| Summary: | 碩士 === 長庚大學 === 化學工程研究所 === 90 === Incorporation of a palladium membrane into a catalytic reactor enables the steam reforming reaction to have higher conversion than the limitation of thermodynamic equilibrium even with 10 fold shortening of space velocity according to literature report. In this study, we found that it could also speed up the forward reaction rate and turnover frequency when a palladium membrane was used in the catalytic reactor.
This experiment used G-66B as the basic catalyst and a 1%Pd/CuOZnO catalyst to study reaction rate of steam reforming of methanol with the WHSV ranging from 426 to 460hr-1 to control the conversion level between 5 to 14%. Palladium was chosen as the promoter in this study to explore the effect of reverse hydrogen spillover from active copper on sites to palladium in the steam reforming of methanol. It was reported that palladium or platinum dopant on silica supported catalyst promoted chemisorption through hydrogen spillover from these precious metal to the support; therefore, we expect the hydrogen atoms produced from methanol steam reforming reaction will reverse-spiltover to the palladium site and desorbs to the gas phase. This way the catalyst sites will be utilized in higher frequency, and the activation energy of reaction may also be decreased by the more efficient utilization of catalyst sites.
The above expectation was first born out by the experimental finding of lower activation energy with 1%Pd/CuOZnO catalyst over the parent CuOZnO catalyst, 22.8 KJ/mol vs. 57 KJ/mol. Furthermore, the turnover frequency of the reaction at 310℃ with the Pd-promoted catalyst was indeed found to be 7.6×10-3(1/sec-m2) compared with 6.2×10-3(1/sec-m2) using the parent catalyst. The reaction rate at 270℃ was found to be 85×10-3sec-1 and 49×10-3sec-1 with the 1%Pd/CuOZnO and CuOZnO, respectively. However, at 350℃, the rate constant became 162×10-3sec-1 and 245×10-3sec-1 with 1%Pd/CuOZnO and CuOZnO, respectively, this reversal of the reaction rate was attributed to the higher activation energy of the parent catalyst causing greater sensitivity of rate to the temperature hike.
Finally, we undertook a rate comparison of the convention steam reforming reaction with the Pd-assisted reforming reaction. The result was astonishing. Using CuOZnO as the catalyst in a catalytic reactor incorporating a Pd-membrane [nominal thickness:33mm, H2/N2 selectivity = 99.93 and permeance = 34mol/M2-hr-P1/2] the reaction rate increase from 151×10-3 to 228×10-3sec-1. Moreover, when 1%Pd/CuOZnO was used as the catalyst, the reaction rate increased from 136×10-3 to 278×10-3sec-1 when the catalytic reactor was inserted with the Pd-membrane.
In conclusion, we have found that hydrogen spillover bridging the gap between the catalytic sites with the surface of Pd-dopant or that of Pd-membrane. This unification of two separate entities with the low activation energized hydrogen spillover results in the rate enhancement or turnover frequency by allowing the active sites to be utilized in higher frequency and lower activation energy. We have found these phenomena, not only in the kinetic study shown in this thesis but also in the increased hydrogen chemisorption of catalysts incorporating Pd-dopant or enhanced permeation with the catalyst connecting to a Pd-membrane in a separate study of this laboratory.
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