Pt for rapid capture and catalytic oxidation of formaldehyde at room temperature

Journal Pre-proof Three-dimensional carbon foam supported MnO2 /Pt for rapid capture and catalytic oxidation of formaldehyde at room temperature Jiawei Ye (Investigation), Minghua Zhou (Investigation) (Methodology), Yao Le (Investigation) (Funding acquisition) (Methodology), Bei Cheng (Funding acquisition) (Investigation), Jiaguo Yu (Funding acquisition) (Writing - review and editing)

PII:

S0926-3373(20)30104-1

DOI:

https://doi.org/10.1016/j.apcatb.2020.118689

Reference:

APCATB 118689

To appear in:

Applied Catalysis B: Environmental

Received Date:

24 December 2019

Revised Date:

19 January 2020

Accepted Date:

24 January 2020

Please cite this article as: Ye J, Zhou M, Le Y, Cheng B, Yu J, Three-dimensional carbon foam supported MnO2 /Pt for rapid capture and catalytic oxidation of formaldehyde at room temperature, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118689

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Three-dimensional carbon foam supported MnO2/Pt for rapid capture and catalytic oxidation of formaldehyde at room temperature

Jiawei Ye a, Minghua Zhou b,*, Yao Le c, Bei Cheng a, Jiaguo Yu a,d,* State Key Laboratory of Advanced Technology for Materials Synthesis and

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a

Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, P. R. China

Hubei Key Laboratory of Wudang Local Chinese Medicine Research, Hubei

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b

c

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University of Medicine, Shiyan, 442000, PR China

College of Architecture and Materials Engineering, Hubei University of Education,

School of Materials Science and Engineering, Zhengzhou University, Zhengzhou,

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d

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Gaoxin Road 129, Wuhan 430205, PR China

450001, P. R. China.

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* Corresponding authors.

Tel.: 0086-27-87871029, Fax: 0086-27-87879468, E-mail: [email protected]

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(J. Yu); [email protected] (M. Zhou),

Graphical Abstract

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Highlights: 

3D carbon foam-MnO2 core-shell structure prepared by in-situ growth method



Combination of adsorption and oxidation for HCHO removal at room

Abundant surface active oxygen species resulted from oxygen vacancies

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

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temperature

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Abstract

Catalytic oxidation of formaldehyde (HCHO) at room temperature is one of the most

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viable approaches to indoor HCHO pollution abatement. Herein, three-dimensional (3D) carbon foam decorated with Pt/MnO2 nanosheets (Pt/MnO2-CF) was in-situ

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synthesized for room-temperature catalytic oxidation of HCHO. The 3D Pt/MnO2-CF

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with a low platinum content of 0.3 wt% exhibited excellent catalytic activity because of its hierarchically porous structure facilitating the diffusion of reactant molecules. Moreover, an abundance of active oxygen species resulting from oxygen vacancies are favorable for HCHO oxidation. In addition, the carbon foam substrate exhibited a very good HCHO adsorption capability, which helps achieve prompt reduction in `2

HCHO concentration in the gas-phase and subsequent complete oxidation of adsorbed HCHO. The combination of adsorption and oxidation was more favorable for oxidative decomposition of HCHO. This work demonstrates that such 3D nanocomposites with low noble metal loading have promising application for indoor HCHO removal and air purification. Keywords: formaldehyde oxidation; carbon foam; air purification; platinum;

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manganese oxide

1. Introduction

Air quality in indoor environments has received increasing concern for its ever greater

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importance in determining airborne pollutant exposure and related human health risks.

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Particularly, due to the extensive use of decorative and building materials, formaldehyde (HCHO) has become a major indoor air pollutant worldwide [1-3].

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Longstanding exposure to even extremely low concentration of HCHO may cause

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various illnesses from respiratory tract irritation to cancer [4]. Therefore, the development and deployment of devices and materials which can efficiently remove

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airborne HCHO is essential for meeting indoor air quality requirement and promoting human health.

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In the last two decades, several categories of methods including physical adsorption

[5], chemical adsorption [6,7], photocatalytic degradation [8-10] and thermal catalytic oxidation [11-17] have been investigated for eliminating indoor HCHO. Among these methods, adsorption is the most convenient method which can reduce the concentration of gaseous HCHO to low levels within a short period of time using `3

adsorbents with large surface area and specific surface functionality. However, the finite adsorption capacity and typically incomplete regeneration of adsorbents after multiple use have limited the development and application of this method. By contrast, well-designed catalysts can continuously and completely oxidize gaseous HCHO into harmless CO2 and H2O with no need for extra energy input. Therefore, room-temperature catalytic oxidation is generally considered as the most viable

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approach to removing HCHO form indoor air [18]. In recent years, intensive research has been carried out to develop catalysts for oxidation of indoor HCHO, and metal

oxide-supported noble metal catalysts have frequently demonstrated superior

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ambient-temperature HCHO oxidation performance [19], such as Pt/TiO2 [20-24],

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Pd/TiO2 [25-27], Pt/Al2O3 [28,29], Pt/Fe3O4 [30], Pt/MnO2 [31], Au/CeO2 [32] and Pt/Co3O4 [33]. In particular, supported Pt catalysts typically exhibited prominent

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catalytic abilities toward HCHO oxidation. Nonetheless, the majority of the reported

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Pt-loaded catalysts are in powdered form, which cannot be directly used in practical application [34]. Extra procedures are needed to adhere such powdered catalysts to

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monolithic substrates, in which process the voids of the catalyst may become blocked and specific morphology destroyed, leading to drastic decrease in the catalytic

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activities of the final products. Therefore, it is desirable to prepare monolithic catalysts by in-situ loading the catalytically active components onto suitable substrate material. Recently, monolithic catalysts such as Pt/MnO2-decorated TiO2 nanotube array [35], Pt-decorated Al2O3 molecular sieve [28] and nickel foam coated with Pt/NiO [36] were reported with excellent HCHO degradation efficiency at ambient `4

temperature. Moreover, in contrast to powdery catalysts, these monolithic catalysts have great workability, holding good prospect of practical application in indoor air purification. Despite these advantages, the development of monolithic supported Pt catalysts has been limited possibly due to the lack of facile in-situ fabrication methods employing low-cost substrates. Carbon materials, such as carbon hollow spheres [37], MOF-derived porous carbon

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dodecahedra [38,39], graphene oxide [40,41] and carbon foam [42,43], have been extensively explored owing to its unique properties such as low bulk density, good workability, high electrical conductivities, suitable pore size distribution and chemical

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stability [44]. Among them, Carbon foam (CF), with a three-dimensional (3D)

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network structure, has been widely used in many research fields, especially in lithium ion battery [42], adsorptive removal of water pollutant [43], supercapacitors [45],

[47].

On

the

other

hand,

manganese

oxide

(MnOx),

as

an

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pollutants

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support material for electrocatalysts [46] and photocatalytic degradation of indoor air

environment-friendly material, has recently been widely investigated as catalyst for

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HCHO decomposition [48] or as support material for Pt-based HCHO oxidation catalysts [16,17,31,49-51] due to its distinct redox property, various crystal structures

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and morphologies [52]. Thus, in-situ loading MnOx nanomaterials on 3D porous CF materials may provide a low-cost yet suitable support material for preparing monolithic Pt-based HCHO oxidation catalysts. Herein, 3D manganese dioxide/carbon foam (MnO2-CF) composites with porous framework and a core-shell structure were prepared via an in-situ hydrothermal `5

growth method, in which process the δ-MnO2 nanosheet shell was formed on the surface of CF as a result of the redox reaction between the C substrate and potassium permanganate (KMnO4) as the Mn precursor. The Pt/MnO2-CF catalyst was then fabricated by loading low level of Pt through the impregnation-NaBH4 reduction method. This 3D monolithic catalyst could overcome the disadvantages of powder MnOx in practical use and achieved high catalytic efficiency toward HCHO oxidation

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at ambient temperature. The carbon foam skeleton in the Pt/MnO2-CF catalyst acts not merely as the substrate of MnO2 nanosheets but also as an adsorbent for enriching

the HCHO at the interface of CF and Pt/MnO2. The combination of adsorption and

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oxidation might contribute new understanding for the design of practical monolithic

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2. Experimental Section

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catalysts for efficient removal of HCHO at room temperature.

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2.1. Preparation of carbon foam

The as-purchased melamine resin sponge was washed by ethanol three times and

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dried at 80 °C for 6 h. Then, the dried sponge with the size of 10 cm  5 cm  2 cm was put in the middle of a tube furnace and heated to 900 °C in N2 at 10 °C/min for 1

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h, yielding 3D carbon foam, which is denoted as the CF sample. 2.2. Preparation of MnO2-carbon foam The MnO2-CF sample was fabricated by an in-situ growth method using a hydrothermal technique. In a typical synthesis, a piece of carbon foam with the size of 4 cm  2 cm  0.8 cm (ca. 0.1 g) was submerged in 80 mL of KMnO4 solution (20 `6

mmol/L). After sonication for 10 min, the solution with a piece of carbon foam was transferred to a 100-mL stainless-steel autoclave with Teflon-lining, heated to 140 °C and kept at this temperature for 10 min. After cooling, the MnO2-carbon foam was washed with deionized (DI) water until the purple color of KMnO4 disappeared, and then air-dried at 80 °C for 6 h. 2.3. Preparation of Pt/MnO2-CF

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In a typical preparation procedure, the obtained MnO2-carbon foam was submerged in 50 mL of H2PtCl6 aqueous solution and shaken for 30 min at 25 °C in a thermostatic

shaker. Then 2 mL of NaBH4 (0.1 M) and NaOH (0.1 M) mixed solution was added to

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the above suspension and kept shaking for 10 min. After that, the Pt-loaded

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MnO2-carbon foam was washed with DI water three times and dried at 80 °C overnight.

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by the above method.

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For the purpose of comparison, Pt-loaded carbon foam (Pt/CF) was also prepared

2.4. Preparation of Pt/MnO2 microsphere

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For comparison, Pt-loaded MnO2 nanospheres were prepared as follows. 0.5 g of KMnO4 and 1 mL of HCl (37 wt%) aqueous solution were dissolved into 80 mL of DI

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water. After stirring, the solution was sealed in a Teflon-lined stainless-steel autoclave (100 mL) and then heated to 140 °C and kept at this temperature for 1 h. After cooling, the products were collected by centrifuging and washed with DI water until the purple color of KMnO4 disappeared. Then the products (denoted as MnO2-MS) were dried and loaded Pt nanoparticles by the above impregnation-NaBH4 reduction method `7

(denoted as Pt/MnO2-MS). 2.5. Characterization The phase structure of CF, MnO2-MS and MnO2-CF was determined by X-ray diffraction (XRD). The morphologies of CF, MnO2-MS, MnO2-CF and Pt/MnO2-CF were observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS)

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analysis was performed for MnO2-CF and Pt/MnO2-CF to probe the surface chemistry

of the samples. The Brunauer-Emmett-Teller (BET) specific surface area (SBET),

single point pore volume (Vp) and the average pore size (dp) were analyzed from

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nitrogen adsorption–desorption isotherms. Hydrogen temperature-programmed

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reduction (H2-TPR) and oxygen temperature-programmed desorption (O2-TPD) tests were performed to analyze the redox properties. In-situ diffuse reflectance infrared

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Fourier transform spectroscopy (DRIFTS) for the Pt/MnO2-CF sample was performed

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in a mixed gas of O2 and 80 ppm HCHO at room temperature. The Pt content in the prepared samples was measured by inductively coupled plasma atomic emission (ICP-AES).

Detailed

characterization

methods

are

given

in

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spectrometry

Supplementary Materials.

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2.6. Catalytic activity test Catalytic HCHO oxidation tests were performed at room temperature (25 °C) in an organic glass box reactor equipped with a photoacoustic field gas monitor (INNOVA Air Tech Instruments, model 1412i). In each test, 0.1 g of sample in a glass culture dish covered with a glass slide was placed in the reactor. Ten microliters of condensed `8

HCHO solution (38 wt%) was injected into the reactor and quickly volatilized by an electrical fan on the bottom of the reactor. When initial concentration of HCHO for each test was adjusted to approximately 200 ppm, HCHO removal test was started immediately after the glass slide was removed. The adsorption efficiency or catalytic oxidation activity of HCHO was evaluated by the decrease of HCHO concentration

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and the increase of CO2 concentration (ΔCO2).

3. Results and discussion 3.1. Phase structure and morphology

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Crystallographic structures of MnO2-MS, CF and MnO2-CF were investigated by

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XRD (Fig. 1), and the broad X-ray diffraction peaks located at about 25° for CF and MnO2-CF samples can be assigned to the (002) plane of highly graphitized carbon

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[37]. The diffraction peaks located around 12.2°, 24.4°, 36.7° and 65.8° for MnO2-MS

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and MnO2-CF samples can be assigned to (001), (002), (110) and (020) planes of the hexagonal-phase δ-MnO2 (JCPDS No. 80-1098), respectively [53]. These results

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indicate that δ-MnO2 was successfully grown on the carbon foam and the carbon skeleton in CF was preserved after the in-situ growth of δ-MnO2 via the hydrothermal

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KMnO4 oxidation process.

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Fig. 1. XRD patterns of MnO2-MS, MnO2-CF and CF samples.

The morphologies of the samples were observed by electron microscopies (Fig. 2).

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The carbonized melamine foam showed good 3D interconnected network with

macroscopic porous structure (Fig. 2a). After the KMnO4 oxidation reaction,

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MnO2-CF (Fig. 2b) showed a relatively rough surface compared to CF, and uniform

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MnO2 nanosheet layers (inset of Fig. 2b) can be seen on the surface of the carbon foam, confirming the successful growth of MnO2. It is noteworthy that the smooth

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carbon surface can be observed beneath the MnO2 layers (inset of Fig. 2b). This indicates that the skeleton of carbon foam was maintained after the hydrothermal

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process, which is consistent with the XRD result. Comparing the FESEM images of

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Pt/MnO2-CF (Fig. 2c) with that of MnO2-CF (Fig. 2b), no apparent morphological change was observed, suggesting that NaBH4 reduction did not remarkably change the microstructure and morphology of MnO2-CF. The MnO2-MS sample had a morphology of porous flower-like microspheres with diameters of 1-2 μm, which were constructed by tightly packed MnO2 nanosheets (Fig. 2d). `10

TEM images (Fig. 2e) of the Pt/MnO2-CF sample further demonstrated that MnO2 nanosheets with lateral sizes of ca. 50 nm (inset) were uniformly grown over the carbon foam, forming a core-shell structure. The HRTEM image of Pt/MnO2-CF (Fig. 2f) clearly demonstrated d-spacing of 0.48 and 0.29 nm, matching the interlayer distance of (110) and (020) facets of δ-MnO2, respectively. In addition, a lattice fringe with spacing of ca. 0.22 nm was evident, which corresponds to the (111) lattice plane

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of metallic Pt [36].

The growth of MnO2 nanosheets can be ascribed to the redox reaction between CF

–

2–

–

4MnO4 + 3C + H2 O → 4MnO2 + CO3 + 2HCO3

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and KMnO4, according to the following chemical equation (Eq. 1).

(1)

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MnO2 nanosheets were in-situ grown on the carbon foam by reacting with the surface carbon. It is noted that, when prolonging the hydrothermal reaction time to

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products were obtained.

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more than 1 h, the carbon foam skeleton was totally consumed and only powdery

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Fig. 2. FESEM and high-magnification FESEM (inset) images of (a) CF, (b) MnO2-CF, (c) Pt/MnO2-CF and (d) MnO2-MS. (e) TEM and high-magnification TEM images (inset), and (f) HRTEM image of Pt/MnO2-CF

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3.2. XPS Analysis

The XPS survey spectra (Fig. 3a) of MnO2-CF and Pt/MnO2-CF reveal peaks with binding energies of 645 (Mn 2p), 398 (N 1s), 532 (O 1s), and 285 eV (C 1s), respectively. The existence of N can be ascribed to the residuals from carbonization of

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the melamine foam. Pt 4f peak was not obvious in the survey spectra due to the low Pt loading amounts (0.27 wt%, Table 1). Fig. 3b displays the high-resolution Mn 2p spectra of MnO2-CF and Pt/MnO2-CF samples. Two peaks at ~642.4 and ~654 eV can be assigned to Mn 2p3/2 and Mn 2p1/2, respectively. After curve-fitting analysis of the Mn 2p3/2 spectra, peaks at 641.7 and 642.9 eV are respectively ascribed to Mn3+ and Mn4+ [16,54]. It can be observed that

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the ratio of Mn3+/Mn4+ for Pt/ MnO2-CF was much higher than that of MnO2-CF. This could be because some surface Mn4+ was reduced to Mn3+ during the NaBH4 reduction process. At the same time, oxygen vacancies would be generated to

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maintain charge balance according to the following chemical equation (Eq. 2), which

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is similar to the oxygen vacancy generation during NaBH4 reduction of TiO2 [55]. 2Mn4+ + [OL ]2– + H– → 2Mn3+ + OH– + [OV ]

(2)

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where [OL] is a lattice oxygen atom and [OV] is an oxygen vacancy.

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The higher ratio of Mn3+/Mn4+ implies the presence of more oxygen vacancies which are favorable for oxidation reactions.

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The deconvoluted O 1s spectra of MnO2-CF and Pt/MnO2-CF samples (Fig. 3c) display three peaks at 529.8-530.0, 531.2 and 532.9 eV, which can be assigned to

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lattice O atom (OL), hydroxyl (OH) and adsorbed O species (OA), respectively [54,56,57]. Notably, the percentage of OH and OA increased after Pt loading, indicating the larger content of surface adsorbed oxygen species in Pt/MnO2-CF. Surface active O species such as O2–, O– and –OH group, which result from surface

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oxygen vacancies, play a significant role in oxidation reaction and are beneficial to HCHO oxidation [54,57]. The Pt 4f spectra of Pt/MnO2-CF are shown in Fig. 3d. Peak doublets are attributed to the Pt 4f7/2 and Pt 4f5/2 levels, which can be deconvoluted to components corresponding to metallic Pt and Pt2+ [31]. Specifically, the Pt 4f7/2 and Pt 4f5/2 peaks of Pt0 were centered at 71.6 and 74.9 eV, whereas those of Pt2+ were centered at 72.9

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and 76.2 eV. Metallic Pt was dominant, indicating the successful deposition of Pt

nanoparticles. The appearance of Pt2+ can be explained by the generation of Pt-O-Mn

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bond, which implies the great interaction force between Pt and MnO2.

Fig. 3. Survey (a) and high-resolution XPS spectra for (b) Mn 2p and (c) O 1s of MnO2-CF and Pt/MnO2-CF samples. (d) High-resolution XPS spectrum for Pt 4f of Pt/MnO2-CF sample. `14

3.3. Textural properties The N2 sorption isotherms and pore size distribution (PSD) curves of the samples are presented in Fig. 4. All the isotherms are type IV with type H3 hysteresis loops, indicative of slit-like mesopores resulting from assembled MnO2 nanosheets [58] and consistent with the FESEM results. From the PSD curves (inset of Fig. 4), MnO2-CF and Pt/MnO2-CF samples possess similar hierarchical pore structures, which are

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mainly composed of micropores and smaller mesopores (<20 nm), while MnO2-MS and Pt/MnO2-MS samples are largely composed of micropores and larger mesopores

(>20 nm). Though micropores (<2 nm) cannot be directly observed in the PSD curves,

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the high value at 2 nm as well as the high N2 adsorption at low relative pressure

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implies the existence of micropores. Notably, the Pt/MnO2-CF sample contains less micropores and more mesopores at ca. 8 nm compared to MnO2-CF sample. This is

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likely due to partial reduction of MnO2 by NaBH4 in the Pt deposition process, thus

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generating small mesopores as well as oxygen vacancies mentioned above. The generation of mesopores gives rise to the increase of SBET, which can be seen in Table

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1. On the contrary, the SBET of MnO2-MS sample decreased after Pt loading process. This is because of partial blocking of large mesopores by NaOH particles generated

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from NaBH4 during the reduction process. Besides, less small mesopores are generated in MnO2-MS compared to MnO2-CF after NaBH4 reduction, which may be due to the better crystallinity and chemical stability of MnO2-MS compared to MnO2-CF.

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Fig. 4. N2 sorption isotherms and PSD curves (inset) of MnO2-CF, Pt/MnO2-CF, MnO2-MS and Pt/MnO2-MS samples. Table 1. Textural properties and Pt loading amount of the samples a. SBET (m2/g)

Vpore (cm3/g)

dpore (nm)

Pt wt%

CF

4

0.007

6.2

–

3

0.003

4.9

0.25

54

0.094

7.0

–

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Pt/CF

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Samples

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MnO2-CF

64

0.12

7.5

0.27

MnO2-MS

42

0.13

17.4

–

Pt/MnO2-MS

36

0.12

13.7

0.29

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Pt/MnO2-CF

3.4. H2-TPR and O2-TPD measurements The reducibility of the prepared materials was characterized by H2-TPR to differentiate the activity of surface adsorbed oxygen. As shown in Fig. 5a, two main `16

peaks at higher temperature in the TPR profiles were actually composed of three reduction processes, which represent the sequential reduction reaction of KxMnO2 → Mn2O3 → Mn3O4 → MnO [16,59]. The weak peak in the lower range of 150-300 °C is assigned to the reduction of surface O species (e.g., O–, O2–, –OH) resulting from surface oxygen vacancies [50]. Among all samples, this peak appeared at the lowest temperature for Pt/MnO2-CF, indicating that the adsorbed O species in Pt/MnO2-CF is

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most prone to reduction. In other words, the surface oxygen species in Pt/MnO2-CF have the highest oxidation activity at a certain temperature (for example, room

temperature). Besides, the reduction peaks of lattice oxygen in the Pt-loaded samples

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(Pt/MnO2-CF and Pt/MnO2-MS) shifted to lower temperature compared to the Pt-free

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ones. This is caused by hydrogen spillover from Pt to MnO2, confirming the existence of Pt NPs [60].

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The O2-TPD profiles of Pt/MnO2-CF and Pt/MnO2-MS are depicted in Fig. 5b. The

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physically adsorbed O2 was desorbed at 50 °C in the flow of He purging. As shown in Fig. 5b, two desorption peaks in the ranges of 400-450 and 500-550 °C were detected,

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which correspond to chemically adsorbed oxygen species and bulk lattice oxygen, respectively. Compared to Pt/MnO2-MS, the Pt/MnO2-CF sample exhibited lower

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desorption peak for the chemically adsorbed oxygen species, implying the higher mobility and reactivity of surface active oxygen species in this sample [50]. The above results indicated that Pt/MnO2-CF can provide more mobile and active surface oxygen species to participate in the catalytic reaction than Pt/MnO2-MS, likely leading to a better catalytic performance for HCHO oxidation. `17

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Fig. 5. (a) H2-TPR profiles of MnO2-MS, Pt/MnO2-MS, MnO2-CF and Pt/MnO2-CF samples and (b) O2-TPD profiles of Pt/MnO2-MS and Pt/ MnO2-CF samples. 3.5. HCHO removal performance The HCHO removal activities of CF, Pt/CF, MnO2-CF, Pt/MnO2-CF and Pt/MnO2-MS at room temperature are shown in Fig. 6. For CF and MnO2-CF, HCHO concentration decreased rapidly but stabilized at 59 and 57 ppm after 40 min, while CO2 `18

concentration kept nearly unchanged during the whole removal test, suggesting that these two samples have negligible catalytic activity for complete HCHO oxidation at 25 °C. On the other hand, Pt/MnO2-CF, Pt/CF and Pt/MnO2-MS samples exhibited HCHO oxidation abilities, since the concentration of CO2 increased with time. This result indicates that Pt NPs are vital for room-temperature HCHO oxidation in our catalyst system. Compared to Pt/MnO2-CF sample, Pt/CF sample showed very poor

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HCHO oxidation activity due to small SBET, which inhibited the dispersion of Pt NPs, and the lack of surface O species, which played important roles in HCHO oxidation process, suggesting that carbon-supported Pt catalysts without metal oxides are not

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favorable for room-temperature HCHO oxidation. Meanwhile, Pt/MnO2-MS exhibited

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much lower oxidation activity than Pt/MnO2-CF, as reflected by the much slower decline of HCHO concentration and increase in CO2 concentration. As a powdery

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catalyst, the inevitable aggregation of MnO2 microspheres may lead to the decline in

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HCHO-oxidation ability of Pt/MnO2-MS sample. On the other hand, the 3D carbon foam skeleton in Pt/MnO2-CF can restrain the aggregation of MnO2 nanosheets, and

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hierarchically porous structure can also accelerate the exchange of reactants [61]. In addition, the CF-containing samples exhibited similar HCHO uptakes in the first 15

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minutes, which indicates that adsorption of HCHO by CF takes the predominant role for HCHO removal in this period of time. The concentration of HCHO almost reached adsorption equilibrium for CF, MnO2-CF and Pt/CF samples, and a smaller amount of HCHO was oxidized for Pt/CF sample. In the following 45 minutes, the HCHO concentration remarkably decreased for Pt/MnO2-CF because HCHO can be `19

efficiently oxidized into H2O and CO2, and a HCHO removal of 91% was achieved after 60 min. The adsorption of HCHO by CF can enrich the formaldehyde at the interface of CF and Pt/MnO2 [47]. Therefore, the combination of adsorption and oxidation can more quickly and efficiently remove indoor HCHO compared to adsorption or oxidation alone. In summary, Pt/MnO2-CF displayed the best HCHO oxidation activity owing to its large SBET, abundant surface oxygen species, 3D

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hierarchical porous structure and combined effects of efficient adsorption and

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oxidation.

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Fig. 6. (a) Real-time HCHO concentration and (b) increase of CO2 concentration of prepared samples.

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For practical usage, the stability of Pt/MnO2-CF is also of great significance. The recyclability of the sample for catalytic oxidation of HCHO was evaluated by 5

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recycling tests (Fig. 7). Apparently, the Pt/MnO2-CF sample exhibited excellent

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repeatability, and nearly 85% HCHO removal was obtained even for the 5th cycle.

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Fig. 7. Real-time HCHO concentration and corresponding ΔCO2 of Pt/MnO2-CF sample in recycling test.

3.6. Intermediates and mechanism To study the catalytic reaction mechanism of HCHO over 3D Pt/MnO2-CF, in situ DRIFTS tests were performed (Fig. 8a). The bands located at 2896, 2861 and 2801 cm–1, which were observed after the exposure of HCHO and O2 for only 1 min and

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kept nearly unchanged in the following 60 min, could be ascribed to ν(CH) stretching of adsorbed HCHO [62], indicating the quick adsorption and saturation of gaseous HCHO on Pt/MnO2-CF sample.

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Besides, bands associated with ν(CH) (2963 cm-1), νas(OCO) (1558 cm–1) and νs(OCO)

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(1368 cm–1) vibration modes of formate species were observed [50,63,64], whereas

the bands at 1457 and 1240 cm–1 can be assigned to δ(CH2) and ν(CH2) modes of

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dioxymethylene (DOM) [63]. The bands for formate species and DOM appeared

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within 1 min and became stronger with time, indicating that the catalytic HCHO oxidation reaction was continuing and the intermediates (formate species and DOM)

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were accumulated on the surface of Pt/MnO2-CF. The broad negative band at 3419 cm-1 can be attribute to the consumption of surface OH groups. Moreover, the band at

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1653 cm–1 associated with δ(H2O) mode of adsorbed water implies that H2O is one product of HCHO oxidation reaction [63].

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Fig. 8. In situ DRIFTS spectra of (a) Pt/MnO2-CF and (b) MnO2-CF samples under an atmosphere of HCHO and O2 at ambient temperature. On account of the DRIFTS analysis, a plausible mechanism for HCHO oxidation

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over Pt/MnO2-CF sample is put forward and illustrated in Fig. 9. First, HCHO

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molecules (reactant) are adsorbed onto OH groups by hydrogen bonding, while O2 molecules (reactant) are adsorbed onto and activated by Pt nanoparticles

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simultaneously (step I) [36,65,66]. Then, the adsorbed HCHO is oxidized to DOM (intermediate) by activated O2, and the reacted O2 molecules are supplemented

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immediately (step II). Subsequently, DOM is quickly oxidized and transformed into formate species (intermediate) by activated O2 (step III). After that, formate species are further oxidized into unstable carbonic acid (step IV), which will quickly decompose into CO2 and H2O (products) and be easily released from the surface of Pt/MnO2-CF catalyst (step V). Meanwhile, the active sites of the catalyst are `23

regenerated. In summary, noxious formaldehyde can be totally mineralized into

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harmless carbon dioxide and water on the surface of Pt/MnO2-CF catalyst.

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4. Conclusions

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Fig. 9. Proposed reaction mechanism for HCHO oxidation over Pt/MnO2-CF.

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Three-dimensional Pt/MnO2-CF catalyst was synthesized via in-situ growth of MnO2 nanosheets by hydrothermal treatment of carbon foam in KMnO4 solution, followed

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by Pt loading through NaBH4 reduction method. The hierarchical porous structure is favorable for the transfer of reactants and products, which can accelerate the oxidation

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rate of HCHO. Meanwhile, abundant surface active oxygen species originating from oxygen vacancies are beneficial for the HCHO oxidation process. Moreover, the combination strategy of adsorption and oxidation can more quickly reduce the HCHO concentration in air. On the other hand, as a monolithic 3D catalyst, Pt/MnO2-CF is easier to be reused and recycled compared to powdery ones. Considering the low Pt `24

loading amount, this monolithic porous catalyst is very promising in practical use and may shed light on the development of highly effective and economical catalysts for room-temperature HCHO removal. Credit author statement

Minghua Zhou: Investigation; Methodology. Yao Le: Investigation; Funding acquisition; Methodology. Bei Cheng: Funding acquisition, Investigation.

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Acknowledgements

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Jiaguo Yu: Funding acquisition, Writing - review & editing.

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Jiawei Ye: Investigation

This work was supported by NSFC (51602098, 21871217, U1905215 and U1705251),

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and Self-determined and Innovative Research Funds of SKLWUT (2017-ZD-4).

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