Stacked etched aluminum flow-through membranes for methanol steam reforming

https://doi.org/10.1016/j.ijhydene.2017.01.106Get rights and content

Highlights

  • Stacked membranes can be used without limited air permeation.

  • The performance of reactors with different numbers of membranes was evaluated.

  • The stacked membrane catalyst reacted more effectively than the granular one.

Abstract

Electrochemical etching has been used to obtain aluminum foil with high surface area for use as electrodes in electrolytic capacitors. In this approach, direct current etching first generates straight penetrating microchannels, and then a second etching step enlarges the microchannel diameter. In the present work, we developed catalyst supports using aluminum etched with microchannels as a microreactor. The metal aluminum foil catalyst support obtained by etching contained microchannels with a diameter of 1.0–3.0 μm (10,000–15,000 microchannels/mm2). We stacked membrane layers and evaluated their performance in methanol steam reforming. The performance of the reactors containing stacked membranes improved as the layer number increased. The microchannels in this catalytic membrane could be used as reaction channels, were easy to fabricate at low cost, and could be mass-produced continuously. This novel catalytic membrane support opens up new possibilities for practical fabrication of industrial materials.

Introduction

Electrochemical etching with a direct current (DC) has previously been employed to obtain aluminum foil with high surface area for use as electrodes in electrolytic capacitors. Such anisotropic etching provides a rough metal surface with numerous tunnel-shaped etch pits [1], [2], [3]. In the manufacture of foil electrodes for use in electrolytic capacitors, the first stage of etching forms the tunnel-shaped pits in the aluminum foil, and the second etching step increases the pit diameter. In the present study, we use this anisotropic etching technique to produce tunnel-shaped penetrating microchannels that act as a catalyst support; that is, a membrane reactor. The characteristic dimensions of such catalytic membrane reactors are smaller than those of general microreactors, which results in excellent heat transfer between the fluid and membrane.

The motivation for the development of a flow-through catalytic etched aluminum membrane reactor is to reach complete conversion in minimum time or space, achieve high catalytic efficiency, or realize maximum selectivity for a given reaction because of the narrow contact time distribution. If a catalyst is placed inside the membrane microchannels and the reactants flow convectively through the microchannels, the resulting intensive contact between reactants and catalyst results in high catalytic activity. Several materials and reaction systems using flow-through catalyst membrane reactors have been studied [4]. The mean microchannel size of the membranes in such reactors varies; furthermore, the choice of appropriate microchannel size always represents a trade-off between intensive reactant–catalyst contact and a small pressure drop. For gas-phase reactions in microchannel structures below the micrometer scale, improved understanding of flow processes and microeffects in Knudsen and transition regimes is required. Similar-sized microchannels of flow-through catalyst membrane reactors exist in microporous silicon [5], [6], but such examples are limited.

Our research group has developed anodized porous alumina layers as catalyst supports and investigated their application [7], [8], [9], [10]. Anodized porous alumina layers can be formed on aluminum plate surfaces by anodic oxidation. These layers have many advantages over conventional catalysts, including high thermal conductivity and ready formation of various shapes and structures.

Steam reforming of methanol (MeOH) was carried out to measure the activity of the stacked membranes. The microreactors fabricated by several methods for steam reforming of MeOH have been studied [11], [12], [13], [14]. It is a significant challenge to fabricate materials with microchannels cost-effectively, and catalyst deposition on microchannels remains a major challenge.

Previous research has suggested that an alumina membrane with penetrating microchannels formed by etching can act as an effective catalyst support [15], [16]. However, etched aluminum has some limitations as a membrane material. For example, it is difficult to form microchannels more dense than those in present membranes while maintaining the strength of the membrane material. It is also difficult to produce thick aluminum foil suitable for anisotropic etching. These limitations may be overcome by stacking flow-through catalyst membranes. Thus, the purpose of this study is to examine how stacking etched aluminum membranes affects their performance.

Section snippets

Catalyst preparation

Aluminum foil was treated by electrochemical DC etching to form penetrating microchannels. Conditions for electrochemical etching, anodization, hydration, and calcination were as reported previously [15]. Electrochemical etching was carried out to generate penetrating microchannels. The surface of the microchannels was treated by anodization and hydration. The hydrated alumina filled in the pores of the anodized alumina and covered the surface of the anodized alumina. The membranes contained

Size distribution of etched microchannels

The membrane structure was investigated by SEM. The pit diameter of 2.6–4.6 μm and pit size distribution of 2 μm determined from the SEM images of the membrane were consistent with those found in our previous research [15]. It remains a challenge for future research to reveal the influence of the pit size distribution on catalytic performance and obtain a membrane with more regular pore geometries to improve the performance of the membrane; e.g., residence time distribution.

TEM, STEM, and EDS analyses

Fig. 2(a), (b), and

Conclusions

We investigated a novel catalyst support consisting of stacked membranes with penetrating microchannels formed by electrochemical etching. Limited air permeability of the stacked membranes was not a problem because air permeability was directly proportional to the number of membrane layers. However, the pressure loss increased with membrane layer number. Activity tests revealed that stacking the membranes improved the performance of flow-through catalyst membrane reactors. We deduced that

References (16)

There are more references available in the full text version of this article.

Cited by (13)

  • Proposal for effective stacking method of structured catalyst

    2021, International Journal of Hydrogen Energy
    Citation Excerpt :

    It is frequently used to clean the exhaust gas from automobiles and chemical plants, and to treat high flow rates, because of its small pressure drop [24]. Other structured catalysts in the form of fibers [25,26], foams [27,28], and etching materials [29,30] have also been studied. The structured catalysts have gained particular attention because of the recent advances in computer performance and the development of additive manufacturing technology.

  • Hydrogen production in microreactors

    2020, Current Trends and Future Developments on (Bio-) Membranes: New Perspectives on Hydrogen Production, Separation, and Utilization
  • Numerical modeling of an automotive derivative polymer electrolyte membrane fuel cell cogeneration system with selective membranes

    2019, International Journal of Hydrogen Energy
    Citation Excerpt :

    Moreover, we retrieve part load performance, in order to lay the foundation of a study considering the CHP system in a real energy management scenario. Many studies focus on the single component development for membrane reactors using several experimental or modeling techniques [38–41,47–54], however here we are interested in retrieving the performance of the whole energy system. We assume that the hydrogen has to be locally produced by NG coming from the distribution grid.

  • Performance assessment and evaluation of catalytic membrane reactor for pure hydrogen production via steam reforming of methanol

    2017, International Journal of Hydrogen Energy
    Citation Excerpt :

    The use of catalytic membrane reactor (CMR) as a novel technology can significantly improve the efficiency of this process [17–19]. Hiramatsu et al. [20] applied the stacked etched aluminum flow-through membranes for SRM and concluded that this novel catalytic membrane support reacted more effectively than the regular one. Also techno-economic assessments of energy systems represent that the use of CMR can be beneficial under specific market and regulatory conditions.

  • Numerical analysis of performance enhancement and non-isothermal reactant transport of a cylindrical methanol reformer wrapped with a porous sheath under steam reforming

    2017, International Journal of Hydrogen Energy
    Citation Excerpt :

    Therefore, the carbon monoxide (CO) production from hydrogen-rich gas can be low at low temperature, and then avoids poisoning the catalyst on the anode to decline the PEMFC performance [6,7]. The methanol steam reformer (MSR) connected with PEMFC is considered as a promising candidate of the power sources at the moment [8–13]. Accordingly, the design of methanol reformer plays a major role for enhancing the MSR performance.

View all citing articles on Scopus
View full text