Direct synthesis of hydrogen peroxide using monometallic and bimetallic Pd based catalysts supported on TiO2

This thesis explores the direct synthesis of hydrogen peroxide (H2O2) from molecular hydrogen and oxygen using Pd-based supported catalysts, under conditions considered unfavourable to H2O2 stability, namely in the absence of external additives and at ambient temperature. The direct synthesis of H2O2 represents an attractive, more atomically efficient, and environmentally friendly alternative to the current industrial route to H2O2 via the anthraquinone oxidation process. However, several issues must be addressed before the direct synthesis process can be considered industrially viable. The main challenge of the process is associated with poor H2O2 selectivity of the highly active catalysts as they often display high rates of H2O2 degradation, through hydrogenation and decomposition pathway. To inhibit unwanted side reaction and increase selectivity of the catalyst, acid and halide additives are often applied to reaction solution, which leads to additional costs affiliated with removal of these additives, corrosion of the reactor vessel, and metal leaching from the catalyst; which decreases the lifetime and activity of the catalyst. In recent years a growing library of catalysts that display high H2O2 selectivity have been reported; however, these earlier studies have utilised complicated catalyst synthesis procedures and still rely on high precious metal loading and the use of sub-ambient temperatures to improve H2O2 stability. The aim of this work is focused on utilising conditions that are more likely to be industrially applicable and could result in a decrease in overall process costs. Additionally, novel catalysts were designed and synthesised with a focus on more effective and efficient utilisation of precious metals. The first part of this work sets out to optimise catalyst formulation and reaction conditions with a focus on monometallic Pd catalysts, prepared by an industrially relevant wet impregnation procedure. It was previously shown that monometallic Pd catalysts are highly active towards synthesis of H2O2, although they also typically demonstrate high activity towards competitive degradation reactions. The optimal metal loaded catalyst produced in the work achieved high H2O2 synthesis activity of 65 molH2O2 kgcat -1 h -1 , with no activity to the subsequent degradation of H2O2, while avoiding costly secondary metals or the need for external additives previously required to improve catalytic performance. The activity of the catalyst can be attributed to highly dispersed PdO species with mean particle size of 0.7 nm. The second part of this work is centred around bimetallic supported catalysts and how the addition of secondary metals (Au, Sn, Ni) affect catalytic performance towards H2O2 synthesis. The addition of secondary metals to monometallic Pd catalyst is well known to significantly enhance synthesis activity and selectivity of the catalyst towards H2O2 and inhibit sequential degradation reactions. To the date, AuPd supported catalyst have been extensively studied in the direct synthesis of H2O2, achieving enhanced catalytic performance through II synergistic effects and avoiding the requirement of external additives typically required for Pd-only catalyst. In a similar manner to AuPd systems, SnPd and NiPd catalysts have been reported to achieve high H2O2 synthesis activities. However, these catalysts have been hindered by the need for extensive and complicated preparation procedures and have typically utilised high metal loadings. Within this work, it is demonstrated that it is possible to synthesise SnPd and NiPd catalysts with high activities of 111 molH2O2 kgcat -1 h -1 and 55 molH2O2 kgcat -1 h -1 , respectively, offering comparable synthesis activities to well established AuPd catalysts, under reaction conditions that are unfavourable to H2O2 stability