(23f) Metal-Support Interactions In High-Temperature Stabilized Nanocomposites

Metal-oxide nanocomposite materials have gained much attention in recent years as a possible configuration for stabilization of metal nanoparticles. In particular, high-temperature reactive environments, such as in high-temperature catalysis, pose a severe challenge for the utilization of nano-sized and/or nanostructured materials. However, such environments are of great importance, for example, for new and emerging energy technologies, ranging from hydrogen generation to clean combustion.

Chemical looping combustion (CLC) is an emerging technology for clean energy-production from fossil and renewable fuels. In CLC, an oxygen carrier (typically a metal) is first oxidized with air. The hot metal oxide is then reduced in contact with a fuel in a second reactor, thus combusting the fuel. Finally, the reduced metal is transferred back to the oxidizer, closing the materials ?loop?. CLC is an emerging ?green' combustion technology which allows for flame-less, NOx-free combustion without requiring expensive air separation, and produces sequestration-ready CO2-streams without significant energy penalty. However, the very demanding high-temperature, cyclic redox conditions impose a significant constraint on the selection of suitable oxygen carriers for CLC.

We have previously demonstrated that the embedding of metal nanoparticles into a ceramic matrix can result in unusually active and sinter-resistant nanocomposite materials which combine the high reactivity of metals with the high-temperature (~1000°C) stability of ceramics. Here, we extended our previous work onto a range of metal-oxide combinations to test the flexibility of our synthesis approach. In particular, the role of metal-support interactions in these nanocomposites and their effect on the stability of the resulting materials in multiple high-temperature redox cycles was investigated.

A range of different supports (hexaaluminates, MgO and SiO2) and metals (Ni, Cu and Fe) were synthesized via a reverse-microemulsion template sol-gel approach. The nanocomposites were characterized via TEM, XRD, EDX, BET, and chemisorption, and then analyzed with regard to their stability in reduction-oxidation cycles at temperatures between 700 and 900oC, mimicking the CLC environment. We find a wide range of different behaviors, depending on the specific metal-support combination. The formation of solid solutions results in complete deactivation of some nanocomposites by making the metal unavailable for reduction/oxidation, while for other metal-support combinations the formation of solid solutions appears to be reversible, keeping the metal available for the redox process. Furthermore, the melting point of the metal component has a significant impact on the stability of the resulting nanocomposite.

Synthesis, characterization, and results from the high-temperature redox tests will be discussed in detail in the presentation.