Irradiation surface catalysts

Several spectroscopic, microscopic and diffraction techniques are used to investigate catalysts. As Fig. 4.2 illustrates, such techniques are based on some type of excitation (in-going arrows in Fig. 4.2) to which the catalyst responds (symbolized by the outgoing arrows). For example, irradiating a catalyst with X-ray photons generates photoelectrons, which are employed in X-ray photoelectron spectroscopy (XPS) -one of the most useful characterization tools. One can also heat a spent catalyst and look at what temperatures reaction intermediates and products desorb from the surface (temperature-programmed desorption, TPD). 

The irradiation of surface catalysts alters the properties of these catalysts through defect production on their surface. These defects have been observed to enhance and inhibit catalytic activity in specific cases. For example, the irradiation of silica gel enhances the rate of H2-D2 exchange on it. 

The reaction mechanism of the photocatalytic oxidation of ethylene over titania has been investigated using a purpose-built in situ reactor [25] using 12 UV LEDs (310-400 nm) to irradiate the catalyst with an intensity of about 4.9 mW cm. This reactor allowed the inspection of the catalytic surface at the moment the reactions actually occur, providing new exciting insights into the reaction pathway of the photocatalytic ethylene oxidation. 

All photocatalytic reactions are carried out with an immobilized heterogeneous photocatalyst. Slurries are difficult to handle in microstructures and often lead to clogging problems. Immobilized catalysts in microstructures, however, have the advantage that no separation from the reaction mixture in an additional costly and time-intensive separation step is required as for conventional slurry-type batch reactors. In contrast to conventional immobilized systems, a high interfacial irradiated surface area of the catalyst can be maintained despite its immobilization due to the large surface-to-volume ratio of the microchannels. 

On photolyzing CoziCOg in the matrix (20), a number of photoproducts could be observed. The results of these experiments are summarized in Scheme 4, which illustrates the various species formed. Of particular interest is the formation of Co2(CO)7 on irradiation of Co2(CO)g in CO (254 nm), as this species had not been characterized in the metal-atom study of Hanlan et al. (129). Passage of Co2(CO)g over an active, cobalt-metal surface before matrix isolation causes complete decomposition. On using a less active catalyst, the IR spectrum of Co(CO)4 could be observed. An absorption due to a second decomposition product, possibly Co2(CO)g, was also noted. 

We have also tried the trapping reactor system, in which ammonia is trapped on the catalyst/adsorbent and microwave is irradiated intermittently. However, due to the small specific surface area and the small ammonia adsorption capacity on the employed CuO, the trapping system was not effective compared to the continuous irradiation. Further study should be made to develop a material having high ammonia adsorption capacity and high efficiency for microwave absorption. Supported CuO on high surface area material or preparation of high surface area CuO can be effective. 

In a biphasic solid-liquid medium irradiated by power ultrasound, major mechanical effects are the reduction of particles size leading to an increased surface area and the formation of liquid jets at solid surfaces by the asymmetrical inrush of the fluid into the collapsing voids. These liquid jets not only provide surface cleaning but also induce pitting and surface activation effects and increase the rate of phase mixing, mass transfer and catalyst activation. 

A similar reaction is the methylenation of 3,4-dihydroxybenzaldehyde in the presence of a phase-transfer catalyst on a benign calcium carbonate surface [26]. Presumably, bonding of the vicinal hydroxyl groups is low thereby enhancing the reaction with the alkylating agent under the action of solvent-free microwave irradiation (Eq. 15). 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

Toshima et al. obtained colloidal dispersions of platinum by hydrogen- and photo-reduction of chloroplatinic acid in an aqueous solution in the presence of various types of surfactants such as dodecyltrimethylammonium (DTAC) and sodium dodecylsulfate (SDS) [60]. The nanoparticles produced by hydrogen reduction are bigger and more widely distributed in size than those resulting from the photo-irradiation method. Hydrogenation of vinylacetate was chosen as a catalytic reaction to test the activity of these surfactant-stabilized colloids. The reaction was performed in water under atmospheric pressure of hydrogen at 30 °C. The photo-reduced colloidal platinum catalysts proved to be best in terms of activity, a fact explained by their higher surface area as a consequence of their smaller size. 

Highly enantioselective hydrogenation of / -keto esters is achieved by using a Raney Ni catalyst modified by tartaric add and NaBr (Fig. 32.14) [9, 55]. The catalyst should be prepared under controlled conditions induding suitable pH (3-4), temperature (100°C), and concentration of the modifier (1%) to achieve an optimum stereoselectivity. The addition of NaBr, an achiral modifier, is important. Furthermore, ultrasonic irradiation of the catalyst tends to increase the activity and enantioselectivity [9f,g]. Ultrasonication may remove nonselective sites of the catalyst surface. 

The surface areas of the three catalysts were 58 (Ti02), 360 (TS-2), and 550 (TS-2h) m2/g, respectively. UV-irradiation of solutions (10-4 M) containing 4-chlorophenol (4-CP) in the presence of suspended Ti02, TS-2, or TS-2h yielded time-dependent spectra from which the concentration of unconverted 4-CP was estimated. Figure 43 is a plot of the relative concentration of 4-CP as a function... 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

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