= !_,

The oxygen potential (~OX)s at the steady-state results from the condition !+ = !_, as illustrated in Fig. 4.29, and is determined by the kinetics. Different solid phases will show different parameters of Eqs. (4.17) and (4.18), and the value of (~OX)s at a given nonequilibrium composition of the gas phase will depend on the phase composition of the solid. If (~OX)s is plotted as function of f.0 2 ' all other partial pressures being constant, one line will be obtained for each solid phase. For two phases a and ~ two possibilities exist for the relative location of these lines as illustrated schematically in Fig. 4.30, where ~* represents the value of (~OX)s corresponding to the coexistence of the two phases a and ~. In the situation

a / /

FIGURE 4.30 (~ox)s at steady state in two solid phases a and ~ as a function of f.0 2 ' at constant I and constant partial pressures of other components in the gas phase. Phase a is stable at (~ox)s < ~*, phase ~ is stable at (~ox)s ) ~*. The two solid phases a and ~ and 02(g) coexist in an equilibrium system at f.o 2 = ~*. (After [46].)

and

shown in Fig. 4. 30a, phase a will be oxidized to ~ only when E02 > CEo2) 2, whereas phase ~ will be reduced to phase a only when E02 < CEo2)1· A hysteresis of the phase composition versus oxygen pressure cycle is to be expected when oxygen pressure will be changed in this range. Let us suppose that we are increasing oxygen pressure from the value CEo2) lover the system composed of phase a. When it reaches the value of CEo2) 2, oxidation to phase ~ takes place. However, when the pressure is lowered, phase ~ will exist until the pressure drops below the value of CEo2)1, when reduction of ~ to a will occur. Thus, in the pressure range CEo2)2CEo2) 1 either phase a or phase ~ will exist depending on whether we approach this pressure range from the side of lower or higher pressures, respectively. The state of the system depends on its pretreatment. In the case represented by Fig. 4. 30b, the two phases a and ~ will coexist in the pressure range ~E02 and the phase composition of the system will be independent of its history. The experimental results obtained by Rieckert et ale [46] on studying the oxidation of propene to acrolein over copper oxide catalyst are shown in Fig. 4.31, in which the ratio K of selectivities to acrolein and carbon oxides is plotted as a function of the variation of oxygen partial pressure, the arrows indicating the direction of this variation. When E02 is decreased from point e, in which the catalyst is composed of CuO, mainly total oxidation takes place and the value of K

a

FIGURE 4.31 The ratio K of the rate of partial oxidation of propene to acrolein to the rate of oxidation of propene to carbon oxides, as a function of the changes of oxygen pressure Eo2. The arrows indicate the direction of the change of Eo2. CAfter [46].)

is low. When point a is reached, suddenly the selectivity to acrolein increases considerably. Chemical analysis showed that CuO was reduced to CU20. This phase exists on further decrease of oxygen pressure and its subsequent increase to point d, well above the pressure corresponding to point a, selectivity to acrolein being now at maximum. Only on raising further the oxygen pressure does oxidation of CU20 to CuO start and does selectivity to acrolein drop until the system attains the initial state at e. Such cycles could then be repeated. The results show that the steady state of a solid catalyst can be indeterminate, the latter showing a distinct memory of its history.