Microfabrication and Functionalization of an Aluminum Gas-Phase Micro-Reactor via Through-Mask Electrochemical Micromachining
Authors: Baldhoff Tobias , Nock Volker , Marshall Aaron Timothy 
Published: February 2020

Gas-phase micro-reactors have been the focus of much research related to their use for the distributed and decentralized production of chemicals and electricity [1]. Micro-reactor applications include the production of hydrogen-rich synthesis gas from fuels (steam reforming), CO removal from fuel cell feedstocks (Water-Gas-Shift reaction, preferential CO oxidation), and the synthesis of hydrocarbons (Fischer-Tropsch reaction), methanol, dimethyl ether, and ethanol. The advantages of micro-reactors in these applications stem from their small characteristic dimensions leading to a high specific surface area and low volume [2]. As a result, the driving forces for heat and mass transfer increase by several orders of magnitude, mixing times lie on the order of milliseconds, and both product selectivity as well as process safety are vastly improved.
Among the available metals and alloys, aluminum is frequently chosen for the fabrication of gas-phase micro-reactors [1]. Despite its low melting point (660°C), aluminum may be safely used for the Water-Gas-Shift reaction, preferential CO oxidation, or the Fischer-Tropsch reaction, as these processes typically operate below 400°C. Advantageously, its high thermal conductivity (237 W/m·K) assists in removing excess heat released by these exothermic reactions. In addition, the surface structure of aluminum surfaces may be altered via the formation of porous oxide films, which subsequently serve as protective coatings, nano-structure templates, or catalyst support layers [3,4]. In particular, this enables noble metal catalysts, such as palladium and gold, to be deposited within the pores via wash-coating or electrodeposition methods [4,5].
The combination of photolithographic masking with electrochemical dissolution processes, termed Through-Mask Electrochemical Micromachining (TMEMM), is an unconventional machining process enabling localized material removal from conductive substrates [6]. Compared to conventional micro-machining techniques utilizing chemical etchants, reactive plasmas, cutting tools, or laser beams for micro-structuring, TMEMM offers several advantages: (i) enhanced process safety, (ii) improved surface finish, (iii) accelerated machining rate, (iv) and increased machining selectivity [6-8]. In the case of aluminum, concentrated phosphoric acid at elevated temperatures is a suitable electrolyte for TMEMM. This is because anodic dissolution proceeds due to the random removal of metal cations through a thin barrier oxide film, resulting in a polished surface finish [9]. In this way, the fabrication of an aluminum micro-mixer was successfully demonstrated, avoiding issues related to electrolyte reactivity, flammability, and stability [10].
In the present work, results on TMEMM of an aluminum gas-phase micro-reactor are presented (Figure 1). To this end, square aluminum platelets were dissolved through a photoresist mask in 85% phosphoric acid at 75°C using a custom-made, jacketed glass cell. In the process, the uniformity of mass transfer along the mask was studied using different designs for the photoresist mask as well as agitation. The use of surfactants as electrolyte additives was examined to retain a polished surface finish over the whole mask design. Using the same set-up, TMEMM was compared with chemical etching in several aluminum wet etchants. The machined micro-reactor structures were subsequently examined via light microscopy, scanning electron microscopy, and profilometry to investigate their surface structure, roughness, and shape profile. This work is complemented by a preliminary study of the formation of catalyst support layers via anodization as well as of gold catalyst nanoparticles via electrodeposition along the channel walls of the micro-reactor.
References:
[1] G. Kolb, Chem. Eng. Process., 65 (2013) 1-44.
[2] T. R. Dietrich, Microchemical Engineering in Practice, Wiley, Hoboken, NJ (2009).
[3] W. Lee, and S.-J. Park, Chem. Rev., 114 (2014), 7487-7556.
[4] G. Wießmeier, and D. Hönicke, J. Micromech. Microeng., 6 (1996) 285-289.
[5] P. Forrer, F. Schlottig, H. Siegenthaler, and M. Textor, J. Appl. Electrochem., 30 (2000), 533-541.
[6] M. Datta, and D. Harris, Electrochim. Acta, 42 (1997), 3007-3013.
[7] B. Chatterjee, Prec. Eng., 8 (1986), 131-138.
[8] M. Datta, J. Electrochem. Soc., 142 (1995), 3801-3805.
[9] T. Baldhoff, and A. T. Marshall, J. Electrochem. Soc, 164 (2017) C46-C53.
[10] T. Baldhoff, V. Nock, and A. T. Marshall, J. Electrochem. Soc., 164 (2017) E194-E202.

Figure
1
: Gas-phase micro-reactor machined into the surface of a 50 mm wide aluminum plate containing 59 parallel channels and two flow manifolds: (a) photograph, (b) surface height map, (c) and channel profile.

Figure 1