Unraveling the Mechanism of High N₂ Selectivity in Ammonia Selective Catalytic Oxidation on Pt-V Tandem Catalyst

Abstract

V_0.5/Pt/TiO₂ tandem catalysts exhibit both an outstanding low-temperature NH₃ conversion rate and high N₂ selectivity in NH₃-SCO reactions, but the mechanism of high N₂ selectivity remains unclear. In this work, V_x/Pt/TiO₂ tandem catalysts were synthesized through a two-step impregnation–deposition method. The modulating effect of the V loading mount on NH₃-SCO performance was evaluated, and the relevant reaction mechanism was explored systematically. The results demonstrated that the synergistic effect of tandem NH₃ oxidation and NH₃-SCR reactions could be regulated by changing the V loading amount, thereby modulating N₂ selectivity. Compared with other V_x/Pt/TiO₂ catalysts and previously reported SCO catalysts, the V_0.5/Pt/TiO₂ catalyst with a V loading amount of 0.5 wt.% exhibited outstanding NH₃-SCO performance, which achieved complete NH₃ conversion and >90% of N₂ selectivity within a range of 250–450 °C. XPS, NH₃-TPD, and O₂-TPD results suggested that the increase in the V loading amount from 0.1 wt.% to 0.5 wt.% was conducive to increasing the relative contents of Pt⁰ and V⁵⁺ species, as well as the amount of acid sites, oxygen species, and oxygen vacancies. Consequently, the synergistic effect of tandem NH₃ oxidation and NH₃-SCR reactions was significantly enhanced, enabling the catalyst to exhibit excellent N₂ selectivity. A further increase in the V loading amount from 0.5 wt.% to 0.9 wt.% would bring about the opposite effect to the above, resulting in a decline in catalytic performance. In situ DRIFTS results showed that a V loading amount of 0.5 wt.% was beneficial for -NH₂ species to participate in NH₃-SCO reactions, thereby boosting N₂ selectivity.

1. Introduction

In response to increasingly stringent CO₂ emission reduction regulations, the international shipping industry is exploring the application of NH₃-fuel engines as the main engines for large merchant ships [1,2,3]. However, the serious environmental risks caused by NH₃ emissions from NH₃-fuel engines have become one of the main obstacles limiting the application of NH₃-fuel engines on board [3,4,5,6,7,8]. It is urgent to explore practical and efficient technologies for removing NH₃ from high-temperature flue gas to meet the needs of NH₃-powered ships. At present, the selective catalytic oxidation (SCO) technique is considered more promising than other techniques in meeting the above needs [9,10,11]. This is because SCO technology can effectively convert NH₃ into N₂ through Equation (1) at relatively high temperatures and space velocities.

$$4\mathrm{N}{\mathrm{H}}_3+{3\mathrm{O}}_2\to {2\mathrm{N}}_2+{6\mathrm{H}}_2\mathrm{O}$$

(1)

The NH₃ conversion performance of SCO technology mainly depends on the design of the catalyst. During the past several decades, numerous types of NH₃-SCO catalysts have been reported, which can be mainly classified into noble metal catalysts and transition metal oxide catalysts. Noble metal catalysts exhibit an outstanding NH₃ conversion performance at low temperatures, which is attributed to their strong catalytic oxidation activity for NH₃ [11,12,13,14,15]. But noble metal catalysts are prone to excessively oxidizing NH₃ into NO_x, resulting in a low N₂ selectivity [13,14,15]. Transition metal oxide catalysts exhibit high N₂ selectivity due to their moderate catalytic oxidation activity for NH₃, but this makes it difficult to achieve a good NH₃ conversion performance at low temperatures [15,16,17].

It is considered that the mainstream approach to designing SCO catalysts with excellent performance is to simultaneously pursue good NH₃ conversion performance and high N₂ selectivity over a broad temperature range. But this remains difficult to achieve at present. Aiming to synthesize SCO catalysts with excellent performance, the combination of transition metal oxides with a small amount of noble metal components was proposed as a strategy [18,19,20,21,22,23]. Its operation relies on an internal selective catalytic reduction (i-SCR) mechanism, which is mainly composed of the following two tandem reactions: Under catalysis of noble metal components, NH₃ is oxidized to NO by O₂ (Equation (2)). Subsequently, NO reacts with NH₃ under the action of transition metal oxide components through SCR reaction (Equation (3)).

$$4\mathrm{N}{\mathrm{H}}_3+5{\mathrm{O}}_2\to 4\mathrm{NO}+6{\mathrm{H}}_2\mathrm{O}$$

(2)

$$4\mathrm{NO}+4\mathrm{N}{\mathrm{H}}_3+{\mathrm{O}}_2\to {4\mathrm{N}}_2+{6\mathrm{H}}_2\mathrm{O}$$

(3)

For the design of catalysts employing the above strategy, the selection of suitable noble metal and transition metal components is deemed to be of great importance. In recent years, Pt-V tandem catalysts have received extensive attention due to their high potential for achieving good NH₃ conversion performance and high N₂ selectivity simultaneously. Byun et al. introduced 0.3 wt.% Pt into the V₃W₂/TiO₂ catalyst. The results showed that, within the temperature range of 330–380 °C, the Pt_0.3V₃W₂/TiO₂ catalyst could achieve complete conversion of NH₃ and the corresponding N₂ selectivity was higher than 90.0% [20]. The main reason for this was the synergistic effect between NH₃-SCO and CO-SCR reactions. Kim et al. reported that the addition of 2 wt.% V into the Pt_0.1/TiO₂ catalyst could enhance the NH₃-SCO performance. The Pt_0.1/V₂/TiO₂ catalyst derived through optimization completely converted NH₃ within the range of 250–350 °C, and the corresponding N₂ selectivity was ~81% [12].

In our previous work, a Pt-V tandem catalyst was synthesized by directly mixing and grinding a certain mass of the VW/TiO₂ catalyst with a small quantity of the Pt/Al₂O₃ catalyst [9]. Results of the experiments indicated that NH₃-SCO and NH₃-SCR reactions over the mixed catalysts displayed a synergistic effect, enabling them to exhibit excellent low-temperature redox performance. In situ DRIFTS results showed that NH₃ conversion reactions on the surface of mixed catalysts followed both the i-SCR mechanism and hydrazine mechanism. Moreover, authors synthesized a series of Pt-modified V/TiO₂ catalysts (0.04 wt.% Pt and 0.5 wt.% V) by using the impregnation–deposition method [22]. The effect of the V and Pt deposition sequence on NH₃-SCO performance was explored. The results displayed that V_0.5/Pt_0.04/TiO₂, Pt_0.04/V_0.5/TiO₂, and Pt_0.04V_0.5/TiO₂ catalysts all possessed outstanding NH₃ conversion performance. They could achieve complete NH₃ conversion at around 250 °C. But the V_0.5/Pt_0.04/TiO₂ catalyst exhibited much better N₂ selectivity than the other two catalysts. However, the mechanism by which the V_0.5/Pt_0.04/TiO₂ tandem catalyst achieves high N₂ selectivity remains unclear. According to the i-SCR mechanism, N₂ selectivity is mainly determined by the V component. Thus, an exploration of the modulating effect of the V loading amount on NH₃-SCO performance may be a promising approach to revealing the mechanism of the high N₂ selectivity of the V_0.5/Pt/TiO₂ tandem catalyst. But similar studies have not been reported yet.

Herein, a series of V_x/Pt/TiO₂ tandem catalysts were prepared via a two-step impregnation–deposition process. The influence of the V loading amount on the NH₃-SCO performance of the V_x/Pt/TiO₂ catalysts was systematically investigated. The results showed that, compared with other V_x/Pt/TiO₂ catalysts, the V_0.5/Pt/TiO₂ catalyst exhibited outstanding comprehensive SCO performance. It could completely remove 3000 ppm of NH₃ within the range of 250–450 °C and corresponding N₂ selectivity was higher than 90%. Compared to previously reported NH₃-SCO catalysts (Table S1), the V_0.5/Pt/TiO₂ catalyst also possessed a remarkable performance. The operating temperature window of the V_0.5/Pt/TiO₂ catalyst was well-matched with the range of exhaust temperatures in NH₃-fuel engines (280–450 °C). In order to explore the mechanism underlying the excellent N₂ selectivity of the V_0.5/Pt/TiO₂ catalyst, some characterization tests including N₂ adsorption–desorption, X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), a High-Resolution Transmission Electron Microscope (HRTEM), Energy Dispersive X-Ray Spectroscopy (EDX), O₂ Temperature Programmed Desorption (O₂-TPD), H₂ Temperature-Programmed Reduction (H₂-TPR), and NH₃ Temperature-Programmed Desorption (NH₃-TPD), and in situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (in-situ DRIFTS) were carried out. The structure–efficiency relation and surface reaction pathways were discussed in depth based on characterization results. It is well known that for NH₃-SCO catalysts, simultaneously achieving high NH₃ conversion efficiency and N₂ selectivity still remains a great challenge. This work is novel in uncovering the significant regulatory effect of V loading on N₂ selectivity of V_x/Pt/TiO₂ catalysts, elucidating the relevant regulatory mechanism, and revealing the mechanism by which the V_0.5/Pt/TiO₂ catalyst achieves a high NH₃ conversion rate and N₂ selectivity simultaneously.

2. Experimental Section

2.1. Catalyst Preparation

Based on our previous research [22], the V_x/Pt/TiO₂ catalysts with 0.04 wt.% Pt and x wt.% V were prepared using a two-step impregnation–deposition method. Different mass fractions of V (x = 0.1, 0.3, 0.5, 0.7, 0.9 wt.%) were incorporated via impregnation in the second step. The detailed preparation processes of V_x/Pt/TiO₂ catalysts were given in the Supplementary Materials. The Pt/TiO₂ (0.04 wt.% Pt) and V/TiO₂ (0.5 wt.% V) catalysts were synthesized by the impregnation method for better benchmarking. The specific synthesis procedures can be found in the Supplementary Materials.

2.2. Catalyst Characterization

Various techniques including XRD, N₂ adsorption–desorption, XPS, HRTEM, EDX, NH₃-TPD, O₂-TPD, and H₂-TPR were employed to characterize the prepared catalysts so as to reveal the physical and chemical properties. Additionally, in situ DRIFTS was used to investigate key intermediates in NH₃-SCO reactions. Detailed information on catalyst characterization can be found in the Supplementary Materials.

2.3. Catalytic Performance Test

The NH₃-SCO performance of the V_x/Pt/TiO₂ catalysts was tested using a fixed-bed reactor (Ming Yang Company, Nanjing, China). A catalyst dosage of 0.2 mL was used in each test. Prior to testing, a N₂ stream was used to purge the catalyst at 150 °C for 1 h, removing any residual contaminants. The catalyst was then exposed to a reactant gas flow of 200 mL/min, consisting of 3000 ppm NH₃, 5 vol.% O₂, and N₂ as the balance gas. Then, reaction temperature was gradually increased from 200 to 450 °C at a rate of 2 °C/min. During this process, the concentrations of NH₃, NO, NO₂, and N₂O were monitored by using a FT-IR spectrometer (IGS, ThermoFisher Scientific, Waltham, MA, USA). Based on Equations (4) and (5), the NH₃ conversion efficiency and N₂ selectivity could be calculated:

$${\mathrm{NH}}_3 \mathrm{conversion}={\displaystyle \frac{{\mathrm{NH}}_{3,\mathrm{in}}-{\mathrm{NH}}_{3,\mathrm{out}}}{{\mathrm{NH}}_{3,\mathrm{in}}}}\times 100\%$$

(4)

$${\mathrm{N}}_2 \mathrm{selectivity}={\displaystyle \frac{{\mathrm{NH}}_{3,\mathrm{in}}-{\mathrm{NH}}_{3,\mathrm{out}}-{\mathrm{NO}}_{\mathrm{out}}-{\mathrm{NO}}_{2,\mathrm{out}}-{2\times \mathrm{N}}_2{\mathrm{O}}_{\mathrm{out}}}{{\mathrm{NH}}_{3,\mathrm{in}}-{\mathrm{NH}}_{3,\mathrm{out}}}}\times 100\%$$

(5)

3. Results

3.1. NH₃-SCO Performance

The NH₃ conversion efficiencies of V_x/Pt/TiO₂ catalysts are illustrated in Figure 1a. It can be seen that with the reaction temperature increased from 200 to 250 °C, the NH₃ conversion efficiencies of the V_x/Pt/TiO₂ catalysts increased significantly. When the reaction temperature was 250 °C, the NH₃ conversion efficiencies of the V_0.1/Pt/TiO₂, V_0.3/Pt/TiO₂, and V_0.3/Pt/TiO₂ catalysts were 100%, while the NH₃ conversion efficiencies of the V_0.7/Pt/TiO₂ and V_0.9/Pt/TiO₂ catalysts were 89.2% and 83.5%, respectively. When the reaction temperature was further increased from 250 to 300 °C, the NH₃ conversion efficiencies of the V_0.7/Pt/TiO₂ and V_0.9/Pt/TiO₂ catalysts reached 100%. The above results indicate that, compared to V_0.7/Pt/TiO₂ and V_0.9/Pt/TiO₂ catalysts, the V_0.1/Pt/TiO₂, V_0.3/Pt/TiO₂, and V_0.5/Pt/TiO₂ catalysts exhibit better NH₃ catalytic conversion performance.

The N₂ selectivity of the V_x/Pt/TiO₂ catalysts within the temperature range of 200–450 °C is shown in Figure 1b. It can be observed that when the loading amount of V was increased from 0.1 wt.% to 0.5 wt.%, the N₂ selectivities of the V_x/Pt/TiO₂ catalysts were distinctly enhanced. It is noteworthy that, compared with other V_x/Pt/TiO₂ catalysts, the V_0.5/Pt/TiO₂ catalyst exhibited outstanding N₂ selectivity. The N₂ selectivity of the V_0.5/Pt/TiO₂ catalyst exceeded 90% across the entire temperature range. The above results imply that a moderate increase in the loading amount of V might be beneficial for enhancing NH₃-SCR reactions over V_x/Pt/TiO₂ catalysts, thereby promoting the generation of N₂ via Equations (3), (6) and (7)

$$2\mathrm{N}{\mathrm{H}}_3+\mathrm{NO}+{\mathrm{NO}}_2\to {2\mathrm{N}}_2+{3\mathrm{H}}_2\mathrm{O}$$

(6)

$${\mathrm{N}}_2\mathrm{O}+2\mathrm{N}{\mathrm{H}}_3+{\mathrm{O}}_2\to {2\mathrm{N}}_2+{3\mathrm{H}}_2\mathrm{O}$$

(7)

When the V loading amount was further increased from 0.5 wt.% to 0.9 wt.%, the N₂ selectivity of the V_x/Pt/TiO₂ catalysts gradually decreased. According to previous studies [22,23,24], this might be related to the excessive catalytic oxidation of NH₃ by the V species.

The NH₃-SCO performance of Pt/TiO₂ (0.04 wt.% Pt) and V/TiO₂ (0.5 wt.% V) catalysts and the bare TiO₂ support were evaluated for benchmarking. The results are shown in Figure 1. It could be seen that the Pt/TiO₂ catalyst exhibited excellent NH₃ conversion performance. It could achieve complete conversion of 3000 ppm NH₃ within the temperature range of 250–450 °C. Its NH₃ conversion performance was close to that of the V_0.1/Pt/TiO₂, V_0.3/Pt/TiO₂, and V_0.5/Pt/TiO₂ catalysts, and was significantly better than that of the V_0.7/Pt/TiO₂ and V_0.9/Pt/TiO₂ catalysts. But the N₂ selectivity of the Pt/TiO₂ catalyst within the above temperature range was lower than 70%, which was significantly lower than that of the V_x/Pt/TiO₂ catalysts. Compared with the V_x/Pt/TiO₂ and Pt/TiO₂ catalysts, the NH₃ conversion performance of the V/TiO₂ catalyst was relatively poor. It only shows the obvious NH₃ conversion performance when the reaction temperature was higher than 250 °C. However, the V/TiO₂ catalyst exhibited excellent N₂ selectivity, which exceeded 90% within the temperature range of 275–450 °C. The bare TiO₂ support did not show the catalytic activity within the whole temperature range.

In order to more intuitively display the influence of the V loading amount on the performance of the V_x/Pt/TiO₂ catalyst, a summary schematic comparing the catalyst performance versus the V loading amount was drawn, which is shown in Figure 2. The reaction temperature corresponding to the schematic is 350 °C. As shown in Figure 2, with the V loading amount increased from 0.1 wt.% to 0.5 wt.%, the N₂ selectivity of the V_x/Pt/TiO₂ catalysts significantly improved from 72.3% to 91.3%. When the V loading amount further increased from 0.5 wt.% to 0.9 wt.%, the N₂ selectivity of the V_x/Pt/TiO₂ catalyst gradually decreased from 91.3% to 82.5%. During the process of increasing the V loading amount from 0.1 wt.% to 0.9 wt.%, the NH₃ conversion efficiency of the V_x/Pt/TiO₂ catalysts always remains at 100%. The above results indicated that, in this work, the V_0.5/Pt/TiO₂ catalyst with a V loading amount of 0.5 wt.% exhibited the best comprehensive SCO performance.

The NH₃-SCO experimental results demonstrated that, compared with other V_x/Pt/TiO₂ catalysts, the V_0.5/Pt/TiO₂ catalyst exhibited excellent N₂ selectivity. In comparison with the NH₃-SCO catalysts reported previously (Table S1), the performance of the V_0.5/Pt/TiO₂ catalyst was also quite outstanding. Therefore, the V_0.5/Pt/TiO₂ catalyst was selected for further study.

In order to evaluate the repeatability of the catalyst synthesis, three batches of the V_0.5/Pt/TiO₂ catalyst were, respectively, synthesized using the same process, and performance tests were carried out. The results are shown in Figure S1. The data of Test 1 in Figure S1 are the performance test data of the V_0.5/Pt/TiO₂ catalyst mentioned in the manuscript. The data of Test 2 and Test 3 are the performance test data of the other two batches of the V_0.5/Pt/TiO₂ catalyst synthesized under the same conditions. It can be seen from Figure S1 that the NH₃ conversion performances of the three batches of the V_0.5/Pt/TiO₂ catalyst in the temperature range of 200–450 °C are quite similar, and so is the N₂ selectivity. This indicated that the catalyst synthesis process has high reproducibility.

The common causes of catalyst aging or deactivation after long-term use include sintering of active components, changes in structure of active components, collapse of pore structure, and catalyst poisoning caused by external impurities [24,25,26]. The humid conditions may promote aging or deactivation of catalysts. This is because under certain conditions, H₂O molecules may react with active components, thus resulting in a reduction in catalytic activity or even deactivation [25,27]. H₂O molecules may also react with supports of the catalyst, resulting in collapse of the pore structure, thus leading to a decline in catalytic activity [28]. Since the combustion of NH₃ in NH₃-fuel engines would produce H₂O vapor, it is advisable to conduct a preliminary test to obtain a brief understanding of the H₂O resistance of the V_0.5/Pt/TiO₂ catalyst. The H₂O resistance test was carried out under the following conditions: the concentration of the H₂O vapor was 10 vol.%, the reaction temperature was set at 300 °C, and the total experimental duration was 8 h. The results are shown in Figure 3. It can be observed that the introduction of water vapor has no significant impact on the NH₃ conversion rate and N₂ selectivity of the V_0.5/Pt/TiO₂ catalyst, indicating that the catalyst possessed good water resistance.

For the purpose of investigating the mechanism for outstanding N₂ selectivity of the V_0.5/Pt/TiO₂ catalyst, some characterization techniques have been employed to characterize physicochemical properties of V_x/Pt/TiO₂ catalysts. The results of the catalyst characterization tests will be presented below, and they will be discussed in detail.

3.2. N₂ Adsorption and Desorption

The specific surface area of a catalyst is usually closely related to the adsorption of reactants and the dispersion of active species on the catalyst surface, thereby having a significant impact on the performance of the catalyst [20,21]. Generally, a high specific surface area of the catalyst is conducive to adsorbing more NH₃ and O₂, which is beneficial for promoting NH₃-SCO performance [22,23]. A high specific surface area of the catalyst is also conducive to active species being more evenly dispersed on the catalyst surface, thereby enriching active sites and improving the NH₃-SCO performance [24]. To characterize the surface physical properties of the V_x/Pt/TiO₂ catalysts, N₂ adsorption and desorption experiments were performed, and the results are presented in Figure 4. Figure 4a shows the N₂ adsorption–desorption isotherms of the V_x/Pt/TiO₂ catalysts. According to previous studies, the isotherms could be categorized as type IV(a) with H3 hysteresis loops, which implied the existence of a large number of mesopores on the surface of the tested catalysts [22,23,24]. It can be seen from Figure 4b that the surface pore sizes of the V_x/Pt/TiO₂ catalysts were predominantly around 46 nm, which is in agreement with the results shown in Figure 4a. It can also be observed from Figure 4b that, compared with other V_x/Pt/TiO₂ catalysts, the V_0.5/Pt/TiO₂ catalyst possessed more abundant 45 nm pores on the surface. As shown in Table S2, the specific surface area of the V_0.5/Pt/TiO₂ catalyst was higher than those of other V_x/Pt/TiO₂ catalysts. But there was little difference in surface pore volume and pore diameter between the V_0.5/Pt/TiO₂ catalyst and each of the other V_x/Pt/TiO₂ catalysts. The results shown in Figure 4b and Table S2 indicate that 0.5 wt.% of the V loading amount might be conducive to the formation of mesopores on the surface of the V_0.5/Pt/TiO₂ catalyst, thus resulting in a relatively higher specific surface area of the V_0.5/Pt/TiO₂ catalyst. Generally, a higher specific surface area is conducive to the dispersion of active metal components and the adsorption of reactants on the catalyst surface, thus improving the catalytic activity [29]. Thus, the relatively larger specific surface area of the V_0.5/Pt/TiO₂ catalyst contributed to its excellent NH₃-SCO performance.

3.3. XRD

The crystal structures of active species and support are closely related to the performance of the catalyst. The XRD technique is considered a common technique for obtaining the above information. XRD tests were conducted on the V_x/Pt/TiO₂ catalyst, and the results are presented in Figure 5. Several obvious diffraction peaks can be observed in the XRD spectra of the V_x/Pt/TiO₂ catalysts. Diffraction peaks centered at 25.0°, 36.6°, 38.0°, 38.8°, 48.1°, 54.3°, 55.2°, 62.8°, 68.9°, 70.6°, and 75.3° could be attributed to anatase TiO₂ phases (ICDD #21-1276) [9,22,23]. The diffraction peak centered at 27.5° could be attributed to rutile TiO₂ phases (ICDD #21-1272) [24]. It can be seen from Figure 5 that there were no peaks attributed to the VO_x phase in the XRD spectra of the V_0.1/Pt/TiO₂, V_0.3/Pt/TiO₂, and V_0.5/Pt/TiO₂ catalysts. This suggested that the VO_x species over the above three catalysts were in a highly dispersed state or an amorphous state. As shown in Figure 5, a small diffraction peak (56.8°) emerged in each of the XRD spectra of the V_0.7/Pt/TiO₂ and V_0.9/Pt/TiO₂ catalysts. These peaks were ascribed to the VO_x species (ICDD #19-1401), indicating that there were tiny VO_x crystals over the V_0.7/Pt/TiO₂ and V_0.9/Pt/TiO₂ catalysts [22,23,24]. The above results show that when the V loading amounts were 0.1 wt.%, 0.3 wt.%, and 0.5 wt.%, the VO_x species could be well dispersed on the catalyst surface. When the V loading amounts were 0.7 and 0.9 wt.%, the VO_x species on the surface of the catalyst agglomerated slightly. It can be seen from Figure 5 that no diffraction peaks that could be attributed to the Pt species appeared in the XRD spectra of the V_x/Pt/TiO₂ catalysts. This phenomenon could be primarily ascribed to the extremely low Pt contents in the V_x/Pt/TiO₂ catalysts.

3.4. XPS

The NH₃-SCO performance of the catalyst is greatly influenced by the valence states and relative contents of the elements on the surface. XPS experiments were performed to analyze the valence states and relative contents of the main elements over the V_x/Pt/TiO₂ catalysts. The test results are presented in Figure 6 and Table S3. As shown in Figure 6a, the V 2p spectrum of each V_x/Pt/TiO₂ catalyst could be split into three sub-peaks. Sub-peaks centered at 515.6, 516.5, and 517.2 eV were attributable to the V³⁺, V⁴⁺, and V⁵⁺ species, respectively [22,23,24]. As shown in Table S3, with the V loading amount increasing from 0.1 wt.% to 0.5 wt.%, the V⁵⁺/(V³⁺+V⁴⁺+V⁵⁺) ratios of the V_x/Pt/TiO₂ catalysts increased from 28.20% to 32.65%. When the V loading amount was further increased from 0.5 wt.% to 0.9 wt.%, the V⁵⁺/(V³⁺+V⁴⁺+V⁵⁺) ratios of the V_x/Pt/TiO₂ catalysts gradually decreased from 32.65% to 25.22%. It has been reported that the V⁵⁺ species possesses good NH₃-SCR catalytic activity [24]. Thus, the relatively high V⁵⁺/(V³⁺+V⁴⁺+V⁵⁺) ratio of the V_0.5/Pt/TiO₂ catalyst was conducive to promoting the generation of N₂, enabling the V_0.5/Pt/TiO₂ catalyst to exhibit excellent N₂ selectivity.

The Pt 4f spectra of the V_x/Pt/TiO₂ catalyst are presented in Figure 6b. It can be seen that each of spectra could be resolved into four sub-peaks. As marked in Figure 6b, sub-peaks in the range of 73.6 to 77.9 eV could be ascribed to Pt⁰, Pt²⁺, and Pt⁴⁺ species [27,28]. As shown in Table S3, when the V loading amount was increased from 0.1 wt.% to 0.5 wt.%, the Pt⁰/(Pt⁰+Pt²⁺+Pt⁴⁺) ratio of the V_x/Pt/TiO₂ catalyst increased from 35.56% to 37.66%. When the V loading amount was further increased from 0.5 wt.% to 0.9 wt.%, the Pt⁰/(Pt⁰+Pt²⁺+Pt⁴⁺) ratio of the V_x/Pt/TiO₂ catalyst decreased from 37.66% to 35.23%. This suggested that adopting a V loading amount of 0.5 wt.% was conducive to the generation of the Pt⁰ species. Compared to the Pt²⁺ and Pt⁴⁺ species, the Pt⁰ species possess much higher catalytic oxidation activity, which is mainly attributed to its high activity in O₂ dissociation [22,23]. Therefore, the existence of a relatively larger amount of Pt⁰ species was beneficial for the V_0.5/Pt/TiO₂ catalyst to oxidize more NH₃ into NO_x, providing more reactants for the NH₃-SCR reaction, thus promoting the generation of N₂.

Figure 6c displays the O 1s spectra of the V_x/Pt/TiO₂ catalysts. It can be seen that the spectrum of each V_x/Pt/TiO₂ catalyst could be resolved into two sub-peaks. Sub-peaks centered at 531.2 eV and 529.8 eV could be ascribed to chemisorbed oxygen species (O_α) and lattice oxygen species (O_β), respectively. Table S3 shows that the O_α/(O_α+O_β) ratio of the V_0.5/Pt/TiO₂ catalyst is 15.23%, which is higher than those of other V_x/Pt/TiO₂ catalysts. Compared with O_β species, the O_α species possess higher activity in the NH₃ oxidation reaction [29,30,31,32]. Thus, a relatively larger amount of O_α species was conducive to oxidation of more NH₃ into NO_x and N₂. Subsequently, NO_x could be converted into N₂ through NH₃-SCR reactions, enabling the V_0.5/Pt/TiO₂ catalyst to exhibit a good N₂ selectivity.

3.5. O₂-TPD

The surface oxygen species can participate in NH₃ catalytic oxidation reactions, thereby influencing the SCO performance of the catalysts. For the purpose of characterizing the active oxygen species over the V_x/Pt/TiO₂ catalysts, O₂-TPD experiments were performed, and the results are shown in Figure 7. Previous studies pointed out that O₂ desorption below 250 °C mainly resulted from the desorption of physically adsorbed oxygen [9,22,23]. O₂ desorption between 250 °C and 600 °C was mainly caused by the desorption of chemically adsorbed oxygen from oxygen vacancies. Desorption of O₂ between 600 °C and 900 °C was mainly related to the desorption of surface lattice oxygen.

It can be seen from Figure 7 that for the O₂-TPD profile of each V_x/Pt/TiO₂ catalyst, there is a single peak within the range of 100–250 °C, which implies that a great amount of physically adsorbed oxygen desorbed from the surface of the V_x/Pt/TiO₂ catalyst in this temperature range. As shown in Figure 7, each of the O₂-TPD profiles of the V_x/Pt/TiO₂ catalysts had one peak in the range of 250 to 600 °C. The peak of the V_0.5/Pt/TiO₂ catalyst within the above temperature range was centered at 371.1 °C, which was significantly lower than the peak temperatures of the other V_x/Pt/TiO₂ catalysts. This indicates that, compared with other V_x/Pt/TiO₂ catalysts, the oxygen vacancies on the surface of the V_0.5/Pt/TiO₂ catalyst exhibit higher activity within the range of 250 to 600 °C [33]. This was beneficial for the V_0.5/Pt/TiO₂ catalyst to adsorb and activate O₂, enabling it to exhibit excellent NH₃-SCO performance.

3.6. Redox Performance

The NH₃-SCO performance of the catalysts is strongly associated with their redox performance. In order to investigate the redox performance of the V_x/Pt/TiO₂ catalysts, H₂-TPR tests were carried out over a temperature range of 100 to 800 °C, and the results are shown in Figure 8. The H₂-TPR profiles of each V_x/Pt/TiO₂ catalyst contained three reduction peaks. The reduction peaks at 268.7, 230.0, 188.7, 226.9, and 262.7 °C were attributed to the reduction processes of the Pt²⁺ and V⁵⁺ species. During these processes, Pt²⁺ was reduced to Pt⁰ and V⁵⁺ was reduced to V⁴⁺ [31,32,33,34]. The reduction processes of the V⁴⁺ species to the V³⁺ species and subsequent reduction in the V³⁺ species accounted for reduction peaks at 505.9, 416.0, 375.0, 479.8, and 430.1 °C [31,32,33,34,35]. As TiO₂ was difficult to react with H₂ under 600 °C, reduction peaks at 746.9, 728.9, 688.3, 742.1, and 727.8 °C were attributed to the reduction in the TiO₂ species [34].

It is worth noting that the temperatures corresponding to three reduction peaks in the H₂-TPR profile of the V_0.5/Pt/TiO₂ catalyst were lower than those of the other V_x/Pt/TiO₂ catalysts. This might be ascribed to the strong interaction between the active components and TiO₂ support [36]. The above results indicate that throughout the entire temperature, the V_0.5/Pt/TiO₂ catalyst exhibited better redox performance compared to other V_x/Pt/TiO₂ catalysts. In addition, as shown in Table S4, the H₂ consumption value (0.75 mmol/g) of the V_0.5/Pt/TiO₂ catalyst was higher than those of other V_x/Pt/TiO₂ catalysts. This indicated that compared with other V_x/Pt/TiO₂ catalysts, there were more abundant reducible species on the surface of the V_0.5/Pt/TiO₂ catalyst. Generally, outstanding redox performance and abundant reducible species are beneficial for the catalyst to exhibit excellent activity in the NH₃-SCR reaction. Thus, the V_0.5/Pt/TiO₂ catalyst could achieve higher N₂ selectivity via the i-SCR mechanism.

3.7. NH₃-TPD

Surface acidity played a pivotal role in facilitating adsorption and activation of NH₃ on the catalyst surface, which was critical for the NH₃-SCO reaction. The surface acidity of the V_x/Pt/TiO₂ catalysts was tested through NH₃-TPD experiments, and the results are displayed in Figure 9. Figure 9a displays the NH₃-TPD profiles of the V_x/Pt/TiO₂ catalysts. Based on previous studies, the NH₃-TPD profiles in the temperature range of 50 to 200 °C were ascribed to NH₃ desorption from weak acid sites. NH₃-TPD profiles between 200 and 350 °C were attributed to the desorption of NH₃ from medium acid sites [36,37,38]. The NH₃-TPD profiles in the temperature range of 350 to 500 °C were ascribed to the NH₃ desorption from strong acid sites [39,40,41]. The quantities of acid sites on the surface of V_x/Pt/TiO₂ catalysts were calculated by integrating the areas of sub-peaks in NH₃-TPD profiles, and the results are shown in Figure 9b. It can be seen that with the V loading amount increasing from 0.1 wt.% to 0.5 wt.%, the total acid quantities of V_x/Pt/TiO₂ catalysts gradually increased from 4.2 to 5.2 mmol/g. When the V loading amount was further increased from 0.5 wt.% to 0.9 wt.%, the total acid quantities of V_x/Pt/TiO₂ catalysts decreased from 5.2 to 3.2 mmol/g. This suggested that compared with other V_x/Pt/TiO₂ catalysts, the V_0.5/Pt/TiO₂ catalyst had more abundant acid sites on its surface. It can be seen from Figure 9b that the quantities of the weak and medium acid sites on the surface of the V_0.5/Pt/TiO₂ catalyst were higher than those of other V_x/Pt/TiO₂ catalysts. This was conducive to the adsorption and activation of NH₃ on the surface of the V_0.5/Pt/TiO₂ catalyst [34,35,36,37,38,39,40,41,42,43,44,45,46], enabling the V_0.5/Pt/TiO₂ catalyst to exhibit excellent N₂ selectivity.

3.8. HRTEM and EDX

HRTEM tests were conducted on Vₓ/Pt/TiO₂ catalysts, and EDX tests were carried out on the V_0.5/Pt/TiO₂ catalyst. The results are shown in Figure 10 and Figure 11. It can be seen from Figure 10 that the V_x/Pt/TiO₂ catalysts all exhibited a granular microscopic morphology, with a particle size of approximately 20–50 nm. This was mainly because in this work, P25 TiO₂ nanoparticles were used as the catalyst support. It could be observed that distinct lattice fringes existed in the highly magnified images. As marked in Figure 10, these fringes could be attributed to anatase TiO₂ species (ICDD 21-1276), rutile TiO₂ species (ICPD 21-1272), the Pt species (ICDD 43-1100), and the V₂O₅ species (ICDD 19-1401) [27,46]. This indicated that there were nanocrystals of the above-mentioned species on the surface of the catalysts.

The EDX test was employed to characterize the dispersion state of O, Pt, V, and Ti elements on the surface of the V_0.5/Pt/TiO₂ catalyst, and the results are shown in Figure 11. The results suggested that O, Pt, V, and Ti elements were uniformly distributed on the surface of the V_0.5/Pt/TiO₂ catalyst. Pt and V elements were in intimate contact with one another, which contributed to facilitating the interaction between the Pt and V species.

3.9. In Situ DRIFTS

The surface reaction pathways of NH₃-SCO catalysts are closely related to their N₂ selectivity. With the aim of exploring reaction pathways over V_x/Pt/TiO₂ catalysts, in situ DRIFTS experiments were carried out for the reaction between NH₃ and O₂ at 350 °C, and the results are shown in Figure 12. The results of in situ DRIFTS experiments have been discussed as follows. Detailed information about in situ DRIFTS experiments is presented in the Supplementary Materials.

As shown in Figure 12, the NH₃ species (3383, 3150 cm⁻¹), NH₃ species coordinated at Lewis acid sites (3250 cm⁻¹), NO₂ species (1650 cm⁻¹), bidentate nitrate species (1585 cm⁻¹), and NH₄⁺ species coordinated at Brønsted acid sites (1436 cm⁻¹) on the surface of V_x/Pt/TiO₂ catalysts were capable of taking part in the NH₃-SCO reaction [47,48,49,50,51,52,53]. It can be seen from Figure 12 that when both NH₃ and O₂ were introduced into reaction cell, a distinct signal of -NH₂ species (1539 cm⁻¹) appeared in the in situ DRIFTS spectrum of the V_0.5/Pt/TiO₂ catalyst [35]. But during the same reaction process, the signal of the -NH₂ species did not appear in the in situ DRIFTS spectra of other V_x/Pt/TiO₂ catalysts. This implied that the -NH₂ species could be involved in the NH₃-SCO reaction over the V_0.5/Pt/TiO₂ catalyst. Since the -NH₂ species was a key intermediate in the i-SCR mechanism [53,54], the appearance of the -NH₂ species over the V_0.5/Pt/TiO₂ catalyst might be associated with the promotion of the i-SCR reaction. This facilitated the formation of N₂, consequently enhancing the N₂ selectivity.

4. Discussion

HRTEM, EDX, XRD, XPS, and H₂-TPR results indicated that there were Pt⁰ and V₂O₅ nanocrystals on the surface of the V_0.5/Pt/TiO₂ catalyst. The Pt⁰ species had a strong ability for catalytic oxidation of NH₃. V₂O₅ species possessed remarkable catalytic performance for NH₃-SCR reactions. Thus, both NH₃-SCO and NH₃-SCR reactions existed on the surface of the Vₓ/Pt/TiO₂ catalyst, as shown in Figure 13. This enabled NH₃ to be converted into N₂ through the i-SCR mechanism. XPS and H₂-TPR results implied that the valence states of the V and Pt species could be regulated by varying the V loading amount, thereby regulating the synergistic effect between NH₃-SCO and NH₃-SCR reactions. When the V loading amount was 0.5 wt.%, there were abundant Pt⁰ and V⁵⁺ species on the surface of the V_0.5/Pt/TiO₂ catalyst. This was beneficial for making the synergistic effect between NH₃-SCO and NH₃-SCR reactions more efficient, thus enabling the V₀.₅/Pt/TiO₂ catalyst to exhibit excellent N₂ selectivity. N₂ adsorption–desorption, NH₃-TPD, O₂-TPD, and in situ DRIFTS results suggested that the V_0.5/Pt/TiO₂ catalyst possessed a higher specific surface area, abundant surface acid sites, and oxygen vacancies, as well as better reaction pathways. This also contributed to enabling the V_0.5/Pt/TiO₂ catalyst to exhibit excellent N₂ selectivity.

Due to the limitation of the article length, a systematic exploration of hydrothermal aging characteristics of the V_0.5/Pt/TiO₂ catalyst has not been carried out in this work. In future work, the NH₃-SCO performance of the V_0.5/Pt/TiO₂ catalyst under different hydrothermal aging conditions will be systematically explored. The influence mechanism of the hydrothermal aging process on the catalytic performance will be analyzed, and corresponding strategies for improving the catalytic performance will be proposed.

5. Conclusions

In this work, the V_x/Pt/TiO₂ tandem catalysts were synthesized for SCO of NH₃ by the two-step impregnation–deposition method. The synergistic effect of NH₃-SCO and NH₃-SCR reactions over V_x/Pt/TiO₂ tandem catalysts could be regulated by changing the V loading amount, thereby modulating the N₂ selectivity. When the Pt loading was 0.5 wt.%, there were relatively abundant Pt⁰ species and V⁵⁺ species on the surface of the V_0.5/Pt/TiO₂ catalyst. These two species, respectively, possessed excellent NH₃ catalytic oxidation performance and NH₃-SCR performance, thus enabling NH₃-SCO and NH₃-SCR reactions to achieve highly efficient synergy. In addition, the V_0.5/Pt/TiO₂ catalyst had a relatively high specific surface area, abundant surface acid sites, good redox properties, and reaction pathways that are more favorable for the generation of N₂, which are also the reasons for its excellent N₂ selectivity.