Entrapment of nanostructured palladium clusters in hydrophobic sol–gel materials

The fluoride-catalysed hydrolysis of mixtures of CH₃Si(OCH₃)₃ and Mg(OC₂H₅)₂ in the presence of preformed nanosized R₄N⁺Br⁻-stabilised Pd colloids results in the formation of micro/mesoporous hydrophobic sol–gel materials in which the Pd clusters are entrapped individually in the solid matrix. The materials are active catalysts in the hydrogenation of 1,5-cyclooctadiene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Complete Oxidation of Methane at Low Temperature over Pt Catalysts Supported on High Surface Area SnO2

The catalytic activity of Pt catalysts supported on high surface area tin(IV) oxide in the complete oxidation of CH₄ traces under lean conditions at low temperature was studied in the absence and in the presence of water (10 vol.%) or H₂S (100 vol.ppm). Their catalytic properties were compared to those of Pd/Al₂O₃ and Pt/Al₂O₃. In the absence of H₂S in the feed, Pt/SnO₂appears as a very promising catalyst for CH₄ oxidation, being even significantly more active under wet conditions than the best reference catalyst, Pd/Al₂O₃. Catalysts steamed-aged at 873 K were also studied in order to simulate long term ageing in real lean-burn NGV exhaust conditions. To this respect, Pt/SnO₂ is slightly less resistant than Pd/Al₂O₃. In the presence of H₂S, Pt/SnO₂catalysts are rapidly and almost completely poisoned, comparably to Pd/Al₂O₃and the catalytic activity is hardly restored upon oxidising treatment below 773 K. A synergetic effect between Pt and specific surface SnO₂sites active in CH₄oxidation is proposed to explain the superior catalytic behaviour of Pt/SnO₂.     

Oxygen-enhanced water gas shift on ceria-supported Pd–Cu and Pt–Cu bimetallic catalysts

Aiming at enhancing H₂ production in water gas shift (WGS) for fuel cell application, a small amount of oxygen was added to WGS reaction toward oxygen-enhanced water gas shift (OWGS) on ceria-supported bimetallic Pd–Cu and Pt–Cu catalysts. Both CO conversion and H₂ yield were found to increase by the oxygen addition. The remarkable enhancement of H₂ production by O₂ addition in short contact time was attributed to the enhanced shift reaction, rather than the oxidation of CO on catalyst surface. The strong dependence of H₂ production rate on CO concentration in OWGS kinetic study suggested O₂ lowers the CO surface coverage. It was proposed that O₂ breaks down the domain structure of chemisorbed CO into smaller domains to increase the chance for coreactant (H₂O) to participate in the reaction and the heat of exothermic surface reaction helping to enhance WGS kinetics. Pt–Cu and Pd–Cu bimetallic catalysts were found to be superior to monometallic catalysts for both CO conversion and H₂ production for OWGS at 300 °C or lower, while the superiority of bimetallic catalysts was not as pronounced in WGS. These catalytic properties were correlated with the structure of the bimetallic catalysts. EXAFS spectra indicated that Cu forms alloys with Pt and with Pd. TPR demonstrated the strong interaction between the two metals causing the reduction temperature of Cu to decrease upon Pd or Pt addition. The transient pulse desorption rate of CO₂ from Pd–Cu supported on CeO₂ is faster than that of Pd, suggesting the presence of Cu in Pd–Cu facilitate CO₂ desorption from Pd catalyst. The oxygen storage capacity (OSC) of CeO₂ in the bimetallic catalysts indicates that Cu is much less pyrophoric in the bimetallic catalysts due to lower O₂ uptake compared to monometallic Cu. These significant changes in structure and electronic properties of the bimetallic catalysts are the result of highly dispersed Pt or Pd in the Cu nanoparticles.     

Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects

A systematic study of the comparative performances of supported Pt, Pd, Ru and conventional CoMo/Al₂O₃, NiMo/Al₂O₃, NiW/Al₂O₃ catalysts as well as the effects of solvent, H₂ pressure and temperature on the hydroprocessing activity of a representative model bio-oil compound (e.g., p-cresol) is presented. With water as solvent, Pt/C catalyst shows the highest activity and selectivity towards hydrocarbons (toluene and methylcyclohexane), followed by Pt/Al₂O₃, Pd and Ru catalysts. Calculations indicate that the reactions in aqueous phase are hindered by mass-transfer limitations at the investigated conditions. In contrast, with supercritical n-heptane as solvent at identical pressure and temperature, the reactant and H₂ are completely miscible and calculations indicate that mass-transfer limitations are eliminated. All the noble metal catalysts (Pt, Pd and Ru) show nearly total conversion but low selectivity to toluene in supercritical n-heptane. Further, conventional CoMo/Al₂O₃, NiMo/Al₂O₃ and NiW/Al₂O₃ catalysts do not show any hydrodeoxygenation activity in water, but in supercritical n-heptane, CoMo/Al₂O₃ shows the highest activity among the tested conventional catalysts with 97?% selectivity to toluene. Systematic parametric investigations with Pt/C and Pt/Al₂O₃ catalysts indicate that with water as the solvent, the reaction occurs in a liquid phase with low H₂ availability (i.e., low H₂ surface coverage) and toluene formation is favored. In supercritical n-heptane with high H₂ availability (i.e., high H₂ surface coverage), the ring hydrogenation pathway is favored leading to the high selectivity to 4-methylcyclohexanol. In addition to differences in H₂ surface coverage, the starkly different selectivities between the two solvents may also be due to the influence of solvent polarity on p-cresol adsorption characteristics.     

Heterobimetallic MIr (M=Pt or Pd) dithiolato bridge complexes

New palladium–iridium and platinum–iridium heterobimetallic bridged dithiolato complexes of general formula [(P-P)M(μ-S-S)Ir(COD)]PF₆ (COD=1,5-cyclooctadiene; M=Pt; P-P=(PPh₃)₂ and S-S=EDT (1,2-ethanedithiolato) (1), PDT (1,3-propanedithiolato) (2) and BDT (1,4-buthanedithiolato) (3); M=Pd; P-P=dppb and S-S=EDT (4), P-P=dppp and S-S=EDT (5) have been prepared.     

Pt–Pd bimetallic catalysts for methane emissions abatement

Bimetallic Pd–Pt catalysts with constant 2 wt% metal loading and varying Pt/Pd ratios were prepared, characterized and studied in the catalytic combustion of methane at low temperature under lean conditions in view of their use for CH₄ abatement from lean-burn NGV heavy duty vehicles exhausts. The influence of mild steam ageing featuring long-term use of the catalysts was also addressed together with their tolerance to H₂S. Catalysts were characterised by Transmission Electron Microscopy and Temperature Programmed Oxidation experiments. Experimental data agreed to suggest an interaction between Pd and Pt in Pd-rich catalysts, thus explaining their improved catalytic activity, even after mild ageing, compared to reference Pd/Al₂O₃. This interaction has no effect on the sulfur tolerance.     

Pt promotional effects on Pd–Pt alloy catalysts for hydrogen peroxide synthesis directly from hydrogen and oxygen

H₂O₂ synthesis directly from H₂ and O₂ over supported Pd–Pt alloy catalysts was carried out using a semibatch reactor under ambient conditions. As compared to pure Pd, the performance of Pd–Pt catalysts was enhanced significantly. The promotional role of Pt was studied systematically by using in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (DRIFTS), quantitative powder X-ray diffraction (XRD), X-rays photoelectron spectroscopy (XPS), and temperature-programmed desorption of H₂/O₂ (H₂/O₂-TPD). The spectra of DRIFT, XPS, and XRD demonstrate the formation of Pd–Pt alloy particles, which surfaces are enriched by Pt accompanying with possible electron transfer from Pd to Pt. The addition of Pt into Pd phase was proposed to impact on reactants adsorption, stabilization of intermediates such as OOH and OH radicals, and the formation and decomposition of H₂O₂.     

Confirmation of sulfur tolerance of bimetallic Pd–Pt supported on highly acidic USY zeolite by EXAFS

The sulfur tolerance (i.e., degree of sulfidation) of Pd and Pt in sulfided bimetallic Pd–Pt catalysts (Pd : Pt mole ratio of 4 : 1) supported on USY (ultrastable Y) zeolites (SiO₂/Al₂O₃ = 10.7, 48, and 310) was investigated using an extended X‐ray absorption fine structure (EXAFS) method. The sulfidation of the catalysts was done in a 1000 ppm H₂S–2% H₂/N₂ stream at 573 K for 0.5 h. In the Fourier transforms of Pd K‐edge and Pt L_III‐edge EXAFS spectra, both of the peaks due to metallic Pd and to metallic Pt for the Pd–Pt/USY (SiO₂/Al₂O₃ = 10.7) catalyst remained most after sulfidation. Further, the results of the Fourier transforms confirmed that the sulfur tolerance of both Pd and Pt decreased with increasing SiO₂/Al₂O₃ ratio, suggesting that Pd and Pt become sulfur‐tolerant when Pd–Pt bimetallic particles are supported on highly acidic USY zeolite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pd and Pt catalysts supported on carbon-coated monoliths for low-temperature combustion of xylenes

Carbon-coated monoliths were prepared from polyfurfuryl alcohol coated cordierite structures obtained by dip-coating. In this way, thin, homogeneous, consistent and good adhered carbon layers were obtained. The different steps followed in the preparation of these catalyst supports were studied by scanning electron microscopy. Pd and Pt catalysts were prepared by equilibrium impregnation of the monolithic supports with an aqueous solution of the corresponding tetraammine metal (II) nitrates. The catalysts were pretreated in H₂ at 300 °C before their characterization by chemisorption or before testing their catalytic activity. This pretreatment was monitored by temperature programme reduction. Catalytic activities in xylene combustion were evaluated as a function of the reaction temperature as well as against time on stream. The monolithic catalysts were thermally stable during the reaction. The Pt catalysts were more active than the Pd ones. The Pd catalysts with smaller Pd-particle sizes were more active. In the case of Pt catalysts however, the opposite was observed, which might be due to a structure sensitivity effect. Complete xylene combustion was reached in the range between 150 and 180 °C with a total selectivity to CO₂ and H₂O. Combustion of m-xylene was easier than p-xylene over Pt.     

New Supported Pd and Pt Alloy Catalysts for Steam Reforming and Dehydrogenation of Methanol

The catalytic performances of supported Group 810 metal (Co, Ni, Ru, Pd, Ir and Pt) catalysts for steam reforming of methanol, CH₃OH + H₂O CO₂ + 3H₂, and dehydrogenation of methanol to methyl formate, 2CH₃ OH HCOOCH₃ + 2H₂, are markedly affected by the kinds of supports as well as the metals used. The selectivity for steam reforming and the formation of methyl formate was markedly improved when Pd or Pt were supported on ZnO, In₂O₃ and Ga₂O₃. The combined results of temperature-programmed reduction, XRD, XPS and AES revealed that Pd-Zn, Pd-In, Pd-Ga, Pt-Zn, Pt-In and Pt-Ga alloys were formed upon reduction. Over the catalysts having an alloy phase, the reactions proceeded selectively, whereas over the catalysts having a metallic phase, methanol was decomposed to carbon monoxide and hydrogen predominantly. It was shown that the reactivity of formaldehyde intermediate over the Pd and Pt alloys was markedly different from that over metallic Pd and Pt. Over Pd and Pt alloys, aldehyde species were stabilized and transformed into carbon dioxide and hydrogen or methyl formate by nucleophilic addition of water or methanol, respectively. By contrast, over metallic Pd and Pt, aldehyde species were rapidly decarbonylated to carbon monoxide and hydrogen.     

Skeletal Isomerization of n-Pentane over Platinum-Promoted Tungstophosphoric Acid Supported on MCM-41

Skeletal isomerization of n-pentane in the presence of hydrogen has been studied over Pt-promoted H₃PW₁₂O₄₀ (TPA)/MCM-41 bifunctional catalyst. A series of solid acid catalysts with different loading amount of TPA and Pt were prepared and characterized by XRD, FT-IR and XPS. The optimal catalytic activity of Pt-TPA/MCM-41 was observed with 2% Pt and 30% TPA. According to the cracked products distribution, this is typical of a monomolecular bifunctional metal-acid mechanism. Further, catalysts with different combination of noble metals (Pt, Pd and Ru), heteropoly acids (HPAs) (TPA, tungstosilicic acid (TSA), and molybdophosphoric acid (MPA)) and supports (MCM-41, SBA-1 and SiO₂) were also synthesized and their catalytic performances were compared.     

Synergistic effect of Pt or Pd and perovskite oxide for water gas shift reaction

The water gas shift (WGS) reaction over Pt and Pd catalysts supported on various perovskite oxides has been investigated at 573 K without catalyst pretreatment. The Pt and Pd catalysts on LaCoO₃ support showed high catalytic activity. Interaction between Pt or Pd and the support is considered to promote the WGS reaction: Pt/LaCoO₃ had high initial activity but deactivated immediately; Pd/LaCoO₃ was less active than Pt/LaCoO₃, but had superior stability. Catalysts were characterized using XRD, STEM, XPS, and H₂-temperature programmed reduction (TPR). Results of this study showed that reduction of the support decreased the CO conversion on Pt/LaCoO₃. On the other hand, Pd/LaCoO₃ showed stable activity for the WGS reaction. Therefore, Pd was added to Pt/LaCoO₃ for stabilizing the catalyst activity, and 0.5 wt.% Pd/1 wt.% Pt/LaCoO₃ catalyst showed higher activity and stability.     

Efficient Anchorage of Palladium Nanoparticles on the Multi‐Walled Carbon Nanotubes as Electrocatalyst for the Hydrazine Electrooxidation in Strong Acidic Solutions

A facile homogeneous precipitation–reduction reaction method, which involves PdCl₂ → PdO · H₂O → Pd⁰ reaction path, is used to synthesize the multi‐walled carbon nanotubes (MWCNTs) supported Pd nanoparticles (Pd/MWCNTs) catalysts. The particle size of Pd/MWCNTs catalysts can be easily tuned by controlling the hydrolysis temperature of PdCl₂. X‐ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show the particle size of Pd/MWCNTs catalysts increases with hydrolysis temperature of PdCl₂, which is ascribed to the fact that the particle size of PdO · H₂O nanoparticles increases with hydrolysis temperature of PdCl₂. At the lower hydrolysis temperature, the as‐prepared Pd/MWCNTs catalyst possesses the higher dispersion and the smaller particle size. Consequently, the resultant Pd/MWCNTs catalyst exhibits the big electrochemical active surface area and the excellent electrocatalytic performance for hydrazine electrooxidation in strong acidic solutions. In addition, the electrochemical measurement indicate that particle size effect of Pd‐NPs occurs during the N₂H₄ electrooxidation. In brief, the mass activity and specific activity of the Pd/MWCNTs catalyst increases and decreases with decreasing the particle size of Pd‐NPs for the N₂H₄ electrooxidation, respectively.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

Combustion of Methane at Low Temperature over Pd and Pt Catalysts Supported on Al2O3, SnO2 and Al2O3-Grafted SnO2

Pd and Pt catalysts supported on alumina, tin(IV) oxide and tin(IV) oxide grafted on alumina were prepared, characterised and tested with respect to the low-temperature combustion of methane after reduction in H₂ and ageing under reactants at 600°C. In the case of Pd, the use of SnO₂ or SnO₂-based supports led to catalysts slightly less active than Pd/Al₂O₃. In contrast, SnO₂ was found to strongly promote the oxidation of methane over Pt catalysts with respect to Pt/Al₂O₃, even after ageing under reactants. When Pt was supported on SnO₂ grafted on Al₂O₃, the activity was found at most similar to or, after ageing, lower than Pt/Al₂O₃. This negative effect was discussed, being partly related to the sintering of SnO₂ under reactants observed by FTIR and XRD.     

Aqueous-phase reforming of ethylene glycol over supported Pt and Pd bimetallic catalysts

More than 130 Pt and Pd bimetallic catalysts were screened for hydrogen production by aqueous-phase reforming (APR) of ethylene glycol solutions using a high-throughput reactor. Promising catalysts were characterized by CO chemisorption and tested further in a fixed bed reactor. Bimetallic PtNi, PtCo, PtFe and PdFe catalysts were significantly more active per gram of catalyst and had higher turnover frequencies for hydrogen production (TOF_H2) than monometallic Pt and Pd catalysts. The PtNi/Al₂O₃ and PtCo/Al₂O₃ catalysts, with Pt to Co or Ni atomic ratios ranging from 1:1 to 1:9, had TOF_H2 values (based on CO chemisorption uptake) equal to 2.8–5.2 min⁻¹ at 483 K for APR of ethylene glycol solutions, compared to 1.9 min⁻¹ for Pt/Al₂O₃ under similar reaction conditions. A Pt₁Fe₉/Al₂O₃ catalyst showed TOF_H2 values of 0.3–4.3 min⁻¹ at 453–483 K, about three times higher than Pt/Al₂O₃ under identical reaction conditions. A Pd₁Fe₉/Al₂O₃ catalyst had values of TOF_H2 equal to 1.4 and 4.3 min⁻¹ at temperatures of 453 and 483 K, respectively, and these values are 39–46 times higher than Pd/Al₂O₃ at the same reaction conditions. Catalysts consisting of Pd supported on high surface area Fe₂O₃ (Nanocat) showed the highest turnover frequencies for H₂ production among those catalysts tested, with values of TOF_H2 equal to 14.6, 39.1 and 60.1 min⁻¹ at temperatures of 453, 483 and 498 K, respectively. These results suggest that the activity of Pt-based catalysts for APR can be increased by alloying Pt with a metal (Ni or Co) that decreases the strengths with which CO and hydrogen interact with the surface (because these species inhibit the reaction), thereby increasing the fraction of catalytic sites available for reaction with ethylene glycol. The activity of Pd-based catalysts for APR can be increased by adding a water-gas shift promoter (e.g. Fe₂O₃).     

Characterisation and microstructure of Pd and bimetallic Pd–Pt catalysts during methane oxidation

The catalytic oxidation of methane was studied over Pd/Al₂O₃ and Pd–Pt/Al₂O₃. It was found that the activity of Pd/Al₂O₃ gradually decreases with time at temperatures well below that of PdO decomposition. The opposite was observed for Pd–Pt/Al₂O₃, of which the activity decreases slightly with time. Morphological studies of the two catalysts showed major changes during operation. The palladium particles in Pd/Al₂O₃ are initially composed of smaller, randomly oriented crystals of both PdO and Pd. In oxidising atmospheres, the crystals become more oxidised and form larger crystals. The activity increase of Pd–Pt/Al₂O₃ is probably related to more PdO being formed during operation. The particles in Pd–Pt/Al₂O₃ are split into two different domains: one with PdO and the other likely consisting of an alloy between Pd and Pt. The alloy is initially rich in palladium, but the composition changes to a more equalmolar Pd–Pt structure during operation. The ejected Pd is oxidised into PdO, which is more active than its metallic phase. The amount of PdO formed depends on the oxidation time and temperature.     

The ROMP catalysts Mo(CO)5PPh3/alkylaluminum with unstrained cyclic and linear dienes as initiators of the addition polymerisation

Summary We study the polymerisation of a series of unstrained cyclic and acyclic dienes with the catalytic system of Mo(CO)₅PPh₃/alkylaluminum. The Mo(CO)₅PPh₃/alkylaluminum catalyse the nonmetathesis polymerisation, through a pair of ionic complexes similar to that of Ziegler-Natta mechanism, and not through the ROMP (Ring-Opening-Metathesis-Polymerisation) mechanism followed by strained olefins. Because of the nature of these complexes the reactivity is lower with respect to that of a common Ziegler-Natta catalyst, such as Cp₂TiCl₂/Et₃Al₂Cl₃, as it is demonstrated, by means of kinetics and ESR measurements, in the case of the polymerisation of of 1,5-cyclooctadiene. Similarly to the Ziegler-Natta catalysts, they give cyclised products in the polymerisation of 1,5-hexadiene and 1,7-octadiene, and the 1,4-cis unit in the case of the polypentadienes. Finally we find that they are active catalysts for the polymerisation of styrene. Received: 12 March 1998/Revised version: 20 April 1998/Accepted: 22 April 1998     

The effect of steam and hydrogen in promoting the oxidation of carbon monoxide over a platinum on alumina catalyst

Steam- and hydrogen-induced effects on carbon monoxide oxidation at temperatures below 200°C were studied over an alumina-supported Pt catalyst. These studies were complemented by measurements of rate expressions and IR spectroscopy. CO oxidation activity over alumina-supported Pt, Pd and Rh catalysts was found to be strongly promoted by the presence of steam and H₂ in the feed gas. The promotion effect decreased in the order Pi>Pd> Rh. The water-gas shift reaction did not occur under these experimental conditions. The steam and H₂ enhancement effects are attributed to the weakening of self-poisoning by CO.     

The Mechanism of N2O Decomposition on Fe-ZSM-5: An Isotope Labeling Study

The role of Fe promoters has been investigated on Pd/ceria, Pt/ceria and Rh/ceria catalysts for the water–gas shift (WGS) reaction in 25 torr of CO and H₂O under differential reaction conditions. While no enhancement was observed with Pt and Rh, the activity of Pd/ceria increased by as much as an order of magnitude upon the addition of an optimal amount of Fe. Similarly, the addition of 1 wt% Pd to an Fe₂O₃ catalyst increased the WGS rate at 453 K by a factor of 10 over that measured on Fe₂O₃ alone, while the addition of Pt or Rh to Fe₂O₃ had no effect on rates. The amount of Fe that was necessary to optimize the rates increased with Pd loading but was independent of the order in which Fe and Pd were added to the ceria. Increased WGS activity was also observed upon the addition of Fe to Pd supported on Ce_0.5Zr_0.5O₂. XRD measurements, performed after running the catalyst under WGS conditions, show the formation of a Fe–Pd alloy, even though similar measurements on an Fe/ceria catalyst showed that Fe₃O₄ was the stable phase for Fe in the absence of Pd. Possible implications of these results on the development of new WGS catalysts are discussed.