An attempt to tailor methane oxidation behaviors over Pd/Al2O3 systems through the concerted fashion of Zn and alkaline-metal promoters

Aiming to manipulate the properties of Pd/Al2O3 catalysts towards CH4 oxidation, trace of Zn and alkaline-earth metal (X = Mg, Ca, Sr and Ba) were dual-doped into mesoporous alumina. This was mainly inspired by their multi-functionality, in terms of the feature of semiconductor structure from amphoteric oxide ZnO, the higher metallicity and electron-donating effect from alkaline-earth metals, and the ability from both Zn and alkaline-earth metals to generate spinel phase with alumina. Eventually, the electron density throughout oxygen and metal atoms underwent rearrangement, the interfacial interaction and the surface work function of catalysts were altered. This in turn gave the distinction in the nanoparticle sizes and reducibility of palladium oxides, surface-concentration and stability of active PdO, distribution of surface oxygen species, and surface acid-base functionalities along the series of catalysts. Results demonstrated that the trade-off of these entire properties allowed Pd/ZnMg-MA with maximum light-off activity, whilst affording minimum drop in methane conversion over both Pd/ZnMg-MA and Pd/ZnBa-MA not only during long-term online test but also when water was injected into the feed.