October 2002 Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders -80 DTA EE 15 121 TEMPERATURE(℃) TEMPERATURE(℃) E5a E8上 10 15 +120 2505007501000 7501000 TEMPERATURE(℃C) TEMPERATURE(℃C) Fig. 7. TG-DTA spectra of the used precursor(A)and the hydrothermal powders synthesized in the basic medium (B), the neutral medium (C), and the acidic medium (D). The reaction temperature and the time were fixed at 200'C and 18 h. c0 toward the right and the concentrations of Ce(SO4),or reaction of Eq (13). Because K24 and K2s were quite small and the OHCO decrease while the concentration of CeOH in- concentration of the hydroxide ion was high, the dissolution of the creases, so the equilibrium of dissolution-recrystallization shifts grains of the precursor was very limited and the amount of SOa and toward crystallization as in reaction (5 CO, which transferred from solid-phase grains to liquid-phase With the combination of reactions(5) and (8), the following medium, was very low, so the SO4 and CO, content in the solid quation can be obtained: phase was greater. Because Ostwald ripening proceeded to a very 2Ce4+H2O=[ CroCe+2H'(K2=4.5×10 slight extent, the size of the grains increased only slightly When the hydrothermal medium was adjusted to neutrality, the 3) equilibria of reactions (24) and (25) shifted toward the right because the concentration of the hydroxide ion was decreased, and Reaction (23) shows of reaction (23)moves to the left and the concentration of CeOH, HCO,, and HSO, were increased Therefore, the CO, and SO, contents of the powder obtained in the movement facilit ution of the precursor. The key step neutral hydrothermal medium were lower. It took a longer time to for the hydrothermal sy of CeO, powders is the dissolution grow grains and reach dissolution-recrystallization equilibrium for of the precursor. smaller K,4 and K2s When the hydrothermal medium was adjusted to basicity, with When the hydrothermal medium was adjusted to acidity, the n increasing temperature the following reactions occurred until following reactions took place equilibrium was reached Ce(SO4)+ 2H,0= 2HSOA OH+ CeOH Ce(SO)+H+ H,O= CeOH+ 2HSO, (Ky=7.42×10-) (K21=742×10-°) CeOHCO,+ H,O= HCO, OH+ CeOH CeOHCO,+ H= CeOH+ HCO, (Kn=1.1 10) (25) (K3=1.1×10-5) (22) Reaction(24) can be obtained by combining the backward reactions Because K2 and K22 were respectively greater than K24 and K2s an of Eqs. (10)and (11)and reaction (12). Reaction (25)is the backward the concentration of the hydrogen ion was increased greatly, theOctober 2002 +5 Hxdrothermai Synthesis of Nanocrystalline Cerium(}V) Oxide Powders +5 - -80 2467 0 - B < -5 -10 - -15 \- •20 TG ' DTA ; • : i / w - -40 - 0 -+40 -+80 250 500 750 TEMPERATURE (°C) 250 500 750 1000 TEMPERATURE ("C) 250 500 750 1000 TEMPERATURE {°C) -80 - -40 -15 - -20 250 500 750 1000 TEMPERATURE {°C) - +40 - +80 +120 Fig. 7. TG-DTA spectra of the used precursor (A) and the hydrolhermal powders synlhesized in Ihe basic medium (B). the neutral medium (C), and Ihe acidic medium (D). The reaction temperature and the time were fixed at 200°C and 18 h. shift toward the right and tbe concentrations of Ce(SO4)3 or CeOHCO, decrease while the eoncentration of CeOH-"^ in￾ereases. so tbe equilibrium of dissolution-recrystallization shifts toward crystallization as in reaction (5). With the combination of reactions (5) and (8), the following equation can be obtained: + H.O = + - 4.5 x (23) Reaction (23) shows that with an increase in the concentration of hydrogen ion the equilibrium of reaction (23) moves to the left and the movement facilitates dissolution of the precursor. The key step tor the hydrothermal synthesis of CeOj powders is the dissolution of the precursor. When the hydrothermal medium was adjusted to basicity, with an increasing temperature the following reactions occurred until equilibrium was reached: + 2H;0 = 2HSO4" + OH" + {K2, = 7.42 X 10-^; + HjO = HCO3 + OH" + (A:« = 1.1 X 10-5; (24) (25) Reaction (24) can be obtained by combining the backward reactions oi" Eqs. (10) and (11) and reaction (12). Reaction (25) is the backward reaction of Eq. (13). Because 7^24 and Kj^ were quite small and the concentration of the hydroxide ion was higb. the dissolution itf the grains of the precursor was very limited and the amount of SO^ and CO^-. which transferred fiom solid-phase grains to liquid-phase medium, was very low, so the SO4 ' and CO^ content in the solid phase was greater. Because Ostwald ripening pnKeeded to a very slight extent, the size of the grains increa.sed only slightly. When the hydrothermal medium was adjusted to neutrality, the equilibria of reactions (24) and (25) shifted toward the right because the concentration of the hydroxide ion was decreased, and lhe concentration of CeOH'^. HCO7, and HSO4 were increased. Therefore, the CO, and SO, contents of the powder obtained in neutral hydrothermal medium were lower. It took a longer time to grow grains and reach dissolution-recrystallization equilibrium for smaller K24 and ^25' When the hydrotherma! medium was adjusted to acidity, the following reactions took place: + H,O = (^3, = 7.42 X + 2HSO; (21) CeOHCOl + H* - CeOH^^ + HC0.7 (K.. = I.I x 10') (22) Because AT^i •^f"' ^•'2 ^^^^ respectively greater than K24 and Kj^ and the concentration of the hydrogen ion was increased greatly, the