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On the conditions and mechanism of PtO2 formation in the photoinduced conversion of H2PtCl6
f
Photochem.
Photobtil.
A: Chem.,
81 (1994) 177-182
177
On the conditions and mechanism of PtO, formation photoinduced conversion of H,PtCl, Zhensheng
Jint, Zhengshi Chen,
LanzhouInstiha (Received
October
of Chemical
Physics,
Qinglin
Chinese Academy
15, 1993; accepted
January
Li, Chanjuan of Sciences, LnnzJzou,
Xi and Xinhua 730000
in the
Zheng
(China)
18, 1994)
Abstract X-ray photoelectron spectroscopy (XPS) and UV-visible absorption spectroscopy were used to investigate the conditions and mechanism of PtOz formation during the photoinduced conversion of PtCl,‘- in a Pt/CdS aqueous dispersion. The experimental results indicate that the critical factors affecting PtOz formation are the pH value of the dispersion and the initial H,PtCI, concentration. At pH13 and an initial H,PtCI, concentration of lOA2 M, the deposits detected on the illuminated Pt/CdS surface were Pt02 and Pt(OH),. It is suggested that PtO, is formed by the photoinduced oxidation of the hydrolysate of PtCI,‘- (i.e. Pt(OH),C12’-) on the Pt/CdS surface. The formation of oxygen was also observed in the process.
1. Introduction The photoinduced chemical reaction of H,PtCI, in homogeneous medium has been investigated extensively, but little work has been carried out in dispersions of semiconductor powders (such as CdS, TiOz, etc). In a previous paper [l, 21, we have reported that the photoreduction of H,PtCI, at low concentration on CdS produces PtS (at pH 2.3) or Pt(OH), (at pH 13) rather than Pt’. When CdS is treated at high temperature in air for a short time, only Pt(OH), is obtained in both basic and acidic dispersions. Recently, Mills and Williams [3] and Lauermann et al. [4] have found that 0, is evolved when H,PtCl, (initial concentration range, 0.5 - 1.0 X lo-* M) is irradiated with visible light in the presence of Pt”-loaded CdS (the pH value of the Pt/CdS dispersion was more than 12). However, if the Pt/CdS catalyst is used repeatedly, its activity for oxygen evolution decreases progressively even when a consistent initial H2PtC16 concentration is used. When the initial H,PtCI, concentration is less than 0.1~10~~ M, the rate of oxygen evolution decreases shbrply. No oxygen is evolved if PtCl,‘- is replac d by PtCl,*or Pt(OH),‘-. All the results obt ined ”,comconfirm that H,PtCl, is an indispensable ponent in the production of O2 from the photoirradiation of Pt” (or Rh&, RuO,)/CdS distAuthor
to whom correspondence
1010.4030/94/$07.00 SSDI
0 1994 Elsevier
1010-6030(94)03792-S
should be addressed.
Science S.A. All rights reserved
persions. The results refute the conclusion that, on these catalysts, water can be photodecomposed directly to yield 0, [5]. Unfortunately, these workers [3, 41 did not analyse the deposit obtained after photoconversion of H,PtCI, and so their suggested mechanisms (eqn. (1) [4]; eqn. (2) [3]) e- + PtCls2- 4PQ3
PtQ3-
- + 6H,O 4Pt0+12H*
PtCl,* - + 2H,O -
+24Cl-
+30,
(1)
Pt” + 6ClF + 4H+ + O2
(2)
are lacking in experimental basis. Recently, under similar experimental conditions, we found that the photodeposits on the Pt/CdS surface were PtO, and Pt(OH), using X-ray photoelectron spectroscopy (XPS). We also studied the absorption spectrum of the dispersion, detected the existence of SO,*- and analysed the product in the gas phase. Based on the results obtained, we propose a new photoinduced reaction mechanism for PtO, and O2 formation.
2. Experimental
details
2.1. Preparation of PtlCdS CdS powder (4 g) and aqueous H,PtCl, solution (0.077 M; 1.32 ml) (Pt:CdS (w/w) =OS:lOO) were added to 40 ml of distilled water. High purity N, was bubbled through the solution for 15 min. The
2. lin et al. / PtO, formation during photoinduced conversion oj H,hC16
178
whole system was then closed and irradiated with visible light for 5 h. The light emitted by a 750 W Xe lamp was passed through 10 cm of &CrO, aqueous solution to filter out wavelengths shorter than 450 nm and longer than the red. The distance from the reaction cell to the Xe lamp was 30 cm and the intensity of light incident on the cell surface was 2.6 mW/cm-‘. After irradiation, the powder was filtered and washed to remove residual Cl- ions and dried in a vacuum oven at 100 “C for 4 h. XPS measurement showed the deposit on the CdS surface to be PtS (Fig. l(a), Ptdfln, 72.4 eV). The powder was air treated at about 510 “C in ambient air for 45 min, washed to remove SO,“- and dried. The deposit on the prepared PtiCdS catalyst was Pt’+PtO (Fig. l(b)) [l].
Distilled water, pho~~atal~t and 0.077 M H,PtCl, aqueous sohtion were added to a 70 ml flask to give a dispersion of pH 2.3_ T%e pH of the dispersion was adjusted to a value of I3 with 50 wt.% NaOH. The light source and irradiative process were the same as described above. During the course of irradiation, a gas sample f0.2-0.3 ml) was taken at fixed intervals with a syringe from the head space of the Bask for HZ and O2 analysis by gas chromatography (type 100 gas chromatograph, Shanghai Analytical Instrument Plant, China). The packing material was 5 10 molecular sieve and the carrier gas was Ar. After irradiation the dispersion was filtered. The filtrate was reacted with saturated BaCI, to detect the presence of SOd2- and its UV-visible absorption spectrum was measured with a UV-240 spectrophotometer {Shi-
madzu, Japan). XPS analysis of the photodeposit on the Pt/CdS surface was obtained using a PHI550 multifunctional spectrometer (P-E Co., USA); spectral deconvolution was achieved by a computer. 2.3. Reagents The specification of CdS is given in ref. 2. H2PtC16 and K2PtCls were analytically pure reagents obtained in China, 3. Results and discussion
Figure 2 shows the Xp spectra of photodeposited platinum on the treated PtlCdS surface for different conditions: before heating (A) and after heating at 200 “C (B) and 600 “C (C) for 30 min. The results indicate that the catalyst surface is covered with a layer of deposit. The de~onvolutio~ of the spectrum in Fig. Z(A) shows the existence of two com~unds: one in which Pt,, is 72.7 eV, i.e. Pt(OI& [a], and one in which Ptqfln is 74.8 eV which should be a IY’compound. When the sample is heated in an ultrahigh vacuum chamber, the Pt’“:Pt’“:Pt” peak area ratio decreases, as shown in Fig. 2 and Table 1. This indicates that the PtKV compound is thermally unstable and decomposes to I??. Ph,,*
C (6OO’Cl
Pt(OH),
b
I
77
,
I
75
I
I
73
,
I
71
,
1
69
Eb feV)
Fig. 1. XP spectra of photodeposited platinum on raw CdS: (a) untreated, (b) air-treated (approximately 510 “C!).
Eb (ev)
Fig. 2. XP spectra of photodeposited platinum on PtiCdS (treated): (A) before heating; (B) after beating at 200 “C for 30 min; (C) after heating at 600 “C for 30 min.
Z. Jin et al. I PtOJ formation druing photoinduced conversion
TABLE 1. XPS analysis of platinized Cd‘? (pf-f 13; [HPCI,] M, catalyst in solution) 5 mg ml-‘)
Room
= lo-’
zoo “Cb
600 Y?
temperature PtW.PtT’-PtO . . peak area ratio
1.94:1:0
0.68:1:0.12
o:1:2.9sc
CUPt atomic ratio
1.0
1.1
0.1
“Free Ci- washed off. bHeating in ultrahigh vacuum chamber in situ for 30 min. ‘PtAoi2 of Pt” changed (72.7-72.3 eV).
pt,w2
’ 75.5
pt4t
I
,cr
79
El
75
77
73
71
EbleV)
Fig. 3. XP spectrum CdS). TABLE PtO
2. Ptdni2 binding PiS
71.0 71.2
70.9 70.7 71.1 71.0
of K,PtCI,
energy
(calibrated
(eV)
WOW2
Pto
Pto,
72.4
72.2 72.5
74.4 74.6
72.4
with the S,, line of
K,PtC&
[61
72.2 74.2
72.4
72.7
72.3
74.8
Reference
75.7 75.5
171 181 191 1101 This work
Figure 3 shows the XP spectrum of solid K,PtCl,. Pt,f7/2 (75.5 eV) is about 0.7 eV higher than that of the PttV compound shown in Fig. 2. The Cl/ Pt atomic ratio of 6.8 (theoretical value, 6.0) is much higher than the Cl/R atomic ratio given in Table 1. This indicates that the PtIV compound cannot be the unreacted PtCL*,‘. The value of 74.8 eV is close to the Pt,, binding energy of PtOz reported in the literature [6-lo] (Table 2). The fact that small amounts of Clstill exist on the washed sample surface (see Table 1, Cl/R atomic ratio= 1) may result from ion exchange between OH- on the air-treated CdS surface and Cl- in the solution during irradiation Cd-OH-
+Cl-
=Cd-Cl-
+OH-
(3)
of H,PtC&
179
In the ultrahigh vacuum chamber, there was no change in surface Cl- concentration after the sample was heated at 200 “C. When the sample was heated at 600 “C, chlorine was liberated. Based on these results, it is concluded that the Pt’” compound shown in Fig. 2(A) is PtO,. 3.2. Factors affecting the formation of PtO, In order to ascertain the factors affecting the formation of PtO,, we designed several experiments and compared the results with those given in Table 1, as shown in Table 3. In experiment 2, the pH of the dispersion was kept at 13, and the initial [H,PtCl,] value was 7.5~ lop4 M rather than lo-’ M used in experiment 1. After irradiation, the deposit on the catalyst surface was Pt(OH), (Fig. 4(a)). In experiment 3, the pH value was 2.2 (not adjusted with NaOH) and the initial [H,PtCl,] value was lo-’ M. After irradiation, the deposit on the catalyst surface was Pt” (Fig. 4(b)). In experiment 4, CdS (treated) was used instead of Pt/CdS (treated) with the other conditions identical to those used in experiment 1. The deposit was the same as that in experiment 1 (Fig. 4(c)). From the results of these experiments, it is clear that the main factors affecting the formation of PtO, include the pH value of the dispersion and the initial H,PtCl, concentration. The results shown in Table 3 also indicate the absence of Hz generation during illumination of the dispersion. In gas chromatographic analysis, it was difficult to avoid the interference of air during sampling, and so a firm conclusion on whether or not gaseous O2 is produced cannot be obtained. However, a milky BaSO, precipitate was formed when the filtrates of experiments 1, 2 and 4 were treated with BaCl, solution. Obviously, secondary photocorrosion took place in experiments 1, 2 and 4 O,+e-
-
02-,
-
Cd*+ + SOd2-
(or OH- +h+ -
20,-+CdS+2h‘ (4)
‘OH, 4OH+CdS+4h+ Cd*+ +SO,‘-
+4H+)
In experiment 3, in which the pH value of the dispersion was 2.2, no SO,‘- was detected. This indicates that 0, was not produced during the photoreaction. Since the surface deposit obtained was PtO, it can be deduced that there is no direct relation between O2 formation and PtCb*- reduction to Pt”. Thus it seems that the mechanisms expressed by eqns. (1) and (2) are incorrect.
2. Jk et aI. / FtO, formation
180 TABLE
3. Comparison
Experiment
of experiments
during photoinduced conversion of H2PtC16
(visible light; illumination
Catalyst
time, 5 h)
Initial [H,PtCl,lb (IO-’ M)
PH”
Substance
detected
Gas HZ
01
Solution
Surface deposit
1
(PtKdS) (treated)
13
1.0
NO
?
so,z-
P%, PtK’H),
2
PtKdS (treated)
13
0.075
No
?
sod=-
PtKJH),
3
Pt/CdS {treated)
2.2
1.0
No
?
NO
PtO
4
CdS (treated)
13
1.0
No
?
SO,‘_
PtOz, Pt(OH)z
*No apparent
change
after illumination.
bVolume of solution,
36 ml.
solution, 20% of PtCl,*- is hydroIysed to give Pt(OH)Cl,‘-; in 0.01 N HCl solution to neutral medium, Pt(OH),C1,2m will form in addition to Pt(OH)Cl,‘-; in neutral to basic solution (0.1 N NaOH or greater), the final hydrolytic product will contain Pt(OH),Cl’and Pt(OH),2-. The hydrolysis equilibrium wilt be accelerated by W illumination. The examination of the absorption spectra of PtCl,‘- aqueous solutions confirms this hydrolysis mechanism reported by Cox et al. [ll] and Blasius et al. [12]. The UV-visible absorption spectra are shown in Figs. 5 and 6(a). The experimental conditions were pH 2.3, [H,PtC&] = 7.5X 10d4 M and pH 13, [H,PtCl,] =7.5 X 10e4 M respectively. Octahedral PtClh2- has four absorption bands [ll, 13, 141 (Table 4). Compared with the data in Table 4, the 445, 360 and 255-260 0.25
Eb feV)
225 255
Fig. 4. Photodeposit on Pt/CdS (treated) surface under different photoreaction conditions: (a) pH 13, initial [HIPtCl,] =7.5 X 10e4 M; (b) pH 2.2, initial [HZPtC16]=10~’ M, (c) pH 13, initial [H,PtCl,] = lo-’ M, PtiCdS (treated) replaced by CdS (treated).
3.3. Hydrolysis of PtC162-
In aqueous solution, occur [ll, 121 PtCI,‘-
+OH-
Pt(OH)C&-
hydrolysis
=Pt(OH)C&+ OH-
+ Cl-
= Pt(OH),ClA2-
360
of PtC16*- will
0.125
(5) + Cl-
(6)
Pt(OH),Cl,=-
+ OH- =Pt(OH)&-
+ Cl-
(7)
Pt(OH)3C132-
+ OH- = Pt(OH),Cl,*-
-I-Cl-
(8)
The hydrolysis begins when the concentration of HCI is between 3.0 and 0.1 N. In 0.05 N HCl
445
;\‘-; 0
300
400
:
Wovslengt h (nm) Fig. 5. UV-visible (pH 2.3).
absorption
spectrum
of 7.5X 10m4 M HQtCld
Z. Jinet al. / PtO, formationduring
photoinduced
conversion of H,PtCl,
181
However, when the initial H,PtCl, concentration is as low as 7.5 x 10 -4 M, all the absorption peaks disappear. With a low initial H,PtCl, concentration, PtClb2 - and its hydrolysates are converted into a solid deposit.
I
i
\
I
‘\
Wavelenglh
fnm)
Fig. 6. UV-visible absorption spectra of 7.5x10-’ M H,PtCl, (pH 13): (a) after 3 h; (b) kept in the dark for 1 month.
3.4. Mechanism of pt02 formation From the results obtained in this study, it can be concluded that the photocatalytic conversion of PtCl,‘in aqueous solution involves the conversion of its hydrolysates. In aqueous solution at pH 13, different hyincluding drolysates exist, Pt(OH)XC16_,2(x= l-6). When the initial [H,PtC&] value is low, the solid product detected after photocatalytic conversion is mainly Pt(OH), (Table 3, experiment 2), indicating that Pt(OH),C&receives two photogenerated electrons to form Pt(OH)2 + 2e- -
Pt(OH),Cl,‘nm bands of our two aqueous samples, which represent ‘A,, -+ ‘Tlg, ‘Aig -+ IT,, and ‘A,, * b’T,, transitions respectiveIy, are slightly shifted. After keeping the sample of Fig. 6(a) in the dark for 1 month, these three peaks (460, 360 and 260 nm) disappear, but the peak around 210 nm remains (Fig. 6(b)). This suggests that the peak around 210 nm belongs to the hydrolysate. From the results given in Table 4, this peak probably represents the transition of Pt(OH),C16-,2Lm+M charge transfer. The correlation between the variation of the transition wavelength within certain limits (210-225 nm) and the degree of PtCl,‘- hydrolysis has not been studied. This is due to the fact that it is very difficult to identify accurately the different hydrolytic products. If the La+ M charge transfer transition of PtCl,2- in Table 4 is slightly blue shifted, the corresponding wavelength will be less than 200 nm, which is out of the scanning range (200-500 run) and cannot be observed. The hydrolysis of PtCl,‘- was also studied by measuring the UV-visible spectra of the filtrate after illumination (Fig. 7). If the initial concentration of H2PtC16 is lo-’ M, an absorption peak at about 210 nm can be observed even if the filtrate is diluted 20 times, indicating the existence of a large amount of hydrolysates Pt(OH)XCl,_,2-. TABLE
4. Excitation
energy (nm) of octahedral
l&--t~~g [Ill 450
aI~aceto&ile,
365 approximately
300 K.
(9)
Pt(OH)2C142-
+ 4h+ Pt02+2H+
+4Cll
(10)
can be eliminated. When the initial [H,PtCl,] value is increased by one order of magnitude, the concentrations of the different hydrolysates are increased and PtO, is deposited together with Pt(OH), (Table 3, experiments 1 and 4). This means that the reaction shown in eqn. (11) takes place where Pt(OH),ClZ2receives four hf in sequence to give PtO, Pt(OH),Cl,‘-
+ 4h+ PtO,+4H+
+0,+2Cl-
(11)
In this way, the relationship between PtO, formation and 0, generation can be explained. As shown in experiment 3 of Table 3, at pH 2.2 and an initial [H,PtCl,] value of lo-’ M, PtC1b2 - hydrolysis may stop at steps (5) or (6) and the hydrolysate is acidic, e.g. Pt(OH)C1,2+H+ Pt(OHZ)C1,-. It is possible that, on the catalyst surface, such a hydrolysate can receive
PtCl,‘-
Singlet ligand-field
band
+ 4ClI
Meanwhile, the photogenerated hole can oxidize OH- to ‘OH, which reacts further with CdS to yield SOa-. Thus the formation of PtO, via
‘Al8--t‘TI, [l 11
Triplet ligand-field
Pt(OH),
*A,g--*C’TI” [13] Lrr+M
band 269.5”
202.0~
Z. Jin et al. f PtO, formation duringphotoinduced conversion of H,PtCI,
182
.25
3.9
photocatalytic RuO,)/CdS.
reaction
of water on Pt” (or Rhz03,
Acknowledgments 8 b p 1.9
.I25
H e
This work was supported by the Natural Science Foundation of Gansu Province The authors express their gratitude to Professor Linghui Tong for help with the measurement of W-visible absorption spectra. References
1.00
0.0 2 Wovelengfh
Cnml
Fig. 7. UV-visible spectra of filtrates of a Pt/CdS (treated) dispersion (illuminated for 5 h): (a) initial [H,PtCl,] = lo-’ M; (b) initial [H,PtCI,] = 7.5 x 10e4 M.
four photogenerated electrons in sequence resulting in the reduction of PtrV to Pt’, and the photogenerated holes on the CdS surface can react with SH- to give So and H*. Therefore no is formed (CdS+H’+Cdz++SH--; sod2 SH- +Zh+ +S’+H+).
1 2. Jin, Q. Li, L. Feng and Z. Chen, 3. MoI. CataL, 50 (1989) 315. 2 Q. Li, Z. Cben, X. Zheng and Z. Jin, J. Phys. Chem., 96 (1992) 5959. 3 A. Mills and C. Williams, 1 Cfiem Sot., Famday Trans. I, 85 (1989) 503. 4 I. Lauermann, D. Meissner and R. Memming, /. Electroonal. Chem., 228 (1987) 45. 5 N.M. Dimitrijevic, S. Li and M. Gratzel, /. Am Chem. Sot., 106 (1984) 6565. 6 J.S. Hammond and N. Winograd, L Eiechaamzl. Chem., 78 (1977) 5.5. 7 G.C. Allen and P.M. Tucker,
8 9 IO
Electroanal. Chem. Interfacial Elecrrochem., 50 (1974) 335. T. Wang, A. Vazguez, A. Kato and L.D. Schmidt, L Calal., 78 (1982) 3%. KS. Kim, N. Winograd and R.E. Dairs, I. Am. Chem. Sot., 93 (1971) 6296. M. Koudelka, J. Sanchez and J. Augustynski, J. Phys. Chem., 86 (1982) 4277. L. Cox, D.C. Peters and E.L. Wehty, /. Irwq. Nucl. Chem.,
4. Conclusions
11
PtO, is the photocatalytic oxidation product of the hydrolysate of PtCl,‘- (Pt(OH),Cl,‘-) on Pt/ CdS (treated). 0, is the associated product of this reaction and is not produced through the direct
12 E. Blasius, W. Preetz and R. Schmilt, 1. Jnorg. Nucl. Chem., I9 (1961) 115. 13 D.L. Swihart and W.R. Mason, Inorg. Cfiem., 9 (1970) 1749. 14 C.M. Davidson and R.F. Jameson. Trans. Faraday Sot., 61 (1965) . , 2462.
34 (1972) 297.
Photochem.
Photobtil.
A: Chem.,
81 (1994) 177-182
177
On the conditions and mechanism of PtO, formation photoinduced conversion of H,PtCl, Zhensheng
Jint, Zhengshi Chen,
LanzhouInstiha (Received
October
of Chemical
Physics,
Qinglin
Chinese Academy
15, 1993; accepted
January
Li, Chanjuan of Sciences, LnnzJzou,
Xi and Xinhua 730000
in the
Zheng
(China)
18, 1994)
Abstract X-ray photoelectron spectroscopy (XPS) and UV-visible absorption spectroscopy were used to investigate the conditions and mechanism of PtOz formation during the photoinduced conversion of PtCl,‘- in a Pt/CdS aqueous dispersion. The experimental results indicate that the critical factors affecting PtOz formation are the pH value of the dispersion and the initial H,PtCI, concentration. At pH13 and an initial H,PtCI, concentration of lOA2 M, the deposits detected on the illuminated Pt/CdS surface were Pt02 and Pt(OH),. It is suggested that PtO, is formed by the photoinduced oxidation of the hydrolysate of PtCI,‘- (i.e. Pt(OH),C12’-) on the Pt/CdS surface. The formation of oxygen was also observed in the process.
1. Introduction The photoinduced chemical reaction of H,PtCI, in homogeneous medium has been investigated extensively, but little work has been carried out in dispersions of semiconductor powders (such as CdS, TiOz, etc). In a previous paper [l, 21, we have reported that the photoreduction of H,PtCI, at low concentration on CdS produces PtS (at pH 2.3) or Pt(OH), (at pH 13) rather than Pt’. When CdS is treated at high temperature in air for a short time, only Pt(OH), is obtained in both basic and acidic dispersions. Recently, Mills and Williams [3] and Lauermann et al. [4] have found that 0, is evolved when H,PtCl, (initial concentration range, 0.5 - 1.0 X lo-* M) is irradiated with visible light in the presence of Pt”-loaded CdS (the pH value of the Pt/CdS dispersion was more than 12). However, if the Pt/CdS catalyst is used repeatedly, its activity for oxygen evolution decreases progressively even when a consistent initial H2PtC16 concentration is used. When the initial H,PtCI, concentration is less than 0.1~10~~ M, the rate of oxygen evolution decreases shbrply. No oxygen is evolved if PtCl,‘- is replac d by PtCl,*or Pt(OH),‘-. All the results obt ined ”,comconfirm that H,PtCl, is an indispensable ponent in the production of O2 from the photoirradiation of Pt” (or Rh&, RuO,)/CdS distAuthor
to whom correspondence
1010.4030/94/$07.00 SSDI
0 1994 Elsevier
1010-6030(94)03792-S
should be addressed.
Science S.A. All rights reserved
persions. The results refute the conclusion that, on these catalysts, water can be photodecomposed directly to yield 0, [5]. Unfortunately, these workers [3, 41 did not analyse the deposit obtained after photoconversion of H,PtCI, and so their suggested mechanisms (eqn. (1) [4]; eqn. (2) [3]) e- + PtCls2- 4PQ3
PtQ3-
- + 6H,O 4Pt0+12H*
PtCl,* - + 2H,O -
+24Cl-
+30,
(1)
Pt” + 6ClF + 4H+ + O2
(2)
are lacking in experimental basis. Recently, under similar experimental conditions, we found that the photodeposits on the Pt/CdS surface were PtO, and Pt(OH), using X-ray photoelectron spectroscopy (XPS). We also studied the absorption spectrum of the dispersion, detected the existence of SO,*- and analysed the product in the gas phase. Based on the results obtained, we propose a new photoinduced reaction mechanism for PtO, and O2 formation.
2. Experimental
details
2.1. Preparation of PtlCdS CdS powder (4 g) and aqueous H,PtCl, solution (0.077 M; 1.32 ml) (Pt:CdS (w/w) =OS:lOO) were added to 40 ml of distilled water. High purity N, was bubbled through the solution for 15 min. The
2. lin et al. / PtO, formation during photoinduced conversion oj H,hC16
178
whole system was then closed and irradiated with visible light for 5 h. The light emitted by a 750 W Xe lamp was passed through 10 cm of &CrO, aqueous solution to filter out wavelengths shorter than 450 nm and longer than the red. The distance from the reaction cell to the Xe lamp was 30 cm and the intensity of light incident on the cell surface was 2.6 mW/cm-‘. After irradiation, the powder was filtered and washed to remove residual Cl- ions and dried in a vacuum oven at 100 “C for 4 h. XPS measurement showed the deposit on the CdS surface to be PtS (Fig. l(a), Ptdfln, 72.4 eV). The powder was air treated at about 510 “C in ambient air for 45 min, washed to remove SO,“- and dried. The deposit on the prepared PtiCdS catalyst was Pt’+PtO (Fig. l(b)) [l].
Distilled water, pho~~atal~t and 0.077 M H,PtCl, aqueous sohtion were added to a 70 ml flask to give a dispersion of pH 2.3_ T%e pH of the dispersion was adjusted to a value of I3 with 50 wt.% NaOH. The light source and irradiative process were the same as described above. During the course of irradiation, a gas sample f0.2-0.3 ml) was taken at fixed intervals with a syringe from the head space of the Bask for HZ and O2 analysis by gas chromatography (type 100 gas chromatograph, Shanghai Analytical Instrument Plant, China). The packing material was 5 10 molecular sieve and the carrier gas was Ar. After irradiation the dispersion was filtered. The filtrate was reacted with saturated BaCI, to detect the presence of SOd2- and its UV-visible absorption spectrum was measured with a UV-240 spectrophotometer {Shi-
madzu, Japan). XPS analysis of the photodeposit on the Pt/CdS surface was obtained using a PHI550 multifunctional spectrometer (P-E Co., USA); spectral deconvolution was achieved by a computer. 2.3. Reagents The specification of CdS is given in ref. 2. H2PtC16 and K2PtCls were analytically pure reagents obtained in China, 3. Results and discussion
Figure 2 shows the Xp spectra of photodeposited platinum on the treated PtlCdS surface for different conditions: before heating (A) and after heating at 200 “C (B) and 600 “C (C) for 30 min. The results indicate that the catalyst surface is covered with a layer of deposit. The de~onvolutio~ of the spectrum in Fig. Z(A) shows the existence of two com~unds: one in which Pt,, is 72.7 eV, i.e. Pt(OI& [a], and one in which Ptqfln is 74.8 eV which should be a IY’compound. When the sample is heated in an ultrahigh vacuum chamber, the Pt’“:Pt’“:Pt” peak area ratio decreases, as shown in Fig. 2 and Table 1. This indicates that the PtKV compound is thermally unstable and decomposes to I??. Ph,,*
C (6OO’Cl
Pt(OH),
b
I
77
,
I
75
I
I
73
,
I
71
,
1
69
Eb feV)
Fig. 1. XP spectra of photodeposited platinum on raw CdS: (a) untreated, (b) air-treated (approximately 510 “C!).
Eb (ev)
Fig. 2. XP spectra of photodeposited platinum on PtiCdS (treated): (A) before heating; (B) after beating at 200 “C for 30 min; (C) after heating at 600 “C for 30 min.
Z. Jin et al. I PtOJ formation druing photoinduced conversion
TABLE 1. XPS analysis of platinized Cd‘? (pf-f 13; [HPCI,] M, catalyst in solution) 5 mg ml-‘)
Room
= lo-’
zoo “Cb
600 Y?
temperature PtW.PtT’-PtO . . peak area ratio
1.94:1:0
0.68:1:0.12
o:1:2.9sc
CUPt atomic ratio
1.0
1.1
0.1
“Free Ci- washed off. bHeating in ultrahigh vacuum chamber in situ for 30 min. ‘PtAoi2 of Pt” changed (72.7-72.3 eV).
pt,w2
’ 75.5
pt4t
I
,cr
79
El
75
77
73
71
EbleV)
Fig. 3. XP spectrum CdS). TABLE PtO
2. Ptdni2 binding PiS
71.0 71.2
70.9 70.7 71.1 71.0
of K,PtCI,
energy
(calibrated
(eV)
WOW2
Pto
Pto,
72.4
72.2 72.5
74.4 74.6
72.4
with the S,, line of
K,PtC&
[61
72.2 74.2
72.4
72.7
72.3
74.8
Reference
75.7 75.5
171 181 191 1101 This work
Figure 3 shows the XP spectrum of solid K,PtCl,. Pt,f7/2 (75.5 eV) is about 0.7 eV higher than that of the PttV compound shown in Fig. 2. The Cl/ Pt atomic ratio of 6.8 (theoretical value, 6.0) is much higher than the Cl/R atomic ratio given in Table 1. This indicates that the PtIV compound cannot be the unreacted PtCL*,‘. The value of 74.8 eV is close to the Pt,, binding energy of PtOz reported in the literature [6-lo] (Table 2). The fact that small amounts of Clstill exist on the washed sample surface (see Table 1, Cl/R atomic ratio= 1) may result from ion exchange between OH- on the air-treated CdS surface and Cl- in the solution during irradiation Cd-OH-
+Cl-
=Cd-Cl-
+OH-
(3)
of H,PtC&
179
In the ultrahigh vacuum chamber, there was no change in surface Cl- concentration after the sample was heated at 200 “C. When the sample was heated at 600 “C, chlorine was liberated. Based on these results, it is concluded that the Pt’” compound shown in Fig. 2(A) is PtO,. 3.2. Factors affecting the formation of PtO, In order to ascertain the factors affecting the formation of PtO,, we designed several experiments and compared the results with those given in Table 1, as shown in Table 3. In experiment 2, the pH of the dispersion was kept at 13, and the initial [H,PtCl,] value was 7.5~ lop4 M rather than lo-’ M used in experiment 1. After irradiation, the deposit on the catalyst surface was Pt(OH), (Fig. 4(a)). In experiment 3, the pH value was 2.2 (not adjusted with NaOH) and the initial [H,PtCl,] value was lo-’ M. After irradiation, the deposit on the catalyst surface was Pt” (Fig. 4(b)). In experiment 4, CdS (treated) was used instead of Pt/CdS (treated) with the other conditions identical to those used in experiment 1. The deposit was the same as that in experiment 1 (Fig. 4(c)). From the results of these experiments, it is clear that the main factors affecting the formation of PtO, include the pH value of the dispersion and the initial H,PtCl, concentration. The results shown in Table 3 also indicate the absence of Hz generation during illumination of the dispersion. In gas chromatographic analysis, it was difficult to avoid the interference of air during sampling, and so a firm conclusion on whether or not gaseous O2 is produced cannot be obtained. However, a milky BaSO, precipitate was formed when the filtrates of experiments 1, 2 and 4 were treated with BaCl, solution. Obviously, secondary photocorrosion took place in experiments 1, 2 and 4 O,+e-
-
02-,
-
Cd*+ + SOd2-
(or OH- +h+ -
20,-+CdS+2h‘ (4)
‘OH, 4OH+CdS+4h+ Cd*+ +SO,‘-
+4H+)
In experiment 3, in which the pH value of the dispersion was 2.2, no SO,‘- was detected. This indicates that 0, was not produced during the photoreaction. Since the surface deposit obtained was PtO, it can be deduced that there is no direct relation between O2 formation and PtCb*- reduction to Pt”. Thus it seems that the mechanisms expressed by eqns. (1) and (2) are incorrect.
2. Jk et aI. / FtO, formation
180 TABLE
3. Comparison
Experiment
of experiments
during photoinduced conversion of H2PtC16
(visible light; illumination
Catalyst
time, 5 h)
Initial [H,PtCl,lb (IO-’ M)
PH”
Substance
detected
Gas HZ
01
Solution
Surface deposit
1
(PtKdS) (treated)
13
1.0
NO
?
so,z-
P%, PtK’H),
2
PtKdS (treated)
13
0.075
No
?
sod=-
PtKJH),
3
Pt/CdS {treated)
2.2
1.0
No
?
NO
PtO
4
CdS (treated)
13
1.0
No
?
SO,‘_
PtOz, Pt(OH)z
*No apparent
change
after illumination.
bVolume of solution,
36 ml.
solution, 20% of PtCl,*- is hydroIysed to give Pt(OH)Cl,‘-; in 0.01 N HCl solution to neutral medium, Pt(OH),C1,2m will form in addition to Pt(OH)Cl,‘-; in neutral to basic solution (0.1 N NaOH or greater), the final hydrolytic product will contain Pt(OH),Cl’and Pt(OH),2-. The hydrolysis equilibrium wilt be accelerated by W illumination. The examination of the absorption spectra of PtCl,‘- aqueous solutions confirms this hydrolysis mechanism reported by Cox et al. [ll] and Blasius et al. [12]. The UV-visible absorption spectra are shown in Figs. 5 and 6(a). The experimental conditions were pH 2.3, [H,PtC&] = 7.5X 10d4 M and pH 13, [H,PtCl,] =7.5 X 10e4 M respectively. Octahedral PtClh2- has four absorption bands [ll, 13, 141 (Table 4). Compared with the data in Table 4, the 445, 360 and 255-260 0.25
Eb feV)
225 255
Fig. 4. Photodeposit on Pt/CdS (treated) surface under different photoreaction conditions: (a) pH 13, initial [HIPtCl,] =7.5 X 10e4 M; (b) pH 2.2, initial [HZPtC16]=10~’ M, (c) pH 13, initial [H,PtCl,] = lo-’ M, PtiCdS (treated) replaced by CdS (treated).
3.3. Hydrolysis of PtC162-
In aqueous solution, occur [ll, 121 PtCI,‘-
+OH-
Pt(OH)C&-
hydrolysis
=Pt(OH)C&+ OH-
+ Cl-
= Pt(OH),ClA2-
360
of PtC16*- will
0.125
(5) + Cl-
(6)
Pt(OH),Cl,=-
+ OH- =Pt(OH)&-
+ Cl-
(7)
Pt(OH)3C132-
+ OH- = Pt(OH),Cl,*-
-I-Cl-
(8)
The hydrolysis begins when the concentration of HCI is between 3.0 and 0.1 N. In 0.05 N HCl
445
;\‘-; 0
300
400
:
Wovslengt h (nm) Fig. 5. UV-visible (pH 2.3).
absorption
spectrum
of 7.5X 10m4 M HQtCld
Z. Jinet al. / PtO, formationduring
photoinduced
conversion of H,PtCl,
181
However, when the initial H,PtCl, concentration is as low as 7.5 x 10 -4 M, all the absorption peaks disappear. With a low initial H,PtCl, concentration, PtClb2 - and its hydrolysates are converted into a solid deposit.
I
i
\
I
‘\
Wavelenglh
fnm)
Fig. 6. UV-visible absorption spectra of 7.5x10-’ M H,PtCl, (pH 13): (a) after 3 h; (b) kept in the dark for 1 month.
3.4. Mechanism of pt02 formation From the results obtained in this study, it can be concluded that the photocatalytic conversion of PtCl,‘in aqueous solution involves the conversion of its hydrolysates. In aqueous solution at pH 13, different hyincluding drolysates exist, Pt(OH)XC16_,2(x= l-6). When the initial [H,PtC&] value is low, the solid product detected after photocatalytic conversion is mainly Pt(OH), (Table 3, experiment 2), indicating that Pt(OH),C&receives two photogenerated electrons to form Pt(OH)2 + 2e- -
Pt(OH),Cl,‘nm bands of our two aqueous samples, which represent ‘A,, -+ ‘Tlg, ‘Aig -+ IT,, and ‘A,, * b’T,, transitions respectiveIy, are slightly shifted. After keeping the sample of Fig. 6(a) in the dark for 1 month, these three peaks (460, 360 and 260 nm) disappear, but the peak around 210 nm remains (Fig. 6(b)). This suggests that the peak around 210 nm belongs to the hydrolysate. From the results given in Table 4, this peak probably represents the transition of Pt(OH),C16-,2Lm+M charge transfer. The correlation between the variation of the transition wavelength within certain limits (210-225 nm) and the degree of PtCl,‘- hydrolysis has not been studied. This is due to the fact that it is very difficult to identify accurately the different hydrolytic products. If the La+ M charge transfer transition of PtCl,2- in Table 4 is slightly blue shifted, the corresponding wavelength will be less than 200 nm, which is out of the scanning range (200-500 run) and cannot be observed. The hydrolysis of PtCl,‘- was also studied by measuring the UV-visible spectra of the filtrate after illumination (Fig. 7). If the initial concentration of H2PtC16 is lo-’ M, an absorption peak at about 210 nm can be observed even if the filtrate is diluted 20 times, indicating the existence of a large amount of hydrolysates Pt(OH)XCl,_,2-. TABLE
4. Excitation
energy (nm) of octahedral
l&--t~~g [Ill 450
aI~aceto&ile,
365 approximately
300 K.
(9)
Pt(OH)2C142-
+ 4h+ Pt02+2H+
+4Cll
(10)
can be eliminated. When the initial [H,PtCl,] value is increased by one order of magnitude, the concentrations of the different hydrolysates are increased and PtO, is deposited together with Pt(OH), (Table 3, experiments 1 and 4). This means that the reaction shown in eqn. (11) takes place where Pt(OH),ClZ2receives four hf in sequence to give PtO, Pt(OH),Cl,‘-
+ 4h+ PtO,+4H+
+0,+2Cl-
(11)
In this way, the relationship between PtO, formation and 0, generation can be explained. As shown in experiment 3 of Table 3, at pH 2.2 and an initial [H,PtCl,] value of lo-’ M, PtC1b2 - hydrolysis may stop at steps (5) or (6) and the hydrolysate is acidic, e.g. Pt(OH)C1,2+H+ Pt(OHZ)C1,-. It is possible that, on the catalyst surface, such a hydrolysate can receive
PtCl,‘-
Singlet ligand-field
band
+ 4ClI
Meanwhile, the photogenerated hole can oxidize OH- to ‘OH, which reacts further with CdS to yield SOa-. Thus the formation of PtO, via
‘Al8--t‘TI, [l 11
Triplet ligand-field
Pt(OH),
*A,g--*C’TI” [13] Lrr+M
band 269.5”
202.0~
Z. Jin et al. f PtO, formation duringphotoinduced conversion of H,PtCI,
182
.25
3.9
photocatalytic RuO,)/CdS.
reaction
of water on Pt” (or Rhz03,
Acknowledgments 8 b p 1.9
.I25
H e
This work was supported by the Natural Science Foundation of Gansu Province The authors express their gratitude to Professor Linghui Tong for help with the measurement of W-visible absorption spectra. References
1.00
0.0 2 Wovelengfh
Cnml
Fig. 7. UV-visible spectra of filtrates of a Pt/CdS (treated) dispersion (illuminated for 5 h): (a) initial [H,PtCl,] = lo-’ M; (b) initial [H,PtCI,] = 7.5 x 10e4 M.
four photogenerated electrons in sequence resulting in the reduction of PtrV to Pt’, and the photogenerated holes on the CdS surface can react with SH- to give So and H*. Therefore no is formed (CdS+H’+Cdz++SH--; sod2 SH- +Zh+ +S’+H+).
1 2. Jin, Q. Li, L. Feng and Z. Chen, 3. MoI. CataL, 50 (1989) 315. 2 Q. Li, Z. Cben, X. Zheng and Z. Jin, J. Phys. Chem., 96 (1992) 5959. 3 A. Mills and C. Williams, 1 Cfiem Sot., Famday Trans. I, 85 (1989) 503. 4 I. Lauermann, D. Meissner and R. Memming, /. Electroonal. Chem., 228 (1987) 45. 5 N.M. Dimitrijevic, S. Li and M. Gratzel, /. Am Chem. Sot., 106 (1984) 6565. 6 J.S. Hammond and N. Winograd, L Eiechaamzl. Chem., 78 (1977) 5.5. 7 G.C. Allen and P.M. Tucker,
8 9 IO
Electroanal. Chem. Interfacial Elecrrochem., 50 (1974) 335. T. Wang, A. Vazguez, A. Kato and L.D. Schmidt, L Calal., 78 (1982) 3%. KS. Kim, N. Winograd and R.E. Dairs, I. Am. Chem. Sot., 93 (1971) 6296. M. Koudelka, J. Sanchez and J. Augustynski, J. Phys. Chem., 86 (1982) 4277. L. Cox, D.C. Peters and E.L. Wehty, /. Irwq. Nucl. Chem.,
4. Conclusions
11
PtO, is the photocatalytic oxidation product of the hydrolysate of PtCl,‘- (Pt(OH),Cl,‘-) on Pt/ CdS (treated). 0, is the associated product of this reaction and is not produced through the direct
12 E. Blasius, W. Preetz and R. Schmilt, 1. Jnorg. Nucl. Chem., I9 (1961) 115. 13 D.L. Swihart and W.R. Mason, Inorg. Cfiem., 9 (1970) 1749. 14 C.M. Davidson and R.F. Jameson. Trans. Faraday Sot., 61 (1965) . , 2462.
34 (1972) 297.