CH4, reaction

Recently, the CH4+-CH4 reaction has been investigated (9) by measuring the CH4 + disappearance cross-section rather than CH5 + formation cross-sections. Results of this work are shown in Figure 9. Two mechanisms cause a loss of CH4 + ions from the total ion yield in the methane mass spectrum. There are loss processes in the ion source which generate new ions, CH5 +, and possibly other products. Other loss... 

Methane-to-methanol conversion by gas-phase transition metal oxide cations has been extensively studied by experiment and theory see reviews by Schroder, Schwarz, and co-workers [18, 23, 134, 135] and by Metz [25, 136]. We have used photofragment spectroscopy to study the electronic spectroscopy of FeO" " [47, 137], NiO [25], and PtO [68], as well as the electronic and vibrational spectroscopy of intermediates of the FeO - - CH4 reaction. [45, 136] We have also used photoionization of FeO to characterize low lying, low spin electronic states of FeO [39]. Our results on the iron-containing molecules are presented in this section. 

Photoionization can also access excited electronic states of the ion that are difficult to study by optical methods. The photoionization yield of FeO increases dramatically 0.36 eV above the ionzation energy. This result corresponds to the threshold for producing low spin quartet states of FeO. These states had not been previously observed, as transitions to them are spin forbidden and occur at inconveniently low energy. Because the FeO + CH4 reaction occurs via low spin intermediates, accurately predicting the energies of high and low spin states is critical. 

Figure 9. Photodissociation spectra of the insertion intermediate of the FeO + CH4 reaction. Top [HO—Fe—CDs], middle [HO—Fe—CHs], bottom (dashed) Franck-Condon simulation of the [HO—Fe—CHs] spectrum. The spectrum shows a long progression in the Fe-C stretch (Vii = 478 cm ) and short progressions in the Fe—O stretch (vg = 861 cm ) and O—Fe—C bend (V14 = 132 cm ).

Figure 5. Plots of rj,p and rj for the NH3+CH4 reaction on clean polycrystalllne Rh. Data can be fit accurately using a model In which surface carbon blocks NH3 decomposition to N2,...

Figure 6. TEM images of cubic Pt nanoparticles supported on alumina (A) before reaction and (B) aged in NO/CH4 reaction mixture for 4h at 950 °C.

Figure 8. Comparison between catalytic properties of Pt(poly-crystalline)/Al203 (Engelhard) and Pt(l 00)/Al203 (morphologically controlled Pt nanoparticles) the NO/CH4 reaction conversion (X) and yield (Y), and the reaction products at 500 °C.

Once more, the beginning of C02 production matches with the first maximum of N02 desorption, suggesting that methane still reacts with N02. Moreover, at 350°C, a second NO high intense peak is observed, due to the NO2-CH4 reaction (in this temperature range, reaction 2 prevails toward reaction 3). From 390°C on, when all N02 is converted, NO reduction to N2 prevails. At 480 °C, the maximum of NOx conversion to N2 is reached (80% of NOx conversion) and, simultaneously, 85% of CH4 is converted to C02 at 500°C. 

The objective of tiie research described here is to explore synthesis gas generation by direct oxidation of CH4 (reaction 3). A reactor giving complete conversion to a 2/1 mixture of H2 and CO would be the ideal upstream process for the production of CH3OH or for the Fischer-Tropsch process. As discussed above, currently implemented or proposed processes utilize a combination of oxidation and reforming reactions to generate synthesis gas from CH4 and O2. In this work, we seek a faster, more efficient route of syngas generation in which H2 and CO are the primary products of CH4 oxidation. It is expected that this may be difficult because... 

Using the heats of formation given in Appendix I, calculate A 7/(298 K) for the reactions of CH4 with Cl and Br, respectively. As discussed in Chapter 12, the Cl + CH4 reaction plays an important role in the stratosphere whereas the analogous reaction of bromine atoms does not. Comment on whether this difference is due to enthalpy. 

Evaluation of these observations is complicated by the presence of ketene in high concentration so that the products of the CH2+CH4 reaction may constitute a small fraction of the total product formation However, absence of CH2DCH3 and CH3CD3 argues against the reaction (at 27 °C.)... 

In a second step, CO (both from the CH4 reaction and that initially present in the mixture) reacts to form C02 ... 

Figure 24 Hydrothermal durability of Pd-exchanged zeolites in the selective reduction of NO by CH4. Reaction conditions NO 150 ppm, CH4 2400 ppm, O210%, H2O 9%, GHSV = 15,000 h- and T = 450°C (adopted from ref 50)...

About 50% of these species can use formate as substrate. Formate conversion to CH4 (Reaction 2) involves the oxidation of formate to CO2 by formate dehydrogenase, generating reducing equivalents which are subsequently used to reduce CO2 to CH4 ... 

In this study, COj reduction with methane was investigated on the second step reaction [reaction (2) and (3)]. Comparing with the reduction of CO2 to carbon via CO, high yield of carbon was expected in the low temperature region by the methanation of CO2 followed by decomposition of it. In this process, decomposition of CH4 [reaction (2)] is a rate determining step due to a chemical equilibrium. If the formed Hj is removed immediately from the reaction system , the chemical equilibrium in CH4 decomposition can be shifted to the product side. 

CRi reacts both on the oxidised surface of the support giving CO and on the reduced metal producing CHx specie The conversion is lower than CO2 when co-feeded and, on the contrary, higher than CO2 when it is feeded alone Really other reactions have to be considered La203 easily forms carbonate specie or, at reduction conditions, cover the metal surface thus, due to the stability of carbonate groups or the excessive metal decoration, inhibiting the CH4 reaction. 

Th( nsactions show a non-linear Arrhenius behaviour. For the O + CH4 reaction, this is attributed to spin-orbit interactions and tunneling. For the H + CH4 reaction, tunneling plays the major role. At high temperature, the curvature is also caused by the vibrational excitations of the CH4 reactant, which can significantly enhance the reaction. 

The RBU model can be used to study the effect of exciting the vibrational modes treated within the model. For the reactions X (X=C1, 0 and H) + CH4 HX + CH3 we find that exciting a vibrational inode results in a lower threshold to reaction. It was also found that exciting the reactive C-H stretch enhances the reactivity more than exciting the CH4 umbrella mode. Vibrational enhancements for the umbrella and C-H stretch vibrations have also been found in other studies of the dynamics[75, 80] and in canonical variational transition state theory (C T) calculations [84]. Enhancement of the Cl + CH4 reaction due to vibrational excitation of the H-CH3 stretch has also been confirmed b experimental measurements by Zare and coworkers[85]. 

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