In many catalytic systems, the catalytic performance of a metal supported catalyst would be affected by the size and shape of the metals, and this phenomena is called structure sensitivity. Generally, the structure sensitivity effect is considered being led by a combination of geometric property change and electronic property change of the surface metals. The particle size variation is an effective way to change the surface structure of the supported metal catalyst, leading to different fractions of the active sites exposing on the support that would take effect on catalyzing the reaction.
In this project, a series of Ir/MgAl₂O₄ catalysts with different particle sizes that less than 2nm were utilized for ethane and n-butane hydrogenolysis reactions to study the structure sensitivity effect as well as the potential reaction mechanism. The results show that the activity on the catalysts with nanoparticles and mostly single atoms is evidently higher than that with the subnanometer clusters in both reactions, but the selectivity to the target product of ethane is not quite dependent on the particle size in the n-butane hydrogenolysis. After the fundamental analysis, it is proposed that the reaction mechanism of alkanes hydrogenolysis on the single atom catalysts including single active sites is probably distinctive from that generally accepted on the large particles containing multiple active sites from literature. For n-butane hydrogenolysis, the parallel reaction pathway of central C-C bond cleavage is dominant at low temperature or in the low conversion range. As the temperature going up or the conversion increasing at a certain temperature, the parallel reaction pathway of terminal C-C bond cleavage becomes more and more competitive. The series reaction pathway of hydrogenolysis on propane intermediate would always take place, but the level would be drastically enhanced when the conversion keeps increasing in the very high range. The C-C bond cleavage on the ethane product would not easily happen unless the conversion is close to 100%. / M. S. / Shale gas is natural gas trapped in shale rocks. Among all the countries that have abundant shale gas reserves, the US, benefited from advanced extraction technology, has the largest production of it. What’s more, the production rate will keep increasing at least for the coming 20 years, and shale gas will eventually become the largest source for natural gas. After extraction, there is a series of treatments shale gas has to go through before it can be utilized, catalytic reaction of alkanes (molecules found in most fuels) is one of these essential procedures. Although they are among the most important compositions of shale gas, different types of alkanes are difficult to separate and purify through traditional methods like condensation. To overcome this obstacle, this thesis focuses on exploring efficient catalysts to convert the n-butane (a straight chain alkane with 4 carbon atoms) to ethane (alkane with 2 carbon atoms). Two reactions are involved: n-butane hydrogenolysis and ethane hydrogenolysis.
Catalysts are some specific materials that can accelerate certain chemical reactions. The catalysts discussed in this thesis are tiny metal (iridium) particles attached to the support material (magnesium aluminate). In this study, the performance of these catalysts with different particle sizes were tested for the above mentioned hydrogenolysis reactions. The results show that changing the particle size of the catalysts considerably affects the rate of these catalytic reactions. The fundamentals of the catalytic system presented in this work can also help the researchers to rationally design the catalysts aiming at higher efficiency and lower cost in the future work.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/96605 |
| Date | 07 August 2018 |
| Creators | Zhang, Xiwen |
| Contributors | Chemical Engineering, Karim, Ayman M., Cox, David F., Xin, Hongliang |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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