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

Commercial Mn2O3,Mn3O4 and MnO2 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 S2O8 2- as sacrificial electron acceptor. Tests were performed in a bubbling reactor, allowing the calculation of the actual rate of O2 evolution as a function of time from raw data (oxygen flow, concen- tration 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 Mn2O3 > MnO2>Mn3O4, 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 Mn3O4, whereas had no effect with Mn2O3. The markedly higher specific surface of Mn2O3 and Mn3O4 samples obtained by thermal treatment of MnO2 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 MnO2 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.