Microfluidic Synthesis of Ultrasmall AuPd Nanoparticles with a

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Microfluidic Synthesis of Ultrasmall AuPd Nanoparticles with a Homogeneously Mixed Alloy Structure in Fast Continuous Flow for Catalytic Applications Ghazal Tofighi, Abhijeet Gaur, Dmitry E. Doronkin, Henning Lichtenberg, Wu Wang, Di Wang, Guenter Rinke, Angela Ewinger, Roland Dittmeyer, and Jan-Dierk Grunwaldt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11383 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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The Journal of Physical Chemistry

Microfluidic Synthesis of Ultrasmall AuPd Nanoparticles with a Homogeneously Mixed Alloy Structure in Fast Continuous Flow for Catalytic Applications

Ghazal Tofighi1, Abhijeet Gaur1, Dmitry E. Doronkin1,2, Henning Lichtenberg1,2, Wu Wang3, Di Wang3,4, Günter Rinke5, Angela Ewinger5, Roland Dittmeyer5and Jan-Dierk Grunwaldt1,2,*

1

Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany

2

Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen, Germany

3

Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen, Germany

4

Karlsruhe Nano Micro Facility (KNMF), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany

5

Institute for Micro Process Engineering (IMVT), Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen, Germany

* Corresponding author: [email protected] Tel.: +49 721 608-42120 Fax: +49 721 608-44820

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ABSTRACT: Ultrasmall Au, Pd and AuPd nanoparticles (NPs) stabilized by PVP were prepared in a microfluidic reactor with cyclone micromixers for rapid mixing of reactants. In this system, pulsation-free flow of reactants was achieved at a total flow rate of 2.6 L h-1. A rapid homogeneous mixing within 2 ms was obtained with three cyclone micromixers. Controlled NP nucleation and growth occur in a following meandering microchannel. The resulting colloidal NPs were characterized thoroughly by various complementary techniques e.g. high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) and Ultraviolet-visible spectroscopy. The average NP diameter was about 1 nm with a narrow size distribution and electron microscopy showed homogeneously alloyed NPs. Moreover, the particles were supported on TiO2 for catalytic tests and further structural characterization. Electron microscopy showed a uniform distribution of NPs on the support with some aggregation. X-ray absorption spectroscopy (XAS) confirmed the formation of well-mixed AuPd alloys in NP cores with Pd-rich surfaces. Finally, 1 wt.% metal loaded supports showed catalytic activities in CO oxidation in the following order: Au/TiO2 ≥ AuxPdy/TiO2 ≥ Pd/TiO2. Hence, the physical and chemical properties of these catalysts can be fine-tuned.


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1. INTRODUCTION Transition metal nanoparticles (NPs) have attracted increasing attention in the fields of nanoscience and nanotechnology and applications in catalysis1-9, sensing10-12, electronics and optics13-15, imaging16 and biology17-19 due to their large surface area and size dependent properties. Including bimetallic systems in such applications opens new perspectives to improve the performance of functional materials by synergizing the properties of both metals, taking advantage of tunable electronic interactions and geometric structures of the NPs and, if applicable, metal-support interactions7, 20-21. Synergistic effects are hard to explain based on literature comparison due to differences in preparation methods, synthesis conditions, surface composition of the NPs, inhomogeneous size distributions and a lack of in-depth structural characterization. Chemical reduction is one of the most straightforward methods for synthesizing colloidal metal NPs, and proceeds in four principal steps: (1) mixing of metal precursors and reducing agents and their diffusion, (2) reduction of metal ions to atoms, (3) nucleation of NPs and (4) NP growth

21-22

. In conventional stirred batch reactors, insufficient instantaneous mixing

results in concentration gradients of reactants in the medium and poor control over the reaction conditions. Hence, often poly-disperse NPs are formed, lacking the desired properties. This challenge is even more complicated in case of bimetallic systems (especially solid-solution alloys) featuring two metals with different redox potentials (successive reduction)7, 23. Microfluidic reactors allow for microscopic control over reaction conditions and spatially and temporarily homogeneous mixing of metal precursors and reducing agents within milliseconds. Thus, nucleation and growth steps can be separated from each other to achieve narrow NP size distributions24-26. This technology also allows for precise control over temperature profiles during different stages of the reaction, e.g. by warming up the 3 ACS Paragon Plus Environment


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microchannels at the beginning of the reaction to facilitate the nucleation, and cooling down the products downstream from the microfluidic device to control NP growth, resulting in highly monodisperse bimetallic nanoclusters21. Recently, the possibility to perform in situ spectroscopic and scattering experiments during NP formation in microfluidic channels has emerged as an additional advantage of microfluidic technology27-30. Bimetallic AuPd systems have received growing attention with regard to catalytic applications such as direct synthesis of hydrogen peroxide31-33, alcohol and glucose oxidation34-36, oxidation of primary C-H bonds37 and CO oxidation38-40. In many cases, the catalytic activity, selectivity and durability of the catalyst are improved if palladium is alloyed with gold7. Moreover, since Au and Pd are fully miscible as bulk alloy, it is possible to tune the interatomic distances and the electronic structure in AuPd. This allows to adjust the adsorption of the reactants and to improve the catalytic performance in structure-sensitive reactions41-42. As the properties are also dependent on the particle size, synthesis techniques are required that allow control over the production quality while minimizing random nucleation and growth, therefore yielding reproducible, uniformly alloyed bimetallic NPs. For this study, a recently built novel microfluidic setup30 was used to synthesize highly monodispersed ultrasmall43-45 AuPd NPs (1-2 nm) stabilized with polyvinylpyrrolidone (PVP) with the aim to obtain homogeneously mixed alloy structures. The setup operates under flow conditions approaching turbulent mixing and plug flow. The resulting mixed nanoalloys were analyzed by various complementary characterization techniques in particular, X-ray absorption spectroscopy (XAS), electron microscopy with elemental mapping and UV-vis spectroscopy. Additionally, the produced Au, Pd, and AuxPdy NPs were supported on TiO2 and tested in CO oxidation as model reaction.


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2. EXPERIMENTAL METHODS 2.1. Materials. Tetrachloroauric (III) acid (HAuCl4·3H2O, Roth, 99.5% purity), potassium tetrachloro palladate (III) (K2PdCl4, Alfa Aesar, 99.99% purity), polyvinylpyrrolidone (PVP, (C6H9ON)x, Sigma-Aldrich, average molecular weight 40 kDa), sodium borohydride (NaBH4, SigmaAldrich, 99.99% purity), sulphuric acid (H2SO4, Sigma-Aldrich, 95% solution) and high surface area titania (TiO2, CristalACTIVTM, >99% purity, anatase, 370 m2/g surface area) were used without further purification. 2.2. Microfluidic Setup. In order to synthesize highly monodisperse metal NPs via fast reduction reactions, a novel microfluidic setup capable of generating a continuous and pulsation-free flow of reactants at high flow rates was used30, 46 (Figure 1). The fluid delivery rack consists of 4 L corrosionresistant stainless steel vessels (polyethylene inner coating for metal precursor vessel) pressurized by 13 bar N2 gas to deliver the reactants at 2.6 L h-1 total flow rate to 3 cyclone micromixers47 integrated in a microfluidic chip for fast (2 ms) and homogeneous mixing. The microfluidic chip, fabricated in collaboration with GeSiM GmbH and the Institute of Semiconductors and Microsystems at Technische Universität Dresden (TUD-IHM), is made of silicon-bonded glass providing an observation window for the meandering microchannel (300 x 300 µm2 cross section) specifically designed for in situ X-ray-based characterization. According to the pressure drop estimation in the microchannel and micromixers and computational fluid dynamics (CFD) calculations30, 47, for each reactant a flow rate of 1.3 L h1

(total flow rate 2.6 L h-1) was required to achieve Reynolds number exceeding 2400 in order

to approach turbulent mixing within 2 ms, which is necessary for fast reduction reactions30. The total residence time of the reactants in the microchannel was about 20 ms.



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Figure 1. Schematic of the microfluidic setup for synthesis of PVP-stabilized monometallic and bimetallic NPs in continuous flow (P, T: pressure transducer and temperature sensor), inset: schematic of the inlets and micromixers integrated in the chip to achieve instantaneous (< 2 ms) and homogeneous mixing. 2.3. Microfluidic Synthesis of Au, Pd and AuxPdy Nanoparticles Reduced by NaBH4. In Table 1 the preparation conditions of the reactant solutions for synthesis of Au, Pd and AuxPdy NPs (x:y = 7:3, 5:5, 3:7) are summarized. Aqueous solutions of HAuCl4 and K2PdCl4 precursors were prepared at a constant total metal precursor concentration (7.5 mM) for all samples. An aqueous solution of NaBH4 (37.5 mM) was prepared in a separate flask. Afterwards, 666 mg PVP was added to each of these two solutions as stabilizer, following a procedure reported by Hayashi et al.21. Prior to the microfluidic synthesis, the microchannel was flushed with aqua regia, and then several times washed with deionized water in order to remove impurities. The metal precursor and NaBH4 solutions were filled into the vessels of the fluid delivery rack. N2 gas at 13 bar pressure was used to push the reactants through the inlet channels at a total flow rate of 2.6 L h-1 in order to achieve turbulent flow conditions in 6 ACS Paragon Plus Environment

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the micromixers for efficient mixing of the reactants. The products were collected and stirred for 1 h in a round-bottom flask in an ice/water bath. Table 1. Microfluidic Synthesis Conditions for Au, Pd and AuxPdy:PVP Colloids solution Aa solution Bb HAuCl4 (mg) K2PdCl4 (mg) NaBH4 (mg) Au 236.4 0.0 114 Au7Pd3 165.5 58.7 114 Au5Pd5 118.2 97.9 114 Au3Pd7 71 137 114 Pd 0.0 200 114 a )7.5 mM aqueous solution with 666 mg PVP, b)37.5 mM aqueous solution with 666 mg PVP sample

2.4. Batch Synthesis of AuxPdy Nanoparticles Reduced by NaBH4. The same molar metal:NaBH4 ratio used for microfluidic NP synthesis was applied in batch synthesis of Au7Pd3 and Au3Pd7 NPs to compare the size distributions of the materials prepared by the two different methods (summarized in Table 2). The gold and palladium precursors and 166.6 mg PVP were dissolved in 60 mL deionized water. 28.5 mg NaBH4 and 166.6 mg PVP were dissolved in 15 mL deionized water and added to gold precursor solution stirred rapidly at room temperature. The fast reduction reaction was evident by a color change from yellow/orange to dark brown/grey. Table 2. Conventional Batch Synthesis Conditions for Selected AuxPdy:PVP Colloids solution Bb HAuCl4 (mg) K2PdCl4 (mg) NaBH4 (mg) Au7Pd3 41.4 14.7 28.5 Au3Pd7 17.7 34.2 28.5 a b )2.5 mM aqueous solution with 166.6 mg PVP, )50.2 mM aqueous solution with 166.6 mg PVP sample

solution Aa

2.5. Preparation of Au, Pd and AuxPdy Supported on TiO2. The PVP-stabilized Au, Pd and AuxPdy colloidal solutions prepared in the microfluidic reactor were added to 1 g TiO2 suspended in 80 mL deionized water in an ultrasonic bath at room temperature and stirred for one hour. Prior to the impregnation of titania with NPs, the titania 7 ACS Paragon Plus Environment


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support was acidified with 8 mL H2SO4 solution (0.2 M), decreasing the suspension pH to 2. After 1 h of stirring, the suspension was centrifuged four times (4500 rpm, each cycle 5 min) and washed with water in between. Eventually, the catalysts were dried at 80 °C overnight and calcined at 380 °C for 3 h. This method was applied for obtaining 1.0 and 2.4 wt.% AuxPdy/TiO2 as well as monometallic Au and Pd NPs on TiO2. 2.6. Characterization of Au, Pd and AuxPdy Nanoparticles.

2.6.1. Transmission Electron Microscopy. The Au/TiO2, Pd/TiO2 and AuxPdy/TiO2 powder samples were directly dispersed on Cu grids covered with holey carbon film. For colloid samples, 5 µl of the diluted colloidal solution was dropped on a holey carbon Cu grid covered by 2 nm carbon film and then dried at room temperature. Morphology and microstructure of the catalysts were characterized by high angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM), and their composition was analyzed by energy dispersive X-ray spectroscopy (EDX) using an EDAX S-UTW EDX detector in a FEI Titan 80-300 microscope operating at 300 kV. Analysis of STEM-EDX spectrum imaging was carried out by using the TEM Image & Analysis (TIA 4.7 SP3 version) software. Particle size statistics of the specimens were performed on HAADF-STEM images by using the ImageJ 1.49v software48.

2.6.2. UV-vis Spectroscopy. A Perkin-Elmer Lambda 650 UV-vis spectrometer was used to study the optical absorption of Au, Pd and AuxPdy colloids with equal atomic metal concentration for all samples. For this purpose, 300 µL of produced metal colloid was dispersed in 1.5 mL deionized water and measured ex situ using UV cuvettes under ambient conditions. The experiments were performed using deionized water as a reference. 8 ACS Paragon Plus Environment

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2.6.3. XAS Analysis. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of Pd/TiO2 and AuxPdy/TiO2 pellets at the Pd K-edge (24.350 keV) were recorded at room temperature in transmission mode at the P65 beamline of the PETRA III synchrotron radiation source (DESY, Hamburg) using undulator radiation (11 period undulator, flux about 1011 photons s-1) and a double crystal monochromator (DCM) equipped with Si (311) crystals (beam size 1.2 x 0.5 mm2). Higher harmonics were rejected by Ptcoated plane mirrors in front of the monochromator. XANES and EXAFS spectra of the samples at the Au L3-edge (11.918 keV) were recorded in transmission using ionization chambers and in fluorescence mode (using a PIPS diode) at the undulator beamline P64 (60 period tapered undulator, flux about 1012 photons s-1) at PETRA III using a DCM equipped with Si (111) crystals (beam size of 2 x 1 mm2). Higher harmonics were rejected by Rh-coated mirrors upstream the monochromator. For data processing the Athena and Artemis interfaces of the IFEFFIT software package49 were used (further details of EXAFS analysis are summarized in the supporting information, Figures S1 and S2).

2.6.4. ICP-OES Analysis. The elemental composition of the samples with respect to Pd and Au was analyzed by ICPOES spectroscopy (iCAP 7600, Thermo Fisher Scientific). 10 mg of each sample were dissolved in 4 mL nitric acid and 4 mL sulfuric acid at 513 K for 10 h in a pressure digestion vessel (DAB-2 Berghof). Elemental analysis was performed using four different calibration solutions and an internal standard (Y) based on three characteristic Au and Pd emission lines.

2.6.5. Procedure for CO Oxidation Tests. CO oxidation was selected as a relatively well-understood test reaction to study the catalytic properties of the metal NPs. The tests were carried out in a fixed-bed quartz flow reactor 9 ACS Paragon Plus Environment


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(quartz tube, inner diameter 8 mm) in a temperature programmed mode from 30 °C to 250 °C at a ramp rate of 1 °C min-1 in the exhaust gas center Karlsruhe50. 300 mg of 1 wt.% Au/TiO2, Pd/TiO2 and AuxPdy/TiO2 catalyst (sieve fraction 125–250 µm) were fixed with glass wool plugs in the quartz tube. The catalyst bed length was about 10 mm, and the gas flow 600 ml min-1 (GHSV: 72000 h−1). The gas mixture contained 1000 ppm CO and 10% O2 in nitrogen (CO feed rate: 13.3 · 10-7 mol·s-1·g-1cat). Before reaching the reactor the gas feed passed through a Messer Hydrosorb® cartridge to remove traces of water. Reaction products were analyzed using an URAS 26 NDIR CO/CO2 analyzer. Prior to the testing, catalysts were dried in N2 flow for 2 h to remove residual water and then pre-treated in 5% H2 in N2 at 250 °C for 1 h. Each heating and cooling cycle was performed twice. The results from the second cycle are reported in this paper. CO conversion was calculated using the following equations:

X CO = 1 −

outlet outlet CCO CCO 2 = inlet inlet CCO CCO

where CCOinlet , CCOoutlet and CCO2outlet denote CO and CO2 concentrations at the inlet and outlet of the reactor. Turnover frequencies (TOF) were determined in the region of CO conversion below 20% (for Au/TiO2 below 25%) based on the assumption that the reaction rate (r) did not depend on the CO concentration51 (differential conditions apply in case of O2). Metal dispersion for TOF calculation was obtained from the average particle size (determined by STEM) assuming spherical particles.

3. RESULTS AND DISCUSSION 3.1. Colloidal Au, Pd and AuxPdy Nanoparticles Produced in the Microfluidic Reactor. The STEM images of the monometallic Au, Pd and bimetallic AuxPdy colloids are presented in Figure 2, indicating mostly spherical ultrasmall NPs (average diameter < 2 nm). The 10 ACS Paragon Plus Environment

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corresponding histograms support that highly monodisperse metal NPs were obtained by mixing the metal precursors, PVP and NaBH4 in the microreactor operating in continuous and turbulent flow.

Figure 2. STEM images and size distribution histograms of PVP-stabilized (a) Au7Pd3, (b) Au5Pd5, (c) Au3Pd7, (d) Au and (e) Pd NPs produced in the microfluidic reactor (about 330 particles analyzed for each size distribution).



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STEM images of two selected bimetallic Au7Pd3 and Au3Pd7 samples produced for comparison in a conventional batch reactor (Figure 3) show a broader size distribution with average particle sizes of 1.4 nm and 1.6 nm, respectively. These observations are comparable to those reported by Hayashi et al.21, who concluded that microfluidic synthesis is superior to batch reactor synthesis in terms of tunable NP size and their narrow size distribution. Hence, the efficient mixing of the reactants and control over the initial states of nucleation and growth in the microfluidic reactors are advantageous. According to literature21, 52-54, reduction of metal ions in microfluidic reactors equipped with micromixers proceeds in a more homogeneous way, faster, and liberation of metal atoms with oxidation state zero occurs in shorter time intervals compared to synthesis in conventional batch reactors. This is due to fast molecular diffusion occurring in the microfluidic reactor compared to batch reactor. NP formation in the batch reactor first requires mixing of the reactant followed by the molecular diffusion. Hence, in batch reactors the initially-formed NPs consume the available metal atoms in the solution during growth, and the newly-formed NPs remain small due to the absence or low concentration of metal atoms which finally leads to broader size distributions.

Figure 3. STEM images and size distributions of PVP-stabilized (a) Au7Pd3 and (b) Au3Pd7 produced in the batch reactor (about 400 particles analyzed for each size distribution).


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The HRTEM images of AuxPdy NPs in Figure 4 show a single crystal-like structure and the absence of other structures such as core-shell or segregated subcluster. The measured lattice spacings (red marks in Figure 4) for AuxPdy NPs range between 2.35 Å (Au (111)) and 2.24 Å (Pd (111))55-56 indicating that a uniform distribution/mixing of Au/Pd precursors and reducing agent was achieved by approaching turbulent mixing conditions using the cyclone micromixers, resulting in the formation of colloidal AuxPdy nanoalloy structures. According to HRTEM analysis of about 10-20 particles, the lattice parameters change upon changes in composition in agreement with Vegard’s law. The average unit cell parameters for Au3Pd7, Au5Pd5 and Au7Pd3 were 4.015 Å, 4.020 Å and 4.041 Å, respectively. This trend indicates that homogeneous alloys are formed following Vegard’s law.

Figure 4. HRTEM images of PVP-stabilized (a) Au7Pd3, (b) Au5Pd5 and (c) Au3Pd7 NPs produced in the microfluidic reactor. The d-spacing is marked by red lines. Figure 5 shows STEM images of PVP-stabilized Au7Pd3, Au5Pd5 and Au3Pd7 colloids and composition profiles across single NPs obtained from STEM-EDX mapping via spectrum imaging. The step scan was set to 1 nm but not smaller to avoid any beam damage. These composition maps indicate that Au and Pd are uniformly distributed in the particles representing the three bimetallic AuxPdy colloids. Along with the HRTEM analysis, these results demonstrate that Au-Pd alloy NPs were formed in all bimetallic AuxPdy colloids produced in the microfluidic reactor. 13 ACS Paragon Plus Environment


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Figure 5. STEM images of PVP-stabilized (a) Au7Pd3, (b) Au5Pd5 and (c) Au3Pd7 colloids and corresponding elemental maps obtained from STEM-EDX spectrum imaging in the areas marked by orange rectangles.

Formation of ultrasmall43-45 Au, Pd and AuxPdy colloidal NPs was also confirmed by UV-vis spectra (Figure 6), which exhibit a strong suppression of the surface plasmon resonance (SPR) band of gold due to the dominance of surface scattering from NPs with diameters below 3 nm1,

25, 57

. Furthermore, since the total concentrations of metal atoms in all samples were

similar, the change in the absorption behavior could be related to different Au:Pd molar ratios. At higher wavelengths (above 500 nm), the absorption increased with increasing Pd content in the NPs probably due to overlapping 4d → 5sp and 5sp → 5sp transitions typically occuring in pure Pd, and intraband 6sp → 6sp transitions in pure gold (wavelengths above 400 nm)21, 58

. Additionally, the absence of the absorption peak of palladium chloride at 425 nm points to

complete reduction of the Pd precursor to metallic Pd59. At shorter wavelengths (below 500 14 ACS Paragon Plus Environment

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nm) the absorption increases with increasing Au content in the NPs, indicating interband 5d → 6sp transitions in pure gold21, 60-62. These results suggest different electronic structures of NPs with changing Au:Pd molar ratios.

Figure 6. UV-vis spectra of monometallic Au and Pd and bimetallic AuxPdy NP colloids produced in the microfluidic reactor. (The abrupt increase in the spectra at 319 nm results from switching between deuterium and tungsten lamps.) 3.2. Characterization of Au, Pd and AuxPdy NPs Deposited on TiO2. For CO oxidation tests the colloidal NPs produced in the microreactor were deposited on TiO2 powder with 1 wt.% total metal (Au+Pd) loading. NP morphology and size distributions of the nanoalloys after calcination at 380 °C were analyzed by STEM (Figure 7). Size distributions were determined by measuring the diameters of hundreds of particles resulting in average diameters of about 5.3, 4.9 and 7.3 nm for Au7Pd3, Au5Pd5 and Au3Pd7, respectively. Obviously, deposition on TiO2 and post treatment (drying and calcination) led to an increase in the NP size. Different phases, surface area and pre-treatment of the support, deposition method and applied conditions as well as post-treatments such as calcination are known to have a strong influence on NP aggregation and on the interaction between the metal NPs and the ceramic supports, which can have a significant impact on the catalytic performance63-64.



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Figure 7. STEM images of calcined (a) Au7Pd3/TiO2, (b) Au5Pd5/TiO2 and (c) Au3Pd7/TiO2.

Among the UV-vis spectra of Au/TiO2, Pd/TiO2 and AuxPdy/TiO2, only the Au/TiO2 spectrum shows the characteristic gold SPR band (cf. Figure S3), i.e. there is no evidence of individual (segregated) Au NPs in the three bimetallic samples. Table 3 shows elemental compositions of selected AuxPdy/TiO2 catalysts obtained from ICP-OES. These values are in good agreement with the targeted Au:Pd molar ratios during colloidal synthesis and the desired metal loadings of the supported catalysts for CO oxidation and ex situ XAS. Table 3. ICP-OES Results for AuxPdy/TiO2 Catalysts

Au Pd Total metal loading Au:Pd molar ratio a : Nominal value

1a wt.% Au3Pd7/TiO2 (wt.%) 0.41 0.55 0.96 2:5

3a wt.% Au3Pd7/TiO2 (wt.%) 1.04 1.41 2.45 2:5

3a wt.% Au5Pd5/TiO2 (wt.%) 1.34 0.94 2.28 5:6

Figure 8 shows k3-weighted Fourier transformed (FT) Au L3 EXAFS data of the 2.4 wt.% AuxPdy and Au NPs deposited on TiO2 and the corresponding near edge spectra (XANES). The EXAFS oscillations of AuxPdy NPs differ significantly from those of Au foil indicating the presence of an alloy phase (Figure 8a). Accordingly, decreasing Au (i.e. increasing Pd) concentration in the bimetallic NPs leads to an increase in the average number of Pd atoms surrounding the Au absorber atoms. The FT data shown in Figure 8b follow the same trend in which the AuxPdy NPs exhibit intense peaks, at 2.0–2.2 Å and 2.6–2.9 Å. These peaks both correspond to the first shell metal–metal contribution20. The splitting of the first shell peak is due to interference between Au–Au and Au–Pd backscattering, with different phase shift and amplitude20, 42. 16 ACS Paragon Plus Environment

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Figure 8. Au L3-edge XAS data of Au/TiO2 and AuxPdy/TiO2: a) k3-weighted EXAFS oscillations, b) corresponding magnitude of the Fourier transformed EXAFS data, and c) XANES spectra (inset: magnified white line region). With decreasing Au content, the contribution at lower R-values increases, suggesting a growing number of Pd neighbors around the absorbing gold atoms. Furthermore, the peak shift toward shorter bond distances with decreasing amount of Au in the alloyed NPs indicates that a larger fraction of gold atoms are now coordinated to palladium, in agreement with the 17 ACS Paragon Plus Environment


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linear dependence of lattice parameters on the ratio of the two metals in bimetallic alloys (Vegard’s law)42,

65

. The corresponding structural parameters extracted from Au L3-edge

EXAFS spectra of the bimetallic AuxPdy NPs are summarized in Table 4. The fitting curves in the corresponding real and imaginary parts of χ(R) are shown in Figure S1. Table 4. EXAFS Fitting Results Obtained from Au L3-Edge X-ray Absorption Spectra of Au/TiO2 and AuxPdy/TiO2; (N: Coordination Number, R: Distance (Å), σ2: Debye– Waller Factor (Å2), ∆E0: Inner Core Correction, S02 = 0.72 ± 0.03: Amplitude Reduction Factor) sample NAu-Au Au foil 12 Au/TiO2 12.9±1.1 Au7Pd3/TiO2 9.4±1.0 Au5Pd5/TiO2 7.2±1.3 Au3Pd7/TiO2 5.7±0.8 f : Fixed during fitting

R (Å) 2.85±0.03 2.85±0.03 2.81±0.07 2.81±0.07 2.79±0.10

σ2 (Å2)x10-3 7.7±0.3 8.6±0.5 8.5±0.7 8.5±0.7f 8.5±0.7f

NAu-Pd 3.3±0.7 5.0±1.5 6.5±0.9

R (Å) 2.78±0.03 2.79±0.03 2.77±0.01

σ2 (Å2)x10-3 7.4±1.1 6.3±1.6 6.5±0.8

∆E0(eV) -4.3±0.0 -5.7±0.7 4.1±0.7 4.5±1.4 4.5±0.7

NTotal 12 12.9 12.7 12.2 12.2

The coordination number of the first shell in bulk fcc metals (e.g. Au and Pd) is 12. In case of bimetallic AuxPdy NPs, two coordination numbers are obtained from the analysis of the Au L3-EXAFS data: NAu–Au and NAu–Pd. For AuxPdy NPs with homogeneous mixed alloy structure, the ratio of NAu–Au to NAu–Pd should be close to the molar ratio of Au:Pd in the NPs, whereas, NAu-Au to NAu–Pd ratios smaller than the Au:Pd molar ratios indicate inhomogeneous alloying mixture20. Moreover, the value of NTotal (i.e., NAu–Au + NAu–Pd) can provide information about the size of the bimetallic NPs. Based on these results, the alloy structure of AuxPdy NPs can be unraveled. The coordination numbers in Table 4 lead to the conclusion that homogeneous mixed nanoalloys were formed for all the bimetallic AuxPdy NPs as for the three AuxPdy samples the NAu–Au / NAu–Pd ratios are comparable to the corresponding Au:Pd molar ratios. The NAu–Au / NAu–Pd ratios are 2.3, 1.4 and 0.87, and the corresponding molar ratios are 2.5, 1 and 0.42 for Au7Pd3, Au5Pd5 and Au3Pd7, respectively. Accordingly, the core of the particles is slightly enriched in Au. 18 ACS Paragon Plus Environment

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NAu–Au decreases from 9.4 in Au7Pd3 to 7.2 in Au5Pd5 and then finally to 5.7 in Au3Pd7. A reverse trend was observed for NAu–Pd, which increases from 3.3 in Au7Pd3 to 5.0 in Au5Pd5 and to 6.5 in Au3Pd7. In all samples NTotal is close to 12, i.e. no significant contribution of surface Au atoms to the EXAFS signal. The Au–Au distance also decreases from 2.81 Å (in Au7Pd3) to 2.79 (in Au3Pd7) showing that the influence of the shorter Pd-Pd length grows stronger with increasing Pd content. Figure 8c shows Au L3 XANES spectra of the Au/TiO2 and AuxPdy/TiO2 catalysts. The first strong resonance in the spectra (white line)66-67 arises from electronic 2p → 5d transitions. The white line intensity increases with increasing number of unoccupied 5d states (d-holes). In bulk Au 5d electrons are transferred to s–p states due to s–p–d hybridization, while in case of Au NPs, a decrease in NP size leads to narrower 5d bands and a lower degree of s–p–d hybridization42. In Figure 8c-inset, the white line intensities of bulk Au, monometallic Au NPs and the three bimetallic AuxPdy NPs supported on TiO2 are compared. The white line intensity decreases in the order of bulk Au > Au NPs > Au7Pd3 NPs > Au5Pd5 NPs > Au3Pd7 NPs, indicating that the number of d-holes decreases with decreasing particle size and Au concentration (i.e. increasing Pd concentration). Liu et al.20 discussed the effect of particle size and alloying on white line intensities in Au L3edge spectra of bimetallic AuxPdy NPs. Using FEFF 8 simulations of Au L3-edge XANES spectra, they reported that the white line intensities decreases in the order of bulk Au > Au55 > Au55@Pd, but it has been proposed that the Au–Pd alloying effect has a stronger influence on the decrease in white line intensity compared to the size effect. This is in line with the present data where the particle size of the supported particles is > 2 nm. Hence, the change in the XANES spectra of AuxPdy samples in the present study (white line intensity decreases with increasing Pd fraction) is mostly due to a higher degree of Au-Pd alloying. This supports



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the EXAFS results where a higher degree of alloying with increasing Pd content was concluded from the increase in NAu-Pd. The corresponding Pd K-edge EXAFS data of AuxPdy/TiO2, Pd/TiO2 and a Pd reference foil are shown in Figure 9a-c. The amplitude of the EXAFS oscillations decreases in the order of Pd foil > Pd/TiO2 > Au3Pd7/TiO2 > Au5Pd5/TiO2 > Au7Pd3/TiO2. The amplitude and shape of the EXAFS oscillations in AuxPdy/TiO2 are clearly different from those in bulk Pd and Pd/TiO2. The structural parameters obtained from fitting the first coordination shell for these samples are presented in Table 5. The fitting curves in the corresponding real and imaginary parts of χ(R) are shown in Figure S2. For Pd/TiO2 the value of NPd-Pd is 8.3, suggesting formation of small Pd entities on TiO2. In case of Au3Pd7, NPd-Pd strongly decreased to 4.0 and NPd-Au was 4.1, while NTotal (8.1) is still close to 8. The lower value of NTotal compared to the bulk material indicates that some Pd atoms are located on the surface and therefore not completely surrounded by neighboring Pd/Au atoms. In case of Au5Pd5, NPd-Pd decreases to 2.8 and NPd-Au increases to 6.0 showing that Au atoms have replaced almost 70% of the Pd atoms in the first coordination shell of Pd. The value of NTotal slightly increases to 8.8, suggesting that a larger fraction of Pd atoms is now coordinated to neighboring Au/Pd atoms. In the Au7Pd3 sample, the Pd content is too low, leading only to a weak rise to EXAFS oscillations. Although, values of NPd-Pd and NAu-Au obtained in this sample show significant error margins, still a further decrease in NPd-Pd was observed with lower Pd content. For the AuxPdy samples, the Pd-Pd distance slightly increases with increasing gold content compared to bulk Pd, probably due to lattice expansion. Furthermore, in all samples Pd is in a reduced state as indicated by the XANES spectra (Figure 9d) in the produced Pd/TiO2 and AuxPdy/TiO2 samples if compared with the Pd foil.


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In case of the Au L3-edge EXAFS results, the value of NTotal is always close to 12, indicating that most Au absorber atoms are completely surrounded by Au/Pd atoms. However, the value of NTotal obtained from Pd K-edge EXAFS fitting was about 8 as compared to 12 in the bulk material. Thus, in the Au-Pd system synthesized in the microfluidic reactor and deposited and aggregated on TiO2, the Au atoms are mixed homogeneously with Pd atoms mostly in the core of the NPs, but Pd is the dominating element in the surface composition of the NPs. However, in the present case it is also possible that Pd is segregated from the alloyed particles.

Figure 9. Pd K-edge XAS data: a) k3-weighted EXAFS oscillations and b) corresponding magnitude of the Fourier transformed EXAFS data of Pd foil, Pd/TiO2 and AuxPdy/TiO2, c) corresponding magnitude of the Fourier transformed EXAFS data of AuxPdy/TiO2 and d) comparison of the XANES spectra of the produced Pd/TiO2 and AuxPdy/TiO2 samples with a reference Pd foil.



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Table 5. EXAFS Fitting Results for Pd/TiO2 and AuxPdy/TiO2 (Pd K-Edge), S02 = 0.82 ± 0.11. sample NPd-Pd Pd foil 12 Pd/TiO2 8.3±0.9 Au3Pd7/TiO2 4.1±0.5 Au5Pd5/TiO2 2.8±0.5 Au7Pd3/TiO2 2.3±1.1 f : Fixed during fitting

R (Å) 2.74±0.00 2.73±0.02 2.75±0.00 2.76±0.01 2.79±0.05

σ2 (Å2)x10-3 5.6±0.3 6.8±0.5 6.8±0.5f 6.8±0.5f 6.8±0.5f

NAu-Pd 4.1±0.9 6.0±0.9 5.7±1.9

R (Å) 2.76±0.00 2.77±0.01 2.80±0.04

σ2 (Å2)x10-3 7.4±1.1f 7.4±1.1f 7.4±1.1f

∆E0(eV) -2.2±0.5 -2.1±1.0 -2.3±2.0 -2.5±2.0 -1.5±3.7

NTotal 12 8.3 8.1 8.8 8.0

3.3. CO Oxidation Test on Au/TiO2, Pd/TiO2 and AuxPdy/TiO2. The titania supported monometallic and bimetallic particles were tested in CO oxidation as model reaction. Figure 10a shows the CO conversion over the corresponding powders as function of time (recorded during heating), and Figure 10b the corresponding Arrhenius plots. The CO conversion profiles follow the typical behavior of Au catalysts39, 68-69, with Au/TiO2 converting 20% of CO already at room temperature and a moderate light-off curve reaching full conversion at 120 °C. An increasing Pd content in the AuxPdy/TiO2 catalysts resulted in lower activity at room temperature and a steeper light-off. Among the tested samples the monometallic Pd/TiO2 catalyst exhibits the lowest activity. The temperatures for 50% CO conversion (T50) obtained from Figure 10a are given in Table 6. The reaction rate in terms of turnover frequencies (TOFs, cf. experimental section) measured over the Au/TiO2 catalyst (Table 6) are in the same range as TOFs over Au/TiO2 catalysts determined by Tsubota et al.70. The TOF values reveal a gradual increase in CO oxidation activity with increasing Au content in the order of Pd/TiO2 < Au5Pd5/TiO2 ≤ Au3Pd7/TiO2 < Au7Pd3/TiO2 < Au/TiO2, in good agreement with a series previously reported by Xu et al.39. Hence, the CO oxidation activity of homogeneously mixed nanoalloys of AuxPdy can be described as a sum of the activities of the individual Au and Pd components (cf. Figure S4) as also observed by Guczi et al38. Gibson et al.71 used in situ XAS and DRIFTS to study structural changes in AuPd NPs (2.5 wt.% of each metal, synthesized via sol-immobilization in a batch reactor) supported on


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γ-Al2O3 during CO oxidation and concluded that during reaction the particles are restructured resulting in a core-shell like structure with a gold core and a Pd shell.

Figure 10. (a) CO conversion and (b) corresponding Arrhenius plots obtained during CO oxidation. Conditions: 1000 ppm CO, 10% O2 in N2, 300 mg of catalyst, 600 mL min-1 flow, 1 °C min-1 temperature ramp rate. Table 6. Apparent Activation Energies for CO Oxidation and Turnover Frequencies Obtained over Calcined Pure TiO2, Au/TiO2, Pd/TiO2 and AuxPdy/TiO2 Catalysts catalyst

Ea TOF273 K TOF313 K surface areab (kJ mol-1) (s-1) (s-1) (m2 g-1) -2a -2a 27 1.9 x 10 8.9 x 10 178 36 6.0 x 10-3 4.5 x 10-2 178 49 1.2 x 10-3 1.9 x 10-2 194 53 1.8 x 10-3 3.5 x 10-2 162 55 4.4 x 10-4a 9.7 x 10-3a 189 154 diameter as bimetallic catalysts and b: from Brunauer–Emmett–Teller (BET)

T50(°C)

Au/TiO2 Au7Pd3/TiO2 Au5Pd5/TiO2 Au3Pd7/TiO2 Pd/TiO2 TiO2 a : Assuming comparable measurements.

62 70 78 68 81 NP



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Apparent activation energies decrease in the order of Pd/TiO2 > Au3Pd7/TiO2 > Au5Pd5/TiO2 > Au7Pd3/TiO2 > Au/TiO2. The activation energies of monometallic Au and Pd catalysts are similar to those published by Xu et al.39 and follow the same qualitative trend as reported for Au (100) and Pd (110) surfaces72. However, in contrast to the work of Xu et al.39, the Ea values of bimetallic catalysts prepared for this study lie in between those for the monometallic samples, which rules out significant synergistic effects. 4. CONCLUSIONS Alloyed AuPd nanoparticles were produced via fast reduction with NaBH4 as reducing agent and PVP as surfactant using efficient mixing in a microfluidic reactor. For this purpose, 3 cyclone micromixers integrated into this chip immediately downstream the reactant inlets allow homogeneous mixing of the reactants in a very short time interval (2 ms) and a Reynolds number of about 2400. In the following meandering microchannel (20 ms residence time), highly monodispersed ultrasmall Au, Pd and AuxPdy colloidal NPs were produced. STEM images unraveled the formation of Au, Pd and AuxPdy NPs with diameters of ~ 1 nm in the microfluidic reactor with narrower size distributions compared to NPs obtained from a batch reactor. In HRTEM the bimetallic AuxPdy particles exhibit lattice spacings between those of pure Au and pure Pd. EDX mapping revealed a uniform distribution of Au and Pd. The strong suppression of the SPR band of gold in UV-vis spectra corroborates the formation of ultrasmall Au and AuxPdy NPs, and also full reduction of the Pd2+ precursor to metallic Pd0. After deposition on titania and calcination the produced colloidal NPs increased in size to about 5-7 nm. EXAFS and XANES analysis supported the homogeneous alloy formation but indicated a slightly gold-enriched core whereas on the surface Pd was present in higher concentration. 24 ACS Paragon Plus Environment

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The different white line intensities in the Au L3 XANES spectra of Au/TiO2, Pd/TiO2 and AuxPdy/TiO2 samples correlated to the degree of alloying between Au and Pd. The ratio of Au-Au to Au-Pd coordination numbers extracted from fitted Au L3 and Pd K EXAFS data suggests the formation of a homogeneous mixture of Au and Pd in these alloyed NPs with either Pd dominating at the NP surface or some being segregated as smaller monometallic particles. CO oxidation as a test reaction showed that the catalytic activity of the produced catalysts was altered in comparison to monometallic samples. Au/TiO2 was the most active catalyst, whereas Pd/TiO2 exhibited the lowest activity, and the reaction rate over the AuxPdy/TiO2 samples ranged in between, showing that the catalytic activity can be tuned by the degree of alloying. Due to the special design of the microreactor, such synthesis process can be also monitored by a combination of in situ X-ray absorption spectroscopy and small angle X-ray scattering in the future. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: EXAFS fitting curves in Real and Imaginary parts of χ(R) for the NPs at Au L3-edge and Pd K-edge in R-space; UV-vis spectra of the catalysts; comparison of turn over frequencies and apparent activation energies of the catalysts for CO oxidation. ACKNOWLEDGEMENTS The Virtual Institute VI-403 “In-situ Nano Imaging of Biological and Chemical Processes”, and the Helmholtz Research Program “Storage and Cross-linked Infrastructures” (SCI) and “Science and Technology of Nanosystems” (STN) and KIT are gratefully acknowledged for 25 ACS Paragon Plus Environment


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financial support. We thank Dr. Georg Hofmann, Dr. Andreas Jahn (IHM TUD) and Dr. Steffen Howitz (GeSiM) for constructing the microfluidic chips. In addition, we would like to appreciate the Karlsruhe Nano Micro Facility (KNMF), a Helmholtz research infrastructure at KIT for providing TEM and ICP-OES measurements especially Dr. Thomas Bergfeldt, and also Jan Pesek (ITCP) for the technical supports during CO oxidation tests, Angela Beilmann for BET measurements and Darma Yuda for designing the reactor holder. Finally, we thank DESY (P64 and P65 beamlines) for providing beamtime and Dr. Roman Chernikov and Dr. Vadim Murzin for the support during the measurements. REFERENCES 1.

2.

3. 4.

5. 6.

7. 8.

9.

10.

11.

Grunwaldt, J.-D.; Kiener, C.; Wögerbauer, C.; Baiker, A. Preparation of Supported Gold Catalysts for Low-Temperature CO Oxidation Via “Size-Controlled” Gold Colloids. J. Catal. 1999, 181, 223-232. Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Colloidal Gold Nanoparticles as Catalyst for Carbon-Carbon Bond Formation: Application to Aerobic Homocoupling of Phenylboronic Acid in Water. Langmuir 2004, 20, 11293-11296. Chen, M.; Cai, Y.; Yan, Z.; Gath, K.; Axnanda, S.; Goodman, D. W. Highly Active Surfaces for CO Oxidation on Rh, Pd, and Pt. Surf. Sci. 2007, 601, 5326-5331. Hossain, M. J.; Tsunoyama, H.; Yamauchi, M.; Ichikuni, N.; Tsukuda, T. High-Yield Synthesis of PVP-Stabilized Small Pt Clusters by Microfluidic Method. Catal. Today 2012, 183, 101-107. Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Nano-Gold Catalysis in Fine Chemical Synthesis. Chem. Rev. 2012, 112, 2467-2505. Behrens, M.; Schlögl, R. How to Prepare a Good Cu/ZnO Catalyst or the Role of Solid State Chemistry for the Synthesis of Nanostructured Catalysts. Z. anorg. Allg. Chem. 2013, 639, 2683-2695. Villa, A.; Wang, D.; Su, D. S.; Prati, L. New Challenges in Gold Catalysis: Bimetallic Systems. Catal. Sci. Technol. 2015, 5, 55-68. Schrader, I.; Neumann, S.; Sulce, A.; Schmidt, F.; Azov, V. A.; Kunz, S. Asymmetric Heterogeneous Catalysis–Transfer of Molecular Principles to Nanoparticles by Ligand Functionalization. ACS Catal. 2017, 7, 3979–3987. Kunz, S.; Schweinberger, F. F.; Habibpour, V.; Röttgen, M.; Harding, C.; Arenz, M.; Heiz, U. Temperature Dependent CO Oxidation Mechanisms on Size-Selected Clusters. J. Phys. Chem. C 2009, 114, 1651-1654. Degler, D.; de Carvalho, H. W. P.; Weimar, U.; Barsan, N.; Pham, D.; Mädler, L.; Grunwaldt, J.-D. Structure–Function Relationships of Conventionally and Flame Made Pd-Doped Sensors Studied by X-Ray Absorption Spectroscopy and DC-Resistance. Sens. Actuators B-Chem. 2015, 219, 315-323. Degler, D.; Rank, S.; Mueller, S.; Pereira de Carvalho, H. W.; Grunwaldt, J.-D.; Weimar, U.; Barsan, N. Gold-Loaded Tin Dioxide Gas Sensing Materials: Mechanistic Insights and the Role of Gold Dispersion. ACS Sens. 2016, 1, 1322-1329. 26 ACS Paragon Plus Environment

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


12. Morsbach, E.; Kunz, S.; Bäumer, M. Novel Nanoparticle Catalysts for Catalytic Gas Sensing. Catal. Sci. Technol. 2016, 6, 339-348. 13. Shipway, A. N.; Katz, E.; Willner, I. Nanoparticle Arrays on Surfaces for Electronic, Optical, and Sensor Applications. ChemPhysChem 2000, 1, 18-52. 14. El-Sayed, M. A. Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes. Acc. Chem. Res. 2001, 34, 257-264. 15. Paul, S.; Pearson, C.; Molloy, A.; Cousins, M.; Green, M.; Kolliopoulou, S.; Dimitrakis, P.; Normand, P.; Tsoukalas, D.; Petty, M. Langmuir−Blodgett Film Deposition of Metallic Nanoparticles and Their Application to Electronic Memory Structures. Nano Lett. 2003, 3, 533-536. 16. Lee, K.-S.; El-Sayed, M. A. Gold and Silver Nanoparticles in Sensing and Imaging: Sensitivity of Plasmon Response to Size, Shape, and Metal Composition. J. Phys. Chem. B 2006, 110, 19220-19225. 17. Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293-346. 18. Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine. J. Phys. Chem. B 2006, 110, 72387248. 19. Jain, P. K.; El-Sayed, I. H.; El-Sayed, M. A. Au Nanoparticles Target Cancer. Nano Today 2007, 2, 18-29. 20. Liu, F.; Wechsler, D.; Zhang, P. Alloy-Structure-Dependent Electronic Behavior and Surface Properties of Au–Pd Nanoparticles. Chem. Phys. Lett. 2008, 461, 254-259. 21. Hayashi, N.; Sakai, Y.; Tsunoyama, H.; Nakajima, A. Development of Ultrafine Multichannel Microfluidic Mixer for Synthesis of Bimetallic Nanoclusters: Catalytic Application of Highly Monodisperse AuPd Nanoclusters Stabilized by Poly (NVinylpyrrolidone). Langmuir 2014, 30, 10539-10547. 22. Polte, J.; Erler, R.; Thunemann, A. F.; Sokolov, S.; Ahner, T. T.; Rademann, K.; Emmerling, F.; Kraehnert, R. Nucleation and Growth of Gold Nanoparticles Studied Via in Situ Small Angle X-Ray Scattering at Millisecond Time Resolution. ACS Nano 2010, 4, 1076-1082. 23. Knauer, A.; Thete, A.; Li, S.; Romanus, H.; Csaki, A.; Fritzsche, W.; Köhler, J. Au/Ag/Au Double Shell Nanoparticles with Narrow Size Distribution Obtained by Continuous Micro Segmented Flow Synthesis. Chem. Eng. J. 2011, 166, 1164-1169. 24. Günther, A.; Jensen, K. F. Multiphase Microfluidics: From Flow Characteristics to Chemical and Materials Synthesis. Lab Chip 2006, 6, 1487-1503. 25. Tsunoyama, H.; Ichikuni, N.; Tsukuda, T. Microfluidic Synthesis and Catalytic Application of PVP-Stabilized ∼ 1 nm Gold Clusters. Langmuir 2008, 24, 11327-11330. 26. Luty-Błocho, M.; Wojnicki, M.; Pacławski, K.; Fitzner, K. The Synthesis of Platinum Nanoparticles and Their Deposition on the Active Carbon Fibers in One Microreactor Cycle. Chem. Eng. J. 2013, 226, 46-51. 27. Abécassis, B.; Testard, F.; Spalla, O.; Barboux, P. Probing in Situ the Nucleation and Growth of Gold Nanoparticles by Small-Angle X-Ray Scattering. Nano Lett. 2007, 7, 1723-1727. 28. Yao, T.; Sun, Z.; Li, Y.; Pan, Z.; Wei, H.; Xie, Y.; Nomura, M.; Niwa, Y.; Yan, W.; Wu, Z. Insights into Initial Kinetic Nucleation of Gold Nanocrystals. J. Am. Chem. Soc. 2010, 132, 7696-7701. 29. Abécassis, B.; Testard, F.; Kong, Q.; Francois, B.; Spalla, O. Influence of Monomer Feeding on a Fast Gold Nanoparticles Synthesis: Time-Resolved XANES and SAXS Experiments. Langmuir 2010, 26, 13847-13854. 27 ACS Paragon Plus Environment


Page 28 of 31

30. Tofighi, G.; Lichtenberg, H.; Pesek, J.; Sheppard, T. L.; Wang, W.; Schottner, L.; Rinke, G.; Dittmeyer, R.; Grunwaldt, J.-D. Continuous Microfluidic Synthesis of Colloidal Ultrasmall Gold Nanoparticles: In Situ Study of the Early Reaction Stages and Application for Catalysis. React. Chem. Eng. 2017, 2, 876-884. 31. Edwards, J. K.; Solsona, B. E.; Landon, P.; Carley, A. F.; Herzing, A.; Kiely, C. J.; Hutchings, G. J. Direct Synthesis of Hydrogen Peroxide from H2 and O2 Using TiO2Supported Au–Pd Catalysts. J. Catal. 2005, 236, 69-79. 32. Edwards, J. K.; Solsona, B.; Ntainjua, E.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Switching Off Hydrogen Peroxide Hydrogenation in the Direct Synthesis Process. Science 2009, 323, 1037-1041. 33. Edwards, J. K.; Freakley, S. J.; Carley, A. F.; Kiely, C. J.; Hutchings, G. J. Strategies for Designing Supported Gold–Palladium Bimetallic Catalysts for the Direct Synthesis of Hydrogen Peroxide. Acc. Chem. Res. 2013, 47, 845-854. 34. Dimitratos, N.; Lopez-Sanchez, J. A.; Hutchings, G. J. Selective Liquid Phase Oxidation with Supported Metal Nanoparticles. Chem. Sci. 2012, 3, 20-44. 35. Griffin, M. B.; Rodriguez, A. A.; Montemore, M. M.; Monnier, J. R.; Williams, C. T.; Medlin, J. W. The Selective Oxidation of Ethylene Glycol and 1, 2-Propanediol on Au, Pd, and Au–Pd Bimetallic Catalysts. J. Catal. 2013, 307, 111-120. 36. Davis, S. E.; Ide, M. S.; Davis, R. J. Selective Oxidation of Alcohols and Aldehydes over Supported Metal Nanoparticles. Green Chem. 2013, 15, 17-45. 37. Kesavan, L.; Tiruvalam, R.; Ab Rahim, M. H.; bin Saiman, M. I.; Enache, D. I.; Jenkins, R. L.; Dimitratos, N.; Lopez-Sanchez, J. A.; Taylor, S. H.; Knight, D. W. Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles. Science 2011, 331, 195-199. 38. Guczi, L.; Beck, A.; Horváth, A.; Koppány, Z.; Stefler, G.; Frey, K.; Sajó, I.; Geszti, O.; Bazin, D.; Lynch, J. AuPd Bimetallic Nanoparticles on TiO2: XRD, TEM, in Situ EXAFS Studies and Catalytic Activity in CO Oxidation. J. Mol. Catal. A: Chem. 2003, 204–205, 545-552. 39. Xu, J.; White, T.; Li, P.; He, C.; Yu, J.; Yuan, W.; Han, Y.-F. Biphasic Pd− Au Alloy Catalyst for Low-Temperature CO Oxidation. J. Am. Chem. Soc. 2010, 132, 1039810406. 40. Qian, K.; Huang, W. Au–Pd Alloying-Promoted Thermal Decomposition of PdO Supported on SiO2 and Its Effect on the Catalytic Performance in CO Oxidation. Catal. Today 2011, 164, 320-324. 41. Tsen, S.-C.; Crozier, P.; Liu, J. Lattice Measurement and Alloy Compositions in Metal and Bimetallic Nanoparticles. Ultramicroscopy 2003, 98, 63-72. 42. Marx, S.; Baiker, A. Beneficial Interaction of Gold and Palladium in Bimetallic Catalysts for the Selective Oxidation of Benzyl Alcohol. J. Phys. Chem. C 2009, 113, 6191-6201. 43. McBride, J. R.; Dukes, A. D.; Schreuder, M. A.; Rosenthal, S. J. On Ultrasmall Nanocrystals. Chem. Phys. Lett. 2010, 498, 1-9. 44. Kim, B. H.; Hackett, M. J.; Park, J.; Hyeon, T. Synthesis, Characterization, and Application of Ultrasmall Nanoparticles. Chem. Mater. 2013, 26, 59-71. 45. Zarschler, K.; Rocks, L.; Licciardello, N.; Boselli, L.; Polo, E.; Garcia, K. P.; De Cola, L.; Stephan, H.; Dawson, K. A. Ultrasmall Inorganic Nanoparticles: State-of-the-Art and Perspectives for Biomedical Applications. Nanomedicine: NBM 2016, 12, 1663-1701. 46. Hofmann, G.; Tofighi, G.; Rinke, G.; Baier, S.; Ewinger, A.; Urban, A.; Wenka, A.; Heideker, S.; Jahn, A.; Dittmeyer, R., et al. A Microfluidic Device for the Investigation of Rapid Gold Nanoparticle Formation in Continuous Turbulent Flow. J. Phys.: Conf. Ser. 2016, 012072. 47. Kölbl, A.; Kraut, M.; Wenka, A. Design Parameter Studies on Cyclone Type Mixers. Chem. Eng. J. 2011, 167, 444-454. 28 ACS Paragon Plus Environment

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48. Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. methods 2012, 9, 671-675. 49. Ravel, B.; Newville, M. Athena, Artemis, Hephaestus: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537-541. 50. Deutschmann, O.; Grunwaldt, J.-D. Abgasnachbehandlung in Mobilen Systemen: Stand Der Technik, Herausforderungen Und Perspektiven. Chem. Ing. Tech. 2013, 85, 595-617. 51. Tsubota, S.; Nakamura, T.; Tanaka, K.; Haruta, M. Effect of Calcination Temperature on the Catalytic Activity of Au Colloids Mechanically Mixed with TiO2 Powder for CO Oxidation. Catal. Lett. 1998, 56, 131-135. 52. Rahman, M. T.; Rebrov, E. V. Microreactors for Gold Nanoparticles Synthesis: From Faraday to Flow. Processes 2014, 2, 466-493. 53. Park, J. I.; Saffari, A.; Kumar, S.; Günther, A.; Kumacheva, E. Microfluidic Synthesis of Polymer and Inorganic Particulate Materials. Annu. Rev. Mater. Res. 2010, 40, 415-443. 54. Karim, A. M.; Al Hasan, N.; Ivanov, S.; Siefert, S.; Kelly, R. T.; Hallfors, N. G.; Benavidez, A.; Kovarik, L.; Jenkins, A.; Winans, R. E. Synthesis of 1 nm Pd Nanoparticles in a Microfluidic Reactor: Insights from in Situ X-Ray Absorption Fine Structure Spectroscopy and Small-Angle X-Ray Scattering. J. Phys. Chem. C 2015, 119, 13257-13267. 55. Maeland, A.; Flanagan, T. B. Lattice Spacings of Gold–Palladium Alloys. Can. J. Phys. 1964, 42, 2364-2366. 56. King, H.; Manchester, F. A Low-Temperature X-Ray Diffraction Study of Pd and Some Pd-H Alloys. J. Phys. F: Met. Phys. 1978, 8, 15. 57. Ftouni, J.; Penhoat, M.; Addad, A.; Payen, E.; Rolando, C.; Girardon, J.-S. Highly Controlled Synthesis of Nanometric Gold Particles by Citrate Reduction Using the Short Mixing, Heating and Quenching Times Achievable in a Microfluidic Device. Nanoscale 2012, 4, 4450-4454. 58. Kuhrt, C.; Harsdorff, M. Photoemission and Electron Microscopy of Small Supported Palladium Clusters. Surf. Sci. 1991, 245, 173-179. 59. Kumar, K. M.; Mandal, B. K.; Kumar, K. S.; Reddy, P. S.; Sreedhar, B. Biobased Green Method to Synthesise Palladium and Iron Nanoparticles Using Terminalia Chebula Aqueous Extract. Spectrochim. Acta A 2013, 102, 128-133. 60. Lopez-Acevedo, O.; Tsunoyama, H.; Tsukuda, T.; Hakkinen, H.; Aikens, C. M. Chirality and Electronic Structure of the Thiolate-Protected Au38 Nanocluster. J. Am. Chem. Soc. 2010, 132, 8210-8218. 61. Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating the Crystal Structure of a Thiol-Protected Au25 Cluster and Optical Properties. J. Am. Chem. Soc.2008, 130, 5883-5885. 62. Iwasa, T.; Nobusada, K. Gold-Thiolate Core-in-Cage Cluster Au25 (SCH3)18 Shows Localized Spins in Charged States. Chem. Phys. Lett. 2007, 441, 268-272. 63. Zwijnenburg, A.; Goossens, A.; Sloof, W. G.; Crajé, M. W.; van der Kraan, A. M.; Jos de Jongh, L.; Makkee, M.; Moulijn, J. A. XPS and Mössbauer Characterization of Au/TiO2 Propene Epoxidation Catalysts. J. Phys. Chem. B 2002, 106, 9853-9862. 64. Ruzicka, J.-Y.; Abu Bakar, F.; Hoeck, C.; Adnan, R.; McNicoll, C.; Kemmitt, T.; Cowie, B. C.; Metha, G. F.; Andersson, G. G.; Golovko, V. B. Toward Control of Gold Cluster Aggregation on TiO2 Via Surface Treatments. J. Phys. Chem. C 2015, 119, 24465-24474. 65. Krumeich, F.; Marx, S.; Baiker, A.; Nesper, R. Characterization of AuPd Nanoparticles by Probe‐Corrected Scanning Transmission Electron Microscopy and X‐Ray Absorption Spectroscopy. Z. anorg. allg. Chem. 2011, 637, 875-881. 66. Teng, X.; Wang, Q.; Liu, P.; Han, W.; Frenkel, A. I.; Wen, W.; Marinkovic, N.; Hanson, J. C.; Rodriguez, J. A. Formation of Pd/Au Nanostructures from Pd Nanowires Via Galvanic Replacement Reaction. J. Am. Chem. Soc. 2008, 130, 1093-1101. 29 ACS Paragon Plus Environment


Page 30 of 31

67. Kaiser, J.; Szczerba, W.; Riesemeier, H.; Reinholz, U.; Radtke, M.; Albrecht, M.; Lu, Y.; Ballauff, M. The Structure of AuPd Nanoalloys Anchored on Spherical Polyelectrolyte Brushes Determined by X-Ray Absorption Spectroscopy. Faraday Discuss. 2013, 162, 45-55. 68. Ishida, T.; Koga, H.; Okumura, M.; Haruta, M. Advances in Gold Catalysis and Understanding the Catalytic Mechanism. Chem. Rec. 2016, 16, 2278-2293. 69. Okumura, M.; Fujitani, T.; Huang, J.; Ishida, T. A Career in Catalysis: Masatake Haruta. ACS Catal. 2015, 5, 4699-4707. 70. Tsubota, S.; Nakamura, T.; Tanaka, K.; Haruta, M. Effect of Calcination Temperature on the Catalytic Activity of Au Colloids Mechanically Mixed with TiO2 Powder for CO Oxidation. Catal. Lett. 1998, 56, 131-135. 71. Gibson, E. K.; Beale, A. M.; Catlow, C. R. A.; Chutia, A.; Gianolio, D.; Gould, A.; Kroner, A.; Mohammed, K. M.; Perdjon, M.; Rogers, S. M. Restructuring of AuPd Nanoparticles Studied by a Combined XAFS/DRIFTS Approach. Chem. Mater. 2015, 27, 3714-3720. 72. Gao, F.; Wang, Y.; Goodman, D. W. CO Oxidation over AuPd (100) from Ultrahigh Vacuum to near-Atmospheric Pressures: CO Adsorption-Induced Surface Segregation and Reaction Kinetics. J. Phys. Chem. C 2009, 113, 14993-15000.


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Microfluidic Synthesis of Ultrasmall AuPd Nanoparticles with a

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