Direct determination of Mn valence states in mixed‐valent manganates by photoluminescence spectroscopy

Abstract

Mn‐based materials are excellent catalysts with applications in many complex heterogeneous charge transfer processes because of their ability to exist in several stable oxidation states. However, accurate detection of Mn valence states is challenging due to the limitations of X‐ray‐based spectroscopic techniques to successfully resolve individual oxidation states. In this work, we performed a combined photoluminescence (PL) and X‐ray photoelectron spectroscopy (XPS) analysis of many Mn compounds to show that PL spectroscopy can consistently detect Mn in different oxidation states and their composition can be determined through a simple fitting procedure. The results suggest that PL spectroscopy can be a relatively simpler, benchtop alternative to X‐ray based techniques for direct metal cation detection without the need for multiplet fitting analysis.

Environmental Factors Controlling the Electronic Properties and Oxidative Activities of Birnessite Minerals

“…Comparison of the relative concentration of Mn cations of various cation-exchanged birnessites determined by PL spectroscopy. Some of the data was previously reported in ref .…”

“…57,75 For some powders, the composition was also determined by multiplet fitting analysis of the Mn 2p 3/2 peak of the XPS spectrum. 75 In all cases, the spectrum was obtained from dry cation-exchanged birnessite powders. Figure 2 shows the relative Mn cation compositions of various birnessites obtained in this study.…”

“…The surface Mn cation composition of all investigated birnessites was determined by PL spectroscopy using the procedure detailed in previous studies. , For some powders, the composition was also determined by multiplet fitting analysis of the Mn 2p 3/2 peak of the XPS spectrum . In all cases, the spectrum was obtained from dry cation-exchanged birnessite powders.…”

Understanding the role of manganese oxides in retaining harmful metals: Insights into oxidation and adsorption mechanisms at microstructure level

“…Cationic agents are capable of passivating birnessite minerals, and in some cases, anionic agents are also capable of doing so [ 5 , 173 , 186 ]. The net positive charge on the edge sites of birnessite particles, especially evident at pH 4, due to proton adsorption, is attributed to the PZC of these sites [ 6 , 7 ]. Consequently, these positively charged edge sites have the ability to attract cationic agents, leading to passivation due to the accumulation of a substantial quantity of negative charges [ 78 , 186 ].…”

“…The common valence states of Mn encompass II, III, and IV. Mn(II) and Mn(IV) exhibit great stability in natural environments, capable of transforming between soluble low-valence states and insoluble high-valence states [ [5] , [6] , [7] , [8] , [9] ]. Mn(III) is thermodynamically unstable, existing stably only in specific soluble organic complexes or minerals; it often disproportionates into Mn(II) and Mn(IV) [ 10 , 11 ].…”