Yanfeng Shen

Mingrui Xu

Jingyang Li

Zhengxing Qin

Chunzheng Wang

Svetlana Mintova

Xinmei Liu

The preparation of core-shell-like zeolite by diffusion controlled chemical etching

The present work reports on the preparation of core-shell-like zeolites with Si-rich mesoporous shell. This is achieved by diffusion controlled chemical etching of zeolite crystals with a mixture containing oxalic acid and ammonium fluoride (OA-NH 4 F). The OA-NH 4 F served as a bifunctional etchant extracting both surface Si and Al and further dealuminating the periphery of the zeolite crystals. The fast dissolution kinetics and the relatively slow intra-crystalline diffusion of the OA-NH 4 F etching solutions resulted in a distinct core-shell-like structure with porous and Si-rich shell keeping the unique advantages of single crystals with interconnected micro-and mesopores.

Introduction

Zeolites, crystalline microporous aluminosilicates, are widely used in industrial processes as shape-selective catalysts thanks to their well-defined structure, variable chemical composition and surface acidity and outstanding thermal and hydrothermal stability [START_REF] Vermeiren | Impact of zeolites on the petroleum and petrochemical industry[END_REF][START_REF] Martínez | Inorganic molecular sieves: Preparation, modification and industrial application in catalytic processes[END_REF] . The sub-nanometer sized micropores are the unique advantage of zeolites for shape-selective catalysis. On the other hand, however, for reactions involving bulky molecules with size greater than zeolite micropores, the intra-particle diffusion of reactants and intermediate products in and out zeolite crystals become an important concern for zeolites scientists. To address this mass transport issue, several characteristic approaches have been developed, which including nano zeolites [START_REF] Mintova | Mechanism of zeolite A nanocrystal growth from colloids at room temperature[END_REF] , extra-large micropore zeolites [START_REF] Corma | High-throughput synthesis and catalytic properties of a molecular sieve with 18-and 10-member rings[END_REF] , and hierarchical zeolites [START_REF] Pérez-Ramírez | Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design[END_REF] . As the most versatile method for improving the intra-particle diffusion efficiency of existing zeolites, hierarchization has been developed as the most popular treatment for the post-synthetic engineering of zeolite that are prepared in large scale at industrial level [START_REF] Chen | Hierarchically Structured Zeolites: From Design to Application[END_REF] .

During the preparation of hierarchical zeolites, the persistence of crystal surface is commonly observed but often ignored [START_REF] Ohsuna | Dealumination of hexagonal (EMT)/cubic (FAU) zeolite intergrowth materials: A SEM and HRTEM study[END_REF][START_REF] Qin | Mesoporous Y zeolite with homogeneous aluminum distribution obtained by sequential desilication-dealumination and its performance in the catalytic cracking of cumene and 1, 3, 5-triisopropylbenzene[END_REF][START_REF] Yuan | Creation of mesostructured hollow Y zeolite by selective demetallation of an artificial heterogeneous Al distributed zeolite crystal[END_REF][START_REF] Li | Defects in AHFS-dealuminated Y zeolite: A crucial factor for mesopores formation in the following base treatment procedure[END_REF][START_REF] Qin | Preparation of Single Crystals "House-of-Cards"-like ZSM-5 and Their Performance in Ethanol-to-Hydrocarbons Conversion[END_REF] . This surface of zeolite crystals is characterized by the universal formation of a "crust" on the outermost layer, no matter the post synthesis treatment including desilication-biased caustic leaching [START_REF] Groen | Creation of hollow zeolite architectures by controlled desilication of Al-zoned ZSM-5 crystals[END_REF][START_REF] Mei | Regular HZSM-5 microboxes prepared via a mild alkaline treatment[END_REF][START_REF] Wang | Nanoporous zeolite single crystals: ZSM-5 nanoboxes with uniform intracrystalline hollow structures[END_REF][START_REF] Dai | Synthesis of hollow nanocubes and macroporous monoliths of silicalite-1 by alkaline treatment[END_REF][START_REF] Xia | Hollow Titanium Silicalite Zeolite: From Fundamental Research to Commercial Application in Environmental-Friendly Catalytic Oxidation Processes[END_REF] , dealumination-biased acidic treatment [START_REF] Ohsuna | Dealumination of hexagonal (EMT)/cubic (FAU) zeolite intergrowth materials: A SEM and HRTEM study[END_REF][START_REF] Qin | Mesoporous Y zeolite with homogeneous aluminum distribution obtained by sequential desilication-dealumination and its performance in the catalytic cracking of cumene and 1, 3, 5-triisopropylbenzene[END_REF][START_REF] Yuan | Creation of mesostructured hollow Y zeolite by selective demetallation of an artificial heterogeneous Al distributed zeolite crystal[END_REF] , or Si and Al unbiased NH 4 F etching [START_REF] Qin | Preparation of Single Crystals "House-of-Cards"-like ZSM-5 and Their Performance in Ethanol-to-Hydrocarbons Conversion[END_REF] .

Typically, the formation of such a crust was attributed to an Al-zoning induced inhomogeneous dissolution of zeolite crystals [START_REF] Dessau | Aluminum zoning in ZSM-5 as revealed by selective silica removal[END_REF] . This is originated from the interpretation that the preferential sitting of Al on zeolite rim protects the outermost shell from dissolution. The preferential dissolution of Si-rich core collaboratively leads to the formation of hollow zeolite structure. Alternatively, the difficulty to engineer the outermost shell of zeolite crystals is related to its less defective nature [START_REF] Qin | Preparation of Single Crystals "House-of-Cards"-like ZSM-5 and Their Performance in Ethanol-to-Hydrocarbons Conversion[END_REF] .

The "coat" is formed at the very end of a zeolite crystallization process [START_REF] Anderson | Crystal Growth in Zeolite Y Revealed by Atomic Force Microscopy[END_REF][START_REF] Cubillas | AFM and HRSEM Invesitigation of Zeolite A Crystal Growth. Part 1: In the Absence of Organic Additives[END_REF][START_REF] Agger | Silicalite Crystal Growth Investigated by Atomic Force Microscopy[END_REF][START_REF] Schmidt | Diagnosing the Internal Architecture of Zeolite Ferrierite[END_REF] . While at this step, the crystallization medium shapes the final surface of zeolites, it also creates a less-defective outermost shell resistive to chemical treatments [START_REF] Qin | Preparation of Single Crystals "House-of-Cards"-like ZSM-5 and Their Performance in Ethanol-to-Hydrocarbons Conversion[END_REF][START_REF] Shen | The inner heterogeneity of ZSM-5 zeolite crystals[END_REF] . The surface resistance of zeolites to chemical etching has not been decisively studied. We anticipate that the transformation of the outermost layer of zeolites to a highly porous shell can be highly favorable for the adsorption of bulky molecules.

The research interest in engineering of zeolite surface is further stimulated by the recent results showing that transport of molecules through microporous crystals can be significantly influenced by diffusion [START_REF] Kaerger | In-depth study of surface resistances in nanoporous materials by microscopic diffusion measurement[END_REF][START_REF] Zhou | The essential mass transfer step in hierarchical/nano zeolite: surface diffusion[END_REF] . The crystal periphery is the first path the molecules encounter during the diffusion into zeolite crystals. To alleviate the surface barriers, post-synthetic modification of the zeolite surface was conducted. In this respect, the covering of a mesoporous and inert over-layer on top of zeolite surface has been proven successful in enhancing the sorption rate because of an increased sticking probability [START_REF] Reitmeier | Enhancement of Sorption Processes in the Zeolite H-ZSM5 by Postsynthetic Surface Modification[END_REF] . The preparation of zeolitic core-shell materials with zeolite core and mesoporous shell was reported [START_REF] Guerrero-Martinez | Recent Progress on Silica Coating of Nanoparticles and Related Nanomaterials[END_REF][START_REF] Enterria | Preparation of hierarchical micro-mesoporous aluminosilicate composites by simple Y zeolite/MCM-48 silica assembly[END_REF][START_REF] Qian | Controllable fabrication of uniform core-shell structured zeolite@ SBA-15 composites[END_REF][START_REF] Zhao | Morphological control of highly ordered mesoporous silica SBA-15[END_REF][START_REF] Vartuli | Effect of surfactant/silica molar ratios on the formation of mesoporous molecular sieves: inorganic mimicry of surfactant liquid-crystal phases and mechanistic implications[END_REF][START_REF] Qian | Exploring Meso-/Microporous Composite Molecular Sieves with Core-Shell Structures[END_REF] . The preparation of core-shell zeolite composites with an integral core and shell parts free of interface boundary are rare. The engineering of zeolite crystals with core-shell structures consisting of a rough mesoporous shell and a zeolitic core would be interesting due to the simultaneous enhancement of surface diffusion and the large preservation of the intrinsic zeolite properties.

In recent years, we focused on the fluoride etching approach (see Figure 1 below) towards post-synthetic engineering of zeolites crystals. The uniqueness in the preparation of mesoporous zeolites with intact microporosity and acidity was demonstrated [START_REF] Qin | The mosaic structure of zeolite crystals[END_REF] . Additionally, we revealed the formation of extended intergrown subunits in zeolite crystals with MFI type structure [START_REF] Qin | Chemical equilibrium controlled etching of MFI-type zeolite and its influence on zeolite structure, acidity, and catalytic activity[END_REF] , the uncovering of the inner mosaic structure of zeolite crystals [START_REF] Qin | The mosaic structure of zeolite crystals[END_REF] , the opening of previously inaccessible zeolite cages [START_REF] Qin | Opening the cages of Faujasite-type zeolite[END_REF] and side pockets [START_REF] Qin | Defect-engineered zeolite porosity and accessibility[END_REF] . The inner heterogeneity of ZSM-5 zeolite crystals was recently revealed by our group [START_REF] Shen | The inner heterogeneity of ZSM-5 zeolite crystals[END_REF] . The studies above were performed taking the unique advantage of fluoride etching approach (NH 4 F etching, "III" in Figure R1) leading to unbiased removal of Si and Al from the zeolite framework. This sharply contrasts the Si-biased dissolution of zeolite crystals in alkaline media, and the Al-biased dealumination by steaming and/or acid leaching.

Herein, we report a new etchant for the "top-down" preparation of a core-shell-like hierarchical zeolite. Zeolite Y (FAU type framework structure), the most commercially used zeolite in heterogeneous catalysis was prepared as a core-shell-like hierarchical material comprised of a rough mesoporous shell and a microporous core, both of which possess the same cohesive zeolite structure.

Experimental

Preparation of single core-shell-like zeolite crystals

Commercial zeolite Y (LZY-62 from UOP, Si/Al = 2.6) was used in this study.

Before chemical etching, the starting material was ion-exchanged three times with a 1 mol/L aqueous ammonium chloride (99.5%, Sinopharm Chemical Reagent Corporation) solution at liquid/solid ratio of 20. The ammonium exchange was carried out at 363 K for 3 h each time. The sample was thoroughly washed using deionized water after each ion exchange. The resulting zeolite was used as the parent sample (PY) for the subsequent treatments.

The acidic etching of PY with oxalic acid was performed as follow: 5 g PY and 5 g ammonium sulfate (99.0%, Sinopharm Chemical Reagent Corporation) were dispersed in 50 g deionized water under stirring at room temperature. Then 17.5 g of 1 mol/l oxalic acid (99.5%, Sinopharm Chemical Reagent Corporation) solution was added dropwise into the zeolite suspension within 20 min. The acid treatment was conducted for 2 h under stirring at room temperature. After that, the acid treated product was recovered by vacuum filtration, washed thoroughly with deionized water and dried at 373 K overnight. The resulting sample was denoted as AY.

The other sample abbreviated as FY was prepared by treating the parent zeolite (PY) with NH 4 F solution. 5 g PY and 5 g ammonium sulfate (99.0%, Sinopharm Chemical Reagent Corporation) were dispersed into 50 g deionized water. 5 g NH 4 F was then added into the mixture under magnetic stirring. The process was kept at room temperature for 2 h. The product was separated by filtration, washed with deionized water thoroughly and dried under 373 K overnight.

The treatment of sample PY with oxalic acid-ammonium fluoride (OA-NH 4 F) was carried as follows: 5 g of PY and 5 g ammonium sulfate was dispersed in 50 g deionized water under stirring at room temperature. Then an oxalic acid-ammonium fluoride mixture made of 5 g ammonium fluoride and 17.5 g or 10 g 1M oxalic acid solution, was added. The obtained suspension was kept at room temperature for 2 h under stirring. After that, the products were recovered by vacuum filtration, washed thoroughly with water and dried at 373 K overnight. The sample treated by OA-NH 4 F with a higher (17.5 g) or lower (10 g) concentration of oxalic acid were denoted as AFY1 and AFY2 respectively.

Characterization

X-ray diffraction measurements were performed on an X`pert Pro/PANalytical Diffractometer using Cu Kα radiation (λ = 1.5418 Å, 45 kV, 40 mA). The samples were studied in the 5-50° 2θ range with a scanning step of 0.0167° s -1 .

Electron micrographs were taken on a JSM-7900F (JEOL) low-voltage high-resolution scanning electron microscope (SEM) equipped with a field emission gun. The cross-section of the AFY1 and AFY2 zeolites were obtained by using an ion Beam cross-section polisher (IB-19510CP, JEOL). Transmission electron microscopy (TEM) images were recorded using a JEOL 2100F microscope at 200 kV.

Nitrogen adsorption/desorption isotherms were recorded with a Micromeritics ASAP 2020 automated gas adsorption analyzer. Prior to analysis, the samples were out gassed at 373 K for 1 h and 573 K for 10 h. Specific surface areas were determined from the BET equation applied to the 0.01 -0.06 p/p 0 range. The total pore volume was taken from nitrogen adsorbed volume at p/p 0 = 0.97. The t-plot method, applied in the 0.4 -0.8 range, was used to distinguish the micropores from the mesopores in the samples. [START_REF] Enterria | Preparation of hierarchical micro-mesoporous aluminosilicate composites by simple Y zeolite/MCM-48 silica assembly[END_REF] Al MAS NMR spectra were recorded on a Bruker Advance 500 MHz spectrometer using a 4 mm rotor. The [START_REF] Enterria | Preparation of hierarchical micro-mesoporous aluminosilicate composites by simple Y zeolite/MCM-48 silica assembly[END_REF] Al MAS NMR spectra were recorded at 104.3 MHz with a 12 pulse length of 2.2 μs, a spinning rate of 14 kHz and a recycle delay of 1 s. 1 M Al(NO 3 ) 3 was used as reference for chemical shifts of [START_REF] Enterria | Preparation of hierarchical micro-mesoporous aluminosilicate composites by simple Y zeolite/MCM-48 silica assembly[END_REF] Al.

Infrared spectra were recorded using a Nicolet Magna 550-FT-IR spectrometer with 2 cm -1 optical resolution. Prior to the measurements, the samples were pressed into self-supporting discs (diameter: 1.6 cm, ~17 mg) and preheated in the IR cell attached to a vacuum line at 723 K (2 K /min) for 5 h down to 10 -6 Torr. The adsorption of pyridine was performed at 373 K. After establishing a pressure of 1 Torr at equilibrium, the cell was then evacuated at 423 -673 K and the amount of pyridine remaining adsorbed on Brønsted and Lewis sites determined using the integrated area of the bands observed at 1545 cm -1 and 1454 cm -1 , respectively. All spectra were normalized to 20 mg wafers. The extinction coefficients used were ɛ(B) 1545 = 1.3 and ɛ(L) 1455 = 1.5. The desorption temperature selected for the quantitative measurement of acidity by pyridine was 473 K.

The chemical composition of parent and treated samples was measured by XRF.

The surface component of zeolites was probed by X-ray photoelectron spectroscopy (XPS) with a PHI 500 spectrometer using Al Ka radiation.

Catalytic test

The conversion of 1,3,5-triisopropylbenzene (TiPBz) was performed in a tubular flow reactor operating in gas phase at atmospheric pressure (1×10 5 Pa). For each experiment, 20 mg of zeolite were loaded in the reactor enclosed between two layers of inert SiC (200 ~ 400 μm size range). The samples were carefully activated in a dry-air stream (50 mL•min -1 ), first at 373 K for 1 h, and then the temperature raised to 450 °C (ramp of 1.5 °C•min -1 ) and maintained 3 h at this temperature. N 2 was subsequently introduced and the reactor cooled-down to the reaction temperature (498 K). A stream of 200 mL•min -1 N 2 was diverted to a saturator filled with TiPBz maintained at 72 °C (P TiPBz = 192 Pa). A unique space time, W/F 0 , of 0.022 kg•h•mol -1 was used. Stable conditions were established in less than 5 minutes; the products were analyzed after 5 min, thereafter at 15 min intervals on a CPG (VARIAN Cp 3800; HP-Pona 50 m, 0.2 mm, 0.5 μm column) with a FID detector.

Results and discussion

Dynamic chemical equilibrium in aqueous oxalic acid-NH 4 F medium

In order to transfer the tough zeolite "coat" into a porous shell, an intensive dissolution of the surface zeolite layer is a prerequisite. Therefore, a highly reactive etchant that is both active for Si and Al removal was applied. The aqueous oxalic acid-NH 4 F mixture (OA-NH 4 F) was used. This is based on our previous finding that NH 4 F-HF mixed solution is able to extract both framework Si and Al unbiasedly [START_REF] Qin | Chemical equilibrium controlled etching of MFI-type zeolite and its influence on zeolite structure, acidity, and catalytic activity[END_REF] .

The co-presence of F -and HF shifts the chemical equilibria and produce in-situ the active fluoride species such as HF, H 2 F 2 , HF 2 -that are active for Si and Al removal [START_REF] Qin | The mosaic structure of zeolite crystals[END_REF] .

Oxalic acid was used here to avoid the direct use of hazardous HF. The higher acid dissociation constant of oxalic acid (K a1 = 5.9 × 10 -2 ) suppresses the dissociation of HF (K a = 6.8 × 10 -4 ) into H + and F -(① and ② in Scheme 1), thus favoring the equilibrium loop towards HF, H 2 F 2 and HF 2 -formation (④, ⑤ and ⑥ in Scheme 1).

What is of additional interest is the chelating properties of oxalic acid [START_REF] Apelian | Dealumination of zeolite β via dicarboxylic acid treatment[END_REF][START_REF] Yan | On the acid-dealumination of USY zeolite: a solid state NMR investigation[END_REF] . As will

Characterization of core-shell-like zeolites

The addition of OA-NH 4 F into the zeolite suspension (Experimental part) increased the pH immediately from 5.5 to above 7, then it continued to increase slowly and stabilized at around 8 (Figure S1). This distinct increase in pH and the release of ammonia are obvious signs of the ongoing hydrolysis of NH 4 F and progressive consumption of HF ( ② , ④ , ⑤ and ⑥ in Scheme 1). Hence a progressive etching of zeolite crystals can be expected. The parent zeolite Y exhibits a higher adsorption capacity for nitrogen at lower relative pressure p/p 0 (Figure 2a, Table 1), which is characteristic of a highly crystalline microporous zeolite. The almost complete absence of mesoporosity for sample FY clearly indicates that the applied NH 4 F etching alone did not lead to the considerable dissolution of the parent zeolite crystals. This is because the concentration of NH 4 F used in the present work is much lower than the one reported before [START_REF] Qin | Opening the cages of Faujasite-type zeolite[END_REF] . On the other side, the chemical treatment with pure oxalic acid resulted in a sample (AY) with a marked low microporosity (Figure 2a and Table 1), which is in agreement with the XRD (Figure 1). However, the loss of microporosity did not lead to an increase in mesoporosity (Table 1). In contrast, the AFY1 sample treated by oxalic acid -NH 4 F mixed solution, shows high adsorption of nitrogen at lower p/p 0 (Figure 2a). The micropore volume of AFY1 is 0.22 cm 3 g -1 (Table 1). In addition, the N 2 adsorption isotherm exhibits a continuous uptake between ca. 0.01 and 0.8 p/p 0 and a hysteresis loop at p/p 0 of ca. 0.4 (Figure 2a). This result indicates that there are small mesopores or even large secondary micropores formed in the AFY1 zeolite crystals [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF] . The pore size distribution (PSD) curves calculated from both the adsorption and desorption branches of the isotherms show noticeable peaks in 2 -10 nm range (Figure 2b,c). Thus, the XRD and N 2 physisorption results provide the first sign that pure NH 4 F, pure oxalic acid and their mixture etched differently the zeolite crystals. The parent and treated zeolites were further studied by SEM, TEM and SEM-EDX mapping. High resolution low voltage SEM show unambiguously that the etching by OA-NH 4 F leads to a remarkably roughened external surface in the case of AFY-1 zeolite sample (Figure 3b-d). Such a rough surface sharply contrasts the smooth feature of the parent sample (PY, Figure 3a) and the one treated by oxalic acid (AY, Figure S3a) or NH 4 F (FY, Figure S3b). Noting that both AY and FY were prepared by chemical etching of PY with the same amount of oxalic acid and NH 4 F as that for AFY1 (see experimental section). In addition, the surface of AFY1 looks also different from that of ultra-stable Y [START_REF] Yokoi | Characterization of zeolites by advanced SEM/STEM techniques[END_REF] and surfactant-templated mesostructured Y zeolites [START_REF] Prasomsri | Mesostructured zeolites: bridging the gap between zeolites and MCM-41[END_REF] . In contrast to the homogeneous and dense PY zeolite crystals seen by TEM (Figure S4), the AFY1 shows a distinct core-shell-like structure with a highly porous shell (Figure 4). The porous shell has a thickness of about 100 nm. No dense outermost crust similar to those reported in previous studies was observed [START_REF] Ohsuna | Dealumination of hexagonal (EMT)/cubic (FAU) zeolite intergrowth materials: A SEM and HRTEM study[END_REF][START_REF] Li | The Link between ZSM-5 Zeolite Crystallization and Mesopore Formation by Leaching[END_REF] .

m 2 •g -1 m 2 •g -1 m 3 •g -1 m 3 •g -1 μmol•g -1 μmol•g -
Compared with the classical core-shell materials [START_REF] Qian | Exploring Meso-/Microporous Composite Molecular Sieves with Core-Shell Structures[END_REF][START_REF] Bouizi | Core-shell zeolite microcomposites[END_REF] , a clear-cut separation between the core and the shell was not observed in the present work. The hierarchical shell part increases its density progressively from the periphery to the central part of zeolite crystals. High resolution TEM reveals the presence of lattice fringes near the exterior of the particles (Figure S5), suggesting the partial preservation of the crystalline zeolite Y. In order to get more comprehensive information about the internal structural characteristics of the core-shell-like zeolite, AFY1 was cut with an ion beam cross-section polisher and the SEM images are depicted in Figure 5. As shown, most mesopores are distributed in the periphery of the cut crystals no matter the size of the zeolite crystals, while the inner part is less porous. In addition to the mesopores that can be distinguished under the applied conditions for SEM observation, there are many other small mesopores which are "invisible" due to the SEM resolution limit.

The presence of small mesopores is indicated by the nitrogen physisorption results

(Figure 2a,2b). Whether they can be "seen" or not, the chemically induced mesopores all together contributed to the formation of a porous shell in the AFY1 zeolite. This mesopore-zoning is clearly indicated by the darker contrast of the crystal rim in the cross-section SEM images. Noting that the intensity of the generated secondary electrons varies depending on the density of the substance, i.e. darker contrast corresponds to less intense secondary electrons, hence to a less dense zone [START_REF] Qin | Preparation of Single Crystals "House-of-Cards"-like ZSM-5 and Their Performance in Ethanol-to-Hydrocarbons Conversion[END_REF] . The distinct difference showing the core-shell-like characteristic of AFY1 zeolite is further provided by SEM-EDX mapping results (Figure 6), where a bright ring indicating a substantial Si-zoning in the crystal periphery is clearly distinguished. In contrast, the core part is richer in Al and the spatial distribution of Si and Al in this part is homogeneous. The bulk Si/Al ratio of 5.5 for the hierarchical core-shell-like zeolite is determined by XRF. While the surface Si/Al ratio of 14.3 for the same sample was determined by XPS analysis. These observations related with size and spatial distribution of mesopores and the thickness of the shell (~150 nm) of the zeolite crystals are in a good agreement with the TEM results (Figure 4).

The combined information from XRD, SEM, TEM and N 2 physisorption confirmed that a crystalline core-shell-like hierarchical zeolite with a rough mesoporous shell was prepared. This was achieved by chemical etching with OA-NH 4 F solutions, which resulted in the extensive removal of Si and Al from the surface of zeolite crystals. The 27 Al MAS NMR spectrum of AFY1 shows a single peak at ca. 60 ppm highlighting the sole presence of tetrahedral aluminum (Figure 2d).

The chemical etching leaded to a noticeable increase of Si/Al ratio in crystal periphery (shell); the substantial removal of both Si and Al from zeolite surface is presented in Figure 6. The Si and Al unbiased chemical etching characteristics for other fluorine-containing media was reported earlier [START_REF] Qin | The mosaic structure of zeolite crystals[END_REF][START_REF] Qin | Chemical equilibrium controlled etching of MFI-type zeolite and its influence on zeolite structure, acidity, and catalytic activity[END_REF][START_REF] Qin | Mesoporous zeolites by fluoride etching[END_REF] . The substantial dealumination behavior is most probably related to the high acid dissociation constant and chelating effect of the oxalic acid.

In agreement with the observed increase of the Si/Al ratio, the IR spectroscopy using pyridine as a probe molecule shows that the Brønsted acidity of sample OA-NH 4 F dropped by half as compared with the parent zeolite (Table 1, Figure S6).

The Brønsted acidity of AFY1 is much higher than that of the sample treated with oxalic acid (Table 1, Figure S6). Thus, the presence of NH 4 F in the OA-NH 4 F is not only crucial for the substantial removal of surface Si and Al leading to the core-shell-like structure formation, but is essential for preserving the crystallinity of the sample (Figure 1). Note that the leaching in the sole presence of oxalic acid indeed results in a substantial loss of crystallinity (Figure 1) without any core-shell hierarchical zeolite formation (Figure 2a).

We further emphasize on the single-crystal architecture and the excellent interconnectivity between the core and shell of the zeolite samples reported here.

While many core-shell zeolites have been synthesized [START_REF] Bouizi | Core-shell zeolite microcomposites[END_REF][START_REF] Mochizuki | Catalytic reactions enhanced under microwave-induced local thermal non-equilibrium in a core-shell, carbon-filled zeolite@ zeolite[END_REF][START_REF] Bouizi | Factors controlling the formation of coreshell zeolite-zeolite composites[END_REF][START_REF] Ghorbanpour | Epitaxial growth of ZSM-5@ Silicalite-1: A core-shell zeolite designed with passivated surface acidity[END_REF][START_REF] Bouizi | Beads comprising a hierarchical porous core and a microporous shell[END_REF][START_REF] Pirngruber | Core-shell zeolite composite with enhanced selectivity for the separation of branched paraffin isomers[END_REF][START_REF] Jin | Methane dehydroaromatization by Mo-supported MFI-type zeolite with core-shell structure[END_REF] , a zeolite with well interconnected micro-and/or meso-porous core and a rough mesoporous shell is rare [28, 42, 50] . Core-shell boundary and misaligned shell growth are some of the undesired consequences of many previously reported techniques applied for the preparation of core-shell materials. The interfaces between the core and shell may constitute diffusion boundaries due to possible mismatching in the alignment of different microporous domains [START_REF] Karwacki | Morphology-dependent zeolite intergrowth structures leading to distinct internal and outer-surface molecular diffusion barriers[END_REF] . In our case, the shell is created by the intensive dissolution of the outermost shell of the same crystals. It can, therefore, be expected that the continuous channels with mesoporous openings will permit unhindered access guest molecules to diffuse in the intrinsic 3D porous network of zeolites. S7). The core-shell-like characteristic of this sample was further confirmed by advanced SEM characterization showing the enrichment of mesopores in the crystals periphery (Figure S8). The EDX mapping of the AFY2 shows a distinct core-shell morphology with a Si-rich shell (Figure 7). In line with this data, the surface Si/Al as determined by XPS is 11.7, and the bulk Si/Al is 4.0. These results confirmed that that the intensive surface etching is controlled by the overall ion strength of the active fluoride species (both Si and Al active), but not by the concentration of oxalic acid along. Noting that the AY sample treated by the oxalic acid has the same smooth surface as the PY (Figure S3a). On the other hand, both the surface and bulk Si/Al ratios of sample AFY2 are lower than that of AFY1. AFY2 also shows higher microporosity (+0.08 cm 3 g -1 , Table 1) and a measurable increase in the crystallinity (Figure 1) without losing the hierarchical core-shell-like characteristics. A careful comparison of the EDX mapping indicates that the acid etching depth decreased in the case of AFY2 (Figures 6 and7). This is a clear sign that the concentration of oxalic acid has an impact on the microporosity of the samples, their chemical composition and on the thickness of the Si-rich shell of zeolite crystals.

Tuning the zeolite surface: turning trade-offs into synergies

We believe that the mixture OA-NH 4 F works as a bifunctional reagent during the chemical etching process. Namely, NH 4 F and more specifically its fluoride derivatives are responsible for framework demetallation (simultaneous Si and Al removal), while the oxalic acid is responsible for the additional framework dealumination. The OA-NH 4 F etching is a diffusion-controlled process, and the intrinsic activity of the NH 4 F-oxalic acid etchant is expected to be very high. This can be rationalized by considering that the dissolution of zeolite crystals should be more homogeneous over space and time, if the dissolution is a kinetically controlled process [START_REF] Qin | Opening the cages of Faujasite-type zeolite[END_REF] . Because of the different kinetics between surface dissolution and intra particle transport of etchants, extensive etching of zeolite was limited on crystal surface. This facilitates the permeation of the chelating agent and favors surface layer dealumination. As a supporting argument, both AFY1 and AFY2 zeolites show higher bulk Si/Al ratios than their oxalic acid treated counterparts (5.5 vs 5.4 and 4.0 vs 3.7, respectively). The Si/Al surface ratios of AFY1 (14.3) and AFY2 (11.7) are also higher than that of AY (11.4). Noting that the concentration of oxalic acid used for AY treatment is equal to that used for AFY1 and 1.6 times more than what used for AFY2. Additional argument comes from the molecular simulation results, which showed that the aluminum trioxalate complex has a longest dimension of 0.64 nm [START_REF] Apelian | Dealumination of zeolite β via dicarboxylic acid treatment[END_REF] . Although this is barely smaller than the pore opening of FAU, the countercurrent diffusion of dissolved aluminum trioxalate coming out should compete with other incoming etchant species and solvent molecules for micropore space. Thus, the chemical etching process itself serves as the first evidence that the "digging" of crystal surface has successfully turned the underperforming surface wall into a high permeable shell.

This may facilitate the contact between Al in zeolite framework and the oxalic acid chelating agent. As a second example highlighting the improved permeability of the zeolite periphery (shell), the dealkylation of 1,3,5-triisopropylbenzene (TiPBz) as a model reaction was carried out (Figure 8). The bulky TiPBz is known to probe specifically the external or mesoporous surfaces of zeolites [START_REF] Qin | Opening the cages of Faujasite-type zeolite[END_REF][START_REF] Awala | Template-free nanosized faujasite-type zeolites[END_REF] . As expected, TiPBz conversion is lower on the parent zeolite PY but increases significantly for the AFY1. The surface Si/Al ratio of sample AFY1 is 14.3, while the Si/Al ratio of PY is 2.6, which unambiguously demonstrates that the accessibility of the inner structure of sample AFY1 is substantially improved. Noticeably, AFY2 showed higher initial activity for dealkylation of TiPBz than AFY1, which corresponds to the higher crystallinity (Figure 1) and acidity of the former core-shell-like structure (Table 1, Figure S6).

Apparently, higher crystallinity (hence higher acidity) in the porous shell is responsible for the higher initial activity of the AFY2 zeolite. AY showed lower TiPBz conversion with time-on-stream than PY. The lower conversion is attributed to the substantially decreased surface active sites and poor surface permeability.

Conclusion

In conclusion, we have developed a simple top-down approach for the preparation of core-shell-like zeolites. This was achieved by diffusion-controlled chemical etching of zeolite crystals with oxalic acid and NH 4 F mixed solution (OA-NH 4 F). The fast dissolution kinetics and the relatively slower intra-crystalline diffusion of the solutions limited the intensive dissolution of zeolite crystals only at the periphery. OA-NH 4 F served as a bifunctional etchant, extracted both surface Si and Al, and further caused dealumination. This resulted in a distinct core-shell-like structure with a rough, porous, and Si-rich shell. Such a hierarchical core-shell-like zeolite is analogous to a bottom-up core-shell zeolite but with the unique advantages of preserving the single crystals characteristic, completely interconnected micro-and

  Scheme 1. (I -IV) The evolution of the fluoride etching toolkit for the post-synthetic engineering of zeolite crystals. (IV) The hydrolysis of oxalic acid (①) and fluoride species (② -⑥) in aqueous oxalic acid and NH 4 F mixture.

  After the chemical treatment, the samples were characterized by XRD. The XRD patterns of the parent and treated zeolites are shown in Figure l. All samples show well-defined Bragg peaks characteristic of FAU-type zeolite, regardless of the crystallinity. These samples were subjected to nitrogen physisorption analysis. FY, the NH 4 F etched zeolite Y, displays a typical type I isotherms similar to parent Y (PY).

Figure 1 .

 1 Figure 1. XRD patterns of the parent Y zeolite (PY) and its derivatives prepared by chemical etching using NH 4 F (FY), oxalic acid (AY) and oxalic acid -NH 4 F mixture (AFY1, 2).

Figure 2 .

 2 Figure 2. N 2 sorption isotherms (a), BJH pore size distribution (b: adsorption, c: desorption) and 27 Al NMR spectra (d) of the parent Y, NH 4 F (FY), oxalic acid (AY) and OA-NH 4 F mixed solution (AFY1 and AFY2) treated zeolites.

Figure 3 .

 3 Figure 3. SEM images of the parent zeolite Y sample (a) and sample AFY1 after treatment with OA-NH4F solution at different magnifications (b-d).

Figure 4 .

 4 Figure 4. TEM images of the AFY1 zeolite crystals at different magnifications (a-d).

Figure 5 .

 5 Figure 5. Cross-section SEM images of different AFY1 zeolite crystals (a-d).

Figure 6 .

 6 Figure 6. SEM and EDX mapping of AFY1 zeolite crystal (a: SEM image, b: EDX mapping image of overlapping Al with Si, c: EDX mapping image of Al, d: EDX

3. 3 Figure 7 .

 37 Figure 7. SEM EDX mapping of AFY2 zeolite (a: SEM image, b: EDX mapping image of overlapping Al with Si, c: EDX mapping image of Al, d: EDX mapping

Figure 8 .

 8 Figure 8. Dealkylation of 1,3,5-triisopropylbenzene on PY (parent zeolite Y), AY (oxalic acid treated zeolite Y) and hierarchical core-shell-like zeolites AFY1 and AFY2 (oxalic acid and NH 4 F. Reaction conditions: T = 498 K, W/F 0 = 0.022 kg h mol -1 , P = 0.1 MPa.

Table 1 .

 1 Physicochemical characteristics of the series of zeolite Y samples subjected

	to chemical etching.
	S BET b	S ext	c	V mic	c	V meso d	B Py e	L Py e
	samples Si/Al a

mesopores and rough mesoporous surface. The core-shell-like zeolites can serve as a model system for surface permeability and diffusion studies, and to evaluate the effect of Al gradient on the catalytic performance due to the Si-rich shell.

Acknowledgements

The authors acknowledge funding from the Sino-French joint laboratory "Zeolites", and the funding from NSFC (21978326, 21706285, 21975285 21991090 and 21991091), Qingdao Applied Basic Research Project (19-6-2-70-cg), the Fundamental Research Funds for the Central Universities (18CX02013A), and the Natural Science Foundation of Shandong Province (ZR2019MB029). S.M. acknowledges the support from Thousand Talents Program for Foreign Experts (WQ20152100316). The authors thank Desheng Xi from JEOL for his support on the preparation of cross-sections and SEM-EDX mappings of the samples.

Conflicts of interest

The authors declare no competing financial interest.


Pingping Wu

Zhengke He

Yonghui Liu

Lei Song

Chunzheng Wang

Edgar Muhumuza

Peng Bai

Lianming Zhao

Svetlana Mintova

Zifeng Yan

Compatibility between Activity and Selectivity in Catalytic Oxidation of Benzyl Alcohol with Au-Pd Nanoparticle through Redox Switching of SnO x

Keywords: SnO x, Au-Pd, oxygen transfer, alloying, benzyl alcohol oxidation

published or not. The documents may come

INTRODUCTION

The catalytic partial oxidation to produce various fine chemicals is a vital research area for the pharmaceutical, food, and cosmetic industries. [START_REF] He | Switching-off Toluene Formation in the Solvent-free Oxidation of Benzyl Alcohol using Supported Trimetallic Au-Pd-Pt Nanoparticles[END_REF] One of such processes in the modern synthetic chemical industry is the catalytic partial oxidation of primary alcohols to their corresponding intermediates, [START_REF] Al-Saeedi | Catalytic Oxidation of Benzyl Alcohol Using Nanosized Cu/Ni Schiff-Base Complexes and Their Metal Oxide Nanoparticles[END_REF] such as benzyl alcohol oxidation to benzaldehyde, benzyl benzoate, benzoic acid and/or benzyl ether depending on the catalytic system and reaction conditions used. [START_REF] Campisi | Metal-Support Cooperative Effects in Au/VPO for the Aerobic Oxidation of Benzyl Alcohol to Benzyl Benzoate[END_REF][START_REF] Xiao | High Selective Oxidation of Benzyl Alcohol to Benzylaldehyde and Benzoic Acid with Surface Oxygen Vacancies on W 18 O 49 /Holey Ultrathin g-C 3 N 4 Nanosheets[END_REF] Among which benzaldehyde is considered as the most valuable product and widely used in the manufacture of flavors, pharmaceuticals, dyes, spices, etc. [START_REF] Ding | Nitrogen Vacancy Engineered Graphitic C 3 N 4 -based Polymers for Photocatalytic Oxidation of Aromatic Alcohols to Aldehydes[END_REF] Oxidizing a primary alcohol to an aldehyde using molecular oxygen/air as an oxidant conforms with the principles of green and sustainable chemical processes since water is the only by-product. [START_REF] He | Switching-off Toluene Formation in the Solvent-free Oxidation of Benzyl Alcohol using Supported Trimetallic Au-Pd-Pt Nanoparticles[END_REF] However, the desired products are highly reactive and susceptible to over oxidization generating low-value by-products, which results in low product selectivity during selective oxidation with oxygen/air as oxidant. In the industrial process, high product selectivity is generally achieved through the control of conversion, which results in low single-pass conversion, excessive recovery of unreacted reactant and high energy consumption. Therefore, it has been a long-standing dilemma between the high activity and high selectivity for the partial oxidation catalysts.

Au-Pd bimetallic catalysts are the most frequently reported for benzyl alcohol partial oxidation; [START_REF] Olmos | Selective Oxidation of Veratryl Alcohol over Au-Pd/Ce 0[END_REF][START_REF] Xin | Methanobactin-mediated Synthesis of Bimetallic Au-Pd/Al 2 O 3 Toward an Efficient Catalyst for Glucose Oxidation[END_REF][START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF] and a catalytic synergy between these two metals has been observed. This catalytic synergy was attributed to the formation of Au-Pd alloy NPs and electrons transfer between surface Au and Pd atoms, preventing the formation of inert surface PdO species. [START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF][START_REF] Xu | Biphasic Pd-Au Alloy Catalyst for Low-Temperature CO Oxidation[END_REF][START_REF] Li | Adsorption of Carbon Monoxide Au/Pd (100) Alloys in Ultrahigh Vacuum: Identification of Adsorption Sites[END_REF] Metallic Pd 0 -enriched clusters/surface has been reported as active sites for O 2 activation, [START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF][START_REF] Xu | Biphasic Pd-Au Alloy Catalyst for Low-Temperature CO Oxidation[END_REF][START_REF] Li | Adsorption of Carbon Monoxide Au/Pd (100) Alloys in Ultrahigh Vacuum: Identification of Adsorption Sites[END_REF][START_REF] Ghazal Tofighi | Chemical Nature of Microfluidically Synthesized AuPd Nanoalloys Supported on TiO 2[END_REF][START_REF] Han | Surface Alloy Composition Controlled O 2 Activation on Pd-Au Bimetallic Model Catalysts[END_REF][START_REF] Freakley | Palladium-tin Catalysts for the Direct Synthesis of H 2 O 2 with High Selectivity[END_REF][START_REF] Savara | Molecular Origin of Selectivity Differences Between Pd and AuPd in Benzyl Alcohol Oxidation: Different Oxygen Adsorption Properties[END_REF] however, Pd-enriched surfaces are usually coated with a layer of inert PdO. [START_REF] Hilaire | Interaction of Oxygen and Hydrogen with Pd-Au Alloys: An AES and XPS Study[END_REF][START_REF] Wilson | From Core-Shell to Alloys: The Preparation and Characterization of Solution-Synthesized AuPd Nanoparticle Catalysts[END_REF] This contradictive condition has been improved to a certain extent by introducing Au to Pd with H 2 reduction process. Electrons rearrangement between Au and Pd atoms facilitates the formation of metallic Pd 0 -enriched clusters/surface on H 2 reduced Au-Pd bimetallic NP catalyst. [START_REF] Wilson | From Core-Shell to Alloys: The Preparation and Characterization of Solution-Synthesized AuPd Nanoparticle Catalysts[END_REF][START_REF] Fu | Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions[END_REF] However, Wiley and co-workers observed a surface enrichment of Au on Au-Pd alloy NPs after H 2 reduction, resulting in a decrease of Pd 0 -enriched surface-active sites. [START_REF] Wilson | From Core-Shell to Alloys: The Preparation and Characterization of Solution-Synthesized AuPd Nanoparticle Catalysts[END_REF] These results pointed out to the need to find another route to reduce the surface inert PdO layers on Au-Pd NP catalyst. Pd has a relative high electron negativity and a low oxygen affinity, [START_REF] Fu | Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions[END_REF][START_REF] Huda | Aerobic Toluene Oxidation Catalyzed by Subnano Metal Particles[END_REF] while Sn is considered as an oxophilic metal. [START_REF] Fu | Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions[END_REF][START_REF] Kumar Puthiyapura | Biobutanol as Fuel for Direct Alcohol Fuel Cells Investigation of Sn-Modified Pt Catalyst for Butanol Electro-oxidation[END_REF][START_REF] Bai | Exclusive Formation of Formic Acid from CO 2 Electroreduction by Tunable Pd-Sn Alloy[END_REF] Thus, this reminds us to consider the possibility of applying tin as a potential promoter for in-situ autogenous reduction of surface PdO layer instead of high temperature H 2 reduction.

Meanwhile as we mentioned above, the dilemma between the high activity and high selectivity for benzyl alcohol partial oxidation also persists on Au-Pd bimetallic NP catalyst. A high benzyl alcohol conversion (> 90%) was obtained on Au-Pd bimetallic catalysts but with a relative low benzaldehyde selectivity of 50%~60%. [START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF] Studies have shown that the introduction of auxiliaries can regulate the surface affinity to reactants, products or intermediates to reduce side reactions and improve product selectivity. [START_REF] Freakley | Palladium-tin Catalysts for the Direct Synthesis of H 2 O 2 with High Selectivity[END_REF][START_REF] Clarke | Selectivity in Catalysis by Alloys[END_REF][START_REF] Vicente | The Relationship Between the Structural Properties of Bimetallic Pd-Sn/SiO 2 Catalysts and Their Performance for Selective Citral Hydrogenation[END_REF] He et al. demonstrated that when a small amount of Pt metal was alloyed with Au-Pd sol, a high selectivity to benzaldehyde was achieved on the solvent-free oxidation of benzyl alcohol. [START_REF] He | Switching-off Toluene Formation in the Solvent-free Oxidation of Benzyl Alcohol using Supported Trimetallic Au-Pd-Pt Nanoparticles[END_REF] Hutchings et al. applied SnO x promoted Pd/TiO 2 catalyst for hydrogen peroxide synthesis. [START_REF] Freakley | Palladium-tin Catalysts for the Direct Synthesis of H 2 O 2 with High Selectivity[END_REF] The promoted catalyst exhibited high H 2 O 2 selectivity attributable to the presence of the SnO x surface layer which prevented further hydrogenation and decomposition of H 2 O 2 . In our previous work, [START_REF] Xu | What is the Effect of Sn and Mo Oxides on Gold Catalysts for Selective Oxidation of Benzyl Alcohol?[END_REF] we found that applying SnO x promoted Au nanoparticle (NP) catalyst for benzyl alcohol oxidation can effectively reduce the disproportionation reaction, achieving a high benzaldehyde selectivity. In summary, tin is a potential promoter for in-situ reduction of surface PdO species because it is more oxophilic and it can also effectively enhance the benzaldehyde selectivity on Au NP catalyst by regulating the surface affinity. [START_REF] Xu | What is the Effect of Sn and Mo Oxides on Gold Catalysts for Selective Oxidation of Benzyl Alcohol?[END_REF] In this paper we report the preparation of Au-Pd bimetallic NP catalyst modified with Sn/SnO x . First, the mesostructured cellular foam silica (MCF) supported Au-Pd NP catalyst (AP-ox) was prepared by a one-pot method. [START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF] Second, the SnO x was deposited on the surface of Au-Pd NPs to obtain the SnO x @AP-ox catalysts. The results showed that the SnOx significantly enhanced the benzaldehyde yield thus broking the dilemma between the high activity and high selectivity of Au-Pd NPs catalyst. The role of SnO x in modulating the catalyst performance was approved experimentally and theoretically. 20 (P123, Aldrich), mercaptopropyltrimethoxysilane (MPTMS, 97%, Aldrich), 1,3,5-trimethylbenzene (99%, Sinopharm Chemical), hydrochloric acid (37%, Sinopharm Chemical), benzyl alcohol (99.99%, Sinopharm Chemical), benzaldehyde (99.99%, Sinopharm Chemical), benzoic acid (99.99%, Sinopharm Chemical) and absolute ethanol (99.98%, Sinopharm Chemical) were used as received without further purification.

EXPERIMENTAL SECTION

Preparation of Unmodified

Catalysts. Supported Au-Pd NP catalyst was prepared following the procedure reported in our previous work. [START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF] In a typical synthesis, 8.0 g of P123 was dissolved in 60 mL deionized water, followed by introduction of 20 mL of concentrated hydrochloric acid (37%) at room temperature.

Then, 8 g of 1,3,5-trimethylbenzene was added into the clear solution as a pore swelling agent and stirred at 38 o C for 2 h, followed by the dropwise addition of 18.4 g TEOS and 1.14 g MPTMS into the mixture. Finally, the HAuCl 4 and PdCl 2 solutions were added into the synthesis mixtures. After stirring at 38 o C for 24 h, the mixtures were transferred to a Teflon-lined stainless-steel autoclave to undergo a hydrothermal treatment at 100 o C for 24 h. The obtained solids were purified with deionized water, dried at 80 o C overnight, and eventually calcined in air at 550 o C for 6 h to remove the templating and coupling agent. Catalysts with a total noble metal (Au + Pd) loading of 0.75 wt.% with an Au loading of 0.25 wt.% and a Pd loading of 0.5 wt.% were abbreviated as AP-ox and the reduced samples were denoted as AP-H 2 .

Monometallic catalysts Au-ox with an Au weight ratio of 0.25% and Pd-ox with a Pd weight ratio of 0.5% were prepared as references. The catalysts 2Pd-ox with a Pd weight ratio of 1.0% and 2AP-ox with Pd and Au weight ratio of 1.0%, respectively, were also prepared for comparison purpose.

Preparation of SnO

x @AP Catalysts. 2 g of AP-ox solid was dispersed in a stannous chloride solution and stirred at room temperature for 4 h until a slurry was formed. The obtained slurry was dried at 100 o C for 12 h and finally calcined at 300 o C for 4 h under air atmosphere. Different amounts of SnO x were introduced and the obtained catalysts were denoted as ySnO x @AP-ox (y represents the nominal weight percentage of SnO x, such as 0%, 0.2%, 0.5%, 1.0%, x=1~2). These catalysts were reduced at 300 o C for 2 h under H 2 atmosphere to investigate the effect of reduction on the catalytic performance; the obtained catalysts were denoted as ySnO x @AP-H 2 .

For comparison, catalysts 1%SnO x @2Pd-ox and 1%SnO x @2AP-ox with higher noble metal loadings were prepared following the same procedure and reduced under H 2 atmosphere to obtain samples 1%SnO x @2Pd-H 2 and 1%SnO x @2AP-H 2 , respectively.

2.3 Characterization. N 2 adsorption/desorption was carried out to analyze the textural properties of catalysts after degassing at 300 o C for 4 h under vacuum using an automatic volumetric sorption analyzer (Micromeritics, TriStar 3000). X-ray powder diffraction (XRD) patterns of catalysts were recorded on a X'Pert PRO MPD system (Dutch company Panaco) with a Cu Kα radiation (λ = 0.15418 nm) at 35 kV and 40 mA. High angle annular dark field (HAADF) images and X-ray energy dispersive spectroscopy (XEDS) of the catalysts were obtained using a scanning transmission electron microscope (STEM) (JEOL JEM-2100F) with an acceleration voltage of 200 kV. HAADF images of nanoparticles were collected on HITACHI HF5000 field emission transmission electron microscope equipped with a spherical aberration corrector (AC-TEM) and energy dispersive spectroscopy (EDS) from Oxford operated at 200 kV. Actual noble metal loadings in the catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a VISTA-MPX Varian system. The solid ultraviolet-visible (UV-vis) spectra of catalysts were collected by a Hitachi U-4100 UV-vis-NIR spectrophotometer using BaSO 4 as an internal reference. The diffuse reflectance spectra from 350 to 800 nm were recorded. The surface chemical properties of the catalysts were analyzed using a K-alpha X-ray spectrometer (XPS) manufactured by Thermo Fisher Scientific. Taking the Al electrode as the anode, the electron kinetic energy was 1486.6 eV. The spectra were calibrated with C1s binding energy (284.5 eV) as a calibration binding energy (BE) reference. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption on the fresh catalysts (pretreated in H 2 at 573 K at a flow rate of 20 mL min -1 for 60 min, cooled down to 303 K and purged with He for 30 min) was conducted in a high-temperature diffuse reflection reaction cell (PIKE). The spectra were recorded on a BRUKER VERTEX70V FT-IR spectrometer at room temperature with a resolution of 4 cm -1 . H 2 -temperature programmed reduction (H 2 -TPR) experiments were conducted on a chemisorption apparatus (Micromeritics AutoChem II 2920). Typically, the sample (100 mg) was pre-treated at 300 °C for 1 h under an Ar stream (30 mL/min). Afterwards, the system was cooled to 50 °C, and then a 10 vol% H 2 /Ar stream (30 mL/min) was introduced. The temperature was raised linearly from 100 °C to 800 °C at a rate of 10 °C/min, and the signal was recorded by a thermal conductivity detector (TCD).

Selective Oxidation of Benzyl Alcohol.

The selective oxidation of benzyl alcohol was carried out in a 100 mL autoclave with a polytetrafluoroethylene liner (Model: SLM100, Beijing Easychem Science and Technology Development Company, China). Typically, 10.8 g of benzyl alcohol and 50 mg of solid catalyst were charged into the reaction vessel. After purging with O 2 for 3 times, the temperature was raised to 110 o C and the O 2 pressure was maintained at 0.8 MPa for 2 h under stirring. Then, the products were separated and analyzed using a gas chromatograph (Agilent 6870) equipped with a FID detector and a DB-1 column (30*0.32*0.25). The turnover frequency (TOF) was calculated based on the moles of benzyl alcohol converted per mole of noble metals (Au + Pd) per hour.

Theoretical

Calculation.The spin-polarized density functional theory (DFT) calculations were implemented using the DMol 3 program of Materials Studio 2018 software package. [START_REF] Delley | An All-electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules[END_REF][START_REF] Delley | Fast Calculation of Electrostatics in Crystals and Large Molecules[END_REF] The exchange-correlation term was described within the general gradient approximation developed by Perdew, Burke, and Ernzerhof (GGA-PBE). [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] Particularly, Grimme's PBE+D2 method was used to correct the long-range dispersion forces, [START_REF] Grimme | Semiempirical GGA-type Density Functional Constructed with a Long-range Dispersion Correction[END_REF] which shows a comparable reliability with the PBE0 hybrid XC functional including an ab initio Van der Waals correction. [START_REF] Zhao | Density Functional Study of Hydrogen Evolution on Cobalt-Embedded Carbon Nanotubes: Effects of Doping and Surface Curvature[END_REF] The ion cores of metal atoms were treated with the density functional semicore pseudopotential (DSPP), while the valence electron functions were represented by the localized double-numerical basis with polarization (DNP) function. The transition state structures were located by using the linear synchronous transition/quadratic synchronous transit method, [START_REF] Halgren | The Synchronous-transit Method for Determining Reaction Pathways and Locating Molecular Transition States[END_REF] confirmed by the frequency calculations. A real-space global orbital cutoff of 4.9 Å was adopted to keep the balance between high quality of results and high computational cost. The convergence tolerances for energy change, max force, and displacement were 1 × 10 -5 Ha, 2 × 10 -3 Ha Å -1 , and 5 × 10 -3 Å, respectively. According to the experimental results, a homogeneous Pd 3 Au alloy with a Pd-Au atomic ratio of 3:1 was selected as the palladium-gold alloy model, and a stable close-packed (111) surface was chosen as the model surface (Pd 3 Au(111)). The 111), the number of Sn, SnO, and SnO 2 units was half of the atom number of the top-layer Pd 3 Au(111), where the unit cell contained 8 Sn, SnO, and SnO 2 units, respectively. In the slab calculations, the atoms in the two bottom layers were fixed in their bulk positions, but all other atoms were allowed to be fully relaxed.

The adsorption energy (E ads ) of an adsorbate on catalyst surfaces was defined as

E ads = E adsorbate/substrate -(E adsorbate + E substrate ),
where E adsorbate/substrate and E substrate correspond to the total energy of the catalyst substrate with and without an adsorbed molecule, respectively, while E adsorbate represents the energy of the free adsorbate.

RESULTS AND DISCUSSION

3.1 Catalyst Characterization. N 2 adsorption-desorption isotherms and pore size distribution curves of catalysts ySnO x @AP-ox(H 2 ) are shown in Figure S1A, B and the results are summarized in Table 1. A typical type IV isotherm was measured on all catalysts, which contains a H-2 hysteresis loop, indicating an ink-bottle mesoporous structure. Reference catalysts Pd-ox and AP-ox exhibited a high specific surface area (~800 m 2 /g) and a large pore volume (1.8~2.3 cm 3 /g). A small amount of SnO x introduced in catalysts 0.2%SnO x @AP-ox and 0.5%SnO x @AP-ox leads to a slight decrease of the specific surface area (700 -750 m 2 /g). While the specific surface area of catalyst 1.0%SnO x @AP-ox significantly decreases to~650 m 2 /g when 1wt.% of SnO x was added. The average pore diameter increased from 5.6 nm for catalyst 0.2%SnO x @AP-ox to 7.8 nm for catalyst 1.0%SnO x @AP-ox. The decrease of specific surface area and pore volume on catalyst 1.0%SnO x @AP-ox was attributed to the partial blocking of small pores after introducing a relatively large amount of SnO x .

Table 1. Physical Properties of Catalysts.

a The metal loading was evaluated by ICP-OES. S BET , surface area calculated by the BET method.

V total , total pore volume calculated at P/P o = 0.998. Pore diameter calculated from the adsorption branch using BJH method.

Catalysts

PdSn

Au Pd Sn patterns of catalysts in the range of 19.5-70.5° 2θ reveal the weak diffraction peaks due to the presence of Pd, Au-Pd and PdO (Figure S1C). Catalysts with high metal loadings were prepared to clarify the peak positions of metals and oxides (Figure 1). Both 2Pd-ox and 2AP-ox catalysts contain a diffraction peak at 33.69 o 2θ corresponding to the PdO (110) (PDF#06-0515). No reduction of the catalysts was carried out and therefore the PdO is present. [START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF] After introducing the SnO x on catalyst 1%SnO x @2AP-ox, the intensity of the characteristic PdO phase peak at 2θ = 33.69 o was reduced. The peaks at 40.1 o and 46.6 o 2θ corresponding to (111) and (200) planes of cubic Pd (PDF#65-2867) shifted to lower 38.9 o and 45.3 o 2θ for catalyst 1%SnO x @2AP-ox, respectively, which was attributed to the formation of Au-Pd(111) phase. [START_REF] Alwan | Efficient Modified Bimetallic Alloy Nanoparticles Porous Silicon Gas Sensors for CO Gas Detection Process[END_REF] This result is in accordance with XRD pattern of catalyst 2AP-H 2 shown in Figure 1. After reduction, two diffraction peaks at 39.9 o and 41.0º 2θ were observed for catalyst 1%SnO x @2AP-H 2 , similar to the diffraction peaks detected for the 1%SnO x @2Pd-H 2 . The peak at 39.9º 2θ can be assigned to the cubic Pd (111) plane (40.1º), while the peak at 41.0º 2θ is due to PdSn (111) phase. These results revealed that the H 2 reduction treatment weakens the interactions between Au and Pd leading to the formation of Pd-Sn alloy. No obvious Sn or SnO x species were detected on these catalysts, implying either a good dispersion or amorphous Sn species. [START_REF] Dong | Multifunctional Pd-Sn Electrocatalysts Enabled by In-situ Formed SnO x and TiC Triple Junctions[END_REF] Concisely, Pd exists as PdO species on unpromoted catalysts 2Pd-ox and 2AP-ox, but partially as Au-Pd alloy species on SnO x promoted 1%SnO x @2AP-ox catalyst. This phase variation indicated that the introduction of SnO x led to partial reduction of PdO and promoted the formation of the Au-Pd alloy, which is probably due to the electron transfer from SnO x to PdO, [START_REF] Tsud | ISS and TPD Study of Pd-Sn Interactions on Pd-SnO x Systems[END_REF][START_REF] Pushkarev | Pt/C and Pt/SnO x /C Catalysts for Ethanol Electrooxidation: Rotating Disk Electrode Study[END_REF] due to the lower electronegativity and stronger oxophilicity of Sn than Pd. [START_REF] Wu | Multisource Synergistic Electrocatalytic Oxidation Effect of Strongly Coupled PdM (M= Sn, Pb)/N-doped Graphene Nanocomposite on Small Organic Molecules[END_REF] As shown in Figure S1C, the same trend was observed for catalysts ySnO x @AP-ox(H 2 ) with different SnO x concentrations; low intensity of the Bragg peaks is due to the low metal loadings.

The UV-vis spectra of catalysts ySnO x @AP-ox and ySnO x @AP-H 2 are shown in Figure 2 and Figure S2, respectively. A broad peak at around 520 nm for monometallic catalyst Au-ox is attributed to the surface plasma resonance (SPR) of Au 0 NPs. [START_REF] Elnaggar | Eco-friendly Microwave-assisted Green and Rapid Synthesis of Well-stabilized Gold and Core-shell Silver-gold Nanoparticles[END_REF] However, on bimetallic AP-ox and SnO x promoted ySnO x @AP-ox catalysts the characteristic peak of Au NPs is not observed, indicating the formation of Au-Pd alloy NPs on these catalysts (Figure 1) and also Au was no longer present as a pure metallic particle. [START_REF] He | Switching-off Toluene Formation in the Solvent-free Oxidation of Benzyl Alcohol using Supported Trimetallic Au-Pd-Pt Nanoparticles[END_REF] Even after H 2 reduction at 300 o C (ySnO x @AP-H 2 ), no bands corresponding to the Au NPs were observed in the UV-vis spectra, revealing that Au still exists as alloy species (Au-Pd-Sn) (Figure S2). Further elevated reduction temperature to 400 o C and 500 o C did not lead to obvious Au phase formation (Figure S3), no SPR of Au NPs (clusters) were detected even after reduction at 500 o C for 2h.

Based on the above results, we could conclude that H 2 reduction weakens the interaction between Au and Pd and no pure Au phase was present, implying that probably Au-Pd-Sn alloy is the dominant phase in the sample. Figure 3 shows representative STEM-HAADF images and EDS spectral maps, and the histograms of the particle size distribution of catalysts AP-ox, 0.5%SnO x @AP-ox and 0.5%SnO x @AP-H 2 . The size effect of the NP catalyst and the distribution of metal elements play important roles in their catalytic performance. [START_REF] Xu | What is the Effect of Sn and Mo Oxides on Gold Catalysts for Selective Oxidation of Benzyl Alcohol?[END_REF][START_REF] Tiruvalam | Aberration Corrected Analytical Electron Microscopy Studies of Sol-immobilized Au+Pd, Au {Pd} and Pd {Au} Catalysts Used for Benzyl Alcohol Oxidation and Hydrogen Peroxide Production[END_REF] The representative STEM-HAADF images and the corresponding EDS spectral maps of catalyst AP-ox indicated the presence of well-dispersed Au and Pd on the MCF support. Moreover, the energy dispersive line scan across NPs revealed the consistent dispersion of these two elements, indicating the formation of Au-Pd nano-alloys, which is in agreement with the UV-vis results. After introducing SnO x , a slight increase of NPs size (2-6nm) was observed on catalyst 0.5%SnO x @AP-ox comparing with that (2-4nm) on catalyst AP-ox, which was probably due to the dispersion of SnO x on Au-Pd NPs. The energy dispersive line scan across the NPs indicated that the dispersion of Sn atoms was identical to that of Au-Pd. [START_REF] Silva | Volcano-like Behavior of Au-Pd Core-shell Nanoparticles in the Selective Oxidation of Alcohols[END_REF] After the reduction, the intensity of Sn signal was significantly enhanced on catalyst 0.5%SnO x @AP-H 2 and this result is indicating the aggregation of Sn on Au-Pd NPs and formation of Au-Pd-Sn NPs with uniform composition. The introduction of SnO x resulted in a slight increase of metal particle size for catalyst 0.5%SnO x @AP-ox and a further increase of particle size on reduced catalyst 0.5%SnO x @AP-H 2 (3-7 nm), which is probably due to the reduction process leading to enhanced interaction between Au-Pd NPs and Sn and aggregation into larger NPs.

To further study the chemical composition of alloy NPs before and after H 2 reduction, the high-resolution HAADF images and corresponding element analysis spectra of catalysts 0.5%SnO x @AP-ox and 0.5%SnO x @AP-H 2 were recorded by AC-TEM (Figure 4). Energy-dispersive line-scanning spectra (Figure 4b) show a much higher signal intensity of Au and Pd than that of Sn on catalyst 0.5%SnO x @AP-ox. EDS elemental mapping (Figure 4d-f) further verified the homogenous dispersion of Au and Pd, confirming the formation of Au-Pd alloy NPs.

Correlating the line-scanning spectra shown in Figure 4b with EDS mapping of Sn element (Figure 4f), it can be concluded that SnO x was successfully deposited on Au-Pd NPs of the un-reduced catalyst 0.5%SnO x @AP-ox. The Sn element evenly distributed on the surface of catalyst 0.5%SnO x @AP-ox (Figure 4b). By contrast, an obvious accumulation of Sn element on Au-Pd alloy NPs was observed through line scan (Figure 4b') and elemental mapping (Figure 4f') on the H 2 -reduced catalyst 0.5%SnO x @AP-H 2 , and the nanoparticles were morphologically structured with an Au-Pd rich core and a Sn-rich shell. These results revealed the structure reconstruction of alloy NPs and presence of strong interaction between Au-Pd NPs and Sn after H 2 reduction. These observations are in line with the XRD and TEM results. images of catalysts 0.5%SnO x @AP-ox (a-f) and 0.5%SnO x @AP-H 2 (a'-f') . Scale bar, M=5 nm. 2. The XPS spectra of Au 4f indicated the presence of two types of Au species (Au δ-and Au 0 ) on the catalysts. [START_REF] Kruse | XPS Characterization of Au/TiO 2 Catalysts: Binding Energy Assessment and Irradiation Effects[END_REF][START_REF] Wang | CeO 2 -modified Au@ SBA-15 Nanocatalysts for Liquid-phase Selective Oxidation of Benzyl Alcohol[END_REF] The two characteristic peaks with binding energies (BE) of 84.0 eV (Au4f 7/2 ) and 87.9 eV (Au4f 5/2 ) were observed on the bimetallic catalyst AP-ox, corresponding to the typical Au 0 species. [START_REF] Arrii | Oxidation of CO on Gold Supported Catalysts Prepared by Laser Vaporization: Direct Evidence of Support Contribution[END_REF] While the other two peaks with BEs of 83.5 eV and 87.0 eV are attributed to the presence of negatively charged Au δ- species. [START_REF] Liu | Au δ--O v -Ti 3+ Interfacial Site: Catalytic Active Center toward Low-Temperature Water Gas Shift Reaction[END_REF] The formation of Au δ-species on bimetallic catalysts was due to the alloying effect of Au and Pd, the electrons can easily transfer from Pd to Au to form negatively charged Au δ-. [START_REF] Mohammad | In Selective Aerobic Oxidation of 1, 3-propanediol to 3-hydroxypropanoic Acid Using Hydrotalcite Supported Bimetallic Gold Nanoparticle Catalyst in Water[END_REF] These four peaks were also observed on catalyst 0.5%SnO x @AP-ox, indicating the existence of Au 0 and Au δ-species, [START_REF] Arrii | Oxidation of CO on Gold Supported Catalysts Prepared by Laser Vaporization: Direct Evidence of Support Contribution[END_REF][START_REF] Liu | Au δ--O v -Ti 3+ Interfacial Site: Catalytic Active Center toward Low-Temperature Water Gas Shift Reaction[END_REF] however, the ratio of surface Au δ-/Au 0 changed drastically from 0.6 for catalyst AP-ox to 4.5 for catalyst 0.5%SnO x @AP-ox. The low surface Au δ-content for catalyst AP-ox was due to the presence of surface PdO species as shown by XRD; the Au cannot alloy with PdO. [START_REF] Ghazal Tofighi | Chemical Nature of Microfluidically Synthesized AuPd Nanoalloys Supported on TiO 2[END_REF] Whereas the introduction of SnO x promoted the reduction of PdO and formation of Au-Pd alloy NPs and increase the amount of surface Au δ-species. On the reduced catalyst 0.5%SnO x @AP-H 2 , the Au δ-/Au 0 ratio decreased to 1.7 due to the partial reduction of SnO x to Sn and its alloying with Au-Pd to form Au-Pd-Sn alloy NPs. The latest weakened the interaction between Au and Pd and resulted in a decrease of electron density of the Au surface.

For 3d Pd XPS spectra contain the 3d 5/2 with BE of 335.5±0.3eV corresponds to Pd 0 species [START_REF] Brun | AES and Auger Parameter of Pd and PdO[END_REF][START_REF] Cao | Structural Characterization of Pd-doped SnO 2 Thin Films Using XPS[END_REF][START_REF] Gabasch | In situ XPS Study of Pd(1 1 1) Oxidation at Elevated Pressure, Part 2: Palladium Oxidation in the 10 -1 mbar Range[END_REF] and the 3d 5/2 with BE of 337.5±0.3eV corresponds to Pd 2+ (PdO) species. [START_REF] Brun | AES and Auger Parameter of Pd and PdO[END_REF][START_REF] Cao | Structural Characterization of Pd-doped SnO 2 Thin Films Using XPS[END_REF] The Pd XPS spectrum (Figure 5B) of the AP-ox bimetallic catalyst contains a peak with BE of 337.5 eV corresponding to PdO species. [START_REF] Wang | PdO/SnO 2 Heterostructure for Low-temperature Detection of CO with Fast Response and Recovery[END_REF] On SnO x promoted catalyst (0.5%SnO x @AP-ox), two Pd 3d 5/2 characterization peaks with BE of 335.9 eV and 337.6 eV corresponding to Pd 0 and PdO, respectively, are present. [START_REF] Wang | PdO/SnO 2 Heterostructure for Low-temperature Detection of CO with Fast Response and Recovery[END_REF] These results are in accordance with the XRD data, revealing that the introduction of SnO x leads to a partial reduction of PdO to Pd 0 species, which is due to the higher oxophilicity of Sn with lower electronegativity than the Pd (Sn=1.96, Pd=2.20). [START_REF] Wu | Multisource Synergistic Electrocatalytic Oxidation Effect of Strongly Coupled PdM (M= Sn, Pb)/N-doped Graphene Nanocomposite on Small Organic Molecules[END_REF] On the H 2 reduced catalyst (0.5%SnO x @AP-H 2 ), the Pd 0 species with BE of 336.2 eV, [START_REF] Priolkar | Formation of Ce 1-x Pd x O 2-δ Solid Solution in Combustion-Synthesized Pd/CeO 2 Catalyst: XRD, XPS, and EXAFS Investigation[END_REF] with a slight positive shift (0.3 eV) probably due to the strong interaction between Sn and Pd is observed.

The Sn 3d XPS spectrum of the reference sample (SnO x /MCF-ox) contains peaks at 487.3 eV(3d 5/2 ) and 495.7 eV(3d 3/2 ) corresponding to the presence of SnO 2 species (Figure 5C). The spectrum of catalyst 0.5%SnO x @AP-ox contains a peak slightly shifted to higher binding energy (487.5 eV) confirming the presence of SnO 2 . This positive BE shift is generally attributed to the strong interaction between Au-Pd NPs and SnO 2 in which an electron transfer from SnO 2 to Au-Pd occurred, [START_REF] Liu | The Effect of Noble Metal (Au, Pd and Pt) Nanoparticles on the Gas Sensing Performance of SnO 2 -based Sensors: A Case Study on the {221} High-index Faceted SnO 2 Octahedra[END_REF] resulting in partial reduction of PdO to Pd 0 and formation of more surface Au δ-species as shown in Figure 5A, B. However, on the reduced catalyst 0.5%SnO x @AP-H 2, typical Sn 0 species with BE of 485.5 eV were detected in addition to the SnO 2 , indicating that SnO 2 was partially reduced under H 2 at 300 o C. In summary, electrons transfer from SnO 2 species to Au and Pd on catalyst 0.5%SnO x @AP-ox, leading to the formation of Au-Pd alloy NPs was observed. As low-coordinated Pd 0 -enriched clusters are generally considered as active sites for benzyl alcohol oxidation, [START_REF] Freakley | Palladium-tin Catalysts for the Direct Synthesis of H 2 O 2 with High Selectivity[END_REF][START_REF] Chen | Formation of Monometallic Au and Pd and Bimetallic Au-Pd Nanoparticles Confined in Mesopores via Ar Glow-discharge Plasma Reduction and Their Catalytic Applications in Aerobic Oxidation of Benzyl Alcohol[END_REF] introducing SnO x to AP-ox catalyst increased the active sites of catalyst 0.5%SnO x @AP-ox, which could cause an enhancement of catalytic activity for benzyl alcohol oxidation.

After the reduction, the PdO was completely reduced to Pd 0 , and SnO x was partially reduced to Sn. The strong interaction between Pd and Sn species leaded to the formation of Au-Pd-Sn alloy as confirmed by correlating the UV-Vis and XRD results. The O 1s XPS spectrum of sample 1%SnO x /MCF-ox contains a lower energy band (532.8 eV) implying that there are more electron-rich oxygen atoms near the surface of the metal oxide (Figure 5D). While for the 0.5%SnO x @AP-ox, the binding energy shifted to higher value (533.0 eV), indicating the weakening of surface metal oxygen (M-O) interactions.

Table 2. Surface Composition of Catalysts.

a Catalyst 0.5%SnO x @AP-ox was analyzed after a control reaction in N 2 atmosphere.

The bulk atomic ratio of Au/Pd/Sn based on ICP and surface chemical composition based on XPS of different catalysts are summarized in Table 2. The atomic ratio of Au/Pd/Sn in the bulk of the 0.5%SnO x @AP-ox catalyst is 1: 3.6: 2.8, while the surface composition was found to be 1: 6.0: 23.8 on the same catalyst. This result confirmed that most of Sn is distributed on the catalysts surface, which is consistent with EDX analysis (see line-scans and mapping in Figure 4). After the reduction, the Au/Pd/Sn ratio on the surface was dramatically changed to 1: 4.1: 6.3, indicating that Sn partially entered in the bulk of NPs. This observation is consistent with the STEM-HADDF results showing that the reduction led to the transformation of surface Sn to the bulk, which may result in the formation of Au-Pd-Sn alloy NPs, Furthermore, the strong interaction between Pd and Sn weakened the interaction between Au and Pd, thus leading to the electron redistribution between Au-Pd-Sn atoms.

H 2 -TPR profiles of the catalysts are depicted in Figure 6A. The spectrum of monometallic catalyst Pd-ox contains a peak at 106 °C which is commonly attributed to the reduction of surface PdO species and a peak at 267°C ascribed to the reduction of bulk PdO. While in the spectrum of bimetallic catalyst AP-ox, the bulk reduction peak at 267°C disappeared, indicating alloying of Au and Pd that prevented the formation of bulk PdO phase. 8-10 A negative reduction peak at 59 °C was observed for catalysts 0.2%SnO x @AP-ox, 0.5%SnO x @AP-ox and 1.0%SnO x @AP-ox, which could be attributed to the decomposition of β-PdH x . Since PdO species are easily reduced in a hydrogen atmosphere to metallic Pd, which further interacts with hydrogen to form β-PdH x species. [START_REF] Li | Shape-controlled Synthesis of Pd Nanocrystals with Exposed {110} Facets and Their Catalytic Applications[END_REF][START_REF] Agostini | Effect of Pre-reduction on the Properties and the Catalytic Activity of Pd/carbon Catalysts: A Comparison with Pd/Al 2 O 3[END_REF] The presence of this peak indicated that PdO species on SnO x promoted catalysts are easily reduced to active Pd 0 species or the Pd 0 species are present. A reduction peak around 525°C is observed for sample SnO x @MCF-ox, which is due to the reduction of surface SnO x species, however, this peak shifts to 370°C for catalysts 0.5%SnO x @AP-ox and 1.0%SnO x @AP-ox, revealing that the interaction between SnO x and Au-Pd NPs promotes the reduction of surface SnO x species. Combined the H 2 -TPR with XPS results, one can conclude that the strong interaction between SnO x and Au-Pd NPs helps the reduction of surface 0.2%SnO x @AP-ox, (d) 0.5%SnO x @AP-ox, (e) 1.0%SnO x @AP-ox and (f) SnO x @MCF-ox. Time-resolved CO-desorption DRIFTS spectra of catalysts (B) Au -ox, (C) Pd -ox, (D) AP-ox, (E) 0.5%SnO x @AP-ox and (F) 0.5%SnO x @AP-H 2 at 25ºC.

The time-resolved CO adsorption DRIFTS spectra were conducted for all catalysts and presented in Figure 6B-F and Figure S4-5 (full spectral range). The bands at 2171 cm -1 and ~2117 cm -1 are observed in the spectra for all samples and their intensity decreased with the CO desorption, which are ascribed to the physically adsorbed gaseous CO. [START_REF] Wu | Effect of Preparation Method on ZrO 2 -based Catalysts Performance for Isobutanol Synthesis from Syngas[END_REF] The typical CO adsorption on Au at 2100~2000 cm -1 was not obvious [START_REF] Takashi Fujita | CO Oxidation over Au/ZnO: Unprecedented Change of the Reaction Mechanism at Low Temperature Caused by a Different O 2 Activation Process[END_REF] on catalyst Au -ox (Figure 6B), which is probably due to the strong interaction between Au NPs and silica support. [START_REF] Wu | Formation of PdO on Au-Pd Bimetallic Catalysts and the Effect on Benzyl Alcohol Oxidation[END_REF] For catalyst Pd-ox (Figure 6C), the carbonyls on Pd 2+ at 2117 cm -1 overlapping with adsorption bands of gaseous CO are confirmed, [START_REF] Wang | Preparation and Characterization of SBA-15 Supported Pd Catalyst for CO Oxidation[END_REF] and this band does not disappear during CO desorption. A new band at ~2100cm -1 was observed on the bimetallic catalyst AP-ox (Figure 6D). The intensity of the latest band was enhanced for catalyst 0.5%SnO x @AP-ox (Figure 6E), associated with the linearly adsorbed CO on Pd δ+ located in the corner or edge sites of Pd nanoparticles [START_REF] Dai | Role of Brønsted Acid Site During Catalytic Combustion of Methane Over PdO/ZSM-5: Dominant or Negligible?[END_REF][START_REF] Yu | Hydrogen Adsorption and Absorption with Pd-Au Bimetallic Surfaces[END_REF] . A weak adsorption band centered at 1985 cm -1 is present in the spectrum of catalysts AP-ox (Figure 6D) due to bridged CO adsorption on contiguous Pd 0 sites or Pd 0 -enriched clusters (ν CO : 1900 -2000 cm -1 ) [START_REF] Yang | Enhanced Electrokinetics of C-C Bond Splitting During Ethanol Oxidation by Using a Pt/Rh/Sn Catalyst with a Partially Oxidized Pt and Rh Core and a SnO 2 Shell[END_REF] that was further enhanced for catalyst 0.5%SnO x @AP-ox (Figure 6E), indicating the formation of Pd 0 -enriched clusters after introducing SnO x . Whereas, the enhanced intensity of the band at ~2100cm -1 reflecting the increase of linearly adsorbed CO on Pd 0 sites. These results reveal the increase of both Pd 0 -enriched clusters and single Pd 0 sites on catalyst 0.5%SnO x @AP-ox, which is consistent with XRD and XPS results. After H 2 reduction (Figure 6F), the band corresponding to the bridged adsorption of CO on Pd 0 at 1985 cm -1 disappeared, indicating the reconstruction of Pd 0 -enriched clusters to Au-Pd-Sn or Pd-Sn alloy NPs. The CO 2 vibration peaks (~2360 and ~2340 cm -1 ) [START_REF] Wu | Effect of Preparation Method on ZrO 2 -based Catalysts Performance for Isobutanol Synthesis from Syngas[END_REF] were detected on SnO x promoted catalysts 0.5%SnO x @AP-ox and 0.5%SnO x @AP-H 2 , revealing that CO was readily oxidized to CO 2 on these catalysts at room temperature (25°C). Especially on unreduced catalyst 0.5%SnO x @AP-ox, the intensity of CO 2 adsorption bands increased as the desorption proceeds (Figure 6E, black frame), indicating that the adsorbed gaseous CO was oxidized to CO 2 . These results indicated that SnO x could serve as an oxygen transfer medium which would enhance the oxygen transfer during partial oxidation of benzyl alcohol over Au-Pd NPs catalyst.

Catalytic Evaluation.

The activity for benzyl alcohol oxidation and benzaldehyde selectivity of different catalysts are presented in Figure 7A; detailed oxidation results are summarized in Table S1. The external and internal diffusion limitations have been neglected based on the results from our recent study. [START_REF] Wu | Mesostructured Cellular Foam Silica Supported Au-Pt Nanoalloy: Enrichment of D-state Electrons for Promoting the Catalytic Synergy[END_REF] Therefore, the TOFs is used to represent the intrinsic activity of catalysts in the current study. As is shown (Figure 7A and Table S1), the catalytic activity on unreduced monometallic catalysts (Au, Pd, Sn) and bimetallic catalysts (Au-Pd, Au-Sn, Pd-Sn) are low. But a significant enhancement of catalytic activity was achieved on SnO x promoted catalyst 0.5%SnO x @AP-ox, indicating a synergy among different Au, SnO x and Pd species. Catalyst AP-ox exhibited a low catalytic activity (TOF=4365h -1 ) for benzyl alcohol oxidation because inert PdO species exist as the dominant surface species (see XPS spectra, Figure 4B). After introducing 0.5wt.% SnO x , a higher catalytic activity (TOF=13662 h -1 ) was achieved on catalyst 0.5%SnO x @AP-ox, due to the presence of active Pd 0 species on the catalyst surface.

Thus, the introduction of SnO x promoted the partial reduction of PdO to Pd 0 and formation of more Pd 0 -enriched clusters on catalyst 0.5%SnO x @AP-ox, leading to catalytic activity enhancement. However, the catalytic activity decreases on the H 2 reduced catalyst 0.5%SnO x @AP-H 2 , where most of PdO species are believed to be reduced to Pd 0 . The low catalytic activity of catalyst 0.5%SnO x @AP-H 2 is due to the formation of Au-Pd-Sn alloy NPs with uniform composition, which results in the decrease of active Pd 0 -enriched clusters as shown by CO adsorption FT-IR. [START_REF] Freakley | Palladium-tin Catalysts for the Direct Synthesis of H 2 O 2 with High Selectivity[END_REF] It is also noted that no obvious enhancement of catalytic activity was observed on SnO x promoted Pd NP catalysts no matter whether were or not treated under H 2 (catalysts 0.5%SnO x @Pd -ox and 0.5%SnO x @Pd-H 2 , TOF=7500-7800 h -1 ), Table S1. These results indicate that the effect of Au on the catalyst performance should not be ignored. The role of Au on the Au-Pd alloy NP catalyst was usually considered as a structural and electronic promoter, which fostered the formation of active Pd 0 -enriched sites. It is worth noting that a significant enhancement of benzaldehyde selectivity (from 60.1% on catalyst AP-ox to 73.2% on catalyst 0.5%SnO x @AP-ox) was achieved after introducing the SnO x promoter, and especially the benzaldehyde yield increased from 15.7% (AP-ox) to 59.8% (0.5%SnO x @AP-ox) after 2h reaction.

The benzaldehyde selectivity increased with the increase of SnO x content, which should be related to the surface properties of the catalyst. Similar results were also 29 reported by Hutchings and coworkers for H 2 O 2 synthesis. [START_REF] Freakley | Palladium-tin Catalysts for the Direct Synthesis of H 2 O 2 with High Selectivity[END_REF] SnO x promoted Pd/TiO 2 catalyst exhibited high H 2 O 2 selectivity attributable to the surface modification of SnO x to Pd sites. In order to study the catalytic mechanism of benzyl alcohol oxidation on SnO x promoted Au-Pd catalyst, the control reaction was carried out on catalyst 0.5%SnO x @AP-ox under the same reaction conditions except for using N 2 atmosphere. A benzyl alcohol conversion of 5.8% with benzaldehyde selectivity of 85.3% were obtained after 2h reaction. The surface properties of the used catalyst were characterized by XPS, Figure 7B-D. A significant increase of Pd 0 /Pd 2+ ratio from 1:1.2 to 1:0.25 was observed after the reaction under N 2 atmosphere, indicating that PdO participates in the oxygen transfer during the oxidation reaction. The BE of Sn 3d shifted from 487.5eV to 487.1eV revealing the partial reduction of SnO x to lower valance state SnO x-δ (Figure 7C). However, no surface Au species are detected after the reaction. ICP results revealed no obvious leaching of Au during the control reaction (Table 2). These results indicated that both PdO and SnO 2 were reduced in the control reaction, namely participating in the oxygen transfer during the oxidation, which is in good accordance with time resolved CO-DRIFTS data. The surface Pd/Sn ratio decreased from 0.25 to 0.12 after the reaction, indicating the surface accumulation of Sn on Au-Pd-Sn NPs, while Au transferred to the inner bulk or covered by Sn-Pd.

Correlating the characterization results with the activity of the catalyst (0.5%SnO x @AP-ox), we could conclude that the introduction of SnO x promotes the partial reduction of surface PdO to active Pd 0 species. Both PdO and SnO x participated in the oxygen transfer reaction, leading to a high catalytic activity. After H 2 reduction, the PdO was completely reduced to Pd 0 , while SnO x was partially reduced and migrated to the bulk of Au-Pd NPs to form Au-Pd-Sn alloy NPs. A low catalytic activity was observed on catalyst 0.5%SnO x @AP-H 2 , which is due to the formation of surface Au-Pd-Sn alloy. The catalytic activity and useful product selectivity of benzyl alcohol partial oxidation were closely related to the adsorption properties and surface O species. [START_REF] Savara | Benzyl Alcohol Oxidation on Carbon-Supported Pd Nanoparticles: Elucidating the Reaction Mechanism[END_REF][START_REF] Savara | Microkinetic Modeling of Benzyl Alcohol Oxidation on Carbon-Supported Palladium Nanoparticles[END_REF] Thus, the adsorption/desorption energies and configurations of benzyl alcohol, O 2 and benzaldehyde on Pd

CONCLUSIONS

In this study, SnO x was introduced into the Au-Pd bimetal catalyst as a promoter.

Based on the comprehensive experimental and theoretical results we found out that the introduction of SnO x promotes the partial reduction of PdO to active Pd 0 -enriched clusters on unreduced catalyst, which is due to the high oxophilicity of Sn. The presence of more active Pd 0 -enriched clusters, together with the enhancement of O transfer by PdO and SnO x , results in the highest catalytic activity (13662 h -1 ) for benzyl alcohol oxidation on catalyst 0.5%SnO x @AP-ox, while the benzaldehyde selectivity increased with SnO x ratio. The enhanced benzaldehyde selectivity on SnO x promoted catalysts ySnO x @AP-ox was due to the in-situ regulation of reactants and products affinity on the catalyst surface through redox switching of SnO x with SnO x-δ during oxidation reaction. After H 2 reduction, the SnO x was partially reduced and Au-Pd-Sn alloy was formed leading to a reduced activity of the catalyst. The theoretical results revealed that the formation of Pd-Sn/Au-Pd-Sn alloy weakens the adsorption of benzyl alcohol on catalysts SnO x @AP-H 2 . In summary, the SnO x promoted autogenous reduction of PdO to active Pd 0 species together with the enhancement of O transfer by PdO and SnO x leading to high catalytic activity, while the SnO x redox switching during the oxidation reaction modulated the adsorption properties of reactants and products which enhanced the benzaldehyde selectivity.

Pd 3

 3 Au(111) model was established by a four-layers periodic slab model with a p(44) unit cell and a vacuum region of 15 Å. The Pd 3 Sn 3 Au(111) model was created by replacing the Pd atoms on the top surface layer of Pd 3 Au(111) with Sn atoms in a ratio of 1:1, and the other bottom three layers still maintained as Pd 3 Au. The Pd 3 Au(111) surface was covered by a layer of Sn, SnO, and SnO 2 species to form Sn/Pd 3 Au(111), SnO/Pd 3 Au(111), SnO 2 /Pd 3 Au(111) models, respectively. Sn, SnO, and SnO 2 species were doped on the Pd 3 Au(111) surface to form Sn/Pd 3 Au(111), SnO/Pd 3 Au(111), SnO 2 /Pd 3 Au(111) models, respectively. In Sn/Pd 3 Au(111), SnO/Pd 3 Au(111), SnO 2 /Pd 3 Au(

Figure 3 .

 3 Figure 3. STEM-HAADF images of catalysts (Insets: energy dispersive line scan

Figure 4 .

 4 Figure 4. High resolution HAADF-STEM images with energy-dispersive line-scans

Figure 5 .

 5 Figure 5. Au 4f XPS spectra (A), Pd 3d XPS spectra (B), Sn 3d XPS spectra (C) and

  SnO x and PdO species, which may act as oxygen transfer intermediate during the oxidation reaction.

Figure 6 .

 6 Figure 6. H 2 -TPR spectra (A) of catalysts (a) Pd-ox, (b) AP-ox, (c)

Figure 7 .

 7 Figure 7. (A) Performance (TOF) of catalysts, XPS spectra of Pd 3d (B), Sn 3d (C),

Figure 8 .

 8 Figure 8. Adsorption/desorption energies and configurations of benzyl alcohol, O 2

  the adsorption energy of benzaldehyde on the reduced surfaces (SnO/Pd 3 Au(111) and Sn/Pd 3 Au(111)) is lower than that on the surface of SnO 2 /Pd 3 Au(111), which indicates benzaldehyde easily desorbed from the SnO/Pd 3 Au(111) or Sn/Pd 3 Au(111) interface.The characterization results revealed that SnO 2 participates in the oxygen transfer reaction and is partially reduced during the reaction (Figure7C). Benzyl alcohol was easily adsorbed on the SnO 2 /Pd 3 Au (111) surface and oxidized by SnO 2 , and at the same time, the SnO 2 /Pd 3 Au (111) interface was reduced to SnO/Pd 3 Au(111) or Sn/Pd 3 Au(111) which are beneficial for the benzaldehyde desorption. The reduced interfaces SnO/Pd 3 Au (111) and Sn/Pd 3 Au(111) continue to adsorb oxygen and participate in the next reaction cycle (Figure9A). However, when the catalyst was completely reduced (Pd 3 Sn 3 Au alloy formation), the adsorption energies of benzyl alcohol decreased (from 16.2 kcal/mol on AuPd 3 alloy to 10.5 kcal/mol on Pd 3 Sn 3 Au alloy), the adsorption distance increased (2.502Å→2.698Å), thus, leading to a low catalytic activity on H 2 -reduced catalyst 0.5%SnO x @AP-H 2 .Based on the above results, a reaction mechanism was proposed (Figure9B). The enhanced catalytic activity on SnO x promoted AP catalyst was due to the presence of high oxophilic Sn, which promoted the partial reduction of PdO to active Pd 0 . Both PdO and SnO x participated in the partial oxidation reaction and enhanced the O transfer, thus leading to a high catalytic activity. The redox switching of SnO x with SnO x-δ changed the catalyst surface affinity to benzyl alcohol and benzaldehyde. Benzyl alcohol was easily adsorbed on SnO 2 /Pd 3 Au(111) surface, and SnO 2 participated in the oxygen transfer via reduction to SnO/Pd 3 Au(111) or Sn/Pd 3 Au(111), which is crucial for the benzaldehyde desorption, leading to a high benzaldehyde selectivity. The SnO/Pd 3 Au or Sn/Pd 3 Au reduced surfaces readily adsorb and react with oxygen to form SnO 2 /Pd 3 Au and participate in the next reaction cycle.

Figure 9 .

 9 Figure 9. (A) Adsorption energy of benzyl alcohol and benzaldehyde on

2.1. Chemicals. HAuCl

  4 •xH 2 O (Sinopharm Chemical), PdCl 2 (Sinopharm Chemical), SnCl 2 •H 2 O (Sinopharm Chemical), tetraethyl orthosilicate (TEOS, 98%, Sinopharm Chemical), triblock co-polymer PEO 20 PPO 70 PEO

  /or a reduced SnO x shell with different spreading on the Au-Pd particles.

		ICP	XPS		XPS
	Samples
		Au: Pd: Sn	Au: Pd: Sn	Au δ : Au 0	Pd 0 : Pd 2+	Sn 0 : Sn δ+
	AP-ox	1: 3.6: 0	1: 2.3: 0	0.6: 1	0: 1:	---
	0.5%SnO x @AP-ox	1: 3.6: 2.8	1: 6.0: 23.8	4.5: 1	1: 1.2	0: 1
	0.5%SnO x @AP-H 2	1: 3.6: 2.8	1: 4.1: 6.3	1.7: 1	1: 0	0.2: 1
	0.5%SnO x @AP-ox a	1: 3.7: 2.6	0: 1: 8.6	undetectable	1: 0.25	0: 1

and

  The DFT calculations were applied to further explain the reconstruction of active sites on SnO x promoted Au-Pd catalysts and the results are shown in Table3. It was found that the bond energy of Sn-O is 141.1 kcal/mol, which is significantly higher than that of the Pd-O bond (86.8 kcal/mol), therefore, part of PdO species would be reduced by Sn and more Pd 0 sites were generated after introducing Sn (XPS results). Simultaneously, the formation energy of different alloys was calculated to verify the reconstruction of active sites after H 2 reduction. The results indicated that the absolute value of Pd 3 Au formation energy (-0.43 eV) is quite smaller than that of Pd 3 Sn (-1.84 eV), which explained the easy formation of Pd-Sn/Au-Pd-Sn alloy after H 2 reduction and justified the experimental results.

3.3 DFT Calculation.

Table 3 .

 3 The bond energies of different M-O bonds and formation energies of Pd i M j ) represents the total energy of the Pd i M j alloy unit cell, E (Pd) and E (M) represent the energy of each atom in Pd and M simplex, respectively, i and j represent the number of Pd and M atoms in the alloy unit cell, respectively.

	different alloys.
	M-O	Bond energy (kcal/mol)	Alloys	E f (eV)
	Sn-O	-141.2	Pd 3 Au	-0.43
	OSn-O	-76.3	Pd 3 Sn	-1.84
	Pd-O	-86.8
	E f = [E(Pd i M

j ) -iE(Pd) -jE(M)]/(i + j), E (

  3 Au(111), Sn 3 Pd 3 Au(111), Sn/Pd 3 Au(111), SnO/Pd 3 Au(111), and SnO 2 /Pd 3 Au(111), whereas the detailed energy barrier for the O 2 decomposition into 2 O, the adsorption energy of benzyl alcohol and oxygen, and the desorption energy of the products are presented in Figure 8 and listed in Table S2. Compared with the bimetallic Pd 3 Au(111) surface, the Sn containing surfaces (Sn/Pd 3 Au (111) and SnO/Pd 3 Au(111)) are conducive for the adsorption and activation of oxygen molecules, since the decomposition energy barrier of the O 2 decomposition is significantly reduced from 42.5 kcal/mol on the Pd 3 Au(111) surface to 19.2 kcal/mol on the Sn/Pd 3 Au(111) surface and to 33.5 kcal/mol on the SnO/Pd 3 Au(111) surface. The adsorption energies of benzyl alcohol on the SnO/Pd 3 Au(111) and SnO 2 /Pd 3 Au(111) surfaces are 32.0 kcal/mol and 39.2 kcal/mol, respectively, much higher than those of other surfaces, indicating that benzyl alcohol could be easily adsorbed on the SnO/Pd 3 Au(111) and SnO 2 /Pd 3 Au(111) interfaces. On the other hand,

This work was financially supported by the Natural Science Foundation of China (51601223, 21908246, 21991090), the Fundamental Research Funds for the Central Universities (17CX05018, 17CX02056), Shandong Provincial Natural Science Foundation (ZR201702160196), State Key Laboratory of Heavy Oil Processing( SKLZZ-2017008) and the Sino-French International Laboratory (LIA) "Zeolites".

 ASSOCIATED CONTENT

Supporting Information: N 2 adsorption/desorption isotherms; XRD spectra; Solid UV-vis spectra of reduced catalysts; DRIFT spectra using CO

 ACKNOWLEDGEMENTS

For Table of Content use only