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.

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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. The catalytic property of A-I catalyst is also shown in Table 3. Compared with A-I catalyst, the B-I catalyst has higher catalytic activity and a high selectivity for carbon monoxide and hydrogen, while formation of methane and dimethyl ether decreased.

Hydrogen is not included in selectivity. Further, results of durability test of the B-I catalyst at 350℃ showed that no changes in catalytic Fig. 2 Catalytic Properties of A-I activity and product composition had occurred

Table 3 Effects of Composition of Bath on Catalytic Properties

石油学会誌 Sekiyu Gakkaishi, Vol. 37, No. 2, 1994 116

Table 4 Catalytic Properties of Plated Catalysts Prepared without Displacement Operation and a Reducing Agent in Nickel Plating

within 5h. An analysis of the surface of catalysts activity and low selectivity for carbon monoxide after reaction showed that adhesion and thickness because of the formation of dimethyl ether at lower of the plate layer of B-I catalyst was much better temperatures and methane at higher temperatures. than that of A-I catalyst. After the reaction, exfoliation of the plated layer of On the other hand, activity of the C-I catalyst the I and III catalysts were observed, although the declined compared to that of the A-I catalyst. plated layer of each of these catalysts was thin and With respect to selectivity on the C-I catalyst, an smooth. Thus, in the case of directly plating increase in dimethyl ether was observed at higher nickel on the aluminum base, without prior zinc temperatures. The plated layer of the C-I catalyst plating, it is thought that adhesion between the was thinner compared to that of the A-I and B-I plated layer and aluminum base is weakened by an catalysts, although the C-I catalyst had a uniformly aluminum oxide film formed on the aluminum plated layer. The plated layer of the C-I catalyst after activation treatment. Namely, the zinc after the reaction had appreciably exofoliated. plating is an important operation to prevent the These results show that the use of an alkaline zinc formation of an aluminum oxide film on the plating bath improves the performance of a plated aluminum, and to form a strong adhesion layer on catalyst, and the use of a zinc nitrate plating bath the aluminum base. Moreover, the B-III catalyst results in undesirable side-effects. Furthermore, had higher activity at lower temperatures and the activity of the A-II catalyst was much higher nearly equal formation of carbon monoxide and than that of the A-I catalyst, but the A-II catalyst hydrogen compared with the A-I catalyst. had a lower selectivity for carbon monoxide Nevertheless, adhesion between the plated layer because of the formation of dimethyl ether at lower and aluminum base of the B-III catalyst was so temperatures and methane at higher temperatures. weak that exfoliation occurred on most of the These results indicate that the use of an alkaline plated layer after the reaction. This indicates that nickel plating bath improves the catalytic activity the use of a reducing agent in the nickel plating and causes negative effects on the selectivity of affects the adhesion of the plated layer. carbon monoxide. Thus, it is evident that the Based on these results, it is concluded that the catalytic property for methanol decomposition displacement of aluminum by zinc and the use of a over the plated catalyst prepared by electroless reducing agent in the nickel plating are needed to plating is significantly influenced by the plating obtain a uniform and strongly adhering plated conditions, and it is possible to control the layer on the aluminum base and higher selectivity catalytic property with selected plating conditions. for carbon monoxide. Next, the effects on the catalytic properties of the 3.2. Effect of Alkali on the Displacement Plating zinc plating operation and reducing agent in the Shown in Fig. 3 are SEM photographs of nickel plating bath were examined. Shown in sections of B-I and A-I catalysts. It should be Table 4 are the catalytic properties of the I and III observed that the plated layer of the A-I catalyst is catalysts, which were prepared together without densely deposited on the aluminum base and the the zinc plating operation, and the B-III catalyst, thickness of this film is ca.15μm. On the other which was prepared by using the nickel plating hand, the thickness of the film on B-I catalyst, bath without a reducing agent. The activity of which is 40 to 60μm, is more than three times that the I catalyst is lower than that of the A-I catalyst at of the A-I catalyst, and the plated layer of the B-I the test temperature range and the methanol catalyst appears to be porous. Mallory7) reported conversion is not 100%, even at 400℃. Further, that the solubility of zinc oxide in an alkaline the formation of dimethyl ether over the I catalyst solution increases because tetrahydroxozincate(II) increased. Furthermore, the III catalyst had high ion formed from zinc oxide, although zinc oxide is

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Fig. 3 SEM Photographs of Sections, (a) B-I and (b) A-I only slightly soluble in water. Measuring the using the alkaline zinc plating bath and neutral concentrations of free zinc ion in the neutral (A) nickel plating bath, which included sodium and alkaline (B) baths, using an ion chromato- hypophosphite as a reducing agent, had the graph, the following interesting results were highest methanol conversion and the highest revealed. Namely, before zinc plating, the con- selectivity for formation of carbon monoxide and centration of free zinc ion in bath A was 8.6ppm hydrogen. and that in bath B was approximately 10,000ppm. Furthermore, it was shown that the displace- Moreover, after zinc plating, the zinc ion con- ment operation and the use of a reducing agent in centration in bath A was 5.4ppm and that in bath the nickel plating bath was important for B was approximately 7,900ppm. This means obtaining a uniform and strongly adhered plated that the use of an alkaline zinc plating bath layer on the aluminum base. increases the free zinc ion and increases the displacement of aluminum by zinc. At the same Acknowledgment time, aluminum, which is an amphoteric metal, is This research was partially supported by a strongly corroded by the alkaline solution. It is Grant-in-Aid for Scientific Research No. 03650662 suggested that this results in the formation of a from the Ministry of Education, Science and more porous and more thickly plated layer as Culture of Japan. shown in Fig. 3(a) and leads to the improvement in catalytic activity. References

4. Conclusions 1) Igarashi, A., Kagaku Keizai, April, (4), 68 (1992). 2) Fukuhara, C., Igarashi, A., Kagaku Kogaku Ronbunshu, 19, Electroless plating, which occurred as a result of (2), 295 (1993). 3) Imai, T., Moriga, T., Karasaki, M., Mitubishi Juko Giho, the displacement of aluminum by zinc and the 28, (6), 563 (1991). deposition of nickel by chemical reduction, was 4) Shimizu, M., Nobori, K., Takeoka, S., Kagaku Kogaku useful for obtaining a catalytic wall material for Ronbunshu, 14, (1), 114 (1988). the tube-wall reactor. The catalytic properties of 5) Murata, K., Yamamoto, K., Kameyama, H., Kagaku the plated Ni/Al catalysts for methanol decom- Kogaku Ronbunshu, 19, (1), 103 (1993). 6) For example, "Mudenkai Mekki Gijutsu", Sogo Gijutsu position were significantly affected by the plating Center, Tokyo (1986), p. 57. conditions. In particular, the catalyst prepared 7) Mallory, G. O., Plat. and Surf. Fin, June, 86 (1985).

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要旨

アルミニウム板上に無電解めっきにより調製したニッケル触媒のメタノール分解特性

福原長寿, 笹原信明, 五十嵐哲

工学院大学工学部化学工学科, 192東京都八王子市中野町2665-1

高伝熱性管壁型触媒反応器の開発を目的として, 亜鉛置換操亜リン酸ナトリウムを還元剤とする中性のニッケル化学めっき作とニッケルの化学還元めっき操作からなる無電解めっきによ浴を用いて調製したプレート触媒の活性と選択性が最も高い値り, アルミニウム板上にニッケル触媒を調製した。得られたプを示した。また, 基板上に均一で付着力の高いめっき層を形成レート触媒はメタノール分解反応に対して高い活性と高い一酸するためには, 亜鉛置換操作と還元剤を含むニッケルめっき浴化炭素生成の選択性を示した。その触媒性能はめっきの条件にの使用が重要であることも明らかとなった。よって大きく影響された。特に, アルカリ性の亜鉛置換浴と次

Keywords Electroless plating, Tube-wall reactor, Nickel catalyst, Aluminum plate, Methanol decomposition

石油学会誌 Sekiyu Gakkaishi, Vol. 37, No. 2, 1994