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Photocatalyzed Production of Urea Using TiO2–Based Materials: Comparison
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Please note this is a comparison between Version 2 by Dean Liu and Version 1 by Felipe Matamala-Troncoso.

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 at ambient temperature and pressure is an attractive alternative to conventional synthesis. 

  • TiO2–based materials
  • photocatalysis
  • urea
  • hydrogen storage

1. Introduction

Synthetic nitrogen–based fertilizers such as urea (CO2(NH2)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 [41][9]. This research group presented the simultaneous reduction of CO2 and nitrate ion (NO3−) using titanium dioxide nanocrystals (Q–TiO2) with sizes ranging from 1.0 to 7.5 nm, which were immobilized in a film of polyvinylpyrrolidone gel (Q–TiO2/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. [41][9] 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 CO2 in the presence of LiNO3 and using 2–Propanol as an electron donor species. However, when using Q–TiO2 colloidal and TiO2 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–TiO2/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–TiO2 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. [41][9] 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–TiO2, degrading and decreasing their concentration in the solution. According to research, using Q–TiO2 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. [41][9].
Catalyst Solvent Amount of Products (µmol)
Urea Acetone Methanol NH4+ H2 Ti3+
Q–TiO2/PVPD PC 5.6 61.0 1.2 2.7 0.18 0.14
Q–TiO2 coloidal PC 0.22 2.5 0.12 0 0 0
P–25 TiO2/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 TiO2/PVPD H2O 0 0.07 0 0 0.01 –
S. Kuwabata et al. [41][9] used reduced species such as NH2OH or NO2, replicating the experimental conditions for the Q–TiO2/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 NO3− ions species. B.-J. Liu, T. Torimoto, and H. Yoneyama [42][10] subsequently generated urea reaching concentrations of 3 mM using a film of TiO2 nanocrystals embedded in SiO2 matrices (Q–TiO2/SiO2). This experiment proposed the photochemical reduction of CO2 saturated in a LiNO3 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. [42][10] reported that urea selectivity regarding other products, such as formate (HCO2−) 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. [43].
 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].
Solvent Dielectric Constant,

ε Amount of Products (mM) (a)
Urea NH3 HCO2− 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
(a) Calculated from data reported in diagrams and considering 5 cm3 as solution volume used in the cell.
The results presented by B.-J. Liu et al. [42][10] showed that the NO3− ion reduction reaction is the determining step in producing urea and NH3 as synthesis products. Urea and NH3 concentrations increase by increasing the concentration of NO3− as opposed to the concentration of HCO2− that remained relatively constant. Similar results were presented by S. Kuwabata et al. [41][9] using TiO2 particles supported on a surface, justifying that the Q–TiO2/SiO2 film has a higher photocatalytic activity since the particles have a negative change in the edge of the conduction band caused by the effect of the quantization and the specific surface area estimated at 290 m2 g−1. In 2005, a new proposal for the photochemical synthesis of urea arose at the hands of D.G. Shchukin and H. Möhwald [44][12]. They proposed using microenvironments to synthesize complex compounds, allowing dimensions, structure, and morphology specificity. These microenvironments correspond to polyelectrolyte capsules, consisting of polyamine hydrochloride (PAH) and polystyrene sulfonate (PSS), which allow permeability to inorganic macromolecules and nanoparticles, depending on the solvent, pH, or ionic strength of the means. Under this methodology, it was possible to perform the photocatalyzed urea synthesis on TiO2 nanoparticles, using CO2 and NO3− ions as a precursor, where polyvinyl alcohol (PVA) fulfills the role of electron donor. Figure 1 shows an assembly scheme for these spherical microreactors.
Figure 1. Schematic illustration of the photocatalytic spherical microreactor assembly. Adapted from D.G. Shchukin et al. [44][12].
D.G. Shchukin and H. Möhwald [44][12] reported a concentration close to 1.1 mM of urea, using spherical microreactors with a 2.2 μm diameter in an aqueous solution saturated with CO2 in the presence of 0.1 M NaNO3, followed by 5 h of irradiation with a 200 W Xg–He lamp. It was possible to observe a slight increase in urea concentration at 1.7 mM when incorporating copper (Cu) nanodeposits on TiO2 particles. A simultaneous photocatalysis was expected both on the TiO2 particles and on the surface of the Cu, which increased the reduction of the NO3− to NH4+ ions, considered the determining step for urea formation. The results of D.G. Shchukin and H. Möhwald [44][12] suggest that the main advantage of using microreactors is controlling reagents in the volume determined for the reaction. Furthermore, the confinement and the ability to control the input of substances give the possibility of modeling biochemical processes of living cells. Later, E.A. Ustinovich et al. [45][13] reported urea synthesis by the photoinduced reduction of CO2 in the presence of NO3− ions, using TiO2 stabilized in perfluorodecalin (PFD:TiO2) emulsions and 2–propanol as electron donor species. A 120 W high–pressure mercury vapor lamp was used as a light source. After one hour of irradiation, the concentration of urea was 0.54 mM and 1.1 mM using PFD:TiO2 and PFD:TiO2/Cu, respectively. The high efficiency and selectivity observed in urea formation, as observed in Table 3, are attributed to a high concentration of CO2 in the oleic phase, granting favorable conditions to stabilize intermediary species, allowing the formation of C–N bonds [46][14].
Table 3.
 Photoproduction ratio of urea and formate from 0.1 M NaNO
3
 solution with CO
2
 saturated using different TiO
2 photocatalysts. Adapted from E.A. Ustinovich et al. [45].
 photocatalysts. Adapted from E.A. Ustinovich et al. [13].
Catalyst Amount of Products (mMh) (a) Urea–Formate Ratio
Urea Formate
TiO2 0.28 0.11 2.5
PFD:TiO
2 0.58 0.15 3.9
PFD:TiO2/Cu 1.12 0.025 45
(a) Calculated from data reported in diagrams.
E.A. Ustinovich et al. [45][13] demonstrated that PFD emulsions could dissolve a considerable amount of O2 and CO2, 45 mL/100 mL and 134 mL/100 mL, respectively, which usually demand high pressures. This methodology allowed the accumulation of different organic compounds, whether substrates, intermediates, or products, favoring light–induced reactions. It is important to note that E.A. Ustinovich et al. [45][13] established that using concentrations higher than 1.0 M NaNO3 as a source of N does not significantly increase urea concentration. Hence, this concentration is the saturation limit value of NaNO3 for solutions used in photocatalyzed urea production. One of the latest publications on photocatalyzed urea production using TiO2 was presented by B. Srinivas et al. [21][15]. The research group studied urea synthesis using KNO3 solutions as a source of N in the presence of 2–propanol or oxalic acid as electron donor, producing CO2 in–situ as a reaction product. After 6 h of UV irradiation, using a 250 W high–pressure mercury lamp, it was possible to obtain urea concentrations close to 0.20, 0.10, and 0.31 mM, using TiO2, iron titanate (Fe2TiO5), and iron titanate supported on proton exchange zeolite Socony Mobil–5 (Fe2TiO5/HZSM–5) as a photocatalyst, respectively. B. Srinivas et al. [21][15] showed that using iron titanates supported in HZSM–5 promotes the separation of photoinduced charges, enhancing urea selectivity. Additionally, zeolites presented high adsorption of CO2 and NH3, which, according to the research, was produced in–situ on the catalyst surface. The condensation for urea formation is facilitated by the adsorption property in the zeolite that inhibits the polymerization of the products. In a recent report by H. Maimaiti et al. [47][16], the photocatalyzed synthesis of urea was performed on carbon nanotubes (CNTs) with Fe/Ti3+–TiO2 composite as catalyst (Ti3+–TiO2/Fe–CNTs). This research group has established that urea synthesis from the simultaneous reduction of N2 and CO2 in H2O is related to the arrangement of Ti3+ sites and oxygen vacancies on the catalyst surface. The Ti3+ sites act as the active center of N2 and oxygen vacancies acts as the active center for CO2 on the Ti3+–TiO2 surface. H. Maimaiti et al. [47][16] state that urea formation is achieved by adsorption and activation, converting the N2 and CO2 molecules into a cyclic intermediate that is transformed into two urea molecules. This research reports a urea yield of 710.1 μmol L−1 g−1 after 4 h of reaction using a 300 W high–pressure mercury lamp as the light source. Although the trend indicates the use of photocatalysts fixed on a surface which enables better control over the reaction solution, and then the separation of the catalyst from the reaction solution, H. Maimaiti et al. [47][16] propose using a catalyst supported on CNTs to improve the dispersion in water. However, the catalysts used have an Fe core, which allows their rapid and simple separation from the reaction solution using an external magnetic field. Table 4 summarizes the highest urea concentration obtained by each reported work, according to the experimental set–up proposed by each author mentioned above. Since the photocatalyzed urea syntheses were performed under different experimental conditions, a direct comparison of the results obtained is limited. Table 4 shows the urea concentration normalized by the irradiation time used in each work. The reaction yields or quantum yields could not be considered for comparing results because these data were not reported in most of the works studied.
Table 4. Amount of urea synthesized by photocatalysis.
Author Catalyst C Source N Source Solvent Illumination Time, h Urea,

mM		 Urea,

mM h		−1
S. Kuwabata et al. [41]S. Kuwabata et al. [9] Q–TiO2/PVPD CO2, sat. NH2OH

0.020 M 		(a) PC 1 5.7 (a,c) 5.7
TiO2/Cu 0.40 0.050 8.0
B.-J. Liu et al. [42]B.-J. Liu et al. [10] Q–TiO2/SiO2 CO2, sat. LiNO3

0.020 M		 H2O 5 2.75 (b) 0.55
D.G. Shchukin et al. [44]D.G. Shchukin et al. [12] Cu/TiO2–PVA–PAH/PSS

(2.2 μm diameter)		 CO2, sat. NaNO3

0.1 M		 H2O 5 1.72 (c) 0.34
E.A. Ustinovich et al. [45]E.A. Ustinovich et al. [13] Cu/TiO2:PFD CO2, sat. NaNO3

1.0 M		 PDF:H2O 1 1.1 (c,d) 1.1
B. Srinivas et al. [21]B. Srinivas et al. [15] Fe2TiO5(10wt%)/HZSM–5 2–propanol

1 		v/v% KNO3

0.016 M		 H2O 6 0.31 (e) 0.052
H. Maimaiti et al. [47]H. Maimaiti et al. [16] Ti3+–TiO2/Fe–CNTs CO2 (100 mL min−1 flow rate) N2 (100 mL min−1 flow rate) H2O 4 0.710 (f) 0.178
(a) Calculated from 3 cm3 volume of propylene carbonate solution used in the cell. (b) Calculated from data reported in charts and considering 5 cm3 as solution volume used in the cell. (c) Estimated value from data reported in charts. (d) Reported in mmol L−1 h−1, the normalized value per hour of irradiation is presented. (e) Converted from ppm to mmol L−1. (f) Converted from μmol L−1 g−1 and considered 1 g of catalyst.
When urea concentration is normalized by hours of irradiation, it is possible to observe how the solution or support used plays an essential role in stabilizing the reagents and intermediate species. For example, PFD emulsions allow the dissolving of high concentrations of gaseous substrates at room pressure, increasing the availability of CO2, favoring urea formation, and stabilizing TiO2 particles. Moreover, using reduced species such as NO or NH2OH as a nitrogen source favors the generation of intermediate species, modifying selectivity by by–products, as S. Kuwabata et al. [41][9] reported. Contrary to what was thought, using supported TiO2 particles, in some cases, favors urea synthesis. Modifying the size of the supported particles allows changing the semiconductor band–gap. It should be considered that reactions on supported catalysts are governed by diffusion phenomena and can be controlled more easily. In addition, the use of support surfaces makes it possible to avoid particle aggregation in solution, reduce irradiation shielding, and prevent reactions depending on particle collision phenomena, allowing, in turn, better adsorption and even increasing the spatial selectivity of reactants and products. The above is critical in scaling these technologies because separating and recovering a suspended catalyst from an aqueous solution using filtration systems or more complex unit operations could increase production costs. The use of catalysts with magnetic properties that could be removed from the reaction solution by applying a magnetic field is a possible solution for the use of suspension catalysts. The confinement of the reaction space in low–scale photoreactors such as microcapsules or microreactors allows the control of the diffusion, input, or output flow of reactants and products and the specificity of the synthesis, stereochemistry, and functionalization of the products formed. The spherical microreactors evaluated produce lower mobility of the compounds, enhancing the interaction between the reactant species and the photocatalyst particles and increasing the concentration of the desired products. A constant flow of the gases used as a source of C and N is a relevant factor. Considering the low solubility of gases in aqueous media and the time necessary to perform the photocatalytic reduction studied, a high and constant concentration of C and N sources is required to optimize the urea production flow.  Even though each experimental set–up proposed has an improvement or an innovation to the photocatalysis technology, there are still many edges to explore in urea synthesis photocatalyzed by TiO2–based materials.

References

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