Supported Rhodium Nanoparticles Research Articles

Fabrication of porous rhodium nanotube catalysts by electroless plating

A versatile electroless plating procedure for the fabrication of rhodium nanomaterials was developed, leading to deposits consisting of loosely agglomerated metal nanoparticles. By using carbon black as the substrate, supported rhodium nanoparticle clusters were obtained. In combination with ion track etched polymer templates, the deposition protocol allowed the first direct synthesis of rhodium nanotubes. Polymer dissolution provided access to well defined, supportless and free-standing rhodium nanotubes of nearly cylindrical shape, 300 nm opening diameter, 28 μm length and 50 nm wall thickness. The characterization by SEM, TEM, EDS and XRD confirmed the purity of the deposit, displayed a small particle size of approximately 3 nm and revealed gaps in the range of a few nanometers between the rhodium particles. BET analysis verified the presence of pores of <5 nm. To evaluate the electrocatalytic potential of the rhodium nanotubes, they were applied in the amperometric detection of hydrogen peroxide. Compared to classical nanoparticle-based sensing concepts, improved performance parameters (sensitivity, detection limit, and linear range) could be achieved.

Metal vapour derived supported rhodium nanoparticles in the synthesis of β-lactams and β-lactones derivatives

Functionalised β-lactones and β-lactams were prepared starting from propargyl alcohols and propargyl tosyl amides by means of efficient silylcarbocyclisation reactions catalysed by rhodium nanoparticles derived from mesitylene-solvated Rh atoms (Metal Vapour Synthesis technique, MVS) and deposited on inorganic (C, γ-Al2O3 Fe2O3) and organic matrices (PBI). All the MVS supported nanoclusters resulted more active than the analogous commercial Rh/C and Rh/γ-Al2O3, as well as homogeneous Rh4(CO)12 used as reference catalyst. In particular, metal vapour derived Rh/C afforded the β-lactones and β-lactams in high yields and chemoselectivity. Preliminary investigations on the nature of the real active metal species involved in the catalytic process showed that rhodium(0) naked nanoparticles are leached by the support. The high catalytic activity encountered with Rh/C could be ascribed to the easy leaching of metal nanoparticles from carbon into solution. Indeed, the presence of a more polar matrix determined a minor catalytic efficiency probably due to a stronger interaction between the metal and the support. Therefore MVS Rh/C species represents a source, stable with ageing at room temperature, of highly active metal nanoparticles.

Stereoselective Hydrogenation of Dibenzo-18-crown-6 Catalyzed by Carbon Nanotube–Supported Rhodium Nanoparticles

Multiwalled carbon nanotube–supported Rh nanoparticles prepared by a simple sonochemical method are highly active for catalytic hydrogenation of dibenzo-18-crown-6 to dicyclohexyl-18-crown-6. The catalytic hydrogenation reaction shows a selectivity favoring the cis-syn-cis isomer over cis-anti-cis isomer of the product by a factor of ≥6. Conventional Rh catalysts supported by activated wood carbon, SiO2, or Al2O3 can only convert dibenzo-18-crown-6 to dicyclohexyl-18-crown-6 with an cis-syn-cis/cis-anti-cis ratio of <1.7. The Rh nanoparticle catalyst can be reused at least six times without losing its catalytic activity.

Planar oxide supported rhodium nanoparticles as model catalysts

C(2)H(4)/CO/H(2) reaction is investigated on Rh/SiO(2) model catalyst surfaces. Kinetic reactivity and infrared spectroscopic measurements are investigated as a function of Rh particle size under near atmospheric reaction conditions. Results show that propionaldehyde turnover frequency (TOF) (CO insertion pathway) exhibits a maximum activity near <d(p)> = 2.5 nm. Polarization modulation infrared reflection absorption spectroscopy under CO and reaction (C(2)H(4)/CO/H(2)) conditions indicate the presence of Rh carbonyl species (Rh(CO)(2), Rh(CO)H) on small Rh particles, whereas larger particles appear resistant to dispersion and carbonyl formation. Combined these observations suggest the observed particle size dependence for propionaldehyde production via CO insertion is driven by two factors: (i) an increase in propionaldehyde formation on undercoordinated Rh sites and (ii) creation of carbonyl hydride species (Rh(CO)H)) on smaller Rh particles, whose presence correlates with the lower activity for propionaldehyde formation for <d(p)> < 2.5 nm.

Tailored supported metal nanoparticles by CVD: an easy and efficient scale-up by a rotary bed OMCVD device

A rotary-bed OMCVD device was developed and successfully applied to the scale-up of the preparation of supported rhodium nanoparticles. The device revealed suitability for the preparation of up to ten grams of material. A multistep CVD protocol was established and allowed a catalytic material with tailored structural/morphological properties of the supported nanoparticles to be obtained. As the result of this kind of catalyst design the supported Rh nanoparticles showed outstanding performance in the hydrogen production by methane catalytic partial oxidation at low contact times. Materials produced were fully characterized by a pool of techniques including HRTEM, CO-DRIFTS, TPRD, etc.

Interaction of small gas phase molecules with alumina supported rhodium nanoparticles: anin situ spectroscopic study

Supported nanoparticulate Rh systems are studied as a model system forthe important three way catalysts as used in the combustion engines of cars.Small Rh nanoparticles with a small particle size distribution can be easilysynthesized and their morphology is studied using x-ray absorption fine structure(XAFS) spectroscopy. The interaction of the supported rhodium nanoparticles onγ-Al2O3 with small gasphase molecules like H2, O2, CO,NO, H2S and SO2 is investigated, in situ and time resolved, using a combination of techniques, i.e. XAFS,diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and massspectrometry. The surface species formed upon exposure of the metal particles to theadsorbing molecules, and their sometimes disruptive interaction with the metal particles,are identified as a function of temperature and time. Dynamic equilibria are observedwhich change the oxidation state and the nuclearity of the supported rhodiumparticles under operational conditions. Rather than merely adsorb on a catalystparticle, these gases have a substantial role in defining the nature of the particle.

Supported rhodium nanoparticles obtained by Metal Vapour Synthesis as catalysts in the preparation of valuable organic compounds

Abstract Rhodium nanoparticles, derived from mesitylene-solvated rhodium atoms, deposited on γ-Al 2 O 3 , are excellent catalysts for the selective hydrogenation of the double bond in α,β-unsaturated carbonyl compounds. The low metal loading Rh on γ-Al 2 O 3 catalyst, containing trioctylamine, TOA, as stabilizing agent of the metal nanoparticles, Rh(TOA)/γ-Al 2 O 3 , 0.1 wt.% Rh, showed the highest catalytic activity. Using this catalyst 4-(6,-methoxy-2,-naphthyl)-3-buten-2-one was reduced to the anti-inflammatory drug 4-(6,-methoxy-2,-naphthyl)-butan-2-one, Nabumetone™, with complete selectivity and under mild reaction conditions (room temperature, atmospheric pressure of hydrogen). Similarly, 2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene was hydrogenated with high selectivity (85%) to 2-acetyl-5,8-dimethoxy-1,2,3,4-tetrahydronaphthalene, precursor of antitumor anthracyclinic compounds, which was obtained chemically pure by crystallization. No leaching of rhodium was observed. The catalyst was completely recovered and, if reused, it works without loss of activity. 1 H NMR DOSY analysis of the Rh(TOA)/mesitylene solution evidenced the presence of nanoparticles with a diameter of about 1.1 nm.

State of Supported Rhodium Nanoparticles for Methane Catalytic Partial Oxidation (CPO): FT-IR Studies

The effect of pretreatments as well as of rhodium precursor and of the support over the morphology of Rh nanoparticles were investigated by Fourier transform infrared (FT-IR) spectroscopy of adsorbed CO. Over a Rh/alumina catalyst, both metallic Rh particles, characterized by IR bands in the range 2070-2060 cm-1 and 1820-1850 cm-1, and highly dispersed rhodium species, characterized by symmetric and asymmetric stretching bands of RhI(CO)2 gem-dicarbonyl species, are present. Their relative amount changes following pretreatments with gaseous mixtures, representative of the catalytic partial oxidation (CPO) reaction process. The Rh metal particle fraction decreases with respect to the Rh highly dispersed fraction in the order CO approximately CO/H2 > CH4/H2O, CH4/O2 > CH4 > H2. The metal particle dimensions decrease in the order CH4/O2 > H2 > CH4/H2O > CO > CO/H2. Grafting from a carbonyl rhodium complex also increases the amount and the dimensions of Rh0 particles at the catalyst surface. Increasing the ratio (extended rhodium metal particles/highly dispersed Rh species) allows a shorter conditioning process. The surface reconstruction phenomena going on during catalytic activity are related to this effect.

Supported rhodium nanoparticles in catalysis: the role of stabilizers on catalytic activity and structural features

Rhodium nanoparticles supported on γ-Al 2O 3, derived from arene-solvated Rh atoms stabilized by trioctylamine (TOA), are valuable catalysts in hydrogenation and silylformylation reactions. They are largely more active than the analogous commercial catalyst, as well as of a sample similarly prepared in the absence of TOA. HRTEM measurements, IR studies on adsorbed CO species and extended X-ray absorption fine structure analyses evidenced the role of TOA in controlling the particle growth in the preparative process and in stabilizing the resulting Rh particles against erosion by CO and oxidation.