Mesoporous Bimetallic RhCu Alloy Nanospheres Using a

The University of Queensland, Brisbane QLD 4072, Australia. Chem. Mater. , 2018, 30 (2), pp 428–435. DOI: 10.1021/acs.chemmater.7b04307. Publica...
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Mesoporous Bimetallic RhCu Alloy Nanospheres Using a Sophisticated Soft-Templating Strategy Bo Jiang,† Kenya Kani,†,‡ Muhammad Iqbal,†,§ Hideki Abe,*,¶ Tatsuo Kimura,□ Md. Shahriar A. Hossain,‡ Oruganti Anjaneyulu,¶ Joel Henzie,*,† and Yusuke Yamauchi*,‡,§,○ †

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Australian Institute for Innovative Materials (AIIM), University of Wollongong (UOW), Squires Way, North Wollongong, NSW 2500, Australia § Department of Nanoscience and Nanoengineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan ¶ Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan □ National Institute of Advanced Industrial Science and Technology (AIST), Shimoshidami, Moriyama, Nagoya 463-8560, Japan ○ School of Chemical Engineering & Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane QLD 4072, Australia S Supporting Information *

ABSTRACT: Mesoporous metals are useful for heterogeneous catalysis because their ultrapermeable architecture promotes mass transport to abundant active sites. Our group recently developed a method to manipulate the interior space of nanocrystals by synthesizing mesoporous Rh nanospheres that serve as high performance catalysts for methanol oxidation and nitric oxide (NO) remediation (Jiang, B.; et al. Nat. Commun. 2017, 8, 15581). Here, we extend this concept and propose a sophisticated soft-templating strategy to synthesize three-dimensional mesoporous bimetallic RhCu nanospheres. The nanospheres are alloys of Rh and Cu, and their compositions can be continuously tuned by varying the amount of metal precursors. The mesoporous RhCu nanospheres were examined for catalytic remediation of NO and show good catalytic performance considering the reduced economic cost of the alloyed material. This method offers a general approach for the precise design of high surface area Rh-based metal catalysts.

1. INTRODUCTION Rh is an important precious metal because it can catalyze a diverse range of chemical reactions, including the selective hydrogenation of fine chemicals, energy generation via fuel cells, and remediation of toxic gases.1−3 Increasing the surface area of Rh-based heterogeneous catalysts has a dual advantage of increasing material utilization efficiency and presenting additional catalytically active sites at the atomic steps, corners, and defects of the crystal.4−6 For example, Huang et al. provided a platform to carry out the structure-dependent catalytic investigation toward electrocatalytic application via systematically demonstrating three types of Rh nanocrystals (tetrahedron, concave tetrahedron, and nanosheet).7 Our group recently developed a method to manipulate the interior space of nanocrystals by synthesizing mesoporous Rh nanospheres which serve as high performance catalysts for methanol oxidation and nitric oxide (NO) remediation.8 Additionally, ultrathin Rh nanosheets with abundant exposed Rh atoms © XXXX American Chemical Society

showed excellent performance for hydrogenation and hydroformylation reactions.9 Yet, despite the progress on developing nanostructured Rh catalysts, the costliness and lack of earth abundance of Rh metal is an unavoidable drawback that limits large-scale applications. Combining noble metals with industrial metals in alloyed form or as core−shell nanostructures can help mitigate cost by substituting a portion of noble metals with an inexpensive component.10−14 Besides the lower economic cost, the properties of bimetallic alloys can be continuously adjusted via the atomic composition. Additionally, electronic and strain effects caused by the different chemical characteristics of the metal atoms yield surfaces with more favorable reactant adsorption, activation, and product desorption properties that Received: October 13, 2017 Revised: November 29, 2017

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DOI: 10.1021/acs.chemmater.7b04307 Chem. Mater. XXXX, XXX, XXX−XXX

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Scheme 1. Mesoporous RhCu Nanospheres Form by the Chemical Reduction of Rh and Cu Precursors on Self-Assembled Polymeric PEO-b-PMMA Micelle Templatesa

a Ascorbic acid serves as the reducing agent, while DMF and H2O are the cosolvents. The synthetic process can be divided into five main steps: (1) Addition of water causes the PEO-b-PMMA copolymers to self-assemble into spherical micelles with a PMMA core and a PEO shell. (2) Na3RhCl6, CuCl2, and ascorbic acid are dissolved into the reaction solution. The metal ions form aqua-complexes with the PEO moieties via hydrogen bonding. (3) The Rh and Cu species are coreduced to begin to nucleate. (4) The particles grow and eventually envelope the micelle templates. (5) The templates are finally removed by solvent extraction.

help facilitate catalytic reactions.15,16 Balancing the catalytic activity and selectivity of alloyed precious metals from the perspective of economic cost is a challenging and important consideration for the application of nanomaterials in realistic technologies. The high surface area and connectivity of mesoporous/ nanoporous architectures enable several improvements over conventional catalysts.17−19 Making bimetallic interconnected nanostructured networks is the next logical step to realize more efficient utilization of precious metals in high surface area; and reagent permeable catalysts.20−22 Hard-templating, soft-templating, and dealloying are just some of the many ways to generate mesoporous/nanoporous metals.23 In most cases, however, the recent success in synthesizing bimetallic metals has been limited to Pt- and Pd-based catalysts.24−28 Rh-based alloys are particularly challenging in the context of nanoporous Rh-based alloys because the surface energy of Rh is much higher than those of similar noble metals (e.g., Pt, Au, and Pd). Therefore, a more refined approach is required to generate Rhbased alloys with nanoporous structures. Here, we describe a simple method to synthesize mesoporous bimetallic RhCu nanospheres via a soft-templating method using polymeric micelles made of diblock copolymer, poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-bPMMA), as illustrated in Scheme 1. Na3RhCl6 and CuCl2 were used as metal precursors, and ascorbic acid (AA) served as the reducing agent. According to this synthetic strategy, the atomic composition and catalytic activity of the products can be tuned by adjusting the ratio of metal precursors.

in 0.8 mL of N,N-dimethylformamide (DMF). The following reagents were added to the DMF solution in this order: 0.2 mL of hydrochloric acid (HCl: 1 M), 1 mL of aqueous sodium hexachlororhodate (Na3RhCl6: 40 mM), 1 mL of aqueous copper chloride (CuCl2: 40 mM), and 2 mL of aqueous ascorbic acid (AA: 100 mM), resulting in a solution with a transparent light-brown color. The solution was kept in a water bath at 80 °C for 2 h until the color changed from light-brown to black. Then, the samples were recovered by centrifugation at 14 000 rpm for 20 min, and the residual polymer was removed by five consecutive washing/centrifugation cycles with acetone and water. Characterization. The morphology and structure of the mesoporous Rh-based bimetallic catalysts was initially examined with field emission scanning electron microscopy (FESEM; HITACHI SU8000; accelerating voltage = 5 kV). The interior structure of the catalysts was observed with high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100F; accelerating voltage = 200 kV). The samples for TEM and HRTEM observations were prepared by depositing a drop of the diluted colloidal suspension on a TEM grid. Powder X-ray diffraction (XRD) measurements were conducted on a SmartLab X-ray diffractometer (RIGAKU) at a scanning rate of 2° min−1 with a Cu Kα radiation (40 kV, 30 mA) source. X-ray photoelectron spectroscopy (XPS) was performed on a JPS-9010TR (JEOL) instrument with an Mg Kα X-ray source. All the binding energies were calibrated using the C 1s binding energy peak (284.6 eV) as a reference. Inductively coupled plasma optical emission spectrometer (ICP-OES) was performed on a Hitachi model SPS3520UV-DD. Nitrogen adsorption−desorption isotherms were acquired by using a BELSORP-mini (BEL, Japan) at 77 K, and surface area was estimated using the multipoint Brunauer−Emmett−Teller (BET) method. NO Remediation Tests. Five milligrams of the Rh-based bimetallic catalyst was mixed with 95 mg of Al2O3 powder to test its performance for NO remediation. Commercial Al2O3-supported Rh nanoparticles with a 5 wt% loading fraction (Rh diameter = 2−3 nm) were used as a reference for comparison. Each catalyst was exposed to a steady flow of simulated exhaust containing equimolar amounts of nitrogen oxide (NO), carbon monoxide (CO), and helium (He) gas (NO: CO: He = 1:1:98; total flow = 50 mL min−1) at temperatures

2. EXPERIMENTAL SECTION Preparation of Mesoporous RhCu Nanospheres. In a typical synthesis, 5 mg of PEO-b-PMMA block copolymer with a molecular weight of 10 500 and 18 000 for each block was completely dissolved B

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Chemistry of Materials spanning room temperature to 350 °C. The chemical composition of the effluent gas was analyzed with a Fourier transform infrared spectrometer (FT-IR, Shimadzu Prestige 21) equipped with a gas cell.

prior to the growth phase, resulting in smaller particles in the final products.29,30 Interestingly, large differences in reduction potential can lead to the formation of core−shell nanoparticles, as was observed in the case of mesoporous Pd@Pt,31 but we will show below that RhCu actually forms alloys. The reduction rates can be roughly assessed by the progression of color in the reaction because nanospheres composed of optically lossy metals such as Rh are typically black in solution (Figure S5). Reactions prepared with a high ratio of Rh:Cu precursor (i.e., Rh:Cu = 80:20) quickly turned from light to dark brown in ∼6 min and then to totally black in ∼10 min, indicating a fast nucleation time (Figure S5a). Reactions with a higher ratio of Cu (i.e., Rh:Cu = 20:80) were much slower and changed from brown to black in ∼30 min (Figure S5b). Reactions that nucleate fast and generate more nuclei tend to yield smaller particles. Thus, we should expect that higher Cu concentration (i.e., decreasing the Rh concentration) will cause the average size of the RhCu nanospheres to increase, which will become clear in SEM/ TEM. Wide-angle XRD measurements were used to investigate the crystal structure and level of alloying of the RhCu samples (Figure 2a). The mesoporous RhCu nanospheres have the typical face centered cubic (fcc) structure with four strong diffraction peaks. The diffraction peaks were located between the typical positions for single-phase Rh and Cu metals, indicating that the partial substitution of Rh atoms by smaller Cu atoms. Additionally, there were no observable diffraction peaks from single-component Rh, Cu or corresponding oxides. As the molar ratio of Cu was increased in the samples the diffraction peaks gradually shifted to larger 2θ values. A linear relationship between the crystal lattice parameter and the content of the constituent elements strongly suggests the formation of an alloyed structure instead of a core−shell structure (Figure 2b). The average sizes of the constituent particles that make up the nanospheres can be calculated from the (111) diffraction peak using the Scherrer equation. The sizes are 9.4, 10.3, 9.6, 9.6, 8.8, and 13.1 nm for the Rh100, Rh 82 Cu 18 , Rh 69 Cu 31 , Rh55 Cu 45, Rh 43 Cu 57 , and Rh 24Cu 76 samples, respectively. The specific surface areas of the samples were determined by the nitrogen adsorption−desorption isotherms and are summarized in Figure S6. The structural details and elemental distributions were further examined by TEM. TEM images (Figure 3a and Figure S7a) show that the porous structure of Rh82Cu18 nanospheres consisted of many tiny subnanoparticles. The selected-area electron diffraction (SAED) pattern had concentric, spotty diffraction rings, confirming that the nanoparticles are polycrystalline (Figure S7b). Closer observation with highangle annular dark-field scanning TEM (HAADF-STEM) shows porous structure can be distinguished by the difference in contrast at the edges of the particles (Figure 3b). It is expected that the higher reduction potential of Rh compared to Cu species will lead to nanospheres with Rh-rich cores and Curich shells because the kinetics of the chemical reaction favors a greater proportion of Rh being reduced faster during nucleation, leaving more Cu precursor to deposit during the growth phase. Surprisingly, this was not the case, and elemental mapping with STEM-EDS revealed that the Rh and Cu elements were homogeneously distributed throughout the entire particle (Figures 3c−f). Both Rh55Cu45 and Rh82Cu18 had similar homogeneity in TEM (Figure 3 and S8). Additional experiments show that the reduction of the Cu precursor

3. RESULTS AND DISCUSSION Synthesis of mesoporous RhCu nanospheres is based on the chemical reduction of Rh and Cu species on polymeric micelle templates (Scheme 1). The block polymer PEO-b-PMMA dissolves as a unimer in neat DMF because both hydrophilic PEO and hydrophobic PMMA segments are compatible with aprotic polar solvents (Figure S1a). Adding water to the mixture causes the hydrophobic PMMA segments to aggregate and drives the assembly of spherical PEO-b-PMMA micelles composed of PMMA cores surrounded by PEO shells. This process of assembly is compatible with aqueous solutions of metal precursors and reducing agents (e.g., Na3RhCl6(aq), CuCl2(aq), AA(aq)). The initial formation of micelles in solution is illustrated by the Tyndall effect (Figure S1b). The polymeric micelles can be imaged by TEM using the phosphotungstic (PW) acid negative staining technique. PW acid localizes in the PEO, resulting in an outline around the PMMA micellar cores because PMMA now has an average electron density much lower than that of the PW−PEO conjugate (Figure S2). The average size of the PMMA cores is ca. 15.5 nm. TEM also shows that there is no significant change in the shape of the micelle after addition of the metal precursors and reducing agent, indicating that the ionic Rh and Cu precursors did not interfere with the micellar superstructure during the formation of the mesoporous RhCu nanospheres. Determining the composition of the RhCu nanospheres is important because it will impact performance in catalysis. ICPOES was used to measure the amount of Rh and Cu in a series of samples designated as RhxCu100−x, where x and 100−x refer to the atomic ratio of Rh and Cu, respectively. These measurements show that the Rh:Cu ratio is slightly different than the Rh:Cu ratio of the precursor solutions (Table 1). Cu2+ Table 1. Molar Ratios of Precursors and Products and Their Parameters in the Colloidal Reactions samples

molar ratio of Rh:Cu in the precursors

molar ratio of Rh:Cu in the products

Rh100 Rh82Cu18 Rh69Cu31 Rh55Cu45 Rh43Cu57 Rh24Cu76

100:0 80:20 65:35 50:50 35:65 15:85

100:0 82:18 69:31 55:45 43:57 24:76

average particle size (nm) 74 73 81 91 107 135

± ± ± ± ± ±

3 3 3 3 3 3

average pore size (nm) 12 12 13 14 14 14

± ± ± ± ± ±

1 1 1 1 1 1

species have a reduction potential (Cu2+/Cu: +0.34 V vs SHE), which is lower than that of Rh3+ (Rh3+/Rh: +0.44 V vs SHE), indicating that some portion of the Cu species is not reduced in the reaction. The shape and mesoporous structure of the RhCu nanospheres were initially characterized by SEM. The samples were composed of well-dispersed, uniformly sized spheres with well-defined mesopores distributed over the entire external surface (Figure 1 and Figure S3). Increasing the Cu concentration caused the average size of the RhCu nanospheres to increase from 74 to 135 nm (Figure S4 and Table 1). Obviously, metal species with higher reduction potentials will reduce more quickly than ones with lower reduction potentials, so it is expected that the Rh species will create more nuclei C

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Figure 1. SEM images of (a) mesoporous Rh100, (b) mesoporous Rh82Cu18, (c) mesoporous Rh69Cu31, (d) mesoporous Rh55Cu45, (e) mesoporous Rh43Cu57, and (f) mesoporous Rh24Cu76 samples.

Figure 2. (a) Wide-angle XRD patterns of mesoporous Rh100, Rh82Cu18, Rh69Cu31, Rh55Cu45, Rh43Cu57, and Rh24Cu76 samples and (b) the relationship between the (111) d-spacing and Cu molar fraction.

particles taken from 6 min into the reaction have an average size of ∼32 nm (Figure S10a). At this stage, the pores on the surface of the particles could not be observed. The XRD data confirm that the alloyed RhCu nanocrystals already formed via the coreduction of Rh and Cu species (Figure S11). With longer reaction times, the particle sizes of RhCu increase from ∼43 and ∼52 nm at 8 and 10 min, respectively. This size increase is accompanied by the formation of distinct porous

cannot occur in the absence of Rh precursor (Figure S9), indicating that the initial tiny Rh seeds induce the coreduction of Rh and Cu species for the formation of the alloy structure, and phase separation, if any, is too small to observe in STEMEDS.32,33 The Rh82Cu18 stoichiometry was used to investigate the formation of the mesoporous nanospheres. At this stoichiometry, the reaction proceeds very quickly, and SEM images of D

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Figure 3. (a) TEM image, (b) HAADF-STEM, (c−e) elemental mapping images, and (f) line-scanning compositional profile of mesoporous Rh82Cu18 nanospheres.

Rh oxides, were present as well. The metallic Rh0 has binding energies of 307.2 and 312.0 eV in the doublets of 3d5/2 and 3d3/2, respectively (Figure 4a).34 The binding energy of the Cu 2p3/2 and Cu 2p1/2 peaks were located at 932.1 and 951.9 eV, respectively, and match metallic Cu0 (Figure 4b).13 However, one satellite could be observed between the Cu doublets, suggesting that Cu2+ (CuO) is present on the surface of the RhCu nanospheres. Additionally, the Rh 3d binding energy in the mesoporous RhCu nanospheres negatively shifts compared to Rh 3d in pure Rh nanospheres (Figure 4c), indicating that there are electronic interactions between Rh and Cu.35 The surface composition ratio of Rh:Cu is 85:15 according to XPS, roughly matching the ICP data (Table 1), and offering further proof that there is no phase segregation of the two component metals. The catalytic performance of the mesoporous RhCu nanospheres was compared to the following catalysts for the remediation of automobile exhaust: (i) mesoporous Rh nanospheres (supported over Al2O3, Rh loading = 5 wt%) and (ii) commercial Al2O3-supported Rh nanoparticles (2−3 nm Rh nanoparticles) (Rh/Al2O3; Aldrich, Rh loading = 5 wt %) (Figure S13). Each of the catalysts was subjected to a steady stream of simulated gasoline engine exhaust consisting of NO, CO, and He gas as the balance (NO:CO:He = 1:1:98 volumetric ratio) at temperatures ranging from room temperature to 350 °C (Figure 5).

network extending through the nanospheres (Figures S10b and c). The shape and porosity of the nanospheres did not change between 18 and 60 min, but the size increased to 63 nm and then to 73 nm (Figures S10d and e). Synthesizing the nanospheres in the absence of PEO-b-PMMA resulted in nonporous RhCu, showing that PEO-b-PMMA served as poredirecting agent (Figure S12). On the basis of the above results, the mechanism formation of RhCu can be explained as follows: Rh and Cu precursors initially adsorb on the PEO group of the PEO-b-PMMA micelles via hydrogen bonding. After addition of the reducing agent (i.e., ascorbic acid) at 80 °C, the Rh ions are reduced to form tiny nuclei that trigger the coreduction of Rh and Cu ions. At this stage, the PEO−PMMA micelles serve as protecting agent to stabilize the primary RhCu clusters through the interactions with the PEO chains of the micelles. As the reaction proceeds, the reduced RhCu clusters from the solution continue to deposit on the spherical PEO-b-PMMA micelles via autocatalytic growth mechanism. At this stage, the micelles acted as pore-directing agent. After removal of the template, the porous RhCu nanospheres were obtained. Assessing the oxidation states of Rh and Cu in the alloy is useful in discovering the structure of the surface and potential for catalysis. XPS measurements on the Rh82Cu18 sample show two 3d5/2 and 3d3/2 doublets in the high-resolution Rh0 3d spectrum (Figure 4a). Two species, including metallic Rh0 and E

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Figure 4. (a and b) High resolution XPS spectra of mesoporous Rh82Cu18 nanospheres: (a) Rh 3d and (b) Cu 2p. (c) Comparison of the Rh 3d spectra for mesoporous Rh82Cu18 and Rh100 nanospheres.

Figure 5. NO conversation through (a) mesoporous Rh100 (b), commercial Rh100, (c) mesoporous Rh82Cu18, (d) mesoporous Rh69Cu31, (e) mesoporous Rh55Cu45, and (f) mesoporous Rh24Cu76 samples, respectively.

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