Resettable Heterogeneous Catalyst: (Re)Generation and (Re

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Resettable heterogeneous catalyst: (re)generation and (re)adsorption of Ni nanoparticles for repeated synthesis of carbon nanotubes on Ni-Al-O thin films Bin Liang, Eongyu Yi, Toshihiro Sato, Suguru Noda, Kai Sun, Dechang Jia, Yu Zhou, and Richard M. Laine ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00847 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on October 1, 2018

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ACS Applied Nano Materials

Resettable Heterogeneous Catalyst: (Re)generation and (Re)adsorption of Ni Nanoparticles for Repeated Synthesis of Carbon Nanotubes on Ni-Al-O Thin Films Bin Liang,†,‡ Eongyu Yi,† Toshihiro Sato,§ Suguru Noda,*,§ Kai Sun,† Dechang Jia,‡ Yu Zhou,‡ Richard M. Laine*,† †

Department of Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States ‡ Institute for Advanced Ceramics, School of MSE Harbin Institute of Technology, Harbin 150080, China § Department of Applied Chemistry, School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan *E-mail: [email protected] *E-mail: [email protected]

Abstract Liquid-feed flame spray pyrolysis (LF-FSP) of selected metalloorganics provides [NiO]0.25[Al2O3]0.75 and [NiO]0.5[Al2O3]0.5 spinel nanopowders (NPs) without aggregation and with good stoichiometric control. Both compositions were processed to thin films as a first step in preparing Ni:Al2O3 nanocomposite films for catalytic synthesis of CNTs. NPs dispersed in polymer were cast giving 30±10 µm green films that were debindered (370 ˚C), sintered at 1000-1500 ˚C/air, then heated at 1000-1100 ˚C in H2/N2 5:95 to generate Ni particles as catalyst sites. These Ni particles promote the growth of CNTs. The CNTs were then oxidatively removed coincident with regeneration of the [NiO]1-x[Al2O3]x surfaces. Thereafter, reduction, CNT syntheses and catalyst regeneration were repeated 2x more. The quality and quantity of CNTs were the same after each cycle. This represents what we believe is a new, general approach to catalyst recycling/regeneration.

Keywords: nickel aluminate spinel; carbon nanotubes; thin films; flexible; metal-ceramic composites, regenerable catalyst

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Introduction A general problem with the use of heterogeneous catalysts for reactions run at higher temperatures is loss of catalytic activity. If we ignore poisoning, activity loss can arise from three distinct processes.1-8 In one, the active metal catalyst particles undergo Ostwald ripening, coalescing (sintering) over time reducing the number of active species as catalyst surface area is lost.2-4 In the second, catalyst particles located in micropores on the substrate surface become encapsulated as these pores sinter closed, again reducing the number of active catalyst sites.1-6 The third process arises when catalyst particles react with the substrate forming inactive species.6-8 Key to the work reported here is the concept that the latter process might actually be reversible allowing one to regenerate a given catalyst to its original activity. A “gedanken” experiment suggests that to: (1) avoid substrate pores sintering closed encapsulating active metal particles, the substrate cannot be porous. (2) ensure that highly active metal particles can be introduced to this non-porous substrate, catalyst particles must be generated after sintering. Two ways to reproducibly obtain finely dispersed particles employ gas or solution phase deposition.9-11 However, both approaches require additional and typically involved second and third steps. Alternately, one could choose an as-prepared substrate that can serve as the source of active metal particles coincident with formation of a suitable new substrate. Phase separation of well-dispersed and active metal particles from the originally formed substrate offers a prospective approach. Given that one would want to use a substrate that does not “waste” the metal that could be active, the non-porous substrate should be as thin as possible. Furthermore, mechanically strong substrates could offer utility permitting elimination of a mechanical support that cannot contribute to catalytic activity. Liquid-feed flame spray pyrolysis (LF-FSP) 11-28 processing provides unaggregated nanopowders (NPs) whose elemental mixture can be defined by the initial precursors used in the

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flame process. In some instances, the particles are crystalline and in others they remain amorphous12,17,18 depending on the elemental makeup. The high temperature flame process followed by rapid quenching causes the resultant NPs to crystallize in the simplest crystal structures accessible often giving kinetic rather than thermodynamic

phases

typically

found

in

their

phase

diagrams.

For

example

[MO]1-x[Al2O3]1+x NPs (M = Mg, Ni, Co…) where x > 0.6 often exhibit single spinel phase morphologies despite having compositions that would normally form MO.Al2O3 + [α-Al2O3] at these synthesis temperatures.15,16 This latter result prompts the work reported here. We recently demonstrated that LF-FSP derived NPs allow processing thin, flexible and dense ceramic films (20-40 µm) for a number of materials including highly Li+ conducting electrolytes.17,18 We separately demonstrated the use of LF-FSP NPs to promote multiple catalytic

reactions

including

the

synthesis

of

carbon

nanotubes,

CNTs

using

[CoO]1-x[Al2O3]1+x.29 In this latter case, thermal treatment in flowing H2 generates Co metal particles supported on CoAl2O4 NPs for rapid in situ generation of active catalyst. Thus, catalyst activation and CNT syntheses all occur in a time scale of ten seconds; with [CoO]1-x[Al2O3]1+x (x < 0.5) providing excellent catalytic activity. In this instance, the catalyst particles are not recoverable and are, like many other nanoparticles used as CNT catalysts, retained with the isolated CNTs. Given that we would like to use thin films to generate CNTs and reuse the catalyst system, we used the gedanken experiment above to design substrates for this purpose. The work reported here on [NiO]1-x[Al2O3]1+x thin films represents a test case for our approach and is the first of more focused efforts to be submitted in the near future describing our approach to regenerable catalysts. In these studies, the catalyst is first generated by selective reduction of the transition metal oxide that is a component of the alumina spinel phase to produce a metal particle and a sub-stoichiometic [NiO]1-x[Al2O3]x substrate.

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CNT synthesis was followed until the catalyst lost some significant activity. Then, the catalyst + substrate was reoxidized such that the metal particles were reabsorbed into the substrate reforming spinel phase. Thereafter, the catalyst was regenerated by a second reduction in flowing H2 and used again. Our rationale for choosing [NiO]1-x[Al2O3]1+x spinel is based on the extensive literature wherein nickel aluminates have been extensively explored as catalysts, see below. 15,16,31-35 A later effort will describe [CoO]1-x[Al2O3]1+x thin film CNT processing.30 Note that in a truly reusable system, we would use shear from gas flow to remove the CNTs rather than oxidation. It is clear that a porous substrate would be less amenable to the removal of CNTs by shearing because the catalyst and many of the CNTs might be embedded within the substrate. Of the many compounds with spinel structures, nickel aluminates (NiAl2O4) are found to be primarily inverse spinels with the nickel ions preferentially occupying the octahedral sites.36 NiAl2O4 spinels have been widely explored due to their multiple potential applications.37-43 For example, they can be used as electrode materials in high temperature fuel cells due to their unusual electrical conductivity. Dense ceramics with good mechanical properties (including good flexibility) can also provide excellent thermal stability ensuring safety and reliability when used in solid oxide fuel cells (SOFCs).37,38 NiAl2O4 spinels also have widespread commercial value in catalytic applications including methane- and methanol-steam reforming, hydrocarbon cracking, dehydrogenation, hydrodenitrogenation, etc.40-42 Steam reforming catalysts usually consist of fine nickel particles on supported substrates where catalytic activity can be related directly to Ni average particle sizes (APSs).43 NiAl2O4 is an attractive candidate anode support for SOFC because it exhibits autogenerated catalysis due to the formation of Ni under reducing conditions. The resulting highly catalytically active Ni NPs can extract H2 efficiently from fuel at the electrode.44,45 Nickel aluminate spinels are prepared via many methods including solid-state reac-

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tion,35,36,38,46-48 sol-gel processing,49,50 ultrasound irradiation-assisted precursor processing,51 and ion exchange in zeolites.52,53 The most widely used method is solid-state reaction where mixtures of the individual metal oxides are co-sintered. However, phase-pure, high surface area nickel aluminate spinels with controlled stoichiometries are very difficult to obtain especially by solid-state reactions due to the high temperatures required for sufficient solid-state diffusion.15,16,36,54 Fortunately, liquid-feed flame spray pyrolysis (LF-FSP) makes it possible to synthesize a wide variety of mixed-metal oxide NPs with close to atomic mixing, high purity, limited aggregation, with good control of stoichiometries and phase compositions, as discussed above.11-28 The above noted utility in various catalyst applications provided the motivation to explore its utility in generating NPs along the NiO-Al2O3 tie line, as described elsewhere.15,16 LF-FSP generated [NiO]0.5[Al2O3]0.5 NPs form as inverse spinel, whereas [NiO]0.25[Al2O3]0.75 NPs consist of a mixed spinel with some Al3+ occupying octahedral sites. Both can be used as catalysts.15,16 An extension of this original work explored their utility in processing dense α-Al2O3/Ni2AlO4 composite monoliths providing an understanding of thermal behavior and microstructural changes during sintering for a bottom up approach.32,33 The current effort extends our processing methods to NiO-Al2O3 thin films of potential use as regenerable catalysts for processing CNTs. We recognize that our catalytically active systems will not have the surface areas of traditional catalysts; however, ease of synthesis and mechanical robustness coupled with regeneration may offer a competing approach to developing new types of commercially viable catalysts. We have also simultaneously looked at the hydrogen reduction of these NPs to generate Ni particles on nickel depleted NiAlOx as a prelude to the catalytic studies presented here as discussed briefly below and in a separate manuscript.55

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The current effort extends LF-FSP methods to NiO-Al2O3 thin films of potential use as regenerable CNT catalysts. We recognize that our catalytically active systems do not have the SSAs of traditional catalysts; however, ease of synthesis, mechanical robustness coupled with regeneration offers a competing approach to new types of commercially viable catalysts.

2. Experimental 2.1. Sample preparation Precursor synthesis Alumatrane, Al(OCH2CH2)3N, was synthesized as described elsewhere.13 Anhydrous ethanol was purchased from Decon Labs (King of Prussia, PA). Nickel acetate tetrahydrate, C4H6NiO4·4H2O, was purchased from Sigma Aldrich (Milwaukee, WI) and used as received. Powder fabrication [NiO]0.25[Al2O3]0.75 NPs were synthesized by liquid-feed flame spray pyrolysis (LF-FSP). The LF-FSP process is detailed in our previous work.11 Alumatrane and nickel acetate tetrahydrate were dissolved in ethanol with a selected molar ratio giving a 3 wt % ceramic yield solution. The precursor solution was aerosolized with oxygen and combusted in a chamber with methane/oxygen torches and shield oxygen. NPs were collected mainly in rod-in-tube electrostatic precipitators operated at 10 kV. The as-produced powders were dispersed in EtOH with 2 wt % bicine as a dispersant via ultrasonication (Vibra-cell VC-505, Sonics & Mater. Inc.). After sedimentation for 1 h, the supernatant suspension was decanted into a beaker, dried in an oven (60 °C) and collected for use. Film processing [NiO]0.25[Al2O3]0.75 NPs were mixed with polyvinylbutyral, benzyl butyl phthalate, acetone, and ethanol in specific ratios (Table S1) in a 30 mL vial and ball-milled for 24 h with Al2O3 beads (3.0 mm in dia.) obtaining a homogenous suspension. The suspension was cast

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using a wire-wound rod coater (Automatic Film Applicator-1137, Sheen Instrument, Ltd). The thickness of the as-cast film was controlled by adjusting the gap between the rod and the Mylar substrate. Dried green films (30±10 µm in thickness) were manually cut into small pieces, then uniaxially pressed at 50 MPa/140 ˚C/5 min using a heated bench-top press (Carver, Inc), and then peeled off the substrate. [NiO]0.5[Al2O3]0.5 NPs and films were processed using the same procedures. Film sintering These green films were placed between Al2O3 plates and debindered at 370 ˚C for 1 h in air, then heated to the target temperatures at a ramp rate of 5 ˚C/min. Al2O3 plates were used to prevent warping of films throughout the process. Film reduction Sintered films (1 cm×1 cm×20 µm) were placed on an Al2O3 plate and heated to target temperatures at 10 ˚C/min/7 h in 5/95 H2:N2 at 100 mL/min. Thermal etching Sintered films were manually broken to generate fresh fracture surfaces, then heated to designated temperatures for 30 min in air. Thermal etching temperatures were generally 100 ˚C lower than sintering temperatures. Carbon nanotube (CNT) synthesis Catalytic performance of the sintered films was evaluated for CNT syntheses. As-sintered films were placed on a quartz stage in a tubular quartz reactor, heated to 1100 ˚C at ~70 ˚C/min, held for 30 min in 10/90 H2:Ar at 500 mL/min, cooled to ambient temperature, and taken out from the reactor. The as-heated films were then set in the reactor, heated to a target temperature (800, 900, 1000 ˚C) at ~60 ˚C/min held for 3 min in 25/75 H2:Ar at 92 mL/min, 8 Torr. Then CNTs were grown by chemical vapor deposition (CVD) by flowing 1.3/13.3/85.3 C2H2/H2/Ar at 181 mL/min, 15 Torr for 10 min.

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CNT regrowth experiment was performed for three cycles at the fixed temperature of 1100 ˚C. Ni nanoparticles were formed by heating in 10/90 H2:Ar at 500 mL/min and 760 Torr for 10 min and in 25/75 H2:Ar at 92 mL/min and 8 Torr for 3 min, and then CNTs were grown by CVD by flowing 1.3/13.3/85.3 C2H2/H2/Ar at 181 mL/min and 15 Torr for 10 min. CNTs were removed by oxidation in 5/95 O2/N2 at 760 Torr for 30 min and then the cycle was restarted from Ni particle formation.

2.2. Sample characterization X-ray diffraction (XRD) Sample phases were identified using a Rigaku Rotating Anode Goniometer (Rigaku Denki., LTD., Tokyo, Japan) operating at 40 kV and 100 mA. Samples were scanned at 2°/min within the range of 10-70° 2θ with 0.02° intervals. As-detected XRD patterns were analyzed using Jade 2010 software (Version 1.1.5 from Mater. Data, Inc.) where JCPDS files used include Ni (04-001-1136), NiAl2O4 (98-000-9338) and α-Al2O3 (04-004-2852). The catalytic film used for the oxidation-reduction-CVD cycle was scanned within the range of 5-110° 2θ with 0.02° step using XRD (Rigaku SmartLab, Tokyo, Japan) operating at 40 kV and 40 mA. Scanning electron microscopy (SEM) Sample morphologies were observed by SEM (NOVA Nanolab, FEI Inc.). All the samples were gold sputter coated using a Technics Hummer VI sputtering system (Anatech Ltd., Alexandria, VA) to prevent charging. CNT samples were observed by SEM (S-4800, Hitachi) without any coating. Thermogravimetric/differential thermal analysis (TG/DTA) Q600 simultaneous TGA/DTA (TA Instruments, Inc.) was used to analyze thermal decomposition and phase transformations of as-cast green films. Samples (20 mg) were loaded

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in an alumina pan and an empty pan was used as a reference. The samples were heated to a target temperature with a ramp rate of 10 ˚C/min in a flowing synthetic air (60 mL/min). Raman spectroscopy The as-synthesized CNT samples were analyzed by Raman spectroscopy (HORIBA HR800, Kyoto, Japan) using a wavelength of 488 nm for excitation.

3. Results and discussion The first step in the process of generating regenerable catalysts resides in learning to make very thin, flexible films along the NiO-Al2O3 tieline that form single phase spinel and that offer the opportunity to tailor the Ni content in subsequently generated catalysts. Thus, initial discussion focuses on reproducibly generating and characterizing such films. 3.1. [NiO]0.25[Al2O3]0.75 film processing and sintering The Figure S1a XRD indicates as-shot [NiO]0.25[Al2O3]0.75 is off-stoichiometric, single-phase spinel, as quenching provides access to kinetic phases.15 The broad diffraction peaks are indicative of nanoscale particles. The Figure S1b SEM shows spherical morphologies for as-produced [NiO]0.25[Al2O3]0.75 NPs and suggests some agglomeration. Almost all particles appear to be 500 nm form within the [NiO]0.5[Al2O3]0.5 spinel matrix and show trans-granular fracture. These islands were proven to be α-Al2O3 by EDS (not shown here) and XRD (Figure S3). This is reflected in the TGA-DTA (Figure 1, slight exotherm 1234 °C), and also is in good agreement with our previous report.16 At 1400 ˚C, all spherical morphologies disappear and obvious necks form between grains. Higher temperatures and longer dwell times induce more extensive sintering improving ceramic densification and promoting grain growth. Fully dense NiAl2O4-Al2O3 composite films with thicknesses of 30±2 µm are obtained at 1500 ˚C/3 h. They offer densities of 95±2 % theoretical density (TD) using Archimedes’ method. Additionally, trans- and inter-granular fractures are both present in film fracture surfaces.

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Figure 2. SEMs of fracture surface morphologies for [NiO]0.25[Al2O3]0.75 films sintered in air at (a) 1000 ˚C/1 h, (b) 1100 ˚C/1 h, (c) 1200 ˚C/1 h, (d) 1400 ˚C/1 h, (e) 1500 ˚C/1 h, (f) 1500 ˚C/3 h, inset shows thickness of full film.

Figure S4 provides thermally etched morphologies of [NiO]0.25[Al2O3]0.75 films (as-sintered at 1500 ˚C/3 h). Obvious grain boundaries are observed on film surfaces and fracture surfaces giving statistical average grain sizes of 1.1±0.3 µm.

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3.2. [NiO]0.25[Al2O3]0.75 film reduction As-sintered, fully dense [NiO]0.25[Al2O3]0.75 films were reduced in 5:95 H2/N2 to produce Ni particles on film surfaces targeting potential catalytic applications. Figure S5 shows no discernible XRD peaks for Ni at 1000 ˚C and very weak peaks at 1050 ˚C indicating limited formation of Ni particles. In contrast, at 1100 ˚C/7 h obvious Ni peaks arise at approximately 44.5˚ and 52˚ 2θ suggesting the formation of more Ni. Films after heating at 1100 ˚C turn completely black (see below) also indicating formation of Ni. XRD results show such films consist of 81.6 wt % (83.9 mole %) Al2O3, 13.9 wt % (8.2 mole %) [NiO]0.5[Al2O3]0.5 and 4.4 wt % (7.9 mole %) Ni. The Figure S6 TGA-DTA allows one to calculate formation of 4.2±1.0 wt % Ni metal in as-reduced films. This represents the Ni metal accessible on the surface. The fact that the XRD evaluation indicates 4.4 wt. % Ni metal and the TGA data also show 4.4 wt. % Ni indicates that all of the Ni metal is accessible suggesting its potential involvement in all catalytic processes. As in the Figure S5 XRDs and Table 1, [NiO]0.25[Al2O3]0.75 films sintered at 1500 ˚C contain only 29 wt. % [NiO]0.5[Al2O3]0.5 phase acting as the only source of Ni. Therefore, only small amounts of Ni NPs form in reduced films. Clearly more Ni particles can be produced in reduced [NiO]0.5[Al2O3]0.5 films after sintering.

Figure 3. SEM images of (a) fracture surface and (b) surface morphologies of [NiO]0.25[Al2O3]0.75 films after heating at 1100 ˚C/7 h/H2. Inset shows pore formation.

In Figure 3a, no obvious morphology changes can be observed on film fracture surfaces 13



before reduction. In contrast, white, spherical particles appear on Figure 3b film surfaces after reduction, confirmed to be Ni by EDS (not shown). Most Ni particles on film surfaces are < 100 nm suggesting that most will be catalytically active.11,34 Additionally, some larger particles (100-300 nm) appear mainly around [NiO]0.5[Al2O3]0.5 grains that seem to be porous due to Ni reduction. The inset shows pore formation that occurs with Ni reduction. Thinner ( 500 nm and after the second reduction AGSs of 40±3 nm (Figure S11). Reaction conditions for repeated CNT synthesis by CVD at 1100 ˚C (Figure S12). Low-magnification SEMs of CNT growth and catalyst reset second and third times. Raman spectra of the CNTs taken at three different positions for each sample are also shown (Figure S13).

Acknowledgments We are grateful for the financial support of this work by NSF through DMR-0723032. Bin Liang, as an international exchange visitor, would also like to thank the Harbin Institute

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of Technology for support. The Noda group thanks JSPS for support through Kakenhi grant JP25107002.

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Resettable Heterogeneous Catalyst: (Re)Generation and (Re

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