One-pot synthesis of colloidally robust rhodium(0) nanoparticles and their catalytic activity in the dehydrogenation of ammonia-borane for chemical hydrogen storage

Rhodium(0) nanoparticles stabilized by tert-butylammonium octanoate were prepared reproducibly from the reduction of rhodium(II) octanoate with tert-butylamine-borane in toluene at room temperature and characterized by ICP-OES, TEM, HRTEM, STEM, EDX, XRD, XPS, FTIR, UV-vis, (11)B, (13)C and (1)H NMR spectroscopy and elemental analysis. These new rhodium(0) nanoparticles show unprecedented catalytic activity, lifetime and reusability as a heterogeneous catalyst in room temperature dehydrogenation of ammonia-borane, which is under significant investigation as a potential hydrogen storage material.     

Vibrational spectroscopy of CO in gas-phase rhodium cluster-CO complexes

Infrared spectra of isolated unsaturated rhodium cluster-CO complexes in the region of the CO stretching vibration, nu(CO), are measured using a molecular beam depletion technique. These spectra provide benchmarks for interpreting values of nu(CO) that are found when CO is used to probe Rh surfaces and supported Rh nanoparticles. Supported nanoparticles have shifts of nu(CO) of as much as +100 cm-1 compared to the free clusters measured here, indicative of significant charge transfer to the support.     

Hydrogenation of arenes over silica-supported catalysts that combine a grafted rhodium complex and palladium nanoparticles: evidence for substrate activation on Rh(single-site)-Pd(metal) moieties

The complex Rh(cod)(sulfos) (Rh(I); sulfos = (-)O(3)S(C(6)H(4))CH(2)C(CH(2)PPh(2))(3); cod = cycloocta-1,5-diene), either free or supported on silica, does not catalyze the hydrogenation of benzene in either homogeneous or heterogeneous phase. However, when silica contains supported Pd metal nanoparticles (Pd(0)/SiO(2)), a hybrid catalyst (Rh(I)-Pd(0)/SiO(2)) is formed that hydrogenates benzene 4 times faster than does Pd(0)/SiO(2) alone. EXAFS and DRIFT measurements of in situ and ex situ prepared samples, batch catalytic reactions under different conditions, deuterium labeling experiments, and model organometallic studies, taken together, have shown that the rhodium single sites and the palladium nanoparticles cooperate with each other in promoting the hydrogenation of benzene through the formation of a unique entity throughout the catalytic cycle. Besides decreasing the extent of cyclohexa-1,3-diene disproportionation at palladium, the combined action of the two metals activates the arene so as to allow the rhodium sites to enter the catalytic cycle and speed up the overall hydrogenation process by rapidly reducing benzene to cyclohexa-1,3-diene.     

Rhodium complex with ethylene ligands supported on highly dehydroxylated MgO: synthesis, characterization, and reactivity

Mononuclear rhodium complexes with reactive olefin ligands, supported on MgO powder, were synthesized by chemisorption of Rh(C(2)H(4))(2)(C(5)H(7)O(2)) and characterized by infrared (IR), (13)C MAS NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopies. IR spectra show that the precursor adsorbed on MgO with dissociation of acetylacetonate ligand from rhodium, with the ethylene ligands remaining bound to the rhodium, as confirmed by the NMR spectra. EXAFS spectra give no evidence of Rh-Rh contributions, indicating that site-isolated mononuclear rhodium species formed on the support. The EXAFS data also show that the mononuclear complex was bonded to the support by two Rh-O bonds, at a distance of 2.18 A, which is typical of group 8 metals bonded to oxide supports. This is the first simple and nearly uniform supported mononuclear rhodium-olefin complex, and it appears to be a close analogue of molecular catalysts for olefin hydrogenation in solution. Correspondingly, the ethylene ligands bonded to rhodium in the supported complex were observed to react with H(2) to form ethane, and the supported complex was catalytically active for the ethylene hydrogenation at 298 K. The ethylene ligands also underwent facile exchange with C(2)D(4), and exposure of the sample to carbon monoxide led to the formation of rhodium gem dicarbonyls.     

反应条件对固相合成负载羰基铑原子簇催化剂及其性能的影响

本文报告了由金属盐直接固相合成负载铑络合物或原子簇催化剂的新方法及IR谱表征。CO容易使表面吸附有水分子的RhCl_3/SiO_2还原并生成表面羰基物Rh~+-(CO)_2/SiO_2;CO、CO/H_2和CO/2H_2等不同还原气对表面络合物的生成没有影响。采用H_2还原只能得到金属Rh催化剂。水是重要影响因素,如果RhCl_3/SiO_2先抽空脱水,再用含水的CO还原,就会使Rh~+(CO)_2/SiO_2转化为Rh_6(CO)_(16)/SiO_2。此外,还考察了负载原子簇的CO加氢和热分解反应性能。采用CO还原RhCl_3/SiO_2制备的催化剂同负载原子簇催化剂的反应行为非常相近,而且比传统催化剂具有更高的反应活性。     

Stabilized rhodium(0) nanoparticles: a reusable hydrogenation catalyst for arene derivatives in a biphasic water-liquid system

A colloidal system based on an aqueous suspension of rhodium(o) nanoparticles proved to be an efficient catalyst for the hydrogenation of arene derivatives under biphasic conditions. The rhodium nanoparticles (2-2.5 nm) were synthesized by the reduction of RhCl3 x 3H2O with sodium borohydride and were stabilized by highly water-soluble N-alkyl-N-(2-hydroxyethyl)ammonium salts (HEA-Cn). These surfactant molecules were characterized by measurements of the surface tension and the aqueous dispersions with rhodium were observed by transmission electron cryomicroscopy. The catalytic system is efficient under ultramild conditions, namely room temperature and 1 atm H2 pressure. The aqueous phase which contains the protected rhodium(0) colloids can be reused without significant loss of activity. The microheterogeneous behavior of this catalytic system was confirmed on a mercury poisoning experiment.     

Determination of phenolic compounds using spectral and color transitions of rhodium nanoparticles

This work reports a new approach for the determination of phenolic compounds based on their interaction with citrate-capped rhodium nanoparticles. Phenolic compounds (i.e., catechins, gallates, cinnamates, and dihydroxybenzoic acids) were found to cause changes in the size and localized surface plasmon resonance of rhodium nanoparticles, and therefore, give rise to analyte-specific spectral and color transitions in the rhodium nanoparticle suspensions. Upon reaction with phenolic compounds (mainly dithydroxybenzoate derivatives, and trihydroxybenzoate derivatives), new absorbance peaks at 350 nm and 450 nm were observed. Upon reaction with trihydroxybenzoate derivatives, however, an additional absorbance peak at 580 nm was observed facilitating the speciation of phenolic compounds in the sample. Both absorbance peaks at 450 nm and 580 nm increased with increasing concentration of phenolic compounds over a linear range of 0–500 μM. Detection limits at the mid-micromolar levels were achieved, depending on the phenolic compound involved, and with satisfactory reproducibility (<7.3%). On the basis of these findings, two rhodium nanoparticles-based assays for the determination of the total phenolic content and total catechin content were developed and applied in tea samples. The obtained results correlated favorably with commonly used methods (i.e., Folin-Ciocalteu and aluminum complexation assay). Not the least, the finding that rhodium nanoparticles can react with analytes and exhibit unique localized surface plasmon resonance bands in the visible region, can open new opportunities for developing new optical and sensing analytical applications.     

Rhodium(0) nanoparticles supported on nanocrystalline hydroxyapatite: highly effective catalytic system for the solvent-free hydrogenation of aromatics at room temperature

The hydrogenation of aromatics under mild conditions remains a challenge in the fields of synthetic and petroleum chemistry. Described herein is a new catalytic material that shows excellent catalytic performance in terms of activity, selectivity, and reusability in the hydrogenation of aromatics in solvent-free systems under mild conditions. The catalyst, consisting of rhodium nanoparticles supported on nanocrystalline hydroxyapatite, can quantitatively hydrogenate neat benzene to cyclohexane with exceptionally high rates (initial TOF > 10(3) h(-1)) at 298 K and 3 bars of initial H(2) pressure. This new material maintains its inherent catalytic activity after several reuses. Importantly, catalyst preparation does not require elaborate procedures because the active metal nanoparticles are readily formed from the in situ reduction of Rh(3+)-exchanged hydroxyapatite while submerged in the aromatic solvent at room temperature under 3 bars of H(2) pressure.     

Room-Temperature Interaction of Nitrogen Dioxide with Rhodium Nanoparticles Supported on the Surface of Highly Oriented Pyrolytic Graphite (HOPG)

Kinetics and Catalysis - Samples of rhodium nanoparticles supported on the surface of highly oriented pyrolytic graphite (HOPG) are prepared by vacuum deposition; their interaction with nitrogen...     

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.     

Synthesis and characterization of rhodium nanoparticles using HREM techniques

In the present paper, we report the growth of rhodium nanoparticles passivated with n-alkyl thiol molecules using a new synthesis method which is a variation of the Brust phase transfer method.Our method yields extremely small nanoparticles ranging from 1-3 nm in diameter. Depending on the preparation, rhodium nanoparticles are very interesting for many potential applications; however, they are among the less understood noble metal nanoparticles.The synthesized nanoparticles were characterized by high resolution electron microscopy (HREM).This is the first report on the formation of many of the rhodium nanoparticle structures described herein.     

Stabilisation of water-soluble platinum nanoparticles by phosphonic acid derivatives

Sodium 2-(diphenylphosphino)ethyl phosphonate (1) was investigated as a stabilising agent for platinum nanoparticles (Pt-NPs) in aqueous solution. This phosphino phosphonate is known to stabilise rhodium nanoparticles (NPs) in water. Here we report that in the case of Pt-NPs this ligand is indirectly involved in the stabilisation mechanism and the actual stabilisation agent is the platinum complex Na(2)[Pt(1)(2)] (2). The reduction of platinum(II) salts in the presence of the phosphonates 1, 2, sodium 2-(diphenylphosphoryl)ethyl phosphonate (3) and 3,3,3-triphenylpropyl phosphonate (4) leads to stable platinum NPs with a remarkably narrow particle size distribution. These platinum NPs show high catalytic activity in the hydrogenation of 1-hexene and 1-chloro-3-nitrobenzene under biphasic as well as heterogeneous (supported on charcoal) conditions. The activity of the supported NPs was 30 times higher than the commercially available catalyst Pt(0) EnCat?. Furthermore, the single-crystal X-ray structures of (1)(MeOH)(2)(H(2)O)(2), (3)(H(2)O)(4), and (4)(2)(H(2)O)(17) have been determined.     

Surface‐Mediated Synthesis of Dimeric Rhodium Catalysts on MgO: Tracking Changes in the Nuclearity and Ligand Environment of the Catalytically Active Sites by X‐ray Absorption and Infrared Spectroscopies

The preparation of dinuclear rhodium clusters and their use as catalysts is challenging because these clusters are unstable, evolving readily into species with higher nuclearities. We now present a novel synthetic route to generate rhodium dimers on the surface of MgO by a stoichiometrically simple surface‐mediated reaction involving [Rh(C₂H₄)₂] species and H₂. X‐ray absorption and IR spectra were used to characterize the changes in the nuclearity of the essentially molecular surface species as they formed, including the ligands on the rhodium and the metal‐support interactions. The support plays a key role in stabilizing the dinuclear rhodium species, allowing the incorporation of small ligands (ethyl, hydride, and/or CO) and enabling a characterization of the catalytic performance of the supported species for the hydrogenation of ethylene as a function of the metal nuclearity and ligand environment. A change in the nuclearity from one to two Rh atoms leads to a 58‐fold increase in the catalytic activity for ethylene hydrogenation, a reaction involving unsaturated, but stable, dimeric rhodium species.     

One-step in situ synthesis of NHx-adsorbed rhodium nanocrystals at liquid-liquid interfaces for possible electrocatalytic applications

Nearly monodisperse rhodium nanoparticles with adsorbed NH(x) were synthesized at the CCl(4)-water interface. The presence of NH(x)-adsorbed species was confirmed by energy-dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) studies. The synthesis of controlled size 2-38 nm rhodium particles was studied as a function of reducing agent concentration by transmission electron microscopy (TEM). HRTEM confirmed the formation of rhodium nanoparticles having fringe spacing consistent with reported Rh (111) planes. The continuity of these films over an area of 1×1 μm was revealed by atomic force microscopy (AFM) studies. The electrocatalytic application of these nanostructure Rh-NH(x) thin films for formaldehyde oxidation in 0.5M NaOH was investigated by cyclic voltammetry. The Rh nanoparticles formed by the present strategy are expected to be useful for other catalytic applications also.     

Morphological characterization of poly(phenylacetylene) nanospheres prepared by homogeneous and heterogeneous catalysis

Scanning electron microscopy characterization of the materials obtained by homogeneous and heterogeneous catalytic polymerization of phenylacetylene is described. The catalysts used are β‐dioxygenato rhodium(I) complexes. The effects of the reaction medium, presence of a cocatalyst and the type of catalysis (homogeneous or heterogeneous) on the morphology of the polymers obtained have been studied and discussed. Using a supported complex at 0 °C, nanoparticles with a diameter distribution as narrow as 30 to 70 nm were obtained. Polymer nanopowders were found to be unaffected by ageing. Copyright © 2003 John Wiley & Sons, Ltd.     

Strong metal-support interactions between gold nanoparticles and ZnO nanorods in CO oxidation

The catalytic performances of supported gold nanoparticles depend critically on the nature of support. Here, we report the first evidence of strong metal-support interactions (SMSI) between gold nanoparticles and ZnO nanorods based on results of structural and spectroscopic characterization. The catalyst shows encapsulation of gold nanoparticles by ZnO and the electron transfer between gold and the support. Detailed characterizations of the interaction between Au nanoparticles and ZnO were done with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and FTIR study of adsorbed CO. The significance of the SMSI effect is further investigated by probing the efficiency of CO oxidation over the Au/ZnO-nanorod. In contrast to the classical reductive SMSI in the TiO(2) supported group VIII metals which appears after high temperature reduction in H(2) with electron transfer from the support to metals, the oxidative SMSI in Au/ZnO-nanorod system gives oxygen-induced burial and electron transfer from gold to support. In CO oxidation, we found that the oxidative SMSI state is associated with positively charged gold nanoparticles with strong effect on its catalytic activity before and after encapsulation. The oxidative SMSI can be reversed by hydrogen treatment to induce AuZn alloy formation, de-encapsulation, and electron transfer from support to Au. Our discovery of the SMSI effects in Au/ZnO nanorods gives new understandings of the interaction between gold and support and provides new way to control the interaction between gold and the support as well as catalytic activity.     

IR STUDIES ON SURFACE CHEMISTRY OF RHODIUM IN HYDROFORMYLATION CATALYSTS

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Synthesis,Solid‐State NMR Characterization,and Application for Hydrogenation Reactions of a Novel Wilkinson’s‐Type Immobilized Catalyst

Silica nanoparticles (SiNPs) were chosen as a solid support material for the immobilization of a new Wilkinson’s‐type catalyst. In a first step, polymer molecules (poly(triphenylphosphine)ethylene (PTPPE); 4‐diphenylphosphine styrene as monomer) were grafted onto the silica nanoparticles by surface‐initiated photoinferter‐mediated polymerization (SI‐PIMP). The catalyst was then created by binding rhodium (Rh) to the polymer side chains, with RhCl₃ ? x H₂O as a precursor. The triphenylphosphine units and rhodium as Rh^I provide an environment to form Wilkinson’s catalyst‐like structures. Employing multinuclear (³¹P, ²⁹Si, and ¹³C) solid‐state NMR spectroscopy (SSNMR), the structure of the catalyst bound to the polymer and the intermediates of the grafting reaction have been characterized. Finally, first applications of this catalyst in hydrogenation reactions employing para‐enriched hydrogen gas (PHIP experiments) and an assessment of its leaching properties are presented.     

Surface photochemistry of Rh(CO)2 on zeolite Y-production of a stable coordinatively unsaturated rhodium monocarbonyl surface species at room temperature

The photochemical production and chemical reactivity of a new coordinatively unsaturated rhodium monocarbonyl species on the surface of dealuminated zeolite Y over a temperature range of 300-420 K and a pressure range from 10(-5) to 20 Torr has been studied. Using high vacuum techniques and transmission infrared spectroscopy, ultraviolet irradiation (350 +/- 50 nm) of supported Rh(CO)(2) surface species led to the production of stable, but reactive, =Rh(CO) surface species, characterized by an infrared band at 2023 cm(-1). The coordinatively unsaturated =Rh(CO) species convert to less reactive and coordinatively saturated Rh(CO) by thermal treatment above 370 K. The Rh(CO) species were characterized by an infrared band at 2013 cm(-1). An explanation of the mode of bonding of the rhodium monocarbonyl species to the zeolite surface is provided. Coordinatively unsaturated =Rh(CO) species captured N(2), H(2), and O(2) gas molecules near room temperature to produce a variety of mixed ligand rhodium surface complexes of the form Rh(CO)(N(2)), Rh(CO)(H(2)), Rh(CO)(H)(2), Rh(CO)(H), Rh(CO)(O), and Rh(O). Infrared band assignments for the new species are provided. The work provides new insight into the photochemical behavior of Rh(CO)(2) species supported on high-area zeolite materials and may improve our understanding of the role of active rhodium monocarbonyl species in the development of heterogeneous photocatalysts.     

Metal–organic framework ZIF-8 loaded with rhodium nanoparticles as a catalyst for hydroformylation

Porous metal–organic framework ZIF-8 materials containing incorporated rhodium (Rh@ZIF-8) and rhodium chloride (RhCl₃@ZIF-8) nanoparticles were obtained and tested as a catalytic reaction vessel in the reactions of hydroformylation of 1-decene and styrene. Catalytic tests with Rh@ZIF-8 showed that the selectivity and conversion of the hydroformylation reaction depended on the size of the substrate molecule. The incorporation of catalytic nanoparticles into the pores of a metal–organic framework opens up new possibilities for regioselective hydroformylation.