Efficient Removal of Formaldehyde by Nanosized Gold on Well

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Efficient Removal of Formaldehyde by Nanosized Gold on WellDefined CeO2 Nanorods at Room Temperature Quanlong Xu,†,‡ Wanying Lei,‡ Xinyang Li,† Xiaoying Qi,‡ Jiaguo Yu,*,† Gang Liu,*,‡ Jinlong Wang,§ and Pengyi Zhang*,§ †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China ‡ National Center for Nanoscience and Technology, Bejing, 100190, China § State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China ABSTRACT: Gold (Au) nanoparticles (NPs) supported on well-defined ceria (CeO2) nanorods with exposed {110} and {100} facets were prepared by a deposition−precipitation method and characterized by powder X-ray diffraction, micro-Raman spectroscopy, X-ray photoelectron spectroscopy, nitrogen adsorption−desorption, transmission electron microscopy, high-resolution transmission electron microscopy, and high-angle annular dark-field scanning transmission electron microscopy. Both nanometer and subnanometer gold particles were found to coexist on ceria supports with various Au contents (0.01−5.4 wt %). The catalytic performance of Au/CeO2 catalysts was examined for formaldehyde (HCHO) oxidation into CO2 and H2O at room temperature and shown to be Au content dependent, with 1.8 wt % Au/CeO2 displaying the best performance. On the basis of the results from hydrogen temperature-programmed reduction and in situ Fourier transform infrared spectroscopy observations, the high reactivity and stability of Au/CeO2 catalysts is mainly attributed to the well-defined ceria nanorods with {110} and {100} facets which present a relatively low energy for oxygen vacancy formation. Furthermore, gold NPs could induce the weakened Ce−O bond which in turn promotes HCHO oxidation.

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INTRODUCTION Emitting from widely used construction and decoration materials and consumer products, formaldehyde (HCHO) is one of the most common and harmful indoor volatile organic compounds (VOCs).1 It has been classified as a carcinogenic species in that a long-term exposure to HCHO for human beings can cause upper respiratory tract and eye irritation, nasopharyngeal cancer, leukemia, etc.2 In order to meet more rigorous environmental regulations than ever before, great efforts have been made to eliminate indoor HCHO. In general, HCHO removal can be achieved by a variety of methods, such as adsorption,3−9 plasma technology,10 photocatalytic oxidation,11−13 and thermal catalytic oxidation.14−18 As an energy efficient and environmental benign approach, thermal catalytic oxidation under ambient conditions is receiving great attention. To date, several studies have been carried out on removing HCHO at mediate temperature or even at ambient temperature using metal oxide (MnO219) or supported noble metal (Pt/ TiO2,14,20 Au/Co3O4,16 and Au/CeO221,22) catalysts. Despite significant advances, it is still a challenge to develop highperformance catalysts for completely oxidizing HCHO into CO2 and H2O under ambient conditions. Owing to its unique redox properties dictated by the facile conversion between Ce3+ and Ce4+ oxidation states, ceria © 2014 American Chemical Society

(CeO2) is a crucial component in three-way automobile converters.23 In addition, CeO2 has effective catalysis in many reactions like water−gas shift,24−26 CO oxidation,27−29 and methanol steam reforming.30 Compared to the bulk CeO2 counterpart, nanoscaled CeO2 can offer relatively larger specific surface area and more active sites that are essential for anchoring noble metal nanoparticles (NPs), which in turn promote the catalytic performance in a synergetic approach.24 Since Haruta et al.31 reported that nanosized gold deposited on various transition metal oxides show excellent performance for low temperature CO oxidation, numerous studies have been directed toward the unique catalytic performance of Au NPs in a variety of catalytic reactions.29,32−34 The catalytic activity is greatly enhanced when gold NPs are deposited on particle surfaces to form composite nanocatalysts like Au/CeO2, in which ceria particle size, morphology, and termination planes play key roles in tuning the catalytic performance. In recent years, size and shape control of a variety of catalytic materials with enhanced performance has seen rapid growth.35 For Received: Revised: Accepted: Published: 9702

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Tecnai G2 F20 U-TWIN microscope equipped with an energydispersive X-ray spectrometer. The Brunauer−Emmett−Teller (BET) specific surface area was measured via nitrogen adsorption at 77 K by using a Micromeritics ASAP 2020 apparatus. Micro-Raman spectroscopy measurement was conducted using a Renishaw Micro-Raman Spectroscopy System (Renishaw in Via plus) with a Renishaw 514 nm laser. X-ray photoelectron spectroscopy (XPS) data were obtained using an ESCALab 250 Xi electron spectrometer from Thermo Scientific Corporation. Monochromatic 150 W Al Kα radiation was utilized with a pass energy of 30 eV. Lowenergy electrons were used for charge compensation. The binding energies were referenced to the adventitious C 1s line at 284.8 eV. Hydrogen temperature-programmed reduction (H2-TPR) was performed on the BELCAT-B (Japan) instrument. In a typical experiment, 50 mg of sample was loaded into the Uscheme quartz tube reactor. For the H2-TPR experiment, the sample was pretreated in He (50 mL min−1) at 150 °C for 1 h and then the temperature was ramped from room temperature to 550 °C at 10 °C min−1 with the introduction of the reducing gas (10% H2/Ar) at a flow rate of 30 mL min−1. In situ diffused Fourier transform infrared spectroscopy (DFTIR) was recorded in Thermo Fisher 6700. The catalyst was pretreated in a dried air gas flow at 150 °C for 1 h in an in situ cell reactor, and then, the reactant gas mixture (78 ppm of HCHO/O2) was introduced into the DFTIR cell at room temperature via separate mass flow controllers at a flow rate of 30 mL min−1. All spectra were recorded with a resolution of 4 cm−1 (32 scans), and the background spectrum was subtracted from each spectrum, respectively. Scans were collected from 4000 to 1000 cm−1. Catalytic Activity. The catalytic activity of Au/CeO2 catalysts toward HCHO oxidation was evaluated in a dark organic glass reactor covered by a layer of aluminum foil on its inner wall. The temperature inside the reactor was maintained at 25 °C during the reaction process. HCHO, CO2, and water vapor were analyzed online with a Photoacoustic IR Multigas Monitor (INNOVA air Tech Instruments Model 1412). Typically, 0.15 g of Au/CeO2 sample was dispersed on the bottom of glass Petri dish with a diameter of 14 cm. After placing the sample-containing dish on the bottom of reactor with a glass slide cover, 6 μL of HCHO (38%) solution was injected into the reactor, while a 5 W fan worked in the bottom of the reactor during the adsorption process. After 2 h, formaldehyde was volatilized completely and the HCHO concentration was stabilized. Thus, the HCHO vapor was allowed to reach adsorption/desorption equilibrium inside the reactor prior to the catalytic activity measurement. The initial concentration of HCHO after adsorption/desorption equilibrium was approximately equal to 100 ppm, which was kept constant until the glass cover on the Petri dish was removed to start the catalytic oxidation. The HCHO conversion was calculated on the basis of the following equation,

instance, rod-like ceria particles with exposed {110} and {100} facets show much greater catalytic activity than cube-like ceria with {100} facet and polyhedra ceria with {111} and {100} facets toward CO oxidation.29 In this study, gold NPs highly dispersed on well-defined single-crystalline CeO2 nanorods with {110} and {100} facets were prepared. Both catalytic reactivity and stability of the as-prepared samples toward HCHO oxidation under ambient conditions were examined. To the best of our knowledge, this work is the first report about well-defined ceria nanorods with supported Au NPs toward HCHO oxidation at room temperature.

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EXPERIMENTAL SECTION Catalyst Synthesis. All reagents used in the synthesis were purchased from Alfa Aesar without any further purification. Well-defined CeO2 nanorods were hydrothermally synthesized with cerium nitride hexahydrate (Ce(NO3)3·6H2O) as the precursor. In a typical synthesis process, Ce(NO3)3·6H2O (99.5%, Alfa Aesar, 4 mmol) was dissolved in 10 mL of deionized water and then added dropwise into 70 mL of NaOH (Alfa Aesar, 6 M) under vigorous magnetic stirring at room temperature for 30 min. The as-obtained white slurry was then transferred into a Teflon-lined autoclave and hydrothermally heated at 120 °C for 24 h. After cooling naturally, the fresh precipitates were separated by centrifugation and washed with Milli-Q water (18.2 MΩ cm in resistivity, Millipore) and ethanol six times, followed by drying at 80 °C overnight. The dried yellow powders were calcined in air at 550 °C for 2 h with a heating ramp of 4 °C min−1. Au/CeO2 catalysts with various Au loadings (0.01−5.4 wt %) were prepared by a deposition−precipitation (DP) method. Briefly, the as-obtained CeO2 nanorods (0.2 g) were dispersed into 100 mL of Milli-Q water (18.2 MΩ cm, Millipore). Subsequently, a certain amount of an aqueous solution of hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·3H2O, 99.99%, 10 mgAu mL−1) was added dropwise to the above solution, followed by adjusting the pH value of the mixed aqueous solution to 9 with 0.2 M NaOH. After impregnation, excessive water was removed in a rotary evaporator at 80 °C under vacuum until dryness. The as-obtained products were washed with 1.25 L of Milli-Q water to remove chloride anions and then dried at 100 °C overnight. Finally, the Au/CeO2 catalysts were calcined at 200 °C in air for 2 h. Au/CeO2−C catalyst was obtained with a similar process where C denotes to the commercial ceria (99.9%, Alfa Aesar). The actual Au contents of the final catalysts were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using an Optima 4300 DV spectrometer (PerkinElmer). Characterization. The crystalline structure of the asprepared CeO2 nanorods was characterized by powder X-ray diffraction (XRD) using a Shimadzu X-ray diffractometer (XRD-6000) with Cu Kα radiation (λ = 1. 54178 Å) at a scanning rate of 4° min−1 in the 2θ range from 20° to 85°. Additionally, the crystallite size of ceria particles can be calculated according to the following Scherrer’s equation, d = Kλ /β cos θ

HCHO conversion (%) = ([HCHO]i − [HCHO]t )

(1)

where d is the crystal size, β is the full width at half-maximum (fwhm) peak, K is 0.89, θ is the Bragg angle, and λ is the X-ray wavelength. Transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed using a 200 kV

/[HCHO]i × 100

(2)

where [HCHO]i (ppm) is the initial HCHO concentration after adsorption/desorption equilibrium and [HCHO]t (ppm) is the HCHO concentration measured as a function of time during the test. 9703

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RESULTS AND DISCUSSION Catalyst Characterization. Figure 1 (line a) shows that all peaks in the XRD patterns can be indexed to the face-centered

The results of TEM, HRTEM, and HAADF-STEM imaging of the as-prepared CeO2 nanorods and Au/CeO2 samples are shown in Figure 3. The representative morphology of elongated

Figure 1. XRD patterns of CeO2 nanorods (a), 0.6 wt % Au/CeO2 nanorods (b), and 1.8 wt % Au/CeO2 catalyst (c).

cubic (fcc) fluorite structure [space group, Fm3m (225)] of ceria (JCPDS No. 43-1002). No additional diffraction peaks were observed, revealing the high crystalline purity of CeO2. On the basis of the Scherrer equation, the crystallite size of ceria is calculated to be 9.3 nm from the prominent (111) diffraction peak. In the case of Au/CeO2 catalysts, Figure 1 (lines b and c) displays that no diffraction peaks of Au NPs are present for 0.6 and 1.8 wt % Au/CeO2 catalysts, due to the low Au contents and small gold particles as well as high dispersion of Au NPs. Nevertheless, the intensities of the diffraction peaks for ceria were slightly decreased. The corresponding microRaman spectra are displayed in Figure 2. The typical band

Figure 3. TEM image (a) and HRTEM image (b) of CeO2 nanorods, HRTEM image of a Au NP for 1.8 wt % Au/CeO2 catalysts (c), and (d−f) HAADF-STEM images of Au/CeO2 composites with different gold content. Inset: the distribution of Au NPs: (d) 1.8 wt %, (e) 3.0 wt %, and (f) 5.4 wt %.

CeO2 nanorods is shown in Figure 3a. The width and the length are in the range of 7−13 and 30−370 nm, respectively. Figure 3b displays the HRTEM image of a typical CeO2 nanorod. Clearly, there are two sets of lattice fringes that are ascribed to (200) and (220), with the respective measured d spacing of 0.27 and 0.19 nm. Accordingly, the growth long-axis of the CeO2 nanorods is determined to be along the [110] direction. Figure 3c shows that the measured d spacing of typical Au NPs supported on CeO2 nanorods with gold content of 1.8 wt % is 0.24 nm, consistent with the (111) plane of fcc Au. On the basis of statistical analysis of Au NPs revealed by HAADF-STEM images (Figure 3d), both nanometer and subnanometer gold particles are found to coexist on ceria. The average size of Au NPs decorated on CeO2 nanorods is ca. 2.4 nm. The Au NPs size of Au/CeO2 composites with gold content of 3.0 and 5.4 wt % is also calculated (Figure 3e,f), and the corresponding Au NP average sizes are 2.7 and 3.1 nm, respectively, indicating that the Au NP size increases with increasing gold content loaded on the ceria support. In addition, the BET specific surface areas of the as-synthesized CeO2 nanorods and 1.8 wt % Au/CeO2 catalysts are 95 and 88 m2 g−1, respectively. There is no great change in BET specific surface areas observed among the catalysts due to low Au loading. In order to obtain the surface chemical states, XPS measurements were performed over 1.8 wt % Au/CeO2 catalysts. Figure 4a depicts the XPS survey spectra. Core-levels like Ce 3d, O 1s, C 1s, and Au 4f can be identified. The detailed oxidation states of the as-prepared samples are revealed in high-

Figure 2. Micro-Raman spectra of CeO2 nanorods (a) and 1.8 wt % Au/CeO2 (b) at the excitation of 514 nm.

located at 461.8 cm−1 is attributed to the first-order vibrational mode with F2g symmetry in a cubic fluorite-type structure. This band essentially results from a symmetrical stretching vibration of the oxygen atoms enclosed cerium ions, and the secondorder bands are present at 1180 and 258 cm−1, which are ascribed to the transverse acoustic (2TA) mode and longitudinal optical (2LO) mode, respectively.29,36,37 In addition, the bands at 595.9 and 575.7 cm−1, which belong to CeO2 nanorods and 1.8 wt % Au/CeO2 catalysts, are due to the oxygen vacancies (i.e., defects) in ceria. Different from CeO2 nanorods, a band at 831.3 cm−1 can be identified in the 1.8 wt % Au/CeO2 catalyst. In previous studies, Lee et al.29 and Wu et al.36 obtained similar results and proposed that the band at 831.3 cm−1 is ascribed to the presence of peroxide species absorbed on the isolated two-electron defect sites, and the peak shift of oxygen vacancies between the CeO2 nanorods and Au/ CeO2 catalyst may be due to the change of surface structure of CeO2 supports upon loading gold. 9704

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Figure 4. XPS spectra of 1.8 wt % Au/CeO2 catalyst. Survey (a) and high-resolution XPS spectra of Ce 3d (b) and Au 4f (c).

resolution XPS. The high-resolution XPS spectra of Ce 3d (Figure 4b) present complex multiple spin−orbit split doublets due to various initial and final states, with u and v referring to Ce 3d3/2 and Ce 3d5/2 components, respectively.38,39 In detail, the results of the curve-fitting show that the peaks (denoted as u1, u0, v1, and v0) are due to emission from Ce3+, whereas other signatures (denoted as u3, u2, u, v3, v2, and v) are due to Ce4+. The u3/v3 doublets with the highest binding energy represent the Ce 3d10 O 2p6 Ce 4f0 initial electronic state of Ce4+. The u2/v2 and u/v doublets are indicative of electron transfer from a filled O 2p to an empty Ce 4f orbital, consistent with a mixture of Ce 3d9 O 2p5 Ce 4f1 and Ce 3d9 O 2p4 Ce 4f2 final states, respectively. As to Ce3+, u1/v1 and u0/v0 doublets result from a mixture of the Ce 3d9 O 2p6 Ce 4f1 and Ce 3d9 O 2p5 Ce 4f2 final states. Apparently, the multiple peaks in the Ce 3d corelevels are ascribed to a mixture of the Ce3+ and Ce 4+ oxidation states, consistent with the aforementioned Raman spectra in Figure 2. The amount of Ce3+ is calculated to be ca. 21.3% according to the following equations,38 Ce(III) = v0 + v1 + u0 + u1

(3)

Ce(IV) = v + v2 + v3 + u + u 2 + u3

(4)

[Ce(III)] =

Ce(III) [Ce(III) + Ce(IV)]

Figure 5. Respective concentration changes of formaldehyde and carbon dioxide as a function of time over CeO2 nanorods (a) and 1.8 wt % Au/CeO2 catalyst (b).

ceria at room temperature is negligible. In the presence of 1.8 wt % Au/CeO2 catalyst, the concentration of HCHO decreases dramatically in half an hour, as shown in Figure 5b. Meanwhile, the CO2 concentration increases remarkably, indicating that formaldehyde decomposition indeed occurs. Compared to previous studies, the Au/CeO2 catalysts investigated in this work show better performance. Chen et al.12 prepared nanosized gold in the range of 3−9 nm on commercial ceria by impregnation and found that HCHO decomposition over Au/CeO2 catalysts only occurs under visible-light irradiation with an intensity of 0.159 W cm−2 via surface plasmon resonance (SPR). Li et al.21 synthesized a series of Au/CeO2 catalysts with various specific surface areas by template and sol−gel methods, indicating that the activity of HCHO oxidation increased with increasing specific surface areas and HCHO can only be decomposed into CO2 and H2O at relatively high temperature (50 to 150 °C). In the current work, HCHO can be completely oxidized into CO2 and H2O under ambient conditions, keeping in mind that the well-defined ceria nanorods used in this work present exposed {110} and {100} facets. Prior studies41,42 reported that the order of vacancy formation energy on the low-index ceria surfaces follows the order of {110} < {100} < {111} due to the various atomic relaxation around the vacancy (2.69, 2.97, and 3.30 eV for {110}, {100}, and {111} planes, respectively). In general, oxygen vacancies are indispensable for stabilizing metal NPs.24 Compared with ceria with other types of exposed facets, CeO2 nanorods with {110} and {100} facets should be a unique support for anchoring and dispersing very fine gold NPs, as evidenced by aforementioned HAADF-STEM imaging. Additionally, the Ce−O bond upon Au deposition can be further weakened by the adjacent gold clusters, as confirmed by previous experimental and theoretical studies.22,43,44 After nucleation at the vacancy, the Au atom bonded with O atom at the surface would transfer an electron to Ce via O and weaken the Ce−O bond, taking into account that the empty f state of Ce4+ is just above the Fermi level and can accept electrons.45,46 The weakened Ce−O bond in turn facilitates

(5)

where Ce(III) and Ce(IV) represent the respective sums of the total peak areas from the Ce3+ and Ce4+ XPS signals. Figure 4c displays the corresponding high-resolution Au 4f core-level spectrum. The Au spectrum is well fitted by three sets of doublets with the respective 4f7/2 component at 83.8 (Au0), 84.6 (Au1+), and 86.2 eV (Au3+), in agreement with previous studies.24,33 The metallic gold (Au0) is the dominant species (66.0%), while oxidized Au1+ and Au3+ are 27.2% and 6.8%, respectively. Catalytic Activity. The catalytic activity of CeO2 and Au/ CeO2 catalysts toward HCHO oxidation at room temperature is illustrated in Figure 5. In the case of CeO2 nanorods, Figure 5a shows that the concentration of HCHO decreases from the initial 109.3 to 50.6 ppm within 1 h. However, no change of CO2 concentration is seen as a function of time. Therefore, HCHO adsorption is dominant and HCHO oxidation by pure 9705

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reducibility as well as catalytic activity of Au/CeO2 catalysts toward HCHO oxidation.46 To gain the new insights into the decomposition mechanism of formaldehyde catalytic oxidation over Au/CeO2 catalysts, H2-TPR of CeO2 and 1.8 wt % Au/CeO2 was performed and the results are shown in Figure 6. In the detected temperature

Figure 8. Schematic diagram illustrating the catalytic oxidation of formaldehyde by Au/CeO2 catalysts at room temperature. Figure 6. H2-TPR profiles of CeO2 and 1.8 wt % Au/CeO2 catalysts.

nucleophilic attack of O atom from lattice oxygen of ceria in which the Ce−O bond adjacent to gold NPs is weakened by Au NPs. Meanwhile, an oxygen vacancy is formed at the Au−CeO2 interface and a gas phase O2 molecule (V−O2*) is subsequently decomposed on the oxygen vacancy site to fill it. 47 Subsequently, a second attack would take place at the C−H bond of the generated formate species by another excess active surface oxygen atom and turn it into carbonic acid due to the charge redistribution between the V−O2*. Finally, the formed carbonic acid species is decomposed into CO2 and H2O. The catalytic properties toward HCHO oxidation over Au/ CeO2 catalysts with various Au contents were monitored and shown in Figure 9. Herein, the reactivity is denoted as the

range, both CeO2 and 1.8 wt % Au/CeO2 catalysts show three reduction peaks. However, the reduction peaks of gold loaded ceria sample shifts to lower temperature compared to the bare ceria, indicating that gold can catalyze the reduction of surface oxygen species as well as the weakening of the Ce−O bond adjacent to the gold.22 The two reduction peaks at low temperature are attributed to surface active oxygen species like O− and O2−. Meanwhile, the peak at high temperature is ascribed to the reduction of lattice oxygen of ceria.40 To detect the intermediates in catalyzing formaldehyde oxidation, in situ DFTIR spectra of the Au/CeO2 catalysts upon exposure to HCHO/O2 at room temperature were obtained, as shown in Figure 7. Transient reaction results show that formate species

Figure 9. Specific reaction rate of CO2 formation with different catalysts: CeO2 nanorods (a), 0.1 wt % Au/CeO2 (b), 0.6 wt % Au/ CeO2 (c), 1.8 wt % Au/CeO2 (d), 3.0 wt % Au/CeO2 (e), 5.4 wt % Au/CeO2 (f), and 0.1 wt % Au/CeO2−C (g).

Figure 7. In situ DFTIR spectra of 1.8 wt % Au/CeO2 catalysts under a flow of HCHO/O2 at room temperature.

and water are the main formed species. The bands appearing at 1360 cm−1 and 1527 and 1579 cm−1 are ascribed to the symmetric (vs) and asymmetric stretching (vas) of COO, respectively.16,22 The band located at 2840 cm−1 is attributed to CH stretching.44 In addition, the bands at 1672 and 3420 cm−1 are due to water formed in the formaldehyde oxidation process.40 No peaks corresponding to HCHO were detected, indicating that HCHO was instantly decomposed over the surface of Au/CeO2, implying the prepared catalyst had excellent catalytic activity. Herein, a possible mechanism for formaldehyde catalytic oxidation over Au/CeO2 catalyst is proposed. The detailed steps are illustrated in Figure 8. The initial stage is HCHO preadsorption on the catalyst surface. Then, the preadsorbed formaldehyde is oxidized into formate species by the

specific reaction rate of CO2 generation and plotted as a function of Au contents. As shown in Figure 9, the catalytic performance for formaldehyde decomposition was shown to be dependent on the Au contents. It is clear that the catalytic performance of Au/CeO2 catalysts with various Au contents (0.1−5.4 wt %) increases with increasing Au contents up to 1.8 wt %. However, the conversion of HCHO decreases with a further increase of Au contents. In general, increasing gold contents often results in the aggregation of gold NPs and further decreases the specific surface area. Nevertheless, the gold−ceria perimeters could be the dominant active sites for catalytic reaction while other gold atoms within Au NPs are not directly involved.48 Thus, the catalytic performance is not increased with increasing gold contents from 1.8 to 5.4 wt %. In 9706

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(5) Domingo-García, M.; Fernández-Morales, I.; López-Garzón, F. J.; Moreno-Castilla, C.; Pérez-Mendoza, M. On the adsorption of formaldehyde at high temperatures and zero surface coverage. Langmuir 1999, 15, 3226−3231. (6) Le, Y.; Guo, D.; Cheng, B.; Yu, J. Bio-template-assisted synthesis of hierarchically hollow SiO2 microtubes and their enhanced formaldehyde adsorption performance. Appl. Surf. Sci. 2013, 274, 110−116. (7) Xu, Z.; Yu, J.; Xiao, W. Microemulsion-assisted preparation of a mesoporous ferrihydrite/SiO2 composite for the efficient removal of formaldehyde from air. Chem.Eur. J. 2013, 19 (29), 9592−9598. (8) Yu, J.; Li, X.; Xu, Z.; Xiao, W. NaOH-modified ceramic honeycomb with enhanced formaldehyde adsorption and removal performance. Environ. Sci. Technol. 2013, 47 (17), 9928−9933. (9) Zhou, P.; Zhu, X.; Yu, J.; Xiao, W. Effects of adsorbed F, OH, and Cl ions on formaldehyde adsorption performance and mechanism of anatase TiO2 nanosheets with exposed {001} facets. ACS Appl. Mater. Interfaces 2013, 5 (16), 8165−8172. (10) Liang, W. J.; Li, J.; Li, J. X.; Zhu, T.; Jin, Y. Q. Formaldehyde removal from gas streams by means of NaNO2 dielectric barrier discharge plasma. J. Hazard. Mater. 2010, 175 (1−3), 1090−1095. (11) Fu, P.; Zhang, P.; Li, J. Photocatalytic degradation of low concentration formaldehyde and simultaneous elimination of ozone by-product using palladium modified TiO2 films under UV254 + 185 nm irradiation. Appl. Catal. B: Environ. 2011, 105 (1−2), 220−228. (12) Chen, X.; Zhu, H. Y.; Zhao, J. C.; Zheng, Z. F.; Gao, X. P. Visible-light-driven oxidation of organic contaminants in air with gold nanoparticle catalysts on oxide supports. Angew. Chem., Int. Ed. 2008, 47 (29), 5353−5356. (13) Yu, J.; Wang, S.; Low, J.; Xiao, W. Enhanced photocatalytic performance of direct Z-scheme g-C3N4-TiO2 photocatalysts for the decomposition of formaldehyde in air. Phys. Chem. Chem. Phys. 2013, 15 (39), 16883−16890. (14) Zhang, C.; Liu, F.; Zhai, Y.; Ariga, H.; Yi, N.; Liu, Y.; Asakura, K.; Flytzani-Stephanopoulos, M.; He, H. Alkali-metal-promoted Pt/ TiO2 opens a more efficient pathway to formaldehyde oxidation at ambient temperatures. Angew. Chem., Int. Ed. 2012, 51 (38), 9628− 9632. (15) Yu, X.; He, J.; Wang, D.; Hu, Y.; Tian, H.; He, Z. Facile controlled synthesis of Pt/MnO2 nanostructured catalysts and their catalytic performance for oxidative decomposition of formaldehyde. J. Phys. Chem. C 2012, 116 (1), 851−860. (16) Ma, C.; Wang, D.; Xue, W.; Dou, B.; Wang, H.; Hao, Z. Investigation of formaldehyde oxidation over Co3O4-CeO2 and Au/ Co3O4-CeO2 catalysts at room temperature: Effective removal and determination of reaction mechanism. Environ. Sci. Technol. 2011, 45 (8), 3628−3634. (17) Nie, L.; Yu, J.; Li, X.; Cheng, B.; Liu, G.; Jaroniec, M. Enhanced performance of NaOH-modified Pt/TiO2 toward room temperature selective oxidation of formaldehyde. Environ. Sci. Technol. 2013, 47 (6), 2777−2783. (18) Hong, Y. C.; Sun, K. Q.; Han, K. H.; Liu, G.; Xu, B. Q. Comparison of catalytic combustion of carbon monoxide and formaldehyde over Au/ZrO2 catalysts. Catal. Today 2010, 158 (3− 4), 415−422. (19) Sekine, Y. Oxidative decomposition of formaldehyde by metal oxides at room temperature. Atmos. Environ. 2002, 36, 5543−5547. (20) Huang, H.; Leung, D. Y. C. Complete elimination of indoor formaldehyde over supported Pt catalysts with extremely low Pt content at ambient temperature. J. Catal. 2011, 280 (1), 60−67. (21) Li, H. F.; Zhang, N.; Chen, P.; Luo, M. F.; Lu, J. Q. High surface area Au/CeO2 catalysts for low temperature formaldehyde oxidation. Appl. Catal. B: Environ. 2011, 110, 279−285. (22) Liu, B.; Li, C.; Zhang, Y.; Liu, Y.; Hu, W.; Wang, Q.; Han, L.; Zhang, J. Investigation of catalytic mechanism of formaldehyde oxidation over three-dimensionally ordered macroporous Au/CeO2 catalyst. Appl. Catal. B: Environ. 2012, 111−112, 467−475. (23) Klm, G. Ceria-promoted three-way catalysts for auto exhaust emission control. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 267−274.

this work, the comparison of formaldehyde catalytic oxidation over Au NPs supported on CeO2 nanorods and commercially available CeO2 was carried out. As shown in Figure 9, the specific reaction rate of CO2 formation over 0.1 wt % Au/CeO2 is nearly three times as high as 0.1 wt % Au/CeO2−C catalyst. This can be attributed to the commercial ceria terminated with stability (111) facet with high oxygen vacancy formation energy. (The HRTEM image of commercial ceria is not shown here). The recyclability of 1.8 wt % Au/CeO2 catalyst in the oxidative decomposition of HCHO at room temperature was examined. Figure 10 shows that, after 6 cycles of catalytic

Figure 10. Durability test of formaldehyde oxidation over 1.8 wt % Au/CeO2 catalyst.

reaction, no significant loss of activity was observed. Our results demonstrate that Au/CeO2 catalysts can be used to catalyze multiple cycles of HCHO oxidation with recyclability at ambient temperature.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: (+86) 27-87871029; fax: (+86) 27-87879468; e-mail: [email protected]. *Tel: (+86) 10-82545613; fax: (+86) 10-62656765; e-mail: [email protected]. *Tel: (+86) 10-62796840; fax: (+86) 10-62797760; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 863 Program (2012AA062701), 973 Program (2013CB632402), NSFC (51272048, 21177100, and 51272199), Fundamental Research Funds for the Central Universities (WUT: 2014-VII-010), and Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1).

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