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Surface-Confined Atomic Silver Centers Catalyzing Formaldehyde Oxidation Pingping Hu,† Zakariae Amghouz,‡ Zhiwei Huang,† Fei Xu,† Yaxin Chen,† and Xingfu Tang*,† †

Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China ‡ Unidad de Microscopia Electrónica de los Servicios Científico-Técnicos, Universidad de Oviedo-CINN, Oviedo 33006, Spain S Supporting Information *

ABSTRACT: Formaldehyde (HCHO) is a prior pollutant in both indoor and outdoor air, and catalytic oxidation proves the most promising technology for HCHO abatement. For this purpose, supported metal catalysts with single silver atoms confined at 4-fold O4-terminated surface hollow sites of a hollandite manganese oxide (HMO) as catalytic centers were synthesized and investigated in the complete oxidation of HCHO. Synchrotron X-ray diffraction patterns, X-ray absorption spectra, and electron diffraction tomography revealed that geometric structures and electronic states of the catalytic centers were tuned by the changes of HMO structures via controllable metal− support interactions. The catalytic tests demonstrated that the catalytically active centers with high electronic density of states and strong redox ability are favorable for enhancement of the catalytic efficiency in the HCHO oxidation. This work provides a strategy for designing efficient oxidation catalysts for controlling air pollution.

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INTRODUCTION Formaldehyde (HCHO) became known as an indoor pollutant in 19621 and was classified as a Group 1 human carcinogen by the International Agency for Research on Cancer in 2004.2 In 2010, the World Health Organization considered an indoor value of 0.1 mg m−3 for HCHO as sufficient.3 Presently, the HCHO concentrations in the outdoor air, particularly in metropolitan areas with high photochemical air pollution,4 gradually increase due partly to the increasing use of biofuels,5 and even reach indoor levels typically in summer months.6 Hence, HCHO is a typical pollutant in both indoor and outdoor air,7,8 which is a hazard to human health and induces photochemical pollution.9,10 Enormous efforts have been devoted to reducing the HCHO concentrations in the air to satisfy the stringent environmental regulations. Adsorption technology with efficient adsorbents such as potassium permanganate and organic amines was often used for HCHO purification,11,12 but adsorbent efficiency is inevitably limited by the temperature-dependent adsorption− desorption balance. Heterogeneous catalytic oxidation is one of the most promising technologies for reducing HCHO pollution,13 because catalysts can convert HCHO into CO2 at low temperatures, and thus transition metal oxides or supported noble metals have been developed as catalysts for this purpose. Among transition metal oxides, manganese oxides showed the best catalytic performance,14 and in particular hollandite manganese oxides (HMO) had high catalytic activity toward the HCHO oxidation at low temperatures.15 Generally, © 2015 American Chemical Society

noble metals such as palladium, gold, and platinum (Pt) supported on metal oxides showed higher catalytic activity in the HCHO oxidation than transition metal oxides. These supported noble metal catalysts could oxidize HCHO at low temperatures or even at room temperature.16−18 Likewise, supported silver (Ag) catalysts, much cheaper than the above noble metal catalysts, were proven to have the high lowtemperature activities for catalyzing HCHO oxidation.19−21 In fact, the microstructures of catalytically active centers play an important role in determining catalytic efficiency apart from the nature of metals and the intrinsic metal effects, such as the electronic quantum size effect and the structure-sensitivity geometrical effect.22,23 To improve catalytic activity, several methods can be used to optimize geometric and electronic structures of catalytic centers. One feasible method is to add another metal to metal particle/cluster catalyst to perturb the electronic states of active centers, resulting in an increase of the reaction rates.16,18 The adding of alkalis, such as sodium or potassium to Pt-based catalysts, increased the number of electron-withdrawing groups, such as O2− and OH−, that modified electronic structures of catalytic metal sites, leading to the enhancement of catalytic performance.18,24 Another alternative is to tune electronic metal−support interactions to Received: Revised: Accepted: Published: 2384

September 18, 2014 December 26, 2014 January 15, 2015 January 15, 2015 DOI: 10.1021/es504570n Environ. Sci. Technol. 2015, 49, 2384−2390

Article

Environmental Science & Technology

with STEM control unit (Gatan), EDX detector (80 mm2 XMax SDD) and CCD camera (14-bit Gatan Orius SC600). Diffraction data (DT) acquisition was performed with high-tilt tomography holder and collected in steps of 1° in a range of ±26°. In order to avoid beam damage for the samples, selected area electron diffraction patterns were collected under mild illumination setting by CCD camera using Digital Micrograph software and ADT3D package for DT processing. Fine powders of the materials were dispersed in ethanol, sonified, and sprayed on a carbon coated copper grid, and then allowed to air-dry. Finally, Gatan SOLARUS 950 was used before observation. The Ag K-edge extended X-ray absorption fine structure (EXAFS) spectra were measured at the BL14W of the SSRF with an electron beam energy of 3.5 GeV and a ring current of 200−300 mA. The data were collected with a fixed exit monochromator using two flat Si (311) crystals. The EXAFS spectra were collected in a transmission mode using ion chambers filled with N2. The raw data were analyzed using the IFEFFIT 1.2.11 software package. X-ray photoelectron spectra (XPS) were recorded at 25 °C on a PHI15300/ESCA spectrometer with a magnesium anode for Kα (hν = 1253.6 eV) radiation. Charging effects were corrected by adjusting binding energy of C 1s to 284.6 eV. The temperature-programmed reduction of hydrogen (H2TPR) measurements were carried out on a 2720 adsorption instrument (Micromeritics, U.S.A.) equipped with a TCD detector. 20−30 mg samples were loaded and were reduced in the stream of 10.0 vol % H2/Ar (50 mL min−1) at a ramp rate of 10 °C min−1. Catalytic Evaluation. The complete oxidation of HCHO was performed in a fixed-bed quartz reactor (i.d. = 8 mm) under atmospheric pressure. Fresh catalyst (200 mg, 40−60 mesh) was charged for each run. HCHO was generated by passing a N2 gas flow over paraformaldehyde (96%, Acros) in an incubator kept at 45 °C. The HCHO gas was mixed with another O2 flow to get a feed gas, which consists of 400 ppm of HCHO and 10.0 vol % O2 balanced by N2. The total flow rate was 100 mL min−1. The effluents from the reactor were analyzed by an online Agilent 7890A gas chromatograph equipped with a TCD detector. The data were recorded at the steady state for each run.

achieve catalysts with excellent active centers. Recently, we supported Ag on HMO to achieve supported single-atom catalysts, and single Ag atoms were stably confined on the surfaces of HMO as catalytic centers.21 An electronic perturbation of the catalytic sites was induced by a subtle change in the structure of HMO, which led to easier redox ability and higher catalytic activity toward the HCHO oxidation.25 Here we systematically investigated the geometric and electronic structures of catalytic centers of two supported single-atom Ag catalysts (Ag1/HMO, denoted as Ag1/HMO-A and Ag1/HMO-I) (Scheme 1) and their catalytic performance Scheme 1. Models Illustrating Four-Fold O4-Terminated Hollow Sites on The HMO (001) Plane and SurfaceConfined Atomic Ag Centers of Two Ag1/HMO Catalystsa

a

The yellow and blue balls are Ag atoms, red and creamy balls are O atoms, and gray balls represent Mn atoms.

in the HCHO oxidation. To correlate the structure of catalytic centers with catalytic activity, the synchrotron X-ray diffraction (SXRD) pattern and the electron diffraction tomography were conducted to determine the crystal structures of the catalysts, and the local structures of catalytic centers were determined by the X-ray absorption spectroscopy (XAS). The relationship between the geometric and electronic microstructures of surface-confined atomic Ag catalytic centers and the enhanced efficiency for HCHO abatement was established.

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EXPERIMENTAL SECTION Materials Preparation. The synthesis of the samples was reported in our recent work.25 Briefly, HMO was prepared by the hydrothermal method and the resulting powder was calcined at 500 °C in air for 6 h. Ag1/HMO-A was prepared by annealing supported Ag particles on HMO at 500 °C in air for 6 h. Ag1/HMO-I was prepared by impregnating the HMO powder in an aqueous solution of AgNO3, and then dried at 80 °C for 24 h and calcined at 500 °C in air for 6 h. The Ag loadings (9.7 wt % for Ag1/HMO-A and 9.8 wt % for Ag1/ HMO-I) were determined by the inductively coupled plasmaatomic emission spectroscopy on a Plasam-Spec-I spectrometer. Materials Characterization. The SXRD patterns were performed at the BL14B of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 1.2398 Å. The beam was monochromatized using Si (111), and a Rh/Si mirror was used for the beam focusing to a size of around 0.5 × 0.5 mm2. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), dark-field scanning TEM (DF-STEM) studies, and energy-dispersive X-ray spectroscopy (EDX) microanalysis (point/line scan analysis) were carried out with a JEOL JEM-2100F field-emission gun transmission electron microscope operating at an accelerating voltage of 200 kV and equipped with an ultrahigh resolution pole-piece that provided a point-resolution better than 0.19 nm. It was also equipped

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RESULTS AND DISCUSSION The crystal structure of HMO can be indexed to a hollandite manganese oxide with the tetragonal structure and space group of I4/m, as shown in the SXRD pattern of HMO in Supporting Information (SI) Figure S1, and thus HMO has a onedimensional square pore (4.7 × 4.7 Å 2 ), which can accommodate various metal atoms.26 Depending on the nature of metal atoms, the structure of HMO can be transformed from tetragonal structure into monoclinic structure.27 The SXRD patterns of Ag1/HMO (SI Figure S1) illustrated that the Ag loading made the crystal structure of HMO changed, possibly from the tetragonal structure (I4/m) into a monoclinic structure (C2/m) due to the relatively small size of Ag atoms.27 Note that the subtle differences between the structures of two Ag1/HMO catalysts can be readily observed from the differential SXRD patterns (ΔSXRD), which demonstrated that two Ag1/HMO catalysts have different crystal structures. After the Ag loading, the morphology of HMO remains unchanged regardless of the different methods of loading, as observed in the TEM and HRTEM images in SI Figure S2. The Ag species of Ag1/HMO were substantiated by the EDX shown 2385

DOI: 10.1021/es504570n Environ. Sci. Technol. 2015, 49, 2384−2390

Article


Figure 1. DF-STEM images (a,e,i) with EDX line-scan along the yellow lines, TEM images (b,f,j) of each selected nanorods for electron diffraction tomography imaging and the corresponding selected area EDX spectra (c,g,k) in the orange circles and 3D reciprocal space reconstruction (d,h,l) from electron diffraction data of the individual nanorods of HMO (a,b,c,d), Ag1/HMO-A (e,f,g,h), and Ag1/HMO-I (i,j,k,l).

loading. The results agree fairly with the above SXRD data. Although two Ag1/HMO catalysts have the monoclinic structures with the same space group of C2/m, their lattice parameters are significantly different from each other, as listed in Table 1, reflecting that their Ag atoms have different local structures and immediate environments. We used the EXAFS spectra to determine the accurate immediate structures of Ag atoms in Ag1/HMO samples.29 Figure 2 shows the Fourier transform (FT) amplitudes of the χ(R) k3-weighted EXAFS spectra at the Ag K-edge of two Ag1/ HMO samples together with Ag foil and Ag2O as references (SI Figure S3). The structural parameters obtained by fitting the spectra with the corresponding models30 are listed in SI Table S1. The FT EXAFS spectra of Ag1/HMO are significantly different from those of Ag foil and Ag2O, which elucidate the different local environments of Ag1/HMO from those of Ag foil or Ag2O. The FT EXAFS spectrum of Ag1/HMO-A is essentially different from that of Ag1/HMO-I, though they are similar to each other. Accordingly, the coordination numbers of Ag at the nearest two shells are the same to each other, while the interatomic distances in the nearest-neighbor shells (AgO and AgAg) for the Ag1/HMO-A are slightly shorter than those of the Ag1/HMO-I because the former has stronger interaction between Ag and O atoms than the latter.25 Therefore, the Ag atoms in two Ag1/HMO samples have different local structures, which will lead to different electronic structures (states) of Ag and O atoms.

in Figure 1, and the ratios of Ag to manganese (Mn) are almost same for two Ag1/HMO samples, approaching to the designed values.25 To determine the structural differences between two Ag1/HMO samples, the electron diffraction tomography, as a complement to the SXRD crystallographic technique, is used as an ideal technique for imaging the three-dimensional (3D) local structure of materials at high resolution.28 The structures of the typical individual nanorods of HMO and two Ag1/HMO samples were determined by the electron diffraction tomography, as shown in Figure 1, from which the lattice parameters were obtained and listed in Table 1. The tetragonal structure of HMO with the space group of I4/m has transformed into the monoclinic structure with the space group of C2/m after the Ag Table 1. Lattice Parameters of HMO and Two Ag1/HMO Catalysts Determined by the Electron Diffraction Tomography sample

HMO

Ag1/HMO-A

Ag1/HMO-I

crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg)

tetragonal I4/m 10.16 10.16 2.94 90.0 90.0 90.0

monoclinic C2/m 10.66 10.22 2.94 90.0 105.5 90.0

monoclinic C2/m 14.23 2.91 10.28 90.0 135.0 90.0 2386


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difference of the Ag electronic states of two Ag1/HMO catalysts mainly originates from the different density states of the Ag d orbitals.25 The Ag atoms with different d electronic states have different interactions with O atoms at the perimeter of Ag atoms through the hybridization of the Ag d orbitals with the O p orbitals, which can be described as the electronic metal− support interaction.25,33 The electronic density of O was investigated by the O 1s XPS, and an intensive peak with a shoulder at the high binding energy side is observed for two Ag1/HMO catalysts in Figure 3b. The main peak at about 528.5−530.5 eV can be attributed to surface lattice oxygen atoms (Os2−), and a discernible shift to lower binding energy for Ag1/HMO-A is observed in comparison to Ag1/HMO-I. A lowering in binding energy of Os2− is in general assigned to an increase of its negative charge.34 Furthermore, the average oxidation states (AOS) of Mn were estimated by the Mn 2p and 3s XPS. For two Ag1/ HMO catalysts, the Mn 2p3/2 and Mn 2p1/2 peaks are centered at about 642.3 and 654.0 eV, respectively, with an energy span of about 11.7 eV, as shown in SI Figure S5, implying that the Mn species are predominantly Mn4+.35 To more accurately determine the AOS of Mn, the Mn 3s XPS were performed and are also shown in SI Figure S5. The same AOS of Mn for both cases can be calculated to be about 3.8, same to the Mn species as MnO1.9, according to an energy difference (ΔE3s) between the main peak and its satellite of the Mn 3s XPS and a relationship between the ΔE3s and the AOS of Mn.35,36 These results indicated that the different electronic states of lattice oxygen atoms predominantly originate from interactions of O with Ag more than those with Mn. On the basis of the above evidence, the electronic states of both Ag and O of Ag1/HMO-A are different from those of Ag1/ HMO-I. The Ag of Ag1/HMO-A have less positive charge than those of Ag1/HMO-I, and the Os2− of Ag1/HMO-A have more negative charge than those of Ag1/HMO-I. The different electronic states of Ag and Os2− should exhibit different chemical reactivity, such as redox ability and catalytic activity. The redox abilities of two Ag1/HMO catalysts were investigated using molecular hydrogen (H2) probe by a

Figure 2. Ag K-edge χ(R) k3-weighted FT EXAFS spectra of two Ag1/ HMO catalysts, Ag2O and Ag foil.

The electronic states of Ag were determined by the Ag 3d XPS and the Ag Auger spectra, as shown in SI Figure S4 and Figure 3a, respectively. It is substantially difficult to use Ag 3d XPS to determine the oxidation states because Ag has essentially no chemical shift in the 3d5/2 XPS at all, but the Auger line (Ag M4VV) of Auger spectra are very sensitive to the oxidation states of Ag.31 As shown in Figure 3a, the kinetic energy of the Ag M4VV Auger lines of Ag1/HMO-A and Ag1/ HMO-I are 356.8 and 356.5 eV, respectively, and both are between those of Ag2O and Ag foil, reflecting that the Ag atoms in two Ag1/HMO catalysts are still at the metallic states (Agδ+, 0 < δ < 1).25,32 Subtly, the Ag atoms of Ag1/HMO-A have relatively higher electronic density of states than those of Ag1/ HMO-I according to their Ag M4VV Auger spectra. The

Figure 3. (a) Ag M4VV Auger spectra of two Ag1/HMO catalysts, Ag foil and Ag2O. (b) O 1s XPS of two Ag1/HMO catalysts. 2387


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Environmental Science & Technology temperature-programmed procedure, shown in the H2-TPR profiles of Figure 4. Two main reduction peaks are observed for

Ea = ln(ν0)kBT

(1)

where kB is the Boltzmann constant, and ν0 is the usual prefactor. By taking ν0 = 6 × 1012,39 and T = 350 K (∼80 °C) or 400 K (∼130 °C), as shown in Figure 4, the calculated Ea are 0.9 and 1.0 eV for the dissociation of H2 on the CASs of Ag1/ HMO-A and Ag1/HMO-I, respectively, which approach to that (1.11 eV) on Ag (111) planes.40 As a consequence, the CASs of Ag1/HMO-A have the stronger redox ability than those of Ag1/ HMO-I. Catalysts with stronger redox ability are often more favorable for enhancing catalytic activity in oxidation reactions such as the HCHO oxidation.21,25 The catalytic properties of Ag1/ HMO together with HMO were investigated by the HCHO oxidation, and the results are shown in SI Figure S7. The catalytic activity was improved after Ag loadings in comparison to HMO. Note that Ag1/HMO-A shows higher catalytic activity than Ag1/HMO-I under the same reaction conditions. The reaction rates in terms of turnover frequencies (TOF, the number of converted HCHO molecules per CAS per second) as a function of temperatures are shown in Figure 5, and the

Figure 4. H2-TPR profiles of Ag1/HMO-A (a) and Ag1/HMO-I (b).

two Ag1/HMO samples, and a shoulder in the low temperature side is also discernible for both cases. The shoulders are associated with the activation and dissociation of H2 on Ag atoms due to the absence of reduction of HMO in this temperature window, as confirmed in SI Figure S6,37 and thus two main peaks result from the spillover of H atoms from Ag atoms. The dissociated H atoms preferentially spill along the atomic Ag chains and react with the O atoms near the Ag atoms, and then spill onto manganese oxides and reduce the O atoms far from Ag atoms.21 Therefore, the H2-TPR profiles of Ag1/HMO can be well deconvoluted into two main reduction processes with typical three-step reduction characteristics of manganese oxides,38 as shown in Figure 4, from which the AOS of Mn are also calculated to be about 3.8 for two Ag1/HMO samples by integrating the corresponding reduction curves, consistent with the XPS results above. The reduction shoulders in the H2-TPR profiles of two Ag1/ HMO catalysts can be attributed to the dissociation of H2 by surface Ag atoms and the subsequent consumption of H atoms by the lattice oxygen at the perimeters of the Ag atoms.21 During this process, the dissociation of H2 is a rate-determining step due to the relatively high barriers for H2 activation on Ag,37 and hence the shoulders (the first reduction peaks) represent the redox ability of surface Ag atoms (catalytically active sites, CASs). To evaluate the redox ability of the CASs, we calculated the activation energy (Ea) required to dissociate a H2 molecule on the CASs by using the following formula:39

Figure 5. TOF of the HCHO oxidation over two Ag1/HMO catalysts and HMO.

detailed calculation of TOF is given in the SI. Two distinct reaction regimes, low-reactivity and high-reactivity steady states (separated by a horizontal dash line), are observed, and temperature-triggered kinetic transitions occur at the same TOF of HCHO for three samples despite the different reaction temperatures, indicating that “single-site ignition” occurs at the same TOF of 0.05 s−1,41 since the openings of the HMO pores (Scheme 1) can be also regarded as single active sites.15 At the single-site ignition temperature, the TOF of HCHO over Ag1/ HMO-A is approximately 20 times as that (2.5 × 10−3 s−1) of Ag powder at 220 °C.20 The HCHO oxidation in the low-reactivity steady state regime often follows a metal-assisted Mars-van Krevelen mechanism,20,21 and dissociation of molecular oxygen (O2) (2) and oxidation of adsorbed formate (the stable intermediate in the HCHO oxidation reaction) (3) are regarded as two critical steps to govern the apparent reaction rate:20

2388

1/2O2(g) + 2e− → Os 2 −

(2)

HCOO−(a) + Os 2 − → CO2(g) + OH−(a) + 2e−

(3)



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where HCOO−(a) and OH−(a) are adsorbed formate and adsorbed hydroxyl, respectively. For the above reaction 2, the activation of O2 is one of the main factors for determining the reaction rate, and Ag often possesses the strong ability to activate O2. Depending on the electronic states of confined atomic Ag active sites, Ag1/HMO-A with higher electronic density of states and better redox ability exhibits stronger activation of O2 in eq 2, and thus gives a lower apparent activation energy of 0.9 eV required for the HCHO oxidation than that (1.1 eV) of Ag1/HMO-I. For reaction 3 above, the oxidation ability of Os2− specifically at the perimeter of the Ag active sites has a crucial influence on the HCOO−(a) oxidation into CO2. Os2− with binding energy of about 528.4−530.4 eV are proven to have nucleophilic properties, which are favorable for complete oxidation reactions.42 The Os2− of Ag1/HMO-A have more negative charge, as evidenced by XPS, and thus have stronger nucleophilic properties than those of Ag1/HMO-I, which lead to an easier reaction with the electrophilic carbonyl group of HCOO−(a) and further facilitate the oxidation of HCHO. As shown in the H2-TPR profiles in Figure 4a, Os2− are about 2.7% of total lattice oxygen atoms for two Ag1/HMO catalysts, approaching to 3% of total lattice atoms as reported.43 For Ag1/ HMO-A, about 0.4% Os2− can be reduced by H2 at temperatures no higher than 70 °C, but only 50% of this part of Os2− (0.2% Os2−, about 12 Os2− per Ag active site25), are reversible Os2− that attend catalytic cycles at this temperature. Under the same conditions, Ag1/HMO-I has much less reversible Os2− responsible for catalytic cycles. Therefore, the catalytic activities of two Ag1/HMO catalysts for the HCHO oxidation are attributed to the surface-confined atomic Ag catalytic centers including single-atom Ag active sites and reversible lattice oxygen atoms at the perimeters of Ag active sites, and the electronic properties of catalytically active centers play important roles in determining catalytic performance. The high activity of Ag1/HMO-A is attributed to highly active atomic Ag sites and abundant reversible Os2− with the strong nucleophilic property. In summary, surface-confined single Ag atoms on the O4terminated hollow sites on the HMO surface as catalytic centers were investigated for the HCHO oxidation at low temperatures. The local structures and electronic states of catalytic centers were readily controlled by the synthesis methods. The results demonstrated that the catalytic center includes the surface atomic Ag site and the vicinal lattice oxygen atoms, and that the electronic states of the catalytic centers play a critical role in determining the catalytic performance in the HCHO oxidation. The Ag sites of the catalytic centers with higher electronic density of states facilitated the activation of O2, and the surface lattice oxygen atoms with more negative charge had stronger nucleophilic property and thus exhibited the higher efficiency for the HCHO abatement. These results may assist in developing an excellent catalyst to efficiently control the emissions of air pollutants including HCHO.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-21-65642997; fax: +86-21-65643597; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the NSFC (21277032, 21477023), the STCSM (14JC1400400) and the SCAPC (201306). The SXRD and XAS measurements were conducted at the SSRF, Chinese Academy of Sciences. Spanish Ministerio de Economiá y Competitividad (MAT2010-15094, MAT201340950-R, and grant Técnicos de Inf raestructuras Cientif́ icoTecnológicas PTA2011-4903-I to Z.A.) and FEDER were also acknowledged.

(1) Wittmann, O. The subsequent dissociation of formaldehyde from particle board. Eur. J. Wood Wood Prod. 1962, 20 (6), 221−224. (2) Formaldehyde, 2-butoxyethanol and 1-tert-butoxy-2-propanol, IARC monographs on the evaluation of carcinogenic risks to humans; World Health Organization: Lyon, France, 2006. (3) WHO guidelines for indoor air quality: Selected pollutants; World Health Organization Regional Office for Europe: Copenhagen, Denmark, 2010. (4) Dasgupta, P. K.; Li, J.; Zhang, G.; Luke, W. T.; Mcclenny, W. A.; Stutz, J.; Fried, A. Summertime ambient formaldehyde in five U.S. metropolitan areas: Nashville, Atlanta, Houston, Philadelphia, and Tampa. Environ. Sci. Technol. 2005, 39 (13), 4767−4783. (5) Kohse-Höinghaus, K.; Oβswald, P.; Cool, T. A.; Kasper, T.; Hansen, N.; Qi, F.; Westbrook, C. K.; Westmoreland, P. R. Biofuel combustion chemistry: From ethanol to biodiesel. Angew. Chem., Int. Ed. 2010, 49 (21), 3572−3597. (6) Duan, J.; Tan, J.; Yang, L.; Wu, S.; Hao, J. Concentration, sources and ozone formation potential of volatile organic compounds (VOCs) during ozone episode in Beijing. Atmos. Res. 2008, 88 (1), 25−35. (7) Salthammer, T. Formaldehyde in the ambient atmosphere: From an indoor pollutant to an outdoor pollutant? Angew. Chem., Int. Ed. 2013, 52 (12), 3320−3327. (8) Toda, K.; Yunoki, S.; Yanaga, A.; Takeuchi, M.; Ohira, S. I.; Dasgupta, P. K. Formaldehyde content of atmospheric aerosol. Environ. Sci. Technol. 2014, 48 (12), 6636−6643. (9) Kuwabara, Y.; Alexeeff, G. V.; Broadwin, R.; Salmon, A. G. Evaluation and application of the RD50 for determining acceptable exposure levels of airborne sensory irritants for the general public. Environ. Health Persp. 2007, 115 (11), 1609−1616. (10) Pinto, J. P.; Gladstone, G. R.; Yung, Y. L. Photochemical production of formaldehyde in earth’s primitive atmosphere. Science 1980, 210 (4466), 183−184. (11) Shaughnessy, R. J.; Levetin, E.; Blocker, J.; Sublette, K. L. Effectiveness of portable indoor air cleaners-sensory testing results. Indoor Air 1994, 4 (3), 179−188. (12) Gesser, H. D.; Fu, S. Removal of aldehydes and acidic pollutants from indoor air. Environ. Sci. Technol. 1990, 24 (4), 495−497. (13) Torres, J. Q.; Royer, S.; Bellat, J. P.; Giraudon, J. M.; Lamonier, J. F. Formaldehyde: Catalytic oxidation as a promising soft way of elimination. ChemSusChem 2013, 6 (4), 578−592. (14) Sekine, Y. Oxidative decomposition of formaldehyde by metal oxides at room temperature. Atmos. Environ. 2002, 36 (35), 5543− 5547. (15) Chen, T.; Dou, H.; Li, X.; Tang, X.; Li, J.; Hao, J. Tunnel structure effect of manganese oxides in complete oxidation of formaldehyde. Microporous Mesoporous Mater. 2009, 122 (1−3), 270−274.

ASSOCIATED CONTENT

S Supporting Information *

Some related calculation details, tables, and figures. This material is available free of charge via the Internet at http:// pubs.acs.org. 2389


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cryptomelane: Highly active in low-temperature CO oxidation. J. Catal. 2014, 309, 58−65. (36) Galakhov, V. R.; Demeter, M.; Bartkowski, S.; Neumann, M.; Ovechkina, N. A.; Kurmaev, E. Z.; Logachevskaya, N. I.; Mukovskii, Y. M.; Mitchell, J.; Ederer, D. L. Mn 3s exchange splitting in mixedvalence manganites. Phys. Rev. B 2002, 65 (11310211). (37) Acerbi, N.; Tsang, S. C. E.; Jones, G.; Golunski, S.; Collier, P. Rationalization of interactions in precious metal/ceria catalysts using the d-band center model. Angew. Chem., Int. Ed. 2013, 52 (30), 7737− 7741. (38) Kapteijn, F.; Singoredjo, L.; Andreini, A.; Moulijn, J. A. Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric-oxide with ammonia. Appl. Catal., B 1994, 3 (2−3), 173−189. (39) Bogicevic, A.; Strömquist, J.; Lundqvist, B. I. Low-symmetry diffusion barriers in homoepitaxial growth of Al(111). Phys. Rev. Lett. 1998, 81 (3), 637−640. (40) Xu, Y.; Greeley, J.; Mavrikakis, M. Effect of subsurface oxygen on the reactivity of the Ag(111) surface. J. Am. Chem. Soc. 2005, 127 (37), 12823−12827. (41) Vogel, D.; Spiel, C.; Suchorski, Y.; Trinchero, A.; Schlögl, R.; Grönbeck, H.; Rupprechter, G. Local catalytic ignition during CO oxidation on low-index Pt and Pd surfaces: A combined PEEM, MS, and DFT study. Angew. Chem., Int. Ed. 2012, 51 (40), 10041−10044. (42) Bukhtiyarov, V. I.; Boronin, A. I.; Savchenko, V. I. Stages in the modification of a silver surface for catalysis of the partial oxidation of ethylene: I. Action of oxygen. J. Catal. 1994, 150 (2), 262−267. (43) Luo, J.; Zhang, Q.; Garcia-Martinez, J.; Suib, S. L. Adsorptive and acidic properties, reversible lattice oxygen evolution, and catalytic mechanism of cryptomelane-type manganese oxides as oxidation catalysts. J. Am. Chem. Soc. 2008, 130 (10), 3198−3207.

(16) Zhang, C.; Li, Y.; Wang, Y.; He, H. Sodium-promoted Pd/TiO2 for catalytic oxidation of formaldehyde at ambient temperature. Environ. Sci. Technol. 2014, 48 (10), 5816−5822. (17) 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. (18) 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. (19) Imamura, S.; Uchihori, D.; Utani, K.; Ito, T. Oxidative decomposition of formaldehyde on silver-cerium composite oxide catalyst. Catal. Lett. 1994, 24 (3−4), 377−384. (20) Mao, C. F.; Vannice, M. A. Formaldehyde oxidation over Ag catalysts. J. Catal. 1995, 154 (2), 230−244. (21) Huang, Z.; Gu, X.; Cao, Q.; Hu, P.; Hao, J.; Li, J.; Tang, X. Catalytically active single-atom sites fabricated from silver particles. Angew. Chem., Int. Ed. 2012, 51 (17), 4198−4203. (22) Lopez-Acevedo, O.; Kacprzak, K. A.; Akola, J.; Häkkinen, H. Quantum size effects in ambient CO oxidation catalysed by ligandprotected gold clusters. Nat. Chem. 2010, 2 (4), 329−334. (23) Nørskov, J. K.; Bligaard, T.; Hvolbæk, B.; Abild-Pedersen, F.; Chorkendorff, I.; Christensen, C. H. The nature of the active site in heterogeneous metal catalysis. Chem. Soc. Rev. 2008, 37 (10), 2163− 2171. (24) Zhai, Y.; Pierre, D.; Si, R.; Deng, W.; Ferrin, P.; Nilekar, A. U.; Peng, G.; Herron, J. A.; Bell, D. C.; Saltsburg, H.; Mavrikakis, M.; Flytzani-Stephanopoulos, M. Alkali-stabilized Pt-OHx species catalyze low-temperature water-gas shift reactions. Science 2010, 329 (5999), 1633−1636. (25) Hu, P.; Huang, Z.; Amghouz, Z.; Makkee, M.; Xu, F.; Kapteijn, F.; Dikhtiarenko, A.; Chen, Y.; Gu, X.; Tang, X. Electronic metalsupport interactions in single-atom catalysts. Angew. Chem., Int. Ed. 2014, 53 (13), 3418−3421. (26) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Alkali-metal ions insertion/extraction reactions with hollandite-type manganese oxide in the aqueous-phase. Chem. Mater. 1995, 7 (1), 148−153. (27) Vicat, J.; Fanchon, E.; Strobel, P.; Qui, D. T. The structure of K1.33Mn8O16 and cation ordering in hollandite-type structures. Acta Crystallogr. B 1986, 42 (2), 162−167. (28) Chen, C. C.; Zhu, C.; White, E. R.; Chiu, C. Y.; Scott, M. C.; Regan, B. C.; Marks, L. D.; Huang, Y.; Miao, J. Three-dimensional imaging of dislocations in a nanoparticle at atomic resolution. Nature 2013, 496 (7443), 74−77. (29) Bordiga, S.; Groppo, E.; Agostini, G.; van Bokhoven, J. A.; Lamberti, C. Reactivity of surface species in heterogeneous catalysts probed by in situ X-ray absorption techniques. Chem. Rev. 2013, 113 (3), 1736−1850. (30) Chang, F. M.; Jansen, M. Ag1.8Mn8O16-square-planar coordinated Ag⊕ ions in the channels of a novel hollandite variant. Angew. Chem., Int. Ed. 1984, 23 (11), 906−907. (31) Wagner, C. D.; Gale, L. H.; Raymond, R. H. Two-dimensional chemical-state plots-standardized data set for use in identifying chemical-states by X-ray photoelectron-spectroscopy. Anal. Chem. 1979, 51 (4), 466−482. (32) Yoshida, H.; Ahlert, S.; Jansen, M.; Okamoto, Y.; Yamaura, J. I.; Hiroi, Z. Unique phase transition on spin-2 triangular lattice of Ag2MnO2. J. Phys. Soc. Jpn. 2008, 77 (0747197). (33) Campbell, C. T. Catalyst-support interactions: Electronic perturbations. Nat. Chem. 2012, 4 (8), 597−598. (34) Dupin, J. C.; Gonbeau, D.; Vinatier, P.; Levasseur, A. Systematic XPS studies of metal oxides, hydroxides and peroxides. Phys. Chem. Chem. Phys. 2000, 2 (6), 1319−1324. (35) Santos, V. P.; Carabineiro, S. A. C.; Bakker, J. J. W.; Soares, O. S. G. P.; Chen, X.; Pereira, M. F. R.; Ó rfão, J. J. M.; Figueiredo, J. L.; Gascon, J.; Kapteijn, F. Stabilized gold on cerium-modified 2390


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