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Hierarchical Pt/MnO2-Ni(OH)2 Hybrid Nanoflakes with Enhanced Room-Temperature Formaldehyde Oxidation Activity Shuying Huang, Bei Cheng, Jiaguo Yu, and Chuanjia Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03139 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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Hierarchical Pt/MnO2-Ni(OH)2 Hybrid Nanoflakes with Enhanced Room-Temperature Formaldehyde Oxidation Activity

Shuying Huang †, Bei Cheng †, Jiaguo Yu *,†,‡ Chuanjia Jiang *,‖

† State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, Luoshi Road 122, Wuhan 430070, P.R. China. ‡ Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‖ College of Environmental Science and Engineering, Nankai University, Tongyan Road 38, Jinnan District, Tianjin 300350, China

*Corresponding authors. Tel: 0086-27-87871029, Fax: 0086-27-87879468, E-mail addresses: [email protected] (J. Yu); [email protected] (C. Jiang).

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ABSTRACT: Nickel foam (NF)-supported Pt/MnNi@NF catalyst was prepared via in situ hydrothermal growth of manganese dioxide-nickel(II) hydroxide (MnO2-Ni(OH)2) hierarchical nanosheets and subsequent impregnation-borohydride-reduction to deposit well-dispersed platinum (Pt) nanoparticles (NPs). The free-standing composite catalyst exhibited improved performance for room-temperature formaldehyde (HCHO) oxidation due to synergistic effect of Pt, MnNi nanosheets and the NF substrate, considering the lack of HCHO oxidation activity in the absence of Pt or MnO2-Ni(OH)2 and low activity of powder-like Pt/MnO2-Ni(OH)2 in the absence of NF. Surface hydroxyl (OH) groups of MnO2 and Ni(OH)2 were helpful for the adsorption of HCHO, while Pt NPs facilitated the formation of active oxygen species. Besides, oxygen vacancies on the surface of MnO2 were beneficial for the migration of active oxygen species. The NF not only served as a substrate, but also contributed to the formation of thinner nanosheets with lager lateral size and smaller grain size, thus exposing more active sites and improving the homogeneous dispersion of Pt nanoparticles. Therefore, this NF-supported freestanding composite catalyst holds great promise for application in indoor HCHO abatement. Keywords: Nickel foam, Platinum, Room temperature, Formaldehyde, Catalytic oxidation


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INTRODUCTION Formaldehyde (HCHO) is a common indoor air pollutant, which is mostly emitted from building and decorative materials and indoor combustion processes.1-3 Prolonged exposure to HCHO even at a very low concentration will do great harm to human health.1,4 Over the past decade, significant efforts have been devoted to indoor HCHO abatement to meet indoor air quality standards and protect human health. Among the reported HCHO removal methods (i.e., adsorption,5-8 photocatalytic decomposition,9-11 thermal degradation12-14 and room-temperature catalytic oxidation15-19), room-temperature catalytic oxidation has been regarded as the most promising solution due to its excellent HCHO removal efficiency, no extra energy consumption and no toxic byproduct formation. Usually, catalysts with excellent room-temperature HCHO oxidation activity contain active catalytic sites constructed around nanoparticles (NPs) of noble metals such as platinum (Pt). According to the components of support materials, the myriad Ptbased catalysts developed so far can be classified into two categories, i.e., Pt NPs supported on single- and multiple-component materials. To date, Pt-based catalysts supported on singlecomponent materials, such as Pt/AlOOH,20 Pt/Al2O3,21 Pt/CeO2,22 Pt/Co3O4,23 Pt/Fe2O3,24 Pt/MnO2,25 Pt/NiO,26 Pt/SnOx,4 Pt/TiO227,28 and Pt/Bi2WO6,29 have been widely explored and exhibited high room-temperature HCHO catalytic oxidation activities. Their outstanding performance can usually be ascribed to large specific surface area, abundant surface OH groups as well as the activation of oxygen over highly dispersed Pt NPs.19 On the other hand, only a few studies reported the room-temperature HCHO oxidation performance of Pt catalysts with multicomponent support materials, including Pt/MnOx-CeO2,30 Pt/CeO2-AlOOH31 and Pt/α-MnO2@δMnO2,25 all of which exhibited excellent HCHO oxidation activity due to the synergistic effect of different components. As an example, for Pt/CeO2-AlOOH, the structural hydroxyl groups of



AlOOH were conducive to the adsorption of HCHO, while the cyclic reduction/oxidation reaction of Ce4+/Ce3+ generated abundant active oxygen species, which contributed to the superior activities of the catalyst. For the Pt/α-MnO2@δ-MnO2 nanocomposite, the (100) exposed facet in α-MnO2 was beneficial for the adsorption and activation of oxygen molecule (O2), while the (001) facet of δ-MnO2 facilitated desorption of water, thus achieving superior HCHO oxidation activity at ambient temperature. Based on the above discussion, we propose that loading Pt on multi-component support materials with synergistic effect between the components is a promising way to fabricate catalysts with excellent HCHO oxidation activity at room temperature. Herein, hierarchical MnO2-Ni(OH)2 composite nanosheets were firstly in situ grown on nickel foam via a hydrothermal method, and then loaded with Pt NPs as catalyst for HCHO oxidation at room temperature. Manganese dioxide (MnO2) has been widely studied as an effective material for HCHO oxidation under low temperature or ambient conditions, which originated from the abundant oxygen vacancy on the surface.32-34 The OH groups in nickel(II) hydroxide (Ni(OH)2) structure are able to adsorb a large number of HCHO molecules, which is indispensable for the catalytic reaction.20,35 Thus, HCHO oxidation activity of Pt/MnO2-Ni(OH)2 is expected to be remarkably enhanced due to the synergistic effect of MnO2 and Ni(OH)2. Moreover, with the flexible and bendable Ni foam as the substrate, the free-standing composite catalyst reported herein holds great potential for application in indoor air purification.

EXPERIMENTAL SECTION Preparation of Catalysts. All reagents were of analytical grade and used as received without any purification. Nickel foam served as the flexible substrate for the coating of catalysts.


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Prior to use, the Ni foam (4 cm × 2 cm × 2 mm) was firstly pretreated with a mixture of acetone and ethanol (volume ratio 1:1) under ultrasonication for 20 min, and then immersed into a 2 M HCl solution overnight followed by washing with deionized water. The hierarchical MnO2-Ni(OH)2 composite was prepared by a facile one-step hydrothermal method,36 with the Ni foam and potassium permanganate (KMnO4) utilized as nickel and manganese sources, respectively. In the typical synthesis, 0.3169 g of KMnO4 was dissolved in 30 mL of deionized water, stirred for 20 min, and then transferred into a 50-mL autoclave with Teflon liner. Afterwards, a piece of the as-pretreated Ni foam was immersed in the solution, and the autoclave was kept at 120 °C for 10 h. During the hydrothermal treatment, the redox reaction between zero-valent Ni and permanganate ion (MnO4–) yields Ni2+ and Mn4+ cations and hydroxide anion (OH–), which in turn precipitate to form hydroxide or oxide,37 according to reaction 1. 3 Ni + 2 MnO4– + 4 H2O = 2 MnO2 + 3 Ni(OH)2 + 2 OH−

(1)

After cooling the autoclave to room temperature in air, the obtained Ni foam-based brown product (denoted as MnNi@NF) was washed with deionized water and ethanol thoroughly and then dried at 80 °C for 12 h. At the same time, the brown powders suspended in the autoclave, denoted as MnNi, were collected by centrifugation and post-treated in the same process as MnNi@NF. Pt-loaded catalysts (i.e., Pt/MnNi@NF and Pt/MnNi) were obtained via the impregnation method. Typically, four pieces of the MnNi@NF or 0.05 g powder-like MnNi sample was immersed into 20 mL of H2PtCl6 solution (19.3 mmol L−1), and the mixture was kept at ambient temperature for 6 h. Subsequently, 1 mL of the mixed solution of sodium borohydride (NaBH4) (0.1 mol L−1) and NaOH (0.1 mol L−1) was added into the above mixture. After 10 min, the



Pt/MnNi@NF or Pt/MnNi was taken out with a pair of tweezers or collected by centrifugation, washed with deionized water and ethanol, and then dried at 80 °C overnight. To investigate the roles of Ni foam and Pt in HCHO oxidation, the Pt/NF sample, which is Pt loaded on pre-cleaned Ni foam, was also prepared through the same deposition procedure except that no KMnO4 was added. Characterization. The X-ray diffraction (XRD) patterns of the products were obtained using a X-ray diffractometer (Rigaku, Japan). Field emission scanning electron microscopy (FESEM) images and energy dispersive X-ray spectroscopy (EDS) were collected by utilizing a JSM-7500 electron microscope (JEOL, Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained on a Tecnai G2 scanning transmission electron microscope (FEI, U.S.). The Brunauer–Emmett–Teller (BET) specific surface area (SBET) was analyzed on an ASAP 2020 surface area and porosity analyzer (Micromeritics, U.S.). X-ray photoelectron spectroscopy (XPS) was performed on a XSAM800 XPS system (Kratos, UK). All binding energies were calibrated by C 1s peak at 284.8 eV of the surface adventitious carbon. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurement was conducted on a Thermo Fisher 6700 instrument, with the sample placed into an in situ cell reactor and then exposed to three kinds of gas streams, respectively. Firstly, HCHO/O2 was continuously introduced for 60 min to explore the reaction mechanism under oxygen-enriched atmosphere. In a second set of experiment, the sample was exposed in HCHO/N2 atmosphere for 30 min and then in pure O2 flow for 30 min, to study the role of surface OH groups during HCHO oxidation reaction. The actual Pt loading of each sample was measured on an AVANTA M (GBC, Australia) inductively coupled plasma atomic emission spectrometer (ICP-AES). The Pt dispersion of the


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samples were measured through the carbon monoxide (CO) pulse chemisorption method on a BELCAT-B-293 Catalyst Analyzer (Bel, Japan) with a thermal conductivity detector (TCD), and 0.05 g of sample was loaded into a U-shaped quartz tube in each measurement.21 HCHO Removal Experiment. The catalytic HCHO oxidation performance of each sample was evaluated in a 6-L organic glass reactor with Al foil covering its inner walls. In a typical measurement, a certain quantity of NF-supported or powder-like sample (with the weight of active components being approximately 0.05 g) were placed on the bottom of a glass petri dish with a glass slide cover, and then placed in the reactor. Subsequently, 10 µL of HCHO solution (38%) was quickly injected into the reactor and volatilized completely. When HCHO concentration maintained at around 200 ppm (i.e., the initial HCHO concentration for each experiment), the glass slide was lifted so that the HCHO removal reaction took place. The test conditions were maintained the same for all samples (25 °C, relative humidity of 50%, and typically for durations of 60 min). A Photoacoustic Field Gas Monitor (INNOVA AirTech Instruments, Model 1412) was utilized to on-line detect the concentrations of HCHO, CO and CO2. The HCHO decomposition activity of each sample was evaluated by examining the temporal changes of HCHO concentration and CO2 concentration increase (∆CO2, relative to CO2 concentration at the start time, in ppm). RESULTS AND DISCUSSION Phase Structure, Morphology and Composition. The XRD pattern of Pt/NF is featured with three sharp peaks at 44.4°, 51.6° and 76.1° (Fig. S1), which can be ascribed to the (111), (200) and (220) planes of metallic Ni, implying that the Pt/NF mainly consisted of elemental Ni. The XRD pattern of the powder-like Pt/MnNi displayed diffraction peaks at 12.5°, 25.2°, 39.6° and 65.5° assigned to δ-MnO2 (JCPDS No. 80-1098) as well as peaks at 11.3° and 22.7° ascribed



to hexagonal Ni(OH)2 (JCPDS No. 38-0715), respectively. Besides, the weak intensity of the peaks suggested poor crystallinity of the MnNi composites. For both Pt/MnNi@NF and MnNi@NF, weak peaks of MnNi composites and sharp peaks of metallic Ni were observed, indicating that the MnNi composites were successfully loaded onto the Ni foam via the hydrothermal method. Moreover, no diffraction peaks of metallic Pt were detected for the Ptloaded samples, suggesting the low content and well-dispersed characteristics of the Pt species. There were no obvious differences between the XRD patterns of MnNi@NF and its corresponding Pt-loaded sample Pt/MnNi@NF, indicating that the phase structure of MnNi composite was not changed during the loading of Pt. The morphologies of the prepared samples were characterized by FESEM (Fig. 1). In Fig. 1a, the Pt/NF sample exhibited smooth surface, together with well-defined hexagonal grains (inset of Fig. 1a). The powder-like Pt/MnNi composite (Fig. 1b) showed a morphology of aggregated polydisperse nanospheres with diameters of 50–200 nm, which were constituted by numerous small nanoflakes stacked together. As to MnNi@NF (Fig. 1c), interconnected nanosheets vertically arrayed on the Ni foam surface, forming numerous irregular macro/mesopores with size ranging from 20 to 300 nm. It is noteworthy that the morphologies of MnNi nanosheets loaded on Ni foam (Fig. 1c) exhibited significantly different morphology from those on the powder-like sample (Fig. 1b). This observation implies that the Ni foam substrate is helpful to the formation of nanostructures with smaller thickness and larger lateral size, thus likely improving the uniform dispersion of Pt NPs and providing more reactive sites for the catalytic reaction. Additionally, a comparison between Fig. 1c and 1d demonstrates that the morphology of the MnNi composite was not changed after the loading of Pt.


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Fig. 1 Top-view FESEM images of (a) Pt/NF, (b) Pt/MnNi, (c) MnNi@NF and (d) Pt/MnNi@NF.

The low-magnification cross-sectional view (Fig. S2a) of Pt/MnNi@NF revealed its spongy nature. The high-magnification view of the selected region (Fig. S2b) together with corresponding EDS line scan view (Fig. S2c) demonstrated the elemental distribution of the sample. The coating depth of the MnNi composite nanosheets was approximately 1.6 µm. Besides, the Ni content decreased along the scanning direction from the Ni foam substrate to the surface MnNi oxide coating, while the contents of Mn and O increased along the same direction. For the MnNi coating, the atomic ratio of Mn, Ni and O was about 1:1:4. The morphological and compositional properties of the Pt/MnNi@NF catalyst were further 9 ACS Paragon Plus Environment


investigated by TEM. Pt NPs with particle size of approximately 5 nm were uniformly dispersed on nanosheets (Fig. S2d), and the dark ribbons correspond to the joint or folded parts of the interconnected nanosheets, which is consistent with the FESEM observation (Fig. 1d). In the HRTEM image (Fig. S2e), the lattice distances of 0.44, 0.35 and 0.23 nm were assigned to the (006) planes of Ni(OH)2, (002) planes of MnO2 and (111) planes of elemental Pt, respectively, thus confirming that the hybrid nanosheets were composed of Ni(OH)2 and MnO2 and that the Pt NPs were uniformly dispersed on the surface. HCHO Degradation Activities. The room-temperature catalytic HCHO oxidation activities of all samples are presented in Fig. 2. Among these samples, Pt/MnNi@NF showed remarkably enhanced HCHO degradation performance, with 88% of HCHO removed within 60 min, while the powder-like Pt/MnNi could only remove about 50 ppm of gaseous HCHO (corresponding to 25% removal) after 60 min of reaction. It is noted that the HCHO oxidation efficiency typically increases with Pt loading until an optimum Pt loading.27,38 Herein, the Pt loading of Pt/MnNi@NF was approximately 20% higher than Pt/MnNi (Table 1). However, the HCHO oxidation rate of Pt/MnNi@NF was approximately three-fold that of Pt/MnNi, which indicated that the slightly higher Pt loading is not responsible for the enhanced activity of Pt/MnNi@NF and that the Ni foam substrate was a key factor to improve HCHO oxidation activity. The CO2 concentration increased accordingly in the presence of these two catalysts, while no CO was detected, indicating complete oxidation of HCHO. As comparison, HCHO removal over MnNi@NF was very low, and no CO2 generation was detected, suggesting that Pt NPs are essential for complete oxidation of HCHO over this series of catalysts at room temperature. For the Pt/NF sample, neither HCHO adsorption nor HCHO oxidation was observed. This result implied that the MnNi (hydr)oxides played an important role in HCHO adsorption, which is


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widely recognized as the first and indispensable step for HCHO mineralization.29,31,39 Thereby, we can conclude that the excellent HCHO catalytic degradation activity of Pt/MnNi@NF was due to the synergistic effects of Ni foam, Pt NPs and NiMn composite.

Fig. 2 (a) HCHO removal performance of the samples, and (b) corresponding CO2 evolution profiles.



The reusability and durability of Pt/MnNi@NF were evaluated by cycling HCHO removal tests, and no regeneration treatment was conducted between two tests. As shown in Fig. S3, Pt/MnNi@NF could efficiently oxidize HCHO into CO2 and H2O during the five consecutive tests, indicating the long-term stability and reusability of Pt/MnNi@NF catalyst. Therefore, the catalyst is of great potential for practical application.

Textural Properties. N2 adsorption-desorption isotherms together with corresponding poresize distribution curves (inset) of Pt/MnNi, Pt/MnNi@NF and MnNi@NF are summarized in Fig. S4. The N2 adsorption-desorption isotherms of all three samples could be ascribed to type IV, suggesting the existence of mesopores in the three samples.40 For the powder-like Pt/MnNi, the hysteresis loop was assigned to type III (Fig. S4a), which indicated a large amount of slit-like mesopores formed between aggregated nanoflakes. Meanwhile, the hysteresis loop of both Pt/MnNi@NF and MnNi@NF were the overlap of types II and III, suggesting ink bottle-like mesopores and slit-like mesopores coexisting in these two Ni foam-based catalysts,41 which is consistent with their pore-size distribution curves (inset of Fig. S4b). It was noted that the proportion of ink bottle-like mesopores for the Ni foam-based samples were much higher than the powder-like sample, implying that the nanoflakes formed on Ni foam substrate possessed looser structure. In other words, grains in such nanoflakes did not stack compactly, so that more active sites were expected to be exposed. From this textural analysis and the aforementioned FESEM results, we can conclude that Ni foam substrate is beneficial for the formation of thinner nanoflakes with larger lateral size, looser structure and more reactive sites exposed. Interestingly, Pt dispersion for Pt/MnNi@NF was much higher than Pt/MnNi (Table 1), which demonstrated that thinner nanoflakes with larger lateral size and looser structure were helpful for the dispersion of Pt NPs. The more abundant exposed reactive sites and better dispersion of Pt NPs may have 12 ACS Paragon Plus Environment

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contributed to the enhanced HCHO oxidation performance of Pt/MnNi@NF. In addition, NaBH4 and NaOH used in Pt deposition process would greatly affect specific surface area as well as pore-size distribution of corresponding samples (Table 1). After the loading of Pt, SBET of the Pt/MnNi@NF sample increased when compared with MnNi@NF. Besides, the amount of mesopores with size less than 10 nm decreased after the loading of Pt, which was assigned to partial crystallization of MnO2 due to the use of NaBH4.42 Moreover, the decrease of the mesopore content in the 3-10 nm size range suggested that Pt NPs preferred depositing over these smaller mesopores in the nanoflakes. By contrast, the amount of larger mesopores (＞10 nm) increased after Pt deposition, which could be ascribed to the erosion of the MnNi by NaOH. Table 1 Physical parameters of as-prepared samples.a

sample

SBET

Vpore

dpore

Pt content

Pt dispersion

（m2 g-1）

（cm3 g-

（nm）

（wt%）

（%）

）

1

Pt/NF

N/A

N/A

N/A

0.11

10.5

Pt/MnNi

147

0.38

9.9

0.05

17.2

Pt/MnNi@NF

9.6

0.03

13.9

0.06

35.2

MnNi@NF

5.9

0.02

12.0

N/A

N/A

a

SBET, specific surface area; Vpore, pore volume; dpore, average pore size. Surface Chemistry. The surface elemental components, chemical states and surface defects

of the Pt/MnNi@NF and MnNi@NF were analyzed via XPS. Fig. 3a depicts the high-resolution XPS spectra of Ni 2p, in which peaks at 854.8, 856.2, 872.4 and 873.8 eV could be observed for Pt/MnNi@NF, ascribed to the Ni0 2p3/2, Ni2+ 2p3/2, Ni0 2p1/2 and Ni2+ 2p1/2 split orbits,



respectively.43 The Ni2+ and Ni0 signal originated from Ni(OH)2 and Ni foam substrate, respectively. Similarly, characteristic peaks of Ni2+ and Ni0 were observed in the MnNi@NF sample, further suggesting that Ni(OH)2 was one of the components of the coating, which is consistent with the XRD and HRTEM results. In Fig. 3b, two broad bands ascribed to the 2p3/2 and 2p1/2 split orbits of Mn were observed, and each could be deconvoluted into three peaks, which were assigned to Mn2+, Mn3+ and Mn4+, respectively.43 The contents of Mn2+, Mn3+ and Mn4+ of Pt/MnNi@NF and MnNi@NF are listed in Table 2. Strikingly, after Pt deposition process, the content of Mn4+ increased, whereas those of Mn3+ and Mn2+ decreased, despite the fact that NaBH4 used in the Pt loading process is a strong reductant. Under the Pt deposition condition, NaBH4 cannot reduce high-valence Mn4+ cation into low-valence Mn3+ and Mn2+ cations,42 since this heterogeneous reaction is kinetically unfavorable compared with the homogeneous reactions of Pt(IV) reduction and NaBH4 selfdecomposition. On the contrary, a portion of the low-valence Mn3+ and Mn2+ cations, which are located between MnO6 octahedral layers of δ-MnO2, can be leached out during the Pt loading process in the basic solution. As a result, the relative contents of Mn3+ and Mn2+ decreased,42 while that of Mn4+ increased for Pt/MnNi@NF as compared with MnNi@NF. The XPS spectra of O 1s could be deconvoluted into three characteristic peaks located at 529.4, 530.7 and 531.7 eV (Fig. 3c), which could be assigned to lattice oxygen (Olat), hydroxyl (OH) and surface adsorbed oxygen (Oads), respectively.44-47 Note that Oads essentially reflected the existence of surface oxygen vacancies.48,49 In this case, the surface oxygen vacancies may result from the loss of oxygen atoms from the δ-MnO2 surface during Pt loading, which occurred in an alkaline and reducing aqueous environment. Such oxygen vacancies would promote the adsorption of O2, and the activation and migration of adsorbed O2 on the surface of the catalyst,


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thus contributing to the enhanced performance of the catalyst.34 After the deposition of Pt, the content of Olat reduced, and the contents of OH and Oads increased. In general, oxygen vacancies on the surface are intrinsic active sites for the catalytic reaction and are helpful to the formation of sufficient active oxygen species.50 On the other hand, the much higher OH content of Pt/MnNi@NF than MnNi@NF is due to the introduction of OH groups on the surface of δ-MnO2 during the Pt loading process, which was performed in an alkaline solution. The more abundant OH groups provided ample adsorption sites, while the increase of the surface oxygen vacancy content resulted in more active oxygen species, synergistically contributing to the superb HCHO oxidation activity of Pt/MnNi@NF. In section 3.3, it was demonstrated that elemental Pt was necessary for complete oxidation of HCHO over Pt/MnNi@NF. In the Pt 4f XPS spectrum of this catalyst (Fig. 3d), the peak at the lowest binding energy represented Ni 3p3/2 orbit and could be deconvoluted into two peaks, located at 66.2 and 67.8 eV, which are assigned to Ni0 and Ni2+, respectively. Besides, characteristic peaks of Pt 4f5/2 and Pt 4f7/2 were observed at 70.8 and 74.1 eV, respectively, with split energy of 3.3 eV, suggesting the existence of elemental Pt. Furthermore, the binding energy of Pt 4f5/2 here was lower compared with that of bulk Pt (71.2 eV), indicating strong interaction between the Pt NPs and the (hydr)oxide support.39



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Fig. 3 XPS spectra of (a) Ni 2p, (b) Mn 2p, and (c) O 1s for samples MnNi@NF and Pt/MnNi@NF, respectively, and (d) Pt 4f spectrum of Pt/MnNi@NF.

Table 2 The proportions of different surface Mn and O species of Pt/MnNi@NF and MnNi@NF. Sample

Mn2+

Mn3+

Mn4+

Olat

OH

Oads

MnNi@NF

15.9%

51.3%

32.8%

57.2%

31.3%

11.5%

Pt/MnNi@NF

14.3%

42.4%

43.3%

29.5%

50.5%

20.0%

Reaction Mechanism. To elucidate the reaction mechanism, the main intermediates under oxygen-enriched atmosphere were investigated by in situ DRIFTS (Fig. S5). Upon exposure to the HCHO/O2 mixture, characteristic bands of molecularly adsorbed HCHO at 1060 and 1000 cm-1, together with the ν(CO) vibration mode of HCHO at 1730 cm-1 were observed.51-53 Besides, a broad negative band at approximately 3500 cm-1 ascribed to surface OH group 16 ACS Paragon Plus Environment

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gradually intensified. The above results indicated that HCHO adsorbed on the surface via OH groups. As reaction time prolonged, intensities of other characteristic bands gradually strengthened, implying the formation of adsorbed intermediates during the process. Bands located at 3837 and 3736 cm-1 were ascribed to active OH groups and ν(OH) of formic acid (HCOOH), respectively.21,54 Besides, formate species (HCOO-) accumulated, with characteristic bands of ν(CH) (i.e., 2894，2807 and 2757 cm-1)，νas(COO) (1570 cm-1) and νs(COO) (1367 cm-1) observed.16,54 Note that the gap between νas(COO) and νs(COO) was nearly 200 cm-1, and ν(CH) occurred as doublets at about 2800 cm-1, which suggested that monodentate and bridging configuration of HCOOH and HCOO- coexisted on the surface of the catalyst.51,55 It was reported that the bridging configuration of adsorbed HCOO- preferentially occurs on MnO2 surface with plentiful oxygen vacancies.51 Therefore, the in situ DRIFTS results suggest again that surface oxygen vacancies exist on the surface of MnO2, which is consistent with the O 1s XPS spectrum. In general, formic acid tends to form on hydroxide.31 Thus, the presence of both HCOOH and bridging HCOO- is consistent with the coexistence of both Ni(OH)2 and MnO2 in Pt/MnNi@NF. In order to investigate the unstable intermediates in catalytic reaction, the following measurements were conducted by inducing different atmospheres into the reaction cell. Firstly, when exposed in HCHO/N2 stream (Fig. 4a), the consumption of OH (3635 cm-1) as well as the formation of dioxymethylene (DOM) species (1460 cm-1), HCOOH/HCOO- species (3837, 3736, 2894, 2807 and 1570 cm-1) were observed.56,57 Moreover, the bands at 1060 and 1000 cm-1 strengthened evidently, implying the accumulation of adsorbed HCHO on the sample. As a result, we can conclude that HCHO can be oxidized by surface OH groups to generate DOM when O2 is absent, and a fraction of the DOM species quickly transform into HCOOH and 17 ACS Paragon Plus Environment


HCOO-. When oxygen was introduced into the reaction cell, the band intensities of HCHO and HCOO- gradually decreased, the characteristic band of DOM disappeared, and the negative band corresponding to consumed active OH groups gradually recovered. The results confirmed that DOM was also a kind of main intermediate during the reaction, which was unstable and could be quickly oxidized into HCOO- and HCOOH by adsorbed oxygen species.


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Fig. 4 In situ DRIFTS spectra of Pt/MnNi@NF exposed to: (a) HCHO/N2 and (b) subsequently O2 respectively.

CONCLUSIONS The hierarchical Ni(OH)2/MnO2 nanostructure with large lateral size was prepared via facile one-step hydrothermal method. Its sponge-like structure not only facilitated the deposition of highly-dispersed Pt NPs, but also exposed more active sties. After Pt loading, more surface OH groups and oxygen vacancies were formed on the surface of MnO2, thus enhancing the amounts of HCHO adsorption and reactive active sites. Therefore, HCHO catalytic oxidization activity of Pt/MnNi@NF was remarkably enhanced. In situ DRIFTS results indicated that DOM, HCOOH and HCOO- were main intermediates. This work not only provided new enlightenment into fabricating practical room-temperature HCHO oxidization catalysts with abundant surface active sites, but also deeply discussed the HCHO oxidation reaction mechanism on multicomponent 19 ACS Paragon Plus Environment


catalyst.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Additional information related to the XRD spectra (Fig. S1), cross-sectional FESEM images, EDS spectrum and TEM images (Fig. S2), catalytic stability test results (Fig. S3), N2 adsorption-desorption data (Fig. S4) and in situ DRIFTS spectra (Fig. S5).

ACKNOWLEDGMENTS This work was partially supported by NSFC (51702248, 51602098, U1705251 and 21573170), Natural Science Foundation of Hubei Province of China (No. 2015CFA001) and Self-determined and Innovative Research Funds of SKLWUT (2017-ZD-4).

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TOC/Abstract Graphic

Free-standing Pt/MnO2-Ni(OH)2 composite catalyst on flexible and bendable Ni foam exhibits enhanced HCHO oxidation activity.


MnO2-Ni(OH)2 Hybrid Nanoflakes ... - ACS Publications

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