Patsakol Prayoonpunratn,Trin Jedsukontorn,Mali Hunsom,2,3,*

1 Fuels Research Center,Department of Chemical Technology,Faculty of Science,Chulalongkorn University,Bangkok 10330,Thailand

2 Current address:Department of Chemical Engineering,Faculty of Engineering,Mahidol University,Nakorn Pathom 73170,Thailand

3 Associate Fellow of Royal Society of Thailand(AFRST),Bangkok 10300,Thailand

Keywords:Hydrogen production Wastewater Remediation Photochemistry Metal nanoparticle-decorated titanium dioxide

ABSTRACT A set of metal nanoparticle-decorated titanium dioxide (Mx/TiO2;where x is the percent by mass,%)photocatalysts was prepared via the sol-immobilization in order to enhance the simultaneous hydrogen(H2)production and pollutant reduction from real biodiesel wastewater.Effect of the metal nanoparticle(NP)type(M=Ni,Au,Pt or Pd)and,for Pd,the amount(1%–4%)decorated on the surface of thermal treated commercial TiO2(T400)was evaluated.The obtained results demonstrated that both the type and amount of decorated metal NPs did not significantly affect the pollutant reduction,measured in terms of the reduction of chemical oxygen demand(COD),biological oxygen demand(BOD)and oil&grease levels,but they affected the H2 production rate from both deionized water and biodiesel wastewater,which can be ranked in the order of Pt1/T400>Pd1/T400>Au1/T400>Ni1/T400.This was attributed to the high difference in work function between Pt and the parent T400.However,the difference between Pt1/T400 and Pd1/T400 was not great and so from an economic consideration,Pd/TiO2 was selected as appropriate for further evaluation.Among the four different Pdx/TiO2 photocatalysts,the Pd3/TiO2 demonstrated the highest activity and gave a high rate of H2 production(up to 135 mmol·h?1)with a COD,BOD and oil&grease reduction of 30.3%,73.7%and 58.0%,respectively.

1.Introduction

Hydrogen(H2)is now recognized as a clean energy carrier[1],which has a high energy content per mass compared to liquid fossil fuel[2].Besides,it is clean,storable and environmental friendly[3].Currently,around 90%–95%of H2used for energy is generated from natural gas and petroleum by the steam reforming[4,5].However,the quantity of natural gas and petroleum are lessening quickly.Thus,many attempts have been performed to develop the technology for producing H2from the renewable resources such as water,glycerol and biomass via the chemical,biological,electrolytic,photolytic and thermo-chemical process[2,6].Although the biomass reforming is one of the most energy efficient method to produce H2,it is too complex and requires sever operating conditions[7,8].Thus,attention has increasingly shifted towards the utilization of water as a source of H2because of its high abundant and process simplicity.

The production of H2from water can be achieved by thermochemical splitting,photovoltaic cell and photochemical reactions,of which photo-electrolysis (water splitting) is an effective and inexpensive method [2] that takes place via n-type semiconductors or photocatalysts.When they were irradiated by the light having a photon energy equal to or higher than their band gap energy(Ebg),an electron(e?)is excited from valence band(VB)to conduction band(CB),leaving a positive hole(h+)in VB[9].The photogenerated h+is able to oxidize the water to form O2and H+,while simultaneously the generated H+readily reacts with the photogenerated e?to from H2according to Eqs.(1)and(2):

The efficiency of the reduction and oxidation reaction depends on the band potential of the VB and/or CB of the photocatalyst and the water redox potential [9,10].The production of H2can be achieved when the CB potential is more negative than the reduction potential of proton,whilst the O2production can be proceed when the VB potential is more positive than oxidation potential of water[11].Therefore,only specific semiconductors which have a Ebgbetween the H2and O2production potentials,such as TiO2,ZnO,CdS and TaNO can split water effectively to produce H2and O2[9].Among all mentioned semiconductors,TiO2is frequently used in the water splitting process because of its cheap,high stability as well as its low-toxicity[12].Nevertheless,the efficiency of water splitting via TiO2is quite low due to its visible light inactive and a short lift of e?-h+pairs[13,14].

The two main strategies have been carried out to improve the TiO2property including(i)the modification of TiO2NPs and(ii)the use of electron donors.The former strategy can be done by coupling TiO2NPs with a narrow band gap semiconductor or carbonaceous material or by doping it with metal/non-metal ions[15–21].The latter strategy is performed via the addition of some electron donors,such as alcohols,polyalcohols,sugars and organic/inorganic acids,to react irreversibly with the photoinduced h+and/or photogenerated O2[22].Thus,various types of modification of TiO2NPs have been developed to convert solution containing organic/inorganic substances to H2such as Pt/TiO2from the water containing ethanol[5],Pt/TiO2from oxalic acid solution[22],Pt/TiO2/Nafion from Rhodamine B solution[23],Pd/TiO2from polyol compound solution[24],N-TiO2/MgO from water[25],TiO2/CdS/CNT from pure water and seawater[26],or Au/TiO2from a fruit juice producing plant[27].

Besides,an attempt on the simultaneous H2production and wastewater treatment was carried out recently.The organic pollutants in olive mill wastewater were reported to enhance H2production via preventing the recombination of photogenerated e?-h+pairs through combining them with a proton to produce a H2molecule via nanostructure mesoporous TiO2[25],while the photocatalytic activity was strongly affected by the wastewater pH and TiO2dosage.The H2production via a graphene-modified TiO2photocatalyst was not achieved using terephthalic acid wastewater under UV light,but was successful when using wastewater and acetic acid as sacrificial reagents[29].

Nowadays,biodiesel is gaining in global popularity due to the depletion of fossil fuel energy sources,the huge increase in energy demand,and the rising awareness of the environmental impact from the combustion of conventional fossil fuels.In Thailand,the biodiesel production capacity is more than 109L·year?1leading to the generation of wastewater from the wet washing process of around 108L·year?1[30].This wastewater usually has a high content of glycerin,soap,alcohol,catalyst residuals,un-reacted or residual free fatty acids(FFAs)and some biodiesel as well as some saturated/unsaturated fatty acids,such as methyl caprate,methyl laurate,lauric acid,methyl palmitoleate and methyl 9-octadecenoate [31],which would create a significant environmental problem if discharged directly into the environment.

Based on the research of our group,it was found that the photocatalytic H2production and pollutant reduction from such wastewater was feasible using a thermal-treated commercial TiO2(P25) at temperature of 400°C and ambient pressure[32].The co-presence of the rutile-anatase phases of TiO2positively affected the simultaneous H2production and pollutant reduction.However,at the optimum condition,only 228 μmol H2(57 μmol H2·h?1) was produced with the low reduction of COD,BOD and oil&grease levels[33].Therefore,to achieve both a high H2production rate and high pollutant reduction,the commercial P25 TiO2treated at 400°C(T400)was decorated with Ni,Au,Pd or Pt NPs via the sol-immobilization in order to serve as electron trapping sites and improve the light absorption property of TiO2towards the visible region.

2.Experimental

2.1.Property and characterization of utilized biodiesel wastewater

The work was carried out with the real biodiesel wastewater collected from the biodiesel industry in Thailand.Initially,part of some contaminants was removed by reducing the pH to 2.5 with the addition of concentrated sulfuric acid(H2SO4;98%,QRec)[31]to allow the phase separation.In order to allow a deep penetration of the incident light through the organic molecules and/or photocatalyst surface,the pretreated biodiesel wastewater was diluted with deionized water for 3.3-fold[32].The physical and chemical properties of the fresh and utilized biodiesel wastewater were analyzed in terms of their pH,chemical oxygen demand(COD),biological oxygen demand(BOD),oil&grease,total dissolved solids(TDS)and total suspended solids(TSS)levels according to standard methods[34].The soap content was analyzed by titration according to the modified version of the AOCS method Cc 17–79[35].The free fatty acid(FFA)content was estimated from the ratio of the acid value to 2.19 using potentiometric titration according to the ASTM D 664[36].

2.2.Preparation and characterization of the Mx/T400 photocatalysts

A series of Mx/T400photocatalysts,where M was Ni,Au,Pd or Pt NPs at x percent by mass(%),as 1%for Ni,Au and Pt and 1%–4%for Pd,was prepared by the sol-immobilization.The commercial TiO2(P25,Degussa)was heat-treated at 400°C in an air atmosphere for 3 h prior to use as the parent photocatalyst material as denoted as T400[32].To prepare 1%Ni NPs-decorated T400(Ni1/T400),approximately 1.98 g of T400was dispersed in 20 ml deionized water under constant agitation at 200 r?min?1at room temperature.Meanwhile,around 0.0835 g of NiCl2?6 H2O(QR?C)was dissolved in 20 ml deionized water and slowly added drop-wise into the suspended T400solution at the same agitation rate.Afterwards,to stabilize the Ni dispersion on the T400surface and prevent agglomeration,1.0 ml of 0.2%(mass)polyvinyl alcohol(99%hydrolyzed,Sigma Aldrich)was added slowly under constant agitation for 10 min.Then,20 ml of 0.2 mol·L?1sodium borohydride(NaBH4;Loba Chemie)was slowly added to the solution to reduce some Ni2+ions to metallic nickel(Ni0).To get a complete solimmobilization,the reaction was left for a day at atmospheric pressure(0.1 MPa)and temperature(～30°C).The obtained mixture was then filtered and rinsed several times with deionized water until no chloride ions were detected.The obtained solid portion was dried at 65°C for 6 h to remove the organic scaffold residue and thermally treated at 350°C under a nitrogen(N2;Linde)flow for 3 h followed by a hydrogen(H2;Praxair)flow for 3 h to yield the ready-touse Ni1/T400photocatalyst.

The same procedure was done for the preparation of the Au1/T400,Pd1/T400and Pt1/T400photocatalysts using 0.0399 g of HAuCl4?3H2O(Sigma Aldrich),0.0340 g of PdCl2(Sigma Aldrich) and 0.0531 g of H2PtCl6?6H2O(Sigma Aldrich),respectively.Also,to prepare the Pdx/T400(x is the percent by mass of Pd)photocatalyst,the same procedures were used except using 0.068,0.102 and 0.136 g of PdCl2for Pd2/T400,Pd3/T400and Pd4/T400photocatalysts,respectively.

The crystallinity of all prepared photocatalysts was analyzed by X-ray diffraction(XRD)using a D8 Discover-Bruker AXS X-ray diffractometer equipped with Cu Kα operated at 40 mA and 40 kV.The quantity of the decorated metal on the T400surface was estimated by scanning electron microscopy (SEM;JSM-6610LV) equipped with energy dispersive X-ray spectrometry (EDX) to perform elemental analysis at the atomic resolution.The textural properties of the prepared photocatalysts were characterized by a surface area analyzer(Quantachrome,Autosorb-1)according to the BET method.The diffuse reflectance spectra were monitored via UV–visible near infrared spectrometry (UV–Vis;Perkin Elmer,Lambda 950) over a wavelength of 320–820 nm.The particle size and dispersion of some decorated metal NPs along the T400surface were observed by transmission electron microscopy (TEM;Philips Tecnai f20) at an electron acceleration of 200 kV.The rate of the e?-h+pair recombination of all prepared photocatalysts was monitored via a Perkin-Elmer LS-55 Luminescence Spectrometer in air at room temperature using a 290 nm cut-off filter.Spectra were excited at 310 nm,and the photoluminescence(PL)spectra were recorded over the range of 375–550 nm using a standard photomultiplier.The elemental oxidation states of all the prepared photocatalysts were assessed by X-ray photoelectron spectroscopy(XPS;PHI 5000 VersaProbeII)with a monochromatized Al Kα source(hv=1486.6 eV).Accurate binding energies (BE),±0.1 eV,were established with respect to the position of the adventitious carbon C1s peak at 284.8 eV,with peak fitting performed using the XPSPEAK41 software package.

Fig.1.Visual appearance of the(a)fresh biodiesel wastewater,(b)3.3-fold dilution of acid-pretreated biodiesel wastewater and(c)treated biodiesel wastewater.

2.3.Simultaneous H2 production and pollutant reduction

The photocatalytic activity of the prepared Mx/T400photocatalysts was tested comparatively for the simultaneous H2production and pollutant reduction from biodiesel wastewater in a hollow,closed glass cylinder,which was placed in a UV-protected box.In each experiment,0.4 g of the selected photocatalyst was dispersed in 100 ml of the pretreated and 3.3-fold diluted biodiesel wastewater(Section 2.1)under constant agitation at 300 r?min?1.Prior to start the photocatalytic reaction,the system was flushed with argon(Ar;Linde)for 1 h at constant flow rate of 400 ml·min?1in order to remove air from system.Then,the system was irradiated at a light intensity of 5.93 mW·cm?2for 4 h with a UV high-pressure mercury lamp(RUV 533 BC,Holland),which was positioned above the glass reactor.The utilized UV high-pressure mercury lamp can generate the electrical power of 120 W in a wide spectrum range of 100–600 nm.When the reaction was over,Ar was fed into the reactor at constant flow rate of 400 ml·min?1to serve as a carrier medium to move the produced gas to the analysis system,while the liquid product(processed wastewater)was collected and separated from the solid catalyst by filtration.The concentration of pollutants in the processed wastewater was analyzed in terms of the COD,BOD and oil&grease levels.Meanwhile,the quantity of H2generated from the photocatalytic oxidation was determined from the calibrated gas chromatography(GC)signal,using a Shimadzu 2014 instrument with Ar as the carrier gas.

3.Results and Discussion

3.1.Properties of raw,utilized and treated biodiesel wastewater

The appearance of the fresh biodiesel wastewater used in this study is shown in Fig.1(a).It had a pale-yellow color with a slight turbidity,with slightly acidic(pH 4.71±0.01)and contained a high content of soap and a trace of FFA.The gross amount of organic matter was extremely high (Table 1) as monitored in terms of COD,BOD,oil &grease,TDS and TSS levels being around 216-,10.8-,49.3-,5.4-and 10.1-fold,respectively,higher than the limitation values set by Thai government for discharge into the environment.After pretreatment by acidification,the gross amount of soap decreased by almost 1.8-fold,while the FFA level increased 7.8-fold,presumably due to the protonation of FFA salt via H+during the pretreatment stage[37].The amount of organic substances,in terms of the COD,BOD,oil&grease,TDS and TSS levels were decreased(Table 1)but were still around 200-,3.1-,24.3-,7.1-and 9.3-fold,respectively,higher than the acceptable value.

Table1 Properties of the biodiesel wastewater

3.2.Effect of photocatalyst type

The crystallinity of the parent T400and all the Mx/T400photocatalysts was characterized using XRD analysis,with representative diffractograms shown in Fig.2(a).The XRD peaks of the parent T400displayed the diffraction peaks of TiO2in the anatase phase at 2θ of 25.3°,37.9°,48.1°,53.9°,55.1°,62.7°,68.8°,70.3°,75.1°and 82.55o,assigned to the(101),(004),(200),(105),(221),(204),(116),(220),(215)and(224)crystal planes,respectively(JCPDS No.21–1272).In addition,it also exhibited the diffraction peaks of the rutile phase at a 2θ of 27.5°,36.09oand 40.36o,corresponding to the (110),(101) and (111) crystal planes,respectively,(JCPDS No.04–0802).This suggested that the utilized parent T400was of a mixed anatase-rutile phase.The anatase fraction of the parent T400photocatalysts was 0.8383,as calculated from the ratio of peak area between the anatase and rutile phases using the equation adopted from the Spurr's equation[38].

For the respective M1/T400photocatalysts,their XRD patterns also exhibited the main characteristic peaks of the parent T400without a shift in the peak position.This suggested that the decorated metal NPs did not incorporate into the structure of the parent T400,but existed as a separate phase along the T400surface.The crystallite sizes of the T400in all M1/T400photocatalysts,calculated from the Debye–Scherrerequation using the diffraction peaks of anatase at plane(101),fluctuated in the narrow range between 20.2 and 21.4 nm(Table 2).Their anatase contents(0.8352–0.8377)were close to that of the parent T400.

Fig.2.Representative XRD patterns of the(a)M1/T400 and(b)Pdx/T400 photocatalysts.

No characteristic peaks of the respective decorated metal NPs were observed in the XRD peaks,probably because of their presence in small quantities.Thus,SEM–EDX analysis was performed to trace the presence of the decorated metal NPs.As shown in Fig.3,the spectra of all the decorated metal NPs were clearly observed,with an estimated quantity of doped metal NPs on the M1/T400surface of around 1.0 %(mass)(Table 2),confirming the presence of the respective decorated metal NPs along the parent T400surface.

The N2physisorption isotherms of all photocatalysts exhibited the characteristic loop of the mesoporous materials [39],as showed in Fig.4.Their pore sizes were distributed in a narrow range of 2–7 nm(Inset of Fig.4).The BET surface areas of all the M1/T400photocatalysts were lower than that of the parent T400,due to the partial loss of the T400surface area where the metal NPs were decorated.The BET surface area of the prepared M1/T400photocatalysts can be ranked in the order of Pd/T400>Au/T400>Pt/T400>Ni/T400.The particle sizes distribution of the decorated metal NPs was also evaluated from the TEM images,as shown in Fig.5.The particle size of the decorated metal NPs,obtained from randomly averaging not less than 150 particles,were ranked in the order of Ni>Pd>Au>Pt(Table 2 and right side of Fig.5).

Table2 Properties of the parent T400 and M/T400 photocatalysts

Representative UV–Vis absorption spectra of the prepared M1/T400photocatalysts and the parent T400are shown in Fig.6(a).The T400photocatalysts did not possess an absorption ability in the visible light region(λ>400 nm),probably due to its large intrinsic band gap energy.All the M1/T400photocatalysts showed a shifted absorption band edge towards the wavelength of 440–460 nm,resulting in a visible light absorption ability.Interestingly,the Au1/T400photocatalyst exhibited abroad band absorption centered at around 540 nm,which differed from the typical flat pattern of the other M1/T400photocatalysts.This was caused by the localized surface plasmon resonance(LSPR)effect of the decorated Au NPs,which can absorb visible light through the polarization and oscillation of the conduction electrons in the metal structure[40].The absorption band of the Au1/T400photocatalyst deviated from the typical features of well-dispersed spherical Au nanocrystals,in which the LSPR band is generally sharp and appears at a wavelength of 520 nm [41],was probably because of the presence of the nonspherical Au nanocrystals[42].

Fig.3.Representative(left)SEM images and(right)EDX spectra(from the box shown in the left panel)of the M1/T400 photocatalysts.

Fig.4.Representative N2 physisorption isotherms and pore size distribution of the parent T400 and M1/T400 photocatalysts.

The Ebgof the prepared M1/T400photocatalysts was estimated from the plot between the incident photon energy(hν)and absorption coefficient(α),as written as Eq.(3);

where A is the absorption constant,Ebgis the band gap energy and n is equal to?for the direct gap semiconductor.

As shown in the inset of Fig.6(a),an extrapolation of the linear part of spectral data curve gives the Ebgvalue of the respective M1/T400photocatalyst.As summarized in Table 2,the Ebgof the T400photocatalyst was around 3.32 eV,close to the well-known Ebgof commercial TiO2(Degussa).The decoration of T400with metal NPs significantly reduced the Ebgof the T400photocatalyst by almost 0.3 eV,acknowledged by the ability to absorb the solar light,which has a maximum irradiance at a wavelength region of 450–480 nm [43–45].

Fig.5.(Left)representative(250,000×magnification)TEM images of the as-prepared M1/T400 photocatalysts with(right)the derived metal NPs size distribution.

Moreover,it has been widely reported that the decorated metal NPs can serve as an electron trapping center and consequently reduce the e?-h+recombination rate,and so enhance the photocatalytic activity.To ascertain the efficiency of electron trapping by the utilized decorated metal NPs,the PL spectra of the M1/T400photocatalysts were explored over a wavelength of 375–550 nm.The T400and Ni1/T400photocatalysts exhibited a PL spectrum in the investigated wavelength range with the main peak at 420 nm(Fig.7(a)).This peak is primarily related to the electron transition from the CB and the VB.The shoulder peak of a PL spectrum at a higher wavelength was probably due to the electron transition by the state of oxygen vacancies and/or defect of the T400support[24].No PL spectra were observed for the Au1/T400,Pd1/T400and Pt1/T400photocatalysts,indicating a low rate of e?-h+recombination.Thus,the decoration of Au,Pt and Pd NPs on the treated T400attenuated the high e?-h+recombination rate in the fresh T400.

The chemical states of the M1/T400photocatalysts were evaluated by XPS analysis.As shown in Fig.8,the photocatalysts displayed the O1s,Ti2p and C1s peak of the O,Ti and C(from background carbon tape),respectively.No peaks for the decorated metal NPs were observed,which were probably due to the small amount of decorated metal NPs (1% (mass)) making them below the detection limit of their photoemission.

Fig.6.Representative UV–visible spectra and the dependence of(αhv)2 on the photon energy of the(a)M1/T400 and(b)Pdx/T400 photocatalysts.

The high resolution(HR)-XPS spectra of the M1/T400photocatalysts showed asymmetric Ti2p spectra (Fig.9(a)).After fitting with Gaussian-Lorentzian function,the photocatalysts exhibited two main peaks with their shoulder at a lower binding energy.The two main peaks arising from the spin orbit-splitting of the doublet Ti 2p1/2(～465.0 eV) and Ti 2p3/2(～459.3 eV) were assigned to the Ti4+on T400lattice.Also,satellite peaks at the lower binding energy of both doublets were observed in all samples,indicating the existence of Ti3+species in their structure.This is probably caused by the loss of lattice oxygen from the surface of T400during the thermal treatment process.The defective structures of the M1/T400photocatalysts,as determined in terms of the ratio of peak area between Ti3+and Ti4+(Ti3+/Ti4+),are summarized in Table 3.It was clearly seen that the T400photocatalyst had a very low defective structure(Ti3+/Ti4+ratio).The decoration of T400with Ni,Au or Pt NPs did not alter the defective structure of the T400photocatalyst,but,surprisingly,the decoration with Pd NPs increased the Ti3+/Ti4+ratio,suggesting an enhanced formation of the defective structure of the T400.For the asymmetric O1s spectra(Fig.9(b)),the HR-XPS of the M1/T400samples can be fitted with three symmetric peaks corresponding to the lattice oxygen in the T400crystalline network (～530.5 eV),non-lattice oxygen/OH?species (～531.7 eV)and surface-adsorbed water molecules(～533.1 eV).

With respect to the chemical states of decorated metals,the HR-XPS spectra of Ni 2p,Au 4f,Pd 3d,and Pt 4f were characterized and are exhibited in Fig.9(c).The fitted XPS spectrum of Ni contained main peaks at 856.4,862.3 and 874.3 eV corresponding to Ni2+(Ni-O form)of Ni 2p3/2,Ni2+(Ni-(OH)2form)of Ni 2p3/2and Ni2+(Ni-O form)of Ni 2p1/2,respectively.The appearance of these peaks indicated the major presence of the Ni2+in the as-synthesized Ni1/T400sample.The two small satellite peaks of both Ni 2p3/2and Ni 2p1/2,observed at a binding energy of around 853.7 and 871.6 eV,respectively,indicating the partial existence of the metallic nickel(Ni0)in the Ni1/T400.Meanwhile,the main chemical state of Au in the Au1/T400photocatalyst was the metallic form (Au0),as shown by the presence of the two main fitting peaks of Au 4f5/2(87.5 eV) and Au 4f7/2(83.9 eV).The small shoulder peaks with the higher binding energy of Au 4f5/2and Au 4f7/2were assigned to the presence of the Au2+form,but it was present at a trivial content.Likewise,in the Pt1/T400and Pd1/T400photocatalysts,the decorated metal NPs mainly existed in the metallic form on the surface of those prepared catalysts;Pd0of Pd 3d3/2(340.7 eV)and Pd0of Pd 3d5/2(335.4 eV)for the Pd1/T400photocatalyst and Pt0of Pt 4f5/2(74.5 eV) and Pt0of Pt 4f7/2(71.1 eV) for the Pt1/T400photocatalyst.The forms of decorated metal NPs on the surface of the M1/T400photocatalysts are summarized in Table 3.The predominant presence of Au0,Pd0or Pt0compared with Ni0could be attributed to the property of these noble metals that are less susceptible to oxidation by atmospheric oxygen,while the nickel is more susceptible and is oxidized to Ni2+.The presence of these metallic states is expected to act as the reactive sites and provides much more actives sites than their ionic states[46].

Table3 Defective structure and chemical states content of the decorated metal NPs of the parent T400 and M1/T400 photocatalysts

Fig.7.Representative photoluminescence spectral distribution of the(a)M1/T400 and(b)Pdx/T400 photocatalysts.

Fig.10 shows the photocatalytic activity of the M1/T400photocatalysts for the simultaneous H2production and pollutant reduction from the biodiesel wastewater,using the 3.3-fold dilution of the acid-pretreated biodiesel wastewater at an initial pH of 3.10,catalyst loading of 4 g·L?1,light intensity of 5.93 mW·cm?2and reaction time of 4 h.The parent T400photocatalyst reduced the COD,BOD and oil&grease levels in the biodiesel wastewater by around 27.9%,77.6%and 51.0%,respectively,(Fig.10(a)).The decoration of Ni,Au,Pt or Pd NPs on the T400did not significantly improve the photocatalytic activity of T400in terms of the COD,BOD and oil&grease reduction,with their reduction being 12.1%–29.1%,75.3%–78.8%and 33.4%–4.3%,respectively.However,they did enhance the H2production rate above that of the T400photocatalyst for both deionized water and biodiesel wastewater(Fig.10(b)).This was probably due to their short Ebgthat can harvest a wide spectrum of irradiated light.In addition,the decorated metals enhanced the e?-h+separation at the interface between the semiconductor and decorated metal,and/or functioned as the co-catalyst for H2evolution[47].

A higher H2production rate was achieved in biodiesel wastewater than deionized water was probably due to the presence of organic compounds that acted as electron donors resulting in different H2production reactions.That is,the H2production from the deionized water might originate from conventional water splitting reactions(Eqs.(1)and(2)),while that from biodiesel wastewater came from both water splitting and photocatalytic oxidation of organic compounds(Eqs.(4)–(9))[48].The organic substances in biodiesel wastewater can act as a hole scavenger to rapidly remove the photogenerated h+,hydroxyl radicals and/or photo-generated oxygen in an irreversible fashion,thereby alleviating the e?-h+recombination and/or H2-O2back reaction[3].

Fig.8.Representative survey XPS spectra of the parent T400 and M1/T400 photocatalysts.

The H+produced from Eq.(8)can react with the excited e?to form H2;

where the RCH2OH and R'CH2OH are denoted as the organic substances in the wastewater.

Among the M1/T400photocatalysts,the photocatalytic H2production from both deionized water and biodiesel wastewater was ranked in the order of Pt1/T400>Pd1/T400>Au1/T400>Ni1/T400.Considering the properties of these photocatalysts,listed in Tables 2 and 3,it was noticed that the trend in all their properties,including the BET surface area,size of decorated metal NPs,Ebg,defective structure and content of metallic state did not relate to the observed H2production rates.This could suggest that all the above photocatalyst properties did not alone play a crucial role on the simultaneous H2production and pollutant reduction.As mentioned in the literature,another possible factor that might affect the photocatalytic activity of the metal-decorated TiO2is the work functions[49].Theoretically,any metal-decorated TiO2that exhibits a large difference in the work function between the decorated metal NPs and the parent TiO2can initiate a high Schottky barrier at the decorated-metal and TiO2interface [24],causing an efficient e?-h+separation [49],which can prolong the lifetime of e?-h+pairs and consequently positively affect the photocatalytic activity.In our case,the relative work functions of Ni,Au,Pd and Pt and T400are 0.8,1.1,1.4 and 1.5 eV,respectively.Thus,the height of the Schottky barrier formed at the TiO2surface in the presence of the decorated metal NPs can be ranked in the order of Pt1/T400>Pd1/T400>Au1/T400>Ni1/T400,which,surprisingly,is consistent with the trend of H2production.Therefore,it can be said that the work function of the decorated metal NPs may play a significant role on the H2production rate from biodiesel wastewater.

From the obtained results and considering the economic viewpoint as well as the photocatalytic activity,although the Pd1/T400photocatalyst gave slightly lower photocatalytic activity for H2production than Pt1/T400photocatalyst,it was selected for further study because Pd is cheaper than Pt.

3.3.Effect of the amount of decorated Pd NPs on T400

To achieve a high H2production rate simultaneously with pollutant reduction,the effects of varying the amount(1%–4%)of decorated Pd NPs on T400was evaluated.Fig.2(b)displayed the XRD patterns of the prepared Pdx/T400photocatalysts.It was evident that the main diffraction peaks of the Pdx/T400photocatalysts still appeared at the position of both the anatase and rutile phase structures,suggesting that all the Pdx/T400photocatalysts were in the mixed anatase-rutile phase.Increasing the Pd NPs content on the surface of T400did not significantly affect the anatase fraction and crystallinity of the parent substrate,but rather were all nearly similar at 0.8244–0.8371 and 21.1–22.2 nm,respectively,(Table 2).

The N2physisorption isotherms of the Pdx/T400photocatalysts still exhibited type IV isotherms with hysteresis loops at a high relative pressure and pore sizes distributed in a narrow range of 2–7 nm(data not shown).Their BET surface areas decreased as the Pd NPs content increased(Table 2),which was attributed to the agglomeration of Pd NPs at a high Pd content,as supported by the TEM analysis(Fig.11).

With respect to the optical spectra of the Pdx/T400photocatalysts,shown in Fig.6(b),they possessed an absorption ability in both the UV(λ<400 nm)and visible light regions(λ>400 nm).The normalized absorbance increased slightly with increasing Pd NPs mass content from 1.0%to 4.0%,probably due to the LSPR effect of the decorated Pd NPs.However,the Tauc's plots(inset of Fig.6(b)and Table 2)revealed that increasing the decorated Pd NPs content slightly decreased the Ebgenergy value,which was because the decorated Pd NPs can induce the formation of the Ti3+defect structure,which can alter the principle electronic state position of the O2p valence orbital and the localized band bending of the O2p valence band edge maximum [50].The PL spectra of the Pdx/T400photocatalysts as displayed in Fig.7(b)indicates a low rate of e?-h+recombination compared with the parent T400photocatalyst.In addition,a Pd content in the range of 1%–4%had no significant effect on the e?-h+recombination rate.

Fig.9.(a)Representative HR-XPS spectra of the(a) Ti2p,(b)O1s and(b)(c)decorated metal NPs of the parent T400 and M1/T400(c)photocatalysts.

Fig.10.(a)Pollutant reduction and(b)H2 production level from biodiesel wastewater with the M1/T400 photocatalysts at 4.0 g·L?1 and a light intensity of 5.93 mW·cm?2 for 4 h.

The pollutant reduction and H2production from biodiesel wastewater by the Pdx/T400photocatalysts,using the 3.3-fold diluted acidpretreated biodiesel wastewater at an initial pH of 3.10,catalyst loading of 4 g·L?1,light intensity of 5.93 mW·cm?2and reaction time of 4 h,are displayed in Fig.12.All the Pdx/T400photocatalysts provided broadly similar COD,BOD and oil&grease reduction levels from the biodiesel wastewater at 22.7%–30.2%,62.8%–73.7%and 33.4%–57.9%,respectively(Fig.12(a)).However,the different Pdx/T400photocatalysts gave different H2production rates from the same biodiesel wastewater(Fig.12(b)).That is,the H2production rate was increased from 110 to 135 mmol?h?1as the decorated Pd NPs mass content increased from 1%to 3%and then decreased to 113 mmol?h?1at a Pd content of 4%.It is indicated that the decorated Pd can serve as electron sink which can trap photogenerated e?and reduce the recombination of h+and e?.Moreover,the height of Schottky barriers at the interface of the decorated Pd and T400could be increased as the Pd mass content increased between 1%–3%,resulting in an increased photocatalytic activity[51].However,the higher(excess)Pd content(4%)induced the accumulation of Pd NPs on the surface of parent T400,preventing the light absorption capacity of the parent T400and scattering the incident light and so resulting in less active e?being generated to participate the photocatalytic reaction[52,53].In addition,a higher Pd content(4 wt%)than the optimum condition would serve as an e?-h+recombination center leading to a decrease of H2production rate.Another possible reason is the isolation of the anatase-rutile crystallite in the presence of a high Pd content.As mentioned previously,a high content of deposited metal(ex.Au)created a large quantity of hotspot sites,which are able to suppress the photocatalytic activity by increasing the metal/rutile sites[54].

The properties of the biodiesel wastewater after the photocatalytic process are summarized in Table 1.This resulted in an improvement in the biodiesel wastewater quality,as assessed from the decreased level of all evaluated wastewater parameters(COD,BOD,oil&grease,TDS and TSS).However,some of them were still markedly higher than the acceptable values set by the Thai Government for discharging into the environment,especially the level of COD and TDS.Thus,retreatment or another physico-chemical process would be required.

Table 4 shows a comparison of the photocatalytic efficiency of the simultaneous H2production and pollutant reduction from this work compared to the two other reported works.This work provided a 3.04-fold lower COD reduction efficiency than a previous study using photocatalytic oxidation with mesoporous TiO2[28].However,the biodiesel wastewater utilized in this study had a higher level of organic molecules,which can hinder the penetration of the incident light to the photocatalyst surface.Nevertheless,this system can achieve an extremely high H2production rate than the other works that used acetic acid with a graphene-modified TiO2photocatalyst [29] or olive mill wastewater with mesoporous TiO2[28].

Table4 Photocatalytic efficiency,in terms of the simultaneous H2 production and pollutant reduction from wastewater

Fig.11.(Left)Representative TEM images(250,000×magnification)of the Pdx/T400 photocatalysts and(right)their deduced Pd metal NPs size distribution.

4.Conclusions

The Mx/TiO2photocatalysts prepared via the sol-immobilization were used to produce H2simultaneously with the pollutant reduction from biodiesel wastewater at an ambient atmosphere.The different types of decorated metal NPs (Ni,Au,Pt and Pd) and the amount of Pd NPs (1%–4 %) did not significantly affected the pollutant reduction level,as monitored in terms of the COD,BOD and oil &grease levels,but significantly affected the rate of H2production.The difference in the work function between the decorated metal NPs and parent TiO2had a seemingly more important effect on the rate of H2production than the BET surface area,size of decorated metal NPs,Ebgor the defective structure of the M1/TiO2photocatalysts.Considering the economic point of view as well as the photocatalytic activity,the Pd3/TiO2photocatalyst was recommended as an appropriate photocatalyst for the simultaneous H2production and pollutant reduction from real biodiesel wastewater,which can achieve a high rate of H2production(up to 135 mmol·h?1)with a COD,BOD and oil &grease reduction of 30.3,73.7 and 58.0%,respectively.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig.12.Variation in the(a)pollutant reduction and(b)H2 production levels from biodiesel wastewater using the Pdx/T400 photocatalysts at 4.0 g·L?1 with a light intensity of 5.93 mW·cm?2 and irradiation for 4 h.

Acknowledgements

The authors thank the TRF-CHE Research Career Development Grant(RSA5980015),the CU Graduate School Thesis Grant,Chulalongkorn University and the Center of Excellence on Petrochemical and Materials Technology(PETRO-MAT),Chulalongkorn University,Bangkok 10330,Thailand.