Rh Metal Particles Research Articles

Structure-dependent activity and stability of Al2O3 supported Rh catalysts for the steam reforming of n-dodecane

Steam reforming of liquid hydrocarbon fuels is an appealing way for the production of hydrogen. In this work, the Rh/Al2O3 catalysts with nanorod (NR), nanofiber (NF) and sponge-shaped (SP) alumina supports were successfully designed for the steam reforming of n-dodecane as a surrogate compound for diesel/jet fuels. The catalysts before and after reaction were well characterized by using ICP, XRD, N2 adsorption, TEM, HAADF-STEM, H2-TPR, CO chemisorption, NH3-TPD, CO2-TPD, XPS, Al27 NMR and TG. The results confirmed that the dispersion and surface structure of Rh species is quite dependent on the enclosed various morphologies. Rh/Al2O3-NR possesses highly dispersed, uniform and accessible Rh particles with the highest percentage of surface electron deficient Rh0 active species, which due to the unique properties of Al2O3 nanorod including high crystallinity, relatively large alumina particle size, thermal stability, and large pore volume and size. As a consequent, Rh/Al2O3-NR catalyst exhibited superior catalytic activity towards steam reforming reactions and hydrogen production rate over other two catalysts. Especially, Rh/Al2O3-NR catalyst showed the highest hydrogen production rate of 87,600 mmol gfuel−1 gRh−1min−1 among any Rh-based catalysts and other noble metal-based catalysts to date. After long-term reaction, a significant deactivation occurred on Rh/Al2O3–NF and Rh/Al2O3-SP catalysts, due to aggregation and sintering of Rh metal particles, coke deposition and poor hydrothermal stability of nanofibrous structure. In contrast, the Rh/Al2O3-NR catalyst shows excellent reforming stability with negligible coke formation. No significantly sintering and aggregation of the Rh particles is observed after long-term reaction. Such great catalyst stability can be explained by the role of hydrothermal stable nanorod alumina support, which not only provides a unique environment for the stabilization of uniform and small-size Rh particles but also affords strong surface basic sites.

Adsorption of Keggin-Type Polyoxometalates on Rh Metal Particles under Reductive Conditions.

The adsorption of POMs on Rh/SiO2 in water solvent under strongly reductive conditions was investigated. Aqueous solutions of α-Keggin type silicotungstate and silicovanadotungstates were mixed with Rh/SiO2 at 393-473 K under 1 MPa of H2. Monovanadium-substituted silicotungstate, α-SiVW11O405- (SiVW11), was more readily adsorbed than nonsubstituted silicotungstate, α-SiW12O404- (SiW12). After adsorption at 433 K, SiVW11 was desorbed from Rh/SiO2 by oxidation with Br2 water without change of the Keggin structure, as evidenced by 51V NMR. Trivanadium-substituted silicotungstate, α-1,2,3-SiV3W9O407-, was not stable, and the desorbed species from Rh/SiO2 by oxidation with Br2 did not maintain the Keggin structure. The very high temperature for adsorption (473 K) also led to the decomposition of the Keggin structure of SiVW11. An increase in the concentration of SiVW11 in the liquid phase gave a saturation of the amount of desorbable SiVW11, up to five SiVW11 anions per one Rh particle with a 3 nm size. The elemental analysis and W L3-edge extended X-ray absorption fine structure of Rh/SiO2 after the adsorption of SiVW11 showed that a part of SiVW11 was decomposed and irreversibly adsorbed as metallic W species incorporated into the surface of Rh metal particles. The amount of decomposed SiVW11 was almost the same as that of SiVW11 adsorbed as the original Keggin structure. The desorbable SiVW11 was probably bonded on the W atom incorporated on the Rh metal particles as the two-electron-reduced form (α-SiVIIIW11O407-).

Detailed Characterization of MoOx-Modified Rh Metal Particles by Ambient-Pressure XPS and DFT Calculations

Characterization of Mo species on Rh metal particles in Rh–MoOx/C catalyst for C–O hydrogenolysis reactions was carried out with in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) a...

Hydrogenolysis of tetrahydrofuran-2-carboxylic acid over tungsten-modified rhodium catalyst

Catalysts for reduction of tetrahydrofuran-2-carboxylic acid (THFCA), which can be synthesized from furfural via oxidation and hydrogenation, were explored among the combinations of noble metal and reducible metal oxide supported on SiO2. Rh-WOx/SiO2 catalysts showed activity in C-O hydrogenolysis at 2-position of THFCA (to δ-valerolactone and 5-hydroxyvaleric acid) and higher yield ratio of these C-O hydrogenolysis products to carboxylic acid hydrogenation products than other bimetallic catalysts. The activity of Rh-WOx/SiO2 catalysts was highest at W/Rh = 0.25 mol/mol. XRD, TPR, CO adsorption and XAFS characterizations showed that the Rh-WOx/SiO2 (W/Rh = 0.25) catalyst contained Rh metal particles with surface modification with isolated W2+ oxide species. The mechanism that hydride-like species formed on Rh atom attacks the C atom at the α-position (2-position) of adsorbed carboxylate on W atom is proposed based on the similar kinetics and similar catalyst structure to Rh-MOx/SiO2 (M = Re, Mo) which is known to be active in THFA hydrogenolysis to 1,5-pentanediol.

Structure of catalytically active Rh–In bimetallic phase for amination of alcohols

The structure of Rh–In bimetallic catalysts supported on carbon for amination of alcohols was determined by XRD, TEM-EDX, XPS, CO adsorption and EXAFS. At low In/Rh ratio (In/Rh ≤ 0.2), Rh metal particles with sizes of 1) on the high Rh loading (20 wt%) catalyst, the cubic RhIn phase with a CsCl structure was observed instead of the tetragonal RhIn phase, and the catalytic activity was much decreased.

Stable Low-Valence ReOx Cluster Attached on Rh Metal Particles Formed by Hydrogen Reduction and Its Formation Mechanism

The structural change of ReOx/SiO2, Rh/SiO2, and Rh–ReOx/SiO2 during the temperature programmed reduction with H2 was investigated using in situ Re L3-edge and Rh K-edge quick-scanning X-ray absorption fine structure. Monometallic ReOx/SiO2 was reduced at about 600 K to form Re metal particles covered with partially oxidized Re species. In contrast, in the case of Rh–ReOx/SiO2 (Re/Rh = 0.5), the reduction of Rh proceeded to give highly dispersed Rh metal particles at first (∼325 K), and then Re was reduced mildly (∼365 K). The reduced Re species interacted with the Rh metal surface and were highly dispersed. At higher reduction temperature (∼595 K), these Re-modified Rh metal particles aggregated and further reduction of Re to low-valence state proceeded on the Rh metal surface. The low-valence Re interacting with Rh metal surface comprises two-dimensional Re oxide clusters. No detection of Re metal or Rh–Re alloy formation even after reduction at high temperature (595 K) suggests that the two-dimensional ReOx clusters on the Rh metal surface are highly stable.

Comparative study of Rh–MoOx and Rh–ReOx supported on SiO2 for the hydrogenolysis of ethers and polyols

Rh–MOx/SiO2 (M=Mo or Re) catalysts shows the high activity of the hydrogenolysis of C–O bond in the substrates with CH2OH group such as tetrahydrofurfuryl alcohol, various diols and their ethers. Characterization results suggest the catalyst structure were isolated MoOx species and small ReOx cluster with low valence attached on the surface of Rh metal particles. It is characteristic that the reactivity of the C–O bond in the –O–C–C–CH2OH is higher than that in the –O–C–CH2OH over Rh–MoOx/SiO2, and Rh–ReOx/SiO2 has the opposite tendency to Rh–MoOx/SiO2. The difference in the regioselectivity of the hydrogenolysis over Rh–MoOx/SiO2 and Rh–ReOx/SiO2 can be related to the different structure of isolated MoOx species and small ReOx clusters.

Supported Rhodium Oxide Nanoparticles as Highly Active CO Oxidation Catalysts

Bigger is not always better: Rhodium metal particles smaller than 2.5 nm are oxidized and stabilized by reducible supports such as ceria under CO oxidation conditions, whereas metal particles larger than 4 nm remain metallic. The very small Rh oxide particles are more active by two orders of magnitude in CO oxidation than Rh metal particles (see picture; TOF=turnover frequency).

Modification of Metal Nanoparticles with TiO2 and Metal−Support Interaction in Photodeposition

Rh, Pt, and Au metals photodeposited on TiO2 with an anatase phase (M−TiO2(p), M: Rh, Pt, Au) were studied using transmission electron microscopy (TEM), scanning transmission electron microscopy, energy dispersive X-ray (EDX) analysis, X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and XPS depth profile analysis. The TEM images of Rh−TiO2(p) and Pt−TiO2(p) did not clearly show the grain boundary of metal particles, although the Rh and Pt species of Rh−TiO2(p) and Pt−TiO2(p) were observed by EDX analysis to show the metal particles 10−20 nm in size. These results suggest that the photodeposited Rh and Pt metal particles have a disorganized structure, which is also supported by extended XAFS spectral analysis. It is speculated that a strong metal−support interaction takes place in the case of Rh and Pt, resulting in the disorganized structure. In the case of Au, however, metal particles with a well-organized and spherical structure were observed, indicating that...

Investigation of the Formation Process of Photodeposited Rh Nanoparticles on TiO2 by In Situ Time-Resolved Energy-Dispersive XAFS Analysis

The photodeposition process of Rh metal nanoparticles on a TiO(2) photocatalyst from RhCl(3) aqueous solution in the presence of methanol as a sacrificial oxidant, which consists of the direct reduction of Rh(3+) ions to Rh metal and the formation of Rh nanoparticles, was uncovered by in situ time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) analysis in a liquid-solid suspension state. The fractions of Rh metal particles and Rh(3+) precursor were estimated by the least-squares fitting of each X-ray absorption near-edge structure (XANES) spectrum by a linear combination of authentic spectra corresponding to Rh(0) and Rh(3+). The fraction of Rh metal linearly increased with photoirradiation time and saturated after 90 min of photoirradiation. The coordination number (Rh-Rh pair) was evaluated by the curve fitting of the Rh-Rh scattering at 2.45 A in the Fourier transforms (FT) of extended XAFS (EXAFS) spectra. The coordination number linearly increased with photoirradiation time and attained a constant value of 10 after 90 min of photoirradiation. This value is lower than that for the Rh foil (12). These suggest the formation of fine Rh metal nanoparticles on TiO(2). In addition, the diminution rate of Rh(3+) as determined by ICP analysis was in good agreement with the increased rates for the fraction of Rh metal particles estimated by XANES spectra and the coordination number (Rh-Rh pair) evaluated by FT-EXAFS spectra. This result strongly supports the fact that electrons generated by charge separation reduce the Rh(3+) precursor to an Rh metal particle at a moment in time and at a constant rate. The Rh particles do not grow in incremental steps, but Rh particles with a uniform size appear one after another on the surface.

Hydrogenolysis of 1,2‐Propanediol for the Production of Biopropanols from Glycerol

Production of propanols from glycerol, which are known as biopropanols, requires catalysts for the hydrogenolysis of 1,2-propanediol, which has been easily derived from glycerol. It is found that the Rh/SiO(2) catalysts modified with ReO(x) species exhibited high activity and selectivity in the hydrogenolysis of 1,2-propanediol to propanols with low selectivity to degradation products and high stability. The optimized Rh--ReO(x)/SiO(2) (Re/Rh=0.5) catalyst gave high yields of 1-propanol (66 %) and propanols (1-propanol +2-propanol) (85 %) in the hydrogenolysis of 1,2-propanediol. In addition, the catalyst was applicable to the one-pot conversion of glycerol to propanols. The structure of Rh metal particles with attached ReO(x) clusters is suggested from the catalyst characterization. It is proposed that 1,2-propanediol hydrogenolysis proceeds by the hydrogenolysis of the alkoxide species on Re with hydrogen species on the Rh metal surface.

Chemoselective Hydrogenolysis of Tetrahydropyran‐2‐methanol to 1,6‐Hexanediol over Rhenium‐Modified Carbon‐Supported Rhodium Catalysts

AbstractModification of Rh/C with Re species enabled chemoselective hydrogenolysis of tetrahydropyran‐2‐methanol to 1,6‐hexanediol. In particular, the Rh–ReOx/C (Re/Rh ratio=0.25) catalyst gave a high yield of 1,6‐hexanediol (84 %) and showed a very low activity for overhydrogenolysis to 1‐hexanol. Using the same catalyst in the hydrogenolysis of tetrahydrofurfuryl alcohol also gave a high yield of 1,5‐pentanediol (94 %). Characterization results indicate the formation of ReOx clusters attached on the surface of Rh metal particles. This can cause the synergy between ReOx and Rh, and the tetrahydropyran‐2‐methanol hydrogenolysis proceeds on the interface between Rh metal surface and attached ReOx species.

Modification of Rh/SiO2 catalyst for the hydrogenolysis of glycerol in water

Addition of Re, Mo and W to Rh/SiO2 enhanced the catalytic activity of the glycerol hydrogenolysis using water as a solvent. The modification with Re gave the highest conversion and yield of 1,3-propanediol (1,3-PrD). The optimized Rh-ReOx/SiO2 (Re/Rh=0.5) catalyst maintained high selectivity to propanediols and suppressed C–C bond breaking even under low H2 pressure and high reaction temperature, where Rh/SiO2 is rather active to C–C bond breaking. Characterization results indicate the formation of ReOx clusters attached to the surface of Rh metal particles. This can cause the synergy between ReOx and Rh, and the glycerol hydrogenolysis proceeds on the interface between Rh metal surface and attached ReOx species.

Promoting effect of Mo on the hydrogenolysis of tetrahydrofurfuryl alcohol to 1,5-pentanediol over Rh/SiO 2

Addition of Mo to Rh/SiO 2 promoted the hydrogenolysis of tetrahydrofurfuryl alcohol to 1,5-pentanediol drastically. Addition of Re and W to Rh/SiO 2 was also effective, but the promoting effect of Mo was more remarkable in terms of catalyst stability and high activity even at low reactant concentrations. Characterization of Rh–MoO x /SiO 2 suggests that a partially reduced Mo species is attached to the surface of Rh metal particles. The synergy between the MoO x and the Rh metal surface results in a high chemoselectivity and activity in the hydrogenolysis of tetrahydrofurfuryl alcohol.

Fundamental Investigation of Oxygen Reduction Reaction on Rhodium Sulfide-Based Chalcogenides

Synchrotron-based X-ray absorption spectroscopy (XAS), including the surface-specific ΔXANES technique, is used to investigate the active reaction site for water activation and the oxygen reduction reaction (ORR) on the novel, mixed-phase chalcogenide electrocatalyst RhxSy/C (De Nora). The specific adsorption of water, OH, and O as a function of overpotential is reported. This study builds on a prior communication based solely on interpreting the XAS spectra of RhxSy with respect to the metallic Rh3S4 phase. Here, a more extensive overview of the electrocatalysis is provided on RhxSy/C, the thermally grown Rh2S3/C and Rh3S4/C preferential phases and a standard 30 wt % Rh/C electrocatalyst, including results obtained by X-ray diffraction (XRD), XAS, high-resolution transmission electron imaging, microanalysis, and electrochemical investigations. Heating of the RhxSy catalysts to prepare the two preferential phases causes Rh segregation and the formation of Rh metal particles, and immersion in TFMSA causes S dissolution and the formation of a Rh skin on the RhxSy samples. It is shown that some Rh−Rh interactions are needed to carry out the ORR. This is present on the Rh6 moieties in both the Rh3S4 and RhxSy catalysts, but a partial Rh skin (present from acid dissolution) is also contributing to the ORR observed on RhxSy. This to our knowledge is the first time a reaction site in a multiphase inorganic framework structure has been investigated in terms of electrocatalytic pathway for oxygen reduction.

Supported Rh catalysts for methane partial oxidation prepared by OM-CVD of Rh(acac)(CO)2

Abstract The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO) 2 as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al 2 O 3 , ZrO 2 , MgO and La 2 O 3 ). Prepared systems were tested in the methane catalytic partial oxidation (CH 4 -CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh 4 (CO) 12 . Catalysts supported on Al 2 O 3 , ZrO 2 and MgO show better or comparable performances with respect to the reference system. Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data. Supported Rh phase was characterized by CO and H 2 chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size. Catalytic performances and Rh species balance were found to be dependent on the support material.

In Situ Time-Resolved Energy-Dispersive XAFS Study on Photodeposition of Rh Particles on a TiO2 Photocatalyst

It is the first report to observe the real-time behavior of photodeposition and the continuous structural change of Rh metal particles anchored on a TiO2 photocatalyst from an aqueous solution of Rh trivalent ions in the liquid—solid suspension by an in situ time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) analysis. The DXAFS analysis uncovered that the Rh trivalent ions are adsorbed on the surface of the TiO2 photocatalyst, followed by reducing to the Rh metal particles by acceptance of electrons because the coordination number of a Rh—Rh bond evaluated by the Fourier transforms of EXAFS spectra and the amount of the Rh trivalent ions determined by the ICP analysis linearly increased and decreased with elongation of photoirradiation time, respectively. The generation rates of the Rh metal particles and the diminution rates of the Rh trivalent ions depended on the type of sacrificial reagents and tends to become slower in the order corresponding to methanol > ethanol ≫ 1-propanol >2...

Unique adsorption and catalytic behavior of H2 and CO over hollow Silica-Rh, -Ir, and -Ru nanocomposites prepared by Reversed Micelle Technique

Hollow silica nano spheres containing Rh, Ir or Ru metal particles were synthesized by Rh(NH3)6Cl3 aq, Ir(NH3)3Cl3 aq or Ru(NH3)6Cl3 aq/NP-6/cyclohexane reversed micelle system. Hydrolysis of TEOS surrounding metal ammine complex crystals inside the micelle caused the formation of the hollow, which contained small metal particles inside and tiny metal clusters in the silica network. The amounts of H2 adsorption over Rh and Ir nanocomposites were two to three times more in the cases of hollow-SiO2 catalysts compared with those of non-hollow ones, suggesting the occlusion of hydrogen inside the hollows of Rh–SiO2 or Ir–SiO2. CO molecules could also permeate into the silica wall and be adsorbed on the metal clusters in the silica wall after 573 K pretreatment. Especially in the case of Ru nanocomposite the amount of adsorbed CO was much more than that of H2, suggesting some unique character of Ru metal nanoparticles. After 773 K pretreatment, however, the amount of CO(a) decreased drastically to less than 1/10 of H(a), indicating the densification of Si–O–Si bonds and the formation of ultra-micropores in the silica wall where only H2 can selectively permeate. Selective formation of methane was observed in the CO–H2 reaction over these nanocomposite catalysts, provably because of the higher concentration of hydrogen inside the hollow and silica network.

TWC deactivation by lead: A study of the Rh/CeO2 system

The effect of the incorporation of low amounts of Pb to a Rh/CeO2 catalyst on the typical reactions involved in Three Way Catalysts converters was investigated. The catalytic tests showed no deactivation for CO oxidation and NO reduction whereas for propylene oxidation the deactivation was evident and more intense with the increasing of Pb concentration. The interpretation of this behaviour is discussed on the basis of HREM combined with XEDS analysis, TPR and DRIFT spectroscopy of chemisorbed CO studies. The results provided clear evidence that Pb is not covering the Rh particles neither forming a new phase with Rh nor altering the dispersion of the Rh particles. They also suggest that a mixed oxide is formed in which lead cations are dissolved within the fluorite-like ceria structure. Thus, CeO2 support may act as sink of Pb preventing the formation of a Rh–Pb compound that explains why the CO oxidation and the NO reduction were only slightly affected. However, an electronic interaction between the Rh particles and the Pb-containing CeO2 support is evidenced by TPR and DRIFTS-CO chemisorption data: the Pb present in the Rh-CeO2 interphase as a result of the Pb dissolution in the ceria framework is proposed to be responsible for the electronic donation to Rh metal particles. The possibility that the electronic interaction and/or the modification of the oxygen storage capacity (OSC) of the ceria, both caused by the Pb, bring about the deactivation in the oxidation of propylene is also discussed.

The PPh 3 ligand modified Rh/SiO 2 catalyst for hydroformylation of olefins

A novel family of hybrid catalyst with high turnover frequency (TOF) and high selectivity towards aldehydes for hydroformylation of olefins could be successfully approached through direct coordination with the PPh 3 ligand to the highly dispersed Rh metal particle precursors. A further advantage is that the catalyst is easily prepared and recyclable. The results revealed that hydroformylation of olefins to aldehydes dominantly took place on the surface of PPh 3 ligand modified Rh metal particles of the hybrid catalyst. It was found that the formation of chemical coordination bond between the Rh metal particles and the lone-pair electron of PPh 3 was evident through the TG and 31P NMR measurement. Moreover, the addition of PPh 3 onto the Rh/SiO 2 exert a significant influence on the adsorption state of reactant CO, H 2 and C 2H 4 on the PPh 3–Rh/SiO 2 sample, which probably lead to good catalytic performances for hydroformylation of olefins.