In Situ DRIFTS Studies of NH₃-SCR Mechanism over V₂O₅-CeO₂/TiO₂-ZrO₂ Catalysts for Selective Catalytic Reduction of NO_x

Abstract

TiO₂-ZrO₂ (Ti-Zr) carrier was prepared by a co-precipitation method and 1 wt. % V₂O₅ and 0.2 CeO₂ (the Mole ratio of Ce to Ti-Zr) was impregnated to obtain the V₂O₅-CeO₂/TiO₂-ZrO₂ catalyst for the selective catalytic reduction of NO_x by NH₃. The transient activity tests and the in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) analyses were employed to explore the NH₃-SCR (selective catalytic reduction) mechanism systematically, and by designing various conditions of single or mixing feeding gas and pre-treatment ways, a possible pathway of NO_x reduction was proposed. It was found that NH₃ exhibited a competitive advantage over NO in its adsorption on the catalyst surface, and could form an active intermediate substance of -NH₂. More acid sites and intermediate reaction species (-NH₂), at lower temperatures, significantly promoted the SCR activity of the V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst. The presence of O₂ could promote the conversion of NO to NO₂, while NO₂ was easier to reduce. The co-existence of NH₃ and O₂ resulted in the NH₃ adsorption strength being lower, as compared to tests without O₂, since O₂ could occupy a part of the active site. Due to CeO_2’s excellent oxygen storage-release capacity, NH₃ adsorption was weakened, in comparison to the 1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst. If NO_x were to be pre-adsorbed in the catalyst, the formation of nitrate and nitro species would be difficult to desorb, which would greatly hinder the SCR reaction. All the findings concluded that NH₃-SCR worked mainly through the Eley-Rideal (E-R) mechanism.

1. Introduction

Generally, nitrogen oxides (NO_x), which may cause environmental problems, such as: Photochemical smog; acid rain; ozone depletion; and, health hazards, are mainly emitted from the industrial combustion of fossil fuels. Therefore, the reduction of NO_x has become an important research field for atmospheric environmental control. Currently, the selective catalytic reduction (SCR) is the most promising method to reduce the emissions of NO_x [1,2]. The temperature window of the traditional V-W (Mo)/Ti catalyst is 300–400 °C, but in some coal-fired power plants, the temperature of exhaust gas is lower. In order to enhance the NO_x conversion rate, the exhaust gases were reheated, which caused a large waste of energy. The narrow temperature window restrained its application. Thus, many researchers redirected their study to focus on the catalyst, which has superior low-temperature activity.

The VO_x/TiO₂ system for SCR has been studied extensively in the past and a number of reaction mechanisms have been proposed. It is generally accepted that the Brønsted and Lewis acid sites are essential for the reaction mechanism. Topsoe et al. [3] proposed a “Brønsted NH₄⁺” mechanism over a V₂O₅-based catalyst, which has gained the majority of support in the literature. Arnarson et al. [4] observed the SCR reaction over the VO₃H/TiO₂ catalyst and demonstrated that the Brønsted acid site served to capture the NH₃ and increased the NH₄⁺ stability (increased Brønsted acid strength), which impacted the catalytic rate in a negative direction. Marberger et al. [5] had a similar conclusion for the V₂O₅-WO₃/TiO₂ catalyst. That is, the Brønsted acid sites hardly contributed to the SCR activity and mainly served as an NH₃ pool to replenish the Lewis sites. NO reacted predominantly with NH₃ adsorbed in the Lewis acid sites at low temperatures. SCR reactions over Ce-based catalysts mainly followed two mechanisms, one is the Eley–Rideal mechanism (i.e., the reaction of gaseous NO with adsorbed NH₃ species), and the other is the Langmuir–Hinshelwood mechanism (i.e., the reaction of adsorbed NO_x with adsorbed NH₃ species on adjacent sites) [6]. While these two reaction pathways probably do not exclude each other, it is essential to understand whether either or both species are relevant. Vuong et al. [7] reported that NH₃-SCR proceeded from a Langmuir–Hinshelwood mechanism on bare supports (TiO₂), while an Eley–Rideal mechanism operated on V-containing catalysts.

In recent years, cerium oxides have attracted extensive attention due to their outstanding oxygen storage-release capacity and excellent redox properties in the low-temperature NH₃-SCR reactions [8,9,10]. In V/Ce_1−xTi_xO₂ catalysts, Ce-O sites are effectively covered by VO_x species, which hinder the formation of surface nitrates and cause the switch in the reaction mechanism. Zhang et al. [11] observed the adsorption and reaction processes in DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) spectra and concluded that the cis-N₂O₂²⁻ formed on CeO₂ reacted more favorably with NH₃ than with other nitrate species. Galvez et al. [12] demonstrated that the SCR reaction over activated carbon supported the V₂O₅ catalysts (V₂O₅/AC) that took place between the adsorbed species of NH₃ on the Brønsted acid sites, and the NO molecules in the gaseous phase, following an Eley–Rideal (E–R) mechanism. In Yu et al. [13], the study proposed that the SCR reaction over Zr₃ (PO₄)₂/CeO₂-ZrO₂ proceeded via the combination of the adjacent, surface N_xO_y species, and the ads-NH₃ species by Langmuir-Hinshelwood (L-H) mechanism. Ma et al. [14] also observed the enhanced NH₃ activation and NO₃-formation. The latter promoted the reaction of ads-NH₃ and ads-NO₃-species for the SCR reaction over N_bO_x/CeO₂-ZrO₂ catalysts—according to the “L-H” mechanism. Getting to know the reaction pathway and proposing reaction mechanisms is helpful in guiding the design and preparation of the catalysts [15].

In our previous study [9,16], a series of 1 wt. % V₂O₅-CeO₂/TiO₂-ZrO₂ catalysts with different contents of CeO₂were prepared by an impregnation method. It was found that the sample of Ce/Ti = 0.2 (the molar ratio) exhibited a favorable performance with a 92% NO_x conversion rate at 250 °C. In addition, the effect of Ce modification on microscopic properties and the catalytic performance of V₂O₅/TiO₂-ZrO₂ were investigated in more detail. It concluded that the promotional effect of adding Cemainly laid in the intensified interaction between the metal oxide components and the larger amount of Brønsted and Lewis acid sites, as well as the formation of active intermediates (-NH₂). In this study, we further investigated the NH₃-SCR mechanism over the optimal 1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst, and by carrying out transient activity tests and in situ DRIFTS analyses under various conditions of single or mixing feeding gas and pre-treatment ways, proposed a possible reaction pathway.

2. Results and Discussion

2.1. Adsorption and Desorption Properties of NO_x and NH₃ on the Catalysts

The adsorption-desorption behavior of the catalyst is considered to be a crucial step to a heterogeneous catalysis system. To study the desorption status of the reactant gas on the catalyst surface, the desorption of NO on 1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ was studied. As shown in Figure 1, the band at 3670 cm⁻¹ was attributed to O-H, and it decreased with the increased temperature until the negative peak appeared. The band at 3203 cm⁻¹ was the result of the hydroxyl vibration. The catalysts contained a little bit of water at a normal temperature, and the water evaporated as the temperature rose; adsorption bands then disappeared. The bands (1618 cm⁻¹, 1367–1378 cm⁻¹, 1245–1288 cm⁻¹, 1130 cm⁻¹, and 1058 cm⁻¹) were ascribed to the adsorbed NO_x, especially the band of 1618 cm⁻¹ which was related to weak adsorption of NO and NO₂ [16,17,18]. In the case of cis-N₂O₂²⁻, bands should appear in the 1300–1400 cm⁻¹ [19]. When the temperature exceeded 200 °C, N₂O₂²⁻ appeared in the region of 1367–1378 cm⁻¹ and the intensity of peaks increased as the temperature rose. This confirmed that it could exist stably on the surface of the catalyst. The band at 1245 cm⁻¹ was due to bridging nitrate; the adsorption intensity receded as the temperature rose, and the band region moved to 1288 cm⁻¹ with the generation of monodentate nitrate [17,20,21]. Subsequently, this peak disappeared as the temperature reached 400 °C. The band at 1130 cm⁻¹ was assigned to nitrosyl NO⁻, which could be oxidized to nitrite and nitrate with the existence of oxygen, and it sharply decreased as the temperature increased [22,23]. The band at 1054 cm⁻¹ corresponded to nitrate species, which could exist on the surface of the catalyst stably and was hard to desorb even when the temperature was raised.

As shown in Figure 2a, as N₂ was steadily purged on, it was clear that the adsorption of NO was very weak at 250 °C, and N₂O₂²⁻ and nitrate species only appeared at the band of 1371 cm⁻¹ and 1052 cm⁻¹. In addition, there was no significant change in peak intensity by increasing the adsorption and desorption time, indicating that it could exist stably on the surface of the catalyst. As shown in Figure 2b, it was observed that the presence of O₂ obviously strengthened the adsorption intensity of NO_x on the surface of catalysts. After being exposed to NO + O₂ for 60 min, weak adsorption of NO and NO₂ appeared at the band of 1630 cm⁻¹. The bands at 1365 cm⁻¹ and 1108 cm⁻¹ could be assigned to cis- and trans-N₂O₂²⁻ [19,24], respectively. Simultaneously, the bands at 1284 cm⁻¹ and 1038 cm⁻¹ were attributed to monodentate nitrate and nitrate species, respectively.

As shown in Figure 3, the adsorption peaks of free O-H appeared at 3662 cm⁻¹ and 3700 cm⁻¹, and the band at 3100–3400 cm⁻¹ was associated with the N-H stretching vibrations, which are linked to Lewis acid sites. The peaks at 1556 cm⁻¹, 1548 cm⁻¹, and 1505 cm⁻¹ corresponded to the formation of intermediate species (-NH₂) in SCR reactions [21]. In addition, there was no significant change in peak intensity as adsorption and desorption time increased. According to our previous study [16], intermediate species (-NH₂) were detected above 300 °C over the V₂O₅/TiO₂-ZrO₂ catalyst. The V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst exhibited more -NH₂ at lower temperatures, which explained its higher activity in comparison to other catalysts. Vuong et al. [11] collected different DRIFTS spectra of bare supports (CeO₂, TiO₂ and CeO₂-TiO₂) and supported vanadium catalysts (V/CeO₂, V/CeO₂-TiO₂ and V/TiO₂) at 200 °C. They reported additional bands at 1510–1520 cm⁻¹ of NH₂, which were only observed on pure CeO₂ and CeO₂-TiO₂. It could be speculated that the addition of Ce was the key factor to affect the surface adsorbed NH₃ species. At the same time, the peaks at 1605 cm⁻¹, 1357 cm⁻¹, 1321 cm⁻¹, 1282 cm⁻¹, 1180 cm⁻¹, and 1133 cm⁻¹ were associated with NH₃ cooperating vibration—linked to Lewis acid sites [25,26]. According to our previous study [16], with the addition of Ce, the acid sites of the catalysts increased and the optimal V₂O₅-0.2CeO₂/TiO₂-ZrO₂ sample possessed the largest amount of surface acid sites, which greatly promoted the SCR reaction. The same trend was observed in Vuong et al. [7]. They demonstrated that the relative amount of Lewis acid sites in the V-containing catalysts decreased in the order V/Ce_0.5Ti0_0.5O₂ > V/CeO₂ > V/TiO₂. The band at 1180 cm⁻¹ in Figure 3a split into two NH₃ adsorption peaks (1085 cm⁻¹ and 1044 cm⁻¹), and the band at 1133 cm⁻¹ in Figure 3b corresponded to the peak at 1085 cm⁻¹. The band at 1678 cm⁻¹ was associated with NH⁴⁺ symmetric vibration and is linked to Brønsted acid sites [27,28]. Comparing Figure 3a with Figure 3b, it can be found that the presence of O₂ hindered the adsorption of NH₃. However, in Figure 3b, after the feeding of NH₃ + O₂ was stopped, the intensity of the NH₃ adsorption peak, linked to Lewis acid sites, was stronger than in Figure 3a. It might have been caused by the re-adsorption of desorbed ammonia or weak ammonia adsorption on Lewis acid sites, because CeO₂ had the capacity of oxygen storage-release and O₂ occupied some active sites.

2.2. Transient Response Experiment Analysis

In order to illuminate the difference between NO_x species and explain the SCR reaction mechanism, transient reaction studies by in situ DRIFTS spectra were performed. As shown in Figure 4, the NH₃ adsorption peak could be found after NH₃ and NO were introduced for two min. After adsorption was saturated, the bands at 3400 cm⁻¹, 3100 cm⁻¹, and 1198 cm⁻¹ were associated with NH₃ adsorption and was linked to Lewis acid sites. The intermediate species (-NH₂) appeared at 1591 cm⁻¹, which implied more active intermediates for the NH₃ oxidation reaction. No obvious NO_x adsorption was observed; it was a preliminary inference that SCR reactions mainly followed from the Eley–Rideal mechanism.

As shown in Figure 5, catalysts were exposed to the flow of NO and NO + O₂ at 250 °C for 60 min. The O-H adsorption peaks appeared at 3510 cm⁻¹ and 3528 cm⁻¹. The N-H stretching vibration peaks appeared in the range of 3400–3100 cm⁻¹ after NO was introduced for two min. However, in Figure 5b, NH₃ adsorption peaks appeared after NH₃ was introduced for 10 min in the same region. NO₂ asymmetric vibration adsorption peaks appeared at 1610 cm⁻¹ and 1620 cm⁻¹ in Figure 5a,b, respectively. The bands at 1583 cm⁻¹, 1226 cm⁻¹, and 1231 cm⁻¹ were ascribed to bridging nitrates. cis-N₂O₂²⁻ appeared at 1353 cm⁻¹ in Figure 5b, and it shifted to the region of 1335 cm⁻¹ with the introduction of NH₃, which then weakened the adsorption. When introducing NO + O₂ again, the adsorption peak recovered to 1353 cm⁻¹.

NH₃ had no obvious influence on the NO_x adsorption peak at 2000–1000 cm⁻¹, especially after being exposed to NO + O₂ where the influence became tinier. After the pre-adsorption of NO, the intensity of the NO_x adsorption peak was obvious, but NH₃ adsorption could barely be found. With the introduction of NH₃, the N-H stretching vibration was present in the range of 3400–3100 cm⁻¹, as seen in both Figure 5a,b. The results showed that when NO + O₂ was injected separately, NO + O₂ occupies SCR active reaction sites and restrains the adsorption of NH₃, before hindering the SCR reaction. When NO and NO + O₂ was reintroduced, respectively, peaks located at 3519 cm⁻¹ and assigned to O-H adsorption were observed. The intensity of the NO_x adsorption peak had no decrement; on the contrary, NH₃ adsorption, which was linked to Lewis acid sites, disappeared. These results indicated that the gas-phase NO_x had reacted with NH₃ on Lewis acid sites, which verified the Eley–Rideal mechanism on catalysts. However, in Chen et al. [29], a Langmuir–Hinshelwood mechanism operated on the CeTi catalyst, and adsorbed NH₃ and NH₄⁺ that reacted with NO/O₂ from the gas phase. Vuong et al. [7] demonstrated that the switch in reaction mechanisms has its roots in the structural differences of catalysts and supports. In V/Ce_1−xTi_xO₂ catalysts, Ce-O sites are effectively covered by VO_x species, which hinders the formation of surface nitrates and causes the switch in the reaction mechanism.

As shown in Figure 6a, when NO + O₂ is introduced, the adsorption peaks at the region of 3400–3100 cm⁻¹ and 1189 cm⁻¹ disappeared, while the O-H adsorption peak (1618 cm⁻¹), the N₂O₂²⁻ adsorption peak (1371 cm⁻¹ and 1112 cm⁻¹), and the nitrate species peak (1024 cm⁻¹) appeared. When NH₃ was introduced again, the NO₂ adsorption peak disappeared. Moreover, a strong adsorption of NH₃ appeared at the region of 3400−3100 cm⁻¹ and 1259 cm⁻¹. Brønsted acid adsorption appeared at 1698 cm⁻¹ and 1428 cm⁻¹ and considerably intensified, while other NO_x adsorption had no obvious change.

As shown in Figure 6b, NO was introduced after being exposed to NH₃ + O₂. Bridging nitrate and monodentate nitrate appeared at 1575 cm⁻¹ and O-H vibration appeared at 3566 cm⁻¹. When NH₃ + O₂were introduced again, the O-H vibration became stronger and the NH₃ adsorption peak at the region of 3400–3100 cm⁻¹ was heavily weakened, as compared with that in Figure 6a. As a result, it can be concluded that O₂ reacted with NO first.

In Figure 7, NH₃ was introduced first, and then NO was introduced in combination with NH_3. Lastly, O₂ was also introduced with NH₃, NO, and O₂ being presented at the same time. In these three different atmospheres, the intensity of NH₃ adsorption on Lewis acid sites had no change. Meanwhile, the active intermediate species of -NH₂ appeared at 1588 cm⁻¹, indicating that NH₃ molecules continued to be adsorbed on the catalytic surface with the process of reaction. The stable existence of intermediate species (-NH₂) explained the high SCR activity of the V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst at low temperatures. Simultaneously, the intensity of the O-H negative peak receded gradually, which might have been caused by the H₂O produced in the SCR reaction.

2.3. Steady-State Response Experiments

As shown in Figure 8, catalysts were saturated at 25 °C after 60 min pre-adsorption. -NO₂ adsorption appeared at 1839 cm⁻¹ and 1843 cm⁻¹. The bands at 1692 cm⁻¹, 1682 cm⁻¹, 1443 cm⁻¹, and 1419 cm⁻¹ were associated with NH₄⁺ adsorption, linked to Brønsted acid sites, and the bands of 3400–3100 cm⁻¹,1197 cm⁻¹, and 1215 cm⁻¹ were associated with NH₃ adsorption, linked to Lewis acid sites. As shown in Figure 8b, N₂O₂²⁻ species appeared at 1106 cm⁻¹ with the presence of O₂, which indicated that the existence of O₂ would promote NO adsorption. Comparing Figure 8a with Figure 8b, NH₃ adsorption became much stronger with the existence of O₂. At the same time, the combination of NO_x and NH₃ appeared at 1248 cm⁻¹, and N₂O₂²⁻ species decreased with increasing temperatures, indicating that O₂ is essential for SCR reactions.

2.4. Transient SCR Activity Test Experiments

As shown in Figure 9a, NO was introduced after the pre-adsorption of NH₃ for 2 h. The initial conversion of NO_x, NO, and NO₂ was 61%, 56%, and 97%, respectively. With a steady flow of NO_x, adsorbed ammonia was consumed gradually and the conversion of NO_x and NO decreased, while NO₂ conversion went down-up-down. According to the in situ DRIFTS results, it might be that NO₂ is easier to be adsorbed on the catalysts surface, thus leading to the decrease of NH₃ adsorption; the conversion of NO₂ dropped correspondingly. Until NH₃ was completely consumed, NO₂ started to be adsorbed on catalysts and the conversion went up, and decreased again after adsorption saturation. By feeding NH₃ and NO simultaneously, the conversion rate of NO_x and NO was lower than if only NO was fed, suggesting that NO₂ occupied active reaction sites resulting in its poor performance. In the case of feeding NH₃ and NO at the same time, we found that all the three conversion rates showed the same trend, namely, that the conversion rate reduced after the first rose, which is associated with the promotion of NH₃ for SCR reaction. When the three gases: NH₃; NO; and, O₂ were fed synchronously, the NO_x conversion rate reached a stable level of 80%. Simultaneously, the conversion rate of NO and NO₂ stabilized at 73% and 92%, respectively. What is more, both conversion rates obviously increased, indicating that NO₂ was easier to be reduced. We can conclude that O₂ was essential for the SCR reaction.

In Figure 9b, NO was introduced after NH₃ and O₂ was pre-adsorbed for 2 h. It was obvious that the conversion rate of NO_x reduced compared to Figure 9a. However, the conversion rate of NO₂ increased and the conversion rate of NO decreased. This might have been caused by the pre-adsorbed O₂ reacting with NO and producing NO₂, which was easier to react with, and be adsorbed by, the catalysts. The conversion rate increased rapidly when NH₃, NO, and O₂ was present at the same time.

In Figure 9c, after the pre-adsorption of NO + O₂ for 2 h, the denitration efficiency declined continuously with the existence of NH₃, NO, and O₂. When introducing NH₃ and NO together, the conversion rate of NO_x and NO went down-up-down, while the conversion rate of NO₂ went down-up-down-up. This could be ascribed to the oxygen storage-release capacity of CeO₂. NO adsorbed on the catalysts, reacted with O₂, and produced NO₂, resulting in the ascended e-conversion rate of NO₂. When O₂ was completely consumed, the conversion rate went down again. In this process, the SCR reaction was very weak. As a result, most of the NO₂ adsorbed on the catalysts, so its conversion rate went up. After this, O₂ reacted with NO and produced more NO₂, and its conversion rate declined after the adsorption of NO₂ was saturated. When O₂ reacted with NO completely, NO₂ occupied the adsorption sites of O₂, leading to the conversion rate going up. The combined effect of the NO₂ and NO conversion rate resulted in the conversion of NO_x going down-up-down-up.

2.5. Low-Temperature SCR Reaction Pathway

The above analyses of in situ DRIFTS have demonstrated the relatively high ability of 1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst on NH₃ adsorption and oxidation. At the reaction temperature (250 °C), the Lewis acid sites were much more stable than were the Brønsted acid sites and the quantity of coordinated NH₃ was larger than that of the NH⁴⁺ ions. The gaseous NH₃ was adsorbed on the catalytic surface, followed by a reaction with the gas phase NO to form the intermediate of NH₂NO, which was unstable and would decompose into N₂ and H₂O (Eley–Rideal mechanism). Based on the combination of in situ DRIFTS experiments and transient SCR activity tests, the mechanism of NH₃-SCR reaction over V₂O₅-CeO₂/TiO₂-ZrO₂ catalysts are mainly as followed:

$${\mathrm{O}}_2{+2}^{*}\to {2\mathrm{O}}^{-*}\left(*:\mathrm{surface}\mathrm{activesites}\right)$$

(1)

$${\mathrm{NH}}_3\left(\mathrm{g}\right)\stackrel{{\mathrm{Ce}}^{4+}}{\to }{\mathrm{NH}}_3\left(\mathsf{\alpha }\right)\left(\mathrm{Lewis}\mathrm{acid}\mathrm{site}\right)$$

(2)

$${\mathrm{NH}}_3\left(\mathsf{\alpha }\right)+{\mathrm{O}}^{-*}\to {\mathrm{NH}}_2\left(\mathsf{\alpha }\right)+\mathrm{OH}\left(\mathsf{\alpha }\right)$$

(3)

$${\mathrm{NH}}_2\left(\mathsf{\alpha }\right)+\mathrm{NO}\left(\mathrm{g}\right)\to {\mathrm{NH}}_2\mathrm{NO}\left(\mathsf{\alpha }\right)\to {\mathrm{N}}_2\left(\mathrm{g}\right){+\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$$

(4)

3. Materials and Methods

3.1. Catalyst Preparation

The Ti-Zr support (molar ratio of Ti:Zr = 1:1) was prepared by a co-precipitation method. Typically, an equal molar amount of TiCl₄ solution and ZrOCl₂·8H₂O was dissolved in the deionized water. NH₃·H₂O solution was dropped into a stoichiometric solution of TiCl₄ and ZrOCl₂·8H₂O with steady stirring until the pH reached 10. The obtained precipitation solution was aged in air for 24h at room temperature, and then washed with deionized water until the supernatant was free from Cl⁻. Subsequently, the resulting paste was dried at 110 °C for 12 h and then calcined at 450 °C for 4 h in a muffle stove.

1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ samples were prepared by the step-by-step impregnation of Ti-Zr and CeNO₃∙6H₂O (Ce/Ti = 0.2, molar ratio). The obtained mixture was stirred for 2 h at 25 °C, and then for about 4 h at 85 °C until the water boiled away. The resulting precipitate was dried at 110 °C for 12 h, followed by being calcined at 450 °C for 4 h in a muffle stove to obtain intermediate CeO₂/Ti-Zr, which was then impregnated with a NH₄VO₃ solution. The obtained mixture was dried and calcined in the same process of preparing CeO₂/Ti-Zr samples to finally acquire 1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ samples.

3.2. In situ DRIFTS Experiments

In situ DRIFTS investigations were carried out on a Nicolet 6700 spectrometer (Thermo Electron Corporation, Waltham, MA, USA), running in the wavenumber range of 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹. A thin, intact and self-supporting wafer of adsorbents were prepared and mounted inside a high temperature cell (HTC-3, Harrick Scientific Corporation, Ithaca, NY, USA). Prior to each experiment, the catalyst was heated to 400 °C under an N₂ atmosphere for 1h to remove any adsorbed species, then cooled down to the reaction temperature. The background spectrum was recorded in N₂ flow and was automatically subtracted from the sample spectrum during the experiment. Then the N₂ flow was switched to a stream containing one or more reactants, such as NH₃, NO, and O₂. In situ DRIFTS experiments included transient response and steady-state response experiments. It should be noted that new catalyst samples pretreated under the same conditions and were used in each in situ DRIFTS experiments.

3.3. Transient SCR Activity Tests

As shown in

Table 1. The working conditions of transient SCR activity tests.

Gas Composition	1	2	3	4
I	NH3	NO	NH3 + NO	NH3 + NO + O2
II	NH3 + O2	NO	NH3 + NO + O2	-
III	NO + O2	NH3 + NO + O2	NH3 + NO	-

, in order to coordinate the in situ DRIFTS experiments, catalyst activity test experiments were designed under different conditions of feeding gases. A total of 0.3 g of catalyst (screening through 40 to 60 mesh sieve) was tested on a fixed-bed quartz tube reactor (Nanjing University of Technology, Nanjing, China)with an internal diameter of 7 mm at the temperature of 250 °C. The total flow rate was 100 mL/min, which was pre-mixed in a gas mixer to obtain the simulated gas containing 0.08% NO, 0.08% NH₃, and 5% O_2, with a balance of N₂, NO, NO₂, and NO_x in the outlet, which was continually monitored by a flue gas analyzer (Testo 330-2 LL, Shanghai, China). Typically, during the experiments, about 5% NO was converted to NO₂. In other words, 5% NO_x existed in the form of NO₂.

4. Conclusions

In situ DRIFTS experiments and transient SCR activity tests were used coordinately to observe active and intermediate species and to describe the possible reaction path of 1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ at low temperature. The results are as followed:

(1)

NH₃ held a dominant position in the competitive adsorption between NH₃ and NO. Transient SCR activity tests showed that the NH₃ pre-adsorbed catalyst exhibited better SCR activity than its NO_x pre-adsorbed counterpart.

(2)

NO might be adsorbed on the catalyst surface and be converted to monodentate nitrite and nitrate species, which is more obvious in the presence of O₂, and dramatically restrains the adsorption of NH₃, hindering the SCR reaction.

(3)

More acid sites and reaction intermediate species -NH₂ at lower temperatures mainly led to the higher activity of the V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst.

(4)

Transient SCR activity tests and steady-state response experiments both confirmed that NH₃-SCR activity was enhanced by the presence of O₂. NH₃ adsorption intensity had no obvious difference, whether NO or O₂ was introduced or not, indicating that the adsorption and consumption of NH₃ was in dynamic equilibrium, which promoted SCR reaction.

(5)

NH₃-SCR reaction over 1 wt. % V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst mainly follows the E-R mechanism.