Rh Particles Research Articles

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.

The effect of Fe on the catalytic behavior of model Pd-Rh/CeO2-Al2O3 three-way catalyst

The present work attempts to address the issue whether iron (Fe) which is accumulated on the surface of “three-way” catalysts (TWCs) used in gasoline-driven cars is a true chemical poison of their catalytic activity. This important issue from a scientific and technological point of view is addressed via catalytic activity, temperature-programmed surface reaction (TPSR), and X-ray photoelectron spectroscopy (XPS) measurements over a model TWC (1wt% Pd–Rh/20wt% CeO2–Al2O3). It was found that deposition of Fe up to the level of 0.4wt% (an average concentration found in aged commercial TWCs) on the model TWC does not deteriorate its activity towards CO and C3H6 oxidation, and reduction of NO by H2. Instead it was found that iron improves significantly the T50 parameter in the activity versus temperature profile. Small Fe clusters in contact with the noble metal (Pd and Rh) particles due to the lower work function of Fe compared to Pd and Rh act likely as a source of electron flow towards the noble metals (as evidenced by XPS measurements), thus altering their surface work function and adsorption energetics of reaction intermediates. The latter have increased significantly the activity of the model TWC towards oxidation of CO and propylene, and to a lesser extent the activity towards the reduction of NO by H2. The presence of Fe on the surface of the model TWC provided and/or created also new active catalytic sites for the reactions investigated. According to previous work from this laboratory, iron up to the level of 0.4wt% was shown not to deteriorate the oxygen storage capacity (OSC) of the same model TWC used in the present work. Thus, it could be concluded that Fe when deposited on a commercial TWC at least up to the level of 0.4wt% acts likely as a promoter than a poison of its catalytic activity.

Hydrogen production from ethanol on Rh/MgO based catalysts: The influence of rhodium precursor on catalytic performance

The influence of the metal precursor on the catalytic behaviour of Rh/MgO catalysts in steam reforming of ethanol was investigated. Rhodium nitrate and acetylacetonate, having different chemical properties, have been used as hydrophilic and hydrophobic precursors, respectively. More highly dispersed catalysts were obtained using Rh-nitrate, while catalysts prepared with Rh-acetylacetonate show higher stability being generally less affected by coke formation. Catalytic tests performed at molten carbonate fuel cell (MCFC) operative conditions ( T R = 923 K ) clearly show a structure-sensitive reaction since the turnover frequency (TOF) significantly increases with the mean Rh particle size. Thermodynamic considerations on the basis of the outlet composition allowed to calculate the experimental equilibrium constant ( K exp ) for (i) methane steam reforming (MSR), (ii) water–gas shift (WGS), (iii) methane pyrolysis and (iv) Boudouard reactions. By comparing K exp with the equilibrium provided mass action ratios ( K eq ) , we suggest that only WGS is fully equilibrated and that coke is formed mainly through methane pyrolysis.

Metal Nanoparticles on Stainless Steel Surfaces as Novel Heterogeneous Catalysts

The goal of this work was the development of a novel type of heterogeneous catalyst, consisting of bare metal nanoparticles on stainless steel foils, which can be shaped to any kind of architecture and, if necessary, heated electrically. Solutions of pre-prepared, ligand protected and monodispersed gold, palladium, platinum and rhodium nanoparticles were sprayed onto stainless steel foils, followed by the careful removal of the ligand molecules by an oxygen plasma treatment. Due to this, bare particles become irreversibly fixed on the steel support. It could be shown that the original particle sizes do not change during the plasma treatment. Foils, densely coated with the nanoparticles, were used for gas phase catalyses in a self-made reactor at room temperature or at 60 °C. Hydrogenation of 1,3-butadiene at 15 nm Pd and 2 nm Pt, CO oxidation at 16 nm, 8 nm and 1.4 nm gold and NO reduction with NH3 at 2 nm Rh particles were performed, indicating that the novel catalysts might in principle be applicable in technical processes if the experimental conditions like form and temperature would be optimized.

Enhanced ethanol production inside carbon-nanotube reactors containing catalytic particles

Carbon nanotubes (CNTs) have well-defined hollow interiors and exhibit unusual mechanical and thermal stability as well as electron conductivity. This opens intriguing possibilities to introduce other matter into the cavities, which may lead to nanocomposite materials with interesting properties or behaviour different from the bulk. Here, we report a striking enhancement of the catalytic activity of Rh particles confined inside nanotubes for the conversion of CO and H2 to ethanol. The overall formation rate of ethanol (30.0 mol mol(-1)Rh h(-1)) inside the nanotubes exceeds that on the outside of the nanotubes by more than an order of magnitude, although the latter is much more accessible. Such an effect with synergetic confinement has not been observed before in catalysis involving CNTs. We believe that our discovery may be of a quite general nature and could apply to many other processes. It is anticipated that this will motivate theoretical and experimental studies to further the fundamental understanding of the host-guest interaction within carbon and other nanotube systems.

Particle size effects in Rh/Al2O3 catalysts as viewed from a structural, functional, and reactive perspective: the case of the reactive adsorption of NO

The structural-dynamic behaviour of γ-Al2O3 supported Rh nanoparticles under He, H2/He, and NO/He has been investigated using a newly developed methodology that permits dispersive EXAFS (EDE), diffuse reflectance infra red spectroscopy (DRIFTS), and mass spectrometry (MS) to be applied simultaneously to the study of gas-solid interactions. This reveals a considerably variability in nanoparticle habit (for 11 A diameter nanoparticles as a function of temperature), and between 8 A and 11 A particles in their response to NO. The selectivity (N2/(N2 + N2O)) of the reactive interaction between NO and the supported Rh shows essentially no particle size dependence above 473 K: it is apparent, however, that considerable differences in some aspects of the structural behaviour of the 8 A and 11 A Rh particles do nonetheless, exist. At 373 < T < 473 K a clear divergence in structural, functional, and reactive response of the different sized supported Rh nanoparticles toward NO is observed. These observations are discussed in terms of the ability of different sized Rh particles to change structure in response to the reactive environment, the subsequent effect this has on the nitrosyl functionality that different phases may support, and the reactive pathways for NO conversion that may therefore arise.

Roles of chlorine in the CO hydrogenation to C2-oxygenates over Rh-Mn-Li/SiO2 catalysts

The activity and selectivity towards C 2 -oxygenates over Rh-Mn-Li/SiO 2 catalyst obviously increased with the increase of chlorine content of dried catalyst. The chlorine seems to function by increasing the amount of Rh species in intimate contact with Mn species and increasing the intensity of Rh–Mn interaction during reduction of the catalyst, therefore improving the Rh particle geometries, sizes and size distribution and the ratio of Rh + /Rh 0 on the reduced catalyst. Rh-Mn-Li/SiO 2 catalysts were prepared by transforming nitrate and chloride precursors from one form to another stepwise, in an attempt to investigate the roles of chlorine in determining the catalytic properties of Rh-Mn-Li/SiO 2 . The space time yield (STY) of C 2 -oxygenates and CO conversion increased from 270.1 g/(kg-cat h) and 4.3% to 597.1 g/(kg-cat h) and 8.5%, respectively, and the selectivity towards C 2 -oxygenates increased from 65.5 to 73.6%, when the precursors of Rh, Mn and Li were completely transformed from nitrates to chlorides. The chlorine seems to function by increasing the amount of Rh species in intimate contact with Mn species and increasing the intensity of Rh–Mn interaction during reduction of the catalyst, therefore improving the Rh particle geometries, sizes and size distribution and the ratio of Rh + /Rh 0 on the reduced catalyst. Such properties are responsible for the activity and selectivity towards C 2 -oxygenates over Rh-Mn-Li/SiO 2 catalyst.

Preparation of highly dispersed PEM fuel cell catalysts using electroless deposition methods

A methodology for the electroless deposition (ED) of PtCl62− using dimethylamine borane (DMAB) on a Rh-seeded carbon support has been developed for electrochemical and fuel cell applications. This procedure required seeding the carbon with a Rh-precursor catalyst via wet impregnation prior to the exposure of an aqueous ED bath containing PtCl62−, DMAB, and sodium citrate (complexing/stabilizing agent). Kinetic parameters that affect the extent and rate of PtCl62− deposition include concentrations of PtCl62−, DMAB, and sodium citrate as well as pH and concentrations of Rh seed sites. A linear relationship between rate and extent of PtCl62− deposition and DMAB and Rh concentrations was found while the citrate concentration had little effect on rate and a modest effect on extent. Lastly, extent of PtCl62− deposition showed a maximum with respect to pH. Characterization of the Rh-seeded, carbon support by transmission electron microscopy (TEM) shows that the Rh particle diameters remain constant at 33–43Å as the Rh weight loading increases from 0.4% to 2.2% to 4.4%. Further, after deposition of similar loadings of Pt, TEM analysis shows Pt particle diameters decrease with increasing Rh loading, since equal amounts of Pt were deposited on greater numbers of Rh seed particles. This pattern suggests a shell-core geometry, where Pt is deposited more or less uniformly around a Rh core.

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.

Methylcyclopentane transformation on Ge–Rh bimetallic catalysts prepared by organometallic grafting

Ge–Rh/Al 2O 3 catalysts were prepared by organometallic grafting, with nominal Ge loadings of 1/8, 1/4, 1/2 and 1 monolayer and actual loading of 300, 600, 2000 and 2100 ppm. The dispersion decreased proportionally to the Ge loading up to 600 ppm Ge. Methylcyclopentane was used as catalytic test reaction. The selectivity of ring opening was 70–90%. Addition of 300 and 600 ppm Ge divided the contiguous Rh sites to smaller ensembles. Here mostly 2-methylpentane gave C 1–C 5 fragments, its surface intermediate undergoing fragmentation without desorption, like on Rh with D < 60–70%. More Ge (2000 ppm) divided further Rh and also may have penetrated into subsurface layers. Here all primary ring opening products gave fragments, similarly to small Rh particles.

On the H-exchange of ammonia and silica hydroxyls in the presence of Rh nanoparticles

Attenuated total reflection infrared spectroscopy (ATR-FT-IR) is employed to study the H/D-exchange of planar hydroxylated silica during ND3 and D2 dosing, and catalyzed by Rh nanoparticles. For ND3 dosing, it is observed that the H/D-exchange is about 10 times more efficient in the presence of Rh nanoparticles on the hydroxylated silica than for bare hydroxylated silica. When the Rh adsorption sites are blocked by CO, the H/D-exchange is similar to the case without Rh nanoparticles. No H/D-exchange is observed for exposure to D2 regardless of the presence of Rh nanoparticles. Hydrogen spillover, involving the decomposition of D2 on the Rh and subsequent transfer of atomic D to the oxide support, therefore does not explain the observed effects. Alternatively, we conjecture that for ND3, the exchange is through a mechanism in which ND3 adsorption on the edge of the Rh particles takes place, followed by direct H/D exchange with the hydroxyls of the support. This exchange is possibly aided by the formation of ammonium complexes with the help of hydrogen from the hydroxyls.

Rh-Promoted Methanol Decomposition on Cerium Oxide Thin Films

Methanol adsorption and reaction have been studied on Rh-deposited cerium oxide thin films under UHV conditions using temperature-programmed desorption and synchrotron soft X-ray photoelectron spectroscopy. The methanol behavior was examined as a function of the Ce oxidation state, methanol exposure, and Rh particle size and coverage. When Rh nanoparticles were deposited on the ceria films, methanol decomposed on Rh to CO and H below 200 K. H atoms recombined and desorbed between 200 and 300 K. CO evolved from Rh deposited on fully oxidized ceria between 400 and 500 K. However, on reduced ceria films, the CO on Rh further decomposed to atomic C. Methanol adsorbed on the ceria films deprotonated to form methoxy as the only intermediate on the surface. This methoxy decomposed and desorbed as CO and H2 at higher temperatures regardless of the ceria oxidation state. Compared with the methanol reaction on Rh-free ceria thin films, formaldehyde formation from methoxy was completely suppressed after Rh deposition. Our results indicate that Rh can promote the decomposition of methoxy adsorbed on the ceria and that decomposition of methoxy intermediates occurred at the metal/oxide interfaces. On the other hand, the reduced ceria can promote total methanol decomposition on Rh.

Structure–activity correlation in thin film model catalysts: CO hydrogenation on Rh/VO x: Part II. Catalytic activity as a function of oxidation and reduction

The structure and morphology of thin film catalysts, consisting of well-defined Rh particles in contact with promoting and supporting vanadium oxides and subjected to oxidative and reductive treatments at elevated temperature, were correlated with the associated activity changes in CO hydrogenation. All systems exhibit a steady decrease of the catalytic activity with reduction temperature (373–823 K), which is in part due to vanadia covering free metal surface area and in part to the formation of stable bulk alloy phases. By comparison with the results of electron microscopy, the initial decrease of catalytic activity (between 373 and 473 K) is primarily caused by decoration of Rh particles by reduced VO x species. A strong decrease of catalytic activity after reduction above 573 K is related the formation of distinct Rh–V alloys. A selectivity shift towards methane at high reduction temperatures goes along with this activity loss.

Adsorption of CO and C 2H 4 on Rh-loaded thin-film dysprosium oxide

A dysprosium oxide thin film was deposited on Ru(0 0 0 1) by vapor depositing Dy in 2 × 10 −7 torr O 2 while the Ru was at 700 K. The film was ca. 5 nm thick and produced a p(1.4 × 1.4) LEED pattern relative to the Ru(0 0 0 1) substrate. The adsorption and reaction of CO and C 2H 4 adsorbed on Rh supported on the Dy 2O 3 film were studied by TPD and SXPS. The CO initially reacted with loosely bound oxygen in the substrate to produce CO 2. After the loosely bound oxygen was removed, the CO adsorbed non-dissociatively in a manner similar to what is seen on Rh(1 1 1). C 2H 4 adsorbed on the Rh particles and underwent progressive dehydrogenation to produce H 2 during TPD. The C from the C 2H 4 reacted with the O in Dy 2O 3 to produce CO. CO dissociation on the Rh particles could be promoted by treating the Dy 2O 3 with C 2H 4 before CO exposure.

Structure–activity correlations in thin film model catalysts: CO hydrogenation on Rh/VO x: Part I. The morphology, composition and structure of vanadia-supported and -promoted Rh particles upon oxidation and reduction

The combination of (high-resolution) electron microscopy and electron diffraction was applied to study the structural and morphological alterations of a number of Rh/VO x -model systems upon oxidation and reduction, and to discriminate between different phenomena of metal–support interaction. Well-defined Rh particles (mean size 10–15 nm) were grown epitaxially on NaCl(0 0 1) surfaces and subsequently covered by layers of VO x of varying thickness (0.07–25 nm), prepared by reactive deposition of V metal in 10 −2 Pa O 2. Most films were covered with a stabilizing layer of amorphous alumina. The resulting model catalysts were subjected to an oxidative treatment at 673 K in O 2 for 1 h and to subsequent reduction in the temperature range 373–873 K. While higher VO x exposures (mean VO x coverage ≥ 3 nm) favour the formation of crystalline V 2O 3 phases in partial epitaxial orientation to the Rh particles in the as-deposited state, lower exposures result in less ordered layers of cubic VO. Similarly, after a treatment in 1 bar O 2 at 673 K the oxidation states of vanadium vary between V 5+ and V 2+, depending on the film thickness. Decoration of Rh by reduced VO x species was found to be the dominant feature of metal–support interaction upon reduction at low temperatures ( T < 573 K), whereas at increasing reduction temperature the formation of distinct Rh–V alloys (V 3Rh 5, Rh 3V, V 3Rh and VRh, respectively) was observed. On a “VO x /Rh/Al 2O 3” catalyst, prepared by depositing 1ML VO x prior to Rh deposition alloy formation was not detected, and decoration of the metal particles was the dominant effect of reduction at 673 K. A counterpart to Rh/V “subsurface” or “surface” alloys, known to be formed on bulk Rh surfaces under similar conditions, could not be observed.

2D-Mapping of the Catalyst Structure Inside a Catalytic Microreactor at Work: Partial Oxidation of Methane over Rh/Al2O3

Tremendous changes of the structure of Rh particles occurred during partial methane oxidation to hydrogen and carbon monoxide over a 2.5 wt % Rh/Al(2)O(3) catalyst upon ignition of the catalytic reaction. Furthermore, near the ignition temperature a variation in the Rh-valence state along the catalyst bed was observed. By combining hard X-ray absorption spectroscopy (X-ray absorption near edge structure, XANES) with a charged coupled device (CCD) camera and using a suitable spectroscopic cell with gas supply and on-line mass spectrometry, we demonstrate that 2D-mapping of the Rh-oxidation state in a catalyst bed can be achieved during the catalytic reaction. For this purpose, X-ray images were recorded with the CCD camera at each energy around the Rh K-edge with and without the spectroscopic cell. This resulted effectively in the transmitted and incident intensity at each energy and at each pixel of the spectroscopic cell. Reconstruction of the full Rh K-edge XANES spectra at each pixel revealed the local distribution of oxidized and metallic Rh-species in the catalyst bed. Along the catalyst bed, structural changes were found with a steep gradient within less than 100 microm. Furthermore, a characteristic cone toward the inlet of the spectroscopic cell was observed.

Adsorption and catalytic reactions of acetonitrile and acetonitrile–oxygen mixture on TiO 2-supported rhodium catalysts

The adsorption and surface reactions of acetonitrile and acetonitrile–oxygen gas mixture were studied on TiO 2-supported Rh catalysts at 300–673 K. FTIR spectra show different kinds of molecularly adsorbed CH 3CN; acetonitrile can be bonded to weak Lewis acid sites (2295 cm −1), to strong Lewis acid sites (2319 cm −1), to very strong Lewis acid centres (2347 cm −1) of titania; it can be coordinated linearly through the lone electron pair of the N atom on Rh sites (2193 cm −1) and η 2 (C,N) CH 3CN species can be formed on Rh particles (1691–1708 cm −1). CH 3CN dissociates on Rh sites, the resulting CN (a) can be oxidized into NCO surface species. CN (a) can be dissociated only on Rh particles into N (a) and C (a). The hydrogenation of N (a) resulted in the appearance of NH 3 among the gaseous products from Rh/TiO 2 catalysts. The formation of other products (CH 3NH 2, H 2, CO 2, CH 4, C 2H 4 and CO) was demonstrated and discussed.

Sulphate-promotion and structure-sensitivity in hydrocarbon combustion over Rh/Al 2O 3 catalysts

A series of Rh/Al 2O 3 catalysts have been prepared using untreated or pre-sulphated alumina supports. The effect of support sulphation on catalyst activity towards propene and propane combustion has been explored as a function of Rh loading. Light-off temperatures for the total oxidation of both hydrocarbons decrease with increasing Rh content, associated with a transition from small oxidic clusters to large metallic Rh particles. Sulphate promotes both propene and propane combustion equally, with the magnitude of promotion exhibiting only a weak loading dependence. Enhanced catalytic performance is accompanied by Rh reduction and sintering.

Rh–V alloy formation in Rh–VOx thin films after high-temperature reduction studied by electron microscopy

Rh nanoparticles (mean size 10 and 15 nm), prepared by epitaxial growth on NaCl surfaces, were covered with layers of crystalline vanadium oxide (mean thickness 1.5 and 25 nm) by reactive deposition in 10(-2) mbar O2. The 1.5 nm film was further stabilized with a coating layer of 25 nm amorphous alumina. The so-obtained Rh/vanadia films, containing vanadium in the V3+ and V2+ state, were treated in 1 bar O2 at 673 K for 1 h and thereafter reduced in 1 bar H2 at increased temperatures, particularly between 723 and 873 K. The structural and morphological changes were followed by (high-resolution) transmission electron microscopy and selected area diffraction. Oxidation at 673 K transforms the purely vanadia-supported samples into Rh/V2O5, while in the alumina-supported films containing only small amounts of VOx, the formation of topotactic V2O3 is observed. The formation of Rh-V alloys during the subsequent reduction is strongly determined by the intimate contact and the structural and orientational relationship between Rh particles and the surrounding VOx phase. Reduction above 473 K transforms the support into substoichiometric vanadium oxides of composition VO and V2O. Analysis of high-resolution images and diffraction patterns reveals the presence of different alloy phases after reduction with increasing T (from 573 up to 823 K). In the alumina-supported film (low V/Rh ratio) the epitaxial alignment between the Rh particles and the surrounding V2O3 phase apparently favours the primary formation of defined alloys of type V3Rh and VRh3, followed by VRh at higher temperature. On the contrary, mainly V3Rh5 is formed in the purely VOx-supported Rh/films, due to different epitaxial relations in the initial state. Possible pathways of alloy formation are discussed.

An Effective Method of Controlling Metal Particle Size on Impregnated Rh-Mn-Li/SiO2 Catalyst

A convenient and effective impregnation method was found to prepare Rh-Mn-Li/SiO2 catalyst by using large granularity of silica as support, of which Rh particle size is at 2–4 nm with narrow distribution and the number of it accessible for reagents is enhanced. As expected, the space-time yield (STY) and selectivity of C2+oxygenates were significantly increased from 338.6 g/kg-cat.h, 49.2% to 618.4 g/kg-cat.h, 54.6% respectively, when 14–20 mesh, instead of 20–40 mesh of silica, was employed.