Selective photocatalytic oxidation of gaseous ammonia to dinitrogen in a continuous flow reactor

Abstract

Commercial titanium dioxide nanomaterials were found to be active in selective photocatalytic oxidation of ammonia to dinitrogen (photo-NH₃-SCO). Photoactivated conversion of 1000 ppm ammonia in simulated air with high selectivity for dinitrogen was achieved over a thin layer of TiO₂ in a flow reactor at 150 °C, 300 h⁻¹ volume hourly space velocity and 1.1 mW cm⁻² UVA illumination.

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Advanced photocatalytic oxidation processes based on TiO₂ are widely investigated in literature.¹ The potential of TiO₂ in photocatalytic elimination of trace amounts of ammonia in connection with purification of ambient air has already been demonstrated.² Such photocatalytic oxidation of ammonia into dinitrogen (photo-NH₃-SCO) at ambient temperature is typically performed by contacting a volume of ammonia bearing gas with an illuminated photocatalyst. Besides ambient air decontamination there are other applications in which ammonia needs to be removed from gas streams. NH₃-SCO is, for example, needed for the clean-up of ammonia slip from selective catalytic reduction of NO_x using NH₃ (NH₃-SCR) in automotive applications. Pt based catalysts are currently being used for achieving NH₃-SCO downstream of NH₃-SCR, but Pt catalysts tend to produce undesired N₂O as a side product.³ There are several types of catalysts that perform NH₃-SCO such as Fe-ZSM-5 zeolites or Ag supported on Al₂O₃.⁴ Often these catalysts are active at high temperatures up to 400 °C. RuO₂/Na-Y zeolite was recently identified as a new low temperature NH₃-oxidation catalyst working already at 175 °C.⁵ In this temperature range photocatalysis may be an alternative way to achieve the NH₃-SCO reaction. Here we report the potential of commercial TiO₂ photocatalysts for NH₃-SCO in a flow reactor using a synthetic gas mixture with traces of ammonia in the presence of oxygen, water and carbon dioxide.

Relevant properties of the investigated commercial TiO₂ photocatalysts are presented in Table 1.

Table 1 Properties of commercial TiO₂ photocatalysts⁶

Catalyst	Anatase content (%)	Specific surface area (m^2 g^−1)	Primary particle size (nm)
P25 (Evonik)	80	50	21
UV100 (Sachtleben Hombikat)	100	250	10
PC 500(Cristal Global)	100	350	9

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The TiO₂ samples were drop cast from a 20/80 wt% H₂O/isopropanol mixture on a glass plate (300 mm × 50 mm).⁷ The weight of the TiO₂ catalyst coated was 30 mg. The glass plate with the TiO₂ catalyst was mounted on a home built flat photoreactor, enabling homogeneous irradiation with a UVA-lamp (Rayonet) at 1.1 mW cm⁻² light intensity from the top through a quartz cover plate. The free space between TiO₂ coated glass and quartz cover plate was ca. 2 mm and the reactor volume 30 cm³. The glass plate with TiO₂ coating was heated from beneath and the temperature monitored with a thermocouple in contact with the TiO₂ layer. The inlet gas had the following composition at 150 °C: 1000 ppm NH₃, 5% O₂, 3% H₂O, optionally 5% CO₂ and N₂. The standard gas flow rate was 150 ml min⁻¹ and corresponded to a VHSV of 300 h⁻¹. Analysis of the reaction products was done on-line via UV spectroscopy for NH₃, NO and NO₂ (ABB Limas 11HW instrument) and NDIR for N₂O, CO and CO₂ (ABB Uras 26 instrument). The selectivity for N₂ was estimated from the nitrogen atom balance.

The ammonia conversion was dependent on the nature of the TiO₂ photocatalyst (Table 2). PC500 was most active achieving 99% NH₃ conversion at 150 °C with 92% selectivity to N₂. P25 and UV100 photocatalysts were less active with 60% and 40% conversion, respectively (Table 2). NO was the main by-product on PC500 and UV100, while P25 favoured NO and NO₂ formation. Lowering the reaction temperature of PC500 to 100 °C and 50 °C lowered the NH₃ conversion to 81% and 23%, respectively, and enhanced the selectivity to NO. In all experiments conversion and selectivity remained constant during 24 h of continuous operation.

Table 2 Photo-NH₃-SCO over commercial TiO₂ samples

Catalyst	Temp (°C)	NH3-conv (%)	Selectivity (%)
			NO	NO2	N2O	N2
Reaction conditions: 1000 ppm NH3, 5% O2, 3% H2O and N2; flow rate: 150 ml min^−1; UVA lamp 1.1 mW cm^−2.
PC500	150	99	8	0	0	92
	100	81	23	0	0	77
	50	23	32	0	0	68
P25	150	60	22	47	3	28
UV100	150	40	20	0	3	77

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Under the investigated reaction conditions without light there was no conversion. When the UV lamp was switched off after 55 min of continuous NH₃-SCO the ammonia conversion dropped immediately confirming that the nature of the measured NH₃ conversion was photocatalytic (Fig. 1). When the UV lamp was switched on again, the NH₃ conversion and the high selectivity for N₂ deduced from the low selectivity for NO_x were quickly re-established (Fig. 1). The formation of nitrate on PC500 was investigated by rinsing the used photocatalyst with water and analysing nitrate content with a dedicated detection kit (Hach Lange LCK 339 test kit). After running the photocatalyst under standard conditions until 150 mg NH₃ was fed, the nitrate content on the catalyst was ca. 0.2 mg. The converted amount of ammonia was significantly higher leading to the conclusion that nitrate formation was negligible.

Fig. 1 Evolution of concentration levels in the reactor outlet of the PC500 photocatalyst at 150 °C: NH₃ (full line), total NO_x (small dots). After 55 min the UV lamp was switched off and at 62 min on again. Gas composition: 1000 ppm NH₃, 5% O₂, 3% H₂O and N₂. N₂O concentration was below the detection limit (1 ppm).

Under the presently investigated photo-NH₃-SCO conditions the best results for the PC500 based TiO₂ material were obtained at 150 °C with 99% NH₃ conversion and formation of only minor amounts of NO (8%). When the VHSV was decreased to 150 h⁻¹ the N₂ selectivity reached 98% at 100% NH₃ conversion. In applications targeting NH₃ elimination such high N₂ selectivity is desired.

Significant amounts of NO (32%) were detected at 50 °C and lower NH₃ conversion levels (Table 2). This observation hints at the formation of N₂via an internal SCR (iSCR) mechanism. First NO is formed by oxidation of NH₃, which reacts with NH₃ to form a nitrosamide intermediate, which is transformed into N₂.² Such a mechanism has been observed on thermal NH₃-SCO catalysts.⁸

Doubling the volume hourly space velocity (VHSV) to 600 h⁻¹ leads to 90% NH₃ conversion and selectivities for N₂ and NO of 90% and 10%, respectively. Increasing the amount of catalyst (up to 60 mg) had only a slight influence on the NH₃ conversion and product distribution.

The ammonia concentration was lowered to 100 ppm and the VHSV varied (Table 3). Lowering the NH₃ inlet concentration to 100 ppm allowed us to perform photo-NH₃-SCO at higher VHSV. However, the N₂ selectivity was lower than in the experiment with 1000 ppm NH₃ and lower space velocity (Table 2). Under these reaction conditions with high space velocity the second step of the iSCR mechanism converting NO with NH₃ into N₂ seems to be the rate limiting step.

Table 3 Photo-NH₃-SCO over commercial PC500 TiO₂ samples with 100 ppm NH₃ as feed

Catalyst	VHSV h^−1	NH3 conv (%)	Selectivity (%)
			NO	NO2	N2O	N2
Reaction conditions: 1000 ppm NH3, 5% O2, 3% H2O and N2; flow rate: 150–1200 ml min^−1; UVA lamp 1.1 mW cm^−2.
PC500	300	100	18	1	0	81
	600	100	10	17	0	73
	1200	100	10	12	0	78
	2400	84	32	47	0	68

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The influence of the presence of CO₂ on photo-NH₃-SCO was investigated. The addition of 5% CO₂ to the gas mixture had no influence on the catalytic activity nor on the selectivity under the optimum reaction conditions (1000 ppm NH₃; 150 °C; VHSV = 300 h⁻¹). Increasing the water concentration in the feed up to 5% and 7% and leaving out water both at 100 and 150 °C had no significant influence on NH₃ conversion and selectivity.

Conversion and selectivity were strongly dependent on the nature of the TiO₂ catalyst. On the P25 catalyst substantial amounts of NO₂ and NO were formed at a NH₃ conversion level of 60% at 150 °C (Table 2). The N₂ selectivity was only 28%. Doubling of the space time on the P25 catalyst led to a higher NH₃ conversion (84%) but hardly there was a change of selectivity. The UV100 catalyst behaved similar to PC500 but was less active (Table 2). P25 has a larger particle size and smaller specific surface area, UV100 is quite similar to PC500 (Table 1) but shows inferior photo-NH₃-SCO activity (Table 2). A possible explanation for this difference between the three catalysts comes from TGA experiments.

The three catalysts were characterized in order to find the reason for the better performance of PC500. TGA (Q500 instrument, TA Instruments) was performed at a heating rate of 5 °C min⁻¹ under N₂ from room temperature to 500 °C (Fig. 2). P25 and UV100 TiO₂ samples showed little weight loss (less than 1 wt% over the whole temperature range). PC500 showed a much larger weight loss (ca. 12 wt%) ascribed to desorption of physisorbed water. From the TGA trace it can be expected that at the reaction temperature of 150 °C at which photo-NH₃-SCO was performed more water will be adsorbed on the surface of PC500 compared to P25 and UV100.

Fig. 2 TGA profiles of PC500 (full line), P25 (dashed line), and UV100 (dots) TiO₂ samples. Conditions: N₂ gas stream and heating rate 5 °C min⁻¹.

The ammonia adsorption capacity of the three photocatalysts was determined by saturation of catalyst powder in a packed bed under a gas stream with 1000 ppm NH₃ in N₂ with and without water at 150 °C (Table 4). On weight basis PC500 has about 10 times higher ammonia adsorption capacity compared to P25 and UV100. In competition with water the ammonia adsorption capacity is lower but the adsorption capacity order is maintained (Table 4).

Table 4 NH₃ adsorption capacity (mmol g⁻¹) of photocatalysts in the presence and absence of water

Water content of gas mixture (%)	P25	UV100	PC500
0	0.45	0.37	3.29
3	0.18	0.08	1.31

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The most active PC500 photocatalyst has the highest adsorption capacity for ammonia and water. Photocatalytic activity seems to be dependent on ammonia adsorption capacity.

Adsorption and photocatalytic degradation were performed in a two step cycle as follows. PC500 was first saturated with ammonia in the absence of oxygen for 15 min and subsequently photooxidized by feeding oxygen containing gas without ammonia for another 15 min (Fig. 3). The cycle was repeated 6 times (Fig. 3).

Fig. 3 Cyclic operation of the PC500 photocatalyst: 15 min periods of NH₃ adsorption in the absence of O₂ (A) alternated with periods in the presence of O₂ without NH₃ (B). There was continuous illumination. NH₃ (full line), total NO_x (dots). T = 150 °C; 1000 ppm NH₃ in A; 5% O₂ and 3% H₂O in B, VHSV = 300 h⁻¹.

The absence of NH₃ in the reactor outlet in the first minutes of the adsorption phases (A) revealed that ammonia was stored on the photocatalyst. After saturation of the NH₃ adsorption sites on the titania catalyst the outlet NH₃ concentration regained the inlet value of 1000 ppm. Thereafter oxygen was admitted and the NH₃ supply interrupted (B). The NH₃ concentration quickly dropped to very low amounts and only trace amounts of NO (up to 200 ppm) were detected in the reactor outlet. The amount of NH₃ adsorbed estimated from the missing NH₃ in the reactor outlet during adsorption was significantly larger than the amount of NO_x produced during the oxidation phase. N₂O formation was below the detection limit. The undetected N-atoms were assumed to be transformed into N₂. The ammonia adsorption and photooxidation cycle was repeated 6 times (Fig. 3). As the cycles were fairly reproducible, this experiment showed that NH₃ can be stored on the photocatalysts at 150 °C and converted into N₂ upon admission of O₂ under constant illumination.

Conclusions

Commercial TiO₂ samples were found to be active in photocatalytic NH₃ oxidation (photo-NH₃-SCO). At relatively low temperature of 150 °C nearly complete conversion of NH₃ into N₂ was obtained on 30 mg of the Cristal Global PC500 TiO₂ photocatalyst spread on glass, illuminated with UVA at 1.1 mW cm⁻² and using a space time of 300 h⁻¹. Evidence was provided that photo-NH₃-SCO proceeds via the iSCR mechanism. The superior activity of Cristal Global PC500 is ascribed to its high adsorption affinity for ammonia. The here presented photo-NH₃-SCO is a low temperature process to remove ammonia from a gas stream containing O₂, CO₂ and H₂O and could be applied, for example, to NH₃ clean-up from intensive livestock farming or to exhaust gas aftertreatment systems.

Acknowledgements

S.H. is grateful to FWO-Vlaanderen (Research Foundation-Flanders) for a PhD research grant. This work is supported by long-term structural funding by the Flemish Government (Methusalem Funding). Cristal Global is acknowledged for providing the PC500 sample.

Notes and references

1. A. Di Paola, E. GarcíaLópez, G. Marcí and L. Palmisano, J. Hazard. Mater., 2012, 211–212, 3; S. Devahasdin, C. Fan, K. Li and D. H. Chen, J. Photochem. Photobiol., A, 2003, 156, 161; A. Folli, S. B. Campbell, J. A. Anderson and D. E. Macphee, J. Photochem. Photobiol., A, 2011, 220, 85; S. Poulston, M. V. Twigg and A. P. Walker, Appl. Catal., B, 2009, 89, 335; Y. Ishibai, J. Sato, S. Akita, T. Nishikawa and S. Miyagishi, J. Photochem. Photobiol., A, 2007, 188, 106.
2. S. Yamazoe, Y. Hitomi, T. Shishido and T. Tanaka, Appl. Catal., B, 2008, 82, 67; S. Yamazoe, T. Okumura and T. Tanaka, Catal. Today, 2007, 120, 220; K. Teramura, T. Tanaka, S. Yamazoe, K. Arakaki and T. Funabiki, Appl. Catal., B, 2004, 53, 29; S. Yamazoe, T. Okumura, K. Teramura and T. Tanaka, Catal. Today, 2006, 111, 266; K. Teramura, T. Tanaka and T. Funabiki, Langmuir, 2003, 19, 1209.
3. A. Scheuer, W. Hauptmann, A. Drochner, J. Gieshoff, H. Vogel and M. Votsmeier, Appl. Catal., B, 2012, 111–112, 445.
4. R. Q. Long and R. T. Yang, Chem. Commun., 2000, 1651; R. Q. Long and R. T. Yang, J. Catal., 2001, 201, 145; D. P. Sobczyk, E. J. M. Hensen, A. M. de Jong and R. A. van Santen, Top. Catal., 2003, 23, 109; S. D. Lin, A. C. Gluhoi and B. E. Nieuwenhuys, Catal. Today, 2004, 90, 3; L. Zhang, C. Zhang and H. He, J. Catal., 2009, 261, 101; L. Gang, B. G. Anderson, J. van Grondelle and R. A. van Santen, Catal. Today, 2000, 61, 179.
5. S. Heylen, N. Delcour, C. E. A. Kirschhock and J. A. Martens, ChemCatChem DOI:10.1002/cctc.201100489 , accepted for publication.
6. J. Arana, A. Pena Alonso, J. M. Dona Rodriguez, G. Colon, J. A. Navio and J. Perez Pena, Appl. Catal., B, 2009, 89, 204.
7. S. W. Verbruggen, S. Ribbens, T. Tytgat, B. Hauchecorne, M. Smits, V. Meynen, P. Cool, J. A. Martens and S. Lenaerts, Chem. Eng. J., 2011, 174, 318.
8. G. G. Ramis, G. Busca, V. Lorenzelli and P. Forzatti, Appl. Catal., 1990, 64, 243; N. Y. Topsøe, H. Topsøe and J. A. Dumesic, J. Catal., 1995, 151, 226.

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