Oxalic acid at the TiO2/water interface under UV(A) illumination: Surface reaction mechanisms

MENDIVE C.; BREDOW T.; SCHNEIDER J.; BLESA M.A.; BAHNEMANN D.

JOURNAL OF CATALYSIS

ACADEMIC PRESS INC ELSEVIER SCIENCE

Lugar: Amsterdam; Año: 2015 vol. 322 p. 60 - 72

0021-9517

It is through the comparison of experimental results and theoreticalcalculations that the mechanistic details of several surface photoreactions initiatedupon UV(A)-illumination of adsorbed oxalic acid on rutile and anatase can beproposed. The absorption of light is found to be rather localized at surface Tiatoms and at the adsorbed species on both TiO2 polymorphs,respectively. Different surface complexes exhibit different photoreactivitiesand, consequently, each of them may follow a different reaction mechanism. Experimentaldata can be explained involving reactions such as the interconvertion of monodentateinto bidentate species which may further oxidize to CO2 or may even produce·OH radicals, while the reduction of monodentate species to the respective aldehyderesults in combination with the oxidation of a neighbouring adsorbed OH groupinto the formation of an adsorbed ·OOH radical. On the basis of the results presentedherein it is concluded that the direct action of the photocatalyticallyproduced electron/hole pairs on the adsorbed species is the primary step of thephotocatalytic reaction, whilst the intermediate formation of free radicalspecies followed by their reaction with an oxalate molecule can be regarded as asecondary process. Within the system described in this work, ·OH radicals only appearto be produced following the direct interaction of a hole with the adsorbedorganic compound but not with chemisorbed water molecules.