Catalysis in Combustion

Investigation of Parameters Influencing the Activation of a Pd/γ-Alumina Catalyst during Methane Combustion

The progressive activation of a Pd(2 wt %)/gamma-alumina catalyst under the reaction conditions of catalytic combustion of methane (CCM) was studied. The reasons of this activation were investigated by XPS, CO-chemisorption, and HR-TEM. The removal of carbon from the surface cannot explain the observed activation process. Sintering of the palladium particles was detected but this parameter alone does not fully explain the activation process of the catalyst. HR-TEM imaging evidences (i) that PdO is present both in the fresh and the active catalyst and (ii) that the PdO nanoparticles sinter and restructure (surface roughening) during the reaction. Development of preferential faces was not observed. It is suggested that this restructuring may be responsible for the activation process by facilitating the formation of an active oxygen layer on the PdO surface. CCM on Pd/gamma-Al(2)O(3) depends on the thermal history of the catalyst and is a structure-sensitive reaction.

Thermal Coarsening of Supported Palladium Combustion Catalysts

An essential property of combustion catalysts is long-term (>8000 h) stability at high temperatures in an environment (approximately 1 atm of both oxygen and water vapor) that aggressively promotes sintering of the supporting oxide and coarsening of the active component. Extrapolation of accelerated coarsening rate measurements, determined from shorter exposures at higher temperatures, can be made with more confidence if the physical processes of the coarsening and sintering processes were well understood. The current work examines in detail the coarsening of a high-weight-loaded palladium catalyst supported by silica-stabilized alumina at 900 degrees C in such an aggressive environment. The results of this investigation showed that the Pd particle size distribution was consistently log-normal for time periods from 100 to 4000 h, the mean particle growth rate was roughly inverse second-order in mean particle diameter, and the support not only sintered but also underwent phase transformation. The results implicate both coalescence and Ostwald ripening as important coarsening processes.

Reverse microemulsion synthesis of nanostructured complex oxides for catalytic combustion

Catalysts play an important role in many industrial processes, but their use in high-temperature applications-such as energy generation through natural gas combustion, steam reforming and the partial oxidation of hydrocarbons to produce feedstock chemicals--is problematic. The need for catalytic materials that remain stable and active over long periods at high operation temperatures, often in the presence of deactivating or even poisoning compounds, presents a challenge. For example, catalytic methane combustion, which generates power with reduced greenhouse-gas and nitrogen-oxide emissions, is limited by the availability of catalysts that are sufficiently active at low temperatures for start-up and are then able to sustain activity and mechanical integrity at flame temperatures as high as 1,300 degrees C. Here we use sol-gel processing in reverse microemulsions to produce discrete barium hexa-aluminate nanoparticles that display excellent methane combustion activity, owing to their high surface area, high thermal stability and the ultrahigh dispersion of cerium oxide on the their surfaces. Our synthesis method provides a general route to the production of a wide range of thermally stable nanostructured composite materials with large surface-to-volume ratios and an ultrahigh component dispersion that gives rise to synergistic chemical and electronic effects, thus paving the way to the development of catalysts suitable for high-temperature industrial applications.