*2.1. Catalytic Activity and SO2/H2O Durability*

The catalytic performances of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts in the absence and presence of H2O or SO2, or both, are displayed in Figure 1. It was found that Pt/CeTi showed a much higher CO oxidation activity in the absence of H2O and SO2, and the CO conversion was up to 100% at 150 ◦C, which is consistent with the literature result [38]. Meanwhile, the intrinsic activities of Pt/Keg-CeTi and Pt/MoP-CeTi were much lower compared to Pt/CeTi (Figure 1a), and their CO complete conversion temperatures were 210 ◦C and 220 ◦C, respectively. It is known that the presence of ceria in the support can improve the catalytic performance of noble metal catalysts for CO oxidation by storing oxygen during oxidation and releasing it during reduction [39]. The introduction of molybdophosphate led to the partial covering of the CeTi surface, which restrained the synergism between Pt and cerium species. When a small amount of SO<sup>2</sup> was introduced into the feed mixture, obvious decreases in the activities of the three samples were observed, particularly for Pt/CeTi, due to the significantly detrimental effect of SO2. In the presence of SO<sup>2</sup> and O2, Ce(IV) ions could transform into Ce(III) ions according to the following reaction [40,41]: 2CeO<sup>2</sup> + 3SO<sup>2</sup> + O<sup>2</sup> → Ce2(SO4)3, which caused the redox cycle between Ce (IV) and Ce (III) to be terminated. The Pt/Keg-CeTi showed the highest activity in the presence of SO<sup>2</sup> compared to the two others (Figure 1b). On the other hand, the introduction of H2O had, to a certain extent, a positive effect on the activities of the Pt/CeTi and Pt/Keg-CeTi catalysts (Figure 1c), and their CO complete conversion temperatures decreased to 130 ◦C and 180 ◦C, respectively. However, the activity of Pt/MoP-CeTi declined significantly, and the CO could not convert completely

even under high temperature. This might be explained by the adsorption of H2O molecules on the active centers, inhibiting the catalytic process due to the structure change of the support used. In the presence of both H2O and SO2, the activity of Pt/Keg-CeTi was also slightly higher than that of Pt/CeTi and was much better than that of Pt/MoP-CeTi (Figure 1d), revealing the better sulfur and water resistance. might be explained by the adsorption of H2O molecules on the active centers, inhibiting the catalytic process due to the structure change of the support used. In the presence of both H2O and SO2, the activity of Pt/Keg-CeTi was also slightly higher than that of Pt/CeTi and was much better than that of Pt/MoP-CeTi (Figure 1d), revealing the better sulfur and water resistance.

Pt/Keg-CeTi catalysts (Figure 1c), and their CO complete conversion temperatures decreased to 130 °C and 180 °C, respectively. However, the activity of Pt/MoP-CeTi declined significantly, and the CO could not convert completely even under high temperature. This

*Catalysts* **2022**, *11*, x FOR PEER REVIEW 3 of 16

**Figure 1.** CO conversions as a function of reaction temperature over the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts in the absence of H2O and SO2 (**a**), and in the presence of SO2 (**b**), H2O, (**c**) or both H2O and SO2 (**d**), respectively. Reaction conditions: [CO] = 1%, [O2] = 6 vol%, [H2O] = 10% (when used), [SO2] = 100 ppm (when used), balance He, total flow rate = 667 mL/min, GHSV = 4 × 105 h−1. **Figure 1.** CO conversions as a function of reaction temperature over the Pt/CeTi, Pt/Keg-CeTi,and Pt/MoP-CeTi catalysts in the absence of H2O and SO<sup>2</sup> (**a**), and in the presence of SO<sup>2</sup> (**b**), H2O, (**c**) or both H2O and SO<sup>2</sup> (**d**), respectively. Reaction conditions: [CO] = 1%, [O<sup>2</sup> ] = 6 vol%, [H2O] = 10% (when used), [SO<sup>2</sup> ] = 100 ppm (when used), balance He, total flow rate = 667 mL/min, GHSV = 4 <sup>×</sup> <sup>10</sup><sup>5</sup> <sup>h</sup> −1 .

The SO2/H2O durability of the three catalysts was also tested (Figure 2). The activities of the Pt/CeTi and Pt/MoP-CeTi catalysts at 250 °C in the presence of SO2 decreased rapidly from 100% to 95% within the first two hours, and then decreased gradually (Figure 2a). Moreover, the deactivation rate of Pt/MoP-CeTi was faster than that of Pt/CeTi. The CO conversions over Pt/CeTi and Pt/MoP-CeTi after continuing the test on the reaction stream for 30 h were 87% and 74%, respectively. However, the durability of Pt/Keg-CeTi was much better than those of the two others, and its activity data after 30 h were still maintained at above 95%. In the presence of 10% H2O only, the Pt/CeTi and Pt/Keg-CeTi The SO2/H2O durability of the three catalysts was also tested (Figure 2). The activities of the Pt/CeTi and Pt/MoP-CeTi catalysts at 250 ◦C in the presence of SO<sup>2</sup> decreased rapidly from 100% to 95% within the first two hours, and then decreased gradually (Figure 2a). Moreover, the deactivation rate of Pt/MoP-CeTi was faster than that of Pt/CeTi. The CO conversions over Pt/CeTi and Pt/MoP-CeTi after continuing the test on the reaction stream for 30 h were 87% and 74%, respectively. However, the durability of Pt/Keg-CeTi was much better than those of the two others, and its activity data after 30 h were still maintained at above 95%. In the presence of 10% H2O only, the Pt/CeTi and Pt/Keg-CeTi catalysts showed a high stability, and their CO oxidation activities at 200 ◦C only showed a slightly decrease when the reaction time extended to 30 h. Nevertheless, the Pt/MoP-CeTi catalyst deactivated much faster under similar conditions, and its CO conversion decreased

quickly from 67% to 31% in the first 5 h on stream (Figure 2b). When both H2O and SO<sup>2</sup> were added, a similar deactivation phenomenon for Pt/MoP-CeTi was also observed. The CO conversion dropped rapidly to less than 30% in the first 5 h (Figure 2c). The Pt/CeTi catalyst showed a middle stability. Its CO conversion was maintained at above 95% within the first 5 h, and then decreased gradually to 61%. However, for Pt/Keg-CeTi, no obvious deactivation was observed within the test time, and the activity data were maintained at above 93% after 30 h. The results further showed that Pt/Keg-CeTi had a higher resistance to SO<sup>2</sup> and H2O poisoning than the two others. CeTi catalyst deactivated much faster under similar conditions, and its CO conversion decreased quickly from 67% to 31% in the first 5 h on stream (Figure 2b). When both H2O and SO2 were added, a similar deactivation phenomenon for Pt/MoP-CeTi was also observed. The CO conversion dropped rapidly to less than 30% in the first 5 h (Figure 2c). The Pt/CeTi catalyst showed a middle stability. Its CO conversion was maintained at above 95% within the first 5 h, and then decreased gradually to 61%. However, for Pt/Keg-CeTi, no obvious deactivation was observed within the test time, and the activity data were maintained at above 93% after 30 h. The results further showed that Pt/Keg-CeTi had a higher resistance to SO2 and H2O poisoning than the two others.

catalysts showed a high stability, and their CO oxidation activities at 200 °C only showed a slightly decrease when the reaction time extended to 30 h. Nevertheless, the Pt/MoP-

*Catalysts* **2022**, *11*, x FOR PEER REVIEW 4 of 16

**Figure 2.** Durability of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts at constant temperatures in the presence of SO2 (**a**), H2O (**b**), and both of them (**c**). Reaction conditions: [CO] = 1%, [O2] = 6 vol%, [H2O] = 10% (when used), [SO2] = 100 ppm (when used), balance He, total flow rate = 667 mL/min, GHSV = 4 × 105 h−1. **Figure 2.** Durability of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts at constant temperatures in the presence of SO<sup>2</sup> (**a**), H2O (**b**), and both of them (**c**). Reaction conditions: [CO] = 1%, [O<sup>2</sup> ] = 6 vol%, [H2O] = 10% (when used), [SO<sup>2</sup> ] = 100 ppm (when used), balance He, total flow rate = 667 mL/min, GHSV = 4 <sup>×</sup> <sup>10</sup><sup>5</sup> <sup>h</sup> −1 .

#### *2.2. Structure and Morphology*  The XRD patterns of the three samples are illustrated in Figure 3. The diffraction *2.2. Structure and Morphology*

peaks at 2θ = 25.2, 37.7, 47.8, 53.7, 55.9, and 62.5° were attributed to anatase TiO2, and the peaks at 28.5, 33.0, and 56.3° were assigned to the cubic fluorite-type CeO2. Moreover, the intensities of the corresponding peaks for the three catalysts were similar, implying that the addition of molybdophosphate with the Keggin structure or molybdophosphate without the Keggin structure had no obvious effect on the structure of cerium and titanium composite oxide. For the Pt/Keg-CeTi sample, several additional sharp peaks at 2θ = 10.5, 15.5, 26.5, 30.5, 36.1, and 40.0° were also detected, demonstrating the formation of a Keggin structure. Meanwhile no similar diffraction peaks were observed on the Pt/MoP-CeTi catalyst, indicating the destruction of the Keggin structure after the high-temperature treatment. In addition, no peak that could be assigned to metallic platinum or its oxides was observed in the XRD curves of the three samples, implying the high dispersion of Pt. This was also corroborated by the TEM data (Figure 4). The XRD patterns of the three samples are illustrated in Figure 3. The diffraction peaks at 2θ = 25.2, 37.7, 47.8, 53.7, 55.9, and 62.5◦ were attributed to anatase TiO2, and the peaks at 28.5, 33.0, and 56.3◦ were assigned to the cubic fluorite-type CeO2. Moreover, the intensities of the corresponding peaks for the three catalysts were similar, implying that the addition of molybdophosphate with the Keggin structure or molybdophosphate without the Keggin structure had no obvious effect on the structure of cerium and titanium composite oxide. For the Pt/Keg-CeTi sample, several additional sharp peaks at 2θ = 10.5, 15.5, 26.5, 30.5, 36.1, and 40.0◦ were also detected, demonstrating the formation of a Keggin structure. Meanwhile no similar diffraction peaks were observed on the Pt/MoP-CeTi catalyst, indicating the destruction of the Keggin structure after the high-temperature treatment. In addition, no peak that could be assigned to metallic platinum or its oxides was observed in the XRD curves of the three samples, implying the high dispersion of Pt. This was also corroborated by the TEM data (Figure 4). *Catalysts* **2022**, *11*, x FOR PEER REVIEW 5 of 16

**Figure 3.** X-ray diffraction patterns of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts. **Figure 3.** X-ray diffraction patterns of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts.

The porosities of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts were characterized by N2 adsorption-desorption isotherms (Figure S1). The three samples all showed

P/P0 > 0.5, indicating the existence of stacking channels and the sorption behavior of mesopores. Meanwhile, there was no obvious saturated adsorption platform in the low-pressure range, revealing that there were few micropores on the catalyst surface (Figure S1). This was in agreement with their relatively low specific surface area (41–63 m2/g) (Table 1). The pore volumes of Pt/Keg-CeTi and Pt/MoP-CeTi were comparatively smaller than that of Pt/CeTi possibly due to the blocking of partial pores of CeTi by the molybdophosphate, and the pore diameter of Pt/Keg-CeTi was slightly larger than those of the two

**Samples SBET (m2/g) Smic (m2/g) Vtot (cm3/g) Pore Size (nm)**  Pt/CeTi 63 0.0 0.210 9.6 Pt/Keg-CeTi 53 7.4 0.152 12.3 Pt/MoP-CeTi 41 3.5 0.152 9.6

The SEM images clearly showed that the three catalysts had a similar morphology, which were comprised of an irregular conglomeration of particles that were formed by many fine particles with different diameters (Figure 4d–f). TEM images of the Pt/CeTi and Pt/MoP-CeTi catalysts showed that the platinum species were well dispersed on the surface of the support (Figure 4a,c), and most of the Pt particle sizes were in the ranges of <5 nm. Meanwhile, on the Pt/Keg-CeTi sample, few clear Pt particles could be observed, indicating a better dispersion of platinum species on the Keggin structure-modified CeTi

others, which might be related to the difference in particles' size.

**Table 1.** Textural and structural properties of the catalyst samples.

composite oxide.

The porosities of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts were characterized by N<sup>2</sup> adsorption-desorption isotherms (Figure S1). The three samples all showed typical type II isotherms and type H3 hysteresis loops in a high relative pressure range of P/P<sup>0</sup> > 0.5, indicating the existence of stacking channels and the sorption behavior of mesopores. Meanwhile, there was no obvious saturated adsorption platform in the low-pressure range, revealing that there were few micropores on the catalyst surface (Figure S1). This was in agreement with their relatively low specific surface area (41–63 m2/g) (Table 1). The pore volumes of Pt/Keg-CeTi and Pt/MoP-CeTi were comparatively smaller than that of Pt/CeTi possibly due to the blocking of partial pores of CeTi by the molybdophosphate, and the pore diameter of Pt/Keg-CeTi was slightly larger than those of the two others, which might be related to the difference in particles' size.

**Table 1.** Textural and structural properties of the catalyst samples.


The SEM images clearly showed that the three catalysts had a similar morphology, which were comprised of an irregular conglomeration of particles that were formed by many fine particles with different diameters (Figure 4d–f). TEM images of the Pt/CeTi and Pt/MoP-CeTi catalysts showed that the platinum species were well dispersed on the surface of the support (Figure 4a,c), and most of the Pt particle sizes were in the ranges of <5 nm. Meanwhile, on the Pt/Keg-CeTi sample, few clear Pt particles could be observed, indicating a better dispersion of platinum species on the Keggin structure-modified CeTi composite oxide. *Catalysts* **2022**, *11*, x FOR PEER REVIEW 6 of 16

**Figure 4.** TEM and SEM images of the Pt/CeTi (**a**,**d**), Pt/Keg-CeTi (**b**,**e**), and Pt/MoP-CeTi (**c**,**f**) catalysts. **Figure 4.** TEM and SEM images of the Pt/CeTi (**a**,**d**), Pt/Keg-CeTi (**b**,**e**), and Pt/MoP-CeTi (**c**,**f**) catalysts.

#### *2.3. Adsorption and Desorption of SO2 on the Catalysts 2.3. Adsorption and Desorption of SO<sup>2</sup> on the Catalysts*

In order to understand the difference in sulfur resistance of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts, the adsorption of SO2 on the three catalysts under the same conditions was investigated in detail by in situ IR spectroscopy and temperature-programmed desorption experiments. The DRIFTS results of SO2 adsorption experiments in the presence of 150 ppm of SO2 and a large excess of O2 at 250 °C are shown in Figure 5. In order to understand the difference in sulfur resistance of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts, the adsorption of SO<sup>2</sup> on the three catalysts under the same conditions was investigated in detail by in situ IR spectroscopy and temperature-programmed desorption experiments. The DRIFTS results of SO<sup>2</sup> adsorption experiments in the presence of 150 ppm of SO<sup>2</sup> and a large excess of O<sup>2</sup> at 250 ◦C are shown in Figure 5. For the

For the Pt/CeTi sample, after the introduction of SO2, four adsorption bands at 1451, 1350, 1294, and 1145 cm−1 were observed and their intensities increased with time. According to

vealing that the surface and bulk sulfate species were formed nearly simultaneously on the Pt/CeTi surface. For the Pt/Keg-CeTi sample, a peak at 1065 cm−1 was detected in the first 5 min, which was attributed to symmetrical oscillations of sulfites [47], which could be detected in the whole experimental time. The signals attributed to the surface and bulk sulfates were much weaker, implying that the accumulation of sulfate species on the catalyst was inhibited significantly due to the modification of the Keggin structure. For the Pt/MoP-CeTi sample, two wide and weak peaks at 1400 and 1168 cm−1 were detected, which were assigned to surface sulfate and bulk sulfate species [48], respectively. It can be seen that both surface sulfate and bulk sulfate species were detected on the three catalysts, but the corresponding peak intensities were quite different. Obviously, the amount of sulfate species on Pt/Keg-CeTi was much lower than those of Pt/CeTi and Pt/MoP-CeTi.

Pt/CeTi sample, after the introduction of SO2, four adsorption bands at 1451, 1350, 1294, and 1145 cm−<sup>1</sup> were observed and their intensities increased with time. According to other studies [42–46], the peaks at 1451, 1350, and 1294 cm−<sup>1</sup> were attributed to the surface sulfate species. The band centered at 1145 cm−<sup>1</sup> was assigned to the bulk-like sulfate, revealing that the surface and bulk sulfate species were formed nearly simultaneously on the Pt/CeTi surface. For the Pt/Keg-CeTi sample, a peak at 1065 cm−<sup>1</sup> was detected in the first 5 min, which was attributed to symmetrical oscillations of sulfites [47], which could be detected in the whole experimental time. The signals attributed to the surface and bulk sulfates were much weaker, implying that the accumulation of sulfate species on the catalyst was inhibited significantly due to the modification of the Keggin structure. For the Pt/MoP-CeTi sample, two wide and weak peaks at 1400 and 1168 cm−<sup>1</sup> were detected, which were assigned to surface sulfate and bulk sulfate species [48], respectively. It can be seen that both surface sulfate and bulk sulfate species were detected on the three catalysts, but the corresponding peak intensities were quite different. Obviously, the amount of sulfate species on Pt/Keg-CeTi was much lower than those of Pt/CeTi and Pt/MoP-CeTi. *Catalysts* **2022**, *11*, x FOR PEER REVIEW 7 of 16

**Figure 5.** Changes in DRIFTS spectra of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi with time. Conditions: [O2] = 16 vol%, [SO2] = 150 ppm, balance N2, total flow rate = 50 mL/min, at 250 °C. tions: [O<sup>2</sup> ] = 16 vol%, [SO<sup>2</sup> ] = 150 ppm, balance N<sup>2</sup> , total flow rate = 50 mL/min, at 250 ◦C.

To further understand the nature and content of sulfate species formed on the poisoned samples better, TPD analysis was examined (Figure 6). It clearly showed that two peaks at 675 and 765 °C were observed for Pt/CeTi, which were attributed to the decomposition of Ce(SO4)2 and Ce2(SO4)3 [49], respectively. There was one main SO2 desorption peak at 705 °C for Pt/MoP-CeTi, while the peak that appeared at 695 °C for Pt/Keg-CeTi transformation. **Figure 5.** Changes in DRIFTS spectra of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi with time. Condi-To further understand the nature and content of sulfate species formed on the poisoned samples better, TPD analysis was examined (Figure 6). It clearly showed that two peaks at 675 and 765 ◦C were observed for Pt/CeTi, which were attributed to the decomposition of Ce(SO4)<sup>2</sup> and Ce2(SO4)<sup>3</sup> [49], respectively. There was one main SO<sup>2</sup> desorption peak at 705 ◦C for Pt/MoP-CeTi, while the peak that appeared at 695 ◦C for Pt/Keg-CeTi

was much weaker compared to the two others. It was clear that the amount of sulfate species accumulated on Pt/Keg-CeTi was much lower than that on Pt/MoP-CeTi and

CeTi were highly inhibited, which might be one main reason why it showed good sulfur resistance. However, the low activity and sulfur resistance of Pt/MoP-CeTi may also relate to the low surface Pt concentrations (see Table 2) except its moderate SO2 adsorption and

**Intensity(a.u.)**

was much weaker compared to the two others. It was clear that the amount of sulfate species accumulated on Pt/Keg-CeTi was much lower than that on Pt/MoP-CeTi and Pt/CeTi, which was also in agreement with the ICP data (Table S1) and the DRIFTS spectra (Figure 5). The results revealed that the SO<sup>2</sup> adsorption and transformation on Pt/Keg-CeTi were highly inhibited, which might be one main reason why it showed good sulfur resistance. However, the low activity and sulfur resistance of Pt/MoP-CeTi may also relate to the low surface Pt concentrations (see Table 2) except its moderate SO<sup>2</sup> adsorption and transformation. *Catalysts* **2022**, *11*, x FOR PEER REVIEW 8 of 16

**Figure 6.** SO2-TPD of Pt/MoP-CeTi, Pt/Keg-CeTi, and Pt/CeTi after exposure to 1000 ppm of SO2 with 16% O2 in Ar atmosphere for 1 h at 250 °C. **Figure 6.** SO<sup>2</sup> -TPD of Pt/MoP-CeTi, Pt/Keg-CeTi, and Pt/CeTi after exposure to 1000 ppm of SO<sup>2</sup> with 16% O<sup>2</sup> in Ar atmosphere for 1 h at 250 ◦C.

*2.4. Surface Properties and Redox Property*  The Pt 4f and Ce 3d XPS spectra of the three samples are illustrated in Figure 7, and **Table 2.** Surface atomic concentration and atomic ratio of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts.


*2.4. Surface Properties and Redox Property*

Pt/MoP-CeTi

lysts.

**65 70 75 80 85**

**Binding Energy**(**eV**)

Pt/Keg-CeTi Pt/Keg-CeTi **Intensity(a.u.)**The Pt 4f and Ce 3d XPS spectra of the three samples are illustrated in Figure 7, and the surface atomic compositions, Pt4+/(Pt2+ + Pt4+), Ce3+/(Ce3+ + Ce4+), and Oads/(Oads + Olatt) molar ratios, and binding energies are summarized in Table 2 and Table S2.

**Figure 7.** XPS spectra of Pt 4f (**a**) and Ce 3d (**b**) in the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi cata-

It was observed that the binding energy values of Pt 4f in the three samples were different (Figure 7a). The peaks were labeled as "a" and "b," representing Pt2+ and Pt4+ [50], respectively. This suggested that Pt2+ and Pt4+ coexisted on the three catalysts' surface with different surface atomic concentrations and atomic ratios, and no Pt0 species were detected. The surface atomic concentrations of Pt for Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi were 0.23%, 0.16%, and 0.11% (Table 2), respectively, revealing that the addition of molybdophosphate with or without the Keggin structure resulted in a decrease in the surface active sites. This might be one reason why Pt/CeTi has the best catalytic activity in the absence of H2O and SO2. The atomic ratios of Pt4+/(Pt2+ + Pt4+) for Pt/CeTi, Pt/Keg-CeTi,

**880 900 920**

**Binding Energy**(**eV**)

Pt/MoP-CeTi

molar ratios, and binding energies are summarized in Tables 2 and S2.

Pt/CeTi

 Pt/MoP-CeTi Pt/Keg-CeTi

with 16% O2 in Ar atmosphere for 1 h at 250 °C.

*2.4. Surface Properties and Redox Property* 

**0.0**

**1.2×10¯**

**1.0×10¯**

**8.0×10¯**

**6.0×10¯**

**Vacuum Degree(torr)**

**4.0×10¯**

**2.0×10¯**

**10**

**10**

**11**

**11**

**11**

**11**

**Figure 7.** XPS spectra of Pt 4f (**a**) and Ce 3d (**b**) in the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi cata-**Figure 7.** XPS spectra of Pt 4f (**a**) and Ce 3d (**b**) in the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts.

**Figure 6.** SO2-TPD of Pt/MoP-CeTi, Pt/Keg-CeTi, and Pt/CeTi after exposure to 1000 ppm of SO2

**100 200 300 400 500 600 700 800**

**Temperature(<sup>o</sup>**

The Pt 4f and Ce 3d XPS spectra of the three samples are illustrated in Figure 7, and the surface atomic compositions, Pt4+/(Pt2+ + Pt4+), Ce3+/(Ce3+ + Ce4+), and Oads/(Oads + Olatt)

**C)**

lysts. It was observed that the binding energy values of Pt 4f in the three samples were different (Figure 7a). The peaks were labeled as "a" and "b," representing Pt2+ and Pt4+ [50], respectively. This suggested that Pt2+ and Pt4+ coexisted on the three catalysts' surface with different surface atomic concentrations and atomic ratios, and no Pt0 species were detected. The surface atomic concentrations of Pt for Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi were 0.23%, 0.16%, and 0.11% (Table 2), respectively, revealing that the addition of molybdophosphate with or without the Keggin structure resulted in a decrease in the surface active sites. This might be one reason why Pt/CeTi has the best catalytic activity in the absence of H2O and SO2. The atomic ratios of Pt4+/(Pt2+ + Pt4+) for Pt/CeTi, Pt/Keg-CeTi, It was observed that the binding energy values of Pt 4f in the three samples were different (Figure 7a). The peaks were labeled as "a" and "b," representing Pt2+ and Pt4+ [50], respectively. This suggested that Pt2+ and Pt4+ coexisted on the three catalysts' surface with different surface atomic concentrations and atomic ratios, and no Pt<sup>0</sup> species were detected. The surface atomic concentrations of Pt for Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi were 0.23%, 0.16%, and 0.11% (Table 2), respectively, revealing that the addition of molybdophosphate with or without the Keggin structure resulted in a decrease in the surface active sites. This might be one reason why Pt/CeTi has the best catalytic activity in the absence of H2O and SO2. The atomic ratios of Pt4+/(Pt2+ + Pt4+) for Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi were 23.3%, 24.7%, and 55.7% (Table 2), indicating that the introduction of molybdophosphate with the Keggin structure had no obvious effect on the Pt valence state. However, the Pt valence state of Pt/MoP-CeTi changed a lot. During the destruction of the Keggin structure under high temperature, more Ce3+ species were generated, resulting in more adsorbed oxygen species on the catalyst surface compared to Pt/CeTi, which might oxidize partial Pt2+ to Pt4+. The XPS spectra of Pt 4f in the recovered Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts indicated that the valence state distribution of Pt species changed significantly during the reaction (Figure S2), and the Pt4+ species in the used Pt/Keg-CeTi and Pt/MoP-CeTi samples disappeared, unlike the corresponding Pt/CeTi. The photoelectron spectrum of Ce 3d is also given in Figure 7b, in which the peaks labeled "u" represented Ce 3d3/2 contribution, and those labeled "v" were assigned to 3d5/2 [51]. The spectral lines denoted as v, v", v"' and u, u", u"' were characteristic of the Ce4+, while v' and u' were assigned to the Ce3+, suggesting the coexistence of Ce3+ and Ce4+ on the three catalyst samples. It is known that oxygen vacancies and unsaturated chemical bonds are related to the presence of Ce3+ and benefit the formation of chemisorbed oxygen on the surface [52]. The atomic ratios of Ce3+/(Ce3+ + Ce4+) for Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi were 18.0%, 31.0%, and 42.3%, respectively (Table 2). The relatively high Ce3+ ratio of Pt/Keg-CeTi might be attributed to the interaction between CeO<sup>2</sup> and molybdophosphate, while the highest Ce3+ ratio of Pt/MoP-CeTi was possibly also related to the high-temperature treatment, except the interaction of CeO<sup>2</sup> and molybdophosphate. The O 1s XPS (Figure S3) signals of the three catalysts could be fitted into two groups referred to the lattice oxygen (Olatt) at around 529.8 eV and the surface chemisorbed oxygen (Oads) in the range of 530.9–531.8 eV [53]. Compared to Pt/CeTi, the concentrations of chemisorbed oxygen species in Pt/Keg-CeTi and Pt/MoP-CeTi were higher due to the existence of more oxygen vacancies on their surface (Table 2). The difference in binding energy of oxygen species also revealed the diversity of the surface microenvironment of the catalysts.

The surface acidity of the catalysts was characterized using the NH3-TPD technique. From the TPD profile (Figure 8), two broad NH<sup>3</sup> desorption peaks centered at 98 and 273 ◦C, 98 and 246 ◦C, and 289 and 419 ◦C could be observed, respectively, for the Pt/MoP-CeTi, Pt/CeTi, and Pt/Keg-CeTi samples between 50 ◦C and 600 ◦C. The signal in the low-temperature range (50–200 ◦C) was assigned to the desorption of physiosorbed NH<sup>3</sup> on the weak acid sites, and the signal in the high-temperature range (200–600 ◦C) was ascribed to the desorption of chemisorbed NH<sup>3</sup> on the strong acid sites [54]. It was known that the high surface acidity could inhibit the adsorption of SO<sup>2</sup> on the catalyst surface [55]. It could be seen that there were many more acid sites, particularly for strong acid sites on the surface of Pt/Keg-CeTi. The calculated data showed that the number of total acid sites was in the order of Pt/Keg-CeTi > Pt/CeTi > Pt/MoP-CeTi (Table S3), which was consistent with their sulfur resistance, implying that the surface acidity of the catalyst might be one factor influencing its sulfur resistance in CO oxidation. *Catalysts* **2022**, *11*, x FOR PEER REVIEW 10 of 16

**Figure 8.** NH3-TPD profiles of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts. **Figure 8.** NH<sup>3</sup> -TPD profiles of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts.

The H2-TPR technique was employed to investigate the reduction property of the catalysts. Figure 9 illustrates the H2-TPR profiles of the three catalysts performed under an Ar atmosphere (Table S4). One could see that there were five reduction peaks at 84, 224, 334, 585, and 640 °C for Pt/Keg-CeTi, as well as five peaks at 76, 193, 366, 613, and 657 °C for Pt/MoP-CeTi. For Pt/CeTi, only three reduction peaks at 83, 379, and 645 °C could be observed. As a result of the hard reduction of titanium oxides below 700 °C, all the signals of the various catalysts were attributed to the reduction of corresponding platinum, cerium, and molybdenum species. The reduction peak below 100 °C could be assigned to the reduction of Pt oxides. The Pt reduction temperatures of Pt/CeTi (83 °C), Pt/Keg-CeTi (84 °C), and Pt/MoP-CeTi (76 °C) are comparably higher than the data of bulk Pt oxides' reduction, which is normally below room temperature, possibly due to the strong interaction between Pt and CeO2 and the formation of Pt−O−Ce species [56]. The reduction peaks in the ranges of 150–500 °C and 550–700 °C originated from the reduction of the surface and bulk of CeO2, respectively [57,58]. For the Pt/Keg-CeTi and Pt/MoP-CeTi catalysts, there were two reduction peaks of surface cerium species between 150 and 500 °C, and their position moved to a lower temperature range compared to the data of The H2-TPR technique was employed to investigate the reduction property of the catalysts. Figure 9 illustrates the H2-TPR profiles of the three catalysts performed under an Ar atmosphere (Table S4). One could see that there were five reduction peaks at 84, 224, 334, 585, and 640 ◦C for Pt/Keg-CeTi, as well as five peaks at 76, 193, 366, 613, and 657 ◦C for Pt/MoP-CeTi. For Pt/CeTi, only three reduction peaks at 83, 379, and 645 ◦C could be observed. As a result of the hard reduction of titanium oxides below 700 ◦C, all the signals of the various catalysts were attributed to the reduction of corresponding platinum, cerium, and molybdenum species. The reduction peak below 100 ◦C could be assigned to the reduction of Pt oxides. The Pt reduction temperatures of Pt/CeTi (83 ◦C), Pt/Keg-CeTi (84 ◦C), and Pt/MoP-CeTi (76 ◦C) are comparably higher than the data of bulk Pt oxides' reduction, which is normally below room temperature, possibly due to the strong interaction between Pt and CeO<sup>2</sup> and the formation of Pt−O−Ce species [56]. The reduction peaks in the ranges of 150–500 ◦C and 550–700 ◦C originated from the reduction of the surface and bulk of CeO2, respectively [57,58]. For the Pt/Keg-CeTi and Pt/MoP-CeTi catalysts, there were two reduction peaks of surface cerium species between 150 and 500 ◦C, and their position moved to a lower temperature range compared to the data of Pt/CeTi, possibly due to the modification of molybdophosphate and the interaction between Pt and CeO2. In addition, the shoulder peaks at 585 and 613 ◦C were attributed to the reduction of molybdenum species in Pt/Keg-CeTi and Pt/MoP-CeTi [59], respectively, implying that it

Pt/CeTi, possibly due to the modification of molybdophosphate and the interaction between Pt and CeO2. In addition, the shoulder peaks at 585 and 613 °C were attributed to

compared to the MoO3 in Pt/MoP-CeTi. The better reduction of surface cerium and molybdenum species for Pt/Keg-CeTi was beneficial to improve its catalytic performance for

CO oxidation.

was easier to reduce the molybdenum species within the Keggin structure compared to the MoO<sup>3</sup> in Pt/MoP-CeTi. The better reduction of surface cerium and molybdenum species for Pt/Keg-CeTi was beneficial to improve its catalytic performance for CO oxidation. *Catalysts* **2022**, *11*, x FOR PEER REVIEW 11 of 16

**Figure 9.** H2-TPR profiles of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts. **Figure 9.** H<sup>2</sup> -TPR profiles of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts.

#### *2.5. Water Adsorption on the Catalysts*

presence of H2O.

1620

**3500 3000 2000 1500 1000**

**1**

**Wavenumber/cm¯**

(a)

**Absorbance/a.u**

3440

80 min

40 min

20 min 0 min

60 min(cut off H2O)

*2.5. Water Adsorption on the Catalysts*  According to Feng's study [60], H2O could enhance the catalytic oxidation of CO, and the promoting effect of H2O was greater than the inhibiting effect of SO2 when H2O and SO2 coexisted in the atmosphere. Similar phenomena were observed over Pt/CeTi and Pt/Keg-CeTi, but it was different for Pt/MoP-CeTi (Figures 1 and 2). In order to explain the phenomena, H2O-DRIFTS experiments over Pt/Keg-CeTi and Pt/MoP-CeTi were conducted at 250 °C with 3% H2O (Figure 10). During the experiment, 3% H2O was supplied in the first 60 min, and then it was cut off. For the Pt/Keg-CeTi sample, with H2O exposure, two peaks at 1620 and 3440 cm−1 grew in intensity with time (Figure 10a). Moreover, their intensities remained after H2O was cut off, indicating a stable adsorption of H2O molecules on the Pt/Keg-CeTi surface at 250 °C. For the Pt/MoP-CeTi sample, two peaks 1650 and 3200 cm−1 were detected in the first 35 min. However, after that, the H2O adsorption peaks almost disappeared and two new peaks centered at 1843 and 2650 cm−1 appeared, which remained even after cutting off the water. The bands at 1843 and 2650 cm−1 could be attributed to the PO-H stretching vibration of hydrogen phosphate and dihydrogen phosphate, revealing the change in the support chemical structure. The formation of hydrogen phosphate and dihydrogen phosphate on the Pt/MoP-CeTi surface in the presence According to Feng's study [60], H2O could enhance the catalytic oxidation of CO, and the promoting effect of H2O was greater than the inhibiting effect of SO<sup>2</sup> when H2O and SO<sup>2</sup> coexisted in the atmosphere. Similar phenomena were observed over Pt/CeTi and Pt/Keg-CeTi, but it was different for Pt/MoP-CeTi (Figures 1 and 2). In order to explain the phenomena, H2O-DRIFTS experiments over Pt/Keg-CeTi and Pt/MoP-CeTi were conducted at 250 ◦C with 3% H2O (Figure 10). During the experiment, 3% H2O was supplied in the first 60 min, and then it was cut off. For the Pt/Keg-CeTi sample, with H2O exposure, two peaks at 1620 and 3440 cm−<sup>1</sup> grew in intensity with time (Figure 10a). Moreover, their intensities remained after H2O was cut off, indicating a stable adsorption of H2O molecules on the Pt/Keg-CeTi surface at 250 ◦C. For the Pt/MoP-CeTi sample, two peaks 1650 and 3200 cm−<sup>1</sup> were detected in the first 35 min. However, after that, the H2O adsorption peaks almost disappeared and two new peaks centered at 1843 and 2650 cm−<sup>1</sup> appeared, which remained even after cutting off the water. The bands at 1843 and 2650 cm−<sup>1</sup> could be attributed to the PO-H stretching vibration of hydrogen phosphate and dihydrogen phosphate, revealing the change in the support chemical structure. The formation of hydrogen phosphate and dihydrogen phosphate on the Pt/MoP-CeTi surface in the presence of moisture at 250 ◦C could lead to the inhibition of H2O adsorption and inaccessibility of active sites, which might be the main reason for the deactivation of Pt/MoP-CeTi in the presence of H2O.

of moisture at 250 °C could lead to the inhibition of H2O adsorption and inaccessibility of active sites, which might be the main reason for the deactivation of Pt/MoP-CeTi in the

80 min

40 min 35 min

20 min

0 min

**Absorbance/a.u**

60 min(cut off H2O)

(b) 2650 1843

**3500 3000 2000 1500 1000 Wavenumber/cm¯**

3200 1650

**1**

**Figure 9.** H2-TPR profiles of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts.

**Temperature/o**

**100 200 300 400 500 600 700 800**

<sup>193</sup> <sup>366</sup> <sup>613</sup>

379

<sup>640</sup> <sup>224</sup> <sup>585</sup> <sup>334</sup>

According to Feng's study [60], H2O could enhance the catalytic oxidation of CO, and the promoting effect of H2O was greater than the inhibiting effect of SO2 when H2O and SO2 coexisted in the atmosphere. Similar phenomena were observed over Pt/CeTi and Pt/Keg-CeTi, but it was different for Pt/MoP-CeTi (Figures 1 and 2). In order to explain the phenomena, H2O-DRIFTS experiments over Pt/Keg-CeTi and Pt/MoP-CeTi were conducted at 250 °C with 3% H2O (Figure 10). During the experiment, 3% H2O was supplied in the first 60 min, and then it was cut off. For the Pt/Keg-CeTi sample, with H2O exposure, two peaks at 1620 and 3440 cm−1 grew in intensity with time (Figure 10a). Moreover, their intensities remained after H2O was cut off, indicating a stable adsorption of H2O molecules on the Pt/Keg-CeTi surface at 250 °C. For the Pt/MoP-CeTi sample, two peaks 1650 and 3200 cm−1 were detected in the first 35 min. However, after that, the H2O adsorption peaks almost disappeared and two new peaks centered at 1843 and 2650 cm−1 appeared, which remained even after cutting off the water. The bands at 1843 and 2650 cm−1 could be attributed to the PO-H stretching vibration of hydrogen phosphate and dihydrogen phosphate, revealing the change in the support chemical structure. The formation of hydrogen phosphate and dihydrogen phosphate on the Pt/MoP-CeTi surface in the presence of moisture at 250 °C could lead to the inhibition of H2O adsorption and inaccessibility of active sites, which might be the main reason for the deactivation of Pt/MoP-CeTi in the

**C**

657

645

*2.5. Water Adsorption on the Catalysts* 

76

84

**H2 Consumption/a.u**

83

 Pt/CeTi Pt/Keg-CeTi Pt/MoP-CeTi

**Figure 10.** Changes in H2O-DRIFTS spectra of Pt/Keg-CeTi (**a**) and Pt/MoP-CeTi (**b**) with time. Conditions: [H2O] = 3 vol%, balance N<sup>2</sup> , total flow rate = 50 mL/min, at 250 ◦C.
