New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Pd and Pt supported on ZnO, Ga₂O₃ and In₂O₃ exhibit high catalytic performance for the steam reforming of methanol, CH₃OH+H₂OCO₂+3HH₂, and the dehydrogenation of methanol to HCOOCH₃, 2CH₃OHHCOOCH₃+2HH₂. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.     

Pd oxidation under UHV in a model Pd/ceria–zirconia catalyst

A model planar catalyst was prepared by depositing Pd onto a thick (few m) film of ceria–zirconia in ultrahigh vacuum (UHV), and the oxidation state of Pd and its support was determined by X-ray photoelectron spectroscopy, following thermal treatments in UHV, oxygen, or carbon monoxide. It was found that Pd could be oxidized simply by heating the catalyst in UHV, indicating that transfer of oxygen from the support to the metal is both thermodynamically favorable and facile.     

Corrosion of Au–Pd–In alloy in simulated physiological solutions

The corrosion of Au–Pd–In alloy, which is of great importance in dentistry, has been studied using an electrochemical quartz crystal microbalance (EQCM) in simulated physiological solutions. The alloy was deposited on quartz substrates by means of magnetron sputtering (MS). Analysis performed using X-ray photoelectron spectroscopy showed that the chemical composition of the sputtered deposit was similar to that of the MS target made of conventional casting alloy. Investigations by X-ray diffraction indicated a crystalline structure of the MS alloy. The electrochemical and corrosion behaviour of the Au–Pd–In alloy was studied in three simulated physiological solutions: 0.9 M NaCl, 0.1 M NaCl + 0.1 M lactic acid and artificial saliva. Determination of break down potential was complicated by the anodic gold dissolution due to formation of a chloride complex. The onset of anodic currents, therefore, indicated not the potential at which the passive layer starts to be destroyed, but the exceeding of the Au/AuCl₄ ^– equilibrium potential, which does not directly reflect corrosion resistance. The EQCM measurements under open circuit conditions indicated corrosion as an increase in mass, caused by the accumulation of corrosion products on the alloy surface. The increase in mass in acidic solution (pH 2.2) was similar to that in neutral solution (pH 6.5), which implies dissolution of corrosion products to be insignificant.     

Modeling of Hydrogen Permeation through Pd Membrane on Ceramic Supports Activated via Pd Nanoparticles–TiO2–Bohemite Suspension

Theoretical Foundations of Chemical Engineering - Thin dense Pd composite membrane was prepared via electroless plating method. Pd nanoparticles embedded polyethylene glycol (PEG) was used in...     

Sol–Gel Pd Exhaust Catalysts and Hydrocarbon Speciation

Sol–gel Pd/alumina promoted by oxygen-storing CeO₂ or CeO₂–Tb₄O₇ catalysts has higher hydrocarbon oxidation activity than commercial three-way catalysts. It is active in hydrocarbon (and CO) oxidation even under net reducing conditions albeit over short periods of time. This observation is relevant to cold start conditions. Cryo-focussing GCMS and pulse RGA experiments suggest that mechanistically interesting by-product hydrocarbon species evolve from the catalysts under just these conditions, even when the fuel is LPG in type. Further catalyst developments and analytical refinements are likely to go hand in hand as the mode of operation of these environmental catalysts is better understood from their by-product fingerprint.     

A calorimetric study of oxygen-storage in Pd/ceria and Pd/ceria–zirconia catalysts

Heats of adsorption were measured calorimetrically for O₂ adsorption on reduced Pd/alumina, Pd/ceria, and Pd/ceria–zirconia catalysts, all with 1 wt% Pd. Significantly more O₂ adsorbed on the ceria-containing catalysts due to oxidation of the support. For Pd/alumina, the heats were found to be between 180 and 220 kJ/mol, only slightly higher in magnitude than the heat of reaction for bulk oxidation of Pd. However, the heats of adsorption for both ceria and ceria–zirconia were also 200 kJ/mol, much lower than the heat of reaction for Ce₂O₃ oxidation to CeO₂, but in reasonable agreement with estimates from O₂ desorption studies on model ceria films. The implications of these results for understanding oxygen-storage properties on ceria-based catalysts are discussed.     

Interaction of carbon monoxide with Cu–Pd and Cu–Ni bimetallic clusters

Interaction of CO with Cu–Pd and Cu–Ni bimetallic clusters deposited on a ZnO substrate has been investigated by core-level spectroscopy. The surface reactivity of both these alloy clusters increases with the decrease in cluster size, giving rise to dissociative adsorption at small cluster size. The surface reactivity also increases with the increase in Pd or Ni content and the reactivity of the alloy clusters is unlike that of either component metal. Thus, dissociative adsorption occurs on small Cu–Pd clusters unlike on either Cu or Pd clusters of comparable size. The reactivity of the Cu–Ni clusters, on the other hand, falls somewhere between those of Cu and Ni clusters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solvothermal synthesis route to ternary chalcogenides Cu(Ag)–Pd–S

A mild route to ternary chalcogenidometalates Cu₃Pd₁₃S₇ and Ag₅Pd₁₀S₅ nanocrystals is proposed in this paper. Both compounds can be directly synthesized from the reaction of PdCl₂, sulfur and CuCl₂ or AgNO₃ through solvothermal processes in diethylamine or ethylenediamine (en) at 140–160°C for 10 h. The final products have been characterized by X-ray diffraction (XRD) patterns, transmission electron microscope (TEM) images, and X-ray photoelectron spectra (XPS). The formation mechanism of the nanocrystalline ternary chalcogenidometalates in the solvothermal process is also investigated.     

Electrochemical fabrication of Rh–Pd particles and electrocatalytic applications

Electrochemical deposition method was employed for the fabrication of rhodium–palladium (Rh–Pd) particles on the glassy carbon electrode (GCE) and indium tin oxide (ITO) electrode surface. Surface morphological analysis of Rh–Pd film has been carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Here, the electrodeposited Rh–Pd particles were found in the average size range of 30–200 nm. The electrochemical activities of the Rh–Pd film have been investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopic (EIS) analysis. The Rh–Pd particles-modified GCE successfully detects the hydrogen peroxide (H₂O₂) (in pH 7.0 phosphate buffer solution (PBS)) in the linear range in the lab (10–460 μM) and real samples (10–340 μM). The Rh–Pd particles-modified GCE possesses the good sensitivity and selectivity for the detection of H₂O₂ in lab and real samples.     

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.     

Recyclable NiFe2O4–APTES/Pd Magnetic Nanocatalyst

A new magnetically recyclable catalyst, NiFe₂O₄–APTES–Pd(0) MRC, as highly effective catalysts for reduction reactions in liquid phase was fabricated and characterized. The chemical characterization of the product was done with X-ray powder diffractometer, Infrared spectroscopy, transmission electron microscopy, UV–Vis spectroscopy, thermal gravimeter and inductively coupled plasma. Magnetic character of the product was analyzed with vibrating sample magnetometer. The synthesized NiFe₂O₄–APTES–Pd(0) MRCs showed a very high activity in reduction reactions of 4-nitro aniline and 1,3-dinitrobenzene in liquid phase. NiFe₂O₄–APTES–Pd(0) MRC could be recovered by magnet and reused for ten runs for hydrogenetaion reaction of 4-nitro aniline and, 1,3-dinitrobenzene without significant degradation in the catalytic activity which shows the indicative of a potential applications of these catalyst in industry.     

Sol–Gel Pd Exhaust Catalysts and N2O Production

Al₂O₃, CeO₂–Al₂O₃, CeO₂–Tb₄O₇–Al₂O₃ and ZrO₂–Al₂O₃ supported Pd samples have been prepared by sol–gel methods. Extents and mechanisms of N₂O production in CO–NO and CO–NO–O₂ reactions on these have been considered. This occurs most selectively under oxidising (lean-burn) conditions or in the presence of CeO₂ and CeO₂–Tb₄O₇ promoters near the CO–NO light off temperature. Over Pd/ZrO₂–Al₂O₃ the CO–NO reaction at 573 K has CO and NO conversions that are second order with respect to p _CO and p _NO. Over this catalyst NO conversion is faster than that of CO until O_2(g) is added, causing CO conversion and N₂O production at 573 K to rise simultaneously. CeO₂ or CeO₂–Tb₄O₇ incorporation into a Pd/Al₂O₃ catalyst enhances N₂O production near the CO–NO light-off temperature in the absence of added O₂ without CO conversion being raised. There is current attention on pollution control opportunities through lean-burn conditions, Pd catalysts and oxygen storage capacity enhancement. The present work suggests that their role in N₂O production may need to be better understood and controlled. For the moment N₂O formation provides a window on mechanisms of TWC operation.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

Structure and Composition of Au–Cu and Pd–Cu Bimetallic Catalysts Affecting Acetylene Reactivity

Au–Cu and Pd–Cu bimetallic model catalysts were prepared on native SiO₂/Si(100) substrate under ultra high vacuum (UHV) by employing buffer layer assisted growth procedure with amorphous solid water as the buffer material. The effect of the bimetallic nanoclusters (NCs) surface composition and morphology on their chemical reactivity has been studied with acetylene decomposition and conversion to ethylene and benzene as the chemical probe. It was found that among the Au–Cu NCs compositions, Au_0.5Cu₃ NCs revealed outstanding catalytic selectivity towards ethylene formation. These NCs were further characterized by employing TEM, XPS and HAADF-STEM coupled EDX analysis. With CO molecule as a probe, CO temperature programmed desorption has been used to investigate the distribution of gold on the top-most surface of the supported clusters. Surface segregation at high relative elemental fraction of gold leads to a decreased activity of the Au–Cu NCs towards ethylene formation. In contrast to the Au–Cu NCs, the Pd–Cu bimetallic system reveals reduced sensitivity to the relative elemental composition with respect to selectivity of the acetylene transformation toward ethylene formation. On the other hand, remarkable activity towards benzene formation has been observed at elemental composition of Cu₃Pd, at comparable rates to those for ethylene formation on clean Pd NCs.     

Pd(II)/Pd(0) Anchored on Magnetic Organic–Inorganic Hybrid Mesoporous Silica Nanoparticles: A Nanocatalyst for Suzuki–Miyaura and Heck–Mizoroki Coupling Reactions

Journal of Inorganic and Organometallic Polymers and Materials - 3-Chloropropyltrimethoxysilane (CPTMS) was grafted on the surface of silica coated Fe3O4 core (Fe3O4@MCM-41) and then condensed with...     

Joining of Cf/SiC composite with a Cu–Au–Pd–V brazing filler and interfacial reactions

A new Cu–Au–Pd–V filler alloy was designed for the joining of C_f/SiC composite. Its wettability on the composite was studied with the sessile drop method. After heating at 1473 K for 10 min the filler alloy showed a low contact angle of 5°. The interfacial reactions under the brazing condition of 1443 K/10 min resulted in the formation of VC_0.75 reaction band at the surface of the composite, and the microstructure in the central part of the joint is composed of Cu(Au, Pd) solid solution and Pd₂Si compound. The average three-point bend strength of the C_f/SiC–C_f/SiC joints at room temperature is 135 MPa. The joints also exhibit stable strengths at high temperatures of 873–1073 K. The presence of refractory Pd₂Si compound within the Cu(Au, Pd) solid solution matrix throughout the joint should contribute to the stable high-temperature property.     

Pd–Ru/C as the electrocatalyst for hydrogen peroxide reduction

Pd–Ru, Pd and Ru nanoparticles supported on Vulcan XC-72 carbon were prepared by chemical reduction of PdCl₂ and/or RuCl₃ in aqueous solution using NaBH₄ as the reducing agent. Transmission electron microscopy measurements showed that Pd–Ru particles were uniformly dispersed on carbon. The particle size of Pd–Ru is around 5–9 nm. X-ray diffraction analysis indicated that Ru formed alloy with Pd in Pd–Ru/C catalyst. The electroreduction of hydrogen peroxide on Pd–Ru/C, Pd/C and Ru/C in H₂SO₄ solution was examined by linear sweep voltammetry and chronoamperometry measurements. Results revealed that Pd–Ru/C catalyst exhibited higher electrocatalytic activity for hydrogen peroxide reduction than Pd/C and Ru/C. All the catalysts showed good stability for hydrogen peroxide electroreduction in H₂SO₄ electrolyte.     

Hydrogen Production from Ethanol over Rh–Pd/CeO2 Catalysts

The work presents a study by temperature programmed desorption, in situ infra red spectroscopy and catalytic steam reforming of ethanol (SRE) over CeO₂ and the bimetallic Pd-Rh/CeO₂; comparison with the monometallic catalysts (Rh/CeO₂ and Pd/CeO₂) was also made for the steam reforming study. Comparing TPD of ethanol over CeO₂ and the bimetallic catalysts indicated two main differences: the direct oxidation route to acetate over CeO₂ is suppressed by the presence of the metal and the lowering of the dehydrogenation reaction temperature by about 100 K. In situ IR study indicated that the bimetallic catalyst breaks the carbon–carbon bond of ethanol at low temperature <400 K, as evidenced by the presence of adsorbed CO species. SRE over ½ wt.% Rh–½ wt.% Pd/CeO₂, at 770 K at realistic conditions showed that maximum conversion and selectivity could be achieved. This high activity considering the very small amounts of transition metals on CeO₂ is discussed in light of their electronic and geometric effects.     

Carbon–chlorine bond cleavage in chlorofluoroethanes on Pd(111)

The kinetics of carbon–chlorine bond cleavage on a Pd(111) surface have been investigated using several dichloroethanes with varying fluorine content (CF₃CFCl₂, CH₃CFCl₂, CH₂FCFCl₂, CH₃CHCl₂, CH₂ClCH₂Cl). Preliminary results show that the dechlorination rates in these molecules exhibit trends that are similar to those observed in catalytic hydrodechlorination over supported Pd catalysts. Fluorination of the 1,1‐dichloroethanes decreases the rate constant for dechlorination. The presence of only one chlorine atom on each carbon atom (CH₂ClCH₂Cl vs. CH₃CHCl₂) also reduces the rate constant for dechlorination. A Hammett correlation using field substituent parameters was established for the rate of dechlorination in an attempt to probe the nature of the transition state to carbon–chlorine bond cleavage ( ). The dechlorination rate constant for 1,1‐dichloroethanes is slightly reduced with increasing field effects (increasing fluorination) and the Hammett correlation reveals a reaction constant of ρ = -1.0 ± 0.2. The interpretation of this result is that there is little polarization of the C–Cl bond in the transition state for dechlorination, which is consistent with a transition state that is early in the reaction coordinate. This revised version was published online in July 2006 with corrections to the Cover Date.