In situ Exsolution of Bimetallic Rh-Ni Nano-alloys: Highly Efficient

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Energy, Environmental, and Catalysis Applications

In situ Exsolution of Bimetallic Rh-Ni Nano-alloys: Highly Efficient Catalyst for CO2 Methanation Hamidreza Arandiyan, Yuan Wang, Jason Scott, Sara Mesgari, Hongxing Dai, and Rose Amal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00889 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 10, 2018

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ACS Applied Materials & Interfaces

In situ Exsolution of Bimetallic Rh-Ni Nano-alloys: Highly Efficient Catalyst for CO2 Methanation Hamidreza Arandiyan,*a Yuan Wang,a Jason Scott,*a Sara Mesgari,b Hongxing Dai,c and Rose Amal*a

Dr. Hamidreza Arandiyan, Ms. Yuan Wang, Associate Prof. Jason Scott, and Prof. Rose Amal a

Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South

Wales, Sydney, NSW 2052, Australia

Dr. Sara Mesgari b

School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney,

NSW 2052, Australia

Prof. Hongxing Dai c

Beijing Key Laboratory for Green Catalysis and Separation, and Laboratory of Catalysis Chemistry and

Nanoscience, Beijing University of Technology, Beijing 100124, China

KEYWORDS: In situ exsolution; bimetallic Rh-Ni; perovskite catalyst; three-dimensionally ordered macropore; CO2 methanation.

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ABSTRACT

Unique CO2 methanation catalysts comprising bimetallic Ni-Rh nanoalloy/3DOM LaAlO3 have been successfully prepared via a poly(methyl methacrylate) microsphere colloidal crystal-templating route, followed by the in situ growth of Ni nanoparticles (NPs). Here we show that, unlike traditional Ni particles deposited on a perovskite support, the exsolution of Ni occurs on both the external and internal surface of the porous perovskite substrate, leading to strong metal−support interaction. Owing to the exsolution of Ni and the formation of Ni-Rh nanoalloys, a 52% enhancement in methanation turnover frequency (TOFcat) was obtained over the Ni-Rh/3DOM LaAlO3 (13.9 mol/(mol h)) compared to Rh/3DOM LaNi0.08Al0.92O3 (9.16 mol/(mol h)) before reduction treatment. The results show that the lowtemperature reducibility, rich surface adsorbed oxygen species and basic sites of the catalyst greatly improve its activity towards CO2 methanation. The hierarchically porous structure of the perovskite support provides a high dispersion of bimetallic Ni-Rh NPs.

INTRODUCTION Aspects of surface nanoparticles (NPs), including shape, size, texture, and morphology define the activity, selectivity, and stability of supported metal catalysts, with one of the key elements governing catalyst performance being the metal−support interaction. Accordingly, utilizing these characteristics is of great interest for fundamental and application reasons

1-6

. The broadly applicable method for preparing

supported metal catalysts is the deposition route (e.g., vapor infiltration and impregnation) 7-10. However, the deposition method for preparing metal NPs on supports often leads to large or asymmetric metal particle-size distributions and has limited control over the interaction between the metal NPs and the support 11. Recently, this problem has been addressed by using the inside–outside exsolution NP process to grow the metal NPs from the host metal oxide by reduction treatment. The resulting supported metal catalyst possesses a high metal dispersion at relatively low noble metal loading, which is excellent for 2 Environment ACS Paragon Plus

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restricting metal NP agglomeration. Researchers at Daihatsu and Toyota

12

first proposed the exsolution

of active metal NPs from a host oxide lattice, whereby palladium (Pd) reversibly moved into and out of a LaFe0.57Co0.38Pd0.05O3 perovskite lattice in the presence of redox atmospheres. The authors stated that the three-way catalyst could be maintained even when the amount of noble metal was reduced by 70–90%. Following the above promising result, Neagu et al.

13

recently used the exsolution strategy to grow Ni

NPs from an A-site-rich perovskite (La0.52Sr0.28Ni0.06Ti0.94O3) under a reducing environment at 930 °C in 5 % H2/Ar. The ensuing supported Ni catalyst (average NP size of 20nm) exhibited enhanced stability and good resistance to hydrocarbon coking, which was attributed to the Ni NPs socketed into the parent perovskite, providing a stronger metal-oxide interaction than Ni NPs deposited on the perovskite. Cation size mismatching and charge compensation often means the A-site in a perovskite structure tends to be over stoichiometric (A1+αBO3+γ), which typically inhibits the redox reactions

14

. The B-site, which is

generally occupied by a catalytically active cation, can be crucial for catalytic activity by it being selectively extruded out from an ABO3 structure. Consequently, in situ growth of the bimetallic B-site in an ABO3 material and the related catalytic performance studies are significantly desirable. Herein, we have designed and prepared highly efficient Rh-Ni alloys on a three-dimensionally ordered macroporous (3DOM) lanthanum aluminate perovskite catalyst (Rh-Ni/LAO) using the exsolution approach. The catalysts were assessed for the CO2 methanation reaction. The newly-designed Rh-Ni/LAO catalyst shows a high Rh-Ni NP dispersion on both the external and internal surface of the 3DOM perovskite support, exhibiting good activity and stability towards CO2 methanation. The performance improvements derive from a combination of the advantages of 3D porous materials, metal-support interaction and a Ni-Rh alloy effect. The interconnected porous network enables control of the small size and homogeneous dispersion of the Rh NPs, with the exsolution process resulting in the Ni being exsolved from the perovskite lattice and Rh-Ni alloy formation after the reduction pretreatment of LaAl0.92Ni0.08O3. The alloying effect of the Rh and Ni boosted the TOF to be higher than that of the monometallic Rh/3DOM LaNi0.08Al0.92O3. Consequently, exsolution may be regarded as a well-designed 3 Environment ACS Paragon Plus


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fabrication process that provides a new dimension for tailoring particle-substrate interactions with advanced functionality across a range of possible application domains.

RESULTS AND DISCUSSION

Scheme 1. Schematic illustration of in situ exsolution of the catalyst from an ABO3 perovskite structure.

The reduction route for partial exsolution of the B-site Ni from the ABO3 structure is illustrated in Scheme 1. The 3DOM LaAl0.92Ni0.08O3 (3DOM LNAO) and Rh/3DOM LNAO are fabricated utilizing the poly(methyl methacrylate) (PMMA) microsphere-templating and bubble-assisted reduction methods


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according to a previously reported procedure 15-18. To exsolve the Ni and form Ni-Rh alloyed particles on the surface, a reduction process is conducted in a controlled manner in a H2 atmosphere. The RhNi/3DOM LAO sample prepared by the in situ exsolution strategy provides small and highly dispersed Rh-Ni NPs on the surface of the hierarchically ordered macroporous LAO. For comparison purposes, we also prepared a conventional one-dimensional disordered nonporous (1DDN) LNAO sample by the solgel method. Details on sample preparation, physicochemical property characterization, analytical techniques, and in situ redox exsolution route are described in the Supporting Information.

Fig. 1. FE-HRSEM and 3D-eAFM images of (a, b) PMMA, (c−e) 3DOM LNAO, (f, g) Rh/3DOM LNAO, and (h, i) Rh-Ni/3DOM LAO.



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It can be clearly seen from Fig. 1 that the high-quality 3DOM architecture of Rh/3DOM LNAO possesses uniform close-packed periodic macropores with an average diameter of 145 ± 10 nm estimated from the field-emission high-resolution scanning electron microscopic (FE-HRSEM) images. The pores shrink by approximately 20–35%, as compared with the initial size of the PMMA microspheres (ca. 175 nm). Fig. 1b and c provide the 3D-environmental atomic force microscopic (3D-eAFM) observations of PMMA and 3DOM LNAO after removing the PMMA by thermal treatment in air flow at 850 °C for 4 h (Figs. S1-3), respectively. It can be noted that a uniform wall thickness (15–22 nm) and randomly dispersed voids (ca. 30-40 nm) are observable throughout the open windows structure. Before and after reduction treatment in 5% H2/Ar at 900 °C for 20 h, the obtained Rh-Ni/3DOM LAO sample displays a 3DOM structure with a uniform morphology (Fig. 1d-i). Due to the low relative sensitivity factor, the metal NPs are unable to be detected by the SEM technique. To obtain specific information of the Rh NPs and the exsolved Ni NPs, high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) was employed.


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Fig. 2. HAADF-STEM images and EDS elemental mapping of (a–c) 3DOM LNAO, (d) 3D visualization of 3DOM LNAO, (e−h) Rh/3DOM LNAO, (i−l) Rh-Ni/3DOM LAO, and (m−q) EDS elemental mapping of Rh-Ni/3DOM LAO.

The HAADF-STEM images (Fig. 2) reveal that the Rh/LNAO catalyst before reduction treatment has well dispersed Rh NPs on the interconnected porous structure of the LANO support (Fig. 2e−g) where the average particle size is 3.2 nm (Fig. 2h). The HAADF-STEM images (Fig. 2i−k) and the EDS mapping (Fig. 2m−q) of Ni-Rh/3DOM LAO after reducing the Rh/3DOM LNAO confirm the successful



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exsolution of Ni from the LNAO perovskite structure (the red coloring is Ni on the surface (Fig. 2o−p)). It can also be observed that, along with its exsolution, the Ni has partially diffused into the Rh NPs to form the bimetallic Ni-Rh alloy. Table 1. BET surface areas, catalytic activities (reaction temperatures T10%, T50%, and T90%), turnover frequencies (TOFcat) at 300 °C of the Rh-Ni/3DOM LAO, Rh/3DOM LNAO, 3DOM LNAO, and 1DNN LNAO samples.

Catalyst

BET surface area (m2/g)

T10% (°C)

T50% (°C)

T90% (°C)

TOFcat (mol/(mol h))

9.2 56.8 54.8 43.2

291 267 249 238

351 316 298 280

398 368 333 308

2.24 5.59 9.16 13.9

1DDN LNAO 3DOM LNAO Rh/3DOM LNAO Rh-Ni/3DOM LAO

Upon the inclusion of Ni into the Rh NPs, the average particle size grows from 3.2 nm to 4.1 nm. This suggests that the interfacial interaction between the 3DOM LAO and Rh-Ni NPs occurs at the atomic level and part of the oxidized metal NP species may migrate into the bulk structure of the support to form metal−support interactions

19

. According to the HAADF-STEM image (Figs. 2k and S4), the

characteristic ABO3 lattice fringes of LNAO exhibit a d spacing of 0.26 nm, which is consistent with the (110) spacing of rhombohedral LaAlO3 perovskite. As seen in Table 1, the 3DOM LNAO and supported Rh catalysts possess BET surface areas of 54.8−56.8 m2/g, which are much higher than that of the 1DDN LNAO sample (9.2 m2/g), providing a large available surface for the dispersion of the metal NPs. It is noticeable that Rh-Ni/3DOM LAO experienced a moderate decrease in surface area (43.2 m2/g) which is attributable to the high temperature thermal treatment. Further evidence of the porous structure is apparent in the N2 adsorption-desorption isotherms in Fig. S6. To further explore Ni particle exsolution from the parent perovskite, X-ray diffraction (XRD) measurements were conducted. As is shown in Fig. 3A, the 3DOM LNAO and Rh/3DOM LNAO samples exhibit a highly crystalline character which can be indexed as rhombohedral LaAlO3 perovskite (Ref. No. 04-012-6129, see Fig. S5). No XRD signals corresponding to Ni and/or Rh are discernible due to the small amount and high dispersion of the Ni in LNAO bulk and Rh on the surface (black and red


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patterns in Fig. 3A). This can be assigned to a small particle size effect which is confirmed by the HAADF-STEM images (Fig. 2e−l). After the reduction treatments, the diffraction peaks corresponding to the perovskite LAO crystal appear to be unchanged, however, a Ni phase is formed (see inset of Fig. 3A). This suggests that the (110) facets are especially suitable for the B-site cation diffusion and the abundance of the A-site cations (high La/Ni ratios (12.5)) can improve distribution of the metal NPs in the parent oxide support, which is consistent with the EDS elemental mapping analyses.

Fig. 3. (A) XRD patterns, (B) H2-TPR, (C) O 1s XPS spectra, and (D) CO2-TPD profiles of (a) 1DDN LNAO, (b) 3DOM LNAO, (c) Rh/3DOM LNAO and (d) Rh-Ni/3DOM LAO.



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The reducibility of the samples is confirmed by H2-TPR analysis (Fig. 3B). Generally, La and Al oxides are irreducible below 1000 °C under the used TPR conditions. The profiles show that LNAO reduction occurs in three steps 20: the first reduction peak at 243–292 °C corresponds to the formation of La4Ni3O10 ((LaNiO3)nLaO Ruddlesden–Popper (n = 3)); the second reduction peak at 300–338 °C is related to the formation of La2NiO4 due to the reduction of Ni3+ to Ni2+; and the third peak at a higher temperature (~630 °C) is associated with the reduction of Ni2+ in La2NiO4 to metallic nickel (Ni0) and La2O3

21-23

. It

can be seen that there is no isolated peak for Rh reduction after loading Rh on 3DOM LNAO. However, the Rh NPs promote reduction of the sample such that it occurs at lower temperatures (271 °C and 318 °C), which can be attributed to hydrogen activation induced by the Rh. The TPR profile of Ni-Rh/3DOM LAO exhibits an intensity increase for the reduction peaks which indicates an increase in hydrogen consumption. Ni exsolution and Ni-Rh alloy formation results in the Rh-Ni/3DOM LAO sample possessing superior low-temperature reducibility, which can contribute to a better catalytic performance. XPS was used to examine the chemical states of oxygen and the electronic properties of LNAO after the reduction treatment. As can be seen in Fig. 3C , the major peak at the lower binding energy (BE) of 529.3 eV is associated with surface lattice oxygen (Olatt) on the obtained samples, illustrating the chemical bonding between oxygen and metal, while the smaller peak at the higher BE (530.9 eV) is associated with the surface adsorbed oxygen (Oads, such as O2–, O22– or O–) on the perovskite

24

. Oxygen vacancies

normally play an important role in the dissociation of oxygen-containing chemical bonds. Thus, by measuring the relative intensity ratio (RIR) of Oads/Olatt, the relative density of oxygen vacancies in different samples can be semi-quantitatively estimated. The Rh-Ni/3DOM LAO after reduction shows a significant increase in RIR (1.35) as compared to the 1DDN LNAO (0.813), indicating a richness in surface oxygen vacancies that are believed to form by lattice defecting during Ni exsolution from the LNAO perovskite lattice. As shown in Fig. S7 a blue shift of 0.20 eV is observed for the Rh 3d XPS profile of Ni-Rh/3DOM LAO after reductive treatment compared with Rh/3DOM LNAO. The shift indicates an electron modification and reconstruction of the electron environment between Ni and Rh, 10 Environment ACS Paragon Plus

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suggesting they interact within the Ni-Rh alloy 5. This interaction effect within a series of NiM (M=Ru, Rh, and Pd) bimetallic has been reported and the study has correlated the alloy synergistic effect with the better catalytic performance of the alloy catalysts through increasing fraction of surface active atoms, enhancing H2 and substrate activation and inhibiting hydrogenation to control the selectivity as well.

The basicity of the samples was investigated using CO2-TPD experiments. The profiles illustrating CO2 desorption from the LNAO sites possessing basic character are shown in Fig. 3D. The basic sites of the 1DDN LNAO and 3DOM LNAO samples are distributed in two regions located at 300–400 and ~700°C, which are associated with moderate and strong basic sites, respectively. Loading Rh on the 3DOM LNAO introduces weaker and moderate basic sites. After reduction, the moderate basic sites almost disappear while one broad and high intensity peak appears at a high temperature (733–791 °C), indicating the strong interaction of CO2 species with strong basic sites. The findings highlight the strong basicity of the RhNi/3DOM LAO catalyst, which is an advantageous property for CO2 adsorption.

Fig. 4. (A) Catalytic activities and (B) the TOF values at 330 °C obtained for the different catalysts.



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Evaluation of the catalytic activities of the samples was conducted according to the process described in Fig. S9. The T50% values, corresponding to the temperature at which 50% conversion is achieved, are 280 °C, 298 °C, 316 °C, and 351 °C for Rh-Ni/3DOM LAO, Rh/3DOM LNAO, 3DOM LNAO, and 1DDN LNAO, respectively. The result demonstrates there is a significant enhancement in activity for the RhNi/3DOM LAO sample relative to the other catalysts. CO2 conversion by Rh-Ni/3DOM LAO exceeds 90% at 308 °C and complete conversion of CO2 is achieved at 328 °C (Fig. 4A), which is superior to the catalytic activity exhibited by 1DDN LNAO (34% at 328 °C) as well as Rh and Ni catalysts supported on ZrO2, Al2O3, and SiO2 as reported by others

25-29

. It should be noted that both Rh-Ni/3DOM LAO and

Rh/3DOM LNAO have a high selectivity toward CH4. To establish the structure–activity relationship, the turnover frequency (TOFcat) was calculated at a reaction temperature at 300 °C. The TOFcat results reveal an enhanced performance by Rh-Ni/3DOM LAO (17.3 h-1) when compared with Rh/3DOM LNAO (15.8 h-1), 3DOM LNAO (11.5 h-1), and 1DDN LNAO (5.96 h-1) for CO2 methanation at 330 °C (Fig. 4B). The differences in catalytic performance might be attributed to the electronic interaction between the bimetallic Ni and Rh as was demonstrated by the earlier XPS findings.

CONCLUSIONS In summary, highly efficient Rh-Ni alloys on a three-dimensionally ordered macroporous (3DOM) lanthanum aluminate perovskite catalyst (Rh-Ni/LAO) were designed and prepared using the exsolution approach. The catalysts were assessed for the CO2 methanation reaction. The newly-designed Rh-Ni/LAO catalyst shows a high Rh-Ni NP dispersion on both the external and internal surface of the 3DOM perovskite support, exhibiting good activity and stability towards CO2 methanation. We suggest that a richness in surface adsorbed oxygen species and basic sites play an important role in strongly adsorbing CO2 on the Rh-Ni/3DOM LAO catalyst which greatly contributes to its activity toward CO2 methanation.. This efficient and robust metal exsolution strategy is a promising catalyst design pathway for preparing highly dispersed bimetallic catalysts on perovskites and could be readily extended to other catalytic


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systems with different compositions. As there is enormous interest in strong metal-support interaction catalysts the exsolution method may serve as a powerful tool to augment synthesis strategies for fundamental and applied studies.

ASSOCIATED CONTENT

Supporting Information Catalyst characterization procedures, synthesis of the PMMA microspheres, preparation of 3DOM LNAO, catalytic activity measurements, XRD patterns of 3DOM LNAO, FE-HRTEM and 3D-eAFM images of PMMA microspheres, HRTEM/HRSEM images of 3DOM LNAO, HAADF-STEM-EDS images of RhNi/3DOM LAO, XPS spectra of the samples and element spectra of Rh-Ni/3DOM LAO. This is available free of charge through the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author Email ID: *E-mail: [email protected] (H. A.) *E-mail: [email protected] (J. S.) *E-mail: [email protected] (R. A.)

The authors declare no competing financial interest.

Acknowledgements H.A. acknowledges financial support through the Vice Chancellor Research Fellowship (RG142406) from The University of New South Wales. The authors also acknowledge Dr. David Mitchell of University of Wollongong Electron Microscopy Centre for his generous help with the HAADF-STEMEDS analysis.



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Table of Contents Highly efficient Rh-Ni alloys on a three-dimensionally ordered macroporous (3DOM) lanthanum aluminate perovskite catalyst (Rh-Ni/LAO) were designed and prepared using the exsolution approach. The catalysts were assessed for the CO2 methanation reaction. The newly-designed Rh-Ni/LAO catalyst shows a high Rh-Ni NP dispersion on both the external and internal surface of the 3DOM perovskite support, exhibiting good activity and stability towards CO2 methanation

In situ Exsolution of Bimetallic Rh-Ni Nano-alloys: Highly Efficient Catalyst for CO2 Methanation Hamidreza Arandiyan,*a Yuan Wang,a Jason Scott,*a Sara Mesgari,b Hongxing Dai,c and Rose Amal*a


In situ Exsolution of Bimetallic Rh-Ni Nano-alloys: Highly Efficient

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