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Remarks on “photocatalytic oxidation of water to hydrogen peroxide by irradiation of aqueous suspensions of TiO2”
J. Electroanal. Chem., 190 (1985) 279-281 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
279
Short communication REMARKS ON "PHOTOCATALYTIC OXIDATION OF WATER TO HYDROGEN PEROXIDE BY IRRADIATION OF AQUEOUS SUSPENSIONS OF TiO2"
VICENTE RIVES-ARNAU Departamento de Quimica lnorgimica, Facultad de Farmacia, Apartado 449, 37071-Salamanca (Spain) (Received 26th November 1984; in revised form 7th February 1985)
Muraki et al. [1] have recently proposed a mechanism to explain the generation of 02 and H202 during photocatalytic oxidation of water on TiO2 particles, suggesting that only H202 is formed initially, eqn. (1), while 02 evolution follows from H202 decomposition through two alternative reactions, eqns. (2) and (3): 2 h++ 2 0 H - ~
H20 2
(1)
2 h++ H202 + 2 OH---* 0 2 + 2 H 2 0
(2)
2 H202
(3)
--~ 0 2 +
2 H20
(eqns. (9), (11), and (12), respectively, in Muraki et al.'s paper). These authors state that conventional methods are usually unable to detect both H202 and 02 because the former is very easily decomposed into the latter. The aim of this communication is to point out that this mechanism also holds during 02 photodesorption from O2/H20/TiO 2 surfaces in gas-solid systems previously studied by us [2-11], in addition to the solution-solid system studied by these authors. Fully hydroxylated TiO2 (Degussa P-25) surfaces were obtained by room-temperature outgassing of TiO2 [2], leading to a surface having enough water to complete an "aqueous monolayer", ca. 50% of which is dissociated into acid and basic hydroxyl groups (Ha+ and OHb) , a situation close to that existing in aqueous suspensions of TiO2. Outgassing at 673 K leads to removal of molecular water and of most of the hydroxyl groups, and "basic" and "acid" surfaces (hereafter OH/TiO 2 and H/TiO2) are obtained by equilibrating the outgassed samples with aqueous solutions containing NaOH or HC1 at pHs 8.5 and 2.0 [3,4], i.e., above or below the pzc of the oxide. Oxygen photoadsorption is fast on the fully hydroxylated and on the OH/TiO 2 surfaces, and if sufficiently low initial oxygen pressures (ca. 1.5 N m -2) are used, a slow oxygen photodesorption, which persists for several hours and which follows a diffusion-controlled kinetic law, is observed [5]. With dehydroxylated (outgassed at temperatures above 470 K) and H/TiO 2 samples, only a slow photodesorption is 0022-0728/85/$03.30
© 1985 Elsevier Sequoia S.A.
280
observed, i.e., oxygen photodesorption is related to the presence of basic hydroxyl groups at the TiO 2 surface. Photodesorption of oxygen is greatly enhanced if H202 is preadsorbed on the TiO 2 surface [6], indicating that hydrogen peroxide should be formed before oxygen evolution, in agreement with the results reported by Muraki et al. [1] in solution-solid systems. The nature of the radical species formed during the process has been assessed by epr [6,7]; in the case of the hydroxylated TiO 2 surfaces (room-temperature outgassed and OH/TiO 2 samples), HO 2 radicals ( ( g ) = 2.0106) have been detected at 77 K, while for the dehydroxylated surfaces (high-temperature outgassed and H/TiO 2 samples), 0 2 , O~- and O 3- species have been detected. On increasing the temperature to 300 K, the signal corresponding to the HO 2 species vanishes, indicating that reaction (4) occurs readily at room temperature: HO~ + HO 2 -, H202 + 02
(4)
With these results, the mechanism represented by eqns. (5) to (10) has been proposed to explain oxygen photodesorption from OH-rich TiO 2 surfaces [5,8-10]: h +~ e - + OHm- ~ e - + OH" OH'+ O H ' ~ H202 H 202 + e - ~ OH'+ OHH202 +
OH'---, HO 2 + H 2 0
HO 2 + OH'--* H20 + 02 HO 2 + HO 2 --* H 202 + 02
(5) (6) (7) (8) (9) (10)
Following Fujihira et al.'s paper [1], conversion of solar energy into chemical energy would proceed more efficiently through eqn. (11) than through eqn. (12): 2hp
2 H20 ~ H 2 + H202 TiO 2
(11)
4hp
2 H20 ~ 2 H 2 + 02 TiO 2
(12)
However, it should be taken into account that further decomposition of H202 formed by reaction (11) would lead to removal of both electrons and holes according to reactions (13) and (14), thus decreasing the efficiency of the overall process: H 2 0 2 -I- h + H20 2 +
e-~ 2 OH-+ 2 h÷
2 h + ~ 2 H++ 0 2
(13) (14)
As stated by Muraki et al., the addition of noble metals to promote H 2 evolution simultaneously accelerates reaction (3) decomposing H202. Perhaps adsorption of fluoride ions on the TiO2 surface replacing hydroxyl groups would prevent the recombination of the charge carriers [3,11], and thus an increase in the efficiency of hydrogen evolution would be observed.
281 W i t h this, it m a y b e c o n c l u d e d t h a t a l t h o u g h eqn. (11) i m p l i e s a b e t t e r e f f i c i e n c y t h a n r e a c t i o n (12) (i.e., t h e c h e m i c a l e n e r g y s t o r e d in t h e p r o d u c t s o f r e a c t i o n (11) is l a r g e r b y ca. 50% t h a n t h a t in the p r o d u c t s o f r e a c t i o n (12), in b o t h cases p e r p h o t o n c o n s u m e d ) , t h e y i e l d o f t h e f o r m e r m a y b e d e c r e a s e d b y r e a c t i o n s (13) a n d (14) a n d so r e d u c t i o n o f H 2 0 2 b y t h e s e last r e a c t i o n s s h o u l d b e e l i m i n a t e d in o r d e r to m a x i m i z e t h e o v e r a l l yield. REFERENCES 1 H. Muraki, T. Saji, M. Fujihira and S. Aoyagui, J. Electroanal. Chem., 169 (1984) 319; in H. Tsubomura (Ed.), Abstracts of the Fifth International Conference on Photochemical Conversion and Storage of Solar Energy, Osaka, 1984, p. 138. 2 G. Munuera, V. Rives-Arnau and A. Saucedo, J. Chem. SOc. Faraday Trans. 1, 75 (1979) 736. 3 G. Munuera, A.R. GonzMez-Elipe, V. Rives-Arnau, A. Navio, P. Malet, J. Sofia, J.C. Conesa and J. Sanz in M. Che and G.C. Bond (Eds.), Studies in Surface Science and Catalysis, Vol. 21: Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 113. 4 G. Munuera, J.A. Navio and V. Rives-Arnau in J.S. Connolly (Ed.), Abstracts of the Third International Conference on Photochemical Conversion and Storage of Solar Energy, Boulder, 1980, p. 417. 5 G. Munuera, A.R. Gonzalez-Elipe, J. Sofia and J. Sanz, J. Chem. Soc. Faraday Trans. 1, 76 (1980) 1535. 6 A.R. GonzMez-Eiipe, G. Munuera and J. Sofia in M.D. Archer (Ed.), Abstracts of the Second International Conference on Photochemical Conversion and Storage of Solar Energy, Cambridge, 1978, p. 87. 7 A.R. Gonz/flez-Elipe, G. Munuera and J. Sofia, J. Chem. Soc. Faraday Trans. 1, 75 (1979) 748. 8 G. Munuera, J.A. Navio, V. Rives-Arnau, J. Sofia and A.R. Gonzb.lez-Elipe, Rev. Real Acad. Cienc. Exactas Fis. Nat. (Madrid, Spain), 76 (1982) 201. 9 G. Munuera, J.A. Navio, V. Rives-Arnau. J. Sofia, A.R, Gonzb.lez-Elipe and J. Sanz in M. Collares Pereira, A. Luque and S. Silverio (Eds.), First Iberian Cong. Solar Energy, Madrid, 1982, Vol. 1, p. III. 42. 10 G. Munuera, J.A. Navio, V. Rives-Arnau, J. Soria, A.R. Gonz~ez-Elipe and J. Sanz in M.A. Alario and L. Puebla (Eds.), Aspectos Recientes en la Quimica del Estado Solido, CSIC, Madrid, 1985, in press. 11 G. Munuera, J.A. Navio and V. Rives-Arnau in J. Rabani (Ed.), Fourth International Conference on Photochemical Conversion and Storage of Solar Energy, Jerusalem, 1982, p. 141.
279
Short communication REMARKS ON "PHOTOCATALYTIC OXIDATION OF WATER TO HYDROGEN PEROXIDE BY IRRADIATION OF AQUEOUS SUSPENSIONS OF TiO2"
VICENTE RIVES-ARNAU Departamento de Quimica lnorgimica, Facultad de Farmacia, Apartado 449, 37071-Salamanca (Spain) (Received 26th November 1984; in revised form 7th February 1985)
Muraki et al. [1] have recently proposed a mechanism to explain the generation of 02 and H202 during photocatalytic oxidation of water on TiO2 particles, suggesting that only H202 is formed initially, eqn. (1), while 02 evolution follows from H202 decomposition through two alternative reactions, eqns. (2) and (3): 2 h++ 2 0 H - ~
H20 2
(1)
2 h++ H202 + 2 OH---* 0 2 + 2 H 2 0
(2)
2 H202
(3)
--~ 0 2 +
2 H20
(eqns. (9), (11), and (12), respectively, in Muraki et al.'s paper). These authors state that conventional methods are usually unable to detect both H202 and 02 because the former is very easily decomposed into the latter. The aim of this communication is to point out that this mechanism also holds during 02 photodesorption from O2/H20/TiO 2 surfaces in gas-solid systems previously studied by us [2-11], in addition to the solution-solid system studied by these authors. Fully hydroxylated TiO2 (Degussa P-25) surfaces were obtained by room-temperature outgassing of TiO2 [2], leading to a surface having enough water to complete an "aqueous monolayer", ca. 50% of which is dissociated into acid and basic hydroxyl groups (Ha+ and OHb) , a situation close to that existing in aqueous suspensions of TiO2. Outgassing at 673 K leads to removal of molecular water and of most of the hydroxyl groups, and "basic" and "acid" surfaces (hereafter OH/TiO 2 and H/TiO2) are obtained by equilibrating the outgassed samples with aqueous solutions containing NaOH or HC1 at pHs 8.5 and 2.0 [3,4], i.e., above or below the pzc of the oxide. Oxygen photoadsorption is fast on the fully hydroxylated and on the OH/TiO 2 surfaces, and if sufficiently low initial oxygen pressures (ca. 1.5 N m -2) are used, a slow oxygen photodesorption, which persists for several hours and which follows a diffusion-controlled kinetic law, is observed [5]. With dehydroxylated (outgassed at temperatures above 470 K) and H/TiO 2 samples, only a slow photodesorption is 0022-0728/85/$03.30
© 1985 Elsevier Sequoia S.A.
280
observed, i.e., oxygen photodesorption is related to the presence of basic hydroxyl groups at the TiO 2 surface. Photodesorption of oxygen is greatly enhanced if H202 is preadsorbed on the TiO 2 surface [6], indicating that hydrogen peroxide should be formed before oxygen evolution, in agreement with the results reported by Muraki et al. [1] in solution-solid systems. The nature of the radical species formed during the process has been assessed by epr [6,7]; in the case of the hydroxylated TiO 2 surfaces (room-temperature outgassed and OH/TiO 2 samples), HO 2 radicals ( ( g ) = 2.0106) have been detected at 77 K, while for the dehydroxylated surfaces (high-temperature outgassed and H/TiO 2 samples), 0 2 , O~- and O 3- species have been detected. On increasing the temperature to 300 K, the signal corresponding to the HO 2 species vanishes, indicating that reaction (4) occurs readily at room temperature: HO~ + HO 2 -, H202 + 02
(4)
With these results, the mechanism represented by eqns. (5) to (10) has been proposed to explain oxygen photodesorption from OH-rich TiO 2 surfaces [5,8-10]: h +~ e - + OHm- ~ e - + OH" OH'+ O H ' ~ H202 H 202 + e - ~ OH'+ OHH202 +
OH'---, HO 2 + H 2 0
HO 2 + OH'--* H20 + 02 HO 2 + HO 2 --* H 202 + 02
(5) (6) (7) (8) (9) (10)
Following Fujihira et al.'s paper [1], conversion of solar energy into chemical energy would proceed more efficiently through eqn. (11) than through eqn. (12): 2hp
2 H20 ~ H 2 + H202 TiO 2
(11)
4hp
2 H20 ~ 2 H 2 + 02 TiO 2
(12)
However, it should be taken into account that further decomposition of H202 formed by reaction (11) would lead to removal of both electrons and holes according to reactions (13) and (14), thus decreasing the efficiency of the overall process: H 2 0 2 -I- h + H20 2 +
e-~ 2 OH-+ 2 h÷
2 h + ~ 2 H++ 0 2
(13) (14)
As stated by Muraki et al., the addition of noble metals to promote H 2 evolution simultaneously accelerates reaction (3) decomposing H202. Perhaps adsorption of fluoride ions on the TiO2 surface replacing hydroxyl groups would prevent the recombination of the charge carriers [3,11], and thus an increase in the efficiency of hydrogen evolution would be observed.
281 W i t h this, it m a y b e c o n c l u d e d t h a t a l t h o u g h eqn. (11) i m p l i e s a b e t t e r e f f i c i e n c y t h a n r e a c t i o n (12) (i.e., t h e c h e m i c a l e n e r g y s t o r e d in t h e p r o d u c t s o f r e a c t i o n (11) is l a r g e r b y ca. 50% t h a n t h a t in the p r o d u c t s o f r e a c t i o n (12), in b o t h cases p e r p h o t o n c o n s u m e d ) , t h e y i e l d o f t h e f o r m e r m a y b e d e c r e a s e d b y r e a c t i o n s (13) a n d (14) a n d so r e d u c t i o n o f H 2 0 2 b y t h e s e last r e a c t i o n s s h o u l d b e e l i m i n a t e d in o r d e r to m a x i m i z e t h e o v e r a l l yield. REFERENCES 1 H. Muraki, T. Saji, M. Fujihira and S. Aoyagui, J. Electroanal. Chem., 169 (1984) 319; in H. Tsubomura (Ed.), Abstracts of the Fifth International Conference on Photochemical Conversion and Storage of Solar Energy, Osaka, 1984, p. 138. 2 G. Munuera, V. Rives-Arnau and A. Saucedo, J. Chem. SOc. Faraday Trans. 1, 75 (1979) 736. 3 G. Munuera, A.R. GonzMez-Elipe, V. Rives-Arnau, A. Navio, P. Malet, J. Sofia, J.C. Conesa and J. Sanz in M. Che and G.C. Bond (Eds.), Studies in Surface Science and Catalysis, Vol. 21: Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 113. 4 G. Munuera, J.A. Navio and V. Rives-Arnau in J.S. Connolly (Ed.), Abstracts of the Third International Conference on Photochemical Conversion and Storage of Solar Energy, Boulder, 1980, p. 417. 5 G. Munuera, A.R. Gonzalez-Elipe, J. Sofia and J. Sanz, J. Chem. Soc. Faraday Trans. 1, 76 (1980) 1535. 6 A.R. GonzMez-Eiipe, G. Munuera and J. Sofia in M.D. Archer (Ed.), Abstracts of the Second International Conference on Photochemical Conversion and Storage of Solar Energy, Cambridge, 1978, p. 87. 7 A.R. Gonz/flez-Elipe, G. Munuera and J. Sofia, J. Chem. Soc. Faraday Trans. 1, 75 (1979) 748. 8 G. Munuera, J.A. Navio, V. Rives-Arnau, J. Sofia and A.R. Gonzb.lez-Elipe, Rev. Real Acad. Cienc. Exactas Fis. Nat. (Madrid, Spain), 76 (1982) 201. 9 G. Munuera, J.A. Navio, V. Rives-Arnau. J. Sofia, A.R, Gonzb.lez-Elipe and J. Sanz in M. Collares Pereira, A. Luque and S. Silverio (Eds.), First Iberian Cong. Solar Energy, Madrid, 1982, Vol. 1, p. III. 42. 10 G. Munuera, J.A. Navio, V. Rives-Arnau, J. Soria, A.R. Gonz~ez-Elipe and J. Sanz in M.A. Alario and L. Puebla (Eds.), Aspectos Recientes en la Quimica del Estado Solido, CSIC, Madrid, 1985, in press. 11 G. Munuera, J.A. Navio and V. Rives-Arnau in J. Rabani (Ed.), Fourth International Conference on Photochemical Conversion and Storage of Solar Energy, Jerusalem, 1982, p. 141.