Correlation between Photocatalytic Activities and Structural and Physical Properties of Titanium(IV)...

Correlation between Photocatalytic Activities and Structuraland Physical Properties of Titanium(IV) Oxide Powders

Orlando-Omar Prieto-Mahaney1 Naoya Murakami1 Ryu Abe12 and Bunsho Ohtani121Graduate School of Environmental Science Hokkaido University Sapporo 060-0810

2Catalysis Research Center Hokkaido University Sapporo 001-0021

(Received November 14 2008 CL-081076 E-mail ohtanicathokudaiacjp)

REPRINTED FROM

The Chemical Society of Japan

Vol38 No3 2009 p238239CMLTAG

March 5 2009

Correlation between structural and physicalproperties and photocatalytic activities for five kindsof reactions of 35 commercial and home-madesamples of pristine titanium(IV) oxide (titania) wasobtained through multivariable analysesphotocatalytic activities were empirically reproducedby a linear combination of six properties with fairreliability An example for oxygen liberation isshown here While a part of results could beinterpreted using the conventional mechanismsignificant activity dependence on properties notdisclosed yet was suggested

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pred

icte

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otoc

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observed photocatalytic activity(standardized)

4Ag+ + 2H2O4Ag + O2 + 4H+

(Reaction a)

Published on the web (Advance View) January 31 2009 doi101246cl2009238





Correlation between structural and physical properties andphotocatalytic activities for five kinds of reactions of 35 titaniasamples was obtained through multivariable analyses photocata-lytic activities were empirically reproduced by a linear combina-tion of six properties with fair reliability While a portion of resultscould be interpreted using a conventional mechanism significantactivity dependence on properties not disclosed yet was sug-gested

Photocatalytic reaction is induced by photoexcited electrons(e) and positive holes (hthorn) generated in a solid photocatalystfollowed by redox reaction with surface-adsorbed substratesTitanium(IV) oxide (titania) is one of the most promising photo-catalysts because of its high chemical stability sufficient energyof its e and hthorn to drive various photocatalytic reactions negli-gible toxicity inexpensiveness and ease of preparation Onlyone plausible weakness of the titania photocatalyst is that it ab-sorbs only ultraviolet light though such transparency is prefera-ble when titania is coated on colored materials It is known thatthe photocatalytic activity ie rate of photocatalytic reactiondepends on the structural and physical properties of titania Itis believed that there is a relationship between properties and ac-tivities ie a structurendashactivity correlation It has often beenclaimed that the smaller the particle size ie the larger the spe-cific surface area the higher the photocatalytic activity or thatanatase is better than rutile1 However such discussion on thecorrelations has been limited to a certain series of samples pre-pared in a similar way or small number of commercial samples2

and there seem no comprehensive correlationsThe present study aims at obtaining the structurendashactivity

correlations for a large number of titania photocatalysts coveringcommercially available titania samples in Japan (see Table S1)14

by statistical multivariable analysis Five representative reac-tions were chosen and their relative rates were analyzed usingsix properties3 specific surface area (BET) density of latticedefects (DEF) primary (PPS) and secondary (SPS) particle sizeand existence of anatase (ANA) and rutile (RUT) phases to ob-tain intrinsic dependence of photocatalytic activities on the prop-erties

Five test photocatalytic reactions were as follows a Oxy-gen (O2) liberation and silver deposition from a deaerated aque-ous silver sulfate solution b Dehydrogenation of methanol in adeaerated aqueous solution c Oxidative decomposition of ace-tic acid to liberate carbon dioxide (CO2) from an aerated aque-ous solution d Decomposition of acetaldehyde into CO2 inair and e Synthesis of pipecolinic acid from L-lysine in a dea-erated aqueous solution (see SI)

First the rates of reactions were compared with each struc-tural property In general there were no significant relationsbetween them while in some cases dependence of the rate on

a parameter was observed (see SI Tables S2 and S4) This seemsreasonable since the titania samples used in this study were of awide range and synthesized or treated in different ways Thenthe data were analyzed statistically by solving the followingmatrix equation frac12rate351 frac14 frac12property356 frac12coefficient61for each reaction The experimental results rates and propertieswere standardized using mean of data and standard deviation inorder to make the calculated coefficients have the same weightbeing independent of properties ie enabling direct comparisonof partial regression coefficients (k)

Table 1 shows the summary of results of analyses Squaredmultiple correlation coefficient R2 was also listed It was foundthat the best R2 values were obtained when ANA and RUT weredefined to be 1 or 0 (dummy variables) when the most intensivepeak of anatase and rutile was detected in the XRD patterns andotherwise respectively Although it is known that increase in thenumber of variables improves the fitting ie makes R2 to beclose to unity no other appropriate properties have been foundto be added to the list As a general trend a and e gave relativelylarger R2s ie higher reproducibility of the results fitting to alinear combination of properties while those for the others werealso fairly high4 Figure 1 shows this linearity for a as an exam-ple the calculated (predicted) relative activities of each sampleusing obtained coefficients (k) were plotted against the observedactivity Another significant feature is that coefficient of kANAhas large positive value in all the cases except for a This isthe first example within the authorsrsquo knowledge of supportfor a general understanding in this field that anatase is moreactive than rutile since previously reported data were of onlya limited number of samples and neglected the influence of theother properties On the other hand kRUT was relatively smalland it was negative in d suggesting that rutile phase in the photo-catalysts is rather inert compared with anatase Although largepositive and negative kBET and kDEF respectively are expectedsince large surface area is advantageous to adsorb reaction sub-strate(s) and defective sites accelerate recombination of e andhthorn large positive kBET was observed only for b and even largepositive kDEF was obtained for c d and e as discussed laterkPPS and kSPS depended on the type of reactions

Table 1 Squared multiple correlation coefficient (R2) and partial regres-sion coefficients (k)

Coefficient a b c d e

R2 086 052 058 060 085

kBET 001 043 009 013 019

kDEF 015 025 019 043 032

kPPS 012 020 018 020 052

kSPS 057 008 020 004 007kRUT 014 028 011 006 002

kANA 004 040 057 055 063

238 Chemistry Letters Vol38 No3 (2009)

Copyright 2009 The Chemical Society of Japan

For a kANA was small but positive which means that ana-tase does not prohibit the reaction while kRUT was not so largesuggesting that it is not indispensable to be rutile for higher ac-tivity It seems that large secondary particles (large positive kSPS)composed of large primary particles of less crystalline defects(negative kDEF) are preferable which agrees with the generalrecognition reported so far5 A significant point which has beenrevealed in this study first is that the reported higher activity ofrutile powders6 for a is mainly attributable to their secondaryparticle size7 but not to rutile crystal and it is expected that ana-tase would be active when its secondary particle size is large

Reaction b was performed using in situ platinized titaniaparticles since bare samples show negligible activities4 Thepositive kRUT and kANA coefficients seem reasonable since thebottom of the conduction band (CB) of anatase (020V vsNHE) and rutile (+004V) crystals is reported8 to be almostthe same or a little negative compared with the standard elec-trode potential of hydrogen evolution (HthornH2 0 V) kANA a lit-tle larger than kRUT might be related to a slightly more negativeCB level of anatase Comparable activity of rutile phase has beenshown previously through action spectrum analyses9 It has beenshown for hydrogenation of 2-propanol that the rate depends onthe amount of surface-adsorbed alcohol10 Large positive kBETas well as negative kPPS and large negative kDEF suggested thatthis reaction requires both a large amount of adsorbed methanoland less probability of endashhthorn recombination

Reactions c and d exhibiting a similar trend of coefficientswere conducted under aerated conditions and their mechanismmust contain O2 reduction by e Preference of anatase crystal-lites (large positive kANA) may be caused by the above-men-tioned CB position The potential of one-electron reduction ofO2 O2

O2 (005V) or HO2O2 (028V) is a little morenegative than that for H2 evolution and thereby small difference(ca 016V) in CB position may be decisive Another feature ofthese oxidative decompositions was a relatively large positivekDEF which must accelerate the endashhthorn recombination A possi-ble interpretation for this inconsistency is that the defective sitesact as an adsorption site for organic compounds andor O2 andthe influence of enhanced endashhthorn recombination on the rate wassmall presumably owing to efficient radical chain reaction11

Taking into account negative kDEF for b in which organic com-pound methanol is also oxidized enhanced adsorption of O2 atthe defective sites might occur though at present we have noevidence supporting this hypothesis Positive kBET for d in con-

trast to the negative value for c suggests that this gasndashsolidphase reaction requires diffusion of substrates acetaldehydeand O2 and therefore the larger the surface area the fasterthe diffusion for d

Reaction e has been known to proceed through redox-com-bined mechanisms Lys is oxidized by hthorn and hydrolyzed andcyclized to a Schiff base intermediate which undergoes reduc-tion at surface-loaded Pt deposits by e to yield PCA12 Simi-larity of the trend with b suggests that the reaction rate is alsogoverned by the first oxidation step The positive kDEF is attrib-utable to the preferable adsorption of Lys at the defective sitesThe larger and smaller kANA and kRUT respectively comparedwith b might reflect a difference in reduction process betweenb and e reduction of the Schiff base intermediate13 is thoughtto require a little more negative potential of e than that forH2 liberation

In conclusion statistical multivariable analyses of photoca-talytic activities depending on the properties of titania powderssupported the conventional wisdom eg anatase is more activethan rutile especially for photocatalytic oxidative decompositionof organic compounds and lattice defects reduce activity inreactions conducted under deaerated conditions On the otherhand unexpected results were also observed eg lattice defectsare beneficial especially for reactions under aerated conditionsand the activity for O2 liberation is governed by secondaryparticle size rather than crystal form Since each propertychanges depending on each other to some extent (see Tables S2and S3) eg rutile crystallites are often large compared withanatase intrinsic dependence of activity on each property hasbeen ambiguous The intrinsic dependence of activity as shownin this study could be demonstrated for the first time by statis-tical multivariable analyses

References and Notes1 Problems appearing in such discussions have been pointed out in a recent

article B Ohtani Chem Lett 2008 37 2162 A recent paper has reported the activity of 6 samples depending on the type

of organic compounds to be degraded in aerated aqueous solutions J RyuW Choi Environ Sci Technol 2008 42 294

3 Some of the properties have been already reported N Murakami O OPrieto-Mahaney R Abe T Torimoto B Ohtani J Phys Chem C 2007111 11927

4 Relatively small R2 for reaction b might be due to possible differences insize and morphologies of platinum deposits and this will be discussed else-where

5 For example S Nishimoto B Ohtani H Kajiwara T Kagiya J ChemSoc Faraday Trans 1 1983 79 2685


7 Nosaka et al reported a relation between secondary particle size and rate ofphotocatalytic superoxide anion radical production but the reason has notbeen fully clarified yet Y Nosaka M Nakamura T Hirakawa PhysChem Chem Phys 2002 4 1088

8 G Rothenberger J Moser M Gratzel N Serpone D K Sharma J AmChem Soc 1985 107 8054

9 T Torimoto N Nakamura S Ikeda B Ohtani Phys Chem Chem Phys2002 4 5910

10 S Nishimoto B Ohtani T Kagiya J Chem Soc Faraday Trans 1 198581 2467

11 B Ohtani Y Nohara R Abe Electrochemistry 2008 76 14712 B Ohtani S Tsuru S Nishimoto T Kagiya K Izawa J Org Chem

1990 55 555113 B Ohtani K Iwai S Nishimoto S Sato J Phys Chem B 1997 101

334914 Supporting Information is available electronically on the CSJ-Journal Web

site httpwwwcsjjpjournalschem-lettindexhtml

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Figure 1 Relation between standardized photocatalytic activities for re-action a observed and calculated using obtained partial regression coeffi-cients in Table 1 This linearity indicates that the activity of samples canbe estimated from their physical properties

Chemistry Letters Vol38 No3 (2009) 239


Supporting Information

Orlando-Omar Prieto-Mahaney1 Naoya Murakami1 Ryu Abe12 and Bunsho Ohtani12

1Graduate School of Environmental Science Hokkaido University Sapporo 060-0810 2Catalysis Research Center Hokkaido University Sapporo 001-0021


Copyright copy The Chemical Society of Japan

Correlation between Photocatalytic Activities and Structural and Physical Properties of Titanium(IV) Oxide Powders

Supporting Information Experimental Details Samples Samples were supplied by several sources mainly Catalysis Society of Japan (JRC-TIO-1 to TIO-13)

Showa Denko (Supertitania series F1-F5 G2) and Ishihara Sangyo (CR-EL ST-21 ST-01) Other samples were named after their suppliers name (Merck Kanto etc) and finally a home made sample (TUF-01) Degussa P25 was provided by Nippon Aerosil Co All samples were used as received without further processing

Although JRC-TIO-4 is to be identical to Degussa P25 their measured properties were a little different

This may be caused by possible heterogeneity of this samplei and they were handled as independent samples Similarly another set of samples ST-01 (Ishihara) and JRC-TIO-8 was considered independent

Photocatalytic reaction A standard amount of the powder photocatalyst (50 mg) was suspended in an aqueous solution (50 mL) in

a borosilicate glass tube (transparent for the wavelength gt 290 nm 18 mm in inner diameter and 180 mm in length) containing silver fluoride (50 mmol L-1) methanol (50 vol) and chloroplatinic acid (H2PtCl66H2O corresponding to 2-wt platinum (Pt) loading) acetic acid (50 vol) in reactions a b and c respectively Air was purged off from the systems by passing argon (Ar) through the suspensions for at least 15 min for reactions a b and e and tightly sealing the sample tubes using a double-capped rubber septum and a sheet of Parafilm to prevent leakage of gas andor contamination during the measurements for the liquid-solid phase reactions (a-c e) In the case of reaction e Pt-preloaded (2 wt) titania powders were used alongside a neutralized solution of L-lysine hydrochloride (Lys 20 mmol L-1) as a substrate The samples were irradiated by a 400-W high-pressure mercury arc (Eiko-sha) at 298 K under vigorous magnetic stirring (1000 rpm) The reactions were monitored for 60 min (reactions a b and e) and 90 min (reaction c) by analyzing liberation of oxygen (O2) hydrogen (H2) a deaminocyclized product pipecolinic acid (PCA) and carbon dioxide (CO2) respectively The gas phase products and PCA were analyzed by gas chromatography (a Shimadzu GC-8A gas chromatograph equipped with a TCD and columns of molecular sieve 5A for H2 and O2 and Porapak Q for CO2) and HPLC (Shimadzu LC-6A a Sumichiral A6000 column with 20 mmol L-1 copper sulfate eluent detected by photoabsorption at 254 nm) respectively Photocatalytic oxidative decomposition of gaseous acetaldehyde in air (reaction d) was carried out in a cylindrical glass vessel with a volume of 660 mL Photocatalyst powder (50 mg) was uniformly spread on a glass plate (10 cm times 10 cm) The plate was placed on the bottom of the cylindrical vessel Gaseous acetaldehyde (066 mL corresponding to ca 0027 mmol) was injected into the vessel filled with ambient air to adjust the initial concentration to be 1000ppm Concentrations of acetaldehyde and CO2 were measured by an Agilent 3000 MicroGC After adsorption of acetaldehyde had reached an equilibrium in the dark photoirradiation (gt290 nm) through a top window of the vessel was performed using a 300-W xenon lamp (ILC Technology CERMAX-LX300F) Principle and details of these photocatalytic activity tests have been reported elsewhere a silver metal deposition and O2 liberationiiiii b methanol dehydrogenationiv c oxidative decomposition of acetic acidv vi d gas-phase oxidative decomposition of acetaldehydevii e conversion of Lys to PCAviii

Photocatalytic activity the rate of reaction was evaluated by the rate of monitored products in an

irradiation time range where the constant rate was observed except for reaction e The yield of PCA a sum of stereoisomers by the photoirradiation for 60 min was used for the calculation of activity in reaction e Raw data were listed in Table S1

Structural and physical properties Specific surface area (BET) of the photocatalysts was evaluated according to a Brunauer-Emmett-Teller

equation with data of nitrogen (N2) adsorption isotherms at 77 K on a Yuasa-Quantachrome NOVA 1200e surface and pore size analyzer Adsorption cross section of N2 0162 nm2 was used for calculation At least 6 data points in the relative pressure range between 005 and 03 were collected

Density of lattice defects (DEF) was estimated through a quantitative analysis of trivalent titanium species

(Ti3+) formed in titania suspended in a deaerated aqueous triethanolamine solution (TEOA 10 vol) After the irradiation a deaerated aqueous solution of methyl viologen (MV2+) was injected to the suspension and the resulting pale blue supernatant involving cation radical of MV2+ was separated and its photoabsorption was measured by a spectrophotometer (Agilent 8453) ix Alternatively double-beam photoacoustic spectroscopy (DB-PAS) in which titania powders are continuously irradiated by a light-emitting diode (LED) with an emission peak at 365 nm (Nichia NCCU033) and monitored by modulated visible light at 530 nm using light-emitting diode (Nichia NSPG500S) after passing a controlled N2 flow containing methanol vapor Since no absolute molar amount of defects could be obtained by DB-PAS the collected data were converted using a correlation curve which has been confirmed in the previous studiesxxi

Primary particle size (PPS) was determined through X-ray diffraction measurements using Cu-Kα radiation

in a Rigaku RINT 2500 equipped with a carbon monochromator Photocatalysts were loaded in a glass sample holder of ca 05-mm depth The crystallite size was estimated by Scherrer equation using the corrected (line-broadening by Cu-Kα2 radiation and emanation in the optical path of a diffractometer) full width at half maximum (FWHM) of the most intense XRD peaks of anatase or rutile at ca 253 and 274 deg respectively As a constant in the Scherrer equation 089 was usedxii Using XRD patterns presence and absence of crystalline phases of anatase and rutile (ANA and RUT respectively) was checked The borderline between presence and absence is the detection limit of XRD peak ca 1 peak height compared with that of single-phase well-crystallized samples One of the reasons why dummy values (1 or 0) were used for ANA and RUT was that it seems that there are no incredible methods for crystalline composition analysis especially for the samples of small crystallitesxii and heterogeneity of crystalline composition as described above Another reason is that the activity can not be a linear combination of activities of anatase and rutile (and possibly amorphous) eg it has been clarified that in certain reaction systems one of the crystalline phase shows rather higher activity even when the composition is smallxiii

Secondary particle size (SPS) was determined by a laser diffraction particle analyzer (Shimadzu SALD-

7000) as a volume-average particle size The sample was suspended in water by ultrasonication before the measurement

Part of data used in this study have been reported elsewherex Multivariable Analyses Data standardization All the data including ANA and RUT were standardized to give standard deviation v to be unity using a

equation zi = (xi - xave)v where zi xi and xave are standardized datum raw datum and average so that direct comparison is possible for calculated partial regression coefficients The standard deviations were calculated using n number of data

Check of multicollinearity When there are strong correlations betweens variables properties multivariable analysis should be made

using one of variable out of strongly correlated variables As shown in Table S3 judging from the calculated correlation coefficient between a set of properties there were some significant correlation but actual coefficients are at most lt 06 We considered that there was no multicollinearity in the data used in

this study As described in the text BET and PPS should be strongly related each other assuming spherical particles of the same diameter However the corresponding correlation coefficient was not so large suggesting that they behave independently

Dependence of photocatalytic activity on each property Table S4 shows the correlation between photocatalytic activities with each one of properties A few rather

strong correlations were observed kPPS and kSPS for reaction a and kPPS for reaction e The kSPS coefficient for reaction a and the kPPS coefficient for reaction e were also evident in the multivariable analyses (see Table 1) reflecting this dependence The reason of a relatively small kPPS coefficient for reaction a in Table 1 in spite of the large correlation coefficient in this table was ambiguous at present

Calculation Multivariable analyses were performed with the above-mentioned standardized data using least-square

method to minimize sum of residues (square of deviation of calculated value from actual value) on Excel 2003 (Microsoft) and MATLAB (The MathWorks)

i O O Prieto-Mahaney R Abe D Li Y Azuma B Ohtani to be submitted ii S-i Nishimoto B Ohtani H Kajiwara T Kagiya J Chem Soc Faraday Trans1 1983 79 2685 iii B Ohtani Y Okugawa S-i Nishimoto T Kagiya J Phys Chem 1987 91 3550 iv S-i Nishimoto B Ohtani T Kagiya J Chem Soc Faraday Trans 1 1985 81 2467 v S Ikeda H Kobayashi Y Ikoma T Harada T Torimoto B Ohtani M Matsumura Phys Chem Chem

Phys 2007 9 6319 vi X Yan T Ohno K Nishijima R Abe B Ohtani Chem Phys Lett 2006 429 606 vii F Amano K Nogami R Abe B Ohtani J Phys Chem C 2008 112 9320 viii B Pal S Ikeda H Kominami Y Kera B Ohtani J Catal 2003 217 152 ix S Ikeda N Sugiyama S-y Murakami H Kominami Y Kera H Noguchi K Uosaki T Torimoto B

Ohtani Phys Chem Chem Phys 2003 5 778 x N Murakami O O Prieto-Mahaney R Abe T Torimoto B Ohtani J Phys Chem C 2007 111 11927 xi N Murakami R Abe O O Prieto-Mahaney T Torimoto B Ohtani Stud Surf Sci Catal 2007 172 429 xii B Ohtani Chem Lett 2008 37 216 xiii T Torimoto N Nakamura S Ikeda B Ohtani Phys Chem Chem Phys 2002 4 5910

Table S1 Actual reaction rates obtained for the five photocatalytic systems Products monitored for the calculation of rates are indicated

n Photocatalyst a O2mmol h-1

b H2mmol h-1

c CO2mmol h-1

d CO2mmol h-1

e PCAmmol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

TIO-1 TIO-2 TIO-3 TIO-4 TIO-5 TIO-6 TIO-7 TIO-8 TIO-9 TIO-10 TIO-11 TIO-12 TIO-13 ST-G2 ST-F1 ST-F2 ST-F3 ST-F4 ST-F5 VP-P90 TUF-01 PC-101 PC-102 Hombikat Merck(A) Aldrich(R) Aldrich(A) Kanto Wako CR-EL ST-01 ST-21 TKP-101 TKP-102 P25

0003 0003 0021 0030 0131 0001 0004 0005 0003 0008 0011 0004 0017 0043 0005 0029 0034 0024 0017 0015 0017 0012 0011 0006 0014 0083 0019 0020 0018 0106 0006 0009 0006 0009 0030

0180 0086 0187 0778 0505 0243 0593 0635 0788 0699 0520 0648 0538 0674 0428 0490 0563 0592 0487 0601 0209 0726 0575 0605 0190 0675 0175 0223 0927 0545 0704 0465 0420 0603 0859

0016 0025 0007 0038 0015 0007 0031 0020 0028 0026 0042 0028 0047 0029 0027 0032 0038 0051 0034 0043 0013 0033 0032 0030 0018 0029 0028 0026 0023 0017 0027 0035 0024 0035 0053

0220 0159 0063 0272 0127 0049 0257 0366 0235 0291 0289 0261 0323 0156 0111 0129 0152 0235 0224 0336 0193 0271 0308 0261 0143 0123 0125 0231 0263 0067 0339 0283 0195 0301 0261

0036 0009 0006 0044 0012 0017 0052 0052 0055 0054 0054 0055 0048 0015 0028 0039 0041 0045 0049 0059 0018 0048 0048 0045 0016 0015 0026 0038 0042 0008 0049 0050 0050 0048 0041

Table S2 Actual experimental predictor values of titania powders The variables ANA and RUT represent the presence (1) or absence (0) of the indicated crystalline phase

n Photocatalyst BETm2g-1 DEFμmol g-1 PPSnm SPSμm RUT ANA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

TIO-1 TIO-2 TIO-3 TIO-4 TIO-5 TIO-6 TIO-7 TIO-8 TIO-9

TIO-10 TIO-11 TIO-12 TIO-13 ST-G2 ST-F1 ST-F2 ST-F3 ST-F4 ST-F5

VP-P90 TUF-01 PC-101 PC-102

Hombikat Merck(A) Aldrich(R) Aldrich(A)

Kanto Wako

CR-EL ST-01 ST-21

TKP-101 TKP-102

P25

73 18 40 50

3 100 270 338 300 100 97

290 59

34 19 29 37 39 73 97

114 301 157 300 11

4 8

11 59

7 298 67

306 114 59

10928485014

242119118105106156111724384

1121081341861403441111189825181738212184

101869450

214004021

57015

84

951515

630

5009060502020

558

129

16951721716929

2008

259

2028

5610512406

1812516

0240921380290791020860410360370670391042316

144139044

14033

043046

1542586689139096094

0 0 1 1 1 1 0 0 0 0 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 1 1 0 0 0 0 1

1 1 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Table S3 Correlation coefficients for 6 properties of titania photocatalysts

Table S4 Correlation coefficients of each property for five kinds of photocatalytic reactions

property BET DEF PPS SPS RUT ANA

BET 1 0291 ndash04982 ndash02358 ndash05930 00705

DEF mdash 1 -05194 ndash02980 00734 ndash05385

PPS mdash mdash 1 05791 02535 01471

SPS mdash mdash mdash 1 01908 00255

RUT mdash mdash mdash mdash 1 ndash02976

ANA mdash mdash mdash mdash mdash 1

property a b c d e

BET ndash046 034 ndash002 052 059

DEF ndash040 ndash014 ndash006 015 024

PPS 069 ndash016 ndash021 ndash049 ndash068

SPS 083 ndash001 ndash032 ndash036 ndash046

ANA 010 044 056 042 046

RUT 048 004 004 ndash051 ndash041





Correlation between structural and physical properties andphotocatalytic activities for five kinds of reactions of 35 titaniasamples was obtained through multivariable analyses photocata-lytic activities were empirically reproduced by a linear combina-tion of six properties with fair reliability While a portion of resultscould be interpreted using a conventional mechanism significantactivity dependence on properties not disclosed yet was sug-gested

Photocatalytic reaction is induced by photoexcited electrons(e) and positive holes (hthorn) generated in a solid photocatalystfollowed by redox reaction with surface-adsorbed substratesTitanium(IV) oxide (titania) is one of the most promising photo-catalysts because of its high chemical stability sufficient energyof its e and hthorn to drive various photocatalytic reactions negli-gible toxicity inexpensiveness and ease of preparation Onlyone plausible weakness of the titania photocatalyst is that it ab-sorbs only ultraviolet light though such transparency is prefera-ble when titania is coated on colored materials It is known thatthe photocatalytic activity ie rate of photocatalytic reactiondepends on the structural and physical properties of titania Itis believed that there is a relationship between properties and ac-tivities ie a structurendashactivity correlation It has often beenclaimed that the smaller the particle size ie the larger the spe-cific surface area the higher the photocatalytic activity or thatanatase is better than rutile1 However such discussion on thecorrelations has been limited to a certain series of samples pre-pared in a similar way or small number of commercial samples2

and there seem no comprehensive correlationsThe present study aims at obtaining the structurendashactivity

correlations for a large number of titania photocatalysts coveringcommercially available titania samples in Japan (see Table S1)14

by statistical multivariable analysis Five representative reac-tions were chosen and their relative rates were analyzed usingsix properties3 specific surface area (BET) density of latticedefects (DEF) primary (PPS) and secondary (SPS) particle sizeand existence of anatase (ANA) and rutile (RUT) phases to ob-tain intrinsic dependence of photocatalytic activities on the prop-erties

Five test photocatalytic reactions were as follows a Oxy-gen (O2) liberation and silver deposition from a deaerated aque-ous silver sulfate solution b Dehydrogenation of methanol in adeaerated aqueous solution c Oxidative decomposition of ace-tic acid to liberate carbon dioxide (CO2) from an aerated aque-ous solution d Decomposition of acetaldehyde into CO2 inair and e Synthesis of pipecolinic acid from L-lysine in a dea-erated aqueous solution (see SI)

First the rates of reactions were compared with each struc-tural property In general there were no significant relationsbetween them while in some cases dependence of the rate on

a parameter was observed (see SI Tables S2 and S4) This seemsreasonable since the titania samples used in this study were of awide range and synthesized or treated in different ways Thenthe data were analyzed statistically by solving the followingmatrix equation frac12rate351 frac14 frac12property356 frac12coefficient61for each reaction The experimental results rates and propertieswere standardized using mean of data and standard deviation inorder to make the calculated coefficients have the same weightbeing independent of properties ie enabling direct comparisonof partial regression coefficients (k)

Table 1 shows the summary of results of analyses Squaredmultiple correlation coefficient R2 was also listed It was foundthat the best R2 values were obtained when ANA and RUT weredefined to be 1 or 0 (dummy variables) when the most intensivepeak of anatase and rutile was detected in the XRD patterns andotherwise respectively Although it is known that increase in thenumber of variables improves the fitting ie makes R2 to beclose to unity no other appropriate properties have been foundto be added to the list As a general trend a and e gave relativelylarger R2s ie higher reproducibility of the results fitting to alinear combination of properties while those for the others werealso fairly high4 Figure 1 shows this linearity for a as an exam-ple the calculated (predicted) relative activities of each sampleusing obtained coefficients (k) were plotted against the observedactivity Another significant feature is that coefficient of kANAhas large positive value in all the cases except for a This isthe first example within the authorsrsquo knowledge of supportfor a general understanding in this field that anatase is moreactive than rutile since previously reported data were of onlya limited number of samples and neglected the influence of theother properties On the other hand kRUT was relatively smalland it was negative in d suggesting that rutile phase in the photo-catalysts is rather inert compared with anatase Although largepositive and negative kBET and kDEF respectively are expectedsince large surface area is advantageous to adsorb reaction sub-strate(s) and defective sites accelerate recombination of e andhthorn large positive kBET was observed only for b and even largepositive kDEF was obtained for c d and e as discussed laterkPPS and kSPS depended on the type of reactions

Table 1 Squared multiple correlation coefficient (R2) and partial regres-sion coefficients (k)

Coefficient a b c d e

R2 086 052 058 060 085

kBET 001 043 009 013 019

kDEF 015 025 019 043 032

kPPS 012 020 018 020 052

kSPS 057 008 020 004 007kRUT 014 028 011 006 002

kANA 004 040 057 055 063

238 Chemistry Letters Vol38 No3 (2009)

Copyright 2009 The Chemical Society of Japan
























0

02

04

06

08

1

0 02 04 06 08 1

calc

ulat

ed p

hoto

cata

lytic

activ

ity fo

r re

actio

n a









































b H2mmol h-1

c CO2mmol h-1

d CO2mmol h-1

e PCAmmol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35


0003 0003 0021 0030 0131 0001 0004 0005 0003 0008 0011 0004 0017 0043 0005 0029 0034 0024 0017 0015 0017 0012 0011 0006 0014 0083 0019 0020 0018 0106 0006 0009 0006 0009 0030

0180 0086 0187 0778 0505 0243 0593 0635 0788 0699 0520 0648 0538 0674 0428 0490 0563 0592 0487 0601 0209 0726 0575 0605 0190 0675 0175 0223 0927 0545 0704 0465 0420 0603 0859

0016 0025 0007 0038 0015 0007 0031 0020 0028 0026 0042 0028 0047 0029 0027 0032 0038 0051 0034 0043 0013 0033 0032 0030 0018 0029 0028 0026 0023 0017 0027 0035 0024 0035 0053

0220 0159 0063 0272 0127 0049 0257 0366 0235 0291 0289 0261 0323 0156 0111 0129 0152 0235 0224 0336 0193 0271 0308 0261 0143 0123 0125 0231 0263 0067 0339 0283 0195 0301 0261

0036 0009 0006 0044 0012 0017 0052 0052 0055 0054 0054 0055 0048 0015 0028 0039 0041 0045 0049 0059 0018 0048 0048 0045 0016 0015 0026 0038 0042 0008 0049 0050 0050 0048 0041



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35



VP-P90 TUF-01 PC-101 PC-102


Kanto Wako

CR-EL ST-01 ST-21

TKP-101 TKP-102

P25

73 18 40 50

3 100 270 338 300 100 97

290 59

34 19 29 37 39 73 97

114 301 157 300 11

4 8

11 59

7 298 67

306 114 59

10928485014

242119118105106156111724384

1121081341861403441111189825181738212184

101869450

214004021

57015

84

951515

630

5009060502020

558

129

16951721716929

2008

259

2028

5610512406

1812516

0240921380290791020860410360370670391042316

144139044

14033

043046

1542586689139096094

0 0 1 1 1 1 0 0 0 0 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 1 1 0 0 0 0 1

1 1 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1






PPS mdash mdash 1 05791 02535 01471




property a b c d e

BET ndash046 034 ndash002 052 059




ANA 010 044 056 042 046

RUT 048 004 004 ndash051 ndash041
























0

02

04

06

08

1

0 02 04 06 08 1

calc

ulat

ed p

hoto

cata

lytic

activ

ity fo

r re

actio

n a









































b H2mmol h-1

c CO2mmol h-1

d CO2mmol h-1

e PCAmmol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35


0003 0003 0021 0030 0131 0001 0004 0005 0003 0008 0011 0004 0017 0043 0005 0029 0034 0024 0017 0015 0017 0012 0011 0006 0014 0083 0019 0020 0018 0106 0006 0009 0006 0009 0030

0180 0086 0187 0778 0505 0243 0593 0635 0788 0699 0520 0648 0538 0674 0428 0490 0563 0592 0487 0601 0209 0726 0575 0605 0190 0675 0175 0223 0927 0545 0704 0465 0420 0603 0859

0016 0025 0007 0038 0015 0007 0031 0020 0028 0026 0042 0028 0047 0029 0027 0032 0038 0051 0034 0043 0013 0033 0032 0030 0018 0029 0028 0026 0023 0017 0027 0035 0024 0035 0053

0220 0159 0063 0272 0127 0049 0257 0366 0235 0291 0289 0261 0323 0156 0111 0129 0152 0235 0224 0336 0193 0271 0308 0261 0143 0123 0125 0231 0263 0067 0339 0283 0195 0301 0261

0036 0009 0006 0044 0012 0017 0052 0052 0055 0054 0054 0055 0048 0015 0028 0039 0041 0045 0049 0059 0018 0048 0048 0045 0016 0015 0026 0038 0042 0008 0049 0050 0050 0048 0041



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35



VP-P90 TUF-01 PC-101 PC-102


Kanto Wako

CR-EL ST-01 ST-21

TKP-101 TKP-102

P25

73 18 40 50

3 100 270 338 300 100 97

290 59

34 19 29 37 39 73 97

114 301 157 300 11

4 8

11 59

7 298 67

306 114 59

10928485014

242119118105106156111724384

1121081341861403441111189825181738212184

101869450

214004021

57015

84

951515

630

5009060502020

558

129

16951721716929

2008

259

2028

5610512406

1812516

0240921380290791020860410360370670391042316

144139044

14033

043046

1542586689139096094

0 0 1 1 1 1 0 0 0 0 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 1 1 0 0 0 0 1

1 1 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1






PPS mdash mdash 1 05791 02535 01471




property a b c d e

BET ndash046 034 ndash002 052 059




ANA 010 044 056 042 046

RUT 048 004 004 ndash051 ndash041





































b H2mmol h-1

c CO2mmol h-1

d CO2mmol h-1

e PCAmmol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35


0003 0003 0021 0030 0131 0001 0004 0005 0003 0008 0011 0004 0017 0043 0005 0029 0034 0024 0017 0015 0017 0012 0011 0006 0014 0083 0019 0020 0018 0106 0006 0009 0006 0009 0030

0180 0086 0187 0778 0505 0243 0593 0635 0788 0699 0520 0648 0538 0674 0428 0490 0563 0592 0487 0601 0209 0726 0575 0605 0190 0675 0175 0223 0927 0545 0704 0465 0420 0603 0859

0016 0025 0007 0038 0015 0007 0031 0020 0028 0026 0042 0028 0047 0029 0027 0032 0038 0051 0034 0043 0013 0033 0032 0030 0018 0029 0028 0026 0023 0017 0027 0035 0024 0035 0053

0220 0159 0063 0272 0127 0049 0257 0366 0235 0291 0289 0261 0323 0156 0111 0129 0152 0235 0224 0336 0193 0271 0308 0261 0143 0123 0125 0231 0263 0067 0339 0283 0195 0301 0261

0036 0009 0006 0044 0012 0017 0052 0052 0055 0054 0054 0055 0048 0015 0028 0039 0041 0045 0049 0059 0018 0048 0048 0045 0016 0015 0026 0038 0042 0008 0049 0050 0050 0048 0041



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35



VP-P90 TUF-01 PC-101 PC-102


Kanto Wako

CR-EL ST-01 ST-21

TKP-101 TKP-102

P25

73 18 40 50

3 100 270 338 300 100 97

290 59

34 19 29 37 39 73 97

114 301 157 300 11

4 8

11 59

7 298 67

306 114 59

10928485014

242119118105106156111724384

1121081341861403441111189825181738212184

101869450

214004021

57015

84

951515

630

5009060502020

558

129

16951721716929

2008

259

2028

5610512406

1812516

0240921380290791020860410360370670391042316

144139044

14033

043046

1542586689139096094

0 0 1 1 1 1 0 0 0 0 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 1 1 0 0 0 0 1

1 1 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1






PPS mdash mdash 1 05791 02535 01471




property a b c d e

BET ndash046 034 ndash002 052 059




ANA 010 044 056 042 046

RUT 048 004 004 ndash051 ndash041



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35



VP-P90 TUF-01 PC-101 PC-102


Kanto Wako

CR-EL ST-01 ST-21

TKP-101 TKP-102

P25

73 18 40 50

3 100 270 338 300 100 97

290 59

34 19 29 37 39 73 97

114 301 157 300 11

4 8

11 59

7 298 67

306 114 59

10928485014

242119118105106156111724384

1121081341861403441111189825181738212184

101869450

214004021

57015

84

951515

630

5009060502020

558

129

16951721716929

2008

259

2028

5610512406

1812516

0240921380290791020860410360370670391042316

144139044

14033

043046

1542586689139096094

0 0 1 1 1 1 0 0 0 0 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 1 1 0 0 0 0 1

1 1 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1






PPS mdash mdash 1 05791 02535 01471




property a b c d e

BET ndash046 034 ndash002 052 059




ANA 010 044 056 042 046

RUT 048 004 004 ndash051 ndash041






PPS mdash mdash 1 05791 02535 01471




property a b c d e

BET ndash046 034 ndash002 052 059




ANA 010 044 056 042 046

RUT 048 004 004 ndash051 ndash041

Correlation between Photocatalytic Activities and Structural and Physical Properties of Titanium(IV)...

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Transcript of Correlation between Photocatalytic Activities and Structural and Physical Properties of Titanium(IV)...