H2-O2 reaction

Figure 3.54 Measured gas exit temperatures for a catalytic H2/O2 reaction with varying H2 content in 0.5 sipm synthetic air (A) and oxygen (T) enriched air (0.1 sipm oxygen -r 0.2 sipm nitrogen) [115].

Because the entropy change for the H2/O2 reaction is negative, the reversible potential of the H2/O2 fuel cell decreases with an increase in temperature by 0.84 mV/°C (assuming reaction product is liquid water). For the same reaction, the volume change is negative therefore, the reversible potential increases with an increase in pressure. 

The explosion limits of the H2 + O2 reaction have been studied thoroughly, and these limits exhibit an interesting variation with pressure. At low pressures the reaction is stable because radicals diffuse to the walls of the chamber and prevent chain branching, which leads to an explosion. At very high pressure radical quenching by diffusion to the walls is slowed down, and the heat released by the reaction leads to a thermal explosion (Figure 1 0-8). 

Figure 10-8 Explosion limits of the H2 + O2 reaction, At low pressures explosion is determined by the quenching of chain branching of radicals at the walls of the vessel, while at high pressures the larger rate of heat generation leads to a thermal explosion,...

As one might guess from our previous example of H2 oxidation, this reaction is extremely complex. The standard model involves over 300 reaction steps among approximately 30 chemical species. Here even the stable product species can be complex, with CO, HCHO (formaldehyde), and soot (carbon) being among the highly undesired pollutants in CH4 combustion. Since H2, O2, and H2O are involved as intermediates and products, the 38 reaction steps in the H2 + O2 reaction hsted previously set of the reactions that be involved in CH4 oxidation. 

Hamilton G.L. Schott, Post-Induction Kinetics in Shock-Initiated H2-O2 Reactions , Ibid, pp 635-43 86) R.I. Solou-... 

B. Koch, BerBunsengesPhysChem 70 (9-10)(1966) CA 66, 1995(1966), "Studies of Detonation and Shock Wave Fronts by Using Microwaves . Its abstract is given in Ref 83 under DETONATION (AND EXPLOSION) WAVES 85) C.W. Hamilton 8c G.L. Schott, "Post-Induction Kinetics in Shock Initiated H2-O2 Reactions , 11th-SympCombstn (19o7), pp 635-43 86) P.A. 

Develop a reaction mechanism for iodine (I2-O2-H2 system) from the information in the NIST Chemical Kinetics Database [256], Start with the H2-O2 reaction subset hydrogen.mec. Using the database, identify the relevant reactions with I2. Add these reactions to the starting mechanism, including product channels and rate constants. List the additional I-containing species formed in reactions of I2. Extend the reaction mechanism with reactions of these species. Continue this procedure until reactions of all relevant iodine species in the I2-O2-H2 system is included in the mechanism. 

Table 7.4 compares with experiment [82] the effect of functionals and of basis set size on the reaction enthalpies of the important H2/CI2 and H2/O2 reactions. 

It is possible to determine the amount adsorbed by a titration method. For example, the amount of hydrogen adsorbed on a Pt surface may be titrated with pulses of oxygen. The oxygen adsorbed can in turn be further titrated by pulses of hydrogen. From the stoichiometry of the H2/O2 reaction a measure of the number of surface metal atoms can be obtained. 

Perhaps the most important of the various chain types is the chain step that is necessary to achieve nonthermal explosions. This chain step, called chain branching, is one in which two radicals are created for each radical consumed. Two typical chain branching steps that occur in the H2-O2 reaction system are... 

Thus, one observes that the rate expression can be written in terms of readily measurable stable species. One must, however, exercise care in applying this assumption. Equilibria do not always exist among the H2-O2 reactions in a hydrocarbon combustion system—indeed, there is a question if equilibrium exists during CO oxidation in a hydrocarbon system. Nevertheless, it is interesting to note the availability of experimental evidence that shows the rate of formation of CO2 to be (l/4)-order with respect to O2, (l/2)-order with respect to water, and first-order with respect to CO [17,18]. The partial equilibrium assumption is more appropriately applied to NO formation in flames, as will be discussed in Chapter 8. 

Recall that in the discussion of kinetic processes it was emphasized that the H2-O2 reaction contains an important, characteristic chain branching step, namely,... 

It is apparent that the fate of the H atom (radical) is crucial in determining the rate of the H2-O2 reaction or, for that matter, the rate of any hydrocarbon oxidation mechanism. From the data in Appendix B one observes that at temperatures encountered in flames the rates of reaction between H atoms and many hydrocarbon species are considerably larger than the rate of the chain branching reaction (17). Note the comparisons in Table 1. Thus, these reactions compete very effectively with reaction (17) for H atoms and reduce the chain branching rate. For this reason. 

At high pressures or in the initial stages of hydrocarbon oxidation, high concentrations of HO2 can make reaction (45) competitive to reaction (44), so reaction (45) is rarely as important as reaction (44) in most combustion situations [4]. Nevertheless, any complete mechanism for wet CO oxidation must contain all the H2-O2 reaction steps. Again, a complete mechanism means both the forward and backward reactions of the appropriate reactions in Appendix B. In developing an understanding of hydrocarbon oxidation, it is important to realize that any high-temperature hydrocarbon mechanism involves H2 and CO oxidation kinetics, and that most, if not all, of the CO2 that is formed results from reaction (44). 

Just as the fate of H radicals is crucial in determining the rate of the H2-O2 reaction sequence in any hydrogen-containing combustion system, the concentration... 

The shift of curves, as shown in Fig. 7, is unsurprising since the larger fuel molecules and their intermediates tend to break down more readily to form radicals that initiate fast reactions. The shape of the propane curve suggests that branched chain mechanisms are possible for hydrocarbons. One can conclude that the character of the propane mechanisms is different from that of the H2-O2 reaction when one compares this explosion curve with the H2-O2 pressure peninsula. The island in the propane-air curve drops and goes slightly to the left for higher-order paraffins e.g., for hexane it occurs at 1 atm. For the reaction of propane with pure oxygen, the curve drops to about 0.5 atm. 

Recapitulating for convenience, the complete mechanism developed to account for the kinetic features of the H2 + O2 reaction in boric acid... 

A more recent study of the Dj + O2 reaction by Baldwin et al. [246] has involved measurements of the second limits, and the induction periods and maximum rates of the slow reaction in an aged boric acid coated vessel of 52 mm diameter. Maximum concentrations of D2 O2 in the slow reaction were also determined. The kinetic parameters of the oxidation process were then determined by a computer optimization treatment similar to that described in Sect. 4.3.3 for the H2 + O2 reaction. Excluding the primary initiation rate 6 which is necessary for the calculation of induction periods, but which needs to be only approximately defined, there are a minimum of seven significant parameters (cf. Table 18). 

In order to make their optimization procedure more realistic in the case of the H2 + O2 reaction, Baldwin et al. [72] used independent measurements of (a) second limits in KCl coated vessels in order to give fe2/fe4 (after correction if necessary for reaction (xi) and for surface termination of H atoms (Sect. 3 and Table 14)), and (b) the homogeneous decomposition of H2 O2 in the presence of hydrogen. The latter measurements give accurate values for 7, and 143/ 14 or fe, j/fe, at high or low hydrogen concentrations respectively, and the combined measurements leave just three adjustable parameters to be determined by the results with the B2O3 coated vessels. 

It has already been noted that the presence of small quantities of hydrogenous impurities expands the explosion peninsula. Such sensitization allows easier experimentation and provides for more reproducible results. The effect of hydrogen on the ignition limits is shown in Fig. 61. As was observed when considering the effect on the first limit, addition of sufficient hydrogen causes the reaction system to behave in essentially the same way as the H2 /O2 reaction. Dixon-Lewis and Linnett [30] found that, on replacing more than about 10 % of the CO by H2 in a KCl coated vessel at 510—570 °C, the second limit pressure could be extrapolated... 

The basic mechanism of the H2 + O2 reaction used by Baldwin and Walker is... 

Rg. 4. Spatial patterns on a Pt foil during the H2/O2 reaction. Thermograms are in time sequence from left to right beginning with the top row. The temperature scale runs from T < 68°C (1) to T > 122°C (8), with the maximum temperature region numbered on each panel. (From Ref. 155.)... 

A stability analysis for such a system was performed by Wicke et al. (98), who modeled the H2/O2 reaction on Pt catalysts. The reaction was simplified to two differential equations that are easily treated analytically ... 

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