1970
J. Phys. Chem. 1993,97, 1970-1973
Visible Light Induced Photocatalytic Behavior of a Layered Perovskite Type Niobate, RbPb2Nb3010 J. Yosbimura, Y. Ebina, J. Kondo, and K. Domen' Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori- ku, Yokohama 227, Japan
A. Tanaka Nikon Company, 1 773 Asamizodai, Sagamihara 228, Japan Received: December 7, 1992
Photocatalytic behaviors of some modified layered compounds derived from RbPb2Nb3O1o which was an ion exchanger with perovskite structure were examined. Under visible light irradiation (>420 nm) RbPb2Nb3010based catalysts were found to evolve H2 from an aqueous methanol solution. The rate of H2 evolution was greatly enhanced by replacing Rb+ ions with H+ ions. Further increase of the activity was obtained by Ptloading. When [Pt(NH3)4]Cl2 was used as a precursor for Pt-loading instead of HzPtC14, much higher activity was observed. It was interpreted that in the former case reduced Pt particles were highly dispersed at interlayer spaces due to the intercalation of [Pt(NH3)42+]ions prior to the photoreduction, while Pt particles existed only at the external surface of the catalyst in the latter case. The proposed structures for both catalysts were directly supported by TEM and XPS measurements. Several kinds of alcohols (ethanol, 1-propanol, and 1-butanol) were also used as sacrificial reagents. Among them methanol was by far the most suitable one for H2 evolution. This remarkable shape selectivity was attributed to the intercalation of reactants into the catalyst.
Introduction
Recently the present authors reported that some ion-exchangeable layered oxides showed a noticeable photocatalytic activity.' The family of the layered perovskite type niobates is one of the groups of those materials.' These compounds are generally formulated as A(M,,Nbn03,+l), (A = K, Rb, Cs; M = La, Ca, etc.).2-9 For example, KLaNb307 and CsCa2Nb3010 correspond to themembersof n = 2 and 3, respectively.I0 Alkaline metal ions at the interlayer space are possible to be exchanged by other cations. The H+-exchangedforms are known to be easily hydrated even under the atmospheric condition while the original ones are not. By the H+ replacement the rates of photocatalytic H2 evolution from an aqueous methanol solution over these niobates increased by ca. three orders of magnitude.' Since the band gaps of most of the niobates such as KLaNb207and KCa2Nb3Olo are wide (ca. 3.3-3.5 eV), no response under visiblelight irradiation was obtained. Among them, however, RbPb2Nb3010 has been found to show somephotocatalytic activityunder visibile light irradiation. The structure of RbPbzNb3Olois schematically depicted in Figure 1. The purpose of this work is to reveal the photocatalytic properties of RbPb2NbjOl0-based catalysts especially under the irradiation of visible light (>420 nm).
Experimental Section RbPb2Nb3Olo was prepared by calcination of the mixture of RbzCO3, PbO and Nb205 in air at 1273 K for 2 days with a grinding in between. About 10% excess Rb2C03 was added to compensate the loss due to the volatilization. After the heating the product was washed with distilled water and dried in air at room temperature. The structure was confirmed by X-ray diffraction pattern which coincided with that in the literature.9 The size of the powder was 1-10 r m in diameter. H+-exchange reaction was carried out in H N 0 3 ( 5 N) solution for 3 days at room temperat~re.~ Degree of H+-exchangewas almost loo%, which was determined by atomic absorption analysis of eluted K+ ion. Therefore, the H+-exchanged form of RbPb2Nb3010is To whom correspondence should be addressed. 0022-3654/93/2097-1970S04.00/0
NbO6 at b = O
C
0
Pb
at b =
0
Rb
1 at b = 2
t
k
a Figure 1. Schematic structure of RbPb2Nb~Olo.
referred to as HPbsNb3Olo. Photocatalytic reaction was carried out in a completely air-free closed gas circulation system made of Pyrex (250 mL). The catalyst ( 1g) was dispersedin an aqueous solution (300 mL) by magnetic stirring and was irradiated under Ar atmosphere (ca. 100 Torr) by Xe lamp (500 W) through a cutoff filter (>420 nm, UV-42 Toshiba). H2 evolution reaction was carried out in an aqueousmethanol (or other alcohols) solution (H20(250 mL) + CH30H (50 mL)), and 0 2 evolution reaction in an aqueous silver nitrate solution (0.01 M, 300 mL). The amount of evolved gas was determined by gas chromatography (MS-SA column, Ar carrier) sampler (10 mL) of which was directly connected to the closed gas circulation system to avoid any contamination from air. Ultraviolet-visible diffuse reflectancespectra(UV-DRS) were measured by Jasco UVDEC 505 spectrometer. XPS was measured by Shimadzu ESCA 750. TEM photograph was obtained by Hitachi HF-2000 (200 kV). Results md Discussion (i) UV-DRS. Ultraviolet-visible diffuse reflectance spectra (UV-DRS) of RbPb2Nb3010and KCa2Nb3010as a reference are shown in Figure 2. Both of them possess the same layered perovskite structure. While the absorption bandof KCatNb3010 was observed only in the UV region, RbPb2Nb3010 showed absorption band extended in visible light region; a stronger CQ 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1971
Photocatalytic Behavior of RbPb2Nb3010 1.o
1
"
200
400
600
'f
E
9 -
4
800
wavelength I nm FigureX Ultraviolet-visible diffuse reflectance spectra of RbPb2Nb3Olo and KCa2Nb3010:(solid line) KCa2NbjOlo;(dashedline) RbPb2Nb3Olo.
150
-
0
0
5
10
Ion-exchange time / day Figure 4. Rate of H2 evolution as a function of the treatment time of HPb2Nb3Olo in an aqueous solution containing a Pt precursor: (0)[Pt(NH3)4]C12; ( 0 )H2PtCi6; Pt(0.l wt %) fixed on the catalyst; 500-WXe lamp (>420 nm); catalyst 1 g; CH3OH (50 mL) + H2O (250 mL).
5 . 0
I"
- I
r
100
10 20 tlh Figure 3. Time courses of visible light induced H2 evolution from an aqueous methanol solution over several modified RbPbzNb3Olo catalysts: ( 0 )RbPb2Nb3010;(v)Pt(0.l wt %)/RbPb2Nb3010(H2PtC16); (0)HPb2NblOio; (A)Pt(0.l wt %)/HPb2Nb3Olo(H2PtC16); (0)Pt(0.l wt %)/HPb2Nb3Olo ([Pt(NH3)4]C12); 500-W Xe lamp (>420 nm); catalyst 1 g; CH3OH (50 mL) + H20 (250 mL).
0
absorption in wavelength region shorter than ca. 500 nm and a weaker broad absorption tail in longer wavelength region. The broad absorption tail may be due to the defects in niobate sheets because its absorbance changed when the sample was calcined at differenttemperatures. The assignment of the absorption band 420 nm) over several modifiedRbPb2Nb3OIocatalysts. RbPbzNb3010itself or Pt-loaded RbPbzNbjOlo showed very low activity for Hz evolution. However, the remarkable enhancement of the activity was observed when Rb+ ions were exchanged by H+ ions. As is mentioned above HPb2Nb3010is easily hydrated under the ambient condition, and the c-axis length corresponding to the interlayer spacing increases upon hydration.8 Actually, 0.1-A of increase of the interlayer spacing due to H+-exchange was observed. On the contrary, RbPb2Nb3010is not hydrated even in an aqueous solution. It is, therefore, suggested that the increase of the H2 evolution rate is due to the migration of reactants, i.e. HzO and CH3OH, into the interlayer space of HPb2Nb3010. Methanol was oxidized into
COS and no detectable amount of HCHO or HCOOH was observed in solution by gas chromatography. This suggests that once methanol begins to be oxidized at the interlayer space, it is not released from the catalyst until it is thoroughly oxidized to C02. The similar result was previously reported for this type of layered compounds under ultraviolet irradiation.1 (iii) Effects of Platinum h d ~ Further . increase of the activity was acquired when Pt was loaded on HPb2Nb3010.Two differentprecursors, H2PtCl6 and [Pt(NH3)4]C12,were employed for Pt-loading. In the former case an aqueous methanol solution containing H2PtC16and the catalyst was irradiated to deposit Pt.llIn thelattercase,atfirst [Pt(NH3)4]2+ion~weresubstituted for H+ ions of HPbzNb3010followed by irradiation in an aqueous methanol solution. In both cases, the yellowish color of the catalyststurned into light gray during the initialirradiationperiod of a few hours, which indicated the reduction of platinum. The catalysts used through this period showed the constant rate of H2 evolution as shown in Figure 3. The higher rate by about an order of magnitude of H2 evolution was observed when [Pt(NH&]C12 was used as a precursor than when H2PtC16was used. Since HPbzNb3Olo is a cation exchanger? it is supposed that anions of PtC162-are not able to intercalate into the interlayer space but are reduced and fixed only on the external surface of the catalyst during the Pt photodeposition period. On the other hand, [Pt(NH3w2+ions are possible to intercalate into the interlayer space by ion-exchange with H+ ions and to be reduced intoultrafinePt particles in theinterlayerspace. ThesePt particles in the interlayer space are expected to work as efficient catalytic sites for H2 evolution. To examine this, the following study was carried out. (iv) Effect of Treatment Time for Platinum Loading. In Figure 4 the rate of H2 evolution is presented as a function of the treatment time of HPb2Nb3010of 1 g in an aqueous solution containing a Pt precursor. HPb2Nb3010was immersed at 363 K with stirring in an aqueous H2PtC16or [Pt(NH3)4]C12solution containing 0.1 wt 9% of Pt. After the treatment, the solution was dried up on a water bath to keep the amount of Pt-loading constant. The catalyst was then photoreduced in an aqueous methanol solution as mentioned above. No effect of the treatment time on the rate of H2 evolution was observed when HzPtC16 was used as a precursor. This indicated that PtCls2-ions could not intercalate into the interlayer space and that Pt was photoreduced on the external surface of the catalyst. On the other hand, when [Pt(NHp)d]C12 was used as a precursor the activity increased with the increase of the ion-exchanging time. It took a week for the ion-exchange reaction to reach maximum rate (24 pmol/h) of H2 evolution. The quantum efficiency roughly estimated by using
1972 The Journal of Physical Chemistry, Vol. 97, No. 9, 1993
Yoshimura et al.
Pt 4d 5/2 314 eV
J.
Pt 4d 3/2 331 eV
A
1
"[
&
0 0 7
I
I
I
I
330
320 310 300 Binding Energy / eV Figure 5. XP spectra of Pt 4dsp (314 eV) and 4d3/2 (331 eV) on Pt(0.l wt %) /HPb2Nb30 10.
a cutoff filter was ca. 5% between 420 and 500 nm. This result was interpreted that [Pt(NH3)4I2+ions were slowly exchanged with H+ ions, and the intercalated [Pt(NH3)4I2+ions were presumably reduced in the interlayer space to form ultrafine Pt particles (v) XPS Measurement, To confirm the structure of both Ptloaded catalysts,XPS and TEM measurements were carried out. The amount of loaded Pt was fixed to 0.1 wt %. The pretreatment was performed for a week at 363 K in each solution,H2PtCl6 and [Pt(NH3)4]C12(see Figure 4). The samples were photoreduced and showed constantrates of H2 evolution in an aqueousmethanol solution under irradiation. Figure 5 shows XP spectra of Pt 4dsp (314 eV) and 4d3p (331 eV) of both catalysts. Spectra a and b of Figure 5 represent the sample prepared from H2PtC16and [Pt(NH3)4]C12, respectively. It was found that in Figure Sa Pt 4d peaks were clearly observed but not in Figure 5b. As XPS is surface sensitive, this result suggests that the amount of Pt located at the external surface of the catalyst is much larger for the sample prepared from H2PtC16than that from [Pt(NH3)4]c12. (vi) TEM Observation. The same samples used in Figure 5 were observed by TEM. Parts a and b of Figure 6 are corresponding to Figure 5a,b, respectively. In Figure 6a, many Pt particles of ca. 50-100 A in diameter were observed, while no metal particles were observed in Figure 6b. The existence of Pt at positions indicated as 1, 2, 3 in Figure 6a was confirmed by high resolution local analysis of EDX ( 20 A). No clear signal of Pt was observed at positions 4 in Figure 6a and 1,2 in Figure 6b. The results of XPS and TEM clearly support the structures of both catalystsproposed above. The similar result was reported in the case of Pt-loaded &Nb6017 prepared from [Pt(NH3)4]2+ precursor for an overallwater splitting.12 The differencein activity between two precursors are interpretedas follows. When Pt exists only on the external surface, photoexcited electrons in the bulk of the catalyst of 1-10 pm have to migrate to the Pt particles at the external surface in order to efficiently reduce H+ ions into H2. During the transfer of photoexcited electrons, however, some of them should be consumed by recombination with simultaneously photoexcited positive holes. If Pt particles are dispersed in the interlayer,the diffusion length of electrons become much shorter compared with the case where Pt exists only on the external surface. Therefore it is considered that many excited electrons could be transferred to Pt at interlayer space and reduce H+ into H2. (vii) Dependenceon Alcohols. The increaseof the H2 evolution rate due to the replacement of Rb+ by H+ ions suggested the migration mechanism of reactants into the interlayer space. To further examine this process, the rates of H2 evolution from several aqueous alcohol solutions over HPb2Nb3010 were compared in
.
-
Figure 6. TEM photographs of Pt(0.l wt %)/HPb2Nb3Olo: (a, top) prepared from H2PtC16; (b, bottom) prepared from [Pt(NH3)4]C12.
Table I. Pt was loaded by photodeposition of H2PtC16in this case. The drastic decrease of the H2evolution rate was observed with the increase of the alkyl chain length. This suggests that the interlayer space is used for the oxidation of alcohol whose rates are dependenton the amount of intercalationof each alcohol. (viii) Reaction Mechanism. From the results obtained above, catalyst preparation process and H2 evolution mechanisms over Pt/HPb2Nb3010are depicted in Figure 7. For H2 evolution, the niobate macro anion sheets absorbphotons (>420 nm) and excited electronsand holes are produced. Then, the intercalatedmethanol molecules are oxidized with holes at the interlayer space. When
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1973
Photocatalytic Behavior of RbPbzNb3010
-
TABLE I: Rates of Hi Evolution from Several Aqueous A l ~ ~ b SOIU~~OIIS ol (Fd*h-')' HPb2Nb30iob
E" .
HPb2NbjOlob
alcohol
alone
Pt-loaded'
methanol ethanol
0.6
3.2
0.1
0.5
alcohol 1-propanol 1-butanol
alone 0
0
3. 10.0
Pt-loadedc 0.2 0
I
I ' O /
Catalyst 1 g; 500-WXe lamp (>420 nm); alcohol (50 mL) + H20 (250mL). H+-exchangedegree 100%. Pt wasloaded from HlPtCldaq) by photodeposition method. The amount of loading was 0.1 wt % for all samples. PtClf i
"0
5
10
tlh Figure 8. Time course of 0 2 evolution over RbPb2NboOlo from aqueous silver nitrate solution under visibile light irradiation: 500-W Xe lamp (>420 nm); catalyst 1 g; 0.01 M AgNOj(aq) (300 mL).
(1) H+-exchanged RbPbzNb3010,i.e. HPbzNb3010,showed H2evolution activity under visible light irradiation (>420 nm). (2) Pt/HPbzNbsOlo prepared from [Pt(NH3)&12 exhibited much higher activity than that from H2PtCls for Hz evolution from an aqueous methanol solution, which was attributed to the catalyst structure containing Pt metal particles at the interlayer space. The structure was supported by XPS and TEM measurements. (3) Methanol worked as the most efficient sacrifical reagent among several alcohols for H2 evolution. (4) RbPb2Nb3010evolved 0 2 in an aqueous AgNO3 solution under visible light irradiation. (a) (b) Figure 7. Schematic mechanisms of H2 evolution over Pt/HPb2Nb~Olo: (a) Pt exists in the interlayer; (b) Pt exists only on the external surface.
Pt exists in the interlayerspace (b), photoexcited electronsreduce H+ on Pt at the interlayer to produce H2, while when Pt exists only on the external surface (including edges) (a), the electrons have to be transferred a long distance through a two-dimensional niobatesheet toreduce H+ontheextemal Pt. Thelargedifference of the H2 evolution rates are attributable to the different H2 evolution sites to which electrons have to migrate. (ix)OzEvdution. It isanother interesting point ofview whether this catalyst evolves 02 under visible light irradiation. The following water oxidation reaction using Ag+ ions as sacrifical reagent was examined. 40H- + 4Ag+
-
0,+ H 2 0 + 4Ag0
The time course of 02 evolution over RbPb2Nb3Om under irradiation of visible light (>420 nm) is shown in Figure 8. The reaction proceeded steadily from the beginning with a constant rate of 0 2 evolution (1.1 pmollh). When we used HPb2Nb3010, however, no 02 evolution was observed which might due to the ion-exchanged Ag+ cations at the interlayer space which worked as recombination centers of excited electrons and holes.
Summary 4 In this study, photocatalytic behaviors of RbPb2Nb3OIo-based catalystswere examined. Main results are summarized as follows:
Acknowledgment. This research was supported by a Grantin-Aid for Developmental Scientific Research (No. 03203 119) from the Ministry of Education, Science and Culture, Japan. The authors thank Hitachi Co. for help in obtaining the TEM micrographs. References and Notes (1) Domen, K.; Yoshimura, J.; Sekine, T.; Tanaka, A,; Onishi, T. Coral. Lerr. 1990, 4, 339. (2) Dion, M.;Ganne, M.;Tournoux, M. Mar. Res. Bull. 1981,16,1429. (3) Dion, M.; Ganne, M.; Tournoux, M. Reo. Chim. Min. 1984,21,92. (4) Jacobson, A. J.; Jhonson, J. W.;Lewandowski, J. T. Inorg. Chem. 1985, 24, 3127. ( 5 ) Dion, M.; Ganne, M.; Tournoux, M. Reu. Chim. M i n . 1986,23,61. (6) Jacobson, A. J.; Lewandowski, J. T.; Johnson,J. W.J . Less-Common Mer., 1986, 116, 131. ( 7 ) Gopalakrishnan, J.; Bhat, V.;Raveau, B.; Mar. Res. Bull 1987,22, 413. (E) Jacobson, A. J.; Lewandowski, J. T.; Jacobson, J. W . Mar. Res. Bull. 1990, 25, 619. (9) Subramanian, M. A,; Gopalakrishnan, J.; Sleight, A. W. Mar. Res. Bull. 1988, 23, 837. (10) The thickness of each perovskite layer is given by the value of n that
determines the number of Nb06 corner-shared octahedra that are connected along a direction perpendicular to the layers. The value of n is typically 2 or 3, but the thickness of the layer is possible to be increased upto 7. See ref 4.
(11) Krarutler, B.; Bard, A. J. J . Am. Chem. Soc. 1978, 100, 5985. (12) Sayama,S.;Tanaka,A.;Domen,K.; Maruya,K.;Onishi.T.;J.Phys. Chem. 1991, 95, 1345.
