*3.1. Chemicals*

All chemical reagents were from commercial sources and were used directly without any further purification. The CeO2, (NH4)6Mo7O<sup>24</sup> (99%), and NH4H2PO<sup>4</sup> (99%) were of analytical grade and were purchased from Fuchen (Tianjin, China). Platinum nitrate solution (14.99%) was from Helishi, (Shanghai, China). TiO<sup>2</sup> was an industrial product from Xinhua, (Chongqing, China).

#### *3.2. Catalyst Preparation*

The CeTi composite oxide support was prepared by the ball milling method. Commercial anatase TiO<sup>2</sup> (16.0 g) and CeO<sup>2</sup> (4.0 g) were placed into a 500 cm<sup>3</sup> sintered zirconium oxide grinding jar with agate balls (20, 15, and 10 mm in diameter). The ball-to-powder mass ratio was 10:1, and the rotation speed and time were 500 rpm and 1 h, respectively. The received powder was calcined at 500 ◦C for 2 h, giving the CeTi support. The Pt/CeTi catalyst was fabricated by an impregnation method. In a typical procedure, a 4 g CeTi support was mixed with a calculated amount of platinum nitrate solution of 0.02 mol/L according to the Pt loading (1 wt%), stirred at 80 ◦C for 2 h, and then dried at 80 ◦C for 6 h. The resulting solid was ground into powder and calcined at 350 ◦C for 2 h, giving the Pt/CeTi catalyst. Certain amounts of CeTi support, (NH4)6Mo7O24, and NH4H2PO<sup>4</sup> were added to 200 mL of distilled water, stirred, and then nitric acid solution was added dropwise to adjust the system pH = 1 at room temperature, leading to the formation of (NH4)3PMo12O<sup>40</sup> with the Keggin structure. After 2 h, the mixture was dried at 80 ◦C for 6 h and calcined at 350 ◦C for 2 h, giving the molybdophosphate with Keggin structure-modified CeTi (Keg-CeTi), in which the (NH4)3PMo12O<sup>40</sup> loading was 10 wt%. The corresponding catalyst, denoted as Pt/Keg-CeTi, was prepared using a similar impregnation method as above. In addition, the Keg-CeTi support was calcinated at 500 ◦C for 2 h to fully destroy the Keggin structure, and then the active Pt species was loaded by a similar impregnation method. The received catalyst was denoted as Pt/MoP-CeTi.

#### *3.3. Catalyst Characterization*

The X-ray diffraction (XRD) instrument was a Bruker D8 ADVANCE (Karlsruhe, Germany). The X-ray radiation source was Cu Kα (λ = 1.54 A), and the voltage between the cathode and anode and the current were 50 kV and 35 mA, respectively. The 2θ angle was in the range of 10~80◦ , and the scanning speed was 8 s/step with a step of 0.02◦ . The N<sup>2</sup> adsorption was detected by using Autosorb iQ automatic physical adsorption made by Quantachrome Instruments (Boynton Beach, FL, USA). The sample was pretreated at 200 ◦C for 4 h under vacuum conditions, and the N<sup>2</sup> adsorption isotherms were performed at 77 K. The BET (Brunauer–Emmett–Teller) method was used to calculate the specific surface

area of the catalyst, and the Barrett–Joyner–Halenda (BJH) method was used to calculate the pore size distribution and pore volume of the catalyst. The profiles of H2-temperature programmed reduction (H2-TPR) was performed on an Autochem II 2920, Micromeritics (Norcross, PA, USA) chemisorption apparatus. Before experiments, 50 mg of catalyst powder was pretreated in pure N<sup>2</sup> at 200 ◦C for 60 min. After cooling to room temperature, a 10% H2/Ar mixture was introduced to purge the sample. When the baseline was stable, the temperature was programmed to 900 ◦C with a heating rate of 10 ◦C/min; meanwhile, the H<sup>2</sup> signal was analyzed with a TCD detector. Temperature-programmed desorption of ammonia (NH3-TPD) was used to investigate the surface acidities of the catalysts by a ChemBET Pulsar TPR/TPD, Quantachrome company (Boynton Beach, FL, USA). First, 100 mg of catalyst powder was pretreated at 200 ◦C for 1 h in a helium atmosphere. After that, when it was cooled to 30 ◦C, 2% NH3/He gas was switched on for purging for 1 h, and then it was purged with helium gas for 1 h. After the baseline was stable, the desorbed NH<sup>3</sup> signal was detected by a thermal conductivity detector (TCD) under a heating rate of 10 ◦C/min. The total gas flow was 20 mL/min in each step. The temperature-programmed desorption of sulfur dioxide (SO2-TPD) was performed on the same apparatus as for NH3-TPD. An amount of 50 mg of catalyst powder was pretreated at 200 ◦C for 1 h in an Ar atmosphere. After that, 1000 ppm SO<sup>2</sup> + 16 vol% O<sup>2</sup> was switched on for 1 h at 250 ◦C. When it was cooled to 30 ◦C, Ar gas was used to purge for 1 h. After the baseline was stable, the desorbed SO<sup>2</sup> signal was detected under a heating rate of 10 ◦C/min. The total gas flow was 50 mL/min in each step. Transmission electron microscopy (TEM) images were recorded over a JEM 2100, JEOL (Tokyo, Japan) microscope and operated at an acceleration voltage of 200 kV and an electric current of 20 mA. Scanning electron microscopy (SEM) images were collected with a JEOL JSM-35C (Tokyo, Japan) instrument and operated at 20 kV acceleration voltages. X-ray photoelectron spectra (XPS) were carried out on an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha, Waltham, MA, USA), using monochromatic Al Kα radiation (1486.6 eV). Inductively coupled plasma–atomic emission spectrometry (ICP–AES) was used to accurately determine the accumulation of sulfur on the catalysts by ICP-AES: Aglient 7800 (Palo Alto, CA, USA). Certain amounts of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi were treated with SO<sup>2</sup> (1000 ppm) + O<sup>2</sup> (16%) at 250 ◦C for 1 h. After cooling, one part of the samples was used to measure the content of sulfur, and another part of the samples was treated at 900 ◦C for 1 h before the sulfur measurement. The adsorption and transformation of sulfur species and water on the catalysts under different conditions were investigated by in situ DRIFTS experiments using a FTIR spectrometer (Nicolet 6700, Madison WI, USA) with a diffuse reflectance chamber and a KBr window. The high-sensitivity mercury-cadmium-telluride (MCT) detector was cooled by liquid nitrogen. The sample (about 120 mg) was pretreated in an N<sup>2</sup> flow (50 mL/min) at 300 ◦C for 1 h. All the IR spectra were recorded in 32 accumulative scans with a resolution of 4 cm−<sup>1</sup> in the range of 4000–400 cm−<sup>1</sup> . The background spectra were collected at corresponding temperatures after pretreatment. For SO2-DRIFTS, 150 ppm SO<sup>2</sup> and 16 vol% O<sup>2</sup> were introduced, and the balance gas was N2. For H2O-DRIFTS, 3% H2O was introduced and balanced with N<sup>2</sup> too. The total flow rate was 50 mL/min.

#### *3.4. Catalytic Activity Test*

CO oxidation activity was measured in a fixed-bed quartz tube reactor (10 mm internal diameter) containing 1 mL of catalyst (40–60 mesh). The FTIR data of the three catalysts at 250 ◦C in the presence of CO and oxygen revealed that CO<sup>2</sup> was the only product of CO oxidation, and there were few or no carbonate species on the catalysts' surface (Figure S4). In order to demonstrate the reproducibility of the preparation method, three parallel samples were prepared and tested (Figure S5). The activity was detected from 80 ◦C to 300 ◦C under a heating rate of 10 ◦C/min. The typical composition of reactant gas was as follows: [CO] = 1%, [O2] = 6%, [SO2] = 100 ppm (when used), [H2O] = 10% (when used), and He as balance. The total flow rate was 667 mL/min, which corresponded to an hourly space velocity (GHSV) of approximately 4 <sup>×</sup> <sup>10</sup><sup>5</sup> <sup>h</sup> −1 . The SO2/H2O durability experiment was evaluated at 200 or 250 ◦C under similar conditions. The reaction was carried out under atmospheric pressure, and the CO conversion was calculated as follows:

$$\chi = \frac{[\text{CO}]\_{\text{in}} - [\text{CO}]\_{\text{out}}}{[\text{CO}]\_{\text{in}}} \times 100\%$$

#### **4. Conclusions**

In this work, three catalysts, Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi, were prepared by similar impregnation methods and used for CO oxidation. It was found that the Pt/Keg-CeTi catalyst showed a higher resistance to SO<sup>2</sup> and H2O compared to Pt/CeTi and Pt/MoP-CeTi, which could be associated with its stronger surface acidity, better reduction of surface cerium and molybdenum species, and much lower SO<sup>2</sup> adsorption and transformation than the two others due to the modification of molybdophosphate with the Keggin structure. However, the Pt/MoP-CeTi catalyst displayed a much lower resistance to SO<sup>2</sup> and H2O, which might be attributed to the low stability of molybdophosphate without the Keggin structure as a result of the formation of hydrogen phosphate and dihydrogen phosphate in the presence of H2O under the reaction temperature, as well as the low surface Pt concentrations and moderate SO<sup>2</sup> adsorption and transformation. This work may offer a simple strategy to improve the catalyst performance for CO oxidation.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/catal12010004/s1, Figure S1: Nitrogen adsorption/desorption isotherms of the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts, Figure S2: Results of XPS of O 1s, Ti 2p, and Mo 3d in the Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi catalysts, Figure S3: Results of XPS of O 1s, Ti 2p and Mo 3d in the Pt/CeTi, Pt/Keg-CeTi and Pt/MoP-CeTi catalysts, Figure S4: Changes of FTIR spectra of Pt/CeTi, Pt/Keg-CeTi and Pt/MoP-CeTi with time under the following conditions: [CO] = 2%, [O<sup>2</sup> ] = 10 vol %, balance N<sup>2</sup> , total flow rate = 50mL/min, T=250 ◦C, Figure S5: CO conversions as a function of reaction temperature over the three parallel samples of Pt/CeTi (a), Pt/Keg-CeTi (b) and Pt/MoP-CeTi (c), respectively. Reaction conditions: [CO]= 1%, [O<sup>2</sup> ] = 6 vol %, balance He, total flow rate = 667 ml/min, GHSV = 4 <sup>×</sup> <sup>10</sup><sup>5</sup> <sup>h</sup> −1 , Table S1: ICP results of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi after being poisoned by SO<sup>2</sup> and the poisoned samples treated at 900 ◦C, Table S2: XPS binding energies (eV) of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi, Table S3: Relative area of NH<sup>3</sup> -TPD desorption peak of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi, Table S4: H<sup>2</sup> consumption of Pt/CeTi, Pt/Keg-CeTi, and Pt/MoP-CeTi calculated from the H<sup>2</sup> -TPR curves.

**Author Contributions:** Conceptualization, T.Z., H.H. and W.Q.; methodology, T.Z. and W.Q.; validation, T.Z. and H.Z.; formal analysis, T.Z., H.Z. and R.W.; investigation, T.Z., X.D. and W.Q.; resources, W.Q.; data curation, T.Z. and W.Q.; writing—original draft preparation, T.Z.; writing—review and editing, W.Q.; visualization, T.Z. and W.Q.; supervision, W.Q. and H.H.; project administration, W.Q.; funding acquisition, W.Q. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (21577005; 22075005).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

