Methane reduction, mechanism

While Table 8 includes reactions for the formation of thermal NO, it does not include those for prompt NO. Mechanisms and reaction rate data for prompt NO formation and various methods for the reduction of NO have been described by Millerand Bowman (Prog. Energy Combust. 5ci., 15,287,1989). The GRIMECH reaction set (http //www.me.berkeley.edu/grijnech) is an example of a high temperature methane oxidation mechanism that includes both thermal and prompt NO production. 

Zsely, I.G., Turmyi, T. Investigation and reduction of two methane combustion mechanisms. Arch. Combust. 21, 173-177 (2001)... 

We have previously examined the mutual dependence of [HO ] upon [NO,jj and [NMHC] as calculated by a combined clean/polluted air chemical mechanism (76), and extend those calculations here to the more modem CAL mechanism of Lurmann et al. (182). To do this we have combined the NO,/NMHC chemical reactions of the CAL mechanism with the methane chemistry of Logan et al. (58). The results of these calculations are shown as contour maps or isopleths for [O3], [HO ], and [HO2 ] in Figure 7. Figure 7a, for ozone, is similar to isopleths used to determine reductions in NMHC and/or... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

In situ infrared observations show that the primary species present during the reduction of NO by CH4 over Co-ZSM-5 are adsorbed NO 2 and CN. When O2 is present in the feed NO2 is formed by the homogeneous and catalyzed oxidation of NO. In the absence of O2, NO2 is presumed to be formed via the reaction 3 NO = NO2 + N2O. The CN species observed are produced via the reaction of methane with adsorbed NO2, and transient response studies suggest that CN species are precursors to N2 and CO2. A mechanism for the SCR of NO is proposed (see Figure 10). This mechanism explains the means by which NO2 is formed from adsorbed NO and the subsequent reaction sequence by which adsorbed NO2 reacts with CH4 and O2 to form CN species. N2 and CO or CO2 are believed to form via the reaction of CN with NO or NO2. CH3NO is presumed to be formed as a product of the reaction of CH4 with adsorbed NO2. The proposed mechanism explains the role of O2 in facilitating the reduction of NO by CH4 and the role of NO in facilitating the oxidation of CH4 by O2. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Relatively soon after the discovery that aqueous solutions containing PtCl - and PtClg- can functionalize methane to form chloromethane and methanol, a mechanistic scheme for this conversion was proposed (16,17). As shown in Scheme 4, a methylplatinum(II) intermediate is formed (step I), and this intermediate is oxidized to a methylplatinum(IV) complex (step II). Either reductive elimination involving the Pt(IV) methyl group and coordinated water or chloride or, alternatively, nucleophilic attack at the carbon by an external nucleophile (H20 or Cl-) was proposed to generate the functionalized product and reduce the Pt center back to Pt(II) (step III) (17). This general mechanism has received convincing support over the last two decades (comprehensive reviews can be found in Refs. (2,14,15)). Carbon-heteroatom bond formation from Pt(IV) (step III) has been shown to occur via nucleophilic attack at a Pt-bonded methyl, as discussed in detail below (Section V. A). 

Dissolved arsenic is correlated with ammonia (Fig. 4), consistent with a release mechanism associated with the oxidation of organic carbon. Other chemical data not shown here provide clear evidence of iron, manganese and sulfate reduction and abundant methane in some samples indicates that methanogenesis is also occurring. It is not clear however if arsenic is released primarily by a desorption process associated with reduction of sorbed arsenic or by release after the reductive dissolution of the iron oxide sorbent. Phreeqc analysis shows PC02 between 10"12 and 10"° bars and that high arsenic waters are supersaturated with both siderite and vivianite. 

The history of the development of methane conversion to synthesis gas is summarized as an introduction to the partial oxidation of methane, which is reviewed with emphasis on hot spots in reactors, major developments in the reduction of O2 separation costs, and reaction mechanisms. The various catalysts employed in CO2 reforming are examined, with emphasis on inhibition of carbon deposition. 2004 Elsevier Inc. 

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