Hierarchically macro-mesoporous flowerlike Pt/NiO composite microspheres for efficient formaldehyde oxidation at room temperature

Abstract

Hierarchically macro-mesoporous flowerlike NiO microspheres (NiO-F) were first prepared by a hydrothermal method using Ni(NO₃)₂ and ethylenediamine as the precursors, and then Pt nanoparticles (NPs) were deposited onto the surface of NiO-F to obtain the Pt/NiO-F composite catalyst by a NaBH₄ reduction method. The prepared Pt/NiO-F sample was used for oxidative decomposition of formaldehyde (HCHO) at room temperature. The results indicate that the Pt/NiO-F composite catalyst shows higher catalytic activity than the Pt/NiO catalyst using NiO particles as the support (Pt/NiO-P), which is due to its high surface area, high-dispersion and accessible Pt NPs as well as hierarchically macro-mesoporous structure facilitating diffusion of reactants and products. The Pt/NiO-F catalyst also exhibited good catalytic stability in recycled experiments. This work may contribute to the development of high-performance catalysts for indoor air purification and related catalytic processes.

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1. Introduction

Formaldehyde emitted mainly from decoration and construction materials is one of the major indoor air pollutants. A long-time exposure to HCHO even with low concentrations for mankind may cause health problems such as nasal tumors, eye and skin irritation, and nasopharyngeal cancer etc.^1,2 In order to satisfy the acquirement of improving indoor environmental standards, a lot of efforts have been made to remove indoor HCHO. For example, HCHO can be removed by adsorption,^3–7 photocatalytic oxidation,^8–10 plasma technology,¹¹ thermal catalytic oxidation^12–35 and so on. Relatively speaking, room-temperature thermal catalytic decomposition of HCHO to CO₂ and H₂O has more advantages and better application potential than other techniques for the removal of HCHO because this technique is energy conservation and eco-friendly.^11–17 In the case of thermal catalytic oxidation, many supported noble metal catalysts (such as Pt,^12–24 Pd,^26–28 and Au^29–33) have been synthesized for the removal of indoor HCHO at low temperature even at room temperature. For example, HCHO can be partly oxidized into CO₂ and H₂O on Au/Co₃O₄–CeO₂ at 25 °C.²⁹ HCHO can be completely oxidized into CO₂ and H₂O on Au/CeO₂,^31–33 Pd/TiO₂ ²⁶ and Pt/TiO₂ ^{12–17,34,35} at room temperature. Comparatively, the Pt-supported catalysts exhibited higher catalytic performance for decomposition of HCHO at room temperatures. However, the Pt-supported catalysts with further enhanced performance must be developed from the view point of practical use and commerce.

The catalytic performance of supported catalysts is associated not only with the properties of catalysts but also with the morphology and structure of supports. In recent years, the fabrication of hierarchically macro-mesoporous materials has received great attention due to their high surface areas and mesopore–macropore system being able to efficiently transport reactants to catalytic sites.^36–40 For example, hierarchical macro-/mesoporous titania prepared by a simple hydrolysis and then calcination at 300 °C shows higher photocatalytic activity than P25.³⁸ In our previous work, hierarchically macro-mesoporous Pt/γ–Al₂O₃ composite catalyst was prepared and used for formaldehyde oxidation at room temperature. It was found that such hierarchically macro-mesoporous Pt/γ–Al₂O₃ catalyst showed enhanced catalytic activity than Pt supported on nanoparticle supports.⁴¹

In this study, the flowerlike NiO microspheres with hierarchically porous structure were first prepared and then used as the support to prepare the Pt/NiO-F catalyst. The as-prepared Pt/NiO-F catalyst also showed enhanced catalytic activity in oxidative decomposition of HCHO at room temperature than the Pt/NiO-P catalyst using NiO particles as the support.

2. Experimental

2.1 Sample preparation

Preparation of the NiO-F sample. In this work, all chemicals with analytical grade were used and without further purification. The preparation of hierarchically macro-mesoporous flowerlike NiO microspheres is similar to the previous work.⁴² In a typical preparation, Ni(NO₃)₂ aqueous solution (16 mL, 0.5 M) and ethylenediamine (C₂H₄(NH₂)₂, EDA, 3.2 mL) were mixed thoroughly, and then the mixed solution was added to a 200 mL Teflon-lined autoclave that contained NaOH aqueous solution (96 mL, 7 M). The solution was magnetically stirred for 10 min, and then the autoclave was sealed and placed in an electric oven set at 100 °C for 150 min. After this reaction, the light-green precipitate was collected, washed with distilled water for three times followed by a rinse with ethanol, and then dried in an oven at 80 °C for 2 h. Finally, the dried powder was calcined in air at 400 °C for 5 h to obtain the NiO-F sample. For the purpose of comparison, the NiO particles (denoted as NiO-P) were prepared by the same procedure only without adding ethylenediamine.

Preparation of the Pt/NiO-F composite catalyst. In a typical preparation procedure, 1 g of NiO-F was added into an H₂PtCl₆ solution (10 mL, 2.56 mmol L⁻¹) under magnetic stirring. After impregnation for 5 min, 2.5 mL of NaBH₄ (0.1 mol L⁻¹) and NaOH (0.5 mol L⁻¹) mixed solution was quickly added into the suspension under stirring for 5 min. After reduction, the suspension was dried at 80 °C for 12 h to obtain the Pt/NiO-F composite catalyst. The nominal weight ratio of Pt to NiO-F was fixed to be 0.5 wt%. For the purpose of comparison, 0.5 wt% Pt/NiO-P catalyst was also prepared with the same Pt deposition process using NiO particles as the support. The real content of Pt in the catalysts measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) was about 0.41 wt% (see Table 1).

Table 1 The basic parameters for the as-synthesized samples^a

Samples	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)	dpore (nm)	Pt wt%	Pt dispersion (%)	TOF min^−1
a Note: the TOF (the turnover frequency) value of the catalyst is calculated on the basis of surface Pt atoms per minute as following formula: TOF value = NHCHO/(NPt × t), NHCHO is the mole of HCHO conversion molecules, NPt is the mole of surface Pt atoms of catalyst, t is reaction time, here t is 21 min.
NiO-F	108	0.10	3.8
Pt/NiO-F	83	0.10	4.6	0.41	24.1	1.08
NiO-P	60	0.25	16.8
Pt/NiO-P	32	0.10	12.1	0.43	20.5	0.63

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2.2 Characterization

The X-ray diffraction (XRD) measurements were performed on a D/Max-RB X-ray diffractometer (Rigaku, Japan) with Cu K_α radiation at a scan rate (2θ) of 0.05° s⁻¹. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected on a JEM-2100F microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on VG ESCALAB250xi with X-ray monochromatisation. All binding energies (BE) were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The Brunauer–Emmett–Teller (BET) surface area (S_BET) of powders was evaluated from nitrogen adsorption data recorded by using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All the samples were degassed at 180 °C prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint method using adsorption data in the relative pressure (P/P₀) range of 0.05–0.3. The desorption isotherms were used to determine the pore size distributions by the Barrett–Joyner–Halenda (BJH) method, and assuming a cylindrical pore model.⁴³ The nitrogen adsorption pore volume at the relative pressure (P/P₀) of 0.97 was used to determine the pore volume and average pore size. The dispersion of Pt was measured and calculated on the basis of CO chemisorption performed by a BELCAT catalyst analyzer (BELCAT-B-293, BEL Japan). The sample was first pretreated in hydrogen (50 SCCM) at 200 °C for 15 min and purged with helium (50 SCCM) for 15 min at the same temperature. Then, the catalyst was cooled to room temperature and CO pulses were injected from a calibrated on-line sampling valve. CO adsorption was assumed to be complete after three successive peaks showed the same peak areas. A CO/Pt stoichiometry of 1 was used for calculations.

2.3 Catalytic activity evaluation

The catalytic activity test for HCHO oxidation was carried out in a dark condition at 20 °C in the same way as it is reported in our previous work.¹¹ 0.3 g of catalyst was dispersed on the bottom of glass Petri dish with a diameter of 14 cm. After placing the sample-coated dishes with a glass slide cover in the bottom of reactor, 8 μL of condensed HCHO solution (38%) was injected into reactor and a 5 watt fan on the bottom of the reactor began to run. After 2 h, the HCHO solution was volatilized completely and the concentration of HCHO was stabilized. At that time, a adsorption equilibrium was reached between the HCHO vapor and the reactor. The HCHO and CO₂ concentrations were on-line monitored by a Photoacoustic IR Multigas Monitor (INNOVA air Tech Instruments Model 1412). The initial concentration of HCHO after adsorption equilibrium was controlled at about 130 ppm, and then the glass slide cover on the Petri dish was removed to start the catalytic oxidation reaction of HCHO. Each set of experiment time was fixed to be 60 min. The CO₂ concentration increase (ΔCO₂, which is the difference of CO₂ concentration at t reaction time and at initial time, ppm) and HCHO concentration decrease were used to evaluate the catalytic performance.

3. Results and discussions

3.1 Phase structures and morphology

Fig. 1 shows the XRD patterns of the NiO-F, Pt/NiO-F, NiO-P and Pt/NiO-P samples. It can be seen that all diffraction peaks in XRD patterns can be indexed to the rhombohedral crystalline structure of NiO (JCPDS no. 44-1159). No additional diffraction peak can be observed for the Pt/NiO-F and Pt/NiO-P samples after Pt deposition. No observation of Pt diffraction peaks in the XRD patterns for the Pt/NiO-F and Pt/NiO-P samples is due to their low loading (0.5 wt%), small particle size, and good dispersion.^12,17 Compared with the NiO-F sample, the peaks in XRD pattern of the NiO-P sample are higher and narrower, which indicates that the latter has better crystallinity and the bigger crystallize size. According to the Scherrer's equation, the crystallize sizes of the NiO-F and NiO-P samples are 9.8 and 16.2 nm, respectively. After Pt deposition, the crystallize sizes of the NiO-F and NiO-P samples do not show obvious change and are 10.0 and 16.8 nm, respectively.

Fig. 1 XRD patterns of the NiO-F, Pt/NiO-F, NiO-P and Pt/NiO-P samples.

The SEM and TEM images of the as-synthesized NiO and Pt/NiO samples are showed in Fig. 2. It can be seen from the SEM image of the NiO-F sample in Fig. 2a that the obtained NiO-F products are composed of many uniform flowerlike microspheres with a hierarchical structure. The diameter of NiO-F microspheres is around 4–6 μm. Further observation indicates that flowerlike microspheres are consisting of thin nanosheets with a thickness of 70–100 nm (see inset of Fig. 2a) and widths and lengths of 1–3 μm. These nanosheets are aligned perpendicularly to the spherical surface, pointing toward centre of microsphere. Fig. 2b shows the SEM image of the Pt/NiO-F sample, indicating that the flowerlike morphology of the sample was preserved after Pt deposition. TEM and HRTEM results of the Pt/NiO-F sample are revealed in Fig. 2c and d, confirming the existence of Pt NPs. TEM image (Fig. 2c) of the Pt/NiO-F sample shows that the Pt NPs of ca. 2–6 nm are presented on the surface of the NiO-F sample. HRTEM image (Fig. 2d) of the Pt/NiO-F sample reveals that the lattice spacing in white circle is around 0.224 nm, which is the same as the lattice spacing of (111) planes of metallic Pt.⁴⁴ Fig. 2e shows the SEM image of the Pt/NiO-P sample. It can be seen that the NiO NPs size is about 10–20 nm and some small black dots (2–10 nm) are presented on the surface of NiO particles, which are confirmed to be Pt NPs by HRTEM (see Fig. 2f).

Fig. 2 SEM images of the NiO-F (a) and Pt/NiO-F (b) samples, TEM (c and e) and HRTEM (d and f) images of the Pt/NiO-F (c and d) and Pt/NiO-P (e and f) samples.

3.2 BET surface areas and pore size distributions

The texture properties of the samples were further elucidated by N₂ adsorption–desorption analysis. Fig. 3a and b shows N₂ adsorption–desorption isotherms and the corresponding pore-size distribution curves for the NiO-F and Pt/NiO-F samples. The above two samples show isotherms of type IV (International Union of Pure and Applied Chemistry (IUPAC) classification),⁴³ suggesting the existence of mesopores. Compared with the isotherms of the NiO-F sample, the isotherms of the Pt/NiO-F sample move downwards slightly, which indicates a decrease of the specific surface area. Also, two hysteresis loops with different shape can be observed for the two samples at a high relative pressure range between 0.4 and 1.0, which suggests bimodal pore size distributions existing in the mesoporous and macroporous regions.²² The pores formed between primary NiO crystallites within nanosheets probably results in the presence of hysteresis loop at low relative pressure range of 0.4–0.7. However, the type H3 of the hysteresis loops at a high relative pressure range of 0.8–1.0 indicates the existence of larger slitlike pores, which are formed by aggregation of NiO nanosheets into the flowerlike superstructures.⁴² The pore-size distribution curves (Fig. 3b) calculated from the desorption branch of the isotherms further confirm the existence of mesopores and macropores. A wide pore size distribution (2–100 nm) is observed for the NiO-F and Pt/NiO-F samples with smaller mesopores (peak pore at approximately 4 nm for NiO-F and 3.5 nm for Pt/NiO-F, respectively) and larger mesopores (peak pore at approximately 20 nm for the NiO-F sample and 40 nm for the Pt/NiO-F sample, respectively). The N₂ adsorption–desorption analysis cannot give macroporous information (>100 nm), but it can be acquired from the SEM images. The SEM image of as-prepared NiO-F sample (inset of Fig. 2a) exhibits the existence of a lot of open irregular slit-shaped pores with a size of about 0.5–2 μm. After Pt deposition, the macroporous structures were well preserved (see Fig. 2b). These open macroporous channels may serve as ideal gas-transport routes for gas molecules into the interior space of NiO.^22,41 Table 1 lists the BET surface area (S_BET), pore volume (V_pore) and pore size (d_pore) of the NiO-F and Pt/NiO-F samples, confirming a slight decrease of the specific surface area for the Pt/NiO-F sample as compared to the NiO-F sample. The larger density of Pt (21.45 g cm⁻³) than NiO (6.60 g cm⁻³, rhombohedral, note that the specific surface area is expressed per gram of the sample) and a partial coverage of NiO's surface by the Pt NPs and Na⁺ from NaBH₄ and NaOH probably cause the smaller specific surface area for the Pt/NiO-F sample.¹²

Fig. 3 Nitrogen adsorption–desorption isotherms (a and c) and the corresponding pore-size distribution curves (b and d) for the NiO-F and Pt/NiO-F samples (a and b), and the NiO-P and Pt/NiO-P samples (c and d), respectively.

However, N₂ adsorption–desorption isotherms and the corresponding pore size distribution curves for the NiO-P and Pt/NiO-P samples (Fig. 3c and d) are different from those of the NiO-F and Pt/NiO-F sample. Although N₂ adsorption–desorption isotherms for the NiO-P and Pt/NiO-P samples are also type IV, but only one hysteresis loop is found. From their pore size distribution curves, a peak pore diameter of approximately 12 nm is observed for both samples, which indicates that the NiO-P and Pt/NiO-P samples are of the typical materials with a monomodal pore structure.

3.3 XPS analysis

The chemical state of atoms in the obtained samples was investigated by XPS. The high-resolution XPS spectra of Ni 2p, Pt 4f, Ni 3p and O 1s regions for the NiO-F and Pt/NiO-F samples are shown in Fig. 4. The high-resolution XPS spectra of Ni 2p are shown in Fig. 4a. The Ni 2p signal can be fitted into six peaks. The binding energies at 853.8, 855.7, 856.9 and 861.1 eV for the NiO-F sample are assigned to the Ni 2p_3/2 peaks with one main peak (855.7 eV) and three peaks satellite peaks (853.8, 856.9 and 861.1 eV).^42,45–47 The 873.1 and 879.3 eV peaks are assigned to the main peak (873.1 eV) and the satellite peak (879.3 eV) of Ni 2p_1/2. Compared with the NiO-F sample, a negative shift of the binding energy (0.5 eV) is observed for the main peak of Ni 2p_3/2 for the Pt/NiO-F sample. These observed Ni 2p peaks are attributed to Ni²⁺ of NiO,^42,45–47 which is consistent with the results of the XRD analysis. Fig. 4b shows the high-resolution Pt 4f and Ni 3p spectra of the NiO-F and Pt/NiO-F samples. The main peaks at 67.4–67.9 eV with two shoulder peaks at 70.6 and 73.7–73.8 eV are attributed to Ni 3p of NiO.⁴⁵ As compared to the NiO sample, a negative shift (about 0.5 eV) for the main peak (67.4 eV) of Ni 3p of the Pt/NiO-F sample can also be observed. One possible explanation for the negative shift of Ni 2p_3/2 and Ni 3p is that NiO of the Pt/NiO-F sample is partially reduced into Ni⁺ during the NaBH₄ reduction process, which results in the negative shift of binding energies of Ni 3p of the Pt/NiO-F sample. The similar negative shift was also observed for Ti 2p of the Pt/TiO₂ sample with the same NaBH₄ reduction process.⁸ One extra peak at ca. 68.8 eV is observed for the Pt/NiO-F sample, which can be assigned to Pt 4f_7/2 of metallic Pt. A negative shift for Pt 4f_7/2 is also observed as compared to the binding energy of Pt 4f_7/2 for bulk metallic Pt⁰ (71.2 eV),^45,48 which can be caused by the electron transfer from NiO to Pt due to strong metal–support interactions (SMSI).¹² However, the Pt 4f_5/2 peak of metallic Pt is not observed, which is due to the weak peak of Pt 4f_5/2 (at about 74.0 eV) overlapped by the strong shoulder peak of Ni 3p (at 73.7–73.8 eV) of NiO.

Fig. 4 High-resolution XPS spectra for Pt 4f and Ni 3p (a), O1s (b) of the NiO-F and Pt/NiO-F samples.

From the high-resolution XPS spectrum of O 1s in Fig. 4c, it shows that the oxygen on the sample surface is fitted into three peaks with a distinct peak at 530.0 eV and two shoulder peaks at 530.7/531.3 and 531.8 eV. The main peak at 530.0 eV with shoulder peak at 530.7/531.3 eV corresponds to the O 1s core level of the O²⁻ anions in the NiO.⁴⁹ The peak at 531.8 eV is attributed to surface hydroxyl groups on the surface of the Pt/NiO-F sample.

3.4 Catalytic activity

Changes of formaldehyde and CO₂ concentrations dependence on reaction time over the NiO-F, NiO-P, Pt/NiO-F, and Pt/NiO-P samples are shown in Fig. 5. As can be seen from this figure, when the NiO-F and NiO-P samples were used, the HCHO concentration was observed to decrease quickly in the initial 3–9 min and then changed little in the subsequent time. Also, no obvious change was observed for the CO₂ concentration in the whole process, suggesting that HCHO was not decomposed and was mainly adsorbed on the surface of the NiO-F and NiO-P samples. Further observation indicates that the HCHO concentration after adsorption equilibrium for the NiO-F sample is lower than that for the NiO-P sample, indicating more HCHO molecules were adsorbed on the surface of the NiO-F sample. It is easy to understand because the NiO-F sample has a larger specific surface area (108 m² g⁻¹) than the NiO-P sample (60 m² g⁻¹, see Table 1), and therefore more surface atoms are exposed and more surface hydroxy groups are contained in the NiO-F sample, which can absorb HCHO molecules. However, in the case of the Pt/NiO-F and Pt/NiO-P samples for HCHO oxidation, a dramatic decrease of the HCHO concentration and a great increase of the CO₂ concentration were observed with increasing reaction time, suggesting that HCHO was oxidized into CO₂ and H₂O. Also, it is noteworthy that the rates of HCHO concentration decrease and CO₂ concentration increase over the Pt/NiO-F sample are different from those over the Pt/NiO-P sample, indicating their different catalytic activity in HCHO oxidation. The HCHO concentration decrease faster over the former than over the later; accordingly, a faster increase of the CO₂ concentration was observed for the former, indicating the Pt/NiO-F sample is the more active catalyst towards HCHO oxidation. There are three main reasons to explain the different catalytic performance of both samples towards HCHO oxidation. First, the Pt/NiO-F sample has a bigger surface area than the Pt/NiO-P sample, which is beneficial for the HCHO adsorption and the deposition of Pt NPs with high dispersion. Second, the Pt dispersion (24.1%) of the Pt/NiO-F sample is slightly higher than that (20.5%) of the Pt/NiO-P sample (see Table 1), which means more surface Pt atoms existing in the Pt/NiO-F sample. It must be emphasized that only the surface Pt atoms can activate O₂ molecules to active oxygen species for HCHO oxidation. The third possible explanation is that Pt NPs on the surface of the Pt/NiO-F sample can be more easily accessed by HCHO molecules than those on the surface of the Pt/NiO-P sample. This is because the hierarchically macro-/mesoporous structure of the Pt/NiO-F sample is more beneficial for the transport of gas molecules in the catalyst (see Fig. 6) than the only mesoporous structure of the Pt/NiO-P sample.^22,41 The TOF (the turnover frequency) values of the Pt/NiO-F and Pt/NiO-P samples at reaction time of 21 min were also calculated and shown in Table 1. It can be seen the TOF value (1.08 min⁻¹) of the Pt/NiO-F sample is bigger than that (0.63 min⁻¹) of the Pt/NiO-P sample, which partially verifies that the hierarchically macro-/mesoporous structure of the Pt/NiO-F sample is more beneficial for HCHO oxidation than the only mesoporous structure of the Pt/NiO-P sample. Because the above two samples have the same Pt deposition method and the same support element composition, so each Pt atom on the surface of the two samples should have the same or similar oxidation ability for HCHO molecules. But the different TOF values for the Pt/NiO-F and Pt/NiO-P samples mean their different oxidation abilities for HCHO molecules, which is mainly due to their different pore structure and the resulted different diffusion rates for reactants and products. So, three factors account for the high catalytic activity of the Pt/NiO-F sample: high surface area, high-dispersion and accessible Pt NPs as well as hierarchically porous structure. And among them, the hierarchically porous structure may be the most important one. It is notable that the larger increase value of the CO₂ concentration (about 300 ppm in 60 min) than the decrease value of the HCHO concentration (about 130 ppm in 60 min) over the Pt/NiO-F sample is observed (Fig. 5), which is due to the oxidation of some HCHO molecules desorbed from the reactor surface into CO₂.²² The catalytic oxidation decomposition mechanism of HCHO over Pt/NiO can be understood as following steps according to the previous reports.^12,15,16,21 Firstly, HCHO and O₂ molecules are adsorbed onto the surface of Pt/NiO (M) (step (1) and (2)), respectively. And the adsorbed active O atom species can be formed by the dissociation of O₂ molecule on the surface of Pt NPs.^21,50 After that, adsorbed HCHO is oxidized into adsorbed formate species by adsorbed active O atom (step (3)). Then, the surface formate species are further oxidized into carbonic acid species (H₂CO₃) (step (4)) or decompose into adsorbed CO and H₂O (step (5)). And finally, carbonic acid will decompose into CO₂ and H₂O (step (6)) or adsorbed CO is oxidized into CO₂ (step (7)).

HCHO + M → HCHO(ads) + M (1)

O₂ + Pt → 2[O]_ads + Pt (2)

HCHO(ads) + [O]_ads → HCOOH(ads) (3)

HCOOH(ads) + [O]_ads → H₂CO(ads) (4)

HCOOH(ads) → CO(ads) + H₂O (5)

H₂CO(ads) → CO₂ + H₂O (6)

CO(ads) + [O]_ads → CO₂ (7)

Fig. 5 Changes in formaldehyde concentration (a) and ΔCO₂ (the difference between CO₂ concentration at t reaction time and initial time, ppm) (b) as a function of reaction time for the NiO-F, NiO-P, Pt/NiO-F, and Pt/NiO-P samples.

Fig. 6 Illustration of the fast diffusion of reactants (O₂ and HCHO) and products (CO₂ and H₂O) in the hierarchically porous channel of the Pt/NiO-F catalyst.

The stability of catalysts and their efficiency are of great importance from the view point of practical use. Therefore, the stability of the Pt/NiO-F and Pt/NiO-P catalysts was further evaluated by reusing these catalysts in the recycle experiments, and the recycled experiments were carried out for five times and the results are shown in Fig. 7. After five recycles, the Pt/NiO-F and Pt/NiO-P samples did not show any obvious decline of catalytic activity, indicating their relatively high stability during catalytic degradation of HCHO molecules.

Fig. 7 Changes in formaldehyde concentration (a) and ΔCO₂ (b) as a function of reaction time for the Pt/NiO-F sample in five repeated tests.

4. Conclusions

In summary, the Pt/NiO-F catalyst with hierarchically macro-mesoporous structure was prepared by a hydrothermal and following NaBH₄ reduction method. The flowerlike morphology of the NiO-F sample was preserved after Pt deposition. The flowerlike Pt/NiO-F sample showed higher catalytic activity for HCHO oxidation at room temperature than the particle-like Pt/NiO-P sample, which is due to the former's high surface area, high-dispersion and accessible Pt NPs as well as hierarchically macro-mesoporous structure. The Pt/NiO-F sample also exhibited good catalytic stability in recycled experiments. This work will provide some new insights into the design and fabrication of advanced catalysts for indoor air purification.

Acknowledgements

The work was partially supported by the 863 Program (2012AA062701), 973 Program (2013CB632402), NSFC (51572074, 21177100 and 51272199), Fundamental Research Funds for the Central Universities and Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1), Project from Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control of Nanjing University of Information Science and Technology, Jiangsu Province Innovation Platform for Superiority Subject of Environmental Science and Engineering (KHK1207), and Project from China Scholarship Council.

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