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Title: Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths

Abstract

Surface energy is a key parameter to understand and predict the stability of catalysts. In this work, the surface energy of MgAl 2O 4, an important base material for catalyst support, was reduced by using dopants prone to form surface excess (surface segregation): Y 3+, Gd 3+, and La 3+. The energy reduction was predicted by atomistic simulations of spinel surfaces and experimentally demonstrated by using microcalorimetry. The surface energy of undoped MgAl 2O 4 was directly measured as 1.65 ± 0.04 J/m 2 and was reduced by adding 2 mol % of the dopants to 1.55 ± 0.04 J/m 2 for Y-doping, 1.45 ± 0.05 J/m 2 for Gd-doping, and 1.26 ± 0.06 J/m 2 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 mapping in a scanning transmission electron microscopy image. Finally, 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,more » 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.« less

Authors:
 [1];  [1];  [1];  [2];  [3]; ORCiD logo [3];  [1]
  1. Univ. of California, Davis, CA (United States). Dept. of Materials Science and Engineering, and Nanomaterials in the Environment, Agriculture, and Technology- Organized Research Unit (NEAT ORU)
  2. Arizona State Univ., Tempe, AZ (United States). John Cowley Center for High Resolution Electron Microscopy, LeRoy Eyring Center for Solid State Science (HREM, LE-CSSS)
  3. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE Office of Science (SC). Basic Energy Sciences (BES) (SC-22); USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1369180
Report Number(s):
LA-UR-16-22939
Journal ID: ISSN 0897-4756 ; 1520-5002 (Electronic)
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Chemistry of Materials
Additional Journal Information:
Journal Volume: 28; Journal Issue: 14; Journal ID: ISSN 0897-4756
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Hasan, M. M., Dey, Sanchita, Nafsin, Nazia, Mardinly, John, Dholabhai, Pratik, Uberuaga, Blas P., and Castro, Ricardo H. R. Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths. United States: N. p., 2016. Web. doi:10.1021/acs.chemmater.6b02577.
Hasan, M. M., Dey, Sanchita, Nafsin, Nazia, Mardinly, John, Dholabhai, Pratik, Uberuaga, Blas P., & Castro, Ricardo H. R. Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths. United States. doi:10.1021/acs.chemmater.6b02577.
Hasan, M. M., Dey, Sanchita, Nafsin, Nazia, Mardinly, John, Dholabhai, Pratik, Uberuaga, Blas P., and Castro, Ricardo H. R. 2016. "Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths". United States. doi:10.1021/acs.chemmater.6b02577. https://www.osti.gov/servlets/purl/1369180.
@article{osti_1369180,
title = {Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths},
author = {Hasan, M. M. and Dey, Sanchita and Nafsin, Nazia and Mardinly, John and Dholabhai, Pratik and Uberuaga, Blas P. and Castro, Ricardo H. R.},
abstractNote = {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 mapping in a scanning transmission electron microscopy image. Finally, 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.},
doi = {10.1021/acs.chemmater.6b02577},
journal = {Chemistry of Materials},
number = 14,
volume = 28,
place = {United States},
year = 2016,
month = 6
}

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  • Density functional theory calculations are performed within the generalized gradient approximation (GGA+U) to determine stable terminations of both low- and high-index spinel LiMn2O4 (LMO) surfaces. A grand canonical thermodynamic approach is employed, permitting a direct comparison of offstoichiometric surfaces with previously reported stoichiometric surface terminations at various environmental conditions. Within this formalism, we have identified trends in the structure of the low-index surfaces as a function of the Li and O chemical potentials. The results suggest that, under a range of chemical potentials for which bulk LMO is stable, Li/O and Li-rich (111) surface terminations are favored, neither of whichmore » adopts an inverse spinel structure in the subsurface region. This thermodynamic analysis is extended to identify stable structures for certain high-index surfaces, including (311), (331), (511), and (531), which constitute simple models for steps or defects that may be present on real LMO particles. The low- and high-index results are combined to determine the relative stability of each surface facet under a range of environmental conditions. The relative surface energies are further employed to predict LMO particle shapes through a Wulff construction approach, which suggests that LMO particles will adopt either an octahedron or a truncated octahedron shape at conditions in which LMO is thermodynamically stable. These results are in agreement with the experimental observations of LMO particle shapes.« less
  • The design of a molten metal-molten salt based chemical and electrochemical process for separation of actinides from plutonium-uranium extraction waste requires a consistent set of thermodynamic properties for the actinides and rare earths present in nuclear waste. Standard potential data for Y, La, Ce, Pr, and Gd in molten LiCl-KCl were obtained. Americium data obtained were standard potentials in molten LiCl-KCl and activity coefficients for Cd and Bi. Data were obtained between 400 and 500 C. Results for the rare earth chlorides using an improved experimental technique were consistent with theory, with standard free energy of formation values somewhat moremore » negative than those found in the literature. Special attention was given to Am in the LiCl-KCl/Cd system because it can exist as the +2 and/or +3 ion in this system. Americium ions existed only as the +3 ion in LiCl-KCl/Bi. Standard electrochemical potentials for Am/Am{sup +2} in LiCl-KCl eutectic at 400, 450, and 500 C were {minus}2.893, {minus}2.853, and {minus}2.838 V, respectively, relative to Cl{sup 2}/Cl{sup {minus}}. Standard electrochemical potentials vs. Cl{sub 2}/Cl{sup {minus}} for Am/Am{sup +3} in LiCl-KCl eutectic were {minus}2.83 V at 450 C and {minus}2.78 V at 500 C. Activity coefficients for Am in molten Cd were 1 {times} 10{sup {minus}5} and 8 {times} 10{sup {minus}5} at 450 and 500 C.« less