Multicomponent Azide-Alkyne Cycloaddition Catalyzed by Impregnated Bimetallic Nickel and Copper on Magnetite

A new bimetallic catalyst derived from nickel and copper has been used successfully for the first time in the multicomponent reaction of terminal alkynes, sodium azide, and benzyl bromide derivatives. The presence of both metallic species on the surface of magnetite seems to have a positive and synergetic effect. The catalyst loading is the lowest ever published for a catalyst of copper anchored on any type of iron support. The catalyst could be easily removed from the reaction media just by magnetic decantation and it could be reused up to ten times without any negative effect on the initial results.

Steam reforming of ethanol over bimetallic RhPt/La2O3: Long-term stability under favorable reaction conditions

Recently, the steam reforming of biofuels has been presented as a potential hydrogen source for fuel cells. Because this scenario represents an interesting opportunity for Colombia (South America), which produces large amounts of bioethanol, the steam reforming of ethanol was studied over a bimetallic RhPt/La2O3 catalyst under bulk mass transfer conditions. The effect of temperature and the initial concentrations of ethanol and water were evaluated at space velocities above 55,000 h−1 to determine the conditions that maximize the H2/CO ratio and reduce CH4 production while maintaining 100% conversion of ethanol. These requirements were accomplished when 21 mol% H2O and 3 mol% C2H5OH (steam/ethanol molar ratio = 7) were reacted at 600 °C. The catalyst stability was assessed under these reaction conditions during 120 h on stream, obtaining ethanol conversions above 99% during the entire test. The effect of both H2 and air flows as catalyst regeneration treatments were evaluated after 44 and 67 h on stream, respectively. The results showed that H2 treatment accelerated catalyst deactivation, and air regeneration increased both the catalyst stability and the H2 selectivity while decreasing CH4 generation. Fresh and spent catalyst samples were characterized by TEM/EDX, XPS, TPR, and TGA. Although the Rh and Pt in the fresh catalyst were completely reduced, the spent samples showed a partial oxidation of Rh and small amounts of carbonaceous residue. A possible Rh–Pt–Rh2O3 structure was proposed as the active site on the catalyst, which was regenerated by air treatment.

Palladium and Bimetallic Palladium–Nickel Nanoparticles Supported on Multiwalled Carbon Nanotubes: Application to Carbon–Carbon Bond-Forming Reactions in Water

Palladium and bimetallic Pd–Ni nanoparticles (NPs) protected by polyvinylpyrrolidone were prepared by the reduction-by-solvent method and deposited on multiwalled carbon nanotubes (MWCNTs). The catalytic activity of these NPs to carbon–carbon bond-forming reactions was studied by using 0.1 mol % Pd loading, at 120 °C for 1 h and water as a solvent under ligand-free conditions. The Suzuki–Miyaura reaction took place quantitatively for the cross-coupling of 4-bromoanisole with phenylboronic acid, better than those obtained with potassium phenyltrifluoroborate, with Pd50Ni50/MWCNTs as a catalyst and K2CO3 as a base and TBAB as an additive, with good recyclability during 4 cycles with some Ni leaching. The Hiyama reaction of 4-iodoanisole with trimethoxyphenylsilane, under fluoride-free conditions using 50 % aqueous NaOH solution, was performed with Pd/MWCNTs as a catalyst in 83 % yield with low recyclability. For the Mizoroki-Heck reaction 4-iodoanisole and styrene gave the corresponding 4-methoxystilbene quantitatively with Pd50Ni50/MWCNTs using K2CO3 as a base and TBAB as an additive although the recycle failed. In the case of the Sonogashira-Hagihara reaction, Pd/MWCNTs had to be used as a catalyst and pyrrolidine as a base for the coupling of 4-iodoanisole with phenylacetylene under copper-free conditions. The corresponding 4-methoxytolane was quantitatively obtained allowing the recycling of the catalyst during 3 cycles.

Silica Microparticles Supported Gold and Copper Ferrite Nanoparticles: A Magnetically Recyclable Bimetallic Catalyst for Sonogashira Reaction

Novel silica supported gold and copper ferrite nanoparticles (NPs) have been synthesized, characterized and used as a separable dual catalyst in Sonogashira type reaction. These Au.CuFe2O4@Silica NPs show a high efficiency as catalyst in the alkynylation not only of aryl iodides but also aryl bromides. By using only 0.5 mol% loading and t-BuOK as base in N,N-dimethylacetamide as solvent, aryl iodides react at 115 ºC in 1 d, whereas for aryl bromides the cross-coupling takes place at 130 ºC in 2 d. The catalyst can be successfully recycled using an external magnet for four consecutive runs.

Bimetallic PtSn/C catalysts obtained via SOMC/M for glycerol steam reforming

A detailed study on the preparation of bimetallic PtSn/C catalysts using surface-controlled synthesis methods, and on their catalytic performance in the glycerol steam reforming reaction has been carried out. In order to obtain these well-defined bimetallic phases, techniques derived from Surface Organometallic Chemistry on Metals (SOMC/M) were used. The preparation process involved the reaction between an organometallic compound ((C4H9)4Sn) and a supported transition metal (Pt) in a H2 atmosphere. Catalysts with Sn/Pt atomic ratios of 0.2, 0.3, 0.5, and 0.7 were obtained, and characterized using several techniques: ICP, H2 chemisorption, TEM and XPS. These systems were tested in the glycerol steam reforming varying the reaction conditions (glycerol concentration and reaction temperature). The best performance was observed for the catalysts with the lowest tin contents (PtSn0.2/C and PtSn0.3/C). It was observed that the presence of tin increased the catalysts’ stability when working under more severe reaction conditions.

Single-walled carbon nanotube buckypapers as electrocatalyst supports for methanol oxidation

This work studies the use of various single-walled carbon nanotube (SWCNT) buckypapers as catalyst supports for methanol electro-oxidation in acid media. Buckypapers were obtained by vacuum filtration from pristine and oxidized SWCNT suspensions in different liquid media. Pt–Ru catalysts supported on the buckypapers were prepared by multiple potentiostatic pulses using a diluted solution of Pt and Ru salts (2 mM H2PtCl6 + 2 mM RuCl3) in acid media. The resulting materials were characterized via SEM, TEM, EDX and ICP-OES analysis. Well dispersed rounded nanoparticles between 2 and 15 nm were successfully electrodeposited on the SWCNT buckypapers. The ruthenium content in the bimetallic deposits was between 32 and 48 at. %, while the specific surface areas of the catalysts were in the range of 72–113 m2 g−1. It was found that the solvent used to prepare the SWCNT buckypaper films has a strong influence on the catalyst dispersion, particle size and metal loading. Cyclic voltammetry and chronoamperometry experiments point out that the most active electrodes for methanol electro-oxidation were prepared with the buckypaper supports that were obtained from SWCNT dispersions in N-methyl-pyrrolidone.

Sulfur resistance of Pt-W catalysts

The sulfur resistance of low-loaded monometallic Pt catalysts and bimetallic Pt-W catalysts during the partial selective hydrogenation of styrene, a model compound of Pygas streams, was studied. The effect of metal impregnation sequence on the activity and selectivity was also evaluated. Catalysts were characterized by ICP, TPR, XRD, and XPS techniques. Catalytic tests with sulfur-free and sulfur-doped feeds were performed. All catalysts showed high selectivities (>98%) to ethylbenzene. Activity differences between the catalysts were mainly attributed to electronic effects due to the presence of different electron-rich species of Pt0 and electron-deficient species of Ptδ+. Pt0 promotes the cleavage of H2 while Ptδ+ the adsorption of styrene. The catalyst successively impregnated with W and Pt (WPt/Al) was more active and sulfur resistant than the catalyst prepared with an inverse impregnation order (PtW/Al). The higher poison resistance of WPt/Al was attributed to both steric and electronic effects.

Single wall carbon nanotubes loaded with Pd and NiPd nanoparticles for H2 sensing at room temperature

Pd and bimetallic Ni50Pd50 nanoparticles protected by polyvinylpyrrolidone (PVP) have been synthesized by the reduction-by-solvent method and deposited on single wall carbon nanotubes (SWCNTs) to be tested as H2 sensors. The SWCNTs were deposited by drop casting from different suspensions. The Pd nanoparticles-based sensors show a very reproducible performance with good sensitivity and very low response times (few seconds) for different H2 concentrations, ranging from 0.2% to 5% vol. H2 in air at atmospheric pressure. The influence of the metal nanoparticle composition, the quality of SWCNTs suspension and the metal loading have been studied, observing that all these parameters play an important role in the H2 sensor performance. Evidence for water formation during the H2 detection on Pd nanoparticles has been found, and its repercussion on the behaviour of the assembled sensors is discussed. The sensor preparation procedure detailed in this work has proven to be simple and reproducible to prepare cost-effective and highly efficient H2 sensors that perform very well under real application conditions.

Growth of Carbon Nanotubes on Surfaces: The Effects of Catalyst and Substrate

We report a study of synthesising air-stable, nearly monodispersed bimetallic colloids of Co/Pd and Fe/Mo of varying compositions as active catalysts for the growth of carbon nanotubes. Using these catalysts we have investigated the effects of catalyst and substrate on the carbon nanostructures formed in a plasma-enhanced chemical vapour deposition (PECVD) process. We will show how it is possible to assess the influence of both the catalyst and the support on the controlled growth of carbon nanotube and nanofiber arrays. The importance of the composition of the catalytic nuclei will be put into perspective with other results from the literature. Furthermore, the influence of other synthetic parameters such as the nature of the nanoparticle catalysts will also be analysed and discussed in detail.

Sonochemical synthesis of graphene oxide supported Pt–Pd alloy nanocrystals as efficient electrocatalysts for methanol oxidation

Pt–Pd bimetallic nanoparticles supported on graphene oxide (GO) nanosheets were prepared by a sonochemical reduction method in the presence of polyethylene glycol as a stabilizing agent. The synthetic method allowed for a fine tuning of the particle composition without significant changes in their size and degree of aggregation. Detailed characterization of GO-supported Pt–Pd catalysts was carried out by transmission electron microscopy (TEM), AFM, XPS, and electrochemical techniques. Uniform deposition of Pt–Pd nanoparticles with an average diameter of 3 nm was achieved on graphene nanosheets using a novel dual-frequency sonication approach. GO-supported bimetallic catalyst showed significant electrocatalytic activity for methanol oxidation. The influence of different molar compositions of Pt and Pd (1:1, 2:1, and 3:1) on the methanol oxidation efficiency was also evaluated. Among the different Pt/Pd ratios, the 1:1 ratio material showed the lowest onset potential and generated the highest peak current density. The effect of catalyst loading on carbon paper (working electrode) was also studied. Increasing the catalyst loading beyond a certain amount lowered the catalytic activity due to the aggregation of metal particle-loaded GO nanosheets.

Nanoparticles of tungsten as low-cost monometallic catalyst for selective hydrogenation of 3-hexyne

Low-cost tungsten monometallic catalysts containing variable amounts of metal (4.5, 7.1 and 8.5%W) were prepared by impregnating alumina with ammonium metatungstate as an inexpensive precursor. The catalysts were characterized using ICP, XPS, XRD, TPR and hydrogen chemisorption. These techniques revealed mainly WO3-Al2O3 (W6+) species on the surface. The effects of the content of W nanoparticles and reaction temperature on activity and selectivity for the partial hydrogenation of 3-hexyne, a non-terminal alkyne, were assessed under moderate conditions of temperature and pressure. The monometallic catalysts prepared were found to be active and stereoselective for the production of (Z )-3-hexene, had the following order: 7.1WN/A > 8.5 WN/A ≥ 4.5 WN/A. Additionally, the performance of the synthesized xWN/A catalysts exhibited high sensitivity to temperature variation. In all cases, the maximum 3-hexyne total conversion and selectivity was achieved at 323 K. The performance of the catalysts was considered to be a consequence of two phenomena: a) the electronic effects, related to the high charge of W (+6), causing an intensive dipole moment in the hydrogen molecule (van der Waals forces) and leading to heterolytic bond rupture; the H+ and H- species generated approach a 3-hexyne adsorbate molecule and cause heterolytic rupture of the C≡C bond into C- = C+; and b) steric effects related to the high concentration of WO3 on 8.5WN/A that block the Al2O3 support. Catalyst deactivation was detected, starting at about 50 min of reaction time. Electrodeficient W6+ species are responsible for the formation of green oil at the surface level, blocking pores and active sites of the catalyst, particularly at low reaction temperatures (293 and 303 K). The resulting best catalyst, 7.1WN/A, has low fabrication cost and high selectivity for (Z )-3-hexene (94%) at 323 K. This selectivity is comparable to that of the classical and more expensive industrial Lindlar catalyst (5 wt% Pd). The alumina supported tungsten catalysts are low-cost potential replacements for the Lindlar industrial catalyst. These catalysts could also be used for preparing bimetallic W-Pd catalysts for selective hydrogenation of terminal and non-terminal alkynes.

NOx storage and reduction over copper-based catalysts. Part 2: Ce0.8M0.2Oδ supports (M = Zr, La, Ce, Pr or Nd)

5% copper catalysts with Ce0.8M0.2Oδ supports (M = Zr, La, Ce, Pr or Nd) have been studied by rapid-scan operando DRIFTS for NOx Storage and Reduction (NSR) with high frequency (30 s) CO, H2 and 50%CO + 50%H2 micropulses. In the absence of reductant pulses, below 200–250 °C NOx was stored on the catalysts as nitrite and nitro groups, and above this temperature nitrates were the main species identified. The thermal stability of the NOx species stored on the catalysts depended on the acid/basic character of the dopant (M more acidic = NOx stored less stable ⇒ Zr4+ < none < Nd3+ < Pr3+ < La3+ ⇐ M more basic = NOx stored more stable). Catalysts regeneration was more efficient with H2 than with CO, and the CO + H2 mixture presented an intermediate behavior, but with smaller differences among the series of catalyst than observed using CO alone. N2 is the main NOx reduction product upon H2 regeneration. The highest NOx removal in NSR experiments performed at 400 °C with CO + H2 pulses was achieved with the catalyst with the most basic dopant (CuO/Ce0.8La0.2Oδ) while the poorest performing catalyst was that with the most acidic dopant (CuO/Ce0.8Zr0.2Oδ). The poor performance of CuO/Ce0.8Zr0.2Oδ in NSR experiments with CO pulses was attributed to its lower oxidation capacity compared to the other catalysts.

Platinum–tin/carbon catalysts for ethanol oxidation: Influence of Sn content on the electroactivity and structural characteristics

Carbon-supported Pt–Sn catalysts commonly contain Pt–Sn alloy and/or Pt–Sn bimetallic systems (Sn oxides). Nevertheless, the origin of the promotion effect due to the presence of Sn in the Pt–Sn/C catalyst towards ethanol oxidation in acid media is still under debate and some contradictions. Herein, a series of Ptx–Sny/C catalysts with different atomic ratios are synthesized by a deposition process using formic acid as the reducing agent. Catalysts structure and chemical compositions are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and their relationship with catalytic behavior towards ethanol electro-oxidation was established. Geometric structural changes are producing by highest Sn content (Pt1–Sn1/C) promoted the interaction of Pt and Sn forming a solid solution of Pt–Sn alloy phase, whereas, the intermediate and lowest Sn content (Pt2–Sn1/C and Pt3–Sn1/C, respectively) promoted the electronic structure modifications of Pt by Sn addition without the formation of a solid solution. The amount of Sn added affects the physical and chemical characteristics of the bimetallic catalysts as well as reducing the amount of Pt in the catalyst composition and maintaining the electrocatalytic activities at the anode. However, the influence of the Sn oxidation state in Pt–Sn/C catalysts surfaces and the alloy formation between Pt and Sn as well as with the atomic ratio on their catalytic activity towards ethanol oxidation appears minimal. Similar methodologies applied for synthesis of Ptx–Sny/C catalysts with a small change show differences with the results obtained, thus highlighting the importance of the conditions of the preparation method.