Rh Particles Research Articles

Catalytic steam reforming of methane using Rh supported on Sr-substituted hexaaluminate

This paper reports the synthesis, characterization, and performance of steam-reforming catalysts based upon dispersed Rh particles on Sr-substituted hexaaluminate supports. As confirmed by electron microscopy and X-ray diffraction, the Sr-substituted hexaaluminate provides a plate-like support structure that resists sintering and occlusion of the Rh. The hexaaluminate is synthesized using an alumoxane process, with the cation substitution accomplished by exchange with metal acetylacetonates. The Rh is dispersed using impregnation with metal-nitrate salts. A stagnation-flow reactor is used to measure catalytic activity. In these experiments, the catalyst is applied to a flat surface that is held at a fixed temperature. Reactive gases (methane, steam, and diluent) impinge on the catalytic stagnation surface. Microprobe mass spectrometry is used to measure gas-phase species profiles in the boundary layer normal to the catalyst surface. The experiments are interpreted using a chemically reacting flow model, including an elementary heterogeneous chemical reaction mechanism. Results confirm that the Rh on Sr-substituted hexaaluminate is a highly stable and active reforming catalyst.

Formation and characterization of Rh–Mo bimetallic layers on the TiO 2(1 1 0) surface

The investigation of Rh, Mo and Rh–Mo nanosized clusters formed by physical vapor deposition on TiO 2(1 1 0) single crystal was performed by X-ray Photoelectron Spectroscopy (XPS), Low Energy Ion Scattering (LEIS) and Auger Electron Spectroscopy (AES). There was no sign for site-exchange between Mo and Rh atoms during deposition of Mo onto Rh particles at 330 K. Mixing between Ti and Mo ions was facilitated at the Mo particle–titania interface due to reaction at 550–700 K. The redox process between titania and Mo deposit was hindered at 330 K by forming predeposited rhodium layer ( Θ Rh = 2.0 ML), but reached nearly the same extent as without Rh after moderate heating to 600 K. The encapsulation of Rh by titania was complete by about 700 K in the presence of 1.2 ML Mo, in case of Mo-predeposition and Mo-postdeposition as well. Elevating the temperature of TiO 2/Rh–Mo layers above 700 K, these metals form alloy at the Mo–Rh interface irrespective of deposition sequences.

Dendrimer-mediated synthesis of subnanometer-sized Rh particles supported on ZrO 2

We have achieved the synthesis of ZrO 2-supported Rh particles with sub-nanometer dimensions and very narrow particle size distributions using G4OH-PAMAM dendrimers as templates. The different steps of this process were characterized by EXAFS and HRTEM measurements. The results indicate that partially hydrolyzed Rh 3+ species in an aqueous solution can interact with amide/amine groups in the interior of G4OH dendrimers to form (Rh 3+) x -G4OH complexes. Rh 3 clusters were subsequently formed in the same solution following room temperature reduction of the (Rh 3+) x -G4OH complexes with NaBH 4. These Rh 3 clusters can be delivered intact on the surface of the ZrO 2 support. The formation of highly dispersed and nearly uniform Rh particles with sizes below 1 nm was observed following thermal removal of the dendrimer component. The structure-sensitive reaction of ethane hydrogenolysis was used for the evaluation of Rh/ZrO 2 samples with different metal particle sizes. A substantial decrease in turnover frequency was observed when the particle diameter decreased below 1.5 nm, consistent with mechanistic explanations advanced previously in the literature for this reaction.

CO Oxidation on Rh/SiO2/Mo(112) Model Catalysts at Elevated Pressures

Well-defined model catalyst samples, prepared under ultra-high-vacuum (UHV) conditions, hold the potential of bridging the gap between traditional surface science and technical catalysis studies. However, work remains to understand and characterize reaction kinetics of such samples under more realistic pressure conditions, specifically, how reaction kinetics of a well-studied reaction (such as CO oxidation) over model surfaces compares to single-crystal surfaces studied under identical conditions. Here, the CO oxidation reaction under elevated-pressure conditions (P = 8.0 Torr) and CO-rich reaction conditions is used as a probe reaction to characterize Rh/SiO2 model catalyst samples, prepared on a Mo(112) substrate under UHV conditions, in a contiguous high-pressure reactor cell-UHV surface analysis chamber. CO oxidation reaction kinetics are studied as a function of O2/CO partial pressures (from 1/1 to 1/10 O2/CO) and Rh coverage (θRh = 0.25−10 ML), along with measurements on a Rh(111) single crystal for direct comparison. CO desorption measurements on the Rh/SiO2 films and STM measurements of Rh particles on ultrathin SiO2 films at T = 300 K have been obtained at various Rh coverages to provide two additional methods to estimate reactive sites. Results demonstrate that, under the reaction conditions employed, CO oxidation reaction on Rh/SiO2/Mo(112) exhibits O2/CO dependencies and reaction activation energies similar to those observed on single-crystal samples. These data show that model catalyst samples can be used to gain qualitative and quantitative reactivity data over a range of elevated-pressure reaction conditions. Measurements of CO oxidation under CO-rich conditions can provide a reasonable estimate of the number of active sites on Rh model catalyst surfaces, thus providing a useful starting point for future studies of particle- and structure-dependent reactions on model catalyst surfaces.

In Situ Time-Resolved XAFS Studies of Metal Particle Formation by Photoreduction in Polymer Solutions

Formation mechanisms of metal particles (rhodium (Rh) and palladium (Pd) particles) in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by photoreduction were investigated by UV-vis, transmission electron microscopy (TEM), and in situ X-ray absorption fine structure (in situ XAFS). The average diameter of the Rh and Pd particles prepared by the photoreduction was estimated from TEM to be 2.2 and 2.0 nm, respectively. In situ energy-dispersive XAFS (in situ DXAFS) measurements were performed to investigate the reduction process of Rh(III) and Pd(II) aqua chlorocomplexes as well as their particle formation process. The reduction rate of these aqua chlorocomplexes could be quantitatively evaluated from the change of X-ray absorbance assigned to the contribution of metal-chloride bonds in these complexes. The reduction rate of Rh(III) aqua chlorocomplexes was found to be slower than that of Pd(II). It was also demonstrated that the reduction process of Rh(III) complexes possessed an induction period before the onset of Rh particle formation, although the Pd(II) complexes displayed no induction period, since the reduction of Pd(II) occurred immediately after mixing of an ethanol solution of Pd(II) complexes with aqueous PVP solutions.

The role of acidic sites and the catalytic reaction pathways on the Rh/ZrO2catalysts for ethanol steam reforming

Rh catalysts supported on ZrO(2)-based oxides were studied for ethanol steam reforming (SR) reaction. Pure ZrO(2) as the support resulted in higher H(2) production yield compared to the ZrO(2) oxide decorated with CeO(2), Al(2)O(3), La(2)O(3) or Li(2)O at the reaction temperature of 300 degrees C. Above 450 degrees C, all the catalysts exhibited similar catalytic activity. However, at low reaction temperatures (below 400 degrees C), a significant enhancement in the catalytic activity, selectivity and stability was achieved by replacing the ZrO(2) support prepared by a precipitation method (ZrO(2)-CP) with that prepared by a hydrothermal method (ZrO(2)-HT). A deactivation was observed during the EtOH SR reaction at 300 degrees C on the two catalysts of Rh/ZrO(2)-CP and Rh/ZrO(2)-HT. NH(3)-TPD experiments confirmed that the ZrO(2)-HT support had two types of acidic sites while the ZrO(2)-CP support had only one type of weak acidic sites. DRIFTS studies showed that the absorption of EtOH molecules was strong on the Rh/ZrO(2)-HT catalyst and a number of C(2) oxygenates were accumulated on the catalyst surface. Meanwhile, the EtOH absorption on the Rh/ZrO(2)-CP catalyst was weak and the accumulation of CO, carbonate and CH(x) was observed. It is concluded that the relatively strong Lewis acidic sites in the Rh/ZrO(2)-HT catalyst is responsible for the strong absorption of EtOH molecules, and the subsequent C-H breakage step (formation of acetaldehyde or called as dehydrogenation reaction) is a fast reaction on it; on the Rh/ZrO(2)-CP catalyst, the EtOH adsorption was weak and the C-C breakage was the dominating reaction which led to the accumulation of surface CO, CH(x) and CO(2) species. Therefore, it is believed that, in order to promote the absorption of EtOH molecules and to reduce the formation of metastable carbonaceous species (C(2) oxygenates) during the reaction, the catalyst should be enhanced both with Lewis acidity and with C-C bond breakage function. Also, it was found that the Rh particle size and distribution, as well as the surface area of the catalyst, were not important factors in determining the catalytic performance.

New insights into catalytic CO oxidation on Pt-group metals at elevated pressures

Abstract Producing a definitive picture of the CO oxidation reaction (CO + O 2 → CO 2 ) on Pt-group metals (Rh, Pd, Pt, and Ru) across the ‘pressure gap’ has proved to be a challenging task. Surface-sensitive techniques amenable to high pressure environments (e.g. PM-IRAS) have sparked a renewed interest in this reaction under realistic pressures. Here, we review recent work in our laboratory examining CO oxidation kinetics on Pt-group single crystals using PM-IRAS, XPS, and mass spectrometry from low (10 −8 –10 −3 Torr) to high (1–10 2 Torr) pressures. These studies have shown that at both low and high pressures (a) Langmuir–Hinshelwood kinetics adequately describe CO oxidation kinetics on Pt-group metals (Pt, Pd, Rh) (i.e. there is no pressure gap) and (b) the most active surface is one with minimal CO coverage. Additionally, recent investigations of high pressure CO oxidation kinetics on SiO 2 film supported Rh particles prepared in situ are discussed.

Remarkable particle size effect in Rh-catalyzed enantioselective hydrogenations

A series of 0.5–4.3 wt% Rh/Al 2O 3 catalysts were prepared by flame synthesis. STEM indicated relatively narrow particle size distributions for all catalysts and the mean particle size increased almost linearly with the Rh content in the range 0.96–1.65 nm. A DRIFTS study of CO adsorption on as prepared Rh/Al 2O 3 and after heat treatment in hydrogen at 400 °C revealed that there was no Rh oxide present at the catalyst surface after the high temperature reduction, which procedure is commonly used prior to enantioselective hydrogenation. In the hydrogenation of ethyl pyruvate and ethyl 3-methyl-2-oxobutyrate the cinchona-modified 4.3 wt% Rh/Al 2O 3 gave considerably higher ee than those achieved with the best known Rh catalyst. A decrease of the metal loading and thus the mean Rh particle size, led to a loss of ee to ( R)-lactate by a factor of up to seven at 1 bar and up to two at 10–100 bar. Our interpretation is that the performance of Rh/Al 2O 3 is strongly distorted at atmospheric pressure by catalyst deactivation due to the Al 2O 3-catalyzed aldol condensation of the substrate. During the fast reactions at 100 bar the contribution of strongly adsorbed impurities is small and the variation of ee is mainly due to an intrinsic particle size effect. The structure sensitivity observed under optimal conditions, at high surface hydrogen concentration, is mainly due to steric effects: a small, ca. 1 nm Rh particle cannot accommodate the enantiodifferentiating diastereomeric substrate–modifier complex and the hydrogenation on its surface leads to racemic product. A practical conclusion is that there is no advantage of using small nanoparticles and low metal loading in the enantioselective hydrogenation of α-ketoesters.

Synthesis and characterization of Rh and RhO particles supported on zeolites with high activity of lean NO –CO–H2 reaction

Rh and RhO x nanoparticles supported on zeolites were synthesized in a hydrothermal process using a gel mixture of zeolite and Rh precursors. The morphology of the nanoparticles was varied depending on the kind of zeolite and synthesis conditions. Samples were characterized with XRD, TEM, CO chemisorption and BET analysis. In the hydrothermal process, the nanoparticles were loaded on Na-beta zeolite and Na-ZSM-5 in the form of dispersed particles and agglomerated particles, respectively. The nanoparticles supported on Na-ZSM-5 more efficiently catalyze NO x reduction in lean conditions using hydrogen and CO as a main reductant than those supported on Na-beta zeolite.

Application of Conjugated Polymer–Platinum Group Metal Composites as Heterogeneous Catalysts

Composites of polyaniline (PANI) or polypyrrole (PPy) doped with chloride ions and Pt or Rh particles were prepared by chemical reduction of metal ions conducted in the presence of the polymers. Based on X-ray diffraction studies it was established that the composites contained metal nanoparticles (5–9 in size). However, according to SEM investigations metal particles were agglomerated (40 nm–1.1 μm in size). Redox activity of the composites in the catalytic isopropyl alcohol conversion was ca. ten times higher than the acid–base one. Pt dispersed in polymer matrices showed higher catalytic activity than Rh. PPy doped with chloride ions had a promoting effect on the activity of Pt catalysts.

Performance and SEM characterization of Rh impregnated microchannel reactors in the catalytic partial oxidation of methane and propane

Abstract Two different reactor materials (Fecralloy, Nicrofer) have been investigated in the catalytic partial oxidation of methane and propane. The focus of the study was on the stability and applicability of the different reactor materials, in particular their alumina coating related to their catalytic performance. Both reactors were impregnated using a RhCl 3 solution, yielding 6.3 and 4.6 mg of Rh deposited on the Fecralloy and Nicrofer, respectively. It is shown that high temperature calcination of Fecralloy established a stable alumina coating. This effectively increased the surface area of the reactor providing sites for the Rh particles. The reactor performance was stable with both methane and propane feeds. SEM images of the Fecralloy reactor revealed significantly larger Rh particles near the reactor exit as compared to inside the reactor. The Nicrofer was wash-coated with alumina using a sol–gel technique. SEM images of the Nicrofer reactor revealed the formation of Cr-layers and Cr-oxide structures covering the impregnated Rh particles. The Cr rich structures coating the Rh particles were found to be detrimental to the reactor performance, and the alumina wash-coating was consequently not successful at stabilizing the Nicrofer reactor material. Methane was found to be more difficult to activate than propane, as expected, but provided the benefit of no formation of C 2+ components, with H 2 and CO selectivities almost as high as with propane.

Evidence of High-Pressure Rhodium Sesquioxide in the Rhodium/γ-Alumina Catalytic System

We report an investigation of Rh-containing nanostructures dispersed on γ-alumina. This system is representative of many common heterogeneous catalysts that consist of transition metals dispersed on a high surface area support. Previous atomic-resolution Z-contrast STEM observations have shown the Rh particles to exist as thin “rafts” on the (100) surface of γ-alumina. This finding is intriguing given that the preferred surface exposure of γ-alumina is (110). Here we present first-principles density functional studies and simulated Z-STEM imaging suggesting that these Rh-containing structures consist of the high-pressure rhodium sesquioxide (II) phase growing on the surface.

Autothermal reforming of ethanol for hydrogen production over an Rh/CeO 2 catalyst

Hydrogen production from ethanol by autothermal reforming over an Rh/CeO 2 catalyst was investigated with a stoichiometric feed composition. Ethanol as well as the reaction intermediates like acetaldehyde and acetone was entirely converted to hydrogen and C1 products at 673 K, and methane steam reforming and reverse water gas shift were the major reactions above 823 K. The Rh/CeO 2 catalyst exhibited stable activity and selectivity during 70 h on-stream operation at 823–923 K without obvious deactivation evidenced by the constant effluent gas composition. Structural analysis of the used catalyst revealed that CeO 2 prevented effectively the highly dispersed Rh particles with sizes of 1–3 nm from sintering and thus maintained sufficient Rh–CeO 2 interfacial areas, which facilitated coke gasification through the high oxygen storage-release capacity.

Dendrimer Templated Synthesis of One Nanometer Rh and Pt Particles Supported on Mesoporous Silica: Catalytic Activity for Ethylene and Pyrrole Hydrogenation

Monodisperse rhodium (Rh) and platinum (Pt) nanoparticles as small as approximately 1 nm were synthesized within a fourth generation polyaminoamide (PAMAM) dendrimer, a hyperbranched polymer, in aqueous solution and immobilized by depositing onto a high-surface-area SBA-15 mesoporous support. X-ray photoelectron spectroscopy indicated that the as-synthesized Rh and Pt nanoparticles were mostly oxidized. Catalytic activity of the SBA-15 supported Rh and Pt nanoparticles was studied with ethylene hydrogenation at 273 and 293 K in 10 torr of ethylene and 100 torr of H 2 after reduction (76 torr of H 2 mixed with 690 torr of He) at different temperatures. Catalysts were active without removing the dendrimer capping but reached their highest activity after hydrogen reduction at a moderate temperature (423 K). When treated at a higher temperature (473, 573, and 673 K) in hydrogen, catalytic activity decreased. By using the same treatment that led to maximum ethylene hydrogenation activity, catalytic activity was also evaluated for pyrrole hydrogenation.

Growth and Characterization of Rh and Pd Nanoparticles on Oxidized and Reduced CeOx(111) Thin Films by Scanning Tunneling Microscopy

The growth and structure of Rh and Pd nanoparticles on vapor-deposited ceria thin films grown epitaxially on Ru(0001) were investigated by scanning tunneling microscopy as a function of coverage, postdeposition annealing temperatures, as well as ceria oxidation states. Both metals grow as three-dimensional nanoparticles on the fully oxidized CeO2(111) thin films at room temperature. At low coverage, Rh particles preferentially decorate step edges on ceria, forming particles with smaller average size than those on the terraces. With increasing coverage, the number of Rh particles increases until near 2.3 monolayers, where the Rh particles cover most of the ceria surface and the particle size becomes relatively uniform. Larger Rh or Pd particles can be prepared by annealing the surface after deposition at 300 K; however, the particle size distribution becomes broader during this process. Similar growth behavior was observed on the reduced ceria surfaces. It is concluded that metal particle size or morphology is not responsible for previously reported differences in surface chemistry observed when Rh is deposited on reduced CeOx compared to fully oxidized CeO2. Instead, it is proposed that the enhanced dissociation occurs at the interface between the metal and the reduced support, which, coupled with rapid O diffusion, may lead to high dissociation fractions.

Promotion Effects in the Oxidation of CO over Zeolite-Supported Rh Nanoparticles

Rh particles with an average diameter smaller than 1.5 nm have been supported on a series of zeolite Y samples. These zeolite materials contained different monovalent (H+, Na+, K+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations and were used as model systems to investigate the effect of promoter elements in the oxidation of CO over supported Rh particles in excess of oxygen. Infrared (IR) spectroscopy was carried out to monitor the electronic changes in the local environment of Rh-adsorbed CO. It was found that the bands corresponding to two Rh gem-dicarbonyl species, Rh+(CO)2−(Oz)2 and Rh+(CO)2−(Oz)(H2O), shift to lower wavenumbers with increasing ionic radius/charge ratio of the cation. In addition, the relative intensity of the bridge bonded CO as compared to the total absorbance of Rh-bonded CO species decreases with increasing Lewis acidity, as expressed by the Kamlet−Taft parameter α of the cation. This trend could be directly correlated to the Rh CO oxidation activity, since low temperatures at 50% CO conversion corresponded with catalyst materials with a high contribution of bridge-bonded CO species and hence with small α values. A lower Lewis acidity causes an increased electron density on the framework oxygen atoms and thus an increased electron density on the zeolite-supported Rh particles. Comparable trends have been observed previously on a similar series of cation containing zeolite supported Pt catalyst materials.

MgO-Supported Rhodium Particles and Films: Size, Morphology, and Reactivity

The chemical transformations of supported Rh particles, ranging in size from a few micrometers to a few nanometers, and nanocrystalline Rh films have been studied under identical oxidizing and reduction conditions by means of scanning photoelectron microscopy (SPEM), which allows determination of the chemical state of single Rh microparticles. Comparing the oxidation states attained by the Rh particles revealed substantial reactivity differences with particle size, as well as variations in the reactivity of particles with similar dimensions and within the same particle. The results are interpreted in terms of the variable morphology of the particles as verified by secondary electron microscopy and atomic force microscopy.

Testing in annular micro-reactor and characterization of supported Rh nanoparticles for the catalytic partial oxidation of methane: Effect of the preparation procedure

This work extends a previous study on the conditioning of Rh/Al2O3 during CH4-CPO experiments at high space velocity. The effect of the preparation procedure was investigated by comparing well-known preparation techniques such as incipient wetness impregnation and grafting, and a novel CVD technique. Catalysts prepared by impregnation of the support with a solution of Rh(NO3)3 showed a very slow activation process, starting from an initially poor syngas activity and ending up to outstanding performances. Catalysts prepared by grafting of Rh4(CO)12 onto alumina showed a similar behaviour, but the initial activity was closer to final activity (similarly high). Catalysts prepared by CVD of Rh(acac)(CO)2 showed very good and stable performances from the very initial CPO run. To better understand the morphological changes which occur during reaction conditions, H2 chemisorption, CO chemisorption, CO-DRIFT and HRTEM were applied in concert to analyze the surface of samples prior and after catalytic testing. Characterization results support the hypothesis that the conditioning process is related to a reconstruction of the Rh particles which tends to eliminate small defective clusters and particles, wherein C-forming reactions are presumably highly favoured. The extent of reconstruction is thus strictly related to the heterogeneity of the surface, which is affected by the preparation procedure; heterogeneity is high over samples prepared via impregnation and grafting, low over sample prepared by CVD.

Hydrogen production from ethanol over bimetallic Rh-M/CeO 2 (M = Pd or Pt)

The reaction of ethanol for the production of hydrogen has been studied over a series of metal supported CeO 2 catalysts. The study is conducted by TPD, steady state reaction, XPS, TEM, and infrared spectroscopy. TPD gave evidence for the role of Rh in dissociating the carbon–carbon bond needed for efficient production of hydrogen molecules. IR of CO adsorption at 90 K revealed that Rh particles are most likely in very small clusters as evidenced by a single O C–Rh IR band at 2020 cm −1. TEM did not show conclusive evidence for the presence of the metal on-top of the CeO 2 support, yet the Rh-Pd/CeO 2 used catalyst has features that might be attributed to epitaxial growth of the noble metal along the (1 1 1) surface of the CeO 2 support. Considerable reconstruction of the CeO 2 support is seen for the used catalysts, in addition. Reforming of ethanol to hydrogen using (3 moles of water per mole of ethanol) was very efficient particularly above 650 K where hydrogen selectivity reaches 60 vol.%. At these temperatures hydrogen production from reforming of methane takes place.

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.