Joumal of the American Ceramic Sociery-Wu et al. Vo.85.No.10 Equations(10).(11), and(12) can be obtained respectively from Eqs. (6),(7), and (8)combined with Eq (9). When the concentra tion of CeOH increases, other reactions take place 50 CeOh'+HCo:+OH- H,O CeOHCO,+ H,O (Kn=9.02X 10) 8az (C) CeCo,+ H (13 Ce-OH or SO.2 2CeOH-[CeOCe]*+H,O (Ks= 16.5) HO CO2- Under certain conditions, the following reactions may take place: 8 OH CO, HCO5=H+CO3(K4=4.7×10-1) (14) 0 (B) Ce-OH CeOH++H= Ce"+H,O (KIs=0. 192) or SOa Reaction (15) is the backward reaction of reaction(8). In Ref. 23 50 H,O CO3 the following reaction is proposed CO 2 Ce++NO3+HO=Ce(OH)(NO3)2++H+(K16=102) In Refs. 24 and 25. some evidence otherwise shows that Ce(O Ce-OH H)(CO,)is stable, so it is suggested that the following reaction can H,ODor SO2 occur OH CO, 2-CO.2- Ce"+CO:+H,0-CeOHCO, +H(Kn,= 10) 40003000 2000 CoCO,+ H In fact, because CO, has two negative charges and will produce WAVENUMBER (cm") greater crystal lattice energy than NO,, we think that a Ce complex with CO is more stable than a Ce complex with NO3. Moreover, it appears that CeOCO, can be stable as Fig. 6. IR spectra of the used precursor (curve A)and the hydrothermal Ce(OH)(CO, ). By combining Eqs. (9).(14).(15), and(17),E powders synthesized in the basic medium (curve B), the neutral medium(13) can be obtained, as shown above (curve C), and the acidic medium(curve D). The reaction temperature and the time were fixed at 200C and 18 h Hydrolysis and dimerization reactions took place successively (Eqs.(8)and (5)), leading to the formation of CeO, grai Because the concentration of CeOH increased rapidly, a great number of CeO, nuclei formed in a very short time, equations CeOH+, CeSO2*, CeOHCO', etc, in each chemical equation (10). (ID), and(3)show that in basic solution CeSO4, Ce(Soa)e, for simplicity. In this paper, the equilibrium constants at 25C re applied. The effect of the temperature on the equilibrium complex with SO: or CO to be resolved, so that it is difficult constant of the reaction limited only to Eqs. (5)and(8)are for the nuclei which combine with SO or CO on the surface discussed: the effect of the temperature on other reactions is not to grow further. discussed for lack of thermodynamic data. Under certain Under certain conditions. the following equilibria can be estab conditions, all reactions discussed in this paper are reversible lished: In Ce(SO1), solution some chemical equilibria are estab- Ce(SO)+H'= CeSO"+HSO, (KIs=5X 10) lished:22-21 (18) Ceso:+ HSO:=Ce(SO)+H(K-200 CeSO+H'=Ce4+HSO(K19=286×10)(19) Ce"+ HSO:= CeSO4+H(K,=3500 CeOHCO,+H= Ce+CO:+H,O(K0=10) Ce+H O= CeOH'+H(K=5.2) 8) Reactions(18),(19), and (20)are the backward reactions of egs (6),(7), and (17), respectively. With the sum of reactions(18) H+OH=H,O(K,=10 4) (19) and(8), an overall reaction can be obtained Ce(SO4)+H+HO- CeOH++2HSON When the Ce(SO4), solution was added to the NaOH solution to (K21=7.42×10) (21) prepare precursor, the following reactions took place CeSO:+OH"+ HSO: =Ce(SO)2+H,O With the sum of reactions(8) and (20) and the backward reaction of Eq.(14), an overall reaction can be obtained (K10=200×10) CeOHCO3+H=CeOH+HCO3(K2=1.1×10° Ce++OH"+ HSO.=CeSOf'+HO (K1=3500×10 (I) Under hydrothermal conditions, with increasing temperature the quilibrium of reaction (8)shifts toward the right, so that the concentration of the hydrogen ion is increased. This leads to a Ce+OH- CeOH (K=5.2 X 10) (12) transformation such that the equilibria of reactions(21)and(22)2466 Journal of the American Ceramic Society—Wu et al. Vol. 85. No. 10 i 0 4000 3000 2000 1000 WAVENUMBER (cm Fig. 6. IR spectra of [lie used precursor (curve A) and [he hydrothermal powders syntht'si/.eij in the hasic medium (curve B). ihe neuirai medium (curve C). iinJ ihe acidic medium (curve D). The reaction temperature and the lime were fixed at 200"C and 18 h. CeOH '. CeSO^^, CeOHCOt. etc., in each chemical equation lor simplicity. In this paper, the equilibrium constants at 25''C are applied. The effect of the temperature on the equilibrium constant of the reaction limited only to Eqs. (5) and (8) are discussed; tbe effect of the temperature on other reactions is not discussed for lack of thermodynamic data. Under certain conditions, all reactions discussed in tbis paper are reversible. In CefSOj)-, solution some chemical equilibria are estab￾lisbed:--'''" ^ HSO; = J: + H' = 200) + HSO4 = CeSOf + H' [Ky = 3500) Ce^' + H:0 H* + OH" - H.O + H" {K^ = 5.2) - 10'*) (6) (7) (8) (9) When the Ce(S04)2 solution was added to the NaOH solution to prepare precursor, the following reactions took place: CeSOf + OH" + HSO; - CelSOJ; + (/Tu, = 200 X lO'-*) e'^ + OH + HSO4 - H.O 10' OH - CeOH^" (^,2 - 5.2 x (10) (II) (12) Equations (10), (II), and (12) can be obtained respectively from Eqs. (6). (7). and (8) combined witb Eq. (9). When the concentra￾tion of CeOH"'"" increases, other reactions take place: CeOH'' +HCO," +0 H = CeOHCo; + H2O (A:,, = 9.02 x 10") U CeOCO., + H' (13) 2CeOH-^ = [CeOCe]*^^ + H2O {K, = 16.5) (5) Under certain conditions, the following reactions may take place:'** HCO3 - H* + CO^" (A:|4 - 4.7 X 10"") (14) CeOH'' + H" =Ce-" + H2O (/f,, = 0.192) (15) Reaction (15) is the backward reaction of reaction (8). In Ref. 23 the following reaction is proposed: Ce-*" + NO3" + H2O = Ce(0H)(N03)^^ + H* (A'.fi = 10') (16) In Refs. 24 and 25. some evidence otherwise shows that Ce(O￾H)(CO,) is stable, so it is suggested that tbe following reaction can occur: + col- + H2O - CeOHCO," + u CeOCO, + H' (17) In fact, because CO3 has two negative charges and will produce greater crystal lattice energy than NO^". we think that a Ce"^ complex with CO3" is more stable than a Ce"" complex with NO,^. Moreover, it appears that CeOCO, can be stable as Ce(bH)(CO,). By combining Eqs. (9). (14). (15). and (17). Eq. (13) can be obtained, as shown above. Hydrolysis and dimerizatJon reactions took place successively (Eqs. (8) and (5)). leading to the formation of CeO^ grains. Because the concentration of CeOH'^ increased rapidly, a great number of CeO^ nuclei formed in a very short time. Equations (10), (I I). and (1*3) show that in basic solution CeSO'^, Ce(SO4)2. and CeOHCO,' are quite stable, and It is quite difficult for a Ce"*"^ complex with SO]" or CO^ to be resolved, so that it is difficult for the nuclei which combine witb SO^ or CO^ on the surface to grow further. Under certain conditions, tbe following equilibria can be estab￾lished: 4)2 + H' - CeSOj' + HSO; = 5 X 10') (18) CeS(^^ + H* = Ce""" + HSO4' (^T,, = 2.86 x 10'') (19) CeOHCO; + H^ - Ce"^ + COf + H.O (Z^,,, = 10"') (20) Reactions (18), (19). and (20) are the backward reactions of Eqs. (6), (7), and (17), respectively. With the sum of reactions (18), (19), and (8), an overall reaction can be obtained: 4)2 + H^ + H.O = CeOH'' + 2HS0; (/r,, = 7.42 X 10-") (21) With the sum of reactions (8) and (20) and the backward reaction of Eq. (14), an overall reaction can be obtained: CeOHCOl + H" - CeOH'^ + HCO," (K22= 10") (22) Under hydrothermal conditions, with increasing temperature the equilibrium of reaction (8) sbifts toward the rights' so that the concentration of the hydrogen ion is increased. This leads to a transformation such that the equilibria of reactit)ns (21) and (22)