Photoreduction of nitrogen

Nitrogen is reduced to ammonia, hydrazine, and mixtures thereof by reaction with water on a solid metal oxide irradiated with ultraviolet light. Useful metal oxides include titanium dioxide and metal-doped titanium dioxide. Titanium dioxide doped with iron, cobalt, molybdenum or nickel is preferred. The metal oxide is irradiated in the presence of nitrogen and water with ultraviolet light from a source such as a mercury arc lamp or sunlight. The nitrogen and the water may be in the vapor phase, or they may be adsorbed on the metal oxide. In addition, liquid water may be present. The ammonia which is formed may be recovered in a number of ways, for example, by heating the metal oxide gently under vacuum, by water extraction, or by adsorption in an acid acceptor. The recovered ammonia may be used as an agricultural fertilizer either directly or in the form of ammonium nitrate, sulfate, or phosphate salts.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the fixation of nitrogen by reduction to ammonia 
and hydrazine. It converts elemental nitrogen, for example, gaseous 
nitrogen from the air, to a form useable as or in the preparation of 
agricultural fertilizers. It provides a way to synthesize ammonia from 
nitrogen and water at ambient pressures and temperatures without the use 
of elemental hydrogen. It also provides a method for using solar radiation 
in the preparation of agricultural fertilizers. 
Ammonia is an important starting material in the production of nitrogen 
fertilizers. It is presently synthesized almost exclusively by the 
reaction of molecular nitrogen and hydrogen at high temperatures and 
pressures in the presence of an iron catalyst. The nitrogen is obtained 
from the air. The hydrogen is synthesized by reactions involving fossil 
fuels such as coal or natural gas. This invention provides a method for 
making ammonia which is not dependent on the use of fossil fuels. 
2. Description of the Prior Art 
The use of iron oxide catalyst in the synthesis of ammonia has been 
disclosed. U.S. Pat. No. 2,500,008 describes the synthesis of ammonia from 
hydrogen and nitrogen which are contacted with finely divided iron oxide 
catalyst and subjected to ultrasonic vibrations. However, the use of water 
instead of hydrogen and the use of ultraviolet light are not disclosed. 
U.S. Pat. No. 3,378,475 describes the oxidation of nitrogen from air by 
passing air through an aqueous suspension of a catalyst such as iron oxide 
while irradiating with high energy ionizing radiation such as nuclear 
radiation, gamma rays, or x-rays. The reduction of nitrogen with the use 
of ultraviolet light is not described. 
U.S. Pat. No. 3,925,212 describes the use of titanium dioxide as a 
semiconductor electrode in a solar photoelectric cell for the 
decomposition of water to hydrogen and oxygen. 
The photoreduction of acetylene to reduced hydrocarbons with the use of 
titanium dioxide catalyst and ultraviolet light has been described by 
Boonstra and Mutsaers in the Journal of Physical Chemistry, Volume 79, 
page 2025 (1975). 
SUMMARY OF THE INVENTION 
The present invention provides an improved method for fixing nitrogen, for 
example, nitrogen from air, by reduction to ammonia and hydrazine, and 
also provides a method for preparing catalysts useful for reduction of 
nitrogen. The nitrogen is reduced by reaction with water on certain solid 
metal oxide catalysts, i.e., titanium dioxide and metaldoped titanium 
dioxide catalysts, under the influence of ultraviolet light. The method 
may be conveniently practiced at relatively low pressures, for example, 
one atmosphere, and moderate temperatures between about 0.degree. C. and 
200.degree. C., typically between about 20.degree. C. and 60.degree. C. 
The use of gaseous hydrogen is avoided and solar radiation may be used as 
the source of ultraviolet light. 
In the practice of this invention the metal oxide is irradiated with 
ultraviolet light in the presence of nitrogen and water. The reaction 
takes place at the catalyst surface between nitrogen and water, producing 
ammonia, some hydrazine, and oxygen. Under proper conditions little or no 
hydrazine is produced. The ammonia may be recovered for use directly as an 
agricultural fertilizer or for use in the preparation of fertilizer salts 
such as ammonia nitrate, ammonium phosphate, and ammonium sulfate. 
DETAILED DESCRIPTION OF THE INVENTION 
Metal oxides useful in the practice of this invention include titanium 
dioxide and titanium dioxide doped with iron, cobalt, molybdenum, or 
nickel. Iron, cobalt and molybdenum are preferred dopants, particularly 
iron. 
The metal oxides are preferably used in a form which has a high surface 
area for contact with nitrogen and water and for exposure to ultraviolet 
light. Finely divided powders having particle sizes from about 0.5 to 
about 5 microns in diameter are preferred. The metal oxide may also be 
placed on a support having high surface area, such as a panel or matrix of 
a material inert under the reaction conditions employed. Glass, stainless 
steel, polypropylene and the like may be used. Unsupported metal oxide 
powders may be agitated or tumbled during irradiation to increase their 
effective surface area. They may be suspended in a stream of gaseous 
nitrogen and water vapor during irradiation, or they may be suspended in 
water through which nitrogen is bubbled. 
The metal oxides are prepared for use by heating in air or under an inert 
atmosphere such as argon or in a vacuum at temperatures on the order of 
250.degree. C. to 1500.degree. C. to drive off harmful impurities such as 
adsorbed oxygen and carbon monoxide. The heated metal oxide is cooled in 
an inert atmosphere such as argon, in vacuum, or, preferably, in an 
atmosphere consisting essentially of nitrogen and water vapor. Cooling the 
metal oxide in the presence of nitrogen and water vapor provides a 
catalyst with a higher initial activity because the reactants are already 
present on the surface of the catalyst. 
The rutile form of titanium dioxide is substantially more active than the 
anatase form. Titanium dioxide of high purity, for example 99.9 percent 
pure, is available in fine powder, such as two micron powder, in the 
anatase form. Anatase titanium dioxide is converted partly or entirely to 
the rutile form by heating for between about 0.1 hour to 10 hours at 
temperatures between about 750.degree. C. and about 1500.degree. C. 
Typically, the anatase is heated for between 1 and 5 hours at about 
1000.degree. C. After about 5 hours at 1000.degree. C., the conversion 
from anatase to rutile is nearly complete, for example, about 95 percent 
complete. The heated titanium dioxide is then cooled, preferably in an 
atmosphere of nitrogen and water vapor. 
Titanium dioxide doped with certain metals, including iron, cobalt, 
molybdenum and nickel, has higher activity than undoped titanium dioxide. 
Particularly useful as a dopant is iron. Cobalt and molybdenum are nearly 
as good. Nickel provides some increase in activity but is not as desirable 
as the other three. The metal-doped titanium dioxide may contain between 
about 0.05 and about 5 weight percent or more of the metal dopant, 
calculated as the free metal. Minor amounts between about 0.1 and 0.5 
weight percent have given more desirable results. For example, titanium 
dioxide containing 0.14 weight percent iron (0.2 weight percent Fe.sub.2 
O.sub.3) appeared to be more active than titanium dioxide containing less 
or more iron. 
Doped titanium dioxide may be prepared by treating TiO.sub.2 powder with an 
aqueous solution of a salt of the desired metal, such as a bromide, 
chloride, fluoride, iodide, nitrate, sulfate, acetate, trifluoroacetic 
acetylacetonate, or formate, and then heating in an oxidizing atmosphere 
to drive off the moisture and convert the metal salt to the corresponding 
oxide. Sufficient metal salt is used to provide the desired concentration 
of metal in the doped titanium dioxide product. Techniques for doping as 
such are in general well known and need not be further described. 
The prepared catalyst tends to lose activity over a period of several weeks 
upon exposure to oxygen, air, or carbon monoxide. Therefore, the catalyst 
is stored under inert atmosphere such as argon or helium or, preferably, 
under nitrogen saturated with water vapor. 
The ultraviolet light may be derived from any source which is rich in 
ultraviolet, especially near ultraviolet in wave lengths from about 390 to 
about 420 nanometers. Useful sources include mercury arc lamps, carbon arc 
lamps, and sunlight. The higher the intensity of the ultraviolet light, 
the higher the yield from a given reactor. The choice of intensity level 
is thus a matter of economics rather than of operability. 
In the practice of the invention the metal oxide is irradiated with 
ultraviolet light in the presence of nitrogen and water. The nitrogen may 
be present as gaseous nitrogen in contact with the metal oxide or it may 
be adsorbed or chemisorbed on the metal oxide. Similarly, the water may be 
present as vapor or liquid in contact with the metal oxide or it may be 
adsorbed or chemisorbed on the surface of the metal oxide. If a metal 
oxide catalyst is heated, then exposed to nitrogen and water vapor, either 
together or separately in any order, and is then irradiated under inert 
atmosphere such as argon, it has been found that some ammonia will be 
produced. However, the reactants present on the catalyst are quickly 
exhausted. Thus it is preferable to have the metal oxide catalyst in 
contact with nitrogen and water before and during irradiation. 
In the best mode of practicing this invention which is presently 
contemplated, a gas-solid system is employed in which the metal oxide 
catalyst is maintained in contact with gaseous nitrogen and water vapor in 
the substantial absence of liquid water during irradiation. However, 
liquid water may be used in other embodiments. Thus, the catalyst is 
suspended in liquid water through which nitrogen is bubbled during 
irradiation. Catalyst having a thin surface film of water may be contacted 
with nitrogen gas during irradiation. In a further embodiment, water is 
trickled over metal oxide catalyst supported on a mesh or plate in a solar 
cell exposed to sunlight. The trickling water then functions both as a 
reactant and as an extractant to remove ammonia as it is produced, thus 
providing a dilute solution of aqueous ammonia for simultaneously 
irrigating and fertilizing fields. 
When the preferred gas-solid reaction system is used, the ammonia products 
may be recovered in a number of ways. For example, the catalyst is heated 
gently in vacuum at temperatures of from 100.degree. to 200.degree. C. to 
remove adsorbed ammonia, which is then condensed to liquid form. 
Alternatively, the ammonia is extracted from the catalyst with liquid 
water and the catalyst then dried and used again. The catalyst may be 
reused without loss of activity after ammonia is extracted with water, 
provided the catalyst surface is protected from contaminants. The ammonia 
may also be absorbed into an acid acceptor such as phosphoric acid or 
sulfuric acid, which is present in the irradiation chamber or in a chamber 
in gas communication with the irradiation chamber. 
The present invention may also be practiced as a continuous process in 
which a gas mixture such as air or nitrogen and water vapor is 
continuously passed over the metal oxide catalyst during irradiation, the 
gas now containing ammonia is removed, and the ammonia is extracted 
therefrom. Desirably, after the ammonia is extracted, the gas is recycled 
over the metal oxide catalyst. Recycling would be particularly desirable 
where pure nitrogen rather than air is used. 
The metal oxide catalyst is preferably contacted with a gas mixture 
consisting essentially of nitrogen and water vapor during irradiation. 
Contaminants such as oxygen, carbon monoxide, etc., which tend to decrease 
the activity of the catalyst are excluded, but inert gases which do not 
interfere with the desired reaction, such as argon and helium, may be 
present. If desired, air may be used instead of pure nitrogen, but some 
lessening of catalyst activity and therefore lower yield may be 
experienced. 
In a gas-solid reaction system, the gas mixture must contain a substantial 
proportion of water vapor. Desirably, it is saturated with water vapor at 
the temperature and pressure of operation. Since the vapor pressure of 
water is fairly low at moderate temperatures, it is evident that in 
nitrogen or air saturated with water vapor, the ratio of water vapor to 
nitrogen will often be much less than the stoichiometric ratio of 3 to 1. 
For example, the vapor pressure of water at 60.degree. C. is 149 
millimeters of mercury, so that the ratio of water vapor to nitrogen in 
saturated nitrogen is about 0.25 to 1 at 60.degree. C. An auxiliary source 
of water vapor is therefore preferred to insure an adequate amount of 
hydrogen to react with the nitrogen. 
During irradiation the metal oxide is kept at a temperature between about 
0.degree. C. and about 200.degree. C., for example, between about 
10.degree. C. and about 100.degree. C. With titanium dioxide catalysts, 
particularly iron-doped titanium dioxide, the yield of ammonia is best at 
temperatures between about 20.degree. C. and about 60.degree. C. 
Any convenient pressure may be used, such as atmospheric pressure or higher 
or lower pressures, for example, between 1 and 10 atmospheres. The yield 
of ammonia appears to be proportional to the partial pressure of nitrogen 
at pressures near one atmosphere. A significant advantage of this process 
is that it may be practiced at ambient atmospheric pressures. 
Contact times are widely variable, depending upon temperature, pressure, 
intensity of radiation, surface area of the catalyst, and the design of 
the reactor. In batch processes, contact times as long as several hours or 
days may be used. In continuous processes involving continuous extraction 
of ammonia, shorter contact times, on the order of several seconds or 
minutes, are useful.

The following examples will more fully illustrate the practice of this 
invention. 
EXAMPLE I 
Titanium dioxide in the anatase form having a purity of 99.9 percent and a 
particle size of 2 microns was heated at 1000.degree. C. for 2 hours in 
air and was then cooled in a container filled with nitrogen saturated with 
water vapor. A 0.2 gram sample of the cooled titanium dioxide was placed 
in a glass bottle of 38 milliliter capacity. The bottle was closed with a 
rubber serum cap and flushed with pure acid-washed nitrogen gas for twenty 
minutes. As a control, a second bottle containing 0.2 grams of the cooled 
titanium dioxide was filled with pure argon. The bottles were irradiated 
for three hours at 30.degree. C. to 40.degree. C. with ultraviolet light 
from a 360 watt mercury-arc lamp located approximately 20 centimeters 
away. Then, a 10 milliliter portion of 1 N hydrochloric acid was injected 
into each bottle. After ten minutes at room temperature, 5 milliliters of 
the supernatant solution was transferred to a Kjeldahl flask, made 
alkaline with 5 milliliters of 5 N sodium hydroxide and distilled. The 
amount of the ammonia in the distillate was determined colorimetrically by 
the method of Kruse and Mellon, J. Water Pollut.Control Fed., 24 1098 
(1952). In the sample from the nitrogen-filled reaction flask, 0.7 to 0.9 
micromole of ammonia was detected. In the sample from the argon-filled 
flask, 0.05 to 0.1 micromole of ammonia was detected. The remainder of the 
hydrochloric acid extract from each flask was used for the determination 
of hydrazine according to the method of Schrauzer, et al., J.Am.Chem.Soc. 
96, 641 (1974). In the sample from the nitrogen-filled flask 0.1 to 0.2 
micromole of hydrazine was detected. No hydrazine was detected in the 
sample from the argon-filled flask. 
EXAMPLE II 
Titanium dioxide powder having a purity of 99.9 percent and a particle size 
of 2 microns was treated with an aqueous solution of ferric sulfate. The 
resulting slurry was dried in a rotary evaporator, and the residue was 
heated in air at 1000.degree. C. for 2 hours and then cooled and 
equilibrated with gaseous nitrogen and water vapor at room temperature for 
six hours. Samples of titanium dioxide containing 0.1, 0.2, 0.3, 0.4, 0.5 
and 1 weight percent Fe.sub.2 O.sub.3 were prepared. Samples of 0.2 gram 
of titanium dioxide and the iron-doped titanium dioxides were placed in 
glass bottles flushed with nitrogen and irradiated as described in Example 
I. The ammonia produced was determined as an Example I. 
Table I below shows that doping with iron substantially increases the 
activity of the titanium dioxide catalyst, up to about 0.5% Fe.sub.2 
O.sub.3. 
TABLE I 
______________________________________ 
PHOTOREDUCTION OF NITROGEN ON TiO.sub.2 AND 
IRON-DOPED TiO.sub.2 AT 40.degree. and 1 ATM, OF NITROGEN 
PRESSURE YIELDS AFTER 3 HRS. OF IRRADIATION, 
EXPRESSED PER GRAM OF TiO.sub.2. 
NH.sub.3, 
Micrograms/g TiO.sub.2 
No.: Solid: Mean: Range: 
______________________________________ 
1 TiO.sub.2 65 45 - 70 
2 TiO.sub.2 
,0.1% Fe.sub.2 O.sub.3 
550 300 - 600 
3 TiO.sub.2 
,0.2% Fe.sub.2 O.sub.3 
660 600 - 1000 
4 TiO.sub.2 
,0.3% Fe.sub.2 O.sub.3 
555 300 - 600 
5 TiO.sub.2 
,0.4% Fe.sub.2 O.sub.3 
530 280 - 580 
6 TiO.sub.2 
,0.5% Fe.sub.2 O.sub.3 
520 280 - 550 
7 TiO.sub.2 
,1.0% Fe.sub.2 O.sub.3 
230 170 - 260 
______________________________________ 
EXAMPLE III 
Titanium dioxide doped with 0.2 weight percent of ferric oxide was prepared 
as outlined in Example II. Samples of 0.2 grams of the doped titanium 
dioxide were placed into glass reaction flasks filled with nitrogen, 
nitrogen mixed with argon, and air, respectively. The bottles were 
irradiated for 3 hours at 40.degree. C. as described in Example I. The 
results of the ammonia determinations, shown in Table 2 below, illustrate 
the effect of the partial pressure of nitrogen on the yield of ammonia and 
also show that air is less efficient than a mixture of nitrogen and argon 
having a similar partial pressure of nitrogen. 
TABLE 2 
______________________________________ 
PHOTOREDUCTION OF NITROGEN ON IRON-DOPED 
TiO.sub.2 AT 40.degree. AFTER THREE HOURS OF IRRADIATON 
AND DIFFERENT PRESSURES OF NITROGEN 
AS WELL AS IN AIR. 
No.: N.sub.2 Pressure (atm.) 
NH.sub.3, Micrograms/g TiO.sub.2 
______________________________________ 
1 0 (argon)* 0 
2 0.3 (argon) 175 
3 0.6 (argon) 320 
4 1.0 (N.sub.2 only) 
550 
5 2.0 (N.sub.2 only) 
750 
6 ca. 0.75 (air) 145 
______________________________________ 
*Entry in parentheses indicates additional component in the gas phase. 
EXAMPLE IV 
Samples of 0.2 gram of titanium dioxide and titanium dioxide doped with 
varying amounts of iron oxide were placed in glass nitrogen-filled 
reaction flasks and exposed to intermittent sunlight near sea level at 
temperatures between 25.degree. C. and 35.degree. C. for a period of two 
weeks. Flasks containing catalyst in the presence of argon or air were 
also exposed. The results of ammonia and hydrazine determinations are 
shown in Table 3. 
TABLE 3 
______________________________________ 
YIELDS OF AMMONIA AND OF HYDRAZINE FROM 
NITROGEN ON EXPOSURE OF UNDOPED AND 
Fe-DOPED TiO.sub.2 TO INTERMITTENT SUNLIGHT AT 
TEMPERATURES BETWEEN 25 and 35.degree.. 
CONDITIONS AND YIELDS, .mu.MOLES 
______________________________________ 
No. CATALYST.sup.[a] NH.sub.3 N.sub.2 H.sub.4 
______________________________________ 
1 TiO.sub.2, 1 atm. Ar 0 0 
2 TiO.sub.2, .5 atm. N.sub.2 
0.74 0.05 
3 TiO.sub.2, 1 atm. N.sub.2 
1.55 0.17 
4 TiO.sub.2 /.05 wt.-% Fe.sub.2 O.sub.3, 1 atm. Ar 
0 0 
5 TiO.sub.2 /.05 wt.-% Fe.sub.2 O.sub.3, 1 atm. N.sub.2 
3.20 0.09 
6 TiO.sub.2 /.2 wt.-% Fe.sub.2 O.sub.3, 1 atm. N.sub.2 
4.98 0.12 
7 TiO.sub.2 /.2 wt.-% Fe.sub.2 O.sub.3, 1 atm. Air 
1.89 0.07 
8 TiO.sub.2 /.5 Wt.-% Fe.sub.2 O.sub.3, 1 atm. Ar 
0 0 
9 TiO.sub.2 /.5 Wt.-% Fe.sub.2 O.sub.3, 1 atm. N.sub.2 
4.65 0.19 
10 TiO.sub.2 /.5 Wt.-% Fe.sub.2 O.sub.3, 1 atm. Air 
2.0 0.11 
______________________________________ 
.sup. [a] Sample weight: 0.2 grams. All photocatalysts were heated for 1 
hr at 1000.degree. in air and stored in H.sub.2 O-saturated N.sub.2 
-filled containers. Rutile/anatase ratios for samples 1-3: 10/90; 4,5: 
10/90; 6,7: 15/85; 8-10 ca. 50/50. Sample bottles were exposed to La 
Jolla, California sunlight in June close to sea-level for two weeks. 
EXAMPLE V 
Samples of titanium dioxide were impregnated with solutions of various 
metal salts and subsequently heated at 1000.degree. C. for 2 hours. The 
resulting metal-doped samples were used for the photoreduction of nitrogen 
as outlined in Example II. The yields of ammonia detected are given in 
Table 4. 
TABLE 4 
______________________________________ 
PHOTOREDUCTION OF NITROGEN ON METAL- 
DOPED TiO.sub.2 ON EXPOSURE WITH A 360 Hg ARC 
LAMP AT THE IRRADIATION TEMPERATURE 
OF 40.degree.. IRRADIATION TIME: 3 HOURS. 
NH.sub.3, Micrograms/gr. 
No.: TiO.sub.2 doped with: 
TiO.sub.2 
______________________________________ 
1 0.5% Fe 525 
2 0.4% Co 375 
3 0.4% Mo 405 
4 0.4% Ni 57 
5 0.4% Pd 39 
6 0.4% Pt 40 
7 0.4% V 25 
8 0.4% Cr 20 
9 0.4% Cu 15 
10 0.4% Pb 17 
______________________________________ 
EXAMPLE VI 
Another group of doped titanium dioxide samples were prepared as described 
in the previous example. In addition to the ammonia yields from these 
doped catalysts, the ratio of rutile to anatase in each was determined by 
x-ray analysis. The results are summarized in Table 5. 
TABLE 5 
______________________________________ 
EFFECTS OF METALS ON NITROGEN PHOTO- 
REDUCTION. YIELDS OF NH.sub.3 DETERMINED AFTER 3 
HRS OF UV-IRRADIATION AT 40.degree. IN GLASS VESSELS 
CONTAINING 0.2 GRAMS OF THE DOPED 
TITANIAS AND N.sub.2 AT 1 ATM. 
NH.sub.3 YIELDS 
No. Metal.sup.[a] 
Rutile/Anatase 
MICROMOLES 
______________________________________ 
1 Fe ca. 95:5 6.4 
2 Co 30:70 3.8 
3 Mo 30:70 4.0 
4 Ni 10:90 1.76 
5 none 95:5.sup.[b] 
1.35 
6 Pd ca. 5:95 0.65 
7 Pt 5:95 0.43 
8 Ag 5:95 0.21 
9 Au 5:95 0.25 
10 V 5:95 0.25 
11 Cr 5:95 0.22 
12 Pb 5:95 0.19 
13 Cu 5:95 0.17 
______________________________________ 
.sup.[a] Metal concentration 0.4wt.-% in all cases. All samples were 
heated in air to 1000.degree. for 2 hrs prior to rehumidification and 
irradiation except where indicated. 
.sup.[b] After 5 hrs of heating to 1000.degree.. 
A comparison of the results of Examples V and VI shows that while iron, 
cobalt, and molybdenum are clearly superior, nickel also provides some 
increase in catalyst activity.