Excellent catalytic performance, thermal stability, and water resistance of 3DOM Mn2O3-supported Au–Pd alloy nanoparticles for the complete oxidation of toluene

Accepted Manuscript Title: Excellent catalytic performance, thermal stability, and water resistance of 3DOM Mn2 O3 -supported Au-Pd alloy nanoparticles for the complete oxidation of toluene Author: Shaohua Xie Jiguang Deng Yuxi Liu Zhenhua Zhang Huanggen Yang Yang Jiang Hamidreza Arandiyan Hongxing Dai Chak Tong Au PII: DOI: Reference:

S0926-860X(15)30159-9 http://dx.doi.org/doi:10.1016/j.apcata.2015.09.026 APCATA 15558

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

7-4-2015 22-8-2015 21-9-2015

Please cite this article as: Shaohua Xie, Jiguang Deng, Yuxi Liu, Zhenhua Zhang, Huanggen Yang, Yang Jiang, Hamidreza Arandiyan, Hongxing Dai, Chak Tong Au, Excellent catalytic performance, thermal stability, and water resistance of 3DOM Mn2O3-supported Au-Pd alloy nanoparticles for the complete oxidation of toluene, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.09.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Excellent catalytic performance, thermal stability, and water resistance

of

3DOM

Mn2O3-supported

AuPd

alloy

nanoparticles for the complete oxidation of toluene Shaohua Xiea, Jiguang Denga,*, Yuxi Liua, Zhenhua Zhangb, Huanggen Yanga, Yang Jianga, Hamidreza Arandiyanc, Hongxing Daia,*, Chak Tong Aud,* a

Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of

Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China b

Institute of Microstructure and Property of Advanced Materials, Beijing University

of Technology, Beijing 100124, China c

Particles and Catalysis Research Group, School of Chemical Engineering, The

University of New South Wales, Sydney, NSW 2052, Australia d

Department of Chemistry, Center for Surface Analysis and Research, Hong Kong

Baptist University, Kowloon Tong, Hong Kong, China Corresponding Author *

Dr.

Jiguang

Deng

([email protected])

and

Prof.

Hongxing

Dai

([email protected]), Tel. No.: +86-10-6739-6118, Fax: +86-10-6739-1983 * Prof. Chak Tong Au ([email protected]), Tel. No.: +852 3411 7067; Fax: +852 3411 7348.

1

Graphical abstract

Research Highlights ► Au-Pd alloy nanoparticles are uniformly dispersed on the 3DOM Mn2O3 surface. ► Au-Pd/3DOM Mn2O3 nanocatalysts exhibit high activity for toluene oxidation. ► Au-Pd/3DOM Mn2O3 shows high thermal stability during toluene oxidation. ► Water vapor introduction induces a positive effect on toluene oxidation. ► There is a strong interaction between Au-Pd nanoparticles and 3DOM Mn2O3.

ABSTRACT Three-dimensionally ordered macroporous (3DOM) Mn2O3-supported AuPdy alloy (xAuPdy/3DOM Mn2O3; x = 1.03.8 wt%; Pd/Au molar ratio : y = 1.85, 1.92) catalysts were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods, respectively. Physicochemical properties of the

2

samples were characterized by means of numerous techniques, and their catalytic activities were evaluated for toluene oxidation. It is found that the AuPdy alloy nanoparticles (NPs) with an particle size of 24 nm were uniformly dispersed on the surface of 3DOM Mn2O3, and the 3.8AuPd1.92/3DOM Mn2O3 sample performed the best: the temperature at 90 % toluene conversion was 162 oC at 40,000 mL/(g h). Furthermore, the 3.8AuPd1.92/3DOM Mn2O3 sample was highly active even after calcination at 700 oC. The introduction of water vapor to the feedstock induced a positive effect on toluene oxidation over 3.8AuPd1.92/3DOM Mn2O3, but a negative effect over 1.9Au/3DOM Mn2O3 or 1.9Pd/3DOM Mn2O3. The excellent catalytic activity, thermal stability, and water resistance of 3.8AuPd1.92/3DOM Mn2O3 were associated with its good activation adsorption of oxygen on AuPd1.92 NPs and strong interaction between noble metal NPs and 3DOM Mn2O3. The 3DOM Mn2O3supported AuPdy alloy NPs are promising industrial catalysts for efficient removal of volatile organic compounds.

Keywords: Three-dimensionally ordered macroporous manganese oxide; AuPd alloy nanoparticle; toluene oxidation; thermal stability; water resistance.

1. Introduction Most of volatile organic compounds (VOCs) are harmful to the atmosphere and human health. Catalytic oxidation is one of the most promising approaches for VOCs removal [1–8]. Although transition-metal oxides exhibit good catalytic performance for VOCs oxidation at high temperatures [2–5], supported noble metals perform well 3

at low temperatures [6–10]. For example, the T90% (temperature required for achieving a 90 % conversion of toluene) was 234 oC over Pd/Co3AlO [8], much lower than that (340 oC) over Co3AlO. Recently, gold-based catalysts have been found to be effective in toluene oxidation [10–15]. For instance, the T90% for toluene oxidation was 182 and 170 oC over Au/γ-MnO2 [11] and Au/La0.6Sr0.4MnO3 [12], respectively. When water vapor is introduced into the reaction system, however, supported Pd or Au nanoparticle (NP) catalysts are not catalytically durable, thus limiting their applications. Some researchers reported that due to the synergistic effect between Au and Pd, the supported Au–Pd NPs exhibited high catalytic activity for toluene oxidation [16,17]. Pd alloyed with Au could give rise to a catalyst that showed high activity and stability in the production of hydrogen peroxide [18,19], in which the Pd/Au atomic ratio played an important role in determining the performance. Pritchard et al. [19] observed that the optimal Pd/Au atomic ratio was 1.85 : 1.00 for the direct synthesis of H2O2. A number of AuPd alloy NPs were generated via different routes [20–23]. Hutchings et al. [23] used a modified impregnation method to fabricate the TiO2supported AuPd materials, and found that such a strategy could tailor-make size and composition of individual noble metal NP. Controlling the size and composition, however, is still a big challenge in preparing supported bimetallic NPs. Compared to the nonporous catalysts, the three-dimensionally ordered macroporous (3DOM) counterparts exhibit better catalytic activity due to their high surface areas and easy diffusion nature. Previously, we observed that Au/3DOM Mn2O3 were active

4

for toluene oxidation. However, these materials deactivated due to the sintering of Au NPs after 40 h of on-stream reaction [24]. It is well known that thermal stability and water resistance of a catalyst are important in practical application. Very recently, we have found that 3DOM Co3O4-supported AuPd alloy NPs exhibited high activity and stability for toluene oxidation [25]. Compared to Co3O4, Mn2O3 possesses different physicochemical properties. We here extend our attention on the investigation of the thermal stability and water resistance as well as the effect of Au/Pd molar ratio on catalytic performance of 3DOM Mn2O3-supported AuPdy alloy NPs for toluene oxidation. Preliminarily, we prepared the xAuPdy/Bulk Mn2O3 (x = 3.63.8 wt%; Pd/Au molar ratio (y) = 0.87, 1.96, and 3.76) samples to examine the effect of Pd/Au atomic ratio on toluene oxidation, and the results showed that the optimal Pd/Au atomic ratio was 1.96 and an AuPd alloy was formed (Figs. S1 and S2), in agreement with the result reported by Pritchard et al. [19].

2. Experimental 2.1. Catalyst preparation 3DOM Mn2O3 was fabricated according to the procedures described elsewhere [24]. The 3DOM Mn2O3-supported noble metal alloy NPs (xAuPdy/3DOM Mn2O3; x = 1.03.8 wt%; y = 0.85 and 1.92) were prepared via a gas bubble-assisted polyvinyl alcohol (PVA)-protected reduction route with NaBH4 as reducing agent. The typical preparation procedure (Scheme S1) is as follows: A desired amount of PVA (noble metal/PVA mass ratio = 1 : 1.2) was added to a HAuCl4 and PdCl2 mixed aqueous solution (1.5 mmol/L) at room temperature (RT) under vigorous stirring. A desired

5

amount of 3DOM Mn2O3 was then added to the mixed solution, with the theoretical AuPdy loading being 1.0, 2.0, and 4.0 wt%, respectively. The obtained suspension was subjected to ultrasonic (60 kHz) treatment for 10 min. The gas bubble-assisted stirring operation was adopted to make the reaction homogenous. After bubbling the suspension with N2 (200 mL/min) for 20 min, a 0.1 mol/L NaBH4 aqueous solution (noble metal/NaBH4 molar ratio = 1 : 5) was rapidly injected to form a dark brown suspension, i.e., the formation of AuPdy NPs on the 3DOM Mn2O3 surface, and the reaction system was further vigorously bubbled with N2 (200 mL/min) for 3 h. The wet solid was filtered, washed with 2.0 L of deionized water and 500 mL of ethanol for the removal of the adsorbed Cl, Na+, and PVA, dried at 80 oC for 12 h, and calcined in air at 300 oC for 2 h, thus obtaining the xAuPdy/3DOM Mn2O3 samples. The results (Table 1) of inductively coupled plasma atomic emission spectroscopic (ICPAES) investigations reveal that the real AuPdy loading (x) was 1.0, 1.9, and 3.8 wt%, and the real Pd/Au molar ratio (y) was 1.85 and 1.92 in the xAuPdy/3DOM Mn2O3 samples, respectively. For comparison purposes, the Bulk Mn2O3 sample was prepared via the thermal decomposition of manganese nitrate at 500 oC for 10 h; the 3.6AuPd1.96/Bulk Mn2O3, 1.9Au/3DOM Mn2O3, and 1.9Pd/3DOM Mn2O3 samples were also prepared using the gas bubble-assisted PVA-protected reduction method. The 1.9Au&1.9Pd/3DOM Mn2O3 sample was prepared as follows: Firstly, single Au and Pd NPs were synthesized via the gas bubble-assisted PVA-protected reduction route, respectively; after homogenously mixing the desired amounts of the Au and Pd NPs, the Au and Pd

6

NPs were then loaded on the surface of 3DOM Mn2O3, thus obtaining the 1.9Au&1.9Pd/3DOM Mn2O3 sample. 2.2. Catalyst characterization All of the samples were characterized by means of techniques, such as ICPAES, Xray diffraction (XRD), N2 adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high angle annular dark field (HAADF) and element mapping, X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2TPR), and oxygen temperature-programmed desorption (O2-TPD). The detailed procedures are described in the Supplementary material. 2.3. Catalytic evaluation Catalytic activities of the samples were evaluated in a continuous flow fixed-bed quartz microreactor (i.d. = 4 mm). To minimize the effect of hot spots, 50 mg of the sample (4060 mesh) was diluted with 0.25 g of quartz sands (4060 mesh). Prior to the test, the sample was treated in O2 (30 mL/min) at 250 oC for 1 h. After being cooled to a given temperature, the reactant mixture was passed through the catalyst bed. The total flow rate of the reactant mixture (1000 ppm toluene + O2 + N2 (balance)) was 33.4 mL/min, giving a toluene/O2 molar ratio of 1/400 and a space velocity (SV) of ca. 40,000 mL/(g h). The 1000-ppm toluene was generated by passing a N2 flow through a bottle containing pure toluene chilled in an ice-water isothermal bath. In the case of water vapor introduction, 0.5, 1.0, 2.0, and 5.0 vol% H2O was introduced by passing a N2 flow of 5.3, 10.7, and 13.1 mL/min and a total

7

reactant flow of 33.4 mL/min through a water saturator at 25, 25, 34, and 34 oC, respectively. Reactants and products were analyzed online by a gas chromatograph (GC-2010, Shimadzu) equipped with flame ionization and thermal conductivity detectors, using a stabilwax@-DA column (30 m in length) for VOCs separation and a Carboxen 1000 column (1/8 inch in diameter and 3 m in length) for permanent gas separation. We estimated the carbon balance by analyzing the concentrations of toluene and carbon dioxide in the reaction mixture before and after the microreactor (CO2 and H2O were the only products of toluene oxidation, which were identified by a mass spectrometer (Hiden HPR20)). The balance of carbon throughout the investigation was estimated to be 99.5 %.

3. Results and discussion 3.1. Crystal phase, particle size, and surface area Fig. S3 shows the XRD patterns of the as-prepared samples. All of the XRD peaks could be indexed to the cubic Mn2O3 phase (JCPDS PDF# 41-1442). The weak diffraction peaks corresponding to the Au and/or Pd phases were not detected due to the low Au and/or Pd loadings and good dispersion on the Mn2O3 surface. According to the Scherrer equation, the Mn2O3 crystallite sizes were estimated to be 4552 nm in xAuPdy/3DOM Mn2O3 and 8687 nm in xAuPdy/Bulk Mn2O3. Figs. S4 and S5 show the SEM and TEM images of the samples, respectively. The xAuPdy/3DOM Mn2O3 samples displayed a high-quality 3DOM structure (Figs. S4, S5, and S7). The macropore sizes of the xAuPdy/3DOM Mn2O3 samples were about 150 nm, and the mean sizes (2.72.9 nm) of AuPdy alloy NPs were smaller than those

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(3.33.7 nm) of Au or Pd NPs. All of the noble metal NPs were highly dispersed on the surface of 3DOM Mn2O3. As shown in the high-resolution TEM images of noble metal NPs (Fig. S5b, d, and f), the intraplanar spacing (d value) of Au was 0.235– 0.236 nm, in good agreement with that (0.235 nm) of the (111) crystal plane of the standard Au sample; the d value of Pd was 0.226–0.227 nm, rather close to that (0.227 nm) of the (111) crystal plane of the standard Pd sample. The recording of multiple bright electron diffraction rings in the SAED patterns (insets of Fig. S5) of xAuPdy/3DOM Mn2O3 suggests formation of a polycrystalline structure. Au and Pd species in 3.8AuPd1.92/3DOM Mn2O3 were evenly distributed on the surface of 3DOM Mn2O3 (Figs. 1 and S6). From Fig. S6(e), one can observe that the signal intensity of the Au element was almost the same as that of the Pd element, implying that Au and Pd mixed well in AuPd NPs. It has been generally accepted that such a well mixed structure was an indication of Au alloyed with Pd in AuPd NPs [19,23]. Surface areas of the xAuPdy/3DOM Mn2O3 samples were 34.537.7 m2/g, much higher than those (7.27.5 m2/g) of the xAuPdy/Bulk Mn2O3 samples (Table 1). 3.2. Catalytic performance Fig. 2 shows the toluene conversion and reaction rate as a function of temperature. Obviously, the catalytic activity was remarkably enhanced by loading AuPdy NPs on Mn2O3 (Fig. 2A and Table 2). 1.9AuPd1.92/3DOM Mn2O3 performed better than 1.9Au/3DOM Mn2O3, 1.9Pd/3DOM Mn2O3, and 1.9Au&1.9Pd/3DOM Mn2O3. It should be noted that there were 1.9 wt% Au and 1.9 wt% Pd in 1.9Au&1.9Pd/3DOM Mn2O3, whereas the content of total AuPd alloy in 1.9AuPd1.92/3DOM Mn2O3 was

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1.9 wt%. The T90% over 1.9AuPd1.92/3DOM Mn2 O3 was 184 oC, whereas that over 1.9Au&1.9Pd/3DOM Mn2O3 was 203 oC. Toluene oxidation rate at 160 oC (15.9× 106 mol/(gnoble metal s)) over 1.9AuPd1.92/3DOM Mn2O3 was much higher than that at 160

C (4.18 × 106 mol/(gnoble

o

metal

s) over 1.9Au&1.9Pd/3DOM Mn2O3.

Significantly, the alloying of Au and Pd in the sample was beneficial for enhancement in activity. With a rise in AuPdy loading from 1.0 to 3.8 wt%, the T90% decreased from 209 to 162 oC (Fig. 2B). Among these samples, 3.8AuPd1.92/3DOM Mn2O3 performed the best, giving the T10%, T50%, and T90% of 96, 146, and 162 oC (Table 2), respectively. Such an activity was much higher than that (T90% = 210 oC) over 3.6AuPd1.96/Bulk Mn2O3. Therefore, 3.8AuPd1.92/3DOM Mn2O3 was regard as the best-performing sample. Compared to Bulk Mn2O3, 3DOM Mn2O3 possessed a larger surface area and an ordered porous structure, which would facilitate the dispersion of noble metal NPs and the diffusion of reactant molecules. Therefore, it is understandable that a 3DOM structure was beneficial for toluene oxidation. Toluene oxidation rates at 160 oC normalized per gram of noble metal of the samples are summarized in Table 2. Among all of the samples, 1.9AuPd1.92/3DOM Mn2O3 showed the highest toluene oxidation rate (15.9 μmol/(gnoble metal s)). Table S1 lists the catalytic activities at 160 oC of various samples reported in the literature for toluene oxidation under

different

conditions.

Apparently,

toluene

oxidation

rate

over

3.8AuPd1.92/3DOM Mn2O3 (10.5 μmol/(gnoble metal s)) was much higher than that (1.80 μmol/(gnoble metal s)) over 3.4Au/3DOM La0.6Sr0.4MnO3 [12], that (1.14 μmol/(gnoble metal s)) over 1.0Pt/Al2O3CeO2 [26], that (4.54 μmol/(gnoble metal s)) over 0.3Pd/Al2O3 [27],

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that (0.00059 μmol/(gnoble metal s)) over 1.6Au/Co3O4 [28], that (4.91 μmol/(gnoble metal s)) over 3.7Au/meso-Co3O4 [29], and that (5.70 μmol/(gnoble

metal

s)) over

1.99AuPd/3DOM Co3O4 [25], but much lower to that (22.7 μmol/(gnoble metal s)) over 0.5Au−0.5Pd/CeO2 [16]. In the reducible metal oxide-supported noble metal catalysts, there are several kinds of active sites, such as noble metal, transition-metal oxide, and interface between noble metal and transition-metal oxide, causing it difficult to identify a single active site. Therefore, accurate calculation of turnover frequencies (TOFs) is hard. In the present study, we calculated TOFnoble

metal

and TOFMn2O3 (Table 2) for toluene

oxidation at 140 oC according to TOFM = xC0/nM, where x is the toluene conversion, C0 is the initial toluene concentration, and nM is the molar amount of Mn2O3 or noble metal atoms exposed on the sample surface. The TOFnoble metal (0.90 × 103 s1) over 1.9AuPd1.92/3DOM Mn2O3 was the highest, but the highest TOFMn2O3 was achieved over 3.8AuPd1.92/3DOM Mn2O3. According to the Weisz-Prater criterion, when the effectiveness factor η  0.95 and reaction order n = 1, the dimensionless Weisz-Prater parameter (WP) value is less than 0.3, which can be considered a sufficient condition for the absence of significant pore diffusion limitations [30]. At a toluene oxidation temperature of 140 ºC, we carried out the Weisz-Prater analysis, the calculated WP values were 0.0040–0.033, much less than 0.3. Therefore, no significant mass transfer limitations existed in our catalytic system. In the past years, there have been reports on the kinetics of catalytic oxidation of VOCs. For instance, the oxidation of butyl acetate over AgZSM-5 was proven to be first-order toward butyl acetate concentration

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and zero-order toward oxygen concentration [31]. Good linear Arrhenius plots were obtained over the ceria–zirconia-supported LaCoO3 catalysts for toluene combustion when the reaction was first- and zero-order kinetics with respect to toluene and oxygen concentrations [32], respectively. Therefore, we can reasonably suppose that the oxidation of toluene in the presence of excessive oxygen (toluene/O2 molar ratio = 1/400) would obey a first-order reaction mechanism with respect to toluene concentration (c): r = k c = (A exp(Ea/RT)) c, where r, k, A, and Ea are the reaction rate (mol/s), rate constant (s1), pre-exponential factor, and apparent activation energy (kJ/mol), respectively. Fig. S8 shows the Arrhenius plots for toluene oxidation over the samples, and their apparent activation energies are summarized in Table 2. The Ea decreased in a sequence similar to that of catalytic activity, and the best-performing 3.8AuPd1.92/3DOM Mn2O3 sample gave the lowest Ea (26 kJ/mol). These samples showed

different

activation

energies,

because

they

possessed

different

physicochemical properties, such as noble metal dispersion and particle size, noble metal loading, metal oxidation state, and interface between noble metals and support. The discrepancy in activation energy of the samples could be reflected from their different catalytic activities. In order to examine the catalytic stability, we carried out the on-stream toluene oxidation over 3.8AuPd1.92/3DOM Mn2O3 at 260 oC and SV = 120,000 mL/(g h), over 1.9Pd/3DOM Mn2O3 at 250 oC and SV = 40,000 mL/(g h), and over 1.9Au/3DOM Mn2O3 at 270 oC and SV = 40,000 mL/(g h). As shown in Fig. 3A, no significant loss in activity was observed over 3.8AuPd1.92/3DOM Mn2O3 after 60 h of on-stream

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reaction, but a considerable activity loss was observed over the supported Pd or Au sample. Furthermore, deactivation of the supported Au sample was irreversible. To further study the thermal stability, we calcined the 3.8AuPd1.92/3DOM Mn2O3 sample in a N2 flow (200 mL/min) at 300, 500 or 700 oC for 3 h. Compared to the fresh sample, only 8 % loss in toluene conversion at 180 oC was detected over the 700 oCcalcined sample (Fig. 3B). These results demonstrate that the 3.8AuPd1.92/3DOM Mn2O3 sample exhibited excellent thermal stability in toluene oxidation. As shown in Fig. S9, the Au NPs in the supported Au sample grew to bigger ones (540 nm), which partially explains the irreversible activity loss after 30 h of on-stream reaction [33]. However, almost no changes in size of Pd or AuPdy alloy NPs in the supported Pd or AuPdy samples took place. It is known that the changes in crystal structure and noble metal particle size and the competitive adsorption of water, CO2, and reactants (toluene and O2) on the sample surface are the factors leading to the deactivation of a catalyst. We did not detect the significant changes in crystal structure and noble metal particle size. After the used supported Pd sample was treated in O2 (50 mL/min) at 250 oC for 1 h, the activity was recovered, indicating that the recovering of activity might be due to the removal of adsorbed H2O and CO2 [34]. It has been reported that activity of a catalyst was negatively affected by water vapor [29,35]. Therefore, it is highly desired to develop a water-resistant catalyst for practical applications. We conducted toluene oxidation in the presence of 0.5, 1.0, 2.0 or 5.0 vol% water vapor over 3.8AuPd1.92/3DOM Mn2O3 (Fig. 4A) and in the presence of 1.0 vol% water vapor over the supported Au or Pd sample (Fig. 4B).

13

Interestingly, when moisture concentration was below 2.0 vol%, a positive effect of water vapor on toluene oxidation was detected over 3.8AuPd1.92/3DOM Mn2O3 ; when moisture concentration rose to 5.0 vol%, a small drop in toluene conversion took place at higher temperatures (> 150 oC). As shown in Fig. 4B, a negative effect of water vapor on toluene oxidation was observed over 1.9Au/3DOM Mn2O3 or 1.9Pd/3DOM Mn2O3, while a positive effect was detected over 3.8AuPd1.92/3DOM Mn2O3. Compared to the supported Au or Pd sample, the supported AuPd alloy sample possessed much better water-resistant ability in toluene oxidation. 3.3. Oxygen species, surface element composition, metal oxidation state, and reducibility Generally speaking, the ability to activate oxygen correlates with the activity of a catalyst for organic oxidation. As shown in Fig. 5, the O2-TPD profile of 3.8AuPd1.92/3DOM Mn2O3 displayed three oxygen desorption peaks at 98, 350, and 770 oC, corresponding the , , and  desorption. In the O2-TPD study, the O2−, O22− or O− species could not be differentiated according to the desorption temperatures due to their desorption at almost similar temperatures. Based on the results reported in the literature [9,33], we tentatively attributed the low- and high-temperature peaks to the desorption of chemisorbed oxygen (Oads, e.g., O2ˉ, O22ˉ or Oˉ) and lattice oxygen (Olatt) species, respectively. The loading of precious metal NPs on the surface of the metal oxide usually enhances oxygen adsorption and desorption. As shown in Table 3, the total O2 desorption (3.55−3.71 mmol/g) of the 3DOM Mn2O3-supported AuPd alloy NPs was larger than that (3.26 mmol/g) of 3DOM Mn2O3. The interaction

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between AuPd alloy NPs and 3DOM Mn2O3 might improve the lattice oxygen mobility, thus increasing the low-temperature O2 desorption of the 3DOM Mn2O3supported AuPd alloy NPs. Above 550 oC, all of the samples showed an oxygen desorption of 2.532.95 mmol/g; below 550 oC, the oxygen desorption (0.831.14 mmol/g) of xAuPdy/3DOM Mn2O3 was much higher than that (0.490.64 mmol/g) of 1.9Au/3DOM Mn2O3 and 1.9Pd/3DOM Mn2O3, with the highest oxygen desorption being achieved over 1.9AuPd1.92/3DOM Mn2O3. It has been generally believed that the O2 activation ability is related to the surface oxygen vacancy density [29]. The rise in surface oxygen vacancy density of 3DOM Mn2O3 induced by loading of AuPdy alloy NPs was due to the presence of strong interactions between Pd in AuPdy NPs and 3DOM Mn2O3 and/or between Au in AuPdy NPs and 3DOM Mn2O3. The two reactions indicate that AuPdy alloy NPs had a strong electron-donating ability, which would result in easier activation of molecular oxygen, thus enhancing the amount of adsorbed oxygen. As can be seen from Table 3, the amount of desorbed oxygen species below 550 oC decreased in the order of 3.8AuPd1.92/3DOM Mn2O3 > 1.9AuPd1.92/3DOM Mn2O3 > 1.0AuPd1.85/3DOM Mn2O3 > 3.6AuPd1.96/3DOM Mn2O3 > 1.9Pd/3DOM Mn2O3 > 1.9Au/3DOM Mn2O3 > 3DOM Mn2O3, and the oxygen desorption of 3.8AuPd1.92/3DOM Mn2 O3 was much higher than that of 3.6AuPd1.96/Bulk Mn2O3, implying that the 3DOM Mn2O3-supported AuPd sample had a stronger interaction between AuPdy alloy NPs and Mn2O3. Fig. S11 illustrates the Mn 2p3/2, O 1s, Au 4f, and Pd 3d XPS spectra of the samples, and the surface element compositions are summarized in Table 3. Each of the Mn

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2p3/2 spectra was asymmetrical and could be decomposed into two components at binding energy (BE) = 641.6 and 640.1eV as well a satellite at BE = 643.8 eV (Fig. S11 A), ascribable to the surface Mn3+ and Mn2+ species [36,37], respectively. After deposition of Au, Pd or AuPd NPs on 3DOM Mn2O3, the surface Mn3+/Mn2+ molar ratio slightly decreased from 0.93 to 0.630.90 (Table 3), indicating that the surface Mn2+ concentration increased on the surface. Such a decrease in Mn3+/Mn2+ molar ratio was due to the interaction between noble NPs and support, thus increasing in the surface oxygen vacancies on the 3DOM Mn2O3 surface. In other words, 3DOM Mn2O3 in the supported noble metal samples possessed a higher oxygen vacancy concentration in the vicinity of the Mn2O3 surface, as compared to the 3DOM Mn2O3 support. As for the O 1s XPS spectra (Fig. S11 B), one can decompose the asymmetrical O 1s spectrum of each of the samples into four components at BE = 529.6, 530.6, 532.2, and 533.8 eV, assignable to the surface lattice oxygen, adsorbed oxygen (Oads, e.g., O2, O22 or O), hydroxyl and/or carbonate, and adsorbed molecular water species [33,38,39], respectively. Since the samples were pretreated in O2 at 450 oC before XPS measurement, the surface carbonate species would be minimized. C 1s XPS spectra (Fig. S10) of the samples are shown in the supplementary material. The signal due to the surface carbonate species [40] was rather weak, indicating that there was only a trace amount of surface carbonate species on the surface of the samples. In other words, the Oads species were mainly O2, O22 or O species. Such oxygen species were active for the oxidation of hydrocarbons at low temperatures [38]. It has

16

been generally established that the higher the surface oxygen vacancy density, the easier the activation adsorption of O2 molecules, and hence the better is the performance of a catalyst for VOCs oxidation [29]. Although the Oads/Olatt molar ratios of 3DOM Mn2O3-supported 1.0AuPd1.85 (0.65), 1.9AuPd1.92 (0.67), and 1.9Pd (0.63) as well as that of 3.6AuPd1.96/Bulk Mn2O3 (0.65) were close, the change trend in surface Oads/Olatt molar ratio of the samples (Table 3) was in good agreement with that in oxygen desorption obtained in the O2-TPD studies. In other words, the supported Au–Pd alloy samples exhibited a stronger ability in activating oxygen than the supported Au or Pd sample, hence resulting in excellent performance for toluene oxidation. Using the curve-fitting approach, the Au 4f spectrum of each of the samples could be decomposed into several components (Fig. S11 C): the ones at BE = 83.4 and 87.4 eV were due to the surface Au0 species, whereas the ones at BE = 84.4, 85.7, 88.4 and 89.7 eV were due to the surface Auδ+ species [29,33]. Apparently, the Auδ+/Au0 molar ratios of the supported Au–Pd alloy samples were much higher than that of the supported Au sample, and it increased with the loading of Au–Pd alloy NPs. It has been reported that a higher Auδ+/Au0 molar ratio is an indication of a better ability in activating oxygen molecules [12,33], thus inducing a beneficial effect on VOCs oxidation. Similarly, the asymmetrical Pd 3d XPS spectrum (Fig. S11 D) of each of the samples could also be decomposed into four components: the ones at BE = 335.6 and 340.6 eV were assignable to the surface Pd0 species, whereas the ones at BE = 337.6 and 342.6 eV were attributable to the surface Pd2+ species [41,42]. The Pd2+/Pd0

17

molar ratios (1.15–1.72) of the 3DOM Mn2O3-supported AuPd alloy samples were higher than that (0.97) of the 1.9Pd/3DOM Mn2O3 sample, suggesting that the addition of Au could enhance the concentration of Pd2+ species. In other words, Au could act as an electronic promoter for Pd. A similar finding was reported by Hutchings and coworkers who investigated the Au–Pd/TiO2 catalysts for the solventfree oxidation of primary alcohols to aldehydes [43]. Fig. S12 illustrates the H2-TPR profiles of the samples. The 3DOM Mn2O3 sample displayed two reduction peaks, assignable to the reduction of surface Mn3+ to Mn2+ and of bulk Mn3+ to Mn2+ [4], respectively. When noble metal NPs were loaded on the surface of 3DOM Mn2O3, all of the reduction peaks shifted to lower temperatures, suggesting that loading noble metal NPs improved the low-temperature reducibility of the samples. Since the oxidation of toluene have been reported to proceed via a Mars– van Krevelen (redox) mechanism [29], the improvement in low-temperature reducibility would be beneficial for the enhancement in catalytic performance of the samples. This result also reveals that there was presence of a strong interaction between noble metal NPs and Mn2O3 support. It should be noted that the two peaks observed for 3DOM Mn2O3 at 275 and 388 ºC (Fig. S12) became weak in intensity when the noble metals were loaded. The H2 consumption (15.416.7 mmol/g) of the supported samples was slightly lower than that (16.9 mmol/g) of the unsupported sample (Table 3). The supported Pd or Au1.00Pdy samples showed lower reduction temperatures than the supported Au sample (Fig. S12 A). It is reported that the PdOx species can be more readily reduced than the AuOx species [17]. Therefore, the

18

AuPdy/3DOM Mn2O3 samples possessed better low-temperature reducibility than the supported gold sample. 3.4. Reactant activation In one of our previous work on the catalysis of Ce0.6Zr0.3Y0.1O2 nanorods-supported Au1Pd2 catalysts for toluene oxidation [44], we examined the effect of pretreatment condition on catalytic activity. We pretreated the 0.90Au1Pd2/Ce0.6Zr0.3Y0.1O2 sample in N2, H2 or O2 at different temperatures, and found that the catalytic activity had no significant change with N2 treatment, increased with O2 treatment, but decreased with H2 treatment. This result demonstrates that the active sites of the sample were mainly oxidized noble metal NPs. A similar scenario would exist in the present study. Therefore, we believe that toluene oxidation mainly took place on the sites of oxidized noble metal NPs as well as the interfaces between noble metal Au and Pd NPs and oxygen vacancy-rich Mn2O3. From the STEM images (Fig. 6) of the 3.8AuPd1.92/3DOM Mn2O3 sample, one can find that the AuPd1.92 alloy NPs were supported on the steps and terraces of 3DOM Mn2O3 (Fig. 6f). Shen and co-workers [45] reported that a strong interaction between Au NPs (2−4 nm in size) and rod-shaped CeO2 could stabilize Au NPs, and rendered the catalyst to show high activity and stability for low-temperature CO oxidation and water-gas shift reaction under realistic reaction conditions. In the present study, the strong interaction between AuPdy alloy NPs and 3DOM Mn2O3 could also stabilize the surface Au+ and Pd2+ species on the supported AuPdy samples. XPS spectra (Fig. S13) of the used samples reveal that the concentrations of surface Oads, Au+, and Pd2+

19

species on the supported AuPdy samples were higher than those on the supported Au or Pd sample, and significantly decreased on the used 1.9Pd/3DOM Mn2O3 or 1.9Au/3DOM Mn2O3 sample. However, only small decreases in surface Oads and Au+ and Pd2+ species concentrations were detected on the 3.8AuPd1.92/3DOM Mn2O3 sample (Table 4). Therefore, the strong interaction between AuPdy alloy NPs and 3DOM Mn2O3 might render 3.8AuPd1.92/3DOM Mn2O3 to exhibit better thermal stability. Recent studies reported that O2 could be readily activated on a Au NP cluster by formation of hydroperoxyl via the H-transfer reactions (O2* + H2O*  OOH* + OH* and OOH*  O* + OH*) [46,47]. The presence of water molecules could greatly promote the hydrogen-transfer reactions, thus enhancing the oxidation of alcohol on a Au(111) surface [47]. In one of our previous investigations [29], however, we concluded that activity inhibition by water vapor in the oxidation of toluene was due to competitive adsorption of H2O and toluene on Au NPs. That is to say, the presence of water vapor had a positive effect (beneficial for oxygen activation) and a negative effect (competitive adsorption of H2O and reactant molecules) on toluene oxidation over the Au-based catalysts. In the present study, the presence of water vapor decreased the catalytic activity of 1.9Au/3DOM Mn2O3 due to the negative effect, i.e., water vapor played a more important role in determining the catalytic activity of 1.9Au/3DOM Mn2O3. The O 1s XPS peaks due to the surface hydroxyl and adsorbed molecular water species on 1.9Pd/3DOM Mn2O3 were high in intensity, indicating that the 1.9Pd/3DOM Mn2O3 sample had a strong ability to adsorb water molecules

20

(Fig. S11). A strong ability to adsorb water molecules would lead to decease in O2 adsorption on the Pd species, thus inducing a negative effect on toluene oxidation over 1.9Pd/3DOM Mn2O3. As mentioned above, the Pd NP cluster has a good ability to adsorb water molecules, and the Au NP cluster shows a good capability of activating O2 molecules in the presence of water [47]. There are Au and Pd species that acted as a unique partner in the 3.8AuPd1.92/3DOM Mn2O3 sample in the presence of water. Therefore, introduction of water vapor to the feedstock could facilitate the easy activation adsorption of O2 molecules over the AuPd alloy sample. As for single Au or Pd sample, the only site for activating oxygen or water vapor adsorption are very hard to met the need of the reaction, thus a negative effects of water vapor played on toluene oxidation. Actually, such a positive effect might play a more important role in governing the catalytic behavior of 3.8AuPd1.92/3DOM Mn2O3. However, due to the competing adsorption of water and reactant molecules, the presence of a higher water vapor concentration (5.0 vol%) decreased the activity of 3.8AuPd1.92/3DOM Mn2O3 above 150 oC. From Fig. 4A, one can see a maximum in toluene conversion at 120130 oC over 3.8AuPd1.92/3DOM Mn2O3 in the presence of water vapor, a behavior different from that over the 3DOM Co3O4-supported AuPd alloy nanocatalysts due to the discrepancy in physicochemical property of Mn2O3 and Co3O4 [25]. Previously, some researchers found that there was a large desorption of water centered at ca. 110 oC in the H2O-TPD profile of the copper- and manganesebased catalyst [48]. Therefore, we believe that at ca. 120 oC, a large amount of water might be desorbed from the surface of 3.80AuPd1.92/3DOM Mn2O3, and then more

21

amounts of oxygen and/or toluene molecules would be adsorbed, thus resulting in a positive effect on toluene oxidation. It should be pointed out that the real promotion mechanism of water vapor on toluene oxidation over supported AuPd NPs needs further investigations.

4. Conclusions 3DOM Mn2O3 and xAuPdy/3DOM Mn2O3 (x = 1.03.8 wt%; y = 1.85, 1.92) with controlled AuPdy particle sizes (24 nm) and good noble metal dispersion were prepared using the PMMA-templating and bubble-assisted PVA-protected reduction methods, respectively. The 3.8AuPd1.92/3DOM Mn2O3 sample showed excellent catalytic activity (T90% = 162 oC at SV= 40,000 mL/(g h)), thermal stability, and water vapor resistance in toluene oxidation. The introduction of water vapor to the feedstock had a positive effect on toluene oxidation over 3.8AuPd1.92/3DOM Mn2O3, but a negative effect over 1.9Au/3DOM Mn2O3 or 1.9Pd/3DOM Mn2O3. The good oxygen activation adsorption of AuPd1.92 NPs and strong interaction between noble metal NPs and 3DOM Mn2O3 were factors governing the excellent performance of 3.8AuPd1.92/3DOM Mn2O3. It is envisioned that the 3DOM-structured transitionmetal oxide-supported noble metal alloy NPs are promising catalysts for the removal of VOCs.

Acknowledgements This work was supported by the NSF of China (21377008), National High Technology Research and Development Program ("863" Program) of China (2015AA034603), Foundation on the Creative Research Team Construction

22

Promotion Project of Beijing Municipal Institutions, and Scientific Research Base ConstructionScience and Technology Creation PlatformNational Materials Research Base Construction.

References [1] P.O. Larsson, A. Anderson, J. Catal. 179 (1998) 72–89. [2] S.C. Kim, W.G. Shim, Appl. Catal. B: Environ. 98 (2010) 180–185. [3] Y.X. Liu, H.X. Dai, Y.C. Du, J.G. Deng, L. Zhang, Z.X. Zhao, C.T. Au, J. Catal. 287 (2012) 149–160. [4] F. Wang, H.X. Dai, J.G. Deng, G.M. Bai, K.M. Ji, Y.X. Liu, Environ. Sci. Technol. 46 (2012) 4034−4041. [5] C.H. Zhang, Y.L. Guo, Y. Guo, G.Z. Lu, A. Boreave, L. Retailleau, A. Baylet, A. G. Fendler, Appl. Catal. B: Environ. 148–149 (2014) 490–498. [6] C.B. Zhang, Y.B. Li, Y.F. Wang, H. He, Environ. Sci. Technol. 48 (2014) 5816−5822. [7] L.H. Nie, J.G. Yu, X.Y. Li, B. Cheng, G. Liu, M. Jaroniec, Environ. Sci. Technol. 47 (2013) 2777−2783. [8] Q.L. Xu, W.Y. Lei, X.Y. Li, X.Y. Qi, J.G. Yu, G. Liu, J.L. Wang, P.Y. Zhang, Environ. Sci. Technol. 48 (2014) 9702–9708. [9] P. Li, C. He, J. Cheng, C.Y. Ma, B.J. Dou, Z.P. Hao, Appl. Catal. B: Environ. 101 (2011) 570–579. [10] M.I.B. Saiman, G.L. Brett, R. Tiruvalam, M.M. Forde, K. Sharples, A. Thetford, R.L. Jenkins, N. Dimitratos, J.A. Lopez-Sanchez, D.M. Murphy, D. Bethell, D.J.

23

Willock, S.H. Taylor, D.W. Knight, C.J. Kiely, G.J. Hutchings, Angew. Chem. 124 (2012) 6083–6087. [11] A.K. Sinha, K. Suzuki, M. Takahara, H. Azuma, T. Nonaka, K. Fukumoto, Angew. Chem. Int. Ed. 46 (2007) 28912894. [12] Y.X. Liu, H.X. Dai, J.G. Deng, X.W. Li, Y. Wang, H. Arandiyan, S.H. Xie, H.G.Yang, G.S. Guo, J. Catal. 305 (2013) 146–153. [13] Y.W. Chen, D.S. Lee, J. Nanosci. Nanotechnol. 13 (2013) 2171−2178. [14] S.S.T. Bastos, S.A.C. Carabineiro, J.J.M. Órfão, M.F.R. Pereira, J.J. Delgado, J.L. Figueiredo, Catal. Today 180 (2012) 148−154. [15] S.A.C. Carabineiro, X. Chen, O. Martynyuk, N. Bogdanchikova, M. AvalosBorja, A. Pestryakov, P.B. Tavares, J.J.M. Órfão, M.F.R. Pereira, J.L. Figueiredo, Catal. Today 244 (2015) 103−114. [16] Y.W. Chen, D.S. Lee, J. Taiwan Inst. Chem. Eng. 44 (2013) 40–44. [17] M. Hosseini, S. Siffert, R. Cousin, A. Aboukaïs, Z. Hadj-Sadok, B.L. Su, Comptes Rendus Chimie 12 (2009) 654–659. [18] J.K. Edwards, J.B. Solsona, E.N. Ntainjua, A.F. Carley, A.A. Herzing, C.J. Kiely, G.J. Hutchings, Science 323 (2009) 1037–1041. [19] J. Pritchard, L. Kesavan, M. Piccinini, Q. He, R. Tiruvalam, N. Dimitratos, J.A. Lopez-Sanchez, A.F. Carley, J.K. Edwards, C.J. Kiely, G.J. Hutchings, Langmuir 26 (2010) 16568–16577. [20] J.G. Xu, A.R. Wilson, A.R. Rathmell, J. Howe, M.F. Chi, B.J. Wiley, ACS Nano 5 (2011) 6119−6127.

24

[21] Y.W. Lee, M.J. Kim, Z.H. Kim, S.W. Han, J. Am. Chem. Soc. 131 (2009) 17036–17037. [22] L.F. Zhang, S.L. Zhong, A.W. Xu, Angew. Chem. 125 (2013) 673–677. [23] G.J. Hutchings, C.J. Kiely, Accounts Chem. Res. 46 (2013) 1759–1772. [24] S.H. Xie, H.X. Dai, J.G. Deng, Y.C. Du, H.G. Yang, W. Han, H. Arandiyan, G.S. Guo, J. Hazard. Mater. 279 (2014) 392–401. [25] S.H. Xie, J.G. Deng, S.M. Zang, H.G. Yang, G.S. Guo, H. Arandiyan, H.X. Dai, J. Catal. 322 (2015) 38–48. [26] Z. Abbasi, M. Haghighi, E. Fatehifar, S. Saedy, J. Hazard. Mater. 186 (2011) 1445–1454. [27] W.G. Shim, J.W. Lee, S.C. Kim, Appl. Catal. B 84 (2008) 133–141. [28] H. Wu, L. Wang, Z. Shen, J. Zhao, J. Mol. Catal. A 351 (2011) 188–195. [29] Y.X. Liu, H.X. Dai, J.G. Deng, S.H. Xie, H.G. Yang, W. Tan, W. Han, Y. Jiang, G.S. Guo, J. Catal. 309 (2014) 408–418. [30] P.B. Weisz, C.D. Prater, Adv. Catal. Relat. Subj. 6 (1954) 143–196. [31] C.T. Wong, A.Z. Abdullah, S. Bhatia, J. Hazard. Mater. 157 (2008) 480–489. [32] M. Alifanti, M. Florea, S. Somacescu, V.I. Pârvulescu, Appl. Catal. B 60 (2005) 33–39. [33] Y.C. Wei, J. Liu, Z. Zhao, Y.S. Chen, C.M. Xu, A.J. Duan, G.Y. Jiang, H. He, Angew. Chem. Int. Ed. 50 (2011) 2326–2329. [34] L.C. Wang, X.S. Huang, Q. Liu, Y.M. Liu, Y. Cao, H.Y. He, K.N. Fan, J.H. Zhuang, J. Catal. 259 (2008) 66–74.

25

[35] R. Kikuchi, S. Maeda, K. Sasaki, S. Wennerström, K. Eguchi, Appl. Catal. A 232 (2002) 23–28. [36] V. Di Castro, G. Polzonetti, J. Electron. Spectrosc. Relat. Phenom. 48 (1989) 117–123. [37] M. Piumetti, D. Fino, N. Russo, Appl. Catal. B 163 (2015) 277–287. [38] H. Arandiyan, H.X. Dai, J.G. Deng, Y.X. Liu, B.Y. Bai, Y. Wang, X.W. Li, S.H. Xie, J.H. Li, J. Catal. 307 (2013) 327–339. [39] S. Rousseau, S. Loridant, P. Delichere, A. Boreave, J.P. Deloume, P. Vernoux, Appl. Catal. B 88 (2009) 438–447. [40] H. Falcón, J.A. Barbero, J.A. Alonso, M.J. Martínez-Lope, J.L.G. Fierro, Chem. Mater. 14 (2002) 23252333. [41] S.Y. Huang, C. B. Zhang, H. He, J. Environ. Sci. 25 (2013) 1206–1212. [42] K.R. Priolkar, P. Bera, P.R. Sarode, M.S. Hegde, S. Emura, R. Kumashiro, N.P. Lalla, Chem. Mater. 14 (2002) 2120–2128. [43] D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362–365. [44] W. Tan, J.G. Deng, S.H. Xie, H.G. Yang, Y. Jiang, G.S. Guo, H.X. Dai, Nanoscale 7 (2015) 85108523. [45] N. Ta, J.Y. Liu, S. Chenna, P.A. Crozier, Y. Li, A. Chen, W.J. Shen, J. Am. Chem. Soc. 134 (2012) 20585−20588. [46] C.R. Chang, Y.G. Wang, J. Li, Nano Res. 4 (2011) 131−142.

26

[47] C.R. Chang, X.F. Yang, B. Long, J. Li, ACS Catal. 3 (2013) 1693−1699. [48] X. Li, L.J. Wang, Q.B. Xia, Z.M. Liu, Z. Li, Catal. Commun. 14 (2011) 15–19.

27

28

Table 1. Macropore diameters, Mn2O3 particle sizes (DMn2O3), mean noble metal particle sizes, and real noble metal contents of the samples.



DMn O b (nm) 87

Mean particle sizec (nm) 2.7

Real noble metal Pd/Au molar ratiod contentd (wt%) (mol/mol) 3.7 0.87

7.2



86

2.8

3.6

1.96

3.8AuPd3.76/Bulk Mn2O3

7.4



87

3.2

3.8

3.76

3DOM Mn2O3

37.7

160170

48







1.9Au/3DOM Mn2O3

36.2

160170

45

3.3

1.9

0

1.0AuPd1.85/3DOM Mn2O3

36.6

150160

47

2.7

1.0

1.85

1.9AuPd1.92/3DOM Mn2O3

34.5

150160

47

2.7

1.9

1.92

3.8AuPd1.92/3DOM Mn2O3

35.2

145155

45

2.9

3.8

1.92

1.9Pd/3DOM Mn2O3

35.3

155165

52

3.7

1.9



1.9Au&1.9Pd/3DOM Mn2O3

34.8

160170

46

3.7

3.8

1.00

Macropore diametera (nm)

3.7AuPd0.87/Bulk Mn2O3

BET surface area (m2/g) 7.5

3.6AuPd1.96/Bulk Mn2O3

Sample

2

3

a

Estimated according to the SEM images;

b

The data were determined based on the XRD results according to the Scherrer equation using the FWHM of the (222) line of Mn2O3;

c

The data were estimated according to the TEM images;

d

The data were determined by the ICPAES technique.

29

Table 2. Catalytic activities, TOF, reaction rates, and apparent activation energies (Ea) at SV = 40,000 mL/(g h) of the samples. Catalytic activity

Toluene oxidation at 140 oC

T10% (oC)

T50% ( oC ) T90% ( oC )

TOFnoble metal (× 103 s1)

TOFMn2O3 (× 103 s1)

3DOM Mn2O3

257

278

297

﹣

1.9Au/3DOM Mn2O3

221

242

258

1.0AuPd1.85/3DOM Mn2O3

141

178

1.9AuPd1.92/3DOM Mn2O3

115

3.8AuPd1.92/3DOM Mn2O3

Reaction rate at 160 oC (× 106 mol/(gnoble metal s))

Ea (kJ/mol)

﹣

﹣

98

﹣

﹣

﹣

73

209

0.56

0.0065

7.93

42

151

184

0.90

0.020

15.9

34

96

146

162

0.55

0.025

10.5

26

1.9Pd/3DOM Mn2O3

160

217

240

0.15

0.0043

2.32

43

3.6AuPd1.96/Bulk Mn2O3

159

188

210

0.12

0.0052

1.64

43

1.9Au&1.9Pd/3DOM Mn2O3

132

166

203

0.21

0.0097

4.18

40

Sample

30

Table 3. Oxygen desorption, surface element compositions, and H2 consumption of the samples. Sample 3DOM Mn2O3 1.9Au/3DOM Mn2O3 1.0AuPd1.85/3DOM Mn2O3 1.9AuPd1.92/3DOM Mn2O3 3.8AuPd1.92/3DOM Mn2O3 1.9Pd/3DOM Mn2O3 3.6AuPd1.96/Bulk Mn2O3

O2 desorption a (mmol/g) 50550 oC 0.31 0.49 0.83 1.05 1.14 0.64 0.75

550850 oC 2.95 2.61 2.72 2.66 2.53 2.62 2.56

Surface element composition b Oads/Olatt molar ratio

Mn3+/Mn2+ molar ratio

Auδ+/Au0 molar ratio

Pd2+/Pd0 molar ratio

0.46 0.59 0.65 0.67 0.74 0.63 0.65

0.93 0.89 0.90 0.86 0.87 0.88 0.63

 0.43 0.64 0.76 0.79  0.90

  1.15 1.21 1.72 0.97 0.61

a

The data were calculated by quantitatively analyzing the O2-TPD profiles of the samples; The data were estimated by quantitatively analyzing the XPS spectra of the samples; c The data were estimated by quantitatively analyzing the H2-TPR profiles of the samples. b

Table 4. Surface element compositions of the fresh and used samples.

31

H2 consumption c (mmol/g) 16.9 15.8 16.3 15.4 15.1 16.7 14.9

Surface element composition a Sample Oads/Olatt molar ratio

Mn3+/Mn2+ molar ratio

Auδ+/Au0 molar ratio

Pd2+/Pd0 molar ratio

Fresh 1.9Au/3DOM Mn2O3

0.59

0.89

0.48



Used 1.9Au/3DOM Mn2O3

0.49

0.91

0.32



Fresh 1.9Pd/3DOM Mn2O3

0.63

0.88



0.97

Used 1.9Pd/3DOM Mn2O3

0.51

0.90



0.30

Fresh 3.8AuPd1.92/3DOM Mn2O3

0.74

0.87

0.79

1.72

Used 3.8AuPd1.92/3DOM Mn2O3

0.72

0.86

0.80

1.64

a

The data were estimated by quantitatively analyzing the XPS spectra of the samples.

32

(a)

(b)

(c)

(d)

(f)

(g)

(h)

Au

Pd

(a)

(e)

Pd

Fig. 1. HAADF-STEM and elemental scanning images of the 3.8AuPd1.92/3DOM Mn2O3 sample.

33

Au

100

0.48

100

0.48

(B) 0.42 80 0.36

 1.9Au&1.9Pd/3DOM Mn2O3

60

 1.9AuPd1.92/3DOM Mn2O3

0.30

Solid curves: conversion Dotted curves: reaction rate

0.24

40

0.18 0.12

20

Toluene conversion (%)

 1.9Pd/3DOM Mn2O3

Reaction rate (mol/(gcat s))

Toluene conversion (%)

80

60

0.42

3DOM Mn2O3 1.0AuPd1.85/3DOM Mn2O3

0.36

1.9AuPd1.92/3DOM Mn2O3 3.8AuPd1.92/3DOM Mn2O3

0.30

 3.6AuPd1.96/Bulk Mn2O3 0.24

Solid curves: conversion 40 Dotted curves: reaction rate

0.18 0.12

20

0.06 0 0

40

80

120

160

200

240

Reaction rate (mol/(gcat s))

(A) 1.9Au/3DOM Mn2O3

0.06

0.00 280

0 -40

o

0

40

80

120

160

200

240

280

0.00 320

o

Temperature ( C)

Temperature ( C)

Fig. 2. Toluene conversion and reaction rate versus temperature over (A) 1.9Au/3DOM Mn2O3, 1.9Pd/3DOM Mn2O3, 1.9Au&1.9Pd/3DOM Mn2O3, and 1.9AuPd1.92/3DOM Mn2O3, and (B) 3DOM Mn2O3, 1.0AuPd1.85/3DOM Mn2O3, 1.9AuPd1.92/3DOM Mn2O3, 3.8AuPd1.92/3DOM Mn2O3, and 3.6AuPd1.96/Bulk Mn2O3 at SV = 40,000 mL/(g h).

34

B)

97% 96% 93%

80

3.8AuPd1.92

o

 3.8AuPd1.92/3DOM Mn2O3 at 260 C

Toluene conversion (%)

70

100

89%

/3DOM Mn2O3

SV = 120,000 mL/(g h)

80

100

60 80

2nd run

1st run

3rd run

40 60 o

20

1.9Pd/3DOM Mn2O3 at 250 C SV = 40,000 mL/(g h) 100

2nd run

Ca lc

1st run

90

3rd run o

80

 1.9Au/3DOM Mn O at 270 C SV = 40,000 mL/(g h) 2 3

0

10

20

30

40

50

60

On-stream reaction time (h)

18 0

esh Fr

ina

tio 300 nt 0 em 50 pe ra tu re (

14 70 o

C)

0

10 0 R

cti ea

0

on

r pe m te

0 o

( re u t a

ersion (%)

90

Toluene con v

A)

C)

Fig. 3. (A) Toluene conversion over 1.9Au/3DOM Mn2O3 at 270 oC, 1.9Pd/3DOM Mn2O3 at 250 oC, and 3.8AuPd1.92/3DOM Mn2O3 at 260 oC within 60 h of on-stream toluene oxidation under different SV, and (B) toluene conversion versus temperature over the fresh 3.8AuPd1.92/3DOM Mn2O3 sample and the 3.8AuPd1.92/3DOM Mn2O3 sample calcined in N2 at 300, 500, and 700 oC for 3 h, respectively.

35

100

100

(B)

(A)  H2O-free  1.0 vol% H2O

 2.0 vol% H2O

60

Solid curves:

80

 0.5 vol% H2O

Toluene conversion (%)

Toluene conversion (%)

80

 5.0 vol% H2O 40

20

H2O-free

Dotted curves: 1.0 vol% H2O

60

40

20

3.8AuPd1.92/3DOM Mn2O3 0

0 40

60

80

100

120

140

160

180

40

200

80

120

160

200

240

280

o

o

Temperature ( C)

Temperature ( C)

Fig. 4. (A) Effects of water vapor with different concentrations on toluene oxidation over 3.8AuPd 1.92/3DOM Mn2O3, and (B) effect of water vapor on toluene oxidation over (●, ○) 1.9Au/3DOM Mn2O3, (■, □) 1.9Pd/3DOM Mn2O3, and (▲, △) 3.8AuPd1.92/3DOM Mn2O3 at SV = 40,000 mL/(g h).

36

A)

 o



B)

o

772 C

o

o

730 C

744 C

o





98 C



o

98 C

o

o

350 C

O2 desorbed

O2 desorbed

350 C

(e) (c)

(f)

(d)

(b)

50

150

250

350

450

550

650

770 C

(a)

750

850

50

o

150

250

350

450

550

650

750

850

o

Temperature ( C)

Temperature ( C)

Fig. 5. O2-TPD profiles of (a) 3DOM Mn2O3, (b) 1.9Au/3DOM Mn2O3, (c)1.9Pd/3DOM Mn2O3, (d) 1.0AuPd1.85/3DOM Mn2O3, (e) 1.9AuPd1.92/3DOM Mn2O3, and (f) 3.8AuPd1.92/3DOM Mn2O3.

37

(a)

(b)

(c)

2 nm

(d)

(f)

(e)

Interfacial anchoring pattern AuPd alloy NPs

3DOM Mn2O3 support

Fig. 6. (ae) STEM images of 3.8AuPd1.92/3DOM Mn2O3 and (f) schematic illustration of AuPd alloy NPs anchored on 3DOM Mn2O3.

38