ABSTRACT
Catalysts can undergo structural changes during the reaction, affecting the number and/or the shape of active sites. For example, Rh can undergo interconversion between nanoparticles and single atoms when CO is present in the reaction mixture. Therefore, calculating a turnover frequency in such cases can be challenging as the number of active sites can change depending on the reaction conditions. Here, we use CO oxidation kinetics to track Rh structural changes occurring during the reaction. The apparent activation energy, considering the nanoparticles as the active sites, was constant in different temperature regimes. However, in a stoichiometric excess of O2, there were observed changes in the pre-exponential factor, which we link to changes in the number of active Rh sites. An excess of O2 enhanced CO-induced Rh nanoparticle disintegration into single atoms, affecting catalyst activity. The temperature at which these structural changes occur depend on Rh particle size, with small particle sizes disintegrating at higher temperature, relative to the temperature required to break apart bigger particles. Rh structural changes were also observed during in situ infrared spectroscopic studies. Combining CO oxidation kinetics and spectroscopic studies allowed us to calculate the turnover frequency before and after nanoparticle redispersion into single atoms.
ABSTRACT
In this study, a facile and effective route to prepare hybrid photocatalysts (paper-TiO2, paper-TiO2-AgBr and paper-AgBr-TiO2) has been reported. The preparation procedure consisted of the direct adsorption of the previously synthesized titania nanoparticles (TiO2 sol) to generate the TiO2 nanosphere and the immersion process in an aqueous suspension of AgBr to form the AgBr nanoclusters on paper fibers. The synthesis technology is economic, efficient, environmentally friendly and easy to implement even at industrial scale. A cellulose-based structure with well dispersed TiO2 particles of around 1 µm and a pseudo-liquid coating of Ag+ and AgBr species was obtained. All the prepared photocatalysts demonstrated effective photocatalytic performance in gaseous phase ethanol degradation with simulated sunlight illumination, through the direct mineralization to CO2 and the parallel reaction via acetaldehyde degradation. A relevant improvement in the photocatalytic activity was noticed when TiO2 was associated with AgBr nanocrystals, with a higher effect observed when AgBr was loaded onto the paper surface prior to TiO2. Ag-Ti interaction reduces the pair recombination rate and increases the available charge carriers generating reactive OH- radicals from both Ag-species and TiO2, and O2- radicals from Ag+-AgBr species, which would be involved in the ethanol degradation process.
