Ind. Eng. Chem. Res. 2004, 43, 2073-2078
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Continuous Production of Nickel Fine Particles by Hydrogen Reduction in Near-Critical Water Kiwamu Sue,† Nobuyoshi Kakinuma,† Tadafumi Adschiri,‡ and Kunio Arai*,† Graduate School of Environmental Studies, Tohoku University, Aramaki Aza Aoba-07, Sendai, 980-8579, Japan, and Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
Nickel fine particles with diameters under 600 nm were synthesized rapidly and continuously in water at temperatures ranging from 300 to 380 °C and at 30 MPa. Nickel particles formed through the following two steps: formation of Fe3O4 nuclei by hydrothermal synthesis from FeSO4 aqueous solution and precipitation of nickel on the surfaces of the nuclei (i.e., Ni coating) by hydrogen reduction from Ni(CH3COO)2 aqueous solution. The substance 1,10-phenanthroline was used as a complex for preventing hydrolysis of Ni2+ because hydrolysis promotes formation of nickel oxide particles at higher temperatures. Thickness of Ni shell on the Fe3O4 nuclei increased with increasing reaction temperature and Ni/Fe molar ratio in the feed solution. The 1,10-phenanthroline was thermally stable up to 380 °C for residence times of ca. 19 s, which means that recycle after product formation is probably possible. Introduction Nickel fine particles having diameters under 1 µm are used in laminated ceramic capacitor electrode materials. Several chemical processes for production of Ni powder have been proposed such as decomposition of nickel carbonyl,1 hydrogen reduction of Ni salt aqueous solutions,1 chemical vapor deposition of NiCl2, 2 and rapid expansion of supercritical solutions.3 Saarinen et al.4 provided a review of the precipitation of nickel from its salt solution by hydrogen reduction at 150-200 °C. As nickel cannot nucleate homogeneously from a solution, a nuclei particle is needed onto which Ni can precipitate.5,6 Therefore, iron(II) salt solution has been generally used as a catalyst for nickel precipitation by hydrogen reduction. Although details of the mechanism are not yet clear, particle nuclei formation of Fe(OH)2 or Fe3O4 has been proposed as one possible explanation,4 onto which Ni precipitates (i.e., Ni coating). Then, at higher temperatures, similar to that for hydrogen reduction of Ni2+, its hydrolysis occurs as a competitive reaction,7 and as a result NiO particles are produced through the dehydration from Ni(OH)2. In Ni synthesis from aqueous solution, to prevent this hydrolysis, researchers have investigated methods such as the formation of ammonia complexes, [Ni(NH3)n]2+ 5, and controlling pH to acidic condition with acetic acid buffer solutions.6 In this work, we aim to take advantage of the following features of near-critical and supercritical water in producing nickel fine particles: homogeneous reduction atmosphere with hydrogen gas due to low dielectric field,8 and formation of monodispersed metal oxide nuclei and short growth period due to high hydrothermal reaction and reduction rates and low ionic solubility.9-12 In the last 10 years, hydrothermal syn* To whom correspondence should be addressed. Phone/Fax: +81-22-217-7245/7246. E-mail:
[email protected]. † Graduate School of Environmental Studies. ‡ Institute of Multidisciplinary Research for Advanced Materials.
thesis has become established for producing metal oxide fine particles in near-critical and supercritical water with a flow-through technique. In those studies, for conditions near the critical temperature of water, the reaction rate is several orders of magnitude higher than that around room temperature.9 Metal oxide solubility above the critical temperature (TC ) 374 °C) is a few orders of magnitude lower than that below TC,9,12 which means that there is a good possibility for producing monodispersed micrometer or sub-nanosized metal oxide fine particles. This can be expected as a method to prepare size-controlled nuclei for Ni coating on its surface. As already described, some techniques such as ammonia complex formation and pH control with acetic acid buffer solution have been proposed for prevention of hydrolysis because ammonia complex is stable in water and at acidic conditions metal cations do not form hydroxide complexes, respectively. However, ammonia causes corrosion of reactor materials at higher temperatures13 and results in contamination of corrosion products in particles. Further, because CH3COO- forms complexes with metal cations and its association degree increases with increasing temperature,14 an effective hydrogen reduction of metal cations cannot be established. Therefore, it is necessary to improve the reduction method for aqueous systems. In the present study, we use magnetite (Fe3O4) as a nuclei for Ni coating because Fe3O4 is stable in higher temperatures and its electric resistivity is relatively low compared with that of other metal oxides,15 which is favorable for laminated ceramic capacitor electrode materials. As the compound 1,10-phenanthroline (phen) is used in aqueous systems for reagents of (fluorescent) colorimetry, oxidoreduction, and masking in chelatometric titration, phen strongly associates with metal cations such as Ni2+ and Fe2+ at room temperature16 which inhibits hydrolysis of metal cations. Phen’s complex stability constant decreases with increasing temperature, which means that rapid heating of solutions should provide dissociation of Ni2+ complexes and these can be immediately reduced to Ni by H2.17 For these features, we used phen as a complex for inhibiting
10.1021/ie030638u CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004
2074 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004
Figure 2. pH potential diagram at 340 °C, 30 MPa. Figure 1. Estimated conditions from stability constant of (phen)3 and Ni2+(phen)3 at ambient conditions.
Fe2+-
hydrolysis at ambient conditions. In this work, we examined the synthesis of Fe3O4 and Ni from FeSO4 + phen and Ni(CH3COO)2 + phen complexes solution as a starting solution and H2 gas in near-critical water. Thermodynamic Analysis In this section, to ensure appropriate experimental conditions for producing Fe3O4 and Ni, theoretical analyses of chemical equilibrium were performed on the basis of literature stability constants at ambient temperature and potential pH diagrams at high temperatures. First, condition stability constants for iron and nickel were calculated. In the Fe2+ + phen system, the main reactions at ambient condition are formation of Fe2+ complexes and hydrolysis of Fe2+ with water dissociation.16
Fe2+(OH-)n + OH- ) Fe2+(OH-)n+1 n ) 0, 1 (1) Fe2+(phen)m + phen ) Fe2+(phen)m+1 m ) 0, 1, 2 (2) Condition stability constant β of Fe2+(phen)3 was estimated from literature data16,18 and the assumption of ideal solution (γ ) 1) with initial concentrations of 0.005 M FeSO4 and 0.015 M phen. In the Ni2+ + phen system, the condition stability constant β of Ni2+(phen)3 was estimated at initial concentrations of 0.05 M Ni(CH3COO)2 and 0.15 M phen. These concentrations were the same as those used for our experimental conditions described in the Materials section. Results are shown in Figure 1. Because the estimated condition stability constants gave higher values around a pH of 5-7, we adopted a pH value of about 5 for both systems and used 0.2 M acetic acid + 0.2 M potassium acetate solution as a buffer for pH control. Secondary, potential pH diagrams for iron and nickel were calculated. For confirmation of single-phase Fe3O4 and Ni production, potential pH diagrams for Fe and Ni at higher temperatures were calculated on the basis of the Nernst equation and the standard Gibbs free energy (∆G0i) of main half cell reaction on each system which was obtained from SUPCRT92.18 The potential pH diagrams for Fe and Ni at 340 °C and 30 MPa are shown in Figure 2(a) and (b), respectively. For temperatures ranging from 300 to 380 °C and 30 MPa, similar diagrams were obtained. The parallel slopes of dotted lines in the diagrams limit the stability area of water at atmospheric pressure of gaseous species. The lower dotted line represents the hydrogen equilibrium line (H+/H2(g)) and it is available to distinguish the chemical
species produced by hydrogen reduction. In 0.2 M acetic acid and 0.2 M potassium acetate solution, the pH at 340 °C and 30 MPa is estimated to be around 7.0 by literature data of acetic acid,19 sodium acetate,20 and water21 dissociation constants and formulation of activity coefficient.22 From the diagrams, it can be considered that our experimental conditions are located as P1 and P2, respectively, in the figure panels (a) and (b), and Fe3O4(s)/Fe(s) and Ni(s) are the predicted primary species under equilibrium conditions. Experimental Section A. Materials. Ni2+ + phen complex acetate buffer solution (Ni-phenAc) was prepared by dissolving nickel acetate (Ni(CH3COO)2‚4H2O), phen (C12H8N2‚H2O), potassium acetate (CH3COOK), and acetic acid (CH3COOH) into distilled water. Fe2+ + phen complexes acetate buffer solution (Fe-phenAc) was prepared by dissolving iron sulfate (FeSO4‚7H2O), phen, potassium acetate, and acetic acid into distilled water. The nickel acetate concentration was 0.05 M and 0.015 M. The iron sulfate concentration was 0.005 M. The phen/Ni molar ratio was held constant at 3. Concentrations of potassium acetate and acetic acid were 0.2 M. The pH of the solution was measured by a pH meter (model PH82, Yokogawa Electrics Co.) and the pH of these prepared solutions was approximately 4.8. Concentration of formic acid aqueous solution was 0.1 M and it was prepared by diluting formic acid aqueous solutions (90% HCOOH). Formic acid was used to provide H2 into the reactor through its thermal decomposition at higher temperatures.23 The oxygen in the starting solutions was measured with a dissolved oxygen meter (OE270AA, TOA DKK). In Fe3O4 synthesis, distilled water was used instead of Ni-phenAc. All reagents were purchased from Wako Pure Chemicals. B. Experimental Apparatus. The experimental apparatus is shown in Figure 3. Dissolved oxygen probably in the solutions was reduced to below 0.1 ppm by bubbling with Ar gas. Fe-phenAc was fed at a flow rate of 1 g/min by HPLC pump 1. Either distilled water or Ni-phenAc was fed at a flow rate of 1 g/min by HPLC pump 2. Formic acid aqueous solution was fed at a flow rate of 8 g/min by HPLC pump 3. When the solution passed through the electric furnace (model FTO-6, Seiwa Riko Co.) formic acid was heated and decomposed into H2 and CO2, completely.23 This was confirmed with separate experiments with other reactants. At the mixing point, Fe-phenAc solution and Ni-phenAc solution (or distilled water) were mixed with preheated aqueous solution and rapidly heated to the reaction temperature. The feed line for the Fe-phenAc solution and Ni-phenAc solutions had a 0.59 mm i.d. and that for the preheated solution had a 7.1 mm i.d. A three-
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Figure 3. Experimental apparatus. Table 1. Experimental Conditions and Results
run
T2/°C
P/MPa
residence time/sec
1 2 3 4 5
300 320 340 360 380
30 30 30 30 30
27 26 24 22 19
6 7 8 9 10
320 340 360 380 360
30 30 30 30 30
26 24 22 19 22
Ni/Fe molar ratio
conversion/% Fe Ni
productsa
crystallite size/nm Fe3O4 (440) Ni (111)
average particle size/nm
SD/nm
Fe3O4 Syntheses 0 65 92 95 97 10 10 10 10 3
8 45 57 79 66
Fe3O4 Fe3O4 Fe3O4 Fe3O4 Ni Syntheses 32 Ni(Fe3O4) 53 Ni(Fe3O4) 79 Ni(Fe3O4) 92 Ni/NiO(Ni(Fe3O4) 90 Ni(Fe3O4)
20 26 46 50 21 21 21 b 21
22 26 40 98 32
59 87 83 77
19 33 28 28
2180 600 580 c 450
1310 290 180 c 190
a Species within parentheses indicate minor products. b Crystallite size of Fe O (440) not determined due to peak overlap with NiO 3 4 (110). c Particle sizes not determined due to aggregation.
zone temperature control was employed to ensure that the preheated solution temperature at the inlet (T1), the reaction temperature inside the reactor (T2), and the outside temperature (T3) were kept constant. The reaction temperature (T2) was controlledat 40 °C lower than T1 using a PID controller with a K-type thermocouple. The mixed solution was passed through a tube reactor (volume of 6 × 103 mm3, 6.3 mm i.d.) and then cooled by an external water jacket at the end of the reactor. The produced particles were collected with an in-line filter. The system pressure was maintained at 30 MPa by a back-pressure regulator (model 26-172224, Tescom Co.). The solution was separated into gas and liquid after depressurization through the backpressure regulator. Gas was exhausted to a hood and liquid was recovered in a filtered reservoir. The Ni/Fe molar ratios were 10 for most runs but for one run this ratio was reduced to 3 in the Ni synthesis. Experimental conditions are summarized in Table 1. C. Analysis. Produced particles were washed with distilled water and acetone, and then dried in a vacuum oven at 60 °C for 24 h. The products were characterized by X-ray powder diffraction (XRD) (RINT 2200VK/PC, Rigaku) with Cu KR radiation. The crystallite size was estimated by application of the Scherrer equation to the XRD patterns (JEOL software version 5.0). To evaluate
the crystallite size we chose the peaks of (440) reflection for Fe3O4 and (111) reflection for Ni.11 Observation of particle morphology and size was performed by a transmission electron microscope (TEM; LEO-912-OMEGA, Karl Zeiss) and scanning electron microscope (SEM; LEO-1420-OMEGA, Karl Zeiss). Specimens for TEM were prepared by collecting the solutionparticle suspensions and applying them to a copper mesh fitted with a collodion membrane, following by drying. Specimens for SEM were prepared by an Au evaporation coating (IC-50 ion coater, Shimadzu). Average particle size and standard deviation (SD) were evaluated on the basis of 200-300 particles analyzed by TEM and SEM. The concentration of remaining metal ions in the recovered solution was measured by inductively coupled plasma emission spectroscopy (ICP-SPS 7800, Seiko). The conversion X of each metal ion to solid product was calculated as follows:
(
X) 1-
C C0
)
(3)
where C and C0 are molar concentrations of aqueous Ni and Fe species in the recovered and feed solutions, respectively. The phen was analyzed by gas chroma-
2076 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004
Figure 4. X-ray diffraction pattern of product at 340 °C, 30 MPa (Run 3) (2: Fe3O4).
Figure 6. X-ray diffraction patterns of products at (a) Ni/Fe molar ratio 10, 360 °C, 30 MPa, (Run 8); (b) Ni/Fe molar ratio 10, 380 °C, 30 MPa (Run 9); (c) Ni/Fe molar ratio 3, 360 °C, 30 MPa, (Run 10); (2, Fe3O4; O, Ni; b, NiO).
Figure 5. TEM photograph of Fe3O4 particles obtained at 320 °C, 30 MPa (Run 2).
tography with flame ionization detector (GC-FID; HP6890 Series GC system, Hewlett-Packard). Residence time, τ, was calculated by eq 4, from the density of pure water24 at the reaction temperature and pressure as follows:
τ)
V F298 F FT2
( )
(4)
where F is the total flow rate, V is the reactor volume, and F298 and FT2 are the densities of pure water at room and given temperatures, respectively. Results are shown in Table 1. Results Experimental results are summarized in Table 1. The phen could be recovered above 99.4% at all experimental conditions, which implies that this complex was stable at the high-temperature conditions for each residence time. A. Fe3O4 Synthesis. The conversion of Fe at 300 °C (Table 1) was zero, implying that no reaction occurred. In contrast, the conversion of Fe at 320 °C increased to 65%. At higher temperatures, the conversions of Fe were over 90%, and they increased with increasing temperature. Figure 4 shows a typical XRD pattern of products obtained at 340 °C and 30 MPa. All peaks could be assigned to the XRD pattern of Fe3O4 as a single phase. The crystallite sizes of Fe3O4 (440) were below 100 nm and increased with increasing temperature. Figure 5 shows a typical TEM photograph of the particles obtained. The shape of Fe3O4 particles was sphere-like, and chain formation due to magnetic forces was ob-
served. The average particle size and SD at 320 °C were relatively small compared with those of other runs (Table 1). B. Ni Synthesis. Conversion of Fe and Ni (Table 1) increased with increasing temperature. Then that of Ni was higher than that of Fe. The XRD pattern of product prepared at 360 °C at a Ni/Fe molar ratio of 10 is shown in Figure 6(a). This product was identified as Ni including a small amount of Fe3O4. The patterns of the products prepared at 320 and 340 °C were similar to those obtained at 360 °C. In contrast, the pattern at 380 °C shown in Figure 6(b) was assigned to the mixture of Ni and NiO including small amounts of Fe3O4. The pattern at Ni/Fe molar ratio of 3 and 360 °C is shown in Figure 6(c). In this chart, the peaks assigned to the XRD pattern of Fe3O4 were detected more clearly than those at the ratio of 10. The crystallite size of Ni (111) increased with increasing temperature. On the other hand, no significant temperature dependence could be observed in crystallite sizes of Fe3O4 (440). Figure 7(a) shows a SEM photograph of particles obtained at 320 °C and an Ni/Fe molar ratio of 10. The surfaces of these particles were rough and aggregation was observed. Figure 7(b) shows a SEM photograph of particles obtained at 360 °C and at the same molar ratio as Figure 7(a). The average particle sizes at 340 and 360 °C were approximately 600 nm. The standard deviation at 360 °C was small compared with that at 340 °C. With increasing Ni/Fe molar ratio, the particle size increased at 360 °C. Discussion A. Crystallite Size. In Fe3O4 synthesis, the crystallite sizes of Fe3O4 progressed with increasing temperature. However, in Ni synthesis, the crystallite sizes of Fe3O4 (440) seemed to be constant at all experimental conditions. This indicates that Ni coating was achieved rapidly after Fe3O4 nucleation and this probably inhibited further Fe3O4 particle growth. These conclusions are supported by results for conversions of Fe in the Ni synthesis that were lower than those for the Fe3O4 synthesis. In the Ni synthesis, the particles prepared at 320 °C had rough surfaces and aggregated as shown in Figure
Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2077
Figure 7. SEM photographs of Ni particles obtained at (a) 320 °C, 30 MPa (Run 6) and (b) 360 °C, 30 MPa (Run 8).
7(a). At those conditions, the number of nucleated Fe3O4 particles was probably not enough to form small particles for Ni coating, considering that the conversion of Fe was only 8%. As shown in Figure 7(b) at 360 °C, fine particles were produced and with decreasing Ni/Fe molar ratio the average particle size and crystallite size decreased. Differences of the intensity of the XRD peaks assigned to Fe3O4 indicated that the Ni/Fe3O4 composition ratio of these products was different. We consider that the Ni/Fe molar ratio most likely affected the bulk volume of Ni coating on the surface of Fe3O4. B. Reaction Rate. In the Fe3O4 synthesis, Fe2+ + phen complex strongly associated up to 300 °C within 27 s. This might be caused by a low reaction rate in the hydrothermal region. Above 300 °C, dissociation and hydrolysis of Fe proceeded and Fe3O4 nucleation occurred in addition to a large increase in the reaction rate around the critical temperature. In the Ni synthesis, the hydrothermal synthesis of Fe3O4 and Ni formation by hydrogen reduction were assumed as the main reaction at temperatures ranging from 320 to 360 °C, and the reaction rate increased with increasing temperature. At higher temperatures, Ni2+ hydrolysis and NiO production through its dehydration occurred in addition to this reaction. These results implied that at these near critical conditions the rate of synthesis of NiO and Ni became reversed. Adschiri et al.9 conducted hydrothermal synthesis of Fe3O4 using a flow type apparatus by decomposition of Fe(NH4)2H(C6H5O7)2. Those authors suggested that partial reduction of Fe3+ by the CO gas produced by thermal decomposition of the citrate contributed to this result. Cabanas and Poliakoff 11 reported that the products were mixtures of Fe3O4 and Fe by the hydrothermal reactions of Fe(CH3COO)2 solutions at 200345 °C and 25 MPa without oxidant or reductant. They suggested that this might be the result of a self-redox reaction of Fe2+. In the present study, however, no significant peak for the XRD pattern of Fe was detected. Therefore it can be assumed that the partial oxidation of Fe2+ occurred under the conditions studied in this work. The reduction of proton (H+) probably affected these reactions considering the acidic conditions. Fi-
nally, a possible reaction scheme can be considered as follows: Hydrothermal Reaction + Partial Oxidation
3[Fe(phen)3]2+ 98 Fe3O4(s) + 9phen (5) [Ni(phen)3]2+ + H2 f Ni(s) + 3phen + 2H+ (6) In the scheme, hydrothermal synthesis of Fe3O4 nuclei occurs as in eq 5 and then Ni formation occurs on nuclei surfaces by hydrogen reduction as in eq 6. Conclusion Ni fine particles having diameters less than 600 nm were produced continuously and rapidly within a reaction time of 1 min. Combination of the hydrothermal synthesis method for Fe3O4 nuclei with Ni formation through hydrogen reduction with 1,10-phenanthrolinecomplex Fe(II) and Ni(II) and H2 gas in near-critical water was found to be key to the method. Because the rate of hydrothermal reaction and reduction is controlled by changing temperature and materials concentration including oxidant or reductant, this flow-through technique can be expected to be applicable to a wide range of fine particles processes. Acknowledgment This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research and the New Energy and Industrial Technology Development Organization (NEDO). Literature Cited (1) Eisen, W. B.; Ferguson, B. L.; German, R. M.; Iacocca, R.; Lee, P. W.; Madan, D.; Moyer, K.; Sanderow, T.; Hardbound, Y. ASM Handbook, Vol. 7, Powder Metal Technologies and Applications; ASM International: Materials Park, OH, 1998. (2) Sato, N.; Katayama, H.; Ogasawara, S. Ni Fine Powder for Multi-layer Ceramic Capacitors Manufactured by Chemical Vapor Deposition Method. Kawasaki Seitetsu Giho 2002, 34, 120.
2078 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 (3) Sun, Y.; Rollins, H. W.; Guduru, R. Preparation of Nickel, Cobalt, and Iron Nanoparticles through the Rapid Expansion of Supercritical Fluid Solutions (RESS) and Chemical Reduction. Chem. Mater. 1999, 11, 7. (4) Saarinen, T.; Lindfors, L.; Fugleberg, S. A Review of The Precipitation of Nickel from Salt Solutions by Hydrogen Reduction. Hydrometallurgy 1998, 47, 309. (5) Mackiw, V. N.; Lin, W. C.; Kunda, W. Reduction of Nickel by Hydrogen from Ammoniacal Nickel Sulfate Solutions. J. Met. 1957, 786. (6) Wo´dka, J.; Charewicz, W. A. Reduction of Aqueous Nickel(II) from Acetate Buffered Solution by Hydrogen Under Pressure. Hydrometallurgy 1991, 27, 191. (7) Sue, K.; Murata, K.; Kimura, K.; Arai, K. Continuous Synthesis of Zinc Oxide Nanoparticles in Supercritical Water. Green Chem. 2003, 5, 659. (8) Seward, T. M.; Franck, E. U. The System Hydrogen-Water up to 400 °C and 2500 bar Pressure. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 2. (9) Adschiri, T.; Hakuta, Y.; Sue, K.; Arai, K. Hydrothermal Synthesis of Metal Oxide Nanoparticles at Supercritical Conditions. J. Nanopart. Res. 2001, 3, 227. (10) Hao, Y.; Teja, A. S. Continuous Hydrothermal Crystallization of R-Fe2O3 and Co3O4 Nanoparticles. J. Mater. Res. 2003, 18 (2), 415. (11) Cabanas, A.; Poliakoff, M. The Continuous Hydrothermal Synthesis of Nano-Particulate Ferrites in Near Critical and Supercritical Water. J. Mater. Chem. 2001, 11, 1408. (12) Sue, K.; Hakuta, Y.; Smith, R. L., Jr.; Adschiri, T.; Arai, K. Solubility of Lead(II) Oxide and Copper(II) Oxide in Subcritical and Supercritical Water. J. Chem. Eng. Data 1999, 44 (6), 1422. (13) Kalakhan, O. S.; Pokhmurs’kyi, V. I. Corrosion and Corrosion Crack Resistance of The PT3V Titanium Alloy in Aqueous Solutions of Ammonia. Mater. Sci. 2001, 37 (5), 718. (14) Giordano, T. H.; Drummond, S. E. The Potentiometric Determination of Stability Constants for Zinc Acetate Complexes in Aqueous Solutions to 295 °C. Geochim. Cosmochim. Acta 1991, 55, 2401.
(15) Weast, R. C. Handbook of Chemistry and Physics; Chemical Rubber: Cleveland, OH, 1980. (16) Hogfeldt, H. IUPAC Chemical Data Series: No. 22, Stability Constants of Metal-Ion Complexes Part B; Pergamon Press: New York, 1979. (17) Christensen, J. J.; Izatt, R. M. Handbook of Metal Ligand Heats and Related Thermodynamic Quantities; Marcel Dekker: New York, 1970. (18) Johnson, J. W.; Oelkers, E. H.; Helgeson, H. C. SUPCRT92: A Software Package for Calculating The Standard Molal Properties of Minerals, Gases, Aqueous Species, and Reactions from 1 to 5000 bar and 0 to 1000 °C. Comput. Geosci. 1992, 7, 899. (19) Sue, K.; Usami, T.; Arai, K. Determination of Acetic Acid Dissociation Constants to 400 °C and 32 MPa by Potentiometric pH Measurements. J. Chem. Eng. Data 2003, 48, 1081. (20) Oscarson, J. L.; Gillespie, S. E.; Christensen, J. J.; Izatt, R. M. Thermodynamic quantities for the interaction of H+ and Na+ with C2H3O2- and Cl- in aqueous solution from 275 to 320 °C. J. Solution Chem. 1988, 17, 865. (21) Marshall, W. L.; Franck, E. U. Ion Product of Water Substance, 0-1000 °C, 1-10 000 bar; New International Formulation and Its Background. J. Phys. Chem. Ref. Data 1981, 10, 295. (22) Lvov, S. N.; Zhou, X. Y.; Macdonald, D. D. Flow-through Electrochemical Cell for Accurate pH Measurements at Temperatures up to 400 °C. J. Electroanal. Chem. 1999, 463, 146. (23) Yu, J.; Savage, P. E. Decomposition of Formic Acid under Hydrothermal Conditions. Ind. Eng. Chem. Res. 1998, 37, 2. (24) Wagner, W.; Purb, A. The IAPWS Formulation 1995 for The Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use. J. Phys. Chem. Ref. Data 2002, 31 (2), 387.
Received for review August 1, 2003 Revised manuscript received December 8, 2003 Accepted December 13, 2003 IE030638U
