TiO2 toward Room

Article pubs.acs.org/est

Enhanced Performance of NaOH-Modified Pt/TiO2 toward Room Temperature Selective Oxidation of Formaldehyde Longhui Nie,† Jiaguo Yu,*,† Xinyang Li,† Bei Cheng,† Gang Liu,‡ and Mietek Jaroniec§ †

State Key Laboratory of Advance Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China ‡ National Center for Nanoscience and Technology, Beijing 100190, China § Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio, 44242, United States . S Supporting Information *

ABSTRACT: Pt/TiO2 catalysts with various Pt loadings (0.05−2 wt %) were prepared by a combined NaOH-assisted impregnation of titania with Pt precursor and NaBH4-reduction. The thermal catalytic activity was evaluated toward catalytic decomposition of formaldehyde (HCHO) vapor in the presence of toluene under ambient conditions. HCHO could be selectively oxidized into CO2 and H2O over Pt/TiO2 catalysts and toluene had no change. Pt/ TiO2 catalysts prepared with the assistance of NaOH showed higher HCHO oxidation activity than those without NaOH due to the introduction of additional surface hydroxyl groups, the enhanced adsorption capacity toward HCHO, and larger mesopores and macropores facilitating diffusion and transport of reactants and products. The as-prepared Pt/TiO2 catalysts with an optimal Pt loading of 1 wt % exhibited high catalytic stability. Considering the versatile combination of noble-metal nanoparticles and supports, this work will provide new insights to the design of high-performance catalysts for indoor air purification.

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nation too. Similarly, Huang and his co-workers5,6 reported a complete oxidation of HCHO on the NaBH4-reduced Pt (0.1− 1%)-impregnated TiO2 catalysts. However, there is a need for a highly active catalyst that could completely oxidize HCHO at room temperature at high concentrations. In addition, selective HCHO oxidation in the presence of other VOCs on Pt/TiO2 has not been reported. It is well-known that alkali-modification of catalysts is an effective strategy for enhancing the activity in CO oxidation, methane dehydroaromatization, and cumene cracking.22−24 For example, Han et al.22 investigated the reactivity of nanoporous gold catalysts modified by alkali toward CO oxidation and found that hydroxyl ion adlayers (Au−OH−ads) formed on gold can promote catalytic activity. Furthermore, ZSM-5 zeolite modified by alkali was shown to have better catalytic activity due to the formation of additional mesopores as well as to improvement of catalytic reaction kinetics and mass transfer.23,24 Thus, a successful alkali modification of the aforementioned catalysts stimulated us to apply analogous modification for Pt/TiO2 catalyst used in the room-temperature thermal catalytic oxidation of HCHO. In this study, we first demonstrates that alkali modification of Pt/TiO2 catalyst has a significant impact on its room-temperature thermal catalytic performance for the

INTRODUCTION The indoor air quality is crucial for human health taking into account that people often spend more than 80% of their time in houses, offices, and cars. Volatile organic compounds (VOCs) are among the most abundant indoor air pollutants. Among VOCs, formaldehyde (HCHO) is a major pollutant and longterm exposure to HCHO may cause health problems such as nasal tumors and skin irritation.1 Room-temperature thermal catalytic oxidation is considered as the most promising strategy for the removal of HCHO due to its environmental-friendly reaction conditions and energy-saving consideration.2−6 It overcomes disadvantages of a relatively short lifetime of adsorbents2−4 and an extra apparatus and additional operating cost of high-temperature thermal catalytic oxidation7 and photocatalytic oxidation.8,9 In the thermal catalytic oxidation, a variety of supported noble metals like Pt and Au5,10−12 and transition metal oxides13−16 containing Ti, Pd, Cu, Ce, and Mn were used as the catalysts for HCHO oxidation. In general, the supported noble metal catalysts were shown to exhibit an excellent catalytic performance at relatively low temperatures. For instance, HCHO can be completely oxidized on Ru/CeO2 at 100 °C,17 Au/CeO2, and Au/iron-oxide at 75−80 °C18,19 and Pd−Mn/Al2O3 at 90 °C.20 Recent studies have shown that Pt/ TiO2 catalysts can oxidize HCHO to CO2 and H2O at room temperature.2−6 Zhang et al.2−4,11 reported that a Pt/TiO2 catalyst prepared by impregnation can completely oxidize HCHO. Tang et al.21 found that HCHO can be oxidized completely over Pt/MnOx-CeO2 catalyst prepared by impreg© XXXX American Chemical Society

Received: November 10, 2012 Revised: January 23, 2013 Accepted: February 25, 2013

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dx.doi.org/10.1021/es3045949 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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Table 1. Experimental Conditions for the As-Synthesized Samples and Their Physical Properties no.

RPt

adding NaOH

composition

color

SBET/ m2/g

Vpore/ cm3/g

dpore/ nm

Pt0 Pt0.05 Pt0.1 Pt0.5 Pt1 Pt2 Pt1-R

0 0.05 0.1 0.5 1 2 1

no yes yes yes yes yes no

TiO2 TiO2/Pt TiO2/Pt TiO2/Pt TiO2/Pt TiO2/Pt TiO2/Pt

white light gray light gray gray gray dark gray gray

47 45 42 41 38 34 46

0.10 0.24 0.23 0.24 0.19 0.20 0.11

8.6 21 22 24 20 23 9.3

glass box covered by a layer of aluminum foil on its inner wall at 25 °C. The experimental setup is shown in Figure S1 of the Supporting Information. Catalyst (0.3 g) was dispersed in 20 mL water under ultrasonic stirring. The as-prepared suspension was coated on the bottom of glass Petri dish with a diameter of 14 cm and dried in an oven at 80 °C for 1 h. The weight of the catalyst used for each experiment was kept at 0.3 g, and the samples were cooled to room temperature before measurement. After placing the sample-coated dishes in the bottom of reactor with a glass slide cover, 12 μL of condensed HCHO (38%) and 12 μL toluene solution (>99.5%) was respectively injected into the reactor and a 5 W fan was placed in the bottom of reactor in the whole reaction process. After 2 h, the HCHO/toluene solution was volatilized completely and the concentration of HCHO and toluene was stabilized. The analysis of HCHO, toluene, CO2, CO and water vapor was online conducted with a Photoacoustic IR Multigas Monitor (INNOVA air Tech Instruments Model 1412). The HCHO and toluene vapor was allowed to reach adsorption equilibrium within the reactor prior to catalytic activity experiment. The initial concentration of HCHO after adsorption equilibrium was controlled at about 105 or 253 ppm, which remained constant until the glass slide cover on the Petri dish was removed to start the catalytic oxidation reaction of HCHO/toluene. During the catalytic oxidation reaction, a near 1:1 ratio of carbon dioxide products to HCHO destroyed was observed, and the HCHO concentration decreased steadily with time. Each set of experiments was followed for about 60 min. The CO2 concentration increase (ppm, ΔCO2, which is the difference between CO2 concentration at t reaction time and initial time) and HCHO concentration decrease were used to evaluate the catalytic performance.

oxidation decomposition of HCHO with high concentrations, which was not achievable on Pt/TiO2 catalysts without this surface modification in the past. The prepared Pt/TiO2 catalysts in the presence of NaOH exhibit enhanced catalytic activity as well as better stability. Furthermore, it is found that surface hydroxyls can remarkably enhance room-temperature thermal catalytic activity of Pt/TiO2 toward HCHO oxidation.

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EXPERIMENTAL SECTION Preparation of Catalysts. Pt/TiO2 catalysts were prepared using a combined NaOH-assisted impregnation of titania with Pt precursor and NaBH4-reduction process. In a typical synthesis, 1 g of TiO2 (P25, Degussa) was added into an H2PtCl6 solution under magnetic stirring. After impregnation for 1 h, 2.5 mL of the mixed solution of NaBH4 solution (0.1 mol/L) and NaOH solution (0.5 mol/L) were quickly added into the suspension under vigorous stirring for 30 min. After reduction, the suspension was evaporated at 100 °C under stirring. Finally, the samples were dried at 80 °C for 6 h. The nominal weight of Pt was designated as RPt, which was 0, 0.05, 0.1, 0.5, 1, 2 wt %, respectively. The corresponding samples are denoted as Pt0, Pt0.05, Pt0.1, Pt0.5, Pt1, and Pt2 (shown in Table 1). For the purpose of comparison, 1% Pt/TiO2 catalyst (denoted as Pt1-R) was also prepared by the above process without NaOH addition in NaBH4 solution. Characterization. Pt/TiO2 catalysts were analyzed by a D/ Max-RB X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation at a scan rate (2θ) of 0.05° s−1. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected on a JEM-2100F microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on VG ESCALAB210 with Mg Kα source. All binding energies (BE) were referenced to the C 1s peak at 285.0 eV of the surface adventitious carbon. The Brunauer−Emmett−Teller (BET) surface area (SBET) of powders was evaluated from nitrogen adsorption data recorded by using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All of the samples were degassed at 180 °C prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint method using adsorption data in the relative pressure (P/P0) range of 0.05−0.3. The pore size distributions were determined using desorption data by the Barret-Joyner-Halender (BJH) method. The single-point pore volume was obtained from nitrogen adsorption volume at the relative pressure of 0.97. The relation between the surface area, pore volume and pore width for cylindrical pore model was used to estimate the average value of the latter. Fourier-transform infrared (FTIR) spectra were collected using a Shimadzu IRAffinity-1 FTIR spectrometer in the frequency range of 4000−500 cm−1. Catalytic Activity Test. The room-temperature thermal catalytic oxidation of HCHO was performed in a dark organic

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RESULTS AND DISCUSSION Phase Structures and Morphology. XRD was used to investigate the phase structure and crystallite size of the prepared samples. Figure S2 of the Supporting Information shows the XRD patterns of the Pt0 and Pt1 samples. All of the peaks can be assigned to anatase (JCPDS, No. 21−1272) and rutile phases (JCPDS, No. 21−1276) of the Pt0 and Pt1 TiO2 samples. No diffraction peaks of Pt (the most strongest peak at 39.8°) was observed in the case of Pt1 due to low loading amount of Pt, small particle size and good dispersion.5,25 The average crystallite size of anatase and rutile phases of in Pt0 and Pt1, calculated using Scherrer’s equation (d = Kλ/Bcosθ, where d is the crystal size, B is the width in radians of the peak, K is equal to 0.89, θ is the Bragg angle, λ is the X-ray wavelength), from the main diffraction peaks of anatase (101) and rutile (110), are about 22.2 and 44.1 nm for Pt0 and 22.4 and 44.2 nm for Pt1, respectively. TEM and HRTEM (as shown in Figure 1) analyses confirmed the presence of Pt nanoparticles, and their particle size and dispersion status on the surface of TiO2. Parts a and b of Figure 1 display that many small Pt nanoparticles are uniformly deposited on the surface of B



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their respective photoelectron peaks appear at binding energies of 458.8 (Ti 2p), 529.8 (O 1s), 70.6 (Pt 4f), 1071.9 (Na 1s), 192.4 (Cl 2p), and 285.0 eV (C 1s). Sodium and chlorine species originate from the residual Na+ and Cl − because of using NaOH, NaBH4 and H2PtCl4 reactants, respectively. High-resolution XPS spectra of Pt 4f, Ti 2p and O 1s regions are shown in Figure 2. The high-resolution Pt 4f spectra of Pt1 and Pt1-R (part a of Figure 2) show two peaks at 70.6 and 74.0 eV, which can be assigned to Pt 4f 7/2 and Pt 4f5/2 of metallic Pt, respectively.27−29 It is known that the Pt 4f 7/2 binding energies (BE) of Pt0, Pt2+, and Pt4+ are around 71.2, 72.4, and 74.2 eV, respectively. A negative shift for Pt 4f 7/2 is seen as compared to the binding energy of Pt 4f 7/2 of bulk metallic Pt0 (71.2 eV).30 This is due to the fact that the electron transfer from TiO2 to Pt can result in this negative shift due to a strong metal−support interaction (SMSI).5,31,32 For a number of HCHO catalytic oxidation reactions, metallic Pt can act as reactive sites and provide more sites than other oxidation states of Pt like PtO.5 A significant negative shift is also observed for Ti 2p in part b of Figure 2 and O1s in part c of Figure 2 when Pt was deposited on TiO2 by NaBH4-reduction process. The measured binding energies of Ti 2p3/2 and O 1s for Pt0 are 459.3 and 530.5 eV, respectively. As compared to Pt0, the binding energies of Ti 2p3/2 for the Pt1 and Pt1-R samples are shifted to 458.6 and 458.8 eV, respectively. The binding energies of O 1s of TiO2 lattice oxygen (Ti−O−Ti) for the Pt1 and Pt1-R samples are also shifted to 529.8 and 530.1 eV, respectively. One possible explanation is that TiO2 is partially reduced into Ti3+, which results in the negative shift of binding energies of Ti 2p3/2 for the Pt1 and Pt1-R samples.33 Meanwhile, oxygen vacancies are generated at the metal−support interface of Pt/TiO2 during the reduction process, and oxygen from the gas phase could be dissociatively adsorbed on such defects, thus

Figure 1. TEM images (a,b) of the Pt0.1 (a) and Pt1 (b) samples and high-resolution TEM image (c) of Pt1.

TiO2 with a high dispersion. The size of Pt nanoparticles is around 1 and 2−3 nm for Pt0.1 and Pt1, respectively. Highresolution TEM image of Pt1 (part c of Figure 1) shows that the lattice spacing in white circle is 0.224 nm, consistent with the lattice spacing of (111) planes of metallic Pt.26 The energydispersive X-ray (EDX) spectrum (not shown here) further confirms that the Pt1 sample contains titanium, oxygen, platinum, sodium, and chlorine elements. XPS Analysis. XPS survey spectrum of Pt1 (not shown here) indicates the presence of Ti, O, Pt, Na, Cl, and C elements and

Figure 2. High-resolution XPS spectra for Pt 4f (a) of the Pt1 and Pt1-R samples, Ti 2p (b), and O1s (c) of the Pt0, Pt1-R, and Pt1 samples. Schematic diagram of electron transfer on TiO2 with adsorbed hydroxyl species (d). C



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increase in the pore volume and average pore size of Pt/TiO2 catalysts with adding NaOH during the preparation may originate from the good dispersion of P25 and the preparation process for Pt/TiO2 catalysts. Degussa P25 was prepared by a gas-phase flame synthesis method with high dispersion. To prepare titania-supported Pt, P25 was dispersed in NaOH aqueous solution. Hydroxyl groups in the solution could be chemisorbed onto the surface of P25. After evaporating the water, the paste was dried in air. During the drying process, particle aggregation would occur by interparticle dehydration along with the formation of hydrogen bonds between hydroxyl groups, which finally result in larger porosity. The overall process is illustrated in Figure S5 of the Supporting Information. The resulting large mesopores and macropores are expected to benefit the diffusion of the reactants and products during the oxidation of HCHO.24 Evaluation of Catalytic Activity. Selective Oxidation of HCHO. Two model pollutants (105 ppm formaldehyde and 315 ppm toluene) were injected into the same reactor with Pt/TiO2 catalyst (Pt1) at room temperature. When the glass slide cover on the dish was removed, these pollutants were exposed to the catalyst and the oxidation reaction occurred. The concentrations of formaldehyde and toluene as a function of time are shown in part a of Figure 3. As can be seen from this figure, the concentration of formaldehyde decreased with reaction time, but the concentration of toluene was almost unchanged. These results demonstrate highly selective oxidation of formaldehyde over Pt/TiO2. The concentrations of the resulting CO2 and CO

resulting in the decrease of binding energies of O 1s of TiO2 lattice oxygen (Ti−O−Ti).5,33−35 Besides the Ti−O−Ti peak, a significant shoulder peak from Ti−OH is observed at 532.1 eV for Pt1 and Pt1-R,36,37 and the surface atomic ratio of Ti−OH to Ti−O−Ti for the Pt1 sample is larger than that of Pt1-R. The FTIR spectra (Figure S3 of the Supporting Information) of the Pt1 and Pt1-R samples also indicate the presence of Ti−OH, and two strong absorption bands centered around 3440 and 1626 cm−1 are attributed to the stretching and bending vibrations of Ti−OH, respectively. Further observation indicates that the Pt1 sample contains more surface hydroxyl groups than the Pt1-R sample. Why does the sample Pt1 contain more surface hydroxyls than Pt1-R? This is easy to understand because Pt1 was prepared under alkaline conditions, thus more OH groups would be chemisorbed on the surface of TiO2. Because the surface OH groups have many lone electron pairs, these electrons would transfer from O of OH groups to Ti and O of TiO2 (part d of Figure 2) inducing a significant negative shift in the binding energies for Ti 2p and O1s. Furthermore, this negative shift is enhanced with increasing surface atomic ratio of Ti−OH to Ti− O−Ti, as confirmed by more negative binding energies of Ti 2p and O1s (Ti−O−Ti) of Pt1 than those of Pt1-R. Nitrogen Adsorption. Figure S4 of the Supporting Information shows nitrogen adsorption−desorption isotherms and the corresponding pore size distribution curves (insert) for the Pt0 and Pt1 samples. Nitrogen adsorption−desorption isotherms for the above two samples are of type IV with a hysteresis loops according to International Union of Pure and Applied Chemistry (IUPAC) classification38 indicating the existence of mesopores. The adsorption branches of two isotherms resemble type II, suggesting the presence of macropores. The shapes of loops resemble type H3 in a high relative pressure (P/P0) range of 0.8−1.0, indicating the presence of slitlike pores. The isotherms show high adsorption values at relative pressures approaching 1.0, which is typical for materials with large mesopores and macropores.39 As compared to Pt0, there is a dramatic increase in the amount of adsorbed N2 on Pt1 at high relative pressures, 0.9−1.0, implying the larger pore volume for the later. The pore-size distributions (insert of Figure S4 of the Supporting Information) calculated from the desorption branch of the nitrogen isotherms by the BJH method are in the range of 2−100 nm with a peak at the pore diameter of about 40 nm for Pt0 and 30 nm for Pt1, further confirming the presence of mesopores and macropores. Furthermore, a dramatic increase in the pore volume for Pt1 is observed in comparison to Pt0. The BET surface area (SBET), pore volume (Vpore), and pore size (dpore) of Pt/TiO2 catalysts together with those for pure TiO2 are listed in Table 1. As can be seen from this table, the specific surface areas of Pt/TiO2 catalysts, exposed to NaOH during preparation, decrease with increasing Pt loadings, and the pore volume and average pore sizes of the aforementioned samples increase almost twice in relation to those of Pt0 and Pt1R. The observed reduction in the specific surface area of Pt/TiO2 catalysts is probably caused by two reasons: (i) The density of the Pt/TiO2 samples increases with increasing Pt loading due to the larger density of Pt (21.45 g/cm3) than TiO2 (3.84 g/cm3, anatase); note that the specific surface area is expressed per gram of the sample; (ii) Pt nanoparticles and Na+ from NaBH4 and NaOH could cover the surface of TiO2 and partially block pores, leading to the observed decrease in the surface area.5,40 The smaller specific surface area of Pt1 than that of Pt1-R is also observed, mainly due to more Na+ species from NaOH covering the surface of TiO2 and partially blocking pores. A dramatic

Figure 3. (a) Concentration changes of formaldehyde and toluene as a function of reaction time for Pt1 catalyst, (b) ΔCO2 and ΔCO concentration dependence on reaction time for Pt1 catalyst. (ΔCO is defined in the same way as ΔCO2). D



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are also shown in part b of Figure 3. This figure shows that the concentration of CO2 increases with reaction time, and the concentration of CO does not change implying that HCHO can be selectively oxidized in the presence of toluene and the dominant product is CO2. Effect of Adding NaOH. The catalytic performance of 1 wt % Pt/TiO2 catalysts without (Pt1-R) and with addition of NaOH during the preparation (Pt1) is shown in Figure 4. As can be seen

bond interactions between HCHO and hydroxyl groups present on TiO2.41 The catalytic oxidation activity of Pt1 toward HCHO can be understood by the following suggested mechanism (Figure 5) based on the previously reported results.11,42 In the

Figure 5. Proposed mechanism for the enhanced oxidation of HCHO over Pt/TiO2 modified by NaOH.

case of catalytic oxidation of HCHO over Pt/TiO2,11 HCHO ,and O2 are first adsorbed onto TiO2 and Pt surface (step 1), respectively. After that, HCHO is oxidized to formate species at the surface (step 2). The surface formate species are then directly decomposed into adsorbed CO species and H2O (step 3), and CO species finally reacts with O2 to generate gaseous CO2 (step 4). Lavalley et al.42 also reported the formation of dioxymethylene, formate, and methoxide species after adsorption of formaldehyde on pure titania (anatase) at 300 K. However, for Pt supported on TiO2, formate and CO species were confirmed by others as the main intermediates.11 This implies that dioxymethylene and methoxide are easily oxidized to formate and further CO species in the presence of Pt. Since the Pt1 sample was synthesized under the basic conditions, the surface of the resulting Pt1 has numerous hydroxyl groups which facilitate HCHO adsorption. As compared to Pt1-R, a larger amount of HCHO was adsorbed on the surface of Pt1 and could be oxidized into formate, CO species, and finally CO2. The underlying mechanism mediated by hydroxyl-groups is presented in Figure 5. Meanwhile, the presence of large mesopores and macropores in Pt1 (confirmed by adsorption studies) facilitate diffusion of the reactants and products during HCHO oxidation. Thus, Pt1 showed higher room-temperature oxidation capacity toward HCHO than Pt1-R. Huang et al.6 also reported that the surface hydroxyls (Ti−OH) were very important in the oxidation of HCHO over Pt/TiO2 and their higher concentration was favorable for the enhancement of catalytic activity. Effect of Pt Loading. The effect of Pt loading on the oxidation activity of the prepared samples toward HCHO decomposition was also investigated (Figure 6). Figure 6 and Figure S8 of the Supporting Information show formaldehyde/ΔCO2 concentration dependence on time for the catalysts with different Pt loadings. For Pt0, the reduced HCHO was mainly adsorbed on TiO2 and could not be oxidized into CO2. In the case of Pt supported on TiO2, HCHO can be oxidized into CO2 at different reaction rate over the Pt/TiO2 catalysts. In the range of 0.05−1 wt % Pt loadings, the decreased rate of HCHO concentration and the increased rate of CO2 concentration increase with increasing Pt loading. When the Pt loading reaches 2 wt %, no further increase in oxidation activity is observed. This is because a further increase in the Pt loading would result in the aggregation of Pt nanoparticles and growth of Pt grains during the preparation, thus there is no obvious increase in the reactive sites on Pt.

Figure 4. Change in the formaldehyde concentration as a function of reaction time for Pt1 and Pt1-R samples.

from this figure, the HCHO concentration decreases with increasing reaction time for both Pt1 and Pt1-R, but the HCHO concentration on Pt1 decreases faster than that on Pt1-R; namely, the HCHO concentration after 60 min decreased from 253 to 15 and 59 ppm for the Pt1 and Pt1-R samples, respectively. Accordingly, the CO2 concentrations achieved within 60 min were about 379 and 276 ppm for Pt1 and Pt1-R, respectively (shown in Figure S6 of the Supporting Information). The above results indicate that Pt1 has higher catalytic activity than Pt1-R in HCHO oxidation at room temperature. To rationalize the observed difference, HCHO adsorption test was performed on P25 and P25-NaOH (prepared under similar conditions as Pt1, but without adding H2PtCl6 solution) and the results are shown in Figure S7 of the Supporting Information. This figure shows that the HCHO concentration decreases quickly in the initial 5 min period and then gradually increases in the subsequent 55 min for both P25 and P25-NaOH. In contrast, the CO2 concentration was almost unchanged in the whole process indicating that HCHO was mainly adsorbed on TiO2. A rapid decrease in the HCHO concentration is due to the HCHO physical adsorption on P25 and P25-NaOH; however, the slow increase in the HCHO concentration during subsequent 55 min is due to the HCHO desorption from the catalyst surface. Also, Figure 4 and Figure S6 of the Supporting Information indicate that the observed increase in the CO2 concentration is slightly larger than the observed decrease in the HCHO concentration. This observation is easy to understand because of desorption of some HCHO molecules from the reactor surface during experiment, and subsequent their oxidation to CO2, which result in the increase of CO2 concentration. At the same time, one can see that the HCHO concentration decreased faster for P25NaOH than for P25 in the first 5 min, and the HCHO concentration after 60 min was lower for P25-NaOH than that for P25. The above results suggest that the HCHO adsorption capacity for P25-NaOH is higher than that for P25, and the basic TiO2 surface favors HCHO adsorption due to strong hydrogenE



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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 863 Program (2012AA062701), 973 Program (2013CB632402), NSFC (51072154, 21177100, and 51272199), the National Science Fund for Postdoctoral Scientists of China (Grant No.2012M511292), Fundamental Research Funds for the Central Universities and Self-determined and Innovative Research Funds of SKLWUT.

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Figure 6. Change in the formaldehyde concentration with reaction time over Pt/TiO2 catalysts with different Pt loadings.

Therefore, the optimal Pt loadings is 1 wt % for Pt/TiO2 catalysts. Catalytic Stability. The stability of catalysts and their efficiency are also very important in their practical applications. In order to investigate the stability of Pt/TiO2 in oxidative decomposition of HCHO, the room temperature oxidation was repeated fourteen times within 7 days (two times a day) over Pt1 and the results are shown in Figure 7. The oxidation rate of

Figure 7. Change in the formaldehyde concentration with reaction time over recycled Pt1 catalyst in 14 repeated tests within 7 days (2 times a day).

HCHO shown in Figure 7 and the generation rate of CO2 shown in Figure S9 of the Supporting Information over Pt1 for 14 repeated cycles do not change as compared with those obtained in the first-cycle indicating that the Pt1 catalyst can maintain a stable and efficient catalytic performance.

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ASSOCIATED CONTENT

S Supporting Information *

Additional Figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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