CeO2–TiO2 Photocatalyst for Nitric

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Highly Efficient and Stable Au/CeO2−TiO2 Photocatalyst for Nitric Oxide Abatement: Potential Application in Flue Gas Treatment Wei Zhu, Shuning Xiao, Dieqing Zhang,* Peijue Liu, Hongjun Zhou, Wenrui Dai, Fanfan Liu, and Hexing Li* Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materials, College of Life and Environmental Science, Shanghai Normal University, Shanghai 200234, China (PRC) S Supporting Information *

ABSTRACT: In the present work, highly efficient and stable Au/CeO2−TiO2 photocatalysts were prepared by a microwave-assisted solution approach. The Au/CeO2−TiO2 composites with optimal molar ratio of Au/Ce/Ti of 0.004:0.1:1 delivered a remarkably high and stable NO conversion rate of 85% in a continuous flow reactor system under simulated solar light irradiation, which far exceeded the rate of 48% over pure TiO2. The tiny Au nanocrystals (∼1.1 nm) were well stabilized by CeO2 via strong metal−support bonding even it was subjected to calcinations at 550 °C for 6 h. These Au nanocrystals served as the very active sites for activating the molecule of nitric oxide and reducing the transmission time of the photogenerated electrons to accelerate O2 transforming to reactive oxygen species. Moreover, the Au−Ce3+ interface formed and served as an anchoring site of O2 molecule. Then more adsorbed oxygen could react with photogenerated electrons on TiO2 surfaces to produce more superoxide radicals for NO oxidation, resulting in the improved efficiency. Meanwhile, O2 was also captured at the Au/TiO2 perimeter site and the NO molecules on TiO2 sites were initially delivered to the active perimeter site via diffusion on the TiO2 surface, where they assisted O−O bond dissociation and reacted with oxygen at these perimeter sites. Therefore, these finite Au nanocrystals can consecutively expose active sites for oxidizing NO. These synergistic effects created an efficient and stable system for breaking down NO pollutants. Furthermore, the excellent antisintering property of the catalyst will allow them for the potential application in photocatalytic treatment of high-temperature flue gas from power plant.

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INTRODUCTION

Titanium oxide (TiO2), as a widely utilized photocatalyst, could generate electron−hole pairs under UV light irradiation9,10 with the further formation of superoxide anions and hydroxyl radicals for oxidizing NOx at room temperature.11 This DeNOx technology involving photocatalytic processes on semiconductor materials is among the most promising possibilities owing to the simplicity of the concept and its technical feasibility. However, suffering from the low efficiency of TiO2 for photocatalytic oxidation of NOx, various routes for enhancing the efficiency, such as doping metal/nonmetal element and coupling TiO2 with narrow-band gap semiconductors have been investigated.12−14 Furthermore, NOx was usually oxidized to nitrate species which did not spontaneously desorb and therefore deactivated the catalyst.15 Therefore, it is urgent to develop highly efficient and stable photocatalytic system for facile NOx abatement. Nanoparticles of late transition metals adsorbed on oxide surfaces form the basis for many catalysts important in energy

Over the past few decades, atmospheric NOx concentrations have greatly increased because of the growing number of automobiles and growing industrial activities.1,2 This is the reason for concern, since the emission of NOx induces the formation of toxic smog, acid rain, and PM2.5 (particles less than 2.5 μm in diameter3), causing serious respiratory problems. The performance of commercial catalytic posttreatment systems are not optimized to fulfill the forthcoming China and United States standard legislation and those that will be implemented in Europe in 2015, particularly the low limit of NOx emissions from flue gas.4 Nowadays, flue gas treatment for NOx control are actually available and can be categorized into three areas: selective noncatalytic reduction (SNCR),5,6 selective catalytic reduction (SCR),7 and hybrid SNCR/SCR systems.8 Nevertheless, all these methods suffered from high temperature applied, secondary pollution emission, and low efficiency. And the strong kinetic and thermodynamic limitations make a suitable solution difficult. Thus, it is highly required to explore an effective route to removing NOx at facile and mild conditions. © XXXX American Chemical Society

Received: June 23, 2015 Revised: August 26, 2015

A

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Figure 1. (a) XRD patterns and (b) N2 adsorption−desorption isotherms of the pure TiO2, Au0.004Ti, Ce0.1Ti, and Au0.004Ce0.1Ti calcined at 550 °C. (c) FESEM and (d) TEM images of Au0.004Ce0.1Ti-550 and the size distribution of Au particles on the photocatalyst (d, inset). certain amount (0, 0.2, 0.4, 0.6, 0.8 g) of Ce(NO3)3·6H2O was added into a mixture containing 13.0 g of glycerin and 20.0 mL of ethyl alcohol under vigorous stirring at room temperature to form a transparent solution. Then, a certain volume (0, 0.38, 0.76, 1.14, 1.52 mL) of 0.048 mol/L tetrachloroauric acid ethanol solution was added into the above solution under vigorous stirring. After stirring for 30 min, a yellow and transparent solution was obtained. Subsequently, 1.0 mL of TiCl4 was dropwise added into above solution under vigorous stirring at room temperature, followed by transferring into a 40 mL Teflon reactor. Then the precursor was heated to 110 °C using a programmed microwave digestion system (Ultrawave, Milestone) and kept at 110 °C for 30 min. The solid product was centrifuged and washed thoroughly with deionized water and ethanol. Finally, it was dried at 80 °C overnight, followed by calcination in air for 6 h. The asprepared Au/CeO2−TiO2 was denoted as AuxCeyTi-T, where x and y refer to the Au/Ti and Ce/Ti molar ratios in the initial preparing solution while T refers to the calcination temperature. For comparison, TiO2, Au/TiO2 and CeO2−TiO2 were also prepared followed the similar method in the absence of either Au or Ce. All those samples were calcined in air at 550 °C for 6 h. Characterization. The composition was determined by inductive coupled plasma emission spectromettry (ICP, Varian, VISTAMPXICP). The crystal structure was characterized using X-ray diffraction (XRD, D/MAX-2000 with Cu Kα radiation). The morphology was observed by field-emission scanning electron microscopy (FESEM, HITACHIS4800) and transmission electron microscopy (TEM, JEOL JEM-2100) coupled with energy dispersive X-ray spectroscopy (EDX). N 2 adsorption−desorption isotherms were determined on a Quantachrome NOVA 4000 instrument at 77 K. Based on the adsorption branches, the Brunauer−Emmett−Teller (BET) method was used to calculate specific surface area (SBET) and the Barrett− Joyner−Halenda (BJH) model was used to calculate pore volume (VP) and pore diameter (DP). Surface electronic state was analyzed by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000). All the binding energy values were calibrated by using C1s = 284.6 eV as the reference. Raman spectra were detected by using a confocal laser Raman spectrum analysis system (Dilor, LabRam II). UV−vis diffuse reflectance spectra (DRS) were obtained on a UV−vis spectropho-

technology, pollution prevention, and environmental cleanup.16 In the case of Au/TiO2 system, Au catalysts with finite nanoparticle size have been found to be very active for specific reactions in dark conditions such as CO oxidation17 and the catalytic activity per surface metal atom and selectivity can depend strongly on the particle size below 6 nm and the choice of oxide support.18 Yet, their potential as active materials for photocatalytic NO oxidation has only received sporadic attention. Herein, we developed a synergistic Au/CeO2−TiO2 system for photocatalytic NOx abatement, in which the tiny Au nanocrystals (∼1.1 nm) were stabilized by CeO2 via strong metal−support bonding. The Au−Ce3+ interface served as an anchoring site of O2 molecule. The adsorbed oxygen can react with photogenerated electrons on TiO2 surfaces to produce more superoxide radicals for NO oxidation. Meanwhile, O2 was also captured at the Au/TiO2 perimeter site and the NO molecules on TiO2 sites were initially delivered to the active perimeter site via diffusion on the TiO2 surface, where they assisted O−O bond dissociation and reacted with oxygen at these perimeter sites. Therefore, these unique Au nanocrystals can expose more active sites for photocatalytic NO removal. These synergistic effects created an efficient and stable system for breaking down NO. The Au/CeO2−TiO2 composites with optimal molar ratio of Au/Ce/Ti of 0.004:0.1:1 delivered a high and stable NO conversion rate of 85% under simulated solar light irradiation, which far exceeded the rate of 48% over pure TiO2. Moreover, the Au nanoparticles could be well maintained at ∼1.1 nm even it was subjected to calcination at 550 °C for 6 h. Such excellent antisintering property of the photocatalyst will allow them for the potential application in photocatalytic treatment of high-temperature flue gas from power plant.

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EXPERIMENTAL SECTION

Synthesis. In this work, all of the reagents employed were analytical grade and used without further purification. Typically, a B

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Langmuir Table 1. Structural Parameters of Different Photocatalysts and NO Conversion Rate under Tungsten Lamp samples

x (±0.02%)a

y (±0.2%)b

T (°C)c

SBET (m2·g−1)d

VP (cm3·g−1)e

DP (nm)f

NO conv (%)

η (mmol·g−1·h−1)g

Au0.004Ce0.1Ti-450 Au0.004Ce0.1Ti-500 Au0.004Ce0.1Ti-550 Au0.004Ce0.1Ti-600 Au0.004Ce0.1Ti-650 Au0.004Ce0.1Ti-800 Au0.002Ce0.1Ti-550 Au0.006Ce0.1Ti-550 Au0.008Ce0.1Ti-550 Au0.004Ce0.05Ti-550 Au0.004Ce0.15Ti-550 Au0.004Ce0.2Ti-550 TiO2-550 Ce0.1Ti-550 Au0.004Ti-550

0.23% 0.23% 0.23% 0.23% 0.23% 0.23% 0.09% 0.35% 0.67% 0.22% 0.23% 0.22%

1.3% 1.3% 1.3% 1.3% 1.3% 1.3% 1.3% 1.3% 1.3% 0.8% 2.0% 3.2%

450 500 550 600 650 800 550 550 550 550 550 550 550 550 550

63.6 54.9 46.2 36.4 25.5 12.6 49.1 44.2 41.2 41.4 53.3 58.9 27.0 50.0 32.4

0.09 0.09 0.11 0.12 0.08 0.04 0.18 0.15 0.10 0.10 0.08 0.12 0.10 0.14 0.14

8.6 9.4 10.2 14.4 11.9 4.8 16.4 13 12.6 9.7 6.0 8.4 15.0 11.1 16.8

71 78 85 70 54 10 73 74 62 67 80 77 49 58 55

8.269 9.084 9.899 8.152 6.289 1.165 21.726 5.663 2.479 8.157 9.317 9.375 0 0 5.893

1.3% 0.25%

a

Actual mass fraction of Au in the samples. bActual mass fraction of CeO2 in the samples. cCalcination temperature. dSpecific surface area determined by the BET method. ePore volume determined by BJH mode. fPore diameter determined by BJH mode. gRemoval quantity of NO which was normalized accordingly to Au loading amounts in per hour. tometer (DRS, MC-2530). The photocurrent responses were determined by electrochemical workstation (CHI 660D) in a homemade three electrode quartz cell containing 0.50 mol/L Na2SO4 aqueous solution under UV light (365 nm) irradiation at an applied potential of 0.5 V vs SCE. The O2 sorption was done with an intelligent gravimetric analyzer (IGA 100B, Hiden). Activity Test. The gas-phase photocatalytic NO oxidation activity test was carried out at ambient temperature in a continuous flow reactor with volume of about 18 L (420 × 260 × 166 mm3). The initial NO concentration was adjusted with dry purified air (Thermo Scientific, model 111) to 500 ppb, and the flow rate of the mixed gas was controlled at 4.0 L·min−1 via a dynamic gas calibrator (Thermo Scientific, model 146i). The relative humidity was about 83%, obtained with a humidifier (HTC-1). In each run of experiments, a 0.20 g sample was homogeneously dispersed on two 15 cm culture dishes with 20 mL of anhydrous ethanol, followed by transferring into the reactor after being vacuum-dried at 80 °C for 30 min to remove the ethanol. The thickness of the homogeneous dispersion of the photocatalysts as obtained by evaporation onto the culture dishes was less than 0.2 mm. Then, NO-contained gas was allowed to pass through the photocatalyst until reaching adsorption−desorption equilibrium. Subsequently, the photocatalytic reaction was started by turning either two 150 W tungsten halogen lamps (simulated sunlight) or eight 4 W mercury lamps with characteristic wavelength of 365 nm. The concentration of NO was continuously measured by using a chemiluminescence NO−NO2−NOx analyzer (Thermo Scientific, model 42i), and the NO conversion rate (%) was calculated as follows:

NO conversion rate (%) =

confirmed by the ICP results that the mass fraction of Au and Ce in Au0.004Ce0.1Ti-550 were only about 0.2 and 1.3 wt %. By controlling the calcination temperature (Figure S1), it could be found that, as for Au0.004Ce0.1Ti sample, pure anatase TiO2 formed below 450 °C and the amount of rutile TiO2 increased with the increasing calcination temperature. And there were still no significant diffraction peaks of either Au or Ce species even after being calcined at 800 °C, which might be attributed to the high dispersion without aggregation. The N2 adsorption−desorption isotherms of pure TiO2-550, Au0.004Ti-550, Ce0.1Ti-550, and Au0.004Ce0.1Ti-550 are shown in Figure 1b. It can be seen that the isotherms of pure TiO2-550 and Au0.004Ti-550 were type I, while the isotherms of Ce0.1Ti550 and Au0.004Ce0.1Ti-550 were type IV. This change indicated that the mesopores were formed after introducing CeO2 into TiO2. This could be ascribed to the stabilization effect of the CeO2 species for maintaining the mesoporosity under calcination treatment.20,21 The isotherms of Au0.004Ce0.1Ti-800 changed to type I (Figure S2), suggesting the collapse of mesopores at high calcination temperature. The BET specific surface area (SBET), pore volume (Vp), and pore size (Dp) were calculated and are shown in Table 1. It could be seen that the SBET gradually decreased with the increasing temperature, while CeO2 doping resulted a larger specific surface area due to the mesoporous structure. The SEM analysis (Figure 1c) further confirmed that the Au0.004Ce0.1Ti-550 sample was composed of hierarchical flowerlike spheres, assembled with numerous nanosheets. Such nanosheets owned the wormlike mesoporous architecture shown by the TEM image (Figure 1d), which was agreed with the BET results. The EDX and mapping results (Figure S3) showed that lots of white points, ascribed to the presence of Au nanocrystals, were highly dispersed on the surface of CeO2/TiO2 nanosheets. The average size (Da) of the Au nanocrystals was calculated to be about 1.1 nm, based on the size distribution analysis (Figure 1d, inset). From the TEM image of pure TiO2-550, Au0.004Ti-550, and Ce0.1Ti-550 (Figure S4a−c), we found that Au nanoparticles and CeO2 species were well dispersed in the TiO2 framework. The average size of Au nanoparticles embedded in Au0.004Ce0.1Ti-550 is 1.1 nm, which is much smaller than that of the Au0.004Ti-550 (Figure S5). This was attributed to the

C0 − C × 100% C0

where C0 is the initial balanced concentration of NO and C refers to the NO concentration determined during reaction process.

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RESULTS AND DISCUSSION Structural Characteristics. As shown in Figure 1a, the XRD patterns of pure TiO 2 , Au 0.004 Ti, Ce 0.1 Ti, and Au0.004Ce0.1Ti, which were calcined at 550 °C, displayed peaks at 2θ of 25.3°, 37.8°, 48.1°, 55.0°, and 62.7° indicative of anatase TiO2 (PDF 21-1272), together with trace of rutile TiO2 (PDF 21-1276). These results could suggest the existence of the surface-phase (anatase/rutile) juncion.19 Nevertheless, there were no significant diffraction peaks of either Au or Ce species in the Au0.004Ti, Ce0.1Ti, and Au0.004Ce0.1Ti-550, which could be attributed to the relatively low contents. This was C

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limit the reunion of Au nanoparticles under the same high calcination temperature, owing to formation of stronger metal− support bonding (Au−CeO2)22,23 located on the interface of between CeO2 and Au. The strong interaction between Au, CeO2, and TiO2 was further proved by the X-ray photoelectron spectroscopy (XPS). The characteristic spin−orbit split peaks of Au 4f, Ce 3d, Ti 2p3, and O 1s signals were shown in Figure 3. As shown in Figure 3a, the Ce 3d photoelectron spectra of Ce0.1Ti-550 and Au0.004Ce0.1Ti-550 could be assigned to be 3d3/2 spin−orbit states (labeled u) and 3d5/2 (labeled (v).20,21 The u‴/v‴ doublet was due to the primary photoemission of Ce(IV)-O2 in Ce0.1Ti-550 sample.24 The u/v and u″/v″ doublets are shakedown features resulting from the transfer of one or two electrons from a filled O 2p orbital to an empty Ce 4f orbital. The u′/v′ doublet is due to photoemission from Ce(III) cations.25,26 Thus, it was reasonable that a mixture of Ce(III)/ Ce(IV) oxidation states existed on the surface of the Ce0.1Ti550. It should be pointed out that no obvious signals of u‴/v‴ could be observed in Au0.004Ce0.1Ti-550 sample upon loading Au on the framework of CeO2/TiO2. Such decrease of u‴/v‴ peaks could be ascribed to the strong interaction between Au and Ce3+, weakening the interaction between Ce (IV) and O2.27 As shown in the O 1s photoelectron spectra (Figure 3b), pure TiO2 exhibited two peaks at ∼529.6 and 531.3 eV based on peak modeling. The main peak at ∼529.6 eV can be ascribed to lattice oxygen in TiO2, while the signal at ∼531.3 eV can be associated with surface hydroxyl groups.28,29 A positive shift (about 0.2 eV) was observed to the case of the main peak after loading Au in TiO2. On the contrary, a negative shift (about 0.1 eV) was obtained after loading CeO2 into the framework of

stabilizing effect of CeO2. The HRTEM images (Figure 2) were further utilized to analyze the surface crystal structure of the as-

Figure 2. HRTEM images of (a) Ce0.1Ti-550 and (b) Au0.004Ce0.1Ti550.

prepared samples. As shown in Figure 2a, the nanocrystalline nature of both anatase TiO2 and CeO2 was well-defined in the sample of Ce0.1Ti-550. Ultrafine CeO2 nanocrystals with an average size of about 2 nm were incorporated into the anatase TiO2 framework with formation of CeO2−TiO2 heterojunctions. In the case of Au0.004Ce0.1Ti-550 sample (Figures 2b and S4d), it could be seen that the Au nanocrystals were mainly deposited on the interface between CeO2 and TiO2, forming the strong metal−support bonding between Au−CeO2 and Au−TiO 2. To further clarify this view, Au0.004 Ti and Au0.004Ce0.1Ti were calcined under higher temperature of 800 °C. It was noted that the Au average size (3.5 nm) in Au0.004Ti800 sample was much larger than that (2.5 nm) of Au0.004%Ce0.1Ti-800, as shown in Figure S5. All the results could suggested that the presence of CeO2 could effectively

Figure 3. XPS analysis of (a) Ce 3d for Ce1.0Ti-550 and Au0.04Ce0.1Ti-550; (b) O 1s and (c) Ti 2p3 for pure TiO2-550, Au0.004Ti-550, Ce0.1Ti-550, and Au0.004Ce0.1Ti-550; and (d) Au 4f forAu0.004Ti-550, Au0.004Ce0.1Ti-550. D

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Figure 4. Raman spectra of pure TiO2-550, Au0.004Ti-550, Ce0.1Ti-550, and Au0.004Ce0.1Ti-550.

Figure 5. (a) UV−vis DRS spectra and (b) PL spectra (excited by 280 nm) of pure TiO2-550, Au0.004Ti-550, Ce0.1Ti-550, and Au0.004Ce0.1Ti-550 sample.

pure TiO2. Thus, it was not strange that the main peak in Au0.004Ce0.1Ti-550 sample could shift back to about 529.7 eV in the presence of both Au and CeO2. Similar results could be observed to the case of binding energy of Ti 2p3 in various samples as shown in Figure 3c. These results indicated that strong interaction existed between various species in the composites of Au/CeO2−TiO2. From Figure 3d, it could be seen that Au was present in the metallic state. Comparing with the Au0.004Ti-550, the binding energy of Au in Au0.004Ce0.1Ti550 showed a little offset to high binding energy, attributing to the strong interaction of Au and CeO2. The Raman spectra further confirmed such interaction among CeO2, Au and TiO2. As shown in Figure 4a, the pure TiO2-550, Au0.004Ti-550, Ce0.1Ti-550 and Au0.004Ce0.1Ti-550 displayed five peaks indicative of the anatase TiO2 at about 143, 195, 396, 516, and 638 cm−1 and an additional peak around 445 cm−1 corresponding to trace of rutile TiO2.19,21 This results was in good accordance with the XRD patterns. It was known that the CeO2 displayed a strong band around 460 cm−1 due to the Raman active mode characteristic of fluorite-structured materials.21 The absence of this peak should be attributed to both the extremely low content and the high dispersion of CeO2. It was also found that the principal peak at 143.3 cm−1 characteristic of the Ti−O stretching mode in the pure TiO2 positively shifted in the Au0.004Ti-550, Ce0.1Ti-550, and Au0.004Ce0.1Ti-550 as shown in Figure 4b. This further confirmed the strong interaction between either Au and TiO2

or CeO2 and TiO2, leading to the formation of oxygen vacancies.30 The UV−vis DRS spectra in Figures 5a and S6 revealed that all the Au- or/and CeO2-doped TiO2 exhibited absorbance in visible region owing to the photosensitizing effect from CeO2 with low energy band gap and/or the plasmon effect from Au nanoparticles.31,32 Noting that when the content of CeO2 was low, the intrinsic absorption of the material was blue shift (Figure S6b). This may be attributed to the decrease of the particle size, which could be in conformity with the BET results. Figure 5b showed the photoluminescence (PL) spectra of the catalysts. It could be observed that the introduction of Au and CeO2 into TiO2 could greatly reduce the intensity of the PL peak around 560 nm, indicating the longer lifetime of the photogenerated carriers. Such inhibited rate of photogenerated electron−hole recombination was ascribed to the heterojunction of CeO2/TiO2 and the electric conduction of Au. However, excess doping of Au or CeO2 would be harmful for the separation of photogenerated carriers (see Figure S7), which may be due to the production of new recombination centers on the surface of the photocatalysts. Meanwhile, the electrochemical impedance spectroscopy (EIS) responses (Figure S8) demonstrated the smaller semicircles of Au0.004Ce0.1Ti-550 than that of pure TiO2-550 and Ce0.1Ti550, indicating a more effective separation of photogenerated electron/hole pairs and faster interfacial charge transfer. As a result, the AuxCeyTi-550 displayed stronger photocurrent E

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Figure 6. Plots of the dependence of NO concentration and ln(C/C0) on irradiation time in the presence of different photocatalysts under (a, b) UV lamp and (c, d) tungsten lamp.

The NO conversion rate began to decrease at 12 min during the reaction. Such decrease of activity could be attributed to the fast adsorption of nitric acid on the active sites of CeO2/ TiO2.33,34 It was also noted that loading Au on TiO2 (Au0.004Ti550) produced a relatively good NO conversion rate (about 58%); however, its initial rate of photocatalytic removal of NO was only about 0.03, much lower than that (0.06) of Ce0.1Ti550 and even lower than pure TiO2 (0.05) (see Figure 6d). Nevertheless, it should be pointed out that the photocatalytic performance of Au0.004Ti-550 for breaking down NO could be well maintained for a long time as shown in Figure S10. Such high and stable photocatalytic efficiency of Au0.004Ti-550 could be attributed to the low photogenerated electron−hole recombination rates and activated oxygen generated at the interface of Au/TiO2.35 The influence of the calcination temperature and the doping amount of Au or/and Ce was also investigated under UV and simulated solar-light (Figures S11−S13). It can be found that the optimal anatase/rutile phase composition and initial molar ratio of Au/Ce/Ti of 0.004:0.1:1 of sample derived from calcination at 550 °C delivered a highest and most stable NO removal performance under UV or solar light irradiation. Too much of Au or CeO2 loading was harmful because they could become the recombination centers of photogenerated electron−hole pairs, inhibiting the photocatalytic reaction. In addition, the lower water adsorption capacity of CeO2 than TiO2 made the sample overloading with CeO2 disfavoring the NO removal due to the decreased active hydroxyl radicals.10,36,37 Importantly, the Au0.004Ce0.1Ti-550 sample also showed strong durability without significant decrease in

response than pure TiO2-550 under Xe lamp (see Figure S9) owing to both the enhanced light harvesting ability for generating more photocarriers and the facilitated electrontransfer for diminishing photoelectron−hole recombination rate. It is worth noting that the Au0.004Ti-550 showed the lower photoelectron−hole recombination rate (see Figure 5b) and displayed stronger photocurrent response than Au0.004Ce0.1Ti550 (see Figure S9). This could be explained by the superior electrical conductivity due to the presence of Au with proper amount (see Figure S8). Photocatalytic NO Abatement. For evaluating the photocatalytic performance of the as-prepared Au/CeO2− TiO2 composites, the photocatalytic oxidation of NO was utilized as a probe reaction. The UV-light driven photocatalytic performance for NO abatement was investigated as shown in Figure 6a. The conversion rate could be up to about 85% over the doped TiO2 photocatalysts. However, from Figure 6b, it could be observed that the initial rate of photocatalytic removal of NO increased from 0.06 to 0.11 after introducing CeO2 and Au into the framework of TiO2. Furthermore, a simulated solarlight (tungsten lamp) was also used as a light source. As shown in Figure 6c, the activity of the aforementioned catalysts except for Au0.004Ce0.1Ti-550 sample decreased obviously due to less UV light in the simulated solar-light. And it can be found that the pure TiO2 exhibited about 46% NO conversion rate under solar-light irradiation. Upon loading CeO2 into the framework of TiO2, the photocatalytic NO conversion rate over the Ce0.1Ti-550 catalyst was greatly increased to about 58%. And this could be attributed to both the increase of the specific surface area and the heterojunction of CeO2/TiO2. However, its photocatalytic activity could not be maintained very well. F

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conversion rate (see Figure S14) even after 20 h UV light irradiation. Proposed Mechanism. Combining with the results of the photocatalytic test, we know that both Au and CeO2 played an important role in photocatalytic NO abatement system. Figure 7 showed the schematic and mechanism for photocatalytic NO

(3)

Ti(H 2O2 )ads + Ti( ·O2−)ads → Ti(·HO)ads + Ti(OH−)ads + Ti(O2 )ads

Ti(OH−)ads + h+ → Ti( ·HO)ads

(4) (5)

TiO2

NO(g) ⎯⎯⎯⎯→ Ti(NO)ads

(6)

Ti(· O2−)ads + Ti(NO)ads → Ti(NO3−)ads

(7)

Ti(· OH)ads + Ti(NO)ads → Ti(HNO2 )ads

(8)

Ti(HNO2 )ads + Ti( ·OH)ads → Ti(NO2 )ads + Ti(H 2O)ads (9)

Figure 7. Schematic for the enhanced photocatalytic activity of Au/ CeO2−TiO2 for NO photocatalytic oxidation.

Ti(NO2 )ads + Ti(· OH)ads → Ti(HNO3)ads

(10)

Ti(· O2−)ads + Ti(HNO2 )ads → Ti(HNO3)ads

(11)

In addition, both of the O2 and NO could be adsorbed on the surface of Au/TiO2:34,35,40

abatement over Au/CeO 2−TiO2 catalyst. The Au−Ce 3+ interface served as an anchoring site of O2 molecule, where more adsorbed oxygen could react with photogenerated electrons on TiO2 surfaces to produce more superoxide radicals for NO oxidation. Meanwhile, O2 was also captured at the Au/ TiO2 perimeter site and the NO molecules on TiO2 sites were initially delivered to the active perimeter site via diffusion on the TiO2 surface,38 where they assisted O−O bond dissociation and reacted with oxygen at these perimeter sites. This mechanism was also supported by the results of greatly enhanced oxygen adsorption capacity on the Au0.004Ce0.1Ti-550 sample (Figure 8). Therefore, these tiny Au nanocrystals can expose more active sites for stable NO oxidizing performance.

Au

NO(g) → Au(NO)ads

(12)

Au

O2(g) → Au(O2 )ads

(13)

Meanwhile, the CeO2 was another key factor. It not only could it inhibit the agglomeration of particles at high temperature resulting in large specific surface area, but also it could adsorb and dissociate O2 to O*, which could diffuse to the surface of Au and TiO2 though their interface.41 That means more activated oxygen species could participate the photocatalytic NO oxidation process, leading to the high removal rate of NO.

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CONCLUSION A novel Au/CeO2−TiO2 photocatalyst with excellent and stable photocatalytic activity for NO abatement was prepared by a microwave-assisted solution approach. The small Au nanoparticle size of 1.1 nm was vital to the reaction, which was stabilized by CeO2 under high-temperature treatment via metal−oxide bonding. The high activity and stability could be attributed to the increase of the specific surface area, heterojunction, and charge transfer as well as oxygen activation by introducing CeO2 and Au into the framework of TiO2, resulting low photoelectron−hole recombination rate and more reactive oxygen species on the surfaces of catalyst. Such unique properties of the catalyst will allow them for the potential application in photocatalytic treatment of high-temperature flue gas from power plant.

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Figure 8. O2 adsorption capacity of different photocatalysts.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02232. XRD patterns, N2 adsorption-desorption isotherms, EDX analysis, TEM images, particle size distribution, UV−vis DRS spectra, PL spectra EIS data, photocurrent response data, and conversion versus irradiation time data (PDF)

Combining the previous work has been reported,7,33,34,39 we suggested that the reaction pathway could be described as following: hν

TiO2 → e− + h+ −

(1) −

Ti(O2 )ads + e → Ti( ·O2 )ads

(2) G

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel/fax: +(86)21-64322272. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21237003, 21261140333, 21477079, 21207090), Shanghai Government (15QA1403300, S30406), PCSIRT (IRT1269), and the doctoral program of higher education (20123127120009).

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DOI: 10.1021/acs.langmuir.5b02232 Langmuir XXXX, XXX, XXX−XXX