Abstract
We report on the synthesis and testing of active and stable nano-catalysts for methane oxidation. The nano-catalyst was palladium/ceria supported on alumina prepared via a one-step solution-combustion synthesis (SCS) method. As confirmed by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HTEM), SCS preparative methodology resulted in segregating both Pd and Ce on the surface of the Al2O3 support. Furthermore, HTEM showed that bigger Pd particles (5 nm and more) were surrounded by CeO2, resembling a core shell structure, while smaller Pd particles (1 nm and less) were not associated with CeO2. The intimate Pd-CeO2 attachment resulted in insertion of Pd ions into the ceria lattice, and associated with the reduction of Ce4+ into Ce3+ ions; consequently, the formation of oxygen vacancies. XPS showed also that Pd had three oxidation states corresponding to Pd0, Pd2+ due to PdO, and highly ionized Pd ions (Pd(2+x)+) which might originate from the insertion of Pd ions into the ceria lattice. The formation of intrinsic Ce3+ ions, highly ionized (Pd2+ species inserted into the lattice of CeO2) Pd ions (Pd(2+x)+) and oxygen vacancies is suggested to play a major role in the unique catalytic activity. The results indicated that the Pd-SCS nano-catalysts were exceptionally more active and stable than conventional catalysts. Under similar reaction conditions, the methane combustion rate over the SCS catalyst was ~18 times greater than that of conventional catalysts. Full methane conversions over the SCS catalysts occurred at around 400 °C but were not shown at all with conventional catalysts. In addition, contrary to the conventional catalysts, the SCS catalysts exhibited superior activity with no sign of deactivation in the temperature range between ~400 and 800 °C.
Highlights
Methane (CH4 ), the main constituent of natural gas plays an increasingly important role in meeting future global energy demands [1]
Energy dispersive X-ray (EDX) analysis were conducted on the 5P5CA sample, Figure 2
Direct evaluation of the high-resolution transmission electron microscopy (HTEM) and the High-Angle Annular Dark-Field (HAADF) images showed that crystalline nanoparticle (NPs) of Pd/CeO2, in the range 1–50 nm, were observed on the Al2 O3 support, Figure 1b
Summary
Methane (CH4 ), the main constituent of natural gas plays an increasingly important role in meeting future global energy demands [1]. It has been concluded that the catalytic activities of Pd-based catalysts in CH4 oxidation depend on the redox properties of the support and the nature of interaction of Pd with the support [10]. For low-temperature applications, Pd-based catalysts supported on alumina or zirconia have been recognized as possessing high catalytic activities [18]. The development of the core-shell catalysts may satisfy the twin goals of high activity at lower temperatures and stability at higher temperatures. It has been reported that steam deactivation is due to oxide surface hydroxylation, which might slow down oxygen mobility, and reduce the methane decomposition catalytic activity [28,29]. The present study aimed to synthesize an economic methane oxidation catalyst that is active at low temperature and stable at high temperature under dry and wet conditions. The study aimed to quantify catalytic activity of the solution-combustion synthesis (SCS) catalyst and benchmark to that of a traditional catalyst
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