Arsenomolybdates for Photocatalytic Degradation of Organic Dyes

Polyoxometalates (POMs) have fascinating structures and promising properties. The arsenomolybdates, as an important branch of POMs, are outstanding photocatalysts for organic dyes. In this work, we selected organic dyes to evaluate the photocatalytic activity of arsenomolybdates under UV light, containing compared with photocatalytic activity of different structural arsenomolybdates, stability, and the photocatalytic reaction mechanism of arsenomolybdates as photocatalyst. The arsenomolybdates may be used to as environmental photocatalysts for the degrading of organic dyes and solving the problem of environmental

POMs are subdivided into isopolyoxometalates, which feature addenda metal and oxygen atoms, and heteropolyoxometalates, where a central heteroatom provides added structural stabilization and enables reactivity tuning [21]. In recent years, the research of POMs is mainly focused on heteropolyoxometalates. The arsenomolybdates are essential member of the heteropolymolybdates family [22], because of the redox properties of Mo and As atoms. The discoveries of many excellent articles on arsenomolybdates for ferromagnetic, antitumor activity, electrocatalysis properties, and lithium-ion battery performance have been reported in the last years [23][24][25][26]. However, there is no stress and discuss on the progress of arsenomolybdates for degradation of organic dyes. Arsenomolybdates possess high-efficient proton delivery, fast multi-electron transfer, strong solid acidity and excellent reversible redox activity [27], which may result to prominent photocatalytic activities. In particular, the integration of metal-organic frameworks (MOFs) into arsenomolybdates for photocatalysis has attracted widespread attention over the past decade, since MOFs combine porous structural and ultrahigh internal surface areas.
Based on these results, we provide a summary of recent works in the synthesis, structure, the photocatalytic activity, reaction kinetics and mechanism mechanisms of arsenomolybdates, which aim at finding the direction followed with the opportunities and challenges for the arsenomolybdates photocatalysis to accelerate the step to realize its practical application in degradation of organic dyes.

Syntheses of arsenomolybdates
Arsenomolybdates crystals reported were almost synthesized via self-assembly processes using hydrothermal method (Figure 2  arsenomolybdates crystals. Therefore, further exploration of synthetic conditions is necessary, which can provide more experimental data for arsenomolybdates.

Structure of classical arsenomolybdates
Up to now, various structures of arsenomolybdates were reported and discussed in detail. The following types are classical arsenomolybdates clusters: (i) {As 2 Mo 6 } type, Pope's group reported the first {As 2 Mo 6 } cluster [28], in which the Mo 6 [41], and so on. The novel arsenomolybdate structure is gaining more and more attention.

Photodegradation process
In recent years, POMs have attracted a lot of attention as photocatalysts for the decomposition of wastewater [42]. Organic dyes, such as methylene blue (MB), rhodamine B (RhB), azon phloxine (AP), and so on, is a typical organic pollutant in waste water. In this work, the photocatalytic activities of arsenomolybdates are investigated via the photodecomposition of organic dyes under UV light irradiation (Figure 3). The photocatalytic reactions were conducted using a common process [27]: arsenomolybdates and organic dyes solution were mixed and dispersed by ultrasonic. The suspension was stirred until reached the surface-adsorption equilibrium. Then, a high pressure Hg lamp was used as light source to irradiate the mixture, which was till stirred for keeping the mixture in suspension. At regular intervals, the sample was withdrawn from the vessel and arsenomolybdates was removed by several centrifugations, and the clear liquid was analyzed by using UV-Vis spectrophotometer.

Photocatalytic degradation of MB
The common arsenomolybdates photocatalysis are shown in Figure 4. The photocatalytic activities of arsenomolybdates are review via the photodecomposition of MB under UV light irradiation ( Figure 5). Su groups reported six compounds with [H x As 2 Mo 6 O 26 ] (6 À x)À clusters and copper-organic complexes. Six {As 2 Mo 6 } compounds were irradiated for 135 min under, the photocatalytic decomposition rates are 94.5%, 93.0%, 92.1%, 92.2%, 93.6%, and 96.5%, respectively [43]. Then the {Co(btb)(H 2 O) 2 } 2 {H 2 As 2 Mo 6 O 26 }Á2H 2 O exhibited better photocatalytic activity in the degradation of MB at the same process, the photocatalytic decomposition rate is 94.27% [44]. Su groups synthesized two {As 2 Mo 6 } compounds with [H x As 2 Mo 6 O 26 ] (6 À x)À clusters and free organic ligands, photocatalytic activities of they are detected, the conversion rate of MB is 91.8% and 92.2% when adding two {As 2 Mo 6 } compounds as the catalyst 160 min later, respectively [45].   The above data show that the photocatalytic activity of the compound composed of [H x As 2 Mo 6 O 26 ] (6 À x)À clusters and metal-organic complexes is higher than supramolecular assemblies based on isomers [H x As 2 Mo 6 O 26 ] (6 À x)À clusters in the degradation of MB under UV irradiation, which maybe that the polyoxoanions can connect with transition metals in diverse modes, which enhanced the contact area between catalysts and substrates availing charge-transfer.
The three {As 6

Photocatalytic degradation of RhB
The photocatalytic activities of arsenomolybdates as photocatalysts are review via the photodecomposition of RhB under UV light irradiation. The photocatalytic decomposition rates of RhB are about 96. 34

Photocatalytic degradation of AP
AP, as one of the azo dyes, is relatively difficult to degrade, and so it was used as target molecules to evaluate the photocatalytic activity of arsenomolybdates under UV irradiation. The photocatalytic activity of {pyr}{Hbib} 2 {As III 2 (OH) 2 As V 2 Mo 18 O 62 } was evaluated for the degradation of AP under UV irradiation [51], the degradation rate is 91.02% after UV light irradiation 90 min. In addition, the photocatalytic activity of noncapped 0D analog (H 2 bimyb) 3 [49], which indicates that the photocatalytic degradation effect of the bi-arsenic capped Dawson compound on AP is much better than that of noncapped analog. The 3D Dawson organic-inorganic hybrid arsenomolybdate, {Ag(diz) 2 } 3 [{Ag(diz) 2 } 3 (As 2 Mo 18 O 62 )]Á H 2 O exhibits merit photocatalytic properties for degradation of refractory dyes AP under UV light [52], the photocatalytic decomposition rate is 93.24% after 80 min.
The photocatalytic activities of (imi) 2  On the basis of the aforementioned points, {As 2 Mo 18 } type arsenomolybdates with 3D networks possess the highest photocatalytic activities for photodecomposition of MB, RhB and AP under UV light irradiation. The following factors are maybe considered: First, quantity of Mo and O atoms in unit cell is a factor, which can increases the amount of charge-transfer from HOMO of O to LUMO of Mo, generating more electron-hole pairs. Second, the enhanced photocatalytic activity may have arisen from the 3D architecture, more extended 3D frameworks favor the migration of excited holes/electrons to the surfaces of {As 2 Mo 18 } type to initiate the photocatalytic degradation reaction with organic dyes.

Reaction mechanisms of photocatalytic performance
Experimental and theoretical studies of arsenomolybdates photocatalysis have revealed that it typically proceeds based on the following mechanism [41,42,48,52]: Irradiated of arsenomolybdates by UV light with energy equal to or greater than the E g value of itself, which induces intramolecular charge-transfer from the HOMO of O to the LUMO of Mo, leading to the formation of photoexcited states, subsequently photogenerated electron-hole pairs were generated. The O 2 captures electron to form • O 2ˉa nd the hole reacts with H 2 O or OH À ions to form • OH. The • O 2ˉa nd • OH radical decompose organic dyes' molecules into the final product, the detail of photocatalytic reaction is shown in Eqs. (1)- (4).

Stability
Some research data show that the samples were washed and dried after the arsenomolybdates as photocatalysis several cycles, and the infrared or X-ray diffraction test were carried out, the infrared spectra or X-ray diffraction data of arsenomolybdates demonstrate that there are almost unchanged before and after photocatalytic reaction [44][45][46][47][48], which indicate that arsenomolybdates photocatalysis have excellent structural stability.

Conclusions
In this chapter, the arsenomolybdates are presented, and the attention is mainly focus on photocatalytic degradation of organic dyes. Various strategies are summarized and discussed based on the knowledge of synthesis, structure and photocatalytic properties for arsenomolybdates, which reflects the major directions of recent research in this field. There are vast research opportunities as new arsenomolybdates architectures are discovered in future; the great effort to promote the development of arsenomolybdates is needed to reduce the gap with commercial applications.