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ELSEVIER
Applied Surface Science 121 / 122 (1997) 525-529
Characterization of photoreduced Pt/TiO 2 and decomposition of dichloroacetic acid over photoreduced Pt/TiO 2 catalysts J.C. Yang a.*, Y.C. Kim a, Y.G. Shul a, C.H. Shin b, T.K. Lee c " Department of Chemical Engineering, Yonsei Unit,ers'iO', Sinchondong, Sudaemungu, 120-749 Seoul, South Korea b Korea Research Institute of Chemical Technology, Daeduck Science Town, Taejon, South Korea c Korea Institute of Energy Research, Jangdong 71-2, Taejon, South Korea Received 19 September 1996; accepted 12 February 1997
Abstract The effect of photoreduction on the properties of Pt/TiO 2 catalysts was investigated. A decrease of the white-line area in the XANES spectra of Pt/TiO 2 catalysts at the Pt LIII edge was observed with increasing photoreduction time, which denoted the photoreduction of Pt/TiO 2 with UV illumination. From the EXAFS fitting results, the growth of Pt particles with the photoreduction time could be monitored. The XPS spectra of Pt/TiO 2 catalysts at the Pt4f5/2 and Pt4f7/2 bands showed the presence of Pt ° and Pt 2+ states after photoreduction and the fraction of metallic Pt U was increased with increasing photoreduction time. The optimum photoreduction time for DCA (dichloroacetic acid) decomposition was 6 h. The partially reduced state of Pt seems to be effective to obtain a high quantum yield in DCA decomposition with minimizing the agglomeration of Pt clusters on the TiO 2 surface. © 1997 Elsevier Science B.V. Keywords: Pt/TiO2; Photoreduction; EXAFS/XANES; XPS; DCA; Characterization
1. Introduction Recently, the photocatalytic reduction of noble metals on various semiconductors has received great interest in metal recovery, photoimaging processes, and preparation of photocatalysts [1-6]. Bard et al. applied this method to the preparation of supported metal catalysts and the photoreduced catalyst was effective in the photo-Kolbe reaction [1]. Borgarello et al. have investigated the irradiation of aqueous TiO 2 dispersions containing Pd or Rh salts with simulated sunlight [4]. They showed that pH, oxygen and TiO 2 concentration could vary the rate of depo-
* Corresponding author. Fax: + 82-2-3126401.
sition of Pd and Rh on TiO 2, which could lead to differently coated TiO 2 powders. Recently, Fernandez et al. have studied the photoreduction of Au on colloidal TiO 2 and they showed the formation of metallic crystallites during the photoreduction of Au metal cations on semiconductor materials [6]. These results indicate that the photoreduction method can be applied to prepare highly dispersed metal supported catalysts and have many advantages for waste water treatment. In the present paper we have used the photoreduction method for the preparation of Pt supported TiO 2 catalysts in aqueous solution. The effects of photoreduction on the properties of P t / T i O 2 catalysts were characterized by X A F S and XPS. The
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 3 5 9 - 0
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J.C. Yang et al. /Applied SurJ~we Science 121 / 122 (1997) 525-529
photo-oxidation of DCA (dichloroacetic acid) was also attempted.
i
Pt foil
Pt*Pt
2. Experimental The anatase form of Hombikat TiO 2 supplied by Sachtleben Chemie was used as TiO 2 support. For the photoreduction experiments, the weighted amount of TiO 2 was suspended in a mixture of platinizing solution (H2PtC16.nH20 in 0.1M methanol solution). The suspensions were irradiated with a 75 W high-pressure mercury lamp. The photocatalytic decomposition of DCA was carried out in a quartz reactor with the photoreduced Pt/TiO 2 catalysts suspended in l mM aqueous DCA solution. For the in situ measurement of protons in the reaction, the pH-stat technique was used to measure the photocatalytic activity [14]. The samples were characterized by EXAFS spectroscopy using synchrotron radiation at Photon Factory (BL-7C, KEK). XPS spectra were collected using a VG ESCA LAB-200D instrument with Mg K a X-rays.
3. Results and discussion
3.1. Characterization of photoreduced Pt / TiO 2 catalysts Fig. 1A shows the XANES spectra of photoreduced Pt/TiO 2 catalysts at the Pt L m edge. Pt/TiO 2 catalysts without photoreduction have a very high intensity of white line above the absorption edge. The decrease of white-line intensity can be clearly observed in photoreduced Pt/TiO 2 catalysts. It has been reported that the intensity of white line of the Pt LHI edge is related to the transition probability of exciting the inner core 2p3/2 to the 5d5/2 and 5d3/2 valence level [7,8]. Therefore, the lower the electron density of the Pt metal, the greater the number of vacancies in the valence level and the higher the white line intensity. The white-line area of Pt/TiO 2 catalysts is decreasing with increasing photoreduction time (Table 1), which shows the progressive reduction of initial Pt "+ of PtCI~, into Pt ° by photoreduction. The absorption edge is a function of
8 g J~ <
I¸
A
[
ohr l
/,. , i ,
11560
, , j , i
11580
. . . .
11600
0h.!
E . . . .
11620
Photon Energy(eV)
(A)
.,c,
11640
0
1
2
3
4
5
6
7
8
R(Angstrom)
(B)
Fig. 1. (A)XANES spectra of photoreduced Pt/TiO_, catalysts with photoreduction time at the Pt L m edge and (B) Fourier transform of k~-weighted EXAFS spectra of Pt/TiO 2 catalysts with photoreduction time (without phase shift correction).
the oxidation state and shifts to higher energy as the oxidation state of the element becomes greater than zero [13]. The edge position of Pt/TiO 2 is shifted to lower energy as the photoreduction time increases (Table 1), which denotes the reduction of the Pt state with photoreduction time. The oscillation feature of Pt foil is somewhat different from that of Pt/TiO 2 after 24 h of photoreduction. This implies that Pt/TiO 2 is not fully reduced into the Pt ° state even after 24 h of photoreduction, and a partially reduced species between Pt "+ and Pt ° can be obtained under our experimental conditions. The Fourier transforms of the EXAFS spectra of Pt/TiO2 with photoreduction time are denoted in Fig. lB. A phase shift correction was not considered in Fig. lB. In the Fourier transform of Pt/TiO 2 without photoreduction (0 h), one main peak is apparently seen in the radial structure function around 2.2 A. While, in the Fourier transforms of photoreduced Pt/TiO 2 catalysts, the main peak is seen at 2.5 A. As the p hotoreduction proceeds, the peak intensity at 2.5 A is increases with photoreduction time. The first peak around 2.2 ,~ in Pt/TiO 2 can be attributed to the interaction of Pt-CI, and the peak around 2.5 ,~ can be assigned as Pt-Pt interaction [9,10]. In the EXAFS fitting results of Pt/TiO_~
J.C. Yang et al./Applied Surjbce Science 121 / 122 (1997) 525 529
527
Table 1 Best fitting parameters for the EXAFS and XPS spectra of P t / T i O 2 catalysts with photoreduction time P t / T i O 2 sample photoreduction time (h)
White line
0
E value a
backscatterer
C.N. b
11567.0
CI
6
2.32
Pt
0
-
CI Pt C1 Pt C1 Pt
2.55 5.01 0.3 8.57 0.0 9.58
2.34 2.74 2.34 2.75 2.75
6
11566.5
12
11565.4
24
11565.4
R (A) ~
~¢r d
area ~ (au)
l(Pt°)/l(Pt 2) g
% Pt ° f
0.000
2.89
0
0.019 0.006 -0.004 (I.008 0.008
2.29
41.7
0.12
1.43
71.4
1.03
1.12
79.8
1.44
-
E value: the position of the first infection point of the absorption edge. u,,.d Estimated precision: C.N. + 20%, R _+ 0.02 ,~, Act_+ 20%. The whiteqine area can be calculated from the difference area between the areas of sample and reference Pt foil. r % pt 0 = (C.N. of Pt Pt bond)/(C.N, of reference bulk Pt) × 100. l(Pt°)/l(Pt 2+): the ratio of Pt ° to Pt 2+ calculated from the XPS spectra of photoreduced P t / T i O 2 at the P t 4 f doublet using the parameters (A E(4fs/2 - 4f7/2) = 3.3 _+ 0.3 eV and l(4f7/2)/l(4fs/2) = 4 / 3 ) .
~
Ohr
~
ehr
¢1
"E E
,,
,hE,
82
, ~ k f : l l l l l l , , I
80
78
, = = = l
76
74
, ~ , l , I
72
,ll
70
,
68
,~
82
66
80
78
76
~ . . . .
I
80
. . . .
il
78
,LI
[
76
. . . .
72
70
68
66
Binding Energy(eV)
Binding Energy(eV)
82
74
r
hr
~E
I
74
. . . .
I
. . . .
72
Binding Energy(eV)
I
70
. . . .
I
68
. . . .
,~L
66
82
,I
80
. . . .
I
78
. . . .
[
76
. . . .
q
74
. . . .
/
. . . .
72
I
70
. . . .
k
68
,
,
,
,
66
Binding Energy(eV)
Fig. 2. XPS spectra of P t / T i O 2 catalysts at the P t 4 f bands with varying photoreduction time. (XPS spectra of P t / T i O 2 catalysts have been deconvotuted into two sets of Pt 4f doublets using the parameters A E(4fs/2 - 4f7/2 ) = 3.3 _+ 0.3 eV and I ( 4 f 7 / 2 ) / I ( 4 f 5 / 2 ) = 4 / 3 for the individual sets of 4|'5/2,7/2 doublets.)
J.C. Yang et al./Applied Surface Science 121 / 122 (1997) 525-529
528
without photoreduction (Table 1), the Pt-CI bond existed at 2.32 A with a coordination number of 6. In the 6 h photoreduced P t / T i O 2, the coordination numbers of the Pt-CI and P t - P t bonds are 2.55 and 5.01, respectively. As photoreduction proceeds, the coordination number of the Pt-C1 bond decreases and reaches 0. The coordination number of P t - P t interaction increases up to 9.58 after 24 h of photoreduction. The EXAFS fitting results of photoreduced P t / T i O 2 denotes the Pt-C1 bonds diminish in intensity and the interaction of the P t - P t bond is increasing as the photoreduction proceeds. It implies the initial PtCI"~- species is reduced into P t z + / P t ° states and agglomeration of Pt particles has occurred on the TiO 2 surface by UV irradiation. Fig. 2 shows the Pt4f photoelectron peaks of P t / T i O 2. It has been reported that Pt foil has Pt4f7/2 and Pt4fs/2 bands at 70.7 and 74.0 eV, respectively [11]. The binding energy of Pt 2+ (4f7/2) and Pt 4+ (4f5/2) is 73.3 eV and 75.5 eV, respectively [12]. In our results, Pt 4+ and Pt 2+ species are seen in the XPS spectrum of the initial P t / T i O 2 sample (0 h). As the photoreduction proceeds (6 h), the Pt ° state begins to occur after 6 h of photoreduction and the relative area of Pt ° to Pt 2+ (l(Pt°)/l(pt2+)) is increasing (Table 1) with photoreduction time. Our XPS result shows the initial Pt 2+ and Pt 4+ are changed into the metallic Pt ° state by photoreduction. On the basis of our E X A F S / X A N E S and XPS
results, a possible mechanism of photoreduction of P t / T i O 2 can be proposed as the following steps. The formation of electron-hole pairs by UV illumination is the first step in the photoreduction of P t / T i O 2. The second step is the reduction of Pt "+ into Pt 2+ or Pt ° atoms and the agglomeration of Pt ions into Pt metal clusters. A two-step mechanism (nucleation and growth) of photocatalytic reduction of Au metals on TiO 2 has been suggested [6]. The mixed state of our photoreduced P t / T i O 2 (Pt 2+ and Pt °) after 24 h of photoreduction denotes the agglomeration of Pt atoms and cathode-like reduction might be simultaneously occurring during the particle growth of Pt metal. On the basis of our EXAFS and XPS results, we can suggest the particle growth mechanism of Pt by the following method: mPt ° + Pt,,, 2e
80
(2)
Considering the coordination numbers of Pt and the % Pt ° of P t / T i O 2 (Table 1), Pt is not completely reduced into a fully crystalline Pt phase even after 24 h of photoreduction, which is different from the results of A u / T i O 2 catalysts [6]. In addition highly dispersed small Pt clusters are formed under our preparation conditions in comparison with A u / T i O 2 catalysts.
- 0 - photoreduction(Pt/HomNkat) --III-- H2 reduction(Pt/Hombikat) ~ Hombikat
4O
60 ~
20
40 0
~ Pt,,,
50 _
.(D
Pt -"+
Pt ° + p t 2 + ~ Pt~ + ~ Pt 2 ~ Pt~ + " "
100
>.. E
(1)
8
20 I P I I I I 500 1000 1500 2000 2500 3000 3500 Irradiation Time(sec)
(A)
10 0
I
I
I
I
10
15
20
25
Photoreduction
30
time(hr)
(B)
Fig. 3. (A) Quantum yield of DCA decomposition with reaction time over photoreduced Pt/TiO 2 catalyst, hydrogen reduced Pt/TiO 2 and Hombikat TiO 2 and (B) quantum yield of DCA decomposition with varying the photoreduction time of Pt/TiO 2 catalysts.
J.C. Yang et al. /Applied Surface Science 121 / 122 (1997) 525-529 3.2. DCA photo-oxidation with photoreduced Pt / Ti02 Fig. 3A shows the photocatalytic activity of D C A decomposition over P t / T i O 2 catalysts and Hombikat TiO 2. In those results, the quantum yield of Hombikat TiO 2 was 33% at 300 s, and the quantum yield of Hombikat was decreased to 13% after 3000 s of reaction time. The hydrogen reduced P t / T i O 2 had a relatively low quantum yield (14%) at the initial reaction time, and a low quantum yield (8%) was maintained after 3000 s. On the contrary, photoreduced P t / T i O ~ had a high initial quantum yield (62%) and 36% after 1000 s. These results show the photoreduced P t / T i O 2 had a higher quantum yield than the hydrogen reduced P t / T i O 2 or Hombikat TiO 2. It suggests that the photoreduction is an effective method to enhance the photocatalytic activity in the D C A decomposition reaction. Fig. 3B shows the quantum yields of D C A decomposition with varying photoreduction time. As the photoreduction time increases, the 6 h photoreduced P t / T i O 2 has a maximum quantum yield (37%) in D C A decomposition. However, beyond this point, the quantum yield is decreasing with photoreduction time. Large Pt particles were formed as the photoreduction proceeds after 6 h of UV illumination and D C A decomposition also decreased concurrently. The maximum photocatalytic activity of D C A decomposition can be attributed to the partially reduced state of Pt and the formation of small Pt clusters by 6 h of photoreduction. The generation of a partially reduced species of Pt seems to be more critical to enhance the quantum yield of D C A decomposition as observed in Fig. 3A. A detailed study of the partially reduced state and the effect of metal
529
dispersion on the quantum yield are now under study by using TEM and chemisorption.
Acknowledgements The authors thank Professor M. Nomura for the EXAFS measurements (Photon Factory Proposal No. 95-G009).
References [1] B. Kraeutler, A. Bard, J. Am. Chem. Soc. 100 (1978) 4317. [2] S. Sato, J. Catal. 92 (1985) 11. [3] J.M. Herrmann, J. Disdier, P. Pichat, C. Leclercq, in: B. Delmon, P. Grange, P.A. Jacobs, G. Poncelet (Eds.), preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 337. [4] E. Borgarello, N. Serpone, G. Emo, R. Harris, E. Pelizzetti, C. Minero, Inorg. Chem, 25 (1986) 4499. [5] Y. Inel, D. Ertek, J. Chem. Soc. Faraday Trans. 89 (1993) 129. [6] A. Fernandez, A. Caballero, A.R. Gonzalez-Elipe, J.M. Herrmann, H. Dexpert, F. Villain, J. Phys. Chem. 99 (1995) 3303. [7] A. Jentys, B.J. Mchugh, G.L. Haller, J.A. Lercher, J. Phys. Chem. 96 (1992) 1324. [8] S.K. Purnell, K.M. Sanchez, R. Patrini, J.R. Chang, B.C. Gates, J. Phys. Chem. 98 (1994) 1205. [9] A. Jentys, G.L. Hailer, J.A. Lercher, J. Phys. Chem. 97 (1993) 484. [10] A. Caballero, F. Villain, H. Dexpert, F. Lepeltier, B. Didillon, J. Lynch, Catal. Lett. 20 (1993) 1. [11] K.S. Kim, N. Winorgrad, R.E. Davis, J. Am. Chem. Soc. 93 (1971) 6296. [12] L. Bornsten, in: Zahlenwerte und Funktionen aus Naturwissenschaft und Technik, Springer, Berlin, 1982. [13] P.A. Lee, P.H. Citrin, P. Eisenberger, B.M. Kincaid; Rev. Mod. Phys. 53 (1981) 769. [14] Y.C. Kim, M.S. Thesis, Yonsei University, 1995.