(240e) Synthesis of Carbon Nanofibers as Support Layer for Metal Catalysts in a Microreactor for Three-Phase Reactions

The main goal of one of the projects within our microreactor-research-program is to develop a microstructured multiphase microreactor, viz. a gas-liquid contactor with specially modified catalytic coatings on its microstructured internals, for studying three-phase (G-L-S) reactions. The microreactor will be fabricated using micromachining (cleanroom) technologies in silicon and vitreous materials (i.e., fused silica and/or Borofloat glass). Figure 1a shows the top view of the reactor channel containing silicon pillars. The schematic cross-sectional view, depicted in figure 1b, explains the working principle of the proposed multiphase microreactor. Gas can be injected into a liquid stream via the hollow pillars with porous walls. On the outside of these porous pillars Carbon Nano Fibers (CNFs) will be synthesized that act as a support layer for the catalytic active phase. Figure 2a shows, schematically, an enlarged view of porous pillars with CNFs anchored on the outside. Figure 2b shows an entangled jungle of such a CNF layer which can be further utilized as structural support for metal catalyst [2], as illustrated in figure 2c. This abstract focuses on the synthesis of CNFs on the microreactor interior.

Currently, there is a trend to use structured catalyst supports, i.e., that the catalyst is deposited on a rigid, orderly arranged support. Catalytic support layers based on CNFs offer a novel option to facilitate this task [1, 3]. With their inherent high surface area-to-volume ratio, CNFs provide more catalytic surface area [1, 2], thereby obtaining sufficient activity per unit volume of catalyst, while maintaining a low fluidic resistance (which is important in microfluidic devices). Additionally, diameter and length of the fibers and hence the bulk density of the CNF layer, can be manipulated to tailor porosity and overcome tortuosity problems. This also eliminates the internal diffusion limitations by preventing concentration gradients inside the CNF layer.

A CVD-process is applied to synthesize CNFs using hydrocarbon gas (e.g. ethylene) and a metal catalyst (such as nickel). Preparation of nickel thin-films along with various adhesion layers (Ti, Ta and TiW) is done by evaporation (thickness of films vary from 10 nm to several hundreds of nanometers). Various compositions or layer configurations (dual-layer and sandwich-structured) are currently investigated in order to understand their influence on the morphology and stability of the synthesized CNF layers. The fundamental growth/synthesis of CNFs ? structure, morphology, length, diameter etc. ? is studied as function of CVD conditions (growth temperature, and time). Different stages in CNF-growth can be discriminated based on the growth time. After 5 min. of reaction, formation of nickel nanoparticles initiated along with very few and thin CNFs as depicted in figure 3a. Whereas, significant growth took place after 15 min. achieving thick layer of CNFs (8 to 15 µm Long) as shown in figure 3b. Longer reaction times (30 min. to 1h.) leads to formation of cauliflower resembling structures along with CNFs as illustrated in figure 3c.

It is expected that this novel approach of constructing microstructured reactors with stable and well-defined carbon nanofibers upon porous silicon pillars as structured catalyst support, results in accomplishing the goal of designing smart and efficient multi-phase microreactors.

References:

[1] M.-J. Ledoux , C. Pham-Huu, Catal. Today 102-103 (2005) 2.

[2] M. L. Toebes, F. F. Prinsloo, J. H. Bitter, A. J. van Dillen, K. P. de Jong, J. Catal. 214 (2003) 78.

[3] J. K. Chinthaginjala, K. Seshan, L. Lefferts, Ind. Eng. Chem. Res. 46 (2007) 3968.