CVD catalyst

The CVD catalyst exhibits good catalytic performance for the selective oxidation/ammoxida-tion of propene as shown in Table 8.5. Propene is converted selectively to acrolein (major) and acrylonitrile (minor) in the presence of NH3, whereas cracking to CxHy and complete oxidation to C02 proceeds under the propene+02 reaction conditions without NH3. The difference is obvious. HZ has no catalytic activity for the selective oxidation. A conventional impregnation Re/HZ catalyst and a physically mixed Re/HZ catalyst are not selective for the reaction (Table 8.5). Note that NH3 opened a reaction path to convert propene to acrolein. Catalysts prepared by impregnation and physical mixing methods also catalyzed the reaction but the selectivity was much lower than that for the CVD catalyst. Other zeolites are much less effective as supports for ReOx species in the selective oxidation because active Re clusters cannot be produced effectively in the pores of those zeolites, probably owing to its inappropriate pore structure and acidity. 

XPS data suggest that the surface of the CVD catalysts is finely covered by a homogeneous silica layer except for Pt particles, where small holes are formed. Since hydrogenation exclusively occurs on Pt particles, selectivity is brought about by steric hindrance around the Pt site in the holes. Less hindered double bonds, consequently, are hydrogenated preferentially. 

Impregnation and physically mixed Re catalysts were much less active and much less selective for the phenol synthesis (Table 2.4). The CVD catalyst was almost 18 times more active than the conventional impregnation catalyst. In the physically mixed and impregnated catalysts, the Re7 + precursors partly aggregated as ReOx like Re02 in the presence of the NH3 reductant and such ill-defined Re aggregates decreased both activity and phenol selectivity as shown in Table 2.4. 

In eonventional gas-phase CVD, catalyst particle formation takes place simultaneously with carbon source deeomposition and SWCNT formation in the reactor. Ferrocene decomposition (465 °C) and CO disproportionation (preferably 400-900 °C) take place at similar temperatures, whieh prevents excessive catalyst particle growth before disproportionation takes place. The most efficient SWCNT formation from CO in the gas-phase CVD method was obtained with ferrocene as the catalyst precursor at 700-900 °C, i.e., at eonditions corresponding to the smallest mean catalyst particle size (as deseribed in more detail later), and most efficient CO disproportionation reaetion. The morphology of the catalyst nanoparticles depended on the gas atmosphere in the reactor. The presence of H2 during ferrocene decomposition resulted in the formation of catalyst particles with larger primary sizes. Figure 20.10 shows the sehematie of a chemical vapor process in whieh a thin-film metal catalyst is used. 

Metal carbonyls form a large and important group of compounds which are used widely in the chemical industry, particularly in the preparation of heterogeneous catalysts and as precursors in CVD and metallo-organic CVD (MOCVD). 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

Chemical vapor deposition (CVD) [27] of hydrocarbons over a metal catalyst is a method that has been used to synthesize carbon fibers, filaments, etc. for over 20 years. Large amounts of CNTs can be formed by catalytic CVD of acetylene over Co and Fe catalysts on silica or zeolite. 

For this purpose, all three catalyst supports were initially synthesized by a chemical vapor deposition (CVD) process and thereafter, using a wet impregnation method, loaded with cobalt as the active component for FTS. The as-synthesized Co/nanocatalysts were then characterized by applying electron microscopic analysis as well as temperature-programmed desorption, chemi- and physisorption measurements, thermogravimetric analysis, and inductively coupled plasma... 

The drawback of the CVD method is eliminated in ROMP, which is based on a catalytic (e.g., molybdenum carbene catalyst) reaction, occurring in rather mild conditions (Scheme 2.3). A living ROMP reaction ofp-cyclophanc 3 or bicyclooctadiene 5 results in soluble precursors of PPV, polymers 4 [31] and 6 [32], respectively, with rather low polydispersity. In spite of all cis (for 4) and cis and trans (for 6) configuration, these polymers can be converted into aW-trans PPV by moderate heating under acid-base catalysis. However, the film-forming properties of ROMP precursors are usually rather poor, resulting in poor uniformity of the PPV films. 

The Re0 7HZ catalyst was prepared by the following procedure. Methyl trioxorhenium (MTO) was sublimed under vacuum at 333 K and the vapor was allowed to enter the chamber, where the zeolites were pretreated in situ at 673 K under vacuum. After the chemical vapor deposition (CVD) into zeolite pores, undeposited MTO was removed by evacuation at RT. The catalyst was treated at 673 K in He before using. 

Table 8.5 Performance of ReOx/Zeolite Catalysts in Selective Oxidation/Ammoxidation of Propene on a CVD HZcvd Catalyst, an Impregnated HZimp Catalyst, and a Physically Mixed HZphys Catalyst at 673 K...

Keywords Chemical Vapour Deposition (CVD), Single Walled Carbon Nanotubes (SWCNTs), Catalyst. 

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