Methane combustion was conducted over Pd catalysts supported on medium (ZSM-5, ferrierite, TNU-9, TNU-10) and large (USY, mordenite, beta) pore zeolites. Among the catalysts studied here, Pd/H-TNU-10 exhibited the highest combustion activity, together with excellent durability in the presence of water vapor. A combination of TEM and Pd K-edge XAFS measurements reveals the formation of highly dispersed PdO particles within the TNU-10 pores. This is also the case of Pd/H-ferrierite, while frequently there are large particles (60-100 Å ) on its outer surface. In contrast, most of PdO on zeolites other than these two medium pore materials were found to exist as aggregated particles ([50 Å ) on their outer crystallite surface. It appears that zeolite structures with intersecting 10-and 8-ring pores may be better supports for stabilizing nanometric PdO particles than those with the uniform 10-ring size or with the 12-ring pore system.
The synthetic details of a novel potassium gallosilicate natrolite with Si/Ga = 1.28, denoted PST-1, are described. The presence of K(+) and Ga with well-defined levels of concentration in the synthesis mixture is essential for directing its crystallisation. PST-1 transforms rapidly into TNU-6 under hydrothermal conditions, behaviour that contrasts sharply with its very high thermal and hydrothermal stability, which is unusual for a material of such a high Ga content. These stability issues are discussed and rationalized based on chemical composition, likely violations of Loewenstein's rule and the temperature of dehydration of as-made K-PST-1. The crystal structure of TNU-6 has been resolved through the combined use of synchrotron X-ray and electron diffraction data; it has the BaFeGaO(4) structure type with an additional radical 3a x radical 3a "GeAlO(4)" superstructure that arises from tilting of some of the tetrahedral units in all of the 6-rings.
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