Exceptional Antisintering Gold Nanocatalyst for Diesel Exhaust

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Exceptional anti-sintering gold nanocatalyst for diesel exhaust oxidation Guo-Qing Ren, Yan Tang, Kai-Peng Liu, Yang Su, Shu Miao, Wei Liu, Wei-Min Cong, Xiaodong Wang, Wei-Zhen Li, Jun Li, and Tao Zhang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03003 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

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Exceptional anti-sintering gold nanocatalyst for diesel exhaust oxidation Guo-Qing Ren†, § , #, Yan Tang‡, #, Kai-Peng Liu†, §, Yang Su†, Shu Miao†, Wei Liu†, Wei-Min Cong†, Xiao-Dong Wang†, §, Wei-Zhen Li†, §, *, Jun Li‡, *, Tao Zhang†, § †

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing, 100084, China § University of Chinese Academy of Sciences, Beijing, 100049, China ‡

Supporting Information Placeholder ABSTRACT: The poor thermodynamic stability of gold nanoparticles (NPs) makes it very challenging to stabilize them in small sizes at elevated temperatures. Herein, we report the preparation of anti-sintering Au nanocatalyst by rationally selecting the sublattice matched MgGa2O4 spinel as support based on theoretical predictions. Au/MgGa2O4 retains Au NPs of 2-5 nm even after aging over the melting temperature of bulk gold (1064 ºC)! By identifying the stable structure, the extraordinary stability is found to arise from the formation of a new phase structure, namely AuMgGa2O4 metal-oxide “hetero-bicrystal” that remains as crystallite without melting even at 1100 ºC. More than 80% of the loaded Au can be efficiently stabilized so that the catalysts can exhibit excellent low-temperature activities for diesel exhaust (CO and C3H6) oxidation after severely thermal and hydrothermal aging. These results may pave ways for constructing anti-sintering gold nanocatalysts for industrial applications. KEYWORDS: gold nanocatalyst, anti-sintering, metal-oxide hetero-bicrystal, diesel oxidation catalyst, CO oxidation, propene combustion

Supported gold NPs as heterogeneous catalysts have attracted considerable attention since the discovery of Haruta and Hutchings in 1980s,1, 2 leading to the “gold rush” in catalysis in the past three decades. It is now recognized that gold deposited as NPs, clusters or single atomic species on supports have unique catalytic performance for oxidation, hydrochlorination, hydrogenation, and carbon-carbon coupling reactions, etc.3-6 Considerable Au particle size- and support-sensitivity and specificity and poor thermal stability when comparing to the platinum-groupmetal (PGM) catalysts constitute the bottleneck of current gold catalysts,7-14 which are partly the reasons for the late arrival of heterogeneous gold catalysis and the few commercial use so far.1518 The catalytic CO oxidation at room temperature or below is a

characteristic function of supported gold catalysts, which is highly desired for automotive pollution control as PGM catalysts are not efficient at temperatures below 200 ºC during the “cold start-up” period.18, 19 However, application of the gold catalysts in conjunction with the Pt-based three-way catalysts or diesel oxidation catalysts (DOC) requires substantial improvement in their thermal stability. It remains a challenge to stabilize gold NPs due to the relative low melting point, 1064.2 ºC for bulk and ~330-380 ºC for naked particles of 2 nm in diameter.3,19-21 The readily melting behavior of gold NPs at elevated temperatures makes them sintering via particles migration and coalescence. Au NPs tend to form thermodynamically stable cuboctahedrons with {111} and {100} facets so that the critical issue for stabilizing Au NPs then is to comfort these terminations. Although several strategies have been applied, including enhancing metal-support interaction, confining metal in pores or channels, and encapsulating metal or supported metal by porous oxide shells, limited enhancement of stability is achieved possibly due to little change in the intrinsic metalsupport interactions and in the melting behaviors of small Au NPs in these processes.20-32 Recently, we reported that MgAl2O4 spinel and PGMs such as Pt, Ir and Rh can form epitaxially matched metal-oxide (111) interfaces so as to resist against long-term sintering at high temperature of 800 ºC.33, 34 Unfortunately, gold cannot form stable metal/oxide interfacial structure with MgAl2O4, presumably because the lattice parameter of Au (PDF#04-0784, a=4.0786 Å) is slightly larger than that of oxygen sublattice of MgAl2O4 spinel (PDF#02-1086, ½a=4.0430 Å), in contrast to those for Pt, Ir and Rh. Hence, the MgGa2O4 (PDF#10-0113, ½a=4.1400 Å) that has similar structure to MgAl2O4 but larger oxygen sublattice than Au could be a potential candidate for stabilizing Au in small sizes. In this work, we report that MgGa2O4 is indeed able to stabilize Au NPs at ~3 nm against aging at 800 ºC for more

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Figure 1. Theoretical predictions. (a) Optimized structures of Au NP supported on MgGa2O4(111) and (b) MgAl2O4(111) surfaces. (c) Calculated charge density differences of Au NP supported on MgGa2O4(111) and (d) MgAl2O4(111) surfaces. Color code: gold (yellow), oxygen (red), magnesium (green), gallium (grey), and aluminum (magenta). Yellow and blue areas in (c) and (d) represent charge increase and reduction, respectively (densitydifference isosurfaces equal to 0.006 electrons /Å3). than 28 days. Remarkably, we discover that the stabilized Au NPs can even withstand aging beyond the Au melting point at 1100 ºC! We further disclose that the stabilized Au NPs remain as nanocrystallites attaching on MgGa2O4 without melting even at temperatures over Au melting point. This is due to the formation of a new kind of phase, namely, metal-oxide “hetero-bicrystal” phase which is designated as Au₲MgGa2O4, where “₲” denotes “growing on”. Moreover, we quantify the stabilization efficiency of Au and evaluate its potential as DOC by testing the catalytic performance for CO oxidation and C3H6 combustion. Figure 1 displays the optimized structures of Au/MgGa2O4 and Au/MgAl2O4 by relaxing the initial configurations (Figure S1). Au metallic layers maintained reasonably good face-centered cubic (FCC) structure on the MgGa2O4 (111), but significantly distorted and lost the FCC structure on the MgAl2O4 (111) (Figure 1a, b). The Au-Au distance in Au layer (2.94 Å) is very close to the Ga-Ga distance in Ga layer (2.91 Å), but is much larger than the Al-Al distance in Al layer (2.84 Å), indicating a better lattice match between Au and MgGa2O4. By calculating the charge density differences of Au and supports (Figure 1 c, d), we found that charge transfer from Au layers to oxide supports in both MgGa2O4 and MgAl2O4, suggesting that the interfacial Au atoms

are positively charged. The averaged energy required to break the interfacial Au-O bond for Au/MgGa2O4(111) (1.56 eV) is much larger than that for Au/MgAl2O4(111) (1.29 eV). These results clearly demonstrate that significant enhancement on the thermal stability of Au NPs can be achieved on MgGa2O4 support. Au₲MgGa2O4 samples with Au of 0.47 wt% and 2.91 wt% were prepared by soaking MgGa2O4 support powders into aqueous chlorauric acid followed by filtrating and washing with concentrated ammonia solution to remove chloride ions. The samples were denoted as xAu₲MgGa2O4-T-t, where “x” is weight loading of Au determined by ICP-AES and “T” is calcination temperature and “t” time. The as-prepared samples after drying at 80 ºC without calcination were denoted as xAu₲MgGa2O4-uncal. Au was highly dispersed on the support with particle size smaller than 1 nm for Au₲MgGa2O4-uncal irrespective of Au loadings (Figure S2). The 0.47Au₲MgGa2O4-300ºC-5h presents highly dispersed Au particles in bright with high particle density in the STEM image (Figure 2a). Most of the Au particles are below 2 nm and few in 3~5 nm with a mean size of 1.6 nm and deviation of 0.7 nm. The 0.47Au₲MgGa2O4-800ºC-5h also displays lots of small Au NPs of ~2.7 nm with deviation of 1.0 nm and few of large Au particles of 20~30 nm (Figure 2b and inset). Even after calcining at 1100 ºC for 4 h, dense Au NPs of ~3.6 nm with deviation of 1.1 nm are still evident accompanied by few Au of ~50 nm on the 0.47Au₲MgGa2O4-1100ºC-4h sample (Figure 2c and inset). Further elevating the aging temperatures to 1200 ºC, Au NPs are dominantly of 9.1 nm with minor larger than 60 nm (Figure 2d and inset). With prolonged aging at 800 ºC for 1, 7 and 28 days, small Au NPs of sizes of ~3 nm retain with high particle density on the 0.47Au₲MgGa2O4-800ºC-1day, -800ºC-7days, and 800ºC-28days samples (Figure 2 e, f, and g), indicating that the structures of Au₲MgGa2O4 samples are thermodynamically stable. Similar phenomena are visible on Au₲MgGa2O4 sample with high Au content. Stable small Au NPs are also evident over the 2.91Au₲MgGa2O4 samples after similar aging treatments (Figure S2). For example, the 2.91Au₲MgGa2O4-800ºC-7days displays

Figure 2. Dispersion and thermal stability. (a-g) STEM images for 0.47Au₲MgGa2O4-T-t and (h) 2.91Au₲MgGa2O4-800ºC-7days. Superimposed size statistic histograms are the small-sized branches of bimodal size distributions of Au particles (~200 Au particles were measured) and inset images (b-d) present Au particles in the large-sized branches. Large Au particles in (e-h) are not shown.

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Figure 3. Thermally stable structure and the mechanism. (a) aberration-corrected HAADF-STEM image for the 2.91Au₲MgGa2O41100ºC-5h taken at room temperature (RT). (b-d), an area series and (e-h), a time series of in-situ HRTEM images for the 0.47Au₲MgGa2O4-1100ºC-4h taken at 1100ºC. Insets in (a) are the fast Fourier transform (FFT) images for the Au and MgGa2O4 particles. small Au NPs of 3.1 nm with deviation of 0.9 nm (Figure 2h). of Au could be (1̅ 1 2̅) as illustrated in Figure 3a albeit the real Figure S2 presents more STEM and TEM images showing the packing manners of Au and Ga at the interface remains unclear. small Au NPs in the 0.47Au₲MgGa2O4 and 2.91Au₲MgGa2O4 Notably, the angle for the normal lines for Au(1 1̅ 1̅) and Au(1̅ 1 samples. The size increases of partial Au NPs with aging were 1̅) is 114.5° which is larger than the one for ideal Au FCC strucalso detected by X-Ray diffractions, in line with the STEM obserture of 110° presumably due to the deviation from direcvations on the large Au NPs (Figure S3, Table S1). The formation tion and/or the deformation of Au FCC structure. Interestingly, of large Au NPs suggests that not all the Au NPs can occupy the unlike Pt(111) interacting with MgAl2O4(111),34 it is found that capable surface sites of the support during the initial randomly Au formed interface with MgGa2O4(111) using Au(1̅ 1 2̅) instead dispersing of Au. The surface areas of the Au₲MgGa2O4 samples of Au(111) as expected. The reason would be worthy of disclosing in the future. It is worthy of pointing out that the existence of decrease from 114.7 to 42.3 and 4.9 m2 g-1 with thermal treatAu(111)/MgGa2O4(111) interface cannot be excluded as not all ments at 300 ºC, 800 ºC for 5 h and 1100 ºC for 4 h, and the average crystallite sizes of MgGa2O4 increase from 5.4 to 15.0 stable ensembles had been clearly imaged. and >100 nm (Table S1). There was no detectable Au loss and the Figure 3 b-d display the structures and states of Au NPs of Au content maintained at 0.47 wt% even after aging at 800 ºC for large, medium and small sizes for the 0.47Au₲MgGa2O4-1100ºC7 days. These results are significantly different from the conven4h sample under in-situ heating at 1100 ºC, a temperature which is tional supported Au catalysts. For example, 0.93Au/MgO showed above the melting point of bulk Au (1064 oC). For the Au NPs a typical sintering process as the Au NPs increased from 1.3 to larger than 100 nm without being directly supported by MgGa2O4, 44.6 nm when the calcination temperature elevated from 400 to smoothly round projections with arching contact angles are evi800 ºC for 5 h (Figure S4). Small Au NPs of ~3 nm are very few dent, clearly indicating that the large particles are melted and on the 0.79Au/MgAl2O4-800ºC-5h counterpart while are almost exhibit the intrinsic thermal property of bulk gold (Figure 3b). not visible on the 0.79Au/MgAl2O4-1100ºC-5h (Figure S2, S3). This melting phenomenon of large Au NPs confirms that the inThe remarkable stability of Au₲MgGa2O4 implies the formation situ heating temperature indeed exceeded the melting point temof a special interfacial structure between Au and MgGa2O4. perature of gold bulk since the heating chips have 5% temperature accuracy. For the Au NP with medium size of ~12 nm (Figure 3c), The fine structures of stable Au NPs are disclosed by aberrathe lattice fringes retain clear while half of the edges in its projection-corrected HAADF-STEM images. Figure 3a displays a white tion becomes arching, showing a partial melting of the Au NP in particle and a gray particle located in an edge on structure for the the part far from the interface. Surprisingly, the small Au NPs 2.91Au₲MgGa2O4-1100ºC-5h sample. According to the EDX (2~5 nm) present clear lattice fringes and highly faceted terminaspectra shown in Figure S2, the white particle should be Au and tions, indicating they remain as crystallites with high crystallinity the gray one is MgGa2O4. The projection of the Au particle preeven at temperature over the Au melting point (Figure 3d, S5). sents at least three sets of planes, i.e., (1 1̅ 1̅), (1̅ 1 1̅) and (002) as During tracking the irregular movements of small Au NPs at 1100 labeled in the inset FFT image, indicating that the crystallite is in ºC in 540 seconds (Figure S5), clear fringes of Au NPs appeared zone axis orientation. The MgGa2O4 spinel particle, howin the HRTEM images every now and then when they were occaever, is not in zone axis orientation as only one set of plane of sionally in certain orientations. The images taken at the 0, 30, 180 (111) with Ga layer as surface cations is visible in the projection. and 300 second are zoomed in on a Au NP and shown as Figure 3 The normal lines for the Au(1 1̅ 1) and MgGa2O4(111) have an e-h, in which the fringes distinctly evidence the Au NP to be crysangle of 24°, suggesting that these two planes are not in parallel. tallite with high crystallinity. Obviously, the stabilized Au NPs on It is clear that the Au particle is locating on the surface of MgGa2O4(111) with non-Au(111) surface. The interacting surface

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Figure 4. Stabilization efficiency and catalytic performances. (a) Au mass specific derivate heat flow profiles. (b) Stabilization efficiency and dispersion of gold over the Au₲MgGa2O4 samples. (c) Temperature needed for 50% CO and C3H6 conversion over the 2.91Au₲MgGa2O4 catalysts (CO only: 1 vol% CO and 20 vol% O2 in He; CO + H2O: 1 vol% CO, 20 vol% O2 and 10 vol% H2O in He; C3H6 only: 0.1 vol% C3H6 and 20 vol% O2 in N2; C3H6 + H2O: 0.1vol% C3H6, 20 vol% O2 and 10 vol% H2O in N2; Space velocity: 12,000 mL/(g-h)). The values for the Au₲MgGa2O4-300ºC-5h are displayed at time before 0 h. MgGa2O4 spinel demonstrate thermodynamic properties more and h in flowing air with 10 vol% H2O, the resulted catalyst displayed more far away from the Au bulk with minimizing Au NP sizes. T50 as 102 and 288 ºC for CO oxidation and C3H6 combustion, This change in the melting behavior of the MgGa2O4 stabilized respectively (Figure S8). During five consecutive catalytic runs, Au NPs indicates the formation of a new crystal phase since meltthe 2.91Au₲MgGa2O4 catalysts displayed almost same light-off ing point is a characteristic property of crystalline materials. The profiles for CO oxidation and C3H6 combustion under reaction feature that the FCC Au and the FCC-like oxygen in MgGa2O4 conditions with and without H2O (Figure S9, S10), indicating that spinel sharing an interface is very similar to the characteristic of these catalysts are stable during CO oxidation and C3H6 combusbicrystal in which two grains of same chemical specie (usually tion reactions. However, the 0.79Au/MgAl2O4 and 0.93Au/MgO metal-metal or oxide-oxide) share a twin boundary. Such special displayed significant deactivation upon aging (Figure S7). Conmetal-oxide ensemble could be called as “hetero-bicrystal” and a sidering that the T50 is highly reaction-condition dependent, we specific symbol “₲” is used here to emphasize the proper strucalso calculated the total molar specific reaction rates and turnover ture. Notably, the melting behaviors of Au NPs in the frequencies (TOFs) at various temperatures and compared with Au₲MgGa2O4 ensembles are size or distance dependent as partial those in literatures (Table S3, S4). According to the Arrhenius plots (Figure S11), we found that the activation energies for CO melting of Au NP occurred when its size reaches ~12 nm. oxidation or C3H6 combustion over different Au₲MgGa2O4 cataThe difference in the melting behaviors between stabilized lysts are very close and comparable to the values in literature, and sintered gold NPs is also evidenced by differential scanning presumably due to the similar sizes of Au NPs and the wellcalorimeter (DSC). As shown in Figure 4a, each known size effect (Table S5, S6). Even for the Au₲MgGa2O42.91Au₲MgGa2O4 sample presents only one endothermic peak 1100ºC catalysts, the activation energies for CO oxidation or C3H6 related to the fusion of large Au NPs. The peak temperature shifts combustion are also comparable to those over the Au₲MgGa2O4gradually from 998 ºC to 1027, 1046, 1048 ºC and 1062 ºC and 800ºC catalysts, albeit the T50s are much higher (Table S5, S6, finally to 1063 ºC with aging, presumably due to the size increase Figure S12, S13). In brief, the Au₲MgGa2O4 catalysts have reof sintered Au NPs. By using the 0.93Au/MgO-1200ºC-5h which tained the high catalytic activities due to little changes in Au sizes has only sintered large Au NPs as reference (Figure 4a, Figure S4), even after severely aging. Notably, the low T50s for CO oxidation the amount of sintered Au can be calculated from the peak area and C3H6 combustion over the Au₲MgGa2O4 catalysts makes and the stabilized Au is the rest (Table S2). Figure 4b shows the them highly promising as diesel oxidation catalyst components for stabilization efficiency, i.e., the percentage of stabilized Au to automotive pollution abatement during “cold start-up” period. total Au over MgGa2O4 support. Even after aging at 800 ºC for 7 In conclusion, we have synthesized thermodynamically stadays, the 0.47Au₲MgGa2O4 and 2.91Au₲MgGa2O4 still retained ble Au nanocatalysts by the rational selection of MgGa2O4 spinel stabilization efficiencies higher than 60% and 80%, respectively (Figure S6, Table S2). The 0.79Au/MgAl2O4-800ºC-5h counteras support based on theoretical predictions. We discover that the stabilized Au NPs on the MgGa2O4 support can survive extremely part displayed low stabilization efficiency of only 28%. If assuming the stabilized small Au NPs having hemispherical geometry, severe thermal aging beyond the melting point of Au bulk and the dispersions of Au estimated from the particle size (DAu = 1/Dm identify the formation of metal-oxide hetero-bicrystal phase by Au growing on the MgGa2O4 (111) as the origin. Such stable * stabilization efficiency) are as high as 20% and 26% for the 0.47Au₲MgGa2O4-800ºC-7days and 2.91Au₲MgGa2O4-800ºCAu₲MgGa2O4 hetero-bicrystal catalysts demonstrate extraordi7days, respectively (Figure 4b). The excellent anti-sintering propnary low-temperature activities for CO oxidation and C3H6 comerty of Au₲MgGa2O4 samples makes them very attractive as bustion reaction. These discoveries pave the way for construction of active Au nanocatalyst with high thermal stability and may DOC for CO and C3H6 oxidation. open up the possibilities of transferring nanoscale Au catalysts Based on the light-off profiles for CO oxidation and C3H6 from the laboratory to industry. combustion (Figure S7), we present the temperatures needed for 50% conversion (T50, a key indicator of DOC) over the ASSOCIATED CONTENT 2.91Au₲MgGa2O4 catalysts in Figure 4c. For CO oxidation, the T50s for the 2.91Au₲MgGa2O4-300ºC-5h, -800ºC-5h, -800ºCSupporting Information 1day, and -800ºC-7days are 32, 86, 127, and 158 ºC, respectively. Experimental and methods, DFT models, STEM/TEM/in-situAnd for C3H6 combustion, they are 213, 299, and 322, and 323 ºC, HRTEM, XRD, DSC results, catalytic performance, and physical respectively. Moreover, the 2.91Au₲MgGa2O4 catalysts demondata, and comparisons of catalytic performance with literature are strate high steam resistance. The T50s increased no more than 20 included in the supporting information. This material is available ºC for CO oxidation and changed little for C3H6 combustion when free of charge on the ACS Publications website. 10 vol% H2O was introduced into the reactants (Figure 4c, Figure S7). Upon aging the 2.91Au₲MgGa2O4-300ºC-5h at 800 ºC for 5

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AUTHOR INFORMATION Corresponding Author * [email protected], [email protected]

Author Contributions # These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Key R&D Program of China (2016YFA0202801), the National Natural Science Foundation of China (21673226, 21403213, 91645203, 21590792), the "Transformational Technologies for Clean Energy and Demonstration", Strategic Priority Research Program of the Chinese Academy of Sciences, (XDA21040200), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17000000), the “Hundred Talents Programme” of the Chinese Academy of Sciences, and the Department of Science and Technology of Liaoning province under contract of 2015020086101. We thank Dr. Jing Ju (Peking University) and Dr. Qiang Xu (Denssolutions) for the in-situ heating TEM analysis. The calculations were performed by using supercomputers at Tsinghua National Laboratory for Information Science and Technology, the Supercomputer Center of the Computer Network Information Center at the Chinese Academy of Sciences, and the Computational Chemistry Laboratory of Department of Chemistry at Tsinghua University, which is supported by Tsinghua Xuetang Talents Program.

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