Abstract
In this study, xCuO-CeO2 mixed oxide catalysts (Cu weight ratio x = 1.5, 3, 4.5, 6 and 15 wt.%) were prepared using solution combustion synthesis (SCS) and their catalytic activities towards the methane (CH4) oxidation reaction were studied. The combustion synthesis of the pure CeO2 and the CuO-CeO2 solid solution catalysts was performed using copper and/or cerium nitrate salt as an oxidizer and citric acid as a fuel. A variety of standard techniques, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were employed to reveal the microstructural, crystal, thermal and electronic properties that may affect the performance of CH4 oxidation. The CuO subphase was detected in the prepared solid solution and confirmed with XRD and Raman spectroscopy, as indicated by the XRD peaks at diffraction angles of 35.3° and 38.5° and the Ag Raman mode at 289 cm−1, which are characteristics of tenorite CuO. A profound influence of Cu content was evident, not only affecting the structural and electronic properties of the catalysts, but also the performance of catalysts in the CH4 oxidation. The presence of Cu in the CeO2 lattice obviously promoted its catalytic activity for CH4 catalytic oxidation. Among the prepared catalysts, the 6% CuO-CeO2 catalyst demonstrated the highest performance, with T50 = 502 °C and T80 = 556 °C, an activity that is associated with the availability of a fine porous structure and the enhanced surface area of this catalyst. The results demonstrate that nanocrystalline copper-ceria mixed oxide catalysts could serve as an inexpensive and active material for CH4 combustion.
Highlights
The catalytic oxidation of methane (CH4 ) is an important combustion reaction that has received great attention in recent years for its importance in energy and heat production [1,2,3,4,5].the capability to find nonconventional oxidation processes to achieve the combustion of CH4 into water and carbon dioxide at lower temperatures and reduce gaseous pollutants, such as carbon monoxide, is important for the environment [6,7]
Solution combustion synthesis (SCS) has received a great deal of interest because of its ability to yield high-surface area materials, with ease of scalability, minimal preparation steps and almost no post-synthesis treatment is needed, significantly reducing the time needed for preparation and processing, allowing a simple and rapid obtainment of solid products [44,45,46]
A series of CuO-CeO2 solid solution catalysts was prepared using the solution combustion synthesis method with Cu inserted into CeO2 lattice
Summary
The catalytic oxidation of methane (CH4 ) is an important combustion reaction that has received great attention in recent years for its importance in energy and heat production [1,2,3,4,5].the capability to find nonconventional oxidation processes to achieve the combustion of CH4 into water and carbon dioxide at lower temperatures and reduce gaseous pollutants, such as carbon monoxide, is important for the environment [6,7]. Solid-solution materials based on transition metals and rare-earth oxides, such as copper and ceria, have drawn great attention as heterogenous catalysts due to their low cost and good activity for catalytic oxidation reactions [16,17]. Ceria (CeO2 ) is a rare-earth metal oxide with unique versatile properties. It has been exploited in a wide variety of applications, such as in gas sensing, solid electrolytes, oxygen membranes, environmental chemistry and heterogenous catalysis [18]. CeO2 exhibits significant oxygen release/storage capabilities because of the fast redox interchange of the associated Ce4+ and Ce3+ ionic species, which are related to the nature of the surface defects [19]. The incorporation of a foreign metal such as lanthanide or transition metals onto the CeO2 lattice can change its chemical and electronic properties and is expected to enrich the redox properties of CeO2 in the final metal/oxide composition, leading to higher oxygen mobility and active sites, as well as improved catalytic activity [22,23]
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