The Activation of Oxygen Species on the Pt/CeO₂ Catalyst by H₂ for NO Oxidation

Abstract

The Pt/CeO₂ catalyst has attracted significant attention due to its exceptional performance in NO oxidation. This study comprehensively examines the effects of calcination temperature and H₂ pretreatment on the structure and activity of the Pt/CeO₂ catalyst. Experimental findings indicate that the calcination temperature significantly affects the catalyst’s redox performance, thereby modulating its efficacy in NO oxidation reactions. H₂ pretreatment facilitates the creation of oxygen vacancies on the catalyst, assisted by the reduction in PtO_x to Pt, enhancing the formation of activated oxygen and thereby improving NO oxidation. This study offers valuable insights into the design and optimization of Pt/CeO₂ catalysts for environmental applications, particularly in the development of exhaust gas after-treatment technologies.

1. Introduction

Nitrogen oxides (NO_x) are among the primary atmospheric contaminants, contributing to various environmental hazards, including photochemical smog, acid rain, the greenhouse effect, and depletion of the stratospheric ozone layer [1,2]. NO oxidation plays a critical role in advanced exhaust gas treatment technologies, particularly in selective catalytic reduction (SCR) [3], non-selective catalytic reduction (NSCR) [4], and NO_x storage reduction (NSR) [5]. These technologies are predominantly employed to mitigate nitrogen oxide emissions, especially from automotive exhaust and power plants. It is widely recognized that the conversion of NO to NO₂ is a critical step in the rapid SCR reaction, significantly enhancing the rate of the SCR reaction [6]

Currently, a common challenge faced by all NO oxidation catalysts is their activity, which is restricted to a relatively narrow high-temperature range. This limitation not only reduces their efficiency in low-temperature environments but also increases energy consumption and operational costs. Consequently, there is an imperative need to innovate and develop catalysts that can perform effectively at lower temperatures. Such advancements would broaden their applicability and significantly enhance overall performance. To address this challenge, researchers are actively exploring various strategies to lower the activation temperature of catalysts. These approaches include optimizing the chemical composition and microstructure of catalysts, such as introducing novel active components, modifying the properties of the support material, incorporating promoters to alter surface and electronic characteristics, and utilizing nanotechnology to fabricate materials with enhanced surface activity and distinctive catalytic performance. Given the complex operating conditions of diesel vehicles, it is essential to investigate composite catalysts that improve the overall efficiency of NO_x reduction. Under fuel-rich conditions, the CO water–gas shift reaction can be employed to generate H₂, which then reacts with NO to produce NH₃, a key component for the downstream SCR unit. This approach aims to achieve a higher NO_x conversion rate than that observed under steady-state experimental conditions, effectively addressing the complexity of diesel exhaust emissions. During lean-burn conditions, diesel engines attain complete combustion, and molecular sieve catalysts adsorb the resulting NO_x. In the subsequent fuel-rich phase, NH₃ produced through the Cu/CeO₂ catalyst can then reduce the adsorbed NO_x. This innovative strategy eliminates the need for urea and platinum-group metals, significantly reducing operational costs. By employing such dual-function catalysts, this approach not only improves NO_x conversion efficiency but also offers a cost-effective solution for managing the fluctuating exhaust conditions of diesel engines. This intricate interplay of reactions ensures the system dynamically adapts to varying conditions, thereby optimizing catalytic performance and minimizing environmental impact [7].

For NO oxidation catalysts, the main categories include multi-metal oxide catalysts, perovskite catalysts, carbon-based catalysts, fast SCR catalysts, and supported catalysts [8]. Additionally, Wu et al. [9] introduced MnO_x-CeO₂ mixed oxide into a Pt/Al₂O₃ catalyst to enhance its NO oxidation activity. The superior redox properties of the MnO_x-CeO₂ mixed oxide significantly improved the catalyst’s performance, achieving a maximum NO conversion rate of 80% at 350 °C. Nickolov et al. [10] and Zhuo et al. [11] demonstrated that TiO₂ promotes the superior dispersion of active components on its surface, effectively preventing the agglomeration of catalyst particles. Kim et al. [12] found that Sr-doped perovskite catalysts (La_0.9Sr_0.1CoO₃) exhibit excellent NO oxidation activity, achieving a maximum NO conversion rate of 86% at 300 °C. Guo et al. [13] studied the oxidation of NO using activated carbon (AC) derived from coconut shell, coal tar, and polyacrylonitrile. Under identical conditions, coconut shell-derived AC exhibited the highest activity, maintaining robust NO oxidation even in low concentrations of O₂. Additionally, noble metals exhibit excellent catalytic activity for NH₃-SCR at low temperatures. Kim et al. [14] evaluated the NO oxidation and NH₃-SCR of Pt/Al₂O₃, Cu/Al₂O₃, and Pt-Cu/Al₂O₃ catalysts. The Pt/Al₂O₃ catalyst exhibited the highest efficiency for both NO oxidation efficiency and NO_x removal. However, a common challenge associated with these metal oxide catalysts is their limited capacity to generate and migrate oxygen vacancies, as well as their poor thermal stability. Furthermore, achieving optimal catalytic performance often requires a high loading of precious metals, which limits their activity in NO oxidation reactions.

In the case of supported catalysts, which include noble metals and metal oxides, the active component predominantly resides on the catalyst surface, while the support offers a large surface area for separating the active phase and providing the necessary space for the catalytic reaction. Hernánove et al. [15] found that an Au-TiO₂ photocatalyst with a gold content of 0.5% exhibited the best catalytic performance, attaining an NO conversion rate of 85% within 60 min under UV irradiation. Single-atom catalysts (SACs) are synthesized using atomic-level dispersion techniques to create atomically dispersed catalysts (ADCs). In these catalysts, active metal atoms are uniformly distributed across the support, with each metal atom individually dispersed rather than forming metal nanoparticles [16]. Liu et al. utilized atomic dispersion techniques to load Ru metal onto MgAl_1.2Fe_0.8O₄ and compared its activity with that of conventional Ru nanocatalysts supported on MgAl_1.2Fe_0.8O₄. The results revealed that the atomically dispersed catalyst demonstrated superior N₂O decomposition performance under both low and high N₂O concentration conditions. Characterization data indicated that the atomically dispersed Ru acted as the primary active site responsible for this enhanced performance [17]. Due to its high reducibility and exceptional oxygen storage capacity, CeO₂ has been extensively used as a catalyst support or promoter [18]. Olsson’s study confirms that the reaction rate of NO oxidation decreases with decreasing platinum particle sizes [19]. They explain that smaller platinum particles are more prone to the formation of PtO and PtO₂, which exhibit lower activity compared to metallic platinum. When Pt is loaded onto CeO₂, the strong interaction at the Pt-Ce interface often results in significant electron perturbation, thereby greatly enhancing the catalytic activity of the catalyst [20,21]. It is widely recognized that the CO oxidation process on Pt/CeO₂ surfaces involves a reaction between CO adsorbed on Pt and active oxygen species supplied by CeO₂ [22]. This process does not involve competitive adsorption between CO and O₂. Zhang et al. found that the composition of Au species in Au/CeO₂ catalysts remains relatively stable after redox reactions. In this context, Ce⁴⁺-OH species are reduced to Ce³⁺ by CO, which are subsequently re-oxidized to Ce⁴⁺-OH by H₂O, generating oxygen vacancies (Ce³⁺-O_v-Ce³⁺) in the process. They proposed an associative redox mechanism to explain the redox dynamics at the metal-support interface of the catalyst [23]. However, current research has not yet fully elucidated the state of Pt, the interactions between Pt and Ce, the redox cycling mechanisms between Ce³⁺ and Ce⁴⁺, and their collective influence on NO oxidation activity.

In this study, a series of 1 wt% Pt/CeO₂ catalysts with varying calcination temperatures were prepared using the impregnation method. The NO oxidation activity of the Pt/CeO₂ catalysts, both before and after H₂ pretreatment, was evaluated. Characterization techniques such as XRD, TPD, TPR, and in situ DRIFTS were employed to investigate the impact of H₂ treatment on the activity of the catalyst annealed at different temperatures. The primary focus was on examining the influence of H₂ reduction treatment on catalyst activity and the synergistic effects between the Pt and CeO₂ surfaces. The role of active oxygen species on the Pt/CeO₂ surface was studied, and the catalytic sites and mechanisms of the NO oxidation reaction were elucidated based on proposed mechanisms in the literature.

2. Results and Discussion

2.1. NO Oxidation Performance

The NO oxidation performance of the Pt/CeO₂ catalyst gradually decreased with the increasing calcination temperature. This trend persisted even after the catalyst underwent H₂ pretreatment. During the H₂ pretreatment process, the total flow rate of 10% H₂/N₂ was 200 mL/min. The catalyst was thermally treated at a heating rate of 5 °C/min at 200 °C for 30 min. Moreover, NO conversion of the catalyst increased following H₂ pretreatment, with a particularly marked improvement observed in the catalysts calcined at 500 and 600 °C. Compared to the NO reaction rate of 0.13 μmol/(g·h) at 325 °C over the 1 wt% Pt/CeO₂ catalyst reported in the literature [24], those over the 1 wt% Pt/CeO₂-500 catalysts before and after H₂ reduction treatment obtained in the present work were 0.76 and 0.87 μmol/(g·h), respectively. Obviously, our catalyst outperformed the catalyst reported in the literature [24].

The catalytic stability test of the 1 wt% Pt/CeO₂-500 catalyst after H₂ pretreatment was carried out at 325 °C for 10 h of NO oxidation, and its result is shown in Figure 1c. Apparently, no significant decreases in NO conversion were observed in 10 h of NO oxidation under the adopted reaction condition.

2.2. Kinetic Study

The kinetics of the NO reaction on a 1 wt% Pt/CeO₂ catalyst were studied at various temperatures. The effects of calcination temperature and H₂ reduction treatment on the kinetics of NO oxidation were investigated. As the calcination temperature increased, the activation energy (E_a) of the catalyst also rose. The E_a of the catalyst decreased from 23.7 to 19.5 kJ/mol after H₂ pretreatment, which is comparable to the reported E_a value of 31.4 kJ/mol for 0.5 wt% Pt/CeO₂ [25].

Table 1. Summary of kinetic parameters for NO oxidation over Pt/CeO₂.

Catalyst	Ea (kJ/mol)		Feed Gas Composition	Ref.
	Fresh	H2 Pretreatment
1 wt% Pt/CeO2-500	23.7	19.5	500 ppm NO, 10% O2, N2 (balance)	This work
1 wt% Pt/CeO2-600	26.7	20.9
1 wt% Pt/CeO2-700	31.1	30.6
1 wt% Pt/CeO2-800	32.2	31.8
0.5 wt% Pt/CeO2-500	31.4	−	500 ppm NO, 10% O2, N2 (balance)	[25]
1.7 wt% Pt/SiO2-500	57.7	−	250 ppm NO, 3.5%O2, N2 (balance)	[26]
0.35 wt% Pt/CeZrO2-500	34.6	−	500 ppm NO, 8% O2, N2 (balance)	[26]

presents the activation energies of the catalysts with different calcination temperatures. As the calcination temperature increased, the E_a of the catalyst gradually rose, which is inconsistent with the activity test results. This indicates that the calcination temperature influences various properties of the catalyst, including the specific surface area, crystalline phase composition, metal dispersion, and metal-support interactions. These properties can affect the catalytic activity, and even if the E_a is higher, the overall performance of the catalyst may still improve. In comparison to Pt/SiO₂ (57.7 kJ/mol) and Pt/CeZrO₂ (34.6 kJ/mol) [26], the Pt/CeO₂ catalyst displays a lower activation energy, indicating a reduction in the reaction barrier. This ultimately facilitates the NO oxidation reaction, making it more favorable.

During the kinetic studies, it was observed that adjusting the O₂ concentration while keeping the NO concentration constant led to significant changes in the NO conversion of the catalyst. In contrast, varying NO concentration while maintaining a fixed O₂ concentration resulted in only minor changes in NO conversion. The results demonstrate a significant impact of O₂ concentration on the NO conversion efficiency of the catalyst, highlighting the dominant role of the oxygen vacancy mechanism. Notably, the concentration and replenishment of oxygen exhibit a direct correlation with O₂ concentration. When the O₂ concentration is increased, more oxygen molecules are adsorbed and dissociated, generating a greater number of active oxygen atoms. These atoms can quickly fill the oxygen vacancies, thereby enhancing NO conversion [27]. This finding highlights the critical importance of controlling oxygen vacancy dynamics to maximize catalytic performance, suggesting that optimizing O₂ concentration is essential for achieving higher NO conversions.

2.3. Catalyst Characterization

2.3.1. XRD Analysis

The XRD patterns of the catalysts exhibit characteristic peaks at 2θ values of 28.55°, 33.08°, 47.48°, 56.33°, 59.09°, 69.40°, 76.70°, 79.07°, and 88.41°, corresponding to the CeO₂ crystal planes with Miller indices (111), (200), (220), (311), (222), (400), (331), (420), and (422) (JCPDS PDF# 34-0394), respectively.

Figure 2 reveals that the XRD patterns of the Pt-loaded catalysts lack characteristic Pt diffraction peaks at 2θ angles of 39.8°, 46.2°, 67.5°, 81.2°, and 85.7° [28], indicating high dispersion of Pt species on the CeO₂ surface, potentially in an amorphous state or obscured by CeO₂ peaks. This suggests the absence of Pt clusters or particles. Notably, increasing the calcination temperature enhances the sharpness of CeO₂ diffraction peaks.

In practical applications, catalysts must withstand high temperatures over extended periods to prevent deactivation or performance degradation due to phase transformations. CeO₂ facilitates effective oxygen storage and release at high temperatures, enhancing the catalyst’s redox properties. The stable crystal phase of CeO₂ provides an ideal carrier environment, promoting the uniform dispersion of Pt nanoparticles and strong metal-support interactions. This synergy enhances the catalyst’s selectivity, stability, and overall performance.

2.3.2. TEM Analysis

The cubic structure of CeO₂ was identified by the lattice fringes of the (111) and (200) crystal planes with interplanar spacings of d = 0.32 and 0.29 nm, respectively. The lattice fringe of the Pt particles was measured at 0.22 nm, corresponding to the Pt (111) crystal plane. Figure 3e–i displays representative elemental distribution maps obtained via EDS, illustrating the distribution of Pt, Ce, and O elements in the 1 wt% Pt/CeO₂ catalyst. Notably, all three elements are uniformly distributed along the TEM cross-section. The density of Pt species on the CeO₂ support is relatively uniform, and the complementary profiles of O and Ce elements further indicate the uniform distribution of Pt particles on the surface of CeO₂, leading to the formation of more Pt-O-Ce bonds [29]. This enhances both the structural stability and catalytic activity of the catalyst. It can be found from the TEM images (Figure 3a,b) that the lattice fringes of CeO₂ and PtO₂ species are clearly seen in the Pt/CeO₂ catalysts calcined at 500 and 800 °C, indicating the possible formation of a Pt-O-Ce-like bond. Such a deduction was confirmed by the subsequent EDS mappings of Pt, O, and Ce elements, in which the Pt species are uniformly dispersed on the CeO₂ support (i.e., there might be the formation of a Pt-O-Ce-like bond) [30,31]. The high-resolution TEM images of the 1 wt% Pt/CeO₂-500 catalysts before and after H₂ treatment are shown in Figure 3c,d. Due to the similar contrast of Pt and Ce, it was difficult to distinguish them in the high-resolution TEM images. Therefore, we did not provide the sizes of Pt particles before and after H₂ treatment.

2.3.3. H₂-TPR Analysis

Three principal reduction peaks are observed in the Pt/CeO₂ sample shown in Figure 4: a low-temperature reduction peak (α peak) at below 200 °C, attributed to the reduction in PtO_x [32]; a peak at approximately 266 °C (β peak), associated with the reduction in surface-active oxygen species near the Pt/Ce interface [33]; and a peak around 390 °C (γ peak), attributed to the reduction in surface lattice oxygen on CeO₂, away from Pt. Furthermore, this peak gradually shifts to lower temperatures as the catalyst calcination temperature increases. At lower temperatures, the formation of Pt-O-Ce bonds activates the O-Ce bonds through electron transfer, facilitating the reduction in surface oxygen species on the catalyst [27]. Characterization results exhibit a significant decrease in the low-temperature peak area of the catalyst following H₂ reduction treatment. This observation indicates that the H₂ reduction process effectively reduces some of the more readily reducible oxygen species or active sites on the catalyst surface, resulting in a diminished presence of oxygen species on the catalyst.

To further compare the low-temperature reducibility of the Pt/CeO₂ catalysts, the H₂ consumption of the catalysts was calculated, as summarized in

Table 2. H₂ consumption (μmol) of the catalysts after H₂ or air pretreatment.

Catalyst	1 wt% Pt/CeO2-500	1 wt% Pt/CeO2-600	1 wt% Pt/CeO2-800
Fresh	75.81	81.20	108.62
H2 pretreatment	51.60	74.94	50.03

. After air pre-treatment, H₂ consumption increased with the rising calcination temperature, although the increase was not significant. However, for catalysts subjected to H₂ reduction treatment, the changes in H₂ consumption were more pronounced and did not follow a clear trend with the increasing calcination temperature. The 1 wt% Pt/CeO₂ catalyst subjected to H₂ reduction at 600 °C exhibited increased H₂ consumption compared with its 500 °C calcined counterpart, indicating a higher density of oxygen vacancies on its surface. However, its catalytic activity was surprisingly lower, suggesting that the mere presence of oxygen vacancies alone is insufficient to enhance NO oxidation. Efficient oxygen migration and supply mechanisms are essential for facilitating this reaction, highlighting that the oxygen vacancy quantity alone is not the sole determining factor.

2.3.4. NO-TPSR Analysis

The variation in NO₂ product signal intensity on the Pt/CeO₂ catalyst and the temperature are depicted in the graph. As shown in the NO-TPSR profiles of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-800 samples (Figure 5), the intensity of the NO₂ signal was stronger in the sample treated in H₂ at 500 or 800 °C than that in the sample treated in air at 500 or 800 °C, indicating that H₂ treatment was beneficial for the improvement in catalytic activity of the sample. The worse activity of the sample treated at 800 °C than the sample treated at 500 °C might be due to its sintering. The most significant change in the low-temperature peak intensity suggests that the catalyst displays an increased number of surface Pt active sites and oxygen vacancies following H₂ reduction treatment. This enables the availability of a greater number of oxygen species at lower temperatures, thereby facilitating NO dissociation [34]. Consequently, the catalyst treated with H₂ reduction exhibits enhanced NO oxidation activity, aligning with the findings from catalyst activity testing.

2.3.5. O₂-TPD Analysis

As shown in Figure 6, the peak at approximately 108 °C, within the low-temperature range (<200 °C), is attributed to the desorption of weakly adsorbed oxygen species, O₂⁻ (ads), from the catalyst surface. The peak centered at 282 °C is associated with the desorption of chemisorbed oxygen, O⁻ (sur), which is predominantly bound to the active sites on the CeO₂ surface. Furthermore, the incorporation of Pt enhances the activation and adsorption of oxygen species, although the primary adsorption sites remain on the CeO₂ surface. Notably, the peak at approximately 490 °C is attributed to the desorption of lattice oxygen, O²⁻ (latt), which is strongly bound to the surface and requires elevated temperatures for release [35]. The catalytic activity of metal oxides is strongly influenced by surface-adsorbed oxygen, which possesses greater mobility and reactivity than lattice oxygen, facilitating its desorption and subsequent participation in catalytic reactions.

Oxygen vacancies are typically regarded as centers for oxygen adsorption and desorption, acting as a bridge for the adsorption of active gaseous oxygen [36]. Compared to catalysts calcined at 800 °C, those calcined at 500 °C exhibit stronger desorption peaks and larger peak areas for chemisorbed oxygen, indicating that a greater amount of adsorbed oxygen is released from the catalyst. This suggests an increased desorption of adsorbed oxygen from the catalyst, implying an enhanced oxygen supply capability. However, at 800 °C, Pt particle sintering on the catalyst surface likely occurs, leading to increased particle size, reduced surface area, and decreased active sites. Consequently, the O₂ adsorption capacity decreases, yielding a smaller desorption peak area. Furthermore, H₂ reduction treatment modifies the surface structure, leading to reduced active sites and decreased adsorbed oxygen species. The greater the amount of desorbed oxygen, the higher the oxygen migration rate, indicating a larger number of oxygen vacancies and a richer abundance of active sites, which promotes the catalytic oxidation of NO. This is consistent with the catalyst’s activity in NO oxidation.

2.3.6. XPS Analysis

The metal chemical valence and adsorbed oxygen species of the catalysts play important roles in catalyzing NO oxidation. XPS characterization experiments for the Pt/CeO₂ catalysts were performed, and the results are shown in Figure 7 and

Table 3. The surface element compositions of the 1 wt% Pt/CeO₂ catalysts obtained through the XPS technique.

Catalyst	Molar Ratio
	Pt0/(Pt2+ + Pt4+)	Ce3+/Ce4+	Oads/Olatt
1 wt% Pt/CeO2-500	0.86	0.18	0.35
1 wt% Pt/CeO2-600	0.48	0.25	0.32
1 wt% Pt/CeO2-500-H2	0.98	0.49	0.51
1 wt% Pt/CeO2-600-H2	0.86	0.37	0.35

. Each of the asymmetrical O 1s XPS spectra of the catalysts were divided into three components at binding energies of 529.3, 531.3, and 533.1 eV (Figure 7a), which were attributed to the surface lattice oxygen (O_latt), adsorbed oxygen (O_ads), and adsorbed water or carbonate species [37], respectively. The O_ads/O_latt molar ratios of Pt/CeO₂-500 and Pt/CeO₂-600 increased from 0.35 and 0.32 to 0.51 and 0.35 after H₂ pretreatment, respectively. The increase in the O_ads/O_latt molar ratio enhanced the catalytic activity of the Pt/CeO₂ catalyst for NO oxidation.

The Pt 4f XPS spectra of the Pt/CeO₂-500 and Pt/CeO₂-600 catalysts are shown in Figure 7b. Pt was present mainly in a metallic state (Pt⁰) and as platinum oxide (Ptⁿ⁺, i.e., Pt²⁺ and Pt⁴⁺). The deconvoluted components at binding energies of 71.5 and 74.8 eV were related to the 4f_7/2 and 4f_5/2 states of the surface Pt⁰ species, those at binding energies of 72.6 and 75.9 eV were assigned to the 4f_7/2 and 4f_5/2 of the surface Pt²⁺ species, and the those at binding energies of 74.2 and 77.5 eV were ascribed to 4f_7/2 and 4f_5/2 states of the surface Pt⁴⁺ species [27]. The signals of Pt 4f_5/2 were 3.3 eV higher than those of Pt 4f_7/2. After H₂ treatment, the Pt⁴⁺ species nearly disappeared. The Pt⁰/(Pt²⁺ + Pt⁴⁺) molar ratios on the surface of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-600 catalysts before and after H₂ treatment were from 0.86 and 0.48 to 0.98 and 0.86, respectively. In other words, the rise in the Pt⁰/(Pt²⁺ + Pt⁴⁺) molar ratio increased the activity of the Pt/CeO₂ catalyst after H₂ treatment for NO oxidation [31].

The deconvolution of Ce 3d XPS spectra (Figure 7c) indicates that both Ce³⁺ and Ce⁴⁺ species exist on the surface of Pt/CeO₂. It is generally believed that oxygen vacancies are generated to maintain the electroneutrality of a catalyst, owing to the existence of Ce³⁺. The higher the content of Ce³⁺, the more oxygen vacancies. The H₂ pretreatment increased the content of Ce³⁺, leading to an increase in the oxygen vacancy concentration on the catalyst surface. The Ce³⁺/Ce⁴⁺ molar ratios of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-600 catalysts before and after H₂ treatment were from 0.18 and 0.25 to 0.49 and 0.37, respectively. It is well known that oxygen vacancies on the surface of a catalyst can adsorb O₂ to generate the reactive adsorbed oxygen species [37]. Hence, the more oxygen vacancies, the better the catalytic activity of the Pt/CeO₂ catalyst.

The XPS spectra of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-600 catalysts before and after H₂ pretreatment are shown Figure 7. The positions of the O 1s, Pt 4f, and Ce 3d peaks of the two catalysts after H₂ pretreatment were significantly shifted, revealing the formation of a Pt-O-Ce-like bond in the Pt/CeO₂ catalysts.

2.3.7. In Situ DRIFTS Analysis

NO Adsorption and Desorption

Figure 8 presents the DRIFTS spectra of NO adsorption on the Pt/CeO₂ catalyst. The prominent band at 1608 cm⁻¹ is typically attributed to NO₂ species on the catalyst surface. As the adsorption time increases, the band intensity also increases, indicating the reaction of adsorbed NO with lattice oxygen on the catalyst to form NO₂ species. The bands at 1369 and 1536 cm⁻¹ are attributed to monodentate nitro (-NO₂) and bidentate nitrate species, respectively. The band at 1450 cm⁻¹ corresponds to linear nitrite [25]. Bands at 1273 and 1238 cm⁻¹ are attributed to monodentate nitrate species. The band at 1012 cm⁻¹ is associated with N-O stretching vibrations of nitrate or nitrite species, formed through the reaction of NO with surface oxygen. Under continuous N₂ purging, the positions and intensities of these bands remain unchanged, indicating the stability of the relevant surface species and chemical states.

As shown in Figure 8b and Figure S1, the NO adsorption spectra following hydrogen and air treatments exhibit similar trends. After air pretreatment, the Pt nanoparticles on the catalyst are oxidized to higher oxidation states, such as Pt²⁺ or Pt⁴⁺, resulting in weaker CO adsorption by the oxidized Pt species. This weaker adsorption is typically indicated by higher-frequency vibrational modes. Additionally, air pretreatment creates oxygen vacancies on the surface of CeO₂, which serve as active sites and influence the CO adsorption behavior. Following air treatment, the intensity of the band attributed to NO₂ increases, which may be associated with the rise in surface-adsorbed oxygen. Notably, after H₂ treatment, the bands at 1539 and 1369 cm⁻¹ weaken or disappear, while the band attributed to linear nitrite (1450 cm⁻¹) intensifies significantly. The H₂ reduction treatment increases oxygen vacancies on the catalyst surface and enhances surface reducibility, potentially leading to the reduction or conversion of nitrate species to nitrite (NO₂⁻).

Pt and Oxygen Species Confirmation Probed by CO over Pt/CeO₂

To clearly identify Pt species and oxygen species on the Pt/CeO₂ catalyst, CO was introduced to a chamber as a probe molecule due to its strong adsorption capability on various active sites of the catalyst surface. This technique facilitates the identification of intermediate species involved in CO-related chemical reactions on the catalyst surface, thereby enhancing the understanding of surface chemical reaction processes. It also enables the inference of the presence and distribution of different active sites and indirectly provides insights into changes in the surface oxidation states of CeO₂.

The CO-DRIFTS spectra of CO adsorption on the catalysts are shown in Figure 9. The band at 2086 cm⁻¹ is associated with the linear adsorption of CO on Pt nanoparticles. For the catalyst calcined at 500 °C, the band observed at 2117 cm⁻¹ is attributed to the linear adsorption of CO on the oxidized Pt species, the band at 2166 cm⁻¹ is linked to CO adsorbed on the Ce³⁺ ions [34,38], and the bands at 1481 and 1566 cm⁻¹ are related to the products (e.g., carbonates) of CO oxidation [30].

With continuous purging by N₂, the CO adsorption bands on Pt²⁺ and Ce³⁺ gradually disappear, while the CO adsorption band on Pt⁰ weakens but remains at a certain intensity. This suggests that the CO adsorption on Ce³⁺ and Pt²⁺ is relatively weak, making it more easily removed during N₂ purging [39]. Therefore, it can be inferred that the CO adsorption bonds at these sites have lower binding energies. The interaction between CO molecules and the surface is relatively weak, making them more susceptible to disruption during N₂ purging. In contrast, CO adsorption on Pt⁰ is stronger and persists even during N₂ purging, indicating that the CO adsorption bond energy on Pt⁰ is higher and less easily disrupted. This suggests that different adsorption sites exhibit varying selectivity toward CO adsorption. CO adsorption sites on Pt⁰ may possess higher stability and selectivity, while those on Ce³⁺ and Pt²⁺ are more vulnerable to environmental changes, such as N₂ purging.

Compared to the catalyst calcined at 500 °C, the CO-DRIFTS spectrum bands of the catalyst calcined at 800 °C shifted to higher wavenumbers. The bands at 1592 and 1528 cm⁻¹ were attributed to the products (e.g., carbonates) of CO oxidation [40]. The bands within the 800–1200 cm⁻¹ range were likely associated with adsorption modes linked to oxygen vacancies on the CeO₂ surface or Pt/CeO₂ interface, suggesting that oxygen vacancies generated by H₂ pretreatment significantly impact CO adsorption. Figure S2 shows that after 30 min N₂ purging, the bands at 2117 and 2166 cm⁻¹ for the 500 °C-calcined catalyst significantly diminish, whereas those for the 800 °C-calcined catalyst remain largely unchanged [41]. This indicates that the 500 °C-calcined catalyst has a weaker affinity for CO, and the catalyst calcined at 800 °C maintains its surface species more effectively, likely due to stronger binding sites or a more stable surface structure.

H₂ pretreatment can modify the surface chemistry and structure of the catalyst, thereby influencing its adsorption behavior towards different species. For Pt/CeO₂ catalysts, H₂ pretreatment typically reduces a portion of the Pt and CeO₂, leading to the formation of more metallic Pt nanoparticles and oxygen vacancies. These metallic Pt nanoparticles exhibit higher surface activity and stronger CO adsorption capability, which influence the adsorption mode and vibrational frequencies of CO. As the adsorption time increases, CO gradually accumulates onto the catalyst surface, resulting in an increased adsorption amount.

O₂ and Pre-Adsorbed NO

The transient reaction of O₂ with pre-adsorbed NO was investigated, and the corresponding DRIFTS spectra for this process over the 1 wt% Pt/CeO₂-500 catalyst are presented in Figure 10. As shown in Figure 10a, NO pretreatment yields various adsorbed NO species on the catalyst. Specifically, the intense band at ca. 1176 cm⁻¹ is attributed to nitrite species, while the weaker band around 1296 cm⁻¹ is assigned to monodentate nitrite species [25]. As time progresses, significant changes occur in the intensities of these bands. This process is believed to involve the reaction of NO with oxygen species on the catalyst surface, oxygen vacancies on CeO_2, or oxygen associated with PtO_x. Notably, the spectral bands corresponding to bridging bidentate nitrate species (1569 cm⁻¹) progressively decrease in intensity over time, indicating a possible conversion to nitrite species. The NO or nitrite species at 1473 and 1386 cm⁻¹ gradually transform into nitrate species at 1435 cm⁻¹ after the introduction of O₂. The nitrite species at 1176 cm⁻¹ are progressively converted into monodentate nitrate or bridging nitrate species at 1227 and 1257 cm⁻¹, as well as bidentate nitrate species at 1015 cm⁻¹. The nitrate species are relatively stable and did not signify notable changes during subsequent cyclic adsorption processes. These observations suggest that NO is adsorbed on Pt/CeO₂ as a mixture of nitrite and nitrate species, a finding that aligns with the results reported by Philipp et al. [42] The decomposition of nitrate species occurs within the temperature range of 300–500 °C, releasing NO into the gas phase. These NO molecules can undergo further oxidation to form NO₂ on the catalyst surface [43]. At appropriate temperatures, the decomposition of nitrate species and the oxidation of NO can synergistically enhance the efficiency of the catalyst. This observation is consistent with the results obtained from activity tests conducted at different temperatures.

Upon air introduction, the reaction between O₂ and pre-adsorbed NO occurs. At 200 °C, a distinct spectral band emerges at 1543 cm⁻¹, indicating the predominant formation of monodentate nitrate species as a result of NO oxidation. Notably, the monodentate nitrate species exhibit relatively low stability, thereby facilitating NO oxidation. In the second cycle of NO adsorption and O₂ adsorption, the DRIFTS spectra remain largely unchanged, with consistent band positions and intensities. This suggests that the adsorbed species, including nitrate and nitrite, have attained a dynamic equilibrium on the catalyst surface. Subsequent NO and O₂ adsorption does not yield additional reaction product intermediates, indicating equilibrium stabilization.

2.4. Catalytic Mechanism

The kinetic test results indicate that NO oxidation on the catalyst surface proceeds via the oxygen vacancy mechanism. Consequently, the catalytic mechanism for the 1 wt% Pt/CeO₂ catalyst is illustrated in Figure 11. During the initial adsorption phase, NO is more readily adsorbed onto the metallic Pt nanoparticles than on the Pt oxides. This adsorption is notably stronger, resulting in the formation of stable chemical bonds that are essential for the initial activation of NO. After H₂ pretreatment, the Ce⁴⁺ species in CeO₂ are partially reduced to the Ce³⁺ species, forming a large number of oxygen vacancies on the catalyst surface. Molecular oxygen are first adsorbed at the oxygen vacancies and Pt sites, which then react with the adsorbed NO. The active adsorbed oxygen species might be the most effective factor for the high activity of the Pt/CeO₂ catalyst. This unique property enables efficient oxygen extraction and migration from the Pt-Ce interface to the Pt nanoparticles, resulting in the formation of abundant oxygen vacancies on the catalyst surface. The surface oxygen vacancies play a vital role in enhancing the Pt/CeO₂ system’s catalytic performance, especially after H₂ reduction, which enhances reactive oxygen species generation and reactant adsorptions, thereby optimizing NO oxidation.

3. Experimental Section

3.1. Catalyst Synthesis

A total of 1 wt% Pt/CeO₂ noble metal catalysts were prepared at different calcination temperatures using the impregnation method. First, cerium nitrate was calcined at 550 °C in a muffle furnace with a heating rate of 5 °C/min for 4 h to obtain CeO₂. A specific amount of chloroplatinic acid and CeO₂ was then dissolved in water according to the calculated ratio. The mixture was stirred in a water bath until the water completely evaporated and was subsequently dried in an oven at 110 °C. After drying, the solid was ground into a powder using an agate mortar and pestle and then calcined at different temperatures in a muffle furnace with a heating rate of 10 °C/min for 4 h. The mass fraction of Pt was 1 wt%. The resulting catalysts were labeled as 1 wt% Pt/CeO₂-500, 1 wt% Pt/CeO₂-600, 1 wt% Pt/CeO₂-700, and 1 wt% Pt/CeO₂-800, respectively.

3.2. Catalytic Testing

An activity evaluation was conducted in a fixed-bed quartz tubular reactor, with the tested temperature range set between 100 and 500 °C. The NO oxidation performance was assessed using 1.0 g of catalyst in a simulated feed gas containing 500 ppm NO, 10% O₂, and N₂ as the balance gas. The total gas flow rate was 1000 mL/min, and the weight hourly space velocity (WHSV) was controlled at 60,000 mL/(g·h) using mass flow meters. The gas mixture at the reactor outlet included NO, NO₂, and NO_x, which were monitored using an exhaust gas analyzer (Thermo 42i-HL, Waltham, MA, USA). The NO conversion was analyzed using the following equations:

$$\mathrm{NO} \mathrm{conversion}={\displaystyle \frac{{\mathrm{NO}}_{2_{\mathrm{out}}}}{{\mathrm{NO}}_{x_{\mathrm{out}}}}}\times 100\%$$

(1)

where [NO_x] = [NO] + [NO₂]. The subscript “out” represents the concentrations of the corresponding substances at the inlet and outlet, respectively.

Kinetic measurements were carried out at different temperatures while maintaining the NO conversion below 20% to eliminate the effects of heat and diffusion. The feed gas consisted of NO (300–800 ppm), O₂ (5%), and N₂ as the balance gas, with a total flow rate of 400 mL/min. The gas concentrations were continuously monitored using a NO_x analyzer (Thermo, 42i-HL). To better evaluate the catalytic activity of the catalyst, the following equation was used to calculate the kinetic parameters for NO oxidation:

$$\mathrm{k}=-{\displaystyle \frac{\mathrm{V}}{\mathrm{W}}}\times \ln (1-\mathrm{x})$$

(2)

where k is the reaction rate constant (cm³/(g·s)), V is the total gas flow rate (cm³/s), W is the actual mass of the catalyst (g), and x is the NO conversion.

The apparent activation energy was calculated using the Arrhenius equation, given by the following equation:

$$\mathrm{K}=\mathrm{A}\cdot \exp \hspace{0.17em}(-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{RT}}})$$

(3)

where k is the rate constant, R is the molar gas constant, T is the thermodynamic temperature, E_a is the apparent activation energy, and A is the pre-exponential factor. This equation can also be expressed as lnk = lnA − E_a/RT. By plotting the data, a straight line could be obtained. The apparent activation energy was determined from the slope of this line, and the pre-exponential factor was calculated from the intercept.

3.3. Catalyst Characterization Methods

X-ray diffraction (XRD) was conducted using a Bruker/AXS D8 Advance X-ray diffractometer (Bruker, Billerica, MA, USA) with Cu Kα radiation (λ = 1.54 Å) over a 2θ range of 10 to 80°. Detailed morphological and structural information about the Pt/CeO₂ catalyst was obtained through transmission electron microscopy (TEM).

Hydrogen temperature programmed reduction (H₂-TPR) was conducted using an Auto Chem II 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA). The testing method involved weighing 0.1 g of the catalyst sample with a particle size of 0.3–0.45 mm into a U-shaped reaction tube. The sample was heated under air conditions at a rate of 10 °C/min to 300 °C and maintained at the temperature for 1 h. It was then purged with helium (He) and cooled to room temperature. Subsequently, the sample was exposed to 10% H₂/Ar and heated at a rate of 10 °C/min to 800 °C, with signal detection carried out using a thermal conductivity detector (TCD).

For the nitric oxide temperature-programmed surface reaction (NO-TPSR), 50 mg of the catalyst was preheated in air at 500 °C for 30 min, cooled to room temperature, and then purged with 900 ppm NO/N₂ gas until the baseline stabilized. The temperature was subsequently increased from room temperature to 800 °C at a rate of 10 °C/min. The testing conditions for the air reduction in the samples at 500 and 800 °C are as follows: 50 mg of the catalyst was heated under air conditions at 500 °C for 30 min, then cooled to 200 °C in the same atmosphere. The catalyst was subsequently exposed to 5% H₂/Ar for 30 min, followed by cooling to room temperature. The sample was then purged with 5% NO/N₂ gas until the baseline stabilized, after which the temperature was increased from room temperature to 800 °C at a rate of 20 °C/min. The desorbed gas was detected using a mass spectrometer (QGA, Hiden, Warrington, UK).

For oxygen temperature-programmed desorption (O₂-TPD), a 100 mg sample was pre-treated at 300 °C for 1 h. After cooling to 30 °C, the sample was exposed to 10% synthetic air for 1 h until saturation was achieved. The Ar flow (40 mL/min) was then activated for 1 h to remove weakly adsorbed O₂ from the surface. Finally, the desorption process was conducted by ramping the temperature from 30 to 800 °C at a rate of 10 °C/min under an Ar atmosphere, and the desorbed gas was analyzed using a mass spectrometer (MS).

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface elemental compositions of the catalysts using a Thermo Scientific ESCALAB 250xi (Thermo Fisher Scientific, Waltham, MA, USA) with a 0.1 eV step and a 46.95 eV pass energy (PE) rate. The X-ray source was monochromatized with Al Kα radiation (hν = 1486.6 eV), and the contaminant carbon peak (C 1s = 284.8 eV) was used to calibrate the shift in binding energy.

In situ diffuse reflectance infrared Fourier transform spectroscopic (In situ DRIFTS) experiments of the catalysts were conducted using an infrared spectrometer (Nicolet 6700, Thermo Fisher Company, Waltham, MA, USA) with a liquid-nitrogen-cooled MCT detector. The samples were preheated at 300 °C in an N₂ atmosphere for 1 h to remove water and adsorbed gases from the surface. As the sample cooled to 50 °C, the background spectrum was collected. All spectra were recorded by accumulating 32 scans with a resolution of 4 cm⁻¹.

4. Conclusions

This study explores the influence of calcination temperature and H₂ pretreatment on NO oxidation catalysis, revealing the critical role of oxygen vacancies in enhancing reaction rates. However, excessive temperature exposure is detrimental, as it depletes surface-active oxygen and inhibits NO oxidation performance. The H₂ pretreatment-induced augmentation of surface oxygen vacancies in 1 wt% Pt/CeO₂-800 and 1 wt% Pt/CeO₂-500 catalysts substantially enhances NO oxidation. This highlights the pivotal role of oxygen vacancy dynamics in driving catalytic performance, emphasizing the importance of optimizing calcination temperature and oxygen vacancy availability to achieve maximum NO conversion, as the presence and renewal of oxygen vacancies are integral to the catalyst’s efficacy.