Continuous synthesis of hollow silver-palladium nanoparticles for catalytic applications

Hollow bimetallic nanoparticles exhibit unique surface plasmonic properties, enhanced catalytic activities and high photo(cid:5)thermal conversion efficiencies amongst other properties, however, their research and further deployment are currently limited by their complicated multi(cid:5)step syntheses. This paper presents a novel approach for their continuous synthesis with controllable and tuneable sizes and compositions. This robust manufacturing tool, consisting of coiled flow inverter (CFI) reactors connected in series, allows for the first time the tempo(cid:5) and spatial(cid:5) separation of the initial formation of silver seeds and their subsequent galvanic displacement reaction in the presence of a palladium precursor, leading to the full control of both steps separately. We have also demonstrated that coupling the galvanic replacement and co(cid:5)reduction leads to a great kinetic enhancement of the system leading to a high yield process of hollow bimetallic nanoparticles, directly applicable to other metal combinations. we the flow regime of micro(cid:5)devices allows the production of nanoparticles in the absence of steric stabilizing agents 18 . In this paper, we present a new approach for the synthesis of Ag(cid:5)Pd bimetallic hollow nanoparticles in a system consisting of a number of coiled flow inverter (CFI) microreactors connected in series. This approach allows the tempo and spatial separation of the formation of silver nanoparticles seeds with controllable sizes and the consequent galvanic replacement reaction in the presence of a palladium precursor. We also demonstrate that the co(cid:5)presence of hydroquinone as mild reducing agent during the galvanic displacement step, leads to its kinetic enhancement due to the simultaneous co(cid:5)reduction of silver and palladium. Finally, 4(cid:5)nitrophenol reduction reaction was used to show the enhanced catalytic activity of the hollow Ag(cid:5)Pd bimetallic nanoparticles respect to the monometallic and solid counterparts. The catalytic activities of synthesized Ag/Pd bimetallic nanoparticles were evaluated using the reduction of 4(cid:5)nitrophenol with NaBH 4 as a model reaction 19 . The reaction was carried out in 4.5 mL cuvettes with a path length of 1 cm. The total volume was fixed as 3 mL with 1mL 4(cid:5)nitrophenol concentration of 10 (cid:5)4 M and 2 mL NaBH 4 concentration of 10 (cid:5)1 M. The reaction concentrations of 4(cid:5)NP and NaBH 4 were 3.3×10 (cid:5)5 M and 6.6×10 (cid:5)2 M, respectively. The reaction was started with the addition of 5 QL of as(cid:5)prepared nanoparticles at room temperature. Immediately after particle addition, time(cid:5)dependent ultraviolet(cid:5)visible (UV(cid:5)vis) absorbance spectra were recorded with a time interval of 8 seconds. The background correction was done with deionized water as reference.

1 Ke Jun Wu, Yunhu Gao, and Laura Torrente Murciano* microreactors, continuous synthesis, hollow particles, Ag Pd bimetallic nanoparticles, 4 nitrophenol reduction Hollow bimetallic nanoparticles exhibit unique surface plasmonic properties, enhanced catalytic activities and high photo thermal conversion efficiencies amongst other properties, however, their research and further deployment are currently limited by their complicated multi step syntheses. This paper presents a novel approach for their continuous synthesis with controllable and tuneable sizes and compositions. This robust manufacturing tool, consisting of coiled flow inverter (CFI) reactors connected in series, allows for the first time the tempo and spatial separation of the initial formation of silver seeds and their subsequent galvanic displacement reaction in the presence of a palladium precursor, leading to the full control of both steps separately. We have also demonstrated that coupling the galvanic replacement and co reduction leads to a great kinetic enhancement of the system leading to a high yield process of hollow bimetallic nanoparticles, directly applicable to other metal combinations. 2 Palladium nanoparticles have been widely used as catalysts for a variety of reactions including oxidation, hydrogenation, reduction, C C coupling, among others 1, 2, 3 . Moreover, the applications of palladium based bimetallic nanostructures, e.g. Pt/Pd and Au/Pd, for oxidation and reduction reactions have been widely reported 4 8 . Among various metals, silver has been a very promising choice to form Ag Pd bimetallic nanostructures with improved catalytic performance due to the unique synergistic interaction between Ag and Pd 9 . It is well known that the catalytic performance of Ag Pd NPs strongly depends on their size, composition, surface modification (i.e. surfactants, ligands, and coordinating solvents) and morphology (i.e. hollow and solid). In recent years, bimetallic nanomaterials with hollow structures have also attracted the research focus as they exhibit unique surface plasmonic and catalytic properties, which differ from their non hollow counterpart structures 10,11 . While bimetallic compositions allow for the combination and/or synergy of catalytic properties between their two metal components, their hollow interiors offer enhanced plasmonic properties, higher photo thermal conversion and higher surface to volume ratios relative to solid structures 12,13 . Galvanic replacement reaction has emerged as a powerful and versatile route for the synthesis of nanomaterials with hollow structures as the size and morphology of the final product can be readily manipulated by using different types of sacrificial templates and precisely controlling the extent of replacement 14 . Ag based nanocrystals have been frequently used as sacrificial templates to produce Au, Pd, and Pt hollow nanostructures 1, 15 .
However, conventional synthesis of these hollow structures in batch processes present a number of difficulties to achieve a controlled system, especially in the case of bimetallic nanoparticles due to their multi step synthetic methods. In addition, the presence of organic stabilizers or ligands, which are usually required in batch processes, limit the size control of the resulting particles as well as potentially interfering during their catalytic applications by blocking their active sites 16 . Consequently, reliable synthetic protocols for Ag Pd bimetallic nanoparticles (NPs) with well defined and tuneable size, composition, and morphology without any organic stabilizing agent are highly demanded.

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In this context, microreactors are presented as a continuous manufacturing platform for the controllable synthesis of nanoparticles 17 . Nanoparticles with defined size, composition, and morphology can be synthesized by readily adjusting the operation parameters such as flow rate, reactor length, reactant concentration and reaction temperature. In addition, and even more importantly, we have recently demonstrated that the laminar flow regime characteristic of micro devices allows the production of nanoparticles in the absence of steric stabilizing agents 18 . In this paper, we present a new approach for the synthesis of Ag Pd bimetallic hollow nanoparticles in a system consisting of a number of coiled flow inverter (CFI) microreactors connected in series. This approach allows the tempo and spatial separation of the formation of silver nanoparticles seeds with controllable sizes and the consequent galvanic replacement reaction in the presence of a palladium precursor. We also demonstrate that the co presence of hydroquinone as mild reducing agent during the galvanic displacement step, leads to its kinetic enhancement due to the simultaneous co reduction of silver and palladium. Finally, 4 nitrophenol reduction reaction was used to show the enhanced catalytic activity of the hollow Ag Pd bimetallic nanoparticles respect to the monometallic and solid counterparts.
Reagents and chemicals used in this work, including silver nitrate solution (AgNO 3   The catalytic activities of synthesized Ag/Pd bimetallic nanoparticles were evaluated using the reduction of 4 nitrophenol with NaBH 4 as a model reaction 19 . The reaction was carried out Reactor 1 was optimised as 1:2 to ensure the synthesis of narrow sized silver seeds and the full consumption of NaBH 4 by silver reduction and hydrolysis, reactions (1) and (2) respectively.
Due to the absence of organic capping ligands, Na 3 CA was added in Reactor 1 to stabilize the silver seeds electrostatically. It is important to note that under these conditions (60°C), sodium citrate does not reduce silver however, it releases OH ions due its dissolution in In this way, silver seeds with particle size of 5.4 ± 1.0 nm (Figure 2a c) were synthesized in Reactor 1 at 60 ˚C with a AgNO 3 :NaBH 4 :Na 3 CA ratio of 2:1:7. This initial seeds were grown in Reactor 2 at 90°C adopting a seed mediated method by mixing with additional AgNO 3 solution (2 mM ) and using Na 3 CA as mild reducing agent to avoid secondary nucleation. Ag As the galvanic displacement reaction (5) takes place in the presence of HQ, additional reduction of Pd 2+ and re dissolved Ag + takes place simultaneously, further promoting the galvanic displacement reaction and leading to bimetallic particles through alloying and dealloying reactions (6 8).
Ag x (s) + yPd(s) → Ag x Pd y (s) (alloying) Ag x Pd y (s) + zPd(s) → Ag x Pd y+z (s) (alloying) Ag x Pd y (s) + Pd 2+ (aq) → Ag x 2 Pd y+1 (s) + 2Ag + (aq) (dealloying) In addition, re dissolved Ag + can rapidly react with Cl ions to form AgCl precipitates following reaction (9)       In order to gain further insights of the effect of coupling the galvanic displacement and co reduction reactions, an additional CFI microreactor was added into the system described above. In this case, silver seeds with average size of 9.4 ± 1.8 nm were formed in Reactors 1 and 2 as described before. However, only Pd(NO 3 ) 2 was added into Reactor 3 (residence time 12 min, 60°C) and HQ was added in the new Reactor 4 (residence time 12 min, 60°C) to separate the galvanic displacement reaction from further reduction respectively. The concentrations and flowrate of reactants were kept equal to the previous systems in all cases.
The UV vis spectra after Reactors 2, 3 and 4 are shown in (Figure 8). When Pd(NO 3 ) 2 was introduced in Reactor 3, the galvanic replacement reaction (5) took place as indicated by the disappearance of the absorbance peak at 402 nm characteristic of the Ag seeds formed in Reactor 2. The appearance of a new peak at 225 nm is due to the Pd(NO 3 ) 2 unreacted in solution however, the shoulder of this peak is likely to be caused by the presence of Pd nanoparticles with absorbance at ~240 nm. As these particles have a hollow structure, their extinction coefficient is larger than their solid counterparts 13 . Finally, when HQ was added in Reactor 4, a clear Ag Pd NP absorbance peak at 246 nm appears. It is important to note that this absorbance peak is not as sharp as the one observed in the previous case where the galvanic displacement reaction and the HQ reduction took place simultaneously from the beginning in Reactor 3 suggesting that re reduction of Ag + ions promotes the galvanic displacement kinetics, considerably quicker than the reduction of Pd 2+ by HQ. In addition, the Ag Pd peak is much broader than in Figure 8    14 Further understanding of the formation of the hollow bimetallic nanoparticles was gained by carrying out a simultaneous reduction of Ag(NO 3 ) and Pd(NO 3 ) 2 by HQ in a single reactor at 60°C as depicted in Figure 10. Similarly to above, the product was characterised by UV vis spectroscopy. Figure 11 shows the appearance of two absorbance peaks at 288 nm and 246 nm assigned to Ag Pd nanoparticles and HQ in solution, respectively. Negligible absorbance at ~400 nm was observed indicating that monometallic silver NPs were not formed under these conditions.
The saturation of the detector at low wavelengths (< 230 nm) was due to the presence of   The formation of a Ag Pd alloy by simultaneous reduction was confirmed by EDS mapping.
As shown in the scan line analysis, the total metal intensity, Pd and Ag, across the particle increases in the middle of the particle in agreement to its solid nature. At the edges of the particle, Pd element has a higher intensity than Ag due to the Pd:Ag ratio (4:1), however, in the centre of the particle, there is a higher presence of Ag, suggesting an Ag rich core. As shown in Figure 13c d, Ag and Pd are uniformly dispersed in the entire nanoparticles.
Moreover, EDX elemental line scanning of the nanoparticles (Figure 13b) also verified the presence of the alloy structure in these NPs.

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To evaluate the catalytic performance of the prepared Ag Pd bimetallic NPs, the reduction of 4 NP reaction was used here as model reaction ( Figure 14). The absorbance peak of the 4 NP at 400 nm is constant and stable in the absence of a catalyst. Upon the addition of the colloidal particles, the absorbance peak at 400 nm significantly decreased with time and a new peak around 300 nm corresponding to 4 aminophenol (4 AP) gradually developed. A typical evolution of the UV vis spectra as the reaction time progresses using hollow Ag Pd NPs synthesized using 3 CFI reactors in series and Pd(NO 3 ) 2 as precursor is shown in Figure   15a. The reduction of 4 NP is considered to be a pseudo first order reaction respect to the concentration of 4 NP when an excess of NaBH 4 (2000 fold excess in the present study) is used 24 . Thus, the rate of consumption of 4 NP, t , is often defined as: where A is the concentration of 4 NP, and app is the apparent rate constant in s 1 .
As the absorbance of the solution is proportional to the 4 NP concentration (according to Beer Lambert law), simple derivation of equation (10) leads to a linear correlation between the initial absorbance ( 0 ), the absorbance at a given time ( t ) and the apparent rate constant

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Hollow bimetallic silver palladium nanoparticles present an enhanced catalytic activity for the reduction of 4 nitrophenol reaction in comparison to their solid or monometallic counterparts. They can be selectively synthesised in a continuous system consisting of a number of coiled flow inverter (CFI) microreactors connected in series. In this way, the initial formation of silver seeds with tuneable sizes can be separated from the galvanic displacement reaction gaining full control of both steps. The nature of the palladium precursor has a key effect in the system, not only determining the feasibility of the galvanic displacement reaction depending on its reduction potential but also, in the formation of by products such as AgCl which greatly reduces the yield. Finally, the galvanic displacement reaction can be greatly enhanced in the presence of a mild reducing agent such as hydroquinone.