Preparation And Catalytic Performance Of Cerium-supported Co-based Catalysts And The Mechanism Of Co-catalyzed Ethanol Steam Reforming

Ethanol steam reforming（ESR）reaction is a promising route of hydrogen production due to its sustainability and carbon neutrality.The hydrogen-production efficiency from ESR reaction depends mainly on the nature of the catalyst,developing low-cost and high-efficiency catalysts for ESR reaction is the focus of curent research.Non-noble Co-based catalysts have been widely studied due to their remarkable C-C bond cleavage capacities.However,there is still great demand for improving the catalytic performances and durability of most Co-based catalysts,meanwhile,the reaction mechanism of ESR reaction catalyzed by Co-based catalysts is still a lack of in-depth and systematic understanding.Therefore,developing effective strategies to improve the catalytic performance and stability and exploring the reaction mechanism of ESR reaction catalyzed by Co-based catalysts are the key to the development of high-effciency Co-based catalysts.In this dissertation,the catalytic performances of Co catalysts supported on different supports for hydrogen production from ESR reaction were studied,and they were improved by regulating the dispersion,valence and reducibility of Co species.The relationship between catalytic performance and structure of catalyst was studied systematically.The reaction mechanism was also explored by in situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）,in situ synchrotron radiation photoionization mass spectrometry（SR-PIMS）and density functional theory（DFT）calculations.1.The effects of supports and Co precursors on the catalytic performances for ESR reaction were investigated.The result showed that Co/CeO₂-3w catalyst supported on CeO₂ and Co/CeO₂-（N）catalyst prepared from Co（NO₃）₂·6H₂O exhibited excellent catalytic performances and stability due to the lower Co valence state.In situ DRIFTS studies indicated that supports and Co precursors had significant influence on the intermediates of ESR reaction catalized by Co-based catalyst,and Co/CeO₂-3w and Co/CeO₂-（N）were favorable for producing C1 products,signifying that the Co species in lower valence state had higher C-C cleavage capacities.2.The effects of preferred exposure of CeO₂ facets on catalytic performance of Co-based catalysts supported on CeO₂ were investigated.The result showed that the Co species supported on CeO₂-（M）with exposure preference of（111）facet had lower valence and higher dispersion than those supported on CeO₂-（U）with exposure preference of（100）facet.Therefore Co/CeO₂-（M）exhibited higher C-C bond cleavage capability and more excellent catalytic performance,the hydrogen selectivity reached as high as 97%at ethanol conversion of 100%,with an excellent durability during long-time（42 h）catalytic experiment.In situ SR-PIMS and DFT calculations revealed that Co site in lower oxidation state could effciently catalyze the C-C cleavage reaction,leading to a promoted the ethanol reforming efficiency for hydrogen production,thus improving the hydrogen-production efficiency of ESR reaction and stability of catalyst.3.The catalytic performance of supported Co catalyst for ESR reaction was adjusted by introducing different heteroatoms（Cu,La and Mn）into CeO₂ lattice to change the electron structure of CeO₂ support.Catalytic ESR reaction experiments demonstrated that the catalytic performance of Co-based catalyst was significantly improved by heteroatoms doping in CeO₂,and Co/Ce La O showed the highest catalytic activity and durability among the investigated catalysts.XPS and H₂-TPR studies indicated that remarkable electron-donating effect occured between the surface active oxygen Os from Ce La O support and the supported Co,resulting in the lower valence state and the higher reducibility of Co species in Co/Ce La O,thus achieving the higher catalytic performance for ESR reaction.4.To further improve the catalytic performance,La_xCe_1-xO_2-x/2 solid solutions were used as supports to fabricate Co catalysts for ESR reaction by optimizing Ce/La molar ratios.The result showed that Co/La_0.25Ce_0.75O_1.88 exhibited the highest catalytic performance and durability,the hydrogen yield reached as high as 80%at ethanol conversion of 100%,with an outstanding durability during 58 h stability test.XPS and H₂-TPR studies demonstrated that the valence state and reducibility of the Co species in Co/La_0.25Ce_0.75O_1.88 were well regulated via donation of electron density from the concomitant peroxide anions in La_0.25Ce_0.75O_1.88.In situ DRIFTS studies revealed that Co/La_0.25Ce_0.75O_1.88 showed a remarkable preference for producing C1products,the Co species in low valence state and with high reducibility resulted in highly efficient C-C cleavage and excellent catalytic durability.