Rh Nanoparticles Research Articles

Sub-3 nm Rh nanoclusters confined within a metal-organic framework for enhanced hydrogen generation.

Ultrafine sub-3 nm Rh nanoclusters confined within a metal-organic framework (UIO-66) were constructed through a double-solvent host-guest strategy and achieved superior hydrogen generation via ammonia borane hydrolysis compared with Rh nanoclusters dispersed on the exterior walls.

Ultra-small Rh nanoparticles supported on WO3−x nanowires as efficient catalysts for visible-light-enhanced hydrogen evolution from ammonia borane

Hydrolysis of ammonia borane (AB) is a safe and convenient means of H2 production when efficient catalysts are used. Here we report a facile one-pot solvothermal method to synthesize Rh/WO3−x hybrid nanowires. Ultra-small Rh nanoparticles with an average size of ∼1.7 nm were tightly anchored on WO3−x nanowires. Rh/WO3−x catalysts exhibited substantially enhanced activity for hydrolytic dehydrogenation of AB under both dark and visible light irradiation conditions relative to mixed Rh nanoparticles and WO3−x nanowires (Rh + WO3−x), and Rh/C and WO3−x nanowires. X-ray photoelectron spectroscopy (XPS) analysis indicated that the synergistic effect between Rh nanoparticles and WO3−x nanowires was responsible for such an enhancement in activity. Specifically, Rh/WO3−x achieved the highest turnover frequency (TOF) with a value of 805.0 molH2 molRh−1 min−1 at room temperature under visible light irradiation. The H2 release rate as a function of reaction time exhibited a volcano plot under visible light irradiation, indicating that a self-activation process occurred in the hydrolytic dehydrogenation of AB due to additional oxygen vacancies arising from in situ reduction of WO3−x nanowires by AB, and thus an enhanced localized surface plasmon resonance (LSPR). Such a self-activation process was responsible for the enhanced catalytic activity under visible light irradiation relative to that under dark conditions, which was supported by the lower activation energy (45.2 vs. 50.5 kJ mol−1). In addition, Rh/WO3−x catalysts were relatively stable with only little loss in activity after five cycles due to the tight attachment between two components.

Green and facile synthesis of Rh/GO nanocomposites for high catalytic performance

We report a facile and green route to prepare ligand-free ultrafine rhodium (Rh) nanoparticles decorated on graphene oxide (GO) nanosheets. The ultrafine Rh nanoparticles with average size 1.8 ± 0.4 nm were synthesized by pulsed laser ablation in liquid. The Rh nanoparticles could be uniformly supported on the surface of GO through the electrostatic interaction. The reduction of 4-nitrophenol in presence of NaBH4 was performed as a model reaction to evaluate the catalytic properties of Rh/GO nanocomposites. The Rh/GO nanocomposites showed remarkable high catalytic activities with rapid and complete conversion of 4-nitrophenol to 4-aminophenol. The kinetic constant rate of the Rh/GO nanocomposites was 7.62 × 10−3 s−1, which is 27 times higher than commercial Rh/C catalyst, revealing the high catalytic activity in catalytic hydrogenation. The Rh/GO nanocomposites showed good stability and high catalytic activity due to the small size Rh nanoparticles, uniformly dispersed on GO nanosheets. This work may provide a green and alternative method to prepare ligand-free supported ultrafine noble metal heterogeneous catalysis.

Nanowire morphology and particle phase control by tuning the In concentration of the foreign metal nanoparticle

Controllable particle assisted growth (PAG) of III–V nanowires is today almost exclusively done with Au, Ga or In nanoparticles, whereas other metals often yield nanowires with uncontrolled growth directions. To improve the control of the initial growth direction in PAG, independent of choice of metal, we propose to initiate nanowire growth from a group-III-rich foreign metal particle. For III–V nanowire growth, the group III concentration of the particle can be made to increase or decrease with the relative supply of group III and group V material, which can be used to promote the liquid phase that is necessary for vapor–liquid–solid growth. In this paper, 30 nm Pd nanoparticles are used to develop growth conditions for In-rich PAG of InAs nanowires. The particle size evolution for different growth times and V/III ratios is correlated with changes in nanowire density and morphology. In addition, we demonstrate In-rich Co, Pd, Pt and Rh nanoparticles and optimized In-rich PAG from Au and Pd seeds. The Au and Pd seeded nanowires are remarkably similar and by tuning the particle composition we trigger a morphological change. The vertical nanowire morphology is associated with In-rich nanoparticles that contain a liquid phase. The curly nanowire morphology, with random growth directions have an In concentration less than or equal to that of the most In rich compound of the seed metal–In system.

Ultrahigh Catalytic Activity of l-Proline-Functionalized Rh Nanoparticles for Methanolysis of Ammonia Borane.

The synthesis of ultrafine and well-distributed rhodium nanoparticles (NPs) with high efficiency toward methanolysis of ammonia borane (AB) is crucially important but challenging. A facile approach has been developed for synthesizing ultrafine and uniform Rh NPs deposited on carbon by using the small soluble organic molecule (SOM) of l-proline (PRO) as capping agent (Rh-PRO/C). The enrichment of N,O-coordination sites for the metal precursor by using PRO was found to be the key to the synthesis Rh-PRO/C. The as-prepared Rh-PRO/C showed high catalytic activity for ammonia borane methanolysis with the highest total turnover frequency (TOF) of 1035 mol (molRh min)-1 under basic conditions, which was three times higher than that of the state-of-the-art Rh-based catalysts. The excellent catalytic performance of Rh-PRO/C was ascribed to the well-dispersed Rh NPs and the PRO-functionalized metal surface, which can provide more active sites for the reaction. The merit of size-controlled synthesis combined with metal NP surface modification by SOMs is likely to be beneficial in various catalytic fields.

Synthesis of Pt/M (M = Au, Rh) Nanoparticles in Microemulsions: Controlling the Metal Distribution in Pt/M Catalysts

The synthesis of Au/Pt and Pt/Rh nanoparticles in microemulsions by a one-pot method has been studied by computer simulation using stoichiometric and in-excess reducing agent. Our study demonstrates that Pt is reduced faster in the presence of Rh than in the presence of Au, and this acceleration is attributed to the higher availability of reducing agent in the Pt/Rh than in Au/Pt synthesis. Final bimetallic nanoparticles show a better metal segregation in Pt/Rh when compared with Au/Pt nanoparticles. In Au/Pt synthesis, Pt reaction rate depends on the ease with which reactants can be exchanged between micelles and the availability of reducing agent. When Pt/Rh nanoparticles are synthesized using a rigid film, the slow intermicellar exchange runs together with the slow reduction rates of the metals. It results in a very slow consumption of reducing agent, so the amount of reducing agent does not affect Pt reaction rate.

Preparation and detailed characterization of zirconia nanopowder supported rhodium (0) nanoparticles for hydrogen production from the methanolysis of methylamine-borane in room conditions

Uniformly dispersed Rh (0) nanoparticles supported on zirconia nanopowder were synthesized by a two-step and simple ex-situ method summarized by mixing rhodium (III) chloride hydrate with zirconia (nano-ZrO2) aqueous solution in ambient conditions followed by reduction with NaBH4. The ex-situ prepared nano-ZrO2 supported Rh (0) nanoparticles (Rh/nano-ZrO2) were characterized by various spectroscopic methods, including TEM, TEM-EDX, HR-TEM, P-XRD, XPS and ICP-OES. The catalytic activity of Rh (0) nanoparticles is 1050 h−1 in terms of initial turnover frequency (TOF), which is the first study in the literature to produce hydrogen by catalytic methanolysis of methylamine-borane. In addition, the catalytic methanolysis of methylamine-borane by using Rh (0) nanoparticles was carried out in different catalyst/substrate concentrations and different temperatures to reveal rate equation and kinetic parameters. Consequently, Rh (0) nanoparticles are taken into account as an encouraging catalyst for the methanolysis of methylamine-borane and for providing a more fertile hydrogen storage gain in fuel cell operations.

Novel rhodium on carbon catalysts for the oxidation of benzyl alcohol to benzaldehyde: A study of the modification of metal/support interactions by acid pre-treatments

Rhodium nanoparticles or rhodium organometallic complexes are mainly used in catalysis for reduction or hydroformylation reactions. In this work instead, we explored the capabilities of Rh nanoparticles as an oxidation catalyst, applied to the oxidation of benzyl alcohol to benzaldehyde under very mild conditions (100 °C, and atmospheric pressure) as a model reaction. Here we report the preparation of novel Rh/C catalysts by using an impregnation protocol, with particular emphasis on the pre-treatment of the carbon supports by using HNO3 and HCl, as well as the characterization of these materials by using an array of methods involving TEM, XPS and XRPD. Our preparation method led to a wide Rh particle size distribution ranging from 20 to 100 nm, and we estimate an upper limit diameter of Rh nanoparticles for their activity towards benzyl alcohol oxidation to be ca. 30 nm. Furthermore, a HNO3 pre-treatment of the activated carbon support was able to induce a smaller and narrower particle size distribution of Rh nanoparticles, whereas a HCl pre-treatment had no effect or sintered the Rh nanoparticles. We rationalise these results by HNO3 as an acid able to create new nucleation sites for Rh on the carbon surface, with the final effect of smaller nanoparticles, whereas for HCl the effect of sintering was most likely due to site blocking of the nucleation sites over the carbon surface. The roles of acid centres on the carbon surfaces for the oxidation reaction was also investigated, and the larger their amounts the larger the amounts of by-products. However, by treatment with HNO3 we were able to convert neutral or basic carbons into supports capable to enhance the catalytic activity of Rh, and yet minimised detrimental effects on the selectivity of the oxidation to benzaldehyde.

Catalytically active rhodium nanoparticles stabilized by nitrogen doped carbon for the hydrolysis of ammonia borane

We report the preparation of an ammonia borane hydrolysis catalyst for use in hydrogen production by dispersing Rh nanoparticles on a nitrogen-doped carbon (NPC) support. The resulting Rh/NPC catalyst had a measured turnover frequency of 473.5 min−1, higher than that of many previously reported Rh-based catalysts. This catalyst could also be reused eight times. The large surface area and abundant nitrogen-functional species of NPCs facilitate dispersion of Rh nanoparticles on their surface, providing numerous catalytically active sites for ammonia borane hydrolysis, thereby leading to high catalytic activity. This study demonstrates that NPC support can be used to prepare highly active catalysts.

Comparative biological effects of spherical noble metal nanoparticles (Rh, Pd, Ag, Pt, Au) with 4-8 nm diameter.

For a comparative cytotoxicity study, nanoparticles of the noble metals Rh, Pd, Ag, Pt, and Au (spherical, average diameter 4 to 8 nm) were prepared by reduction in water and colloidally stabilized with poly(N-vinyl pyrrolidone) (PVP). Thus, their shape, size, and surface functionalization were all the same. Size and morphology of the nanoparticles were determined by dynamic light scattering (DLS), analytical disc centrifugation (differential centrifugal sedimentation, DCS), and high-resolution transmission electron microscopy (HRTEM). Cell-biological experiments were performed to determine the effect of particle exposure on the viability of human mesenchymal stem cells (hMSCs). Except for silver, no adverse effect of any of the metal nanoparticles was observed for concentrations up to 50 ppm (50 mg L−1) incubated for 24 h, indicating that noble metal nanoparticles (rhodium, palladium, platinum, gold) that do not release ions are not cytotoxic under these conditions.

Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane

The effects of the metal oxide support on the activity, selectivity, resistance to carbon deposition and high temperature oxidative aging on the Rh-catalyzed dry reforming of methane (DRM) were investigated. Three Rh catalysts supported on oxides characterized by very different oxygen storage capacities and labilities (γ-Al2O3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) were studied in the temperature interval 400–750 °C under both integral and differential reaction conditions. ACZ and CZ promoted CO2 conversion, yielding CO-enriched synthesis gas. Detailed characterization of these materials, including state of the art XPS measurements obtained via sample transfer between reaction cell and spectrometer chamber, provided clear insight into the factors that determine catalytic performance. The principal Rh species detected by post reaction XPS was Rh0, its relative content decreasing in the order Rh/CZ(100%)>Rh/ACZ(72%)>Rh/γ-Al2O3(55%). The catalytic activity followed the same order, demonstrating unambiguously that Rh0 is indeed the key active site. Moreover, the presence of CZ in the support served to maintain Rh in the metallic state and minimize carbon deposition under reaction conditions. Carbon deposition, low in all cases, increased in the order Rh/CZ < Rh/ACZ < Rh/γ-Al2O3 consistent with a bi-functional reaction mechanism whereby backspillover of labile lattice O2− contributes to carbon oxidation, stabilization of Rh0 and modification of its surface chemistry; the resulting O vacancies in the support providing centers for dissociative adsorption of CO2. The lower apparent activation energy observed with CZ-containing samples suggests that CZ is a promising support component for use in low temperature DRM.

XAS/DRIFTS/MS spectroscopy for time-resolved operando investigations at high temperature.

The combination of complementary techniques in the characterization of catalysts under working conditions is a very powerful tool for an accurate and in-depth comprehension of the system investigated. In particular, X-ray absorption spectroscopy (XAS) coupled with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectroscopy (MS) is a powerful combination since XAS characterizes the main elements of the catalytic system (selecting the absorption edge) and DRIFTS monitors surface adsorbates while MS enables product identification and quantification. In the present manuscript, a new reactor cell and an experimental setup optimized to perform time-resolved experiments on heterogeneous catalysts under working conditions are reported. A key feature of this setup is the possibility to work at high temperature and pressure, with a small cell dead volume. To demonstrate these capabilities, performance tests with and without X-rays are performed. The effective temperature at the sample surface, the speed to purge the gas volume inside the cell and catalytic activity have been evaluated to demonstrate the reliability and usefulness of the cell. The setup capability of combining XAS, DRIFTS and MS spectroscopies is demonstrated in a time-resolved experiment, following the reduction of NO by Rh nanoparticles supported on alumina.

Two dimensional Rh/Fe3O4/g-C3N4-N enabled hydrazine mediated catalytic transfer hydrogenation of nitroaromatics: A predictable catalyst model with adjoining Rh

A magnetically separable 2D Rh/Fe3O4/g-C3N4-N catalyst has been synthesized for the catalytic transfer hydrogenation of nitroarenes to afford corresponding anilines. The highly dispersed Rh nanoparticles are the dominant active species. The reaction is second order for rhodium according to the kinetic study, and this means the activity of different Rh-containing catalyst can be precisely predicted and the active site contains two adjoining rhodium atoms. The strong interaction between Rh and Fe3O4 contributes to the catalytic performance, therefore the obtained Rh/Fe3O4/g-C3N4-N catalyst exhibits excellent catalytic activity and chemoselectivity for the reduction of various nitroaromatics, and the turnover frequencies (TOF) can reach as high as 12,666 h−1 for 4-nitrophenol. Moreover, this catalytic system is environmentally friendly because of its green solvent water and harmless byproducts (H2O and N2), and the catalyst shows outstanding stability and recyclability with at least 12 cycles.

Development and Application of Carbon-Layer-Stabilized, Nitrogen-Doped, Bamboo-Like Carbon Nanotube Catalysts in CO2 Hydrogenation.

Nitrogen‐doped, bamboo‐like carbon nanotubes (BCNTs) were synthesized from butylamine by catalytic chemical vapor deposition (CCVD method). The nanotubes were oxidized by H2SO4/HNO3 treatment and used to prepare calcium alginate gelled BCNT spheres. These beads were first carbonized and then Pd, Rh and Ni nanoparticles were anchored on the surface of the spheres. These systems were then applied as catalysts in CO2 hydrogenation. The BCNT support was examined by Raman spectroscopy, dynamic light scattering (DLS) and X‐ray photoelectron spectroscopy (XPS). The prepared catalysts were characterized by HRTEM and SEM. The oxidation pretreatment of BCNTs was successful, with the electrokinetic potential of the water‐based dispersion of BCNTs measuring −59.9 mV, meaning the nanotube dispersion is stable. Pyridinic and graphitic types of incorporated nitrogen centers were identified in the structure of the nanotubes, according to the XPS measurements. The Pd‐containing BCNT sphere catalyst was the most efficient in the catalytic studies. The highest conversion was reached on the Pd catalyst at 723 K, as well as at 873 K. The difference in the formation rate of CO was much less at 873 K between the Pd and Rh compared to the 723 K values. Accordingly, the application of Pd‐containing BCNT/carbon‐supported catalyst favored the generation of CO. However, the Ni‐BCNT/carbon catalyst leads to the formation of CH4 as the major product.

Synthesis of ultra-small Rh nanoparticles congregated over DNA for catalysis and SERS applications.

Exploration of rare earth metals for the Surface Enhanced Raman Scattering (SERS) is greatly preferred to identify probe molecules even at nano molar level. Highly stable Rh nanoparticles (NPs) which are ultra-small size have been prepared within 20 min of reaction time as a colloidal solution using a bio-molecular scaffold DNA and NaBH4 as a reducing agent under room temperature. While keeping metal ion concentration fixed and by making difference in DNA concentration, three different sets of Rh@DNA such as 0.08, 0.085 and 0.09 M were formed as nanochains like structure with varying diameters. The average chain length of Rh NPs for varying concentrations of DNA is ∼98 nm and the Rh particles size is below 5 nm in all the cases. These ultra-small Rh NPs have been utilized for two distinct potential applications such as in catalysis and SERS studies. From the catalysis reaction, reduction of 4-Nitro benzaldehyde to 4-amino benzaldehyde, Rh@DNA (0.08 M) has shown rate constant value of 0.26 min-1 which is highest among other concentrations studied. SERS study revealed that the calculated Enhancement Factor (EF) value was 1.19 × 105 for Rh@DNA (0.08 M) which is highest while compared with other concentrations. Apart from catalysis and SERS, the as-synthesized Rh NPs can find applications in other interdisciplinary fields such as organic catalysis, electro-catalysis and so on in near future.

Synthesis and Characterization of Rh/B⁻TNTs as a Recyclable Catalyst for Hydroformylation of Olefin Containing ⁻CN Functional Group.

The TiO2-based nanotubes (TNTs, B–TNTs) of different surface acidities and their supported Rh catalysts were designed and synthesized. The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectrometer (XPS), tempera–ture–programmed desorption of ammonia (NH3–TPD), atomic emission spectrometer (ICP), and Brunauer–Emmett–Tellerv (BET) surface-area analyzers. Images of SEM and TEM showed that the boron-decorated TiO2 nanotubes (B–TNTs) had a perfect multiwalled tubular structure; their length was up to hundreds of nanometers and inner diameter was about 7 nm. The results of NH3-TPD analyses showed that B–TNTs had a stronger acid site compared with TNTs. For Rh/TNTs and Rh/B–TNTs, Rh nanoparticles highly dispersed on B–TNTs were about 2.79 nm in average diameter and much smaller than those on TNTs, which were about 4.94 nm. The catalytic performances of catalysts for the hydroformylation of 2-methyl-3-butennitrile (2M3BN) were also evaluated, and results showed that the existence of B in Rh/B–TNTs had a great influence on the catalytic performance of the catalysts. The Rh/B–TNTs displayed higher catalytic activity, selectivity for aldehydes, and stability than the Rh/TNTs.

Nanosheets of MoSe2@M (M = Pd and Rh) function as widespread pH tolerable hydrogen evolution catalyst

In this present study we have developed method for the synthesis of MoSe2 nanosheets following a simple hydrothermal technique. Palladium (Pd) and rhodium (Rh) nanoparticles were decorated on the surface of MoSe2 following a simple wet-chemical route. Pd and Rh nanoparticles decorated MoSe2 were applied for hydrogen evolution reaction (HER) in different pH conditions like acidic (0.5 M H2SO4), neutral (pH-7 buffer) and in alkaline (1 M KOH) medium and 3.5 wt% of saline water. Pd and Rh decorated MoSe2 show efficient activity towards HER irrespective of the applied electrolyte. In 0.5 M H2SO4, MoSe2 can produce 10 mA/cm2 current density with applied potential of −0.256 V vs. RHE. Rh decorated MoSe2 shows more shift in the onset potential. Upon applied potential of −0.192 V vs. RHE, MoSe2/Rh can produce 10 mA/cm2 current density. MoSe2/Rh is electrocatalytically more active than MoSe2/Pd which is established from the calculated electrochemically active surface area (ECSA) value. Significantly lower (47 mV/decade) Tafel value is observed for MoSe2/Rh in 0.5 M H2SO4 which indicates the superior activity. MoSe2/Rh is more stable in neutral and alkaline medium compared to acidic medium and it can retain its own activity even after continuous 12 h reaction.

Single-layer Rh nanosheets with ultrahigh peroxidase-like activity for colorimetric biosensing

When the dimensionality of layered compounds decreases to the physical limit, ultimate two-dimensional (2D) anisotropy and/or quantum confinement effects may lead to extraordinary physicochemical attributes. Here, we report single-layer Rh nanosheets (NSs) exhibiting ultrahigh peroxidase-like activity, far exceeding that of horseradish peroxidase (HRP) and of most known layered nanomaterial-based peroxidase mimics. Considering per NS as an active subunit, the Rh NSs displayed a catalytic rate constant (Kcat) as high as 4.45 × 105 s–1 to H2O2, two orders of magnitude higher than those of HRP and Rh nanoparticles. The high atom efficiency of the Rh NSs can be attributed to the full exposure of surface-active Rh atoms, which greatly facilitates electron transfer and formation of superoxide anions, representing reactive oxygen species in the catalytic process. As a proof-of-concept application, the Rh NSs were successfully used as peroxidase mimics for the colorimetric detection of H2O2 and xanthine, with high sensitivity and selectivity. Moreover, a simple, rapid, and sensitive Rh-based paper sensor for ascorbic acid was also developed. In summary, this work provides a novel example of single-layer metallic NSs for biosensing.

Linear polystyrene-stabilized Rh(III) nanoparticles for oxidative coupling of arylboronic acids with alkenes in water

Linear polystyrene-stabilized Rh(III) nanoparticles (PS-Rh(III)NPs) were obtained when an aqueous solution of RhCl3 was stirred at 90 °C in the presence of KOH, 4-methylphenylboronic acid, and linear polystyrene, as indicated by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). PS-Rh(III)NPs exhibited high catalytic activity for the oxidative coupling of arylboronic acids with alkenes. In contrast, PS-Rh(0)NPs prepared with NaBH4 had little activity for the same reaction.

Subsurface Oxygen Formation during H2 Oxidation over Rh, Pt and Pt-Rh Model Nanoparticles.

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