Catalytic Properties of Nickel Catalysts, for Methanol Decomposition, on Aluminum Plate Prepared by Electroless Plating

石油学会誌 Sekiyu Gakkaishi, 37, (2), 173-178 (1994) 173 [Regular Paper] Catalytic Properties of Nickel Catalysts, for Methanol Decomposition, on Aluminum Plate Prepared by Electroless Plating Choji FUKUHARA, Nobuaki SASAHARA, and Akira IGARASHI* Dept. of Chemical Engineering, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji, Tokyo 192 (Received June 28, 1993) In order to obtain a tube-wall reactor with high thermal conductivity, nickel catalysts on an aluminum plate were prepared by electroless plating which consisted of displacement of aluminum by zinc and the deposition of nickel by chemical reduction. Thus plated catalysts showed high activities and high selectivities when forming carbon monoxide and hydrogen by methanol decomposition. The catalytic properties of the plated catalysts were significantly affected by the plating conditions. In particular, the plated catalyst prepared using an alkaline zinc plating bath and the neutral nickel plating bath that included sodium hypophosphite as the reducing agent had the highest activity and the highest selectivity. It was also shown that the displacement operation and the use of the nickel plating bath including a reducing agent was important factors forming a uniform plated layer having strong adhesion to the aluminum base. 1. Introduction methane by comparison with that of a packed-bed type reactor in which a granular catalyst was At present, environmental problems such as the packed2). The simulation results inferred that the effect of green gas is currently of great concern. In rectangular wall reactor has excellent thermal the reformed gas industries, to reduce energy conductivity between the heating area and the consumption for reaction processes, catalysts now reacting area, and gives a uniform temperature play important roles. In addition to improve- distribution in the reactor. Furthermore, it was ments in catalysts, various reaction apparatus and also demonstrated that the performance of the reaction methods have been proposed to attain rectangular wall reactor would gain power by improvement of reaction yields, thermal efficiency laying a small channel. and catalyst life, resulting from improved respon- Incidentally, a method to deposit catalytic com- siveness to load fluctuation, downsizing and more ponents on a metallic wall is the most important uniform temperature distribution of the reformed technique for constructing the preferable tube- reactor1). wall reactor. As a method to hold a catalyst on the We have recently theorized that a tube-wall wall of the reactor, a spray coating gun with reactor, otherwise called "a heat exchangeable flame3), a high-temperature melt coating4), and reactor with corrugated fins on which catalytic anodic oxidation5) were tested. These methods, components are deposited", is a suitable reactor to however, are not always industrially desirable maintain temperature necessary for the reforming because of limitations met, attributed to applicable reaction to take place on the reforming catalyst, as shape and space of the reactor which are affected by well as the instantaneous supplementation of heat the use of high temperatures and the limited extent for the heat consumed by the endothermic re- of freedom during construction and fabrication of forming reaction. We also simulated the per- the corrugated or laminated sheets required for formance of a reaction and heat transfer of a keeping the reactor compact. rectangular wall reactor, which consists of In order to deposit the catalyst on the boundary alternating piles for a catalyst wall channel and a wall surface of the tube-wall reactor, we adopted heat medium channel, for the endothermic the deposition of catalytic components by elec- reforming reaction, such as steam reforming of troless plating. Electroless plating consists of displacement plating by ion exchange, and * To whom correspondence should be addressed. chemical reduction plating using metal ions and a 石油学会誌 Sekiyu Gakkaishi, Vol. 37, No. 2, 1994 174 reducing agent6). Displacement plating is a solution for about 20min. It was then immersed method wherein electrons are transferred between a in a zinc oxide plating bath to displace aluminum plating metal and the metal to be plated, where the with zinc for 1min and then washed in a water bath plating metal becomes the ions and the platable for 30sec. The displacement and washing proc- metal is the metal respectively, then the platable esses were repeated two times, although the metal dissolves and the plating metal will immersion time of the second displacement was deposit on the platable metal. Chemical reduc- only 30sec. Subsequently, the plate was im- tion plating is also called the chemical plating mersed in a nickel plating bath for 7min to deposit method, wherein metal ions in a plating solution nickel on the surface by chemical reduction. are deposited on a platable material by the action of After washing in a water bath under ultrasonic a reducing agent. waves, it was dried for about 40min in a drying The tube-wall reactor may be regenerated when oven at about 70℃. catalytic activity is reduced after a certain period of In Tables 1 and 2 are shown the compositions of operation by depositing fresh catalytic compo- the plating bath and the pH of various zinc plating nents on the wall surface by electroless plating. and nickel plating baths, respectively. Three In this paper, we report the catalytic properties kinds of zinc plating baths were used: (A) a neutral of plated nickel catalysts on an aluminum plate for bath which component is only zinc oxide, (B) an methanol decomposition being highly endother- alkaline bath in which sodium hydroxide was mic. Further, the catalytic properties of the added, and (C) a weak acid bath which component plated catalysts, which are considerably affected by is zinc nitrate. Furthermore, three kinds of nickel plating conditions, are described. plating baths were also used: (I) a neutral bath of nickel chloride which reducing agent was sodium 2. Experimental hypophosphite, (II) an alkaline bath of nickel chloride which agent was sodium borohydride, 2.1. Preparation of Plated Catalyst and (III) a neutral bath containing no reducing The procedure of preparing a catalyst by agent. electroless plating is shown in Fig. 1. A 0.5mm 2.2. Reaction Conditions thick aluminum plate (JIS Al 100P-H24) was used The methanol decomposition reaction was as the supporting base for the catalytic com- conducted at atmospheric pressure using a ponents. The aluminum plate was formed into a conventional flow reactor. After placing the pentagonal prism shape, which sectional view plated Ni/Al catalyst in the reactor, the catalyst resembled a star. Its maximum diameter was was reduced in a H2 stream flowing at the rate of 21mm and its length was 120mm. The apparent 100ml/min for 1h at 400℃. Methanol gas was total surface area of the electrolessly plated catalyst then pumped into the reactor. The flow rate of composition was 330cm2. In order to remove methanol was 7.9×10-3mol/min and the partial impurities and activate the surface of the alu- pressure of methanol was adjusted to 0.8atm by minum plate, it was first immersed in 3N HCl adding He as a diluent gas. The effluent gas were analyzed by means of TCD gas chromatography. Total conversion and selectivities for products were calculated on the basis of carbon. Table 1 Composition of Bath for Displacement of Aluminum by Zinc Table 2 Composition of Bath for Reduction of Nickel Fig. 1 Procedures of Electroless Plating a) pH adjusted with NaOH. 石油学会誌 Sekiyu Gakkaishi, Vol. 37, No. 2, 1994 175 2.3. SEM Observation properties of a commercial granular catalyst, Ni/ In order to evaluate the adhesion of the plated SiO2 (N111, Nikki Chemical Co., Ltd., 0.2g), for layer, sections of the plated catalysts were observed methanol decomposition, are also shown in Fig. 2 by SEM. for comparison with A-I catalyst. Since the void fraction of the plated catalyst is much greater than 3. Results and Discussion that of the granular catalyst, it may be concluded that the feed gas passed through the reactor 3.1. Catalytic Properties of Plated Catalysts without reacting. As shown in Fig. 2, however, Shown in Fig. 2 are the catalytic properties of the A-I catalyst has high activity, which indicates A-I catalyst prepared by using (A), the neutral zinc almost 100% methanol conversion at 400℃. plating bath, and (I), the nickel plating bath With respect to selectivities of the A-I catalyst, containing sodium hydrophosphite. Hereafter, a selectivity for carbon monoxide is more than 90% at letter and a Roman numeral will denote the kind of temperature range of 300-400℃ and only a few plated catalyst corresponding to the specific zinc by-products of methane and carbon dioxide are plating bath shown in Table 1 and nickel plating present. Only a slight amount of dimethyl ether, bath shown in Table 2, respectively. Catalytic probably attributed to an acidic site over aluminum oxide, which may be partly exposed on the plated surface, was observed. Generally, the nickel catalyst is mainly effective to produce methane at higher temperatures, such as the Ni/ SiO2 catalyst shown in Fig. 2. It is significant to note that the A-I catalyst has high selectivity for carbon monoxide. The slight formation of by- products such as methane, carbon dioxide, and dimethyl ether show that the coating of plated layer on aluminum base was almost complete. These results suggest that electroless plating is a useful method to form a catalytic layer for the fabrication of tube-wall reactor. In Table 3 are shown the catalytic properties of B-I catalyst prepared by using (B), the alkaline zinc plating bath, C-I catalyst using (C), the zinc nitrate plating bath, and A-II catalyst using (II), the alkaline nickel chloride bath containing sodium borohydride as a reducing agent.

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