Metal substitution in a Lindqvist polyoxometalate leads to improved photocatalytic performance

Abstract

The photochemical properties of homo- and heterometallic molybdate-based Lindqvist polyoxometalate clusters are investigated in a comparative study and it is shown that vanadium substitution can be used as a facile synthetic tool to optimize the visible light absorption and photocatalytic activity of the cluster. The mono-vanadium substituted unit, [VMo(5)O(19)](3-) shows light absorption up to 480 nm whereas the light absorption of the molybdate analogue [Mo(6)O(19)](2-) is mainly in the UV region below 400 nm. The electronic absorption properties of both clusters are further investigated using TD-DFT calculations which show that vanadium incorporation leads to the formation of low-energy O → V LMCT transitions. In comparative photochemical dye decomposition test reactions under UV and Vis irradiation, a higher reactivity is observed for [VMo(5)O(19)](3-) together with turnover numbers of more than 1600. In addition it is shown that under anaerobic conditions, the photoreaction proceeds faster than in the presence of oxygen, suggesting that oxygen acts as a quencher in one of the photoredox steps.

Photocatalytic reactivity tuning by heterometal and addenda metal variation in Lindqvist polyoxometalates

“…In a previous report we showed that the visible light absorption is mainly arising from O-V LMCT transitions. 24 Notably, {VMo 5 } shows the lowest light absorption in the visible range between λ = 400-500 nm amongst the compounds investigated, see Fig. 2.…”

“…POMs were reported. 24 To this end, the standard photooxidation conditions (see above) were used, but the samples were de-aerated by flushing with argon for 10 min prior to the experiment. During the photooxidation, the samples were kept under argon.…”

“…1. 24 Photocatalytic tests showed that under UV and visible light irradiation, higher photoactivity was found for {VMo 5 } than for {Mo 6 }. At high wavelengths (λ > 450 nm), {Mo 6 } was photochemically inactive whereas {VMo 5 } still showed photooxidative activity, confirming that metal substitution is a powerful tool for tuning the visible light photoactivity of small POM clusters.…”

“…6,7 This approach can also be extended to the development of POM photocatalysts, 14,15 as the electronic properties of the clusters (and therefore their visible-light photoactivity) are controlled by the underlying cluster structure. [21][22][23][24][25][26] Based on this concept, functionalization routes were developed where selective metal substitution is used to photosensitize clusters in the visible region. 23,24,26,27 Thus far, the concept of metal-substituted POM photocatalysts has mainly been focused on Keggin derivatives [XM 12 O 40 ] n− (M = V, Mo, W) 28,29 whereas the development of other heterometallic, photoactive POM clusters is still in its infancy.…”

“…[21][22][23][24][25][26] Based on this concept, functionalization routes were developed where selective metal substitution is used to photosensitize clusters in the visible region. 23,24,26,27 Thus far, the concept of metal-substituted POM photocatalysts has mainly been focused on Keggin derivatives [XM 12 O 40 ] n− (M = V, Mo, W) 28,29 whereas the development of other heterometallic, photoactive POM clusters is still in its infancy. 30,31 Based on this background, we started to investigate small prototype clusters 24 where selective metal exchange can be used to tune the electronic cluster properties whilst retaining the physical cluster architecture.…”