Photodissolution of iron oxides 11: The lack of efficiency of thiocyanate MARTA I. LITTER AND MIGUEL A. BLESA' Departamento de Quimica de Reactores, Comisidn Nacional de Energia Atdmica, Avenida del Libertador 8250, 1429 Buenos Aires, Argentina Received August 3, 1989 MARTA I. LITTER and MIGUEL A. BLESA. Can. J. Chem. 68,728 (1990). Thiocyanate does not enhance, and even inhibits, the photochemical dissolution of iron oxides at 254 nm. At low SCN- concentrations, surface complexes )F~"'-scN- are formed only to a small extent due to the low affinity for the surface by the monovalent anion, and electron-hole pairs essentially recombine. At higher ligand concentrations, only the homogeneous photolysis of SCN- is observed. At 450 nm, there is no appreciable light absorption by the oxide but homogeneous photolysis of Fel"-SCN- complexes produces Fe", which dissolves the oxide by a reductive mechanism. Key words: iron oxides photodissolution, thiocyanate heterogeneous photochemistry, metal oxides surface complexation. MARTA I. LITTER et M~GUEL A. BLESA. Can. J. Chem. 68, 728 (1990). Le thiocyanate loin d'augmenter, inhibe plutbt, la dissolution photochimique des oxydes de fer, a 254 nm. A des concentrations faibles de SCN-, la quantitd de complexes superficiels de )Fe1"--SCN qui re forme est faible cause de la faible affinitC de,l'anion monovalent pour la surface et du fait que les paires de trous Clectroniques ne font essentiellement que se recombiner. A des concentrations plus ClevCes de ligand, on n'observe que la photolyse homogkne du SCN-. A 450 nm, l'oxyde n'absorbe pas beaucoup la lumiitre; toutefois, la photolyse homogkne des complexes F~["-scN- conduit a la formation de ~ e" qui dissou~l'oxyde parun mCcanisme rkducteur. Mots cle's : photodissolution d'oxydes de fer, photochimie hCtCiogene du thiocyanate, complexation superficielle d'oxydes mktalliques. [Traduit par la revue] Introduction Reductive dissolution of iron oxides in the presence of a variety of complexing agents has been the subject of a number of papers (1-7). The proposed mechanisms involve an interfacial electron transfer, and it is therefore not surprising that light absorption by the oxide may substitute for the reducing agent in thermal dissolutions, in the presence of adequate complexants and hole scavengers (8-14). In fact, organic carboxylate anions may play the role of both complexing agents and hole scavengers, and it is only because of kinetic factors that the implicit reducing power does not give rise to fast thermal dissolution at room temperature. Reductive thermal dissolution does, however, take place at higher temperatures (4, 5). In this view there is a close parallel between the thermal and photochemical behaviour of aqueous and surface )Fel"-L complexes (1 5). Electron transfer, either thermal or photo- chemical, may take place, generating Fe" + L' pairs that may either revert to Fe"' + L-, or further evolve to Feaq2+ + oxidation products of L. In this context, thiocyanate appears almost as a paradigmatic case. It is a very simple species, which forms well-characterized complexes with iron(II1) ( 16-1 9), and these complexes are known to exhibit both the thermal (20) and photochemical (21) reactivity mentioned above. Furthermore, another study from our laboratory2 showed its ability to promote thermal dissolu- tion of iron oxides through a reductive mechanism. In this paper, the photochemical behaviour of y-Fe203 suspended in thiocyanate solutions is described, and shown not to correspond to the expectations mentioned above. The photochemistry of thiocyanate ion in aqueous solutions has been explored in several papers. Ultraviolet irradiation of '~uthor to whom correspondence may be addressed. 'A. E. Regazzoni and M. Blesa. Unpublished results. SCN- solutions results in the production of cyanide, sulfur, and lesser amounts of sulfate. Two main reaction paths have been proposed (23-25): 111 SCN- + SCN' + e,,- followed by complex reactions in which cage recombination and further reaction with SCN- are central: [3] SCN' + SCN- (scN)~: [5] 3(SCN), + 4H20 -+ 7H+ + 5SCN- + HCN + ~ 0 4 ~ - The photochemistry and thermal decomposition of iron(II1)- thiocyanate complexes has also been much studied (20, 21). At 436 nm, absorption of light in the charge-transfer band of the complex produces Fe2+ and SCN', which then reacts with SCN- according to eq. [3] (21). The same intermediate (SCN),~ was proposed in the thermal dark reaction (20). The photooxidation of thiocyanate by semiconducting oxides has also been described (26-28). In the case of colloidal titanium dioxide, band-gap excitation by a laser leads to electron-hole pair formation (26); the hole migrates to the semiconductor interface and is transferred to SCN- , producing SCN'. In this system, Kamat (28) was able to observe directly the association of SCN- with the Ti02 surface from an absorption band at 295 nm. SCN- anions complex with Ti02 with an association constant of 350 mol-' dm3. In the case of iron oxides, Schumacher et al. (27) report an increase in the photocurrent of thin film electrodes after boiling in 0.5 mol dmp3 KSCN. They suggest that the complexing agent stabilizes surface Fe2+ ions on the oxide surface, thereby enhancing the charge-transfer step at the interface. Can. J. Chem. Downloaded from www.nrcresearchpress.com by 186.124.20.169 on 12/25/13 For personal use only.