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

Abstract

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.

1. 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]. MnO₂ is widely used in catalytic oxidation reactions due to its variable valence states and good oxygen-storage and oxygen-release capacities [7,8,9]. MnO₂ can form a variety of crystal structures, such as α-, β-, γ-, and δ-MnO₂, according to [MnO₆] as a backbone arrangement [7]. Among them, β-MnO₂ has a thermodynamically stable phase and high crystallinity, which has become one of the current research hotspots [10]. It is still a challenge for MnO₂ 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/Al₂O₃ catalyst to convert formaldehyde at room temperature [14]. Huang et al. prepared a Pt/CeO₂-TiO₂ catalyst and found that it has high catalytic activity and selectivity for CO + NO reactions [15]. Cui et al. prepared an octahedral Fe₃O₄ 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-) MnO₂ catalysts for methanol oxidation, and showed that the temperatures for methanol oxidation on Pt/(α-, γ-, and δ-) MnO₂ catalysts can be reduced in comparison with MnO₂ [17].

MnO₂ 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 MnO₂ 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 MnO₂ [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 O₂ 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 MnO₂ 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 H₂ as a probe molecule to study the role of oxygen species on the surface oxygen species of β-MnO₂ 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 SO₂) on mercury oxidation over CeO₂-WO₃/TiO₂ catalysts through in situ DRIFTS and showed that NO₂ 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 Al₂O₃ and TiO₂ using Fourier transform infrared spectroscopy (FTIR) [24].

In this study, 1 wt% Pt NPs loaded on β-MnO₂ prepared with deposition–precipitation is used to explore the metal–support interaction of Pt and β-MnO₂ during CO oxidation. The physicochemical properties of β-MnO₂ and Pt/MnO₂ 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-MnO₂ and Pt/β-MnO₂ catalysts in CO oxidation is revealed using operando DRIFTS-MS. The interaction of Pt and β-MnO₂ in CO oxidation is proposed.

2. Results and Discussion

2.1. Physical Phase Characterization

Figure 1 shows the XRD patterns of two catalysts. The diffraction peaks of β-MnO₂ 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-MnO₂ exhibits sharp and narrow diffraction peaks, suggesting that β-MnO₂ 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-MnO₂ are consistent with that of β-MnO₂, indicating that the loading Pt did not change the crystal structure of β-MnO₂. However, after loading Pt, the intensities of the diffraction peaks decreased, indicating that the crystallinity of β-MnO₂ can be affected by Pt loading [30].

Figure 2 shows the HRTEM images of the two catalysts. The lattice spacing of β-MnO₂ is 0.311 nm (Figure 2a), which is consistent with the (110) crystal face of β-MnO₂. 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 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. Specific surface areas, pore volumes, and pore diameters.

Catalyst	β-MnO2	Pt/β-MnO2
BET surface (m2 g−1)	1.29	1.29
Pore volume (cm3 g−1)	0.00354	0.00174
Pore diameter (nm)	5.83	32.74
T10 (°C)	140	182
T50 (°C)	235	212
T90 (°C)	310	222

. 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.

2.2. 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-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.

2.3. 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. XPS and Raman analysis results.

Catalyst	XPS				Raman
	(Mn2+ + Mn3+)/Mn4+	Oads/Ototal	Pt2+/Pttotal	Pt4+/Pttotal	Mn=O Ratio *
β-MnO2	3.55	0.30			0.45
Pt/β-MnO2-fersh	4.00	0.33	0.32	0.68	0.66
Pt/β-MnO2-used-150 °C **			0.48	0.52
Pt/β-MnO2-used-200 °C **			0.51	0.49
Pt/β-MnO2-used-250 °C **			0.64	0.36

* Mn=O ratio: M=O/(M=O + M-O-M). ** Pt/β-MnO₂ catalysts after CO oxidation in 1% CO/20%/N₂ atmosphere at 150 °C, 200 °C, and 250 °C for 1 h, respectively.

. The peaks of 641.6, 642.5, and 643.6 eV (Figure 5a) correspond to Mn²⁺, Mn³⁺, and Mn⁴⁺, respectively [10]. The peak-area ratios of Mn cations with different valence states are shown in Table 2. Before and after Pt loading, the (Mn²⁺ + Mn³⁺)/Mn⁴⁺ ratio increased from 3.55 to 4.00, proving that Pt NPs on β-MnO₂ can promote the formation of Mn²⁺ and Mn³⁺; those contribute to the formation of oxygen vacancies [33]. Moreover, the increase in the Mn²⁺ and Mn³⁺ 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 (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-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 (Mn²⁺ + Mn³⁺)/Mn⁴⁺ 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), indicating that Pt has different valence states. The peaks of 72.6 and 74.5 eV correspond to Pt²⁺ and Pt⁴⁺, respectively [13,20]. The peak-area ratios of different valence states before and after CO oxidation are given in Table 2. The Pt/β-MnO₂ catalyst before CO oxidation mainly has Pt⁺⁴ and Pt⁺², among which the Pt⁴⁺ ratio is as high as 0.68, showing that Pt⁴⁺ is the mainstay. The emergence of Pt²⁺ can be explained by the generation of Pt-O-Mn bonds, meaning that there is an interaction between Pt and β-MnO₂ [33]. After the reaction at 150, 200, and 250 °C, the ratio of Pt²⁺/Pt_total gradually increased to 0.64. At the same time, the ratio of Pt⁴⁺/Pt_total decreased to 0.36. This finding clearly explores that Pt⁴⁺ was continuously reduced to Pt²⁺ during CO oxidation. Since Pt²⁺ 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-MnO₂ below 210 °C, it is concluded that Pt/β-MnO₂ containing more Pt⁴⁺ 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 [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]. Compared with β-MnO₂, the vibration intensities of Mn=O and Mn-O-Mn after Pt loading have been weakened. This indicates that the mobility of lattice oxygen (Mn-O-Mn) increases after Pt loading [20]. The peak areas of Mn=O (A_Mn=O) and Mn-O-Mn (A_Mn-O-Mn) were calculated using LabSpec software. Pt/β-MnO₂ has a higher A_Mn=O/(A_Mn-O-Mn + A_Mn-O) ratio than β-MnO₂ (Table 2), implying that Pt/β-MnO₂ has more adsorbed oxygen (Mn=O) than β-MnO₂.

2.4. 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 R_I (200–600 °C). Two reduction peaks (319 and 455 °C) were observed on β-MnO₂, which were attributed to the reduction of Mn⁴⁺ to Mn³⁺ and then to Mn²⁺ [13,20]. After loading Pt, the R₀ 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 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 = 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].

2.5. Surface Reactions Using Operando DRIFTS-MS

2.5.1. Surface Reactions in CO Atmosphere

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-Pt⁰ [39,40]. Peaks at 1300, 1112, 947, and 770 cm⁻¹ are attributed to Mn=O, Mn⁺-O₂⁻, Mn⁺-O₂²⁻, and Mn⁺-O²⁻-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(CO₃²⁻)) [19].

Figure 9 shows the normalized peak heights of the surface species and MS signals during CO oxidation on β-MnO₂ and Pt/β-MnO₂ in a CO atmosphere. As the temperature increased, the normalized intensities of Mn=O and Mn-O-Mn on β-MnO₂ and Pt/β-MnO₂ decreased (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-MnO₂ is 50 °C, obviously lower than that (150 °C) on Pt/β-MnO₂, suggesting that Mn=O on β-MnO₂ can react more easily with CO than that on Pt/β-MnO₂. 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-MnO₂ and Pt/β-MnO₂ are higher than 100 °C, but the normalized peak height of Mn-O-Mn on β-MnO₂ is less than that on Pt/β-MnO₂ (Figure 9b), also suggesting that Mn-O-Mn on β-MnO₂ can react easier with CO than that on Pt/β-MnO₂; Pt inhibited CO oxidation by the O in Mn-O-Mn. The reactions of Mn=O and Mn-O-Mn with CO result in the formation of Mn=□ and Mn²⁺-□-Mn, where □ denotes an oxygen vacancy. As the initial adsorbed O₂ in (O₂⁻)Mn²⁺-O-Mn is reduced to (O₂²⁻)Mn²⁺-□-Mn by CO (Equations (5) and (6)), the decrease in the normalized peak height of (O²⁻)Mn²⁺-O-Mn and increase in Mn²⁺-□-Mn above 50 °C (Figure 9c) proved the reactions in Equations (5) and (6). The CO oxidation products include surface carbonate and gaseous CO₂. Figure 9d shows the normalized peak heights of monodentate carbonate (v(CO₃²⁻)) on β-MnO₂ and Pt/β-MnO₂. The remarkable differences between β-MnO₂ and Pt/β-MnO₂ can be found within the temperature range of 100 and 300 °C, in which the normalized peak heights of monodentate carbonate on β-MnO₂ are higher than that on Pt/β-MnO₂, 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-MnO₂ and Pt/β-MnO₂; thus, Pt has no influence on bidentate carbonate formation following Equation (9) [41].

The CO₂ MS signal from β-MnO₂ is higher than that from Pt/β-MnO₂ in the temperature range of 50–210 °C (Figure 9f), as Pt inhibited CO oxidation with O atoms in Mn=O and Mn-O-Mn.

Mn=O + CO → Mn=□ + CO₂

(1)

Mn=O + CO-Pt^δ+ → Mn=□ + CO₂

(2)

Mn-O-Mn + CO → Mn-□-Mn + CO₂

(3)

Mn-O-Mn + CO-Pt^δ+ → Mn-□-Mn+CO₂

(4)

(O₂⁻)Mn-O-Mn + CO⁺ → (O₂²⁻)Mn²⁺-□-Mn + CO₂

(5)

(O₂⁻)Mn-O-Mn + CO-Pt^δ+ → (O₂²⁻)Mn²⁺-□-Mn + Pt^δ+ + CO₂

(6)

Mn-O-Mn=O + CO → Mn-□-Mn-O-CO₂

(7)

Mn=O + CO₂ → Mn-O-CO₂

(8)

Mn²⁺-O₂²⁻+CO₂ → Mn=O₂-CO

(9)

2.5.2. Surface Reactions in CO/O₂ Atmosphere

Figure 10 shows the DRIFTS-MS spectra of two catalysts during CO oxidation in a 1%CO/20%O₂/Ar atmosphere. The normalized intensities of Mn=O and Mn-O-Mn obviously decreased above 150 and 100 °C, respectively (Figure 11a,b), which are similar to those (Figure 9a,b) during CO oxidation in the absence of O₂. The normalized intensities of (O₂⁻)Mn-O-Mn and Mn²⁺-□-Mn on Pt/β-MnO₂ changed at 50 °C, lower than that (100 °C) on β-MnO₂. Since the changes of (O₂⁻)Mn-O-Mn and Mn²⁺-□-Mn are related to the formation of oxygen vacancy, Pt can promote the formation of oxygen vacancy in the presence of O₂. The normalized intensities of monodentate carbonate β-MnO₂ and Pt/β-MnO₂ (Figure 11e) are similar to those (Figure 9e), explaining that O₂ and Pt cannot influence the formation of monodentate carbonate.

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.

2.5.3. 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 (Figure 9 and Figure 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,b show the DRIFTS spectra on β-MnO₂ and Pt/β-MnO₂ during regeneration. Figure 12c,d demonstrate that the normalized intensities of Mn=O and Mn-O-Mn increased to 0 at 300 °C, indicating that O₂ decomposition at Mn=□ and Mn-□-Mn happened, resulting in the complete regeneration of Mn=O and Mn-O-Mn, where the O₂ decomposition obeys Epling–Xu mechanism (Equation (10)) [42].

(O₂²⁻)Mn-□-Mn → O=Mn-O-Mn

(10)

3. Interaction of Pt and β-MnO₂ in CO Oxidation

Figure 13 shows the interaction of Pt NPs and β-MnO₂ in CO oxidation. Pt NPs are anchored on MnO₂ by Pt-O-Mn bonding, resulting in Pt⁺⁴ and Pt⁺² valence states. CO is mainly adsorbed at Mn=O sites as the Pt loading is as low as 1 wt%. CO oxidation required additional O, which is from Mn=O or Pt-O-Mn and Mn-O-Mn. Step (a) indicates that O from Pt⁺⁴-O-Mn is difficult, causing the inhibition of CO oxidation. Step (b) is the easy process of O supplement from Pt⁺²-O-Mn to CO for its oxidation to CO₂. 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 MnO₂ 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.

4. Materials and Methods

4.1. Catalyst Preparation

β-MnO₂ was purchased from Aladdin, Shanghai, China. Chloroauric acid (H₂Cl₆Pt·xH₂O) was from the Shanghai McLean Company (Shanghai, China).

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.

4.2. 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), respectively.

$$x=\frac{{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{i}\mathrm{n}}-{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{i}\mathrm{n}}}\times 100$$

(11)

where [CO]_in and [CO]_out are the CO concentrations in gas streams from the inlet and outlet of the reactor, respectively.

$$TOF=\frac{F·{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{i}\mathrm{n}}·x}{m_{cat}{·m}_{Pt}/M_{Pt}}$$

(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).

4.3. 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].

5. Conclusions

In this study, 1 wt% Pt NPs loaded on β-MnO₂ using deposition–precipitation was used to explore the effect of the metal–support interaction of Pt and β-MnO₂ in CO oxidation. The main conclusions are as follows:

(1)

After loading Pt on β-MnO₂, it was found that Pt NPs (1–2 nm) were anchored on β-MnO₂ 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 °C were Pt/β-MnO₂-fresh (0.32) < Pt/β-MnO₂-used-150 °C (0.48) < Pt/β-MnO₂-used-200 °C (0.51) < Pt/β-MnO₂-used-250 °C (0.64). Pt⁴⁺ can convert to Pt⁺² by heating;

(3)

The Operando DRIFTS-MS results show that Mn=O plays an important role in CO oxidation. Below 210 °C, more Mn=O was consumed on β-MnO₂ than on Pt/β-MnO₂. When the temperature is higher than 210 °C, more Mn=O was consumed on Pt/β-MnO₂ than on β-MnO₂. 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-MnO₂ interaction is proposed, where the Pt⁺⁴ and Pt⁺² functions are clearly illustrated.