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Currently, for the industrial ethylene epoxidation α-alumina supported silver catalysts are the only catalyst of choice. We demonstrate a novel method to produce these catalysts with different silver particle sizes, but without changing other key parameters that may affect the catalytic performance such as support specific surface area or metal precursor. α-Alumina was impregnated with a silver oxalate solution, and was subsequently dried and treated in different gas atmospheres and at different temperatures to tune the silver particle sizes in the range of 20–500 nm. Particles of 20 nm exhibited a lower turnover frequency than particles of 70 nm and larger, which exhibit a constant turnover frequency, in accordance with results in literature. However, the selectivity, when measured at constant conversion, was particle size independent. This is the first time that the effect of the particle size on the selectivity of ethylene epoxidation is reported at constant conversion. This was made possible by a new method of producing supported silver catalysts, which we expect that is also applicable for silver catalysts with other supports and for the preparation of other supported metal catalysts.

The decomposition of hydrogen peroxide (H2O2) on Pt, PtO and Au catalysts has been investigated in the presence of nitrogen, propylene and hydrogen. H2O2 formation on the catalyst is known to be a key intermediate step for the direct synthesis of propylene oxide (PO) from hydrogen, propylene and oxygen. Therefore, during this reaction, H2O2 is in contact with the catalyst on which it is produced, propylene and hydrogen. In this work we investigate the effect of the simultaneous presence of a metal catalyst (Pt, Au) and these gases on the H2O2 decomposition. The presence of hydrogen favors the decomposition of H2O2 over all the studied catalysts. This is attributed to the combination of direct decomposition and hydrogenation reactions. Furthermore, hydrogen changes the catalyst from an oxidized to a more metallic state accelerating the H2O2 decomposition. We also observed the positive effect of propylene on decreasing the decomposition activity of Pt and Au. The experimental results were used to estimate reaction rate constants. The obtained rate constants hinted to changes of the catalyst during the decomposition as the main reason for reaction rate changes. This was confirmed to happen for Pt but not for the Au catalyst. The decrease of H2O2 decomposition on Au in the presence of propylene is due to their interaction where propylene blocks the Au active sites responsible for the H2O2 decomposition.

Gold nanoparticles supported on a MgCuCr2O4 spinel catalyze the aerobic oxidation of propylene to acrolein. At 200 °C, the selectivity is 83% at a propylene conversion of 1.6%. At temperatures above 220 °C, propylene combustion dominates. The good performance of Au/MgCuCr2O4 in selective propylene oxidation is due to the synergy between metallic Au and surface Cu+ sites. Kinetic experiments (H2 addition, N2O replacing O2) show that the reaction involves molecular oxygen. DFT calculations help to identify the reaction mechanism that leads to acrolein. Propylene adsorbs on a single Au atom. The adsorption of propylene via its p-bond on gold is very strong and can lead to the dissociation of the involved Au atom from the initial Au cluster. This is, however, not essential to the reaction mechanism. The oxidation of propylene to acrolein involves the oxidation of an allylic C–H bond in adsorbed propylene by adsorbed O2. It results in OOH formation. The resulting CH2–CH–CH2 intermediate coordinates to the Au atom and a support O atom. A second C–H oxidation step by a surface O atom yields adsorbed acrolein and an OH group. The hydrogen atom of the OH group recombines with OOH to form water and a lattice O atom. The desorption of acrolein is the most difficult step in the reaction mechanism. It results in a surface oxygen vacancy in which O2 can adsorb. The role of Cu in the support surface is to lower the desorption energy of acrolein.