Toward effective membranes for hydrogen separation: Multichannel composite palladium membranes

Composite palladium membranes can be used as a hydrogen separator because of their excellent permeability and permselectivity. The total membrane area in a hydrogen separator must be reasonably large for industrial use, and it is important that each membrane provides a large enough area. Such a demand can be well met by introducing multichannel composite membranes. In this work, a commercially available microporous ceramic filter with 19 channels was used as a membrane substrate, and the diameter of each channel was 4 mm. A uniform thin palladium layer was fabricated inside the narrow channels by using an electroless plating method, and the resulting membranes were highly permeable and selective. This membrane concept provides a high surface-to-volume ratio without causing significant pressure loss, making the hydrogen separator compact and capable. However, special attention should be paid to cleaning the membrane after electroless plating.     

塞曼石墨炉原子吸收法中用钯消除氯化物干扰的机理

本文研究了塞曼石墨炉原子吸收法中采用钯消除环境水样基体氯化物干扰的机理。在最佳条件下,用10μg钯可消除5μg氯化钠基体对1.0ng铜的测定干扰。在10μg钯改进剂中加入5μg镁,可改善测定水样中铜的回收率。测定结果表明,铜的检出限(3σ)为5.0×10~(-11)g,回收率为97%～103%。     

A novel route for the synthesis of nanosize particles of metallic palladium

A novel route for the synthesis of metallic palladium consisting of nanosize particles has been reported. The synthesis is based on (a) the addition of tetramethylammonium hydroxide (TMAH) to the aqueous solution of PdCl₂, and (b) autoclaving of this precipitation system at 160 °C. The distribution of the nanosize particles of metallic palladium has its mean value at about 18 nm. The mechanism of the metallic palladium formation is briefly discussed.     

Characterization of palladium supported on sulfated zirconia catalysts by DRIFTS, XAS and n-butane isomerization reaction in the presence of hydrogen

Palladium supported on sulfated zirconia (PdSZ) has been characterized by the n-butane isomerization reaction in the presence of hydrogen, X-ray absorption spectroscopy (XAS) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) of adsorbed carbon monoxide. Catalyst calcination at 873 K followed by hydrogen reduction at 513 K results in the formation of 30–40 Å Pd metal clusters, but the surface can only weakly adsorb CO, though stronger than Pd-free, sulfated zirconia catalysts. In the presence of hydrogen, PdSZ has a lower n-butane isomerization activity than SZ, and the Pd function cannot stabilize the reaction at low H₂/n-butane ratios.     

Catalytic combustion technology to achieve ultra low NOx, emissions: Catalyst design and performance characteristics

A catalytic combustion system has been developed which feeds full fuel and air to the catalyst but avoids exposure of the catalyst to the high temperatures responsible for deactivation and thermal shock fracture of the supporting substrate. The combustion process is initiated by the catalyst and is completed by homogeneous combustion in the post catalyst region where the highest temperatures are obtained. Catalysts have been demonstrated that operate at inlet temperatures as low as 320°C at 11 atm total pressure and conditions typical of high performance industrial gas turbines. The ignition temperature is shown to correlate with the specific catalytic activity of the washcoat layer over a rather broad range of activities. A reaction model has been developed that can predict ignition behavior from the measured catalytic activity.     

Hydrogen generation in a Pd membrane fuel processor: Productivity effects during methanol steam reforming

Performance analyses are carried out for the palladium membrane fuel processor for catalytic generation of high purity hydrogen. The reactor model includes detailed particle-scale multi-component diffusion, multiple reversible reactions, flow, and membrane transport. Using methanol steam reforming on Cu/ZnO/Al₂O₃ catalyst as the test reaction, a systematic examination of the effects of operating and reactor design parameters on key performance metrics is presented. Single particle simulations reveal a complex interplay between nonisobaric transport and the reversible reactions (methanol reforming and decomposition, and water-gas shift), which impact overall reactor performance. An analysis of characteristic times helps to identify four different productivity controlling regimes: (i) permeation control, encountered with thick membranes and/or insufficient membrane area; (ii) catalyst pore diffusion control encountered with diffusion of reacting species in larger particles; (iii) reaction control, encountered when intrinsic catalytic rates are too low because of inadequate activity or catalyst loading; and (iv) feed control, encountered when the limiting reactant feed rate is inadequate. The simulations reveal that a maximum in the hydrogen productivity occurs at an intermediate space velocity, while the hydrogen utilization is a decreasing function of space velocity, implying a trade-off between productivity and hydrogen utilization. The locus of productivity maxima itself exhibits a maximum at an intermediate membrane surface to volume ratio, the specific value of which is dependent on the particle size, membrane thickness and reaction conditions. At moderate temperature and total pressure (, 10 bar), particles smaller than 2 mm diameter, Pd membranes with thickness less than , and membrane surface to volume ratio exceeding are needed to achieve viable productivity . A comparison between the packed-bed membrane reactor and conventional packed-bed reactor indicates a modest improvement in the conversion and productivity due to in situ hydrogen removal.     

Direct <Emphasis Type="SmallCaps">d</Emphasis>-Glucose Oxidation over Noble Metal Nanoparticles Introduced on Polymer and Inorganic Supports

In the present work, nanocatalysts prepared on inorganic supports (zeolites) were investigated in d-glucose oxidation and compared to systems supported on polymer (hypercrosslinked polystyrene (HPS)) previously described. Catalytic activities and selectivities were measured under various reaction conditions. The selectivity of d-glucose oxidation and activity of both Pd-containing zeolites and HPS-Ru were similar (99.7% and TOF 0.013–0.014 mol/(mol Me s)). Physicochemical analysis X-ray photoelectron spectroscopy, liquid nitrogen physisorption, diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO for metal sites evaluation, CD₃CN for acid sites evaluation showed that Pd species were in oxidic form, while Ru species were in oxidic and reduced form. The catalytic activity decreased when acidic sites were present in Pd-containing zeolites.     

Fabrication of palladium coated nanoporous gold film electrode via underpotential deposition and spontaneous metal replacement: A low palladium loading electrode with electrocatalytic activity

An electrochemical approach to nanoporous film-based gold catalyst design using the underpotential deposition and redox replacement technique is presented. The procedure consisted of the underpotential deposition (UPD) of copper on the gold nanoporous film, with subsequent replacement of the copper by palladium at open circuit in a palladium containing solution. The resulting electrode was studied using cyclic voltammetry and scanning electron microscopy. The electrocatalytic activity of as-prepared palladium nanoporous gold film electrodes toward the oxygen reduction reaction is presented.     

Studies on the feasibility of electrochemical recovery of palladium from high-level liquid waste

The electrochemical behavior of palladium (II) in nitric acid medium has been studied at platinum and stainless steel electrodes by cyclic voltammetry. The cyclic voltammogram consisted of a surge in cathodic current occurring at platinum electrode at a potential of −0.1 V (vs. Pd), which culminates in a peak at −0.3 V was due to the reduction of Pd(II) to Pd. This was accompanied by a broad scant anodic peak at 0.25 V during scan reversal. Reduction of Pd(II) was irreversible and the diffusion coefficient was found to be 2.35 × 10⁻⁸ cm²/s at 298 K. At stainless steel electrode, a surge in the cathodic current occurring at −0.4 V (vs. Pd) was due to palladium deposition, which was immediately followed by a steep increase in cathodic current at −0.66 V due to H⁺ reduction. Electrolysis of palladium nitrate from 1 M to 4 M nitric acid medium at stainless steel electrode resulted in complete recovery of palladium with reasonably high Faradaic efficiency depending upon nitric acid concentration. However, the recovery and Faradaic efficiency were significantly lowered (to 40%) in the case of electrolysis from simulated high-level liquid waste due to other interfering competitive reactions.     

Pd deposition onto Au(111) from nitrate solution

The initial stages of palladium deposition onto Au(111) from 0.1 M HNO₃ + 0.2 mM Pd(NO₃)₂ have been studied by cyclic voltammetry and in situ scanning tunnelling microscopy. It is demonstrated that nucleation starts exclusively at surface defects such as monoatomic high steps, which is at variance with recently published work. From this and our previous work it thus appears that surface defects are the preferred nucleation sites indeed for nitrate, sulphate and chloride containing solutions.