| Hydrogen peroxide (H2O2), as one of the most important bulk chemicals, has been regarded as an environmentally friendly alternative applied in industry, such as pulp and paper bleaching, textile, foodstuff, pharmacy, waste water treatment, exhaust control, chemicals synthesis, aerospace and electronics industry. China has become the leading country of H2O2 consumption in the world. The research and development of environmental chemical process based on the application of H2O2 has been one of the most important issues in China. Nowadays, more than 95% of world’s H2O2 is produced by an indirect process involving sequential hydrogenation and oxidation of an anthraquinone (AO process). However, the AO process suffers from several drawbacks, such as use of complex process involving multiple sections, high energy consumption and periodic replacement of costly quinone-derivative due to non-selective hydrogenation, which potentially leads to the pollution problem. The AO process is economically viable only for a large-scale production (>40,000 tons per year), consequently, H2O2 has to be concentrated (50-70 vol%) and then transported to the point of use, when it has danger of explosion due to the instability of high concentration of H2O2. For economic and environmental concerns, H2O2 synthesis directly from H2 and O2 (the DS process) is regarded as a promising alternative to the AO process and the H2O2 product with concentration of 1-10% perfectly meets the requirements from the most of practical applications. The on-site realization of the DS process can benefit the other chemical process, such as propylene epoxidation and waste water treatment. However, the DS process is still not commercialized, due to the low efficiency of catalysts and the security concerns with the mixing of H2 and O2 in feedstock. Large numbers of papers have been concentrated on the study of relationship between the structure and performance of catalysts. Moreover, the DS process is proposed as a model reaction, used for the studies for structure and properties of catalysts and activation of O2 and H2.The aim of this dissertation is to establish the corresponding relationships of structure and performance over different active sites of the catalysts in the DS process. For the supported heterogeneous catalysts, the active sites can be classified as single site of metal, surface site of single or several metal atoms layer, surface site of metal alloy, interface site between metal and oxide and interface site between two oxides. Based on the mechanism of activation of H2 and O2, H2O2 synthesis and the physical-chemical properties of Pd and supports, three catalysts including Pd/TiO2, Pd-Au/TiO2 and Pd/HAp were selected to study the relationships between structure and performance over different active sites. The catalytic performance was tested by using a semi-batch reaction system under the ambient temperature and pressure. Combined with multiple characterization techniques, the geometric and electronic structure was systematically studied. Conclusions are described as follows:1. Pd/TiO2 catalysts were prepared by an incipient wetness impregnation method with different Pd loading amounts. The H2O2 selectivity increased with a decrease in Pd loading amounts under the reaction conditions without diffusion resistance, which indicates that the performance difference resulting from the intrinsic structure of the catalysts. The loading amount shows strong effects on the nucleation and growth of Pd particles, leading to the difference of short-range and surface structure. In the presence of O2, the metallic Pd can be partially oxidized on the TiO2 surface by forming a Pd-PdO-TiO2 three phase structure. The relationship of structure and performance indicates that H2O2 formation likely occur at the interfaces of Pd and PdO domains where the Pd and PdO species act as the active sites for the dissociation of H2 and the non-dissociative activation of O2, respectively. The optimization of Pd/PdO ratio would enhance the selectivity and productivity of H2O2. Therefore, the reducible properties of support had strong effects on the interaction between Pd and supports. Moreover, the electronic structure of the surface Pd atoms had proved to change dynamically in different atmospheres.2. Based on the study of DS over Pd/TiO2 catalysts, the effects of Pd/Au ratio on DS over Pd-Au/TiO2 catalysts were studied. Pd upon alloying with Au could enhance the selectivity and productivity of H2O2, which is remarkably reduced with the excessive amount of Au. Pd monomer surrounded by Au atoms has proved to be the primary active site for H2O2 formation. Continuous Pd ensembles were assumed to be more active for H2O2 hydrogenation than monomeric Pd sites. The over-oxidation of H2 to H2O likely occurred at the interface between Au and TiO2. The reduction of PdOx at the interface domains could also produce H2O.3. The active sites for H2O formation existed on both Pd/TiO2 and Pd-Au/TiO2 catalysts, leading to a decrease in the H2O2 selectivity. In order to further increase the H2O2 selectivity, a novel catalyst system was developed by using the functional material hydroxyapatite as the support. Pd/HAp catalysts were prepared by combining with incipient wetness impregnation and ion-exchange methods. The catalysts with different Pd structures were characterized, including the mono dispersed Pd (zero dimensional structure), subnano-nano Pd clusters (size of 0.3-5.0 nm, single layer Pd clusters with 2D structure and Pd nanoparticles with corner and edge sites as dominant sites) and Pd nano crystals (>5.0 nm, slab sites as dominant sites). Due to the absence of adsorption sites for O2 on the Pd monomers and limited sites for simultaneously activating H2 and O2 on Pd dimers, both the Pd monomers and the Pd dimers show no activity for H2O2 synthesis. The electronic structure of the single layer and the double layer Pd clusters was strongly affected by the support, leading to the positively charged Pd clusters. It weakened the interaction between Pd and O2, but enhanced the H2O2 selectivity by the activation of O2 without dissociation. The DFT calculation demonstrates that the formation of OOH is more thermodynamically favorable on Pd10/HAp than Pda/HAp, which prefers the formation of H2O2 on the former. When the subnano Pd clusters grew up to larger Pd nano particles, the relationship between structure and performance for Pd/HAp is close to the DS over Pd/TiO2, while the H2O2 formed on the interface domains between Pd and PdO. When the particle size of Pd was larger than 5 nm, the H2O2 formation was determined simultaneously by the exposed facets and the modification effects of species adsorbed on the Pd surface. |
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