Urea has been widely used in the agricultural industry as a fertilizer. It represents more than 50% of the nitrogen fertilizer market, and its global demand has increased more than 100 times in the last decades. In energy terms, urea has been considered a hydrogen–storage (6.71 wt.%) and ammonia–storage (56.7 wt.%) compound, giving it fuel potential. Urea properties meet the requirements of the US Department of Energy for hydrogen–storage substances, meanly because urea crystalizes, allowing storage and safe transportation. Conventional industrial urea synthesis is energy–intensive (3.2–5.5 GJ ton-1) since it requires high pressures and temperatures, so developing a photocatalyzed synthesis using TiO2 (Titanium dioxide) at ambient temperature and pressure is an attractive alternative to conventional synthesis.
1. Introduction
Synthetic nitrogen–based fertilizers such as urea (CO
2(NH
2)
2) have become essential for intensive agriculture and global food production. The industrial production of urea began in the 1920s with the development of the Bosch–Meiser process, and since then, its global demand has been increasing year after year. Approximately 90% of urea is used as a fertilizer, and the remaining percentage represents its uses as an additive in different industries. Urea can be used as a ruminant feed additive
[1][2][1,2] or chemically synthesized to make urea–based herbicides or pesticides
[3][4][5][3,4,5]. Other non–fertilizer uses of urea include manufacturing plastic materials, additives in fire retardants, tobacco products, some wines, and the cosmetic industry
[6]. Urea is even used in fuels, helping reduce NOx gases in combustion engines through selective catalytic reduction (SCR) systems
[7][8][7,8].
2. Developments in the Photocatalyzed Production of Urea Using TiO2–Based Materials
Urea production by photocatalysis was first reported in 1998 by S. Kuwabata, H. Yamauchi, and H. Yoneyama
[9][41]. This research group presented the simultaneous reduction of CO
2 and nitrate ion (NO
3−) using titanium dioxide nanocrystals (Q–TiO
2) with sizes ranging from 1.0 to 7.5 nm, which were immobilized in a film of polyvinylpyrrolidone gel (Q–TiO
2/PVPD). A 500 W high–pressure mercury arc lamp was used as a light source for the experiments, in which wavelengths below 300 nm were removed with a glass filter.
S. Kuwabata et al.
[9][41] reported a concentration of about 1.9 mM urea in a 5 h photocatalysis using a Q–TiO2/PVPD film in polypropylene carbonate (PC) solutions saturated with CO
2 in the presence of LiNO
3 and using 2–Propanol as an electron donor species. However, when using Q–TiO
2 colloidal and TiO
2 P–25 colloidal, under the same conditions, they obtained an approximate concentration of 7.3 × 10
−2 mM and 3.3 × 10
−2 mM, respectively.
The results obtained by the Q–TiO
2/PVPD film were promising since a lower concentration of urea was expected as a product because of the belief that the use of the Q–TiO
2 photocatalyst particles fixed in a polymeric film reduces the area exposed to the solution as opposed to using suspended photocatalyst particles. S. Kuwabata et al.
[9][41] justified their results by studying the photo–oxidation of the urea formed, determining that urea and 2–propanol compete for active sites on the surface of Q–TiO
2, degrading and decreasing their concentration in the solution. According to research, using Q–TiO
2 in suspension increases the collision frequency and interaction between urea and the catalyst surface. The interface interaction generates a more significant reaction and increases the degradation of the already–formed products.
However, the concentration of the products formed by the urea degradation via photo–oxidation does not contribute directly to the secondary reactions leading to by–products such as methanol, ammonia, acetone, and hydrogen. The concentrations observed for the by–products remain proportional to urea formation, as shown in
Table 1, whose results were obtained after photoreductions were made for 5 h.
Table 1. Amounts of products obtained by the photoinduced reduction of nitrate ions and carbon dioxide using different kinds of titanium dioxide photocatalysts. Adapted from S. Kuwabata et al.
[9][41].
Catalyst |
Solvent |
Amount of Products (µmol) |
Urea |
Acetone |
Methanol |
NH | 4+ |
H | 2 |
Ti | 3+ |
Q–TiO | 2 | /PVPD |
PC |
5.6 |
61.0 |
1.2 |
2.7 |
0.18 |
0.14 |
Q–TiO | 2 | coloidal |
PC |
0.22 |
2.5 |
0.12 |
0 |
0 |
0 |
P–25 TiO | 2 | /PVPD |
PC |
2.3 |
30.1 |
0.04 |
1.2 |
0.08 |
– |
P–25 Coloidal |
PC |
0.1 |
1.1 |
0.05 |
0 |
0.06 |
– |
P–25 TiO | 2 | /PVPD |
H | 2 | O |
0 |
0.07 |
0 |
0 |
0.01 |
– |
S. Kuwabata et al.
[9][41] used reduced species such as NH
2OH or NO
2, replicating the experimental conditions for the Q–TiO
2/PVPD film, reaching urea concentrations close to 5.6 and 2.9 mM, respectively, in 1 h. These results concluded that the limiting step of photocatalyzed urea production is the formation of reduced NO
3− ions species.
B.-J. Liu, T. Torimoto, and H. Yoneyama
[10][42] subsequently generated urea reaching concentrations of 3 mM using a film of TiO
2 nanocrystals embedded in SiO
2 matrices (Q–TiO
2/SiO
2). This experiment proposed the photochemical reduction of CO
2 saturated in a LiNO
3 solution, using 2–propanol as an electron donor. The system was irradiated for 5 h with a 500 W mercury arc lamp.
B.-J. Liu et al.
[10][42] reported that urea selectivity regarding other products, such as formate (HCO
2−) and carbon monoxide (CO), is influenced by the type of solvent used in the reaction and its dielectric constant (see
Table 2).
Table 2. Amount of product obtained by the photoinduced reduction reaction of lithium nitrate in solvents saturated with carbon dioxide. Adapted from T. Torimoto et al. [11]. Amount of product obtained by the photoinduced reduction reaction of lithium nitrate in solvents saturated with carbon dioxide. Adapted from T. Torimoto et al. [43].
Solvent |
Dielectric Constant,
| ε |
Amount of Products (mM) | (a) |
Urea |
NH | 3 |
HCO | 2− |
CO |
Ethylene glycol monoethyl ether |
29.6 |
1.00 |
0.20 |
0.80 |
0.50 |
Acetonitrile |
37.5 |
1.15 |
0.15 |
0.70 |
0.20 |
Sulfolane |
43.0 |
1.00 |
0.20 |
0.40 |
0.25 |
PC |
69.0 |
0.85 |
0.25 |
0.10 |
0.05 |
Water |
78.5 |
2.75 |
0.75 |
0.10 |
0.05 |