Influence of LaNiO3 Shape on Its Solid-Phase Crystallization into

May 26, 2016 - Catalysts derived from LaNiO3 spheres and rods were found to be free of carbon accumulation after 100 h of reforming, while those deriv...
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Influence of LaNiO3 Shape on Its Solid-Phase Crystallization into Coke-Free Reforming Catalysts Sarika Singh, Daria Zubenko, and Brian A. Rosen* Department of Materials Science and Engineering, Tel Aviv University, 55 Haim Levanon Street, Ramat Aviv 6139001, Israel S Supporting Information *

ABSTRACT: Shape-controlled LaNiO3 nanoparticles were prepared by modified hydrothermal and precipitation routes resulting in cubes, spheres, and rods. The solid-phase crystallization of LaNiO3 into its active catalyst form, Ni/La2O3, was found to be highly dependent on the shape and structure of the parent nanoparticle. Factors such as the crystallization pathway and Ni2+-ion depletion are considered as key factors influencing the final material. Catalysts derived from LaNiO3 spheres and rods were found to be free of carbon accumulation after 100 h of reforming, while those derived from cubes showed excessive carbon accumulation and signs of sintering. All three catalysts are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), and thermogravimetric analyses (TGA). The presence of defects, particularly stacking faults within the perovskite, may impact the reduction pathway and subsequent catalytic properties. Stable and active catalysts can therefore be designed and tuned by controlling the shape and structure of perovskite precursors. KEYWORDS: perovskite, shape control, dry reforming of methane, solid-phase crystallization, carbon formation, defects

1. INTRODUCTION The exploitation and control of structure in methane reforming catalysts remains a significant challenge to chemists and engineers worldwide.1 Currently, methane conversion catalysts are limited by their ability to resist surface carbon accumulation, sintering, and unwanted oxidation.2,3 These mechanisms are detrimental to catalyst lifetime and have prevented processes such as methane dry reforming from large scale industrialization. All of these deactivation mechanisms are highly influenced by the strength of the interaction between the active catalyst and its support, as well as the size and shape of the active catalyst phase.4,5 The use of wetness impregnation and solution-phase crystallization as ways to disperse the active catalyst (e.g., Ni) onto a high surface-area support generally leads to weak and superficial catalyst−support interactions.6,7 These weak interactions result in a high susceptibility to sintering, poor redox properties, and carbon accumulation. Solid-phase crystallization of well-ordered materials has been proposed to be an effective single-step method for forming catalysts with strong metal−support interactions. So far, investigation of this method has largely focused on bulk and supported perovskites (ABO3) as the parent phase and the influence of nonstoichiometric formulations on the ability to exsolve metal (B atom) crystals.8−10 These studies revealed that A-site deficiencies in conjunction with oxygen vacancies beyond a limiting concentration (δ,lim) destabilized the perovskite structure sufficiently such that the B atom would exsolve in order to maintain the original stoichiometry shown below in eq 1. A1 − αBO3 − δ ,lim → (1−α)(ABO3 − δ ) + α B © 2016 American Chemical Society

This work investigates how shape-control of the parent LaNiO3 perovskite nanoparticle can be exploited to imbue a favorable size and spatial distribution of exsolved Ni crystals, a strong catalyst−support interaction, and provide for coke- and sinterfree catalytic methane oxidation. We show how selection of parent particle shape can lead to differences in methane conversion and carbon resistance over several orders of magnitude. These results are considered in light of how depletion within the parent perovskite nanoparticle and the pathway taken during crystallization influence the final material to enable sustainable and intelligent catalyst design.

2. RESULTS AND DISCUSSION 2.1. Parent Perovskites and Catalytic Activity. Modified hydrothermal and chemical precipitation methods were used in order to synthesize LaNiO3 nanoparticles with cubic, quasispherical (hereafter referred to as “spheres” for simplicity), and rod morphologies. Cubic particles became porous after calcination, the final step in the perovskite synthesis, and are not the result of spherical particles sintered together (Figure S1). Figure 1 shows SEM micrographs of fresh cube-, sphere-, and rod-shaped perovskites as well as the XRD pattern for both fresh and spent catalysts after 100 h of methane dry reforming. X-ray diffraction of the fresh sample shows that the obtained materials were 97% perovskite and 3% nickel(II) oxide (NiO) (Figure S2) in all cases. Table 1 shows the characteristic structural parameters for each system. Statistics about the size Received: March 7, 2016 Revised: May 25, 2016 Published: May 26, 2016

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DOI: 10.1021/acscatal.6b00673 ACS Catal. 2016, 6, 4199−4205

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Figure 1. (left) Scanning electron micrographs of fresh cube-, sphere-, and rod-shaped LaNiO3 nanoparticles. (right) XRD patterns for all three shapes showing both fresh LaNiO3 and spent catalysts after 100 h of methane reforming.

For each system, solid-phase crystallization was carried out by heating the LaNiO3 nanoparticles in a 50% hydrogen atmosphere and resulted in Ni nanoparticles supported by La2O3. XRD patterns of spent catalyst revealed that no perovskite phase remained after reduction but that the particles maintained the shape of their parent after crystallization. XRD also reveals that the La2O3 support was converted to the oxycarbonate La2O2CO3 phase after exposure to CO2 from the dry reforming process. Such a conversion has been found to be beneficial as the oxycarbonate is an excellent oxidizing agent to gasify accumulated carbon at the catalyst−support interface. Figure 2 shows how the dry reforming of methane proceeded over Ni/La2O3 catalysts derived from cube-, sphere-, and rodshaped parent perovskites. Crystallographic data and size distribution statistics in Figures 1 and 2 allowed us to estimate the methane turnover numbers to be 0.5 s−1, 0.21 s−1, and 0.35 s−1 for the cubes, spheres, and rods, respectively. These numbers match well with turnover numbers for other industrial dehydrogenation reactions carried out at 650 °C (generally between 0.05 s−1 and 1 s−1). The reaction was carried out at 650 °C where carbon accumulation is highly favored thermodynamically. Nikoo et al. calculated the equilibrium conversion for methane dry reforming while considering solid carbon formation using the Gibbs free-energy minimization technique employing the Soave−Redlich−Kwong (SRK) equation-of-state.12 At 650

Table 1. Geometric Data for the Perovskites and the Ni Crystals Exsolved from a Perovskite of a Particular Shape LaNiO3 shape cubes

spheres

rods

LaNiO3 phase composition rhombohedral − 58% cubic − 42% rhombohedral − 65% cubic − 35% rhombohedral − 100%

BET surface area (m2/g)

av perovskite size (nm)

av Ni crystal size (nm)

10.44

367

21

10.69

89

18

9.20

109 (L) × 38 (H)

13

distribution of the parent perovskites and exsolved Ni particles were measured by analyzing approximately 200 particles over many TEM images using ImageJ analysis software. Details about the crystal size distribution of all three shapes can be found in Figures S3, S4, and S5. Here, methane dry reforming was selected as the probe reaction as it is the most susceptible to carbon accumulation.11 The methane dry reforming reaction proceeds via eq 2, ideally producing a 1:1 product ratio, which is favorable for downstream processing into synthetic fuels. CH4 + CO2 ↔ 2CO + 2H 2

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Figure 2. (a) Methane conversion, (b) carbon dioxide conversion, and (c) H2:CO product ratio over Ni/La2O3 catalysts derived from cube-, sphere-, and rod-shaped perovskites for dry reforming of methane at 650 °C, 1:1:8 CH4:CO2:Ar, and a gas-hourly space velocity of 125,000 mL/(gcat·h).

°C, the equilibrium conversions of CH4 and CO2 were calculated to be 85% and 55%, respectively. By contrast, a similar calculation by Yanbing et al. did not consider solid carbon formation and reported the equilibrium conversion of CH4 and CO2 to be 60% and 75%, respectively.13 The equilibrium H2:CO product ratio using this later model was calculated to be 80%. Our measured conversions and productratio for the rods, on which no accumulated carbon was detected, agree well with the thermodynamic model that did not consider solid carbon formation. The equilibrium conversions of both CH4 and CO2 for the cubes were larger (CO2 more than CH4) than calculated by Yanbing, suggesting that the decomposition of CO2 and CO via eqs 3 and 4 is mostly responsible for the excess carbon formation on the cubes.

2CO ↔ C + CO2

(3)

CO2 + 2H 2 ↔ C + 2H 2O

(4)

nanotubes (MWCNTs). The growth of the MWCNTs proceeds largely by the tip-growth mechanism, where carbon accumulates within the Ni crystal, proceeds to grain boundaries, and ultimately begins forming a MWNT which detaches and lifts the Ni particle from its support.14 This mechanism has been reported to be characteristic of large Ni particles which are prone to carbon accumulation and have weaker interactions with their support.15 By contrast, HR-TEM imaging (Figures 3c-f) shows that Ni particles exsolved from sphere- and rod-shaped perovskites are buried tightly into their supports. Dashed lines in the HR-TEM images clearly show the interface of the Ni crystal with the La2O2CO3 support and that it is firmly buried within the support. Ni crystals were identified by measuring the lattice spacing and/or energy dispersive spectroscopy (EDS) analysis (Figure S5). The small size and strong interaction between the Ni particles and the support for the spheres and rods enabled methane reforming to proceed for 100 h with no detectable form of carbon accumulation in the spent samples as the tipgrowth mechanism was unable to proceed. By comparison, carbon accumulation on the cubes was so large that the weight of carbon in the spent sample was almost 4 times the weight of the original catalyst (Figure S7). The fact that the methane conversion rate on the cube-derived catalyst was stable throughout the entire test period, despite severe coking, means that most of the area of the Ni crystals was still exposed to the gas phase. In this case, we would expect complete deactivation of this material once the surface and pores become completely blocked to the gas-phase.16 SEM images of spent catalysts derived from perovskite cubes, spheres, and rods are shown in Figure S8. We observed that the weak catalyst−support interaction found with the cubes was not simply a function of Ni crystal size, as previously thought. The Ni crystal lifted from the surface of the cube in Figure 3b is about 30 nm. The Ni particle size distribution for the spheres and rods (Figures S3, S4, and

Fresh catalyst derived from the cube perovskites were subjected to a mixture of CO, CO2, and H2 at 650 °C and showed significant carbon accumulation (Figure S6) after 30 h, supporting this coking mechanism. The larger CO2 conversion compared to CH4 can also be attributed to the reverse water− gas shift reaction (eq 5) and the ability for the basic lanthanum oxide support to dissociate CO2, which exhibits acidic character. CO2 + H 2 ↔ H 2O + CO

(5)

2.2. Sensitivity of Performance to Perovskite Structure. Figure 3a shows an SEM image of the catalyst derived from a cubic perovskite particle. EDX measurements confirm that each face of the cubic perovskite exsolved 50−70 Ni particles ranging in size from 15 to 35 nm. Figure 3b shows a TEM image of the same particle after 100 h of dry reforming. Severe carbon accumulation is shown on the surface of the cubes, present as both graphite and multiwalled carbon 4201

DOI: 10.1021/acscatal.6b00673 ACS Catal. 2016, 6, 4199−4205

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ACS Catalysis 2LaNiO3 + H 2 → La 2Ni 2O5 + H 2O 0

(9)

La 2Ni 2O5 + 2H 2 → 2Ni + La 2O3 + 2H 2O

(10)

Figure 4 shows temperature-programmed reduction (TPR) of all three perovskite shapes and revealed that the cubes were

Figure 4. Temperature-programmed reduction (TPR) of the cube-, sphere-, and rod-shaped LaNiO3 parent nanoparticles in 5% H2 in N2 at a heating rate of 10 K/min.

reduced via the 2-step pathway, whereas the spheres and rods were reduced by the 3-step mechanism. Although not universally true, TPR peaks observed in the reduction of LaNiO3 have been shown to be associated with individual reduction events.17−20 The key difference in distinguishing between the 2 pathways using TPR is comparing the ratio of the area under the second peak to that of the first. For the 2step mechanism, the second peak should have an area 2× larger than the first peak, and for the 3-step mechanism the second peak should have an area 1.5× larger than the first. After deconvolution, we found that the area ratio for the cubes (2step mechanism) was 1.85, and the area ratio for the spheres and rods (3-step mechanism) was 1.52 and 1.49, respectively. Furthermore, the molar hydrogen uptake in all three cases matched the expected stoichiometric values for complete Ni reduction, namely 1.5 moles of H2 for every mole of LaNiO3. In all cases, the final reduction peak represents the reduction of Ni2+ to metallic Ni. The position of the final reduction peak shows that the Ni crystals exsolved from the cubic parent had a weaker interaction with their support and came about from the La2Ni2O5 phase. By comparison, Ni exsolution from the spinel phase via the 3-step mechanism was observed at higher temperatures, suggesting that Ni crystals formed by this route are more stable. To emphasize this point, rod-shaped parent particles with a spinel structure were synthesized and showed no carbon accumulation after 30 h of methane dry reforming (Figure S9). This result also corroborates the strongly socketed Ni-crystals observed in TEM and the lack of tip-growth carbon accumulation on the sphere- and rod-derived catalysts. We suggest that the reason that the cube perovskites took the 2-step pathway and spheres and rods took the 3-step pathway is linked to structural defects, specifically stacking faults and dislocations. The 3-step pathway goes through a series of Ruddlesden−Popper (RP) structures which are naturally layered; by comparison, the 2-step mechanism involves a “defective” perovskite. Features such as stacking faults may be precursory to the formation of the layered RP structure and force the transition down the 3-step pathway.21,22 Figures 5a and 5c show HR-TEM images of fresh cube- and rod-shaped perovskites, respectively, along with their diffraction patterns. Compared to the cubes, the diffraction pattern for the rods

Figure 3. (a) SEM image of cubic perovskite after solid-phase crystallization via reduction in 50% hydrogen/argon. (b) TEM image of spent catalyst derived from cube perovskite where tip-growth MWCNT is shown lifting a nickel crystal off of the surface. (c) TEM image of a nickel crystal exsolved from a perovskite sphere. (d) HRTEM of the same. (e) TEM image of a nickel crystal exsolved from a perovskite rod. (f) HR-TEM image of the same. Yellow and red dashed lines show the edge of the support and Ni crystal, respectively, to emphasize how socketing provides a strong catalyst−support interaction.

S5) showed that 5−10% of the Ni crystals exceeded this size, yet no MWCNT growth or lifting was observed. The source of this added stability is likely connected to the solid-phase crystallization pathway. There is significant confusion in the literature about which reduction pathway LaNiO3 takes to form the final Ni/La2O3 phase or what factors influence the reduction pathway. Moreover, papers that confirm a particular pathway rarely mention the alternatives or give insight as to why seemingly similar materials would give different reduction profiles.17−20 In one such pathway, LaNiO3 reduction takes place as a 3-step process as shown below 4LaNiO3 + 2H 2 → La4Ni3O10 + Ni0 + 2H 2O

(6)

La4Ni3O10 + 3H 2 → La 2NiO4 + 2Ni0 + La 2O3 + 3H 2O (7) 0

La 2NiO4 + H 2 → Ni + La 2O3 + H 2O

(8)

An alternative 2-step mechanism is also reported 4202

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38 nm, and the Ni crystals exsolved on their surface had an average diameter of 12.5 nm. A spherical Ni crystal with this diameter contains approximately 25,000 unit cells of FCC Ni (100,000 Ni atoms). The perovskite rods were found to be composed entirely of the rhombohedral phase with a lattice parameter of 5.45 Å. With the unidirectional diffusion condition, the perovskite would be able to supply at most 38,000 Ni atoms to the growing crystal (= max number of LaNiO3 unit cells underneath the nickel crystal throughout the thickness of the rod, 1 Ni atom/cell) accounting for only 1/3 of the Ni crystal size. As a result, Ni2+ transport during the crystallization process necessarily occurs in 3 dimensions. The resulting depletion regions should also extend in 3 dimensions (e.g., hemispheres) as depicted in Figure 6. Such hemispherical depletion may limit the lateral proximity that one growing Ni crystal can have with its neighbor in perovskites whose depletion regions are comparable to the dimensions of the parent particle. This mechanism is advantageous for resistance against sintering as the exsolved particles are separated by some minimum distance at the time of their creation in a process limited by depletion. By contrast, if the Ni2+ depletion region created by the formation of a single Ni crystal were small relative to the size of its parent particle, this region could be replenished by Ni2+ ions deeper in the bulk, and the second nickel crystal could exsolve in close proximity to the first. Small lateral distances between exsolved Ni-crystals in conjunction with a weak Ni-support interaction (as observed with the Ni-crystals exsolved from the cubes) can lead to sintering. Sintering of Ni particles on the cubes is evidenced in this work by the larger Ni crystals, their highly nonspherical morphology, and the many instances of close proximity as seen in Figure 1. The superiority of the rods over the spheres for catalytic activity is likely due to the fact that the Ni particles exsolved from the rods are both higher in number and smaller in size. Highly anisotropic perovskites with a large number of defects may therefore be an ideal parent nanoparticle design, because it gives rise to reforming catalysts which provide a large number of stable nickel crystals capable of resisting sintering and carbon accumulation.

Figure 5. (a) HRTEM image of a LaNiO3 cube with the diffraction pattern of the boxed region inset in the upper left corner. (b) Inverse FFT of the (012) diffraction regions. (c) HRTEM image of a LaNiO3 with the diffraction pattern of the boxed region inset in the upper left corner. (d) Inverse FFT of the (012) diffraction regions where arrows indicate possible stacking faults. (e) Crystal model of rod-shaped perovskites showing the La-Ox/Ni-Ox stacking in the (012) direction where faults are observed; La-green, Ni-white, O-red, (012) plane shown in orange.

shows a pattern of featureless streaks, typical of a faulted structure. Figures 5b and 5d show an inverse FFT of the (012) plane for both the cubes and the rods. The arrows in Figure 5d indicate the location of stacking faults in the rods, a feature absent in the cubes. Stacking faults were distinguished from dislocations by rotation of the sample in the TEM. We therefore see that defects can impact the phase from which the final Ni reduction step occurs. This effect, in addition to the Ni crystals size, plays an important role in the ultimate stability of the catalyst. An additional characteristic providing stability to the catalyst is resistance against Ni-particle sintering. This can be achieved by strong Ni-support interactions in addition to limiting the proximity of surface Ni crystals.23 When the characteristic dimensions of the parent perovskite particle are small, depletion of Ni2+ within the lattice during solid-phase crystallization can play an impactful role. Such depletion regions can be shown to be 3-D in nature, since unidirectional diffusion of Ni-ions cannot account for the size of exsolved Ni crystals. For example, the rod-shaped perovskites synthesized in this work had an average thickness of

3. CONCLUSIONS Hydrothermal and chemical precipitation routes were adopted for synthesis of cube-, sphere-, and rod-shaped LaNiO3 nanoparticles. Utilization of spheres and rods as the parent LaNiO3 shape was found to be highly promising for application in the dry reforming of methane at 650 °C. By contrast, catalysts derived from the cubes exhibited extreme amounts of carbon accumulation and evidence of Ni particle sintering. The rod- and sphere-like perovskite exhibited excellent catalytic activity with no carbon formation even after 100 h. Stacking faults are suggested to play a key role in determining the reduction pathway taken by the parent perovskite nanoparticle in order to form the final catalyst phase. The remarkable stability of the spheres and rods is explained by small size and high stability of the exsolved Ni crystals brought about by the 3step reduction pathway. Utilization of high-aspect ratio parent shapes (e.g., rods) may prove to be an ideal structure as many Ni crystals can be formed whose minimum separation distance is influenced by ion-depletion during the reduction step. 4203

DOI: 10.1021/acscatal.6b00673 ACS Catal. 2016, 6, 4199−4205

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Figure 6. (top-left) Schematic diagram of Ni particle formation and growth of hemispherical Ni2+ depletion regions within a rod shaped LaNiO3 particle. The large depletion regions, being on the same length scale as the parent particle, help to maintain an even size and distance between exsolved particles as new particles cannot form over regions depleted of Ni ions. (bottom-left) Schematic diagram of Ni particle formation in a parent perovskite where the Ni-ion depletion region is small relative to the perovskite size. This permits the depletion regions to be regenerated by Ni-ions deeper in the bulk and for Ni particles to form in closer proximity, making them susceptible to sintering. (right) TEM image showing wellspaced and spherical Ni particles formed from a rod-shaped perovskite.

calcined at 800 °C for 2 h to obtain finally a black powder sample of LaNiO3. A well-defined cube shape LaNiO3 was synthesized by a hydrothermal method reported elsewhere.24 In the typical hydrothermal synthesis, 0.433 g of La(NO3)3·6H2O and 0.290 g of Ni(NO3)2·6H2O were dissolved in 70 mL of Milli-Q water. Then, 0.30 g of PVP and 0.375 g of glycine were added into the solution mixture. The pH of the solution mixture was adjusted to 7.7 by NH3 solution. The solution mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 180 °C temperature for 12 h. The product was collected by centrifugation and washed 4−5 times with Milli-Q water and ethanol. This powder was calcined at 650 °C for 2 h in air atmosphere to form a nanocube of LaNiO3. 4.2. Characterization Techniques. X-ray diffraction (XRD) patterns were recorded on a Scintag−powder diffractometer equipped with a liquid nitrogen cooled germanium solid state detector with Cu Kα radiation. X-ray photoelectron analysis (XPS) was performed on a 5600 AES/ XPS system (PHI, USA) using Al Kα (hυ = 1486.6 eV) as the excitation source. The scanning electron micrographs were taken by a Quanta 200 FEG (field-emission gun) Environmental Scanning Electron Microscope (ESEM). The transmission electron micrographs (TEM) were taken by Field Emission TEM (Tecnai F20, Philips) operated at 200 kV. Thermogravimetric analyses (TGA) were carried out by a TA instruments TGA 2950 thermogravimetric analyzer. The surface area and temperature program reduction (TPR) were recorded by a PulsarBET gas analyzer (Quantachrome) instruments. Prepared perovskites were placed in a quartz tube (11 mm OD) fixed between two pieces of quartz wool. Dry reforming performance of each catalyst was tested on an

4. MATERIALS AND METHODS 4.1. Synthesis of Shape Controlled LaNiO3 Nanostructures. A facile coprecipitation route was adopted to synthesize well dispersed spherical LaNiO3 perovskite nanoparticles. In a typical synthesis, 0.433 g of La(NO3)3·6H2O and 0.290 g of Ni(NO3)2·6H2O were dispersed in 80 mL of Milli-Q water in a round bottle flask by ultrasonication for 5 min. Afterward, a 20 mL NaOH solution (0.3 g of NaOH pearls in 20 mL of Milli-Q water) was directly added into an 80 mL solution mixture. The solution mixture quickly turned turbid, and formation of a greenish-white precipitate is instantly observed. Finally, a 100 mL solution mixture was kept on a hot plate under continuous stirring, and the solution temperature was raised to 60 °C in refluxing conditions for 4 h. The obtained sample was centrifuged and washed 4−5 times with Milli-Q water and ethanol. The sample was dried at 70 °C overnight and calcined at 650 °C for 2 h to obtain finally a black powder sample of LaNiO3. The only difference in the synthesis procedure of the rods was the addition of surfactant and the adding manner of NaOH. Briefly, a solution was prepared by dissolving 0.433 g of La(NO3)3·6H2O and 0.290 g of Ni((NO3)2·6H2O in 100 mL of Milli-Q water in the round bottle flask. Then, 150 mg of polyvinylpyrrolidone (PVP) surfactant was added in the solution mixture to tune the size of particles. Afterward, 10 mM NaOH is added drop by drop into the solution mixture to maintain pH 12 under constant stirring. Finally, the solution was kept on a heating mantle with a magnetic stirrer under refluxing conditions at 75 °C for 6 h. The resulting product was centrifuged and washed 4−5 times with Milli-Q water and ethanol. The solid sample was dried at 70 °C overnight and 4204

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ACS Catalysis identical surface area of perovskite (0.3 m2) under the same gas hourly space velocity 25,000 mL/(g·h). Prior to dry reforming catalysis, the perovskite nanoparticles were heated to 800 °C for 1 h in 50 vol % H2 atmosphere in order to drive the solidphase crystallization process. The resultant material, Ni/La2O3, was then cooled to 650 °C, and the catalytic reaction was initiated by feeding a mixture of CH4 and CO2 with a molar ratio of unity over the catalyst. The conversion of methane and carbon dioxide into carbon monoxide and hydrogen was monitored using an SRI gas chromatograph fitted with MS-13X and Haysep-C packed columns. Hydrogen was detected using a nitrogen carrier and an TCD detector. CO2, CO, and CH4 were detected by an FID detector with a methanizer placed directly upstream.

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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/acscatal.6b00673. SEM images of cubes before and after calcination, TEM micrographs of fresh LaNiO3 and spent cubes, spheres, and rods, size distribution of LaNiO3 and Ni crystals, TGA plots of spent cubes, spheres, and rods (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.A.R. would like to acknowledge the Israeli Department of Energy (Grant# 215-11-033) for partial funding. S.S. would like to thank the Israeli Planning and Budgeting Committee (PBC) for partial funding. The authors want to thank Dr. Yuri Rosenberg for collection of the XRD data, Dr. George Levi and Prof. Amit Kohn for assistance with TEM imaging, and Prof. Noam Eliaz for the use of laboratory space. The authors also want to thank Gil Hayoun for assisting with the construction of the experimental assemblies. The authors also want to thank James Rondinelli (Northwestern) for providing insights into the LNO reduction process.

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DOI: 10.1021/acscatal.6b00673 ACS Catal. 2016, 6, 4199−4205