Introduction

Ever increasing demand for the reduction of greenhouse gas results in the more stringent regulation on its emission and also the corresponding research and development to capture or convert into inert molecule1. Compared to that of a major greenhouse, CO2, N2O has a high greenhouse gas effect up to 300 times2. Therefore, the impact of N2O emission can be comparable to that of CO2 though the emission concentration of N2O is relatively low. Most recent diesel engine emission regulation is now started to include N2O because of its high impact and stability in stratosphere3. For diesel engine emission control under lean condition, urea SCR (selective catalytic reduction) system is the state of art technology for the reduction of NOx in most engine companies4. Under lean condition where the air to fuel ratio is far beyond the stoichiometric condition, the N2O formation can be suppressed readily while the system has been maintained under oxidizing condition. However, the emission of N2O from diesel engine can be increased when the reducing agent for NOx is introduced in the SCR system following the reactions, such as 2NH3 + 2NO + O2 = N2O + N2 + 3H2O, 2NH3 + 2O2 = N2O + 3H2O and NH4NO3 = N2O + 2H2O3. The former two reactions were believed to be the major pathway for N2O formation in which bimolecular reaction can occur.

For NOx abatement, VOx catalyst supported on TiO2 has been used widely in most diesel engines5,6,7,8. There are numerous investigations on the improvement of the catalytic performance using additives such as Ce or W and also using peculiar TiO2 synthesized using sol-gel method or organic or inorganic templating method9,10,11,12,13,14,15. However, VOx supported on TiO2 of relatively low surface area is the state of art technology catalyst. Indeed the current VOx/TiO2 catalyst emits N2O when the reducing agent is present in the stream. The current emission level of N2O is 50 mg per mile, which also depends on the catalyst composition and the system configuration such as diesel oxidation catalyst-selective catalytic reduction-diesel particulate filter3. The N2O emission characteristics of the VOx/TiO2 catalyst should be improved under reducing condition. For this purpose, the economically viable catalyst has to be developed in near future.

It has been demonstrated that the hydrothermal conversion of commercially available TiO2 in the presence of alkaline hydroxide produces unique TiO2 structures differently depending on the species of alkaline hydroxide16,17,18,19. Recently, the addition of LiOH, NaOH and KOH to the hydrothermal medium was reported to be resulted in the formation of zeolitic microporous TiO2, nanotube and nanorod, respectively19,20,21, which seems to be a cost effective process. The obtained microporous nanocrystalline TiO2 showed large surface area of 250 m2g−1 with the pore volume of 0.15–0.20 ccg−1, which was similar to those of zeolites and also suitable for catalyst preparation.

The formation mechanism of TiO2 nanotube by hydrothermal synthesis in the presence of NaOH has been studied extensively but it is not clarified yet16,22. Initial work on the formation mechanism of TiO2 nanotube suggested that the nanotube is obtained with an acid washing and subsequent Na+ ion exchange after the formation of amorphous TiO2 during hydrothermal reaction between NaOH and bulk TiO216,20,22,23,24. The other mechanism proposed that the bond breaking of 3-dimensional TiO2 structure formed layered 2-dimensional structure and finally 1-dimensional nanotubes through sheet folding mechanism22. The removal of Na+ cation from the nanotube also deteriorates readily the thermal stability when it is heated at high temperature.

However, the use of zeolitic microporous TiO2 prepared from alkaline condition has not explored yet. Thus, it is interesting to know whether the zeolitic microporous TiO2 has a high thermal stability or not and also is suitable for catalytic application as a substrate. For the first time in the present work, the zeolitic microporous nanocrystalline TiO2 has been demonstrated as a catalyst support for VOx over selective catalytic reduction of NOx using ammonia in order to decrease the N2O formation.

Results

Figure 1 shows the scanning electron micrographs and transmission electron micrographs of the zeolitic microporous TiO2 after the hydrothermal treatment. The morphology of the particle after the hydrothermal conversion contained sharp edges, suggesting the formation of the well-crystallized TiO2, which was consistent with the literature19,20,21. The obtained TiO2 had the typical argon adsorption-desorption isotherm consistent with the Langmuir isotherm type containing micropore mostly where the micropore area estimated from t-plot was ~200 m2g−1, corresponding to 80% of the total surface area of which the pore size was estimated to be ~7 Å. The corresponding surface area and pore volume were controlled to be 250 ± 20 m2g−1 and 0.20 ± 0.05 ccg−1, respectively, depending on the hydrothermal reaction condition. Also, the presence of the mesopore was observed above P/P0 > 0.9 but its portion can be decreased with the increase of hydrothermal reaction time. Such mesopore formation was also shown clearly in Fig. 1(c) where the mesopore was formed with several interconnecting crystalline TiO2 frameworks of which the thickness was 3–4 nm.

Figure 1
figure 1

Morphology of the zeolitic microporous TiO2.

(a) Scanning electron micrographs, (b,c) transmission electron micrographs and (d) the argon adsorption-desorption isotherm of the zeolitic microporous TiO2 with the pore size distribution (inset) obtained from Horvath-Kawazoe method. The solid and open symbols indicate the adsorption and desorption branch, respectively.

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Therefore, the results of the transmission electron micrograph observation supported the corresponding unique TiO2 structure containing the micropore and also the mesopore of which the size was 5–7 nm, which was consistent with the result of argon and nitrogen adsorption-desorption measurement.

The hydrothermal conversion of the bulk TiO2 into zeolitic microporous TiO2 in the presence of LiOH seems not to follow the sheet folding mechanism like TiO2 nanotube. The intercalation of Li+ ion into the TiO2 structure leads to the formation of the Li+-O-Ti bond similar to that of Na+ case, resulting in the partial delamination of the TiO2 layer where the interaction between the layers is high enough to induce the combination of the corresponding layers25,26.

The thermal stability of the zeolitic microporous TiO2 under ambient condition either with the presence or absence of the saturated water was also investigated using the X-ray diffraction pattern (XRD) and N2 adsorption and desorption isotherms as a function of heating temperature. Nearly up to 773 K, the microporous structure was retained as evidenced from the results of the nitrogen adsorption-desorption isotherm for the sample shown in Fig. 2 even though the microporous structure was collapsed significantly with the increase of temperature in the presence of water. The XRD of the sample also showed that the crystalline anatase structure appeared to be the major phase when the samples were heated above 773 K. The combined results from XRD and nitrogen adsorption-desorption isotherm confirmed the transformation of microporous TiO2 to macroporous TiO2. The moderate thermal stability of the zeolitic microporous TiO2 due to the crystal growth into anatase in the presence of water may limit the catalytic application at higher temperature. However, many catalytic applications including photocatalysis or solar energy harvesting can adopt the present zeolitic microporous TiO221.

Figure 2
figure 2

Phase and surface properties of the zeolitic microporous TiO2.

X-ray powder diffraction pattern of the zeolitic microporous TiO2 heated at (a,e) 673 K, (b,f ) 773 K, (c,g) 873 K and (d,h) 973 K for 4 h, respectively, in ambient air with (upper right panel) and without saturated water (upper left panel). The tick mark corresponded to anatase phase. Nitrogen adsorption-desorption isotherm for the zeolitic microporous TiO2 heated at (○,•) 673 K, (▽,▾) 773 K, (□,■) 873 K and (◇, ◆) 973 K, respectively, in ambient air with (lower left panel, A) and without saturated water (lower right panel, B). The open and solid symbols indicate the adsorption and desorption branch, respectively.

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VOx was supported onto the corresponding unique TiO2 structure following the procedure reported in the literature27,28,29,30,31,32. However, the supporting VOx catalyst up to 5 wt% resulted in the lower surface area of 105 m2g−1 with 0.19 ccg−1 because of the high calcination temperature at 773 K though the sample still contained large surface area. These textural properties were maintained before and after the catalytic reaction measurement. The TEM observation of VOx incorporated TiO2 as shown in Fig. 3 suggested that most VOx particle are located inside the pores of the zeolitic microporous TiO2, ~7 Å without changing the corresponding morphologies. Thereby, the VOx particle was observed clearly as a spot in Fig. 3(c) while there was no particle on the external surface of the TiO2 though the elemental analysis showed the presence of V in the same region as shown in Fig. 3(d), which can be beneficial for the catalytic reaction. Further, increasing V content in the present also did not alter the location of V.

Figure 3
figure 3

VOx encapsulated in the zeolitic microporous TiO2.

Transmission electron micrographs of the microporous TiO2 containing 5 wt% heated at (a,c) 673 K and (b) 773 K, respectively. The elemental analysis in (d) showed the presence of V.

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Figure 4 shows the catalytic performance of the V/TiO2 catalyst after the calcination at different temperatures also under various reaction conditions. Increasing V content in the catalyst improved the catalytic performance of the NOx reduction comparable to that of conventional V/TiO2 catalyst containing 5 wt% V over the whole reaction temperature range while the N2O formation was much lower than that of conventional catalyst. The N2O formation from the present V/TiO2 was increased with the increase in the temperature where NH3 was combined with NOx to produce N2O. The effect of the calcination temperature was also pronounced to increase the N2O formation at high temperature but the N2O formation was still lowered than that of the conventional catalyst by more than 80%. Also, the V/TiO2 catalyst prepared from the microporous TiO2 resulted in the superior catalytic performance over the SCR reaction both in the presence of water in the reactant stream and after the aging in the presence of water at 773 K for 12 h, as shown in Fig. 4(c,d). In the presence of water in the stream the low temperature catalytic activity was decreased while the high temperature catalytic activity was increased slightly because of competitive adsorption of NH3 and H2O suppressing the NH3 oxidation.

Figure 4
figure 4

SCR activity of VOx encapsulated in the zeolitic microporous TiO2.

Catalytic activity of V/TiO2 catalyst calcined at (a) 673 K and (b) 773 K for over NOx reduction using ammonia: (○,•) 1wt%, (▽,▾) 3 wt% and (□,■) 5 wt%. The long and short dashed lines also represent the N2O concentration and NOx conversion from conventional V/TiO2, respectively. The open and solid symbols were corresponded to NOx conversion and N2O formation, respectively. The dashed line was the catalytic performance of commercial V/TiO2. The reactant consisting of 500 ppm NO, 500 ppm NH3, 2% O2 balanced with N2 was flowed through the V/TiO2 catalyst bed containing 0.15 g at GHSV = 40,000 h−1. The catalytic activity of (c) 5 wt% V/TiO2 catalyst and (d) conventional V/TiO2 catalyst calcined at 773 K were measured in the different reaction conditions : (○,•) dry reaction condition, (▽,▾) wet reaction condition and (□,■) dry reaction condition with hydrothermal aging at 773 K for 12 h in the presence of 5% water. In order to achieve the wet condition, the reactant consisting of 500 ppm NO, 500 ppm NH3, 2% O2 and 3% H2O balanced with N2 was flowed through the V/TiO2 catalyst bed at GHSV = 40,000 h−1.

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Under the present condition, the main reaction for N2O formation is believed to be 2NH3 + 2NO + O2 = N2O + N2 + 3H2O following the literature3. The catalytically active VOx inside the pore, ~7 Å was believed to have a strong metal support interaction with TiO2, resulting the smaller VOx particle size as referred from Fig. 3. Thus, the strong metal-support interaction between TiO2 and VOx led to the formation of Bronsted acid site with high strength, which is beneficial for selective catalytic reduction of NO by NH3. The N2O formation can be suppressed up to ~80% because of the increased Bronsted acidity of the VOx small particle in the microporous zeolitic TiO2 where the superior SCR activity can be maintained as illustrated in Fig. 4(c). This result was partly consistent with the increased N2O formation on the V/TiO2 catalyst when the catalyst deteriorates because of the sintering3. Also, it was possible to include NH3 oxidation by O2 as potential pathway for the following N2O formation reaction: 2NH3 + 2O2 = N2O + 3H2O where the catalyst deactivation was severe like the commercial V/TiO2 catalyst in the presence of water or after hydrothermal aging. One possibility to explain the superior catalytic performance V supported on zeolitic microporous TiO2 over the SCR reaction was that the growth of the vanadium oxide particle size can be limited due to the pore size, implying the encapsulation of vanadium oxide particle surrounded by TiO2 matrix.

We have demonstrated that the zeolitic microporous TiO2 with moderate thermal stability can be prepared from the simple hydrothermal conversion from commercially available bulk TiO2 of low grade, 98% or lower in the presence of LiOH at 400–440 K, which can be scaled up easily for industrial process. The obtained zeolitic microporous nanocrystalline TiO2 contains the micropore up to 80% referred from the t-plot method. For the first time, it was proved that that the supporting VOx into such zeolitic microporous TiO2 resulted in the high NOx reduction activity with lower N2O formation, which was ascribed to the location of catalytically active VOx particles in the microporous TiO2, resulting the strong metal-support interaction and consequently the increased Bronsted acidity. Therefore, the zeolitic microporous TiO2 has potential as a substrate for the SCR reaction below 773 K while the thermal stability of the microporous TiO2 was retained.

Methods

Synthesis of zeolitic microporous TiO2

TiO2 anatase (Aldrich, 98%) of 2–8 g was added to the solution containing 10 M or more LiOH in the Teflon lined autoclave for hydrothermal heating at 400–440 K for 72 hr under rotating condition at 40 rpm. After cool down to room temperature, the slurry was neutralized with 0.1 N HCl under stirring for 6 hr. The solution was filtered and washed with deionized water thoroughly. The acidification and filtration was repeated three times to remove the residual trace metal hydroxides. The obtained product was dried at 330 K in an oven and calcined under flowing oxygen at 673 K for 4 h. The inductively coupled plasma analysis of the obtained sample showed that the residual Li was ~6 ppm level, indicating the complete removal of Li+ by the neutralization and subsequent thorough washing. The scale up to ~100 g per batch was also demonstrated to give the same textural properties.

Preparation of VOx in zeolitic microporous TiO2

All catalysts were prepared by applying wet impregnation of vanadium precursor solution on titania. Ammonium metavanadate (99%, Sigma Aldrich) was dissolved in diluted oxalic acid solution (0.5 M) to produce the solution of vanadium precursor. Anatase TiO2 powder (DT-51 Millennium Chemicals) was used as support to prepare the conventional catalyst containing 5 wt% V. The samples with 1 wt%, 3 wt% and 5 wt% V2O5 loading on TiO2 were prepared. After impregnation process in a rotary evaporator, catalysts were dried and then calcined at 673 K or 773 K for 4 h in air.

SCR activity measurement of VOx in zeolitic microporous TiO2

SCR activity was measured in a fixed-bed quartz tubular reactor. Catalysts were sieved to 300–500 μm in diameter then loaded in the reactor. 500 ppm NO, 500 ppm NH3, 2% O2 and balanced with N2 were introduced as reactants. In order to examine the catalytic activity in the presence of water, the reactant containing 500 ppm NO, 500 ppm NH3, 2% O2, 3% H2O balanced with N2 was used. The catalyst was further aged in the presence of 10% O2, 5% H2O balanced with N2 at 500 oC for 12 h before catalytic reaction.

Space velocity of inlet gas was maintained to be 40,000 h−1. We raised reaction temperature from 423 K to 673 K by 50 K. NOx concentration of outlet gas by using NOx chemiluminescence analyzer (Model 42i High level, Thermo Scientific). Also, Fourier Transform Infrared (FT-IR) spectroscopy was applied to observe the N2O concentration in the gas. We used the average data of 16 scans at a resolution of 1.0 cm−1. A Nicolet 6700 (Thermo Scientific) with 2 m gas analysis cell heated to 120 °C to exclude the effect of H2O, was used for gas phase analysis.

Additional Information

How to cite this article: Lee, S. G. et al. Suppressed N2O formation during NH3 selective catalytic reduction using vanadium on zeolitic microporous TiO2. Sci. Rep. 5, 12702; doi: 10.1038/srep12702 (2015).