Boreskov Institute of Catalysis of the Siberian Branch of Russian ...

OP-I-11KINETIC ASPECTS OF METHANE OXIDATION BY O 2 AND N 2 OOVER V 2 O 5 /SiO 2Yu.I. Pyatntsky, L.N. Raevskaya, A.I. BostanL.V. Pisarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine,pr. Nauki, 31, 03087 Kiev, Ukraine, e-mail: yupyat@ukr.netNitrous oxide has been explored as an oxidant in the different catalytic reactions [1]including methane partial oxidation [2-4]. According to [4], the relationship between the ratesof methane oxidation by O 2 and N 2 O over V 2 O 5 /SiO 2 depends on the reaction temperature.The aim of the present work is to study the mechanistic causes of the temperature affect onthe relative rates of methane oxidation by oxygen and N 2 O.Steady-state catalytic runs were performed in fixed-bed quartz reactor (11 mm i.d.) over1.4%V 2 O 5 /SiO 2 prepared with chemical vapour deposition method and 3.6%V 2 O 5 /SiO 2prepared with the impregnation. The support was fumed silica A-380 (380 m 2 /g). The catalystgranules (0.5-1mm) were diluted with quartz granules to improve a temperature control. Inletconcentration of methane was 30 vol.%, N 2 O 30 vol.%, O 2 15 vol.%.Figs. 1a and 1b show the dependence of reaction selectivity on methane conversion over1.4%V 2 O 5 /SiO 2 catalyst. As seen, the selectivity dependencies for the main reaction products(formaldehyde and CO) on methane conversion are similar; the CH 2 O/CO ratio is in a littledependence on the oxidant.Fig.1a. 550 o CFig.1b. 575 o C100100Selectivity, %80604020CO(O2)CO2(O2)CH2O(O2)CH3OH(O2)CO(N2O)CO2(N2O)CH2O(N2O)CH3OH(N2O)Selectivity, %80604020CO(O2)CO2(O2)CH2O(O2)CH3OH(O2)CO(N2O)CO2(N2O)CH2O(N2O)CH3OH(N2O)00 2 4 6 8Conversion, %00 2 4 6 8 10 12 14Conversion, %The temperature increasing leads to inversion of the levels of the reaction rates (Fig. 2a).The obtained activation energies for N 2 O and O 2 are equal to 116 and 232 kJ/mole,correspondingly. The qualitatively same result was obtained for 3.6%V 2 O 5 /SiO 2 catalyst withenergy activation 158 kJ/mole for N 2 O and 227 kJ/mole for O 2 .Attempts to explain the different action of N 2 O and О 2 using different versions of the«two-step» mechanism of the Mars-van Krevelen type had no success. Quite satisfactoryexplanation can be made using the following scheme of the surface chain reaction mechanismlike mechanism proposed by Kazansky for CO oxidation over V 2 O 5 /SiO 2 catalyst [5]:46