Enhanced Formaldehyde Oxidation Performance of the Mesoporous TiO2(B)-Supported Pt Catalyst: The Role of Hydroxyls

As one of the crystal phases of titania, TiO2(B) was first utilized as a catalyst carrier for the oxidation of formaldehyde (HCHO). The mesoporous TiO2(B) loaded with Pt nanoparticles enhanced the HCHO oxidation reaction whose reaction rate was 4.5–8.4 times those of other crystalline TiO2-supported Pt catalysts. Simultaneously, Pt/TiO2(B) exhibited long-term stable HCHO oxidation performance. The structural characterization results showed that in comparison with Pt/anatase, Pt/TiO2(B) had more abundant hydroxyls, facilitating increasing the content of oxygen species. Studies on the role of hydroxyls in HCHO oxidation of Pt/TiO2(B) illustrated that synergistic involvement of terminally bound hydroxyls and bridging hydroxyls in HCHO oxidation accelerated the transformation from HCHO to formate via dioxymethylene. Moreover, hydroxyls could avoid the accumulation of excessive formate on Pt/TiO2(B) and promote the rapid oxidation of CO. Accordingly, the hydroxyl groups could accelerate each substep of formaldehyde oxidation, which enabled Pt/TiO2(B) to exhibit excellent formaldehyde oxidation performance.


INTRODUCTION
Improvement in the lives of the masses arouses great public concern about the importance of health. Formaldehyde (HCHO) is considered as one of the most representative pollutants that can cause several diseases. 1,2 Among developed treatments, the oxidation of HCHO into nontoxic H 2 O and CO 2 is an environmentally friendly and sustainable technology for HCHO removal. 3−5 To date, the supported noble metal catalysts have demonstrated more advanced catalytic performance compared with other types of HCHO oxidation catalysts. However, the high cost of noble metals severely limits the wide application of corresponding types of catalysts. Therefore, an attempt to promote the HCHO oxidation performance of catalysts is a feasible strategy for reducing the usage of noble metals. 6−8 In the development course of catalysts, the regulation of the catalytic support is a relatively facile strategy to improve the catalytic performance of catalysts. TiO 2 , Al 2 O 3 , CeO 2 , activated carbon, cellulose triacetate, and so on were chosen as the support to investigate their HCHO oxidation performances. 9−12 It was found that the HCHO oxidation mechanisms on catalysts with various supports were distinct from each other. On this basis, several aforementioned materials were compounded together to give full play to their advantages. 13,14 Moreover, the approaches of enlarging specific areas, constructing hierarchical pores, and modifying surface properties have been proved to promote the dispersion of noble metals, adjust the electronic structure of active sites, accelerate the transfer of reactants, or enrich the hydroxyls of the support. 7,15−17 Although great progress has been made in the past, the strategy on the support regulation still attracts increasing attention for enhancing the HCHO oxidation performance of the catalyst. TiO 2 is the most commonly used support in HCHO oxidation, which is usually regarded as the research benchmark in this area. 6,18,19 Anatase, rutile, and brookite are three crystal phases of TiO 2 . Jiang et al. comparatively studied the turnover frequency (TOF) value for HCHO oxidation of the rutile TiO 2 -supported Pt catalyst, which was five times as high as that of the anatase TiO 2 -supported counterpart. 20 However, most research studies were carried out on anatase or anatase-rutile mixed phases because the anatase TiO 2 with a large surface area was easy to be synthesized. As for the brookite TiO 2 , Yu et al. constructed hierarchical porous TiO 2 -supported Pt catalysts. They found that the presence of a small amount of brookite TiO 2 in the support made negligible contribution to the HCHO oxidation performance of the catalyst. 21 In addition to the aforementioned crystal phases, TiO 2 (B) represents the metastable phase of TiO 2 , which has the potential to be applied as a prospective lithium electrode, sensor material, and photocatalyst. 22−24 To the best of our knowledge, TiO 2 (B) has not been used as a catalyst support for HCHO oxidation until now. Compared to the three aforementioned TiO 2 polymorphs in structure that have only one type of oxygen atom (i.e., three-coordinated one), TiO 2 (B) features two, three, or four-coordinated oxygen atoms on the surface. 25−27 This leads to different surface properties between TiO 2 (B) and other crystalline phases of titania. Previous research studies have demonstrated that the adsorbed water molecules could readily dissociate into hydroxyls on the TiO 2 (B) surface. 25 It is well acknowledged that hydroxyl groups play a crucial role in the oxidation of HCHO by supported noble metal catalysts, such as promoting the adsorption and transfer of oxygen, assisting the catalyst in adsorbing HCHO, directly reacting with formate to generate CO 2 and H 2 O, and so on. 28−31 Accordingly, considering the unique properties of TiO 2 (B), it is urgent and meaningful to conduct related research on the TiO 2 (B)-supported catalyst for HCHO oxidation.
In the present work, mesoporous TiO 2 (B) was used as the support to load Pt nanoparticles (NPs) for synthesizing the HCHO oxidation catalyst (denoted as Pt/TiO 2 (B)). As expected, the Pt/TiO 2 (B) catalyst exhibited better HCHO oxidation performance than Pt NPs supported on anatase, rutile TiO 2 , and P25. The structural properties of various catalysts were systematically studied by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer−Emmett− Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), CO-diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS), and so on. Simultaneously, the structure−performance relationship discussion was supported by HCHO-DRIFTS analyses.

Overall Structural Properties of Catalysts.
The XRD pattern and Raman spectrum of the Pt/TiO 2 (B) catalyst are displayed in Figure 1. There are six obvious peaks at 2θ = 25.0°, 28.7°, 29.9°, 33.3°, 37.8°, 43.6°, and 48.5°, as shown from the XRD pattern in Figure 1a. These peaks are ascribed to the (110), (002), (−401), (310), (003), and (020) reflections of the TiO 2 (B) phase, respectively (PDF#35-0088). The characteristic peak of Pt species is not observed in the XRD pattern, suggesting the good dispersion of Pt NPs in the Pt/TiO 2 (B) catalyst. 32 It is acknowledged that the Raman technique is more sensitive than XRD in detecting the crystal information of TiO 2 (B). 33 In the Raman spectrum of Pt/TiO 2 (B) presented in Figure 1b, the main modes at 122, 197, 241, 248, 364, 407, 431, 468, 636, and 654 cm −1 are the direct expression of the corresponding characteristic Raman vibrations of the TiO 2 (B) phase. 26,33 The signal related to the anatase phase could not be observed in the Raman spectrum, which indicates that the support of Pt/TiO 2 (B) consists of pure TiO 2 (B) only. Figure 2 displays nitrogen adsorption−desorption isotherms and pore size distributions of Pt/TiO 2 (B). The catalyst Pt/ TiO 2 (B) has a type IV isotherm with a H3 hysteresis loop in a relative pressure p/p 0 range of 0.4−0.6, suggesting the mesoporous structure of the Pt/TiO 2 (B) catalyst. As shown in Figure S1, all control catalysts exhibited type IV isotherms and possessed mesoporous structures. 34 The inset in Figure 2 displays that the most probable pore size of the Pt/TiO 2 (B) catalyst is about 3.9 nm and its pore size distribution is relatively uniform. Furthermore, the detailed pore structural data of catalysts are shown in Table 1. The specific surface area of Pt/TiO 2 (B) is 53.0 m 2 g −1 , slightly higher than that of the control catalysts (40.1 m 2 g −1 for Pt/anatase, 30.6 m 2 g −1 for Pt/rutile, and 41.0 m 2 g −1 for Pt/P25). In addition, the pore volume of the Pt/TiO 2 (B) catalyst is 0.16 cm 3 g −1 , comparable to those of the control catalysts (0.14−0.40 cm 3 g −1 ).   Figure 3a shows the HCHO conversion of various catalysts as a function of reaction temperature. From 30 to 140°C, the HCHO conversion of Pt/TiO 2 (B) is invariably higher than those of anatase, rutile, and P25 TiO 2 -supported Pt catalysts. Note that the HCHO conversion of Pt/TiO 2 (B) can reach 65% at 30°C (i.e., room temperature), while those of the three control samples are only between 5 and 15%, indicating the excellent room-temperature HCHO oxidation performance of Pt/TiO 2 (B). As listed in Table 1, the corresponding HCHO oxidation rate of Pt/TiO 2 (B) is 4.5−8.4 times as high as that of Pt/anatase, Pt/rutile, and Pt/P25. Pt/TiO 2 (B) can achieve complete conversion of HCHO at 80°C under the present experimental conditions. By comparison, the reaction temperatures of HCHO complete conversion on all control catalysts are above 140°C. The HCHO conversion of Pt/anatase is slightly higher than that of Pt/rutile, but both of which are obviously lower than that of Pt/TiO 2 (B). Apparently, mesoporous TiO 2 (B) as the support has a significant advantage in catalytically oxidizing HCHO at room temperature, as confirmed by the superior reaction rate of Pt/ TiO 2 (B) to those in the reported literature (as seen in Table  1).

Formaldehyde Decomposition Performance.
In general, the pore structure of catalysts directly influences the transport of reactants/resultants and the dispersion state of active species during catalyst preparation. 21,45,46 Accordingly, the catalytic performance of Pt/TiO 2 (B) was preliminarily analyzed with the pore structure data of catalysts. The porous structural data in Table 1 show that the specific surface area of Pt/TiO 2 (B) is somewhat larger than those of Pt/anatase, Pt/ rutile, and Pt/P25, while the pore volume value of Pt/TiO 2 (B) is between them. According to the similar porous properties of Pt/TiO 2 (B) and control samples, it could be determined that the geometrical factor of Pt/TiO 2 (B) is not the main factor for its excellent room-temperature HCHO oxidation performance.
Moreover, it can also be concluded from the above data that the structural properties and catalytic performance between the anatase TiO 2 -supported Pt catalyst and rutile TiO 2 -supported one are different but not significant. The catalytic differences between them have been discussed previously. 20,47 Thus, in the following content, Pt/anatase is selected as the primary control sample for comparatively studying the unique structural and  catalytic characteristics of Pt/TiO 2 (B). Figure 3b displays the results of the long-term reaction of HCHO oxidation of Pt/ TiO 2 (B) and Pt/anatase. Generally, there is only slight fluctuation in HCHO conversion of Pt/TiO 2 (B) and no significant deactivation during the process of stability evaluation, which is consistent with the results of Pt/anatase. This manifests the excellent long-term HCHO oxidation performance of Pt/TiO 2 (B) at room temperature.  Figure 4c clearly shows that numerous dark spots are evenly dispersed in the Pt/TiO 2 (B) catalyst. One of the spots is randomly chosen to analyze its fringe patterns ( Figure 4d). The lattice fringe spacing is 0.23 nm, which matches with the (111) plane of metallic Pt. Hence, all the dark spots can be identified as the Pt NPs. The element mapping shown in Figure S3 also confirms the good dispersion of Pt NPs on the catalyst surface. Moreover, the lattice fringe spacing of the support nearby Pt NP is 0.35 nm, which is assigned to the (110) plane of TiO 2 (B), 22,26 further confirming the Raman and XRD analyses.
The statistical result of the Pt NP size of Pt/TiO 2 (B) inserted in Figure 4c shows that the most probable particle size of Pt NPs of Pt/TiO 2 (B) is ca. 1.8 nm, slightly smaller than that of Pt/anatase (2.2 nm, as seen in Figure S2). This indicates that Pt NPs can be well dispersed on both TiO 2 (B) and anatase, which is consistent with the above XRD analyses. The detailed structural properties of Pt NPs are further analyzed by CO-DRIFTS. Figure 5 displays that both Pt/ TiO 2 (B) and Pt/anatase have two distinct bands at 2056 and 2078 cm −1 , which are assigned to the linear adsorption of CO on step and terrace sites of Pt NPs, respectively. 48−50 The similar shape in CO-DRIFT spectra between Pt/TiO 2 (B) and Pt/anatase indicates their analogous morphology of Pt NPs, 51 which is consistent with the observation by HRTEM analyses. Furthermore, the degree of Pt dispersion could be calculated according to the sphere model, 52 and the detailed process is described in the Supporting Information. Based on this, the TOF values of catalysts can be obtained and those of Pt/ TiO 2 (B) and Pt/anatase are 4.4 × 10 −3 and 0.65 × 10 −3 s −1 , respectively. 53 The dramatically higher TOF value indicates that Pt/TiO 2 (B) possesses higher intrinsic activity in roomtemperature HCHO oxidation than Pt/anatase, which suggests the presence of different active sites for HCHO oxidation between Pt/TiO 2 (B) and Pt/anatase.
The analyses in chemical valence of Pt NPs of Pt/TiO 2 (B) and Pt/anatase are presented in Figure 6a. All the XPS Pt 4f spectra could be divided into three 4f7/2-4f5/2 doublets at binding energies of 71.1−74.4, 72.3−75.6, and 73.8−77.1 eV assigned to Pt 0 , Pt 2+ , and Pt 4+ species, respectively. 54,55 The content of corresponding species can be estimated by the area of the resolved peak, which is used to identify the chemical valent state of Pt NPs of the two catalysts. The percentage contents of Pt 0 , Pt 2+ , and Pt 4+ species of Pt/TiO 2 (B) are 10.3, 64.0, and 25.6%, respectively, while those of Pt/anatase are 37.7, 41.7, and 20.6%, respectively. Pt/TiO 2 (B) contains a higher content of oxidized Pt δ+ species than Pt/anatase does, especially for Pt 2+ .
As seen in Figure 7, temperature-programmed reduction (TPR) patterns of both Pt/TiO 2 (B) and Pt/anatase display two reduction stages from room temperature to 150°C and from 200 to 300°C, which are ascribed to the reduction of oxidized Pt species and Pt-support boundary species, respectively. 19,56 According to the reduction temperature of various species provided by TPR patterns, it is confirmed that all oxidized Pt species of two catalysts can be completely reduced to metallic Pt species under the preparation conditions. Moreover, the reduction temperature of Pt/ TiO 2 (B) is not dramatically different from those of Pt/anatase, which demonstrates that the interaction of Pt species with TiO 2 (B) is the same as that with anatase. Additionally, the XPS spectra of Ti 2p of the catalysts in Figure 6c show that the binding energies of Ti ions in both Pt/TiO 2 (B) and Pt/anatase are the same; that is, a similar state of oxygen vacancies around Pt species to the Pt/anatase should exist in the Pt/TiO 2 (B). 32 Thus, it is inferred that the presence of adsorbed species on Pt species may be the main reason for the different chemical valent states of Pt NPs between Pt/TiO 2 (B) and Pt/anatase, because the reduced catalysts inevitably come into contact with air and moisture during the transfer and preservation. It is required to comparatively analyze the state of adsorbed species on the surface of Pt/TiO 2 (B) and Pt/anatase. Figure 6b shows the XPS O 1s spectra of Pt/TiO 2 (B) and Pt/anatase. Both of them have three deconvoluted peaks at  and Pt/anatase as mentioned above can be explained.
The amount of oxygen species is generally considered to be one of the key factors directly influencing the HCHO oxidation performance of catalysts. 20,50,59 The large amount of oxygen species must be one of the pieces of evidence of highly efficient HCHO oxidation on Pt/TiO 2 (B). As for the increment in the amount of oxygen species of Pt/TiO 2 (B) relative to Pt/anatase, it is unlikely to be related to their dispersion states of Pt NPs as they have the same   morphological structure of Pt NPs according to the HRTEM and CO-DRIFTS analyses. Alternatively, the increasing content of oxygen species on Pt/TiO 2 (B) could be attributed to the possession of a large number of hydroxyls. With the hydroxyl-rich supports or in a high-humidity environment, the corresponding catalysts could achieve the markedly increased content of oxygen species. 28,31,60 It is well accepted that hydroxyls can facilitate the adsorption and diffusion of O 2 to active sites and consequently improve the oxygen activation. 30,61 In addition to assisting oxygen activation, the hydroxyls can enhance the adsorption ability of HCHO on the catalyst due to their good affinity to HCHO molecules by the formation of hydrogen bonds. 62,63 In addition, the hydroxyls themselves can participate in the HCHO oxidation process. Hong et al. found that under anaerobic conditions (where oxygen was absent), the reaction of HCHO oxidation can still be performed in the presence of moisture. 31 37,64 According to data analyzed above, although a high overall reaction rate of HCHO oxidation can be obtained on Pt/TiO 2 (B), it is still unclear which step of HCHO oxidation is improved. The understanding of this question is crucial for exploring the structure−activity relationship of catalysts and their further development. Herein, in situ HCHO-DRIFTS was used to detect the intermediate species of HCHO oxidation on the surface of various catalysts to study the differences in the reaction mechanisms between Pt/TiO 2 (B) and Pt/anatase.
In the initial stage, a mixed gas flow of HCHO, H 2 O, and N 2 is injected into the in situ chamber. As shown in Figure 8a, the bands at around 1361 and 1576 cm −1 are assigned to the symmetric stretching vibration and the asymmetric stretching vibration of formate. 37,65 Moreover, the symmetric vibration of the −CH 2 − group at around 2861 cm −1 corresponding to DOM can also be observed, 66 demonstrating that the HCHO decomposition process from HCHO to formate via DOM also occurs on Pt/TiO 2 (B), which accords with that on Pt/anatase (as seen in Figure 8b). It is noteworthy that the signal intensity of the CO band at 2073 cm −1 is very weak in various DRIFT spectra of Pt/TiO 2 (B), while the intensity of the CO signal on Pt/anatase is obviously strong. After N 2 purging in the second stage and O 2 purging in the third stage, no signal about other new species arises in the DRIFT spectra of Pt/TiO 2 (B) and Pt/anatase. Apparently, the appearance of the signal of all intermediate species in DRIFT spectra demonstrates that the oxidation of HCHO on Pt/TiO 2 (B) follows the reaction pathway of HCHO → DOM → HCOO − → CO → CO 2 , which is consistent with that of Pt/anatase. Subsequently, the state of hydroxyls of two catalysts is further analyzed by comparing the hydroxyl-related signals in HCHO-DRIFT spectra. The magnified DRIFT spectra in Figure 9 display that the bending vibrations of adsorbed water molecules (δ(H 2 O)) on Pt/TiO 2 (B) appear at a wavenumber of 1623 cm −1 , while the corresponding wavenumber of Pt/ anatase is 1652 cm −1 . The red shift in the wavenumber of δ(H 2 O) is attributed to the increased interaction of H 2 O molecules with the substrate. 25,68 This suggests that Pt/ TiO 2 (B) could have better affinity to water molecules than Pt/ anatase, which is beneficial to increasing the adsorption of water on Pt/TiO 2 (B), as also demonstrated by its obviously higher signal intensity of bending vibrations of δ(H 2 O). As mentioned above, H 2 O is liable to dissociate into hydroxyl on TiO 2 (B). 25 Hence, the characteristics of enhanced H 2 O adsorption and dissociation can endow Pt/TiO 2 (B) with a large number of hydroxyls, echoing with the above XPS analyses.

ACS Omega
http://pubs.acs.org/journal/acsodf Article Moreover, Figure 8 shows that the DRIFT spectra of both Pt/TiO 2 (B) and Pt/anatase have the negative peaks in the range of 3600−3800 cm −1 , which are assigned to the stretching vibration of hydroxyls. These negative peaks have been regarded as evidence of hydroxyl involvement in HCHO oxidation. 6,31,69 As clearly displayed in Figure 9b, there is one negative peak at 3669 cm −1 in this region of DRIFT spectra of Pt/anatase, corresponding to bridging hydroxyls. 68,70 This indicates that the bridging hydroxyl-involved HCHO oxidation mechanism is followed on Pt/anatase, in accordance with many previous reports. 6,13,15 Dissimilarly, the DRIFT spectra of Pt/TiO 2 (B) (as seen in Figure 9a) show two negative peaks at 3729 and 3664 cm −1 , ascribed to terminally bound hydroxyls and bridging hydroxyls, respectively. 68,70 It is confirmed that two types of hydroxyls participate in the HCHO oxidation on Pt/TiO 2 (B). Moreover, the intensity of terminally bound hydroxyls is obviously stronger than that of bridging hydroxyls, suggesting the dominant role played by the terminally bound hydroxyls in the HCHO oxidation of Pt/TiO 2 (B).
To the best of our knowledge (see the survey results in Table S1), the reports about the bridging hydroxyl involvement in the HCHO oxidation account for the majority, while very rare reports record the terminally bound hydroxyls or both. The type of hydroxyl as a factor influencing the HCHO oxidation performance receives little attention. Several studies have shown that the bridging hydroxyls interacted with HCHO to form DOM, and subsequently the terminally bound hydroxyls promoted the formation of formate from DOM through the Cannizzaro mechanism, and the latter reaction was considered to be thermodynamically and kinetically favorable. 66,67 This indicates that the synergetic involvement of bridging hydroxyls and terminally bound hydroxyls in HCHO oxidation is essential for elevating the reaction rates of HCHO → DOM → HCOO − . That is to say, the first two steps of HCHO oxidation can be accelerated over Pt/TiO 2 (B).
Further comparative analysis is conducted on the signal intensity of the other two key intermediate species (about 1580 cm −1 for formate and 2075 cm −1 for CO) as a function of experimental time, as plotted in Figure 10. During the initial stage of HCHO + H 2 O + N 2 injection, the signal ascribed to formate is gradually enhanced, indicating that the formate increasingly accumulates on the surface of the two catalysts. As reported previously, the formate decomposition was the slowest step in formaldehyde oxidation, so the signal of formate was the most obvious in HCHO-DRIFT spectra compared with other intermediate species, and its rate was considered to be the rate-limiting step of HCHO oxidation. 6,37 In the second and third stages, the signal intensity of formate of both catalysts decreases slightly, indicating difficulty in decomposing the formate under the condition of only oxygen but no hydroxyl. 31 Thus, hydroxyls are essential for the fast formate decomposition. By comparison, the signal intensity of formate of Pt/TiO 2 (B) is always weaker than that of Pt/ anatase during the whole experimental process, suggesting that  the formate is not easy to accumulate on Pt/TiO 2 (B). As mentioned above, the coparticipation of two hydroxyl groups in HCHO oxidation can accelerate the formation of formate. Therefore, the weak signal intensity of formate in DRIFT spectra of Pt/TiO 2 (B) could be caused by the rapid decomposition of formate rather than the slow generation.
Moreover, as seen in Figure 10b, the CO signal intensity of Pt/anatase increases slowly with the emergence of formate and then decreases after O 2 is introduced. The slow increase in CO signal intensity demonstrates the difficult decomposition of formate into CO. Additionally, the CO signal intensity begins to decrease with N 2 purge, corresponding to the oxidation of CO by oxygen species on Pt/anatase. This indicates that the further reaction of CO has actually proceeded from the first stage. After pure O 2 injection, the CO signal intensity decreases to a low value. Different from Pt/anatase, the CO signal of Pt/TiO 2 (B) has insignificant intensity at the first stage and instantly disappears upon stopping the HCHO injection. Obviously, the consumption rate of CO on Pt/TiO 2 (B) significantly exceeds the generation rate of CO from formate. This could be attributed to the existence of more oxygen species on Pt/TiO 2 (B) as analyzed by the above XPS measurement. Moreover, the hydroxyls on Pt/TiO 2 (B) can also contribute to the transport of CO species due to the weakened interaction between CO and the hydroxylated surface. 71,72 Based on the above analyses and discussion, the role of hydroxyls in HCHO oxidation of Pt/TiO 2 (B) is clarified. As shown in Figure 11, the presence of abundant hydroxyls is beneficial to oxygen activation for formation of more active species on Pt/TiO 2 (B). Moreover, two types of hydroxyls (bridging hydroxyl and terminally bound hydroxyl) are synergistically involved in the HCHO oxidation, which effectively improves the reactions of formaldehyde to DOM and DOM to formate with Pt as the active site. Additionally, hydroxyls also contribute to the fast decomposition of formate and consequently avoid the accumulation of excessive formate on Pt/TiO 2 (B). Finally, the formed CO is liable to be transported to active sites on the hydroxylated surface and rapidly oxidized by a large number of reactive oxygen species on the Pt. It is noteworthy that all the steps of HCHO oxidation are connected in series, and consequently the delay in any step will result in a decrease in the overall reaction rate of HCHO oxidation. By consecutively analyzing each HCHO oxidation step, it can be determined that the reaction rate of all steps of HCHO oxidation on Pt/TiO 2 (B) is accelerated, which explains the excellent catalytic performance of Pt/TiO 2 (B).

CONCLUSIONS
In the present work, a mesoporous TiO 2 (B)-supported Pt catalyst was prepared to first apply in the HCHO oxidation. The as-synthesized Pt/TiO 2 (B) catalyst not only exhibited superior HCHO oxidation performance to other crystalline TiO 2 -supported Pt catalysts, but also demonstrated excellent catalytic stability. The roles of hydroxyls in different steps of HCHO oxidation were analyzed successively, and it was found that all steps could be accelerated. The hydroxyls on Pt/ TiO 2 (B) significantly contributed to improving the HCHO oxidation performance.
Mesoporous TiO 2 (B) was synthesized according to the procedures as described in our previous work. 26,73 Briefly, potassium titanate (K 2 Ti 2 O 5 ) was preliminarily prepared by calcination from a mixture of K 2 CO 3 and TiO 2 ·nH 2 O in a Ti/ K molar ratio of 0.95 at 860°C for 4 h. Then, the K 2 Ti 2 O 5 powders were separately placed in an autoclave filled with CaCl 2 saturated solution for being treated at 180°C. After 24 h, the powders were ion-exchanged to remove potassium ions by HNO 3 aqueous solution (pH = 2). The collected powders were calcined at 500°C for 2 h to achieve the mesoporous TiO 2 (B). The content of potassium residual was below 0.2 wt % according to previous work, 22 most of which were distributed in the subsurface of mesoporous TiO 2 (B) due to no detection on potassium by XPS analyses.
Loading of Pt NPs (0.5 wt %) on mesoporous TiO 2 (B) was carried out by wet impregnation using H 2 PtCl 6 as the precursor. The impregnated powders were calcined in air at 500°C for 2 h and subsequently reduced in H 2 at 300°C for 2 h to obtain the Pt/TiO 2 (B) catalyst. For comparison, anatase TiO 2 , rutile TiO 2 , and P25 were used as control samples and their corresponding catalysts were denoted as Pt/anatase, Pt/ rutile, and Pt/P25, respectively.

Characterization.
The Raman spectrum of TiO 2 (B) was provided by a Horiba HR 800 spectrometer equipped with a CCD camera detector with the wavelength of excitation light of λ = 514 nm. The signal was sampled in the range from 100 to 800 cm −1 . Nitrogen adsorption−desorption isotherms at −196°C of various catalysts were measured on an ASAP2020 physisorption analyzer (Micromeritics, USA). The surface area (S BET ) of catalysts was calculated using the BET model, and the pore volume (V P ) was obtained by nitrogen adsorption with a relative pressure of 0.99. Pore size distribution was determined by the Barrett−Joyner−Halenda model. Nanoscale morphology of catalysts was provided by field-emission scanning electron microscopy (FESEM, JEM-7600 F) and high-resolution transmission electron microscopy (HRTEM, JEM-2010). X-ray photoelectron spectroscopy (XPS, ESCA-LAB 250) equipped with a monochromatic Al Kα X-ray beam was utilized to study the chemical state of various elements on the surface of the catalyst. The binding energies of elements were referenced to the C 1s peak (284.6 eV) of adventitious hydrocarbons. H 2 -TPR experiments were carried out in a TP-5000 micro fixed-bed flow system. Before the test, the catalyst powders (0.1 g) were pretreated in a flow of air (30 mL·min −1 ) at 500°C for 0.5 h followed by cooling to room temperature. The atmosphere of the system was switched to a mixed gas of 10% H 2 /N 2 . After stable flow for 0.5 h, the system started to be heated to 500°C at a heating rate of 10°C·min −1 . The H 2 consumption during the test was monitored by the TCD detector.

ACS Omega
http://pubs.acs.org/journal/acsodf Article In situ diffuse reflectance infrared Fourier transformed spectroscopy (in situ DRIFTS) was performed on a Nicolet-6700 spectroscope apparatus (Thermo Electron) equipped with a diffuse reflectance accessory and an MCT detector. Herein, the DRIFTS test contained HCHO-DRIFTS and CO-DRIFTS depending on different adsorbents. The operational processes of HCHO-DRIFTS were taken to investigate the reaction path of HCHO oxidation over various catalysts. The as-prepared catalyst powders were placed in the in situ chamber and pretreated at 300°C for 0.5 h in the atmosphere of nitrogen with a flowing rate of 30 mL·min −1 . After being cooled down to 30°C, the chamber was introduced with a mixed gas stream of HCHO + H 2 O + N 2 , which was produced by bubbling. After 1 h, pure N 2 and pure O 2 passed through the chamber sequentially to study the consumption and oxidation of different intermediates. The spectra under reaction conditions were recorded after 32 scans with a resolution of 4 cm −1 . Moreover, CO-DRIFTS was used to analyze the dispersion state of Pt NPs. The catalyst was in situ reduced in 10% H 2 /N 2 at 300°C for 0.5 h and cooled down to 30°C in an N 2 gas flow. Then, CO was introduced into the chamber. After 0.5 h, the excessive and weakly adsorbed CO was removed by being purged with a flowing pure N 2 stream. The spectra from 1900 to 2200 cm −1 were recorded after 32 scans with a resolution of 4 cm −1 .

Catalytic Evaluation.
The HCHO oxidation reaction was carried out in a fixed-bed flow reactor. Catalyst powders were added in a quartz tube with an inner diameter of 6 mm. The feed gas consisted of 180 ppm HCHO, H 2 O vapor (relative humidity of 35%), 20 vol % O 2 , and balance N 2 , produced by passing air stream through a bubbler containing a 37% HCHO aqueous solution in bath at 4°C. The gas hourly space velocity for each experiment was 54,000 h −1 . During the reaction, the reaction temperature of HCHO oxidation was controlled between 30 and 140°C by a temperatureprogrammed furnace. The HCHO concentrations of feed gas and exhaust gas were analyzed online by a gas chromatograph, which were synchronously checked by the CO 2 detector. No CO was detected in exhaust gas by the CO detector. The sample was analyzed after at least 30 min of stable reaction. HCHO conversion of various catalysts was calculated according to the change of HCHO concentration in the feed gas and exhaust gas.