The state of Rh during the partial oxidation of methane into synthesis gas

The partial oxidation of methane to synthesis gas over an - and a -supported Rh catalyst has been studied at atmospheric pressure using in situ DRIFTS between 823 and 973 K. A surface intermediate species with IR band at 2000 cm^-1, correlating with the CO formation, was observed during the partial oxidation. DRIFT spectra of adsorbed CO at 323 K were used to study the state of Rh during the partial oxidation. The state of Rh at 973 K is proposed to be a matrix of metallic rhodium with clusters of partially reduced oxide phase with isolated Rh⁺ atoms dispersed on the support. Rh oxide with Rh⁺ cations is the state of Rh during partial oxidation of methane at 823 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Immobilized Co/Rh Heterobimetallic Nanoparticle‐Catalyzed Pauson–Khand‐Type Reaction

Co/Rh heterobimetallic nanoparticles were prepared from cobalt‐rhodium carbonyl clusters [Co₂Rh₂(CO)₁₂ and Co₃Rh(CO)₁₂] and immobilized on charcoal. HR‐TEM revealed that the size of the heterobimetallic nanoparticles was ca. 2 nm and ICP‐AES analysis showed a 2 : 2 and a 3 : 1 cobalt‐rhodium stoichiometry (Co₂Rh₂ and Co₃Rh₁) in the heterobimetallic nanoparticles. The Co/Rh heterobimetallic nanoparticles immobilized on charcoal were used as a catalyst in the Pauson–Khand‐type reaction under 1 atm of CO. The catalytic reactivity was highly dependent upon the ratio of Co : Rh with the highest reactivity being observed when the ratio was 2 : 2 (Co₂Rh₂). The Co₂Rh₂ immobilized catalyst is quite an effective catalyst for intra‐ and intermolecular Pauson–Khand‐type reactions. When the immobilized Co₂Rh₂ catalyst was used as a catalyst in the Pauson–Khand‐type reaction in the presence of an aldehyde instead of carbon monoxide, the catalytic system was highly efficient. When the reaction was carried out in the presence of chiral diphosphines, ee values up to 87% were observed. The catalytic system can be reused at least five times in the presence of chiral diphosphines without loss of catalytic activity and enantioselectivity. The addition of Hg(0), a known heterogeneous catalyst poison, completely inhibits further catalysis. Thus, an environmentally friendly and sustainable process was developed.     

Electrochemically Induced Oscillations of C2H4 Oxidation Over Thin Sputtered Rh Catalyst Films

The electrochemically promoted induction of self-sustained catalytic rate and potential oscillations during C₂H₄ oxidation was studied over sputtered Rh thin (40 nm catalyst films interfaced with ZrO₂ (8 mol% Y₂O₃). The reaction rate oscillates simultaneously with the catalyst potential, and always in the opposite direction. The electrochemically induced oscillatory rate is typically 60 times larger than the open-circuit catalytic rate and 1000 times larger than the electrochemical rate of O²⁻ supply to the catalyst. The phenomenon is completely reversible and only observed under anodic polarization where the rate oscillates between the values corresponding to metallic Rh and surface Rh₂O₃. The oscillations are caused by the electrochemically controlled backspillover of O²⁻ to the catalyst surface and the concomitant, via repulsive lateral interactions, decomposition of surface rhodium oxide followed by surface reoxidation to Rh₂O₃ by gaseous O₂.     

Novel Catalytic Properties of Rh Sulfide for the Synthesis of Methanol from CO + H2 in the Presence of H2S

Rh sulfide yielded 800 gkg-cat^–1h^–1 of methanol at 593 K and 5.1 MPa from CO + H₂ (syngas) even in the presence of H₂S 100 ppm in concentration. The obtained space-time yield of methanol was comparable with that obtained with a commercial Cu/Zn/Al catalyst at a conventional reaction condition (523 K and 5.1 MPa) from a feed containing both syngas and CO₂.     

Comparative Study on the Mechanisms of Partial Oxidation of Methane to Syngas Over Rhodium Supported on SiO2 and γ-Al2O3

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO₂- and -Al₂O₃-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the in situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH₄ to syngas is the main pathway of the POM reaction over Rh/SiO₂ catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/-Al₂O₃ catalysts. The results of TPR characterization indicate that Rh supported on -Al₂O₃ is more difficult to reduce than Rh supported on SiO₂. The IR experiments of CO adsorption over Rh/SiO₂ and Rh/-Al₂O₃ after the POM reaction reveal that the surface of the Rh/-Al₂O₃ catalyst contains more partially oxidized rhodium (Rh⁺) species as compared to the Rh/SiO₂ catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO₂ and Rh/-Al₂O₃ catalysts can be related to the difference in the surface concentration of O^2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.     

Simulation of kinetics of nitrogen desorption from Rh(111)

Associative desorption of N atoms from the Rh(111) surface is simulated in the framework of the lattice-gas model. The Arrhenius parameters and nearest-neighbour lateral interaction employed to describe the measured thermal desorption spectra are as follows:v=10¹³ s^–1,E _d=40 kcal/mol, and ₁=1.7 kcal/mol. The results obtained are used to clarify the role of nitrogen desorption in the NO + CO reaction on Rh(111) atT=400–700 K andP _NOP _CO0.01 atm.     

EXAFS investigations of lanthana-promoted Rh/SiO2 catalysts

EXAFS investigations at the Rh K edge of lanthana-promoted Rh/SiO₂ catalysts showed that the local environment of the Rh ions in the oxidic catalyst precursor state did not depend on the La₂O₃ content and resembled that of Rh₂O₃. No LaRhO₃ formation could be detected. In the reduced state, EXAFS as well as H₂ and CO chemisorption demonstrated that La₂O₃ increased the Rh dispersion. Covering of the Rh metal particles by La₂O₃ was minor, because during catalyst preparation, La was impregnated prior to Rh.     

M2YSi (M=Rh,Ir): Theoretically predicted damage‐tolerant MAX phase‐like layered silicides

Searching for layered MAX phase‐like materials with properties of both ceramics and metals is a topic in its infancy. Herein, through a combination of crystal structure, electronic structure, chemical bonding, and elastic property investigations, we report two MAX phase‐like layered materials Rh₂YSi and Ir₂YSi. Rh₂YSi and Ir₂YSi have bulk modulus B of 150 and 185 GPa, respectively, which are comparable to the typical MAX phases like Ti₂AlC, Ti₃AlC₂, and Ti₃SiC₂, but much lower shear modulus G (82 and 97 GPa for Rh₂YSi and Ir₂YSi, respectively) than MAX phases. The high stiffness is due to the presence of rigid Si2–M–Si3–M (M = Ir, Rh) units, while the low shear deformation resistance is due to the presence of metallic bonds and the weak bonds that link the rigid Si2–M–Si3–M (M = Ir, Rh) units. Based on the low shear deformation resistance and low Pugh's ratio, Rh₂YSi and Ir₂YSi are predicted as damage‐tolerant silicides and promising water vapor‐resistant interphase materials for SiC_f/SiC composites if yttria or yttrium silicates are formed to protect the SiC fibers in oxygen containing environments. The possible slip systems are {0001} <> and {} <0001> for both Rh₂YSi and Ir₂YSi.     

FTIR study of the rearrangement of adsorbed CO species on Al2O3-supported rhodium catalysts

The adsorption of CO at low temperatures (130–293 K) has been investigated on Rh/Al₂O₃ catalysts of low (0.001–1 wt%) Rh loadings by means of Fourier transform infrared spectroscopy. The surface structure of Rh produced at different reduction temperatures (573 and 1173 K) was shock-cooled to 130 K, where the addition of CO caused the appearance of the band due to bridge-bonded CO ((Rh⁰)₂–CO) on all samples. The appearance of the bands due to gem-dicarbonyl (Rh⁺(CO)₂) and linearly bonded CO (Rh_x–CO) depended on the Rh content and the reduction temperature of the catalysts. The positions and the integrated absorbances of the symmetric and asymmetric stretchings of the Rh⁺(CO)₂ changed with temperature. On the basis of the above findings the rearrangement of the adsorbed CO species (indirectly that of surface Rh) is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reactions of methylcyclopentane on Rh–Pt catalyst prepared by underpotential deposition of Rh on Pt/SiO2

Rh was deposited on to a well-characterized 3.1% Pt/SiO₂ (InCat-1) parent catalyst by underpotential deposition method to obtain a model Rh–Pt bimetallic catalyst. TEM and EDS was used to determine its mean particle size and bulk composition: the particles of ca. 3 nm contained ca. 60% Pt and 40% Rh. The Rh–Pt catalyst was tested in methylcyclopentane (MCP) reaction between 513 K and 603 K and 60–480 Torr H₂ pressure (with 10 Torr MCP). The parent Pt/SiO₂ as well as a 5% Rh/SiO₂ catalyst were also studied for comparison. Four subsequent treatments with O₂ and H₂ up to T = 673 K were applied on the bimetallic catalyst before the catalytic runs. The overall activity showed positive hydrogen order on all samples, bimetallic Rh–Pt resulting in the lowest TOF values. Ring opening and hydrogenolysis products, as well as unsaturated hydrocarbons were formed from MCP. The selectivity of ring opening products and fragments over Rh–Pt catalyst was between the values observed on Pt and Rh, while the selectivity towards benzene formation was highest on the bimetallic sample, especially at higher temperatures. “Selective” ring opening occurred on all samples, resulting mostly in 2 and 3-methylpentane and less hexane. Different pretreatments with H₂ and O₂ affected slightly the dispersion values and the catalytic behavior of Rh–Pt sample. The selectivities of the Rh–Pt catalyst being between the values observed for Pt/SiO₂ and Rh/SiO₂ indicates that the sample studied represented a real bimetallic catalyst, resembling both components and exhibiting at the same time, new properties in addition to those, characteristic of Pt or Rh. Dedicated to Konrad Hayek.     

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

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-Al₂O₃, ZrO₂, MgO and La₂O₃). Prepared systems were tested in the methane catalytic partial oxidation (CH₄-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh₄(CO)₁₂.Catalysts supported on Al₂O₃, ZrO₂ 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 H₂ 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.     

Redox chemistry of highly dispersed rhodium in zeolite NaY

NaY zeolite exchanged with [Rh(NH₃)₅Cl]²⁺ ions have been studied using temperature programmed oxidation (TPO), temperature programmed reduction (TPR), and Fourier transformed infrared spectroscopy. The TPO profiles show that ammine ligands in NaY encaged [Rh(NH₃)₅Cl]²⁺ are destroyed above 300 °C, whereas the Rh precursor ion remains intact after calcination at 200 °C. TPR profiles in conjunction with the CO_ads IR spectra show that the reducibility of Rh by H₂ is largely controlled by the concentration of the surface protons, i.e. Rh³⁺+H₂Rh⁺+2H⁺ Rh⁺ + 1/2H₂Rh⁰+H⁺ In the presence of ammonia, the protons are neutralized and Rh³⁺ is reduced to Rh⁰. However, reduction remains incomplete when the concentration of protons is high. The ammonia was provided either by NH₃ admission or by conservation of ammine ligands by controlled calcination. CO adsorption does not lead to reoxidation of Rh⁰ particles to Rh⁺ ions.     

Structure and catalytic reactivity of Rh oxides

Using a combination of experimental and theoretical techniques, we show that a thin RhO₂ surface oxide film forms prior to the bulk Rh₂O₃ corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO₂ surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh₂O₃ corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(1 1 1), Rh(1 0 0) and Pt₂₅Rh₇₅(1 0 0) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO₂ production coincides with the formation of the thin RhO₂ surface oxide film. In the case of Pt₂₅Rh₇₅(1 0 0), our measurements demonstrate that the formation of bulk Rh₂O₃ corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase.     

Synthesis gas formation by direct oxidation of methane over Rh monoliths

The production of H₂ and CO by catalytic partial oxidation of CH₄ in air or O₂ at atmospheric pressure has been examined over Rh-coated monoliths at residence times between 10^–4 and 10^–2 s and compared to previously reported results for Pt-coated monoliths. Using O₂, selectivities for H₂ ( ) as high as 90% and CO selectivities (S _CO) of 96% can be obtained with Rh catalysts. With room temperature feeds using air, Rh catalysts give of about 70% compared to only about 40% for Pt catalysts. The optimal selectivities for either Pt or Rh can be improved by increasing the adiabatic reaction temperature by preheating the reactant gases or using O₂ instead of air. The superiority of Rh over Pt for H₂ generation can be explained by a methane pyrolysis surface reaction mechanism of oxidation at high temperatures on these noble metals. Because of the higher activation energy for OH formation on Rh (20 kcal/mol) than on Pt (2.5 kcal/mol), H adatoms are more likely to combine and desorb as H₂ than on Pt, on which the O+ H OH reaction is much faster.This research was partially supported by DOE under Grant No. DE-FG02-88ER13878-AO2.     

Recyclable plant tannin‐chelated Rh(III) complex catalysts for aqueous–organic biphasic hydrogenation of quinoline

BACKGROUND: Synthetic ligands have conventionally been used for the preparation of homogenous Rh complex catalyst but biomass has rarely been utilized for this purpose. In the present investigation, plant tannins (natural polyphenols) were used as water‐soluble ligands for the preparation of homogenous Rh³⁺ complex catalysts. RESULTS: Based on X‐ray photoelectron spectroscopy (XPS) and proton nuclear magnetic resonance (HNMR) analyses, the preparation mechanism of these complex catalysts was proven to involve chelating interactions between Rh³⁺ and the adjacent phenolic hydroxyls of plant tannins. As a result, the use of plant tannin promoted aqueous‐organic biphasic interactions and the plant tannin‐chelated Rh³⁺ complex catalysts exhibited much higher catalytic activity than commercial Rh complex catalysts in the biphasic hydrogenation of quinoline. Furthermore, the plant tannin‐chelated Rh³⁺ complex can be reused three times without significant loss of catalytic activity CONCLUSION: Our experimental results suggested that black wattle tannin (BWT) can be used as water‐soluble ligands for the preparation of highly active and recyclable Rh³⁺ complex catalysts. Copyright © 2012 Society of Chemical Industry     

Formation of the rhodium oxides Rh2O3 and RhO2 in Rh/NaY

The oxidation states of Rh in NaY supported catalysts have been studied by temperature programmed reduction (TPR). After calcination of the exchanged catalyst to 380°C, both RhO₂ and Rh₂O₃ are identified, besides small amounts of RhO⁺ and Rh³⁺. Quantitative reduction is possible for samples calcined at temperatures not exceeding 500°C. Re-oxidation of the reduced samples leads to formation of RhO₂ and Rh₂O₃, with negligible protonolysis to Rh³⁺. The dioxide prevails after re-oxidation at 320°C, but the sesquioxide after oxidation at 500°C. In the temperature regime where both oxides coexist the reduction of NO with propane is catalyzed even at an O₂/C₃H₈ ratio of 10. Total oxidation of propane reaches 80% at 350°C.     

A Fixed-Bed Reactor Study of Catalytic Partial Oxidation over Rh/Al2O3: An Indication of a Direct Pathway to CO

Catalytic partial oxidation (CPO) of CH₄ in air was investigated over Rh/Al₂O₃ catalysts (0.01, 0.05, 0.1 and 1 wt% Rh⁰) in co-feed modus in laboratory scale fixed-bed reactors. Main focus was on catalyst stability and selectivity at low temperatures (<700 °C). A particularly high selectivity to CO was observed, indicating existence of a direct pathway.     

Small Amounts of Rh-Promoted Ni Catalysts for Methane Reforming with CO2

Small amounts of Rh-promoted Ni/-Al₂O₃ catalysts possessed higher activity than pure Ni/-Al₂O₃, Rh-Al₂O₃ catalysts and exhibited excellent coke resistance ability in methane reforming with CO₂. XRD, H₂-TPR, CO₂-TPD and coking reaction (via CH₄ temperature-programmed decomposition) indicated that Rh improved the dispersion of Ni, retarded the sintering of Ni and increased the activation of CO₂ and CH₄ on the surface of catalyst.     

Acetate formation and explosive decomposition during ethanol oxidation on Rh

The adsorption and reaction of ethanol with the Rh(110) surface has been studied using a thermal molecular beam system and temperature programmed desorption. On the clean surface, ethanol shows a very simple dehydrogenation, producing hydrogen in the gas phase, adsorbed CO (which is desorbed by heating to 550 K) and carbon. Since in alcohol synthesis reactions it is likely that the surface will be partially oxidised, the reaction with predosed oxygen was also investigated. The reaction pathway then becomes much more complex. The main changes are (i) CH₄ and H₂O evolution during adsorption, and (ii)Acetate formation by oxygen insertion in the molecule. The acetate shows very unusual decomposition kinetics — a surface explosion with a very narrow peak-yielding CO₂ and H₂ in the gas phase and adsorbed C. The acetate is always seen on Rh catalysts which are selective for alcohol synthesis from CO and H₂, and it is proposed that oxidic promoters such as vanadia may act to stabilise this intermediate.     

Catalysis of the oxidation of 1,4-cyclohexadiene to benzene by electroactve binuclear rhodium complexes

Electrochemical oxidation of Rh₂(TMB) ₄ ²⁺ (TMB = 2,5-diisocyano-2,5-dimethylhexane) in the presence of 1,4-cyclohexadiene produces benzene and two protons. It is probable that a key step in the reaction is hydrogen-atom transfer from 1,4-cyclohexadiene to electrochemically generated Rh₂(TMB) ₄ ³⁺ . The H-atom abstraction apparently is facilitated by the presence of a d* hole in the Rh ₂ ³⁺ ₂ complex.