Revealing the Electronic Effects Between Pt and W on the Performance of Selective Catalytic Reduction of NO_x with H₂ over Pt-W/SSZ-13

Abstract

Selective catalytic reduction of NO_x with H₂ (H₂-SCR) is crucial for eliminating NO_x emissions from hydrogen internal combustion engines (H₂-ICE). Although 1 wt.% Pt/SSZ-13 (Pt/SZ) is a promising H₂-SCR catalyst, it faces challenges such as a narrow operating window and low N₂ selectivity. Herein, the effects of WO₃ on improving the H₂-SCR performance of Pt/SZ was investigated. Results showed that incorporating 5 wt.% WO₃ significantly widened the temperature window for 80% NO_x conversion and enhanced N₂ selectivity at 90–180 °C. Several characterizations revealed that electrons transfer from W to Pt, so more active Pt⁰ species were formed on 1 wt.% Pt-5 wt.% W/SZ (Pt-5W/SZ). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis indicated that more active monodentate nitrates, nitrites, and NH₄⁺ species were generated on Pt-5W/SZ, which are key intermediates for N₂ formation. Consequently, the temperature windows for NO_x conversion (over 80%) and N₂ selectivity (over 70%) were widened by 65 °C and 66 °C, respectively. This work provides insights into the developing H₂-SCR catalysts with broader operating windows and higher N₂ selectivity.

1. Introduction

Hydrogen, as a zero-carbon energy source, has attracted significant attention and is expected to replace traditional fossil fuels [1,2]. Recently, the hydrogen internal combustion engine(H₂-ICE) has developed rapidly and is anticipated to supplant conventional internal combustion engines to avoid the emissions of CO₂ and pollutant emissions (CO, hydrocarbons, NO_x, and particulate matter). H₂O is the only product from the combustion of H₂ in theory. However, thermal NO_x generated from the reaction between N₂ and O₂ at high pressures and temperatures is the main pollutant emitted from the H₂-ICE with the dilute combustion mode [3]. Especially, high-concentration NO_x would be emitted under the high-duty conditions, so it is intensively necessary to control NO_x emission. Currently, NH₃-SCR is considered as the most effective and mature de-NO_x technology, which needs to install the system of injecting urea solution to supply NH₃ as the reductant but usually suffers from NH₃ slip [4,5]. Selective catalytic reduction of NO_x with H₂ (H₂-SCR) is more suitable to the aftertreatment of H₂-ICE, which would simplify the conventional SCR system due to directly using the fuel H₂ as the reductant [6,7]. The key of H₂-SCR technology is the development of high-efficiency H₂-SCR catalysts.

Pt with unique d-electronic structure and better oxidation resistance would effectively activate and dissociate H₂ as the active component of H₂-SCR catalysts [8,9,10]. Metal oxides and zeolites are the main supports of H₂-SCR catalysts [11,12]. Machida et al. [13] prepared the Pt/ZrO₂-TiO₂ catalyst, which was able to achieve 89% NO conversion at 90 °C after pre-treatment with H₂. However, most Pt-based oxide catalysts that achieve high NO conversion but exhibit low N₂ selectivity convert NO to NO₂ and N₂O [14], with N₂O being 273 times more potent as a greenhouse gas than CO₂ [15], thus defeating the purpose of NO_x removal. Li et al. [16] conducted an investigation into the selective catalytic reduction of NO_x by H₂ over Pt/MgO, Pt/γ-Al₂O₃, Pt/ZrO₂ and Pt/HZSM-5. The findings indicated that Pt was more likely to remain in the metallic state on HZSM-5 as the acidic support with elevated active H, in conjunction with the effective conversion of NO to N₂ rather than N₂O by NH₄⁺ adsorbed on the Brønsted acidic sites.

SSZ-13, as the support, often displays higher catalytic performance among the reported zeolites. For example, Cu-SSZ-13 is a successful commercial NH₃-SCR catalyst to meet China/European VI emission standards for diesel engines [17,18]. Hong et al. [19] demonstrated that the substantial specific surface area of SSZ-13 has been shown to enhance the dispersion of platinum and increase the number of active sites, while its optimal acidity contributes to improved catalytic activity. However, the narrow operational temperature window and low N₂ selectivity of H₂-SCR reaction were observed on Pt/SSZ-13 [19,20]. Pt⁰ has been proved to be the active component for numerous catalytic reactions [21,22]. Enhancing the amount of Pt⁰ would widen the operational temperature window of H₂-SCR reaction over Pt/SSZ-13 [23,24,25]. WO₃, as an electronic and acidic promoter, has been employed to modulate the Pt valence states and promote the formation of Brønsted acid sites on Pt-based catalysts for the total oxidation of propane and glycerol hydrogenolysis [26,27]. NH₄⁺ species formed from NH₃ adsorbed on Brønsted acid sites of the zeolite are commonly considered as key intermediates to enhance N₂ selectivity of NH₃-SCR reaction, which have been detected in the H₂-SCR reaction [20,28]. However, detailed reports on the mechanism of H₂-SCR reaction on Pt-based zeolites are still lacking, including the types of the possible reaction pathways and the effects of the operational temperature on the reaction pathways. Therefore, this work primarily investigates the effects of introducing WO₃ on the catalytic activity and N₂ selectivity of Pt/SSZ-13, as well as the possible reaction pathways of H₂-SCR reaction.

2. Results and Discussion

2.1. H₂-SCR Performance

The 1 wt.% Pt-x wt.% W/SSZ-13 (x = 0, 1, 3, 5, 7, 10, 15) catalysts were prepared using the co-impregnation method and designated as Pt-xW/SZ. For comparison, monometallic catalysts were synthesized using the incipient wetness impregnation method, while the Pt/SZ@5W catalyst was specifically prepared by mechanically mixing Pt/SZ with WO₃. The catalytic activities of Pt-xW/SZ catalysts are illustrated in Figure 1a,b. A maximum NO_x conversion of 86% with only 54% N₂ selectivity was obtained on Pt/SZ at 80 °C, but the NO_x conversion was rapidly declined with further elevated the reaction temperatures, which might be due to the undesirable reaction between H₂ and O₂ at higher temperatures [19,20]. With increasing W loading, the NO_x conversion and N₂ selectivity were both significantly improved on catalysts, especially in the range of 90–250 °C. Among the investigated catalysts, Pt-5W/SZ exhibited the highest NO_x conversion and N₂ selectivity of H₂-SCR reaction. Exceeding 80% NO_x conversion and 70% N₂ selectivity were obtained on Pt-5W/SZ within 90–170 °C, as described in Figure S1. In contrast, the 5W/SZ catalyst (Figure S1f) displayed little H₂-SCR activity, suggesting that Pt was the active site for the H₂-SCR reaction, while W was employed as the promoter to modify Pt and enhance the catalytic activity. In addition, the maximum by-product NO₂ generation was significantly decreased from 179 ppm for Pt/SZ to 68 ppm for Pt-5W/SZ at 220 °C, as shown in Figure 1c. However, the H₂-SCR activity and N₂ selectivity of Pt/SZ@5W were both higher than those of Pt/SZ but still significantly lower than those of Pt-xW/SZ, which implied that there was the interaction between Pt and W on Pt-xW/SZ.

2.2. Structural Properties

XRD was employed to investigate the effect of introducing W on the structures of Pt/SZ, as shown in Figure 1d. The characteristic diffraction peaks of SSZ-13 (9.5°, 14.0°, 16.1°, 17.8°, 20.7°, 25.0°, and 30.7°) [29] were observed on all SSZ-13 supported catalysts. The loading of Pt and W had a negligible effect on the location of diffraction peaks. Additionally, the typical monoclinic WO₃ (PDF#71-2141) located at 2θ = 23.1, 23.6, 24.3, and 33.6° [30] was observed on the Pt-5W/SZ, Pt/SZ@5W, and Pt-10W/SZ. Due to the overlap between the Pt(111) peak at 39.7° and the SSZ-13 (502) peak at 39.6°, the Pt(200) peak at 46.2° was used to analyze the state of Pt species. As shown in the enlarged view of Figure 1d, the intensity of the Pt(200) peak increases with higher W loading, suggesting potential aggregation of Pt species. This observation is further supported by CO chemisorption results (

Table 1. Textural properties and Pt dispersion of the SSZ-13, Pt/SZ and Pt-xW/SZ (x = 1, 5, 10) catalysts.

Catalysts	Surface Area (m2/g)	Total Pore Volume (mL/g)	Average Pore Size (nm)	Dispersion a
SSZ-13	695	0.31	1.87	-
Pt/SZ	668	0.30	1.68	27%
Pt-1W/SZ	663	0.28	1.81	21%
Pt-5W/SZ	635	0.28	1.78	16%
Pt-10W/SZ	596	0.27	1.87	11%

^a measured by CO chemisorption.

), which reveal that the dispersion of Pt decreased on the SSZ-13 surface with increasing the loading of W. Notably, when the W content exceeds 5%, the Pt dispersion decreases significantly, consistent with the XRD findings. Moreover, the decline in the H₂-SCR catalytic performance may be attributed to the coverage of Pt active sites by an excessive amount of WO_x, which reduces the accessibility of reactants to the active sites [31]. Furthermore, as shown in Table 1, the specific surface area (S_BET) of the catalysts was decreased with increasing the loading of metals. A particularly severe decrease in S_BET was observed on Pt-10W/SZ due to the high loading of metal oxides occupying the pores of SSZ-13. In general, the specific surface area of the catalyst remains approximately 600 m²/g, which is adequate for the dispersion of Pt and W.

2.3. Chemical States of Pt Species

X-ray photoelectron spectroscopy (XPS) was employed to illustrate the chemical states of Pt and W species on catalysts. Herein, XPS spectra of Pt 4d were used to distinguish the chemical states of Pt species due to the overlap of Pt 4f and Al 2p orbitals [22]. As shown in Figure 2a,b, the characteristic peak of Pt 4d_5/2 at 316.6 eV on Pt/SZ decreased to 316.2 eV on Pt-5W/SZ, while the characteristic peak of W 4f_7/2 centered at 35.0 eV on 5W/SZ was shifted to 35.4 eV on Pt-5W/SZ. However, the differences in binding energies for both Pt and W species between Pt/SZ and Pt/SZ@5W were neglected. The increased binding energy of W 4f and the decreased binding energy of Pt 4d on the Pt-5W/SZ catalyst directly demonstrate the electronic transfer from W to Pt [32,33]. Additionally, Figure S2 shows the Pt 4d peak shifting to lower binding energies with higher W content, while the W 4f peak gradually shifts toward higher binding energy, indicating stronger Pt-W interactions. Below 5% W, weaker interactions partially modify Pt electronically, enhancing activity at 1% W. Above 5%, strong Pt-W interactions and W coverage of Pt sites reduce H₂-SCR activity. When the W content exceeds 5%, although Pt-W interactions are significantly enhanced, the excessive coverage of Pt active sites by WO_x species results in reduced H₂-SCR catalytic activity. Thus, optimal performance at 5% W results from balanced Pt-W interactions that enhance activity without excessive Pt site blockage.

The fitted XPS spectra of Pt 4d and W 4f were shown in Figure 2c,d. The characteristic peaks assigned to Pt⁰ (315.3 eV, 332.5 eV) and Pt²⁺ (318.3 eV, 335.0 eV) species were observed on the three catalysts [34]. The relative concentration of Pt⁰ was increased from 51% of Pt/SZ to 71% of Pt-5W/SZ, as listed in

Table 2. XPS fitting results of Pt and W species in the three catalysts.

Catalysts	Pt0/(Pt0 + Pt2+)	W5+/(W5+ + W6+)	Pt (At%)	W (At%)
Pt/SZ	51%	-	0.78%	-
Pt-5W/SZ	71%	31%	0.75%	1.42%
5W/SZ	-	48%	-	1.15%

. Furthermore, the characteristic peaks assigned to W⁵⁺ and W⁶⁺ species were observed on all W-contained catalysts, with the relative concentration of W⁶⁺ decreasing from 48% on 5W/SZ to 31% on Pt-5W/SZ [35]. Thus, the strong interactions between Pt and W, due to electron transfer from W to Pt, resulted in the formation of more Pt⁰ species on Pt-5W/SZ. More Pt⁰ is favorable for the adsorption and dissociation of H₂ and the generation of NO_x⁻ intermediates [23,24], which contributed to improving the H₂-SCR activity of Pt-5W/SZ catalyst.

Pt located in different coordination environments tends to exhibit different chemical states and play different roles in catalytic reactions. Therefore, the locations of Pt species in SSZ-13 zeolite for different catalysts were investigated by FTIR spectra within the -OH vibrational region. As illustrated in Figure 3a, the vibrational peak at 3612 cm⁻¹ is attributed to the O-H stretching vibration of Si-OH-Al in the frameworks of zeolites [36]. Compared with SSZ-13, the corresponding band intensity of 5W/SZ was essentially unchanged, indicating that W was not entered into the framework of SSZ-13, because the ionic radius of H₂W₁₂O₄₀⁶⁻ as the precursor of W species is much larger than the pore size of the SSZ-13 (0.38 nm). But for the Pt/SZ catalyst, the relative intensity of the band assigned to Si-OH-Al was remarkedly decreased with respect to SSZ-13, indicating that a substantial number of Pt had entered into the framework of SSZ-13 and interacted with the -OH sites to form Pt²⁺ species [37]. After the introduction of 5 wt.% W into Pt/SZ, the band intensity ascribed to Si-OH-Al structures on Pt-5W/SZ catalyst was lower than that of Pt/SZ but higher than that of SSZ-13, suggesting that, while some Pt species remained within the SSZ-13 framework, others migrated to the surface of Pt-5W/SZ to form Pt⁰ species. It could be explained by the electrostatic interaction between H₂W₁₂O₄₀⁶⁻ and Pt²⁺ during the co-impregnation process. To further confirm this point, Pt-5W/SZ-SI and 5W-Pt/SZ-SI catalysts were prepared via the stepwise impregnation method. The band intensities of Si-OH-Al structures for both catalysts were similar as that of the Pt/SZ. Therefore, the interactions between Pt and W were in the presence of Pt-5W/SZ prepared by the co-impregnation method to induce the formation of Pt⁰ species rather than the stepwise impregnation method, which is in agreement with the results of XPS.

2.4. Formation of NO_x⁻ Species

The adsorption of NO + O₂ on the catalysts was examined by NO_x-TPD, as illustrated in Figure 3b and Figure S4. There was a similar adsorption amount of NO_x on 5W/SZ and SSZ-13, which suggested that NO_x would not be adsorbed on W species. Nevertheless, the adsorption amount of NO_x on Pt/SZ was markedly larger than that of SSZ-13, and the desorption temperature wi ndow was considerably widened, indicating that Pt was the primary adsorbed site for NO_x. Meanwhile, more of the total desorption amount of NO_x was achieved on Pt-5W/SZ than Pt/SZ, especially the desorption amount of NO₂ was notably increased from 149 μmol/g to 295 μmol/g. This indicated that the addition of W into Pt/SZ was in favor of generating more NO₂⁻/NO₃⁻ species on Pt-5W/SZ rather than Pt/SZ. Active NO₃⁻ and NO₂⁻ would react more rapidly with H₂ to produce N₂ and H₂O, thereby improving both the NO_x conversion and N₂ selectivity [23,24]. Subsequently, in situ DRIFTS was employed to further confirm the formation of NO_x⁻ intermediates during NO_x adsorption.

NO + O₂ co-adsorption DRIFTS on the catalysts for 30 min was conducted as a function of temperature (30–250 °C). As illustrated in Figure 3c, the NO_x adsorption peaks observed on the catalyst were attributed to Pt-NO⁺ (1746 cm⁻¹) [38], bridged bidentate nitrate (1628 cm⁻¹) [39], chelated bidentate nitrate (1609 cm⁻¹) [40], and nitrite species (1440 cm⁻¹) [41], respectively. The intensity of adsorption bands on the Pt/SZ catalyst gradually decreased with increasing the temperature. Nitrite species (1440 cm⁻¹) and chelated nitrate (1609 cm⁻¹) species weakly adsorbed on the surface of catalysts disappeared at 100 °C, while other adsorption bands disappeared until at 180 °C. The lower NO_x adsorption on Pt/SZ resulted in smaller adsorbed NO_x that can react with H₂ on the surface of the catalyst, which led to the decline in NO_x conversion between 140 and 250 °C. However, more Pt-NO⁺, nitrates, and nitrites species were observed on Pt-5W/SZ at higher temperatures. Furthermore, new bands attributed to monodentate nitrate (1504 cm⁻¹) [42] and nitrite (1445 cm⁻¹) species were observed on Pt-5W/SZ in the whole range of 30–200 °C. Similarly, the enhanced adsorption of NO_x⁻ is observed even at 200 °C on the Pt-5W/SZ catalyst, indicating that it possessed more stable NO_x⁻ species to participate in the H₂-SCR reaction at high temperatures. The combined NO_x−TPD and DRIFTS analyses demonstrated that the incorporation of W increased the population of NO_x adsorption sites through enhanced formation of metallic Pt⁰ species, which serve as adsorption and activation sites of NO_x [23]. This elevated Pt⁰ content consequently facilitated the generation of NO_x⁻ intermediates, which undergo efficient hydrogenation by H₂ to yield N₂ as the final product.

2.5. Reactivity of the Adsorbed NO_x⁻ with H₂

To reveal the reactivity of adsorbed NO_x⁻ with H₂, the dynamic changes of DRIFTS spectra with preabsorbed NO + O₂ followed by H₂ were studied on catalysts at 180 °C, as shown in Figure 4a,b. Co-adsorption of NO and O₂ at 180 °C showed adsorption bands similar to those in Figure 3c over Pt/SZ. When H₂ was introduced into the reaction chamber of preabsorbed NO + O₂, the band intensities of the adsorbed NO_x⁻ species were gradually decreased until they disappeared, indicating that NO_x⁻ species were reduced by H₂ and gradually converted to NH₄⁺ adsorbed on the Brønsted acid site. After 140 s, an adsorption band located at 1441 cm⁻¹ assigned to the NH₄⁺ species was observed [43]. Thus, both Pt-NO⁺ and bridged bidentate nitrate species on Pt/SZ are reactive at 180 °C and serve as the active intermediates of the H₂-SCR reaction. Figure 3d showed that all bands assigned to nitrate species on Pt-5W/SZ exhibited a decrease after the introduction of H₂. Differently, the nitrite and monodentate nitrate species were quickly consumed within 20 s of H₂ reaction on Pt-5W/SZ, indicating that the two types of nitrate species were more reactive than Pt-NO⁺ and bridged bidentate nitrate species. With the consumption of nitrate species, the characteristic band attributed to NH₄⁺ on the Brønsted acid site was detected at 100 s. Consequently, the Pt-5W/SZ catalyst possessed more active NO_x⁻ intermediates species, which would be more quickly consumed by H₂ than those of Pt/SZ, consistent with higher NO_x conversion, as shown in Figure 1. Furthermore, more formed NH₄⁺ species on Pt-5W/SZ would serve as a pivotal intermediate involved in the NH₃-SCR reaction, thereby enhancing N₂ selectivity of H₂-SCR reaction on Pt-5W/SZ.

2.6. In Situ NH₄⁺ Formation and Reactions

For a better understanding of the evolution of intermediates during the H₂-SCR reaction, in situ DRIFTS of NO + H₂ + O₂ from 40 to 250 °C was performed, as shown in Figure 4c,d. In contrast to the co-adsorption DRIFTS of NO + O₂, Pt-NO⁺ and bidentate nitrate species were observed on Pt/SZ at the initial reaction temperature (40 °C). New N₂O₄ species (1695 cm⁻¹) [44] were detected with increasing the temperature, leading to an enhancement of byproduct NO₂ in the H₂-SCR reaction on Pt/SZ (Figure 1). Simultaneously, NH₄⁺ (1457 cm⁻¹) adsorbed on the Brønsted acid sites of the catalyst was observed until in 140–160 °C, indicating that a small amount of NH₄⁺ species as key intermediates was generated on Pt/SZ. However, Pt-NO⁺ and bridged nitrate species accumulated on Pt-5W/SZ from 100 °C and were gradually consumed due to participation in the H₂-SCR reaction. When above 120 °C, NO₂ was also generated on Pt-5W/SZ, but its amount was significantly less than that on Pt/SZ. Meanwhile, NH₄⁺ (1460 cm⁻¹) was detected from 100 °C, and the intensity of its band was increased with rising temperature. It was interesting that no NH₃ was detected during the process of the H₂-SCR reaction, implying that the intermediate NH₄⁺ would be reacted with adsorbed NO_x⁻ species to be N₂ rather than NO₂ at higher temperatures in Figure 1c. Furthermore, more Brønsted acid sites were formed on Pt-5W/SZ, allowing more NH₄⁺ species to be adsorbed on Pt-5W/SZ rather than Pt/SZ (Figure S5). As a result, N₂ selectivity was obviously enhanced on Pt-5W/SZ by introducing WO₃ into Pt/SZ.

Since the amount of in situ-generated NH₃ in the catalyst could not be determined, NH₃-SCR activity tests were conducted at 200 °C by adjusting the NH₃ feed concentration. As shown in Figure 4e, the N₂ selectivity was gradually improved by reducing the NH₃ concentration, with a particularly significant improvement being observed under low NH₃ concentration. In Figure 4f, further NH₃-SCR tests under the NH₃/NO = 0.1 demonstrated that both catalysts achieved higher NH₃-SCR activity rather than under the standard SCR conditions [45] within 90–300 °C. Importantly, higher NH₃-SCR activity and N₂ selectivity were achieved on Pt-5W/SZ rather than Pt/SZ within the H₂-SCR conversion window. On the other hand, pre-adsorbing NH₃ on the catalysts prior to H₂-SCR testing (Figure S6) revealed that NH₄⁺ species formed and were stored on Brønsted acid sites of Pt/SZ, significantly enhancing N₂ selectivity above 130 °C. In contrast, similar treatment on Pt-5W/SZ slightly reduced H₂-SCR performance. These results indicate that, in the mid-to-high temperature range, a small amount of in situ-generated NH₄⁺ adsorbed on Brønsted acid sites is critical for improving N₂ selectivity on Pt-5W/SZ. More Pt⁰ species on the surface of Pt-5W/SZ catalyst promoted the adsorption and oxidation of NO, allowing more NO₂ or NO_x⁻ species to be reduced by the fast SCR reaction at low temperatures. Thus, the NH₃-SCR activity of Pt-5W/SZ was higher than that of Pt/SZ. Moreover, NH₄⁺ and adsorbed nitrite species on the Pt-5W/SZ catalyst reacted to form NH₄NO₂, which could directly decompose into N₂ and H₂O at 90–180 °C [36,46], thereby improving the N₂ selectivity of the H₂-SCR reaction through the NH₃-SCR pathway. More NH₄⁺ and nitrite species were formed on Pt-5W/SZ, and they participated in the NH₃-SCR reaction pathway, thereby contributing to higher N₂ yields on Pt-5W/SZ compared to Pt/SZ [20,47].

Based on the characterization and reactivity results, a bifunctional catalytic mechanism is proposed for the Pt-5W/SZ catalyst. The synergistic interaction between Pt and W promotes electron transfer from W to Pt, enhancing the formation of active metallic Pt0 sites. Initially, adsorbed H₂ dissociates on Pt⁰ to form active hydrogen species, while NO and O₂ are co-adsorbed and activated as NO_x⁻ species, including Pt-NO⁺, nitrates, and nitrites. These NO_x⁻ species react with active hydrogen to yield N₂ and H₂O. Simultaneously, adsorbed NO combines with active hydrogen to produce NH₃, which is subsequently adsorbed on Brønsted acid sites to form NH₄⁺. The NH₄⁺ species further react with NO and O₂ via NH₄⁺ + 4NO + O₂ → 4N₂ + 6H₂O + 4H⁺, improving N₂ selectivity. Moreover, NH₄⁺ may react with adsorbed nitrites on the Pt-5W/SZ catalyst to generate NH₄NO₂, which decomposes directly into N₂ and H₂O at 90–180 °C. This bifunctional mechanism enables the Pt-5W/SZ catalyst to exhibit a wide H₂-SCR reaction window and high N₂ selectivity.

3. Materials and Methods

3.1. Catalyst Preparation

A 1 wt.% Pt-x wt.% W/SSZ-13 (x = 0, 1, 3, 5, 7, 10, 15) catalyst was prepared using the co-impregnation method, and the details were as follows. Firstly, Pt(NO₃)₂ and (NH₄)₆H₂W₁₂O₄₀·xH₂O as precursors were dissolved in a certain amount of distilled water and then poured into the commercial SSZ-13 (SiO₂/Al₂O₃ = 22). Secondly, the mixture was stirred homogeneously and dried at 90 °C for 12 h. Thirdly, the serial 1 wt.% Pt-x wt.% W/SSZ-13 catalysts were obtained after calcining the dried power at 550 °C for 3 h, denoted as Pt-xW/SZ. The same procedure was used to prepare 1 wt.% Pt/SSZ-13 and 5 wt.% W/SSZ-13 catalysts, which were denoted as Pt/SZ and xW/SZ, respectively. WO₃ was obtained by calcining (NH₄)₆H₂W₁₂O₄₀·xH₂O at 550 °C. For comparison, Pt/SZ@5W catalysts were prepared with mechanical mixing of Pt/SZ and WO₃. Similarly, Pt-5W/SZ-SI and 5W-Pt/SZ-SI catalysts were prepared using the stepwise impregnation method. In detail, 5W/SZ (or Pt/SZ) catalysts were prepared, and then 1 wt.% Pt (or 5 wt.% W) continued to be loaded on the as-synthesized 5W/SZ (or Pt/SZ).

Finally, all the prepared catalyst powders were coated on the honeycomb cordierite (400 meshes, 2.5 mL, Corning Ltd., Corning, NY, USA) as homogeneous slurry (350 mg) and carried the same roasting process as the powders to obtain the monolithic catalysts. Furthermore, the catalytic performance measurement and catalytic characterization of the catalysts can be found in the Support Information.

3.2. Catalytic Performance Measurement

The H₂-SCR catalytic performance was evaluated in a self-assembled fixed-bed quartz tube reactor with a reaction gas consisting of 300 ppm NO, 2400 ppm H₂, and 5 vol.% O₂ in N₂ balance. The gas flow rate was 1500 mL·min⁻¹ (GHSV = 36,000 h⁻¹). The outlet gas concentration at different temperatures was monitored using a Fourier transform infrared spectrometer (Nicolet Antaris IGS-6700, Thermo Fisher Scientific, Waltham, MA, USA). After the reaction reached a steady state, the associated NO_x conversion and N₂ selectivity were calculated as follows:

$${\mathrm{NO}}_{\mathrm{x}} \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)={\displaystyle \frac{{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{in}}-{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{out}}}{{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{in}}}}\times 100$$

(1)

$${\mathrm{S}}_{{\mathrm{N}}_2}\left(\%\right)={\displaystyle \frac{{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{in}}- {{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{out}}-2{{[\mathrm{N}}_2\mathrm{O}]}_{\mathrm{out}}}{{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{in}}- {\left[\mathrm{NO}\right]}_{\mathrm{out}}}}\times 100$$

(2)

where [NO_x]_in and [NO_x]_out represent the NO_x concentration at the tail gas inlet and outlet, respectively.

The catalyst activity of NH₃-SCR was evaluated in a fixed-bed quartz tube flow reactor. The gas flow was controlled using a mass flowmeter before entering the reactor. The simulated reaction conditions were as follows: 30–300 ppm NH₃, 300 ppm NO, 5 vol.% O₂, and N₂ as balance gas. The GHSV was 36,000 h⁻¹, corresponding to total flow rate of 1500 mL·min⁻¹. The related method of the gas detection method and the calculation for NO_x conversion were consistent as above, while the NH₃ conversion and N₂ selectivity were calculated according to the following equation:

$${\mathrm{N}\mathrm{H}}_3 \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)={\displaystyle \frac{{{[\mathrm{N}\mathrm{H}}_3]}_{\mathrm{in}}-{{[\mathrm{N}\mathrm{H}}_3]}_{\mathrm{out}}}{{{[\mathrm{N}\mathrm{H}}_3]}_{\mathrm{in}}}}\times 100$$

(3)

$${\mathrm{S}}_{{\mathrm{N}}_2}\left(\%\right)={\displaystyle \frac{{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{in}}-{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{out}}+{{[\mathrm{NH}}_3]}_{\mathrm{in}}-{{[\mathrm{NH}}_3]}_{\mathrm{out}}-2{{[\mathrm{N}}_2\mathrm{O}]}_{\mathrm{out}}}{{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{in}}-{{[\mathrm{NO}}_{\mathrm{x}}]}_{\mathrm{out}}+{{[\mathrm{NH}}_3]}_{\mathrm{in}}-{{[\mathrm{NH}}_3]}_{\mathrm{out}}}}\times 100$$

(4)

3.3. Catalyst Characterization

X-ray diffraction (XRD) characterization was performed on a Rigaku Ultima IV diffractometer (Rigaku Corporation, Tokyo, Japan) with a Cu-K radiation source (λ = 1.5406 Å), 2θ from 5 to 80° at a step of 8°/min. The specific surface area was calculated from the N₂ adsorption isotherm using the BET method, and the samples were pretreated with N₂ at 300 °C for 3 h.

The dispersion of Pt was determined using CO chemisorption. Prior to the measurement, the catalyst sample underwent a pretreatment process to ensure complete reduction of Pt species and removal of surface-adsorbed impurities. Specifically, 200 mg of the sample was placed in a U-shaped tube and reduced under a H₂ flow (30 mL/min) at 450 °C for 1 h. After reduction, the sample was cooled to room temperature under a He atmosphere. Subsequently, CO pulses were introduced into the sample until the thermal conductivity detector (TCD) signal stabilized. The Pt dispersion was determined based on the amount of CO consumption (assuming a CO/Pt = 0.8).

X-ray photoelectron spectra (XPS) were collected on a Thermo Scientific K-Alpha electron spectrometer. Al Kα was used as the radiation source, and the binding energy of all samples was calibrated with C1s binding energy of 284.6 eV.

Temperature-programmed desorption experiments of NO_x (NO_x-TPD) were measured in a fixed-bed quartz tube flow reactor. The concentrations of NO_x were monitored with FT-IR in Antaris IGS (Nicolet, Green Bay, WI, USA). All sample masses of about 200  mg in a quartz tubular mico-reactor were used. The experiment included four stages: (1) the samples were activated in N₂ at 450 °C for 1  h; (2) a flow of 300 ppm NO and 5  vol.% O₂ was induced at 30 °C for 30  min; (3) isothermal desorption in N₂ at 30 °C until no N_xO_y was detected; and (4) temperature programmed was performed desorption in N₂ (TPD stage) at 10 °C/min up to 200 °C.

All in situ diffuse reflectance infrared Fourier transform (DRIFT) experiments were carried out on the Thermo Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) after being purified in He for 0.5 h at 450 °C. (1) For in situ DRIFT spectra after the adsorption of NO + O₂, the samples were saturated at 30 °C under 300 ppm NO + 5 vol.% O₂. Then, the temperature was heated up stepwise from 30 to 250 °C to record the adsorption spectra. (2) For the in situ DRIFT reaction after the adsorption of NH₃, the sample was pre-adsorbed with 500 ppm NH₃ at 30 °C, followed by N₂ purging to remove weakly adsorbed species, and then the spectra were recorded at 250 °C. (3) For the in situ DRIFT reaction of the adsorbed NO_x⁻ with H₂, the sample was first co-adsorbed with NO and O₂ at 180 °C for 30 min—a temperature regime where the Pt-5W/SZ catalyst maintained superior catalytic performance, while the Pt/SZ catalyst demonstrated marked activity discrepancies under identical conditions—followed by controlled H₂ pulse injection to initiate the surface reactivity evaluation. Similarly, the H₂-SCR reaction flow containing NO, O₂, and H₂ were also injected into the sample and heated to explore the formation of intermediate species in the reaction process.

4. Conclusions

Pt-xW/SZ catalysts with different loadings of W were prepared using the co-impregnation method for de-NO_x via H₂-SCR reaction. The results indicated that NO_x conversion, the reaction temperature window, and N₂ selectivity were simultaneously improved by introducing W into Pt/SZ, and the best H₂-SCR performance was achieved on Pt-5W/SZ with 5 wt.% WO₃. Several characterizations showed that the above enhancements were attributed to more Pt⁰ formed on Pt-5W/SZ rather than Pt/SZ. The IR spectra of -OH regions and XPS results indicated that a strong interaction between Pt and W existed on Pt-5W/SZ prepared by the co-impregnation method, which resulted from the electron transfer from W to Pt to form more Pt⁰ on Pt-5W/SZ than Pt/SZ. In situ DFTIRS studies suggested that more reactive NO_x⁻ species on Pt-5W/SZ were conductive to improving the NO_x conversion in 90–180 °C. Moreover, the amount of Brønsted acid sites was increased by the addition of W into Pt/SZ, which facilitated the formation of NH₄⁺ species that subsequently participated in the NH₃-SCR pathway to improve N₂ selectivity. As a result, the H₂-SCR performance of the Pt-5W/SZ catalyst was significantly better than that of Pt/SZ.