Influence of different fuels on the properties of solution-combustion synthesized palladium/ceria catalysts for low-temperature methane combustion.

Abstract

Catalytic combustion of methane is a leading technology in energy production, emission prevention and gas clean-up. Its main advantage over traditional flame combustion is to carry out complete oxidation of fuel at low temperatures. Noble metals supported over high surface area supports are known to combust methane at low-temperatures. However, noble metals supported on oxide supports has been observed to result in high methane combustion activity at low temperatures. In recent studies on methane combustion, it was observed that the use of ceria as a support can significantly improve the catalyst activity. PdO supported on ceria is known to be a very active catalyst for methane combustion. However, this catalyst still suffers from poor activity at low temperature (below 673 K) and deactivation at high temperature (above 973 K) owing to the formation of metallic Pd from PdO particles. In this study, a comparison between solution combustion synthesis (SCS) and conventional incipient wet-impregnation catalysts was made and discussed for low temperature methane combustion. The PdO/CeO2 catalysts was prepared by the solution combustion synthesis method (SCS) with different fuels including oxalylhydrazide (ODH), citric acid monohydrate, urea, β-alanine, and tartaric acid were subsequently evaluated for low-temperature methane combustion. Each fuel is known to affects the physical and chemical properties of the catalyst which further influences the catalytic performances. To the best of our knowledge, the effect of fuels on the properties of SCS synthesised PdO/CeO2 catalysts, for low temperature methane combustion has not been reported. To evaluate these effects, several fuels such as oxalylhydrazide (ODH), citric acid monohydrate, urea, β-alanine, and tartaric acid were used. Furthermore, all prepared catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), laser Raman spectroscopy (LRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2-physisorption analysis and X-ray photoelectron spectroscopy (XPS). Furthermore, all prepared catalysts were tested for methane combustion activity at 4 vol% methane in air and GHSV of 15 000 h-1. The N2-Physisorption analysis revealed that SCS ceria had a high surface area and a smaller crystallite size when compared to the commercial ceria. From Raman spectroscopy, the SCS ceria was established to contain more defects and thus, contained a higher amount of lattice oxygen incomparison to the commercial ceria. The SCS ceria provided almost 2.5-fold higher methane conversion than the commercial ceria at 600 ̊C. When the 2 wt.% PdO/CeO2 catalysts were compared, it was revealed that the SCS synthesized catalysts had a higher surface area and more oxygen vacancies and thus, higher catalytic activity in comparison to the catalysts prepared using wet impregnation. The study of the effect of fuels revealed, that different fuels result in catalysts with different physical and chemical properties. The surface areas of the prepared catalysts were observed to decrease according to the following trend, Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-tartaric acid > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH. Raman spectroscopy, revealed that the amount of oxygen vacancies decreased in the order of Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid > Pd0.03Ce0.97O2-δ-δurea. The H2-TPR and XPS studies, revealed that the citric acid and ODH synthesized catalysts contained supported PdO in comparison to the other SCS synthesized catalysts. The T50 was observed to decrease in the following order Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid. However, the T100 was observed to decrease in following order Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid.