Experimental and Computational Study of the Apparent Absorption Spectra of Nanostructured Manganese Tungstate Catalysts

Abstract

The UV-Vis spectra of a series of MnWO4 nanorods with different Mn/W ratio represent the subject of the present study. Since the surfaces of the obtained catalysts are damaged to understand the observed spectra ransformations with the change of the Mn/W content homometalic binuclear, trinuclear and tetranuclear linear manganese clusters, binuclear tungsten clusters and mixed manganese-tungsten clusters of different nuclearity are examined. The choice of the linear manganese clusters of different nuclearity for examination is dictated by the revelation of MnOx units on the surface of the MnWO4 nanorods. The structures of these different nuclearity species have been optimized, and their absorption have been calculated with the aid of density functional theory (DFT). It has been shown that homonuclear tungsten clusters demonstrate only ligand-metal charge transfer, while manganese clusters demonstrate both ligand-metal and metal-ligand charge transfer. At the same time ligand-metal and manganese-tungsten electron transfers are characteristic for mixed manganese-tungsten clusters. A comparison of the calculated absorption spectra of the modeled clusters with those observed for MnWO4 nanorods showed, that chain-like manganese clusters mainly contribute to the high-energy part of the UV-Vis spectra, while the fall in the intensity of the observed spectra in the range of the longer wavelengths is connected not only with the decrease of the relative manganese content, but it also manifested due to decrease of the concentration of the mixed manganese-tungsten clusters, mainly contributing to this part of the spectra.