Doping effects on the electro-degradation o(3) - 电镀与精饰杂志社投稿_期刊论文发表|版面费|电话|编辑部|论文发表- 电镀与精饰

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Doping effects on the electro-degradation o(3)

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and degradation mechanism

The electro-degradation mechanism of phenol was investigated by many researchers[2,12,21].Usually,hydroquinone and/or benzoquinone were the first-step intermediates in the degradation progress,and then broke to open-ring carboxylic carboxylic acids were further degraded to oxalate and/or formic ,organic pieces are thoroughly mineralized to CO2 and H2O.

In this article,the intermediates during the electro-degradation of phenol were identified by HPLC–MS,as Table 3 intermediates included a series of aliphatic dicarboxylic acid and unsaturated dicarboxylic to the list,pure reagents of the key intermediates were quantitatively injected to calibrate the individual peaks in HPLC–UV chromatograms.

From the HPLC chromatograms(Fig.7),typical intermediates were found during the electro-degradation on the pure Ti4O7 anode,including benzoquinone and hydroquinone(carbon-six,C6),oxalate and maleic acid(C2+C4),formic acid and α-ketoglutaric acid(C1+C5)[1,5,10,22,23].The isomers of benzoquinone and hydroquinone were the first order intermediates during the electro-oxidation of phenol[5,22,23].It is believed that phenol firstly reacted with various radicals,especially with OH radical,resulting in a series of phenoxy radical reactions and then transformed to benzoquinone[1,11,12].As the key intermediate,benzoquinone reacted prior with free radicals to transform to carboxylic acids but was not oxidized by ,during the experiments of the Y-doped anode,the peak of benzoquinone and hydroquinone did not appear in the HPLC chromatograms,implying these intermediates might be not necessary for degradation.

The concentrations of the intermediates were integrated from the HPLC chromatograms of the solution displays the concentration curves of the intermediates during the electro-degradation on the pure Ti4O7 concentrations increased at the early stage and then decreased due to the balance between generations and the apex concentration of α-ketoglutaric acid(C5)was much higher(15 t)than that of maleic acid(C4),the main routes were suggested to be the reaction C6→C5+,C5 and C4 were decomposed to final,oxalate and formic acid were thoroughly oxidized to CO2 and H2O.

The concentrations of intermediates on the Ga-doped Ti4O7 anode were measured and are shown in and hydroquinone(C6),maleic acid and oxalate(C4+C2)were acid(C1)and α-ketoglutaric acid(C5)were not found,implying thereaction of C6→C5+C1 was probably not first-step degradation route was only C6→C4+C2.The apex concentration of benzoquinone was much higher(3–5 t)than that on the pure Ti4O7 apex concentration of oxalate reached to 1.41 mmol·L-1 at the eighth hour, five times higher than thaton the pure Ti4O7 anode and three times higher than that on the Y-doped anode.

Table 1 The average currents and COD removal ratios in the electro-degradations(5 h)①Since the average currents were calculated by the discrete measured value,some deviations had to be introduced,which resulted thatthe two currentefficiencies were slightly higher than 100%.In fact,the efficiency could not be higher than 100%.Voltage/V Ti4O7 Ti4O7–Ti5O9 Ga-doped Ti4O7 Y-doped Ti4O7 I a/mA·cm-2 COD removal ratio/%CE/% I a/mA·cm-2 COD removal ratio/%CE/% I a/mA·cm-2 COD removal ratio/%CE/% I a/mA·cm-2 COD removal ratio/%CE/%2.5 0.54 43 105① 0.65 30 72 0.23 17 106① 0.79 40 67 3.0 1.14 49 57 0.99 49 65 1.05 42 55 0.94 46 57 3.5 2.79 58 27 2.66 52 26 2.12 46 29 2.48 72 38 4.0 3.04 57 25 4.05 43 14 3.17 35 15 4.91 62 16.6

concentration curves of phenol(A)and the kinetics plot(B)on the different anodes.

Table 2 Kinetics data of the phenol degradation on the different anodesAnodes k/h-1 t1/2/h R Pure Ti4O7 0.129 5.37 0.997 Ga-doped Ti4O7 0.120 5.78 0.998 Y-doped Ti4O7 0.102 6.82 0.995

Table 3 MS peaks and retention time of the possible intermediatesSubstance Retention time on UV detector NIST MS number Oxalate 1.65 min 60 α-Ketoglutaric acid 2.51 min 102 Maleic acid 3.12 min 99 Hydroquinone 4.00 min 110,81 Pyrocatechol 4.97 min 110,91 Benzoquinone 6.49 min 107 The peaks on MS detector

chromatograms of the solution after a one hour degradation on the pure Ti4O7 anode and the peaks of the pure intermediates at different UV wavelengths(245 nm and 275 nm).

concentrations of intermediates during the degradation on the pure Ti4O7 anode.

It is known that the concentration of residual phenol was 0.41 mmol·L-1 at the eighth hour during the electro-degradation(Fig.6).As the original concentration was 1.04 mmol·L-1,0.63 mmol·L-1 phenolhad been the reaction from phenol(C6)to H2C2O4(C2)could ideally give three times of oxalate,0.63 mmol·L-1 phenol equals to 1.89 mmol·L-1 ,the concentration of the accumulated oxalate(1.41 mmol·L-1)means that the surface yield of oxalate was as high as 74.6%.Due to the minimum COD removal ratio on the Ga-doped anode,as Table 1 demonstrates,it was reasonably speculated that the anode was too weak to oxidize organic compounds,especially the result,oxalate could be accumulated.

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