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Transition Metal Dopant Cation Distributions in MgO and CaO: New Inferences from Paramagnetically Shifted Resonances in O, Mg and Ca NMR Spectra 17

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Ryan Jeffrey McCarty, and Jonathan F. Stebbins J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02710 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016

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The Journal of Physical Chemistry C 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.

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The Journal of Physical Chemistry

Transition Metal Dopant Cation Distributions in MgO and CaO: New Inferences from Paramagnetically Shifted Resonances in 17O, 25Mg and 43Ca NMR Spectra Ryan J. McCarty and Jonathan F. Stebbins* Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA. ABSTRACT We report on paramagnetic shifts identified in 17O, 25Mg and 43Ca NMR spectra of CaO and MgO doped with 0.3 to 2.2 mol % NiO or CoO. Systematically shifted features were identified for both paramagnetic dopants (Ni2+ or Co2+) and both bulk materials (CaO and MgO), and in some cases spectral features could be assigned to paramagnetic cations at specific sites within 5 bonds of the observed nucleus. We compare the empirical peak areas to theoreticallyderived peak areas expected under conditions of random mixing, and observe that Ni2+ and Co2+ doped MgO systems agree well with random mixing but doped CaO systems do not. In spite of the very low natural isotopic abundances of 17O and 43Ca, moderate natural abundance of 25Mg, and relatively low Larmor frequencies of all three nuclides, the cubic crystal structures and the resulting narrow NMR peaks allowed a first look at the potential of investigating dopant cation distributions in group 2 oxides though NMR.

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INTRODUCTION Understanding how transition metals are incorporated into cubic (rock salt or halite structured) group 2 oxides has potential value for geologic and catalytic research, and provides empirical data for what may be an ideal system to test theoretical calculations. Geologically, ferropericlase (a solid solution of FeO in MgO) is the second most abundant mineral in the Earth’s lower mantle, and research on this phase has developed our understanding of the mantle and its processes.1 Technologically, group 2 oxides serve as solid base catalysts, with solid solutions often doped with additional elements that directly affect catalytic function or supporting catalytic materials.2–5 The simple symmetry and electronic structure of periclase has been noted in DFT research, making it a good oxide material to test and refine simulations of more complex bulk and surfaces properties.6–10 The interaction of unpaired electron spins from paramagnetic cations with nearby nuclear spins can in some cases produce distinct resonances in observed NMR spectra, at frequencies distinct from normal chemical shifts. These are often described as “hyperfine” or “paramagnetic” shifts. Two mechanisms can contribute to such shifts: a through-space dipolar coupling (“pseudo-contact interaction”) and a through-bond electronic interaction (“Fermi contact interaction”). In most oxide systems the latter is indicated as the major shift contributor, although both are usually present.11 In this regard rock salt-structured oxides are unique, as the cubic symmetry of both cation and anion sites (both in octahedral coordination) suggests that the pseudo-contact effect, which requires site asymmetry, should be absent unless local site distortion results from solid solution. The magnitude and sign of paramagnetic shifts are highly dependent on the crystallographic relationship between the observed NMR


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nucleus and the paramagnetic cation. This can provide new information about local structures and cation distributions in a wide variety of materials, including both solids and molecules in liquids, such as organic compounds, battery materials, and minerals.11–14 Paramagnetic shifts for

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O NMR in an oxide material were observed in 1971 for cubic

UO2.15 Recent work identified hyperfine shifts produced by distinct Np/U-O bond paths or distances in actinide oxides.16,17 Results were compared to a solid solution model to confirm random actinide distribution within the first cation shell. Paramagnetic NMR shifts for oxygens directly bonded to lanthanide cations in aqueous solutions of organic compounds have been described as well.12 For quadrupolar nuclides (17O, 25Mg, 43Ca, etc.) NMR experiments on cubic materials are particularly advantageous as high site symmetry results in low quadrupolar coupling constants (ideally equal to 0) and correspondingly narrow NMR peak widths. In part because of this, spectra of both cations and anions in MgO and CaO were collected in relatively early solid state NMR experiments, and in some cases the pure phase is used as a frequency reference.18–21 Collecting 25Mg and 43Ca NMR spectra is typically quite challenging in solid materials because of low resonant frequencies and large quadrupolar broadening.21 Additionally all three nuclides have low natural abundances, with 17O exceptionally low at 0.035%,

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Ca marginally higher at

0.135%, and 25Mg at 10%. Although both materials, but in particular MgO, should be sensitive to effects of coupling of electronic with nuclear spins, paramagnetic shifts have not been reported. Furthermore this report may be the first observation of paramagnetic shifts for 25Mg and 43Ca in solids.19,21


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Our investigation was in part motivated by an early 17O and 25Mg NMR study (at natural isotopic abundances) of Ni2+ bearing polycrystalline MgO powder.22 The authors observed decreased spin-lattice relaxation times in Ni2+ bearing samples, line widths inversely correlated with sample temperature, and a loss of signal from observed nuclei within one or 2 shells of paramagnetic cations. Paramagnetic shifts probably contributed to the observed broadening of the resonances, but were unresolvable in spectra collected at a low magnetic field without magic angle spinning. Our new study investigates transition metals incorporated into group 2 oxides, with 17O, 25

Mg and 43Ca MAS NMR spectra exhibiting paramagnetic shifts produced by Ni2+ and Co2+

within the lattices of MgO and CaO. In an octahedral environment Ni2+ (d8) has only one spin state, and Co2+ (d7) can have either a high spin or low spin state.23 Unlike many recent studies, especially for 17O in oxides and silicates, our samples are not enriched over natural abundance, except for one with minor 17O enrichment. We identify 17O, 25Mg and 43Ca NMR resonances with paramagnetic shifts and are able to assign some of them to specific bond paths/distances between the paramagnetic cations and the observed nuclei, based on the number of peaks and their relative areas. We compare site populations implied by these results with random models of cation distributions in the known crystal structures, and comment on transition metal distributions at percent-level concentrations. These preliminary observations and our initial analyses serve as a starting point for future investigations of paramagnetic cations in isotopically enriched group 2 oxides.


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EXPERIMENTAL SECTION To prepare Co- and Ni-doped MgO and CaO samples without isotopic enrichment, 99.95% MgO and reagent grade CaO were thoroughly mixed with Co3O4 or with NiO in an agate mortar. Powders were pressed with 6 metric tons into 0.8 cm diameter pellets and fired to 1550 °C for one day in ambient atmosphere and pressure. The samples were quenched in air at about 60 °C/s. Synthesis conditions should have maintained the Ni2+ oxidation state and converted all cobalt to Co2+.24 To minimize hydration or carbonation, the samples were immediately placed into desiccators, and many samples were packed into NMR rotors within 10 minutes of being removed from the furnace. 17O enriched Co-doped CaO was produced as an impurity phase in a sample of Ca3SiO5 made with 45%

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O enriched silica, which was quenched

from 1400 °C.25 The Co2+ concentration in the CaO portion of this sample is unknown, but the bulk, total Co content was 0.2%. Because the source of the 17O in this synthesis was Si17O2, the CaO impurity phase had some isotopic enrichment by partial exchange, which is estimated very roughly as 0.5% 17O based on calibrated absolute peak areas in the spectra. For the pure-phase Co-CaO, Ni-CaO, Co-MgO and Ni-MgO, nominal compositions are listed in cation percent in Table 1. 17

O NMR data were acquired using Varian/Chemagnetics “T3” probes with a Varian

Infinity Plus 400 spectrometer at 9.4 Tesla (17O at 54.2 MHz) and a Varian Unity/Inova 600 spectrometer at 14.1 Tesla (17O at 81.3 MHz, 25Mg at 36.7 MHz, 43Ca at 40.3 MHz) with an external “low gamma” tuning attachment for 25Mg and 43Ca. 17O spectra were referenced to 17O enriched H2O at 0 ppm. An effective radiofrequency (RF) power, measured on the liquid, was 93 kHz, with a single pulse of 0.33 μs (ca. 13˚ RF tip angle) being used for all 17O experiments. 25Mg


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spectra were referenced to 1 M Mg(NO3)2 at 0 ppm. Effective RF power, measured on the liquid, was 53 kHz, with a single pulse of 5 μs (ca. 90˚ RF tip angle) being used for all 25Mg experiments. 43Ca spectra were referenced to 1 M CaCl2 at 0 ppm, as recommended as best practice.20 Effective RF power, measured on the liquid, was 43 kHz, with a single pulse of 3 μs (ca. 45˚ RF tip angle) used for all 43Ca experiments. Spectral windows of 400 kHz were used. To change the sample temperature from approximately 42 ˚C to 27 ˚C, spinning speeds were decreased from 20 kHz to 9 kHz to reduce air friction. The 3.2 mm zirconia rotors were not airtight, but weight gain due to hydration or carbonation during experiments increased the sample weight by less than 1 wt. %; changes in the spectra over time were undetectable. A minor 17O background signal from the zirconia rotors, centered at 378 and/or 392 ppm, was observable but did not overlap with the region of interest. Due to the low isotopic abundances, experiments lasting up to 5 days were necessary to resolve small features (

Transition Metal Dopant Cation Distributions in ... - ACS Publications

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