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Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths Md. M. Hasan, Sanchita Dey, Nazia Nafzin, John Mardinly, Pratik P Dholabhai, Blas P. Uberuaga, and Ricardo H. R. Castro Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02577 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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Chemistry of Materials

Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths Md M. Hasan1, Sanchita Dey1, Nazia Nafsin1, John Mardinly2, Pratik P. Dholabhai3, Blas P. Uberuaga3, and Ricardo H. R. Castro1,* 1

Department of Materials Science and Engineering & NEAT ORU, University of California,

Davis, CA 95616, USA.; 2 John Cowley Center for HREM, LE-CSSS, Arizona State University, Tempe, AZ 85281, USA; 3 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. *Corresponding author: [email protected]; FAX: (530) 752-9307; Tel.: (530) 752-3724 Abstract. Surface energy is a key parameter to understand and predict the stability of catalysts. In this work, the surface energy of MgAl2O4, an important base material for catalyst support, was reduced by using dopants prone to form surface excess (surface segregation): Y3+, Gd3+, and La3+. The energy reduction was predicted by atomistic simulations of spinel surfaces and experimentally demonstrated by using microcalorimetry. The surface energy of undoped MgAl2O4 was directly measured as 1.65±0.04 J/m2 and was reduced by adding 2 mol% of the dopants to 1.55±0.04 J/m2 for Y-doping, 1.45±0.05 J/m2 for Gd-doping and 1.26±0.06 J/m2 for La-doping. Atomistic simulations are qualitatively consistent with the experiments, reinforcing the link between the role of dopants in stabilizing the surface and the energy of segregation. Surface segregation was experimentally assessed using electron energy loss spectroscopy 1 ACS Paragon Plus Environment


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mapping in a scanning transmission electron microscopy image. The reduced energy resulted in coarsening inhibition for the doped samples and, hence, systematically smaller particle sizes (larger surface areas), meaning increased stability for catalytic applications. Moreover, both experiment and modeling reveal preferential dopant segregation to specific surfaces, which leads to the preponderance of {111} surface planes, and suggests a strategy to enhance the area of desired surfaces in nanoparticles for better catalyst support activity.

Introduction Nanoparticles are inherently interesting for chemical catalysis due to their enlarged specific surface areas.

1-5

However, nanostructures are generally unstable due to the increased energy

coming from the excess surface term, resulting in a strong tendency for growth or agglomeration when exposed to elevated or moderate temperatures (high enough to activate atomic mobility) during operation or processing. This is especially problematic for oxide nanoparticles, where the synthesis and processing temperatures may be quite high – in particular for multi-elemental compositions.

6

Several approaches have been reported to effectively reduce coarsening in

nanoparticles by using organic caps as diffusion barriers.

7-9

However, high temperature

environments limit the effectiveness of strategies relying on organic additives and require other solutions. Ionic dopants have shown to be successful in stabilizing nanoparticles on a thermodynamic basis.

10-12

Ions prone to segregation to the surface could decrease the surface energy of the

particles, decreasing the driving force for coarsening, as demonstrated in SnO2, CeO2 and other oxides.

6, 13

Using a thermodynamic model proposed by Hillert,

14

Krill et al. derived an

expression for segregation-induced reduction in grain boundary energy which is applicable for

2 ACS Paragon Plus Environment

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surface segregation.

15

In the model, the surface energy decreases as the surface becomes

enriched with the dopant, following the expression: γ = γ − Γ∆H

(1)

Here, γ is the surface energy of the doped material, γ0 is the surface energy of the undoped material, and ∆Hseg is the enthalpy of segregation, which is multiplied by the specific solute excess at the surface, Γ. The negative sign in the equation assumes that a positive enthalpy of segregation indicates a tendency for spontaneous segregation. This expression assumes a dilute concentration of dopant such that dopants are not interacting with one another at the surface. Although primarily used by assuming an average surface energy of the system and identical segregation to all surfaces, this equation is valid when applied individually to each surface plane exposed in the system. In this case, each plane should have a distinct energy of segregation. This concept suggests an anisotropic distribution of dopants in a nanoparticle that can be optimized to design particular surface morphologies, as will be further explored in this paper. MgAl2O4 is isostructural to gamma-alumina but does not show the detrimental phase transformation to the alpha polymorph at elevated temperatures, being therefore an important support material for high temperature catalysts. However, obtaining the spinel phase requires relatively high temperatures to avoid formation of constituent MgO and Al2O3 phases

16, 17

and

such temperatures may activate coarsening mechanisms, leading to undesirable grain enlargement. We hypothesize it is possible to increase nanostability of MgAl2O4 by using dopants prone to segregate to its surfaces, lowering surface energies and decreasing coarsening driving force. Most doped MgAl2O4 studies found in the literature focus on enhancement of optical properties with the dopant in solid-solution in the matrix.


18-20

However, grain boundary


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segregation has been observed in rare-earth doped spinel. 21, 22 Although the reports do not focus on the effects of the dopants on the system energetics, segregation was clearly observed, suggesting a consequent decrease in grain boundary energy. Based on these previous observations, in this work we selected three rare earth elements (Y, Gd, La) as isovalent dopants in MgAl2O4 nanoparticles, hypothesizing surface segregation would equally occur. The dopants have different ionic radii, so that the effect of ionic radius on the segregation behavior and energetic trends can be evaluated. Surface energies of the undoped and doped MgAl2O4 were directly assessed using microcalorimetric techniques

6, 13, 23, 24

and correlated with dopant

segregation profiles found from atomistic simulations and nanostructure stability. Theoretical description of surfaces and grain boundaries can also provide important insights on the effect of dopants on the overall system behavior.

25-28

Atomistic simulations and theoretical

calculations using density functional theory on MgAl2O4 surface structures and surface energies have been previously reported in the literature.

29-31

Recently, using atomistic calculations, we

reported the relative segregation tendency of several dopants on low index surfaces of stoichiometric MgAl2O4 spinel.

32

Assuming an arbitrary surface dopant concentration, we also

reported the surface energies of the doped surfaces, and using those energies we predicted the equilibrium morphologies of doped spinel particles. As the surface energy of doped surfaces is dependent on the surface dopant concentration, the surface energy of doped nanoparticles should depend on the nanoparticle size and the percentage of doping. Hence, using the segregation energies and surface energies of undoped surfaces from our previous work, here we predicted the equilibrium morphologies of the doped spinel nanoparticles assuming dimensions and doping concentrations similar to our experimental nanoparticles. From the equilibrium morphologies, the overall surface energies of the undoped and doped nanoparticles were calculated. Direct


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correlation with the experimental assessment of energetics was found, and it proves that rare earths can be used to increase stability of the nanoparticles on a thermodynamic basis. The results also provide valuable insights on effects of dopants on the control of nanoparticle morphology.

Experimental Procedures Synthesis of undoped and doped spinel nanoparticles The reverse-strike co-precipitation method was used to synthesize pure (undoped) and doped spinel nanoparticles. The technique has been reported to provide isotropic particle morphology and compositional homogeneity.

33

None of the cations used in this work is a transition metal

ion, so the problem associated with the stoichiometry reported elsewhere for this technique is not encountered here.

33

Metal nitrates from Sigma Aldrich Inc. were used as the precursors. The

water contents of the nitrates were measured using differential scanning calorimetry coupled with thermogravimetry (DSC/TG) before the synthesis. 1M solution of metal nitrates in stoichiometric ratio of 1 mol Mg to 2 mol Al ions in water was added drop wise into the 5M NH4OH aqueous solution. Excess NH4OH was used to maintain a high pH (~10) throughout the whole co-precipitation process. The precipitate (metal hydroxides) was separated from the supernatant using a centrifuge operated at 3000 rpm. The obtained powder was washed using deionized water and ethanol followed by centrifugation at 3000 rpm. The precipitate was dried overnight at 100 °C and later calcined at 800°C for 6 h under air. The calcination schedule was determined from DSC/TG run of the precipitate using synthetic air flow at 20ml/min. For the doped spinel, metal nitrate of the corresponding dopant was used as a precursor and was added to the Mg- and Al-nitrate mixture before the co-precipitation. Y, Gd, and La were



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used separately to obtain three doped spinels where 2 mol% of the trivalent ions are dopant ions and 98 mol% are Al ions. The calcination schedule was the same as for the undoped spinel. Sample Characterization Powder X-ray diffraction patterns were obtained using a Bruker AXS D8 Advance powder diffractometer (Bruker AXS, Madison, Wisconsin) (CuKα radiation, λ= 1.5406 Å) operated at 40 kV and 40 mA. JADE 6.1 (MDI) software was used to perform a whole pattern profile fitting to determine crystal structure, phase purity, and crystallite size. Lattice constants were calculated from XRD full spectra of the samples mixed with a standard silicon (JCPDS # 27-1402) powder sample. Surface area was calculated by the Brunauer−Emmett−Teller (BET) technique 34 using a Micromeritics Gemini VII Surface Area Analyzer equipped with VacPrep 061 degas station (Micromeritics Instrument Corporation, Norcross, Georgia). The degassing temperature of the spinel for the BET measurement was determined from the thermogravimetry analysis of the spinel nanopowders. Particle size distributions and particle morphologies were determined through transmission electron microscopy (TEM) using a JEOL JEM-2500SE transmission electron microscope (Jeol Ltd., Tokyo, Japan) operated at 200 kV. Surface segregation of La in MgAl2O4 nanoparticles was determined by electron energy loss spectroscopy (EELS) using a JEOL ARM 200F transmission electron microscope. Spectrum images were recorded using an Enfinium™ spectrometer. The EELS spectra were recorded with a pixel dwell time of 0.2 s and a dispersion of 0.25 eV/channel. To facilitate imaging, the nanoparticles were coarsened before mapping, and therefore they show larger grains as compared to the as-synthesized nanoparticles. Surface Energy Measurements Water adsorption microcalorimetry was used to measure the surface energy of the undoped and doped spinels. It is important to note that surface energy is a very small thermodynamic quantity,


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and to distinguish between surface energies of surfaces doped with different dopants, highly sensitive instrumentation is required for reliable assessment. Castro and Quach 23 have proposed the use of the microcalorimetry of water adsorption to probe the surface energy of oxide nanoparticles in both hydrated and anhydrous states, where hydrated refers to the state where the surface is covered water molecules to some degree and the anhydrous state refers to the complete absence of water attached to the surface. This method has been proven effective for oxides 6, 13, 23, 24

and is based on a thermodynamic correlation between the heat of water adsorption on the

surface and the surface energy itself. In this work, this technique was used to measure the surface energies of doped and undoped spinel. In a typical procedure, the amount of adsorbed water at the powder surface and the corresponding enthalpy change is simultaneously measured combining two instruments: a Micromeretics ASAP 2020 and a Setaram Sensys Evolution Calvet microcalorimeter. In this method, one uses the heat of water adsorption as a function of the total coverage to calculate the surface energy of the anhydrous state using the following equation: γ = γ + θ ∙ ΔH

(2)

Here, θ is the surface water coverage, ∆H is the enthalpy of water adsorption, γ is the surface energy of the anhydrous surface, and γ is the surface energy at the coverage of θ. The experiment is performed up to the point where the heat of adsorption converges to -44 kJ.mol-1 (heat of liquefaction of water vapor), which suggests that the surface of the particle is converging to that of liquid-water itself, which has a defined surface energy, 0.072 J.m-2. Equation 2 gives a pathway to measure the surface energy of an anhydrous solid from a set of known coverages and surface energies. Defining the exact first point where the heat of adsorption reaches -44 kJ/mol-1 is critical for this method. This is because further adsorption will result in the same heat of



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adsorption but no changes in the surface energy as the method assumes negligible contribution of the chemical potential change of water molecules themselves to the system. This is a good assumption for relatively low coverage studies, but higher pressures will invalidate equation 2 for accurate surface energy determinations. The reader is referred to recent publications regarding this topic for further clarifications. 35 A total surface area of about 2 m2 was used for each sample, i.e. based on BET measurements, a total of powder mass enough to provide a total of 2 m2 of area was used. This is important as this area has been demonstrated to be enough to provide reliable adsorption-heat signals in the calorimetric measurements. Prior to analysis, the sample was degassed under vacuum for 12 h at 750 °C using the Micromeritics ASAP 2020 to get an anhydrous surface condition (confirmed by thermogravimetric analysis). After that, the sample was kept at 25 °C for the rest of the experiment (controlled by the calorimeter), and 100 m2.g-1 at 1000 °C. This effect is reflected also in the crystallite sizes, as shown in Figure 8b, with the La doped spinel maintaining


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8nm crystallite size at 1000 °C. There is definitely some level of coarsening still occurring in all the samples, with reduction of the surface area and increase in crystallite size, but it is evident that rare-earth doping increases stability by decreasing the surface energy. It is important though that the stability in terms of the effect of the dopants on the crystall size is much more impressive as compared to the effect on the surface area itself. This suggests that the rare earths are also acting in the solid-solid interfaces, decreasing the energies of those interfaces and hence facilitating its formation. 51,52 This implies that the dopants are enabling small sizes but inducing agglomeration as well. Similar effects have been studied for other dopants in CeO2 and SnO2 6, 53, 54

and should be considered in further studies for rare-earth doping in spinels.

CONCLUSION In this work, spinel nanoparticles were stabilized by using trivalent dopant segregation to surfaces. While pure spinel had a particle size of ~ 6 nm, 2 mol% La doped spinel exhibited a reduced particle size of ~ 4.5 nm and a 45% increase in surface area. Among the three dopants studied, La was the most effective in reducing the nanoparticle size. Surface energies of the pure and doped spinel surfaces were measured experimentally using a microcalorimetric technique and the nanostability could be attributed to surface energy reduction caused by dopant segregation to the surface. The surface energy of the pure spinel was 1.65±0.04 J/m2 while that of La-doped spinel was 1.26±0.04 J/m2. Based on previously reported atomistic calculations of rare earth dopant segregation on low index spinel surfaces, equilibrium morphologies were constructed for 2 mol% doped spinel nanoparticles with dimension around 5 nm. It was found that, because dopant segregation to different surfaces is highly anisotropic, the dopant segregation changes the morphology from one in which {100} surfaces dominate to one in which



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the dominant facets are {111}. Surface energies of those nanoparticles were also calculated and the physical trends describing how the different dopants decreased the surface energy are qualitatively consistent with experimental results. The decreased surface energy caused a remarkable improvement in the stability against coarsening, maintaining high surface areas >100 m2.g-1 even after long exposure at 1000 °C.


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Acknowledgements RHRC would like to thank US Department of Energy - BES ER46795 for support of this work. BPU acknowledges support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the (U.S.) Department of Energy under contract DE-AC52-06NA25396.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.



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Table 1. Lattice constants from XRD patterns (experimental) for undoped and doped MgAl2O4 spinels with crystallite size calculated from whole pattern fitting (WPF) refinement, and surface area from BET. Theoretical lattice constants are from equivalent dopant concentration using molecular dynamics.

Trivalent Ion Radius (Å)

Experimental Lattice Const. (Å)

MgAl2O4

0.535

8.085±0.0013

6.1±0.6

121.66±0.15

8.138

Y-doped

0.900

8.093±0.0015

5.2±0.6

138.22±0.13

8.159

Gd-doped 0.938

8.090±0.0013

5.1±0.6

142.54±0.13

8.164

1.032

8.092±0.0017

4.6±0.3

175.30±0.30

8.172

La-doped

Crystallite Size (nm)


Surface Area Theoretical (m2/g) Lattice Const.(Å)

(a)

(440)

1.6

(400)

1.8

0.6

(444)

(533)

(530)

0.8

(620)

(422)

1.0

(511)

(222)

1.2

(220)

1.4

Intensity, a.u.

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


(311)

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La-doped Gd-doped

0.4 Y-doped 0.2

Undoped

0.0 30

40

50

60

2θ, degree

70

80

90

Figure 1. (a) X-ray diffraction patterns of undoped and doped MgAl2O4 spinels (b) TEM micrograph of a nanoparticle shows the lattice fringes extend to surface.



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Figure 2. (a) Annular Dark Field (ADF) image of the MgAl2O4 nanoparticles doped with La that served as a survey image for subsequent EELS acquisition. The ADF image already showing bright contrast in the surface indicates the presence of high atomic number element in those places. (b) RGB composite image of both La and O map confirms preferential segregation of La on the surface. (c) HRTEM micrograph images for undoped MgAl2O4 nanoparticles and (b) coarsen La doped MgAl2O4 nanoparticles showing the lattice spacing of spinel (200) planes.


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Figure 3. Predicted equilibrium morphologies for (a) undoped and doped MgAl2O4 nanoparticle nanoparticles with evenly distributed dopants, and (b) undoped and La doped MgAl2O4 nanoparticles where all dopant atoms are segregated to {111} surface with three different segregation energies. Note that the doped particle morphology is size dependent and the presented morphologies are obtained using experimental particle dimensions. In the particles, Red coloring is for {100} surface, Green is for {111} surface; Blue is for {110} surface.



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Figure 4. Surface energies of undoped and doped spinel nanoparticles from atomistic calculations.


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30

(a)

Undoped Y-doped Gd-doped La-doped

Water Coverage, H2O/nm

2

25

20

15

10

5

0 0.0

0.2

0.4

0.6

Relative Pressure, P/Po Differential Enthalpy of adsorption, kJ/mol H2O

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


-40

(b)

-60 -80 -100 Undoped Y-doped Gd-doped La-doped

-120 -140 -160 -180 0

5

10

15

20


25

30

2

Figure 5. (a) Water adsorption isotherm curves for undoped and doped MgAl2O4 spinels. (b) Differential heat of adsorption as a function of water coverage for undoped and doped MgAl2O4 spinels. The solid line indicates the enthalpy of water condensation, -44 kJ/mol.



2

1.5

- Integral ∆Hads, J/m

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1.0

Undoped Y-doped Gd-doped La-doped

0.5

0.0 0

5

10


15 2

Figure 6. Integral heats of water adsorption per unit surface area as a function of water coverage on pure and doped MgAl2O4 spinel. The dotted lines are the best fitted straight lines that represent the linear portion of each curve. The R squared measure of goodness of fit was at least 0.999 for all of the best fit lines in the linear region of the curves.


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Al 1.7

3+

3+

Y

Gd

3+

La

3+

Undoped

Y-doped

Surface Energy, J/m

2

1.6

Gd-doped

1.5

1.4 La-doped 1.3

1.2 0.5

0.6

0.7

0.8

0.9

1.0

Å

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


1.1

Trivalent Ion Radius in Octahedral Coordination,

Figure 7. Surface Energy measured by water adsorption microcalorimetry of pure MgAl2O4 and Y-, Gd-, La-doped MgAl2O4 as a function of trivalent ion radius.



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Figure 8. Surface area (a) and crystallite sizes (b) of undoped and rare-earth doped MgAl2O4 after exposure for 1h at different temperatures. Significant improvement in stability is observed with the dopant, as predicted by the surface energy trend.


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TOC Figure



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