Mechanism Study And Kinetic Modeling For Steam Reforming Of Ethanol On ZnO/Al₂O₃ Catalyst

Steam reforming of ethanol (SRE) has considered as a promising alternative sources for H2 production. This is probabily due to following reasons: (i) ethanol can be produced from renewable resources; (ii) ethanol represents net-zero CO2 emission. At present, many researchers are working on the catalysts for SRE. However, the mechanism of SRE is very complicated and always affected by many other factors (Heat and mass transport, e. g.), so that different researchers report diiferent results. In this paper, we prepared series of ZnO/Al₂O₃ and Pd and Ag promoted ZnO/Al₂O₃ catalysts by incipient wetness impregnation method, and investigated the factors which affect the structures and activities of the catalysts using X-ray diffraction and BET methods. The SRE results of the catalysts were evaluated at different reaction conditions, and the mechanism of SRE on the catalysts was also provided. The kinetic model was developed according to the provided mechanism and the parameters were solved based on the experimental results. Different types of?-Al₂O₃ were used as carrier and they were calcined at different temperatures prior to the impregnation. The results showed that the effect of catalyst preparing conditions on catalyst performance was:?-Al₂O₃ powder >?-Al₂O₃ pellet, calcination temperature: 700�C > 500�C. For the particle sizes of catalysts in the range of 60-120 mesh (125-250?m), the mass transport effects on the catalyst performance could be ignored, and the flow conditions in the reactor could be assumed to be plug flow as well.The 10 wt% ZnO/Al₂O₃ catalyst contained ZnO crystallites while ZnO containing domains on the catalysts with lower loadings were amorphous or smaller than ⁵ nm. The SRE results showed that H₂, CH₃CHO, C₂H₄, CH₃COOH, CO₂ and CH₃COCH₃ were the major products. All the catalysts were deactivated 160 min after the reaction began, and TG-DSC-MS results suggested that it was caused by site blockage by coking. For ZnO loading lower than 6 wt%, C₂H₅OH conversion decreased while ZnO loading was increasing. For ZnO loading higher than 6 wt%, C₂H₅OH conversion increased along with ZnO loading. C₂H₅OH conversion and H₂ selectivity reached 40% when ZnO loading was 10 wt%. C₂H₅OH conversion decreased as the H₂O:C₂H₅OH ratio was increased, suggesting that the SRE reaction had a positive order dependence on C₂H₅OH concentration. The highest H₂ production rate was occurred when H₂O:C₂H₅OH ratio was 3:1. C₂H₅OH conversion and H₂ selectivity increased as contact time was increased, and H₂ production rate reached 55?mol/kg/s while contact time was 0.38 kg/ (mol/s). Higher reaction temperature could have higher C₂H₅OH conversion and H₂ selectivity, however, it inhanced side reations and coking as well.Crystallites of noble metals were amorphous or smaller than ⁵ nm, indicating the dispersion of noble metals was well on ZnO/Al₂O₃ catalysts. The SRE results showed that Pd and Ag promoted catalysts had higher C₂H₅OH conversion and H₂ selectivity than unpromoted ZnO/Al₂O₃ catalysts, and CH₃COOH formation and C₂H₅OH dehydration were suppressed as well. For Pd and Ag promoted catalysts, Pd showed higher catalytic activity than Ag. All the catalysts were deactivated 160 min after the reaction began. TG-DSC-MS results showed that two types of coking were occurred on Ag catalyst, and Ag catalyst also suffered higher coke deposition that Pd catalyst. H₂, CH₃CHO, C₂H₄, CH₃COOH, CO and CH₃COCH₃ were the major products on Pd and Ag promoted catalysts, suggesting that C-C scission of CH₃CHO was favored on promoted catalysts which could produce CH₄ and CO.The results of steam reforming of intermediate on ZnO/Al₂O₃ catalyst showed that CH₃CHO was inactive when reation temperature was lower than 500�C, and it was converted to CH₄, CO and CH₃COCH₃ while temperature rised. CH₃COOH was converted to CH₃COCH₃ and CO₂ at 450�C, and CH₃COCH₃ was inactive even temperature was increased to 550�C. In situ FTIR showed that during SRE, ethoxy species, bidentate or bridge absorbed acetate species and C=O and C=C contained species were detected on ZnO/Al₂O₃ catalyst. The mechanism of SRE on ZnO/Al₂O₃ catalyst, involving C₂H₅OH dehydrogenation, CH₃CHO oxidation and CH₃COOH ketonization, was provided based on the experimental results. 2 wt% Ag/Al₂O₃ catalyst was inactive for H₂ production during SRE, indicating that ZnO played a very important role in Ag/ZnO/Al₂O₃ catalyst during SRE.A kinetic model was developed based on the reaction mechanism, and the overall SRE reaction rate expression was solved according to Langmuir-Hinshelwood assumption. Parameters in the rate expression were estimated by nonlinear regression, and the comparison between experimentally measured data and model predicted data showed that correlation coefficient was greater than 0.93.