Photocatalytic reforming of glycerol for H 2 evolution on Pt/TiO 2 : fundamental understanding the effect of co-catalyst Pt and the Pt deposition route

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Photocatalytic reforming of glycerol for H2 evolution on Pt/TiO2:fundamental understanding the effect of co-catalyst Pt and the Ptdeposition routeXiaoliang Jianga, Xianliang Fua*, Li Zhanga, Sugang Menga, Shifu Chena,b*

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX5

DOI: 10.1039/b000000x

To fundamental understanding the effects of the deposited Pt and its deposition route on thephotocatalytic reforming (PR) of glycerol for H2 evolution over Pt/TiO2, several 1 wt% Pt/P25 (PT)samples were prepared by photo-deposition (PD, glycerol as hole scavenger) and impregnation-reductiondeposition routes (IRD, NaBH4 or H2/Ar as reductant) using H2PtCl6 as the precursor. The samples were10

characterized by XRD, UV-vis DRS, TEM and XPS, and the PR activities were examined and comparedunder ambient condition. The formation of photo-induced charge carriers (CCs) over PT was measured byphotoluminescence technique using terephthalic acid as a probe molecule. The results indicated thereforming activity depends on both the nature of the light harvesting of P25 and the characteristics of co-catalyst Pt, including its chemical state, size, and the interaction with P25, which serves as the active sites15

for H2 evolution. Uniform Pt particles could be selectively and intimately deposited on P25 in Pt(0) statevia in situ PD process (PT-S), while by IRD routes, Pt were randomly loaded with surface in Pt(0) and thebulk in Pt(II/IV) states. Unlike the Pt chemical state, the Pt sizes are less impacted by the depositionroutes and are about 2 nm. Compared to P25, a low generation efficiency of CCs was observed onplatinized samples due to the covering of the photo-active sites by Pt. Pt(0) exhibits higher light shielding20

effect than Pt(II/IV). Meanwhile, the separation of CCs was promoted by Schottky barriers formed at Pt-TiO2 interface. Photo-induced electrons can be trapped by the barriers and the process was favored bywell-contacted Pt(0) and obstructed by the bulk Pt(II/IV) component. The promotion effect of Pt(0)prevails over its adverse effect. Thus, PT-S exhibited the highest PR activity as it only possesses Pt(0)demonstrating the advantage of PD process. Control test suggests Pt with this kind of feature can only be25

achieved in a dilute suspension by PD route.

1. IntroductionStimulated by the depletion of fossil fuel and its derivedenvironmental issues, considerable efforts are being devoted torenewable energy production to meet the rising fuel demand.30

Biodiesel which produced by transesterification of triglyceridesin renewable vegetable and soya bean oils is regarded as an idealsustainable energy source to replace petroleum diesel1-3. It alsooffers a way to reduce the net CO2 emission and air pollution4. Itis predicted that as much as 20% of all transportation fuels will be35

provided by biodiesel by 20205. The increasing production ofbiodiesel leads to a huge quantity of crude glycerol (CG), aninevitable byproduct accounts for ca. 10% (wt.) of the totalproducts. Although high purity glycerol is an important industrialfeedstock, CG possesses very low value due to containing of40

unreacted methanol, water, and alkali ions. Considering thedropping of glycerol price and the saturation of market, therefining of CG becomes unprofitable at present. It is rapidlybecoming a waste product with an attached managing, storage,and disposal cost6. Thus, developing a feasible and profitable45

route for the conversion of CG to useful products, such as H2 richgas, has great practical significance6-8. As a viable biomass feedstock, the renewable CG can beconverted into H2 rich gas instead of the steam reforming ofnatural gas, which currently accounts for about 90% of produced50

H2. The conversion of CG to H2 is mainly achieved by pyrolysis,steam reforming, aqueous phase reforming, and theirmodifications6,7. Unfortunately, most of the processes arecomplex and energy intensive for the high operating temperature(350-900 oC) or pressure (2.5-4.5 MPa). Compared to these55

traditional routes, photocatalytic reforming (PR) process, exhibitsdistinct advantages for it can be carried out under ambientconditions and driven by sunlight. The reaction can be describedby Eq. 1.

(Eq. 1)60

The validation of PR has been approved not only for theconversion of glycerol9-13, but also for the treatment of otherbiomass-derived compounds13-16. Generally, the reaction is

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View Article OnlineDOI: 10.1039/C4TA06052K


2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

carried out under anaerobic condition over irradiatedphotocatalyst. Photo-induced holes (h+) serve as oxidant for thedeprotonation of H2O, glycerol and the mineralization of glycerol,while the induced electrons (e-) serve as reductant for thereduction of H+ to H2

6,17,18. Undoubtedly, photocatalyst plays a5

crucial role in the conversion process. In the reportedworks9,11,13,18,19, TiO2 is the most studied material due to itsinertness, stability. Furthermore, a small amount of noble metalssuch as Pt, Pd, and Au15,18 is also essential for the reaction whichserves as co-catalyst. Due to their large work function and the10

low activation energy for H2, the co-catalyst can effectivelysuppress the recombination of e- and h+ and promote theevolution of H2

18,20. Pt with the largest work function and thelowest over-potential for H2 production is the best candidate18.Without the co-catalyst, TiO2 almost shows no activity for H215

evolution20. Thus, except the light harvesting semiconductor, thePR reaction also highly depends on the nature of the co-catalyst.A comprehensive investigation of this catalytic system will notonly benefit the production of H2 from biomass-derivatives orH2O, but also contribute to understanding the fundamental of PR20

process. However, related works are still limited10,21,22. Herein, Pt/TiO2 (PT) was selected as a model to disclose therelationship between the reforming activity and the characteristicsof Pt, including its size, chemical state, distribution, and theinteraction with TiO2. Due to the research effort is concentrated25

on Pt, standard Degussa P25 (TiO2) was used as substrate. As theproperty of the deposited Pt is mainly determined by thepreparation method21,23, photo-deposition (PD) and impregnation-reduction deposition (IRD), the most popular methods were usedto deposit Pt. The effects of the Pt deposition route on the30

characteristics of Pt and the consequent reforming activity werethen investigated. The aim of this study is to identify the keyfeatures of Pt and the optimal Pt deposition conditions whichfavor the PR of CG for H2 production. Furthermore, the kineticand mechanism of the reforming reaction were tentatively35

investigated.

2. Experimental2.1 Chemicals and photocatalysts preparation

P25 used in this work was kindly provided by Evonik DegussaCo. (70% anatase and 30% rutile, prepared by flame hydrolysis of40

TiCl4 at temperatures greater than 1200 °C in the presence ofhydrogen and oxygen, i.e. Aerosol process24). Analytically pureglycerol, H2PtCl6 6H2O, NaBH4 and terephthalic acid (TA) werepurchased from Sinopharm Chemical Reagent Co. and used asreceived. High purity water was used throughout the sample45

preparation and photocatalytic reaction. Four different reduction routes, including in situ PD, PD, IRDwith H2/Ar and NaBH4 as reductant were used to deposit Pt onP25. 1.0 wt.% was taken as the optimal loading amount accordingto our former works18,25. The resulted samples are labeled as PT-50

S, PT-G, PT-H, and PT-N, respectively. As for PT-S, thepreparation procedure was similar to the PR of glycerol (seesection 2.3) except 100 L H2PtCl6 solution (containing 1 mg Pt)was added. At the beginning of the PR reaction, Pt was depositedonto P25. The resulted PT-S then utilized in situ to reform55

glycerol for H2 evolution. PT-G was prepared by a similar PD

process as that of PT-S except the suspension is quite dense andperformed separately. 2 g P25 was suspended in 80 mL glycerolsolution containing of 10 mL glycerol. Then, required amount ofH2PtCl6 was added into the suspension. Before irradiation (300 W60

Xe lamp, full arc, PLS-SXE300, Beijing Trusttech Co.), thesuspension was purged with Ar (100 mL/min, 99.99%) for 15min to remove the dissolved O2 and ventilated during thedeposition reaction. The solution was kept cool by an ice-waterbath. 6h later, the resulted precipitates were collected by65

centrifugation and rinsed thoroughly with deionized water, andfinally dried in air at 90 oC for 10 h. PT-H and PT-N wereprepared by impregnation and subsequent reduction procedure. 2g P25 was first impregnated with H2PtCl6 solution under stirringand then dried at 110 oC for 10 h. The brown powders were then70

reduced by H2/Ar (20 vol. % H2, 100 mL/min) at 400 oC for 3 hin a tube furnace according to reported work (PT-H)26. As for PT-N, the powders were then reduced by 0.1 M NaBH4/0.1 M NaOHmixed solution following reported method18. To investigate the effects of the contact condition between Pt75

and P25, and the chemical state of Pt on the H2 evolution, PT-Gand PT-H undergo a post heat treatment. The former was treatedin H2/Ar (20 vol. % H2, 100 mL/min) at elevated temperature for4 h to enhance the interaction between Pt and TiO2, while thelatter treated in O2 (40 mL/min) at 400 oC for 2 h to convert Pt(0)80

to Pt(II or IV) (labeled as PT-H-O). Here, PT-G was chosen asthe model is due to its high yield and the simple depositionprocedure, while for PT-H, the reason is it has been suffered asimilar heat treatment process. Thus, the further treatment in O2

atmosphere will mainly influence the chemical state of Pt, rather85

than other structures. The preparation conditions for PT arebriefly summarized in Tab. 1 for easy reference.

2.2 Characterizations

X-ray diffraction (XRD) patterns of the samples were obtained ona Bruker D8 Advance X-ray diffract meter with Cu K radiation90

=1.5406A). UV-visible diffuse reflection spectra (UV-Vis DRS)were recorded using a Hitachi UV-365 spectrophotometer withBaSO4 as reference. Transmission electron microscopy (TEM)and high-resolution transmission electron microscopy (HR-TEM)images were performed on a JEM-ARM200F electron95

microscope operated at an acceleration voltage of 200 kV. Theelemental compositions were detected by an energy-dispersive X-ray (EDS) spectrometer attached to the TEM instrument. At leastthree EDX measurements were performed on each sample. X-rayphotoelectron spectroscopy (XPS) analysis was conducted on an100

ESCALAB 250 photoelectron spectrometer (Thermo FisherScientific) at 3.0×10-10 mbar using Al K X-ray beam (1486.6eV).All binding energies were corrected with reference to the C 1speak of the surface adventitious carbon at 284.6 eV. N2

physisorption measurements were carried out at 77 K using a105

Micromeritics Tristar II 3020 surface area analyzer. Prior to theadsorption measurements, the samples were degassed at 523 Kand 1×10-3 Pa for 5 h. Multipoint Brunauer-Emmett-Teller (BET)specific surface areas were then determined from the adsorptionisotherms.110

2.3 PR of glycerol for H2 evolution

PR of glycerol for H2 evolution was performed at roomtemperature in a commercial gas circulation system (LabSolar H2,

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This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3

Trusttech Co, Beijing). A Pyrex top-irradiation reaction cell wasused as a reactor which can be connected to the system. In atypical experiment, 0.1 g catalyst was suspended in a mixture of150 mL water and 5 mL glycerol. The whole reaction system wasthen evacuated by a mechanical pump several times to remove5

system air and the suspension was stirred in the dark for 30 minto establish an adsorption-desorption equilibrium beforeirradiation. Aforementioned 300 W Xe lamp was used as the lightsource. The solution temperature was controlled at ca. 10 oC bycooling water. The evolved H2 was measured by an online gas10

chromatograph (GC7900, TianMei, Shanghai) equipped with aTCD detector and molecular sieve 5A column using Ar as carriergas.

2.4 Measurements the formation of photo-induced chargecarriers15

Photoluminescence (PL) technique was used to determine theformation of ·OH with TA as a probe molecule. The separationefficiency of photo-induced charge carriers (CCs) can beestimated indirectly by the formation of 2 hydroxyterephthalicacid (TAOH)27. The measurements were carried out in a tubular20

reactor which surrounded by a water cooling jacket. A Julabo F12heating/cooling bath (Julabo Labortechnik, Germany) was used tocontrol the reactor temperature at 20 oC. A 9 W H-shaped UVlamp (Philips TUV PLS-9W) assembled at the center of thereactor was used as light source. In a typical run, 100 mg sample25

was dispersed in 200 mL mixed aqueous solution of 5×10 4 MTA and 2×10 3 M NaOH. 3 mL of suspension was collected afterirradiation and then centrifuged and filtrated. The fluorescencesignal of TAOH was measured by a fluorescencespectrophotometer (JASCO FP 6500) excited by 315 nm light at30

room temperature.

3. Results3.1 XRD and UV-Vis DRS analysis

Fig. 1 XRD patterns of prepared PT samples and P25.35

Fig. 1 shows the XRD patterns of prepared PT samples. Thesamples exhibit a similar XRD pattern to bare P25. A mixedphase of anatase and rutile can be indexed. The diffraction peaksat 27.5, 36.2, 41.4° can be assigned to (110), (101), (111) planesof rutile, while the others can be ascribed to anatase. No Pt-40

derived peak is perceived due to low loading amount or small Ptparticle size, which are more likely to locate at 40.0 (111) and46.5° (200) (indicated by the dash lines)28. The result suggests theintrinsic crystalline structure of P25 cannot be modified by themild loading processes. A careful examination indicates, with the45

deposition of Pt, the intensity of (101) peak slightly decreases(especially for PT-S), while the other peaks are almost untouched.It seems the Pt particles are preferentially deposited on the (101)surface of anatase by in situ PD method.

50

Fig. 2 XRD patterns of (a) PT-G treated at different temperature for 4 h inH2/Ar (20 vol. % H2, 100 mL/min); (b) PT-H treated at 400 oC for 2 h in

O2 (40 mL/min)

Fig. 2a shows the XRD patterns of heat treated PT-G.55

Apparently, the crystal structure of P25 can be largely preservedwhen it was treated below 400 oC. Further increasing thetemperature to 500 oC, a decline of anatase peaks can be observed.The transition of anatase to rutile accounts to this change. Themagnified pattern shown in the right side of Fig. 2a reveals the60

(111) plane of Pt emerges after the treatment, even it was onlyperformed at 200 oC. It suggests the loaded Pt particles are notfirmly anchored on P25 in PT-G. An immigration andconjunction of the small Pt particles was trigged by the heattreatment29. In the range of investigated temperature, the average65

size of the aggregated Pt particles maintains at about 13 nmestimated by the Debye–Scherrer equation suggesting the


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conjunction of the small Pt particles is accomplished at 200 oC.The particles are then immobilized on P25 surface. Furtherincreasing the temperate can only favor the interdiffusion of Ptinto the substrate, which consequently improves the interactionbetween Pt and P25 rather than Pt particle size. This deduction5

will be confirmed by the photocatalytic activity test and the PLmeasurements which are demonstrated below (Fig. 9b and 12b).Fig. 2b shows the XRD patterns of PT-H-O and the untreated PT-H. No apparent difference can be perceived between the twopatterns. Unlike treated PT-G, the Pt diffraction peak cannot be10

found in PT-H-O (shown in the right side of Fig. 2b) suggestingPt are firmly loaded on P25 in PT-H. It is not surprising to seethis result considering a similar reducing process (400 oC for 3 hin H2) has been performed on PT-H. Thus, instead of Pt particlesize and the microstructure of P25, the post treatment of PT-H15

would more likely alter the chemical state of Pt.

Fig. 3 UV-Vis DRS of prepared PT samples and P25.

Fig. 3 shows the UV-Vis DRS of prepared PT. A strongabsorption edge at ca. 390 nm can be found in all PT samples due20

to the band gap absorption of P25. An apparent visible absorption > 400 nm) can also be observed for the deposition of Pt, which

is consistent with the dark gray color of PT. The absorption canbe assigned to the localized surface plasmon resonance (L-SPR)of Pt nanoparticles which depends on their size, shape, and25

dielectric environment30,31 or to a d-d transition of Pt(IV)29,32. Theabsorption coefficients increase in order PT-G<PT-N<PT-H<PT-S. The different absorption feature suggests Pt particles maypossess a different size or a different chemical state in theprepared PT. This will be revealed by the TEM and XPS results.30

As for the post treated PT-G and PT-H, the visible absorptionsbecome less intense after the treatment. For PT-G (Fig. 4a), theabsorption generally decreases with temperature. The increasingof Pt size and the decreasing of Pt(IV) component (reduced bythe treated process, as shown in Tab. 1) may account for this35

change, which reduces the absorption of L-SPR and the d-dtransition of Pt(IV), respectively. A slight decrease of the UVabsorption is observed in the samples treated at 400 and 500 oC.It is caused by the phase transition of P25, which has beenconfirmed by the XRD result in Fig. 2a. For the treated PT-H (Fig.40

4b), the decrease of the visible absorption should be induced bythe changing of the Pt chemical state33.

Fig. 4 UV-Vis DRS of (a) PT-G treated at different temperature for 4 h in45

H2/Ar (20 vol. % H2, 100 mL/min) and (b) PT-H treated at 400 oC for 2 hin O2 (40 mL/min)

3.2 TEM analysis

Fig. 5 (a) TEM image, (b) HAADF-STEM image, (c and d) HR-TEM50

images, (e) elemental mapping of PT-S and (f) particle size distributionhistogram of Pt in PT-S.

The morphologies of prepared PT were studied by TEM toanalyze the distribution and the size of Pt. Fig. 5 shows the


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typical results of PT-S. As pointed by the arrows in Fig. 5a, theP25 surface is dotted uniformly by some black dots. The highdensity contrast suggests they should be Pt. The distribution andthe size of Pt become more visible in the high-angle annular darkfield scanning image (HAADF-STEM) in Fig. 5b. Apparently,5

most of Pt particles are mono-dispersed with size about 2 nm.The crystal lattice fringes of TiO2 and Pt can be disclosed in theHR-TEM images (Fig. 5c and d). The fringes with d spacing of0.23 nm can be ascribed to the (111) crystalline plane of the facecentered cubic structure Pt29, while the d spacing of 0.35 nm10

correspond to the (101) plane of TiO222. An intersection of the

two lattice fringes can be found in Fig. 5d indicating an intimatecontact between the Pt particle and the substrate P25. Pt particleswith sizes ca. 2 nm was further approved by HR-TEM. Fig. 5eshows the EDS mapping result around Pt loading site. The red,15

yellow, and green spots are assigned to Pt, Ti and O element,respectively. Undoubtedly, the bright spot (left side in Fig. 5e) isPt particle for the Pt mapping image shows the same outline asthe spot. Ti and O elements disperse uniformly in the scanningarea. Fig. 5f displays the size distribution histogram of the Pt20

particles in PT-S, which calculated from HAADF-STEM image(Fig. 5b) by randomly picking out 50 bright dots. The Gaussiancurve indicates Pt particles are well dispersed and the averagesize is 2.0 ± 0.27 nm.

25

Fig. 6 HAADF-STEM images of (a) PT-H, (b) PT-N, and (c) Pt-G andthe corresponding particle size distribution histogram of Pt in these

samples.

Other PT samples show a similar morphology to that of PT-S

except the distribution and the size of Pt are slightly different (Fig.30

6). Generally, Pt can be well-dispersed on P25 surface via thesepreparation methods, except PT-G prepared by PD in densesuspension. The aggregation of some Pt particles can be observedin PT-G (Fig. 6c, indicated by the arrows). As shown in the rightside of Fig. 6, the Pt size in these samples is less uniform than in35

PT-S due to a large deviation from Gaussian distribution curvecan be observed. The Pt sizes in PT-H, PT-N, and PT-G are 1.8 ±0.45, 2.0 ± 0.36, and 2.2 ± 0.55 nm, respectively. PT-G exhibitsthe highest standard deviation. The compositions of the PT samples were analyzed by EDX.40

The presence of Ti, O, and Pt were confirmed by the results (Tab.1) and the weight contents are close to the theoretical values of60%, 40%, and 1%.

3.3 XPS analysis

45

Fig.7 High resolution XPS spectra of Pt 4f for prepared PT samples.

The surface composition and the chemical states of the elementsin PT were investigated by XPS. Fig. S1-a (supportinginformation) shows the survey spectra of PT samples. Thepresences of Ti, O, Pt, and C elements can be found. Consistent50

with the EDX results, the content of Ti, O, and Pt are close to thetheoretical values. The core level peaks of the elements appear at


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the binding energy (BE) of 459 (Ti2p), 529 (O1s), 71 (Pt 4f), and285 eV (C1S), respectively. The signal of C element comes fromthe adventitious carbon in the instrument. The high resolutionspectra of these elements are shown in Fig. S1-b (Ti and O) andFig. 7 (Pt). The Ti2p and O1s core peaks for prepared PT locate5

at the same BE. As shown in Fig. S1-b, Ti 2p shows twoindividual symmetric peaks positioned at 459.0 (Ti 2p3/2) and464.8 eV (Ti 2p1/2,) with a peak separation of 5.8 eV, suggestingthe state of Ti4+ 34. The O1s peak can be deconvoluted into twopeaks, one positioned at 529.8, and the other at 532.2 eV. They10

correspond to the lattice O (Ti-O-Ti) and the surface hydroxyloxygen (-OH), respectively35. The OH peak observed in PT-G isobviously larger than that in other samples. That means PT-Gsurface possesses a high population of OH group. The spectrumof PT-G-400 indicates these OH group can be preserved even15

after treated at 400 oC for 2 h. The deposition procedureperformed in a dense glycerol solution may responsible for thisphenomenon considering the report17 that the under-coordinatedTi4+ on surface can be stabilized and repaired by hydroxylicmolecules. A high concentration of glycerol may favor this20

repairing process. The chemical state of the deposited Pt species was disclosedby Pt 4f XPS spectra. Theoretically, the spectrum of Pt 4f iscomposed of Pt 4f7/2 and Pt 4f5/2 with a separation of 3.3 eV dueto the spin orbit coupling. The BE value of Pt 4f7/2 mainly25

depends on the chemical state of Pt. Generally, the spectrum isthe superposition of two or three doublet peaks originated fromdifferent valence state of Pt. To resolve the overlapped peaks (Fig.7), the spectra were treated by a Shirley-type backgroundsubtraction and a subsequent weighted least-squares fitting30

procedure (70% Gaussian, 30% Lorentzian) to optimize the peakpositions and areas. As shown in Fig. 7, including Pt(0), Pt(II),and Pt(IV) can be found in the prepared samples. Thecorresponding BE of Pt 4f7/2 locates at 71.2, 72.7, and 74.1 eV.The values agree well with the reported works22,35,36. For PT-S,35

only Pt(0) exists, suggesting Pt precursor can be effectivelyreduced to metallic state by the in situ PD process. However, forother deposition routes, metallic and oxidized Pt co-exists in thesamples. Specifically, both Pt(0) and Pt(II) exist in PT-N, whilePt(0), Pt(II) and Pt(IV) exist in the rest of PT samples. By40

comparing the spectrum of PT-H and PT-H-O, it can be foundpartial of Pt(0) was oxidized to Pt(II) after treatment. However,for PT-G, the chemical state of Pt is less impacted by the posttreatment because PT-G-400 shows almost the same Pt 4fspectrum as that of PT-G. The quantification of the different45

chemical state of Pt was performed based on Scofield’s relativesensitivity factor37 and the results summarized in Tab.1. Thecontent of Pt(0) which commonly considered as an active site20,31

for H2 evolution decreases in order PT-S > PT-N > PT-H > PT-G.The results indicate the chemical state of Pt depends on the50

synthesis route. In situ photo-reduction and NaBH4 reductionroutes show high deposition efficiency of Pt(0). Here, it must benoted that, due to the probing depth of XPS is about 3 nm38 and islarger than the average Pt particle size, the collected Pt 4f7/2signals should originate from both the Pt surface and its bulk.55

Thus, the oxidized Pt component may locate on the surface orinside the Pt particles. Science PT-G-400 shows almost the sameamount of Pt(0) as that of PT-G and PT-H (Tab. 1), the oxidized

Pt component is more likely in bulk. Otherwise, the content ofPt(0) will be improved substantially after the post treatment.60

Based on the N2 isotherm data, the BET surface area of PTsamples can be derived. The linear fits to the corresponding BETtransform plot of 1/Q[(P0/P)-1] versus P/P0 (where Q is thequantity of adsorbed N2 (cm3/g, STP), P/P0<0.3) are shown in Fig.S2. The BET results derived from the linear fits are listed in Tab.65

1. The PT samples show almost the same surface area (40-44m2·g-1) and it is slightly lower than that of P25 (ca. 50 m2·g-1).The decrease is caused by the deposition of Pt. Similarphenomenon has been reported10.

3.4 PR of glycerol for H2 evolution70

Fig. 8 (a) PR of different concentration of glycerol solution for H2

evolution over PT-S; (b) initial H2 evolution rate vs the glycerol addingamount. The insert shows the fitting plot of 1/rH2 vs 1/C (1/glycerol75

concentration).

Glycerol essentially serves as a sacrificial reagent in thereforming reaction. To investigate the influence of glyceroladding amount on the H2 production efficiency, a seriesreforming reactions with different adding amount of glycerol was80

performed on PT-S. As shown in Fig. 8a, a linearly increase of H2

with illumination time can be found for all runs. Fig. 8b showsthe initial H2 production rate (rH2, calculated according to thefitting lines’ slopes) as a function of glycerol adding amount. Itindicates rH2 increases with glycerol amount and then hesitates85

when the amount is larger than 5 mL. Therefore, a characteristic


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of Langmuir-Hinshelwood reaction mechanism can be expected.According to the reaction mode, rH2 can be expressed as:

rH2=kH2KC0/(1+KC0) (Eq. 2)

where the C0 is the initial concentration of glycerol, kH2 and Kare the rate constant of hydrogen evolution and the adsorption5

constant of glycerol, respectively. At low glycerol adding amount,rH2 kH2KC0 (1>> KC0) leading to a first order kinetics. At highadding amount, rH2 kH2 (1<< KC0) and a zero-order kinetics canbe observed. Inversion of Eq. 2 provides a linear plot,1/rH2=1/kH2+1/ (kH2KC0). As shown in the insert of Fig. 8b, the10

obtained rH2 values fitting well with the linear relationship andthe kH2 and K are calculated to be 4.76 mmol h-1 g-1 and 8.4

mol-1, respectively. According to Eq. 1, both water and glycerol are the source ofH2. To confirm this assumption, a separate reaction was carefully15

performed with a dosage of 0.3 g PT-S and 0.01 mL glycerol.Under this reaction condition, the depletion of glycerol can beachieved in a short time. As shown in Fig. S3, the evolution of H2

ceases after irradiation for 10 h. About 0.84 mmol H2 is produced.The value is close to the theoretical value of 0.96 mmol estimated20

by Eq. 1. The selectivity of H2 is 87.5 %. Some other reformingproducts such as CH4 and CO39 may account for the smalldiscrepancy, which cannot be detected by our present analysisconditions due to the low detection limit. Thus, the PR ofglycerol for H2 production can be largely described by Eq. 1.25

The activity of PT samples was compared with adding of 5 mLglycerol. Fig. 9 shows the H2 evolution amount with irradiationtime. Control test indicates no H2 was produced without usingany photocatalyst. A negligible amount of H2 was produced onthe bare P25 after irradiation for 5 h. However, after loading with30

Pt, a substantial generation of H2 was observed and its amountincreased almost linearly with time. rH2 decreases in order PT-S(4.28) > PT-H (3.30) > PT-N (2.93) > PT-G (2.04) > PT-H-O(1.61 mmol h-1 g-1). PT-S exhibits the highest activity. As shownin the insert of Fig. 9a, an induction period about 1 h can be35

found over PT-S at the beginning of the reaction. After then, theevolution of H2 proceeded steadily. The photo-deposition of Ptoccurred at this stage slows down the efficiency. The activity ofPT-H-O is only half the value of PT-H demonstrating theoxidation of Pt(0) is detrimental to the activity. Fig. 9b shows rH240

of the heat treated PT-G. It indicates rH2 increases gradually withthe temperature and then reaches a maximum rate of 2.60mmol h-1 g-1 at 400 oC. Further increasing the temperature to 500oC, the rate drops sharply to 2.08 mmol h-1 g-1, a value almostidentical to that of PT-G (2.04 mmol h-1 g-1). It suggests the45

activity of PT-G can be improved by a heat treatment of PT-G inH2 at a moderate temperature. The increasing of the interactionbetween Pt and P25 is responsible for the improvement. These results suggest the PR performance of PT depends on Ptdeposition method which potentially influences Pt distribution,50

size, chemical state, and its contact with P25. This will beaddressed in the discussion section.

Fig. 9 (a) PR of glycerol for H2 evolution over prepared PT and P25; (b)55

effect of the heat treatment of PT-G on its reforming activity.

The stability of PT was evaluated by cyclic experiments bytaking PT-S as a model. Four successive runs (5 h/run) wereperformed without replacing the solution. After each run, theproduced H2 was removed by evacuation. During the first run, the60

light was turned off twice intentionally (Fig. 10) and thegeneration of H2 ceases immediately. It clearly suggests thereforming of glycerol is driven by the photon. A slight drop of rH2

can be observed in the subsequent runs. It may be caused by theconsumption of glycerol and the decline of solution pH. The first65

possibility has been revealed by Fig. 8b which indicates whenglycerol adding amount lower than 5 mL, rH2 is proportional tothe concentration of glycerol amount. Another possibility thatdisturbs the H2 production is the acidification of the solutioncaused by the accumulation of intermediates (mainly carboxylic70

acid), which can affect both the surface properties of TiO2 and theadsorption behavior of glycerol as observed for ethanol andglucose18,40. As shown in Fig. S4, the solution pH decreasesgradually with irradiation time and then approaches to a stablevalue of ca. 3.5. Similar acidification of the solution has been75

observed during PR of ethanol and methanol in reportedworks39,40.


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Fig. 10 PR of glycerol for H2 production on PT-S for four successive runs.

4. DiscussionsPR of glycerol for H2 production not only depends on the natureof the light harvesting photocatalyst, but also highly relies on the5

property of the co-catalyst. Here, Pt which shows the lowest H2

evolution potential was used as the co-catalyst and famous P25was used as the substrate photocatalyst. The properties of Pt, suchas its size, distribution, chemical state, and the interaction withthe P25, are affected by the deposition condition. The10

characteristics of loaded Pt, on one hand, directly influence theevolution of H2. On the other hand, they also affect the lightabsorption of P25 and the separation of CCs, which influencesthe production of H2 indirectly. Therefore, based on the aboveresults, a comprehensive discussion about these interactions is15

indispensable to reveal the key factors of Pt that contribute tohigh PR activity and to identify the optimal Pt deposition method.The discussion will be focused on two aspects: (1) the effect of Ptcharacteristics on H2 production, and (2) the effect of Ptdeposition route on its characteristics.20

The TEM and XPS results indicated Pt are well dispersed inPT-S in form of Pt(0), while in other PT samples, the particle sizeis less uniform than in PT-S and both Pt(0) and Pt(II, IV) can befound. These Pt particles account for the visible absorptionobserved in Fig. 3 and 4. The results also suggest Pt in small size25

and in elemental state show higher absorption coefficient than thebig ones in oxidation state. However, a control test indicates thegeneration of H2 is essentially derived from the band gaptransition of P25. The visible absorption did not contribute to theactivity. Unfortunately, the photo-active sites of P25 will be30

covered to a certain extent by the dark brown Pt particles41. Theshielding effect increases in order PT-G < PT-N < PT-H < PT-Sestimated by the visible absorption of Pt shown in Fig. 3. In thisregard, the loaded Pt has a side effect on the formation of photo-induced CCs. On the other hand, Pt can suppress the35

recombination of CCs due to the formation of Schottky barriers atPt-TiO2 interface. As the work function of Pt (5.65 eV) is largerthan that of TiO2 (4.2 eV)18, photo-induced e- will be driven fromTiO2 to Pt until their Fermi levels are aligned. The interplay ofthese two competing effects determines the available quantity of40

CCs on P25 surface. Their amount can be measured indirectly byPL technique using TA as a probe molecule. OH radicals derivedfrom h+ can react readily with TA to produce TAOH which has amaximum PL peak located at ca. 425 nm. Unlike the generation

of H2, the formation of TAOH does not require any special active45

sites. Thus, the PL intensity of TAOH is in proportion to theseparation efficiency of CCs. Fig. 11 shows the intensitydecreases in order P25 > PT-S > PT-H > PT-N > PT-G afterirradiation for 15 min. Surprisingly, all of the PT samples showlower formation of TAOH than that of P25. Thus, the light50

shielding effect caused by loaded Pt prevails to its promotioneffect induced by Schottky barriers. Although more amount ofCCs are available on P25 surface, the production of H2 on P25 isnegligible for the lack of Pt (Fig. 9a). It approves Pt isindispensable for the evolution of H2, which is essentially the55

active site for H2 evolution.

Fig. 11 The PL intensity of the formed TAOH in the suspension of (a) PTsamples and P25; (b) post treated PT-G. The inserts show the relative line60

between PL intensity of TAOH and the H2 evolution rate

For prepared PT, the rH2 and the PL intensities of formedTAOH show a same decreasing trend. A linear correlationbetween them can be revealed through the association analysisshown in the insert of Fig. 11a. The result suggests the activity of65

PT is determined directly by the amount of surface CCs andseems not affected by the different chemical state of Pt (Tab. 1).In theory, it should be controlled by both the amounts of CCs andthe active sites of Pt(0). Considering the average Pt particle sizesare almost the same (ca. 2 nm) in these samples, the irrelevance70

observed here suggests they have the same amount of Pt(0) sites.Namely, the oxidized Pt components are located in the particles’bulk rather than on the surface. On the other hand, even someoxidized Pt are survived after the deposition reactions, the


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components will be reduced to Pt(0) inevitably by photo-inducede- in the subsequent PR of glycerol, a reduction process occurredon PT-S. Thus, for prepared PT samples, the surface of the Ptparticles is in Pt(0) state during the PR of CG. Only in this case,the linear correlation can be observed. The impact of the different5

valence state Pt on the production of H2 is more likely achievedby influencing of the CCs’ separation, rather than the amount ofactive sites. Although the light shielding effect of Pt increases in order PT-G < PT-N < PT-H < PT-S, the formation of TAOH has not shown10

a reverse order and also increases in the same order. That meansthe separation of CCs in PT-S and PT-H is more effective than inPT-N and PT-G. It can be ascribed to the high amount of Pt(0)and/or the strong interaction of Pt with P25. The explanationarises from two facts: (1) only Pt(0) exists in PT-S, which favors15

the formation of Schottky barriers than oxidized Pt, and (2) forPT-H, it has been reduced at an elevated temperature which canstrength the contact between Pt and P25 and consequentlybenefits the immigration of e- from P25 to Pt. The explanationcan be further validated by the results derived from the post20

treated PT-H and PT-G. As shown in Fig. 11a, the formation ofTAOH on treated PT-H (i.e. PT-H-O) is substantially decreasedwith the oxidation of Pt(0) to Pt(II) (shown in Tab. 1), as well asthe production of H2 (Fig. 9a). The changes are caused by thetransformation of Pt chemical state suggesting Pt(0) is more25

effective for the separation of CCs. Similar results have beenobserved33,41. On the other hand, a well-contacted junctionbetween Pt and P25 benefits the transfer of e- across the twophase interface20. To strength this contact, a heat treatment of PT-G at elevated temperate was performed in H2 atmosphere.30

Although the Pt size was increased substantially (from 2.2 to 13nm, Fig. 2a) after the treatment, the calcination does improve theevolution of H2 (Fig. 9b), as well as the formation of TAOH (Fig.11b). Except PT-G-500, the resulted samples show higher TAOHformation efficiency than that of PT-G and it increases with the35

treated temperature. The reason why PT-G-500 shows noimprovement may due to the phase transition of anatase to rutilewhose band structure and surface property is unsuitable for H2

evolution42-44. However, for treated PT-G, no linear relationshipbetween the production rates of H2 and the PL intensities of40

formed TAOH can be observed (insert of Fig. 11b). According tothe relationship shown in PT samples (insert of Fig. 11a), theactivities of treated PT-G seem disproportionate to the formationof TAOH and are lower than they should be. It suggests adisadvantage is also introduced by the treatment, which results in45

the CCs cannot be effectively involved in the reforming reaction.Because the chemical state of Pt almost un-affected by thetreatment (by comparing the data of PT-G and PT-G-400 in Tab.1), the disadvantage should be the increasing of Pt size, whichhas been observed in Fig. 2a. Due to the Pt content is fixed at 1%,50

the amount of the active sites for H2 production will be reducedby the increasing of Pt size. PD and IRD are the most popular methods to deposit Pt onTiO2. Both of them were used to prepare Pt/P25 in this work. Inagreement with some reported works45,46, the convenient in situ55

PD method yields well dispersed Pt in only elemental state andsubsequently exhibits higher H2 evolution rate than the samplesprepared by IRD methods (Fig. 9a). However, the reason why the

performance of PT benefits from the in situ PD route is stillobscure and lack of discussion. The rest of the discussion will60

refer to this point.

Fig. 12 Schematic illustration the deposition process of Pt: (a) PD and (b)IRD routes. [Pt] and [Pt]x stand for Pt precursor and its cluster,

respectively.65

Comparing with IRD, the advantage of PD is that Pt isselectively deposited on the e- trapping sites through an internal-to-external reduction process. This is the key to Pt highly andintimately deposited on P25 in Pt(0) state. Specifically, theprocess can be described as follow (Fig. 12a): (1) after activated70

by UV lights, the photo-induced e- will immigrate from bulk P25to its surface and then be trapped by surface trapping sites; (2) Ptprecursor (PtClx(OH)6-x

2- 41,47) near the e- trapping sites is thenreduced to Pt(0) and loaded exactly on the sites; (3) instead of theformer e- trapping sites, the resulted Pt clusters will serve as new75

e- sinks and loading sites to carry on the deposition of Pt. Via thisroute, Pt can be selectively deposited on some special sites whichcan shorten the transferring pathway of e- from bulk P25 to itssurface and then to Pt. Experiment and theory works48-53 indicatethe e- trapping sites are mainly on the {101} facets for the80

different surface energy level, which results in Pt arepreferentially deposited on {101} facets. This may be the reasonwhy the diffraction intensity of (101) peak is apparentlydecreases in PT-S (Fig. 2a). Due to the Pt particles are negativelycharged during the reduction process, the aggregation of Pt85

particles can be suppressed, leading to a uniform distribution ofPt as shown in Fig. 5. Furthermore, as the reduction of Ptproceeds gradually from internal to external, all of the loaded Ptprecursor can be effectively reduced to Pt(0) and depositedintimately on P25. As discussed above, these characteristics of Pt90

favor the charges separation and the formation of H2,consequently leading to a high H2 evolution rate. The finding isconsistent with the general idea that Pt(0) particle is the activesite for H2 evolution. We noted a contrary result reported recentlyby Yang’s group who thought the oxidized Pt species are the key95

active sites22,54. It must point out that the conclusion is drawnfrom the assumption that Pt(0) was leached by cyanide. OxidizedPt components were then exposed and were found to be effectivefor H2 evolution54. However, the high performance may notascribe to the oxidized Pt species because their surface should be100

already reduce to Pt(0) by photo-induced e- before the evolutionof H2.


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As for IRD (Fig. 12b), Pt precursor is first loaded on P25 byimpregnation and drying processes. With the evaporation ofwater, the precursor will be randomly loaded on P25 andaggregates in the late stage due to the concentration ofimpregnation solution, which leads to the Pt size less uniform5

(Fig. 6). The precursor cluster is then reduced to metallic Pt byexternal reductant such as H2 and NaBH4. On contrary to PD, thereduction is occurred from external to internal. Once the outerlayer is reduced to Pt(0), the reduction of the interior will behampered by the layer. A thorough reduction of the cluster will10

be difficult. Thus, as disclosed by XPS results (Fig. 7), bothelemental and oxidized states Pt are possessed in these samples.Although the H2 evolution reaction can still run smoothly on Pt(0)surface, the attraction and immigration of e- will be hampered bythe oxidized Pt components, as well as the interaction between Pt15

and P25. Similar phenomenon has been observed in Rh/TiO2

sample34. PT-G was prepared by a similar PD process as that of PT-Sexcept the suspension is very dense and performed separately.However, the deposited Pt is quite different from that in PT-S.20

The dense suspension accounts for the discrepancy. XPS result

(Fig. S1) indicates the glycerol-rich solution (almost four timeshigher than that in preparation of PT-S) leads to a hydroxylationof P25 surface. Compared with PT-S, the density of OH group onPT-G is substantially improved after the platinization leading to25

the high content of O as shown in Tab. 1. Both the EDX and XPSmeasurements indicate PT-G shows high content of O than othersamples. An interaction between the OH group and the Ptprecursor may be then formed35, which disturbs the reduction ofPt and its anchoring on P25. This may be the reason why the30

immigration and conjunction of Pt cluster is readily trigged inPT-G by a heat treatment (Fig. 2a). Furthermore, a quickdeposition of Pt has been observed in the dense suspension for 2g P25 was used. A through reduction of Pt cannot be achieved bythe quick reaction. The precursor may deposit too quickly to35

reduce to Pt(0). Although a further study is required to ascertainthese assumptions, one thing is certain, that the PD should beperformed in a dilute suspension, like the condition used forpreparing of PT-S.

40

Tab. 1 Synthetic conditions and characterization of prepared PT samples.

Sample notation Prepare condition Pt size (nm)b

Content (wt.%) Pt ratio (Pt4f7/2 eV)BET(m2

g-1)

rH2

(mmolh-1g-1

)

EDX XPS 0 II IV

Ti O Pt Ti O Pt 71.2 72.7 74.1

PT-S in situ PD 2.0 56.8 42.2 0.95 59.8 39.3 1.3 1.0 - - 44.2 4.28

PT-H IRD, H2/Ar 1.8 57.3 41.8 0.93 55.4 43.7 1.1 0.57 0.19 0.24 40.1 3.30

PT-H-O PT-H/O2a - - - - 57.4 41.7 1.0 0.42 0.38 0.20 - 1.61

PT-N IRD, NaBH4 2.0 61.8 37.3 0.95 57.0 42.0 1.4 0.66 0.34 - 41.2 2.93

PT-G PD 2.2 54.0 45.0 0.97 55.6 43.5 0.9 0.48 0.25 0.27 43.0 2.04

PT-G-400 PT-G/H2a 13.0c - - - 57.9 41.1 1.0 0.54 0.22 0.24 - 2.6

a Post treated at 400 oC in O2 (PT-H-O) or H2 (PT-G-400);b Determined by TEM; c Determined by XRD.

45

5. ConclusionsThe following conclusions can be reached from the above resultsand discussion. By in situ Photo-deposition (PD) process,uniform Pt particles can be selectively deposited on P25 in Pt(0)state (i.e. sample PT-S). However, as for impregnation-reduction50

deposition (IRD) routes, Pt particles are randomly loaded on P25with the surface in Pt(0) and the bulk in Pt(II/IV) states (PT-H,PT-N). The average Pt sizes are about 2 nm and less impacted bythe deposition route. Photocatalytic reforming (PR) of glycerolfor H2 evolution can be achieved over the platinized samples55

under ambient condition. The activity of the samples depends onboth the nature of the light harvesting of P25 and thecharacteristics of co-catalyst Pt, including its chemical state, size,and the interaction with P25. A side effect to cover the photo-active sites of P25 and a promotion effect to improve the60

separation of CCs are induced by the loaded Pt. The interplay of

these two competing effects determines the available quantity ofcharge carriers (CCs) on P25. Pt(0) in small size exhibits highlight shielding effect than oxidized Pt component. However, it ismore effective than the latter for the separation of CCs and is the65

key active site for H2 evolution. The promotion effect prevailsover the side effect and, consequently, PT-S shows higher PRactivity for H2 evolution than other samples prepared by IRDroutes. Furthermore, the evolution of H2 is favored by the smallPt particle and the well contact between Pt and P25 favors.70

Although the performance of PT-S benefits from the PD route,comparing test suggests the deposition should be carried out in adilute suspension. This work reveals the optimal deposition of Pton TiO2 and the key characteristics of Pt for PR of CG for H2

production. The work provides a valuable reference for75

platinization of photocatalyst which can be potentially used forsolar energy conversion and environmental remediation.

Acknowledgements


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This work was financially supported by the Natural ScienceFoundation of China (Grant Nos. 21473066, and 21103060), theNatural Science Foundation of Anhui Province, China (Grant Nos.1408085QB38), and the High Education Revitalization Plan ofAnhui province, China. Prof. Shifu Chen thanks the support of5

the Natural Science Foundation of China (Grant Nos. 51472005,51172086 and 51272081).

Notes and referencesa College of Chemistry and Material Science, Huaibei Normal University,Huaibei, Anhui, China, 235000. Fax:+86 561 3090518; Tel: +86 56110

3803235; E-mail: [email protected] Department of Chemistry, Anhui Science and Technology University,Fengyang, Anhui, China, 233100. Fax: +86 561 3090518; Tel: +86 5613806611; E-mail: [email protected]

15

† Electronic Supplementary Information (ESI) available: [Survey andhigh resolution XPS spectra of PT samples; The BET transform plots of1/Q[(P0/P)-1] versus P/P0; The change of the reaction solution pH duringPR of glycerol over PT-S]. See DOI: 10.1039/b000000x/

20

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Fundamental understanding the effects of the deposited Pt and its deposition route on the

photocatalytic reforming of glycerol for H2 evolution over Pt/TiO2 was investigated. Intimately

loaded Pt(0) is the key active sites for the reaction, which’s formation was favored by an in situ

photo-deposition route.


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Documents

Photocatalytic reforming of glycerol for H 2 evolution on Pt/TiO 2 : fundamental understanding the effect of co-catalyst Pt and the Pt deposition route