| Hydrogen peroxide is an important chemical, which is widely applied in many industries, such as textile, bleaching, paper and pulp, waste treatment, etc. The traditional manufacturing is based on the anthraquinone autoxidation process (AO process). However, the AO process using anthraquinone as the carrier for H2O2 formation by H2 and O2, is complex and not benign to the environment. The direct formation of hydrogen peroxide by hydrogen and oxygen in liquid phase has been attracting attention as a potential replacement. This is a clean technology since only water is produced as a by-product, and moreover it is amenable to on-site small scale production which avoids the problems from transportation and storage of hydrogen peroxide. However, the yields of hydrogen peroxide are low because of low feed gas solubility and side reactions including decomposition and hydrogenation of hydrogen peroxide. Most of the work in the current literature focuses on ways to improve the productivity of hydrogen peroxide by overcoming these problems.; In this thesis work, the formation of hydrogen peroxide was carried out in a gas mixture of H2-O2 (H2:O2:He=90:2:8, 90ml/min of H2 and 10ml/min of 20% O2 in He) in methanol containing 0.02M H2SO4 at atmospheric pressure. Two types of nanoscale gold catalysts supported on cerium oxide and titanium dioxide, Au/CeLaOx and Au/TiO2, were investigated for the direct formation of hydrogen peroxide. Gold was found to be essential for the formation of hydrogen peroxide, but it also catalyzes the competing decomposition and hydrogenation reactions. The bare supports, CeLaOx and TiO 2, did not show any activity. Compared to room temperature, low temperature (2-4°C) operation was found to give a higher productivity of hydrogen peroxide, but at the cost of a lower rate of hydrogen peroxide formation. The productivity improvement derived from the suppression of the rates of the side reactions (decomposition and hydrogenation) at low temperature.; In tests conducted in methanol, Au/TiO2 was consistently a more active catalyst for the formation of hydrogen peroxide than Au/CeLaO x containing the same loading of gold by weight. Extensive structural investigation of the fresh, used, pre-reduced, and uncalcined catalyst samples was performed to understand the reason for the higher activity of titania-supported gold particles. We found that the presence of gold nanoparticles was necessary for an active catalyst. Ceria is known to stabilize part of the gold as non-metallic oxidized clusters. This form is not active for the formation of hydrogen peroxide. However, this was not enough to explain the large difference in activity between the two systems, because only a small fraction (∼10%) of the total gold is in oxidized cluster form tightly bound to ceria. Characterization of the used catalysts identified that the ceria surface had changed during reaction and gold particles grew. The destabilization was caused by the combined effect of hydrogen peroxide and sulfuric acid, which dissolved the ceria locally. Extensive dissolution was not observed because of low solubility of cerium sulfate in the methanol medium. STEM/EDS analysis found amorphous cerium sulfate on the surface blocking the pores, as a result of which the surface area of ceria in the used samples was greatly reduced. Gold particle growth took place fast under these conditions and the activity dropped. Au/TiO2, on the other hand, was not destabilized in the reaction media and preserved a small gold particle size after reaction.; The overall conclusion of this thesis is that Au-titania is active for H2O2 and can be further developed for practical applications. The particle size of gold is important to be less than ∼5 nm for high activity. There was no effect of support on the observed activity of gold. Because of the partial dissolution of ceria in the methanol, hydrogen peroxide and sulfuric acid solution, ceria is not a practical support. However,... |
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