Microfabricated catalyst

Ajmera, S. K., Deiattre, C., Schmidt, M. A., Jensen, K. F., Microfabricated cross-flow chemical reactor for catalyst testing. Sens. Actuators 82, 2-3 (2002) 297-306. 

Figure 3.17 Microfabrication sequence for the silicon component of the catalyst membrane micro reactor [57],...

GP 8[ [R 7[ The structure of the rhodium catalyst changed during operation. Owing to the microfabrication process (thin-wire pEDM), the surface of the micro channels was rough before catalytic use [3]. After extended operational use, small crystallites are formed, especially in oxygen-rich zones such as the micro channels inlet. Thereby, the surface area is enlarged by a factor of 1-1.5. 

Cao et al., 2006 recently used microfabricated reactors with online GC for a Raman investigation of silver catalysts during methanol oxidation catalysis. Their results are consistent with those described above. 

FIGURE 6 An example of a microfabricated packed bed in which pillars take on the function of catalyst particles. These pillars can be fabricated with extremely high precisions, resulting in excellent control of flow properties (8). 

EBL was used to fabricate uniform platinum nanoparticle arrays on Si02 (mean platinum particle diameter 30-1000 nm 52,53,106,107,398)), and evaporation techniques were used to prepare smaller particles and a continuous platinum film. The EBL microfabrication technique allows the production of model catalysts consisting of supported metal nanoparticles of uniform size, shape, and interparticle distance. Apart from allowing investigations of the effects of particle size, morphology, and surface structure (roughness) on catalytic activity and selectivity, these model catalysts are particularly well suited to examination of diffusion effects by systematic variations of the particle separation (interparticle distance) or particle size. The preparation process (see Fig. 1 in Reference 106)) is described only briefly here, and detailed descriptions can be found in References 53,106,399). 

Transmission electron microscopy (TEM) is probably the most powerful technique for obtaining structural information of supported nanoparticles [115-118], Complementary methods are STM, AFM, and SEM. Both the latter and TEM analysis provide more or less detailed size, shape, and morphology information, i.e., imaging in real space. TEM has the great additional advantage to provide information in Fourier transform space, i.e., diffraction information, which can be transformed to crystal structure information. From a practical point of view, considering the kinds of planar model catalysts discussed above, STM, AFM, and SEM are more easily applied for analysis than TEM, since the former three can be applied without additional sample preparation, once the model catalyst is made. In contrast, TEM usually requires one or more additional preparation steps. In this section, we concentrate on recent developments of microfabrication methods to prepare flat TEM membrane supports, or windows, by lithographic methods, which eliminate the requirement of postfabrication preparation of model catalysts for TEM analysis. For a more comprehensive treatment of other, more conventional, procedures to make flat TEM supports, and also similar microfabrication procedures as described here, we refer to previous reviews [118-120]. 

Microfabrication processes have been used successfully to form micro-fuel cells on silicon wafers. Aspects of the design, materials, and forming of a micro-fabricated methanol fuel cell have been presented. The processes yielded reproducible, controlled structures that performed well for liquid feed, direct methanol/Oj saturated solution (1.4 mW cm ) and direct methanol/H O systems (8 mA cm" ). In addition to optimizing micro-fuel cell operating performance, there are many system-level issues to be considered when developing a complete micro power system. These issues include electro-deposition procedure, catalyst loading, channel depth, oxidants supply, and system integration. The micro-fabrication processes that have... 

As with the development of new catalysts, effective new materials benefit from a thorough understanding of structure/property relationships. This involves multiscale modeling and experimental efforts in surface science, including morphology. Enabling the use of new materials will also require extensive development of new nano- and microfabrication techniques, including biodirected or self-assembly syntheses. 

FIGURE 17 Microfabricated structured catalyst packing inside a microchannel [31]. (Adapted with permission from Elsevier.)... 

S.K. Ajmera, C. Delattre, M.A. Schmidt, K.F. Jensen, Microfabricated differential reactor for heterogeneous gas phase catalyst testing, J. Catal. 209 (2002) 401. 

Due to their cost, instability, and limited longevity, enzymes are not widely employed in production-scale syntheses however, through their immobilization and incorporation into flow reactors, biocatalysts have the potential to be employed in the synthesis of high-value products. Although the use of microfabricated reactors for the screening of biocatalysts for organic synthesis is a relatively new area of research, the field has been quick to employ those techniques developed for the use of solid-supported catalysts under continuous flow, a feature that is illustrated by the diverse array of immobilization techniques reported to date. 

In Takahashi et al. (2005) a suspended MEMS based micro-fuel reformer was designed and manufactured, and the performance of the reformer evaluated. In this study, in-situ chemical vapour deposition (CVD) of the alumina catalyst bed on a membrane was used as the preparation method for better mechanical and thermal isolation of the reaction zone on the membrane. Most of the microfabricated Pd-based MMs are much more efficient than the conventional thicker or large scale devices, as reported by many authors. 

B. A. Wilhite, S. E. Weiss, J. Y. Ying, M. A. Schmidt, K. F. Jensen, High-purity hydrogen generation in a microfabricated 23 wt% Ag-Pd membrane device integrated with 8 1 LaNio. sCoo.osOj/ Al20j catalyst. Adv. Mater., 2006, 18, 1701-1704. 

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