Pd-Rh Catalysts Research Articles

Stabilization of active sites in alloyed Pd–Rh catalysts on γ-Al2O3 support

Active components for three-way catalysts containing alloyed Pd–Rh particles with the concentration of the components 0.12 and 0.08wt.%, respectively, were synthesized and studied. The properties of these materials were compared with those of supported catalysts containing one or two precious metals (PM) prepared by mechanical mixing of the components with the same concentrations of PM. The stability of the samples was estimated by in situ prompt thermal aging (PTA) procedure. Photoluminescence (PL) and electron paramagnetic resonance (EPR) were used to show the importance of intrinsic sites of the support in stabilization of single-metal and alloyed Pd-containing clusters with high catalytic activity in CO oxidation. Substantial mutual effect of Pd and Rh in the alloyed particles was revealed. It prevents Pd-containing particles from sintering and hinders diffusion of Rh3+ ions into the bulk of the support at high temperatures. The spin probe technique was used to demonstrate that the alloy formation permits one to improve significantly the catalyst stability to reduction.

Metal foam-supported Pd–Rh catalyst for steam methane reforming and its application to SOFC fuel processing

Pd–Rh/metal foam catalyst was studied for steam methane reforming and application to SOFC fuel processing. Performance of 0.068 wt% Pd–Rh/metal foam catalyst was compared with 13 wt% Ni/Al2O3 and 8 wt% Ru/Al2O3 catalysts in a tubular reactor. At 1023 K with GHSV 2000 h−1 and S/C ratio 2.5, CH4 conversion and H2 yield were 96.7% and 3.16 mol per mole of CH4 input for Pd–Rh/metal foam, better than the alumina-supported catalysts. In 200 h stability test, Pd–Rh/metal foam catalyst exhibited steady activity. Pd–Rh/metal foam catalyst performed efficiently in a heat exchanger platform reactor to be used as prototype SOFC fuel processor: at 983 K with GHSV 1200 h−1 and S/C ratio 2.5, CH4 conversion was nearly the same as that in the tubular reactor, except for more H2 and CO2 yields. Used Pd–Rh/metal foam catalyst was characterized by SEM, TEM, BET and CO chemisorption measurements, which provided evidence for thermal stability of the catalyst.

Influence of the Brønsted acidity, SiO2/Al2O3 ratio and Rh–Pd content on the ring opening. Part II. Selective ring opening of methylcyclohexane

Monometallic and bimetallic Pd–Rh catalysts with various Rh/Pd atomic ratios (=0.5, 1 and 2) and a total metal charge of 1wt% supported on SiO2–Al2O3 (SIRAL 40, 20, 5) were studied for the ring opening of methylcyclohexane (MCH). The results were compared with those obtained previously for ring opening of decalin. It was found that the total acidity and the Brønsted acidity of the support have a higher influence on the activity for ring opening of naphthenic bicycles than on the activity for opening single rings. The influence of the metal charge is more important on the reaction of MCH. All the catalysts display a high MCH conversion, which increases with the reaction temperature. The bimetallic catalyst with the Rh/Pd ratio equal to 2 and supported on SIRAL 40 has the highest yield to ring opening products.

Enhanced Pd pillared clays by Rh inclusion for the catalytic hydrodechlorination of chlorophenols in water

Bimetallic Pd-Rh catalysts supported on pillared clays have been prepared and tested for aqueous-phase hydrodechlorination (HDC) using 4-chlorophenol (4-CP) as target compound. These bimetallic catalysts combine the higher activity of Rh with the better stability of Pd found in previous works with monometallic catalysts. Different combinations of Pd and Rh amounting to 1 wt% total metal load were tested and the catalyst with 0.75 wt% Pd and 0.25 wt% Rh yielded the best results. Ecotoxicity and biodegradability (measured as BOD(5)/COD ratio) of the effluents were checked. A significant decrease of ecotoxicity was observed while biodegradability was dramatically improved from 0.02 for the initial 4-CP solution up to values higher than 0.6.

Kinetic study of three-way catalyst of automotive exhaust gas: Modeling and application

The kinetics of carbon monoxide and propylene oxidation as well as nitrous oxide reduction on a Pd–Rh catalyst were determined with synthetic gas mixtures between 500 and 700 K. The overall reaction rate expression was constructed from elementary reaction steps and the kinetic parameters were determined from the experiments. A transient mathematical model has been developed to simulate automobile exhaust gas conversion based on the kinetics model equations. A comparison of simulated results with engine experiments proved to be successful. Results indicate that the reaction mechanisms proposed in this study are capable to describe the behavior of automotive exhaust gas converters, if mutual interactions of gaseous components and surface species are taken into account via elementary kinetic steps.

Novel mesoporous aluminosilicate supported palladium-rhodium catalysts for diesel upgrading

Abstract Bimetallic PdRh (with Pd/Rh mole ratio = 2/1) catalysts supported on a mesoporous aluminosilicate have been prepared and three methods of metal incorporation in the support have been compared: direct incorporation into the synthesis gel, impregnation and ion-exchange. Physico-chemical characterisations (nitrogen adsorption–desorption, hydrogen chemisorption, transmission electron microscopy, X-ray photoelectron and extended X-ray absorption fine structure spectroscopies), as well as simulated coking and regeneration tests are described. The dispersion, metal particle size and the formation of bimetallic particles within the material depend on the method of metal incorporation. Direct incorporation and ion-exchange methods lead to co-existence of PdRh intermetallic aggregates and segregated Pd; on the other hand, the impregnation method leads to the formation of very small and well dispersed PdRh alloy particles, with Pd preferentially located on the alloy particle surface.

Novel mesoporous aluminosilicate supported palladium-rhodium catalysts for diesel upgrading

Abstract Bimetallic PdRh catalysts (molar ratio Pd/Rh = 1 and 2) were prepared by impregnation of a mesoporous aluminosilicate (molar ratio Si/Al = 10 and 20). These materials are destined to be used in industrial processes aiming to improve diesel quality by hydrogenation and ring-opening of aromatic components. The four catalysts were examined for their activity in hydrogenation of naphthalene and tetralin model feedstocks, at 6 MPa, including in the presence of sulfur containing compounds. The capacity of one of these catalysts to improve the quality of hydrogenated industrial light cycle oil containing ≤50 wt ppm of sulfur was evaluated in a pilot plant. In these industrial conditions, the catalyst has a higher catalytic activity at lower temperature than a reference state of-the-art catalyst, giving a seven-point improvement of the cetane number at 280–300 °C and with formation of less than 10% of non-selective cracking products.

The effect of Rh addition on Pd/γ-Al2O3 catalysts deposited on FeCrAlloy fibers for total combustion of methane

Bimetallic Pd-Rh/Al2O3 catalysts were deposited by dip coating on FeCrAlloy type fibers. The effect of different gaseous pretreatments (under O-2/H-2 or H-2 flows) on the physico-chemical properties and the catalytic activity in the total combustion of methane was investigated. The gaseous pretreatments did not seem to exert any particular influence on the catalytic activity of these systems. When palladium is partially replaced by rhodium, the catalytic performances of the so-obtained bimetallic catalysts are lower than those of the monometallic Pd-based one. However it should be remarked that bimetallic catalysts with a Pd loading of 1% at least, show better methane conversion than the monometallic Rh-based samples. The long duration tests demonstrated that a bimetallic Pd-Rh catalyst has a better catalytic behaviour than a Pd-only catalyst. Such a behaviour could be due to the presence of Rh2O3, in the bimetallic catalyst, which avoids PdO reduction to Pd-0 and consequent sintering. It is worth noticing that no direct interaction between I'd and Rh was observed. A cooperation between PdO and Rh2O3 seems to explain the higher catalytic performances of bimetallic Pd-Rh catalysts. (c) 2007 Elsevier B.V. All rights reserved.

Conversion of chlorophenols into cyclohexane by a recyclable Pd-Rh catalyst

Mono and polychlorinated phenols that undergo only partial detoxification by conventional hydrogenation methods undergo exhaustive hydrogenation in the presence of a combined silica sol–gel entrapped catalyst composed of metallic palladium and chloro(1,5-cyclooctadiene)rhodium dimer. While the mono-, di-, tri- and tetrachlorophenols are converted into cyclohexane in a variety of solvents at 120 °C under 27.6 bar H 2, the pentachlorophenol requires the presence of toluene. Experiments conducted under milder conditions revealed that the hydrodechlorination of the chlorophenols proceeds via gradual evolvement of the chlorine atoms forming HCl and phenol. The C C bonds of the latter compound are saturated stepwise. The 1-cyclohexenol accumulates as cyclohexanone and is further hydrogenated to cyclohexanol. HCl-catalyzed dehydration yields cyclohexene which is readily transformed to cyclohexane. The initial hydrodechlorination of 2-chlorophenol follows a first order rate law with rate constant k = 5.0 × 10 −4 s −1 at 100 °C. The ceramic combined catalyst is leach-proof and recyclable and can be used in at lest six runs without loss in catalyst activity.

CO oxidation and NO reduction over supported Pt-Rh and Pd-Rh nanocatalysts: a comparative study

The surface compositions of supported Pt 50Rh 50 and Pd 50Rh 50 three-way catalysts (TWC) have been simulated by Monte-Carlo (MC) technique and used for calculating the activity of the catalysts for CO oxidation and NO reduction. Pd-Rh shows a higher activity than Pt-Rh for CO oxidation in presence of O 2, while Pt-Rh is a better CO oxidation catalyst in absence of O 2. For NO reduction, Pt-Rh nanoparticles are better catalysts both in presence and absence of O 2. β-N 2 is found to be the most dominant channel for N 2 formation on Pt-Rh nanoparticles, while, δ-N 2 is found to be the most dominant pathway for N 2 production on Pd-Rh. The poisoning effect of sulphur is found to be more on Pd-Rh catalyst than on Pt-Rh catalyst.

Exhaustive hydrodechlorination of chlorinated aromatic environmental pollutants to alicyclic compounds

A combined Pd–Rh catalyst obtained by co-entrapment of metallic palladium and soluble [Rh(cod)Cl]2 within a silica sol–gel matrix exhaustively detoxifies chlorinated aromatic pollutants not only by total removal of the chlorine, but also by the total reduction of the aromatic moieties to alicyclic rings. The list of chloroarenes which have been successfully destroyed by the catalysts includes most of the major pollutants which are of public concern, namely chlorinated dibenzo[b,e]dioxin and dibenzofuran, PCBs, DDT, Hexachlorophene and 2,4,5-T. The reactions take place under the relatively mild conditions of 80–100 °C under 27 atm H2. The immobilised catalyst is leach proof in a variety of solvents and can be reused in several runs. The high activity of the combined Pd–Rh system is attributed to synergism between the two different metal atoms.

Titration by Oxygen of the Spill-over Hydrogen Adsorbed on Ceria–Zirconia Supported Palladium–Rhodium Catalysts

To find a method for estimating the surface area of ceria–zirconia in composite three-way catalysts, successive O2/H2/O2 chemisorption measurements were performed at room temperature on model Pd and/or Rh/Ce0.63Zr0.37O2 catalysts previously reduced at 573 K by hydrogen, according to a methodology developed for ceria or ceria–alumina supported metal catalysts. The quantity adsorbed during the first oxygen chemisorption was found to correspond to the reoxidation of several layers of the support. The number of layers increases when the catalysts are aged at 1323 or 1423 K, which clearly indicates the great mobility of bulk oxygen in CeO2–ZrO2 supports, as already shown in the literature. After a subsequent H2 chemisorption, the titration by oxygen of the hydrogen adsorbed on metal particles and on the support leads to a lower quantity of chemisorbed oxygen which is closely related to the surface area of ceria–zirconia. This relationship was observed in fresh and aged catalysts and was used to calculate a mean calibration coefficient of 5.1 μatom O m−2. Thus, contrary to temperature-programmed reduction measurements, this method of quantification by oxygen of the spill-over hydrogen can be used to estimate the surface area of ceria–zirconia in composite catalysts in which CeO2–ZrO2 is one of the components of the support. This latter point is illustrated with model Pd and PdRh catalysts deposited on a ‘ceria–zirconia + alumina’ support.

Effects of Pretreatments on the Surface Composition of Alumina-Supported Pd–Rh Catalysts

The surface composition of alumina-supported Pd, Rh, and Pd–Rh catalysts fresh or after high temperature treatments under oxidizing (O2 in N2) or reducing (H2 in N2) mixture has been characterized using various complementary techniques: chemisorption and titration of H2 and O2 probe molecules, 18O2/16O2 isotopic equilibration (OIE), FTIR spectroscopy of adsorbed CO and NO, and electron microscopy with X analysis. From both techniques it is concluded that the surface composition is close to the bulk composition for all the fresh bimetallics, whatever the Rh content. The surface state of the bimetallics treated at 1173 K is strongly dependent on the nature of the gas mixture. There is a relative increase in surface Pd after treatment in an oxidative medium, because of a Rh3+ migration into alumina, and in surface Rh after treatment in a reducing medium. The specific role of alumina can explain these results, in contradiction with thermodynamic models predicting the reverse situation (enrichment in Rh in O2 and in Pd in H2). Activity for propane oxidation at 573 K and for propane steam reforming at 673 K was also determined over fresh and sintered catalysts. Because Pd is more active than Rh in oxidation and Rh is more active than Pd in steam reforming, activity changes with the Rh content, before and after sintering, may reflect modifications of surface composition. The results obtained with the series of Pd–Rh catalysts are in agreement with the measurements made by OIE and FTIR spectroscopy. The hypothesis and the conditions of applications of the methods are discussed.

Reducibility of Bimetallic Pd–Rh Catalysts and Its Relation with Synergism in the Oxidation Reaction of CO with O2†

A synergistic effect related to the greater stability of the Pd–Rh bimetallic catalysts in the reduced state in the oxidation reaction of CO is observed.

Activity during reduction of NO by CO over bimetallic palladium-rhodium/silica catalysts

A study of the activity of bimetallic Pd-Rh catalysts supported on silica in the reduction of NO by CO is presented. The catalysts were prepared by three different methods: (1) Pd and Rh were coimpregnated on the support, (2) Rh was impregnated first and, after calcining, the sample was impregnated with Pd, (3) the monometallic Pd and Rh catalysts were physically mixed. The results showed that the activity of the catalysts prepared by coimpregnation was much lower than that of the other two catalysts.

Propylene oxide synthesis via propene acetoxylation over supported palladium and platinum catalysts followed by cracking of glycol acetates in a melt of potassium acetate

We have investigated the process of propylene oxide synthesis via propene acetoxylation to propylene glycol (PG) and its acetates (PGA) followed by cracking of propylene glycol monoacetates (PGMA). We studied the acetoxylation of propene in acetic acid solutions of nitric acid over Pd, Pd-Pt and Pd-Rh catalysts supported on oxide (TiO 2, SiO 2) and carbon carriers. Palladium catalysts on carbon exhibited high activity and selectivity for PG and PGA (total yield up to 90%). The introduction of promoters (platinum or rhodium) into palladium catalysts improved their properties; in particular increasing their stability towards metal leaching and poisoning under the reaction conditions. The formation of cyanide metal complexes was found to be one of the reasons for catalyst deactivation. Unlike monometallic and homogeneous systems, bimetallic supported catalysts can be effectively reactivated many times. A melt of potassium acetate was shown to catalyze the cracking of PGMA into propylene oxide and acetic acid. The accompanying reactions of the decomposition of potassium acetate and acetic acid were studied. The most favorable synthesis conditions were found, leading to the avoidance of undesirable reactions. The yields of propylene oxide and acetic acid reached the values of 80 and 90% (based on PGMA), respectively, at 40% conversion of PGMA.

Activity and infrared studies during carbon monoxide oxidation over bimetallic palladium-rhodium/silica catalysts

Abstract A study of the activity of bimetallic Pd-Rh catalysts supported on silica in the oxidation of carbon monoxide by oxygen is presented. The catalysts were prepared by three different methods (1) palladium and rhodium were coimpregnated on the support, (2) rhodium was impregnated first and, after calcining, the sample was impregnated with palladium, (3) the monometallic palladium and rhodium catalysts were physically mixed. The results showed that the activity of the catalysts prepared by coimpregnation was much lower than that of the other two catalysts. The low activity of the coimpregnated catalyst can be explained by the formation of alloys which affect the surface concentration of each metal and the dispersion of rhodium. The most active catalyst was the one prepared by physical mixture, which also shows the highest concentration of linearly adsorbed carbon monoxide detected by FT-IR.