Synthesis, Noble Metal Hybrids, and Photocatalytic Applications

Mar 26, 2018 - of Fujian Province (Grant Nos. 2017J05031 and 2018I0021), and the Key Research Program of Frontier Sciences of the. Chinese Academy of ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A New Defect Pyrochlore Oxide Sn1.06Nb2O5.59F0.97: Synthesis, Noble Metal Hybrids, and Photocatalytic Applications Xiaoyang Pan,† Chao Li,‡ Jing Zheng,† Shijing Liang,§ Rong Huang,*,‡ and Zhiguo Yi*,†,∥ †

Key Laboratory of Design and Assembly of Functional Nanostructures & Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200062, China § Department of Environmental Science and Engineering, Fuzhou University, Fuzhou 350108, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Noble metal nanoparticles have attracted considerable attention due to their useful capabilities as heterogeneous catalysts. However, they are usually prepared using various organic stabilizing agents that negatively affect their catalytic activities. Herein, we report a facile, clean, and effective method for synthesizing supported ultrafine noble metal nanoparticles by utilizing the reductive property of a new pyrochlore oxide: Sn1.06Nb2O5.59F0.97 (SnNbOF). Ultrafine Au, Pd, and Pt nanoparticles or clusters are homogeneously distributed on the SnNbOF surface. In addition, the atomic cavities and ionexchange properties of pyrochlore-type SnNbOF can facilitate the synthesis of atomic Ag dispersed within the framework of SnNbOF. Noble metal− SnNbOF hybrids can be obtained in one step at room temperature, and no foreign reducing agents or stabilizing organics are required for the synthesis. We also show that the fabricated hybrids exhibit promising photocatalytic properties for ethylene oxidation and CO2 reduction.

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INTRODUCTION Nanoparticles of noble metals such as Au, Pd, Pt, and Ag are of great interest in the field of catalysis for diverse applications.1−12 The ability to control the size, shape, and distribution of these nanoparticles is a primary goal of catalyst design.1,2,4,5 However, noble metal nanoparticles used in isolation suffer from two serious problems in catalytic reactions. First, the organic stabilizing agents on the metal surface may block the active sites of the noble metal and attenuate its catalytic activity.13,14 Second, the organic agents are not stable during the catalytic process, resulting in aggregation or sintering of the noble metal during catalytic reactions.15 To overcome these disadvantages, an effective method is to anchor the noble metals to specific supports with less or no organic agents. Of all the available support materials, metal oxides are the most widely used.16 To prepare oxide-supported noble metals, various strategies have been developed, including coprecipitation and impregnation,16 photodeposition,17 chemical vapor deposition (CVD),18 and colloidal methods.16 The conventional coprecipitation,19 impregnation,20 and photodeposition21,22 methods are relatively simple but usually do not provide precise control over the size and distribution of the noble metal nanoparticles. The CVD method can be used to produce ultrafine noble metal particles with good dispersion, but this method requires expensive organic metal precursors and complex synthetic procedures.18 In the colloidal method, © XXXX American Chemical Society

presynthesized metal nanoparticles are stabilized by organic agents and mixed with oxide supports, allowing precise control over the size and morphology of the noble metal nanoparticles.16 However, the removal of the organic stabilizing agent by thermal or oxidative treatments usually results in significant changes in the sizes and shapes of the nanoparticles.23 Therefore, it remains an important challenge to develop a simple and effective method to synthesize small and homogeneously distributed oxide-supported noble metals. Herein, a new defect pyrochlore oxide, Sn1.06Nb2O5.59F0.97 (SnNbOF), is synthesized and used as a reactive support for in situ homogeneous deposition of ultrafine noble metal nanoparticles without organic reagents or additional reducing agents. The redox reaction between the reductive Sn(II) ions in SnNbOF and noble metal ions facilitates the in situ growth of the noble metal (Au, Pd, Pt, or Ag) on SnNbOF at room temperature. In the proposed technique, by utilizing the unique properties of SnNbOF, ultrafine Au, Pd, and Pt nanoparticles or atomically dispersed Ag can be readily prepared. The promising photocatalytic properties of the fabricated catalysts for various reactions are also demonstrated. Received: March 26, 2018

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DOI: 10.1021/acs.inorgchem.8b00818 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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RESULTS AND DISCUSSION Synthesis and Characterization of SnNbOF. A new pyrochlore-type oxide Sn1.06Nb2O5.59F0.97 (SnNbOF) was first synthesized by a hydrothermal method. Figure S1 shows the Xray diffraction (XRD) patterns of the SnNbOF samples obtained at various pH values. The XRD profiles varied with the pH of the solution. The samples prepared at pH 8−10 exhibited sharp XRD peaks that can be attributed to the cubic cell of the pyrochlore-type structure (space group Fd3m). Furthermore, a pH of 8 resulted in the SnNbOF sample with the best crystallinity. Therefore, this sample was chosen for further investigation. The composition of the sample of the pyrochlore-type compound obtained at pH 8 and 353 K for 24 h was investigated. The total amounts of Sn and Nb were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis. The amount of fluorine in the SnNbOF was determined by measuring the concentration of residual F− ions in the reaction solution following the SnNbOF synthesis by ion chromatography. A negligible amount of nitrogen was detected by elemental analysis. The molar ratio of Sn/Nb/F in the sample was found to be 1.06:2:0.97. In addition, the valence states of Sn, Nb, O, and F elements in SnNbOF are determined to be +2, +5, −2, and −1, respectively (Figure S2a−d). Therefore, the chemical formula of this material can be expressed as Sn1.06Nb2O5.59F0.97, which is known as a defect pyrochlore-type compound. The A, O, and O′ sites in Sn1.06Nb2O5.59F0.97 are partially vacant, unlike those in typical pyrochlore oxides, which have a general formula of A2B2O7 or A2B2O6O′.24 The vacant sites in the defect pyrochlore endow it with interesting ion-exchange properties because of the mobility of the An+ ions.25 The results of the Rietveld refinement of Sn1.06Nb2O5.59F0.97 (SnNbOF) are shown in Figure 1a. All the observed peaks for the sample can be attributed to a face-centered cubic crystal (Tables S1). The crystal structure of the sample is a threedimensional framework with corner-shared NbO6 octahedra in which the interstitial sites are filled with Sn2+ and F−/O2− ions (Figure 1b and Table S2), consistent with the conclusion based on the XRD patterns of the pyrochlore compounds such as Pb1.2Ti0.4Ta1.6O6 (JCPDS No. 76-0177) and Sr0.4H1.2Nb2O6 (JCPDS No. 77-1165) shown in Figure S3.26 To understand the stability of the reductive Sn2+ ions of SnNbOF in air atmosphere or under simulated sunlight irradiation, XPS analysis is performed. The results show that the Sn2+ ions are stable under these conditions (Figure S4a). In addition, the crystal structure of SnNbOF is also not changed (Figure S4b). The scanning electron microscopy (SEM) images in Figure S5 and Figure 1c show that SnNbOF is octahedral in shape with the octahedron size ranging from 60 to 100 nm. The transmission electron microscopy (TEM) image in Figure 1d further confirms the presence of an octahedral morphology in SnNbOF. The high-resolution transmission electron microscopy (HR-TEM) image shows a distinct lattice spacing of 0.602 nm for the (111) facet of the pyrochlore compound (Figure 1e).27 The Brunauer−Emmett−Teller (BET) surface area of SnNbOF is determined to be 31.6 m2/g (Figure S6), while the average pore size of SnNbOF is 13.6 nm (inset of Figure S6). In Situ Growth of Noble Metals on SnNbOF. The procedure used to synthesize noble metal−SnNbOF composites is shown in Scheme S1. Briefly, the as-obtained SnNbOF

Figure 1. Physical characterization of the Sn 1.06 Nb 2 O 5.59 F0.97 (SnNbOF). (a) Observed and calculated powder XRD profiles. (b) Crystal structure. (c) SEM image. (d) TEM image. (e) HR-TEM image.

was first dispersed in water, and then metal salts were added to this solution dropwise at room temperature. Once in contact with SnNbOF, the noble metal salts were immediately reduced by the Sn2+ ions (Figure S7) and subsequently nucleated on the surface of SnNbOF, forming clusters and, eventually, nanoparticles. Figure 2a shows the XRD patterns of SnNbOF and of the SnNbOF composites loaded with 1 wt % noble metal (1 wt %

Figure 2. XRD patterns (a) and UV−vis diffuse reflectance spectra (b) of the 1 wt % noble metal loaded SnNbOF (1 wt % M-SnNbOF).

M-SnNbOF). The results show that the 1 wt % M-SnNbOF samples with different loaded metals have XRD patterns that are similar to those of SnNbOF. Notably, however, the XRD peaks decreased in intensity after the noble metals were deposited on the surface when compared to the XRD pattern of as-prepared SnNbOF. This may be attributed to the oxidation of the Sn(II) ions in SnNbOF. In addition, the characteristic XRD peaks corresponding to Ag, Au, Pd, and Pt were not B

DOI: 10.1021/acs.inorgchem.8b00818 Inorg. Chem. XXXX, XXX, XXX−XXX

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indicates the presence of strong interactions between Au and SnNbOF in 1 wt % Au-SnNbOF.29 The coexistence of Sn(II) and Sn(IV) in 1 wt % Au-SnNbOF suggests that some of the Sn(II) ions in SnNbOF were oxidized by the Au ions (Figure S10a). That is, the redox reaction between Sn(II) and Au(III) results in the formation of Au(0) nanoparticles dispersed on the surface of SnNbOF as well as the partial oxidation of Sn(II) in SnNbOF. In addition, XPS peak characteristic of O 1s, Nb 3d, and F were observed in the sample (Figure S10b−d). We also synthesized 1 wt % Pd-SnNbOF, 1 wt % PtSnNbOF, and 1 wt % Ag-SnNbOF nanocomposites using the same method. Here, 1 wt % represents the nominal weight ratio of noble metals. The actual weight ratios of Au, Pd, and Pt of 1 wt % M-SnNbOF are determined to be 1.05, 0.95, and 0.93 wt %, respectively (Table S3). Figure 4a,b shows representative

observed in the 1 wt % M-SnNbOF samples. This is because of the low weight ratio of the metal particles in 1 wt % MSnNbOF.28 The ultraviolet−visible diffuse reflectance spectra of the samples are shown in Figure 2b. The spectra indicate that SnNbOF exhibits strong absorption in the UV and visible light regions and that the absorption edge is located at ∼530 nm. The bandgap of SnNbOF is estimated to be ∼2.33 eV (Figure S8a). A Mott−Schottky analysis revealed that the conduction band minimum is −0.19 V versus normal hydrogen electrode (NHE; Figure S8b). Thus, the corresponding valence band maximum was calculated to be +2.14 V versus NHE. The decoration of different noble metals has a significant influence on the optical properties of 1 wt % M-SnNbOF. As shown in Figure 2b, the addition of noble metals increases the light absorption intensity in the range of 550−800 nm. Specifically, a distinct absorption peak was observed in 1 wt % Au-SnNbOF around 550 nm due to the surface plasmon resonance of the metallic gold nanoparticles. Figure 3a shows a TEM image of 1 wt % Au-SnNbOF (here, 1 wt % represents the nominal weight ratio of Au), revealing

Figure 3. Characterization of 1 wt % Au-SnNbOF. (a) TEM image. (b) Size distribution of Au nanoparticles. (c) HR-TEM image. (d) XPS spectrum of Au 4f.

that the SnNbOF sample retained its octahedral shape, whereas the Au nanoparticles were homogeneously dispersed on the surface and had an average particle size of 3.8 nm (Figure 3b). The elemental mapping analysis also shows that the Au nanoparticles are uniformly distributed onto the surface of 1 wt % Au-SnNbOF (Figure S9a−f). The HR-TEM image in Figure 3c shows distinct lattice spacings of 0.235 and 0.202 nm, which can be attributed to the (111) facet for Au and the (511) plane for SnNbOF, respectively.29 The X-ray photoelectron spectroscopy (XPS) analysis (Figure 3d) reveals that most of Au in 1 wt % Au-SnNbOF is in the metallic state, whereas only a small amount of Au(+1) can be detected. The shift in Au 4f7/2 in 1 wt % Au-SnNbOF (83.5 eV) when compared with that in pure Au foil (84.0 eV)

Figure 4. TEM images and elemental mapping analyses of the 1 wt % M-SnNbOF. (a−i) 1 wt % Pd-SnNbOF. (j−r) 1 wt % Pt-SnNbOF. (s−x) 1 wt % Ag-SnNbOF. (y) Schematic illustration of the formation of 1 wt % M-SnNbOF.

TEM and HR-TEM images of 1 wt % Pd-SnNbOF. These images clearly show a uniform Pd distribution through the SnNbOF support. The elemental mapping analysis (Figure 4c− i) also confirmed the homogeneous dispersion of Pd on the support. The XPS spectrum of 1 wt % Pd-SnNbOF shows peaks corresponding to Pd with a Pd 3d5/2 apparent binding energy of ∼335.8 eV, which is higher than that of bulk Pd (335 eV), as shown in Figure S11a.28 This phenomenon can be C

DOI: 10.1021/acs.inorgchem.8b00818 Inorg. Chem. XXXX, XXX, XXX−XXX

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compared to the well-known narrow-band gap semiconductor photocatalysts C3N4, WO3, and Ag3PO4 (Figure S14a). The results show that the SnNbOF exhibits obviously enhanced photoactivity than that of C3N4 and WO3 under simulated sunlight irradiation (Figure S14b), while the photoactivity of SnNbOF is lower than that of Ag3PO4. After noble metal decoration, the photoactivity of SnNbOF is significantly enhanced (Figure 5a). Furthermore, the photoactivities of

attributed to the relatively small size of the metal particles, which has been reported in previous studies.30 TEM and elemental mapping analyses of 1 wt % Pt-SnNbOF show that small nanoparticles were homogeneously deposited on the surface of SnNbOF (Figure 4j−r). In addition, the Pt 4f7/2 binding energy (71.95 eV) was higher in 1 wt % Pt-SnNbOF than in bulk Pt (71.2 eV), further supporting the small metal particle size (Figure S11b).31 Compared to the 1 wt % Au−SnNbOF, no obvious noble metal nanoparticles with distinct lattice spacing could be found on the surface of 1 wt % Pd and Pt-SnNbOF nanocomposites, while the elemental mapping analysis indicates the uniform distribution of Pd and Pt elements. This result suggests that the Pd and Pt elements may be doping into the lattice of SnNbOF. To examine this, Ar+ sputtering is used to remove the surface layer of 1 wt % M-SnNbOF (M = Pd or Pt), and the samples were analyzed by XPS analysis. The results show that the Pd and Pt elements are mainly distributed on the surface of 1 wt % M-SnNbOF (Figure S12a,b). In addition, we also found that the smooth surface of SnNbOF became rough after Pd and Pt decoration. These results together suggest that Pd and Pt are in the form of nanoclusters or atomically doped into the surface layer of SnNbOF. Unlike the SnNbOF hybrids with 1 wt % Au, Pd, and Pt, 1 wt % Ag-SnNbOF was found to have a smooth surface without obvious Ag nanoparticles or clusters (Figure 4s,t). However, the elemental mapping analysis showed that Ag was homogeneously dispersed within the SnNbOF support (Figure 4u,v,w,x). Furthermore, the XPS analysis indicated that only a small amount (∼0.3 wt %) of Ag(0) was present on the surface layer of SnNbOF (Figure S13a). The ICP-OES analysis showed that the actual weight ratio of Ag in 1 wt % Ag-SnNbOF is 1.12 wt % (Table S3). To further investigate the distribution of Ag in the 1 wt % Ag-SnNbOF sample, Ar+ sputtering was used to remove the surface layer of the sample, and the sample was then characterized by XPS analysis. The results showed that Ag(0) was also presented in the lattice of SnNbOF (Figure S13b). Together, these results indicate that metallic Ag was dispersed within the framework of SnNbOF. This difference in 1 wt % Ag-SnNbOF compared to the Au, Pd, and Pt nanocomposite hybrids is mainly attributed to the differences in the precursors that are used for noble metal preparation: the Au, Pd, and Pt precursors were negatively charged MClyx−, whereas Ag+ ions were used as the precursor for Ag (Figure 4y). Because of the ion-exchange properties of SnNbOF, the Ag+ ions would be exchanged with the Sn2+ ions and be incorporated into the SnNbOF lattice. The exchanged Ag+ ions would be simultaneously reduced by Sn2+ and therefore trapped in the SnNbOF crystal lattice. In contrast, for 1 wt % Au, Pd, and Pt-SnNbOF nanocomposites, noble metal elements are mainly distributed on the surface of 1 wt % M-SnNbOF. This is because the negatively charged noble metal precursors (AuCl4−, PdCl42−, and PtCl62−) could not exchange with the positively charged Sn2+ ions. Photocatalytic Properties. As a proof-of-concept for the use of this technique for fabricating photocatalysts, we first demonstrate the application of the SnNbOF and the 1 wt % MSnNbOF hybrids for photocatalytic C2H4 oxidation under simulated sunlight irradiation, which is important for environmental remediation due to the harmful effects of ethylene on the fresh-keeping.32,33 Initially, the photoactivity of SnNbOF (Eg = 2.33 eV) for ethylene oxidation was investigated and

Figure 5. Photocatalytic oxidation of C2H4. (a) Photo-oxidation of C2H4 over SnNbOF and 1 wt % M-SnNbOF under simulated sunlight irradiation. (b) Rate constants of photocatalytic C2H4 oxidation over different catalysts and under different light sources. (c) Stability of 1 wt % Pt-SnNbOF for C2H4 oxidation under simulated sunlight irradiation. (d) Photocatalytic oxidation of C2H4 over 1 wt % PtSnNbOF in a flow mode under simulated sunlight irradiation.

these samples were investigated under irradiation from different light sources (Figure S15a−d, Figure S16a−d, and Figure S17a,b). The results indicate that 1 wt % M-SnNbOF exhibits a much higher photoactivity than SnNbOF under UV light, visible light, and simulated sunlight irradiation (Figure 5b). Specifically, 1 wt % Pt-SnNbOF exhibits the best photoactivity of all of the samples tested under simulated sunlight irradiation. The rate constant of 1 wt % Pt-SnNbOF (0.17 min−1) was found to be 73 times higher than that of the asprepared SnNbOF (0.0024 min−1) under simulated sunlight irradiation and is stable during the recycled photocatalytic reaction (Figure 5c). To further examine the rate of conversion and the selectivity for ethylene oxidation, the performance of 1 wt % Pt-SnNbOF was investigated in a flow mode. As shown in Figure 5d, before the light irradiation, the C2H4 concentration was 200 ppm, and no CO2 was present. When the light was turned on, the amount of C2H4 decreased rapidly to ∼5 ppm, and the concentration of CO2 simultaneously increased rapidly to ∼390 ppm. These results confirm that ethylene oxidation was driven by a photocatalytic process. The mineralization ratio of ethylene in this reaction was found to be ∼100%. To understand the effect of noble metal on the improvement of photoactivities of 1 wt % M-SnNbOF, photoluminescence analysis was first performed (Figure S18). The results show that the noble metal decoration can obviously improve the charge separation efficiency on SnNbOF and thus enhance the photoactivity of it. We also noted that the blank noble metal nanoparticles decorated on SiO2 (Figures S19a−c and S20) also exhibit photoactivities for ethylene oxidation under D

DOI: 10.1021/acs.inorgchem.8b00818 Inorg. Chem. XXXX, XXX, XXX−XXX

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subsequently captured by molecular oxygen to form superoxide radicals. These superoxide radicals oxidize C2H4 to generate CO2 and H2O. Note that the photogenerated holes can also participate in the C2H4 oxidation reaction. According to previous findings, the reaction between photoexcited holes and C2H4 could proceed through two reaction pathways:35−37 The photogenerated holes react with the water adsorbed on the surface to form hydroxyl radicals, which are further oxidized by C2H4 into CO2 or directly interact with C2H4 to break its C C bond and facilitate its complete oxidation.38 In this case, the C2H4 oxidation by photoexcited holes tends to proceed through the latter pathway, because the photogenerated holes, which have an anodic potential of 2.1 V versus NHE, are unable to oxidize H2O to form hydroxyl radicals, which have an anodic potential of 2.8 V versus NHE. Therefore, the main reaction process of ethylene oxidation herein can be described by the following steps:

simulated sunlight irradiation (Figure S21). However, the photoactivities are very weak, which may be ascribed to their surface plasmonic resonance properties or photothermal effects. The light irradiation would increase the temperature of the reaction system, which could facilitate the thermcatalytic oxidation of ethylene over noble metal (Figure S22). Note that the enhanced light absorption properties of SnNbOF (Figure 2b) caused by noble metal decoration also contributed to the improved photoactivity of 1 wt % M-SnNbOF. To investigate the mechanism of the C2H4 oxidation over MSnNbOF, in situ electron spin resonance (ESR) analysis was conducted. Figure 6 shows the ESR spectra obtained from the 1

SnNbOF + hv → SnNbOF(h+ + e−) −

e + O2 →

Figure 6. Electron spin resonance spectra collected upon 1 wt % PtSnNbOF sample at 100 K under various conditions. (a) Dark and air. (b) Under irradiation and air. (c) Under irradiation and after injecting C2H4.

· − O2

(1) (2)

· − − C2H4 + 3O 2 → 2CO2 + 2H 2O + 3e

(3)

C2H4 + 6O−O → 2CO2 + 2H 2O + 6V −O

(4)

2− 2V −O + O2 + 2e− → 2OO

(5)

It was also found that 1 wt % Pt-SnNbOF shows obvious photoactivity for CO2 reduction, unlike SnNbOF, which exhibits almost no activity for this reaction (Figure S23). However, the reason for this behavior remains uncertain.

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wt % Pt-SnNbOF sample under various conditions. Under dark conditions and in an air atmosphere, the sample exhibited a distinct signal at g = 2.005, which can be attributed to a single electron trapped in an oxygen vacancy site (Figure 6a). Under simulated sunlight irradiation (Figure 6b), an ESR signal that is characteristic of O2− emerges (g1 = 2.032, g2 = 2.005, and g3 = 1.999).34 After C2H4 was injected into the reactor, the O2− signal almost disappeared, as shown in Figure 6c, indicating O2− was consumed during the photocatalytic reaction. On the basis of these results, a possible reaction mechanism for the photooxidation of C2H4 is proposed. As shown in Figure 7, under the simulated sunlight irradiation, SnNbOF is excited, photogenerating electrons and forming holes. The photoexcited electrons are trapped by the noble metal and

CONCLUSION In conclusion, a novel and facile yet effective method has been developed for the in situ growth of noble metals on a novel pyrochlore oxide support, SnNbOF, under ambient conditions. By utilizing the unique properties of SnNbOF, a variety of noble metal−oxide hybrids with small metal particles were uniformly dispersed on SnNbOF. The metal−oxide hybrids exhibit promising properties for photocatalytic applications.

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METHODS

Materials. Niobium(V) oxide (Nb2O5), hydrofluoric acid (HF), ammonium hydroxide (NH4OH), tin(II) fluoride (SnF2), silicon dioxide (SiO2), sodium dihydrogen phosphate (NaH2PO4), urea (CH4N2O), poly(vinylpyrrolidone) (PVP), chloroauric acid tetrahydrate (AuCl3·HCl·4H2O), silver nitrate (AgNO3), hexachloroplatinic (IV) acid hexahydrate (H2PtCl6·6H2O), and palladium(II) chloride (PdCl2) were purchased from Sinopharm Chemical Regent Co., Ltd. Deionized water was supplied from local sources. All of the materials were used as received without further purification. Synthesis. First, Nb2O5·nH2O was prepared as established in a previous study.27 Then, Nb2O5·nH2O was mixed with SnF2 in a Nb5+/ Sn2+ molar ratio of 2:1. This mixture was dispersed into 70 mL of deionized water and then transferred to a 100 mL Teflon-lined stainless steel autoclave. The pH of the mixture was adjusted with NH4OH while stirring vigorously. The autoclave was kept in an oven at 473 K for 24 h. The product SnNbOF was filtered and washed with distilled water several times and dried in an oven at 353 K overnight. The Au, Ag, and Pt precursors were dissolved directly in deionized water at 10 mM. Because PdCl2 is only slightly soluble in water, hydrochloric acid was used to dissolve it such that the resulting H2PdCl4 solution had the same 10 mM concentration as that of the precursor solutions of Au, Ag, and Pt. In situ growth of noble metals on the surface of SnNbOF was conducted as follows: 0.2 g of the asobtained SnNbOF was dispersed in 100 mL of deionized water with

Figure 7. Schematic illustration of C2H4 oxidation over noble metal loaded SnNbOF (M-SnNbOF) under simulated sunlight irradiation. E

DOI: 10.1021/acs.inorgchem.8b00818 Inorg. Chem. XXXX, XXX, XXX−XXX

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21.1% O2, and small quantity of C2H4 was flowed over the sample and analyzed directly by gas chromatography (GC9720 Fuli). During the reaction, a 300 W Xe lamp was used without filters to provide simulated sunlight. The photocatalytic reduction of CO2 was performed in a gas−solid heterogeneous reaction mode under atmospheric pressure at ambient temperature. A 40 mL Schlenk flask with a silicone rubber septum was used as a reactor. The photocatalyst sample (20 mg) was loaded into the reactor. The system was evacuated by a mechanical pump and filled with pure CO2 gas; this evacuation/filling operation was repeated three times. A bar of CO2 and 6 μL of liquid water were introduced with a syringe via the septum. A 300 W commercial Xe lamp was used as an irradiation source and placed vertically outside the reactor. The temperature of the reactor was kept at 298 K by an electronic fan. After 4 h of irradiation, 0.5 mL of the gas was taken from the reactor with a syringe and analyzed by a GC-7890A gas chromatograph equipped with a flame-ionized detector and a chromatographic column (GASPRO).

the aid of ultrasonication. Then, a given quantity of metal ions (HAuCl4, AgNO3, H2PtCl6, or H2PdCl4) in an aqueous solution was mixed with the SnNbOF suspension under strong stirring at room temperature. After they were stirred for 48 h, the final products were collected, washed with distilled water, and dried at 353 K in an oven. C3N4, Ag3PO4, and WO3 were prepared according to our previous report.39 Au, Pd, and Pt nanoparticles were prepared via the NaBH4 reduction method in the presence of PVP. Typically, 15 mL of noble metal precursors (2 mM) were mixed with 21 mL of H2O and 0.0889 g of PVP, and the solution was stirred for 15 min at room temperature. Then 2.4 mL of NaBH4 solution (10 mM) was added, and the solution was stirred for 5 h. The resulting product is PVP-capped noble metal solution. SiO2-supported noble metal composite was prepared by the rotary evaporation of commercially available SiO2 with appropriated amount of colloidal noble metal nanoparticles solution followed by a sample drying at 373 K in an oven. Characterization. The optical properties of the samples were characterized by a Cary 500 UV−visible ultraviolet/visible diffuse reflectance spectrophotometer (DRS), during which BaSO4 was employed as the internal reflectance standard. Crystal structures of the as-prepared samples were analyzed on a Rigaku Miniflex II X-ray powder diffractometer using Cu Kα radiation. A field-emission scanning electron microscope (FESEM, JSM-6700F) and transmission electron microscope (JEM-2010, FEI, Tecnai G2 F20 FEG TEM) were used to identify the morphology and microscopic structure of the assynthesized samples. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Scientific ESCA Lab250 spectrometer consisting of monochromatic Al Kα as the X-ray source, a hemispherical analyzer, and a sample stage with multiaxial adjustability to obtain the composition on the surface of samples. All the binding energies were calibrated by the C 1s peak of the surface adventitious carbon at 284.6 eV. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on Ultima 2 to determine the weight ratio of Sn and Nb. Ion chromatography analysis (IC-2010) was performed to determine the content of residual F element in the reaction solution for SnNbOF synthesis. Photoluminescence analysis was performed with a Varian Cary Eclipse spectrometer at an excitation wavelength of 325 nm. Photoelectrochemical Measurements. The photoelectrochemical analysis was performed in a conventional three-electrode cell. Ag/ AgCl electrode was used as reference electrode, and Pt electrode acted as the counter electrode. The work electrode was prepared according to our previous report.40 Mott−Schottky analyses were performed in a homemade three-electrode quartz cell with a CHI660D workstation. The electrolyte was 0.2 M aqueous Na2SO4 solution (pH = 6.8) without additive. Catalytic Activities. Photocatalytic ethylene degradation was performed in a custom-modified Pyrex reaction cell with a volume of 450 mL under irradiation by a 300 W Xe lamp with different light filters. First, 0.1 g of the photocatalyst was spread uniformly over the bottom of the reactor. Then, the reactor was flushed with N2 repeatedly to remove the water, and CO2 adsorbed on the catalyst and the interior walls of the reactor. Subsequently, 5 mL of O2 and 90 μL of C2H4 were injected into the reactor using a microsyringe; the initial concentration of C2H4 was 200 ppm by volume. Prior to the irradiation, the reactor was kept in the dark for 2 h to ensure that an adsorption−desorption equilibrium between the photocatalyst and the reactants was attained. Then, the reactor was irradiated by a 300 W Xe lamp with various light filters. At fixed time intervals, 4 mL of the gas was sampled from the reactor and analyzed by gas chromatography (GC9720 Fuli) with an HP-Plot/U capillary column, a molecular sieve 13X column, a flame ionization detector, and a thermal conductivity detector. The percentage of ethylene that had degraded at each sampling was calculated as C/C0, where C is the concentration of ethylene in the sample, and C0 is the initial concentration of ethylene injected. The flow-mode experiments were conducted as follows: 1 g of the metal particles was placed into a 28 × 18 × 1 mm quartz reactor, and N2 was flowed to expel CO2 and other species that were adsorbed on the surface of the catalyst. Then, a mixed gas comprising 78.9% N2,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00818. XRD, crystal data, TEM, XPS spectra, photocatalytic activity, and photoluminescence analysis of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.H.) *E-mail: [email protected]. (Z.Y.) ORCID

Chao Li: 0000-0002-4212-3098 Shijing Liang: 0000-0001-9963-5935 Zhiguo Yi: 0000-0002-3828-1712 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (Grant Nos. 21607153, 21577143, 51702317, and 21677036), the Natural Science Foundation of Fujian Province (Grant Nos. 2017J05031 and 2018I0021), and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant No. QYZDB-SSWJSC027).

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DOI: 10.1021/acs.inorgchem.8b00818 Inorg. Chem. XXXX, XXX, XXX−XXX