Synergistic Effects of Au and FeOx Nanocomposites in Catalytic NO

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Synergistic Effects of Au and FeOx Nanocomposites in Catalytic NO Reduction with CO Xianwei Wang,† Nobutaka Maeda,*,†,‡ and Alfons Baiker*,§ †

Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Gold Catalysis Research Center, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China § Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, CH-8093 Zurich, Switzerland S Supporting Information *

ABSTRACT: Promotion of Au/TiO2 by FeOx addition is shown to result in a very strong enhancement of activity and selectivity in the NO reduction with CO. The nanocomposite Au-FeOx/TiO2 catalysts were prepared by deposition−precipitation (Au/TiO2) and subsequent impregnation (FeOx). Structural characterization of the nanocomposite catalysts using XRD, TEM, TPR, XPS, and DRIFTS revealed that they consist of titania-supported Au particles of about 4.4−4.8 nm size, which were partially covered with a thin layer of FeOx,, mainly made up of Fe2+ (FeO), in addition to a small fraction of Fe3+ (Fe2O3). In situ DRIFTS studies showed that, in comparison to Au/TiO2, on the Au-FeOx/TiO2 nanocomposites new adsorption sites were created, which led to enrichment of CO and NO molecules on the Au surface and at its interfaces with FeOx and TiO2. At the reaction temperature (250 °C), a new surface NO− CO complex, where both Fe and Au are proposed to be involved in the adsorptive interaction, was discovered. The titaniasupported Au-FeOx nanocomposites considerably facilitated the formation of this complex and contributed to the striking enhancement of the catalytic performance (activity and selectivity) of the Au-based catalyst. KEYWORDS: gold catalyst, FeOx promotion, in situ DRIFTS, NO reduction with CO, synergistic effect rate.16 On a reconstructed Au(100) surface, the activation energy for the NO desorption is >57 kJ/mol.17 On the other hand, NO decomposes to N2O on a Au(310) surface at temperatures higher than −193 °C.21 At temperatures above 227 °C, the formation of N2O and N2 was found to prevail.8 The reduction of NO with CO over platinum-group-metal (Pt, Pd, Rh) catalysts has been widely studied.22−26 On the Pd(111) surface, NO molecules were proposed to take over the hollow sites and push CO molecules away onto bridge sites.26 The reaction of NO and CO to give N2, (N2O), and CO2 has been proposed to occur via the following steps27

1. INTRODUCTION Gold came into the spotlight when its nanoparticles were found to catalyze the oxidation of carbon monoxide at ambient or even lower temperature.1 Only small Au particles (di < 5 nm) well dispersed over metal oxide supports show high catalytic activity in the low-temperature range.2 Synergistic effects between small Au particles and oxidic supports play a key role in controlling the activity and selectivity of such catalysts in various reactions.3 These unique properties of gold catalysts gave access to a variety of applications such as hydrogenation,3 water-gas shift reaction,4 methanol synthesis from CO or CO2,5 and NOx reduction.6−8 Au catalysts also show catalytic activity for NOx abatement to a certain extent.9−13 The adsorption and decomposition of NOx molecules on Au surfaces have been extensively investigated up to the present time.9,14−20 Theoretical studies showed that the adsorption of NO on a supported single gold atom results in a higher binding energy in comparison to that on Au2 dimers.19 NO molecules begin to adsorb on Au(111) already at −178 °C15,16 with a calculated adsorption energy of >20 kJ/mol.18,20 The adsorbed NO easily reacts with dissociated atomic O on Au(111) to form NO2 below −73 °C.16,18,20 Therefore, the lifetime of the adsorbed NO species affects the NO2 formation © XXXX American Chemical Society

NO + CO → 1/2N2 + CO2

(1)

2NO + CO → N2O + CO2

(2)

N2O + CO → N2 + CO2

(3)

CO2 formation occurs via two distinct pathways, i.e., by direct reaction (reaction 1) and by parallel and subsequent reactions (reactions 2 and 3). Although Pt and Pd catalysts Received: August 22, 2016 Revised: October 11, 2016

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the parent Au/TiO2 catalyst described above. The iron precursor, iron(III) nitrate nonahydrate, was dissolved in 100 mL of deionized water, and the solution was stirred for 30 min. After Au/TiO2 powder was added, the suspension was stirred for 1 h at room temperature. Subsequently, the suspension was heated to 60 °C in a rotary evaporator and aged for 1 h, followed by solvent evaporation at 35 °C under vacuum. The obtained powders were dried at 60 °C overnight and calcined in air at 300 °C for 1 h at a ramping rate of 1 °C/min. The FeOx/TiO2 reference catalyst containing the same molar amount of Fe as used in the preparation of Au-Fe(1:1)/TiO2 was synthesized in the same manner, employing the corresponding amount of iron(III) nitrate nonahydrate. The different TiO2-supported catalysts are designated as Au, AuFe(0.5:1), Au-Fe(1:1), Au-Fe(2:1) and Fe, where the ratios in parentheses indicate the molar ratios of the metals. Note that in all Fe-containing catalysts the iron component was present as FeOx. 2.2. Catalyst Characterization. BET specific surface areas were estimated by N2 adsorption at liquid N2 temperature using a Micromeritics instrument. Before each measurement, the samples were degassed at 120 °C for 1 h. X-ray diffraction (XRD) measurements were performed on a PANalytical X’Pert diffractometer using Pd-filtered Cu Kα radiation at 60 kV and 55 mA. The weight ratio R of anatase to rutile in the samples was calculated using the relation R = 0.886IA/IR, where IA and IR are the integrated intensities of the reflections of anatase (101) at 25.4° and rutile (110) at 27.5°, respectively.45,46 The average grain sizes of anatase and rutile, were estimated using these reflections and the Scherrer formula:

exhibit high catalytic performance, the activity deteriorates rapidly at low temperatures because of poisoning by oxygen atoms formed via NO dissociation.28 The use of gold provides an opportunity to mitigate this deficiency due to lower oxygen coverage on the metal surface, thereby enhancing the lowtemperature activity.6,7 CO-covered surfaces were reported to enhance the adsorption of NO to form dinitrosyl species on Au-TiO2.6 Isocyanate species (−NCO) were also detected by in situ FTIR on Au catalysts at temperatures higher than 400 °C. The formed isocyanate species were proposed to migrate from the Au surface to the oxide supports.29,30 By alteration of the local atomic geometry on the Au-TiO2 catalyst, irreversible and thermally stable surface “AuNO” complexes can be formed, leading to better stability on the Au surface.31 However, the interaction between NO and Au−TiO2 also depends on the pretreatment.32 On the other hand, CO adsorption on Au catalysts is still subject to an ongoing debate.33 Outka and Madix reported that CO does not adsorb on a gold film.34 However, density functional theory (DFT) calculations showed a different result with Au(100), Au(211), and Au(221) surfaces, and the adsorption on bridge sites was proposed to be favored.35 Some studies showed that a small amount of CO can chemisorb on gold powders, whereas substantial chemisorption can occur on Au nanoparticles supported on metal oxide supports.36 Despite numerous reports on the chemisorption of NO and CO on Au single crystals and supported Au catalysts,37−39 the catalytic behavior of the reduction of NO with CO on Au-based catalysts has to our knowledge only rarely been reported.40,41 This is because the N2O conversions on Au catalysts are quite low below 400 °C42 and the NOx removal occurs efficiently only in the presence of H2.43 In our research on catalytic materials, which achieve high activity and selectivity in the NO−CO reaction, we prepared Au-FeOx/TiO2 catalysts and herein we demonstrate that synergistic effects of Au and FeOx nanocomposites can be highly beneficial for the reduction of NO with CO.

d = 0.89λ /β cos θ

where d is the average particle size, λ = 0.15406 nm, θ is the diffraction angle, and β is the full width at half-maximum. The surface compositions and oxidation states of the metal components in the calcined catalysts were examined by X-ray photoelectron spectroscopy (XPS) using a Thermo ESCALAB 250Xi instrument. Al Kα radiation (1486.6 eV, 15 kV, 10.8 mA) was employed to excite photoelectrons at 50 eV pass energy at an energy scale calibrated versus Ag 3d5/2 at 1 eV. Transmission electron microscopy (JEOL, JEM-2100, 200 kV) was employed to estimate the mean size of the Au particles. Temperature-programmed reduction (TPR) with H2 was performed using an automated chemisorption apparatus (Quantachrome, ChemBET Pulsar TPR/TPD). A 100 mg portion of the samples loaded in a U-shaped quartz microreactor was heated at a ramping rate of 10 °C/min in 10 vol % H2 in Ar (total flow rate of 15 mL/min). In situ IR studies were carried out using a Fourier transform infrared spectrometer (Bruker Optics, Vertex 70) equipped with a liquid-N2-cooled mercury cadmium telluride (MCT) detector at a spectral resolution of 4 cm−1, scanning velocity of 60 kHz, and aperture size of 8 mm. A 20 mg portion of each sample was loaded into a ceramic cup. The cup was then placed in an in situ diffuse reflectance IR spectroscopy (DRIFTS) cell with ZnSe windows, connected to an automated four-way Valco valve (VICI AG International) and mass flow controllers (Azbil Control Solution Co., Ltd.). The DRIFTS compartment and all of the IR beamlines were purged with dry air supplied by a compressor−dryer combined system (EKOM) to remove H2O vapor, whose IR bands overwhelm bands of our interests

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Commercially available titanium(IV) oxide (Sigma-Aldrich, mixture of rutile and anatase, 99.5%, BET surface area of 70.5 m2/g) was used in all catalyst preparations as a support. Chloroauric acid (HAuCl4·H2O, Energy Chemical, 98%), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, Sigma-Aldrich, ≥98%), and ammonia solution (Aladdin, 25−28%) were used for the deposition−precipitation (DP) method and impregnation. The ammonia solution was diluted to 1.0 mol/L by adding deionized water. The titania-supported gold catalyst was prepared by a DP method using NH4OH as follows.44 HAuCl4·H2O was dissolved in 100 mL of deionized water. The solution was heated to 70 °C, and then the pH was adjusted to 7.0 by adding NH4OH solution. Afterward, the TiO2 powder was added with constant stirring. The gold loading was adjusted to a nominal value of 3 wt % as Au metal (97 wt % TiO2). The resulting slurry was aged for 1 h, while the pH was maintained at around 7.0 and then filtered and washed with deionized water. The obtained material was dried at 60 °C overnight and calcined in air at 300 °C for 1 h at a ramping rate of 1 °C/min. Au-FeOx/TiO2 catalysts with Au:Fe molar ratios of 1:0.5, 1:1, and 1:2 were prepared by an impregnation method using 7899

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ACS Catalysis (ca. 1500−1800 and 3200−3600 cm−1). Prior to each measurement, the samples were pretreated with 20 vol % O2 in He balance at a total flow rate of 100 mL/min for 1 h at 300 °C (ramping rate 5 °C/min). 2.3. Catalytic Reaction. Catalytic performance was tested using a fixed bed microflow reactor system. A 300 mg portion of each catalyst was loaded into a stainless steel tube reactor with an inner diameter of 8.2 mm. Prior to the NO−CO reaction, the catalysts were pretreated with 20 vol % O2 in He balance (total flow rate of 150 mL/min) at 300 °C for 1 h at a ramping rate of 5 °C/min. After 1 h, the catalyst was kept in a He stream at 300 °C for 10 min to remove gaseous O2 in the gas lines and adsorbed oxygen on the catalyst surface. Subsequently the reactor was cooled to 50 °C. Then a gas mixture of 2000 ppm of CO and 2000 ppm of NO in He balance was introduced into the reactor at a total flow rate of 150 mL/min, corresponding to a gas hourly space velocity (GHSV) of 18000 h−1. The gaseous compositions were measured with a gas chromatograph (Fili, 9750 GC) equipped with a TCD detector and a molecular sieve 5A column and also with an NO−NO2−NOx analyzer (Thermo Scientific, 42i-HL). All of the data were recorded when the gaseous compositions were stabilized for 30−70 min at each measurement temperature. The NO conversion and N2 selectivity were averaged on the basis of repeated experiments (five times) with reproducible data. Blind test runs performed with the empty tube reactor did not show any activity.

on the surface or formed an amorphous layer. The average sizes of anatase and rutile particles estimated using the Scherrer equation (see the Experimental Section) were 17.5 and 15.8 nm, and the amount of the Fe addition had virtually no influence on the grain size of TiO2. Figure 2 shows XPS spectra of Au 4f, Fe 2p, and Ti 2p of the different Au-FeOx/TiO2 catalysts. The Au 4f7/2 photoelectron peak was located at a binding energy (BE) between 82.9 and 83.3 eV (see Table 1), which is assigned to metallic Au0 species.47 The Au 4f7/2 signal showed a negative shift in the spectra. The decrease in the Au 4f7/2 BE for Au/TiO2 catalysts can be explained by a Au−support interaction; electrons on the Ti3+ surface defects are transferred to the Au particles.47 A continuous shift to higher BE with increasing Fe content demonstrates that the FeOx decreases the initial state effect due to its interaction with the Au particles. The Ti 2p peak located at 458.3 eV with a core level spectrum corresponding to Ti4+ 2p3/2 at 464.0 eV was assigned to Ti4+ 2p3/2.48 The intense Fe 2p3/2 peak at 709.5 eV is assigned to Fe2+ species in the form of FeO as a major component. However, its asymmetric shape implies the presence of a small fraction of Fe3+ (Fe2O3) at ca. 710.2 eV. The peak at ca. 715 eV is attributed to the characteristic shakeup satellites of Fe2+ and Fe3+ cations.49 Normally bulk FeOx components transform to Fe2O3 (Fe3+) after calcination. Therefore, the impregnation of FeOx onto Au nanoparticles produces a unique form of Fe oxide species. The signal intensity (peak area) of Au 4f changed depending on the amount of FeOx added. As shown in Table 1, the addition of FeOx dramatically decreased the Au/(Au + Fe) ratio in the near-surface region as probed by XPS. For example, the bulk Au ratio for the Au-Fe(1:1) catalyst should be 0.5, whereas the corresponding surface ratio estimated by XPS was only 0.27. This suggests that FeOx species, particularly FeO, covered the Au particles as mono- or multilayers, forming Au-FeOx nanocomposites, in line with XRD results, where no reflections due to FeOx were observed (see Figure 1). TEM was employed to investigate the morphology of Au-Fe/ TiO2 catalysts and the mean particle size of Au (Figure 3). The TEM images show a rather uniform dispersion of spherical Au particles on TiO2. Au particles tend to be homogeneously deposited on the defect sites of the TiO2 surface.50 The mean particle size of Au was estimated to be 4.4−4.8 nm (see Table 1). The BET surface area of Au/TiO2 was 56.1 m2/g. By addition of FeOx, the surface area slightly decreased, as indicated in Table 1. TPR profiles of the different catalysts are shown in Figure 4. The reduction peak centered at 420 °C is assigned to the partial reduction of Ti4+ to Ti3+.51 The low reduction temperature, in comparison to the bulk reduction at 1300 °C,52 is ascribed to the Au-catalyzed reduction at the Au-TiO2 perimeters.53 The peak at 140 °C observed on Au-Fe(1:1) and Au-Fe(1:2) is assigned to the reduction of Fe2O3 to Fe3O4 (magnetite). The TPR profile at higher temperature and up to 750 °C can be related to the final transformation of FeO into metallic Fe.54 The broad peak at 100−250 °C for Au/TiO2 is assignable to the reduction of Au2O3 to AuO and AuO to metallic Au.55 The small peak at 300 °C originates from the reduction of Fe2O3, interacting with Au nanoparticles, to metallic Fe.56 For the Fe/ TiO2 reference catalyst, the reduction peaks of Fe2O3, Fe3O4, and FeO were observed at 400, 490, and 770 °C (Figure 4). Thus, all reduction peaks shifted to higher temperature in comparison to the corresponding peaks of the Au-Fe/TiO2

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. XRD patterns of Au/TiO2 and Au-Fe/TiO2 catalysts with different Au:Fe molar ratios are shown in Figure 1. The diffraction patterns of anatase (JCPDS

Figure 1. XRD patterns of Au/TiO2 (Au) and Au-FeOx/TiO2 catalysts with different Au:Fe molar ratios.

21-1272) and rutile (JCPDS 21-1276) structures were confirmed61 and did not change by addition of Fe (FeOx). The weight ratio of anatase to rutile was about 7.3. The small shoulder at 38° is due to the (111) reflection of Au. No diffraction patterns attributable to Fe species were observed in any samples, suggesting that iron oxides were finely dispersed 7900

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Figure 2. XPS analyses of Au/TiO2 (Au) and Au-FeOx/TiO2 catalysts with different Au:Fe molar ratios.

Table 1. BET Surface Area, Au Particle Size, and XPS Results of Au/TiO2 and Au-FeOx/TiO2 Catalysts with Different Fe Loadinga catalyst

surface area (m2/g)

Au particle size (nm)

BE Au 4f7/2 (eV)

Au/(Au + Fe)

Fe/(Au + Fe)

Au/TiO2 Au-Fe(1:0.5)/TiO2 Au-Fe(1:1)/TiO2 Au-Fe(1:2)/TiO2

56.1 53.4 52.6 52.1

4.4 4.8 4.4 4.5

82.9 83.0 83.2 83.3

1.0 0.35 0.27 0.23

0.65 0.73 0.77

a

Catalyst designations are given in the Experimental Section. Note that Au/(Au + Fe) and Fe(Au + Fe) represent ratios of elemental compositions in the near-surface region as probed by XPS.

Figure 3. TEM images of (A) Au/TiO2, (B) Au-Fe(1:0.5)/TiO2, (C) Au-Fe(1:1)/TiO2, and (D) Au-Fe(1:2)/TiO2.

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temperature range. The reference Fe/TiO2 catalyst outperformed the other catalysts in terms of NO conversion at temperatures higher than 350 °C. However, FeOx/TiO2 showed low N2 selectivity even at high temperatures, indicating that the reduction of N2O with CO, that is reaction 3 (as described in the Introduction), occurred less efficiently. Strikingly, the addition of Fe to Au/TiO2 dramatically enhanced both the activity and selectivity at lower temperatures. As shown in Table 2, the conversion of Au-Fe(1:2) of Table 2. Catalytic Performance of Catalysts in NO Reduction by CO at 250 °Ca

a

catalyst

NO conversn (%)

N2 selectivity (%)

Au/TiO2 Au-Fe(1:0.5)/TiO2 Au-Fe(1:1)/TiO2 Au-Fe(1:2)/TiO2 FeOx/TiO2

16.8 76.0 84.4 91.0 23.3

0.0 85.0 90.6 74.2 0.0

Conditions are specified in the Experimental Section.

91.0% was much higher than the sum of the expected contributions from the individual components of the catalyst (ca. 40%) (Au/TiO2, 16.8%; Fe/TiO2, 23.3%) at 250 °C. In addition, all of the Au-Fe catalysts showed high N2 selectivity (74.2−90.6%) while Au/TiO2 and Fe/TiO2 were not selective. This synergistic effect of Au-FeOx nanocomposites on TiO2 is striking. The highest conversion was achieved with Au-Fe(1:2), but the N2 selectivity was higher for Au-Fe(1:1). This behavior implies that the addition of Fe enhances reactions 1 and 2, i.e., the NO−CO reaction, but further addition causes the deterioration of the activity for reaction 3, i.e., the N2O−CO reaction, leading to low N2 selectivity. Au, Au-Fe(1:0.5), and Au-Fe(1:1) catalysts showed slightly higher conversion at 50−100 °C, followed by a decrease in conversion at 150 °C and then re-enhancement of the activity at 250 °C. Such a behavior can be observed for some metal alloys56 and metal−metal oxide nanocomposites.57 The hightemperature mechanism involves the dissociative adsorption of NO resulting in surface N and O atoms, and the former subsequently recombines to N2. The surface O atoms are removed by oxidizing CO to CO2. The low-temperature activity arises from a direct bimolecular mechanism involving

Figure 4. TPR profiles of Au/TiO2 (Au), FeOx/TiO2 (Fe), and AuFeOx/TiO2 catalysts with different Fe (FeOx) loading. Conditions: catalyst weight, 100 mg; gas mixture, 10 vol % H2 in Ar balance; ramping rate, 10 °C/min; total flow rate, 15 mL/min.

catalysts. This implies that the presence of Au nanoparticles facilitates the reduction of iron oxides. 3.2. Catalytic Performance. The catalytic performance (NO conversion and N2 selectivity) of the Au-FeOx/TiO2 catalysts with different Fe loadings in the reduction of NO with CO are shown in Figure 5. Au/TiO2 showed extremely low activity and no significant N2 selectivity over the entire

Figure 5. NO conversion and N2 selectivity of Au/TiO2 (Au), FeOx/TiO2 (Fe), and Au-FeOx/TiO2 catalysts with different Fe loadings. Conditions: gas mixture, 2000 ppm of NO, 2000 ppm of CO, He balance; total flow rate, 150 mL/min; GHSV, 18000 h−1; reaction temperature, 50−400 °C. 7902

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Aup0-CO. Bands at 1920 and 1891 cm−1 are assigned to bridged and 3-fold bonded CO. Bands due to carbonate species on TiO2 were also observed at 1500−1700 cm−1.61,62 The addition of FeOx gave rise to an intense band at ca. 1810 cm−1. To our knowledge such a sharp band in this wavenumber region for Au catalysts has hitherto never been reported. One example helping in the interpretation of the sharp band at low wavenumbers is the CO adsorption on Pt-K/Al2O3, where oxygen atoms of adsorbed CO interact with potassium oxide.63 Since our catalysts consist of Fe2+ as the main Fe species, the band at 1810 cm−1 might originate from bridged CO, where C atoms are adsorb on Au and O atoms, which interact with Fe2+ located at the perimeter, similar to that reported previously.63 Upon the emergence of this band, the intensity of on-top Aup0CO at the perimeter decreased significantly for Au-Fe (1:0.5), which is plausible on the basis of the above assumption of AuFe2+ bridged sites. The band at 2040 cm−1 due to linearly adsorbed CO on metallic Fe sites was not observed.61,64 Further addition of FeOx contributed to the reappearance of on-top Aup0-CO at 2075 cm−1, probably due to the increased perimeter between Au and FeOx. However, too much FeOx addition suppressed the CO adsorption for Au-Fe(1:2) due to high FeOx coverage on the Au particles, which is evident from the XPS data (see Table 1). This might explain why the NO conversion and N2 selectivity were well-balanced for AuFe(1:1), as shown in Table 2. For FeOx/TiO2, all absorption bands were approximately 10 times less intense, and on-top CO on TiO2 (2172 cm−1), FeOx (2116 cm−1), bridged CO (2005 cm−1) on FeOx, and carbonate species (1400−1700 cm−1) were observed. Therefore, we conclude that Au-FeOx nanocomposites on TiO2 provide new and unique adsorption sites. FT-IR spectra measured during NO adsorption (2000 ppm of NO in He balance; 100 mL/min; 30 °C; 5 min) for all of the catalysts are summarized in Figure 7. There was no band observed at 1810 cm−1 due to on-top NO− species (Au-NO−), which is typically detected on Au/TiO2,6,8 while bands due to bridged NO (1643 cm−1), nitrates (1400−1600 cm−1), and nitrites (1200 cm−1) were present.65,66 Interestingly, the band at 1810−1820 cm−1 assignable to NO− species (Au-NO−)6,8 grew considerably by adding FeOx. Thus, we infer that the FeOx addition leads to some enrichment of CO and NO molecules on the Au surface and its perimeters, causing a strong promotional effect on the NO−CO reaction. In the region of OH stretching vibrations at 3400−3800 cm−1, sharp bands at 3728, 3696, and 3686 cm−1 (isolated OH groups) and a broad band centered at 3545 cm−1 (H-bonded OH groups) on TiO2 were observed. However, these bands did not change significantly by the FeOx addition. Figure 8 shows IR spectra monitored during the NO−CO reaction (2000 ppm of CO, 2000 ppm of NO in He balance; 100 mL/min) at 250 °C. After 2 min, on-top NO at 1822 cm−1 was saturated while bridged NO at 1707 cm−1 and nitrates at 1601 and 1540 cm−1 were accumulated gradually on Au/ TiO2.30,36 The band at 2203 cm−1 is assigned to isocyanate species (−NCO). The isocyanate and nitrate species all behaved with kinetics similar to that of the isolated and Hbonded hydroxyl groups of TiO2. Therefore, we may assume that these species are accumulated on the TiO2 surface. The isocyanate species are expected to migrate from the Au surface to the TiO2 support, where they are stabilized and agglomerate, as previously demonstrated.39 In contrast, Au-Fe catalysts only showed one sharp and intense band at 1810−1816 cm−1, whose intensity was at least 10 times higher than those of CO and NO

adjacent NO and CO forming surface N atoms and gaseous CO2.56 Therefore, we suggest that the addition of FeOx facilitates both bimolecular and dissociative mechanisms, but its further addition, as represented with catalyst Au-Fe(1:2), suppresses the bimolecular mechanism at 50−150 °C. 3.3. In Situ DRIFTS Study. To gain deeper insight into the effect of Fe (FeOx) promotion of the Au/TiO2 catalyst, we carried out in situ DRIFTS studies. IR spectra measured during the adsorption of 2000 ppm of CO in He balance at 30 °C after 5 min are shown in Figure 6. Upon adsorption (within 10 s,

Figure 6. In situ DRIFT spectra measured during CO adsorption on Au/TiO2 (Au), FeOx/TiO2 (Fe), and Au-FeOx/TiO2 catalysts with different Fe loadings at 30 °C for 5 min (from blue to red line at intervals of 1 min). Conditions: gas mixture, 2000 ppm of CO, He balance; total flow rate, 100 mL/min.

blue line), a small band at 2140 cm−1 appeared, which is assigned to on-top Auδ+-CO and was slightly red shifted upon addition of FeOx.58−60 As the adsorption proceeded on Au/ TiO2, two new intense bands emerged at 2110 and 2073 cm−1, assignable to CO on Au0 particles (Aus0-CO)10,58,59 and CO at the perimeter of Au0 and TiO2 (Aup0-CO).10 The CO molecules tend to adsorb on the Auδ+ sites at the beginning and then on the Aus0 sites, followed by a gradual emergence of 7903

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Figure 7. In situ DRIFT spectra measured during NO adsorption on Au/TiO2 (Au), FeOx/TiO2 (Fe), and Au-FeOx/TiO2 catalysts with different Fe loadings at 30 °C for 5 min (from blue to red line at intervals of 1 min). Conditions: gas mixture, 2000 ppm of NO, He balance; total flow rate, 100 mL/min.

Figure 8. In situ DRIFT spectra measured during NO−CO reaction on Au/TiO2 (Au), FeOx/TiO2 (Fe), and Au-FeOx/TiO2 catalysts with different Fe loadings at 250 °C for 10 min (from blue to red line at intervals of 1 min). Conditions: gas mixture, 2000 ppm of NO, 2000 ppm of CO, He balance; total flow rate, 100 mL/min.

adsorption at 30 °C (see Figures 6 and 7). For exothermic adsorption, higher temperature should result in lower coverage of adsorbed species and consequently there should be less adsorbed CO and NO at 250 °C. Therefore, this band is likely originating from a completely different species, which can be formed at higher temperatures. This band was extremely weak for Fe/TiO2, indicating that the Au-FeOx nanocomposites on TiO2 facilitate the formation of new surface species. To uncover

the nature of this unknown species, we measured NO and CO coadsorption at 250 °C on Fe/TiO2, followed by switching to a flow of NO or CO alone at the same temperature. As shown in Figure 9A, the absorption band at 1819 cm−1 almost completely disappeared upon switching to NO for 5 min (blue line) and stayed unchanged by further switching to CO (red line). However, reintroduction of NO and CO together let this band reappear (green line). This behavior suggests that the 7904

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Figure 9. In situ DRIFT spectra: (A) measured during NO−CO reaction on FeOx/TiO2 at 250 °C for 10 min (black line), followed by switching to NO (blue line) and then CO (red line), and reswitching to NO−CO (green line); (B) measured during NO−CO reaction on Fe/TiO2 at 250 °C for 1 min (blue line) and 10 min (solid read line) and at 350 °C for 10 min (dotted red line).

adsorption sites for NO and CO are created, leading to enrichment of these molecules on the surface. In addition, a new surface complex formed between NO and CO was detected at the reaction temperature, giving rise to a band at around 1810 cm−1. The formation of this complex on AuFeOx/TiO2 correlates with the catalytic behavior and thus is proposed to contribute to high NO conversion and N2 selectivity.

coexistence of NO and CO is a necessary prerequisite for the formation of the new species. When the reaction temperature was increased, the same band was intensified considerably (Figure 9B). Au-FeOx/TiO2 catalysts also showed the same behavior. These results led us to conclude that the band at ca. 1810 cm−1 can be assigned to a surface NO−CO complex. According to our results and literature data, a conceivable reaction pathway could be expressed as follows.67 For the sake of brevity the TiO2 support is omitted in this reaction sequence. Au−FeOx + NO + CO → Au−FeOx (···NO−CO)

(4)

Au−FeOx ···(NO−CO) → Au−FeOx ···N + CO2

(5)

Au−FeOx ···N + CO → Au−FeOx ···NCO

(6)

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02401. TPR profile of spent Au-Fe(1:2)/TiO2 catalyst and catalytic performance of Au-Fe(1:2)/TiO2 catalyst at different CO:N2 ratios (PDF)

Finally, Au···NCO reacts with NO to produce N2 and CO2:53

■

2Au−FeOx ···NCO + NO → N2 + CO2 + 2Au−FeOx (7)

AUTHOR INFORMATION

Corresponding Authors

Au-FeOx/TiO2 catalysts showed no band associated with isocyanate species probably because reaction 7 occurs rapidly. The formation of the surface (NO−CO) complex is enhanced by Au-FeOx nanocomposites and proposed to lead to enrichment of NO and CO on the catalyst surface, resulting in high NO conversion and N2 selectivity.

*E-mail for N.M.: [email protected]. *E-mail for A.B.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Yan Xie, Dr. Zhang Shaohua, and Prof. Jiahui Huang at the Gold Catalysis Research Center (GCRC) of the Dalian Institute of Chemical Physics (DICP) are acknowledged for measuring BET, XRD, XPS, and TEM. This work was financially supported by the Natural Science Foundation of China (NSFC) (No. 21377017), start-up grants from the Dalian University of Technology (Nos. DUT13RC(3)04 and DUT13RC(3)26), the National Thousand Talents Program of China, and the Fundamental Research Funds for the Central Universities of China.

4. CONCLUSIONS We prepared Au-FeOx/TiO2 catalysts by deposition−precipitation (Au/TiO2) and subsequent impregnation (FeOx). The addition of FeOx to Au/TiO2 showed striking promotional effects in the catalytic NO reduction with CO. XRD and XPS indicated that the FeOx component covering part of the Au surface is amorphous and that the iron is present mainly as Fe2+ (FeO) even after calcination at 300 °C. In comparison to Au/ TiO2 and FeOx/TiO2, the Au-FeOx/TiO2 catalysts showed a tremendous enhancement of activity and selectivity to N2, indicating a striking synergistic effect caused by the promotion with FeOx. The performance of the nanocomposite Au-FeOx/ TiO2 catalyst is influenced by the Au:Fe molar ratio. Catalysts with an equimolar Au:Fe ratio show the highest selectivity to N2 (90.6% at 84.4% conversion at 250 °C). In particular, the addition of FeOx to Au/TiO2 facilitated the reduction of N2O. In situ DRIFTS revealed that on Au-FeO x/TiO2 new

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