The Journal of
Physical Chemistry ~~
~~~
VOLUME 97, NUMBER 3, JANUARY 21,1993
0 Copyright 1993 by the American Chemical Society
LETTERS Photocatalytic Decomposition of Water and Photocatalytic Reduction of Carbon Dioxide over ZrOz Catalyst K. Sayama and H. Arakawa’ National Chemical Laboratory for Industry, Higashi 1-1, Tsukuba, Ibaraki, 305, Japan Received: August 31, 1992; In Final Form: November 9, I992
It was found for the first time that the photocatalytic decomposition of pure water proceeded over ZrO2 powder without any loaded metals under UV irradiation. The rate of H2 and 0 2 evolution increased upon addition of Na2CO3 and NaHCO3. Moreover, the evolution of CO (the photocatalytic reduction product of COz) was observed from NaHCO3 solutions. The special characteristics of Zr02 semiconductor are believed to be associated with its highly negative flat-band potential and wide bandgap. In the case of Cu(1 wt 96)-ZrO2 catalyst suspended in NaHCO3 aqueous solution, the rates of gas evolutions were 19.5 pmol/h of H2, 10.8 pmollh of 0 2 , and 2.5 pmollh of CO. These mass balances were indicative of a stoichiometric and catalytic reaction.
Introduction
small compared to the stoichiometric ratio, and the production rates decreased immediately with irradiation time. In the present study, the evolution of 0 2 and the steady-state reaction for long periods were evident during the experiment. Here, we would like to report the successful photocatalytic decomposition of water and the photocatalytic reduction of COz using ZrO2 semiconductor which has a wide bandgap and a highly negative flat-band potential.
As a model reaction of the artificial photosynthesis, photodecomposition of water (eq 1) and photoreduction of C02 (for example, eq 2) using semiconductorpowders loaded with several kinds of metals have been widely studied. The AG values of both reactions are highly positive, and they hardly proceed at ambient temperature.
- + - + +
2H,O
+
C02 H 2 0
0,
2H2
CO
H,
AG = 56.7 kcal/mol
(1)
0, AG = 59.1 kcal/mol (2)
In the case of the decomposition of water, successful photocatalysts using semiconductors such as TiO2,’” SrTi03,5-7 kNbt,Ol7,8 Na2Ti6013,9 and BaTi4091° were reported. These photocatalysts consist of semiconductor and loaded materials which are metals and metal oxides-R, NiO,, and RuO2, for example. The stoichiometricevolution of H2 and 0 2 was observed in these catalyst systems. On the other hand, the photoreduction of C02 was rep0rt~dll-l~ Over several semiconductors such as Sic, Gap, CdS, ZnO, TiO2, and SrTiO3. Methanol, HCHO, and HCOOH were detected as the reduction products from C02 in these catalyst systems. In most of the papers, however, the evolution of 0 2 was not detected or the amount of 0 2 was negligibly To whom correspondence should be addressed.
0022-3654/58/2097-053 1$04.00/0
ExperiwsW Section
The photocatalytic reaction was performed using a closed gascirculating system with an inner irradiation quartz reactor equipped with 400-Whigh-pressure Hg lamp (Riko Kagaku). Semiconductor powders of ZrOz and Ti02 were supplied from Soekawa Chemicals (99.9%) and Nihon Aerosil (P-25,anatase), respectively. Precursorsof noble metal were chlorides, and those of other metals were nitrates. Catalysts loaded with Pt or Au were prepared by an in situ photochemical deposition method. A reaction mixture was prepared by the introduction of semiconductor powder and precursor of noble metal (and a certainamount of bicarbonate in the case of C02 reduction) into distilled water (350 mL) in a quartz reactor. Then, they were mixed well and deaerated. Argon gas was introduced up to 4.6 kPa into the circulatingsystem, and then the first run was started by irradiation. After several hours of irradiation, the evolved gases in the first run were pumped away and argon was introduced again, and (b
1993 American Chemical Society
Letters
532 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
time I h Fipw 1. Time course of H2,02, and CO evolution over Zr02: Hz evolution (O), 0 2 evolution (0),CO evolution (A). After 20- and 454 irradiation, the evolved gases were pumped away. Catalyst, 1 g; water, 350 mL; an inner irradiation type quartz cell; high-pressure Hg lamp (400W).
-8
3 %
ki
1200 1OOo-
B3
800-
b C 8
400-
8
0
2
600-
4
200 100
50
0
150
time I h Figure 2. Time course of H2, 02, and CO evolution over Cu(1 wt %)-ZrOz: H2 evolution (O), 0 2 evolution (0),CO evolution (A), After 100-h irradiation, the evolved gases were pumped away. Catalyst, 1 g; water, 350 mL; an inner irradiation type quartz cell; high-pressure Hg lamp (400
W). then the next run was started. Catalysts loaded with Cu or Ru02 were prepared by the impregnation method. Catalyst loaded with Cu was reduced by H2 at 723 K for 3 h. RuOz loaded catalystwasoxiddbyO2at 773Kfor3 h. Therateofevolutions of H2,02, and CO were measured by anon-line gas chromatograph (TCD, molecular sieve 5A, Ar carrier gas) with an error of about 2%. The production of HCOOH and CH3OH was analyzed by ion chromatography and gas chromatgraphy (FID).
R d Q d Mscussion All semiconductors reported so far for the photocatalytic decomposition of water need loaded metals, such as Pt, Rh, NiO,, RuO2 etc.; it was reported that the semiconductor itself without loaded metal could not produce catalytic decomposition of water.'-10 The loaded metals seem to work for the acceleration of chargeseparation, the formation of reaction sites, the decrease J ' we report for the first of overpotential, and so O ~ . ~ ~However, time that thecatalyticdecompositionofwaterproceedsoverZrO2 powder without any loaded metals, as shown in Table I. The stoichiometric and stable evolutions of H2 and 0 2 were observed over ZrO2 catalyst suspended in pure water. It is very significant that the water splittingproceedswithout any loaded metal, because problems with a back-reaction (2H2 + 0 2 2H20) in the case of Pt-loaded catalyst, and deactivation in case of Ni-loaded catalyst, are possible to avoid. Also, it is useful system for understanding the reaction mechanism through a comparison with other photocatalysts. When Na2CO3 was added to ZrOz suspension, the rates of H2 and 0 2 evolution were doubled. There was no CO evolution. In the case of addition of NaHCO3 or KHCO,, the gas evolution rates increasedmore when compared with the addition of Na2CO3, and theratioof H2and02wasslightlylargerthantwo. Moreover, the evolution of CO was detected. Figure 1 shows the time course of H2, 02,and CO evolution over ZrOz powder suspended in NaHCO3 aqueous solution. These evolution rates were stable
-
TABLE I: wOtoaeeOmposition of Water rad Photoredoetioa of COZover V8dops Z a rad Ti02 CafdysQ. rate of gas amount/ evolution/Gol ti1 photocatalyst additive mol H2 0 2 CO pH none none 0 0 0 7.9 72 36 0 7.9 none zro2 0.38 142 75 0 Na2CO3 11.1 0.12 285 145 1.1 8.3 NaHCO3 0.33 309 167 3.0 8.3 KHCO, 0.33 231 119 1.8 8.4 1.00 294 157 2.0 8.4 R(0.l wt %)-Zr02 none tr 0 0 7.9 NaHCO3 0.33 120 61 0 8.3 Ru02( 1 wt %)-Zr0~ none 11 5 0 7.9 0.33 103 54 0.9 8.3 NaHCO3 Au(O.1 wt %)-ZrO2 none 17 6 0 7.9 NaHCO3 0.33 310 166 tr 8.3 Cu( 1 wt %)-ZrOz none 14 6 0 7.9 NaHCO, 0.33 19 11 2.5 8.3 Ti02 none tr 0 0 7.9 NaHCO3 0.33 tr 0 0 8.3 Pt(0.3 wt %)-TiOzb none tr 0 0 7.9
.
NaHCO3
0.33
4
2
0
8.3
Catalysts ZrO2 (1.0 g) and Ti02 (0.3 g); water, 350 mL; an inner irradiation type quartz cell; high-pressure Hg lamp (400W). See ref a
4.
for more than 70 h. In this reaction, both the decomposition of water (eq 1) and the reduction of C02 (eq 2) proceeded at the same time, and the rate of water decomposition was mom than 100 times faster than that of C02 reduction. The production of HCOOH, CH4, and CH3OH could not be observed. The difference of evolution rate between CO and H2 was too large to agree on the mass balance with the error of 2%. Why did the rates of H2 and 0 2 evolution increase by the addition of Na2C03, NaHCOp, and KHCOp? In a previous paper,4 it was reported first that the addition of carbonate into
Letters
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 533
a Pt-Ti02 suspension led to the efficient photocatalytic d e " positionof water;thiseffect was associated withC032-and HC03ions and not with cations or pH. It is considered that the surface of the semiconductor powder is covered with several kinds of carbonate species, and they seem to help with the desorption of 0 2 from surface of catalyst. However, it is not understood how C03z-and HC03-promote gas evolution. In caseof ZrO2 catalyst, it is suggested that the increase of the activity was mainly associated with the C032- and HC03- ions just as the case of Pt-TiO2. The evolution of CO could be observed only in case of using NaHC03and KHCO3; there was no CO evolution using Na2C03. The rate of CO evolution was larger with higher concentrations of HC03-. There seems to be the following equilibrium in the reaction solution:
+
+
2HCO; C02 H20 C0,2(3) It is considered that the COZ molecule evolved from HC03solution was reduced into CO on the ZrO2 catalyst. A similar mechanism is considered in the field of the electrochemistry.25-2'j The behavior of ZrOz catalyst is very different from that of Ti02. Ti02 photocatalysts cannot reduce C02, and the Ti02 catalyst without loadedmetalcoddnot decomposewater asshown in Table I. The differencesbetween the Z r 0 2catalyst and Ti02 catalyst without loaded metal could be explained on the basis of overvoltage of H2 evolution. Special characteristics of Z r 0 2 semiconductor are associated with the highly negative flat-band potential ( E b -1 .O eV NHE, pH = 0) and the wide bandgap (Eg 5.0 eV)I8compared to Ti02(& +0.05 eV NHE, pH = 0, E, 3.0 eV).I9 The lowest potential in the conduction band is estimated to be the same as the flat-band potential, and the highest potentials in the valence band of ZrO2 and Ti02 are estimated at ca. +4.0 and +2.95 eV, respectively (NHE, pH = 0). The reduction potential of e- in the Ti02 conduction band is almost same as the potential of H2/H20 (E = 0 eV NHE, pH = 0) and is more positive than the potential of CO/C02 (E = -0.1 1 eV NHE, pH = 0). In contrast, the reduction potential of e- in the ZrO2 conduction band was more negative than the potentials of H2/ H20 and CO/C02, and the oxidation potential of h+ in the Zr02 valence band was more positive than the potential of 0 2 / H 2 0(E = +1.23 eV NHE, pH = 0). Next, the Z r 0 2 catalysts loaded with several kinds of metals were studied. In the case of Ti02 and SrTiO3catalysts the activity of the photocatalyst was improved by loading metals such as platinum and ruthenium oxide. According to electrochemical studiesusing severalelectrodes,22,23selectivityof reduced products from NaHCO3 aqueous solution varied with electrode materials; only H2 evolved from the Pt electrode, CO evolved from the Au electrode, and CH4 evolved from the Cu electrode. Therefore, it is expected that the rate of gas evolution would increase and the selectivity of reduced products from C02 and H 2 0 would vary with loaded metals. However, the activities of the ZrO2 catalysts in pure water decreased by loading metal as shown in Table I. The reason why the activity of ZrOz was not improved by loading metal might be explained in terms of the barrier height of the semiconductor-metal junction. The barrier of ZrOrmetal junction isvery highcompared with that of other semiconductormetal such as TiO2-1netal,~~ and it is presumed that the e- in the ZrO2 can thus scarcely migrate to metal. In the case of the ZrO2 catalyst loaded with Pt, reaction in pure water could not proceed. Then, the addition of NaHC03 intoa Pt-ZrO2 suspension led to the photocatalyticdecomposition of water, with behavior similar to the Pt-Ti02 catalyst.* The rate of HZ and 0 2 evolution over Z r 0 2 catalyst decreased by loading platinum, and the CO evolution was prohibited. This might be because of the back-reaction of eqs 1 and 2 on the platinum site. On the other hand, the addition of carbonate led to the efficient decomposition of water even in the presence of
platinum. This phenomenon is very interesting; however, the mechanism is not clear at this moment. It is now under investigation. As for the RuOrZrO2 catalyst in pure water, the rate of gas evolution decreased by loading metal. In the case of Cu-ZrO2 catalyst, the rate of H2 evolution was much suppressed, but CO evolution was not much changed. It was found that the selectivity of products could be varied by several kinds of loaded materials. Figure 2 shows a time course of gas evolution over Cu( 1 wt %)-Zr02 suspended in NaHC03 aqueous solution. This reaction was very stable after more than 150 h. The exact rates of gas evolutions were 19.5 f 0.3 pmol/h of H2, 10.8 f 0.2 pmol/h of 0 2 , and 2.5 f 0.1 pmol/h of CO. These mass balances were consistent with a stoichiometric and catalytic reaction. The production of CH4 which was expected over Cu loaded catalyst, could not be observed in anyexperiments. The reason why the activity decreased by loading metal such as RuO2 and Cu was different from the case of Pt-Zr02, because the back-reactionon RuO2 and Cu was very slow. It is presumed that these loaded metals might block a reaction site on Zr02. However, there is still much room for improvement in the preparation method.
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