Abstract
In the present work, the thermal stability of metal - in particular platinum - supported catalysts and their catalytic application in CPOM as a high temperature test reaction was investigated. Aiming for the synthesis of a thermally stable supported nano-catalyst system, barium hexaaluminate (BHA) – a stabilized alumina – was chosen as support material, building the basis for a matrix in which platinum particles were incorporated. To generate such catalysts, a reverse microemulsion was used as a tool. The encapsulated water droplets in the micelles act as nano reactors in which particles are formed. Therefore, the reverse microemulsion provide an extraordinary environment for the synthesis of nano particles and a variety of possibilities to influence the properties of the generated material. The BHA support particles as well as the active platinum particles were generated at the same time to result in a unique catalyst system. The generated Pt-BHA consists of agglomerated spherical BHA particles building a network with a textural porosity in which the platinum particles are anchored. The material was investigated by sorption measurements, XRD and TEM to obtain information about the thermal stability of the material and the platinum particles in particular. An amorphous microstructure was found up to more than 1100 °C, helping to maintain a surface area of 100 m2/g after calcination at 1100 °C.
Varying the water content in the reverse microemulsion the particle shape of the supporting BHA was changed from needle like to spherical. Also the crystallization point of the BHA and the porosity of the BHA particle network were influenced by changes in the composition of the microemulsion. Upon heating, the platinum particles were found to be thermally stabilized by the supporting framework. High temperature in-situ XRD measurements reveal no sintering of single particles up to at least 900°C. By TEM and sorption measurements it was found that the platinum particles adapt to the size of the pores. By this the platinum particles can be stabilized to more that 1100°C resulting in a mean particle size not larger than 18 nm. Therefore, the Pt-BHA outranges any impregnated material with its extraordinary thermal stability.
Other metals were incorporated into the BHA matrix as well. Rhodium showed a similar thermal stability. Other materials formed oxide particles (e.g. Mo, Ru) and again other metals or metal oxides were incorporated into the BHA structure (e.g. Co, Mn).
Beside the reverse microemulsion, an alternative synthesis method – the metal oxide concept (MOC) – was applied, using surfactants to stabilize nanoparticles in an aqueous environment. The MOC resulted in supporting materials with large surface areas having a good thermal stability. However, platinum particles could not be stabilized to high temperature exposure.
The catalysts were tested for their stability and activity in CPOM at autothermal conditions and at temperatures up to 1100 °C. A good catalytic activity was found for the Pt-BHA, being higher than those of an impregnated monolith. As indicated by the catalyst investigations, the thermal stability of the Pt-BHA was much higher than that of impregnated systems. No deactivation was observed for the Pt-BHA under ‘normal’ conditions (CH4/O2 =2.0). Beside the improved stability, a larger number of active sites (per volume) and an improved heat management in the catalyst were found to be the main driving forces for the better activity of the Pt-BHA catalyst.
In comparison to other metals-BHAs Pt-BHA is the leading CPOM catalyst being on par with Rh-BHA and followed by Ru-BHA and Ir-BHA and Ni-BHA. Other metals - e.g. Co, Fe, Cr - and partially also nickel (only low Ni concentrations) were found to deactivate due to the incorporation of the metal into the BHA. Coking was only observed for Pd-BHA.
Overall, the extraordinarily good thermal stability makes the Pt-BHA an ideal catalyst for high temperature applications - perhaps beyond CPOM.
