Catalyst coating chemical vapor deposition

The wafers were coated with silicon dioxide (400 nm thickness) and silicon nitride by low pressure chemical vapor deposition (LPCVD) alternately. The chips were fabricated by photolithography and etching. The catalyst (for the application Pt) was introduced as a wire (150 pm thickness), which was heated resistively for igniting the reaction. The ignition of the reaction occurred at 100 °C and complete conversion was achieved at a stochiometric ratio of the reacting species generating a thermal power of 72 W (Figure 2.28). 

Control of pore sizes of known catalysts like zeolites has been known for some time although the use of chemical vapor deposition (CVD) of organosilanes to control pore sizes has been the focus of recent research.7 Other catalysts like silica have been treated with methods like CVD and sol-gel in order to deposit thin films. Monolayer coatings of titanium oxide prepared by sol-gel methods have been recently used to coat silica and such films are active in alcohol dehydrogenation reactions.8... 

The methods proposed in the literature to do so, e.g. spin-coating [9], thermal evaporation [10], chemical vapor deposition [11], flash evaporation [12], laser deposition [13] and r.f. reactive sputtering [14], are rather scarce and complex. Moreover, they are often more dedicated to the deposition of active phase on flat and/or monolithic supports (to produce model catalysts for surface science purposes) than on powder supports. These methods thus usually only allow the production of samples at a small scale, so that they are often inadequate for the production of pulverulent real catalysts in large amounts. 

The thermal decomposition of organic compounds can also be employed to generate small carbon clusters or atoms. The borderline with chemical vapor deposition (CVD) as presented in the next section is not really fix. In both cases, the method is based on the thermal decomposition of organic precursors. Processes both with and without catalyst have been reported. Contrary to the chemical vapor deposition, however, the catalyst (if applied) is not coated onto a substrate, but the substance or a precursor is added directly to the starting material ( floating catalyst ). The resulting mixture is then introduced into the reactor either in solid or in liquid state by a gas stream. From this point of view the HiPCo-process could also be considered a pyrolytic preparation of SWNT, but due to its importance it is usually regarded as autonomous method. 

In this study we summarize the recent developments in catalyst development in which nano-porous catalytic sites are accessible through a network of arterial micro-pores. These catalysts are obtained through a solution deposition of metals on a micro-porous polymeric template which is subsequently heat-treated to obtain porous metallic structures where the size of the pores ranged from tens of micrometers to tens of nanometers thus eliminating the problems of accessibility and rapid pore fouling and closure. The technique differs fundamentally from the compression-based systems where the porosity is reduced as a result of compaction. It also differs from the well-known wash-coating or chemical vapor deposition techniques. Furthermore, the mechanisms of metal deposition within micro-pores and nano-structure formation are novel. The importance and current fabrication techniques of porous metallic systems can be found in Refs. l... 

Very recently Chen and coworkers [56] demonstrated the use of ElS for label-free electrochemical detection of DNA sequences relevant to anthrax lethal factor on gallium nitride (GaN) nanowires. The GaN nanowires were grown on a silicon substrate coated with Au catalyst using Ga as the source material and NH3 as the reactant gas in a tubular furnace via air pressure chemical vapor deposition. ElS measurements of the "as grown" GaN nanowires, observed in the Nyquist plot in Fig. 14.12A, exhibited a semicircle and a straight vertical line, indicative of finite impedance at the GaN/electrol3Ae... 

The catalysts can be prepared by coelectrospinning of poly(amido amine) dendrimers and poly(ethylene oxide). These nanofibers can be coated with poly(/ -xylylene) by chemical vapor deposition. 

Cobalt complexes among other metals such as iron and nickel are known to catalyze the growth of carbon nanotube (CNT) by chemical vapor deposition (CVD) [79-81]. Thanks to the thermal stability of the hyperbranched polyyne backbone, spin-coated films of the organometalhc polymer were successfully probed to function as catalyst and arrays of CNT bundles could be prepared (Figure 2.4). This preliminary result already suggests potential application in the field of paUemable tailor-made catalysts. 

Figure Metal particle catalyzed and laser assisted chemical vapor deposition. Left Chemical vapor deposition causes the formation of a film or coating on a hot surface. Center and right Metal catalyzed and laser assisted chemical vapor deposition causes the formation of a potentially continuous fiber with a diameter corresponding to the hot metal catalyst particle or laser focus respectively. Redrawn from F. T. Wallenberger, P. C. Nordine and M. Boman, Inorganic fibers and microstructures directly from the vapor phase, Composites Science Technology, 5,193-222 (1994).

An important topic of research is the introduction of the catalyst in the microreactor. In brief solid catalysts can be incorporated on the interior of micromachined reaction channels, prior to or after closure of the channel, by a variety of strategies anodic oxidation, plasma-chemical oxidation, flame combustion synthesis, sol-gel techniques, impregnation, wash coating, (electro-)plating, aerosols, brushing, chemical vapor deposition, physical vapor deposition and nanoparticle deposition or self-assembly. Some of these methods can be applied in combination with photolithography or shadow masking. 

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