Investigation of the Performances of TiO2 and Pd@TiO2 in Photocatalytic Hydrogen Evolution and Hydrogenation of Acetylenic Compounds for Application in Photocatalytic Transfer Hydrogenation

Abstract

The development of effective bifunctional catalysts demonstrating high performance in both photocatalytic hydrogen evolution and selective hydrogenation of unsaturated compounds is of great interest for photocatalytic transfer hydrogenation. In this work, TiO2 and Pd@TiO2 catalysts were studied in two separate processes: photocatalytic H2 evolution and conventional hydrogenation reactions. Photocatalytic properties of titanium dioxide synthesized by a simple precipitation method were compared with those of commercial ones. Commercial anatase with a lower agglomeration degree showed better activity in H2 evolution. Further modification of the commercial anatase with Pd resulted in increasing its activity, achieving an H2 evolution rate of 760 μmol/h gcat. The Pd catalysts supported on different TiO2 samples were tested in hydrogenation of acetylenic compounds. The activity of the Pd@TiO2 catalysts was found to be dependent on the photocatalytic properties of TiO2 supports. XPS studies of Pd catalysts indicated that commercial anatase with better photocatalytic properties provided easier reduction of Pd2+ to active Pd0 particles. The Pd catalyst supported on commercial anatase demonstrated the highest activity in the hydrogenation process. The WC≡C rate achieved 2.6 × 10–6, 9.0 × 10–6 and 35.7 × 10–6 mol/s for hydrogenation of 2-hexyne-1-ol, 5-hexyne-1-ol and 2-hexyne, respectively. The selectivity of the catalyst to target olefinic compounds was 94–96%. In addition, the hydrogenation rate was found to be significantly affected by reaction conditions such as hydrogen concentration and solvent composition. The WC≡C rate decreased linearly with decreasing hydrogen concentration in a H2:He gas mixture (30–100 vol%). Performing the reaction in 0.10 M NaOH ethanolic solution resulted in increasing the WC≡C rate and selectivity of the process. The Pd catalyst was reused in an alkali medium (NaOH in ethanol) for 35 runs without significant degradation in its catalytic activity. Thus, the results obtained in this work can be useful in photocatalytic transfer hydrogenation.