No abstract

Methane in the form of natural gas is increasingly used as a transportation fuel, but the treatment of methane in the exhaust is a challenge since methane is a potent greenhouse gas. Pd is one of the most active catalysts for methane oxidation. Previous work has shown that transformation of Pd into the oxide, and decomposition of the oxide to metallic Pd can occur as temperature is raised in an oxidizing atmosphere, causing profound changes in catalytic reactivity. Equilibrium thermodynamics predict that the phases Pd and PdO must be in equilibrium at a well-defined temperature and oxygen pressure, since the two phases are immiscible and do not form solid solutions. But catalytic data suggests the existence of metallic Pd under conditions where only PdO should be thermodynamically stable. In this study we have explored the Pd ↔ PdO transition at high temperature using in situ XRD, TGA and from TEM examination of Pd catalysts that were quenched in liquid nitrogen or in a heating TEM holder to prevent any changes in microstructure during cooling. Corresponding data was obtained during methane oxidation, helping shed light on the nature of the working catalyst. The results show that the oxidation of metallic Pd to PdO is kinetically-controlled at high temperatures, allowing Pd to co-exist along with PdO. We refer to these as metastable Pd ↔ PdO structures. TEM shows that Pd and PdO domains can co-exist within a single particle, forming a phase boundary but allowing both Pd and PdO to be exposed to the gas phase. This kinetically controlled oxidation of Pd explains why we do not see core–shell PdO–Pd structures at elevated temperatures

It is shown that self-supporting graphitic structures of specific shape can be grown in a variety of forms, from nanoscale to macroscale, on metal templates, in a fuel-rich mixture of ethylene and oxygen at temperatures between 750 and 900 K. The evidence presented suggests graphite can be grown in any shape created from catalytic metals (e.g., Ni) under the proper conditions of temperature and gas composition. Structures produced include macroscale bodies, centimeters in dimension, composed of micrometer-scale graphite elements such as graphite "foam" and regular graphite "lattices". Nanoscale hollow graphite spheres were also produced. The production rate in the apparatus employed was roughly shown to be 1 layer/s and was steady with time over several hours. The process of producing self-supporting bodies generally produces hollow graphite structures, as the underlying metal template must be removed by acid following the completion of graphite growth. The process is believed to be possible only in an environment, such as combustion, in which a high concentration of particular radical species is present in the vicinity of the template surface. The following process is postulated: (i) a single layer of graphite is formed from gas-phase radicals by the catalytic action of the metal template, (ii) additional graphite growth is "autocatalytic" and occurs via the decomposition of radicals on the surface and the incorporation of "free" carbon atoms, or other radical fragments, into "edge sites" on the graphite surface.