Continuous microfluidic synthesis of Pd nanocubes and PdPt core

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Surfaces, Interfaces, and Applications

Continuous microfluidic synthesis of Pd nanocubes and PdPt core-shell nanoparticles and their catalysis of NO2 reduction Anna Pekkari, Zafer Say, Arturo Susarrey-Arce, Christoph Langhammer, Hanna Härelind, Victor Sebastian, and Kasper Moth-Poulsen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09701 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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ACS Applied Materials & Interfaces

Continuous microfluidic synthesis of Pd nanocubes and PdPt core-shell nanoparticles and their catalysis of NO2 reduction Anna Pekkari,† Zafer Say,‡ Arturo Susarrey-Arce,‡ Christoph Langhammer,‡ Hanna H¨arelind,† Victor Sebastian,∗,¶,§ and Kasper Moth-Poulsen∗,† †Applied Chemistry, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 41296 Gothenburg, Sweden ‡Chemical Physics, Department of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden ¶Department of Chemical Engineering, Aragon Institute of Nanoscience (INA), University of Zaragoza, Campus Ro Ebro-Edificio I+D, c/Poeta Mariano Esquillor s/n, 50018 Zaragoza, Spain §Networking Research Center on Bioengineering, Biomaterials and Nanomedicine, CIBER-BBN, 28029-Madrid, Spain E-mail: [email protected]; [email protected]

Abstract Faceted colloidal nanoparticles are currently of immense interest due to their unique electronic, optical and catalytic properties. However, continuous flow synthesis that enables rapid formation of faceted nanoparticles of single or multi-elemental composition is not trivial. We present a continuous flow synthesis route for the synthesis of uniformly sized Pd nanocubes and PdPt core-shell nanoparticles in a single-phase

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microfluidic reactor, which enables rapid formation of shaped nanoparticles with a reaction time of three minutes. The PdPt core-shell nanoparticles features a dendritic, high surface area with the Pt shell covering the Pd core, as verified using high resolution scanning transmission electron microscopy (STEM) and Energy Dispersive X-ray Spectroscopy (EDX). The Pd nanocubes and PdPt core-shell particles are catalytically tested during NO2 reduction in presence of H2 in a flow pocket reactor. The Pd nanocubes exhibited low-temperature activity (i.e.< 136 ◦ C) and poor selectivity performance towards production of N2 O or N2 , whereas PdPt core-shell nanoparticles showed higher activity and were found to achieve better selectivity during NO2 reduction together with retaining its basic structure at relatively elevated temperatures, making the PdPt core-shell particles a unique, desirable synergic catalyst material for the potential use in NOx abatement processes.

Keywords Flow chemistry, microreactor, core-shell, Palladium, Platinum

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Introduction

The colloidal synthesis of shaped noble metal nanoparticles has received growing attention in recent years because of their unique physical and chemical properties making them highly interesting in the fields of sensing, plasmonics and heterogeneous catalysis. 1,2 The possibility to tailor nanoparticles using shape-controlled synthesis provides control of morphology and crystal facets, properties that strongly influences the activity and selectivity in catalytic reactions. 1–5 Particularly, catalysts based on Pd and Pt are used in several industrial processes, and shaped nanoparticles of these metals have been studied for numerous catalytic reactions showing high activity in carbon-carbon bond formation, reduction and oxidation reactions in fuel cells, and hydrogenation reactions. 4

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Bimetallic nanoparticles are an interesting group of materials since they can exhibit not only the properties of each individual metal but the performance is enhanced due to synergy effects of the two elements creating new properties. 6–8 The combination of Pd and Pt are especially interesting because the low lattice mismatch of 0.77 % facilitates combination into alloys and core-shell structures and the strong coupling between Pd and Pt gives rise to synergy effects that may enhance the catalytic activity. 9 The formation of core-shell nanoparticles is normally achieved by seed-growth methods where preformed shaped Pd nanocrystals are overgrown with a Pt shell, and has provided good control of final particle morphology but requires multiple synthesis steps. 9 A more facile approach is synthesis through co-reduction of Pd and Pt precursors that have been explored with excellent shape control to form alloys, 10–12 porous nanoparticles, 13–15 dendritic nanoparticles, 16–20 and coreshell structures. 19,20 However, these methods require prolonged reaction in no less than three minutes. Solution based colloidal synthesis methods of faceted noble metal nanoparticles are typically performed in batch reactors and have extensively been explored creating a wide range of complex structures. 1,2,21 However, the inefficient mixing and poor control of the reaction conditions in this type of reactor typically gives variability between batches, thus yielding polydisperse particles, not fully attaining the desired properties. 22 Additionally, the mass and heat transfer restraints of the batch reactor limits the application to small scale production thus limiting possible application areas. Flow chemistry using continuous microfluidic reactors, potentially being a cost effective solution for large scale synthesis, provides a way to circumvent challenges with reproducibility and scalability. By providing excellent mixing, heat and mass transport and improved control over nucleation and crystal growth, faster production of more shape- and size-uniform nanoparticles is possible, 22,23 opening up to large scale production which may lead to more widespread use of complex nanoparticle systems in industrial applications. Generally, the synthesis of noble metal nanoparticles in flow using continuous microfluidic

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reactors are divided into two types; i) the single-phase reactor where reactants are mixed in one phase via diffusion in a micro-sized fluidic channel followed by crystal growth in a heated reaction zone at a certain residence time. ii) the two-phase continuous reactor where a liquid is infused in segments into a second immiscible liquid or carrier gas, producing micro sized droplets. Micro- and milli sized two-phase reactors have produced monodisperse Pd nanoparticles with a range of morphologies; including cubes, 24,25 cuboctahedra, 25 octahedral 25 , nanoplates, 26 rods, 26 and bimetallic PdPt structures with different shapes (alloys, dendritic particles, Pt-decorated Pd particles). 26 However, the reactor requires time consuming separation of the two phases. 22 Single phase flow reactors benefit from their facile setup, but the design of faceted Pd nanoparticles and PdPt nanoparticles in this type of reactor are so far limited. In this study, we synthesized faceted Pd nanocubes and PdPt core-shell nanoparticles in a single-phase continuous microfluidic reactor. Systematic evaluation of a large range of reaction parameters and their influence on the particle shape is investigated to find optimum conditions that produce uniform nanoparticles. Pd nanocubes synthesized in the microreactor are compared to equivalent nanoparticles produced in batch, and the scale-up of particle production in a continuous millifluidic reactor. The metal composition in bimetallic PdPt core-shell nanoparticles is varied to study the shape transformation to the dendritic structure with increased Pt content in the particles. Further, the Pd nanocubes and PdPt core-shell nanoparticles are tested in a catalytic model reaction to evaluate the catalytic activity during NO2 reduction in the presence of H2 , conceptually demonstrating an alternative approach for low-temperature NOx abatement.

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Results and discussion

In this work, we have translated the synthesis of Pd nanocubes in a batch reactor to a continuous flow microfluidic tube reactor producing Pd nanocubes with a narrow size distribution.

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A schematic illustration of the continuous microfluidic reactor is presented in Figure 1. Briefly, an injection stream containing Pd precursor solution and surfactant (Cetyltrimethylammonium bromide, CTAB) is infused in polymeric tubing and is mixed with a second stream containing a solution of reducting agent (L-Ascorbic acid), and the streams are then heated for a certain time, forming the Pd nanocubes. Details of the synthesis can be found in the Experimental section.

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Figure 1: Schematic setup of the microfluidic reactor used to synthesize Pd nanocubes and PdPt core-shell nanoparticles.

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Nanoparticle synthesis

Batch synthesized Pd nanocubes were produced according to the synhtesis method by Niu et al. 27 The formed crystalline Pd nanocubes comprised of [100] facets 27 are uniform in size and slightly elongated with an aspect ratio of 1.1, as can be seen in the transmission electron microscope (TEM) image in Figure 2a. The histogram of size distribution in Figure 2b shows an average edge length of 27 nm (standard deviation 2.9 nm). When the synthesis of Pd nanocubes is transferred to a microfluidic reactor, keeping all synthesis parameters constant, here it is possible to reduce the residence time from 30 to 3 minutes. Cubic Pd nanoparticles are formed, with the same aspect ratio as previous, seen in Figure 2c and a high 5



resolution image using scanning transmission electron microscopy with a high-angle annular dark field detector (STEM-HAADF) inset in Figure 2c image shows the crystallinity of the Pd nanocubes. It can be seen in the size distribution histogram in Figure 2d that the microfluidic synthesized particles are smaller, with a mean edge length of 14 nm (standard deviation 1.6 nm). The efficient mixing and heat transfer in the microfluidic reactor enable a reduction of the residence time, thus shortening the growth time of particle seeds which leads to the formation of smaller Pd nanocubes. Additionally, the microfluidic reactor provides rapid and uniform nucleation of nanocrystal seeds, considered important factors in controlling the size and shape homogeneity of the final nanoparticles. 22,28 When comparing the size distributions in Figure 2a-b and Figure 2c-d, batch synthesized Pd nanocubes appear more polydisperse in size. However, when the size deviation is presented in percent (± 11 % for the batch and microfluidic reactor), the two methods show similar size uniformity of the synthesized Pd nanocubes, and no major difference in quality can be concluded. However, at the chosen synthesis conditions, the productivity of the two methods differ. The microfluidic reactor has the capacity to produce 35 µg Pd nanocubes /min, which is an improvement compared to the equivalent production in batch (10.57 mL Pd nanocube solution, 17.6 µg Pd nanocubes /min). In the development of the microfluidic system for the synthesis of cubic Pd nanoparticles, a systematic evaluation of reaction parameters was performed to find the optimum conditions resulting in narrow size distributed cubic particles. A set of reaction parameters and its influence on the shape and morphology of the formed particles like residence time, CTAB concentration and temperature were investigated. Figure S1 shows TEM images of the Pd nanoparticles synthesized under the different conditions, revealing that inhomogeneous nanoparticles are formed. The increase of residence time to 10 and 20 minutes, see Figure S1a and S1b, resulted in a shape transformation from cubic particles to a mixture of rods, triangles and spheres. It could be estimated that longer residence times would yield larger cubic particles due to increased growth time, but instead, the Pd nanocubes

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Figure 2: TEM images of cubic Pd nanoparticles produced in a) batch reactor and c) microfluidic reactor. Inset in c) shows a high-resolution STEM-HAADF image of a Pd nanocube. Histograms to the right show particle size distributions and Gaussian curve fit to data for Pd nanocubes from b) batch with 27 ± 2.9 nm (± 11 %) in size, and d) microfluidic with 14 ± 1.6 nm(± 11 %) size. grow inhomogeneously into different shapes. Reducing the CTAB concentration to a third of standard (4.17 mM) seen in Figure S1c, fewer cubes were formed, and more rods, triangles and spheres were present. However, when increasing CTAB concentration 3 times (37.5 mM), see Figure S1d, fewer cubic shaped particles formed in favour of larger spherical shapes and rods. The high CTAB concentration slows down the reduction rate of the Pd precursor resulting in fewer crystal seeds that subsequently grow into larger nanoparticles. Reduction of the reaction temperature from 96◦ C to 60◦ C, as can be seen in Figure S1e, favoured the growth of rods, with cubes, triangles and other shapes also present. When the temperature is increased to 130◦ C, shown in Figure S1f, particles with a range of sizes and

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morphologies are formed. From these results we can suggest that the temperature affects the kinetics of seed formation and growth, altering the crystal structure and homogeneity of the nanocrystal seeds and growth into final shapes. We have scaled up the continuous microfluidic synthesis of Pd nanocubes to a continuous millifluidic reactor. The latest due to the current need to translate the synthesis of Pd nanocubes into large scale, and still retain particle size and shape control for applications requiring larger amounts of nanoparticles such as catalysis. Therefore, a microfluidic sized nucleation stage was set up to allow for homogeneous nucleation of Pd seeds, followed by a growth section in millifluidic sized tubing. The formed palladium nanocubes, shown in Figure S2a, are cubic, and the size histogram of particles in Figure S2b, present a mean edge length of 14 nm (standard deviation 1.6 nm, 11 %), corresponding well to the microfluidic synthesized Pd nanocubes. From TEM results and the calculation of size distribution, it is clear that the scale-up of the reactor to millifluidic dimensions and a possible change in mixing, heating and growth kinetics does not impact the uniformity of the formed Pd nanocubes. Compared to the microfluidic reactor, the millifluidic reactor provides an eightfold increase of productivity (0.28 mg Pd nanoparticles/min). Evaluation of particle yield of the Pd nanocube synthesis was performed by analysis of Pd content in nanoparticle solutions using microwave plasma-atomic emission spectrometry (MP-AES). The particle yield is determined by comparing the amount of Pd in metal form with the total Pd content (Pd(s) and Pd2+ ) in the solution, hence the amount of metal precursor that has been reduced and formed nanoparticles. Pd ions are removed from the product solutions by dialysis (24 h) and the Pd content is then measured and compared to untreated nanoparticle solutions. A significant improvement in the synthesis yield is seen when transferring from batch reactor (63 %) to the continuous microfluidic setup (94 %). The microfluidic reactor generally improves particle yields by the more efficient heat and mass transfer. 22 On the contrary, when the synthesis is upscaled to the millifluidic reactor the yield is reduced (33 %). An encountered problem with single-phase microfluidic reactors

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is the parabolic velocity profile giving rise to residence time distributions in the reactor. 23 This effect seems to be enhanced when scaling up to the millifluidic reactor negatively impacting the synthesis yield. Additionally, nanoparticle precipitation on the reactor walls, a common problem in single-phase continuous reactors 22 was observed in the millifluidic reactor. Comparison of the amount of added Pd precursor to Pd measured in the nanoparticle solution showed a significant loss (35 %) in the millifluidic reactor. For the batch and microfluidic reactor no loss due to precipitation could be measured. These results indicate that the scale-up to millifluidic dimension affects the mixing properties, thus contributing to a lower yield. The problem with fouling could be mitigated in droplet millifluidic reactors, where improved mixing inside the reaction droplets and less contact between droplets and reactor walls minimizes the risk of particle precipitation. Zhang et al. synthesized several shaped nanoparticles with high yield, using a continuous millifluidic liquid-liquid phase droplet reactor. 25 However, this reactor requires time consuming product separation. This could possibly be avoided by using the gas-liquid droplet reactor, that has been used to synthesize various shaped nanoparticles. 26,29 For the production of bimetallic PdPt core-shell nanoparticles, the microfluidic reactor setup used for Pd nanocube synthesis was modified for the incorporation of Pt, following a one-pot approach combining hexachloroplatinic acid (H2 Cl6 Pt) solution with the Pd precursor solution stream (Figure 1), keeping other synthesis parameters constant. Here we vary the relative molar ratios of Pd:Pt in the precursor solutions (6:1, 3:1, 1:1), and the morphology of the formed particles can be controlled which is visualised in the TEM images in Figure 3. Figure 3a shows PdPt nanoparticles with the highest Pd content (6:1) with a mean diameter of 28 nm (standard deviation 4.2 nm). The particles are mostly cubic shaped with a wavy surface morphology resembling the structure of the Pd nanocubes. As Pd content decreases from PdPt (6:1) to (3:1), seen in Figure 3b, slightly spherical and rough shaped particles of 31 nm (standard deviation 4.2 nm) are formed. Particles with the lowest Pd content, i.e Pd:Pt (1:1), in Figure 3c, are 31 nm in diameter (standard deviation 4.3 nm), with

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a dendritic surface topography attributed to the high Pt content. To this end, it is found that as the PdPt increases the diameter of the particles gradually increase and gradually morphing from cubic wavy surface morphology to more spherical dendritic particles.

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Figure 3: TEM images of microfluidic synthesized PdPt core-shell nanoparticles. The addition of Pt to the reaction influences the surface morphology of the formed nanoparticles by increasing surface roughness with higher Pt content. The molar ratio of Pd:Pt in the particles was varied: a) 6:1, b) 3:1, c) 1:1. Scale bars are 50 nm, in inset images scale bars are 10 nm. To visualise the surface morphology, high-resolution STEM-HAADF was performed on PdPt (1:1) nanoparticles showing the highly dendritic surface in Figure 4a, c, d. Investigation of the elemental distribution of Pd and Pt in the particles was performed by a scanning transmission electron microscope and energy dispersive x-ray spectroscopy (STEM-EDX) line scan of a nanoparticle. As shown in Figure 4b, the line scan reveals a core-shell structure consisting of Pd enriched in the core of the particle and Pt dominating at the particle surface. The formation of bimetallic nanoparticles is a complex interplay between kinetics and reduction potentials of the two metals. In our case, we observe that Pd predominantly forms the core whereas Pt forms a dendritic shell structure, despite the lower reduction potential of the Pd-precursor. These observations are in line with findings by Kim et al, 16 where a coreduction of Pd- and Pt-precursor yields core-shell PdPt nanoparticles. The Pt-precursor exhibit stronger complexation with Br- and CTA+ in CTAB, and a stronger binding affinity for nitrogen groups in CTAB, compared to Pd(II) ions. This leads to a slower reduction rate of the Pt-precursor, thus resulting in the formation of core-shell PdPt nanoparticles. 16 Conversely, when Zhou et. al. 30 synhtesised PdPt nanoparticles by coreduction in the pres-

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ence of Br- (by addition of KBr), and without CTAB, the reduction rate of the Pd-precursor was slowed down and alloy nanocubes formed. Thus, it seems like the strong binding of Pt-precursor to the nitrogen groups in CTAB plays a critical role in lowering the reduction rate of the Pt ions, yielding the core-shell structure.

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Figure 4: a) STEM-HAADF image of PdPt (1:1) nanoparticles. b) EDS line scan of one PdPt 1:1 nanoparticle showing the relative distribution of Pd and Pt in the particle. c), d) high resolution STEM-HAADF images of PdPt (1:1) nanoparticles. The influence of reaction temperature on the morphology of PdPt (1:1) particles was investigated at nanoparticle synthesis at 60◦ C and 130◦ C. When the temperature was reduced from 96◦ C to 60◦ C, seen in Figure S4a, the particle yield is very low and the formed particles are highly dendritic with a size around 50 nm in diameter. The increase of temperature to 130◦ C modifies the nucleation and growth of PdPt core-shell nanoparticles and yields

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particles with a range of sizes and morphologies. These experiments demonstrate the effect of temperature on the particle morphology, and it is observed that 96◦ C is the optimum synthesis temperature to produce uniform PdPt core-shell nanoparticles with high yield.

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Catalytic evaluation and the effect on particle shape

For evaluation of catalytic properties using NO2 -reduction as a model reaction, larger amounts of Pd nanocubes and PdPt (1:1) core-shell nanoparticles were synthesized by running the microfluidic reactor for a longer time (two hours). The longer synthesis times resulted in lower quality of the nanoparticles, which can be seen in the scanning electron microscopy (SEM) image in Figure 5a, where Pd nanoparticles include cubes and rods, spherical and triangular particles. The SEM image of large-scale synthesized PdPt (1:1) core-shell nanoparticles, shown in Figure 5c, reveal rough particles consisting of cubic and spherical shapes of different sizes. A possible reason for the shape variations when scaling up the microfluidic reactor could be attributed to slight particle precipitation in the polymeric tube walls, thus influencing the nucleation and growth of the nanoparticles leading to some shape variability. The nanoparticle solutions of Pd nanocubes and PdPt (1:1) particles were stored for one year in the fridge before catalytic evaluation. Analysis with SEM shows no significant particle growth and Ostwald ripening indicating good stability of the solutions after storage over longer time periods. Treatment in reaction conditions (2.2 % H2 , 0.22 % NO2 ) with temperature ramps from 48◦ C to 136◦ C (including pre-treatment) slightly affect the shape of the Pd nanoparticles, giving cubes with rounded edges as can be seen in Figure 5b. When Pd nanoparticles are subjected to reaction conditions with higher temperatures (up to 220◦ C), they undergo sintering and form large agglomerates as seen in Figure S4a. The PdPt (1:1) core-shell nanoparticles on the other hand, showed high stability under the operating conditions (48◦ C to 136◦ C), see Figure 5c, retaining their shape and size. Additionally, at increased temperature (220◦ C), seen in Figure S4c, PdPt (1:1) core-shell nanoparticles exhibit good stability. The combi12


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nation of metals in the design of bimetallic nanoparticles is a way to improve the thermal stability, where the introduction of a metal with higher melting temperature enables less sintering of the nanoparticles. 31 This indicates that the combination of Pd with Pt improves the thermal stability of the core-shell structures. Furthermore, the crystal arrangement and shape might also influence the particle stability, where the dendritic structure of the PdPt core-shell nanoparticles may have contributed to the improved thermal stability.

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Figure 5: SEM images of untreated a) Pd nanocubes and c) PdPt (1:1) core-shell nanoparticles, deposited on Si substrates. b) Pd nanocubes and d) PdPt (1:1) core-shell nanoparticles, treated in reaction condition (48-136◦ C in 0.22 % NO2, 2.2 % H2 in Ar(g) including pretreatment). Scale bars are 100 nm. The restructuring behaviour exhibited by the Pd nanocubes at elevated temperatures (220◦ C and 390◦ C), see Figure S4a and S4b, with the loss of shape and surface area by crystal growth can be attributed to sintering, which arise from two mechanisms; atom migration (Ostwald ripening) or particle migration and coalescence. 31,32 These two processes can occur individually or interact simultaneously in the sintering process. 31,32 It can be seen from the results that after pre-treatment and reaction, Pd nanoparticles are affected by one or a combination of the effects, that lead to growth of the nanoparticles into larger agglomerate films. Sintering effects are also observed when PdPt (1:1) core-shell nanoparticles are treated 13



at high temperature (390◦ C), see Figure S4d, where particles sinter and form large clusters. The sintering behaviour of nanoparticles depends on temperature and particle loading and the inter-particle distance. If the particle distance is sufficiently long it prevents sintering caused by particle migration and coalescence, 31 and for Pd nanoparticles treated at higher temperature (390◦ C) with lower particle loading, see Figure S4b, the particles sinter into larger structures that are still in the nano size range despite the higher reaction temperature.

2.3

Model catalytic evaluation: NO2 -reduction

The microfluidic synthesized Pd nanocubes and PdPt (1:1) core-shell nanoparticles were catalytically tested by evaluation of the catalytic activity and selectivity in the reduction of NO2 .

2.4

Direct H2 -deNO2 activity of Pd nanocubes and PdPt coreshell nanoparticles

As already mentioned in experimental section, catalyst particles in the form of colloidal solutions were dispersed onto a glass substrate in the form of 2D flat catalyst rather than high surface area porous metal oxide support materials. Their reactivities for NO2 reduction were tested in a direct comparative manner on bare catalysts as proof of principle type of work. Indeed, the current reaction parameters in this work is not expected as an ideal steady state condition. In this context, temperature dependent direct H2 -deNO2 reaction were studied on both Pd nanocubes and PdPt (1:1) core-shell nanoparticles. Figure 6 exhibits outlet QMS ion current (right y-axis) and temperature (left y-axis) values for the entire catalysis work from conditioning to the reduction process at gradually elevated temperatures. Here one can easily follow the H2 -deNO2 reaction by monitoring the decrease in NO2 outlet ion current towards the end of the process in time interval between 140 - 200 min. The corresponding % NO2 conversion values were calculated based on calibration equations and are illustrated

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in Figure 7. Here, both of the materials had limited NO2 conversion efficiencies below 70◦ C, with sizeable activities closely reaching 100 % at temperatures between 91◦ C and ca. 136◦ C. When directly comparing the two systems at hand, it was also found that the Pd nanocubes exhibited relatively lower activity for the H2 de-NO2 process compared to the PdPt (1:1) core-shell nanoparticles. This lower activity of Pd can possibly be attributed to its cubic morphology, which is enclosed by low-index [100] facets that are known to exhibit lower catalytic performance compared to high-index facet counterparts in general. 33–36 On the other hand, the PdPt (1:1) core-shell nanoparticles exhibit a rough dendritic surface structure that most likely has a higher abundance of high-index facets and thus exhibits higher activity emphasizing the importance of the bimetallic catalyst synergy improving NO2 decomposition.

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Figure 7: Relative NO2 conversion efficiencies of Pd nanocubes, PdPt core-shell nanoparticles calculated based on the data depicted in Figure 6, and blank sample.

2.5

Product selectivity

A second important aspect to consider is the product selectivity. To this end, NO, N2 O and N2 are the three products formed during NO2 reduction by H2 . Among these three products, N2 is the most desired one due to its non-toxic characteristics, whereas the other two molecules are major greenhouse gases and/or toxic. Figure 8 shows the QMS ion current values (right y-axis) for these three major detectable products at elevated temperatures (left y-axis) during the entire catalysis process up to 136◦ C. In Figure 8a, it is clear to observe that NO production took place beyond 48 ◦ C and gradually became more dominant above this temperature for Pd nanocube catalyst. On the other hand, while N2 formation from Pd nanocube catalyst was found negligible for the entire temperature regime, N2 O formation was rather elusive at relatively higher temperature as shown in Figure 8a. However, as clearly seen in Figure 8b, NO production trend for PdPt (1:1) core-shell nanoparticles has changed above 115◦ C and became less abundant. In contrast to Pd nanocubes, PdPt (1:1) core-shell nanoparticles exhibited much better selectivity towards N2 and N2 O production which can 16


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only be detected by QMS outlet signals in corresponding temperature regime. Here one can also consider NH3 as another potential NO2 reduction product, but we have not observed any detectable signal based on its 15 a.m.u fragment.

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Figure 8: Temperature dependent NO2 , NO, N2 O and N2 MS ion current readings for a) Pd nanocubes and b) PdPt (1:1) core-shell nanoparticles. The reaction takes place in a mixture of 2200 ppm NO2 and 2.22 % H2 in Ar carrier gas , supplied at a 6 mL/min flow rate. Prior to temperature ramp up, the catalysts were exposed to the reaction mixture for conditioning at 48◦ C for 3 hours. To further quantify the selectivity and compare the two catalysts in this respect, the relative selectivity for the two systems were calculated, shown in Figure 9, by integrating the area under the ion current curves for each product in the temperature range between 48 ◦ C and 136 ◦ C. It becomes apparent that the PdPt (1:1) core-shell nanoparticles exhibit higher selectivity towards N2 formation, as well as lower selectivity towards NO production compared to the Pd nanocubes which product abundance is 98 % or higher at identical conditions. Based on these findings, it is important to note that catalysis was only performed 17



Page 18 of 39

in narrow temperature range up to 150 ◦ C by considering the loss in structural integrity and well-defined shape of Pd nanocubes at higher temperatures. Therefore, we have not included product selectivity distributions beyond this temperature which could be more likely to get lower NO selectivity. As previously stated, we have merely compared the reactivity and selectivity performance of two different microfluidic synthesized structures under identical conditions, but not the ideal. NO

PdPt particles

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Pd nanocubes

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10

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Figure 9: Calculated relative percent selectivity of three major conversion products of NO, N2 O and N2 in 48 136◦ C temperature interval for Pd nanocubes and PdPt (1:1) core-shell nanoparticles.

3

Conclusions

In summary, a continuous flow microfluidic synthesis method in single-phase, has been developed for the synthesis of Pd nanocubes and PdPt core-shell nanoparticles with narrow size distributions. Uniformly sized and shaped nanoparticles could be synthesized in just three minutes of residence time. Compared to the equivalent method in a batch reactor, Pd nanocubes are equally uniform in size and the synthesis yield significantly improves. Furthermore, the synthesis was upscaled to a millifluidic sized continuous reactor. Uniformily sized bimetallic PdPt core-shell nanoparticles are formed by co-reduction in the microfluidic reactor, forming Pd nanoparticles with a dendritic Pt shell where the particle morphology 18


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is controlled by the molar composition of Pd:Pt. These nanoparticles were subsequently tested in a flow pocket-reactor using a model H2 -deNO2 direct reduction reaction. The Pd nanocubes and PdPt (1:1) core-shell nanoparticles exhibited high catalytic activity at low temperature (< 136 ◦ C) and demonstrated thermal stability by preserving their original shape. Furthermore, the PdPt (1:1) core-shell nanoparticles exhibited good thermal stability at higher temperatures and showed high selectivity towards the formation of N2 O and N2 as compared to Pd nanocubes that had NO product abundance close to 98 %. This work thus illustrates that functional core-shell nanoparticles can be prepared using simple single-phase flow chemistry methods.

4 4.1

Experimental Materials

Palladium(II)chloride ≥ 99.9 %, PdCl2 , Sigma), Hydrogen hexachloroplatinate (IV) solution (8 wt % in H2 O, H2 Cl6 Pt, Sigma), L-ascorbic acid (AA) (99 %, Sigma), Cetyltrimethylammonium bromide (CTAB) (≥ 99 %, Acros) were used as received without further purification. Deionized (DI) water (MilliQ, 18.2 MΩ) was used in all experiments. Aqueous H2 PdCl4 solution (10 mM) was prepared by dissolving 35.6 mg PdCl2 in 20 mL HCl (20 mM) at 60◦ C. Aqueous H2 Cl6 Pt solution (10 mM) was prepared by diluting 1.024 mL H2 Cl6 Pt solution with DI water up to a volume of 20 mL.

4.2

Batch synthesis

Synthesis of cubic Pd nanoparticles was performed using a batch reactor, in accordance with previous work by Niu et al. 27 In brief, 0.5 mL H2 PdCl4 solution (10 mM) was added to 10 mL CTAB solution (12.5 mM) and the solution was heated in a water bath to 96◦ C for 20 min. Freshly prepared, 0.08 mL AA solution (100 mM) was then quickly added under stirring, and the resulting solution was left undisturbed for 30 min. Cubic Pd nanoparticles 19



were purified by two consecutive cycles of centrifugation (14000 rpm, 20 min), cleaned with DI water between cycles and redispersed in the same volume (10mL) of DI water.

4.3

Microfluidic synthesis

Figure 1 shows the experimental setup of the microfluidic reactor used in this work. Two inlet streams are interfaced in a Polyether ether ketone (PEEK) based y-shaped junction connected to Polytetrafluoroethylene (PTFE) tubing (Inner diameter = 800 µm) with a certain length (4.2 m). The two solutions were infused with the same flow rate, maintained by the use of a syringe pump (Harvard Apparatus) and the reaction temperature was kept constant by immersing the PTFE tubing in a temperature-controlled water bath. For experiments performed at temperatures above 100◦ C, a back-pressure regulator (Zaiput Flow Technologies, 6 bar) was used and the PTFE tubing was heated in an oil bath. The reaction parameters used in the synthesis are summarized in Table 1. A first injection solution containing Pd precursor was prepared by mixing 4.79 mL CTAB solution with 0.5 mL H2 PdCl4 (10 mM) solution. Various concentrations of CTAB were tested, as presented in Table 1. For making bimetallic PdPt core-shell nanoparticles, solutions with different molar ratios of the metals were prepared by mixing H2 PdCl4 (10 mM) and H2 Cl6 Pt (10 mM) solutions of different amounts to a total volume of 0.5 mL, experimental details are presented in Table 2. The resulting metal precursor solution was then heated to 96◦ C in a water bath for 20 min under stirring. PdPt mixtures appeared as suspensions and were sonicated for 30 s until homogeneous turbidity was observed in the suspension. The turbid orange suspension was subsequently transferred to an injection syringe. The second solution was composed of freshly prepared 0.08 mL AA solution (100 mM) dissolved in 5.21 mL DI water. The formed Pd nanocubes and PdPt core-shell nanoparticles were purified by three consecutive cycles of centrifugation (8000-13000 rpm, 10-20 min), cleaned with DI water between cycles and redispersed in 10 mL water.

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Table 1: Reaction conditions used in the synthesis of Pd nanocubes in the continuous microfluidic reactor. Residence time Flow rate per stream Temperature Concentration (min) (µL/min) (◦ C) CTAB (mM) 3 700 96 26.1 10 210 96 26.1 20 105 96 26.1 10 210 96 8.7 10 210 96 78.4 3 700 60 26.1 3 700 130 26.1 Table 2: Compositional details of the metal precursor solutions and reaction conditions for the microfluidic synthesis of PdPt core-shell nanoparticles with varying molar ratio. Pd:Pt molar ratio H2 PdCl4 10 mM H2 Cl6 Pt 10 mM Residence time (mL) (mL) (min) 6:1 0.857 0.143 3 3:1 0.375 0.125 3 1:1 0.25 0.25 3

4.4

Millifluidic synthesis

The continuous flow synthesis was scaled up in a millifluidic reactor by connecting the microfluidic reactor setup of PTFE microfluidic tubing (Inner diameter = 800 µm, 2.1 m), connected to millilitre sized PTFE tubing (Inner diameter = 1.4 mm, length = 8.1 m). Infusion flow rates were kept constant (2.84 mL/min per stream), providing a residence time of three minutes in the reactor and the reaction temperature was kept at 96◦ C. The formed Pd nanocubes were cleaned by centrifugation as described in the Microfluidic synthesis section.

4.5

Characterization

Particle morphology and size distribution were investigated using a transmission electron microscope (TEM) FEI Tecnai T20 operating at 200 kV. Scanning transmission imaging was performed using a high-angle annular dark field detector (STEM-HAADF) and energy dispersive x-ray spectroscopy (EDX) in a FEI XFEG TITAN electron microscope operating

21



at 300 kV. For Pd nanocubes the side length of 100-300 nanocubes were measured to evaluate the size distribution as well as the mean aspect ratio (Length/Width, L/W). Samples for TEM-analysis were prepared by drop casting 5 µL concentrated particle solution onto a carbon coated TEM-grid. The nanoparticle synthesis yield for Pd nanocubes was determined using a microwave plasma-atomic emission spectrometer (Agilent 4100 MP-AES). Nanoparticle samples were cleaned from metal cations and CTAB by a dialysis membrane (D9652 Sigma) immersed in water for 24 h. Both dialysed solutions and untreated particle solutions were analysed and compared. Particle samples were dissolved by addition of a mixture of fresh aqua regia (HNO3 : HCl, molar ratio 1:3) and DI water (1:5 v/v, 12 mL), and left at room temperature for 2 h. The solutions were shaken a few times to ensure complete dissolution. The dissolved particle samples were diluted with DI water to a final volume of 25 mL for spectrometric analysis. The shape and morphology of nanoparticles deposited on Si substrates were evaluated with a Scanning Electron Microscope (SEM) Zeiss Supra 60 VP operating at 15 kV, using an In-Lens detector.

4.6

Preparation of catalyst samples

Nanoparticle solution (12 mL, 0.47 mM) was divided in Eppendorf tubes (1.5 mL each) and cleaned by two consecutive cycles of centrifugation (12 000-13 000 rpm, 20-30 min) with 1 mL DI water between cycles. The water was removed and the particle pellet (100 µL) was sonicated for 1 min, drop casted onto an O2 -plasma treated (5 min, 50 W) fused silica substrate (2 cm2 ) and left to dry overnight.

4.7

Direct NO2 reduction by H2

To evaluate the catalytic activity for the direct H2 -deNO2 reaction, the flat nanoparticle samples were mounted in a pocket reactor of the type reported by Bu et al. 37 and were exposed to a gas mixture of 2200 ppm NO2 and 2.22 % H2 gas mixture in Ar with 100 mL/min gas flow through a tubular reactor, which then was reduced down to 6 mL/min 22


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flow rate in the pocket where the sample was mounted. The samples were subsequently pre-treated in pure Ar, 5 % O2 and 1.5 % H2 for 60 min each at 158◦ C. Then, the reactor was cooled down to 48◦ C and was exposed to the reaction mixture (2200 ppm NO2 and 2.22 % H2 in Ar) at 48◦ C for 3 hours for conditioning and stabilization of mass spectrometer response. Finally, the sample was gradually heated up to 136◦ C at a 10◦ C/min heating rate and kept for 10 minutes at each temperature.

Acknowledgement We acknowledge financial support from the Knut and Alice Wallenberg Foundation via project 2015.0055, and the Swedish Foundation for Strategic Research via project RMA150052. V. S. acknowledges the support of the People Program (CIG-Marie Curie Actions, REA grant agreement no. 321642) to develop this research.

Supporting Information Available Experimental data and TEM-images from the nanoparticle synthesis, and data from catalytic characterization. This material is available free of charge via the Internet at http://pubs.acs.org/.

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tion of CO + NO on Pd(111): An In Situ Infrared Spectroscopic Study at Elevated Pressures. Journal of the American Chemical Society 2002, 124, 8524–8525. (37) Bu, Y.; Niemantsverdriet, J. W.; Fredriksson, H. O. Cu Model Catalyst Dynamics and CO Oxidation Kinetics Studied by Simultaneous in Situ UV-Vis and Mass Spectroscopy. ACS Catalysis 2016, 6, 2867–2876.

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Graphical TOC Entry Metal precursor and stabilizer solution

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3 min!

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CTAB and metal precursor solution

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