DRIFT Study on Promotion Effect of the Keggin Structure over V 2 O 5 -MoO 3 /TiO 2 Catalysts for Low Temperature NH 3 -SCR Reaction

: Heteropoly acids (HPAs) with the Keggin structure have been widely used in NO x removal. Two kinds of catalysts (those with and without the Keggin structure) are prepared for studying the effect of the Keggin structure on the NH 3 -SCR reaction. A series of in situ diffuse reﬂectance infrared Fourier transform spectroscopy (DRIFT) analyses are conducted to investigate the surface-adsorbed species on the catalysts during the SCR reaction. The mechanism for enhancing low-temperature activity of the catalysts is proposed. Furthermore, the effect of NH 4+ in the Keggin structure is also investigated. Results indicate that both the Langmuir–Hinshelwood (L-H) and Eley–Rideal (E-R) mechanisms occurred in the NH 3 -SCR reaction over the catalyst with the Keggin structure (Cat-A); in addition, when more acid sites are provided, NO x species activity is improved and more NH 4+ ions participate in reaction over Cat-A, thus promoting SCR activity.


Introduction
NO x is an air pollutant that can result in a variet of pollution, such acid rain, optical chemical smog, and the greenhouse effect.NO x is emitted when using fossil fuels in industrial facilities and automobiles [1,2].Selective catalytic reduction (SCR) has been extensively used to remove NO x emissions from stationary sources such as coal-fired boilers [3].The commercialized catalyst used in SCR has mainly been V 2 O 5 -WO 3 /TiO 2 (or V 2 O 5 -MoO 3 /TiO 2 ), which has high activity and good SO 2 tolerance.However, it has a very narrow operating temperature (300-400 • C), high conversion of SO 2 to SO 3 , and low N 2 selectivity at high temperatures [4,5].Therefore, developing an NH 3 -SCR catalyst with good catalytic performance at low temperature is desirable.
Heteropoly acids (HPAs) have unique structure and extraordinarily strong acidity, and, as such, HPAs have attracted the attention of many researchers [6,7].The main kind of HPAs used in catalytic applications is Keggin HPAs, such as H 3 PW 12 O 40 , H 3 PMo 12 O 40 , and H 3 SiW 12 O 40 [8].It has been found that NO can be effectively absorbed on 12-tungstophosphoric acid at 150 • C and decomposed at about 450 • C upon rapid heating.It has been reported that small polar molecules (such as NO, NH 3 , and H 2 O) can access the secondary structure of Keggin HPAs and that SO 2 cannot influence adsorption of NO.Supplementary experiments showed that NO can replace the structural water present between Keggin units of HPAs [9,10].Later, some researchers used aromatic hydrocarbons loaded with Pt and Pd to improve the activity of NO reduction [11,12].Putluru et al. reported that HPAs can promote SCR activity and shows excellent alkali deactivation resistance [13].Weng et al. used 12-tungstaphosphoric acid loaded on CeO 2 and found that the catalyst had significantly improved performance in the SCR reaction and improved SO 2 poisoning resistance [14].Ammonium salts are more available than HPAs, and using 12-tungstophosphoric acid ammonium salt for NO x removal has been reported.Moffat et al. found an interesting phenomenon; specifically, that the ammonium salt of 12-tungstophosphoric acid exhibited better NO x removal capacity in the case without NH 3 because bound NH 4 + on the solid can react with absorbed NO 2 to produce N 2 , and ammonium salt can be regenerated from gaseous NH 3 [9].However, there are few reports about using ammonium salts of HPAs for SCR, and further research on the effect of the Keggin structure and that of the NH 4 + of ammonium salts on SCR is lacking.
In this research, V 2 O 5 -(NH 4 ) 3 PMo 12 O 40 /TiO 2 and a conventional oxide catalyst are prepared for studying the effect of the Keggin structure on the NH 3 -SCR reaction.A series of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analyses are conducted to investigate surface-adsorbed species.Fourier Transform infrared spectroscopy (FT-IR) is used to evaluate the effect of NH 4 + of ammonium salts on the catalysts.

Catalytic Activity
Catalytic performances of the catalysts with and without the Keggin structure at elevated temperature are shown in Figure 1.Structures of the two samples were determined using X-ray diffraction (XRD) and Raman spectroscopy (Figures S1 and S2, respectively).X-ray Fluorescence Spectrometer (XRF) results of elemental analysis and the BET specific surface area data are shown in Table S1.Results show that Cat-A, which has the Keggin structure, had the same composition and BET surface area as Cat-B, which has an oxide phase.As shown in Figure 1, Cat-A had much higher SCR activity than Cat-B.NO conversion over Cat-A reached 93% at 220 • C and showed no obvious decline until 350 • C. In contrast, NO conversion over Cat-B was only 67% at 220 • C. Clearly, the catalyst with the Keggin structure had improved NO conversion in low temperature.Both catalysts had excellent N 2 selectivity of 99% below 250 • C. The time-on-stream stability was investigated (Figure S5).The NO x conversion over Cat-A decreased from 100% to 95% in 20 h, while, for Cat-B, it decreased from 99% to 92%.Therefore, Cat-A exhibited better time-on-stream stability than Cat-B.As seen in Figure 2A, there were six bands: 1211, 1391, 1454, 1596, 1685, and 1737 cm −1 .The peaks centered at 1454 cm −1 and in the range of 1750-1650 cm −1 can be, respectively, attributed to asymmetric and symmetric bending (δ as and δ s ) vibrations of NH 4 + species on Brönsted acid sites [15][16][17].
The bands at 1211 and 1391 cm −1 can be assigned to N-H bonds in NH 3 that is chemisorbed on Lewis acid sites [18].) and δ as NH 3 species (1605 cm −1 ) on Lewis acid sites were also observed [20,21].It is evident that the intensity of the band assigned to Lewis acid sites (1230 cm −1 ) is much weaker than that over Cat-A.It is known that HPAs have stronger (Brönsted) acidity, but it was reported that (NH 4 ) 3 PMo 12 O 40 provides Lewis acid sites that were strong at high temperature because of the oxygen-deficient Keggin structure [22].The NH 3 -desorption peaks in the temperature programmed desorption of ammonia (NH 3 -TPD) was interconnected to the concentration of acid sites [23].Combined with the results for NH 3 -TPD (Figure S3), the peak area of Cat-A was more than Cat-B, which suggested that the concentration of Lewis acid sites was more than Cat-B.As reported, the high amount of Lewis acid site could improve the low temperature activity [24].Therefore, the Keggin structure not only provides more Brönsted acid sites, but also offered Lewis acid sites that are beneficial for low-temperature SCR catalytic activity.

Co-Adsorption of NO + O 2 on the Surface of the Catalysts
Figure 3 shows DRIFT results of NO + O 2 co-adsorption on the surface of Cat-A and Cat-B.Adsorbed species on the two catalysts are clearly very different.As seen in Figure 3A, there are several distinct bands at 1347, 1513, 1640, 1668, 1786, 1751, 1860, and 1976 cm −1 .A broad band at 1347 cm −1 appeared first and can be ascribed to free nitrate ions [25].Later, smaller bands at 1513, 1640, and 1668 cm −1 were observed, and these can be assigned to bidentate nitrate, adsorbed NO 2 , and adsorbed N 2 O 4 , respectively [20,26].Bands at 1751 and 1786 cm −1 can be ascribed to adsorption of trans-(NO) 2 [25,27].A weaker band at 1860 cm −1 can be assigned to surface bound NO [28].The band at 1976 cm −1 increased with time and can be ascribed to NO in the secondary structure of Keggin anions [28].For Cat-B (Figure 3B), a series of peaks can be ascribed to monodentate nitrates (1317 and 1490 cm −1 ), free nitrate ions (1340 cm −1 ), and adsorbed NO 2 (1647 cm −1 ) [29,30].For Cat-B, there were fewer kinds of adsorbed NO x species than for Cat-A.Over Cat-B, there were more monodentate nitrates than free nitrate ions.It is reported that, for NSR, NO x was primarily trapped by the metal ions (such as K or Li) in free nitrate ions, thus leading to increased trapping capacity of NO x [31,32].It is likely that NH 4 + in Cat-A plays the same role in trapping NO x , and this indicates that the NO x storage behavior of Cat-A is better than that of Cat-B.The catalysts were pre-treated with NO + O 2 at 250 • C for 60 min, and then NH 3 was introduced to investigate the reaction of NH 3 with nitrate species over the catalysts.As seen in Figure 5A, several peaks that correspond to nitrate species were found in the spectra after treatment with NO + O 2 .With the introduction of NH 3 , the amounts of free nitrate ions (1347 cm −1 ), bidentate nitrates (1513 cm −1 ), NO 2 (1640 cm −1 ), and N 2 O 4 (1668 cm −1 ) decreased quickly within 1 min.Surface NO (1840 cm −1 ) and NOH + (1976 cm −1 ) in the Keggin structure gradually decreased in 5 min, implying that NO in the secondary structure of Keggin anions participates in the reaction.However, the peak at 1786 cm −1 , which corresponds to trans-(NO) 2 , is still present after purging with NH 3 for 30 min, and this suggests that the Eley-Rideal (E-R) mechanism occurred [26].For Cat-B (Figure 5B), a series of peaks that correspond to nitrate species were observed in the spectra.After NH 3 was introduced for 1 min, NO 2 (1647 cm −1 ) disappeared, and this can enhance the process of the "fast-SCR" reaction.Compared with Cat-A, free nitrate ions of Cat-B decreased more slowly and monodentate nitrates disappeared gradually within 5 min.It has been reported that monodentate nitrates and NO 2 molecules had higher activity in the SCR reaction than free nitrate ions.In our results, free nitrate ions for Cat-A, which had the Keggin structure, also easily participated in the reaction, and this was possibly because NO x was trapped by the NH 4 + of the Keggin structure in free nitrate ions, such as in the case of NO x storage/reduction (NSR) catalysts where NO x can easily be trapped.If NH 4 + of the Keggin structure participates in the reaction, the SCR reaction on Cat-A would continue longer than that on Cat-B without NH 3 .To verify this assumption, NO conversion of Cat-A and Cat-B after cutting off NH 3 was investigated, and the results are shown in Figure 6.
As seen in Figure 6, NO conversions on the two catalysts decreased after cutting off NH 3 .For Cat-A, NO conversion remained 100% for 14 min, whereas NO conversion over Cat-B kept remained 100% for only 4 min.Thus, it is obvious that Cat-A, which has the Keggin structure, had higher NH 3 storage than Cat-B, and this suggests that NH 4 + of the Keggin structure participates in the reaction.

Reaction Mechanism
From DRIFT spectra for NH 3 adsorption on the two samples, it is observed that NH 3 was adsorbed on Brönsted and Lewis acid sites in the forms of NH 4 + and NH 3 , respectively.After NO + O 2 was passed over the catalyst surface that was pretreated with NH 3 , the adsorbed NH 3 species gradually vanished in 10 min, and this indicates that both Brönsted and Lewis acid sites were active centers.
As shown in DRIFT spectra of NO + O 2 adsorption, NO was adsorbed and oxidized to NO 2 and other nitrate species on the two samples.There were more kinds of NO x species adsorbed on Cat-A than on Cat-B.Different results were observed when NH 3 was passed over the surface of each of the two samples that were pretreated with NO + O 2 at 250 • C. Compared with Cat-B, not all of the adsorbed NO x species participated in the reaction over Cat-A, and most of the adsorbed NO x species quickly vanished in 1 min, which indicates that both the E-R and L-H mechanisms occurred [35][36][37][38].Noticeably, the peaks of NOH + disappeared after 5 min, and this suggests that NO in the secondary structure of the Keggin anions (NOH + ) also participated in reaction.The effect of NH 4 + in the Keggin structure for the SCR reaction was investigated, and the results reveal that NH 4 + of the Keggin structure participates in the SCR reaction as an NH 3 pool and can be regenerated from ammonia gas.Based on the above discussion, a mechanism for improving the low-temperature activity of Cat-A was proposed; the proposed mechanism is shown in Figure 8 and is described as follows.Both the E-R and L-H mechanisms occurred in the NH 3 -SCR reaction.For the E-R mechanism, NH 3 was adsorbed on Lewis acid sites and reacted directly with gaseous NO and NO 2 .This was followed by decomposition into N 2 and H 2 O.For the L-H mechanism, NO was adsorbed, and then most of it was oxidized to nitrate species (NO 3 − and NO 2 -adsorbed), which reacted with NH 3 -adsorbed species.It is noted that the catalyst with Keggin structure had more Brönsted acid Lewis acid sites than the catalyst with oxide phase.In addition, NH 4 + of the Keggin structure reacted by the L-H mechanism and was regenerated during the reaction; thus, low-temperature SCR activity was promoted via the L-H mechanism.
For Cat-B, the mechanisms were similar to Cat-A.Both the E-R and L-H mechanisms also occurred in the reaction.For the E-R mechanism, gaseous NO and NO 2 could react with NH 3 on Lewis acid sites.For the L-H mechanism, NO could be adsorbed and then oxidized to nitrate species.Most of the nitrate species were monodentate nitrates and NO 2 -adsorbed not free NO 3 − , and the nitrate species could react with NH 3 -adsorbed species.The results were in agreement with many SCR catalysts with oxide phase [25,26,39].

Catalyst Preparation
Two kinds of catalysts were prepared via impregnation, and the main chemical compositions were the same.Specifically, (NH 4 ) 3 PMo 12 O 40 loading was 20 wt % and V 2 O 5 loading was 1 wt %.
The catalyst with the Keggin structure was prepared as follows.NH 4 H 2 PO 4 (0.6 g, Fuchen, Tianjin, China, 99%) and (NH 4 ) 6 Mo 7 O 24 (11.5 g, Fuchen, Tianjin, China, 99%) were dissolved in distilled water (100 mL).Solution pH was adjusted to a value of about 1. TiO 2 anatase powder (40.0 g, Xinhua, Chongqing, China, 99%) was added to the precursor solution and solid to liquid ratio (g/mL) was 2:5.The mixture was stirred at 80 • C for 5 h and then dried at 120 • C for 3 h.The solid was ground into a powder and calcined at 400 • C for 5 h.NH 4 VO 3 (0.6 g, Fuchen, Tianjin, China, 99%) was then added to a solution of oxalic acid, and the weight of H 2 C 2 O 4 •2H 2 O (Fuchen, Tianjin, China, 99%) was double that of NH 4 VO 3 .Desiccation and calcination conditions were the same as above, and the obtained catalyst was denoted as Cat-A.
Catalysts with an oxide phase were prepared via multiple impregnation methods to avoid generating the Keggin structure.(NH 4 ) 6 Mo 7 O 24 (11.5 g) was dissolved in 100 mL of distilled water.TiO 2 anatase powder was impregnated with the precursor solution and stirred at 80 • C for 5 h.The mixture was then dried at 120 • C and was calcined at 400 • C for 5 h.NH 4 H 2 PO 4 (0.6 g) was then dissolved in 100 mL of distilled water.V 2 O 5 was loaded using the above conditions, and this sample was denoted as Cat-B.

Catalytic Activity Test
Catalytic activities of the samples were measured in a fixed bed quartz reactor (9 mm i.d.) with 0.4 mL of catalysts (40-60 mesh).Typical reactant gas was a mixture of 1000 ppm NO, 1000 ppm NH 3 , 5% O 2 , 5% water vapor, and 350 ppm SO 2 with a balance of N 2 under a flow of 500 mL/min.The space velocity was 40,000 h −1 .Concentrations of NO and NH 3 were continuously detected using a Thermo Scientific 17i NO x chemiluminescence analyzer.N 2 O was monitored using a Bruker Tensor 27 FTIR spectrometer.The reaction system was maintained at each reaction temperature for 30 min before analysis.The equations used to calculate NO x conversion and N 2 selectivity are as follows:

Conclusions
Two kinds of catalysts (one with and one without the Keggin structure) are synthesized to investigate the effect of the Keggin structure on the NH 3 -SCR reaction.Cat-A, which had the Keggin structure, exhibits better low-temperature SCR performance.NH 3 is adsorbed on both Brönsted and Lewis acid sites over the two catalysts but the catalyst with Keggin structure has more Brönsted and Lewis acid sites.Moreover, there are more kinds of adsorbed NO x species on Cat-A than on Cat-B, and the adsorbed NO x species are mainly free nitrate ions.The two catalysts follow both E-R and L-H mechanisms.Compared with Cat-B, most of the adsorbed NO x species for Cat-A react quickly with gaseous NH 3 .In addition, NH 4 + of the Keggin structure participates in the reaction via the L-H mechanism and is recovered by ammonia gas in the flow.Thereby, more acid sites are provided, adsorbed NO x species activity is improved, and more NH 4 + ions participate in the L-H mechanism over Cat-A, which had the Keggin structure, thus promoting SCR activity.S1); NH 3 -TPD results (Figure S3); in situ sequential Raman spectra results (Figure S4); the time-on-stream stability of the catalysts (Figure S5).

2. 2 .
Adsorption Behaviors of Reactants on the Surface of the Catalysts (In Situ DRIFT) 2.2.1.NH 3 Adsorption on the Surface of the Catalysts In situ DRIFT spectra of NH 3 adsorption on Cat-A and Cat-B at 250 • C are shown in Figure 2.

Figure 4 .
Figure 4. DRIFT spectra of NO + O 2 reacted with pre-adsorbed NH 3 species at 250 • C on: (A) Cat-A; and (B) Cat-B.

Figure 5 .
Figure 5. DRIFT spectra of NH 3 reacted with pre-adsorbed NO + O 2 species at 250 • C on: (A) Cat-A; and (B) Cat-B.

Figure 7 .
Figure 7. FT-IR spectra of Cat-A with: (A) no adsorption; (B) adsorption of NO + O 2 ; and (C) after cutting off NO + O 2 , adsorption of NH 3 .

Figure 8 .
Figure 8. Schematic of the proposed NH 3 -SCR reaction mechanism over Cat-A.