Catalytically Active PdâAg Alloy Nanoparticles Synthesized in

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Catalytically active Pd-Ag alloy nanoparticles synthesized in microemulsion template Linda Ström, Henrik Ström, Per-Anders Carlsson, Magnus Skoglundh, and Hanna Härelind Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01838 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Catalytically active Pd-Ag alloy nanoparticles synthesized in microemulsion template Linda Ström1*, Henrik Ström2, Per-Anders Carlsson1, Magnus Skoglundh1 and Hanna Härelind1 1. Competence Centre for Catalysis, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96, Göteborg, Sweden; 2. Division of Fluid Dynamics, Department of Mechanics and Maritime Sciences, Chalmers University of Technology, SE-412 96, Göteborg, Sweden; * Correspondence: [email protected]; Tel.: +46-31-772-1000

Abstract: This work investigates the possibility to form catalytically active bimetallic Pd-Ag nanoparticles synthesized in the water pools of a reversed microemulsion using methanol, a more environmental- and userfriendly reductant compared to hydrazine or sodium borohydride, which are commonly used for this type of synthesis. The nanoparticles were characterized with regards to crystallinity and size by X-ray diffraction and transmission electron microscopy. CO chemisorption and oxidation followed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used for investigating the elemental composition of the surface and catalytic activity, respectively. Moreover, the structural composition of the bimetallic particles was determined by scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy. The particles were shown to be crystalline nanoalloys of around 5-12 nm. CO adsorption followed by in situ DRIFTS suggests that the particle surfaces are composed of the same Pd-Ag ratios as the entire particles, regardless of elemental ratio, i.e. no core-shell structures can be detected. This is also shown by numerical simulations using a Monte Carlo based model. Furthermore, CO oxidation confirms that the synthesized particles are catalytically active.

Keywords: Microemulsion, Nanoreactor, Catalysis, Nanoalloys, Metallic nanoparticles, Pd-Ag, Monte Carlo simulation

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Introduction Efficient use of (bio-based) feedstocks for production of chemicals and fuels requires improved or new chemical technologies. This holds as well for applications for environmental protection, e.g., emission control for mobile and stationary sources. An important area for reaching these goals is to increase the efficiency of catalysts, i.e., catalytic activity and selectivity, by the design of the catalytic material. The catalytic activity of nano-sized particles consisting of bimetallic structures can differ significantly from those of the separate elements,1 which is important for catalysts in several processes.2 Attenuation and promotion of the catalytic activity may be obtained depending on the synthesis conditions, which enables the tailoring of welldefined structures with specific, controllable properties.1, 3-8 More specifically, appropriately designed bimetallic nanoparticles may under certain conditions exhibit improved activity and selectivity in heterogeneous catalysis. Pd-Ag nanoparticles have received attention for their potential use as catalysts in different types of fuel cells9 and for on-board hydrogen production.10 Other possible applications include catalysts for selective oxidation reactions such as oxidation of formic acid, glycerol and ethanol9,11-14 and hydrogenation reactions, for example of methyl acrylate.15 All these applications would benefit from the possibility to design and tune the available active sites, which can be done by adequately balancing the surface exposure to the promoting effect of the participating atomic species.9, 12 Synthesis of bimetallic nanoparticles with highly specific properties requires extraordinary control of the synthesis conditions, as the particle properties are affected by the resulting intraparticle structure. Bimetallic nanoparticles can be classified according to four main mixing patterns: i) core-shell structured particles with a core of, say, element A surrounded by an outer layer of another element, say, B, ii) sub-cluster segregated structures where A and B atoms share a mixed interphase or just have a minor number of A-B bonds, iii) mixed structure, either randomly, which is often referred to as an ‘alloy’ in literature, or ordered and iv) multi-layer structure, for example A-B-A, which has been theoretically investigated by simulations and observed experimentally.1 In this work, a randomly mixed particle is referred to as an ‘alloy’, as opposed to a core-shell structure. The preparation of well-dispersed supported catalysts has traditionally been performed by impregnation of the support by the active phase precursor dissolved in water, followed by calcination. Catalysts prepared by this method receive high dispersion and good thermal stability. However, the method results in poor control over size, shape and composition of the particles. One possibility of tailoring nanoparticles is the use of water pools of a reversed (w/o) microemulsion, which are created thanks to the ability of surfactants to self-assemble into welldefined structures.16-21 Macroscopically, microemulsions are single-phase systems where nanosized droplets of oil are dispersed into a continuous water phase, shortly oil-in-water (o/w). The opposite system, water-in-oil (w/o), is denoted reversed microemulsion. Moreover, the droplets of a microemulsion are highly dynamic and will repeatedly fuse and redisperse such that the content of the droplets becomes mixed. Metallic nanoparticles can be formed by combining a w/o microemulsion containing metal salt solution(s) with a microemulsion containing a reductant. The traditionally used reducing agents, however, are hydrazine (N2H4)17, 18, 22 or sodium borohydride (NaBH4)18 that are toxic.23 In this work, we investigate synthesis of well-defined catalytically active Pd-Ag bimetallic nanoparticles using the water pools of a reversed microemulsion as template and methanol as a reducing agent. Methanol is a relatively green reducing agent that has the advantage of being


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biodegradable. We characterize the size and structure, i.e. core-shell or alloy, of the nanoparticles using transmission electron microscopy (TEM), scanning TEM coupled with energy-dispersive X-ray spectroscopy (STEM-EDX) and X-ray diffraction (XRD), and compare these results to numerical Monte Carlo simulations. Further, we characterize the bimetallic nanoparticles with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using CO as probe molecule and investigate their catalytic properties by oxidation of CO also using in situ DRIFTS. Experimental Section Synthesis and characterization The nanoparticles were synthesized in reversed (w/o) microemulsions consisting of 75 wt.% nheptane (Reag. Ph. Eur. Riedel-de Haën), 20 wt.% of the surfactant dioctyl sulfosuccinate sodium salt (AOT, 98% Aldrich Chemistry) and 5 wt.% water. Aqueous solutions of silver nitrate (>99% Sigma-Aldrich) and palladium(II) nitrate (15% Alfa Aesar), both holding 2 wt.% metal, were used as the water phases. Samples containing solutions of palladium salt, silver salt and mixtures of each precursor (20/80, 40/60 and 50/50 wt. percentage Ag/Pd), were prepared. In order to reduce the metal salt solutions into metallic nanoparticles, a second microemulsion containing the equivalent oil/water/surfactant ratios, but with methanol (50 wt.%, anhydrous, 99.8%, Sigma-Aldrich) as the aqueous phase, was added. Subsequently, the samples were allowed to react at 70 ˚C for 24 h. The synthesis procedure is illustrated in Figure 1. c)

a)

70∘C, 24h

AOT

+ Metal precursors Heptane b)

Methanol Nanoparticles Figure 1. Illustration of the particle synthesis method. The metal salt-containing microemulsions are mixed (a), subsequently they are mixed with the reductant (b), and finally they are allowed to react at 70 ˚C for 24 h (c).

In order to study the crystalline phases of the resulting particles, X-ray diffraction was carried out using a Bruker XRD D8 Advance X-ray diffractometer with monochromatic CuΚα1 radiation. Prior to the analysis, the particles were deposited onto a carbon powder (2-12µm, 99.95%, Aldrich Chemistry) by dropwise addition of tetrahydrofuran (THF) to a


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microemulsion/carbon slurry, subsequently the samples were mixed under vigorous stirring overnight and filtered off. After drying in room temperature, the samples were dried at 150 ˚C for 2 h. XRD scans were executed in the range 2q = 20 to 80˚ with a step size and time of 0.02˚ and 1.5 s, respectively. Particles containing pure palladium, pure silver and a 50/50 wt. percentage mixture were analyzed, all containing a total of 2 wt.% metal. In order to distribute the nanoparticles over g-alumina (PURALOXÒ SBa 200, Sasol), the support powder was mixed with the microemulsions, and thereafter THF was added dropwise in a volume corresponding to three times the microemulsion volume, in order to destabilize the microemulsion. After stirring overnight, the samples were centrifuged, washed with ethanol during ultra-sonication and then centrifuged once more. Excess solvent was removed after each centrifugation. Finally, the samples were allowed to dry in room temperature and were then slowly (2 ˚C/min) ramped up to 550 ˚C, where they were calcined for 4 h. All samples contained 2 wt.% metal. In situ DRIFT spectra were recorded for alumina-supported samples using a Bruker Vertex70 spectrometer equipped with a high-temperature reaction cell (Harrick Scientific) with KBr windows. Prior to the CO-adsorption, the samples were pretreated in O2 (0.5 vol.%, Ar-bal.) at 500 ˚C for 1 h, followed by reduction in H2 (0.5 vol.%, Ar-bal.) at 500 ˚C for 15 minutes and during cooling to 25 ˚C. At this temperature, the samples were exposed to CO (0.5 vol.%, Arbal.) for 10 minutes. Spectra were recorded after removal of excess CO. Subsequently, CO oxidation experiments were performed by flowing CO and O2 (0.5 and 2.5 vol.%, respectively) through the sample while raising the temperature in steps of 25 ˚C until reaching 400 ˚C. Carbon monoxide oxidation spectra were recorded at steady-state. TEM images of the microemulsions were received using a FEI Tecnai T20 electron microscope operating at 200 kV. The distribution of the Pd and Ag atoms within a particle was investigated using scanning TEM coupled with energy-dispersive X-ray spectroscopy, using a FEI Titan 80300 microscope operating at 300 kV. This microscope was also used for analysis of the samples supported on g-alumina in Scanning TEM mode using a high-angle annular dark-field (HAADF) detector. The STEM images display a combined mass-thickness contrast. The images of the supported samples were received both after drying the samples at 150 ˚C and after calcination at 550 ˚C. For deposition of the alumina-supported particles onto TEM grids, the powder was dissolved in ethanol during ultra-sonication. Monte Carlo simulations Numerical simulations of nanoparticle nucleation and growth were performed using the Monte Carlo method developed by Tojo et al.24-32 The microemulsion was described as containing 15,000 droplets, of which half contained methanol and the remaining either Ag or Pd in some proportion (wt. percentage ratio 20/80, 50/50 or 80/20). The initial concentrations of the reactants in each droplet were sampled from a Poission distribution with a mean-value obtained via an approximation of the hydrodynamic radius of the water droplets.16 The droplet motion is assumed to be governed by Brownian motion in such a way that energetic collisions may give rise to the establishment of an inter-droplet channel, which allows material exchange. In each Monte Carlo time step, 10% of the droplets in the microemulsion were therefore randomly chosen to collide, fuse and re-disperse. The droplet interaction model permits the following processes:


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1) Inter-droplet exchange: If the two interacting droplets contain the same component(s), redistribution is assumed to occur according to a concentration gradient principle. The maximum number of entities that can be transferred from one droplet to another (kex) depends on the material and its size and charge, as well as on the film flexibility (f, expressed as a maximum number of aggregated atoms for which particle transfer between droplets is still possible).32 For water/AOT/n-heptane, it has previously been established that appropriate values are kex = 1 and f = 5.25 However, due to the presence of large quantities of methanol, which may act as a co-surfactant 33 and thereby increase the film flexibility,34, 35 we also explore other values in a sensitivity study (kex = 1 and f = 1, which correspond to an extremely rigid film, and kex = 5 and f = 30, corresponding to a highly flexible film 32). 2) Chemical reaction: Pd2+ and Ag+ have relatively similar reduction potentials (De0 = 0.915 - 0.7996 = 0.12 V) and may therefore be expected to have similar reduction rates, which promotes nanoalloy formation.31, 32 Whenever the reducing agent and a metal salt are present in the same droplet, reduction is assumed to occur with a 10% probability. Each reduction reaction implies transfer of kex (= 1) components (reductant or metal, chosen according to rule 3 below) through the inter-droplet channel. To investigate the influence from the fact that the reduction of Pd2+ is somewhat faster than that of Ag+, we also perform simulations where the reaction probability is 20% for Pd2+ and 10% for Ag+. 3) Nucleation and ripening: Upon collisions between a droplet containing reactants and a droplet containing a growing bimetallic aggregate, the droplet carrying the nucleus acts as an autocatalytic nucleation point. If both droplets contain growing aggregates, the growth is assumed to continue on the larger particle due to its larger surface area (socalled Ostwald ripening).31 Continued aggregation may however only proceed if allowed according to film flexibility considerations, i.e. only particles of f (= 5) or fewer atoms may be transmitted through the inter-droplet channel. Each simulation was run for 10,000 Monte Carlo time steps for the resulting particle size and property distribution to stabilize. At the end of a simulation, each aggregate was divided into ten concentric layers and the stored order of aggregation was used to generate spatially resolved data on the metal distribution inside each aggregate. By employing an average atomic volume weighted from the number count of Ag and Pd atoms in the aggregate, it was also possible to obtain an estimation of the particle size distribution.27 Results and Discussion The samples were characterized with regards to appearance of the microemulsions, the size of the synthesized nanoparticles, and the crystalline phases. The atomic structure of the nanoparticles was investigated by STEM-EDX as well as adsorption and oxidation of CO. In addition, the experimental work was compared to a numeric simulation using the Monte Carlo model. In this section, we will present and discuss the results. Characterization of particle size and crystalline phases After mixing the metal precursor-containing microemulsion with the methanol-containing microemulsion, the appearance of the samples is light colored/transparent as shown in Figure


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2a. After allowing the samples to react for 24 h at 70 ˚C, the colors of the samples shift to significantly darker, indicating a significant content of reduced particles. All samples remained stable during the experiments, i.e. no cloudiness or phase separation could be observed. TEM micrographs of the microemulsion samples show that crystal shaped particles of around 5-12 nm have been formed. XRD patterns of the pure Pd particles, Ag/Pd and pure Ag particles supported on carbon are shown in Figure 2b. The peaks at 2q = 40 and 46˚ are associated with the Pd crystal planes (111) and (200), respectively, confirming the presence of face centered cubic structured Pd.36-40 The supported Ag sample exhibits peaks at 38 and 43.6˚, confirming cubic close-packed structure.41, 42 The carbon support is known to give rise to a peak at 44˚, corresponding to C(100).38, 43, 44 Ag

a)

Pd

Ag-Pd

C(100)

b)

Pd(111)

Pd(200)

70 ∘C, 24h Pd

Intensity (a.u.)

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Ag

Ag-Pd

Ag(111) Pd/Ag

Ag(200)

Pd

Ag

20 nm

20 nm

20 nm

36

40 44 48 2 Theta (º)

Figure 2. a) The appearance of the samples before and after reduction, with TEM images showing partly polyhedral shaped nanoparticles. b) XRD patterns of carbon-supported Pd (top), Pd-Ag in 1:1 wt. ratio (middle) and Ag (bottom) particles.

Figure 3 shows the STEM-images of the alumina-supported samples. The figure shows that throughout the deposition and drying process, the nanoparticles to a high degree remain dispersed while calcination at 550 ˚C results in somewhat sintered particles.


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a)

b)

e)

50 nm

Pd

100 nm

50 nm

100 nm

d)

c)

100 nm

f)

50 nm

200 nm

Figure 3. STEM images of alumina supported samples: a) Ag/Al2O3, b) Ag(50%)Pd(50%)/Al2O3 c) Pd/Al2O3, d) Ag(80%)Pd(20%)/Al2O3, e) Ag(40%)Pd(60%)/Al2O3, f) Pd/Al2O3. All ratios are based on weight. The top three samples are dried at 150 ˚C while the bottom three samples are calcined at 550 ˚C.

Conceptual characterization of particle structures The simulation results, illustrated in Figure 4, show the formation of approximately 2,000 nanoparticle aggregates from the initial droplet distribution. The innermost core of the aggregates consists of either Ag or Pd with a probability matching the proportions in the microemulsion mixture. It should be emphasized that the use of concentric layers at equidistant radial spacing in the visualization implies that the innermost layer represents only one or a few atoms, whereas the number of atoms in the outermost layer can be orders of magnitude higher. From the middle of the aggregate and towards the surface, the composition is gradually more mixed, pointing to the formation of nano-alloys rather than core-shell structures. The average composition for the nanoparticle aggregates resulting in the three different cases (in Figure 4) also consistently correspond to the proportions in the mixture, indicating minimal tendency to metal segregation. The simulations thus show that the particles synthesized from the microemulsions are nano-alloys with both metals present at the aggregate surface.

Figure 4. Simulation result histograms for the spatial data on Ag/Pd-nanoparticles for three different Ag/Pd-wt. ratios (from left to right: 20/80, 50/50 and 80/20). The colored spheres represent the average composition. Pd is represented in blue, Ag in red and a 50/50 mixture in grey. As the color turns to lighter tonalities, the proportion of the corresponding pure metal is higher.


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Table 1. Ag weight-% at the nanoparticle surface predicted in the simulations, as a function of the Ag/Pd ratio in the precursors used for the microemulsion, with different assumptions for the film flexibility parameters. The main results reported in this work were obtained with kex = 1 and f =5.

Film flexibility kex = 1, f = 1 kex = 1, f = 5 kex = 5, f = 30

Ag weight-% in nanoparticle precursor 20 50 80 23.8 49.4 75.7 24.0 48.7 75.8 20.6 50.7 78.4

The effect of varying the film flexibility parameters on the simulation results are shown in Table 1. For the current conditions, the particle surfaces exhibit mixed composition with Ag present in approximately the same proportion as in the precursor mixture, irrespective of the degree of film flexibility. This observation implies that the overall conclusion that nano-alloys were formed is insensitive to whether methanol acts as a co-surfactant or not. Otherwise, such an effect could be assumed to vary with methanol concentration and thereby in effect require a more advanced model also accounting for variations in the film flexibility during the nucleation and growth phases. Similarly, these results also imply that if the microemulsion properties are affected by the oxidation of methanol into other compounds, the effect on the characteristics of the synthesized particles will still not be significant at the present conditions. It is also reasonable to assume that the effects of temperature on the film flexibility for the employed water/AOT/n-heptane system fall within the parameter space investigated here, and thus do not impart significant changes to the nanoparticle surface composition. Finally, an increase in the reduction probability of Pd2+ to double that of Ag+ was observed to result in an increase of the proportion of Ag at the particle surface. This effect appears as the faster reacting metal precursor will be depleted first, leaving proportionally more of the slower metal to be reduced at the particle surface by the reductant present in excess. The surface, although enriched in Ag, however retains a mixed character, supporting the conclusion that the difference in reduction potential is still small enough to produce nano-alloys.31

Figure 5. Predicted particle size distribution for Ag/Pd-nanoparticles at 50/50 wt. proportion.

The predicted particle size distribution is similar for all tested Ag/Pd ratios, and the result from the 50/50 case is shown in Figure 5. The distribution is unimodal and the mean particle size is


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approximately 1 nm. This simulation method have been shown to under predict the aggregate size by up to an order of magnitude for other systems.27 Nevertheless, Wei et al. 45 experimentally obtained similarly sized aggregates (1-2 nm) using Brij-30 and hydrazine. The qualitative agreement found for the size distribution with the results reported in the literature for the present type of model lends credibility to the current implementation and further supports the conclusions on the degree of atomic mixing, presented in Figure 6. Furthermore, a shift towards larger aggregate sizes was observed for increasing film flexibility: from a mean particle diameter of approximately 0.63 nm for an extremely rigid film (kex = 1, f = 1) to 1.3 nm for a very flexible film (kex = 5, f = 30). This trend is in agreement with previous observations on the relation between film flexibility and aggregate sizes in microemulsion synthesis.35, 46 In situ DRIFTS using CO as a probe molecule was performed in order to investigate how the catalytic properties of the nanoparticles are affected by the particle composition. Furthermore, CO adsorption may give an indication upon the surface elemental structure (whether the particles have formed alloys or core-shell systems). Figure 6 shows the absorption bands of CO adsorbed on the alumina-supported nanoparticles with different Ag/Pd compositions, ranging from 100 wt.% Pd to 100 wt.% Ag, with steps of 20%. In agreement with literature,11, 47 CO does not adsorb onto Ag at ambient conditions. Absorption peaks below 2000 cm-1 are attributed to the adsorption of bridge-bonded CO, and the bands above 2000 cm-1 are assigned to CO linearly bound to the surface.11, 47-51 However, the exact position of absorption bands on Pd have been recognized to be a function of the particle preparation method, which normally results in a variety of particle size and crystal imperfections. An absorption band assigned to CO linearly bound to Pd is detected at 2099 cm-1 on the pure Pd sample, while the Agcontaining samples exhibit a weak redshift of this signal to lower wavenumber, relative the pure Pd sample (from 2099 to 2095 cm-1). However, weaker adsorption of CO would result in a blueshift towards the vibration of gas phase CO at 2143 cm-1, i.e. shift to higher wavenumber. Therefore, other effects such as change in dipole-dipole interaction between the linearly bound CO molecules should be responsible for this redshift.2, 48 Furthermore, this signal decreases steadily in intensity as the Pd-Ag ratio of the samples move towards increased Ag content. The sample containing pure Pd particles exhibits a signal at 1960 cm-1, which shows the presence of bridge-bonded CO. According to Soma-Noto et al.,47 the diadsorbed bridge complex CO prevails on pure Pd. However, Childers et al. 52 show that the ratio between linearly bound and bridge-bonded CO on Pd strongly depends on particle size. In our study, the signal corresponding to the single-bonded CO vibrations is significantly more pronounced, compared to the bridge-bonded CO vibrations, also for the sample containing Pd only. The peak assigned to bridge-bonded CO exhibits a red shift (towards lower wavenumbers) as the silver content increases (from 1960 to 1930 cm-1), as shown in Figure 6. If the nanoparticles would have higher Ag ratio at the surface, compared to the overall particle, the absorbance corresponding to bridge-bonded CO would decrease rapidly with increased Ag ratio, since each bridge-bonded CO molecule needs to occupy two surface Pd atoms. A Pd-prevailed surface would, on the other hand, result in similar CO absorbance regardless of Ag concentration, since the Ag core would not affect the CO adsorbed at the surface. The gradual redshift of the bridge-bonded CO, with increasing Ag ratio, might be due to electronic and/or geometric impact of Ag atoms in the Pd lattice. A particle surface with agglomeration of Ag and Pd atoms separately should only result in a gradual decrease of this signal, since the Pd lattice would not be affected. In summary, the results of the CO adsorption study indicate that the surface of the particles consists of the same Pd-Ag ratio as the entire particle, i.e. no core-shell structure can be detected. This is also illustrated in Figure 6b, showing the STEM-EDX mapping of a particle containing equal amounts of Pd and Ag, confirming an even distribution of the elements throughout the nanoparticles.


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2099

a) 2081 2095 2065

b)

1960

Intensity (a.u.)

Ag

Pd

1945 100% Pd

50 nm 80% Pd 20% Ag 1930

60% Pd 40% Ag

40% Pd 60% Ag 20% Pd 80% Ag 100% Ag 2200

2100

2000

1900

-1

1800

Wavelength (cm Wavenumber (cm-1))

Figure 6. a) DRIFT spectra of CO adsorption (0.5 vol.% flowing for 10 minutes at 25 ˚C) followed by evacuation of gas phase CO by Ar flow. b) STEM-EDX maps showing the location of Ag (orange) and Pd (red) in a nanoparticle. The Pd-Ag wt. ratio of this sample is 1:1. 100% Pd

80% Pd 20% Ag

60% Pd 40% Ag

40% Pd 60% Ag

20% Pd 80% Ag 400 °C 375 °C 350 °C 325 °C

Intensity (a.u.)

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300 °C 275 °C 250 °C 225 °C 200 °C 175 °C 150 °C 125 °C 100 °C 75 °C 50 °C 25 °C 2400

2200

2000 2400

2200

2000 2400

2200

2000 2400

2200

2000 2400

2200

2000

Wavenumber (cm-1-1) ) Wavelength (cm

Figure 7. DRIFT spectra of CO oxidation over Ag/Pd particles of varying ratios. The inlet gas composition was 0.5 vol.% CO and 2.5 vol.% O2 (Ar-bal).


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The DRIFTS spectra of CO oxidation over the alumina-supported metal nanoparticles are shown in Figure 7. The double absorption peaks around 2150 cm-1 represents the presence of gas-phase CO, while the peaks evolving around 2350 cm-1 are owing to formation of CO2.53 The figure shows that the formation of CO2 starts at a lower temperature over the sample containing pure Pd nanoparticles, as illustrated by the stronger CO2/CO signal (light blue spectrum at 225 ˚C) for the Pd sample, compared to the other samples. Furthermore, the amount of chemisorbed CO (at 25 ˚C), is higher for the samples with high Pd content, compared to the samples containing more Ag. This absorption peak is positioned at 2098 cm-1 and is overlapped by the right part of the gas phase-CO double peak. Moreover, the samples containing a high ratio of Ag exhibit lower CO oxidation at high temperature. Taken together, in the reversed microemulsion templated synthesis, the metal salts were reduced by the methanol into crystalline particles, which was shown by XRD measurements. Results provided by STEM-EDX, DRIFTS and the numeric simulations all show that the particles formed in this synthesis exhibit an alloy structure with the Ag and Pd atoms mixed, regardless of the Ag/Pd-ratio. Furthermore, the particles were shown to be active for CO oxidation, where the pure Pd-particle sample was active at a lower temperature, compared to the other ratios. Conclusions We show that catalytically active crystalline nano-alloys can be synthesized by a simple route with methanol as reducing agent, which provides a considerably greener alternative to traditional reductants. Visually, it can be seen that the microemulsions change from light transparent color to a darker shade after reduction at 70 ˚C for 24 h. X-ray diffractograms and TEM micrographs show that crystalline particles of around 5-12 nm were formed. Furthermore, a mixed atom-alloy structure, regardless of Pd-Ag ratio, was confirmed experimentally by STEM-EDX as well as in numeric simulations. Following adsorption of CO at particles of different Pd-Ag ratios by in situ DRIFTS indicates that the surface of the formed particles consists of equal Pd-Ag ratio as the entire particle, i.e. no core-shell structure can be detected. In addition, CO oxidation confirms catalytically active particles. Acknowledgments: This work has been financially supported by the Swedish Research Council and was performed within the Competence Centre for Catalysis, which is hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and the member companies: AB Volvo, ECAPS AB, Johnson Matthey AB, Preem AB, Scania CV AB, Umicore Denmark ApS and Volvo Car Corporation AB. Thanks also to Stefan Gustafsson at Chalmers Materials Analysis Laboratory for providing the TEM images.


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References 1. Ferrando, R.; Jellinek, J.; Johnston, R. L., Nanoalloys: From theory to applications of alloy clusters and nanoparticles. Chemical Reviews 2008, 108 (3), 845-910. 2. Li, M. S.; Shen, J. Y., Microcalorimetric and infrared spectroscopic studies of CO and C2H4 adsorption on Pd/SiO2 and Pd-Ag/SiO2 catalysts. Materials Chemistry and Physics 2001, 68 (1-3), 204-209. 3. Celorrio, V.; Quaino, P. M.; Santos, E.; Florez-Montano, J.; Humphrey, J. J. L.; Guillen-Villafuerte, O.; Plana, D.; Lazaro, M. J.; Pastor, E.; Fermin, D. J., Strain Effects on the Oxidation of CO and HCOOH on Au-Pd Core-Shell Nanoparticles. Acs Catalysis 2017, 7 (3), 1673-1680. 4. Antonyraj, C. A.; Huynh, N. T. T.; Park, S. K.; Shin, S.; Kim, Y. J.; Kim, S.; Lee, K. Y.; Cho, J. K., Basic anion-exchange resin (AER)-supported Au-Pd alloy nanoparticles for the oxidation of 5-hydroxymethyl-2-furfural (HMF) into 2,5-furan dicarboxylic acid (FDCA). Applied Catalysis a-General 2017, 547, 230-236. 5. Han, D. X.; Bao, Z. B.; Xing, H. B.; Yang, Y. W.; Ren, Q. L.; Zhang, Z. G., Fabrication of plasmonic Au-Pd alloy nanoparticles for photocatalytic Suzuki-Miyaura reactions under ambient conditions. Nanoscale 2017, 9 (18), 6026-6032. 6. Semaltianos, N. G.; Chassagnon, R.; Moutarlier, V.; Blondeau-Patissier, V.; Assoul, M.; Monteil, G., Nanoparticles alloying in liquids: Laser-ablation-generated Ag or Pd nanoparticles and laser irradiation-induced AgPd nanoparticle alloying. Nanotechnology 2017, 28 (15). 7. Wang, Z. W.; Deng, J. G.; Liu, Y. X.; Yang, H. G.; Xie, S. H.; Wu, Z. X.; Dai, H. X., Three-dimensionally ordered macroporous CoCr2O4-supported Au-Pd alloy nanoparticles: Highly active catalysts for methane combustion. Catalysis Today 2017, 281, 467-476. 8. Lin, W. F.; Iwasita, T.; Vielstich, W., Catalysis of CO electrooxidation at Pt, Ru, and PtRu alloy. An in situ FTIR study. Journal of Physical Chemistry B 1999, 103 (16), 3250-3257. 9. Liu, D.; Xie, M. L.; Wang, C. M.; Liao, L. W.; Qiu, L.; Ma, J.; Huang, H.; Long, R.; Jiang, J.; Xiong, Y. J., Pd-Ag alloy hollow nanostructures with interatomic charge polarization for enhanced electrocatalytic formic acid oxidation. Nano Research 2016, 9 (6), 1590-1599. 10. Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.; Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E., Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst. Nature Nanotechnology 2011, 6 (5), 302-307. 11. Hirasawa, S.; Watanabe, H.; Kizuka, T.; Nakagawa, Y.; Tomishige, K., Performance, structure and mechanism of Pd-Ag alloy catalyst for selective oxidation of glycerol to dihydroxyacetone. Journal of Catalysis 2013, 300, 205-216. 12. Oliveira, M. C.; Rego, R.; Fernandes, L. S.; Tavares, P. B., Evaluation of the catalytic activity of Pd-Ag alloys on ethanol oxidation and oxygen reduction reactions in alkaline medium. Journal of Power Sources 2011, 196 (15), 6092-6098. 13. Kakaei, K.; Dorraji, M., One-pot synthesis of Palladium Silver nanoparticles decorated reduced graphene oxide and their application for ethanol oxidation in alkaline media. Electrochimica Acta 2014, 143, 207-215. 14. Fu, S. F.; Zhu, C. Z.; Du, D.; Lin, Y. H., Facile One-Step Synthesis of ThreeDimensional Pd-Ag Bimetallic Alloy Networks and Their Electrocatalytic Activity toward Ethanol Oxidation. Acs Applied Materials & Interfaces 2015, 7 (25), 1384213848.


Page 12 of 18

Page 13 of 18 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

Langmuir

15.

16. 17. 18. 19.

20. 21. 22.

23. 24. 25. 26. 27. 28. 29.

30. 31.

He, J. H.; Ichinose, I.; Kunitake, T.; Nakao, A.; Shiraishi, Y.; Toshima, N., Facile fabrication of Ag-Pd bimetallic nanoparticles in ultrathin TiO2-gel films: Nanoparticle morphology and catalytic activity. Journal of the American Chemical Society 2003, 125 (36), 11034-11040. Holmberg, K.; Jönsson, B.; Kronberg, B.; Lindaman, B., Surfactants and Polymers in Aqueous Solutions. Second edition ed.; John Wiley & sons: Chichester, West Sussex, England, 2003. Holmberg, K., Surfactant-templated nanomaterials synthesis. Journal of Colloid and Interface Science 2004, 274 (2), 355-364. Boutonnet, M.; Sanchez-Dominguez, M., Microemulsion droplets to catalytically active nanoparticles. How the application of colloidal tools in catalysis aims to well designed and efficient catalysts. Catalysis Today 2017, 285, 89-103. Yashima, M.; Falk, L. K. L.; Palmqvist, A. E. C.; Holmberg, K., Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion. Journal of Colloid and Interface Science 2003, 268 (2), 348-356. Boutonnet, M.; Kizling, J.; Stenius, P., The preparation of monodisperse colloidal metal particles from micro-emulsions. Colloids and Surfaces 1982, 5 (3), 209-225. Eriksson, S.; Nylen, U.; Rojas, S.; Boutonnet, M., Preparation of catalysts from microemulsions and their applications in heterogeneous catalysis. Applied Catalysis aGeneral 2004, 265 (2), 207-219. Sanchez-Dominguez, M.; Koleilat, H.; Boutonnet, M.; Solans, C., Synthesis of Pt Nanoparticles in Oil-in-Water Microemulsions: Phase Behavior and Effect of Formulation Parameters on Nanoparticle Characteristics. Journal of Dispersion Science and Technology 2011, 32 (12), 1765-1770. United States Environmental Protection Agency, w. e. g. (accessed 2018-04-15). Tojo, C.; Blanco, M. C.; Rivadulla, F.; Lopez Quintela, M. A., Kinetics of the formation of particles in microemulsions. Langmuir 1997, 13 (7), 1970-1977. Tojo, C.; Blanco, M. C.; LopezQuintela, M. A., Preparation of nanoparticles in microemulsions: A Monte Carlo study of the influence of the synthesis variables. Langmuir 1997, 13 (17), 4527-4534. Quintillan, S.; Tojo, C.; Blanco, M. C.; Lopez-Quintela, M. A., Effects of the intermicellar exchange on the size control of nanoparticles synthesized in microemulsions. Langmuir 2001, 17 (23), 7251-7254. Tojo, C.; Barroso, E.; de Dios, A., Critical nucleus size effects on nanoparticle formation in microemulsions: A comparison study between experimental and simulation results. Journal of Colloid and Interface Science 2006, 296 (2), 591-598. Tojo, C.; de Dios, M.; Lopez-Quintela, M. A., On the Structure of Bimetallic Nanoparticles Synthesized in Microemulsions. Journal of Physical Chemistry C 2009, 113 (44), 19145-19154. Buceta, D.; Tojo, C.; Vukmirovic, M. B.; Deepak, F. L.; Lopez-Quintela, M. A., Controlling Bimetallic Nanostructures by the Microemulsion Method with Subnanometer Resolution Using a Prediction Model. Langmuir 2015, 31 (27), 74357439. Tojo, C.; Buceta, D.; Lopez-Quintela, M. A., Understanding the Metal Distribution in Core-Shell Nanoparticles Prepared in Micellar Media. Nanoscale Research Letters 2015, 10. Tojo, C.; Buceta, D.; Lopez-Quintela, M. A., On Metal Segregation of Bimetallic Nanocatalysts Prepared by a One-Pot Method in Microemulsions. Catalysts 2017, 7 (2).


Langmuir 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

32. 33. 34. 35. 36. 37.

38.

39. 40. 41. 42. 43.

44. 45.

46.

Tojo, C.; Buceta, D.; Lopez-Quintela, M. A., Bimetallic nanoparticles synthesized in microemulsions: A computer simulation study on relationship between kinetics and metal segregation. Journal of Colloid and Interface Science 2018, 510, 152-161. Stilbs, P.; Rapacki, K.; Lindman, B., Effect of alcohol cosurfactant length on microemulsion structure. Journal of Colloid and Interface Science 1983, 95 (2), 583585. De Gennes, P. G.; Taupin, C., Microemulsions and the flexibility of oil/water interfaces. The Journal of Physical Chemistry 1982, 86 (13), 2294-2304. Lopez-Quintela, M. A., Synthesis of nanomaterials in microemulsions: formation mechanisms and growth control. Current Opinion in Colloid & Interface Science 2003, 8 (2), 137-144. Barrera, A.; Viniegra, M.; Bosch, P.; Lara, V. H.; Fuentes, S., Pd/Al2O3-La2O3 catalysts prepared by sol-gel: characterization and catalytic activity in the NO reduction by H-2. Applied Catalysis B-Environmental 2001, 34 (2), 97-111. Lorenz, H.; Turner, S.; Lebedev, O. I.; Van Tendeloo, G.; Klotzer, B.; Rameshan, C.; Pfaller, K.; Penner, S., Pd-In2O3 interaction due to reduction in hydrogen: Consequences for methanol steam reforming. Applied Catalysis a-General 2010, 374 (1-2), 180-188. Szumelda, T.; Drelinkiewicz, A.; Lalik, E.; Kosydar, R.; Duraczynska, D.; Gurgul, J., Carbon-supported Pd100-XAuX alloy nanoparticles for the electrocatalytic oxidation of formic acid: Influence of metal particles composition on activity enhancement. Applied Catalysis B-Environmental 2018, 221, 393-405. Basavegowda, N.; Mishra, K.; Lee, Y. R., Ultrasonic-assisted green synthesis of palladium nanoparticles and their nanocatalytic application in multicomponent reaction. New Journal of Chemistry 2015, 39 (2), 972-977. Cha, J. H.; Kim, K. S.; Lee, H., Size-selective Pd nanoparticles stabilized by dialkylmorpholinium ionic liquids. Korean Journal of Chemical Engineering 2009, 26 (3), 760-764. Jaroenapibal, P.; Boonma, P.; Saksilaporn, N.; Horprathum, M.; Amornkitbamrung, V.; Triroj, N., Improved NO2 sensing performance of electrospun WO3 nanofibers with silver doping. Sensors and Actuators B-Chemical 2018, 255, 1831-1840. Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N., Uniform silver nanowires synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly(vinyl pyrrolidone). Chemistry of Materials 2002, 14 (11), 4736-4745. Jiang, A. N.; Zhang, B. H.; Xue, Y. G.; Cheng, Y.; Li, Z. H.; Hao, J. C., Pt electrocatalyst supported on metal ion-templated hierarchical porous nitrogen-doped carbon from chitosan for methanol electrooxidation. Microporous and Mesoporous Materials 2017, 248, 99-107. Sljukic, B.; Martins, M.; Kayhan, E.; Balciunaite, A.; Sener, T.; Sequeira, C. A. C.; Santos, D. M. F., SnO2-C supported PdNi nanoparticles for oxygen reduction and borohydride oxidation. Journal of Electroanalytical Chemistry 2017, 797, 23-30. Wei, G.; Dai, W.; Qian, L.; Cao, W.; Zhang, J. Reverse microemulsions synthesis and characterization of Pd-Ag bimetallic alloy catalysts supported on Al2O3 for acetylene hydrogenation China Petroleum Processing & Petrochemical Technology, 2012, p. 59-67. Tojo, C.; Blanco, M. C.; Lopez-Quintela, M. A., The influence of reactant excess and film flexibility on the mechanism of nanoparticle formation in microemulsions: A Monte Carlo simulation. Langmuir 1998, 14 (24), 6835-6839.


Page 14 of 18

Page 15 of 18 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

Langmuir

47. 48.

49. 50. 51.

52.

53.

Somanoto, Y.; Sachtler, W. M., Infrared-spectra of carbon-monoxide adsorbed on supported palladium and palladium-silver alloys. Journal of Catalysis 1974, 32 (2), 315-324. Sun, J. Y.; Han, Y. X.; Fu, H. Y.; Wan, H. Q.; Xu, Z. Y.; Zheng, S. R., Selective hydrodechlorination of 1,2-dichloroethane catalyzed by trace Pd decorated Ag/Al2O3 catalysts prepared by galvanic replacement. Applied Surface Science 2018, 428, 703709. Adams, E. C.; Skoglundh, M.; Folic, M.; Bendixen, E. C.; Gabrielsson, P.; Carlsson, P.-A., Ammonia formation over supported platinum and palladium catalysts. Applied Catalysis B-Environmental 2015, 165, 10-19. Terekhina, O.; Roduner, E., FTIR Spectroscopic Investigation of Zeolite-Supported Pd-Ag Bimetallic Clusters. Journal of Physical Chemistry C 2012, 116 (12), 69736979. Alexeev, O. S.; Krishnamoorthy, S.; Jensen, C.; Ziebarth, M. S.; Yaluris, G.; Roberie, T. G.; Amiridis, M. D., In situ FTIR characterization of the adsorption of CO and its reaction with NO on Pd-based FCC low NOx combustion promoters. Catalysis Today 2007, 127 (1-4), 189-198. Childers, D.; Saha, A.; Schweitzer, N.; Rioux, R. M.; Miller, J. T.; Meyer, R. J., Correlating Heat of Adsorption of CO to Reaction Selectivity: Geometric Effects vs Electronic Effects in Neopentane Isomerization over Pt and Pd Catalysts. Acs Catalysis 2013, 3 (11), 2487-2496. Szanyi, J.; Kwak, J. H., Dissecting the steps of CO2 reduction: 2. The interaction of CO and CO2 with Pd/gamma-Al2O3: an in situ FTIR study. Physical Chemistry Chemical Physics 2014, 16 (29), 15126-15138.


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TOC Graphic Pd + Ag salts

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Pd/Ag alloys Methanol


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Catalytically Active PdâAg Alloy Nanoparticles Synthesized in

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