Active Site-Directed Tandem Catalysis on Single Platinum

(32) No typical peaks of Pt can be detected for the samples deposited with Pt, ..... catalyst of our study may be applicable to the oxidation of other...
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Active Site Directed Tandem Catalysis on Single Platinum Nanoparticles for Efficient and Stable Oxidation of Formaldehyde at Room Temperature Mengmeng Huang, Yingxuan Li, Mengwei Li, Jie Zhao, Yunqing Zhu, Chuanyi Wang, and Virender K. Sharma Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01176 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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Active Site Directed Tandem Catalysis on Single Platinum Nanoparticles for

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Efficient and Stable Oxidation of Formaldehyde at Room Temperature

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Mengmeng Huang,† Yingxuan Li,†* Mengwei Li,† Jie Zhao,† Yunqing Zhu,† Chuanyi Wang,†

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Virender K. Sharma‡*

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†School

of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China

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‡Program

for the Environment and Sustainability, Department of Occupational and

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Environmental Health, School of Public Health, Texas A&M University, College Station, Texas

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77843, USA

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*To whom correspondence should be addressed.

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Email: [email protected] (Y. Li) and [email protected] (V.K. Sharma)

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ABSTRACT: The application of tandem catalysis is rarely investigated in degrading organic

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pollutants in environment. Herein, a tandem catalyst on single platinum (Pt) nanoparticles (Pt0

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NPs) is prepared for sequential degrading formaldehyde (HCHO) to carbon dioxide gas (CO2(g))

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at room temperature. The synthesis approach includes coating of uniform Pt NPs on SrBi2Ta2O9

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platelets using a photoreduction process, followed by calcination of the sample in atmosphere to

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tune partial transformation of Pt0 atoms to Pt2+ ions in the tandem catalyst. The conversion of

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HCHO to CO2(g) is monitored by in situ Fourier transform infrared spectroscopy, which shows

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first conversion of HCHO to CO32- ions onto Pt0 active sites and subsequently the conversion of

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CO32- ions to CO2(g) by neighboring Pt2+ species of the catalyst. The later process by Pt2+ species

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do not allow CO32- poisoning of the catalyst. The enhanced activity of the prepared tandem

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catalyst to oxidize HCHO is maintained continuously for 680 min. Comparatively, the catalyst

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without Pt2+ shows the activity only for 40 min. Additionally, the tandem catalyst presented in

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this paper has better performance than Pt/titanium dioxide (TiO2) catalyst to degrade HCHO.

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Overall, the tandem catalyst may be applied to degrade organic pollutants efficiently.

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INTRODUCTION

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Formaldehyde (HCHO) is widely recognized as a harmful indoor volatile organic

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compound because it is commonly released from various building materials, furnishings, and

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consumer products, causing serious health problems such as eye irritation, respiratory tract

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irritation, nasal tumors, and even cancer.1,2 To date, various strategies have been shown to

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remove HCHO at room-temperature, which included physical adsorption, plasma technology,

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and thermal catalytic oxidation.3−9 The application of oxidation processes using catalysts must

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have practical aspects such as need of low energy consumption, high activity, and persistent

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stability.10 Among the various catalysts, supported platinum (Pt) catalysts consist of Pt metallic

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(Pt0) nanoparticles (NPs) has recently triggered extraordinary interest to oxidize HCHO because

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of their effectiveness at room temperature that save energy and also generate environmental

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friendly oxidized products (CO2(g) and H2O).11−15

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The efficiency of the Pt0 NPs is largely limited because of the strict requirement of small

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average size and narrow size distribution to adsorb substrate (i.e., HCHO). However,

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synthesizing such Pt0 NPs by traditional methods is still a great challenge.16 Recently, the

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positive effect of ferroelectric polarization on photochemical growth of metal NPs has attracted

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interest.17−24 The spontaneous polarization in ferroelectric materials such as SrBi2Nb2O9 has a

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significant impact on the nature of photoreduction process.25,26 The polarization bound charges

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on the surface of the ferroelectric materials can provide uniform sites for the reduction and

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nucleation of the metal NPs, which is beneficial for monodispersely loading metal NPs.

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Furthermore, the preparatory methods of Pt0 NPs always yield both Pt0 and Pt oxides (or Pt2+

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species).14,15 Pt2+ species on Pt0 NPs give negative effect to oxidize HCHO because Pt0 is

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generally provides catalytic sites to activate O2 to perform oxidation process.27,28 In the present

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paper, we have coated uniform Pt0 NPs on ferroelectric SrBi2Ta2O9 (SBT) platelets using a

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photoreduction process, and a strategy of tandem catalysis was developed for the first time to

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oxidize HCHO efficiently by selectively introducing Pt2+ species into Pt0 NPs.

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In literature, the tandem catalysis mechanism has received sustained attention in chemical

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synthesis, in which multiple reactions occur sequentially and selectively in one operation.29–31

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Traditionally, the tandem catalyst is a complex system containing multiple components and the

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different interfaces with the corresponding active sites can carry out sequentially coupling the

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multiple reactions.29–31 However, effectively integrating active sites with different functions into

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a single nanoparticle that work independently is critical. Additionally, fabricating a catalyst with

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such properties is an enormous challenge. In the current paper, we used an approach of a

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photoreduction process under ferroelectric polarization, which enabled us to synthesize tandem

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catalyst (Pt/SBT) successfully that consisted integrating Pt0 and Pt2+ species to degrade HCHO

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efficiently. The tandem catalytic mechanism was demonstrated by in situ Fourier transform

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infrared spectroscopy studies, which could establish Pt0 and Pt2+ species acted as cooperative

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active sites to carry out two distinct sequential steps of HCHO → CO32− and CO32− → CO2(g),

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respectively. The study presented herein provides new insights into the fundamental

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understanding of the microscopic mechanism of the oxidation of HCHO by Pt-based catalysts.

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MATERIALS AND METHODS

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Synthesis and characterization. Synthesis of SrBi2Ta2O9 (SBT) was carried out by a

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molten salt method according to the previously reported procedure.32 The details are given in

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Text S1 (Supplementary Information). Pt0 NPs were synthesized by using a photodeposition

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method (Text S2, Supplementary Information). The nominal weight percentage ratios of Pt-to-

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SBT in Pt/SBT samples were 0.5%, 1%, 2%, and 2.5%. The resultant catalysts are denoted as X4 Environment ACS Paragon Plus

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Pt/SBT (X is the actual weight percentage ratios of Pt-to-SBT, which were 0.44%, 0.9%, 1.5%,

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and 1.63%) and 1.5%-Pt/SBT-x (x represent the further calcination temperature of the 1.5%-

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Pt/SBT sample, which were 300, 400, or 500 °C).

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Characterization. The Pt content in the catalyst was determined by inductively coupled plasma-

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atomic emission spectroscopy (ICP-6000 SERIES). X-ray diffraction (XRD) patterns of the

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samples were measured using a MSAL-XD2 X-ray diffractometer equipped with Ni-filtered Cu

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Kα radiation (λ = 0.1541 nm), and the data were recorded at a scan rate of 4 degrees min−1.

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Morphology images of the samples were acquired utilizing scanning electron microscopy (SEM)

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(JSM-6510 microscope, Japan). High-resolution transmission electron microscopy (HRTEM)

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imaging and high-angle annular dark-field scanning transmission electron microscopy (HAADF-

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STEM) measurements were performed on a FEI Tecnai F20 microscope at 200 kV. The

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corresponding particle size distribution was carried out by taking the geometric mean of two

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orthogonal measurements, based on TEM images. To construct the corresponding particle size

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distribution histogram of Pt NPs, sizes of 100 particles were measured. Fractional frequency was

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calculated by dividing the particle count within a size range by the total particle count of the

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sample. The chemical states of surface elements on the catalysts were measured with X-ray

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photoelectron spectroscopy (Kratos AXIS Supra, USA). The reference binding energy was the C

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1s signal at 284.6 eV. In situ Fourier transform infrared (FT-IR) spectroscopy (VERTEX 70v,

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Bruker) was used to detect the intermediates during the oxidation of HCHO. Background signals

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were subtracted at the beginning of the in situ FT-IR tests. The BET surface area of SBT

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powders was determined by nitrogen adsorption using a Micromeritics ASAP 2460 nitrogen

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adsorption apparatus (USA). The sample was degassed at 150 °C prior to nitrogen adsorption

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measurements.

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Degradation of HCHO. An airtight reactor of 500 mL was used to evaluate the catalytic

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performance of the as-prepared samples toward HCHO degradation. Fresh catalyst powders (0.3

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g) were uniformly dispersed on a glass dish with a diameter of 45 mm. After the sample-

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containing dish was fixed in the reactor, a mixture gas of O2 + H2O + 100 ppm HCHO was

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injected into the reactor. HCHO oxidation was performed under constant stirring with a

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polytetrafluoroethylene stirrer 20 mm in diameter (1000 rpm/min). During the reaction process,

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the temperature inside the reactor was maintained at 25 °C. The concentrations of HCHO and

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CO2 (ppm) were determined by an online Photoacoustic IR Multigas Monitor (INNOVA Air

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Tech Instruments Model 1412). In the recycling experiments, the catalyst powders were heated

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at 100 °C for 60 min to remove the adsorbed molecules prior to carrying out the subsequent

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oxidation of HCHO.

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RESULTS AND DISCUSSION

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Oxidation activity of catalysts. Initially, oxidation of HCHO over Pt/SBT catalysts containing

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different weight percentage ratios of Pt (0.44%, 0.9%, 1.5%, and 1.63%) was investigated at

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room temperature by monitoring decay of HCHO and concomitant formation of CO2(g) (Figures

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1a,b). The highest degradation of HCHO (or formation of CO2(g)) was ~20% (i.e., from 101.5

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ppm to 78.1 ppm), which was for the 1.5%-Pt/SBT catalyst. Next, the increased catalytical

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activity was sought by calcinating the 1.5%-Pt/SBT catalyst at 300, 400, and 500 °C,

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respectively, for 3 h and corresponding prepared catalysts were designated as 1.5%-Pt/SBT-300,

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1.5%-Pt/SBT-400, and 1.5%-Pt/SBT-500. As shown in Figure 1c, the HCHO concentration onto

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1.5%-Pt/SBT-400 decreases faster than that onto 1.5%-Pt/SBT-300 and 1.5%-Pt/SBT-500. More

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than 90 % of HCHO is degraded in 20 min. The simultaneous formation of CO2(g) increases to

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116.1 ppm onto 1.5%-Pt/SBT-400 (Figure 1d), indicating that the HCHO is completely oxidized

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to CO2(g) and H2O onto 1.5%-Pt/SBT-400 catalyst at room temperature.

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Figure 1. Oxidation of HCHO (a, c) and formation of CO2(g) (b, d) using prepared catalysts at room temperature. (a) and (b) Un-calcinated prepared Pt/SBT catalyst of varying weight percent composition of Pt. (c) and (d) Calcinated Pt/SBT containing 1.5% Pt (i.e., 1.5%-Pt/SBT-x where x is temperature of calcination in °C).

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of HCHO oxidation over 1.5%-Pt/SBT-400 and 1.5%-Pt/SBT (i.e., no calcination) catalysts

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suggest that catalytic activity of 1.5%-Pt/SBT-400 is ~3.6 times higher than that of 1.5%-Pt/SBT

The role of calcination is further demonstrated in Figure S1. The kinetic linear simulation

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without calcination. The results of the influence of catalysts in Figure 1 in oxidizing HCHO was

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understood by subjecting the prepared catalysts to surface analysis.

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Surface characterization. The XRD patterns of pure SBT, 1.5%-Pt/SBT, 1.5%-Pt/SBT-300,

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1.5%-Pt/SBT-400, and 1.5%-Pt/SBT-500 are shown in Figure S2. The XRD patterns can be

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attributed to the pure SBT (JCPDS No. 01-081-0557), which is consistent with a previous

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study.32 No typical peaks of Pt can be detected for the samples deposited with Pt, suggesting an

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amorphous state of the Pt NPs in catalysts. Next, phase structure and morphology of catalysts

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were determined.

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Figure S3 shows SEM, TEM, and HAADF-STEM images of SBT sample and prepared

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catalysts. The SBT sample is composed of platelet particles with thicknesses in the range of 60-

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140 nm (see SEM image of Figure S3a). Low resolution and high resolution TEM images of a

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single SBT platelet are shown in Figure S4a, b. In case no Pt NPs onto SBT, the distinct (013)

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lattice fringe with a measured d spacing of 0.46 nm is seen in HRTEM images (Figure S4b),

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indicating the single crystal nature of the platelets. The corresponding particle size distribution of

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the Pt0 NPs in 1.5%-Pt/SBT-400 shows that the average particle diameter is ~2 nm (Figure S3c).

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The TEM images of 1.5%-Pt/SBT-400 (Figure S3b) and 1.5%-Pt/SBT (Figure S5a) suggest the

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high dispersion of Pt0 NPs. An average particle diameter of the Pt NPs in 1.5%-Pt/SBT is also ~2

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nm (Figure S5b), indicating that the calcination treatment did not affect the size of the Pt NPs.

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The dispersion of Pt0 NPs on the SBT platelet was also confirmed by HAADF-STEM images.

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Figure S3d shows the overall morphology of the 1.5%-Pt/SBT-400 platelet. The whiter

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spots represent the Pt NPs, which are densely and monodispersely dispersed on the surface of the

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SBT platelet. The closer observation in Figure S3e indicates that Pt NPs with small average size

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and narrow size distributions are formed onto the SBT surface. Considering the small specific

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surface areas of SBT (~2 m2/g), the formation of the uniform dispersion of Pt NPs with such

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small sizes is possibly related to the ferroelectric property of SBT.32 The corresponding STEM-

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energy-dispersive X-ray spectrometry (STEM-EDAX) mapping of a single 1.5%-Pt/SBT-400

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platelet is shown in Figures S3f and S3g, which confirms the homogeneous distribution of Pt

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over the entire surface of the SBT platelet. The mappings of Sr, Bi, Ta and O elements are

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presented in Figure S6. Images and particle distributions of other calcinated catalysts (1.5%-

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Pt/SBT-300 and 1.5%-Pt/SBT-500) were like image of 1.5%-Pt/SBT-400 (Figure S3 and Figure

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S7). It is therefore concluded that the trend of the oxidation of HCHO by different calcinated

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1.5%-Pt/SBT-x cannot be described by variation in the morphology of the catalysts. Next,

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speciation of Pt in Pt/SBT with and without calcination was explored by X-ray photoelectron

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spectroscopy (XPS) technique.

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XPS measurements. After calcination in air, the proportions of Pt species (or different oxidation

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states of Pt) in Pt/SBT catalysts may be changed33 and therefore, XPS studies were performed on

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1.5%-Pt/SBT, 1.5%-Pt/SBT-300, 1.5%-Pt/SBT-400, and 1.5%-Pt/SBT-500 catalysts. The

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obtained results are shown in Figure 2. As shown in Figure 2a, the Pt 4f core-level XPS spectrum

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of the 1.5%-Pt/SBT sample can be deconvoluted into three peaks located at 78.0, 74.8, and 72.5

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eV, which correspond to Pt4+ and Pt0 species in 1.5%-Pt/SBT samples.34−36 This result indicates

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that originally presented adsorbed Pt4+ species in Pt-SBT samples are not reduced to Pt0 in the

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photoreduction process.37 Figures 2b, c, and d demonstrate the Pt 4f core-level XPS spectra of

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the samples calcined at 300, 400, and 500 °C, respectively. As shown in Figure 2b-d, there is a

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decrease in the Pt0 peaks at approximately 74.2 and 72.1 eV, and a simultaneous emergence of

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peaks at approximately 75.7 and 72.8 eV, corresponding to Pt2+ species.38,39 This result suggests

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that Pt2+ species are transformed from Pt0 species during calcination of samples.

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The variation of platinum species (or contents of Pt0, Pt2+, and Pt4+), obtained from XPS

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spectra, with temperature in different samples is presented in Figure 2e. The content of Pt4+

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almost remained unchanged with the heat treatment. However, levels of Pt2+ increase and

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simultaneous decrease in Pt0 species are seen (Figure 2e). For example, content of Pt2+ gradually

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increases from undetectable to 49% for the 1.5%-Pt/SBT-500 sample, accompanied by a

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decrease of Pt0 (from 69% to 21%). Significantly, linear relationships between the contents of Pt0

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and Pt2+ and temperature are observed (Figure 2e). The formed Pt2+ species is most likely the

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platinum oxide (PtO), resulted in from the oxidation of Pt0 in calcination of the 1.5%-Pt/SBT

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sample in air. The formation of Pt-O bond (or PtO) is confirmed by measuring XPS spectra of

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1.5%-Pt/SBT and 1.5%-Pt/SBT-400 samples (Figure 2f). The shoulder peaks at 529.6 eV can be

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ascribed to the Pt−O bond,40 which increases significantly after treatment at 400 °C (1.5%-

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Pt/SBT-400 (calcination) versus 1.5%-Pt/SBT (no calcination)). This indicates the formation of

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PtO species is from the calcination of the catalyst. The peaks at 532.9 eV and 531.6 eV for both

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samples are attributed to the dissociated and chemisorbed oxygen species (O2- or O-), OH, or

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lattice oxygen on the surface of the SBT substrate.41 The XPS results clearly demonstrate the

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partial conversion of Pt0 to Pt2+ species in calcination. Furthermore, the conversion can be

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controlled by temperature, giving a new tool to tune the composition of Pt0 and Pt2+ in the

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Pt/SBT catalysts. Furthermore, the enhanced degradation of HCHO of calcinated 1.5%-Pt/SBT-x

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compared to un-calcinated 1.5%-Pt/SBT sample (see Figure 1c) may be related to the formation

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of PtO species in calcination. This is described in next section.

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Figure 2. (a-d) Pt 4f XPS spectra for 1.5%-Pt/SBT, 1.5%-Pt/SBT-300, 1.5%-Pt/SBT-400, and 1.5%-Pt/SBT-500 samples. (e) The relative intensity changes of Pt0, Pt2+, and Pt4+. (f) O 1s XPS spectra of 1.5%-Pt/SBT and 1.5%-Pt/SBT-400 samples.

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Tandem catalytic mechanism. The oxidation of HCHO over catalysts was investigated using

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1.5%-Pt/SBT and 1.5%-Pt/SBT-400 samples. The former sample is un-calcinated catalyst and

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the later sample corresponds to calcinated catalysts that gives the highest oxidation performance

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(see Figure 1c). In initial study, oxidation of HCHO was monitored using FT-IR technique,

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which can observe bands associated with HCHO and its oxidized products. Both catalysts were

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first exposed to gas mixture of O2, HCHO, H2O, and Ar for 60 min and subsequently to only Ar

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gas (or purging with Ar gas). Figures 3a and 3b give FT-IR spectra obtained in exposures to

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1.5%-Pt/SBT-400 and 1.5%-Pt/SBT, respectively. Observed peaks in the spectra after initial 60

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min exposure are similar for oxidation of HCHO onto both catalysts. However, visible difference

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in bands is seen when Ar purging is carried out for next 60 min.

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The band at 1720 cm−1 is assigned to physically adsorbed formic acid (HCOOH) (Figure

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3).42 Bands at 1610, 1556, and 1450 cm−1 correspond to the vibration of formate species

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(HCOO−).43-44 The characteristic band of the carbonyl group (C=O, at 1771 cm−1) in HCOO- also

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appears after exposing catalysts to the gas mixture.45 The spiculate band at 1745 cm−1 belongs to

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the carbonyl group (C=O) of HCHO,46 which suggests that HCHO molecules are oxidized to

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HCOO− species over catalysts in the presence of O2 and H2O. The band at 1530 cm−1 can be

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ascribed to the formation of carbonate species (CO32-),47 indicating that CO32- species are formed

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and accumulated in the oxidation of HCHO over catalysts. Two bands located at 3850 and 3735

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cm−1 are assigned to the characteristic vibrations of hydroxyl (OH) groups onto catalysts.48,49

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When Ar was purged for next 60 min, the peak belonging to CO32- ion at 1530 cm−1

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disappears in oxidation over 1.5%-Pt/SBT-400 (Figure 3a, red line). Simultaneously, two bands

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of OH groups, located at 3850 and 3735 cm−1, also disappear after Ar purging (Figure 3a, red

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line). Comparatively, all these peaks remain when 1.5%-Pt/SBT catalyst is used (Figure 3b, red

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line). Results of Figure 3 conclude the role of Pt2+ species in oxidation of HCHO because 1.5%-

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Pt/SBT catalyst is without the Pt2+ species (see Figure 2a and Figure 2e). Furthermore, OH

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groups of 1.5%-Pt/SBT-400 have a role of the disappearance of CO32- ions.

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Figure 3. FT-IR spectra observed during the oxidation of HCHO over catalysts: (a) calcinated sample (1.5%-Pt/SBT-400) and (b) un-calcinated sample (1.5%-Pt/SBT). Black lines are the FTIR spectra of the sample treated in a mixture gas of O2 + HCHO + H2O + Ar for 60 min. Red lines are the FT-IR spectra of the sample treated in a mixture gas of O2 + HCHO + H2O + Ar for 60 min followed by Ar purging for 60 min. Reaction conditions: HCHO (~100 ppm), O2 (20 vol%), H2O (34 vol%), and Ar as the balance.

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different time intervals of the oxidation of HCHO over both catalysts (Figure 4). Figures 4a and

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4b show in situ FT-IR spectra obtained at different time during oxidation of HCHO over 1.5%-

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Pt/SBT-400 and 1.5%-Pt/SBT catalysts exposed to gas mixture of O2, HCHO, H2O, and Ar at

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room temperature. Characteristics of different peaks correspond to HCOOH, HCOO-, HCHO,

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CO32-, and OH groups are like bands seen in Figure 3. Attention was paid to evolution of CO32-

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and OH groups on surfaces of the catalysts. Peak areas of FT-IR peaks at 1530 cm−1 (i.e., CO32-

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and 3850−3610 cm−1 (i.e., OH groups) as a function time are depicted in Figures 4c and 4d. The

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concentrations of CO32- and OH onto 1.5%-Pt/SBT-400 attained to a dynamic equilibrium of

The steps of the mechanism were further examined by monitoring of FT-IR peaks at

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formation and consumption in 10 min of reaction (i.e., no further increase in concentrations of

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both species with time). Comparatively, levels of CO32- and OH groups on the surface of 1.5%-

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Pt/SBT increase with the reaction time. The CO32- ions on surfaces poison the activity of catalyst

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by reducing the number of active sites, hence low oxidation of the HCHO over 1.5%-Pt/SBT is

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observed (see Figure 1a). It seems that calcinated catalysts, 1.5%-Pt/SBT-400 has desorption

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mechanism to avoid decrease in active sites to carry oxidation of HCHO (Figure 1c versus

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Figure 1a). This favorable mechanism to perform oxidation is related to the formation of Pt2+

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species in calcinated sample (or 1.5%-Pt/SBT-400).

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Figure 4. In situ FT-IR spectra of (a) 1.5%-Pt/SBT-400 and (b) 1.5%-Pt/SBT in a mixture gas of O2 + HCHO + H2O + Ar at room temperature. The enlarged peaks belonging to OH and CO32are shown on the right sides of Figures 4a and b. (c, d) Areas of the dominant peak belonging to the CO32- (at 1530 cm−1) and a series of dense peaks belonging to OH (ranging from 3850 cm−1 to 3610 cm−1) versus time during reaction in an O2 + HCHO + H2O + Ar gas mixture. Reaction conditions: HCHO (~100 ppm), O2 (20 vol%), H2O (34 vol%), and Ar as the balance. 14 Environment ACS Paragon Plus

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The role of O2 in the catalytic mechanism was explored by eliminating O2 from the gas

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mixture. Results of collected in situ FT-IR spectra after exposing the gas mixture of H2O, HCHO,

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and Ar to 1.5%-Pt/SBT-400 and 1.5%-Pt/SBT catalysts are shown in Figure S8. After 20 min

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exposure, no peaks of OH, HCOO−, and CO32- are observed, implying O2 is a critical component

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of gas mixture to oxidize HCHO by both catalysts.

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Next, participation of water in the steps of mechanism was investigated by exposing gas

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mixture of O2, HCHO, and Ar (i.e., no H2O) to 1.5%-Pt/SBT-400 catalyst and the degradation of

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HCHO and the formation of CO2(g) (Figure 5). Without H2O, the degradation of HCHO is

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slower than with H2O in gas mixture (Figure 5a). This suggests that water in gas mixture

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facilitates the activity of catalyst better than no water involved in the oxidation of HCHO.

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Formed CO32- ions in the oxidation of HCHO likely adsorb on the catalyst to poison the activity

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of 1.5%-Pt/SBT-400. However, this poisoning of the catalyst is diminished by water via the

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formation of OH groups at the catalyst surface, which ultimately converts CO32- ions to CO2(g).

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Formation of CO2(g) was monitored during the degradation of HCHO. As shown in Figure 5b,

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H2O was able to stoichiometrically convert HCHO to CO2(g). Comparatively, the oxidation of

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HCHO without H2O results in smaller amount of CO2(g) than that of expected concentration

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from the conversion of HCHO in oxidation over 1.5%-Pt/SBT-400 catalyst. This further implies

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H2O is participating in oxidation by not allowing CO32- ion to interfere in the activity of the

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prepared calcinated catalyst (i.e., 1.5%-Pt/SBT-400) by completing the oxidation process to

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convert HCHO to CO2(g).

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Figure 5. Degradation of HCHO and formation of CO2(g) with and without H2O using the 1.5%Pt/SBT-400 catalyst: (a) HCHO and (b) CO2(g) (Reaction conditions: HCHO (~100 ppm), O2 (20 vol%), H2O (34 vol%), temperature (25 °C), and Ar as the balance).

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Results of Figures 3-5 demonstrate the operation of tandem catalytical mechanism in the

303

oxidation of HCHO to CO2(g) by prepared catalyst, 1.5%-Pt/SBT-400 in our study. The

304

mechanism is schematically presented in Figure 6. Initially, HCHO is adsorbed onto the surface

305

of 1.5%-Pt/SBT catalyst (or on Pt0 NPs), possible via hydrogen bonding.50 Molecules of O2 are

306

also simultaneously adsorb onto Pt0 NPs and decompose into active oxygen radicals (O-). The

307

activated oxygen species oxidize HCHO to formate (HCOO-) ions. The HCOO- ions are also

308

oxidized by activated oxygen species to yield CO32- ions. Both prepared un-calcinated catalyst

309

(1.5%-Pt/SBT) and calcinated catalyst (1.5%-Pt/SBT-400) have same steps of oxidizing HCHO

310

to CO32- through intermediate HCOO- ions (Figures 6a, b). The similarity of the steps was

311

explored by conducting catalytic oxidation of HCHO using both catalysts at different

312

temperature. The rate constants (k) of the degradation of HCHO at different temperature were

313

determined. The plots of lnk versus 1/T(K) give apparent activation energies (Figure S9), which

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are 10.45 ± 0.81 kJ/mol and 11.09 ± 0.47 kJ/mol for degradation of HCHO by applying 1.5%-Pt-

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SBT-400 and 1.5%-Pt-SBT, respectively. Similar values of apparent activation energy suggest

316

that the similar step may be involved in mechanism of the oxidation of HCHO in using both

317

catalysts. However, the active site of Pt0 to participate in oxidation of HCHO likely poisoned by

318

CO32- (or decrease in number of active sites) in using 1.5%-Pt-SBT to decrease the overall

319

oxidation rate. Significantly, the presence of Pt2+ (or PtO) species in 1.5%-Pt-SBT/400 can

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convert poisoned adsorbed CO32- ions to CO2(g) (or desorption of CO32- ions) to diminish

321

poisoning of the catalyst. This conversion step may be presented by reactions (1) and (2). These

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reactions are not possible in applying un-calcinated 1.5%-Pt-SBT catalyst to oxidize HCHO.

323 324

It has been reported that oxygen atoms on the PtO surface can abstract hydrogen from water to create OH groups:28

325

2PtO + H2O →Pt2+-OH + Pt2+-O-OH

(1)

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2Pt2+-OH + Pt0-CO3 → 2PtO + Pt0 + CO2(g) + H2O

(2)

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In presence of H2O, formation of Pt2+-OH and Pt2+-O-OH species onto 1.5%-Pt/SBT-400 occur

328

(reaction 1).51 The Pt2+-OH species may supply protons, either directly or indirectly.52,53 Protons

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on surfaces are able to convert adsorbed CO32- ions to CO2(g) (reaction 2). The formed Pt2+-O-

330

OH species may also be involved in oxidation of HCHO by reacting with an intermediate,

331

HCOO- species to directly produce CO2(g).51 Overall, an active site directed tandem catalysis

332

mechanism of 1.5%-Pt/SBT-400 is operational. However, the 1.5%-Pt/SBT catalyst is not able to

333

carry out reactions (1) and (2) because of absence of Pt2+ species (see Figures 2a and 2e).

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Therefore, CO32- ions onto 1.5%-Pt/SBT surfaces cannot be desorb (or no tandem catalytic

335

activity) and poisoning of the catalyst happens to decrease the activity to oxidize HCHO (see

336

Figure 1).

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The role of Pt2+ species in tandem catalytic mechanism of 1.5%-Pt-SBT-400 is also

338

supported indirectly by the oxidation of HCHO by Pt/TiO2 catalyst under same conditions. The

339

Pt/TiO2 catalyst was prepared using a previous report (Text S3).54 The Pt/TiO2 catalyst is without

340

Pt2+ species while the tandem catalyst, 1.5%-Pt-SBT-400, contains Pt2+ species. When exposing

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gas mixture of O2, HCHO, H2O, and Ar to Pt/TiO2 catalyst, the degradation of HCHO takes

342

place (Figure S10). However, the oxidation was incomplete in 20 min (101.2 ppm to 47.5 ppm).

343

Also, the activity of Pt/TiO2 catalyst appears to be almost stopped at ~10 min. Furthermore,

344

degraded HCHO does not correspond to equal amount of the formation of CO2(g) (Figure S10).

345

This suggests the deficiency in the Pt/TiO2 catalyst, which could be overcome in the

346

development of a tandem catalyst, 1.5%-Pt/SBT-400 in our study (see Figure 1c).

347

Significantly, 1.5%-Pt/SBT-400 had high catalytical activity than that the observed

348

activity using 1.5%-Pt/SBT-300 and 1.5%-Pt/SBT-500 catalysts (see Figure 1c). This may be

349

understood by considering the cooperative effect between Pt0 and Pt2+ species in the tandem

350

catalysis process during the oxidation of HCHO. The increase of Pt2+ due to increase in

351

temperature of calcination inevitably causes the decrease of Pt0 (see Figure 2e), which decreases

352

active sites for activating O2 in the first step of oxidation of HCHO to from CO32- ions. After

353

introducing the Pt2+ species into calcinated Pt/SBT at different temperatures, the

354

synergistic effect between Pt0 and Pt2+ occurs during the degradation of HCHO to CO2(g). It

355

appears that the optimum levels of both Pt0 and Pt2+ exist in 1.5%-Pt/SBT-400 to yield higher

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catalytic activity than the activity generated by 1.5%-Pt/SBT-300 and 1.5%-Pt/SBT-500.

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Figure 6. Plausible reaction pathway for oxidation of HCHO over (a) 1.5%-Pt/SBT catalyst, and (b) 1.5%-Pt/SBT-400 catalyst at room temperature.

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Significance. The newly developed tandem catalyst in our study has demonstrated high

362

effectiveness to fully degrade HCHO to CO2(g) at room temperature. The 1.5%-Pt/SBT-400

363

catalyst exhibits favorable ability to oxidize HCHO completely for six cycling tests (Figure 7a).

364

No obvious decline in HCHO oxidation occurs after six cycling tests; indicating high stability of

365

the 1.5%-Pt/SBT-400 catalyst. The suitability of the 1.5%-Pt/SBT-400 catalyst was also tested in

366

continuous oxidation process. The continuous oxidation of HCHO over 1.5%-Pt/SBT-400

367

catalyst (0.3 g) was carried out in a fixed bed reactor system. Online Photoacoustic IR Multigas

368

Monitor was connected to a tail gas collector to determine the concentration of HCHO. The

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standard HCHO feed gas contains ~100 ppm of HCHO, 20% O2, 34% H2O, and Ar (as the

370

balance). The concentration of HCHO in the feed gas was adjusted by changing the flow rate of

371

Ar using a rotameter. The total flow rate was approximately 40 mL min−1, corresponding to a gas

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hourly space velocity (GHSV) of 12600 h−1. As shown in Figure 7b, ~5% activity loss was lost

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conversion. This supports the reasonably high stability of the 1.5%-Pt/SBT-400 catalyst under

375

continuous oxidation of HCHO. Comparatively, 1.5%-Pt/SBT catalyst, which does not carry

376

tandem catalytical activity, oxidation of HCHO was completely inhibited in 40 min (Figure 7c).

377

This suggests that complete deactivation of 1.5%-Pt/SBT catalyst in a short period of time and

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this catalyst is not appropriate to carry oxidation of HCHO. Overall, high effectiveness of the

379

tandem catalyst of our study may be applicable to oxidize other organic compounds.

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Figure 7. Durability test of HCHO oxidation over 1.5%-Pt/SBT-400 and 1.5%-Pt/SBT catalysts: (a) cycling test for 1.5%-Pt/SBT-400, (b) continuous test for 1.5%-Pt/SBT-400, and (c) continuous test for 1.5%-Pt/SBT.

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ASSOCIATED CONTENT

385

Supporting Information

386

Synthesis of SrBi2Ta2O9 (Text S1), loading of the Pt nanoparticles (Text S2), preparation of

387

Pt/TiO2 catalyst (Text S3), kinetic linear simulation for HCHO oxidation (Figure S1), XRD

388

patterns (Figure S2), SEM, TEM, and HAADF-STEM images (Figures S3-S7), in situ FT-IR

389

spectra in gas mixture of HCHO, H2O, and Ar (Figure S8), Plots of lnk versus 1/T(K) for the

390

oxidation of HCHO (Figure S9), and the HCHO oxidation performance over Pt/TiO2 catalyst

391

(Figure S10) (PDF)

392

AUTHOR INFORMATION

393

Corresponding Authors

394

*Emails: [email protected] and [email protected]

395 396

Notes

397

The authors declare no competing financial interest.

398

ACKNOWLEDGMENT

399

This work was supported by the National Natural Science Foundation of China (Grant Nos.

400

21643012 and U1403193). We thank anonymous reviewers for their comments, which improved

401

the paper greatly.

402 403 404 405

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