Complete oxidation of methane over Ag- and Cu-modified cerium and zirconium oxides

Low temperature oxidation of methane is an area in catalysis that, despite the large number of catalytic systems studied, does not yet have an adequate solution. The exhaust gas from natural gas burning devices (gas turbines and natural gas vehicles), contains unconverted methane which is emitted into the atmosphere (currently unregulated). Methane is the most refractory of all hydrocarbons, and its activation requires temperatures higher than the typical exhaust temperatures (350-400{dollar}spcirc{dollar}C).; In this work, ceria- and zirconia-supported Ag and CuO catalysts were studied for the complete oxidation of methane. Catalysts were typically tested under excess oxygen and high space velocity (72,000 h{dollar}sp{lcub}-1{rcub}).{dollar} The reaction kinetics were measured over selected catalysts. H{dollar}sb2{dollar}- and CH{dollar}sb4{dollar}- TPR and oxygen uptake measurements were used to characterize the catalyst reduction properties. XRD, STEM/EDX and HRTEM were used to characterize the catalyst structure. The oxidation state of various active species present was identified by XPS and UV-VIS DR spectrometry.; Ceria- and zirconia-supported Ag and CuO catalysts are very active and stable catalysts for the complete oxidation of carbon monoxide and methane. When ceria is used as an active catalyst support, its activity depends strongly on its structure. Nanocrystalline ceria, stabilized by dopants such as La or Zr, is highly reducible and structurally defective. Our results indicate that surface oxygen species present under reaction conditions are the active sites for methane activation. Activity in the complete oxidation of methane is related to ceria crystal size and reducibility of surface oxygen species. The addition of a transition metal (Ag) or a metal oxide (CuO) in low amounts increases the low temperature reducibility of ceria and enhances the catalyst oxidation activity.; On the other hand, both Ag and CuO are active catalysts for the complete oxidation of methane. These were studied separately on inert zirconia support. A strong correlation of the transition metal/metal oxide state and dispersion with the catalyst activity was observed. Isolated Ag ions and small Ag nanoparticles ({dollar}<{dollar}5 nm) are both poor catalysts for the complete oxidation of methane. The turnover frequency for methane oxidation was increased by an order of magnitude as the Ag particle size increased from 5 to 10 nm, as seen by HRTEM. Similar catalytic behavior was found in copper oxide-containing zirconia. Small copper clusters (few nm) which are highly defective and do not have the identity of CuO are less active than larger CuO particles for the complete oxidation of methane. In view of the importance of surface oxygen species in methane activation, the observed structure sensitivity of methane oxidation in both the supported metal (Ag) and metal oxide (CuOx) systems is discussed in terms of oxygen adsorption on the metal and reduction properties of the metal oxide, respectively.