Rh Sites Research Articles

An EPR study of rhodium oxihydroxide

Rhodium oxihydroxide has been synthesized by means of hydrothermal synthesis at 1500 bars and 600°C in acidic media. Powder X-ray diffraction shows small variations in cell parameters which appear to be related to the oxidizing power of the reaction media. On the basis of EPR work such variations can be attributed to the presence of small amounts of either Rh(II) or Rh(IV). This last species had not been fully characterized before by EPR in oxide-like materials. The paramagnetic species seem to be noninteracting and randomly distributed in the Rh(III) sites. These results can be understood on the basis of two different kinds of nonstoichiometry for rhodium oxihydroxide: either proton or hydroxyl ion defects.

Effects of preparation procedure on the surface properties of rhodium supported on γ-alumina

The decomposition of rhodium(III) nitrate, chloropentamminerhodium(III) chloride and rhodium(III) sulphite in atmospheres of hydrogen, nitrogen and nitrogen (80%)+ oxygen (20%) have been investigated by thermogravimetric analysis with the salts either pure or impregnated onto a γ-alumina support. The decomposition of rhodium(III) sulphite failed to give dispersions of rhodium metal on alumina but the reduction of the other salts to Rh0 was complete in nitrogen or hydrogen at temperatures between 400 and 690 K.Hydrogen and CO uptake data have shown that rhodium(III) nitrate led to a more finely divided dispersion of metal than did chloropentamminerhodium(III) chloride. Infrared spectra of CO absorbed on the Rh/Al2O3 dispersions were consistent with this conclusion. The effects of sintering and oxygen adsorption on the infrared spectra are reported. Linear and bridge-bonded CO were formed on exposed metal planes or two-dimensional rafts at Rh0 sites which were also active for the chemisorption of oxygen. Gem-dicarbonyl species were formed at monatomically dispersed rhodium sites or edge sites in rafts, both of which had cationic character through interaction with the alumina support. These sites were not active for the adsorption of oxygen.

Effect of support material on rhodium catalysts

The effect of support material on Rh/X catalysts has been studied by using CO as a probe molecule for chemisorption and ir spectroscopy as the analytical method. Support materials including TiO/sub 2/, Al/sub 2/O/sub 3/, SiO/sub 2/, kaolinite, and montmoroillonite have been compared as to their tendencies to produce the various CO/RH/X species generally attributed to this catalytic system. The TiO/sub 2/ and SiO/sub 2/ supports enable the most facile reduction of a rhodium precursor material to rhodium metal. Although all of the Rh/X catalysts contain some Rh(I) sites, alumina-supported ones contain the most nonmetallic rhodium. Kaolinite and montmorillonite may contain impurities which tend to poison the Rh sites for CO chemisorption. The CO/Rh/SiO/sub 2/ infrared bands are much less intense than for alumina- or titania-supported rhodium. This is probably due to weak metal/support interaction for Rh/SiO/sub 2/ because the silica employed was of high purity. 3 figures.

Site distribution studies of Rh supported on Al2O3—An infrared study of chemisorbed CO

Using infrared spectroscopy, chemisorbed CO has been employed as a surface probe to observe differences in Rh site distributions on Al2O3-supported Rh. It has been found that higher loadings of Rh lead to Rh site distributions resembling those found on Rh(111) single crystals, whereas low Rh loadings produce isolated Rh sites preferentially. The isolated Rh sites yield infrared spectra characteristic of a species Rh(CO)2. Chemisorption of oxygen onto the CO-covered surface effectively eliminates crystalline Rh sites, leaving only the isolated Rh sites which maintain their bonding to CO as seen by infrared spectroscopy. Estimates of the infrared extinction coefficient for Rh(CO)2 have been made. The results are consistent with data from other laboratories which indicate that the Rh is in a Rhd+ oxidation state.

Chemical Modification of Tryptophan Residues in Ribonuclease from a Rhizopus sp.1

Tryptophan residues in ribonuclease from a Rhizopus sp. (RNase Rh) were modified by NBS, H2O2-dioxane, o-nitrophenylsulfenyl chloride (NPS-Cl) and the relation between the extent of modification and enzymatic activity was studied in each case. By extrapolation of the modified tryptophan residue-enzymatic activity curve to a completely inactive state, it was found that modification of 1-2 tryptophan residues is responsible for loss of enzymatic activity. RNase Rh was partly protected from modification by H2O2-dioxane (pH 8.4) and NPS-Cl (pH 3.5) when in the presence of 2'-AMP and the fluorescence emission spectrum of RNase Rh was quenched by adding 2'-AMP. It seems, therefore, that 1 or 2 tryptophan residues are involved in the active site of RNase Rh or are located near the active site. The solvent perturbation difference spectra of RNase Rh were measured using ethylene glycol and D2O as perturbants. The results indicated that 1.2 tryptophan residues for D2O and 1.9 tryptophan residues for ethylene glycol were exposed to the solvents. These data show that about 1.2-1.9 tryptophan residues are exposed to the solvent and their modification causes loss in enzymatic activity.

Infrared spectroscopic study of activated surface processes: CO chemisorption on supported Rh

The infrared spectrum of CO chemisorbed on alumina-supported Rh surfaces has been studied following chemisorption at cryogenic temperatures. Major differences are observed in the distribution and spectroscopic character of chemisorbed CO species produced at low temperatures (110–170 K) compared to chemisorbed CO species produced on Rh at 295 K. It has been found that the species Rh(CO)2, formed on isolated Rh sites, is produced rapidly via an activated chemisorption process above ∼200 K. On more ’’crystalline’’ Rhx sites, containing chemisorbed CO, an activated CO adsorbate-conversion process has been detected in which ?CO decreases by ∼50 cm−1 on warming the adsorbed layer above ∼265 K. Isotopic exchange between 13CO(g) and 12CO(ads) has been shown to occur rapidly at low temperature (∼200 K) for Rh(CO)2 species, whereas Rhx(CO) species exchange rapidly only at higher temperatures (≳250 K). These results, taken together, serve to confirm a model in which isolated Rh sites coexist on the alumina support with crystalline Rhx sites; the two kinds of sites are separable on the basis of the spectroscopic character of the chemisorbed CO species they adsorb as well as by means of their chemical properties.

Vapor phase carbonylation of methanol over RhY zeolite

The carbonylation of methanol over Rh Y zeolite catalyst in the presence of methyl iodide was studied at 423 to 473 K under atmospheric pressure. The catalytic activity of Rh Y was much superior to other supported rhodium chloride catalysts such as RhCl 3 /Al 2 O 3 , RhCl 3 /SiO 2 Al 2 O 3 , RhCl 3 /SiO 2 , and Rh-cation exchanged resin. Methyl acetate was the main product of this carbonylation, although the yield of acetic acid increased with contact time and the partial pressure of methyl iodide. The rate of methyl acetate formation increased with the content of Rh on the zeolite up to 0.6 wt%. For Rh Y with more than 0.6 wt% of Rh, however, the rate decreased with the process time, and it was observed that the rhoidum species fell off from the catalyst support system and the most zeolite framework was broken. During the first stage of the reaction, an induction period was observed which could be eliminated by calcining in nitrogen or helium at 473 K. However, the catalytic activity of Rh Y was reduced by calcination at 573 K. When Rh Y was calcined in hydrogen at 573 K, the catalyst had no activity. Therefore, it was concluded that the precursor of the active rhodium site was a trivalent rhodium which was firmly anchored on the zeolite by an ion exchange process, and that water molecules adsorbed in the zeolite prevented the formation of the active Rh site. The active Rh sites on Y zeolite were stable up to 473 K, but over 483 K was inactivated with an irreversible change.

Kinetics and mechanism of methanol carbonylation over RhY zeolite

Methanol carbonylation over RhY zeolite was studied kinetically at 428 to 473 K under atmospheric pressure. For low conversions of methanol, methyl acetate was the only carbonylation product observed. The reaction was first order in methyl iodide and zero order in both methanol and carbon monoxide. The apparent activation energy was 5.65 × 104 J/mol in the temperature range studied. Carbonylations in the ethanol-methyl iodide, methanol-ethyl iodide, and deuterated methanol-methyl iodide systems were studied to clarify the reaction mechanism. From these results, the reaction mechanism for this carbonylation is proposed as follows: (i) the oxidative addition of methyl iodide to an active Rh site, (ii) the insertion of carbon monoxide into the methyl-Rh bond, (iii) the formation of methyl acetate by methanolysis of the acetyl-Rh bond, (iv) the reductive dissociation of hydrogen iodide to reform the active site, and (v) the regeneration of methyl iodide by the reaction of hydrogen iodide with methanol. This reaction mechanism is quite similar to that proposed for the homogeneous rhodium complex catalyst.

Electrochemical reduction of CO2 and oxidation of adsorbed species on the rhodium electrode

The adsorption of CO 2 on the rhodium electrode and the oxidation of the adsorption products were investigated by the use of a radiochemical method, by cyclic voltammetry and by chronoamperometry. The adsorption process occurs in the hydrogen region and the oxidation of the resulting organic species takes place in the oxygen region of the electrode potentials, respectively. Making use of the charges involved in the hydrogen asorption and in the oxidation of the organic adsorbate, the net composition of the CO 2 adsorption products was found to be best represented (for E ads >0.000 V) by the carbon atom bonded to four adsorption sites. It was also found that a surface oxidant is needed for the r.d.s. of the oxidation process. This report refers to some earlier works concerning the interactions of CO 2 with noble metal surfaces. In particular it is shown that the CO 2 adsorption probably does not pass through the activated CO 2 intermediates which are weakly bonded to the bare Rh sites. It is more likely that two Rh-H sites and the CO 2 molecule from the bulk of solution are involved in the adsorption mechanism (r.d.s.)

Electrochemical reduction of CO 2 and oxidation of adsorbed species on the rhodium electrode

The adsorption of CO 2 on the rhodium electrode and the oxidation of the adsorption products were investigated by the use of a radiochemical method, by cyclic voltammetry and by chronoamperometry. The adsorption process occurs in the hydrogen region and the oxidation of the resulting organic species takes place in the oxygen region of the electrode potentials, respectively. Making use of the charges involved in the hydrogen asorption and in the oxidation of the organic adsorbate, the net composition of the CO 2 adsorption products was found to be best represented (for E ads >0.000 V) by the carbon atom bonded to four adsorption sites. It was also found that a surface oxidant is needed for the r.d.s. of the oxidation process. This report refers to some earlier works concerning the interactions of CO 2 with noble metal surfaces. In particular it is shown that the CO 2 adsorption probably does not pass through the activated CO 2 intermediates which are weakly bonded to the bare Rh sites. It is more likely that two Rh-H sites and the CO 2 molecule from the bulk of solution are involved in the adsorption mechanism (r.d.s.)

Hyperfine magnetic field measurement in Heusler alloys with rhodium-99 and indium-111 by TDPAC technique

The hyperfine magnetic field (hmf) at nonmagnetic impurities Ru and Cd in Heusler alloys Rh2MnGe and Rh2MnSn has been measured as a function of temperature by the Time Differential Perturbed Angular Correlation (TDPAC) technique. Measurement of the hmf at the Ru utilized the 20.7-nsec 90 keV state in 99Ru populated in the decay of 16-d 99Rh; alloys were prepared using Rh metal powder bombarded with ?60 MeV protons. Measurement of hmf at Cd in Rh2MnSn utilized the 84-nsec 247 keV state in 111Cd populated in the decay of 2.8-d 111In; the alloy was prepared by substituting for 2 atomic per cent of Sn with In metal bombarded with 37 MeV protons. The measured hmf’s in kG at Ru impurity at Rh site are (with temperatures in parentheses) in Rh2MnGe: 195±11 (77 K), 126±4 (293 K), and 49±4 (355 K); in Rh2MnSn: 137±2 (293 K) and 37±6 (355 K). The measured hmf’s at Cd impurity (presumably at Sn site) in Rh2MnSn are 189±5 (77 K), 113±5 (293 K), and 33±3 (373 K).

Evidence for the presence of a histidine residue having pKa 7 in the active site of a ribonuclease from a Rhizopus sp.

The kinetic parameters, Km and log Vmax, of the cleavage of dinucleoside phosphates, GpU, GpC and ApU, by RNase Rh from a Rhizopus sp. were measured at various pH's. Analysis of the pH profiles of Km and Vmax of these dinucleoside phosphates according to Dixon's theory suggested that two functional groups having pKa values of 7.0-7.3 and 3.25-3.5 are present in the active site of RNase Rh. The former value may correspond to a histidine residue. The pH dependence of the rates of inactivation of RNase Rh by photoxidation and carbethoxylation also indicated that a functional group having pKa about 7.0 was involved in the active site. Amino acid analysis of photooxidized RNase Rh showed that only a histidine residue had been destroyed. In the carbethoxylation of RNase Rh, the formation of carbethoxyhistidine caused a corresponding loss of activity of RNase Rh without modification of tyrosyl residues. It was concluded that a histidine residue having pKa about 7.0 is involved in the active site of RNase Rh.

The quadrupole interaction of100Rh in various intermetallic compounds of palladium

The nuclear quadrupole interaction of the 75 keV excited state of100Rh in the ordered intermetallic compounds PdHg, PdPb2 PdSb and PdTe was measured. Using an estimate for the nuclear quadrupole moment of the 75 keV state in100Rh and point ion lattice sums for the lattice electric field gradient (EFG) at the Rh site, the electronic contributionV zz el to the total EFG was derived.V zz el was found to be correlated with the average valence electron concentration in the compounds studied. In the case of PdPb2, the temperature dependence of the quadrupole interaction was also studied and found not to follow theT 3/2 rule which is observed in virtually all pure noncubic metals.

Surface-supported metal cluster carbonyls. Chemisorption, decomposition and reactivity of Rh 6(C0) 16 supported on silica

A partially decarbonylated metal cluster is quickly formed on the surface of silica by oxidation at room temperature; it is possible to regenerate the initial cluster compound under a carbon monoxide atmosphere at 200°C. Decarbonylation of Rh 6(CO) 16 at higher temperature produces a new metallic material on the surface, characterized by two v(CO) vibration bands at 2048 ± 7 cm −1 and 1893 ± 10 cm −1. These two bands have been respectively assigned to a terminal carbonyl group and a bridged carbonyl group bonded to two rhodium atoms. Oxidation of this compound occurs very easily under oxygen at room temperature and gives an oxidized material presumably of the same nuclearity; adsorption of carbon monoxide produces two intense sharp bands at 2093 and 2038 cm −1 which have been assigned to the symmetric and asymmetric stretching modes of two CO molecules bonded to a single oxidized Rh site as Rh I(CO) 2. The conversion from the oxidized surface species to the metallic one can be performed under mild conditions, but attempts to regenerate the initial cluster compound were unsuccessful.

Magnetic moment distribution of ferromagnetic Ni-Rh alloys

Abstract The diffuse scattering of polarized and unpolarized neutrons was used to determine the spatial distribution of the magnetic moment for ferromagnetic NiRh alloys. The average Ni moment remains near 0.6 μ B to 12 at.% Rh and then decreases toward zero at the critical concentration of 37 at.% Rh. There is an initial rapid decrease in the Rh moment that follows a P 12 dependence and corresponds to a moment of 2 μ B for isolated Rh atoms. The data indicate moment fluctuations at both the Ni and the Rh sites that are associated with local environment.

Nuclear resonance and susceptibility study of Pt-Rh alloys

Low-temperature [$T\ensuremath{\approx}(1\ensuremath{-}4)$\ifmmode^\circ\else\textdegree\fi{}K] measurements of the Knight shift and spin-lattice relaxation time for $^{103}\mathrm{Rh}$ and $^{195}\mathrm{Pt}$ in ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}$ for $0\ensuremath{\le}x\ensuremath{\le}1.0$ have been carried out using transient NMR methods. Bulk magnetic susceptibilities of these alloys as a function of temperature were also measured. The magnitude of the $^{103}\mathrm{Rh}$ Knight shift and relaxation rate increased monotonically with decreasing $x$ to the point that exchange enhancement effects dominate the NMR at $x=0.03$. In this low Rh content or impurity region the local susceptibility at the Rh site was approximately five times the Pt susceptibility. The localization is gradually increased with concentration as evidenced by the smooth monotonic increase in the ratio of $^{103}\mathrm{Rh}$ to $^{195}\mathrm{Pt}$ Knight shift as $x$ decreased. The magnitude of the $^{195}\mathrm{Pt}$ resonance shift was found to track the bulk susceptibility with both quantities exhibiting maxima near $x\ensuremath{\approx}0.2$. Here, in contrast to the Rh NMR, rigid-band considerations are qualitatively applicable in that the maxima reflect the peak in density of states of Pt known to occur just below the Fermi energy. Extended polarization of the Pt sublattice around Rh impurities probably accounts for the rigid-band nature of the Pt NMR. The experimental nuclear-nuclear exchange constants for $^{195}\mathrm{Pt}$ were approximately 4.5 kHz and very weakly concentration dependent.

NMR Studies of Pd–Rh and Ni–Rh Alloys

Low-temperature (T ≈1°–4°K) measurements of the Knight shift (K), spin-lattice relaxation time (T1), and inhomogeneous linewidth (Δ) of 103Rh in the two alloy systems Pd1−x–Rhx (0.03 ≤ x ≤ 1.00) and Ni1−x–Rhx (0.38 ≤x ≤1.00) have been carried out using transient NMR techniques. In Pd1−x–Rhx both T1T and K vary monotonically with decreasing x, K decreasing from +0.43% to −14.4% and T1T from 9.2 to 0.8 sec°K. The minimum K value agrees with the infinite dilution value K = −14.7% obtained by Rao, Matthias, and Shirley using PAC techniques, and indicates that K is essentially independent of x for Rh concentrations up to ∼5 at.%. The threefold increase in K for 103Rh as a dilute impurity in Pd over that of 105Pd in the elemental metal indicates that the local susceptibility at the Rh sites exceeds that of the Pd sites. Similar results are found in Ni1−x–Rhx with K decreasing from +0.43% to −8.1% and T1T from 9.2 to 1.2 sec°K. However, the susceptibility at the Rh sites appears to be smaller than at the Ni sites in this case. Both alloy systems display severe inhomogeneous broadening of the NMR with Δ ≈ K in some cases. This result demonstrates an extreme sensitivity of the local susceptibility to the near-neighbor environment.

The Specificity of pH and Ionic Strength Effects on the Kinetics of the Rh (D)-Anti-Rh (D) System

Summary Under physiologic conditions of pH and ionic strength, the I* anti-D appears to react with intact Rh(D) red cells and their Rh(D) stroma according to reversible, second-order kinetics. At equilibrium the influence of hydrogen ion concentration in the pH range 6.4 to 8.4 on the I* anti-D system is slight. Beyond this pH range the profound depression of the I* anti-D system is consistent with marked conformational changes in the I* anti-D molecule. The effect of lowered ionic strength on the kinetics of the nonspecific reaction between I* anti-D and rh(d) stroma appears electrostatically controlled if a first-order reaction is assumed. In contrast, the specific reaction between intact Rh(D) red cells or Rh(D) stroma appears to follow a reversible, second-order reaction when the ionic strength of the system is lowered. Consequently, the electrostatic effect of ionic strength on the I* anti-D system seems to be exerted predominantly at the specific Rh(D) site in the case of the intact Rh(D) red cell, but both specifically and nonspecifically in the case of Rh(D) stroma. These results are considered in view of the known effects of pH and ionic strength on other immunologic systems and their components.

Mössbauer Study of Hyperfine Fields and Isomer Shifts in the Fe-Rh Alloys

Hyperfine fields and isomer shifts of Fe/sup 57/ in the Fe-Rh system were investigated by the Mossbauer technique in the composition range between 0 and 52 at.% Rh. When the Rh concentration exceeds 20%, chemical ordering of CsCl type develops and two distinct hyperfine fields are observed, corresponding to Fe on Fe sites and Fe on Rh sites. For example, H/sub i/(I) = 277 kOe and H/sub i/ (II) = 384 kOe in an alloy containing 48% Rh. Neutron diffraction study of this alloy gave FeI = 3.2 /sub B/, but no direct measurement is available for FeII. It is suggested that Fe II, which is surrounded by 8 Fe nearest neighbors, may have a smalier magnetic moment than FeI despite its higher hyperfine field. The alloy with 50% Rh transforms, on cooling, from ferromagnetic to antiferromagnetic at 65 c- C. At this phase transition the hyperfine field shows a disconti nuous decrease of 14 kOe. (auth)

Hyperfine Fields and Magnetic Moments in the Fe–Rh System

Hyperfine fields and isomer shifts of Fe57 in the Fe–Rh system have been measured by the Mössbauer technique in the composition range between 0 and 52 at. % Rh. When the Rh concentration exceeds 25%, chemical ordering of CsCl type develops and two distinct hyperfine fields are observed, corresponding to Fe on Fe sites (Fe I) and Fe on Rh sites (Fe II). For example, Hi(I) = 277 kOe and Hi(II) = 384 kOe in an alloy containing 48% Rh. Neutron diffraction study of this alloy gives μFeI = 3.2 μB and μRh = 0.9 μB. It is suggested that Fe II, which is surrounded by 8 Fe nearest neighbors, may have a smaller magnetic moment than Fe I despite its higher hyperfine field. These results are consistent with a model in which the polarization of conduction electrons is negative and its magnitude is reduced when rhodium replaces iron as nearest neighbors.