Rh Sites Research Articles

Insights into dynamic surface processes occurring in Rh supported on Zr-grafted γ-Al2O3 during dry reforming of methane

γ-Al2O3 support was modified with zirconia by the grafting method, using zirconium(IV) n-propoxide as precursor. Rh catalysts were prepared by wet impregnation of pure γ-Al2O3 (Rh/A), Zr modified γ-Al2O3 (Rh/100Z-A), and pure ZrO2 (Rh/Z) supports. Rh/A presents a good activity in dry reforming of methane and exhibits a high amount of surface oxidized rhodium (92% of surface Rh atoms) after the reaction. Rh/Z is poorly active and 45% of Rh surface atoms remain reduced during the reaction. The catalytic activity significantly increased when using Rh/100Z-A. Tetragonal ZrO2 crystallites are formed on the surface of modified alumina. It was obtained that, after Rh deposition, the grafted catalyst is composed by Rh/A and Rh/Z, as separated catalytic particles in good contact. To further understand the high activity of the grafted catalyst, mechanical mixtures of Rh/A and Rh/Z catalysts were synthesized. Compared with the single catalysts, an important synergistic effect in methane conversion and yields to CO and H2 was observed when high amounts of Rh/A were mixed with Rh/Z catalyst, along with a promotion of rhodium oxidation in Rh/Z, which leads to a high surface fraction of RhOx (88%) in the mixture. The formation of mixed phases between the supports and the migration of Rh atoms from one phase to the other were discarded. It is suggested that a catalytic cooperation occurs due to the presence and mobility of oxygen species generated by the dissociation of CO2 on Rh/A, which migrate to the Rh/Z catalyst promoting the oxidation of rhodium at its surface, and the catalytic activity. This cooperation mechanism is also expected to operate in the Rh/100Z-A catalyst, between the Rh/A and Rh/Z particles present in the surface. The importance of oxidized Rh for dry reforming of methane is evident from the high catalytic activity achieved when highly oxidized rhodium atoms are present together with the metallic Rh sites where methane activation occurs.

Establishment phase greenhouse gas emissions in short rotation woody biomass plantations in the Northern Lake States, USA

Uncertainty exists over the magnitude of greenhouse gas (GHG) emissions associated with open land conversion to short-rotation woody biomass crops (SRWC) for bioenergy in the Northern U.S. Lake States. GHG debts incurred at the plantation establishment phase may delay the climate mitigation benefits of SRWC production. To better understand GHG debts associated with converting open lands to SRWC, we established research plantations with willow (Salix spp.), hybrid-poplar (Populus spp.), and control plots in spring 2010 at two sites in northern Michigan (ES) and Wisconsin (RH). These sites had similar climates, but differed in time since last cultivation: 5 vs. 42 years. To address the short-term effects of plantation establishment, we compared two-year biomass production and GHG emissions. We hypothesized that the long-idle ES site, with higher initial soil C and N stocks, would have higher GHG emissions following conversion compared to the recently-idle RH site, but that this would be balanced in part by greater SRWC productivity at the ES site. As hypothesized, grassland conversion resulted in two-year net GHG emissions due to land conversion of 43.21 and 33.02 Mg-CO2eq ha−1 for poplar and willow at ES that was far greater than the 4.81 and −1.54 Mg-CO2eq ha−1 for poplar and willow at RH. Contrary to our hypothesis, we did not observe greater SRWC productivity at ES, which will take longer than RH to reach C neutrality and begin mitigating GHG emissions. Our results show that site-specific soil and management factors determine the magnitude of GHG emissions.

Spectral evidence for hydrogen-induced reversible segregation of CO adsorbed on titania-supported rhodium.

The reduction of a 1.3% Rh/TiO2 sample with carbon monoxide leads to the formation of uniform Rh nanoparticles with a mean diameter of dp ≈ 2.2 nm. Adsorption of CO on the reduced Rh/TiO2 produces linear and bridged carbonyls bound to metallic Rh(0) sites and only a few geminal dicarbonyls of Rh(I). The ν(CO) of linear Rh(0)-CO complexes is strongly coverage dependent: it is observed at 2078 cm(-1) at full coverage and at ca. 2025 cm(-1) at approximated zero coverage. At low coverage, this shift is mainly caused by a dipole-dipole interaction between the adsorbed CO molecules while at high coverage, the chemical shift also becomes important. Hydrogen hardly affects the CO adlayer at high CO coverages. However, on a partially CO-covered surface (θCO ≈ 0.5), the adsorption of H2 at increasing pressure leads to a gradual shift in the band of linear Rh(0)-CO from 2041 to 2062 cm(-1). Subsequent evacuation almost restores the original spectrum, demonstrating the reversibility of the hydrogen effect. Through the use of (12)CO + (13)CO isotopic mixtures, it is established that the addition of hydrogen to the CO-Rh/TiO2 system leads to an increase in the dynamic interaction between the adsorbed CO molecules. This evidences an increase in the density of the adsorbed CO molecules and indicates segregation of the CO and hydrogen adlayers. When CO is adsorbed on a hydrogen-precovered surface, the carbonyl band maximum is practically coverage independent and is observed at 2175-2173 cm(-1). These results are explained by a model according to which CO successively occupies different rhodium nanoparticles.

Characterization and activity study of the Rh-substituted pyrochlores for CO2 (dry) reforming of CH4

Isomorphic substitution of Rh at varying levels on the B site of lanthanum zirconate pyrochlore (La2Zr2O7; designated LZ) resulted in the formation of thermally stable catalysts suitable for fuel reforming reactions operating at 900 °C. Three specific catalysts are reported here: (a) unsubstituted lanthanum zirconate (LZ), (b) LZ with 2 wt% substituted Rh (L2RhZ), and (c) LZ with 5 wt% substituted Rh (L5RhZ). These catalysts were characterized by XRD, XPS, and H2-TPR. XRD of the fresh, calcined catalysts showed the formation of the pyrochlore phase (La2Zr2O7) in all three materials. In L5RhZ, the relatively high level of Rh substitution led to the formation of LaRhO3 perovskite phase which was not observed in the L2RhZ and LZ pyrochlores. TPR results show that the L5RhZ consumed 1.57 mg H2/gcat, which is much greater than the 0.508 H2/gcat and 0.155 mg H2/gcat for L2RhZ and LZ, respectively, suggesting that the reducibility of the pyrochlore structure increases with increasing Rh-substitution. DRM was studied on these three catalysts at three different temperatures of 550, 575, and 600 °C. The results showed that CH4 and CO2 conversion was significantly greater for L5RhZ compared to L2RhZ and no activity was observed for LZ, suggesting that the surface Rh sites are required for the DRM reaction. Temperature programmed surface reaction showed that L5RhZ had light-off temperature 80 °C lower than L2RhZ. The spent catalysts after runs at each temperature were characterized by temperature programmed oxidation (TPO) followed by temperature programmed reduction and XRD. The TPO results showed that the amount of carbon formed over L5RhZ is almost half of that formed on L2RhZ.

New Supramolecular Structural Motif Coupling a Ruthenium(II) Polyazine Light Absorber to a Rhodium(I) Center

Two new complexes, [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 and [(Me2bpy)2Ru(dpp)Rh(I)(COD)](PF6)2(BF4) (bpy = 2,2'-bipyridine, Me2bpy = 4,4'-dimethyl-2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, and COD = 1,5-cyclooctadiene), representing a new Ru(II),Rh(I) structural motif, have been prepared and characterized by mass spectrometry, (1)H NMR spectroscopy, electrochemistry, electronic absorption spectroscopy, and emission spectroscopy. These two complexes represent a new type of supramolecular complex with a [(TL)2Ru(dpp)](2+) (TL = terminal ligand) light absorber (LA) coupled to a Rh(I) center and are models for Ru(II),Rh(I) intermediates in the photochemical reduction of water using dpp-bridged Ru(II),Rh(III) photocatalysts. Electrochemical study reveals overlapping reversible Ru(II/III) and irreversible Rh(I/II/III) oxidations and a quasi-reversible dpp(0/-) reduction, demonstrating that the lowest unoccupied molecular orbital (LUMO) is dpp(π*) based. The COD ligand is sterically bulky, displaying steric repulsions between hydrogen atoms on the alkene of COD and dpp about the square planar Rh(I) center. An interesting reactivity occurs in coordinating solvents such as CH3CN, where Rh(I) substitution leads to an equilibrium between the Ru(II),Rh(I) bimetallic and [(TL)2Ru(dpp)](2+) and [Rh(I)(COD)(solvent)2](+) monometallic species. The electronic absorption spectra of both complexes feature transitions at ca. 500 nm attributed to a Ru(dπ) → dpp(π*) metal-to-ligand charge transfer (MLCT) transition that is slightly red-shifted from the Ru synthon upon Rh(I) complexation. The methylation of TL on the Ru impacts the electrochemical and optical properties in a minor but predictable manner. The photophysical studies, by comparison with the model complex [{Ru(bpy)2}2(dpp)](PF6)4 and related Rh(III) complex [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3, reveal the expected absence of a Ru(dπ) → Rh(dσ*) (3)MMCT state (metal-to-metal charge transfer) in the title complexes, which is present in Rh(III) systems. The absence of this (3)MMCT state in Ru(II),Rh(I) complexes results in a longer lifetime and higher emission quantum yield for the Ru(dπ) → dpp(π*) (3)MLCT state than [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3. Both complexes display photocatalytic hydrogen production activity in the presence of water and a sacrificial electron donor, with the [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 possessing a higher catalytic activity than the methyl analogue. Both display low activities, hypothesized to occur due to steric crowding about the Rh(I) site.

ChemInform Abstract: Kondo Resonance in Magnetic and Non‐Magnetic Ce‐Intermetallics

AbstractReview: 26 refs.

Comparative structural study of decagonal quasicrystals in the systems Al–Cu–Me (Me = Co, Rh, Ir)

A comparative single-crystal X-ray diffraction structure analysis of the family of Al-Cu-Me (Me = Co, Rh and Ir) decagonal quasicrystals is presented. In contrast to decagonal Al-Cu-Co, the other two decagonal phases do not show any structured disorder diffuse scattering indicating a higher degree of order. Furthermore, the atomic sites of Rh and Ir can be clearly identified, while Cu and Co cannot be distinguished because of their too similar atomic scattering factors. The structure models, derived from charge-flipping/low-density elimination results, were refined within the tiling-decoration method but also discussed in the five-dimensional embedding approach. The basic structural building units of the closely related structures are decagonal clusters with 33 Å diameter, which are consistent with the available electron-microscopic images. The refined structure models agree very well with the experimental data.

Facile Synthesis of Chiral Diphosphine‐Containing Multiple Dendrimeric Catalysts for Enantioselective Hydrogenation

AbstractA new kind of chiral diphosphine PyrPhos‐functionalized codendrimers have been synthesized via a liquid‐phase strategy in high yields. The resulting dendrimeric PyrPhos ligands were purified by a simple solvent precipitation without the need for chromatographic separation, and well characterized by 1H, 13C and 31P NMR, MALDI‐TOF mass spectroscopy as well as elemental analysis. Their rhodium complexes were applied to the asymmetric hydrogenation of α‐acetamido cinnamic acids. Excellent enantioselectivities were achieved, which are comparable to those with the corresponding small molecular catalysts. In addition, these codendrimeric catalysts showed better catalytic performance than the dendrimeric catalysts with Rh(PyrPhos) sites located in the focal point of poly(aryl ether) dendrons or in the periphery of poly(propyleneimine) dendrimers.

Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate pyrochlores for higher alcohol synthesis

Two Rh-based lanthanum zirconate pyrochlores (La2Zr2O7; LZ) were prepared by the Pechini method and tested for the synthesis of higher alcohols via CO hydrogenation. In one, Rh was substituted into the pyrochlore lattice (LRZ, 1.7wt%) while for the second, Rh was supported on an unsubstituted La2Zr2O7 (R/LZ, 1.8wt%). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) results show that the surface reducibility depends on whether the Rh is in (or supported on) the LZ pyrochlore. The total hydrogen consumption in TPR for LRZ (0.28mg H2/gcat) is much greater than R/LZ (0.07mg H2/gcat), likely due to the presence of a perovskite phase in the LRZ (LaRhO3; identified by XRD), in which rhodium is more reducible than in R/LZ. The formation of the perovskite accompanies that of the pyrochlore in the synthesis process, which includes heat treatment up to 1000°C. CO hydrogenation results show higher ethanol selectivity for R/LZ than LRZ, possibly due to the strong interaction between Rh and LZ on the R/LZ, forming atomically close Rh+/Rh0 sites, which have been suggested to favor ethanol production.

Structural investigations on YbRh2Si2: from the atomic to the macroscopic length scale

YbRh2Si2 has advanced to a prototype material for investigating physics related to the Kondo effect. An optimization of the synthesis resulted in single crystals of extraordinary crystalline quality. At the atomic scale, we utilize scanning tunneling microscopy to study the topography of cleaved single crystals. A structural and chemical analysis was conducted by highly accurate x-ray diffraction and wavelength dispersive x-ray spectroscopy measurements. The latter indicate a homogeneity range of the YbRh2Si2 phase between approximately 40.0–40.2 at.% Rh. For our high-quality samples the number of defects found on the atomic scale (of the order of 0.3% of the visible lattice sites) is in quantitative agreement with a very small off-stoichiometry within this homogeneity range. Comparing our results for these samples allows an assignment of the structural defects observed at the cleaved surfaces to Rh occupying Si sites and, even less numerous Si in Rh sites. Such an analysis is hampered for samples of lesser quality, but there seem to be numerous empty Si-sites. Based on these observations the results of scanning tunneling spectroscopy can be analyzed in further detail and provide insight into the Kondo physics.

Electro-oxidation of ethanol on Pt/C, Rh/C, and Pt/Rh/C-based electrocatalysts investigated by on-line DEMS

The ethanol electro-oxidation reaction was studied on carbon-supported Pt, Rh, and on Pt overlayers deposited on Rh nanoparticles. The synthesized electrocatalysts were characterized by TEM and XRD. The reaction products were monitored by on-line DEMS experiments. Potentiodynamic curves showed higher overall reaction rate for Pt/C when compared to that for Rh/C. However, on-line DEMS measurements revealed higher average current efficiencies for complete ethanol electro-oxidation to CO2 on Rh/C. The average current efficiencies for CO2 formation increased with temperature and with the decrease in the ethanol concentration. The total amount of CO2, on the other hand, was slightly affected by the temperature and ethanol concentration. Additionally, the CO2 signal was observed only in the positive-going scan, none being observed in the negative-going scan, evidencing that the C–C bond breaking occurs only at lower potentials. Thus, the formation of CO2 mainly resulted from oxidative removal of adsorbed CO and CHx,ad species generated at the lower potentials, instead of the electrochemical oxidation of bulk ethanol molecules. The acetaldehyde mass signal, however, was greatly favored after increasing the ethanol concentration from 0.01 to 0.1molL−1, on both electrocatalysts, indicating that it is the major reaction product. For the Pt/Rh/C-based electrocatalysts, the Faradaic current and the conversion efficiency for CO2 formation was increased by adjusting the amount of Pt on the surface of the Rh/C nanoparticles. The higher conversion efficiency for CO2 formation on the Pt1Rh/C material was ascribed to its faster and more extensive ethanol deprotonation on the Pt–Rh sites, producing adsorbed intermediates in which the C–C bond cleavage is facilitated.

First-principles study of CaCu3B4O12 (B=Co, Rh, Ir)

Employing first-principles density functional theory based calculations we investigated the change in electronic structure of CaCu3B4O12 compounds as one moves from 3d (Co) to 4d (Rh) to 5d (Ir) element at B site. Our study sheds light on valences of Cu and B ions as one moves from 3d to 4d to 5d based compounds. The valence of Cu in Co and Rh compound turn out to be that of less known 3+ state, while that in Ir compound turn out to be commonly known 2+ state. Our first-principles study provide microscopic understanding of these different valences of Cu, in terms of changes in the mixing of Cu x2 − y2 and B-a1g states, driven by changes in the crystal field and spin splitting. The stronger crystal field splitting for 4d and 5d elements compared to 3d at B site drive the low-spin state at Rh and Ir site as opposed to intermediate spin in case of Co.

Operando infrared spectroscopy of the reduction of NO by H2 over rhodium based catalysts

An operando spectroscopic study of the NO/H2 reaction has been achieved to relate spectral features to kinetic data in order to identify key intermediates involved in the production of N2, N2O and NH3 as the main reaction products. Transient experiments show that different nitrosyl species, essentially neutral, positively and negatively NO adsorbed over metallic Rh sites exhibit different reactivity below the light-off temperature at low conversion and temperature. In particular, stable positively charged nitrosyl species inhibit the re-adsorption and further dissociation of N2O predominantly formed during the cold start engine. This can be related to a major drawback in the use of Rh for further developments of compressed natural gas engine catalysts running typically in three-way conditions. Hydrogen was found to enhance the conversion of NOx and the transformation to nitrogen mainly by destabilizing those species further enhancing the subsequent reduction of N2O. The participation of the fast H2/O2 reaction at high temperature that restores the metallic character of rhodium could be an indirect effect in the selectivity enhancement changing the nature and the relative rate of steps related to nitrogen production.

Efficient Photocatalytic Hydrogen Production in a Single‐Component System Using Ru,Rh,Ru Supramolecules Containing 4,7‐Diphenyl‐1,10‐Phenanthroline

Small changes go a long way: Modification of the terminal ligand to incorporate 4,7-diphenyl-1,10-phenanthroline has generated superior Ru,Rh,Ru photocatalysts (see scheme; ochre Ru, red Rh, blue N, yellow Cl or Br) displaying significantly enhanced photocatalytic H2 production from H2O with long-term functioning: H2O is reduced to H2 with over 1300 turnovers per Rh site and a maximum efficiency of 7.3 %.

Promotion effect of support calcination on ethanol production from CO hydrogenation over Rh/Fe/Al 2O 3 catalysts

A series of Rh/Fe/Al 2O 3 catalysts, in which the Al 2O 3 supports were calcined at different temperatures, were prepared by a sequential impregnation method. The catalysts were tested by CO hydrogenation and characterized by N 2 adsorption–desorption, X-ray diffraction (XRD), CO pulse chemisorption, temperature programmed surface reaction (TPSR) and temperature programmed reduction (TPR) techniques. The activity of ethanol formation was the highest when the catalyst support was calcined at 800 °C, while that of methanol formation increased continuously with the calcination temperature of the support. According to the commonly accepted mechanism of C 2-oxygenates formation, CO conversion followed three separate pathways after CO dissociation, and our results suggested that the activity towards CO insertion and dissociation increased gradually with the calcination temperature, but began to decrease at 900 °C. On the other hand, direct hydrogenation of CO to methanol was still increasing at 900 °C. These observations were in agreement with TPSR results. Dispersion of the Rh or Fe species was not impaired, and even improved, with the declining of the surface area of the support due to high temperature calcination. TPR results revealed that Rh–Fe interaction was strengthened after calcination, due to a lowering in surface hydroxyl reactivity of the support and an increase of the Rh–Fe interface area. As a result, the amount of Rh–Fe–O sites for CO dissociation and insertion increased with the calcination temperature, giving rise to the increase in ethanol formation activity. However, an over-strong Rh–Fe interaction would be resulted when the catalyst support was calcined at 900 °C, and this would cause more Fe species to be reduced, which would then cover the Rh sites. Consequently, CO uptake as well as dissociation and insertion of CO would decrease, leading to more CO molecules being hydrogenated directly to methanol, thus causing a decrease in the selectivity of ethanol formation.

Size and Topological Effects of Rhodium Surfaces, Clusters and Nanoparticles on the Dissociation of CO

The present density functional theory study provides insight into the reactivity of the surface metal atoms of extended/periodic Rh surfaces, clusters, and nanoparticles toward CO adsorption and dissociation. Our results demonstrate that the defect site in a B5 configuration is the most active one for CO dissociation on all three considered systems. However, the reactivity of the B5 site for CO dissociation depends critically on the size of the system. The barrier for CO dissociation barrier on the B5 site increases for smaller particles. The lowest barrier is found for the B5 site of a stepped Rh (211) surface. CO dissociation on this site occurred with a barrier below the desorption energy of CO.

Electron Correlations in Superconductor Rh17S15 Studied by 103Rh NMR and Specific Heat Measurements

The electronic state of superconductor Rh 17 S 15 has been investigated by 103 Rh nuclear magnetic resonance (NMR) and specific heat measurements. We have identified the observed NMR lines corresponding to four inequivalent Rh sites in the cubic unit cell and obtained the site-selective information of each site. The spin–lattice relaxation rate 1/ T 1 indicates that the 4 d electrons from the Rh 24 m site are the most responsible for the large electronic specific heat coefficient γ. This conclusion is supported by the energy band calculation. Considering the experimental results and the band calculation, we concluded that the γ value is enhanced because of the electron correlations of Rh 4 d bands. In the superconducting state, an exponential decrease of both 1/ T 1 and the electron specific heat are observed, revealing the isotropic superconducting gap with 2Δ/ k B T c =4.0. In addition, the decrease of the spin part of the Knight shift indicates the spin-singlet Cooper pair formation. These results demo...

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.

Pyridyl Conjugated 1,2,3-Triazole is a Versatile Coordination Ability Ligand Enabling Supramolecular Associations

Four new complexes with 2-((4-phenyl-1H-1,2,3-triazol-1-yl)methyl)pyridine (ptmp) ligand: [Rh2(O2CCH3)4(ptmp)2] (1), [PdCl3(Hptmp)]·H2O (2), (Hptmp)2[AuCl4]Cl (3), and [Hg4Br8(ptmp)2]n (4) were prepared and their crystal structures were determined by single crystal X-ray diffraction analyses. Three nitrogen atoms of the ligand ptmp are prone to coordinate to the metal ions. In complex 1 ligand ptmp coordinates monodentately through the pyridyl N4 atom to the axial site of Rh. In compound 2 the N4 protonated ptmp, Hptmp+, coordinates Pd by the triazole N3 nitrogen atom. Ionic compound 3 is composed of three types of ions, Hptmp+, AuCl4−, and Cl−, interconnected by hydrogen bonds. Mercury complex 4 is a coordination polymer in which two ptmp ligands stabilize the Hg4Br8 chain, and the coordination involves N2 and N3 nitrogen atoms of the triazole and the pyridyl nitrogen atom N4. The crystal structures of 1−4 are dominated by hydrogen-bonding, C−H···π, and π−π stacking interactions. The potential of ptmp to...

Carbon monoxide-free hydrogen production via low-temperature steam reforming of ethanol over iron-promoted Rh catalyst

For the first time, a novel iron-promoted Rh catalyst is developed to produce CO-free H 2 through steam reforming of ethanol at low temperatures, between 623 and 673 K. The iron oxides in the vicinity of Rh sites reduce the CO adsorption on Rh sites and transfer the adsorbed CO from Rh to COO-formate species on Fe x O y for the subsequent water–gas shift reaction, resulting in a high H 2 yield, extremely low CO selectivity, and a long Rh life span.