Dynamic Phenomena during Reduction of α-NiMoO₄ in Different Atmospheres:  In-Situ Thermo-Raman Spectroscopy Study

Abstract

A study by in-situ thermo-Raman spectroscopy was carried out to understand the structural changes occurring during the reduction of R-NiMoO 4 in different atmospheres (viz., H 2 , CO, and C 3 H 6 ) at different temperatures and time. In addition, the dynamics of R-NiMoO 4 during reduction/reoxidation were also investigated. The products from reoxidation of reduced R-NiMoO 4 in different atmospheres were analyzed by XRD. During the reaction of H 2 with R-NiMoO 4 at 400 °C a new band was detected at 455 cm -1 , which is characteristic of reduced surface molybdenum oxide species. Thus, the formation NiMoO 3 and/or Ni 2 Mo 3 O 8 suboxide intermediates was suggested. On the other hand, reversible H 2 reduction/O 2 reoxidation dynamics was observed, where the main Raman features of H 2 -reduced R-NiMoO 4 were restored completely after reoxidation by oxygen. On the contrary, during the reaction of C 3 H 6 with R-NiMoO 4 the formation of a stable suboxide was difficult to detect, and irreversible C 3 H 6 reduction/O 2 reoxidation dynamics was suggested as the extensive heating of the C 3 H 6 -reduced sample in 10% O 2 /He for 2 h at 400 °C did not restore the Raman features of the R-NiMoO 4 observed prior to reduction. No significant change was observed in the Raman spectra of R-NiMoO 4 during heating in a flow of 10% CO/He, except for the steady decrease in the intensities of all bands with increasing temperature. This may be due to the fact that at high temperatures CO decomposed and the produced carbon deposited on the sample surface.

Structural, Optical, and Magnetic Properties of NiMoO4 Nanorods Prepared by Microwave Sintering

“…There is also a band at around 261 cm −1 related to deformation modes of Mo-O-Mo linkage. Our results are in very good agreement with other published studies [36, 37, 40, 41]. …”

Photoluminescent Properties of CoMoO<sub>4</sub> Nanorods Quickly Synthesized and Annealed in a Domestic Microwave Oven

“…Figure 5 illustrates that all Raman spectra have the same features. The results obtained are in agreement with the literature [28], and the small variations in the positions of the vibrational modes can be associated with the method of preparation, crystal size, morphology and strength of interaction between the ions and the degree of structural order-disorder of these materials. …”

Insight Into Malachite Green Degradation, Mechanism and Pathways by Morphology‐Tuned α‐NiMoO4 Photocatalyst

“…Figure . The results suggested that nanorod revealed higher amount of oxygen defects or vacancy than spherical quantum-sized and layered square microsheet (5,14). Hence, from the XPS analysis, it was confirmed that sample consists of nickel, molybdenum and oxygen element.…”

“…The strong Raman peak was observed at 818.31 cm À1 that is related to distorted MoO 6 octahedral present in sample NM-M. The Raman peak is associated with the symmetric (992.02 and 665.28 cm À1 ) and asymmetric (957.11 cm À1 ) stretching modes of the (5,6,14). The Raman peaks were slightly varied and shifted due to variation of morphology of samples.…”

“…The peaks 941.52 and 957.11 cm −1 were observed for sample NM‐N and NM‐Q, respectively, which are associated with symmetric and asymmetric stretching modes of the terminal Mo=O bond as well as distorted octahedral. The Raman band at 704 cm −1 is associated with the Ni‐O‐Mo symmetric stretch in sample NM‐N . The Raman peaks were slightly varied and shifted due to variation of morphology of samples.…”