Preparation and characterisation of gold and palladium based catalysts for the direct synthesis of hydrogen peroxide

I The research presented in this thesis describes the direct synthesis of hydrogen peroxide from H2 and O2 using supported gold-palladium based catalysts. The direct synthesis process offers a green and sustainable approach compared to the anthraquinone autoxidation (AO) process, which is currently used on an industrial scale to produce >95% H2O2 worldwide. The work presented in this thesis is an attempt to examine the direct synthesis process in terms of determining optimum catalyst compositions for potential scale-up in the near-future. The primary aim of this investigation is centred on catalyst design and characterisation. The first part of this work is a catalyst optimisation study for 2.5 wt% Au-2.5 wt% Pd/TiO2, and involved changing the amount of water used in the catalyst preparation, in this case wet impregnation. It was found that the addition of small amounts of water resulted in approximately 100% enhancement in activity for TiO2-supported catalysts but not for carbonsupported Au-Pd catalysts. The rate of Au/Pd uptake was contrasted and it was determined that the isoelectric point of the support was highly influential. While the activity can be enhanced for TiO2-supported catalysts, both catalyst nanostructure and stability were detrimentally affected by the addition of water during the impregnation step. The second part of this work is focussed on understanding the precise nature of the acid pre-treatment effect, where treatment of a carbon support in dilute nitric acid prior to the impregnation of Au and Pd precursors can result in the complete switching-off of sequential H2O2 hydrogenation activity over the catalyst. Characterisation and heat treatment studies gave an improved understanding of the relationship between Au/Pd and the carbon support. The next part of the study addresses the use of a colloidal immobilisation method to pre-fabricate Au-Pd ‘designer’ nanoparticles onto supports and is accompanied by extensive advanced aberration corrected electron microscopy studies. The effect of acid pre-treating silica based supports is then considered for catalysts prepared by wet-impregnation, specifically the fact that acid pre-treatment of silica is required to induce synergy between Au and Pd metals for the direct synthesis of hydrogen peroxide. The final part of this work considers the effect of introducing a third metal into the catalyst design, specifically the addition of Pt to Au/Pd compositions. An extensive catalyst screening study is undertaken for Au-Pd-Pt/CeO2 catalysts