J. Chem. SOC., Faraday Trans. 1, 1982, 78, 1065-1069 Temperature Dependence for the Reduction of Water to Hydrogen by Reduced Methyl Viologen on Platinum BY MILICA T. NENADOVIC,* OLGA I. M I ~ I ~ AND RADOSLAV R. A D ~ I C ~ Boris KidriE Institute of Nuclear Sciences, 11001 Belgrade, P.O. Box 522, Yugoslavia Received 9th April, 198 1 The electron-transfer reactions from methyl viologen monocation radicals to colloidal platinum hydrosols were studied using the pulse-radiolysis technique. Besides the reduction of water, hydrogenation of methyl viologen on platinum also takes place in these reactions. Evolution of hydrogen is enhanced at higher temperatures (65 "C) and reduction of MV+ is hindered. The reaction shows complex behaviour in the presence of hydrogen because of the formation of reduction products which decrease the catalytic activity of platinum.The reaction rate decreases from 1.3 x lo3 s-' in an argon-saturated solution to 0.6 s-' in a hydrogen-saturated solution in the presence of 2.5 x mol dm-3 Pt hydrosols. The photoinduced reduction of methyl viologen (NN'-dimethyl-4,4'-bipyridine dication, MV2+) by a suitable sensitizer can be employed to achieve the reduction of water to hydrogen over redox catalysts.' The resulting one-electron acceptor couple MV2+/MV+ has a low potential E, = -0.44 V, sufficient to reduce water: MV+ + 2H20 + iH2 + MV2+ +OH-. cat However, it has been shown recently that MV2+ irreversibly decomposes to some extent during irradiation in the presence of catalyst2? due to a hydrogenation process on platinum, which leads to a completely reduced bipiperidine derivative :3 H* MV2+ (or MV+) __+ reduction product.Pt In this work pulse radiolysis has been used to study the kinetic behaviour of reaction (1). This reaction was observed both at room temperature and 65 OC in order to determine the optimum conditions for hydrogen evolution. EXPERIMENTAL All the reagents used were commercial products of highest purity available. Solutions were prepared with triply distilled water. Oxygen was removed by bubbling with argon. The pH was adjusted with H,SO,, NaOH or phosphate buffers. Colloidal platinum was prepared as follows :, 0.02 g hexachloroplatinic acid was dissolved in 142.5 cm3 distilled water and the solution brought to the boiling point. 7.5 cm3 of 1 % sodium citrate solution was then added and the solution boiled for 1 h, water being added to supply the quantity lost by evaporation.After approximately 15 min the yellow colour began to deepen and after 30 min turned to deep brown. The solution was then stirred with a ion-exchange resin to remove the citrate. The liquid contained small particles of colloidal platinum (average size t Permanent address : Institute of Electrochemistry, ICTM and Centre for Multidisciplinary Studies, University of Belgrade, Belgrade, Yugoslavia. 10651066 REDUCTION OF WATER BY REDUCED METHYL VIOLOGEN 3 nm, measured by electron microscopy). EDTA was determined by reduction with arsenious acid.5 For determination of citric acid, an improved pyridine-acetic anhydride method was used.6 For pulse radiolysis a Febetron 707 (Field Emission Corp.) electron accelerator was used and the operation conditions were similar to those in the previous work.’ The total light path through the cell was 5.1 cm.The absorbed doses were in the range of 10-1 5 Gy per pulse. The measurements were taken at 19 f 1 O C . The gas-chromatographic method was employed for hydrogen detection using a Perkin-Elmer 154D instrument with a column of silica gel at 50 OC. RESULTS AND DISCUSSION FORMATION OF MVf IONS The reduced form of methyl viologen was produced by the reaction of hydrated electrons with MV2+ during pulse radiolysis H,O +w+ eLq, H, OH, H,O,, H,, H30+, OH- (3) eiq + MV2+ + MV+ + H,O k = 8.4 x 1Olo dm3 mol-1 s-l [ref. (S)]. (4) The nature and absorption spectrum of stable radical MV+ (Amax = 605 nm; Esos = 8.6 x lo3 mol-l cm-l) have been established previously.*.EDTA (ethylenediamine tetra-acetic acid) ( lo-, mol dm-3) was added to the solution (lov4 mol dm-3 MV2+, pH 5, unbuffered) to scavenge OH and H radicals. In the radiolysis of amino acids it is known that the a-carbon atom to the -COOH group is prone to attack by OH or H radical leading to the abstraction of hydrogenlo ( 5 ) (6) OH + RR’NCH,COOH + H,O + RR’NCHCOOH H + RR’NCH,COOH + H, + RR’NCHCOOH k = 1.2 x lo9 dm3 mol-l s-l, pH 7 [ref. (1 l)] k = 6.5 x lo7 dm3 mol-l s-l, pH 1 [ref. (12)]. It is assumed that a similar set of radicals is formed during photoinduced oxidation of EDTA in the presence of dye.13 The pK values of these radicals are probably very close to those of EDTA (pK, = 10.23, pK, = 6.16, pK3 = 2.67, pK4 = 1.99) since the pKvalues of carboxylic radicals seem to be the same as those of the parent molecule^.^^ We have found that the radical formed in reactions (5) and (6) reduces MV2+ to MV+.The yield for MV+ formation in aqueous EDTA solution was found to be 6.0 [G(MV+) = G(~L~)+G(H)+G(OH)]. In an irradiated solution of MV2+ (5 x rnol dm-3) and EDTA (0.1 mol dmd3) saturated with N,O (2.5 x lo-, mol dm-3), formation of MV+ was observed MV2+ + RR’NCHCOOH + MV+ + RR’N=CHCOOH (7) and k, = 1.4 x lo9 dm3 mol-1 s-l at pH 5 was obtained. All rate constants are summarized in table 1. MV+ is not stable at concentrations > 5 x lop5 mol dm-3, implying over-reduction of MV+ to dihydrobipyridyl. This reaction is known to occur with powerful chemical agents such as zinc or sodium dithionite.15 REACTION WITH PLATINUM HYDROSOLS MV+ radicals reduce water on a Pt catalyst according eqn (1).The first step is an electron transfer to platinum MV+ + Pt -P MV2+ + Pt-. (8)M. T. NENADOVIC, 0. I. MICIC AND R. R. ADZIC 1067 TABLE 1 .-RATE CONSTANTS FOR ELECTRON-TRANSFER REACTIONS IN METHYL VIOLOGEN-PLATINUM HYDROSOL AQUEOUS SOLUTION AT pH 5 platinum concentration species present reaction /mol dm-3 in solution rate constant RRNCHCOOH + MV2+ --* MV+ + RR'N=CHCOOHa - MV+ + Pt -+ Pt- + MVz+ 1 x 10-5-1 x 10-4 2.5 x 2.5 x MV++ Pb2+ -+ MVz+ + Pb+ e&+Pt -+ Pt-+H,O - 1 x 10-6-5 x 2.5 x - Ar (0.6-6.5) x lo3 s-I 1.5 x los dm3 mol-I s-l 0.6 s-l 8 x lod4 rnol dm-3 H, 5 x mol dm-s PbZ+ 1 x 10+ mol Pbz+, pH 5, Ar 8 x 104 mol dm-3 H, + 8 x mol dm-3 H, 3 x 103 s-1 < 10 dm3 mol-l s-' 7 x lo* dm3 mol-I s-' < los dma mol-l s-' Ar Radicals RRNCHCOOH form in reaction (5).I 1 I (a) 0 / O ' I I I I I 2 4 6 8 10 [PtJ/10-5 rnol dm-3 T/"C FIG. 1.-Rate constant for reaction of MV+ with Pt hydrosols: (a) effect of the Pt hydrosol concentration on the decay of MV+ absorption at 600 nm; (b) effect of temperature on reaction rate.1068 REDUCTION OF WATER BY REDUCED METHYL VIOLOGEN We have used colloidal platinum hydrosols stabilized with citrate ions4 This catalyst shows better reproducibility and smaller activity loss than Pt-poly(viny1 alcohol). The decay rate of MV+ by reaction (8) does not depend on the initial MV+ concentration (3 x 10-'j-5 x mol dm-3) but increases sharply with Pt concentra- tion, fig. 1 (a).The same behaviour has been noticed earlieP for centrifuged colloidal Pt stabilized by poly(viny1 alcohol). A platinum-electron adduct formed in reaction (8) is the same as the intermediate for the reaction of hydrated electrons with Pt particles (9) The rate constant for reaction (9) is 7 x lo9 dm3 mol-l s-l. We were not able to identify the transient absorption spectrum of Pt-. In the experiments where EDTA was added to scavenge OH radicals, and after the recombination of EDTA radicals (k = 1.5 x lo9 dm3 mol-1 s-l), only very weak absorption below 500 nm was obtained with a lifetime of ca. 1 s. We assumed that this absorption is probably derived from intermediates on the platinum. However, since the transient absorption is very weak, the procedure used in this study does not permit unequivocal identification of the intermediates and observation of their kinetics.The rate of decay of MV+ by reaction (8) increases with increasing solution temperature [fig. 1 (b)]. From the temperature dependence, the activation energy for reaction (8) was calculated to be 12.5 kJ mol-I, which corresponds to the diffusion- limited reaction. We have found that the hydrogen evolution in reaction (1) is enhanced at higher temperature. Hydrogen formationincreases by a factor two withincreasing temperature from 20 to 70 O C . However, even at 70 O C only 60% of MV+ ions produce hydrogen according to the stoichiometric relation of reaction (1). The rest probably disappears in the hydrogenation process on platinum [eqn (2)].eLq + Pt + Pt- + H20. HYDROGEN CONCENTRATION EFFECT In the presence of hydrogen, reaction (8) decreases markedly. The increase of hydrogen concentration decreases the rate constant value. The rate of MV+ decay at a concentration of 2.5 x loh5 mol dm-3 Pt in a hydrogen-saturated solution was found to be 0.6 s-l. Temperature appears to affect remarkably the rate of this reaction and the increase of the rate constant is not only a result of the temperature effect on the reaction activation, but also due to the nature of platinum surface in the presence of hydrogen. The inhibition of electron transfer to Pt most likely reflects the formation of some products which are strong poisons for Pt or effect the coagulation of Pt particles. MV+ and MVO are produced upon saturating an MV2+ and Pt colloid containing solution with H,: (10) (1 1) We were able to detect a MV+ steady-state concentration of 3 x mol dm-3 by measuring the absorbance at 605 nm; the hydrogenation process of MV2+ proceeds via formation of MV+ ions in a solution of 5 x mol dm-3 MV2+, 1 x mol dm-3 EDTA and 2.5 x lo-* mol dm-3 Pt saturated with hydrogen.This hydrogenation process leads to complete destruction of MV2+ ions. On the other hand, the formation of MV+ and MVO decreases the catalytic activity of the platinum. Coagulation of Pt hydrosols can be observed when a colloidal solution of Pt containing MV2+ stands for more then 30 min after saturation with H,. MV2+ + iH2 + MV+ + H+ MV+ + $H2 + MVO + H+.M. T. NENADOVIC, 0.I. MICIC AND R. R. ADZIC 1069 On principle, the presence in the solutions of any species more strongly adsorbed than hydrogen will change the mechanism of reactions (1) and (2). We chose Pb2+ ions since they adsorb strongly on the Pt surface17 and modify the catalytic properties of platinum.18 MV+ ions do not react with Pb2+ when Pt hydrosols are absent (table 1). However, we found that Pb2+ ions affect electron transfer from MV+ to Pt. The presence of 5 x mol dm-3Pb2+ ions increases the reaction rate from 0.6 to 3 x lo3 s-l in a hydrogen-saturated solution. Though the second step, hydrogen evolution, is inhibited when Pb is adsorbed on platinum,18 our experiments show that reduction of water on Pt by MV+ ions is a complex process sensitive to many factors.Electron transfer from MV+ to Pt in the presence of hydrogen is a real condition which can be expected in the case of continuous evolution of hydrogen from water. Note that under this condition, undesirable reactions, such as the hydrogenation of MV2+, take place and drastically decrease the catalytic activity of platinum. B. V. Koryakin, T. S. Dzhabiev and A. E. Shilov, Dokl. Akad. 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