Effect of surface area on the rate of photocatalytic water oxidation as promoted by different manganese oxides.

Commercial Mn 2 O 3 , Mn 3 O 4 and MnO 2 and the same after thermal or ball-milling treatments have been examined as catalysts for the photocatalytic water oxidation reaction, using [Ru(bpy) 3 ] 2+ as photosensitizer and S 2 O 8 2− as sacrificial electron acceptor. Tests were performed in a bubbling reactor, allowing the calculation of the actual rate of O 2 evolution as a function of time from raw data (oxygen flow, concentration of dissolved oxygen, DO) through a model able to take into account mass transfer phenomena Hernández et al. [19]. A few parameters are proposed for measuring activity, and comparison among them is made. The activity per unit mass of commercial samples is Mn 2 O 3 > MnO 2 > Mn 3 O 4 , in agreement with the literature. Increase in the surface area brought about by milling correspond, as expected, to a steady increase in activity in the case of Mn 3 O 4 , whereas had no effect with Mn 2 O 3 . The markedly higher specific surface of Mn 2 O 3 and Mn 3 O 4 samples obtained by thermal treatment of MnO 2 and a home-made sample, respectively, correspond surprisingly to activities lower than low surface area ball milled samples. Reasons for this are proposed to be a different nature of the surfaces arrived at, because of the different preparation route. A similar study of the effect of surface area for MnO 2 specimens is prevented by their largely amorphous nature. Comparison of present data with those already reported gives further support to the bounty of the model taking into account mass transfer. [ABSTRACT FROM AUTHOR]