| dc.description.abstract | Diesel Oxidation Catalysts (DOC’s) are Pt-based catalysts that are known to convert toxic exhaust gases, such as CO, into the less harmful CO2 at 250 °C. At lower temperatures, the surface of the DOC is adsorbed with only CO, resulting in poisoning of the system. One solution to counter the poisoning, is to increase the temperature. This results in a more easily desorption of CO, to create more active sites for the adsorption of O2. However, advancements in technology (such as hybrid cars), have led to a decrease in the average temperature at which exhaust gases are released. This results in the lose of efficiency of the DOC’s and resulting in a poisoning of CO on the surface. Therefore, a new model catalytic system must be developed to facilitate CO oxidation at a lower temperature without the chance for CO poisoning. This could be achieved by adding a promotor element such as tin (Sn).
First in this study, catalytic powder samples were analyzed using operando Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFTS), to observe if Sn has a promoting effect. Then, the CO oxidation reaction was monitored using Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS), a gold core with a protective layer to amplify the Raman signal. A platinum- (Pt) and a Pt-Sn layer were used to protect the gold core to examine the effect of Sn as a promotor element on the CO oxidation reaction. Besides that, an alumina coating was also applied using the ALD-method to assess its effect on the CO oxidation reaction.
The results of the operando DRIFTS show a promoting effect of Sn on the decrease in initial CO oxidation temperature, 139 °C compared with 172 °C for the catalyst without Sn. The results of the operando Raman Spectroscopy also show a promoting effect of Sn on lowering the initial CO oxidation temperature, 195 °C compared with 215 °C for the catalyst without Sn. However, the formation of a core-shell structure was not observed with TEM and SEM-EDX analysis. In the TEM-image of the system with Sn, a grey spot was found next to the nanoparticles, which can be attributed to a metallic state of Sn. Besides that, the adsorption of CO was observed at higher temperatures for the catalyst with Sn compared without Sn. This is contradicted to the promoting effect of Sn on the initial CO oxidation temperature because if CO2 is formed at a lower temperature, CO should desorb more easily at a lower temperature, which is not observed. Finally, the promoting effect of Sn decreases as the thickness of the Al2O3-coating increases. A coating of 1.4 nm shows a promoting effect with Sn, but a coating of 2.8 nm does not.
In conclusion, Sn shows a promoting effect on decreasing the initial CO oxidation temperature in both operando Raman Spectroscopy as operando DRIFTS, even though the core-shell structure has not been observed. However, to protect the Au core and therefore the enhancement of the Raman signal, it is important to have a shell around the core. Therefore, more research needs to be done to optimize the synthesis method such as the use of different ratio concentrations Pt-and Sn-precursors. Additional information could also be obtained using techniques such as TEM-EDX or ICP element analysis. | |
