No abstract

followed by agglomeration of the M n 0 2 particles formed nMn02 -(Mn02)" ( 5 ) or, via 2[Mn(HzO)4(OH)21' -followed by [ Mn( H20)4(0H)OMn( H20)4( OH)] 2+ -[ M I I ( H~O )~]~+ + M n 0 2 + 3 H 2 0 (7)and reaction 5 .Equations 4 and 5 describe a process where the disproportionation occurs between two primary product molecules and is then followed by the agglomeration of MnOz formed. On the other hand, as described by eq 6 and 7, disproportionation may take place after condensation. It is even conceivable that condensation of more than two Mn(II1) species takes place before disproportionation. The induction period observed in the pulse experiments (Figure 7) may be caused by condensation to a certain size until disproportionation takes place. On the other hand, the mechanism of eq 4 and 5 also could explain the induction period, assuming that Mn02 particles of smaller size do not absorb. These reasons may also be given to explain the nonlinear behavior of the curves in Figure 2.Manganese(II1) oxide would according to these mechanisms be formed if condensation occurs much more rapidly than disproportionation. At a high degree of condensation, i.e., when colloidal manganese(II1) oxide is formed, there is no dispropor-[Mn(H20)4(OH)OMn(H20)4(OH)]2+ + H 2 0 (6) tionation. (On the contrary, in MnOl colloid containing Mn(I1) ions, even conproportionation takes place.6) According to the mechanism of eq 6 and 7, condensation would be a process of an order higher than one, while disproportionation would be a first-order-like reaction as the rate would probably not depend on particle size. High intensity of radiation should lead to a higher ratio of rates of condensation to disproportionation, Le., to a colloid of higher Mn(II1) content. This may explain why the colloid formed in neutral solution by pulse radiolysis (Figure 7, inset) contains Mn(III), while that produced by y-irradiation ( Figure 1) consists of pure Mn02. Furthermore, the presence of OH-ions (pH 8.8 in Figure 9) seems to promote efficiently the condensation process as practically pure manganese(II1) oxide is produced in pulse radiolysis. This effect may be explained by OH-addition to the [Mn(H20)4(OH)OMn(H20)4(OH)]2+ species (eq 6) or/and of higher condensation products. This also explains the decrease in conductivity during the formation of manganese(II1) oxide. An even better promotion of condensation or inhibition of disproportionation is caused by hexametaphosphate as manganese(II1) oxide is formed also in neutral solution (Figure 3). Concluding RemarksIn this work, the nature of the first product of oxidation of manganese(I1) by hydroxyl radicals was investigated in more detail than in the previous studies. The nature of the final colloidal products was also recognized. It was not possible to formulate in detail the mechanism for colloid formation, although we think that our more general outline has led to some understanding of the processes involved.The photocatalytic deposition of metallic platinum has been carried out with powder titania in aqueous susp...

No abstract

No abstract