Insight into the Metal–Support Interaction of Pt and β-MnO2 in CO Oxidation

Pt-based catalysts exhibit unique catalytic properties in many chemical reactions. In particular, metal–support interactions (MSI) greatly improve catalytic activity. However, the current MSI mechanism between platinum (Pt) and the support is not clear enough. In this paper, the interaction of 1 wt% Pt nanoparticles (NPs) on β-MnO2 in carbon monoxide (CO) oxidation was studied. The Pt on β-MnO2 inhibited CO oxidation below 210 °C but promoted it above 210 °C. A Pt/β-MnO2 catalyst contains more Pt4+ and less Pt2+. The results of operando DRIFTS-MS show that surface-terminal-type oxygen (M=O) plays an important role in CO oxidation. When the temperature was below 210 °C, Mn=O consumption on Pt/β-MnO2 was less than β-MnO2 due to Pt4+ inhibition on CO oxidation. When the temperature was above 210 °C, Pt4+ was reduced to Pt2+, and Mn=O consumption due to CO oxidation was greater than β-MnO2. The interaction of Pt and β-MnO2 is proposed.


Introduction
CO is a colorless, odorless, but toxic, gas [1]. CO is mainly emitted from vehicle combustion and industrial processes [2,3]. It is not only hazardous to human health, but also indirectly contributes to global warming [4]. Catalytic oxidation technology is a considerably effective method for reducing CO emission because it has high economic feasibility, lower cost, and lower secondary pollution generation [5]. Transition metal oxide catalysts have shown promising applications due to their low cost and good poisoning and sintering resistance [6]. MnO 2 is widely used in catalytic oxidation reactions due to its variable valence states and good oxygen-storage and oxygen-release capacities [7][8][9]. MnO 2 can form a variety of crystal structures, such as α-, β-, γ-, and δ-MnO 2 , according to [MnO 6 ] as a backbone arrangement [7]. Among them, β-MnO 2 has a thermodynamically stable phase and high crystallinity, which has become one of the current research hotspots [10]. It is still a challenge for MnO 2 to perform CO oxidation at low temperatures [11,12].
Loading a noble metal onto a support is an effective way to improve catalytic activity. Noble metal-supported catalysts have been widely used in CO oxidation due to the MSI [2]. Among them, platinum (Pt)-based catalysts have attracted wide attention for their excellent catalytic activity [13]. Zhu et al. used a Pt/Al 2 O 3 catalyst to convert formaldehyde at room temperature [14]. Huang et al. prepared a Pt/CeO 2 -TiO 2 catalyst and found that it has high catalytic activity and selectivity for CO + NO reactions [15]. Cui et al. prepared an octahedral Fe 3 O 4 to support Pt to remove formaldehyde and found that the catalyst has a high activity and stability [16]. Ru et al. synthesized Pt/(α-, γ-, and δ-) MnO 2 catalysts for methanol oxidation, and showed that the temperatures for methanol oxidation on Pt/(α-, γ-, and δ-) MnO 2 catalysts can be reduced in comparison with MnO 2 [17]. MnO 2 has two typical oxygen species: terminal-type oxygen (Mn=O) and bridge-type oxygen (Mn-O-Mn) [18]. Mn=O plays an important role in the CO oxidation process [19]. MSI can inhibit the growth of small noble metal nanoparticles and optimize the bonding structure and electron transfer, and, thus, improve catalytic performance [13]. Dispersion of Pt NPs onto MnO 2 is an efficient method to weaken Mn=O and Mn-O-Mn bonds and generate oxygen vacancies through MSI between interfaces of Pt NPs and MnO 2 [20]. However, some studies have shown that the strong interactions may limit catalytic reactions and weaken catalytic activity [13]. Miao et al. found that excessively strong MSI does not promote O 2 adsorption [13,21]. At present, the interaction between Pt and the support is not clear enough. This study aims to gain insight into the interaction of Pt and MnO 2 in CO oxidation.
Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) can monitor surface species and oxygen vacancies under a real reaction condition and is often used to study catalytic reaction mechanisms [10]. Xu et al. used H 2 as a probe molecule to study the role of oxygen species on the surface oxygen species of β-MnO 2 of a low amount of M=O using operando DRIFTS coupled with mass spectrometry (MS) [10]. Li et al. studied the effects of acid gases (NO, HCl, and SO 2 ) on mercury oxidation over CeO 2 -WO 3 /TiO 2 catalysts through in situ DRIFTS and showed that NO 2 and nitrate are active centers of mercury oxidation [22]. Using CO as a probe molecule and in situ DRIFTS, Jang et al. quantified a part of the catalytic active sites on PdOx, further elucidating the surface reconstruction of Pt-Pd bimetallic particles during the reaction [23]. Zhang et al. confirmed that Rh atoms are dispersed on Al 2 O 3 and TiO 2 using Fourier transform infrared spectroscopy (FTIR) [24].
In this study, 1 wt% Pt NPs loaded on β-MnO 2 prepared with deposition-precipitation is used to explore the metal-support interaction of Pt and β-MnO 2 during CO oxidation. The physicochemical properties of β-MnO 2 and Pt/MnO 2 catalysts are characterized using an X-ray diffractometer (XRD), high-resolution transmission microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and paramagnetic resonance spectroscopy (EPR). The role of oxygen species on the surfaces of β-MnO 2 and Pt/β-MnO 2 catalysts in CO oxidation is revealed using operando DRIFTS-MS. The interaction of Pt and β-MnO 2 in CO oxidation is proposed. Figure 1 shows the XRD patterns of two catalysts. The diffraction peaks of β-MnO 2 correspond to JCPDS 24-0735, and 2θ = 28.7 • , 37.4 • , 42.8 • , 56.7 • , and 59.5 • correspond to (110), (101), (111), (211), and (220) crystal planes, respectively. β-MnO 2 exhibits sharp and narrow diffraction peaks, suggesting that β-MnO 2 has a high degree of crystallinity [25]. No diffraction peaks of Pt have been observed from XRD patterns, which may be due to small particle size, low Pt loading, or high dispersion of the Pt NPs [13,[26][27][28][29]. The positions of the diffraction peaks of Pt/β-MnO 2 are consistent with that of β-MnO 2 , indicating that the loading Pt did not change the crystal structure of β-MnO 2 . However, after loading Pt, the intensities of the diffraction peaks decreased, indicating that the crystallinity of β-MnO 2 can be affected by Pt loading [30]. Molecules 2023, 28, x FOR PEER REVIEW 3 of 15  After loading Pt on β-MnO2, it was found that there are some crystal-plane blurs between the normal lattice stripes (inside the red circles in Figure 2b). This fact implied that the loading of Pt NPs caused defects in the catalyst [31]. Although no crystal planes of Pt species were observed in HRTEM, the presence of Pt NPs could be confirmed from HRTEM images (Figure 2c,d). The sizes of the Pt NPs were between 1.0 and 2.0 nm.   After loading Pt on β-MnO 2 , it was found that there are some crystal-plane blurs between the normal lattice stripes (inside the red circles in Figure 2b). This fact implied that the loading of Pt NPs caused defects in the catalyst [31]. Although no crystal planes of Pt species were observed in HRTEM, the presence of Pt NPs could be confirmed from HRTEM images (Figure 2c  After loading Pt on β-MnO2, it was found that there are some crystal-plane blurs between the normal lattice stripes (inside the red circles in Figure 2b). This fact implied that the loading of Pt NPs caused defects in the catalyst [31]. Although no crystal planes of Pt species were observed in HRTEM, the presence of Pt NPs could be confirmed from HRTEM images (Figure 2c,d). The sizes of the Pt NPs were between 1.0 and 2.0 nm.  The N 2 adsorption isotherm and pore-size distribution curves of the two catalysts are shown in Figure 3. The physical and chemical properties of the two catalysts are summarized in Table 1. According to the IUPAC classification, the N 2 adsorption isotherms of both catalysts are type IV isotherms. The specific surfaces before and after Pt loading are the same. However, the BJH pore-size distribution showed that β-MnO 2 mainly has a microporous structure (<2 nm), while Pt/β-MnO 2 is mainly of a mesoporous structure (2-50 nm). After loading Pt NPs, the pore volume of β-MnO 2 decreased and the pore diameter increased, indicating that the small pores in β-MnO 2 were blocked by Pt NPs. The N2 adsorption isotherm and pore-size distribution curves of the two catalysts are shown in Figure 3. The physical and chemical properties of the two catalysts are summarized in Table 1. According to the IUPAC classification, the N2 adsorption isotherms of both catalysts are type IV isotherms. The specific surfaces before and after Pt loading are the same. However, the BJH pore-size distribution showed that β-MnO2 mainly has a microporous structure (<2 nm), while Pt/β-MnO2 is mainly of a mesoporous structure (2-50 nm). After loading Pt NPs, the pore volume of β-MnO2 decreased and the pore diameter increased, indicating that the small pores in β-MnO2 were blocked by Pt NPs.

Catalytic Activity
The catalytic activities of the two catalysts are shown in Table 1 and Figure 4. Pt/β-MnO2 has T50 and T90 values, lower than β-MnO2. This means that loading Pt can enhance the activity of β-MnO2. The TOF value of Pt/β-MnO2 at 222 °C (T90) was 0.06 s -1 ; this TOF value is around the order of Pt NPs on SiO2 [32].

Catalytic Activity
The catalytic activities of the two catalysts are shown in Table 1 and Figure 4. Pt/β-MnO 2 has T 50 and T 90 values, lower than β-MnO 2 . This means that loading Pt can enhance the activity of β-MnO 2 . The TOF value of Pt/β-MnO 2 at 222 • C (T 90 ) was 0.06 s −1 ; this TOF value is around the order of Pt NPs on SiO 2 [32]. The N2 adsorption isotherm and pore-size distribution curves of the two catalysts are shown in Figure 3. The physical and chemical properties of the two catalysts are summarized in Table 1. According to the IUPAC classification, the N2 adsorption isotherms of both catalysts are type IV isotherms. The specific surfaces before and after Pt loading are the same. However, the BJH pore-size distribution showed that β-MnO2 mainly has a microporous structure (<2 nm), while Pt/β-MnO2 is mainly of a mesoporous structure (2-50 nm). After loading Pt NPs, the pore volume of β-MnO2 decreased and the pore diameter increased, indicating that the small pores in β-MnO2 were blocked by Pt NPs.

Catalytic Activity
The catalytic activities of the two catalysts are shown in Table 1 and Figure 4. Pt/β-MnO2 has T50 and T90 values, lower than β-MnO2. This means that loading Pt can enhance the activity of β-MnO2. The TOF value of Pt/β-MnO2 at 222 °C (T90) was 0.06 s -1 ; this TOF value is around the order of Pt NPs on SiO2 [32]. It should be noted that CO oxidation on Pt/β-MnO 2 is lower than that of β-MnO 2 at a temperature below 210 • C. This result suggested that Pt NPs inhibited CO oxidation below 210 • C by Pt NPs alone or in combination with β-MnO 2 . The inhibition mechanism is discussed later in Section 3.

Surface Chemical Structures
The surface chemical structures of the two catalysts were characterized using XPS and Raman. Figure 5 shows the XPS spectra of the two catalysts. Each element is qualitatively identified and semiquantitatively calculated using the peak areas of each element [28,29]. The results are listed in Table 2. The peaks of 641.6, 642.5, and 643.6 eV ( Figure 5a) correspond to Mn 2+ , Mn 3+ , and Mn 4+ , respectively [10]. The peak-area ratios of Mn cations with different valence states are shown in Table 2. Before and after Pt loading, the (Mn 2+ + Mn 3+ )/Mn 4+ ratio increased from 3.55 to 4.00, proving that Pt NPs on β-MnO 2 can promote the formation of Mn 2+ and Mn 3+ ; those contribute to the formation of oxygen vacancies [33]. Moreover, the increase in the Mn 2+ and Mn 3+ ratio is also related to the formation of Pt-O-Mn [20]. It should be noted that CO oxidation on Pt/β-MnO2 is lower than that of β-MnO2 at a temperature below 210 °C. This result suggested that Pt NPs inhibited CO oxidation below 210 °C by Pt NPs alone or in combination with β-MnO2. The inhibition mechanism is discussed later in Section 3.

Surface Chemical Structures
The surface chemical structures of the two catalysts were characterized using XPS and Raman. Figure 5 shows the XPS spectra of the two catalysts. Each element is qualitatively identified and semiquantitatively calculated using the peak areas of each element [28,29]. The results are listed in Table 2. The peaks of 641.6, 642.5, and 643.6 eV ( Figure 5a) correspond to Mn 2+ , Mn 3+ , and Mn 4+ , respectively [10]. The peak-area ratios of Mn cations with different valence states are shown in Table 2. Before and after Pt loading, the (Mn 2+ + Mn 3+ )/Mn 4+ ratio increased from 3.55 to 4.00, proving that Pt NPs on β-MnO2 can promote the formation of Mn 2+ and Mn 3+ ; those contribute to the formation of oxygen vacancies [33]. Moreover, the increase in the Mn 2+ and Mn 3+ ratio is also related to the formation of Pt-O-Mn [20].  The XPS spectra of O1s are shown in Figure 5b. The peaks at 529.3 and 531.5 eV correspond to lattice oxygen (Olatt) and surface-adsorbed oxygen (Oads), respectively [10]. The peak-area ratios of the surface oxygen species on the two catalysts are listed in Table 2. The Oads/(Oads + Olatt) ratios of β-MnO2 and Pt/β-MnO2 are 0.30 and 0.33, respectively. Pt/β-MnO2 has more adsorbed oxygen than β-MnO2, which supports the fact that a higher (Mn 2+ + Mn 3+ )/Mn 4+ ratio can generate more oxygen vacancies on Pt/β-MnO2. The more surface-adsorbed oxygen is essential for catalytic oxidation [13]. Our finding also confirmed that catalysts containing more surface-adsorbed oxygen have better catalytic activity (Figure 4a). The XPS spectra of Pt 4f have broad peaks in the Pt 4f region (Figure 5c),  The XPS spectra of O1s are shown in Figure 5b. The peaks at 529.3 and 531.5 eV correspond to lattice oxygen (O latt ) and surface-adsorbed oxygen (O ads ), respectively [10]. The peak-area ratios of the surface oxygen species on the two catalysts are listed in Table 2.
The O ads /(O ads + O latt ) ratios of β-MnO 2 and Pt/β-MnO 2 are 0.30 and 0.33, respectively. Pt/β-MnO 2 has more adsorbed oxygen than β-MnO 2 , which supports the fact that a higher (Mn 2+ + Mn 3+ )/Mn 4+ ratio can generate more oxygen vacancies on Pt/β-MnO 2 . The more surface-adsorbed oxygen is essential for catalytic oxidation [13]. Our finding also confirmed that catalysts containing more surface-adsorbed oxygen have better catalytic activity (Figure 4a). The XPS spectra of Pt 4f have broad peaks in the Pt 4f region (Figure 5c), indicating that Pt has different valence states. The peaks of 72.6 and 74.5 eV correspond to Pt 2+ and Pt 4+ , respectively [13,20]. The peak-area ratios of different valence states before and after CO oxidation are given in Table 2. The Pt/β-MnO 2 catalyst before CO oxidation mainly has Pt +4 and Pt +2 , among which the Pt 4+ ratio is as high as 0.68, showing that Pt 4+ is the mainstay. The emergence of Pt 2+ can be explained by the generation of Pt-O-Mn bonds, meaning that there is an interaction between Pt and β-MnO 2 [33]. After the reaction at 150, 200, and 250 • C, the ratio of Pt 2+ /Pt total gradually increased to 0.64. At the same time, the ratio of Pt 4+ /Pt total decreased to 0.36. This finding clearly explores that Pt 4+ was continuously reduced to Pt 2+ during CO oxidation. Since Pt 2+ is associated with the formation of Pt-O-Mn, it is speculated that Pt-O-Mn can promote CO oxidation. Combined with the low activity of Pt/β-MnO 2 below 210 • C, it is concluded that Pt/β-MnO 2 containing more Pt 4+ and less Pt-O-Mn is not in favor for CO oxidation. Figure 6 shows the Raman profiles of the two catalysts. The peak located in 500-700 cm −1 is attributed to the stretching of the [MnO 6 ] octahedron [34].  indicating that Pt has different valence states. The peaks of 72.6 and 74.5 eV correspond to Pt 2+ and Pt 4+ , respectively [13,20]. The peak-area ratios of different valence states before and after CO oxidation are given in  Figure 6 shows the Raman profiles of the two catalysts. The peak located in 500-700 cm -1 is attributed to the stretching of the [MnO6] octahedron [34]. The peak around 637 cm -1 is the symmetrical Mn=O tensile vibration perpendicular to the [MnO6] octahedral double strand. The low-intensity peak with a low wavenumber is attributed to the deformation mode of the metal-oxygen chain (Mn-O-Mn) in the MnO2 octahedral lattice [35].

Reduction, Oxygen-Vacancy, and Electron-Transfer Analyses
To further understand the interaction of Pt and β-MnO2, H2-TPR and EPR were performed. Figure 7a shows the H2-TPR profiles for two catalysts. The reduction temperature can be divided into two regions, R0 (50-200 °C) and RI (200-600 °C). Two reduction peaks (319 and 455 °C) were observed on β-MnO2, which were attributed to the reduction of Mn 4+ to Mn 3+ and then to Mn 2+ [13,20]. After loading Pt, the R0 region has the reduction peak of PtOx at 88 °C and surface-adsorbed oxygen (Oads) at 143 °C [36]. Since the reduction peak in the R0 region is attributed to H2 spillover effects, which are caused by the formation of Pt-O-Mn bonds between β-MnO2 and adjacent Pt NPs [17,37], the Pt/β-MnO2 of peaks in the R0 region interacts between Pt and β-MnO2. Figure 7b shows the EPR spectra of the two catalysts. β-MnO2 has obvious electron paramagnetic resonance signal peaks at g =

Reduction, Oxygen-Vacancy, and Electron-Transfer Analyses
To further understand the interaction of Pt and β-MnO 2 , H 2 -TPR and EPR were performed. Figure 7a shows the H 2 -TPR profiles for two catalysts. The reduction temperature can be divided into two regions, R 0 (50-200 • C) and R I (200-600 • C). Two reduction peaks (319 and 455 • C) were observed on β-MnO 2 , which were attributed to the reduction of Mn 4+ to Mn 3+ and then to Mn 2+ [13,20]. After loading Pt, the R 0 region has the reduction peak of PtOx at 88 • C and surface-adsorbed oxygen (O ads ) at 143 • C [36]. Since the reduction peak in the R 0 region is attributed to H 2 spillover effects, which are caused by the formation of Pt-O-Mn bonds between β-MnO 2 and adjacent Pt NPs [17,37], the Pt/β-MnO 2 of peaks in the R 0 region interacts between Pt and β-MnO 2 . Figure 7b shows the EPR spectra of the two catalysts. β-MnO 2 has obvious electron paramagnetic resonance signal peaks at g = 2.005 [36], and the peak intensity can also reflect the oxygen-vacancy concentration. After loading Pt on β-MnO 2 , the signal at g = 2.005 is offset. This proves that there is electron transfer between Pt NPs and β-MnO 2 , which is consistent with typical MSI phenomena [38]. In addition, it is clear that the oxygen-vacancy concentration order is Pt/β-MnO 2 > β-MnO 2 . Higher oxygen-vacancy concentrations correspond to better oxygen-mobility capacity, which is consistent with the characterization of H 2 -TPR. The oxygen vacancies of Pt/β-MnO 2 increased significantly, mainly due to the fact that MSI affects the Mn=O bonds of the vector and forms more oxygen vacancies [36].

Surface Reactions Using Operando DRIFTS-MS 2.5.1. Surface Reactions in CO Atmosphere
Molecules 2023, 28, x FOR PEER REVIEW 7 of 15 2.005 [36], and the peak intensity can also reflect the oxygen-vacancy concentration. After loading Pt on β-MnO2, the signal at g = 2.005 is offset. This proves that there is electron transfer between Pt NPs and β-MnO2, which is consistent with typical MSI phenomena [38]. In addition, it is clear that the oxygen-vacancy concentration order is Pt/β-MnO2 > β-MnO2. Higher oxygen-vacancy concentrations correspond to better oxygen-mobility capacity, which is consistent with the characterization of H2-TPR. The oxygen vacancies of Pt/β-MnO2 increased significantly, mainly due to the fact that MSI affects the Mn=O bonds of the vector and forms more oxygen vacancies [36].  Figure 8 shows the operando DRIFTS spectra during CO oxidation on the two catalysts in a 1% CO/Ar atmosphere without O2. The peaks on β-MnO2 at 2173 and 2114 cm -1 belong to the gas-phase CO. On Pt/β-MnO2, the peak at 2173 cm -1 belongs to the gas-phase CO [19], the peak at 2121 cm -1 belongs to CO-Pt δ+ [39,40], and the peak at 2068 cm -1 is assigned to CO-Pt 0 [39,40]. Peaks at 1300, 1112, 947, and 770 cm -1 are attributed to Mn=O, Mn + -O2 -, Mn + -O2 2-, and Mn + -O 2--Mn + , respectively [10,19]. The peak in 1605-1688 cm -1 is attributed to bidentate carbonate (v(OCO)), and the peak in 1463-1560 cm -1 belongs to monodentate carbonate (v(CO3 2-)) [19].    (Figure 9a,b), indicating that oxygen species are continuously consumed due to their reactions with CO following Equations (1)- (4). The initial consumption temperature of Mn=O on β-MnO 2 is 50 • C, obviously lower than that (150 • C) on Pt/β-MnO 2 , suggesting that Mn=O on β-MnO 2 can react more easily with CO than that on Pt/β-MnO 2 . This fact also implied that Pt inhibited CO oxidation by the O in Mn=O. The initial consumption temperatures of Mn-O-Mn on β-MnO 2 and Pt/β-MnO 2 are higher than 100 • C, but the normalized peak height of Mn-O-Mn on β-MnO 2 is less than that on Pt/β-MnO 2 (Figure 9b (5) and (6)), the decrease in the normalized peak height of (O 2− )Mn 2+ -O-Mn and increase in Mn 2+ --Mn above 50 • C (Figure 9c) proved the reactions in Equations (5) and (6). The CO oxidation products include surface carbonate and gaseous CO 2 . Figure 9d shows the normalized peak heights of monodentate carbonate (v(CO 3 2− )) on β-MnO 2 and Pt/β-MnO 2 . The remarkable differences between β-MnO 2 and Pt/β-MnO 2 can be found within the temperature range of 100 and 300 • C, in which the normalized peak heights of monodentate carbonate on β-MnO 2 are higher than that on Pt/β-MnO 2 , indicating that Pt inhibited CO oxidation to monodentate carbonate (Equation (7)). Figure 9e illustrates no obvious difference in the normalized peak heights of bidentate carbonate on β-MnO 2 and Pt/β-MnO 2 ; thus, Pt has no influence on bidentate carbonate formation following Equation (9) [41]. The CO 2 MS signal from β-MnO 2 is higher than that from Pt/β-MnO 2 in the temperature range of 50-210 • C (Figure 9f (Figures 9 and 11), indicating that the regeneration rates of oxygen vacancies (Mn= and Mn--Mn) are lower than their consumption rates. To determine whether the oxygen vacancies can be regenerated, the catalysts with oxygen vacancies (generated via CO oxidation in a 1%CO/Ar atmosphere for 30 min at 300 • C) were heated from 25 • C to 300 • C with a ramp of 10 • C min −1 in a 20%O 2 /Ar atmosphere. The regeneration results are shown in Figure 12. Figure 12a The trend of the CO2 MS signal (Figure 11f) is basically similar to those during CO oxidation in the absence of O2, further proving that Pt can inhibit CO oxidation in the presence of O2 below 225 °C. This result is consistent with that in Figure 4a.

Surface Reactions in O2 Atmosphere
The normalized intensities of Mn=O and Mn-O-Mn after CO oxidation with or without O2 are all negative (Figures 9 and 11), indicating that the regeneration rates of oxygen vacancies (Mn=□ and Mn-□-Mn) are lower than their consumption rates. To determine whether the oxygen vacancies can be regenerated, the catalysts with oxygen vacancies (generated via CO oxidation in a 1%CO/Ar atmosphere for 30 min at 300 °C) were heated from 25 °C to 300 °C with a ramp of 10 °C min -1 in a 20%O2/Ar atmosphere. The regeneration results are shown in Figure 12. Figure 12a    Step (b) is the easy process of O supplement from Pt +2 -O-Mn to CO for its oxidation to CO 2 . At a temperature below 210 • C, the O in Pt +4 -O-Mn cannot function as an O source, and Pt loading reduces the active sites of MnO 2 for CO oxidation, as Pt is anchored at places having defects. Those are active sites for CO oxidation, and this results in the inhibition of Pt on CO oxidation. When the temperature is above 210 • C, the ratio of Pt +4 decreases, and the Pt +2 ratio increases, resulting in an increase in CO oxidation activity. When the temperature is above 210 °C, the ratio of Pt +4 decreases, and the Pt +2 ratio increases, resulting in an increase in CO oxidation activity. Figure 13. The interaction of Pt and β-MnO2 in CO oxidation.
The 1 wt% Pt/β-MnO2 was prepared using the deposition-precipitation method according to the method reported by Koo et al. [43]. Add H2Cl6Pt·xH2O (0.1 g mL -1 ), β-MnO2 (4 g), and distilled water (100 mL) to a beaker and mix thoroughly. Place the beaker in a 60 °C water bath and stir for 1 h; NH3·H2O was used to adjust the pH of the mixture to around 9 and then the mixture was stirred at 60 °C for 8 h. After stirring, the mixture was filtered and washed with hot distilled water and then dried at 60 °C for 12 h. After drying, the catalyst was put into a muffle furnace and calcined in an air atmosphere at 300 °C for 2 h.

Evaluation of Catalytic Activity
The CO oxidation activity of two catalysts (200 mg, 40-60 mesh) was measured in a fixed-bed flow reactor fed with 1% CO/20% O2 balanced with N2 at 100 mL min -1 , with a gas-space velocity of 60,000 mL g -1 h -1 [27]. CO and CO2 concentrations were analyzed online using a gas chromatograph (GC9790II, Fuli, Taizhou, China) equipped with a flame ion detector with a catalyst convert to reduce CO and CO2 to CH4. Each catalyst was pretreated in 20% O2 with an N2 balance (80 mL min -1 ) atmosphere at 300 °C for 30 min. The CO conversion (x) and turnover frequency (TOF, s -1 ) were calculated using Equation (11) and Equation (12) where F is the total gas flow rate in mol s -1 , mcat is the amount of catalyst in g, mPt is the weight ratio of Pt in the catalyst, and MPt is the molar atomic weight of Pt (195.08).

Catalyst Characterization
The operando DRIFTS-MS was used to monitor surface species and gaseous products during CO oxidation using the method reported elsewhere [10]. The catalyst characterization was performed as per [10].  The 1 wt% Pt/β-MnO 2 was prepared using the deposition-precipitation method according to the method reported by Koo et al. [43]. Add H 2 Cl 6 Pt·xH 2 O (0.1 g mL −1 ), β-MnO 2 (4 g), and distilled water (100 mL) to a beaker and mix thoroughly. Place the beaker in a 60 • C water bath and stir for 1 h; NH 3 ·H 2 O was used to adjust the pH of the mixture to around 9 and then the mixture was stirred at 60 • C for 8 h. After stirring, the mixture was filtered and washed with hot distilled water and then dried at 60 • C for 12 h. After drying, the catalyst was put into a muffle furnace and calcined in an air atmosphere at 300 • C for 2 h.

Evaluation of Catalytic Activity
The CO oxidation activity of two catalysts (200 mg, 40-60 mesh) was measured in a fixed-bed flow reactor fed with 1% CO/20% O 2 balanced with N 2 at 100 mL min −1 , with a gas-space velocity of 60,000 mL g −1 h −1 [27]. CO and CO 2 concentrations were analyzed online using a gas chromatograph (GC9790II, Fuli, Taizhou, China) equipped with a flame ion detector with a catalyst convert to reduce CO and CO 2 to CH 4 . Each catalyst was pretreated in 20% O 2 with an N 2 balance (80 mL min −1 ) atmosphere at 300 • C for 30 min. The CO conversion (x) and turnover frequency (TOF, s −1 ) were calculated using Equation (11) and Equation (12) where F is the total gas flow rate in mol s −1 , m cat is the amount of catalyst in g, m Pt is the weight ratio of Pt in the catalyst, and M Pt is the molar atomic weight of Pt (195.08).

Catalyst Characterization
The operando DRIFTS-MS was used to monitor surface species and gaseous products during CO oxidation using the method reported elsewhere [10]. The catalyst characterization was performed as per [10].

Conclusions
In this study, 1 wt% Pt NPs loaded on β-MnO 2 using deposition-precipitation was used to explore the effect of the metal-support interaction of Pt and β-MnO 2 in CO oxidation. The main conclusions are as follows: (1) After loading Pt on β-MnO 2 , it was found that Pt NPs (1-2 nm) were anchored on β-MnO 2 and formed Pt-O-Mn interfaces. CO oxidation was inhibited at a temperature below 210 • C and promoted above 210 • C; (2) The ratio of Pt +2 dominates CO oxidation. The ratios of Pt +2 before the reaction and after the reaction at 150, 200, and 250 When the temperature is higher than 210 • C, more Mn=O was consumed on Pt/β-MnO 2 than on β-MnO 2 . It is speculated that the inhibition of activity below 200 • C after Pt loading is due to Pt +4 , which causes less Mn=O consumption for CO oxidation; (4) The mechanism of Pt and β-MnO 2 interaction is proposed, where the Pt +4 and Pt +2 functions are clearly illustrated. Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: The raw data are available from the corresponding author upon reasonable request.