Manipulating Gold Spatial Location on Titanium Silicalite-1 To

Engineering Au spatial locations on support harbors tremendous potential to boost catalytic performance but still remains a scientific and technologic...
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Manipulating Gold Spatial Location on Titanium Silicalite-1 to Enhance the Catalytic Performance for Direct Propene Epoxidation with H2 and O2 Xiang Feng, Song Zhaoning, Yibin Liu, Xiaobo Chen, Xin Jin, Wenjuan Yan, Chaohe Yang, Jun Luo, Xinggui Zhou, and De Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02836 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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Manipulating Manipulating Gold Spatial Location on Titanium Titanium Silicaliteilicalite-1 to Enhance the Catalytic Performance for Direct Propene Epoxidation with H2 and O2 Xiang Feng†,‡, Zhaoning Song†, Yibin Liu†, Xiaobo Chen†, Xin Jin†, Wenjuan Yan†, Chaohe Yang*,†, Jun Luoǁ, Xinggui Zhou*,§ and De Chen*,‡ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, China. Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway § State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, China ǁ Center for Electron Microscopy, Tianjin University of Technology, Tianjin 300384, China. ‡

ABSTRACT: Engineering Au spatial locations on support harbours tremendous potential to boost catalytic performance, but still remains a scientific and technological challenge. Herein, we devise a strategy to manipulate Au spatial location inside micropores of TS-1 support by tuning Au hydrolysis process. By multi-techniques (e.g., quench molecular dynamics simulations, aberration-corrected HAADF-STEM, in-situ UV-vis and quantitative model calculation), the correlation between Au dynamic location and average x in [AuClx(OH)4-x]- complexes is established. It is found that compositions and locations of Au complexes can be effectively manipulated using different preparation temperature. This results in more effective Au clusters in TS-1 microporous channels and smaller-sized Au NPs on external surfaces. Gratifyingly, the optimum Au/TS-1 catalyst shows the reported highest initial PO formation rate without adding promoters and high stable activity for direct propene epoxidation with H2 and O2. Moreover, single Au atoms and Au clusters on Au/TS-1 catalyst are directly observed for the first time. However, overmuch Au clusters inside TS-1 only result in diffusion limit and poor performance, indicating that “effectively accommodated Au clusters” play pivotal role. This strategy together with mechanistic results should allow significant progress in the design of efficient Au catalysts in the close future. Key Words: Spatial location, Au complexes, propene epoxidation, effectively Au, deposition-precipitation method.

14, 20, 27-28

■ INTRODUCTION The discovery that suitably designed Au catalyst is catalytically active towards CO oxidation has stimulated extensive 1-2 work on Au catalysts . Since then, well-dispersed Au nanoparticles, clusters and even atoms have been demonstrated 3-4 to be active for a series of reactions, such as CO oxidation , 5-6 6-7 selective hydrogenation , hydrochlorination , H2O2 syn8-9 10-11 thesis and propene epoxidation . It should be noted that these Au atoms, clusters and nanoparticles usually have 10-11 strong Au-support interaction , and their locations on support make Au catalysis much more complex and intriguing. In the past few years, Au spatial location on support is attracting enormous research interests and also poses huge scientific and technological challenges. Take propene epoxidation with H2 and O2 for example, this is a typical structure-sensitive reaction which is also the 12-13 greener, simpler and more sustainable route to synthesize propylene oxide (PO). The catalytic performance is highly relevant to the synergy between Au nanoparticles and isolat4+ ed Ti -containing supports. It is thus reported that the catalytic performance can be significantly affect by Au particle 14-15 11, 13, 16-20 15, 21-22 size , type of support (e.g., TS-1 , Ti-SiO2 , 3d23 24 25 titanosilicate , TS-1@mesoporous silica , Ti-TUD , S-1/TS26 1 ), Au location on the internal or external surfaces of sup10 port , etc. This is mainly because of the unique reaction

which involves the synergy between Au mechanism and Ti-containing supports. Therefore, this special requirement makes most of Au/Ti-containing catalysts for propene epoxidation prepared by deposition-precipitation (DP) 14, 17-20, 23-25, 29-38 method , which could change the Au spatial location (including internal/external surfaces and Ti/Si sites). DP method is one of the most widely employed methods 39to prepare well-dispersed Au nanoparticles in Au catalysis 43 . By adjusting pH values of preparation process, the support shows distinct electric charges based on the isoelectric point 39 of support , and [AuClx(OH)4-x] could be selectively deposited close to the active titanium sites rather than the inactive silicon sites when the pH of solution is higher than the isoe20 lectric point of silicon sites . Besides the location of Au near Ti species, the Au spatial location also includes Au inside or 10, 20 outside microporous channels of TS-1 support . This usually leads to significantly different catalytic activity and stability. However, few works have reported the strategy to change Au spatial location inside or outside TS-1 microporous channels. There is also no answer on detailed manipulation mechanism and whether it is better to make as much as Au inside micropores. Moreover, it is also highly desirable to develop novel strategy to tune Au spatial location, and also to elucidate the intrinsic interaction between

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Au spatial location and performance, which will undoubtedly be beneficial to the rational design of effective Au catalysts. Herein, we envision a convergent strategy to manipulate the spatial location of Au inside or outside TS-1 micropores. The correlation between average x in [AuClx(OH)4-x] complexes and dynamic location of the Au on the TS-1 support is established for the first time by in-situ UV-vis, aberrationcorrected HAADF-STEM, quench molecular dynamics simulations together with quantitative model calculation. Moreover, the underlying structure-performance relationship for propene epoxidation with H2 and O2 is also elucidated. It is o found that lower temperature (i.e., 5 C) leads to slower hydrolysis rate and different compositions of Au complexes, resulting in more highly active tiny Au complexes on the internal TS-1 surfaces and smaller-sized Au nanoparticles on external surfaces. For the first time, single Au atoms and Au clusters on Au/TS-1 catalyst are directly observed. Gratifyingly, the catalyst exhibits the reported highest rate without adding promoters. The catalyst also has higher stable activity. Moreover, the concept of “effectively accommodated Au clusters” inside TS-1 is proposed and the strategy to calculate Au distribution is provided. The insights reported here not only unravel the structure-performance relationship both experimentally and theoretically, but also shed new light on the synthesis of efficient Au catalysts by DP method for a variety of reactions in Au catalysis.

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absorption spectroscopy values of Au-Cl (2.26 Å) and Au-O 49 (1.98 Å) . The size of Au complex (R) is equal to the length

Figure 1 UV-vis spectra of Au complexes at different pH values (a) and size/structure of Au complexes determined by Quench Molecular Dynamics Simulations (b).

■ RESULTS AND DISCUSSION Evolution of Au complexes during depositionprecipitation process. For the deposition-precipitation process of Au precursor (i.e., chloroauric acid), the involved chemistry is mainly about the hydrolysis of [AuCl4] with water to form different Au complexes. Therefore, it can be assumed that at different preparation temperatures, the hydrolysis rate may be different. This may lead to distinct compositions of Au complexes. After reduction, the Au size distribution and location on internal or external surfaces of TS-1 support may also be different. Therefore, the Au hydrolysis process during deposition-precipitation method is first investigated using in-situ UV-vis spectroscopy. As shown in Figure 1a, two UV-vis bands p → d and p → d related to σ

x2 − y 2

π

x2 − y 2

the ligand-to-metal charge transfer transitions from chlorine p to gold d orbitals show up at ca. 240 and 310 nm for low pH 44-45 value of 2.2, respectively . In the hydrolysis progress, there is the replacement of -Cl by -OH as described in equations 14, leading to the blue-shift of both bands and reduction in 46 band intensity, in accordance with finding of Baatz . At each pH, different compositions of Au complexes are shown in 47-48 Table S1 according to literature . (1) [ AuCl4 ] + H 2 O →  AuCl3 ( OH ) + H + +Cl−  AuCl3 ( OH )  + H 2 O →  AuCl 2 ( OH )2  + H + +Cl −

(2)

 AuCl 2 ( OH )2  + H 2 O →  AuCl ( OH )3  + H + + Cl-

(3)

 AuCl ( OH )3  + H 2 O →  Au ( OH ) 4  + H + + Cl-

(4)

-

-

-

-

-

-

Figure 1b illustrates the calculated sizes and lowest energy configurations of [AuClx(OH)4-x] by quench molecular 39 dynamic simulation according our previous work . The calculated bond lengths of Au-Cl and Au-O are 2.26 Å and 1.88 Å, respectively, which are quite similar to the reported X-ray

Figure 2 In-situ UV-vis spectra of Au complexes at different o temperature of 5 (a), 28 (b) and 50 C (c); Relation between peak location and calculated average x in [AuClx(OH)4-x] (d); Peak location at different aging time (e). between two furthest A and B atoms (RAB) plus the covalent radii of A (rA) and B (rB) atoms, respectively. The calculated size of [AuCl4] is 6.51 Å, slightly larger than the 6.50 Å of [AuCl3(OH)] and [AuCl2(OH)2] . The sizes of [AuCl(OH)3] and [Au(OH)4] are 5.81 and 4.90 Å, respectively. Therefore, the change of Au complexes sizes is as follows: [AuCl4] > [AuCl3(OH)] = [AuCl2(OH)2] > [AuCl(OH)3] > [Au(OH)4] . Although the size of Au complex are slightly larger than 0.55 nm, the Au complexes could still enter inside the micropores of TS-1 because molecular size can show stretching and 50-51 bending vibrations . In general, the more extensively hydrolyzed [AuClx(OH)4-x] with smaller x has higher ability to enter into the micropores of TS-1 due to smaller size and steric hindrance. Figure 2(a-c) show the in-situ UV-vis spectra of Au complexes as a function of aging time at various preparation time, and the peak locations at three different temperature are summerized in Figure 2e. At 0 h, the Au complexes is

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already mixed with water for 30 min, which is mainly designed to represent the real real deposition-precipitation process. From the spectra at different pH (Figure 1a) and detailed Au compositions at different pH in Table S1, the average x of Au complexes at different temperature can be calculated, as shown in Figure 2d, e. It is thus quite clear that the hydrolysis is more extensive at higher temperature. For o the hydrolysis of Au complexes at 28 C (Figure 2b), there is a slow blue-shift of the main p → d band in the initial 18 h σ

9 h, indicating the high loading rate of Au complexes originates from the intense hydrolysis or possibly the fast formation of Au NPs. In comparison, the Au loading shows a volcanic-shape relationship with aging time for both Au/TS-

x2 − y 2

and the band keeps slightly changed afterwards. This shows that the hydrolysis is a slow process and long time of ca. 18 h is still not enough to reach hydrolysis equilibrium. In o comparison, the hydrolysis process at 5 C (Figure 2a) is much o slower than that at 28 C. After 44h, the blue-shift of peak o location is still not comparable to that at 28 C, indicating that the Au complexes are still not extensively hydrolyzed. Moreover, it is noted that the peak location is almost unchanged from 4 to 30h, which suggests that the composition of Au complex is not greatly changed due to slow hydrolysis at low temperature. In contrast, the o hydrolysis at 50 C is much more intense, because only 1h is needed to reach the extent of blue-shift in comparison to the o hydrolysis at 28 C. After 1 h, the peak location is almost unchanged, suggesting that the euqlibrium is almost reached. Manipulation of Au location on Au/TS-1 catalyst. It is already known from literature that Au complexes with differ52 ent sizes have distinct affinities to the support and thus the abilities of Au complexes into the micropores of TS-1 support are different. In addition, at different hydrolysis temperatures, the growth of Au nanoparticles may also be different. Therefore, the physico-chemical properties of Au/TS-1 at different preparation conditions are subsequently investigated. Figure S1a shows the XRD pattern of TS-1. The presence o of a single peak at 24.5 indicates the presence of orthoo o rhombic unit cell. The XRD peaks at 2-theta of 7.9 , 8.8 , o o o 23.1 , 23.9 and 24.5 with high intensity not only coincide with the typical MFI structure of TS-1, but also show high 43 crystallinity of TS-1 . Figure S1b displays the typical type I 53 isotherms according to the IUPAC classification . The pore size distribution in the inset of Figure S1b shows that the micropore size of TS-1 is ca. 0.55 nm. With respect to the use of TS-1 in epoxidation reaction, the coordination states of Ti species are one of the most essential parameters. Figure S1c illustrates the FT-IR spectrum of TS-1. The characteristic -1 band of framework Ti species at 960 cm indicates the incorporation of titanium into MFI framework. The adsorption -1 bands at 550 and 1230 cm are regarded as vibration of the double five-membered ring unit and asymmetrical stretching 54-55 vibration of MFI framework structure, respectively , confirming the XRD results. Figure S1d shows the UV-vis spectrum of TS-1, and the sharp adsorption peak at 220 nm is the 24+ charge transfer from O to Ti and is ascribed to isolated 53, 56-57 4+ tetrahedally coordinated Ti . This type of Ti is indis14, 20, 28, 39 pensable for propene epoxidation . No other adsorption peaks show up at 260 and 330 nm, demonstrating the absence of octahedrally coordinated Ti and anatase phase, respectively. Au/TS-1 catalysts are then prepared by depositionprecipitation method at different preparation temperature. Figure 3a shows the Au loading as a function of slurry aging time. For Au/TS-1-50C catalyst, Au loading increases from 0.10 to 1.8wt% with the increase of slurry aging time from 1 to

Figure 3 Au loading (a) and PO formation rate (b) of Au/TS-1 catalysts prepared at different temperature as a function of aging time. 0.120 wt%. Compared with Au/TS-1-50C catalyst, longer aging time is required for Au/TS-1-28C to get the same Au loading. The reduction of Au loading after 15h aging time can be originated from the lower affinity between Au complexes and 48, 52 TS-1, as reported by Nechayev et al . It is demonstrated in Figure 2e that the Au composition has more extensively hy- 48, 52 drolyzed Au complexes (e.g., [Au(OH)4] ) , which have smaller adsorption constant and could be easily removed from TS-1 under vigorous stirring. At lower temperature of o 5 C, the slowest loading rate is observed and Au loading reaches the highest value of 0.15wt% at 28 h and then decreases afterwards. During the initial 28h, the Au complexes do not change greatly, as confirmed by in-situ UV-vis (Figure 2). However, it can also be seen from Figure 2 that the composition of Au complexes greatly changes after 28h. This leads to the decrease of Au loading on TS-1 support from 0.15 to 0.05wt%. From the above analysis, it can be inferred that the process of Au NPs formation on TS-1 support involves hydrolysis, diffusion, adsorption and aggregation to form the clusters or NPs. The Au loading should be affected by the competition between adsorption rate and removal rate of Au complexes, which are affected by hydrolysis temperature and adsorption ability of Au complexes, respectively. At high o temperature of 50 C, intense hydrolysis dominates, and the interaction between Au complex and support is quite strong, resulting in larger Au particle size of 4.5 nm at aging time of 9 h. In contrast, at medium and low temperature, the change of Au adsorption ability together with lower Au loading rate lead to the volcanic-shape relationship between Au loading and slurry aging time. Figure 3b shows the catalytic performance of Au/TS-1 catalysts at different preparation temperatures. The linear decrease of PO formation rate is observed for Au/TS-1-50C catalyst, nevertheless, volcanic-shape curves are shown for Au/TS-1-5C and Au/TS-1-28C catalysts. Interestingly, the PO formation rate does not change monotonically with Au loading because the optimum PO formation rate for all catalysts have the same loading of 0.10wt%. The highest PO formation rate for propene epoxidation without adding -1 -1 promoters (e.g., Cs) was ca. 160gPOh kgCat , as reported by 17 Delgass . However, the Au/TS-1-5C catalyst exhibits superior -1 -1 performance of 205 gPOh kgCat . This enhanced catalytic performance could be mainly affected by the physicochemical properties of Au nanoparticle and TS-1 support. TS-1

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supports are the same for all of Au catalysts. Therefore, the only difference is related to Au and Au-Ti synergy.

N2 physisorption is subsequently employed to study the

Au clusters inside TS-1. As shown in Table S2, the micropore volume of Au/TS-1-5C catalyst reduces from 0.138 to 0.117 3 -1 cm g with the increase of aging time from 9.5 to 28 h. In this period, the composition of Au complexes is not greatly changed (Figure 2e), and the Au complexes can gradually enter into the micropores of TS-1, resulting in the reduction of micropore volume. It is also noted that the PO formation rate -1 -1 (gPOh gAu ) of Au/TS-1-5C catalysts are almost the same in the same period, indicating that the Au distribution may be

because the slow hydrolysis leads to retarded growth rate of Au crystal.

the same due to the similar Au complexes. From 28 to 36 h, 3 -1 the micropore volume stops reducing, but rises to 0.134 cm g . This is consistent with the in-situ UV-vis results (Figure 2) 48 and the finding of Nechayev et al. that extensively hydrolyzed Au complexes after a longer aging time could have smaller adsorption constant and be more easily removed from TS-1 under vigorous stirring. Similar phenomenon is also observed for Au/TS-1-28C catalyst. The decreasing trend followed by increasing micropore volume during the 24 h indicates that more extensively hydrolyzed Au complexes first enter into and then move out of the micropores of TS-1. The size of Au nanoparticles of Au/TS-1 catalysts at different preparation temperature are then investigated. The observable Au nanoparticle size distribution and average particle size of Au/TS-1-28C by HAADF-STEM are shown in Figure S2. The average Au nanoparticle size is determined as 3.0 nm. It should be noted that the tiny Au clusters inside the

Figure 4 Aberration-corrected HAADF-STEM images of Au/TS-1-5C catalyst with Au single atoms (a), clusters and nanoparticles (b). The insets shows the particle size distribution and the scale bar represents 5 nm. microporous channels of TS-1 (ca. 0.55 nm) is still not well determined. The aberration-corrected high-angle annular, dark-field scanning transmission with much lower detection 58 limit (ca. 0.07 nm) should be the best choice. Therefore, we further use the aberration-corrected HAADF-STEM characterizations (Figure 4). Notably, there indeed exist the Au single atoms (0.2-0.3 nm) and Au clusters. Combined with the decrease of microporous volume, it can be demonstrated that the Au single atoms and clusters can be confined inside micropores of TS-1 (ca. 0.6 nm). Since 1998, this is the first direct evidence that these Au species exist on TS-1 catalysts by DP method. Besides this Au catalyst, the averaged Au nanoparticle sizes of all catalysts are shown in Table S2. There is a gradual increase of observable Au particle size with the rise of aging time. With similar Au loadings, the Au/TS-1 catalyst prepared at lower DP temperature tends to have smaller observable Au nanoparticle size. This is possibly

Figure 5 PO formation rate as a function of average x in o o [AuClx(OH)4-x] at final aging time (Blue: 50 C; Green: 28 C; o Red: 5 C. The different triangles represent different locations). Figure 5 shows the relationship between PO formation rate and average x in [AuClx(OH)4-x] . When the Au complexes are the same (x of ca. 0.14 or 1.2), the PO formation rate greatly changes. This offers another evidence that Au location is indispensable for the activity. In addition, different Au complexes may also lead to distinct catalytic performance and [AuClx(OH)4-x] with x of 1.2 seems to show highest performance during these catalysts. In order to better analyze the interaction between Au location and catalytic performance, we introduce two parameters, i.e., Vau and Vna, which are the volume of gold and the volume not accessible to probe molecules (N2). The volume of gold is the weight of gold (i.e., Au loading%*0.15g) divided by the 3 gold density (19.3cm /g). The volume not accessible to N2 is the difference between the micropore volumes of TS-1 support and Au/TS-1 catalyst. If the volume of gold is equal to the volume not accessible to N2, the gold should only cover the sites on TS-1 support and do not lead to blocking phenomenon. In this circumstance, Vau/Vna=1. In contrast, if the volume of gold is smaller than the volume not accessible to N2, some of microporous channels should be blocked by Au nanoclusters, and Vau/Vna Au/TS-1-28C ≥ Au/TS-1-50C. Moreover, the catalytic selectivity for each catalyst is shown in Figure 9. The PO selectivities for Au/TS-1-5C, Au/TS-1-28C are similar. In comparison, the Au/TS-1-50C catalyst has slightly lower PO selectivity, possibly due to larger particle size. The H2 efficiency for Au/TS-1-5C, Au/TS-1-28C and Au/TS-1-50C catalysts are 26, 22 and 20%, respectively. 39 This could be possible due to size effect .

To sum up, we first devise a new route for the synthesis of effective Au catalyst by low-temperature depositionprecipitation (DP) method, and then elucidate by both theoretical (quench molecular dynamics simulations, model calculation) and experimental (in-situ UV-vis, HAADF-STEM, etc.) methods that temperature mainly affect the Au spatial location and also Au particle size. The process of Au NPs formation on TS-1 support involves hydrolysis, diffusion, adsorption and aggregation to form the clusters or NPs. The correlation between average x in [AuClx(OH)4-x] and aging time is established. At lower preparation temperature (i.e., o 5 C), more Au complexes tend to enter into the micropores of TS-1, and Au nanoparticles on external surfaces of TS-1 are better dispersed (ca. 2.4 nm). As a consequence, the resulted o Au/TS-1 catalyst prepared at 5 C shows greatly enhanced PO -1 -1 formation rate of 205 gPOh kgCat which is the reported best Au catalysts without adding promoters. For the first time, single Au atoms and Au clusters on Au/TS-1 catalyst are directly observed, offering a strong evidence that Au could reside into the micropores of TS-1. In addition, effectively

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accommodated Au clusters inside TS-1 are critical for PO synthesis because too much Au clusters inside TS-1 only result in pore diffusion resistance and reduced catalytic performance. Although preparation temperature can tune the spatial location of Au clusters, more Au species are still on the external surfaces. The stable PO formation also follows the Au size-dependent activity. These findings not only offer a promising avenue for the rational design of highly efficient Au catalysts for propene epoxidation, but also show considerable influence on improvement of deposition-precipitation method in the fields of Au catalysis.

■ EXPERIMENTAL SECTION Synthesis of TS-1 supports and different Au/TS-1 catalysts. Microporous titanium silicalite-1 (TS-1) supports with Si/Ti molar ratio of 100 was synthesized according to classical 17, 43 hydrothermal method . In a typical process, 3.5 g tween20 was added to 45 mL deionized water, and the solution was added to the mixture of 44.8 g tetrapropylammonium hydroxide (TPAOH, 25 wt%) and 66.4 g tetraethylorthosilicate o (TEOS, 95 wt%). The solution was stirred at 28 C for more than 30 min. Subsequently, titanium (IV) tetrabutoxide (TBOT, 99 wt%) dissolved in 20 mL isopropanol (99.5 wt%) was then added drop-wise to avoid the formation of TiO2. The final solution containing template, Ti and Si source was further stirred for more than 1 h and crystallized in a Teflon o autoclave at 170 C for more than 18 h. The as-obtained uncalcined TS-1 was thoroughly washed and dried overnight at o room temperature. This is followed by calcination at 550 C for 5.5 h. Au/TS-1 catalysts were prepared by the depositionprecipitation (DP) method as outlined in our previous re14, 20, 28 ports with different preparation temperatures. 0.5 g TS-1 was first mixed with 0.1 g HAuCl4⋅3H2O and 50 mL H2O o for 30 min at different temperatures (i.e., 5, 28, 50 C). The pH of Au precursor and TS-1 slurry was adjusted to 7.3-7.5 by 1 M and 0.1 M NaOH and then aged for different hours to tune Au loadings. The solid was then centrifuged, washed o and dried at 28 C under vacuum. The as-prepared Au/TS-1 o catalyst prepared at x C is denoted as Au/TS-1-xC catalyst. For example, Au/TS-1-5C represents Au/TS-1 catalyst preo pared at 5 C. Characterization and quench molecular dynamics simulation. X-ray diffraction (XRD, Rigaku D/Max2550VB/PC, Cu Kα radiation) was used to characterize the crystal phases of the TS-1 supports. In situ ultraviolet-visable spectroscopy was recorded on a spectrometer (AvaSpec-2048) equipped with a transmission dip probe in order to determine the Au complexes. N2 physisorption (Micromeritics ASAP 2020) was taken to measure the pore diameters and pore volumes of the Au/TS-1 catalysts prepared at different temperature. Atomic absorption spectroscopy (AAS, ZEEnit 600) was employed to determine weight percentages of elements. Highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed on a Tecnai G2 F20 S-Twin equipped with a digitally processed STEM imaging system. Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (FEI Titan Cubed Themis G2 300) was operated at 200kV. More than 150 nanoparticles were counted to make an accurate Au particle size distribution. X-ray photoelectron spectroscopy (XPS) on a Kratos XSAM-800 instrument using Al Kα X-ray with 1486.6

eV as the excitation source was used to give the surface Au/Si molar ratios. Stable structures of Au complexes with the lowest energy conformation were obtained by quench molecular dynamics simulation using Materials Studio package. The UFF (Universal Force Field) was used and the simulation was conducted at 298 K in the microcanonical ensemble (NVE). The time step and total simulation time were set as 1 fs and 100 ps, respectively. Geometry optimization calculations were performed every 5 ps. Catalytic testing. Direct gas-phase propene epoxidation with H2 and O2 was carried out in a quartz tubular reactor (i.d. 8 mm) using 0.15 g catalyst of 60-80 mesh particle size at 1 bar. The reactor was heated from room temperature to 200 o C with C3H6, H2, O2 and N2 of 10/10/10/70 vol.%, respectively. The feedstock was fixed at a space velocity of 14,000 mLh 1 -1 gcat . The concentration of reactants (C3H6, H2 and O2) and products (PO, CO2, acetaldehyde, acetone, propanal, acrolein and so on) were measured online by two gas chromatographs (Agilent 6890) equipped with TCD (Porapak Q and 5A columns) and FID (Porapak T column) detectors. The carbon balance, conversion and selectivity were defined as follows: Carbon balance = (3×(propene + PO + Propanal + Acetone) + 2×ethanal+CO2) / (3×propene) Propene conversion = moles of (C3-oxygenates + 2/3ethanal + CO2/3)/moles of propylene in the feed. H2 conversion = moles of H2 converted/moles of H2 in the feed. C3-oxygenate selectivity = moles of C3-oxygenate/moles of (C3-oxygenates + 2/3ethanal + CO2/3). Ethanal selectivity = 2/3(mole of ethanal)/moles of (C3oxygenates + 2/3ethanal + CO2/3). H2 efficiency = moles of PO/moles of H2 converted. It should be noted that the CO2 can be well-analyzed in this work, and carbon balance is larger than 95%.

■ AUTHOR INFORMATION Corresponding Author * De Chen: [email protected] * Xinggui Zhou: [email protected] * Chaohe Yang: [email protected]

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Representative HRTEM image of catalysts and the composition of Au complexes at different pH levels. This material is available free of charge via the Internet at http://pubs.acs.org.

■ACKNOWLEDGMENT The present work was financially supported by the Natural Science Foundation of China (21606254), Key research and development plan of Shandong Province (2017GSF17126), Natural Science Foundation of Shandong Province (ZR2016BB16), Fundamental Research Funds for the Central Universities (18CX02014A) and Independent innovation foundation of Qingdao (17-1-1-18-jch).

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