Partial oxidation of methane to syngas over Rh/SiO2 catalyst
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Partial oxidation of methane (POM) over Rh/SiO2 Catalyst was investigated by using several techniques, such as TPD, TPR, TPSR and trapping agent, combined with MS. At the beginning of POM reaction, only gaseous CO2 can be detected over the catalyst. With the increase in space velocity, the conversion of CH4 and the selectivity for CO and H-2 increase, while the selectivity for CO2 decreases. During the pulse reaction with CH4 as reactant, over the catalyst prereduced at 700 degreesC, CO and H-2 can be detected as main products with trace C2H6 and C2H4. When the catalyst is exposed to CH4-He, there are two kinds of carbonaceous species formed, and they are designated CHalpha and CHbeta, as identified by their hydrogenation temperature of 210 similar to 260 degreesC and 450 similar to 800 degreesC, respectively. The CHalpha is assigned to H-rich form and the CHbeta is assigned to H-deficient form. When the catalyst is exposed to CH4-O-2-He, the carbonaceous species are mainly CHalpha with trace CHbeta. The two kinds of carbonaceous species may play different roles in POM reaction. The CHbeta accumulated during CH4 activation is the possible cause for catalyst deactivation, and the CHalpha may be responsible for CO formation. The CHx may be the intermediate of POM reaction. In the trapping reaction, a series of ions with M-r/z = 2 similar to 46 have been detected at 300 similar to 600 degreesC. The CHxO (x = 1 similar to 3) may be the O-containing intermediate of POM reaction. Based on the above results, the POM mechanism has been proposed. Over the reduced catalyst, CH4 is firstly dissociated, forming the surface species CHx. By reacting with the active species OH-, the CHalpha is oxidized to O-containing intermediate, CHxO, which can be dehydrogenated to give the adsorbed and gaseous CO.