13541
J. Phys. Chem. 1994,98, 13541-13545
Thermosplitting of Hydrochloric Acid for Hydrogen Production with Cerium Dioxide as the Recycle Reagent E. I. Onstott Chemical Science and Technology Division, LQS Alamos National Laboratory and University of Califomia,
LQS Alamos, New Mexico 87545 Received: June 13, 1994; In Final Form: August 30, 1994@
CeO2 (solid) reacted with pressurized HCl(g) at 475-625 K to yield CeOCl(solid), Cl2(g), and H20(g). The solids surface was flooded with HCl(g), and the reaction rate was linear. Normalized gas quotients were calculated for an extent of reaction of 0.212 to yield values within the limits of 0.279 167-0.279 314. Enthalpy changes from second law calculations were -9.43 to -6.60 J mol-' of Cl2, and entropy changes were -10.624 to -10.618 J K-'mol-! CeOCl was hydrolyzed at 1050-1270 K to yield H2(g) and HCl(g). Data normalized for 1 atm total pressure conformed to the conventional plot of log(gas quotient) vs UT. H2 production rates and enthalpy changes varied with input rate of H2O(g). At 1270 K the mol fraction of H2 in the output gas was highest at 0.069 with an enthalpy change of 206.20 kJ mol-' and an entropy change of 110.64 J K-' mol-! The chemical work of producing H2 decreased with increasing T and extrapolated to zero at about 1865 K. At this T,two of three H2O(g) molecules would be converted to Hz(g). All of the AH requirement would be TAS.
Introduction In a previous paper by Hollabaugh et al.,l some of the thermochemistry of hydrochlorination of Ce02 to yield CeCl3 and the thermochemistry of its hydrolysis at higher temperatures was discussed in combination with other thermochemistry and computer simulations of transport mechanisms in a thermochemical cycle. In this paper a simpler cycle for H2 production is evaluated, in which only two redox chemical reactions are utilized. C12 is a waste product (for disposal or other uses) by splitting 2HC1 instead of splitting H20 to yield 0 2 . The redox reaction for producing CeOCl and C4 at 383-625 K is2 2Ce02(cr)
+ 4HCl(g) -2CeOCl(cr) + 2H20(g) +
HCI FLOW CONTROLLER
CAPILLARY TUBE FOR PRESSURE DROP
HCI AND CI, ANALYSIS
Figure 1. Apparatus for reaction 1.
Cl,(g) (1) The Ce02(solid) shrinks as it is converted to CeOCl(so1id). Cl2 production should be affected by reactant pressure and also by the small increase in reactor volume when CeOCl is formed. By physically separating the Clz(g) and H20 at 383 K (where constant boiling HCl condenses at 1 atm), reaction 1 can be reversed for H2 production at > 1050 K:
Reaction 1 and reaction 2, each operating separately, split 2HC1 into the elements
+
2HC1- C12(g, 383 K) H2(g, 1270 K)
(3)
The mechanisms are the solid-state changes in valence in Ce, and phase exchanges of oxide and chloride, and separation of Cl2(g) and H2(g) at widely different temperatures. HCl(g) provides C1- for Clz(g) formation and placement of C1- in the solid phase, where Ce(Iv) as dioxide is converted to Ce(II1) as CD Abstract
published in Advance ACS Abstracts, November 15, 1994.
OO22-3654/94/2098-13541$O4.50/0
the oxychloride. One 02-in the solids is transported to the gas phase as steam.
Experimental Section Rate studies of the reaction of HCl(g) with CeOz(cr) were done with the apparatus shown in Figure 1. The quartz reactor was 1 cm i.d. by 14 cm long. The exit capillary, 99.9% purity with respect to other rare earths). HCl input (99.99% pure) was metered through a calibrated electronic flow meter, and the pressure was measured with a Matheson dial gauge. The C12 samples were collected in KI solutions to yield 13-, which was titrated with standard Na2S203. Unreacted HCl was collected in distilled water and titrated with standard NaOH. Rates of HCl consumption were measured immediately after steady-state operation was achieved at the desired T. Pressure remained stable for about 1 h and then slowly increased. Data points for CeOCl formation were taken from experiments showing linear behavior for 60?
From extrapolation.
too small for beneficial H2 production. Excess heat as entropy change was required for splitting HZ from H20. These data show that there would be an economic compromise between HZ production rate and waste heat input at higher T. There would be an optimum H20 input for economic HZproduction. Possibly at 1270 K the H20 input rate could be increased profitably to '60 pmol s-l. In reaction 2 the waste product was HC1 in unreacted H20 that could be recycled as constant boiling HC1:8H20(g) for regeneration of CeOCl as shown in Table 6. The heat of formation of 2HCl(g) from the elements is - 189.7 kJ at 1270 K.3 For HzO the heat of formation at the same Tis
-249.3 kJ mol-'. These data show that the minimum heat requirement for producing H2 from 2HC1 could be as much as 40 kJ less than for a thermochemical cycle for production of HZ l/202 from H2O.
+
Solids Redox Behavior
The constancy of normalized AH and AS data in Table 2 suggest that the properties of the CeOz and CeOCl changed very little at 475-625 K for Cl2 formation and are well represented by eq 1. Also, the propensity for CeO2 formation by hydrolysis of CeOCl at high T is well documented in Tables 3 and 4.
Thermosplitting of Hydrochloric Acid
J. Phys. Chem., Vol. 98, No. 51, 1994 13545
TABLE 6: Calculations Based on Constant Boiling (HCI:8H20)(g) Instead of Anhydrous HCI(g) for Formation of ZCeOCl(cr), According to the Following Stoichiometry:'
2Ce02(cr)
-
+ 4(HC1:8H20)(g)
2CeOCl(cr)
T, K
HCl in @mols-l)
Clz out @mols-I)
CeOCl yield'
383 383 383
60.0 60.0 60.0
6.00 15.00 24.00
0.200 0.500 0.800
+ 34H20(g) + C12(g) gas out HCld H20/HCl 0.0894 0.0563 0.0227
10.12 16.50 42.00
(4)
k9
ArGb (J)
18350 1208.1 303.18
-51 016 -36 878 -18 197
a For reaction 4, kq = [Clz mol frac out][HzO mol frac outI2/[HC1mol frac outI4. Spontaneous work done (joules) in producing 1 mol of Clz at the specified K,rate, and partial pressure. The chemical reaction did the necessary work. Same as the fraction of CeOz consumed. Partial pressure at the outlet of the reactor, with total pressure at 1 am.
The thermochemical property of entropy of formation of each solid should be a good way of expressing differences in behavior of each solid at various temperature intervals. The solids difference equation for reaction 1 is3,4 AfS"[CeOCl(cr) - CeO,(cr)] = 1/2(AH- A G ) T ' '/,AS0(JANAF)[2H2O(g)
+ C12(g)- 4HCl(g)]
For reaction (2) AfS[CeOCl(cr) - CeO,(cr)] = '/,AS0(JANAF)[2HCl(g)
+
H2(g) - 2H20(g)] - 1/2(AH- A G ) T ' Results of calculations were as follows:
T (K)
500 600 1100 1200 1300
redox products Clz, Ce(IV-III) Hz, Ce(1II-IV)
AfS[CeOCl- CeOz] solids (J K-' mol-') 71.625 72.467 36.082 19.479 19.199
The difference in entropy of formation of CeOCl from CeO2 by hydrochlorination at 500-600 K was much larger than the difference by its conversion to CeO2 by hydrolysis at high T. Conclusions
H2 production from H20 at high T (vs lower r ) by hydrolyzing CeOCl at '1200 K provides better rates, better product quality, higher partial pressures and efficient utilization of heat energy. A pressurized reactor is not required. The waste HCl from hydrolysis can be recycled at 383 K for production of Cl2(g) and regeneration of CeOC1. Additional HCl must be added for this recycle operation. The heat energy required for making Clz(g) with CeO2 and HC1 is small compared to the heat requirement for Hz(g) production. Anhydrous HCl(g) is not practical for use because of the high energy required for desolvation, and a work requirement of 7 M mol-', as listed in Table 1. Table 6 shows
that Cl2(g) can be produced with a mixture of HCl(g) and HzO(g) at 383 K (or higher r ) without added heat. Production of 1 mol of H2 by hydrolysis at 1270 K would require (ideally) a minimum of heat energy of 206 M (Table 5 ) . In addition to the heat load would be other processing needs of purifying the HZfor immediate use or storage. The method of condensing the HCl(g) and HzO(g) in a closed vessel would upgrade the H2(g) purity and increase the pressure to near ambient. Splitting of 2 mol of HCl in this thermochemical cycle for H2 production is fundamentally less demanding of heat energy than any cycle that requires splitting of 1 mol of H2O into the elements. The enthalpy of formation of 2HCl(g) from the elements is -189.7 kJ at 1270 K, whereas the value for formation of 1 H20 is -249.3 kT. 0 2 is available anywhere on Earth for H2 as a fuel. Acknowledgment. Many of the early experiments in this research project were sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. Assistance was provided also by the Cooperative Education Program of Los Alamos National Laboratory, University of New Mexico, Eastern New Mexico University, and New Mexico State University. Mary Pretzel and W. G . Witteman helped with the X-ray diffraction analyses. E. J. Peterson, E. M. Foltyn, and M. Fletcher helped with related experiments on cerium chemistry. The author thanks W. C. Danen and the Physical Chemistry Group for support and encouragement. References and Notes (1) Hollabaugh, C. M.; Onstott, E. I.; Wallace, T. C . ; Bowman, M. G. Study on the Cerium-Chlorine System for Hydrogen Production; Los Alamos National Laboratory Document LAUR-78-1019, 1978. (2) Onstott, E. I. Thermochemistry of Hydrochlorination of Cerium Dioxide for Chlorine Production; Los Alamos National Laboratory Document LAUR-93-4381, 1993. (3) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; FrUrip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed. J. Phys. Chem. Ref. Data 14, 1985, Sup. No. 1 . (4) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, Iva; Bailey, S. M.; Churney, K. L.; Nuttal, R. L. The NBS tables of chemical thermodynamic properties of inorganic and C1 and C2 organic substances, J . Phys. Chem. Ref. Data 11, 1982, Sup. No. 2.
