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Zn-based catalysts for steam reforming of methanol, prepared by electroless plating
Applied Catalysis A: General 330 (2007) 108–116 www.elsevier.com/locate/apcata
Catalytic performance of plate-type Pd/Zn-based catalysts for steam reforming of methanol, prepared by electroless plating Choji Fukuhara a,*, Yoshiyuki Kamata b, Akira Igarashi c a
Department of Materials Science and Chemical Engineering, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Nakaku Hamamatsu, Shizuoka 432-8561, Japan b Graduate School of Mechanical Engineering Systems, Hachinohe Institute of Technology, 88-1 Myo Ohbiraki, Hachinohe, Aomori 031-8501, Japan c Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji, Tokyo 192-0015, Japan Received 1 February 2007; received in revised form 24 May 2007; accepted 4 June 2007 Available online 12 July 2007
Abstract It has been reported that a Pd/ZnO catalyst, which consists of palladium and zinc oxide, shows high performance in steam reforming of methanol. By processing this granular catalyst into a plate-type catalyst, a hybrid wall-type methanol reformer with high heat resistance and high reforming performance would be constructed. In this study, such a plate-type Pd-ZnO catalyst was prepared on an aluminum substrate by electroless plating, which consisted of a displacement plating of zinc and a chemical reduction plating of palladium. The catalytic properties of the prepared Pd-ZnO catalyst for steam reforming of methanol were then investigated. The reforming properties of the prepared catalyst varied depending on the reducing agent used in the palladium plating. The plate-type catalyst prepared with a trimethylamine-borane (TMBA) as the reducing agent exhibited high reforming properties. The measurements of physicochemical properties for such plated layer proved that a PdZn alloy was formed on the surface layer. The PdZn alloy, which was thought to be the active-site of methanol reforming, formed in the plated layer when a continuous treatment of hydrogen reduction followed by oxidation was carried out prior to the reaction. In contrast, when either reduction or oxidation treatments individually were carried out prior to the reaction, little PdZn alloy was formed. The continuous treatment of hydrogen reduction followed by oxidation was thought to be an important operation in proceeding the reforming activity. In addition, the durability in reforming properties of the prepared Pd-ZnO catalyst was examined at 350 8C. The prepared catalyst was less prone to deterioration, and restored and progressed its initial performance when a reoxidation treatment was carried out on the deteriorated surface. # 2007 Elsevier B.V. All rights reserved. Keywords: Wall-type catalyst; Structured catalyst; Palladium catalyst; Electroless plating; Steam reforming of methanol
1. Introduction There is a growing social interest in the construction of a hydrogen-oriented society based on the electric generation technique by fuel cells. Such trend has led to a focusing of attention on steam reforming of methanol as one of the reactions for hydrogen production. Over the past few years, a large number of researchers have made studies on steam reforming of methanol. In these reports, a large proportion used catalysts with copper and zinc as the main component of
* Corresponding author. Tel.: +81 53 478 1171; fax: +81 53 476 0095. E-mail address: [email protected] (C. Fukuhara). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.06.037
reforming catalyst [1–9]. However, as is commonly known, although copper-based catalysts show high activity and selectivity for methanol reforming, their heat and oxidation resistances are poor. Therefore, copper-based catalysts are susceptible to deterioration in activity due to sintering and partial oxidation of copper particles. To solve such problems, Iwasa et al. [10–13] noted noble metals, such as palladium and platinum, as alternative to copper component. They prepared palladium and platinum catalysts on zinc oxide support (Pd/ZnO and Pt/ZnO) by a coprecipitation method, and investigated the reforming properties over these catalysts. They found that the active-site species of PdZn alloy and PtZn alloy were formed on the surfaces of the catalysts through a hydrogen reduction treatment, and that the reforming properties
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of the Pd/ZnO and Pt/ZnO catalysts were comparable to that of the Cu/ZnO catalyst. The high reforming performance of the Pd/ZnO catalyst was also reported by Chin et al. [14,15]. The steam reforming of methanol is a reaction that involves a relatively large amount of endothermic energy. Thus, the efficient supply of thermal energy to reaction fields is necessary for achieving high reforming efficiency. The autothermal reforming (ATR) is one of the techniques proposed for achieving such efficiency. In the ATR, thermal energy for the reforming reaction is obtained by partial oxidation or combustion of methanol while oxygen coexists with the raw materials. Because the oxidation and the combustion cause significant amount of heating energy, the use of a catalyst with high heat resistance is essential. The Pd/ZnO and Pt/ZnO catalysts described above are thought to meet these requirements. Agrell et al. [16] and Liu et al. [17–19] investigated the reforming performance of the Pd/ZnO catalyst under the ATR condition. Another technique for efficiently supplying the reforming energy to reaction fields would be proposed from the viewpoint of the reaction system. The conventional fixed-bed reaction system has limitations with regard to the efficient supply of thermal energy because the heat exchange with the reaction fields is conducted by convective heat transfer. Instead of the fixed-bed reaction system, the authors [20–23] have proposed a wall-type reaction system equipped with a platetype catalyst as a new reaction system for reactions involving a considerable heat transfer. The wall-type reaction system allows the efficient supply of thermal energy to the reaction fields by conductive heat transfer. It is expected that, in reforming operated in the wall-type reaction system, there will be an improvement in heat transfer characteristics that cannot be achieved in the fixed-bed reaction system. Therefore, by combining the wall-type reaction system with the Pd/ZnO catalyst or the Pt/ZnO catalyst, it will be possible that a hybrid type of methanol reforming system will be constructed, providing high heat resistance and efficient heat exchangeability. Such a system could have better reaction properties than ever before, as well as stable operation in the ATR reforming. For the purpose of constructing such a reaction system, this study examined the preparation of some plate-type Pd-Zn catalysts, where palladium and zinc components were deposited on an aluminum substrate by electroless plating, and investigated its catalytic properties for steam reforming of methanol. The electroless plating technique is thought to enable both the uniform, homogeneous deposition and ‘‘insitu regeneration’’ of catalyst components, then this technique is considered to be an effective method for preparing a plate-type catalyst for a wall-type methanol reformer. Furthermore, combining with the measurement of physicochemical properties of the prepared catalysts, the influences of plating conditions and pretreatment conditions prior to reaction on the reforming properties of the catalyst were investigated and the suitable condition was determined. Finally, the durability performance change of the prepared catalyst was investigated at 350 8C, comparing with that of the copper-based catalyst.
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2. Experimental 2.1. Preparation of plated palladium catalysts When a palladium component is plated on the surface of an aluminum substrate, for achieving a strong adhesion of palladium, it is necessary to carry out an intermediate plating of a component with an electrode potential between aluminum and palladium. Because zinc (standard electrode potential: 0.76 V) has an electrode potential between aluminum ( 1.7 V) and palladium (+0.82 V), the combination of zinc plating and palladium plating is a convenient method to provide both strong adhesion and support for the palladium catalyst. Thus, an electroless plating, which consisted of a displacement plating of zinc and a chemical plating of palladium, was used for preparing a plate-type palladium-based catalyst on an aluminum plate. The aluminum plate (JIS A1100P-H24, thickness: 0.4 mm) was formed into a pentagonal prism shape, the sectional view of which resembled a star. Its maximum diameter was 21 mm and its length was 120 mm. The apparent total surface area of electrolessly plated catalyst composition was 330 cm2. The procedure of preparing a catalyst by electroless plating is shown in Fig. 1. In order to remove impurities and activate the surface, the aluminum plate was firstly immersed in 3N hydrogen chloride solution for about 15 min. The plate was then immersed in a zinc oxide plating bath (ZnO:50 g/l, NaOH:90 g/l, alkaline, bath temperature: 20 8C, time: 3 min) to displace the surface aluminum with zinc, and the plate was washed in a water bath. The displacement and washing procedures were repeated two times, although the immersion time of the second displacement was only 1.5 min. Subsequently, the plate was immersed in a palladium plating bath (PdCl2: 1.8 g/l, NH2CH2CH2NH2: 5.4 ml/l, S(CH2COOH)2: 0.05 g/l, reducing agent, alkaline, bath temperature: 30 8C, time: 60 min) to deposit palladium component on the surface by chemical reduction. Three kinds of
Fig. 1. Procedures of electroless plating.
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reducing agents were used in the palladium plating bath: (I) a trimethylamine-borane ((CH3)3NBH3, 4.4 g/l), (II) a disodium hydrogenphosphite pentahydrate (Na2HPO35H2O, 4.3 g/l), and (III) a sodium phosphinate monohydrate (NaPH2O2H2O, 6.4 g/l). In order to complex a palladium component, the palladium plating bath was used after agitation for one day and one night. The lower bath temperature of palladium plating was adopted, which was 30 8C, so as to avoid exfoliation of the plated component. After being washed in a water bath, the plated catalyst was dried in an air at room temperature overnight to prepare a plate-type palladium-based catalyst. 2.2. Steam reforming of methanol over plated catalysts The steam reforming of methanol over the prepared catalyst was conducted at atmospheric pressure using a conventional flow reactor. After being placed in the reactor, the catalyst was pretreated by a hydrogen reduction in a hydrogen stream (100 ml/min) at 500 8C for 3.0 h or an oxidation in an oxygen stream (50 ml/min) at 500 8C for 2.0 h, and/or a hydrogen reduction continuously followed by oxidation (300–500 8C). Methanol and water were then pumped into the reactor. The flow rate of methanol was 8.0 10 3 mol/min, partial pressure of feed 0.9 atm (diluted by helium), ratio of steam to carbon (S/C) 1.0, and reaction temperature 250–400 8C. Conversion and selectivity of products were calculated on the basis of carbon. 2.3. Characterization of plated layers The crystal structures for each plated layers on the catalyst were measured by X-ray diffraction (XRD, Rigaku RINT2000/ PC) with Cu Ka radiation. The surface morphology and the sectional view of the plated layers were observed using scanning electron microscopy (SEM, HITACHI S-4300), and the elemental profiles in the same fields analyzed using energy dispersion X-rays (EDX, HORIBA EMAX-7000). The specific surface areas of the plated catalysts were measured by the BET method (COULTER SA3100-PLUS) using nitrogen at its liquid temperature. The net weight of the plated components on the aluminum substrate was estimated as described in the previous study [24].
3. Results and discussion 3.1. Reforming performance and morphology of plated catalysts Three types of palladium plating baths with different reducing agents were reported in some researches [25–27]. It is generally known that different reducing agents used in electroless plating lead to variations in plating rates, because the electron donating ability are differ from each reducing agents. The difference in plating rate eventually brings about variations in physicochemical properties such as the structural state and the elemental state of deposited layer. In addition, it is predicted that the variation in the physicochemical properties of plated layer leads to differences in its catalytic property. So we firstly examined the effect of differences in reducing agents used in the palladium plating bath on methanol reforming properties. The obtained results are shown in Table 1. Prior to the reforming, the continuous treatment of hydrogen reduction followed by oxidation (400 8C) with oxygen was applied to all examined catalysts. As shown in Table 1, all plate-type palladium-based catalysts exhibited activity in the reforming. It was assumed that a catalytic component with reforming some activity was formed on each plated layer through the electroless plating. Also, the selectivity of carbon dioxide was high. In catalysts based on noble metals of group VIII including palladium, methanol decomposition was used to occur and the selectivity of carbon monoxide increases [28–31]. In the case of the prepared plate-type catalysts, however, the steam reforming mainly progressed. It was considered that a palladium-based catalytic component as reported by Iwasa et al. [10–13] and Liu et al. [14,15], which would promote the steam reforming, was formed on the aluminum substrate. In addition, their reforming properties varied depending on the reducing agents used in the palladium plating. It was found that the use of disodium hydrogenphosphite pentahydrate (DSHPP) was preferable with regard to catalytic activity, while trimethylamine-borane (TMAB) with regard to selectivity of carbon dioxide. When sodium phosphinate monohydrate (SPM) was used, the activity was lower than those of other catalysts and a large amount of carbon monoxide was produced at lower temperature. For all
Table 1 Reforming properties of various plate-type Pd-Zn catalysts prepared using different reducing agents Reducing agent used in Pd bath
Reaction temperature (8C)
Conversion (%)
Selectivity (%) CO2
CO
Trimethylamine-borane
250 300 350
5.4 27.9 78.6
92.2 93.9 93.0
7.8 6.1 7.0
Disodium hydrogenphosphite pentahydrate
250 300 350
10.1 46.8 96.9
89.8 90.7 81.7
10.2 9.3 18.2
Sodium phosphinate monohydrate
250 300 350
4.0 17.4 43.4
79.6 83.7 86.1
20.4 16.3 13.9
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Fig. 2. SEM photographs and EDX analyses for surface of the Pd-Zn catalyst prepared using the DSHPP in the palladium plating.
catalysts a plated layer with high strength was formed on the substrate, preventing the catalytic components from dropping off the substrate in the reaction test. Figs. 2 and 3 represent the typical SEM photographs of the catalyst surfaces prepared with the DSHPP and the TMAB and the results of EDX elemental analysis. While there were rough asperities on both surfaces, higher magnifications showed differences between the surfaces. Namely, some spherical particles were observed in the surface of the catalyst prepared with the DSHPP, while some plane crystals in the surface of the
catalyst prepared with the TMAB. The BET specific surface area of the catalyst prepared with the DSHPP was 40.4 m2/gdeposit, and that of the catalyst prepared with the TMAB was 51.2 m2/g-deposit. Such differences in surface morphology and surface area of the material making up the plated layers are thought to cause difference in growth rate of each plated layer, which depends on the kind of reducing agent. The elemental analyses suggest that the surface layers of each plated catalysts included both a palladium and a zinc components. The weight percent of palladium in the catalyst prepared with the DSHPP
Fig. 3. SEM photographs and EDX analyses for surface of the Pd-Zn catalyst prepared using the TMAB in the palladium plating.
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was somewhat higher than that prepared with the TMAB. As inferred in the previously study on the copper-based and nickel-based plate-type catalysts [24,32], the detected zinc component was thought to be derived from the zinc oxide used in the displacement plating of zinc. The amount of palladium component in both layers was estimated about 2 mg/cm2-appearance, and that of zinc component about 2–4 mg/cm2-appearance, respectively. Fig. 4 represents the SEM photographs of a cross-section of the plated layers prepared with the DSHPP and the TMAB and the results of EDX elemental distribution (the analyzed areas are the parts enclosed with white lines in the SEM photographs). The photographs show that, in both layers, plating component was in close contact with the substrate, the thickness of the layer was approximately 20–40 mm, and the surface was an undulated morphology. Furthermore, in the plated layer prepared with the DSHPP, a palladium component and a zinc component were widely distributed from the surface layer near to the substrate, and the detected concentration of the zinc component was relatively high. On the other hand, in the case of the plated layer prepared with the TMAB, although a zinc component was distributed throughout the plated layer, the palladium component was mainly distributed near the surface of the plated layer than near the substrate. Considering the observed results for the surfaces and the cross-sections of the plated layers, factors contributing to the difference in reforming properties as shown in Table 1 were thought to be the difference in the structural state or elemental distribution in the plated layers. The palladium content in the plated layer prepared with the SPM was the lowest of the three catalysts, which was thought to be one of the causes of low catalytic activity. 3.2. Formation of PdZn alloy on the plated layer Although the prepared plate-type catalyst consisted of palladium, it showed high activity and high selectivity of
Fig. 5. XRD profiles of plated catalysts prepared using different reducing agents in the palladium plating.
products in the steam reforming of methanol, as described in Section 3.1. The results suggest the possibility of constructing a reforming active-site as reported by Iwasa et al. [10–13] and Liu et al. [14,15] on the plated surface layer. In this section, the crystalline structure of the plated layer was measured by XRD so as to examine the construction of such reforming active-site. Fig. 5 represents the XRD profiles of each plate-type catalysts prepared by using different reducing agents in the palladium plating. All spectra were measured after the steam reforming of methanol, of which results are shown in Table 1.
Fig. 4. SEM photographs and elemental profiles for cross-section of the Pd-Zn layer prepared using (a) the DSHPP and (b) the TMAB.
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The examined catalysts were pretreated by the hydrogen reduction followed by oxidation prior to the reaction. The formation of the PdZn alloy, which was thought to be the activesite for methanol reforming, was recognized on all the prepared catalysts. However, the shape of the spectra varied depending on the reducing agents used in the palladium plating. Specifically for the catalyst prepared with the TMAB, the peaks assigned to PdZn alloy were definitely observed in 2u = 30.88, 41.28, and 44.18, and the peaks assigned to ZnO were observed in the range of 31.8–36.38. For the catalyst prepared with the DSHPP, although the PdZn alloy peaks were observed, the peak assigned to metallic palladium was clearly found at 2u = 40.18. The ZnO peaks were very small. With the catalyst prepared with SPM, the PdZn alloy peak was very weak, but the ZnO peaks were the highest of the three catalysts. In comparison with the reforming properties shown in Table 1, the selectivity of carbon dioxide was higher for the catalyst prepared with the TMAB, in which the formation of the PdZn alloy was clearly recognized, while the amount of carbon monoxide for the catalyst prepared with the DSHPP was somewhat higher than that with the TMAB, in which the peak assigned to the metallic palladium was detected. In addition, for the catalyst prepared with the SPM, in which the PdZn alloy peak was weak, the amount of formation of carbon monoxide was larger at lower temperature and the reforming activity was also low. From these results, it was considered that there was a correlation between the crystal structure and the reforming properties of the plated layer deposited by electroless plating. The otherwise factor that affects the reforming property was pointed out contamination of boron and/or phosphorus components in the plated layer, which originated in the reducing agent used in the plating process. A deduction that the phosphorus component accelerated the formation of carbon monoxide rather than the boron component might be caused from the reforming properties as shown in Table 1. More accumulated data, however, is needed to discuss about such the deduction. 3.3. Effect of pretreatment condition on formation of PdZn alloy The previous results were obtained by conducting the continuous treatment of hydrogen reduction followed by oxidation after preparing the catalyst. In this section, the influences of differences in pretreatment conditions prior to the reaction on the crystal structure of the plated layers were examined as well as on the reforming properties of methanol. Fig. 6 represents the XRD profiles about the plated layer with no treatment after plating, the plated layer that was oxidized or reduced, and the plated layer that was hydrogenreduced followed by oxidation subsequently. The reducing agent used in the palladium plating was the TMAB. Table 2 shows the methanol reforming properties of the catalysts to which oxidation or reduction treatment was individually applied prior to reaction. Only peaks assigned to the metallic Zn and ZnO were recognized on the XRD profile for the plated layer with no treatment. No peaks for the palladium species
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Fig. 6. XRD profiles of plated layers with different treatment conditions. The TMAB was used in the palladium plating.
were found. The result shows that there was no crystallization of the palladium phase after plating. The palladium phases were detected in the plated layers to which each treatment was applied. Namely, when the plated layer was oxidized, peaks assigned to the metallic Pd and PdO were recognized. As shown in Table 2, because of the existence of the metallic Pd in this layer, a decomposition of methanol progressed to a considerable degree, and the selectivity of carbon monoxide was high at all reaction temperatures. In addition, when the oxidation treatment was conducted, the metallic Zn disappeared and the ZnO peaks became larger instead, indicating the growth of ZnO phase. With regard to the plated layer reduced by hydrogen, although the PdZn alloy was observed, its peak intensity was weak, whereas the peak assigned to a Pd3.9Zn6.1 alloy was clearly observed. The formation of the PdZn alloy, which was thought to be the active-site of methanol reforming, was insufficient. For this reason, its activity was very low regardless of the high selectivity of carbon dioxide. On the other hand, in the case where a continuous treatment of hydrogen reduction followed by oxidation was carried out, no formation of the Pd3.9Zn6.1 alloy was observed, and only the PdZn alloy was formed. Furthermore, its peak intensity was higher than that of the reduced layer. There was a sufficiently crystallized PdZn alloy in this plated layer. As shown in Fig. 5, the crystallinity of the PdZn alloy was sufficiently maintained after the reaction. The crystallinity of the ZnO phase was also well. Consequently, this plated catalyst showed high reforming properties as shown
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Table 2 Reforming performance change of the Pd-Zn catalysts by different treatment condition after plating Treatment condition after plating
Reaction temperature (8C)
Conversion (%)
Selectivity (%) CO2
CO
Oxidized
250 300 350
10.5 43.9 91.1
32.1 41.9 56.4
67.9 58.1 43.6
Reduced
250 300 350
0.2 4.3 35.4
92.7 96.5 97.3
7.3 3.5 2.7
The TMAB was used in the palladium plating.
in Table 1. Judging from a comprehensive overview of the results of Fig. 6 and Tables 1 and 2, it was considered that the crystal state of Pd-Zn layer prepared by electroless plating was very sensitive to the treatment condition after plating and has a significant effect on its catalytic property. This knowledge might lead to the deduction that the reforming performance of the plated Pd-Zn catalyst would be still promoted by finely controlling the treatment condition, such as a rate and composition of flowing gas and a pattern of heating the layer. At any rate, the continuous treatment of the hydrogen reduction and subsequent oxidation after electroless plating would be the suitable pretreatment for preparing the plate-type Pd-Zn catalysts with high reforming property. Table 3 shows the reforming properties of the plated Pd-Zn catalysts oxidized at various treatment temperatures after the hydrogen reduction. Fig. 7 shows the XRD profiles of these catalysts after the reaction. From the table, the catalytic activity improved at a treatment temperature of 500 8C, but the selectivity of carbon dioxide decreased. Because the formation of metallic Pd other than the PdZn alloy was also observed in the XRD spectrum of this layer, a decomposition reaction progressed and the selectivity of carbon dioxide was lowered. In case of the catalyst oxidized at 300 8C, the activities at each reaction temperature were lower than at a treatment temperature of 400 8C, although the reforming selectively proceeded. In the XRD spectrum of a treatment temperature of 300 8C, the
Fig. 7. Effect of oxidation temperature on XRD profiles of the plated Pd-Zn layer prepared using the TMAB. Each layer was reduced at 500 8C and subsequently oxidized at (a) 300 8C, (b) 400 8C and (c) 500 8C.
peak profiles of the PdZn alloy observed at 2u = 41.28 and 44.18 were different from those of at a treatment temperature of 400 8C. Basing on the peak at 41.28 for each profiles, the crystallite diameters of the PdZn alloy calculated using the Scherrer equation were about 59 nm at a treatment temperature of 400 8C and about 27 nm at 300 8C. The crystallite diameter of the PdZn alloy would be smaller at the treatment temperature of 300 8C. The BET specific surface area, however, was 33.7 m2/g-deposit at 300 8C, smaller to the 51.2 m2/g-deposit at 400 8C. From these results, it was considered that the crystallite diameter of the PdZn alloy would have little effect on the reforming activity of the plated layer, rather than the surface area. Furthermore, when the hydrogen reduction was carried out at various range of temperature with a constant oxidation temperature (400 8C), 500 8C was found to be the most suitable for the hydrogen reduction as a treatment temperature. 3.4. Durability performance of the plate-type Pd-Zn catalyst
Table 3 Reforming properties of various plate-type Pd-Zn catalysts oxidized at different temperature after reducing Temperature in oxidizing (8C)
Reaction temperature (8C)
Conversion (%)
Selectivity (%)
300
300 350 400
11.4 68.5 91.4
97.3 98.5 96.9
2.7 1.5 3.1
400
300 350 400
27.9 78.6 96.8
93.9 93.0 96.1
6.1 7.0 3.9
500
300 350 400
32.0 86.7 96.9
89.4 91.7 89.9
10.6 8.3 10.1
The TMAB was used in the palladium plating
CO2
CO
It is generally known that a deterioration of activity with time is low for catalyst that consists of a noble metal. Finally, we examined a durability performance with time of the Pd-Zn catalyst prepared by the electroless plating. Fig. 8 represents the result of the variation of reforming activity and carbon dioxide selectivity with time at 350 8C for the prepared Pd-Zn catalyst. The catalyst was prepared using the TMAB as a reducing agent, and was conducted by the continuous treatment of hydrogen reduction followed by oxidation at 400 8C prior to reaction. For comparison, the figure also represents the activity and selectivity change for the plate-type copper-based catalyst prepared by electroless plating [24] under the same reaction conditions. In case of the plate-type copper-based catalyst, the activity rapidly declined until 700 min after the
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Fig. 8. Durability performance of steam reforming at 350 8C for the Pd-Zn catalyst prepared using the TMAB.
beginning of reaction, which was caused by the sintering of the copper particle at high temperature (When the reforming was performed at 300 8C, it was clarified that the degree of deterioration with time for this copper-based catalyst was quite low [24]). In contrast, the activity for the plate-type Pd-Zn catalyst was up-drifted until 100 min after the beginning of reaction and maintained constant to 1200 min and then gradually declined after that. Even at high temperature of 350 8C, the prepared Pd-Zn catalyst was less prone to deterioration of activity, basing on its noble metal component. The up-drift in activity observed until 100 min was thought to be a stabilization period of reforming active-site on the Pd-Zn layer. As a great feature of the Pd-Zn catalyst, when the catalyst was reoxidized at 400 8C after deteriorating, it restored and progressed its initial activity, indicating 100% conversion. Such a high activity was constantly maintained until 24 h and gradually declined with a less deterioration. For the selectivity of carbon dioxide, the Pd-Zn catalyst maintained a high and stable value over 90%, though slightly changes occurred during a stabilization period and when the catalyst was reoxidized. One can judge that the prepared Pd-Zn catalyst is a catalyst with great advantage from the viewpoint of practical use, for it has a less deterioration and it restores and progresses its initial activity by reoxidation. Such tendency of the Pd-Zn catalyst by reoxidation was also observed when the catalyst was reoxidized in 1200 min after the beginning of reaction. It seems that the oxidation after a stabilization period of reforming active-site might have a relation to such a progressed restoration of activity. The detailed relation between these points, however, is not clarified in this study. Details will be given in subsequent papers, basing on the measurements of physicochemical properties of the plated Pd-Zn layer. 4. Conclusion The plate-type Pd-Zn catalysts were prepared on the aluminum substrate by electroless plating, and their catalytic
properties for steam reforming of methanol were examined. The prepared catalysts had different reforming properties depending on the reducing agent used in the palladium plating bath. The plate-type Pd-Zn catalyst prepared with the TMAB exhibited the best reforming property. From the measurement of physicochemical properties for the plated layer, this catalyst formed the PdZn alloy in the plated layer when a continuous treatment of reduction followed by oxidation was carried out prior to the reaction. The continuous treatment of reduction followed by oxidation was thought to be an important operation in proceeding reforming activity. In addition, the prepared PdZn catalyst was less prone to deterioration, and restored and progressed its initial activity when the reoxidation treatment was carried out on the deteriorated surface. Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science and Technology (13126221). The authors are deeply indebted to Dr. Nobuhiro Iwasa for his helpful advices. References [1] T.L. Reitz, S. Ahmed, M. Krumpelt, R. Kumar, H.H. Kung, J. Mol. Catal. A: Chem. 162 (2000) 275. [2] B. Lindstro¨m, L.J. Petterson, Int. J. Hydrogen Energy 26 (2001) 923. [3] Y. Choi, H.G. Stenger, Appl. Catal. B: Environ. 38 (2002) 259. [4] P.J.D. Wild, M.J.F.M. Verhaak, Catal. Today 60 (2002) 3. [5] B. Lindstro¨m, L.J. Pettersson, J. Power Sources 106 (2002) 264. [6] X. Zhang, P. Shi, J. Mol. Catal. A: Chem. 194 (2003) 99. [7] X.R. Zhang, P. Shi, J. Zhao, M. Zhao, C. Liu, Fuel Process. Technol. 83 (2003) 183. [8] Y. Men, H. Gnaser, R. Zapf, V. Hessel, C. Ziegler, G. Kolb, Appl. Catal. A: Gen. 277 (2004) 83. [9] Y. Kawamura, K. Yamamoto, N. Ogura, T. Katsumata, A. Igarashi, J. Power Sources 150 (2005) 20.
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Catalytic performance of plate-type Pd/Zn-based catalysts for steam reforming of methanol, prepared by electroless plating Choji Fukuhara a,*, Yoshiyuki Kamata b, Akira Igarashi c a
Department of Materials Science and Chemical Engineering, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Nakaku Hamamatsu, Shizuoka 432-8561, Japan b Graduate School of Mechanical Engineering Systems, Hachinohe Institute of Technology, 88-1 Myo Ohbiraki, Hachinohe, Aomori 031-8501, Japan c Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji, Tokyo 192-0015, Japan Received 1 February 2007; received in revised form 24 May 2007; accepted 4 June 2007 Available online 12 July 2007
Abstract It has been reported that a Pd/ZnO catalyst, which consists of palladium and zinc oxide, shows high performance in steam reforming of methanol. By processing this granular catalyst into a plate-type catalyst, a hybrid wall-type methanol reformer with high heat resistance and high reforming performance would be constructed. In this study, such a plate-type Pd-ZnO catalyst was prepared on an aluminum substrate by electroless plating, which consisted of a displacement plating of zinc and a chemical reduction plating of palladium. The catalytic properties of the prepared Pd-ZnO catalyst for steam reforming of methanol were then investigated. The reforming properties of the prepared catalyst varied depending on the reducing agent used in the palladium plating. The plate-type catalyst prepared with a trimethylamine-borane (TMBA) as the reducing agent exhibited high reforming properties. The measurements of physicochemical properties for such plated layer proved that a PdZn alloy was formed on the surface layer. The PdZn alloy, which was thought to be the active-site of methanol reforming, formed in the plated layer when a continuous treatment of hydrogen reduction followed by oxidation was carried out prior to the reaction. In contrast, when either reduction or oxidation treatments individually were carried out prior to the reaction, little PdZn alloy was formed. The continuous treatment of hydrogen reduction followed by oxidation was thought to be an important operation in proceeding the reforming activity. In addition, the durability in reforming properties of the prepared Pd-ZnO catalyst was examined at 350 8C. The prepared catalyst was less prone to deterioration, and restored and progressed its initial performance when a reoxidation treatment was carried out on the deteriorated surface. # 2007 Elsevier B.V. All rights reserved. Keywords: Wall-type catalyst; Structured catalyst; Palladium catalyst; Electroless plating; Steam reforming of methanol
1. Introduction There is a growing social interest in the construction of a hydrogen-oriented society based on the electric generation technique by fuel cells. Such trend has led to a focusing of attention on steam reforming of methanol as one of the reactions for hydrogen production. Over the past few years, a large number of researchers have made studies on steam reforming of methanol. In these reports, a large proportion used catalysts with copper and zinc as the main component of
* Corresponding author. Tel.: +81 53 478 1171; fax: +81 53 476 0095. E-mail address: [email protected] (C. Fukuhara). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.06.037
reforming catalyst [1–9]. However, as is commonly known, although copper-based catalysts show high activity and selectivity for methanol reforming, their heat and oxidation resistances are poor. Therefore, copper-based catalysts are susceptible to deterioration in activity due to sintering and partial oxidation of copper particles. To solve such problems, Iwasa et al. [10–13] noted noble metals, such as palladium and platinum, as alternative to copper component. They prepared palladium and platinum catalysts on zinc oxide support (Pd/ZnO and Pt/ZnO) by a coprecipitation method, and investigated the reforming properties over these catalysts. They found that the active-site species of PdZn alloy and PtZn alloy were formed on the surfaces of the catalysts through a hydrogen reduction treatment, and that the reforming properties
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of the Pd/ZnO and Pt/ZnO catalysts were comparable to that of the Cu/ZnO catalyst. The high reforming performance of the Pd/ZnO catalyst was also reported by Chin et al. [14,15]. The steam reforming of methanol is a reaction that involves a relatively large amount of endothermic energy. Thus, the efficient supply of thermal energy to reaction fields is necessary for achieving high reforming efficiency. The autothermal reforming (ATR) is one of the techniques proposed for achieving such efficiency. In the ATR, thermal energy for the reforming reaction is obtained by partial oxidation or combustion of methanol while oxygen coexists with the raw materials. Because the oxidation and the combustion cause significant amount of heating energy, the use of a catalyst with high heat resistance is essential. The Pd/ZnO and Pt/ZnO catalysts described above are thought to meet these requirements. Agrell et al. [16] and Liu et al. [17–19] investigated the reforming performance of the Pd/ZnO catalyst under the ATR condition. Another technique for efficiently supplying the reforming energy to reaction fields would be proposed from the viewpoint of the reaction system. The conventional fixed-bed reaction system has limitations with regard to the efficient supply of thermal energy because the heat exchange with the reaction fields is conducted by convective heat transfer. Instead of the fixed-bed reaction system, the authors [20–23] have proposed a wall-type reaction system equipped with a platetype catalyst as a new reaction system for reactions involving a considerable heat transfer. The wall-type reaction system allows the efficient supply of thermal energy to the reaction fields by conductive heat transfer. It is expected that, in reforming operated in the wall-type reaction system, there will be an improvement in heat transfer characteristics that cannot be achieved in the fixed-bed reaction system. Therefore, by combining the wall-type reaction system with the Pd/ZnO catalyst or the Pt/ZnO catalyst, it will be possible that a hybrid type of methanol reforming system will be constructed, providing high heat resistance and efficient heat exchangeability. Such a system could have better reaction properties than ever before, as well as stable operation in the ATR reforming. For the purpose of constructing such a reaction system, this study examined the preparation of some plate-type Pd-Zn catalysts, where palladium and zinc components were deposited on an aluminum substrate by electroless plating, and investigated its catalytic properties for steam reforming of methanol. The electroless plating technique is thought to enable both the uniform, homogeneous deposition and ‘‘insitu regeneration’’ of catalyst components, then this technique is considered to be an effective method for preparing a plate-type catalyst for a wall-type methanol reformer. Furthermore, combining with the measurement of physicochemical properties of the prepared catalysts, the influences of plating conditions and pretreatment conditions prior to reaction on the reforming properties of the catalyst were investigated and the suitable condition was determined. Finally, the durability performance change of the prepared catalyst was investigated at 350 8C, comparing with that of the copper-based catalyst.
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2. Experimental 2.1. Preparation of plated palladium catalysts When a palladium component is plated on the surface of an aluminum substrate, for achieving a strong adhesion of palladium, it is necessary to carry out an intermediate plating of a component with an electrode potential between aluminum and palladium. Because zinc (standard electrode potential: 0.76 V) has an electrode potential between aluminum ( 1.7 V) and palladium (+0.82 V), the combination of zinc plating and palladium plating is a convenient method to provide both strong adhesion and support for the palladium catalyst. Thus, an electroless plating, which consisted of a displacement plating of zinc and a chemical plating of palladium, was used for preparing a plate-type palladium-based catalyst on an aluminum plate. The aluminum plate (JIS A1100P-H24, thickness: 0.4 mm) was formed into a pentagonal prism shape, the sectional view of which resembled a star. Its maximum diameter was 21 mm and its length was 120 mm. The apparent total surface area of electrolessly plated catalyst composition was 330 cm2. The procedure of preparing a catalyst by electroless plating is shown in Fig. 1. In order to remove impurities and activate the surface, the aluminum plate was firstly immersed in 3N hydrogen chloride solution for about 15 min. The plate was then immersed in a zinc oxide plating bath (ZnO:50 g/l, NaOH:90 g/l, alkaline, bath temperature: 20 8C, time: 3 min) to displace the surface aluminum with zinc, and the plate was washed in a water bath. The displacement and washing procedures were repeated two times, although the immersion time of the second displacement was only 1.5 min. Subsequently, the plate was immersed in a palladium plating bath (PdCl2: 1.8 g/l, NH2CH2CH2NH2: 5.4 ml/l, S(CH2COOH)2: 0.05 g/l, reducing agent, alkaline, bath temperature: 30 8C, time: 60 min) to deposit palladium component on the surface by chemical reduction. Three kinds of
Fig. 1. Procedures of electroless plating.
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reducing agents were used in the palladium plating bath: (I) a trimethylamine-borane ((CH3)3NBH3, 4.4 g/l), (II) a disodium hydrogenphosphite pentahydrate (Na2HPO35H2O, 4.3 g/l), and (III) a sodium phosphinate monohydrate (NaPH2O2H2O, 6.4 g/l). In order to complex a palladium component, the palladium plating bath was used after agitation for one day and one night. The lower bath temperature of palladium plating was adopted, which was 30 8C, so as to avoid exfoliation of the plated component. After being washed in a water bath, the plated catalyst was dried in an air at room temperature overnight to prepare a plate-type palladium-based catalyst. 2.2. Steam reforming of methanol over plated catalysts The steam reforming of methanol over the prepared catalyst was conducted at atmospheric pressure using a conventional flow reactor. After being placed in the reactor, the catalyst was pretreated by a hydrogen reduction in a hydrogen stream (100 ml/min) at 500 8C for 3.0 h or an oxidation in an oxygen stream (50 ml/min) at 500 8C for 2.0 h, and/or a hydrogen reduction continuously followed by oxidation (300–500 8C). Methanol and water were then pumped into the reactor. The flow rate of methanol was 8.0 10 3 mol/min, partial pressure of feed 0.9 atm (diluted by helium), ratio of steam to carbon (S/C) 1.0, and reaction temperature 250–400 8C. Conversion and selectivity of products were calculated on the basis of carbon. 2.3. Characterization of plated layers The crystal structures for each plated layers on the catalyst were measured by X-ray diffraction (XRD, Rigaku RINT2000/ PC) with Cu Ka radiation. The surface morphology and the sectional view of the plated layers were observed using scanning electron microscopy (SEM, HITACHI S-4300), and the elemental profiles in the same fields analyzed using energy dispersion X-rays (EDX, HORIBA EMAX-7000). The specific surface areas of the plated catalysts were measured by the BET method (COULTER SA3100-PLUS) using nitrogen at its liquid temperature. The net weight of the plated components on the aluminum substrate was estimated as described in the previous study [24].
3. Results and discussion 3.1. Reforming performance and morphology of plated catalysts Three types of palladium plating baths with different reducing agents were reported in some researches [25–27]. It is generally known that different reducing agents used in electroless plating lead to variations in plating rates, because the electron donating ability are differ from each reducing agents. The difference in plating rate eventually brings about variations in physicochemical properties such as the structural state and the elemental state of deposited layer. In addition, it is predicted that the variation in the physicochemical properties of plated layer leads to differences in its catalytic property. So we firstly examined the effect of differences in reducing agents used in the palladium plating bath on methanol reforming properties. The obtained results are shown in Table 1. Prior to the reforming, the continuous treatment of hydrogen reduction followed by oxidation (400 8C) with oxygen was applied to all examined catalysts. As shown in Table 1, all plate-type palladium-based catalysts exhibited activity in the reforming. It was assumed that a catalytic component with reforming some activity was formed on each plated layer through the electroless plating. Also, the selectivity of carbon dioxide was high. In catalysts based on noble metals of group VIII including palladium, methanol decomposition was used to occur and the selectivity of carbon monoxide increases [28–31]. In the case of the prepared plate-type catalysts, however, the steam reforming mainly progressed. It was considered that a palladium-based catalytic component as reported by Iwasa et al. [10–13] and Liu et al. [14,15], which would promote the steam reforming, was formed on the aluminum substrate. In addition, their reforming properties varied depending on the reducing agents used in the palladium plating. It was found that the use of disodium hydrogenphosphite pentahydrate (DSHPP) was preferable with regard to catalytic activity, while trimethylamine-borane (TMAB) with regard to selectivity of carbon dioxide. When sodium phosphinate monohydrate (SPM) was used, the activity was lower than those of other catalysts and a large amount of carbon monoxide was produced at lower temperature. For all
Table 1 Reforming properties of various plate-type Pd-Zn catalysts prepared using different reducing agents Reducing agent used in Pd bath
Reaction temperature (8C)
Conversion (%)
Selectivity (%) CO2
CO
Trimethylamine-borane
250 300 350
5.4 27.9 78.6
92.2 93.9 93.0
7.8 6.1 7.0
Disodium hydrogenphosphite pentahydrate
250 300 350
10.1 46.8 96.9
89.8 90.7 81.7
10.2 9.3 18.2
Sodium phosphinate monohydrate
250 300 350
4.0 17.4 43.4
79.6 83.7 86.1
20.4 16.3 13.9
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Fig. 2. SEM photographs and EDX analyses for surface of the Pd-Zn catalyst prepared using the DSHPP in the palladium plating.
catalysts a plated layer with high strength was formed on the substrate, preventing the catalytic components from dropping off the substrate in the reaction test. Figs. 2 and 3 represent the typical SEM photographs of the catalyst surfaces prepared with the DSHPP and the TMAB and the results of EDX elemental analysis. While there were rough asperities on both surfaces, higher magnifications showed differences between the surfaces. Namely, some spherical particles were observed in the surface of the catalyst prepared with the DSHPP, while some plane crystals in the surface of the
catalyst prepared with the TMAB. The BET specific surface area of the catalyst prepared with the DSHPP was 40.4 m2/gdeposit, and that of the catalyst prepared with the TMAB was 51.2 m2/g-deposit. Such differences in surface morphology and surface area of the material making up the plated layers are thought to cause difference in growth rate of each plated layer, which depends on the kind of reducing agent. The elemental analyses suggest that the surface layers of each plated catalysts included both a palladium and a zinc components. The weight percent of palladium in the catalyst prepared with the DSHPP
Fig. 3. SEM photographs and EDX analyses for surface of the Pd-Zn catalyst prepared using the TMAB in the palladium plating.
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was somewhat higher than that prepared with the TMAB. As inferred in the previously study on the copper-based and nickel-based plate-type catalysts [24,32], the detected zinc component was thought to be derived from the zinc oxide used in the displacement plating of zinc. The amount of palladium component in both layers was estimated about 2 mg/cm2-appearance, and that of zinc component about 2–4 mg/cm2-appearance, respectively. Fig. 4 represents the SEM photographs of a cross-section of the plated layers prepared with the DSHPP and the TMAB and the results of EDX elemental distribution (the analyzed areas are the parts enclosed with white lines in the SEM photographs). The photographs show that, in both layers, plating component was in close contact with the substrate, the thickness of the layer was approximately 20–40 mm, and the surface was an undulated morphology. Furthermore, in the plated layer prepared with the DSHPP, a palladium component and a zinc component were widely distributed from the surface layer near to the substrate, and the detected concentration of the zinc component was relatively high. On the other hand, in the case of the plated layer prepared with the TMAB, although a zinc component was distributed throughout the plated layer, the palladium component was mainly distributed near the surface of the plated layer than near the substrate. Considering the observed results for the surfaces and the cross-sections of the plated layers, factors contributing to the difference in reforming properties as shown in Table 1 were thought to be the difference in the structural state or elemental distribution in the plated layers. The palladium content in the plated layer prepared with the SPM was the lowest of the three catalysts, which was thought to be one of the causes of low catalytic activity. 3.2. Formation of PdZn alloy on the plated layer Although the prepared plate-type catalyst consisted of palladium, it showed high activity and high selectivity of
Fig. 5. XRD profiles of plated catalysts prepared using different reducing agents in the palladium plating.
products in the steam reforming of methanol, as described in Section 3.1. The results suggest the possibility of constructing a reforming active-site as reported by Iwasa et al. [10–13] and Liu et al. [14,15] on the plated surface layer. In this section, the crystalline structure of the plated layer was measured by XRD so as to examine the construction of such reforming active-site. Fig. 5 represents the XRD profiles of each plate-type catalysts prepared by using different reducing agents in the palladium plating. All spectra were measured after the steam reforming of methanol, of which results are shown in Table 1.
Fig. 4. SEM photographs and elemental profiles for cross-section of the Pd-Zn layer prepared using (a) the DSHPP and (b) the TMAB.
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The examined catalysts were pretreated by the hydrogen reduction followed by oxidation prior to the reaction. The formation of the PdZn alloy, which was thought to be the activesite for methanol reforming, was recognized on all the prepared catalysts. However, the shape of the spectra varied depending on the reducing agents used in the palladium plating. Specifically for the catalyst prepared with the TMAB, the peaks assigned to PdZn alloy were definitely observed in 2u = 30.88, 41.28, and 44.18, and the peaks assigned to ZnO were observed in the range of 31.8–36.38. For the catalyst prepared with the DSHPP, although the PdZn alloy peaks were observed, the peak assigned to metallic palladium was clearly found at 2u = 40.18. The ZnO peaks were very small. With the catalyst prepared with SPM, the PdZn alloy peak was very weak, but the ZnO peaks were the highest of the three catalysts. In comparison with the reforming properties shown in Table 1, the selectivity of carbon dioxide was higher for the catalyst prepared with the TMAB, in which the formation of the PdZn alloy was clearly recognized, while the amount of carbon monoxide for the catalyst prepared with the DSHPP was somewhat higher than that with the TMAB, in which the peak assigned to the metallic palladium was detected. In addition, for the catalyst prepared with the SPM, in which the PdZn alloy peak was weak, the amount of formation of carbon monoxide was larger at lower temperature and the reforming activity was also low. From these results, it was considered that there was a correlation between the crystal structure and the reforming properties of the plated layer deposited by electroless plating. The otherwise factor that affects the reforming property was pointed out contamination of boron and/or phosphorus components in the plated layer, which originated in the reducing agent used in the plating process. A deduction that the phosphorus component accelerated the formation of carbon monoxide rather than the boron component might be caused from the reforming properties as shown in Table 1. More accumulated data, however, is needed to discuss about such the deduction. 3.3. Effect of pretreatment condition on formation of PdZn alloy The previous results were obtained by conducting the continuous treatment of hydrogen reduction followed by oxidation after preparing the catalyst. In this section, the influences of differences in pretreatment conditions prior to the reaction on the crystal structure of the plated layers were examined as well as on the reforming properties of methanol. Fig. 6 represents the XRD profiles about the plated layer with no treatment after plating, the plated layer that was oxidized or reduced, and the plated layer that was hydrogenreduced followed by oxidation subsequently. The reducing agent used in the palladium plating was the TMAB. Table 2 shows the methanol reforming properties of the catalysts to which oxidation or reduction treatment was individually applied prior to reaction. Only peaks assigned to the metallic Zn and ZnO were recognized on the XRD profile for the plated layer with no treatment. No peaks for the palladium species
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Fig. 6. XRD profiles of plated layers with different treatment conditions. The TMAB was used in the palladium plating.
were found. The result shows that there was no crystallization of the palladium phase after plating. The palladium phases were detected in the plated layers to which each treatment was applied. Namely, when the plated layer was oxidized, peaks assigned to the metallic Pd and PdO were recognized. As shown in Table 2, because of the existence of the metallic Pd in this layer, a decomposition of methanol progressed to a considerable degree, and the selectivity of carbon monoxide was high at all reaction temperatures. In addition, when the oxidation treatment was conducted, the metallic Zn disappeared and the ZnO peaks became larger instead, indicating the growth of ZnO phase. With regard to the plated layer reduced by hydrogen, although the PdZn alloy was observed, its peak intensity was weak, whereas the peak assigned to a Pd3.9Zn6.1 alloy was clearly observed. The formation of the PdZn alloy, which was thought to be the active-site of methanol reforming, was insufficient. For this reason, its activity was very low regardless of the high selectivity of carbon dioxide. On the other hand, in the case where a continuous treatment of hydrogen reduction followed by oxidation was carried out, no formation of the Pd3.9Zn6.1 alloy was observed, and only the PdZn alloy was formed. Furthermore, its peak intensity was higher than that of the reduced layer. There was a sufficiently crystallized PdZn alloy in this plated layer. As shown in Fig. 5, the crystallinity of the PdZn alloy was sufficiently maintained after the reaction. The crystallinity of the ZnO phase was also well. Consequently, this plated catalyst showed high reforming properties as shown
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Table 2 Reforming performance change of the Pd-Zn catalysts by different treatment condition after plating Treatment condition after plating
Reaction temperature (8C)
Conversion (%)
Selectivity (%) CO2
CO
Oxidized
250 300 350
10.5 43.9 91.1
32.1 41.9 56.4
67.9 58.1 43.6
Reduced
250 300 350
0.2 4.3 35.4
92.7 96.5 97.3
7.3 3.5 2.7
The TMAB was used in the palladium plating.
in Table 1. Judging from a comprehensive overview of the results of Fig. 6 and Tables 1 and 2, it was considered that the crystal state of Pd-Zn layer prepared by electroless plating was very sensitive to the treatment condition after plating and has a significant effect on its catalytic property. This knowledge might lead to the deduction that the reforming performance of the plated Pd-Zn catalyst would be still promoted by finely controlling the treatment condition, such as a rate and composition of flowing gas and a pattern of heating the layer. At any rate, the continuous treatment of the hydrogen reduction and subsequent oxidation after electroless plating would be the suitable pretreatment for preparing the plate-type Pd-Zn catalysts with high reforming property. Table 3 shows the reforming properties of the plated Pd-Zn catalysts oxidized at various treatment temperatures after the hydrogen reduction. Fig. 7 shows the XRD profiles of these catalysts after the reaction. From the table, the catalytic activity improved at a treatment temperature of 500 8C, but the selectivity of carbon dioxide decreased. Because the formation of metallic Pd other than the PdZn alloy was also observed in the XRD spectrum of this layer, a decomposition reaction progressed and the selectivity of carbon dioxide was lowered. In case of the catalyst oxidized at 300 8C, the activities at each reaction temperature were lower than at a treatment temperature of 400 8C, although the reforming selectively proceeded. In the XRD spectrum of a treatment temperature of 300 8C, the
Fig. 7. Effect of oxidation temperature on XRD profiles of the plated Pd-Zn layer prepared using the TMAB. Each layer was reduced at 500 8C and subsequently oxidized at (a) 300 8C, (b) 400 8C and (c) 500 8C.
peak profiles of the PdZn alloy observed at 2u = 41.28 and 44.18 were different from those of at a treatment temperature of 400 8C. Basing on the peak at 41.28 for each profiles, the crystallite diameters of the PdZn alloy calculated using the Scherrer equation were about 59 nm at a treatment temperature of 400 8C and about 27 nm at 300 8C. The crystallite diameter of the PdZn alloy would be smaller at the treatment temperature of 300 8C. The BET specific surface area, however, was 33.7 m2/g-deposit at 300 8C, smaller to the 51.2 m2/g-deposit at 400 8C. From these results, it was considered that the crystallite diameter of the PdZn alloy would have little effect on the reforming activity of the plated layer, rather than the surface area. Furthermore, when the hydrogen reduction was carried out at various range of temperature with a constant oxidation temperature (400 8C), 500 8C was found to be the most suitable for the hydrogen reduction as a treatment temperature. 3.4. Durability performance of the plate-type Pd-Zn catalyst
Table 3 Reforming properties of various plate-type Pd-Zn catalysts oxidized at different temperature after reducing Temperature in oxidizing (8C)
Reaction temperature (8C)
Conversion (%)
Selectivity (%)
300
300 350 400
11.4 68.5 91.4
97.3 98.5 96.9
2.7 1.5 3.1
400
300 350 400
27.9 78.6 96.8
93.9 93.0 96.1
6.1 7.0 3.9
500
300 350 400
32.0 86.7 96.9
89.4 91.7 89.9
10.6 8.3 10.1
The TMAB was used in the palladium plating
CO2
CO
It is generally known that a deterioration of activity with time is low for catalyst that consists of a noble metal. Finally, we examined a durability performance with time of the Pd-Zn catalyst prepared by the electroless plating. Fig. 8 represents the result of the variation of reforming activity and carbon dioxide selectivity with time at 350 8C for the prepared Pd-Zn catalyst. The catalyst was prepared using the TMAB as a reducing agent, and was conducted by the continuous treatment of hydrogen reduction followed by oxidation at 400 8C prior to reaction. For comparison, the figure also represents the activity and selectivity change for the plate-type copper-based catalyst prepared by electroless plating [24] under the same reaction conditions. In case of the plate-type copper-based catalyst, the activity rapidly declined until 700 min after the
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Fig. 8. Durability performance of steam reforming at 350 8C for the Pd-Zn catalyst prepared using the TMAB.
beginning of reaction, which was caused by the sintering of the copper particle at high temperature (When the reforming was performed at 300 8C, it was clarified that the degree of deterioration with time for this copper-based catalyst was quite low [24]). In contrast, the activity for the plate-type Pd-Zn catalyst was up-drifted until 100 min after the beginning of reaction and maintained constant to 1200 min and then gradually declined after that. Even at high temperature of 350 8C, the prepared Pd-Zn catalyst was less prone to deterioration of activity, basing on its noble metal component. The up-drift in activity observed until 100 min was thought to be a stabilization period of reforming active-site on the Pd-Zn layer. As a great feature of the Pd-Zn catalyst, when the catalyst was reoxidized at 400 8C after deteriorating, it restored and progressed its initial activity, indicating 100% conversion. Such a high activity was constantly maintained until 24 h and gradually declined with a less deterioration. For the selectivity of carbon dioxide, the Pd-Zn catalyst maintained a high and stable value over 90%, though slightly changes occurred during a stabilization period and when the catalyst was reoxidized. One can judge that the prepared Pd-Zn catalyst is a catalyst with great advantage from the viewpoint of practical use, for it has a less deterioration and it restores and progresses its initial activity by reoxidation. Such tendency of the Pd-Zn catalyst by reoxidation was also observed when the catalyst was reoxidized in 1200 min after the beginning of reaction. It seems that the oxidation after a stabilization period of reforming active-site might have a relation to such a progressed restoration of activity. The detailed relation between these points, however, is not clarified in this study. Details will be given in subsequent papers, basing on the measurements of physicochemical properties of the plated Pd-Zn layer. 4. Conclusion The plate-type Pd-Zn catalysts were prepared on the aluminum substrate by electroless plating, and their catalytic
properties for steam reforming of methanol were examined. The prepared catalysts had different reforming properties depending on the reducing agent used in the palladium plating bath. The plate-type Pd-Zn catalyst prepared with the TMAB exhibited the best reforming property. From the measurement of physicochemical properties for the plated layer, this catalyst formed the PdZn alloy in the plated layer when a continuous treatment of reduction followed by oxidation was carried out prior to the reaction. The continuous treatment of reduction followed by oxidation was thought to be an important operation in proceeding reforming activity. In addition, the prepared PdZn catalyst was less prone to deterioration, and restored and progressed its initial activity when the reoxidation treatment was carried out on the deteriorated surface. Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science and Technology (13126221). The authors are deeply indebted to Dr. Nobuhiro Iwasa for his helpful advices. References [1] T.L. Reitz, S. Ahmed, M. Krumpelt, R. Kumar, H.H. Kung, J. Mol. Catal. A: Chem. 162 (2000) 275. [2] B. Lindstro¨m, L.J. Petterson, Int. J. Hydrogen Energy 26 (2001) 923. [3] Y. Choi, H.G. Stenger, Appl. Catal. B: Environ. 38 (2002) 259. [4] P.J.D. Wild, M.J.F.M. Verhaak, Catal. Today 60 (2002) 3. [5] B. Lindstro¨m, L.J. Pettersson, J. Power Sources 106 (2002) 264. [6] X. Zhang, P. Shi, J. Mol. Catal. A: Chem. 194 (2003) 99. [7] X.R. Zhang, P. Shi, J. Zhao, M. Zhao, C. Liu, Fuel Process. Technol. 83 (2003) 183. [8] Y. Men, H. Gnaser, R. Zapf, V. Hessel, C. Ziegler, G. Kolb, Appl. Catal. A: Gen. 277 (2004) 83. [9] Y. Kawamura, K. Yamamoto, N. Ogura, T. Katsumata, A. Igarashi, J. Power Sources 150 (2005) 20.
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