In many catalytic systems, the activity and selectivity of supported metal catalysts or extended metal surface catalysts would be affected by the metal surface structure, and this phenomenon is called structure sensitivity. Generally, structure sensitivity is led by the change of geometric and electronic properties of the metal on the surface. The variation of metal nuclearity and metal-support interactions are effective ways to change the geometric and electronic properties of the supported metal catalyst, leading to different types of the active sites exposing on the support that would take effect on catalyzing the reaction.
In this work, a series of supported Ir catalysts (on MgAl2O4 and SiO2) with different particle sizes less than 3 nm were utilized for hydrogenolysis of n-butane and ethane to study the structure sensitivity as well as the potential reaction pathways. The results indicate that the activity of n-butane hydrogenolysis increases as Ir particle size increases in the small particle size range (0.7–1.4 nm) and then drops when the Ir particle size further increases and the Ir single atoms might be inactive for hydrogenolysis after the post-reaction analysis. The selectivity of n-butane hydrogenolysis is dominated by central and one terminal C–C bond cleavage on the n-butane molecules at low temperature range. The selectivity to central C–C bond cleavage is highly dependent on the size of Ir and increases with a decrease in particle size down to ~1.4 nm but remains constant with further decrease in size. The hydrogenolysis of ethane shows a similar trend in the small size range but the activity is much lower than n-butane, which supports the low level of series reaction pathway in the case of n-butane hydrogenolysis.
In addition to Ir nuclearity, the effect of electronic properties was also studied on another series of Ir catalysts supported on ZnAl2O4, in which zinc replace the magnesium within the same spinel structure. The characterization results including HAADF-STEM and volumetric CO chemisorption show the difference of Ir nuclearity in the subnanometer regime and nanoparticles (~1.4 nm), while XPS and DRIFTS indicate the difference of electronic properties from metal-support interaction on the two Ir catalysts with the same nuclearity but reduced at different temperatures. Acetylene hydrogenation is structure sensitive on Ir/ZnAl2O4 catalysts and the activity and selectivity are mainly determined by Ir nuclearity instead of the difference in electronic properties. The Ir single atoms and subnanometer clusters are more selective to the target product of C2H4 but less active than large Ir nanoparticles as there might be more π-bonded adsorption than di-σ bonded adsorption for C2H2 on the Ir single atoms and subnanometer clusters. / Doctor of Philosophy / The supported metal catalyst is a kind of effective substance that could help increase the reaction rate when being properly utilized in the reaction. From the industry point of view, the best thing is to maximize the catalyst productivity and minimize the expense so that the economic benefit could be magnified. The catalyst effectiveness in a certain reaction might be different when the surface structure of the catalyst varies. Usually, only the fraction of the surface metals could take effect. As the particle size of the catalyst decreases, the fraction of the surface atoms that contain active sites drastically changes, leading to a different catalytic performance and probably lower cost with improved efficiency for metal utilization. Therefore, it is very significant for the researchers to study the reaction structure sensitivity on the same series of catalysts with different particle sizes. Also, by understanding the reaction mechanism and fundamentals of the catalytic system, it would be possible for the researchers to rationally design the catalysts aiming at higher efficiency and lower cost.
In this work, the reaction of hydrogenolysis that cleaves the C–C bonds within the alkanes molecules was studied on the supported Ir catalysts (Ir/MgAl2O4 and Ir/SiO2) with different particle sizes ranging from mostly single atoms, subnanometer clusters to nanoparticles. For n-butane hydrogenolysis, it is found that the selectivity to the target product of ethane is weakly dependent on particle size when smaller than 1.4 nm but decreases as the size further increases. Meantime, the activity is highest on the catalyst with surface-average particle size of 1.4 nm. Therefore, Ir size of ~1.4 nm is optimum for activity and selectivity to ethane.
The series of Ir/ZnAl2O4 catalysts was tested for structure sensitivity by another probe reaction, semi-hydrogenation of acetylene. The adsorbed acetylene molecules could be hydrogenated by adding two hydrogen to form the adsorbed ethylene before desorption or further hydrogenation to form ethane. Our results show the Ir single atoms and subnanometer clusters are more selective to the target product of ethylene but less active than the large nanoparticles. With the understanding of structure sensitivity, researchers are able to rationally design the catalysts based on their necessity for certain reactions.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111829 |
| Date | 23 March 2021 |
| Creators | Zhang, Xiwen |
| Contributors | Chemical Engineering, Karim, Ayman M., Xin, Hongliang, Nagaki, Dick A., Cox, David F. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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