**3. Photocatalytic activity of arsenomolybdates**

### **3.1 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 At regular intervals, the sample was withdrawn from the vessel and arsenomolybdates was removed by several centrifugations, and the clear liquid was analyzed by using UV–Vis spectrophotometer.

#### **3.2 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 [HxAs2Mo6O26] (6 x) clusters and copper-organic complexes. Six {As2Mo6} 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)(H2O)2}2{H2As2Mo6O26}2H2O 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 {As2Mo6} compounds with [HxAs2Mo6O26] (6 x) 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 {As2Mo6} compounds as the catalyst 160 min later, respectively [45].

**Figure 4.**

**Figure 5.**

**121**

*The common arsenomolybdates photocatalysis polyoxoanion.*

*Arsenomolybdates for Photocatalytic Degradation of Organic Dyes*

*DOI: http://dx.doi.org/10.5772/intechopen.92878*

*The arsenomolybdates photocatalytic decomposition rates of MB under UV irradiation.*

**Figure 3.** *The structure of dyes and photodecomposed product.*

*Arsenomolybdates for Photocatalytic Degradation of Organic Dyes DOI: http://dx.doi.org/10.5772/intechopen.92878*

**3. Photocatalytic activity of arsenomolybdates**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

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

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 com-

{As2Mo6} 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)(H2O)2}2{H2As2Mo6O26}2H2O 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

photocatalytic activities of they are detected, the conversion rate of MB is 91.8% and 92.2% when adding two {As2Mo6} compounds as the catalyst 160 min later,

(6 x) clusters and copper-organic complexes. Six

(6 x) clusters and free organic ligands,

**3.1 Photodegradation process**

UV–Vis spectrophotometer.

pounds with [HxAs2Mo6O26]

respectively [45].

**Figure 3.**

**120**

**3.2 Photocatalytic degradation of MB**

{As2Mo6} compounds with [HxAs2Mo6O26]

*The structure of dyes and photodecomposed product.*

**Figure 4.** *The common arsenomolybdates photocatalysis polyoxoanion.*

**Figure 5.** *The arsenomolybdates photocatalytic decomposition rates of MB under UV irradiation.*

The above data show that the photocatalytic activity of the compound composed of [HxAs2Mo6O26] (6 � x)� clusters and metal-organic complexes is higher than supramolecular assemblies based on isomers [HxAs2Mo6O26] (6 � x)� 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.

UV irradiation. The photocatalytic activity of {pyr}{Hbib}2{AsIII

the same condition. Compared with {pyr}{Hbib}2 {AsIII

*Arsenomolybdates for Photocatalytic Degradation of Organic Dyes*

*DOI: http://dx.doi.org/10.5772/intechopen.92878*

The photocatalytic activities of (imi)2[{CuI

photocatalytic degradation reaction with organic dyes.

**3.5 Reaction mechanisms of photocatalytic performance**

detail of photocatalytic reaction is shown in Eqs. (1)–(4).

*dye* <sup>þ</sup> •

*O*� <sup>2</sup> <sup>þ</sup> •

0.5(trz)}6[{CuI

2H2O and {CuI

ation 135 min [25].

electron to form •

**3.6 Stability**

**123**

O2 <sup>ˉ</sup> and • O2

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 (H2bimyb)3(As2Mo18O62) was also studied under

32.76% of AP was degraded by (H2bimyb)3(As2Mo18O62) after 90 min [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(As2Mo18O62)]� H2O exhibits merit photocatalytic properties for degradation of refractory dyes AP under UV light [52], the photocatalytic decomposition rate is 93.24% after 80 min.

of AP under UV irradiation. The photocatalytic decomposition rates are 89.06 and 96.38% after 80 min [53]. The photocatalytic decomposition rates are 92.49% of AP for [Cu(pyr)2]6[As2Mo18O62] and 92.25% of AP for [Ag(pyr)2]6[As2Mo18O62] after irradi-

On the basis of the aforementioned points, {As2Mo18} 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 {As2Mo18} type to initiate the

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 Eg 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 O2 captures

OH. The •

� <sup>þ</sup> *<sup>O</sup>*<sup>2</sup> ) •

*<sup>h</sup>*<sup>þ</sup> <sup>þ</sup> *<sup>H</sup>*2*<sup>O</sup>* ) *<sup>H</sup>*<sup>þ</sup> <sup>þ</sup> •

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

*arsenomolybdate* þ *hv* ) *arsenomolybdate* ∗ *e*

*e*

<sup>ˉ</sup> and the hole reacts with H2O or OH� ions to form •

OH radical decompose organic dyes' molecules into the final product, the

*O*�

2(OH)2AsV

2Mo18O62}, only

� <sup>þ</sup> *<sup>h</sup>*<sup>þ</sup> (1)

<sup>2</sup> (2)

*OH* ) *H*2*O* þ *CO2* þ *other* (4)

*OH* (3)

2(OH)2AsV

(imi)2}2{Na(imi)2} {AsIIIAs2

0.5(trz)}6(As2Mo18O62)] were evaluated for degradation

2Mo18O62}

VMo18O62}]�

The three {As6Mo6} compounds, ((phen)(H2O)4]2 [(CoO6)(As3O3)2Mo6O18]� 2H2O,{[Co(phen)2(H2O)]2[(CoO6)(As3O3)2Mo6O18]}�4H2O and {[Zn(biim)2 (H2O)]2[(ZnO6)(As3O3)2Mo6O18]}�4H2O), as catalysts under UV light irradiation after 180 min [46], the photocatalytic decomposition rates of MB are about 92.64, 93.40, and 94.13%.

Yu groups prepared three Keggin arsenomolybdates, the photocatalytic decomposition rates of MB are 94.2% for (Hbimb)(H2bimb)[AsMo8 VIVV 4Co2O40], 96.1% for (Hbimb)2(H2bimb)0.5 [AsMo8 VIVV 4Cu2O40]�1.5H2O, 99.8% for [CuI (imi)2] [{CuI (imi)2}4{AsMo6 <sup>V</sup> Mo6 VIO40(VIV 2O2)}] after 90 min irradiation, respectively [47].

Four biarsenate(III) capped Keggin arsenomolybdates with tetravanadium(IV) substituted were prepared, which exhibit excellent degradation activity for MB under UV light. The absorption peaks of MB reduced obviously after 120 min in the presence of four Keggin arsenomolybdates, the degradation rates for MB are 92.9%, 95.8%, 96.6%, and 97.7%, respectively [48].

The photocatalytic decomposition rate of MB is about 96% for [{Cu (btp)2}3{As2Mo18O62}] after 40 min [26], and the photocatalytic decomposition rates were 96.32% and 95.57% for [Cu(pyr)2]6[As2Mo18O62] and [Ag (pyr)2]6[As2Mo18O62] after irradiation for 45 min [25]. Yu reported that (H2bimyb)3(As2Mo18O62) exhibits high-efficient degradation ability for MB under UV light. After UV light irradiation of (H2bimyb)3(As2Mo18O62) for 70 min, the photocatalytic decomposition rate is 95.82% [49].

It is reported that the conversion rate of MB is 94.6% when adding compound [Cu(imi)2]5Na[(AsO4)Mo9O27(AsO3)]�5H2O as the photocatalyst after 105 min [50]. {Cu(2,2<sup>0</sup> -bpy)}2{H2As2Mo2O14} as photocatalyst was investigated via the photodecomposition of MB under UV light irradiation and the same conditions. The photocatalytic decomposition rate of MB that is 96.7% after 180 min [40].

#### **3.3 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 and 95.7% for {Co(btb) (H2O)2}2{H2As2Mo6O26}�2H2O and [{Cu(abi){H4AsIIIAsVMo9O34}](abi)4[Cu (abi)2]�H2O as photocatalysts under UV light irradiation after 135 and 140 min, respectively [27, 44]. [{Cu(btp)2}3{As2Mo18O62}] as photocatalyst was investigated decomposition rate of RhB after 40 min is about 96% [26]. The photocatalytic decomposition rates of RhB are 94.42 and 95.07% for [M(pyr)2]6[As2Mo18O62] (M = Cu,Ag) under UV light irradiation after 45 min [25].

The photocatalytic decomposition rates of RhB are 95.9% for (Hbimb)(H2bimb) [AsMo8 VIVV 4Co2O40], 94.3% for (Hbimb)2(H2bimb)0.5[AsMo8 VIVV 4Cu2O40]� 1.5H2O, 95.8% for [Cu<sup>I</sup> (imi)2][{Cu<sup>I</sup> (imi)2}4{AsMo6 VMo6 VIO40(VIV 2O2)}] after 108 min irradiation, respectively [47].

#### **3.4 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

*Arsenomolybdates for Photocatalytic Degradation of Organic Dyes DOI: http://dx.doi.org/10.5772/intechopen.92878*

The above data show that the photocatalytic activity of the compound composed

degradation of MB under UV irradiation, which maybe that the polyoxoanions can connect with transition metals in diverse modes, which enhanced the contact area

The three {As6Mo6} compounds, ((phen)(H2O)4]2 [(CoO6)(As3O3)2Mo6O18]� 2H2O,{[Co(phen)2(H2O)]2[(CoO6)(As3O3)2Mo6O18]}�4H2O and {[Zn(biim)2 (H2O)]2[(ZnO6)(As3O3)2Mo6O18]}�4H2O), as catalysts under UV light irradiation after 180 min [46], the photocatalytic decomposition rates of MB are about 92.64,

Yu groups prepared three Keggin arsenomolybdates, the photocatalytic decompo-

Four biarsenate(III) capped Keggin arsenomolybdates with tetravanadium(IV) substituted were prepared, which exhibit excellent degradation activity for MB under UV light. The absorption peaks of MB reduced obviously after 120 min in the presence of four Keggin arsenomolybdates, the degradation rates for MB are 92.9%,

The photocatalytic decomposition rate of MB is about 96% for [{Cu (btp)2}3{As2Mo18O62}] after 40 min [26], and the photocatalytic decomposition

(H2bimyb)3(As2Mo18O62) exhibits high-efficient degradation ability for MB under UV light. After UV light irradiation of (H2bimyb)3(As2Mo18O62) for 70 min, the

It is reported that the conversion rate of MB is 94.6% when adding compound [Cu(imi)2]5Na[(AsO4)Mo9O27(AsO3)]�5H2O as the photocatalyst after 105 min

The photocatalytic activities of arsenomolybdates as photocatalysts are review via the photodecomposition of RhB under UV light irradiation. The photocatalytic

The photocatalytic decomposition rates of RhB are 95.9% for (Hbimb)(H2bimb)

(imi)2}4{AsMo6

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

VMo6

VIVV

VIO40(VIV

4Cu2O40]�

2O2)}] after

todecomposition of MB under UV light irradiation and the same conditions. The photocatalytic decomposition rate of MB that is 96.7% after 180 min [40].


rates were 96.32% and 95.57% for [Cu(pyr)2]6[As2Mo18O62] and [Ag (pyr)2]6[As2Mo18O62] after irradiation for 45 min [25]. Yu reported that

decomposition rates of RhB are about 96.34 and 95.7% for {Co(btb)

(M = Cu,Ag) under UV light irradiation after 45 min [25].

(H2O)2}2{H2As2Mo6O26}�2H2O and [{Cu(abi){H4AsIIIAsVMo9O34}](abi)4[Cu (abi)2]�H2O as photocatalysts under UV light irradiation after 135 and 140 min, respectively [27, 44]. [{Cu(btp)2}3{As2Mo18O62}] as photocatalyst was investigated decomposition rate of RhB after 40 min is about 96% [26]. The photocatalytic decomposition rates of RhB are 94.42 and 95.07% for [M(pyr)2]6[As2Mo18O62]

4Co2O40], 94.3% for (Hbimb)2(H2bimb)0.5[AsMo8

VIVV

2O2)}] after 90 min irradiation, respectively [47].

4Cu2O40]�1.5H2O, 99.8% for [CuI (imi)2]

supramolecular assemblies based on isomers [HxAs2Mo6O26]

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

between catalysts and substrates availing charge-transfer.

sition rates of MB are 94.2% for (Hbimb)(H2bimb)[AsMo8

<sup>V</sup> Mo6

95.8%, 96.6%, and 97.7%, respectively [48].

photocatalytic decomposition rate is 95.82% [49].

**3.3 Photocatalytic degradation of RhB**

VIVV

VIO40(VIV

(6 � x)� clusters and metal-organic complexes is higher than

(6 � x)� clusters in the

4Co2O40], 96.1% for

of [HxAs2Mo6O26]

93.40, and 94.13%.

[{CuI

[50]. {Cu(2,2<sup>0</sup>

[AsMo8

**122**

VIVV

1.5H2O, 95.8% for [Cu<sup>I</sup> (imi)2][{Cu<sup>I</sup>

108 min irradiation, respectively [47].

**3.4 Photocatalytic degradation of AP**

(Hbimb)2(H2bimb)0.5 [AsMo8

(imi)2}4{AsMo6

UV irradiation. The photocatalytic activity of {pyr}{Hbib}2{AsIII 2(OH)2AsV 2Mo18O62} 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 (H2bimyb)3(As2Mo18O62) was also studied under the same condition. Compared with {pyr}{Hbib}2 {AsIII 2(OH)2AsV 2Mo18O62}, only 32.76% of AP was degraded by (H2bimyb)3(As2Mo18O62) after 90 min [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(As2Mo18O62)]� H2O 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[{CuI (imi)2}2{Na(imi)2} {AsIIIAs2 VMo18O62}]� 2H2O and {CuI 0.5(trz)}6[{CuI 0.5(trz)}6(As2Mo18O62)] were evaluated for degradation of AP under UV irradiation. The photocatalytic decomposition rates are 89.06 and 96.38% after 80 min [53]. The photocatalytic decomposition rates are 92.49% of AP for [Cu(pyr)2]6[As2Mo18O62] and 92.25% of AP for [Ag(pyr)2]6[As2Mo18O62] after irradiation 135 min [25].

On the basis of the aforementioned points, {As2Mo18} 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 {As2Mo18} type to initiate the photocatalytic degradation reaction with organic dyes.

#### **3.5 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 Eg 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 O2 captures electron to form • O2 <sup>ˉ</sup> and the hole reacts with H2O or OH� ions to form • OH. The • O2 <sup>ˉ</sup> and • OH radical decompose organic dyes' molecules into the final product, the detail of photocatalytic reaction is shown in Eqs. (1)–(4).

$$
\epsilon \text{ arsenomolybdate} + hv \Rightarrow \text{ arsenomolybdate} \ast \left(e^- + h^+\right) \tag{1}
$$

$$\text{e}^- + \text{O}\_2 \Rightarrow \text{"O}\_2^- \tag{2}$$

$$h^+ + H\_2O \Rightarrow H^+ + ^\bullet OH \tag{3}$$

$$^\bullet dry + ^\bullet \text{O}\_2^- + ^\bullet \text{OH} \Rightarrow H\_2\text{O} + \text{CO}\_2 + other \tag{4}$$

#### **3.6 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 [44–48], which indicate that arsenomolybdates photocatalysis have excellent structural stability.
