Interaction-Induced Self-Assembly of Au@La2O3 CoreâShell

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Interaction-induced Self-assembly of Au@La2O3 Coreshell Nanoparticles on La2O2CO3 Nanorods with Enhanced Catalytic Activity and Stability for Soot Oxidation Qiangqiang Wu, Jing Xiong, Yilin Zhang, Xuelei Mei, Yuechang Wei, Zhen Zhao, Jian Liu, and Jianmei Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00107 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Interaction-induced Self-assembly of Au@La2O3 Core-shell Nanoparticles on La2O2CO3 Nanorods with Enhanced Catalytic Activity and Stability for Soot Oxidation Qiangqiang Wu, Jing Xiong, Yilin Zhang, Xuelei Mei, Yuechang Wei*, Zhen Zhao, Jian Liu, and Jianmei Li State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, China

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ABSTRACT: Sintering resistance of supported Au nanoparticle (NP) catalysts is crucial to practical application in heterogeneous catalysis reaction. Herein, a series of the catalysts of Au@La2O3 core-shell nanoparticles (NPs) supported on the surfaces of La2O2CO3 nanorod (Aun@La2O3/LOC-R) were successfully synthesized via the method of interaction-induced self-assembly. Supported Au NPs with uniform size are deposited on the surfaces of La2O2CO3 nanorods by the gas bubbling-assisted membrane reduction (GBMR) method. La2O3 shell layers spontaneously formed and then partially coated the surface of Au NPs to interaction-induced self-assembly of Au@La2O3 core-shell NPs during the process of calcination at 600 oC. The strong interaction between Au NPs and La2O3 oxides increases the density of active sites (oxygen vacancy) for enhancing adsorption-activation property of O2. Aun@La2O3/LOC-R catalysts show high catalytic activity and stability for soot oxidation under condition of loose contact between soot and catalyst. Such as, the catalytic activities (T50, TOF) of Au4@La2O3/LOC-R catalyst for soot oxidation are 375 oC and 1.15 S-1 ×10-3, respectively. Based on the results of various physicochemical characterizations, the strong metal (Au)-oxide (La2O3) interaction and the increasing active oxygen species of Aun@La2O3/LOC-R catalysts are responsible for enhancing catalytic activity of soot oxidation. Au@La2O3 core-shell nanostructure can improve catalytic stability and suppress sintering of supported Au NPs during catalytic soot oxidation. The catalytic mechanism of soot oxidation is proposed and discussed that the catalytic oxidation of NO to NO2 over Au@La2O3 core-shell NPs is the key step for catalytic soot oxidation, the active sites at the interface of Au core and La2O3 shell can promote catalytic NO oxidation. Aun@La2O3/LOCR catalysts are promising to the practical applications for diesel soot oxidation. KEYWORDS: La2O2CO3 Nanorod; Core-shell Nanostructure; Strong Au-La2O3 Interaction; Sintering Resistant; Soot Oxidation


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1. INTRODUCTION Particulate matter (PM, consisting mainly of soot particles) emitted from diesel engines has caused serious environment and human health problems.1-3 The combination of diesel particulate filters (DPFs) and high active oxidation catalysts was considered as one of the most efficient techniques for soot purification, and the purification efficiency for soot particles is strong related to the catalytic oxidation activity of the catalysts.4-7 The essence of catalytic soot oxidation is a complex deep oxidation reaction which occurs at the interfaces of triple-phase contact boundary among the solid soot particle, the solid catalyst and the gaseous reactants (O2, NO).8-10 The adsorption and activation properties of the catalysts for O2 play a central role in acquiring the high catalytic activity for soot oxidation, but it still is a challenge. Supported Au nanoparticles (NPs) as active components have exhibited good catalytic activities in many important oxidation reactions because of theirs excellent ability for the adsorption and activation of gaseous O2.11-15 The high catalytic performances (activity, selectivity and stability) of Au-based catalysts for oxidation reaction are strongly associated with its physicochemical characteristics, such as the particle size of supported Au, the nature of the support and the metal-oxide/support interaction and so on.16-21 In our previous work, supported Au nanoparticle (NP) catalysts with average particle size of 3.0 nm exhibited highefficient catalytic activity for soot oxidation because of its strong adsorption-activation properties for gaseous O2.22 Unfortunately, the sintering of supported Au NPs during the reaction process leads to rapid deactivation of Au-based catalysts due to its low Tammann temperature (395 oC) and high surface energies.15,23 The poor thermodynamic stability of Aubased catalysts limits the application in the field of deep oxidation at high temperatures. Thus, the improving stabilization of supported Au NPs with the optimal particle size is crucial for the development of highly efficient Au-based catalysts.


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For enhancing stabilization of supported Au NPs, the sintering mechanism must be understood which is derived from the Ostwald ripening process and Brownian-like motion of Au NPs on the surface of oxide support.24 A number of methods have been developed to enhance the catalytic activity and sintering resistance of Au-based catalysts, such as the optimized preparation process of Au NPs, the selection and pretreatment of support materials before Au loading, the confinement effect of porous channels and materials for Au NPs.25-27 The essence of the strategies and methods for suppressing sintering of supported Au NPs is the use of the strong metal-support interaction (SMSI) between Au NPs and oxide supports. Thus, the increasing interface area between Au and oxide supports via the fabrication of Au@metal oxides core-shell or yolk-shell nanostructure is regarded as a facile method to develop novel Au-based catalysts with enhanced catalytic performances.28-30 On the one hand, the core-shell structure constructs a physical barrier to separate Au NPs from each other by oxide shell, which prevents the aggregation and growth of Au NPs at high temperatures.31 On the other hand, the SMSI effect of Au NP core and oxide shell provides abundant interface active sites to enhance the catalytic activity for deep oxidation.32,33 For example, supported Au@ZnO core-shell NPs did not only enhance the sintering resistance of Au NPs, but also improved their catalytic activity in CO oxidation reaction.34 The active sites for activated O2 are often oxygen vacancy or coordinatively unsaturated metal cations located at the interface between Au and oxide. Therefore, the structure of oxides shell and the SMSI effect of Auoxides are crucial to improve the activity and stability for soot oxidation. However, it is still a great challenge to the fabrication of supported Au@oxides core-shell NPs with suitable oxide shell layers by the facile method. The selection of oxide shell layers is highly essential to effectively suppress sintering of Au NPs and enhance catalytic activity of Au-based catalysts. The structure and property of oxide support at the nanometre scale have an important influence on the dispersion and the


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electronic features of supported Au species. La2O2CO3-supported metal nanoparticle catalysts exhibit the excellent anti-sintering performances in many heterogeneous catalysis reactions.3537

The good sintering resistance of supported metal nanocatalysts is related to the reversible

phase transition between La2O2CO3 and La2O3, and the presence of La2O3 can anchor the metal NPs on the surface of La2O2CO3.38,39 La2O2CO3 also shows the excellent performance for catalytic soot oxidation by the surface reaction of La2O2CO3 with carbon to La2O3 and CO products.40 The unique morphology structure (e.g. rods) of La2O2CO3 is beneficial for enhance the transformation of La2O2CO3 to La2O3 oxides induced by strong metalLa2O3/La2O2CO3 interaction, which is considered as one reason for excellent anti-sintering ability of metal NPs.41,42 In terms of both the enhancing catalytic activity and sintering resistance of Au-based catalysts for soot oxidation, La2O2CO3-supported Au NP catalysts with SMSI effect may be an ideal strategy for soot oxidation. Herein, a series of novel catalysts of Au@La2O3 core-shell NPs supported on the surfaces of La2O2CO3 nanorod (Aun@La2O3/LOC-R) were designed and successfully synthesized by the method of interaction-induced self-assembly. The strong metal-support interaction between Au NPs and La2O2CO3 nanorods is responsible for the formation of supported Au@La2O3 core-shell NPs, and La2O3 oxides shell originated from the partial decomposition of La2O2CO3 at the interface of Au-La2O2CO3. The Aun@La2O3/LOC-R catalysts exhibit excellent catalytic activity and sintering resistance of Au NPs during the soot-TPO tests. The mechanisms of self-assembly of Au@La2O3 core-shell NPs and catalytic soot oxidation were systemically investigated and proposed. Aun@La2O3/LOC-R catalysts are not only excellent and heritable systems for soot oxidation, but also are promising to apply in the practical applications. 2. EXPERIMENTAL SECTIONS


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2.1. Catalyst Preparation. The schematic representation for the preparation and catalysis of Aun@La2O3/LOC-R catalysts is shown in Scheme 1, which includes two procedures in detail as follows. 2.1.1. Synthesis of La2O2CO3 Nanorods. La2O2CO3 nanorods were synthesized via a hydrothermal method, which is similar to the reference reported by Shen and co-authors.35 NaOH (24.0 g) was dissolved in 120 mL of deionized water to produce precipitant solution, which was added into the solution containing of La(NO3)3·6H2O (2.0 g) and deionized water (40 mL) dropwise under agitation, and then a white mixture solution was obatined. Subsequently, the white slurry was deposited into PTEF autoclave (100 mL) and heated at 180 oC for 12 h. The obtained sample was filtered, and further washed by distilled water and absolute ethanol, and then dried at 50 oC for 12 h to obtain La(OH)3 nanorods. After the process of calcination at 500, 550, 600, 650 and 700 oC for 2 h in air with the CO2 concentration of ~420 ppm, the samples with different calcination temperature were obtained. 2.1.2. Synthesis of Aun@La2O3/LOC-R Catalysts. The fabrication of Aun@La2O3/LOCR catalysts was carried out by the method of interaction-induced self-assembly. Firstly, the Au NPs supported on the surface of La2O2CO3-500 nanorods with calcination of 500 oC was performed by the gas bubbling-assisted membrane reduction (GBMR) method, whose detailed preparation mechanism is shown in Supporting Information (Figure S1-S4). La2O2CO3-500 nanorods (0.5 g) were dispersed into the deionized water (200 mL) under magnetic stirring at room temperature, and the stoichiometric amount of HAuCl4 solution was added into above solution dropwise (denoted as Beaker I). The stabilizer (Poly N-vinyl2-pyrrolidone, [PVPunit]/[Au]=100) was then transferred to Beaker I. A peristaltic pump with rotation speed of 200 rpm was developed to form a tubal cycling flow of above mixture solution between the membrane reactor and Beaker I at a flow rate of 360 ml min-1. In the membrane reactor, the mixture solution flowed in the glass tube and outside the ceramic


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tubes. NaBH4 solution as the reductant (the molar ratio of [NaBH4]/[Au] of 5) was injected into the membrane reactor by a constant flow pump at a flow rate of 0.8 ml min-1. When NaBH4 solution infiltrated through the abundant holes (d = 40 nm) on the walls of the two ceramic tubes (Ø 3 mm × 160 mm) into the glass tube, the reduction of Au ions occurred immediately. To create the highly homogeneous dispersion of reductant and provide a reducing atmosphere, which is crucial to the size and distribution of supported Au NPs, the reaction system was further bubbled by a hydrogen gas bubble-assisted stirring operation. The hydrogen gas (40 ml min-1) was developed to stir the mixture solution via two other ceramic tubes until all the NaBH4 in the solution was completely consumed. The product was then filtered and washed with deionized water until Cl- was undetected by AgNO3 test. The obtained solid was further dried in an oven at 50 oC overnight (denoted as Aun/La2O2CO3500). Aun/La2O2CO3-500 catalysts were calcined in an air atmosphere at 600 oC for 0.5 h. During the process of calcination, H2O and stability reagent were not only removed, but also supported Au@La2O3 core-shell NPs were self-assembled on the surface of La2O2CO3 nanorods induced by strong Au-La2O3/La2O2CO3 interaction. Finally, the desired La2O2CO3 nanorods-supported Au@La2O3 core-shell nanoparticle catalysts were obtained. The obtained catalysts are generically named as Aun@La2O3/LOC-R, where n is the initial weight percent of Au in the catalysts, i.e., n wt%, and LOC denotes the La2O2CO3 support. For comparison, La2O2CO3-500 nanorods were calcined in an air at 600 oC for 0.5 h, and then used as substrate to support Au NPs (the initial loading amount of Au is 4 wt%) by GBMR method. Finally, the obtained sample is denoted as Au4/LOC-R. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns were measured on a X-ray diffractometer (Shimadzu XRD 6000) operating at 10 mA and 40 kV using Cu Kα (λ=0.15406 nm) radiation with a Nickel filter, and the scanning rate is 4° min-1 in the 2θ range of 10-70 °. The obtained XRD patterns were compared with JCPDS reference data to


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explore the phase identification of samples. Raman spectra of the catalysts were collected in the anti-Stokes range of 100-1200 cm-1 using an inVia Reflex-Renishaw spectrometer. The He-Cd laser with wavelength of 325 nm was used as the exciting source. The surface morphology of the catalysts was investigated by means of transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) mapping, which were recorded on the JEO1 JEM 2100 electron microscope equipped with a field emission source, operating at an accelerating voltage of 200 kV. The average particle diameter (d) of all samples was calculated by the following equation: d = ∑nidi/∑ni, where ni is the number of particle diameter (di) in a certain range, and the values of ∑ni is more than 100 in TEM images of each catalyst. The nitrogen (N2) adsorption-desorption isotherms were performed on an automated gas sorption analyzer (Quantachrome Autosorb-iQ, USA) at -196 oC. In order to remove water and other atmospheric contaminants, the catalysts were pretreated at 300 oC for 4 h. The specific area was determined from the Brunauer-Emmett-Teller (BET) equation in relative pressure (P/P0) range of 0.05-0.30, and the pore-size distribution was obtained by BJH method using the desorption branch. The actual contents of Au in all samples were determined by inductive coupled plasma atomic emission spectrometry (ICPAES, PE, OPTIMA 5300DV). The temperature-programmed reduction of H2 (H2-TPR) measurements were carried out on a conventional flow apparatus equipped with a thermal conductivity detector (TCD) to monitor the consumption of H2, and a cooling trap and a filter packed with molecular sieve 5A (60-80 meshes) were used to remove H2O and CO2 before the outlet gases entering the TCD. The sample (100 mg) was pretreated at 300 oC for 1 h and then cooled to 30 oC in the N2 flow (30 mL min-1). H2-TPR analysis was performed from 30 to 800 oC with a heating rate of 10 oC min-1 under a mixture gas flow (10% molar H2 in Ar, 30 ml min -1). X-ray photoelectron spectroscopy (XPS) studies were taken on a PerkinElmer PHI-1600 ESCA spectrometer using a Mg Kα X-ray excitation source. The XPS analyses


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were performed at ambient temperature and the pressure in the vacuum chamber during the test was less than 10-7 mbar. The binding energies were corrected by the C1s peak of contaminant carbon (BE = 284.6 eV). The temperature-programmed oxidation of NO (NOTPO) was carried out on a fixed-bed tubular quartz reactor. The catalyst (100 mg) was pretreated with N2 at 200 °C for 0.5 h. Then, the catalyst was heated from 150 to 500 oC at a heating rate of 2 oC min-1 under a gases flow (50 ml min-1) of 5 vol% O2, 0.2 vol% NO balanced with N2. Finally, the FT-IR with TQ analyst software was used to monitor gaseous products. 2.3. Catalytic Activity Evaluation. The catalytic activities of all the catalysts for soot oxidation were evaluated by temperature-programmed oxidation of soot particles (soot-TPO) tests, and the commercial carbon (Printex-U, diameter ~25 nm) purchased from Degussa was used as the model soot. The soot-TPO tests were performed on a fixed-bed tubular quartz micro-reactor with an inner diameter of 6 mm. The reaction temperature was accurately measured by a thermocouple placed in the middle of the catalyst bed. The catalysts (100 mg) were mixed with soot particles (10 mg) by using a spatula in order to reproduce the loose contact mode. The mixture was heated from 150 to 600 oC at a heating rate of 2 oC min-1 under a reactant gases flow (50 ml min-1) of O2 (5 vol%), NO (0.2 vol%) and/or SO2 (0.01 vol%) balanced with Ar. An on-line gas chromatograph (GC, Sp-3420, Beijing) with a flame ionization detector (FID) was employed to monitor the concentrations of CO and CO2 in outlet gas. The temperatures at 10 %, 50 % and 90 % of soot conversion from TPO tests (denoted as T10, T50, and T90, respectively) were evaluated as indices of catalytic activity for soot oxidation. The selectivity to CO2 formation (SCO2) was defined as the percentage of CO2 concentration (CCO2) in the sum of the CO2 and CO concentration, i.e., SCO2 = CCO2/(CCO + CCO2). SmCO2 was defined as SCO2 at which the CCO2 was the maximum.


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The turnover frequency (TOF) was taken as a parameter of the intrinsic activity of catalysts for soot oxidation, which was defined as the ratios of the relative reaction rates to the amounts of catalytic active sites. The relative reaction rates (R) of all the catalysts were obtained through a same isothermal reaction for soot oxidation. The isothermal reaction temperature was selected at 300 oC in an approximate kinetic regime because the conversion of soot oxidation was low (< 10 %) and nearly constant over time. The reaction gas composition was same as that of soot-TPO tests, and the total flow rate was 50 ml min-1. Furthermore, in order to exclude mass transport limitations for isothermal reaction, the small particle diameter (~ 40 μm) of catalysts and the relatively high flow rate were performed to maximally reduce the influence of internal and external mass transport diffusion on R values.7,43 Finally, the R values were calculated by the slope lines of conversion rate over time based on the low and similar limitations of mass transport diffusion. The active site amounts of all the catalysts were tested by isothermal anaerobic titrations. During the process, soot particles were regarded as the probe molecules. When the concentration of produced CO2 kept stable, O2 was instantaneously removed from the reactant. The transient decay in concentration from the steady state was monitored with an online gas chromatograph by using a FID detector. The amount and density of active oxygen species can be quantified by integrating the diminishing rate of CO2 formation over time. Finally, the TOF values of all the catalysts for soot oxidation were obtained. 3. RESULTS 3.1. XRD Patterns. In order to investigate the effect of calcination temperature on phase structure, Figure 1 shows the XRD patterns of La(OH)3 nanorods calcined at 50, 500, 550, 600, 650 and 700 oC for 2 h in air. The diffraction peaks of the samples calcined at 50 oC can be assigned to the hexagonal phase structure of La(OH)3 (JCPDS 36-1481). The sample with calcination of 500 oC shows pure the hexagonal phase structure of La2O2CO3 (JCPDS 37-


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0804), while one sample with calcination of 700 oC shows pure the hexagonal phase structure of La2O3 (JCPDS 05-0602). For the sample calcined at 550, 600 and 650 oC, the main Bragg diffraction peaks in the 2θ range of 10-70o can be indexed to a hexagonal La2O2CO3 phase (JCPDS 37-0804), and a diffraction peak (2θ) centered at 39.5 ° is also observed, which can be assigned to (102) lattice planes of La2O3 phase with the hexagonal phase structure (JCPDS 05-0602).44 It indicates that the crystal phase structures of hexagonal La2O2CO3 and La2O3 coexist in the samples, which may be ascribed to the partial transformation of La2O2CO3 to La2O3 via the dissolution and exsolution of CO2 on the surface of La2O2CO3 support during the calcination process.27,45 Based on the results of XRD pattern, the calcination temperature of 600 oC is the optimal to the interaction-induced self-assembly of supported Au@La2O3 core-shell nanoparticles. Figure 2 shows the XRD patterns of La2O2CO3 support and Aun@La2O3/LOC-R catalysts obtained by interaction-induced self-assembly. After introduction of Au NPs with different amounts, the most of diffraction peaks of Aun@La2O3/LOC-R catalysts are similar to those of La2O2CO3 support, indicating that the supported Au NP catalysts maintain a hexagonal La2O2CO3 phase structure. It is worth noted that the relative intensity of characteristic diffraction peak at 39.5° corresponding to La2O3 phase is higher than that of La2O2CO3 support, indicating that supported Au NPs can induce the increasing formation of La2O3 on the surface of La2O2CO3 support. It may be related to promote the transformation of La2O2CO3 to La2O3 by the strong metal-support interaction at phase interfaces between Au and La2O2CO3. For Au4/LOC-R catalyst, the characteristic diffraction peaks are similar to those of La2O2CO3 support, and its relative peak intensity at 39.5° is lower than those of Aun@La2O3/LOC-R catalysts. It suggests that the phase transformation from La2O2CO3 to La2O3 is strongly related to the synthesis processes of supported Au NP catalysts. The calcination treatment for Au NPs supported on La2O2CO3500 with pure La2O2CO3 phase synthesized by GBMR method can promote the formation of


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La2O3, while the process has unchanged the phase composition of La2O2CO3 with the coexisting crystal phases of La2O2CO3 and La2O3 in Au4/LOC-R catalyst. Therefore, the presence of La2O3 component depends on the strong Au-La2O2CO3 interaction. In addition, no characteristic diffraction peak (∼38.1o) associated with FCC (111) crystal plane of Au NPs is observed, which may be attributed to the small crystalline size and good dispersion of Au NPs outside the detection limit of XRD spectra. 3.2.

Raman

Spectra.

The

phase

structures

of

La2O2CO3,

Au4/LOC-R

and

Au4@La2O3/LOC-R catalysts were further investigated by means of UV-Raman spectra with an excitation wavelength of 325 nm, and the results are shown in Figure 3. For comparison, the results of UV-Raman spectra of La(OH)3, La2O2CO3-500 and La2O3 catalysts are also included. It is noted that the two strong bands of La2O2CO3-based catalysts at 1062 and 1087 cm-1 are presented in comparison with La(OH)3, which is originated from the CO3vibration.46 For La2O2CO3 support, five Raman bands appeared at 372, 393, 842, 1062 and 1087 cm-1 can be attributed to the hexagonal La2O2CO3, while other two bands centered at 345 and 455 cm-1 are related to the hexagonal La2O3.35 It suggests that there is the presence of partial La2O3 crystal phases on the La2O2CO3, which is in accord with the result of XRD. After introduction of Au NPs supported on the surface of La2O2CO3 support, a shoulder band shifts from 393 to 401 cm-1, which is attributed to the lattice contraction of La2O2CO3 at the surface interfaces of Au-La2O2CO3 induced by the strong metal-support interaction. For Au4@La2O3/LOC-R catalyst, the intensities of two peaks at 345 and 455 cm-1 corresponding to La2O3 phase increase obviously compared with those of Au4/LOC-R, indicating that the strong Au-La2O2CO3 interaction is beneficial for phase transformation from La2O2CO3 to La2O3 and promoting the self-assembly of La2O3 over Au NPs during calcination process. And its relative intensity of the band at 401 cm-1 is remarkably lower than those of La2O2CO3 and Au4/LOC-R catalysts, further suggesting that the percent of hexagonal La2O2CO3 phase


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ACS Catalysis

decreases. It is attributed to the induced effect of strong Au-La2O2CO3 interaction by Au-OLa bonds, which is beneficial to the release of CO2 species on the surface of La2O2CO3 support to form La2O3. Based on the above results of Raman and XRD spectra, it can be concluded that the phase structures of all the catalysts calcined at 600 oC are coexistence of dominating hexagonal La2O2CO3 and subordinating hexagonal La2O3, and supported Au NPs on the surface of La2O2CO3 can promote the transformation of La2O2CO3 to La2O3. The strong Au-La2O2CO3 interaction should be responsible for the self-assembly formation of Au@La2O3 core-shell nanostructures (shown in section 3.3). 3.3. TEM Images. Figure 4 shows TEM and HRTEM images of La2O2CO3-500 sample obtained by the calcination of nanorod-shaped La(OH)3 at 500 oC for 2 h in air. The nanorodshaped La(OH)3 was synthesized by a hydrothermal method, and its size distribution is obviously uniform in the length of 200-500 nm and the width of about 20-50 nm in Figure S5. As shown in Figure 4A, La2O2CO3-500 maintains a nanorod-shaped structure similar to the morphological features of La(OH)3 nanorods after high temperature treatment, and the width and length of La2O2CO3-500 are same as those of La(OH)3. The unique morphology of La2O2CO3-500 nanorod is considered as a perfect structure to support metal nanoparticles. The HRTEM images of La2O2CO3-500 nanorod are showed in Figure 4B and Figure 4C. -

La2O2CO3-500 nanorod is a hexagonal crystallite by the observation from the [001] and [110] directions, respectively, which is consistent with the results reported by Shen et al.35 La2O2CO3-500 nanorod is ended with two obtuse ends in 120 o formed by the (010) and (100) surfaces. From the inset of Figure 4B, the fringes correspond well to the distances (0.353 nm) between (100) or (010) planes of hexagonal La2O2CO3. As shown in Figure 4C, the interplanar crystal spacing of 1.595 nm is consistent with the (001) planes of hexagonal system La2O2CO3. Around the long axis [110] direction, the La2O2CO3-500 nanorod is -

enclosed by two (001) flat planes and two (110) side planes. The width of the (001) plane is


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-

about 40 nm, while the width of the side (110) plane is about 20 nm. Among exposed planes, -

the high energy (110) surfaces play an important role in the self-assembly of Au@La2O3 core-shell nanostructures because the transformation of La2O2CO3 to La2O3 oxides is prefer to occur on the {110} surfaces of La2O2CO3.36 Taking all these observations into account, as illustrated in Figure 4D, the model of La2O2CO3-500 nanorod is assumed to be a square block -

with obtuse ends, which is terminated by two (001) flat planes, two (110) side planes, two (100) and two (010) end planes. Figure 5 shows TEM, HRTEM and HAADF-STEM images of Aun@La2O3/LOC-R and Au4/LOC-R catalysts. As shown in Figures 5A, 5C, 5E, S7A and S8A, all the catalysts maintain the rod-shaped morphology, indicating that the synthesis processes of supported Au NPs and calcination treatment rarely influence the nanorod structure. For compared with La2O2CO3-500 nanorods, their length and width of La2O2CO3 support drop slightly, and their surface roughness is improved obviously, which is attributed to the presence of La2O3 proved by the results of XRD and Raman. The uniform Au NPs are dispersed evenly on the surface of La2O2CO3 support. The size distributions of supported Au NPs are counted by the statistical analyses of more than 100 NPs in each catalyst, and the results are shown in Figure S6 and Table S1. Aun@La2O3/LOC-R catalysts have a narrower distribution in the range 1-7 nm, and the mean diameters of Au NPs over Au2@La2O3/LOC-R, Au4@La2O3/LOC-R catalysts are 3.8 and 3.9 nm, respectively. The Au4@La2O3/LOC-R catalyst only shows a slight increase in size compared with Au4/La2O2CO3-500 precursor (3.6 nm) without calcination at 600 oC in Figure S9. It indicates that the synthesis process of interactioninduced self-assembly plays a decisive role in controlling the high dispersion and thermal stability of supported Au NPs at a confined size range. As shown in Figures 5B, 5D, S7B and S8B, some semi-coated Au NPs are observed that the hemispherical shapes of supported Au NPs rooted into the support, which is direct proof


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ACS Catalysis

for Au@La2O3 core-shell nanostructures. The lattice fringes in HRTEM images are measured to be 0.23 nm indexed as (111) planes of fcc-structured Au NPs, and the interplanar crystal spacing of shell oxides with the thicknesses of 1-2 nm is 0.30 nm which is assigned to the (101) crystal face of hexagonal La2O3. For Au4/LOC-R catalyst, one spherical Au NP can be observed in Figure 5F that its particle size is 5.4 nm which is bigger than that of Au4@La2O3/LOC-R catalyst (3.9 nm). The contact interface between Au and support is loose, and the shell of La2O3 oxides is unobserved. It indicates that Au@La2O3 core-shell nanostructures are derived from the transformation of La2O2CO3 to La2O3 induced by the strong Au-La2O2CO3 interaction at phase interfaces during calcination process at 600 °C, and it can suppress sintering of supported Au NPs at high temperatures. The core-shell nanostructures of Au@La2O3 can be further demonstrated by the HAADF-STEM-EDS elemental mapping analyses of Au4@La2O3/LOC-R catalyst, and the result is shown in Figure 5G. The colors of orange, yellow and green images indicate the dispersion mapping of C, Au and La elements, respectively. It is noted that the intensity of C element mapping in the area of Au NP is lower than that of support, and the intensity of La element mapping at the interfaces between Au and support increases obviously, indicating the formation of Au@La2O3 core-shell nanostructures. From the above, the stabilization of Au NPs on the surface of La2O2CO3 nanorods may be related the formation of La2O3 shell located at around Au NPs. For the Aun@La2O3/LOC-R catalysts, it is most likely that the partial covering of La2O3 oxides onto the surface of Au NPs occurs during the thermal treatment to selfassembly form the Au@La2O3 core-shell nanostructures, which is attributed to the surface reconstruction of La2O2CO3 induced by the strong interaction between La2O2CO3 and Au. In contrast, Au4/LOC-R catalyst has not been observed the formation of Au@La2O3 core-shell nanostructures, suggesting that the surface La2O3 species on the surface of La2O2CO3 nanorods with calcination treatment at 600 oC can prevent the contact between Au and


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La2O2CO3. Thus, the self-assembly of Au@La2O3 core-shell nanostructures is strong dependent on the interaction between Au and La2O2CO3. In addition, the N2 adsorption-desorption isotherms of all the catalysts are shown in Figure S10. All catalysts clearly display a type IV N2 adsorption-desorption isotherms with a type H3 hysteresis loop in the relative pressure (P/P0) range from 0.8 to 1.0, indicating the presence of texture mesoporous structures formed by La2O2CO3 nanorods. As shown in Table S1, the average mesopore sizes of all the catalysts are in the range of 9.7-11.3 nm. The total pore volume (VP) of La2O2CO3 nanorods and Aun@La2O3/LOC-R catalysts are in the ranges of 0.15-0.19 cm3 g-1, and the values of BET surface area (SBET) are located in the ranges of 4045 m2 g-1. After introduction of supported Au NPs on the surface of La2O2CO3 nanorods, there is no significant difference to BET surface areas. Table S1 also shows the actual amounts of Au in the catalysts determined by ICP-OES, which are close to the theoretical values. It suggests that Au NPs are well supported on the surface of La2O2CO3 support. 3.4. H2-TPR Profiles. The temperature-programmed reduction of H2 (H2-TPR) measurements were used to evaluate the reducibility of all the prepared catalysts, and the results are shown in Figure 6. La2O2CO3 nanorods have not been observed the reduction peak below 500 oC. And one peak centered at 722 oC is attributed to the decomposition of La2O2CO3 to La2O3 and CO2.36,39 It indicates that La2O2CO3 nanorods have poor redox property. After introduction of Au NPs for obtaining Aun@La2O3/LOC-R catalysts, two new reduction peaks during the reduction temperatures of ~200 and 500 oC are observed. The first peaks located at 167-215 oC are assigned to the reduction of oxidized AuOx species or surface chemisorbed (active) oxygen species.47 It suggests that the introduction of Au NPs can improve the redox ability of support dramatically, which is associated with the strong Auoxide/support interaction in supported Au@La2O3 core-shell nanoparticle catalysts. For Aun@La2O3/LOC-R catalysts with different contents of Au NPs, it shows the similar


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ACS Catalysis

temperature of reduction peaks in low temperature range (below 250 oC), which reflects the same essence of Au-oxide interaction. It further indicates that Aun@La2O3/LOC-R catalysts possess the identical interfaces between metal (Au) and oxide (La2O3) via the Au@La2O3 core-shell nanostructures, which is in accordance with the results of HRTEM images. With the increasing of Au loading amount in Aun@La2O3/LOC-R catalysts, the positions of the reduction peaks increase from 167 to 215 oC, and their peak areas of H2 consumptions increase gradually. It indicates that the amounts of active oxygen species on the surface of catalysts improve, which is attributed to increasing the loading amounts of Au active component with Au@La2O3 core-shell nanostructures. In order to observe the effect of Au@La2O3 core-shell NPs on the redox property, the H2-TPR result of Au4/LOC-R catalyst is also included in Figure 6. The reduction peak area of Au4@La2O3/LOC-R catalyst is larger than that of Au4/LOC-R catalyst, indicating that the active site of deep oxidation locates at the interface area between Au and La2O3, and the Au@La2O3 core-shell nanostructures improve the amounts of active site for activated oxygen, which would be beneficial for enhancing catalytic performance during soot oxidation. It might be attributed to the strong Au-La2O3 interaction effect to improve the mobility of lattice oxygen and promote the formation of surface vacancies via Au-O-La bonds at interfaces of metal-oxide/support. The reduction peaks of supported Au NP catalysts at ~500 oC are assigned to the reduction of surface La2O2CO3, suggesting that the surface reduction of support is obviously enhanced compared with vacant support. It suggests that the strong metal-oxide/support interaction can improve the redox property of catalysts, which is related to the hydrogen spillover on the surface of supported Au NPs.27 The position and intensity of bulk-phase reduction peaks in all catalysts located at 722 oC are almost unchanged in comparison to that of La2O2CO3 support, showing a strong structural stability.


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3.5. XPS Characterization. The technique of XPS was used to investigate the effect of supported Au NPs on the surface elemental composition and chemical state for the asprepared catalysts, which is correlated with the catalytic performances for soot oxidation. Figure S11A shows the C 1s spectra of La2O2CO3 and Au4@La2O3/LOC-R catalysts, two peaks around 284.3 and 288.8-289.0 eV are attributed to two kinds of carbon species, respectively. The C 1s peak located at the lower binding energy is assigned as reference. The peak located at high binding energy can be attributed to carbonate species.48 It is noted that the peak corresponding to carbonate species in the supported Au catalyst shows an obvious positive shift in comparison to that of La2O2CO3 support.49 As shown in Figure S11B, the La 3d5/2 spectra for La2O2CO3 and Au4@La2O3/LOC-R catalysts are composed by a main peak at about 835.3 eV and its accompanying satellite peak (~839.1eV), which is consistent with that of pure La2O2CO3.50 It is believed that the binding energy of the satellite peak is closely related to local chemical environment of La3+ that is affected by hybridization of La4f and O2p orbitals.51 After the introduction of Au@La2O3 core-shell NPs over La2O2CO3 surface, the satellite peak of La 3d5/2 in the Au4@La2O3/LOC-R catalyst shifts apparently to the lower binding energy, which is attributed to the strong interaction between Au core and La2O3 shell performed by the La-O-Au bonds.49 Figure 7 exhibits the XPS spectra of Au 4f and O 1s regions for Aun@La2O3/LOC-R catalysts. As shown in Figure 7A, the Au 4f spectra of each sample can be decomposed into three components using the standard procedure: the metallic Au0 species at 83.5 and 86.9 eV, the low-valent Au+ species at 84.2 and 87.6 eV, and the high-valent Au3+ species at 85.6 and 89.1 eV, respectively. It indicates that both the metallic (majority) and ionic species coexist on the surface of supported Au catalysts. The formation of ionic Au species is derived from the electron transfer from Au0 to La2O3/La2O2CO3 at metal-oxide/support interfaces because of the strong Au-La2O3/La2O2CO3 interaction. It is well known that the ionic metal species


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ACS Catalysis

are the active sites for adsorption-activation of O2 and NO.52 Therefore, the relative amount of each metal species in all the catalysts is estimated using the integrated area of each peak in the deconvolution spectra, and the values are shown in Table 1. The Au species ratio (Ra) of Au&+ (Au+ and Au3+) to Au0 over Au2@La2O3/LOC-R catalyst is about 0.536. With the increasing of Au content, the Ra value increases from 0.536 to 0.802 for Au4@La2O3/LOC-R catalyst, suggesting that the increasing of Au content is beneficial for the formation of active Au&+ species. For Au4/LOC-R catalyst, the peak area percentage of Au&+ species reduces in comparison with Au4@La2O3/LOC-R catalyst. For instance, the Ra value of Au4/LOC-R catalyst is only 0.642, which is probably attributed to the bigger particle size of supported Au NPs (Figure 5) and the lower interface area between Au NPs and La2O3 oxides. It indicates that the nanostructures of Au@La2O3 core-shell are beneficial for the formation of active Au&+ species. For Au4@La2O3/LOC-R catalyst, there is a strong interaction between Au core and La2O3 shell, which is consistent with the H2-TPR results, and the increased Au-oxide interface areas may lead to the larger amounts of Auδ+ species due to the electron transfer from Au core to La2O3 shell at metal-oxide interfaces. Therefore, the introduction of Au species and the formation of La2O3 shell over Au NPs surface are beneficial to increase the active interface areas between Au and oxide support and the active site amounts for adsorption-activation of O2. In order to observe the active oxygen species over La2O2CO3 and Aun@La2O3/LOC-R catalysts, XPS analyses of oxygen element were performed. As shown in Figure 7B, the asymmetrical O 1s spectra of each catalyst can be divided into four types of oxygen species by peak fitting, which are assigned to lattice oxygen species (O2-) at 528.7 eV, chemisorbed oxygen species (O22-) at 530.1 eV, surface oxygen species (O2-) at 531.1 eV and oxygencontaining contamination CO32- at 532.2 eV, respectively.32 The peak areas of these oxygen species are summarized in Table 1. The ratio (Rb) of surface adsorbed active oxygen (O22-


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and O2-) species to lattice oxygen (O2-) species is estimated from the relative amounts of oxygen species calculated by the areas of each peak. The Rb values are used to understand the catalytic performance of each catalyst for soot oxidation because the catalyst activity mainly depends on the density of surface active oxygen species.53,54 As shown in Table 1, the ratio of (O22-+O2-) to O2- over La2O2CO3-500 is only 5.9, while the percentage of La2O2CO3 support increases to 8.2. After introduction of the Au NPs, the Rb values of Aun@La2O3/LOC-R catalysts are remarkably higher than that of La2O2CO3 support. For example, the Rb value of Au4@La2O3/LOC-R catalyst is 17.4. The Rb values over all the catalysts can be arranged in the following order: Au4@La2O3/LOC-R (17.4) > Au4/LOC-R (13.3) > Au2@La2O3/LOC-R (10.7) > La2O2CO3 (8.2) > La2O2CO3-500 (5.9). It suggests that La2O3 shell can obviously enhance the amounts of surface active oxygen species compared with shell-free Au4/LOC-R catalyst on the basis of the same amount of Au components. It is attributed to that the strong Au (core)-La2O3 (shell) interaction can weaken La-O bonds and improve the mobility of lattice oxygen in La2O3 shell via the electron transfer from Au NPs to La2O3 oxides. Therefore, Au atoms located at the interfaces of metal-oxide should bear a positive charge, which is in accordance with the results of Au 4f XPS and results in the formation of oxygen vacancies over La2O3 oxides. It would be favorable for improving the adsorption-activation capacity of gaseous O2, and may be conducive to the production of active oxygen species on the surface of Aun@La2O3/LOC-R catalysts. In addition, the abundant active oxygen species are greatly beneficial to the oxidation of NO to NO2 intermediate, which is an important oxidant for catalytic soot oxidation. Therefore, Aun@La2O3/LOC-R catalysts should show excellent catalytic activity for soot oxidation. 3.6. Catalytic Performances for Soot Oxidation. The catalytic performances of all the catalysts for soot oxidation were evaluated by TPO method under the condition of loose contact between soot particles and catalysts, and the results are shown in Figure 8, Figure S12


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ACS Catalysis

and Table 2. The uncatalyzed soot oxidation was also performed for comparison, i.e., its values of T10, T50 and T90 are 462, 586 and 645 °C, respectively. After introduction of each as-prepared catalyst into the reaction system, the soot conversion curve shifts to lower temperature compared with that of without catalyst, indicating that all the catalysts can promote soot oxidation. Compared with La2O2CO3-500 (T50=448 oC), La2O2CO3 catalyst shows higher catalytic activity (T50=431 oC) for soot oxidation, which is attributed to the good redox property of impure La2O3 formed by calcination at 600 oC. The formation of Au@La2O3 core-shell NPs on the surface of La2O2CO3 nanorods can further enhance the catalytic activity of soot oxidation compared with La2O2CO3 catalyst. With the increasing of Au amount in catalysts, the position of CO2 concentration peak shifts to the lower temperature in Figure S12. For example, Au4@La2O3/LOC-R catalyst exhibits excellent soot oxidation activity among the prepared catalysts: the values of T10, T50, and T90 are 278, 375, and 424 oC, respectively. It is related to the promotion effect of supported Au NPs and the strong Au (core)-La2O3 (shell) interaction. For Au4/LOC-R catalyst, the catalytic activity (T50 = 388 oC) for soot oxidation is lower than that of Au4@La2O3/LOC-R catalyst with same Au loading amount. It could be ascribed to the lower metal-oxide interface areas to reduce the density of active oxygen species in comparison to Au4@La2O3/LOC-R catalyst with La2O3 shell over Au core. In a word, the catalytic performances of supported Au catalysts could be strongly related to the amount of supported Au NPs and the metal (Au)-oxide (La2O3) coreshell nanostructures. In addition, it is also noted that the highest selectivities of CO2 (SCO2m) are remarkably enhanced in the oxidation process of soot after introduction of Au NPs, and it is close to 100% in Table 2 and Figure S13. It is attributed to the self-assembled Au@La2O3 core-shell NPs with the strong oxidation capacity (abundant active oxygen species) for CO molecules. It suggests that CO originated from the initial oxidation of soot particles is


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immediately oxidized under the practical conditions of motor vehicle operation, which avoids secondary pollution and suppresses emission of CO. The turnover frequency (TOF), denoted by the ratios of relative reaction rates (R) to the active site densities of the catalysts, is taken as an index of the intrinsic activity for catalyst. In order to obtain the R values of all the catalysts for soot oxidation, an isothermal oxidation reaction at 300 oC in the kinetic regime was carried out. A series of procedures, such as the different the flow rates of reactant gas (10, 30, 50 and 100 ml min-1), were performed to maximally reduce the influence of internal and external mass transport diffusion on relative reaction rates of catalytic soot oxidation, and the results are shown in Figure S14. The mass transport limitations could be excluded under conditions of the small particle diameter (~40 μm) of catalyst and the gas flow rates of 50 ml min-1. Figure 9A and Table 2 show the R values (the concentration of CO2 per unit time) of all the catalysts for soot oxidation according to the slope of lines. The slopes of Aun@La2O3/LOC-R catalysts are much higher than those of La2O2CO3-500, La2O2CO3 and Au4/LOC-R catalysts, indicating that the catalytic activities for soot oxidation are improved by supported Au@La2O3 core-shell NPs. For example, the R values of Aun@La2O3/LOC-R catalysts are larger than 0.057 μmol g-1 s-1, while that of La2O2CO3 nanorod is only 0.038 μmol g-1 s-1. And the R value of Au4@La2O3/LOC-R catalyst is 0.104 μmol g-1 s-1, which is nearly 3-fold of La2O2CO3 support. The quantified density of active oxygen species were measured by the isothermal anaerobic titrations at 300 oC, and the results are shown in Figures 9B, S15 and Table 2. Aun@La2O3/LOC-R catalysts exhibit higher the density of surface active oxygen species in comparison with La2O2CO3-500, La2O2CO3 and Au4/LOC-R catalysts. With increasing of Au loading amount in the Aun@La2O3/LOC-R catalysts, the densities of surface active oxygen species increase gradually. On the basis of relative reaction rate values and the quantified density of active oxygen sites, TOF values of all catalysts are obtained and shown in Table 2.


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Aun@La2O3/LOC-R and Au4/LOC-R catalysts with higher TOF values exhibit higher intrinsic activity than La2O2CO3 nanorods, indicating that supported Au NPs are very important to enhance the catalytic activity. The formation of metal-oxide interfaces can ensure the sufficient density of active sites for enhancing O2 adsorption-activation capacity. In addition, TOF values (1.05-1.15 × 10-3 s-1) of supported Au catalysts are nearly close, indicating that the nature of active sites of all the Au-based catalysts is identical, i.e., the oxygen vacancies derived from the Au-La2O3 interaction are the active sites for O2 adsorption-activation. Au@La2O3 core-shell nanostructures formed by La2O3 oxides covering on the surface of Au NPs in Au4@La2O3/LOC-R catalyst is beneficial for increasing the density of active sites in comparison with Au NPs directly supported on the surface of La2O2CO3 nanorods with same Au amount. Thus, the catalytic activity of Aun@La2O3/LOCR catalysts for soot oxidation is dependent on the strong Au (core)-La2O3 (shell) interaction, which is also in accordance with the soot-TPO results. The influence of NO and SO2 in reactant gas on the catalytic activity of Au4@La2O3/LOCR catalyst for soot oxidation is also investigated, and the results are shown in Figure S16. It suggests that NO can remarkably enhance the catalytic activity of Au4@La2O3/LOC-R catalyst for soot oxidation, indicating that NO plays an important role during catalytic soot oxidation. It also exhibits that the catalytic activity for soot oxidation is strongly related to the concentrations of NO in reactant gas. For example, Au4@La2O3/LOC-R catalyst (NO: 2000 ppm) has the higher catalytic activity for soot oxidation in comparison with that of in only presence of O2, i.e., the T50 value of Au4@La2O3/LOC-R catalyst decreases from 428 to 375 oC.

It is attributed to the strong oxidation capacity of NO2 intermediates for soot particles

originating from the oxidation of NO molecules at Au-La2O3 active interfaces, indicating that the oxidation of NO to NO2 reaction plays an important role for enhancing the catalytic activity of Aun@La2O3/LOC-R catalysts for soot oxidation.16,17 Figure S17 shows the curves


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of NO2 concentration over Au4@La2O3/LOC-R catalyst during NO-TPO and soot-TPO catalytic tests. After introduction of soot particles, the NO2 concentration originated from NO oxidation decreases obviously, indicating that soot particles can react with NO2 and consume it. Thus, it suggests that the catalytic oxidation of NO to NO2 reaction is the key step for catalytic soot oxidation under the conditions of presence of NO. Figure S16 also shows that the catalytic activity of Au4@La2O3/LOC-R catalyst decreases slightly after the introduction of SO2 gas into reaction system, indicating that Au4@La2O3/LOC-R catalyst has excellent SO2 resistance ability, which is consistent with the results reported by Merraudeau P. and coauthors.40 It suggests that the La2O2CO3 nanorods supported Au@La2O3 core-shell nanoparticle catalysts are the excellent material system for soot oxidation. It may be assigned to that the strong interaction between Au NPs (core) and La2O3 oxides (shell) could enhance the adsorption-activation ability of gaseous O2 to produce abundant active oxygen species during the process of soot oxidation, which has been confirmed by the Raman, H2-TPR and XPS measurements. 3.7. Stability Testing of the Catalysts. The stability of heterogeneous catalysts is very important for their potential applications in real conditions. To study stability of the catalysts, Au4@La2O3/LOC-R and Au4/LOC-R catalysts are further examined by eight cycles of sootTPO experiments, and the results are shown in Figure 10. It can be seen that Au4@La2O3/LOC-R catalyst has not been observed appreciable deactivation during eight cycles of soot-TPO tests, its T10, T50 and T90 values almost unchanged. In contrast, the activity of Au4/LOC-R catalyst decreased greatly during the recycling tests. For example, the T50 value increases from 388 oC to 433 oC, which is close to the activity of La2O2CO3 support for soot oxidation. It demonstrates that Au4@La2O3/LOC-R catalyst displays the better stability in comparison with Au4/LOC-R catalyst during soot oxidation, indicating that Au4@La2O3/LOC-R catalyst is an excellent material for soot oxidation in real conditions.


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Figure S18 shows XRD patterns of fresh and used Au4@La2O3/LOC-R catalysts after eight soot-TPO cycles. The positions and intensities of diffraction peaks of used Au4@La2O3/LOCR catalyst did not change substantially after eight cycles of soot-TPO measurements compared with the fresh catalyst, revealing that Au4@La2O3/LOC-R catalyst possess a good crystalline stability. In addition, the TEM images of used Au4@La2O3/LOC-R catalyst are shown in Figure 11. As shown in Figure 11A, the nanorod-shaped morphology of used Au4@La2O3/LOC-R catalyst has not changed obviously. Figure 11B shows the TEM image of supported Au NPs with highly dispersion on the surface of La2O2CO3 support. The nanostructure of one supported Au@La2O3 core-shell NP in Figure 11C has not changed compared with the fresh catalyst, and the mean diameter of Au core is about 4.0 nm in Figure 11D from the statistical analyses of more than 100 Au@La2O3 core-shell NPs in used Au4@La2O3/LOC-R catalyst. It is close to the mean diameter (3.9 nm) of Au core in fresh Au4@La2O3/LOC-R catalyst. It indicates that Au@La2O3 core-shell nanostructures will effectively

stabilize

Au

NPs

to

inhibit

sintering

at

high

temperatures.

Thus,

Au4@La2O3/LOC-R catalyst retains relatively high catalytic activity for soot oxidation. In other words, the strong metal (Au)-oxide (La2O3) interaction strengthens the confinement of Au core by La2O3 shell, thereby preventing the aggregation and growth of Au NPs. Unfortunately, Au NPs of Au4/LOC-R catalyst clearly sintered under the same soot-TPO tests shown in Figure S19, which shows supported Au NPs with mean sizes at 9.5 nm after eight successive soot-TPO recycles. In comparison with Au4@La2O3/LOC-R catalyst, the direct deposition of Au NPs over La2O2CO3 support fails to construct a shell (barrier) due to the lack of the migration of La2O3 oxides onto the surface of Au NPs. It is concluded that the La2O3 shell, which derived from the partial decomposition of La2O2CO3 induced by the strong Au-La2O2CO3 interaction during calcination treatment, plays an essential role in the strong stability of Au-based catalysts.


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4. DISCUSSIONS 4.1. The Interaction-induced Self-assembly of Au@La2O3 Core-shell NPs over La2O2CO3 Nanorods. The rapid advancements in the preparation of nanomaterials enabled us to regulate and control the microstructure at nanometre scale of a catalyst applied in several important reactions. It is well known that the sintering of supported Au-based nanocatalysts severely affects their practical application, and the structure and property of support have important influences on the geometric and electronic features of supported Au components. La2O2CO3-500 nanorods with widths of 20-50 nm and lengths of 200-500 nm are synthesized by hydrothermal method and following calcination at 500 oC for 2 h. As shown in Figure 4, a single La2O2CO3-500 nanorod with two obtuse ends growing along the -

[110] direction, and two (110) side planes and two (001) flat planes are selectively exposed. The La2O2CO3-500 nanorods can be employed to fabricate a La2O3 shell over Au NPs for enhancing the activity and stability of supported Au-based nanocatalysts. For example, the phase structures of Aun@La2O3/LOC-R catalysts are coexistence of dominating hexagonal La2O2CO3 and subordinate hexagonal La2O3 according to the results of XRD (Figure 2) and Raman (Figure 3) spectra. The formation of hexagonal La2O3 is attributed to the partial transformation of La2O2CO3 to La2O3 on the surface of La2O2CO3 nanorods during calcination treatment. The phase transformation of La2O2CO3 is more likely to occur on the {110} surfaces because CO2 can leave solid surfaces from void defects between two (La2O2)2+ layers via exchanging between O2- and CO32-.36 Under this conditions, La2O3 oxides may be formed and then partially cover the Au NPs surface to form Au@La2O3 coreshell nanostructures on the {110} surfaces induced by the strong interaction between Au and La2O3. The self-assembled Au@La2O3 core-shell nanostructures can optimize metal-support interface areas and improve metal-support contact, and the annealing at high temperatures further strengthen electronic interaction between Au NPs and La2O3 oxides.55 It implies that


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Aun@La2O3/LOC-R catalysts exhibit the excellent activity and stability during the soot-TPO tests (Figures 8 and 10). In contrast, Au4/LOC-R catalyst fails to form the core-shell structured Au@La2O3 NPs due to the lack of the transformation of La2O2CO3 to La2O3 oxides in Figure 5. In addition, the La2O2CO3 support has an excellent ability for the elimination of soot particles by the surface reaction of La2O2CO3 with carbon to La2O3 and CO products. The La3+-O2- pairs and coordinative unsaturated O2- sites, which locate on the {110} planes of La2O2CO3 nanorods, are active sites for soot particles in comparison with {001} surfaces containing C and O atoms because the former possesses greater mobility than oxygen species on the {001} surfaces.41 It is concluded that the fabrication of Au@La2O3 core-shell NPs over La2O2CO3 nanorods are successfully carried out by the interactioninduced self-assembly combined with GBMR method and calcination treatment. 4.2. The Role of Active Oxygen Species during Soot Oxidation. It is well known that the catalytic oxidation of soot particles is a complicated heterogeneous reaction involving the solid soot particle, the solid catalyst and the gaseous reactants (O2, NO). The active oxygen (Oa) species originated from the surface of catalyst is an important factor for soot oxidation, which consists of direct (catalyst-Oa-soot) and indirect (catalyst-Oa-NO-NO2-soot) pathways.56,57 In order to verify the role of active oxygen species during soot oxidation, the effect of NO in reactant gas on catalytic activity for soot oxidation is investigated. Figure S16 shows the catalytic activity of Au4@La2O3/LOC-R catalyst for soot oxidation in the presence or absence of NO. It is worth noting that NO can remarkably enhance the catalytic activity for soot oxidation, indicating that the NO plays an important role during catalytic soot oxidation. In other words, the indirect NO2-assisted oxidation mechanism is the dominant react pathway of active oxygen species for catalytic soot oxidation. It is very beneficial for the catalytic activity of soot oxidation in loose contact between catalyst and soot particle under practical engine operation conditions because NO2 acts as an efficient mobile oxidizing


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agent. Thus, the active oxygen species at the Au-La2O3 interfaces can catalyze soot particles by using NO2 as an intermediate oxidant which can oxidize soot to CO2, i.e., the indirect pathway (overall reaction: NO+1/2O2→NO2 and NO2+soot→COx+NO+N2/N2O). It is concluded that the catalytic oxidation of NO to NO2 reaction is the key step for catalytic soot oxidation in the indirect NO2-assisted oxidation mechanism. 4.3. The Effect of Strong Au-La2O3 Interaction on Catalytic Performances for Soot Oxidation. The reaction nature of catalytic soot oxidation is complex deep oxidation process. The catalytic activity and stability of Aun@La2O3/LOC-R catalysts are strongly dependent on the density of active oxygen species and structural feature of themselves. The XPS results (Figure 7) demonstrate that supported Au nanoparticle catalysts have higher amounts of active oxygen species compared with La2O2CO3 nanorods. The larger fraction of active oxygen species may readily take part in the oxidation process of NO, and significantly contribute to the formation of NO2 oxidant, which has an important influence for soot oxidation. Figure 12A shows NO2 concentration derived from NO oxidation over Aun@La2O3/LOC-R catalysts. For comparison, NO2 concentration curves of La2O2CO3-500 and La2O2CO3 catalysts are also included. For La2O2CO3-500 sample, NO2 is detected at a low concentration below 500 C, while NO2 peak of La2O2CO3 support increases slightly, which is in accord with the results of XPS. NO2 concentration cannot increase with temperature rising until the thermodynamic equilibrium of the equation (NO+1/2O2←→NO2), and then decreases at higher temperatures. After introduction of supported Au NPs, NO2 concentrations of Au-based catalysts remarkably increase due to the strong interaction effect of Au-La2O3 on promoting the efficiency of NO oxidation. Thus, Aun@La2O3/LOC-R and Au4/LOC-R catalysts exhibit higher catalytic activity for soot oxidation because the NO2assisted mechanism occupies a dominant position in soot oxidation. For example, the T50 value of La2O2CO3 support is higher (nearly 60 oC) than those of Au4@La2O3/LOC-R


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catalyst in Table 2. Furthermore, larger reduction peak areas (H2 consumption amounts) can reflect the more active oxygen species derived from the adsorption-activation of O2 at AuLa2O3 interfaces. The T50 values have good correlation with the H2 consumption amounts (reduction peaks at 160-220 oC) shown in Figure 12B, indicating that the catalytic activity for soot oxidation is strongly dependent on the different nanostructures of active components (Figure 5) and distinct interface areas between metal (Au) and oxide (La2O3) (Figure 6). For example, Au4@La2O3/LOC-R catalyst with larger Au-La2O3 interface areas is easier to provide oxygen species towards H2 oxidation compared with Au4/LOC-R catalyst, indicating better catalytic activity for soot oxidation. For Au4@La2O3/LOC-R catalyst, the strong interaction between Au core and La2O3 shell creates a lattice strain and disorder in La2O3 shell via La-O-Au bonds compared with simple Au-La2O3 interface structures in Au4/LOC-R catalyst, which could lead to abundant oxygen vacancies on the Au4@La2O3/LOC-R catalyst.58 It is well known that the oxygen vacancies or coordinatively unsaturated metal cations located at the metal-oxide interfaces are active sites for promoting the adsorption-activation of O2 in oxidation reaction. Thus, the Au@La2O3 core-shell interfaces act as a reservoir for active oxygen species in soot oxidation, which could improve the formation of NO2 to enhance the catalytic activity of Aun@La2O3/LOC-R for soot oxidation. On the other hand, the partial covering of Au NPs by La2O3 oxides, which originated from partial decomposition and reconstruction of La2O2CO3 via CO2 dissolution on the surface of nanorods, is very important for achieving Au NPs with strong stability. Compared with Au4/LOC-R catalyst, the constructed strong core (Au NPs)shell (La2O3 oxides) interaction maintains the dispersion and stabilization of Au NPs, thereby preventing the aggregation and growth of Au NPs in soot oxidation at elevated temperatures (Figure 11). It indicates that Aun@La2O3/LOC-R catalysts display the outstanding stability (Figure 10), which benefits long-life reactions, and the self-assembled Au@La2O3 core-shell


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NPs with strong metal-oxide interaction result in the enhanced catalytic activity and stability for soot oxidation in real conditions. 4.4. Catalytic Mechanism of Aun@La2O3/LOC-R Catalysts for Soot Oxidation. Based on the above discussion and experiment results, the nano-composite structure catalysts consisting of both active Au@La2O3 core-shell NPs and nanorod-shaped La2O2CO3 support, possess the good catalytic performances for soot oxidation. The catalytic pathways of Aun@La2O3/LOC-R catalysts for soot oxidation are described below, and the schematic diagram is displayed in Figure 13. First, the La2O2CO3 support has a strong ability to eliminate soot via the surface reaction of La2O2CO3 with carbon to La2O3 and CO products. The formed La2O3 oxides then partially cover the Au NPs surface to interaction-induced selfassembly form Au@La2O3 core-shell nanostructures on the surface of La2O2CO3 nanorods. Second, the self-assembled Au@La2O3 core-shell NPs can increase the density of active sites located at the core (Au)-shell (La2O3) interfaces. It is beneficial to the formation of surface active oxygen species by improving the adsorption-activation of reactant molecules (O2 and NO) over active sites due to strong metal-oxide interaction. The abundant active oxygen species can dramatically enhance the oxidation of NO to NO2 intermediate, which is an important oxidant for catalytic soot oxidation. Finally, the NO2 can migrate to the surface of soot particles and oxidize them to CO and CO2. The CO originated from the initial oxidation of soot particles is adsorbed and oxidized immediately by abundant active oxygen species over the Au@La2O3 core-shell interfaces (overall reaction: NO+1/2O2→NO2 and NO2+soot →COx+NO+N2/N2O). Thus, Aun@La2O3/LOC-R catalysts show high-efficient catalytic performances for soot oxidation. 5. CONCLUSIONS La2O2CO3 nanorods with diameters of 20-50 nm and lengths of 200-500 nm were successfully prepared by a hydrothermal method, and the interaction-induced self-assembly


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of Au@La2O3 core-shell NPs over the surface of La2O2CO3 nanorods were achieved by the combined methods of GBMR and following calcination treatment. The La2O2CO3 as precursor of La2O3 shell plays an important role in the fabrication of self-assembled Au@La2O3 core-shell NPs with the strong metal-oxide interaction. The sizes of Au core have a narrow distribution in the range 1-7 nm, while the thicknesses of La2O3 shell are 1-2 nm. The Au@La2O3 core-shell NPs are clearly observed and highly dispersed on the surface of La2O2CO3 nanorods in Aun@La2O3/LOC-R catalysts. The strong metal (Au)-oxide (La2O3) interaction is favorable for increasing amount of surface active oxygen species by improving the adsorption-activation of O2 molecules at the interfaces of Au core and La2O3 shell. The abundant surface active oxygen species are beneficial for the oxidation of NO to NO2 as an intermediate oxidant which can oxidize soot to CO2, and the oxidation of NO to NO2 reaction is the key step for catalytic soot oxidation. Thus, Aun@La2O3/LOC-R catalysts exhibit highefficient catalytic activity for soot oxidation because the indirect path (NO2-assisted oxidation mechanism) occupies a dominant position in soot oxidation reaction. In addition, the confinement effect of La2O3 shell and strong metal (Au)-oxide (La2O3) interaction will stabilize Au NPs effectively to inhibit sintering at high temperatures, the Aun@La2O3/LOC-R catalysts exhibit also excellent stability compared with Au4/LOC-R catalyst without forming a shell over Au surface. In summary, the work not only offers a new strategy to enhance activity and stability of Au-based nanocatalysts by the strong Au (core)-La2O3 (shell) interaction, but also could be further applied to the development of other advanced catalysts for soot oxidation. ASSOCIATED CONTENT Supporting Information. Detailed processes of synthesized catalysts of La2O2CO3-500 nanorods supported Au nanoparticles; Schematic and digital photo of GBMR method; XRD, TEM images and size distribution of supported Au NPs; N2 Adsorption-desorption isotherms


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and pore-size distributions; XPS spectra of C 1s and La 3d5/2 regions; Isothermal anaerobic titrations and isothermal soot oxidation experiments; Catalytic activity for soot oxidation under different gaseous conditions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21673142), National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2017A05), PetroChina Innovation Foundation (2018D-5007-0505) and Science Foundation

of

China

University

of

Petroleum,

Beijing

(242017QNXZ02

and

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Support Interaction for Ultrastable Au Nanocatalysts. J. Am. Chem. Soc. 2016, 138, 1613016139. (56) Davies, C.; Thompson, K.; Cooper, A.; Golunski, S.; Taylor, S. H.; Macias, M. B.; Doustdar, O.; Tsolakis, A. Simultaneous Removal of NOx and Soot Particulate from Diesel Exhaust by In-Situ Catalytic Generation and Utilisation of N2O. Appl. Catal. B 2018, 239, 10-15. (57) Xing, L.; Yang, Y.; Cao, C.; Zhao, D.; Gao, Z.; Ren, W.; Tian, Y.; Ding, T.; Li, X. Decorating CeO2 Nanoparticles on Mn2O3 Nanosheets to Improve Catalytic Soot Combustion. ACS Sustainable Chem. Eng. 2018, 6, 16544-16554. (58) Putla, S.; Amin, M. H.; Reddy, B. M.; Nafady, A.; Al Farhan, K. A.; Bhargava, S. K. MnOx Nanoparticle-Dispersed

CeO2 Nanocubes:

A

Remarkable

Heteronanostructured

System with Unusual Structural Characteristics and Superior Catalytic Performance. ACS Appl. Mater. Interfaces 2015, 7, 16525-16535.


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ACS Catalysis

Tables Table 1 Surface compositions and oxidation states of Au and O species over Aun@La2O3/LOC-R catalysts derived from XPS analyses. Au species Catalysts

O species

Au0/%

Au+/%

Au3+/%

Ra

O2-/%

O22-/%

O2-/%

CO32-/%

Rb

La2O2CO3-500

-

-

-

-

13.4

23.3

56.1

7.2

5.9

La2O2CO3

-

-

-

-

10.1

17.0

66.0

6.9

8.2

Au2@La2O3/LOC-R

65.1

25.4

9.5

0.536

8.0

17.4

68.1

6.5

10.7

Au4@La2O3/LOC-R

55.5

34.6

9.9

0.802

5.1

20.3

68.2

6.4

17.4

Au4/LOC-R

60.9

29.2

9.9

0.642

6.5

18.7

68.0

6.8

13.3

a The noble metal species ratio of the Au&+ (Au++Au3+) to Au0. b Determined by the oxygen species ratio of the adsorbed oxygen (O22-+O2-) to lattice oxygen (O2-).


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 40 of 55

Table 2 Catalytic activity, selectivity, the density of active oxygen, relative reaction rate (R) and TOF values of Aun@La2O3/LOC-R catalysts for soot oxidation under the conditions of loose contact.

Catalysts

a (oC)

T10

a (oC)

T50

a (oC)

T90

Sm CO2

Oxygen amountb

(%)

(μmol g-1)

Density of oxygenc (μmol

g-1)

Rd

TOFe

(μmol g-1 s-1)

(× 10-3 s-1)

Soot (without catalyst)

462

586

645

55.0

-

-

-

-

La2O2CO3-500

307

448

504

89.1

20.1

40.2

0.027

0.67

La2O2CO3

306

431

481

92.0

25.3

44.6

0.038

0.85

Au1@La2O3/LOC-R

303

407

455

99.6

26.9

53.8

0.057

1.06

Au2@La2O3/LOC-R

302

397

445

99.7

31.2

62.4

0.068

1.09

Au4@La2O3/LOC-R

278

375

424

99.8

45.2

90.4

0.104

1.15

Au6@La2O3/LOC-R

273

372

423

99.8

49.1

98.2

0.112

1.14

Au4/LOC-R

293

388

438

99.2

37.6

75.2

0.079

1.05

a Reaction condition: 5 % O2, 0.2 % NO in Ar, 50 ml min-1. b Determined by isothermal anaerobic titrations at 300 °C. c The density of active oxygen (DO) is equal to 2 times the consumed active oxygen amount. d Relative reaction rates (R) at 300 oC; Reaction gas: 5 % O2 and 0.2 % NO in Ar, 50 ml min-1. e TOF= R/DO.


Page 41 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figures

Scheme 1. Schematic processes for the preparation of Aun@La2O3/LOC-R and Au4/LOC-R catalysts.


ACS Catalysis

f La2O3 (JCPDS 05-0602) e

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 55

d c b La2O2CO3 (JCPDS 37-0804) a La(OH)3 (JCPDS 36-1481)

10

20

30

2

40

50

60

70

Figure 1 XRD patterns of La(OH)3 nanorods calcined at 50 (a), 500 (b), 550 (c), 600 (d), 650 (e) and 700 (f) oC for 2 h in air.


La2O3 (JCPDS 05-0602)

f e d c b a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(102)

Page 43 of 55

La2O2CO3 (JCPDS 37-0804)

10

20

30

40

2   

50

60

70

Figure 2. XRD patterns of La2O2CO3 and Aun@La2O3/LOC-R catalysts calcined at 600 oC. a. La2O2CO3; b. Au1@La2O3/LOC-R; c. Au2@La2O3/LOC-R; d. Au4@La2O3/LOC-R; e. Au6@La2O3/LOC-R; f. Au4/LOC-R.


ACS Catalysis

285

345

455 1078

372 401

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 55

842

1087 1062

e d

393

c b

779 287 337

200 `

f

455

400

a

599

600

800 -1

1000

1200

Raman shift (cm )

Figure 3. Raman spectra of La2O2CO3 and Aun@La2O3/LOC-R catalysts. a. La(OH)3; b. La2O2CO3-500; c. La2O2CO3; d. Au4@La2O3/LOC-R; e. Au4/LOC-R; f. La2O3.


Page 45 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 4. TEM (A), HRTEM (B-C) images and the shape (D) of La2O2CO3-500 nanorods. -

The different facets are marked with symbols, revealing the main exposure of (110) and (001) -

facets. HRTEM images of (B) and (C) show the viewed along [001] and [110] directions, respectively. Part (D) illustrates the shape of La2O2CO3-500 nanorod.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. TEM, HRTEM and HAADF-STEM images of Aun@La2O3/LOC-R and Au4/LOCR catalysts. The (G) image shows HR-STEM image of one Au NP and its HAADF-STEMEDX element-mapping analyses of Carbon (orange), Au (yellow) and La (green). (A-B) Au2@La2O3/LOC-R; (C-D) Au4@La2O3/LOC-R; (E-F) Au4/LOC-R; (G) Au4@La2O3/LOCR.


Page 46 of 55

Page 47 of 55

722 215

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

f e d c b a 100

167

200

300

400

500 o

600

700

Temperature ( C) Figure 6. The H2-TPR profiles of La2O2CO3 nanorods and supported Au nanoparticle catalysts. a. La2O2CO3; b. Au1@La2O3/LOC-R; c. Au2@La2O3/LOC-R; d. Au4/LOC-R; e. Au4@La2O3/LOC-R; f. Au6@La2O3/LOC-R.


ACS Catalysis

A

Au 4f

3+

Au

+

Au

0

Intensity (a.u.)

Au

1

e

1

d

2

c

92

90

B

88

86

Binding Energy (eV)

84

82

O 1s

-

2-

CO3

O2

2-

O2

2-

O

e

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 55

d c b a 534

532

530

Binding Energy (eV)

528

Figure 7. X-ray photoelectron spectra (XPS) of Au 4f (A) and O 1s (B) regions for La2O2CO3 and Aun@La2O3/LOC-R catalysts. a. La2O2CO3-500; b. La2O2CO3; Au2@La2O3/LOC-R; d. Au4/LOC-R; e. Au4@La2O3/LOC-R.


c.

Page 49 of 55

100

Conversion rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

80 60 40

Without catalyst

La2O2CO3-500 La2O2CO3 Au1@La2O3/LOC-R Au2@La2O3/LOC-R Au4/LOC-R Au4@La2O3/LOC-R Au6@La2O3/LOC-R

20 0 200

300

400

500 o

Temperature ( C)

600

700

Figure 8. The profiles of soot conversion over La2O2CO3 nonorods and supported Au nanoparticle catalysts under conditions of loose contact between soot particles and catalysts.


ACS Catalysis

A 45 -6

Soot conversion amount (mol*10 )

f e

40 35

g

30

d c

25 20

b a

15 10 5 0 0

10

20

30

Time (min)

40

50

60

50 45 40 35 30 25 20 15 10 5 0

B

Active oxygen amounts (umol g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 55

a

b

c

d

e

Catalysts

f

g

Figure 9. Soot conversion amount as a function of time (A) and active oxygen amount (B) over La2O2CO3 and Aun@La2O3/LOC-R catalysts at 300 oC. a. La2O2CO3-500; b. La2O2CO3; c. Au1@La2O3/LOC-R; d. Au2@La2O3/LOC-R; e. Au4@La2O3/LOC-R; f. Au6@La2O3/LOCR; g. Au4/LOC-R.


Page 51 of 55

480

T90

460

b

440 440 o

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

420

a T50

400

b

380

a

310

T10

300

b

290

a

280 1

2

3

4

5

Cycle Times

6

7

8

Figure 10. Stability test of the Au4@La2O3/LOC-R (square) and Au4/LOC-R (triangle) catalysts for soot oxidation.


ACS Catalysis

D 40

Frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 55

D=4.0 nm

30 20 10

1

2

3

4

D (nm)

5

6

7

Figure 11. The TEM (A-B), HRTEM (C) images and size distribution (D) of Au NPs in Au4@La2O3/LOC-R catalyst after eight cycles of soot-TPO tests.


Page 53 of 55

200

250

300

350

o

Temperature ( C)

400

450

B 420

500

440

410

420

400 o

400 380

390

360

380 340

370 360

320

c

d

e Catalysts

f

g

-1

150

a b c d e f g

H2 consumption amounts mol g )

NO2 concentration (a.u.)

A

T50 values ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

300

Figure 12. NO2 concentration curves of NO temperature-programmed oxidation (A) and the relationship between T50 values and H2 consumption amounts (reduction peaks at 160-220 oC) (B). a. La2O2CO3-500; b. La2O2CO3; c. Au1@La2O3/LOC-R; d. Au2@La2O3/LOC-R; e. Au4@La2O3/LOC-R; f. Au6@La2O3/LOC-R; g. Au4/LOC-R.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 13. Schematic diagram of soot oxidation over Aun@La2O3/LOC-R catalysts.


Page 54 of 55

Page 55 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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