In our previous research it was demonstrated that 5% palladium deposited onto alumina nanoparticles by precipitation was not only active, but one of the best catalysts reported for this reaction to date. Our hypothesis was that the high activity of this catalyst is due to the high number of low coordination sites, such as corners and edges that many nano-particles and some traditional porous supports have. Alumina was a good prospective nanoparticle-support due to its thermal stability and high surface area. However, there is no expectation that there is an alumina-specific interaction with the active palladium phase. The main objective of our current work is to determine if palladium supported on other nanoparticle oxides have activity in this reaction and determine which support properties are important for high catalytic activity.
5% loadings of Pd precipitated onto nano-particle CaO or Al(OH)3 had little or no activity. Nanoparticles of ZrO2 (with and without CeO2 doping), CeO2, ZnO, TiO2, MgO and SiO2 had varying degrees of activity. Reduction and CO adsorption was used to measure the dispersion of Pd in the catalyst. All of the active catalyst gave dispersions in the range of a commercially prepared 5% palladium on alumina to twice that of a commercial 5% palladium on carbon. In contrast, the CaO and Al(OH)3 catalyst had low dispersions, which may partially explain their lack of activity. CO adsorption measurements on the ZnO, CeO2, ZrO2, and CeO2 doped ZrO2 supports Pd catalysts gave high Pd dispersions relative to their surface areas. This is likely due to very strong Pd/support interactions for these materials.
The supports were titrated with CO2 and NH3 gas to determine the number of basic and acidic sites. No simple correlation between the number of acidic or basic sites and the catalytic activity or Pd dispersion could be found. This indicates that the catalyst activities are related to other chemical or structural properties.
While none of these supports consistently outperformed the nano-particle alumina catalyst, several, such as nanoparticle ZrO2 and MgO, had activities, that, if optimized, could match or exceed the performance of the nano alumina catalyst. Also, numerous other test catalysts have been found that can be probed to better understand the requirements for an active catalyst. Additionally, several of the supports gave high dispersions that may be suitable to other reaction systems.