandgap of N-doped TiO2 was narrowed by 0.5–1.02 eV. IR studies revealed that illumination of the HCOOH–TiO2 system initiated the decomposition of adsorbed formate species. On the IR spectra of metal-promoted TiO2 adsorbed CO attached to the metals was also detected. The photodecomposition of formic acid on pure TiO2 occurs to only a limited extent to yield H2 and CO2 as the major products with a small amount of CO. Depending on the origin of TiO2 and on the preparation, N-doped TiO2 exhibited higher activity. Its efficiency is increased with the narrowing of the bandgap, a feature attributed to the prevention of electron–hole recombination. The deposition of noble metals on pure and N-modified TiO2 dramatically enhanced the extent of photodecomposition of formic acid. Pd/TiO2 was found to be the most active catalyst. Addition of water to formic acid completely eliminated the small amount of CO formed. Both the N-doped TiO2 and metal-promoted TiO2 + N samples exhibited photocatalytic effects even in visible light. The promoting effect of metals was explained by a better separation of charge carriers induced by illumination and by improved electronic communication between metal particles and TiO2.

Metal-assisted electroless etching of silicon in aqueous NH4HF2 solution

One-step and two-step metal-assisted electroless chemical etchings of p-type silicon substrate in new solutions were investigated. In the one-step etching process, the etching is performed in NH4HF2/AgNO3 solution. On the other hand, the two-step etching process involves chemical deposition of noble metal onto silicon substrate surface followed by electroless etching in NH4HF2/H2O2 solution. The effect of several parameters on the morphology of etched layer was studied namely: pH of etching solution for the two cases and the etching temperature, the concentration of NH4HF2 and the type of metal deposited on silicon surface for second case. It is shown that the morphology depends strongly on etching parameters where different nanostructure shapes can be formed. An important result is that silicon nanowires are formed at pH=4 and pH≤2 for the first and second case, respectively.

In situ synthesis of CdS modified CdWO4 nanorods and their application in photocatalytic H2 evolution

CdWO4 is a wide band gap semiconductor with relatively high chemical stability and has been widely used as a scintillator material. However, the photocatalytic activity of CdWO4 for H2 evolution from water is low due to the fast electron–hole combination. In this study, CdS nanoparticles have been grown on the surface of CdWO4 nanorods via an in situ approach and the high photocatalytic H2 evolution has been realized. The structure, optical properties and morphologies of CdWO4 with different amount of CdS were systematically investigated. Element mapping and high resolution transmission electron microscopy (HRTEM) analysis proved the existence of an interfacial structure between CdS and CdWO4 nanorods and the effect of CdS modification was investigated by photoluminescence spectroscopy. Under UV and visible light irradiation, H2 was evolved steadily from an aqueous solution containing Na2S and Na2SO3 over this photocatalyst even without noble metal deposition. Moreover, the rate of photocatalytic H2 evolution over CdS modified CdWO4 is ca. 3.4 and 34 times higher than that of sole CdS or CdWO4 under the same conditions, respectively. The encouraging results presented in this study demonstrate the potential of CdWO4 as a stable photocatalyst for hydrogen evolution from water.

Electric-field-actuation of in situ composites that contain silver-coated carbon fibers in sodium sulfonate ionomers

In this research contribution, an electric-field-stimulated actuation system was developed by impregnating sodium-sulfonated ionomers, poly(styrene-co-butyl acrylate-co-sodium allyl sulfonate, PSBS) with in situ deposited silver-modified carbon fibers (SCCF). In the actuator, PSBS acts as an electrolytic medium for metal ion migration, and the SCCFs act as surface conductive electrodes on both sides of a sandwich configuration. The maximum bending angle of 98° is achieved after 11 s of stimulation when the electric potential difference is 5 V. This novel actuator has two outstanding characteristics: (1) in situ deposited SCCFs in PSBS form double-sided surface electrodes with high conductivity (10−3 Ω cm) and good interfacial binding with the PSBS matrix, which minimizes the drawbacks of electrochemical plating or electrode-less deposition of noble metals like Pt on the membrane surface of ionic polymer metal composites (IPMCs); and (2) it has the highest bending rate compared to currently existing IPMCs including Nafion and novel ones published recently. Additionally, the storage E′ and loss E′′ moduli of the PSBS–SCCF actuation system can be adjusted by selecting the appropriate ratio of butyl acrylate to styrene, where the latter comonomer increases the rigidity of the composite. The bending angle of this actuator can be controlled by the electric potential difference, water-uptake, and terpolymer composition. This fabrication technology exhibits significant advantages, such as process simplicity using non-toxic and low cost materials, rapid response, large bending angles, and reproducibility.

Mono- and bimetallic Rh and Pt NSR-catalysts prepared by controlled deposition of noble metals on support or storage component

Mono- and bimetallic Rh and Pt based NOx storage-reduction (NSR) catalysts, where the noble metals were deposited on the Al2O3 support or BaCO3 storage component, have been prepared using a twin flame spray pyrolysis setup. The catalysts were characterized by nitrogen adsorption, CO chemisorption combined with diffuse reflectance infrared Fourier transform spectroscopy, X-ray diffraction, and scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy. The NSR performance of the catalysts was investigated by fuel lean/rich cycling in the absence and presence of SO2 (25ppm) as well as after H2 desulfation at 750°C. The performance increased when Rh was located on BaCO3 enabling good catalyst regeneration during the fuel rich phase. Best performance was observed for bimetallic catalysts where the noble metals were separated, with Pt on Al2O3 and Rh on BaCO3. The Rh-containing catalysts generally showed much higher tolerance to SO2 during fuel rich conditions and lost only little activity during thermal aging at 750°C.

Review on Modified TiO2Photocatalysis under UV/Visible Light: Selected Results and Related Mechanisms on Interfacial Charge Carrier Transfer Dynamics

Titania is one of the most widely used benchmark standard photocatalysts in the field of environmental applications. However, the large band gap of titania and massive recombination of photogenerated charge carriers limit its overall photocatalytic efficiency. The former can be overcome by modifying the electronic band structure of titania including various strategies like coupling with a narrow band gap semiconductor, metal ion/nonmetal ion doping, codoping with two or more foreign ions, surface sensitization by organic dyes or metal complexes, and noble metal deposition. The latter can be corrected by changing the surface properties of titania by fluorination or sulfation or by the addition of suitable electron acceptors besides molecular oxygen in the reaction medium. This review encompasses several advancements made in these aspects, and also some of the new physical insights related to the charge transfer events like charge carrier generation, trapping, detrapping, and their transfer to surface are discussed for each strategy of the modified titania to support the conclusions derived. The synergistic effects in the mixed polymorphs of titania and also the theories proposed for their enhanced activity are reported. A recent venture on the synthesis and applications of anatase titania with a large percentage of reactive {001} facets and their band gap extension to the visible region via nonmetal ion doping which is a current hot topic is briefly outlined.

Noble‐Metal‐Based Catalysts Supported on Zeolites and Macro‐Mesoporous Metal Oxide Supports for the Total Oxidation of Volatile Organic Compounds

The use of porous materials to eliminate volatile organic compounds (VOCs) has proven very effective towards achieving sustainability and environmental protection goals. The activity of zeolites and macro-mesoporous metal-oxide supports in the total oxidation of VOCs has been investigated, with and without noble-metal deposition, to develop highly active catalyst systems where the formation of by-products was minimal. The first catalysts employed were zeolites, which offered a good activity in the oxidation of VOCs, but were rapidly deactivated by coke deposition. The effects of the acido-basicity and ionic exchange of these zeolites showed that a higher basicity was related to exchanged ions with lower electronegativities, resulting in better catalytic performances in the elimination of VOCs. Following on from this work, noble metals were deposited onto macro-mesoporous metal-oxide supports to form mono and bimetallic catalysts. These were then tested in the oxidation of toluene to study their catalytic performance and their deactivation process. PdAu/TiO(2) and PdAu/TiO(2) -ZrO(2) 80/20 catalysts demonstrated the best activity and life span in the oxidation of toluene and propene and offered the lowest temperatures for a 50 % conversion of VOCs and the lowest coke content after catalytic testing. Different characterization techniques were employed to explain the changes occurring in catalyst structure during the oxidation of toluene and propene.

Nanocomposites of Ag2S and Noble Metals

With a little help from my friends: Successive deposition of different noble metals on Ag2S nanocrystals yields binary, ternary, and quaternary hybrid nanocomposites (see picture). In particular, the platinum-containing systems were found to exhibit superior catalytic activity toward methanol oxidation.

The synthesis of TiO2 and TiO2–Pt and their application in the removal of Cr (VI)

The deposition of noble metal on titanium dioxide (TiO2) has been considered as an effective strategy to improve the activity of TiO2. In this paper, TiO2 nanoparticles were prepared via a sol‐gel route, followed by heat treatment at elevated temperatures. TiO2–Pt catalyst was prepared by deposition of platinum (Pt) on the surface of as‐prepared TiO2 nanoparticles. TiO2 and TiO2–Pt were characterized by X‐ray diffraction, transmission electron microscopy, Raman spectroscopy, ultraviolet‐visible differential reflectance spectra and infrared spectroscopy. TiO2 and TiO2–Pt were used as heterogeneous catalysts for the removal of Cr (VI) under ultraviolet‐light illumination. TiO2 prepared at low temperature had smaller particle size and higher specific surface area, and consequently had higher activity on the removal of Cr (VI). The Pt deposited on the surface of TiO2 favoured the separation of photo‐produced electrons (e−) and holes (h+), inhibited the recombination of e− and h+, and enhanced Cr (VI) removal. However, its blocking of active sites also inhibited the removal of Cr (VI). The deposition of 1% (wt.%) Pt to TiO2 produced the optimum activity for the removal of Cr (VI). A lower pH favoured the adsorption of Cr (VI) on the surface of TiO2, and correspondingly enhanced the removal of Cr (VI).

Photocatalytic synthesis of M/Cu2O (M = Ag, Au) heterogeneous nanocrystals and their photocatalytic properties

M/Cu2O (M = Ag, Au) heterogeneous nanocrystals are successfully prepared by depositing noble metal nanoparticles onto the surfaces of Cu2O octahedral nanocrystals through a simple photocatalytic process. The samples are characterized by means of X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). The influence of the light source and solution temperature on the deposition of noble metal (taking Ag as an example) nanoparticles has been studied. The experimental results show that visible light is more favorable for the deposition of Ag nanoparticles onto Cu2O nanocrystals, and a solution temperature of more than 30 °C can prevent the erosion of Cu2O. The photocatalytic properties of the prepared M/Cu2O heterogeneous nanocrystals are studied, showing enhanced photocatalytic activities.

Attachment of noble metal nanoparticles to conducting polymers containing sulphur – preparation conditions for enhanced electrocatalytic activity

Taking advantage of the spontaneous deposition of noble metals on polymers containing sulphur, the inclusion of gold and platinum in poly(3-methylthiophene) and poly(3,4-ehylenedioxythiophene) (PEDOTh) layers, achieved by immersion of the polymer into the metal nanoparticles suspension, is reported in the present work. Platinum and gold nanoparticles (NPs), with diameters between 3 and 17 nm, have been prepared from colloidal methods (citrate or borohydride reduction in the presence of citrate capping agent) and characterized by transmission electron microscopy, ultraviolet–visible spectrophotometry and X-ray diffraction (XRD). The electropolymerization was carried out under potentiostatic and potentiodynamic conditions, imparting distinct morphologies, as revealed by atomic force microscopy. After polymer films immersion in the colloidal solutions, evidence of the NPs confinement and distribution was provided by XRD analysis and scanning electron microscopy. For thin layers, the quantity of attached metal NPs could be estimated from quartz crystal microbalance data collected throughout the films immersion.The influence of the polymer type and morphology, NPs nature, size and incorporated amount on the electrocatalytic activity of the so-prepared modified electrodes towards the hydrazine oxidation, in phosphate buffer solution, has been investigated by cyclic voltammetry. The results clearly show the superior properties of potentiodynamically prepared PEDOTh films attaching very small (3 nm) freshly prepared Pt-NPs.

Correction to Mechanism, Products, and Growth Rate of Atomic Layer Deposition of Noble Metals

Correction to Mechanism, Products, and Growth Rate of Atomic Layer Deposition of Noble Metals

Mechanism, Products, and Growth Rate of Atomic Layer Deposition of Noble Metals

We present mechanisms for atomic layer deposition of Ru, Rh, Pd, Os, Ir, or Pt metal from homoleptic precursors and oxygen. The novel mechanistic feature is that combustion of ligands produces transient hydroxyl groups on the surface, which can undergo Brønsted-type elimination of a further ligand or water from the surface. Each ligand therefore releases one electron for reduction of the metal. The growth reaction may be described as oxide-catalyzed redox decomposition of the precursor. To validate the mechanism against experiment, we derive analytical expressions for product ratios and the growth rate in terms of saturating coverages.

Room-Temperature Synthesis and Their Applications of Noble Metal Nanoparticles by Metal Ion-Reducing Bacteria

Although chemical and physical synthetic routes to noble metal nanoparticles have been extensively developed, another possibility is biological synthesis by reduction of noble metal ions using microorganism. We focused on the Fe(III)-reducing bacteria Shewanella algae and Shewanella oneidensis as a candidate microbe for rapid reductive deposition of noble metal at room temperature, because the reduction potential of Fe(III) ions is almost equal to that of noble metal ions. Intracellular synthesis of noble metal (Au, Pd, Pt) nanoparticles was achieved at room temperature using resting cells of the Fe(III)-reducing bacteria, when either formate or lactate was provided as the electron donor. The bioreductive deposition of noble metal was a fast process: 1-10 mol/m3 aqueous Pd(II) ions were completely reduced to crystalline Pd(0) nanoparticles within 60 min. Biogenic metal nanoparticles 5-10 nm in size were located in the periplasmic space of bacterial cells. The dried biomass-supported palladium was applicable to a heterogeneous catalyst in a chemical reaction and an anode catalyst in a fuel cell for power production. Our methodology for the room-temperature synthesis of noble metal nanoparticles using the Fe(III)-reducing bacteria is an attractive green process that is cost-effective and environmentally benign.