Dispersed Rhodium Research Articles

Sustainable syngas production from ethylene glycol reforming processes using Rh-based catalysts in microreactors

In this work the ethylene glycol (EG) as feed for syngas production by steam reforming (SR) and oxidative steam reforming (OSR) was studied applying microchannel testing reactors. The product composition was determined at a steam to carbon ratio (S/C) of 4.0, reaction temperatures between 625°C and 725°C and atmospheric pressure at volume hourly space velocities (VHSV) of the feed between 100 and 300NL/(gcath). Catalysts with different rhodium loading were prepared by two different preparation methods (conventional impregnation and separate nanoparticle synthesis) and compared with regard to their activity and selectivity. All the catalysts were characterized using the following techniques: N2 physisorption, temperature programmed reduction (TPR), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Under all experimental conditions full conversion of ethylene glycol was achieved over the samples containing 2.5% and 5% Rh prepared by the impregnation method, equilibrium selectivity towards CO and CO2, lowest selectivity towards CH4 and lowest formation of by-products such as CH3CHO and C2 hydrocarbons. The other catalysts showed lower activity. The superior performance of the catalysts prepared by the impregnation method is supported by the measured higher rhodium dispersion of these samples.

Oxidation State of Ce in CeO2-Promoted Rh/Al2O3Catalysts during Methane Steam Reforming: H2O Activation and Alumina Stabilization

The cerium oxidation state in ceria-doped Rh/Al2O3 catalysts during the methane steam reforming (MSR) reaction was determined by in situ X-ray absorption spectroscopy and online mass spectrometry at 773 K. The catalysts were characterized by electron microscopy and X-ray diffraction. The oxidation states of rhodium and cerium during MSR are related. A total of 25% of Ce4+ reduces to Ce3+ under a CH4/H2O/He flow when the catalyst is active in MSR (773 K). A slight reoxidation occurs when the catalyst is sequentially exposed to steam diluted in helium at high temperatures, showing that only a small fraction of CeO2–x may be involved in the water activation step by reoxidation. The main role of ceria during reaction, besides the activation of water, is to stabilize the structure of alumina by forming CeAlO3 and to maintain the dispersion of rhodium.

Hydrogenation of 3-nitroacetophenone over rhodium/silica catalysts: Effect of metal dispersion and catalyst support

The effect of physical properties of rhodium/silica catalysts on the hydrogenation of 3-nitroacetophenone was investigated using a series of catalysts with different particle sizes, pore sizes and dispersions. The hydrogenation reaction was carried out in the liquid phase at different temperatures (313–343K) and hydrogen pressures (2–5barg). The hydrogenation of 3-nitroacetophenone was found to be a consecutive reaction with 3-aminoacetophenone, 1-(3-aminophenyl) ethanol, and 1-(3-aminocyclohexyl) ethanol the main products; traces of 3-aminoaniline were also detected. The reaction was zero order with respect to the concentration of 3-nitroacetophenone and half order with respect to hydrogen. The rate of formation and disappearance of intermediates increased with increasing temperature and hydrogen pressure. The rate of ring hydrogenation to form 1-(3-aminocyclohexyl) ethanol shows a marked increase at high temperatures. The catalytic activity varies with both the pore size of silica support and the rhodium dispersion with no significant influence of catalyst particle size. The turnover frequency increases with decreasing metal dispersion (or with increasing average metal crystallite size) revealing an antipathetic particle size effect. This behaviour can be related to changes in the electronic and structural make up of small metal crystallites but suggests that the reaction takes place on the plane face surface of the FCC close-pack structure of rhodium in which the number of face surface atoms increases at the expense of the edge and corner sites as the metal crystallite size increases.

Effect of the nature of TiO2 support over the performances of Rh/TiO2 catalysts in the partial oxidation of methane

The effect of TiO2 phase composition on the catalytic and physico-chemical properties of supported Rh catalysts in the partial oxidation of methane were studied. Two types of TiO2 with different rutile/anatase crystalline phase compositions were used as support. The performance of supported Rh catalysts depends on the nature of the TiO2 support. CH4 conversion and selectivity towards H2 and CO are improved when using an anatase-rich TiO2 support, probably because it improves dispersion and reducibility of rhodium particles compared with the rutile-rich carrier. Both Rh/TiO2 catalysts produced more CO than H2, which suggests the involvement of other reactions such as dry reforming, steam reforming and water gas shift reactions. The anatase-rich catalyst is remarkably stable but its rutile-rich counterpart becomes deactivated after a few hours. This loss of activity is attributed to the oxidation of rhodium. With anatase-rich TiO2 catalyst the dispersion of rhodium is stabilized and its oxidation is avoided.

A facile one-step synthesis of polymer supported rhodium nanoparticles in organic medium and their catalytic performance in the dehydrogenation of ammonia-borane

A new type of supported rhodium nanoparticles were reproducibly prepared from N(2)H(4)BH(3) reduction of [Rh(μ-Cl)(1,5-cod)](2) without using any solid support and pre-treatment technique. Their characterization shows the formation of well dispersed rhodium(0) nanoparticles within the framework of a polyaminoborane based polymeric support. These new rhodium(0) nanoparticles were found to be the most active supported catalyst in the catalytic dehydrogenation of ammonia-borane in water at room temperature.

Thermochemical Stability of Alumina-Supported Rh-LaCoO3 Catalysts for Tar Conversion

The thermal and chemical stability of alumina-supported catalysts (Rh/Al2O3, LaCoO3/Al2O3, Rh-LaCoO3/Al2O3) have been investigated by performing cycles of tar conversion at 700 °C followed by regeneration of the catalyst by oxidation of coke deposited on the surface up to 800 °C. Structural and chemical modifications underwent by the catalysts upon conversion/regeneration cycles have been studied by BET, TPR/TPO, and DRIFT analyses. The catalysts containing rhodium are highly stable maintaining the original performance and chemical properties of the fresh sample also after several cycles, dispersion of rhodium on Al2O3 surface being sufficiently preserved also at 800 °C. On the contrary, LaCoO3/Al2O3 catalyst undergoes a partial deactivation after the first cycles related to the irreversible migration of cobalt into the alumina lattice.

Effect of sulphur on the performance of Rh–LaCoO3 based catalyst for tar conversion to syngas

The effect of sulphur addition on the catalytic activity of 1%Rh/Al2O3, LaCoO3/Al2O3 and 1%Rh–LaCoO3/Al2O3 catalysts in reforming of tar produced by biomass pyrolysis has been investigated by adding 20 or 200ppm H2S in a fixed bed reactor at 700°C using a S-free biomass slowly pyrolyzed. The catalysts have been characterized by ICP-MS and BET analyses. The poisoning effect of sulphur has been also studied by in situ DRIFT and TPR analyses. A very high sulphur exposure of the catalysts leads to the partial inhibition of the reforming properties related to the poisoning of dispersed rhodium sites. The LaCoO3 layer effectively preserves rhodium from sulphur poisoning in the 1%Rh–LaCoO3/Al2O3, the catalyst containing both phases showing mostly unchanged catalytic properties also upon 200ppm S-treatment under reaction conditions. The LaCoO3/Al2O3 catalyst shows reduced reforming properties coupled to an increase of cracking and total oxidation reactions associated to the partial destruction of the perovskite structure.

Microemulsion and incipient wetness prepared Rh-based catalyst for diesel reforming

The role of the catalyst preparation technique was investigated for diesel reforming. Reverse microemulsion (ME) and incipient wetness (IW) techniques were used for the preparation of Rh-based monolithic catalysts that were employed for hydrogen generation of low-sulfur diesel via autothermal reforming (ATR). The washcoat of the tested catalysts consisted of 0.5 wt% Rh, 1 wt% Rh, and 1:1 wt% Rh:Pt supported on γ-alumina. All washcoats were deposited on 400 cpsi cordierite monoliths. The reaction condition was T feed = 650 °C, H 2O/C ∼ 2.5, O 2/C ∼ 0.49, TOS = 3 h, GHSV ∼ 13 000 h −1 and P = 1 atm. Fresh and aged powder samples of the catalyst were characterized by N 2-BET, H 2 chemisorption, XRD, H 2-TPR, O 2-TPO and TEM. The activity results established that Rh and RhPt formulations, prepared by ME and IW, are highly active for ATR of diesel where fuel conversions above 92% were obtained. FTIR and NDIR analysis also showed that the highest formation of ethylene was found in the product gas stream from the bimetallic samples indicating that RhPt/Al 2O 3 is less resistant towards carbon deposition. The latter observation was confirmed by O 2-TPO analysis of the aged samples where high loads of coke were found both on the active metals and on the support. Interestingly, these effects were less significant on the ME samples. The characterization results clearly showed differences in morphology between the ME and the IW samples. N 2-BET analysis showed that higher surface area, ∼268–285 m 2/g, was obtained with the ME samples. Also, H 2 chemisorption analysis showed that the rhodium dispersion was ∼10% higher for the ME samples (H/Rh ∼ 60–66%). XRD analysis showed that crystalline phases of γ-alumina were present on all samples. The diffractograms also showed small traces of metallic Pt (∼16–30 nm) in the bimetallic samples. H 2-TPR analysis, showed peaks ascribed to bulk rhodium oxides and rhodium aluminates. It was also noted that the addition of Pt on the support lowered the reducibility of the different rhodium species. TEM analysis performed on the fresh and aged ME and IW bimetallic samples showed mainly Rh x Pt 1− x alloys with an average particle size of ∼20–50 nm were present on the alumina support. Also, for the aged samples, no sintering effects were noted. Furthermore, rhodium was found to switch oxidation state from e.g. Rh 3+ to Rh 0 while Pt remained in the metallic state.

Reforming of CH4 with CO2 over Rh/H‐Beta: Effect of Rhodium Dispersion on the Catalytic Activity and Coke Resistance

AbstractThe effect of Rh dispersion on reforming of CH4 with CO2 over H‐Beta supported Rh catalysts has been investigated. The CH4 and CO2 conversion over the catalysts increase with increasing Rh loading from 0.5 wt% to 4.0 wt% in the reaction temperature range of 823–1123 K. The high TOF of CH4 over 0.5 wt% and 1.0 wt% Rh/H‐beta may be attributed to high dispersion of rhodium species. The catalysts before and after the reaction were characterized by XRD, TEM, and TG‐DTA and the results indicate the catalysts with Rh loading of 0.5 wt% and 1.0 wt% exhibiting high resistance to coke. Under controllable conditions, we confirm that the coke is originated from methane dissociation and can be substantially oxidized by active oxygen species dissociated from the adsorbed carbon dioxide on the catalyst with high dispersion of Rh species.

Promoting effect of CeO2 in the electrocatalytic activity of rhodium for ethanol electro-oxidation

The promoting effect of ceria in the electrocatalytic activity of rhodium for ethanol electro-oxidation in alkali media has been studied. Rh/C, CeO2/C and RhCeO2/C catalysts were synthesized and characterized by TEM, XRD, XPS, TG-MS, H2-TPR and XAS. The electrocatalytic activity was studied by Cyclic Voltammetry (CV) and chronoamperometry. The onset potential of oxidation on RhCeO2/C was shifted negatively as compared to that on Rh/C, despite ceria itself does not show any electrocatalytic activity. The promoting effect of ceria has been attributed to the improved rhodium dispersion, and differences in the oxidation state of rhodium between Rh/C and RhCeO2/C were not found. The carbon support reduces rhodium species to Rh0, and also partially reduces ceria, during the samples preparation, and the surface of the carbon support is oxidised.

A surfactant-assisted preparation of well dispersed rhodium nanoparticles within the mesopores of AlSBA-15: characterization and use in catalysis

Well dispersed and efficient Rh(0) hydrogenation catalysts were obtained by the reduction of Rh(III)-exchanged mesoporous aluminosilicates by sodium borohydride in the presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride.

Asymmetric hydroformylation of olefins catalyzed by rhodium nanoparticles chirally stabilized with ( R)-BINAP ligand

Rhodium nanoparticles have been conveniently synthesized by one-pot chemical reduction of aqueous rhodium chloride (RhCl 3·3H 2O) dispersed in toluene solution in the presence of amphiphilic tetraoctylammonium bromide (TOAB) and chiral ( R)-BINAP at ambient conditions. The resulting highly dispersed rhodium nanoparticles show small and narrowly distributed core sizes (1.5–2.0 nm). The chirally stabilized rhodium nanoparticles were also immobilized on silica by impregnation to give the corresponding supported catalysts. 31P MAS NMR results and IR spectra of adsorbed CO confirm that chiral ( R)-BINAP ligands stabilize the nanoparticles by coordinative interaction between phosphine and rhodium, and produce chirally catalytic active sites on the rhodium nanocatalysts. Chirally stabilized catalysts exhibit high regioselectivity and chiral induction ability for the asymmetric hydroformylation of olefins. The supported rhodium nanocatalysts show the increased activities compared to the unsupported catalysts (e.g. from 12 to 22 h −1 for the hydroformylation of styrene). One hundred percentage regioselectivity of branched aldehyde and up to 59% ee were obtained for the hydroformylation of vinyl acetate.

State of Supported Rhodium Nanoparticles for Methane Catalytic Partial Oxidation (CPO): FT-IR Studies

The effect of pretreatments as well as of rhodium precursor and of the support over the morphology of Rh nanoparticles were investigated by Fourier transform infrared (FT-IR) spectroscopy of adsorbed CO. Over a Rh/alumina catalyst, both metallic Rh particles, characterized by IR bands in the range 2070-2060 cm-1 and 1820-1850 cm-1, and highly dispersed rhodium species, characterized by symmetric and asymmetric stretching bands of RhI(CO)2 gem-dicarbonyl species, are present. Their relative amount changes following pretreatments with gaseous mixtures, representative of the catalytic partial oxidation (CPO) reaction process. The Rh metal particle fraction decreases with respect to the Rh highly dispersed fraction in the order CO approximately CO/H2 > CH4/H2O, CH4/O2 > CH4 > H2. The metal particle dimensions decrease in the order CH4/O2 > H2 > CH4/H2O > CO > CO/H2. Grafting from a carbonyl rhodium complex also increases the amount and the dimensions of Rh0 particles at the catalyst surface. Increasing the ratio (extended rhodium metal particles/highly dispersed Rh species) allows a shorter conditioning process. The surface reconstruction phenomena going on during catalytic activity are related to this effect.

Alkali-metal promoted rhodium-on-alumina catalysts for nitrous oxide decomposition

Catalytic activity of γ-alumina supported rhodium catalysts in nitrous oxide decomposition into dinitrogen and dioxygen has been determined, the total conversion being reached at 450°C. Rhodium is present at alumina surface in three forms: metal particles, Rh2O3 and rhodium zero valent atoms interacting with oxygen ions at the metal/support interface. Linear dependence of the catalytic activity on rhodium dispersion has been found. Deposition of alkali metal cations: Li, Na, K and Cs as promoters at the surface of alumina results in a considerable increase of rhodium dispersion and hence catalytic activity. The effect of promoters depends strongly on the speciation of alkali metals and rhodium used in the preparation of the catalyst. Both alkali metal cations and rhodium compete for the same OH groups at the alumina surface. The electronegativity of alkali metal oxides is much greater than that of alumina and their deposition increases the negative charge of surface oxide ions hindering the diffusion of rhodium and preventing the growth of its larger particles.

Rhodium Dispersion during NO/CO Conversions

Change and change again: Under catalytic conditions for NO/CO conversion, simultaneous time-resolved extended X-ray absorption fine structure spectroscopy and IR studies show that the rhodium atoms in 5 wt % Rh/Al2O3 rapidly migrate between CO-covered Rh particles and mononuclear Rh–NO sites depending on the changing gas composition. At lower temperatures (473 K), there is interconversion between mononuclear Rh(CO)2 and Rh(NO) centers.

Rhodium Dispersion during NO/CO Conversions

Rhodium Dispersion during NO/CO Conversions

Role of the oxide support on the performance of Rh catalysts for the autothermal reforming of gasoline and gasoline surrogates to hydrogen

Catalysts consisting of 2 wt% Rh deposited on Gd-CeO 2, Y-ZrO 2, γ-Al 2O 3, La-Al 2O 3, and CaAl 12O 19 were tested for the autothermal reforming (ATR) of isobutane and sulfur-free gasoline. The catalysts were characterized for activity by microreactor tests and temperature-programmed reduction (TPR), and for physical properties by surface area measurements, CO chemisorption, X-ray diffraction (XRD), and extended X-ray absorption fine structure (EXAFS) analyses. The order of activity for ATR of isobutane was Rh/La-Al 2O 3 > Rh/Y-ZrO 2 > Rh/Gd-CeO 2 > Rh/γ-Al 2O 3 > Rh/CaAl 12O 19, which paralleled the ranking of the catalysts based on the dispersion and reducibility of Rh, with Rh/La-Al 2O 3 having the highest dispersion and the lowest reduction temperature. After subjecting the catalysts to an accelerated ageing process, the activity for ATR of isobutane was similar for all catalysts except for Rh/Gd-CeO 2, which experienced a significant loss in steam reforming activity. Rhodium dispersion measurements confirmed that the ageing process had caused Rh to sinter on all of the supports. A major conclusion from this study is that the primary effect of the support was on the dispersion of Rh.

Rh/Ti–SiO 2 catalysts prepared by organic grafting: A novel class of catalysts towards hydrogen production by partial oxidation of methane

The influence of grafting Ti over SiO 2 surface, the Ti loading and the catalytic performance of Rh supported on Ti–SiO 2 catalysts in the partial oxidation of methane have been studied. Ti-grafting led to modifications in the surface of the catalysts that affect significantly their catalytic behaviour. The grafting method gives rise to two different Ti-species: (i) titanium in the Ti–O–Si bonds; (ii) anatase nanoparticles on the support surface for the highest Ti amount. Ti-grafted catalysts led to a significant improvement in the activity, as well as in the selectivity towards H 2 and CO, related to bare supports (SiO 2 and TiO 2). The performance of such catalysts depends on the Ti loading. These positive effects could arise from the ability of titania to modify redox properties as well as rhodium dispersion. Results suggest guidelines for the design of very active and selective Rh-based catalysts in POM for producing hydrogen.

Spatial near‐infrared imaging of hydroxyl band coverage on ceria‐based catalysts

AbstractHigh‐throughput near‐infrared imaging was used to distinguish catalyst activity for low‐temperature methane steam‐reforming. Geminal hydroxyls of reduced ceria were depicted during methane reforming at 673 K. The changes in absorbance maps under various water partial pressures showed evidence of formate intermediate formations without redox exchanges. Higher resolution was observed in absorbance change images than that of thermal images obtained from catalyst surface self‐emissions. The experimental results illustrated higher activity of pure rhodium catalyst than that of bimetallic ones, likely because of the high dispersion of rhodium on the catalyst support. Moreover, the reaction was accelerated when high surface area silica was added because more reduced sites were exposed. Our filter bandwidths limited our interest in band‐shift distribution of geminal hydroxyl band during the reduction process. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Preparation and characterisation of Rh/Al 2O 3 catalysts and their application in the adiponitrile partial hydrogenation and styrene hydroformylation

Rh/Al 2O 3 catalysts with a low metal loading were prepared using different preparation methods (ion exchange and homogeneous precipitation through urea decomposition). Catalysts with very high rhodium dispersion were obtained by properly choosing the preparation parameters. Moreover the kinetic parameters for the adsorption of the precursor on the support were determined. The catalysts were tested in two industrially important reactions: the adiponitrile hydrogenation and hydroformylation reaction of styrene. The hydrogenation of adiponitrile was carried out in mild conditions in a slurry reactor, at temperatures between 70 and 100 °C and under a hydrogen pressure of 3 MPa. The catalyst prepared with the ion exchange technique gave aminocapronitrile with a conversion of 60% and a selectivity of 99%. The catalyst was very active in the hydroformylation of styrene, but it suffered leaching problems such as many supported catalysts.