Selective heating of the catalyst

These results suggest that selective heating of the catalyst to temperatures higher than that of the liquid phase is responsible for the modest increases in rate in the presence of a heterogeneous catalyst. 

Similar selective heating of the catalyst particles was observed by Harutyunyan [33] in the first step of a general purification procedure for single-wall CNT produced by the arc-discharge technique with an Ni-Y catalyst added to the electrodes. Microwave heating as an initial step for metal removal was compared with selective oxidation , which refers to selective oxidation of undesirable minority phases (Figs. 5.13 and 5.14). 

A selective and relatively quick - although not parallelized - method of analysis was developed by the Symyx team using mass spectrometry (Fig. 15.6) [20], This technique is suited for small amounts of catalyst deposited on a flat substrate. The reagents flow towards a catalytically active spot via the outer of two coaxial capillaries, and the products are brought into a mass spectrometer by the inner capillary. Heating of the catalyst spot is carried out from the back via a laser, and the temperature is also monitored at the back of the substrate via an IR sensor. The whole substrate with the catalyst library can be moved in x-, y-, and z-directions to position the capillaries. This set-up has been used to analyze catalysts for the CO-oxidation [20] and for oxidative ethane coupling [21], and it was claimed that, when compared to an industrially optimized catalyst, an improvement had been achieved. 

The selective heating of the active areas of the monolith by the CHC-concept can also be demonstrated experimentally. An automotive catalyst was aged artificially in a way that the first 3 cm were completely deactivated. With this aged catalyst the cold-start experiment from Figure 7 was repeated. 

Molybdate-Based Catalysts. The first catalyst commercialized by SOHIO for the propylene ammoxidation process was bismuth phosphomolybdate, Bi9PMoi2052, supported on silica (9). The catalytically active and selective component of the catalyst is bismuth molybdate. In commercial fluid-bed operation, the bismuth molybdate catalyst is supported on silica to provide hardness and attrition resistance in the fluidizing environment. Bismuth molybdate catalysts can be prepared by a coprecipitation procedure using aqueous solutions of bismuth nitrate and ammonium molybdate (10). The catal3ret is produced by drying the precipitate and heat treating the dried particles to crystallize the bismuth molybdate phase. Heat treatment temperature for bismuth molybdate catalysts is generally arovmd 500°C. 

Another interesting possibility is die selective deposition of copper on the structures. We are using an electro less deposition process for this purpose. First of all we have to define the parts of the copper deposition by activating them with palladium, i ch acts as a catalyst. Because palladium is not able to coordinate direcdy to hydroxy groups it is necessary to heat the surface with a SnCla solution. In a first step Sn will coordinate to the hydroxy groups and then it will reduce the PdCl2-solution in a second step. After the selective deposition of the catalyst the substrate is exposed to a mixture of a solution consistent of potasium-sodium tartrat copper(II)sulfate and potassium hydroxide in deionized water and a solution of formaldehyde in water for the reduction of the copper solution. The deposition of the copper is very fast and so tiie substrate has to be removed fixim the solution after 30 seconds (Figure 7). 

It is estimated that the heat produced at the entrance of the catalyst bed is reduced by approximately 80% when the present Ru/Ti07 catalyst is employed, due to suppression of combustion reactions. It should be stated that the quantity of heat produced depends on conversion and selectivity. Thus, regardless of the reaction scheme which is followed, if high... 

Microwave radiation can be used to prepare new catalysts, enhance the rates of chemical reactions, by microwave activation, and improve their selectivity, by selective heating. The heating of the catalytic material generally depends on several factors including the size and shape of the material and the exact location of the material in the microwave field. Its location depends on the type of the microwave cavity used [2]. 

To elucidate the cause of the microwave-induced enhancement of the rate of this reaction in more detail the transformation of 2-t-butylphenol was performed at low temperatures (up to -176 °C). At temperatures below zero the reaction did not proceed under conventional conditions. When the reaction was performed under micro-wave conditions in this low temperature region, however, product formation was always detected (conversion ranged from 0.5 to 31.4%). It was assumed that the catalyst was superheated or selectively heated by microwaves to a temperature calculated to be more than 105-115 °C above the low bulk temperature. Limited heat transfer in the solidified reaction mixture caused superheating of the catalyst particles and this was responsible for initiation of the reaction even at very low temperatures. If superheating of the catalyst was eliminated by the use of a nonpolar solvent, no reaction products were detected at temperatures below zero (see also Sect. 10.3.3). 

Selective heating can occur as selective heating of catalyst particles or in the extreme case as selective heating of active sites. 

If microwave heating leads to enhanced reactions rates, it is plausible to assume that the active sites on the surface of the catalyst (micro hot spots) are exposed to selective heating which causes some pathways to predominate. In the case of metal supported catalysts, the metal can be heated without heating of the support due to different dielectric properties of both catalyst components. The nonisothermal nature of the microwave-heated catalyst and the lower reaction temperature affects favorably not only reaction rate but also selectivity of such reactions. 

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