Continuous Flow Synthesis of Rh and RhAg Alloy ... - ACS Publications

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Continuous Flow Synthesis of Rh and RhAg Alloy Nanoparticle Catalysts Enables Scalable Production and Improved Morphological Control Pranaw Kunal,†,⊥ Emily J. Roberts,‡,⊥ Carson T. Riche,§ Karalee Jarvis,∥ Noah Malmstadt,*,‡,§ Richard L. Brutchey,*,‡ and Simon M. Humphrey*,† †

Department of Chemistry, The University of Texas at Austin, 6.336 Norman Hackerman Building, 100 East 24th Street, Stop A1590, Austin, Texas 78712, United States ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States § Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, United States ∥ Texas Materials Institute, The University of Texas at Austin, 204 East Dean Keeton Street, Stop C2201, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: The pursuit of scalable methods for the preparation of well-defined metallic nanoparticles (MNPs) is addressed in this work via a novel microwave-assisted continuous flow synthesis technique. It is shown that single- and two-phase flow synthesis methods provide access to morphologically well-defined and near-monodisperse RhNPs. The RhNPs can be prepared in shorter reaction times and at lower temperatures than are commonly required in conventional batch reactions. Under single-phase flow conditions, in which Rh(III) is reduced in ethylene glycol, near-monodisperse cuboctahedral RhNPs are obtained; the average NP size can be controlled as a function of the residence time of the reactant stream within the microwave cavity. In contrast, a two-phase microfluidic droplet flow method leads to the highly selective formation of Rh multipods. When compared to cuboctahedral RhNPs of comparable size, the Rh multipods are found to exhibit significantly higher catalytic activity in the vapor-phase hydrogenation of cyclohexene. The potential versatility of this new two-phase flow method coupled with microwave-assisted heating is further demonstrated in the synthesis of well-defined isotropic NPs comprised of classically immiscible RhAg random alloys.

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INTRODUCTION

approach is especially valuable when preparing colloidal catalysts with noble metals (e.g., Rh, Ir, Pd, and Pt) because relationships between surface structure, reactivity, and selectivity for these face-centered-cubic (FCC) metals are well-understood from single-crystal studies.3 Noble metals are employed as catalysts in a large number of industrial-scale catalytic processes.4 The thermodynamically most stable morphology for small (2−10 nm) MNPs are cuboctahedra, which display a mixture of (111) and (100) faces. It is wellknown that metal surface structure can favor different catalytic pathways (even for simple, small molecule reactions) due to significant differences in reactant binding energies associated with each face.5,6 For example, alkenes can undergo hydro-

The synthesis of well-defined metallic nanoparticles (MNPs) is becoming increasingly central to the field of heterogeneous catalysis.1 Small (2−10 nm) MNPs display large surface areato-volume ratios and exhibit other advantageous chemical and physical properties that are manifested at the nanoscale. Such properties can result in increased catalytic activity, selectivity, and/or stability. A distinct advantage of MNP catalysts is their potential to be synthesized with well-defined morphologies via solution-phase “bottom-up” approaches; this is in stark contrast to the poorly defined morphologies typically achieved by traditional methods (e.g., incipient wetness impregnation of metal precursors followed by in situ nucleation and growth).2 Morphological control is important because it dictates the MNP surface structure (i.e., the types of facets that are presented in individual nanocrystals). The ability to engineer MNP morphologies can ultimately engender control over catalytic reactivity and selectivity for specific reactions. This © 2017 American Chemical Society

Received: February 17, 2017 Revised: April 12, 2017 Published: April 16, 2017 4341

DOI: 10.1021/acs.chemmater.7b00694 Chem. Mater. 2017, 29, 4341−4350

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continuous flow methods are highly amenable to automation, which affords improved product fidelity and higher throughput. Scale-up can be further enhanced by reaction parallelization, since the local reaction conditions are invariant in terms of mixing uniformity and temperature distribution.23,32 Here, we present highly encouraging initial results from studies of the μwI-assisted synthesis of RhNPs and randomly alloyed RhAgNPs under continuous flow synthesis, using both one-phase and two-phase flow. Interestingly, stark differences in morphology of the MNPs were observed for the two different flow configurations. Single-phase continuous flow reactions led to monodisperse cuboctahedral MNPs, which directly mirrors the outcome of the previously studied batch reactions. However, reactions carried out using two-phase droplet flow resulted in very disparate products: well-defined multipods were obtained that have greatly improved surface area-tovolume ratios vs cuboctahedra and were found to predominantly display (111) surface structure. Variation of heat transfer in the two-phase flow regime resulted in the ability to access desirable Rh multipod structures with a high (>90%) degree of morphological selectivity. A model assessment of the comparative catalytic activity of the RhNPs showed that multipods have superior activity in the vapor-phase hydrogenation of alkenes. The kinetic multipod structures were surprisingly stable over long on-stream catalysis reaction times and upon prolonged heating at 175 °C, resisting conversion to thermodynamically more favorable cuboctahedra.

genation or dehydrogenation as a function of surface structure effects.7−10 Over the past 2 decades, a great deal of effort has been made to gain a better understanding of how a careful choice of solvents and ligands used in MNP synthesis can directly influence particle size11 and morphology.12,13 However, inherent issues that prevent such syntheses from being easily scaled-up is an unsolved problem that presently impedes the utilization of well-defined MNPs in large-scale industrial catalytic processes. For example, batch-type syntheses of colloidal MNPs are extremely sensitive to heat and mass transport limitations, which become increasingly problematic for larger reactor volumes and/or higher reagent concentrations.14 These scaling issues negatively influence nucleation and growth and result in lower monodispersity, inferior process reproducibility, and morphological inconsistency. One emerging solution to this problem is to conduct multiple, parallel syntheses using established laboratory-scale batch chemistries, but in a continuous flow arrangement. Flow chemistry conducted in micro- or millifluidic channels provides excellent control over heat and mass transport. Thus, large quantities of products can be obtained without compromising the quality of products.15,16 Micro- and millifluidic continuous flow strategies for the synthesis of NPs fall into two categories: (a) single-phase flow approaches, in which the reactants are simply mixed and flowed through narrow (typically 0.6 depending on specific morphology but only 0.15 for the 12 nm Rh cuboctahedra (Scheme 3). The smaller difference in measured SS-TOF values between μwI- and CvH-Rh multipods can be explained by the fact that the percent occurrence of multipods was higher in the μwI reaction (94% vs 72% for CvH). An alternative explanation is that the different heating methods lead to differences in the number of accessible surface sites; however, the measured activation energy (Ea) values for the two multipod samples were found to be within experimental error (μwI = 44.4 kJ mol−1, cf. CvH = 46.3 kJ mol−1). By comparison, the measured Ea for the RhNP

selectivity of RhNPs are intrinsically governed by surface structure, as well as the ratio of corner and edge sites to face sites. Since Rh multipods display higher surface area-to-volume ratios and have a significantly larger number of edge sites compared to isotropic (spherical) NPs, they are intriguing species for heterogeneous catalysis. Surprisingly, however, previous studies of this type of catalyst are scarce.40 In this work, we chose to assess the vapor-phase alkene hydrogenation activity of SiO2-supported Rh multipods and cuboctahedra obtained from the different continuous flow syntheses, to facilitate a direct comparison of their catalytic performance. These were also compared to the catalytic properties of RhNPs prepared by batch synthesis using otherwise identical synthetic conditions and reagents. The vapor-phase hydrogenation of cyclohexene is a useful model reaction that can be studied to extract important kinetic information.41 In this work, three separate catalysts were prepared: Rh multipods prepared in two-phase flow under both μwI and CvH, and a third control sample consisting of 12 nm Rh cuboctahedra prepared under μwI from a batch reaction. Chosen as the control were 12 nm RhNPs because the average effective diameter of the Rh multipods (appendage tip-to-tip separation distance) was close to 12 nm. Thus, the Rh multipods can be considered as hybrid structures that have been etched from 12 nm cuboctahedra. The as-synthesized RhNPs were deposited on amorphous SiO2 by simple incipient wetness impregnation, isolated by filtration, washed with ethanol, and finally dried in a static oven at 70 °C for 12 h. The composites were used in catalytic studies without harsh pretreatment (e.g., calcination) that could otherwise cause restructuring of the multipods; it is therefore assumed that the capping polymer PVP was still present in each case.42,43 The use of SiO2 as an inert substrate in this work was chosen to minimize any potential metal−support interactions. TEM analysis of the as-synthesized catalysts supported on amorphous SiO2 (Figures S9−S12) did not reveal any discernible particle aggregation or morphological changes (Rh loading, μwI = 1.4 wt %, 94% multipods; CvH = 3.3 wt %, 72% multipods; 4347

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affects the nature of the binding interactions toward PVP monomers. It is known that Ag atoms bind PVP much more weakly than Rh atoms,43 and therefore selective surface passivation effects that favor the formation of Rh multipods may be absent for mixed RhAg surfaces.

Scheme 3. Cuboctahedral and Tetrapod Geometries for Cyclohexene Hydrogenation Reaction

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CONCLUSIONS We have demonstrated for the first time the continuous flow synthesis of Rh and RhAgNPs under microwave-assisted heating. It was discovered that particle morphology and monodispersity could be finely controlled as a function of the flow synthesis method and the total residence time of reactants within the microwave cavity. Single-phase, continuous flow reactions were found to closely emulate the products obtained from batch syntheses, yielding well-defined and near-monodisperse RhNP cuboctahedra in which the average NP size was directly affected by residence time. In stark contrast, two-phase flow synthesis conducted in microfluidic droplets resulted in the isolation of well-defined Rh multipods. The same technique yielded isotropic RhAgNPs, in which Rh and Ag were randomly alloyed. Model vapor-phase cyclohexene hydrogenation studies revealed that the Rh multipods are catalytically significantly more active than cuboctahedral RhNPs, due to an increased proportion of surface atoms and lower coordinate (edge and corner) sites. Such flow synthetic methods show great potential to be used for large-scale synthesis of various types of NPs, as well as to provide access to kinetic morphologies that cannot be as easily obtained under conventional heating and from batch syntheses.

cuboctahedra catalyst was 57.2 kJ mol−1,26 which is in line with previous measurements.25 Theoretical calculations performed by our group in previous work suggested that desorption of the hydrogenated product cyclohexane product is in fact the ratelimiting step on a RhNP(111) surface.26 The significant increase in TOF values for multipods may therefore be ascribed to faster cyclohexane desorption, which is necessary to generate new vacant surface sites for adsorption of new reactants. This is not unreasonable, since both cyclohexene and cyclohexane preferentially adsorb face-on to the Rh(111) surface and occupy a 3 × 3 atom slab (Scheme 3). There are many fewer extended surfaces present in Rh multipods with the appendage dimensions studied here; on average, a 12 nm FCC Rh cuboctahedron displays (111) planes comprised of 24 × 24 atoms, whereas multipods with appendages of a 3 nm width have (111) planes limited to only 7−9 atoms (Scheme 3). Thus, desorption of cyclohexane should be statistically more favored for the multipods. In addition, an increased proportion of corner and edge sites present on the multipods should accelerate the rate of H2 dissociative chemisorption. Such low activation energy values for the multipods can be explained owing to their higher surface area-to-volume ratios and the presence of a larger proportion of highly active facets. Synthesis of Alloy RhAgNPs by Two-Phase Flow Synthesis. Given the interesting morphological control obtained for RhNPs in flowing microfluidic droplets, the synthesis of RhAg alloys was also studied. Rh and Ag are classically immiscible metals in the bulk, but it was recently shown that RhxAg100−x (x = 10−90) alloy NPs can be obtained as metastable solid solutions on the nanoscale using μwI heating.25 Based on this work, Rh70Ag30 NPs were targeted using a two-phase flow synthesis. A single solution of RhCl3· xH2O and AgNO3 dissolved in EG was prepared with a 70:30 molar ratio of Rh:Ag. The same microfluidic arrangement was utilized as for the pure Rh multipods, wherein the RhCl3· xH2O/AgNO3 solution was flowed through the μw cavity at a flow rate = 8 cm3 h−1 (0.912 mmol of Rh + Ag h−1) at 120 °C to give a total residence time of 9 min. The resulting RhAgNPs showed a clear intermediate peak shift in the PXRD pattern for the (111) reflection compared to what is observed for pure Rh or Ag NPs, which is indicative of alloying (Figure S13). Additional 2D mapping of the alloy NP surfaces by EDS and HAADF-STEM confirmed the formation of RhAg alloys (Figure S14). TEM analysis showed that the alloyed NPs were not multipods but rather a mixture of more isotropic morphologies (Figure 2I). Some particles appeared to have undergone etching, due to the presence of truncated cuboctahedra with concave faces (Figure S15).One plausible explanation for the difference in morphology between the pure Rh multipods and the RhAgNPs produced under two-phase flow conditions with μwI heating stems from the markedly different electronic properties of the NP surfaces, which in turn

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00694. Flow reaction parameters, ratios of intensity values, statistical and ICP analyses, thermal treatment and TEM images, PXRD plot, HAADF-STEM image and EDS mapping, and kinetic studies and determination of activation energies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(N.M.) E-mail: [email protected]. *(R.L.B.) E-mail: [email protected]. *(S.M.H.) E-mail: [email protected]. ORCID

Richard L. Brutchey: 0000-0002-7781-5596 Simon M. Humphrey: 0000-0001-5379-4623 Author Contributions ⊥

P.K. and E.J.R. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Dwight Romanovicz (U.T. Austin; TEM), and Dr. Vincent M. Lynch (U.T. Austin; PXRD) for analytical assistance. Funding for this research was provided by the National Science Foundation under Grant Nos. CHE-1505135 and CMMI-1068212 and the Welch Foundation (Grant No. F1738). 4348

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