Abstract
Molybdenum nitrides have been employed in a variety of applications. For use in catalysis, the cubic γphase with the nominal stoichiometry Mo2N and the space group Fm3¯ m is typically prepared by high-temperature reaction of MoO3 with NH3. The literature presents conflicting reports of the possible presence of residual oxygen from typical ammonolysis reactions and whether such species influence the crystal structure and morphology. With the aim of resolving these open questions, a comprehensive study of the chemistry, crystal structure, and electronic structure of molybdenum nitride materials prepared by ammonolysis has been undertaken here, with particular focus on the role of reaction temperature. Ammonolysis of MoO3 was carried out at 973 and 1073 K and yielded single-phase cubic products. Using electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, prompt gamma-ray neutron activation analysis, and combustion analysis, significant concentrations of oxygen and, to a lesser extent, hydrogen were found in both materials. The crystal structure of each phase was refined by Rietveld analysis using combined synchrotron X-ray diffraction and neutron diffraction data. The structures were found to be derivatives of the B1 rock salt (halite) structure, as is often reported for "γ-Mo2N."However, both materials adopt the space group Pm3¯ m, as opposed to the typically presumed space group of Fm3¯ m, and both have much higher anion content than implied by the stoichiometry Mo2N. Ordering of cation vacancies and of anion species is responsible for the loss of the translational symmetry expected for the space group Fm3¯ m. X-ray absorption spectroscopy studies, along with the EELS and XPS results, showed the Mo oxidation state to be diminished with higher temperature synthesis, consistent with the retention of a lower concentration of anions and in particular oxygen. The difficulty in differentiating oxygen and nitrogen and the impossibility of detecting hydrogen by X-ray and electron diffraction methods, especially in the presence of the heavy element Mo, have likely inhibited accurate identification of Mo1-x(N1-yOy)Hz as the product of MoO3 ammonolysis. The findings reported here offer a critical assessment for understanding the properties of molybdenum "nitrides"in electronic and catalytic applications.
Original language | English (US) |
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Pages (from-to) | 6671-6684 |
Number of pages | 14 |
Journal | Chemistry of Materials |
Volume | 33 |
Issue number | 17 |
DOIs | |
State | Published - Sep 14 2021 |
Funding
MRSEC-IRG2 (NSF, no. DMR-1720139) is acknowledged for financial support of this work. This work made use of the J.B. Cohen X-Ray Diffraction Facility for lab XRD. EPIC and Keck-II facilities of Northwestern University’s NUANCE Center were used for TEM, SEM, and XPS analysis, which have received support from the SHyNE Resource (NSF, no. ECCS-2025633), the IIN, and the Northwestern University’s MRSEC program (NSF, no. DMR-1720139). The CleanCat Core facility at the REACT center of Northwestern University was used for BET measurements. Heather Chen-Mayer from the NIST is acknowledged for assistance with PGAA data analysis. Midwest Microlabs is acknowledged for combustion analysis. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357. Synchrotron diffraction data were collected at the APS beamline 11 BM using the mail-in program. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory, under Contract no. DE-AC05 00OR22725. Neutron diffraction data were collected at POWGEN using the mail-in program. Commercial products are mentioned only to specify the procedure in sufficient detail. Their inclusion in the paper does not imply product endorsement, nor does it imply that the mentioned products are the best for such use.
ASJC Scopus subject areas
- General Chemistry
- General Chemical Engineering
- Materials Chemistry