Loading Eu₂O₃ Enhances the CO Oxidation Activity and SO₂ Resistance of the Pt/TiO₂ Catalyst

Abstract

Pt/TiO₂ and Pt-Eu₂O₃/TiO₂ catalysts were prepared via the impregnation method for catalytic oxidation of CO. The Pt-2Eu₂O₃/TiO₂ catalyst exhibited better CO oxidation activity as well as greater SO₂ resistance than the Pt/TiO₂ catalyst. For the inlet gas consisting of 0.8% CO, 5% O₂, and balanced N₂, the lowest complete conversion temperatures (T₁₀₀) of CO were 120 °C and 140 °C for the Pt-2Eu₂O₃/TiO₂ and Pt/TiO₂ catalysts, respectively. During the 72 h SO₂-resistance test at 200 °C under an inlet gas composition of 0.8% CO, 5% O₂, 15% H₂O, 50 ppm SO₂, and balanced N₂, the CO conversion on the Pt-2Eu₂O₃/TiO₂ catalyst remained >99%, while that on the Pt/TiO₂ catalyst gradually decreased to 77.8%. Pre-loading 2 wt% Eu₂O₃ on TiO₂ enhanced the dispersion of Pt, increased the proportion of Pt⁰, and facilitated the adsorption and dissociation of H₂O, all of which promoted CO oxidation. SO₂ preferentially occupied the Eu₂O₃ sites by forming stable sulfates on the Pt-2Eu₂O₃/TiO₂ catalyst, which protected the Pt active sites from poisoning. The OH* species produced from the dissociation of H₂O played a significant role in promoting CO oxidation through the formation of COOH* as the key reaction intermediate. The developed Pt-2Eu₂O₃/TiO₂ catalyst has great application potential in terms of the removal of CO from industrial flue gases.

1. Introduction

Carbon monoxide (CO) is considered one of the most dangerous air pollutants because it can cause tissue hypoxia and fatal asphyxiation [1]. As a typical byproduct of the incomplete combustion of fossil fuels, CO is primarily emitted from energy-intensive industries. For example, exhaust gas from the sintering process in the steel industry includes about 1 vol% of CO [2]. Catalytic oxidation emerges as the most viable approach to remove CO at the low temperatures (<200 °C) of industrial flue gases, which has attracted considerable research interest [3].

A wide range of catalysts have been studied for CO oxidation, including noble-metal-based catalysts such as Pt [4], Pd [5], and Au [6] and non-noble metal catalysts such as Co [7], Cu [8], and Mn [9]. Non-noble metal catalysts are highly susceptible to the H₂O and SO₂ contained in industrial flue gases, hindering their widespread application. Among all the investigated noble metal catalysts, Pt-based catalysts exhibit promising potential for industrial applications due to their excellent low-temperature activity in the presence of H₂O [10]. It is well recognized that H₂O promotes CO oxidation over Pt-based catalysts, mainly by weakening the CO adsorption on Pt surfaces (avoiding CO self-poisoning) and generating highly reactive OH species that facilitate CO oxidation [11]. However, the presence of SO₂ in flue gases, even at low concentrations (tens of ppm), has been shown to cause serious deactivation of Pt-based catalysts in low-temperature conditions. The sulfur poisoning can be attributed to H₂SO₄-induced pore blockage and sulfation processes affecting both the catalyst support (e.g., TiO₂, Al₂O₃) and the Pt active sites [2,12,13,14]. Therefore, it is meaningful to further enhance the sulfur resistance of Pt-based catalysts.

Among the Pt-based catalysts used for CO oxidation, modified Pt/TiO₂ catalysts have received broad attention due to their superior sulfur resistance. The modification strategies explored in previous studies include modifying the TiO₂ support with hydrophobic graphite and introducing WS₂, WO₃, MoO₃, and CeO₂ [15,16,17,18,19]. Notably, loading 0.1% CeO₂ was found to significantly enhance the CO oxidation activity and SO₂ resistance of the 0.1% Pt/TiO₂ catalyst at temperatures as low as 160 °C, an effect attributed to increased adsorbed oxygen species and preferential binding with SO₂ by CeO₂ [19]. Introducing various rare earth oxides (LnO_x, Ln = La, Ce, Nd, Sm and Dy) was shown to enhance the CO oxidation activity of the Pt/SiO₂ catalyst as well [20]. These findings suggest that modifying Pt/TiO₂ catalysts with rare earth oxides constitutes a promising strategy to simultaneously enhance the catalytic activity and sulfur resistance at low temperatures. To the best of our knowledge, no studies have investigated the effects of Eu₂O₃ loading on the performance of Pt-based catalysts used for CO oxidation, although introduction of Eu₂O₃ into Pt/CeO₂ catalyst was proved to facilitate the oxidation of toluene [21].

The main objective of this study is to investigate the effects of Eu₂O₃ addition on the activity and SO₂ resistance of the Pt/TiO₂ catalyst for CO oxidation. A series of Pt-Eu₂O₃/TiO₂ catalysts were prepared by a two-step impregnation method. The physiochemical characteristics of typical catalysts were investigated by the SEM, TEM, XRD, N₂ adsorption, CO chemisorption, H₂-TPR, XPS, NH₃-TPD, SO₂-TPD and H₂O-TPD techniques to establish the catalyst structure–performance relationships. Additionally, in situ DRIFTS studies of the CO oxidation were conducted to elucidate the catalytic promotion mechanisms of Eu₂O₃.

2. Results and Discussion

2.1. Catalyst Activity and SO₂ Resistance

As shown in Figure 1a, the Pt/TiO₂ and Pt-Eu₂O₃/TiO₂ catalysts exhibited characteristic S-type conversion profiles for CO oxidation over noble metal catalysts, which can be attributed to the rapid increase in the catalyst surface temperature owing to the release of a large amount of chemical heat after the ignition of CO oxidation [22]. The TiO₂ and 2Eu₂O₃/TiO₂ samples demonstrated negligible activity at the temperatures investigated (Figure 1a), indicating that Pt species are essential for catalytic CO oxidation. Introducing different contents of Eu₂O₃ all enhanced the catalytic activity of Pt/TiO₂. As the Eu₂O₃ content increased, the catalytic activity first increased and then decreased, with the Pt-2Eu₂O₃/TiO₂ catalyst exhibiting the highest activity. The lowest complete conversion temperature (T₁₀₀) decreased from 140 °C for the Pt/TiO₂ catalyst to 120 °C for the Pt-2Eu₂O₃/TiO₂ catalyst. The TOF of the Pt-2Eu₂O₃/TiO₂ catalyst was calculated as 0.13 s⁻¹ at 100 °C, higher than most values reported in previous studies for Pt/TiO₂-based catalysts (Table S1) [23,24,25,26,27]. This result identifies Eu₂O₃ as a promising modifier for enhancing catalytic activity. As shown in Table S1, the test temperatures of the SO₂ resistance for the Pt/10EG-TiO₂-10 [15], 0.1Pt-5WS₂/TiO₂-400 [16], 0.1Pt-5W/Ti-A [17], and 0.1Pt/3Mo/Ti [18] catalysts ranged from 220 to 300 °C, exceeding the typical flue gas temperatures and thus presenting challenges for industrial implementation. Although the Pt_0.1%Ce_0.1%/TiO₂ [19], Pt/TiO₂ [23], Pt/TiO₂-Na [25], and Pt-0.125P&M/TiO₂ [26] exhibited some SO₂ resistance at temperatures below 200 °C, their CO conversion activities after sulfur poisoning remained lower than that of the Pt-2Eu₂O₃/TiO₂ catalyst developed in this study.

Figure 1b shows the Arrhenius plots of the CO oxidation over the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts. In the tested temperature range, the reaction rates on the Pt-2Eu₂O₃/TiO₂ catalyst were higher than those observed on the Pt/TiO₂ catalyst. Furthermore, the apparent activation energy (E_a) of the Pt-2Eu₂O₃/TiO₂ catalyst was 36.2 kJ/mol, much lower than that of 49.2 kJ/mol for the Pt/TiO₂ catalyst. These results demonstrate that the introduction of Eu₂O₃ increased the reaction rate and lowered the energy barrier for CO oxidation, which is likely attributable to the enhanced dispersion and elevated intrinsic activity of Pt species when Eu₂O₃ is present on the TiO₂ support.

Figure 1c presents the CO oxidation performance under reaction gas containing 3% H₂O. A comparison of Figure 1a,c reveals that the presence of 3% H₂O significantly enhanced the CO oxidation over all the catalysts. The Pt-2Eu₂O₃/TiO₂ catalyst still exhibited the highest activity under H₂O-containing conditions, achieving a T₁₀₀ value of 100 °C. Xu et al. [17] also reported a similar water-promoting effect for CO oxidation over the Pt-W/TiO₂ catalyst.

Figure 1d illustrates the conversion of CO during the 72 h stability test in the presence of 50 ppm SO₂ and 15% H₂O at 200 °C. The CO conversion over the Pt/TiO₂ catalyst showed a slightly fluctuating but generally declining trend, decreasing from an initial 100% to the final 77.8%. In contrast, the Pt-2Eu₂O₃/TiO₂ catalyst maintained >99% CO conversion throughout the entire 72 h stability test. Obviously, the Pt-2Eu₂O₃/TiO₂ catalyst exhibited superior SO₂ resistance compared to the Pt/TiO₂ catalyst.

2.2. SEM and TEM Results

The SEM images in Figure 2a,d depict the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ samples, respectively. It can be seen that the two samples have similar surface morphologies, characterized by particles of varying sizes and irregular shapes. Figure 2b,c,e,f are TEM images of the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ samples, respectively. As shown in Figure 2b,e, the Pt nanoparticles exhibit uniform dispersion on both catalysts. The mean particle sizes were measured as 2.70 ± 0.13 nm and 1.78 ± 0.04 nm for Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂, respectively, indicating that the Eu₂O₃ loading enhanced the dispersion of Pt.

High-resolution TEM analysis confirmed the lattice fringes of anatase TiO₂ and PtO in the Pt/TiO₂ catalyst (Figure 2c) and the additional Eu₂O₃ lattice fringes in the Pt-2Eu₂O₃/TiO₂ catalyst (Figure 2f). SEM-EDS elemental maps of the Pt-2Eu₂O₃/TiO₂ catalyst (Figure 2g–j) further confirmed that the Pt and Eu species were uniformly dispersed on the surface of TiO₂.

2.3. XRD Results

The XRD patterns of the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ samples are shown in Figure S2. Both samples exhibited XRD peaks at 2θ = 25.40°, 37.06°, 37.75°, 38.54°, 47.98°, 54.00°, 55.03°, 62.63°, 68.73°, 70.23°, 75.1°, and 76.1°, which are indexed to anatase TiO₂ (PDF#21-1272) [28]. No diffraction peaks corresponding to Eu or Pt species were detected in either catalyst, which could be due to the low contents and/or highly dispersed states of Eu and Pt.

2.4. Specific Surface Area and Pore Structure Characteristics

Figure S3 shows the N₂ adsorption–desorption isotherms and pore size distributions for the Pt/TiO₂, Pt/TiO₂-WS, Pt-2Eu₂O₃/TiO₂, and Pt-2Eu₂O₃/TiO₂-WS samples. All the samples presented type V isotherms according to the IUPAC classification (Figure S3a), characteristic of mesoporous structures [29]. As shown in Figure S3b, the pore size distributions span 2–50 nm across all the samples, with the average pore diameters being 21.7, 22.7, 22.2, and 23.9 nm for Pt/TiO₂, Pt/TiO₂-WS, Pt-2Eu₂O₃/TiO₂, and Pt-2Eu₂O₃/TiO₂-WS, respectively (

Table 1. Surface structure parameters and Pt dispersion of different catalysts.

Samples	BET Specific Surface Area (m2·g−1)	Pore Volume (cm3·g−1)	Average Pore Diameter (nm)	Pt Dispersion (%)
Pt/TiO2	69.4	0.39	21.7	47.5
Pt/TiO2-WS	67.0	0.39	22.7	/
Pt-2Eu2O3/TiO2	72.1	0.41	22.2	53.5
Pt-2Eu2O3/TiO2-WS	61.4	0.38	23.9	/

). Notably, the average pore diameters of the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts slightly increased following the 72 h SO₂-resistance test.

Table 1 also lists the BET specific surface areas and pore volumes of the four samples. It can be seen that compared with Pt/TiO₂, Pt-2Eu₂O₃/TiO₂ has a slightly higher specific surface area and larger pore volume, providing more reaction surface and space [30]. After the 72 h SO₂-resistance test, the specific surface area of Pt/TiO₂ decreased marginally from 69.4 to 67.0 m²·g⁻¹, while Pt-2Eu₂O₃/TiO₂ showed a more pronounced decrease in specific surface area, from 72.1 to 61.4 m²·g⁻¹, with a concurrent decrease in the pore volume from 0.41 to 0.38 cm³·g⁻¹. These results might suggest the preferential deposition of sulfur species on the Eu₂O₃ surface.

2.5. CO Chemisorption Results

As shown in Table 1, the dispersion of Pt increased from 47.5% for the Pt/TiO₂ catalyst to 53.5% for the Pt-2Eu₂O₃/TiO₂ catalyst, which is consistent with the observed decrease in the average Pt particle size after introduction of Eu₂O₃ (Figure 2). This enhanced Pt dispersion on Pt-2Eu₂O₃/TiO₂ may be attributed to the formation of Pt-O-Eu bonds. Nagai et al. [31] claimed that Pt-O-Ce bond formation in the Pt/Ce-Zr-Y catalyst suppressed the migration of Pt nanoparticles, thereby enhancing the Pt dispersion. The enhanced Pt dispersion indicates increased availability of Pt active sites, which could partially explain the higher CO oxidation activity of Pt-2Eu₂O₃/TiO₂ compared to Pt/TiO₂ (Figure 1).

2.6. H₂-TPR Results

Figure 3 displays the H₂-TPR profiles of the Pt/TiO₂ and Pt-xEu₂O₃/TiO₂ catalysts. All the samples exhibit a minor H₂ consumption peak at 50–125 °C (peak I) corresponding to the reduction of PtO_x and Eu₂O₃ species and a prominent peak around 300 °C (peak II) due to the reduction of TiO₂ by hydrogen spillover from the Pt sites [26,32,33]. As the Eu₂O₃ content increased, the peak temperature of peak I first decreased and then increased, reaching a minimum of 79.8 °C for Pt-2Eu₂O₃/TiO₂. This indicates the best reducibility of PtO_x and Eu₂O₃ species in Pt-2Eu₂O₃/TiO₂, explaining its superior CO oxidation activity (Figure 1a). The enhanced reducibility of Pt-2Eu₂O₃/TiO₂ is likely due to the formation of Pt-O-Eu bonds, which facilitates the electron transfer from Eu to Pt due to the lower electronegativity of Eu (compared to Pt) [34].

The reduction peak II shifted to a higher temperature range with increasing Eu₂O₃ content, indicating that strong Pt-Eu interactions impede hydrogen spillover from Pt to TiO₂, thereby elevating the reduction temperatures of the surface TiO₂.

Table 2. H₂ consumption amounts of Pt/TiO₂ and Pt-xEu₂O₃/TiO₂ catalysts.

Samples	H2 Consumption (μmol/gcat)
	Peak I	Peak II	Total
Pt/TiO2	9.7	673.2	682.9
Pt-1Eu2O3/TiO2	13.1	491.1	504.2
Pt-1.5Eu2O3/TiO2	16.6	509.8	526.4
Pt-2Eu2O3/TiO2	19.0	498.8	517.8
Pt-2.5Eu2O3/TiO2	23.0	492.0	515

lists the H₂ consumption amounts of different catalysts. It can be observed that as the Eu₂O₃ content increased, the H₂ consumption at low temperature (peak I) increased due to the increased H₂ consumption by more Eu₂O₃. Meanwhile, the H₂ consumption at high temperature (peak II) notably decreased after introducing Eu₂O₃, but the Eu₂O₃ content did not largely affect the H₂ consumption amount, suggesting that the hindering effects of Pt–Eu interactions on the reduction of TiO₂ did not enhance with increasing Eu₂O₃ loadings.

2.7. Surface Element Valences

2.7.1. XPS Results

Figure 4a presents the Pt 4f XPS spectra of the six samples. The characteristic peaks observed at binding energies of around 75.7 and 72.4 eV are attributed to surface Pt²⁺ species [23], while those at 74.2 and 71.3 eV correspond to surface Pt⁰ species [35]. The proportion of Pt⁰ (Pt⁰/(Pt⁰ + Pt²⁺)) was determined by integrating the areas under these peaks, with the results listed in

Table 3. Surface element compositions of different catalysts.

Samples	Oads/(Olat + Oads) (%)	Pt0/(Pt0 + Pt2+) (%)
Pt/TiO2	18.7	20.0
Pt-2Eu2O3/TiO2	18.0	38.7
Pt/TiO2-used	22.1	52.0
Pt-2Eu2O3/TiO2-used	22.2	56.5
Pt/TiO2-WS	24.5	62.3
Pt-2Eu2O3/TiO2-WS	27.6	63.3

. It can be seen that the proportion of Pt⁰ was 20.0% and 38.7% on the surface of Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂, respectively. These findings confirm that Pt²⁺ species predominate on both catalysts, with Eu₂O₃ addition promoting the formation of Pt⁰. As previously reported, Pt⁰ has higher catalytic activity for CO oxidation compared to PtO_x species [36]. The elevated Pt⁰ proportion on the Pt-2Eu₂O₃/TiO₂ catalyst could explain its higher activity compared to Pt/TiO₂ (Figure 1a). The electron transfer from Eu to Pt via the Pt-O-Eu bonds may explain the enhanced formation of Pt⁰ on the Pt-2Eu₂O₃/TiO₂ catalyst, which also accounts for the aforementioned weakened Pt-O bonds in the catalyst.

Furthermore, as shown in Table 3, the Pt⁰ proportion increased to 52.0% and 56.5% for the Pt/TiO₂-used and Pt-2Eu₂O₃/TiO₂-used catalysts, respectively, which is primarily attributed to the reduction of PtO_x by CO during the catalytic reaction. After the 72 h SO₂-resistance test, the Pt⁰ proportion showed a further increase to 62.3% and 63.3% for the Pt/TiO₂-WS and Pt-2Eu₂O₃/TiO₂-WS catalysts, respectively.

Figure 4b shows the O 1s spectra of the six samples, which were divided into two peaks. The main peak around 529.9 eV is assigned to the lattice oxygen (denoted as O_lat) on the catalyst surface, while the broad peak around 531.6 eV corresponds to the surface adsorbed oxygen and hydroxyl oxygen (denoted as O_ads) [25]. These surface O_ads play a significant role in the catalytic oxidation of CO [37,38]. The proportion of O_ads (O_ads/(O_lat + O_ads)) was calculated to be 18.7% and 18.0% for the fresh Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts, respectively, and it increased to 22.1% and 22.2% for the Pt/TiO₂-used and Pt-2Eu₂O₃/TiO₂-used catalysts, respectively (Table 3). This enhancement in terms of the O_ads proportion can be attributed to the replenishment of oxygen vacancies by molecular oxygen during the CO oxidation process, leading to an increased concentration of adsorbed oxygen species. After the SO₂-resistance test, the O_ads/(O_lat + O_ads) ratios showed a further increase to 24.5% and 27.6% for the Pt/TiO₂-WS and Pt-2Eu₂O₃/TiO₂-WS catalysts, respectively, which can be attributed to the accumulation of surface OH* generated from the dissociation of H₂O during the reaction process [39]. Moreover, the Pt-2Eu₂O₃/TiO₂-WS sample displayed the highest O_ads/(O_lat + O_ads) ratio (27.6%), suggesting that Eu₂O₃ addition enhanced the dissociation of H₂O under reaction conditions.

Figure 4c presents the Eu 3d XPS spectra of the Pt-2Eu₂O₃/TiO₂ and Pt-2Eu₂O₃/TiO₂-WS samples. Obviously, there are two different Eu valence states in the Pt-2Eu₂O₃/TiO₂ catalyst, namely Eu²⁺ at 1126.1 eV and Eu³⁺ at 1135.3 eV [40,41,42]. The proportion of Eu³⁺ (Eu³⁺/(Eu³⁺ + Eu²⁺)) was determined to be 59.5%, indicating a predominant presence of Eu³⁺ (Eu₂O₃) over Eu²⁺ (EuO) in the as-prepared catalyst. After the SO₂-resistance test, the Eu 3d XPS signals became indistinguishable (Figure 4c), probably due to the deposition of sulfur-containing species on the Eu₂O₃ domains.

The S 2p XPS spectra of the Pt/TiO₂-WS and Pt-2Eu₂O₃/TiO₂-WS catalysts are shown in Figure 4d. The peaks at 168.5 eV (S 2p_3/2) and 169.7 eV (S 2p_1/2) are attributed to sulfate species (SO₄²⁻) [17]. Obviously, sulfur accumulated on both catalysts after the SO₂-resistance tests, mainly in the form of sulfates. However, the deposition sites of these sulfates probably differ between the two catalysts, as evidenced by the distinct changes in the Eu 3d XPS signals. Specifically, sulfates primarily occupied the Pt sites on the Pt/TiO₂ catalyst, whereas they preferentially deposited on the Eu sites on the Pt-2Eu₂O₃/TiO₂ catalyst.

2.7.2. In Situ DRIFTS Results of CO Adsorption

Figure 5a shows that characteristic bands appeared at 2174, 2120, 2089, 2046 and 1831 cm⁻¹ during CO adsorption on the Pt/TiO₂ catalyst at 25 °C. The peaks at 2174 cm⁻¹ and 2120 cm⁻¹ were assigned to gaseous CO [43]. The peak at 2089 cm⁻¹ was assigned to CO linearly adsorbed on the Ptⁿ⁺ (1 < n<2) sites. The peaks at 2046 cm⁻¹ and 1831 cm⁻¹ were attributed to CO coordinated in a linear and a bridging manner on the Pt⁰ sites, respectively [38,39,44]. After the N₂ purge, the peaks for the CO adsorbed on the Pt⁰ sites remained unchanged, while those for the gaseous CO and CO adsorbed on the Ptⁿ⁺ sites disappeared, which confirms the strong adsorption of CO on the Pt⁰ sites and the weak adsorption on the Ptⁿ⁺ sites [44].

Figure 5b shows characteristic bands at similar positions (2174, 2120, 2085, 2064 and 1831 cm⁻¹) for CO adsorption on the Pt-2Eu₂O₃/TiO₂ catalyst. It is also similar that the peaks for the gaseous CO and CO adsorbed on the Ptⁿ⁺ sites vanished after the N₂ purge, whereas the peaks corresponding to the CO adsorbed on the Pt⁰ sites remained unchanged. Despite these similarities, a comparison of the CO adsorption spectra acquired after 60 min reveals significantly enhanced CO adsorption on the Pt-2Eu₂O₃/TiO₂ catalyst. This observation aligns with the finding that Eu₂O₃ addition promotes the dispersion of Pt.

2.8. Catalyst Acidity and Basicity

2.8.1. NH₃-TPD Results

NH₃-TPD was conducted to titrate the acidity of the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts. As shown in Figure 6a, both catalysts have NH₃ desorption peaks at low (<200 °C), medium (200–400 °C) and high temperatures (>400 °C), which correspond to weak, medium, and strong acid sites, respectively. Figure 6b compares the number of acid sites of different strengths as well as the total number of acid sites on the two catalysts, which were calculated based on the peak areas. It can be seen that the number of strong acid sites significantly decreased after introducing Eu₂O₃, leading to the lower total acidity of the Pt-2Eu₂O₃/TiO₂ catalyst compared to Pt/TiO₂. Notably, the decrease in strong acidity with the introduction of Eu₂O₃ did not compromise the catalyst’s SO₂ resistance.

2.8.2. SO₂-TPD Results

To better understand the different SO₂ resistance of the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts, SO₂-TPD analysis of TiO₂, 2Eu₂O₃/TiO₂, Pt/TiO₂, and Pt-2Eu₂O₃/TiO₂ was performed. As shown in Figure 7a, two SO₂ desorption peaks, one at temperatures lower than 300 °C and the other at higher temperatures, were observed for all the investigated samples, corresponding to weak and strong adsorption sites for SO₂, respectively. Compared to TiO₂, the desorption peaks became larger and shifted to higher temperature ranges after loading Eu₂O₃ and/or Pt species, indicating that Eu₂O₃ and Pt species have stronger affinity for SO₂ than the TiO₂ support.

For the weakly adsorbed SO₂, the desorption temperature decreased in the order of Pt/TiO₂ (242 °C) > Pt-2Eu₂O₃/TiO₂ (230 °C) > 2Eu₂O₃/TiO₂ (205 °C) (Figure 7a). Obviously, in the low-temperature region, Pt species exhibited stronger SO₂ adsorption affinity than Eu₂O₃. The presence of Eu₂O₃ led to the lower SO₂ desorption temperature for Pt-2Eu₂O₃/TiO₂ compared to Pt/TiO₂. For the strongly adsorbed SO₂, the desorption temperature followed the order of Pt-2Eu₂O₃/TiO₂ (475 °C) ≈ 2Eu₂O₃/TiO₂ (473 °C) > Pt/TiO₂ (442 °C) (Figure 7a). Notably, in the high-temperature region, SO₂ exhibited stronger adsorption on Eu₂O₃ than on Pt species. Eu₂O₃ acted as the primary sites for strong SO₂ adsorption on the Pt-2Eu₂O₃/TiO₂ catalyst, thereby explaining the observed disappearance of the Eu XPS signals in Pt-2Eu₂O₃/TiO₂-WS (Figure 4c).

Figure 7b compares the numbers of weak, strong, and total SO₂ adsorption sites calculated based on the peak areas. It can be seen that loading Eu₂O₃ and/or Pt on TiO₂ increased the SO₂ adsorption at both weak and strong sites. The total adsorbed amount of SO₂ decreased in the order of Pt-2Eu₂O₃/TiO₂ > Pt/TiO₂ > 2Eu₂O₃/TiO₂ > TiO₂ (Figure 7b). Compared to Pt/TiO₂, only slightly higher amounts of SO₂ were adsorbed on Pt-2Eu₂O₃/TiO₂, due to the slight increase in the number of weak adsorption sites for SO₂.

Overall, the Eu₂O₃ in Pt-2Eu₂O₃/TiO₂ contributed to the increased adsorption sites for weakly adsorbed SO₂ while lowering its desorption temperature, and the Eu₂O₃ functioned as strong SO₂ adsorption sites, thereby protecting the Pt species. These synergistic effects provide a plausible explanation for the enhanced SO₂ resistance of Pt-2Eu₂O₃/TiO₂ compared to Pt/TiO₂ (Figure 1d).

2.9. H₂O-TPD Results

The H₂O adsorption–desorption behaviors of the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts were investigated using the H₂O-TPD. As shown in Figure 8a, both samples displayed several H₂O desorption peaks, which were classified into three types (I, II, and III). The peaks appearing below 150 °C (type I) were attributed to H₂O physically adsorbed on the catalyst surface [45]. The peaks at 231/236 °C (type II) and 402/406 °C (type III) corresponded to hydroxyl species formed via the dissociative adsorption of H₂O and structural hydroxides, respectively [46,47]. Notably, the loading of Eu₂O₃ significantly increased the desorption temperature of type I H₂O, indicating stronger physisorption of H₂O on Pt-2Eu₂O₃/TiO₂ than on Pt/TiO₂. In contrast, both surface hydroxyl species (type II H₂O) and structural hydroxides (type III H₂O) desorbed at similar temperatures for the two catalysts.

Figure 8b compares the desorbed amount of H₂O calculated based on the peak areas. It can be seen that introducing Eu₂O₃ increased the desorbed amounts of all the types of H₂O, indicating that Eu₂O₃ enhanced the physisorption of H₂O, the dissociation of H₂O into hydroxyl groups, and the formation of structural hydroxides. These finding might be explained by the hygroscopic nature of Eu₂O₃ [48], resulting in the much higher CO oxidation activity of Pt-2Eu₂O₃/TiO₂ than Pt/TiO₂ in the presence of H₂O (Figure 1c).

2.10. In Situ DRIFTS Results of CO Oxidation

To further elucidate the influence mechanisms of SO₂ and H₂O on the oxidation of CO, the in situ DRIFTS spectra of CO oxidation in the presence SO₂ and H₂O were recorded at 250 °C, as shown in Figure 9. Figure 9a shows that characteristic peaks appeared at 3671, 2361, 2336, 1524, 1450, 1373, 1304, 1276 and 1187 cm⁻¹ during CO oxidation on the Pt/TiO₂ catalyst. The peak at 3671 cm⁻¹ is assigned to bridge-bonding OH, which originates from the dissociation of H₂O on the oxygen vacancies [49]. The peaks at 2361 and 2336 cm⁻¹ correspond to gas phase CO₂ [39]. The peaks at 1524, 1450, 1373, 1304, 1276 and 1187 cm⁻¹ are related to sulfur-containing compounds. Specifically, the peak at 1524 cm⁻¹ is attributed to the stretching vibration of HSO₄⁻. The peaks at 1450, 1373 and 1304 cm⁻¹ correspond to surface sulfate species [50,51,52]. The peak at 1276 cm⁻¹ is ascribed to the stretching vibration of the bridging bidentate sulfate species [15]. The peak at 1187 cm⁻¹ is assigned to the stretching vibration of bulk sulfate species [51].

As the reaction time extended, the peaks correlated with sulfate and OH* species increased in intensity (Figure 9a), indicating that these species accumulated on the Pt/TiO₂ catalyst’s surface. The accumulation of sulfate species led to the gradual deactivation of the catalyst, explaining the weakening of the CO₂ peaks observed during the oxidation of CO on the Pt/TiO₂ catalyst (Figure 9a).

Figure 9b shows that, in addition to the characteristic peaks corresponding to sulfate species, OH* species, and CO₂, new peaks attributable to adsorbed COOH* species were observed at 1700 and 1649 cm⁻¹ during CO oxidation on the Pt-2Eu₂O₃/TiO₂ catalyst [53]. The intensity of all the peaks increased progressively with the reaction time, and the OH* peak (3671 cm⁻¹) exhibited a significant higher intensity on Pt-2Eu₂O₃/TiO₂ than on Pt/TiO₂. This observation indicates that H₂O dissociation into OH* species is more favorable on the Pt-2Eu₂O₃/TiO₂ surface, which aligns with the XPS and H₂O-TPD results.

The enhanced generation of OH* species, the detection of COOH* species, and the promoted production of CO₂ on the Pt-2Eu₂O₃/TiO₂ catalyst collectively suggest that Eu₂O₃ facilitates water dissociation, thereby promoting CO oxidation via forming COOH* as a key reaction intermediate. Notably, more sulfate species were accumulated on the Pt-2Eu₂O₃/TiO₂ surface compared to Pt/TiO₂, which is consistent with the SO₂-TPD results (Figure 7). However, these sulfate species had little effect on the long-term stability of the Pt-2Eu₂O₃/TiO₂ catalyst (Figure 1d), which might be due to the preferential adsorption of sulfate species on Eu₂O₃ sites, thereby protecting the Pt active sites from deactivation.

2.11. CO Oxidation Pathway

Figure 10 illustrates the H₂O-mediated CO oxidation pathway on the Pt-2Eu₂O₃/TiO₂ catalyst. The major steps include the adsorption of CO and H₂O (R1, R2), the dissociation of H₂O (R3), and the oxidation of CO accompanied by the consumption and replenishment of the lattice oxygen (R4~R9). The formation of sulfate species on the Eu₂O₃ sites (R10~R12) is also presented in Figure 10.

$$\mathrm{CO}\to \mathrm{CO}*$$

(R1)

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2\mathrm{O}*$$

(R2)

$${\mathrm{H}}_2\mathrm{O}*\to \mathrm{OH}*+\mathrm{H}*$$

(R3)

$$\mathrm{OH}*+\mathrm{CO}*\to \mathrm{COOH}*$$

(R4)

$$\mathrm{H}*+{\mathrm{O}}_{\mathrm{lat}}\to {\mathrm{O}}_{\mathrm{lat}}\mathrm{H}*$$

(R5)

$$\mathrm{COOH}*+{\mathrm{O}}_{\mathrm{lat}}\mathrm{H}*\to {\mathrm{CO}}_2*+{\mathrm{H}}_2\mathrm{O}*+( )$$

(R6)

$${\mathrm{CO}}_2*\to {\mathrm{CO}}_2$$

(R7)

$${\mathrm{H}}_2\mathrm{O}*\to {\mathrm{H}}_2\mathrm{O}$$

(R8)

$$( )+1/2{\mathrm{O}}_2\to {\mathrm{O}}_{\mathrm{lat}}$$

(R9)

$${\mathrm{SO}}_2\to {\mathrm{SO}}_2*$$

(R10)

$${\mathrm{SO}}_2*+\mathrm{O}\to {\mathrm{SO}}_3*$$

(R11)

$${\mathrm{SO}}_3*+{\mathrm{O}}^{2-}\to {{\mathrm{SO}}_4}^{2-}$$

(R12)

where *, O_lat and ( ) indicate the adsorption state, lattice oxygen and oxygen vacancy, respectively.

3. Experimental Section

3.1. Catalyst Preparation

The Pt-Eu₂O₃/TiO₂ catalysts were prepared via a two-step impregnation method. First, calculated amounts of TiO₂ and Eu(NO₃)₃·6H₂O were added to 200 mL of deionized water and ultrasonically agitated at 75 °C for 4 h. After drying at 110 °C for 6 h, the solid powder was calcined in a muffle furnace at 500 °C for 3 h to obtain the Eu₂O₃/TiO₂ composite. Subsequently, this composite was impregnated with aqueous Pt(NO₃)₂ solution, followed by identical ultrasonic treatment (75 °C/4 h), drying (110 °C/6 h), and calcination (500 °C/3 h) to yield the final catalyst. For comparison, Pt/TiO₂ was prepared by impregnating TiO₂ with aqueous Pt(NO₃)₂ solution, following the same ultrasonic treatment–drying–calcination procedures. The obtained catalysts were pressed, crushed, and sieved into particles of 20–40 meshes before testing.

The mass fraction of Pt in all the catalysts was fixed at 0.5%, while that of Eu₂O₃ in the Pt-Eu₂O₃/TiO₂ catalysts was varied (1%, 1.5%, 2%, and 2.5%). The catalysts with different Eu₂O₃ contents were denoted as Pt-xEu₂O₃/TiO₂ (x = 1, 1.5, 2, 2.5). The Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts that had been tested under conditions without H₂O and SO₂ were designated as Pt/TiO₂-used and Pt-2Eu₂O₃/TiO₂-used, respectively. The Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts that had been tested under conditions containing 15% H₂O and 50 ppm SO₂ at 200 °C for 72 h were designated as Pt/TiO₂-WS and Pt-2Eu₂O₃/TiO₂-WS, respectively.

3.2. Catalytic Activity Test

The catalytic CO oxidation activity was tested using a continuous-flow fixed-bed quartz microreactor. The standard inlet gas composition was 0.8% CO, 5% O₂, and N₂ balance. The influence of H₂O on CO catalytic oxidation was studied by injecting deionized water at a given flow rate into the inlet gas using a syringe pump. All the pipes before the reactor inlet were kept at around 110 °C to avoid condensation of the water vapor. The total flow rate of the feed gas was 1500 mL·min⁻¹, corresponding to a space velocity of ca. 45,000 mL·g⁻¹·h⁻¹. The long-term (72 h) SO₂ resistance of the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts was evaluated in the presence of 15% H₂O and 50 ppm SO₂ at 200 °C. The CO concentration in the inlet and outlet gases was analyzed using an MGA6 Plus infrared flue gas analyzer (MRU, Heidelberg, Germany).

The CO conversion was calculated by the following equation.

$$\mathrm{CO} \mathrm{conversion}={\displaystyle \frac{{\left[\mathrm{CO}\right]}_{\mathrm{in}}—{\left[\mathrm{CO}\right]}_{\mathrm{out}}\to }{{\left[\mathrm{CO}\right]}_{\mathrm{in}}}}\times 100\%$$

(1)

where [CO]_in and [CO]_out indicate the inlet and outlet concentration of CO, respectively.

The turnover frequency (TOF) was calculated by the following equation.

$$\mathrm{TOF}={\displaystyle \frac{\mathrm{x}{\mathrm{C}}_0}{{}_{\mathrm{nPt}}\mathrm{D}_{\mathrm{Pt}}}}$$

(2)

where x, C₀ (mol/s), n_Pt (mol), and D_Pt represent the CO conversion, the inlet molar flow rate of CO, the molar amount of Pt, and Pt dispersion, respectively.

3.3. Kinetic Measurements

The CO oxidation rates under standard inlet gas composition (0.8% CO, 5% O₂, and N₂ balance) over the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts were measured using the same apparatus as used for the CO oxidation activity test. Prior to the kinetic measurements, CO oxidation experiments were conducted at increasing WHSV to search for reaction conditions free from external mass-transfer limitations. The experimental details and results (Figure S1) of these preliminary experiments are described in the Supporting Information. To ensure that the collected data fell within kinetically relevant regimes (CO conversion < 20% and free from external diffusion limitations), the loading amount of the catalyst was fixed at 0.30 g and the total flow rate of the inlet gas was kept at 1.6 L/min to provide a WHSV of 320,000 mL·g⁻¹·h⁻¹ for the CO oxidation kinetic tests, which were conducted in the temperature range of 120–180 °C.

3.4. Catalyst Characterization

Scanning electron microscopy (SEM) analysis was conducted on a JSM-7401F instrument (JEOL, Tokyo, Japan). Transmission electron microscopy (TEM) images were captured using a JEM-2100F instrument (JEOL, Tokyo, Japan).

The X-ray diffraction (XRD) patterns of the catalysts were recorded with a Bruker D8 Advance instrument (Germany) using nickel-filtered Cu Kα radiation (λ = 1.54 Å) at 40 kV and 40 mA. The samples were scanned from 10° to 80° (2θ) with a step size of 0.02°.

The N₂ adsorption–desorption isotherms of the catalysts were obtained using an ASAP 2050 automatic gas adsorption analyzer (Micromeritics, Norcross, GA, USA). The catalysts were degassed under a vacuum at 250 °C for 5 h before the measurement. The specific surface area of the catalyst was calculated by the Brunauer–Emmett–Teller (BET) method, and the pore volume and pore size distribution were determined by the Barrett–Joyner–Halenda (BJH) method.

CO chemisorption experiments were performed on an Auto Chem II 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA) to analyze the Pt dispersion of the catalysts. The dispersion of Pt was determined by dividing the molar amount of CO adsorbed by the molar amount of Pt contained in the tested samples.

Hydrogen temperature-programmed reduction (H₂-TPR) was performed on an Auto Chem II 2920 chemisorption analyzer (Micromeritics, USA). The sample (100 mg) was pre-treated in air at 300 °C for 30 min before it was cooled down to room temperature. The sample was then purged in an Ar atmosphere until the baseline was stable. The TPR profile was obtained by heating the sample from 30 °C to 600 °C in a flow of 5% H₂/Ar at a rate of 10 °C min⁻¹. The H₂ consumption was detected by a thermal conductivity detector (TCD). The CuO standard sample was used to quantify the H₂ consumption amount.

X-ray photoelectron spectroscopy (XPS) data were collected on a Thermo Scientific ESCALAB 250xi equipment (Thermo Fisher Scientific, Waltham, MA, USA) using an Al Kα X-ray source (1486.7 eV) at 15 kV and 25 W, with the binding energy calibrated by C 1s at 284.8 eV.

The NH₃ temperature-programmed desorption (NH₃-TPD) was tested on a TP-5080 chemisorption analyzer (Tianjin Xianquan Industry and Trade Development Co., LTD, Tianjin, China). The sample (100 mg) was pre-treated in He flow at 300 °C for 1 h, cooled down to room temperature, and then exposed to 5% NH₃/He for 1 h. The gas was then switched to He, and the sample were heated to 600 °C at a rate of 10 °C min⁻¹ to detect the desorbed NH₃ by a TCD.

The SO₂ temperature-programmed desorption (SO₂-TPD) was performed on an Auto Chem II 2920 chemisorption analyzer (Micromeritics, USA). The SO₂-TPD test procedure was identical to that of NH₃-TPD, except that the adsorbate was changed from NH₃ to SO₂.

The H₂O temperature-programmed desorption (H₂O-TPD) was tested on an Auto Chem II 2920 chemisorption analyzer (Micromeritics, USA). Approximately 100 mg of the sample (40–60 mesh) was pre-treated in He flow at 300 °C for 1 h, cooled down to 50 °C under the same He flow, and then exposed to saturated water vapor for 1 h. The gas was then switched to He, and the samples were heated to 500 °C at a rate of 10 °C min⁻¹ to detect the desorbed H₂O by a TCD.

In situ diffuse reflectance infrared Fourier transform spectroscopic (in situ DRIFTS) tests were conducted on a Tensor II Fourier transform infrared spectrometer (Bruker Optics, Ettlingen, Germany) equipped with an MCT detector. The in situ DRIFTS spectra of CO chemisorption on the Pt/TiO₂ and Pt-2Eu₂O₃/TiO₂ catalysts at 25 °C, and the spectra of CO oxidation in the presence of SO₂ and H₂O at 250 °C, were recorded. The samples were pre-treated in N₂ at 300 °C for 1 h before being cooled down to the target temperature (25 or 250 °C). During the CO chemisorption test, 5% CO/N₂ was introduced into the chamber for 1 h, followed by a 20 min N₂ purge. During the CO oxidation test, a reaction gas mixture composed of 0.8% CO, 5% O₂, 1% H₂O, 50 ppm SO₂, and N₂ as the balance gas was fed into the chamber for 1 h.

4. Conclusions

In this study, Pt/TiO₂ and Pt-Eu₂O₃/TiO₂ catalysts were prepared via the impregnation method for catalytic oxidation of CO. The catalytic activity, SO₂ resistance, physicochemical properties of representative catalysts, and reaction mechanism of CO oxidation were systematically investigated. The main findings can be summarized as follows.

(1) Introducing different contents of Eu₂O₃ enhanced the catalytic activity of Pt/TiO₂ and the presence of H₂O favored CO oxidation. The Pt-2Eu₂O₃/TiO₂ catalyst exhibited the highest catalytic activity no matter whether H₂O was present or not. Under the inlet gas composition of 0.8% CO, 5% O₂, 3% H₂O, and balanced N₂, the lowest complete conversion temperature (T₁₀₀) of CO decreased from 120 °C for the Pt/TiO₂ catalyst to 100 °C for the Pt-2Eu₂O₃/TiO₂ catalyst.

(2) The Pt-2%Eu₂O₃/TiO₂ catalyst exhibited superior SO₂ resistance to the Pt/TiO₂ catalyst. During the 72 h SO₂-resistance test at 200 °C under an inlet gas composition of 0.8% CO, 5% O₂, 15% H₂O, 50 ppm SO₂, and balanced N₂, the CO conversion on the Pt-2%Eu₂O₃/TiO₂ catalyst remained >99% while that on the Pt/TiO₂ catalyst gradually decreased to 77.8%.

(3) Compared with the Pt/TiO₂ catalyst, the Pt-2Eu₂O₃/TiO₂ catalyst exhibited enhanced dispersion of Pt species, an elevated proportion of metallic Pt⁰, and facilitated adsorption and dissociation of H₂O. These collectively accounted for the superior catalytic performance of Pt-2Eu₂O₃/TiO₂ in terms of CO oxidation. The OH* species derived from H₂O dissociation played a crucial role in the CO oxidation by forming COOH* as the key reaction intermediate.

(4) Introducing 2 wt% Eu₂O₃ decreased the desorption temperature of weakly adsorbed SO₂ while increasing that of strongly adsorbed SO₂. SO₂ preferentially occupied the Eu₂O₃ sites by forming stable sulfates on the Pt-2Eu₂O₃/TiO₂ catalyst, thereby protecting the Pt active sites from sulfur poisoning.