Activation of Pt Nanoclusters on TiO 2 via Tuning the Metallic Sites to Promote Low-Temperature CO Oxidation

: Metallic Pt sites are imperative in the CO oxidation reaction. Herein, we demonstrate the tuning of Pt sites by treating a Pt catalyst in various reductive atmospheres, inﬂuencing the catalyst activities in low-temperature CO oxidation. The H 2 pretreatment of Pt clusters at 200 ◦ C decreases the T 50 from 208 ◦ C to 183 ◦ C in the 0.1 wt % Pt/TiO 2 catalyst. The T 50 shows a remarkable improvement using a CO pretreatment, which decreases the T 50 further to 135 ◦ C. A comprehensive characterization study reveals the integrated reasons behind this phenomenon: (i) the extent of PtO transition to metallic Pt sites, (ii) the ample surface active oxygen triggered by metallic Pt, (iii) the CO selectively adsorbs on metallic Pt sites which participate in low-temperature CO oxidation, and (iv) the formation of the unstable intermediate such as bicarbonate, contributes together to the enhanced activity of CO pretreated Pt/TiO 2 .


Introduction
Low-temperature CO oxidation is one of the most investigated model reactions in the field of heterogeneous catalysis [1]. The CO oxidation is not only important in practical applications (e.g., automotive emission control [2,3] and remediation of flue gas [4,5]) but also acts as a probe reaction guiding the design of catalysts' surface properties [6]. The adsorption of CO on active sites of catalyst surface is a crucial step that controls CO oxidation reaction rates, and it is especially true for noble metal catalysts [7]. The linear CO adsorbed on metallic Pt sites that can facilely react with atmospheric O 2 is responsible for the high activity in the low-temperature CO oxidation [8]. The ionic Pt reacts with linear CO when the temperature is higher than 100 • C. This required reduction pretreatment of Pt/TiO 2 to obtain high activity in the low-temperature CO oxidation. On the other hand, the pretreatment conditions influence the peroxide and superoxide generated by O 2 adsorption at the metal-support interface that determines the catalytic performance [9,10]. As a critical step of CO oxidation, pretreatment can modify the chemical properties of catalyst and alter the reaction pathway [11,12]. Maldonado-Hodar's group [13] studied the influence of treatments in the reductive atmosphere (i.e., H 2 ) and inert atmosphere (i.e., He) on the performance of Pt/TiO 2 . They found that the reduction pretreatment by H 2 favored the formation of oxygen vacancies and facilitated the diffusion of Pt species into the TiO 2 . The Pt-TiO 2 interface and the active surface sites were suppressed by He pretreatment, leading to unsatisfactory activity in the selective citral hydrogenation. Reduction pretreatment in CO also promotes the activity, as evidenced by the CO oxidation over Pt/CeO 2 -Al 2 O 3 catalyst. The CO pretreatment induces a more reduced Pt with a low Pt-O coordination number than the catalysts pretreated in H 2 or C 3 H 6 , leading to enhanced activity in diesel oxidation reaction [14]. Moreover, investigations on Au/CeO 2 catalyst pretreated in O 2 , N 2 , CO, and H 2 suggest that the long-term activity is dominated by the reducibility of the support and the particle size [15]. It is generally accepted that the reductive atmosphere has a significant impact on both the chemical status and morphology of noble metal and thus varies the activity of the catalysts. However, the difference of the reductant atmospheres (e.g., CO and H 2 ) on the transition of active sites (e.g., Pt 0 , Pt δ+ sites) and surface oxygen species (e.g., O 2 − ads , O − ads ) and roles they play in the reaction are still unclear and require detailed investigations.
In this work, the tuning of Pt sites in Pt/TiO 2 was carried out by a mild reduction in the atmospheres of H 2 and CO, respectively. The influences on the chemical and surface properties of Pt/TiO 2 catalyst were investigated by a combination of surface characterizations (e.g., X-ray photoelectron spectroscopy (XPS), H 2 temperature-programmed reduction (H 2 -TPR), O 2 temperature-programmed desorption (O 2 -TPD)). The impact of reduction pretreatment on the structural characters of Pt/TiO 2 was studied by the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The transition of Pt active adsorption sites and the reactive intermediates were investigated by the CO adsorption using an in situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTs). Furthermore, the catalytic performance of Pt/TiO 2 catalysts was studied by both in situ and ex situ CO oxidation to identify the influence of pretreatments on catalyst surface species.

Chemical and Structural Modifications
The Pt/TiO 2 -H 2 and Pt/TiO 2 -CO catalysts have XRD patterns similar to that of Pt/TiO 2 (Figure 1a  . The absence of Pt characteristic peak in the XRD patterns indicates a good dispersion of Pt nanoclusters [16,17]. A slight shift of anatase TiO 2 (101) from 25.30 • to 25.34 • is detected on Pt/TiO 2 -H 2 , implying a distortion of TiO 2 lattice caused by the interaction of Pt species [18,19]. The crystal sizes of TiO 2 for Pt/TiO 2 catalysts are 17.1 ± 0.3 nm, as calculated in Table S1 by the Debye-Scherrer equation from the XRD patterns in Figure 1a.
Catalysts 2021, 11, x FOR PEER REVIEW 2 of 14 over Pt/CeO2-Al2O3 catalyst. The CO pretreatment induces a more reduced Pt with a low Pt-O coordination number than the catalysts pretreated in H2 or C3H6, leading to enhanced activity in diesel oxidation reaction [14]. Moreover, investigations on Au/CeO2 catalyst pretreated in O2, N2, CO, and H2 suggest that the long-term activity is dominated by the reducibility of the support and the particle size [15]. It is generally accepted that the reductive atmosphere has a significant impact on both the chemical status and morphology of noble metal and thus varies the activity of the catalysts. However, the difference of the reductant atmospheres (e.g., CO and H2) on the transition of active sites (e.g., Pt 0 , Pt δ+ sites) and surface oxygen species (e.g., O2 − ads, O − ads) and roles they play in the reaction are still unclear and require detailed investigations. In this work, the tuning of Pt sites in Pt/TiO2 was carried out by a mild reduction in the atmospheres of H2 and CO, respectively. The influences on the chemical and surface properties of Pt/TiO2 catalyst were investigated by a combination of surface characterizations (e.g., X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), O2 temperature-programmed desorption (O2-TPD)). The impact of reduction pretreatment on the structural characters of Pt/TiO2 was studied by the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The transition of Pt active adsorption sites and the reactive intermediates were investigated by the CO adsorption using an in situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTs). Furthermore, the catalytic performance of Pt/TiO2 catalysts was studied by both in situ and ex situ CO oxidation to identify the influence of pretreatments on catalyst surface species.

Chemical and Structural Modifications
The Pt/TiO2-H2 and Pt/TiO2-CO catalysts have XRD patterns similar to that of Pt/TiO2 (Figure 1a  . The absence of Pt characteristic peak in the XRD patterns indicates a good dispersion of Pt nanoclusters [16,17]. A slight shift of anatase TiO2 (101) from 25.30° to 25.34° is detected on Pt/TiO2-H2, implying a distortion of TiO2 lattice caused by the interaction of Pt species [18,19]. The crystal sizes of TiO2 for Pt/TiO2 catalysts are 17.1 ± 0.3 nm, as calculated in Table S1 by the Debye-Scherrer equation from the XRD patterns in Figure 1a. The micro-Raman spectra of Pt/TiO2, Pt/TiO2-H2, and Pt/TiO2-CO catalysts in Figure  1b are characterized by the vibrational modes of 144 cm −1 (Eg(1)), 197 cm −1 (Eg(2)), 396 cm −1 (B1g), 516 cm −1 (A1g), and 639 cm −1 (Eg(3)) of TiO2 anatase phase, and 453 cm −1 (Eg) of TiO2 rutile phase (Figure 1b) [20]. The blue shifts of Eg(1) modes at 144 cm −1 are observed for Pt/TiO2-H2 and Pt/TiO2-CO that shift to 146 cm −1 and 148 cm −1 , respectively. Meanwhile, The micro-Raman spectra of Pt/TiO 2 , Pt/TiO 2 -H 2 , and Pt/TiO 2 -CO catalysts in Figure 1b are characterized by the vibrational modes of 144 cm −1 (E g(1) ), 197 cm −1 (E g (2) ), 396 cm −1 (B 1g ), 516 cm −1 (A 1g ), and 639 cm −1 (E g(3) ) of TiO 2 anatase phase, and 453 cm −1 (E g ) of TiO 2 rutile phase (Figure 1b) [20]. The blue shifts of E g(1) modes at 144 cm −1 are observed for Pt/TiO 2 -H 2 and Pt/TiO 2 -CO that shift to 146 cm −1 and 148 cm −1 , respectively. Meanwhile, peak broadening is found for both the Pt/TiO 2 -H 2 and Pt/TiO 2 -CO catalysts. This is determined by a full width at half maximum (FWHM) of E g(1) modes of 14 cm −1 and 16 cm −1 for Pt/TiO 2 -H 2 and Pt/TiO 2 -CO catalysts, respectively, in comparison to an FWHM of 12 cm −1 for Pt/TiO 2 (Table S2). The peak shift and broadening indicate the lattice distortion and the defect formation [16] trigger by reduction pretreatments [21]. Additionally, the porous structure of the three Pt/TiO 2 catalysts is comparable with a surface area of 64.0 ± 1.8 m 2 /g and a pore volume of 0.30 ± 0.02 cm 3 /g ( Figure 1c and Table 1). The chemical status of Pt and surface composition of Pt/TiO 2 catalysts are characterized by the X-ray photoelectron spectroscopy (XPS) spectra with Pt 4f, O 1s, and Ti 2p narrow scan spectra in Figure 2a-c. The influence of air exposure on a sample before the XPS test is negligible for it happened at room temperature (ca., 25 • C), which is much lower than the reduction pretreatment temperature of 200 • C. In the presence of an overlapping Ti loss peak at around 75 eV [22], the Pt 4f spectra of Pt/TiO 2 catalysts exhibits Pt 0 at 71.00 eV, 74.35 eV [23] and Pt 2+ at 72.40 eV, 75.75 eV [24]. Though the signal-to-noise ratio (S/N) problem occurs for the catalyst with a low loading ratio of Pt on TiO 2 (i.e., 0.1 w.t.%) [22], the tendency of Pt valence state change over reduction pretreatments can be recognized. The ratio of metallic Pt 0 of Pt/TiO 2 catalyst without pretreatment is 16.7% (Table 1), which is lower than the H 2 -pretreated (i.e., 23.7%) and CO-pretreated catalysts (46.9%). Almost a half of Pt in Pt/TiO 2 -CO being metallic Pt suggests that CO reduced Pt effectively. Meanwhile, the O 1s spectra in Figure S1 exhibits two peaks belonging to lattice oxygen (O Latt ) at 529.78 eV and oxygen species adjacent to oxygen vacancies (O Ads ) at 531.44 eV [25,26]. The ratio O Ads /(O Latt + O Ads ) of reduced Pt/TiO 2 catalysts in Table 1 is close to 17.0%, which is higher than that of un-pretreated Pt/TiO 2 (i.e., 12.5%). This indicates that the reduction pretreatment facilitates the adsorption of oxygen on the catalyst surface by producing metallic Pt, as schemed in Figure 2 [27,28].
The structural modifications of Pt/TiO 2 catalysts are studied using high-resolution, high-angle annular dark-field (HAADF) scanning transmission electron microscopy. All three catalysts present a spherical morphology with a TiO 2 particle size of 20 ± 2 nm that is similar to the P25 TiO 2 support. The bright points of 2 nm in the dark-field STEM micrographs and elemental map of Pt in Figure 2d-f indicate the existence of Pt nanoclusters in all the Pt/TiO 2 catalysts. A slight aggregation of Pt nanoparticles is observed on the Pt/TiO 2 pretreated by H 2 in Figure 2e. This might be due to the particle migration and coalescence in H 2 during heating treatment [29].

Surface Reduction Properties
The temperature-programmed studies (i.e., O2-TPD, H2-TPR) are performed to analyze the surface properties of the Pt/TiO2 catalysts prior to and after the reduction pretreatments. The surface adsorbed oxygen is determined by the O2-TPD, and the desorption profiles for the three Pt/TiO2 catalysts are shown in Figure 3a. The desorption peak at 120 °C to 250 °C is attributed to the superoxide O2 − ads [30]. The amount of O2 − ads is improved after the reduction pretreatment in CO and H2, and the desorption temperatures of O2 − ads on Pt/TiO2-CO (i.e., 187 °C) and Pt/TiO2-H2 (i.e., 190 °C) are lower than the one of unpretreated Pt/TiO2 (i.e., 202 °C). The desorption peak from 350 °C to 550 °C is assigned to the dissociatively adsorbed peroxide O − ads [30] on Pt/TiO2 catalysts that participate in lowtemperature CO oxidation [31]. The O − ads peak area of the Pt/TiO2-CO at 350 °C to 550 °C is the largest, followed by Pt/TiO2-H2, and Pt/TiO2 shows the smallest O − area, similar to pristine TiO2. The superoxide and peroxide in Pt/TiO2-CO facilitate the oxidation of CO at low temperatures, indicating an elevated activity of the catalyst.
The redox property of the Pt/TiO2, Pt/TiO2-CO, and Pt/TiO2-H2 catalysts is evaluated by H2-TPR profiles shown in Figure 3b, and the H2 consumption summarized in Table S3. The reduction peak at 119 °C that is only observed on the Pt/TiO2 catalyst is ascribed to the reduction of PtOx to metallic Pt [32]. The absence of PtOx reduction on Pt/TiO2-CO, and Pt/TiO2-H2 suggests that the majority of Pt on the catalyst surface are in the form of metallic Pt. The reduction that occurs at 314 °C is assigned to the partial reduction of surface oxygen, as induced by hydrogen spillover from Pt to the TiO2 surface [21,32,33]. The pure TiO2 only presents a reduction of lattice oxygen at a temperature higher than 600 °C [33]. These results show that the reduction peak of surface oxygen only presents in the presence of Pt, which is indicative of a strong Pt-TiO2 interaction that produces oxygen

Surface Reduction Properties
The temperature-programmed studies (i.e., O 2 -TPD, H 2 -TPR) are performed to analyze the surface properties of the Pt/TiO 2 catalysts prior to and after the reduction pretreatments. The surface adsorbed oxygen is determined by the O 2 -TPD, and the desorption profiles for the three Pt/TiO 2 catalysts are shown in Figure 3a. The desorption peak at 120 • C to 250 • C is attributed to the superoxide O 2 − ads [30]. The amount of O 2 − ads is improved after the reduction pretreatment in CO and H 2 , and the desorption temperatures of O 2 − ads on Pt/TiO 2 -CO (i.e., 187 • C) and Pt/TiO 2 -H 2 (i.e., 190 • C) are lower than the one of unpretreated Pt/TiO 2 (i.e., 202 • C). The desorption peak from 350 • C to 550 • C is assigned to the dissociatively adsorbed peroxide O − ads [30] on Pt/TiO 2 catalysts that participate in low-temperature CO oxidation [31]. The O − ads peak area of the Pt/TiO 2 -CO at 350 • C to 550 • C is the largest, followed by Pt/TiO 2 -H 2 , and Pt/TiO 2 shows the smallest O − area, similar to pristine TiO 2 . The superoxide and peroxide in Pt/TiO 2 -CO facilitate the oxidation of CO at low temperatures, indicating an elevated activity of the catalyst.

Catalytic Performance
The catalytic performance of un-pretreated Pt/TiO2 and the two pretreated Pt/TiO2 catalysts in H2 (i.e., Pt/TiO2-H2) and CO (i.e., Pt/TiO2-CO) are evaluated by the CO oxidation in Figure 4. It is noteworthy that the Pt/TiO2-CO exhibits a significantly enhanced activity, with a much lower temperature for 50% conversion (i.e., T50) of 135 °C, as compared to 184 °C of Pt/TiO2-H2 and to 203 °C of the catalyst without pretreatment. Complete oxidation of CO is achieved at 150 °C (i.e., T100) by Pt/TiO2-CO, while the Pt/TiO2 and Pt/TiO2-H2 require high temperatures of 210 °C and 190 °C, respectively. Figure 4b presents the Arrhenius plots of the three Pt/TiO2 catalysts. The apparent activation energies calculated from the slopes of the fitting lines are 63.4 kJ/mol, 56.1 kJ/mol, 41.2 kJ/mol for Pt/TiO2, Pt/TiO2-H2, and Pt/TiO2-CO, respectively. It is found that the CO reduction pretreatment produces a high ratio of metallic Pt sites over ionic Pt sites (i.e., 46.9%) as indicated by the XPS spectra in Figure 2, and abundant surface oxygen species (e.g., O2 − ads) as suggested by the temperature-programmed studies in Figure 3, which promotes the oxidation at low-temperatures [34]. The related mechanism is discussed in Section 2.4 by in situ DRIFTs that explore the adsorbed intermediates of CO adsorption and the reaction pathway of CO oxidation.  Table S3. The reduction peak at 119 • C that is only observed on the Pt/TiO 2 catalyst is ascribed to the reduction of PtO x to metallic Pt [32]. The absence of PtO x reduction on Pt/TiO 2 -CO, and Pt/TiO 2 -H 2 suggests that the majority of Pt on the catalyst surface are in the form of metallic Pt. The reduction that occurs at 314 • C is assigned to the partial reduction of surface oxygen, as induced by hydrogen spillover from Pt to the TiO 2 surface [21,32,33]. The pure TiO 2 only presents a reduction of lattice oxygen at a temperature higher than 600 • C [33]. These results show that the reduction peak of surface oxygen only presents in the presence of Pt, which is indicative of a strong Pt-TiO 2 interaction that produces oxygen vacancy for the dissociative oxygen adsorption. Furthermore, Pt/TiO 2 consumes more H 2 (i.e., 0.40 mmol/g) during H 2 -TPR than the H 2 required by Pt/TiO 2 -CO (i.e., 0.32 mmol/g) and Pt/TiO 2 -H 2 (i.e., 0.29 mmol/g). The suppression of PtO x reduction peak and low H 2 consumption for Pt/TiO 2 -H 2 and Pt/TiO 2 -CO catalysts confirm the transition from ionic Pt to metallic Pt by H 2 or CO pretreatment, which is in line with XPS results (Figure 2).

Catalytic Performance
The catalytic performance of un-pretreated Pt/TiO 2 and the two pretreated Pt/TiO 2 catalysts in H 2 (i.e., Pt/TiO 2 -H 2 ) and CO (i.e., Pt/TiO 2 -CO) are evaluated by the CO oxidation in Figure 4. It is noteworthy that the Pt/TiO 2 -CO exhibits a significantly enhanced activity, with a much lower temperature for 50% conversion (i.e., T 50 ) of 135 • C, as compared to 184 • C of Pt/TiO 2 -H 2 and to 203 • C of the catalyst without pretreatment. Complete oxidation of CO is achieved at 150 • C (i.e., T 100 ) by Pt/TiO 2 -CO, while the Pt/TiO 2 and Pt/TiO 2 -H 2 require high temperatures of 210 • C and 190 • C, respectively. Figure 4b presents the Arrhenius plots of the three Pt/TiO 2 catalysts. The apparent activation energies calculated from the slopes of the fitting lines are 63.4 kJ/mol, 56.1 kJ/mol, 41.2 kJ/mol for Pt/TiO 2 , Pt/TiO 2 -H 2 , and Pt/TiO 2 -CO, respectively. It is found that the CO reduction pretreatment produces a high ratio of metallic Pt sites over ionic Pt sites (i.e., 46.9%) as indicated by the XPS spectra in Figure 2, and abundant surface oxygen species (e.g., O 2 − ads ) as suggested by the temperature-programmed studies in Figure 3, which promotes the oxidation at low-temperatures [34]. The related mechanism is discussed in Section 2.4 by in situ DRIFTs that explore the adsorbed intermediates of CO adsorption and the reaction pathway of CO oxidation.
Catalysts 2021, 11, 1280 6 of 14 treatment produces a high ratio of metallic Pt sites over ionic Pt sites (i.e., 46.9%) as indicated by the XPS spectra in Figure 2, and abundant surface oxygen species (e.g., O2 − ads) as suggested by the temperature-programmed studies in Figure 3, which promotes the oxidation at low-temperatures [34]. The related mechanism is discussed in Section 2.4 by in situ DRIFTs that explore the adsorbed intermediates of CO adsorption and the reaction pathway of CO oxidation.

In Situ DRIFTS Study
To determine the adsorbed intermediates and to investigate the reaction pathway of CO oxidation, two groups of in situ DRIFTs studies are performed by feeding CO and then O 2 (i.e., CO + O 2 ), and by feeding the mixture gas of CO and O 2 (i.e., CO/O 2 ), respectively. Figure 5 plots in situ DRIFT spectra of CO + O 2 condition at 200 • C, where the CO adsorption is carried out in 2 vol.% CO/N 2 for 10 min, followed by purging in N 2 for 15 min, and the oxidation is carried out by introducing O 2 for 6 min. The CO exhibits four types of IR vibrations that centered at 2171 cm −1 for gaseous CO [32,34,35], at 2117 cm −1 for CO adsorbed on ionic Pt sites (Pt 2+ -CO) [34][35][36], at 2090 cm −1 for CO adsorbed on metallic Pt with Pt(O x ) neighbor (Pt δ+ -CO) [32,34,35,37], and at 2078 cm −1 for linear CO adsorbed on metallic Pt sites (Pt 0 -CO) [34][35][36]38] (Table S1). CO adsorbed on Pt species is barely observed on unpretreated Pt/TiO 2 catalyst at 1 min (Figure 5a), implying that CO adsorption on predominating Pt 2+ sites takes place slowly at 200 • C. A fast CO adsorption occurred on the pretreated catalysts that have Pt 0 sites. Both Pt/TiO 2 -H 2 and Pt/TiO 2 -CO exhibit the adsorbed CO at 1 min, with Pt 0 -CO and Pt 2+ -CO on Pt/TiO 2 -H 2 (Figure 5b), and only Pt 0 -CO on Pt/TiO 2 -CO ( Figure 5c). The absence of Pt 2+ -CO on Pt/TiO 2 -CO at 1 min demonstrates that CO pretreatment converted the surface Pt species to Pt 0 more effectively than H 2 pretreatment, and Pt 0 adsorbed CO faster than Pt 2+ . When the adsorption is launched for 2 min, the CO is adsorbed on unpretreated Pt/TiO 2 primarily in the form of Pt 2+ -CO, followed by Pt δ+ -CO and Pt 0 -CO (Figure 5a). The CO adsorbed on Pt 0 sites accumulates on the surface of Pt/TiO 2 catalyst for 2 min, and the intensity of Pt 2+ -CO declines simultaneously (Figure 5a,d). This indicates that the fed CO in the CO adsorption triggers the reduction of the unpretreated Pt/TiO 2 [24,34].

In Situ CO Adsorption-Desorption-Oxidation
CO assists the transition of higher coordination number of Pt at larger wavenumber to lower coordination number of metallic Pt (i.e., Pt 0 -CO) at smaller wavenumber more effectively than H2 [14]. It provides active sites for CO adsorption and low-temperature oxidation [39], considering that the Pt 2+ sites are less active in low-temperature CO oxidation and the oxygen-surrounding Pt δ+ partially deactivates the sites [34]. The species that formed during the CO adsorption, desorption and oxidation are presented in Figure 6 for Pt/TiO2, Pt/TiO2-H2, and Pt/TiO2-CO catalysts. Carboxylate, which has the structure of -CO2 − , with C atom or C and O atoms binding on the support, is found on all the three Pt/TiO2 catalysts with characteristic IR vibrations of asymmetric stretching mode at 1542 cm −1 and symmetric stretching mode at 1361 cm −1 [40]. Carbonate species at 1438 cm −1 [41] is only observed on unpretreated Pt/TiO2 after the CO adsorption ( Figure  6a). The carboxylate requires a lower energy barrier to be reacted to CO2 than that of carbonate species [42]. Moreover, the bidentate carbonate at 1315 cm −1 [41] is produced at 31 min on Pt/TiO2 (Figure 6a) and Pt/TiO2-H2 (Figure 6b), along with the partial oxidation of Pt 0 to Pt 2+ (Figure 5a,b). It implies that adsorbed CO is transformed to bidentate carbonates in O2. No carbonates species are found on in situ DRIFT spectra of Pt/TiO2-CO in Figure  6c for the whole CO adsorption-desorption-oxidation process, which is in line with its high activity in the CO oxidation in Figure 4.  (Figure 5a,d), indicating that the Pt 0 sites created by the CO adsorption for only 10 min are unstable that facilely transfer to Pt 2+ . In contrast, a transition from Pt 0 to Pt 2+ is not observed on the pretreated samples (Figure 5b,c,e,f), instead, the intensity of Pt 2+ -CO decreases because of the desorption of CO in N 2 . In the spectra obtained after CO adsorption and N 2 purging ( Figure S2), the ratio of the CO on metallic Pt peak area of Pt/TiO 2 -H 2 and Pt/TiO 2 -CO are 99% and 93%, respectively, both higher than that of Pt/TiO 2 (i.e., 72%), indicating that H 2 and CO reduction triggers the transition of Pt 2+ to Pt δ+ and Pt 0 , which is consistent with the XPS (Figure 2) and H 2 -TPR (Figure 4b) analysis.
The CO adsorbed on metallic Pt (i.e., Pt 0 -CO and Pt δ+ -CO) for all the three catalysts is completely disappeared after feeding O 2 for 6 min, as shown in Figure 5. The CO adsorbed on ionic Pt accumulates on the catalyst surface of unpretreated Pt/TiO 2 (Figure 5d) and H 2 pretreated Pt/TiO 2 (Figure 5e), especially on the unpretreated Pt/TiO 2 , which is in line with its low activity in Figure 4. This confirms that the metallic Pt sites act as the active sites for the low-temperature CO oxidation [34]. When introducing oxygen at 25 min after the saturation adsorption of CO on the catalysts, a change of CO 2 intensity is observed for the three catalysts ( Figure S4). The Pt/TiO 2 -H 2 and Pt/TiO 2 -CO produced more CO 2 than Pt/TiO 2 as their CO 2 intensities increase more than Pt/TiO 2 . The weaker CO 2 peak intensity of Pt/TiO 2 at 31 min indicates that not all CO adsorbed on Pt 0 converts to CO 2 , but also transfers to Pt 2+ to form Pt 2+ -CO, where the Pt 2+ is originated from the re-oxidation of Pt 0 under the oxidative condition. The disappearance of adsorbed CO is monitored on Pt/TiO 2 -CO when introducing O 2 only for 2 min (Figure 5f). The high activity of Pt/TiO 2 -CO in CO oxidation in Figure 4 is due to the plentiful metallic Pt sites (Figure 2) created by CO pretreatment, which promotes both CO adsorption and CO oxidation at low temperature [34,35]. Furthermore, studies of Pt catalysts pretreated in CO have shown that CO assists the transition of higher coordination number of Pt at larger wavenumber to lower coordination number of metallic Pt (i.e., Pt 0 -CO) at smaller wavenumber more effectively than H 2 [14]. It provides active sites for CO adsorption and low-temperature oxidation [39], considering that the Pt 2+ sites are less active in low-temperature CO oxidation and the oxygen-surrounding Pt δ+ partially deactivates the sites [34].
The species that formed during the CO adsorption, desorption and oxidation are presented in Figure 6 for Pt/TiO 2 , Pt/TiO 2 -H 2 , and Pt/TiO 2 -CO catalysts. Carboxylate, which has the structure of -CO 2 − , with C atom or C and O atoms binding on the support, is found on all the three Pt/TiO 2 catalysts with characteristic IR vibrations of asymmetric stretching mode at 1542 cm −1 and symmetric stretching mode at 1361 cm −1 [40]. Carbonate species at 1438 cm −1 [41] is only observed on unpretreated Pt/TiO 2 after the CO adsorption (Figure 6a). The carboxylate requires a lower energy barrier to be reacted to CO 2 than that of carbonate species [42]. Moreover, the bidentate carbonate at 1315 cm −1 [41] is produced at 31 min on Pt/TiO 2 (Figure 6a) and Pt/TiO 2 -H 2 (Figure 6b), along with the partial oxidation of Pt 0 to Pt 2+ (Figure 5a,b). It implies that adsorbed CO is transformed to bidentate carbonates in O 2 . No carbonates species are found on in situ DRIFT spectra of Pt/TiO 2 -CO in Figure 6c for the whole CO adsorption-desorption-oxidation process, which is in line with its high activity in the CO oxidation in Figure 4.

In Situ CO Oxidation
In situ DRIFT spectra of the CO oxidation (i.e., CO/O2 condition) in Figure 7 are obtained in an atmosphere containing 0.7 vol.% CO and 16 vol.% O2. The temperature program with an interval of 15 °C is set up for each catalyst according to the corresponding light-off curve in Figure 5. The unpretreated Pt/TiO2 in Figure 7a displays both Pt 2+ -CO at 2117 cm −1 and Pt 0 -CO at 2071 cm −1 at 150 °C, and consumption of the Pt 0 -CO occurs when the temperature is higher than 180 °C, implying that CO oxidation takes place at the metallic Pt sites with CO adsorbed. The Pt 0 -CO disappears as it is fully converted to CO2, leaving CO adsorbed on Pt 2+ participates in the high-temperature oxidation. Similarly, it is also true for the pretreated Pt/TiO2 catalysts in Figure 7b,c that the ratio of Pt 2+ -CO to Pt 0 -CO becomes predominant along with increasing temperatures. Bicarbonates at 1608 cm −1 (υas) and 1627 cm −1 (υs) are reactive intermediates in the CO oxidation for Pt/TiO2 catalysts, implying a reaction pathway that CO converted to bicarbonates before transiting to CO2 ( Figure S5) [40]. Figure 7d,e summarizes the IR intensity of Pt 0 -CO and Pt 2+ -CO as a function of reac-

In Situ CO Oxidation
In situ DRIFT spectra of the CO oxidation (i.e., CO/O 2 condition) in Figure 7 are obtained in an atmosphere containing 0.7 vol.% CO and 16 vol.% O 2 . The temperature program with an interval of 15 • C is set up for each catalyst according to the corresponding light-off curve in Figure 5. The unpretreated Pt/TiO 2 in Figure 7a displays both Pt 2+ -CO at 2117 cm −1 and Pt 0 -CO at 2071 cm −1 at 150 • C, and consumption of the Pt 0 -CO occurs when the temperature is higher than 180 • C, implying that CO oxidation takes place at the metallic Pt sites with CO adsorbed. The Pt 0 -CO disappears as it is fully converted to CO 2 , leaving CO adsorbed on Pt 2+ participates in the high-temperature oxidation. Similarly, it is also true for the pretreated Pt/TiO 2 catalysts in Figure 7b,c that the ratio of Pt 2+ -CO to Pt 0 -CO becomes predominant along with increasing temperatures. Bicarbonates at 1608 cm −1 (υ as ) and 1627 cm −1 (υ s ) are reactive intermediates in the CO oxidation for Pt/TiO 2 catalysts, implying a reaction pathway that CO converted to bicarbonates before transiting to CO 2 ( Figure S5) [40].  Figure 7d,e summarizes the IR intensity of Pt 0 -CO and Pt 2+ -CO as a function of reaction temperature. The Pt 0 -CO peak intensity of Pt/TiO 2 , Pt/TiO 2 -H 2 , and Pt/TiO 2 -CO reaches their summit at 180 • C, 180 • C, and 150 • C, respectively, which correspond with their activity tests in Figure 4a (i.e., the T 100 of 210 • C, 190 • C, 150 • C, respectively). The Pt 0 -CO peak intensity of Pt/TiO 2 -CO at low temperature is higher than those of the Pt/TiO 2 -H 2 and Pt/TiO 2 -CO, indicating that Pt/TiO 2 -CO owns high CO adsorption capacity. The Pt 2+ -CO accumulates up to a temperature of 255 • C in Figure 7e, which is much higher than Pt 0 -CO, implying that CO is adsorbed more strongly on Pt 2+ than Pt 0 -CO [35,36] upon exposure to the reactant gases. These results are consistent with the literature [35,43] that linear CO adsorbed on metallic Pt is responsible for reacting in the low-temperature CO oxidation, while the CO adsorbed on ionic Pt participates in the CO oxidation along with the increasing temperature. The abundant metallic Pt sites introduced by the reduction pretreatment, especially the CO pretreatment, are responsible for the high activity in the low-temperature CO oxidation.

Discussion
The reduction assists the formation of the metallic Pt, which is responsible for the oxygen activation at the metal-support interface. This is related to the electron transfer from platinum to titanium, forming Pt-O-Ti 3+ sites, leading to more oxygen vacancy for the dissociative oxygen adsorption (Figure 3) [27]. The abundant adsorbed superoxide and peroxide produced on the catalyst surface participate in the Langmuir-Hinshelwood (L-H) mechanism at low temperatures. The oxygen activation capacity of Pt/TiO 2 is elevated by the CO and H 2 pretreatment. Moreover, in the reaction gas, the adsorbed oxygen on the catalyst surface assists the direct transition of bicarbonate intermediates to CO 2 , instead of forming carbonate, which could inhibit CO oxidation [44]. On the basis of the above considerations, the reductive pretreatment of Pt/TiO 2 catalyst by H 2 and CO enhances the catalytic performance over CO oxidation reaction.
CO adsorption is also influenced by the factors such as dispersion of Pt on TiO 2 support and the coordination number of Pt sites to O [34]. The reductive pretreatment at 200 • C in CO can not trigger the particle migration and coalescence that enlarge the particle size [14,29]. The Pt/TiO 2 after CO and H 2 pretreatment varies from pristine Pt/TiO 2 in valence state, presenting more metallic Pt sites. The reduction extent of H 2 and CO in this work is consistent with the results reported on Pt/CeO 2 -Al 2 O 3 catalyst [14]. CO pretreatment generates more metallic Pt with a low Pt-O coordination number than H 2 pretreatment. The strength of CO adsorption on metallic Pt is weaker than ionic Pt [36], and thus the CO on metallic Pt reacts with oxygen at lower temperatures [34], leading to a remarkable CO oxidation activity.

Materials
The PtCl 4 (99%) and TiO 2 (P25, 20 nm, 99%) were purchased from Macklin Inc. (Shanghai, China). All chemicals were used as received without further purification. The deionized water was generated from a Master-Q (Hitech, Shanghai, China) water purification system.

Catalyst Preparation
The 0.1 wt.% Pt/TiO 2 catalyst was synthesized by an incipient wetness impregnation method. The P25 TiO 2 powder (0.1 mol, 8 g) was dispersed in 5.5 mL deionized water by stirring to obtain a homogenous slurry. The slurry was dried at 120 • C for 6 h, then calcinated in a programmable furnace at 450 • C for 4 h with a heating rate of 5 • C/min. The TiO 2 was crushed and sieved to make 40-60 mesh granules. The 1.75 g/L platinum precursor solution was prepared by dissolving 5.25 mg PtCl 4 in 3 mL deionized water, and it was dropwise absorbed by 2.625 g TiO 2 granules under continuous stirring. The wet granules were dried at 120 • C for 6 h and then calcined at 400 • C for 4 h with a ramping rate of 5 • C/min to obtain 0.1 wt.% Pt/TiO 2 catalyst.
Reductive pretreatment of Pt/TiO 2 catalyst was performed in a tube furnace in 2 vol.% H 2 /Ar and in 2 vol.% CO/N 2 at 200 • C for 2 h to obtain Pt/TiO 2 -H 2 and Pt/TiO 2 -CO catalysts, respectively. After pretreatments, the catalysts were swept by N 2 for 15 min to remove the physically adsorbed gas molecules.

Catalysts Characterization
Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 X-ray diffractometer (XRD) using Cu Kα radiation (λ = 1.54178 Å). Micro-Raman spectra of catalysts were generated by a LabRAM ARAMIS micro-Raman spectrometer (HORIBA Scientifics, Kyoto, Japan) with an excitation laser of 532 nm. N 2 physisorption of the granular samples was conducted on a V-sorb 2800 P surface area and porosimetry analyzer (Gold APP Instruments), before which the samples were outgassed in vacuum at 120 • C for 2 h. Highresolution transmission electron microscopy (HRTEM) patterns were collected on an FEI (Hillsboro, OR, USA) Talos F200S and analyzed by the Gatan Digital Micrograph program. X-ray photoelectron spectra (XPS) were analyzed by a Thermo Fisher Scientific (Waltham, MA, USA) K-alpha, with the binding energy calibrated to C 1s at 284.8 eV.
The O 2 temperature-programmed desorption (O 2 -TPD), H 2 temperature-programmed reduction (H 2 -TPR), and CO pulse chemisorption were performed on an AutoChem II 2920 automated chemisorption analyzer equipped with a thermal conductivity detector (TCD) detector. A total of 0.1 g granular catalyst was packed into the reactor of the chemisorption analyzer. The pretreatment was performed in a He atmosphere at 200 • C for 60 min for the O 2 -TPD, H 2 -TPR, and the CO pulse chemisorption measurements. The O 2 adsorption was conducted at 50 • C for 60 min in 10 vol.% O 2 /He flow at a rate of 50 mL/min, and then the temperature program was carried out in He flow from 50 • C to 600 • C at a heating rate of 10 • C/min. A total of 10 vol.% H 2 /Ar was used in H 2 -TPR experiment, with a temperature ramping rate of 10 • C/min from 50 • C to 450 • C. The CO pulse chemisorption was carried out at room temperature to estimate the Pt diameter of the Pt/TiO 2 catalyst, and prior to the CO chemisorption, the sample was reduced by H 2 /Ar at 450 • C for 30 min.
The in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) for CO adsorption (i.e., CO + O 2 ) and CO oxidation (i.e., CO/O 2 ) was performed using a Thermo Fisher Nicolet iS20 Fourier transform infrared (FTIR) spectrometer equipped with a Mercury-Cadmium-Telluride (MCT) detector. Each spectrum was recorded for 8 scans at a resolution of 8 cm −1 . The sample was loaded into the Harrick reaction cell with ZnSe windows and purged at 200 • C in 50 mL/min N 2 for 30 min. A background spectrum was recorded at 200 • C before in situ DRIFTS measurements of the CO adsorption. A total of 2 vol.% CO/N 2 was passed through the catalyst bed for 10 min, followed by N 2 purging for 15 min, and by O 2 feeding for 6 min. In the CO oxidation DRIFTS measurements, the background spectra of Pt/TiO 2 , Pt/TiO 2 -H 2 , and Pt/TiO 2 -CO under N 2 flow were recorded at 150 • C, 135 • C, and 90 • C, respectively. As the reaction gas (0.7 vol.% CO, 16 vol.% O 2 , balanced with N 2 ) passed through the catalyst bed, the spectra were recorded at a temperature interval of 15 • C. For each temperature, 5 min was maintained to reach the equilibrium.

Catalytic Performance Tests
The CO oxidation was carried out in a stainless-steel tubular reactor (ID = 15 mm) encased by a tubular furnace with a thermocouple inserted in the reactor above the catalyst bed. A total of 2.5 g catalyst (40-60 mesh) was packed into the fixed bed reactor to generate a 17 mm-high catalyst bed. The gas hourly space velocity (GHSV) was 32,000/h as the flow rate of the reaction gases was 1600 mL/min. The large GHSV rules out the mass transfer limitation. Typically, the components of the reaction gases were 0.7 vol.% CO, 16 vol.% O 2 , 0 vol.%/ 0.01 vol.% NO, and N 2 for balance. The reactant gases were modulated by the mass flow controller and pre-mixed in a chamber before feeding into the reactor. The catalytic performance was evaluated by analyzing the exhaust gases using a GASMET, DX4000 FTIR gas analyzer.

Conclusions
The Pt/TiO 2 surface composition and textural properties were significantly modified after the reduction treatment in H 2 and CO at 200 • C. Reduction pretreatment improved the catalytic performance of Pt/TiO 2 in low-temperature oxidation by facilitating the formation of metallic Pt sites that can adsorb CO effectively. The unpretreated Pt/TiO 2 with insufficient metallic Pt sites adsorbed CO on Pt 2+ , which participate in the oxidation reaction at elevated temperatures. The carbonate species on the unpretreated catalyst is hard to convert to CO 2 , leading to low activity. CO pretreatment is more effective than H 2 pretreatment to trigger the reduction of Pt 2+ to Pt 0 with low coordination numbers, which exhibits the most remarkable CO adsorption and oxidation property at low temperatures. The metallic Pt sites triggered by the CO reduction are more stable upon oxygen exposure than H 2 , without transiting to Pt 2+ . The copious metallic Pt sites on the CO-pretreated Pt catalyst facilitate the production of active oxygen species (e.g., O 2 − ads ). The CO adsorbed on metallic Pt sites participate in low-temperature CO oxidation with the surface oxygen species via the L-H mechanism, presenting bicarbonate as reactive intermediates. As a result, the Pt/TiO 2 -CO catalyst exhibited high activity with complete oxidation of CO at 150 • C. Thus, it is tentatively concluded that Pt on TiO 2 is more activated with thermal pretreatment in an atmosphere of CO than H 2 .
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/catal11111280/s1, Figure S1: XPS narrow spectra of (a) O 1s and (b) Ti 2p of Pt/TiO 2 , Pt/TiO 2 -H 2 and Pt/TiO 2 -CO catalysts. Figure S2: The deconvoluted peaks of the IR spectra obtained after CO adsorption and N 2 purging at 24 min for (a) Pt/TiO 2 , (b) Pt/TiO 2 -H 2 and (c) Pt/TiO 2 -CO. Figure S3: The plots of CO-Pt 0 and CO-Pt 2+ peaks intensity to time in CO adsorption-desorption-oxidation. Figure S4. The peak intensity of CO 2 in the in situ DRIFT spectra of the CO adsorption-desorptionoxidation for Pt/TiO 2 , Pt/TiO 2 -H 2 , and Pt/TiO 2 -CO catalysts at 200 • C. Figure S5. The peak intensity of CO 2 , the integrated peak area of carboxylate and bicarbonate as a function of temperature for (a) Pt/TiO 2 , (b) Pt/TiO 2 -H 2 , and (c) Pt/TiO 2 -CO. Figure S6: (a) Influence of NO on CO conversion as a function of reaction temperatures, and (b) apparent activation energy (E a ) calculated by the Arrhenius equation of Pt/TiO 2 , Pt/TiO 2 -H 2 , and Pt/TiO 2 -CO catalysts (R = 8.314 × 10 −3 kJ/(mol·K)).