Thermochemical cyclic system for splitting water and/or carbon dioxide by means of cerium compounds and reactions useful therein

A thermochemical cyclic process for producing hydrogen from water comprises reacting ceric oxide with monobasic or dibasic alkali metal phosphate to yield a solid reaction product, oxygen and water. The solid reaction product, alkali metal carbonate or bicarbonate, and water, are reacted to yield hydrogen, ceric oxide, carbon dioxide and trialkali metal phosphate. Ceric oxide is recycled. Trialkali metal phosphate, carbon dioxide and water are reacted to yield monobasic or dibasic alkali metal phosphate and alkali metal bicarbonate, which are recycled. The cylic process can be modified for producing carbon monoxide from carbon dioxide by reacting the alkali metal cerous phosphate and alkali metal carbonate or bicarbonate in the absence of water to produce carbon monoxide, ceric oxide, carbon dioxide and trialkali metal phosphate. Carbon monoxide can be converted to hydrogen by the water gas shift reaction.

It relates generally to the art of thermochemical hydrogen production. 
Hydrogen is presently considered to be an attractive energy agent to be 
developed for future use. Hydrogen has many attributes which make it a 
logical replacement for fossil fuels which are being rapidly consumed and 
are becoming increasingly expensive. The combustion of hydrogen produces 
no obnoxious products and thus no harm to the environment. Existing energy 
transport means and energy consuming equipment can be adapted to a 
hydrogen-based energy system using technology presently available. Natural 
gas pipelines, for example, can be converted to hydrogen carrying 
pipelines with minor modifications. Experimental automobiles are operating 
with modified conventional internal combustion engines using hydrogen as a 
fuel. As the prospect of hydrogen utilization becomes increasingly likely, 
means for producing hydrogen need to be upgraded and increased. 
Description of the Prior Art 
Conventionally, hydrogen has been produced by the electrolysis of water. 
Electrolysis, however, is highly inefficient in view of the less than 40 
percent efficiency for electricity production coupled with an efficiency 
of about 80 percent for electrolysis. Inherent in the electrolytic 
production of hydrogen is the general futility of using one energy source, 
typically fossil fuels at present, to produce electricity which is then 
ultimately used to produce hydrogen at the point of electrolysis. The 
disadvantages of excess consumption of fossil fuels are obviously not 
overcome by such a process. 
Chemical processes for the direct conversion of fossil fuels and water into 
hydrogen are presently feasible technically and overcome many of the 
inefficiencies and disadvantages of electrolysis. However, prudence 
indicates that fossil fuels should be preserved as much as possible for 
long term pharmaceutical, chemical and metallurgical requirements. 
Thermochemical processes present the most attractive method for producing 
hydrogen. Using this technique, water is broken down into hydrogen and 
oxygen by a series of chemical reactions not involving the use of fossil 
fuels. This series of reactions is preferably carried out in a closed 
cyclic manner in which all products except hydrogen and oxygen are reused 
as reactants in other reactions. One such process, disclosed in U.S. Pat. 
No. 3,490,871, utilizes the reaction of cesium with water to release 
hydrogen. 
Another such process, disclosed by Grimes et al in U.S. Pat. No. 3,919,406, 
involves the reaction of copper and magnesium chlorides with water to 
produce hydrogen in a closed cyclic manner. 
Another such process is disclosed by Bamberger et al in U.S. Pat. No. 
3,927,192. The process therein disclosed comprises reacting chromium oxide 
with an alkali metal hydroxide to produce hydrogen, water and alkali metal 
chromate as reaction products. 
Bamberger et al (U.S. Pat. No. 3,929,979) also disclose a cyclic process 
for splitting water wherein magnetite is reacted with an alkali metal 
hydroxide to give hydrogen, alkali metal ferrate and water as products. 
Bamberger et al, in U.S. Pat. No. 3,996,343, disclose the production of 
hydrogen in a closed chemical cycle for the thermal decomposition of water 
by reaction of water with chromium sesquioxide and strontium oxide. 
Bamberger et al (U.S. Pat. No. 4,005,184) employ chromium and barium 
compounds in a thermochemical process for producing hydrogen. 
Bamberger et al in commonly assigned U.S. Pat. No. 4,169,884 issued Oct. 2, 
1979, describe a process for producing hydrogen using copper and barium 
hydroxide. 
Bamberger et al in commonly assigned U.S. Pat. No. 4,180,555 issued Dec. 
25, 1979, describe a process for producing hydrogen from water using 
cobalt and barium compounds. The use of cerium and titanium compounds in a 
thermochemical cycle for producing hydrogen from water is set forth in 
commonly assigned U.S. application Ser. No. 47,447 entitled 
"Thermochemical Cycle for Water Decomposition Based upon Ce-O-Ti 
Compounds," filed in the name of Carlos E. Bamberger. 
Ishii et al (U.S. Pat. No. 4,098,875) produce hydrogen thermochemically 
from water using tri-iron tetraoxide and hydrogen bromide as the main 
cyclic reaction media. The use of barium iodide, carbon dioxide and 
ammonia as cyclic reaction media is disclosed in U.S. Pat. No. 3,996,342. 
OBJECTS OF THE INVENTION 
It is an object of any thermochemical process to use heat directly from an 
energy producing facility requiring no fossil fuels, such as a nuclear 
reactor or solar furnace. The upper temperature limit for these sources is 
about 1300.degree. K. for a high-temperature gas-cooled nuclear reactor 
and about 3500.degree. K. for a solar furnace. 
It is an object of this invention to provide a novel process for producing 
hydrogen from water or carbon monoxide from carbon dioxide. 
It is a further object of this invention to provide a cyclic thermochemical 
process for splitting water into hydrogen and oxygen or for splitting 
carbon dioxide into carbon monoxide and oxygen. 
It is a further object to provide novel chemical reactions useful in such 
processes. Another object is to provide embodiments of the foregoing 
processes wherein all reactions are carried out at temperatures below 
about 1300.degree. K. 
SUMMARY OF THE INVENTION 
In one aspect, this invention comprises a novel method for producing a 
trialkali metal cerous phosphate comprising reacting ceric oxide with a 
compound selected from the group of monobasic alkali metal phosphates, 
dibasic alkali metal phosphates, alkali metal pyrophosphates and alkali 
metal metaphosphates at a temperature above about 650.degree. C. to cause 
the formation of a trialkali metal cerous phosphate. 
In another aspect, this invention comprises a novel method for producing 
hydrogen comprising reacting trialkali metal cerous phosphate with a 
reactant selected from the group of alkali metal carbonates and alkali 
metal bicarbonates in the presence of water at a temperature above about 
650.degree. C. to cause the formation of gaseous hydrogen. 
In another aspect, this invention comprises a novel method for producing 
carbon monoxide comprising reacting trialkali metal cerous phosphate with 
a reactant selected from the group of alkali metal carbonates and alkali 
metal bicarbonates in a substantially water-free environment at a 
temperature above about 650.degree. C. to cause the formation of gaseous 
carbon monoxide. 
In another aspect, this invention comprises a cyclic process for producing 
hydrogen comprising the steps of: (a) reacting ceric oxide with a compound 
selected from the group of monobasic alkali metal phosphates, dibasic 
alkali metal phosphates, alkali metal pyrophosphates and alkali metal 
metaphosphates, to yield oxygen, water and a solid product; (b) reacting 
the thus-produced solid product with a reactant selected from the group of 
alkali metal carbonates and alkali metal bicarbonates in the presence of 
water to yield ceric oxide, trialkali metal phosphate, hydrogen, and 
carbon dioxide; (c) reacting the thus-produced trialkali metal phosphate 
with water and carbon dioxide to yield alkali metal bicarbonate and 
monobasic or dibasic alkali metal phosphate; (d) recycling ceric oxide 
produced in step (b) to step (a); and (e) recycling monobasic or dibasic 
alkali metal phosphate produced in step (c) to step (a). The alkali metal 
bicarbonate produced in step (c) can be recycled to step (b) or decomposed 
into water, alkali metal carbonate, for recycle to step (b), and carbon 
dioxide, for recycle to step (c). 
In still another aspect, this invention comprises a cyclic process for 
producing carbon monoxide, comprising the steps of: (a) reacting ceric 
oxide with a compound selected from the group of monobasic alkali metal 
phosphates and dibasic alkali metal phosphates to yield oxygen, water, and 
a solid product; (b) reacting the thus-produced solid product with a 
reactant selected from the group of alkali metal carbonates and 
bicarbonates in a substantially water-free environment to yield ceric 
oxide, trialkali metal phosphate, carbon monoxide and carbon dioxide; (c) 
reacting the thus-produced trialkali metal phosphate with water and carbon 
dioxide to yield alkali metal bicarbonate and monobasic or dibasic alkali 
metal phosphate; (d) recycling ceric oxide produced in step (b) to step 
(a); and (e) recycling monobasic or dibasic alkali metal phosphate to step 
(a). The alkali metal bicarbonate produced in step (c) can be recycled to 
step (b) or can be thermally decomposed into water, alkali metal carbonate 
for recycle to step (b), and carbon dioxide to recycle to step (c). 
In the cyclic processes for producing hydrogen or carbon monoxide the 
monobasic or dibasic alkali metal phosphate recycled from step (c) to step 
(a) can be recycled either directly or indirectly by first converting 
(e.g. by heating) the dibasic phosphate to an alkali metal pyrophosphate, 
or the monobasic phosphate to an alkali metal metaphosphate. When sodium 
or potassium are the alkali metals used in the cycles, the solid product 
resulting from the first reaction is a mixture of alkali metal cerous 
phosphate and either cerous phosphate or trialkali metal phosphate, as 
explained more fully herein. When the alkali metal used in the cycle is 
lithium, the solid product resulting from the first reaction is a mixture 
of cerous phosphate and trilithium phosphate.

EXAMPLE 1 
Ceric oxide (2.4 g) was reacted with Na.sub.2 HPO.sub.4 (7.4 g), at 
continuously increasing temperature (5.4.degree. C./min) in a platinum 
boat inside a quartz tube heated in a tube furnace. The tube system was 
provided with a thermocouple well and lines for sparging with argon 
carrier gas. The exit gas mixture was dried by passage through a column 
packed with anhydrous CaSO.sub.4 and analyzed for oxygen content with a 
Beckman oxygen analyzer. Oxygen evolution began at about 750.degree. C. 
and continued to about 950.degree. C. Conversion of ceric oxide and 
Na.sub.2 HPO.sub.4 to Na.sub.3 Ce(PO.sub.4).sub.2, trisodium phosphate, 
water and oxygen was essentially complete after 120 minutes. The yield of 
oxygen was 50 ml (88.5%). The solid products were identified by neutron 
activation analysis and X-ray diffraction analysis. 
EXAMPLE 2 
Disodium hydrogen phosphate.2.27 H.sub.2 O (19.8 g) was heated in the 
apparatus used in Example 1 at about 500.degree. C. for one-half hour to 
yield 14.4 g sodium pyrophosphate (Na.sub.4 P.sub.2 O.sub.7) and water. 
The residual pyrophosphate was mixed with 8.8 g of ceric oxide and the 
mixture heated as in Example 1 to give sodium cerous phosphate, sodium 
phosphate and oxygen, identified as in Example 1. Significant oxygen 
evolution began at about 660.degree. C. The reaction was essentially 
complete at about 1000.degree. C. after 80 minutes (79% yield of oxygen). 
EXAMPLE 3 
Sodium cerous phosphate, trisodium phosphate and sodium pyrophosphate (15 g 
total), obtained in Example 1, were mixed in the apparatus described in 
Example 1 with anhydrous sodium carbonate (2.7 g). Water vapor was 
supplied continuously to the reaction described in Example 1, using argon 
carrier gas. The sodium pyrophosphate was excess remaining from the 
formation of the sodium cerous phosphate. The charge was heated at a rate 
of 5.4.degree. C./min. to a maximum of about 950.degree. C. The exit gases 
were dried by passage through a water-cooled condenser and anhydrous 
calcium sulfate and hydrogen was determined quantitatively using a thermal 
conductivity detector (Gow-Mac Analyzer). A solution containing 
Ba(OH).sub.2 was used to trap the evolved CO.sub.2 as solid BaCO.sub.3. 
Hydrogen evolution began at 650.degree. C. and reached a maximum at about 
850.degree. C. The yield of hydrogen was quantitative (154 ml). Ceric 
oxide and trisodium phosphate in the residue were identified by neutron 
activation analysis and X-ray diffraction analysis. 
EXAMPLE 4 
Sodium cerous phosphate and sodium phosphate obtained as in Example 1 (11.3 
g) were mixed with 6.6 g of anhydrous sodium carbonate which had been 
recycled from a run of reaction 3 and heated in the apparatus of Example 1 
at a rate of 5.4.degree. C./min. to a maximum of about 850.degree. C. A 
solution of Ba(OH).sub.2 was used to trap evolved CO.sub.2 as solid 
BaCO.sub.3. Evolved carbon monoxide was detected with a thermal 
conductivity detector. Evolution of carbon monoxide began at about 
700.degree. C., but the maximum rate of gas evolution was at about 
850.degree.-950.degree. C. The CO yield was 98% (189 ml). The presence of 
ceric oxide and trisodium phosphate in the solid residue was verified by 
X-ray diffraction. 
EXAMPLE 5 
The solid residue from Example 3 was removed from the crucible and treated 
with 100 ml of water and saturated with carbon dioxide at room 
temperature, CO.sub.2 pressure about 1 atmosphere. The undissolved 
material (2.4 g) following filtration was ceric oxide. The filtrate 
contained disodium hydrogen phosphate and sodium bicarbonate, which were 
separated by fractional crystallization at 0.degree.-22.degree. C. in two 
crystallization steps. 
EXAMPLE 6 
Sodium bicarbonate (2.6 g) is decomposed at about 200.degree. C. in the 
apparatus of Example 1 to sodium carbonate, which is treated with 5.25 g 
of sodium cerous phosphate and excess water as steam at 
650.degree.-750.degree. C. to give hydrogen. 
EXAMPLE 7 
Recycled sodium carbonate (6.66 g) was mixed with 7.92 g of sodium cerous 
phosphate and 1.7 grams of trilithium phosphate at 650.degree.-850.degree. 
C. to liberate carbon monoxide (88% yield). 
EXAMPLE 8 
The residue from Example 3 was dissolved in a minimal volume (72 ml) of 
water and saturated with carbon dioxide by maintaining a CO.sub.2 vapor 
pressure at one atmosphere. The residue following filtration was a mixture 
of ceric oxide and disodium hydrogen phosphate. The aqueous phase 
contained dissolved sodium bicarbonate. 
EXAMPLE 9 
Lithium pyrophosphate (8.84 g) and ceric oxide (5 g) are heated at 
700.degree.-850.degree. C. in the apparatus described in Example 1 to give 
oxygen, water and a solid mixture of cerous phosphate and trilithium 
phosphate. The residue of insoluble salts is treated with lithium 
bicarbonate (5.95 g) at 650.degree.-800.degree. C. to produce ceric oxide, 
trilithium phosphate, carbon dioxide and carbon monoxide. The recycling 
solids are contacted at 25.degree. C. with 100 g of water at 2 atm. of 
carbon dioxide to yield ceric oxide, trilithium phosphate, lithium 
dihydrogen phosphate and lithium bicarbonate. The latter two relatively 
soluble salts are removed from insoluble trilithium phosphate and ceric 
oxide and separated by fractional crystallization. Residual ceric oxide 
and trilithium phosphate are mixed with 4.55 g of the separated LiH.sub.2 
PO.sub.4 and heated to 400.degree. C. to give cerous phosphate, lithium 
pyrophosphate and water, which are recycled. 
EXAMPLE 10 
A mixture of trilithium phosphate and cerous phosphate obtained as in 
Example 9 is contacted with 5.95 g of lithium bicarbonate and steam at 
about 650.degree.-800.degree. C. to form ceric oxide, trilithium 
phosphate, carbon dioxide and hydrogen. The solid residue is treated as in 
Example 9. 
EXAMPLE 11 
A mixture of dipotassium hydrogen phosphate (13.5 g) and ceric oxide (4.14 
g) is heated in an apparatus as in Example 1. At about 1000.degree. C., 
production of oxygen is about 50% complete. 8.8 g of the solid product 
tripotassium cerous phosphate, and tripotassium phosphate were heated with 
potassium carbonate (3.81 g) in the presence of steam to about 
1000.degree. C. to yield ceric oxide, tripotassium phosphate, carbon 
monoxide and carbon dioxide (about 98% yield of CO). The solid residue is 
reacted in the same manner as used for sodium salts. 
EXAMPLE 12 
Material balances for step 3 of the process were determined experimentally 
by treating Na.sub.3 PO.sub.4 with H.sub.2 O and CO.sub.2. Na.sub.3 
PO.sub.4.12 H.sub.2 O (38 g) was dissolved in 400 ml of H.sub.2 O. Carbon 
dioxide was bubbled through the solution at a rate of about 155 ml/min (1 
atm CO.sub.2, 22.degree. C.) for 12 hours to assure completion of the 
reaction. 
The resulting solution was evaporated to a volume of 100 ml and cooled to 
22.degree. C. The precipitate that formed was separated from the solution 
by filtration. Analysis of the solids by acidimetric titration showed the 
composition to be 19.0 g of Na.sub.2 HPO.sub.4.10 H.sub.2 O, 0.11 g of 
NaH.sub.2 PO.sub.4 and 42 g of NaHCO.sub.3. Evaporation of the filtrate to 
20 ml and crystallization of 24.degree. C. gave 0.48 g of Na.sub.2 
HPO.sub.4.18 H.sub.2 O and 2.18 g of NaHCO.sub.3 (acidimetric titration). 
A third solid fraction, obtained from the remaining filtrate by partial 
evaporation and cooling to 0.degree. C., contained 4.82 g of Na.sub.2 
HPO.sub.4, 3.7 g of water and 1.92 g of NaHCO.sub.3. The final solid 
fraction was obtained by evaporating the remaining filtrate to dryness. 
This solid fraction contained 0.27 g of Na.sub.2 HPO.sub.4.4 H.sub.2 O and 
1.87 g of NaHCO.sub.3. The total recovered product, exclusive of 
analytical samples, was 24.57 g of Na.sub.2 HPO.sub.4, 0.11 g of NaH.sub.2 
PO.sub.4 and 6.39 g of NaHCO.sub.3. 
The preceding examples can be repeated with similar success by substituting 
the generically and specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention and, without departing 
from the spirit and scope of the invention herein described, can make 
modifications of the reaction conditions of the various steps and can 
alter the product separations and recycle the material in the cycle in 
various ways. Such modifications are contemplated as equivalents of the 
invention herein claimed and described.