Rh Sites Research Articles

H 2 production for MC fuel cell by steam reforming of ethanol over MgO supported Pd, Rh, Ni and Co catalysts

The performance of MgO supported Pd, Rh, Ni and Co catalysts in the steam reforming of simulated bio-ethanol at MC fuel cell operative conditions ( T = 650 °C) has been investigated. Rh/MgO shows the best performance both in terms of activity and stability, however, it seems not to be so selective towards H 2 production. Ni, Co and Pd catalysts are affected by deactivation mainly due to metal sintering. Ni/MgO displays the best performance in terms of H 2 selectivity (> 95%). A very low coke formation rate was observed on Rh/MgO catalyst, however, even on Ni/MgO catalyst coke formation occurs with modest rate. Kinetics measurements have shown that a significant difference in terms of metals specific activity exist: Rh sites resulted to be 2.2, 3.7 and 5.8 times more active than Pd, Co and Ni, respectively.

Multifunctional Aspects of Three-Way Catalyst: Effects of Complex Washcoat Composition

In this paper multifunctional aspects in operation of Pt/Rh/Ce/γ-Al 2 O 3 three-way catalyst (TWC) are discussed on the basis of computed spatiotemporal patterns of reacting and deposited species on specific active sites distributed in the catalytic washcoat layer. A spatially pseudo-one-dimensional, heterogeneous model of a TWC reactor has been developed, with well-mixed gas phase and description of diffusion in the porous catalytic washcoat. The model includes a detailed non-stationary kinetic scheme for CO and C 2 H 2 oxidation, oxygen storage on CeO 2 , CO 2 adsorption on the support and NO x transformation and reduction. Pt, Rh, Ce and γ-Al 2 O 3 sites are considered with different kinetic parameters. A methodology is developed for the studies of the effects of internal diffusion and washcoat composition (i.e. concentration and distribution of active Pt, Rh and Ce catalytic sites) on the conversion of main pollutants in automobile exhaust gas under transient conditions. Several cases of variable distribution of the catalytic sites in the washcoat layer are discussed with respect to the reactor performance. Effects of oscillations in the inlet oxygen concentration are studied. Complex interactions, particularly between reaction intermediates and the stored oxygen within the washcoat, cause the increase of conversions in the periodic operation when compared with the stationary one.

Anisotropic elastic properties ofCeRhIn5

The structure of the quasi-two-dimensional heavy fermion antiferromagnet ${\mathrm{CeRhIn}}_{5}$ has been investigated as a function of pressure up to 13 GPa using a diamond anvil cell under both hydrostatic and quasihydrostatic conditions at room $(T=295\mathrm{K})$ and low $(T=10\mathrm{K})$ temperatures. Complementary resonant ultrasound measurements were performed to obtain the complete elastic moduli. The bulk modulus $(B\ensuremath{\approx}78\mathrm{GPa})$ and uniaxial compressibilities $({\ensuremath{\kappa}}_{a}=3.96\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{GPa}}^{\ensuremath{-}1}$ and ${\ensuremath{\kappa}}_{c}=4.22\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{GPa}}^{\ensuremath{-}1})$ found from pressure-dependent x-ray diffraction are in good agreement with the ultrasound measurements. Unlike doping on the Rh site where ${T}_{c}$ increases linearly with the ratio of the tetragonal lattice parameters $c/a,$ no such correlation is observed under pressure; instead, a double-peaked structure with a local minimum around 4--5 GPa is observed at both room and low temperatures.

In Situ IR Study of Transient CO2 Reforming of CH4 over Rh/Al2O3

The CO2−CH4 reaction on Rh/Al2O3 was studied by in situ infrared spectroscopy coupled with pulse and step transient techniques. Steady-state isotopic 13CO transient studies at 773 K and 0.1 MPa show that the formation of gaseous 13CO2 closely follows that of linear 13CO, indicating that linear CO is an active adsorbate. Pulsing CH4 into CO2 flow and step switching from He to CO2/CH4 flow showed that the formation of H2 led that of CO, revealing that the first step of the reaction sequence is the decomposition of CH4 into *CHx species and hydrogen. Hydrogen activated adsorbed CO2 to produce linear CO. The linear CO was found to be the major species on Rh/Al2O3 during the reaction by in situ infrared spectroscopy. The accumulation of the linear CO on Rh0 sites revealed that the surface of Rh crystallites on Al2O3 remained in a reduced state throughout the study. The O2 pulse into CO2/CH4 resulted in (i) a total oxidation of CH4 to CO2 and H2O and then (ii) a net increase in the formation of the desired prod...

Theoretical study of the chemisorption of CO on bimetallic RhCu surfaces and nanoparticles

The CO interaction with bimetallic RhCu surface models representing several compositions has been studied by first principles density functional theory calculations. The analysis of the bare bimetallic clusters Rh(4s) and Cu(3s) core-level binding energies indicates that is not possible to extract information about the oxidation state of the alloy components. The present calculations predict that CO does always sit on top sites, the influence of the alloy composition on the equilibrium geometry and vibrational frequency of CO chemisorbed at a given Rh or Cu site being very small. However, there is a large difference in the structural properties corresponding to CO chemisorption above either Rh or Cu. Therefore, the absolute value of the vibrational frequency of chemisorbed CO does not permit to extract any information about the alloy composition but afford to assign the chemisorption site. Finally, the CO adsorption energy does not follow a monotonic trend with composition. The use of the Constrained Space Orbital Variation analysis permits one to firmly establish that the difference in adsorption energy for different compositions cannot be explained through differences in the σ-donation and π-backdonation mechanisms.

Evidence for ordered moments on the Rh sites inCeRhIn5

The magnetic structure of the tetragonal heavy fermion compound ${\mathrm{CeRhIn}}_{5}$ ${(T}_{N}=3.8\mathrm{K})$ has been studied by zero-field muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ in the range 1.8--4.3 K in a single crystal. In addition, the anisotropic muon $({\ensuremath{\mu}}^{+})$ Knight shift has been measured in the paramagnetic regime with the aim to determine the ${\ensuremath{\mu}}^{+}$ site or sites. It is found that about 70% of the implanted ${\ensuremath{\mu}}^{+}$ are located at the a site $(\frac{1}{2},\frac{1}{2},\frac{1}{2}).$ The site of the other 30% is not unambigeously determined but may be the f sites $(0,\frac{1}{2},0)$ and $(\frac{1}{2},0,0).$ The data also imply that a small negative moment is induced on the Rh sites. Below ${T}_{N}$ we find five components in the $\ensuremath{\mu}\mathrm{SR}$ signal reflecting spontaneous, relatively sharp internal fields of zero, 13, 37, 69, and 98 G at $T=0\mathrm{K},$ which are confined to the surface of cones oriented along the c axis with apertures ranging from $14\ifmmode^\circ\else\textdegree\fi{}$ to $90\ifmmode^\circ\else\textdegree\fi{}.$ These results are not reproducible by the known incommensurate helical magnetic structure of the Ce sublattice but can be explained if, in addition, a small moment order of unusual type on the Rh sublattice is assumed.

Photoelectron diffraction study of the low-temperature low-coverage oxygen layer onRh(110)

The local adsorption geometry of the low-coverage, short-range-ordered layer formed upon oxygen adsorption on the Rh(110) surface at 120 K has been quantitatively characterized by photoelectron diffraction measurements and multiple-scattering cluster calculations. The results of the structural analysis are compatible with a model where the oxygen atoms are located in the symmetric or slightly asymmetric short-bridge sites of the unreconstructed Rh(110) surface. O adsorption induces a small increase of the first substrate interlayer distance with respect to the relaxation of the clean Rh(110) surface.

NMR properties of CoRhP: mixing between Co and Rh sites

NMR properties of 59Co and 31P in CoRhP with orthorhombic TiNiSi structure are reported. Field-swept NMR spectra of 59Co show very broad ones from nonmagnetic Co sites where satellite peaks due to quadrupole interaction are smeared out. The Fourier transformed spectrum of 31P NMR has two peaks. It is suggested that the Co and Rh atoms do not order but their sites are mixed. The mixing is consistent with a Rietvelt analysis of X-ray diffraction powder patterns where a mixing of 10% of Co atoms into Rh sites is estimated. This kind of mixing seems to be one of the origins of the variety of magnetic and electric properties of MRuP and MRhP (M=3d transition metal) systems. The nuclear spin-lattice relaxation rates of 59Co and 31P in CoRhP obey Korringa relations at high temperatures and they show stepwise decreases with decreasing temperature at 11 K where the static susceptibility has a peak. The behaviors are interpreted as an onset of a charge density wave (CDW) at 11 K rather than an onset of an antiferromagnetism.

N2O Decomposition Catalysed by Transition Metal Ions

The direct N2O decomposition catalysed by 5-coordinated transition metal ions Fe, Co, and Rh was studied using the DFT method. The cluster model has the formula M(OH)3(H2O)2. Energies of intermediates and transition states were computed. The results show that Co and Rh sites are more active than Fe, the former two being very similar.

Ethylene hydroformylation and carbon monoxide hydrogenation over modified and unmodified silica supported rhodium catalysts

Ethylene hydroformylation and carbon monoxide hydrogenation (leading to methanol and C 2-oxygenates) over Rh/SiO 2 catalysts share several important common mechanistic features, namely, CO insertion and metal–carbon (acyl or alkyl) bond hydrogenation. However, these processes are differentiated in that the CO hydrogenation also requires an initial CO dissociation before catalysis can proceed. In this study, the catalytic response to changes in particle size and to the addition of metal additives was studied to elucidate the differences in the two processes. In the hydroformylation process, both hydroformylation and hydrogenation of ethylene occurred concurrently. The desirable hydroformylation was enhanced over fine Rh particles with maximum activity observed at a particle diameter of 3.5 nm and hydrogenation was favored over large particles. CO hydrogenation was favored by larger particles. These results suggest that hydroformylation occurs at the edge and corner Rh sites, but that the key step in CO hydrogenation is different from that in hydroformylation and occurs on the surface. The addition of group II–VIII metal oxides, such as MoO 3, Sc 2O 3, TiO 2, V 2O 5, and Mn 2O 3, which are expected to enhance CO dissociation, leads to increased rates in CO hydrogenation, but only served to slow the hydroformylation process slightly without any effect on the selectivity. Similar comparisons using basic metals, such as the alkali and alkaline earths, which should enhance selectivity for insertion of CO over hydrogenation, increased the selectivity for the hydroformylation over hydrogenation as expected, although catalytic activity was reduced. Similarly, the selectivity toward organic oxygenates (a reflection of the degree of CO insertion) in CO hydrogenation was also increased.

Catalytic Reduction of NO by CO over Rhodium Catalysts: 2. Effect of Oxygen on the Nature, Population, and Reactivity of Surface Species Formed under Reaction Conditions

The effect of oxygen on the nature, population, and reactivity of surface species formed during reduction of NO by CO over Rh/TiO2 catalysts has been examined employing FTIR and transient MS techniques. It has been found that the activity of Rh is hindered by accumulation of surface oxygen originating from NO decomposition and gas-phase oxygen in the feed. Adsorbed CO and reduced TiO2−x species in the vicinity of Rh particles act as oxygen atom scavengers and, under fuel-rich conditions, remove atomic oxygen from the surface and restore the catalytic properties. Results of the present study provide additional evidence that production of N2 is related to dissociation of adsorbed Rh–NO− while production of N2O is related to the presence of Rh(NO)2. The presence of reduced Rh0 sites is necessary for the formation of both reduction products. In the absence of oxygen in the feed, surface isocyanate species are also observed under reaction conditions. Their formation requires the presence of adjacent Rh0–CO and reduced Rh0 sites. Although these species are favored under conditions in which NO conversion to reduction products is observed, there is no evidence that they are catalytically active species.

Preparation of Rh-containing polycarbonate films and the study of their chemical properties in the polymer

RhCl[P(C 6H 5) 3] 3 complexes have been incorporated in polycarbonate (PC) as a dispersion medium using cosolvent (THF). The interactions between Rh(I) complexes and polycarbonate polymer molecules are studied by infrared spectroscopy and thermal analysis. The reaction chemistry of Rh in PC films has been investigated by reacting Rh sites in PC with small gaseous molecules like CO, H 2, D 2, O 2, NO, C 2H 2, and C 2H 4 in the temperature range 25∼150°C. Various Rh–carbonyls, –hydride, –nitrosyl, and –superoxo dioxygen species formed in PC films are characterized by infrared spectroscopy. The Rh complexes in PC are easily reduced by reacting them with H 2 gas and such reduction results in the formation of small Rh metal particles of 20∼30 Å in diameter in PC. The Rh complexes in PC show interesting catalytic reactivities such as hydrogenation of olefin and acetylene, oxidation of CO, reduction of NO, methanol synthesis from CO or CO 2, and oxidation of alcohol under relatively mild conditions.

Enzymatic properties of phenylalanine101 mutant enzyme of ribonuclease rh from Rhizopus niveus.

To investigate the role of Phe101, a component of a base recognition site (B2 site) of a base-nonspecific RNase Rh from Rhizopus niveus, we prepared several enzymes mutated at this position, F101W, F101L, F101I, F101A, F101Q, F101R, and F101K, and their enzymatic activities towards RNA, 16 dinucleoside phosphates, and 2', 3'-cyclic pyrimidine nucleotides were measured. Enzymatic activity toward RNA of F101W, F101L, and F101I were about 7, 20, and 3.8% of the native enzyme, respectively, and those of the other mutants were less than 1% of the RNase Rh. Similar results were also obtained with GpG as substrate. Thus, it was concluded that Phe101 is a very important residue as a component of the B2 site of RNase Rh, and its role could be replaced by Leu, then Trp and Ile, though in less effectively. The results suggested that some kind of interaction between B2 base and the side chain of amino acid residue at the 101th position, such as pi/pi or CH/pi interaction is very important for the enzymatic activity of RNase Rh. The mutation of Phe101 markedly affected the enzymatic activity toward dinucleoside phosphates and polymer substrates, but only moderately the rate of hydrolysis of cyclic nucleotides, indicating the presence of secondary effect of the mutation on B1 site.

Spectroscopic Identification of the Active Site for CO Oxidation on Rh/Al2O3 by Concentration Modulation in situ DRIFTS

Diffuse reflectance infrared spectroscopy experiments are described in which the concentrations of the reactant gases passing over a solid catalyst are modulated. This simple modification to the normal in situ experiment enables direct correlation of specific surface species with the production of gaseous products. Spectra of solely the active surface intermediates with no contribution from so called “spectator” species are generated. For CO oxidation over a Rh/alumina catalyst at 576 K, the active CO species is identified as linearly adsorbed CO on oxidized Rh sites (υCO=2100 cm−1) on a catalyst surface apparently otherwise dominated by CO adsorbed as unreactive geminal dicarbonyl.

Infrared Spectra of RhCO+, RhCO, and RhCO- in Solid Neon: A Scale for Charge in Supported Rh(CO) Catalyst Systems

Laser-ablated Rh+, Rh, and electrons react with CO on condensation in excess neon at 4 K to form RhCO+, RhCO, RhCO-, and Rh(CO)2+, Rh(CO)2, Rh(CO)2-, and higher carbonyls. These rhodium carbonyls are identified by isotopic substitution (13CO, C18O, and mixtures), electron trapping with added CCl4, and comparison with DFT calculations of isotopic frequencies. This is the first spectroscopy of isolated rhodium carbonyl cations and anions. The isolated monocarbonyl species provide a scale to estimate local charge on Rh(CO) sites in catalyst systems.

Electronic Excitation of RhI(CO)2 Species on Al2O3 Surfaces: Photodissociation of the RhI−CO Bond

The formation of the RhI ion during CO chemisorption on Rh/Al2O3 catalysts has been directly observed using UV−visible spectroscopy. Comparison of infrared and UV−visible spectra indicates that the species RhI(CO)2 is produced by a surface oxidation process operating on metallic Rh0 sites and involving Al−OH groups as oxidizing centers, in agreement with previous postulates.

The versatile nature of [ClAu(L–L)AuCl] (L–L=bis(diphenylphosphino)methane (dppm) or -butane (dppb)) in reactions with [Rh 2Ru 4(CO) 16B] −: crystal structures of [{Rh 2Ru 4(CO) 16B} 2{μ-Au(dppb)Au}] and [RhRu 4(CO) 14B{Au(dppm)Au}

The reaction of the octahedral borido cluster [Rh 2Ru 4(CO) 16B] − (which exists as trans and cis isomers with respect to the Rh sites) with [ClAu(dppb)AuCl] (dppb=bis(diphenylphosphino)butane) leads to the formation of separable isomers [{ cis-Rh 2Ru 4(CO) 16B} 2{μ-Au(dppb)Au}] and [{ trans-Rh 2Ru 4(CO) 16B} 2{μ-Au(dppb)Au}]. The cis-isomer converts rapidly to the trans-isomer. A similar reaction with [ClAu(dppm)AuCl] (dppm=bis(diphenylphosphino)methane) gives the linked dicluster [{Rh 2Ru 4(CO) 16B} 2{μ-Au(dppm)Au}], accompanied by the formation of [Rh 2Ru 4(CO) 15BAu(dppm)] and [RhRu 4(CO) 14BAu 2(dppm)], as well as the unstable [Rh 2Ru 4(CO) 16B{Au(dppm)AuCl}]. The crystal structures of [{ trans-Rh 2Ru 4(CO) 16B} 2{μ-Au(dppb)Au}] and [RhRu 4(CO) 14B{Au(dppm)Au}] have been determined; the former retains octahedral borido Rh 2Ru 4B units, while the latter is the consequence of cluster degradation and contains a square-based pyramidal RhRu 4B unit capped on the square-face by the two gold atoms (Au–Au=2.878(1) Å) of a {Au(dppm)Au} unit.

Chemical Bond Activation on Surface Sites Generated Photochemically from RhI(CO)2Species

The activation of chemical bonds in molecules on photochemically produced RhI sites is reviewed in this article. The focus is primarily on the RhI(CO) species generated during the ultraviolet photolysis of the well-known RhI(CO)2 species supported on an aluminum oxide surface (designated RhI(CO)2/Al2O3). The RhI(CO)2 surface species is the heterogeneous analogue of the homogeneous phase CpRh(CO)2 and Cp*Rh(CO)2 (Cp* = η5-C5(CH3)5; Cp = η5-C5H5) known for its ability to activate the C−H bond in alkanes during ultraviolet irradiation. Here we review some of the key studies that have led to the molecular understanding of the activation of the RhI(CO)2/Al2O3 species by photochemical methods. Relevant examples of the homogeneous-phase bond activation studies utilizing a rhodium center will be discussed as well as theoretical studies that have provided valuable information regarding the active nature of the photoproduced Rh site. This will be followed by a review of recent photochemical studies of surface-bound...

Rh/C catalysts in ethene hydroformylation: the effect of different supports and pretreatments

The activity, selectivity and stability of Rh/C catalysts prepared from nitrate precursor on three different active carbon supports was determined in ethene hydroformylation. The different active carbon supports yielded catalysts, which had different extents of reduction (62–94%) and dispersion (4–27%) after hydrogen reduction. The best activity and selectivity in the formation of propanal was obtained with the coconut shell based Rh/C(C) catalyst, presumably due to the better metal dispersion and the presence of unreduced Rh sites. Furthermore, without any pretreatment, the Rh/C(C) catalyst exhibited even higher activity and better propanal yields than its hydrogen treated counterpart. Most likely, the active sites remained better dispersed in the absence of pretreatment than after hydrogen reduction, and it was the well-dispersed sites that were active in the formation of propanal. High temperature CO pretreatment partially blocked the catalyst surface with carbonaceous residues, which improved propanal selectivity, but suppressed the overall activity. Thus, the right choice of pretreatment appears to be a key factor in providing an active and selective catalyst for heterogeneous hydroformylation. It should also be noted that the catalysts lost 10–30% of the metal deposited on the support during 24 h on stream.

IR study of CO adsorption on Cu–Rh/SiO 2 catalysts, coked by reaction with methane

Infrared spectra are reported of CO adsorbed on silica-supported Rh, Cu, and Cu–Rh catalysts before and after coking by heat treatment in methane at 673 K, and after subsequent exposure to hydrogen at 293, 373 or 473 K. The surface of Cu–Rh particles was enriched with copper, but all surfaces contained alloy phases. Addition of copper to rhodium blocked sites for the formation of Rh(CO) 2, decreased the proportion of sites which linearly adsorbed CO, but favoured bridge-bonded CO on Rh sites. Cu sites in Cu/SiO 2 were not poisoned by methane treatment, but Cu sites and all types of Rh site in Cu–Rh/SiO 2 underwent poisoning in the sequence Cu–CO>Rh–CO>Rh 2CO≈Rh 2(CO) 3. CO band shifts caused by coking of rhodium sites are attributed to a decrease in dipolar coupling. Dipole coupling changes were more significant when the Rh surface was modified by Cu than by coking. Coke deposits, at least in the presence of CO, aggregated into patches on exposed Rh surfaces. Similar effects of coking occurred for linear CO on copper sites. Rh/SiO 2 gave a high yield of methane, but only a low yield of ethane on coke hydrogenation. Copper largely poisoned the catalytic activity, which it is tentatively proposed primarily involved sites on rhodium which adsorb CO in a bridging configuration.