Doping effects on the electro-degradation o(2)

patterns of the four anodes.

identified peaks of Ga(A)and Y(B)in the XPS spectra of the anodes.

and Discussion

of titanium suboxide anodes

The phases of samples were identified by XRD,as shown in Ti4O7 sample exhibited the typical peaks of Ti4O7(purity≥98%)while the Ti4O7–Ti5O9 sample was estimated to be of 56.3%Ti4O7 and 43.7%Ti5O9 by using XRD quantitatively re fined the two doped samples,only the simple peaks of Ti4O7 were characteristic peaks of Ga2O3 and Y2O3 did not appear in the XRD patterns,implying the doping elements had been highly dispersed without any separated aggregation ,the doping elements gave their identified peaks in the XPS spectra(Fig.2).The identified peak of Ga5d5/2 was found only in the sample of Ga-doped sample,as well as the peaks of Y3d5/2 and Y3d3/2 were found only in the sample of Y-doped sample,as Fig.2 shows.

The SEM images(Fig.3)reveal that the surface of all anodes was similarly coarse and the polyimide particles in the precursor had been burned out,some residual pores were coarse and porous surface would be in favor of electrochemical cauli flower-like aggregates were constructed by spherical primary titanium suboxide aggregates and the pores were of micrometer scale.

oxygen demand(COD)and current efficiency

The electro-oxidation was performed under a certain constant voltage for 5 h.Fig.4 shows the current and COD on the Y-doped anodes during the phenol was observed the higher voltage resulted in the higher ,the current quickly decreased in the first half hours and then smoothly the same time,the COD decreased with the increasing time.The average currents,COD removal ratios,and the surface current ef ficiencies were calculated on the four different anodes under different results were demonstrated in Fig.5.It is obvious the highest COD removal was assigned to the Y-doped anode,and 3.5 V was the optimal efficient voltage to remove COD for any exact data were listed in Table 1.

From Table 1,the Ga-doped anode showed the minimum COD removal ratio under each given voltage,and the Y-doped sample showed the optimum COD removal ratio(72%at 3.5 V).The results implied the doping reagent could greatly affect the electro-degradation each test anode,the highest COD removal ratio was always observed at 3.5 V while the average current was around 2.5 mA·cm-2(see Table 1).Similar phenomenon was reported by Geng[17]that the highest COD removal ratio(95.3%in 3 h)on a Ti4O7 nanotube anode was observed at the current density of 2.5 mA·cm-2(voltage around 2.5 V).Either higher or lower current density resulted in lower removal ratio because a relatively larger percentage of energy was consumed by the oxidation of water companying with oxygen gas releasing[17].It is implied that the diffusion of phenol may play an important role in the rate-limiting steps.A suf ficiently high voltage would result in the much restrictive transport limitation of organic species,as the relevant article revealed[18].During our experiments,it was observed that bubbles appeared on the surface of anode when the voltage was higher than 4.0 V.The bubbles covered on the anode surface would also suppress the electrochemical degradation[19].Considering the conclusions,the following experiments were carried on under the constant voltage of 3.5 V.

analysis of phenol degradation

images of the anodes:(A)pure Ti4O7,(B)Ti4O7–Ti5O9,(C)Ga-doped Ti4O7,(D)Y-doped Ti4O7.

current(A)and COD curves(B)during the electro-degradation on the Y-doped anode.

COD removal ratios in 5 h under different voltages on the four anodes.

The concentrations of phenol were measured by HPLC during the degradation in 9.5 h.The curves are showed in Fig.6.At the end of the experiment,the concentration of phenol was 0.326 mmol·L-1 for the pure Ti4O7 anode and 0.351 mmol·L-1 for the Ga-doped Ti4O7 anode,,for the Y-doped Ti4O7 anode,the concentration of phenol just dropped from 1.04 to 0.425 mmol·L-1.The comparison indicated the Y-doped Ti4O7 anode was relatively not good to destroy phenol.

It is strange that the Y-doped anode had the highest COD removal,as shown in Table 1.The tension probably attributes to the fact that the chemical oxygen demand(COD)is related to the total oxidable substances in solutions,including various intermediate compounds,but not only ,the highest COD removal ratio and the highestconcentration of residualphenolmightimply the leastintermediates accumulated during the electro-degradation on the Y-doped Ti4O7 assumption was supported by the following HPLC analysis.

The kinetics analysis indicated that the phenol electro-degradation was a first-order (A)demonstrates the concentrations of phenol during the concentration and the time in a first-order reaction follow the equation:d C/d t=-kC,which also equals to the equation ln(C0/C)=kt.It means there was a linear relationship between the ln(C0/C)and the time,and the slope is the apparent rate constant(k)[20].As Fig.6B and Table 2 display,the correlation coefficients are very close to 1.0,suggesting that the first order model agreed well with the reaction of phenol degradation rate constants were listed in Table 2.

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