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Enhancement of TiO2-photocatalyzed organic transformation by ZnO and ZnS. Oxidation of diphenylamine

Abstract TiO2 anatase photocatalyzes the oxidative transformation of diphenylamine (DPA) to N-phenyl-p-benzoquinonimine (PBQ) in ethanol. The PBQ formation increases with increase of [DPA], TiO2-loading, airflow rate and light intensity. The catalyzed formation of PBQ is larger with UV-C light illumination than with UV-A light illumination. The catalyst is recyclable. The photocatalytic reaction mechanism has been proposed and the kinetic parameters deduced. ZnO and ZnS enhance the TiO2-photocatalyzed DPA oxidation suggesting interparticle charge transfer in mixed semiconductors.


Introduction
Semiconductor-photocatalysis for environmental cleanup has been extensively studied [1,2] and reports on the application of this process for selective organic transformations are also available [3e5]. The photocatalytic technology is benign and TiO 2 is widely employed as a photocatalyst for both applications [2,3,5]. On band gap excitation, TiO 2 produces electronehole pairs; electrons in the conduction band (CB) and holes in the valence band (VB) [2]. In the presence of moisture and air, the photoproduced charge carriers generate reactive oxidizing species (ROS) such as hydroxyl radical, which mineralize the organic pollutants. The VB hole oxidizes the water molecule and hydroxide ion adsorbed on the TiO 2 surface to produce hydroxyl radical. The oxygen molecule adsorbed on TiO 2 picks up the CB electron to form superoxide radical, which through a series of reactions yield hydroxyl radical. But in non-aqueous media formation of hydroxyl radical is unlikely and the VB hole directly oxidizes the adsorbed substrate and hence photocatalyzed organic transformations are carried out in non-aqueous media. TiO 2 selectively oxidizes alcohols to aldehydes or ketones and functionalized nitroarenes are selectively reduced by N-doped TiO 2 [4]. Propene and cyclopentene are transformed into the corresponding epoxide using TiO 2 eSiO 2 as photocatalyst [4]. TiO 2 Degussa P25 selectively photocatalyzes the oxidation of benzyl amines to the corresponding imines [5]. However, significant amounts of aldehydes are produced on the photooxidation of secondary benzyl amines on TiO 2 [5]. TiO 2 is also useful for the photocatalytic oxidative dehydrogenation of 1,2,3,4-tetrahydroquinoline [5]. Diphenylamine (DPA) is widely used in post-harvest treatment of apple and pear [6] and there is no study on the photocatalytic transformation of DPA. Photosensitized oxidation of DPA has been reported and cyanoanthracenes [7] and benzophenone [8] are some of the photosensitizers used. Photolysis of DPA solution in ethanol slowly yields N-phenyl-p-benzoquinonimine (PBQ) [9]. Hence it is of interest to study the photocatalytic transformation of DPA with nanocrystalline TiO 2 anatase. Generally, the photocatalytic activity could be improved by enhancing the lifetime of the charge carriers in semiconductor nanoparticles. This could be achieved through interparticle charge transfer (IPCT). When a mixture of two nanoparticulate semiconductors is used as a photocatalyst for mineralization of organics enhanced photocatalytic activity has been observed [10,11]. This has been explained on the basis of IPCT. However, such enhancement is not realized in the photocatalytic oxidation of iodide ion suggesting the IPCT to be slower than hole-transfer to iodide ion [12]. Hence it is of interest to study photocatalytic organic transformation using nanoparticulate semiconductor mixture as photocatalyst.
Here we investigate the photocatalytic transformation of DPA on TiO 2 and TiO 2 eZnO and TiO 2 eZnS mixtures; ZnO has also been employed as photocatalyst for organic transformation [5]. Systematic study on TiO 2 -photocatalyzed transformation of DPA has been made to deduce the kinetic law of the reaction. TiO 2 acts as a photocatalyst under natural sunlight utilizing the UV light (5%) present in the solar spectrum and hence the reaction has also been examined with natural sunlight.

Materials and measurements
TiO 2 , ZnS and ZnO (Merck) were used as supplied and their specific surface areas, obtained by BET method, are 14.7, 7.7 and 12.2 m 2 g À1 , respectively. The mean particle sizes (t) of TiO 2 , ZnS and ZnO, deduced using the relationship t ¼ 6/rS, where r is the material density and S is the specific surface area, are 104, 190 and 87 nm, respectively. The UVevisible diffuse reflectance spectra of the semiconductors were obtained using a Shimadzu UV-2600 spectrophotometer with an ISR-2600 integrating sphere attachment. The KubelkaeMunk plots provide the band gap energies of TiO 2 , ZnS and ZnO as 3.18, 3.57 and 3.15 eV, respectively. The recorded powder X-ray diffractograms of TiO 2 , ZnS and ZnO show the crystalline structures as anatase, zinc blende and wurtztie, respectively [13]. Potassium tris(oxalato)ferrate(III), K 3 [Fe(C 2 O 4 ) 3 ].3H 2 O, was prepared following the reported procedure [14]. DPA, AR (Merck) was used as received. Commercial ethanol was distilled with calcium oxide.

Photocatalysis with UV light
Photocatalytic oxidation with UV light was performed in a multilamp photoreactor. The reactor was equipped with eight 8 W mercury UV lamps (Sankyo Denki, Japan) of wavelength 365 nm. The lamps were shielded by highly polished anodized aluminum reflector. Four cooling fans fixed at the bottom of the reactor dissipate the heat produced. A borosilicate glass tube of 15-mm inner diameter was used as the reaction vessel and was placed at the center of the photoreactor. The photoreaction was also carried out in a micro photoreactor fixed with a 6 W 254 nm low-pressure mercury lamp and a 6 W 365 nm mercury lamp. Quartz and borosilicate glass tubes were employed as reaction vessels for 254 and 365 nm lamps, respectively. The light intensity (I) was determined using ferrioxalate actinometer. The volume of the DPA solution was always maintained as 25 mL in the multilamp photoreactor and 10 mL in the micro-photoreactor. Air was bubbled through the DPA solution to keep the photocatalyst powder under suspension and at constant motion. The airflow rate was determined by soap bubble method. The UVevisible spectra were recorded using a Hitachi U-2001 UVevisible spectrophotometer, after diluting the solution 5-times to keep the e g y p t i a n j o u r n a l o f b a s i c a n d a p p l i e d s c i e n c e s 2 ( 2 0 1 5 ) 3 2 e3 8 absorbance within the BeereLambert law limit. The formed PBQ was estimated from its absorbance at 450 nm.

Photocatalysis with sunlight
The photocatalytic transformation with natural sunlight was made in summer (MarcheJuly) under clear sky at 11.30 am e 12.30 pm. The intensity of sunlight (W m À2 ) was determined with a Global pyranometer, supplied by Industrial Meters, Bombay. The solar irradiance (Einstein L À1 s À1 ) was also measured by ferrioxalate actinometry. The measured 440 W m À2 corresponds to 22 mEinstein L À1 s À1 . For each set of experiment, solution of DPA of desired concentration in ethanol was prepared afresh and taken in wide cylindrical glass vessels of uniform diameter. The entire bottom of the vessel was covered by the catalyst powder. Air was bubbled using a micro pump without disturbing the catalyst bed. The volume of DPA solution was 25 mL and the loss of solvent due to evaporation was compensated periodically. The formed PBQ was estimated spectrophotometrically.

TiO 2 -catalyzed DPA oxidation with UV light
The TiO 2 -photocatalyzed oxidation of DPA in ethanol was carried out in the presence of air with a multilamp photoreactor fixed with UV lamps of wavelength 365 nm. The UVevisible spectra of the illuminated solution, recorded at different reaction time, reveal the formation of PBQ (l max ¼ 450 nm). Fig. 1 (a) displays the time scan. The illuminated solution is EPR silent indicating absence of formation of diphenylnitroxide. In addition, thin layer chromatographic analysis shows the formation of a single product. The illuminated DPA solution was evaporated after the separation of TiO 2 powder and the solid was dissolved in chloroform to develop the chromatogram on a silica gel G-coated plate using benzene as eluant. The PBQ formed was estimated from the measured absorbance at 450 nm using its molar extinction coefficient [15,16]. The linear increase of [PBQ] with illumination time, displayed in Fig. 1 (b), provides the initial rate of PBQ formation and the rates are reproducible to ±6%. The direct photooxidation of DPA, the photoformation of PBQ in the absence of TiO 2 , is not negligible [9] and the rate of TiO 2photocatalyzed PBQ formation was obtained by measuring the rates of PBQ formation in the presence and absence of TiO 2 .  Fig. 4. PBQ formation was also measured without bubbling air but the solution was not deoxygenated.
The dissolved oxygen itself brings out the photocatalysis but the PBQ formation is slow. The TiO 2 -catalyzed PBQ formation was examined at different light intensities. The reaction was carried out with two, four and eight lamps; the angles sustained by the adjacent lamps are 180 , 90 and 45 , respectively. Fig. 5 shows the variation of the catalyzed reaction rate with the light intensity. PBQ is not formed in the dark. Investigation of the TiO 2 -catalyzed PBQ formation with UV-A and UV-C light, using a 6 W 365 nm mercury lamp (I ¼ 18.1 meinstein L À1 s À1 ) and a 6 W 254 nm low-pressure mercury lamp (I ¼ 5.22 meinstein L À1 s À1 ), separately in the micro-photoreactor under identical conditions reveals that UV-C light is more efficient than UV-A light in bringing out the organic transformation. PBQ formation with UV-A and UV-C light are 12.8 and 51.7 nmol L À1 s À1 , respectively. TiO 2 does not lose its photocatalytic activity on usage. Reuse of TiO 2 shows sustainable photocatalytic activity. Singlet oxygen quencher azide ion (5 mmol L À1 ) fails to suppress the formation of PBQ indicating the absence of involvement of singlet oxygen in the photocatalytic transformation. This is in agreement with an earlier report; Fox and Chen [17] excluded     Fig. 4 displays the TiO 2photocatalyzed PBQ formation rate at different airflow rates. The observed enhancement of the reaction by oxygen reveals that the organic transformation conforms to Lang-muireHinshelwood kinetics with respect to oxygen also. The double reciprocal plot of reaction rate versus airflow rate is a straight line with a finite y-intercept (figure not given). The TiO 2 -photocatalyzed PBQ formation was studied without bubbling air but the solution was not deoxygenated. The dissolved oxygen itself brings in the photocatalytic transformation. However, the reaction is slow. The TiO 2photocatalyzed reaction enhances linearly with the apparent area of TiO 2 -bed. Fig. 6 presents the results. PBQ is not formed in the dark. TiO 2 does not lose its photocatalytic efficiency on usage. Reuse of the catalyst shows sustainable photocatalytic activity.

Mechanism
Band gap illumination of TiO 2 generates electronehole pairseelectrons in the conduction band (CB) and holes in the valence band (VB). The recombination of the electronehole pairs in semiconductor is so fast (occurring in a picoseconds time scale) and for an effective photocatalysis the reactants are to be adsorbed on the surface of TiO 2 [2]. The hole is likely to take up an electron from the adsorbed DPA molecule to form diphenylamine radical-cation (Ph 2 NH þ ). The oxygen molecule adsorbed on the TiO 2 surface picks up the CB electron. The formed superoxide radical-anion is likely to react with diphenylamine radical-cation yielding PBQ.   e g y p t i a n j o u r n a l o f b a s i c a n d a p p l i e d s c i e n c e s 2 ( 2 0 1 5 ) 3 2 e3 8

Kinetic law
Kinetic treatment could be made using the results obtained with artificial UV light. The heterogeneous photocatalytic reaction taking place in a continuously stirred tank reactor (CSTR) is governed by the kinetic law [18]: where K 1 and K 2 are the adsorption coefficients of DPA and O 2 on illuminated TiO 2 surface, k is the specific rate of oxidation of DPA on TiO 2 surface, g is the airflow rate, S is the specific surface area of TiO 2 , C is the amount of TiO 2 suspended per liter and I is the photon flux. The data-fit to the LangmuireHinshelwood kinetic curve, drawn using a computer program [18], confirms the rate law (Figs. 2 and 4). The linear double reciprocal plots of TiO 2 formation rate versus [DPA] and airflow rate confirm the kinetic law. The data-fit provides the adsorption coefficients K 1 and K 2 as 56 L mol À1 and 0.010 mL À1 s, respectively, and the specific reaction rate k as 110 mmol L m À2 einstein À1 . However, the TiO 2 -photocatalyzed PBQ formation rate fails to increase linearly with the amount of TiO 2 suspended. This is because of the high catalyst loading. At high catalyst loading, the surface area of the catalyst exposed to light does not commensurate with the weight of the catalyst. The amount of TiO 2 employed is beyond the critical amount corresponding to the volume of the reaction solution and reaction vessel; the whole amount of TiO 2 is not exposed to illumination. The photocatalytic reaction lacks linear dependence on illumination intensity; less than first power dependence of surface-photocatalysis rate on light intensity at high photon flux is well known [19].

Enhancement of photooxidation by ZnS and ZnO
Vectorial transfer of photoformed charge carriers from one semiconductor to another is possible in coupled semiconductors. This separation of charges leads to improved photocatalytic efficiency and examples of coupled semiconductors are many [20]. In coupled semiconductors, both the semiconductors exist in the same particle and the charge separation takes place within the particle. But what we observe here is enhanced photocatalytic transformation of DPA to PBQ on mixing ZnO or ZnS powder with TiO 2 powder. Fig. 7 presents the enhancement of photocatalytic formation of PBQ under UV light on mixing ZnO or ZnS with TiO 2 ; the two particulate semiconductors are in suspension and at constant motion. This observed enhanced photocatalytic transformation is because of interparticle charge transfer [10,11]. Nanoparticles in suspension aggregate [21]. Fig. 8 presents the particle size distributions of TiO 2 , ZnS and ZnO in suspension. Examination of Fig. 8 in conjunction with the determined particle sizes shows aggregation of the particles. As observed in individual semiconductor suspension, aggregation is likely in particulate semiconductor mixtures under suspension and both the semiconductor particles are likely to be present in the aggregates. Charge transfer between TiO 2 and ZnS or ZnO particles is likely to take place when both the semiconductors are under band gap-illumination and in contact with each other; electron from CB of a semiconductor may move to another if the latter is of lower energy and so is the hole from VB. The charge transfer between the particulate semiconductors is governed by the CB and VB energy levels. The CB electron of TiO 2 is less cathodic than those of ZnO [22] and ZnS [23]. This enables transfer of electron from the CB of ZnO or ZnS to the CB of TiO 2 in TiO 2 eZnO or TiO 2 eZnS mixtures (Fig. 9). The VB hole of TiO 2 is more anodic than those of ZnO and ZnS. This favors transfer of hole from the VB of TiO 2 to that of ZnO or ZnS. This interparticle charge transfer enhances the photocatalytic process. The energy difference between the CB electrons or VB holes of the two semiconductors is the driving force for the interparticle charge separation and the free energy change is given by eDG ¼ e(E (CBSC1) e E (CBSC2) or e(E (VBSC1) e E (VBSC2) ) [24]. In terms of redox chemistry, the CB and VB refer to the reduced and oxidized states in the semiconductor. In TiO 2 and ZnO or ZnS the CB electrons refer to the reduced forms of Ti 4þ (i.e., Ti 3þ ) and Zn 2þ (i.e., Zn þ ), respectively. Similarly, the VB hole corresponds to the oxidized forms of the respective O 2À (i.e., O À ) or S 2À (i.e., S À ). The interparticle charge-transfer, the transfer of electron from the CB of ZnS or ZnO to that of TiO 2 refers to the electron jump from Zn þ to Ti 4þ . The hole-transfer from the VB of TiO 2 to those of ZnO and ZnS corresponds to the electron-jump from O 2À of ZnO or S 2À of ZnS to O À of TiO 2 . The possibility of crosselectronehole combination, the transfer of electron from the CB of one semiconductor (SC1) to the VB of the other (SC2) is very remote; the very low population of the excited states makes the electron transfer between two excited states highly improbable. A possible reason for not observing the maximum photocatalytic transformation at 50% wt. composition for the semiconductor mixtures is the densities and particle sizes of the semiconductors and also the aggregation.

Conclusions
TiO 2 photocatalyzes the oxidative transformation of DPA to BPQ. The BPQ formation enhances with [DPA] and airflow rate and follows the LangmuireHinshelwood kinetic law. TiO 2 mixed with ZnO or ZnS produces more PBQ than by the individual semiconductor and this is likely because of interparticle charge separation.