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MnO2 Promoted TiO2 Nanotube Array Supported Pt Catalyst for Formaldehyde Oxidation with Enhanced Efficiency Huayao Chen,† Minni Tang,†,‡ Zebao Rui,*,†,‡ and Hongbing Ji*,†,‡ †

Department of Chemical Engineering, School of Chemistry & Chemical Engineering, and The Key Lab of Low-Carbon Chem & Energy Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou 510275, P.R.China ‡ R&D Center of Waste-Gas Cleaning & Control, Huizhou Research Institute of Sun Yat-Sen University, Huizhou 516081, P.R. China

Downloaded by UNIV OF WINNIPEG on September 3, 2015 | http://pubs.acs.org Publication Date (Web): September 2, 2015 | doi: 10.1021/acs.iecr.5b01970

S Supporting Information *

ABSTRACT: Highly ordered pore-through TiO2 nanotube arrays (TiNT) prepared by an electrochemical anodization method were modified with MnO2 and used as the support for a Pt/MnO2/TiNT catalyst. The monolith-like Pt/MnO2/TiNT was then applied to low-concentration HCHO oxidation with enhanced efficiency. The effect of the MnO2 promotion on its performance for HCHO oxidation was studied with respect to the behavior of adsorbed species on the catalyst surface using in situ diffuse reflectance Fourier transform spectroscopy. In comparison with Pt/TiNT, Pt/MnO2/TiNT shows higher activity under parallel preparation and test conditions. A HCHO conversion of 95% with a more than 100 h stable performance is achieved over Pt/ MnO2/TiNT at 30 °C with a low 0.20 wt % Pt loading amount. The superior performance is related to the specific monolith-like structure and its confinement effect, metal−support interaction, and superior HCHO adsorption and storage properties of Pt/ MnO2/TiNT.

1. INTRODUCTION Formaldehyde (HCHO) is one of the most common toxic volatile organic compounds which pose a potential risk to human health even at a very low concentration level. Catalytic oxidation is one of the most effective and economically feasible technologies to remove low concentrations of HCHO.1 Many groups have been working on this subject, and significant progress has been achieved over the supported noble metal catalysts,2−15 especially supported Pt catalysts.2−12 It is found that the performance of a supported Pt catalyst is related to many factors, such as Pt loading amount,2,5 nanoscale features of Pt and metal−support interaction,5−7,9,10 adsorption and storage ability of the support,12,14 pore structure of the support, etc.8,9 The nanoscale features, i.e., dispersion and chemical states of Pt, and the metal−support interaction play an important role in the performance of a supported Pt catalyst, especially over the reducible metal oxide supported Pt catalyst.5−7,9,10 Huang et al.5 proposed that well-dispersed and negatively charged metallic Pt nanoparticles and rich chemisorbed oxygen were probably responsible for the high catalytic activity of Pt/TiO2. Zhang et al.7 attributed the high activity of the NaBH4 reduced Pt/TiO2 samples to the formation of an atomically dispersed Pt− O(OH)x−alkali-metal species on the catalyst surface. Chen et al.10 reported that Pt/P25 prepared by deposition precipitation and reduced with HCHO held dispersed metallic Pt nanoparticles with homogeneous and appropriate Pt particle size, rich chemisorbed oxygen, and high activity for HCHO oxidation. Another point is that the adsorption and storage properties of a catalyst are closely related to its performance considering that the content of HCHO in the practical application is very low, and diffusion would become an important step for the catalytic oxidation reaction at a low HCHO concentration. Benefiting from the understanding of the HCHO oxidation pathway, Shi et al.14 proposed a type of “storage−oxidation” cycling process for © XXXX American Chemical Society

the removal of HCHO in a single unit. In this process, HCHO was first partially oxidized and stored as HCOO− species. When the catalyst reached saturation, the stored HCOO− species were completely oxidized into CO2 and H2O by heating. The effect of HCHO adsorption and storage was further addressed in our recent work.12 A multifunctional Pt/ZSM-5 catalyst for the treatment of low concentration HCHO in air was developed, which could selectively trap HCHO molecules from the environment, efficiently store the adsorbed HCHO as HCOO−, and rapidly oxidize the intermediates to CO2 and H2O. Finally, the effect of mass transfer on the performance of a supported Pt catalyst needs also to be considered especially when the catalyst with high activity is applied.16 A structured catalyst with high surface and low pressure-drop is favorable.17−23 In our recent work,9 highly ordered pore-through TiO2 nanotube arrays (TiNT) prepared by an electrochemical anodization method was used to make up a monolith-like Pt/TiNT catalyst for the lowconcentration HCHO oxidation. In comparison with the commercial TiO2 powders supported Pt catalysts, Pt/TiNT presented better performance because of its specific monolithlike structure and confinement effect in Pt/TiNT. A HCHO conversion of 95% with a more than 100 h stable performance was achieved over Pt/TiNT at 30 °C with a 0.40 wt % Pt loading amount. Despite the great progress made for the catalytic removal of HCHO, there are still many obstacles for practical applications in indoor air quality control. A more efficient catalyst with a low noble metal loading is required. Against the background aforementioned, the aim of the present work is to improve the Received: May 28, 2015 Revised: August 22, 2015 Accepted: August 28, 2015

A

DOI: 10.1021/acs.iecr.5b01970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

10−7 Pa. Charging effects were corrected by adjusting the main C 1s peak to a position of 284.8 eV. In situ diffuse reflectance infrared Fourier transformed spectroscopy was carried out on an EQVINOX-55 FFT spectroscope apparatus (Bruker) equipped with a diffuse reflectance accessory and a MCT detector. The finely ground sample (ca. 10 mg) was placed in a ceramic crucible in the in situ chamber. The total gas flow rate was 100 mL/min. HCHO was bubbled into the chamber with helium (∼35% relative humidity). The spectra under reaction conditions were recorded after 64 scans with a resolution of 4 cm−1. 2.3. Catalytic Experiment and Adsorption Test. The catalytic oxidation of HCHO was performed in a quartz tubular (i.d. = 7 mm) fixed-bed reactor under atmospheric pressure with a homemade setup. Approximately 0.2 g of the catalyst was packed in the reactor. A simulated air stream (N2/O2 = 4; 100 mL/min) containing ∼50 ppm of HCHO and water vapor (∼35% relative humidity) was introduced as the reactants. Gaseous HCHO was generated by passing a stream of simulated air through a bubbler containing an HCHO solution (35 wt % HCHO). The gas hourly space velocity is 30 000 mL·h−1·g−1. HCHO concentration in the reactant or product gas stream was analyzed by phenol spectrophotometric method.10 The conversion of HCHO was calculated based on its concentration change. HCHO equilibrium adsorption amount (Qe, mg/g) over the TiNT or MnO2/TiNT at 30 °C was calculated by measuring the HCHO concentrations (mg/m3) of inlet gas (Cin) and outlet gas (Cout) with the same setup for the activity evaluation, as listed below.

performance of monolith-like Pt/TiNT for the efficient treatment of low-concentration HCHO by enhancing its HCHO adsorption−storage ability through surface modification with manganese oxide. Manganese oxides (MnOx) have been previously demonstrated to be one of the most effective metal oxides for HCHO degradation without releasing harmful byproducts.24,25 It is also regarded as an efficient support for Pt in HCHO oxidation.4,6 These previous findings in the literature indicate the special interaction between the MnOx and HCHO and encourage us to undertake further study. Herein, we provide a strategy to improve the performance of monolith-like Pt/TiNT catalyst for HCHO oxidation by surface modification with MnO2 at no cost of the advantages of the nanotube arrays. A high loading of MnO2 may block the nanotube, while a low amount of MnO2 cannot effectively modify the surface properties of the support. Finally, a 0.5 wt % MnO2 loading amount is chosen and an enhanced performance is obtained. A HCHO conversion of 95% with a more than 100 h stable performance is achieved over Pt/MnO2/TiNT at 30 °C with an extremely low Pt loading amount of 0.20 wt %. The mechanism leading to its high catalytic activity and stability is studied by various characterizations, especially in situ diffuse reflectance Fourier transform spectroscopy (DRIFTS), which is a powerful technology for the investigation of surface species under realistic conditions.

2. EXPERIMENTAL PROCEDURE 2.1. Catalyst Preparation. The procedure for the TiNT preparation by an electrochemical anodization method follows the well-established procedures in our previous study.22 The assynthesized TiNT was dried at 120 °C and subsequently calcined at 500 °C for 5 h with a heating rate of 10 °C/min in air. MnO2 modified TiNT (or MnO2/TiNT) was prepared by immersing TiNT with Mn(NO3)2·4H2O solution, containing an appropriate amount of Mn(NO3)2 to obtain a MnO2 loading amount of 0.50 wt %, which was confirmed by inductively coupled plasma-atomic emission spectrometry (ICP). The sample was then calcined at 500 °C for 5 h. The supported Pt catalysts were prepared via impregnation using the incipient wetness method. The support TiNT and MnO2/TiNT were separately dispersed into the H2PtCl6 solution (5.4 mg/mL, Alfa Aesar) with an appropriate volume to obtain a Pt loading amount of 0.08, 0.16, 0.20, or 0.40 wt %, which were confirmed by ICP. The samples were then dried at 120 °C overnight to evaporate the solvent and finally calcined at 400 °C for 4 h with a heating rate of 10 °C/min in air. The samples were then reduced in the H2 stream at 300 °C for 3 h before reaction and characterization. The as-prepared catalysts were denoted as Pt/TiNT and Pt/MnO2/TiNT, respectively, with a Pt loading amount indicated in front for simplicity. 2.2. Characterization. Metal nanoparticle size distribution was observed by a JEOL 2100F transmission electron microscopy (TEM) instrument. BET surface area of the catalyst was determined by N2 adsorption isotherms at 77 K, operated on ASAP 2020 adsorption equipment. The samples were degassed at 300 °C for 2 h in vacuum before N2 adsorption experiment. The phase purity and crystal structure of the samples were examined by X-ray diffraction (XRD) using a D-MAX diffractometer with Cu Kα radiation at a scanning rate of 10°/ min and a step size of 0.02°. The loading amount of Pt and MnO2 were confirmed by inductively coupled plasma-atomic emission spectrometry (ICP, TJ IRIS). X-ray photoelectron spectra (XPS) were recorded on a ESCALAB 250 spectrometer (Thermo Fisher Scientific, Al Kα, hν = 1486.6 eV) under a vacuum of ∼2 ×

Qe =

∫0

t

Q v(C in − Cout) W

dt

(1) 3

Here, t (min) stands for the adsorption time and Qv (m /min) and W (g) represent the total gas flow rate and the weight of the loaded sample, respectively.

3. RESULTS AND DISCUSSION 3.1. Structural Properties. Figure 1 depicts the top, bottom, and cross-sectional morphologies of the annealed MnO2/TiNT. As shown, the nanotube array structure of TiNT is wellmaintained after MnO2 modification, though the pore wall

Figure 1. SEM images of MnO2/TiNT: (a) top view, (b) bottom view, and (c) cross-sectional view. B


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becomes thick and the pore size decreases from ∼110 to ∼90 nm after modification in comparison with the TiNT morphology presented in the former study.9 The BET surface area of TiNT and MnO2/TiNT are 31.6 m2/g and 36.8 m2/g, respectively, which indicates that the modification process draws slight positive effect on the morphology of the monolith-like TiNT. Figure 2 displays the XRD patterns of TiNT, MnO2/TiNT, Pt/

characteristic peaks of MnO2 or Pt or PtO2 are too weak to be detected during the XRD characterizations for all the samples because of their small loading amount. All the samples are of anatase phase (JCPDS file 21-1272), indicating the MnO2 modification and the subsequent Pt loading process have no effect on the phase structure of TiNT. In other words, monolithlike TiNT has a fair structural stability, as systematically evaluated in a previous study.22 TEM images and Pt particle size distribution of Pt/TiNT and Pt/MnO2/TiNT with a Pt loading amount of 0.40 wt % are compared in Figure 3. It can be found that small and homogeneous Pt nanoparticles uniformly present on both catalysts. Approximately 500 Pt nanoparticles were calculated to obtain the histogram of metal particle size distribution for each sample. As shown in Figure 3b, a Pt nanoparticle size range of 1.0−7.0 nm was observed from Pt/TiNT, with an average value of 3.2 nm. A Pt nanoparticle size range of 1.0−6.0 nm with an average value of 2.6 nm was calculated from Pt/MnO2/TiNT, as listed in Figure 3d. Obviously, the introduction of MnO2 leads to a better Pt dispersion and a smaller average Pt particle size over Pt/MnO2/TiNT. 3.2. XPS Characterization. The XPS analysis was carried out to identify the surface elements chemical states, as shown in Figure 4 and listed in Table 1. Figure 4a shows that the binding energy (BE) negative shift of Pt 4f was observed over both catalysts, with a BE value of 70.2 and 70.8 eV for Pt/TiNT and Pt/MnO2/TiNT, respectively, in comparison with the BE of 71.2 eV for bulk metallic Pt.26 The electron transfer from TiO2 to Pt was proposed to be responsible for this negative shift.27 As

Figure 2. XRD patterns of TiNT, MnO2/TiNT, 0.40% Pt/TiNT, and 0.40% Pt/MnO2/TiNT.

TiNT, and Pt/MnO2/TiNT with a Pt loading amount of 0.40 wt % for all the supported Pt catalysts. As presented, the

Figure 3. TEM images and Pt nanoparticle size distribution of 0.40% Pt/TiNT (a,b) and 0.40% Pt/MnO2/TiNT (c,d). C


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Figure 4. XPS data for 0.40% Pt/TiNT and 0.40% Pt/MnO2/TiNT: Pt 4f (a), Ti 2p (b), O 1s (c), and Mn 2p (d).

Table 1. XPS Data for 0.40% Pt/TiNT and 0.40% Pt/MnO2/TiNT BE (eV)

a

surface atom ratio

catalysts

Pt 4f7/2

OII(OI)

Ti 2p

Mn 2p

OII/OIa

Pt/Ti

Mn/Ti

Pt/TiNT Pt/MnO2/TiNT

70.2 70.8

532.3(530.1) 532.0(530.0)

458.8 458.7

− 642.6

0.10 0.12

0.002 0.004

− 0.015

Calculated from the corresponding areas of fitted peaks calculated by XPSPEAK 4.1 with Shirley background.

presented, the introduction of MnO2 can moderate the negative shift to some extent. In other words, the surface modification of TiNT by the MnO2 layer weakens the interaction between Pt and TiO2 substrate. Figure 4b,c shows that the main peaks of O 1s (OI) and Ti 2p3/2 on the reduced catalysts are located at 530.0− 530.1 and 458.7−458.8 eV, respectively. The main peaks of O 1s (OI) and Ti 2p3/2 on the PtO/TiO2 catalyst were located at 530.3 and 459.0 eV, respectively.5 A BE negative shift for O 1s and Ti 2p3/2 over these reduced samples occurs. In addition, a significant shoulder peak of O 1s (OII) appears at 532.3 and 532.0 eV over Pt/TiNT and Pt/MnO2/TiNT, respectively. The chemisorbed oxygen (OII) can be activated on the metal−support interface, forming highly active oxygen species that are involved in the oxidation reaction. 5,10 The results show that the MnO2

modification has a significant influence on the surface Pt and active oxygen (OII) concentrations. For example, the Pt/Ti and OII/OI ratios are 0.002 and 0.10 for Pt/TiNT, while they are 0.004 and 0.12 for Pt/MnO2/TiNT, respectively. The Mn/Ti surface atom ratio over Pt/MnO2/TiNT is 0.015. The corresponding Mn 2p peak over Pt/MnO2/TiNT is found at 642.6 eV, as shown in Figure 4d, which can be assigned to Mn4+.28 Thus, it can be proposed that the MnO2 surface modification layer has been successfully introduced. 3.3. Catalyst Activity Test. The catalytic activities of TiNT, MnO2/TiNT, Pt/TiNT, and Pt/MnO2/TiNT are compared in Figure 5. As shown, the activity of Pt/MnO2/TiNT is higher than that of Pt/TiNT. When the Pt loading amount is 0.16 wt %, the initial HCHO conversions at 30 °C are 88% and 38% over Pt/ D


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oxidation. The intensities of these bands increase with increasing exposure time and reach a steady level after 60 min of exposure. A semiquantitative analysis by calculating the integrated area of the formate species band at 1570 cm−1 is given to illustrate the activity difference among these samples. As shown in Figure 6d, both the band intensities and the speed of intensity change with exposure time over 0.16%Pt/TiNT are the largest and reach a steady level in 30 min, while both are smallest over 0.16% Pt/ MnO2/TiNT during the whole 60 min on stream time. In situ DRIFTS spectra of O2/HCHO/He gas mixture adsorption as a function of time at 30 °C over Pt/TiNT and Pt/MnO2/TiNT with various Pt loading amount are listed in the Supporting Information (Figures S1 and S2). Similarly, four bands appear around 2062, 1657, 1560−1570, and 1359 cm−1 over both Pt/TiNT and Pt/MnO2/TiNT, and a small band at ∼1720 cm−1 appears over Pt/MnO2/TiNT. Their intensities vary with adsorption time and Pt loading amount. Intensities of the 1570 and 1720 cm−1 bands at various Pt loadings over Pt/ TiNT and Pt/MnO2/TiNT with an adsorption time of 60 min in O2/HCHO/He gas mixture were calculated and are shown in Figure 7. The band intensity of 1720 cm−1 over MnO2/TiNT, which is attributed to the adsorbed HCHO, decreases rapidly upon the loading of Pt, indicating the adsorbed HCHO is effectively transferred to formate species or the deep oxidation products in consideration with the good apparent activity of Pt/ MnO2/TiNT. The band intensity of 1570 cm−1, which is assigned to formate species, over both Pt/TiNT and Pt/MnO2/ TiNT first increases with increasing Pt loading amount and then declines rapidly after reaching a maximum value with further increase in Pt loading amount. 3.5. Discussion. The in situ DRIFTS study indicates that HCHO oxidation over the monolith-like Pt/MnO2/TiNT generally follows the mechanism developed in the previous study for Pt/TiNT.9 In combination with the O2 activation process or the involvement of the chemisorbed oxygen in the process, this process can be generally separated into two parts, i.e., storage step and oxidation step. In the storage step, HCHO is first adsorbed and reacts with the chemisorbed oxygen to form formate surface species and then decomposes into adsorbed CO species and H2O. In the oxidation step, the adsorbed CO species reacts with O2 or chemisorbed oxygen to produce gas-phase CO2, and the gas O2 is reactivated by the active sites to form chemisorbed oxygen. In this route, the storage of the HCHO as the surface formate species in the storage process and its further oxidation in the oxidation process were usually regarded as the crucial steps for HCHO oxidation.3,9,12 To further prove the proposed mechanism, in situ DRIFTS spectra of HCHO/He gas mixture adsorption as a function of time at 30 °C over 0.16%Pt/TiNT and 0.16%Pt/MnO2/TiNT were performed and are presented in the Supporting Information (Figure S3). It is found that four minor bands also appear at 2062, 1657, 1570, and 1359 cm−1, proving the adsorbed HCHO is converted into formate species under the function of chemisorbed oxygen. In addition, a band around 1720 cm−1 attributed to the CO group of adsorbed HCHO appears over 0.16%Pt/MnO2/TiNT and its intensity increases with exposure time, indicating the HCHO storage ability of the modified catalyst. The “storage” here refers to both the adsorbed HCHO and the accumulation of the key reaction intermediates formate species over the catalyst surface, which are formed under the reaction conditions followed by the adsorption of HCHO ability. As for the adsorption, HCHO equilibrium adsorption amount (Qe, milligrams per gram) over the TiNT (32 m2/g) and MnO2/

Figure 5. (a) Dependence of HCHO conversion on reaction temperature for TiNT, MnO2/TiNT, 0.16% Pt/TiNT, 0.16% Pt/ MnO2/TiNT, 0.20% Pt/TiNT, and 0.20% Pt/MnO2/TiNT; (b) longterm test for trace HCHO oxidation over 0.20% Pt/MnO2/TiNT at 30 °C.

MnO2/TiNT and Pt/TiNT, respectively. As the Pt loading amount increases up to 0.20 wt %, a (>) 95% HCHO conversion is observed over Pt/MnO2/TiNT. Figure 5b shows that 0.20% Pt/MnO2/TiNT holds a good stability over the 100 h test at 30 °C, with a HCHO conversion around 95%. These results indicate the good performance and great application potential of Pt/ MnO2/TiO2 catalyst for HCHO oxidation, and the structural properties of the TiO2 support modified by MnO2 affect the performance of the catalyst significantly. 3.4. In Situ DRIFT Study. In situ DRIFTS study was carried out to compare the different performance of MnO2/TiNT, Pt/ TiNT, and Pt/MnO2/TiNT for HCHO oxidation with respect to the behavior of adsorbed species on the catalyst surface. Figure 6 shows the dynamic changes in the DRIFTS spectra of the different catalysts as a function of time in a flow of O2/HCHO/ He at 30 °C. After exposing the catalyst to O2/HCHO/He mixture gas, four bands appear around 2062, 1657, 1560−1570 (referred to 1570 cm−1), and 1359 cm−1. According to previous studies, two strong bands at 1570 and 1359 cm−1 are ascribed to υa(COO) and υs(COO) on the active sites,29−31 which indicate that the adsorbed HCHO is converted into formate species and adsorbed by the catalysts. In addition, a band around 1720 cm−1 appears over the MnO2 modified catalysts, which is attributed to CO group indicating the adsorbed HCHO over the MnO2 modified surface.32,33 The weak band at 2062 cm−1 can be assigned to linear CO adsorbed on Pt, and the band at 1657 cm−1 is due to water adsorbed on the catalyst,3,14,33 which originates from the water in reactant gases or the products of the HCHO E


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Figure 6. In situ DRIFTS spectra of O2/HCHO/He gas mixture adsorption at 30 °C over (a) MnO2/TiNT, (b) 0.16% Pt/TiNT, and (c) 0.16% Pt/ MnO2/TiNT; (d) intensity of 1570 cm−1 peak vs time for different samples.

Supporting Information (Figure S4), MnO2/TiNT has a Qe of 0.97 mg/g, which is larger than that of TiNT (0.77 mg/g). In other words, MnO2/TiNT based catalysts have superior HCHO adsorption capacity over the TiNT based catalysts in the storage process. Upon loading Pt particles, the adsorbed HCHO can be transferred into formate species. In situ DRIFTS study shows that the amount of surface formate species over both Pt/TiNT and Pt/MnO2/TiNT varies with Pt loading amount and reaches a maximum with a Pt loading amount of 0.16%. The amount of formate species over the catalyst surface is determined by two factors, i.e., the formation rate from adsorbed HCHO and its decomposing rate, and the loading of metallic Pt is crucial for both processes. The overall effect of the two factors can increase or decrease the formate species amount, depending on their relative dominance. In combination with the activity test in Figure 5, we can propose that the low formate species amount over TiNT and MnO2/TiNT is due to the slow formate species formation rate. Both the formate species formation rate and decomposing rate increase with increasing Pt loading amount, and it seems that the formate species formation rate is higher when the Pt loading amount is lower than 0.16 wt %, which is the opposite of that when the Pt loading amount is higher than 0.16 wt %. As a result, a maxima formate species amount is observed at a loading of 0.16% over both Pt/TiNT and Pt/MnO2/TiNT. In comparison with 0.16% Pt/TiNT, the lower formate species amount and the higher apparent activity of 0.16%Pt/MnO2/ TiNT indicate its higher formate species decomposing rate. While the formate species amount over both 0.20% Pt/TiNT and 0.20% Pt/MnO2/TiNT is negligible, the higher HCHO conversion (>95%) of 0.20% Pt/MnO2/TiNT implies that both the formate species formation rate and decomposing rate

Figure 7. Intensities of the 1570 cm−1 (a) and 1720 cm−1 (b) bands at various Pt loadings over Pt/TiNT and Pt/MnO2/TiNT with an adsorption time of 60 min in O2/HCHO/He gas mixture.

TiNT (37 m2/g) at 30 °C with an influent HCHO concentration of 45 mg/m3 were measured and calculated. As shown in the F


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Pt/MnO2/TiNT catalyst. The introduction of MnO2 modification layer promoted the HCHO adsorption over the support surface, the dispersion of Pt particles, and the surface enrichment of Pt and chemisorbed oxygen. Finally, the monolith-like Pt/ MnO2/TiNT held an enhanced performance for HCHO oxidation in comparison with that of Pt/TiNT. A HCHO conversion of 95% with a more than 100 h stable performance was achieved over Pt/MnO2/TiNT at 30 °C with a low 0.2 wt % Pt loading amount. In summary, the MnO2 modification is an effective way to further improve the activity of monolith-like Pt/ TiNT catalyst for catalytic oxidation of formaldehyde.

over 0.20% Pt/MnO2/TiNT are rapid, and the performance difference between 0.20% Pt/TiNT and 0.20% Pt/MnO2/TiNT is due to the higher formate species formation rate over 0.20% Pt/MnO2/TiNT. The concentrated HCHO around Pt nanoparticles resulted from the storage by MnO2/TiNT support should be the reason for the promoted formate species formation step of Pt/MnO2/TiNT. At the same time, the introduction of MnO2 modification layer affects significantly the Pt dispersion, Pt chemical state, and adsorbed oxygen (OII) amount. Highly uniform dispersed metallic Pt nanoparticles with smaller size and richer surface Pt and OII are obtained for Pt/MnO2/TiNT in comparison with Pt/TiNT, which closely relate to the performance, especially the oxidation step of the supported Pt catalysts.5−10,34 All these factors together lead to the rapid formation and decomposing of formate species, and finally superior performance of Pt/MnO2/TiNT for HCHO oxidation even with a low Pt loading amount under mild conditions. By now, the complete oxidation of HCHO over Pt/MnO2/ TiNT catalysts is generally illustrated in consideration of both the reported reaction mechanism and the data in this work, as shown in Figure 8. First, HCHO is trapped by the monolith-like

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01970. Additional in situ DRIFTS spectra and the results of the adsorption test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was preliminarily supported by the Natural Science Foundation of China (21576298 and 21425627), the Science and Technology Plan Project (2013B090500029) and Natural Science Foundation (2014A030313135 and 2014A030308012) of Guangdong Province, and the State Key Laboratory of Chemical Resource Engineering (CRE-2015-C-301), China.

Figure 8. Reaction scheme for the catalytic oxidation of HCHO over the Pt/MnO2/TiNT catalyst.

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MnO2/TiNT because of confinement effects. The effects of confinement inside the nanotube structure include electronic interaction of the confined materials with the nanotubes, space restriction of the confined particles, enrichment of reactants inside the nanotubes, and diffusion inside the channels.35 As for the TiNT and MnO2 modified TiNT, the nanotube size is around 90 nm, which is much larger than the kinetic diameter of HCHO (∼0.243 nm 1). Thus, the diffusion phenomena should not have significant effect in this work. Here, the confinement of HCHO refers to the enrichment of HCHO inside the nanotube due to the interaction (or sorption) of HCHO with the inner surface of the nanotubes. This confinement effect makes up a concentrated HCHO reaction atmosphere, which is then stored over the surface of Pt/MnO2/TiNT as surface formate species under the function of Pt particles. Subsequently, deep oxidization of the chemisorbed species with O2 or chemisorbed oxygen on the Pt surface to produce gas-phase CO2 occurs. Both the storage ability and oxidation activity are crucial for the performance of the catalysts. The Pt/MnO2/TiNT developed in this work possesses both high HCHO storage ability and good oxidation activity, leading to its exceptional performance for the complete oxidation of HCHO to CO2.

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4. CONCLUSIONS Highly ordered pore-through TiO2 nanotube arrays were modified with 0.5 wt % MnO2 and used as the support for a G


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