Rhodium Loading Research Articles

The acetyl CoA synthase paradigm for hybrid bio-organometallics: Quantitative measures for resin-bound Ni–Rh complexes

The active site of Acetyl CoA Synthase utilizes a square planar NiN 2S 2 complex in the form of Ni II(CGC) 2− (CGC = the cysteine–glycine–cysteine tripeptide motif within the protein) to serve as a bidentate sulfur-donor ligand to chelate a second, catalytically active Ni atom responsible for the C–C and C–S coupling reactions for the production of Acetyl CoA. Metalloenzymes, such as this, which house stable catalytic complexes within intricately designed pockets accessible by solvent channels, have inspired design of resin-bound complexes. Through the use of TentaGel S-RAM ® resin beads, the O-Ni(CGC) 2− ligand has been synthesized and derivatized with the Rh I(CO) 2 moiety. The identification of the O - Ni(CGC)Rh(CO) 2 1 - adduct on these resin beads is afforded by attenuated total reflectance FTIR spectroscopy in the ν(CO) region and compared to solution analogues. The goal of this study is to establish a quantitative measure of the loading of nickel and rhodium on the tripeptide modified resin beads, O-(CGC). The extent of CGC derivatization was determined by Fmoc cleavage of the Fmoc protected O-(CGC). Nickel and rhodium loading were determined by Neutron Activation Analysis. This work provides evidence that the TentaGel S-RAM ® resin beads greatly decrease the air sensitivity of the Ni–Rh complex as compared to the unsupported solution phase analogue. The derivatized beads have also been studied for their ability to withstand a number of physical stresses, i.e., for leaching.

Partial oxidation of n-hexadecane at short contact times: Catalyst and washcoat loading and catalyst morphology

Rhodium supported on alumina foam is an exceptionally active catalytic partial oxidation catalyst, giving high selectivities to syngas or olefins by changing reaction stoichiometry or catalyst properties such as rhodium loading and washcoat loading. Rhodium catalyst loading and γ-alumina washcoat loading were varied systematically on α-alumina foams to investigate resulting microstructure and products of the partial oxidation of n-hexadecane. At conditions favorable for producing syngas, varying rhodium loading between 0.05 and 10.0 wt.% (a factor of 200) had little affect on H 2 and CO selectivities or fuel conversions. However, non-washcoated catalysts gave low selectivities to syngas. Increasing washcoat loading results in high selectivities of olefins, while varying rhodium loading has little affect on production of olefins. Selectivitiy data from multi-metallic catalysts, Rh–Ce and Rh–Pt, was also investigated. The morphology of catalysts was examined using scanning electron microscopy. The micrographs indicate that increasing rhodium loading above a few percent led to the formation of a rhodium film on the catalyst support. The presence of washcoat seems to keep rhodium spread on the surface at high reaction temperatures enhancing heterogeneous products. Very high washcoat loadings (10 wt.%) decrease pore diameters, increasing the likelihood of reactant contact with the rhodium surface, leading to increased H 2 and decreased olefin selectivities.

Effects of rhodium and platinum on the reactivity of lanthanum phases

Rhodium and platinum catalysts were prepared by wet impregnation of Anedra lanthanum oxide. The polymorph forms of La 2O 2CO 3 have also been prepared by an appropriate treatment of lanthanum compounds in carbon dioxide. The characterisation of the phases present in these solids was performed by XRD, FT-IR, LRS, TPD and TGA. One of the untreated supports (Aldrich) was La(OH) 3, and the other (Anedra) consisted mainly of II-La 2O 2CO 3 with traces of type Ia oxycarbonate and lanthanum hydroxide. Catalysts with a low rhodium loading showed an increase of Ia-La 2O 2CO 3 already present on the support, whereas those with a low platinum loading would not modify the phases of the lanthanum solid. A different behaviour was observed in high metal loading catalysts. In this case, metals would catalyse the decomposition of type Ia polymorph and also prevent the hydration of the support. The treatment with CO 2 provides additional information about the effect of the gas upon the metal containing solids. Pt would favour the carbon dioxide interaction with the support to form monoclinic polymorph (type Ia), while rhodium would prevent the further formation of the oxycarbonate phase.

The Performance of C2 Oxygenates Synthesis from Syngas over Rh−Mn−Li−Fe/SiO2 Catalysts with Various Rh Loadings

The promotion effect of additives on Rh−Mn−Li−Fe/SiO2 catalyst was investigated by means of an in-situ FTIR technique. IR results suggested that the additives Fe, Mn, and Li might be in close contact with Rh, and form new active sites, which promoted CO activation over Rh surface and favored the formation of C2-oxygenated intermediates. The catalysts with rhodium loading of 1.0−5.0 wt % were tested by CO hydrogenation to find the optimized rhodium loading. The results showed that the high-rhodium-loading catalysts exhibited high activity and selectivity toward C2 oxygenates at low temperature, but the selectivity was very sensitive to reaction temperature, low-rhodium-loading catalysts showed low activity, while the selectivity was not so sensitive to reaction temperature, therefore high space time yield (STY) of C2 oxygenates can be obtained by increasing the reaction temperature. Among the catalysts investigated, the 1.5 wt % rhodium catalyst exhibited the highest selectivity and relative higher rhodium efficiency for the synthesis of C2 oxygenates.

Modelling of the metallic phases of different Pt–Rh/Al 2O 3–CeO 2 catalysts: influence of the rhodium loading and nature of the metallic precursors

The activities for propane–propene mixture oxidation under lean conditions were found to be strongly different in the case of three-way Pt–Rh/Al 2O 3–CeO 2 catalysts prepared either by coimpregnation of the metallic salts (CI catalysts) or by an original method of successive impregnations with an intermediary reducing treatment (SI catalysts). Based upon the analysis of results of catalytic tests and physical characterisations including EXAFS at the Pt edge, it was shown previously that such a different behaviour for propane oxidation could easily be explained. The formation of clusters of inactive Pt–Rh alloys of a rather homogeneous composition was suggested for CI catalysts while active bimetallic catalysts obtained by the SI preparation procedure were modelled by Pt and Rh clusters more or less mixed together. To achieve the complete description of the metallic species (especially for SI catalysts), the influence of rhodium loading and the nature of the metallic precursors were examined by studying the different catalysts by EXAFS mainly at the Pt edge but also at the Rh edge.

Structures and performance of Rh–Mo–K/Al 2O 3 catalysts used for mixed alcohol synthesis from synthesis gas

A series of rhodium-modified Mo–K/Al 2O 3 catalyst samples was prepared by varying the rhodium loading between 0 and 1.0 wt.% and maintaining molybdenum and potassium contents as constants. The structures of the samples were characterized by techniques of XRD, LRS, TPR, XPS and EXAFS and correlated to the catalytic properties of the samples for alcohol synthesis from synthesis gas. It was found that, in the oxidic rhodium-modified samples, a strong interaction of the rhodium modifier with the supported K–Mo–O species occurs. This interaction facilitates the sulfidation and reduction of the supported oxo-molybdenum and leads to a decrease in the size of the molybdenum species and stabilization of the cationic rhodium species on the samples during sulfidation. Upon sulfidation, the sulfided molybdenum species in the rhodium-free sample is mainly present as large patches of MoS 2-like slabs with their basal sulfur planes interacting with the support surface. With the modification of rhodium to the samples, the supported MoS 2-like species becomes highly dispersed, as revealed by the decrease in the average size of the sulfided molybdenum species. The interaction of the rhodium species with the molybdenum component may cause the basal planes of the MoS 2-like species to become oriented perpendicular to the support surface due to favorable bonding of the MoS 2edge planes to the support through Mo–O–Al bonds. In comparison with the sulfided sample free of rhodium, the properties of the rhodium-modified samples for alcohol synthesis from synthesis gas are much improved. It most probably results from the synergic interaction of the rhodium with the molybdenum species that gives rise to the appearance of the catalytically active surfaces or sites. The co-existence of cationic and metallic rhodium stabilized by this interaction may be responsible for the increased selectivity for the formation of C 2+ alcohols.

Radiotracer method for quantifying the amount of platinum and rhodium deposited in automotive catalytic converters

Radiotracer methods have been developed to quantify the amount of platinum and rhodium deposited on automotive catalytic converters as a function of production conditions. In particular, this study determined the effects of selective adsorption and evaporation processes on the aqueous impregnation of converters with platinum group metal salts. The radiotracers used in this study, 191Pt and 105Rh, were produced by thermal neutron activation of Pt and Ru. Chemical processing was performed to remove undesired radioactive products and to ensure that each tracer was in its appropriate chemical state. This radiotracer method has been shown to be capable of measuring the platinum and rhodium loading on an entire substrate with a precision better than ±0.5%. At this precision level, the influence of selective adsorption and evaporation were determined.

98/03769 Mechanism of the partial oxidation of methane to synthesis gas over Pd

The decomposition of CH4 and CO2 as well as the reaction between CH4 and CO2 over SiO2-supported rhodium catalysts has been investigated in the temperature range of 600–800°C over a pulse reactor. The initial activities for methane decomposition and CO2 decomposition increased with the increase of rhodium loading; whereas the activity for CO formation in the CH4+CO2 reaction was almost unaffected by the rhodium loading over catalysts with rhodium loading >0.05%. Different reaction behaviors of methane reforming with CO2 were observed at 700°C and 800°C. At 700°C, CO2 conversion was higher than CH4 conversion; whereas at 800°C, CH4 conversion was higher than CO2 conversion. The bond order conservation-Morse potential (BOCMP) calculations indicate that the oxygen at on-top site can promote the dissociation of methane on the Rh(111) surface. Normal deuterium isotope effect was observed to be more noticeable on the methane conversion reaction than on the CO formation reaction, while no such effect was observed on the CO2 conversion reaction. The mechanism for CO and H2 formation in the reforming of methane with CO2 is discussed based on the results of the present work.

Preparation, Characterization, and Activity forn-Hexane Reactions of Alumina-Supported Rhodium–Copper Catalysts

Alumina-supported rhodium–copper catalysts were prepared and characterized by chemisorption of hydrogen, temperature-programmed reduction, X-ray photoelectron spectroscopy, and infrared spectroscopy of adsorbed CO. It is shown that the alumina support interacts with rhodium as well as with copper. None of the bimetallic catalysts have a single phase, but are composed of segregated phases of rhodium and copper and alloyed clusters, the latter being formed at the lowest rhodium loading. The catalytic properties of the catalysts were studied by means of then-hexane reaction. The turnover frequencies remained constant upon incorporation of copper into rhodium catalysts. However, the selectivity pattern changed; i.e., the isomerization reaction was enhanced at the expense of the hydrogenolysis reaction. The hydrogenolysis product distribution also shifted to a lower fragmentation factor. These results were explained by an ensemble effect in which the addition of copper isolates the rhodium in small clusters.

Carbon dioxide reforming of methane to syngas over SiO 2-supported rhodium catalysts

The decomposition of CH 4 and CO 2 as well as the reaction between CH 4 and CO 2 over SiO 2-supported rhodium catalysts has been investigated in the temperature range of 600–800°C over a pulse reactor. The initial activities for methane decomposition and CO 2 decomposition increased with the increase of rhodium loading; whereas the activity for CO formation in the CH 4+CO 2 reaction was almost unaffected by the rhodium loading over catalysts with rhodium loading >0.05%. Different reaction behaviors of methane reforming with CO 2 were observed at 700°C and 800°C. At 700°C, CO 2 conversion was higher than CH 4 conversion; whereas at 800°C, CH 4 conversion was higher than CO 2 conversion. The bond order conservation-Morse potential (BOCMP) calculations indicate that the oxygen at on-top site can promote the dissociation of methane on the Rh(111) surface. Normal deuterium isotope effect was observed to be more noticeable on the methane conversion reaction than on the CO formation reaction, while no such effect was observed on the CO 2 conversion reaction. The mechanism for CO and H 2 formation in the reforming of methane with CO 2 is discussed based on the results of the present work.

Mechanistic Studies of Methane Partial Oxidation to Syngas over SiO2-Supported Rhodium Catalysts

The reaction behaviors of CH4and CH4/O2(2/1) with reduced (Rh/SiO2) and oxidized (Rh(O)/SiO2) SiO2-supported rhodium catalysts were investigated at temperatures ranging from 600 to 800°C over a pulse microreactor. The interaction of methane with Rh/SiO2led to CO, H2and surface carbon formation. The conversion of methane increased with the increase in rhodium loading. During the interaction of methane with Rh(O)/SiO2catalysts having rhodium loading ≥0.5%, besides CO, H2, and surface carbon, CO2and H2O were formed. Total oxidation of methane occurred over rhodium oxide together with the simultaneous reduction of Rh3+to Rh0. The partial oxidation of methane as well as the decomposition of methane took place over the resulting metallic rhodium. Within the first pulse of CH4/O2(2/1), rhodium oxide in the oxidized catalysts was reduced to metallic rhodium. From the second pulse onward, both methane conversion and CO selectivity were similar to those observed over the reduced catalysts. The results indicate that metallic rhodium is the active site for methane partial oxidation and chemisorbed oxygen species have participated in the methane oxidation reaction. We suggest that OH groups and/or certain low coordinated oxygens of SiO2were also involved in the oxidation of methane, especially at low rhodium loadings. Deuterium isotope effects in the methane partial oxidation reaction were investigated at 700°C by performing CH4+O2and CD4+O2reactions alternately over the Rh/SiO2catalysts. Normal deuterium isotope effects of similar magnitude were observed on the overall reaction of methane oxidation as well as on the CO and CO2formation reactions with insignificant change in product selectivities. The results indicate that methane dissociation is a key step and that CO and CO2are formed via some common intermediates, viz. surface CHx(x=0–3) species. The activation energies for methane dissociation with or without the involvement of chemisorbed oxygen on a Rh(111) surface were calculated by means of the bond-order conservation Morse-potential approach.

Observation of in situ reduction of ceria-supported rhodium catalysts by electron beam irradiation

In rhodium catalysts supported on lanthanide oxides, significant difference in rhodium dispersions has been observed between cerium dioxide (CeO2) and sesquioxides (Ln2O3) by high resolution electron microscopy (HREM). This has been attributed to the different behaviors of the oxide supports during their impregnation with aqueous solution of rhodium nitrate (Rh(NO3)3). In the acid media, cerium dioxide has a higher reducibility while the sesquioxides have a higher solubility. So studying the catalyst precursor and the reduction process can provide an understanding of its interaction with the support during the impregnation stage and the metal dispersion on the oxide supports. Results on cerium dioxide support are presented here.The catalyst precursor was prepared by incipient wetness impregnation method as reported elsewhere. The rhodium loading was 2.4 wt%. The HREM work was performed in JEOL 4000EX electron microscope (point resolution of 1.7 Å). The typical current density used during observation was ∼6.5A/cm2 at the specimen. The vacuum in the column of the microscope was ∼10-7 torr.

AlPO4-supported rhodium catalysts: IV. Individual and competitive hydrogenation of styrene and α-methylstyrene

Hydrogenation rates of the olefinic double bond of styrene (S) and α-methylstyrene (MS) have been measured using methanol as the solvent in the temperature range 293–323 K over RhAlPO4 catalysts of different rhodium loading. Both organic reactants exhibited nearly identical areal rates and apparent activation energies independent of the rhodium loading (0.25 to 1 wt%) of the catalyst. The hydrogenation reaction involves a Horiuti-Polanyi mechanism modified by an irreversible adsorption of alkene. Styrene hydrogenation was also studied using other alcohols as solvents. The initial rate of hydrogenation was dependent on the polarity of the solvent. The competitive hydrogenation of both S and MS provides unambiguous evidence that the relative reactivity, RMS,S, and the relative adsorption equilibrium constant, KMS,S, are independent of the reaction temperature, initial hydrogen pressure, and rhodium loading. Thus, the adsorption heat and the activation energy are nearly identical for two organic reactants.

AlPO 4- supported rhodium catalysts V. Liquid phase hydrogenation of cycloalkenes

Liquid phase catalytic hydrogenations of cyclic alkenes, cyclopentene (C5), cyclohexene (C6), cycloheptene (C7) and cyclooctene (C8), on rhodium catalysts supported on two AlPO 4 (F and P) samples prepared by different methods, have been studied, as has an AlPO 4-SiO 2 system (E), at several rhodium loadings (0.25 – 1 wt%) under an initial hydrogen pressure of 0.55 MPa, at temperatures between 293 and 323 K and with methanol as solvent. The hydrogenation rates were in the order C5 > C6 > C7 > C8, independent of catalysts or metal loading, and the activity on each substrate was very different, depending on the catalyst involved. Thus, the sequence Rh/F > Rh/P ~- Rh/E was obtained. The values of relative reactivities between C6 and C8, R C6, C8, were very close to unity, although the values of the relative adsorption constants, K C8, C6, were in the range 2–10 depending on the catalyst, which was in the sequence Rh/F > Rh/P > Rh/E. Apparent activation energies, E a, and Arrhenius constants, A, were obtained, and the existence of a linear correlation between 1g A and E a, known as the “compensation effect”, was determined.

Effects of loading on structure of Rh zeolite catalysts and their activity for methanol carbonylation

Rhodium X zeolites prepared from RhCl 3 and (Rh(NH 3) 5Cl)Cl 2 by ion exchange at room temperature (with Rh loadings from 0.38 to 4.0 wt%) have been studied with ESCA, volumetric CO-adsorption and measurements of activities for carbonylation of methanol. The zeolites are predominantly exchanged with RhCl(H 2O) 5 2+ and Rh(NH 3) 5Cl 2+, respectively. A surface enrichment in rhodium is seen, particularly at low metal loadings. A more homogeneous distribution at higher loadings is associated with a reduced CO/Rh ratio and a lower unit Rh activity in the carbonylation of alcohols. Rh(III) species are present initially but lower valent species can be observed after thermal vacuum treatment of the samples. Samples with the highest unit rhodium activity (low rhodium loading) contain about 2 Rh atoms per unit cell in the surface layers and the CO/Rh ratios are just below 2.0, whereas samples with lower unit activity (higher rhodium loading) contain about 5–10 Rh atoms per unit cell in the surface layers and lower CO/Rh ratios. The Rh(NH 3) 5Cl-NaX and RhCl 3-NaX series exhibit somewhat different detailed behaviour.

AlPO 4-supported rhodium catalysts. II. Determination of metal dispersion of Rh/AlPO 4—SiO 2 catalysts by TEM and XRD

The applications of X-ray diffraction (XRD) line broadening and transmission electron microscopy (TEM) in determining metal crystallite size and size distribution in Rh/AlPO 4—SiO 2 catalysts having wide ranges of rhodium loading were investigated. The average crystallite diameter, d v, and metal surface area were obtained from X-ray measurements, and cross-checked with the crystallite size distribution observed by electron microscopy. The results obtained in both experimental methods were in agreement. The rhodium surface area was linearly dependent on the metal loading up to 4 wt% Rh. This is a result of a change in the number of metal particles without remarkable change in particle dimensions ( d v from TEM). On increasing the metal loading up to 4–5 wt% Rh, a deviation from linearity is observed and the metal surface area increases more gradually.

Infrared study of rhodium clusters entrapped within zeolites

Samples containing 0.5 and 1.5% by wt rhodium were prepared by sublimating Rh/sub 6/(CO)/sub 16/, onto Y zeolite, subjected to various outgassing, oxidizing, and reducing treatments, and studied by IR spectroscopy. The IR spectra were compared with spectra of zeolite-supported (Rh(CO)/sub 2/ Cl)/sub 2/, alumina-supported Rh/sub 6/(C0)/sub 16/, and CO adsorbed on metallic rhodium. At sufficiently low rhodium content, the zeolite-supported Rh/sub 6/(CO)/sub 16/ species could be decarbonylated reversibly by oxygen or evacuation without losing the cluster structure. At higher rhodium loadings, a rhodium(I) dicarbonyl species formed as well as the decarbonylated zero-valent rhodium.