abstracts - Институт катализа им. Г.К. Борескова

OP-III-33a) b)Temperature [K]Pressure [Torr]1120110010801060104010206005004003002001000112CH 443O 20 1 2 3 4 5Time [min]x 3ssx 4sx 1sx 2s1.00.90.80.70.60.30.01.00.50.00.00060.00030.0000Reduced surfaceC coverageO coverage0.0009 CO coverage0.004 H coverage0.00212340.0000 1 2 3 4 51Time [min]Figure 1. a) Calculated variations of temperature and partial pressures of reactants atP CH4 = 570 Torr, P O2 = 190 Torr, T 0 = 1023 K. b) Simulated oscillations of reduced surface sand surface species coverages x i s.The mechanism of oscillations may be presented as follows: during the period of time1-2, the system is in the so-called “oxidised state”. Here the temperature increases due to theexothermic reactions, proceeding on the NiO and the oxygen concentration in the gas phasedecreases. At some temperature the rate of the catalyst reduction becomes larger than the rateof the catalyst oxidation and the system moves to the so-called “reduced state” indicated as3-4. At this point the temperature decreases, because less exothermic reactions producing COand H 2 occur on the reduced Ni surface. Although CH 4 is in the excess in the gas phase, theconcentration of O surface atoms increases, while the concentration of C atoms decreases.This is the result of the competition of CH 4 and O 2 for the free sites. As O 2 adsorption needsless free active sites, at some temperature the surface is covered by oxygen and the systemmoves to the oxidised state.The results of mathematical modelling showed that the thermokinetic oscillations duringmethane oxidation originate on the “reactor level”. This means that the pressure variation inthe reactor is the essential condition for the appearance of the oscillatory behaviour andparameters of the reactor play the important role in the generating of the oscillatorybehaviour.This work was supported by the RFBR (grant N 05-03-33128).References:[1] X.L. Zhang, C.S-M. Lee, D.M.P. Mingos, and D.O. Hayward, Catal.Today, 105 (2005)283-294.[2] V.Yu. Bychkov, Yu.P. Tulenin, V.N. Korchak, and E.L. Aptekar, Appl. Catal. A, to bepublished396