Tailoring the physical and catalytic properties of lanthanum oxycarbonate nanoparticles

Abstract

The synthesis of lanthanum oxide and its carbonate analogues has been performed by flame spray pyrolysis (FSP). Two different feeds have been studied: an organic solution and an aqueous/organic microemulsion. A key experimental parameter of FSP, the O2 dispersion, i.e., the flow rate of the dispersing gas in the FSP nozzle, exhibits an effect on the properties of the materials prepared. Increasing the level of O2 dispersion led to an increase in surface area and a decrease in mean particle size and basicity when a lanthanum containing organic solution was used as FSP feed. Lanthanum can form different phases, such as oxides, hydroxides, oxycarbonates and carbonates. The increase of O2 dispersion also produced a phase change, going from a mixture of type Ia and type II La2O2CO3 and La2O3 to pure La2O3. The use of an aqueous/organic microemulsion feed, which had a higher viscosity than the organic feed, resulted in materials with a lower surface area and a higher mean particle size than those prepared using the organic solution at the same O2 dispersion. In this case a mixture of type II La2O2CO3 and La2O3 was obtained. The materials were tested for oxidative coupling of methane (OCM). We were able to demonstrate that the OCM performance of the materials could be modified by changing the synthesis parameters. For example, lower O2 dispersion produced the highest CH4 and O2 conversions. We also demonstrated that on ageing the mean particle size remain stable; however, the phases do not, showing a new phase, La(OH)3, formation and resulting in an increase in OCM activity. While the OCM performances are modest they do demonstrate the power of this approach for controlled synthesis of lanthanum materials.