Reaction channels

PHOTOLYSIS OF AMMONIA. Restricting the discussion to neutral species only (ionic ones require high energy, and are not important in the 170-220-nm UV range, where ammonia absorbs strongly), the two low-energy reaction channels to ground state products are... 

Many of the species involved in the endogenous metabolism can undergo a multitude of transformations, have many reaction channels open, and by the same token, can be produced in many reactions. In other words, biochemical pathways represent a multi-dimensional space that has to be explored with novel techniques to appreciate and elucidate the full scope of this dynamic reaction system. 

This leads to the possibiUty of state-selective chemistry (101). An excited molecule may undergo chemical reactions different from those if it were not excited. It maybe possible to drive chemical reactions selectively by excitation of reaction channels that are not normally available. Thus one long-term goal of laser chemistry has been to influence the course of chemical reactions so as to yield new products unattainable by conventional methods, or to change the relative yields of the products. 

This reaction has been carried out with a carbon dioxide laser line tuned to the wavelength of 10.61 p.m, which corresponds to the spacing of the lowest few states of the SF ladder. The laser is a high power TEA laser with pulse duration around 100 ns, so that there is no time for energy transfer by coUisions. This example shows the potential for breakup of individual molecules by a tuned laser. As with other laser chemistry, there is interest in driving the dissociation reaction in selected directions, to produce breakup in specific controllable reaction channels. 

As discussed above, by changing the geometry of the lattice, it is possible to change the intrinsic nature of the stochastic process. On the other hand, Meng et al. [80] have shown that by adding a new reaction channel, namely... 

This reaction was considered the only reaction channel because it is the only known channel which is exothermic with ground state CH4+ ions. Reactions yielding C2H5+ and C2H4 + have been observed and are the least endothermic of the possible reactions of CH4+ with CH4. However, ionization efficiency curves establish CH3 + rather than CH4 + as the reactant ion. Reaction 14 ... 

A significant recent experimental advance is the introduction of tandem mass spectrometers for studying ion-molecule reactions. Examining various isotope effects as a function of translational energy can provide detailed information about reaction mechanisms. Tandem experiments can also observe many of the possible reaction channels for a given collision complex. Such information provides valuable clues to the chemical and physical nature of the intermediates in ion-neutral interactions. 

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

For the remaining systems ion-permanent dipole interactions should be negligible. In these systems the experimental rate constants are considerably lower than the calculated values, and this undoubtedly reflects the fact that other reaction channels are available to the collision complex. It might be noted that many of the reactions are of the type ... 

Since it thus appears that reactions other than the atom transfer process are occurring, one must consider the possibility that the low k (thermal)/ (10.5 volts/cm.) ratios may result from a variation of the relative importance of the atom transfer reaction channel with ion energy. Similarly, in some of the cases where (thermal) = (10.5 volts/cm.) the relative importance of the atom transfer process may also change with ion energy. Thus the value of k(thermal)// (10.5 volts/cm.) does not necessarily provide conclusive evidence for the interaction potential between the ion and the neutral molecules. 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

Many other, less obvious physical consequences of miniaturization are a result of the scaling behavior of the governing physical laws, which are usually assumed to be the common macroscopic descriptions of flow, heat and mass transfer [3,107]. There are, however, a few cases where the usual continuum descriptions cease to be valid, which are discussed in Chapter 2. When the size of reaction channels or other generic micro-reactor components decreases, the surface-to-volume ratio increases and the mean distance of the specific fluid volume to the reactor walls or to the domain of a second fluid is reduced. As a consequence, the exchange of heat and matter either with the channel walls or with a second fluid is enhanced. 

The figure indicates the transport of gas bubbles through the reaction channel. In contrast to the standard process, which is of batch type, the micro reactor allows the fluids to be proceeded in a continuous-flow manner. 

Also a simulation of the flow field in the methanol-reforming reactor of Figure 2.21 by means of the finite-volume method shows that recirculation zones are formed in the flow distribution chamber (see Figure 2.22). One of the goals of the work focused on the development of a micro reformer was to design the flow manifold in such a way that the volume flows in the different reaction channels are approximately the same [113]. In spite of the recirculation zones found, for the chosen design a flow variation of about 2% between different channels was predicted from the CFD simulations. In the application under study a washcoat cata-... 

Figure 2.37 Temperature profiles across the membrane covering the reaction channel of the T micro reactor for a silicon and a silicon nitride membrane and two different heater designs, as discussed by Quiram et al. [128].

Figure 2.50 Reaction channels with a smooth surface (left) and a porous medium (right) as catalyst.

The design of multichannel micro reactors for gas-phase reactions is typically based on a stack of micro structured platelets. For strongly endothermic or exothermic reactions, it lends itself to alternate between layers of reaction channels and heat-... 

Micro reactor housing material Nickel Reactor No. 3 total number of reaction channels 49... 

Platelet material Aluminum Pt Reaction channel No. 3 width depth length 280 pm 140 pm 9.0 mm... 

Operating temperature 370 °C Reactor No. 4 total number of reaction channels 20... 

Operating pressure - Reaction channel No. 4 diameter length 145 pm 6.5 mm... 

Heating Forced convection flow Reaction channel (laser-LIGA) width depth length 500 pm 50 pm 9.5 mm... 

T otal number of mixing plates 14 Total number of reaction channels 126... 

The surface of the silver reaction channels was enhanced by means of the oxidation and outgassing reduction (OAOR) process, which relies on oxidation at 250 °C using pure oxygen and subsequent reduction. An increase in surface area by a factor of 2-3 was reached as indicated by chemisorption data. 

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