Hydrothermal synthesis of metal nanoparticles using glycerol as a reducing agent

Metal and metal oxide nanoparticles were synthesized using supercritical water (SCW) as a reaction medium and glycerol as a reducing agent at 400 °C and 300 bar. X-ray diffraction (XRD) patterns confirmed that silver, copper and nickel nitrates were reduced to zero-valent metal nanoparticles. On the other hand, cobalt, iron and manganese nitrates were partially reduced into low-valent metal oxides. Scanning electron microscopy (SEM) images showed that the reduced metals and metal oxides were smaller than the metal oxides formed without glycerol. The difference in reduction behavior of elements is explained using their reduction potentials. Glycerol proved to be an effective reducing agent for hydrothermal applications.     

Catalytic wet oxidation of p-chlorophenol over supported noble metal catalysts

Heterogeneous catalytic wet oxidation of aqueous p-chlorophenol (p-CP) solution was investigated at 453 K and 2.6 MPa in a slurry reactor. The performance of noble metal catalysts was compared with that of traditional manganese catalysts. Activated carbon supported catalysts showed significant higher activities for total organic carbon (TOC) reduction than those supported on alumina or cerium oxide. Pt was found to be the most active metal for p-chlorophenol oxidation. The activities of noble metal catalysts were found to correlate with heat of formation of metal oxides. CO₂ is the predominant oxidation product with formation of minor p-benzoquinone and acetic acid as intermediate compounds. Possible reaction pathway was also discussed.     

Catalytic conversion of olefins on supported cubic platinum nanoparticles: Selectivity of (1 0 0) versus (1 1 1) surfaces

The surface chemistry and catalytic conversion of cis- and trans-2-butenes on platinum (1 0 0) facets were characterized via surface-science and catalytic experiments. Temperature-programed desorption studies on Pt(1 0 0) single crystals pointed to the higher hydrogenation probability of the trans isomer at the expense of a lower extent of CC double-bond isomerization. To test these trends under catalytic conditions, shape selective catalysts were prepared by dispersing cubic platinum colloidal nanoparticles (which expose only (1 0 0) facets) onto a high-surface-area silica xerogel support. Infrared absorption spectroscopy and transmission electron microscopy were used to determine the conditions needed to remove the organic surfactants without loosing the original narrow size distribution and cubic shape of the original metal nanoparticles. Catalytic kinetic measurements with these materials corroborated the surface-science predictions, and pointed to a switch in isomerization selectivity from preferential cis-to-trans conversion with Pt(1 0 0) surfaces to the reverse trans-to-cis reaction with Pt(1 1 1) facets.     

Anodic catalysts for polymer electrolyte fuel cells: the catalytic activity of Pt/C,Ru/C and Pt-Ru/C in oxidation of CO by O2

The catalytic activity for oxidation of CO by O₂ was investigated on commercial Pt/C, Pt-Ru/C (Pt/Ru atomic ratio = 20, 3, 1, 1/3) and Ru/C. All samples contained 20 wt.% metal. Assuming equal surface and bulk composition, the number of surface Pt and Ru atoms was calculated from the average size of the supported metal particle as determined by TEM. On Pt-Ru/C alloys, the turnover frequency per Ru atom, N_Ru/molecules s⁻¹ Ru-atom⁻¹, was independent of chemical composition. This finding suggests that the active site in these alloys is Ru. In the temperature range 300–400 K, the turnover frequency per active metal atom was 50–300 times higher on Pt-Ru/C than on Pt/C. The turnover frequency was 400 times higher on Ru/C than on Pt/C at 313 K and 90 times higher at 353 K. Addition of water vapor to the reactant mixture left the catalytic activity of Ru/C unchanged but slightly increased the activity of Pt/C. On both catalysts the activation energy and reaction orders were nearly the same as in dry atmosphere. Conversely, the addition of water markedly decreased the activation energy for Pt-Ru(1 : 1)/C alloy (from 19 to 11 kcal mol⁻¹). These findings suggest that fuel cells equipped with Pt-Ru/C anodes perform better than cells with Pt/C anodes. They do so because Ru effectively oxidizes the carbon monoxide present as an impurity in the H₂-reformed fuel.     

A facile synthesis of CuO nanowires and nanorods,and their catalytic activity in the oxidative degradation of Rhodamine B with hydrogen peroxide

In this study, a new and facile solution-phase route to prepare CuO nanowires and nanorods with the assistance of salicylic acid was reported. Compared with the commercial CuO nanoparticles, both the CuO nanowires and nanorods exhibited significantly improved catalytic activity in the degradation of Rhodamine B with H₂O₂, which may result from their special one dimensional nanostructures. The apparent activation energy of Rhodamine B oxidation with H₂O₂ in the presence of the CuO nanowires and nanorods was 30.95 kJ·mol^− 1 and 32.07 kJ·mol^− 1, respectively, which was much lower than that in the absence of catalysts.     

Combination of flame synthesis and high-throughput experimentation: The preparation of alumina-supported noble metal particles and their application in the partial oxidation of methane

Mono and multi-noble metal particles on Al₂O₃ were prepared in one step by flame spray pyrolysis (FSP) of the corresponding noble metal precursors dissolved in methanol and acetic acid (v/v 1:1) or xylene. The noble metal loading of the catalysts was close to the theoretical composition as determined by WD-XRF and LA-ICP-MS. The preparation method was combined with high-throughput testing using an experimental setup consisting of eight parallel fixed-bed reactors. Samples containing 0.1–5 wt% noble metals (Ru, Rh, Pt, Pd) on Al₂O₃ were tested in the catalytic partial oxidation of methane. The ignition of the reaction towards carbon monoxide and hydrogen depended on the loading and the noble metal constituents. The selectivity of these noble metal catalysts towards CO and H₂ was similar under the conditions used (methane: oxygen ratio 2:1, temperature from 300 to 500 °C) and exceeded significantly those of gold and silver containing catalysts.Selected catalysts were further analysed using XPS, BET, STEM-EDXS and XANES/EXAFS. The catalysts exhibited generally a specific surface area of more than 100 m²/g, and were made up of ca. 10 nm alumina particles on which the smaller noble metal particles (1–2 nm, partially oxidized state) were discernible. XPS investigation revealed an enrichment of noble metals on the alumina surface of all samples. The question of alloy formation was addressed by STEM-EDXS and EXAFS analysis. In some cases, particularly for Pt–Pd and Pt–Rh, alloying close to the bulk alloys was found, in contrast to Pt–Ru being only partially alloyed. In situ X-ray absorption spectroscopy on selected samples was used to gain insight into the oxidation state during ignition and extinction of the catalytic partial oxidation of methane to hydrogen and carbon monoxide.     

Au–Pt nanoparticles in amine functionalized MCM-41: Catalytic evaluation in hydrogenation reactions

Nano-sized Au–Pt nanoparticles (Au–Pt-bi-MNPs) have been synthesized by the simultaneous reduction of HAuCl₄ and HPtCl₆ by NaBH₄ inside the channels of amine functionalized Si-MCM-41 (NH₂–Si-MCM-41) at ambient conditions. These materials were characterized using chemical analysis, UV–vis, XPS, XRD, FT-IR, Surface area and TEM analysis. The size of these alloyed nanoparticles (bi-MNPs) were found in the range of 2–4 nm. These nanoparticles were evaluated to study their catalytic activities towards hydrogenation of aromatic nitro compounds. The catalytic activity of the Au–Pt bi-MNPs was found to be superior to monometallic Au nanoparticles.     

Low-temperature catalytic oxidation of multi-walled carbon nanotubes

When in a pure form, carbon nanotubes are known to be stable in air up to ∼800 K making them attractive for a large variety of applications. In this work, we report a significant decrease of ignition temperature (in some cases occurring at ∼500 K) and a reduction in the apparent activation energy for oxidation in air as a result of impregnation with nanoparticles (<2 nm) of metal (Pt, Pd, Ni and Co) acetylacetonates or by decoration with corresponding oxides. Surprisingly, defects introduced by partial oxidation of the carbon nanotubes do not in practice have any influence on the enhancement of further oxidation. Reduction temperatures of metal oxides with H₂ were close to those of other carbon supported catalyst materials. However, the carbon nanotubes showed a tendency for low temperature gasification in the presence of hydrogenation catalyst metals (Pt, Pd).     

Metal-assisted hydrogen storage on Pt-decorated single-walled carbon nanohorns

The catalytic dissociation of hydrogen molecules by metal nanoparticles and spillover of atomic hydrogen onto various supports is a well-established phenomenon in catalysis. However, the mechanisms by which metal catalyst nanoparticles can assist in enhanced hydrogen storage on high-surface area supports are still under debate. Experimental measurements of metal-assisted hydrogen storage have been hampered by inaccurate estimation of atomically stored hydrogen deduced from comparative measurements between metal-decorated and undecorated samples. Here we report a temperature cycling technique combined with inelastic neutron scattering (INS) measurements of quantum rotational transitions of molecular H₂ to more accurately quantify adsorbed hydrogen aided by catalytic particles using single samples. Temperature cycling measurements on single-wall carbon nanohorns (SWCNHs) decorated with 2–3 nm Pt nanoparticles showed 0.17% mass fraction of metal-assisted hydrogen storage (at ≈0.5 MPa) at room temperature. Temperature cycling of Pt-decorated SWCNHs using a Sievert’s apparatus also indicated metal-assisted hydrogen adsorption of ≈0.08% mass fraction at 5 MPa at room temperature. No additional metal-assisted hydrogen storage was observed in SWCNH samples without Pt nanoparticles cycled to room temperature. The possible formation of C–H bonds due to spilled-over atomic hydrogen was also investigated using both INS and density functional theory calculations.     

FeVO4 nanorods supported TiO2 as a superior catalyst for NH3–SCR reaction in a broad temperature range

In this work, a catalyst with FeVO₄ nanorods supported on TiO₂ was prepared and applied for NH₃–SCR reaction. A significantly enhanced low temperature catalytic activity has been achieved in the presence of 10% H₂O with the active window shifting by 15 °C to lower temperatures, compared to the classical catalyst with FeVO₄ nanoparticles supported TiO₂. For the catalyst containing FeVO₄ nanorods, enhanced redox ability and enriched surface active oxygen species originated from its unique crystal structure and predominantly exposed reactive crystal facets (− 2 1 0) on the catalyst surface are responsible for its improved low temperature catalytic activity.     

Preparation of supported Pt(0) nanoparticles as efficient recyclable catalysts for hydrogenation of alkenes and ketones

A magnetically recoverable Pt(0) catalyst was prepared by in situ H₂ reduction of Pt²⁺ species bound to an amino modified silica-coated magnetic nanoparticles. Compared to ordinary silica (maximum uptake Pt 0.03 wt%), the amino-functionalized silica surfaces were loaded with 1.95 wt% of metal. The supported Pt(0) nanoparticles exhibit high catalytic activity in the hydrogenation of alkenes and ketones under solventless mild reaction conditions. Partially hydrogenated products could also be isolated. The magnetic property of the catalyst grants a fast and efficient product isolation compared to traditional methods used in heterogeneous systems that generally make use of time- and solvent-consuming procedures.     

Synergistic effect of Pt or Pd and perovskite oxide for water gas shift reaction

The water gas shift (WGS) reaction over Pt and Pd catalysts supported on various perovskite oxides has been investigated at 573 K without catalyst pretreatment. The Pt and Pd catalysts on LaCoO₃ support showed high catalytic activity. Interaction between Pt or Pd and the support is considered to promote the WGS reaction: Pt/LaCoO₃ had high initial activity but deactivated immediately; Pd/LaCoO₃ was less active than Pt/LaCoO₃, but had superior stability. Catalysts were characterized using XRD, STEM, XPS, and H₂-temperature programmed reduction (TPR). Results of this study showed that reduction of the support decreased the CO conversion on Pt/LaCoO₃. On the other hand, Pd/LaCoO₃ showed stable activity for the WGS reaction. Therefore, Pd was added to Pt/LaCoO₃ for stabilizing the catalyst activity, and 0.5 wt.% Pd/1 wt.% Pt/LaCoO₃ catalyst showed higher activity and stability.     

Fabrication of self-binding noble metal/flexible graphene composite paper

Self-binding noble metal (Pt, Au, and Ag)/graphene composite papers as large as 13 cm in diameter were fabricated using a flow-directed method where in situ reduced graphene served as a “binder”. The papers were characterized by X-ray diffraction, scanning and transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. This approach yielded well dispersed metals with various nanostructures both on and between the graphene layers to form papers with good conductivity and flexiblility. The 300 °C-annealed Ag/graphene papers were evaluated as binder-free anodes for lithium ion batteries, delivering a reversible charge capacity of 689 mAh/g at a current density of 20 mA/g.     

Modes of occurrence and origins of noble metals in the Late Permian coals from the Puan Coalfield,Guizhou, southwest China

The concentrations, modes of occurrence, and origins of noble metals in the Late Permian coals from the Puan Coalfield, southwest China were studied using high-resolution inductively coupled-plasma mass spectrometry (ICP-MS) and sequential chemical extraction procedures (SECP). The results show that the minerals in the No. 2 coal from the Puan Coalfield were dominated by pyrite of low-temperature hydrothermal origin and clay minerals of detrital terrigenous origin. As compared with the concentrations of noble metals in the ordinary Chinese and American coals, the No. 2 coal significantly enriched in elements Rh (38 ng/g), Pd (640 ng/g), Ir (9 ng/g), Pt (98 ng/g), Au (16 ng/g), and Ag (1620 ng/g). The concentrations of Pd, Ir, and Au in the No. 2 coal are 4.3, 9, and 5.3 times higher than those of the ordinary Chinese coals, respectively. The SECP results indicate that the noble metals enriched in the No. 2 coal were mainly distributed in the sulfide association. The concentrations of Rh, Pd, Ir, Pt, Au, and Ag in the sulfide association are as high as 720, 15,000, 310, 2380, 360, and 32,300 ng/g, respectively. However, these noble metals in organic and silicate associations were lower than or close to their background values of coal. The low-temperature hydrothermal fluids play a dominant role in the enrichment of noble metals in the coal.     

Platinum supported on reduced graphene oxide as a catalyst for hydrogenation of nitroarenes

Reduced graphene oxide (RGO)-supported platinum (Pt) catalyst was prepared by simple ethylene glycol (EG) reduction and used for hydrogenation of nitroarenes. Characterizations showed that EG as a reductant exhibited more advantages than the widely used hydrazine hydrate to fabricate monodispersed, small sized Pt nanoparticles on the surface of RGO. The yield of aniline over the Pt/RGO-_EG catalyst reached 70.2 mol-_AN/(mol-_Pt min) at 0 ^oC, which is 12.5 and 19.5 times higher than that of multi-walled carbon nanotube- and active carbon-supported Pt catalysts, respectively. When the reaction temperature was increased to 20 ^oC, the catalytic activity of Pt/RGO-_EG jumped to 1138.3 mol-_AN/(mol-_Pt min), and it was also extremely active for the hydrogenation of a series of nitroarenes. The unique catalytic activity of Pt/RGO-_EG is not only related to the well dispersed Pt clusters on the RGO sheets but also the well dispersion of Pt/RGO-_EG in the reaction mixture.     

A novel microwave absorption material of Ni doped cryptomelane type manganese oxides

Cryptomelane type manganese oxide α-MnO₂ and Ni doped KMn₈O₁₆ nanostructures were synthesized by water-bathing methods at 80 °C for 24 h using NiSO₄·H₂O as the dopant sources. The structures, morphologies and physical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results show that the products are Ni doped KMn₈O₁₆ nanorods after the introduction of NiSO₄·H₂O during the reaction process. The electromagnetic characteristics and microwave absorption properties of the materials were carried out with a vector network analyzer (VNA) and the transmission line (TML) theory. The dielectric loss and microwave absorption properties of the cryptomelane materials are improved after Ni doping. The thickness dependent reflection loss shows that the peak frequency and effective absorption bandwidth all decrease with the increasing material thickness. With the increase of Ni doping concentration, the peak frequency shifts to higher frequency bands and the effective absorption bandwidth increases. The electromagnetic performance of cryptomelane can be attributed to its unique tunnel structures and the improvement of Ni doping can be due to the enhanced electromagnetic polarization.     

Microstructure of new composite electrocatalyst and its anodic behavior for chlorine and oxygen evolution

The first layer of active coating made from a rutile-structured solid solution of ruthenium and titanium dioxides having an average crystal grain size of 30 nm was thermally deposited on an adequately prepared titanium metal substrate. Then, at a temperature of 500 °C, the second layer was formed on the first layer from a mixture of amorphous particles of metallic platinum and rutile-structured iridium dioxide nanocrystals having an average crystal grain size of 26 nm.Rutile phase nanocrystals are characterized by a high density of chaotically distributed dislocations and high internal microstrain values. The coatings exhibit a compact granular morphology without cracks on the surface. Their catalytic activity is similar to that of conventional DSAs for the anodic oxidation of chloride ions from both concentrated and dilute sodium chloride solutions. The anodic current efficiency both during chlorate formation and active chlorine production was several percentage points higher in electrolyzers containing these anodes than in those containing DSAs. The catalytic activity of anodes having these coatings is about 50 mV lower than that of DSAs and about 350 mV higher than that of lead/antimony alloy electrodes, for oxygen evolution from acid sulfate solutions (0.5 mol dm^?3 H₂SO₄) characteristic of processes for the production of some metals. An accelerated corrosion test showed that the stability of the double-layer anodes is about twelve-fold higher than that of conventional DSAs.     

Photocatalytic oxidation of cyanide under visible light by Pt doped AgInS2 nanoparticles

AgInS₂ nanoparticles were prepared by a microwave method, while Pt was doped on the surface of AgInS₂ via photoassisted deposition method. The catalytic performances of the AgInS₂ and Pt/AgInS₂ samples were carried out for photocatalytic oxidation of cyanide under visible light. The UV–vis analysis proved a red shift was detected after the loading of Pt into the AgInS₂. The maximum oxidation efficiency achieved was 100% at 1.5 wt% Pt/AgInS₂ photocatalyst after 35 min reaction time. The catalyst could be reused without any loss in activity during the first five cycles.     

Nanostructured perovskite oxides – LaMO3 (M=Al,Co, Fe) prepared by co-precipitation method and their ethanol-sensing characteristics

Nanostructured La-based perovskite oxides − LaMO₃ (M=Al, Co, Fe) were synthesized by a new co-precipitation procedure using metal nitrate and carbonate salts as starting materials. X-ray diffraction and energy dispersive X-ray spectroscopic results confirmed the formation of single-phase nanocrystalline perovskite oxides with high purity. Characterizations by scanning/transmission electron microscopy and nitrogen adsorption revealed that LaAlO₃ was produced in the form of rectangular porous nanorods exhibiting much larger surface area and porosity compared with densely aggregated LaCoO₃ particles and loosely clustered LaFeO₃ nanoparticles with cracked-egg morphologies. The materials were characterized for gas sensing towards ethanol at 200–350 °C. From gas-sensing results, the LaAlO₃ sensor displayed n-type gas-sensing behaviors with considerably higher ethanol response than p-type LaFeO₃ and LaCoO₃ sensors, respectively. In particular, the LaAlO₃ sensor exhibited a high response of 16.45–1000 ppm ethanol and excellent ethanol selectivity against NO₂, SO₂, CO and H₂ at 350 °C. The superior gas-sensing performances could be attributed to the effective receptor function, transducer function and utility factor of LaAlO₃ nanorod structures prepared by the co-precipitation method.     

Hydrogenation of soybean oil over various platinum catalysts: Effects of support materials on trans fatty acid levels

The effects of various support materials on the catalytic performance of supported platinum catalysts for the hydrogenation of soybean oil were examined. There was a linear relationship between the catalytic activity and the platinum dispersion of the platinum catalysts. Among the examined catalysts, Pt/BaSO₄ was effective for the reduction of both trans fatty acid (TFA) and additional saturated fatty acid (ASFA) levels in partially hydrogenated oils (iodine value (IV) = 70). In addition, the relationship between the TFA levels and the electronegativity of the metal ion in the support material was a volcano function.