WO3 nanoflakes assembled hollow microspheres for room-temperature formaldehyde oxidation activity

Journal Pre-proofs Full Length Article Hierarchical Pt/WO3 Nanoflakes Assembled Hollow Microspheres for RoomTemperature Formaldehyde Oxidation Activity Yao Le, Lifang Qi, Chao Wang, Shaoxian Song PII: DOI: Reference:

S0169-4332(20)30519-5 https://doi.org/10.1016/j.apsusc.2020.145763 APSUSC 145763

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Please cite this article as: Y. Le, L. Qi, C. Wang, S. Song, Hierarchical Pt/WO3 Nanoflakes Assembled Hollow Microspheres for Room-Temperature Formaldehyde Oxidation Activity, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145763

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Hierarchical Pt/WO3 Nanoflakes Assembled Hollow Microspheres for Room-Temperature Formaldehyde Oxidation Activity

Yao Lea,b,c,*, Lifang Qia,c, Chao Wang a,c, Shaoxian Songb a

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

Gaoxin Road 129, Wuhan 430205, PR China. b

School of Resources and Environmental Engineering, Wuhan University of

Technology, Luoshi Road 122, Wuhan 430070, P.R. China. c

Hubei Engineering Technology Research Center of Environmental Purification Materials, Hubei University of Education, Gaoxin Road 129, Wuhan 430205, PR China.

*Corresponding authors. Tel: 0086-27-87943983, E-mail addresses: [email protected] (Y. Le).

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Abstract Hierarchical tungsten trioxide (WO3) hollow microspheres were successfully fabricated via a facile solution method and heat treatment, followed by platinum (Pt) deposition. The Pt/WO3 composite showed superior catalytic activity for formaldehyde (HCHO) removal under ambient condition, resulting from the synergistic effect between Pt and WO3. The optimal Pt loading amount is determined to be 1.0 wt%. The hierarchical hollow architecture endowed Pt/WO3 with relatively large SBET and opened porous architecture. These promote the diffusion and adsorption of HCHO on the Pt/WO3 composite. The present work affords new route to design highly effective catalyst for environmental protection. Keywords: Tungsten trioxide; Hierarchical; Hollow microspheres; Formaldehyde removal

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1. Introduction Nowadays, human mental and physical health are greatly impacted by indoor air quality. As a priority indoor pollutant, formaldehyde (HCHO) is often found in buildings and poses a great threat to people’s health even at very low concentration [1]. Over the last few decades, two other indoor air carcinogens, such as radon and PAHs (polycyclic aromatic hydrocarbons), have been concerned lesser, nevertheless, formaldehyde remains the main indoor air pollutant [2]. Thus, it is important to find effective strategies to remove indoor formaldehyde, including adsorption [3,4], photocatalysis [5-7], plasma technology [8] and catalytic thermal decomposition [9]. Catalytic decomposition is used as one of the best technologies to remove HCHO completely [9,10]. Metal oxide is widely used as catalyst support for the removal of formaldehyde [11-18]. Usually, HCHO can be effectively decomposed to CO2 and H2O by using supported noble metal materials at room temperature [19-28], while, generally speaking, HCHO can only be removed by supported non-noble metal materials under higher temperature [29,30]. Whereas, researchers have developed some approaches to improve the noble metal utilization efficiency and reduce the load amount of noble metal in order to manufacture highly effective and low cost supported noble metal catalysts. This can be achieved by adjusting the texture properties, morphology, and surface chemical property of the supports [19]. As an excellent candidate, among metal oxides, WO3 is widely used in different research fields [31], for instance, gas sensors [32,33], solar hydrogen generation [34], photoelectrocatalytic water oxidation [35], heterogeneous catalysis [36], photocatalysis 3

[37], etc. As an important catalyst, WO3 could be very suitable for application in removal of air pollutants, which is due to its advantages, such as remain stable in corrosive and oxidative conditions, safe and low cost. WO3-based catalysts have been turned out to effectively decompose some common indoor air pollutants into CO2. Sun et al [38] confirmed that the m-Ag/WO3 photocatalyst could effectively degrade most of the acetaldehyde into CO2 under visible light irradiation. Gaseous 2-propanol (IPA) could also be oxidized into CO2 by WO3-based photocatalyst [39]. The limited use of WO3 catalyst is due to the ineffective oxidative decomposition of organic air pollutants. Thus, it is necessary to improve its catalytic activity, and loading noble metals is one of the most effective methods [31,40]. Abe et al [41] prepared Pt-loaded WO3 catalyst, which could decompose the organic pollutants efficiently both in the liquid and the gas phases. Morphology control has a great impact on catalytic performance of WO3. Various morphologies of WO3 materials has been reported, such as nanorods [42,43], nanofibers [44], nanotubes [45], nanosheets [46] and 3D porous structures [47-49]. Porous WO3 catalyst regularly exhibits better catalytic properties, which could be assigned to the larger specific surface area, efficient diffusion of pollutants and more reactive sites in the porous structure [31]. In this work, WO3 hollow spheres were prepared by using a low-cost and simple solution method, and loaded Pt NPs (nanoparticles), which possessed superb HCHO removal efficiency under ambient condition. This has certain guiding significance for the development of hierarchical porous catalyst with excellent HCHO conversion efficiency under ambient condition. 4

2. Experimental 2.1 Preparation Preparation of hierarchical WO3 nanoflakes assembled hollow microspheres was according to the previously reported method [50]. SrCl2 (0.27 g) and PMAA (polymethacrylic acid) (50 mg) were dissolved in distilled water (40 mL), followed by adding Na2WO4 (0.33 g) into the above mixture. After adjusting the pH value to 12, the solution was sealed and let it stand for 12 h under ambient temperature. The samples were obtained using centrifugation, then, washed by water and ethanol for 3 times, finally, dried at 60 oC. To synthesize WO3 hollow microspheres, the as-prepared SrWO4 precursor was dipped in HNO3 solution (8 mol/L) for 24 h, and then washed and dried. The WO3 hollow microspheres were obtained by calcining the above samples at 300 ◦C in air for 2 hours. To obtain the Pt/WO3 catalyst, WO3 (0.1 g) hollow microspheres were dispersed in distilled water (30 mL), and trace volume of H2PtCl6 solution was added, then a mixed solution which contains NaOH and NaBH4 was injected into the above solution and then stirring 60 min. subsequently, the gray precipitate was collected, then, water wash 5 times. The washed products were dried at 60 oC for 10 h. The resulting Pt/WO3 catalysts were denoted as WO3-Pt r, where r represents the weight percentage of Pt loaded in the sample. The WO3 reagent was also deposited with Pt NPs, which was labeled as WO3R-Pt r. 2.2 Characterization The phase structure, morphology and surface chemistry composition of the 5

catalysts were analyzed using Rigaku X-ray diffractometer (XRD), JEOL JSM-7500 scanning electron microscopy (SEM), JEM-2100F transmission electron microscopy and VG ESCALAB250 X-ray photoelectron spectroscopy (XPS), respectively. The specific surface area (SBET) and porosity were tested by ASAP 2020 N2 adsorption instrument. H2 temperature-programmed reduction (H2-TPR) was obtained on BELCAT-B equipment. Diffuse reflectance infrared Fourier transform spectra (DRIFT) was characterized on Nicolet IS50 apparatus, and introduction of HCHO+ O2 flows. 2.3 Measurement of catalytic performance The HCHO catalytic performance was measured in a sealed reactor. The temperature and humidity were controlled at 25oC and 50%, respectively. Firstly, the catalyst (0.1 g) was scattered into a clean glass dish, then, put it into the reactor and covered by slide. Subsequently, 10 L of formaldehyde solution (38% mass concentration) was added into the reactor. After 2 hours, formaldehyde concentration reaches equilibrium (approximate 260 ppm). HCHO, CO2, CO and H2O were recorded by INNOVA 1412 Gas Monitor.

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Results and discussion

3.1 Phase and morphology XRD patterns (Fig. 1) confirmed the phase structures of all the catalysts. For SrWO4, all peaks in the pattern correspond to the tetragonal scheelite (JCPDS No. 080490) [51]. For WO3 hollow microspheres, the characteristic peaks in the pattern are attributed to the monoclinic WO3 (JCPDS No. 83-0951) [52]. For the Pt loaded 6

composite catalysts, no obvious diffraction peaks related to Pt can be found, mainly owing to the relatively low Pt content and well dispersed Pt NPs in the samples [52]. Moreover, the peaks intensity for Pt/WO3 catalysts decreased with increasing Pt concentration, majorly because of the synergistic effect between Pt and WO3.

Fig.1 XRD patterns of WO3, WO3-Pt0.1, WO3-Pt0.5, WO3-Pt1.0, WO3-Pt2.0 and SrWO4.

SEM, TEM and HRTEM images illustrated the morphological structure of WO3 and Pt/WO3 samples (Fig. 2). As can be seen in Fig. 2a and b, Pt/WO3 sample appear obviously hollow nature and loading of Pt nanoparticles have no significant effect on the size of WO3 (ca. 5 m). The hollow microspheres are composed of many nanoflakes as the building blocks (Fig. 2c), which assemble to form the hierarchical structure. Further observation of the broken spheres revealed that the thickness of hollow spheres was about 300-400 nm. The nanoflake surface is very rough, which implies that the 7

nanoflakes are composed of small crystallites.

Fig. 2 Low (a) and high (b) magnification SEM, TEM (c) and HRTEM (d) images of WO3-Pt1.0, low magnification SEM (e) of WO3 reagent.

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As shown in Fig 2d, the lattice spacing of 0.376 nm and 0.226 nm match the theoretical interplanar spacing of WO3 (020) planes and metallic Pt (111) planes, respectively. Moreover, the HRTEM results presented that the Pt NPs (ca. 3 nm) were evenly deposited on the surface of WO3 nanoflakes. As is known to all, small sized and well dispersed Pt NPs could be conductive to increasing the oxidation reaction sites and improving the catalytic property of Pt nanoparticles. Contrarily, WO3 reagent (Fig. 2e) was composed of several hundred nanometer particles with irregular morphology. 3.2 Textural properties The porous structures of the WO3 and WO3-Pt1.0 catalysts were measured by nitrogen sorption analysis (Fig. 3). The shape of all the isotherms were type IV, which was related to mesoporous materials [50]. The higher absorption at the high-pressure part suggested the presence of macropores that could be formed between nanoflakes of WO3 hollow spheres. The hysteresis loops for both samples were the combined H2/H3 type, resulting from the ink bottle-like and slit-like mesopores [53]. Hysteresis loop at P/P0 = 0-0.4 gradually disappeared, indicating the overlapping of the pore size distributions of intra- and inter-aggregated pores [50].

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Fig. 3. N2 sorption isotherm and pore size distribution (inset) for WO3 and WO3Pt1.0 samples.

Moreover, compared with WO3 sample, the isotherm of Pt/WO3 sample shifted down slightly. This suggests that SBET and pore volume (Vp) decreased after Pt deposition. The pore size distributions are shown in the inset of Fig. 3. Apparently, both mesopores and macropores existed in the WO3 and Pt/WO3 samples. The WO3 sample contained mesopores (2-50 nm) and macropores (＞50 nm). While, Pt/WO3 sample also contained mesopores and macropores, and a peak pore located at 33 nm. In general, the mesopores could be ascribed to the inter-space of primary grains, the macropores were from aggregation of nanoflakes. It is well known that hierarchical porous structures facilitate the diffusion of reactant and product molecules inside catalysts [54]. The SBET, Vp and average pore size (dp) of the samples are presented in Table 1. The SBET value of WO3 sample was larger than that of WO3-Pt1.0 sample, while the dp value was 10

smaller. It is possible that the surface area and pores were partially covered by Pt nanoparticles. Table 1 Properties of WO3, WO3-Pt1.0 and WO3 reagent. Samples

Pt wt%

SBET (m2/g)

Vp (cm3/g)

dp (nm)

WO3

0

30

0.12

15.7

WO3-Pt1.0

1.0

23

0.12

21.2

WO3 reagents

0

4.1

0.011

11.0

3.3 Reducibility The reducibility of WO3 and WO3-Pt1.0 samples was measured by H2-TPR (Fig. 4). For pure WO3, only one weak peak could be found at 325 oC. While, for WO3-Pt1.0, two peaks could be observed. The first negative peak confirmed the existence of metallic Pt, resulting from desorption of hydrogen from metallic Pt. Additionally, HxWO3 would be formed which was originated from the hydrogen adsorption on the WO3 surface [55].

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Fig. 4. H2-TPR profiles of WO3 and WO3-Pt1.0 samples.

The second peak suggested a larger consumption of hydrogen, which is caused by the reduction of lattice oxygen on WO3. In contrast, pure WO3 did not consume a lot of hydrogen. Accordingly, the Pt loading improved the surface activity of the WO3 hollow microspheres. Nevertheless, it should be noted that the promotion in surface activity could greatly improve the HCHO oxidation performance.

3.4 XPS characterization The surface chemical states of WO3 and WO3-Pt1.0 were measured by XPS analysis (Fig. 5). As can be seen in the survey spectra (Fig. 5a), both samples had W, O, Sr and C elements. The C1s peak (284.8 eV) was ascribed to C species derived from XPS instrument. Sr species derived from Sr2+ because of the SrWO4 precursor. For WO3 sample, the W4f spectrum (Fig. 5b) containing W7/2 (~35.6 eV) and W5/2 (~37.8 12

eV), the peaks were very symmetrical, implying the existence of W6+ state [50]. For WO3-Pt1.0 sample, owing to the strong interaction between Pt and WO3, the binding energies of W4f had a negative displacement (0.2 eV). Fig. 5c showed the O1s spectrum, deconvoluted into two peaks (Oa and Ob). Oa peak (530.2 eV) was derived from the lattice oxygen of W-O-W, the Ob peak (531.5 eV) was due to the surface adsorbed oxygen [15]. After the Pt loading, the surface molar ratio of Ob/Oa increased from 0.117 to 0.378, indicating that more surface adsorbed oxygen was introduced resulting from the Pt deposition [56]. This might be achieved through the proceeding of promoting vacancies transfer on the catalyst surface [57]. Fig. 5d showed the Pt4f spectrum of WO3-Pt1.0 catalyst. Two peaks located at 71.0 and 74.4 eV suggest the presence of metallic Pt, corresponding to the Pt4f7/2 and Pt4f5/2 peaks of Pt [19]. The theoretical value of metallic Pt4f7/2 peak is 71.2 eV. In contrast, the binding energy of Pt4f7/2 peak for WO3-Pt1.0 catalyst shifted negatively by 0.2 eV. It could be assigned to the electron transfer from WO3 to Pt due to the difference of work function. Additionally, during the Pt deposition process, WO3 was partially reduced by NaBH4, the W4f peaks of WO3-Pt1.0 shifted negatively compared with WO3.

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Fig. 5 XPS survey spectra (a), high-resolution W4f (b), O1s (c) and Pt4f (d) XPS spectra for WO3 and WO3-Pt1.0.

3.5 Catalytic activity tests

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Fig. 6 Concentration changes of HCHO (a) and CO2 (b) as a function of reaction time on WO3-Pt0.1, WO3-Pt0.5, WO3-Pt1.0, WO3-Pt2.0, WO3R-Pt1.0 and WO3. CO2 is the change in CO2 concentration relative to the background level.

The HCHO oxidation performances over the samples are shown in Fig. 6. It was 16

discernible that WO3 was inert for catalytic oxidation of HCHO (Fig. 6a), the HCHO conversion efficiency was only 3% within 60 min, and the CO2 concentration of the WO3 sample basically unchanged (Fig. 6b). The results showed that the WO3 sample cannot effectively oxidized HCHO into CO2 and H2O at ambient condition. The slight decreasing of HCHO concentration was caused by adsorption. This implied that the Pt played a key role in the HCHO decomposition. Contrarily, Pt-loaded catalysts possessed high HCHO conversion efficiency. The HCHO conversion efficiency was 57%, 81%, 97%, 95% and 31% for WO3-Pt0.1, WO3-Pt0.5, WO3-Pt1.0, WO3-Pt2.0 and WO3R-Pt1.0, respectively (within 60 min). Moreover, all the as-prepared WO3 hollow microspheres exhibited better catalytic activity than that of WO3 reagent. It is easy to understand that the WO3 hollow microspheres had hierarchical porous structures and relatively larger specific surface areas than WO3 reagent. In addition, the CO2 concentrations (Fig. 6b) of all Pt/WO3 catalysts increased obviously, implying that Pt/WO3 samples could effectively oxidized HCHO to CO2 and H2O. The results implied that the HCHO conversion performance of Pt/WO3 samples improved with increasing Pt content. Whereas, the HCHO conversion efficiency for WO3-Pt2.0 was slightly lower than that of WO3-Pt1.0. This might be due to the introduction of more Pt NPs would lead to the aggregation and the growth of Pt NPs. Thus, the optimal deposition amount of Pt was determined to be 1.0 wt%. As shown in Fig. 6, Δ CO2 for all Pt/WO3 samples are higher than the decreased value of corresponding HCHO. This is because of adsorption of extra HCHO from reactor, and then oxidized to CO2 [58]. Firstly, HCHO adsorbed on the surface of the catalysts 17

during the reaction [59], followed by forming reactive oxygen species originated from the activation of chemisorbed oxygen on the interface between Pt and WO3 [60]. The presence of Pt and oxygen vacancies were beneficial for the electron adsorption, transfer and forming of activated oxygen, resulting in the superior performance of HCHO removal for Pt/WO3 catalysts under room temperature.

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Fig. 7 HCHO concentration decrease (a) and CO2 concentration increase (b) with time over WO3-Pt1.0 catalyst in six repeated tests.

In order to investigate the stability of catalysts, recycling experiments of the WO3Pt1.0 catalyst for oxidation of HCHO were carried out for six times. As can be observed in Fig. 7, the HCHO conversion efficiency for WO3-Pt1.0 catalyst had no obvious changes after six cycles. The superior cycling stability is very important for its practical use.

3.6 Reaction mechanisms To study the decomposition mechanism of formaldehyde, the surface reaction of catalysts was measured using in-situ DRIFTS. The DRIFTS spectra for WO3-Pt1.0 catalyst over time after exposure to HCHO and O2 under ambient condition are shown in Fig. 8. The absorption peaks at 3519 cm-1 are from the —OH group [61]. The declined peaks can be attributed to the consumption of —OH group during the oxidation reaction. Whereas, the formation of surface water leaded to the enhancement of the absorption bands at around 3000-3500 cm-1. Meanwhile, the bands at 2977, 2875, 2790 cm-1 are ascribed to CH of HCOO— species [62]. Moreover, peaks at 1648 and 1593 cm-1 are from the surface water and asCOO— of formate species, respectively [61]. Additionally, adsorption peaks, assigned to dioxymethylene (DOM) species (1475 cm-1 for (CH2) ) and formate species (1357 cm-1 for sCOO— and  cm-1 for COO—), increased with the reaction time. The results illustrated that the main 19

intermediates were formate and DOM during formaldehyde oxidation reaction [54,63].

Fig. 8 In situ DRIFTS spectra change of with time in the presence of HCHO + O2 at room temperature for WO3-Pt1.0. According to above characterization analysis, the decomposition mechanism of formaldehyde is suggested (Fig. 9). After HCHO adsorption on the surface of WO3Pt1.0, the surface-active oxygen is formed due to strong interaction between Pt and WO3. Then, HCHO are decomposed into formate and DOM species by chemisorbed and surface-active oxygen of catalyst. Subsequently, the intermediates are completely decomposed and oxidized to CO2 and H2O [24,64].

Fig. 9 Possible mechanism of HCHO decomposition on the Pt/WO3 catalysts 20

4

Conclusion Hierarchical WO3 hollow microspheres were successfully fabricated via a facile

solution method and calcination treatment, then loaded with Pt. The Pt/WO3 composite catalyst possessed better HCHO conversion efficiency under ambient condition than pure WO3. The catalytic performance of Pt/WO3 samples improved with the increase of Pt content, the optimal deposition amount of Pt was determined to be 1.0 wt%. The hierarchical hollow architecture endowed Pt/WO3 more active sites for diffusion and transfer for reactants. Observed form the H2-TPR profiles, the surface-active oxygen originated from Pt/WO3 promoted the HCHO oxidation performance. Additionally, DRFITS spectroscopy results illustrated that the main intermediates were formate and DOM species, then further decomposed to CO2 and H2O. Thus, the hierarchical Pt/WO3 composite catalyst possesses a promising potential for indoor air purification. Acknowledgement This work was supported by NSFC (51602098, 51508168), Research Project of Hubei Provincial Department of Education (Q20163002), the Natural Science Foundation of Hubei

Province

(2015CFC882),

China

Postdoctoral

Science

Foundation

(2015M580673) and the Technology Creative Project of Excellent Middle & Young Team of Hubei Province (No. T201620).

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Credit Author Statement Yao Le: Conceptualization, Methodology, Resources, Writing - review & editing. Lifang Qi: Investigation, Writing - review & editing. Chao Wang: Writing - review & editing. Shaoxian Song: Writing - review & editing.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Hierarchical WO3 hollow microspheres fabricated then loaded with Pt. Pt/WO3 composite with good HCHO removal efficiency than pure WO3. Optimal deposition amount of Pt was determined to be 1.0 wt%. Hierarchical hollow architecture endowed Pt/WO3 more active sites.

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