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The Investigation of Ethanol Steam Reforming over Hydrotalcite-derived Ni-based Catalysts

Author: ZengGuangMing
Tutor: LiYongDan
School: Tianjin University
Course: Industrial Catalysis
Keywords: Ethanol Steam reforming Hydrotalcite Perovskite Structuralreconstruction Shell-core catalyst Kinetics Intrinsic activity
CLC: TQ426.8
Type: PhD thesis
Year: 2012
Downloads: 21
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Abstract

Syngas （H₂+CO） is a key intermediate in the chemical industry for the productionof a wide range of valuable fuels and chemicals such as clean synthetic gasoline,methanol, NH₃and H₂. Ethanol steam reforming （ESR） is currently being considered asone of the best sustainable and economical alternative to fossil fuel-based processes forsyngas and H₂production. However, the severe environment of high temperature and theco-existence of ethanol and high partial pressure of steam in ESR process create asignificant challenge for the design of an effective Ni-based catalyst. On the other hand,most of the investigations of ESR focused on the development of the catalysts, only a fewones concerned the kinetics and mechanism of this reaction. Thus, this thesis is motivatedfirstly to prepare a novel and efficient core-shell Ni catalyst derived from hydrotalcite（HT）-like precursor. Secondly, effort is devoted to the kinetic study of ESR over aNi/Mg-Al catalyst and the investigation of the effect of Ni loading on the intrinsicactivity of this reaction.In chapter2, a shell-core Ni@Mg-Al catalyst has been prepared via thereconstruction of a HT-derived Mg-Al mixed oxide in a Ni²⁺nitrate solution. Resultsshow that a part of Ni²⁺can incorporate into the layered structure of the reconstructedhydrotalcite-like compound, but the others may be adsorbed and/or blocked by theforming flake-like sheets. At700oC, the H₂yield over one mole of Ni atomsdemonstrates that the shell-core catalysts with much lower Ni contents perform betterreactivities than that of the bulk15wt.%Ni/Mg-Al sample. The effects of temperatureand space-time on the product distribution over the best Ni@Mg-Al catalyst indicate thatmethane steam and dry reforming as well as WGS are the key reactions during ESRunder the conditions investigated. Moreover, the activity evaluations perfomed at700oCand very low space-time confirm the existence of C₂H₄during the reforming process.In chapter3, the innovative idea of combining perovskite-type and HT-derivedmixed oxides in a shell-core structure was explored and succeeded in the preparation of aLaNi（Mg, Al）O₃@Mg-Al catalyst through the reconstruction of a HT-derived Mg-Almixed oxide. Except some Ni²⁺, small amount of La³⁺could also incorporate into the HT- like structure formed during the reconstruction, affecting the electrostatic equilibriumbetween the layer and intra-layer, and further leading to the formation of large flake-likesheets and their flower-like gathering to balance the defect. After calcination at900oC, awell crystalline LaNi（Mg, Al）O₃perovskite-type mixed oxide is formed in the outer layerof the reconstructed particle. Due to the successive preparation procedure and appropriatecalcining temperature, a shell-core LaNi（Mg, Al）O₃@Mg-Al catalyst is accomplishedwith a clear interface between the core and the uniform shell layer. The effects oftemperature and time-on-stream on the product distribution of ESR demonstrate that theshell-core LaNi（Mg, Al）O₃@Mg-Al catalysts calcined at900oC have excellentreactivities and good coking resistance.In chapter4, temperature-programmed （TP） characterizations and kineticinvestigation were carried out over a2wt.%Ni/Mg-Al catalyst to reveal the possiblemechanisms of reaction and deactivation. The results of TP experiments primarilyillustrate the behaviors of CH₃CHO, C₂H₄, CH_xgroup, acetate and carbonate speciesduring ESR reaction. At500oC, the initial rapid deactivation at very low space-time ismainly due to the carbon deposition after the oxidation of Ni active metal. The effect ofspace-time demonstrates that C-C bond scission is significantly restricted without the aidof active metal and dehydrogenation and dehydration of ethanol are dominant reactionsunder the conditions examined. The kinetic investigation shows that an increase of P_H2Ostrongly inhibits the ethanol conversion rate. However, an increasing P_C₂H₅OHfirstlyenhances the ethanol conversion rate at partial pressures <3.0%, while it is insensitive toethanol at higher P_C₂H₅OH. The modeling of the collected kinetic data demonstrates thatthe dehydrogenation of adsorbed C₂H₅OH*to ethoxy species （C₂H₅O*） is the possiblerate-determining step for both ethanol dehydrogenation and dehydration.In chapter5, the effect of Ni loading on the intrinsic activity of ESR was investigatedover Ni/Mg-Al catalysts. The results of characterization show that the variation of theoverall acid-base properties of catalysts with different Ni loading is mainly due to the Nimetal on the catalyst surface. According to the Ni loading, the Ni particle size on thecatalyst surface is varied in the range of3-14nm. At500oC, the activity tests performedat very low space-time （0.03mg min/ml） show that the active Ni metal on catalysts withNi loading≤2wt.%are oxidized during the initial several minutes after reaction started.Therein, the dominant C-containing products produced during ESR are CH₃CHO and C₂H₄. On the other hand, although the carbon deposition which results in the initial rapiddeactivation, the Ni particles are stable on the catalysts with Ni loading≥5wt.%.Therefore, large amounts of CO₂, CO and CH₄are produced, while much less CH₃CHOand C₂H₄are generated during ESR reaction. Moreover, it was found that the quasi-steady state intrinsic activity （TOF） of the catalyst depends strongly on the Ni particlesize over catalysts with Ni loading≥5wt.%. An increase of Ni particle size,corresponding to a decrease of Ni metal dispersion, leads to an exponential decrease ofthe quasi-steady state TOF, indicating that the ESR is a structural sensitive reaction andthe overall conversion rate is limited by the C-C bond scission occurred on the edge andcorner active sites of the Ni particles.

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