Influence of LaNiO3 Shape on Its Solid-Phase Crystallization into

Subscriber access provided by UCL Library Services

Article 3

Influence of LaNiO shape on its solid-phase crystallization into coke-free reforming catalysts Sarika Singh, Daria Zubenko, and Brian A. Rosen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00673 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Influence of LaNiO3 shape on its solid-phase crystallization into coke-free reforming catalysts Sarika Singh1, Daria Zubenko1, Brian A. Rosen1* 1. Department of Materials Science and Engineering, Tel Aviv University, 55 Haim Levanon Street, Ramat Aviv 6139001, ISRAEL *[email protected]

ABSTRACT: Shape-controlled LaNiO3 nanoparticles were prepared by modified hydrothermal and precipitation routes resulting in cubes, spheres, and rods. The solidphase 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 Ni+2-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 hours of reforming while those derived from cubes showed excessive carbon accumulation and signs of sintering. All the three catalysts are characterized by X-ray diffraction (XRD), scanning electron transmission microscopy (SEM), transmission electron microscopy (TEM), hydrogen 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 sape and structure of Perovskite precursors. KEWWORDS: Perovskite, Shape control, Dry reforming of methane, Solid phase crystallization, Carbon formation, Defects

ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.

Page 2 of 22

INRODUCTION:

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 wellordered 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 non-stoichiometric 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 Equation 1. A1-αBO3-δ,lim → (1-α)(ABO3- δ) + αB

(1)

This work investigates how shape-control of the parent LaNiO3 Perovskite nanoparticle can be exploited to imbue a favorable size and spatial distribution of


Page 3 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

exsolved Ni crystals, a strong catalyst-support interaction, and provide for coke- and sinter-free 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 influences 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 a cubic, quasi-spherical (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 hours 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 distribution of the parent Perovskites and exsolved Ni particles were measured by analyzing approximately 200 particles over many TEM images using Image J 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


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 22

via Equation 2, ideally producing a 1:1 product ratio, which is favorable for downstream processing into synthetic fuels. CH4 + CO2 ↔ 2CO + 2H2

(2)

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.

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.


Page 5 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 1: Geometric data for the Perovskites and the Ni crystals exsolved from a Perovskite of a particular shape LaNiO3

LaNiO3 Phase composition

shape Cubes

Rhombohedral – 58%

BET surface area

Avg. Perovskite

Avg. Ni Crystal Size

(m2/g)

Size (nm)

(nm)

10.44

367

21

10.69

89

18

9.20

109 (L) x 38 (H)

13

Cubic – 42% Spheres


Rods


Cubic – 35%

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 rod-shaped parent Perovskites. Crystallographic data and size distribution statistics in Figures 1 and 2 allowed us to estimate the methane turnover number 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).


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 22

(b)

(a)

(c)

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 gashourly space velocity of 125,000 mL/(gcat∙h).

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-ofstate.12 At 650°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


Page 7 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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 Equations 3 and 4 is mostly responsible for the excess carbon formation on the cubes. 2CO ↔ C + CO2

(3)

CO2 + 2H2 ↔ C + 2H2O

(4)

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 hours, supporting this coking mechanism. The larger CO2 conversion compared to CH4 can also be attributed the reverse water-gas shift reaction (Equation 5) and the ability for the basic lanthanum oxide support to dissociate CO2, which exhibits acidic character. CO2 + H2 ↔ H2O + CO

(5)

2.2 Sensitivity of performance to Perovskite structure Figures 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 15-35nm. Figure 3b shows a TEM image of the same particle after 100 hours of dry reforming. Severe carbon accumulation is shown on the surface of the cubes, present as both graphite and multi-walled carbon nano-tubes (MWCNTs). The growth of the MWCNTs proceeds largely by the tipgrowth 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


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 hours with no detectable form of carbon accumulation in the spent samples as the tip-growth 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 gasphase.16 SEM images of spent catalysts derived from Perovskite cubes, spheres, and rods are shown in Figure S8.


Page 8 of 22

Page 9 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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 the how socketing provides a strong catalyst-support interaction.

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 30nm. The Ni particle size distribution for the spheres and rods (Figures S3, S4 and S5) showed that 5-10% of the Ni


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 22

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 + 2H2 → La4Ni3O10 + Ni0 + 2H2O

(6)

La4Ni3O10 + 3H2 → La2NiO4 + 2Ni0 + La2O3 + 3H2O

(7)

La2NiO4 + H2 → Ni0 + La2O3 + H2O

(8)

An alternative 2-step mechanism is also reported: 2LaNiO3 + H2 → La2Ni2O5 + H2O La2Ni2O5 + 2H2 → 2Ni0 + La2O3 + 2H2O

(9) (10)

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


Page 11 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 4 shows temperature programmed reduction (TPR) of all three Perovskite shapes and revealed that the cubes were 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 2-step mechanism, the second peak should have an area 2x larger than the first peak, and for the 3-step mechanism the second peak should have an area 1.5x larger than the first. After deconvolution, we found that the area ratio for the cubes (2-step 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 Ni+2 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 30h (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


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

stacking faults and dislocations. The 3-step pathway goes through a series of Ruddlesden-Popper (RP) structures which are naturally layered; by comparison, the 2step 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 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.


Page 12 of 22

Page 13 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(E)

[012]

Figure 5: (a) HRTEM image of a LaNiO3 cube with the diffraction pattern the of 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 the of boxed region inset in the upper left corner (d) inverse FFT of the (012) diffraction regions where arrows indicate possible stacking faults and (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.

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 Niparticle 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 Ni+2 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 38


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 5.45Å. 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, Ni+2 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 Ni+2 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 Ni+2 ions deeper in the bulk and the second nickel crystal could exsolve in close proximity to the first.


Page 14 of 22

Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 6: (Top-left) Schematic diagram of Ni particle formation and growth of hemispherical Ni+2 depletion regions within a rod shaped LaNiO3 particle. The large depletion regions, being on the same length scale as the parent particle, helps 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 well-spaced and spherical Ni particles formed from a rod-shaped Perovskite.

Small lateral distances between exsolved Ni-crystals in conjunction with a weak Nisupport 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 non-spherical morphology, and the many instances of close proximity from Figure 1.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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. 3.

CONCLUSIONS

Hydrothermal and chemical precipitation routs 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 in the parent Perovskite are suggested 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 3-step 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 iondepletion during the reduction phase.


Page 16 of 22

Page 17 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

4.

MATERIALS AND METHODS

4.1 Synthesis of shape controlled LaNiO3 nanostructures A facile co-precipitation route was adopted to synthesize well dispersed spherical LaNiO3 perovskite nanoparticles. In a typical synthesis, 0.433 gm of La(NO3)3.6H2O and 0.290 gm of Ni(NO3)2.6H2O were dispersed in 80 mL of Milli-Q water in a round bottle flask by ultra-sonication for 5 min. Afterwards, 20 mL NaOH solution (0.3 gm NaOH pearls in 20 mL Milli-Q water) were directly added into 80 mL solution mixture. The solution mixture quickly turned turbid, and formation of greenish-white precipitate is instantly observed. Finally, 100 mL solution mixture was kept on hot plate under continuous stirring and solution temperature was raised at 60 0C in refluxing conditions for 4h. The obtained sample was centrifuged, washed 4-5 times with Milli-Q water and ethanol. The sample was dried at 70 0C overnight and calcined at 650 0C for 2h to obtain finally black powder sample of LaNiO3. The only the 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 gm La(NO3)3.6H2O and 0.290 gm Ni((NO3)2.6H2O in 100 mL MilliQ of water in the round bottle flask. Then, 150 mg of polyvinyl pyrrolidone (PVP) surfactant was added in the solution mixture to tune the size of particles. Afterwards, 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 heating mantle with a magnetic stirrer under refluxing conditions at 75 0C for 6h. The resulted product was centrifuged, washed 4-5 with Milli-Q water and ethanol. The solid sample was dried at 70 0C overnight and calcined at 800 0C for 2h to obtain finally black powder sample of LaNiO3.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A well-defined cube shape LaNiO3 was synthesized by hydrothermal method reported elsewhere.24 In the typical hydrothermal synthesis, 0.433 gm of La(NO3)3.6H2O and 0.290 gm Ni(NO3)2.6H2O, 0.3 gm were dissolved in 70 mL Milli-Q water. Then, 0.30 gm PVP and 0.375 gm of glycine was added in the solution mixture. The pH of solution mixture was adjusted to 7.7 by NH3 solution. The solution mixture are transferred to Teflon-lined stainless steel autoclave and heated at 180 0C temperature for 12h. The product is collect by centrifugation and washed with 4-5 times with Milli-Q water and ethanol. This powder is calcined at 650 0C for 2 h in air atmosphere to form nanocube of LaNiO3. 4.2 Characterization techniques X-ray diffraction (XRD) patterns were recorded on a Scintag – powder diffractometer equipped with liquid nitrogen cooled germanium solid state detector with Cu Kα radiation. X-ray photoelectron analysis (XPS) analysis were performed on a 5600 AES/XPS system (PHI, USA) using Al Kα (h = 1486.6 eV) as the excitation source. The scanning electron micrographs were taken by 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 200KV. Thermo-gravimetric analyses (TGA) were carried out by TGA 2950 thermogravimetric analyzer. The surface area and temperature program reduction (TPR) were recorded by ChemBET Quantachrome instruments. Prepared Perovskites were placed in a quartz tube (11mm OD) fixed between two pieces of quartz wool. Dry reforming performance of each catalyst was tested on an 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 hour in 5 vol% H2 atmosphere in order to drive the solid-phase


Page 18 of 22

Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

crystallization process. The resultant material, Ni/La2O3, was then cooled to 650 °C and the catalytic reaction was initiated by feeding 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 and FID detector with a methanizer placed directly upstream. SUPPORTING INFORMATION 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 cube, sphere and rod. AUTHOR CONTRIBUTIONS BAR conceived of the idea and planned the experiments. SS carried out the catalytic testing, and all materials analysis with the help of DZ. BAR drafted the manuscript with critical comments provided by SS and DZ. ACKNOWLEDGEMENTS BAR would like to acknowledge the Israeli Department of Energy (Grant# 215-11033) for partial funding. SS 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 and 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.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

REFERENCES (1)

Liu, C. j.; Ye, J.; Jiang, J.; Pan, Y. ChemCatChem 2011, 3, 529-541.

(2)

Lu, J.; Fu, B.; Kung, M. C.; Xiao, G.; Elam, J. W., Kung, H. H.; Stair, P. C. Science 2012, 335, 1205-1208.

(3)

Budiman, A. W.; Song, S.-H.; Chang, T.-S.; Shin, C.-H.; Choi, M.-J. Catalysis Surveys from Asia 2012, 16, 183-197.

(4)

Farmer, J. A.; Campbell, C. T. Science 2010, 329, 933-936.

(5)

Zhao, E. W.; Zheng, H.; Ludden, K.; Xin Y.; Hagelin-Weaver, H. E.; Zhao, Bowers, C. R. ACS Catal. 2016, 6, 974-978.

(6)

Meille, V.; Appl. Catal. A: Gen. 2006, 315, 1-17.

(7)

Frontera, P.; Macario, A.; Aloise, A.; Antonucci, P. L.; Giordano, G.; Nagy, J. B. Catal. Today 2013, 218, 18-29.

(8)

Neagu, D.; Oh, T.; Miller, D.N.; Ménard, H.; Bukhari, S.M.; Gamble, S.R.; Gorte, R.J.; Vohs, J.M.; and Irvine, J.T.S. Nat. Commun. 2015, 6:8120, doi:10.1038/ncomms9120.

(9)

Nair, M. M.; Kaliaguine, S.; Kleitz, F. ACS Catal. 2014, 4, 3837−384.

(10) Rivas, I.; Alvarez, J.; Pietri, E.; Pérez-Zurita, M. J.; Goldwasser, M. R. Catal. Today 2010, 149, 388-393. (11) Courson, C.; Makaga, E.; Petit, C.; Kiennemann, A. Catal. Today 2000, 63, 427-42. (12) Nikoo, M. K.; Amin, N. A. S.; Fuel Process. Technol. 2011, 92, 678-691. (13) Li, Y.; Jin, B.; Xiao, R. Korean J. Chem. Eng. 2007, 24, 688-692. (14) Rostrup-Nielsen, J. R.; Sehested, J. Studies in Surf. Sci.Catal. 2001, 139, 1-12. (15) Gohier, A.; Ewels, C.; Minea, T.; Djouadi, M. Carbon 2008, 46, 1331-1338. (16) Rostrum-Nielsen, J.R., PE Højlund Nielsen. Catalyst deactivation in synthesis gas production, and important syntheses. 1985, Marcel Dekker: New York, NY USA. (17) Crespin, M.; Levitz, P.; Gatineau, L. J. Chem. Soc., Faraday Trans. 2: Mol. Chem. Phys. 1983, 79, 1181-1194. (18)

Gallego, S. G.; Batiot-Dupeyrat, C.; Barrault, J.; Mondragón, F. Ind. Eng. Chem. Res. 2008, 47, 9272-9278.

(19)

Batiot-Dupeyrat, C.; Valderrama, G.; Meneses, A; Martinez, F.; Barrault, J.; Tatibouët, J. M. Appl. Catal. A: Gen. 2003, 248, 143-151.


Page 20 of 22

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(20)

Lima, S. M. de; Assaf, J. M. Catal. Lett. 2006, 108, 63-70.

(21)

Zvereva, I. A.; Popova, V. F.; Vagapov, D. A.; Toikka, A. M.; Gusarov, V. V. Russian J. Gen. Chem. 2001, 71, 1181-1185.

(22)

Šturm, S.; Rečnik, A.; Scheu, C.; Čeh, M. J. Mater.Res. 2000, 15, 2131-2139.

(23)

Han, J. W.; Kim, C.; Park, J. S.; Lee, H. ChemSusChem 2014, 7, 451-456.

(24)

Zhang, J.; Zhao, Y.; Zhao, X.; Liu, Z.; Chen, W. Sci. Rep. 2014, 4, 6005.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

**TABLE OF CONTENT FIGURE** 62x46mm (96 x 96 DPI)


Page 22 of 22

Influence of LaNiO3 Shape on Its Solid-Phase Crystallization into

Recommend Documents