Abstract
In this work, we studied how TiO2 and ZrO2 coatings enhance the CO oxidation performance of SiO2-supported Pt catalysts under conditions relevant to automotive emissions control. SiO2 was coated with metal oxides TiO2 or ZrO2 by sol-gel method and the subsequent Pt loading was done by incipient wetness method. The prepared catalysts Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 were compared with Pt/SiO2 and Pt/Al2O3 in fresh, sulfated, and hydrothermally aged states. The structure of the catalysts was characterized with BET, XRD, TEM, EDS, EXAFS, XANES, TPD and TPR to help interpret the CO oxidation performance. Higher dispersion, stability, and oxidation state of Pt were achieved on TiO2-SiO2 and ZrO2-SiO2 supports due to stronger metal-support interaction leading to superior CO oxidation performance compared to Pt/SiO2 and Pt/Al2O3. TiO2 and ZrO2 coatings introduced surface acidity but negligible basicity, which is a likely reason for the weak and low adsorption of SO2. The results suggest that the sol-gel coating of SiO2 with metal oxides could be an attractive strategy for designing automotive oxidation catalysts with enhanced performance such as low-temperature activity, sulfur tolerance, and hydrothermal stability.
Highlights
Diesel oxidation catalysts (DOCs) have been extensively studied due to their various roles in the emission control systems of diesel engine vehicles
The TiO2-coated Pt catalyst had a surface area of 193 m2/g which is almost identical to that of SiO2, indicating that a thin layer of TiO2 was formed on SiO2 surfaces
While the white-line intensity for Pt/Al2O3 and Pt catalysts supported on uncoated SiO2 (Pt/SiO2) increased slightly, that of Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 increased dramatically when exposed to the O2 flow. These results indicate that the Pt atoms dispersed on the TiO2- and ZrO2-coated SiO2 had a greater affinity towards oxygen leading to higher oxidation states of Pt
Summary
Diesel oxidation catalysts (DOCs) have been extensively studied due to their various roles in the emission control systems of diesel engine vehicles. DOCs such as Pt/Al2O3 catalyze the oxidation of carbon monoxide (CO), nitric oxide (NO), unburned hydrocarbons (HC) and the soluble organic fraction (SOF) of particulates [1,2,3,4,5,6,7,8,9,10,11]. DOCs are often exposed to temperatures above 800 °C created to regenerate downstream diesel particulate filters (DPFs) and/or desulfate NOx control devices. Sulfur poisoning of DOCs themselves is another significant technical barrier [2,3]. Alumina is a most widely used support for DOCs due to good thermal stability and interaction with
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