Reversible gas-phase decompositions

The reversible gas-phase decomposition of nitrogen tefroxide, N2O4, to nitrogen dioxide, NOj,... 

Lapidus G, Barton D, Yankwich PE (1966a) Reversing hydrogen isotope effect on the rate of the gas phase decomposition of oxalic acid. J Phys Chem 70 407-411... 

The WGS reaction is a reversible reaction, that is, it attains equilibrium with reverse WGS reaction. Thus the fact that the WGS reaction is promoted by H20(a reactant), in turn, implies that the reverse WGS reaction may also be promoted by a reactant, H2 or CO2. In fact the decomposition of the surface formates produced from H2+CO2 is promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions can conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility[63]. 

The WGS reaction is a reversible reaction that is, the WGS reaction attains equilibrium with the reverse WGS reaction. Thus, the fact that the WGS reaction is promoted by H20 (a reactant), in turn implies that the reverse WGS reaction may also be promoted by a reactant, H2 or C02. In fact, the decomposition of the surface formates produced from H2+C02 was promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility. The activation energy for the decomposition of the formates (produced from H20+CO) in vacuum is 155 kJ/mol, and the activation energy for the decomposition of the formates (produced from H2+C02) in vacuum is 171 kJ/mol. The selectivity for the decomposition of the formates produced from H20+ CO at 533 K is 74% for H20 + CO and 26% for H2+C02, while the selectivity for the decomposition of the formates produced from H2+C02 at 533 K is 71% for H2+C02 and 29% for H20+C0 as shown in Scheme 8.3. The drastic difference in selectivity is not presently understood. It is clear, however, that this should not be ascribed to the difference of the bonding feature in the zinc formate species because v(CH), vav(OCO), and v/OCO) for both bidentate formates produced from H20+C0 and H2+C02 show nearly the same frequencies. Note that the origin (HzO+CO or H2+C02) from which the formate is produced is remembered as a main decomposition path under vacuum, while the origin is forgotten by coadsorbed H20. 

The reduced iron atoms of complex C, being inert to dioxygen, are readily oxidized by nitrous oxide into complex D to give adsorbed species of a-oxygen, Oa. As Figure 7.3 shows, the reversible redox transition Fc" <-> Fe provides the catalytic activity of FeZSM-5 both the oxidation cycle due to the oxygen transfer from N20 to a substrate and the decomposition cycle of N20 into N2 and 02 due to recombination of a-oxygen into the gas phase. The decomposition is an environmentally important process, and FeZSM-5 zeolites are considered to be the best catalysts for this reaction (see review [117] and references therein). 

The kinetic parameters of thermal decomposition of several 2,5-disubstituted tetrazoles 6 in the gas phase and in nitrobenzene solution have been determined using manometric methods. Limiting stages of the stepwise uni-molecular decomposition that determine the experimental rate of nitrogen evolution include reversible formation and subsequent breakdown of the azo-diazo intermediates 186 (Scheme 15) <1996RCB2094>. 

The capillary plasma reactor consists of a Pyrex glass body and mounted electrodes which are not in direct contact with the gas flow in order to eliminate the influence of the cathode and anode region on CO2 decomposition. Analysis of downscaling effects on the plasma chemistry and discharge characteristics showed that the carbon dioxide conversion rate is mainly determined by electron impact dissociation and gas-phase reverse reactions in the capillary microreactor. The extremely high CO2 conversion rate was attributed to an increased current density rather than to surface reactions or an increased electric field. 

CH3)2PH>CH3PH2>CF3(CH3)2P. Adducts of the latter three bases are completely dissociated in the gas phase. The reversible decomposition reaction shown below is observed when either CH3PH2 or CF3(CH3)2P is the ligand. 

This reaction is reversible.16 The temperature at which decomposition occurs depends therefore on the gas phase for example, under vacuum, decomposition becomes noticeable at as low as 500°C. The lack of S° values for this salt prevents any equilibrium calculations for the above reaction, as well as the gef calculation for the paraperiodate. The enthalpy and entropy increments are based on the calorimetric enthalpy increments of David, Mathurin, and Thevenot.12 Thermodynamic data for these salts are given in Tables 9.2 and 9.3. 

Herrmann and co-workers synthesized [Os(0)(Me)4] from 0s04 and dimethylzinc or methyltris(isopropoxy)titanium (180). An alternative route is by methylation of the glycolate osmium(VI) complex [0=0s(0CH2CH26)2] with dimethylzinc (180). The thermally labile ethyl derivative [Os(0)(Et)4] has also been prepared (180). [Os(0)(Me)4] is an orange, air-stable, volatile, crystalline compound that melts at 74°C without decomposition. The gas-phase average molecular structure of [Os(0)(Me)4], determined by electron diffraction techniques, is consistent with a theoretical model of C4 symmetry with d(Os—C) = 2.096(3) A, d(0s=0) = 1.681(4) A, and ZO—Os—C = 112.2(5)° (180). Cyclic voltammetric studies showed that [Os(0)(Me)4] undergoes reversible reduction at - 1.58 V and an irreversible oxidation at -f 2.2 V vs Ag/AgCl in MeCN. 

The respective DSC measurements of the decomposition reactions revealed that reaction (7.5) is exothermic with an enthalpy of about 15 kJ moO and reaction (7.6) is endothermic with an enthalpy of 13 kJ moO. Thus, this system again has rather unfavorable thermodynamic properties and is not reversible by gas-phase H2 loading. 

The gas-phase reaction of nitric oxide with chlorine has received a great deal of attention. Very often it has been studied concurrently with its reverse, the decomposition of nitrosyl chloride, viz. 

The forward cmnene decomposition reaction is a single-site mechanism involving only adsorbed cumene while the reverse reaction of propylene in the gas phase reacting with adsorbed benzene is an Eley-Rideal mechanism. 

Some other results of other TPH experiments can be seen in Table 1. In atmospheric-pressure tests at 900°C with 500 ppm HjS in the gas phase, sulfur was not desorbed from Catalyst Al. The same phenomenon was noticed in the tests performed at 900°C under 5 bar pressure with Catalysts A2 and C. In addition, when the sulfur content of the catalyst beds was analyzed after TPH experiments, it was observed that only a small amount of sulfur was present on the catalyst. This observation indicates that sulfur adsorption is not completely reversible, but that part of the adsorbed sulfur remains on the catalyst. The effect of this phenomenon was also observed when a catalyst was regenerated by removal of HjS to the gas mixture in fixed-bed poisoning tests. The catalyst activity did not reach the original level (with no HjS in the gas) especially in ammonia decomposition. The analysis of the sulfur content of the bed showed that a small amount of sulfur was still present on the catalyst. 

Other reactions important to reforming are also considered in the reaction network in Figure 10, include the water-gas-shift reaction and its reverse, the reversible adsorption and decomposition of water, the desorption and adsorption of reforming products like CO, CO2, and H2, and the formation of hydrocarbons like CH. The formation of dissolved carbon, oxygen, and hydrogen in bulk nickel is also considered. Dissolved C, 0, or H may be important in the transport of those elements to or from interfaces with other solid phase (carbon, carbides, oxides, support). The possible formation of NiO from H2O is also shown. Finally, an important reaction to consider is the formation of a deactivating layer of carbons (6 or e carbon states). 

Removal of a borane group often occurs when a borane is treated with a nucleophilic reagent. The initial product of this type of reaction is an adduct of the borane group, but the nature of the donor part of the adduct controls the subsequent fate of the whole. For all such compounds there is a tendency, varying from complex to complex, to dissociate reversibly into donor and acceptor parts. Thus whereas (CH3)2S BH3 (49) is appreciably dissociated in the gas phase at 50°, the compound (CH3)3P BH3 (48) may be heated to 200° without appearance of significant amounts of borane decomposition products. 

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