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Preparation of a Palladium Composite Membrane by an Improved Electroless Plating Technique Li-Qun Wu, Nanping Xu,* and Jun Shi Membrane Science & Technology Research Center, Nanjing University of Chemical Technology, Nanjing 210009, People’s Republic of China
An ultrathin palladium membrane was prepared by an improved electroless plating technique on the surface of a porous titania ceramic substrate. A simple yet new and valuable activation technique named photocatalytic deposition (PCD) has been presented. The thickness of the resulting palladium membrane was about 0.3-0.4 µm. Hydrogen permeation through the composite membrane was proportional to the hydrogen partial pressure difference. The hydrogen permeability and H2:N2 selectivity through the palladium composite membrane are 6.3 × 10-6 mol m-2 s-1 Pa-1 and 1140 at 773 K, respectively. The activation energy for hydrogen permeation is 21.27 kJ/mol at the temperature range of 673-773 K. Long-term hydrogen permeation operation at 673 K indicated that the flux through the resulting membrane under hydrogen/ nitrogen gradients requires 80 h to reach a steady state. The steady-state hydrogen flux showed no observable changes during the thermal cycles between 673 and 773 K. 1. Introduction Palladium membranes have the potential for the selective removal and separation of hydrogen and can serve in catalytic membrane reactors at high temperatures.1 For preparation of these Pd-based supported membranes, several methods have been used, such as chemical vapor deposition (CVD),2-6 electroless plating,7-11 sputtering deposition,5,12,13 and spray pyrolysis.14 The technique of electroless plating is perhaps one of the most popular methods in preparing palladium membranes, which provides distinct advantages such as deposits on complex shapes, hardness, low cost, use of very simple equipment.15 When this technique is used, membranes can be prepared on microporous glass,7 silver support,16 porous stainless steel,17 and ceramic membranes.8 The mechanism of the electroless plating technique is based on the controlled autocatalytic reduction of metastable metallic salt complexes on the target surface. The reduction of a cathodic metal and anodic oxidation of a reductant occur simultaneously.18 Because the common substrates mentioned above are not active enough to initiate the redox reaction, pretreatment is needed to activate with finely divided palladium nuclei. This catalyzes the electroless plating reaction. Thus, the process involves a necessary activation pretreatment and electroless deposition. The traditional activation procedure consists of two steps, an immersion sequence in an acidic tin chloride (sensitization) bath to adsorb a reducing agent, followed by an acidic palladium(II) chloride (nucleation) bath to preplate a Pd compound adsorbed on the surface of the substrate, with gentle rinsing in deionized water between baths. This activation procedure usually was repeated 10 times to produce enough palladium particles for postdeposition of palladium. According to this autocatalytic mechanism, many disadvantages were found on the traditional activation process of the electroless plating technique. First, the * Corresponding author. Tel.: 0086-25-3319580. Fax: 008625-3300345. E-mail:
[email protected].
repetitious two-step pretreatment is a cumbersome and time-consuming process. Second, because this process employs tin solution and palladium chloride solution, which leads to the presence of tin compounds with a low melting point at the palladium/ceramic interface, the melting and the decomposition of the co-deposited impurities at high temperatures will cause the formation of pinholes and cracks. Finally, because the palladium membrane is built up by packing Pd particles deposited initially from the activation step and then from the plating solution through the autocatalytic decomposition of a palladium complex,1 the uniformity of packed palladium nuclei and the adhesion between the metallic film and the substrate is questionable. Many improvements of the activation procedure were developed in the literatures. The electroless plating technique was always combined with osmosis used to enhance the adhesion between the substrates and the metal;1,19 however, other problems mentioned above have always existed. Paglieri and Way9 reported a new activation procedure not only with osmosis pressure but also with use of palladium acetate instead of palladium chloride. This procedure avoided the impurity of tin deposited on the membrane and obtained a membrane with stable hydrogen flux and H2/N2 ideal selectivity up to 873 K. However, the two-step activation process was just as time-consuming as the traditional electroless plating technique. Zhao et al.11 reported the palladium membrane of about 1 µm thickness with high compactness using a modified electroless plating technique, which activated ceramic substrates by the sol-gel process of a Pd(II)modified boehmite sol. By this method both the twostep activation process and the tin impurities were avoided. However, the boehmite gel coatings must be dried and calcinated, and then the sol-gel-derived substrate must be treated with hydrogen at 500 °C to reduce palladium. So the activated substrates would be obtained with a time-consuming cycle and relatively complex strict conditions.
10.1021/ie9904697 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/07/2000
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 343 Table 1. Properties of a Two-Layer TiO2 Membrane Used for Deposition of Palladium layers
pore size (nm)
porosity (%)
thickness (µm)
R-Al2O3 TiO2
80-100 2-3
30 47
35 1.5-2
In this study, a simple yet new and valuable activation procedure called photocatalytic deposition (PCD) was established. With use of this technique, palladium will be deposited on a semiconductor surface by a photocatalytic reaction between the semiconductor and the solution containing Pd(II) under the irradiation of UV light. Because a TiO2 ceramic membrane in the form of anatase is a good semicondutor and shows many advantages, such as photochemical stability, commercial availability, and relatively low cost, it was selected as the support of the palladium membrane in this study. The formation mechanism of palladium film on a titania membrane was discussed in detail. A Pd composite membrane was subsequently obtained by electroless deposition. X-ray diffraction analysis (XRD), scanning electron micrographs (SEM), and energy-dispersive spectroscopy (EDS) were used to characterize the properties of the prepared membranes. The hydrogen permeation experiment was conducted in a high-temperature membrane permeator, using graphite gaskets as sealing.
Figure 1. Schematic diagram of the experimental setup for a photocatalytic reaction: (1) magnetic stirrer; (2) thermostated Pyrex reactor; (3) stirring rod; (4) glass baffle sheet; (5) supported TiO2 membranes; (6) thermometer; (7) purging gas; (8) gas outlet; (9) cooling water inlet; (10) cooling water outlet; (11) rubber sealing; (12) quartz cover; (13) 150-W gallium lamp.
2. Experimental Section 2.1. Membrane Preparation. The activation of porous TiO2 substrates was performed by PCD. The disk-shaped TiO2 membrane, in the form of anatase with a diameter of 16 mm, a thickness of 3 mm, and an average pore size of 2-4 nm, was prepared by a colloidal route of the sol-gel technique from the hydrolysis of tetrabutyl titanate (TBT), which was given previously.23 An aqueous-based sol with a TiO2 concentration of 0.05 M was employed in the fabrication of the TiO2 membrane by the dip-coating process onto the R-Al2O3 supports at ambient temperature and humidity. The resulting sample was sintered at 450 °C for 2 h to form dominant anatase TiO2 membranes. Table 1 summarized the properties of supported TiO2 membranes. Experimental setup of PCD is shown in Figure 1. An aqueous palladium(II) chloride solution was added to a thermostatic Pyrex reactor with a mesh glass baffle that samples were mounted on. Palladium chloride, a methanol additive, and hydrochloric acid of at least analytical grade were applied for photocatalytic reaction. Water was deionized and doubly distilled. The reactor was closed with a rubber septum and fitted with a quartz optical cover, which was used for filtering the UV light. Irradiation was performed at 35 °C with a 150-W gallium lamp. The solution was purged with N2 prior to and during light exposure. A magnetic stirrer at a high rotating speed was used to ensure uniform mixing of the solution. After the PCD process, the irradiated membrane was rinsed with distilled water to remove the remainder of palladium(II) chloride on the surface of the membrane. After the activation process, the derived TiO2 membrane with Pd nuclei was plated in a palladium bath using the electroless plating technique. The composition and conditions of the electroless bath are listed in Table 2. After a given time, the deposits were washed with deionized water and dried. 2.2. Membrane Characterization. The morphology of the resulting palladium membranes was examined
Figure 2. Apparatus for gas permeation through palladium membranes at high temperatures: (1) gas cylinders; (2) purifying traps; (3) flow control valves; (4) mass flow controllers; (5) gas mixer; (6) pressure gauge; (7) pressure transfer; (8) computer; (9) furnace; (10) stainless steel permeator; (11) GC-TCD; (12) baker pressure regulator. Table 2. Composition and Conditions of an Electroless Plating Bath component palladium chloride concentration ammonium hydroxide (28%) concentration disodium EDTA concentration hydrazine (1 M solution) concentration pH temperature
3.9 g/L 350 mL/L 50 g/L 11 mL/L 11 65 °C
by high-resolution SEM (JEOL JSM-6300). Surface element analysis of the palladium membranes prepared was performed by energy-dispersive X-ray spectroscopy (EDS, U.S. Kevex-Sigma, electron source 15.0 kV, beams current 100.0 pA). The phase development of the palladiumico membrane was studied by XRD (Rigaku D/MAX-rB diffractometer) with Cu KR radiation. 2.3. High-Temperature Gas Permeation Measurement. Gas permeation through Pd/TiO2 composite membranes at high temperatures was measured on the permeation apparatus shown in Figure 2. The membrane was connected with the body of a stainless steel permeator using graphite gasket seals. The permeator was surrounded by a tubular furnace and the temperature was measured by a type K thermocouple encased in an alumina tube. A microprocessor temperature controller (model 708PA, Xiamen Yuguang Electronics
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Technology Research Institute, China) was used to control the temperature to within (1 K of the set points. During the gas permeation measurement, the feed gases (H2 and N2) were supplied from gas cylinders and the flow rate of each gas was controlled by mass flow controllers (models D07-7A/ZM, Beijing Jianzhong Machine Factory, China). Hydrogen and nitrogen were joined together to be mixed and introduced to the Pd side of the disk-type membrane. Both the upstream and the downstream were maintained at atmospheric pressure. Argon as the sweep gas for the permeating gases was fed to the support side of the membrane to adjust the partial pressure of hydrogen. The effluent streams were analyzed by gas chromatography (GC, model Shimabzu GC-7A), which was equipped with a 2-m 5-A molecular sieve operated at 40 °C with helium as the carrier gas. The hydrogen, nitrogen permeation flux and the separation efficiency through the membrane were calculated from the data collected. 3. Results and Discussion 3.1. Activation Mechanism of TiO2 Substrates by PCD. In this study, the TiO2 membrane with nanometer pore size was used as the substrate of the palladium composite membrane. Titania in the form of anatase crystal is a good semiconductor, which was usually used as a photocatalyst to deposit the metals by a photocatalytic reaction. To the best of our knowledge, it appears that we are the first to report the palladium deposition on the surface of a TiO2 membrane by PCD. During this process, under irradiation by UV light on the TiO2 membrane surface, the semiconductor absorbs the bandgap energy, exciting electrons from the valence band to the conduction band. Electron/hole pairs appear, which are able to promote redox reactions at the TiO2-liquid surface. Thus, in the absent of any reducing agent, the fundamental process is given by the following reactions:
TiO2 + hν (
