The exploration of less expensive materials for the direct synthesis of hydrogen peroxide

The research presented in this thesis describes the direct synthesis of hydrogen peroxide from H2 and O2 using supported palladium based catalysts. The direct synthesis of hydrogen peroxide offers a more straightforward and sustainable alternative to the current industrial anthraquinone autoxidation (AO) process. Au-Pd bimetallic catalysts have been proved to be highly active for the direct synthesis process. The work presented in this thesis attempted to produce less expensive catalysts through adding cheap secondary metal to Pd as an effective substitute to Au or using an effective preparation for a low metal loading of Au-Pd nanoparticles. In addition, a comprehension of the actual active sites over bimetallic and Pd monometallic particles for H2O2 direct synthesis was also attempted. The first part of this work aims to explain an interesting phenomenon – an increase of activity for H2O2 direct synthesis and a decrease of hydrogenation of H2O2 over carbon supported Ni-Pd bimetallic and Pd only catalysts after both hydrogen peroxide synthesis and storage under ambient conditions. Based on the results of XPS, XRD and CO-chemisorption integrated with previous publications, it was concluded that (i) both the reaction of hydrogen peroxide direct synthesis and catalyst storage led to an decrease of particle dispersion; (ii) relative to the active sites on high energy surfaces/small particles of Pd (0), those on low energy surfaces/large particles are more selective for H2O2 synthesis, as the latter demonstrates lower activity of dissociative adsorption of O2 and H2O2. The role of secondary metal-Ni added to Pd was also investigated for H2O2 direct synthesis in the thesis. For carbon supported Ni/Pd catalysts (including Ni monometallic, Pd monometallic and Ni-Pd bimetallic catalysts), the addition of Ni to Pd enhanced catalytic activity and selectivity for H2O2 synthesis. The results of MP-AES, XPS, XRD and TPR implied that metallic Pd may sit on the top of Ni oxides with a dissolution of metallic Ni in Pd to some degree. Electron transfer from Ni to Pd probably also occurred which was inferred by XPS analysis. The role of Ni in Pd for H2O2 direct synthesis was Preface III also investigated over TiO2 supported catalysts which led to an enhancement of H2O2 productivity, H2 conversion rate and H2O2 selectivity relative to Pd only catalyst. Based on the results of XPS, TPR and STEM, it was concluded that inactive Ni species diluted Pd sites as individual Pd atoms which are the selective active sites for H2O2 direct formation. The next part of the study addressed a modified impregnation method (MIm) for the preparation of Au-Pd nanoparticles. These nanoparticles have been proved previously by STEM which are well dispersed homogeneous particles because of excess amount of Cl- ions in the preparation. As a consequence, the resulted catalyst demonstrated a superior activity than conventional impregnation method (CIm) analogues even the latter loaded with a quintuple metal loading. Through tuning Pd metal loading in 1 wt% Au-Pd and Pd only catalysts for H2O2 direct synthesis, two typical phenomena were observed in general: (i) an enhanced synergistic effect of Au and Pd by MIm than CIm and (ii) a rise of H2O2 productivity based on the mass of Pd loading with the addition of Au in 1 wt% Au-Pd MIm catalysts. As the possible formation of homogeneous Au-Pd alloy, an increase of H2O2 productivity based on Pd with the increase of Au content is probably out of the ensemble effect from the secondary metal