Chunzheng Wang
email: czwang@upc.edu.cn

Weisong Xu

Zhengxing Qin

Xinmei Liu

Svetlana Mintova

Low-temperature synthesis of -alumina nanosheets on microfibrous-structured Al-fibers for Pd-catalyzed CO oxidative coupling to dimethyl oxalate

Keywords: Structured catalyst, Alpha alumina, Nanosheet, Phase transformation, Aluminum fiber, Dimethyl oxalate

We reported the low-temperature synthesis of -Al2O3 nanosheets on the microfibrous-structured Alfibers at 800 º C. The boehmite (AlOOH) nanosheets were initially formed on the Al-fibers through in situ endogenous growth. Then the AlOOH/Al-fibers was transformed to the -Al2O3/Al-fibers composite at 800 º C by a single heating step. The low-temperature phase transformation was tentatively attributed to the Al metal in the AlOOH/Al-fibers. Palladium was then dispersed on the -Al2O3/Alfibers composite, and the resulting Pd/-Al2O3/Al-fibers catalyst was examined in the strongly exothermic CO oxidative coupling to dimethyl oxalate (DMO) reaction. High CO conversion of 58 % and DMO selectivity of 95 % were obtained and maintained for at least 150 h using a feedgas of CH3ONO / CO / N2 (1 / 1.4 / 7.6, mole) at 150 º C with a gas hourly space velocity of 20000 mL g 1 h 1 . Computational fluid dynamics calculations and experimental results indicated that the Pd/-Al2O3/Al-fibers catalyst remarkably decreased the hot-spot temperature of catalyst bed due to its enhanced thermal conductivity.

Introduction

Alpha alumina (-Al2O3) is of fundamental and technological importance for plenty of applications in the petroleum industries and environmental protection [13]. In general, -Al2O3 is derived from the thermolysis of gibbsite, boehmite, other hydrated alumina and undergoes a lot of transitional phases (-, -, -, -, -, -Al2O3) [4,5]. However, these methods commonly require a high calcination temperature above 1100 º C and thus lead to high energy consumption. This becomes a reason for developing a low-temperature synthesis procedure of -Al2O3. Several groups reported on the use of additives such as seeds, oxidants, chelating agents, mineralizers and special reactors (laser or microwave) to lower the formation temperature of -Al2O3 [6,7]. Suchanek et al. reported the synthesis of -Al2O3 nanosheets using AlOOH under hydrothermal treatment in the presence of -Al2O3 seeds and additives (H2O2, SiO2 and H3BO3) at 380450 º C under 6.914.5 MPa [8,9].

The -Al2O3-based catalysts are usually loaded in the fixed-bed reactors as randomly packed pellets of several millimeters. Such packed beds suffer from the large pressure drop, low intraparticle/interbed heat transfer and irregular flow pattern, which is detrimental to high-throughput, diffusion-determining and strongly exothermic/endothermic reactions [10,11]. To solve these problems, -Al2O3 is directly manufactured into the structured foam and honeycomb monoliths, but the mechanical strength of the structure is very poor [12]. Furthermore, -Al2O3 is deposited on the structured foam (Ni, Al) and honeycomb (cordierite) monoliths by coating techniques, which combines the merits of -Al2O3 support with structured substances [13]. The catalysts using Al2O3-coated structured substances as supports, showed the high catalyst utilization and enhanced heat-transfer performance [14,15]. However, the conventional coating techniques are subjected to the nonuniformity and detachment of the coating layer as well as harmful contamination of the binder [16,17].

Recently, the conversion of non-petroleum syngas (CO/H2) into chemicals and liquid fuels is a promising alternative to the petrochemical-based routes [18]. The gas-phase carbonylation of methyl nitrite (CH3ONO) to dimethyl oxalate (DMO, (CH3OCO)2) is a pivotal step from syngas to ethylene glycol (DMO hydrogenation) [19]. This process is self-closing and includes chemical-looping with two main reactions: (1) Pd-catalyzed CO oxidative coupling to DMO: 2 CH3ONO + 2 CO  2 NO + (CH3OCO)2; and (2) noncatalytic CH3ONO regeneration: 2 NO + 2 CH3OH + 1/2 O2  2 CH3ONO + H2O. The overall reaction (2 CO + 2 CH3OH + 1/2 O2  (CH3OCO)2 + H2O) is highly efficient and environmentally benign. However, the industrial Pd/-Al2O3 catalyst for CO oxidative coupling to DMO, needs a high Pd content of 2 wt.% (the-state-of-the-art), leading to a compromised economy of the DMO production [20]. The on-going efforts have been dedicated to reduce the Pd content by proper tuning of the Pd size/shape, Pd-support interactions and adding of a promoter [2123]. Peng et al. reported a high-performance Pd/-Al2O3 catalyst with Pd content of only 0.13 wt.% [22]. Nevertheless, little attention is paid to the -Al2O3 support used in the strongly exothermic CO oxidative coupling to DMO (H25 º C = 159 kJ mol 1 ). The real application of -Al2O3 a few millimeters in size, may provoke large temperature rise with undesired hot spot in the reactor bed, which decreases the product selectivity and even makes the temperature control difficult [21,23]. To overcome this problem, a structured Pd-Fe/-Al2O3-coated-cordierite catalyst was prepared, but the Pd loading was as high as 1.0 wt.% [21]. Therefore, it is important to develop a highly efficient, low Pd-loaded catalyst with enhanced heat transfer for both fundamental study and commercial application.

Herein, we revealed the low-temperature synthesis method of -Al2O3 nanosheets on the microfibrous-structured Al-fibers through the phase transformation of AlOOH nanosheets at 800 º C.

Both the metal Al-promoted phase transformation of AlOOH and the formation of -Al2O3 were 5/25 carefully studied. The Pd/-Al2O3/Al-fibers catalyst was prepared by incipient-wetness impregnation using the as-obtained -Al2O3/Al-fibers composite, and was tested in the strongly exothermic CO oxidative coupling to DMO reaction. Such microfibrous-structured catalyst showed a combination of improved heat transfer, low pressure drop, good activity, and excellent stability. The computational fluid dynamics (CFD) simulation was carried out to probe the heat-transfer ability of the microfibrousstructured catalyst.

Experimental

Catalyst preparation

A thin fibers felt (2.0 m length  1.0 m width  ~1.3 mm thickness) of 60-μm Al-fibers (99.9 wt.% Al) was employed as a substrate; it was purchased from Shanghai Xincai Network-microstructured Material Co. Ltd. (China). The circular Al-fibers (8 mm diameter; equal to the internal diameter of fixed-bed reactor) were tailored from the purchased fibers felt, and treated with 0.1 wt.% NaOH solution at 25 º C for 2 min to remove the protective oxide layers from the surface. Subsequently, the pre-treated Al-fibers were loaded into a quartz tube, and oxidized in steam flow at 120 º C for 6 h to form the boehmite (AlOOH) nanosheets on the Al-fibers surface (denoted as AlOOH/Al-fibers) [24].

The as-prepared AlOOH/Al-fibers were heated to 800 º C in air with a ramping rate of 2 º C min 1 and kept at that temperature for 2 h to obtain the -Al2O3/Al-fibers composite.

The catalysts with a theoretical Pd content of 0.3 wt.% were prepared, as described elsewhere [23]. 0.0128 g of palladium acetate (Pd(CH3COO)2, 47 wt.% Pd, Sinopharm Chemical Reagent Co., Ltd., China) was dissolved in 3.7 g of toluene (C6H5CH3) and the solution was used to impregnate 2.0 g of the -Al2O3/Al-fibers composite. After drying at 100 º C for 2 h, the pre-impregnated Al-fibers were calcined at 300 º C in air for 2 h to obtain the catalyst denoted as Pd/-Al2O3/Al-fibers. For reference, an -Al2O3 (6080 mesh; Alfa Aesar (China) Chemical Co. Ltd.) support was used to prepare the Pd/-Al2O3 catalyst by the same preparation method.

Characterizations

Scanning electron microscope (SEM) observations were conducted on a Hitachi S-4800 instrument.

Transmission electron microscope (TEM) images were obtained with a FEI TECNAI G 2 F30 instrument at 300 kV. The shell of the -Al2O3/Al-fibers composite was carefully removed by a cutter knife and then the obtained powder (i.e., the shell) was used for the TEM analysis. X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku Ultima IV diffractometer with Cu K radiation (35 kV and 25 mA). Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were recorded in a Bruker Tensor 27 spectrometer equipped with a Harrick Scientific HVC-DRP-4 cell and a liquid N2 cooled mercury-cadmium-telluride (MCT) detector. The actual Pd loading was measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) with a Thermo IRIS Intrepid II XSP apparatus. Prior to the analysis, the sample was dissolved in aqua regia. Pd ICP standard solutions with concentrations of 5, 10 and 20 g g 1 were used to obtain the calibration curve.

Nitrogen physisorption was performed at 196 º C using a Quantachrome Quadrasorb-evo instrument.

The specific surface area and pore size distribution were estimated using the Brunauer-Emmett-Teller (BET) theory and Barrett-Joyner-Halenda (BJH) method based on the desorption branch of the isotherms. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an Escalab 250xi spectrometer with an analyzer pass energy of 30.0 eV and an Al Kα X-ray source (300 W). The results were referenced to the adventitious carbon, C1s peak at 284.6 eV prior to fitting the spectra. H2temperature programmed reduction (H2-TPR) was conducted on a Tianjin XQ TP-5080 apparatus equipped with a thermal conductivity detector (TCD). 100 mg of sample was treated with He (flow of 7/25 40 mL min 1 ) at 300 º C for 1 h and then cooled to 25 º C. Subsequently, a gas mixture of 5 vol.% H2/N2 was introduced into the reactor at 40 mL min 1 . After the baseline of TCD signal was steady, the sample was heated to 300 º C at 10 º C min 1 .

Catalytic tests

The catalytic behavior of the Pd/-Al2O3/Al-fibers and Pd/-Al2O3 catalysts was tested in a vertical fixed-bed, continuous down-flow quartz tube microreactor (internal diameter of 8 mm, length of 760 mm) (Scheme 1) [25]. Three calibrated mass flow controllers were employed to regulate the flow rates of N2, CO, the gas mixture of methyl nitrite (CH3ONO) and N2. The weight of the catalyst was 0.12 g at gas hourly space velocity (GHSV) of 20000 mL g 1 h 1 and 0.04 g at GHSV of 60000 mL g 1 h 1 . The reaction temperature was varied in the range of 120200 º C. Prior to the test, the catalyst was in situ activated by performing the CO oxidative coupling to DMO at 200 º C for 2 h with a gas mixture of CH3ONO / CO / N2 (1 / 1.4 / 7.6, mole) [23]. The outlet gas was analyzed by an online gas chromatography (GC; Shanghai Ouhua 9160, China) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID). The TCD and FID were connected to a ShinCarbon ST packed column (DIKMA) and Innowax PEG-20M capillary column (HP), respectively.

The six-way valve for FID and partial gas pipelines were heated at 120 º C to avoid the condensation of reaction products. The calculations of the turnover frequency (TOF), CO conversion, DMO selectivity and "methanol + methyl formate" (ME + MF) selectivity were presented in the Supporting Information.

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Scheme 1. Schematic diagram of the apparatus used for catalyst testing (the red parts are heated). network, consisting of 10 vol.% 60-m Al-fibers and 90 vol.% void volume were employed as the pristine substrate (Fig. 1B). The Al-fibers had a smooth surface as shown in the SEM images (Fig. S1).

Results and discussion

With the aid of the steam-only oxidation between Al metal and H2O at 120 º C (2 Al + 4 H2O → 2 AlOOH + 3 H2), the uniform and honeycomb-like AlOOH nanosheets were endogenously generated, and aligned to a nanosheet array (shell) on the Al-fibers (core) (Fig. 1C) [24]. The thickness of the AlOOH shell was about 0.7 μm (inset of Fig. 1C) corresponding to an AlOOH content of 7.5 wt.% [24]. The as-prepared AlOOH/Al-fibers composite was calcined in air at 800 º C. The nanosheets were 9/25 preserved (Fig. 1D) but the microstructure of these nanosheets was slightly changed, possibly due to the dehydroxylation of the AlOOH (Fig. S2). It should be noted that the metal Al (core) could not leak from the core-shell AlOOH/Al-fibers composite even at high-temperature treatment at 800 º C (melting point of Al is 660 º C), due to the obstruction of densely packed AlOOH nanosheets (shell). The similar observations were obtained on the Al alloys (Ni-Al and Fe-Al), indicating that the protective alumina scale on the alloys surface could provide excellent oxidation resistance above 1000 º C [26][27][28]. The characteristic diffraction peaks of metal Al (JCPDS 04-0787) were clearly observed in the pristine Alfibers (Fig. 1E, pattern a). The weak diffraction peaks at 28.2 and 49.3 º 2 theta were attributed to the boehmite (JCPDS 21-1307) (Fig. 1E,pattern b). After the calcination at 800 º C, the -Al2O3/Al-fibers composite showed the formation of -Al2O3 phase (JCPDS 10-0173) confirmed by the appearance of the diffraction peaks at 25.6, 35.1, 43.4, 52.6, 57.5, 66.5 and 68.2 º 2 theta (Fig. 1E,pattern c). In addition, the DRIFTS spectrum of AlOOH/Al-fibers composite contained five bands at 3336, 3126, 1066, 857 and 788 cm 1 that correspond to boehmite (Fig. 1F). The broad bands in the region 36003100 cm 1 were assigned to O-H stretching vibrations of water, while the bands at 950600 cm 1 are the Al-O bending stretching mode of -Al2O3 [29,30]. The DRIFTS results were in a good agreement with the XRD results. The as-prepared -Al2O3/Al-fibers composite had a BET specific surface area of 4.0 m 2 g 1 and a total pore volume of 0.012 cm 3 g 1 (including the Al-fibers) (Fig. 1G).

After the Al-fibers content of ~93 wt.% was subtracted, it was calculated that the -Al2O3 shell had a large BET surface area of ~50.0 m 2 g-Al2O3 1 and total pore volume of ~0.150 cm 3 g-Al2O3 1 . The -Al2O3/Al-fibers composite displayed a type-Ⅳ isotherm and mesopores with a size in the range of 419 nm. The appearance of type-H3 hysteresis loop at P/P0 > 0.5 was attributed to the voids formed by stacking of -Al2O3 nanosheets (Fig. 1G). The shell of the -Al2O3/Al-fibers composite was carefully removed by a cutter knife and the obtained powder (the shell) was analyzed by TEM (Fig. 2). TEM images showed that the shell of the -Al2O3/Al-fibers composite was composed of nanosheets (Fig. 2A). This observation was in 11/25 agreement with the nanosheet array observed in the SEM image (Fig. 1D). The nanosheets structure remained intact after calcination at 800 º C, indicating the good thermal stability. The thickness of the nanosheets was 26 nm as measured by TEM (Fig. 2A). From the HRTEM image, the lattice spacing of the nanosheets of 0.25 nm was calculated, which possibly corresponds to the (104) lattice plane of -Al2O3 crystals (JCPDS 10-0173). This result further verified the formation of -Al2O3 on the -Al2O3/Al-fibers composite, which was consistent with the XRD and DRIFTS results (Fig. 1E and Fig. 1F).

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Phase transformation of AlOOH in the Al-fibers

Fig. 3 shows the XRD patterns of the AlOOH/Al-fibers composite calcined at various temperatures.

After calcination at 600 º C, the diffraction peaks at 45.8 and 67.1 º 2 theta were assigned to -Al2O3 (JCPDS 80-0956) (Fig. 3, pattern a). This revealed that the AlOOH on the Al-fibers could convert to -Al2O3 at 600 º C. The peak intensity of 67.1 º 2 theta increased gradually but the -Al2O3 phase remained unchanged with arising the calcination temperature from 600 to 750 º C (Fig. 3, patterns ac).

However, the AlOOH on the Al-fibers was transformed into -Al2O3 at 800 º C (Fig. 3,pattern d). The

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composite after calcination at 600 º C was calcined again at 800 º C, and the XRD pattern was identical with that of the composite calcined at 800 º C (Fig. 3,patterns d,e). This suggested that the -Al2O3 on the Al-fibers converted to -Al2O3 at 800 º C. This result confirmed that the AlOOH on the Al-fibers is transformed according to the following scheme: AlOOH  -Al2O3  -Al2O3. 4A, these four composites had a binding energy (BE) of 74.074.6 eV, which was attributed to Al 3+ in alumina [31]. The absence of Al 2p peak at 72.5 eV indicated that the Al-Al bond (Al metal) was not present on the surface of these four composites [32]. This further confirmed the SEM observation that the metal Al could not leak from the Al-fibers under calcination at 800 º C (Fig. 1D). The full width at half-maximum (FWHM) of Al 2p peak on the AlOOH/Al-fibers (1.5 eV) without calcination was lower than of the other three composites (2.02.3 eV) (Fig. 4A and Table 1). The Al 2p peak with a characteristic FWHM of 1.5 eV was attributed to boehmite [33], suggesting the presence of AlOOH on the surface of the AlOOH/Al-fibers. This result

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was consistent with the XRD and DRIFTS results (Fig. 1E and1F). As shown in Fig. 4B, the broad O 1s spectra were decomposed into two component species: Al-O (530.7 eV) and Al-OH (532.1 eV) [34,35]. From the XPS results one can see that the peak area ratio of Al-O and Al-OH gradually increased, due to the desorption of hydroxyl groups at high temperature (Fig. 4B and Table 1).

Low-temperature synthesis of -Al2O3: promotion of Al metal

The low-temperature phase transformation from AlOOH to -Al2O3 was initially attributed to the presence of Al metal. Firstly, reference AlOOH nanosheets were synthesized from a bayerite precursor as described elsewhere (Fig. 5A, pattern a) [36]. Even though the average thickness of the reference AlOOH nanosheets (15 nm measured by TEM images) was similar to the counterpart of nanosheets of the AlOOH/Al-fibers (Fig. 2A) [36], the former was totally transformed into -Al2O3 after calcination at 800 º C (Fig. 5A, pattern b). This was consistent with the previous reports revealing that the boehmite could not convert to -Al2O3 at 800 º C [37,38]. Secondly, to study the effect of the Al metal, the AlOOH/Al-fibers composite calcined at 600 º C was treated with an excess of NaOH solution (4.0 wt.%) to remove the Al metal. Note that the concentration of NaOH solution should be carefully controlled, otherwise the composite will be completely dissolved in a concentrated solution. After the treatment, the rest of the AlOOH/Al-fibers composite calcined at 600 º C showed the characteristic diffraction peaks corresponding to the -Al2O3 phase (Fig. 5B, pattern a). The rest of the AlOOH/Alfibers composite after the calcination at 600 º C and the subsequent NaOH treatment, was treated at 800 º C but the -Al2O3 crystalline phase remained (Fig. 5B, pattern b). Both -Al2O3 and AlOOH on the Al-fibers could transform to the -Al2O3 at 800 º C as previously stated (Fig. 3, patterns d,e) while only the -Al2O3 without the Al-fibers could not (Fig. 5B). The phase transformation to -Al2O3 is governed by nucleation and grain growth mechanism [39,40]. It has been confirmed experimentally and by modeling that the -Al2O3 can be formed at low temperature (9001100 º C) on Al alloys such as Fe-Al and Ni-Al, due to the promoted -Al2O3 nucleation at the oxide/metal interface [7,41]. It is thus reasonable to infer that the formation of -Al2O3 at 800 º C was possibly attributed to the Al metal in the AlOOH/Al-fibers composite, which lowered the nucleation energy barrier of -Al2O3 at the oxide/Al metal interface.

Microfibrous-structured Pd/-Al2O3/Al-fibers catalyst for CO oxidative coupling to DMO

Features of Pd/-Al2O3/Al-fibers catalyst

The Pd/-Al2O3/Al-fibers catalyst was prepared by incipient-wetness impregnation method using the as-prepared -Al2O3/Al-fibers as support. As shown in Fig. 6A, the nanosheet array of -Al2O3

was well retained after the Pd loading. No diffraction peaks of PdO (JCPDS 88-2434) and Pd (JCPDS 01-1201) were detectable except the peaks corresponding to Al and -Al2O3, possibly due to a good

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Pd dispersion (Fig. 6B). In fact, the Pd was homogeneously dispersed on the -Al2O3/Al-fibers composite, with an average particle size of 4.4 ± 0.9 nm (Fig. 6C). The stacks of nanosheets of the catalyst were clearly observed, consistent with the catalyst support (Fig. 2A and Fig. 6C). The Pd/-Al2O3/Al-fibers catalyst contains mesopores with a size of 419 nm and the BET specific surface area is 3.9 m 2 g 1 with a total pore volume of 0.011 cm 3 g 1 (including the Al-fibers mass), which were almost equal to the -Al2O3/Al-fibers composite (Fig. 1G and Fig. 6D).

Catalytic performance of Pd/-Al2O3/Al-fibers catalyst

The microfibrous-structured Pd/-Al2O3/Al-fibers catalyst was tested in the CO oxidative coupling to DMO using a GHSV of 20000 mL g 1 h 1 and a feedgas of CH3ONO / CO / N2 (1 / 1.4 / 7.6, mole).

Compared to the reference catalyst of Pd/-Al2O3, the Pd/-Al2O3/Al-fibers delivered similar CO conversion, DMO selectivity and "methanol + methyl formate" (ME + MF) selectivity in the range of 120140 º C (Fig. 7). Interestingly, the Pd/-Al2O3/Al-fibers showed higher CO conversion, DMO selectivity, and lower "ME + MF" selectivity than the reference Pd/-Al2O3 catalyst at temperatures ranging from 150 to 170 º C (Fig. 7). This evolution behavior was initially assigned to the different thermal conductivity of microfibrous-structured Al-fibers and the -Al2O3. It should be pointed out that the CO oxidative coupling to DMO is highly exothermic (H25 ºC = 169 kJ mol 1 ), which may induce large temperature rise with unexpected hot spot in the reactor bed [23]. To examine the reaction heat effect, the thermocouple was inserted in the upper side of the catalyst bed. The catalyst characteristics and intrinsic activity (i.e., TOF) of the Pd/-Al2O3/Al-fibers and Pd/-Al2O3 were summarized in Table 2. The Pd loadings of the two catalysts were determined to be 0.25 wt.% and 0.26 wt.% by ICP-AES analysis. The average Pd particle sizes (d) in the Pd/-Al2O3/Alfibers and Pd/-Al2O3 were 4.4 nm and 2.0 nm (TEM images in Fig. 6C and Fig. S3), corresponding to the Pd dispersion (D) of 0.25 and 0.56, respectively (the D was calculated using the formula D = 1.12 / d [23]. Similar H2-temperature programmed reduction (TPR) curves for the two catalysts were measured, which showed the catalysts could be completely reduced at about 120 º C (Fig. S4). As a control experiment, the CO conversion was decreased to be  6% at 120 º C and GHSV of 60000 mL g 1 h 1 for the TOF calculation. The TOFs of 0.7 and 0.5 s 1 for the two catalysts, respectively were obtained, indicating comparable intrinsic activity. Clearly, the Pd/-Al2O3/Al-fibers and Pd/-Al2O3 catalysts have similar Pd contents, reduction behavior, comparable Pd particle size and intrinsic activity.

These results suggested that the -Al2O3/Al-fibers composite is a promising alternative to the conventional -Al2O3 for the CO oxidative coupling to DMO. e TOF based on the CO conversion and surface Pd atoms.

Heat transfer enhancement

As previously stated, our main efforts were focused on the development of a microfibrousstructured catalyst with a combination of high catalytic performance and excellent thermal conductivity.

To probe the heat-transfer ability of our catalyst, the R was conducted to obtain the temperature distribution in the microfibrous-structured bed of Pd/-Al2O3/Al-fibers and randomly packed bed of Pd/-Al2O3 catalysts. As shown in Fig. 8A, the tubular reactor was divided into three parts: one isothermal section and two adiabatic sections. The detailed CFD model and calculation methods were described in the Supporting Information. Fig. 8B shows the steady-state temperature distribution on the microfibrous-structured Pd/-Al2O3/Al-fibers and the Pd/-Al2O3 catalysts. The released reaction heat on the two catalysts was similar due to the comparable CO conversion (58 % vs. 50 % at 150 º C, Fig. 7A) under identical reaction conditions. However, the reactor' temperature distribution of the the microfibrous-structured Pd/-Al2O3/Al-fibers catalyst. The catalyst showed a good stability throughout the 150 h test, exhibiting a CO conversion of ~58% and DMO selectivity of ~95% (Fig. 9A). The diffraction peaks of PdO (JCPDS 88-2434) and Pd (JCPDS 01-1201) were still not detectable for the used catalyst after 150 h test (Fig. S5). The TEM image showed Pd nanoparticles with an average size of 4.5 ± 0.6 nm (Fig. S6), almost equal to that of the catalyst before the test (4.4 ± 0.9 nm, Fig. 6C). This suggested that the Pd/-Al2O3/Al-fibers catalyst was resistant to the sintering of Pd nanoparticles. A noteworthy feature of the used catalyst was that the nanosheet array of -Al2O3 remained unaltered, confirmed by SEM (Fig. 9B,C) and XRD (Fig. S5) characterizations.

Conclusions

The -Al2O3 nanosheets on the microfibrous-structured Al-fibers have been prepared through the phase transformation of boehmite nanosheets at temperature as low as 800 º C. The presence of Al metal

3. 1 .

 1 Figure 1A schematically illustrates the endogenous growth of AlOOH nanosheets on the

Fig. 1 .

 1 Fig. 1. (A) Schematic presentation of the top-down strategy applied for preparation of the core-shell

Fig. 2 .

 2 Fig. 2. TEM images of the -Al2O3/Al-fibers composite: (A) scale bar, M = 50 nm and (B) scale bar,

Fig. 3 .

 3 Fig. 3. XRD patterns of the AlOOH/Al-fibers composite after calcination: (a) 600 º C for 2 h, (b) 700

Fig. 4

 4 Fig.4shows the XPS spectra of the Al 2p and O 1s regions for the AlOOH/Al-fibers composite

Fig. 4 .

 4 Fig. 4. XPS spectra of (A) Al 2p and (B) O 1s regions of the AlOOH/Al-fibers composite (a) without

a

  XPS spectra of the AlOOH/Al-fibers composite without calcination (a) and after calcination for 2 h at (b) 600 º C, (c) 700 º C and (d) 800 º C. b Area ratio of O 1s peak at 530.7 eV to the peak at 532.1 eV.

Fig. 5 .

 5 Fig. 5. (A) XRD patterns of the reference boehmite nanosheets (a) before and (b) after calcination at

Fig. 6 .

 6 Fig. 6. (A) SEM image, (B) XRD pattern, (C) TEM image and (D) N2 sorption isotherm (Inset: BJH

Fig. 7 .

 7 Fig. 7. (A) CO conversion, DMO selectivity and (B) "methanol + methyl formate" (ME + MF)

c

  Pd particle size (d) obtained from TEM images in Fig. 6C and Fig. S3, and Pd dispersion (D) calculated by the formula: D = 1.12 / d [23]. d Surface Pd atoms determined by Pd dispersion (D) and Pd content.

Fig. 9 .

 9 Fig. 9. (A) Durability performance of the Pd/-Al2O3/Al-fibers catalyst, and SEM images of (B) fresh

Table 1 .

 1 Parameters obtained from the XPS spectra a .

	Sample a	Al 2p		O 1s at 530.7 eV	O 1s at 532.1 eV	O 1s
		BE (eV)	FWHM (eV)	FWHM (eV) Area	FWHM (eV) Area	Ratio b
	a	74.4	1.5	1.9	738	1.5	1584	0.46

Table 2

 2 Content, property and activity of the Pd/-Al2O3/Al-fibers and Pd/-Al2O3 catalysts a .

	Sample	Pd (wt.%) b d (nm) c	D c	Surface Pd (mol gcat. 1 ) d TOF (s 1 ) e

b Pd content determined by ICP-AES analysis.

Acknowledgements

This work was supported by the China Postdoctoral Science Foundation (2017M622311), Natural Science Foundation of China (21706285), Fundamental Research Funds for the Central Universities (18CX02148A, 18CX02013A), Postdoctoral Research Project of Qingdao (BY20170209) and the Sino-French International Laboratory (LIA) "Zeolites".

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hot spot with a temperature rise of 8 º C, which was much lower than the latter of 30 º C (the highest temperature of CFD simulation minus the reactor wall temperature of 150 º C). The low temperature rise for the Pd/-Al2O3/Al-fibers catalyst was unambiguously attributed to the high thermal conductivity of the microfibrous-structured Al-fibers, which rapidly dissipated a large amount of reaction heat [44,45]. Notably, the trend of temperature rise by CFD simulation was consistent with the experimental observation using a thermocouple (5 vs.

Stability

To investigate the long-term stability of the catalyst, an experiment was run at 150 º C for 150 h on in the Al-fibers is responsible for the low-temperature phase transformation to -Al2O3. Such -Al2O3/Al-fibers, with enhanced heat transfer and low pressure drop, is a promising alternative to the conventional -Al2O3 support in some industrial and environmental catalysis. As an example, the Pd/-Al2O3/Al-fibers catalyst is prepared via incipient-wetness impregnation, and tested in the strongly exothermic CO oxidative coupling to dimethyl oxalate (DMO). The catalyst with a low Pd loading of 0.25 wt.% exhibits a CO conversion of 58 % and DMO selectivity of 95 % with a feedgas of CH3ONO / CO / N2 (1 / 1.4 / 7.6, mole) at 150 º C with a large gas hourly space velocity of 20000 mL g 1 h 1 . The Pd/-Al2O3/Al-fibers catalyst showed high stability during the continuous test of 150 h. The Pd/-Al2O3/Al-fibers catalyst effectively inhibited the side reaction and showed a higher activity compared to the conventional Pd/-Al2O3 catalyst, resulted from the decreased temperature of the catalyst bed. Furthermore, computational fluid dynamics calculations and experimental measurements demonstrated the large reduction of hot-spot temperature thanks to the enhanced thermal conductivity originated from the microfibrous-structured Al-fibers.


Pingping Wu

Yunxiang Cao

Lianming Zhao

Yue Wang

Zhengke He

Wei Xing

Peng Bai
email: baipeng@upc.edu.cn

Svetlana Mintova

Zifeng Yan

Formation of PdO on Au-Pd bimetallic catalyst and the effect on benzyl alcohol oxidation

Keywords: Au-Pd bimetallic catalyst, surface composition, synergistic effect, benzyl alcohol oxidation

A series of catalysts containing of gold-palladium bimetallic nanoparticles (Au-Pd NPs in the range of 1-6 nm) anchored on foam-like mesoporous silica were used for the aerobic oxidation of benzyl alcohol. A remarkable synergistic effect was observed on these Au-Pd NPs catalysts prepared by one-pot method. Both the experimental and theoretical study revealed a close relationship between the surface PdO species on the catalysts and their catalytic performance, that is, a higher surface PdO content leads to a lower catalytic activity. The surface content of PdO species on the catalysts could be tuned by controlling the Au/Pd ratios, because the formation of Au-Pd alloy NPs and electron transfer between surface Au and Pd atoms prevented the oxidation of surface Pd and retarded the formation of PdO species. An optimal Au/Pd ratio of 1/4.5 on the foam-like mesoporous silica support was obtained, with nearly no surface PdO species formed and resulted the highest benzyl alcohol conversion of 96%.

The bimetallic Au-Pd catalysts exhibited much higher catalytic activity for benzyl alcohol oxidation (TOF = 50000 -60000 h -1 ) than the monometallic Pd catalyst (TOF = 12500 h -1 ) on which surface Pd is easily oxidized to PdO. These results provide direct evidence for the synergistic effect of the Au-Pd bimetallic catalyst in benzyl alcohol oxidation.

Introduction

The selective oxidation of alcohols on metal nanoparticles (NPs) to aldehydes or esters has attracted tremendous attention because the products play an important role in pharmaceuticals, agrochemistry and cosmetics industries [START_REF] Gogoi | Synthesis of Palladium Nanoparticle by Bio-reduction Method and Its Effectiveness as Heterogeneous Catalyst Towards Selective Oxidation of Benzyl Alcohols in Aqueous Media[END_REF][START_REF] Lavenn | Au25 (SPH-pNH2)17 Nanoclusters Deposited on SBA-15 as Catalysts for Aerobic Benzyl Alcohol Oxidation[END_REF]. In contrast to the conventional approach using stoichiometric amounts of chromate or permanganate as oxidants, the solvent-free liquid phase oxidation of alcohols with air, O2 or H2O2 is recognized as a sustainable and environmentally friendly process, for the sole by-product is water. However, the major challenge for the application of solvent-free liquid oxidation in industry relies in the development of highly active and selective catalyst systems. Supported noble metal NPs, such as Au [START_REF] Hong | Gold Nanoparticle Stabilization Within Tailored Cubic Mesoporous Silica: Optimizing Alcohol Oxidation Activity[END_REF][START_REF] Lackmann | A Comparative Study of Alcohol Oxidation over Nanoporous Gold in Gas and Liquid Phase[END_REF][START_REF] Liu | Preparation of Au Nanoparticles with High Dispersion and Thermal Stability by A Controlled Impregnation Method for Alcohol Oxidation[END_REF], Pd [START_REF] Campisi | Selectivity Control in Palladium-Catalyzed Alcohol Oxidation through Selective Blocking of Active Sites[END_REF][START_REF] Savara | Benzyl Alcohol Oxidation on Carbon-Supported Pd Nanoparticles: Elucidating the Reaction Mechanism[END_REF] and bimetallic catalysts (Au-Pd) [START_REF] Gomez-Villarraga | Synergistic Effect in the Oxidation of Benzyl Alcohol Using Citrate-Stabilized Gold Bimetallic Nanoparticles Supported on Alumina[END_REF][START_REF] Olmos | Synergistic Effect of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Selective Oxidation of Glycerol[END_REF][START_REF] Al-Rifai | Hydrodynamic Effects on Three Phase Micro-packed Bed Reactor Performance -Gold-Palladium Catalysed Benzyl Alcohol Oxidation[END_REF][START_REF] Cheong | Au-Pd Core-Shell Nanoparticles as Alcohol Oxidation Catalysts: Effect of Shape and Composition[END_REF][START_REF] Olmos | Influence of Pretreatment Atmospheres on the Performance of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Benzyl Alcohol Oxidation[END_REF] have been extensively studied for selective oxidation of alcohols [START_REF] Jiang | Gold and Gold-palladium Alloy Nanoparticles on Heterostructured TiO2 Nanobelts as Plasmonic Photocatalysts for Benzyl Alcohol Oxidation[END_REF]. Among them, gold (Au) and palladium (Pd) bimetallic catalyst was the most frequently reported for alcohol oxidation; a synergistic effect between these two metals was observed [START_REF] Gomez-Villarraga | Synergistic Effect in the Oxidation of Benzyl Alcohol Using Citrate-Stabilized Gold Bimetallic Nanoparticles Supported on Alumina[END_REF][START_REF] Olmos | Synergistic Effect of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Selective Oxidation of Glycerol[END_REF][START_REF] Carter | Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support[END_REF][START_REF] Silva | Volcano-like Behavior of Au-Pd Core-shell Nanoparticles in the Selective Oxidation of Alcohols[END_REF][START_REF] Wang | Precisely-controlled Synthesis of Au@Pd Core-shell Bimetallic Catalyst via Atomic Layer Deposition for Selective Oxidation of Benzyl Alcohol[END_REF]. A particular "crown-jewel" structured Au-Pd bimetallic catalyst was synthesized by Toshima and co-workers and applied for glucose oxidation [START_REF] Zhang | Catalytically Highly Active Top Gold Atom on Palladium Nanocluster[END_REF]. A much higher catalytic activity than those of monometallic Au and Pd NPs was observed in this "crown-jewel" structured Au-Pd catalyst. The synergistic effect was generally attributed to the collaborative effect (closely related to the local composition of Au/Pd) and electronic effect (electronic modification through hetero-nuclear metal-metal bond formation) [START_REF] Long | Selective Oxidation of Saturated Hydrocarbons Using Au-Pd Alloy Nanoparticles Supported on Metal-Organic Frameworks[END_REF][START_REF] Wei | Bimetallic Au-Pd Alloy Catalysts for N2O Decomposition: Effects of Surface Structures on Catalytic Activity[END_REF][START_REF] Gao | Pd-Au Bimetallic Catalysts: Understanding Alloy Effects from Planar Models and (supported) Nanoparticles[END_REF]. Several works have been devoted to understand the synergistic effect in bimetallic catalysts. Chen and co-worker applied Au-Pd/CeZrO2 catalyst for glycerol oxidation and they attributed the synergistic effect to the degree of compositional homogenization [START_REF] Olmos | Synergistic Effect of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Selective Oxidation of Glycerol[END_REF]. A synergy effect was also observed on Au-Pd/CeZrO2 catalyst for benzyl alcohol oxidation, which was ascribed to the electronic interaction between Au and Pd atoms [START_REF] Carter | Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support[END_REF]. The activity enhancement of Au atoms was also reported primarily by the variation in the occupation state of Pd d-orbitals [START_REF] Liu | Au@ Pd Bimetallic Nanocatalyst for Carbon-Halogen Bond Cleavage: An Old Story with New Insight into How the Activity of Pd is Influenced by Au[END_REF]. Both the compositional homogenization and electronic interaction would lead to the alteration in occupation states of Pd d-orbitals, which seems to be the essence of the synergy effect, however the direct evidence is lacking.

Furthermore, just on the opposite, an antisynergistic effect was observed in the bimetallic system by Hutchings et al. [START_REF] Carter | Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support[END_REF], which implies that the intrinsic mechanism of the synergy effect still remains elusive.

Among some studies on the Au-Pd bimetallic catalysts, the Au/Pd ratio was commonly observed to have significant effect on the catalyst performance. It was reported by Chen and co-workers that Au-Pd bimetallic catalyst with higher Au/Pd ratio exhibited better catalytic activity in the selective oxidation of glycerol [START_REF] Olmos | Synergistic Effect of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Selective Oxidation of Glycerol[END_REF]. A remarkably high catalytic activity using an Au-Pd core-shell catalyst with an Au/Pd ratio of 10:1 in benzyl alcohol oxidation was reported by Silva and coworkers [START_REF] Silva | Volcano-like Behavior of Au-Pd Core-shell Nanoparticles in the Selective Oxidation of Alcohols[END_REF]. Wang et al applied an atomic layer deposition method towards preparation of Au@Pd core-shell bimetallic catalyst. They found that the maximum benzyl alcohol conversion was obtained with a Pd shell thickness of 0.6-0.8 nm [START_REF] Wang | Precisely-controlled Synthesis of Au@Pd Core-shell Bimetallic Catalyst via Atomic Layer Deposition for Selective Oxidation of Benzyl Alcohol[END_REF]. Recently, in Hutchings's work, the most active catalyst for benzyl alcohol oxidation had an Au/Pd ratio of 1 [START_REF] Carter | Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support[END_REF]. Among these works, a volcano-like relationship between catalyst activity and Pd content was recognized, and a maximum activity was observed at a certain Au/Pd ratio, which may vary depending on the different synthesis conditions. However, most of the works did not provide satisfactory explanations on this phenomenon. Wang et al attributed the effect of Pd shell thickness to both collaborative and electronic promotion, but without further mechanistic investigation [START_REF] Wang | Precisely-controlled Synthesis of Au@Pd Core-shell Bimetallic Catalyst via Atomic Layer Deposition for Selective Oxidation of Benzyl Alcohol[END_REF]. Based on the density functional theory (DFT) calculation, Silva et al. [START_REF] Silva | Volcano-like Behavior of Au-Pd Core-shell Nanoparticles in the Selective Oxidation of Alcohols[END_REF] explained that when Pd loading was high, the activity was enhanced because the increased number of Pd ensembles was favorable for the adsorption of benzyl alcohol. While further increase of Pd loading above the monolayer coverage, the adsorption of benzaldehyde was too strong, which impeded its desorption. The model was built based on the Au-Pd alloy without consideration of the possible existence of surface PdO species under oxidation conditions. Indeed, after oxidation at 250 o C, the formation of Pd δ+ species in both monometallic Pd/CeZrO2 and bimetallic AuPd/CeZrO2 catalysts was observed by Chen et al; the concentration of these species is higher in the monometallic Pd/CeZrO2 [START_REF] Olmos | Influence of Pretreatment Atmospheres on the Performance of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Benzyl Alcohol Oxidation[END_REF]. However, no detailed investigation on different Pd δ+ contents in monometallic and bimetallic catalysts as well as their effect on the catalytic activity was carried out.

In this work, a series of Au-Pd bimetallic NPs catalysts supported on foam-like mesoporous silica were prepared with different Au/Pd ratios by a one-pot synthesis method, and applied for benzyl alcohol selective oxidation. In most cases, the formation of PdO species on Au-Pd NPs was identified even at room temperature in air atmosphere. The content of PdO was tuned by varying the Au/Pd ratios.

The relationship between the catalyst activity and the PdO content was experimentally and theoretically studied.

Experimental

Chemicals

HAuCl4• xH2O (Sinopharm Chemical), PdCl2 (Sinopharm Chemical), triblock co-polymer PEO20PPO70PEO20 (P123, Aldrich), tetraethyl orthosilicate (TEOS, 98%, Sinopharm Chemical), 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.

Preparation of catalysts

The mesoporous silica support was synthesized by methods reported previously [START_REF] Han | Spherical Siliceous Mesocellular Foam Particles for High-speed Size Exclusion Chromatography[END_REF][START_REF] An | Preparation of Mesoporous Oxides and Their Support Effects on Pt Nanoparticle Catalysts in Catalytic Hydrogenation of Furfural[END_REF]. The noble metal precursors were introduced during the synthesis of mesoporous silica support, which has been reported in our previous work [START_REF] Wu | Au Nanoparticles Dispersed on Functionalized Mesoporous Silica for Selective Oxidation of Cyclohexane[END_REF][START_REF] Wu | Gold Nanoparticles Supported on Mesoporous Silica: Origin of High Activity and Role of Au NPs in Selective Oxidation of Cyclohexane[END_REF]. Typically, 4 g of P123 was dissolved in a mixture of 65 mL deionized water and 5 mL 37% HCl at room temperature. Then, 3g of 1,3,5-trimethylbenzene was added into the clear solution as a pore swelling agent and stirred at 40 o C for 2 h,followed by addition of 8.32 g of TEOS and 0.78g of MPTMS drop wise into the mixture. Finally, the HAuCl4 and PdCl2 solutions (the metal loading of 1 wt. %) were introduced in the synthesis mixture. After stirring at 40 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. After the crystallization was accomplished, the solids were purified with deionized water, dried under vacuum at 80 o C overnight and finally calcined in air at 500 o C for 6 h. The calcined samples were further treated under H2 at 400 o C for 2h. The catalysts with a total metal loading of 1wt.% were abbreviated as AuxPdy/MCF-H2 (x, y represent the molar percentages of Au and Pd, respectively): samples Au100/MCF, Au8Pd92/MCF-H2, Au18Pd82/MCF-H2, Au26Pd74/MCF-H2, Au61Pd39/MCF-H2 and Pd100/MCF-H2. The catalysts with a total metal loading of 2wt.% were abbreviated as 2wt.%Au/MCF-H2, 1wt.%Au-1wt.%Pd/MCF-H2 and 2wt.%Pd/MCF-H2.

Additionally, a sample under static air calcination at 500 o C for 1 h was prepared to evaluate the effect of treatment under different atmosphere (sample Au26Pd74/MCF-O2) and compared with the original sample Au26Pd74/MCF-H2.

Characterization

N2 adsorption/desorption was carried out to analyze the textural property of catalysts degassed at 300 o C for 4 h under vacuum using an automatic volumetric sorption analyzer (Micromeritics, TriStar 3000). The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) equation at the relative pressure of 0.05-0.25. The total pore volume of catalysts was obtained at the relative pressure P/P0 of 0.989. The Barrett-Joyner-Halenda (BJH) equation was used to obtain the pore size distribution using the desorption branch of the isotherm. X-ray powder diffraction (XRD) patterns of catalysts were recorded on a X'Pert PRO MPD system with a Cu Kα radiation (λ = 0.154 nm) at 35 kV and 40 mA. Wide-angle X-ray scattering was recorded from 20° to 85° 2Theta with a scanning rate of 5° per minute. Noble metal loadings in the catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a VISTA-MPX Varian system. The ultraviolet-visible (UV-vis) spectra analysis of catalysts was performed on a HitachiU-4100 UV-vis-NIR spectrophotometer using BaSO4 as an internal reference. The diffusion reflectance spectra from 350 to 800 nm were recorded.

The microstructure of the catalysts was characterized by high-resolution Transmission Electron Microscope (HRTEM) using a JEM 2010 JEOL operated at 200 kV. Based on the TEM images of the Au-Pd bimetallic catalysts, diameters of more than 150 metal particles randomly selected were measured, and the average particle diameter was calculated based on following equation: d=Σnidi/Σni, where ni ≥150. The metal dispersion was calculated using GAUSS software from Nanomaterials and Catalysis Group of University of Cadiz (Spain) assuming a spherical particle shape [START_REF] López-Haro | Bridging the Gap Between CO Adsorption Studies on Gold Model Surfaces and Supported Nanoparticles[END_REF]. Scanning transmission electron microscopy (STEM) high angle annular dark field (HAADF) images and X-ray energy dispersive spectra (XEDS) of the metallic particles were applied to analyze the distribution and elements dispersion of Au and Pd NPs. Additionally, tomography data were obtained using a FEI Tecnai F30 electron microscope operating at 300 kV. Colloidal gold particles (10 nm) were used as a marker in the TEM study. The tomographic series were carried out by tilting the specimen around a single axis from 50 ° to -50 ° at an increment of 1 o under the electron beam. Data processing was carried out by the IMOD software.

The surface property and relative proportions of elements in the catalysts were investigated by Xray photoelectron spectroscopy (XPS). The XPS analyses were performed on a Thermo-VG Scientific K-Alpha spectrometer equipped with an Al anode (Al Ka = 1486.6 eV). C 1s electron bond energy corresponding to graphitic carbon at 284.5 eV was applied as a calibration binding energy (BE) reference. The reducibility of catalysts was measured by hydrogen temperature programmed reduction using a Chem-BET 3000 TPD/TPR analyzer (Autochem Ⅱ, USA). Before analysis, the samples were degassed at room temperature for 4 h and the signals were recorded from 40 o C to 600 o C with a heating rate of 20 o C /min.

Catalytic test

Solvent-free aerobic oxidation of benzyl alcohol with O2 as oxidant was performed in a 100 mL autoclave liner with a polytetrafluoroethylene (Model: SLM100, Beijing Easychem Science and Technology Development Company, China). Typically, 10.8 g benzyl alcohol and 50 mg solid catalyst were introduced in the reactor. After purging with O2 three times, the reactor was heated to 110 o C and the O2 pressure was adjusted to 0.8 MPa under stirring. The O2 pressure was kept at 0.8 MPa during the entire reaction. After 1 h of reaction, the reactor was cooled down in an ice bath to 30 o C and the pressure was released. Then the reactor was opened slowly and the reaction products were separated by centrifuge. The solid catalyst was washed with ethanol and dried at 80 ℃ prior the next catalytic experiment. The n-pentanol was added to the reaction products as an internal standard. Finally, the reaction products were analysed using a gas chromatogram (Agilent 6870) equipped with an FID detector and a DB-1 column (30*0.32*0.25). The turnover frequency (TOF) was calculated based on moles of benzyl alcohol converted per mole of surface metals (Au + Pd) per hour, e.g., MBA converted h - 1 M surface metal -1 , while the amount of surface metals (Au + Pd) was calculated based on the total moles of metals multiplied by the metal dispersion.

Theoretical study

All first-principle calculations were carried out using the periodic density functional theory (DFT) method with the Perdew-Burke-Ernzerh (PBE) form of the general gradient approximation (GGA) functional in the DMol 3 package. The ion cores of Pd and Au atoms were treated by the density functional semicore pseudopotential (DSPP), while the valence electrons were described by the double-numerical basis with polarization functions (DNP). The Fermi smearing and real space cutoff were set to 0.005 hartree and 4.7 Å, respectively. The convergence tolerances were set to 5 × 10 -3 Å for displacement, 2 × 10 -3 hartree/Å for gradient, and 1 × 10 -5 hartree for total energy. In addition, the spin-polarization was employed in all calculations.

The calculated adsorption energy is presented as follows, Eads = Eadsorbate/substrate -(Eadsorbate +

Esubstrate),

where Eads is the adsorption energy of the adsorbate on metal surfaces, Eadsorbate/substrate is the total energy of the slab with an adsorbed molecule, and Eadsorbate and Esubstrate are the energies of the free adsorbate and the clean slab, respectively. For example, the oxygen adsorption energy was calculated using the following equation: Eads = Esub+O* -Esub -1/2EO2, where Esub+O*, Esub, 1/2EO2 are the total energy after O adsorption, the energy of the catalyst substrate, and one-half of the energy of a free oxygen molecule, respectively. 1. According to IUPAC classification, all catalysts exhibit a type-IV isotherm related to the H-2 hysteresis loop suggesting that all catalysts possess inkbottle mesopores. The shape of the hysteresis loop of the Au-Pd bimetallic catalysts deviates from the H-1 loop which is typically observed for pure MCF-17 and this is probably due to the addition of MPTMS during the synthesis. The MPTMS may disturb the formation and self-assembly process and lead to distortion of the mesopore structure of the catalysts. Additionally the introducing of Au-Pd NPs may change the pore shape of the MCF-17 [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 hysteresis loop for all Au-Pd catalysts (Fig. 1) is observed at a relative pressures, P/P0 of 0.4-0.5, indicating the presence of small mesopores. The pore size distribution of all catalysts are in the similar range of 3~5 nm. A high specific surface area above > 650 m 2 /g was measured for all samples, while sample Au26Pd74Pd/MCF-O2 shows the highest specific surface area (737 m 2 /g). No relationship between the Au/Pd ratios or treatment conditions and porosity of samples was observed.

Results

Textural properties of Au-Pd bimetallic catalysts

Morphology and crystalline structure of Au-Pd bimetallic catalysts

TEM images and particle size distribution histograms of Au-Pd/MCF catalysts are shown in Fig. 2 and Fig. S1. The metal loading, particle diameter and metal dispersion of catalysts are summarized in Table 1. Well dispersed noble metal NPs (1-6 nm) were observed in both monometallic and bimetallic catalysts. Sample Au100/MCF exhibited the highest metal dispersion (46 %) among all catalysts as shown in Fig. 2f and Table 1. With an increase of the Pd ratio, the average particle diameter increased and metal dispersion decreased, indicating that the addition of Au to Pd improved the metal dispersion on the mesoporous silica support.

The crystalline structure of NPs treated under different atmospheres were studied by HRTEM. As shown in the XRD patterns in Fig. S2 indicating that the formation of Au-Pd alloy NPs may prevent the oxidation of Pd in air.

Electron tomography (ET) analysis of Au-Pd bimetallic catalysts

The presence of well-dispersed noble metal NPs in foam-like silica support was confirmed by

STEM-HAADF characterization of Au-Pd bimetallic catalysts

The elemental distribution within the catalysts has a profound effect on their catalytic performance as was previously revealed by STEM [START_REF] Olmos | Influence of Pretreatment Atmospheres on the Performance of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Benzyl Alcohol Oxidation[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]. Fig. 4 shows the representative STEM-HAADF images and the corresponding colour-coded EDS spectral maps of Au26Pd74/MCF-H2 and Au26Pd74/MCF-O2 catalysts. Well dispersed metal NPs on foam-like porous silica were seen on both catalysts subjected to H2 or static air treatments (Fig. 4a and4c). EDS mapping shows that both Au (green) and Pd (yellow) elements are well distributed and aligned thus indicating the formation of Au-Pd alloy NPs. The ratios of the peaks corresponding to the amount of Au and Pd in the bimetallic catalyst reveal that each NP has a similar Au/Pd ratio (see insert in Fig. 4 a,c). The STEM-EDS mapping results of analysed area are presented in Fig. S4 and Fig. S5, the weight percentages of Au and Pd in the analysed area of catalyst Au26Pd74/MCF-H2 are 0.58 wt. % and 0. 62 wt. %, while on catalyst Au26Pd74/MCF-O2 are 0.49 wt. % and 0.63 wt. %, respectively, quite consistent with ICP results (0.39% and 0.58% for Au and Pd, respectively, as shown in Table 1). UV-vis spectra of Au-Pd/MCF catalysts are shown in Fig. 5. The Au100/MCF-H2 sample showed two broad peaks at 420 nm and 520 nm. The latter peak is associated with the surface plasmon resonance (SPR) of Au 0 NPs [START_REF] Sa | Redispersion of Gold Supported on Oxides[END_REF], whereas the former peak is assigned to the partially charged Aun clusters [START_REF] Wu | Selective Oxidation of Cyclohexane over Gold Nanoparticles Supported on Mesoporous Silica Prepared in the Presence of Thioether Functionality[END_REF]. The Pd100/MCF-H2 catalyst does not show any SPR band between 200 nm and 800 nm (see inset of Fig. 5).

Introducing 0.2 wt.% Pd (Au61Pd39/MCF-H2) led to significant broadening or even disappearance of the Au SPR band (520 nm). However, the peak at 420 nm was enhanced and shifted to lower wavelength. As the Pd/Au ratio increased, a decrease of the Au 0 NPs and an increase of partially charged Aun clusters in the bimetallic catalysts was measured. These results indicate the surface electron variation of Au NPs after introducing Pd as a promoter, which is probably due to the formation of Au-Pd alloy NPs and the presence of surface electron transfer between Au and Pd. This statement would be further confirmed by XPS data. The electronic properties of the Au-Pd bimetallic NPs were characterized by XPS. XPS spectra of Au 4f

XPS analysis of Au-Pd bimetallic catalysts

and Pd 3d are shown in Fig. 6 and Fig. 7, respectively. As shown in Fig. 6, typical Au 0 species [START_REF] Venezia | Effect of Gold on the HDS Activity of Supported Palladium Catalysts[END_REF][START_REF] Deki | Synthesis and Characterization of Nano-sized Gold-palladium Bimetallic Particles Dispersed in Polymer Thin Film Matrix[END_REF][START_REF] Venezia | Activity of SiO2 Supported Gold-palladium Catalysts in CO Oxidation[END_REF] with binding energy (BE) of 84.0 eV (Au4f7/2) and 87.8 eV (Au4f5/2) were observed on catalyst Au100/MCF-H2 (Fig. 6a). Comparing to the pure Au sample, the Au 4f peaks for Au-Pd bimetallic catalysts shift downward in BE by 0.2~0.3 eV (4f7/2 83.7 eV and 4f5/2 87.5 eV) with an increase of the Pd introduced. This is attributed to charge transfer between Pd and Au, indicating that the electronic structure of the surface Au atoms was modified upon the addition of Pd, and negatively charged Au species were obtained. It is interesting to note that two doublets at higher BEs of 84.9 eV and 88.3 eV were observed for the Au18Pd82/MCF-H2 catalyst. It was reported that this positive shift of Au BE was due to the presence of sub-nano size Au 0* clusters or positively charged Au δ+ species, which is still under debate [START_REF] Herzing | Identification of Active Gold Nanoclusters on Iron Oxide Supports for CO Oxidation[END_REF]. The Au 0* or Au δ+ species found on the Au catalyst surface was reported to enhance the catalytic activity of supported gold NPs catalyst for low temperature CO oxidation [START_REF] Li | Activation and Deactivation of Au-Cu/SBA-15 Catalyst for Preferential Oxidation of CO in H2-Rich Gas[END_REF]. In our previous work, it was also found that the surface Au δ+ sites acted as active species for O2 activation during non-solvent cyclohexane oxidation [START_REF] Wu | One-Pot Synthesis of Silica Supported Au-Ag Alloy Nanoparticles for Cyclohexane Oxidation[END_REF]. Thus, the presence of these active sites on the Au18Pd82/MCF-H2 catalyst may lead to high catalytic activity for benzyl alcohol oxidation.

The results from the catalytic test are presented below.

The Pd 3d spectra of different catalysts were shown in Fig. 7. Fig. 7a shows the Pd 3d peaks for sample Pd100/MCF-H2 at 337.2 eV and 342.8 eV, which are assigned to the surface Pd 2+ species [START_REF] Sarina | Visible Light Enhanced Oxidant Free Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle Catalysts[END_REF]. The presence of 100 % Pd 2+ in sample Pd100/MCF is due to the easy oxidation of Pd to PdO in air or water at room temperature during storage or characterization step [START_REF] Jiang | Gold and Gold-palladium Alloy Nanoparticles on Heterostructured TiO2 Nanobelts as Plasmonic Photocatalysts for Benzyl Alcohol Oxidation[END_REF]. The presence of PdO on the pure Pd catalyst was also confirmed by the XRD characterization (Fig. S3). The spectra of the Au-Pd bimetallic catalysts were fitted by two doublets: the higher Pd 3d5/2 energy value of Pd 2+ (~337.2 eV) [START_REF] Teranishi | Size Control of Palladium Nanoparticles and Their Crystal Structures[END_REF][START_REF] Farrauto | Thermal Decomposition and Reformation of PdO Catalysts: Support Effects[END_REF] and the lower energy value of metallic Pd 0 (~335.5 eV) [START_REF] Wang | Precisely-controlled Synthesis of Au@Pd Core-shell Bimetallic Catalyst via Atomic Layer Deposition for Selective Oxidation of Benzyl Alcohol[END_REF], showing that the percentage of oxidized Pd 2+ decreased dramatically. Focusing on the Pd 3d5/2 with a BE of 335.5 eV on catalyst Au26Pd74/MCF-H2 (Fig. 7d), it is clearly noted that this peak is gradually shifted to higher BE with increasing the Pd/Au ratio. This positive shift of Pd 0 BE on Au-Pd bimetallic catalysts indicates that the redistribution of electrons is occurred, leading to the formation of positively charged surface Pd δ+ atoms. Comparing the XPS results, the negative shift of Au 4f peaks and positive shift of Pd 3d peaks clearly indicate the transfer of electrons from Pd to Au on the bimetallic catalysts.

In addition, the fraction of Pd 2+ to metallic Pd 0 species in the bimetallic catalysts was calculated by the peak fitting and presented in Table 2. By comparing the relative intensity of the palladium oxide (PdO) and palladium metal (Pd) components in bimetallic catalysts, it was found that the PdO/Pd ratio decreased with the increase of the Au/Pd ratio. This indicates that alloying with Au prevent the surface Pd 0 to be oxidized to Pd 2+ species, which is consistent with Yang's research results [START_REF] Olmos | Synergistic Effect of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Selective Oxidation of Glycerol[END_REF][START_REF] Yang | High-performance Pd-Au Bimetallic Catalyst with Mesoporous Silica Nanoparticles as Support and its Catalysis of Cinnamaldehyde Hydrogenation[END_REF]. The XPS results provide information on the surface metal ratios of Au/Pd and PdO/Pd , while the ICP results indicate the bulk Au/Pd ratios (Table 2).

A relationship between the surface ratios of Au/Pd and PdO/Pd was observed. Catalyst Au18Pd82/MCF exhibited a steady surface and bulk Au/Pd atomic ratio of 0. The results for solvent-free oxidation of benzyl alcohol on the AuxPdy/MCF-H2 catalysts are shown in Fig. 9. The comparison of benzyl alcohol oxidation results on the bimetallic Au-Pd catalysts is summarized in Table S1. In this work, the catalytic activity of bimetallic catalysts exhibited a volcano-shape trend across the range of catalysts tested. As shown in Table S1, pure Au and Pd catalysts (samples Au100/MCF-H2 and Pd100/MCF-H2) exhibited very low benzyl alcohol conversion studied under the same reaction conditions. Only 4.9 % and 16 % of benzyl alcohol conversion with a TOF of 5000 h -1 and 12500h -1 on Au100/MCF-H2 and Pd100/MCF-H2 catalysts, respectively was obtained. However, a dramatic enhancement of catalytic activity for benzyl alcohol oxidation on bimetallic catalysts was observed, with TOF values of 50000 to 60000 h -1 . The benzyl alcohol conversion increased significantly under addition of Au to the Pd catalyst. The highest catalytic activity (TOF = 65700 h -1 ) was achieved for the Au18Pd82/MCF-H2 catalyst, and it decreased with further increase of the Au/Pd ratio (Table S1). Previously, the highest catalytic activity for benzyl alcohol oxidation (TOF = 63800 h -1 ) was reported for Au-Pd/C bimetallic catalysts with an Au/Pd molar ratio of nearly 1/2 under similar reaction conditions at 120 o C [START_REF] He | Switching-off Toluene Formation in the Solvent-free Oxidation of Benzyl Alcohol Using Supported Trimetallic Au-Pd-Pt Nanoparticles[END_REF], as shown in Table S1. In this work, the highest catalytic activity was achieved for the Au18Pd82/MCF-H2 catalyst with an Au/Pd molar ratio of 1/4.5. The low activity of monometallic Au was due to its inert catalytic activity to alcohol oxidative dehydrogenation and the Habstraction, which are the rate-determining steps in benzyl alcohol oxidation [START_REF] Olmos | Influence of Pretreatment Atmospheres on the Performance of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Benzyl Alcohol Oxidation[END_REF][START_REF] Mallat | Oxidation of Alcohols with Molecular Oxygen on Solid Catalysts[END_REF]. And the relative low activity of monometallic Pd catalyst is probably due to the oxidation of surface Pd active sites to PdO. The contiguous Pd islands and Au-Pd interface have been widely accepted as active sites for benzyl alcohol oxidation [START_REF] Han | Surface Alloy Composition Controlled O2 Activation on Pd-Au Bimetallic Model Catalysts[END_REF], and Wang et al. revealed that contiguous Pd islands exhibited higher activity for benzyl alcohol activation than separated Pd atoms [START_REF] Wang | Precisely-controlled Synthesis of Au@Pd Core-shell Bimetallic Catalyst via Atomic Layer Deposition for Selective Oxidation of Benzyl Alcohol[END_REF]. Our results confirmed the synergistic effect between Au and Pd for benzyl alcohol oxidation as reported previously [START_REF] Olmos | Influence of Pretreatment Atmospheres on the Performance of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Benzyl Alcohol Oxidation[END_REF][START_REF] Carter | Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support[END_REF]. The enhanced catalytic activity of bimetallic Au-Pd catalyst in this work (Au18Pd82/MCF-H2 and Au26Pd74/MCF-H2) is probably due to the formation of a large amount of contiguous Pd islands and a certain amount of Au-Pd interfaces with a low Au/Pd ratio. The catalytic activity decreased dramatically on catalyst Au61Pd39/MCF-H2 with further increasing the Au/Pd ratio.

The products distribution on the Au-Pd/MCF catalysts is shown in Table S1. It is clearly seen that different monometallic catalysts lead to completely different products distribution. Catalyst Au100/MCF-H2 exhibited the lowest benzaldehyde selectivity (42%) among all catalysts with a benzyl alcohol conversion of 4.9%, which is probably due to its inert catalytic activity to benzyl alcohol activation and a free radical reaction may occur. A benzaldehyde selectivity of 66% was obtained on monometallic Pd catalyst with a benzyl alcohol conversion of 16%. The highest benzaldehyde selectivity (>70%) was obtained over the Au8Pd92/MCF-H2 with a much higher benzyl alcohol conversion of 83%, indicating that small amount of Au introduced to Pd improved the catalytic activity and benzaldehyde selectivity. However, the benzaldehyde selectivity decreased with further increasing the Au ratio. It was widely accepted that contiguous Pd islands would be active for benzyl alcohol adsorption [START_REF] Wang | Precisely-controlled Synthesis of Au@Pd Core-shell Bimetallic Catalyst via Atomic Layer Deposition for Selective Oxidation of Benzyl Alcohol[END_REF][START_REF] Han | Surface Alloy Composition Controlled O2 Activation on Pd-Au Bimetallic Model Catalysts[END_REF]. Thus, the catalytic activity is proportional to the number of Pd islands. A small amount of Au introduced to Pd created an appropriate amount of Pd islands for benzyl alcohol adsorption, leading to a high benzyl alcohol conversion and benzaldehyde selectivity over catalyst Au8Pd92/MCF-H2. But further increase of the Au ratio leads to higher amount of Pd islands, which promotes benzyl alcohol adsorption, but also impedes the desorption of benzaldehyde [START_REF] Silva | Volcano-like Behavior of Au-Pd Core-shell Nanoparticles in the Selective Oxidation of Alcohols[END_REF]. Thus, a balance between benzyl alcohol adsorption and benzaldehyde desorption on Au-Pd core-shell catalyst must be achieved to produce a satisfactory benzaldehyde yield [START_REF] Silva | Volcano-like Behavior of Au-Pd Core-shell Nanoparticles in the Selective Oxidation of Alcohols[END_REF]. In this work, the optimum catalyst Au8Pd92/MCF-H2 with an Au/Pd ratio of 1/11.5 was obtained.

Theoretical study: DFT calculations

In order to understand the reactivity of catalysts and surface compositions, DFT calculations were surface, the desorption energy of water molecules is still as high as 0.70 eV (the oxygen atoms of water form a bridge adsorption with Pd, the adsorption distance is 2.518 Å and 2.844 Å). This value is much larger than those on Pd(111) and Pd3Au(111), indicating the difficulty of H2O molecule formation and its desorption from the PdO(200). Although the strong hydroxyl adsorption of benzyl alcohol on PdO(002) is beneficial for the activation, the hydrogen produced by hydroxyl cleavage is easy to form a stable -OH on the PdO surface and difficult to form water molecules to desorb, which is detrimental for the following reaction. Therefore, the PdO(002) is an inert surface for catalytic oxidation of benzyl alcohol. This result is consistent with our experimental work demonstrating that the surface enrichment of PdO species leads to a low catalytic activity.

Discussion

A volcano-shape activity trend was observed on Au-Pd bimetallic catalysts in the series of samples for benzyl alcohol oxidation tested in this work. Catalyst Au18Pd82/MCF-H2 with the Au/Pd molar ratio of 1/4.5 exhibited much higher benzyl alcohol conversion (96 %) than those of monometallic Au (4.9 %) and Pd (16 %) catalysts. This catalytic activity enhancement was also reported in previous works [START_REF] Carter | Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support[END_REF][START_REF] Yang | High-performance Pd-Au Bimetallic Catalyst with Mesoporous Silica Nanoparticles as Support and its Catalysis of Cinnamaldehyde Hydrogenation[END_REF], which was explained as a synergy effect of bimetallic catalyst. Our results confirmed the synergistic effect between Au and Pd for benzyl alcohol oxidation [START_REF] Olmos | Influence of Pretreatment Atmospheres on the Performance of Bimetallic Au-Pd Supported on Ceria-zirconia Mixed Oxide Catalysts for Benzyl Alcohol Oxidation[END_REF][START_REF] Carter | Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support[END_REF], but the reason for this promoting effect is still under debate. No direct evidence for this synergy effect is provided yet.

The focus of this work is to reveal the relationship between the structural properties of bimetallic catalysts and their catalytic performance in benzyl alcohol oxidation reaction. Well dispersed Au-Pd bimetallic NPs with a size in the range of 1-6 nm anchored on the pore wall of foam-like mesoporous silica (> 650 m 2 /g) support with addition of MPTMS as a functional ligand were prepared. The Au-Pd alloy NPs were formed under high temperature treatment (500 o C) as verified by XRD, UV-Vis and XPS. The surface composition and electronic properties of different catalysts were characterized by XPS and it was found that electron transfer occurred between surface Au and Pd atoms. It is also worth noting that the presence of Au prevents the oxidation of surface Pd atoms, therefore most of the surface Pd species exist as Pd 0 rather than PdO in the Au-Pd alloys. DFT results revealed that the formation of Au-Pd alloy can effectively inhibit the adsorption of oxygen on the Pd(111) surface to form PdO species, consistent with the experimental data. All bimetallic catalysts exhibited much higher catalytic activity for benzyl alcohol oxidation (TOF = 50000 -60000 h -1 ) than that of monometallic catalyst Pd100/MCF-H2 (TOF = 12500 h -1 ) (Table S1). XPS results also indicated the partial presence and enrichment of PdO species (oxidation of surface Pd atoms) on bimetallic catalysts, leading to a lower Au/Pd ratio at the surface than in the bulk. Correlating the catalytic performance with characterization results, no obvious connection between microstructure of catalysts including specific surface area, pore structure, and NP size with catalytic activity was identified, but a close relationship between surface PdO species and the catalytic activity was observed as shown in Fig. 11. A nearly linear trend indicating that the catalytic activity decreased with the increase of the surface PdO content was observed. The Au18Pd82/MCF-H2 catalyst without any surface PdO species exhibited the highest catalytic activity (TOF = 65700 h -1 ) among all catalysts, while the Pd100/MCF-H2 catalyst exhibited much lower catalytic activity (TOF = 12500 h -1 ) with 100 % surface PdO species. The DFT results confirmed that the Pd3Au(111) surface of the catalyst favours benzyl alcohol activation and benzaldehyde and H2O desorption, resulting in a high catalytic activity.

However, on the PdO(200) surface, the formation and the desorption of H2O molecules is very difficult, leading to a low catalytic activity. These results indicated that the catalytic activity is determined by the state of the surface which is closely related to the Au/Pd ratio. Thus, the relationship between surface PdO content on the Au-Pd bimetallic catalyst and benzyl alcohol oxidation activity is revealed for the first time. A generally accepted mechanism of benzyl alcohol partial oxidation over Pd-containing catalyst is that the first step is to form the Pd alcoholate intermediate through the O-H bond between the alcohols and Pd sites, followed by the β-hydride elimination [START_REF] He | Switching-off Toluene Formation in the Solvent-free Oxidation of Benzyl Alcohol Using Supported Trimetallic Au-Pd-Pt Nanoparticles[END_REF]. Both the Au-Pd interfaces and low-coordinated Pd 0 islands were considered as active sites for benzyl alcohol oxidation [START_REF] Chen | Formation of Monometallic[END_REF]. The surface enrichment of PdO species certainly will decrease or even block the active sites and will affect the catalytic performance. The results of benzyl alcohol oxidation on the Au26Pd74/MCF-H2 and Au26Pd74/MCF-O2 catalysts support this conclusion. After calcination of the catalyst in air (Au26Pd74/MCF-O2), the TOF value decreased dramatically to 55900 h -1 in comparison to the TOF of 65700 h -1 for the Au26Pd74/MCF-H2 catalyst (Table S1). While similar metal loading, textural structure and Au/Pd bulk ratio were obtained for both catalysts as revealed by STEM-HAADF. The only difference lies in their surface composition as shown in Fig. 8 and Table 2. After oxidation in static air, the surface Pd 2+ species increased from 16 % on the Au26Pd74/MCF-H2 to 38 % on the Au26Pd74/MCF-O2 catalyst. Meanwhile, the surface Au/Pd atomic ratio decreased from 0.27 to 0.14, and the PdO/Pd ratio increased significantly from 0.19 to 0.61. These results clearly demonstrated that the enrichment of PdO species on catalyst surface decreased the low-coordinated Pd 0 islands and blocked the Au-Pd interface, leading to decreased catalytic activity on the Au26Pd74/MCF-O2 catalyst.

It has been reported that the activity enhancement of Au to Pd atoms is primarily due to the variation of the occupation state of Pd d-orbitals [START_REF] Liu | Au@ Pd Bimetallic Nanocatalyst for Carbon-Halogen Bond Cleavage: An Old Story with New Insight into How the Activity of Pd is Influenced by Au[END_REF]. The occupation state of Pd d-orbitals can be altered by varying the Au/Pd ratio. Different amount of surface PdO species observed on catalysts with different Au/Pd ratios provided the direct evidence in this study. For benzyl alcohol oxidation, the optimal occupation state of Pd dorbitals was achieved with a Au/Pd ratio of 1/4.5. A significant deviation from this ratio may lead to different occupation states of Pd d-orbitals, resulting in the formation and enrichment of PdO species on the surface with a low catalytic activity.

Conclusions

Well dispersed Au-Pd bimetallic (1-6 nm) catalysts supported on foam-like mesoporous silica (specific surface area > 650 m 2 /g) were obtained by one-pot synthesis. The formation of Au-Pd alloy NPs and the presence of electron transfer between surface Au and Pd atoms were observed. Both the XPS and DFT results revealed that the addition of Au to Pd prevent the oxidation of surface Pd and formation of PdO species. A volcano-shape activity trend on Au-Pd bimetallic catalyst for benzyl alcohol oxidation was observed. The Au18Pd82/MCF-H2 catalyst with a consistent surface and bulk Au/Pd atomic ratio of 1/4.5 exhibited the highest benzyl alcohol partial oxidation activity. The bimetallic catalysts exhibited greater catalytic activity in benzyl alcohol oxidation (TOF = 50000 -60000 h -1 ) compared to the monometallic Pd catalyst (TOF = 12500 h -1 ).

Correlating the catalytic performance with the properties of the catalysts, it was found that high surface PdO content leads to lower catalytic activity. This was further verified by the DFT results revealing that H2O is difficult to form and desorb from the PdO surface. The overall results provide the direct evidence of synergistic

Fig. 1 .

 1 Fig. 1. N2 adsorption-desorption isotherms (left) and pore size distribution curves (right) of Au-Pd

Fig. 2g and

  Fig. 2g and 2h present the Au26Pd74/MCF-H2 and Au26Pd74/MCF-O2 catalysts, respectively. Metal NPs

Fig. 2 .

 2 Fig. 2. TEM images and particles size distribution histograms of catalysts: (a) Au8Pd92/MCF-H2 (b)

  HRTEM. But the exact spatial location of the noble metal NPs either inside the pores or on the external surface of the silica surface is still unknown. It is known that the location and stability of noble metal NPs are directly related with their catalytic activity. The HRTEM only provides a two-dimensional image, resulting often in misleading spatial information. Therefore, electron tomography (ET), a 3D analysis, was employed to reveal the spatial location of noble metal NPs in the porous silica support; the results are shown in Fig.3. Fig.3adisplays the TEM image of sample Au26Pd74/MCF-H2. The large black points in Fig.3aare colloidal gold particles (10 nm) which were used as marker for the TEM study. The well-defined foam-like structure can be clearly seen by the full range tilting video (SI, Video 1: scale bar 100 nm). Fig.3b and 3cshow the ET tomograms sliced at different thicknesses (full range tomogram video slicing from the bottom to the top of the catalyst is provided in SI; Video 2: scale bar 50 nm). No aggregation of noble metal NPs was observed, only separated and well-dispersed dots with an average size of around 3 nm are seen in the porous silica support. This indicates that no aggregation of the NPs treated at 500 o C occurred. The noble metal NPs are observed not only on the silica surface (indicated by red arrows in Fig.3b, 3c) but also embedded within the silica wall (indicated by white cycles in Fig.3b, 3c). The homogeneous distribution of NPs in the silica support is due to the in-situ incorporation of gold during the preparation of foam-like silica support. The ET tomograms sliced at different thicknesses of the sample indicate that the metal NPs are attached on the silica wall surface, which will be conducive to the catalytic stability. The high porosity of the foamlike silica support was retained which is beneficial for the diffusion of reactants into the pores. The restructured 3D structures of three neighboring mesopores containing Au-Pd NPs (colored cycles highlighted in Fig.3c) is also shown in Fig.3d. This result provides a direct visualization of the distribution of metal particles within the silica foam. The Au-Pd NPs without aggregation (pink color) are anchored on the pore walls (yellow blue and green colors).

Fig. 3 .

 3 Fig. 3. (a) TEM images of sample Au26Pd74/MCF-H2, (b-c) electron tomograms sliced at different

Fig. 4 .

 4 Fig. 4. (a) STEM-HAADF image of Au26Pd74/MCF-H2 (insert is the energy dispersive line scan

a 20 3. 5 .Fig. 5 .

 2055 Fig. 5. UV-vis spectra of catalysts with different Au/Pd ratios: (a) Au100/MCF-H2, (b) Au61Pd39/MCF-H2, (c) Au26Pd74/MCF-H2, (d) Au18Pd82/MCF-H2, (e) Au8Pd92/MCF-H2, and Pd100/MCF-H2 (inset).

Fig. 6 .Fig. 7 .

 67 Fig. 6. XPS spectra of Au 4f of different catalysts: (a) Au100/MCF-H2, (b) Au61Pd39/MCF-H2, (c)

Fig. 8 .

 8 Fig. 8. XPS spectra of Pd 3d of samples (a) Au26Pd74/MCF-H2 (H2 treatment) and (b) Au26Pd74/MCF-O2

Fig. 9 .

 9 Fig. 9. Selectivity and conversion on catalysts in the benzyl alcohol partial oxidation reaction.

  performed. The Pd (111), Pd3Au (111), and PdO (200) (Fig.S6) surfaces were selected based on the XRD results and studied as model surfaces. The adsorption and desorption properties of reactants (benzyl alcohol and O2) and products (benzaldehyde and H2O) were evaluated to explore the essence of synergy effect of Au-Pd alloy NPs. The adsorption configurations are shown in Fig.10and Fig.S7-8, the adsorption energies are listed in TableS2.Top and side views of the adsorption configurations of atomic O(1/2 O2) on Pd(111), Pd3Au(111), and PdO(002) are presented in Fig.S7. The obtained adsorption energies of oxygen (adsorption energy of 1/2O2) on Pd(111), Pd3Au(111) and PdO(200) surfaces are -0.63 eV, -0.33 eV and -0.75 eV, respectively (see TableS2). Compared with the Pd(111) surface, the oxygen adsorption energy on Pd3Au(111) is reduced by half, indicating that the formation of gold-palladium alloy can effectively inhibit the adsorption of oxygen on Pd(111) surface to form PdO species, which is consistent with the experimental results. Fig.10shows the adsorption/desorption energies and configurations (inset) of benzyl alcohol and H2O on Pd(111), Pd3Au(111) and PdO(002) surfaces. The adsorption energies of benzyl alcohol on Pd(111), Pd3Au(111) and PdO(002) are -1.13 eV, -0.75 eV and -1.06 eV, respectively. The results suggest that Pd3Au(111) alloy surface has the lowest adsorption energy to benzyl alcohol. If only the adsorption energy is considered, the activity of the alloy surface should be the lowest. However, it should be noted that the adsorption configurations of benzyl alcohol on different surfaces are different too. Although the benzyl alcohol is adsorbed in parallel on both Pd(111) and Pd3Au(111) surfaces, it is mainly adsorbed by the benzene ring (π-adsorption) on Pd(111) and primarily by the hydroxyl group on Pd3Au(111) (Fig.10). The adsorption via the hydroxyl group on Pd3Au(111) is beneficial for further activation of O-H bond, which is considered as the first step of benzyl alcohol activation on Pd sites. In addition, the PdO(002) surface presents a strong hydroxyl adsorption, which is probably beneficial for the activation of benzyl alcohol.

Fig. 10 .

 10 Fig. 10. Adsorption/desorption energies and configurations of benzyl alcohol and H2O on Pd(111),

Fig. 11 .

 11 Fig. 11. Relationship between the amount of surface PdO on Au-Pd bimetallic catalysts and their

Table 1

 1 Physical properties of catalysts.

	Catalysts	ICP Au a	ICP Pd a	SBET	Pore	Pore size	Particle	Average	Metal
		(wt. %)	(wt. %)	(m 2 /g)	volume	(nm)	size b (nm)	particle	dispersion d
					(cm 3 /g)			diameter c	(%)
								(nm)
	Pd100/MCF-H2 e	0.0	0.89	694	0.96	4.0	1-6	3.2±0.1	35
	Au8Pd92/MCF-H2	0.13	0.86	652	0.84	4.0	1-6	3.1±0.2	37
	Au18Pd82/MCF-H2	0.32	0.70	701	0.92	4.0	1-5	2.9±0.1	39
	Au26Pd74/MCF-H2	0.39	0.59	715	1.00	4.0	1-5	2.7±0.1	42
	Au61Pd39/MCF-H2	0.60	0.21	734	0.91	3.9	1-5	2.6±0.1	43
	Au100/MCF-H2	0.94	-	730	0.91	4.0	1-5	2.5±0.2	46
	Au26Pd74/MCF-O2 e	0.39	0.59	737	0.97	3.9	1-5	2.7±0.2	42

  [START_REF] Han | Spherical Siliceous Mesocellular Foam Particles for High-speed Size Exclusion Chromatography[END_REF], and no PdO species were measured (three parallel XPS analyses were performed). However, the Au26Pd74/MCF-H2 catalyst has much lower surface Au/Pd (0.27) than bulk (0.36) ratios with the presence of PdO species. Furthermore, the Au26Pd74/MCF-H2 catalyst was treated in static air at 500

	344	342	340	338	336	334
						Anyhow, compared
	with the pure Pd100/MCF-H2 sample (100 % PdO), the percentage of PdO in the Au26Pd74/MCF-O2 is still
	lower even after oxidation at 500 o C for 1h (38 % PdO), indicating that the formation Au-Pd alloy prevented
	the easy oxidation of Pd 0 atoms.

o C for 1 h (catalyst denoted as Au26Pd74/MCF-O2) to justify the relation between surface PdO species and surface Au/Pd ratio. XPS results shown in Fig.

8

indicate the presence of certain amount of Pd 2+ phase (12 %) in the Au26Pd74/MCF-H2 catalyst, which is probably due to the easy oxidation of surface Pd atoms during the sample treatment. While in the Au26Pd74/MCF-O2, the percentage of Pd 2+ was notably enhanced to 38 %. The PdO/Pd ratio (0.61) for the Au26Pd74/MCF-O2 was much higher than in the Au26Pd74/MCF-H2 (0.19). Simultaneously, the surface Au/Pd ratio decreased from 0.27 to 0.14, implying the enrichment of PdO species on the surface of the bimetallic NPs after oxidation in static air. The presence of PdO species on the bimetallic catalysts leads to a lower amount of surface Au; this is due to the enrichment of PdO species on the surface of Au-Pd alloy NPs and covering of the Au sites.

Table 2

 2 Binding energies of Au and Pd and atomic ratios of Au/Pd and PdO/Pd.

			Pd 3d5/2 (eV)		Atomic ratio
	Samples	Au 4f7/2(eV)
			Pd 0 (%)	Pd 2+ (%)	ICP Au/Pd	XPS Au/Pd	XPS PdO/Pd
	Pd100/MCF-H2	--	--(0%)	337.2 (100%)	-	-	∞
	Au8Pd92/MCF-H2	--	336.1 (70%)	337.9 (30%)	0.08	--	0.42
	Au18Pd82/MCF-H2	83.7 (84.9)	336.0 (100%)	--(0%)	0.22	0.22	0
	Au26Pd74/MCF-H2	83.7	335.6 (84%)	337.5 (16%)	0.36	0.27	0.19
	Au26Pd74/MCF-O2	83.9	335.6 (62%)	337.5 (38%)	0.36	0.14	0.61
	Au61Pd39/MCF-H2	83.8	--	--	1.54	--	--

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (51601223, 21206195), 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).

Au-Pd NPs surfaces result in the decease and blockage of these surface active sites leading to decreased activity. For benzyl alcohol oxidation, an optimal Au/Pd ratio of 1/4.5 was identified and nearly no surface PdO species formation/enrichment was observed, resulting in a high catalytic activity. This study provides new insights on the synergistic effect of Au-Pd bimetallic catalysts and sheds light on the design of bimetallic NPs catalysts for making full use of both Au and Pd atoms.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.X.X.X.