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Highly active mesoporous ferrihydrite supported Pt catalyst for formaldehyde removal at room temperature Zhaoxiong Yan, Zhihua Xu, Jiaguo Yu, and Mietek Jaroniec Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00532 • Publication Date (Web): 11 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015

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Environmental Science & Technology

Highly active mesoporous ferrihydrite supported Pt catalyst for formaldehyde removal at room temperature Zhaoxiong Yan 1,3, Zhihua Xu 1,3, Jiaguo Yu 1,4*, Mietek Jaroniec 2,**

1

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Wuhan 430070, PR China. 2

Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio,

44242, USA. 3

Hubei Key Laboratory for Processing and Application of Catalytic Materials,

Huanggang Normal University, Huanggang 438000, PR China. 4

Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia. * Corresponding author. Tel: +86 27 87871029, fax: +86 2787879468, E-mail: [email protected] (J. Yu) ** Corresponding author. Tel: +1 330 672 3790, fax: +1 330 672 3816, E-mail: [email protected] (M. Jaroniec)

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ACS Paragon Plus Environment


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ABSTRACT

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Ferrihydrite (Fh) supported Pt (Pt/Fh) catalyst was first prepared by combining

3

microemulsion and NaBH4 reduction methods, and investigated for room-temperature

4

removal of formaldehyde (HCHO). It was found that the order of addition of Pt

5

precursor and ferrihydrite in the preparation process has an important effect on the

6

microstructure and performance of the catalyst. Pt/Fh was shown to be an efficient

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catalyst for complete oxidation of HCHO at room temperature, featuring higher

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activity than magnetite supported Pt (Pt/Fe3O4). Pt/Fh and Pt/Fe3O4 exhibited much

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higher catalytic activity than Pt supported over calcined Fh and TiO2. The abundance

10

of surface hydroxyls, high Pt dispersion and excellent adsorption performance of Fh

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are responsible for superior catalytic activity and stability of the Pt/Fh catalyst. This

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work provides some indications into the design and fabrication of the cost-effective

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and environmentally benign catalysts with excellent adsorption and catalytic

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oxidation performances for HCHO removal at room temperature.

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TOC Graphics

KEYWORDS: Ferrihydrite, Adsorption, Pt-ferrihydrite catalyst, Formaldehyde oxidation

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1. Introduction

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The indoor air quality is of great importance for human health because people

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often spend most of their time in houses, offices, schools, hospitals and cars.

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Formaldehyde (HCHO) is a typical indoor pollutant released from wood-based

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building products, furnishings, ornaments, and so on. A long-term exposure to HCHO

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can cause serious health problems such as eye irritation, respiratory tract, headache,

7

pneumonia, and even cancer.1,2 Therefore, the elimination of gaseous HCHO from the

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indoor environment is an important issue. Usually physical adsorption and chemical

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oxidation are used for HCHO removal.3-10 Physical adsorption is a simple and

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effective approach to eliminate HCHO under ambient conditions, however, its wide

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application is largely restricted by the limited adsorption capacity and the need for

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adsorbent regeneration. Catalytic oxidation of HCHO is advantageous as compared to

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physical adsorption, because it can completely decompose HCHO into CO2 and

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H2O.11-17 Catalysts like transition metal oxides and supported noble metals such as Pt,

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Au and Pd have been extensively explored for oxidation of gaseous HCHO.10, 18-20 As

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compared to the transition-metal oxide catalysts, the supported noble metal catalysts

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are usually very efficient for complete oxidation of HCHO at lower temperatures,

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even at room (or ambient) temperature. For instance, HCHO can be completely

19

oxidized on Ru/CeO2 at 100 °C,21 on Pd-Mn/Al2O3 at 90 °C,13 on Au/CeO2 and

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Au/iron-oxide at 75-80 °C.11,22 Among the supported noble metals, the supported Pt

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has attracted a lot of attention because of its ability for decomposition of HCHO into

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CO2 and H2O at room (or ambient) temperature.23-25 For example, Pt/TiO2 catalysts

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prepared by impregnation or NaBH4-reduction method can completely oxidize HCHO

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at ambient (or room) temperature.26,27 Also, Pt/Fe2O3 prepared by colloid-deposition

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method and Pt/MnOx-CeO2 prepared by a modified co-precipitation method showed

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100% conversion of HCHO to CO2 and H2O at room (or ambient) temperature.24,29

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These investigations suggest that the highly active Pt catalysts for room (or

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ambient)-temperature destruction of HCHO can be prepared by Pt deposition on the

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proper supports. Usually, the support is the key factor in fabricating highly active Pt

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and related noble metal catalysts, which affects the formation, dispersion and even the

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surface state of noble metals, as well as the diffusion of the reactants and products in

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the catalytic oxidation process due to its unique microstructure, and the proper surface

33

and electronic properties.30,31

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Recently, porous materials with high surface areas are often used as catalyst

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supports to disperse the active species to provide more active sites for adsorption and

36

reaction.32,33 Moreover, the porous structure with suitable distribution and sizes of

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pores allowing a full accessibility of inner active sites to the reactant molecules, their

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adsorption and/or oxidation. Porous ferrihydrite has been investigated as an adsorbent

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to remove contaminants from wastewaters,34 due to its natural abundance, large

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surface area, high adsorption affinity, and eco-friendliness. In our previous reports,7,35

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mesoporous ferrihydrite and AlOOH prepared by microemulsion-assisted method

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were shown to have an excellent performance for HCHO adsorption owing to their

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porous structure, large specific surface area and abundance of surface hydroxyls.36

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Iron oxide supported Pt catalysts prepared by different methods were extensively

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studied for HCHO decomposition at a temperature from 25 to 100 °C, and Pt/Fe2O3

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prepared by colloid deposition method showed a 100% conversion of HCHO at room

47

temperature.37 The latter result indicates that the importance of the preparation

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method on the performance of the catalysts for HCHO decomposition. However, to

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the best of our knowledge, porous ferrihydrite with high surface area and abundance

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of surface hydroxyls has not been studied yet as a support for Pt nanoparticles with a

51

goal to use it for oxidation of gaseous HCHO at room temperature. Here we report for

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the first time the synthesis of mesoporous ferrihydrite supported Pt catalyst with an

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abundance of surface hydroxyls and plenty of catalytic oxidation sites for fast and

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complete removal of HCHO at room temperature.

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2. Experimental Section

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2.1. Preparation

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Mesoporous ferrihydrite powder was synthesized by using water-in-oil

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microemulsion method, which was analogous to that reported previosuly.7 In the case

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of Pt/Fh, 5 mL of the mixed solution of NaBH4 (0.1 mol/L) and NaOH (0.1 mol/L)

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were added into 30 mL of H2PtCl6 solution under vigorous stirring, and 0.4 g of the

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as-prepared Fh powder was quickly added into the above solution under magnetic

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stirring (the nominal weight of Pt was 0.8 wt%.). Then the suspension was evaporated

64

at 80 °C under continuous stirring. Finally, the resulting powder was dried at 80 °C in

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a drying oven for 3 h. In the case of Pt/Fe3O4, the preparation process was similar to

66

that of Pt/Fh except for the order of Fh addition. Namely, 0.4 g of the as-prepared Fh

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was added into 30 mL of H2PtCl6 solution under magnetic stirring (the nominal

68

weight of Pt was 0.8 wt%.). After impregnation for 20 min, 5 mL of the mixed NaBH4

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(0.1 mol/L) and NaOH solution (0.1 mol/L) was added into the suspension under

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vigorous stirring for 30 min. Then the suspension was evaporated at ca. 80 °C under

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continuous stirring, and finally the resulting powder was dried at 80 °C in a drying

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oven for 3 h.

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For the purpose of comparison, the same nominal weight of Pt supported over

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TiO2 (P25, Degussa) (denoted as Pt/TiO2) and over ferrihydrite calcined at 400 °C for

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2 h with a heating rate of 2 °C min-1 (denoted as Pt/Fh-c) were also prepared using the

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same procedure as that for the preparation of Pt/Fe3O4 catalyst, except for replacing

77

Fh by P25 or calcined ferrihydrite. Commercial Fe2O3 supported Pt (Pt/Fe2O3) was

78

also prepared using the aforementioned process with the nominal Pt content of 1%. It

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is noteworthy that the Fh, calcined ferrihydrite, TiO2 and commercial Fe2O3

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adsorbents used were prepared via the same way but without adding Pt precursor, and

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the samples are designated as Fh-Na, Fh-c, TiO2 and c-Fe2O3, respectively. The

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treatment of the supports is to investigate the effect of NaBH4 solution on the

83

microstructure and their performance.

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2.2 Characterization

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X-ray diffraction (XRD) patterns of the samples were obtained using a Philips

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X’Pert powder X-ray diffractometer with Cu Kα radiation (λ = 0.15419 nm). The

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morphologies of the samples were characterized on a JEM-2100F transmission

88

electron microscope (TEM) (JEOL, Japan). Nitrogen adsorption–desorption isotherms

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were obtained on an ASAP 2020 (Micromeritics Instruments, USA). All samples were

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degassed at 80 °C prior to adsorption measurements. The Brunauer–Emmett–Teller

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(BET) surface area (SBET) was determined by a multipoint BET method by using

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adsorption data in the relative pressure P/P0 range of 0.05–0.2. The single-point pore

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volume (Vpore) was estimated from the amount adsorbed at a relative pressure of 0.98.

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The pore size distributions (PSD) were calculated using adsorption branches of

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nitrogen adsorption-desorption isotherms by the improved KJS method.38 X-ray

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photoelectron

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ESCALAB250xi spectrometer (Thermon Scientific). All the binding energies were

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referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. Fourier

99

transform infrared spectra (FTIR) were collected using a Shimadzu IRAffinity-1 FTIR

100

spectrometer in a range of 4000–400 cm-1. Thermogravimetric analysis (DTA-TGA)

101

was performed on a DTG-60H analyzer (Shimadzu Corp., Tokyo, Japan) in air using a

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heating rate of 5 °C/min. Hydrogen temperature programmed reduction (H2-TPR)

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measurements were performed on the BELCAT-B (Japan) instrument. Measurement

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of the metal dispersion by CO pulse chemisorption was also carried out on the

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BELCAT-B (Japan) instrument at room temperature by a pulse injection method. The

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samples were first reduced under pure hydrogen (50 mL min−1) at 200 °C for 1 h and

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further outgassed under He (50 mL min−1) for 0.5 h at the same temperature.

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Subsequently, CO pluses were injected into the carrier gas intermittently after the

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sample was cooled to room temperature, and the whole process was detected by a

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TCD detector. In situ diffused Fourier transform infrared spectroscopy (DRIFTS)

spectroscopy

(XPS)

measurements

were

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performed

on

an

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spectra were recorded using a Thermo Fisher 6700 instrument. The catalysts were

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pretreated in a dry air gas flow at 150 °C for 1 h in an in situ cell reactor, and then, the

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reactant gas mixture (78 ppm HCHO + O2) was introduced into the DRIFT cell at

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room temperature via separate mass flow meters at flow rates of 30 mL min−1. All

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spectra were recorded with a resolution of 4 cm−1, and the background spectrum was

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subtracted from each spectrum, respectively.

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2.3 HCHO removal testing

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The removal of HCHO at room temperature was carried out in an organic glass

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box covered by a layer of aluminum foil on its inner wall. 0.1 g of catalyst (or

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adsorbent) powder was dispersed on the bottom of a glass petri dish having a diameter

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of 14 cm. After placing the sample-containing dish at the bottom of reactor with a

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glass slide cover, a certain amount of condensed HCHO (38%) was injected into the

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reactor having a 5 watt fan at the bottom of reactor. After 2~3 hours, the HCHO

125

solution was volatilized completely and the concentration of HCHO was stabilized.

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HCHO, CO2, CO and water vapor were analyzed using an on-line Photoacoustic IR

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Multigas Monitor (INNOVA air Tech Instruments Model 1412). The HCHO vapor was

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allowed to reach adsorption/desorption equilibrium within the reactor prior to the

129

experiment. The initial concentration of HCHO after adsorption/desorption

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equilibrium was controlled at ca. 300 ppm, which remained constant until the removal

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of the glass slide cover from the petri dish to start the adsorption or catalytic oxidation

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reaction of HCHO. During the catalytic oxidation reaction, carbon dioxide

133

concentration increased and the HCHO concentration decreased steadily with time.

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The CO2 concentration increase (ppm, CO2, which is the difference between CO2

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135

concentrations at t reaction time and initial time) and HCHO concentration decrease

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were recorded to evaluate the adsorption and catalytic performance.

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The removal of HCHO was determined as follows:

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HCHO removal (%) =

[ HCHO] I − [ HCHO] F ×100% [ HCHO] I

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Where [HCHO]I (ppm) is the initial equilibrium concentration of HCHO before test,

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and [HCHO]F (ppm) is the final concentration of HCHO at the end of the test.

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The calculation of TOF was as follows: TOF = the amount of products (mol)/

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(the amount of the catalyst active sites × Time (min)). Herein, we used the amount of

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HCHO removal at the time of 20 min from the start of the test.

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3. Results and discussion

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Figure 1 shows the XRD patterns of the samples studied. The ferrihydrite

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treated with NaBH4 and NaOH solution (Fh-Na) exhibits two broad bands at ca. 35 °

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and 61 °, featuring a typical two-line ferrihydrite.39 The broad bands with weak

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intensities indicate the amorphous nanostructure and/or very small size of Fh

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crystallites. The XRD pattern of Fh-Na is similar to that of the as-prepared

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ferrihydrite,7 suggesting that the treatment with NaBH4 and NaOH has no apparent

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effect on the phase of ferrihydrite. Fh-c shows a typical XRD pattern of Fe2O3

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(JCPDS No.33-0664), just like the commercial Fe2O3, revealing the transformation

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of ferrihydrite to Fe2O3 during calcination at 400 °C and also indicating that the

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addition of NaBH4 and NaOH has no influence on the phase of the iron-containing

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support in the absence of Pt. As compared to Fh-Na, Pt/Fh also displays a typical 10


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two-line ferrihydrite. Nevertheless, the diffraction peaks of Pt/Fe3O4 are clearly

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different from those of Fh-Na, and a series of characteristic peaks at 2θ = 30.2°,

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35.4°, 43.3°, 53.7°, 57.3°, and 63.0° is observed, which is quite similar to magnetite

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and matches well with JCPDS No. 82-1533.40 It suggests that the order of addition of

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Fh and Pt precursors plays a significant effect on the phase of the ferrihydrite support,

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and subsequently its surface properties and performance of the supported Pt catalyst

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for HCHO removal. In the case of Pt/Fh-c, the diffraction peaks of Fe2O3 and Fe3O4

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are observed, indicating NaBH4 can partly reduce Fe2O3 into Fe3O4 during the

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preparation process. The above result implies that Pt nanoparticles can partially

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catalyze the reduction of the support in the presence of NaBH4. No diffraction peaks

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of Pt are observed for all supported Pt samples, due to the low Pt content and/or

168

small size of Pt nanoparticles as well as their high dispersion.

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TEM images of Pt/Fe3O4, Pt/Fh and Pt/Fh-c are shown in Figure 2. The

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morphologies of the supports are similar and appear to be in the form of aggregated

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particles composed of tiny spherical nanoparticles of approximately 10 nm. Pt species

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are highly dispersed on the supports and possess an average particle size of about 2-3

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nm (shown in Figure 2d). The high resolution images (inset in Figure 2a-c) show that

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the interplanar spacing is between 0.222-0.227 nm, corresponding to the (111) set of

175

plane for metallic Pt.41,42 This result indicates that Pt nanoparticles are successfully

176

deposited on the supports.

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To compare the porous structures and textures of the Pt/Fh and Pt/Fh-c catalysts,

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N2 adsorption/desorption isotherms were measured and used to calculate the

11



179

corresponding pore size distributions, which are shown in Figure 3; the

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corresponding structural parameters obtained from adsorption isotherms are

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summarized in Table 1. Adsorption isotherms shown in Figure 3 are type IV,

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reflecting the mesoporosity of the aforementioned samples. The isotherms measured

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for these two samples show H2-type hysteresis loops in the range of lower relative

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pressures P/P0, which are associated with a constricted porous networks.43 As

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compared to Pt/Fe3O4, Pt/Fh exhibits a visible increase in the amount of adsorbed N2

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at high relative pressures between 0.5 and 1.0, indicating that Pt/Fh has larger pore

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volume. The pore size distributions calculated for these two samples (inset in Figure

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3) are mainly in the range of 2-15 nm, further indicating the mesoporous structure of

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the samples, formed by random aggregation of nanoparticles. The data listed in Table

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1 show that the order of addition of Fh and Pt precursors in the preparation process

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has an effect on the textural structures of Pt/Fh and Pt/Fe3O4. As shown in Table 1,

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Pt/Fh has the specific surface area (SBET) of 222 m2/g and pore volume (Vpore) of 0.28

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cm3/g and pore width (dpore) of 4.4 nm at the maximum of the pore size distribution;

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while Pt/Fe3O4 shows smaller SBET of 84 m2/g and Vpore of 0.20 cm3/g and larger dpore

195

of 8.0 nm. Generally, the larger SBET and Vpore facilitate adsorption and improve the

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accessibility of reaction sites for HCHO molecules, resulting in better catalytic

197

performance.

198

Figure 4a shows the high-resolution XPS spectra for Fe 2p of Fh-Na, Pt/Fh,

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Pt/Fe3O4 and Pt/Fh-c. For Fh-Na sample, Fe 2p3/2 and Fe 2p1/2 binding-energy (BE)

200

peaks are located at 711.9 and 725.3 Ev, respectively, which agrees with the reported

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BE values of Fe (III) species.44 As compared to those obtained for Fh-Na, the BE

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values of Fe 2p3/2 and Fe 2p1/2 for Pt/Fh, Pt/Fe3O4 and Pt/Fh-c shift to lower values,

203

located at 710.4 and 723.9 eV, respectively. This indicates an enlargement in the

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negative charge of Fe(III) atoms in the three samples. This charge enlargement for the

205

Fe(III) atoms is presumably due to the partial reduction of Fe(III) during preparation

206

process and the existence of interaction between Pt and Fe species.

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The deconvolution of the Pt 4f peaks visible on the XPS spectra of Pt/Fh,

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Pt/Fe3O4 and Pt/Fh-c is shown in Figure 4b. The divided peaks at 70.9-71.0 and

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72.4-72.5 eV are attributed to the Pt 4f 7/2 BE of Pt0 and Pt2+,

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further confirms that Pt has been successfully deposited on the support surface. These

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data indicate that the metallic and oxidized Pt co-exist in the samples studied. As

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compared to Pt/Fh-c, the fraction of Pt0 on Pt/Fh and Pt/Fe3O4 was found to be

213

smaller (Table S1 in supporting information), which can be attributed to the formation

214

of numerous Pt-O-Fe linkages.24 This result suggests a stronger interaction between Pt

215

and support in the case of the Pt/Fh and Pt/Fe3O4 catalysts. The interaction between Pt

216

and Fe species reflected by the presence of Pt-O-Fe linkages can lead to the electron

217

transfer from the H atoms of (Pt)-O-Fe-OH to Fe, thus leading to an enlargement in

218

the negative charge of Fe(III) atoms. The electron transfer from the H atoms of the

219

surface O-H to Fe can result in the formation of active H atoms, and subsequently the

220

formed active H atoms can readily bond O atoms of HCHO.7

45-47

respectively; this

221

As shown in Figure 4c, the O 1s signal of Fh-Na, Pt/Fh, Pt/Fe3O4 and Pt/Fh-c

222

could be deconvoluted into two peaks. The peaks at 529.4-530.3 and 531.2-531.3 eV

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223

are attributed to the lattice O (OL) and hydroxyl O and/or surface adsorbed O (OII),

224

respectively.44,48 The BE values of OII for Pt/Fh (531.2 eV), Pt/Fe3O4 (531.2 eV) and

225

Pt/Fh-c (531.1 eV) are similar to that for Fh-Na (531.3 eV), while the BE peaks of

226

OLattice for Pt/Fh (529.7 eV), Pt/Fe3O4 (529.6 eV) and Pt/Fh-c (529.4 eV) samples are

227

shifted to the lower values as compared to Fh-Na (530.3 eV). Moreover, the

228

percentage of OII follows the order: Fh-Na > Pt/Fh > Pt/Fh-c >Pt/Fe3O4. This

229

indicates that Pt/Fh possesses more surface hydroxyls and/or higher amount of

230

adsorbed water as compared to Pt/Fe3O4, and this result is consistent with the TG and

231

FTIR data (Figure S1 and S2 in supporting Information (SI)). Based on the results of

232

TG (Figure S1 in supporting information) and FTIR (Figure S2 in supporting

233

information), the higher OII in the case of Pt/Fh-c as compared to Pt/Fe3O4 partly

234

result from the surface adsorbed oxygen. A larger amount of surface hydroxyls can

235

result in higher dispersion of Pt,36 which was confirmed by the metal dispersion

236

analysis (Table 2). Namely, Pt/Fh showed higher Pt dispersion (22.1%) as compared

237

to that of Pt/Fe3O4 (18.0%) and Pt/Fh-c (16.0%). A higher amount of surface

238

hydroxyls and better Pt dispersion results in better performance of the catalyst for

239

HCHO decomposition at room temperature.

240

Figure 5a and b show the catalytic performance of Pt/Fh, Pt/Fe3O4, Pt/TiO2,

241

Pt/Fh-c and Pt/Fe2O3. The decrease in the HCHO concentration and increase in the

242

CO2 concentration, as well as no change of CO concentration (not shown), are seen

243

as a function of reaction time for all the catalysts, implying a complete

244

decomposition of HCHO into CO2 and water at room temperature. However, the

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HCHO concentration on Pt/Fh decreases much faster than those on Pt/Fe3O4, Pt/TiO2,

246

Pt/Fh-c and Pt/Fe2O3. That is, the HCHO concentration after 50 min decreases from

247

300 ppm to 30.3, 66.4, 104.2, 132.4 and 224.3 ppm for Pt/Fh, Pt/Fe3O4, Pt/TiO2,

248

Pt/Fh-c and Pt/Fe2O3, respectively. Accordingly the CO2 concentrations increase to

249

ca. 367, 318, 297, 229 and 95 ppm for Pt/Fh, Pt/Fe3O4, Pt/TiO2, Pt/Fh-c and Pt/Fe2O3,

250

respectively. The above results indicate that Pt/Fh shows higher catalytic activity

251

than Pt/Fe3O4, Pt/TiO2, Pt/Fh-c or Pt/Fe2O3 in HCHO oxidation at room temperature.

252

In addition, the observed increase in the CO2 concentration is larger than the

253

decrease in the HCHO concentration, mainly due to desorption of some HCHO

254

molecules from the reactor surface during experiment and subsequent their oxidation

255

to CO2, as well as the measurement accuracy of CO2. The fact that Pt/Fh shows

256

higher activity than Pt/Fe3O4 also reveals the effect of the preparation method on the

257

performance of Fh-based catalysts for HCHO removal at room temperature.

258

Furthermore, both Pt/Fh and Pt/Fe3O4 show higher catalytic activities than Pt/TiO2

259

and Pt/Fh-c, which is attributed to the abundance of surface hydroxyl groups, better

260

adsorption performance of Fh (SI Figure S3), larger specific surface area and pore

261

volume of the support. The calculated turnover frequency (TOF) for catalytic

262

decomposition of HCHO on Pt/Fh was 3.21 min-1 (Table 2), smaller than that on

263

Pt/Fe3O4 (3.28 min-1) and larger than that on Pt/Fh-c (2.03 min-1) and on Pt/TiO2

264

(2.82 min-1). The HCHO removal tests and TOF results obtained for Pt/Fh indicate

265

that the surface hydroxyl groups play an important role in the catalytic removal of

266

HCHO. As compared to Pt/Fh and Pt/Fe3O4, the activity of Pt/Fh-c toward HCHO

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decomposition is lower, although the corresponding Pt0/ (Pt0+Pt2+) value is higher

268

(Table S1 in supporting information). This indicates that an appropriate fraction of

269

Pt0/Pt2+ is desirable for enhancing the catalytic activity of the catalysts studied,

270

which is consistent with the previous report.24 The presented data indicate that the

271

three iron-based catalysts show different catalytic activities, which can be attributed

272

to the different surface properties of the supports. The surface –OH groups especially

273

play an important role in the catalytic HCHO removal, besides the beneficial

274

interactions between Pt and support (Figure S6 in supporting information). Since

275

several factors such as RH, surface properties of the support, Pt-support interactions,

276

etc. affect the catalytic HCHO removal, an accurate assessment of the role of the

277

surface –OH groups and the interaction between Pt and support in this process is

278

difficult; therefore, further studies are needed in this area.

279

The in situ DRIFT spectra of the Pt/Fh and Pt/Fe3O4 catalysts obtained upon

280

exposure to HCHO + O2 at room temperature are shown in Figure 6. After exposing

281

the catalysts to the gaseous mixture, the measured spectra exhibit the bands at 2883,

282

2778, 1612, 1450 and 1080 cm-1 in Figure 6a and at 2800, 2701, 1604, 1432 and 1257

283

cm-1 in Figure 6b. The peaks observed at ca. 1080 cm-1 in Figure 6a and at ca. 1257

284

cm-1 in Figure 6b are due to dioxymethylene (DOM).48,49 This observation indicates

285

partial oxidation of HCHO to DOM species, which can further combine with surface

286

oxygen to form formate species.50 Previous studies show that the bands at 2700-2900

287

cm-1 can be attributed to the stretch vibration of C-H, and the bands at 1604-1612 cm-1

288

and at 1432-1450 cm-1 can be assigned to the asymmetric and symmetric COO

16


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Page 17 of 33


289

stretching modes of formate species on the catalysts surface, respectively,49,51

290

suggesting that the surface oxygen and/or hydroxyls are active species in the partial

291

oxidation of HCHO into formate intermediates. No peaks associated with the

292

molecularly adsorbed HCHO are observed on Pt/Fh and Pt/Fe3O4 catalysts in flowing

293

HCHO +O2 at room temperature, indicating an immediate oxidation of adsorbed

294

HCHO into DOM and/or formate species on these two catalysts.

295

Besides the active surface hydroxyls directly participating in the HCHO

296

oxidation

into formate

species,

the

possible

mechanism

297

hydroxyls-assisted HCHO catalytic oxidation over Pt/Fh and Pt/Fe3O4 catalysts is

298

shown in Figure 7, based on the in-situ DRIFTS results and previous data15,26,35.

299

Briefly, oxygen from air is first adsorbed and splits into active oxygen radicals on the

300

surface of Pt nanoparticles. Meanwhile, the O atom of HCHO is adsorbed on the H

301

atom of the surface hydroxyl of the support by hydrogen bonding, which leads to the

302

positive charging of C atom in the HCHO molecule (step I). Then, the active oxygen

303

radical on the Pt surface readily attacks the electrophilic C atom, and the DOM

304

intermediate is formed due to the charge redistribution (step II). Subsequently, another

305

active surface oxygen on the Pt surface quickly attacks the C-H bond of the generated

306

DOM intermediate, and formate acid and active hydroxyl are formed (step III).

307

Meanwhile, the occupied hydroxyl group of the support is regenerated (step III).

308

Subsequently, the adsorbed formate acid is further oxidized into adsorbed CO2

309

(CO2)ads and H2O (H2O)ads by an active hydroxyl radical on the Pt surface (step IV).

310

Finally, the adsorbed CO2 and water desorb from the Pt surface, and Pt active sites are

17


of

the

surface


311

regenerated again (step V). The surface hydroxyls of the support assist the Pt

312

nanoparticles oxidating HCHO at room temperature in two aspects. Firstly, the

313

surface hydroxyls of the supports provide active adsorption sites for HCHO molecules,

314

resulting in much higher HCHO concentration in the vicinity of Pt nanoparticles.

315

Secondly, the hydrogen bond formed between HCHO and the surface hydroxyl of the

316

supports leads to the positive charging of C atom in the HCHO, causing that the

317

electronphilic C atom is easily attacked by the active oxygen radical on Pt

318

nanoparticles. So, it is easy to understand that Pt/Fh exhibits higher catalytic activity

319

toward HCHO oxidation due to the abundance of surface hydroxyls as compared to

320

Pt/Fe3O4 and Pt/TiO2.

321

The durability of the catalyst is of great significance for its practical applications.

322

The stability test of Pt/Fh, Pt/Fe3O4 and Pt/TiO2 in the process of decomposition of

323

HCHO at room temperature is shown in Figure 8. The HCHO removal (%) recorded

324

for six repeated cycles over Pt/Fh, Pt/Fe3O4 and Pt/TiO2 did not change as compared

325

to that obtained in the first cycle, suggesting that these catalysts have excellent

326

stability and efficient catalytic performance. It is clear that the Pt/Fh and Pt/Fe3O4

327

catalysts showed higher catalytic activities than Pt/TiO2, indicating their potential to

328

be efficient catalysts for complete decomposition of HCHO at room temperature. As

329

compared to Pt/Fe3O4, Pt/Fh showed a higher HCHO removal (%), further

330

demonstrating that the preparation process influences the catalytic performance of

331

Fh-based supported Pt catalyst.

332

18


Page 18 of 33

Page 19 of 33


333

Acknowledgements

334

This work was partially supported by the 863 Program (2012AA062701), 973

335

Program (2013CB632402), NSFC (21307038, 21433007, 51320105001 and

336

51272199), Key Project of Chinese Ministry of Education (212115), PSFC

337

(2012M521482 and 2013T60754) and Deanship of Scientific Research (DSR) of

338

King Abdulaziz University (90-130-35-HiCi).

339 340

Supporting Information Available

341

Six figures showing DTA and TGA profiles, FTIR spectra, HCHO adsorption vs.

342

time, the effect of humidity on the catalytic removal of HCHO, DRIFTS spectra and

343

TPR spectra for the catalysts studied together with the corresponding discussion,

344

references and table summarizing the XPS analysis. This information is available free

345

of charge via the Internet at http://pubs.acs.org.

346 347

References

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24


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Figure 1 XRD patterns of c-Fe2O3, Fh, Fh-Na, Pt/Fh, Pt/Fe3O4, Fh-c and Pt/Fh-c.

25



Figure 2 TEM images of (a) Pt/Fh, (b) Pt/Fe3O4 and (c) Pt/Fh-c. Insets in Figure 2a, b and c show the Pt HRTEM images of Pt/Fh, Pt/Fe3O4 and Pt/Fh-c, respectively; (d) Pt particle size distribution of Pt/Fh.

26


Page 26 of 33


300 250 200 150

0.06

dV/dW ( cm 3 . g -1 . nm -1)

3

Volume adsorbed ( STP cm /g)

Page 27 of 33

a 0.04

0.02

a

b

0.00 10

20

30

40

50

60

Pore diameter (nm)

100

b

50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( p/p0) Figure 3 Nitrogen adsorption/desorption isotherms and the corresponding pore size distributions (inset ) for Pt/Fh (a) and Pt/Fe3O4 (b).

27



Figure 4 High-resolution XPS profiles: Fe 2p spectra of Fh-Na, Pt/Fh, Pt/Fe3O4 and Pt/Fh-c (panel a), Pt 4f spectra of Pt/Fh, Pt/Fe3O4 and Pt/Fh-c (panel b), and O 1s spectra of Fh-Na, Pt/Fh, Pt/Fe3O4 and Pt/Fh-c (panel c).

28


Page 28 of 33


HCHO concentration / ppm

Page 29 of 33

300

(A)

250

e

200

d

150

c

100

b

50

a

0 0

10

20

30

40

50

Time / min ∆CO2 concentration / ppm

400

a

(B)

b

300

c 200

d

100

e 0 0

10

20

30

40

50

Time / min Figure 5 The concentration changes of formaldehyde (A) and ∆CO2 (B) as a function of reaction time for Pt/Fh (a), Pt/Fe3O4 (b) Pt/TiO2 (c), Pt/Fh-c (d) and Pt/Fe2O3 (e) catalysts at the initial HCHO concentration of 300 ppm (59% relative humidity).

29



Figure 6 In-situ DRIFTS spectra of (a) Pt/Fh and (b) Pt/Fe3O4 catalysts as a function of time in flowing HCHO + O2 at room temperature.

30


Page 30 of 33

Page 31 of 33


Figure 7 A simplified reaction pathway for complete oxidation of HCHO over the Pt/Fh and Pt/Fe3O4 catalysts

31



HCHO removal /%

100

Pt/Fh

Page 32 of 33

Pt/Fe3O4

Pt/TiO2

80 60 40 20 0 1

2

3

4

5

6

Recycle time Figure 8 Comparison of HCHO removal with recycle times obtained for Pt/Fh, Pt/Fe3O4 and Pt/TiO2.

32


Page 33 of 33


Table 1 Physical properties of the as-synthesized samples Samples

HCHO removal %

SBET (m2/g)

dpore (nm)

Vpore (cm3/g)

Fh-Na

41.2%

270

3.0

0.20

Pt/Fh

89.9%

222

4.4

0.28

Pt/Fe3O4

77.9%

84

8.0

0.20

Fh-c

24.7%

97

9.3

0.25

Pt/Fh-c

55.9 %

77

9.4

0.22

TiO2

14.4%

37

29.5, 52.6

0.15

Pt/TiO2

65.3%

35 a

30.3, 52.1 a

0.16 a

c- Fe2O3

4.0%

3.9

2.7

0.003

Pt/Fe2O3

25.3%

27

2.8, 9.4

0.05

SBET, BET specific surface area; Vpore, single-point pore volume; and dpore, pore width at the maximum of the pore size distribution. a

Data taken from ref (36).

Table 2 Pt dispersion and HCHO turnover frequency (TOF) at room temperature Catalysts

Pt dispersion (%)

TOF (min-1)

Pt/Fh

22.1

3.21

Pt/Fe3O4

18.0

3.28

Pt/Fh-c

16.0

2.82

Pt/TiO2

17.9

2.03

33


Highly Active Mesoporous Ferrihydrite Supported ... - ACS Publications

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