Physicochemical properties of a plate-type copper-based catalyst, prepared on an aluminum plate by electroless plating, for steam reforming of methanol and CO shift reaction

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Abstract

A plate-type copper-based catalyst was prepared by electroless plating on an aluminum substrate. Its physicochemical properties were measured to examine the relation to methanol reforming and CO shift performances. The catalytic activity of the plated copper-based catalyst was improved more by oxidation treatment in air than by reduction treatment prior to the reaction. In this case, the oxidation treatment caused the zinc located in the bulk layer to migrate to the surface and form a CuZn alloy-like compound. While, the reduction treatment made only a small amount of zinc migration, so the CuZn alloy was barely formed. The surface area of metallic copper component on the catalyst was increased by the reduction treatment, and with the number of reductions; however, there was no relationship between such metallic surface area and CO shift activity. The valence of the copper species at the surface layer was metallic after reduction treatment and cationic after oxidation treatment or reforming reaction. It was found that the presence of metallic copper species on the plated catalyst hardly contributes to the formation of active site. The formate species, which are considered as the intermediate of reforming and CO shift reaction, were adsorbed on the catalyst in the form of a monodentate-type or a bridge-type (and/or bidentate-type). On the surface that experienced oxidation treatment, the proportion of the monodentate-type formate group was higher. It was inferred that the formation of a CuZn alloy-like compound accelerates the increase of monodentate-type formate group and contributes to the improvement of catalytic activity.

Graphical abstract

The physicochemical properties of the plate-type copper-based catalyst prepared by electroless plating were measured to examine the relation to methanol reforming and CO shift reaction. When the catalyst was oxidized, zinc located in the layer to migrate to surface and form a CuZn alloy-like compound in the surface layer. The formation of such compound contributed to an improvement of catalytic activity.

Introduction

When using fuel cells on-site type or on-board type, requirements for the reforming system, which produces hydrogen, include compact dimension as the system itself, efficient exchange of thermal energy and quick response to load fluctuation [1], [2]. In case of constructing such a reforming system using a fixed-bed type reactor, a film resistance, a convectional heat transfer and an increment of pressure loss in the catalyst bed make it difficult to achieve these requirements. On the other hand, a plate-type reactor [3], [4], [5], [6], [7], [8], [9], which is equipped with a structured catalyst that metal surface is catalyzed, effectively exchanges heat energy by conductional heat transfer and rapidly responds to load fluctuation due to its low pressure loss. In addition, the size of the reactor can be made compact. The plate-type reactor realizes the above requirements.

In order to construct a steam reformer and a CO shift converter using such plate-type reaction system, we previously prepared a plate-type copper-based catalyst by depositing a copper component on an aluminum substrate by electroless plating [10], [11]. The obtained results proved that the reforming and shift activities of the prepared catalyst was superior to that of commercial copper-based reforming and shift catalysts of the same weight. It was also found that the prepared catalyst is greatly improved in reforming and shift activities by oxidation treatment in air prior to the reaction, and the activity is regenerated by oxidation treatment when it was declined. For ordinary copper-based catalysts, oxidation of the catalyst causes the active sites to deteriorate and the copper particles to be sintered, decreasing the reforming and shift activities. Therefore, oxidation resistance is required for the copper-based catalysts. The prepared plate-type copper-based catalyst, however, makes oxidation produce a positive effect oppositely, which is interesting properties from the viewpoint of the catalytic chemistry. Such catalytic property makes it possible to omit the catalyst reduction treatment using hydrogen and store the catalyst in a state exposed to air, which are convenient from a practical usage as well. One of the reasons for the prepared catalyst indicating such unique properties despite its copper-based component would be thought that the electroless plating was used for preparing the catalyst layer. However, the correlation between them has not been clarified in detail.

As the catalyst preparation using an electroless plating is a novel method for catalyst preparation, there have been few reports on investigating the relation between the physicochemical properties and a development of catalytic performance of the prepared catalyst. The investigations of physicochemical properties of the plate-type nickel-based catalyst performed by Fukuhara and Igarashi [12], [13] were only a few report based on such point. Catalyst preparation by electroless plating enables the catalyst component uniformly and homogeneously to be deposited on the surface of any shape wherever the surface can contact the plating solution. By changing the metallic component in the plating solution, it is possible to prepare a suitable catalyst for various reactions. Therefore, considering the wide application possibilities of the electroless plating, it is important to measure the physicochemical properties of the plated layer and investigate the correlation with catalytic properties. This study used various spectroscopic instruments to measure the physicochemical properties of the plated layer of the plate-type copper-based catalyst prepared by electroless plating, and investigated the developing factors of reforming and shift performances.

Section snippets

Preparation of plate-type copper-based catalyst

A plate-type copper-based catalyst was prepared on an aluminum plate by electroless plating, which consisted of a displacement plating of zinc, an intermediate plating of iron, and a chemical plating of copper. The aluminum plate, plating procedure and composition of each plating bath were the same as those in the previous study [10], [11]. The plating process is as follows. In order to remove impurities and activate the surface, the aluminum plate was first immersed in a hydrogen chloride

Catalytic performance of the plated copper-based catalyst

Table 1 shows the catalytic properties of the plate-type Cu–Fe/Zn catalyst (dried in air after plating) for steam reforming of methanol and CO shift reaction, which were obtained in our previous studies [10], [11]. Table 2 shows the reforming and shift properties, under the same reaction condition, of a catalyst forcedly dried in a helium stream after plating. From the result of Table 1, in both reforming and shift reaction, the catalyst activity is improved by oxidation treatment prior to the

Conclusion

The physicochemical properties of the plate-type Cu–Fe/Zn catalyst prepared by electroless plating were measured to examine the relation to methanol reforming and CO shift properties. When the catalyst was oxidized in air, zinc located in the bulk layer to migrate to the surface and form the CuZn alloy-like compound in the surface layer. Such CuZn alloy was barely formed when the catalyst was reduced by hydrogen and forcedly dried in the non-oxygen atmosphere. The surface area of metallic

Acknowledgment

The authors are deeply indebted to Dr. Yasuyuki Matsumura for his kindly cooperation in the XPS measurement.

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