Selective catalytic reduction of NO by C3H6 over Cu(x)Co(y)Ce(z)O oxides derived from LDHs

Title : Selective catalytic reduction of NO by C3H6 over Cu(x)Co(y)Ce(z)O oxides derived from LDHs

Abstract:

Nitrogen oxide (NO_x) emissions from stationary and mobile sources posd a serious threat to human health and the atmosphere air quality. Selective catalytic reduction (SCR) technology is widely employed to eliminate NO_x due to its demonstrated high efficiency in converting NO_x to nitrogen gas. Since hydrocarbons and NO_x typically coexist in the exhaust flue gas, researchers have focused a great effort on the SCR of NO utilizing C₃H₆ as a reductant (C₃H₆-SCR), which allows for the simultaneous removal of both of the two species of gases.  Two-dimensional layered materials called layered double hydroxides (LDHs) as catalyst precursor have the advantages such as excellent thermal stability and high dispersion of the active metal species with small particle size, etc.  A series of Cu_(x)Co_(y)Ce_(z)-LDH precursors were synthesized using a hydrothermal method in one step, followed by calcination to create Cu_(x)Co_(y)Ce_(z)O catalysts. Subsequently, the C₃H₆-SCR reactivity, catalyst stability, and the resistance to sulfur oxides were investigated in a fixed bed micro-reactor. The surface physicochemical properties of the catalysts were characterized by different techniques, such as XRD, TEM, XPS, BET, H₂-TPR, NH₃-TPD, etc. In-situ DRIFTS study was conducted to check the gas adsorption and the intermediates formed in the SCT reaction.  The C₃H₆-SCR reaction mechanism was finally proposed. Results proved, as compared to the bimetallic catalysts Cu_(0.64)Ce_(0.36)O and Co_(0.70)Ce_(0.30)O, the trimetallic catalysts exhibited superior C₃H₆-SCR performance. Among them, the catalyst Cu_(0.21)Co_(0.48)Ce_(0.31)O attained 90% N₂ selectivity and 95% NO conversion at 225 °C. Meanwhile, the Cu_(0.21)Co_(0.48)Ce_(0.31)O catalyst showed better sulfur resistance than the bimetallic catalyst.  The excellent activity of the catalyst could be attributed to the synergistic effect between Cu, Co, and Ce. The XRD results showed that the synergistic effect between the metals was because of the formation of the solid solution between Cu, Co, and Ce, which promoted the interaction and dispersion of the three active components. EDS mapping further proved this point. According to the XPS data, Cu and Co had a redox reaction that increased the number of oxygen vacancies favorable for NO adsorption and dissociation. The redox properties between Cu and Co were further confirmed to improve the catalytic activity by H2-TPR. Additionally, the results of BET and NH3-TPD demonstrated that the SCR catalytic activity was promoted by the right quantity of acidic sites and the specific surface area. In situ DRIFTS results showed that the predominant adsorbed species produced upon NO and C₃H₆ adsorption were formate and acetate, as well as monodentate and bidentate nitrates. In the SCR-C3H6 reaction, C₃H₆ was first oxidized to formate and acetate, while NO was oxidized to monodentate and bidentate nitrates. The nitrogen-containing organic intermediate R-NCO is then formed by the reaction of the NO oxide species and C₃H₆ oxide species, and it is further hydrolyzed to yield R-NH species. Lastly, R-NH produces N₂, CO₂ by a redox reaction with NO and O₂.

Biography:

Dr Su is a professor at the School of Environmental Science and Engineering, Donghua University, China.   Currently he serves as the chair of Department of Built Environment and Energy Engineering. He received his Ph D from Zhejiang University, China, in 2000. He worked in Department of Chemical Engineering, University of Mississippi, USA as visiting professor during 2006-2007. He has published 3 academic books, 5 textbooks and 1 academic book chapter, more than 100 research articles in SCI (E) journals, covering gas-solid suspension flow and separation, thermo-chemical conversion of solid fuels, selective catalytic reduction of NOx and new catalysts, CO₂ capture, etc.